International Science and Technology Center

Objective

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. At the moment there is a long shutdown (LS1) of LHC. The goal of Long Shutdown 1 (LS1) is to perform the full maintenance of equipment and the necessary consolidation and upgrade activities in order to ensure reliable LHC operation at nominal performance - at the full 14 TeV energy from mid-2014 or begin of 2015. The luminosity will increase in tandem with increased understanding of the safe operation modes of the machine. Physics running started at L = 1.9·10²⁷ cm^?2 s^?1 and then has been increased in several steps to a target of L = 10³² cm^?2 s^?1. Subsequent running will be with 25 ns bunch spacing and luminosity of L = 10³⁴ cm^?2 s^?1 after 2014 for several years. 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 main part in ATLAS physics program, is one of the research issues of the proposed project. Top-antitop quark pairs will be produced at LHC predominantly via QCD processes (gluon-fusion and quark-antiquark annihilation). A calculation that includes NNLL soft-gluon corrections results in a central value for the cross-section of 872.8pb. This large cross section means that LHC will be a prolific source of top quarks and produce about 9 million t?t events per year (at “low” luminosity, 10³³ cm^?2 s^?1), a real “top factory”. Such large event samples will permit precision measurements of the top-quark parameters. The statistical uncertainties on top quark mass m_t will become negligible, and, using essentially the methods developed at the Tevatron (Fermilab, USA), systematic uncertainties, better than ±2 GeV/c² per channel are anticipated. Precise measurements of the top-quark mass provide a crucial test of the consistency of the Standard Model and could indicate a strong hint for physics beyond the Standard Model. The search of Flavour Changing Neutral Current (FCNC) top quark decays have been carried out with the aim of their detection. Most of these measurements are limited by the small sample of top quarks collected at the Tevatron up to now. Kinematically it is accessible to many FCNC decay modes of top quark such as t > cV(V= ?,Z,g). The reason for the interest of top quark rare decays via FCNC is numerous. Flavor changing neutral interactions of the top quark with a light quark q = u, c through gauge (Z, ?, g) or Higgs (H⁰) bosons do not appear in the Standard Model (SM) at tree level and are strongly suppressed in the SM due to the Glashow-Iliopoulos-Maiani (GIM) mechanism. 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, ?, g), to below 10^-10, well out of reach of sensitivity of the Tevatron or 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 supersymmetry (SUSY), dynamical Electroweak Symmetry Breaking (EWSB) model, Technicolor models, multi-Higgs doublet models, and Standard Model extensions with exotic (vector-like) quarks which predict the presence of FCNC contributions already at tree level and significantly enhance the ratios (increase by many orders of magnitude) . In Run II, the CDF Collaboration has performed a search for the FCNC decay t > Zq on a dataset with an integrated luminosity of 1.9 fb^?1. The upper limit is extracted on the branching fraction B (t > cZ) + B (t > uZ) of < 3.7% at 95% C.L. The expected limit in absence of signal is 5%. This is the best limit on B (t > Zq) to date, starting to constrain predictions from a dynamic EWSB model. Thus on the experimental side, a systematic study of the experimental observability for these FCNC top quark decays at the LHC is very important task. The participant of the project T.Djobava together with L Chikovani and the physicists participating in ATLAS Collaboration studied the ATLAS experiment sensitivity to top quark FCNC rare decays t > Zu(c), t > u(c) and t > gu(c) in tt^-pair production (Eur.Phys.J. C52, p.999, 2007). Different types of analyses (the cut-based analysis method and the likelihood analysis method) were used for the estimation of the Branching ratios of these decays at 5? level and at 95% C.L.
Part of this project will focus on FCNC manifestations in heavy quark and lepton rare decays as probes for physics beyond the SM (BSMP. We will explore possible signatures of BSMP scenarios involving Large Extra Dimensions (LED). Furthermore the realization that leptogenesis might be the primary effect underlying the observed baryon number in the Universe makes searches for CP violation in the lepton sector more mandatory than ever. The proposed research aims at making specific suggestions for observable signatures of such BSMP scenarios that could be searched at ATLAS experiment at CERN. As a central element it will be analyzed FCNC ??transitions with respect to their widths, the photon polarizations and CP asymmetries. It will be investigated FCNC processes also in the lepton sector. The analysis will be based on the operator product expansion and heavy quark theory, where appropriate. It will be compared LED predictions with those obtained within the SM and its SUSY extensions, both the SM results and results which were obtained by different authors and by participants of the project A. Liparteliani and G. Devidze in frames of various extensions of the SM. The goal in this direction is to identify those transitions that carry the greatest promise of differentiating between TeV scale BSMP from SUSY and LED.
The ATLAS has a great physics discovery potential. To perform the mentioned above researches successfully, ATLAS needs to have very good electromagnetic and hadronic calorimeters for the identification of electrons, pions, photons and hadronic jets and for measurement of missing transverse energy. The main task of the Tile Hadronic Calorimeter Detector Control System (TileCal DCS) is to enable the coherent and safe operation of the detector. All actions initiated by the operator and all errors, warnings, and alarms concerning the hardware of the detector are handled by the DCS. The DCS has to continuously monitor all system parameters, provide warnings and alarms concerning the hardware of the detector. Because calorimeter will be working for long period during data taking (9-10 months per each year) it is very important to control permanently all main system, accumulate necessary information for fixation of failed component and provide stable functionality of detector elements in future.
The research goals and tasks of the proposed project are:
1. The Tile Cal sub-detector behavior and stability analysis by use of DCS data.
2. Data Processing. Optimization of the MC optics model to better describe the tile calorimeter response along phi.
3. Development, validation and then apply a search for FCNC top quark decays in the initial ATLAS data samples from 2012, 2015-2016, giving sensitivity to FCNC significantly beyond what has been achieved by previous experiments.
4. Investigation of flavour changing neutral current inspired heavy quarks and lepton rare decays in frame of large extra dimension models. We are going to recognize beyond standard model effects’ signatures for such models, which could be discovered at ATLAS experiment.

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