Marina Cobal Lund, Sept 2001 Activities of the Top quark physics group ATLAS Physics Workshop Lund, September 2001 Marina Cobal (University of Udine)

Marina Cobal Lund, Sept 2001  tt reconstruction t-quark mass measurement  Rare decays  Single top production  tt spin correlations  CP violation in tt production Summary and outlook Outline

Marina Cobal Lund, Sept 2001 tt reconstruction and m t In Physics TDR  Inclusive single lepton + jets  High P t single lepton plus jets  Di-lepton events  First look at all jets mode  (m t ) < 2 GeV possible (systematic errors) Updates performed for all of these topologies, and new analyses started

Marina Cobal Lund, Sept 2001 After Physics TDR  Kinematic fit of entire event (P. Roy, see presentation at this meeting)  Systematics of using different MC generators  ATLAS COM-PHYS (M. David, M. Cobal, A. De Angelis, A. Maio)  M t measurement with Herwig 6.1, Pythia 6.115, Isajet 7.49  Herwig, Pythia very similar. More gluon rad in Isajet   M jjb ~ 0.1 ± 0.1 GeV (Herwig- Pythia) ~ -2.2 ± 0.1 GeV (Isajet-Pythia)  High PT subsample (F. Fassi, see presentation at this meeting)

Marina Cobal Lund, Sept 2001 After Physics TDR (II)  All jets final state (Lj. Simic,G. Skoro, I. Mendas, D. Popovic)  Extract signal above the large QCD bkgnd QCD with Pythia and  (P t > 100 GeV) = 1.7 mb N jets >= 6, P t (j) > 40 GeV, N bjet >=2, S/B ~1/19  Subsample with P t (top) > 200 GeV, has 3300 evts per 10 fb -1, with S/B ~ 18/1  Kinematic and event shapes does not help to improve S/B  Instead, perform kinematic fit to tt hypothesis after cut on  2, S/B ~ 5/1 Next: - Use Herwig - Trigger studies

Marina Cobal Lund, Sept 2001 After Physics TDR (III)  Dilepton events (V. Simak, P. Homola, J. Valenta, R. Leitner)  Solving event kinematics leads, typically, to several solutions  Examine mean prob. of solution with heighest weight, according to angle of top in tt CMS system and E  tt events with J/  (P. Grenier, see next transparencies)  “Continuous” jet cone algorithm (V. Kostiukhine, see next transparencies)

Marina Cobal Lund, Sept 2001  Correlation between M(l-J/  ) and Mt  About 2700 evts/year at high L expected  After analysis cuts: 430 evts/year - 1 iso lepton Pt > 30 GeV and |  | < non-iso muons Pt > 3 GeV and |  | < 2.4 (mass of two of them compatible with M J/  ) M t measurement from t  l + J/  + X tt  M(l-J/  ) (stat)  0.5 GeV  M(top) (stat)  0.9 GeV for 5 years of running  W b b  W J/  l

Marina Cobal Lund, Sept 2001   (pp  tt  bX l b)  350 pb  Backgrounds:  W+jets, Z+jets, WW, WZ, ZZ,bb negligible  Wbb (complete analysis)  ( W  l ) ~ 85 pb -> at production level S/B ~ 4 Rec.  ~ 1.2% compared to 16% for signal  S/B ~ 55  Systematic uncertainties:  look at ISR, structure function, B-quark fragmentation < 1 GeV might be possible  P t top dependence  needs good tuning MC to data

Marina Cobal Lund, Sept 2001

Marina Cobal Lund, Sept 2001

Marina Cobal Lund, Sept 2001

Marina Cobal Lund, Sept 2001

Marina Cobal Lund, Sept 2001

Marina Cobal Lund, Sept 2001

Marina Cobal Lund, Sept 2001 Rare decays In Physics TDR  FCNC top decays - according to SM, would be impossible to be detected even at the LHC (Br < O( )) - however, enhanced in many Beyond SM theories - results for t  Zc,  c,gc presented sensitivity to Br ~ O(10 -5 ) ( for gc) After Physics TDR  t  Hc (C. Chikovani, T. Diobava, see next tranparencies)

Marina Cobal Lund, Sept 2001 t -> Hc  Strongly suppressed in SM ( Br (t  Hq) ~ ) Analysis on final states: tt  HqWb  bbj,l b (l=e,  )  Sensitivity of ATLAS requiring >5 signal events, assuming no bckgnd at high L, 100% signal efficiency  Hypothesis: bckgnd for WH channel can be reduced by additional cuts and reconstruction of t  Hq mass ProcessesM H (GeV)Sensitivity tt -> HqWb -> ggj,l b tt -> HqWb -> bbj,l b tt -> HqWb -> ZZj,l b tt -> HqWb -> bbj,l b

Marina Cobal Lund, Sept 2001 Background The most dangerous backgnd is due to the tt process. After all the analysis cuts : ~ tt evts are left, with 56 signal events

Marina Cobal Lund, Sept 2001 Cuts - 1 lepton, Pt > 20 GeV - 4 jets, Pt > 40 GeV - 3 b-tags - M H ± 20 GeV - M t ± (12,16,32) GeV A Br(t  Hq) as low as could be discovered at the 5  level with an integrated luminosity of 100 fb -1 M bbj

Marina Cobal Lund, Sept 2001 RPV SUSY decays of top  If RPV exists can have significant top decay widths for certain 3-body final states (hep-ph/ , Belyaev, Ellis, Lola). (i.e: t   0 ql)  F. Martin used COMPHEP to calculate decay width for t   0 b  as a fcn of RPV coupling ’ 233

Marina Cobal Lund, Sept 2001 Isajet used to generate tt evts with t   0 b   If  0 escape see excess of events at low P t (  )  Analysis under way Other possibilities can be considered: - t   0 ql, followed by  0  qql (can result in same-sign lepton signal) - t   + q, followed by  +   0 W

Marina Cobal Lund, Sept 2001 EW single top In Physics TDR Possible to:  see signals  measure V tb in each prod. mode  see predicted top and W polarization in Wg fusion (at parton level) showed After Physics TDR ATLAS PHYS (B. Gonzalez Pineiro, D. O’ Neil, R. Brock, M. Lefebvre) top, W polarization with detector smearing with 10 fb-1 can measure W and top pol., with errors < 2% varying bkgnd by +-20%  <1% syst. error in polarization.

Marina Cobal Lund, Sept 2001 tt spin correlation In Physics TDR studied lab-frame angles between charged leptons in dilepton events (V. Simak) After Physics TDR  extended study to single lepton + jet events (L. Hinz) Dileptonic channel :  low statistics  Semileptonic channel :  high statistics   dilepton events analysed using event reconstruction (K. Smolek, V. Simak, P. Homola) see presentation of V. Simak at this meeting

Marina Cobal Lund, Sept 2001 Background: W+f, W+g, bb,  * /Z 0 +f,  * /Z 0 +g,  * /Z 0,Wbb, Z 0 Z 0, Z 0 W, WW Selection >= 4 jets with p T j > 20 GeV/c and |  | < 2.5, 2 b | M jj - M W | < 20 GeV/c 2 1 iso lepton with p T l > 20 GeV/c and |  | < 2.5 p T miss > 20 GeV/c tt with S/B=5.5 Observables from lepton angles Comparison SM - SM without spin effects (NC) Maximum in angular distribution : 1.5% For 1 year at low L with BCK, ISR and FSR :

Marina Cobal Lund, Sept 2001  Models distinguishable with 3  after 10 fb¯ 1 for  lj After 3 years at low luminosity sensitivity to a max effect of 0.7% with 2  New physics   necessary to be sensitive to a max effect little than 1%   possible after 3 years at low luminosity Semileptonic channel   easy to reconstruct neutrino   direct measurement of  in top rest frame Dileptonic channel   greater effect but lower statistics compared to semileptonic   Semileptonic is complementary to dileptonic channel  Have to completely simulate new physics models to better quantify ATLAS sensitivity to each one

Marina Cobal Lund, Sept 2001 CP violation Due to CKM, extremely small in top sector  any observation would be a sign of new physics Different observables can be considered -  N LR = (N ll -N RR )/(all tt evts) - A = - / [ - ] In Physics TDR Dedicated studies not yet started After Physics TDR Studies underway (see presentation of J. Sjolin)

Marina Cobal Lund, Sept 2001 Summary and conclusions  Understanding of how to reconstruct tt event topologies and precisely determine m t improved  Several new studies recently started CP violation in tt production FCNC decay t-> Hc novel top decays in RPV SUSY model  Some ideas still to be exploited  tt  HqWb  WW*j,l b  l l j,l b  Determination of the top charge  Searches (i.e: W’ study in EW single top)  Exploit extra-dimensions in top sector