Searches for Physics Beyond the Standard Model with the ATLAS Вetector at LHC

This contribution discusses in brief recent results from the search for new physics beyond the Standard Model obtained by the ATLAS experiment with the analysis of up to 80 fb–1 of 13 TeV LHC data. It is organised over three main themes: the Higgs sector and new physics, searches for dark matter motivated new physics scenarios and relations between QCD and searches for new physics.


THE HIGGS BOSON AND NEW PHYSICS
The discovery of the Higgs boson at a mass of 125 GeV offers an important opportunity to search for new physics through precision measurements of its properties. First, its couplings may deviate from those predicted by the Standard Model (SM) due to the effects of new particles. Then, the observed boson might be only the lightest state of a family of Higgs particles in an extended Higgs sector. The recent observation of the decay is an important step in this program. This decay is crucial to determine the nature of the bosons because of potentially large new physics contribution to the effective coupling by direct vertex corrections, resulting into anti-correlated variations of the branching fractions into all the other decay modes. The contributions from new physics to the effective coupling, have been computed in details in the case of SUSY [1] and can be expressed as: (1) with the term in Eq. (1), arising from loop contributions with bottom or top scalar quarks and gluinos, given by: (2) that highlights the enhancement and the dependence on the SUSY particle masses.
The new ATLAS result for the search for the Higgs boson produced in association with or and decaying into is based on 79.8 fb -1 of 13 TeV data collected by ATLAS [2] in 2015-2017 [3]. The full combination of this analysis with those from , and production in Run1 and Run2 data results in a 5.4 observation with a ratio of the observed yield to the SM expectation of [3]. Similar results have been reported also by CMS Collaboration [4].
Combined measurements of Higgs boson couplings from the study of the Higgs boson decays into and have been obtained by ATLAS using up to ~80 fb -1 of collision data at = 13 TeV. Results are usefully interpreted in terms of modifiers applied to the Higgs boson SM couplings to other particles, and can be used to set exclusion limits on new physics. Results are in agreement with SM predictions within measurement accuracy (see Fig. 1) [3]. However, the expected deviations for many new physics scenarios are still of the order of these accuracies and the scattering of the results in the different channels, or smaller, and sensitive tests will require the full statistics foreseen for the entire LHC program.
The study of the Higgs coupling is also effective for testing whether the observed boson is part of an extended Higgs sector, as predicted by SUSY and nonsupersymmetric two Higgs doublet models (2HDM

PHYSICS OF ELEMENTARY PARTICLES AND ATOMIC NUCLEI. THEORY
like, g huu , and down-like quarks, g hdd , can be expanded to obtain the following tree-level result: (3) The couplings of the lightest boson in a generic Type II 2HDM or MSSM model approach those of the SM boson for a sufficiently heavy CP-odd boson and the limit is reached more quickly at large values of , again as a result of the presence of factors in the denominators of the expansion terms in Eq. (5). The determination of the Higgs decay rates sets non-trivial constraints on the mass, of the heavy Higgs [6]. The ATLAS results on Higgs couplings imply that 520 GeV (with 400 GeV

SEARCHES FOR DARK MATTER-MOTIVATED NEW PHYSICS
If it is indeed the manifestation of a new particle produced in the early universe, dark matter (DM) possibly represents the most intriguing signal of physics beyond the SM, since no SM particle can be responsible for it. DM also sets constraints for models of new physics, since the relic density of particles should not overclose the universe or conflict with bounds from direct DM searches. The range of masses for DM particle candidates is very broad. Even restricting to a specific models, such as SUSY, neutralinos can provide exactly the correct relic density for masses from a few tens of GeV up to 5 TeV and more, well beyond the LHC reach. Still the LHC offers the opportunity to test the same interactions between DM and ordinary matter that might have been responsible for DM annihilation in the early universe and are the underlying assumptions of direct DM searches. ATLAS searches relevant to DM-motivated new physics are directly looking for the production of DM particles or setting constraints to models that may explain DM, in particular SUSY. The first set of analyses is based on processes where a SM particle is radiated by the colliding system resulting in the pair production of DM particles that escape the detector. These "mono"-particle   signatures, characterised by an isolated particle or jet and large missing transverse energy (MET), due to the escaping DM particles, have been applied since the time of LEP and Tevatron and their results can be interpreted in complete models, for example SUSY and Extra Dimension extensions of the SM, but also, in a more model-independent manner, using effective field theory (EFT), where new operators represent the effect of new physics whose dynamics becomes effective at an high scale without specifying the full particle content of the model [10][11][12][13]. ATLAS has recently released new results for mono-jet [9] and mono- [14] searches. Constraints from these searches can be translated into bounds on the masses of the DM particle and their mediator (see Fig. 2). Within the range of EFT applicability, it is also possible to recast the bounds on the strengths of individual operators into bounds on DM scattering cross sections as shown in Fig. 2, thus directly connecting the LHC to searches of cosmic DM scattering in low noise experiments.
Within the searches for SUSY, those for weakly interacting particles, charginos and neutralinos, are of special importance in connection to DM. Recent ATLAS analyses have addressed the direct associated production of charginos and neutralinos with -and -mediated decays [16], the direct chargino pair production with -mediated decays [17] and the search for electro-weak SUSY production in compressed scenarios. The exclusion limits at 95% C.L. on production with -, -or -mediated * decays for pair production cross-sections, for pure wino and pure higgsino, are summarised in Fig. 3.
The search for electro-weak SUSY particle production in compressed scenarios, where the mass difference, between the pair-produced particle and the lightest neutralino (generally the DM candidate of the model) is small, is a difficult program because of the low energy of the SM decay products. Two techniques have been developed to probe these scenarios. The first uses soft leptons and is sensitive to values in the range from a few to 10 GeV [18]. The second technique is based on the search for disappearing tracks in tracklet topologies exploiting decays of charginos travelling through the layers of the ATLAS pixel detector and decaying soon afterwards, corresponding to values from 100 to ∼300 MeV [19]. The combination of the results of these searches constrains the compressed scenarios from the large and low ends of the spectrum, as summarised in Fig. 3. These searches cover part of the "natural" SUSY solutions [20], where the low value of the Higgsino parameter ensures a limited amount of fine tuning in the theory. These solutions are under pressure due to the negative results of direct DM searches, since the neutralino scattering cross section scales with the parameter [21].

QCD AND NEW PHYSICS
Given the main focus of this conference, it is interesting to discuss the role of QCD in the search for new physics at the LHC. Inclusive jet and di-jet events rep- resent a background to many of these searches. There are important QCD effects to be accounted for in the signal acceptance and background rejection. Examples can be found in many of the analyses discussed above. The extraction of the yield of is affected by a number of systematic effects related to the signal acceptance and the description of the backgrounds from QCD processes. The spectrum in + jets, the spectrum in + jets and di-boson events and the acceptance due to the parton shower modelling and the QCD scales rank as the top contributions to the systematic sources of uncertainty of the measured value of [3].
In the mono-jet analysis, the uncertainties on the background at large from + jets, di-boson and multi-jet events are large, of the order of 20 to 40%. These include the effect of the choice of the QCD renormalization and factorization scales, the QCD and electroweak interference terms, and the higher order QCD corrections in the event generator. Control regions for the + jets, + jets and processes have been used to constrain the normalisation of these backgrounds in the search regions. After a likelihood fit to the shape of the distribution in the control regions, the remaining uncertainty on the total background in the signal regions amounts to ~0.4-2%, depending on the and bin [9] and include the effect of the non-universality of QCD corrections across the + jets and + jets processes.
Precise measurements of jet cross-sections are of great importance for LHC searches. The predictive power of fixed-order QCD calculations is therefore relevant in many searches for new physics. Jet cross sections have been measured by ATLAS double-differentially as a function of the jet and absolute rapidity, Di-jet production has been analysed in terms of the double-differential cross-sections as a function of the di-jet invariant mass, and half of the absolute rapidity separation between the two highest-jets, [23]. These have been compared with predictions obtained using NLO and NNLO pQCD calculations [24] using several sets of parton distribution functions (PDF) [25][26][27][28][29]. Overall, a good agreement for jets in individual and bins and di-jets and a tension on full phase-space for inclusive jets have been observed (see Fig. 4).
Searches for stable or metastable gluinos and R-hadrons, bound states formed by a SUSY strongly interacting particles with ordinary quarks and gluons, exploit the production of heavy stable hadrons (stable/long-lived gluinos and these hadronising into heavy bound states) identifiable in ATLAS through their specific energy loss measured in the four pixel layers [30]. This analysis brings QCD at the forefront of the ATLAS searches for new physics focusing on the possible strong interactions between SUSY and ordinary particles. The mass limits are extracted from the in Si and the track momentum reconstructed in the tracker. Results are shown in Fig. 4.   4. CONCLUSIONS Searches for signals of new physics beyond the SM represent a major part of the ATLAS physics program at LHC. The analysis of the 13 TeV Run 2 data with integrated luminosities up to 80 fb -1 have provided us with significant bounds on new phenomena from indirect and direct searches. Despite the negative results of the searches performed so far, significant room for testing these models further still exists, since the sensitivity to new phenomena will increase further with the full Run 2 and Run 3 data sets and the HL-LHC program. The increase in the available statistics and sensitivity makes important to ensure a constantly improving control of SM backgrounds to new physics searches. QCD processes play an important role both in defining the acceptance of signal processes and in the topology and kinematics and background processes and improvements in their theoretical description and experimental bounds will benefit the sensitivity of future new physics searches at LHC.