Max Baak1 Measurement of small time-dependent CP asymmetries in BaBar Max Baak NIKHEF, Amsterdam Iowa State University, April 2004 Max Baak2Outline 1.Motivation and Historical Introduction to CP-violation C,P,T symmetries, Kaon physics, CKM-matrix Basics of B mesons 2.Measurement of sin(2 + ) using B 0 D (*) The BaBar experiment, analysis technique Analysis results 3.Wrong- to right flavor decay ratio for B 0 /B 0 J/ K *0 Motivation, analysis results Max Baak3 Equal amounts of matter & anti-matter Matter dominates ! Max Baak4 Searches for Anti-Matter in the Universe Observable universe is matter dominated: 1.Absence of intense ray emission due to annihilation of distant galaxies in collision with antimatter 2.Absence of anti-nuclei amongst cosmic rays in our galaxy Alpha Magnetic Spectrometer Max Baak5 Searches for Anti-Matter in the Universe Visible universe very much matter dominated Max Baak6 Cosmic Microwave Background Where did all the Anti-matter go? Almost all matter annihilated with anti-matter but not all Angular Power Spectrum WMAP probe Max Baak7 Where did all the anti-matter go? In 1966, Andrei Sakharov showed that the generation of a netto baryon number requires: 1.Baryon number violating processes (e.g. proton decay) 2.Non-equilibrium state during the expansion of the universe 3.Violation of C and CP-symmetry (e.g. protons and anti-protons decay differently) CP-violation interesting place for further investigation! Max Baak8 Three Discrete Symmetries Parity, P Parity reflects a system through the origin. Converts right-handed coordinate systems to left-handed ones. Vectors change sign but axial vectors remain unchanged x x, p p, but L = x p L Charge Conjugation, C Charge conjugation turns a particle into its anti- particle e e K K Time Reversal, T Changes, for example, the direction of motion of particles t t Max Baak9 Weak Force breaks C and P, but what about CP? 1957: In Weak Interactions, both C and P symmetry are broken maximally! The combined operation, CP, seemed conserved W+W+ e+e+ P C In 1964, Cronin, Fitch et al. observed CP violation in decays of neutral Kaons! Left-handed neutrino Right-handed anti-neutrino Spin direction Momentum Max Baak10 The neutral Kaon system Kaons are mesons (qq bound states) with Strangeness = 1. The neutral kaons are: But K 0 and K 0 are not mass eigenstates. It was long thought that those were given by the following states of definite CP : K S and K L decay to 2 or 3 pions, one can show that the 2 final state has CP 1, and the 3 state has CP 1 CP 1 CP 1 Max Baak11 Discovery of CP violation in K 0 decay Discovery of CP violation in K 0 decay KL+-KL+- Effect is tiny: about 2/1000 In 1964, Cronin, Fitch et al. observed the long lived K L (which was presumed to be CP-odd) decaying into , which is a CP -even final state! This decay occurs only ~0.2% of the time The long lived particle is therefore not a CP-eigenstate, implying Weak Interaction violates CP! Max Baak12 CP is a broken symmetry! Max Baak13 The beauty of B mesons B-mesons are like kaons, but are heavier as they have b-quarks instead of s-quarks. The neutral B-mesons are: 1.Many more decay modes to explore than kaons 2.B-system has larger CP-asymmetries to study! Like kaons, neutral B-mesons mix into each other: ~17% of B 0 and B 0 mix before they decay: m ~ 0.5/ps, B ~ 1.5 ps Max Baak14 Requirements for observation of CP Violation 1.Existence of complex coupling-constants give rise to CP-violation 2.All CP-violating observables possible due to interference between different decay amplitudes involving CP-odd phase difference ( ) CP Spin direction Max Baak15 Weak coupling of quarks: the CKM matrix Weak coupling vertex given by: gV ij g is the universal Fermi weak coupling V ij depends on which quarks are involved For 3 generations, the V ij can be written as unitary 3x3 matrix This matrix is referred to as the CKM matrix (Cabibbo, Kobayashi, Maskawa) One can view this matrix as rotating the quarks from mass eigenstates to weak interaction eigenstates b WW c gV cb d s b d s bd s b d s b = u c tu c t Weak interaction CKM matrix Max Baak16 CP Violation in Standard Model: the CKM matrix Standard Model accomodates a complex non-trivial phase in CKM quark-mixing matrix Unitarity of the CKM Matrix CKM unitarity triangle V td V tb * V ud V ub * V cd V cb * sin2 measured in: B 0 J/ K S,L, K * 0 B 0 c K S B 0 c K S and still to be measured Goal in B-physics: Determine sides and angles of CKM- triangle Max Baak17 Compilation of sin2 Measurements sin(2 ) sin(2 ) with Golden decay mode: B 0 J/ K S/L Consistent with Standard Model World average ~15 significant sin2 = 0.74 0.05 CP asymmetry in B J/ K S,L is large Max Baak18 Measurement of sin(2 + ) using B 0 D (*) 1.Motivation and Historical Introduction to CP violation C,P,T symmetries, Kaon physics, CKM-matrix Basics of B mesons, sin(2 ) 2.Measurement of sin(2 + ) using B 0 D (*) The BaBar experiment, analysis technique Analysis results 3.Wrong- to right flavor decay ratio for B 0 /B 0 J/ K *0 Motivation, analysis results Max Baak19 CP violation through B 0 -B 0 mixing and interference of amplitudes: CP violation proportional to ratio r of amplitudes Small: r |V * ub V cd / V cb V * ud | Relative weak phase from b u transition Relative strong phase CP Violation in B 0 D (*) Strong phase difference CKM Unitarity Triangle Favored amplitude Suppressed amplitude through b u transition u,c,t Max Baak20 B D *- + - pure cosine: r = 0 B D *- + - pure cosine: r = 0 - plus sine term, 5x the expected size in data r = 0.1, = 0 sin(2 + ) = 1 B D *- + time-dependent evolution B 0 (t) D *- + B0B0 Initial state Flavor eigenstate No b u transition B 0 (t) B0B0 Initial state Flavor eigenstate D *- + a) unmixedb) mixed B0B0 B0B0 With b u transition b) a) b) a) CP asymmetry: small additional sine term Smallness of amplitude ratio r greatly reduces sensitivity to sin(2 + ) Max Baak21 sin(2 + ) from time-dependent evolution for D (*) Time evolution for B 0 decays and B 0 decays (R mix ) to D (*) : Need S + and S - together to give (2 + ) and SM: sin(2 + ) ~ 1 Factorization theory: is small SMALL sine terms Max Baak22 PEP-II Asymmetric B-Factory at SLAC B B production threshold 9 GeV e - on 3.1 GeV e + e + e - (4S) BB The (4S) boost in lab frame: = 0.55 Only B d and B u ! Max Baak23 BaBar Detector Cerenkov Detector (DIRC) 144 quartz bars PMs 1.5 T solenoid Electromagnetic Calorimeter 6580 CsI(Tl) crystals Drift Chamber 40 stereo layers Silicon Vertex Tracker 5 layers, double sided strips e + (3.1 GeV) e - (9 GeV) SVT: 97% efficiency, 15 m z hit resolution (inner layers, perp. tracks) SVT+DCH: (p T )/p T = 0.13 % p T % DIRC: K- separation GeV/c GeV/c EMC: E /E = 2.3 % E -1/4 1.9 % z Instrumented Flux Return Max Baak24 (4S): Coherent B 0 B 0 production B 0 B 0 system evolves coherently until one of the Bs decays B mesons have opposite flavour at time t=0 CP/Mixing oscillation clock only starts ticking at the time of the first decay, relevant time parameter t: Half of the time B of interest decays first ( t 100%. With 4x statistics (say 300 /fb), need different approach. [3] [3] D.A. Suprun, C.-W. Chiang, J.L. Rosner, Phys.Rev.D65: (2002) Max Baak64 Dependence of sin(2 + ) on the error on r (*) Including or not including the measured and/or theoretical errors on r(Dpi) and r(D*pi) for fully reconstructed events has the following effect on the 68% CL lower limit on |sin(2 + )|: -No uncertainties on r(Dpi) and r(D*pi): |sin(2 + )| > Measured uncertainties (from D s BR):|sin(2 + )| > Measured uncertainties + 30% theoretical errors: |sin(2 + )| > Measured uncertainties + 60% theoretical errors: |sin(2 + )| > 0.60 In each of these scenarios the minimum 2 value lies at |sin(2 + )|=0.98. The uncertainty has only a small impact on the lower limit -Relative errors on the ratios are small compared to the >100% statistical error on the signal parameters. Will become problem once relative errors get comparable. Max Baak65 Small deviation from usual mixing asymmetry pure cosine: r = 0 plus sine term, 5x the expected size in data Smallness of amplitude ratio r greatly reduces sensitivity to sin(2 + ). SMALL mixing asymmetry Mixing Asymmetry CP violation in terms of mixing asymmetry (eg. D - + ): r = 0.1, = 0 sin(2 + ) = 1 Small disturbance from pure cosine CP asymmetry. Max Baak66 Recent Results for fully reconstructed D (*) BaBar, hep-ex/ submitted to PRL (82 /fb) Belle, hep-ex/ (140 /fb) Lepton tags only Tag-side interference - In theory, coefficients [-0.04,+0.04] - BaBar: systematic ~ 0.5 x statistical error - Large statistical errors on measured coefficients limited sensitivity D*D* DD DD D*D*