Fourth%20Nordic%20LHC%20Physics%20Workshop

1
Investigation of the decays B0d,s-gt J/yh in the
ATLAS experiment at the LHC

• C. Driouichi, P.Eerola, Lund
• M. Melcher, F. Ohlsson-Malek, S. Viret, Grenoble

2
Physics background

• Decays B0s-gtJ/yf, B0s -gtJ/yh(), B0s -gtDsD-s
  are analogous, but up to now, only the first one
  has been studied

• Advantages of B0s-gtJ/yh no angular analysis
  needed since the final state is a CP eigenstate
  (V-PS)
• Cross-check of the decay mode B0s-gtJ/yf
• CP asymmetry in the Standard Model, the CP
  asymmetry is predicted to be theoretically
  clean, but very small,
• where lsinqC, qC is the Cabibbo angle, h is the
  heigth of the Unitarity Triangle, and g is one of
  the angles of the UT.

3
Measurements

• Branching ratio
• Decay is not yet observed, exp. upper limit
  BRlt3.8 10-3
• Bs lifetime and other parameters
• If large CP violation is observed gt 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 ?

4
J/y reconstruction

• J/y-gtmm- selected with pT(m1)gt5 GeV, pT(m2)gt3
  GeV gt 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/NDFlt6,
  transverse decay length gt 250 mm gt mass res. 39
  MeV, rec. eff. 79.

5
h reconstruction

• h-gtgg reconstruction in the ATLAS LAr
  calorimeter
• cell size 0.025x0.025 (DhxDf) in the second
  sampling layer

• 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
• gt if E(g)lt 1 GeV gt photon lost
• gt if gg opening angle lt 5o gt two clusters
  merge to 1 gt rejected
• 20 of h-gtgg retained

• further selection cuts on
• energy sharing in the 3 ECAL layers and HCAL
• cluster width
• energy containment in the 3x3 window
• gt mass resolution 35 MeV, efficiency 2.3,
  background 28

6
B0s,(d) reconstruction
J/y and h candidates combined, using mass
constraints. s(B0s,d) 67 MeV.
7
Results for 30 fb-1
qP-10o qP-20o
BR(Bs-gtJ/yh) 8.3 10-4 9.5 10-4
N(Bs) observed 8 500 9 700
BR(Bd-gtJ/yh) 4.1 10-6 1.6 10-6
N(Bd) observed 200 80
N(background) 10 800 10 800

• Background considered b-gtJ/yX
• Clear Bs signal
• Bd CANNOT be observed

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.
• d(sinfM) 0.27 (xs Dms/Gs19), 0.31 (xs30)
• Only very large asymmetries can be observed

9
Comparisons and conclusions

• Simple estimates
• B-factories PEPII, KEKB cannot produce Bs
• Tevatron 2 fb-1 CDF 190 Bs-gtJ/yh
• LHC ATLAS 30 fb-1 9700 Bs-gtJ/yh
• LHC LHCb 10 fb-1 260 000 Bs -gtJ/yh, 2150
  Bd-gtJ/yh
• ATLAS can see a clear signal of Bs-gtJ/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-gtpp-p0 (BR23)
• improve h efficiency by better use of the fine
  granularity of the first ECAL sampling
  (cluster fine structure)