Rare Exclusive Decays. Results from BaBar on the. DPF-2002, College of William and Mary. University of California, Jeffrey Berryhill.

Transcription

1 Results from BaBar on the Rare Exclusive Decays B K l + l and B K* l + l Jeffrey Berryhill University of California, Santa Barbara DPF2002, College of William and Mary May 24, 2002

2 B K (*) l + l in the SM and Beyond Flavor changing neutral current (b to s): proceeds via penguin or box diagrams in the SM. New physics at the EW scale (SUSY, technicolor, 4th generation quarks, etc.) can compete with small SM rate. Complementary to studying b to s γ due to presence of W and Z diagrams. K (*) ll Results from BaBar, J. Berryhill, DPF2002 2

3 Branching Fraction Predictions in the Standard Model New Ali et al. predictions lower by 3040% longdistance contribution from ψ resonances excluded K (*) ll Results from BaBar, J. Berryhill, DPF2002 3

4 Decay rate vs. q 2 in the SM and SUSY B Kµ + µ J/ψΚ * B K µ + µ Pole from K*γ, even in µ + µ SUSY models ψ(2s)k q 2 q 2 SM nonres SM nonres constructive interf. destructive K (*) ll Results from BaBar, J. Berryhill, DPF2002 4

5 Recent Experimental Results Belle: Kll observed in 2001 (29.1 fb 1, PRL 88, (2002) ) + + BB ( Kll ) = (0.75 ± 0.09) BB ( Kµ µ ) = (0.99 ) BB ( K µ µ ) < * + 6 BB ( Kee ) < * + 6 BaBar Run 1 upper limit (20.7 fb 1, accepted by PRL): BB Kll + 6 ( ) < % C.L. BB Kll * + 6 ( ) < % C.L. Belle Kll central value = BaBar Run 1 96% C.L. upper limit This result: Run 1+Run 2 update (56.4 fb 1, preliminary). K (*) ll Results from BaBar, J. Berryhill, DPF2002 5

6 BaBar PEP II Superconducting Coil (1.5T) Silicon Vertex Tracker (SVT) e + (3 GeV) e (9 GeV) Drift Chamber (DCH) Cherenkov Detector (DIRC) CsI Calorimeter (EMC) Instrumented Flux Return (IFR) K (*) ll Results from BaBar, J. Berryhill, DPF2002 6

7 B Meson Reconstruction at Υ(4s) e + e Υ( 4 S ) BB 2 m = ES E beam * ( p 2 i * ) i E = p i * + m 2 i E * i 2 beam Typical resolutions: σ(m ES ) 2.5 MeV σ( E) MeV B full fit region is blind B (*) measured in Υ(4S) rest frame E i E * beam Improve resolution Define 3 regions in E, m ES plane: A Signal region B Fit region C Large Sideband region K (*) ll Results from BaBar, J. Berryhill, DPF2002 7

8 Analysis Strategy: Event Selection Reconstruct candidates for the different decay modes with appropriate particle ID requirements: B + K + l + l, where l is either e or µ B 0 K 0 l + l, where K 0 K 0 s π + π B + K* + l + l, where K* + K 0 sπ + and K 0 s π + π B 0 K* 0 l + l, where K* 0 K + π Backgrounds suppressed using more detailed aspects of the event Continuum events event shape BB events vertexing, E miss B J/ψ( l + l )K(*) decays exclude regions in E / m(l + l ) plane Peaking backgrounds (small) Signal/background optimization with signal simulation and large sideband data. All candidates in the fit region blinded until selection criteria are finalized. K (*) ll Results from BaBar, J. Berryhill, DPF2002 8

9 Analysis Strategy: Signal Fitting 2D fit in the m ES / E plane estimates signal and background yield in the fit region. Background shape and yield float for each decay mode Signal shape fixed from signal MC and J/ψ K(*) data Small residual peaking background fixed from MC. Signal branching fractions obtained from simulated signal efficiencies, total # of BB pairs produced. Control samples in the data check signal efficiencies and background characteristics : B to J/ψ K(*) candidates Large sideband region in m ES / E plane K ( * ) e µ + combinations K (*) ll Results from BaBar, J. Berryhill, DPF2002 9

10 B J/ J/ψ( l + l )K(*) Background These decays do not give us information about the shortdistance physics and must be removed explicitly by a veto in the E vs. M(l+l) plane. + B J / ψ ( e e ) K * B J / ψ ( µ + µ ) K * When the leptons from J/ψ>l + l radiate or are mismeasured, the event shifts in both m(ψ) and in E. Nominal signal region Remove these events from BG region as well: simplify fit in m ES vs. E plane K (*) ll Results from BaBar, J. Berryhill, DPF

11 B J/ J/ψ( l + l )K(*) : Control Sample Kinematics very similar to the signal Verifies efficiencies of essentially all selection criteria + + K ee E.g. study tails in M(l + l ) distribution + + K µ µ Excellent agreement btwn. Data and MC for rates and distributions 0 K S ee + K 0 S µ + µ *0 K ee + *0 K µ + µ Points: data Histo: MC M(l + l ) K (*) ll Results from BaBar, J. Berryhill, DPF

12 J/ψ and Large Sideband Control Sample Study: B Likelihood Variable J/ψ Sample: signallike *0 K e + e keep K *0 µ + µ Large SB Sample: backgroundlike *0 K e + e K *0 µ + µ off resonance B log L B log L 10 4 K (*) ll Results from BaBar, J. Berryhill, DPF

13 E Kl + l Fit Regions, Unblinded Run 1+2 data (56.4 fb1) m ES K (*) ll Results from BaBar, J. Berryhill, DPF

14 E K*l + l Fit Regions, Unblinded Run 1+2 data (56.4 fb 1 ) m ES K (*) ll Results from BaBar, J. Berryhill, DPF

15 Run 1+2 Unblinded: m ES 2D fit projections after E cut: e: 110< E<50 MeV µ: 70< E<50 MeV (a) K + e + e (c) K S (e) K (d) K 0 *0 *+ e e + e e + + e e (b) K (f) K S (f) K (h) K + µ + µ 0 µ + µ *0 µ + µ *+ µ + µ Preliminary Entries/3 MeV/c m ES (GeV/c ) m ES (GeV/c ) K (*) ll Results from BaBar, J. Berryhill, DPF

16 Run 1+2 Unblinded: E 2D fit projections after mes cut <m ES < GeV 5 (a) K + e + e (b) K + µ + µ (c) K S 0 e + e (e) K *0 e + e (d) K S (f) K *0 0 µ µ + + µ µ Entries/20 MeV 0 5 (g) K *+ e + e (h) K *+ µ + µ E (GeV) E (GeV) K (*) ll Results from BaBar, J. Berryhill, DPF

17 Signal Candidate Properties M(ll) no apparent pileup near the J/ψ vetoes Preliminary 2 of these consistent with D mass M(Kl) possible background from B Dπ, D Kπ, both π s misid d as electrons. (Note, this peaking BG is explicitly vetoed in Kµµ channel). Simulation predicts 0.06 events of this background for this channel Studies of electron misid probabilities show no indication of problem. Nevertheless, include systematic error to account for possibility that 2 of these events are BG. K (*) ll Results from BaBar, J. Berryhill, DPF

18 Fit Results I Preliminary Unbinned max. likelihood fit in E m ES plane for the 8 decay modes Mode Signal yield Eff. bkgd ε (%) ( B =B )ε (%) ( B )fit (10 6 ) B (10 6 ) B K e e 9:6 +4:6 3: :1 ±6:8 + +! + +0:11 0:23 0:91 +0:42+0:13 0:32 0:24 B K µ µ 0:8 +2:5 1: :9 ±6:8 ±0:10 0:13 +0:37 0:23 ±0:10 + +! + B 0 K 0 e e 1:8 +2:8 1: :1 ±8:0 ±0:35 0:47 +0:69 0:39 ±0:35 +! B 0 K 0 µ µ 2:9 +2:7 1: :3 ±7:8 ±0:22 1:34 +1:16 0:78 ±0:25 +! B 0 K Λ0 e e 7:3 +4:7 3: :2 ±7:7 ±0:48 1:66 +1:08 0:83 ±0:50 +! B 0 K Λ0 µ µ 4:6 +4:2 2: :6 ±9:3 ±0:39 1:68 +1:57 1:09 ±0:42 +! B K e e 1:5 +4:0 2: :8 ±9:7 + Λ+! + +1:04 1:06 1:07 +2:86+1:04 1:51 1:06 B K µ µ 2:8 +3:5 2: :4 ±11:1 ±1:82 3:68 +4:39 2:88 ±1:86 + Λ+! + K (*) ll Results from BaBar, J. Berryhill, DPF

19 Fit Results II Preliminary Combining channels: m ES and E projections for Kll and K*ll (a) Kl + l (b) Kl + l B(B K*ee)/B(B K*µµ)=1.21 from Ali, et al, is used in combined K*ll fit. (c) K * l + l (d) K * l + l B ( B ( B B + Kl l * + K l l ) ) = = (0.84 ( ) 10 ± 0.31) m ES c 2 ) (GeV/ E (GeV) K (*) ll Results from BaBar, J. Berryhill, DPF

20 Systematic Uncertainties Systematic errors on the efficiency Largest sources Trk eff. Model dependence ~ 7 11 % total per mode Systematic errors on the # of signal events in the fit Signal shape variation Background shape variation o includes peaking background uncertainty ~ events per mode K (*) ll Results from BaBar, J. Berryhill, DPF

21 Signal Statistical Significance What is the probability that background alone would fluctuate to produce the observed signal? Consider change in ln L when fixing the signal component to zero in fit. For Kl + l, equivalent to 5.0σ fluctuation; if systematic uncertainties in signal yield included still > 4σ For K*l + l, equivalent to 3.5σ fluctuation The Kl + l signal yield constitutes a significant observation of this decay. The K*l + l signal yield is not conclusively significant, and we place an upper limit for this channel: ( l l ) < * + 6 B B K 90% C.L. Preliminary K (*) ll Results from BaBar, J. Berryhill, DPF

22 Comparison with Run 1 Result Run 1: BB Kll + 6 ( ) < % C.L. BB Kll * + 6 ( ) < % C.L. Run 1+2: B ( B B ( B + Kl l * + K l l ) = (0.84 ) < ) % C.L. Preliminary All data fully reprocessed for Run 1+2 results: improvements in tracking, vertex detector alignment, etc. resulted in migration of events in/out of signal region. Sensitivity of this analysis is mostly unchanged by the reprocessing (some improvement in Ks modes). Migration of events into/out of signal region checked with control samples results are compatible The probability for a Kll branching fraction at our new value to give our Run 1 result is at the 23% level. K (*) ll Results from BaBar, J. Berryhill, DPF

23 Conclusions We have studied the channels B Kl + l and B K*l + l using 56.4 fb 1 of data at the BaBar experiment at PEPII. We obtain the following results: Preliminary B ( B B ( B + Kl l * + K l l ) = (0.84 ) < ) The statistical significance for B Kl + l is computed to be > 4σ including systematic uncertainties. K (*) ll Results from BaBar, J. Berryhill, DPF

24 Peaking Backgrounds Usually due to particle misidenficiation, e.g.: Β Dπ Kπ Misid d as muons Kµµ background Since misid probability is higher for muons than for electrons, explicit vetoes are applied for the muon modes. Summary of peaking backgrounds as obtained from high statistics Monte Carlo sample. These are included in fit to extract signal. Mode Peaking background B K e e 0:06 +0:7 0:06 + ±! ± B ±! K ± µ + µ 0:5 ± 0:5 B 0 K 0 S e 0:0 +0:1 0:0 e+! B 0! K 0 S µ + µ 0:3 ± 0:3 B 0! K Λ0 e + e 0:3 ± 0:3 B 0! K Λ0 µ + µ 0:8 ± 0:8 B K e e 0:05 +0:3 0:05 + Λ±! ± B ±! K Λ± µ + µ 0:7 ± 0:7 K (*) ll Results from BaBar, J. Berryhill, DPF

25 BaBar Run 1 Analysis (20.7 fb 1 ) Projections of the 2D fit onto m ES after a E cut. K (*) ll Results from BaBar, J. Berryhill, DPF

26 Belle results (29.1 fb 1 ) 4.1 evts evts evts evts evts Bkgd shape fixed from MC K (*) ll Results from BaBar, J. Berryhill, DPF

27 Continuum Background Suppression Continuum suppression: exploit fact that continuum events are more jetlike than BB events R 2 : WF 2 nd moment qq e + e Cos θ thrust : angle of candidate thrust axis Cos θ B : angle of B in CM m Kl : Kl invariant mass Combine optimally using Fisher discriminant Other B e + e Put plot here. Signal B K (*) ll Results from BaBar, J. Berryhill, DPF

28 Generatorlevel q 2 Distributions from B Ke + e FormFactor Models B Kµ + µ Ali et al (solid line) * B Kee + * B K µ + µ Colangelo 1999 (dashed line) Melikhov 1997 (dotted line) Shapes are very similar! K (*) ll Results from BaBar, J. Berryhill, DPF

29 Particle Identification Electrons p* > 0.5 GeV shower shapes in EMC E/p match E/p from E.M.Calorimeter 1 < p < 2 GeV/c Shower Shape e e π π 0.8 < p < 1.2 GeV/c E/p > 0.5 Muons p* > 1 GeV Kaons Penetration in iron of IFR de/dx in SVT, DCH θ C in DRC de/dx from Dch 0.5 < p < 0.55 GeV/c e π e π q c from Cerenkov Detector π 0.8 < p < 1.2 GeV/c e K (*) ll Results from BaBar, J. Berryhill, DPF

30 Kaons with DIRC The DIRC is able to identify particles via a measurement of the cone angle of their emitted Cherenkov light in quartz Quartz bar θ c Active Detector Surface cos θ c = 1 n β p = m βγ Particle Cherenkov light DCH DIRC Provides good π/k separation for wide momentum range (up to ~4 GeV/c) K (*) ll Results from BaBar, J. Berryhill, DPF

31 Quartz bar Particle Particle Identification (DIRC) (Detector of Internally Reflected Cherenkov Light) θ c Active Detector Surface Measure Angle of Cherenkov Cone in quartz cos θ 1 c =, p = nβ mβγ Cherenkov light Transmitted by internal reflection Detected by PMTs K (*) ll Results from BaBar, J. Berryhill, DPF

32 Particle Identification (DIRC) cont. DIRC θ c resolution and Kπ separation measured in data D *+ D 0 π + (K π + )π + decays σ(θ c ) 2.2 mrad >9σ K/π Separation 2.5σ K (*) ll Results from BaBar, J. Berryhill, DPF

33 J/ψ Control Samples: Lepton energy distributions Electron channels Points: data Histo: MC K (*) ll Results from BaBar, J. Berryhill, DPF

34 J/ψ Control Samples: Lepton energy distributions Muon channels Points: data Histo: MC K (*) ll Results from BaBar, J. Berryhill, DPF

35 Data Sample e + e ϒ(4s) BB data used for this talk Run 1: 20.6 fb 1 ( ) 23 million BB events Run 2: 55 fb 1 ( ) 60 million BB events (so far) e + e annihilation 40 MeV below ϒ(4s) Run 1: 2.6 fb 1 Run 2: 6.2 fb 1 This talk: 56.4 fb 1 on peak K (*) ll Results from BaBar, J. Berryhill, DPF

36 Outline Introduction Analysis Overview Control Samples Results K (*) ll Results from BaBar, J. Berryhill, DPF

37 Charmonium Control Samples: Yields in Data vs. Simulation K (*) ll Results from BaBar, J. Berryhill, DPF