Experimental review and perspectives of CP violation in the B system

Transcription

1 Ida Ida Peruzzi Peruzzi Università Università di di Perugia Perugia & L.N.F. L.N.F. Experimental review and perspectives of violation in the B system Introduction The phenomenology of of violation in in B meson decays Present experimental status Future Prospects: Evolution of of present experiments Approved or or planned new new projects

2 The b quark: historical highlights Extension of the Cabibbo theory to 3 quark doublets. (Kobayashi, Maskawa, 1973) Discovery of the τ lepton (Perl, 1976) Discovery of the b quark (Lederman, 1977) Discovery of the B mesons (CLEO, 1979) b lifetime measurement (MAC, MARKII, 1983) Discovery of B mixing (UA1, ARGUS, CLEO,...,1987) Asymmetric B-factories proposal (Oddone, 1987) sin (2β) measurement (BABAR, BELLE, 21)

3 violation in the Standard Model Mixing between the three quark generations expressed by the unitary Cabibbo-Kobayashi-Maskawa (CKM) matrix A ( i ) λ λ λ ρ η Vud Vus Vub V = Vcd Vcs Vcb = λ 1 λ Aλ + O( λ 2 V V V 3 2 td ts tb Aλ (1 ρ iη) Aλ 1 Unitarity relations can be represented by triangles All triangles have same area Only the B triangle has all sizes of the same order Violation non zero area η = No violation in SM 4 )

4 B Physics Measurements of all sides and angles of the unitarity triangle Stringent tests of the SM through a large number of measurements of related variables Possible discrepancies would also point to the SM sector where new Physics should be introduced Main goals for the B-factories: To study the asymmetries from violation To study rare B decays Discover effects due to new Physics! Discover effects due to new Physics!

5 Indirect Constraints on the Angles Höcker et al, hep-ex/1462 (+ many other recent global CKM matrix analyses) -averaged measurements: J > at ~ 1.7 σ Evidence for violation from -averaged observations consistent with violation in kaon system (expressed by ε K ) Constraints on -violating observables (95% c.l.) : sin 2β sin 2α γ [ ] [ 1.5 ] [ ]

6 b Quark Decays Tree Decays: Penguin Decays : B Oscillations :

7 B hadron decays: tree diagrams Cabibbo favored : D - π +, D * - π + +, D π, J/Ψ Κ. D - D s + ( Aλ2 ) Cabibbo suppressed : D s + π -, D - K +, D* - K +, D - D +, π - π +, J/Ψ π, D Κ,D Κ, π π ( Aλ 3,Aλ 3 (ρ iη) ) Cabibbo doubly suppressed: K + π - -, Κ π ( Aλ 4 (ρ iη) ) Several final states have contribution from penguin diagrams, the effect is is larger if if the tree diagram is is suppressed

8 B hadron decays: penguin diagrams Some final states can only be reached via penguin diagrams: Φ π, η π, Φ Κ, η Κ, Κ Κ, π γ, Κ γ, Κ l + l final states with : K*, ρ, ω, η etc BR 1-6 measurement possible at present B-factories Charged B meson decays have similar diagrams

9 A A = Direct violation A e i A e i i i ( δ φ ) i ( δ +φ ) i i i B f B f δ 1 δ 2 strong phases Γ(B f ) Γ(B f ) = 2 A1 A sin δ sin ϕ Γ(B f ) + Γ(B f ) A 2 φ 1 φ 2 weak phases Possible also in charged B decays, example: A if A A 1 A 1 A 2 N(B + K + π ) N(B K π ) BUT: A tiny if A 1 >> A 2 must utilize tree suppressed or penguin only decays

10 violation in the mixing (indirect ) A BUT: A B B B B B B B B Γ(B Γ(B B B l l + = + expected to be tiny For B d m >> Γ Re(ε Bd ) MS expectation: A T νx) Γ(B νx) + Γ(B ALEPH OPAL(1997) OPAL(2) CLEO: BABAR: B B l l Could be measured with same sign lepton pairs: N(l N(l + + l l + + ) ) + N(l N(l l l ) ) # B B 4x1 5 3x1 5 3x1 5 5x νx) νx) q p A = 1 1 ε + ε 4 Re( ε 1+ ε if -.3±.7(stat+syst) B B B 2 B q p.2±.7(stat) ±.3(syst).1±.14(stat) ±.3(syst).35±.13(stat) ±.15(syst).12±.29(stat) ±.36(syst) ) 1

11 violation in the interference decay/mixing t = B B mixing A f A f f t violation arises from interference between the two paths : - direct decay: B f - decay after oscillation: B B f Final state a eigenstate reachable by both B flavors Bphys(t) pure b at t = B phys (t) pure b at t = A f (t) = Γ(B Γ(B phys phys (t) f (t) f ) Γ(B ) +Γ(B phys phys (t) f (t) f ) )

12 A Asymmetry in the interference mixing/decay The observable asymmetry A is f (t) is a function of of the oscillation time tt (t= when the B is is a pure b or or b state f (t) = C A f A = A cos( m B d t) + S f BEST CASE: λ f = 1 f 1 (t) = Im( λ f ) sin( m sin( m IF NO DIRECT VIOLATION (Cabibbo favored tree decays SINGLE phase large BR!) B d t) Mixing eigenvalue B λ d f t) C S f f = η = = f q p 1 λ 1 + λ 1 + λ Ratio of amplitudes f f f 2Imλ A A f f f 2 2 2

13 Measurement of the angles α, β, γ V α tdv arg VudV V β cdv arg VtdV V γ udv arg VcdV * tb * ub * cb * tb * ub * cb B d J/Ψ K S (=-1) V = V V V V V V V V * * * tb td cs cb cd cs λ J / Ψ K S * * * tbvtd csvcb cdvcs B d D + D - V = V V (=-1) V V V * * tb td cd cb λ DD * * tbvtd cdvcb B d J/Ψ K, Ψ K, χk, (K = K S o K L, K*, ) B d DD, DD*, D*D* B d φk, η K, φk* ηk* B s J/Ψ φ B d D*π B d B d Decay π + π -, ρπ,, Aπ φπ, ΚΚ Imλ J/ΨΚ = sin2β Imλ DD = sin2β 7x1-4 4x1-4 Process b ccs b ccd b sss b ccs b cud b uud b ssd BR φ β β β γ 2β+γ Imλ = ±sin2φ, φ = α, β, γ α

14 The Asymmetric B-factory The Y(4S) is the cleanest B d source Each event is made of 2 monochromatic B d s Simple and efficient final state reconstruction and flavor id The BB system evolves coherently until first B decays t (oscillation ) = time difference between the two decays But: a integrated over time is exactly zero! It s necessary to measure the decay distance But: B mesons do not live long enough to measure t Y(4S) boost (asymmetric beams) But: the cross section is only ~ 1nb ( > 1 7 M B s needed) Luminosity must be > 1 33 cm -2 sec -1

15 KEK-B (BELLE) (BABAR) PEP-II Low Energy Ring (LER) for Positron WIGGLER RF RF NIKKO Area HER TSUKUBA Area (Belle) HER LER Interaction Region (TRISTAN Accumulation Ring) Electron Positron e + /e - High Energy Ring (HER) for Electron LER WIGGLER RF RF OHO Area βγ=.45 e - 8 GeV 9GeV 3.5 GeV e GeV βγ=.56 RF RF FUJI Area Linac BELLE L = cm -2 sec -1 1 B /sec!

16 BELLE INTEGRATED LUMINOSITY

17 BABAR INTEGRATED LUMINOSITY LP1 Recorded Lumin.: Shift: pb pb -1-1 Day: pb pb -1-1 Week: 1758 pb pb -1-1 Month: 63 pb pb fb fb -1-1 DATA SET SET FOR LATHUILE & MORIOND 32 pb -1 in the last 3 shifts! 32 pb -1 in the last 3 shifts!

18 Measurement technique e Υ( 4S) rec B rec B flav B = B = B tag B rec z t z/ < βγ > c ( eigenstates) (flavor eigenstates) l K S K µ + µ J/ψ π Exclusive B Meson Reconstruction analysis B-Flavor Tagging + π tag, lifetime, mixing analyse

19 Flavor tagging (BABAR) Lepton Kaon NT1 NT2 electron (muon ) charge, p* > 1. (1.1) GeV/c Net charge of identified K Neural network discriminating variables using leptons and kaons not well identified, soft pions, D*, etc. Mis-tag fraction from B flav sample, with one B completely reconstructed: w i = mistag fraction; D i = Dilution = 1-2w i ε i = fraction of tagged events Q i = quality factor = ε i D i 2 Category ε i (%) w i (%) Q i (%) Lepton 1.9 ± ± ±.5 Kaon 35.8 ± ± ±.9 NT1 7.8 ± ± ±.4 NT ± ± ±.3 ALL 68.4 ± ± 1.2

20 Decay modes for sin 2β J/Ψ K S -J/Ψ K L : No penguin contamination, favorable S/B ratio Same value and opposite sign asymmetry More charmonium states; (and also K* with angular analysis) DD, DD*, D*D*: Penguin pollution Only DD is pure state angular analysis neededf or D* φk, η K, πk : Penguin dominated (or pure penguin) Small BR s Direct violation possible (A/A 1) At At the the beginning the the results from different modes can can be be combined to to improve the the statistical error For For a stringent test test of of the the S.M. S.M. it s it snecessary to to measure the the asymmetry in in in in each channel and and compare the the results

21 SAMPLE (BABAR) Events / 2.5 MeV/c Before tagging: BABAR J/ψ K S K S π + π - Events / 2.5 MeV/c BABAR J/ψ K S K S π π Beam-Energy Substituted Mass (MeV/c ) data 32 x 1 6 BB pairs, 29 fb -1 on peak Events / 2.5 MeV/c Beam-Energy Substituted Mass (MeV/c ) BABAR ψ(2s) K S Beam-Energy Substituted Mass (MeV/c ) Events / 2.5 MeV/c BABAR χ c1 K S Beam-Energy Substituted Mass (MeV/c ) 2 Sample After tagging: tagged events Purity [J/ψ,ψ,χ c1 ] K S 48 96% -1 J/ψ K L % +1 Events / 2.5 MeV/c 25 BABAR J/ψ K* J/ψ K L J/ψ K* (K S π ) Full sample % 8% mi x Beam-Energy Substituted Mass (MeV/c )

22 Systematic Errors (BABAR) Signal resolution and vertex reconstruction =.3 Resolution model, outliers, SVT residual misalignment Tagging =.3 Studies of possible differences between B and B flavor samples Backgrounds =.2 (overall) Signal probability, peaking background, content of background Total.93 for J/Ψ K L channel;.11 for J/Ψ K * Total =.5 for full sample Will improve with more data and more refined analyses

23 IN THE B SYSTEM IS ESTABLISHED!

24 Check for Direct Violation If more than one amplitude matters λ 1 Fitting with λ as a free parameter to: A f (t) = C f cos( m B d t) + S f sin( m B d t) BABAR : λ =.93 ±.9 (stat.) ±.3 (sys.) BELLE : λ = 1.3 ±.9 (stat.) No evidence for direct Violation due to decay amplitude interference; consistent with SM expectation

25 sin 2β World Results OPAL 98 CDF ALEPH Mean Value.79 ±.1 BELLE 1 BABAR 1 AVERAGE.59 ±.14 ±.5.59 ±.14 ±.5.99 ±.14 ±.6.99 ±.14 ±.6

26 Prospects for sin (2β) (Winter 22 Conf.)

27 Measurement of sin 2α In principle α could be measured from b d u u (B π + π -, B ρ + π - ) But: tree diagrams are Cabibbo suppressed and penguin contribution could be sizeable. More phases are introduced by penguin diagrams and by penguin-tree interference DIRECT violation ( A/A 1 is also possible, test with B ± s) cos( m t) term. Effect can be expressed as: sen 2 α eff = sen (2α + 2 δ Ping ) The 2δ Ping angle can be measured from triangular relations between amplitudes into isospin states π + π, π π Experimental problem : B π π

28 BR s of 2-body: B π + π -, Κ + π -, Κ + Κ Consistent picture: KK < ππ <Kπ Consistent picture: KK < ππ <Kπ

29 Violation in B π + π decays b t τ e f± ( t) = f d m 4τ [ 1± S sin( m t) C cos( m t) ] u b u f d d u f f + B B tag tag tree diagram d penguin diagram u Significant penguin contribution expected with different weak phase P/T.3 C ππ, S ππ = sin2α eff

30 Fit and Results (BABAR) Extended ML fit to the BRs and done simultaneously: ~1 pre-selected two prongs candidates in ~3.4 fb -1 5 tagging categories (leptons, K, NT1, NT2, untagged) 8 event species (Sig and Bkg: π + π -, K + π, K - π +, K + K - ) Discriminating variables (m ES, E, F, θ c1, θ c2, t) Dilutions, R( t) for the signal taken from sin2β analysis m d, B lifetime fixed as in sin2β analysis R( t) for the background taken from sidebands in m ES distribution A S( π C( π (K + + ± π π π m ) ) ) =.3 = (stat) ±.11(syst) (stat) ±.14(syst) =.7 ±.8(stat) ±.2(syst) Yields (from fit) N ( π N ( K N ( K π π K ) ) ) = = = ± ± ±

31 Direct Violation in B Κπ decays Self tagged, small Br S If Penguin dominated, small A expected (but theoretically not well understood ) + + ± + + ± + + ± π π π π π π π π π S S S K B K B : K K B K B : K K B K B : K m

32 Measurement of sin 2γ In B s decays (example: ) Several methods suggested to derive γ from B d decays, examples: Triangular relations from decay amplitudes B D K (originally proposed by Gronau e Wiler) B s D s K Ratio of BR (B K + π - ) to BR (B + K π + ) and asymmetry (Fleisher, Mannel et al) The asymmetry in B D* - π + also measures sin (2β+γ)... Measurement still out of reach...

33 Future plans Without long shut down and major machine work, PEP-II and KEK-B luminosity will continue to increase adiabatically up to few 1 34 cm -2 s -1 Extrapolating present performance it appears possble that both detectors will collect data for 5 fb -1 by 25 and 1ab -1 by 21 Several future options under discussion KEK-B plans to shut down in 26 to upgrade to 1 35 cm -2 s -1 Working groups have been set up to study the feasibility and the physics reach of a new machine at L=1 36 cm -2 s -1 (SUPERBABAR )

34 BABAR and BELLE in 25 :.5 ab -1 sin2β to.3 V ub,v cb,v td,v ts better known V ub to 5% Some estimate of γ sin2α eff to.12

35 A LOOK INTO THE FUTURE

36 Hadronic B-factory concept VERY large B production cross section All B mesons are produced (also B s and B c ) B s allow clean sin 2γ measurement Rare decays are more accessible (3-4 orders of magnitude) Decay vertices are more distant (2-3 times) But S/B ratio is much worse (factor ~1) But trigger is much more complex But final states are very crowded excellent detectors are needed! Today: CDF and D at FNAL Tomorrow: BTEV (FNAL) & LHCB (CERN)

37 B Physics at D and CDF Cross section: σ(pp bb) 1µb Run II B Physics program at 1.8 TeV All kind of B s produced: B u B d B s B c Λ b sin2β : B J/ψK s sin2α eff : B ππ B S mixing: m s from B s D s nπ Γ S from B s J/ψ φ in B s J/ψφ Rare Decays sin γ (possible?) B c and B baryons physics CDF B J/ψK s 395 ± 31events, S/B =.7

38 CDF RUN II (2fb -1 ) sin2β: Same Run I technique 1, B J/ψK s Assuming: εd 2 = 6.7% extrapolating present analysis (but detector is improved ) δ(sin2β).9 A in B π + π - -BR(B π + π - ) 5 x1-6 - Expected 5, ev. - Bckg: B Kπ; B s Kπ, KK + Combinatorial - Tagging Efficiency: εd 2 7.8% δ(sin2α).15

39 B-TEV A FERMILAB σ b 1 µb Luminosity: 2x1 32 (<2> interactions/crossing) # of B /(1 7 s) = 1.5x1 11 Trigger Efficiency L=γ β c τ B = 48 µ x p B /m B L=γ β c τ B = 48 µ x p B /m B π/k Identification is crucial!

40 LHCB AL CERN χ.3 B d J/Ψ K s : 4, tagged events/year, σ sin2β =.2 B s J/Ψ Φ : sin 2χ measurement with σ =.4-.6 (x S = 2 4) violation in radiative decays: B d K* µ+ µ 45 events if BR 1.5 x 1-6 B d K* γ 26 events if BR 4.9 x 1-5

41 Super b-factories e + e - GIGA-Z Factory at NLC/TESLA Unique feature: beam polarization (automatic tag!) Signal /background ratio and clean events as in Y(4S) B-factoriesB ALL B particles produced as in hadron colliders ] Examples: b b s Λ b Λγ But will be good enough at the time of turn on?? SUPER-BABAR KEK-B B is planning a luminosity increase up to 1 35 A BABAR-BELLE BELLE working group has been set up for SUPER BABAR at SLAC with 1 36 luminosity and a new detector: complementary to LHC-B / BTeV for rare decays superior for decays with ν, γ, π and for measuring V ub competitive for α, β, γ, clearly inferior for χ. B s maybe at the Y(5S), but no B c & Λ b

42 Rare decays FCNC b b sss, s γ, s l + l - Possible in the SM via box or penguin. Sensitive to physics beyond b sss (es B φk K, ηk K, η K K ) (Measure( independent from sin(2β) ) b s γ (es( B K(*) γ, (Br ~ ) b s s l + l - (es B K(*) l + l - ) EW penguin theoretically clean (Br ~ 1-6 ) B γ γ Br in the SM, strong increase possible for NP b d γ (complementary to b sγ) pure leptonic decays.

43 B s l + l - decays Very difficult to measure: Kinematics regions corresponding to charmonium states must be cut from l + l - invariant mass spectrum; bkg not only from continuum, but also from B semileptonic decays Theorists ask for lepton angular distributions not just BR s

44 CKM REACH BTeV 1 7 s LHC-b 1 7 s BABAR BELLE (25) SUPER BABAR 1 7 s Sin 2β Equal Sin 2 α Equal γ B s (D s K) ~7 Had γ B(DK) ~2 o ~ o Equal Sin2χ Had Br (B π π ) ~2% 6% e+e- V ub ~2.3% ~1% (sys) e+e-

45 Rare Decays

46 CONCLUSIONS More than 2 years after initial discovery, b physics is still exciting and studied by many dedicated experiments! violation in B decays has been measured by BABAR and BELLE, but this is only the beginning... In 25 BABAR & BELLE will have a data base of 1 9 B pairs, improved sin 2β measurement, and rare decays Br s... but still a lot will be left to be done! LHC-B and B-TEV will perform more stringent tests of the SM Up to now the SM does not show failures... But discrepancies could be found with increased precision of the measurements... In the meanwhile work is going on to design and built new machines and new detectors... More work is is needed from the the theorists otherwise the the experimental errors will will become limited by by model uncertainties

47 FINAL COMMENT BABAR CDF B-TEV LHC-B GIGA-Z SUPERBABAR NO MATTER WHO IS GOING TO DO BEST FRASCATI WILL BE THERE!