# Presentation: 522338

## BS Mixing Measurementsat the Tevatron

- Michael Kirby
- Radboud University, Nijmegen
- NIKHEF

- New Trends in High Energy Physics, CRIMEA 2006

## B Meson Flavor Oscillations

**Form factors and B-parameters from Lattice calculations are known at ~15% level**

**Mixing involves CKM elements,**
**measuring ********m****q ****constraints**
**the unitarity triangle **

**New exotic particles may run in the loop **
****** ****mixing sensitive to NP**** **

- Neutral B mesons can spontaneously transform in the corresponding antiparticle

## ms and the side of UT

- many theoretical uncertainties cancel in the ratio
-  |Vts|/|Vtd| can be determined at ~4%

- ms and the side of UT

- md  f2BBB [(1-)2+2]  circle centered in ()=(1,0)
- f2BBB known at 15% from LQCD

*(hep-lat/0510113)*

- Experimental challenge:
- |Vts| >> |Vtd|  ms >> md  needs to resolve > 2.3 THz oscillations

- Previous ms measurements:
-  LEP/SLD/CDF-I: ms > 14.4 ps-1	@ 95% CL                 *HFAG Average for PDG 2006*

## Unitarity Triangle Fit

-   just for illustration, other fits exist
-   CKM Fit :ms: 18.3+6.5 (1)  : +11.4 (2)  ps-1

-  from md

-  from md/ms

- Lower limit on ms

- -1.5

- -2.7

## B Mixing

-  Neutral B Meson system
- mixture of two mass eigenstates (No CP violation case):
- BH and BL may have different mass and decay width
-  m = MH – ML
-      (>0 by definition)
-   =  H -  L

- The case of  = 0

## Measurement .. In a Perfect World

- what about detector effects?

- “Right Sign”

- “Wrong Sign”

## Realistic Effects

- flavor tagging power,
- background

- displacement
- resolution

- momentum
- resolution

- mis-tag rate 40%

- L) ~ 50 m

- p)/p = 5%

## Real Measurement Layout

- momentum resolution
- displacement resolution
- flavor tagging power

- Unbinned
- Likelihood
- Fitter

- Data

- A(ms=15 ps-1)= ?

- ms = ?

- scan for signal:

- measure frequency:

- p ~ e-t/[1±AD cosmt]  R(t)

## Road map to ms measurement

- Collect as many Bs as possible
- Tevatron, Trigger, Reconstruction
- Extract Signal
- Bs flavor at decay inferred from decay products
- Measure proper decay time of the Bs meson
- Si trackers, per event primary vertex, candidate specific decay time resolution
- Determine Bs flavor at production (flavor tagging)
- PID (TOF, dE/dx)
- Flavor tag quantified by Dilution: D=1-2w, w = mistag probability
- Measure asymmetry between unmixed and mixed events
- _In practice_: perform likelihood fit to expected unmixed and mixed distributions

- Road map to ms measurement

**5**

##  This analysis: Feb 2002 – Jan 2006  1 fb-1

- Tevatron Luminosity

## 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**

## 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**** **

## Signal Selection

******+**

**π**** ****-**

- K+

- K-

****

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

******(e)**

**B**

****** **

**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%**

## 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

## Dilution in md 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: D2=2.48±0.21±0.08 %** **

- Input for Bs measurement

## Cross-check on BdXD±()

- Cross-check on BdX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**

## Measure Resolution Using Data

**Ultimately ********m****s**** sensitivity is limited by decay length resolution –  ****very important issue**** **
**Use J/******** sample**
** ****Fit pull distribution for J/******** 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**

## 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**

## 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 **********

## Amplitude Scan of BsXDs()

- Amplitude Scan of BsXDs()

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

## 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)

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

*“**Direct Limits on the B**s** Oscillation Frequency”*
*hep-ex/0603029 – published by Physical Review Letters**  *

## More Amplitude Scans

- New results : Amplitude scans from two additional modes

**Ds ******** K*K**

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

**Bs********Ds ******** X**

**Ds **********

## Combined D0 Result

- 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

## CKM Fit Before D0

## CKM Fit With D0 Limit

## The CDFII Detector

- multi-purpose detector
- excellent momentum resolution (p)/p<0.1%
- Yield:
- SVT based triggers
- Tagging power:
- TOF, dE/dX in COT
- Proper time resolution:
- SVXII, L00

## Event Selection: Fully Hadronic Bs

**used in this analysis**

| Decay mode | Events |
| --- | --- |
| BsDs () | 1600 |
| Bs Ds (K* K) | 800 |
| Bs Ds  (3) | 600 |
| Bs Ds3 ( ) | 400 |
| Bs Ds3 (K*K) | 200 |
| Total | 3600 |

- Cleanest decay mode:
- BsDs[] [KK] 

-  Bs momentum completely reconstructed
-  Excellent decay time resolution, good S/N
-  Small BR  low statistic
-  Good sensitivity at high values of ms

## Event Selection: Semileptonic Bs

-  BG Sources:
-  Ds + fake lepton from PV
-  Bs,dDsDX (DslX)
-  cc

- Event Selection: Semileptonic Bs

-  Missing momentum ()
-  Poorer decay time resolution
-  Large BR  high statistic
-  Good sensitivity at low values of ms

- 48000 l+Ds candidates, 75% are from Bs decay

- Ds Mass

- l+Ds Mass

## Flavor Tagging Performances

- Two types of flavor tags used in CDF
- OST: produce bb pairs: find 2nd b, determine flavor, infer flavor of 1st b
- calibrated on large samples of B0 ad B+ decays
- SST: use charge correlation between the b flavor and the leading product of b hadronization
- performances (D) evaluated in MC, after extensive comparison data VS MC

- Same-side kaon tag increases effective statistics  ~4

|  | εD2 Hadronic (%) | εD2 Semileptonic (%) |
| --- | --- | --- |
| Muon | 0.48  0.06 (stat) | 0.62  0.03 (stat) |
| Electron | 0.09  0.03 (stat) | 0.10  0.01 (stat) |
| JQ/SecVtx | 0.30  0.04 (stat) | 0.27  0.02 (stat) |
| JQ/Displ’d trk | 0.46  0.05 (stat) | 0.34  0.02 (stat) |
| JQ/High pT | 0.14  0.03 (stat) | 0.11  0.01 (stat) |
| Total OST | 1.47  0.10 (stat) | 1.44  0.04 (stat) |
| SSKT | 3.42  0.96 (syst) | 4.00  1.12 (syst) |

## Amplitude Scans

- example: B0 Mixing signal in hadronic decays
- points: A§(A) from likelihood fit for different m
- yellow band: A § 1.645 (A)
- m values where A+1.645 (A) < 1 are excluded at 95% C.L.
- dashed line: 1.645 (A) as function of m
- measurement sensitivity: 1.645 (A) = 1

- narrow ms range

- wide ms range

## Calibrating the Proper Time Resolution

- Calibrating the Proper Time Resolution

- utilize large prompt charm cross section
- construct “B0-like” topologies of prompt D- + prompt track
- calibrate ct resolution by fitting for “lifetime” of “B0-like” objects

- trigger tracks

- period 3

- +

## Proper decay time reconstruction

- Detector length scale and proper treatment of detector/selection biases controlled by performing lifetime measurements

| Decay | CDF [ps]
(stat. only) | PDG 06 [ps] |
| --- | --- | --- |
| B0  D-+ | 1.508  0.017 | 1.530  0.009 |
| B-  D0- | 1.638  0.017 | 1.638  0.011 |
| Bs  Ds() | 1.538  0.040 | 1.466  0.059 |

- PV

## l+Ds ct* Projections 

- l+Ds ct* Projections

- Bs lifetime in 355 pb-1: 1.48   ± 0.03 (stat) ps
- World Average value:  1.469 ± 0.059 ps

## Hadronic Scan: Combined

- Preliminary

- Bs ! Ds / Ds

## Semileptonic Scan: Combined

## Combined Amplitude Scan

- A/A (17.25 ps-1) = 3.5

- How significant is this result?

- Preliminary

- 25.3 ps-1

## Likelihood

- Amplitude method(*): scan ms space: fix ms fit for A:
- A consistent with 1  mixing detected at the given ms

*k*

- k=sig,bg

*sig*

- for each event:

- pdg

- Procedure checked on B0 by fitting for md

*Courtesy of J.Kroll*

*(*)** H-G.Moser, A.Roussarie,*
*NIM A384 (1997)*

- Data fitted with an unbinned likelihood function to the expected unmixed and mixed distributions

## ms = 17.33 +0.42 (stat) ± 0.07 (syst) ps-1

- ms = 17.33 +0.42 (stat) ± 0.07 (syst) ps-1

- Measurement of ms

- -0.21

- limit ms in [16.94, 17.97] ps-1 at 95% CL

- -0.21

*“**Measurement of the B**s**-B**s** Oscillation Frequency”*
*hep-ex/0606027 – published by Physical Review Letters**  *

## |Vtd| / |Vts|

- compare to Belle  bd (hep-ex/0506079):
- |Vtd| / |Vts| = 0.199 +0.026 (stat) +0.018 (syst)

- -0.025

- -0.015

## CKM Fit including CDF ms measurement

## 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, and commissioning advancing quickly**

- D0 Outlook

## CDF Outlook

-  Collecting new integrated luminosity
-  Squeezing maximum information from the data
-   we already have:

- Systematic use of Neural Networks in signal extraction:
- use decays modes previously discarded cause high BG
- more signal in already used modes
- Use partially reconstructed BsDs*/K and Ds:
- large BR
- good momentum resolution
- Improve Flavor taggers:
- OST:  +15% D2
- NN to combine OS taggers
- OSKT
- SSKT: ~+10% D2
- better use of combined  PID and kinematics

- BsDs+-+  (Ds +--)

- NBs = 220

- BsDs+  (Ds-)

## Conclusions

- Frequency of Bs mixing successfully measured at the Tevatron!
- D0 reported the first two-sided limit on ms
- ms [17,21] ps-1 @ 90% C.L.
- CDF confirmed result with measurement
- 	ms = 17.33 +0.42 (stat) ± 0.07 (syst) ps-1
- 		|Vtd / Vts| = 0.208 +0.008 (stat + syst)

- -0.21

- -0.007