Techniques and results in Charged Long-Lived particle searches in ATLAS and CMS in Run 2 NORA PETTERSSON (UNIVERSITY OF MASSACHUSETTS, AMHERST) ON BE HALF OF THE ATLAS AND CMS COLLABORATIONS
Techniques to Search for 2 Charged Long Lived Particles Charged particles that only traverse a certain extent of the 1. e tracking detector and subsequentially disappear e e e e e e e e e Employ non-standard track reconstruction to find short tracks Veto hits in “outer” tracker volume to ensure short tracks Highly ionizing particles leaving abnormal energy losses in the 2. detector – 𝑒𝐹/𝑒𝑦 measurements Utilise the measuring capabilities of the tracking detector Time of flight measurements using timing information available 3. from the calorimeters and muon spectrometer Displaced Vertices inside the tracking volume 4.
ATLAS and CMS Experiments 3 Two Large experiments at CERN! Probably heard all about them in previous talks Long-lived particles yield non standard signals It is vital to understand the performance of the detector!
Disappearing Track (ATLAS) 4 ± (NLSP) Assume a SUSY model where 𝜓 1 0 (LSP) is nearly mass-degenerate with 𝜓 1 ± decays: 𝜓 1 + → 𝜓 1 – Long-lived 𝜓 1 0 𝜌 + (soft) Common to Wino and Higgsino LSP scenarios – vital to a large portion of SUSY dark matter searches Gives a signature of a charged track that seemingly disappears after crossing only few layers of the inner detector Need to reconstruct the short tracks (tracklets) using only measurements (hits) expected for the given lifetime spectrum In this case, restrict to the pixel detector and measurements up to ~120 mm JHEP 06 (2018) 022
Disappearing Track (ATLAS) 5 Track reconstruction is done in two steps for this analysis Standard algorithms – e.g. to find mainly the primary tracks Requiring at least seven measurements in the silicon detector layers A second pass of the tracking Using only leftover measurements from the first pass The hit requirement is significantly looser and aimed at short tracks: at least four hits in the pixel layers Addition of the insertable B-Layer (IBL) improved the efficiency pixel tracklet reconstruction efficiency Up to 60% efficiency to reconstruct tracklets in the pixel detector volume, up to 300 mm Veto is applied to make sure that the tracklets do not SCT have any hits in the silicon tracker (SCT) Pixel Effective background and fake removal JHEP 06 (2018) 022
Disappearing Track (ATLAS) 6 Backgrounds arise from hadrons or leptons that may Run-2 improvement interact with the detector material as well as combinatoric backgrounds of tracklets made out of random hits Producing templates of the tracklet p T distribution varying on the type of expected background Likelihood fit performed on the signal and background templates ± mass as a function lifetime Limits are set on the 𝜓 1 IBL help improving the limits for run-2 due to the increased reconstruction efficiency for pixel tracklets Reinterpretation of this analysis on the Higgsino scenario is covered in ATL-PHYS-PUB-2017-019 JHEP 06 (2018) 022
Disappearing Track (CMS) 7 CMS have a smiliar search for the same model and topology Slightly different analysis strategies The disappearing track candidates are required to be short and to have no hits in the outer layers of the tracking volume This suppresses background from random combinations and from tracking inefficiencies that can create spurious short tracks Require strict quality cuts on the short tracks Restriction on the impact parameters Require no missing hits in the inner layers JHEP 08 (2018) 016
Disappearing Track (CMS) 8 Missing number of outer hits is used to select short track candidates for the analysis Powerful discriminator of signal versus background Reduce QCD background by angle cuts between the jets and the missing p T Remaining backgrounds are: Charged leptons that fail lepton identifications Spurious tracks from random hits Both are estimated in dedicated regions enhancing the contributions JHEP 08 (2018) 016
JHEP 08 (2018) 016 Disappearing Track (CMS) 9 ± as well as a function of the lifetime Limits are set on the cross section of the 𝜓 1 The limits are set on the cross section for lifetimes between 0.1 and 100 ns ± masses up to 715 (695) GeV are excluded for lifetimes of 3 (7) ns, 𝜓 1 This is the range of lifetimes the analysis is most powerful Masses of up to 505 GeV are excluded for the broader range of 0.5 ns to 60 ns
JHEP 08 (2018) 016 Disappearing Track (CMS) 10 NB: CMS results are pre- Different strategies: update and are still using a three layer pixel detector CMS optimised for while ATLAS results are with longer lifetimes a four layer pixel detector while ATLAS for shorter lifetimes ATLAS
Large ionization energy loss (ATLAS) 11 Search for long-lived charged particles traversing the e inner detector (ID) and leaving large 𝑒𝐹/𝑒𝑦 deposits e e e e e e e Interpreted on long lived R-hadrons hypothesised by e e e e split-susy model Charge deposits per track length in the pixel layers provides 𝑒𝐹/𝑒𝑦 measurements Adjacent fired pixels are combined into clusters Cluster size depends on incident angle To reduce the tail fractions, a particle’s 𝑒𝐹/𝑒𝑦 is taken as the average over all the pixel hits, removing one or two measurements with the largest deposits of energy IBL helps improving the capability of measuring the energy loss more precisely Phys. Lett. B 788 (2019) 96
Large ionization energy loss (ATLAS) 12 Energy losses are dependent on the mass and the mass can be calculated for the LLP using the Bethe-Bloch formula Use fit range of 0.3 < 𝛾𝛿 < 0.9 Corresponds well to the LLPs which are expected to be produced at the LHC Fit shown for pions, kaons and protons Estimated masses from applying this method on signal samples of R-hadrons, reproduced the generated mass well up to masses of 1.5 TeV Calibrations on protons in data shows consistent results within 1% of the expectations Phys. Lett. B 788 (2019) 96
Phys. Lett. B 788 (2019) 96 Large ionization energy loss (ATLAS) 13 Fully data-driven background estimation Derive shape and normalisations in control regions defined by inverting selections Limits set on the production cross section and lifetime of the gluino For lifetimes of and above 1 ns: 1290 to 2060 GeV excluded
Heavy Stable Charge Particles (CMS) 14 Search for heavy stable charge particles (HSCP) 𝑒𝐹 with large ionization energies 𝑒𝑦 and non-unit charges Phys. Rev. D 94 (2016) 112004 The search considers two techniques A tracker-only approach and one where the tracker information is combined with the muon system (tracker and time of flight (TOF)) Considering three models that exploits the two different techniques For example, split SUSY with R-hadrons that are either stables or are expected to lose their charge before the muon system Staus postulated in mGMSB Lepton like fermions in a Drell-Yan model EXO-16-036
Heavy Stable Charge 15 Particles (CMS) A particle’s energy loss is measured from ionization deposited in the pixel and silicon tracker layers Exclude the measurement with the smallers charge deposit Increase the quality and reduce instrumental biases Powerful discriminating variable is defined by comparing the measured values with what is expected of a minimum-ionizing particle Provide good separation of SM backgrounds J. High Energy Phys. 03 (2011) 024 EXO-16-036
EXO-16-036 Heavy Stable Charge Particles (CMS) 16 No excess observed in either analysis and limits are set on the three models For split-susy gluino masses below 1850 GeV are excluded Stop masses below 1250 GeV are excluded Stau masses below 660 GeV are excluded for the GMSB and below 360 GeV for direct pair production model Drell-Yan signals with |Q| = 1e (2e) are excluded below 730 (890) GeV
EXO-16-036 Heavy Stable Charge Particles (CMS) 17 No excess observed in either analysis and limits are set on the three models For split-susy gluino masses below 1850 GeV are excluded Stop masses below 1250 GeV are excluded Stau masses below 660 GeV are excluded for the GMSB and below 360 GeV for direct pair production model Drell-Yan signals with |Q| = 1e (2e) are excluded below 730 (890) GeV ATLAS - Gluino at 36.1 fb -1
Multi-charged LLP (ATLAS) 18 ATLAS have a search dedicated to only multi-charge particles (MCP) Results also interpreted on the Drell-Yan production model like the previous CMS analysis Assume the particles decay outside the detector so they appear stable and leave muon-like signatures with large energy loss Measure dE/dx in the pixel, transition radiation tracker (TRT), and in MDT subsystem in the muon spectrometer dE/dx from the pixels is estimated as discussed for previous analyses, in the TRT the dE/dx is a mean of the hit-level energy losses calculate for the each tracks time above threshold, and similar for the MDT an average is taken from all drift tubes crossed Emission of many 𝜀 for higher charge broaden the distribution Miss modelling in simulation due to gas-change in arXiv:1812.03673 the TRT not being propagate to MC
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