nucleon decay simulation in genie - indico-fnal (indico) · 2016. 10. 26. · nucleon decay channel...

Nucleon Decay Simulation in GENIE

Validation and future plans

Michel Sorel

DUNE NDK/Atmospherics/Cosmogenic Physics WGs MeetingOctober 26th, 2016

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Nucleon decay channel additions in GENIE 2.12 Reminder and recent news

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Description: Build a generic structure function interface for DIS calculations, where GENIE reads inpre-calculated structure function values.Developers: Tom Stainer (Liverpool), Costas Andreopoulos (Liverpool/STFC-RAL), Roberto Petti(South Carolina), Hugh Gallagher (Tufts)Reporting: PPWG.Target release: GENIE/Generator v?Documentation: internal wiki

Project: nc_onegammaDescription: Improve modeling of NC1γ production in GENIE.Developers: Teppei Katori (QMUL), Pierre Lasorak (QMUL)Reporting: PPWG.Target release: GENIE/Generator v?Documentation: internal wiki

Project: dis_reevalDescription: Re-evaluate the DIS model for use by experiments with Enu > 100 GeV. Attempt todevelop a fully comprehensive DIS model that can describe DIS processes from TeV to few-GeV.Developers: Jacob Morrison, Joshua Hignight, Kendall Mahn (MSU)Reporting: PPWG.Target release: GENIE/Generator v?Documentation: internal wiki

Project: Geant4-GENIE hadronic interfacesDescription: Develop a new class in Geant4 to expose portions of the hadronic model to computenuclear evaporation and de-excitation. Build and link GENIE to versions of Geant4 that expose thisAPI. Make modifications to a new version of hN INTRANUKE to take advantage of this API alongwith other computations in hN INTRANUKE to produce more realistic and detailed final stateinformation for a detector simulation, including physically reasonable and plausible definitions of thenuclear remnant. Allow coupling between hN INTRANUKE and the remnant state computation, andtarget only one Geant4 evaporation / de-excitation model for the "Minium Viable Product."Re-factoring hN INTRANUKE and expanding to multiple Geant4 models is a project for the nextincubator iteration.Incubation manageer: Gabe Perdue (Fermilab)Developers: Dennis Wright (SLAC), Makoto Asai (SLAC), Robert Hatcher (Fermilab), Steve Dytman(Pittsburgh)Reporting: NPWGTarget release: GENIE/Generator v2.14.00Documentation: internal wiki

Project: ndec_channel_additionsDescription: An upgrade all GENIE's nucleon decay tool to include all 2-body and 3-body decaychannels listed in PDG and adoption of PDG decay mode numbering scheme. All decaysimplemented as phase space decays decays at the moment.Developers: Michel Sorel (IFIC), Elena Gramellini (Yale), Jennifer Raaf (Fermilab).Reporting: TCWG.Target release: GENIE/Generator v2.12?Documentation: internal wiki

Project: lhapdfv6Description: Integrate GENIE with LHAPDF6 and eliminate the optional LHAPDF5 dependencyDevelopers: Costas Andreopoulos (Liverpool/STFC-RAL)Reporting: PPWGTarget release: GENIE/Generator v2.12.00Documentation: internal wiki

Project: martini_mecDescription: A GENIE implementation of the M. Martini, M. Ericson, G. Chanfray, J. Marteau MECmodel.Developers: Sara Bolognesi (CEA Saclay)

GENIE Neutrino Monte Carlo Generator – Hepforge https://genie.hepforge.org/load.php?include=incubator

4 of 7 19/07/2016 11:46

• Since my last update in July: • Code merged into GENIE’s trunk (Gabe Perdue, myself) • Much more extensive validation • Bug fixes • Adapt to PDG-2016 NDK numbering scheme (slightly different from PDG-2015)

Validation procedure

• For validation purposes, a number of 1D histograms have been produced

• Decayed nucleon histograms: PDG code, momentum

• Decayed nucleon daughters histograms: number of daughters per decay, PDG code and momentum for each daughter

• GENIE’s final state particles histograms: number of final state particles per decay, PDG code and momentum of all final state particles

• Histograms have been produced for each NDK mode, both for Ar and H2O targets

• Ar target → gevgen_ndcy -g 1000180400

• H2O target mix → gevgen_ndcy -g 1000080160[0.8879],1000010010[0.1121]

3

16O nucleus [weight fraction in H2O] 1H nucleus [weight fraction in H2O]

Free vs bound decayed nucleons

• For proton decays in H2O target, 20% involved free protons, 80% protons bound in oxygen nuclei

• Note: this is the expected behaviour after a code fix (Oct 2016). Before that, only 1.5% of proton decays in H2O involved free protons

• For protons in Ar, and neutrons in either Ar or H2O, decayed nucleons always bound in Ar/O nuclei

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DecayedNucleonNDaughters

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DecayedNucleonDaughter1Momentum [GeV/c] Entries 1000Mean 0.4552RMS 0.05142Underflow 0Overflow 0


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1000FinalParticlesPdg Entries 2643


FinalParticlesPdg

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FinalParticlesMomentum [GeV/c]

p → e+ π0 decays in H2O

free protons (20%)

bound protons (20%)

Decayed nucleon momentum

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Decay: p --> nubar K+Target: 1000180400[N=2212]

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• For Ar, max momentum for decaying protons 0.235 GeV/c, plus ~10% event fraction in higher momentum tail

• To be compared with RFG Fermi momentum in GENIE manual: 0.242 GeV/c. High-energy tail due to short-range nucleon-nucleon correlations?

• For neutrons, max momentum slightly higher (0.27 GeV/c) and similar higher momentum tail

higher momentum tail (~10%)

Fermi momentum

Kinematically forbidden decays

6

• Several nucleon decays are kinematically forbidden for a fraction of the decays

• Expected behaviour: sum of daughter particles masses close to nucleon mass

• Non-zero nucleon momentum may forbid momentum-energy conservation

Reaction mN (MeV) ∑mdaughter (MeV) Fraction forbidden decays (%)n → μ+ ρ- 939.6 874.7 7.7 ± 0.9p → μ+ ρ0 938.3 874.7 4.2 ± 0.6p → μ+ ω 938.3 888.0 6.6 ± 0.8

p → e+ K∗0 938.3 896.3 16.4 ± 1.3n → ν ̅K∗0 939.6 895.8 30.3 ± 1.7p → ν ̅K∗+ 938.3 889.1 6.8 ± 0.8n → μ- ρ+ 939.6 874.7 7.7 ± 0.9

• When decay kinematically forbidden, GENIE produces a single final state “particle”, 39Ar or 39Cl, and no mesons/leptons

Nucleon decay daughters kinematics

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DecayedNucleonPdg

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p → ν ̅K+ in H2O p → ν ̅K+ in Ar

• Kinematics of nucleon decay daughters looks reasonable

• Explicit check: K+ momentum for free proton decays in p → ν ̅K+ is 339 MeV/c, as it should be

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Decay: p --> nubar K+Target (1st evt): 1000080160[N=2212]

DecayedNucleonPdg

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free protons

Meson decays in GENIE vs decays in Geant4

8

• Few mesons that decay electromagnetically are decayed in GENIE: η, ρ0, ρ±, ω, so that their decay products can undergo FSI. Branching ratios look reasonable

• Exception: π0, which undergoes FSI within GENIE, and decayed in Geant4

• All other mesons are also decayed in Geant4

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Decay: p --> e+ etaTarget: 1000180400[N=2212]

DecayedNucleonPdg

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FinalParticlesMomentum [GeV/c] Entries 3622Mean 0.228RMS 0.1018Underflow 0Overflow 0


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Decay: p --> e+ etaTarget: 1000180400[N=2212]

DecayedNucleonPdg

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FinalParticlesMomentum [GeV/c] Entries 3622Mean 0.228RMS 0.1018Underflow 0Overflow 0


p → e+ η in Ar

e+ + η→3π0, e+ + η→π+π-π0

e+ + η→γγ

π- π+

e+

γ

π0

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Decay: p --> e+ pi0Target: 1000180400[N=2212]

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Meson final state interactions in GENIE

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• Only mesons undergoing FSI in GENIE currently: π±, π0, K+

p → e+ π0 in Ar

No π0 FSI e+

π0 n p


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• Only mesons undergoing FSI in GENIE currently: π±, π0, K+

p → ν ̅K+ in Ar

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• No FSI in GENIE currently for all other mesons: K0, K-, η, etc.

p → μ+ K0 in Ar

No K0 FSI K0μ+

Possible future GENIE projects for NDK physics

12

• Simulation of nuclear de-excitation gamma rays

• Simulation of removal (binding) energy effects in nucleon decay

• Better treatment of K+ FSI. Addition of FSI for K0, K-, and possibly other mesons

Nuclear de-excitation gamma rays in GENIE

13

• If nucleon decays, remaining nucleus (39Cl or 39Ar) would be left in excited state

• Few-MeV energy deposition near the nucleon decay vertex

• Effect on calorimetry, vertex finding

• Accounted for in NEUT for Super-K nucleon decay searches in H2O

4

State Energy of γ Probabilityp3/2 6.3MeV 41%p3/2 9.9MeV 3%s1/2 7.03MeV 2%s1/2 7.01MeV 2%others 3.5MeV 16%Other than γ emissionp/n emission - 11%ground state - 25%

TABLE II: Summary of probabilities of nuclear γ ray emissions atthe de-excitation of the remaining nucleus.

B. Atmospheric Neutrinos

The SK standard atmospheric neutrino MC used in theprevious neutrino oscillation analyses [17] and proton decaysearches [4, 10, 24, 25] is used in this analysis. It is basedon the Honda atmospheric neutrino flux [26] and NEUT [21]neutrino-nucleus interaction model. Some neutrino interac-tions which produce K mesons via resonances could be poten-tial background sources for p → νK+ search. Cross sectionsof the single meson production via resonances are calculatedbased on Rein and Sehgal’s theory [22]. In NEUT, the neu-trino reactions:

ν n → l− Λ K+

ν n → ν Λ K0

ν p → ν Λ K+

ν p → l+ Λ K0

ν n → ν Λ K0

ν p → ν Λ K+

are taken into account assuming the same cross section bothfor νe and νµ. The differential cross sections are shown inFig. 2.We simulate propagation of the produced particles and

Cherenkov light in water by custom code based onGEANT3 [27]. The propagation of charged pions in wateris simulated by custom code based on Ref. [28] for less than500MeV/c and by GCALOR [29] for more than 500MeV/c.The equivalent of 500 years of SK atmospheric neutrino

data is simulated for each SK run period. The generated at-mospheric neutrino samples are weighted to include the ef-fect of νµ disappearance due to νµ-ντ oscillation assuming∆m2 = 2.5×10−3 eV2 and sin22θ= 1.0, ignoring the appear-ance of νe or ντ as a possible background. The final back-ground event rates for each period are normalized by the ob-served total sub-GeV event rate.

0

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0-38 cm

)

ν + n → l- + Λ + K+

ν + n → ν + Λ + K0

ν + p → ν + Λ + K+

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0 1 2 3 4 5 6 7 8 9 10Neutrino Energy (GeV)

σ (1

0-38 cm

) ν_ + p → l+ + Λ + K0

ν_ + n → ν

_ + Λ + K+

ν_ + p → ν

_ + Λ + K+

FIG. 2: (color online) Cross sections of the single K-meson produc-tions via resonances calculated by NEUT. Upper plots show neutrinointeractions and lower show anti-neutrino interactions.

IV. DATA SET, REDUCTION AND RECONSTRUCTION

The vast majority of the triggered events are cosmic raymuons and low energy backgrounds from the radioactivity ofmaterials around the detector wall. Several stages of data re-duction were applied to the events before proceeding to fur-ther detailed event reconstruction processes. Details of thedata reduction and reconstruction can be found in [17].The fully contained (FC) data sample in the fiducial volume

(FV) is defined by the following cuts:

• number of hit PMTs in the largest OD hit cluster is lessthan 10 for SK-I and 16 for other period.

• total visible energy is greater than 30MeV in ID

• distance of the reconstructed vertex from the ID PMTsurface is greater than 2meters (corresponding to22.5 kton of water volume)

The rate of FCFV events is about 8 events per day. The con-tamination of events other than atmospheric neutrinos is esti-mated to be less than 1% and composed of cosmic rays thatevaded the OD veto and events caused by flashing PMTs.Reconstruction algorithms are applied to the events remain-

ing after the reduction process to determine the event vertex,the number of Cherenkov rings, the particle type of each ring,the momentum assigned to each ring, and number of Michelelectrons. As a first step, the event vertex is defined as thepoint at which the timing distribution, after subtraction of thecalculated time of flight of the photon from the vertex (TOFsubtraction), has the sharpest peak. The dominant ring direc-tion is determined from the charge distribution as a function

Gamma-ray emission in H2O (NEUT)

• How to simulate in GENIE? Options:

• Perhaps similar approach as MARLEY event generator, using TALYS nuclear structure database?

• Gabe Perdue: two ongoing GENIE projects to use either Geant4 or INCL++ to help compute these processes

Binding energy effects in GENIE nucleon decay

14

• Phase space available for nucleon decay in GENIE currently does not account for binding energy. It should

• Effectively modifying nucleon mass

• Accounted for in NEUT for Super-K nucleon decay searches in H2O

Binding energy effect in H2O (NEUT)

3

inch PMT is about 2.1 nsec at the single photo-electron level.The PMT response, water quality, and reflections from the de-tector wall are tuned in the SK detector simulation programusing injected light as well as various control data samplessuch as cosmic ray muons.

III. SIMULATION

To determine selection criteria for the proton decay search,and to estimate efficiencies and background rates, we use pro-ton decay and atmospheric neutrino Monte Carlo (MC) simu-lations. Because the configuration of the detector is differentin SK-I through IV, we generated MC samples for each pe-riod. Proton decay MC samples with 50,000 events are gen-erated in an oversized volume which is 1 meter outside thefiducial volume boundary, and therefore 1 meter from the de-tector wall. This allows us to include event migration near thefiducial boundary in our estimates. The selection efficiencyis defined as the number of events fulfilling all requirementsdivided by number of generated events in the fiducial volume.The MC equivalent of 500 years of atmospheric neutrino ex-posure are generated for each period. These atmospheric neu-trino MC samples are used for our studies of neutrino oscilla-tions [17]. Because the background rates for the proton decaystudied in this paper are small (less than one event for the en-tire exposure for two of the analysis techniques), these largeMC background samples provide fewer than 40 atmosphericneutrino events that survive the proton decay selection crite-ria.

A. Proton Decay

A water molecule contains two free protons and eight pro-tons bound in the oxygen nucleus. In the decay of a free pro-ton, the ν and the K+ are emitted opposite each other withmomenta of 339MeV/c. In the case of proton decay in oxy-gen, Fermi momentum, correlation with other nucleons, nu-clear binding energy, and kaon-nucleon interactions are takeninto account as described below.We use the Fermi momentum and nuclear binding energy

measured by electron-12C scattering [18]. Nuclear bindingenergy is taken into account by modifying the proton mass.Ten percent of decaying protons are estimated to have wavefunctions which are correlated with other nucleons within thenucleus [19]. These correlated decays cause the total invariantmass of the decay products to be smaller than the proton massbecause of the momentum carried by the correlated nucleons.Figure 1 shows the invariant mass of the products of the de-caying proton, K+ and ν and the resulting kaon momenta af-ter the simulation of the proton decay for both bound and freeprotons. Correlated decays produce the broad spectrum belowabout 850MeV/c2. In our experiment, the kaon momentum isunobserved because the kaon is always produced below theCherenkov threshold of 749MeV/c in water. The majority ofK+ (89%) are stopped in water and decay at rest. We search

for K+ decay at rest into µ+νµ (64% branching fraction) andπ+π0 (21% branching fraction).

10

10 2

10 3

10 4

400 600 800 1000Invariant proton mass in 16O

Num

ber o

f eve

nts

10

10 2

10 3

10 4

0 200 400 600K+ momentum (MeV/c)

Num

ber o

f eve

nts

FIG. 1: (color online) The upper figure shows the decaying protonmass distribution in 16O and the lower figure shows the K+ mo-mentum distribution from the simulation of p→ νK+. In the upperfigure, the single-bin histogram shows the free proton case and thebroad histogram shows the bound proton case. The rightmost peakin the bound proton case corresponds to the p-state, located slightlylower than the proton mass by 15.5MeV of binding energy; the sec-ond rightmost peak is the s-state (39MeV in binding energy). Thecorrelated nucleon decay makes the longer tail in the lower mass re-gion. In the lower figure, the single-bin histogram shows the freeproton case (339MeV/c) and the broad histogram shows the boundproton case which is smeared by Fermi motion.

The position of the decaying proton in 16O is calculatedaccording to the Woods-Saxon nuclear density model [20].The kaon nucleon interactions which are considered includeelastic scattering and inelastic scattering via charge exchange.The type of interaction is determined using the calculatedmean free path [21]. For kaons, whose momenta are de-scribed by Fig. 1, the probability of charge exchange for K+

in p→ νK+ is 0.14%.If a nucleon decays in the oxygen nucleus, the remain-

ing nucleus can be left in an excited state from which itpromptly de-excites by the emission of gamma rays. Theprompt gamma emission processes are simulated based on ref-erence [23]. The dominant gamma ray is 6.3MeV from thep3/2 state with 41% branching fraction. The probabilities of γemission in this simulation are summarized in Table II. Otherstates emitting low energy gamma rays are averaged and as-signed 3.5MeV γ emission. Nuclear decay into states thatemit protons or neutrons and nuclear decay into the groundstate are taken to have no γ ray emission.

Free pp-state (15.5 MeV BE)

s-state (39 MeV BE)

NN correlations

• Should be relatively straightforward to implement in GENIE nucleon decay

Implementing more/better meson FSI in GENIE

15

K+

• Inelastic interactions producing neutrons and/or protons accounted for, but always with K+ in final state

• We could add K+→K0 charge-exchange (couple % percent effect)

• Anything else?

K0, K-, other mesons

• Problem: little/no existing data

• Steve Dytman: could use theory-inspired model to relate K0, K- FSI to K+ FSI. Better than nothing

• Can something like this be applied to other mesons, such as η, ρ, etc.?

Summary

• GENIE 2.12 will have the most basic upgrade we need for DUNE nucleon decay physics: being able to generate any nucleon decay mode

• Note: we will need to update the LArSoft interface accordingly

• I have extensively validated the GENIE NDK code. Everything looks good

• For the longer term, there are a number of issues that need further work to have a reliable simulation of nucleon decay physics in GENIE

• Nuclear de-excitation, binding energy, better/more FSI

• Something we might want on the timescale of the DUNE Physics TDR in 2019

16

nucleon decay simulation in genie - indico-fnal (indico) · 2016. 10. 26. · nucleon decay channel...

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