A joint Fermilab/SLAC publication

april 2015dimensionsofparticlephysicssymmetry

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Table of contents

Breaking: US scientists celebrate the restart of the LHC

Feature: Our flat universe

Contest: Physics Madness Grand Champion

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breakingApril 02, 2015

US scientists celebrate the restartof the LHCThe Large Hadron Collider circulates the first beam of Run 2.

Earlier today, the worlds most powerful particle accelerator began its second act. Aftertwo years of upgrades and repairs, proton beams once again circulated around the LargeHadron Collider, located at the CERN laboratory near Geneva, Switzerland.

With the collider back in action, the more than 1700 US scientists who work on LHCexperiments are prepared to join thousands of their international colleagues to study thehighest-energy particle collisions ever achieved in the laboratory.

These collisions hundreds of millions of them every second will lead scientists tonew and unexplored realms of physics, and could yield extraordinary insights into thenature of the physical universe.

A highlight of the LHCs first run, which began in 2009, was the discovery of theHiggs boson, the last in the suite of elementary particles that make up scientists bestpicture of the universe and how it works. The discovery of the Higgs was announced inJuly 2012 by two experimental collaborations, ATLAS and CMS. Continuing to measurethe properties of the Higgs will be a major focus of LHC Run 2.

The Higgs discovery was one of the most important scientific achievements of ourtime, says James Siegrist, the US Department of Energys Associate Director ofScience for High Energy Physics. With the LHC operational again, at even higherenergies, the possibilities for new discoveries are endless, and the United States will beat the forefront of those discoveries.

During the LHCs second run, particles will collide at a staggering 13 teraelectronvolts(TeV), which is 60 percent higher than any accelerator has achieved before. The LHCsfour major particle detectors ATLAS, CMS, ALICE and LHCb will collect and analyzedata from these collisions, allowing them to probe new areas of research that were

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previously unattainable.

At 17 miles around, the Large Hadron Collider is one of the largest machines everbuilt. The United States played a vital role in the construction of the LHC and the hugeand intricate detectors for its experiments. Seven US Department of Energy nationallaboratories joined roughly 90 US universities to build key components of the accelerator,detectors and computing infrastructure, with funding from the DOE Office of Science andthe National Science Foundation.

The US contingent was part of an estimated 10,000 people from 113 differentcountries who helped to design, build, and upgrade the LHC accelerator and its fourparticle detectors.

We are on the threshold of an exciting time in particle physics: the LHC will turn onwith the highest energy beam ever achieved," says Fleming Crim, National ScienceFoundation Assistant Director for Mathematical and Physical Sciences. "This energyregime will open the door to new discoveries about our universe that were impossible asrecently as two years ago.

In addition to the scientists pushing toward new discoveries on the four mainexperiments, the US provides a significant portion of the computing and data analysis roughly 23 percent for ATLAS and 33 percent for CMS. US scientists on the ALICEexperiment developed control and tracking systems for the detector and made significantcontributions in software, hardware and computing support. US scientists also helpedimprove trigger software for data analysis for the LHCb experiment.

US institutions will continue to make important contributions to the LHC and itsexperiments, even beyond the second run, which is scheduled to continue through themiddle of 2018. Universities and national laboratories are developing new accelerator anddetector technology for future upgrades of the LHC and its experiments. This ongoingwork encourages a strong partnership between science and industry, and drivestechnological innovation in the United States.

"Operating accelerators for the benefit of the physics community is what CERNshere for, says CERN Director General Rolf Heuer. "Today, CERNs heart beats oncemore to the rhythm of the LHC.

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

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featureApril 07, 2015

Our flat universeNot a curve in sight, as far as the eye can see.By Lauren Biron

Mathematicians, scientists, philosophers and curious minds alike have guessed at theshape of our universe. There are three main options to choose from, in case youd like todo some digging of your own:

The universe could be positively curved, like a sphere.

The universe could be negatively curved, like a saddle.

The universe could be flat, like a sheet of paper.

As far as scientists can tell, this third option is correct. But what do people really meanwhen they talk about flatness? Your high school math teacher would be overjoyed totell you that its all about geometry.

In a flat universe, Euclidean geometry applies at the very largest scales. This meansparallel lines will never meet, and the internal angles of a triangle always add up toexactly 180 degreesjust like youre used to.

But in curved universes, whether finite or infinite, things get weird. In a positivelycurved universe, space bulges, skewing parallel lines toward a single point and inflatingthe sum of angles in a triangle to more than 180 degrees. In a negatively curved space,parallel lines diverge forever and triangle angles get pinched, so the sum is less than 180.

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Armed with this knowledge, how do scientists know the universe is flat? The answer iswritten in the sky, etched on the background radiation streaking at us from everydirection. This so-called cosmic microwave background is a snapshot of our universe atone of its earliest moments, when photons (packets of light) were freed from a hot plasmasoup produced in the big bang. Scientists also study the structure of galaxies over largescales to test their conclusions.

The cosmic microwave background in combination with the distribution of galaxiesreally nails down the flatness, says Josh Frieman, a physicist at the Fermilab Center forParticle Astrophysics. But, he adds, the CMB is the linchpin.

The techniques use similar approaches: Scientists compare the apparent size offeatures (ripples in the CMB or galactic clumps) with how big they actually are. Anydifference indicates distortion caused by the curvature of space. A variety ofexperimentsincluding the game-changing Wilkinson Microwave Anisotropy Probe,launched in 2001, and the more recent Planck surveyorhave lent support to the idea of aflat universe.

Back when we first started with WMAP, we didnt know the geometry at all, saysDavid Spergel, a theoretical astrophysicist at Princeton University who worked on WMAP.Now were doing sub-percent measurements.

Theres also potential to improve the measurement of flatness with current andupcoming experiments including the Atacama Cosmology Telescope, Dark EnergySurvey, Large Synoptic Survey Telescope, Polar Bear Telescope, South Pole Telescopeand Square Kilometer Array. Researchers on these projects have different aims, andmeasuring the curvature of the universe is often just a byproduct of the main scientificgoal. But its an important one.

Besides being a fundamental feature of the universe, curvature helps constrain othermeasurements, such as the influence of dark energy, the mysterious force driving theaccelerating expansion of our universe. It also affects the model scientists have of aninflationary universe. That model predicts the flatness we see today. If more precisemeasurements showed a departure from flatness, they would indicate that theories aboutthe early universe need to be tweaked.

There arent many handles on what happened at 10-35 seconds after the big bang,but curvature is one of them, Frieman says.

The shape of the universe is a clue to its origin and may hold a key to its fate. Theshape and density of matter in the universe and the strength of dark energy ultimatelydecide whether the universe will contract back together in a big crunch or spread out andsuffer a heat death.

On the largest of all scales, it is still possible that the universe is curved, beyond theedge of our perception. Much like standing in the middle of the Great Plains might leadyou to believe the Earth is flat, our understanding of the universe might be limited by our

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vantage point and the horizon of our visible universe. Theres a chance that the universeis a sphere, or a donut, or a saddle, or a dodecahedron, or some kind of twisted manifold.But if it is, Spergel says, its several times larger than our observable universe.

All we really know is the universe is close to flat and its large, he says. Verylarge.

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contestApril 06, 2015

Physics Madness GrandChampionAnd your 2015 winning physics machine isBy Lauren Biron

The Dark Energy Camera! The plucky DECam and heavily favorited Large HadronCollider beat out 14 solid competitors to make it to the final round, but only one physicsmachine could emerge victorious. Perhaps while the LHC was starting up for Run 2,DECam fans were voting for one of the most powerful imaging devices in the world. Ormaybe the recent photobomb by Comet Lovejoy won the hearts of undecided voters,pushing DECam ahead.

Thanks to all those who voted for their favorite and learned a bit more about some ofthe amazing pieces of physics equipment that drive discoveries at the smallest andlargest scales. There are many more that didnt make it into our bracket this yearbuttheres always 2016. Until then, congratulations DECam!

Dark Energy Camera

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Boasting a staggering 570 megapixels, DECam is the most powerful digital camera inthe world. It hunts dark energy, the mysterious force pushing the universe apart, as a partof the Dark Energy Survey. But thats not all it does. While surveying the skies, it alsocomes across goodies such as asteroids, supernovae and trans-Neptunian objects. Froma mountaintop in Chile, DECam can see light from up to 8 billion light-years away andcapture more than 100,000 galaxies in each snapshot.

More info

Out in Round 4

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Large Hadron ColliderThe LHC is the most powerful particle accelerator in the world. The machine

accelerates protons and other ions close to the speed of light and guides them around a17-mile tunnel using magnets that generate fields 100,000 times as strong as theEarths. Scientists smash particles together to recreate in miniature the conditions afterthe big bang. Experiments at the LHC discovered the Higgs boson in 2012. When itrestarts this year, the LHC will search for things like supersymmetry, dark matter andextra dimensions.

More info

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Out in Round 3

Fermi TelescopeThe Fermi Space Telescope peers into the gamma ray universe, gathering

information about objects emitting high-energy light to answer questions about blackholes, pulsars, dark matter, quantum gravity and cosmic rays. The telescope, which orbitsthe Earth every 95 minutes, has already recorded the highest-energy gamma ray burstand solar flare ever observed by scientists. It also discovered and studied more than 150gamma-ray pulsars, several dozen of which are seen to pulse only in gamma rays.

More info

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Hubble Space TelescopeSince it was launched into space in 1990, Hubble has taken stunning images of

galaxies and nebulae in ultraviolet, visible and near-infrared wavelengths. It helpednarrow down the Hubble constant, the rate at which the universe is expanding, whichhelps drive our understanding of dark energy. Scientists used Hubble to discover Plutosfifth moon, find evidence of proto-planetary disks, and identify black holes at the center ofgalaxies.

More info

Out in Round 2

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Planck

This space telescope, run by the European Space Agency with significantparticipation from NASA, gave us the worlds most precise map of the universe in themoments after the big bang. Through Planck, scientists have refined the standard modelof cosmology and homed in on the properties of neutrinos.

More info

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Holometer

Using a laser and mirrors, the Holometer is probing the smallest scale of spacethePlanck scale, 10 trillion trillion times smaller than an atomto determine if the universe islike a hologram. Scientists are checking to see if the information of our universe could becoded in tiny packets in two dimensions, creating a pixelated universe that just appearssmooth and three-dimensional from our everyday perspective.

More info

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Super KSuper-Kamiokande is a massive neutrino experiment containing 50,000 tons of

ultrapure water, housed 1000 meters below a mountain in Japan. Super K helpsresearchers study whether protons decay and is one of the heavy hitters of neutrinoresearch. It collected the largest sample of solar neutrinos in real time and was the firstexperiment to detect oscillations of atmospheric neutrinos. Its observations implied thatthe ghostly particleswhich were predicted to be masslesshave a mass after all, amystery that has yet to be explained.

More info

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IceCubeMade of a cubic kilometer of Antarctic ice, IceCube is the worlds largest neutrino

detector. In 2013, some of the 5,000+ sensors strung down 86 holes drilled into the icepicked up signals from the highest-energy neutrinos ever found, nicknamed Bert, Ernieand Big Bird. IceCube scientists aim to discover the cosmic sources that produce thesehigh-speed intergalactic visitors. The site is also an enormous muon detector that seesmore than 100 billion muons every year.

More info

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Out in Round 1

LUX

The Large Underground Xenon dark-matter detector is the current record-holder formost sensitive experiment searching for the most popular type of dark matter particle. Itssix-foot titanium tank holds liquid xenon at minus 150 degrees Fahrenheit a mile belowground in a former gold mine. LUX made news in 2013 when it released the worlds moststringent constraints on dark-matter particles and shot down potential hints of dark matterreported by other groups.

More info

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Borexino

This massive solar neutrino detector, located in a cavern in the Gran Sassomountains in Italy, contains 300 tonnes of active liquid scintillator within an 11-metersphere surrounded by 2000 photomultiplier tubes. Solar neutrinos interact with thescintillator, letting scientists measure the rate of nuclear reactions powering the sun. In2014, scientists used it to discover that the sun releases as much energy today as it did100,000 years ago. Borexino also found evidence of geoneutrinos, particles created byradioactive decay within the Earth.

More info

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CEBAFThe Continuous Electron Beam Accelerator Facility at Jefferson Lab was the worlds

first large-scale application of superconducting radio-frequency (SRF) technology. Theaccelerator, which has been upgraded and is now running at almost three times itsoriginal design energy, is a powerful tool to investigate the structure of an atoms nucleusdown to the level of quarks and the glue that holds them together.

More info

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Daya BayThe Daya Bay Reactor Neutrino Experiment, based in China, uses 110-ton

antineutrino detectors to track the ghostly particles produced at nearby nuclear powerplants. The experiment is known for discovering a hard-to-measure property of neutrinooscillations, a key piece of the neutrino puzzle scientists had been trying to solve for adecade. While gathering information on how neutrinos morph from one type to another,Daya Bay detectors amassed the most data on antineutrinos from a group of nuclearreactors to date.

More info

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DEAP

The DEAP-3600 detector at Canadas SNOLAB is scheduled to begin taking datalater this spring. Located 2km below ground in Vale Creighton Mine, it will be the mostsensitive particle detector to search for the most sought-after type of dark matter, theWIMP. Its massive sphere, which will contain 565 imperial gallons of liquid argon, will be20 times more sensitive to finding these types of particles than the current best.

More info

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Fermilab Neutrino Beam

When youre trying to study particles as hard to detect as neutrinos, it helps to makeyour own and start with a whole bunch of them. Fermilabs particle accelerators makethe most powerful beam of high-energy neutrinos in the world. Two focusing horns tunethe energy and shape of the beam and then send the neutrinos straight through the earthto detectors 450 and 500 miles away. Upgrades to the accelerator complex will increasethe beams intensity even further.

More info

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RHICBrookhaven Labs Relativistic Heavy Ion Collider was the first machine to create

quark-gluon plasma, a state of matter that existed a fraction of a second after the bigbang. At the time, it was the hottest matter ever produced in a laboratory. RHIC is theonly machine in the world able to collide beams of polarized protons and was the first thatcould collide ions as heavy as gold.

More info

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LIGOThe Laser Interferometer Gravitational-Wave Observatory will use the worlds largest

precision optical instruments to test Einsteins prediction that massive objects moving inspace send out ripples in spacetime. LIGO seeks to measure these ripples as theydisturb beams of light traveling through its 4-kilometer tunnels and to use them to furtherinvestigate the nature of gravity and the cosmos. Its instruments are so sensitive, theycan see a change on the scale of a thousandth the size of a proton.

More info

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