A zoom-in of axion strings in a simulation of axion dark matter in the early universe. The strings throw off axions that go on to become that dark matter. These simulations are used to predict the properties of the axion, which informs laboratory detection efforts. (Ben Sadfi, UC Berkeley)
SCIENTISTS HAVE BEEN STUDYING THE COSMOS for centuries, but we still don’t know what makes up 85% of all matter in the universe. Unlike ordinary matter that we can see and feel, dark matter hasn’t been observed directly by even our most advanced scientific instruments. These invisible particles may be zipping through us all the time without interacting.
But scientists believe our world wouldn’t exist without dark matter. Its gravitational pull holds galaxies together, gathers them into clusters, bends light around them, and affects how they rotate. Dark matter also played a crucial role as galaxies initially formed.
“Lots of observational data show us that dark matter is a real particle, but we don’t know what kind. Its possible mass has a huge range, and there might be multiple types of dark matter particles,” says Berkeley Physics Professor Dan McKinsey, the Georgia Lee Chair in Physics. “We’re working hard to detect dark matter in the lab to open a window into new physics. It’s the only particle we know to exist outside the standard model.”
Berkeley Physics is one of the top places in the world to study dark matter. Experimental and theoretical physicists at Berkeley are leading far-reaching searches—hunting for dark matter candidates ranging from 1 TeV weakly interactive massive particles (WIMPs) to 1 MeV light dark matter particles down to 10 μeV axions.
Berkeley faculty are conducting, building, and designing next-generation dark matter experiments, including the LZ, SuperCDMS, TESSERACT, and ALPHA plasma haloscope. These innovative experiments are guided by models developed by Berkeley Physics theorists, including professors Hitoshi Murayama, Lawrence Hall, and Ben Safdi. We highlight only a few of these comprehensive efforts here.
HUNTING FOR WIMPS WITH LZ
One promising candidate is WIMPs, weakly interacting but heavy dark matter particles with a predicted mass of about 10 GeV to 100 TeV. A GeV is roughly the mass of a proton.
Hunting for WIMPs over 9 GeV is the aim of LZ, the larger and more sensitive successor of the LUX experiment. After 60 days of running, LZ recently became the most sensitive dark matter detector in the world. Berkeley Physics Professor and Berkeley Lab Director Mike Witherell, Emeritus Professor Bob Jacobsen, and McKinsey contributed to this success.
Because dark matter particles rarely interact with ordinary matter, their signal is easily drowned out by background noise. To shield from cosmic rays, LZ is located nearly a mile underground at the Sanford Underground Research Facility (SURF) in South Dakota. To reduce radioactive contamination, it uses ultra-clean detector materials. And to lower environmental backgrounds, it is built in several layers like an onion.
At the center of LZ is a time projection chamber (TPC)—a tank filled with seven tons of highly-purified liquid xenon. If a dark matter particle strikes a xenon nucleus, a flash of light and an electric charge are produced as the nucleus recoils. A strong electric field drifts the charge to the top surface of the TPC, where the electrons create a much larger flash of light that is measured by photomultiplier tubes on top and bottom.
The pattern and timing of the two flashes pinpoint the position and energy of the event. And the ratio of the two scintillation signals determines if the event was caused by a nuclear or electron recoil.
Outside the TPC are two veto detectors—a “skin” hold- ing three tons of liquid xenon and then an “outer detector” of gadolinium-loaded liquid scintillator—which are used to reject signals from gamma rays and neutrons, respectively. The whole thing lives inside a massive tank of water.
LZ is 25 times larger than the previous generation LUX experiment, which helps suppress backgrounds. But this increase also created a major challenge for McKinsey: designing and building a much higher high-voltage system to get the correct drift electric field, without the xenon lighting up like a neon lamp.
McKinsey also led the data analysis effort to reduce “accidental backgrounds” with support on backgrounds from Berkeley Physics postdoc Ibles Olcina and graduate students Jose Soria, Yue Wang, Ryan Gibbons, Ryan Smith, and James “Reed” Watson. “Occasionally, isolated first and second scintillation pulses randomly pair up to look like a dark matter event,” explains McKinsey. “My group combed through data, produced a statistical model, and developed cuts to reduce these accidentals without cutting into our dark matter acceptance.”
So far, LZ has found no evidence of WIMPs, but it set the most stringent limits on WIMP cross-sections and masses to date. And the second 1000-day run is underway.
“LZ is performing to specification, which is a big deal since we’ve been working on it for a decade,” says McKinsey. “We’re now poised to push through more dark matter parameter space over the next few years.”
McKinsey is also helping to design the next-generation of LZ, a scaled-up 80-ton xenon experiment called XLZD.
SEARCHING FOR LOW-MASS WIMPS WITH SUPERCDMS
SuperCDMS, the next-generation of the CDMS experiment, is located deep underground at SNOLAB near Sudbury, Canada. It plans to detect dark matter particles with a mass between 10 GeV and 0.5 GeV. Berkeley Physics Professor Emeritus Bernard Sadoulet led the NSF-funded part of its construction.
Searching for dark matter with lower mass requires more sensitive detectors. SuperCDMS uses germanium or silicon crystals attached to sensors on both faces. When a dark matter particle interacts with either semi-conductor, its nucleus recoils and creates minute crystal vibrations (phonons) and ionization (charge). An electric field causes the charge to drift and shed lots of phonons.
“Measuring both phonons and ionization gives us discrimination capability against backgrounds. And drifting the charges in the high-voltage detector increases our energy sensitivity by a factor of 100, allowing us to search for lower masses,” says Sadoulet.
However, measuring these phonon signals is challenging. Berkeley Physics Assistant Professor Matt Pyle played a major role in developing this unique sensor technology with the help of Berkeley’s associate research physicist Bruno Serfass and former postdoctoral fellow William Page.
At its core are transition-edge sensors (TES)—materials stabilized in the middle of their superconducting transition—attached to aluminum fin antennas. The fin absorbs the energy of the vibrations, concentrates it, and pushes it into the TES. The resulting increase in TES temperature changes its resistance, which is measured by cryogenic electronics.
“These phonon sensors need to be small to reduce their heat capacity. But if they’re attached directly to a giant crystal, an athermal phonon bounces around for a long time before it interacts with the TES. By using fins, we increase the interaction probability and area coverage,” says Pyle, the Michael M. Garland Chair in Physics.
He adds, “Only about 30% of the energy is transferred to the TES using the fins, but that’s more than made up for by collecting the phonons quickly before they thermalize.”
The collaboration is currently installing the experiment at SNOLAB. Meanwhile, they have been commissioning the SuperCDMS detectors, software, and operations at CUTE, a nearby cryogenic underground test facility at SNOLAB.
Sadoulet notes that Berkeley Physics collaborations encourage technology and data analysis transfer between various groups. The core athermal phonon sensor technology and discrimination methods are being used in multiple experiments, including SuperCDMS and TESSERACT.
“We’re giving a single solution that will hopefully be employed many times by many different experiments, all searching for slightly different things,” says Pyle.
SEEKING LIGHT DARK MATTER WITH TESSERACT
TESSERACT intends to take the dark matter search a step further. This umbrella of two experiments is being designed to detect light dark matter particles from both nuclear and electron recoils, in the mass range of the proton to the electron—1 GeV down to 1 keV.
The entire project will use identical Berkeley Physics next-generation sensors, readout technology, and operations—and no electric field for signal amplification.
“What makes TESSERACT unique is that every detector is designed to have multiple signal channels that have to be in coincidence for dark matter events. That’s the secret idea sauce of TESSERACT,” says Pyle. “We’re also eliminating a whole class of backgrounds by going to zero field, which means we need very highly sensitive detectors.”
McKinsey’s group is helping develop TESSERACT’s HeRALD experiment with assistance from Assistant Project Scientist Junsong Lin and graduate students Roger Romani, Will Matava, and Wang. HeRALD uses purified liquid helium as the target for nuclear recoil dark matter. Its silicon athermal phonon detectors are submerged in the vat of liquid helium and suspended in a vacuum above it.
“Using helium provides excellent background discrimination. If a dark matter event occurs in the helium, it lights up multiple pixels in coincidence, whereas a background event or microfracture in the silicon only lights up one pixel.” Helium is also cheap, easy to purify, easy to scale up, and naturally immune to some backgrounds. HeRALD will initially be sensitive to dark matter particles from 1 GeV to 100 MeV, but the scientists hope to reach the keV scale in the future.
Pyle’s group is helping develop TESSERACT’s SPICE experiment. It uses polar crystals—either gallium arsenide (GaAs) or sapphire (Al2O3)—as the target for both nuclear and electron recoil dark matter. A polar crystal has two types of ions with opposite charges. Some dark matter candidates may transform themselves, with low probability, into photons, which then nudge the different ions in opposite directions. This produces phonons that can be detected by the TES.
For the GaAs part of SPICE, the photons and phonons are collected in separate detectors, enabling photon-phonon coincidence to tag the unique dark matter signature, says Pyle. This scheme is designed to detect dark matter between 1 GeV to 1 MeV, caused primarily by electron scattering.
To detect dark matter with even lower mass, the second part of SPICE measures only the athermal phonon signal using sapphire detectors with even better energy sensitivity.
Pyle’s group is unraveling how to read out and calibrate these sapphire crystalline targets using his latest TES. Currently, the detectors are sensitive to dark matter in the GeV to MeV range, but the team hopes to get down to a keV.
McKinsey and Pyle are both enjoying their tabletop experiments. “If the detector is sensitive enough, then you can in principle detect light dark matter in your lab at the surface. It could happen,” McKinsey says. But TESSERACT will be installed underground in the next few years—at SURF or the Modane Underground Laboratory.
PROBING FOR AXIONS
Berkeley Physics Assistant Professor Ben Safdi studies other dark matter candidates, but he finds the ultra-light axions to be the most compelling because they explain more than dark matter. Axions were first theorized to explain the mystery of how neutrons behave in electric fields. And they have deep connections to the consistency of gravity at a quantum level.
“Axions are theoretically very well motivated, and they’re almost completely unexplored experimentally,” says Safdi, who holds the Henry Shenker Professor in Physics. “In the next decade or so, we’ll be able to say definitively whether or not these particles exist in nature.”
Although Safdi is a theoretical physicist, he looks for indirect signatures of dark matter in experimental data with his team, including graduate students Yujin Park and Joshua Benabou. “My work starts theoretically, with pencil and paper and then simulations. For a particular astrophysical system or precision laboratory experiment, we ask how axions would affect it and what data we need to test these predictions. Then, we get and analyze those data sets, determining if we find evidence for axions or not,” explains Safdi.
Additionally, Safdi spends much of his time simulating how axions were produced shortly after the Big Bang to determine what mass gives the correct abundance of dark matter. For this work, he uses advanced supercomputers at the National Energy Research Scientific Computing Center (NERSC) at Berkeley Lab, which just commissioned its faster and bigger Perlmutter supercomputer. “We have lots of jobs up the hill churning away. It’s a game changer for our mass prediction computations,” says Safdi.
One feature he needs to simulate in the early universe is axion strings, which are very violent but narrow regions of space—like tiny tornadoes—that whip around and emit lots of axions.
“During the simulations, a small part of the expanding universe is represented by a 3D grid over which the equations are solved,” explains Safdi. “But the axion strings are moving, so we have to dynamically update the grid. Despite running on supercomputers, computer memory is our limiting factor.”
Luckily, Safdi teamed up with the AMReX collaboration at Berkeley Lab, adapting their code framework designed to solve multi-scale problems. The key was using an adaptive mesh grid with a fine spatial resolution around the axion strings and sparse resolution elsewhere.
Using the biggest simulation of cosmology to date, they more precisely predicted the axion mass to be between 180 to 40 μeV, higher than expected. This claim implies axions from the early universe can’t be detected by the current experiments, which use a microwave resonance chamber to enhance the photon frequency coming from axions. The required chamber would be too small to get a measurable signal.
However, Safdi’s prediction excited Berkeley Nuclear Engineering Professor Karl van Bibber. He is building the ALPHA plasmonic haloscope, which creates resonant enhancement using parallel wires in a strong magnetic field. And van Bibber is waiting to tune his experiment using the more precise predictions Safdi is now calculating with the Perlmutter.
Overall, Berkeley Physics’ search for dark matter is casting an impressively wide net. “Berkeley might be the best place in the world for dark matter research. I can’t think of any place that’s stronger overall and better,” says McKinsey.
This is a reposting of my Berkeley Physics magazine feature, courtesy of UC Berkeley’s Physics Department.
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