# Presentation: 875491

## Recent Run II Electroweak and QCD Results from DØ

**Robert Kehoe**
**Michigan State University**
**for the D****Ø**** collaboration**
**3/12/03**
**Les Recontres de Physique de la Vall****é****e d'Aoste**
**Measurement of the Dijet Mass Cross Section**
**Measurement of the Z      ********  *********BR**
**Search for Z' in Dielectron Decays**

## DØ at the Tevatron

-       a multipurpose collider detector:
**Tracking**
- silicon vertex detector
- fiber tracker
**Calorimetry**
- preshowers
- U/LAr calorimeters
**Muon**
- drift tubes and scint.

**Pipelined 3-level trigger**
**     ****current L1 trigger rate****	****	****1 kHz**
**     ”” ****L2 trigger rate****	****	****	****0.6 kHz**
**     ”” ****L3 trigger rate****	****	****	****50 Hz**
**current operating efficiency: ****	****80% - 85%**

## New Tracking for DØ

- Silicon Microstrip Tracker	(SMT)
- 840k channels
- 4 layer barrels - 3D tracking
- detached vertex detection
- b-tagging
- triggering
- fully commissioned
-                                                       2T superconducting solenoid
-                                                         momentum, charge

## New Tracking for DØ: CFT

- Central Fiber Tracker
- 77k channels with VLPC readout
- first sci-fi/VLPC tracker at collider
- VLPC's
- 9K operating temp.
- 85% QE, excellent S/N
- 8 layers fiber doublets
- tracking trigger
- fully operational
- E/p measurement
- improved calibration

## Forward Proton Detector

- beam

- beam

- x1

- xp16

- v1

- vp20

- up1

- u20

- 	Scintillating fiber detectors in Roman pots near beam used to tag protons and anti-protons

**Proton Detector 1**

**Proton Detector 2**

**EVENT**

******p/p) distribution for **
**a sample of  clean elastic events**

**Notes:**
The top part of the slide shows an event from a Forward Proton Detector standalone run in January. The trigger is the coincidence of trigger scintillators in the anti-proton up and proton down spectrometers, each of which consist of two detectors.  There was also a veto on the luminosity monitor counters in order to provide a clean elastic event. The  pictures are face on views of the active area of the two proton down detectors comprising the proton down spectrometer and shows the fiber segments that were turned on.

The bottom figure shows the xi (delta p/p) distribution from this elastic run.  This data was used to align the detectors and results in the expected xi peak at zero.

## Dijet Mass Cross Section

- probe of
- QCD
- hunting for resonances
- proton structure at large *x*
*quark compositeness*
*data sample:**	**	**	**	**	**34.1 pb**-1*
*     **/ P**Tj1** < 0.7*
*primary vertex: |z**vtx**| < 50 cm, N**trks** > 4*
*selection & sample definitions*
***R = 0.7 cone jets*
*|****jet**| < 0.5*
*N**jet** > 1*
*calculate invariant mass of leading two jets*

**twice the ******** at 1.96 TeV**

- Mjj = 838 GeV

## Jet Energy Scale

- methods currently used
- O: underlying event, noise
- minimum bias events
- R: non-linearities, dead material
- direct photon candidate events
- statistics up to 200 GeV energy
- S: particle showers
- jet transverse shapes in data
- errors
- large statistical errors
- substantial systematic errors
- increase with energy due to extrapolation

- Ecorr = (Euncorr - O)/ R*S

## Trigger Selection

- four triggers utilized
- _L3 E__T__ threshold__	__offline M__jj__ cut_
**25 GeV****	****	****	****	****150 GeV**
**45 GeV****	****	****	****	****180 GeV**
**65 GeV****	****	****	****	****300 GeV**
**95 GeV****	****	****	****	****390 GeV**

## Energy Resolutions

- use dijet events in data
- same sample as Mjj analysis
- asymmetry measurement
- corrected for unfound third jets
- particle jet resolution corrected for
- use to determine dijet mass resolution
- smear PYTHIA events in mass bins
- gaussian fit to Mjj/ Mjj in each bin
- fit to
- determine unsmearing correction

- (stat. only)

*A*

## Observed Cross Section

- calculated by
- cut efficiencies
- estimated from data
- vertex quality: 		~78%
- jet quality: 			~97%
- cross section
- with total error
- luminosity error
- additional 10%
- fully correlated bin-to-bin and not shown
- compare to NLO theory
- CTEQ6 pdf
- Rsep = 1.3

## Comparison to Theory

- agrees within uncertainties
- main uncert.: jet energy scale, PT resolution, jet quality
- dominated by jet energy scale
- 150 to 160 GeV: 	+52% 	- 38%
- 800 to 1400 GeV: 	+190% 	- 73%

**10% luminosity uncertainty not included**

## Z     *BR

- Z     *BR

- data sample:
- 31.8 pb-1
- selection
- || < 1.8
- pair of oppositely charged muons
- PT > 15 GeV (central track required)
- DR > 2.0
- require one be isolated in calorimeter AND tracker
- timing cut to remove cosmics
- di-muon trigger
- efficiency calculated from data
- 1585 events pass cuts

******tag**

******probe**

**L1+L2,**
**central track**

**L1 OR no L1**

**track OR no track**

- Z

## Dimuon Backgrounds

- cosmics negligible
- heavy flavor (     )
- compare dimuon events
- two isolated muons
- one isolated muon
- two samples agree well
- < 1% non-isolated muons
- ∴      1% ± 1% BG
- 
- Drell-Yan
- PYTHIA + fast detector simulation
- Z and Z/*
- muon resolution tuned to data
- correction factor = NZ/NZ,

- Z()

## Measured *BR(Z   )

- calculation of efficiency
-        contributions
- contributes most to error on cross section
- components: 										_effic.__	__error_
- Monte Carlo										0.403	0.012
- Level 1 muon									0.912	0.017
- loose muon identification							0.909	0.01
- track efficiency									0.822	0.014

******Br = 263.8 ****±**** 6.6 (stat) ****±**** 17.3 (sys) ****±**** 26.4 (lum) pb**

## New result!

## Z'    ee Search

- expected in many models, including left-right symmetric models
- data sample
- 50 pb-1
- selection
- single electron trigger
- || < 2.5
- ET > 25 GeV
*electron identification*
*isolation, energy fraction in electromagnetic layers*
- shower shape
- studied using GEANT
- MZ' 300 thru 800 GeV with PYTHIA
- PT dependent shower shape cut

## Backgrounds

- Z + Drell-Yan
- use fast detector simulation
- resolutions tuned to data
- PYTHIA+CTEQ4L
- correct LO cross section vs Mee
- Hamburg, van Neerven, Matsura, Nucl. Phys. B359, 343 (1991)
- QCD fake background
- mainly dijets to leading 0's
- dominates at high mass
- Mee distribution from data
- normalize Z/* + QCD
- 65 GeV < Mee < 115 GeV

**QCD**

## Data at High Mass

- estimated Z     ee cross section
- consistent with existing measurement

- highest mass central-central
- event:  **M****ee**** = 386 GeV**

**data *****vs*****. expected background**
- 	- agree well at high mass
- 	- no excess observed

- Z'

## Results

- reference model
- couplings to quarks and leptons
- assume same as standard model Z
- top decays available when mZ' permits
- assume Z' width scales with mass
- decay to Z is suppressed
- acceptance
- calculated relative to Z
- PYTHIA
- fast detector simulation
- MZ' > 620 GeV @ 95% c.l.

## Summary

- probing highest mass scales with new energy
- QCD
- dijet mass cross section
- agrees with NLO theory, even at high mass
- inclusive jet cross section very soon
- near-term plan
- substantially reduce jet energy scale errors
- expand use of forward pseudorapidity coverage
- ultimately improve errors and E resolution with tracking
- W/Z production
- Z       dimuon
- Br = 263.8 ± 6.6 (stat) ± 17.3 (sys) ± 26.4 (lum) pb
- W      , and W/Z electron channels well-advanced
- Z'     ee search						MZ' > 620 GeV @ 95% c.l.