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Project Description 1. INTRODUCTION 1.1 Underground Science and Dark Matter Search Experiment LUX: Critical questions [1,2,3,4] exist at the core of modern physics: What is dark matter? Does neutrinoless double-beta decay really exist? And do protons decay? To answer these questions, experiments must be conducted over many years in low background environments (such as deep underground or under water/ice) due to the infrequency of the events. A dozen underground laboratories are currently in operation around the world to advance research in this area. See summary of major underground labs and their depths on the left in Figure 1.

As the first step of this great effort, Sanford Lab in Homestake, South Dakota is hosting several early science programs including the Majorana Demonstrator neutrinoless double beta decay experiment and the Large Underground Xenon (LUX) dark matter search experiment [5]. The LUX experiment is primarily designed to look for signals from weakly interacting massive particles (WIMPs) [4]. It consists of a 350 kg liquid Xenon detector that is submerged in a 300-ton water tank. [See the plot on right in Figure 1.] The water tank is designed to shield gamma rays and attenuate neutrons that are produced in the rocks and surrounding materials. With the tank’s water monitored by 20 photomultiplier tubes (PMTs), the water shield itself is also a great detector that is sensitive to local small showers, muons, and other charged particles. The LUX collaboration is currently testing its detector system in the surface lab in Homestake. Deployment to the Davis Cavern at 4850 ft level (~ 4300 meter-water-equivalent (m.w.e.) overburden) is scheduled for the year 2012.

Figure 1. Left: A brief summary of major underground labs. Right: The LUX water shield conceptual diagram. The water shield is a cylinder, eight meters in diameter and six meters high, filled with purified water. Twenty photomultiplier tubes (PMTs) around the sidewall and on the bottom monitor Cherenkov light produced by relativistic charged particles in the tank. The cylinder hanging at the center is the two-phase Xenon detector. The cryogenic and circulating systems are on the upper level and are not shown in this diagram.

The PI joined the LUX collaboration in 2009 and has made substantial contributions in

experiment safety and hardware reviews, PMT calibration, and water shield simulation. LUX is approaching its data-collection phase: Now, more than ever, it is important to understand and precisely calculate the backgrounds in Davis Cavern. This CAREER award will support and advance the work of the PI on an unprecedented systematic calculation of the muon induced backgrounds in the Davis Cavern. The PI is well-prepared to make systematic comparisons between simulations and the data from the liquid Xenon (LXe) detector and the water shield, utilizing the LUX inner detector’s low energy threshold, capability of electron versus nuclear recoil discrimination, and low internal radioactivity level. (See Section 2, Preliminary Supporting Data.) This work not only will provide a benchmark in describing various backgrounds in the Sanford Lab, but also will benefit

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future underground experiments. (The PI is the lead on the LUX Collaboration’s cosmogenics; see letter of support and cooperation.) 1.2 Cosmogenic Background in Deep Underground: Despite the shielding from large overburden mass, two types of backgrounds still exist in deep underground labs: (1) radiations including low energy electrons, gamma rays, and neutrons from the decay of radioactive elements in the detector construction materials, the surrounding air, and rocks, and (2) secondary particles and radioactive isotopes produced by high-energy penetrating particles (mainly muons) originated from cosmic ray interactions in the atmosphere, the so called cosmogenic background.

This CAREER proposal focuses on a more detailed study of high-energy muons and the background radiations induced by them. High-energy muons can generate background signals directly in particle detectors through processes such as ionization, Cherenkov radiation, photonuclear reaction, and capture or decay. Take the Davis Cavern, for example, in which muons on the surface must have more than about one TeV (1012 electron Volts) to reach the cavern. The muon flux was estimated to be about 4.4×10-9 cm-2sec-1 in the cavern, with an expected mean energy of 320 GeV [6] on a spectrum spread over several orders of magnitude. High-energy muons can create complicated backgrounds that affect the experimental design, detector performance, and data analysis. For instance, in the dark matter search experiment LUX, high-energy neutrons produced by multi-GeV muons in the surrounding rocks may enter the LXe detector and mimic dark matter signals. Muons with kinetic energy less than 55 MeV (muon Cherenkov threshold energy in water) can be stopped in the water shield without producing Cherenkov light to trigger the water shield. Depending on their angle and position, muons with energy up to 400 MeV ~ 600 MeV can also be stopped in the inner detector after losing energy in the water shield. These “stopped” muons either decay (µ+/µ- to positron/electron and neutrinos) or are captured through the semileptonic weak interaction by a proton (µ- + p → νµ + n) or by a nucleus (µ- + (Z, N) → νµ + (Z−1, N+1)*). Since the µ- -capture cross-section increases rapidly with Z of the target elements [7], over long periods, µ--capture processes can also make contributions to cosmogenic production [8] in the detector construction materials and in Xenon.

It is well known that neutrons are a very important background in many underground experiments. For example, in double-beta decay experiments, neutrons at a few MeV and above can produce background gamma rays via inelastic scattering. Thermal neutrons also contribute to the gamma ray background through neutron capture. Neutrons at sub-GeV and GeV energies, although rare, constitute a background for proton decay and atmospheric neutrino experiments. Besides the low energy neutrons from heavy element fission, muons can produce neutrons over a wide energy range through processes including muon-induced spallation, photo nuclear interaction, and in muon induced cascades.

For many years, experiments have been conducted in major underground laboratories to measure muon and neutron fluxes and/or spectra [6, 9, 10]. With this data, great progress has been made in the flux parameterization and Monte Carlo simulation [9, 11, 12, 13, 14]. For the muon flux calculation, it is important to know the average rock composition and density between the lab and the surface. To calculate backgrounds in muon-induced local showers, one must know the rock composition, information that varies for different underground sites. Presently, Monte Carlo simulations of muon-induced neutron background using GEANT4 and FLUKA can predict the neutron event rate with an accuracy of about a factor of two below ~100 MeV [12, 13, 14, 15, 16], assuming accurate rock composition and surface muon profile. However, greater uncertainties still exist at higher energies. For example, at the 3650 meter-water-equivalent (m.w.e.) level, the calculated neutron yield starts to fall below the LVD data at about 100 MeV. It decreases further by nearly one order of magnitude at about 400 MeV [12]. Ideally, any simulation of muon propagation (with MUSIC or MMC for example) should be normalized using experimental data. However, compared to other major underground sites, few measurements of muon and neutron fluxes were measured in the Homestake mines except a few gathered three decades ago on the separation between muon pairs, the rates of multiple-muon events (with muon multiplicity from 1 to 4, a total of 7124 vertical muon events) in the Davis Cavern [17], and 37Ar production rates in potassium

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detectors at various depths [18] in the mines. The reported error in 37Ar production rate at 4850 ft level is as large as 100%. On the other hand, many improvements have been made in the simulation of particle interactions since then. Much more powerful computing capabilities also enable scientists to do full Monte Carlo simulations at higher energies. It is possible and important to make more systematic comparisons between simulation results and experimental data.

Most current underground background simulations start from an average muon flux on the surface. The advantage of this approach is that one can alter several parameters so that the muon profile accurately describes the mean muon flux and spectrum on the surface. Nevertheless, this approach does not include the following three important features in high-energy atmospheric muons, which are crucial for estimating cosmogenic systematics and uncertainties: 1) The modulation in high-energy muon flux associated with air shower development in the Earth

atmosphere. This feature is well-approved in several underground experiments including MACRO [19], MINOS [20, 21], IceCube [22], and LVD [23] and causes modulations in the trigger rate in those experiments (see Figure 2 for the example of LVD data). Accordingly, one expects similar modulations in the overall muon and muon-induced cosmogenic backgrounds. A systematic study of these modulations is of great interest also because the well-known annual modulations of event rate observed in the dark matter search experiment DAMA [24], which reported a modulation with relative amplitude of about 1.3±0.1% against an overall background counting rate of about 1 cpd/kg/keV at recoil energies between 2-6 keV. The phase in DAMA modulation is 144±8 days, with the maximum in early June. A careful simulation study will further clarify potential causes for these phenomena.

Figure 2. Annual muon flux modulation for energies great than 1.3 TeV observed by the LVD experiment. The LVD muon modulation had amplitude of 1.5±0.1 %, with a phase of 185±15 days and the maximum in early July [23].

2) The large fluctuations in air shower development and muon production in the atmosphere. The

amplitude and distribution of these fluctuations as function of muon energy play an important role in the calculation of the confidence level of physics events. An example of muon multiplicity in air showers of proton and iron primaries is given in Figure 3. One can see significant fluctuations from shower to shower over the full spectrum.

3) The high-energy muon bundle structure. For a long time, we have known that high-energy air showers can produce high energy muons in the form of muon bundles in which muons are highly collimated and closely packed in space [11, 25]. As shown in Figure 3, although the muon multiplicity formula can approximately describe the average muon bundle size, the bias increases at higher energies. A study has shown that transferring the energy carried by all muons in the bundle to a single muon in the simulation makes a huge difference in IceCube [26]. Nevertheless, almost all muon flux measurements have assumed single uncorrelated muons, which inevitably introduce errors into all cosmogenic calculations using average muon flux.

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Figure 3. Number of muons in the bundles as function of muon energy in 50 PeV proton showers at 30-degree zenith (lower black crosses) and 1 EeV iron showers at zero degree zenith (higher red dots). The open squares and circles along the curves are the average over all 200 showers at each muon energy. The curves represent Elbert formula [25].

The ultimate goal of this proposal is to provide a quantitative description of the important

features mentioned above -- namely, the modulation in high-energy muon flux associated with air shower development in the Earth atmosphere, the large fluctuations in high-energy muon production, and the high-energy muon bundle structure. The proposed work includes a full Monte Carlo simulation study of atmospheric muon flux, energy spectrum, prompt muon signature, and their correlation with atmosphere seasonal variations and cosmic ray composition. The PI will compare the simulation results with data from the LUX and other underground experiments. Doing so will not only achieve a better understanding of the LUX data, but will also identify differences among various simulation models and improve the model-dependant systematics calculation. (See Section 3.) This proposal also includes science education and outreach activities designed to improve the undergraduate astroparticle physics curriculum and to promote the involvement of underrepresented groups in physics (See Section 4). 2. PRELIMINARY SUPPORTING DATA: RESEARCH AND EDUCATION OUTREACH EXPERIENCE WITH PREVIOUS NSF GRANTS PI Bai became a faculty member at South Dakota School of Mines and Technology (SDSMT) in August 2009. Since then, he has received two NSF grants: (1) NSF Grant 1041890: 08/01/2010~07/31/2011, $7,200, PI. This grant supported the “2010

Workshop on Major DUSEL Physics Topics, Rapid City, SD” (October 1-3, 2010, chaired by Bai). Fifty-two (52) participants from six countries participated in the workshop. During the workshop, experimental and theoretical physicists discussed most recent progress in dark matter and neutrino physics. Participants offered views about several important experimental projects including DAMA/LIBRA (dark matter, Italy), LUX (dark matter, Sanford Lab), XMASS (dark matter, Japan), Majorana (neutrinoless double beta decay, Sanford Lab), LBNE (neutrino physics & proton decay, Sanford Lab), Super-Kamiokande (neutrino physics & proton decay, Japan), IceCube (high energy neutrino astronomy, the South Pole), Pierre Auger Observatory (extremely high energy cosmic ray, Argentina). The PI presented the status of background calculations and summarized “next steps” for Sanford Lab and the future US Deep Underground Science and Engineering Laboratory, (DUSEL). The workshop website, including the agenda and presentations are available at http://odessa.phy.sdsmt.edu/~bai/dusel.php. The success of the workshop not only helped the PI establish a direct connection with prominent scientists in this field but also helped him to collect comprehensive comments on key issues in the study of various backgrounds for deep underground experiments.

(2) NSF Grant 0963536: 09/15/2010~09/14/12, $860,000, Co-PI. This project, titled “Research Laboratory Infrastructure Improvements in Electrical Engineering and Physics Building at the South Dakota School of Mines and Technology" resulted in the renovation of several of the PI's

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laboratories (computer, simulation and research) in the Electrical Engineering and Physics Building at SDSMT. A mid-term interim report that includes the Architects Draft Program Study was submitted to NSF in March 2011. When completed in 2012, the renovation will assist the PI and his students in carrying out the simulation work and establish space for an education and outreach satellite station at SDSMT proposed in this CAREER proposal.

Dr. Bai has long and rich research experience in astroparticle physics before his faculty career. In the last decade, PI Bai was supported by two major NSF grants. While not a PI or Co-PI on either grant, his work provided the most important basis for the success of the projects and substantially helped the science publications: 1) SPASE2 experiment (“South Pole Air Shower Experiment-2”, No. 9615101, 05/01/1997-

01/31/2001, $665,000, PI: Thomas Gaisser, Co-PIs: Todor Stanev, Paul Evenson; “Continued Operation of the South Pole Air Shower Experiment-2”, No. 9980801, 07/01/2001-06/30/2004, $719,855, PI: Thomas Gaisser, Co-PIs: Todor Stanev): The SPASE2 experiment [27] was an air shower experiment located at the South Pole. Bai first worked as a winter-over scientist at the South Pole for this experiment and AMANDA (Antarctica Muon and Neutrino Detector Array) from 1998-1999. Later, he worked on virtually every aspect of this experiment, including operation, data analysis and simulation. Importantly, Bai completed the muon survey for AMANDA optical modules with SPASE2-AMANDA coincident events [28]. In the cosmic ray composition study [29] and point source search [30] with SPASE2 data, he made significant contributions by reconstructing air shower events and producing all the Monte Carlo events.

2) IceCube experiment (“IceCube Startup and Construction Project”, No. 0236449, 08/01/2002-03/31/2011, $201,914,198, PI: Francis Halzen; “Air Showers in IceCube”, No. 0602679, 06/01/2006-05/31/2010, $750,000, PI: Thomas Gaisser, Co-PIs: Todor Stanev, David Seckel): Bai joined the inchoate IceCube R&D in 2000, making important contributions to the surface array (IceTop) design, ice Cherenkov detector R&D, simulation [31], and IceCube DOM calibration [32]. After the first IceCube string was successfully deployed, he led several calibration and performance verification tasks using IceTop and in-ice coincident data. Some of the results were reported in journal publications or professional conference proceedings [33, 34, 35, 36]. Bai was the first to measure the muon flux from vertical to nearly horizontal on the surface at the South Pole and compare with Monte Carlo simulations [37]. In order to understand the energy loss of high-energy muons or muon bundles in deep ice, he led a careful Monte Carlo study on muon bundle properties and energy loss. The preliminary results were reported in [38]. Bai also served as a member of the IceCube publication committee (total 6 members) between 2007 and early 2010. Since the cosmogenic background radiations are produced by high energy particles in high

energy cosmic ray air showers, Bai’s experience with both measurement and simulation of cosmic ray air showers and high energy muons is directly related to the proposed research work. Some of his work on muon measurement and simulation [37, 38] and very recently his study of atmosphere effect in air shower development and muon production [39] will benefit the proposed work.

Working with those NSF projects has also made Bai well adept in using and managing computer clusters, which is essential to this CAREER project. He has been using clusters for data analysis and simulation work in IceCube since 2002. He also worked closely with the computer system administrator at Bartol Research Institute, including managing the needs of computing power, memory size, disk space, and the installation and upgrade of analysis and simulation packages.

In addition to his research activities, Bai participated and led several educational outreach activities for projects including SPASE2, AMANDA, IceCube and LUX. For example, at the presentation for IceCube at the “Antarctic Treaty Meeting Displays” at the Maryland Science Center in April 2009, the PI displayed posters and IceCube hardware. He presented the science and construction of IceCube to several hundred visitors. At the “Engineer's Week” in 2010 and 2011 in Rapid City, SD, the PI led seven sessions focusing on dark matter detection in Sanford Lab. More

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than 120 middle school students participated in the interactive sessions. He also lectured on cosmic ray physics, detection of muons, and search for dark matter at the EO programs, including the lecture series for Davis-­‐Bahcall scholars in 2009 and 2010 summers. 3. PROPOSED RESEARCH

In order to obtain a quantitative description of the modulations, fluctuations, and muon bundle structure, a full Monte Carlo study must be carried out starting from cosmic ray primary particles. Although many useful simulation tools have been developed, including CORSIKA [40] for air shower simulation, MMC [41] or MUSIC [42] for muon (and lepton) propagation through matter, and GEANT [12, 43] or FLUKA [44] for detailed detector simulation, systematic work is needed for each experiment site. Besides the high statistics of the Monte Carlo events, the proposed plan will: (1) parameterize local atmosphere and the rock overburden, (2) survey the results of different cosmic ray composition models, various interaction models, and (3) carefully compare them to data. In order to shed light on cosmic ray physics and benefit future deeper site, we must study prompt muons that dominate the muon flux at energies greater than about 50 TeV. The PI proposes to pursue the following research foci over the next five years: 3.1 Systematic study of high energy muons in air showers and their characteristics in Sanford Lab This work involves multiple steps in order to achieve the best resolution in the simulation and provide benchmark results. First, in preparation for the full Monte Carlo simulation, the PI (and his student) is optimizing the parameterization of the atmosphere overburden in the region of Rapid City/Lead and at the South Pole in order to compare with data from LUX and IceCube. To obtain the best resolution, we are using the most recent CORSIKA release that allows variable atmosphere layer boundaries. The improvement can be seen on the left plot in Figure 4. The parameter values for the fixed boundary fitting (red curve on the plot) are cited from CORSIKA user guide. It is obvious that the variable boundary fitting (blue curve) agrees better with the MSIS [45] overburden profile (black curve, almost fully covered by the blue curve) than the fixed boundary fitting. Since the highest energy muons are more likely produced in the first several interactions high in the atmosphere, it is important to have the atmosphere model parameters re-tuned to better fit the data at a higher altitude.

Figure 4. Left: The fitting to overburden extracted from MSIS [45] using fixed (red) and variable (blue) atmosphere layer boundaries. The fitting uses the atmosphere overburden data (black) at the South Pole extracted from MSIS. Right: The overburden at Rapid City in January (blue) and July (red) in 2010. Values on the y-axis are relative to the averaged overburden over 31 days of July data [46].

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It is well known that the atmosphere overburden profile varies not only at different locations but also during different time periods. An example of the difference between January and July overburden data [46, both balloon and satellite data are used] is shown on the right entry in Figure 4 [39]: There is obvious separation between the blue and red lines indicating the overall change in the atmosphere overburden between specified time period. The spread of lines in the same color is due to the day-by-day variations in one month. In order to have the monthly variations and fluctuations within each month included in the simulation, the PI will parameterize the average, the maximum, and the minimum overburden profiles for every month.

Several cosmic ray composition models cover the interested energy range of this study, i.e. from a few tens of TeV up to about 1 EeV. To obtain a quantitative measure of the systematics, close attention will be paid to the variations due to different composition assumptions. We are planning to use the Horandel polygonato model [47] and the parameters in this publication with the following ansatz describing the energy dependence of the flux for particles with charge Z,

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dΦZ

dE0(E0) =ΦZ

0 E0γZ [1+ ( E0

E Z)ε c ](γ c −γ Z ) /ε c

and are the absolute flux and the spectral index. The flux above the cut-off energy

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E Z is

modeled by a second and steeper power law characterized by and . ARGO (one of the best experiments for the composition study in the “Knee” region due to it high altitude); a future LHAASO project will produce more data to improve the composition measurement. An IceCube group in University of Madison-Wisconsin is also improving the cosmic ray composition models using IceCube data. The progress in those projects will be implemented on a timely basis. Letters of cooperation from Professor Zhen Cao (Spokesperson of ARGO and LHAASO) and Professor Teresa Montaruli (IceCube) are appended.

Once enough air showers are produced, we will study the characteristics of high-energy muons including their energy distribution, production processes (i.e. decay from pion, kaon or charmed mesons) and production height. The surface muon rate from this simulation will be compared to available data and the muon monitor at SDSMT a). The muons or muon bundles will then be propagated to deep underground using MUSIC. This requires implementing the local slant depth distribution in the simulation. Professor Larry Stetler (a geologist at SDSMT) and Dr. Vitaly Kudryavtsev (principal developer of MUSIC) will support the PI by providing the geological survey data for Homestake and administer them into the simulation package. [Letters of cooperation from both Drs. Stetler and Kudryavtsev are appended.] In addition to the research, this effort will also provide a standard overburden module in MUSIC that will be useful for all future simulations in Sanford Lab and future DUSEL.

We will carry out the following systematic studies to obtain the quantitative description of muons and muons bundles in the Davis Cavern in the Sanford Lab: (1) Produce detailed energy spectrum of signal muons and muons in bundles using major

interactions models and various composition models. (2) Stud the systematics in muon flux and spectrum and their dependence on cosmic ray

composition and interaction models. The goal is to produce and analyze Monte Carlo events produced with different composition assumptions and major high energy interaction models, including QGSJET, SIBYLL, TARGET, etc. [40].

(3) Examine modulations in muon and muon bundle signals. Since the long-term variation of underground muon rate is rather small and depends on the overburden, we must carefully verify the full simulation with experimental data. The 300-ton water shield of LUX experiment is expected to see muon modulations after ~1-2 years of data collection, which will be analyzed and compared by the PI to this simulation. The PI will also extrapolate the simulations to other

a) The muon flux monitor in PI’s lab at SDSMT was built in early 2011. It consists three plastics scintillate detectors and a CAMAC data acquisition system that records GPS time for each event and the rates, the charge and timing of the signals from the detectors. The detectors and most of the electronics are from decommissioned SPASE2 and AMANDA experiments.

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depths and compare results published by MINOS, IceCube, and LVD. (4) Conduct simulations of the muon induced showers in underground caverns using the GEANT4

package. This is important because a shower contains more and different particles that cover a larger energy range.

(5) Simulate the muon induced neutron spectrum using the GEANT4 simulation package and analyze the contribution of different processes to the neutron production. By producing enough Monte Carlo events, we will also carefully analyze the statistics and uncertainties in neutron flux and energy distribution.

It should be emphasized that data from the LUX Experiment, together with the 300-ton water

shield monitored by 20 10” PMTs, can provide significant information about the background radiations in the Davis Cavern. The through-going and decay muons provide an alternative way to calibrate and monitor the water shield and crosscheck the simulation. The energy loss of muon in water can be described as dE/dX ≈ - a - b×E, with a ~ 0.260 GeV/m.w.e. and b ~ 0.360×10-3/m.w.e. [41]. If the water shield is stable enough, after ~1-2 months, the through-going muons will be statistically sufficient to provide an additional calibration point at a mean energy deposition of about 1.5 GeV in the water shield and with decay muons of about 37 MeV.

The LUX water shield can also be used to measure muon decay rate. Local muons with less than ~1.5 GeV may stop in the water shield. The maximum and average energies of Michel electron from muon decay are 53 MeV and 37 MeV. According to the water shield simulation, one Michel electron may generate 2~6 photoelectrons on average in each PMT. According to data from other experiments [48, page 558 in reference 9] and the parameterization in [page 80 in reference 11], the muon-decay rate is expected to be between 1 and 10 per day in the LUX water shield, with unknown systematic uncertainties related to the local rock composition and other environmental parameters. The decay muon measurement is very important because the stopped µ- can be captured by the nuclei of the detector materials. The muon capture can produce various isotopes, some of which are radioactive [8]. Since the µ- capture cross-section depends on the atomic mass of the target elements, by successfully tagging muon decay events in water, together with detailed detector simulation, the PI can estimate the µ- -capture rate in the LUX detector construction materials and liquid Xenon that have much larger atomic numbers.

The LUX experiment also provides an opportunity to study the muon-induced neutrons around and in a dark matter detector. Immersed in the water shield, the LUX dark matter detector is a two-phase liquid xenon (LXe) detector. It has high sensitivity, low threshold, and electron recoil versus nucleon recoil discrimination capability. More than 99% of electron recoil events in LUX may be rejected above the analysis threshold of 5 keVr (keVr: nuclear recoil equivalent energy). With the LXe PMTs sensitive to single photoelectrons, the analysis threshold can be in the range of 5~10 keVr, which is the crucial energy region in many underground experiments. EDELWEISS-II (4,800 m.w.e. level, Frejus site, muon flux ~4×10-6 /m2/d, fast neutron flux ~1.6×10-6 /cm2/s [49]) for the first time claimed several coincidences between its muon veto and muon-induced neutrons in its 1.1 kg of Ge crystals [50]. However, the rather small rate of ~0.04 coincidences/kg/d for E≤250 keV neutron- and electron-type recoils prevent people from a detailed investigation of µ-induced neutrons in this experiment. The search for muon-induced neutrons by the ZEPLIN-II group [51] has not detected real coincidences between low-energy (<100 keV) events in its Xenon vessel and high-energy events (> 0.5 MeV) in its liquid scintillator [16]. In contrast to the 730 kg liquid scintillator shield and 7.2 kg of liquid Xenon used in ZEPLIN-II, LUX (300-tons of water and 350 kg of LXe) should have a much better chance to see the coincidence between high-energy muons (triggering the water shield) and nuclear recoils in the inner LXe detector. Successful observation of the coincidence between muons and the nuclear recoil signals will give us enormous confidence in the muon-induced neutrons in the LUX experiment, which will further provide an anchoring point in the simulation of the neutron background in the Davis Cavern.

The proposed research work in this CAREER proposal is tightly connected with all those cosmogenic topics. The PI will also coordinate with LUX analysis group during the study of them.

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The PI also plans to verify his simulation using data from other experiments. In particular, he will carry out the simulation for Soudan Underground Lab and compare with the data from the muon veto shield located half a mile underground (2090 m.w.e.). Such comparison will help better understand the data of the experiment and improve the data analysis. [A letter of cooperation from Professor Priscilla Cushman is appended.] Besides its obvious scientific significance, doing so also eliminates the impact of the uncertainty in the DUSEL project and possible delays in the LUX experiment.

New ideas, initiatives and lines of research may emerge during the course of this project. The PI will exchange his ideas with various groups, including Dr. Kudryavtsev (one of the top experts in underground background simulation) and participants of the proposed workshops. 3.2 Simulation study of high energy prompt muon signature in deep underground and a new method to identify them Besides conventional muons produced in air showers, muons with higher energies may come from charm decays. Study of the prompt muons from charm decay has broader interest because they are useful in the study of cosmic ray compositions and heavy quark production in high energy p-N interactions. Although the prompt muons have lower flux than conventional muons at lower energies, they become dominant at energies above ~50 TeV according to some calculations (see Figure 5). Because of the high energies they carry, prompt muons are able to reach larger depths and create more complicated background radiations underground. Unfortunately, little data is available for the study of prompt muons and large uncertainties exist in the simulation of their production [52][53].

Figure 5. Vertical atmospheric muon and neutrino fluxes. Conventional muon and neutrino fluxes by solid and dashed lines marked “conven.” Prompt muon flux predictions from different model calculations are: ZHVa & ZHVe - two charm production models; RVS - empirical model; QGSM - quark-gluon string model; RQPM - recombination quark-parton model; PRS & GGV band & TIG - perturbative QCD. Two experimental bounds on prompt muons are from LVD and AKENO. See [52] and references therein for more details.

Since prompt muons in cosmic ray showers are produced along with multiple conventional

muons in a bundle, identifying prompt muons is a challenging task. A couple of methods have been proposed to identify them in underground experiments, such as using their zenith angle distributions, or the property of their depth dependence at a given zenith angle [54]. In this proposal, the PI will employ another technique that relies on characterizing the catastrophic energy losses by the prompt muons as bursts of light on an otherwise smoother energy deposition from a bundle of lower energy muons.

Preliminary studies by the PI support the utility of this method [38]. Figure 6 shows the probability of certain amounts of energy losses by conventional muon bundles and single muon in five-meter steps in ice. As one can see, for example, the chance to have an energy loss of about 30 TeV (point A in Figure 6) is much higher for single muon (in a bundle or by itself) with energy about 100 TeV than conventional muon bundles from showers below 100 PeV. If one sees a burst energy

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loss above 160 TeV, it is almost certain (P>1×10-3 versus P<3×10-5) that the bundle consists of a single 1 PeV muon rather than a bundle of conventional muons in showers no more than one EeV (point B). However, due to stochastic processes such as pair production, bremsstrahlung, and photonuclear interactions, the energy deposition of conventional muons in a bundle also has large fluctuations. Therefore, identifying prompt muon signals by the energy deposition has to be done on a statistical basis. More careful full Monte Carlo simulation is needed to define the key parameters to distinguish prompt muon signals.

Figure 6. The probability of the energy loss (in five-meter steps) of muon bundles in air showers (solid lines) and muons with fixed energy (dashed lines). Vertical proton and iron showers with five primary energies (500 TeV, 10 PeV, 100 PeV, 600 PeV, and 1EeV) are plotted together with vertical single muons of five fixed energies (5 TeV, 100 TeV, 1 PeV, 6 PeV, and 10 PeV) on the surface. When the energy of the primary particle or single muon goes higher, the probability of larger energy loss also increases [38]. Bundle of conventional muons was generated with CORSIKA. Muon energy loss was calculated with MMC.

The new CORSIKA release (v.6980) includes charmed particles that are treated implicitly in the

hadronic interaction codes (at present DPMJET and QGSJET01c). The PI proposes to: (1) Carry out a full Monte Carlo simulation of the muon bundles consisting of prompt muons and

verify the new method carefully. (2) Characterize the prompt muon spectrum in the Davis Cavern and at larger depths proposed in

DUSEL project. (3) Estimate the event rate of the prompt muons by combining the cosmic ray spectrum up to multi-

EeV region from the Pierre Auger Observatory [55]. This will contribute to the analysis of very high energy events in IceCube and the Long-Baseline Neutrino Experiment (LBNE) being planned for DUSEL.

3.3 Evaluating statistical methods for the identification of events from rare physics processes

Identifying interesting signal events from background noise often requires knowledge of the background and reliable data analysis techniques. This is because, in many cases, signals overlap with the backgrounds on the spectrum. There are different unfolding techniques developed to address this issue. Two methods have shown great success: the Bayes’ unfolding [56] and the direct demodulation [57, first invented for hard X-rays and soft gamma rays imaging to achieve high resolution reconstruction of complicated objects from incomplete and noisy data]. The application of these methods usually requires enough simulation to obtain the smearing matrix and careful control of the iteration process to reach optimum results. To evaluate the simulation sanity and experimental design, it has been a long desire to define a simple parameter that can describe the resolving capability for any given event-background distributions.

In the study of cosmic ray compositions, the PI has invented the resolving power parameters, (A|Y) and (B|Y), for a given event-background distribution Y consisting of two distributions A (noise for example) and B (signal). (A|Y) and (B|Y) are defined as [38]:

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in which Pi

A =1.0 when NiA ≠ 0.0. Or, Pi

A = 0.0 when NiA = 0.0. Ni

A and NiB are the number of event A

and B in the ith bin on dN/dY distributions. See the meaning of these parameters and an example in Figure 7.

Figure 7. Left: Definition of resolving parameters: Two histograms represent the distribution of variable Y for two types of radiations. See the text for details. Right: Distributions of particle A and B are represented by two Gaussian shaped distributions. The values of the amplitude, mean, and σ are in the parentheses. Particle A and B each has 320,000 and 80,000 events in this test.

The resolving power parameters can describe the resolving capability for any given event-

background distributions. (A|Y) and (B|Y) have the value between 0.0 and 1.0. When the two distributions are well separated from the other, no matter what their shapes might be or how many events in each distribution, both (A|Y) and (B|Y) are equal to 1.0. On the other hand, when the two distributions are identical and fully overlap each other, (A|Y) = (B|Y) = 0.5, which means the chance to assign a signal as A (noise) or B (event) is 50%. The resolving parameters can be explained as the possibility of identifying A (as noise in this case) or B (as signal) in the signal-background distribution Y.

Another advantage of this parameter is that its value also depends on the frequency of the signal. This can be seen in the plot on the right in Figure 7, in which (A|Y) = 0.697 and (B|Y) = 0.579. More importantly, this definition can be easily extended to cases in which the event is defined by multiple parameters, which is often the case in modern particle physics experiments.

It would be very much valuable to have more studies on this parameter and to use it to evaluate the impact of background distributions. Accordingly, the PI proposes to: (1) Conduct further studies on the correlation between the resolving power parameters and the

unfolding results by the two strict unfolding techniques. (2) Investigate the resolving power for typical signal events in the muon-induced background

distributions from the simulation work in Section 3.1. (3) Evaluate the application of the results to the search for the dark matter and prompt muon events. 4. EDUCATION ACTIVITIES AND INTEGRATION OF RESEARCH WITH EDUCATION

The PI believes successful science education consists of two primary elements: The first follows well-established procedures and curriculum. The second involves discovering students’ interests in science and helping to strengthen such interests for those with different educational and cultural backgrounds. The education plan of this proposal will balance both, as described below. 4.1 Develop a new course to mainly benefit physics students in South Dakota

There are about 15 underground physics laboratories and more than 25 active underground

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experiments. If completed, DUSEL will become one of the largest operating underground mines in the world. Experiments in Sanford Lab and those planned in DUSEL are designed to answer the most important questions in today’s physics. The new detectors will be significantly larger, more sophisticated, and more sensitive than most existing ones. To develop a new course that timely summarizes the science, detector design and analysis techniques of these experiments is necessary and will benefit the next generation of physicists for many years to come. The PI proposes to develop a new course, “Topics in Modern Astroparticle Physics,” that includes the following three major sections: (1) The physics of cosmic rays, neutrinos, and dark matter; (2) The fundamental detector techniques for cosmic ray, dark matter, and neutrino experiments; (3) Underground radiation and data analysis in cosmic ray and underground experiments.

There are about 75 undergraduate students enrolled in the physics departments in South Dakota.

Additionally, two major universities, SDSMT and the University of South Dakota (USD), have about 12 physics graduate students. The proposed new course will be first designed as junior level course for undergraduate students. Due to the relatively low enrollment in physics in South Dakota public universities, many physics courses are taught over distance video system. Significantly, this course will benefit physics students statewide.

According to the SDSMT development plan, the university is planning to double its physics faculty members and students over the next five years. Moreover, the state has approved the initiation of a physics Ph.D. program in 2010. To advance the work of the Ph.D. program once it is funded, the PI intends to develop this course into a graduate level course to better support physics research programs in South Dakota. 4.2 Outreach to underrepresented groups in South Dakota and young scholars and students in China and Tibet

Unlike many other states in the US, South Dakota is home to nine Native American tribes; they comprise 9% of the country’s population. Native Americans in South Dakota have a rich and colorful culture, ranging from their family structures to their spirituality. Unfortunately, American Indian students have higher high school drop-out rates than the majority population, and only a small number attend college. Even fewer finish college and or pursue degrees in science, technology, engineering, or mathematics (STEM) fields. Although SDSMT is a major science and engineering university in South Dakota, it only has about 2.5% Native Americans on campus and many of them are first generation college students. In order to mitigate this issue, SDSMT hosts several programs including NSF Tiospaye in Engineering Program and the SD GEAR UP Honors Program. Programs like the SD GEAR UP Honors Program are designed to prepare Native American students for college-level science. The PI has given lectures and hosted student visits in physics labs during the summer of 2009 and 2010, which he will continue under this award.

In addition to working with Native American students, the PI is also interested in outreach to students and institutions in China, especially those in Tibet. Because of its booming economy, China is pouring more resources into higher education and research programs. This provides opportunities for the next generation of students in more and more countries. The PI’s interest in pursuing more collaborative work was also sparked by interactions with a Tibetan student. Tibetans are one of the most underrepresented groups in modern society. Their participation in science is much lower than many minority nationalities in Asia. Women in Tibet have even lower participation in science. The Institute of High Energy Physics (IHEP) in Beijing, where the PI spent one year as a postdoc in 1997, runs a large-scale cosmic ray observatory in Tibet and has strong connections with the Tibet University and with a couple of local schools. After talking with his colleagues in IHEP, the PI sees the motivation from both sides and the feasibility of extending his educational activities to young people in Tibet and in IHEP as well.

In this CAREER proposal, the PI will work to reach those groups, try to build solid connections with them, and increase their interest in physics. The PI will fulfill the following goals:

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(1) Establish an education and outreach (EO) station adjacent to the PI’s lab at SDSMT. An effective way to enhance young students’ interests in science is to teach them how to solve science problems and to foster pride in succeeding as a team. Since it is unrealistic to deploy these young students into underground caverns, it is necessary to have opportunities for them to practice in surface labs, which the PI is capable of doing at SDSMT. During the summer of 2010, the PI accepted one Davis-Bahcall student to the LUX detector calibration work on SDSMT campus. It turned out to be a very successful experience. The PI will extend work in this area to provide students opportunities to join group meetings and work with graduate students on actual research projects. To support the PI’s work at SDSMT, SPASE-2 and AMANDA provided some electronics and particle detectors retired from these two experiments. The PI has transformed a 1000 sq ft storage space into a decent astroparticle physics lab at SDSMT. The PI also received a NSF Grant to refurbish his labs in 2011. This provides a good base for developing future research and EO programs. To establish the EO station, the PI will: (a) add an interactive event displayer in the PI’s lab to show the simulated air showers, high energy muons, and muon-induced air showers in underground caverns, (b) open the lab to students in the summer programs and give mini lectures during their visits, and (c) continue accepting the Davis-Bahcall students as assistants to work with SDSMT’s graduate students on a more regular basis.

(2) Develop a series of lectures that cover the science and current research projects in astroparticle physics by making use of the LUX experiment and other ongoing world-class experiments such as LBNE and IceCube. The PI will ensure that the lecture materials are comprehensive, engaging, attractive, and comprehensible for the public and K-12 students.

(3) Arrange a minimum of three trips to Beijing and Tibet during the 5-year period. The PI will give seminar or colloquium and have discussions with K-12 and college Tibetan students. For audiences at higher levels such as the young scholars in IHEP, the PI will develop presentations using materials from the new course and enrich them with the results of his research work.

(4) Provide mentoring / student exchange opportunities. This consists of two parts: one is to make lasting connections among students so they can inspire each other. Peer to peer support is invaluable. To establish a connection among K-12 students from underrepresented groups, the PI will create a website dedicated to posting blogs and other information of the EO activities. The other is to establish a more formal and productive connection with IHEP and Tibet. The PI will explore and pursue programs to exchange students and researchers between the SDSMT physics department and IHEP and Tibet University.

4.3 Integrate research with education

The PI will integrate his research with science education by involving undergraduate students in the project. SDSMT has an excellent tradition of encouraging undergraduate students (including those from other departments) to work with faculty members for their design project credits starting freshman year. The PI has been working with roughly four undergraduate students every year from the departments of physics and computer science on mini projects since he joined SDSMT in 2009. In the next five years, the PI will continue to involve students at least at the same level in the proposed research work. The PI will design some mini projects that are suitable for students in physics and computer science majors, such as data fitting and plotting, monitoring jobs on the computer cluster, and optimizing the management of large amount of data in different formats. Because the CAREER plan is a five-year project, it will also motivate these students to continue their graduate education with the PI. 4.4 EO Assessment and evaluation Plan

To evaluate the performance of the new course, the PI will utilize the South Dakota faculty course questionnaire and teaching evaluation system. Every semester, South Dakota students evaluate each of their courses and instructors using the questionnaire and evaluation system. The results go to individual instructors for use in improving their courses and teaching. They also go to

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department heads/deans for use in course assignments and in promotion, salary, and tenure decisions. The feedback can also be very useful in assessing the development of new courses. The PI will use this system to evaluate his teaching performance and improve his curriculum. In addition, the PI will invite tenured faculty to sit in and evaluate his teaching.

For the outreach activities, the PI will use existing EO evaluation questionnaires developed for EO programs in SDSMT and Sanford Lab Education Outreach Office (which collects the feedback from both the students and their parents/guardians). In addition, the PI will try a web-based network using tools and social media, such as Facebook. This will allow the EO activity to be more interactive for the students. To improve his outreach skills, the PI will analyze the information, consult and discuss the students’ feedbacks with science EO experts like Dr. Peggy Norris, Deputy Director E&O Sanford Lab. A letter of cooperation from Dr. Norris is included. 5. ANNUAL WORK PLAN

As a junior faculty with research, teaching and other responsibilities, the PI is aware of the importance of having a practical work plan that balances his responsibilities. This plan also needs to take into account the actual situation in his institution. Because the proposed work includes significant comparisons between the simulation results and experimental data, the work plan also needs to be synchronized with the pace of the experiments.

The proposed annual work plan follows: Year 1. Purchase and set up the computer cluster. Install software packages for system administration and simulation research work. Continue the systematic studies of seasonal modulations in the atmosphere and parameterize the atmosphere models in CORSIKA simulation using balloon and satellite data. Implement the rock compositions, the overburden depth and local land surface profile into the muon propagation simulation. Start some testing and verification simulation runs. Year 2. Start air shower simulations and muon propagation, which are needed to determine the modulations in both the surface signals and underground muon signals in the Davis Cavern. Monte Carlo data will also be generated for systematic comparisons among different cosmic ray composition models and hadronic interaction models. The research will focus on the energy spectrum, multiplicity, and spatial distribution of muons and muon bundles at 4850 ft level. After LUX data becomes available, the PI will start to compare it with the simulation.

The simulation of muons in the Sudan Mines and the study of prompt muon signals will also be started this year.

To exchange ideas among experts and to learn more about the progress in other experiments, the PI proposes to have a workshop on underground cosmogenics (of about 15~20 participants) at SDSMT or Sanford Lab during Year 2 (the 2nd one in Year 4). A comprehensive summary of the progress will be presented at the workshop. As the cosmogenics lead in the LUX collaboration, the PI will particularly involve students and young scholars from the LUX analysis and simulation groups in the discussions during the workshop. Year 3. Simulate the neutron production in the Davis Cavern using the best-obtained muon spectrum from the simulation (further normalized by data if needed). The PI will compare the simulation with more LUX data and try to obtain the best description of the cosmogenic systematics in LUX. They will also continue comparing their simulation with data from the Sudan Mines. We expect the study of prompt muon signals to be concluded by the end of year 3. Year 4. Carry out more LUX detector simulations and continue comparing the results with more experimental data from LUX and the muon experiment in the Sudan Mines. The PI will start to compare several unfolding techniques and evaluate the resolving power parameters’ application in the identification of rare events. The second comprehensive summary of our research work and results will be presented at the second workshop this year. Year 5. Finish the rest of the work, document the research properly, and write papers for publications.

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Starting Year 1, the PI will develop the new course. Trial lectures will be given in the department from the fall semester of the first funding year. The new course will be fully implemented into the physics curriculum by the end of Year 3. All the materials of the new course will be available online from the PI’s teaching website. They will be open to students, the physics community and the public at no cost. 6. RELATION TO LONG-TERM CAREER GOAL

The PI’s long-term CAREER goal is to establish a competitive research and education program at SDSMT and to become a well-recognized lead in the simulation, physics data analysis, and science education in astroparticle physics focusing on underground experiments for the study of dark matter and neutrino physics.

This CAREER project represents the PI’s first step towards achieving his career goal. Successful performance of the research plan will produce important scientific results and publications, establish a closely coordinated group with prominent scientists in dark matter search, cosmic ray physics, and neutrino physics, and help to spur additional competitive fundings. Through the education and outreach activities, more students in South Dakota public colleges will be trained for future underground science programs. More young students, including women and those from under represented groups, will be inspired to choose science as their career. The outcome will diversify the astroparticle physics pool, strengthen the research force, and enrich future research opportunities at SDSMT, which supports the department and university strategic plan that aims to make crucial contributions to astroparticle physics programs.

Located in Rapid City, SDSMT is the only science and technology university close (50 miles) to the Sanford Lab. Because he was the first and only astroparticle physicist in the history of SDSMT, the PI was hired to initiate new research and education programs reflecting new development in the Sanford Lab and future DUSEL. With very limited resources from the university (a total of $100k startup fund and three odiferous storage rooms), the PI has made rapid progress in setting up a research lab, bringing SDSMT into LUX and LBNE collaborations, recruiting graduate students, and making scientific contributions to LUX. See more information in letters from the department head and from the LUX Collaboration. This award is pivotal to establish the PI as an independent, competitive researcher. . 7. INTELLECTUAL MERIT AND BROADER IMPACTS Intellectual merit: The results from the proposed research work will provide, for the first time, a full Monte Carlo calculation of the cosmogenic radiations in Sanford Lab, including the quantitative description of the systematics of the cosmogenic radiations in the LUX dark matter experiment. The results will be critical for the LUX experiment to make any solid statement of discovery of dark matter. By comparing calculations using various simulation models and with data, this project will identify differences among them and provide guidance to further improve the calculations. The research results and experience will also advance the PI and his team to play long-term leading roles in new experiments and in improving analysis techniques in the search for rare physics events, including dark matter particles, neutrinoless double decay, and proton decay. Broader Impacts: The outcome of the proposed research work will provide precise estimations of the cosmogenic backgrounds in one of the most important candidate sites for future underground experiments. The experience obtained will enable the PI (and his group) to make additional contributions to underground experiments at other sites. The new course on modern topics in astroparticle physics will better prepare physics students in South Dakota public colleges for future research programs. By establishing a satellite education and outreach station on astroparticle physics at SDSMT, about 120 K-12 students will benefit every year. The outreach activities in China and Tibet will establish a strong connection between a US Midwest university and the institutions in East Asia, which will further enrich the research opportunities for students at SDSMT. These combined efforts will motivate general interest in science, attract more students from the under-represented groups, including Native American students, Tibetans, and women to SDSMT, and help make SDSMT a more diverse community.