A joint Fermilab/SLAC publication

july 2015dimensionsofparticlephysicssymmetry

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Table of contents

Deconstruction: The Standard Model of particle physics

Signal to background: Something goes bump in the data

Signal to background: Miraculous WIMPs

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deconstruction

July 21, 2015

The Standard Model of particlephysicsExplore the elementary particles that make up our universe.By Kurt Riesselmann

The Standard Model is a kind of periodic table of the elements for particle physics. Butinstead of listing the chemical elements, it lists the fundamental particles that make up theatoms that make up the chemical elements, along with any other particles that cannot bebroken down into any smaller pieces.

The complete Standard Model took a long time to build. Physicist J.J. Thomsondiscovered the electron in 1897, and scientists at the Large Hadron Collider found thefinal piece of the puzzle, the Higgs boson, in 2012.

Use this interactive model to explore the different particles that make up the buildingblocks of our universe.

Quarks

uctdsb

Up Quark

Discovered in:

1968

Mass:

2.3 MeV

Discovered at:

3

SLAC

Charge:

2/3

Generation:

First

Spin:

1/2

About:

Up and down quarks make up protons and neutrons, which make up the nucleus ofevery atom.

Charm Quark

Discovered in:

1974

Mass:

1.275 GeV

Discovered at:

Brookhaven & SLAC

Charge:

2/3

Generation:

Second

Spin:

1/2

About:

In 1974, two independent research groups conducting experiments at twoindependent labs discovered the charm quark, the fourth quark to be found. Thesurprising discovery forced physicists to reconsider how the universe works at thesmallest scale.

Top Quark

Discovered in:

1995

4

Mass:

173.21 GeV

Discovered at:

Fermilab

Charge:

2/3

Generation:

Third

Spin:

1/2

About:

The top quark is the heaviest quark discovered so far. It has about the same weightas a gold atom. But unlike an atom, it is a fundamental, or elementary, particle; as far aswe know, it is not made of smaller building blocks.

Down Quark

Discovered in:

1968

Mass:

4.8 MeV

Discovered at:

SLAC

Charge:

-1/3

Generation:

First

Spin:

1/2

About:

Nobody knows why, but a down quark is a just a little bit heavier than an up quark. Ifthat weren’t the case, the protons inside every atom would decay and the universe wouldlook very different.

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Strange Quark

Discovered in:

1947

Mass:

95 MeV

Discovered at:

Manchester University

Charge:

-1/3

Generation:

Second

Spin:

1/2

About:

Scientists discovered particles with “strange" properties many years before it becameclear that those strange properties were due to the fact that they all contained a new,“strange” kind of quark. Theorist Murray Gell-Mann was awarded the Nobel Prize forintroducing the concepts of strangeness and quarks.

Bottom Quark

Discovered in:

1977

Mass:

4.18 GeV

Discovered at:

Fermilab

Charge:

-1/3

Generation:

Third

Spin:

6

1/2

About:

This particle is a heavier cousin of the down and strange quarks. Its discoveryconfirmed that all elementary building blocks of ordinary matter come in three differentversions.

Leptons

e???e????

Electron

Discovered in:

1897

Mass:

0.511 MeV

Discovered at:

Cavendish Laboratory

Charge:

-1

Generation:

First

Spin:

1/2

About:

The electron powers the world. It is the lightest particle with an electric charge and abuilding block of all atoms. The electron belongs to the family of charged leptons.

Muon

Discovered in:

1937

Mass:

105.66 MeV

Discovered at:

Caltech & Harvard

Charge:

7

-1

Generation:

Second

Spin:

1/2

About:

The muon is a heavier version of the electron. It rains down on us as it is created incollisions of cosmic rays with the Earth’s atmosphere. When it was discovered in 1937, aphysicist asked, “Who ordered that?”

Tau

Discovered in:

1976

Mass:

1776.82 MeV

Discovered at:

SLAC

Charge:

-1

Generation:

Third

Spin:

1/2

About:

The discovery of this particle in 1976 completely surprised scientists. It was the firstdiscovery of a particle of the so-called third generation. It is the third and heaviest of thecharged leptons, heavier than both the electron and the muon.

Electron Neutrino

Discovered in:

1956

Mass:

Muon Neutrino

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Discovered in:

1962

Mass:

Tau Neutrino

Discovered in:

2000

Mass:

Photon

Discovered in:

1923

Mass:

Gluon

Discovered in:

1979

Mass:

0

Discovered at:

DESY

Charge:

0

Spin:

1

About:

The gluon is the glue that holds together quarks to form protons, neutrons and otherparticles. It mediates the strong nuclear force.

Z Boson

Discovered in:

1983

Mass:

9

91.1876 GeV

Discovered at:

CERN

Charge:

0

Spin:

1

About:

The Z boson is the electrically neutral cousin of the W boson and a heavy relative ofthe photon. Together, these particles explain the electroweak force.

W Boson

Discovered in:

1983

Mass:

80.385 GeV

Discovered at:

CERN

Charge:

±1

Spin:

1

About:

The W boson is the only force carrier that has an electric charge. It’s essential forweak nuclear reactions: Without it, the sun would not shine.

Higgs Boson

Discovered in:

2012

Mass:

10

125.7 GeV

Discovered at:

CERN

Charge:

0

Spin:

0

About:

Discovered in 2012, the Higgs boson was the last missing piece of the StandardModel puzzle. It is a different kind of force carrier from the other elementary forces, and itgives mass to quarks as well as the W and Z bosons. Whether it also gives mass toneutrinos remains to be discovered.

Launch the interactive model »

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signal to background

July 16, 2015

Something goes bump in the dataThe CMS and ATLAS experiments at the LHC see somethingmysterious, but it’s too soon to pop the Champagne.By Katie Elyce Jones and Sarah Charley

An unexpected bump in data gathered during the first run of the Large Hadron Collider isstirring the curiosity of scientists on the two general-purpose LHC experiments, ATLASand CMS.

CMS scientists first published this bump in 2014. But because it was compatible withbeing a statistical fluke, they made no claim that they had observed a new particle.Recently ATLAS confirmed that they also see a bump in roughly the same place, and thistime it’s bigger and stronger.

“Both ATLAS and CMS are developing new search techniques that are greatlyimproving our ability to search for new particles,” says Ayana Arce, an assistantprofessor of physics at Duke University. “We can look for new physics in ways wecouldn’t before.”

Unlike the pronounced peak that recently led to the discovery of pentaquarks, thesetwo studies are in their nascent stages. And scientists aren’t quite sure what they’reseeing yet… or if they’re seeing anything at all.

If this bump matures into a sharp peak during the second run of the LHC, it couldindicate the existence of a new heavy particle with 2000 times the mass of a proton. Thediscovery of a new and unpredicted particle would revolutionize our understanding of thelaws of nature. But first, scientists have to rule out false leads.

“It’s like trying to pick up a radio station,” says theoretical physicist Bogdan Dobrescuof Fermi National Accelerator Laboratory who co-authored a paper on the bump in CMSand ATLAS data. “As you tune the dial, you think you’re beginning to hear voices

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through the static, but you can’t understand what they’re saying, so you keep tuninguntil you hear a clear voice.”

On the heels of the Higgs boson discovery in the first run of the LHC, scientists mustnavigate a tricky environment where people are hungry for new results while relying ondata that is slow to gather and laborious to interpret.

The data physicists are analyzing are particle decay patterns around 2 TeV, or 2000GeV.

“We can’t see short-lived particles directly, but we can reconstruct their mass basedon what they transform into during their decay,” says Jim Olsen, a professor of physics atPrinceton University. “For instance, we found the Higgs boson because we saw morepairs of W bosons, Z bosons and photons at 125 GeV than our background modelspredicted.”

Considering that the heaviest particle of the Standard Model, the top quark, has amass of 173 GeV, if this bump is real and not a fluctuation, it indicates a significantlyheavier particle than those covered in the Standard Model.

While the theories being batted around at this early stage disagree on the particulars,most agree that, so far, this data bump best fits the properties of an extended StandardModel gauge boson.

The gauge bosons are the force-carrying particles that enable matter particles tointeract with each other. The heaviest bosons are the W and Z bosons, which carry theweak force. An extended Standard Model predicts comparable particles at higherenergies, heavier versions known as W prime and Z prime (or W’ and Z’). Severaltheorists suggest the bump at 2 TeV could be a type of W prime.

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ATLAS data shows an increased number of W and Z boson pairs at 2 TeV.

Courtesy of: ATLAS collaboration

But LHC physicists aren’t practicing their Swedish for the Nobel ceremony yet.Unexpected bumps are common and almost always fizzle out with more data. Forinstance, in 2003 an international collaboration working on the Belle experiment at theKEK accelerator laboratory in Japan saw an apparent contradiction to the StandardModel’s predictions in the decay patterns of particles containing bottom quarks.

“It was really striking,” says Olsen. “The probability that the signal was due to sheerstatistical fluctuation was only about one in 10,000.”

Seven years later, after inundating their analysis with heaps of fresh data, the originalcontradiction from the Belle experiment withered and died, and from its ashes arose astronger result that perfectly matched the predictions of the Standard Model.

But scientists also haven’t written off this new bump as a statistical fluctuation. In fact,the closer they look, the more exciting it becomes.

With most anomalies in the data, one experiment will see it while the other onewon’t—a clear indication of a statistical fluctuation. But in this case, both CMS and ATLASindependently reported the same observation. And not only do both experiments see it,

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they see it at roughly the same energy across several different types of analyses.

“This is kind of like what we saw with the Higgs,” says JoAnne Hewett, a theoreticalphysicist at SLAC National Accelerator Laboratory who co-authored a paper theorizingthe bump could be a type of W prime particle. “The Higgs just started showing up as 2- to3-sigma bumps in a few different channels in the two different experiments. But therewere also false leads with the Higgs.”

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CMS data summarizing the search for new heavy particles decaying into severaldecay channels. The search reveals hints of a structure near 2000 GeV (2 TeV).

Courtesy of: CMS collaboration

Scientists are seeing more Z boson and W boson pairs popping up at 2 TeV than theStandard Model predicts. But besides this curious excess of events, they haven’tidentified any sort of clear pattern.

“Theorists come up with the models that predict the patterns we should see if there issome type of new physics influencing our experimental data,” Olsen says. “So if thisbump is new physics, then our models should predict what else we should see.”

Even though this bump is far too small to signify a discovery and presents nopredictable pattern, its presence across multiple different analyses from both CMS andATLAS is intriguing and suspicious. Scientists will have to patiently wait for more databefore they can flesh out what it actually is.

“We will soon have a lot more data from the second run of the LHC, and bothexperiments will be able to look more closely at this anomaly,” Arce says. “But I think itwould almost be too lucky if we discovered a new particle this soon into the second run ofthe LHC.”

The latest results from these two studies will be presented at the European PhysicalSociety conference in Vienna at the end of the month.

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signal to background

July 15, 2015

Miraculous WIMPsWhat are WIMPs, and what makes them such popular dark mattercandidates? By Manuel Gnida

Invisible dark matter accounts for 85 percent of all matter in the universe, affecting themotion of galaxies, bending the path of light and influencing the structure of the entirecosmos. Yet we don’t know much for certain about its nature.

Most dark matter experiments are searching for a type of particles called WIMPs, orweakly interacting massive particles.

“Weakly interacting” means that WIMPs barely ever “talk” to regular matter. Theydon’t often bump into other matter and also don’t emit light—properties that could explainwhy researchers haven’t been able to detect them yet.

Created in the early universe, they would be heavy (“massive”) and slow-movingenough to gravitationally clump together and form structures observed in today’suniverse.

Scientists predict that dark matter is made of particles. But that assumption is basedon what they know about the nature of regular matter, which makes up only about 4percent of the universe.

WIMPs advanced in popularity in the late 1970s and early 1980s when scientistsrealized that particles that naturally pop out in models of Supersymmetry could potentiallyexplain the seemingly unrelated cosmic mystery of dark matter.

Supersymmetry, developed to fill gaps in our understanding of known particles andforces, postulates that each fundamental particle has a yet-to-be-discoveredsuperpartner. It turns out that the lightest one of the bunch has properties that make it a

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top contender for dark matter.

“The lightest supersymmetric WIMP is stable and is not allowed to decay into otherparticles,” says theoretical physicist Tim Tait of the University of California, Irvine. “Oncecreated in the big bang, many of these WIMPs would therefore still be around today andcould have gone unnoticed because they rarely produce a detectable signal.”

When researchers use the properties of the lightest supersymmetric particle tocalculate how many of them would still be around today, they end up with a number thatmatches closely the amount of dark matter experimentally observed—a link referred to asthe “WIMP miracle.” Many researchers believe it could be more than coincidence.

“But WIMPs are also popular because we know how to look for them,” says darkmatter hunter Thomas Shutt of Stanford University and SLAC National AcceleratorLaboratory. “After years of developments, we finally know how to build detectors thathave a chance of catching a glimpse of them.”

Shutt is co-founder of the LUX experiment and one of the key figures in thedevelopment of the next-generation LUX-ZEPLIN experiment. He is one member of thegroup of scientists trying to detect WIMPs as they traverse large, underground detectors.

Other scientists hope to create them in powerful particle collisions at CERN’s LargeHadron Collider. “Most supersymmetric theories estimate the mass of the lightest WIMPto be somewhere above 100 gigaelectronvolts, which is well within LHC’s energyregime,” Tait says. “I myself and others are very excited about the recent LHC restart.There is a lot of hope to create dark matter in the lab.”

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A third way of searching for WIMPs is to look for revealing signals reaching Earth fromspace. Although individual WIMPs are stable, they decay into other particles when two ofthem collide and annihilate each other. This process should leave behind detectableamounts of radiation. Researchers therefore point their instruments at astronomicalobjects rich in dark matter such as dwarf satellite galaxies orbiting our Milky Way or thecenter of the Milky Way itself.

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“Dark matter interacts with regular matter through gravitation, impacting structureformation in the universe,” says Risa Wechsler, a researcher at Stanford and SLAC. “Ifdark matter is made of WIMPs, our predictions of the distribution of dark matter based onthis assumption must also match our observations.”

Wechsler and others calculate, for example, how many dwarf galaxies our Milky Wayshould have and participate in research efforts under way to determine if everythingpredicted can also be found experimentally.

So how would researchers know for sure that dark matter is made of WIMPs? “Wewould need to see conclusive evidence for WIMPs in more than one experiment, ideallyusing all three ways of detection,” Wechsler says.

In the light of today’s mature detection methods, dark matter hunters should be ableto find WIMPs in the next five to 10 years, Shutt, Tait and Wechsler say. Time will tell ifscientists have the right idea about the nature of dark matter.

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