Precision of Francesca Dordei University of Heidelberg, Physikalisches Institut b 3rd IMPRS-PTFS Seminar, Heidelberg - 24 th April 2012 (Heidelberg University) 24-04-2012 1 / 24
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) 24-04-2012 2 / 24
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 2010-2011). The four main s located at the intersection points are: ATLAS, CMS, ALICE and LHCb (Heidelberg University) 24-04-2012 3 / 24
b LHCb Forward single arm spectrometer Acceptance: 15-300(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) 24-04-2012 4 / 24
b detector LHCb was built precisely for this purpose! b (Heidelberg University) 24-04-2012 5 / 24
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) 24-04-2012 6 / 24
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) 24-04-2012 7 / 24
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 ) = ( 0.0363 ± 0.0016)rad [J.Charleset al., Phys.Rev. D84, 033005 (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) 24-04-2012 8 / 24
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) 24-04-2012 9 / 24
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) 24-04-2012 10 / 24
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) 24-04-2012 11 / 24
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) 24-04-2012 12 / 24
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-2012-002] (Heidelberg University) 24-04-2012 13 / 24
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-2012-002] L = 1fb 1 Very pure sample: - 21200 signal candidates. - background O(%) World s largest B 0 s J/ψφ dataset! (Heidelberg University) 24-04-2012 14 / 24
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) 24-04-2012 15 / 24
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) 24-04-2012 16 / 24
B 0 s J/ψφ φ s Γ s profile Likelihood contour plot b [LHCb-CONF-2012-002] Γ s = 0.6580 ± 0.0054 (stat.) ± 0.0066 (syst.) ps 1 Γ s = 0.116 ± 0.018 (stat.) ± 0.006 (syst.) ps 1 φ s = 0.001 ± 0.101 (stat.) ± 0.027 (syst.) rad Two-fold ambiguity resolved in a different! Solution with positive Γ s is preferred at 4.7σ. [LHCb-PAPER-2011-028, arxiv:1202.4717[hep-ex]] (Heidelberg University) 24-04-2012 17 / 24
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) 24-04-2012 18 / 24
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) 24-04-2012 19 / 24
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 0.0003 0.002 0.020 Production asymmetry 0.0002 0.002 0.008 Total 0.0066 0.006 0.027 b (Heidelberg University) 24-04-2012 20 / 24
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) 24-04-2012 21 / 24
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 35 30 25 20 15 10 5 φ s pull distribution Mean = 0.024 ± 0.044 Width = 0.992 ± 0.031 s s 0-5 -4-3 -2-1 0 1 2 3 4 5 fitted generated (φ -φ )/σ ϑ fitted ϑ generated σ(ϑ fitted ) 25 20 15 10 5 pull distribution We can fit for the CP in mixing and/or decay without any bias! λ λ pull distribution Mean = -0.040 ± 0.046 Width = 1.018 ± 0.032 0-5 -4-3 -2-1 0 1 2 3 4 5 fitted -λ (λ generated )/σ b (Heidelberg University) 24-04-2012 22 / 24
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 0.0002 0.003 0.007 Wrong tag asym 0.0003 0.002 0.006 Production asym 0.0001 0.001 0.005 All asimmetries 0.0003 0.002 0.007 CP mix and/or decay 0.0003 0.002 0.020 Production asym 0.0002 0.002 0.008 b (Heidelberg University) 24-04-2012 23 / 24
b [LHCb-CONF-2012-002] 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 = 0.002 ± 0.083 (stat) ± 0.027 (syst) rad * In combination with φ s from B 0 s J/ψπ + π (Heidelberg University) 24-04-2012 24 / 24
More thoery Ambiguity Backup slides More about the pdf (Heidelberg University) 24-04-2012 25 / 24
More theory More thoery Ambiguity More about the pdf (Heidelberg University) 24-04-2012 26 / 24
Ambiguity More thoery Ambiguity More about the pdf (Heidelberg University) 24-04-2012 27 / 24
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) 24-04-2012 28 / 24