# Presentation: 395989

## Sensitivity to PDFs at the Tevatron.

- Mathieu Agelou
- CEA – Saclay
- Low x workshop, Sinaia, Romania

## Outline

- The Tevatron and the detectors (CDF and DØ)
- Top results :
- tt cross-section
- Single top
- Z+jets events
- W charge asymmetry

## Acceleration complex of Fermilab

-      collider
- Two experiments : DØ , CDF
- Run II since mars 2001

## DØ and CDF detectors

-  Silicium tracker, || < 3
-   Fiber tracker, |tck| < 1.6
-  Solenoidal magnet 2 T
-  Calorimeter : U-LAr
-    EM ~ 15%/E,  HAD ~ 80%/E
-  Muons (scint. + drift chambers) || < 2
-   & toroidal magnet 1.8T

-  Silicium tracker, || < 2, R < 30 cm
-   Drift chambers |tck| < 1
-  Calorimeter : scint. tiles + Pb/Fe (EM/HAD)
-   EM ~ 13.5%/E,  HAD ~ 80%/E
-  Time-of-Flight
-  Solenoidal magnet 1.4 T
-  Muons : || < 1.5, 80% in 

- = 1

- = 0

- z

****

## Luminosity

- 4.1031 cm-2s-1

- Jan 02

- Jan 04

- 1032 cm-2s-1

- Numerous runs above 1032cm-2s-1

- Analysis presented here : L up to 350 pb-1

- Efficiency of data taking for
- the 2 experiments > 85%

- Integrated luminosity

-   Peak luminosity

- Jun 05

## top quark production at the Tevatron

- 2 production processes :
- Via the strong interaction
- Via the electroweak interaction
- Strong interaction : pair production

- 85%        annihilation
- 15% gluon fusion

-        (reverse at LHC)

- World only top sample :
-    - measurement of its mass (Jiri’s talk)
-    - studies of its properties :
- 	- W helicity
- 	- Rare decays

- Important to measure the cross-section
- in all channels : good cross-checks,
- different systematics.

## Pair production

- _dilepton channel_ : l = e , 
- - small BR (~6%)
-  - small backgrounds (WW, Z->)
-  - 2 high PT leptons (>15 GeV)
-  - 2 jets (>20 GeV)
-  -      > 15 GeV

- _semileptonic channel_ : l = e , 
- - larger BR (~30%)
-  - Backgrounds (W+jets, multijets)
-  - 1 high PT leptons (>20 GeV)
-  - ≥ 3 jets (>15 GeV)
-  -      > 20 GeV

- L = 318 p-1

- L = 230 p-1

- hep-ex/0504058

## Pair production

- _Hadronic channel_ :
- Large BR : ~45%
- Huge multijet background
- 6 jets (>15 GeV)

- Nobs = 220
- Nbck = 186

- Small sensitivity
- Difficult analysis

- Good agreement with
- NLO predictions

## Single top production

- Electroweak interaction :
- Top is produced singly in the final -> *’’single’’ top*

- 2 main processes
- at the TeVatron

-   Theoritical cross section ~0.4 x cross section
-   Very difficult to observe because of the smaller multiplicity than

- a 3rd mode exists but will be visible only at LHC :

-  Window to new phenomena
-  New vector bosons
-  Anormalous couplings
-  RPV SUSY

-  tests of standard model
-  Direct measurment of |Vtb|
-  V-A coupling
-  top polarisation

- |Vtb|

- |Vtb|

-  Background to Higgs
- searches :
-   same final state as WH

## Probing pQCD with single top

- Recently, theorists (esp. Z.Sullivan) think the single top process as a tool to understand perturbative QCD.
- “single top is a new Drell-Yan and DIS process”
- s-channel : ISR and FSR  color independent
- t-channel : DDIS, 2 different scales

**http://www.hep.vanderbilt.edu/~wjohns/fcp05/recent/Zack%20Sullivan-ZSFCP05.pdf**

- Drell-Yan
- ISR only

- e+e- -> jets
- FSR only

- DIS
- Proton structure

## Single top search

- 2) Multivariate
- techniques

- 1) Basic event selection

- 3) Limit extraction

- s < 4.5/6.4 pb
- t  < 5.8/5.0 pb

- exp/obs

## Z+b / Z+jet cross-sections ratio

- Measurement of Z+b jets cross section :
- Important background for Higgs searches in the ZH process.
- Give insight on b PDF

- Z + 1 b

- We measure ratios of cross-sections
- Z + b / Z + jet so that some uncertainties
- cancel out

- Good agreement with NLO
- predictions using CTEQ6
- (*Campbell et.al *
*PRD 074021 (2004) *) :
-          0.018  0.004

**Phys.Rev.Lett.94:161801,2005**

## W charge asymmetry

- At the Tevatron, W are produced mainly via u and d quark :
- ud̅  W+
- du̅  W-
- In average, u quarks in protons (u̅ in p̅) carry higher momentum fraction than d (d̅ in p̅) quarks. This leads to a boost of :
- the W+ in the proton direction
- the W- in the antiproton direction
- Asymmetry is defined like this :
- Since we do not measure the pz of the neutrino, we use the pseudorapidity of the charged lepton :

- Forward-backward charge
- asymmetry

- Input for ratio of u and d PDF

## CDF electron analysis

- 77930 selected W  e events
- Main background : QCD dijet (<2%)

- Gain in sensitivity if split in different ET bins

- Corrected for charge
- misidentification rate,
- Background subtracted

## Conclusion

- Various measurements at the Tevatron can probe/constraint the PDFs
- Top pair cross-sections in agreement with NLO  expectations
- Single top : once discovered, tool to probe pQCD, b PDF
- Ratio Z+b / Z+jet : first test of b PDF -> good agreement
- W charge asymmetry : input for ratio of u and d quark PDF
- The first 1 fb-1 of Run II data has been delivered !
- 				STAY TUNED for new results !!