B Physics Beyond CP Violation Semileptonic B Decays Masahiro MoriiHarvard University

MIT LNS Colloquium, 2004

M. Morii, Harvard

OutlineIntroduction: Why semileptonic B decays?CP violation Unitarity Triangle |Vub| vs. sin2b|Vub| from inclusive b uv decaysMeasurements: lepton energy, hadron mass, lepton-neutrino massTheoretical challenge: Shape function|Vub| from exclusive b uv decaysMeasurements: B pvTheoretical challenge: Form factorsSummary

M. Morii, Harvard

History of CP Violation (1)1964: Cronin & Fitch discover CPVKL (thought to be CP = 1) decayed into p +p (CP = +1)1973: Kobayashi-Maskawa mechanism proposed

Unitary matrix VCKM translates mass and weak basis3 real parameters + 1 complex phase

1974: charm quark, 1975: t lepton, 1977: bottom quarkThe only source of CPV in the Minimal SM

M. Morii, Harvard

History of CP Violation (2)1970s90s: CPV in K0-K0 mixing (e) studied in great details~1999: Direct CPV in K0 decays (e') confirmedKM mechanism most likely explanation1999: BABAR and Belle start taking data2001: CPV in B0 decays (sin2b) measuredAgrees with expectation from the KM mechanismKobayashi-Maskawa mechanism is likely the dominant source of the CP violation observed in the labIs it the sole source?

M. Morii, Harvard

Quantitative TestCKM matrix has 4 free parameters

All but the two smallest elements Vub and Vtd are well measuredIn order to test the KM-only hypothesis:Interpret measurements assuming the minimal SM is correctEither CPV or non-CPV as long as they are sensitive to Vub and VtdTurn them into constraints on (r, h) and compareIt would be nice to express this graphicallyWolfenstein parameterization

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Unitarity TriangleVCKM is unitaryThis is neatly represented by the familiar Unitarity Triangle

Each measurement constrains the apex position (r, h)The only complex phases of O(1) in the minimal SM

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Consistency TestCompare the measurements (contours) on the (r, h) planeIf the SM is the whole story, they must all overlapThe tells us this is true as of todayStill large enough for New Physics to hidePrecision of sin2b outstripped the other measurementsMust improve the others to make more stringent test

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Next Step: |Vub|Zoom in to see the overlap of the other contoursIts obvious: we must make the green ring thinnerLeft side of the Triangle is

Uncertainty dominated by ~15% on |Vub|Measurement of |Vub| is complementary to sin2bGoal: Accurate determination of both |Vub| and sin2b

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Measuring |Vub|Best probe: semileptonic b u decay

The problem: b cv decay

How can we suppress 50 larger background?Tree leveldecoupled from hadronic effects

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Detecting b uvInclusive: Use mu

Inclusive b uvThere are 3 independent variables in B XvTake E, q2 (lepton-neutrino mass2), and mX (hadronic mass)6%20%70%Where does it come from?

TechniqueEfficiencyTheoretical ErrorEStraightforwardLowLargeq2ComplicatedModerateModeratemXComplicatedHighLarge

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Theoretical IssuesTree level rate must be corrected for QCDOperator Product Expansion gives us the inclusive rateExpansion in as(mb) (perturbative) and 1/mb (non-perturbative)

Main uncertainty (10%) from mb5 5% on |Vub|But we need the accessible fraction (e.g., E > 2.3 GeV) of the rateknown to O(as2)Suppressed by 1/mb2

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Shape FunctionOPE doesnt work everywhere in the phase spaceOK once integratedDoesnt converge, e.g., near the E end pointResumming turns non-perturb. terms into a Shape Function b quark Fermi motion parallel to the u quark velocitySmears the quark-level distribution observed spectraRough features (mean, r.m.s.) are knownDetails, especially the tail, are unknown

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Shape Function What to Do?Measure: Same SF affects (to the first order) b sg decays

Caveat: whole Eg spectrum is neededOnly Eg > 1.8 GeV has been measuredBackground overwhelms lower energiesCompromise: assume functional forms of f(k+)Example:Fit b sg spectrum to determine the parametersTry different functions to assess the systematicsMeasure Eg spectrum in b sgExtract f(k+) Predict E spectrum in b uv1.82 parameters (L and a) to fit

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SF from b sgCLEO and Belle has measured the b sg spectrum

BABAR result on the wayStatistical errors dominate the uncertainty around the peakModel dependence important in the tailCLEO hep-ex/0402009Belle hep-ex/0407052BelleFit3 models tried

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Predicting b uv SpectraOPE + SF can predict triple-differential rateDe Fazio, Neubert (JHEP 9906:017)Every experiment uses DFN for simulating b uv signal

Unreliable in the SF region where OPE converges poorlySmall mX and small q2 X is jet-likeThe right tool: Soft Collinear Effective Theory6%20%70%

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Soft Collinear Effective TheoryDeveloped since 2001 by Bauer, Fleming, Luke, Pirjol, StewartPRD63:014006, PRD63:114020, PRD65:054022Applied to b uv in the SF region by several groupsBauer, Manohar (PRD70:034024) Bosch, Lange, Neubert, Paz (NPB699:335) Lee, Stewart (hep-ph/0409045)Caveat: Works only in the SF regionWe tried implementing an event generator with limited successWanted: Theoretically-sound b uv Monte Carlo generator THAT WORKS

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Lepton EndpointSelect electrons in 2.0 < E < 2.6 GeVAccurate subtraction of background is crucial!Data taken below the U4S resonance for light-flavor backgroundFit the E spectrum with b uv, B Dv, B D*v, B D**v, etc. to measureData (eff. corrected)MCData (continuum sub)MC for BB backgroundBABAR hep-ex/0408075 CLEO PRL 88:231803BELLE-CONF-0325

E (GeV)DB (10-4)BABAR2.02.64.85 0.29stat 0.53sysCLEO2.22.62.30 0.15stat 0.35sysBelle2.32.61.19 0.11stat 0.10sys

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Lepton EndpointTranslate DB into |Vub| using the SF parameters from Belle

Lower E cut-off reduces theoretical uncertainty to ~10%But theorists raise possibilities of additional uncertaintiesSub-leading SFs, 4-quark operators, weak annihilationRecalculated by the Heavy Flavor Averaging GroupBABAR hep-ex/0408075 CLEO PRL 88:231803BELLE-CONF-0325

E (GeV)DB (10-4)|Vub| (10-3)BABAR2.02.64.85 0.29stat 0.53sys4.40 0.15exp 0.44thCLEO2.22.62.30 0.15exp 0.35sys4.69 0.23exp 0.63thBelle2.32.61.19 0.11exp 0.10sys4.46 0.23exp 0.61th

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Measuring mX and q2Must reconstruct all decay products to measure mX or q2E was much easierB mesons produced in pairsReconstruct one B in any mode Rest of the event contains exactly one recoil BFind a lepton in the recoil B Remaining part must be X in B XvCalculate mX and q2Fully reconstructed B hadronsleptonvX

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Recoil B SampleReconstruct B mesons in

~1000 channels usedEfficiency ~0.2%/BYield and purity from mB fit Recoil B is a clean and unbiased sample of B mesonsCharge and 4-momentum knownIdeal for measuring branching fractions

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Recoil B XvFind an = e or m (p > 1GeV) in recoil B and requireTotal event charge = 0If its a B, Q = QBMissing 4-momentum consistent with a massless neutrino2-C kinematical fit to determine pX4-momentum conservationmv = 0, mB = mB mX resolution ~ 350 MeVSample is mostly b cv at this stageB hadronsleptonvX

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Charm SuppressionSuppress b cv by vetoing against D(*) decaysD decays usually produce at least one kaon Reject events with K and KSB0 D*+( D0p +)v has peculiar kinematicsp + almost at rest w.r.t. D*+ D*+ momentum can be estimated from p + aloneCalculate for all p + Reject events consistent with mv = 0Vetoed events are depleted in b uvUsed to validate simulation of background distributionsWeve got (mX, q2) distribution of a signal-enriched sample

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Extracting b uv SignalFit mX to extract B(B Xuv)Best variable for charm rejection Best statistical errorStrong shape-function dependenceFit mX vs. q2 to extract DB(B Xuv)Restrict to, e.g., mX < 1.7 GeV, q2 > 8 GeV2Reduced shape-function dependenceUnfold detector effects to get true mX spectrumLimited statistical powerPotential for constraining shape function

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Fitting mXSimple fit in mX shows clear b uv signalSignal modeled by DFN with Belle SF

Translate to |Vub|

Theoretical error ~8%, but strong dependence on the shape function|Vub| moves by 0.45103 if CLEO SF parameters are usedBABAR 80fb-1 hep-ex/0408068

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Unfolding mXUnfold detector efficiency and resolution true mX spectrumNB: error bars are correlatedMatches simulation with different shape functions (curves)Not enough statistics to extract shape function parametersBABAR has 3 more dataMeasured mX spectrumBackground subtractionDetector unfoldingBABAR 80fb-1 hep-ex/0408068

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Fitting mX vs. q2 BABARSplit b uv signal into {mX < 1.7, q2 > 8} and elsewhere

2-D fit to measure DB in the former region yieldsBABAR 80fb-1 hep-ex/0408068

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Fitting mX vs. q2 BelleBelle has a nearly identical analysisBelle 140fb-1 hep-ex/0408115

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Turning DB into |Vub|From Bauer, Ligeti, Luke (hep-ph/0111387)

Theoretical error ~10%BABAR result moves by 0.06103 with CLEO SF paramsBABAR result moves by 0.20103 with DFN Results are more stable than the mX fitG = 0.282 0.053 using Belle SFBABAR 80fb-1 hep-ex/0408068Belle 140fb-1 hep-ex/0408115

|Vub| (10-3)BABAR4.98 0.40stat 0.39syst 0.47theoBelle5.54 0.42stat 0.50syst 0.54theo

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Status of Inclusive |Vub|mX vs. q2E endpointmX fit

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Exclusive b uvMeasure specific final states, e.g., B pvGood signal-to-background ratioBranching fraction in O(10-4) Statistics limitedSo far B pv and rv have been measuredAlso seen:B(B wv) = (1.30.5)104 [Belle hep-ex/0402023]B(B hv) = (0.840.36)104 [CLEO PRD68:072003]Need Form Factors to extract |Vub|

M. Morii, Harvard

Form FactorsForm Factors are calculated using:Lattice QCD (q2 > 16 GeV2)Existing calculations are quenched ~15% uncertaintyLight Cone Sum Rules (q2 < 16 GeV2)Assumes local quark-hadron duality ~10% uncertaintyOther approachesAll of them have uncontrolled uncertaintiesLQCD and LCSR valid in different q2 ranges No crosscheckUnquenched LQCD starts to appearPreliminary B pv FF from FNAL+MILC (hep-lat/0409116), HPQCD (hep-lat/0408019)Current technique cannot do B rv

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Measuring B pvConcentrate on B pv with q2 binningCLEO [PRD 68:072003]Reconstruct pv using missing 4-momentum as the neutrinoBelle [hep-ex/0408145]Tag B D(*)v and look at mX distribution

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B pv CLEOMissing 4-momentum = neutrinoCLEO has a better solid-angle coverage than BABAR/BelleReconstruct B pv and calculate mB and DE = EB Ebeam/2 Clear signal over backgroundRed: rv, wv, hvYellow:other XuvGreen:continuum (udsc)Black:b cvCLEO PRD 68:072003

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B pv BelleTag B D(*)v and look at the recoil BSimilar to inclusive |Vub| measurements on recoil BD(*)v tag is less pure, but more efficientHadronic mass distribution shows pv and rv signals

Belle hep-ex/0408145pvrvother Xuvq2 < 88 < q2 < 1616 < q2

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DG(B pv)Small model-dependence due to efficiency estimationCLEO PRD 68:072003Belle hep-ex/0408145CLEOBelle

B(B pv) [104]DB(q2 > 16 GeV2) [104]CLEO1.33 0.18 0.130.25 0.09 0.05Belle1.76 0.28 0.200.46 0.17 0.06

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B pv to |Vub|FF from LQCD calculationsAverage of quenched LQCD results: FNAL01, JLQCD01, APE01, UKQCD00Unquenched FNAL+MILCUnquenched HPQCDUncertainty still largeMainly statisticalExpect rapid progress in the next yearUnquenched LQCDMore data from BABAR, BelleCLEO pvBelle pvCLEO PRD 68:072003Belle hep-ex/0408145

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Summary (1)Exclusive b uvB pvInclusive b uvmX-q2wv, hv ?mXEWAdualityunquenched|Vub|SSFs

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Summary (2)Precise determination of |Vub| complements sin2b to test the (in)completeness of the Standard Model

Backup Slides

M. Morii, Harvard

Penguinsb sss decay dominated by the penguin diagramIn the SM, same CP asymmetry as b ccs decays: sin2bNew Physics may modify the loop CP asymmetries may not agreeSeveral decay channels are studiedB0 f KS is pure-penguinSmall BF: 7.610-6B0 hKS has larger BF = 5.510-5Tree diagram affects the asymmetry by

Status of PenguinsPenguins disagree with sin2b by 2.7s (BABAR), 2.4s (Belle)Belle

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Sub-leading Shape FunctionsShape Function represents non-perturb. effects at O(1/mb2)Next order (1/mb3) 4 Sub-leading Shape FunctionsBauer, Luke, Mannel calculated their effects on Eg (PRD68:094001) and E (PLB543:261) spectraNeubert (PLB543:269) estimated impact on |Vub| measurement Errors quoted by HFAGNew calculations using SCET appeared recentlyLee, Stewart (hep-ph/0409045)Bosch, Neubert, Paz (hep-ph/0409115)Significant impact on |Vub| measured with E endpointRe-evaluation of the SSF error is due

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B pv to |Vub|Average of quenched LQCD resultsFNAL01, JLQCD01, APE01, UKQCD00Two preliminary unquenched LQCD resultsCLEO PRD 68:072003Belle hep-ex/0408145LQCD calculation

CLEOBelleQuenchedFNAL04HPQCD

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