Searches for physics beyond the Standard Model at ATLAS and CMS

Abstract Latest results on searches for new physics beyond the Standard Model at the LHC are presented. Results are based on the RUN1 dataset collected by the ATLAS and CMS experiments in years 2011 and 2012, at an energy of 7 and 8 TeV in the proton-proton centre of mass, respectively.


Introduction
The main goals of the Large Hadron Collider (LHC) are to elucidate the electroweak symmetry breaking mechanism and to search for new physics beyond the Standard Model (SM). In July 2012, ATLAS and CMS announced the discovery of a new particle at a mass of 125 GeV [1,2]. The measured properties of the new particle, as reported at the Moriond conference in March 2013 [3,4], were found to be compatible with the ones of the minimal SM scalar boson proposed in 1964 by Brout, Englert and Higgs (H boson) [5][6][7]. In particle physics, the H boson discovery is a main achievement: it was indeed the missing piece of the SM for many years. Is this new particle the only elementary scalar particle of the model or more are to be discovered?
In parallel to the study of the scalar boson(s), the search for other new particles expected from new physics is also crucial. Indeed the SM cannot answer several fundamental questions (the hierarchy problem, matter-antimatter asymmetry, no dark matter candidate...). The SM is then generally considered as a low energy effective model of a more fundamental theory. A well known extension of the SM, supersymmetry (SUSY), introduces a new symmetry between bosons and fermions and allows the stabilisation of the radiative corrections to the H mass. It also generally proposes a candidate for dark matter.
This proceeding reports on searches for physics beyond the standard model (BSM), with a selection of lat-est results from both ATLAS and CMS experiments.The complete public ATLAS and CMS results are available on the web pages given in [8,9].

The LHC machine performance and the CMS and ATLAS detectors
ATLAS and CMS are multipurpose detectors described in detail in [10,11]. Results presented in this proceeding are based on the RUN1 LHC dataset taken during the years 2011 and 2012 at the proton-proton center of mass energy √ s = 7 TeV and 8 TeV, respectively. During these two years, excellent performance were achieved by the LHC machine and the ATLAS and CMS detectors. The integrated luminosities recorded by ATLAS and CMS are of more than 5 and 20 fb −1 at 7 and 8 TeV respectively, as shown in Fig. 1, with a peak instantaneous luminosity up to 7.7 10 33 cm −2 s −1 .
Due to the high instantaneous luminosity provided by the LHC, the number of proton-proton interactions in each beam bunch crossing was in average about 9 in 2011 and 21 in 2012, see Fig. 2(top). These additional (soft) interactions, called as pile-up events (PU), provide an important challenge to experimentalists. Much effort were put in the PU control and correction in particular for the 8 TeV data analyses. As an example, Fig. 2(bottom) presents the CMS resolution on the missing transverse energy (MET) measurement in Drell-Yan events, as a function of the number of reconstructed  vertex, for several MET algorithms using particle flow techniques [12].  cross sections over many orders of magnitude as the topantitop quark pair and single-top quark production, the Z and W boson production, and the di-boson production : WW, WZ and ZZ. These accurate measurements, in agreement with the theory predictions, as shown in Fig. 3 (ATLAS results from [8]), demonstrate the excellent understanding of the detector, which in crucial for the search for BSM physics as the SM processes mentioned above are usually the main backgrounds to new signal searches.

Search for New Physics in the scalar sector
The search for new physics can be done indirectly, looking for deviations in the H boson coupling measurements, or directly, looking for additional scalar particles as predicted in SUSY and exotics models, also possibly as DM candidate. CMS Searches for heavy H boson in the channels : H → WW → lνqq' and H → ZZ → llνν have been updated [13,14] and interpreted in BSM physics as a Higgs singlet extension of the SM, with the mass of the lighter one being around 125 GeV, see also [15]. Figure 4 presents the resulting limits on the coupling parameter as function of the heavy SM-like H mass [14].
Search for charged Higgs are also fundamental as they are predicted in 2 Higgs Doublet Models (2HDM). The main production mode is through top decays (for m H + smaller than the top quark mass), and the branching ratio (BR) H + → τν (or H + → cs) is usually assumed to be 100%. No signal is observed and 95% Confidence Level (CL) limits on the BR(t → H + b) of about 1-3% are derived as a function of the H + mass, in the range

Search for SUSY particles
Supersymmetry is a well known extension of the SM. Its minimal version, the MSSM, contains two scalar doublets, giving rise to five physical states (h, H, A, H + ,H − ). The mass relations between these particles depend in particular on the MSSM parameter tan β, the ratio of the scalar fields vacuum expectation values. Superpartners of the SM particles are introduced : squarks, sleptons, gauginos and higgsinos. The gauginos and higgsinos mix to give charginos and neutralinos. The lightest neutralino particle (LSP) is a possible candidate for dark matter. The

SUSY inclusive searches
At the LHC, the gluino and squark pair production cross sections are typically large and inclusive searches for SUSY are performed, looking for an excess of events in multijet + MET, or multijet + lepton(s) + MET final states. ATLAS has updated the search in six final states, and limits are extracted in the context of MSUGRA as presented in Fig  Dedicated analyses are also performed considering final states with soft leptons in order to increase the sensitivity to SUSY spectra at small mass splitting, as presented in Fig. 5

Searches for Natural SUSY
After the hint of a new scalar boson at 125 GeV end of 2011, ATLAS and CMS focussed on phenomenology oriented approach to target the m H hierarchy problem and several 'Natural SUSY' dedicated searches have been performed. A typical spectrum of natural SUSY is presented in [24] with m(gluino) < 1500  Another possibility is to search directly for stop pair (or sbottom pair) production. The stop can decay directly into a top-LSP pair or via an intermedi-ate chargino/neutralino state. Other channels are also possible if the stop is light (below the top mass), see Fig. 7(top). The final state signature is then a top-antitop pair and MET. The searches focussed on the 1-lepton channel (e/µ). At small ∆M, where ∆M = m(stop)m(LSP), the signal cross section is sizable but the analysis suffers from large tt background contamination; at large ∆M, the signal cross section is low but presents kinematical distributions different from the background.

Dark matter search at the LHC
A search for dark matter (DM) particles in events with an energetic jet or photon and an imbalance in transverse momentum is performed in ATLAS and CMS, the unique object in the event being the jet or the photon from Initial State Radiation (ISR). The ATLAS and CMS analyses are detailed in [36,37] and [38], respectively. The data are in good agreement with the expected contributions from SM processes. Under specific assumptions and using effective operators [39], constraints on the dark matter-nucleon scattering cross sections are determined for spin-independent model as shown in Fig. 8(top), and for spin dependent one, see Fig. 8(bottom). For the spin-independent model, these are the best limits for a dark matter particle with mass below 3.5 GeV, a region unexplored by the direct detection experiments. For the spin-dependent model, these are the most stringent constraints over the entire 1-1000 GeV mass range studied.

Other BSM searches
ATLAS and CMS have performed extensive searches for BSM physics in many different final states. Latest results on the search for heavy resonances and vectorlike quarks are detailed below.

Search for heavy resonances
ATLAS and CMS are searching for new heavy resonances in several final states. Searches for neutral resonances decaying into a dilepton pair (ee and µµ) are performed by ATLAS [40] and CMS [41]. Events with two isolated high p t leptons are selected. The CMS dielectron spectrum is presented in Fig. 9(top). No excess  above SM processes is observed and limits on the production cross section are derived at 95% CL, see the ATLAS results in Fig. 9(down). A SM-like coupling Z is excluded for mass below 2.9 TeV and a superstringinspired Z ψ is excluded below 2.5 TeV.
Heavy W search are preformed in the leptonic final  Figure 8: Comparison of the 90% CL upper limits on the dark matternucleon scattering cross section versus dark matter mass for the spinindependent (top) and spin-dependent (bottom) models, with results from various direct detection experiments. Results from ATLAS [36,37] and CMS [38]. state (W → lν) and hadronic final state (W → tb). The W → lν signal implies high MET in the final state, with a Jacobian peak in the falling transverse mass distribution, as shown in Fig. 10 (top) from CMS (for the electron channel) [42]. Hadronic decay search requires high p t lepton, jets and MET in the final state. The ATLAS results based on a Boosted Decision Tree (BDT) analysis are shown in Fig. 10 (bottom) [43]. CMS results are presented in [44]. The data are compatible with the SM background estimations.

Conclusions
This proceeding has presented a selection of the latest results on searches for new physics at the LHC, performed by the ATLAS and CMS experiments with the RUN1 dataset collected in years 2011 and 2012. Many different signal topologies were investigated. No significant excess has been observed and limits on new physics cross section production or on new particle mass have been put. In April 2015, the LHC will restart for the high energy run (at an energy of 13 TeV) and will search for new physics in a larger phase space, with hopefully discoveries of new particles.
[14] CMS Coll., Search for a heavy Higgs boson in the H to ZZ to 2l2ν channel in pp collisions at √ s = 7 and 8 TeV, CMS PAS HIG-13-014.