Download presentation
Presentation is loading. Please wait.
Published byElvin Todd Modified over 9 years ago
1
CP-Violating Asymmetries in Charmless B Decays: Towards a measurement of James D. Olsen Princeton University International Conference on High Energy Physics Amsterdam, July 24-31, 2002 On behalf of the BaBar Collaboration CP asymmetries in and K + Decay rates for and CP asymmetries in and K Submitted to Phys Rev (hep-ex/0207055) hep-ex/0207065 and hep-ex/0207063 hep-ex/0207068
2
ICHEP 2002 J. Olsen 2 CP Violation in the Standard Model Unitarity Triangle V td V tb * V ud V ub * V cd V cb * B , B J/ K S B DK CP symmetry can be violated in any field theory with at least one irremovable complex phase in the Lagrangian This condition is satisfied in the Standard Model through the three- generation Cabibbo-Kobayashi-Maskawa (CKM) quark-mixing matrix The angles ( , , ) are related to CP-violating asymmetries in specific B decays One down, two to go… ICHEP 2002
3
J. Olsen 3 Observing CP violation at the (4S) At the S), BB pairs are produced in a coherent P-wave Three observable interference effects: CP violation in mixing (|q/p| ≠ 1) (direct) CP violation in decay (|A/A| ≠ 1) (indirect) CP violation in mixing and decay (Im ≠ 0) Observable in time evolution of B B system (assume ) direct CP violation C ≠ 0 indirect CP violation → S ≠ 0
4
ICHEP 2002 J. Olsen 4 CP Violation in B → With Penguins (P): mixing decay Tree (T) Level: Need branching fractions for , , and to get from eff → isospin analysis
5
ICHEP 2002 J. Olsen 5 Overview of Analyses Analysis issues: charmless B decays Rare decays! BR ~ 10 -10 → need lots of data (PEP-II) Backgrounds: Large background from e e → qq → need background suppression Modes with suffer backgrounds from other B decays Ambiguity between and K → need excellent particle ID (DIRC) Time-dependent CP analysis issues: Need to determine vertex position of both B mesons → silicon Need to know the flavor of “other” B → particle ID We use maximum likelihood (ML) fits to extract signal yields and CP-violating asymmetries Kinematic and topological information to separate signal from light- quark background Particle ID to separate pions and kaons The data sample corresponds to 87.9 million BB pairs
6
ICHEP 2002 J. Olsen 6 K/ Separation with the DIRC Cherenkov angle c used in the maximum likelihood fit to distinguish pions and kaons Resolution and K- separation measured in data Kaon sample K hypothesis hypothesis
7
ICHEP 2002 J. Olsen 7 Analysis of B → , K , KK Analysis proceeds in two steps: Time-independent fit for yields and K charge asymmetry Time-dependent fit for S , and C Kinematically select B candidates with m ES, E Suppress qq background with Fisher discriminant Fit yields and charge asymmetry ** p*p*
8
ICHEP 2002 J. Olsen 8 Branching Fraction Results ModeYieldBR (10 -6 ) A CP (K ) B → B → K B → K K KK KK Projections in m ES and E Preliminary KK 87.9 1.1 million BB Submitted to Phys Rev (hep-ex/0207055)
9
ICHEP 2002 J. Olsen 9 Vertex Reconstruction Resolution function parameters obtained from data for both signal and background Signal from sample of fully reconstructed B decays to flavor eigenstates: D * ( , , a ) Background from data sidebands Beam spot Interaction Point B REC Vertex B REC daughters Exclusive B rec reconsctuction B TAG direction TAG tracks, V 0 s z B TAG Vertex B → Example in B → e e → qq z resolution dominated by tag side → same resolution function as charmonium (sin2 ) sample Average z resolution ~ 180 m
10
ICHEP 2002 J. Olsen 10 B Flavor Tagging New tagging algorithm with physics-based neural networks Inputs include leptons, kaons, slow- (from D*), and high-momentum tracks Outputs combined and categorized by mistag prob (w) 5 mutually exclusive categories: Lepton – isolated high-momentum leptons Kaon I – high quality kaons or correlated K and slow- Kaon II – lower quality kaons, or slow- Inclusive – unidentified leptons, poor-quality kaons, high- momentum tracks Untagged – no flavor information is used b sc K-K- ~7% improvement in Q w
11
ICHEP 2002 J. Olsen 11 Tagging in Charmless B Decays Tagging efficiency is very different for signal and bkg Strong bkg suppression in categories with the lowest mistag prob (Lepton/Kaon) Different bkg tagging efficiencies for , K , KK 81/fb B→ h h sample split by tagging category CategorySignal KK KK Lepton9.10.50.40.6 Kaon I16.68.912.77.8 Kaon II19.815.519.414.4 Inclusive20.121.519.221.7 Untagged34.453.648.355.6 Tagging Efficiencies (%) Background
12
ICHEP 2002 J. Olsen 12 Validation of Tagging, Vertexing, and ML Fit K decays are self-tagging T = tag charge Q = kaon charge Float and m d in same sample used to extract CP asymmetries: Fit projection in sample of K -selected events
13
ICHEP 2002 J. Olsen 13 CP Asymmetry Results Fit projection in sample of -selected events qq + K Preliminary Submitted to Phys Rev (hep-ex/0207055)
14
ICHEP 2002 J. Olsen 14 Cross-checks Inspect -selected sample 2-param fit consistent with full fit asymmetry vs. m ES Asymmetry in yields consistent with measured value of C , but does not suggest large direct CP violation Toy MC generated over all allowed values of S and C Expected errors consistent with data No significant bias observed Validated in large samples of signal and background MC events Systematic errors dominated by uncertainty in PDF shapes A vs. m ES in sample of -selected events signal bins
15
ICHEP 2002 J. Olsen 15 Taming the Penguins: Isospin Analysis The decays B , , are related by isospin Central observation is that states can have I = 2 or 0 (gluonic) penguins only contribute to I = 0 I = 1/2) is pure I = 2 I = 1/2) so has only tree amplitude (|A A ) Triangle relations allow determination of penguin- induced shift in Gronau and London, Phys. Rev. Lett. 65, 3381 (1991) But, need branching fractions for all three decay modes, and for B and B separately
16
ICHEP 2002 J. Olsen 16 The Base of the Isospin Triangle: B → Analysis issues: Usual charmless two-body; large qq background, /K separation Potential feeddown from Minimize with tight cut on E ModeYieldBR (10 -6 )A CP B → B → K Simultaneous fit to /K Preliminary Fit region e e → qq hep-ex/0207065
17
ICHEP 2002 J. Olsen 17 Next Side Please: B → Analysis issues: Small signal! feeddown Background suppression: Event shape and flavor tagging to reduce qq Cut on M( ) and E to reduce background, then fix in the fit Significance including systematic errors = 2.5 Data after cut on probability ratio ( ) Preliminary hep-ex/0207063
18
ICHEP 2002 J. Olsen 18 Setting a Bound on Penguin Pollution Can still get information on with only an upper bound on : For example: Grossman-Quinn bound (assume only isospin) Many other bounds on the market Charles, Gronau/London/Sinha/Sinha, etc… Correlations and systematic errors included
19
ICHEP 2002 J. Olsen 19 CP-Violating Asymmetries in B → , Opportunity and challenges In principle, can measure directly, even with penguins Much more difficult than Three-body topology with neutral pion (combinatorics, lower efficiency) Significant fraction of misreconstructed signal events and backgrounds from other B decays Need much larger sample than currently available to extract cleanly We perform a “quasi-two-body” analysis: Select the -dominated region of the K Dalitz plane Use multivariate techniques to suppress qq backgrounds Simultaneous fit for and R. Aleksan et al., Nucl. Phys. B361, 141 (1991)
20
ICHEP 2002 J. Olsen 20 direct CP violation → A CP and C ≠ 0 indirect CP violation → S ≠ 0 C and S are insensitive to CP violation CP Not a CP eigenstate, (at least) four amplitudes contribute: Time evolution includes: CP Time-integrated asymmetry: Q is the charge K is self-tagging: Fit for:
21
ICHEP 2002 J. Olsen 21 Analysis Multi-dimensional ML fit m ES, E, Neural Net (NN), c, t Components Signal and K Misreconstructed signal events Mostly due to wrong photon(s) B backgrounds from b → c and charmless B decays Same lifetime as signal e e → qq Fix B background yields, fit for signal yields and CP asymmetries Validation: signal misreconstructed signal
22
ICHEP 2002 J. Olsen 22 Yields and Charge Asymmetries Preliminary hep-ex/0207068
23
ICHEP 2002 J. Olsen 23 B 0 time-dependent asymmetry Systematic error dominated by uncertainty on B backgrounds Preliminary hep-ex/0207068
24
ICHEP 2002 J. Olsen 24 Summary Hyperactive effort within BaBar to constrain, measure, and otherwise determine Charmless two-body decays: No evidence for large direct or indirect CP violation in Beginning to piece together the necessary inputs to the isospin analysis Measurements of decay rates for and (upper limit) Too early for a significant constraint Charmless three-body decays First measurement of CP asymmetries in and K The next few years will be interesting indeed!
Similar presentations
© 2025 SlidePlayer.com. Inc.
All rights reserved.