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Pos(PP@LHC2016)006 Search for signatures with top, bottom, tau and exotics PoS(PP@LHC2016)006 Andrea Favareto,∗ Camilla Galloni†and Luigi Longo‡ On behalf of the ATLAS and CMS Collaborations E-mail: [email protected], [email protected], [email protected] The Standard Model of particle physics is a sensational success, especially since the discovery of the 125 GeV Higgs boson. However, there are still several open questions that the Stan- dard Model doesn’t address, like the nature of dark matter and dark energy, the matter-antimatter asymmetry, the neutrino oscillations, the inclusion of general relativity and the hierarchy pro- blem. Theories Beyond the Standard Model (BSM), such as Supersymmetry, Little and Compos- ite Higgs, Extra-Dimensions, Extended Gauge models, Technicolor, Left-Right symmetric mod- els, and many other BSM scenarios are trying to answer these questions. In these proceedings we present the most recent results for searches Beyond the Standard Model at the Large Hadron Col- lider (LHC) by the ATLAS and CMS experiments, focusing on signatures with top, bottom, tau and exotics. The data are found to be consistent with the Standard Model. The non-observation of a signal allows to set limits at the 95% confidence level on the production cross section times branching ratio and on the masses of the hypothesized new particles for appropriate benchmark models. PP @ LHC 2016 - VII Workshop italiano sulla fisica pp a LHC 16-18 May 2016 Pisa, Italy ∗Università degli Studi di Genova and INFN, Genova, Italy †Universitaet Zuerich, Zuerich, Switzerland ‡Università del Salento and INFN, Lecce, Italy c Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND 4.0). http://pos.sissa.it/ Search for signatures with top, bottom, tau and exotics Introduction The Standard Model of particle physics has been very successful at describing particles and their interactions. There are however several open questions that the Standard Model doesn’t ad- dress. The unexplained phenomena and problems like the nature of dark matter and dark energy, the matter-antimatter asymmetry, the neutrino oscillations, the inclusion of general relativity and the hierarchy problem, suggest that the universe we observe requires theoretical developments Be- yond the Standard Model (BSM). The Large Hadron Collider (LHC) at CERN has provided the ATLAS [1] and CMS [2] experiments with high energy proton-proton collisions to test the Stan- PoS(PP@LHC2016)006 dard Model and perform searches for physics Beyond the Standard Model. Among the possible extensions of the Standard Model, Supersymmetry (SUSY) [3] provides solutions to some of the open questions. In addition to SUSY, there are many other theoretical approaches grouped under the term of exotic physics (e.g. Little and Composite Higgs, Extra-Dimensions, Extended Gauge models, Technicolor, Left-Right symmetric models, etc.) that address as well some of the open issues mentioned above, and predict a rich phenomenology accessible at the LHC. The ATLAS and CMS experiments has run extensive physics searches to cover as many sce- narios as possible. These proceedings review a selection of results among the many searches for supersymmetry and exotic signatures based on the LHC proton-proton collision data collected in Run 11 at centre of mass energy of 7 and 8 TeV, and Run 22 at centre of mass energy of 13 TeV. 1. Supersymmetry Supersymmetry is one of the most studied framework to extend the Standard Model since it provides a natural solution to the hierarchy problem. In its minimal formulation, Minimal Su- persymmetric Standard Model (MSSM), it predicts a new bosonic (fermionic) partner for each fermionic (bosonic) Standard Model particle with identical quantum number except for a differ- ence of half unit of spin (S). An additional Higgs doublet is also introduced together with its supersymmetric partner. In order to preserve the baryon number (B) and lepton number(L), it is assumed that the multiplicative quantum number R−parity, defined as R = (−1)3(B−L)+2S, is con- served. In this framework SUSY particles are produced in pairs and the lightest supersymmetric particle (LSP) is stable and a candidate for dark matter. The SUSY partners of the Higgs and electroweak gauge bosons mix to form the mass eigen- 3 ± 0 states known as charginos (c˜i=1;2) and neutralinos (c˜i=1;2;3;4); in a large variety of models, the 0 miss LSP is the lightest neutralino c˜1 and events with large missing momentum, ET , are searched as proof of the supersymmetry. Even the scalar partners of the right-handed and left-handed top (bottom), t(b) quark, t˜R and t˜L (b˜R and b˜L ), can mix to form two mass eigenstates, t˜1 and t˜2 (b˜1 and b˜2). In many scenarios t˜1 is assumed to be the lightest supersymmetric quark (squark) and as 1Run 1 corresponds to LHC proton-proton data from years 2011 and 2012 at the centre of mass energies of 7 TeV and 8 TeV. The collision datasets correspond to approximately 5 fb−1 at centre of mass energy of 7 TeV and 20 fb−1 at 8 TeV. 2Run 2 corresponds to LHC proton-proton data from 2015, hence after the machine technical stop of 2013-2014, at the centre of mass energy of 13 TeV. The collision datasets correspond to approximately 3 fb−1 so far. 3All the mass eigenstates are ordered by increasing mass. 1 Search for signatures with top, bottom, tau and exotics Signal Model SUSY masses spectrum Experiment 0 g˜ ! tt¯c˜1 m(g˜) << m(q˜) ATLAS [4, 5] & CMS [6] ¯ 0 g˜ ! bbc˜1 m(g˜) << m(q˜) ATLAS [5] & CMS [6] 0 0 g˜ ! t˜1t¯ ! t¯c˜1 soft term Dm(t˜1; c˜1 ) =5 GeV ATLAS [7] 0 ± t˜1 ! tc˜1 m(t˜1) > m(c˜0 ) ATLAS [7] & CMS [8, 9] ± 0 ± t˜1 ! bc˜1 ! bW c˜1 m(t˜1) = m(c˜1 )+10 GeV ATLAS [10] ± 0 ± ± t˜1 ! bc˜1 ! bW c˜1 m(c˜1 ) = 2 × m(c˜0 ) ATLAS [10] ˜ 0 ˜ ± b1 ! bc˜1 m(b1) > m(c˜0 ) ATLAS [11] & CMS [12] ˜ ± 0 ± 0 PoS(PP@LHC2016)006 b1 ! tc˜1 ! tW c˜1 m(c˜1 ) = m(c˜1 )+100 GeV ATLAS [4] Table 1: SUSY models analyzed by ATLAS and CMS with a final state characterized by a top quark or bottom quark signature. For each of them it has been specified the SUSY masses spectrum assumed and which experiment has developed analys(es). A 100% Branching Ratio (BR) is assumed for all the decays. a consequence bottom squark may also be light, being bound to top squaks by SU(2) invariance leading to a potentially large production cross-sections at the Large Hadron Collider (LHC). Thanks to the increased Run 2 LHC centre of mass energy (13 TeV), the sensitivity to a possible discovery is higher than the one reached in Run 1. For this reason, both ATLAS and CMS experiments were able to explore the possibility of a supersymmetry discovery already with the data collected in 2015, consisting in an integrate luminosity of 3.2 fb−1 and 2.3 fb−1 respectively for the two experiments. In particular, focusing the attention on SUSY processes characterized by signatures with top quarks (t) and bottom quarks (b) in the final state, the studied signal models essentially involve the pair production of gluinos (g ˜), top squaks (t˜1) and bottom squarks (b˜1) as shown in Tab. 1. To better discriminate the supersymmetric processes from the Standard Model ones, in addi- miss tion to the missing transverse momentum, other kinematic variables based on or correlated to ET 0 ¯ 0 were used. For example in the CMS multi-jets analysis [6] that looks atg ˜ ! tt¯c˜1 andg ˜ ! bbc˜1 decay processes, due to the all hadronic-final state request, the target SUSY events are expected miss to exhibit large values of HT and HT ; while HT is the scalar sum of the transverse momenta miss of the jets, HT is the magnitude of the vector sum of the jet pT and can be considered an es- miss 4 timate of the missing transverse momentum when the ET soft term is neglected; in this way a better separation between signal and background should be obtained since only the neutralinos miss ± should contribute to HT . Other useful variables used for other searches, like the t˜1 ! bc˜1 [10] ˜ 0 or b1 ! bc˜1 [11] ATLAS analyses, were the leptonic stransverse mass, mT2, and the contransverse mass, mCT , defined as ~ lept1 ~ lept2 mT2(lept1;lept2) = min fmax[mT (Pt ;~qt );mT (Pt ;~rt )]g (1.1) ~ miss ~qt +~rt =Pt q jet1 jet2 2 ~ jet1 ~ jet2 2 mCT ( jet1; jet2) = [ET + ET ] − [PT − PT ] (1.2) lepti jeti th th where PT and PT are the transverse momentum of the i lepton (lept) and the j jet respec- 4 miss the ET soft term takes into account the calorimeter deposit not associated with any objects. 2 Search for signatures with top, bottom, tau and exotics 5 6 tively while mT indicates the transverse mass . mT2 could be considered to be a variable formed ~ miss from dividing Pt into two parts (~qt ,~rt ) in all possible combinations that satisfy the kinematics of the event and calculating the transverse mass for each decay branch; the resulting value is the best lower limit on the mass of a pair-produced SUSY particle that could have decayed to the observed ~ miss final state with the given ~qt ;~rt and Pt ; for tt¯ and WW decays, mT2 is bounded from above by the mass of the W boson while t˜1 processes are not limited.
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