A joint Fermilab/SLAC publication

october 2015dimensionsofparticlephysicssymmetry

1

Table of contents

Signal to background: Xenon, xenon everywhere

Feature: A measurement to watch

2

signal to background

October 13, 2015

Xenon, xenon everywhereIt’s in the air we breathe, but it’s not so easy to get ahold of 10metric tons of xenon in its liquid form.By Glenn Roberts Jr.

So, you want to buy some xenon to try to detect dark matter deep underground. Not aproblem. There’s a market for that, with a few large-scale suppliers.

Wait, what’s that you say? You need 10 metric tons of incredibly pure, liquid xenonfor the LUX-ZEPLIN dark matter experiment? That’s a bit trickier.

Looking for large amounts of xenon is a bit like searching for dark matter: It’s allaround us, but it’s colorless, odorless and hard to separate from everything else. Xenonis in the air that we breathe, but it’s also one of the rarest elements on Earth.

There is about 1 part xenon in every 11.5 million parts of air. The global industry thatextracts liquid xenon produces a total of about 40 tons of xenon per year, so 10 tons is avery tall order.

“Buying several tons per year won’t perturb the market too much,” says ThomasShutt, a SLAC physicist who, along with physicist Daniel Akerib, left Case WesternReserve University in Ohio last year to join SLAC National Accelerator Laboratory. “If youbuy 10 tons in a year that's a quarter of the market.”

Akerib and Shutt are heading up SLAC’s effort in the planned LUX-ZEPLIN, or LZ,experiment, one of the largest-scale efforts to find dark matter particles. Like its smallerpredecessor experiment, called LUX (for Large Underground Xenon), LZ will be filled withsupercooled liquid xenon.

Xenon, like several other rare gases, can emit flashes of light and electrons when itsatoms are hit by other particles. The LZ detector will sit 1 mile underground in a SouthDakota mine, shielded from most other particles, and wait to see signals from dark matterparticles.

3

“Xenon has really good stopping power,” Akerib says. Its liquid form is so dense thataluminum can float on it. It is particularly sensitive to passing particles.

Xenon is used in more than just dark matter experiments. It is also in demand as acomponent in halogen lights such as the bluish headlights in some vehicles, in the bulbsfor other specialized lighting such as flash lamps that drive lasers, and as a propellant forsatellites and other spacecraft. It is also used in semiconductor manufacturing andmedical imaging, and it has been used as an anesthetic.

Xenon is a by-product of the steel-making process, which uses liquid oxygen to washaway contaminants on the surface of molten iron. Russia, South Africa and Saudi Arabiaare among the major producers of xenon. Russia became a major player in this marketduring the era of the Soviet Union, when steel-making was largely centralized.

Industrially produced xenon isn’t nearly pure enough for the exacting requirements ofLZ, though.

Shutt says extracting its own xenon from air was not an option. “If we had to startfrom scratch in refining xenon, it would be vastly more expensive,” he says.

The LZ team plans to acquire xenon over the next 3 to 4 years.

There is no expiration date on xenon, Shutt said; it just needs to be tightly containedso no venting occurs. “The xenon we use we can put back on the market or put to otherscientific uses after the LZ experiment is complete,” he says. “It’s around forever.”

To ensure that the dark matter detector is ultrasensitive, the LZ team is building apurification system at SLAC National Accelerator Laboratory to remove krypton, anotherrare gas that can get mixed in with liquid xenon. LUX started with xenon that had 100parts of krypton per billion and purified it down to four parts per trillion, and LZ needsxenon purified to a standard of 0.015 parts krypton per trillion—a factor of 300 purer.

Shutt jokes that, while LZ is all about particle physics, “we have become armchairchemical engineers” in the process of putting the experiment together.

The current plan is to purify the xenon in 2018, and to run each batch through thepurification process twice. The process is expected to take several months in total. LZ isscheduled to start running in 2019.

Like what you see? Sign up for a free subscription to symmetry!

4

feature

October 07, 2015

A measurement to watchFinding a small discrepancy in measurements of the properties ofneutrinos could show us how they fit into the bigger picture.By Lauren Biron

Physics, perhaps more so than any other science, relies on measuring the same thing inmultiple ways. Different experiments let scientists narrow in on right answers that satisfyall parties—a scientific system of checks and balances.

That’s why it’s exciting when a difference, even a minute one, appears. It can teachphysicists something about their current model – or physics that extends beyond it. It’spossible that just such a discrepancy exists between a certain measurement of neutrinoscoming out of accelerator experiments and reactor-based experiments.

Neutrinos are minuscule, neutral particles that don’t interact with much of anything.They can happily pass through a light-year of lead without a peep. Trillions pass throughyou every second. In fact, they are the most abundant massive particle in theuniverse—and something scientists are, naturally, quite keen to understand.

The ghostly particles come in three flavors: electron, muon and tau. They transitionbetween these three flavors as they travel. This means that a muon neutrino leaving anaccelerator at Fermi National Accelerator Laboratory in Illinois can show up as anelectron neutrino in an underground detector in South Dakota.

Not complicated enough for you? These neutrino flavors are made of mixtures ofthree different “mass states” of neutrinos, masses 1, 2 and 3.

At the end of the day, neutrinos are weird. They hang out in the quantum realm, aland of probabilities and mixing matrices and other shenanigans. But here’s what youshould know. There are lots of different things we can measure about neutrinos—and oneof them is a parameter called theta13 (pronounced theta one three). Theta13 relates

5

deeply to how neutrinos mix together, and it’s here that scientists have seen the faintesthint of disagreement from different experiments.

Accelerators vs. reactors

There are lots of different ways to learn about neutrinos and things like theta13. Two ofthe most popular involve particle accelerators and nuclear reactors.

The best measurements of theta13 come from nuclear reactor experiments such asDouble Chooz, RENO and Daya Bay Reactor Neutrino Experiment based in China (whichreleased the best measurement to date a few weeks ago).

Detectors located near nuclear reactors provide such wonderful readings of theta13because reactors produce an extremely pure fountain of electron antineutrinos, andtheta13 is closely tied to how electron neutrinos mix. Researchers can calculate theta13based on the number of electron antineutrinos that disappear as they travel from a neardetector to the far detector, transforming into other types.

Accelerators, on the other hand, typically start with a beam of muon neutrinos. Andwhile that beam is fairly pure, it can have a bit of contamination in the form of electronneutrinos. Far detectors can look for both muon neutrinos that have disappeared andelectron neutrinos that have appeared, but that variety comes with a price.

“Both the power and the curse of long-baseline neutrino oscillation is that it’ssensitive to all of neutrino oscillation, not just theta13,” says Dan Dwyer, a scientist atLawrence Berkeley National Laboratory and researcher on Daya Bay.

With that in mind, we come to the source of the disagreement. The results coming outof accelerator-based experiments, such as the United States-based NOvA and Japan-based T2K, see just a few more electron neutrinos than researchers would predict basedon what the reactor experiments are saying.

“The theta13 value that fits the beam experiments, that really describes how muchelectron neutrino you get, is somewhat larger than what Daya Bay, RENO and DoubleChooz measure,” says Kate Scholberg, professor of physics at Duke University andresearcher on T2K. “So there’s a little bit of tension.”

Many grains of salt

Data coming out of the accelerator experiments is still very young compared to the strongreadings from reactor experiments, and it is complicated by the nature of the beam. Noone is jumping on the discrepancy yet because it can be explained in different ways. Mostimportantly, the accelerator experiments just don’t have enough information.

“We have to wait for T2K and NOvA to get sufficient statistics, and that’s going totake a while,” says Stephen Parke, head of the Theoretical Physics Department atFermilab. Parke, Scholberg and Dwyer all estimated that about five more years of datacollection will be required before researchers are able to start saying anything substantial.

“There’s been a lot of pressure on Daya Bay to try to eke out as precise ameasurement as we possibly can,” Dwyer says. “Every bit of increased precision weprovide further improves the ability of NOvA and T2K and eventually [proposed neutrinoexperiment] DUNE to measure the other parameters.”

Finding meaning in neutrinos

If the accelerator experiments gather more data and if a clear discrepancy emerges—a big

6

if—what does it mean?

Turns out there are lots of reasons to love theta13. It’s one of the fundamentalparameters that can define our universe. From a practical standpoint, it helps designfuture experiments to better understand neutrinos. And it could help physicists learnsomething new.

“We don’t expect things not to agree, but we kind of hope that they won’t,” saysAndré de Gouvêa, professor of physics at Northwestern University. “It means that we’remissing something.”

That something could be CP violation, evidence that neutrinos and antineutrinosbehave differently. CP violation has never been seen in neutrinos before, but ifresearchers observed it with accelerator experiments, it could help explain why ouruniverse is made of matter rather than equal parts of matter and antimatter.

Figuring out if CP violation is occurring means nailing down all of the different neutrinomixing parameters, which in turn means building more powerful, next-generationexperiments such as Hyper-K in Japan, JUNO in China and the Deep UndergroundNeutrino Experiment in the United States. DUNE will build on oscillation experiments likeNOvA but will be able to better separate background noise from neutrino events, see abroader energy spectrum of neutrinos and find other neutrino characteristics.

DUNE, which will be built in a repurposed gold mine in South Dakota and detectneutrinos passed 800 miles through the Earth from Fermilab in Illinois, will be one of thebest ways to see CP violation and rely on expertise gained from smaller neutrinoexperiments.

“Developing these types of experiments is very complicated,” de Gouvêa says. Oneof the major challenges of physics experiments is making sure you are measuring whatyou think you are measuring. “That’s part of the reason why we have a significantnumber of neutrino oscillation experiments.”

Ultimately, the neutrino puzzle is still missing many pieces. A variety of experimentsare ramping up to fill in the gaps, making it an exciting time to be a neutrino physicist.

“We have to untangle the mysteries of the neutrino, and it’s not easy,” Parke says.“The neutrino doesn’t give up her secrets very easily.”

Like what you see? Sign up for a free subscription to symmetry!

7

Copyright © 2015 symmetry

Powered by TCPDF (www.tcpdf.org)

8