# Presentation: 363179

## DØ Results and Run II Status

- Brad Abbott
- University of Oklahoma
- On behalf of the D0 collaboration

**9th International Symposium on Heavy Flavor Physics**
**Caltech **
**September 10-13 2001**

##  Large production cross section
 Even larger inelastic cross section 
    (S/B10-3)  specialized triggers

 Single lepton triggers

 Dilepton triggers such as
	J/    +  -

** ****Large production cross section**
** ****Even larger inelastic cross section **
**    ****(S/B********10****-3****) ******** specialized triggers**
** ****Single lepton triggers**
** ****Dilepton triggers such as**
**	****J/ ******** ******** ******** ****+ ******** -**

- B Physics at DO

**At 1.8 TeV**

**At ********(4S)**

**At Z****0**

**All species produced**
**           ****including:**
-       Bs      Bc         b

## B cross section at D0

- Easy measurement to make.
- There are still disagreements between theory and experiment.
- Muon plus jet trigger.
- Vertex tagging of second jet.
- Restrict angle between jets for better comparison with theory.

## B cross section at D0

##       Run II Detector

-       Run II Detector

## DØ Run II B Physics Goals

***B******S****** *****mixing**
**Spectroscopy and lifetimes**
***B*****0****, *****B*****+****, *****B******s***** , *****B******c *****, ***********b***
**	**
**Rare decays**
** **
** **

**DØ Run II B Physics Goals**

**QCD tests**
**cross sections**
**correlations**
**charmonium polarization**
**CP violation and CKM angles**
- sin(2**
-  ****

## DØ Upgraded Detector Performance

- Good Momentum resolution:
- dpT/pT2 = 0.002     (Silicon + Fiber tracker)
- High tracking efficiency:
- at least 95 %  (Silicon disks)
- Vertex Reconstruction:
- primary vertex: vertex = 15-30 m (r-)
- secondary vertex:   vertex = 40 m (r-) , 80 m (r-z)
- Excellent lepton coverage trigger and ID efficiency:
- muons: pT > 1.5 GeV, 
- electrons: pT > 1 GeV
- Impact parameter trigger

##  Towards Sin(2):B J/ KS Reconstruction

** ****Towards Sin(2********):*****B********* *****J*****/*********** K******S *****Reconstruction**

***J*****/*********** ********* *********** *****+*********** *****-**** ****  **require two central tracks with *p**T* > 1.5 GeV/c
***K******S***** ******** *********** *****+*********** *****-****  **** **use long lifetime to reject background:  Lxy/  >  5
** **Perform 4-track fit assuming *B* *J*/*** +K**S*
- constrain ***+** **** **-**  *and ***+** **** **-** *to mass of  *K**S*  and *J/***  respectively
- force* K**S * to point to *B* vertex and *B* to point to primary** **

## B J/ Ks decay length reconstruction

- Two secondary two-track vertices
- Average *B* decay length: 2.3 mm
- Measured decay length resolution:  100 m

## Opposite side tags:
identify the flavor of the other B in the event
soft lepton tags 	 b  l - + X 	
jet charge tags	 Qjet < 0 for  b

Same side tags:
correlation of flavor and charge of closest particle produced in fragmentation or decay

- identify the flavor of the other *B* in the event
- soft lepton tags 	 *b ** l* - + *X*
- jet charge tags	 Qjet < 0 for  *b*
- Same side tags:
- correlation of flavor and charge of closest particle produced in fragmentation or decay

**Flavor Tagging**

- Efficiency (**) and
- dilution factor (*D*)
*D * =  2 *P* - 1
-  *P*  is the correct tag probability
** *D*2  is the tag’s effectiveness

## Flavor Tagging

**Flavor Tagging**

## Number of events expected

## Sin(2  Expectations for 2 fb-1

**D********:**
**(S/B ~ 0.75)**
** ******** *****D*****2**** ****~ 9.8 %**
**Time resolution **
******t **** ~ 100 fs**

- For a ***time dependent*** analysis:

## Run II Plan

- No long shutdowns
- Gradual luminosity improvements as we run
- Run until LHC results tell us to stop
- 3 (or more) fb-1 per year at peak

- L (fb-1)

- Number of
- B     J/ Ks

-  (sin2)

- 2

- 5

- 10

- 20

- 40 K

- 100 K

- 200 K

- 400 K

- 0.04

- 0.03

- 0.02

- 0.01

## BS Mixing at DØ

- Use fully reconstructed events
*B**S  **D**S***100 - 300 events
*B**S  **D**S*******300 - 900 events
*D**S  *****
- final flavor tagged by charge of the *D**S*
- hadronic modes require lepton from opposite *b* for trigger - sign gives initial flavor
*B**S  **J*********300 - 1000 events
- easier trigger
- initial flavor tag*****D*2 ~ 7 %
- final flavor tagged by charge of *K *in          *           ****** ******  -  ****D*2 ~ 90 %

- Expect to measure xs ~ 30 in 2 fb-1

## Semileptonic Decays of Bs

- Trigger on dilepton event
-  Initial flavor tagged by opposite side lepton
- Final flavor tagged by same side lepton
- Expect 40,000 events in 2 fb-1
- Useful for  xs < 30 (momentum uncertain due to missing )

- Other B  semileptonic

## B  + - at DØ

- We must trigger on a lepton from the other B in the event
- require: 1 lepton pT > 3 GeV and two other tracks with pT > 1.5 GeV
- total detection efficiency  is between 0.25 - 0.5 %
- expected tagged events:
- Bd  + -	300 - 600
- Bs  + -	650 - 1300
- Bd  + -	1300 - 2600
- Bs   +	150 - 300

## Rare decays: Bd  + - *(892)

- FCNC process **B****0**** ******** ********+**** ********-**** *********** is forbidden at tree level
- SM branching fraction: BR(**B****0**** ******** ********+**** ********-**** *********** ) = (1-1.5) x 10-6
- Current CDF limit:  BR(**B****0**** ******** ********+**** ********-**** *********** ) < 4 x 10-6   (90% CL)
- Non SM physics will change the branching ratio and decay asymmetry

-  Trigger on muons
-   Find separated vertex with ******+**** ********-**** ***********
-   Cut out resonances
-   Will get about 700 events for 2 fb-1

-     Briefly the method used in the search is:

## Rare decays: b  + - s

- Theoretically the most interesting but experimentally very difficult.
- Very large backgrounds.
- Access to only a small M window
- End of M spectrum can not be accurately predicted

-   Briefly the method used in the search is:

-   Events generation, trigger and cuts similar to **B****d ******** ********+**** ********-**** ************* **
-   Look inside a small M window
-   Will get about 1000 events for 2 fb-1   (BR(SM)=6 x 10-6)
-   Expect 10,000 to 100,000 background events

- Small S/B ~1:100                       It will be very difficult

## Rare decays: Bs  + -

| PT() GeV/c
PT() GeV/c | >2.0
>1.5 | > 5.0
> 1.5 | > 5.0
> 3.0 |
| --- | --- | --- | --- |
| Trigg. Eff. (%) | 32 | 55 | 60 |
| Recorded | 15 | 6 | 3 |
| Secondary vtx separation of 100 m | 6 | 3 | 1.5 |
| Impact parameter significance > 2 |   |   |   |

-   Assuming: BR( Bs  + - ) = 4 x 10-9  and 2 fb-1
-   Bs cone isolation cut: no tracks inside a cone of 0.7 around the muon
-   I = pT(B)/pT(                        ) > 0.6  (this cut is 90% efficient)

- Looks almost hopeless for SM branching ratio.
- Signal could be sign of non SM physics

## Bc ProductionBc J/ l 

- M**** within 2 of the J/ mass.
- PT(J/ *l*) > 8 GeV/c.
- Decay length greater than 50 m.

- 2 pb-1   and  CDF’s

- Leads to an estimate of approximately 600 events

- This sample is large enough to make improvements
- in the lifetime and mass measurements that would
- significantly increase the understanding of the Bc system

## b Lifetime 

- Naive spectator model predicts
- Current data suggests
- Non-spectator processes (final state quark interference and W boson exchange) too small in SM (10%) to account for deviation
- Previous measurements used semileptonic modes to achieve statistical precision
- Disadvantage is that neutrino is lost, carrying with it momentum information

## b Lifetime Measurement

- 20 times the statistics during Run II
- Fully hadronic mode bJ/ 0
- BR(bJ/ 0)=5x10-4
- Trigger on J/ ****** ****+******** ****-** dilepton
- Reconstruct 0p in J/ events
- expect 15 000 events
- No need to rely on Monte Carlo for momentum correction

## b Lifetime Projections

- bJ/
- p() points to +- vertex
- mass/vertex constraints

- b mass resolution 16 MeV/c2
- b lifetime resolution 0.11 ps

## Current Status and Plans

- Silicon system installed and working(Secondary vertices found)
- Muon system working well (Much lower noise due to shielding)
- Calorimeter performing well (Jets and electrons have been seen)
- During shutdown in October install electronics for Fiber Tracker
- After shutdown, begin to take high quality physics data

## Conclusions

- DØ is ready to make significant b physics measurements in Run II. We will begin to take high quality physics data soon and hope in 2 years (2 fb-1) to have:
- continue QCD studies
- measure sin2** to an accuracy of 0.04
- measure (or put a limit on) Bs mixing up to xs ~ 30
- put limits on other CKM angles **, **
- measure ***b* lifetime in exclusive decays
- rare decays, heavy baryons, .......