A search for the rare decay B+ g K+nn Steven Robertson Stanford Linear Accelerator Center

Steven H. Robertson Stanford Linear Accelerator Center Introduction The Standard Model flavour-changing neutral-current process b g s nn occurs via loop and box diagrams: Theory prediction: Potential for significant enhancements from non-SM processes Br( B+ g Xs+nn )SM ~ 4.1 x 10-5 27 December 2018 Steven H. Robertson Stanford Linear Accelerator Center

Steven H. Robertson Stanford Linear Accelerator Center B g Xsnn Inclusive B g Xsnn is theoretically very clean, but is also very difficult experimentally Look instead for exclusive decay modes: Can also look for the corresponding neutral B decay modes: B0 g K0s nn , B0 g K*0 nn Best published experimental limit (CLEO): Plenty of room for new physics! Br( B+ g K+nn )SM ~ 4 x 10-6 Br( B+ g K*+nn )SM ~ 1.3 x 10-5 ~20% of inclusive rate Br( B+ g K+nn ) < 2.4 x 10-4 at 90% CL 27 December 2018 Steven H. Robertson Stanford Linear Accelerator Center

Steven H. Robertson Stanford Linear Accelerator Center B+ g K+nn with BABAR In B-factory environment, B decays are produced via Current BABAR analysis based on 80.1 fb-1 data set (86.9  1.0) x 106 BB pairs e+ e- g Y(4S) g B+ B- at ~10.5 GeV in the CM frame 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 Y(4S) B- B+ n K+ 27 December 2018 Steven H. Robertson Stanford Linear Accelerator Center

Steven H. Robertson Stanford Linear Accelerator Center The BABAR Detector 27 December 2018 Steven H. Robertson Stanford Linear Accelerator Center

Hadronic B reconstruction Exclusively reconstruct “tag-B” decay modes in hadronic final states: etag = (0.131  0.006  0.009)% ~ 114k reconstructed B mesons! Energy substituted mass: difference: B- g D0 X- K- p+ K- p+ p0 K- p+ p- p+ B+ g K+nn simulation X- system: up to three charged tracks (K, p) and two additional p0 Data and background simulation mES (GeV/c2) 27 December 2018 Steven H. Robertson Stanford Linear Accelerator Center

Continuum backgrounds Reduce continuum backgrounds by exploiting topological differences between Y(4S) g BB and “continuum” events f e+ e- B PB~320 MeV Thrust magnitude: |T| <0.925 (reject residual tt background) Require angle between the thrust axes defined by the reconstructed B and by everything else to satisfy: |cos qT|<0.8 B+ g K+nn simulation Data and background simulation 27 December 2018 Steven H. Robertson Stanford Linear Accelerator Center

Steven H. Robertson Stanford Linear Accelerator Center B+ g K+nn selection Exploit the low multiplicity of B+ g K+nn signal events Number of tracks B+ g K+nn simulation Data and background simulation Require exactly one charged track (kaon), recoiling against the reconstructed B Eextra (GeV) B+ g K+nn simulation Limit also the total “extra” energy in the calorimeter: Eextra < 300 MeV (Sum of all clusters with E>30MeV ) 27 December 2018 Steven H. Robertson Stanford Linear Accelerator Center

Steven H. Robertson Stanford Linear Accelerator Center Kaon identification Excellent charged kaon identification using the quartz Cerenkov detector (DIRC) pions >3s K–p separation up to ~4 GeV kaons 27 December 2018 Steven H. Robertson Stanford Linear Accelerator Center

Steven H. Robertson Stanford Linear Accelerator Center Kaon momentum Buchalla et al. (hep-ph/0006136) Faessler et al. (hep-ph/0205287) Phase-space nn invariant mass (q2/mB2) Differential decay rate (arbitrary scale) Effective q2 cut B+ g K+nn signal kaon typically has fairly high momentum: Require PK >1.5 GeV to further reduce backgrounds B+ g K+nn simulation Data and background simulation Some theoretical uncertainty introduced due to modeling of decay form factors “New physics” also potentially has different spectrum 27 December 2018 Steven H. Robertson Stanford Linear Accelerator Center

Steven H. Robertson Stanford Linear Accelerator Center Control samples Use data control samples from several “sideband” regions to validate MC modeling and background estimates mES (and “large” mES) Eextra >0.5 GeV Ntrks=2 Ntrks=3 Various samples test different aspects of the analysis, e.g. peaking vs combinatorial background MC modeling of Eextra endpoint Eextra (GeV) “Blinding box” Signal region mES (GeV/c2) 27 December 2018 Steven H. Robertson Stanford Linear Accelerator Center

Steven H. Robertson Stanford Linear Accelerator Center Control sample yields All control sample yields are consistent with Monte Carlo simulation at the level of the available statistics Estimate combinatorial background in signal region by extrapolating mES sideband into signal region (adds an additional 1.0  0.4 background events) MC/data type Signal region mES Large mES Eextra Ntrks=2 Ntrks=3 B+B- 1.7  0.6 1.1  0.5 7.0  1.4 3.3  0.9 17.4  1.9 54.6  3.4 B0B0 1.4  0.6 0.6 0.4 0.9  0.5 3.5  1.0 uds 1.8  1.0 14.0  2.9 2.4 1.2 0.6 0.6 1.2  0.9 cc 11.1  2.6 1.9 1.0 3.1  1.4 tt Onpeak data Total MC (80.1 fb-1) 3 2.7  0.7 7 4.8  1.7 31 33.5  4.2 10 8.8  2.0 21 20.7  2.3 55 62.4  3.9 Offpeak data Continuum MC (9.58 fb-1) 0.11 0.05 0.4  0.2 1 3.0  0.5 0.6  0.2 0.3  0.1 0.5  0.2 27 December 2018 Steven H. Robertson Stanford Linear Accelerator Center

Hadronic tag-B results Observe three events in signal region in 80.7 fb-1 of data consistent with expected background of 2.7  0.8 events Overall selection efficiency: etot = (0.046 0.005)% Branching ratio limit: B+ g K+nn simulation Data and background simulation Br( B+ g K+ nn ) < 10.5 x 10-5 at 90% CL BABAR PRELIMINARY (hep-ex/0304020, March 2003) 27 December 2018 Steven H. Robertson Stanford Linear Accelerator Center

Semileptonic tag-B reconstruction K+ D0 B+ B- e- n Previous BABAR search for B+ g K+ nn based on a semileptonic B- g D0 l-n X0 reconstruction sample Higher reconstruction efficiency, but fewer kinematic constraints Analysis based on 50.7 fb-1 of BABAR data Observed two events in signal region (treated as signal for limit determination) Eextra (GeV) Br( B+ g K+ nn ) < 9.4 x 10-5 at 90% CL BABAR PRELIMINARY (hep-ex/0207069 Spring 2002) 27 December 2018 Steven H. Robertson Stanford Linear Accelerator Center

Results and future prospects Hadronic and semileptonic B reconstruction methods produce statistically independent samples combine the two analyses to give an improved limit: Plans: Extend existing B+ analysis to include K*+nn final state Perform B0 gK0nn and B0 gK*0nn searches using the corresponding hadronic and semileptonic B0 reconstruction samples Add additional new BABAR data Br( B+ g K+nn ) < 7.0 x 10-5 at 90% CL Factor of ~2.5 improvement on best published exclusive limit a Double the effective sensitivity to B g Xsnn a Double present data set by 2004 27 December 2018 Steven H. Robertson Stanford Linear Accelerator Center

Steven H. Robertson Stanford Linear Accelerator Center Backup slides 27 December 2018 Steven H. Robertson Stanford Linear Accelerator Center

The BABAR Collaboration 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 BABAR Collaboration 9 Countries 72 Institutions 554 Physicists 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 [10/80] 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 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 [5/50] 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 27 December 2018 Steven H. Robertson Stanford Linear Accelerator Center

Steven H. Robertson Stanford Linear Accelerator Center DIRC (Detector of Internally Reflecting Cherenkov) 27 December 2018 Steven H. Robertson Stanford Linear Accelerator Center

Steven H. Robertson Stanford Linear Accelerator Center MC and data samples Analysis based on Run1+2 data set (80.7 fb-1) Use all available generic MC (VubRemoveOrphans for <10.3.1a) MC type Equivalent lumi (fb-1) B+ g K+nn 254k events B+B- 288 B0B0 277 uds 131 cc 130 tt 137 Onpeak data 80.07 Offpeak data 9.58 27 December 2018 Steven H. Robertson Stanford Linear Accelerator Center

Systematic uncertainties Systematic uncertainty estimates derived from MC - data comparison Dominated by MC statistics and tag B reconstruction efficiency Source se / e (%) Signal MC statistics 4.9 Tag B yield 7 Track reconstruction 1.3 Kaon particle ID 2 Eextra modeling MC generator model 3 Total 10 Source s bg/bg (%) Generic MC statistics 27 Tag B yield 7 Track reconstruction 5 Eextra modeling 2 Total 29 Limit computed using modified frequentist approach (Cousins & Highland) Uncertainties modeled in “toy” Monte Carlo by Gaussians Limit set as value at which 10% of toy experiments yield less than observed number of signal candidates 27 December 2018 Steven H. Robertson Stanford Linear Accelerator Center

Steven H. Robertson Stanford Linear Accelerator Center 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 qPmiss|<0.8 Signal-B selection efficiency esig =(35.0 0.5  1.1)% (Buchalla et al. model) 27 December 2018 Steven H. Robertson Stanford Linear Accelerator Center

Calorimeter energy deposition Limit the amount of calorimeter activity which is not associated with the reconstructed B decay products Require no signal-side p0 candidates Limit also the total extra calorimeter energy: Eextra < 300 MeV (Sum of all clusters with E>30MeV ) Eextra (GeV) 27 December 2018 Steven H. Robertson Stanford Linear Accelerator Center

Steven H. Robertson Stanford Linear Accelerator Center Control samples Eextra distribution in mES sideband Dominated by continuum backgrounds Onpeak data Onpeak data Offpeak data Eextra (GeV) Offpeak data Eextra distribution in Ntrks=3 sideband Dominated by peaking BB background Eextra (GeV) 27 December 2018 Steven H. Robertson Stanford Linear Accelerator Center

Semileptonic tag sample Comparatively high statistics due to large b g c l n branching ratio ~5500 events per fb-1 Missing neutrino reduces available kinematic constraints use kinematics of D0 - l combination: Possibility of additional photons from D(*)0 g D0 g/ p0 feeding into signal channel B- n D0 L- K- p+ from Y(4s) 27 December 2018 Steven H. Robertson Stanford Linear Accelerator Center

B+ g K+nn (semileptonic tags) Based on 50.7 fb-1 of BABAR data Will be updated to full data set in the near future Signal region defined as a box in Eextra and the reconstructed invariant mass of the tag-side D0 Observe two events in signal region (treated as signal for limit determination) expected background ~2.2 Signal region Sideband region Br (B+ g K+ nn) < 9.4 x10-5 at 90% CL BABAR PRELIMINARY (Spring 2002) 27 December 2018 Steven H. Robertson Stanford Linear Accelerator Center