# Presentation: 107534

## Gauge Boson Production at the Tevatron

- Recent Results
- V. Daniel Elvira
- (Fermi National Accelerator Laboratory)

-                           **Outline     **
-  The Differential W Cross Section versus pT
-  W & Z Differential Cross Section Ratio
-  Direct Photon Differential Cross sections

**Notes:**
I am going to present results on W and Z production at the Tevatron
I will start telling you why it is important to study Ws and Zs, followed by a description of how we did it at D0 during Run 1.
 I will concentrate in 4 new D0 results, two published in early 2000, and two just submitted for publication (papers 99 and 100).

## Introduction to W & Z Production at the Tevatron

-  Production dominated by qqbarannihilation
-  Due to very large pp JJ production, need to use leptonic decays
** **W** ************l******** (BR ~ 11% per mode)**
** **Z** ************ll (BR ~ 3% per mode)**
-  Distinctive event signatures
** ****High P****T**** isolated leptons (e or ******
** ****One high P****T**** lepton + Missing  E****T ****(W)**
** ****Two high P****T ****leptons (Z)**
-  Large samples, low backgrounds
-  Well understood EW vertex
-                Test QCD

**Introduction to W & Z Production at the Tevatron**

- O(s0)

- W Bosons Detected

**Notes:**
How do we produce W and Z at the Tevatron?

The production is dominated by qqbar annihilation.
Most of the W and Zs decay hadronically. Unfortunately, that decay mode gets buried by the much larger dijet production. So, when we say we do W and Z physics at hadron colliders, we refer to their leptonic decays (e or mu).

## QCD Corrections: O(s )

**W**

*g*

*q*

*q’*

*q*

*q*

*q’*

*q’*

**W**

**W**

*g*

- Modifications due to QCD corrections:

-  Boson produced with transverse momentum  ( < PT > ~ 10 GeV )
-  Boson + jet events possible  ( W + 1 jet ~ 7%, ETjet > 25 GeV )
-  Inclusive cross sections larger (K factor ~ 18%)
-  Boson decay angular distribution modified

- O(s)

**Notes:**
What does QCD do for us?

In the first place W and Zs are no longer produced at rest, but with an average transverse momentum of 10 GeV.

Furthermore, the bosons can be produced with jets (roughly 10% of W events have 1 jet with Et>25GeV).

...

## D0 W and Z candidate samples

- Electron Identification
-  within good fiducial region of detector
-  highly EM Object
-  Longitudinal & Transversal Shower Profile
-  Isolated from Jet activity
-  Central Detector track   Tight electron
- W eevent selection
-  One tight electron, ET>25 GeV
-  Missing ET>25 GeV
-  No second Loose electron with ET>25 GeV
- Z ee event selection
-  One Tight and one Loose electron, both with ET>25 GeV
-  76 GeV < Mee < 106 GeV

- Loose electron

- D0 W e

- D0 Z ee

**D0 W and Z candidate samples**

**Notes:**
Results use data collected by D0 during 94-96 running period.

## Introduction to W, Z PT Theory

**Notes:**
Error on W mass ~5-30 Mev from Pt model

High Pt: neutrino, jets  signatures for new physics
              W+jets, background to top and Higgs

Phenomenological models also used for the Higgs pt calculation.

## D0 W PT measurement

**D0 W P****T**** measurement**
- Accepted by PLB, hep-ex/0010026

- versus pT

- Measurement corrected for acceptance and detector effects using an iterative unfolding method

- Previous measurements by UA1, UA2, CDF (Run 0) and D0 (Run 1A).

## D0 W PT measurement

- Accepted by PLB, hep-ex/0010026

## D0 W & Z PT Ratio Measurement

- We define the ratio of differential cross sections for the scaled W & Z boson transverse momentum, pTW/MW and pTZ/MZ

- _Experiment:_  the 4.3% luminosity error (dominant in both the WpT &                          ZpT measurements)  cancel in the ratio
- _Theory:_  radiative corrections cancel         pure perturbative calculation

- pQCD prediction (PRD 57, 4433, 1998)

## D0 W & Z PT Ratio Measurement

**D0 W & Z P****T**** Ratio Measurement**

- Good agreement with pQCD  (************18.7/ 21)

- Solid line: pQCD calculation from Phys. Rev. D 57, 4433 (1998)

- Dash band: uncertainty in pQCD from MC integration

- Circles: data ratio

## D0 W & Z PT Ratio Measurement

- Use method in  Phys. Rev. D 57, 4433 (1998) to predict W boson transverse momentum from measured Z boson pT and pQCD Ratio:

- The method reduces theoretical uncertainties in the W**pT**  with respect to the standard method:
-  Error now dominated by limited number of Z events (higher   stats expected in Run II)

- with no need to introduce resummation calculations and non-perturbative parameters at low pT

## D0 W & Z PT Ratio Measurement

**D0 W & Z P****T**** Ratio Measurement**

- Points (bars): WpT D0 measurement (error)

- Predicted WpT agrees well with D0 measurement         ( K-S probab  =1)

- Histograms: 68% CL limits in the prediction

- Method in PRD 57, 4433 (1998) should  lead to smaller errors in
- W mass and width compared to current methods (using high stat Z samples in Run II)

## Ratio of Direct Photon Cross Sections at 630 and 1800 GeV

- NLO diagrams contributing to  production

- Probe pQCD without the complications from jet identification and hadronization
- Previous D0 and CDF (1.8 TeV) results reported excess of g production below ~30 GeV

-  Gluon radiation (not included in NLO-QCD), pdf knowledge

- Ratio of dimensionless cross section:
- ET4 E d3/d3p versus xT=2ET/sqrt(s)
- At 630 and 1800 GeV reduces systematic uncertainties and pdf sensitivity of the theory

**Notes:**
Shown here are the L and NL order diagrams for gamma production at the Tevatron. This plot shows the ratio plot (data -NLO theory)/theory for gamma production x-sec vs gamma x_T as measured by various experiments. As you can see, all the measurements show discrepancies between data and NLO QCD.

I’m going to present photon measurements by two Fermilab experiments: CDF and E706, and a new measurement by D0, based on 110 inverse pb of data.

## Photon Cross Sections

- QCD prediction is NLO-QCD (PRD 42, 61 (1990), Nucl. Phys. B453 334 (1995): open triangles (cteq5m, =ETmax)

**Photon Cross Sections**

- 630 GeV Cross Sections

- Ratios 630/1800

- of 11 (4.55)/7 gives 12.2% (71.4%) agreement probability in CC (EC)

- of 6.5 (3)/7 gives 49% (89%) agreement probability in CC (EC)

- Satisfactory agreement over the whole range

**Notes:**
Shown here are the measured differential cross sections for isolated photon production for D0 (left) and CDF right. The D0 measurement is based on
110 inverse pb of data. The photon pseudorapidity is either central of forward.
The curves show the NLO QCD calculated cross sections from Baer, Ohnemus and Owens.

## Summary and Prospects

- Run II has started & we expect to collect 2fb-1  during 2001-2003

-  New results in good/satisfactory agreement with theoretical predictions
-  The study of the W, Z ,  production mechanism is important:        - as a direct test of the Standard Model                                              - to improve understanding of background in top, Higgs production             - to control systematics in precision measurements such as the W mass

- Much larger number of W, Z, (~factor 20 with respect to Run I):
-  reduce systematics in the WpT, ZpT measurements as well as the predictions
-  Reduce uncertainties in  cross sections (dominant error was  purity in Run I), extend pT reach from ~100 GeV to ~<300 GeV, understand photon production mechanism at low pT