LHCb Rare Decays Status WG6 – CKM 2006 Nagoya, Japan Jeremy Dickens on behalf of the LHCb Collaboration
Outline LHCb detector Bs mm Bd K*mm Measurement of RK Signal yield and B/S Forward-backward asymmetry sensitivity Tranversity angles sensitivity Measurement of RK Radiative decays Bd K*g Bs fg Lb Lg 14/12/06 Jeremy Dickens
LHCb Detector Forward peaked, correlated bb-pair production 250 mrad 10 mrad Vertex Locator Dipole magnet Tracking system Calorimeters Muon system RICH detectors (B), rad Forward peaked, correlated bb-pair production sbb 500mb with L 2 x 1032 cm-2 s-1 For 1 nominal year (107 sec) 1012 bb-pairs LHCb is a forward spectrometer 14/12/06 Jeremy Dickens
Bs mm Branching ratio ~ 3.5 x 10-9 in Standard Model Sensitive to New Physics and can be strongly enhanced by SUSY (by up to factor ~100) Current limit from Tevatron CDF + D0 with 1 fb-1 is ~ 7 x 10-8 at 90% CL Expected limit with 8 fb-1 is ~ 2 x 10-8 at 90% CL 14/12/06 Jeremy Dickens
Bs mm: Mis-ID background Extremely low branching ratio issue is background rejection Combinatorial – with muons mainly from b decays Studied with bb and b mX, b mX samples Mis-identified Hadrons – eg Bs pp, Kp and KK For example Br(Bs Kp) 5 x 10-6, so need mis-id rate O(1%) per track For a 1% mis-id rate, LHCb expects of order 1 event per fb-1 in 2s mass window 14/12/06 Jeremy Dickens
Bs mm: N-counting experiment Perform N-counting experiment in 3 variables: Combined geometry variable (impact parameters, distance of closest approach, lifetime, vertex isolation) Particle-ID (likelihood for p or K to be mis-id as m) Invariant mass signal Arbitrary normalization bb inclusive b μX, b μX (component) geometry variable signal Arbitrary normalization bb inclusive b μX, b μX (component) Invariant mass (MeV) For geometry variable > 0.4 background is dominated by b mX, b mX 14/12/06 Jeremy Dickens
Bs mm: N-counting experiment For example, with a cut on geometry (> 0.4) and invariant mass (± 30 MeV) expect: ~19 signal events per fb-1 (SM) [54, 295] background events per fb-1 (90% CL) Better sensitivity from N-counting method Each variable is divided into several bins b m-X, b m+X used to compute expected background in each 3D bin Limited MC statistics many bins have little or zero background Background is shifted upward in each bin so that total background has 90% probability to be below this value Compute confidence levels for observation and exclusion No cut actually applied in the analysis 14/12/06 Jeremy Dickens
Bs mm 5 3 Limit at 90% CL (only bkg is observed) Integrated Luminosity (fb-1) BR (x10-9) Expected CDF+D0 Limit Uncertainty in bkg prediction SM prediction LHCb Sensitivity (signal+bkg is observed) Integrated Luminosity (fb-1) BR (x10-9) 5 3 SM prediction Background assumed to be dominated by combinations of b m- X and b m+ X 14/12/06 Jeremy Dickens
Bd K*mm Branching ratio and forward-backward asymmetry (FBA) sensitive to New Physics Branching ratio (Babar/Belle) = 1.22 x 10-6 FBA defined as asymmetry between m+ (m-) in forward and backward directions in m+m- pair rest frame, with respect to the B (B) direction Position of the zero crossing is sensitive to new physics + 0.38 – 0.32 m+ m- K* B0 q From hep-ph/9910221 Standard Model d Bd b s K* m g m 14/12/06 Jeremy Dickens
Bd K*mm: Backgrounds Background dominated by b mX, b mX Sample LHCb Simulation BR() No. evts per 2fb-1 Bd smm (no K*) 4 x 10-6 44 ± 10 Bu smm 14 ± 4 b mX, b mX 1.2% 1770 ± 320 b m c( m) 2.7% 1310 ± 280 TOTAL 3100 ± 430 Signal 1.22 x 10-6 7700 ± 180 ± 2200 B/S = 0.4 ± 0.1 Statistical From BR() Background dominated by b mX, b mX In the majority of these events, all the particles used to build the “B” came from b-decays Large fraction from b m c ( m ) Also expect some background from non-resonant Bd Kpmm events Rate from this source not certain (not included in table above) 14/12/06 Jeremy Dickens
Bd K*mm: FBA sensitivity Toy model based on LHCb simulation: Large sample of generator level events Smaller sample of fully simulated events (include detector effects) Take resolutions, acceptances and background from the fully simulated events, in bins of Mmm (03 GeV) and qFBA (0p) Mmm (qFBA) resolution ~ 10 MeV (4 mrad) ~3% (~1%) of bin width Sample from (fluctuated) generator level distributions, applying the acceptance functions, to simulate a large number of 2 (10) fb-1 experiments. Calculate and fit FBA for each data set to find the zero-crossing point Quoted sensitivity is then the standard deviation of the fitted zero points from a large number of experiments 14/12/06 Jeremy Dickens
Bd K*mm: FBA sensitivity Neglected non-resonant background for now Consider two types of background Asymmetric in qFBA (expect 240 ± 75 events per 2 fb-1) Fraction of the b m c ( m) sample For example: B+ m+ n D0 ( K+ m- n) and B- m- n D0 ( K- m+ n) For both cases, we make the angle with the muon direct from the b, which tends to have higher momentum than the muon from the charm, so is more likely to be forward in the mm rest frame Symmetric in qFBA (expect 1700 ± 250 events per 2 fb-1) All events in the b mX, b mX sample (muons from different b-decays) Some events in the b m c ( m) sample, where the K and p come from different b-decays to the two muons – the charge of the K is uncorrelated with the charge of the muon direct from the b 14/12/06 Jeremy Dickens
Bd K*mm: FBA sensitivity Generator zero-crossing point: s0 = 4.10 GeV2 From 1000 experiments of 2 fb-1: No background s0 = 4.17 ± 0.38 GeV2 With background s0 = 4.11 ± 0.52 GeV2 With 10 fb-1 (with background) s0 = 4.17 ± 0.28 GeV2 Fitted zero-point (GeV2) With 10 fb-1 Example 2 fb-1 experiment Mmm2 (GeV2) 14/12/06 Jeremy Dickens
Bd K*mm: Non-resonant events Simulation of these events is currently Jetset fragmentation Spectra and rate are very uncertain! LHCb simulation uses BR(Bd Kpmm) = 1 x 10-6 in full mass range Gives non-resonant background of 4550 ± 200 events per 2 fb-1 Probably an overestimate – will eventually measure this Identical from a selection point of view, but without the K* mass constraint For FBA, can be treated as signal, under certain conditions… (hep-ph/0505155) Region I: soft pion, energetic kaon Shifts zero of FBA and larger theory errors Region II: energetic Kp pair Can be treated as B Xmm and X Kp Region III: soft kaon, energetic pion Amplitude suppressed so very few events… Defined by kinematics Drawn for Mmm = 2 GeV 14/12/06 Jeremy Dickens
Bd K*mm: Non-resonant events Simulation of these events is currently Jetset fragmentation Spectra and rate are very uncertain! LHCb simulation uses BR(Bd Kpmm) = 1 x 10-6 in full mass range Gives non-resonant background of 4550 ± 200 events per 2 fb-1 Probably an overestimate – will eventually measure this Identical from a selection point of view, but without the K* mass constraint For FBA, can be treated as signal, under certain conditions… (hep-ph/0505155) Region I: soft pion, energetic kaon Shifts zero of FBA and larger theory errors Region II: energetic Kp pair Can be treated as B Xmm and X Kp Region III: soft kaon, energetic pion Amplitude suppressed so very few events… Defined by kinematics Drawn for Mmm = 2 GeV 14/12/06 Jeremy Dickens
Bd K*mm: Non-resonant events Simulation of these events is currently Jetset fragmentation Spectra and rate are very uncertain! LHCb simulation uses BR(Bd Kpmm) = 1 x 10-6 in full mass range Gives non-resonant background of 4550 ± 200 events per 2 fb-1 Probably an overestimate – will eventually measure this Identical from a selection point of view, but without the K* mass constraint For FBA, can be treated as signal, under certain conditions… (hep-ph/0505155) Region I: soft pion, energetic kaon Shifts zero of FBA and larger theory errors Region II: energetic Kp pair Can be treated as B Xmm and X Kp Region III: soft kaon, energetic pion Amplitude suppressed so very few events… Defined by kinematics Drawn for Mmm = 2 GeV 14/12/06 Jeremy Dickens
Bd K*mm: Non-resonant events Simulation of these events is currently Jetset fragmentation Spectra and rate are very uncertain! LHCb simulation uses BR(Bd Kpmm) = 1 x 10-6 in full mass range Gives non-resonant background of 4550 ± 200 events per 2 fb-1 Probably an overestimate – will eventually measure this Identical from a selection point of view, but without the K* mass constraint For FBA, can be treated as signal, under certain conditions… (hep-ph/0505155) Region I: soft pion, energetic kaon Shifts zero of FBA and larger theory errors Region II: energetic Kp pair Can be treated as B Xmm and X Kp Region III: soft kaon, energetic pion Amplitude suppressed so very few events… Defined by kinematics Drawn for Mmm = 2 GeV 14/12/06 Jeremy Dickens
Bd K*mm: Transversity angles Recent theoretical work has highlighted other asymmetries to study (Phys Rev D71: 094009, 2500) Describe the decay in terms of 4 parameters s = mm mass squared ql = FBA angle (between m and B in mm rest-frame) qK* = equivalent K* angle (between K and B in K* rest-frame) f = angle between K* and mm decay planes FL extracted from ql, qK* and f distributions AT(2) extracted from qK* and f distributions 14/12/06 Jeremy Dickens
Bd K*mm: Transversity angles Toy MC created to describe ql, qK* and f distributions SM predictions used as input values Inputs are averaged (weighted by cross-section in each bin) Background distributions added (without non-resonant component) Analysis performed in 4 bins of s (0 < s < 9 GeV2) Mmm2 range Resolutions AT(2) FL 0.05 0.49 0.180 0.037 0.49 1.96 0.400 0.033 1.96 6.25 0.470 0.018 6.25 9.0 0.31 0.020 FL AT(2) A possible 2fb-1 measurement 14/12/06 Jeremy Dickens
RK b lls suppressed by 1/aEM with respect to b sg Sensitive to SUSY, extra-dimensions… In SM RK = 1 ± 0.001 Neutral Higgs corrections could be O(10%) Measure RK 1 New Physics CDF 95% CL limit on Bs mm LHCb 10 fb-1 yields Bd eeK ~ 10k Bd mmK ~ 19k Gives RK = 1 (fixed) ± 0.043 14/12/06 Jeremy Dickens
Radiative Decays b sg proceeds only via loop diagram Sensitive to New Physics, eg charged Higgs, gluino, neutralino loops Decay 2 fb-1 yield B/S Bd K*g 35000 < 0.7 Bs fg 9000 < 2.4 Bd wg 40 < 3.5 g t W b s Decay 2 fb-1 yield B/S Lb L g 750 < 42 Lb L(1520) g 4200 < 10 Lb L (1670) g 2500 < 18 Lb L (1690) g 2200 Lb decays need special reconstruction since L flies L vertex doesn’t define the Lb vertex Sensitivity under study 14/12/06 Jeremy Dickens
Conclusions Bs mm Bd K*mm Potential to exclude BR between 10-8 and SM with 0.5 fb-1 Potential for 3s (6s) observation with ~ 2 fb-1 (10 fb-1) Bd K*mm Yield per 2 fb-1 of 7700 ± 180(stat) ± 2200(branching ratio) B/S = 0.4 ± 0.1 (without non-resonant events) FBA zero-crossing point s0 = 4.2 ± 0.5 GeV2 for 2 fb-1 (± 0.3 for 10 fb-1) Potential to measure K* polarisation with higher precision than SM errors 10 fb-1 measurement RK = 1 (fixed) ± 0.043 Good potential for study of radiative B-decays LHCb has good sensitivity for new physics discovery 14/12/06 Jeremy Dickens
SPARE 14/12/06 Jeremy Dickens
Bd K*mm: FBA From hep-ph/0106067 From Phys Rev D55 4273 14/12/06 Jeremy Dickens
Bd K*mm Non-resonant background (Bd Kpmm) Current branching ratio in LHCb simulation is 1 x 10-6 Not yet observed – may be too large? (hep-ex/0604007) Recent theoretical work suggests that in some kinematic regions (requires energetic Kp pair), these events should have the same FBA as the signal (resonant) events May be able to treat the events as signal (hep-ph/0505155) 14/12/06 Jeremy Dickens
Bd K*mm: FBA sensitivity Toy model based on LHCb simulation: Large sample of generator level events (5.5M) Smaller sample (550k) of fully simulated events (include detector effects) Take generator level distributions, binned in Mmm (03 GeV) and qFBA (0p) Mmm (qFBA) resolution ~ 10 MeV (4 mrad) ~3% (~1%) of bin width Apply fluctuated acceptance function in each Mmm x qFBA bin Measured using the 550k fully simulated events Now have “Expected distributions” In each bin of Mmm sample X events (Poisson fluctuated) from “Expected distributions” Add background (see later) – now have “Experimental Distributions” Remove (un-fluctuated) acceptance Assume same acceptance for signal and background Calculate and fit FBA to find the zero-crossing point Quoted sensitivity is then the standard deviation of the fitted zero points from a large number of experiments 14/12/06 Jeremy Dickens
Bd K*mm: Transversity Angles f 14/12/06 Jeremy Dickens
Bd K*mm: Transversity Angles qK* 14/12/06 Jeremy Dickens