# Presentation: 702648

## CP Violation in the B Meson System: The Belle Measurement of sin21

*Eric Prebys, Princeton University*
- for the
**BELLE Collaboration**

- SUNY Stony Brook, April 2, 2001

- 4/2/2001

## The BELLE Collaboration

- 300 people from 49 Institutions in 11 Countries:
- Australia, China, India, Korea, Japan, Philippines, Poland, Russia, Taiwan, Ukraine,  and USA

- SUNY Stony Brook, April 2, 2001

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## Parity Violation

- The “parity” operation transforms the universe into its mirror image (*goes from right-handed to left-handed*).
- Maxwell’s equations are totally parity invariant.
- BUT, in the 50’s huge parity violation was observed in weak decays...

-  decay of polarized **Co:**

- electron preferentially emitted opposite spin direction

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## CP (almost) Conservation

- It was found that by applying the **C**[harge Conjugation]  operation to all particles, the overall symmetry seemed to be restored (neutrinos are left-handed, anti-neutrinos are right-handed).
- This symmetry fit nicely into the current algebras, and later the gauge theories being used to describe weak interactions.
- Unfortunately, it wasn’t *quite* exact...

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## CP Violation

- In 1964, Fitch, Cronin, *etal*, showed that physics is not *quite* invariant under the CP operation, essentially by proving that neutral kaons formed mass eigenstates

- where

- This generated great interest (not to mention a Nobel Prize), and has been studied in great detail ever since, but to date has only been conclusively observed in the kaon system.

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## Quark Mixing

- Leptons can only transition *within* a generation

- Although the rate is *suppressed*, quarks can transition *between* generations.

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## The CKM Matrix

- The weak quark eigenstates are related to the strong (or mass) eigenstates through a unitary transformation.

- Cabibbo-Kobayashi-Maskawa (CKM) Matrix

- The only straightforward way to *accommodate** *CP violation in the SM is by means of an irreducible phase in this matrix (requires at least three generations, led to prediction of *t* and *b* quarks)

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## Wolfenstein Parameterization 

- The CKM matrix is an SU(3) transformation, which has four free parameters.  Because of the scale of the elements, this is often represented with the “Wolfenstein Parameterization”

- CP Violating phase

- First two generations *almost* unitary.

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## “The” Unitarity Triangle

- Unitarity imposes several constraints on the matrix, but one...

- Results in a triangle in the complex plane with sides of similar length                , which appears the most interesting for study

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## The Plane

- Remembering the Wolfenstein Parameterization

- we can divide through by the magnitude of the base....

- CP violation is generally discussed in terms of this plane

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##  Direct CP Violation

- CP Violation is manifests itself as a difference between the physics of matter and anti-matter

*Direct* CP Violation is the observation of a difference between two such decay rates; however, the amplitude for one process can in general be written

- Weak phase changes sign

- Strong phase does not

- Since the observed rate is only proportional to the _amplitude_, a difference would only be observed if there were an *interference* between two diagrams with different weak *and* strong phase*.*

*** *Rare and hard to interpret

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## Indirect CP Violation

- Consider the case of B-mixing

- Mixing phase

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## Indirect CP Violation (cont’d)

- If both                  can decay to the same *CP eigenstate** f,* there will be an *interference*

- And a time-dependent asymmetry

- Decay phase

- CP state of* f*

- Mixing phase

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## The Basic Idea

- We can create          pairs at the           resonance.
- Even though both *B’*s are mixing, if we tag the decay of one of them, the other must be the CP conjugate *at that time**.* We therefore measure the time dependent decay of one B relative to the time that the first one was tagged (EPR “paradox”).
- PROBLEM:  At the          resonance, B’s only go about 30 m in the center of mass, making it difficult to measure time-dependent mixing.

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## The Clever Trick

- If the collider is *asymmetric*, then the entire system is Lorentz boosted.
- In the Belle Experiment, 8 GeV e-’s are collided with 3.5 GeV e+’s  so

- 

- So now the time measurement becomes a *z* position measurement.

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## “Gold-Plated” Decay

- Total state CP

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## Predicted Signature

*t* = Time of tagged decays

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## “Tin-Plated” Decay

- Complicated by “penguin pollution”, but still promising

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## What about 3?

- Corresponding decay would be Bs KS,, but...
- Require move to (5s) resonance (messier)
- Time dependent Bs mixing not possible.
-  Have to find another way.

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## Make LOTS of       pairs at the (4S) resonance in an asymmetric collider.
Detect the decay of one B to a CP eigenstate.
Tag the flavor of the other B.
Reconstruct the position of the two vertices.
Measure the z separation between them and calculate proper time separation as
Fit to the functional form

Write papers.

- Detect the decay of one *B* to a CP eigenstate.
- Tag the flavor of the other *B.*
- Reconstruct the position of the two vertices.
- Measure the *z* separation between them and calculate proper time separation as
- Fit to the functional form
- Write papers.

- Review - What B-Factories Do...

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## Are Two B-Factories Too Many?

- These are not discovery machines!
- Any interesting physics would manifest itself as small deviations from SM predictions.
- People would be very skeptical about such claims without independent confirmation.
- Therefore, the answer is NO (two is not *one* too many, anyway).

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## Motivations for Accelerator Parameters

- Must be asymmetric to take advantage of Lorentz boost.
- The decays of interest all have branching ratios on the order of 10-5 or lower.
- Need lots and lots of data!
- Physics projections assume 100 fb-1 = 1yr @ 1034 cm-2s-1
- Would have been pointless if less than 1033 cm-2s-1

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## The KEKB Accelerator

- Asymmetric Rings
- 8.0GeV(HER)
- 3.5GeV(LER)
- Ecm=10.58GeV= M((4S))
- Target Luminosity: 1034s-1cm-2
- Circumference: 3016m
- Crossing angle: 11mr
- RF Buckets: 5120
-  2ns crossing time

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## Motivation for Detector Parameters

- Vertex Measurement
- Need to measure decay vertices to <50m to get proper time distribution.
- Tracking...
- Would like p/p.5% to help distinguish B decays from BK  and BKK decays.
- Provide dE/dx for particle ID.
- EM calorimetry
- Detect ’s from slow, asymmetric ’s   need efficiency down to 20 MeV.
- Hadronic Calorimetry
- Tag muons.
- Tag direction of KL’s from decay BKL .
- Particle ID
- Tag strangeness to distinguish B decays from Bbar decays (low p).
- Tag ’s to distinguish B decays from BK  and BKK decays (high p).

*Rely on mature, robust technologies whenever possible!!!*

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## Particle ID needs

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## The Detector

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## All Finished!!

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## June 1, 1999: Our First Hadronic Event!!

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## Luminosity

- Daily integrated luminosity

- Total integrated luminosity

- Total for first CP Results (Osaka):

- Total for *these* Results:

- Our Records:
- Instantaneous:
- Per (0-24h) day:
- Per (24 hr) day:
- Per week:
- To date:

- (on peak)

- Note: integrated numbers are accumulated!

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## The Pieces of the Analysis

- Event reconstruction and selection
- Flavor Tagging
- Vertex reconstruction
- CP fitting

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## J/ and KS Reconstruction

- J/ and KS Reconstruction

- Mev
- Require mass
-  within 4of PDG

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## BKS Reconstruction

*B****K**S** *Reconstruction

- In the CM, both *energy* and *momentum* of a real B0 are constrained.
- Use “Beam-constrained Mass”:

- Signal

*123 Events*
*3.7 Background*

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## All Fully Reconstructed Modes (i.e. all but L)

| Mode | Events | Background |
| --- | --- | --- |
| BS | 123.0 | 3.7 |
| All Others | 71.0 | 7.3 |
| Total | 194.0 | 10.0 |

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## BKL Reconstruction

- Measure direction (only) of *K**L** *in lab frame
- Scale momentum so that M(*K**L**+***)=M(*B**0*)
- Transform to CM frame and look at *p(B**0**)*.

- KLM Cluster

*J/**** *daughter particles

*K**L*

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## BKL Signal

*0<p**B*****<2 *GeV/c

- Biases spectrum!

*131 Events*
*54 Background*

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## Complete Charmonium Sample

**Total **                                                   325               65

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## Flavor Tagging           

- X

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## Flavor Tagging (Slow Pion)

- Very *slow* pion

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## Event by Event Tagging Quality

- If we tag events wrongly, we’ll measure CP violation as

- So the measurement is *diluted* by a factor

- Ideally, we can determine this on an event by event basis to be used in the CP fit
- Example, for high*-p* lepton

- p*

- right

- wrong

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## Multi-dimensional Flavor Tagging

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## Comparison Between MC and Data

- +   Data
- --- MC

- Events

- Diluted B-Mixing

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## Tagging Efficiency

- Tagging efficiency = 99.4% (vs. 99.3% in MC)
- Effective efficiency eff = (1-2w)2  = **27.0%** (vs. 27.4% in MC)

- Experimentally determined w values in each *r* region

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## Vertex Reconstruction

- Common requirements in vertexing
- # of associated SVD hits > 2 for each track
- IP constraint in vertex reconstruction
*CP* side vertex reconstruction
- Event is rejected if reduced 2 > 100.
- Tag side vertex reconstruction
- Track parameters measured from *CP* vertex must satisfy:
- |*z*|<1.8mm, |*z*|<500m, |*r*|<500m
- Iteration until reduced 2 < 20 while discarding worst track.
- |*z**CP* - *z*tag|<2mm (****10***B*)

- Overall efficiency = ~87%.    In total 282 events for the CP fit.

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## CP Fit (Probability Density Function)

*f**BG* = background fraction.  Determined from a 2D fit of *E* vs *M.*
*R(**** t) = *resolution function.  Determined from *D*****’s* and MC.
*PDF**BG**(**** t) = *probability density function of background. Determined from  sideband (210 events).

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## Resolution Function

- Fit with a double-Gaussian...

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## Test of Vertexing – B Lifetime

- pdg2000

- [ps]

**Lifetime (ps)**

**Mode**

- Combined

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## The Combined Fit (All Charmonium States)

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## Individual Subsamples 

**Fit  ****(stat. err.)**

**Mode**

- CP = -1

- CP = +1

- Non-CP

- All CP

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## Consistency Check

- Plot asymmetry in individual time bins...

- ACP

- Fix at PDG value

- Our fit

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## Sources of Systematic Error

- Bottom Line

- Published in **Phys.Rev.Lett. 86, 2509 (2001)**

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## “Measurement of B0d - B0d-bar Mixing Rate from the Time Evolution of Dilepton Events at Upsilon(4S)” (to appear in PRL)
"A Measurement of the Branching Fraction for the Inclusive B->Xs gamma Decays with Belle“ (submitted to PLB)
"Measurement of Inclusive Production of Neutral Pions from Upsilon(4S) Decays” (submitted to PRL)

- "A Measurement of the Branching Fraction for the Inclusive B->Xs gamma Decays with Belle“ (*submitted to PLB)*
- "Measurement of Inclusive Production of Neutral Pions from Upsilon(4S) Decays” (*submitted to PRL)*

- Other Recent Publications

- + Several More in the Pipeline!!

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## Belle is working very well!!
Our current value of sin2based on 10.5 fb-1 of data is

This is consistent with the BaBar value of 

and with other previous results (CDF, LEP)

The probability of observing this value if CP is conserved is 4.9%
The next few years should be very exciting!

- Our current value of *sin2*******based on 10.5 fb-1 of data is
- This is consistent with the BaBar value of
- and with other previous results (CDF, LEP)
- The probability of observing this value if CP is conserved is 4.9%
- The next few years should be very exciting!

- Summary and Outlook

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