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Rare decays at Belle. Peter Kri ž an University of Ljubljana and J. Stefan Institute. Belle @ KEK-B in Tsukuba. Tsukuba -san. Belle. KEKB. ~ diameter 1 km. Belle spectrometer at KEK-B.  and K L detection system (14/15 layers RPC+Fe). Aerogel Cherenkov Counter

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Rare decays at Belle


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    1. Rare decays at Belle Peter Križan University of Ljubljana and J. Stefan Institute Seminar, SLAC

    2. Belle @ KEK-B in Tsukuba Tsukuba-san Belle KEKB ~diameter 1 km Seminar, SLAC

    3. Belle spectrometer at KEK-B  and KL detection system (14/15 layers RPC+Fe) Aerogel Cherenkov Counter (n=1.015-1.030) 3.5 GeV e+ Silicon Vertex Detector (4 layers DSSD) Electromag. Cal.(CsI crystals, 16X0) 8 GeV e- Central Drift Chamber (small cells, He/C2H6) ToF counter 1.5T SC solenoid Accumulated data sample >600 M BB-pairs Seminar, SLAC

    4. Contents • FCNC bs decays • bsg : CP violation • Measurement of Afb vs q2 in B  K* l+ l- decays • Decays with >1 neutrino • Purely leptonic decay: B-t- nt • B  K(*)n n • Upgrade plans • PID in the forward region Seminar, SLAC

    5. Why FCNC decays? Flavour changing neutral current (FCNC) processes (like bs, bd) are fobidden at the tree level in the Standard Model. Proceed only at low rate via higher-order loop diagrams. Ideal place to search for new physics. Seminar, SLAC

    6. mb C7 ms ms mb p+ p- B vertex IP profile g g BXsg CP Asymmetry • Sensitive to NP – right handed currents • Theoretically clean. • Standard Model “~Zero”. • g is polarized, and the final state is almost flavor specific. • Helicity flip ofgsuppressed by ~ms/mbS ~ 0.02 • QCD corrections  S remains small (Grinstein, Pirjol, PRD 73 014013; Matsumori, Sanda, PRD 73 014013) • Time dependent CPV requires vertex reconstruction with KSp+p- g KS trajectory Vertex recon. eff. at Belle 51% (SVD2), 40% (SVD1) Seminar, SLAC

    7. B0KSp0g time dependent CPV • Atwood, Gershon, Hazumi, Soni, • PRD71, 076003 (2005) M(KSp0) < 1.8GeV/c2 • NP effect is independent of the resonance structure. Belle: data sample 535MBB • Three M(KSp0) regions(MR1:0.8-1.0GeV/c2, MR2:1.3-1.55, MR3: rest with M<1.8GeV/c2) • 112.5+-12.0, 28.7+-7.1, 35.2+-10.0 events in MR1,2,3. All events: S= -0.10±0.31±0.07 A= -0.20±0.20±0.06 Good tag (0.5<r<1.0) Seminar, SLAC

    8. B0KSp0g time dependent CPV Results: Belle (275M BB) hep-ex/0507059 S(BK*g, K*KSp0)=-0.01 ±0.52 ±0.11 S(B KSp0g)=0.08 ±0.41 ±0.10 BaBar (232M BB) PRD 71 (2005) 0501103 S(BK*g, K*KSp0)=-0.21±0.40 ±0.05 Belle (535M BB) hep-ex/0600818 S(BK*g, K*KSp0)=-0.32 +0.36-0.33 ±0.05 S(B KSp0g)=-0.10 ±0.31 ±0.07 Prospects: Add more modes:BKSfg(with angular analysis), higher K resonances, BKShg(recent observation by BaBar),... 5ab-1 50ab-1 Acpmix(BK*g, K*KSp0) 0.14 0.04 Acpdir(BXsg) 0.011 0.005 Seminar, SLAC

    9. Acp(BXsg) vs SUSY models 50ab-1 5ab-1 Mixing CPV Direct CPV U(2) tanb=30 mSUGRA tanb=30 U(2) tanb=30 mSUGRA tanb=30 Acpdir Acpmix SU(5)+nR tanb=30 non-degenerate SU(5)+nR tanb=30 degenerate SU(5)+nR tanb=30 non-degenerate SU(5)+nR tanb=30 degenerate Seminar, SLAC T. Goto, Y.Okada, Y.Shimizu,T.Shindou, M.Tanaka hep-ph/0306093, also in SuperKEKB LoI

    10. B  K* l+ l- Important for further searches for the physics beyond SM Ci: Wilson coefficients Seminar, SLAC

    11. Particularly sensitive: forward-backward asymmetry in K* l+l l- l+ B B q q K* K* l+ l- Backward event Forward event [γ* and Z* contributions in BK* l l interfere and give rise to forward-backward asymmetries c.f. e+e- + -] Seminar, SLAC

    12. Sample used for AFB(BK*ll)(q2) PRL 96, 251801 (2006) Treat q2, cos(θ) dependence of bkgs. Sample for BK* l l: 113±13 events Unbinned fit to the variables q2 (di-lepton invariant mass) and cos(θ) for the BK* l l data. Fit parameters A9/A7 and A10/A7(Ai = leading term in Ci) Seminar, SLAC

    13. Control sample BKll BK l l control sample: 96±12 events Consistent with 0 Integrated asymmetry: Seminar, SLAC

    14. * Constraints on Wilson coefficients from AFB(BK* l l)(q2) Projections of the full fit to q2, cos(θ) Integrated FB asymmetry BaBar: AFB >0.55 (@ 95% CL) J/y y’ Observed integrated AFB rules out some radical New Physics Models with incorrect signs/magnitudes of C9 and C10(red and pink curves) Seminar, SLAC

    15. Results of the unbinned fit to q2 and cos(θ) distributions for ratios of Wilson coefficients A10/A7 SM A9/A7 SM A10/A7 Best fit A9/A7 |A7| constrained from bs to be close to SM at 95% C.L. Seminar, SLAC

    16. AFB(BK* l+ l-)[q2] at Super B Factory q02 Precision with 5ab-1 dC9 ~ 11% dC10 ~14% dq02/q02 ~11% 4AFB zero-crossingq02 will be determined with 5% error with 50ab-1 Seminar, SLAC

    17. Purely leptonic decay Bt n • Challenge: B decay with at least two neutrinos • Proceed via W annihilation in the SM. • Branching fraction • Provide information of fB|Vub| • |Vub| from BXu l n fB cf) Lattice • Br(Btn)/Dmd |Vub| / |Vtd| • Expected branching fraction |Vub| = (4.39 ± 0.33)×10-3(HFAG) fB = (216 ± 22) MeV (lattice) • BF(B  t nt) = (1.59 ± 0.40)×10-4 Seminar, SLAC

    18. H+/W+ t+ Charged Higgs contribution to Bt n ~ e0 =SUSY corrections to b Yukawa coupling Br(SM) ~ 1.59 x 10-4 Seminar, SLAC

    19. Full Reconstruction Method • Fully reconstruct one of the B’s to • Tag B flavor/charge • Determine B momentum • Exclude decay products of one B from further analysis Decays of interest BXu l n, BKn n BDtn, tn B e- (8GeV) e+(3.5GeV) Υ(4S) p B full reconstruction BDpetc. (0.1~0.3%) Offline B meson beam! Powerful tool for B decays with neutrinos Seminar, SLAC

    20. Fully reconstructed sample Belle (447M BB) 4.12x105 B0B0 + 6.80x105 B+B- Seminar, SLAC  SLAC seminar by Ilija Bizjak, Nov 2005

    21. Event candidate B- t- nt Seminar, SLAC

    22. Bt n Submitted to PRL, hep-ex/0604018 • decay modes • Cover 81% of t decays • Efficiency 15.8% Event selection • Main discriminant: extra neutral ECL energy Obtained EECL Fit to Eresidual 17.2+5.3signal events. 3.5σsignificance including systematics -4.7 Seminar, SLAC

    23. Consistency Check with BD*ln • Extra neutral energy EECL validationwith double-tagged sample (control sample): • Btag is fully reconstructed • Bsig is a semileptonic decay Calibration data B+ D(*)0 X+ (full reconstruction) B- D*0 l-n D0p0 K-p+ K-p+ p-p+ Purity ~ 90% Extra energy in the calorimeter Seminar, SLAC

    24. Byields broken down by  decay mode (stat sign. only) For all modes, the background is fitted with a 2nd order polynomial plus a small Gaussian peaking component. MC studies: small peaking bkg in the 0 and 0 modes. Seminar, SLAC

    25. B  t nt  Product of B meson decay constant fB andCKM matrix element |Vub| Using |Vub| = (4.39 ± 0.33)×10-3 from HFAG First measurement of fB! fB = (216 ± 22) MeV (an unquenched lattice calc.) [HPQCD, Phys. Rev. Lett. 95, 212001 (2005) ] 15% 15% = 13%(exp.) + 8%(Vub) Seminar, SLAC

    26. Impact of B- t- nt • Use BF(B  t nt) with Dmd  constraint in the (r,h) plane Seminar, SLAC

    27. Charged Higgs limits from B- t- nt If the theoretical prediction is taken for fB limit on charged Higgs mass vs. tanb rH=1.130.51 Seminar, SLAC

    28. Bt nprospects • Expected precisionat Super-B • 13% at 5 ab-1 • 7% at 50 ab-1 • Search with D(*) l n tag will help.  BaBar 232M BBPRD73 (2006) 057101 • Tag eff ~ 1.75 x 10-3 • Signal selection eff. ~31% • Similar S/N to Belle (full recon. sample) Seminar, SLAC

    29. If D|Vub| = 0 & DfB = 0 Future Prospects: B 95.5%C.L. exclusion boundaries (for BFobs = BFSM) 50ab -1 rH

    30. B  K(*)nn • BK(*)nnis a particularly interesting and challengingmode (with B τνas a small background), theoretically clean • Experimental signature:B  K + nothing • The “nothing” can also be light dark matter with mass oforder 1 GeV. Direct dark-matter searches cannot see the M<10 GeV region. • SM prediction for B+K+nn: (3.8+1.2) x 10-6 • B τν analysis is a proof that such a one prong decay can be studied at a B factory • Present limits: • BaBar (89M BB): BF(B+K+νν) < 52 x 10-6PRL 94 (2005)101801 • Belle (277M BB): BF(B+K+νν) < 36 x 10-6hep-ex/0507034 -0.6 Seminar, SLAC

    31. Motivation for BK*(bs with 2n’s) SM: BF(BK*) ~1.3 x 10-5(Buchalla, Hiller, Isidori) PRD 63, 014015 BSM: New particles in the loop Other weakly coupled particles: light dark matter Seminar, SLAC

    32. Motivation for BK* - 2 DAMA NaI 3s Region CDMS 04 CDMS 05 The experimental signature is BK + Nothing The “nothing” can also be light dark matter (mass of order (1 GeV)) C. Bird et al PRL 93 201803 Direct dark-matter searches cannot see M<10 GeV region Seminar, SLAC

    33. Search for BK* 535 x 106 B Bbar pairs BELLE-CONF-0627 (1.7σ stat. significance) Sideband = 19 MC expectation = 18.73.3 SM (Buchalla, Hiller, Isidori) 1.3 x 10-5 Extra Calorimeter Energy (GeV) (at 90% C.L) Seminar, SLAC

    34. Event display for a BK* candidate due to an identified background (BK*γ) Tag Side B  D+ a1- D+  K-π+π+ a1-  ρ0π- , ρ0 π+π- π+ K- Missing mass ~ 0 (Hard photon is lost in the barrel-endcap calorimeter gap) γ MC: Expected bkg from this source ~0.3 evts. Seminar, SLAC

    35. B- K-n n prospects MC extrapolation to 50 ab-1 5s Observation of B± K±n n SM prediction: G.Buchalla, G.Hiller,G.Isidori (PRD 63 014015) Extra EM calorimeter energy Fig. from SuperKEKB LoI Seminar, SLAC

    36. Summary • Radiative,electroweak and tauonicB decays are of great importance to probe new physics. • We are starting to measure B tn,Knn, Dtn, AFB(K*ll), ACP(Kp0g) etc. at the current B factories. Hot topics in the coming years ! • For precise measurements, we need a Super-B factory! Observe K(*)nn, zero crossing in AFB, D(*)tn  Expected precision (5ab-150ab-1); • Br(tn): 13%7% • Br(D(*)tn): 7.9%2.5% • q02 of AFB(K*ll): 11%5% • ACP(Kp0g) tCPV: 0.140.04  Substaintial upgrade of the detector is mandatory Seminar, SLAC

    37. Belle Upgrade for Super-B CsI(Tl) 16X0 SC solenoid 1.5T g pure CsI (endcap) Aerogel Cherenkov counter + TOF counter Si vtx. det. 4 lyr. DSSD g “TOP” orDIRC + Aerogel RICH g 2 pixel/striplet lyrs. + 4 lyr. DSSD Tracking + dE/dx small cell + He/C2H5 • remove inner lyrs. fast gas+Si r<20 cm m / KL detection 14/15 lyr. RPC+Fe g tile scintillator New readout and computing systems Seminar, SLAC

    38. Belle upgrade – side view Two new particle ID devices, both RICHes: Barrel: TOP or focusing DIRC Endcap: proximity focusing RICH Seminar, SLAC

    39. PID upgrade in the endcap improve K/p separation in the forward (high mom.) regionfor few-body decays of B mesons good K/p separation for b -> dg, b -> sg improve purity in fully reconstructed B decays low momentum (<1GeV/c) e/m/p separation (B ->Kll) keep high the efficiency for tagging kaons Seminar, SLAC

    40. Proximity focusing RICH in the forward region K/p separation at 4 GeV/c qc(p) ~ 308 mrad ( n = 1.05 ) qc(p)– qc(K) ~ 23 mrad dqc(meas.) = s0 ~ 13 mrad With 20mm thick aerogel and 6mm PMT pad size  6s separation with Npe~10 Seminar, SLAC

    41. Beam test: Cherenkov angle resolution and number of photons Beam test results with 2cm thick aerogel tiles: excellent, >4s K/p separation NIM A553 (2005) 58 but: Number of photons has to be increased. Seminar, SLAC

    42. How to increase the number of photons? What is the optimal radiator thickness? Use beam test data on s0 and Npe s0 Npe Minimize the error per track: Optimum is close to 2 cm Seminar, SLAC

    43. Radiator with multiple refractive indices How to increase the number of photonswithout degrading the resolution? measure overlaping rings“focusing” configuration normal Seminar, SLAC

    44. Beam tests Clear rings, little background Photon detector: array of 16 H8500 PMTs Seminar, SLAC

    45. Focusing configuration NIM A565 (2006) 457 4cm aerogel single index 2+2cm aerogel Seminar, SLAC

    46. Focusing configuration – n2-n1 variation Single photon resolution refractive index difference • upstream aerogel: d=11mm, n=1.045 • downstream aerogel layer: vary refractive index • measured resolution in good agreement with prediction • wide minimum allows some tolerance in aerogel production NIM A565 (2006) 457 Seminar, SLAC

    47. Multilayer extensions Resolution per track Single photon resolution Number of detected photons Npe s0 Multiple layer radiators combined from 5mm and 10mm tiles Cherenkov angle resolution per track: around 4.3 mrad →p/K separation at 4 GeV better than 5s Seminar, SLAC

    48. Photon detector candidate: MCP-PMT MCP-PMT multi-anode PMTs BURLE 85011 MCP-PMT: • multi-anode PMT with two MCP steps • 25 mm pores • bialkali photocathode • gain ~ 0.6 x 106 • collection efficiency ~ 60% • box dimensions ~ 71mm square • 64(8x8) anode pads • pitch ~ 6.45mm, gap ~ 0.5mm • active area fraction ~ 52% • Tested in combination with multi-anode PMTs • sJ~13 mrad (single cluster) • number of clusters per track N ~ 4.5 • sJ~ 6 mrad (per track) • -> ~ 4 s p/K separation at 4 GeV/c • 10 mm pores required for 1.5T • collection eff. and active area fraction should be improved • aging study should be carried out Seminar, SLAC

    49. TOF capability 2GeV/c p/K: Dt ~ 180ps 4GeV/c p/K: Dt ~ 45ps Cherenkov photons from aerogel START track STOP Cherenkov photons from PMT window aerogel MCP-PMT With the use of a fast photon detector, a proximity focusing RICH counter can be used also as a time-of-flight counter. Cherenkov photons from two sources can be used: • photons emitted in the aerogel radiator • photons emitted in the PMT window Beam tests: study timing properties of such a configuration. Seminar, SLAC

    50. TOF capability: photons from the ring Time resolution for Cherenkov photons from the aerogel radiator:50psagrees well with the value from the bench tests Resolution for full ring (~10 photons)would be around 20 ps Distribution of hits on the MCP-PMT (13 channels were instrumented) - left Corrected distribution using the tracking information - left Seminar, SLAC