A search for the rare decay B + K + νν

Transcription

1 A search for the rare decay B + K + νν Steven Robertson Stanford Linear Accelerator Center

2 Outline Motivation Hadronic B reconstruction B + K + νν selection Sideband samples and backgrounds Results and future prospects October 29, 7 Steven H. Robertson Stanford Linear Accelerator Center 2

3 Introduction Standard Model b s νν process is a flavour-changing neutral-current process occurring via loop and box diagrams Potential for significant enhancement from additional non-sm diagrams Inclusive experimentally Inclusive B X s νν νν is theoretically clean, but very difficult Look instead for exclusive decay modes Br(B + K + νν) SM ~ 4 x 1-6 Best published limit is from the CLEO experiment: Br(B + K + νν) < 2.4 x at 9% CL October 29, 7 Steven H. Robertson Stanford Linear Accelerator Center 3

4 B + K + νν with BABARAR In B-factory environment, B decays are produced via e + e Y(4S) B + B Current BABARAR analysis based on 8.1 fb -1 data set (86.9 ± 1.) x 1 6 BB pairs ν ν B - Y(4S) B + Need to determine that the Kaon is not accompanied by additional (charged or neutral) particles Exclusively reconstruct hadronic B decays in order to identify tracks and clusters associated with the opposing B Κ + October 29, 7 Steven H. Robertson Stanford Linear Accelerator Center 4

5 B reconstruction Exclusively reconstruct B decay modes in hadronic final states: B - D X - K - π + K - π + π K - π + π - π + X - system: : up to three charged tracks (K, π) ) and two additional π Energy substituted mass: cm 2 cm m ( ) ( ) 2 ES = E mass: beam! pb Energy difference: " E = E! E B + K + νν simulation cm B cm beam ε reco = (.131 ±.6 ±.9)% ~114k reconstructed B mesons! Data and background simulation m ES (GeV/c ) October 29, 7 Steven H. Robertson Stanford Linear Accelerator Center 5

6 Event shape cuts Reduce continuum backgrounds by exploiting topological differences between Y(4S) BB and continuum events f e + e - f P B ~32 MeV B e + e - B Require angle between the thrust axes defined by the reconstructed B and by everything else to satisfy: cos θ T <.8 Thrust magnitude: T <.925 (reject residual ττ background) B + K + νν simulation Data and background simulation October 29, 7 Steven H. Robertson Stanford Linear Accelerator Center 6

7 B + K + νν signal selection Signal signature is a single kaon and nothing else recoiling against the opposing reconstructed B Require exactly one track, identified as a kaon,, with charge opposite that of the reconstructed B October 29, 7 Steven H. Robertson Stanford Linear Accelerator Center 7

8 Kaon momentum spectrum Signal kaon has a fairly hard momentum spectrum Require P K >1.5 GeV to further reduce backgrounds Differential decay rate (arbitrary units) Buchalla et al. (hep-ph/6136) Faessler et al. (hep-ph/25287) Phase-space Effective q 2 cut νν invariant mass (q 2 /m B2 ) Some theoretical uncertainty due to modeling of decay form factors New physics also potentially has different spectrum October 29, 7 Steven H. Robertson Stanford Linear Accelerator Center 8

9 Calorimeter energy deposition Limit the amount of calorimeter activity which is not associated with the reconstructed B decay products E extra (GeV) Require no signal-side π candidates Limit also the total extra calorimeter energy: extra < 3 MeV E extra (Sum of all clusters with E>3MeV ) October 29, 7 Steven H. Robertson Stanford Linear Accelerator Center 9

10 Missing momentum Requiring low charged and neutral multiplicity enhances backgrounds from events with unreconstructed particles (i.e. outside of detector acceptance) Require the event missing momentum vector to satisfy cos θ P miss <.8 Signal-B selection efficiency ε sig =(35. ±.5 (Buchalla et al. model).5 ± 1.1)% October 29, 7 Steven H. Robertson Stanford Linear Accelerator Center 1

11 Control samples Use data control samples from several sideband regions to validate MC modeling and background estimates m ES (and large m ES ) E extra >.5 GeV E extra (GeV GeV) Blinding box N trks =2 N trks =3 Signal region Various samples test different aspects of the analysis, e.g. peaking vs combinatorial background MC modeling of E extra endpoint m ES (GeV/c 2 ) October 29, 7 Steven H. Robertson Stanford Linear Accelerator Center 11

12 Control samples E extra distribution in m ES sideband Dominated by continuum backgrounds Onpeak data Onpeak data Offpeak data Offpeak data E extra (GeV) E extra (GeV) E extra distribution in Ntrks=3 sideband Dominated by peaking BB background October 29, 7 Steven H. Robertson Stanford Linear Accelerator Center 12

13 Control sample yields All control samples consistent with Monte Carlo simulation at the level of the available statistics Estimate combinatoric background in signal region by extrapolating m ES sideband into signal region (adds an additional 1. MC/data type B + B - B B uds cc ττ Signal region 1.7 ±.6 m ES 1.1 ± ± ± 1. (adds an additional 1. ±.4 background events) Large m ES 7. ± ± ± ± 2.6 Onpeak data 2.7 ± ± ± ± ± ± 3.9 Total MC Offpeak (8.1 fb data -1 ) 1 1 Continuum.11 ±.5.4 ±.2 3. ±.5.6 ±.2.3 ±.1.5 ±.2 MC (9.58 fb ) October 29, 7 Steven H. Robertson Stanford Linear Accelerator Center 13 E extra 3.3 ±.9.6 ± ± ±1.2 N trks = ± ±.5.6 ± ±1. N trks = ± ± ± ± 1.4

14 Results Observe three events in signal region in 8.7 fb -1 of data consistent with expected background of 2.7 ±.8 Overall selection efficiency of ε tot = (.46 ±.5)% Limit computed using modified frequentist approach (Cousins & Highland) Systematic uncertainties modeled in toy Monte Carlo by Gaussians Limit set at branching fraction value at which 1% of toy experiments give less than observed number of signal candidates Br (B + K + νν) ) < 1.5 x1-4 at 9% CL BABARAR PRELIMINARY (March 23) October 29, 7 Steven H. Robertson Stanford Linear Accelerator Center 14

15 B + K + νν (semileptonic reco) Previous BABARAR analysis based on a semileptonic B - D l - ν X reconstruction sample Search based on 5.7 fb fb -1 of data -1 of Observed two events in signal region (treated as signal for limit determination) ν ν D B - B + ν e - K + expected background ~2.2 Br (B + K + νν) ) < 9.4 x1-5 at 9% CL BABARAR PRELIMINARY (Spring 22) E extra (GeV) October 29, 7 Steven H. Robertson Stanford Linear Accelerator Center 15

16 Combined B + K + νν result Analyses are statistically independent and so can be combined to give a single result: Semileptonic reconstruction: Br (B + K + νν) ) < 9.4 x 1-5 at 9% CL (5.7 fb fb -1-1 ) Hadronic reconstruction: Br (B + K + νν) ) < 1.5 x 1-5 at 9% CL (8.7 fb fb -1-1 ) Combined limit : Br(B + K + νν) < 7. x at 9% CL Standard Model prediction: Br(B + K + νν) ~ 4 x 1-6 October 29, 7 Steven H. Robertson Stanford Linear Accelerator Center 16

17 Conclusions and prospects New B + K + νν search based on 8.1 fb -1 data set (87M BB pairs) using hadronic B reconstruction Observe three events consistent with background prediction of 2.7 ±.8 Br (B + K + νν) ) < 1.5 x 1-4 at 9% CL (Hadronic reconstruction) Combine this result with previous statistically independent result using semileptonic tags to give combined limit of Br (B + K + νν) ) < 7. x 1-5 at 9% CL (Combined limit) Expect significant increase in BABARAR data set over next several years (factor of ~1 2 by 29) Add also B K*νν and neutral B K (*) νν modes Significantly increased sensitivity to b s νν process October 29, 7 Steven H. Robertson Stanford Linear Accelerator Center 17

18 USA [35/276] California Institute of Technology UC, Irvine UC, Los Angeles UC, San Diego UC, Santa Barbara UC, Santa Cruz U of Cincinnati U of Colorado Colorado State Florida A&M U of Iowa Iowa State U LBNL LLNL U of Louisville U of Maryland U of Massachusetts, Amherst MIT U of Mississippi Mount Holyoke College Northern Kentucky U U of Notre Dame ORNL/Y-12 U of Oregon U of Pennsylvania Prairie View A&M Princeton SLAC U of South Carolina Stanford U U of Tennessee U of Texas at Dallas Vanderbilt U of Wisconsin Yale The BABARAR Collaboration 9 Countries 72 Institutions 554 Physicists Canada [4/16] U of British Columbia McGill U U de Montréal U of Victoria China [1/6] Inst. of High Energy Physics, Beijing France [5/5] LAPP, Annecy LAL Orsay LPNHE des Universités Paris 6/7 Ecole Polytechnique CEA, DAPNIA, CE-Saclay Germany [3/21] U Rostock Ruhr U Bochum Technische U Dresden Italy [12/89] INFN and U Bari INFN and U Ferrara Lab. Nazionali di Frascati dell' INFN INFN and U Genova INFN and U Milano INFN and U Napoli INFN and U Padova INFN and U Pavia INFN, SNS and U Pisa INFN, Roma and U "La Sapienza" INFN and U Torino INFN and U Trieste Norway [1/3] U of Bergen Russia [1/13] Budker Institute, Novosibirsk United Kingdom [1/8] U of Birmingham U of Bristol Brunel University U of Edinburgh U of Liverpool Imperial College Queen Mary & Westfield College Royal Holloway, University of London U of Manchester Rutherford Appleton Laboratory October 29, 7 Steven H. Robertson Stanford Linear Accelerator Center 18

19 The BABAR Detector October 29, 7 Steven H. Robertson Stanford Linear Accelerator Center 19

20 B + K + νν (hadronic tags) Limit also the extra neutral energy: E extra < 3 MeV Observe three events in the signal region Predicted background: (2.7 ±.8) Overall efficiency: (.46 ±.5)% B + K + νν simulation Data Data Br (B + K + νν) ) < 1.5 x1-4 at 9% CL BABARAR PRELIMINARY October 29, 7 Steven H. Robertson Stanford Linear Accelerator Center 2

21 Systematic uncertainties Systematic uncertainty estimates derived from MC - data comparison Dominated by MC statistics and tag B reconstruction efficiency Source Generic MC statistics Tag B yield Track reconstruction E extra modeling Total σ bg/bg (%) Source Signal MC statistics Tag B yield Track reconstruction Kaon particle ID E extra modeling MC generator model Total Limit computed using modified frequentist approach (Cousins & Highland) Uncertainties modeled in toy Monte Carlo by Gaussians Limit set as value at which 1% of toy experiments yield less than observed number of signal candidates σε / ε (%) October 29, 7 Steven H. Robertson Stanford Linear Accelerator Center

22 MC and data samples Analysis based on Run1+2 data set (8.7 fb fb -1 Use all available generic MC (VubRemoveOrphans for <1.3.1a) -1 ) MC type B + K + νν B + B - B B uds cc ττ Onpeak data Offpeak data Equivalent lumi (fb - 1 ) 254k events October 29, 7 Steven H. Robertson Stanford Linear Accelerator Center 22

23 Semileptonic tag sample Comparatively high statistics due to large b c l ν branching ratio ~55 events per fb fb -1 Missing neutrino reduces available kinematic constraints use kinematics of D - l combination: Κ L - D B - π + from Y(4s) ν cos" BDl = (2E B E 2 Dl p! m B p 2 B! Dl m 2 Dl ) Possibility of additional photons from D (*) D γ/ π feeding into signal channel October 29, 7 Steven H. Robertson Stanford Linear Accelerator Center 23

24 B + K + νν (semileptonic tags) Based on 5.7 fb fb -1 of BABARAR data -1 of Will be updated to full data set in the near future Signal region defined as a box in defined as a box in E extra and the reconstructed invariant mass of the tag-side D Signal region Sideband region Observe two events in signal region (treated as signal for limit determination) expected background ~2.2 Br (B + K + νν) ) < 9.4 x1-5 at 9% CL BABARAR PRELIMINARY (Spring 22) October 29, 7 Steven H. Robertson Stanford Linear Accelerator Center 24

25 K π separation with the DIRC pions kaons >3σ K π >3 separation up to ~4 GeV October 29, 7 Steven H. Robertson Stanford Linear Accelerator Center 25