A joint Fermilab/SLAC publication

august 2015dimensionsofparticlephysicssymmetry

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Table of contents

Feature: The age of the universe

Breaking: Dark Energy Survey finds more celestial neighbors

Signal to background: MicroBooNE sees first cosmic muons

Signal to background: Q&A: Underground machinist

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feature

August 18, 2015

The age of the universeHow can we figure out when the universe began?By Amelia Williamson Smith

Looking out from our planet at the vast array of stars, humans have always askedquestions central to our origin: How did all of this come to be? Has it always existed? Ifnot, how and when did it begin?

How can we determine the history of something so complex when we were not around towitness its birth?

Scientists have used several methods: checking the age of the oldest objects in theuniverse, determining the expansion rate of the universe to trace backward in time, andusing measurements of the cosmic microwave background to figure out the initialconditions of the universe and its evolution.

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Hubble and an expanding universe

In the early 1900s, there was no such concept of the age of the universe, says StanfordUniversity associate professor Chao-Lin Kuo of SLAC National Accelerator Laboratory.“Philosophers and physicists thought the universe had no beginning and no end.”

Then in the 1920s, mathematician Alexander Friedmann predicted an expandinguniverse. Edwin Hubble confirmed this when he discovered that many galaxies weremoving away from our own at high speeds. Hubble measured several of these galaxiesand in 1929 published a paper stating the universe is getting bigger.

Scientists then realized that they could wind this expansion back in time to a pointwhen it all began. “So it was not until Friedmann and Hubble that the concept of a birth ofthe universe started,” Kuo says.

Tracing the expansion of the universe back in time is called finding its “dynamicalage,” says Nobel Laureate Adam Riess, professor of astronomy and physics at JohnsHopkins University.

“We know the universe is expanding, and we think we understand the expansionhistory,” he says. “So like a movie, you can run it backwards until everything is on top ofeverything in the big bang.”

The expansion rate of the universe is known as the Hubble constant.

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The Hubble puzzle

The Hubble constant has not been easy to measure, and the number has changedseveral times since the 1930s, Kuo says.

One way to check the Hubble constant is to compare its prediction for the age of theuniverse with the age of the oldest objects we can see. At the very least, the universeshould be older than the objects it contains.

Scientists can estimate the age of very old stars that have burned out—called whitedwarfs—by determining how long they have been cooling. Scientists can also estimate theage of globular clusters, large clusters of old stars that formed at roughly the same time.

They have estimated the oldest objects to be between 12 billion and 13 billion yearsold.

In the 1990s, scientists were puzzled when they found that their estimate of the age ofthe universe—based on their measurement of the Hubble constant—was several billionyears younger than the age of these oldest stars.

However, in 1998, Riess and colleagues Saul Perlmutter of Lawrence BerkeleyNational Laboratory and Brian Schmidt of the Australian National Lab found the root ofthe problem: The universe wasn’t expanding at a steady rate. It was accelerating.

They figured this out by observing a type of supernova, the explosion of a star at theend of its life. Type 1a supernovae explode with uniform brightness, and light travels at aconstant speed. By observing several different Type 1a supernovae, the scientists wereable to calculate their distance from the Earth and how long the light took to get here.

“Supernovae are used to determine how fast the universe is expanding around us,”Riess says. “And by looking at very distant supernovae that exploded in the past andwhose light has taken a long time to reach us, we can also see how the expansion rate

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has recently been changing.”

Using this method, scientists have estimated the age of the universe to be around13.3 billion years.

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Recipe for the universe

Another way to estimate the age of the universe is by using the cosmic microwavebackground, radiation left over from just after the big bang that extends in every direction.

“The CMB tells you the initial conditions and the recipe of the early universe—whatkinds of stuff it had in it,” Riess says. “And if we understand that well enough, inprinciple, we can predict how fast the universe made that stuff with those initial conditionsand how the universe would expand at different points in the future.”

Using NASA’s Wilkinson Microwave Anisotropy Probe, scientists created a detailedmap of the minute temperature fluctuations in the CMB. They then compared thefluctuation pattern with different theoretical models of the universe that predict patterns ofCMB. In 2003 they found a match.

“Using these comparisons, we have been able to figure out the shape of the universe,the density of the universe and its components,” Kuo says. WMAP found that ordinarymatter makes up about 4 percent of the universe; dark matter is about 23 percent; andthe remaining 73 percent is dark energy. Using the WMAP data, scientists estimated theage of the universe to be 13.772 billion years, plus or minus 59 million years.

In 2013, the European Space Agency’s Planck space telescope created an evenmore detailed map of the CMB temperature fluctuations and estimated the universe to be13.82 billion years old, plus or minus 50 million years—slightly older than WMAP’sestimate. Planck also made more detailed measurements of the components of theuniverse and found slightly less dark energy (around 68 percent) and slightly more darkmatter (around 27 percent).

New puzzles

Even with these extremely precise measurements, scientists still have puzzles to solve.The measured current expansion rate of the universe tends to be about 5 percent higher

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than what is predicted from the CMB, and scientists are not sure why, Riess says.

“It could be a sign that we do not totally understand the physics of the universe, or itcould be an error in either of the two measurements,” Riess says.

“It is a sign of tremendous progress in cosmology that we get upset and worried abouta 5 percent difference, whereas 15 or 20 years ago, measurements of the expansion ratecould differ by a factor of two.”

There is also much left to understand about dark matter and dark energy, whichappear to make up about 95 percent of the universe. “Our best chance to understand thenature of these unknown dark components is by making these kinds of precisemeasurements and looking for small disagreements or a loose thread that we can pull onto see if the sweater unravels.”

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breaking

August 17, 2015

Dark Energy Survey finds morecelestial neighborsThe observation of new dwarf galaxy candidates could mean our skyis more crowded than we thought.

Scientists on the Dark Energy Survey, using one of the world’s most powerful digitalcameras, have discovered eight more faint celestial objects hovering near our Milky Waygalaxy. Signs indicate that they—like the objects found by the same team earlier thisyear—are likely dwarf satellite galaxies, the smallest and closest known form of galaxies.

Satellite galaxies are small celestial objects that orbit larger galaxies, such as our ownMilky Way. Dwarf galaxies can be found with fewer than 1000 stars, in contrast to theMilky Way, an average-sized galaxy containing billions of stars. Scientists have predictedthat larger galaxies are built from smaller galaxies, which are thought to be especially richin dark matter, which makes up about 25 percent of the total matter and energy in theuniverse. Dwarf satellite galaxies, therefore, are considered key to understanding darkmatter and the process by which larger galaxies form.

The main goal of the Dark Energy Survey, as its name suggests, is to betterunderstand the nature of dark energy, the mysterious stuff that makes up about 70percent of the matter and energy in the universe. Scientists believe that dark energy isthe key to understanding why the expansion of the universe is speeding up. To carry outits dark energy mission, DES is taking snapshots of hundreds of millions of distantgalaxies. However, some of the DES images also contain stars in dwarf galaxies muchcloser to the Milky Way. The same data can therefore be used to probe both dark energy,which scientists think is driving galaxies apart, and dark matter, which is thought to holdgalaxies together.

Scientists can only see the nearest dwarf galaxies, since they are so faint, and had

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previously found only a handful. If these new discoveries are representative of the entiresky, there could be many more galaxies hiding in our cosmic neighborhood.

“Just this year, more than 20 of these dwarf satellite galaxy candidates have beenspotted, with 17 of those found in Dark Energy Survey data,” says Alex Drlica-Wagner ofFermi National Accelerator Laboratory, one of the leaders of the DES analysis. “We’venearly doubled the number of these objects we know about in just one year, which isremarkable.”

In March, researchers with the Dark Energy Survey and an independent team fromthe University of Cambridge jointly announced the discovery of nine of these objects insnapshots taken by the Dark Energy Camera, the extraordinary instrument at the heart ofthe DES, an experiment funded by the DOE, the National Science Foundation and otherfunding agencies. Two of those have been confirmed as dwarf satellite galaxies so far.

Prior to 2015, scientists had located only about two dozen such galaxies around theMilky Way.

“DES is finding galaxies so faint that they would have been very difficult to recognizein previous surveys,” says Keith Bechtol of the University of Wisconsin-Madison. “Thediscovery of so many new galaxy candidates in one-eighth of the sky could mean thereare more to find around the Milky Way.”

The closest of these newly discovered objects is about 80,000 light years away, andthe furthest roughly 700,000 light years away. These objects are, on average, around abillion times dimmer than the Milky Way and a million times less massive. The faintest ofthe new dwarf galaxy candidates has about 500 stars.

Most of the newly discovered objects are in the southern half of the DES survey area,in close proximity to the Large Magellanic Cloud and the Small Magellanic Cloud. Theseare the two largest satellite galaxies associated with the Milky Way, about 158,000 lightyears and 208,000 light years away, respectively. It is possible that many of these newobjects could be satellite galaxies of these larger satellite galaxies, which would be adiscovery by itself.

“That result would be fascinating,” says Risa Wechsler of SLAC National AcceleratorLaboratory. “Satellites of satellites are predicted by our models of dark matter. Either weare seeing these types of systems for the first time, or there is something we don’tunderstand about how these satellite galaxies are distributed in the sky.”

Since dwarf galaxies are thought to be made mostly of dark matter, with very fewstars, they are excellent targets to explore the properties of dark matter. Further analysiswill confirm whether these new objects are indeed dwarf satellite galaxies, and whethersigns of dark matter can be detected from them.

The 17 dwarf satellite galaxy candidates were discovered in the first two years of datacollected by the Dark Energy Survey, a five-year effort to photograph a portion of thesouthern sky in unprecedented detail. Scientists have now had a first look at most of thesurvey area, but data from the next three years of the survey will likely allow them to findobjects that are even fainter, more diffuse or farther away. The third survey season hasjust begun.

“This exciting discovery is the product of a strong collaborative effort from the entireDES team,” says Basilio Santiago, a DES Milky Way Science Working Group coordinatorand a member of the DES-Brazil Consortium. “We’ve only just begun our probe of thecosmos, and we’re looking forward to more exciting discoveries in the coming years.”

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This article is based on a Fermilab press release.

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signal to background

August 13, 2015

MicroBooNE sees first cosmicmuonsThe experiment will begin collecting data from a neutrino beam inOctober.By Ali Sundermier

A school bus-sized detector packed with 170 tons of liquid argon has seen its first particlefootprints.

On August 6, MicroBooNE, a liquid-argon time projection chamber, or LArTPC,recorded images of the tracks of cosmic muons, particles that shower down on Earthwhen cosmic rays collide with nuclei in our atmosphere.

"This is the first detector of this size and scale we've ever launched in the US for usein a neutrino beam, so it's a very important milestone for the future of neutrino physics,"says Sam Zeller, co-spokesperson for the MicroBooNE collaboration.

Picking up cosmic muons is just one brief stop during MicroBooNE's expedition intoparticle physics. The centerpiece of the three detectors planned for Fermilab's Short-Baseline Neutrino program, or SBN, MicroBooNE will pursue the much more elusiveneutrino, taking data about this weakly interacting particle for about three years.

When beam starts up in October, it will travel 470 meters and then traverse the liquidargon in MicroBooNE, where neutrino interactions will result in tracks that the detectorcan convert into precise three-dimensional images. Scientists will use these images toinvestigate anomalies seen in an earlier experiment called MiniBooNE, with the aim todetermine whether the excess of low-energy events that MiniBooNE saw was due to anew source of background photons or if there could be additional types of neutrinosbeyond the three established flavors.

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One of the first images of cosmic rays recorded by MicroBooNE.

Courtesy of: MicroBooNE collaboration

One of MicroBooNE's goals is to measure how often a neutrino that interacts with anargon atom will produce certain types of particles. A second goal is to conduct R&D forfuture large-scale LArTPCs.

MicroBooNE will carry signals up to 2.5 meters across the detector, the longest driftever for a LArTPC in a neutrino beam. This requires a very high voltage and very pureliquid argon. It is also the first time a detector will operate with its electronics submergedin liquid argon on such a large scale. All of these characteristics will be important forfuture experiments such as the Deep Underground Neutrino Experiment, or DUNE, whichplans to use similar technology to probe neutrinos.

"The entire particle physics community worldwide has identified neutrino physics asone of the key lines of research that could help us understand better how to go beyondwhat we know now," says Matt Toups, run coordinator and co-commissioner forMicroBooNE with Fermilab scientist Bruce Baller. "Those questions that are driving thefield, we hope to answer with a very large LArTPC detector."

Another benefit of the experiment, Zeller said, is training the next generation ofLArTPC experts for future programs and experiments. MicroBooNE is a collaborativeeffort of 25 institutions, with 55 students and postdocs working tirelessly to perfect thetechnology. Collaborators are keeping their eyes on the road toward the future of neutrinophysics and liquid-argon technology.

"It's been a long haul," says Bonnie Fleming, MicroBooNE co-spokesperson. "Eightand a half years ago liquid argon was a total underdog. I used to joke that no one wouldsit next to me at the lunch table. And it's a world of difference now. The field has chosenliquid argon as its future technology, and all eyes are on us to see if our detector willwork."

A version of this article was published in Fermilab Today.

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signal to background

August 12, 2015

Q&A: Underground machinistWhat’s it like being the machinist at the deepest machine shop inthe world?By Lauren Biron

The Majorana Demonstrator searches for a rare decay process a mile below the surfaceat Sanford Lab in Lead, South Dakota. To craft the experiment’s precise copper partsaway from cosmic rays, the lab evolved a unique feature: the deepest machine shop inthe world, complete with lathe, CNC mill, wire EDM (electrical discharge machine), 70-tonpress and a laser engraver to track the parts.

It is here that Randy Hughes comes to work every day and has for the last threeyears. He dons two pairs of booties, full white coveralls, glasses, two pairs of gloves anda facemask before he starts machining the majority of the pieces in the experiment, fromthick shield plates to microscopic pins.

Hughes, a motorcycle enthusiast and baseball fan from Detroit, brings 40 years ofexperience as a machinist and toolmaker to the job. He just happened to be working atAdams ISC in Rapid City when the experiment came around looking for a temporarycleanroom. The rest is history.

S: Had you worked on anything of this scale before?

RH: I don’t think anybody has done anything on this scale. I’ve had experiences thatwere more detailed and demanding as far as the product, but nothing in such anenvironment as this one.

S: How do you feel about working a mile underground?

RH: At first I was wondering if I was capable. They were preparing me to come down,and I’m wondering if I can handle it, which turned out to be a silly question. It’s like

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working anywhere else, but without a view. I can’t step outside and look out the window.And I can’t go out for lunch.

S: What’s a typical day?

RH: Presently, I’m finished with the string parts and working mostly on the largerparts, such as the shield. Two years ago, I was working mainly on string parts, whichwere all the smaller parts. As far as the inventory of what they need, I am winding downto the end of it. The shield parts could wait until the end. They didn’t want the surface tobe machined and then sit around.

S: Is there anything that has surprised you about working down here?

RH: The lack of vitamin D. I feel like a mole. On days where I travel to get here, Inever see the sun until the weekend. Hopefully it’s sunny out. I travel almost an hour toget here one way. I come through the Black Hills here every day. I like to tell people Ihave the most beautiful one-hour commute in the country. It’s along a creek and throughcanyons, and I see elk and eagles and deer.

S: What have been your favorite and least favorite things to work on?

RH: The least favorite thing was the hollow hex rods. Trying to thread them becausethey are so long, and then cutting the threads with no support and no coolant, has madefor a real balancing act as far as not breaking the parts off in the machine. The hex rod isthe main building block of the string. There are three of them where each detector is, andit stacks the build together.

Favorite parts, believe it or not, were little things, like the Vespel pins. It’s my favoritebecause of the sense of pride and the look I get from people when they see how small itis. The parts are no bigger than the ball of a ball-point pen. Vespel pins plug into voltageconnectors to hold the wire in place, so it doesn’t get pulled and tugged on other than atthat particular point.

S: What is unique about working on this job?

RH: Copper is not something that, as a machinist, you have to spend a lot of timemaking parts out of. The entire project, from my point of view, has been copper. Andcopper would be your last choice to make anything out of, for more than one reason.One, you wouldn’t think it would be durable enough, and two, it’s really soft, pliable andgummy, and it creates manufacturing problems.

S: What adaptations have you made?

RH: I’ve had to come up with procedures to accommodate the lack of equipment,tooling, coolant and processability. The way I set the tooling against the part, what kind offeeds and speeds I use to cut and the angle of the tools—it’s a lot of trial and error. I’vedeveloped a few little tricks that have paid off for me.

S: Do you ever practice on normal copper?

RH: There is some commercial copper down here for a few things, such as theprototype. What I was working on at the beginning was that copper. It machines almostexactly the same, but for the most part, I might be overconfident in my ability and I go atit. As a machinist, I have a machinist attitude: I can do anything. I can make anything.

S: What is the back-and-forth communication between the machine shop and thecleanroom?

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RH: I try to build in my head as I’m doing something. If I see something in numbersthat doesn’t make sense to me mechanically, all I have to do is pick up a phone or knockon the door, and I can ask them about it.

S: How do you feel about working in a cleanroom every day?

RH: At first, it was unique. And it was kinda novel. But after three years, it’smonotonous and trying. The first thing I do when I get out of this room is take thisfacemask off. The hood was given up for visibility’s sake. You steam up a lot, and I havemy fingers and hands around the machines. I need any kind of help with being able tosee where my hands are. Forty years, I still have all my digits.

S: Did you know anything about neutrinos or physics or Sanford Lab before this?

RH: Nooo. This was like a trip into outer space for me. It’s taken me all of these threeyears of small talk and side talk and listening to the scientists and physicists andstudents, and a few questions, and having them translate it into layman’s terms. I’velearned quite a bit about it. And that’s another reward from this job. When I start talkingabout this project, everybody stops and listens, because it’s unique, and it’s different,and it’s interesting.

S: What are your plans after this?

RH: I’m still employed with Adams, and they’re anticipating my return. They offeredme my old position back as shop foreman and are starting to pick up business and wouldreally like to have me back.

S: And you won’t have to wear this getup everyday.

RH: That would be a pleasurable change. But I’ll also be getting dirty every day. Theproblem with wearing the gloves every day is I’ve lost all the calluses on my hands. Andthe few times I’ve gone back there and worked, I always seem to get a metal shavingsliver or little cut because my hands are soft. I have to get toughened up again.

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