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B Physics Beyond CP Violation — Semileptonic B Decays —. Masahiro Morii Harvard University MIT LNS Colloquium, 2004. Outline. Introduction: Why semileptonic B decays? CP violation — Unitarity Triangle — | V ub | vs. sin2 b | V ub | from inclusive b → uℓv decays - PowerPoint PPT Presentation
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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
M. Morii, Harvard
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
M. Morii, Harvard
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
M. Morii, Harvard
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
M. Morii, Harvard
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
M. Morii, Harvard
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
M. Morii, Harvard
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
M. Morii, Harvard
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
M. Morii, Harvard
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
M. Morii, Harvard
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
M. Morii, Harvard
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%
M. Morii, Harvard
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
M. Morii, Harvard
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
M. Morii, Harvard
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
M. Morii, Harvard
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
M. Morii, Harvard
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
M. Morii, Harvard
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
M. Morii, Harvard
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
M. Morii, Harvard
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
M. Morii, Harvard
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
M. Morii, Harvard
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
M. Morii, Harvard
Fitting mX vs. q2 BelleBelle has a nearly identical analysisBelle 140fb-1 hep-ex/0408115
M. Morii, Harvard
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
M. Morii, Harvard
Status of Inclusive |Vub|mX vs. q2E endpointmX fit
M. Morii, Harvard
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
M. Morii, Harvard
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
M. Morii, Harvard
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
M. Morii, Harvard
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
M. Morii, Harvard
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
M. Morii, Harvard
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
M. Morii, Harvard
Summary (1)Exclusive b uvB pvInclusive b uvmX-q2wv, hv ?mXEWAdualityunquenched|Vub|SSFs
M. Morii, Harvard
Backup Slides
M. Morii, Harvard
Status of PenguinsPenguins disagree with sin2b by 2.7s (BABAR), 2.4s (Belle)Belle
M. Morii, Harvard
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
M. Morii, Harvard
B pv to |Vub|Average of quenched LQCD resultsFNAL01, JLQCD01, APE01, UKQCD00Two preliminary unquenched LQCD resultsCLEO PRD 68:072003Belle hep-ex/0408145LQCD calculation
CLEOBelleQuenchedFNAL04HPQCD
M. Morii, Harvard
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