Radiative B decays at the BaBar Experiment Minliang Zhao (For the BaBar Collaboration) Massachusetts Institute of Technology Pheno 2008
Introduction • b->(s,d) g are flavor changing neutral currents (FCNC) at the loop level • New particles contribution to the loop are not suppressed by additional factors a/(4p) relative to the standard-model contribution • I will talk about the status of b->sg and b->dg at BaBar respectively
b->sgin the Standard Model and Beyond • The Standard Model diagrams dominated by the top quark (figure a,b) • New physics contributions from new particles (figure c,d) Primary interests Branching Fraction could be enhanced overthe Standard Model rate by newheavy particles. CP asymmetry Standard Model predicts <1%, an enhancement is a sign of new physics. Photon spectrum reveals the QCD effect inside the B meson, in light of the underlying 2-body decay.
b->sg results on Br, photon spectrum • B recoil method • flavor tagging from fully reconstructing one B through one of 1114 exclusive hadronic decay channels • measure photon from other B • ~0.2% efficient for B+ and ~0.1% efficient for B0; but good signal/background ratio • Allows for the determination of the charge, flavor and momentum of both of the B mesons, and thus the photon spectrum can be determined in the rest frame of the signal B. D(*) Xs B B Xhad g Measured number of B (B-bkg subtracted signal yield) vs Eg signal region B-bkg
b->sg B recoil method results arXiv:0711.4889 Accepted in PRD Moments of the photon spectrum: Sensitive to heavy quark parameters b quark mass: mb Fermi motion: mp2 important input for |Vub|and |Vcb|
CP asymmetry measurement in b->sg BaBar new result uses 383M BB samples accumulated through 2007, will reduce a large amount of the statistical error of BaBar '05 result; Current measurements are all statistics limited! BaBar '08 result BaBar '05 result
Preliminary result on ACP(b->sg) Sum of 16 exclusive modes (383M BB pairs) 50% of the B->Xsg Br Hadronic mass (MXs) range: [0.6,2.8] GeV/c2 Good efficiency but huge background: High energy photons that form good p0 and h are vetoed A Boosted DecisionTree with 17 input variables X Xs B B g • Consistent with the SM and the minimal flavor violation model in SUSY (T. Hurth, E. Lunghi hep-ph/0312260)
ACP dependence on MXs • The entire sample was divided into 4 sub-regions according to the hadronic mass MXs, fit separately to check the dependence of ACP on the it; • 4 MXs sub-regions are: [0.6,1.1] [1.1,1.5] [1.5,2.0] [2.0,2.8] GeV/c2 • No dependence was found MXs sub-regions 1-4
b->(r/w)g Motivation Primary interest: Branching fractions: • Search for new physics enhancement: • Constrain the SM: |Vtb/Vts| Measurement Vtd |Vtb/Vts| also obtained from the ratio of Bd and Bs mixing frequencies, but new physics affects them in different ways!
B->r/wg BaBar Result PRL 98, 151802 (2007) (347M BB pairs) Used lepton tagging information to suppress the non-B decay background for the first time 4 dimensional (5 for wg) fit to extract the yields
|Vtb/Vts| Measurement • Experimental uncertainty can be reduced to 5% by B factories • New BaBar r/wgresult will be based on 465M BB samples. • |Vtd/Vts| can also be extracted from the ratio of inclusive b->dg/b->sg BFs |
b->dg sum of exclusive modes • Sum of 7 exclusive final states • B->Xdg (Xd= 2p; 3p; 4p ; ph) • Hadronic mass (MXd) range: [1.0,1.8] GeV/c2 -> Eg [1.9,2.6]GeV Preliminary result presented at LP '07 (arXiv:0708.1652) First evidence for non-resonant hadronic b->dg transitions!
b->dg work in progress • Include BaBar full dataset • Increase hadronic mass upper limit to 2.2GeV/c2 • Including more final modes • Extrapolate measurements to fully inclusive value and obtain limit on |Vtd/Vts|
Summary • Current B-factories provide an indirect search of the physics beyond the Standard Model through the precision measurements in the flavor sector • It is complementary to the direct production of new particles at LHC • The results will place important constraints to both the Standard Model and possible new physics if any are found at LHC