Sergey Burdin FNAL DØ Collaboration 8/12/2005 Chicago Flavor New Bs Mixing Result from DØ

8/12/2005 S.Burdin CF 2  DØ conference notes 4878 & 4881  ∫Ldt=610pb -1 (All available statistics up to June 2005)  Many people contributed to this work Bs mixing with B s  D s μX, D s   π, K * K and opposite-side flavor tagging

8/12/2005 S.Burdin CF 3 History: from Simple to Complex  2003  Reconstruction of semileptonic B decays: μD 0, μD *±, μD ±, μD s  Understanding of sample composition, resolution, K-factor (momentum of non-reconstructed particles) Precise measurement: B + /B 0 lifetime ratio (PRL 94, (2005))PRL 94, (2005)  2004  Bd mixing measurements Opposite-side muon tagging Same-side tagging  2005  Bs mixing measurements First result for Moriond 2005 Update for EPS 2005 Considerable improvement

8/12/2005 S.Burdin CF 4 B Mixing Analyses  Signal Selection  Initial and final state flavor tagging  Study of time evolution of tagged B signal  Use Visible Proper Decay Length for semileptonic decays  Use special variable “Asymmetry”  Fit  Comparison with expected asymmetry gives Δm

8/12/2005 S.Burdin CF 5 Bs data DØ  World largest sample  Data up to end of May 2005 (~610pb -1 ) 15640±190 Ds   π 4349±152 D ±   π 18780± ± ±910 Charge of muon gives the final state tagging

8/12/2005 S.Burdin CF 6 Signal Selection  A set of discriminating variables is constructed for a given event  Cut on combined variable  f s (x i ) and f b (x i ) --- pdf for signal and background

8/12/2005 S.Burdin CF 7 Improvement wrt Moriond

8/12/2005 S.Burdin CF 8 Analyses road map  Binned asymmetry  Asymmetry fitting procedure  Essentially the same as for the lifetime ratio and Bd mixing analyses  Inputs to the fitting procedure  MC sample composition K-factor taking into account non-reconstructed particles Efficiencies Visible Proper Decay Length (VPDL) resolution  Scale factor for VPDL resolution from tuning procedure  Tagging algorithm tested and its dilution determined from Bd and Bu semileptonic samples

8/12/2005 S.Burdin CF 9 Initial State Tagging  Initially bb pair is produced – use decays of b to tag flavor of b  Flavor at production moment determined by sign of opposite side muon (electron), tracks from Secondary Vertex and Jet Charge  For example  +  -  no oscillation  +  + or  -  -  oscillation Beware: Additional dilution from oscillations on the opposite side

8/12/2005 S.Burdin CF 10 Initial state flavour tagging For this analysis, we use opposite-side flavour tagging to determine the flavour of a given B meson at production. b quarks are produced in pairs (b-b); we use the decay products of the “other b” to infer the initial flavour of the B. A method based on likelihood ratios is used to combine different discriminating variables into one continuous tagging variable d (b-like: d>0 ; b-like: d<0). We distinguish different categories of events, and use the following discriminating variables: If an opposite muon [cos  (p ,p B ) < 0.8] is found: Muon jet charge: constructed from p T and charge of muon and tracks within  R < 0.5 of muon. Muon p T rel : transverse momentum of muon w.r.t. nearest track-jet. If secondary vertex is found (e.g. from B decay): Secondary vertex jet charge constructed from charge and momenta of tracks from vertex. If an opposite electron [cos  (p e, p B ) < 0.5] is found: Charge of the electron Otherwise: Secondary vertex jet charge p T of secondary vertex Event jet charge: constructed from charge and momenta of all tracks at  R > 1.5 from B. Distribution of combined variable in data samples enriched in B 0 and in B 0 : B 0 -enriched

8/12/2005 S.Burdin CF 11 Dilution from Δm d measurement  B d oscillation measurement with the same opposite-side tagger as for B s   m d =  0.030±0.016ps -1  Dilutions D(B d )=0.414  0.023±0.017 D(B u )=0.368  0.016±0.008  Used for systematic error MC shows that dilutions for B s and B d are in agreement Dilution for B d agrees in data and MC  Better use of tag variables  εD 2 =2.17±0.13±0.09 % Combined dilution: D=0.384±0.013±0.008 εD 2 =1.94±0.14±0.09 %

8/12/2005 S.Burdin CF 12 Tagged B s →D s μX events  Tagging efficiency %  In agreement with B d and B u 1917± 66 Ds   π candidates 566±55 D ±   π candidates

8/12/2005 S.Burdin CF 13 Measurement of B s Oscillation Frequency Amplitude fit = Fourier analysis + Maximum likelihood fit can be used for the Δm s measurements If A=1, the Δm’ s is a measurement of Bs oscillation frequency, otherwise A=0 Need to know dilution (from Δm d analysis) Amplitude fit for Bd mixing Is not the best method to determine the oscillation frequency Good to establish the oscillation frequency range

8/12/2005 S.Burdin CF 14 Asymmetry in μD s sample (  π mode )  Expected curve is affected by bin width, resolution and K-factor

8/12/2005 S.Burdin CF 15 Asymmetry for K*K decay mode

8/12/2005 S.Burdin CF 16 Asymmetries in μD s and μD ± samples (large bin)  See oscillations in μD ± (D ±   π ) sample

8/12/2005 S.Burdin CF 17 Asymmetry Fitting Procedure Use amplitude method to set a limit on the B s oscillation frequency

8/12/2005 S.Burdin CF 18 Asymmetry Fitting Procedure  For given decay mode j :  For given VPDL interval i :  Minimize χ 2 for given Δm s in range from 1 to 20 ps -1 with step 1 ps -1

8/12/2005 S.Burdin CF 19 Sample Composition  Inputs from MC  Sample composition for signal peak  + 3.5±2.5% contribution of from gluon splitting 1.3%B s →D s τ ν 0.9%B s →D s DX 4.1%B - →D s DX 4.0%B 0 →D s DX 2.9%B s →D s D s X 3.1%B s →D * 1s μ ν 1.4%B s →D * 0s μ ν 60.7%B s →D * s μ ν 21.7%B s →D s μ ν Sample composition Decay Useful signal — 88.3%

8/12/2005 S.Burdin CF 20 contamination  From MC:  tagging suppresses the ccbar by factor of ~3  From lifetime ratio analysis:  10±7% contamination  Result:  3.5±2.5% contribution  VPDL distribution from MC

8/12/2005 S.Burdin CF 21 K-factors 0.687B - →D s DX 0.681B 0 →D s DX 0.762B s →D s D s X 0.830B s →D * 1s μν 0.815B s →D * 0s μν 0.861B s →D * s μν 0.881B s →D s μν Decay

8/12/2005 S.Burdin CF 22 Efficiency vs VPDL  Use MC  Have lifetime cuts in the analysis → efficiency (VPDL)  In the Bs oscillation analysis the asymmetry in the range [-0.01, 0.06] cm is the most important → efficiency shape is a large effect over all sensitivity region  Would cancel out if not the sample composition  Good news : same turn-on shape for different processes Signal Background

8/12/2005 S.Burdin CF 23 VPDL Resolution  Understanding of resolution is crucial for Δm s measurement  Measured and tuned tracking errors in data and MC  Tracking errors depend on  Track momentum and angles  Silicon detector hit configuration and cluster width ~150 configurations are being considered

8/12/2005 S.Burdin CF 24 Tuning VPDL resolution Data before tuning MC before tuning Data after tuningMC after tuning ln(σ 2 IP ) -ln(p 2 sin 3 θ) Track IP errors IP resolution ln(σ 2 IP )

8/12/2005 S.Burdin CF 25 VPDL Resolution  Resolution described by 3 gaussians  One scale factor for all 3 gaussians: 1.142±0.020  Tuning is crucial for event by event fit Dependence of resolution from VPDL MC Before tuning After tuning

8/12/2005 S.Burdin CF 26 Result on Bs oscillations in  π mode  1.7 times better than our Moriond result

8/12/2005 S.Burdin CF 27 Result on Bs oscillations in K*K mode  New Result

8/12/2005 S.Burdin CF 28 Systematic Errors Tagging Purity Resolution Br(B s  D s μX)

8/12/2005 S.Burdin CF 29 Combined DØ result in  π and K * K modes

8/12/2005 S.Burdin CF 30 Sensitivity in Comparison (this analysis, 610 pb -1 ) (prior to this conference, 355 pb -1 ) Jan Stark, EPS 2005

8/12/2005 S.Burdin CF 31 B s Mixing Projections Planned hardware improvement  L3 bandwidth increase from 50 to 100 Hz  Expect considerable increase in signal yield  Tests are successful !  Layer0  Improvement in decay length resolution Layer0 + L3 BW upgrades No upgrades We are here Analysis improvement  event by event fit  better tagging  Improved OST  Same-Side Tagging

8/12/2005 S.Burdin CF 32 New Tevatron Combination  Combined Tevatron average comparable to the best single measurement

8/12/2005 S.Burdin CF 33 New World Combination

8/12/2005 S.Burdin CF 34 Δm s Experimental Status of Unitarity Triangle  Present and future experiments to improve our knowledge of the Unitarity Triangle  B-factories Access to B d mesons Δm d = (0.510 ± 0.005) ps –1  Tevatron and LHC Access to all B hadrons (B d, B s, B c,  b etc) Measurement of  m s /  m d Strong constraint on one of the triangle's sides CKM fit predicts : Δm s = 18.3 ps – – 2.3 [ CKM constraint dominated by theory error ] CKM fit predicts : Δm d = 0.47 ps – – 0.12 HFAG – Winter 2005 Δm s measured

8/12/2005 S.Burdin CF 35 Conclusion  We are entering era when Bs mixing will be defined by the Tevatron results  Our result has the second best sensitivity (after ALEPH inclusive lepton analysis)  Impressive team work of many people  Good prospects  10-fold increase statistics during next 3 years (more lumi + increased bandwidth)  Layer 0  Now it is clear that we will push the sensitivity well beyond 20 ps -1  measure Δm s if it is close to 20 ps -1