The goal of the proposed project is the participation in the researches within the physics program of the ATLAS (A Toroidal LHC ApparatuS) experiment. The ATLAS is general-purpose proton-proton experiment of the Large Hadron Collider (LHC) at European Organization for Nuclear Research (CERN, Geneve, Switzerland). The LHC started its operation in 30-th of March of 2010 with a bunch spacing of 50 ns and collided protons at vs = 7 TeV, then run during 2012 with increased energy at vs = 8 TeV. After the long shutdown (LS1) of LHC during 2013-2014 the machine have been rerun at June of 2015 with bunch spacing of 50 ns at the first stage, then with 25 ns and collided protons at vs = 13 TeV.
ATLAS is the largest collider experiment ever built. Its size is determined by the extent of its muon system which uses 20 m diameter air-core toroids and endcap planes separated by 46 m.
Exciting time is for fundamental physics, after the LHC experiments announced about discovery of neutral Higgs particle. Many in the scientific community expect a new paradigm –(New Physics beyond the Standard Model) to emerge around the TeV scale, be it some variant of SUSY or of Technicolour or something even more radical, like extra (space) dimensions. Those novel structures can manifest themselves directly through the production of new quanta or the topology of events or indirectly by inducing forces that modify rare weak decays. Such indirect searches are not a luxury. We consider it likely that to differentiate between different scenarios of New Physics, one needs to analyze their impact on flavor dynamics.
Processes, which are highly suppressed in the Standard Model (SM), such as decays mediated by flavour changing neutral currents (FCNC) allow stringent tests of our current understanding of particle physics. These transitions are forbidden at tree level in the SM, as all electrically neutral particles have only diagonal couplings in the flavor space. FCNC processes are therefore only allowed through loop contributions and probe the underlying fundamental theory at the quantum level. Historically, many observations have first been indicated by FCNC processes, examples include the existence of the charm quark or the high top quark mass. Enhancements of the decay rates of these FCNC decays are predicted in a variety of different New Physics models.
Top quark physics, which is one of the interesting topics in ATLAS physics program, is one of the research issues of the proposed project. Being the only particle with its Yukawa coupling close to unity also raises the question whether it plays a special role in the process of mass generation. By now, the mass of the top quark is measured to be 173.4 ±0.27(stat) ± 0.71(syst) GeV marking the most precisely measured quark mass and the most massive fundamental particle known to date. The consequent lifetime of the top quark in the Standard Model (SM) of ? 5 · 10?25 s is extremely short and it decays before hadronizing almost exclusively to bW . In addition, top quark events will be the dominant background in many searches for new physics at the TeV scale; extraction of new physics will therefore require detailed measurement and understanding of the productions rate and properties of top quark events. At present the Large Hadron Collider (LHC) at CERN is the machine where top quarks can be produced and studied directly. In the framework of the standard model, top quarks can be produced in pairs (t?t) predominantly via the strong interaction and singly via the electroweak interaction. Top-antitop quark pairs are produced at LHC predominantly via QCD processes (gluon-fusion and quark-antiquark annihilation). The tt? cross section for pp collisions at a centre-of-mass energy of vs =8 TeV is ott- =253+13-15 pb for a top quark mass of 172.5 GeV. This large cross section means that LHC is prolific source of top quarks, a real “top factory”.
Flavor changing neutral currents (FCNC) interactions of the top quark with a light quark q = u, c through gauge (Z, a, g) or Higgs (H0) bosons do not appear in the Standard Model (SM) at tree level and are strongly suppressed in the SM. For the top quark within the framework of the SM, these contributions limit the FCNC decay Branching Ratios to the gauge bosons, BR(t > qX, X = Z, a, g, H), to below 10?10, well out of reach of sensitivity of the LHC. Any observation of such FCNC decays would therefore signal physics beyond the standard model. There are however extensions of the Standard Model like the quark-singlet model (QS) , the two-Higgs doublet model with (FC 2HDM) or without (2HDM) flavour-conservation, the minimal supersymmetric model (MSSM) , SUSY with R-parity violation (R SUSY), the Topcolour- assisted Technicolour model (TC2) or models with warped extra dimensions (RS) which predict the presence of FCNC contributions already at tree level and significantly enhance the FCNC decay Branching Ratios (increase by many orders of magnitude). The present experimental 95% CL upper limits on the branching fractions of the FCNC top quark decay channel t > qZ established (obtained) by experiments at the LEP, HERA, Tevatron and LHC colliders are : BR(t > qZ) < 7.8% (LEP); BR(t > uZ) <30% (LEP); BR(t > qZ) < 3.2% (Tevatron); ); BR(t > qZ) < 0.05% (LHC);
The first ATLAS results on the search for flavour changing neutral current (FCNC) processes
involving the top quark has been obtained (ATLAS collaboration (Georges Aad (Freiburg U.) et al.). Jun 2012. 19 pp. PuPublished in JHEP 1209 (2012) 139) in pp collision data collected with the ATLAS detector at vs=7 and 8 TeV energy during 2011, 2012 and corresponding to an integrated luminosity of 2.1 fb?1, 20.3 fb?1 respectively. The t > qZ and t > qa decay channels were searched for by looking for top quark pair production with one top quark decaying through FCNC (t > qZ) and the other through the Standard Model dominant mode (t > bW). Only the leptonic decays of the Z and W bosons were considered (Z > e+e?,i+i? and W > e?i,i?i) as signal. No evidence for any FCNC signal is found. The upper limit on the t > qZ FCNC top quark decay branching ratios BR(t > qZ) < 0.73% (at vs=7 TeV) and BR(t > qZ) < 0.07% (at vs=8 TeV) were set at the 95% confidence level (CL).
The research goals and tasks of the proposed project are:
1. The Tile Cal sub-detector behavior and stability analysis using the DCS data.
2. ATLAS Tile Calorimeter demonstrator prototype performance investigation based on the test beam data with the purpose of an optimization of the demonstrator developed for the Phase 2 Upgrade of the ATLAS Tile Calorimeter.
3. Investigation of Tile Calorimeter E4 scintillators role in the physics objects (electrons and jets) calibration.
4. To develop, validate and then apply a search for FCNC top quark decays in the initial ATLAS Run 2 data samples giving sensitivity to FCNC process significantly beyond what has been achieved by previous experiments.
5. Investigation of flavour changing neutral current inspired rare decays of heavy quarks and lepton. We are going to recognize beyond standard model effects’ signatures, which could be discovered at ATLAS experiment.
