# Presentation: 706365

## Probing QCD withVector Bosons

- Harry Melanson
- Fermilab
- Joint APS/AAPT MeetingIndianapolis, In
- May 5, 1996

## Probing QCD with Vector Bosons...       ... a BIG topic!

- e,m

- g*

- e,m,n

- W*,Z*

- Deep-Inelastic Scattering

- g

- Direct Photon

- g*

- e+e-

- Z

- e

- e

- W, Z

- This is my talk...

## W and Z Productionat Fermilab Collider Recent results fromCDF and D0

- W and Z Cross Sections
- Cross Section Ratio
- W charge asymmetry
- W + jets, Z + jets
- W + 1 jet / W + 0 jet ratio

## W and Z Productionat Fermilab Collider

- G(x)

- +

- +...

**O(****a****s****0****)**

**O(****a****s****1****)**

- W, Z

- q(x)

**O(****a****s****2****)**

- (W/Z + 0 Jet)

- (W/Z + 1 Jet)

- (W/Z + 2 Jet)

- LO: Sensitive to Quark pdf’s
- NLO: Sensitive to Gluon, as
- Scale ® Q2  =  MW2 ~ 6400 GeV2
- Theoretical calculations:
- Total cross section:
- Complete **O(****a****s****2****)** : Hamberg, van Neerven, Matsura, NP B359, 313; van Neervan, Zijlstra, NP B382, 11.
- W/Z + 1 jet:
- NLO: DYRAD MC, W.T. Giele, et al, NP B403, 633 (1993).
- W/Z + n jet:
- LO: VECBOS MC, F.A. Berends, et al, NP B357, 32 (1991).

## Event Samples

- W ® l n    (l = e or m)
- High pT lepton, large ET , mT
- Z ® l+ l-     (l = e or m)
-  Two High pT leptons, mll

## W and Z Cross Sections

**D0 92-93 sample**

## W and Z Cross Sections

- Data:
- D0 (92): S. Abachi, et al., PRL 75, 1456 (1995)
- D0 (94): PRELIMINARY
- CDF (92,e): F. Abe, et al., PRL 76, 3070 (1996)
- CDF (88, m) F. Abe, et al., PRL 69, 28 (1992)
- Theory: *O*(as2)
- Hamberg, et al., NP B359, 313 (1990).
- van Neerven, Zijlstra, NP B382, 11 (1992).

## Cross Section Ratio (GW)

- s(W)/s(Z) from theory, B(Z) from LEP/SLC
- Assume G(W ® ln) from theory
- Extract G(W )
- G(W) = 2.077 ± 0.014 GeV (pre-1B)
- SM:  G(W) = 2.077 ± 0.014 GeV
- GW-GW(SM)  < 109 MeV (95% CL)

- Theory: O(as2)

## W charge asymmetry

- Sensitive to *u(x)/d(x) *ratio for *0.007<x<0.27*
- CP conservation:  *A(y) = -A(-y)*
- Fold distribution

*W* production is asymmetric: xu> xd

- u

- d

- p

- p

- W+

*V-A* decay introduces opposite asymmetry:

- W+

- l+

- n

**Notes:**
figure:  tasym0.eps, theoretical prediction of charge asymmetry

## W Lepton Charge Asymmetry

- CDF: PRL **74**, 850 (1995).
- compared to pre-94/95 pdf’s
- lepton pseudorapidity = h

- CDF
- Run 1A

## W Lepton Charge Asymmetry

- CDF:  Complete ‘92-93, ‘94-95 sample
- W ® e n, m n (central, plug, FMU)
- Theory:  DYRAD MC, Giele et al, NP* ***B405, **663.

## W Lepton Charge Asymmetry

- D0: W ® m n

## W/Z + Jets

- CDF Analysis (106 pb-1)
- Jets:
- R = 0.4
- ET > 15 GeV
- |h| < 2.4

## Z + Jets

- Normalize theory to data - check shapes of distributions

## Z + Jets

## s(W + 1 jet) / s (W + 0 jet)

- W + 0 jet cross section @ NLO:
- W + 1 jet cross section @ NLO:
- Ratio:

- +

- +

**O(****a****s****0****)**

**O(****a****s****1****)**

**O(****a****s****2****)**

- +...

- These are coupled.

## CTEQ3 PDF’s

## s(W + 1 jet) / s (W + 0 jet)

- D0 Analysis
- Sample: ‘94-95 (~83 pb-1)
- W ® e n (+ jets)
- Jets: DR = 0.7, ETmin = 25 GeV
- # Candidates:
- 33.5K W+0 jets, 2.8K W+1 jet
- Backgrounds:
- W + 0 jet: 2%, W + 1 jet: 8.5%
- Multijet, Z ® e e, Drell Yan
- Systematic errors
- Jet energy scale, background subtraction

## s(W + 1 jet) / s (W + 0 jet)

- Theory: DYRAD, Giele, et al., NP B403, 633 (1993).

## s(W + 1 jet) / s (W + 0 jet)

## Slide 21

## Conclusions

- Large samples of W’s and Z’s @ Tevatron being used to test QCD
- Total cross sections:
- Measured to ~ 5 %,
- Ratio measured to ~ 3%
- Good agreement with NNLO PQCD
- W charge asymmetry:
- Used to constrain u(x)/d(x)
- W + n jets, Z + n jets:
- LO QCD + PS/Frag. describes properties of jets
- Underestimates rates
- Ratio s(W+1jet)/ s(W+0jet):
- NLO QCD underestimates ratio