# Presentation: 352597

## 33rd International Conference in High Energy Physics (Jul 26th   – Aug 2nd, Moscow, Russia) 

- 33rd ***International Conference in High Energy Physics ******(Jul 26******th******   – Aug 2******nd******, Moscow, Russia)****** ***

***Bs Mixing at D0***

*Tania Moulik   (Kansas University) *
- presented by
- Andrei Nomerotski (Fermilab/Oxford)* *

- Speaker, ICHEP 2006

## Mass eigenstates are a mixture of  flavor eigenstates:

BH and BL have a different mass and may have different decay width.
m = MH – ML = 2|M12| , 
 =  H -  L = 2|12|

- BH and BL have a different mass and may have different decay width.
- m = MH – ML = 2|M12| ,
-  =  H -  L = 2|12|

***B mixing***

- Time evolution follows the Schrodinger equation

- Dominant Diagram for the transition :

- Speaker, ICHEP 2006

##  In an Ideal Scenario..

- “Opposite sign”

- “Same sign”

- Oscillations with amplitude = 1.0 and
- Frequency = ms.

- Speaker, ICHEP 2006

## DZero Detector

**Spectrometer : Fiber and Silicon Trackers in 2 T Solenoid **
**Energy Flow : Fine segmentation liquid Ar Calorimeter and Preshower**
**Muons : 3 layer system & absorber in Toroidal field**
**Hermetic : Excellent coverage of Tracking, Calorimeter and Muon Systems**

- Speaker, ICHEP 2006

## Analysis outline

- Signal Selection
- Look for tracks displaced from primary vertex in same jet as /electron
-  Two tracks should form a vertex and be consistent with mass  (  K K) or K* mass (K*K  KK)
-  KK invariant mass should  be consistent with Ds mass

- Identify e/
-    PT (e/) > 2.0
-  | | (e/) < 1.0/2.0

- Speaker, ICHEP 2006

## Signal Selection

**Muons were selected by triggers without lifetime bias **
**= no online/offline Impact Parameter cuts**
**Trigger muon can be used as tag muon : gives access to eD****s**** sample with enhanced tagging purity**** **

- Speaker, ICHEP 2006

## Signal Selection

***Signal Selection***

**μ****+**

**π**** ****-**

- K+

- K-

**φ**

**D****-****S**

**μ****(e)**

**B**

**ν**

**X**

**PV**

- LT(DS)

**Ds lifetime is used to have non-zero selection efficiency at Interaction Point **
**Bs can decay at IP and be reconstructed**** **

**Eff=30%**

- Speaker, ICHEP 2006

## Effect of Neutrino

***Effect of Neutrino***

- Need to correct Decay Length for relativistic contraction  need to know Bs momentum
- Can estimate Bs momentum from MC (through so called k-factor) at expense of additional uncertainty
- k/k uncertainty causes additional smearing of oscillations
- Only few first periods are useful for semileptonic channels
- Sensitivity at DL=0 is crucial
- All above represents the main difference wrt hadronic channels

**200 micron**

**# of periods**

- Speaker, ICHEP 2006

## Flavor Tagging and dilution calibration

**Identify flavor of reconstructed B****S**** candidate using information from B decay in opposite hemisphere**.

- a) _*Lepton Tag*_ :  Use semileptonic b decay :
- Charge of electron/muon identifies b flavor

-  Ds

- cos  (l, Bs) < 0.8

- Bs

- 

- e / 

- 

- b) _*Secondary Vertex Tag*_ :
- Search for secondary vertex on opposite
- Side and loop over tracks assoc. to SV.

- c) _Event charge Tag:_
- All tracks opposide to rec. B

- Secondary Vertex

- Speaker, ICHEP 2006

## Dilution in Δmd measurement

**Combine all tagging variables using likelihood ratios**
**B****d**** oscillation measurement with combined tagger**
******m****d****= ****0.501********0.030±0.016ps****-1**** **

- Combined dilution: εD2=2.48±0.21±0.08 %** **

- Input for Bs measurement

- Speaker, ICHEP 2006

## Bs decay samples after flavor tagging

***Bs decay samples after flavor tagging***

- NBs( ) = 5601 102
- NBs( + e)  = 1012  62 (Muon tagged)
- NBs(K*K + ) = 2997  146

**Bs********Ds e ******** X**

**Ds **********

**Ds ******** K*K**

**Bs********Ds ******** X**

**Bs********Ds ******** X**

**Ds **********

- Speaker, ICHEP 2006

## K*K Fit Components

- Difficult mode due to K* natural width and mass resolution – larger errors wrt  mode

- Speaker, ICHEP 2006

## Results of the Lifetime Fit

- From a fit to signal and background region:

| Decay Mode | cBs (m) | cbkg (m) |
| --- | --- | --- |
| BsDs   X, Ds   | 4049 | 6276 |
| BsDs e  X, Ds   | 44429 | 64518 |
| BsDs  X, Ds  K*K | 40722 | 54910 |

**Bs********Ds e ******** X**

**Ds **********

**Ds ******** K*K**

**Bs********Ds ******** X**

- Speaker, ICHEP 2006

## Amplitude Method

*Amplitude fit = Fourier analysis + Maximum likelihood fit*              often used in oscillation measurements

- If *A=1, the *Δm’s is a measurement of Bs oscillation frequency, otherwise *A*=0

- Need to know dilution (from Δmd analysis)

- Speaker, ICHEP 2006

## Cross-check on BdXμD±()

***Cross-check on B******d************X******μ******D******±******(******************)***

**EXACTLY the same sample & tagger**
**Amplitude Scan shows B****d**** oscillations**
** ****at correct place ******** no lifetime bias**
** ****with correct amplitude ******** correct dilution calibration**
**Same results for two other modes**

- Amplitude Scan

**DØ Run II Preliminary**

- Speaker, ICHEP 2006

## Measure Resolution Using Data

**Ultimately ********m****s**** sensitivity is limited by decay length resolution –  ****very important issue**** **
**Use J/****ψ→****μμ**** sample**
** ****Fit pull distribution for J/****ψ**** Proper Decay Length with 2 Gaussians**
** ****Resolution Scale Factor is 1.0 for 72% of the events and 1.8 for the rest**
** ****Cross-checked by several other methods**

**DØ Run II Preliminary**

- Speaker, ICHEP 2006

## Amplitude Scan of BsXμDs()

***Amplitude Scan of B******s************X******μ******D******s******(******************)***

-  Deviation of the amplitude at 19 ps-1
-  2.5σ from 0  1% probability
-  1.6σ from 1  10% probability

- Speaker, ICHEP 2006

## Log Likelihood Scan

*********m******s****** <  *****21 ps****-1**** @ 90% CL assuming Gaussian errors**
**Most probable value of ***********m******s***** = 19 ps****-1**

- Systematic

-  Resolution
-  K-factor variation
-  BR (BsDsX)
-  VPDL model
-  BR (BsDsDs)

- In agreement with the amplitude scan

**Have no sensitivity above 22 ps****-1**

- Speaker, ICHEP 2006

## Interpretation

***Interpretation***

**Results of ensemble tests:**
**DZero result :**
**Combined with World (before CDF measurement):**

- Speaker, ICHEP 2006

## Impact on the Unitarity Triangle

***Impact on the Unitarity Triangle***

- BeforeBS mixing

- Speaker, ICHEP 2006

## Impact on the Unitarity Triangle

***Impact on the Unitarity Triangle***

- With D0

- Speaker, ICHEP 2006

## Impact on the Unitarity Triangle

***Impact on the Unitarity Triangle***

- With CDF

- Speaker, ICHEP 2006

## “Golden” Events for Visualization

***“******Golden” Events for Visualization***

** ****Weigh events using**

**# of periods**

- Speaker, ICHEP 2006

## Can We See Bs Oscillations By Eye ?

**Weighted asymmetry**
**This plot does not represent full statistical power of our data**

**# of periods**

- Speaker, ICHEP 2006

## More Amplitude Scans

- New results : Amplitude scans from two additional modes

**Ds ******** K*K**

**Bs********Ds (************e ******** X**

**Bs********Ds ******** X**

**Ds **********

- Speaker, ICHEP 2006

## Combination

- Amplitude is centred at 1 now, smaller errors
- Likelihood scan confirms 90% CL ms limits: 17-21 ps-1
- Data with randomized tagger : 8% probability to have a fluctuation (5% before for  mode)
- Detailed ensemble tests in progress

- Speaker, ICHEP 2006

## Add Same Side Tagging
Add hadronic modes triggering on tag muon
Add more data  (4-8 fb-1 in next 3 years) with improved detector – additional layer of silicon between beampipe and Silicon Tracker (Layer0) – better impact parameter resolution

**Add hadronic modes triggering on tag muon**
**Add more data  (4-8 fb****-1**** in next 3 years) with improved detector – additional layer of silicon between beampipe and Silicon Tracker (Layer0) – better impact parameter resolution**

**Layer0 has been successfully installed in April 2006**
** ****S/N = 18:1   &   no pickup noise   **
**  ****First 50 pb****-1 ****of data on tape, first tracks have been reconstructed**

***Outlook ***

- Speaker, ICHEP 2006

## Summary

- Established upper and lower limits on  ms using               Bs  Ds  X mode
- Analysis published in PRL 97 (2006) 021802
- Combined with two other channels
- Bs  Ds  X
- Bs  Ds e X
-    considerable improvement in sensitivity
-    14.1  16.5 ps-1, no improvement for ms interval
- Looking forward to a larger dataset with improved vertex detection
- If ms is indeed below 19 ps-1 expect a robust measurement with the extended dataset

- Speaker, ICHEP 2006

## BACKUP SLIDES

- Speaker, ICHEP 2006

## B Mesons

| Bu+ | B0 | Bs0 | Bc+ |
| --- | --- | --- | --- |

**Matter**

- b

- d

**Anti-Matter**

- Speaker, ICHEP 2006

**Notes:**
A reminder : There are 4 B mesons with their respective anti-particles with the quark contents
As shown here. 

## CKM matrix and B mixing

- Wolfenstein parametrisation - expansion in .

- complex

- Why are we interested to study B meson oscillations

- Speaker, ICHEP 2006

## B Mixing

- In general, probability for unmixed and mixed decays  Pu,m(B)  Pu,m(B). In limit, 12 << M12 ( << M) (Standard model estimate and confirmed by data), the two are equal.

**~ 10****-4**** for Bs system**
**~ 10****-3**** for Bd system **

- Speaker, ICHEP 2006

## Constraing the CKM Matrix from ms

***Constraing the CKM Matrix from ************m******s***

- 
- from Lattice QCD calculations)
-   Ratio suffers from lower theoretical
- Uncertainties – strong constraint Vtd

- And similar expression for
- ms

- 

- CDF+D0 (2006) ms inputs

- Speaker, ICHEP 2006

## Excellent Tevatron Performance

-  Data sample corresponding to over 1 fb-1 of the integrated luminosity used for the Bs mixing analysis
-  Full dataset is ready    (85-90% DAQ efficiency)

- Speaker, ICHEP 2006

## Muon Triggers

- Limitation of data recording. Triggers are needed to select useful physics decay modes. 396 ns bunch crossing rate ~ 2.5 MHz  ~50 Hz for data to be recorded.
- Single inclusive muon Trigger:
-  |η|<2.0, pT > 3,4,5 GeV
-  Muon + track match at Level 1
- Prescaled or turned off depending on inst. lumi.
-  We have B physics triggers at all lumi’s
*** ******Extra tracks at medium lumi’s ***
*** ******Impact parameter requirements***
*** ******Associated invariant mass***
*** ******Track selections at Level 3***
-  Dimuon Trigger : other muon for flavor
-      tagging
-  e.g. at 50·10-30 cm-2s-1, L3 trigger rate :
-  20 Hz  of unbiased single μ
-  1.5 Hz of IP+μ
-  2 Hz    of  di-μ
-  No rate problem at L1/L2

- Speaker, ICHEP 2006

## μ  Sample

***μ************  Sample***

- μDs: 26,710

**Opposite-side flavor**
**tagging**

- Tagging efficiency 21.90.7%

- μD±: 7,422

- μD±: 1,519

- μDs: 5,601±102

- Speaker, ICHEP 2006

## check Using BdXμD±()

***check Using B******d************X******μ******D******±******(******************)***

-  The Amplitude Scan shows Bd  oscillations at 0.5 ps-1
-  no lifetime bias
-  (A=1) : correct dilution calibration

- Speaker, ICHEP 2006

## Detector Effects

*flavor tagging power,*
*background*

*Decay length*
*resolution*

*momentum*
*resolution*

- p)/p = ? %
- l = ?

- SM prediction - ms ~ 20 ps-1
- Trying to measure :
- Tosc~0.3 X 10-12 s !

- 

- Speaker, ICHEP 2006

**Notes:**

SM Prediction Dms = 20 ps-1,   Tosc = 0.3 ps

## Sample Composition

-  Estimate using MC simulation, PDG Br’s, Evtgen exclusive Br’s

**Signal: 85.6%**

- Speaker, ICHEP 2006

## Flavor tag Dilution calibration

- Bd mixing measurement using Bd  D*  X, D* D0 , D0 , and evaluate dilution in various diution bins. Follows similar analysis outline as Bs mixing.
- Form _measured asymmetry_ in 7 bins in _visible proper decay length_ (xM) – Count OS and SS events (compare charge of reconstructed muon with tagger decision)
- Fit the 2:
- Also include B+ D0   X decay asymmetry.

- Speaker, ICHEP 2006

## Dilution calibration : Results

- For final fit, bin the tag variable |d| in 5 bins and do a simultaneuos fit
- 	(i) where i=1,5. Parameters of the fit : m, fcc, 5 Dd, 5 Du = 12

*********m************stat.) ps******-1***
*********D******2 ******= (2.48 ************ 0.21) (%) (stat.)***
*********stat.*********

- Increasing dilution

- Increasing dilution

*B0*

*B+*

- Speaker, ICHEP 2006

## Individual Taggers performance

| Tagger |  | D (%) | D2 (%) |  |
| --- | --- | --- | --- | --- |
| Muon | 6.61  0.12 | 0.473  0.027 |  | 1.48  0.17(stat) |
| Electron | 1.83  0.07 | 0.341  0.058 |  | 0.21  0.07 (stat) |
| SV | 2.77  0.08 | 0.424  0.048 |  | 0.50  0.11 (stat) |
| Total OST | 11.14  0.15 |  |  | 2.19  0.22 (stat) |

- Note :
- To evaluate the individual tagger performance |dpr| > 0.3
- This cut was not imposed for final combined tagger.
- Final eD2 is higher.

- Speaker, ICHEP 2006

## Likelihood minimization to get ms

-  Form Probability Density Functions (PDF) for each source

**Minimize **

- Dilution Calibration
- (From md measurement)
- Signal selection function (y)

- Speaker, ICHEP 2006

**Notes:**
Use variables that discriminate signal and background using M distribution sidebands and signal region. Form yi = fs/fbkg for ith variable. y is product over all yi’s.
Use the signal selection function in the likelihood
 Use the full information to weight the events

## Bs Signal and background

- Signal PDF:
- Background PDF composed of long-lived and prompt components – Evaluated from a lifetime fit.
- Long Lived Background – Described by exponential convoluted with a gaussian resolution function.
***Non-sensitive to the tagging ***
*** ******Non-oscillating***
***Oscillating with ******Δ******m******d****** frequency***
- Prompt Background – Gaussian distribution with resolution as fit parameter.

- Speaker, ICHEP 2006

## Combine individual tag informations to tag the event.
Get tag on opposite side and construct PDF’s for variables discriminating b () and b (+) (Use B+  D0  X  decays in data)

Discriminating variables (xi):

- Get tag on opposite side and construct PDF’s for variables discriminating b () and b (+) (Use B+  D0  X  decays in data)
- Discriminating variables (*x*i):

***Combined flavor tag algorithm***

*Electron/Muon*

*SV Tagger*

- Speaker, ICHEP 2006

## Ensemble Tests

**Using data**
** ****Simulate ****Δ****m****s****=****∞**** by randomizing the sign of flavour tagging**
** ****Probability to observe ****Δ****log(L)>1.9 (as deep as ours) in the range 16 < ****Δ****m****s**** < 22 ps****-1**** is 3.8%**
** ****5% using lower edge of syst. uncertainties band**
**Using MC **
** ****Probability to observe ****Δ****log(L)>1.9 for the true ****Δ****m****s****=19 ps****-1**** in the range 17 < ****Δ****m****s**** < 21 ps****-1**** is 15%**
**Many more parameterized MC cross-checks performed – all consistent with above**

- Speaker, ICHEP 2006