Research

Themes Papers Talks

Themes

My research interests in particle physics concern designing experimental techniques to search for new physics beyond the Standard Model, notably supersymmetry and dark matter. I am opening new frontiers in discovery sensitivity, which is underpinned by my work on the ATLAS silicon tracker, state-of-the-art analysis strategies to phenomenological studies on dark matter and photon collider physics.

Particle physics

What are the fundamental building blocks of the universe? How do they interact? The Standard Model and General Relativity are our current best theories of nature that sit on the core principles of spacetime symmetries and quantum theory. Yet, due to both theoretical arguments and observational inconsistencies, we know they cannot be the whole story. We need new data from experimental tests to extend our understanding of nature.

Supersymmetry

Why is gravity so weak compared to the other forces? Why is the Higgs boson so unexpectedly light? Supersymmetry is a speculative theory that could help answer these questions. It predicts new particles which could be discovered at the Large Hadron Collider at CERN, Switzerland. More technically, I think about how to interpret LHC searches and analyse data for colourless superparticles that decay into low energy final states.

ATLAS

The ATLAS Experiment is essentially a glorified camera, capturing images of proton-proton collisions at CERN with exquisite precision. Thousands of scientists work together to ensure the detector works seamlessly to analyse data at the energy frontier. These could glimpse signatures of new particles created at high energy collisions. On the operations side, I monitor the optical links in the silicon tracker subdetector.

Dark sectors

Arguably the most startling realisation of fundamental science in recent decades is that our universe is mostly dark. There is now overwhelming observational evidence for dark matter, from galaxy cluster collisions to the cosmic microwave background. It makes up over 80% of the matter in the universe, but does not emit, reflect nor absorb light. What particles is it made of? How can we exploit colliders to shine light on the possibility of a rich dark sector?

Papers

Phenomenology

Photon collider search strategy for sleptons and dark matter at the LHC
Lydia Beresford and Jesse Liu
arXiv:1811.06465

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, 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
Submitted Oct 2018
Preliminary results: SCT-2018-003, SCT-2017-003, SCT-2016-002

I am author on all papers signed 'ATLAS Collaboration'. Find a full list of my papers at inspire:

Talks

ICHEP Compressed EWK slide title pMSSM title slide First 13 TeV pMSSM title slide

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
'Phenomenological interpretations of SUSY searches'

Conference Presentations

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

Using the LHC as a photon collider

Slepton sensitivity with photon collider

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

Lydia Beresford and Jesse Liu
arXiv:1811.06465

When LHC beams cross, photons from the proton electromagnetic fields can collide to make new particles. The so-called ultra-peripheral events are exceptionally clean, with only QED interactions involved at production. 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.