A joint Fermilab/SLAC publication

may 2015dimensionsofparticlephysicssymmetry

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Table of contents

Application: The accelerator in the Louvre

Breaking: LHC experiments first to observe rare process

Signal to background: Looking to the heavens for neutrino masses

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application

May 14, 2015

The accelerator in the LouvreThe Accélérateur Grand Louvre d’analyse élémentaire solvesancient mysteries with powerful particle beams.By Glenn Roberts Jr. and Kelen Tuttle

In a basement 15 meters below the towering glass pyramid of the Louvre Museum inParis sits a piece of work the curators have no plans to display: the museum’s particleaccelerator.

This isn’t a Dan Brown novel. The Accélérateur Grand Louvre d’analyse élémentaireis real and has been a part of the museum since 1988.

Researchers use AGLAE’s beams of protons and alpha particles to find out whatartifacts are made of and to verify their authenticity. The amounts and combinations ofelements an object contains can serve as a fingerprint hinting at where minerals weremined and when an item was made.

Scientists have used AGLAE to check whether a saber scabbard gifted to NapoleonBonaparte by the French government was actually cast in solid gold (it was) and toidentify the minerals in the hauntingly lifelike eyes of a 4500-year-old Egyptian sculptureknown as The Seated Scribe (black rock crystal and white magnesium carbonate veinedwith thin red lines of iron oxide).

“What makes the AGLAE facility unique is that our activities are 100 percentdedicated to cultural heritage,” says Claire Pacheco, who leads the team that operatesthe machine. It is the only particle accelerator that has been used solely for this field ofresearch.

Pacheco began working with ion-beam analysis at AGLAE while pursuing a doctoratedegree in ancient materials at France’s University of Bordeaux. She took over as its leadscientist in 2011 and now operates the particle accelerator with a team of threeengineers.

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Jean-Claude Dran, a scientist who worked with AGLAE during its early days andserved for several years as a scientific advisor, says the study methods pioneered forAGLAE are uniquely suited to art and archaeological artifacts. “These techniques arevery powerful, very accurate and very sensitive to trace elements.”

Photo by: V. Fournier, C2RMF

Crucially, they are also non-destructive in most cases, Pacheco says.

“Of course, AGLAE is non-invasive, which is priority No. 1 for cultural heritage” shesays. The techniques used at AGLAE include particle-induced X-ray and gamma-rayemission spectrometries, which can identify the slightest traces of elements ranging fromlithium to uranium.

Before AGLAE, research facilities typically required samples to be placed in apotentially damaging vacuum for similar materials analysis. Researchers hoping to studypieces too large for a vacuum chamber were out of luck. AGLAE, because its beamswork outside the vacuum, allows researchers to study objects of any size and shape.

The physicists and engineers who conduct AGLAE experiments typically work hand-in-hand with curators and art historians.

While AGLAE frequently studies items from the local collection, it has a larger missionto study art and relics from museums all around France. It is also available to outsideresearchers, who have used it on pieces from museums such as the J. Paul GettyMuseum in Los Angeles and the Metropolitan Museum of Art in New York.

AGLAE has been used to study glasses, metals and ceramics. In one case,Pacheco’s team wanted to know the origins of pieces of lusterware, a type of ceramicthat takes on a metallic shine when kiln-fired. The technique emerged in ninth-centuryMesopotamia and was spread all around the Mediterranean during the Muslimconquests. It had mostly faded by the 17th century, but some potters in Spain still carry

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on the tradition.

Pacheco’s team used AGLAE to pinpoint the elements in the lusterware, and thenthey mixed up batches of raw materials from different locations. “What we have tried todo is make a kind of ‘identity card’ for every production center at every period in time,”Pacheco says.

Another, recently published study details how AGLAE was also used to analyze thechemical signature of traces of decorative paint on ivory tusks. Pacheco’s teamdetermined that the tusks were likely painted during the seventh century B.C.

A limitation of the AGLAE particle analysis techniques is that they are not veryeffective for studying paintings because of a slight risk of damage. But Pacheco says thatan upgrade now in progress aims to produce a lower-power beam that, coupled withmore sensitive detectors, could solve this problem.

Dubbed NEW AGLAE, the upgraded setup could boost automation to allow theaccelerator to operate around the clock—it now operates only during the day.

While public tours are not permitted of AGLAE, Pacheco says there are frequent visitsby researchers working in cultural heritage.

“It’s so marvelous,” she says. “We are very, very lucky to work in this environment,to study these objects.”

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breaking

May 13, 2015

LHC experiments first to observerare processA joint result from the CMS and LHCb experiments precludes orlimits several theories of new particles or forces.

Two experiments at the Large Hadron Collider at CERN have combined their results andobserved a previously unseen subatomic process.

As published in the journal Nature this week, a joint analysis by the CMS and LHCbcollaborations has established a new and extremely rare decay of the Bs particle—a heavycomposite particle consisting of a bottom antiquark and a strange quark—into two muons.Theorists had predicted that this decay would only occur about four times out of a billion,and that is roughly what the two experiments observed.

“It’s amazing that this theoretical prediction is so accurate and even more amazingthat we can actually observe it at all,” says Syracuse University Professor SheldonStone, a member of the LHCb collaboration. “This is a great triumph for the LHC andboth experiments.”

LHCb and CMS both study the properties of particles to search for cracks in theStandard Model, our best description so far of the behavior of all directly observablematter in the universe. The Standard Model is known to be incomplete since it does notaddress issues such as the presence of dark matter or the abundance of matter overantimatter in our universe. Any deviations from this model could be evidence of newphysics at play, such as new particles or forces that could provide answers to thesemysteries.

“Many theories that propose to extend the Standard Model also predict an increase inthis Bs decay rate,” says Fermilab’s Joel Butler of the CMS experiment. “This new resultallows us to discount or severely limit the parameters of most of these theories. Anyviable theory must predict a change small enough to be accommodated by the remaining

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uncertainty.”

Courtesy of: LHCb collaboration

Researchers at the LHC are particularly interested in particles containing bottomquarks because they are easy to detect, abundantly produced and have a relatively longlifespan, according to Stone.

“We also know that Bs mesons oscillate between their matter and their antimattercounterparts, a process first discovered at Fermilab in 2006,” Stone says. “Studying theproperties of B mesons will help us understand the imbalance of matter and antimatter inthe universe.”

That imbalance is a mystery scientists are working to unravel. The big bang thatcreated the universe should have resulted in equal amounts of matter and antimatter,annihilating each other on contact. But matter prevails, and scientists have not yetdiscovered the mechanism that made that possible.

“The LHC will soon begin a new run at higher energy and intensity,” Butler says.“The precision with which this decay is measured will improve, further limiting the viableStandard Model extensions. And of course, we always hope to see the new physicsdirectly in the form of new particles or forces.”

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Courtesy of: CMS collaboration

Fermilab published a version of this article as a press release.

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signal to background

May 19, 2015

Looking to the heavens forneutrino massesScientists are using studies of the skies to solve a neutrino mystery.By Matthew R. Francis

Neutrinos may be the lightest of all the particles with mass, weighing in at a tiny fractionof the mass of an electron. And yet, because they are so abundant, they played asignificant role in the evolution and growth of the biggest things in the universe: galaxyclusters, made up of hundreds or thousands of galaxies bound together by mutualgravity.

Thanks to this deep connection, scientists are using these giants to study the tinyparticles that helped form them. In doing so, they may find out more about thefundamental forces that govern the universe.

Curiously light

When neutrinos were first discovered, scientists didn’t know right away if they had anymass. They thought they might be like photons, which carry energy but are intrinsicallyweightless.

But then they discovered that neutrinos came in three different types and that theycan switch from one type to another, something only particles with mass could do.

Scientists know that the masses of neutrinos are extremely light, so light that theywonder whether they come from a source other than the Higgs field, which gives mass tothe other fundamental particles we know. But scientists have yet to pin down the exactsize of these masses.

It’s hard to measure the mass of such a tiny particle with precision.

In fact, it’s hard to measure anything about neutrinos. They are electrically neutral, so

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they are immune to the effects of magnetic fields and related methods physicists use todetect particles. They barely interact with other particles at all: Only a more-or-less directhit with an atomic nucleus can stop a neutrino, and that doesn’t happen often.

Roughly a trillion neutrinos pass through your body each second from the sun alone,and almost none of those end up striking any of your atoms. Even the densest matter isnearly transparent to neutrinos. However, by creating beams of neutrinos and by buildinglarge, sensitive targets to catch neutrinos from nuclear reactors and the sun, scientistshave been able to detect a small portion of the particles as they pass through.

In experiments so far, scientists have estimated that the total mass of the three typesof neutrinos together is roughly between 0.06 electronvolts and 0.2 electronvolts. Forcomparison, an electron’s mass is 511 thousand electronvolts and a proton weighs in at938 million electronvolts.

Because the Standard Model—the theory describing particles and the interactionsgoverning them—predicts massless neutrinos, finding the exact neutrino mass value willhelp physicists modify their models, yielding new insights into the fundamental forces ofnature.

Studying galaxy clusters could provide a more precise answer.

Footprints of a neutrino

One way to study galaxy clusters is to measure the cosmic microwave background, thelight traveling to us from 380,000 years after the big bang. During its 13.8-billion-yearjourney, this light passed through and near all the galaxies and galaxy clusters thatformed. For the most part, these obstacles didn’t have a big effect, but takencumulatively, they filtered the CMB light in a unique way, given the galaxies’ number,size and distribution.

The filtering affected the polarization—the orientation of the electric part of light—andoriginated in the gravitational field of galaxies. As CMB light traveled through thegravitational field, its path curved and its polarization twisted very slightly, an effect knownas gravitational lensing. (This is a less dramatic version of lensing familiar from thebeautiful Hubble Space Telescope images.)

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The effect is similar to the one that got everyone excited in 2014, when researcherswith the BICEP2 telescope announced they had measured the polarization of CMB lightdue to primordial gravitational waves, which subsequent study showed to be more ambiguous.

That ambiguity won’t be a problem here, says Oxford University cosmologist ErminiaCalabrese, who studies the CMB on the Atacama Cosmology Telescope Polarizationproject. “There is one pattern of CMB polarization that is generated only by the deflectionof the CMB radiation.” That means we won’t easily mistake gravitational lensing foranything else.

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Small and mighty

Manoj Kaplinghat, a physicist at the University of California at Irvine, was one of the firstto work out how neutrino mass could be estimated from CMB data alone. Neutrinos movevery quickly relative to stuff like atoms and the invisible dark matter that binds galaxiestogether. That means they don’t clump up like other forms of matter, but their small massstill contributes to the gravitational field.

Enough neutrinos, even fairly low-mass ones, can deprive a newborn galaxy of anoticeable amount of mass as they stream away, possibly throttling the growth ofgalaxies that can form in the early universe. It’s nearly as simple as that: Heavierneutrinos mean galaxies must grow more slowly, while lighter neutrinos mean fastergalaxy growth.

Kaplinghat and colleagues realized the polarization of the CMB provides a measurethe total amount of gravity from galaxies in the form of gravitational lensing, whichworking backward will constrain the mass of neutrinos. “When you put all that together,what you realize is you can do a lot of cool neutrino physics,” he says.

Of course the CMB doesn’t provide a direct measurement of the neutrino mass. Fromthe point of view of cosmology, the three types of neutrinos are indistinguishable. As aresult, what CMB polarization gives us is the total mass of all three types together.

However, other projects are working on the other end of this puzzle. Experimentssuch as the Main Injector Neutrino Oscillation Search, managed by Fermilab, havedetermined the differences in mass between the different neutrino types.

Depending on which neutrino is heaviest, we know how the masses of the other two

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types of neutrinos relate. If we can figure out the total mass, we can figure out themasses of each one. Together, cosmological and terrestrial measurements will get us theindividual neutrino masses that neither is able to alone.

The space-based Planck observatory and POLARBEAR project in northern Chilehave yielded preliminary results in this search already. And scientists at ACTPol, locatedat high elevation in Chile’s Atacama Desert, are working on this as well. They willdetermine the neutrino mass as well as the best estimates we have, down to the lowestpossible values allowed, once the experiments are running at their highest precision,Calabrese says.

Progress is necessarily slow: The gravitational lensing pattern comes from seeingsmall patterns emerging from light captured across a large swath of the sky, much likethe image in an Impressionist painting arises from abstract brushstrokes that look likevery little by themselves.

In more scientific terms, it’s a cumulative, statistical effect, and the more data wehave, the better chance we have to measure the lensing effect—and the mass of aneutrino.

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