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Rare Processes at Experiment of Large Hadron Collider


The Search for and Study of a Rare Processes within and beyond Standard Model at ATLAS Experiment of Large Hadron Collider at CERN

Tech Area / Field

  • PHY-PFA/Particles, Fields and Accelerator Physics/Physics

8 Project completed

Registration date

Completion date

Senior Project Manager
Malakhov Yu I

Leading Institute
Tbilisi State University, Georgia, Tbilisi

Supporting institutes

  • Joint Institute of Nuclear Research, Russia, Moscow reg., Dubna


  • Argonne National Laboratory (ANL), USA, IL, Argonne\nCERN / European Laboratory for Particle Physics, Switzerland, Geneva

Project summary

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 a general-purpose proton-proton experiment, preparing on the Large Hadron Collider (LHC) at CERN, European Laboratory for Particle Physics (Geneva, Switzerland). The LHC is a proton-proton accelerator with 14 TeV center of mass energy and design luminosity of 1034 cm-2s-1.

The origin of mass at the electro-weak scale is one of the major focus of interest for ATLAS. Other important goals of ATLAS are: the searches for Higgs Bosons, Heavy W- and Z- like objects, super symmetric particles, the investigations of CP-violation in B-decays, detailed studies of the top quarks production and decays, checking of the predictions of the ADD (N. Arkani-Hamed, S. Dimopoulos and G. Dvali) extra dimensions theory. New direct experimental insight is required to advance in one of the most fundamental questions of physics which is closely connected to this, namely: What is the origin of the different particle masses? One of the possible manifestations of the spontaneous symmetry-breaking mechanism could be the existence of a Standard Model Higgs boson (H).

Top quark physics is one of the key subjects in ATLAS physics program, is among the research issues of the proposed project. Several properties of the top quark have already been examined at the Tevatron (FNAL, USA). Particularly the search for flavour changing neutral current (FCNC) top quark decays has 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. As it is well known, the LHC can be considered as a “top factory'', producing about 80,000 tt- events per day at L=1033 cm-2s-1, making the LHC an ideal place to explore rare decays of top quark. Kinematicaly it is accessible to many FCNC decay modes of top quark such as t → cV(V= γ,Z,g) and t → ch (h=h0,H0,A0), where h is a Higgs boson. The reason for the interest of top quark rare decays via FCNC is at least twofold. First, the typical branching ratios for the rare top quark FCNC decays predicted within the Standard Model (SM) are so small (Br(t → Zq) ~1.3×10-13; for mH = 100 (160) GeV one gets Br(t → Hq) ~ 0.9 ×10-13 (4 ×10-15), respectively), that the observation of a several events of this kind should be an "instant evidence" so to speak, of new physics, and second, due to its large mass (mt ≈ 175 GeV) the top quark could play a momentous role in the search for Higgs physics beyond of the SM. The Minimal Supersymmetric Standard Model (MSSM), Two Higgs Model (2HDM II) and Technicolor (TC2) Models predict the following values of Branching ratios for t → Zq (q=u,c) and t → Hq (q=u,c): Br(t → Zq) ~ 10-5 ÷ 10-4 and Br(t → Hq) = 5 ×10–4 . Experimentally, little is known about FCNC decays of top quark. The only existing limit comes from a CDF Experiment (FNAL) analysis of its RunI data (CDF Coll., F. Abe et al., Phys. Rev. Lett. 80 (1998) 2525) yielding Br(t → Zq) < 33%(at 95% CL) at integrated luminosity 110 pb-1. The branching ratios Br(t → Zq) < 2% and Br(t → Zq) < 0.1% for Tevatron Run II at luminosity 2 fb-1 and Run III at luminosity 30 fb-1, respectively were given at 95% CL limits. 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 participants of the project L.Chikovani and T.Djobava studied the sensitivity of the ATLAS experiment to the top-quark rare decays via FCNC t → Zq, t → Hq → WW*q (q represents c and u quarks) top anti-top quarks (tt-) pair production. The decay t → Zq has been studied in two decay modes: 1) the pure leptonic decay of gauge bosons 2) the leptonic decay of Z bosons and hadronic decay of W bosons. It have been estimated that combining the results from the two above mentioned decay modes for t → Zq decay the branching ratio 1.8x10-4 can be observed at 5σ level with high luminosity of 100 fb-1 (1034 cm-2s-1) at ATLAS; The branching ratio for t → Hq → WW*q decay as low as 2.0x10-3 for mH = 150 GeV and 2.1x10-3 for mH = 160 GeV can be observed at 5σ level with an integrated luminosity of 100 fb-1 at ATLAS experiment.

Within this project we will study the possibility of SUSY discovery at ATLAS in its minimal extension model with gravitation mediated Supersymmetry breaking mechanism – mSUGRA. We based our study on the called so EGRET (Energetic Gamma Ray Experiment Telescope, E.B. Hughes et al. IEEE Trans. Nucl. Sci. NS-27, 364 (1980)) point (set) of mSUGRA free parameter values with conserved R-parity assumption. Predicted amount of stable neutralinos can fit the observed cold dark matter in the Universe and also naturally explains the excess of diffuse galactic gamma rays observed by EGRET. The EGRET point values for mSUGRA free parameters are the followings:
0 =1400 GeV, m1/2 =180 GeV, sign μ=+1, A0 =0, tanβ=50. Our study is focused on this particular process – gg → ğğ, which has quite large cross-section (for EGRET point) at LHC. The study will include generation, simulation, reconstruction and analysis of the given process in the ATLAS detector.

The research goals and tasks of project are:

  1. Search for and study of FCNC top quark rare decays t → Zq and t → Hq (where q= u, c; H is a Standard Model Higgs boson) at ATLAS experiment (LHC). It will be estimated the ATLAS experiment sensitivity to these FCNC decays at integrated luminosities 10 and 100 fb-1 (MC generation, full simulation, reconstruction). Then analyses will be carried out on real experimental data of ATLAS with the goal to detect mentioned above decays in tt- pair production. The observation of these rare decays will be "instant evidence" of the existence of the “new physics” beyond of Standard Model, which can change basically our understandings concerning to the evolution of Universe, the forces and processes acting in it.
  2. ATLAS calorimeters performance investigation based on the Combined 2004 test beam data with the purpose of an optimization and a final establishment of Tile Calorimeter electromangnetic energy scale, hadronic calibration, energy resolution and linearity. The detailed study of the hadronic shower energy losses in the dead material before and between the calorimeter parts to improve the hadronic calibration precision. ATLAS Calorimeter Hadronic Calibration.
  3. Measurements of the top quark mass in the dilepton and lepton+jet channels using the transverse momentum of the leptons with the ATLAS detector at LHC/CERN.
  4. Further development and implementation of Tile Calorimeter Detector Control System (DCS) and integration with ATLAS central DCS
  5. Theoretical investigation of FCNC inspired B-meson and lepton rare decays in frame of large extra dimension model of Appelquist-Cheng-Dobrescu. We are going to recognize beyond standard model effects’ signatures for such models, which could be catched at ATLAS experiment.
  6. Search for the observation SUSY process – two gluinos production via gluon-gluon fusion gg→ğğ.

The project participants will acquire experience, knowledge, increase their professional level and then will share their experience with young physicists and use this experience in the future experimental and theoretical researches in particle physics. The project scientific leader J.Budagov is performimg measurement of top quark mass based on the new RUN II CDF data at the tevatron World most precise top quark mass now is obtained with J. Budagov and his group contribution of a principal significance. This is a source of a priceless experience for the forthcoming ATLAS data analysis. Project participants L.Chikovani and T. Djobava have experience in the estimation of the Branching ratios of FCNC top quark rare decays t → Zq and t → Hq at the ATLAS experiment at luminosity LL= 100 fb-1. The project participants are experienced in generation, simulation (run products through detector) of any physical processes and analyses of results obtained using ATLAS Software (the generators PPYTHIA, HERWIG, etc, events production fast simulation package ATLFAST and data analyses packages PAW and ROOT, ATHENA software). Project participants M. Mosidze, T. Djobava, G.Khoriauli, Yu. Kulchitsky and P.Tsiareshka have experience of the “offline” processing and analysis of test beam data (which are recorded as Ntuple) within the package ROOT, obtained on the combined set-up (Liquid Argon electromagnetic and hadronic Tile Calorimeters modules) exposed by negative pion beams of energy 1 ÷ 350 GeV on CERN SPS accelerator with the goal of establishment of ATLAS Tile and Combined Calorimeters energy resolution and linearity. Project participants J. Budagov, J. Khubua, I.Minashvili, Yu. Kulchitsky, G. Arabidze, G.Khoriauli, V.Bednyakov and P.Tsiareshka have experience in the construction (universalization) and installation of Tile calorimeter on the surface as well as in the cavern at CERN, in preparation and performance of combined set-up (Liquid Argon electromagnetic and hadronic Tile Calorimeters modules) test beams and analysis of test beam data. The participants of this project J. Budagov, J. Khubua,, V.Bednyakov and G. Khoriauli have the experience of SUSY theoretical and Monte-Carlo study. G.Devidze and A. Liparteliani have a rich experience in the theoretical researches of FCNC processes. Z. Modebadze is mathematics, systems programmer, have experience in installation of ATLAS software. Young participant of the project V. Tsiskaridze is an expert of computational languages and systems at high professional level.

The theoretical and experimental researches proposed in the project are very difficult and laborious, which one can see from the scope of activities of the full project information. For the performance of the project tasks the following technical approaches and methodology will be used. The method of studying of the properties and characteristics of Tile Calorimeter consists in the test exposing of the modules of Tile Calorimeter on SPS accelerator of CERN by the beam of the electrons and negative pions with energies varying from 1 GeV to 350 GeV. Then the “offline” processing and analysis of experimental data obtained by test beams are performed by the C++ based data analyzing package ROOT. Concerning the possibility of observation of different physical processes at ATLAS experiment, before the ATLAS detector will functioning, so far the generation of the processes is carried out by using the different generators such as PYTHIA, TOPREX, HERWIG. Then the full simulation that is running of all produced particles in the interactions through the ATLAS (in all detectors and sub-systems) is performed. The fast simulation package ATLFAST has been created for the simulations of ATLAS characteristics, which uses the parametrizations of the detector resolution functions. The results obtained by the ATLFAST are close to the expected experimental results. This statement has been tested by comparision with the results obtained by full simulation packages GEANT 3 (4). Therefore ATLFAST gives ones the opportunity of fast testing and study of different physical processes, then the packages of full simulation and reconstruction can be used (e.g. GEANT). At present the Software ATHENA is created for ATLAS experiment, which includes the events generation, simulation and reconstruction. ATHENA creates Event Summary Data –ESD which can be used to study different physical processes. Our theoretical study of flavor changing neutral current inspired B-meson decays will be based on the method of operator product expansion and heavy quark effective theory. Calculations of basic amplitudes will be done using clasical method of Feynman graph calculus and modern computer programs. All above mentioned methods and Software (packages) will be used for the fulfillment of the goals and tasks of the proposed project.


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