# Presentation: 661906

## Observation of CP violation in the B0 meson system

***David MacFarlane***
***University of California at San Diego***
***(representing the BABAR Collaboration)***
***Snowmass, July 12th 2001***

- See SLAC-PUB-8904, hep-ex/0107013,
- submitted to PRL July 5

## The BABAR Collaboration

**The *****B******A******B******AR***** Collaboration**
**9 Countries**
**72 Institutions**
**554 Physicists **

**USA   [****35/276****]**
- California Institute of Technology
- UC, Irvine
- UC, Los Angeles
- UC, San Diego
- UC, Santa Barbara
- UC, Santa Cruz
- U of Cincinnati
- U of Colorado
- Colorado State
- Florida A&M
- U of Iowa
- Iowa State U
- LBNL
- LLNL
- U of Louisville
- U of Maryland
- U of Massachusetts, Amherst
- MIT
- U of Mississippi
- Mount Holyoke College
- Northern Kentucky U
- U of Notre Dame
- ORNL/Y-12
- U of Oregon
- U of Pennsylvania
- Prairie View A&M
- Princeton
- SLAC
- U of South Carolina
- Stanford U
- U of Tennessee
- U of Texas at Dallas
- Vanderbilt
- U of Wisconsin
- Yale

**Italy****	****[****12/89****]**
- INFN and U Bari
- INFN and U Ferrara
- Lab. Nazionali di Frascati dell' INFN
- INFN and U Genova
- INFN and U  Milano
- INFN and U Napoli
- INFN and U Padova
- INFN and U Pavia
- INFN, SNS and U Pisa
- INFN, Roma and U "La Sapienza"
- INFN and U Torino
- INFN and U Trieste
**Norway****	****[****1/3****]**
- U of Bergen
**Russia****	****[****1/13****]**
- Budker Institute, Novosibirsk
**United Kingdom   [****10/80****]**
- U of Birmingham
- U of Bristol
- Brunel University
- U of Edinburgh
- U of Liverpool
- Imperial College
- Queen Mary & Westfield College
- Royal Holloway, University of London
- U of Manchester
- Rutherford Appleton Laboratory

**Canada****	****[****4/16****]**
- U of British Columbia
- McGill U
- U de Montréal
- U of Victoria
**China****	****[****1/6****]**
- Inst. of High Energy Physics, Beijing
**France****	****[****5/50****]**
- LAPP, Annecy
- LAL Orsay
- LPNHE des Universités Paris 6/7
- Ecole Polytechnique
- CEA, DAPNIA, CE-Saclay
**Germany****	****[****3/21****]**
- U Rostock
- Ruhr U Bochum
- Technische U Dresden

- D.MacFarlane for the BABAR Collaboration

- July 12, 2001

## BABAR Detector

**DIRC ********PID)**
- 144 quartz bars
- 11000 PMs

**1.5T**** solenoid**

**EMC**
- 6580 CsI(Tl) crystals

**D****rift ****C****hamber**
- 40 stereo layers

**I****nstrumented ****F****lux ****R****eturn**
- iron / RPCs  (muon / neutral hadrons)

**S****ilicon ****V****ertex ****T****racker**
- 5 layers, double sided strips

- e+ (3.1GeV)

- e- (9GeV)

***BABAR Detector***

- SVT:         B vertex  z resolution ~70 microns
- Tracking:  (p**T**)/p**T** = 0.13% **** p**T**  ****  0.45%
- DIRC:       K- separation > 3.4   for P < 3.5GeV/c
- EMC:        **E**/E = 1.33%E**-1/4** **** 2.1%

- D.MacFarlane for the BABAR Collaboration

- July 12, 2001

## ARGUS, 1987

***Mixed full event and dilepton studies demonstrate mixing***
***Integrated luminosity 1983-87:***
- 103 pb-1

***Seeds sown at Snowmass 1988 for asymmetric-energy B Factories***

- D.MacFarlane for the BABAR Collaboration

- July 12, 2001

## B Factories, 2001

_**KEK-B / **__***BELLE***_

_**PEP-II / **__***B***__***A***__***B***__***AR***_

_**CESR / CLEO**_

**This analysis**

***Key factor in today’s results: High luminosity and Factory running at PEP-II and KEK-B***

- D.MacFarlane for the BABAR Collaboration

- July 12, 2001

## Origin of CP Violation

_**Time-dependent CP Observable:**_

- CP violation results from interference between        decays with and without mixing

- mixing

- decay

- CP eigenvalue

- amplitude ratio

- cosine term

- sine term

***Origin of CP Violation***

- D.MacFarlane for the BABAR Collaboration

- July 12, 2001

## Golden Channel: B0 J/K0S/L

- Single weak phase =  no direct CPV

- K0  mixing is required

***Golden Channel: B******0****** ************J/************K******0******S/L***

- D.MacFarlane for the BABAR Collaboration

- July 12, 2001

## Unitarity Triangle

***Present experimental knowledge:***
- (with a range of theor. inputs)

- Bd mixing  md
- b ul  Vub , D*l   Vcb
- Bs mixing  ms / md
- Kaon decays  |K|

*Following H**ö**cker et al, hep-ex/0104062*

- D.MacFarlane for the BABAR Collaboration

- July 12, 2001

## Experimental Technique

** ****B-Flavor Tagging**

**Exclusive****B Meson Reconstruction**

***Experimental Technique***

- D.MacFarlane for the BABAR Collaboration

- July 12, 2001

## Effects of CP Asymmetry

***Perfect Experiment, sin2************ = 0.59***

**Looking for a different ********t spectrum in events where the tag is a B****0**** or a B****0**

- Note: integrated asymmetry is 0

***Visible ***
***asymmetry***
***A******CP***

- D.MacFarlane for the BABAR Collaboration

- July 12, 2001

## CP Analysis: Time Distributions

**realistic **
- mis-tagging probability &
- finite time resolution

**perfect**
- flavor tagging  &
- time resolution

**Mixing & CP**
**Time evolution**

- D.MacFarlane for the BABAR Collaboration

- July 12, 2001

## Recent Developments

***Our first publication was in February 2001***
- Sin2 = 0.34 0.20 (stat)  0.05 (syst)
***Belle also reported a measurement at the same time***
- Sin2 = 0.58 (stat)      (syst)
***What has changed since then?***
*BABAR* has added more data: 23 million BB pairs becomes 32 million now
- Improved reconstruction efficiency
- Improved J/KL selection
- New modes added: c1KS, J/K*0
- Improved resolution of vertex reconstruction for both tag and reconstructed B

- Analysis was re-blinded

**PRL 86 (2001) 2515**

**PRL 86 (2001) 2509**

- D.MacFarlane for the BABAR Collaboration

- July 12, 2001

## Improved Reconstruction

**Run2/Run1**

**J/********K****s****(********+********-****)**

******S********K****s****(********+********-****)**

******c****K****s****(********+********-****)**

**J/********K****s****(********0********0****)**

**J/********K*****0****(********+********-****)**

**J/********K****+**

**J/********K* non CP**

******S********K****+**

******c********K****+**

**K****S**** Golden modes ~30% larger than run 1: efficiency improved**** **

- D.MacFarlane for the BABAR Collaboration

- July 12, 2001

## Additional Channels: C1KS

***Major improvements in ************c1******K******S****** analysis:***
- M(J/)-M(J/) cut tightened / 0 veto (decrease fake photons)
- Cut on E tightened (decrease inclusive J/ background)
- Relaxed continuum requirements (not dominant background)
***Peaking background reduced by factor ~4 (to 3%)***
- With only a 10% efficiency reduction

*********c2******K******s ******shown to be absent***

- D.MacFarlane for the BABAR Collaboration

- July 12, 2001

## Additional Channels: J/K*0(KS0) 

***Improved understanding of the background and its effective CP***

- (Angular analysis paper about to be submitted)

**55 signal events before tagging; 37 after**

- D.MacFarlane for the BABAR Collaboration

- July 12, 2001

## Improved KL Selection

***Improved K******L****** Selection***

***Original analysis was optimized for S******2******/(S+B)***
- Fine for BF measurements, but not for CP
***Need to optimize accounting for the background asymmetry (S+A******B******/A******S******B)******2******/(S+B)***
- Re-optimized with Monte Carlo
- Expect 10% improvement on the error

- D.MacFarlane for the BABAR Collaboration

- July 12, 2001

## Resulting KL Yields: Run1

***For data the improvement is better than expected***

- In the IFR selection the signal yield has not changed while the background is halved

- Run1

- Old

- New

- D.MacFarlane for the BABAR Collaboration

- July 12, 2001

## CP Sample: Non-KL Modes

**N****NOW****=672**

**Present Sample: 725**
**PRL Sample: 425**

- Before tagging and vertexing requirements

- D.MacFarlane for the BABAR Collaboration

- July 12, 2001

## CP Sample: J/ KL

- Run1+Run2

|  | EMC | IFR |
| --- | --- | --- |
| Run1 | 77/52 | 96/68 |
| Run2 | 49/59 | 32/55 |
| Run1+Run2 | 128/56 | 129/65 |

- D.MacFarlane for the BABAR Collaboration

- July 12, 2001

## Fully-Reconstructed B Sample

***Fully-Reconstructed B Sample***

- Cabibbo-favored hadronic decays
-                    “Open Charm” decays
- e.g.
- Charmonium Decays
- e.g.

**Neutral *****B***** Mesons**

**Charged *****B***** Mesons**

_**Flavor eigenstates**_** for mixing and lifetime measurements**

***Here determine mistag rates, ************t resolutions***

- D.MacFarlane for the BABAR Collaboration

- July 12, 2001

## Vertex Algorithm Improvements

***More elaborate tagging vertex algorithm***
***Increased efficiency through inclusion of 1-prong tag vertices (86% becomes 97%)***
***Improves average tagging vertex resolution from 200 to 180 ************m***

- D.MacFarlane for the BABAR Collaboration

- July 12, 2001

## Improved Vertex Performance

***We expect some vertex improvements in Run2 from:***
- Better use of layer 1 SVT hits in the Kalman fit
- Better SVT internal alignment

- NEW

- OLD

- Improvement in resolution leads to

- ... 3-4% on sin2 error

- run2

- run1

******(sin2********)**

- D.MacFarlane for the BABAR Collaboration

- July 12, 2001

## Tagging Algorithm: Unchanged

**Category ****	****  Definition        ****	****   efficiency     Q**_** **_
***Lepton***	Charge of fastest electron (muon) with
-                p*>1.0(1.1) GeV/c.			     10.9  0.4%      7.4  0.5%
***Kaon******	***Total charge of identified kaons	     35.8  0.5%    15.0  0.9%
***NT1*****	**Neural Network			           7.8  0.3%     2.5  0.4%
***NT2***	Neural Network,			         13.8  0.3%     1.2  0.3%
- 				**Total**	      68.4  0.7%   26.1  1.2%

- NT1, 2 recover information from:
- unidentified leptons, soft pions from D*, and the momentum spectrum of B decay products

**Tagging power: ****Q = ********(1-2w)****2****=26.1 ********1.2 %**

- D.MacFarlane for the BABAR Collaboration

- July 12, 2001

## Tagging Performance from Data 

***Tagging Performance from Data ***

***Obtained from “mixing fit” to data samples***

**L**

**K**

**NT1**

**NT2**

**Q = ********D****2****=**
**26.1 ********1.2 %**

**NT1**

**L**

**K**

**NT2**

- D.MacFarlane for the BABAR Collaboration

- July 12, 2001

## Likelihood Fit Method

***Global unbinned maximum likelihood fit to data:***
***Mistag rates, ************t resolutions = tagged flavour sample***
***sin2************ = tagged CP samples***
***45 parameters for mistag rates, ************t resolution & backgrounds: floated to obtain an empirical description from data***

- Largest correlation
- with sin2****** : **13%

- Separate t resolutions for run1 and run2

- B = 1.548 ps and md = 0.472 ps-1 fixed

- D.MacFarlane for the BABAR Collaboration

- July 12, 2001

## Raw Asymmetries

**sin2********=0.56±0.15**

**sin2********=0.59±0.20**

**Kaon tags**

- (Q=15%)

***Raw Asymmetries***

**All tags**

***Visible***
***A******CP***

- D.MacFarlane for the BABAR Collaboration

- July 12, 2001

## “Corrected” Asymmetries

***B******A******B******AR***

***B******A******B******AR***

***“******Corrected” Asymmetries***

**Comparable to similar Belle and CDF plots**

- D.MacFarlane for the BABAR Collaboration

- July 12, 2001

## CP=-1 Time Distributions

- -5

-  0

-  5

- t(ps)

***Visible ***
***asymmetry***
***A******CP***

**sin2********=0.56±0.15**

**Fit projections are from global fit**

- D.MacFarlane for the BABAR Collaboration

- July 12, 2001

## J/ KL Time Distributions

***Visible ***
***asymmetry***
***A******CP***

**sin2********=0.70±0.34**

**Fit projections are from global fit**

- D.MacFarlane for the BABAR Collaboration

- July 12, 2001

## Sin2 Fit Results

***Sin2************ Fit Results***

**Consistency of CP channels P****(********2****) = ****8%**

***sin2************ = 0.59 ******± 0.14***

**Cross-checks:**
**Null result in flavor samples**

**Goodness-of-fit: P(L****max****>L****obs****)>27%**

- D.MacFarlane for the BABAR Collaboration

- July 12, 2001

## Run1 – Run2 Comparison

**Run1**

**Run2**

**Run1-Run2 change for PRL modes: 1.8******

- D.MacFarlane for the BABAR Collaboration

- July 12, 2001

## Run1 – Run2 Comparison

***Run1 – Run2 Comparison***

***Change in central value ~1.8************ in uncorrelated error***
***30% efficiency improvement for all K******S****** modes***
***15% improvement due to vertexing/alignment***

- D.MacFarlane for the BABAR Collaboration

- July 12, 2001

## Systematic Errors

***Signal resolution and vertexing = 0.03***
- Resolution model, outliers, SVT residual misalignment
***Tagging = 0.03***
- Studies of possible differences between BCP and Bflavor samples
***Backgrounds = 0.02 (overall)***
- Signal probability, peaking background, CP content of background
- Total 0.093 for J/ KL channel; 0.11 for J/ K*0
***Total = 0.05 for full sample***

|  | KS | KL | K*0 | Full |
| --- | --- | --- | --- | --- |
| Total Sys | 0.049 | 0.104 | 0.162 | 0.049 |
| Total Stat | 0.151 | 0.340 | 1.01 | 0.137 |

- D.MacFarlane for the BABAR Collaboration

- July 12, 2001

## Search for Direct CP 

- Probing new physics: only use CP=-1 sample (contains no mixing background)

- || = 0.93 ± 0.09 (stat.) ± 0.03 (sys.)

***No evidence of direct CP violation due to decay amplitude interference; none expected***
- Coefficient of the “sine” term unchanged

- If more than one amplitude matters || might be different from 1

- (assuming  = 0)

- D.MacFarlane for the BABAR Collaboration

- July 12, 2001

## Updated World Average

- D.MacFarlane for the BABAR Collaboration

- July 12, 2001

## Interpretation of the Results

*Following H**ö**cker et al, hep-ex/0104062*
*(many other recent global CKM matrix analyses)*

- D.MacFarlane for the BABAR Collaboration

- July 12, 2001

## Summary

***We have established CP violation at 4************ level with sin2************ = 0.59 ± 0.14 ± 0.05***
- Probability is < 3 x 10-5 to observe an equal or larger value if no CP violation exists
- Corresponding probability for the CP = -1 modes is 2 x 10-4
- Probability of J/KL and J/KS having the same CP is < 0.1%
***The study of CP violation in the B system has just started ...***
- With more data becomes both a precision measurement and allows us to look at individual channels for consistency
- Start to look at sin2 and other approaches

- D.MacFarlane for the BABAR Collaboration

- July 12, 2001