Our group recently published in Nature Communications!

Pyle Group

UC Berkeley Pyle Group

Direct dark matter detection at UC Berkeley using low temperature detector technology. We are located on the University of California, Berkeley campus and led by our PI, Matt Pyle.

This is our official group page, scroll down to learn about our scientific collaborations and the goals of our group.

SPICE/HeRALD Collaboration

Leveraging TES technology for Light Dark Matter searches with superfluid helium and polar crystals. Read more about the goals of the collaboration in our Snowmass 2021 Letter of Interest here.

SuperCDMS Collaboration

A cryogenic dark matter search experiment to be run at SNOLAB. SuperCDMS SNOLAB will be the successor to the previous generator of CDMS experiments - located deep underground for shielding from high energy cosmic ray particles and radioactive byproducts. See the link below for more information on the collaboration.

https://supercdms.slac.stanford.edu/

Research

Our universe is made up of three components: dark energy, dark matter, and baryonic (normal) matter.

We want to know: what is the dark matter?

Light Dark Matter

The cosmological evidence for dark matter (DM) has shown that it likely has four general characteristics:

It is not normal matter
It is stable
It is (nearly) noninteracting
It is nonrelativistic

The long-motivated WIMP with a GeV-scale mass has long been the primary DM candidate, but decades of null results have pushed experimentalist to look for other proposed dark matter candidates. Our group is focusing on the light dark matter (LDM) regime, which includes dark matter masses from keV to GeV scales. A standard benchmark model for LDM models in this mass range is the Hidden Sector model, where the coupling of LDM particles to Standard Model particles is via a new force mediator.

To look for these LDM candidates at sub-GeV masses, novel sensor technology is required. Due to the small recoil energies imparted by LDM to the detector target, extremely sensitive detectors are necessary. Our goal is to research and develop ways to achieve sub-eV baseline energy resolution and below in order to perform LDM searches in new DM parameter space. We do this first and foremost via cryogenic (order mK) detectors, taking advantage of the engineering marvel that is a dilution refrigerator.

Figure: From D. Clowe, et al., Astrophys. J. (2006): the Bullet Cluster shows a clear decoupling between the normal matter (the color density map) and the total matter (the green contours).

Athermal Phonons

Our detector technology is primarily based on the collection of athermal phonons due to DM-nucleon scattering. For some DM event that interacts with our detector substrate (e.g. a silicon crystal), that nuclear recoil will create phonons (think of them as vibrations) and electron-hole pairs (which recombine via the release of phonons).

These phonons are athermal, and they bounce around inside the substrate. Eventually, they would down convert to lower energy (acoustic) phonons and then thermalize. However, if we place high-resolution sensors on the surface of the substrate, then we can collect these athermal phonons before the thermalization process, thus giving us the ability to have a measurement of the energy imparted by (hopefully) a DM particle.

The question then becomes: what sensors should we use?

Many different groups will have many different answers, which is the beauty of physics research. For us, we base our detectors on Transition-Edge Sensors (TESs).

Figure: A schematic of a DM-nuclear recoil creating athermal phonons, which we collect and readout via our (blue) high-resolution sensors.

Transition-Edge Sensors

A Transition-Edge Sensor (TES) is a superconducting material that has been stabilized to be in the middle of its superconducting transition.

At this point, any small change in temperature gives a large change in resistance because the superconducting transition is so steep (see the rightmost figure from Irwin and Hilton's Transition-Edge Sensors), . Thinking about our the energy of our athermal phonons as heat, when they hit the TES, this will change the TES temperature, which changes its resistance. Using cryogenic electronics (especially SQUIDs), we can understand this change in resistance, which (with signal processing and detector modeling) allows us to completely understand the energy of the original nuclear recoil.

Figure: Simple TESs (see the four rectangles) we made for R&D

Figure: The superconducting transition of a metal (from Irwin & Hilton)

QETs

Figure: From M. Pyle thesis, drawings of a QET cross section and its superconducting bandgap variation

The problem with TESs is that their energy resolution degrades when you make them larger in area. Why is this a problem? We want to capture those athermal phonons in our sensors before the down convert and thermalize! The simplest way to do this would be to make big TESs, but then we run into the aforementioned problem.

So, what can we do? In comes Quasiparticle-trap-assisted Electrothermal-feedback TESs (QETs). These devices consist of TESs that overlap with superconducting (usually aluminum) fins. With these fins, we can increase the sensor coverage without sacrificing our baseline energy resolution.

The story of the athermal phonon becomes more complex. They bounce in the detector substrate, are collected in the aluminum fins, break superconducting Cooper pairs in the aluminum to create quasiparticles. These quasiparticles then find their way to the TES due in part to its smaller superconducting bandgap (i.e. W has a much smaller superconducting bandgap). The quasiparticles reach the TES and thermalize, giving the resistance change we want. It's important to remember that there are efficiency losses at each interface, but they are worth it for increased sensor coverage.

One of the focuses of our group is optimizing the geometry of these QETs to achieve the best baseline energy resolution for a given detector substrate, which brings us back to our ultimate goal of detecting dark matter.

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