Using more data, more decay modes, and an improved analysis BaBar has just released a new result on the measurement of CP violation in B mesons that is 4.1 standard deviations from zero:
sin2beta = 0.59 +- 0.14 +- 0.05 (July 2001).
If the true value was zero, the probability of observing the measured value above is 3 x 10^-5. The new result is consistent with the Standard Model prediction, which is in the range 0.6-0.8.
As you know we "blind" the value (but not the error) on our sin2beta measurement until we complete a stringent analysis and cross check of the data. Immediately prior to publishing we unblind the result (i.e. we make all decisions on how to do the fit and which decay modes to include before we observe the actual value). Last Friday (June 29) we unblinded the data for the first time since our March 2001 publication.
So how does this compare to our previous measurement?
| Value | March 2001 | July 2001 |
|---|---|---|
| B-Bbar pairs | 23 x 10^6 | 32 x 10^6 |
| sin2beta Total | 0.34 +- 0.20 +- 0.05 | 0.59 +- 0.14 +- 0.05 |
| sin2beta Total | 0.32 +- 0.18* | |
| sin2beta CP- only | 0.25 +- 0.22 | 0.56 +- 0.15 |
| sin2beta CP+ only (Psi KL) | 0.87 +- 0.51 | 0.70 +- 0.34 |
The March publication was based on 1999-2000 data only. Obviously the data collected in 2001 has a larger asymmetry. A separate analysis of the 2001-only data is consistent with the 1999-2000 data at the 2 sigma level.
So what about the rest of the world?
| sin2beta +- (stat+sys) | Experiment |
|---|---|
| 0.59 +- 0.15 | BaBar July 2001 |
| 0.58 +- 0.35 | Belle March 2001 |
| 0.88 +- 0.38 | CDF+OPAL+ALEPH |
| 0.72 +- 0.26 | Belle+CDF+OPAL+ALEPH |
| 0.62 +- 0.13 | BaBar+Belle+CDF+OPAL+ALEPH |
The BaBar result dominates the current world average, and all by itself establishes CP violation beyond 3 sigma for the first time.
At the moment, BaBar has received about 20% more data than Belle, but the Japanese have only published on 1/3 of their current data set. The new BaBar result will undoubtedly be motivational.
The current BaBar run will continue until June 30 2002, and we expect to double the data set by that time.
As it was for the previous BaBar measurement, LLNL postdocs David Lange and Vuko Brigljevic played a key role in producing the new BaBar results (for all channels!). Congratulations to everyone.
As you might suspect, the recent Newsline article about our BaBar effort was written some time ago. If we had anticipated this dramatic improvement in our result, we would have delayed the article!
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