Investigation of the decays B 0 d,s -> J/ yh in the ATLAS experiment at the LHC

Investigation of the decays B0d,s-> J/yh in the ATLAS experiment at the LHC C. Driouichi, P.Eerola, Lund M. Melcher, F. Ohlsson-Malek, S. Viret, Grenoble Fourth Nordic LHC Physics Workshop Stockholm 23 September 2001 P Eerola

Decays B0s->J/yf, B0s ->J/yh(’), B0s ->D+sD-s are analogous, but up to now, only the first one has been studied Physics background • Advantages of B0s->J/yh: no angular analysis needed since the final state is a CP eigenstate (V-PS) • Cross-check of the decay mode B0s->J/yf • CP asymmetry: in the Standard Model, the CP asymmetry is predicted to be theoretically ”clean”, but very small, • where l=sinqC, qC is the Cabibbo angle, h is the heigth of the Unitarity Triangle, and g is one of the angles of the UT. 23 Nov 2001 P Eerola

Branching ratio • Decay is not yet observed, exp. upper limit BR<3.8 10-3 • Bs 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 h-h’ mixing. • Direct measurement of angle g using Bs and Bd decays to J/yh ? Measurements 23 Nov 2001 P Eerola

J/y reconstruction • J/y->m+m- selected with pT(m1)>5 GeV, pT(m2)>3 GeV => passes level-1 and level-2 triggers • efficiency: 85% for m1 (incl. level-1 and reconstruction in muon spectrometer), 78% for m2 (incl. Reconstruction in muon spectrometer and tilecal). Fake rate is very small. • J/y reconstruction: vertexing with c2/NDF<6, transverse decay length > 250 mm => mass res. 39 MeV, rec. eff. 79%. 23 Nov 2001 P Eerola

h->gg reconstruction in the ATLAS LAr calorimeter: • cell size 0.025x0.025 (DhxDf) in the second sampling layer h reconstruction • energy resolution (s(E)/E)2=(0.10/sqrt(E))2 + (0.01)2 (E in GeV) • clusterization of 3x3 cells with 1 GeV threshold • => if E(g)< 1 GeV => photon lost • => if gg opening angle < 5o => two clusters merge to 1 => rejected • 20% of h->gg retained • 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% 23 Nov 2001 P Eerola

B0s,(d) reconstruction J/y and h candidates combined, using mass constraints. s(B0s,d)= 67 MeV. 23 Nov 2001 P Eerola

Results for 30 fb-1 • Background considered: b->J/yX • Clear Bs signal • Bd CANNOT be observed 23 Nov 2001 P Eerola

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. • d(sinfM) = 0.27 (xs = Dms/Gs=19), 0.31 (xs=30) • Only very large asymmetries can be observed 23 Nov 2001 P Eerola

Comparisons and conclusions • Simple estimates: • B-factories PEPII, KEKB cannot produce Bs • Tevatron 2 fb-1: CDF 190 Bs->J/yh • LHC ATLAS 30 fb-1: 9700 Bs->J/yh • LHC LHCb 10 fb-1: 260 000 Bs ->J/yh, 2150 Bd->J/yh • ATLAS can see a clear signal of Bs->J/yh, measure BR and other parameters: measurement and cross-check. • Note ISN-01.68, LUNDF6/(NFFL-7198)2001 submitted for publication. • Improve statistics by • investigating h->p+p-p0 (BR=23%) • improve h efficiency by better use of the fine granularity of the first ECAL sampling (cluster fine structure) 23 Nov 2001 P Eerola

Investigation of the decays B 0 d,s -> J/ yh in the ATLAS experiment at the LHC