# Publications by Chris Hays

## Measurement of colour flow using jet-pull observables in tt¯ events with the ATLAS experiment at s=13TeV.

The European physical journal. C, Particles and fields 78 (2018) 847-

Previous studies have shown that weighted angular moments derived from jet constituents encode the colour connections between partons that seed the jets. This paper presents measurements of two such distributions, the jet-pull angle and jet-pull magnitude, both of which are derived from the jet-pull angular moment. The measurement is performed in tt¯ events with one leptonically decaying W boson and one hadronically decaying W boson, using 36.1fb-1 of pp collision data recorded by the ATLAS detector at s=13TeV delivered by the Large Hadron Collider. The observables are measured for two dijet systems, corresponding to the colour-connected daughters of the W boson and the two b-jets from the top-quark decays, which are not expected to be colour connected. To allow the comparison of the measured distributions to colour model predictions, the measured distributions are unfolded to particle level, after correcting for experimental effects introduced by the detector. While good agreement can be found for some combinations of predictions and observables, none of the predictions describes the data well across all observables.

## Measurement of τ polarisation in Z/γ∗→ττ decays in proton-proton collisions at s=8 TeV with the ATLAS detector.

The European physical journal. C, Particles and fields 78 (2018) 163-

This paper presents a measurement of the polarisation of τ leptons produced in Z/γ∗→ττ decays which is performed with a dataset of proton-proton collisions at s=8 TeV, corresponding to an integrated luminosity of 20.2 fb -1 recorded with the ATLAS detector at the LHC in 2012. The Z/γ∗→ττ decays are reconstructed from a hadronically decaying τ lepton with a single charged particle in the final state, accompanied by a τ lepton that decays leptonically. The τ polarisation is inferred from the relative fraction of energy carried by charged and neutral hadrons in the hadronic τ decays. The polarisation is measured in a fiducial region that corresponds to the kinematic region accessible to this analysis. The τ polarisation extracted over the full phase space within the Z/γ∗ mass range of 66 <mZ/γ∗< 116 GeV is found to be Pτ=-0.14±0.02(stat)±0.04(syst) . It is in agreement with the Standard Model prediction of Pτ=-0.1517±0.0019 , which is obtained from the ALPGEN event generator interfaced with the PYTHIA 6 parton shower modelling and the TAUOLA τ decay library.

## Measurement of the inclusive and fiducial tt¯ production cross-sections in the lepton+jets channel in pp collisions at s=8TeV with the ATLAS detector.

The European physical journal. C, Particles and fields 78 (2018) 487-

The inclusive and fiducial tt¯ production cross-sections are measured in the lepton+jets channel using 20.2fb-1 of proton-proton collision data at a centre-of-mass energy of 8 TeV recorded with the ATLAS detector at the LHC. Major systematic uncertainties due to the modelling of the jet energy scale and b-tagging efficiency are constrained by separating selected events into three disjoint regions. In order to reduce systematic uncertainties in the most important background, the W+\,jets process is modelled using Z + jets events in a data-driven approach. The inclusive tt¯ cross-section is measured with a precision of 5.7% to be σinc(tt¯)=248.3±0.7(stat.)±13.4(syst.)±4.7(lumi.)pb , assuming a top-quark mass of 172.5 GeV. The result is in agreement with the Standard Model prediction. The cross-section is also measured in a phase space close to that of the selected data. The fiducial cross-section is σfid(tt¯)=48.8±0.1(stat.)±2.0(syst.)±0.9(lumi.)pb with a precision of 4.5%.

## Search for dark matter and other new phenomena in events with an energetic jet and large missing transverse momentum using the ATLAS detector

Journal of High Energy Physics Springer Verlag 2018 (2018) 126

G Aad, G Artoni, A Barr, K Becker, L Beresford, D Bortoletto, A Cooper-Sarkar, JA Frost, E Gallas, C Gwenlan, F Giuli, C Hays, T Huffman, C Issever, J Liu, L Marchese, K Nagai, ME Nelson, R Nickerson, I Shipsey, G Viehhauser, J Tseng, A Weidberg, G Zemaityte

Results of a search for new phenomena in final states with an energetic jet and large missing transverse momentum are reported. The search uses proton-proton collision data corresponding to an integrated luminosity of 36.1 fb−1 at a centre-of-mass energy of 13 TeV collected in 2015 and 2016 with the ATLAS detector at the Large Hadron Collider. Events are required to have at least one jet with a transverse momentum above 250 GeV and no leptons (e or μ). Several signal regions are considered with increasing requirements on the missing transverse momentum above 250 GeV. Good agreement is observed between the number of events in data and Standard Model predictions. The results are translated into exclusion limits in models with pair-produced weakly interacting dark-matter candidates, large extra spatial dimensions, and supersymmetric particles in several compressed scenarios.

## A search for pair-produced resonances in four-jet final states at s=13 TeV with the ATLAS detector.

The European physical journal. C, Particles and fields 78 (2018) 250-

A search for massive coloured resonances which are pair-produced and decay into two jets is presented. The analysis uses 36.7 fb -1 of s = 13 TeV pp collision data recorded by the ATLAS experiment at the LHC in 2015 and 2016. No significant deviation from the background prediction is observed. Results are interpreted in a SUSY simplified model where the lightest supersymmetric particle is the top squark, t~ , which decays promptly into two quarks through R-parity-violating couplings. Top squarks with masses in the range 100GeV<mt~<410   GeV are excluded at 95% confidence level. If the decay is into a b-quark and a light quark, a dedicated selection requiring two b-tags is used to exclude masses in the ranges 100GeV<mt~<470   GeV and 480GeV<mt~<610   GeV . Additional limits are set on the pair-production of massive colour-octet resonances.

## Measurement of longitudinal flow decorrelations in Pb+Pb collisions at sNN=2.76 and 5.02 TeV with the ATLAS detector.

The European physical journal. C, Particles and fields 78 (2018) 142-

Measurements of longitudinal flow correlations are presented for charged particles in the pseudorapidity range |η|<2.4 using 7 and 470 μb-1 of Pb+Pb collisions at sNN=2.76 and 5.02 TeV, respectively, recorded by the ATLAS detector at the LHC. It is found that the correlation between the harmonic flow coefficients vn measured in two separated η intervals does not factorise into the product of single-particle coefficients, and this breaking of factorisation, or flow decorrelation, increases linearly with the η separation between the intervals. The flow decorrelation is stronger at 2.76 TeV than at 5.02 TeV. Higher-order moments of the correlations are also measured, and the corresponding linear coefficients for the kth -moment of the vn are found to be proportional to k for v3 , but not for v2 . The decorrelation effect is separated into contributions from the magnitude of vn and the event-plane orientation, each as a function of η . These two contributions are found to be comparable. The longitudinal flow correlations are also measured between vn of different order in n. The decorrelations of v2 and v3 are found to be independent of each other, while the decorrelations of v4 and v5 are found to be driven by the nonlinear contribution from v22 and v2v3 , respectively.

## Search for additional heavy neutral Higgs and gauge bosons in the ditau final state produced in 36 fb(-1) of pp collisions at root s=13 TeV with the ATLAS detector

JOURNAL OF HIGH ENERGY PHYSICS (2018) ARTN 055

## Search for new phenomena using the invariant mass distribution of same-flavour opposite-sign dilepton pairs in events with missing transverse momentum in s=13   Tepp collisions with the ATLAS detector.

The European physical journal. C, Particles and fields 78 (2018) 625-

A search for new phenomena in final states containing an e+e- or μ+μ- pair, jets, and large missing transverse momentum is presented. This analysis makes use of proton-proton collision data with an integrated luminosity of 36.1fb-1 , collected during 2015 and 2016 at a centre-of-mass energy s=13TeV with the ATLAS detector at the Large Hadron Collider. The search targets the pair production of supersymmetric coloured particles (squarks or gluinos) and their decays into final states containing an e+e- or μ+μ- pair and the lightest neutralino ( χ~10 ) via one of two next-to-lightest neutralino ( χ~20 ) decay mechanisms: χ~20→Zχ~10 , where the Z boson decays leptonically leading to a peak in the dilepton invariant mass distribution around the Z boson mass; and χ~20→ℓ+ℓ-χ~10 with no intermediate ℓ+ℓ- resonance, yielding a kinematic endpoint in the dilepton invariant mass spectrum. The data are found to be consistent with the Standard Model expectation. Results are interpreted using simplified models, and exclude gluinos and squarks with masses as large as 1.85 and 1.3  Te at 95% confidence level, respectively.

## Measurement of differential cross sections of isolated-photon plus heavy-flavour jet production in pp collisions at root s=8 TeV using the ATLAS detector

PHYSICS LETTERS B 776 (2018) 295-317

## Measurement of differential cross-sections of a single top quark produced in association with a W boson at s=13TeV with ATLAS.

The European physical journal. C, Particles and fields 78 (2018) 186-

The differential cross-section for the production of a W boson in association with a top quark is measured for several particle-level observables. The measurements are performed using 36.1fb-1 of pp collision data collected with the ATLAS detector at the LHC in 2015 and 2016. Differential cross-sections are measured in a fiducial phase space defined by the presence of two charged leptons and exactly one jet matched to a b-hadron, and are normalised with the fiducial cross-section. Results are found to be in good agreement with predictions from several Monte Carlo event generators.

## Direct top-quark decay width measurement in the tt¯ lepton+jets channel at s=8TeV with the ATLAS experiment.

The European physical journal. C, Particles and fields 78 (2018) 129-

This paper presents a direct measurement of the decay width of the top quark using tt¯ events in the lepton+jets final state. The data sample was collected by the ATLAS detector at the LHC in proton-proton collisions at a centre-of-mass energy of 8 TeV and corresponds to an integrated luminosity of 20.2 fb -1 . The decay width of the top quark is measured using a template fit to distributions of kinematic observables associated with the hadronically and semileptonically decaying top quarks. The result, Γt=1.76±0.33(stat.)-0.68+0.79(syst.)GeV for a top-quark mass of 172.5 GeV, is consistent with the prediction of the Standard Model.

## Search for doubly charged Higgs boson production in multi-lepton final states with the ATLAS detector using proton-proton collisions at s=13TeV.

The European physical journal. C, Particles and fields 78 (2018) 199-

A search for doubly charged Higgs bosons with pairs of prompt, isolated, highly energetic leptons with the same electric charge is presented. The search uses a proton-proton collision data sample at a centre-of-mass energy of 13 TeV corresponding to 36.1 fb-1 of integrated luminosity recorded in 2015 and 2016 by the ATLAS detector at the LHC. This analysis focuses on the decays H±±→e±e± , H±±→e±μ± and H±±→μ±μ± , fitting the dilepton mass spectra in several exclusive signal regions. No significant evidence of a signal is observed and corresponding limits on the production cross-section and consequently a lower limit on m(H±±) are derived at 95% confidence level. With ℓ±ℓ±=e±e±/μ±μ±/e±μ± , the observed lower limit on the mass of a doubly charged Higgs boson only coupling to left-handed leptons varies from 770 to 870 GeV (850 GeV expected) for B(H±±→ℓ±ℓ±)=100% and both the expected and observed mass limits are above 450 GeV for B(H±±→ℓ±ℓ±)=10% and any combination of partial branching ratios.

## Search for dark matter produced in association with bottom or top quarks in s=13  TeV pp collisions with the ATLAS detector.

The European physical journal. C, Particles and fields 78 (2018) 18-

A search for weakly interacting massive dark-matter particles produced in association with bottom or top quarks is presented. Final states containing third-generation quarks and missing transverse momentum are considered. The analysis uses 36.1fb-1 of proton-proton collision data recorded by the ATLAS experiment at s=13  TeV in 2015 and 2016. No significant excess of events above the estimated backgrounds is observed. The results are interpreted in the framework of simplified models of spin-0 dark-matter mediators. For colour-neutral spin-0 mediators produced in association with top quarks and decaying into a pair of dark-matter particles, mediator masses below 50 GeV are excluded assuming a dark-matter candidate mass of 1 GeV and unitary couplings. For scalar and pseudoscalar mediators produced in association with bottom quarks, the search sets limits on the production cross-section of 300 times the predicted rate for mediators with masses between 10 and 50GeV and assuming a dark-matter mass of 1GeV and unitary coupling. Constraints on colour-charged scalar simplified models are also presented. Assuming a dark-matter particle mass of 35GeV , mediator particles with mass below 1.1TeV are excluded for couplings yielding a dark-matter relic density consistent with measurements.

## Electroweak measurements at the tevatron

Proceedings of the 53rd Rencontres de Moriond - 2018 Electroweak Interactions and Unified Theories, EW 2018 (2018) 97-102

CP Hays

© 2018 CERN for the benefit of the ATLAS and CMS Collaborations. The Fermilab Tevatron produced proton-antiproton collisions at a center-of-mass energy of √s = 1.96 TeV between 2001 and 2011. Using an integrated luminosity of ≈ 10 fb−1 collected by each of the general-purpose experiments CDF and D0, many precision measurements were performed. I present the Tevatron's highest-precision electroweak measurements: the final combined measurement of the forward-backward asymmetry of charged leptons from Z-boson decays, and the ongoing measurements of the W boson mass and the charge-asymmetry of leptons from its decays.

## Search for new phenomena in high-mass final states with a photon and a jet from pp collisions at s = 13 TeV with the ATLAS detector.

The European physical journal. C, Particles and fields 78 (2018) 102-

A search is performed for new phenomena in events having a photon with high transverse momentum and a jet collected in 36.7fb-1 of proton-proton collisions at a centre-of-mass energy of s = 13 TeV recorded with the ATLAS detector at the Large Hadron Collider. The invariant mass distribution of the leading photon and jet is examined to look for the resonant production of new particles or the presence of new high-mass states beyond the Standard Model. No significant deviation from the background-only hypothesis is observed and cross-section limits for generic Gaussian-shaped resonances are extracted. Excited quarks hypothesized in quark compositeness models and high-mass states predicted in quantum black hole models with extra dimensions are also examined in the analysis. The observed data exclude, at 95% confidence level, the mass range below 5.3 TeV for excited quarks and 7.1 TeV (4.4 TeV) for quantum black holes in the Arkani-Hamed-Dimopoulos-Dvali (Randall-Sundrum) model with six (one) extra dimensions.

## Measurement of the W-boson mass in pp collisions at root s=7 TeV with the ATLAS detector

European Physical Journal C Springer Verlag 78 (2018) 110

M Aaboud, G Aad, B Abbott, CP Hays

A measurement of the mass of the W boson is presented based on proton–proton collision data recorded in 2011 at a centre-of-mass energy of 7 TeV with the ATLAS detector at the LHC, and corresponding to $4.6~\hbox {fb}^{-1}$ of integrated luminosity. The selected data sample consists of $7.8\times 10^6$ candidates in the $W\rightarrow \mu \nu$ channel and $5.9\times 10^6$ candidates in the $W\rightarrow e \nu$ channel. The $W$-boson mass is obtained from template fits to the reconstructed distributions of the charged lepton transverse momentum and of the W boson transverse mass in the electron and muon decay channels, yielding \begin{aligned} \nonumber m_W = 80370&amp;\pm 7~(\text {stat.}) \pm 11 (\text {exp. syst.}) \\&amp;\pm 14~(\text {mod. syst.})~\text {MeV} \\ = 80370&amp;\pm 19\,\text {MeV}, \end{aligned} where the first uncertainty is statistical, the second corresponds to the experimental systematic uncertainty, and the third to the physics-modelling systematic uncertainty. A measurement of the mass difference between the $W^+$ and $W^-$ bosons yields $m_{W^+}-m_{W^-} = -\,29 \pm 28 MeV$.

## Search for B - L R-parity-violating top squarks in root s=13 TeV pp collisions with the ATLAS experiment

PHYSICAL REVIEW D 97 (2018) ARTN 032003

## Search for heavy resonances decaying into WW in the eνμν final state in pp collisions at s=13 TeV with the ATLAS detector.

The European physical journal. C, Particles and fields 78 (2018) 24-

A search for neutral heavy resonances is performed in the WW→eνμν decay channel using pp collision data corresponding to an integrated luminosity of 36.1fb-1 , collected at a centre-of-mass energy of 13 TeV by the ATLAS detector at the Large Hadron Collider. No evidence of such heavy resonances is found. In the search for production via the quark-antiquark annihilation or gluon-gluon fusion process, upper limits on σX×B(X→WW) as a function of the resonance mass are obtained in the mass range between 200 GeV and up to 5 TeV for various benchmark models: a Higgs-like scalar in different width scenarios, a two-Higgs-doublet model, a heavy vector triplet model, and a warped extra dimensions model. In the vector-boson fusion process, constraints are also obtained on these resonances, as well as on a Higgs boson in the Georgi-Machacek model and a heavy tensor particle coupling only to gauge bosons.

## Measurement of the cross-section for producing a W boson in association with a single top quark in pp collisions at root s=13 TeV with ATLS

JOURNAL OF HIGH ENERGY PHYSICS (2018) ARTN 063