# Presentation: 530797

## Rare Decays At the Tevatron

**Rare Decays At the Tevatron**
**Cheng-Ju S. Lin**
**(Fermilab**)
**BEAUTY 2006**
**   ****OXFORD **
**    ****28 September 2006**

## OUTLINE

-   Experimental Issues For Rare Decay Searches
-   Bs(d)+- Status and Prospects
-   Non-resonant Rare Decays:
- 	-  Bd  K*0
- 	-  B+  K+
- 	-  Bs  

## Tevatron is gold mine for rare B decay searches:

**Tevatron is gold mine for rare B decay searches:**
-   **Enormous *****b***** production cross section,**
**    ****x1000 times larger than e****+****e****-**** B factories**
**  ****All B species are produced (B****0****, B****+****, B****s****, ********b****...)**
**Dataset:**
-   **Di-muon sample, easy to trigger on in**
**   ****hadronic environment**
**  ****Analyses presented today use**
**   ****0.450 to 1 fb****-1**** of data**

**TEVATRON**

## CDF + D0 DETECTORS

**Key elements for rare decay **
**searches:**
-   -  Good muon coverage
-      D0:   ||<2.2
-   -  Good track momentum resolution  mass resolution
-      CDF: can resolve Bs from
-                Bd decays
-   -  Good B vertexing resolution
-       CDF:  L00 (rinner~1.4cm)
-       D0:     L0 upgrade(rinner~1.6cm)
-   -  Particle ID
-       CDF:  dE/dx and TOF

- DO

- CDF

## RARE B TRIGGERS AT TEVATRON

- Inelastic QCD cross section is about 1000x larger than b cross section
- Primary triggers:
- Di-muons + kinematic requirements
- Single muon for calibration
**Main issue:  trigger rate blows up rapidly vs. luminosity ******

**RARE B TRIGGERS AT TEVATRON**

- CDF L2 Dimuon Trigger Cross Section

- For illustration, these  two
- low pT di-muon triggers
- alone would take
- up ~100% of L2 trigger
- bandwidth at 200E30

## KEEPING RARE B TRIGGERS ALIVE

-   Handles to control rates:
-       -  Tighter selection cuts           (e.g. pT of muon)
-       -  Apply prescales
-          (DPS, FPS, UPS, etc.)
-       -  Improving trigger algorithm
-       -  Upgrading trigger hardware
-   We’ve been using a combination of all four handles to    control the trigger rate  trading efficiency for purity
-   It’s been a great challenge keeping B triggers alive at Tevatron
-   It’ll be an even greater challenge at the LHC !!

- Non-optimal for rare searches

## B  SEARCH AT THE TEVATRON

## BRIEF MOTIVATION

-   In the Standard Model, the FCNC decay of B +- is heavily
-    suppressed

-   SM prediction is below the sensitivity of current experiments
-    (CDF+D0):  SM   Expect to see 0 events at the Tevatron

- (Buchalla & Buras, Misiak & Urban)

-   Bd is further suppressed by CKM factor (vtd/vts)2

- SM prediction 

**Any signal at the Tevatron would indicate new physics!!**

-   See Tobias Hurth talk this morning for new physics scenarios

## CDF: 
780 pb-1 di-muon triggered data
Two separate search channels
Central/central muons 
   (CMU-CMU)
Central/forward muons 
    (CMU-CMX)
 CMU ||<0.6, CMX 0.6 < || <1
Extract Bs and Bd limit

DØ: 
First 300 pb-1 di-muon triggered data
    with box opened  limit
400 pb-1 data still blinded
Combined sensitivity for 700 pb-1 of recorded data (300 pb-1 + 400 pb-1 )

- 780 pb-1 di-muon triggered data
- Two separate search channels
- Central/central muons
-    (CMU-CMU)
- Central/forward muons
-     (CMU-CMX)
-  CMU ||<0.6, CMX 0.6 < || <1
- Extract Bs and Bd limit
**DØ:**
- First 300 pb-1 di-muon triggered data
-     with box opened  limit
- 400 pb-1 data still blinded
- Combined sensitivity for 700 pb-1 of recorded data (300 pb-1 + 400 pb-1 )

**RARE B DATASETS**

- S/B is expected to be
- extremely small.  Effective
- bkg rejection is the key
- to this analysis!!

- Search region

## METHODOLOGY

- Motto:  reduce background and keep signal eff high

- Step 1:  pre-selection cuts to reject obvious bkg
- Step 2:  optimization (need to know signal efficiency
-              and expected bkg)
- Step 3:  reconstruct B+ J/ K+ normalization mode
- Step 4: open the box  compute branching ratio or
-              set limit

## CDF PRE-SELECTION

- 4.669 < m< 5.969 GeV/c2
- muon quality cuts
- pT()>2.0 (2.2) GeV/c CMU (CMX)
- pT(Bs cand.)>4.0 GeV/c
- |y(Bs)| < 1
- good vertex
- 3D displacement L3D between primary and secondary vertex
- (L3D)<150 m
- proper decay length 0 <  < 0.3cm

**CDF PRE-SELECTION**

**Bkg substantially reduced but still**
**sizeable at this stage**

## Pre-selection DØ:
4.5 < m< 7.0 GeV/c2
muon quality cuts
 pT()>2.5 GeV/c
 ()| < 2
pT(Bs cand)>5.0 GeV/c
good di-muon vertex

- 4.5 < m< 7.0 GeV/c2
- muon quality cuts
-  pT()>2.5 GeV/c
-  ()| < 2
- pT(Bs cand)>5.0 GeV/c
- good di-muon vertex

**~ 38k events after pre-selection**

***300 pb******-1***

**D0 PRE-SELECTION**

- Potential sources of background:
-  continuum  Drell-Yan
-  sequential semi-leptonic bcs decays
-  double semi-leptonic bb X
-  b/cX+fake
-  fake + fake

## B  SIGNAL VS BKG DISCRIMINATION

-      ~±2.5 mass window
- B vertex displacement:
-     CDF 
-     D0  
- Isolation (Iso):
- (fraction of pT from B within R=(2+2)1/2 cone of 1)
- “pointing ()”:
- (angle between Bs momentum and decay axis)

**B******** ******************** SIGNAL VS BKG DISCRIMINATION**

## CDF OPTIMIZATION

**CDF OPTIMIZATION**

-   CDF constructs a likelihood ratio
-     using discriminating variables  Iso

- Ps/b is the probability for a given sig/bkg to have a
- value of *x*, where *i* runs over all variables.

-   Optimize on expected upper limit
-   LR(optimized)>0.99

##  Random Grid Search

- Pointing angle
- 2D decay length significance
- Isolation
-  Random Grid Search
-  Maximize S/(1+sqrt(B))

**D0 OPTIMIZATION**

## BACKGROUND ESTIMATES

-   Extrapolated bkg from
-    side-bands to signal region
-    assume linear shape
-   CDF signal region is also contaminated
-    by Bh+h- (e.g. BK+K-, K+, )
-    -  K muon fake rates measured from data
-    -  Convolute fake rates with expected Br(Bh+h-) to estimate #
-    	Bs signal window =  0.19 ± 0.06
-             Bd signal window =  1.37 ± 0.16
-    -  Total bkg = combinatoric + (Bhh)

## CDF Bs (780 pb-1): 

- central/central: observe 1, expect 0.88 ± 0.30
- Central/forward: observe 0, expect 0.39 ± 0.21
- DØ Bs (300 pb-1):
- observe 4, expect
-        4.3 ± 1.2
- DØ (blinded, 400 pb-1):
-      - <Nbkg> = 2.2 ± 0.7

**BOXES OPENED**

- 300pb-1

## BRANCHING RATIO LIMITS

| CDF Bs-> | 176 pb-1 | 7.5×10-7 | Published |
| --- | --- | --- | --- |
| DØ Bs-> | 240 pb-1 | 5.1×10-7 | Published |
| DØ Bs-> | 300 pb-1 | 4.0×10-7 | Prelim. |
| DØ <Bs-> | 700 pb-1 | <2.3×10-7> | Prelim.
Sensitivity |
| CDF Bs-> | 364 pb-1 | 2.0×10-7 | Published |
| CDF Bs-> | 780 pb-1 | 1.0×10-7 | Prelim. |

**BRANCHING RATIO LIMITS**

-  Evolution of limits (in 95%CL):

- World’s best
- limits

| Babar Bd-> | 111 fb-1 | 8.3×10-8 | Published |
| --- | --- | --- | --- |
| CDF Bd-> | 364 pb-1 | 4.9×10-8 | Published |
| CDF Bd-> | 780 pb-1 | 3.0×10-8 | Prelim. |

- 90% CL

## TEVATRON REACH ON Bs

**TEVATRON REACH ON B****s**********

- Conservative projection based on sensitivity of current analyses
- Ongoing efforts to significantly improve sensitivity of the analyses
- Tevatron can push down to at least low 10-8 region

**Integrated Luminosity/exp  (fb****-1****)**

## B  h DECAYS AT THE TEVATRON

## Bu,d,s  K+/K*/

- New physics could interfere with the SM amplitudes
- Can look for new physics via decay rates and decay kinematics
- B Rare Decays B  h :
- B+   K+
- B0  K*
- Bs   
- Rare processes:  predicted BR(Bs   )=16.1x10-7

- observed at Babar, Belle

- PRD 73, 092001 (2006)
- hep-ex/0410006

- not seen

- C. Geng and C. Liu, J. Phys. G 29, 1103 (2003)

**B****u,d,s**** ******** ************************K****+****/K*/******

## CDF: 
1 fb-1 di-muon trigger data
Search in all three modes:

 B+   K+
 B0  K*
 Bs   

DØ: 
450 pb-1 di-muon data
Published Bsresult

- 1 fb-1 di-muon trigger data
- Search in all three modes:
-  B+   K+
-  B0  K*
-  Bs   
**DØ:**
- 450 pb-1 di-muon data
- Published Bsresult

**B********J/********h DATASETS**

## METHODOLOGY

-   Experimental method similar to Bs analysis
-   Measure branching ratio (or set limit) relative to the
-     reference BJ/ h resonance decay
-   Exclude  and ’ invariant mass regions for
-     non-resonant decays
-   Relative efficiency determined from a combination of
-    data and Monte Carlo
-   Bkg estimated from mass side-band(s). Feed-down
-    contribution estimated from MC

## NORMALIZATION MODES

- 450 pb-1

- Clean samples
- of norm events

- Apply similar pre-selection requirements
- as B analysis

## B h SIGNAL VS BKG DISCRIMINATION

- Decay length significance
- 2D Pointing | B –  vtx|
- Isolation

**B******** ******************h** SIGNAL VS BKG DISCRIMINATION**

- Optimization:
-    Using data sidebands and MC to avoid
-     introducing biases
-    CDF  f.o.m. = Nsig / sqrt(Nsig+Nbkg)
-    D0     f.o.m. = Nsig / (1 + sqrt(Nbkg) )

- Cut

- Cut

- Cut

- (D**Ø** uses |P|, instead of pT)

## UNBLINDED B0 AND B+ RESULTS

- B+   K+ :
-    Nobs = 107
-    <bkg> = 51.6 ± 6.1
-    Significance = 5.2

- B0   K*0 :
-    Nobs = 35
-    <bkg> = 16.5 ± 3.6
-    Significance = 2.9

## UNBLINDED Bs RESULTS

- CDF Bs    :
-    Nobs = 9
-    <bkg> = 3.5 ± 1.5
-    Significance = 1.8

- D0 Bs    :
-    Nobs = 0
-    <bkg> = 1.6 ± 0.6

- 450 pb-1

**No B****s**** ******** ******** ******** **
**signal observed**

## BJ/h RESULTS

| D0  0.45fb-1 | Bs  |
| --- | --- |
| Rel BR 95% CL Limit x10-3 | 4.4 |
| Rel BR 90% CL limit x10-3 | 3.5 |

**B********J/********h RESULTS**

- PRD 74, 031107 (2006)

| CDF 1fb-1 | B  | B  | Bs  |
| --- | --- | --- | --- |
| Rel BR± stat 
± syst 10-3 | 0.71 ± 0.15 ± 0.04 | 0.62 ± 0.23 ± 0.07 | 0.91 ± 0.55 ± 0.11 |
| Abs BR ± stat 
± syst 10-6 | 0.72 ± 0.15 ± 0.05 | 0.82 ± 0.31 ± 0.10 | 0.85 ± 0.51 ± 0.31 |
| Rel BR 95% CL 
Limit x10-3 | - | - | 2.3 |
| Rel BR 90% CL 
limit x10-3 | - | - | 2.0 |

## SUMMARY

-   CDF and D0 have analyzed first ~800 pb-1 of data to
-    search for B.  No signal is seen in the CDF data.  D0 has   not opened the box for the later half of data.
-   Current limit already severely constrain new physics models
-   Both CDF and DO are significantly improving the sensitivity
-    for the 1 fb-1 update.  The expected combined sensitivity
-    for 1 fb-1 is in the mid 10-8 level.
-   Tevatron is now getting into B+ and B0 h terrority.     Preliminary results are consistent with B factories.
-   No Bs signal.  New CDF result improves limit by x2.
-    Tevatron closing in on SM prediction.
-   Still have ~ x8 more data to be collected.  Plenty of    room for discoveries !!!

## BACKUP

## D0 SENSITIVITY FOR 700 pb-1

- background events

- Cut Values changed
- only slightly!

-   Obtain a sensitivity (w/o unblinding) w/o changing the analysis
-   Combine “old” Limit with obtained sensitivity

**D0 SENSITIVITY FOR 700 pb****-1**

- (400 pb-1)