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Recent Results from the BaBar Experiment. Brian Meadows University of Cincinnati. Outline. CP Violation The BaBar Experiment B 0 – B 0 Mixing, Lifetime and sin 2  Measurements Summary. Why the B Factories Studied CP Violation.

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Recent results from the babar experiment

Recent Results from the BaBar Experiment.

Brian Meadows

University of Cincinnati

Brian Meadows, U. Cincinnati.


  • CP Violation

  • The BaBar Experiment

  • B0 – B0 Mixing, Lifetime and sin 2 Measurements

  • Summary

Brian Meadows, U. Cincinnati

Why the b factories studied cp violation
Why the B Factories Studied CP Violation

  • Understanding its origin is an intrinsically interesting goal

  • It had, thus far, only been seen in K0 decays

    • KL0->p+p-

    • Asymmetry in KL0 p§ l ¨n

      Over 40 years ago !

  • It is an important ingredient in explaining baryon-antibaryon asymmetry in the universe

  • Standard Model has 3 quark generations with CPV built in

    • Unlikely to be sufficient to explain baryon-antibaryon asymmetry

    • The B0-B0 system appeared to be an excellent laboratory for studying CPV in the Standard Model

  • K0’s are known to mix and exhibit CPV:

    • Could CPV be due to a new force that brings about DS = 2 ?

    • B0 mixing had also been discovered by then – seems like a place to look for DB = 2.

Brian Meadows, U. Cincinnati

What is known about matter asymmetry
What is Known About Matter Asymmetry?

  • It exists – well, at least locally

    • No isotropic, high energy g’s from e+e- annihilations

    • Cosmic rays do not contain anti-nuclei

    • Anti-protons abundances consistent with production in atmosphere

    • Baryon to CMB g ratio nB/ng ~ 109

      • If nB = nB initially, for T < mp thermal equilibrium would lead to

        nB/ng = nB/ng ~ 10-20

  • Simplest explanation depends on

    • Mechanism for baryon non-conservation

    • Mechanisms for C and CP violation

    • Departure of universe from thermal equilibrium so that collision time long in comparison with above.

  • Other models are less compelling or too complicated

    • nB = nB when t = “big bang”

    • Baryons and anti-baryons separated spacially.

Brian Meadows, U. Cincinnati

Why a b factory
Why a “B Factory”?

  • Principal aim of experiment – study CP violation, principally in the B0-B0 system.

  • Requires examination of rare decay modes of B0 mesons

    • Need huge samples (at least 108) BB pairs

  • Original goal at SLAC was to build a machine with “luminosity” 3 x 1033 cm-2¢ sec-1¢

    • Should provide 3 x 107 B pairs (30 fb-1) per year (107 secs.)

    • Produces BB pairs at a few Hz and other interesting physics events at ~ 100 Hz.

  • Tuned on the resonance Y (4S) ( BB)

  • Uses 9 GeV/c e- and 3.1 GeV/c e+ beams to provide way to measure time dependence in the laboratory (bg=0.56).

Brian Meadows, U. Cincinnati

The babar detector at slac pep2
The BaBar Detector at SLAC (PEP2)

  • Asymmetric e+e- collisions at (4S).

  •  = 0.56 (3.1 GeV e+, 9.0 GeV e-)

1.5 T superconducting field.

Instrumented Flux Return (IFR)

Resistive Plate Chambers (RPC’s):

Barrel: 19 layers in 65 cm steel

Endcap: 18 “ “ 60 cm “

Brian Meadows, U. Cincinnati

Pep ii performances 2010

  • On

PEP-II performances (2010)

Peak Luminosity ~2 £ 1034 cm-2¢ s-1

  • Approx. 600 fb-1 in runs 1-7



Data taken mostly at Y(4S)

BUT ~ 12% below this:






Brian Meadows, U. Cincinnati

Belle kek performances 2010
Belle (KEK) performances (2010)

Peak Luminosity ~2£1034 cm-2¢ s-1

Integrated luminosity

Approx. 880 fb-1

You can “LIVE” event Displays at

Brian Meadows, U. Cincinnati

Silicon vertex tracker svt
Silicon Vertex Tracker (SVT)

  • 5 Layers double sided AC-coupled Silicon

  • Rad-hard readout IC (2 MRad – replace ~2005)

  • Low mass design

  • Stand alone tracking for slow particles

  • Point resolution z » 20 m

  • Radius 32-140 mm

Brian Meadows, U. Cincinnati

The babar collaboration
The BaBar Collaboration

Brian Meadows, U. Cincinnati

Drift chamber
Drift Chamber

40 layer small cell design

7104 cells

He-Isobutane for low multiple scattering




Mean position


125 m

Brian Meadows, U. Cincinnati

Particle id dirc
Particle ID - DIRC

Detector of



Cherenkov light

  • Measures Cherenkov angle in quartz

    • Photons transported by internal refl.

    • Detected at end by » 10,000 PMT’s

144 quartz bars

Brian Meadows, U. Cincinnati

Particle id dirc1
Particle ID - DIRC

It Works Beautifully!

Provides excellent K/ separation

over the whole kinematic range

Brian Meadows, U. Cincinnati

Particle id dirc2
Particle ID - DIRC



Brian Meadows, U. Cincinnati

Electromagnetic calorimeter
Electromagnetic Calorimeter

  • CsI (doped with Tl) crystals

    • Arranged in 48()£120()

    • » 2.5% gaps in .

  • Forward endcap with 8 more  rings (820 crystals).

Brian Meadows, U. Cincinnati

Weak decays





Weak Decays

  • Two kinds of diagrams are prevalent in weak decays



  • These interfere with one another.


















Brian Meadows, U. Cincinnati

Weak decays1
Weak Decays

  • These decays are actually more complicated:

    • Each diagram represents the weak decay only

    • This occurs over a very short distance scale

    • It is represented by an amplitude – Weif

      This is followed by:

  • “Hadronization”

    • When the quarks emerge, they interact strongly in a “sea” of gluons and form hadrons that scatter off each other.

    • This process is represented by an amplitude – Seid

  • The nett result is represented by the amplitude

    A e i(d+f) where (|A| = W x S)

Brian Meadows, U. Cincinnati

Cp violation
CP Violation

  • CP violation is manifest when a process involving particles occurs at a different rate to that with anti particles:

    (B ! f)  (B ! f)

  • Under CP transformation, amplitudes A have weak phases  that reverse sign but strong ones  that do not

    A = a exp{i(+ )}! A = a e{i(- )}

  • If two amplitudes A1 and A2 contribute to a process, the rates are:

     = |A1+ A2|2 = a12+ a22+ 2a1a2 cos( + )

     = |A1+ A2|2 = a12+ a22+ 2a1a2 cos( - )

  •  (CP Violation)when

     (´2-1)  0 and (´2-1) 0.


CP violation is maximum when a1 = a2 !

Brian Meadows, U. Cincinnati

Cpv in the standard model cabbibo kobayashi maskawa matrix
CPV in the Standard ModelCabbibo-Kobayashi-Maskawa Matrix

  • In the Standard Model, weak decays allow quarks to change flavor in transitions from charge +2/3 to charge –1/3.

  • The couplings are defined by the CKM matrix

  • The matrix in unitary, so is defined by

    • Three angles (real)

    • One complex phase

  • Phase is real – cannot be removed by re-phasing the quark fields.

Brian Meadows, U. Cincinnati

d s b




The Unitarity Triangles

(K system)

d•s* = 0

(Bs system)

s•b* = 0

(Bd system)

d•b* = 0

These three triangles (and the three triangles corresponding to the rows) all have the same area. A nonzero area is a measure of CP violation and is an invariant of the CKM matrix.

apply unitarity constraint to pairs of columns

From P. Burchat

Brian Meadows, U. Cincinnati

d s b




The Usual Unitarity Triangle




Orientation of triangle has no physical significance. Only relative angle between sides is significant.

apply unitarity constraint to these two columns

From P. Burchat

Brian Meadows, U. Cincinnati

The usual unitarity triangle

d s b




(, )





(1, 0)

(0, 0)

The Usual Unitarity Triangle

apply unitarity constraint to these two columns

From P. Burchat

Brian Meadows, U. Cincinnati

Cp violation in the standard model

J=Vqq’ q’  (1- 5) q




CP Violation in the Standard Model

  • The phase in the CKM quark mixing matrix can give rise to CP Violation.

    • CKM imparts a phase to weak currents

      that cannot be removed by re phasing

      the quark fields.

  • Interference between a tree and a penguin process can give direct CP Violation

    but information on strong phases is required to interpret it.

  • Decays of B0 to CP eigenstates f accessible also to B0 can occur directly or through mixing.

Allows interpretation without knowing strong phases

Brian Meadows, U. Cincinnati

B mixing
B Mixing

  • Principal standard-model mechanism

dNB0 exp(–t/B) ( 1 ± cos(mt) )

Brian Meadows, U. Cincinnati

B0 – B0 Mixing

md , B andsin 2


Brian Meadows, U. Cincinnati

An Early BaBar B Mixing Measurement

hep-ex/0207071 (ICHEP)






correlation coefficient (m, B0) = -0.22

  • “In a few years we might:

  • anticipate < 1% uncertainty in B0 mixing

  • possibly measure 

  • (test CPT limits directly).”

Brian Meadows, U. Cincinnati

Most Recent BaBar B Mixing Measurement

Phys.Rev.D 73 012004 (2006)



Dmd=0.511±0.007 ps-1



B0=1.504±0.013 ps

  • “We HAVE:

  • acheived~ 1% uncertainty in B0 mixing

Brian Meadows, U. Cincinnati

Increase in precision of b lifetimes and mixing frequency
Increase in precision of B lifetimes and mixing frequency

B0 Lifetime (ps)

1.548  0.032

1.530  0.009

Ratio of B+ to B0 Lifetime

1.060  0.029

1.071  0.009

B0 Mixing Frequency ( x 1012 s-1)

0.472  0.017

0.507  0.005


18 measurements

12 measurements

10 measurements


New measurements:

5 B Factory

5 TeVatron

3 B Factory

3 TeVatron

6 B Factory

1 TeVatron

  • Uncertainties limited by:

    • knowledge of t resolution function

    • B (for mixing).

  • BABARMeasured both together using the copious B0!D*lmode.

Brian Meadows, U. Cincinnati

Mixing induced cp violation





Mixing Induced CP Violation

  • If final state f is accessible for both B0 and B0 decay then mixing will interfere with direct decays

  • If f is a CP eigen-state, amplitudes

    <f|T|B0> and <f|T|B0> have:

    • identicalstrong phases

    • identical weak phases, but with opposite signs

    • the same magnitudes.

  • Therefore

    • The CP violation is maximized.

    • The strong phases cancel in any interference observed

  • A Bonus:

    • CP violation has a time structure emanating from B0 mixing

Brian Meadows, U. Cincinnati


B0 / B0




B0 / B0

e ±, m ±, K± tag

Dz =c t

Dz ~ 255 mm for PEP-II: 9.0 GeV on 3.1 GeV

~ 200 mm for KEKB: 8.0 GeV on 3.5 GeV

The Asymmetric-Energy B Factories


Brian Meadows, U. Cincinnati

dN/dt/ e - |t|/£ [1 §Ccos(mt) ¨S sin(mt)]

C = (1 - |f|2) / (1 + |f|2)

S = Im{ f }/ (1 + |f|2)

  • For b!ccs decays (in the SM):

    • f = e 2ib



Time-Dependence of B0 Decay Modified by

Interference between two direct decay modes such as P and T

Interference between mixing and decay.

 is one of the angles in a unitarity triangle


  • AND Penguin has the same weak phase

Brian Meadows, U. Cincinnati

D t distributions with no experimental effects
Dt distributions with NO experimental effects

Flavor states sorted by mixing status

CP states sorted by B tag flavor

B0B0 or B0 B0

Btag= B0

Btag= B0

B0B0 or B0 B0

B Mixing

dN exp(–|Dt|/tB) ( 1 ± cos(DmDt) )

CP violation

dN exp(–|Dt|/tB) ( 1 ± sin2b sin(DmDt) )

With NO


Brian Meadows, U. Cincinnati

unmixed – mixed

unmixed + mixed

Asymmetry =

~ (1 – 2w)

 (1 – 2w) cos(DmdDt)

~ p / Dmd

Perfect flavor tagging and time resolution

Realistic mis-tag and finite time resolution

- unmixed

- unmixed

- mixed

- mixed

Brian Meadows, U. Cincinnati

Access to unitarity triangle angles
Access to Unitarity Triangle Angles

  • However, this strategy did not work out

  • Background at Bs is large

  • Bs oscillations too fast for our resolution

Brian Meadows, U. Cincinnati

G from direct cp violation
g from direct CP violation

= rB x

If D0 and D0 can

Decay to same final state

They Interfere !!

Colour suppression

Closer rB is to 1.0, larger interference, betterg experimental precision

Brian Meadows, U. Cincinnati

Results on sin 2 2003
Results on sin 2 (2003)

Brian Meadows, U. Cincinnati

Sin 2
Sin 2

Primary result comes from

charmonium decay modes.

Simultaneously fit flavour specific modes to determine flavour tagging quality and  resolution.

Additional information from modes which include penguin (P) in addition to tree (T) modes.



Brian Meadows, U. Cincinnati

Charmonium modes for sin 2
Charmonium Modes for sin 2



, c, c

One dominant decay amplitude !theoretically clean.

(Penguin has same phase!)




KS , L



Both BABAR and Belle use six charmonium modes:

BJ/Ks0, Ks0p+p-, p0p0




BJ/K*0, K*0 Ks0


Simultaneously measure self tagging modes to determine  and (t).

Brian Meadows, U. Cincinnati

Sin2b Data Samples in BABAR


Mixing sample

ccKs modes

B0D(*)-p+/ r+/ a1+

Ntagged= 23618

Purity= 84%

  • Data

  • Data


J/y KL

J/y Bkg

Fake J/y Bkg


Brian Meadows, U. Cincinnati

hep-ex/ 0207042 (PRL)

Ks modes

KL modes

81 fb-1 (88 M BB)

2641 tagged events with Dt measured (78% purity; 66% tagging e)

sin2b = 0.741  0.067  0.034 || = 0.948  0.051  0.030

effective tagging eff: e=(28.1  0.7)%

Brian Meadows, U. Cincinnati

Golden modes with a lepton tag

The best of the best!

Ntagged = 220

Purity = 98%

Mis-tag fraction 3.3%

sDt 20% better than other tag categories


sin2b = 0.79  0.11

Brian Meadows, U. Cincinnati

sin2b measurement history

  • “Osaka 2000” measurement

  • (hep-ex/0008048)

    • Only J/y Ks and y(2s) Ks.

  • 1st Paper (PRL 86 2515, 2001)

    • Added J/y KL.

    • Simultaneous sin2b and mixing fit.

  • 2nd Paper (PRL 87 201803, 2001)

    • Added J/y K*0 and c Ks.

    • Better vertex reconstruction.

    • Better SVT alignment and higher Ks efficiency for new data.

  • Winter 2002 (hep-ex/0203007)

    • Improved event selection.

    • Reprocessed 1st 20 fb-1.

  • e) Current measurement (hep- ex/0207042, PRL)

    • Improved flavor tagging.

    • One more CP mode: hcKs.

(compiled by Owen Long)






Brian Meadows, U. Cincinnati

Decrease in statistical uncertainty
Decrease in Statistical Uncertainty

  • Curves represent 1/sLdt.

  • Improvements in statistical uncertainty due to

  • adding new B decay modes,

  • improved vertex reconstruction,

  • improved SVT alignment,

  • improved tagging performance.

Brian Meadows, U. Cincinnati

hep-ex/0208025, sub to PRD RC


78 fb-1 (85 M BB) 2958 events (81% purity)

effective tagging efficiency: e=(28.8  0.6)%

sin2b = 0.719  0.074  0.035|| = 0.950  0.049  0.025

Brian Meadows, U. Cincinnati

Constraints on upper vertex of unitarity triangle from all measurements except sin2 b
Constraints on upper vertex of Unitarity Triangle from all measurements EXCEPT sin2b


Regions of >5% CL

A. Höcker, H. Lacker, S. Laplace, F. Le Diberder, Eur. Phys. Jour. C21 (2001) 225, [hep-ph/0104062]

Brian Meadows, U. Cincinnati

World average 2003 sin2 b 0 78 0 08
World Average (2003) measurements EXCEPT sin2sin2b = 0.78  0.08

The Standard Model (and the CKM paradigm, in particular) wins again … at least at the current level of experimental precision, in this decay mode.

Brian Meadows, U. Cincinnati

World average 2010 sin2 b 0 67 0 02
World Average (2010) measurements EXCEPT sin2sin2b = 0.67  0.02

Brian Meadows, U. Cincinnati

B 0 k s

s measurements EXCEPT sin2






Other studies of sin2















  • Pure penguin !

  • time-dependent asymmetry in B0Ks measures sin2.

  • direct charge asymmetry in B+K+ sensitive to new physics.

Brian Meadows, U. Cincinnati

B measurements EXCEPT sin20Ks Old Result (2002)


51 signal events

hep-ex/0207070 (ICHEP2002)


- 0.50

sin2b = -0.19  0.90

  • c.f. world average: sin2 = 0.67 ± 0.02

  • >2 difference.

  • (over) stimulating theoretical interest

Brian Meadows, U. Cincinnati

B measurements EXCEPT sin20fKs Most Recent Result (2005)

 SM is FINE !!

~120 signal events



- 0.04

sin2b = +0.50  0.25

  • c.f. world average: sin2 = 0.67 ± 0.02

  • <1 difference.

Brian Meadows, U. Cincinnati

Motivations for super flavour factories
Motivations for Super Flavour Factories measurements EXCEPT sin2

A new generation of e+e- machines with L~100 x B factories will supplement efforts of LHC:

Measure effects of New Physics (NP) on the decays of heavy quarks (b and c) and of the  lepton

Allow us to look at the pattern of flavour and helicity dependencies of the couplings of NP particles as they are found at LHC

CP asymmetries and rates in rare decays will need to be measured. The consensus is that a sample with integrated luminosity ~50-100 ab-1 will be required

Maximum synergy with the findings of LHC will require collection of this sample in the next decade, requiring luminosity ~1036 cm-2s-1

If the LHC fails to find NP (!!) then Super Flavour factories might help to set the factor by which the scale was missed.

Determination of CKM parameters and New Physics measurements EXCEPT sin2


SuperB+Lattice improvements

r = xxx ± 0.0028

h = yyy ± 0.0024

r = 0.163 ± 0.028

h = 0.344± 0.016

Improving CKM is

crucial to look for NP