1. Marina Cobal Lund, 12-16 Sept 2001 Activities of the Top quark physics group ATLAS Physics Workshop Lund, 12-16 September 2001 Marina Cobal (University of Udine)

2. Marina Cobal Lund, 12-16 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

3. Marina Cobal Lund, 12-16 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

4. Marina Cobal Lund, 12-16 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-2001-004 (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)

5. Marina Cobal Lund, 12-16 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

6. Marina Cobal Lund, 12-16 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)

7. Marina Cobal Lund, 12-16 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 |  | < 2.4 - 3 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

8. Marina Cobal Lund, 12-16 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

9. Marina Cobal Lund, 12-16 Sept 2001

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15. Marina Cobal Lund, 12-16 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(10 -10 )) - however, enhanced in many Beyond SM theories - results for t  Zc,  c,gc presented sensitivity to Br ~ O(10 -5 ) (7. 10 -3 for gc) After Physics TDR  t  Hc (C. Chikovani, T. Diobava, see next tranparencies)

16. Marina Cobal Lund, 12-16 Sept 2001 t -> Hc  Strongly suppressed in SM ( Br (t  Hq) ~ 10 -8 -10 -7 ) 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 150 0.7.10 -3 tt -> HqWb -> bbj,l b 160-170 4.7.10 -6 tt -> HqWb -> ZZj,l b 120-130 0.7.10 -4 tt -> HqWb -> bbj,l b 100-120 2.4.10 -7

17. Marina Cobal Lund, 12-16 Sept 2001 Background The most dangerous backgnd is due to the tt process. After all the analysis cuts : ~ 12000 tt evts are left, with 56 signal events

18. Marina Cobal Lund, 12-16 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 4.5. 10 -3 could be discovered at the 5  level with an integrated luminosity of 100 fb -1 M bbj

19. Marina Cobal Lund, 12-16 Sept 2001 RPV SUSY decays of top  If RPV exists can have significant top decay widths for certain 3-body final states (hep-ph/0002220, 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

20. Marina Cobal Lund, 12-16 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

21. Marina Cobal Lund, 12-16 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-2000-017 (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.

22. Marina Cobal Lund, 12-16 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

23. Marina Cobal Lund, 12-16 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 240000 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 :

24. Marina Cobal Lund, 12-16 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

25. Marina Cobal Lund, 12-16 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)

26. Marina Cobal Lund, 12-16 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