# Presentation: 529559

## Search for Rare Decays of the BS  Meson at the Tevatron

- Ralf Bernhard
- University of Zurich
- Moriond QCD
- 19th March 2006

## Outline

- Introduction
- Tevatron
- CDF & DØ Detector
- Search for the Decay Bs → μ+μ-
- Search for the Decay Bs → μ+μ-
- Summary & Conclusions

## Tevatron performance

- excellent performance of Tevatron in 2005 and early 2006
- machine delivered more than 1500 pb-1 up to now !!
- recorded (DØ/CDF)
- 1.2/1.4 fb-1
- record luminosity of 1.71032 cm-2/s in January 2006
- high data taking efficiency ~85%
- current dataset reconstructed and under analysis
- ~1000 pb-1
- compare  with ~100 pb-1 Run I

## CDF detector

- Silicon Tracker SVX
- up to |<2.0
- SVX fast r- readout for trigger
- Drift Chamber
- 96 layers in ||<1
- particle ID with dE/dx
- r- readout for trigger
- tracking immersed in Solenoid 1.4T
- Time of Flight
- →particle ID

## DØ detector

- 2T Solenoid
- hermetic forward & central muon detectors
- excellent coverage ||<2
- Fiber Tracker
- 8 double layers
- Silicon Detector
- up to |<2.5

## B production at the Tevatron

- bb cross section orders of magnitude larger than at B-factories (4S) or Z
- σ(pp → bb) = 150μb at 2TeV
- σ(e+e- → Z → bb) = 7nb
- σ(e+e- → Υ(4S) → bb) = 1nb
- all kinds of b hadrons produced:
- Bd, Bs, Bc, B**, ******b****, ********b****,** ...
- However:
- QCD background overwhelming, b-hadrons hidden in 103 larger background
- events complicated, efficient trigger and reliable tracking necessary
- crucial for B physics program:
- good vertexing & tracking
- triggers w/ large bandwidth, strong background rejection
- muon system w/ good coverage

**Lots going on**
** ****in Si detector**

- e.g., integrated cross sections
- for |y|<1:
- (B+, pT  6 GeV/c)~4 b

## Triggers for B physics 

- robust and quiet di-muon and single-muon triggers
- working horse for masses, lifetimes, rare decays etc.
- keys to B physics program at DØ
- “advanced” triggers using silicon vertex detectors
- exploit long lifetime of heavy quarks
- displaced track + leptons for semileptonic modes
- two-track trigger (CDF) – all hadronic mode
- two oppositely charged tracks with impact parameter

- pT(B)5 GeV
- Lxy450 m

- Decay length Lxy

- Triggers for B physics

## Purely leptonic B decay

**B->l****+**** l****-** decay is helicity suppressed FCNC
- SM: BR(Bs->) ~ 3.410-9
- depends only on one SM operator in effective Hamiltonian, hadronic uncertainties small
- Bd relative to Bs suppressed by |Vtd/Vts|2 ~ 0.04 if no additional sources of flavor violation
- reaching SM sensitivity: present limit for  Bs -> +- comes closest to SM value

|  | Br(Bdl+l-) | Br(Bsl+l-) |
| --- | --- | --- |
| l = e | 3.4 × 10-15 | 8.0 × 10-14 |
| l=μ | 1.0 × 10-10 | 3.4 × 10-9 |
| l=τ | 3.1 × 10-8 | 7.4 × 10-7 |

- SM expectations:

| C.L. 90% | Br(Bdl+l-) | Br(Bsl+l-) |
| --- | --- | --- |
| l = e | < 6.1 ·10-8 | < 5.4 ·10-5 |
| l=μ | < 8.3 ·10-8 | <1.5 x 10-7 |
| l=τ | < 2.5% | < 5.0% |

- Current published limits:

## Purely leptonic B decay

- excellent probe for many new physics models
- particularly sensitive to models w/ extended Higgs sector
- BR grows ~tan6 in MSSM
- 2HDM models ~ tan4
- mSUGRA: BR enhancement correlated with shift of (g-2)
- also, testing ground for
- minimal SO(10) GUT models
- Rp violating models, contributions at tree level
- (neutralino) dark matter ...

- Two-Higgs Doublet models:

- Rp violating:

## Experimental search

**CDF:**
- 364 pb-1 di-muon triggered data
- two separate search channels
-  central/central muons
-  central/forward muons
- extract Bs and Bd limit
**DØ:**
- 240 pb-1 (update 300 pb-1) di-muon triggered data
- Sensitivity for 1fb-1 of recorded data
**both experiments:**
- blind analysis to avoid experimenter’s bias
- side bands for background determination
- use B+ -> J/ K+ as normalization mode
- J/ ->  cancels selection efficiencies

- blinded signal region:
- DØ:  5.160 < m < 5.520 GeV/c2;
-  ±2 wide, =90 MeV
- CDF: 5.169 < m < 5.469 GeV/c2;
- covering Bd and Bs; =25 MeV

## Pre-selection

**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 vertex
**Pre-Selection CDF:**
- 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
- (Bs)| < 1
- good vertex
- 3D displacement L3D between primary
- and secondary vertex
- (L3D)<150 m
- proper decay length 0 <  < 0.3 cm

**e.g. D****Ø: ****about 38k events after **
**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

## Optimization I

- DØ:
- optimize cuts on three discriminating variables
- angle between  and decay length vector (pointing consistency)
- transverse decay length significance (Bs has lifetime): Lxy/σ(Lxy)** **
- isolation in cone around Bs candidate

-  Random Grid Search
-  maximize /(1.+B)

## Optimization II

- CDF: discriminating variables
- pointing angle between  and decay length vector
- isolation in cone around Bs candidate
- proper decay length probability p() = exp(- Bs)

- construct likelihood ratio to optimize on “expected upper limit”

## Unblinding the signal region

- CDF:
- central/central: observe 0, expect 0.81 ± 0.12
- Central/forward: observe 0, expect 0.66 ± 0.13
- DØ (300 pb-1):
- observe 4, expect 4.3 ± 1.2

- CDF

## DØ Sensitivity 1fb-1

- Expect 2.2 ± 0.7
- background events

- Cut Values changed
- only slightly!

- Used in previous analysis

- Used now in addition!

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

## Normalization

- relative normalization is done to B+ -> J/ K+
- advantages:
- selection efficiency same
- high statistics
- BR well known
- disadvantages:
- fragmentation b->Bu vs. b-> Bs
- DØ: apply same values of discriminating cuts on this mode
- CDF: no likelihood cut on this mode

## Master equation

- R = BR(Bd)/BR(Bs) is small due to |Vtd/Vts|^2
******B****+ /********Bs **relative efficiency of normalization to signal channel
******Bd**** /********Bs **relative efficiency for Bd-> versus Bs-> events in Bs search channel (for CDF~0, for DØ ~0.95)
**f****s****/f****u **fragmentation ratio (in case of Bs limit)  - use world average with 15% uncertainty

## The present (individual) limits

- DØ mass resolution is not sufficient to separate Bs from Bd. Assume no Bd contribution (conservative)
- CDF sets separate limits on Bs & Bd channels
- all limits below are 95% C.L. Bayesian incl. sys. error, DØ also quotes FC limit

- Bd limit x2 better
- than published Babar
- limit w/ 111 fb-1

| 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-> | 1 fb-1 | 2.3×10-7 | Prelim.
Sensitivity |
| CDF Bs-> | 364 pb-1 | 2.0×10-7 | Published |
| CDF Bs-> | 1 fb-1 | 1.0×10-7 | Prelim. |
| CDF Bd-> | 364 pb-1 | 4.9×10-8 | Published |

- updates on limits/sensitivities expected soon

## Tevatron limit combination I

- fragmentation ratio b->Bs/b->Bu,d
- standard PDG value as default
- Tevatron only fragmentation (from CDF) improves limit by 15%
- uncorrelated uncertainties:
- uncertainty on eff. ratio
- uncertainty on background

-  correlated uncertainties:
-   BR of B± -> J/(->) K±
-   fragmentation ratio b->Bs/b->Bu,d
-   quote also an average expected upper limit and single event sensitivity

- hep-ex/0508058

- DØ has larger acceptance due
- to better  coverage,
- CDF has greater sensitivity  due to  lower background expectations

## Combination II

- combined CDF & DØ limit:
- BR(Bs->  ) < 1.2 (1.5) × 10-7           @ 90% (95%) C.L.
- world-best limit, only factor 35 away from SM
- important to constrain models of new physics at tan
- e.g. mSO(10) model is severely constraint

**Example: SO(10) symmetry breaking model**

**Contours of constant Br(B****s********μ****+****μ****-****)**

- R. Dermisek et al.
- hep-ph/0507233

## Future Prospects for Bs->  

- assuming unchanged analysis techniques and reconstruction and trigger efficiencies are unaffected with increasing luminosity
- for 8fb-1/experiment an exclusion at 90%C.L. down to 210-8 is possible
- both experiments pursue further improvements in their analysis

##  Search for Bs -> +-

- long-term goal: investigate b -> s l+ l- FCNC transitions in Bs meson
- exclusive decay: Bs -> +-
- SM prediction:
- short distance BR: ~1.6×10-6
- about 30% uncertainty due to  B-> form factor
- 2HDM: enhancement possible, depending on parameters for tan and MH+
- presently only one published limit
- CDF Run I: 6.7×10-5 @ 95% C.L.

##  Search for Bs ->  +-

- DØ: 300 pb-1 of dimuon data
- normalize to resonant decay Bs -> J/
- cut on mass region 0.5 < M() < 4.4 GeV/c2 excluding J/& ’
- two good muons, pt > 2.5 GeV/c
- two additional oppositely charged tracks pt>0.5 GeV/c for 
-  candidate in mass range 1.008 < M() < 1.032 GeV/c2
- good vertex
- pt(Bs cand.) > 5 GeV/c
- non-resonant decay: cut out J/ and ’

- Dilepton mass spectrum
- in b -> s l l decay

- J/

- ’

## Search for Bs -> +-

- blind analysis: optimization with following variables in random grid search
- Pointing angle
- Decay length significance
- Isolation
- Background modeled from sidebands
- Use resonant decay Bs -> J/with same cuts as normalization
- Gaussian fit with quadratic background: 73 ± 10 ± 4 Bs-> J/resonant decays

##  Limit on Bs ->  + -

- expected background from sidebands: 1.6 ± 0.4 events
- observe zero events in signal region

**BR(B****s ****-> ********************)/BR(B****s ****-> J********) < 4.4 × 10****-3 ****@ 95% C.L.**
- Using central value for **BR(B****s ****-> J********) = 9.3×10****-4 **** PDG2004:**** **
**BR(B****s ****-> ********************) < 4.1×10****-6**** @ 95% C.L.**** **

- x10
- improvement
- w.r.t previous limit

## Conclusions

- CDF & DØ provide world best limits on purely leptonic decays Bd,s ->  limit important to constrain new physics
- with more statistics to come enhance exclusion power/discovery potential for new physics
- improved DØ limit on exclusive Bs -> +- decay shown, about 2x above SM
- Tevatron is doubling statistics every year - stay tuned for many more exciting results on B physics

## SPARE

## FCNC & new physics

- flavor-changing neutral current processes
- in SM forbidden at tree level
- at higher order occur through box- and penguin diagrams
- sensitive to virtual particles in loop, thus can discern new physics
- GIM-suppression for down-type quarks relaxed due to large top mass
- observable SM rates lead to tight constraints of new physics
- corresponding charm decays are less scrutinized and largely unexplored
- smaller BRs,  more suppressed by GIM-mechanism, long-distance effects also dominating
- nevertheless large window to observe new physics beyond SM exists: Rp-violating models, little Higgs models w/ up-like vector quark etc.

## Systematic uncertainties

- systematics for DØ (CDF very similar)
- efficiency ratio determined from MC with checks in data on trigger/tracking etc.
- large uncertainty due to fragmentation ratio
- background uncertainty from interpolating fit

## expected limit Bs -> +-

- expected limit at 95% C.L. for Bs -> +-

## Constraining dark matter

- mSUGRA model: strong correlation between BR(Bs->) with neutralino dark matter cross section especially for large tan
- constrain neutralino cross section with less than, within and greater than 2 of WMAP relic density

- universal
- Higgs mass
- parameters

- non-universal Higgs mass
- Parameters, Hu=1, Hd=-1

- S. Baek et al., JHEP 0502 (2005) 067