A joint Fermilab/SLAC publication

july 2015dimensionsofparticlephysicssymmetry

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Table of contents

Feature: Is this the only universe?

Breaking: W bosons remain left-handed

Breaking: A new first for T2K

Signal to background: Underground plans

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feature

July 28, 2015

Is this the only universe?Our universe could be just one small piece of a bubbling multiverse.By Laura Dattaro

Human history has been a journey toward insignificance.

As we’ve gained more knowledge, we’ve had our planet downgraded from the center ofthe universe to a chunk of rock orbiting an average star in a galaxy that is one amongbillions.

So it only makes sense that many physicists now believe that even our universe mightbe just a small piece of a greater whole. In fact, there may be infinitely many universes,bubbling into existence and growing exponentially. It’s a theory known as the multiverse.

One of the best pieces of evidence for the multiverse was first discovered in 1998,when physicists realized that the universe was expanding at ever increasing speed. Theydubbed the force behind this acceleration dark energy. The value of its energy density,also known as the cosmological constant, is bizarrely tiny: 120 orders of magnitudesmaller than theory says it should be.

For decades, physicists have sought an explanation for this disparity. The best onethey’ve come up with so far, says Yasunori Nomura, a theoretical physicist at theUniversity of California, Berkeley, is that it’s only small in our universe. There may beother universes where the number takes a different value, and it is only here that the rateof expansion is just right to form galaxies and stars and planets where people like us canobserve it. “Only if this vacuum energy stayed to a very special value will we exist,”Nomura says. “There are no good other theories to understand why we observe thisspecific value.”

For further evidence of a multiverse, just look to string theory, which posits that thefundamental laws of physics have their own phases, just like matter can exist as a solid,liquid or gas. If that’s correct, there should be other universes where the laws are indifferent phases from our own—which would affect seemingly fundamental values that we

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observe here in our universe, like the cosmological constant. “In that situation you’ll havea patchwork of regions, some in this phase, some in others,” says Matthew Kleban, atheoretical physicist at New York University.

These regions could take the form of bubbles, with new universes popping intoexistence all the time. One of these bubbles could collide with our own, leaving tracesthat, if discovered, would prove other universes are out there. We haven't seen one ofthese collisions yet, but physicists are hopeful that we might in the not so distant future.

If we can’t find evidence of a collision, Kleban says, it may be possible toexperimentally induce a phase change—an ultra-high-energy version of coaxing water intovapor by boiling it on the stove. You could effectively prove our universe is not the onlyone if you could produce phase-transitioned energy, though you would run the risk of itexpanding out of control and destroying the Earth. “If those phases do exist—if they canbe brought into being by some kind of experiment—then they certainly exist somewhere inthe universe,” Kleban says.

No one is yet trying to do this.

There might be a (relatively) simpler way. Einstein’s general theory of relativityimplies that our universe may have a “shape.” It could be either positively curved, like asphere, or negatively curved, like a saddle. A negatively curved universe would be strongevidence of a multiverse, Nomura says. And a positively curved universe would show thatthere’s something wrong with our current theory of the multiverse, while not necessarilyproving there’s only one. (Proving that is a next-to-impossible task. If there are otheruniverses out there that don’t interact with ours in any sense, we can’t prove whetherthey exist.)

In recent years, physicists have discovered that the universe appears almost entirelyflat. But there’s still a possibility that it’s slightly curved in one direction or the other, andNomura predicts that within the next few decades, measurements of the universe’sshape could be precise enough to detect a slight curvature. That would give physicistsnew evidence about the nature of the multiverse. “In fact, this evidence will bereasonably strong since we do not know any other theory which may naturally lead to anonzero curvature at a level observable in the universe,” Nomura says.

If the curvature turned out to be positive, theorists would face some very difficultquestions. They would still be left without an explanation for why the expansion rate ofthe universe is what it is. The phases within string theory would also need re-examining.“We will face difficult problems,” Nomura says. “Our theory of dark energy is gone if it’sthe wrong curvature.”

But with the right curvature, a curved universe could reframe how physicists look atvalues that, at present, appear to be fundamental. If there were different universes withdifferent phases of laws, we might not need to seek fundamental explanations for someof the properties our universe exhibits.

And it would, of course, mean we are tinier still than we ever imagined. “It’s likeanother step in this kind of existential crisis,” Kleban says. “It would have a huge impacton people’s imaginations.”

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breaking

July 27, 2015

W bosons remain left-handedA new result from the LHCb collaboration weakens previous hints atthe existence of a new type of W boson.By Sarah Charley

A measurement released today by the LHCb collaboration dumped some cold water onprevious results that suggested an expanded cast of characters mediating the weakforce.

The weak force is one of the four fundamental forces, along with the electromagnetic,gravitational and strong forces. The weak force acts on quarks, fundamental buildingblocks of nature, through particles called W and Z bosons.

Just like a pair of gloves, particles can in principle be left-handed or right-handed. Thenew result from LHCb presents evidence that the W bosons that mediate the weak forceare all left-handed; they interact only with left-handed quarks.

This weakens earlier hints from the Belle and BaBar experiments of the existence ofright-handed W bosons.

The LHCb experiment at the Large Hadron Collider examined the decays of a heavyand unstable particle called Lambda-b—a baryon consisting of an up quark, down quarkand bottom quark. Weak decays can change a bottom quark into either a charm quark,about 1 percent of the time, or into a lighter up quark. The LHCb experiment measuredhow often the bottom quark in this particle transformed into an up quark, resulting in aproton, muon and neutrino in the final state.

“We found no evidence for a new right-handed W boson,” says Marina Artuso, aProfessor of Physics at Syracuse University and a scientist working on the LHCbexperiment.

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If the scientists on LHCb had seen bottom quarks turning into up quarks more oftenthan predicted, it could have meant that a new interaction with right-handed W bosonshad been uncovered, Artuso says. “But our measured value agreed with our model’svalue, indicating that the right-handed universe may not be there.”

Earlier experiments by the Belle and BaBar collaborations studied transformations ofbottom quarks into up quarks in two different ways: in studies of a single, specific type oftransformation, and in studies that ideally included all the different ways thetransformation occurs.

If nothing were interfering with the process (like, say, a right-handed W boson), thenthese two types of studies would give the same value of the bottom-to-up transformationparameter. However, that wasn’t the case.

The difference, however, was small enough that it could have come from calculationsused in interpreting the result. Today’s LHCb result makes it seem like right-handed Wbosons might not exist after all, at least not in a way that is revealed in thesemeasurements.

Michael Roney, spokesperson for the BaBar experiment, says, "This result not onlyprovides a new, precise measurement of this important Standard Model parameter, but italso rules out one of the interesting theoretical explanations for the discrepancy... whichstill leaves us with this puzzle to solve."

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breaking

July 23, 2015

A new first for T2KThe Japan-based neutrino experiment has seen its first threecandidate electron antineutrinos.By Kathryn Jepsen

Scientists on the T2K neutrino experiment in Japan announced today that they havespotted their first possible electron antineutrinos.

When the T2K experiment first began taking data in January 2010, it studied a beamof neutrinos traveling 295 kilometers from the J-PARC facility in Tokai, on the east coast,to the Super-Kamiokande detector in Kamioka in western Japan. Neutrinos rarely interactwith matter, so they can stream straight through the earth from source to detector.

From May 2014 to June 2015, scientists used a different beamline configuration toproduce predominantly the antimatter partners of neutrinos, antineutrinos. After scientistseliminated signals that could have come from other particles, three candidate electronantineutrino events remained.

T2K scientists hope to determine if there is a difference in the behavior of neutrinosand antineutrinos.

“That is the holy grail of neutrino physics,” says Chang Kee Jung of State Universityof New York at Stony Brook, who until recently served as international co-spokespersonfor the experiment.

If scientists caught neutrinos and their antiparticles acting differently, it could helpexplain how matter came to dominate over antimatter after the big bang. The big bangshould have produced equal amounts of each, which would have annihilated one anothercompletely, leaving nothing to form our universe. And yet, here we are; scientists arelooking for a way to explain that.

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“In the current paradigm of particle physics, this is the best bet,” Jung says.

Scientists have previously seen differences in the ways that other matter andantimatter particles behave, but the differences have never been enough to explain ouruniverse. Whether neutrinos and antineutrinos act differently is still an open question.

Neutrinos come in three types: electron neutrinos, muon neutrinos and tau neutrinos.As they travel, they morph from one type to another. T2K scientists want to know ifthere’s a difference between the oscillations of muon neutrinos and muon antineutrinos.A possible upgrade to the Super-Kamiokande detector could help with future data-taking.

One other currently operating experiment can look for this matter-antimatterdifference: the NOvA experiment, which studies a beam that originates at Fermilab nearChicago with a detector near the Canadian border in Minnesota.

“This result shows the principle of the experiment is going to work,” says IndianaUniversity physicist Mark Messier, co-spokesperson for the NOvA experiment. “Withmore data, we will be on the path to answering the big questions.”

It might take T2K and NOvA data combined to get scientists closer to the answer,Jung says, and it will likely take until the construction of the even larger DUNE neutrinoexperiment in South Dakota to get a final verdict.

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signal to background

July 22, 2015

Underground plansThe Super-Kamiokande collaboration has approved a project toimprove the sensitivity of the Super-K neutrino detector.By Liz Kruesi

Super-Kamiokande, buried under about 1 kilometer of mountain rock in Kamioka, Japan,is one of the largest neutrino detectors on Earth. Its tank is full of 50,000 tons (about 13million gallons) of ultrapure water, which it uses to search for signs of notoriously difficult-to-catch particles.

Recently members of the Super-K collaboration gave the go-ahead to a plan to makethe detector a thousand times more sensitive with the help of a chemical compoundcalled gadolinium sulfate.

Neutrinos are made in a variety of natural processes. They are also produced innuclear reactors, and scientists can create beams of neutrinos in particle accelerators.These particles are electrically neutral, have little mass and interact only weakly withmatter—characteristics that make them extremely difficult to detect even though trillions flythrough any given detector each second.

Super-K catches about 30 neutrinos that interact with the hydrogen and oxygen in thewater molecules in its tank each day. It keeps its water ultrapure with a filtration systemthat removes bacteria, ions and gases.

Scientists take extra precautions both to keep the ultrapure water clean and to avoidcontact with the highly corrosive substance.

“Somebody once dropped a hammer into the tank,” says experimentalist Mark Vaginsof the University of Tokyo's Kavli Institute for the Physics and Mathematics of theUniverse. “It was chrome-plated to look nice and shiny. Eventually we found the chromeand not the hammer.”

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When a neutrino interacts in the Super-K detector, it creates other particles that travelthrough the water faster than the speed of light, creating a blue flash. The tank is linedwith about 13,000 phototube detectors that can see the light.

Looking for relic neutrinos

On average, several massive stars explode as supernovae every second somewherein the universe. If theory is correct, all supernovae to have exploded throughout theuniverse’s 13.8 billion years have thrown out trillions upon trillions of neutrinos. Thatmeans the cosmos would glow in a faint background of relic neutrinos—if scientists couldjust find a way to see even a fraction of those ghostlike particles.

For about half of the year, the Super-K detector is used in the T2K experiment, whichproduces a beam of neutrinos in Tokai, Japan, some 183 miles (295 kilometers) away,and aims it at Super-K. During the trip to the detector, some of the neutrinos change fromone type of neutrino to another. T2K studies that change, which could give scientists hintsas to why our universe holds so much more matter than antimatter.

But a T2K beam doesn’t run continuously during that half year. Instead, researcherssend a beam pulse every few seconds, and each pulse lasts just a few microsecondslong. Super-K still detects neutrinos from natural processes while scientists are runningT2K.

In 2002, at a neutrino meeting in Munich, Germany, experimentalist Vagins andtheorist John Beacom of The Ohio State University began thinking of how they couldbetter use Super-K to spy the universe’s relic supernova neutrinos.

“For at least a few hours we were standing there in the Munich subway stationsomewhere deep underground, hatching our underground plans,” Beacom says.

To pick out the few signals that come from neutrino events, you have to battle aconstant clatter of background noise of other particles. Other incoming cosmic particlessuch as muons (the electron’s heavier cousin) or even electrons emitted from naturallyoccurring radioactive substances in rock can produce signals that look like the onesscientists hope to find from neutrinos. No one wants to claim a discovery that later turnsout to be a signal from a nearby rock.

Super-K already guards against some of this background noise by being buriedunderground. But some unwanted particles can get through, and so scientists need waysto separate the signals they want from deceiving background signals.

Vagins and Beacom settled on an idea—and a name for the next stage of theexperiment: Gadolinium Antineutrino Detector Zealously Outperforming OldKamiokande, Super! (GADZOOKS!). They proposed to add 100 tons of the compoundgadolinium sulfate—Gd2(SO4)3—to Super-K’s ultrapure water.

When a neutrino interacts with a molecule, it releases a charged lepton (a muon,electron, tau or one of their antiparticles) along with a neutron. Neutrons are thousands oftimes more likely to interact with the gadolinium sulfate than with another water molecule.So when a neutrino traverses Super-K and interacts with a molecule, its muon, electron,or antiparticle (Super-K can’t see tau particles) will generate a first pulse of light, and theneutron will create a second pulse of light: “two pulses, like a knock-knock,” Beacomsays.

By contrast, a background muon or electron will make only one light pulse.

To extract only the neutrino interactions, scientists will use GADZOOKS! to focus on

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the two-signal events and throw out the single-signal events, reducing the backgroundnoise considerably.

The prototype

But you can’t just add 100 tons of a chemical compound to a huge detector withoutdoing some tests first. So Vagins and colleagues built a scaled-down version, which theycalled Evaluating Gadolinium’s Action on Detector Systems (EGADS). At 0.4 percent thesize of Super-K, it uses 240 of the same phototubes and 200 tons (52,000 gallons) ofultrapure water.

Over the past several years, Vagins’ team has worked extensively to show thebenefits of their idea. One aspect of their efforts has been to build a filtration system thatremoves everything from the ultrapure water except for the gadolinium sulfate. Theypresented their results at a collaboration meeting in late June.

On June 27, the Super-K team officially approved the proposal to add gadoliniumsulfate but renamed the project SuperK-Gd. The next steps are to drain Super-K to checkfor leaks and fix them, replace any burned out phototubes, and then refill the tank.

But this process must be coordinated with T2K, says Masayuki Nakahata, the Super-K collaboration spokesperson.

Once the tank is refilled with ultrapure water, scientists will add in the 100 tons ofgadolinium sulfate. Once the compound is added, the current filtration system couldremove it any time researchers would like, Vagins says.

“But I believe that once we get this into Super-K and we see the power of it, it’s goingto become indispensable,” he says. “It’s going to be the kind of thing that peoplewouldn’t want to give up the extra physics once they’re used to it.”

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