# Presentation: 573667

## B0B0 Oscillations and sin2at BABAR

**David Nathan Brown**
**Lawrence Berkeley National Laboratory**
**Representing the B****A****B****A****R Collaboration**

## Outline

**B****0****B****0**** ****mixing and CP in the Standard Model**
**B****A****B****A****R and PEPII**
**Event sample**
**Analysis procedure**
**Signal channel selection**
**Flavor tagging**
**Time measurement resolution modeling**
**Fitting**
**Results**
******m****B****0**
**sin2******
**Systematic error estimation**
**Conclusions**

## Quark mixing in the Standard Model

- Unitarity of the mixing matrix can be shown graphically as a triangle.

- Quark electroweak doublets are composed of mass eigenstate mixtures given by a mixing matrix.

- Non-zero triangle area (,, or  != 0) implies CP violation
- A ‘triangle’ which doesn’t close implies non-SM physics

## B0 B0 Mixing

- mB0 is sensitive to |VtdVtb*|

## CP Violation via Mixing Interference

- Interference between mixed and unmixed B0 decays to CP eigenstates induces a time and flavor-dependent rate

- J/ K0S

- B0

- B0

- Flavor-specific rate vs time depends on sin2

## CP and Mixing at the (4S) 

****(4S) ****B0 B0 proceeds via coherent P-wave.  Tagging the flavor of one B at decay determines the flavor of the other at that instant.

*Tagging* side

*Reco* side

- By boosting the ****(4S) the decay time difference becomes observable.

## PEPII at SLAC

- 3.1 GeV e+

- 9 GeV e-

****(4S)**

## The BaBar Detector

- Silicon Vertex
- Tracker (SVT)

- CsI Calorimeter (EMC)

- Instrumented Flux Return (IFR)

- Cherenkov Detector (DIRC)

- Drift Chamber
- (DCH)

- Superconducting Coil (1.5 Tesla)

## The BaBar Data Sample

- 23 Million B0 B0 pairs recorded

## A Fully-Reconstructed (4S)B0B0 Event

## Event Reconstruction

**‘****Stable’ particle reconstruction**
**Tracking**
- dE/dx and DIRC PID
- Vertexing (ks, 
********** ****and ************ in EMC and IFR**
**Neutrals in EMC and IFR**
- (4S) **BB** **selection**
**Event shape cuts (FW, thrust)**

## J/Ks and Ks Reconstruction

- 363 Events above background

## J/KL Reconstruction

**K****L ****are found in EMC and IFR**
**Cluster with no associated track**
**Inconsistent with ********0**** or ******
**B****0 ****mass used to reconstruct candidates (no cut)**

- 182 Events above background

- EMC clusters

- IFR clusters

## Hadronic (non-CP) B0 Reconstruction

**B****0** ******D****(*********)****-**** ****(********,********,a****1****)****+**
**D****-**** ********K****-********+********+****,K****0********-**
**D****0**** ********K****-********+****, K****-********+ ********0****, K****-********+********-********+****, K****0********-********+**
**D*****- ********D****0********-**
**B****0** ******J/********K*****0**
**K*****0 ********K****-********+**

- 4637 Events above background (CP fit)
- 6368 Events above background (mB0 fit)

## Flavor Tagging

**Tags are assigned hierarchically**
**Leptons from semi-leptonic decay ( b ******** c W****-****, W****-**** ********e********e****)**
**Kaons From Cabibo-favored hadronic cascade ( b ********c ********s )**
**Neural net (1 and 2)**
- Low-momentum leptons, soft pions, additional kaons
**Efficiency and mistag rates are determined from B****0 ****decays**
**Parameters in sin2********, ********m****B****0**** Likelihood fits**
**Cross-checked with B****0 ********D*****l******

**************efficiency**
**w = mistag rate**
**Q = ********(1-2w)****2**

## t Resolution

******t ≈ ********z/********c**
******z from vertex difference**
**Tag B****0 ****z resolution ~180 ********m**
**Signal B****0 ****z resolution ~70 ********m**
**Resolution modeled with 3 Gaussians**
**Core,Tail, and Outliers**
****** ****scaled to vertex error estimate (core and tail)**
**Shifts from 0 allowed (charm vertex flight)**
**Fit Parameters in sin2********, ********m****B****0**** Likelihood fits **

## B0 B0 Mixing Result (Preliminary)

- mB0 = 0.519 ± 0.020 ± 0.016 ps-1

**Fit for ********m****B****0**** + 33 parameters for tagging effects and ********z resolution for signals and backgrounds**

## Comparison with Other Results

- This Result

## CP Rates by Mode and Tag

## sin2 Extraction

- sin(2) = 0.34 ± 0.20 ± 0.05

- CP = -1

- CP = 1

- sin2 = 0.24 ± 0.22 ± 0.05

- sin2 = 0.86 ± 0.51 ± 0.14

**Fit for sin2******** + 33 parameters for tagging effects and ********z resolution for signals and backgrounds**

## Systematic Errors and Cross-Checks

**No significant differences are seen dividing sample by tagging mode, signal mode, decay mode, ...**
**No significant direct CP violation**
**No CP asymmetry in fits to B****±**** samples**

## Comparison with Other Results

- New World
- Average

- This Result

## CKM Constraints

- Vub ≈ A3(-i) (Wolfenstein parameterization)

## Conclusions

**BaBar has measured ********m****B****0**** ****and sin2******** in ~21 fb****-1**** of recorded ********(4S) decays**
**Most precise single measurement of sin2******
**Consistent with other measurements and world average**
**World average sin2******** is now ****3 ******** away from 0**
**BaBar is running again since February 1**
**0.6 fb****-1 ****recorded so far**
**Peak luminosity ~1X10****33****/cm****2****s**
**We expect an additional ~30 fb****-1****of data by August**

## Mixing and sin2measurements are done with the same strategy: do a global fit to all the events that can carry information:

-        Mixing   :  tagged flavour eigenstates
-        sin2      :  tagged flavour and CP eigenstates
- We float in the fit as many parameters as possible

- Only in CP fit

- Only in mixing

- Biggest correlation with sin2 7.6%

## J/KL composition

- Inclusive J/ MC

- J/sidebands

- J/KL composition