The High-Luminosity LHC aims to provide a total integrated luminosity of 3000 fb−1

from

proton-proton collisions at √

s = 14 TeV over the course of ∼ 10 years, reaching instantaneous

luminosities of up to L = 7.5 × 1034cm−2

s

−1

, corresponding to an average of 200 inelastic pp

collisions per bunch crossing (µ = 200) . The upgraded ATLAS detector and trigger system

must be able to cope well with increased occupancies and data rates. The performance of the

upgrade has been estimated in full simulation studies, assuming expected HL-LHC conditions

and a detector configuration intended to maximise physics performance and discovery potential

at the HL-LHC, and is expected to be similar to current performance. Fast simulation studies have

been carried out to evaluate the prospects of various benchmark physics analyses to be performed

using the upgraded ATLAS detector with the full HL-LHC dataset

Duncan, Anna

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PoS(FPCP2017)044Physics prospects at the HL-LHC with ATLASAnna Duncan∗†University of GlasgowE-mail: a.duncan.2@research.gla.ac.ukThe High-Luminosity LHC aims to provide a total integrated luminosity of 3000 fb−1 fromproton-proton collisions at√s = 14 TeV over the course of ∼ 10 years, reaching instantaneousluminosities of up to L = 7.5× 1034cm−2s−1, corresponding to an average of 200 inelastic p-p collisions per bunch crossing (µ = 200) . The upgraded ATLAS detector and trigger systemmust be able to cope well with increased occupancies and data rates. The performance of theupgrade has been estimated in full simulation studies, assuming expected HL-LHC conditionsand a detector configuration intended to maximise physics performance and discovery potentialat the HL-LHC, and is expected to be similar to current performance. Fast simulation studies havebeen carried out to evaluate the prospects of various benchmark physics analyses to be performedusing the upgraded ATLAS detector with the full HL-LHC dataset.The 15th International Conference on Flavor Physics CP Violation5-9 June 2017Prague, Czech Republic∗Speaker.†These proceedings include results from four HL-LHC physics prospects studies carried out by members of theATLAS collaboration and coordinated by the ATLAS Upgrade group.c© Copyright owned by the author(s) under the terms of the Creative CommonsAttribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND 4.0). https://pos.sissa.it/PoS(FPCP2017)044HL-LHC Physics Prospects Anna Duncan1. HL-LHC and the Phase-II ATLAS UpgradeThe 3000 fb−1 total integrated luminosity of proton-proton collisions delivered by the HL-LHCbeginning in 2026 will significantly improve physics capabilities. Analyses will be able to probethe multi TeV scale in searches for new heavy particles, make higher precision Standard Modelmeasurements, and set stronger limits on/ have improved sensitivity to rare processes. In order toachieve the target integrated luminosity, the HL-LHC will reach instantaneous luminosities of up toL = 7.5×1034cm−2s−1, giving rise to a very challenging environment. An average of 200 inelasticproton-proton interactions per bunch crossing (〈µ〉 = 200) is expected. The higher occupancywill degrade jet reconstruction, and event reconstruction in general, and increased data rates willcause a non-linear increase of trigger rates. Superior detector performance in pileup rejection,jet reconstruction and flavour tagging will be essential for maintaining physics performance withATLAS under HL-LHC conditions.The phase-II ATLAS upgrade will involve replacements and modifications of the existing AT-LAS subdetectors. The current inner detector will be replaced with an all silicon tracking system:a pixel detector surrounded by a large-area strip tracking detector. Its will have higher granularityto reduce occupancy, and extended pseudorapidity coverage: (|η | < 4.0). A new trigger/ DAQarchitecture will be implemented to cope with increased data rates.Figure 1: LHC Timeline100 1000110100  gg Σqq qgWJS2012ratios of LHC parton luminosities:8 TeV / 7 TeV and 14 TeV / 7 TeV l umi nosi ty rat ioMX (GeV)MSTW2008NLO_Figure 2: LHC parton luminosity ratios [2]2. Physics Prospects Studies and Detector EffectsFast simulation studies have been carried out to evaluate the prospects of benchmark analyses atHL-LHC. The expected performance of the phase-II upgrade has been estimated for physics objectsand trigger/ DAQ with a fully simulated detector and assuming pile-up of 〈µ〉= 200 [1]. Parame-terised estimates of performance were derived from the full simulation studies and implemented insome of the prospects studies, allowing “smeared truth" simulations of analyses to be carried out.Other studies used extrapolations from Run 1 and Run 2 as performance estimates.The following prospects studies are based on analyses which will be interesting to see at theHL-LHC due to their potential sensitivity to the increased energy and statistics; a search for a heavyparticle, precision measurements in the Higgs sector and a search for a rare process.1PoS(FPCP2017)044HL-LHC Physics Prospects Anna Duncan3. Search for tt¯ ResonancesSeveral theories of physics beyond the Standard Model predict new particles with masses in theTeV scale that decay to a top - anti top quark (tt¯) pair. A search for such particles, or resonances,is a benchmark analysis for evaluating physics prospects at the HL-LHC. The 14 TeV collisionenergy will improve the mass reach of the search and the increase in statistics of high pT eventswill tighten the upper limits that, in the absence of signal, can be set on the cross sections ofhypothesised heavy particles. A tt¯ resonance would cause a local excess or deficit in the tt¯ massspectrum as predicted by the Standard Model. A search for such a deviation is performed on tt¯ pairsselected from events in Monte Carlo simulations of proton-proton collisions with a centre of massenergy of 14 TeV, over a total integrated luminosity of 3000 fb−1. The search is performed in thesemi-leptonic decay channel: both top quarks decay to a b quark and a W boson; one W decays totwo quarks and the other decays to a lepton and a neutrino. Detector effects are accounted for usingparameterised estimates of the performance of an upgraded ATLAS detector. This search relies ona good reconstruction of boosted objects, and is dependent on upgrade tracking performance in adense environment for b-tagging and lepton isolation. The tt¯ invariant mass spectrum is examinedfor local excesses or deficits. A statistical analysis sets expected upper limits on the cross section ofa tt¯ resonance in a benchmark theoretical model for several signal masses. A systematic uncertaintyon the total signal and background yield in each channel is applied: 8.8% on the resolved signalchannel, 18.0% on the boosted signal channel, 10.8% on the resolved background channel and13.4% on the boosted background channel. The uncertainties are taken from the Run 1 analysis[4] and reflect the average impact of the dominant systematic uncertainties on the yields. The massreach of a search for a Z’ boson in a TopColour model at the HL-LHC is estimated to be 4 TeV.This is an increase of ∼1 TeV relative to the estimated limit using 300 fb−1 that is expected fromthe LHC prior to the upgrade [3]. The strongest mass limit on this resonance is currently 2.1 TeV[4]. [TeV]ttm0 1 2 3 4 5 6 7Events1102103104105106107108109101010 50)× σ = 2 TeV (Z’m 50)× σ = 3 TeV (Z’m < 1.1 TeV)tt (truth mtt > 1.1 TeV)tt (truth mttW+jetssingle topZ+jets = 14 TeVs-1 L = 3000 fb∫ATLAS Simulation Preliminary Figure 3: mtt¯ spectrum in the boosted electronchannel [3].Figure 4: Expected upper limits set on the crosssection times branching ratio of a Z’ boson atdifferent mass points [3].2PoS(FPCP2017)044HL-LHC Physics Prospects Anna Duncan4. Higgs Boson Production via Vector Boson FusionVector boson fusion (VBF) is the Higgs Boson production mechanism with the second highestcross-section after gluon-gluon fusion (ggF) (Figure 5). In VBF production, the Higgs boson isproduced along with two jets in the forward region. The unique signature of two forward jets withlarge dijet invariant mass means that a good signal to background ratio can be achieved in the eventselection. Additionally, the theoretical uncertainty on the VBF Higgs production cross-section islow relative to ggF. Therefore a precision measurement on this process is possible with sufficientstatistics. This study [5] estimates the precision with which ATLAS will be able to measure VBFHiggs production at the HL-LHC. The expected precision depends on the inner tracker coverageas this dictates the degree to which the forward jet signature can be exploited. The analysis uses acentral jet veto (CJV), where events are rejected if there is a high pT jet inside the rapidity rangeof the two leading pT jets. Increased tracker acceptance in |η | allows for better pileup rejection,leading to fewer signal events being rejected by the CJV. In this measurement, events are selectedwhere the Higgs decays to two W bosons which in turn each decay to a lepton and a neutrino, soit relies on the performance of lepton and missing transverse energy reconstruction. The analysisrequires a b-jet veto to mitigate the tt¯ background, and so is also sensitive to b-tagging performance.The quoted precision of the measurement is the uncertainty on the signal strength. Figure 7 showsthe expected precision (neglecting theoretical uncertainties on ggF and VBF production) for threedifferent scenarios of inner tracker |η | coverage. The increased tracker coverage allows a precisionmeasurement of VBF Higgs production even in a high pileup environment.Figure 5: Higgs production mechanisms [5].5. Flavour Changing Neutral Current Top Quark DecayFlavour changing neutral currents (FCNC) in top quark decays are highly suppressed in the Stan-dard Model with rates < 1010 and are sensitive to some hypothesised new physical processes; anydetection of this decay would be a sign of new physics. This study [6] estimates the expectedsensitivity to the FCNC top quark decays t → Zq and t → Hq at HL-LHC. The decay channelsconsidered are tt¯ →W (→ `ν)bZ(→ ``)u/c and tt¯ →W (→ `ν)H(→ bb)u/c. Events are selectedfrom Monte Carlo simulations of signal and background, and expected upper 95% CL limits are seton FCNC top quark branching ratios. This is done for no systematic uncertainties, and for two sets3PoS(FPCP2017)044HL-LHC Physics Prospects Anna DuncanFigure 6: Forward dijet invariant mass spec-trum for signal and background events [5].Tracker Coverage Expected Precision|η |< 4.0 14%|η |< 3.2 20%|η |< 2.7 30%Figure 7: Expected precision of VBF HH signalmeasurement [5].Uncertainties t→ Zq channels t→ Hqstatistics only (2.4 - 5.8) ×10−5 (0.6 - 1.2) ×10−4statistics + systematics (A) (12 - 41) ×10−5 (1.1 - 2.4) ×10−4statistics + systematics (B) (8.3 - 24) ×10−5 (1.1 - 2.4) ×10−4Table 1: Expected upper 95% CL limits on FCNC top quark branching ratios [6].of systematic uncertainties: A and B. Systematics A are based on Run 1 estimates where MonteCarlo predictions are compared with data in control regions. Systematics B conservatively accountfor improvements to dominant theoretical and background normalisation uncertainties which canbe achieved with HL-LHC statistics. Detector related systematic uncertainties are considered tobe negligible. Table 1 shows the expected upper limits on FCNC top quark decay for each decaychannel and systematics scenario. The systematics have a lower impact on t→ Hq sensitivity dueto larger expected statistics and multiple fit regions. The strongest observed limits from Run 1 areB(t→ Zq)< 50×10−5,B(t→ Hu)< 45×10−4 andB(t→ Hc)< 46×10−4 [7], [8].6. Measuring the Higgs Trilinear Self-Coupling through Higgs Pair ProductionOne of the primary goals of LHC physics is to establish whether the Higgs mechanism is respon-sible for electroweak symmetry breaking (EWSB). This requires extensive investigation of Higgsboson properties, including the Higgs trilinear self-coupling, λHHH . The processes representedby the diagrams in Figures 8 and 9 interfere destructively, so any deviation in λHHH will modifynon-resonant Higgs pair production. The low Standard Model HH production cross section meansthat it has not yet been observed; the best 95% CL limit on HH production so far is σ/σSM ≤ 29,obtained from an analysis performed on ATLAS Run 2 data searching in the dominant HH→ bbbbdecay channel. This study [9] extrapolates the ATLAS Run 2 results to estimate the prospects HHproduction measurement at HL-LHC, which in turn can be used to place limits on λHHH . The cho-sen HH→ bbbb decay channel means that b-tagging performance of the upgrade is critical to thisanalysis. It is assumed that the upgrade jet reconstruction and b-tagging performance is the same4PoS(FPCP2017)044HL-LHC Physics Prospects Anna Duncanas for the current detector, and that the analysis method remains the same, which a conservativeassumption. The HH → bbbb search translated to constraints on λHHH by exploiting the fact thatλHHH variations change the HH cross section ad the shape of the mHH distribution. The 95% CLconstraints on λHHH are −3.5 < λHHH/λ SMHHH < 11 in the case where systematic uncertainties re-main as they were in Run 2 (Figure 10) and 0.2< λHHH/λ SMHHH < 7.0 in the case when systematicuncertainties are negligible (Figure 11).Figure 8: HH production with Higgs self-coupling vertex [9]. Figure 9: Non-resonant HH production [9].Figure 10: Expected constraints on λHHH withcurrent systematic uncertainties included [9].Figure 11: Expected constraints on λHHH withno systematic uncertainties [9].References[1] ATLAS Collaboration. Expected performance for an upgraded ATLAS detector at High-LuminosityLHC. ATLAS-PUB-2015-026, url: https://cds.cern.ch/record/2055248[2] W.J. Stirling, private communication[3] ATLAS Collaboration. Study on the prospects of a tt¯ resonance search in events with one lepton at aHigh Luminosity LHC. ATL-PHYS-PUB-2017-002, url: https://cds.cern.ch/record/2243753[4] ATLAS Collaboration. A search for tt¯ resonances using lepton-plus-jets events in proton-protoncollisions at√s = 8 TeV with the ATLAS detector. JHEP 08 (2015) 053, arXiv:1505.07018 [hep-ex][5] ATLAS Collaboration. Measurement prospects for VBF H→WW∗→ eνµν production with 3 ab−1of HL-LHC pp-collisions. ATL-PHYS-PUB-2016-018, url: http://cds.cern.ch/record/2209092[6] ATLAS Collaboration Expected sensitivity of ATLAS to FCNC top quark decays t→ Zq and t→ Hq atthe High Luminosity LHC. ATL-PHYS-PUB-2016-019, url: https://cds.cern.ch/record/2209126[7] ATLAS Collaboration. Search for flavour-changing neutral current top quark decays t→ Hq in ppcollisions at√s = 8 TeV with the ATLAS detector. JHEP 12 (2015) 061, arXiv:1509.06047 [hep-ex],5PoS(FPCP2017)044HL-LHC Physics Prospects Anna Duncan[8] CMS Collaboration. Search for flavor-changing neutral currents in top-quark decays t to Zq in ppcollisions at√s = 8 TeV Phys. Rev. Lett. 112 (2014) 171802, arXiv:1312.4194[9] ATLAS Collaboration Projected sensitivity to non-resonant Higgs boson pair production in the bb¯bb¯final state using proton-proton collisions at HL-LHC with the ATLAS detectorATL-PHYS-PUB-2016-024, url: http://cds.cern.ch/record/2221658/6

Physics Prospects at the HL-LHC with ATLAS

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Physics Prospects at the HL-LHC with ATLAS

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