Rare B Decays with Missing Energy Tom Browder
Rare B Decays with “Missing Energy” Tom Browder (University of Hawaii) Representing the Belle Collaboration Will discuss experimental results from Belle on B ν (BELLE-CONF-0671) and B K*νν(BELLE-CONF-0627) All results discussed here are preliminary.
Motivation for B+ +ν Sensitivity to new physics from charged Higgs if the B decay constant is known Most stringent published limit: BF(B+ + ν) < 2. 6 x 10 -4 (Ba. Bar) B. Aubert et al. , PRD 73, 057101 (2006) B decay constant
Why measuring B ν is non-trivial e+ BB- X (4 S) B+ e Most of the sensitivity is from tau modes with 1 prong B+ + , + e+ e The experimental signature is rather difficult: B decays to a single charged track + nothing
Belle’s sample of B tags (447 x 106 BB) 7 modes 6 modes ~ 180 channels reconstructed 2 modes Signal region : -0. 08 < DE < 0. 06 Ge. V, Mbc > 5. 27 Ge. V/c 2 N=680 K N=412 K Eff=0. 29% Eff=0. 19% Purity =57% Purity =52% m ~ 5. 28 Ge. V/c 2 s~ 3 Me. V/c 2 from s(Ebeam) Charged B’s Neutral B’s Beam constrained mass distn’s ~10% feed-across between B+ and B 0
Outline of B νexperimental analysis • Reconstruct one B (Btag) in a charged hadronic b c mode (remove tag’s decay products from consideration. ) • Little or no extra electromagnetic calorimeter energy (EECL). Beam-related backgrounds modeled in MC using random trigger data runs. ·For B X n known EB, m. B, small p. B – narrow missing mass distn. (mn~0) ·Two missing neutrinos, large missing p (cut depends on decay mode 0. 2 Ge. V-1. 8 Ge. V)
Outline of experimental analysis (cont’d) • The lepton is identified in the 5 decay modes: 81% of all decays • Signal-side efficiency including decay BFs) 15. 81 0. 05% • All selection criteria were optimized before examining the signal region (a. k. a. blind analysis) • Fit the extra energy distribution (EECL), the signal peaks near zero
Consistency Check with B D* lν • Extra neutral energy EECL Validation with double tagged sample (control sample); – Btag is fully reconstructed – Bsig is a semileptonic decay Calibration data B+ D(*)0 X+ (fully reconstruction) B- D*0 l- D 0 0 K- + K - + B+ B- 494 18 B 0 7. 9 2. 2 Total 502 18 Data 458 Purity ~ 90% Extra energy in the calorimeter
Example of a B ν candidate Tag: B D 0 , D 0 K
Evidence for B+ ν (Belle) 447 106 B pairs Btag D(*)[ , r, a 1, Ds(*)] 680 k tags, 55% pure. 5 decay modes Find signal events from a fit to a sample of 54 events. 4. 6 s 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 0 and 0 modes.
B yields broken down by decay mode (stat sig only) For the first 3 modes, the background is fitted with a 2 nd order polynomial plus a small Gaussian peaking component.
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 control sample or in the internal review process. New value Previous value (Preliminary)
Direct experimental determination of f. B • Product of B meson decay constant f. B and CKM matrix element |Vub| • Using |Vub| = (4. 39 0. 33)× 10 -3 from HFAG ( Belle) 15% 14% = 12%(exp. ) + 8%(Vub) f. B = 216 22 Me. V (an unquenched lattice calc. ) [HPQCD, Phys. Rev. Lett. 95, 212001 (2005) ]
Constraints on the charged Higgs mass Assume f. B and |Vub | are known, take the ratio to the SM BF. r. H=1. 13 0. 51
Motivation for B K* (b s with 2 neutrinos) BSM: New particles in the loop Other weakly coupled particles: light dark matter SM: BF(B K* ) ~1. 3 x 10 -5 (Buchalla, Hiller, Isidori) PRD 63, 014015 c. f. SM: BF(B K- ) ~4 x 10 -6 [Belle preliminary (275 x 106 B Bbar) : BF(B K- ) <3. 6 x 10 -5] to be updated soon
B K(*)νν are particularly interesting and challenging modes (B ν is even a small background) The experimental signature is B K + Nothing The “nothing” can also be light dark matter (mass of order (1 Ge. V)) (see papers by M. Pospelov et al. ) (But need to optimize p. K cut) DAMA Na. I 3 s Region 04 S M CD 05 S M CD C. Bird et al PRL 93 201803. (T. Adams et al. PRL 87 041801; A. Dedes et al. , PRD 65 015001) Direct dark-matter searches cannot see M<10 Ge. V region
Search for B K* (532 x 106 B Bbar pairs) Result from a blind analysis. BELLE-CONF 0627 (1. 7σ stat. significance) Sideband = 19 MC expectation = 18. 7 3. 3 Extra Calorimeter Energy (Ge. V) SM (Buchalla, Hiller, Isidori) 1. 3 x 10 -5 (at 90% C. L)
Search for B K* (properties of candidates) b c background rare B background (x 15 data) udsc background Signal x 20 combined background Data KπInv. mass
Search for B K* (properties of candidates) b c background rare B background (x 15 data set) udsc background Need more b c MC (only 2 x data) Signal shape combined background Data P*_K* K* momentum distribution
Event display for a B K* candidate due to an identified background (B K*γ) Tag Side B D+ a 1 D+ K - π + π+ a 1 - ρ0 π- , ρ0 π+π- K- Missing mass ~ 0 γ (Hard photon is lost in the barrel-endcap calorimeter gap) MC: Expected bkg from this source ~0. 3 evts.
Future Prospects: B 95. 5%C. L. exclusion boundaries Df. B(LQCD) = 5% Extrapolations (T. Iijima) Lum. 50 ab DB(B ) exp D|Vub| 414 fb-1 36% 7. 5% 5 ab-1 10% 5. 8% 50 ab-1 3% 4. 4% If D|Vub| = 0 & Df. B = 0 -1 r. H 2006/05/15 20
Future Prospects: Other probes of charged Higgs • Semileptonic: B D(*) c b H+/W + Expected BF(SM)~ 8 x 10 -3 Decay amplitude + Multiple neutrinos, low momentum lepton (use e’s), large bkg but still might be possible with enough data. 2006/05/15 21
Some modes are very difficult at hadron colliders MC extrapolation to 50 ab-1 5 s Observation of B± g K± (compare to K+ +ννand KL g p 0 ) Belle result on B ν shows that B to one prong decays can be measured. MC SM pred: G. Buchalla, G. Hiller, G. Isidori (PRD 63 014015 ) Extra EM calorimeter energy Super B Lo. I Fig. 4. 18
Conclusions on “Missing Energy Decays” • Evidence for B νand experimental determination of f. B (preliminary result has been updated) • Search for B K* (UL is still a factor of 10 above the SM range) • Further dramatic progress (e. g. signals for B K(*) νν) will require Super B Factory class luminosity.
Backup Slides
Contributions to systematic error for B
Peaking Backgrounds in B Tau tagging mode
Fits to individual B decay modes (updated for ICHEP 06)
Requirements in B ν analysis • The lepton is identified in the 5 decay modes. 81% of all decay modes • Signal selection criteria. • Signal-side efficiency including decay br. ) 15. 81 0. 05% • All selection criteria were optimized before examining the signal region (blind analysis).
Verification of the Signal (1) • For events in the EECL signal region, distribution of event selection variables other than EECL are verified. • They are consistent with MC expectation for B B signal + background. Background Mbc Pmiss
Verification of the Signal(2) • About 30% of background have neutral cluster in the KLM detector (KL candidates). • The excess remains after requiring KL veto. KL in coincidence. KL in veto EECL • We do not use this cut in the result, to avoid introducing a large systematic error due to the uncertainty in KL detection efficiency.
Selection Requirements for B K* MC signal and bkg distributions,
Tag Side B D+ a 1 D+ K - π + a 1 - ρ0 π- , ρ0 π+ππ+ tag. B K- tag. B γ
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