1. 4/12/05 -Xiaojian Zhang, zhangxj@uiuc.edu 1 UIUC paper review Introduction to Bc Event selection The blind analysis The final result The systematic error A further check - a paper written simply and neatly with full physics concepts associated to the analysis

2. 4/12/05 -Xiaojian Zhang, zhangxj@uiuc.edu 2 Short introduction to B c Meson(bc): The highest combination of two different quark flavors Reco. event : has not been observed in any fully reconstructed decay modes. (CDF RunI, D0 RunII measurement of mass : using semileptonic decay.) Mass : 6.40 .39(stat)  0.13(syst) GeV/c 2 (PDG-2004) The measurement in this analysis results: ~19 fully reconstructed B c  J/  events, J/   . Mass : 6287. .8(stat)  1.2(syst) MeV/c 2

3. 4/12/05 -Xiaojian Zhang, zhangxj@uiuc.edu 3 Online event selection ● 360 pb -1 data sample ● Di-muon triggered sample – Level 1: CMU, CMX stubs are matched to COT axial tracks. A muon pair (CMU-CMU, CMU-CMX) is required with pT > 1.5 and 2.0 GeV/c for the CMU and CMX muons respectively. – Level 2: require the opposite charge and impose requirement on the opening angle. – Level 3: three dimensional tracking is done for the muon pair; require the invariant mass from 2.7 to 4.0 GeV/c 2.

4. 4/12/05 -Xiaojian Zhang, zhangxj@uiuc.edu 4 Reconstruct B c  J/  events offline ● Ensure good vertex resolution – Three axial SVX hits. ● Good matching between the fitted muon tracks and the muon stubs. ● Di-muon invariant mass window [3.042, 3.152] GeV/c 2. ● pion candidate : each charged particle track that is not part of the reconstructed J/ . ● The J/  candidate and pion candidate are constrained to a common 3-D vertex with a vertex fit cut. ● B c search mass window is defined from 5.6 to 7.2 GeV/c 2.

5. 4/12/05 -Xiaojian Zhang, zhangxj@uiuc.edu 5 The blind analysis ● The search mass window [5.6, 7.2] GeV/c 2. – ~ 2 standard deviation region around the CDF Run I measurement of the B c mass. ● The mass values of the events in the search window are temporarily hidden ● Binning the invariant mass in two intervals above or below 6.4 GeV/c 2. ●  function: ● Optimize the  function with MC samples: – A quality requirement on the J/  three-track 3-D vertex fit. – A requirement on the pion track contribution to the 3-D  2 vertex fit. – The impact parameter of the J/  pair with respect to the beam line. – The maximum proper decay time of the J/  pair. – The transverse momentum of the pion. – The 3-D angle between the momentum and decay direction of the J/  pair. – The significance of the transverse decay length of the J/  pair ● Find 390 candidate events in the search window in data sample.  J/   

6. 4/12/05 -Xiaojian Zhang, zhangxj@uiuc.edu 6 Control sample: B u  J/  k ● B u  J/  k vs B c  J/  – Same decay topology – Different masses ● Check the understanding of the reconstruction of the relevant variables in the simulation. – Mass resolution ● Estimate the expected B c yield. – 10 – 50 B c  J/  events in data sample Three Gaussians and a linear function 2378 signal events 11.5  0.3MeV/c 2 Agree with the mass resolution from MC simulation.

7. 4/12/05 -Xiaojian Zhang, zhangxj@uiuc.edu 7 A sliding fit window ● The sliding search window – extends from –100 to +200 MeV/c 2 in mass – around each nominal peak position, m. – seeach mass region [5.7,7.0] ● In each search window, do a binned likelihood fit – Signal : Gaussian with fixed width (13-19 GeV/c 2 ) from MC. – Background: linear function including two components  Partially reconstructed B c.  Combinatorial background. ● Calculate  in each search window

8. 4/12/05 -Xiaojian Zhang, zhangxj@uiuc.edu 8 A sliding fit window to search for maximum  ● Establish a signal or set an upper limit for the production? – Toy MC determines the expected distribution of  for pure bdg samples. ● The overall distribution is consistent with the two mass bins of the search window ● Two types of backgrounds are statistically varied in each toy MC experiment. ●  >3.5 establishes a signal. ●  distribution from the sliding fit window scans – Maximum value 3.6 at a mass of 6.29 GeV/c 2 – The background is consistent to that in the toy MC scans. – The chance for a pure bgd distribution to yield  >3.6 is 0.27%.

9. 4/12/05 -Xiaojian Zhang, zhangxj@uiuc.edu 9 The final fit ● Use unbinned likelihood fit to the mass region containing the peak. – Gaussuan + linear function – Gaussian width is fixed to 15.5 MeV/c 2, mean is allowed to float. ● Results: (within a region of  2 standard deviation of the central value) – Mass:6287.0  4.8 MeV/c 2 – Signal events: 18.9  5.7 – Background: 10.0  1.4 B c  J/  exclusive decay signal Established.

10. 4/12/05 -Xiaojian Zhang, zhangxj@uiuc.edu 10 Systematic error ● The systematic errors are evaluated from the corresponding uncertainties on the B u mass analysis. – The uncertainties on the track parameters. – The uncertainty on the momentum scale. – Differences in the pT spectra of the B u and B c mesons. – Limited knowledge of the background shape used in the final mass fit. ● The total systematic uncertainty: 1.1 MeV/c 2

11. 4/12/05 -Xiaojian Zhang, zhangxj@uiuc.edu 11 A further check to this B c enriched sample ● For data sample, lower mass window include both partially reconstructed B c and the linear combinatorial bgd. The upper mass window contains mostly the linear combinatorial bgd. ● Lose cuts to get more events in the data sample ● The lower mass window should be enriched with events having small impact parameter of the pion wrt the jpsi vertex. Data sample