Dream Machines:Dream Machines:B Physics with eB Physics with e++ee-- Colliders Colliders
2007 Fermilab Academic Lecture Series: Flavor Physics2007 Fermilab Academic Lecture Series: Flavor PhysicsJeffrey Berryhill (FNAL) Jeffrey Berryhill (FNAL)
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OutlineOutline
An Introduction to Y(4S) Experiments and Measurement Techniques
•Dream Machines: Accelerators, Detectors, and the Y(4S) environs
•Exclusive B branching fractions and direct CP violationKinematic constraintsMultivariate background discriminationCase Study: B0 ➔ K+ -
•Time dependent CP violation of exclusive B decaysB decay length differenceB flavor taggingCase Study: B0 ➔ Ks
•B branching fractions to inclusive or neutrino-ful final states Recoil side reconstructionCase Study: B- ➔
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B meson production with eB meson production with e++ee-- collisions collisions
Y(4S) lightest bb bound state decaying to B mesons Mass = 10.58 GeV (B meson mass = 5.279 GeV) Peak production xsec = 1.1 nb Decays exclusively to B+B- (50%) and B0B0 (50%) B pair produced nearly at rest in Y(4S) frame ( = 0.06)
bb threshold
BB threshold
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Other production at 10.58 GeVOther production at 10.58 GeV
Final State Xsec (nb)
Events/fb-1
Y(4S) → BB 1.1 1.1 M
uu, dd, and ss 2.0 2.0 M
cc 1.4 1.4 M
1.0 1.0 M
~2.2 million b quarksper fb-1 =
0.55M B+B- + 0.55M B0B0
Y(4S) signal/bkg = 1:4
Also (very low multiplicity): QED ( ee() , () ) Two-photon collisions
A B factory at Y(4S) is also acharm and tau factory!
At 10.54 GeV, “off-resonance” events collected to study or measure background
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e+3.1 GeV
e-9 GeV
Y(4S)
Y(4S)B0
B0
Asymmetric-energy B FactoriesAsymmetric-energy B Factories
B Meson = 0.56
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PEP-II at SLAC
KEKB at KEK
Belle
BaBar
9GeV (e) 3.1GeV (e+)peak luminosity:
1.121034cm2s1
Asymmetric-energy B FactoriesAsymmetric-energy B Factories
8GeV (e) 3.5GeV (e+) peak luminosity:
1.651034cm2s1
B Meson = 0.56No crossing angle,dipole separates beams
B Meson = 0.42Beams cross at an angle“crab crossing”
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PEP-II at the Stanford Linear PEP-II at the Stanford Linear Accelerator CenterAccelerator Center
BaBar detectorBaBar detector
LinacLinac
PEP-II Storage RingsPEP-II Storage Rings
SF Bay ViewSF Bay View
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PEP-II performancePEP-II performance
Operated continuously since 1999
PEP-II top luminosity: 11.2 x 1033cm-2s-1 (more than 3x design goal 3.0x1033)
Beams carry >1 Amp of current each, injected continuously
Beams collide in a small area of about
100 m X 6 m
In 1 day : created 1.5 million
b quarks !780 million b quarks produced!
Also hundreds of millions each of u, d, s, c, and
Int. Lum. = 391 fb-1
Off resonance = 37 fb-1
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Integrated Luminosity
PEP-IIfor BaBar
KEKBfor Belle
KEKB + PEP-II
World Y(4S) sample > 2 Billion B mesons
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The BaBar detectorThe BaBar detector
Cherenkov Detector (DIRC)
144 quartz barsK, separation
Electromagnetic Calorimeter6580 CsI crystals
e+ ID, and reco
Drift Chamber40 layers
Tracking + dE/dx
Instrumented Flux Return19 layers of RPCs (now LSTs) and KL ID
Silicon Vertex Tracker
5 layers of double sided silicon strips
e+ [3.1 GeV]
e- [9 GeV]
e eff. = 90%→ e fake rate = 0.1%
eff. = 90%→ fake rate = 1-2%
K eff. = 80%→ K fake rate = 0.1% eff. = 80%K → fake rate = 1%
(t) ≈ 1 ps
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Particles producedParticles produced
(Quasi) stable detectable “building blocks”Mesons Leptons Baryons+ 0 ( pair) e p K+ KS ( pair) KL n
Charmed mesons
D+ (K- KS+)D0 (K-+, K- + 0, K-KS)D*0 (D0 0, D0 )D*+ (D0 +, D+ 0)Ds
+ ( +, K+ KS)Ds
+* (Ds+ )
J/e+e-
Light mesons
’
K* (K)(K+ K-, KS KL)
Others: (2S), c, c, K2*(1430), C, f0(980), a0,1,2
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BB++→→ K* K*++ Candidate Candidate
High energy photon absorbedby ECAL
Ks →
and +
Muon from other B decay
Large fraction of stable particles identified by speciesand kinematics measured with high precision
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OutlineOutline
•Dream Machines: Accelerators, Detectors, and the Y(4S) environs
•Exclusive B branching fractions and direct CP violationKinematic constraintsMultivariate background discriminationCase Study: B0 ➔ K+ -
•Time dependent CP violation of exclusive B decaysB decay length differenceB flavor taggingCase Study: B0 ➔ Ks
•B branching fractions to inclusive or neutrino-ful final states Recoil side reconstructionCase Study: B- ➔
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Anatomy of a Branching Fraction Anatomy of a Branching Fraction MeasurementMeasurement
•Signal candidate reconstruction: Collect sample of events with exclusivefinal state X (particle ID and kinematic constraints)
•Background rejection: Cuts which exploit event shape differencesto reduce continuum and other backgrounds
•Signal yield extraction: Estimator of NX for selected events usingmulti-dimensional likelihood fit over kinematic and background rejection variables
•Efficiency estimation: Fraction X of produced events selected (checked against numerous control samples)
•B counting: Total number of B pairs NBB produced in collisions
•Putting it all together:
BF(B0 → X) = NX / X NBB
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Kinematic ConstraintsKinematic Constraints
•For rare decay studies or high precision, eliminating random combinationsof particles as possible B candidates is essential •For a completely reconstructed B meson candidate with daughters (pi, mi), two uncorrelated kinematic constraints:
Beam-constrained mass:
Candidate should have mass mB2 = (5.279 GeV)2 = E2 – p2
AND, Y(4S) decays exclusively to a B pair, so E = E(beam) in Y(4S) frame
pB, E(beam) in Y(4S) frame
pB and E(beam) precisely known, so Mbc should agree with mB to within3 MeV! Independent of mass hypotheses for daughters
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Kinematic ConstraintsKinematic Constraints
Energy difference:
Candidate energy should coincide with beam energy in Y(4S) frame
p, E(beam) in Y(4S) frame
Resolution typically 10-30 MeV
Depends explicitly on mass hypotheses for daughters
Incorrect particle ID or missing particles will shift E from 0(suppresses backgrounds from other B decays: false ID or “cross-feed”)
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B0 →KsJ/(ee)
Kinematic ConstraintsKinematic Constraints
Feed-down fromJ/ K*+
Also: Intermediate particle masses (e.g. composites listed previously)and decay helicity angles can provide additional rejection
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Background SuppressionBackground Suppression
Exploit fact that continuum events are more jet-like than BB events
•Event shape variables: spherical B vs. jet-like continuum(Fox-Wolfram moments, Legendre moments, sphericity, thrust angle)
•B flavor tagging variables: kaons and leptons in the rest of the event•B candidate vertex separation fromrest of the event
Sometimes combined into a single Fisher discriminant or neural net variable
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Signal ExtractionSignal Extraction
Unbinned maximum likelihood fit of (mES, E, et al.): maximizes LH function
i components: signal, peaking backgrounds, combinatorial background, with normalization Ni and shape parameters i
j events: each event with a vector of variables xj
Pi typically multidimensional: Pi = Pi(mES)*Pi(E)*….. (usually uncorrelated)
Signal yield, background yield and background shape parameters are typically free parameters in fit (background systematics absorbed intostatistical error on signal yield)
Signal yield or other parameters of key interest are BLIND until all analysis techniques finalized
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B countingB counting
•Select all on-resonance multi-track events satisfying very loose event shape cuts ( Non(MH) )
•Use same selection on off-resonance data ( Noff(MH) )
•Select ee → events as an estimator of luminosity in on and offresonance samples ( Non() and Noff() )
In practice, is very close to 1
B counting systematic uncertainty is typically 1%
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Case Study: BCase Study: B00 →→K+K+ BF and A BF and ACPCP
BaBar preliminary BF, hep-ex/0608003 227M BBBaBar preliminary ACP, hep-ex/0607106 347M BB
Measurement of rare b →s penguin decay and its direct CP asymmetry
Simultaneous study of K, , KK (pion hypothesis for all tracks)
5D likelihood fit of
F = event shape Fisher discriminant, C = Cherenkov angle in DIRC
background at few % level
C PDFs and asymmetry in efficiencies from large (400k) D*+→ D control sample
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Case Study: BCase Study: B00 →→K+K+ BF and A BF and ACPCP
Signal fit PDFs vs. data
(1660 events)
qq Bkgfit PDFs vs. data
(65000)
“sPlot” method of projecting out of the data each PDF component(a fancy background subtraction using LH function)
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Case Study: BCase Study: B00 →→K+K+ BF and A BF and ACPCP
Signal fit PDFs vs. data
(1660 events)
qq Bkgfit PDFs vs. data
(65000)
“sPlot” method of projecting out of the data each PDF component(a fancy background subtraction using LH function)
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Case Study: BCase Study: B00 →→K+K+ BF and A BF and ACPCP
4.5 evidence for direct CP violation!
K+-
K-+
No contamination
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Case Study: BCase Study: B00 →→K+K+ BF and A BF and ACPCP
CDF is now an evencompetitor
Hard to reconcile withnull result in K+0
(4.9 differencein world average)
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OutlineOutline
•Dream Machines: Accelerators, Detectors, and the Y(4S) environs
•Exclusive B branching fractions and direct CP violationKinematic constraintsMultivariate background discriminationCase Study: B0 ➔ K+ -
•Time dependent CP violation of exclusive B decaysB decay length differenceB flavor taggingCase Study: B0 ➔ Ks
•B branching fractions to inclusive or neutrino-ful final states Recoil side reconstructionCase Study: B- ➔
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BB00 → J/→ J/ K KSS00 and sin 2 and sin 2
Decay dominated by a single “tree-level”Feynman diagram: b → ccs
J/ identified cleanly by decay to a lepton pair; KS identified cleanly by decay to pion pair.Both particles are CP eigenstates → both B0 and B0 decay to them
Time-dependent CP violation has amplitude sin 2 and frequency m
Works for several other b →ccs decays as well; results can be combined
S =
BaBar preliminary: hep-ex/0607107, 347 M BB
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Determine time between decays from vertices
Δz=(βγc)Δt
(t) ≈ 1 ps
Time-Dependent CP Violation: Experimental Time-Dependent CP Violation: Experimental techniquetechnique
Determine flavor and vertex position of other B decay
l-
K-
Asymmetric energiesproduce boosted Υ(4S), decaying into coherent BB pair
e- e+
B0
B0
Fully reconstruct decay to CP eigenstate f
J/ψ
KS
µ+
µ- π+
π-
Naively: compute CP violating asymmetry A(t) = N(f ; t) – N(f ; t) N(f ; t) + N(f ; t)
In reality, extract A from unbinned LH fit over signal & control samples
Signal extraction
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t Measurementt MeasurementTime diff. distributions of CP B decays with a B (B) tag
is average mistag rate (3-40%) is mistag difference between B and B tag (~1%)Observed f’s are convolved with resolution functions
BtagB0
BtagB00BBtag0BBtag
fobs = f± (t ) X R( t ) f±( t ), = = 0
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t Measurementt MeasurementTime diff. distributions of CP B decays with a B (B) tag
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Flavor TaggingFlavor Tagging
Six
Q = 30.4 %!
NQ
1σ(sin(2β))
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B Candidate SamplesB Candidate Samples
Extract control sample of hadronic B decaysthat are self-tagging and negligible CPV
B → D(*)- h+, h+ = +,+, a1+
100,000 events!
For each flavor tagging category, measure , and resolution functions.
Extract signal of CP B decays
11,000 events
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Asymmetry ExtractionAsymmetry ExtractionSimultaneous fit to t distributions of CP events and hadronic eventsover six tagging categories and six decay modes
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CKM Global Fit (Sep.2006)
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CKM Global Fit (Sep.2006)
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Time-Dependent CPV in b →sss Decays
Smaller than bccs in all of 9 modes
Smaller than bccs in all of 9 modes
Naïve average of all b s modes
sin2eff = 0.52 ± 0.052.6 deviation betweenpenguin and tree (b s) (b c)
Naïve average of all b s modes
sin2eff = 0.52 ± 0.052.6 deviation betweenpenguin and tree (b s) (b c)
Hazumi, ICHEP 2006
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OutlineOutline
•Dream Machines: Accelerators, Detectors, and the Y(4S) environs
•Exclusive B branching fractions and direct CP violationKinematic constraintsMultivariate background discriminationCase Study: B0 ➔ K+ -
•Time dependent CP violation of exclusive B decaysB decay length differenceB flavor taggingCase Study: B0 ➔ Ks
•B branching fractions to inclusive or neutrino-ful final states Recoil side reconstructionCase Study: B- ➔
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Recoil Side ReconstructionRecoil Side ReconstructionLesson from sin 2To gain precise control over one of the B mesons,reconstruct the other B meson in high-purity, high-efficiency modes
Efficiency penalty is at least 1%, so signal B process cannot be too rare ( BF >10-6 ) and backgrounds must be large for this to be optimal
Use cases: Inclusive B decay rates and distributions: large rates, but fewer kinematic constraints on the signal decays, due to neutrinos ( b → cl or ul ) ornumerous final states ( b → s )
Exclusive B decays to one or more neutrinos: lack of kinematic constraintson the signal side means recoil side is the only recourse for backgroundrejection ( B → , B → , B → K(*) , B → invisible)
Recoil B(D(*)+n
Signal B(anything!)
Y(4S)
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Case Study: B+ → Case Study: B+ →
Belle PRL 97 (2006) 251802, 449M BB
Simple decay through weak annihilation
Sensitive to B decay constant fB or to charged Higgs boson
SM BF ~ 1 X 10-4
Up to 3 neutrinos in the final state requires recoil reconstruction sample
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Case Study: B+ → Case Study: B+ →
Total Event ReconstructionOn recoil side, require charged B decays to high purity, high rate hadronic decays using previously described signal extraction methods
180 modes, 680k events, Recoil side efficiency = 0.13%
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Case Study: B+ → Case Study: B+ →
Total Event ReconstructionOn signal side, require simplest decay modes and no other tracks
5 modes, 300 events, recoil side efficiency = 15%
qq background eliminated!Remaining background from B decays to neutrals (0, n, KL)
Extra neutral energy (ECL)peaked at 0 for signal
Background broad in (ECL)(shape from MC)
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Case Study: B+ → Case Study: B+ →
Signal extracted from 1D likelihood fit to ECL
24+\- 7 signal events observed!
Significance = 3.5
Consistent excess in all decay modes
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BF sensitive to H+ in Type II 2HDM at largetan
Case Study: B+ → Case Study: B+ →
For MSSM, there are significant SUSY radiative corrections which complicatesinterpretation
Sensitivity comparable torecent MSSM Higgs searchat Tevatron
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SummarySummary
Highlights of B physics capability at Y(4S)
•Awesome accelerator performance results in Giga-B samples
•Unique kinematic constraints and control samples allow for precise/rare studies of exclusive B decays and direct CPV
•Highly efficient flavor tagging makes for world’s best time-dependent CPV
•Recoil side reconstruction allows for detection of B decays to virtually anything
•Can be extended in the future to a Super-B factory (100X) or Bs physics at the Y(5S)
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