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
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.
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.
New physics and tau g–2 using LHC heavy ion collisions
Lydia Beresford and Jesse Liu arXiv:1908.05180
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'
New physics and tau g−2 using LHC heavy ion collisions
Lydia Beresford and Jesse Liu
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.
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
Search for electroweak production of supersymmetric states in scenarios with
compressed mass spectra at sqrt(s)=13 TeV with the ATLAS detector
arXiv:1712.08119, Phys. Rev. D 97 (2018) 052010
I had the privilege of collaborating with an excellent international analysis team for
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
Alan Barr and Jesse Liu
arXiv:1608.05379, Eur. Phys. J. C (2017) 77: 202.
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
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 severalspeakers at major summer conferences, and used by the SUSY-AI Online effort.