arXiv subjects

Alan Barr

Publications and source records attributed to Alan Barr.

6 recordsLinked to original sources

arXiv2021

Testing Bell inequalities in Higgs boson decays

Higgs boson decays produce pairs of $W$ bosons in a maximally entangled state, the spins of which can be expected to violate Bell inequalities. We show that the spin density matrix of the $W^\pm$ pair may be reconstructed experimentally from the directions of the charged lepton decay products, and from it the expectation values of various Bell operators determined. Numerical simulations of $H\rightarrow WW^*$ decays indicate that violation of a generalised CHSH inequality is unlikely to be measurable, however the CGLMP inequality is near-maximally violated. Experimental Bell tests could be performed at a variety of colliders and in different production channels. If reconstruction effects and backgrounds can be controlled then statistically significant violations could be observable even with datasets comparable to those already collected at the LHC.

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arXiv2016

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

This paper examines unexplored correlations in the parameter spaces probed by recent ATLAS analyses for gluinos and squarks, addressing various shortcomings in the literature. Six 13 TeV ATLAS analyses based on 3.2 fb$^{-1}$ of integrated luminosity are interpreted in the 19-parameter R-parity conserving phenomenological minimal supersymmetric extension to the Standard Model (pMSSM). The distinct regions covered by each search are independent of prior, and we reveal particularly striking complementarity between the 2-6 jets and Multi-b searches. In the leptonic searches, we identify better sensitivity to models than those used for analysis optimisation, notably a squark-slepton-wino scenario for the SS/3L search. Further, we show how collider searches for coloured states probe the structure of the pMSSM dark sector more extensively than the Monojet analysis alone, with sensitivity to parameter spaces that are challenging for direct detection experiments.

hep-ph↗

arXiv2016

First interpretation of 13 TeV supersymmetry searches in the pMSSM

The combined constraints from six early Run 2 ATLAS searches for supersymmetry are interpreted in the phenomenological minimal supersymmetric extension to the Standard Model (pMSSM). Each of the searches was based on proton-proton collision data recorded in 2015 at $\sqrt{s} =13$ TeV with 3.2 fb$^{-1}$ of integrated luminosity. Sensitivity to squarks of the first two generations and gluinos are evaluated using fast detector simulation. Results are presented in the 19-parameter R-parity conserving pMSSM with the lightest supersymmetric particle (LSP) being the neutralino. Considering 181.8k points that survived Run 1 constraints, 15.7% are excluded at 95% confidence level. Of those satisfying these Run 2 constraints, 0.5% (1.0%) have sub-TeV gluinos (sub-500 GeV squarks), the lightest of which has a mass of 757 GeV (293 GeV) with a 689 GeV (217 GeV) LSP.

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arXiv2015

A boost for the EW SUSY hunt: monojet-like search for compressed sleptons at LHC14 with 100 fb^-1

Current Large Hadron Collider (LHC) analyses are blind to compressed supersymmetry (SUSY) models with sleptons near the lightest super partner (LSP) in mass: $m_{\tilde{l}} - m_{\tilde{\chi}_1^0} \equiv \Delta m \lesssim 60$ GeV. We present a search sensitive to the very compressed range $3~\text{GeV} < \Delta m < 24~\text{GeV}$ using the channel $p p \rightarrow \tilde{l}^+ \tilde{l}^- +\rm{jet} \rightarrow$ $l^+ l^- \tilde{\chi}_1^0 \tilde{\chi}_1^0 +\rm{jet}$ with soft same-flavor leptons and one hard jet from initial state radiation ($p_{\rm T}^j >100$ GeV). The sleptons recoil against the jet boosting them and their decay products, making the leptons detectable and providing substantial missing transverse momentum. We use the kinematic variable $m_{\rm T 2}$ along with a different-flavor control region to reduce the large standard model backgrounds and control systematic uncertainty. We find the analysis should allow LHC14 with $100~\text{fb}^{-1}$ to search for degenerate left-handed selectrons and smuons in the compressed region up to $m_{\tilde{l}_L} \lesssim 150$ GeV. In addition, it should be sensitive to $m_{\tilde{l}_L} \lesssim 110$ GeV for the very challenging case of auto-concealed SUSY, in which left-handed sleptons decay to the Kaluza-Klein tower of a modulino LSP which lives in $d=6$ extra dimensions. In both the compressed spectrum and auto-concealed SUSY scenarios this analysis will need more data to improve on LEP2 limits for right-handed sleptons due to their smaller cross sections.

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arXiv2009

MTGEN : Mass scale measurements in pair-production at colliders

We introduce a new kinematic event variable MTGEN which can provide information relating to the mass scales of particles pair-produced at hadronic and leptonic colliders. The variable is of particular use in events with a large number of particles in the final state when some of those particles are massive and not detected, such as may arise in R-parity-conserving supersymmetry.

hep-ph↗

arXiv2003

m_T2 : the truth behind the glamour

We present the kinematic variable, m_T2, which is in some ways similar to the more familiar `transverse-mass', but which can be used in events where two or more particles have escaped detection. We define this variable and describe the event topologies to which it applies, then present some of its mathematical properties. We then briefly discuss two case studies which show how m_T2 is vital when reconstructing the masses of supersymmetric particles in mSUGRA-like and AMSB-like scenarios at the Large Hadron Collider.

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