Research

Themes Papers Talks Hardware Analysis

Themes

My research focuses on tests of the Standard Model including the Higgs boson, searches for new particles such as dark matter, and laboratory probes of astroparticle physics. In ATLAS, I pioneer high-energy tests of quantum electrodynamics using the LHC as a photon collider and have growing interests in measurements for cosmic ray science. At Chicago, I am developing broadband detection strategies for axion dark matter using emerging quantum technologies.

Supersymmetry and dark matter

A startling realisation of contemporary science is that 80% of the matter in our universe is dark. Supersymmetry predicts new particles of dark matter that could be discovered at the Large Hadron Collider. My doctoral research identified promising scenarios of supersymmetric dark matter involving partners of the Higgs boson and charged leptons. I then proposed and led new ATLAS searches resulting in world-leading sensitivity.

ATLAS and forward detectors

Particle detectors are our eyes to the microcosm. Recently, I led important performance and data quality studies for the ATLAS Forward Proton (AFP) system to successfully deliver the first physics results using this novel instrument in standard high-luminosity data taking. During my PhD, I led operational radiation damage studies of the ATLAS silicon tracker (SCT) and real-time hardware-based algorithms to select dark matter signatures (L1 Topo trigger).

Colliding light at the LHC

Electromagnetic fields surrounding LHC beams act as an intense beam of photons that collide to make matter-antimatter. I led the first publication using the AFP spectrometer to measure proton scattering when light collides into lepton pairs. This opens a new experimental programme, and I am preparing studies to motivate HL-LHC upgrades. I have also proposed using heavy ion collisions to measure the tau anomalous magnetic moment.

Axions and dark photons

Ultra-light dark matter, such as axions and dark photons, can modify Maxwell's equations and create photons we detect in the lab. Detecting the low predicted signal rates requires pushing quantum sensing technologies into uncharted territory. At Chicago, I am constructing and testing the optical performance of a custom Fourier transform spectrometer. This serves as preparation for developing next-generation dark matter experiments.

The origin of mass

The Higgs boson discovery opens the study of new interactions. I have studied the projected sensitivity of the Higgs self-interaction. By reconstructing decays into four bottom quarks, this exploits the high signal rate. This detailed simulation study of a challenging analysis informs one of the central goals of HL-LHC upgrades, crucial for understanding the shape of the Higgs potential that governs how particles acquired mass in the early universe.

Theoretical physics

Theoretical and experimental developments go hand in hand. My theoretical work focuses on simulation studies to design new ways to interpret and analyze LHC data. My recent research sharpens the case for using the LHC as a photon collider to test QED at the energy frontier. For my master's research, I developed the field theory formalism of so-called continuous-spin particles. As an undergraduate, I studied jet pileup mitigation for LHC upgrades.

Papers

Phenomenology

Higgs self-coupling measurements using deep learning in the bb̅bb̅ final state
Jacob Amacker, William Balunas, Lydia Beresford, Daniela Bortoletto, James Frost, Cigdem Issever, Jesse Liu, James McKee, Alessandro Micheli, Santiago Paredes Saenz, Michael Spannowsky and Beojan Stanislaus
JHEP 12 (2020) 115, arXiv:2004.04240

New physics and tau g−2 using LHC heavy ion collisions
Lydia Beresford and Jesse Liu
Phys. Rev. D 102 (2020) 113008, arXiv:1908.05180

Search Strategy for Sleptons and Dark Matter using the LHC as a Photon Collider
Lydia Beresford and Jesse Liu
Phys. Rev. Lett. 123 (2019) 141801, arXiv:1811.06465

Analysing parameter space correlations of recent 13 TeV gluino and squark searches in the pMSSM
Alan Barr and Jesse Liu
Eur. Phys. J. C 77 (2017) 202, arXiv:1608.05379

First interpretation of 13 TeV supersymmetry searches in the pMSSM
Alan Barr and Jesse Liu
arXiv:1605.09502
Oxford physics highlight

Analysis

Observation and Measurement of Forward Proton Scattering in Association with Lepton Pairs Produced via the Photon Fusion Mechanism at ATLAS
ATLAS Collaboration (JL Editor and Lead Analyzer)
Phys. Rev. Lett. 125 (2020) 261801, arXiv:2009.14537, STDM-2018-16
ATLAS Physics Briefing, ATLAS ICHEP highlights
CERN Courier feature, Press Release

Search for electroweak production of supersymmetric states in scenarios with compressed mass spectra at √s = 13 TeV with the ATLAS detector
ATLAS Collaboration (JL Editor and Lead Analyzer)
Phys. Rev. D 97 (2018) 052010, arXiv:1712.08119, SUSY-2016-25
ATLAS Physics Briefing on Higgsinos, Briefing on sleptons
CERN Courier feature, Oxford physics highlight

Instrumentation

Design and performance of a multi-terahertz Fourier transform spectrometer for axion dark matter experiments
Kristin Dona, Jesse Liu, Noah Kurinsky, David Miller, Pete Barry, Clarence Chang and Andrew Sonnenschein
arXiv:2104.07157

In situ radiation damage studies of optoelectronics in the ATLAS SemiConductor Tracker
Ian Dawson, Bruce Gallop, Jesse Liu, Peter Miyagawa, Peter Phillips, Gavin Pownall, Dave Robinson and Anthony Weidberg
JINST 14 (2019) P07014
Preliminary results: SCT-2018-003, SCT-2017-003, SCT-2016-002

Thesis

Discovery strategies for dark matter and Higgsinos at the LHC
Jesse Liu
CERN-THESIS-2019-088, Oxford Research Archive

I am author on all papers signed 'ATLAS Collaboration' since 2017.
Listed here are public references with leading or major personal contributions.
Particle physics convention lists authors alphabetically.

Find a full list of my papers at inspire:

Talks

ICHEP 2018, Seoul
ICHEP 2018, Seoul
Dalitz Seminar, Oxford
Dalitz Seminar, Oxford
LHC Reinterpretation Workshop, CERN
LHC Reinterpretation Workshop, CERN

Invited Seminars

Cornell University, USA, LEPP Journal Club Seminar, 30 Oct 2020
Stony Brook University, USA, Particle Physics Seminar, 19 Oct 2020
University of Pittsburgh, USA, PITT PACC Seminar, 5 Aug 2020
SLAC, Stanford University, USA, Elementary Particle Physics Theory Seminar, 12 Feb 2020
Fermilab, USA, Cosmic Physics Center Chalk Talk, 14 Nov 2019
Fermilab, USA, LHC Physics Center Physics Forum, 31 Oct 2019
'Colliding light to make dark matter and measure tau g–2'

University of Cambridge, UK, Cavendish HEP Seminar, 19 Feb 2019
LBNL, University of California, Berkeley, USA, Research Progress Meeting Seminar, 20 Nov 2018
'New frontiers in LHC discovery strategies'

SLAC, Stanford University, USA , Joint Theory–Experiment Seminar, 20 Apr 2018
Perimeter Institute for Theoretical Physics, Canada, BSM Seminar, 17 Apr 2018
University of California, Santa Cruz, USA, SCIPP Seminar, 10 Apr 2018
'Supersymmetry: closing the gaps at the LHC'

University of Oxford, UK, Dalitz Seminar in Fundamental Physics, 19 Jan 2017
University of Cambridge, UK, Joint DAMTP–Cavendish Seminar, 13 Jan 2017
'Soft physics and interpretation challenges for LHC supersymmetry searches'

Conference & Workshop Presentations

APS April Meeting, Online, 20 Apr 2021
'Turning Light Into Matter: First Physics Results Using ATLAS Forward Proton'

DIS 2021, Stony Brook, USA, Online, 14 Apr 2021
'Measurements of diffractive physics and soft QCD at ATLAS'

LHC Forward Physics Meeting, CERN, Online, 4 Mar 2021
'Exclusive dilepton production in ATLAS'

Workshop on Forward Physics and QCD with LHC, EIC and Cosmic Rays, Jefferson Lab, Online, 23 Jan 2021
'ATLAS Forward Proton'

Snowmass EF09 meeting, Online, 4 Sep 2020
'Colliding light to search for BSM phenomena'

ICHEP 2020, Online, 31 Jul 2020
'Colliding light to make dark matter at the LHC'
ICHEP 2020, Online, 30 Jul 2020
'The Alignment of the ATLAS Forward Proton Detector'

Snowmass Energy Frontier "Open questions and new ideas", Online, 8 Jul 2020
'Dark matter and tau g-2 using the LHC as a photon collider'

Higgs Couplings 2019, Oxford, UK, 2 Oct 2019
'Colliding light to measure tau g–2'

Young Experimentalists and Theorists Institute, Durham, UK, 8 Jan 2019
LHC Forward Physics Workshop, CERN, Switzerland, 18 Dec 2018
'Photon collider opportunities for new physics: SUSY and dark matter'

SUSY 2018, Barcelona, Spain, 23 Jul 2018
'Reconstruction techniques in ATLAS SUSY searches'

ICHEP 2018, Seoul, South Korea, 6 Jul 2018
ICHEP Prize Talk, 11 Jul 2018
'Innovative strategies in compressed electroweak SUSY searches'

DM@LHC 2018, Heidelberg, Germany, 5 Apr 2018
Institute of Physics Conference 2018, Bristol, UK, 27 Mar 2018
Young Theorists Forum 2018, Durham, UK, 10 Jan 2018
'Opening the soft lepton frontier for new physics at the LHC'

Young Theorists Forum 2017, Durham, UK, 11 Jan 2017
2nd LHC BSM Reinterpretation Workshop, CERN, Switzerland, 14 Dec 2016
'Parameter space correlations of 13 TeV SUSY searches'

BUSSTEPP 2016, Manchester, UK, 31 Aug 2016
1st LHC BSM Reinterpretation Workshop, CERN, Switzerland, 17 Jun 2016
'Phenomenological interpretations of strong SUSY searches'

Collaboration Plenaries

ATLAS SM Group Plenary, Online, 15 Apr 2021
'Cosmic ray physics at ATLAS'

ATLAS Run 4 Roman Pot Meeting, Online, 26 Mar 2021
'AFP at HL-LHC: Physics and experimental overview'

ATLAS Idea Day, Online, 20 Jan 2021
'Cosmic ray physics at ATLAS'

ATLAS Roman Pot General Meeting, Online, 7 Oct 2020
'Lessons learned from dilepton AFP analysis'

ATLAS SM Jamboree, Online, 30 Sep 2020
'Summary of ATLAS Roman Pot exploitation opportunities'

ATLAS SM Group Plenary, Online, 2 Jul 2020
'Exclusive dilepton production with AFP'

ATLAS Week Plenary, Online, 24 Jun 2020
'Photon fusion production with and without ATLAS Forward Proton'

ATLAS Roman Pot General Meeting, CERN, Switzerland, 23 Jan 2020
'Exclusive dilepton production with proton tag'

ATLAS UK Exotics SUSY Meeting, Cambridge, UK, 11 Apr 2019
'Photon collider SUSY/DM searches with forward proton detectors'

ATLAS SUSY Group Plenary, CERN, Switzerland, 19 Jul 2018
'Physics highlights at recent international conferences'

ATLAS Exotics Workshop, Rome, Italy, 29 May 2018
'Opening the monojet + soft lepton frontier for dark matter' (poster)

ATLAS Week Plenary, CERN, Switzerland, 21 Feb 2018
'Latest SUSY results'

ATLAS Analysis Open Presentation, CERN, Switzerland, 1 Nov 2017
'Search for Higgsinos and compressed sleptons'

ATLAS Joint Exotics-SUSY Workshop, Bucharest, Romania, 12 May 2017
'Phenomenological studies of ATLAS SUSY searches'

ATLAS UK Meeting, University of Liverpool, UK, 5 Jan 2017
'New innovative ideas and analyses in supersymmetry'

ATLAS Week Plenary, CERN, Switzerland, 17 Oct 2016
'Semiconductor tracker: status report'

Instrumentation

ATLAS detector performance, UChicago interferometer development

Light
Light
Alignment
Alignment
Mallet
Mallet / Photo: K. Dona
Optomechanics
Optomechanics
Electronics
Electronics
Detection
Detection

Photon collider science

Energy frontier tests of QED, turning light into (dark) matter

ATLAS 2L AFP correlation

Observation and measurement of forward proton scattering in association with lepton pairs produced via the photon fusion mechanism at ATLAS

ATLAS Collaboration
arXiv:2009.14537, Phys. Rev. Lett. 125 (2020) 261801

I co-led a small team to produce this landmark result announced at ICHEP 2020. It is the first physics analysis using the ATLAS Forward Proton (AFP) spectrometer, a new class of LHC instrument installed in 2017 at ATLAS. For me, this result is exciting because not only is it a new analysis, it's also not every day in ATLAS you get a new kind of detector to understand and play with. It's a real treat for experimentalists! The physics itself is sublime: we're seeing light turning into matter and antimatter. This result observes the scattered intact protons when light turns into dielectron or dimuons. We also understand the detector reconstruction to a level that allows us to perform the first cross-section measurements using AFP. With so many novelties in one analysis, this opens up an exciting new programme at the LHC to use proton-tagging for measurements and searches.

New physics and tau g−2 using LHC heavy ion collisions

Lydia Beresford and Jesse Liu
arXiv:1908.05180, Phys. Rev. D 102 (2020) 113008
Lepton magnetic moments

The muon g–2 has a longstanding 3.7 sigma tension with prediction and new physics interpretations such as supersymmetry have been widely studied. With a larger mass, the tau g–2 can be much more sensitive to new physics but is rarely discussed. The strongest constraints come from LEP, which is an order of magnitude away from the central value and even the sign remains elusive. Interestingly, photon collisions using heavy ions could open new advances, given the exceptionally clean environment and huge photon flux. The key to our proposal is introducing the strategy amenable for ATLAS or CMS to implement using data already collected at the LHC.

Photon collider search strategy for sleptons and dark matter at the LHC

Lydia Beresford and Jesse Liu
arXiv:1811.06465, Phys. Rev. Lett. 123 (2019) 141801
Slepton sensitivity with photon collider

When LHC beams cross, photons from the proton electromagnetic fields can collide to make new particles. The protons remain intact, travel down the beampipe, and are detected by very forward detectors. This allows us to reconstruct initital state information and the full missing momentum 4-vector — impossible in usual head-on collisions. My collaborator and I exploit these unique features to propose a search strategy that uncovers the blind spot where the slepton is 15 to 60 GeV heavier than the dark matter. Remarkably, this is the region favoured by non-collider data from cosmology and muon magnetic moment measurements.

Supersymmetry and dark matter

Higgsinos, compressed sleptons, beyond simplified models

ATLAS SUSY EWSummary higgsino

Search for electroweak production of supersymmetric states in scenarios with compressed mass spectra at √s = 13 TeV with the ATLAS detector

ATLAS Collaboration
arXiv:1712.08119, Phys. Rev. D 97 (2018) 052010

I had the privilege of collaborating with an excellent international analysis team for this project. This work presents the first hadron collider sensitivity to some of the most challenging but sought-after scenarios of natural supersymmetry and dark matter involving so-called compressed mass spectra, namely Higgsinos and compressed sleptons. We probed these using the two leptons and missing transverse momentum final state, which were striking blind spots before Run 2 of the LHC. Soft lepton reconstruction down to 4 GeV — among the lowest used by the ATLAS Experiment — was crucial in opening world-leading sensitivity that surpasses nearly two-decade old LEP limits. I led important validation studies during the transition to new reconstruction and simulation software for the improved analysis that uses 4 times as much data collected in the full LHC Run 2 [arXiv:1911.12606]. I also produced the first Higgsino dark matter and slepton-bino SUSY summary plots [ATL-PHYS-PUB-2019-022], which are regularly presented at conferences.

Analysing parameter space correlations of recent 13 TeV gluino and squark searches in the pMSSM

Alan Barr and Jesse Liu
arXiv:1608.05379, Eur. Phys. J. C (2017) 77: 202
Dark matter LHC and direct detection

LHC supersymmetry searches are designed around simplified models. These capture the key experimental kinematic (e.g. jet energies) and structural (e.g. number of electrons) features in a collision. But beyond this model-independent characterisation of signatures, they are toy models for interpretation. If our universe were supersymmetric, how do the sensitivity of these searches map onto realistic scenarios? This is the LHC interpretation challenge, and addressing this is the purpose of our paper.

First interpretation of 13 TeV supersymmetry searches in the pMSSM

Alan Barr and Jesse Liu
arXiv:1605.09502
Squarks and gluinos early 13 TeV

This is the first interpretation of six early 13 TeV ATLAS searches for supersymmetry within the 19-parameter 'phenomenological MSSM' theoretical framework. This work was referenced by several speakers at major summer conferences, and used by the SUSY-AI Online effort.