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Posts Tagged ‘CDMS’

This article appeared in symmetry on July 11, 2014.

Together, the three experiments will search for a variety of types of dark matter particles. Photo: NASA

Together, the three experiments will search for a variety of types of dark matter particles. Photo: NASA

Two US federal funding agencies announced today which experiments they will support in the next generation of the search for dark matter.

The Department of Energy and National Science Foundation will back the Super Cryogenic Dark Matter Search-SNOLAB, or SuperCDMS; the LUX-Zeplin experiment, or LZ; and the next iteration of the Axion Dark Matter eXperiment, ADMX-Gen2.

“We wanted to pool limited resources to put together the most optimal unified national dark matter program we could create,” says Michael Salamon, who manages DOE’s dark matter program.

Second-generation dark matter experiments are defined as experiments that will be at least 10 times as sensitive as the current crop of dark matter detectors.

Program directors from the two federal funding agencies decided which experiments to pursue based on the advice of a panel of outside experts. Both agencies have committed to working to develop the new projects as expeditiously as possible, says Jim Whitmore, program director for particle astrophysics in the division of physics at NSF.

Physicists have seen plenty of evidence of the existence of dark matter through its strong gravitational influence, but they do not know what it looks like as individual particles. That’s why the funding agencies put together a varied particle-hunting team.

Both LZ and SuperCDMS will look for a type of dark matter particles called WIMPs, or weakly interacting massive particles. ADMX-Gen2 will search for a different kind of dark matter particles called axions.

LZ is capable of identifying WIMPs with a wide range of masses, including those much heavier than any particle the Large Hadron Collider at CERN could produce. SuperCDMS will specialize in looking for light WIMPs with masses lower than 10 GeV. (And of course both LZ and SuperCDMS are willing to stretch their boundaries a bit if called upon to double-check one another’s results.)

If a WIMP hits the LZ detector, a high-tech barrel of liquid xenon, it will produce quanta of light, called photons. If a WIMP hits the SuperCDMS detector, a collection of hockey-puck-sized integrated circuits made with silicon or germanium, it will produce quanta of sound, called phonons.

“But if you detect just one kind of signal, light or sound, you can be fooled,” says LZ spokesperson Harry Nelson of the University of California, Santa Barbara. “A number of things can fake it.”

SuperCDMS and LZ will be located underground—SuperCDMS at SNOLAB in Ontario, Canada, and LZ at the Sanford Underground Research Facility in South Dakota—to shield the detectors from some of the most common fakers: cosmic rays. But they will still need to deal with natural radiation from the decay of uranium and thorium in the rock around them: “One member of the decay chain, lead-210, has a half-life of 22 years,” says SuperCDMS spokesperson Blas Cabrera of Stanford University. “It’s a little hard to wait that one out.”

To combat this, both experiments collect a second signal, in addition to light or sound—charge. The ratio of the two signals lets them know whether the light or sound came from a dark matter particle or something else.

SuperCDMS will be especially skilled at this kind of differentiation, which is why the experiment should excel at searching for hard-to-hear low-mass particles.

LZ’s strength, on the other hand, stems from its size.

Dark matter particles are constantly flowing through the Earth, so their interaction points in a dark matter detector should be distributed evenly throughout. Quanta of radiation, however, can be stopped by much less significant barriers—alpha particles by a piece of paper, beta particles by a sandwich. Even gamma ray particles, which are harder to stop, cannot reach the center of LZ’s 7-ton detector. When a particle with the right characteristics interacts in the center of LZ, scientists will know to get excited.

The ADMX detector, on the other hand, approaches the dark matter search with a more delicate touch. The dark matter axions ADMX scientists are looking for are too light for even SuperCDMS to find.

If an axion passed through a magnetic field, it could convert into a photon. The ADMX team encourages this subtle transformation by placing their detector within a strong magnetic field, and then tries to detect the change.

“It’s a lot like an AM radio,” says ADMX-Gen2 co-spokesperson Gray Rybka of the University of Washington in Seattle.

The experiment slowly turns the dial, tuning itself to watch for one axion mass at a time. Its main background noise is heat.

“The more noise there is, the harder it is to hear and the slower you have to tune,” Rybka says.

In its current iteration, it would take around 100 years for the experiment to get through all of the possible channels. But with the addition of a super-cooling refrigerator, ADMX-Gen2 will be able to search all of its current channels, plus many more, in the span of just three years.

With SuperCDMS, LZ and ADMX-Gen2 in the works, the next several years of the dark matter search could be some of its most interesting.

Kathryn Jepsen

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This article appeared in symmetry on February 28, 2014.

The Cryogenic Dark Matter Search has set more stringent limits on light dark matter.

The Cryogenic Dark Matter Search has set more stringent limits on light dark matter.

Scientists looking for dark matter face a serious challenge: No one knows what dark matter particles look like. So their search covers a wide range of possible traits—different masses, different probabilities of interacting with regular matter.

Today, scientists on the Cryogenic Dark Matter Search experiment, or CDMS, announced they have shifted the border of this search down to a dark-matter particle mass and rate of interaction that has never been probed.

“We’re pushing CDMS to as low mass as we can,” says Fermilab physicist Dan Bauer, the project manager for CDMS. “We’re proving the particle detector technology here.”

Their result, which does not claim any hints of dark matter particles, contradicts a result announced in January by another dark matter experiment, CoGeNT, which uses particle detectors made of germanium, the same material as used by CDMS.

To search for dark matter, CDMS scientists cool their detectors to very low temperatures in order to detect the very small energies deposited by the collisions of dark matter particles with the germanium. They operate their detectors half of a mile underground in a former iron ore mine in northern Minnesota. The mine provides shielding from cosmic rays that could clutter the detector as it waits for passing dark matter particles.

Today’s result carves out interesting new dark matter territory for masses below 6 billion electronvolts. The dark matter experiment Large Underground Xenon, or LUX, recently ruled out a wide range of masses and interaction rates above that with the announcement of its first result in October 2013.

Scientists have expressed an increasing amount of interest of late in the search for low-mass dark matter particles, with CDMS and three other experiments—DAMA, CoGeNT and CRESST—all finding their data compatible with the existence of dark matter particles between 5 billion and 20 billion electronvolts. But such light dark-matter particles are hard to pin down. The lower the mass of the dark-matter particles, the less energy they leave in detectors, and the more likely it is that background noise will drown out any signals.

Even more confounding is the fact that scientists don’t know whether dark matter particles interact in the same way in detectors built with different materials. In addition to germanium, scientists use argon, xenon, silicon and other materials to search for dark matter in more than a dozen experiments around the world.

“It’s important to look in as many materials as possible to try to understand whether dark matter interacts in this more complicated way,” says Adam Anderson, a graduate student at MIT who worked on the latest CDMS analysis as part of his thesis. “Some materials might have very weak interactions. If you only picked one, you might miss it.”

Scientists around the world seem to be taking that advice, building different types of detectors and constantly improving their methods.

“Progress is extremely fast,” Anderson says. “The sensitivity of these experiments is increasing by an order of magnitude every few years.”

Kathryn Jepsen

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This article first appeared in Fermilab Today on April 15, 2013.

Scientists around the world are working to understand the nature of dark matter, which accounts for most of the mass of the universe. The earth seems to be moving through a cloud of dark-matter particles that encompasses the visible parts of our galaxy. We should be able to sense this dark matter if we can deploy detectors that are sensitive to the ‘billiard ball’ scatter of a dark matter particle from an atomic nucleus inside these detectors.

Experimental upper limits (90 percent confidence level) for the WIMP-nucleon spin-independent cross section as a function of WIMP mass. The black dotted line is from the present analysis, and the blue solid line includes previous CDMS II silicon-detector data. Also shown are limits from the CDMS II germanium-detector standard and low-threshold analyses (dark and light dashed red), as well as limits from the XENON collaboration (dark and light dash-dotted green). The magenta oval shows a possible WIMP signal region proposed to explain data from CoGeNT. The light and dark blue regions indicate the 68 percent and 90 percent contours obtained if the present result were to be interpreted as a WIMP signal. The asterisk shows the maximum likelihood point under this interpretation.

The Cryogenic Dark Matter Search (CDMS) experiment was designed to do exactly that, using germanium and silicon detectors cooled to very low temperatures in order to detect the electric charge and heat liberated by single dark-matter particle collisions with nuclei and distinguish them from the messier interactions created by normal matter.

At the American Physical Society April meeting in Denver, the CDMS collaboration presented on Saturday its blind-analysis results from data taken with silicon detectors during CDMS II operation at the Soudan Underground Laboratory. Kevin McCarthy, a graduate student from MIT, presented the results, which were submitted to Physical Review Letters.

The blind analysis resulted in three candidate events. Although this number is higher than the expected background of roughly half an event, this is far from a discovery. Simulations of the known backgrounds indicate that a statistical fluctuation could produce three or more events about 5 percent of the time. In other words, if the experiment were done 100 times, five of them would show at least three events in the signal region even if dark-matter particles did not exist.

However, there is more information on the characteristics of the expected background events and the expected signals from weakly interacting massive particles, or WIMPs, which are the favorite particle explanation for dark matter. A likelihood analysis that includes the measured recoil energies of the three events yields a 0.19 percent probability for the background-only hypothesis when tested against a model that includes a WIMP contribution. This translates into roughly a 3-sigma confidence level for the hypothesis that the three events are due to WIMP interactions. This is exciting but still does not meet the scientific standard for a discovery. Further investigations are necessary.

Nevertheless, if one indulges in a “what if” scenario and interprets the result as due to a WIMP signal, the WIMP mass would be around 8.5 times the mass of a proton. For the simplest theories of WIMP interactions and using the Standard Model for dark-matter distribution in our galaxy, the rate found for such interactions is in some conflict with the current results from the XENON experiments. The paper presents more details.

In 2010, the CDMS collaboration published results on dark-matter searches with germanium detectors, which resulted in two events in the signal region and an estimated background of 0.8 events. The conclusion at the time was that these events were likely leakage surface electrons rather than true nuclear recoils, and other experiments have not found any signals in this mass region.

The SuperCDMS Soudan experiment is currently taking data with larger, and better, germanium detectors, and hopes to shed additional light on low-mass WIMPs before the end of the year. The collaboration is considering the use of silicon detectors in future experiments.

—Dan Bauer, Deputy Director, Fermilab Center for Particle Astrophysics

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Fermilab planning a busy 2012

Tuesday, January 3rd, 2012

This column by Fermilab Director Pier Oddone first appeared in Fermilab Today Jan. 3 .

We have a mountain of exciting work coming our way!

In accelerator operations, we need to give enough neutrinos to MINERvA to complete their low-energy run, enough anti-neutrinos to MiniBooNE to complete their run and enough neutrinos to MINOS to enable their independent neutrino velocity measurement that will follow up on last year’s OPERA results. We need to provide test beams to several technology development projects and overcome setbacks due to an aging infrastructure to deliver beam to the SeaQuest nuclear physics experiment. And we need to do all of this in the first few months of the year before a year-long shutdown starts. During the shutdown, we will modify the accelerator complex for the NOvA era and begin the campaign to double the number of protons from the Booster to deliver simultaneous beams to various experiments.

In parallel with accelerator modifications, we will push forward on many new experiments. The NOvA detector is in full construction mode, and we face challenges in the very large number of detector elements and large mechanical systems. Any project of this scale requires a huge effort to achieve the full promise of its design. We have the resources in our FY2012 budget to make a lot of progress toward MicroBooNE, Mu2e and LBNE. We will continue to work with DOE to advance Muon g-2. All these experiments are at an important stage in their development and need to be firmly established this year.

At the Cosmic Frontier, we will commission and start operation of the Dark Energy Survey at the Blanco Telescope in Chile, where the camera has arrived and is being tested. In the dark matter arena we will commission and operate the 60 kg COUPP detector at Canada’s SNOLAB and continue the run of the CDMS 15 kg detector in the Soudan Mine while carrying out R&D on future projects. We continue to have a major role in the operation of the Pierre Auger cosmic-ray observatory. In addition we should complete the first phase of the Fermilab Holometer, which will study the properties of space-time at the Planck scale.

At the Energy Frontier, we play a major role in the LHC detector operations and analysis. It should be a fabulously exciting year at the LHC as we push on the hints that we already see in the data.

Beyond construction and operation of facilities we continue our R&D efforts on the superconducting RF technology necessary for Project X and other future accelerators. We will be building the Illinois Accelerator Research Center and moving forward to connect our advanced accelerator program with industry and universities. Our rich program on theory, computation and detector technology will continue to support our laboratory and the particle physics community.

If we accomplish all that is ahead of us for 2012, it will be a year to remember and celebrate when we hit New Year’s Day 2013!

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The cleanup of the MINOS cavern and the rest of the Soudan Underground Laboratory is complete.

This article first appeared in

Fermilab Today May 25.

Two months after a fire broke out in the access shaft of the Soudan mine, the Soudan Underground Laboratory is again open for operation. Safety officers inspected the mine and laboratory last week and issued a permit for normal occupancy. The officers identified a short list of additional repairs, which will be carried out in the upcoming months. Work also continues on the Soudan mine shaft.

“The cleanup of the laboratory is complete, and the MINOS far detector is ready to take beam data with full magnetic field,” said Fermilab physicist Rob Plunkett, co-spokesperson for the MINOS neutrino experiment. “The small number of components we had to replace was consistent with a normal power outage.”

The 5,000-ton MINOS far detector is located a half mile underground in the Soudan laboratory. In March, fire-fighting foam covered parts of the detector and the lowest part of the magnet coil was partially immersed in water. Laboratory staff gently heated the coil over the past two weeks to dry it out.

The CDMS experiment, located in a cavern adjacent to the MINOS detector, experienced no damage to its equipment except to a backup generator. Its cryogenic system recovered unscathed from the power outage triggered by the fire. CDMS scientists have removed the new particle detectors they were testing before the fire, and they will begin operation of an expanded experiment with more dark-matter detectors in September.

University of Minnesota building code inspectors and ES&H personnel from the university and Fermilab inspected the laboratory last Wednesday. The University of Minnesota manages the Soudan Underground Laboratory.

— Kurt Riesselmann

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Last week was yet another exciting moment for those of us who are researching the nature of dark matter. The long-awaited XENON(100) results were released. XENON, is the biggest rival to my own experiment, the Cryogenic Dark Matter Search, or CDMS.  In the world-wide race to discover dark matter, XENON and CDMS have been leading the pack over the past few years. These two experiments have been taking turns nudging ahead of each other, only to have the other pull ahead within about a year’s time.  This time around, XENON has made a fairly big leap ahead. While the XENON collaboration did not report a discovery, their data does provide significant new constraints on the many theories that aim to explain dark matter.  Their new result has lowered the possibility for dark matter interactions by a factor of ~4 over previous world limits.

I’m certain two of the big questions many now have for CDMS, are: What are our plans for the future and will we be taking the next jump that puts us in the lead? Though we’ve had a setback recently, I’m optimistic about CDMS.  We are in the process of starting a new phase of the experiment named SuperCDMS.  For SuperCDMS, we will implement a new detector design which will significantly increase our sensitivity to WIMPs. Last month, we were in the middle of testing these detectors when a fire broke out in the mine where the experiment resides. We are now waiting while the mine infrastructure is repaired. Once that is completed, we will begin our first physics run with the new detectors, which may be as soon as this summer. In the meantime, we are planning a much bigger version of the experiment at a much deeper underground site, SNOLAB in Canada. Both of these endeavors have planned sensitivities that exceed the current XENON limits.  

In the meantime, of course, the XENON collaboration will be continuing to gather more data and working on their next-generation experiment. However, based on their reported results, it is clear that they cannot simply improve their sensitivity with more of the same data. To push their sensitivity further, they must reduce intrinsic radioactive contaminants in their detector. Though they claim to have started a new run with higher purity levels, it’s unclear how long they can sustain the current conditions of the detector.  

So the worldwide race has tipped toward XENON for the time being, but meanwhile, the future of CDMS is bright. We don’t yet know what nature has to reveal or what the future will bring. This makes the world of dark matter research fascinating. No matter what happens, we all look forward to learning what the final outcome will be. For me, these are great reasons to push forward in the race to understand dark matter.

— Lauren Hsu

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As an undergraduate physics major, I was introduced to the Heisenberg uncertainty principle, which states that it is impossible to measure the exact momentum and position of an object at the same time.  This is not caused by inadequacies in our experiments.  Instead, it implies a fundamental limit to our ability to predict the future of a system because we cannot precisely determine its present state.  Such a conclusion is shocking to any physicist. Even Einstein himself refused to accept it.  

Visible matter accounts for only 5 percent of the universe. CDMS hopes to identify the dark matter contained in the remainder of the universe. Courtesy: SLAC/Nicolle Rager

Shocking as the principle is, my university education at least prepared me for the uncertainty of the subatomic world.  What I wasn’t taught was how much uncertainty is embedded in the day-to-day life of a physicist.   A little over a week ago, the mine where my experiment is housed experienced a fire.  The name of my experiment is the Cryogenic Dark Matter Search, or CDMS.  

Before I tell you about the fire, let me explain the purpose of CDMS. Scientists have gathered a large body of evidence that tells us most of the matter in the universe is not in a form that we can see.   Matter that we can see takes on the form of stars, planets, moons, comets, interstellar dust etc..  Dark matter is instead composed of a form of matter that we have never observed on Earth.  My experiment is attempting to probe this dark matter component of the universe and will help us understand what  dark matter is really made of.  CDMS is located approximately 1 km, or a little more than half a mile, underground inside the Soudan Underground Laboratory – up near the Boundary Waters of northern Minnesota.  This unusual location allows us to use the earth as a barrier to cosmic rays.  These can produce signals that  will confuse our attempts to observe dark matter.

The CDMS with sheilding surrounding the silver cryostat where the detectors are housed. Credit: Fermilab

So while housing the experiment deep underground is necessary for its function, it can make for some unexpected challenges. The day of the fire, I and my colleagues waited anxiously, hour-by-hour for the latest news on the attempts to extinguish it.  Luckily, the fire was not in the lab, but was instead in the mine shaft.  Since this shaft serves as the entry and exit to the laboratory, it was still quite a serious situation.  In the end, the fire was put out after heroic efforts on the part of the Minnesota Department of Natural Resources, which operates the laboratory, and the various emergency responders.  Thankfully no one was injured and the damage to the mine shaft and infrastructure were minimal compared to our initial fears.  Since last week, the laboratory staff has been busy restoring power to the underground lab and assessing damage to the mine infrastructure.   As of this Monday, a few scientists have finally been allowed restricted access to the lab.  They are  beginning to assess the status of CDMS.

Before the fire broke out, we were in the midst of an engineering run.  The purpose of this run was to commission a new design for our detectors.   We were very excited about the results of this run because they would demonstrate the power of the new detector design.  This is a necessary step towards convincing our funding agencies that we are ready for the next step of building a much bigger experiment.   Now everything has come to a screeching halt as we continue to wait to find out when we will be able to resume our work. 

 Even without the drama of the mine fire, these past few weeks are a very tense time for a postdoc, such as myself, who is in the process of applying for faculty positions.   I was one of the lucky few this year who was able to land several interviews at top universities.   These interviews are grueling sessions where one must meet and talk to many people over the course of a few days.  During a packed series of 30-45 minute interviews, where one often doesn’t even get a few minutes break in between sessions, you must simultaneously explain your research and try to find out as much about the university as possible. 

The interview rounds are largely finished for this year.  Now it is the time when the schools begin making offers to their first-choice candidates.  Some of these decisions will make or break the dreams of young physicists.  On the part of the universities, its a very large investment, especially because the recent downturn in the economy has prohibited many schools from making hires in the past few years.

 Anxiety runs high on all sides as I continue to wait for news of my future and that of CDMS…

–Lauren Hsu

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