Radiative and Electroweak Penguin Decays of B Mesons

Transcription

1 Radiative and Electroweak Penguin Decays of B Mesons Jeffrey D. Richman University of California, Santa Barbara BABAR AR Collaboration 11 th International Conference on B Physics at Hadron Machines Oxford, Sept. 28, 2006

2 Outline Overview: a little history, physics goals, and challenges. B + ρ + γ, B 0 ρ 0 γ, B 0 ωγ and measurement of V td /V ts B Kl + l - and B K* l + l - : search for new physics using the lepton forward-backward asymmetry Inclusive B X s γ: branching fraction measurements and extraction of heavy-quark expansion parameters from the E γ spectrum. Conclusions My apologies for not covering all results on radiative/electroweak penguin decays in this talk!

3 Radiative penguin decays of B mesons Observation of B K* γ CLEO II (1993): Loops in B decays! PRL 71, 674 (1993): cited >500 times! Now it s a physics program! BB * ( Kγ ) B(10 ) Rare, but not all that rare! * M( K γ )

4 What can we learn from b s, d transitions? Flavor-changing neutral currents probe SM at 1-loop level. b uct,, d W, γ sd (dominated by t quark) d New physics can affect the amplitudes at leading order! b d γ, Z uct,, W d s (+ W + W - box diagram) + As for b c or b u semileptonic decays, the amplitude in EM/EW penguins is factorizable (only one hadronic current).

5 What can we learn from b s,d transitions? b d V tb W t γ * V td d d Presence of only single hadronic current allows us to isolate non-perturbative QCD parameters in well-defined way. Can be related to same parameters for other decays. Exclusive decays: decay form factors f i (q 2 ). b s transition is similar to b u (heavy to light) Inclusive decays: parameters of heavy-quark expansion (m b, μ π2, ) Can extract information on CKM elements if info on hadronic parameters is available from data, theory, or both. b d V ub W ν u d

6 Observation of b d γ and Measurement of V td /V ts B b uct,, u W γ d u ρ + W annihilation diagram (small) b uct,, W 2 ( 2 2) 3 2 mb m ρ ργ V td ρ T 1 ( 2 2 ) ts mb m * K 1 BB ( ) (0) * = 3 * K +Δ BB ( Kγ ) V T (0) T * (0) K 1 ξ = ± T ρ 1 (0) Ball and Zwicky, JHEP 0604, 046 (2006) 0 B d 2 1/ξ ( 1 R) Δ R = 0.1± 0.1 Ali, Lunghi, Parkhomenko, PLB 595, 323 (2004) ( B ρ γ ) 2 ( B 0 ρ 0 γ ) 2 ( B 0 ωγ ) Γ = Γ Γ I-spin (ρ), quark model (ω). Expect small I-spin violation: (1.1+/-3.9)%. γ d d ρ 0, ω

7 Measurement of b d γ Decays (Belle) Belle, PRL 96, (2006); 386 M BB. N = 8.5 sig N sig = N sig = Signal continuum background B K*γ Good particle ID is critical in this measurement to suppress B K* γ feeddown. N sig = 36.9

8 Measurement of b d γ Decays ( BABAR, hep-ex/ , 347 M BB Decays (BABAR) projections of 4-D fit signal + bkgnd bkgnd B + ρ + γ N = sig 12.6 B + ρ + γ signal B 0 ρ 0 γ B 0 ρ 0 γ N = sig 9.8

9 Comparison of b d γ Branching Fractions CKM fitter includes CDF B s mixing result. Error on CKM Fitter prediction includes uncert. on B Vγ form-factor ratio. I-spin consistency? 6 (10 ) B B 0 B Mode ρ γ + + ρ γ 0 0 ωγ + 0 (,, ) I -avg BABARAR (10-6 ) (6.3 σ signif.) preliminary; hep-ex/ ± ± < 0.84 (90% C.L.) B ρ ρ ω γ 1.01± 0.21± 0.08 Belle (10-6 ) (5.1σ signif.) PRL 96, (2006)

10 V V V V td ts td ts Extracting V td /V ts from b d γ Decays Belle, PRL 96, (2006). = = BABAR, hep-ex/ (preliminary) CDF, hep-ex/ (preliminary) V V td ts = Consistent within errors. Theoretical uncertainties limiting both approaches. courtesy M. Bona (UTfit collab.)

11 B Kl + l - and B K*l + l - in the SM and Beyond Photon penguin b d γ uct,, W s d + Z penguin b d Z uct,, W d s + W + W - box ν + b d W W + uct,, d s Dependence on kinematic variables in 3-body decays can be used to study the different amplitudes and their interference effects. The mode B Kl + l - is allowed as well as B K*l + l - (B Kγ forbidden by conservation of angular momentum).

12 Amplitude for B K*l + l - { * + GFαEM * eff * M( B K ) = VtsV tb C9 K sγ μpb L B 2π mix of Z-penguin, W + W - box Kruger and Matias; PRD 71, (2005) 2 mb eff * 2 C 7 K si q ν σμν P R b B q + C K sγ Pb B ( μ γ γ )} * 10 μ L 5 photon penguin dom. at v. low q 2 ( μ γ ) Short-distance physics encoded in C i s (Wilson coefficients); calculated at NNLO in SM: eff C7 0.3 C C Ali et al., PRD 61, (2000) C 9, C 10 generate asymm. in lepton angular distribution over most of q 2. C i s can be affected by new physics, which enters at same order as SM

13 Form Factors and Observables Long distance QCD physics is mainly described in terms of form 2 2 factors, which are functions of q = ( p + + p ) 4 semileptonic form factors: A 1, A 2, V, A 0 (similar to B D*lν, B ρlν) 3 penguin form factors: T 1, T 2, T 3 Form factor uncertainies 35% uncertainty in rate predictions. θ χ + B K K * θ K π 2 ( s q ) dafb eff m m m bm B K K ( ) K ds s mb mb s { eff * * C10 Re C9 VA ˆ 1 + C7 VT2 1 (1 m * ) + AT ( + ) = 4.07 GeV Precise SM prediction due to ff cancellation A FB 1 0 dγ dcosθ dcosθ dγ dcosθ + dcosθ dγ dcosθ dcosθ dγ dcosθ dcosθ

14 Predictions for A FB in B K*l + l - : SM and beyond q = q min q s * K 2 2 q = q max * K s q + C eff = C (SM) 7 7 Standard Model C C = C (SM) C eff eff (SM) s = + ( ) 4.07 GeV C = C (SM), C C = C (SM) C (SM) eff eff eff

15 B Kl + l - and B K*l + l - : q 2 distributions B Kμ + μ * B K μ + μ SUSY models J/ψΚ ψ(2s)k Pole from K*γ, even in μ + μ - SM nonres SM nonres q 2 q 2 constructive interf. destructive

16 B Kl + l - and B K*l + l - : the J/ψ veto The decays B J/ψ K and B J/ψ K* are huge backgrounds and must be carefully removed (also B ψ(2s)k, ψ(2s)k*). These backgrounds are restricted in q 2, but there is a tail due to bremsstrahlung in the electron modes. But B J/ψ K and B J/ψ K* are valuable control samples; use them to study efficiency of almost any analysis cut. Ali, Kramer, Zhu: J/ψ and ψ(2s) veto: MC B Ke + e - BB ( K ;1 q 7 GeV ) = (2.92 ) 10 * m(e + e - ) projection: MC B Ke + e - m +

17 B Kl + l - BABAR, PRD 73, (2006) N = sig -8.9 Signal from BABARAR summed over all K l + l - modes (K + e + e -, K + μ + μ - K S e + e -, K S μ + μ - ) significance 6.6 σ; rarest observed B decay BB ( K + ) = (0.34 ± 0.07 ± 0.02) 10 6 (averaged)

18 B K* K*l + l - BABAR, PRD 73, (2006) N = sig AR Signal from BABAR 229 M BB BB ( K + ) = ( ± 0.11) 10 *

19 B K ( * ) l + l - Signals from Belle Belle, PRL 96, (2006) 386 M BB N sig = 96.0 ± 12.0 N sig = ± 13.0 (data sample used for study of Wilson coefficients)

20 B Kl + l - and B K*l + l - branching fractions Mode B K + * B K + AR (10-6 ) BABAR PRD 73, (2006) ± ± Belle (10-6 ) preliminary ± ±

21 B K*l + l - : BABAR results on K* polarization and A FB 1 dγ 3 3 = F cos θ + (1 F )sin Γ d cosθ 2 4 K BABAR, PRD 73, (2006) 2 2 L K L K K* polarization θ use in 2 bins of q 2 Data SM C = C (SM) 7 7 Polarization consistent with SM, but doesn t discriminate against new physics scenarios with current data sample. Theory predictions in graphs: Ali et al., PRD 66, (2002); Ball and Zwicky, PRD 71, (2005).

22 FB B K*l + l - : BABAR results on A FB and Γ L 1 dγ 3 3 = F sin θ + (1 F )(1 + cos θ ) + A cosθ Γ d q 2 range (GeV 2 ) > * 2 * * * L l L l FB l cosθl 4 8 A FB > 0.19 (95% C.L.) ± + + ( ) = ( ± 0.08 > 0.1 GeV ) A B K q Data C = C (SM) 7 7 SM C C = C (SM) C eff eff C eff = C (SM), 7 7 eff eff C C = C (SM) C excluded at 3.6σ! F L ± ± use in 2 bins of q 2 (SM) (SM) Any A FB <0 excluded at >2.7σ (A FB =0 in SM and many BSM)

23 Belle results on A FB for B K ( * ) l + l - * B K + SM: A = 0.33, A = 4.07, A = A = A (SM) 7 7 Standard Model A = 0.28, A = 2.22, A = A = 0.28, A = 2.42, A = q * + ( ) + ( ) A B K = 0.50 ± 0.15 ± 0.02 FB A B K = 0.10 ± 0.14 ± 0.01 FB Belle, PRL 96, (2006)

24 Belle results on Wilson coefficients for B K ( * ) l + l - fix A 7 to SM (B X s γ) fit for A 9 /A 7 and A 10 /A 7 data consistent with SM quadrants II, IV allowed SM fit A 10 >0 SM AA 1400 < < % C.L A7 A 9 A 10 < 0 excludes quadrants I,III at 98.2% C.L.

25 Inclusive B X s γ Canonical process for studying b s transition. Theory uncertainties currently at 10% level (NLO); pushing toward 5% (NNLO). Huge theoretical effort to predict branching fractions & photon energy spectrum. Branching fraction measures C 7 ; spectrum is insensitive to new physics but is sensitive to m b and Fermi motion of b-quark ( shape function ). E γ > 1.6 GeV T. Hurth, E. Lunghi, W. Porod, Nucl. Phys. B 704, 56 (2005). ( ) 0.40 CKM param scale BB ( Xγ ) = 3.61 ± 0.02 ± 0.24 ± s m CP s m / m CKM 0.08 c / m A ( B X γ ) = (0.42 ± 0.08 ± 0.03 )% ( ) BB ( Xγ ) = s b c M. Neubert, Eur. Phys. J. C 40, 165 (2005) pert b param scale

26 Inclusive B X s γ: some history CLEO, PRL 74, 2885 (1995); 2.01 fb -1 on Y(4S), 0.96 fb -1 below Y(4S) Backgrounds: B decays, continuum, e + e - qqγ (ISR), e + e - qq π 0 X Event-shape analysis B-reconstruction analysis total background (points w/error bars) scaled off resonance BB ( Xγ ) = (2.32 ± 0.57 (stat.) ± 0.35 (sys.)) 10 s E γ 4

27 Challenges of inclusive B X s γ Weak experimental signature: single high-energy photon + eventshape cuts. Lots of background from π 0 s and η s! Fully inclusive analysis is not able to exploit the kinematic constraints (m Β, ΔE). Difficult to carry analysis down to E γ < 2.0 GeV. Want to push toward 5% precision to match the expected precision of NNLO calculations. (It s amazing that you can do this analysis at all!) Two methods have evolved from initial CLEO approaches. Method Fully inclusive don t reconstruct X s Sum of exclusive B Kn(π) γ Advantages Closest correspondence to inclusive B(B X s γ). Less background due to additional kinematic constraints. Better E γ resolution. Disadvantages Large background; limited sensitivity at low E γ. More model dependence due to finite set of explicitly reconstructed B X s γ decays.

28 Fully inclusive B X s γ: pushing down the energy threshold CLEO, PRL 87, (2001), 9.1 fb-1 Belle, PRL 87, (2004), 140 fb -1 Belle, hep-ex/ BF = (3.21± 0.43 ± ) 10 Measure for E γ >2.0; extrap. to E γ >0.25 GeV BF = (3.55 ± ) 10 Measure for E γ >1.8 GeV; extrap. to full

29 Fully inclusive, lepton-tagged B X s γ (BABAR) Want to suppress large continuum background. Strengthen signature for signal by using decay of 2 nd B in event. Require high energy lepton: p e *>1.25 GeV, p μ >1.9 GeV in addition to event-shape cuts. Tag does not compromise inclusiveness of X s selection. BB dom. BB qq + ττ * 1.9 < E γ < 2.7 GeV (blind) X S γ contin. dom. lepton tag from 2 nd B meson

30 BABARAR Fully Inclusive B X s γ, w/lepton tag hep-ex/ (preliminary, submitted to PRL) BB events Spectrum from best fit to kinetic scheme. Spectrum from best fit to shape function scheme. BB ( Xγ ) = (3.67 ± 0.29 ± 0.34 ± 0.29) 10 not efficiency corrected E γ > 1.9 GeV 4 s Xγ s = ± ± ± 4 E γ > 1.6 GeV BB ( ) ( ) 10 (measured) (extrapolated, kinetic scheme)

31 BABARAR B X s γ with Sum of Exclusive Final States Reconstruct 38 exclusive modes K ( +,,0) + ( 4 π ) ΔE <40 MeV Fit m ES distrib. in bins of m(x s ) Correct for efficiency of each mode and missing modes fraction ( model dependence) summed over all m(x s ): K ( +,,0) + η + ( 2 π ) 3K ( +,,0) + ( 1 π ) comb. BB signal continuum peaking bknd

32 BABARAR B X s γ with Sum of Exclusive Final States BABAR, PRD 72, (2005) K*(890) Energy Range E γ E γ >1.9 GeV >1.6 GeV (extrapolated) Branching Fraction (10-4 ) 3.27 ± ± averages over two shape-function schemes errors: stat, sys, variation of shape fcn params K*(890) E γ Moments E γ Value (GeV or GeV 2 ) ± E 2 γ E γ ± E γ (min) = GeV

33 Summary of B X s γ Branching Fraction Measurements BB ( Xγ ) = (3.55 ± 0.24 ± 0.03) 10 s HFAG average

34 Extraction of heavy-quark expansion parameters from B X s γ Using heavy-quark expansion (HQE), moments of inclusive B decay distributions can be expressed in terms of non-perturbative QCD parameters and quark masses. B X s γ inclusive E γ spectrum B X c l ν inclusive E l spectrum and M(X c ) hadron mass distrib. m b now determined to about 1% and V cb is determined to <2%. E γ m b γ γ μπ E E g( m b,,...) (kinetic energy squared of b-quark)

35 2 μ π Fits to moments of inclusive B X c l ν and B X s γ distributions 2 (GeV ) Buchmüller and Flächer, PRD 73, (2006); Data from BaBar, Belle, CDF, CLEO, & DELPHI b sγ V cb 3 (10 ) all moments all moments b c ν b c ν m b m b kinetic mass scheme m = (4.590 ± ± ) GeV b V cb = (41.96 ± 0.23 ± 0.35 ± 0.59 ) 10 exp exp HQE HQE Γ sl μ = (0.401± ± ) GeV 2 2 π exp HQE 3 m b used for V ub (7.5% error!)

36 Conclusions Studies of radiative/electroweak penguins have moved far beyond B K*γ. Observation of exclusive b d γ decays: B (ρ 0, ρ +, ω) γ Use to extract V td /V ts ; consistent with value from B s mixing. Precision limited by theoretical uncertainties. Electroweak penguins decays B K l + l -, B K* l + l -, and B X s l + l - have been measured. First studies of decay distributions have been performed and exclude some non-sm scenarios. Much more data needed to exploit full potential. Inclusive B X s γ measurements provide information on m b and non-pert. QCD parameters and help improve precision on V cb and V ub. Difficult issues with systematic errors, but goal si to achieve 5% uncertainty on branching fraction. Much more to learn about penguins; we will study them for many years to come at BaBar, Belle, and LHC-b!

37 Backup slides

38 B K*l + l - : K* * polarization vs. q 2 SM

39 M(l + l - ) distributions from B J/ B J/ψ K + control samples: data vs. Monte Carlo BABAR points: data histogram: MC absolute normalization Bremsstrahlung tails well described by MC.

40 da ds Lepton angular distribution in l + l rest frame FB { l θ l ( eff ) C Re C VA ( + ) l + B s q * K mm eff m * m * b B K K + C ˆ 7 VT 2 1 (1 m * ) K AT + s mb mb eff s * * 0 mb C 7 T2( s0) m K T1( s0) mk = Re( eff ( )) + + mb mb C A 9 s0 1( s0) mb V( s0) mb s = 2 2 s q = m GeV use l - if B use l + if B Ali, Kramer, Zhu, hep-ph/

41 B K*l + l - Dalitz plot Can see A FB behavior and q 2 dependence from the Dalitz plot 2 q A FB >0 effect of γ pole cosθ A FB A FB < = 0 at q 4 GeV E Note: B Kl+l- is expected to have very small A FB, even in presence of new physics; effectively provides a crosscheck.

42 Extracting A FB and F L in bins of q 2 1 dγ 3 3 = F cos θ + (1 F )sin Γ d cosθ 2 4 K θ 2 2 L K L K 1 dγ 3 3 = F sin θ + (1 F )(1 + cos θ ) + A cosθ Γ d 2 * 2 * * * L l L l FB l cosθl 4 8 BABAR, PRD 73, (2006) 2 2 q > 0.1 GeV 2 2 q > 0.1 GeV

43