The other window on New Physics: CP violation at the B factories

Transcription

1 The other window on New Physics: CP violation at the B factories Gabriella Sciolla M.I.T. Outline: The physics of CP violation What is CP and why is it interesting? CPV in the B system CPV in the Standard Model and the Unitarity Triangle Constraining the Unitarity Triangle at the B factories Measurements of angles and sides Conclusion How would you like to live in Looking-glass House? L. Carroll Summary & Prospects

2 The matter dominated Universe The Big Bang model predicts: matter and anti-matter produced in equal amounts matter and anti-matter annihilated into pure energy But this goes against experimental evidence: The Universe exists It is made of (almost) only matter How is this possible? A. Sakharov s 3 conditions (1967): - Baryon number non conservation - Thermal non equilibrium - C and CP violation

3 The CP symmetry CP = C P C: Charge Conjugation Particle Anti-particle P: Parity Inverts space coordinates Is Nature CP symmetric? Wu et al., 1957 but CP was expected to be conserved CP ν> L = ν> R

4 CP violation in K decays In 1964 Fitch and Cronin discovered CP violation in the decays of K L mesons: K L π + π - K L π + π π π π π π + π π π } } CP=-1 CP=+1

5 CP violation in the Standard Model In 1973 the Kobayashi-Maskawa mechanism explained CPV and predicted the existence of third quark family. CP violation originates from a complex phase in the quark mixing matrix (CKM matrix). ) ( 1 ) ( ) ( λ λ η ρ λ λ λ λ η ρ λ λ λ O A i A A i A V V V V V V V V V V tb ts td cb cs cd ub us ud + = =

6 Pros: Pros and Cons of CKM Elegant and simple explanation of CPV in SM It is very predictive: only one CPV phase It accommodates all experimental results Indirect CP violation in K ππ and K L πlν Direct CP violation in K ππ CP violation in the B system CKM Cons: n B /n γ predicted by CKM «observed value by orders of magnitude! New sources of CPV must exist besides CKM! CPV as a probe for New Physics:. Any extension of SM provides new sources of CP violation

7 Standard Model or New Physics? Measure CP violation in channels theoretically very well understood and look for deviations w.r.t. Standard Model prediction What can we learn from kaons? Experimental results very hard to interpret theoretically: Loose constraints from ε K measurement No constraints from ε /ε yet or clear theory but very hard to reach experimentally: BF(K L π νν)~1-11! Can B mesons do better?

8 The Unitarity Triangle Unitarity of CKM implies: V V = 1 6 unitarity conditions * * * Of particular interest: V V + V V + V V * VubV V V cd ud * cb (ρ,η) α ud ub cd cb td tb = V V V * tb cd V td * cb γ β (,) (1,) All sides are ~ O(1) possible to measure both sides and angles! CP asymmetries in B meson decays measure α, β and γ Sides from B mixing, rare B decays, Semileptonic B decays

9 Redundancy, redundancy, redundancy! 3 ways to look for New Physics: a) Sides vs angles b) Measurement of same angle using channels with different sensitivity to NP c) Measurement of same sides using channels with different sensitivity to NP α γ β

10 Redundancy, redundancy, redundancy! 3 ways to look for New Physics: a) Sides vs angles b) Measurement of same angle using channels with different sensitivity to NP c) Measurement of same sides using channels with different sensitivity to NP α γ β

11 Redundancy, redundancy, redundancy! 3 ways to look for New Physics: a) Sides vs angles b) Measurement of same angle using channels with different sensitivity to NP c) Measurement of same sides using channels with different sensitivity to NP α γ β

12 How to measure the angles? Time dependent CP asymmetry in B decays. A CP N( B ( t) fcp) N( B ( t) ( t) = N( B ( t) f ) + N( B ( t) CP f f CP CP ) ) Questions: How is A CP (t) related to the UT angles? How can we measure A CP (t)?

13 The B system The B system: The K system: Mass eigenstates Flavor eigenstates B H and B L B and B B mesons = + = ' ' ' ' K q K p K K q K p K L S = + = B q B p B B q B p B H L 2 2 ( 1) p q + = Mass eigenstates Flavor eigenstates K S and K L K and K K mesons

14 Time evolution of B Starting from pure B (B ) state, and after time t mt q mt B B ( t) = e e cos B i sin B 2 p 2 im t Γt p m t m t B B ( t) = e e i sin B + cos B q 2 2 im t Γt Interested in Prob(B (t) f) and Prob(B (t) f) : Calculate amplitudes f H B t f H B t ( ) and ( ) Use A = f H B A = f H B f f Take square to get decay rates 2iβ ~ e t = B B mixing decay A f A f CP CP t f CP

15 CP violation in interference between mixing and decays in B Time dependent CP asymmetry for B f CP : N( B ( t) fcp ) N( B ( t) fcp ) CP f f N( B ( t) fcp ) + N( B ( t) fcp ) A ( t) = = S sin( mt) C cos( mt) where S For B f = 2Imλ 1+ λ f f 2 C f 1 λf = 1+ λ f q ~1, so when only 1 diagram contributes to the final state: λ =1 p A CP (t) = -η f Imλ sin( mt) 2 2 λ f q = p A A f f (CP violation in interference between mixing and decays in B )

16 CP violation in B decays: sin2β For some lucky modes, Imλ is directly and simply related to the angles of the Unitarity Triangle. Example: B J/ΨK S : the golden mode * * * VV tb td VV cs cb VcdV cs λ = * * * VV tb td VV cs cb VcdVcs A CP (t) = sin2β sin mt B b d η γ α c β J/ψ c s d ρ K

17 How to measure the angles? Time dependent CP asymmetry in B decays. A CP N( B ( t) fcp) N( B ( t) ( t) = N( B ( t) f ) + N( B ( t) CP f f CP CP ) ) Questions: How is A CP (t) related to the UT angles? How can we measure A CP (t)?

18 How to measure the CP asymmetry. A CP N( B ( t) fcp) N( B ( t) ( t) = N( B ( t) f ) + N( B ( t) CP f f CP CP ) ) e - Exclusive B reconstruction Br~ Υ(4S) B e - e + µ + B µ - π - π + Flavor tagging (coherent state) Excellent PID z~ βγc t Precise t determination B has boost

19 The B factories: PEP-II and KEK-B Asymmetric beams 9. GeV e - beam 3.1 GeV e + beam Currents: 1-2 A Peak Luminosity: Design: 3 x 1 33 cm -2 s -1 Achieved: 12 x 1 33 cm -2 s -1 Integrated Luminosity >4 fb -1 Belle: ~6 fb -1 >1 ab -1 in total!

20 The BABAR Detector 1.5 T solenoid DIRC (PID) CsI(Tl) EMC e + (3.1GeV) Silicon Vertex Tracker Drift Chamber e - (9GeV) Instrumented Flux Return SVT: 97% efficiency, 15µm z resolution Tracking: σ(p T )/p T =.13% p T.45% DIRC: K-π separation: > p=3 GeV/c EMC: σ E /E = 2.3% E -1/4 1.9%

21 The measurements VV * ub ud * VV cd cb α * VtbV V V cd td * cb The angle β γ β β

22 The golden mode for sin2β: B charmonium K BaBar 26 (347M BB) Theoretically clean Experimentally clean Relatively large BF (~1-4 ) CP sample: J/ ψk, Ψ(2s)K, χ K, η K N CP = 1 s s c s c s = 628 Purity = 92% b d J/ ψ K d L N CP =+ 1 = Purity = c c s % m ES [GeV] E [MeV]

23 Gold plated event at BaBar B J/Ψ K S B K - X Zoom on interaction region

24 The principle: B flavor tagging B D *- D ν l + π (soft) π B D *- K + W + π + (hard) Information combined in a NN-based algorithm. Q=ε tag (1-2w) 2

25 The importance of tagging Flavor tagging is a crucial ingredient in CP measurements since σ (sin 2 β) 1/ obs A () t = (1 2 ω ) A () t CP Extract tagging purity and efficiency directly from data in time dependent B mixing measurement e - B e - Υ 4S B e + Q D - K - CP π + π + Apply tagging algorithm π - B flavor exclusive reconstruction

26 Time dependent mixing fit A mixing mixed unmixed N ( t) N ( t) ( t) = (1 2ω )cos( mt) mixed unmixed N ( t) + N ( t) 1-2ω π/ m d 3 fb -1. m d =.516 ±.16 (stat) ±.1 (syst) hps -1

27 The golden mode for sin2β: CP fit in B charmonium K Unbinned maximum likelihood fit to t distribution hep-ex/6717 (BaBar 26) CP=-1 CP=+1 sin(2β) =.71 ±.34 stat ±.19 syst

28 The golden mode for sin2β: Belle s B charmonium K Belle 26 _B tag B tag _ 532 M BB pairs sin(2β) =.642 ±.31 stat ±.17 syst

29 How well do we know sin(2β)?. BABAR PRL 94, (25) Belle BELLE-CONF ±.4 ± ±.39 ±.2 HFAG Average.685 ±.32 ICHEP sin2β

30 How to look for New Physics Beautiful measurement: Error on sin2β is.26! Error on β < 1 degree! But by itself is useless! To test the SM we need to compare it with other measurements: Opposite side in the UT Independent measurement of sin2β using channels mediated by other Feynman diagrams

31 The left side: R b R b * VV ub VV cd V V ub ud * cb cb α VV * tb VV cd td * cb γ β NB: β is the best measured quantity in the Unitarity Triangle β = 21.2 ± 1. degrees precise measurement of R b is needed for accurate tests of SM

32 Semileptonic B Decays b Parton level V ub, Vc b W u, c 2 GF Γ( b u ν ) = V 2 ub m 192π ν l 2 5 b b B Hadron level V ub u ν X, u Xc X l ν Sensitive to hadronic effects Theory error not negligible Prob(b c)/prob(b u)~5 V cb precisely measured (±2%) V ub is the challenge

33 Two approaches to V ub Inclusive B X u l ν ν Exclusive B π l ν ν B V ub X u l - B V ub W π l - l Inclusive B X u lν Hadronic final state is not specified b c l ν background is suppressed using kinematical variables Partial rate is measured theoretical uncertainties ~5% Exclusive B π lν Better S/B but lower branching fraction (1-4 ) Needs form factor calculation from Lattice QCD uncertainty of ~ 12%

34 V ub from Inclusive B X u l ν Close collaboration between theorists and experimentalists led to Precision on V ub : ±7.3% World Average 4.49 ±.33 χ 2 /dof = 6.1/6 c.f.r.: precision on β: ±4.7% Goal in 28: error on V ub : ±5%

35 Unitarity Triangle constraints η sin2β vs indirect UT constraints: very good agreement! 95% CL from sides CKM mechanism is the dominant source of CPV at low energies New Physics does not show up in the golden mode SM reference Compare with sin2β in independent modes with different sensitivity to NP ρ

36 How to look for New Physics (2) Beautiful measurement: Error on sin2β is.26! Error on β < 1 degree! But by itself is useless! To test the SM we need to compare it with other measurements: Opposite side in the UT Independent measurement of sin2β using channels mediated by other Feynman diagrams

37 Another way to look for New Physics: sin2β in Penguin Modes Decays dominated by gluonic penguin diagrams The typical example: B φk S SM NP b ~ br g ~ + (δ d 23 ~ s R RR ) g s s d d d s d No tree level contributions Impact of New Physics could be significant New particles could participate in the loop new CPV phases NP affects each channel differently; low branching fractions Measure A CP in as many b sqq penguins as possible! φ K, K + K K S, η K S, K S π, K S K S K S, ω K S, f (98) K S

38 The golden penguin: B φk BaBar ICHEP 26 Belle ICHEP 26 Nφ = 37 ± 21 K S Nφ = 114 ± 14 K L March 21, 26 What can we learn from CPV? φk G. Sciolla M.I.T. S C φk = +.5 ±.21 ±.5 =.7 ±.15 ±.6

39 BaBar ICHEP 26 Belle ICHEP 26 The silver penguin: B η K S Relative large BF ~ 6 x 1-5 Theoretically less clean than φk S Tree diagram possible but Cabibbo and color suppressed S = S ΨK -S Penguin ~.1 QCDFact.1 SU(3) Nη ' = 1421 ± 46 K S S η K s =.64 ±.1 ±.4 C η K s =.1±.7±.5

40 BaBar ICHEP 26 Belle ICHEP 26 A new golden penguin: B K S K S K S Theoretically clean Penguin dominated CP=+1 eigenstate Experimentally challenging B decay vertex uses K S pseudo-particles and beam spot constraint N K S K S K S = 176 ± 17 Add babar plot S = +.66 ±.26 ±.8 K K K S S S K K K S S S =.14 ±.22 ±.5 C

41 sin2β in penguins No significant shift observed in each mode although a trend is visible Naïve average:.52±.5 ~2.6σ from J/ΨK s Statistical errors still large

42 The measurements VV * ub ud * VV cd cb α * VtbV V V cd td * cb The side R t γ β

43 The measurement of R t α R t = V 1 td λ V ts b B ds () W t V td () s W ds () B d ( s ) γ β ds ( ) V td () s t b B s /B d oscillations m m Theory error <5% d s V V m d is precisely measured But B s mixing is very hard td ts 2

44 B S mixing at the Tevatron.... Flavor tagging Q SST ~4% Q OST ~1% Time reconstruction σ(ct)~26-7 µm Reconstruction of B S decay in hadronic or SL modes

45 D result B s µd s : 561± < m s < 21 ps 9% CL. assuming Gaussian errors Most probable value of m s = 19 ps -1

46 CDF result 3,7 events V m = ±.7 ps =.28 S td Vts Probability of random fluctuation: ~.5%

47 Impact of m S on Unitarity Triangle

48 But to test the SB we need redundancy R t from B ργ/ωγ vs. B Κ γ Radiative penguin decays with b dγ and b sγ V td,v ts Ratio of BF measures V td /V ts Ali and Parkomenko B K*γ well established; B ρ(ω)γ is the challenge! Standard Model expectation: B ρ γ/ωγ: ~.5 x 1-6 ; B ρ + γ ~ 1 x 1-6 Theory error: 8% for ρ γ 15% for average

49 B ργ/ωγ: analysis In principle a simple analysis Two body decay: p γ ~m B /2 Exclusive meson reconstruction ρ π + π ρ + π + π ω π + π π Pion identification is a must to reject B K*γ background Exclusively reconstruct B meson m ES and E But huge continuum background! Eg: γ from π vs pions in opposite jets NN for continuum suppression is key Shape variables (e.g.: R2) Properties of B decay (e.g.: z) Tagging -like variables (e.g.: p CMS of leptons) + B ργ MC qq ee ee + γ bb B ργ MC π + π m ES (GeV)/c 2 B B E (GeV)/c 2

50 NN Output and performance Black: background Red: signal MC Background Rejection Efficiency Rejects >98% Bgd keeping ~5% Sig NN Output Signal Efficiency

51 SM expectation Recent Results 1. x x x 1-6 Belle - Summer 25: 37 fb -1 Combined BF 5.1(5.4) BaBar - Summer 26: 316 fb Combined BF 6.3 σ 1.1±.21±.8

52 UT constraints From BF(ργ)/BF(K*γ) one can extract V td /V ts : ICHEP 26 BaBar only In agreement with B S mixing results

53 Conclusion Precise and redundant measurements of sides and angles of UT allowed quantitative test of CKM mechanism CKM works beautifully! No New Physics? If Λ NP ~1 TeV effects in CP ~ m W /Λ NP ~ 1% Precision ~ 3% needed: more data coming Not seeing New Physics does mean something CPV is just part of the puzzle Many other NP studies at B factories: B τν, B sγ, B sll Constraints on New Physics models coming from B physics will help interpret the discoveries at the LHC

54 What have we learned? 95% CL UT in 21