# Presentation: 804616

## Observation of CP Violation at Belle

- HEP Seminar
- Brookhaven National Lab
- Daniel Marlow
- Princeton University
- November 8, 2001

- CP Violation at Belle

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## Talk Outline

- Introduction/Review (with apologies to the experts)
- The KEK-B Collider
- The Belle Detector
- Indirect CP Violation Measurement/Analysis
- Conclusions

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## Physics Motivation

*Direct* CP violation is perhaps the simplest case.  Consider the CP mirror processes:

- The CP asymmetry is defined as

- The decay amplitudes are

- Note that the weak phase changes sign.

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## Direct CP Violation

- Note that

- Yielding

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

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## 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.

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## Indirect CP Violation

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

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## Indirect CP Violation

- Thus for a decay                       where    *    *is a CP eigenstate, we have two “indistinguishable” decay paths

- Working through the algebra, yields a time-dependent CP asymmetry

- Where         and           are the weak phases for the mixing and decay diagrams, respectively and

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## The Kobayashi-Maskawa Mixing Scheme

- Where the second matrix is the Wolfenstein parameterization.

- Quark mixing is described via

- The “d b” unitarity relation yields

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## CP Phases in the Gold-Plated Mode

- Decay

- Mixing

- The KM phase comes from the mixing.

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## The Unitarity Triangle

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

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## Constraints

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## 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.

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## The Measurement

- tag

- CP

- Need to:
- Measure momenta
- ID leptons & K’s
- Measure vertices

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## The Measurement

- B0

- B0

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## 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.

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## KEK B Asymmetric Collider

**Two separate rings**
**e****+**** (LER) : 3.5 GeV**
**e******** (HER) : 8.0 GeV**
**E****CM**** : 10.58 GeV at ********(4S)**
**Luminosity **
**target:     ****10****34**** cm****-2****s****-1**** **
**achieved:****5.2****x****10****33****cm****-2****s****-1**
**±****11 mrad crossing angle **
**Small beam sizes:**
******y**** ********3 ********m;  ********x**** ******** 100 ********m**

**asymmetric e****+****e******** ****collider**

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## 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 to
- achieve 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.

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## Machine Performance

**Reported here**

**Summer ’01 shutdown**

- Integrated/day

- Total Integrated

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## Typical~good day.

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## Another Typical Day

- This is probably a bit *worse* than usual.

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## 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-1 | 1865 pb-1
 |
| Integrated | 37.6 fb-1 |  59.6 fb-1 |

- As of ~November 5, 2001

- Comparisons not quite apples to apples.

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## 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.

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## Slide 23

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## The BELLE Collaboration

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## Detector Performance

- Silicon Vertex Detector
- ~55m 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

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##  Mass Reconstruction

-  Mass Reconstruction

- Belle

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## CsI EM Calorimeter Performance

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## Vertex Detector

- ~55m 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

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## 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.

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## Belle: Aerogel

- Barrel Module

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

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## PID Bake Off

- BaBar

- Belle

- Looks like BaBar does better.

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## Muon Identification

- 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.

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## Putting it all together

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## Analysis Flowchart

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## CP Mode Sample

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## 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.

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## B Reconstruction

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## Other Modes

**M****bc**

- Total events = 76
- Bkgd  9 evts (12%)

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## ’ Modes

- ’ Modes

- Both leptons tagged.

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## Other Charmonium Modes

**M(*****l**********l*************) ******** M(*****l**********l*********)**** **

**1st observation**
**of inclusive**
**B*********** *********c2***** *****X**

******c2**

******c1**

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##  CP Even Mode (BJ/KL) Reconstruction

-  CP Even Mode (BJ/KL) Reconstruction

*K**L** *hits

*K**L*

-  *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

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## Detection

- Inclusive “KL’s”

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## BJ/KL Reconstruction

- 61% signal purity

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## CP Mode Summary

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## 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.

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## Flavor Tagging

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## Flavor Tagging

- Data

- Monte Carlo

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## Flavor Tagging

- Two measures of tagging performance:
-   = efficiency
-  *w* = wrong-tag fraction

- Amplitude of mixing oscillation depends on *w*

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## Vertexing

- Reality

- Cartoon

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

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## 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 |*z**CP* - *z*tag|<2 mm (10***B*)

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## Test of Vertex Resolution

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## Fit to Lifetime

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## Fitting

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## Fitting

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## The Data

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

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## The Fitted Result

- PRL 87, 091802 (2001)

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## Sources of Systematic Error

| Vertex 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 |

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## 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.

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## Subsample Dependence

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## Subsample Dependence

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## Comparison to other sin21 Measurements

- It looks like CDF had it right!

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## Comparison to Other CKM Results

- BaBar

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## Future Prospects

- Oide

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## 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.

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## Additional Slides

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## Two Pseudoscalar Modes

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## What if ||1 ? 

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

- The CPV asymmetry is unchanged.

- In general we have

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

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## 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.

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## Slide 70

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

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