Rare B Decays. Yee Bob Hsiung National Taiwan University and Belle Collaboration. Highlights from Belle and Babar

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1 Rare B Decays Yee Bob Hsiung National Taiwan University and Belle Collaboration Highlights from Belle and Babar PASCOS 2006, Sept , OSU, Columbus Ohio

2 What is a rare decay? Experimental definition: One which has not been seen, or only just been seen for the first time Theoretical definition: One which, in the standard model, is either absolutely forbidden or strongly suppressed: FCNC, helicity, small CKM element. (b c is big, so everything else is small)

3 What are rare B decays? Charmless B meson decays Radiative decays Leptonic decays Charmless baryonic decays of B Many, many rare B decay modes - BR, Acp, Pol, etc. See later talks: CP Violation by Chunhui Chen and Elements of CKM Matrix by T. Tsumiyoshi

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5 Why look for rare decays? If new particles are to appear on-shell at high energy phenomenon, they must appear in virtual loops and affect amplitudes b u H + τ + ν Our chance to see them is when the Standard Model amplitudes are small: ==> Rare decays Hadronic rare B decays also study SM, CP violation and NP.

6 What made this possible? Measuring Branching ratios of ~10-6 needs millions of events - 1 fb -1 ~ 1M BB events!! KEKB/Belle 630 fb -1 Physics progress possible thanks to machine physicists at SLAC and KEK designed and built B factories operating in a new highcurrent regime, continually faced and overcame new problems and challenges. continually push the luminosities. Peak Lum ~ 16.5 /nb/s!! PEPII/BaBar

7 Belle Apparatus and data set BB 䠫(4S) (8.0GeV)e- e+(3.5gev) KEKB Data sample for analysis Int. L = 414fb-1 447MBB (Int. L=497fb-1 532MBB)

8 BaBar Detector

9 Finding a needle in haystack. Huge backgrounds from other Bs and e + e - q q Use energy balance ΔE=E B - E i ΔE(GeV) and beam-constraint M bc or M ES = E B2 -( p i ) 2 Use data sidebands (rather than Monte Carlo) to estimate background Blind Analysis Tune cuts without looking in the M bc -ΔE signal box M ES (GeV/c 2 ) or M bc

10 Continuum Background suppression

11 Other Experimental techniques Continuum suppression from combined information from shape variables Single-B beam technique: Reconstruct one tag B in a common decay mode (hadronic or semileptonic). Remaining particles must also form a B Limits from N=ε S + b N small Uncertainties on ε, b

12 B Decays to leptons Proceeds through one or two weak bosons with strong CKM suppression door open for NP particles to contribute b u Free of hadronic uncertainties in final state - Clean W + τ + b W u,c,t ν W d,s ν µ + Plus many µ - other diagrams b H + τ + b ν~ µ + u ν d,s µ - Rare B decay with Missing Energy

13 Motivation for B + τ + Sensitivity to new physics from charged Higgs if the B decay constant is known Most stringent published limit: BF(B + τ + ν) < 2.6 x 10-4 (BaBar) Β decay constant B. Aubert et al., PRD 73, (2006)

14 Measuring Β τ is non-trivial e + ϒ(4S) B - B + ν τ B - X ν τ ν e Most of the sensitivity is from tau modes with 1-prong B + τ + ν τ, τ + e + ν e ν τ The experimental signature is rather difficult: B decays to a single charged track + nothing

15 Belle s sample of B tags (449 x 10 6 BB) B # D +! / " / a / DS + (*) (*) D! / D " D + s! B $ D +! / " / a / DS 0 (*)# (*) + 1 D! / D! 0 " " 0 D + s! 0 D! D! " 7 modes 6 modes D + s! 2 modes ~ 180 channels reconstructed Signal region : < ΔE < 0.06 GeV, M bc > 5.27 GeV/c 2 N=680 K N=412 K Eff=0.29% Eff=0.19% Purity =57% Purity =52% m ~ 5.28 GeV/c 2 σ~ 3 MeV/c 2 from σ(e beam ) Charged B s Neutral B s ~10% feed-across between B + and B 0 Beam constrained mass distn s

16 Outline of B τ experimental analysis Reconstruct one B (B tag ) in a charged hadronic b c mode (remove tag s decay products from consideration.) Little or no extra electromagnetic calorimeter energy (E ECL ). Beam-related backgrounds modeled in MC using random trigger data runs. For B X n known E B, m B, small p B narrow missing mass distn. (m n ~0) Two missing neutrinos, large missing p (cut depends on τ decay mode 0.2 GeV-1.8 GeV)

17 Outline of experimental analysis (cont d) The τ lepton is identified in the 5 decay modes: 81% of all τ decays Signal-side efficiency including τ decay BFs) ±0.05% All selection criteria were optimized before examining the signal region (a.k.a. blind analysis) Fit the extra energy distribution (E ECL ), the signal peaks near zero

18 Consistency Check with B D * l Extra neutral energy E ECL Validation with double tagged sample (control sample); B tag is fully reconstructed B sig is a semileptonic decay B + D (*)0 X + (fully reconstruction) B - D *0 l - ν D 0 π 0 K - π + K - π + π - π + Calibration data B + B ± 18 B 0 B ± 2.2 Total Data 502 ± Purity ~ 90% Extra energy in the calorimeter

19 Example of a B τ ν candidate

20 Evidence for B + τν (Belle) B pairs B tag D (*) [π,ρ,a 1,D s (*) ] 680k tags, 55% pure. 5 τ decay modes ! Find signal events from a fit to a sample of 54 events. 4.6σ stat. significance w/o systematics, After including systematics (dominated by bkg), the significance decreases to 3.5σ Extra Calorimeter Energy MC studies show there is a small peaking bkg in the τ ππ 0 ν and τ πππ 0 ν modes.

21 Error in the efficiency calculation Due to a coding error, the efficiency quoted in the 1 st Belle preliminary result was incorrect. The data plots and event sample are unchanged. However, f B and the branching fraction must be changed. This mistake was not detected when checking the B D* l ν control sample or in the internal review process. New value ! 4 Previous value BF( B BF( B + # $ % $ ) + # $ % $ = (1.79 ) = 1.06! 0.49! 0.46) " ! 4! 0.28! 0.16 " 10 (Preliminary)

22 Results for B + τ + 324M B s (revised). 3.5 σ significance BF( B + + # $ % $ ) = ( ! 4! 0.49! 0.46) " 10 (new) BF(B + τ + ν τ )= ( ±0.11) x BR< %CL Belle and BaBar results are similar. Agree within errors Can be combined to give (1.36 ± 0.48)x10-4 BF(B + τ + ν τ )

23 Direct experimental determination of f B Product of B meson decay constant f B and CKM matrix element V ub f " V = (10.1 )" 10 GeV B ub ! 4! 1.4! 1.3 Using V ub = (4.39 ± 0.33) 10-3 from HFAG f B ! 31! 34 = 229 MeV ( Belle) f B = 216 ± 22 MeV (an unquenched lattice calc.) [HPQCD, Phys. Rev. Lett. 95, (2005) ] 15% 14% = 12%(exp.) + 8%(V ub )

24 Constraints on the charged Higgs mass Assume f B and V ub are known, take the ratio to the SM BF. r H m = (1 " tan! ) m 2 B 2 H 2 2 r H =1.13±0.51 Limits on e.g. 2 Higgs doublet model: W.S.Hou, PRD 48, 2342 (1993)

25 B ± µ ± ν and e ± ν Helicity Suppressed Use hadronic tags: B fully reconstructed as B to D(*) X Lepton is monoenergetic in signal-b rest frame Limits (@ 90% CL) <7.9 x10-6 for eν (SM ~ ) <6.2 x10-6 for µ ν (SM ~ 10-7 ) Lepton momentum in B frame (GeV/)

26 Motivation for B K * νν (b s with 2 neutrinos) BSM: New particles in the loop Other weakly coupled particles: light dark matter SM: BF(B K * νν) ~1.3 x 10-5 (Buchalla, Hiller, Isidori) PRD 63, c.f. SM: BF(B K - νν) ~4 x 10-6 [Belle preliminary (277 x 10 6 B Bbar) : BF(B K - νν) <3.6 x 10-5 ] to be updated soon

27 B K (*) are particularly interesting and challenging modes (B τ is even a small background) The experimental signature is B K + Nothing The nothing can also be light dark matter (mass of order (1 GeV)) (see papers by M. Pospelov et al.) (But need to optimize p K cut) DAMA NaI 3σ Region C. Bird et al PRL CDMS 04 CDMS 05.(T. Adams et al. PRL ;A. Dedes et al., PRD ) Direct dark-matter searches cannot see M<10 GeV region

28 Search for B K * νν (535 x 10 6 B Bbar pairs) BELLE-CONF Result from a blind analysis. Yield = ! 2.6 Sideband = 19 (1.7σ stat. significance) MC expectation = 18.7±3.3 Extra Calorimeter Energy (GeV) SM (Buchalla, Hiller, Isidori) 1.3 x *0 " 4 B( B # K!! ) < 3.4$ 10 (at 90% C.L)

29 Search for B K * νν (properties of candidates) b c background rare B background (x 15 data) udsc background Signal x 20 combined background Data KπInv. mass

30 Event display for a B K * νν candidate due to an identified background (B K * γ) Tag Side B D + a1 - π+ D + K - π + π + a1 - ρ 0 π -, ρ 0 π + π - K- γ Missing mass ~ 0 (Hard photon is lost in the barrel-endcap calorimeter gap) MC: Expected bkg from this source ~0.3 evts.

31 B 0 to l + l - γ (simulated) FCNC and helicity suppressed, but an initial state photon allows helicity flip SM predictions of order (10-15, respectively without the γ ) See 0 events for e, 3 events for µ (but compatible with background) Limits (at 90% CL) BR(B eeγ)< 0.7 x 10-7 BR(B µµγ)< 3.4 x 10-7

32 Radiative B decays FCNC process suppressed in SM: sensitive to new particles in loops b sγ Inclusive and many exclusive measurements b sl + l -: More information from kinematics b dγ : strongly suppressed but open to different physics b d l + l - : on the way

33 B s γ inclusive Branching Fraction now well measured. Theory and experimental error similar NLO calculation (3.61 ) x10-4 result: (Eγ >1.9GeV) = (3.67 ±0.29 ± 0.34 ± 0.29) x10-4 Fully Inclusive and sum of exclusives (38 modes) HFAG average (3.55 ± 0.24 ± 0.03) x Not much room for New Physics here Constrains model builders

34 B s γ exclusive Lots of channels Branching Fractions measured CP violating asymmetries measured If nonzero these would be a signature of New Physics Example: B K 0 sπ 0 γ Δt(ps)

35 Belle: B d γ First Observation in 2005! Combined BF

36 B d γ B + ρ + γ First observation of B + ρ + γ B 0 ρ 0 γ M ES (GeV/c 2 )

37 Compare with B K*γ Same CKM elements as mixing but a non-trivial test Vtd/Vts = b W d, s d,s t W t b t ds

38 Charmless Hadronic Decays Many modes Will present collected branching ratios Will present measurements of time-integrated CP violation A CP : they follow on directly from differences in charge conjugate decay states from the B + /B - difference - trivial From self-tagged neutral modes trivial From C part of CP+mixing fit nontrivial but standard

39 Observation of B to KK M bc (GeV/c 2 ) ΔE(GeV) M ES (GeV/c 2 )

40 2 body π-k combinations

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42 B K + π - /K - π + Direct CP violation Experiments agree: BaBar: A CP = ±0.024± B 0 B Bel A CP = ±0.018±0.008

43 Direct CP in K π Competing amplitudes with different strong and weak phases A CP should be the same for K + π - and K + π 0 (Gronau: hep-ph ) Current averages (HFAG) A CP (K + π - )= ± A CP (K + π 0 )= ± Difference 0.14 ± 0.03 a long way from zero Maybe colour-suppressed trees are responsible Maybe New Physics

44 B K π ratios Can form many ratios, especially (A Buras, R Fleischer et al, Phys J C 45 ( ) 2006) R n =Γ(K + π - ) R c =2 Γ(K + π 0 ) 2 Γ(K 0 π 0 ) Γ(K 0 π + ) Obtain (HFAG averages) R n =0.99 ± 0.07 R c =1.11 ± 0.07 Agree with each other And with SM predictions The Kπ puzzle is no more

45 B ρ 0 ρ 0 See ±22 events - 3 σ significance BR (1.16 ±0.27) M ES (GeV/c 2 ) ΔE(GeV) Fit longitudinal polarisation f l = 0.86 ± Measurement needed for B 0 ρ + ρ -, used for alpha Informs penguin uncertainty in α determination

46 B + φ φ K +, B 0 φ φ K 0 φφk + : Signal Yield in 2D fit: 34.2 Significance: 9.5σ B (B φφk ): (3.18 ± 0.27) x A C P : 0.01 ± φφk 0 : Signal Yield in 2D fit: Significance: 4.7σ M<2.85 GeV B (B φφk 0 ): (2.31 ± 0.24) x10-6

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48 Threshold Enhancement PLB 617, , 2005 ppk + ppk 0 S pλπ - Glueball is ruled out

49 Angular distribution: ppk + ppk signal p X p Ө p K + b s dominant process Fragmentation picture Proton against K - (p against K + ) : flavor dependence!

50 Conclusion-I Tremendous progress in rare B decays for exploring SM parameters and probing physics at the TeV scale. Evidence for B τν and experimental determination of f B (preliminary result has been updated) BF( B + + # $ % $ ) = ( ! 4! 0.49! 0.46) " 10 Search for B K * νν (UL is still a factor of 10 above the SM range) BF( B # K!! ) < 3.4$ 10 0 *0 " 4

51 Conclusion -II Limits on Higgs Masses, tan β, SUSY particles. Many rare B decay modes are now measured, many with upper limits to constraint the SM parameters Direct CP violation in B to Kpi has been seen. Standard Model begin to be stressed heavily, new SuperB factory would stress it even further with a potential for new discovery

52 Future Prospects: B τν Δf B (LQCD) = 5% 95.5%C.L. exclusion boundaries Extrapolations (T.Iijima) Lum. ΔB(B τν) exp Δ V ub 414 fb -1 36% 7.5% 5 ab -1 10% 5.8% 50 ab -1 3% 4.4% tan! / mh 50ab -1 If Δ V ub = 0 & Δf B = 0 rh tan! / mh

53 Some modes are very difficult at hadron colliders MC extrapolation to 50 ab 1 5σ Observation of B ± K ± ν ν (compare to K + π + and K L π 0 νν) Belle result on B τ shows that B to one prong decays can be measured. MC SM pred: G. Buchalla, G. Hiller, G. Isidori (PRD ) Extra EM calorimeter energy Super B LoI Fig.4.18

54 Backups

55 B τν yields broken down by τ decay mode (stat sig only) For all modes, the background is fitted with a 2 nd order polynomial plus a small Gaussian peaking component.

56 Fits to individual B τ ν decay modes (updated for ICHEP06)

57 Search for B K * νν (properties of candidates) b c background rare B background (x 15 data set) udsc background Need more b c MC (only 2 x data) combined background Data Signal shape P*_K* K * momentum distribution

58 Lipkin Sum Rule R Lipkin =2 Γ(B+ K + π 0 )+Γ(B 0 K 0 π 0 ) Γ(B + K 0 π + )+Γ(B 0 K + π - ) From isospin and assuming the b s penguin diagram dominates R should be 1+O(10-2 ) Obtain (HFAG average) R Lipkin =1.06 ± 0.05 (Was 1.25 ± 0.10 in 2003)

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