BABAR Group

Faculty: Masahiro Morii
Graduate student: Corry Lee

We are no longer accepting new students.

Ex-postdocs: Stephen Bailey, Eunil Won, Jinwei Wu
Ex-student: Kris Chaisanguanthum (thesis)

The BABAR Experiment

The BABAR Experiment operated between 1999 and 2008 at the PEP-II Electron Positron Storage Ring at Stanford Linear Accelerator Center (SLAC). PEP-II collided electrons and positrons at the center-of-mass energy of 10.58 GeV, so-called the Υ(4S) resonance, to produce a large number of B mesons, and their decays are observed by the BABAR Detector. The accelerator provided very high luminosity (1.2×10³⁴ cm^-2s^-1), with the electron and positron beams having different energies (9 GeV for electrons, 3.1 GeV for positrons) in the laboratory frame. The final-state B mesons are therefore boosted toward the direction of the electron beam momentum. This “asymmetric” collision scheme makes it possible to measure the proper decay time of the B mesons through their flight lengths, thereby allowing BABAR to detect time-dependent CP asymmetry in various decays of B mesons. Such measurements have provided strong constraints on the imaginary phase of the Cabbibo-Kobayashi-Maskawa matrix, the origin of the CP violation in the Standard Model, for which we had very limited experimental data.

During the 9-year operation, the BABAR Experiment accumulated an integrated luminosity of 531 fb^-1, of which 433 fb^-1 was recorded on the Υ(4S) resonance. Measurements with improved accuracies and in a wider variety of decay modes will follow in the next few years.

Physics Analyses

Members of the Harvard BABAR group have made numerous measurements of the properties of the B meson decays.

• Morii and Chaisanguantum worked on the determination of the CKM matrix element |V_ub| through the rate of the B → πlν and similar decays [1].
• Bailey and Lee measured branching fractions of the "penguin-dominated" rare B decays such as B → η′K^* [2].
• Wu worked on measurements of CP violation in the B⁰ → f₀K_S [3] and B⁰ → (ρπ)⁰ [4] decays.

Level-1 Trigger Upgrade

Several upgrade projects took place during the lifetime of the BABAR Experiment, enabling the detector to take data at increasingly luminosities delivered by PEP-II. The Harvard BABAR group played a leading role in the Level-1 Trigger upgrade project.

Prototype ZPD module

Production ZPD system in the BABAR Electronics House, with Harvard postdocs Bailey (left) and Won.

The Z-P_T Discriminator (ZPD) modules we have developed at Harvard are described in [5].

References

1. B. Aubert et al., Phys. Rev. Lett. 97, 211801 (2006), arXiv:hep-ex/0607089.
   B. Aubert et al., Phys. Rev. Lett. 101, 081801 (2008), arXiv:0805.2408.
   
2. B. Aubert et al., Phys. Rev. D 70, 032006 (2004), arXiv:hep-ex/0403025.
   
3. B. Aubert et al., Phys. Rev. Lett. 94, 041802 (2005), arXiv:hep-ex/0406040.
   
4. B. Aubert et al., Phys. Rev. D 76, 012004 (2007), arXiv:hep-ex/0703008.
   
5. S.J. Bailey et al., IEEE Trans. Nucl. Sci. 51 (2004) 2352 (SPIRES);
   S. Bailey et al., Nucl. Instrum. Meth. A 518 (2004) 544 (SPIRES).

Masahiro Morii, November 17, 2009