For much of the first half of the twentieth century, physicists believed there were just three fundamental particles: the familiar proton, neutron, and electron. By the mid 1960s, however, that picture had changed. Improvements to particle accelerator and detector technology had given way to the discovery of a seemingly endless list of new particles. Simplicity, elegance - these are hallmarks of a good scientific theory, and these were definitively lacking from the so called ‘particle zoo’ of the day. Scientists began to look for a more simple, unified theory to explain these particles on a fundamental level.
Elegant, but Incomplete
Over the course of the next several decades, a theory that became known as the Standard Model of Particle Physics emerged. Now one of the most well supported scientific theories in history, this model does explain the fundamental structure behind the ‘particle zoo’ with incredible accuracy.
The particles of the Standard Model: fermions in red (quarks) and green (leptons), vector bosons in blue and the Higgs boson in yellow.
The theory describes two fundamental types of particles: fermions, which makes up all of the ‘stuff’ around us, and bosons, which mediate how fermions interact with one another. Two familiar examples are the electron (a fermion) and a photon (a boson), the particle of light which carries the electromagnetic force. Fermions are further divided into quarks - which make up protons and neutrons - and leptons - which include electrons in addition to muons, taus, and the elusive, barely-massive neutrinos.
The Standard Model predicts the properties of particles with incredible precision, and for a while, it truly seemed to be the fundamental theory that physicists of the ‘particle zoo’ days sought so ardently. Yet, there remained two major problems - the theory could not explain why any particle has mass, much less predict the masses of individual particles, and it had no candidate particle for Dark Matter, which comrpises 85\% of the mass in the universe.
The Higgs Boson and Beyond
To solve this problem, Peter Higgs, François Englert and others theorized an extension to the Standard Model. They predicted the existence of a fundamental field that exists everywhere, all of the time and gives mass to fundamental particles. Further, they predicted that an excitation of this field could be observed as a particle - the famed Higgs Boson. In July of 2012, nearly fifty years after the Higgs boson was first theorized,CERN confirmed that the elusive particle had been observed by both the CMS and Atlas experiments.
This initial observation of the Higgs lead to almost as many questions as answers. Physicists have learned relatively little about the boson’s properties from experimental data - more data must be taken to confirm to what extent the observed particle matches the predicted one. And, despite it’s successes, the Standard Model has some deficiencies. It cannot account for most of the mass in the universe, which is bound up in so-called Dark Matter. Nor can it explain why the universe is dominated by matter and not made of equal parts matter and anti-matter. And don’t even think about including gravity in the picture! Clearly, there are many questions to explore about the universe and sub atomic particles.
Dark Matter
Dark matter is a form of matter that we can't see through it's interaction with light, or electromagnetism. Nonetheless, its gravitational influence is unmistakable: from the way galaxies rotate, to the way massive clusters bend the path of distant light (gravitational lensing), and even the subtle patterns in the cosmic microwave background. Accounting for roughly 85% of the total matter in the universe, dark matter is instrumental to our understanding of cosmology, helping explain how large-scale structures—such as galaxy clusters and the cosmic web—evolved over billions of years.
There are many possibilities for what comprises dark matter. Light relic dark matter is a compelling candidate because it fits naturally into standard early-universe physics. In this scenario, dark matter particles were once in thermal or kinetic equilibrium with the hot primordial plasma. As the universe expanded and cooled, interactions became too scarce to maintain equilibrium, causing these particles to “freeze out,” leaving the dark matter we see today. The final abundance depends on both the mass of the particles and the strength of their interactions, and if they are relatively light (on the scale of an electron mass to a proton mass), it aligns well with certain astrophysical observations and could still be detectable in upcoming experiments. This thermal freeze-out mechanism provides a neat explanation for how dark matter formed and persists, bridging cosmological evidence with modern particle physics pursuits.
Experiments
ATLAS
The ATLAS experiment at the Large Hadron Collider (LHC) is one of the world’s largest particle detectors, designed to investigate the fundamental building blocks of matter. It examines over a billion of the world's highest energy proton-proton collisions per second and searches for clues to the nature of the Higgs Boson, to the composition of Dark Matter, and to the underlying structure of fundamental interactions in the universe.
Our group's interest lies primarily in searching for evidence of complex dark sectors. These dark sectors can be strongly self-interacting, or they can harbor a suite of bosons. Historically we have made measurements which elucidate the Higgs boson's couplings to b-quarks and we have investigated the strong interaction at low and high scales.
We are contributing in several aspects of the ATLAS experiment's trigger and data acquisition system. We were a key institute for the Fast TracKer (FTK) project, which demonstrated the feasibility of reconstructing all charged particles above 1 GeV at a 100kHz rate using a mix of custom and commercial electronics. We have helped develop and commission gFex, a trigger board which calculates global event variables from the full calorimeter at for every beam crossing. Most recently we are contributing to the heart of the first level trigger for ATLAS's High Luminosity LHC upgrade: the Global Trigger system. We are experts in FGPA-based real-time processing systems.
Light Dark Matter eXperiment (LDMX)
The Light Dark Matter eXperiment (LDMX) is an accelerator-based search for low mass, thermal relic dark matter, using a low-intensity, high repetition electron beam from SLAC's LCLS-II accelerator, directed onto a thin target. By precisely measuring outgoing particles and looking for missing energy, LDMX has broad sensitivity to thermal relic dark matter from the electon mass to a few times the proton mass. LDMX is in a design phase, with construction beginning soon.
Within LDMX our group is responsible for the trigger scintillator system, a subdetector which accurately counts the number of incoming electrons every 26.7 ns. We additionally contribute to the overall LDMX trigger and data acquisition architecture, and are heavily involved in preliminary testbeam efforts. We are also contributing to charged particle tracking for LDMX and overall analysis strategies.
The Heavy Photon Search (HPS)
The Heavy Photon Search (HPS) is an experiment at Jefferson Laboratory's CEBAF accelerator, designed search for the visible decays of massive dark photons to electron-positron pairs. Massive dark photons arise in many thermal relic dark matter models, and CEBAF's high current beam provides an extensive dataset in which to search for these decays. The beam is incident on a thin tungsten target and electron positron pairs are reconstructed using a silicon vertex detector. HPS is a currently experiment, having taken data in 2016, 2019, and 2021, with plans to run again in 2026 or 2027
Within HPS our group works on the charged particle reconstruction software, data analysis, and tracker refurbishment.
Software and Computing
As a part of the NSF Institute for Research and Innovation in Software for High Energy Physics (IRIS-HEP) we work on algorithm development for charged particle reconstruction for particle physics experiments generically. We have worked on the development new methods of automatic parameter tuning for track reconstruction tasks as well as adapted deep learning methods for primary vertex reconstruction. We are also a part of the ACTS open source software project.
Members
Lauren Tompkins is an Associate Professor of Physics at Stanford University. She recieved her BA and PhD from UC Berkeley, and completed a postdoctoral fellowship at the University of Chicago. She is a member of the ATLAS, LDMX and HPS collaborations, as well as IRIS-HEP, and is an expert in FPGA-based real time processing systems for particle physics experiments. In addition to her research, she is passionate about teaching and inclusion in science.
Elizabeth Berzin is a 2nd-year PhD student working on the LDMX and HPS experiments. On LDMX, she works with the trigger scintillator (TS) subsystem, conducting design studies and contributing to test beam activities with a TS prototype installed in the S30XL beam line at SLAC. She also works on tracking software development and strategies for handling multi-electron events. On HPS, she works on improving Monte Carlo simulations of the detector.
Rocky Bala Garg is a research scientist, working with the ATLAS Collaboration. Her primary research interests lie in dark matter searches and the development of tracking software for high-energy physics. She currently serves as a subgroup co-convener within the Tracking CP group at ATLAS. On the dark matter front, her research focuses on unconventional searches that explore complex dark sectors. She currently leads two analyses within ATLAS as the analysis contact. She is also an IRIS-HEP fellow and regularly mentors undergraduate and graduate students. In recognition of her outreach efforts, she has been awarded the US ATLAS Education and Public Outreach Award. Beyond ATLAS, she is actively involved in the development of future muon colliders, working on multiple aspects, including tracking optimization and the physics analysis of simulated data. More details about her work can be found on her LinkedIn page.
Majd Ghrear is a new postdoc in the group working on the LDMX and HPS experiments. He recently joined us from the University of Hawaii where he worked on directional dark matter detection.
Noe Gonzalez is a first-year graduate student working on the ATLAS experiment. His current research and interests are focused on dark sector extensions of the Standard Model and fast trigger hardware using FPGAs. Previously, he was an undergraduate at UC Santa Cruz working on the ATLAS silicon strip tracker.
Sadaf Kadir is a is a 4th year Physics PhD student working in the ATLAS collaboration. She is working in a dark matter search which is studying potential dark sector theories of dark matter, as well as a search for a high mass Higgs particle. In addition, she also works with tracking for ATLAS, working on the algorithms which turn hits in the detector into particle tracks. Outside of physics she is involved with Gender Minorities in Applied Physics and Physics.
Qi Bin Lei is a post-baccalaureate scholar. He is working on deep learning methods for primary vertex finding for the ATLAS Experiment and developing Muon Collider Software. He is an occasional enjoyer of running, hiking, trail mix, and pears.
Rory O'Dwyer is a 5th year Ph.D. student studying heavy photon-mediated dark matter at the HPS and LDMX experiments. Among his contributions to these collaborations include the design and implementation of the LDMX TS triggering firmware, as well as the systematic optimization of HPS' reconstruction in its 2019 and 2021 runs. His analysis aims to exclude or find dark photons coupled to a strongly interacting DM sector in regions of thermal relic model parameter space that are currently inaccessible to traditional beam dump and bump hunt experiments. Outside of his research, Rory is involved in graduate worker advocacy at the SGWU.
Associate Members
Layan Alsaraya is a graduate student at San Fransisco State University working with the group on the LDMX and HPS experiments. She is studying the LDMX trigger scintillator system perforamnce and the HPS experiment's sensitivity to rare processes.
Michal Husejko is a senior FPGA engineer working on ATLAS trigger system for the high luminosity LHC upgrades.
Publications & Presentations
This page highlights select publications where group members have made significant contributions to the work. For a full list of publications for each group member, please see the link to the group member's Inspire HEP pages below. This page is often under construction.
he ATLAS Collaboration. Measurements of Higgs Bosons Decaying to Bottom Quarks from Vector Boson Fusion Production with the ATLAS Experiment at sqrt(s) = 13 TeV. Eur. Phys. J. C. 81 (2021) 537
This is bold and this is strong. This is italic and this is emphasized.
This is superscript text and this is subscript text.
This is underlined and this is code: for (;;) { ... }. Finally, this is a link.
Heading Level 2
Heading Level 3
Heading Level 4
Heading Level 5
Heading Level 6
Blockquote
Fringilla nisl. Donec accumsan interdum nisi, quis tincidunt felis sagittis eget tempus euismod. Vestibulum ante ipsum primis in faucibus vestibulum. Blandit adipiscing eu felis iaculis volutpat ac adipiscing accumsan faucibus. Vestibulum ante ipsum primis in faucibus lorem ipsum dolor sit amet nullam adipiscing eu felis.
Preformatted
i = 0;
while (!deck.isInOrder()) {
print 'Iteration ' + i;
deck.shuffle();
i++;
}
print 'It took ' + i + ' iterations to sort the deck.';
