A.Hicheur EPFL Seminar, 09/07/07

Transcription

1 Analysis of B s J/ J/Ψ(e + e - )Φ(K + K - ) A.Hicheur EPFL Seminar, 09/07/07 1

2 Outline Physics motivation LHCb experiment Analysis Main steps Selection and challenges Conclusion

3 Motivation LHCb Physics program: dedicated to b physics All B s mixing parameters: Δm s, φ s, ΔΓ s B s D s π, b c c s modes: J/ΨΦ ΨΦ,, J/Ψη ( ), η c Φ,, D s D s CKM angles α β and γ Rare B decays Sensitivity to new physics (loops in diagrams) Radiative,, elecoweak, gluonic penguins B s μμ (rare box diagram) B c, b-baryon b baryon physics, charm physics, jets, etc B mixing and decays: for CP eigenstates (e.g:: J/ΨΦ ΨΦ,, J/Ψη Ψη, η c Φ) For B s D s- π +, Combine decays to CP eigen states and non CP eigen states to exact the mixing parameters: 1. Precision measurement for Δm s f f. φ s measurement Small (φ( s ) SM (-0.04): possible early detection of New Physics 3

4 Mixing and decay asymmey for b cc s modes Mixing: flavor eigenstates mass eigenstates B L / H = p B ± q B, ΔM = M M, ΔΓ q q q H L q = Γ L Γ H Decay Mixing phase φ = Arg[V ts* V tb ] for B s +φ NP s with new physics Final phase for asymmey ratio φ s (+φ NP ) Time dependent asymmey: exaction of φ s and ΔΓ s (ΔM s obtained from conol sample B 4 s D s- π + )

5 Channels for mixing parameters determination Admixture of CP eigenstates (η f = -1,+1) B s J/ J/Ψ(μ + μ - )Φ(K + K - ) Large yield but angular analysis to disentangle final states + B s J/ J/Ψ(e + e - )Φ(K + K - ) + Bremssahlung challenge CP Even eigenstates (η f = +1) B s J/ J/Ψ(μ + μ - )η(γγ,π + π - π 0 ), J/Ψ(μ + μ - )η (π + π - η,ρ 0 γ), η c (h + h - h + h - )Φ(K + K - ) Low yield, high background B s D s+ (K + K - π + )D s- (K + K - π - ) Low yield, degradation of proper time resolution Exaction of ΔM s using flavor-specific conol channel B s D s- (K + K - π - )π + Remark: B s J/ J/Ψ(e + e - )Φ(K(K + K - ) is expected to have more weight than the sum of the CP even eigenstates! 5

6 Decay diagrams Dominant ee Penguin conibution λ λ 4 u d π 6

7 pp 14 TeV Correlated forward production σ b 500 μb Forward specometer LHCb experiment bb Need prim. & b vertex reco for time measurements + subsequent decays separation Vertexing RICH system for PID Primary vertex B signal D K π Tracking PID to identify K/ π Muon detector L 1 cm b tag Flavour tagging 7

8 Basic ingredients for the analysis Candidate selection Daughters selection B s mass resolution Proper time Vertex/time resolution and time-dependent efficiency ε(t) Angular disibutions Tagging Mistag rate and tagging efficiency Perform a likelihood fit to the time-dependent decay rate, using angular disibution of the final decay products 8

9 Kaon ID: ΔL(K-π)> ΔL(K-p)> p)>- Vertex χ < 40 Pt>1GeV/c φ K + K - selection μ = 100 MeV/c σ = 3.5 MeV/c 9

10 J/ψ ψ e + e - selection Elecon: Apply bremssahlung reco algorithm (see next slide) ID: ΔL(e-π)>0 pt(e)>500 MeV e ± energy loss in VeLo e + e - vertex χ < 9 10

11 Bremssahlung: : reminder Radiation emitted by a charged particle caused by deceleration when passing through the field of atomic nuclei The energy emitted by an accelerated particle is proportional to 1/m : important for light particles In the relativistic limit, the radiated energy is contained in a narrow cone of average half-angle angle σ θ = 1/γ = mc /E ~ colinear to the particle propagation 11

12 Bremssahlung correction Matching between ack exapolation and ECAL cluster. T stations region Minimize estimator: r r T r r χbr = ( ) C ( ) + r r T r r ( cl ) Ccl ( cl ) r C, : ack exapolation position and error r r C cl, cl : cluster position and error Velo region ( χ br ) min value is then used to maximize #candidates with (p( rec rec -p MC )/p rec rec <10% 1

13 Velo de/dx Full window.1<m( m(e+e-)<4 GeV Side bands.1<m( m(e+e-)<.6 GeV + 3.4<m( m(e+e-)<4 GeV e - from J/ψ ψ e + e e - from J/ψ ψ e + e - - K - from φ K + K - Idea: suppress background e ± from γ e + e -. Bkg deposit in Si should be ~ times signal deposit. Mean de/dx(e) ) ~ Peak at de/dx(e)~.7: conversions Shoulder in the lower part: misid ed acks Suggested cut: 1.4<veloch veloch<1.8 13

14 Impact on m(e + e - ) disibution Pt(e) ) cut de/dx cut! Different evts + different version of the software (VeLo de/dx cut is not applied in what follows) 14

15 Bs signal Combine J/ψ and φ candidates to form a Bs vertex Improvement of Bs mass resolution using kinematical mass consaint of the daughters Use of primary vtx,, Bs vtx and Bs momentum to fit the proper time Background removal Cut on daughters masses.7<m(ee m(ee)<3.3 GeV/c 1.01<m(KK m(kk)<1.03 )<1.03 GeV/c Angle between flight vector and momentum Cos(p,F)>0.995 Prim vtx F Bs vtx p 15

16 B s mass disibution: mass consaint χ E Minimize the following χ : E E r r = p p r r v v0 T E E r r p p r r v v r 0 C 0 λi ( Ei pi i 0 r p, r v Lagrange multiplier m, Energy, momentum and decay vertex i, pdg ) Consaint on daughters parameters With consaint Without consaint Resonances mass consaint drastically improves the disib. Corrects both mean and width (~3 MeV/c for the core gaussian) 16

17 We have: x r r x τ = r r x, v, = r v r v μ =0.03 Σ τ = 1.15 Pull τ r p γ V τ m r x r p Proper time fit Primary & decay vertices + momentum We want: τ,(, p) Minimize: χ = R T W R v r r ~ r v v r ~ r R = p p r ~ r τ x ( v ~ r p) m μ =33.5 σ = 9.1 fs σ τ (ps) ~ ~ v r r Remarks: -Proper time error σ τ depends on τ: : per-event event error Resolution function will change evt per evt - σ τ to be corrected by scaling factor Σ τ 17

18 J/ψ ψ e + e - mass consaint and propertime fit No mass consaint Mass consaint e + Propertime pull bias e - PT bias located at the brems.. tail. Correction to slopes increase of J/ψ ψ ee mass flight length. Problem is ininsic to the physical process (and not due to CALO calibration pbs for example) Have to find another way (variable) to obtain a good signal resolution without biasing PT 18

19 m(b s )-m(e + e - )(+m(j/ψ) PDG ) The disibution of Δm (+ m(j/ψ) PDG ) has similar resolution to m(b s ) with mass consaint fit. Using this variable allow us to preserve an unbiased PT fit Price: throw one variable away m(b s )-m(e + e - )+m(j/ψ) PDG Core gaussian (9.3%): σ = 5 MeV/c m(bs) PDG m(b s ) with daughters mass consaint 19

20 Angular disibutions Differential rate dγ/d(cosθ )dφ d(cosθ φ ) In φ s rest frame θ φ K - expressed in terms of Transversity amplitudes: A 0 (t), A // (t) (CP even) A (t) (odd) Integrated one-angle disibutions: K + φ s dγ/d(cosθ ): ( A 0 + A // ) (1+cos (θ ))+ A (1-cos (θ )) dγ/d(cosθ φ ): A 0 cos (θ φ ) + ( A + A // ) sin (θ φ ) dγ/dφ : 3 A 0 + A + A // + ( A // - A 0 ) cos (φ ) K + K - φ s B s z J/ψ θ e + φ Analysis choice: - One angle: robust, easy to conol but limited access to parameters - Three angle: uses the full info. Ability to fit much more parameters but more complicated to handle e - 0

21 Angular disibutions (obtained with 50k signal evts) ( A 0 + A // ) (1+cos (θ ))+ A (1-cos (θ )) Reconsucted Matched MC 3 A 0 + A + A // + ( A // - A 0 ) cos (φ ) cos(θ ) φ A 0 cos (θ φ ) + ( A + A // ) sin (θ φ ) The obtained disibutions match the expectations cos(θ φ ) 1

22 Opposite side Flavour tagging Charge of the kaon in the b b c s chain Charge of the lepton in semi-leptonic decays Charge of accompanying b jet Same side Charge of the K accompanying B s Charge of the π from B** B*π ± tagging B signal B ε = Definitions: N R : correctly tagged evts N W : wrongly tagged evts N U : untagged evts N R NR + N + N + W W N U ω = N R N + W N W

23 One angle analysis dγ Bs ( t) d cosθ ( f ) ( A0 ( t) + A// ( t) )(1 + cos θ ) + A ( t) (1 cos θ ) Γ ΔΓ t ΔΓ t st s s A0,// ( t) = A0,// (0) e cosh cosφs sinh + Dqi sinφs sin ΔM st A ( t) = A (0) e Γ t s ΔΓst cosh ΔΓst + cosφs sinh = RT f A A (0) f (0) Dq i sinφ sin ΔM s s t CP-odd fraction q i = +1 if tagged as B s -11 if tagged as B s 0 if untagged Parameters to determine: φ s, ΔΓ s, ΔM s, R T Cannot fit for all the parameters independently For example D and φ s come in the same term: 100% correlated ΔM s and dilution factor D= 1-ω tag obtained from conol sample B s D s- π + Some work needed to anslate D from conol sample into D for signal 3

24 B s D - s π + decay rates and ΔM s R R Ideal rate ( ΔΓst e ΔΓst B s ( t) Dsπ ) (cosh ± cos ΔM st) ( ) + dilution from flavour tagging ( ΔΓst e ΔΓst B s ( t) Dsπ ) (cosh ± D cos ΔM st) ( ) + Proper time resolution ~40 fs R ( ue ue rec B s ( t ) D π ) G( t t, Σ τ στ ) ( ) + Background S/B ~ 3 + Acceptance s Events Perfect reconsuction + flavour tagging + proper time resolution + background + acceptance ε ( t rec ( ) ). R( B s ( t ue ) D π ) G( t s ue t rec, Στσ τ ) Proper time (ps) 4

25 dγ( Bs ( t) f ) d cosθ dφ d cosθ sin + θ φ Three angles analysis φ A ( t) [ ] * A ( t) (1 sin θ sin φ ) + A ( t) sin θ I( A ( t) A ( t))sin θ sinφ // 1 sin θ φ cos * * [ R( A ( t) A ( t))sin θ sin φ + I( A ( t) A ( t))sin θ cosφ ] 0 // 0 θ (1 sin φ θ 0 cos φ ) + // Two more observables available One more parameter R 0 ( A 0 fraction) needed for the non-interfering terms Interference terms involve two (relative) song phases between ansversity amplitudes, δ 1 and δ D= 1-ω tag could in principle be exacted from the interference terms if φ s is not too big 5

26 Final fit Given N obs events falling in the acceptance after applying all the cuts, build Likelihood L( B Nobs rec τ rec [ f sig Lsig ( mi ). Lsig ( t, σ i, qi, { αi} ) + fbkglbkg ( mi ). Lbkg ( t,{ αi} )] i i i= 1 s J / ψ φ) = ( N sig + N N obs bkg! ) N obs e ( N Involves B s mass, m i, fitter proper time and associated error, angle(s) ) and tagging output (q( i =0,±1). Background: inclusive b b N sig, N bkg : expected #signal and background events. f sig, f bkg : signal and background fractions sig + N bkg ) 6

27 A glimpse at the background Ran on 78k b b events. Had to relax all the cuts to obtain disibutions m(b s ) (MeV/c ) Signal (for comparison) τ (ps) τ (ps) 7

28 Conclusions Hope that the B s J/ J/Ψ(e + e - )Φ(K + K - ) channel conibutes to the φ s measurement as soon as first data is available Still some improvements to be expected in J/Ψ e + e - selection Optimization of Bremssahlung recovery Background fighting (converted γ,, etc..) Status of the analysis Most of the signal disibutions are known Background disibutions are pending more stat. Still need to study effect of igger cuts + tagging + acceptances (proper time, angles) before performing toy studies on sensitivity to mixing param. 8