# Presentation: 119257

## Bd mixing and    prospects for Bs mixing at D

-    Tulika Bose
- (for the D Collaboration)
*Columbia University*
- DPF 2004

-  Motivation for mixing studies
-  D @ Tevatron
-  Bd mixing measurements
- 	Different approaches
-  Prospects for Bs mixing

## Neutral  meson transition from particle to anti-particle,                                                and vice-versa
Caused by higher order flavor changing weak interactions:

                                                          mq= m(Boheavy) - m(Bolight)
                                                           mq  |VtbVtq|2

Mixing parameters xq=  mq/q and yq=   q/  q where q=d,s 
First oscillations observed at ARGUS (’87) in the Bd system
  signaled a large top quark mass (later verified by CDF and DØ)

- Caused by higher order flavor changing weak interactions:
-                                                           mq= m(Boheavy) - m(Bolight)
-                                                            mq  |VtbVtq|2
- Mixing parameters xq=  mq/q and yq=   q/  q where q=d,s
- First oscillations observed at ARGUS (’87) in the Bd system
-   signaled a large top quark mass (later verified by CDF and DØ)

- What is mixing?

## md  and  Vtd

- md  and  Vtd

*Phys. Rev. D (2004)*

- Large uncertainty

-  Precise measurement of Vtd is important:                  	  properly constrain the CKM matrix              yield info on CP-violating phase

- md has been precisely measured: the world average is

- Large uncertainty

-  consider ratio

-  Measure ms    constrain Vtd

*from Lattice QCD*

*Theoretical error on the ratio expected to drop faster *

- Why do we care about Bs mixing ?

- We *think* we understand the mixing of *B*’s.
- A deviation from this simple diagram
- may be a deviation from SM  (New Physics).

- K  mixing   direct & indirect CPV
- Bd mixing   heavy top mass
-   mixing   neutrino mass  0
- Bs mixing   ????

## The Tevatron B-factory

- Tracking: Solenoid, Silicon,
- Fiber Tracker, Preshowers

- Large production cross-section
- All B species, including Bs, Bc, b

**@1.96 TeV**

**@ ********(4S)**

**@ Z****0**

- Rich B Physics program at DØ
-  benefits from :
-  Large muon acceptance: || < 2
-  Forward tracking coverage:
-    || < 1.7 (tracking), ||  < 3 (Si)
-  Robust muon trigger

## Essential Ingredients

- A typical oscillation analysis involves:
- Proper time reconstruction for each meson candidate
- Selection of final states suitable for the study
- Tagging of the meson flavor at decay time (final state)
- Tagging of the meson flavor at production time (initial state)

- final state particles  provide tag

## Initial state tagging

- Soft lepton tagging (SLT) :
-     Opposite side:  b-
-           Same side tagging   (SST) :
- 		         Same side:
-           Jet charge tagging  (JetQ) :
- 	               Opposite side:

- b-hadron

- Trigger lepton

- Fragmentation
- pion

- B

- PV

- D

- neutrino

- Same side

- Lxy

- Soft lepton

- Jet charge

- Opposite side

## Semileptonic samples

- Reconstruct semileptonic decay  B  μD0X
- Select D0 candidates ( D0  K-+ )
- Search for a pion track which in combination with D0 gives D* invariant mass ( D*+→ D0π+ )
- Divide the μD0X sample into 2 sub-samples:

- No D* was found: D0 sample

- D* was found: D* sample

## Proper time reconstruction

- Bd proper lifetime determined using

- Visible proper decay length
- (VPDL)

- Correction factor
- (for missing )

- K – factors from Monte Carlo

## Soft Lepton Tagging (SLT)

- If Q opposite * Q > 0  B hadron oscillated
- If Q opposite * Q < 0  Not oscillated

- How often the tagging algorithm ‘fires’

- How often the tagging algorithm gives the correct answer

- Maximize tagging power: D2

## Measured Asymmetry

- Obtain # of D* , D0 events tagged as “non-oscillated” &  “oscillated”
- for different VPDL bins:

**D* sample**

**D****0**** sample**

- Expect to see oscillations
- Level is offset by B+ contribution

- Expect to see no oscillations
- Some variation from oscillations due to B0 contribution in
-      sample composition

## Expected Asymmetry 

- Calculate expected value of asymmetry:

- B meson lifetimes and branching rates from PDG
- K-factor distributions, decay length resolution,
- reconstruction efficiencies from MC

**D* sample**

**D****0**** sample**

## md  (SLT)

**                     **
-    md=0.5060.055(stat)0.049(syst) ps-1

- Tagging efficiency:  4.8  0.2 %
-                 Dilution :  46.0  4.2 %

- 250pb-1

- Chief systematics:
-  Fitting procedure for D* candidates
-  VPDL resolution function
-  branching rates of B mesons
-  k-factor variations

- Preliminary results:

- Soft Lepton tagging

## Same Side Tagging (SST)

- B+: Correct tag Qtag·Qμ<0
-        Non-oscillated

- B0: Correct tag Qtag·Qμ>0
-           Non-oscillated

- Charge of fragmentation pion correlates with the B flavor

- tagging track

- Different algorithms for selecting tagging track (in a R < 0.7 cone around B):
- lowest Ptrel (transverse momentum  relative to B) track in a R < 0.7 cone around B
- track with minimum R wrt B-meson

## D** complications

- charged pion from D** may be taken as a tag
-        always gives “correct tag” for both B0 and B+
-  irrespective of oscillations
- evaluated from D** topological analysis

## md  (SST)

**                     **
-        md=0.4880.066(stat)0.044(syst) ps-1

- Preliminary results:

- 250pb-1

- Simultaneous 2 fit to asymmetries in D* and D0 samples:

- Chief systematics:
-  D** pion tagging probability
-  branching rates of B mesons
-  k-factor variations
-  D* and D0 fitting procedure
-  VPDL resolution function

- D0 : Dilution for D* sample
- D : Dilution for D0 sample

- D* sample

- D0 sample

- D0=0.116 0.014(stat)0.016(syst)
- D=0.244 0.016(stat)0.024(syst)

## Combined tags analysis

- Data sample split into two sets:
-  Tagged by soft muons (SLT)
-  Tagged by combined jetQ+SST algorithm
- Combined algorithm produces non-zero answer if:
- Event not tagged by SLT
- At least one of jetQ and SST gives a non-zero answer
- jetQ and SST give same answer  ( better dilution)

- 200pb-1

- SST: track with
- min. R wrt B-meson

## Combined tagger result

- Simultaneous fit to SLT and jetQ+SST asymmetries

- SLT

- jetQ+SST

- md=0.456  0.034 (stat) 0.025 (syst) ps-1
-                  D0 = (44.8  5.1) %  SLT
-                  D0 = (14.9  1.5) %   jetQ+SST
- 	    D= (27.9  1.2) %   jetQ+SST
- 	     = (5.0  0.2) %   SLT
- 	     = (68.3  0.9) % jetQ+SST

- Preliminary results:

- Chief systematics:
-  D*  sample composition
-  D** pion tagging probability
-  Charged B dilution determination

- 200pb-1

## In search of BS oscillations

- (“Amplitude method”)

- Fit data to

- Fit for A as a function of ms

- Measurement: A = 1
- Sensitivity:  1.645A = 1 (95%)
- Limit: A < 1 - 1.645A  (95%)

- Current limit :
- ms > 14.4 ps-1  @95% CL

## Sensitivity 

- Current limits :
- Bsoscillates at least 30 times faster than B0 !
-  A measurement of ms is experimentally very challenging

- Statistical Significance :

- For large m, proper time resolution (t)
- becomes v. imp.

- Flavor tagging

- Signal purity

- Initial-state tagging algorithms
- being verified and optimized using
- md measurements

- Bs  Ds  X decays being reconstructed in different modes. Hadronic modes are being studied too

## BS  DS  X

- Largest semileptonic yield in the world !!

- DS 
- BR= (3.60.9)%

- ~ 9481 events in 250pb-1

-  Large signal yield
-  Cuts being optimized for S/S+B
-  Flavor tagging being tested with md

## BS  DS  X

- BS  DS  X

- Other DS  decays are being studied too

- BR= (3.30.9)%
- (BR comparable to Ds )
- But larger backgrounds
-       D-  K+ - -
-       		D-  K* -
-       		non-resonant D-  K+ - -
- ~ 4933  events in 200 pb-1
- Significant increase in total BS yield

## Summary

- We have preliminary measurements of md  using different tagging techniques (250pb-1)
- md=0.5060.055(stat)0.049(syst) ps-1  (SLT)
- md=0.4880.066(stat)0.044(syst) ps-1  (SST)
- We have started to combine different taggers (200pb-1)
- md=0.4560.034(stat)0.025(syst) ps-1  (SLT+jetQ+SST)
- We are optimizing our taggers for Bs mixing studies
- We have the largest Bs  Ds  X  yields in the world
- Prospects for Bs mixing look good
*                        **Stay tuned...*