Review of BABAR results

1
Review of BABAR results

• G. Finocchiaro
• INFN Laboratori Nazionali di Frascati
• on behalf of the BABAR Collaboration

Pushkinskie Gory May 2004
2
Outline

• Intro
• Super quick (really!) review of CP in the B
  system
• PEP-II and BABAR
• Basics of time dependent analysis _at_ BABAR
• BABAR is producing a very rich harvest of
  interesting results. Today I will focus on
• The angles of the Unitarity Triangle
• b
• a
• g
• Conclusions and outlook

3
The CKM matrix

• Cabibbo-Kobayashi-Maskawa quark mixing matrix
• Relates weak flavour (q?) to mass eigenstates (q)
• The complex Vijs parameters determine the
• electroweak coupling strength of the W to quarks
  qiqj

• VCKM is unitary
• only 4 independent parameters (3 angles, 1
  phase)
• CPV accommodated in the SM through the CKM phase
• Wolfenstein parameterization (expansion in
  powers of l ? 0.22) l , A, r, h

4
The Unitarity Triangle
The unitarity of VCKM is conveniently expressed
by the product db VudVubVcdVcbVtdVtb0

• sides of the triangle of same order, O(l3)
• ? sides and angles directly measurable
• area (A J/2) proportional to amount of CPV
• CP asymmetries in B decays sensitive to the
  angles of the Triangle

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CP violation in interference between decays with
and without mixing

• CP violation can arise from interference between
  two paths

Time dependent CP asymmetry
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The asymmetric B factory PEP-II

• Ingredients to measure time-dependent CP
  asymmetries
• Measurement of the B decay time (ACP 0 when
  time integrated)
• Knowledge of the initial favor (at t0) of the B
  (flavor tagging)
• Very large data sample (the relevant processes
  have low branching fractions)
• PEP-II a 2nd generation ee- B factory
• asymmetric beam energies
• Lpeak gt 8 1033 cm-2 s-1 ( 8 BB pairs/second)
• ( gt 3 x design lumi)

7
The BABAR detector
SVT efficiency 97 , sz 15 mm, sz (inner
layers, perpendicular tracks) Tracking (SVTDCH)
s(pT)/pT ? ( 0.13 pT(GeV) ? 0.45 ) K-p
separation (DIRC) ? 3 s ( plt3.5 GeV ) EMC
energy resolution sE/E ( 2.3 E-1/4 ? 1.8 )

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Principle of analysis technique
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B meson reconstruction at ?(4S)
Kinematical signature for B decays
Typical resolutions s(mES) ? 2.5 MeV s(DE)
? 1040 MeV

• () ? measured in U(4S) rest frame

• Define 2 regions in DE, mES plane
• Signal region DE0,mESmB
• Sideband region

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Measuring sin2?
b ? ccs Golden Modes J/yKS, J/yKL, J/yK,
charmonium precision measurements b ? ccd
Cabibbo-suppressed, possibly penguin-contaminated
D()D(), J/yp0 Consistency of the SM
picture b ? sss, sdd Penguin-dominated fK 0,
KK-KS , f 0K 0, h'K0 , p0 K0, Kg Most
sensitive to New Physics
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B0?J/yKs the Golden Mode

• Large asymmetry
• theoretically cleanest way to measure sin2b
• clear experimental signature
• relatively large BF (4.510-4 )
• similar considerations hold for other charmonium
  modes (y(2S)KS, ?c1KS, hcKs)

ACP(t) directly measure sin2b
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sin2b from B0?J/yKS,L results

• unbinned maximum likelihood fit to data
• models for n signal m background components
• Background PDfs, mis-tag rates and Dt resolution
  from data
• extract signal yield and CP asymmetry parameters
  (S,C,A)

13
sin2b from b?s penguins

• B0? fKs , B0? h'Ks , B0? p0Ks

• If t-loop diagram dominates only 1 phase
• Sf sin2b, Cf 0
• u-quark loop (g) suppressed
• ratio of CKM factors 0.02

(u,c)
(u,c)

• h'Ks and p0 Ks get contributions from tree b?u
• Vus Vub (weak phase g)
• CKM and color suppressed
• Measure sin2beff, which in SM is estimated not to
  differ from sin2b by more than 0.10.3
• New Physics can manifest in penguin decays
• e.g. virtual non-SM particles as propagators in
  the loop

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b?s penguin experimental issues

• small branching fraction

• large continuum background
• In addition to standard DE and mES, use event
  shape discriminating variables in M.L. fit

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B0?fK0

• Dominated by pure b?sss gluonic penguins
• Reconstruct
• B0?fK0s
• B0?fK0L (new result)
• reconstruct Ks? pp-, f?KK-
• Control channel B? fK
• SfK 0.230.24,
• CfK-0.140.18
• consistent with 0

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B0?fK0 results of time dependent analysis

• Combined (B0?fK0s B0?fK0L) result from
  BABAR
• Consistent with prediction from SM

Belles result (B0? fK0s only), based on 152 106
BB pairs 3.5 s away from SM, 2.3 s away from
BABAR
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B0?KK-K0

• same final states as B0?fK0s, but without going
  through the f resonance (f ?KK- are vetoed)
• need to determine CP-even fraction from
• helicity angle distribution of KK-
• confirms Belles result

Moriond EW 04

• fit of ACP(t) on data

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B0?f0(980)K0s

• decay probably dominated by pure b?sss gluonic
  penguins (also uus, but suppressed)

Moriond EW 04
f0 a scalar, f0 K0s CP-even ?S -sin2b in
time-dependent CPV

• f0 reconstructed in pp-. Signal
• extracted through a fit on mES, DE,
• m(pp-) and quantities sensible to
• event shape

• fit of Acp(t) on data

preliminary

• S,C consistent with SM prediction

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B0?K0sp0

• In SM decay dominated by penguin b?sqq diagram.
  Ignoring the tree, Ssin2b
• Position of B decay determined via innovative
  technique
• only info from Ks vertex
• extrapolate back to beam spot region in x, y

hep-ex/0403001 sub. to PRL
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Status of the Sf and Cf measurements

• no hint of direct CPV
• errors still large

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Comments on sin 2b

• World average from charmonium (dominated by BB)
• sin2b 0.7360.049
• CKM fit using only Vub, Vcb, eK, Dmd, Dms
• sin2b 0.6760.090
• One solution in excellent agreement the SM
  passed the first precision test of the CKM matrix
• Building up impressive collection of sin2beff
  results
• Measurements still statistically limited
• will improve in (near) future
• approaching precision of theory
• Overall agreement with SM reasonable
• BABAR consistent with SM
• one single measurement (out of 10) away from SM
• optimistic attitude hint of NP!
• lets wait for new data (see 5. above)

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Measuring a
Textbook example B0?pp-
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Coping with penguins

• Difficult to reliably estimate how much penguins
  contribute
• B(B0?Kp-) (pure penguin) indicates they cannot
  be neglected
• Gronau/London analysis
• Assuming isospin symmetry, these triangular
• relations between the B?hh amplitudes hold
• The B and B triangles do not match, and 2aeff
  2a?pp
• Need to measure 5 BFs, including B(B0?p0p0) from
  tagged samples
• still a 8-fold ambiguity
• Grossman/Quinn bound

(-)

• Do not need to tag B flavour
• Useful if B(B0?p0p0) is small

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Status of pp- analysis
120M BB preliminary Npp265.924.0 NKp873.337.5
NKK 12.510.4 Spp -0.40 0.22 0.03 Cpp
-0.19 0.19 0.05
Belle (140fb-1) Spp -1.00 0.21 0.07 Cpp
0.58 0.15 0.07 (direct CPV _at_ 3.2s !)

• standard time dependent analysis
• particle ID crucial
• info to fight continuum background combined in
  Fisher discriminant

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B0?p0p0 and the GQ bound
B0?p0p0 observed by BABAR with 4.2 s
significance
PRL91, 241801 (2003)

• B(B0?p0p0)
• too small for full isospin analysis
• too large to give a useful G/Q constraint
• compare to B(B0?pp-)(4.60.4) 10-6 (WA)

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B0?rr
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B0?rr-

• Challenging experimentally
• 4 particles (two p0s) in the B final state
• the r is wide
• small BF, lots of background from B decays (209
  B decay modes studied!)
• simultaneous ML fit to DE, mES, NN, cos?hel, mr

Main systematics from uncertainty of CP content
in background
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rr- Isospin analysis
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Summary on a

• Large penguin pollution in B?pp
• B0?p0p0 measured, but
• small makes full Isospin analysis hard
• too large for useful bound on a-aeff
• Situation looks promising in rr
• although VV decay, flong1 (pure CP even)
• small B(B0?r0r0) indicates small penguin
  pollution
• first measurement of ACP(Dt), and direct
  constraints on a (with assumptions)
• Other modes being studied (rp)

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Measuring g
0 1
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sin(2bg) from B0?D()p?

• CPV from interference of direct Cabibbo-favoured
  and mixed b? u decays
• not a CP eigenstate
• relative phase is 2bg
• asymmetry depends on hadronic parameters

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sin(2bg) from B0?D()p?

• 2 complementary analyses in BABAR
• Full reconstruction of B0?Dp? and B0?Dp?
• 2rDpsin(2bg)cosdDp-0.0680.0380.021
• 2rDpsin(2bg)cosdDp -0.0220.0380.021
• Partial reconstruction of B0?Dp?
• Use only fast (2.1ltplt2.4 GeV/c) and slow
  (plt0.250 GeV/c) pion tracks
• compute mass of recoiling object (the D0)
  without reconstructing it
• 2rDpsin(2bg)cosdDp-0.0630.0240.014

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B0?D()p? interpretation

• Need r() to extract limit on sin(2bg)
• Using SU(3) (30 error) and exp. WA
• Derive limits combining fullpartial reco, using
  a Feldman-Cousins method with Toy MC
• sin(2bg) gt 0.87 68 CL
• sin(2bg) gt 0.58 95 CL

We are setting constraints on rh plane from g!
Limit without assumptions on r()
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g from B-?Kp-K- ADS method

• Equalize the interfering amplitudes
  (Atwood-Dunietz-Soni, PRL 78, 3257)
• Extract g from decay rates measurements

35
g from ADS results
RADS0.0040.012 RADSlt0.026 _at_90 CL

• rB lt0.22 _at_ 90 CL
• (or rB lt0.20 if 48oltglt73o)
• implying
• small interference
• difficult measurement of g

36
Summary on g

• Common wisdom (prejudice?) g immeasurable at B
  factories
• Looks not impossible now (increasing size of B
  samples)
• First constraints on r-h plane from B0?D()p?
• Result will depend on size of color suppressed
  b?u amplitude
• B-? D()0K- ADS analysis disfavors optimistic
  scenario
• No single gold-plated channel need to combine
  many measurements

- -
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Concluding remarks

• CPV is now well established in the B meson system
• Interest now moving onto searching for New
  Physics in channels with loop-dominated diagrams
• hints for new physics from sin2b in b?s penguin
  modes?
• a moving target at the moment. B-factories will
  eventually resolve the question.
• a not only through B?pp. B?rr looks the most
  promising channel
• first steps toward a measurement of g
• Success of the effort depends on several unknowns
  - validity of theoretical assumptions, values of
  parameters to be experimentally determined (rB)
• The game started in May 1999, but working in
  BABAR is still very exciting.
• Stay tuned for more results this summer!

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Backupslides
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Flavour tagging

• determine the flavor of BTAG from its charged
• decay products
• lepton,
• kaons,
• soft p from D?D0p
• high momentum tracks
• use neural network techniques to combine PID and
  kinematics info
• 4 different tagging categories
• cleanest tag from leptons (lt5 mistag rate)
• most abundant tag kaons (lt20 mistag rate)

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Dz resolution

• BTAG vertex fit
• use all tracks left over by BREC vertex
• BTAG direction constrained by BREC and beam spot
  information
• long-lived particles (KS, L) explicitly recoed
• photon conversion (g?ee-) removed
• tracks with large c2 iteratively removed

Both w and R measured on data
Efficiency for CP sample 97 Typical
resolutions Dz 180mm, Dt 0.7ps