Observation of CP Violation at Belle - Princeton …marlow/talks/bnl/bnl2.pdfNovember 2001 CP...

November 2001 CP Violation at Belle

Observation of CP Violation at Belle

HEP SeminarBrookhaven National Lab

Daniel Marlow Princeton UniversityNovember 8, 2001


Talk Outline

• Introduction/Review (with apologies to the experts)

• The KEK-B Collider• The Belle Detector• Indirect CP Violation Measurement/Analysis• Conclusions


Physics MotivationDirect CP violation is perhaps the simplest case. Consider the CP mirror processes:

))())(

fBfBfBfBACP →(Γ+→Γ

→(Γ−→Γ≡

fBfB →→ and The CP asymmetry is defined as

The decay amplitudes are

and SWSW iiff

iiff eeAAeeAA φφφφ −==

Note that the weak phase changes sign.


Direct CP ViolationNote that

Yielding

22

ffff AA Γ=Γ⇒=

221121

SWSW iiiif eeAeeAA φφφφ

+=

221121

SWSW iiii

f eeAeeAA φφφφ −−+=

We need some sort of interference, two amplitudes, for example

)cos( 2 212

22

1 SWf AAAA φφ ∆+∆++=Γ

)cos( 2 212

22

1 SWf AAAA φφ ∆−∆++=Γ


Direct CP ViolationDespite its conceptual and experimental simplicity, there are two problems with direct CP violation:

• Cases where there are two comparable amplitudes that are large are (probably) rare.

• The strong phases are poorly understood, making it difficult to extract the weak (KM) phases that are of greatest interest.

One is thus led in the direction of asymmetric e+e- B factories aimed at the study of indirect CP violation.


Indirect CP Violation

( ) ( )[ ]022

02

(0 sincos)( BeiBetB mimtmtimi φ−∆∆)/2Γ−− +×=

In this approach we provide the interference using state mixing, i.e.,

00BB


Indirect CP ViolationThus for a decay where is a CP eigenstate, we have two “indistinguishable” decay paths

fB →0

f0B0B f

Working through the algebra, yields a time-dependent CP asymmetry )(2sin)sin(

)()()()()( 00

00

DMf

CP

tmfBfBfBfBtA

φφη +∆∆−=→Γ+→Γ→Γ−→Γ=∆

Mφ DφWhere and are the weak phases for the mixing and decay diagrams, respectively and 1 ±=

=

f

f ffCPη

η


The Kobayashi-Maskawa Mixing Scheme

Where the second matrix is the Wolfenstein parameterization.

0* =++ ∗∗tbubtsustdud VVVVVV

Quark mixing is described via

The “d b” unitarity relation yields 3* λAVV ubtd ≈+

−−−−−

−−≅

=

1)1(21

)(21

23

22

32

ληρλλλλ

ηρλλλ

AiAA

iA

VVVVVVVVV

M

tbtstd

cbcscd

ubusud


CP Phases in the Gold-Plated Mode

0≈DφSKJB /0 Ψ→

)arg( tdM V≈φ

Decay

Mixing

The KM phase comes from the mixing.


The Unitarity Triangle

η

ρ

3λAVtd

( )ηρ,

( )0,1

3λAVub

∗

3

*

λAVV tsus

1φ

2φ

3φ

The gold-plated mode determines the angle which is also called .

1φ

β

1≈≈ tbud VV


Constraints


Existing Constraints

Höcker et al. hepex/0104062

Eur.Phys.J. C21 (2001) 225-259

sin2β results are average prior to July 2001. They are notincluded in the fit.


The Measurement

0B

+µ−µ

0B

+µtag

CP

z∆

0DNeed to:

•Measure momenta

•ID leptons & K’s

•Measure vertices

−e +e

0K

Ψ/J


B0 B0

SKJB Ψ→ /0

The MeasurementThe time-dependent asymmetry appears mainly as a mean shift in the distribution between events tagged

as decays and

events tagged as

decays.

z∆

0B0B

1

00

00

2sin)sin( )()()()()(

ϕtmfBfBfBfBtACP

∆∆−=→Γ+→Γ→Γ−→Γ=∆


The KEKB Asymmetric e+e- Collider

KEKB is similar to PEP-II in many ways, although there is an important difference in the way in which the beams are brought into collision.

−+ee


KEK B Asymmetric Collider

•Two separate ringse+ (LER) : 3.5 GeVe−−−− (HER) : 8.0 GeV

•ECM : 10.58 GeV at ΥΥΥΥ(4S)•Luminosity

•target: 1034 cm-2s-1

•achieved:5.2x1033cm-2s-1

•±11 mrad crossing angle •Small beam sizes:

σσσσy ≈≈≈≈3 µµµµm; σσσσx ≈≈≈≈ 100 µµµµm

asymmetric e+e−−−− collider


Beam Crossing SchemesBy colliding the beams at an angle, the KEK-B design achieves a simplified interaction region.

Moreover, every RF bucket (2 ns spacing) can be filled toachieve maximum luminosity. However, this approach risks destabilizing couplings between transverse and longitudinal modes of the machines.

A crab-crossing cavity is under development in case this proves to be a problem.


Machine Performance

Reported here

Summer ’01 shutdown

Integrated/day

Total Integrated

November 2001 CP Violation at Belle Typical~good day.


Another Typical Day

This is probably a bit worse than usual.


KEKB/PEP II Luminosity Bakeoff

1865 pb-11661 pb-17 day

59.6 fb-137.6 fb-1Integrated

291 pb-1277 pb-124 hour

103.2 pb-192.8 pb-1Shift

Peak

PEP-IIKEKB

1233 scm 102.5 −−× 1233 scm 103.4 −−×

As of ~November 5, 2001 Comparisons not quite apples to apples.


KEKB Future Prospects• Chief limitation recently has been the photo-electron instability in the LER.

• This has been reduced, although not fully mitigated by installing a solenoidal winding around much of the beam pipe (last ~500 m this summer).

• Past history has shown that the situation slowly improves, in a manner consistent with a reduction in quantum efficiency of the beam-pipe wall brought that is reminiscent of “scrubbing.”

• If that is the case, we can expect additional modest gains in luminosity, but it is hard to know.


An international collaboration involving about 10 Countries, 50 Institutes, & 250 People

The BELLE Collaboration


Detector Performance! Silicon Vertex Detector

! σ ~ 55µm for 1GeV/c @ 90o

! Central Drift Chamber ! σp/p ~ 0.35% @ 1GeV/c! σπ (dE/dx) ~ 7%

! K± id up to plab=3.5 GeV/c! TOF (σ ∼ 95 ps)! Aerogel (n = 1.01 ~ 1.03)

! γ, e± with CsI crystals! σE/Eγ ~ 1.5% @ 1GeV

! KL and µ± with KLM (RPCs)! µ± : effic. > 90% ; ~2% fakes


)(GeV/ 2cM −+ππ

2eff MeV/ 6.4 c=σ

−+→ ππ0SK

Mass Reconstruction

Belle


CsI EM Calorimeter Performance

MeV 50>γE

tionReconstruc 0π

tionReconstruc γγη →


Vertex Detector

σ ~ 55µm for 1GeV/c @ 90o

Occupancy at present luminosity ~4%

Upgraded SVD coming:

• 4 layers

• Radiation hard

• Level 1 trigger

• Summer ’02 installation Excl. mode B lifetime


Belle: ToF & Aerogel

The ToF has a resolution of σ=100 ps, which provides π/K separation up to about 1.2 GeV/c, at which point the ACC array kicks in. The index of refraction of the counters varies as a function of angle, reflecting the boosted CM.


Belle: Aerogel

Barrel Module


Particle ID (dE/dx, ToF, & Aerogel)

++∗ → π0DD+−→ πK


PID Bake Off

BaBar Belle

Looks like BaBar does better.


Muon Identification

µµµ

side- tagplus

/+−+

++

→

Ψ→

KKJB

Muons are important for the same reason. They are identified by the way in which they penetrate the iron return yoke of the magnet.

The gaps are instrumented with RPCs.


Putting it all together

( )( )−+−+→Ψ→ ππµµSKJB /0


Analysis Flowchart


CP Mode Sample


J/Ψ Reconstruction

• Require one lepton to be positively identified and the other to be consistent with lepton hypothesis.

• For e+e-, add any photon within .05 of electron direction.


B Reconstruction

*beam

*cand EEE −≡∆

( )2

cand

2*beam

−= ∑ pEmbc

!


Other Modes

Mbc

Total events = 76Bkgd ≈ 9 evts (12%)

000 ;/ ππ→Ψ→ SS KKJB


Ψ’ Modes

Both leptons tagged.


Other Charmonium Modes

M(l++++l−−−−γγγγ) −−−− M(l++++l−−−−)

1st observationof inclusiveB→→→→ χχχχc2 X

χχχχc2

χχχχc1


CP Even Mode (B→J/ΨKL) Reconstruction

KL hits

KL

LKJB Ψ→ /0• Assume(2-body) kinematics.

• Look for KL recoiling from J/ψ

• hits in RPCs

• cluster in ECL

• Remove positively tagged background modes: J/ψK+ , J/ψK* , etc.

• Plot

LKJB Ψ→ /0

**/

*LKJB ppp += Ψ


0LK Detection

Θ

LKP

missP

∑± =

−=hi

iS PPP,

4missγ

Inclusive “KL’s”


B→J/ΨKL Reconstruction

**/

*LKJB ppp += Ψ

61% signal purity


CP Mode Summary


Flavor TaggingWe need to know the flavor of the spectator B0.

• Track level tags

• High momentum leptons

• Medium momentum K±

• High-momentum π± (from e.g., )

• Low-momentum π± (from D*’s).

• Need to take into account multiple tags and correlations.

+−→ π(*)0 DB


Flavor Tagging

yreliabilit tag 10 ;1 ⇔<<±= rq


Flavor Tagging

rq ⋅

Data

Monte Carlo

ν±→ "#*0 DB


Flavor TaggingTwo measures of tagging performance:

• ε = efficiency

• w = wrong-tag fraction

Amplitude of mixing oscillation depends on w

)21( w−01.027.0eff ±=ε


Vertexing

0B

+µ−µ

0B

+µtag

CP

z∆

0D−e +e

0K

Ψ/J

Reality

Cartoon

The B lifetime is of the same order as the vertex resolution, so the effect is quite subtle.


Vertexing

• Common track requirements– # of associated SVD hits > 2– Use run-dependent IP

• For CP-side, use J/ψ"l+l- tracks– Reject poorly fit events.

• For Tag-side, use tracks with:– |δz|<1.8mm, |σz|<500 µm, |δr|<500 µm– Iterate: discard worst track until fit is acceptable.

• Require |zCP - ztag|<2 mm (≈10τB)


Test of Vertex Resolution0*0 / KJB Ψ→


Fit to Lifetime

ν"*DB →

(ps) t∆

ps 03.064.1

ps 02.055.10

±=

±=

−B

B

ττ


Fitting

)()'( sigsig tRttPLi ∆⊗∆−∆= )1( bgf−×

bgbgbg )()'( ftRttP ×∆⊗∆−∆+

[ ])sin(2sin)21(12

)( 1

/

sig tmwqetP fB

t B

∆∆−−=∆∆−

ϕητ

τSignal

)()1(2

)(/

bkg tfeftPbg

t bg

∆−+=∆∆−

δτ τ

τ

τ Background

Response function


Fitting


The Data

The first order effect is a mean shift between positively and negatively tagged samples.


The Fitted Result

06.014.099.02sin 1 ±±=ϕ

PRL 87, 091802 (2001)


Sources of Systematic Error

±±±±0.06Total

±±±±0.01∆∆∆∆md and ττττB0 errors

±±±±0.01Background shapes

±±±±0.02KL background fraction

±±±±0.02Resolution function

±±±±0.03Flavor tagging

±±±±0.04Vertex algorithm


Time-Dependent AsymmetryOne can plot the excess/deficit on a bin-by-bin basis.

First we start with a non-CP sample to look for false asymmetries.


Subsample Dependence


Comparison to other sin2ϕ1Measurements

It looks like CDF had it right!


Comparison to Other CKM Results

BaBar


Future Prospects

Oide

2.0)2(sin 2 ≈φδ


Conclusions• CP violation in the B system has been observed at the >6σ level.

• Our data are consistent with the SM.

•The KEKB accelerator is working very well and continues to improve.

• There will be lots of good physics to come.

06.014.099.02sin 1 ±±=ϕ


Additional Slides


Two Pseudoscalar Modes


What if |λ|≠1 ?

If |λ| is allowed to float, (i.e. a cos(Δm t) term)

14.099.02sin 1 ±=φThe CPV asymmetry is unchanged.

09.003.1 ±=λ

In general we have

Note: if |λ|≠1, then the gold-plated mode isn’t really gold plated.


KL Details

•Two classes: KLM+ECL and ECL only(397 and 172 entries, respectively)Fitted yield is 346 events.• Background composition: 71% non-CP modes.The remainder is ψKLπ0(13%), ψKs(10%) whichare both CP=-1 and 5%(ψK, χc1Ks, ψπ0) whichare CP=+1.• pB

* background shape is used to find bkg level.


−+→ ""(*)KB613.0 40.0

0.14- 32.0 10)99.0()(

−++−

−+

×=→ µµKBB

CL 90% 101.3)(

6

*

−

−+

×<→ µµKBB

CL 90% 103.1)(

6−

−+

×<→ eeKBB

CL 90% 106.5)(

6

*

−

−+

×<→ eeKBB


Particle ID (dE/dx, ToF, & Aerogel)

Observation of CP Violation at Belle - Princeton …marlow/talks/bnl/bnl2.pdfNovember 2001 CP...

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