Measuring the Unitarity Triangle Angle α with BaBar. Adrian Bevan KEK, 15 th September 2006

Transcription

1 (C) Peter Ginter (2002). Measuring the Unitarity Triangle Angle α with BaBar Adrian Bevan KEK, 15 th September 2006 (α, β, γ) (φ 1, φ 2, φ 3 ) 15 th September 2006 Adrian Bevan 1

2 Overview Motivation PEP-II and the BaBar Detector CKM Matrix. Measuring α through understanding penguins. Ingredients of a time dependent analysis Results B ππ, ρρ, ρπ. combination of results Prospects from uud transitions with a 1 mesons. Summary 15 th September 2006 Adrian Bevan 2

3 Motivation Why is our universe matter dominated? 15 th September 2006 Adrian Bevan 3

4 PEP-II Asymmetric energy e + e - collider Study decay of B meson pairs. σ (e + e - Hadrons)(nb) ϒ(1S) ϒ(2S) ϒ(3S) Υ(4S) off-peak ϒ(4S) Mass (GeV/c 2 ) Record data at the Υ(4S) ~90% of the time. Run below the bb production threshold for background studies. PEP-II tunnel: 2.2km in circumference + 9 GeV e 3.1 GeV e ϒ (4 S ) boost: βγ = 0.55 Head - on collisions 15 th September 2006 Adrian Bevan 4

5 PEP-II performance The accelerator is performing well: Maximum Luminosity of cm 2 s -1 I HER = 1900 ma I LER = 2900 ma Reached over 4 times the design luminosity. Current PEP-II status: Scheduled summer shutdown. Data taking will resume in January th September 2006 Adrian Bevan 5

6 BaBar detector DIRC (PID) 144 quartz bars PMs e - (9GeV) 1.5T solenoid EMC 6580 CsI(Tl) crystals e + (3.1GeV) Drift Chamber 40 layers Instrumented Flux Return iron / RPCs or LSTs (muon / neutral hadrons) Silicon Vertex Tracker 5 layers, double sided strips Upgrading muon system to replace all remaining RPCs with LSTs for run th September 2006 Adrian Bevan 6

7 BaBar integrated luminosity Recorded 390/fb (~410 million B pairs). Expect ~1000/fb by summer DOE funding is approved until summer contributed papers to ICHEP papers submitted to either PRL or PRD as of 12 th September 06. Data sample used for ICHEP th September 2006 Adrian Bevan 7

8 The BaBar Collaboration 80 institutes 11 countries 623 Physicists July 2004 (C) Peter Ginter (2002) 15 th September 2006 Adrian Bevan 8

9 CP violation in meson decay Manifest in couplings between quark generations: W + V ij qi q j = u, c, t = d, s, b V = The couplings V ij form the 3 3 CKM Matrix. Wolfenstein Parameterisation: V λ /2 λ Aλ ( ρ iη) = λ λ λ Aλ (1 ρ iη ) Aλ /2 A O( ) Understanding Standard Model CP Violation means accurate measurements of ρ and η 15 th September 2006 Adrian Bevan 9 λ λ ~0.22 A ~0.8 ρ ~ η ~

10 Studying CP violation in B decay Study decays involving b u and b t transitions to probe the weak phase of V ub and V td. ππ, ρπ, ρρ etc. V V V V ud us = cd cs cb td V V V ts V V V ub tb D ( * ) K ( * ) etc. ccs (e.g. J/ψK 0 ), ccd (e.g. J/ψπ 0 ), η,ω,φk 0 etc. B-factories have measured β to ~1 in ccs, and starting to do precision measurements in charmless b s penguin decays. Now starting to constrain α and γ. 15 th September 2006 Adrian Bevan 10

11 CP violation effects CP violation is manifest through interference. Direct CP violation Pr( B f) Pr( B f ) just number counting like ε /ε: e.g. A CP in B Kπ. B 0 B ( ) f CP violation in interference of mixing and decay (Imλ 0). Time-dependent effect. Need asymmetric e + and e beam energies study in detectors. Phys.Rev.Lett. 94 (2005) t (ps) 15 th September 2006 Adrian Bevan 11

12 Observables Analyse time evolution of B 0 B 0 system (assume Γ=0): / ( / phys /, ) f B B f f t = Γ 4 Γ t [ 1+ ηs sin( ) ηc cos( t) ] e m t m d η =+ d 0 0 1( 1) for B ( B ) λ f CP q = p A A f f CP CP C 2Imλ f CP f CP λ f S = CP 1 λ 2 f CP f = CP λ f CP C = A CP BaBar : Belle 15 th September 2006 Adrian Bevan 12

13 Observables Construct an asymmetry from the time distributions for f + and f. t distribution for a perfect detector f - ( t) f + ( t) A CP ( t) = f+ ( t) f ( t) f ( t) + f ( t) + = S sin( m t) C cos( m t) d d t distribution accounting for resolution t (ps) 15 th September 2006 Adrian Bevan 13

14 CP violation in B h + h -, h=π,ρ V * td : β +Loops (penguins) Vtd : β C S hh hh = 0 = sin(2 α) Vub Measure α eff. P Need to bound α eff -α (shift from loops). Different Penguin/Tree for different decays. : γ C S hh hh sin( δ ) = δ = δ δ 2 1 Chh sin(2 eff ) T α 15 th September 2006 Adrian Bevan 14

15 Isospin analysis SU(2) isospin symmetry ππ: Gronau & London PRL65, 3381 (1990) etc. ρπ Snyder-Quinn: PRD48, 2139 (1993) etc. (ρρ 3 x ππ) relates u and d quarks: m u ~ m d Different B ππ, ρπ and ρρ final states can be related to each other through isospin amplitudes. These amplitude relations can be used to constrain the penguin shift in the time dependent measurement: Bounds α eff -α. Relations are triangles for ππ and ρρ, and pentagons for ρπ. 15 th September 2006 Adrian Bevan 15

16 Isospin analysis A + A = A A + A = A For ππ & ρρ require: 0 + B h h + CC B h h + CC B h h + CC.. 2κ 2κ=α eff -α S + h h There are SU(2) violating corrections to consider, for example electroweak penguins, but these are much smaller than current experimental accuracy and can be incorporated into the isospin analysis. 15 th September 2006 Adrian Bevan 16

17 Ingredients of a time dependent analysis In order to measure S and C we need to be able to Isolate the signal from large samples dominated by background. Measure the proper time difference between the decay vertices of the two B mesons in the event [vertexing]. Determine the flavor of the other B meson in the event [tagging]. 15 th September 2006 Adrian Bevan 17

18 Isolating signal events Beam energy is known very well at an e + e collider like PEP-II / KEK-B use an energy difference and effective mass to select events: σ( E)~ MeV (mode dependent) E = E E B * beam m = ( E ) P * 2 2 ES beam B signal M ES σ ~ 3 MeV B background 15 th September 2006 Adrian Bevan 18

19 More background suppression Use the shape of an event to distinguish between Υ(4S) BB and e + e qq B events tend to be spherical continuum (ee qq) events are jetty Analyses combine several event shape variables in a single discriminating variable: either Fisher or artificial Neural Network. This allows for some discrimination between B and continuum events Arbitrary Units Signal u,d,s,c background Fisher Discriminant 15 th September 2006 Adrian Bevan 19

20 Vertexing e - e + B 0 B 0 Asymmetric energies produce boosted Υ(4S), decaying into coherent BB pair Δz=(βγc)Δt Determine time between decays from vertices Determine flavor and vertex position of other B decay (B TAG ) Then fit the t distribution to determine the amplitude of sine and cosine terms. π + π Fully reconstruct decay to state or admixture under study (B REC ) l- K - 15 th September 2006 Adrian Bevan 20

21 Flavor tagging Decay products of B TAG are used to determine its flavor. At t=0, the flavor of B REC is opposite to that of other B TAG. B REC continues to mix until it decays. Different B TAG final states have different purities and different mistag probabilities. BaBar s flavor tagging algorithm splits events into mutually exclusive categories ranked by signal purity and mis-tag probability. These plots are for the 316fb -1 h + h - data sample. Lepton Kaon 1 Kaon 2 Kaon-Pion 0 B 0 B Pion Other 15 th September 2006 Adrian Bevan 21

22 Search for a needle in a haystack Study the decay of several channels to constrain α: B ππ B ρρ 0 B B 0 f CP VV ud VV cd * ub * cb α α VV td VV cd * tb * cb B B B ρπ aπ a 1 1 ρ 0 B f B 0 1 f 2 γ β 15 th September 2006 Adrian Bevan 22

23 B π + π Updated measurement using 347x10 6 B pairs. hep-ex/ B Kπ 2542±67 events B B 0 tagged 0 Kπ 0 B ππ 675±42 events B 0 tagged e e + qq background All of the plots use the splot technique NIM A555 (2005) th September 2006 Adrian Bevan (physics/ ) 23

24 Evidence for CP Violation in B ππ BaBar data shows evidence for CP violation at 3.6σ using the S and C measurement in B π + π. hep-ex/ (S,C)=(0,0) is excluded at a confidence level of Still a mild discrepancy with Belle s result need more data to resolve this. Belle 15 th September 2006 Adrian Bevan 24

25 Updated sides of the ππ triangle Using 347x10 6 B pairs. π 0 π 0 selection improved use γ e + e conversions, and merged π 0 mesons. B π π B π π hep-ex/ splot 572±53 events 140±25 events 15 th September 2006 Adrian Bevan 25

26 A B 0 π 0 π 0 candidate π 0 π 0 15 th September 2006 Adrian Bevan 26

27 B ππ Isospin analysis hep-ex/ α=0 is disfavoured and is excluded using additional experimental information The measurement of C 00 is starting to distinguish between possible solutions for δα. Need more data before the dip starts to become significant. The precision of the ππ isospin analysis will be something to watch as the B-factories approach 1ab th September 2006 Adrian Bevan 27

28 θ φ θ 1 B Use B ρρ decays to measure α. Theory is slightly more complicated than ππ. Experimentally challenging. But loop contributions are better constrained than ππ. B ρρ 3 d Γ dcos d d u d P J = 0 :π P J =1 :ρ Spin 0 Spin 1 narrow particle broad resonance AY ( θ, Φ) Y ( θ, Φ) m 1, m 1 1, m 2 θ1 cosθ2 Φ m= 1,0, sin θ1sin θ2( A+ 1 + A 1 ) + cos θ1cos θ2 A * * 2 sin θ1sin θ2[cos 2 ( A+ 1A 1) sin 2 ( A+ 1A 1)] 1 * * * + 4 sin 2θ1sin 2 θ2[cos ΦR ( A+ 1A0 + A 1A0) sin ΦI( A+ 1A0 A A * )] ΦR ΦI f L = 2 A0 A m= 1,0,1 m th September 2006 Adrian Bevan 28

29 Measuring α with B ρρ decays B VV decay; Angular correlation has 11 observables (6 amplitudes, 5 phases) need angular analysis to determine CP content. ρ + ρ is almost 100% B 0 longitudinally polarized simplifies analysis a lot! CP even helicity zero state dominates: Longitudinal (CP even) Transverse (Mixed CP state) 15 th September 2006 Adrian Bevan 29

30 B 0 ρ 0 ρ 0 Updated measurement using 347x10 6 B pairs. hep-ex/ BACKGROUND SIGNAL Previous result UL < 1.1x10-6 (central value was 0.54x10-6 ) [ ]x10-6 3σ Evidence for ρ 0 ρ 0 with systematic errors. Leads to a weaker constraint on penguin pollution. N( ρρ) = 98 ± f f 6 N( ρ ) = 12 ± 13 N( f ) = 5 ± th September 2006 Adrian Bevan 30

31 B + ρ + ρ 0 Updated measurement using 232x10 6 B pairs. Simultaneous fit for B + ρ + f 0 (980). Smaller Branching Fraction measured. Leads to a weaker constraint on penguin pollution. Submitted to PRL hep-ex/ ρ 0 (770) f 0 (980) BACKGROUND 15 th September 2006 Adrian Bevan 31

32 B 0 ρ + ρ Updated for ICHEP 06. Using 347x10 6 B pairs. Reduced systematic uncertainty by improving treatment of correlations. Use only the tagged events for all results. hep-ex/ th September 2006 Adrian Bevan 32

33 B 0 ρ + ρ Conservative uncertainty on mis-reconstructed signal fraction which can be reduced. hep-ex/ Improvements in modelling correlations and backgrounds result in a reduced systematic uncertainty on S and C. Improved upper limit for B a 1 ρ also helps to reduce systematic uncertainty. 15 th September 2006 Adrian Bevan 33

34 Updated α from B ρρ decays Penguin pollution is constrained to be <18 (68% CL). Evidence for ρ 0 ρ 0 and a lower branching fraction for ρ + ρ 0 result in a weakened constraint on α. hep-ex/ hep-ex/ Previous Results α = (100 ± 13) 15 th September 2006 Adrian Bevan 34

35 Can use a different approach Beneke et al., Phys.Lett. B638 (2006) Relate the penguin contribution in ρ + ρ to K* 0 ρ + using SU(3) symmetry: r= P/T δ=strong phase difference between P and T F=Correction for SU(3) breaking effects not included in the decay constants It turns out that the error on α and δ don t depend strongly on F and σ(f). 15 th September 2006 Adrian Bevan 35

36 Constraints using the Beneke et al. model Several solutions in For the standard model solution of α, these two are overlapping Points are for CL=1.0 α~90 is preferred for δ <90 1-CL All allowed r, δ and α No uncertainty on F δ <90 F=0.3 F=0.9 F=1.5 α~100 is preferred for δ >90 σ(α)~7 (expt.)±1.5 (th.) 15 th September 2006 Adrian Bevan 36

37 B ρπ Snyder-Quinn method: PRD (1993) Extract α and strong phases using interference between amplitudes in the π + π π 0 Dalitz plot. Amplitude A 3π dominated by ρ + π, ρ π +, ρ 0 π 0 and radial excitations Monte Carlo The differential decay rate across the Dalitz plot is given by ρ 0 π 0 where ρ + π ρ π + A κ and f κ are the amplitudes and kinematic functions of the Dalitz variables for a ρ meson of charge κ. 15 th September 2006 Adrian Bevan 37

38 Fitting the time dependence Fit 26 coefficients of the time dependence of B π + π π 0 decays hep-ex/ Where the amplitudes are written in terms of U and I. The information on α and CP violation is hidden in these variables: S and C are not CP observables 15 th September 2006 Adrian Bevan 38

39 B ρπ Updated result using 347x10 6 B pairs hep-ex/ Correctly reconstructed signal Mis-reconstructed signal ± 69 B π + π π signal 15 th September 2006 Adrian Bevan 39

40 Results Convert the 27 fitted coefficients into observables used in the Q2B analysis (where one analyses the narrow ρ bands in the Dalitz plot) δ + is the strong phase difference between ρ + π and ρ π CL δ + = (34 ± 29) We can also study A CP for B ρ + π B ρ π + as A A + ρπ + ρπ = = A C A C ρπ ρπ 1 C A C ρπ A + C+ A C ρπ ρπ 1+ C+ A C ρπ 15 th September 2006 Adrian Bevan 40

41 1 - CL Constraint on α Data prefer α~90 hep-ex/ No constraint at 95% CL Need more data for a precision measurement. 68% CL hep-ex/ % CL An alternative would be to use SU(3) to obtain a model dependent result with better precision and 15 theory uncertainty [see M. Gronau and J. Zupan PRD70 (2004) ]. 15 th September 2006 Adrian Bevan 41

42 Combination of results Two statistical methods: UT Fit: CKM Fitter: 15 th September 2006 Adrian Bevan 42

43 +11 α B-Factories = [ 93 ] º -9 This constraint does not include Combination of results The constraint on α obtained is dependent on the statistical treatment used. This is a reflection of the fact that we need more data to perform a precision measurement of α. Excluded regions are common to both methods. the latest 15 th result September from Belle s 2006 ρπ Adrian Bevan 43 analysis

44 Finding the apex of the triangle... Inputs: Inputs: V V ub cb m d m s B τν V V ub cb m d m s B τν ε K β sin2 α γ ε K β sin2 α γ This constraint does not include the latest 15 th result September from Belle s 2006 ρπ analysis Adrian Bevan 44

45 B a 1 π something for the future? Not a CP eigenstate like B ρπ. First observation of this channel (9.2σ). First step towards extracting α. PRL 97 (2006) To complete the puzzle we need a time dependent analysis, and to study related decays [K 1 π etc. see Gronau and Zupan PRD 73 (2006) ]. 15 th September 2006 Adrian Bevan 45

46 B a 1 ρ PRD 74 (2006) (R) Motivation analogous to ρρ and a 1 π (this is an AV final state). Also a background to other charmless decays Using a data sample of 110x10 6 B pairs: BR B 0 ( a ± 1 ρ ) < (90% CL) Little prospect to measure α with this decay channel. Improved upon the previous upper limit by more than a factor of 50. SIGNAL e + e qq splots 15 th September 2006 Adrian Bevan 46

47 Conclusions Evidence for CP violation in B π + π. Updated measurements of the inputs to the ρρ isospin analysis weaken the constraint on α from ρρ. 1-2σ constraint on α from π + π π 0 Dalitz analysis very useful when combining the result with other measurements. SU(3) approach is more precise than isospin analysis for both of these. a 1 π may be able to provide an additional check Null result from search for a 1 ρ not encouraging. Combining ππ, ρπ and ρρ gives a measurement of α with precision ~ th September 2006 Adrian Bevan 47

48 Additional Material 15 th September 2006 Adrian Bevan 48

49 splots NIM A555 (2005) (physics/ ) Technique for visually representing fit species in a maximum likelihood fit. likelihood fit Set of discriminating variables: z control variable: x (variable to plot) no correlation between x and y. discriminating variables: y (everything else) Calculate an sweight for each event based on PDFs for variables, y summed over set of species, k sweight covariance matrix PDF for variables, y Number of species, k An splot for a variable is made without using that variable in the likelihood. 15 th September 2006 Adrian Bevan 49

50 Flavor Tagging Lepton Tagged MC Kaon 1 Tagged MC Kaon 2 Tagged MC Amplitude of oscillation decreases for a given S and C as the mistag probability ω c increases. The effective tagging efficiency decreases with increasing ω c Q=ε c (1-2ω c ) 2 15 th September 2006 Adrian Bevan 50