SUSY Searches with Taus at the LHC

The supersymmetric partner of the tau lepton, the stau, is predicted to be relatively light in a range of SUSY models and may be a key for dark matter. This talk presents recent ATLAS and CMS results from proton-proton collisions at √ s = 13 TeV corresponding to an integrated luminosity of 36.1 fb − 1 and 35.9 fb − 1 delivered by the LHC and recorded by the ATLAS and CMS detectors respectively in 2015 and 2016, to search for direct stau pair production, and indirect stau production mediated by other SUSY particles.


Introduction
The Standard Model (SM) of particle physics is very successful so far, however it can only cover about 5% of the universe, and still many phenomena and theoretical problems are unexplained, such as: Dark matter and dark energy; matter/anti-matter asymmetry; neutrino masses/mixing; hierarchy problem; gravity in gauge theory and its unification, and so on. The SUSY theoretical hypothesis, based on a unique symmetry which relates matter and forces particles (fermions and bosons), can solve most of the current puzzles.
SUSY could be a powerful key to unification and cosmology, the discovery of its 'hidden world' can bring a great revolution of modern physics in the 21st century.
Final states with taus are of particular interest in SUSY searches: firstly,τ is a superpartner of the third generation fermion τ , it's a colorless scalar, which is predicted to be light in SUSY scenarios and leads to τ -riched final states; Secondly, Models with light staus can lead to a dark-matter relic density consistent with cosmological observations; Finally, Independent studies of τ s channels are necessary to investigate the coupling structure of the new physics, potentially discovered in leptonic final states, especially with regard to lepton universality. SUSY particles can be created by strong and electroweak processes at the Large Hadron Collider (LHC) [1]. All analyses results in this talk come from proton-proton collisions at √ s = 13 TeV corresponding to an integrated luminosity of 36.  2 SUSY strong production processes 2.1 The first and second squark or gluino pairs to taus An inclusive search for squarks and gluinos produced via the strong interaction in events with jets, at least one hadronically decaying to τ -lepton, and large missing transverse momentum is performed by the ATLAS Collaboration [4]. Two SUSY models are considered: a simplified model [5][6][7]  LSP masses up to 1000 GeV are excluded for gluino masses around 1400 GeV (see Figure   2). In the GMSB model, values of the SUSY-breaking scale Λ below 110 TeV are excluded at 95% CL for all values of tanβ in the range 2 ≤ tanβ ≤ 60, while a stronger limit of 120 TeV is achieved for tanβ > 30 (see Figure 3).

Stop pair to taus
ThE ATLAS analysis in Ref. [11] describes a search for SUSY in a benchmark scenario motivated by gauge-mediated SUSY breaking [8][9][10] and natural gauge mediation [12]. In this scenario, only three sparticles are assumed to be sufficiently light to be relevant for the phenomenology at LHC: the lightest scalar partner of the top quark (top squark, t 1 ), the lightest scalar partner of the tau lepton (tau slepton,τ 1 ), and a nearly massless gravitinoG. The search strategy is optimized using a simplified model [6][7] with this  : Exclusion contours at the 95% CL as a function of tanβ and the SUSYbreaking mass scale Λ for the gauge-mediated supersymmetry-breaking model [4].
limited sparticle content. The relevant parameters are the sfermion masses m(t 1 ) and m(τ 1 ). The process is illustrated in Figure 4. The top squark is assumed to be light [13][14] and directly pair-produced through the strong interaction. Each top squark decays to a b-quark, a tau neutrino, and a tau slepton which in turn decays to a tau lepton and a gravitino. The branching ratios for these decays are set to 100%, and the decays are assumed to be prompt. The tau-slepton mixing matrix is chosen such that the tau slepton is an equal mix of the superpartners of the left-and the right-handed tau lepton.
In this ATLAS analysis, a search is presented for the direct pair production of supersymmetric top squarks in final states with two tau leptons, jets identified as originating from b-hadron decays, and missing transverse momentum. Two exclusive channels are considered, which select events with either two hadronically decaying tau leptons or one hadronically decaying tau lepton and one electron or muon. Good agreement between the Standard Model prediction and the event yield observed in data is found in the signal region of each channel. The analysis results are therefore interpreted in terms of upper limits on the production of supersymmetric particles. In a simplified model with production of two top squarks, each decaying via a tau slepton to a nearly massless gravitino as the lightest supersymmetric particle, masses up to m(t 1 )= 1.16 TeV and m(τ 1 ) = 1.00 TeV are excluded at 95% confidence level (see Figure 5). This result represents an improvement of the previous limits by almost a factor of two. Model-independent limits allow the exclusion of visible cross sections above 0.15 (0.13) fb in the leponic-hadronic (fully hadronic) channel.

Direct/indirect staus
This CMS analysis [15] examines simplified SUSY models in which theτ can be produced either directly, through pair production, or indirectly, in the decay chains of Figure 4: The simplified model for production and decay of SUSY particles considered [11]. Figure 5: Exclusion contours at 95% CL in the plane of top-squark and tau-slepton mass for the simplified model [11]. Figure 6: Diagrams for the simplified models: directτ pair production followed by eachτ decaying to a τ lepton andχ 0 1 (left), and chargino-neutralino (middle) and chargino pair (right) production with subsequent decays leading to τ leptons in the final state [15]. charginos and neutralinos. In all cases, we assume that theτ decays to a τ lepton and χ 0 1 . Diagrams illustrating these simplified models of direct and indirectτ production are shown in Figure 6.
This CMS search for the direct and indirect production of τ sleptons has been performed with a τ lepton pair and significant missing transverse momentum in the final state. Both leptonic and hadronic decay modes of the τ leptons are considered. Search regions are defined using discriminating kinematic observables that exploit expected differences between signal and background. No excess above the expected SM background has been observed. Upper limits on the cross section of directτ pair production are derived for simplified models in which eachτ decays to a τ lepton and the lightest neutralino, with the latter assumed to be the lightest supersymmetric particle (LSP). The analysis is most sensitive to aτ that is purely left-handed. For a left-handedτ of 90 GeV decaying to a nearly massless LSP, the observed limit is 1.26 times the expected production cross section in the simplified model. The limits obtained for directτ pair production represent a considerable improvement in sensitivity for this production mechanism with respect to previous LHC measurements. Exclusion limits are also derived for simplified models of chargino-neutralino and chargino pair production with decays to τ leptons that involve indirectτ production via the chargino and neutralino decay chains. In the charginoneutralino production model, in which the parent chargino and second-lightest neutralino Figure 7: Exclusion limits at 95% CL for chargino-neutralino production with decays throughτ to final states with τ leptons [15]. Figure 8: Exclusion limits at 95% CL for chargino pair production with decays throughτ to final states with τ leptons [15].
are assumed to have the same mass, chargino masses up to 710 GeV are excluded under the hypothesis of a nearly massless LSP (see Figure 7). In the chargino pair production model, chargino masses up to 630 GeV are excluded under the same hypothesis (see Figure   8).

Indirect stau to taus
This ATLAS analysis [16] considers scenarios where the production of charginos, neutralinos, and sleptons may dominate at the LHC with respect to the production of squarks and gluinos which can be realised in the general framework of the phenomenological Minimal Supersymmetric Standard Model (pMSSM) [17][18]. Two simplified models [5][6]19] of − 1 andχ ± 1χ 0 2 production are considered in this work. The models are designed to enhance the probability of experimental observation. In both models, the lightest neutralino is the LSP and purely bino, the stau and tau sneutrino are assumed to be mass-degenerate, and theτ 1 is assumed to be purely left-handed stau (τ L ). The mass of theτ L state is set to be halfway between the masses of theχ ± 1 andχ 0 1 , i.e. m(τ L ) = m(χ 0 1 )+x·(m(χ ± 1 )-m(χ 0 1 )) , with the parameter x = 0.5. Other values of x are also studied for selected benchmark models where x is varied between 0.05 and 0.95 in steps of 0.1. All sparticles other than those explicitely mentioned here are assumed to be inaccessible at the LHC energy. In the model characterised byχ ± 1χ 0 2 production, theχ ± 1 andχ 0 2 are assumed to be pure wino and mass-degenerate. In the model where onlyχ + 1χ − 1 production is considered, theχ ± 1 is pure wino. The above assumptions guarantee large production cross sections and short decay chains forχ ± 1 andχ 0 2 . Charginos and next-tolightest neutralinos decay into the lightest neutralino via an intermediate on-shell stau or tau sneutrino,χ ± 1 −→τ ν τ (ν τ τ ) −→τ ν τ (ν τ τ )χ 0 1 ,χ 0 2 −→τ τ −→ τ τχ 0 1 , andχ 0 2 −→ν τ ν τ −→ ν τ ν τχ 0 1 (see Figure 9). Agreement between data and SM predictions is observed in two optimised signal re-

Gauginos to 3/4 L ( up to 2τ h )
This CMS analysis, described in Ref. [20], performed a search for direct production of charginos and neutralinos, mixtures of the SUSY partners of the electroweak gauge and Figure 11: Chargino and neutralino pair production with decays mediated by sleptons and sneutrinos [20].            GeV. In R-parity-violating simplified models with decays of the lightest supersymmetric particle to charged leptons, lower limits of 1.46 TeV, 1.06 TeV, and 2.25 TeV are placed on wino, slepton and gluino masses, respectively (see Figure 23-27).

Conclusion
The above six results from ATLAS and CMS for SUSY searches with taus at the LHC under √ s = 13 TeV by an integrated luminosity of 36.1 fb −1 and 35.9 fb −1 respectively are included, no significant deviation from SM is observed, so 95% CL limits are set. Modelindependent upper limits on the SUSY-tau cross section are set; SUSY masses exclusion limits are set for the various signal scenarios by two kinds of SUSY productions (strong and electroweak processes), which remarkably extend the exclusion space of SUSY search.