1. Fourth Nordic LHC Physics Workshop Stockholm 23 September 2001 P Eerola 1 Investigation of the decays B 0 d,s -> J/  in the ATLAS experiment at the LHC C. Driouichi, P.Eerola, Lund M. Melcher, F. Ohlsson-Malek, S. Viret, Grenoble

2. 23 Nov 2001 P Eerola 2 Physics background Decays B 0 s ->J/ , B 0 s ->J/  (’), B 0 s ->D + s D - s are analogous, but up to now, only the first one has been studied Advantages of B 0 s ->J/  no angular analysis needed since the final state is a CP eigenstate (V-PS) Cross-check of the decay mode B 0 s ->J/  CP asymmetry: in the Standard Model, the CP asymmetry is predicted to be theoretically ”clean”, but very small, where  sin  C,  C is the Cabibbo angle,  is the heigth of the Unitarity Triangle, and  is one of the angles of the UT.

3. 23 Nov 2001 P Eerola 3 Measurements Branching ratio Decay is not yet observed, exp. upper limit BR<3.8 10 -3 B s lifetime and other parameters If large CP violation is observed => New Physics! For example, LR-symmetric models with spontaneous CP violation predict CP asymmetry O(40%)[Ball, Fleischer] (SM: 4%) BR predictions: BR=(8.3-9.5) 10 -4 [Skands]. Uncertainty from   ’ mixing. Direct measurement of angle  using B s and B d decays to J/  ?

4. 23 Nov 2001 P Eerola 4 J/  reconstruction J/  ->     selected with p T (  1)>5 GeV, p T (  2)>3 GeV => passes level-1 and level-2 triggers efficiency: 85% for  1 (incl. level-1 and reconstruction in muon spectrometer), 78% for  2 (incl. Reconstruction in muon spectrometer and tilecal). Fake rate is very small. J/  reconstruction: vertexing with  2 /NDF 250  m => mass res. 39 MeV, rec. eff. 79%.

5. 23 Nov 2001 P Eerola 5  reconstruction  ->  reconstruction in the ATLAS LAr calorimeter: cell size 0.025x0.025 (  x  ) in the second sampling layer further selection: cuts on energy sharing in the 3 ECAL layers and HCAL cluster width energy containment in the 3x3 window => mass resolution 35 MeV, efficiency 2.3%, background 28% energy resolution (  (E)/E) 2 =(0.10/sqrt(E)) 2 + (0.01) 2 (E in GeV) clusterization of 3x3 cells with 1 GeV threshold => if E(  ) photon lost => if  opening angle two clusters merge to 1 => rejected 20% of  ->  retained

6. 23 Nov 2001 P Eerola 6 B 0 s,(d) reconstruction J/  and  candidates combined, using mass constraints.  (B 0 s,d )= 67 MeV.

7. 23 Nov 2001 P Eerola 7 Results for 30 fb -1  P =-10 o  P =-20 o BR(B s ->J/  )8.3 10 -4 9.5 10 -4 N(B s ) observed8 5009 700 BR(B d ->J/  )4.1 10 -6 1.6 10 -6 N(B d ) observed20080 N(background)10 800 Background considered: b->J/  X  Clear B s signal  B d CANNOT be observed

8. 23 Nov 2001 P Eerola 8 CP asymmetry Events need to be tagged as B or anti-B Use jet-charge tagging with 57% efficiency, 37% wrong-tag fraction Fit: where D is dilution from background and wrong tags.  (sin  M ) = 0.27 (x s =  m s /  s =19), 0.31 (x s =30)  Only very large asymmetries can be observed

9. 23 Nov 2001 P Eerola 9 Comparisons and conclusions Simple estimates: B-factories PEPII, KEKB cannot produce B s Tevatron 2 fb-1: CDF 190 B s ->J/  LHC ATLAS 30 fb-1: 9700 B s ->J/  LHC LHCb 10 fb-1: 260 000 B s ->J/ , 2150 B d ->J/  ATLAS can see a clear signal of B s ->J/ , measure BR and other parameters: measurement and cross-check. Note ISN-01.68, LUNDF6/(NFFL-7198)2001 submitted for publication. Improve statistics by investigating  ->       (BR=23%) improve  efficiency by better use of the fine granularity of the first ECAL sampling (cluster fine structure)