The Mystery of Vus from Tau decays. Swagato Banerjee

Transcription

1 The Mystery of Vus from Tau decays Swagato Banerjee

2 Flavor generations in the Standard Model Vij: Mixing between Weak and Mass Eigenstates 2

3 Unitarity of the CKM matrix 1!! 3! 1! 2! 3! 2 1 V ud = ± (from nuclear β and neutron decays) V ub = (3.93 ± 0.36) 10 3 (from B X u lν decays) V us = ± T. Eronen, et. al., PRL 100, (2008) Particle Data Group, Phys. Lett. B 667, 1(2008) 3

4 Kl3 decays: Approaches to Vus V us f + (0) Kl2 decays: V us V ud F K Fπ Hyperon decays: V us f 1 (0) τ decays: m s, V us 4

5 τ Hadronic Width Vud 2 Vus 2 5

6 τ Hadronic Width Dress the QCD contributions using Optical Theorem: Spectral Moments: R kl τ = 1 0 dz(1 z)k z l dr τ dz, z = q2 m 2 τ Using Finite Energy Sum Rules (FESR), SU(3) Symmetry Breaking Corrections have been calculated upto O(α 3 ): s 6

7 From A. Pich s Kaon07 Talk with fixed m s from Lattice Average 7

8 Vus with Fixed ms QCD Sum rules, Lattice: M. Jamin et. al., PRD74, (2006) Smallest uncertainity on (0,0) moment E. Gamiz et. al. (hep-ph/ ) (Tau06) δr τ,theory << R τ modest (δr τ,theory ) 13% gives 0.5% error on V us 8

9 Route to Vus from τ decays Direct measurement averaged with B e from B µ & τ τ : Be uni = ( ± 0.032)% R τ = (3.640 ± 0.010) Strange τ decays: Measure them all... R τ,non strange = R τ R τ,strange

10 Status before B-Factories 10

11 B-Factories at s ~ GeV PEP-II at SLAC 9 GeV (e! )! 3.1 GeV (e + ) Peak luminosity: 1.2!10 34 cm!2 s!1" Belle BaBar KEKB at KEK 8 GeV (e! )! 3.5 GeV (e + ) Peak luminosity: 1.7!10 34 cm!2 s!1" 11 K. Honscheid, Ohio State University, C2CR 2007

12 B-Factories are also τ-factories Million B Mesons CLEO II CLEO II.5 Integrated Luminosity 850 fb -1 Belle 470 fb -1 BaBar

13 The BaBar Detector Cerenkov Detector (DIRC) 144 Quartz bars and PMTs 1.5T solenoid Electromagnetic Calorimeter 6580 CsI(Tl) Crystals e + (3.1GeV) Drift Chamber 40 layers e - (9GeV) Instrumented Flux Return Resistive Plate Chambers and Limited Streamer Tubes Silicon Vertex Tracker 5 layers, double strip 13

14 Particle Identification at BaBar: Tracking Tracking detectors (SVT & DCH) measure momentum & de/dx 14

15 Particle Identification at BaBar: DIRC DIRC measures velocity of charged particles Excellent K/π separation Index of Refraction =

16 Particle Identification at BaBar: Calorimetry EMC measures photon energy and provides electron identication CsI (Tl) Crystal Pion rejaction factor Longitudinal Shower Depth Improved e/π separation p! (0.2, 0.4) GeV/c E/p, LAT, "L E/p, LAT Electron ID efficiency 16

17 Particle Identification at BaBar: Muons Muon tracks from beam collision IFR Barrel Entire IFR Barrel replaced with Limited Streamer Tubes (2007) Outer layer Brass-slabs Iron slab 17

18 τ-pair Event Topology 18

19 τ K π 0 ν τ decays L = 230 fb 1 PRD76-RC, (2007) 19

20 τ h h h + ν τ decays PRL 1001, (2008) L = 342 fb 1 20

21 τ h h h + ν τ decays K 1 (1270) (K 0 π, ρ 0 K ) a 1 (1260) ρ0 π K 1 (1400) K 0 π (φπ, ρ π, K 0 K ) (φk ) 21

22 τ h h h + ν τ decays 22

23 τ h h h + ν τ decays 23

24 OZI & Cabibbo suppressed modes PRL 1001, (2008) L = 342 fb 1 24 PL B643, 5(2006) L = 401 fb 1

25 τ K 0 π ν τ decays ICHEP08 Preliminary : [hep-ex] L = 385 fb 1 Reconstruct K 0 using K 0 S π+ π mode Efficiency ~ 3.2%, Purity ~ 80% Remaining background Fractions: > 5 PDG'06: B(τ π KS 0K0 L ν τ ) = (0.112 ± 0.030)% } Reduce background Instead, use Isospin relations & B(τ π K K + ν τ ) = ( ± )% error by factor of ~2 25

26 τ K 0 π ν τ decays B(τ K 0 π ν τ ) (0.840 ± ± 0.023)% (Preliminary) [hep-ex] (0.808 ± ± 0.026)% PLB654, 65 (2007) L = 385 fb 1 L = 351 fb 1 New World Average: (0.835 ± 0.022)% (includes S = 1.4) 26

27 (Correlations between measurements of different channels not yet available) Status of Strange τ decays Xus B World Averages (%) K [τ decay] ± ([K µ2 ]) (0.715 ± 0.003) K π ± K 0 π ± (S = 1.4) K π 0 π ± K 0 π 0 π ± K π π ± (S = 1.9) K η ± (S = 1.8) ( K3π) (est d) ± K 1 (1270) K ω ± ( K4π) (est d) ± K η ± (S = 2.0) K φ ± TOTAL ± (2.858 ± 0.071) (Preliminary) ICHEP 08 27

28 Status of Vus from τ Decays using FESR K.Maltman, C.E.Wolfe, S.B., I.M.Nugent, J.M.Roney (arxiv: [hep-ph]) Non-τ Data τ Data nonspectral weights (0,0) weight OPE convergence has sizeable instability [s0 ~ 2.5 GeV 2 ], which can be improved using non-spectral weights (based on ALEPH τ data) Update does not include new B(τ K 0 S π ν τ ) 28 Discrepency w.r.t. Unitarity still ~ 3!

29 Another way to measure Vus from τ decays Electroweak corrections cancel Independent of convergence of OPE All non-perturbative QCD effects encapsulated as ratio of meson decay constants for which precise Lattice QCD prediction exists: fk/fπ = ± E. Follana, et. al., PRL 100, (2008) (HEPQCD + UKQCD Collaboration) Vud = ± used throughout this talk T. Eronen, et. al., PRL 100, (2008) 29

30 Another look at 1-vs-3 τ decays Classify using Particle Identifcation Reduce normalization uncertainity by measuring: 30

31 Reconstruct τ direction [hadronic modes only] signal Reject Reject 31

32 1-prong Leptonic Modes L = 467 fb 1 32

33 1-prong Hadronic Modes L = 467 fb 1 33

34 Ratios of 1-prong modes 34

35 1-prong Branching Fractions 35

36 Lepton Universality and Vus = ( ± ± ) BaBar 08 (Preliminary) V us = ± (agrees with Unitarity) 36

37 Status of Lepton Universality 37

38 Status of Vus 38

39 Future Plans Measure all strange spectral functions for simultaneous Vus & ms extraction Large τ samples available at B-Factories... but significant Experimental Challenges: Luminosity at B-Factories known to ~ 1% as compared to ~ 0.1% at LEP Systematic Uncertainty on π 0 efficiency is quite large ~ 3% PID Systematic Uncertainties are relatively high: limited by control sample statistics New τ sub-group formed in the Heavy Flavour Averaging Group (HFAG) to consolidate experimental efforts and make averages including correlations 39

40 Summary Vus from Rτ, strange lower than other measurements by ~ 3σ: Experimental Caveats: estimates only for (K3π) -, (K4π) - ; missing correlations. Theoretical Caveats: convergence of OPE; calculation of weights need data. Vus from (τ - K - ν/τ - π - ν) consistent with Unitarity prediction, but individually both the branching fractions are slightly lower than Universality predictions. BaBar 1-prong Branching Ratio measurements are consistent with Charged Current Lepton Universality within at most ~ 2.5σ. (τ - µ - νµντ) & (τ - π - ν) Branching Ratios are as precise as PDG average. (τ - K - ν) Branching Ratio is more precise than PDG average by factor of 2. New Physics? Other related (?) indications : The fd + agrees with lattice, which has very small errors, while the fds average (both measurements currently dominated by CLEOc) is 3σ above the lattice predictions Neutral/Charged current ratio in muon (anti-)neutrino nucleon scattering is 3σ lower than the SM prediction (NuTev anomaly) 40