Overview of LHCb Experiment

Transcription

1 Overview of LHCb Experiment Yuanning Gao, Tsinghua University Representing the LHCb Collaboration Detector performance CKM triangles Other topics (selected) Conclusions A very selective review! Sino-French workshop on LHC physics and Associated Grid Computing, Beijing, Dec , 26

2 LHCb is a dedicated B physics experiment at the LHC Mont Blanc Geneva airport LHCb ATLAS CERN CMS ALICE LHC tunnel

3 b Forward spectrometer (running in pp collider mode) b b b doesn't occur θb 3 [ra 2 d] θb [rad] b pt of B-hadron θb b Pythia production cross section 1 2 ATLAS/CMS 1 μb LHCb 23 μb 1 σ b b (14TeV ) ~ 5μb eta of B-hadron ~ 112 b b / L = cm 2s 1 Great potential for B physics!

4 As a next generation B experiment, LHCb provides: high statistics B d and B s samples robust and efficient trigger, even for non-leptonic decays good decay vertex resolution; good tracking; good particle ID

5 Examples for detector performance B s oscillation frequency Fully reconstructed decay excellent momentum resolution Decay length resolution ~ 2 μm Proper time resolution ~ 4 fs 5σ measurement in one year data (2 fb -1 ) for Δm s up to 8 σ 1 = 33± 1 fs σ 2 = 67± 3 fs (31%) D s π t rec t true [fs] 68 ps -1 CDF result: (PRL 97: 623) Δ = ±.7 ps m s LHCb needs 1 month data to confirm!

6 Examples for detector performance Particle Identification B S KK signal Typical event in the RICH1 photon detectors y (cm) x (cm)

7 Examples for detector performance π reconstruction

8 Flavor Tagging opposite side same side Method (For B S ) μ ± e ± K ± same K ± opp Jet charge εd 2 (%) Expect εd 2 ~ 7-9% for B S & 4-5% for B d

9 CKM Triangles The Standard model CP violation is described by the single complex phase in the CKM matrix, two unitarity triangles relevant to B physics at LHCb statistics To test SM and explore possible new physics, The primary goal of the LHCb experiment is to over constraint the unitarity triangles

10 CKM Triangles: φ s Just as B o J/ψ K S measures CPV phase β, B S J/ψ φmeasures CPV B S mixing phase φ S Angular analysis is necessary The width difference ΔΓ S /Γ S also enters in the fit K - θ φ φ K + z θ tr l + 12, events in 2fb -1 J/ψ y σ(φ S ) =.23 x φ tr l -

11 CKM Triangles: α from B d π π π + decays Dalitz plot analysis (Quinn m 2 (π π ) ρ + π ρ π Snyder method) - B d π π π + selection based on multivariate analysis - Use resolved and merged π ρ π + - Expect 14k events per year m 2 (π π + ) Combined discriminant variable 11-parameter likelihood fits performed in timedependent Dalitz space B/S =.8 (flat and resonant bkg) α gen =16 σ(α) ~1 with 2fb -1

12 CKM Triangles: γ 1. Measurement of the angle γ from B s D s± K + CP asymmetry arises from interference between two tree diagrams via B s mixing: B s D s+ K and B s D s K + b c + W + tree B s b s W s u c s K + D s b u + W + tree B s b s W s c u s + D s K Measures γ 2 φ S extract γ

13 The strong phase difference of the two diagrams can be resolved by fit two time-dependent asymmetries: Phase of D s+ K asymmetry is Δ (γ 2 φ S ) Phase of D s K + asymmetry is Δ + (γ 2 φ S ) can extract both Δ and (γ 2 φ S ) Background from Bs D s π is suppressed using PID information from RICH1 & RICH2 remaining contamination ~ 1% Asym (D s K + ) Asymmetries for 5 years of simulated data Reconstruct using D s K K + π 54 signal events/year + Asym (D s K ) σ(γ) ~ 14 in one year Theoretically clean; insensitive to new physics t [ps]

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15 CKM Triangles: γ 2. Measurement of the angle γ from B π + π, B s K + K b u processes with possible large b d(s) penguin contributions Tree diagram Penguin diagram (example) W d(s) u π + (K + ) b W t c u d(s) u π + (K + ) B (B ) s b d(s) u d(s) π (K ) d(s) g u d(s) π (K ) Measure time-dependent CP asymmetries for B π + π and B s Κ + Κ A CP (t) = A dir cos(δmt) + A mix sin(δmt) extract four asymmetries A dir (B π + π ) = f 1 (d, θ, γ) de iθ = ratio of penguin and tree A mix (B π + π ) = f 2 (d, θ, γ, β) amplitudes in B π + π A dir (B s Κ + Κ ) = f 3 (d, θ, γ) d e iθ = ratio of penguin and tree A mix (B s Κ + Κ ) = f 4 (d, θ, γ, φ s ) amplitudes in B s Κ + Κ

16 Assume U-spin flavour symmetry (under interchange of d and s quarks) d=d and θ = θ [R. Fleischer, PLB 459 (1999) 36] Taking β and φ s from other channels can solve for γ σ(γ) ~ 5-1 in one year Theoretical uncertainty from U-spin assumption (can be tested); Sensitive to new physics in the penguin loops

17 CKM Triangles: γ 3. Measurement of the angle γ from B D K *, D K * CP asymmetry arises from interference between two tree diagrams via D -mixing D D mixing

18 Measure six rates ( following 3 + CP-conjugates ) γ can be extracted from triangles [Gronau and Wyler, PLB 265 (1991) 172, Dunietz, PLB 27(1991) 75] (1) B D ( K + π ) + K * (2) B D ( K π + ) + K * (3) B D CP ( K + K ) + K * D CP = (D + D ) / 2 : CP-even eigenstate of D D system 3.4K,.5K and.6k events respectively for one year data taking σ(γ) ~ 7-1 in one year No theoretical uncertainties; sensitive to new physics in D CP

19 CKM Triangles: γ 4. Measurement of the angle γ via ADS method

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23 LHCb performance for CKM Triangles with 2fb-1 1 (1 year)

24 CKM Triangles: Outlook A possible scenario after the LHCb measurement: new physics?

25 Other topics: Bs μ + μ

26 Other topics: B c In addition to B and B s, LHCb also records B C, and b baryons Working in progress on exploring the by-product A joint efforts on B C : LAL-Tsinghua Theoreticians from China and France

27 Potential for Charm physics LHCb can also trigger and collect charm events events are essential for the calibration of RICH detectors High statistics (~1 8 events), with precise vertexes and particle identification, for

28 The power of PID

29 Conclusions LHCb is dedicated to the study of B physics, with a devoted trigger,excellent vertex and momentum resolution, and particle identification LHCb will give unprecedented statistics for B decays, including access to the B s meson, unavailable to the B factories B s B s oscillations will be measured precisely Many measurements of rare decays and CP asymmetries will be performed CP angles determined via channels with different sensitivity to new physics detailed test of the CKM description of the quark sector by-products are under exploring