Precision measurement of

Transcription

1 Precision of Francesca Dordei University of Heidelberg, Physikalisches Institut b 3rd IMPRS-PTFS Seminar, Heidelberg - 24 th April 2012 (Heidelberg University) / 24

2 and the LHCb detector CP violation CP in interference of Bs 0 decay and mixing φ s in Bs 0 J/ψφ Challenges of b (Heidelberg University) / 24

3 The Large Hadron Collider (LHC) b accelerates protons in both directions until they collide at four intersection points with a central mass system energy of 8 TeV (7 TeV in ). The four main s located at the intersection points are: ATLAS, CMS, ALICE and LHCb (Heidelberg University) / 24

4 b LHCb Forward single arm spectrometer Acceptance: (250) mrad Copious source of b,c in the forward region ECAL HCAL Unique eta coverage Tracking Muon Hadron PID Counters bb angular correlation Studies LHCb detector CP s require : Time dependent analysis need good time resolution Flavour tagging needs particle IDentification High statistics, purity and efficiency needed to reach SM predictions. b Lumi (Heidelberg University) / 24

5 b detector LHCb was built precisely for this purpose! b (Heidelberg University) / 24

6 Short introduction to CP The only source of the SM is the Kobayshi-Maskawa mechanism, which predicts the existence of a phase factor in the 3x3 CKM matrix: V CKM = V ud V us V ub V cd V cs V cb V td V ts V tb The request that the CKM matrix is unitary leads to relations between the elements: V ub V ud + V cb V cd + V tb V td = 0 V ub V us + V cb V cs + V tb V ts = 0 b The heigth of the triangle depends on the value of the imaginary phase. In this talk I will focus on CP in interference of B 0 s decay and mixing. (Heidelberg University) / 24

7 B 0 s -B0 s mixing and B0 s J/ψφ decay B 0 s -B0 s mixing s V ts V tb b Time development of the mixing described by phenomenological Schroedinger eq: B s W u, c, t W B s i d ( Bs ) ( dt B = M i s 2 Γ ) ( B s ) B s Diagonalizing it in terms of mass eigenstates: i d dt (B L) = ( M L i 2 Γ L) (BL ) i d dt (B H) = ( M H i 2 Γ H) (BH ) B 0 s J/ψφ decay b V tb u, c, t V ts Phenomenological mixing parameters: Γ = Γ L Γ H M = M L M H Mixing phase: φ M = 2 arg (V ts V tb ) s b Decay dominated by tree level Decay phase: φ D = arg (V cs V cb ) 0 (Heidelberg University) / 24

8 CP in interference of B 0 s decay and mixing The interference between B 0 s decays to J/ψφ with or without B0 s -B0 s oscillation allows the of φ s, via CP violation. The SM prediction is very precise: φ SM s = 2β s = 2 arg ( V ts Vtb /V csvcb ) = ( ± )rad [J.Charleset al., Phys.Rev. D84, (2011)] φ s sensitive to New Physics (eg. 4th generation in the box): B s s b V ts V tb W u, c, t u, c, t W V tb V ts b s B s φ s φ SM s + φ NP s b (Heidelberg University) / 24

9 φ s, the CP phase, a bit of history Tevatron s have looked to B 0 s J/ψφ, deraving confidence intervals for φ s. Originally they found a combined 2σ deviation from 2β s. Deviation has decreased with more data, but σ(φ exp s ) still much larger than φ SM s. Note the 2-fold ambiguity Decay rates invariant under transform: φ s π φ s δ 2π δ Γ s Γ s δ δ b [D0 Prelim, S.Burdin] (Heidelberg University) / 24

10 Analysis strategy CP Asymmetry : What we measure is: A CP = Γ (Bs(t) (J/ψphi) CP Γ B s(t) (J/ψphi) CP Γ (B s(t) (J/ψphi) CP +Γ B s(t) (J/ψphi) CP Decay time: fast Bs 0 oscillation needs to be resolved ( m s = 17.63ps 1 ); Flavour tagging: to separate B 0 s from B0 s Mass: to separate signal from background;!!! B 0 s J/ψφ is not a CP eigenstate. A CP = η CP sin(φ s ) sin( m s t) b (Heidelberg University) / 24

11 Separation of CP eigenstates B 0 s J/ψ( µ + µ )φ( K + K ) Pseudoscalar to vector mesons (J PC = 1 ) decay: final states CP odd and CP even. Three polarisation amplitudes and phases: A 0 2, A 2, δ 0, δ (CP-even) A 2, δ (CP-odd) L = 0, 2 CP = ( 1) L = +1 L = 1 CP = ( 1) L = -1 b Angular analysis in θ, φ, ψ to separate CP=±1 states and extract φ s. (Heidelberg University) / 24

12 B 0 s J/ψφ angular and proper time distributions clear separation of CP even and CP odd states different shapes in angular distributions b (Heidelberg University) / 24

13 Decay time resolution Fast B 0 s oscillation need to be resolved! Time resolution affect the observed A CP: A CP η CP sin(φ s ) D σct sin( m s t) D σct exp[ ( m s σ ct ) 2 /2] We need good proper time resolution σ ct w.r.t. sinusoid period of oscillation 350fs, and excellent knowledge of σ ct in data. How to determine σ ct in data?!! IDEA Reconstructing fake Bs 0 with zero lifetime; Using prompt J/ψ bkg plus 2 random tracks: τ = 0 ± σ ct Decay time resolution σ ct 45 fs b [LHCB-CONF ] (Heidelberg University) / 24

14 B 0 s J/ψφ in LHCb Different lifetimes for CP odd and CP even components Γ s = Γ L Γ H Γ CP odd Γ CP even b [LHCB-CONF ] L = 1fb 1 Very pure sample: signal candidates. - background O(%) World s largest B 0 s J/ψφ dataset! (Heidelberg University) / 24

15 Flavour tagging - B 0 s or B0 s? Tagging: determine flavour of decaying B 0 s -meson. b tagging efficiency ɛ tag = (32.99 ± 0.33)% wrong tag probability ω tag = (36.81 ± 0.18 ± 0.74)% effective tagging power ɛ tag (1 2ω tag ) 2 = (2.29 ± 0.07 ± 0.26)% A CP η CP (1 2ω tag ) sin(φ s ) D σct sin( m s t) (Heidelberg University) / 24

16 Other ingredients There are still other ingredients needed: Proper time acceptance. Angular acceptances. Mass distribution modelling. Background composition and acceptances. S-wave modelling.... b (Heidelberg University) / 24

17 B 0 s J/ψφ φ s Γ s profile Likelihood contour plot b [LHCb-CONF ] Γ s = ± (stat.) ± (syst.) ps 1 Γ s = ± (stat.) ± (syst.) ps 1 φ s = ± (stat.) ± (syst.) rad Two-fold ambiguity resolved in a different! Solution with positive Γ s is preferred at 4.7σ. [LHCb-PAPER , arxiv: [hep-ex]] (Heidelberg University) / 24

18 Systematic Uncertanties b The dominant contribution for Γ s from the proper time acceptance. The dominant contribution for φ s from the CP in mixing and decay. In view of more data, with the goal of separating the observed φ s from the SM prediction to see if New Physics is playing a role, the must be very precise. Systematic uncertainty need to be reduced! (Heidelberg University) / 24

19 Nuisance «CP» There are several effects that may affect the the CP asymmetry: Bs 0-B0 s production. A difference in the production rate of B and B introduces a production asymmetry Tagging efficiency. A different probability to tag B and B may cause a tagging efficiency asymmetry. Wrong-tag probability. A different probability for a wrong tag for B and B can be parametrized by wron tag asymmetry. Additional mixing and/or decay. Can be parametrized by a parameter λ. b (Heidelberg University) / 24

20 Systematic uncertainties In the past this systematic uncertainties due to nuisance CP were: production asymmetry (ν p ) performing a toy study, which includes a ν p = 10% and fitting a decay model without the nuisance asymmetry. The bias in the parameter of interested is the resulting systematic uncertainty; additional mixing and/or decay fitting for it in data to get a feeling of the magnitude and then using the same procedure as before with λ 2 = 1 ± 5%; tagging/wrong tag already covered by the uncertainties on the tagging calibration parameters. The systematic uncertainties quoted are : Γ s Γ s φ s Additional CP in mix and/or decay Production asymmetry Total b (Heidelberg University) / 24

21 CP in mixing and/or decay Can we fit directly in data for the additional CP in mixing and/or decay? Defining λ = q p q,p complex numbers that define Bs 0 mass eigenstates in terms of flavour eigenstates B H,L >= p Bs 0 > q B 0 s > A f and A f the decay amplitudes. Both the Bs 0 mixing q/p 1 or the the decay (direct CP) A f /A f 1 result in λ 1. A f A f b Parametrizing λ as: λ = λ e iφs and allowing for λ 1 is it possible to fit for the CP in mixing and/or decay. (Heidelberg University) / 24

22 Fitting for the CP in mixing and/or decay Allowing for λ 1 is it possible to fit for the CP in mixing and/or decay. Making toy to check if there are bias introduced fitting for λ ; I generated the same statistics as in data and run over 500 toys. looking to the pull distribution for the parameter ϑ: φ pull distribution s φ s pull distribution Mean = ± Width = ± s s fitted generated (φ -φ )/σ ϑ fitted ϑ generated σ(ϑ fitted ) pull distribution We can fit for the CP in mixing and/or decay without any bias! λ λ pull distribution Mean = ± Width = ± fitted -λ (λ generated )/σ b (Heidelberg University) / 24

23 What about the other? Included performing a toy study Adding a tagging efficiency, a wrong tag probability and a production asymmetry in generation fitting using a decay model without any asymmetry. The bias in the parameter of interested is the resulting systematic uncertainty on the parameter: Γ s Γ s φ s Tagging efficiency asym Wrong tag asym Production asym All asimmetries CP mix and/or decay Production asym b (Heidelberg University) / 24

24 b [LHCb-CONF ] First direct observation of a non-zero value for Γ s World s most precise of Bs 0 J/ψφ The of φ s is : φ s = ± (stat) ± (syst) rad * In combination with φ s from B 0 s J/ψπ + π (Heidelberg University) / 24

25 More thoery Ambiguity Backup slides More about the pdf (Heidelberg University) / 24

26 More theory More thoery Ambiguity More about the pdf (Heidelberg University) / 24

27 Ambiguity More thoery Ambiguity More about the pdf (Heidelberg University) / 24

28 More about the pdf Signs in blue are tag dependent and change for B 0 s More thoery Ambiguity More about the pdf (Heidelberg University) / 24