Radiative B decays b qγ

Transcription

1 LHCb week - june 2010 St Petersburg Radiative B decays b qγ status report Olivier Deschamps LPC Clermont-Fd

2 Physics case and expected performance Time-dependent decay width for radiative b qγ penguin : ( ) Γ( B q f CP γ ) = A 2 e Γ τ Within SM for B s one expects : q Δ [ cosh( ΔΓ τ / 2) + A sinh( ΔΓ τ / 2) ± C cos( Δm τ ) S sin( Δm τ )] q Reliable theoretical prediction at NNLO probe for NP in loop C A q s Δ s S s 0 sin(2ψ ) 0.1 q q q q q Expect 11x10 3 selected signal events / 2 fb -1 B/S < 90% CL b sγ L + (m s /m b ) sγ R Unbinned likelihood fit proper lifetime reconstructed mass simultaneous fit of (A Δ, C, S) tagging involved parametrize background from side-bands acceptance function from control channel (K*γ, J/ψφ) Other radiative decays under study : B d K* γ, B u ΦK+γ, Λ b Λ γ 2

3 Physics case Measuring the photon polarization in Bs Φγ # # # # fit of the lifetime distribution A small bias in the lifetime is a serious systematics in the analysis Possible source of bias are : B s 4-momentum (dominated by the calorimetry part) vertex reconstruction provided by the Φ decay Outline 1. Satus calorimeter calibration & gamma reconstruction 2. Charged/neutral (mis)alignment 3. Φ vertex resolution study (Fatima) 4. Triggering radiative decays (Fatima) 5. First look at radiative B decays in 2010 data (Fatima) 3

4 1- Ecal calibration status Ecal precalibration done before 2009 data taking at the 8% level based on the absolute gain measurement from photostatistics π 0 peak observed immediately after start-up at the expected mass resolution ~13 MeV/c 2 First relative inter-calibration on collision data using the energy flow method (Aurélien) smoothing the transverse energy deposit on the basis of the average deposit of neighbour channels Calorimeter standalone method requiring a relatively small statistics (~ 1M events) 2009 data channel equalization at the 4% level π 0 peak improves by 15-20% Outer Absolute calibration using reconstructed π 0 peak Iterative procedure by π 0 mass peak fitting Accumulate π 0 contributing to each cell Require a large statistics (~ 200M events) Preliminary calibration obtained from 80M events 2% intercalibration achieved π 0 peak resolution at the 8 MeV/c 2 level 4

5 1 - Photon reconstuction Energy reconstruction E γ = α E Ecal + β E Prs accounts for leakages in Ecal : Preshower sampling factor trasnversal leakage, dead material parameters (partially) extracted from data per Ecal region includes dependency on barycenter position, position of cluster wrt Ecal module frame momentum reconstruction : Transversal barycenter : Energy-weighted cluster barycenter S-shape corrected Log(energy)-dependent longitudinal barycenter ( ~ 6 X 0 inside Ecal ) Parametrisation inherited from Monte-Carlo, so far 5

6 1 - π 0 / η γγ resonances Neutral pions as pairs of resolved photons From 220M events (~3.4 nb -1 ) From 220M events (~3.4 nb -1 ) Overall π 0 resolution σ= (8.0±0.1) MeV/c 2 Inner Middle Outer ALL N 1.5 M 1.6 M 3.2 M 7.5 M m 0 (MeV/c 2 ) ± ± ± ±0.01 σ (MeV/c 2 ) 7.32± ± ± ±0.02 η γγ resolution consistent with MC expectation ( 17MeV/c 2 ) 6

7 1- Photon reconstruction : conversion Photon conversion after magnet is mostly seen as a single cluster Identified looking at Spd hit(s) in front of Ecal cluster Dedicated energy corrections applied From 220M events (~3.4 nb -1 ) e e e π 0 γ(ee) e π 0 γγ σ= (7.2±0.1) MeV/c 2 π 0 γ(ee) σ= (8.2±0.1) MeV/c 2 π 0 (ee)(ee) σ= (9.5±0.1) MeV/c 2 Resolution degradation wrt unconverted sample : +30% π 0 (ee)(ee) Tracking system to reconstruct conversions before magnet as a pair of tracks standard electron-pair from conversion to be defined 7

8 1 Photon reconstruction dependencies Calo calibration and reconstruction tuning are done and validated in the low energy range so far need to check the validity in the higher energy range where (radiative) physics is E.g π 0 mass dependency with photon P T observed probably not an effect of the photons reconstruction but rather due to a global π 0 effect when the 2 photons get closer effect much smaller when looking at η γγ Energy-dependent loss may occur at high energy range, though Using η γγ allows to increase by a a factor ~2 the explored energy range but need to go even further... Outer calo region Inner calo region a = (2.7±0.1)10-3 E T (MeV) a = (1.6±0.1)10-3 E T (MeV)

9 1 Photon reconstruction dependencies Another dependecy : π 0 mass dependency with photon-id cut π 0 from ω/η π + π - π 0 selection γ-id > 0.75 γ-id < ± ±0.1 possible explanation (to be confirmed) : we are very sensitiv to extra energy deposit in photon cluster cleaner (isolated) cluster with strong photon-id threshold (large χ 2 (track-cluster)) If so, the effect can be more dramatic with multiple interaction events

10 A low mass h + h - γ resonance... η (π + π - )γ From 220M incl. minibias events (~3.4 nb -1 ) courtesy Régis Lefèvre Useful channel to check the photon reconstruction & relative alignment of the charged system with the radiative component η π + π - γ BR PDG =(29.4±0.9)% σ=(18±3)mev/c 2 expect 10 MeV/c 2 This channel is to be stripped together with other calorimeter candles (η/ω/φ π + π - π 0,...) 10

11 2 - tracking/ecal alignment Track/Cluster alignment study using BremStrahlung photon : linear extrapolation of electron track (first state slope) onto Ecal get closest BremStrahlung cluster Δ = cluster position extrapolated position for default z-cluster value (~6 X0 inside Ecal active area) electron track selection : default electron with RichDLLe> 2 && brem-chi2 <9 (electron cluster brem cluster) distance > 3 cells 11

12 2 - tracking/ecal alignment Δy as a function of Θ Y and Θ x ΔY a =(53±3) rad -1 a =(0.9±3.7) rad -1 Θ Y Θ X ΔY/Θ Y slope is consistent with a z -translation of O(5cm) either Ecal mis-positioning or wrong shower Z-barycenter or a bit of both

13 2 - tracking/ecal alignment Results more tricky in the bending (x,z) plane need to separate electron from positron ΔX electrons ΔX positrons a =(29±5) rad -1 a =(33±5) rad -1 Θ X overal +(-) 1cm systematical bias for electron (positron) probably an effect of the origin of the BremStrahlung along the (bended) track? Θ X Current (preliminary) status Δy = 2.0 mm/1.7 mm in Ecal A/C side Δx = 2-3mm in Ecal C-side. global Δz~5cm (cluster barycenter) Work ongoing to be checked/validated with track extrapolation onto electron cluster to be finalized before implemented in condition DataBase for a next re-processing

14 3 - Φ K + K - vertexing Fatima Soomro (june 2010) Purpose : look at the vertex resolution in Φ KK in minibias data Method Assume all Φ KK come from PV (neglect D decays) and assume the error on PV is much smaller than the error on Φ vertex. PV is the true vertex Validation of the method using MC truth 2010 MC simulated minibias events (Sim03Reco03) Request single collision events resolution in Z (mm) resolution in X (μm) resolution in Y (μm) σ core σ wide f core/wide σ core σ wide σ core σ wide MC-truth 1.9± ±0.2 73% 89±4 233±10 83±4 282±10 MC using PV 1.9± ±0.2 70% 88±4 239±10 80±4 282±9 14

15 Results on data 3 - Φ K + K - vertexing Selection Stripping line : calibration stream ΔLL(K-π)>30 Pt(K) max > 500 MeV/c ΔM Φ < 35 MeV/c 2 χ 2 (Φ-vertex)< events selected (80% with 1 candidate) selection efficiency wrt stripping ~2.2% σ z (Φ-vtx) as a function of the polar angle Z (mm) μ core σ core μ wide σ wide f core/wide MC-truth -0.03± ± ± ±0.2 73% MC using PV 0.005± ± ± ±0.2 70% Data ± ± ± ±0.4 79% 15

16 3 - Φ K + K - vertexing Expected resolution on the Φ-vertex in the B decay signal is better than in inclusive Φ decay from minibias sample because of different kinematics polar angle σ z (Φ-vtx) as a function of the polar angle resolution in Z (mm) resolution in X (μm) resolution in Y (μm) σ core σ wide f core/wide σ core σ wide σ core σ wide MC-truth 1.9± ±0.2 73% 89±4 233±10 83±4 282±10 MC using PV 1.9± ±0.2 70% 88±4 239±10 80±4 282±9 MC (Φ from B decay ) 1.22± ± % 70.0± ± ± ±1.4 16

17 4 Triggers for radiative decay Fatima Soomro Trigger components (and stripping stream) in place for radiative decays HLT2 exclusive selection for B s Φγ and B d K*γ, F. Soomro, LHCB-PUB Basic performance on MC09 data HLT1 (Ecal alleys) : 85% efficient on offline selected events factor 3 reduction of the mb rate HLT2 : 89% (77%) efficient for B s Φγ (B d K*γ) passing L0/HLT1 minibias rate ~2.5% of the global HLT2 bandwidth Radiative line stripping : ~19k data available (last week) 17

18 4 Triggers for radiative decay Study the online/offline P T resolution 2M K*γ simulated data Fraction of L0Calo rate important to keep L0γ not too high 18

19 B d K*γ : first look in the 2010 data 13 nb -1 analyzed expect ~0.2 signal event 1 selected candidate in the B d mass region Mass : MeV/c 2 19

20 B d K*γ candidate y γ x 20

21 Summary Ecal calibrated at the 2% level (preliminary) photon reconstruction tuning applied in the latest data re-processing Transversal shower shape and longitudinal barycenter being studied on data further progress in calibration and alignment by the end of summer need to explore/validate in the higher energy range First look at Φ vertexing on data resolution not so bad 20-25% degradation wrt MC expectation First B d K*γ candidate observed Result on first data to be presented at ICHEP 21