November 2001CP Violation at Belle Observation of CP Violation at Belle HEP Seminar Brookhaven National Lab Daniel Marlow Princeton University November

November 2001 CP Violation at Belle

Observation of CP Violation at Belle

HEP Seminar

Brookhaven National Lab

Daniel Marlow

Princeton University

November 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:

))(

))(

fBfB

fBfBACP

fBfB and

The CP asymmetry is defined as

The decay amplitudes are

and SWSW iiff

iiff eeAAeeAA

Note that the weak phase changes sign.


Direct CP Violation

Note that

Yielding

22

ffff AA

221121

SWSW iiii

f eeAeeAA

221121

SWSW iiii

f eeAeeAA

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

)cos( 2 21

2

2

2

1 SWf AAAA

)cos( 2 21

2

2

2

1 SWf AAAA


Direct CP Violation

Despite 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

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

tm

fBfB

fBfBtA

M DWhere and are the weak phases for the mixing and decay diagrams, respectively and

1

f

f ffCP


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* AVV ubtd

1)1(

21

)(21

23

22

32

AiA

A

iA

VVV

VVV

VVV

M

tbtstd

cbcscd

ubusud


CP Phases in the Gold-Plated Mode

0DSKJB /0

)arg( tdM V

Decay

Mixing

The KM phase comes from the mixing.


The Unitarity Triangle

3A

Vtd

,

0,1

3A

Vub

3

*

A

VV tsus

1

2

3

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

1

1 tbud VV


Constraints


Existing Constraints

Höcker et al. hepex/0104062

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

sin2 results are average prior to July 2001. They are not included in the fit.


The Measurement

0B

0B

tag

CP

z

0DNeed to:

•Measure momenta

•ID leptons & K’s

•Measure vertices

e e

0K

/J


B0B0

SKJB /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(

)()(

)()()(

tm

fBfB

fBfBtACP


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 rings

e+ (LER) : 3.5 GeV

e (HER) : 8.0 GeV

•ECM : 10.58 GeV at (4S)

•Luminosity

•target: 1034 cm-2s-1

•achieved:5.2x1033cm-2s-1

•±11 mrad crossing angle •Small beam sizes:

y 3 m; x 100 m

asymmetric e+e collider


Beam Crossing Schemes

By 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

KEKB PEP-II

Peak

Shift 92.8 pb-1 103.2 pb-1

24 hour 277 pb-1 291 pb-1

7 day 1661 pb-11865 pb-1

Integrated 37.6 fb-1 59.6 fb-1

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

~55m for 1GeV/c @ 90o

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

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

, e with CsI crystals E/E ~ 1.5% @ 1GeV

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


)(GeV/ 2cM

2eff MeV/ 6.4 c

0SK

Mass Reconstruction

Belle


CsI EM Calorimeter Performance

MeV 50E

tionReconstruc 0

tionReconstruc


Vertex Detector

~55m 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 =100 ps, which provides /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)

0DD K


PID Bake Off

BaBar Belle

Looks like BaBar does better.


Muon Identification

side- tagplus

/

K

KJB

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

SKJB /0


Analysis Flowchart


CP Mode Sample


J/ 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 ;/ SS KKJB


’ Modes

Both leptons tagged.


Other Charmonium Modes

M(ll) M(ll)

1st observationof inclusiveB c2 X

c2

c1


CP Even Mode (BJ/KL) Reconstruction

KL hits

KL

LKJB /0• Assume (2-body) kinematics.

• Look for KL recoiling from J/

• hits in RPCs

• cluster in ECL

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

• Plot

LKJB /0

**/

*

LKJB ppp


0LK Detection

LKP

missP

hi

iS PPP,

4miss

Inclusive “KL’s”


BJ/KL Reconstruction

**/

*

LKJB ppp

61% signal purity


CP Mode Summary


Flavor Tagging

We need to know the flavor of the spectator B0.

• Track level tags

• High momentum leptons

• Medium momentum K

• High-momentum (from e.g., )

• Low-momentum (from D*’s).

• Need to take into account multiple tags and correlations.

(*)0 DB


Flavor Tagging

yreliabilit tag 10 ;1 rq


Flavor Tagging

rq

Data

Monte Carlo

*0 DB


Flavor TaggingTwo measures of tagging performance:

• = efficiency

• w = wrong-tag fraction

Amplitude of mixing oscillation depends on w

)21( w

01.027.0eff


Vertexing

0B

0B

tag

CP

z

0D

e e

0K

/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/l+l- tracks– Reject poorly fit events.

• For Tag-side, use tracks with:– |z|<1.8mm, |z|<500 m, |r|<500 m– Iterate: discard worst track until fit is acceptable.

• Require |zCP - ztag|<2 mm (10B)


Test of Vertex Resolution0*0 / KJB


Fit to Lifetime

*DB

(ps) t

ps 03.064.1

ps 02.055.10

B

B


Fitting

)()'( sigsig tRttPLi )1( bgf

bgbgbg )()'( ftRttP

)sin(2sin)21(12

)( 1

/

sig tmwqe

tP fB

t B

Signal

)()1(2

)(/

bkg tfe

ftPbg

t bg

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

PRL 87, 091802 (2001)


Sources of Systematic ErrorVertex algorithm 0.04

Flavor tagging 0.03

Resolution function 0.02

KL background fraction 0.02

Background shapes 0.01

md and B0 errors 0.01

Total 0.06


Time-Dependent Asymmetry

One 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


Subsample Dependence


Comparison to other sin21 Measurements

It looks like CDF had it right!


Comparison to Other CKM Results

BaBar


Future Prospects

Oide

2.0)2(sin 2


Conclusions

• CP violation in the B system has been observed at the 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


Additional Slides


Two Pseudoscalar Modes


What if ||1 ?

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

14.099.02sin 1 The CPV asymmetry is unchanged.

09.003.1

In general we have

Note: if ||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%) which

are both CP=-1 and 5%(ψK, χc1Ks, ψπ0) which

are CP=+1.

• pB* background shape is used to find bkg level.


(*)KB613.0 40.0

0.14- 32.0 10)99.0(

)(

KBB

CL 90% 101.3

)(6

*

KBB

CL 90% 103.1

)(6

eeKBB

CL 90% 106.5

)(6

*

eeKBB


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

Documents

November 2001CP Violation at Belle Observation of CP Violation at Belle HEP Seminar Brookhaven National Lab Daniel Marlow Princeton University November