# Presentation: 553442

## Evidence for Bs Mixing and measurement of ms at CDF

- S. Giagu and CDF Collaboration
- University of Rome “La Sapienza”
- INFN Sezione di Roma 1

## Outline

- Introduction
- Search for Bs-Bs oscillations in CDF
- Impact on the overall UT fit
- Work in progress and Outlook

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

- S.Giagu - ICHEP 2006, Moscow

## B Meson Flavor Oscillations

- In the SM generated via F=2 2nd order weak interactions, dominated by the exchange of a top quark

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

**Mixing involves CKM elements,**
**measuring ****Δ****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

- S.Giagu - ICHEP 2006, Moscow

## 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

- Status of ms measurements:
-  LEP/SLD/CDF-I: ms > 14.4 ps-1	@ 95% CL                 *HFAG Average for PDG 2006*
* **D0 Run-II:           *ms  [17,21] ps-1	@ 90% CL          *Phys. Rev. Lett. 97, 021802  (2006)*

## Road map to ms measurement

- Collect as many Bs as possible
- Tevatron, Trigger (SVT)
- Extract Signal
- Bs flavor at decay inferred from decay products
- Measure proper decay time of the Bs meson
- L00, 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**

## 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

## 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

## Decay time resolution

-   Sensitivity maximized by making full use of all available information:
-   layer-00, candidate specific primary vertex and decay time resolution
-   Resolution measured in data in large samples of prompt D meson decays
-   D+ combined with prompt tracks to mimic B0-like topologies

- oscillation period
- @ ms=18 ps-1

- 4 sampling per cycle** **

- Hadronic decays gives CDF sensitivity at much
- larger values of ms than previous experiments

**M(lD****s****) > 3.3 GeV/c**

**first bin of ct**** **

## 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) |

## 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

## Results

- Likelihood ratio:
- A=1 VS A=0 hypothesis

- P-value = 0.2% (>3)

- small systematic uncertainty
- dominated by knowledge of the absolute scale of the decay-time measurement

*hep-ex/0606027 – accepted by PRL*

## Impact on the overall UT Fit

*UTfit Coll.: hep-ph/0605213 *
*and Vincenzo’s talk*

- Similar results from CKMfitter group: _[http://ckmfitter.in2p3.fr](http://ckmfitter.in2p3.fr/)_ and Stephane T’Jampens talk

- SM fit

- SM+NP fit

**C****Bs**** = 0.97 ± 0.27**

_**angles only**_

_**no angles**_

**CDF**
**measurement**

**CKM fit (no Δm****s**)
**(21.5 ± 2.6) ps****-1**

## Work in progress

-  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-)

## Summary and Outlook

- CDF finds evidence for flavor oscillations in the Bs sector
- Probability of a random fluctuation 0.2%
- Measurement of the mixing frequency with <2% precision
- Most precise measurement of |Vtd/Vts|

- An important and precise experimental input for the overall test of the SM and the end of a very long effort to measure ms
- ... but not the end of the CDF B-physics programme

## Random Slides

- S.Giagu - ICHEP 2006, Moscow

## Data Sample

- Bs candidates collected by SVT trigger
-  TTT:      two displaced tracks
-  L+SVT: lepton + displaced track(s)

- Typical inst. Luminosity  1032 cm-2 s-1
- ~1.4 fb-1 collected by CDF
- ~1 fb-1 (good runs) used in this analysis

- used in this analysis

- S.Giagu - ICHEP 2006, Moscow

## Other results on ms

- LEP, SLD, CDF-I

- 1st direct single experiment upper bound
- ms  [17,21] ps-1 @ 90% CL
- Null hypothesis probability: 5%

*D0 Coll.: Phys. Rev. Lett. 97, 021802  (2006)** *

- Recent from D0 collaboration

- ms > 14.4 ps-1@ 95% CL

*HFAG Average for PDG 2006*

- S.Giagu - ICHEP 2006, Moscow

## Example of Specific Trigger for B Physics

**Level 1**
-  - 2 XFT tracks with pT > 1.5 GeV
-  - opposite charge
-  -  < 135o
-  - |pT1| + |pT2| > 5.5 GeV

**Level 2**
-  - confirm L1 requirements
-  - both XFT tracks
-     - SVT 2<15
-     - 120 m< |d0| <1mm
-  - 2o <  < 90o
-  - Decay length Lxy > 200m

**Level 3**
-  - confirm L2 with COT & SVX
-    “offline” quality track reco.

- S.Giagu - ICHEP 2006, Moscow

## Semileptonics: Correction for Missing Momentum

- oscillation period
- @ ms=18 ps-1

- S.Giagu - ICHEP 2006, Moscow

## PID

- 

- K

- Separartion Power

- Combined PID: TOF + dE/dx

- S.Giagu - ICHEP 2006, Moscow

## Systematic Uncertainties

- related to absolute value of amplitude, relevant only when setting limits
- cancel in A/A, folded in in confidence calculation for observation
- systematic uncertainties are very small compared to statistical

- Hadronic

- Semileptonic

- S.Giagu - ICHEP 2006, Moscow

## Incertezze sistematiche su ms

- systematic uncertainties from fit model evaluated on toy Monte Carlo
- have negligible impact
- only relevant systematic: knowledge of lifetime scale

|  | Syst. Unc  |
| --- | --- |
| All other syst. | < 0.01ps-1 |
| SVX Alignment | 0.04 ps-1 |
| Track Fit Bias | 0.05 ps-1 |
| PV bias from tagging | 0.02 ps-1 |
| Total | 0.07 ps-1 |

- All relevant systematic uncertainties are common
- between hadronic and semileptonic samples

- S.Giagu - ICHEP 2006, Moscow

## Amplitude Scan: Hadronic decays

- data period 1

- data period 2

- data periodo 3

- S.Giagu - ICHEP 2006, Moscow

## Amplitude Scan: Semileptonic decays

- data period 1

- data period 2

- data period 3

- S.Giagu - ICHEP 2006, Moscow

## Exploit peculiarity of each tagger to minimize mistag probability 
example: soft muon tag

- example: soft muon tag

- from c decay

- Parameterization of the tagging decision

- jet axis

- from b decay

- S.Giagu - ICHEP 2006, Moscow

## SSKT Calibration

- Dilution measured in high statistic samples of light B meson decays and compared with the results of simulation

- Dominant source of systematic uncertainty: Data/MC agreement ~O(14%)

- S.Giagu - ICHEP 2006, Moscow

## Negative log likelihood ratio

- S.Giagu - ICHEP 2006, Moscow