Research

Themes Papers Talks

Themes

My research sits at the experiment–theory interface, devising strategies to search for new physics beyond the Standard Model, probing the Higgs sector, supersymmetry and dark matter. I collaborate in the ATLAS experiment and perform standalone phenomenology studies. Recently, I am focussing on using the LHC as a photon collider and designing tests of quantum electrodynamics in extreme regimes.

New physics searches

When Galileo upgraded his telescope to study Jupiter more closely, he did not anticipate unveiling new worlds in the process. He discovered the Galilean moons not necessarily because he was looking for them, but because his instruments could see them. Today's science is no exception. I collaborate with scientists worldwide to refine and extend the capability of our instruments to measure what we can see with greater precision. These advances could reveal phenomena we could never have anticipated.

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 PhD thesis identified promising parameter space of supersymmetric dark matter (pMSSM). I then proposed and led analyses in ATLAS to surpass nearly 2-decade old sensitivity (LEP) on scenarios favoured by such phenomenology studies (Higgsino dark matter and compressed sleptons).

ATLAS and forward detectors

Particle detectors are our eyes to the microcosm. I led operational radiation damage studies of the ATLAS silicon tracker (SCT) to ensure the longevity of this crucial subsystem. I also designed new hardware-based algorithms to select dark matter signatures in real time with higher efficiency (L1 Topo trigger). Recently, I am commissioning ATLAS Forward Proton (AFP) detectors, which measure the energy of intact protons, opening novel use of the LHC as a photon collider.

LHC as a photon collider

Quantum electrodynamics (QED) is the theory of everyday life, governing biochemical reactions to the optoelectronics displaying this text. The LHC tests QED at the energy frontier. Electromagnetic fields surrounding protons and heavy ions source an intense beam of high energy photons. I proposed strategies to search for supersymmetry and dark matter production in these photon collisions. I also introduced new directions to use heavy ion collisions to measure the tau anomalous magnetic moment.

The origin of mass

The Higgs boson is an experimental triumph of big science and its discovery opens study of new interactions. One tantalising prediction are Higgs bosons interacting with themselves. I am studying how to probe this Higgs self-coupling using challenging final states of four bottom quarks. Experimentally observing and measuring these new phenomena is crucial for understanding the mechanism that endows all particles with mass.

Theoretical physics

In my master's essay, I developed the field theory of particles whose spin is labelled by an infinite tower of discrete quantum numbers (continuous-spin particles), which may be relevant for gravity and electromagnetism. In recent years, I have turned to phenomenology bridging theory with experiment. Specifically I focus on devising new ways to search for new physics and use the LHC as a photon collider to test QED at the energy frontier.

Papers

Phenomenology

New physics and tau g–2 using LHC heavy ion collisions
Lydia Beresford and Jesse Liu
arXiv:1908.05180

Photon collider search strategy for sleptons and dark matter at the LHC
Lydia Beresford and Jesse Liu
arXiv:1811.06465, Accepted Phys. Rev. Lett.

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

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

Analysis

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

Detector

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) 07, 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'.
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

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 Presentations

Higgs Couplings 2019, Oxford, UK, 2 Oct 2019
'Higgs EFT modifications of tau g–2 using LHC photon collisions'

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'

ATLAS Plenaries

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'

Photon collider searches for new physics

Energy frontier tests of QED

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

Lydia Beresford and Jesse Liu
arXiv:1908.05180
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, Accepted Phys. Rev. Lett.
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.

The soft lepton frontier for new physics

Analysing data collected by ATLAS to search for Higgsinos and compressed sleptons

ATLAS SUSY EWSummary higgsino

Search for electroweak production of supersymmetric states in scenarios with compressed mass spectra at sqrt(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.

The LHC interpretation challenge

How do we interpret the results of searches pursued by LHC experiments?

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.