CP violation at the B factories: experimental aspects. Gabriella Sciolla MIT

Transcription

1 CP violation at the B factories: experimental aspects Gabriella Sciolla MIT New England Particle Physics Summer Retreat August 18-22, 23

2 oal of this lecture ve a concrete example of how a measurement is performed in a modern HEP experiment. What do we want to measure and why Design of accelerator and detectors The many challenges of data analysis The interpretation of the result What s next? I will discuss only one of the many measurements we do in our experiment. Many other measurements are possible! For more information contact your local B representative at MIT, Harvard, Umass, Yale, Princeton,

3 hy CPV? we learned from Gene s lecture: CP is crucial to explain the matter-dominate Universe Andrei Sakarov 1968 CP violation in the B system offers unique opportunity to test CP sector of Standard Model CKM mechanism is precise and predictive Deviations from Standard Model are expected SM not enough to explain observed matter-antimatter asymmetry CPV as a probe sensitive to New Physics

4 ain tool: The Unitarity Triangle VV * ub ud * VV cd cb (ρ,η) α * VtbV V V cd td * cb γ β (,) (1,) l 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, V ub, V cb

5 he measurement of the sides direct constraints on UT apex before B-factories 95% C.L.

6 he measurement of the angles Measuring directly and independently the angles tests the Standard Model in the CP violation sector Scenario I: Agreement with bb

7 he measurement of the angles Measuring directly and independently the angles tests the Standard Model in the CP violation sector Scenario II: Discovery of New P b

8 ow to measure the angles? me 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 ) ) uestions: How is A CP (t) related to the UT angles? How do we measure A CP (t)?

9 hat is a B? neutral particle made of b and d quarks Particle Quarks Mass Charge Spin B bd 5.28 GeV/c 2 B bd 5.28 GeV/c 2 culiarities: B can turn into B, and vice versa (mixing) B and B are flavor eigenstates. Mass eigenstates B H and B L : B B = p B = p B + q B q B L 2 2 H ( p + q = 1)

10 ime 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 mt mt 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 () B Use Af = f H B Af = f H B 2iβ ~ e Take square to get decay rates t = B mixing decay A f A f CP CP

11 P violation in interference between mixing and decays in B me dependent CP asymmetry for B f CP : here r B, so when only 1 diagram contributes to the final state: λ sin( ) cos( ) ) ( ( ) ) ( ( ) ) ( ( ) ) ( ( ) ( m S mt C f t B N f t B N f t B N f t B N t f f CP CP CP CP P + = + = f f f C λ λ = + 2 2Im 1 f f f S λ λ = + f f f f A q?? p A = 1 q p A CP (t) = - Imλ sin( mt)

12 P violation in B decays: sin2β or some lucky modes, Imλis directly and simply related to the angles of the Unitarity Triangle. J/ψ xample: c c B J/ΨK S : the golden mode b s B d d * * * VV tb td VcsV cb VcdV cs = ( 1) * * * VV tb td VcsVcb VcdVcs A CP (t) = sin2β sin mt = e -i2β η α γ b ρ

13 ow to measure the angles? me 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 ) ) uestions: How is A CP (t) related to the UT angles? How can we measure A CP (t)?

14 ow to measure CPV at B factories A N( B ( t) fcp ) N( B ( t) fcp ) t) = sin 2β N( B ( t) f ) + N( B ( t) f ) CP ( CP CP sin( m t) redient #1: clusive onstruction U 4S B CP B e - e + m + B m - Dz~ bgc Dt B tag p - p + Ingredient Flavor tagg (coherent sta Ingredient #3: Dt determination

15 he experimental challenge Rare events: Br(B J/ΨK s ) x Br(J/Ψ l + l - ) ~ 5 x 1-5 Br(B φk s ) x Br(φ K + K - ) ~ 5 x 1-6 a lot of Bs needed! Time dependent analysis B produced with a boost precise tracking around interaction region Tagging of B flavor good particle identification

16 ccelerator Design High luminosity 2 orders of magnitude better than previous experiments Clean environment needed e + e - interactions Y(4S) B B with no extra particles in the final state B e - U 4S e + Bs must have boost Asymmetric collisions needed B

17 The asymmetric B factories Asymmetric e + e - B factories First proposed in 1987 Elegant and conceptually simple Challenging for accelerator builders! 2 beam pipes, 2 sets of magnets, difficult interaction region Many proposals, only 2 survived: SLAC: PEP PEP-II KEK: Tristan KEK-B Similar in design and achievements Will discuss PEP-II and BaBar as an example

18 The PEP-II Accelerator Asymmetric B factory 9 GeV e - beam 3.1 GeV e + beam Currents: 1-2 A Very High Luminosity: 6 M Bs/year

19 Linac I-28 BABAR PEP-II

20 -II straight section. avities provide energy he beams

21 uperb accelerator performance of July, 23: stantaneous luminosity: 6.6 x 1 33 cm -2 s -1 (i.e.: 6 BBbar pairs/second!) Twice the design! tegrated luminosity: 137 fb -1 ( i.e. ~28 M B mesons! ) A lot of interesting thesis opportunities!!!

22 EKB Storage Ring Layout + 8 GeV e 3.5 GeV e (4 S ) boost: βγ=.425 ± 11 mrad crossing angle Located in the Tristan tunnel a

23 e main difference: Interaction Region Head-on collisions Safe design Higher backgrounds crossing a Riskier design Lower backgrounds

24 he BaBar Detector 1.5 T solenoid Electromagnetic Cal ov Detector IRC) e + (3.1 G 9 GeV) Drift Cha mented Flux Return Silicon Vertex T

25 ertex Detector Design Requirements Excellent transverse and longitudinal vertex resolution Typical z ~ 25 µm need σ( z) < 2 µm Resolution on z small compared to average B decay flight Stand-alone tracking capability For slow p from D* + decays High background tolerance Constraints Space is tight (magnets occlude below 35 mrad) $$$

26 icon Vertex Detector at BABAR am e + 5 Layer AC-coupled double-sided silicon detector Radiation hard readout electronics (4-5 Mrad) Hit reconstruction efficiency ~ 97% Hit resolution ~15 µm at 9 o

27 ompleted SVT Detector

28 rift Chamber Design Requirements Pattern-recognition for charged tracks Momentum measurement over maximum possible so angle de/dx measurements for tagging (low momentum) Fast L1 input to tracking trigger Constraints Outside radius balances cost (EMC) and p t resolution Minimize material in front of EMC, DIRC

29 ABAR Drift Chamber layers of wires (714 cells) in 1.5 Tesla magnetic field elium:isobutane 8:2 gas, Al field wires, Beryllium inner wall, an ll readout electronics mounted on rear endplate article identification from ionization loss (7% resolution) σ( pt ) p =.13% p +.45% T T

30 adron PID Detector Design ID Requirements Excellent kaon tagging for momentum range GeV/c Low momentum K: tagging High momentum K: B Kπ onstraints Must be thin and light to maximize tracking volume minimize EMC cost minimal material in front of EMC mplementation Momentum<.7 Gev :de/dx Momentum>.7 GeV: DIRC

31 rinciple of the DIRC DIRC: Detector for Internally Reflected Cherenkov light Novel ring-imaging Cherenkov detector based on quartz radiators and phototube imaging of rings

32 ternal reflection in DIRC quartz bars

33 alorimetry Design Requirements Best possible energy and position resolution 11 photons per (4S) event; 5% below 2 MeV in energy Electron identification down to low momentum Constraints Cost of raw materials and growth of crystals Operation inside magnetic field Implementation Thallium-doped Cesium-Iodide crystals with photodiode readou Thin structural cage to minimize material

34 abar s Electromagnetic Calorimete 658 CsI(Tl) crystals with photodiode readout About 18 X, inside solenoid σ ( E ) (2.32±.3±.3)% = 4 E E (1.85±.7±.1)% π s = 5.%

35 uon Detectors assembly with RPCs at BABAR Up to 21 layers of resistive-p chambers (RPCs) between iro plates of flux return Muon identification > 8 M Neutral Hadrons (K L ) detecti also with EMC/ECL Bakelite RPCs at BABAR Problems with QC, dark curr and stability Barrel replacement in 24-2 Glass RPCs at Belle Possible problems with neutr in forward endcap Probably problems at higher background rates

36 he measurement of the angle VV * ub ud * VV cd cb α V V * tb cd V V td * cb γ β

37 ow to measure CPV at B factories A N( B ( t) fcp ) N( B ( t) fcp ) t) = sin 2β N( B ( t) f ) + N( B ( t) f ) CP ( CP CP sin( m t) redient #1: clusive onstruction U 4S B CP B e - e + m + B m - Dz~ bgc Dt B tag p - p + Ingredient Flavor tagg (coherent sta Ingredient #2: Dt determination

38 redient 1: clusive reconstruction of B CP final state J/ΨK S : the golden channel A CP (t) = sin2β sin mt B b d c J/ψ c s d dvantages: Theoretically well understood Experimentally clean Relatively large BF (~1-4 ) nce 1-4 is not that large, it is crucial to add many CP modes to enhance the statistical significance of the measurement.

39 hat are J/Ψ and Ks? J/Ψ Bound state of (cc), mass 3.1 GeV Very narrow width Decay modes: J/Ψ µ+µ- and e + e - (BR~6% each) Easy and pure final state with excellent vtx resolution Ks Neutral kaon, bound state of (ds) Mass.5 GeV, narrow width, <cτ>~ 2.7 cm Best decay mode: Ks π+π- Pure final state too!

40 d plated event at BaBar B J/Ψ B K - X Zoom on interaction region

41 ow do we see intermediate particle xample: J/Ψ It decays IMMEDIATELY after being produced! Neutral particle so trace in tracking detectors! olution: Use its daughters and reconstruct invariant mass m J/Y p µ =(E,p) p µ p µ = E 2 p 2 = m 2 For J/Ψ e+e- E J/Ψ = E(e+) + E(e-) p J/Ψ = p(e+) + p(e-) relativistic invariant! J / ψ µµ + We have all the necessary ingredients to see it!

42 he B reconstruction wo variables are used to fully reconstruct the B deca B invariant mass Using E and p of daughter particles Better resolution using E beam instead of E B (m ES ) Resolution: 2.6 MeV E = E B -E beam Uses conservation of energy: all the energy of th beams MUST go into the 2 neutral Bs Resolution: 1-2 MeV

43 xample for Hadronic B Decays m ES Signal Region: [ ] mes, E = m B ± 3 σm ES,± 3σ Sideband Region: Defined outside signal region in order to estimate backgrounds E signal region E B J/ψK S sidebands m ES [GeV/c 2 ]

44 The full CP sample are events: add as many channels as possible! Mode ψk s (π + π - ) ψk s (π π ) (2s)K s K s c K s otal CP=-1 ψk L ψk * (K s π ) ull CP sample N events Purity (%)

45 gredient #2: B flavor tagging e principle: e -,µ - W- W + b c s K - any physics processes can be used for tagging: Charge of the lepton coming from B semileptonic decay Charge of the kaon(s) coming from the hadronization of b c Charge of the soft π from B D * - X with D *- D π - Charge of energetic π ± from W hadronization

46 The sub-taggers All information relative to a specific physics process are used to bu a so called sub-tagger Example: The electron sub-tagger exploiting B D* - e + ν e e+ B D Useful quantities: Charge ν e π slow Momentum in B reference frame (p*) Angle between input track and missing momentum vector (cosθ m Sum of energy within 9 o of estimated W direction (E W 9 )

47 puts to NN Tagger for lepton tagger B S NB: log scale! NN Training and * charge B B

48 he tagging algorithm Combined Tagging NN B B Lepton Gold Kaons Silver Kaons a Slow Pio Other b-taggers 4 Physics Categorie

49 The importance of tagging agging the flavor of the B hadron is a crucial ingredient in the measurement since A obs CP ( t) = (1 2ω σ (sin 2β ) 1 / ) A ( t) = 1 / (1 2ω ith ε tag = tagging efficiency ω = mis-tag probability nd ε eff =ε tag (1-2ω) 2 = effective tagging efficiency ε eff CP ε tag 2 ) don t rely on MC but extract tagging purity and efficiency directly from data

50 ow to measure tagging performanc Measured from data using B flavor eigenstates in the context of the time dependent B mixing analysis: - U 4S B e - B e + p + D - p + K - p - Apply standard tagging B flavor exclusive reconstruction

51 he B flavor Sample Open charm decays Cabibbo-favored decays b cud B D (*)- h + N cand =23618 B (*) D π / ρ / a1 Charmonium decays b ccs B J / ΨK ( K K π * * + ) B J/ΨK * N cand =1757

52 ime 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 m d =.516 ±.16 (stat) ±.1 (syst) h ps -1 3 fb -

53 Tagging performance Category ε tag (%) ω (%) ε eff (%) Lepton 9.1 ± ± ±.3 Gold K 16.7 ± ± ±.4 Silver K 19.8 ± ± ±.4 Other 2. ± ± ±.3 ALL 65.6 ± ±.7 ε tag = tagging efficiency ω = mis-tag probability ε e ff = ε tag (1-2ω) 2 = effective tagging efficiency sensitivity

54 gredient #3: t measurement A ( ) dt CP t = time dependent analysis! he technique: B e - e + e - U 4S B m + Dz~ bgc Dt m - p - p +

55 ertex and z Reconstruction construct B CP vertex from daughters construct B tag direction from vertex & momentum, beam ot, and ϒ (4S) momentum = eudotrack construct B tag vertex from eudotrack plus consistent t of tag tracks Beam spot nvert from?z to?t, accounting for mall) B momentum in ϒ (4S) frame Interaction Point z B CP vertex B tag Vertex tag tracks, V s B CP B CP d B tag Performance: σ CP ~ 65 µm σ Tag ~ 18 µm cf. <?z > ~ ß?ct = 26µm

56 The sin2β CP fit sin2β CP Asymmetry vs. time A obs CP ( t) = (1 2w)sin2β sin( B tag B tag Raw t distribution: no binning event by event errors Decay Time Difference (reco-tag) (ps)

57 The sin2β CP fit ombined fit to t distribution of: CP sample sin2β B-flavor sample mistag rates, t resolution B tag B tag Decay Time Difference (reco-tag) (ps) e f (?t) 1? sin2 ß = m 1-2? sin(?m?t) CP, ± 4 t f d?t /t B d B d R

58 ot an easy fit Parameters 2β tag fractions for B and B tags nal resolution function pirical description of background t etime from PDG 22 ing frequency from PDG 22 al parameters # Main Sample Tagged CP sample Tagged flavor samp Tagged flavor samp Sidebands τ B = ps m d =.489 ps -1 Global correlation coefficient for sin2β: 13%

59 e CP fit: sin2β from 88M B B CP=-1 CP=+1 sin(2b) =.741 ±.67 stat ±.33 syst

60 atinum events: Lepton Tags BABAR AR 81.3 fb lepton-tagge h f = -1 events 98% purity 3.3% mistag rate 2% better Dt resolution sin2ß = 79. ±

61 ast but not least e measurement is done: Can we trust it??? e hardest part of a precise analysis: Cross-checks Systematic errors and what you are most likely to spend long night on when working on your thesis

62 heck null Control Sample Repeat same analysi But on the flavor sam Sample sin2β B flav.21± B +.17± No asymmetry expec No asymmetry obser

63 ubsample Checks

64 he systematics CP content of background t resolution Tagging (B CP vs B flav ) All Others Total Systematic Error is is not the ultimate systematic error: Main systematics decreasing with N sin2β will never be dominated by systematics at present B fact

65 ow well do we know sin(2β)? 88 M BB 85 M BB Run 1 Update after LP23:.736 ±.49

66 nitarity Triangle constraints sin2β vs indirect UT constrai Excellent agreement CKM mechanism is the dominant source of at low energies sin2βwa Some SM extensions exclu Approximate CP Minimal L-R symmetric with spontaneous CPV New physics hidden in mixing disfavored Could this be a coincidence? Measure sin2β in other independent and clean mo

67 P violation as a probe for New Physics idea: Measure CP violation in penguin decays such as B Φ r B η K S Two possible situations: sin2β Penguin = sin2β J/ΨKs another triumph of SM sin2β Penguin!= sin2β J/ΨKs clear sign of New Physics

68 New Physics? BaBar Belle Average sin2β(φk S ) +.45 ±.43 ±.7.96 ±.5 ±.1.14 ±.33 sin2β(η K S ) BaBar (old).2 ±.34 ±.3 Belle Average.43 ±.27 ±.6.27 ±.21 Compare with sin2β(j/ψk S )=.73±.5 Β ΦK S : -2.6 σ Β η K S : -2.1 σ Keep an eye on them!

69 he bright future of BaBar Excellent opportunity for precise SM tests or even for discovery New Physics!

70 onclusion he B factories offer a unique opportunity to precise est the CP sector of the Standard Model 21: first unambiguous observation of CPV in the B syste 23: sin2β is almost a precision measurement sin(2β) =.736 ± : discovery of New Physics??? Exciting times ahead!