paper_BLstop2016.bib \addbibresourceATLAS.bib \addbibresourceCMS.bib \addbibresourceConfNotes.bib \addbibresourcePubNotes.bib \addbibresourcesusy_v3.bib \AtlasTitleA search for -parity-violating top squarks in  TeV collisions with the ATLAS experiment \AtlasRefCodeSUSY-2016-29 \PreprintIdNumberCERN-EP-2017-171 \AtlasJournalRefPhys. Rev. D 97 (2018) 032003 \AtlasDOI10.1103/PhysRevD.97.032003 \AtlasAbstractA search is presented for the direct pair production of the stop, the supersymmetric partner of the top quark, that decays through an -parity-violating coupling to a final state with two leptons and two jets, at least one of which is identified as a -jet. The dataset corresponds to an integrated luminosity of 36.1 fb of proton–proton collisions at a center-of-mass energy of  TeV, collected in 2015 and 2016 by the ATLAS detector at the LHC. No significant excess is observed over the Standard Model background, and exclusion limits are set on stop pair production at a 95% confidence level. Lower limits on the stop mass are set between 600 GeV and 1.5 TeV for branching ratios above 10% for decays to an electron or muon and a -quark.

1 Introduction

The extension of the Standard Model (SM) of particle physics with supersymmetry (SUSY) [Golfand:1971iw, Volkov:1973ix, Wess:1974tw, Wess:1974jb, Ferrara:1974pu, Salam:1974ig] leads to processes that violate both baryon number () and lepton number (), such as rapid proton decay. A common theoretical approach to reconcile the strong constraints from the non-observation of these processes is to introduce a multiplicative quantum number called -parity [Farrar:1978xj], defined as where is the spin of the particle. If -parity is conserved, then SUSY particles are produced in pairs, and the lightest supersymmetric particle (LSP) is stable. The LSP cannot carry electric charge or color charge without coming into conflict with astrophysical data [Ellis:1983ew, Pospelov:2006sc].

A number of theoretical models beyond the Standard Model (BSM) predict -parity violation (RPV) [Dreiner:1997uz, Barbier:2004ez, Perez:2013kla, Restrepo:2001me]. The benchmark model for this search considers an additional local symmetry to the Standard Model with right-handed neutrino supermultiplets. The minimal supersymmetric extension then only needs a vacuum expectation value for a right-handed scalar neutrino in order to spontaneously break the \BLsymmetry [FileviezPerez:2008sx, Barger:2008wn, Everett:2009vy, Braun:2005ux, Deen:2016vyh]. This minimal \BLmodel violates lepton number but not baryon number. The couplings for RPV are highly suppressed as they are related to the neutrino masses, and the model is consistent with the experimental bounds on proton decay and lepton number violation. At the LHC, the most noticeable effect is that the LSP is no longer stable and can now decay via RPV processes, and it also may now carry color and electric charge. This leads to unique signatures that are forbidden in conventional models with -parity conservation. A novel possibility is a top squark or \Stop(\StopSym) as the LSP with a rapid RPV decay. The supersymmetric partners of the left- and right-handed top quarks, and , mix to form two mass eigenstates consisting of the lighter and heavier . Given the large top quark mass, the lighter is expected to be significantly lighter than the other squarks due to renormalization group effects [Barbieri:1987fn, deCarlos:1993rbr]. The lighter , denoted \StopSymfor simplicity, is the target of this analysis.

This paper presents a search performed by ATLAS for direct \Stoppair production, with the RPV decay of each \StopSymto a -quark and a charged lepton (), as shown in \figreffeynman_diagram. In contrast to -parity-conserving searches for \StopSym, there is no significant missing transverse momentum in the decay. The \StopSymdecay branching ratios to each lepton flavor are related to the neutrino mass hierarchy [Marshall:2014cwa, Marshall:2014kea], and a large phase space in the branching ratio plane is currently available. With an inverted mass hierarchy the branching ratio to the final state may be as large as 100%, and with a normal mass hierarchy the branching ratio to the final state may be as high as 90%. The experimental signature is therefore two oppositely charged leptons of any flavor and two -jets. In this analysis, only events with electron or muon signatures are selected, and final states are split by flavor into \EE, \EM, and \MMselections. At least one of the two jets is required to be identified as initiated by a -quark, improving the selection efficiency of signal events over a requirement of two -jets. Events are chosen in which the two reconstructed \blpairs have roughly equal mass.

Previous searches with similar final states have targeted the pair production of first-, second-, and third-generation leptoquarks at ATLAS [EXOT-2015-19, EXOT-2014-03] and at CMS [CMS-EXO-16-023, CMS-EXO-12-041]. However, they consider final states within the same generation (, , , where indicates a light-flavor jet) and do not focus on final states with both -jets and electrons or muons (, ), nor consider final states with both electrons and muons (). The results of the Run 1 leptoquark searches were reinterpreted for the \StopSymmass and its decay branching ratios in the \BLmodel [Marshall:2014cwa, Marshall:2014kea], setting lower mass limits between 424 and 900 \GeVat a 95% confidence level.

Figure 1: Feynman diagram for \Stoppair production, with \StopSymand anti-\StopSym() decay to a charged lepton of any flavor and a -quark through an -parity-violating coupling .

The ATLAS detector and the dataset collected during Run 2 of the LHC are described in \secrefdetector, with the corresponding Monte Carlo simulation samples presented in \secrefmc. The identification and reconstruction of jets and leptons is presented in \secrefreco, and the discriminating variables used to construct the signal regions are described in \secrefsel. The method of background estimation is described in \secrefbkg, and the systematic uncertainties are detailed in \secrefsystematics. The results are presented in \secrefresult, and the conclusion given in \secrefconclusion.

2 ATLAS detector and data set

The ATLAS detector [PERF-2007-01] consists of an inner detector tracking system, electromagnetic and hadronic sampling calorimeters, and a muon spectrometer. Charged-particle tracks are reconstructed in the inner detector (ID), which spans the pseudorapidity1 range , and consists of three subdetectors: a silicon pixel tracker, a silicon microstrip tracker, and a straw-tube transition radiation tracker. The ID is surrounded by a thin superconducting solenoid providing an axial magnetic field of 2 T, allowing the measurement of charged-particle momenta. In preparation for Run 2, a new innermost layer of the silicon pixel tracker, the insertable B-layer (IBL) [ATLAS-TDR-19], was introduced at a radial distance of 3.3 cm from the beamline to improve track reconstruction and the identification of jets initiated by -quarks.

The ATLAS calorimeter system consists of high-granularity electromagnetic and hadronic sampling calorimeters covering the region < 4.9. The electromagnetic calorimeter uses liquid argon (LAr) as the active material with lead absorbers in the region < 3.2. The central hadronic calorimeter incorporates plastic scintillator tiles and steel absorbers in the region < 1.7. The hadronic endcap calorimeter (1.5 < < 3.2) and the forward calorimeters (3.1 < < 4.9) use LAr with copper or tungsten absorbers.

The muon spectrometer (MS) surrounds the calorimeters and measures muon tracks within < 2.7 using three layers of precision tracking chambers and dedicated trigger chambers. A system of three superconducting air-core toroidal magnets provides a magnetic field for measuring muon momenta.

The ATLAS trigger system begins with a hardware-based level-1 (L1) trigger followed by a software-based high-level trigger (HLT) [TRIG-2016-01]. The L1 trigger is designed to accept events at an average rate of 100 kHz, and the HLT is designed to accept events to write out to disk at an average rate of 1 kHz. Electrons are triggered in the pseudorapidity range < 2.5, where the electromagnetic calorimeter is finely segmented and track reconstruction is available. Compact electromagnetic energy deposits triggered at L1 are used as the seeds for HLT algorithms that are designed to identify electrons based on calorimeter and fast track reconstruction. The muon trigger at L1 is based on a coincidence of trigger chamber layers. The parameters of muon candidate tracks are then derived in the HLT by fast reconstruction algorithms in both the ID and MS.

The data sample used for this search was collected from proton–proton collisions at a center-of-mass energy of in 2015 and 2016. An integrated luminosity of 36.1 


  1. ATLAS uses a right-handed coordinate system with its origin at the nominal interaction point (IP) in the center of the detector and the -axis along the beam pipe. The -axis points from the IP to the center of the LHC ring, and the -axis points upward. Cylindrical coordinates are used in the transverse plane, being the azimuthal angle around the -axis. The pseudorapidity is defined in terms of the polar angle as . Rapidity is defined as where denotes the energy and is the component of the momentum along the beam direction.
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