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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

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Radiative b decays at the babar experiment l.jpg

Radiative B decays at the BaBar Experiment

Minliang Zhao

(For the BaBar Collaboration)

Massachusetts Institute of Technology

Pheno 2008

Introduction l.jpg

  • 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 s g in the s tandard m odel and beyond l.jpg
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 s g results on br photon spectrum l.jpg
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.







Measured number of B (B-bkg subtracted signal yield) vs Eg

signal region


B s g b recoil method results l.jpg
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|

B s g br summary l.jpg
b->sg Br summary

Cp asymmetry measurement in b s g l.jpg
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 a cp b s g l.jpg
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






  • Consistent with the SM and the minimal flavor violation model in SUSY (T. Hurth, E. Lunghi hep-ph/0312260)

A cp dependence on m xs l.jpg
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


B r w g motivation l.jpg
b->(r/w)g Motivation

Primary interest: Branching fractions:

  • Search for new physics enhancement:

  • Constrain the SM: |Vtb/Vts| Measurement


|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 l.jpg
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

V tb v ts measurement l.jpg
|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 d g sum of exclusive modes l.jpg
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 d g work in progress l.jpg
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 l.jpg

  • 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