Techniques and results in Charged Long-Lived particle searches in ATLAS and CMS in Run 2 - NORA PETTERSSON (UNIVERSITY OF MASSACHUSETTS, AMHERST) ...
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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 1. Charged particles that only traverse a certain extent of the 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 2. Highly ionizing particles leaving abnormal energy losses in the detector – / measurements Utilise the measuring capabilities of the tracking detector 3. Time of flight measurements using timing information available from the calorimeters and muon spectrometer 4. Displaced Vertices inside the tracking volume
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 Assume a SUSY model where 1± (NLSP) is nearly mass-degenerate with 10 (LSP) – Long-lived 1± decays: 1+ → 10 + (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 pT distribution varying on the type of expected background Likelihood fit performed on the signal and background templates Limits are set on the 1± mass as a function lifetime 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 pT 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 Limits are set on the cross section of the 1± as well as a function of the lifetime The limits are set on the cross section for lifetimes between 0.1 and 100 ns 1± masses up to 715 (695) GeV are excluded for lifetimes of 3 (7) ns, 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 / 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 / measurements Adjacent fired pixels are combined into clusters Cluster size depends on incident angle To reduce the tail fractions, a particle’s / 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 < < 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) with large ionization energies 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 for higher charge broaden the distribution Miss modelling in simulation due to gas-change in the TRT not being propagate to MC arXiv:1812.03673
Multi-charged LLP (ATLAS) 19 Two signal regions are defined depending on the expected charge Needed by the different detector responses for z= 2 and z> 2 Expected backgrounds are due to possible high occupancy in the detector and the presence of large amount of -rays Background estimated by ABCD method for z=2 and using the side bands of MDT / TRT dE/dx distributions for z > 2 No significant excess observed and limits are set on DY model and multi-charged lepton-like particles From 50 GeV up to 980-1220 GeV are excluded arXiv:1812.03673
Multi-charged LLP (ATLAS) 20 Two signal regions are defined depending on the expected charge Needed by the different detector responses for z= 2 and z> 2 Expected backgrounds are due to possible high occupancy in the detector and the presence of large amount of -rays Background estimated by ABCD method for z=2 and using the side bands of MDT / TRT dE/dx distributions for z > 2 No significant excess observed and limits are set on DY model and multi-charged lepton-like particles From 50 GeV up to 980-1220 GeV are excluded CMS targeted a CMS z=2 and 12.9 fb-1 wider mass range arXiv:1812.03673
ATLAS-CONF-2019-006 Displaced Vertex (ATLAS) 21 Search for LLP decaying inside the inner detector to several charge particles Results interpreted for R-parity violating SUSY where a stop decays to a quark and a muon Standard track reconstruction limits the efficiency Impose strict cuts on transverse (d0) and longitudinal (z0) impact parameters with respect to the IP Use special track reconstruction A dedicated second pass of the tracking is ran on leftover hits from the standard tracking with looser cut on d0 and z0 The search looks for a displaced vertex with a mass larger than 20 GeV and have a track multiplicity larger than three and a displaced muon is required to be present with large impact parameters of|d0| > 2 Additional quality criteria are imposed to reduce backgrounds from detector effects such as fake muons A cosmic veto is applied to reduce the largest background of displaced muons
ATLAS-CONF-2019-006 Displaced Vertex (ATLAS) 22 Backgrounds are estimated with a fully data driven method Relies on the fact that variables used to reduce SM background for the displaced vertices and the displaced muons are not correlated Control regions are defined by inverting parts of the selection No significant excess observed and limits are set Stop masses up to 1.7 TeV are excluded for lifetimes of 0.1 ns For the range of 10-17 ns the range up to 1.4 TeV is excluded More details on DV+μ in a dedicated talk by Karri Di Petrillo
LLP Searches for ATLAS and CMS 23 Many interesting results from “non” standard search techniques Utilise the ATLAS and CMS detectors’ full potential! Stay tuned for interesting future developments!!! CMS Combined ATLAS Combined
24 BACK-UP SLIDES MORE INFORMATION HERE
ATLAS LLP Summary of Results 25
CMS LLP Summary of Results 26
Large / and time of flight (TOF) (ATLAS) 27 An other search from ATLAS includes in addition to the dE/dx measurement als the time of flight Search for R-hadrons (split-SUSY), directly produced staus (GMSB), and charginos (mAMSB) utilising pixel / measurements and TOF The estimated from the particles energy loss uses the same method as in the previous analysis The velocity β of the particles are determined by time of flight measurements Measurements from the tile calorimeter and from the monitored drift tubes (MDTs) and resistive-plate chambers (RPCs) in the muon spectrometer Both TOF measurements are combined into an average factoring in the resolution of the two systems → events are used to derive the resolution on the β-distribution for the two detectors arXiv:1902.01636
Large / and TOF (ATLAS) 28 Few signal regions are defined and optimised for the different expected signatures of the three models under consideration All five regions are mutually exclusive No excess observed and limits are set on R-hadrons and direct pair-production of staus/charginos Lower limits on the mass of long lived gluinos, sbottom and stop R-hadrons are set at 2000 GeV, 1250 Gev and 1340 GeV Lower limits on the mass of long lived staus and charginos are set at 430 GeV and 1090 GeV arXiv:1902.01636
Disappearing Track (ATLAS) 29 Backgrounds arise from hadrons or leptons that may interact with the detector material as well as combinatoric backgrounds of tracklets made out of random hits Hadronic interactions, multiple scattering, bremsstrahlung, and fakes Minor inefficiency and low purity start to play a role for these types of analysis Red solid (dotted) shows charged (neutral) particles; thick blue represents the reconstructed tracklet Similarly for the emission of A tracklet made up of hits Scattering where the a hard photon, the produced by several extended track is missed extension is judged not to particles or noise hits due to the large kink be part of the same track JHEP 06 (2018) 022
Disappearing Track (CMS) 30 Remaining background consists of charged leptons that fail lepton identifications while the missing pT requirements are still met Contribution in the signal region is estimated by creating the probability of this type of leptons Add extra requirement that to the Poffline that the event also passes the Number of events Estimated via triggers seen in the tag-and-prob for Probability that a single control region → and → lepton event passes the missing pT selection given that the lepton isn’t identified Spurious track background is estimated from a control region enhanced with lower quality tracks by requiring larger impact parameters Number of events is scaled by a transfer factor derived from tracks with three pixel hits but no hits in the middle detector and requiring the impact parameter selections of the signal region JHEP 08 (2018) 016
Disappearing Track (ATLAS) 31 Event selection of disappearing tracks Expecting missing ET due to the neautralino → Rely on missing ET trigger Require a pixel tracklet with pT > 5 GeV and no associated SCT hits Lepton veto is applied to reduce background of V+jet and ҧ events Various quality requirements on the pixel tracklet to ensure good quality Further selection on kinematics optimised for the two channels, e.g. more jets for the strong channel JHEP 06 (2018) 022
Large ionization energy loss (ATLAS) 32 Corrections for / applied to simulation Changes in the measured changes during runs depending on run conditions and accumulated luminosity Effect such as radiation damange on the pixel sensors are not taking into account in simulation and need to be corrected for Calibrations for low momentum particles under 500 MeV corrections are applied for kaons and protons The pion mass is the default hypothesis in track reconstruction and the calibrations are produced from a fit between the simulated mass and the reconstructed mass to account for this effect Variation in the energy losses depends on the particles indicent angle on the sensors After applied corrections / only depend on momentum and mass of the particles Phys. Lett. B 788 (2019) 96
Large / and TOF (ATLAS) 33 Fully data-driven background estimate by producing probability density functions (pdfs) of the key distributions , ( ) / for R-hadrons and momentum and The expected number of events in the signal regions are estimated by randomly sampling from the pdfs using = / Possible correlations between , ( ) / and the momentum are taken into account by binning the pdfs in pseudorapidity η , ( ) / are η-dependent due to that the resolutions varies dependen on the detector region arXiv:1902.01636
Large / and TOF (ATLAS) 34 Few signal regions are defined and optimised for the different expected signatures of the three models under consideration All five regions are mutually exclusive Do not require any MS activity → region less dependent on the hadronization model Take advantage of MS information → better TOF measurements No dE/dx requirement imposed for the pair- produced stau/chargino to be effective for low- masses as well The lower limits on the mass are derived from mass-planes of and / for the R-hadrons while only from for the stau/chargino regions arXiv:1902.01636
Heavy Stable Charge Particles (CMS) 35 Event selections of HSCP High transverse momentum single muon trigger or missing ET trigger used to select events Track candidate with pT > 55 GeV and various quality requirements to ensure good tracks The tracker + TOF analysis also require that the track should be matched to a reconstructed muon and at least eight time measurements Data driven background estimate ABCD method using two non-correlated variables The regions are divided up using pT > 65 GeV and > 0.3 Candidates found in the control regions are used to form binned probability density functions of ℎ and momentum using the mass estimated for SM extrapolated to the signal region 1 For the tracker+TOF analys an additional dimension is added with > 1.25 to form ABCDEFGH method Phys. Rev. D 94 (2016) 112004
Heavy Stable Charge Particles (CMS) 36 Time of flight is measured in the muon system from the Drift Tubes (DT) and Cathode Strip Chambers (CSC) Slow particles can be distinguished from those traveling near speed of light A relativistic particle will produce an aligned pattern of hits in the DT while a slower particle will have a reconstructed position shifted relative to the path The offset of position is proportional to the delay of the particle The CSC measure the delay for each hit separately Phys. Rev. D 94 (2016) 112004
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