# Presentation: 247789

## Precision Measurements of W and Z BosonProduction at the Tevatron 

- Jonathan Hays
- Northwestern University
- On Behalf of the CDF and DØ Collaborations

- XIII International Workshop on Deep Inelastic Scattering, Madison WI, USA
- 27th April to May 1st 2005

## Outline

- Fermilab, CDF and DØ
- Acceptance and PDF uncertainties
- W and Z production cross-sections

- R - Cross-section ratio measurement
- The future – a new way to measure R
- Summary

## Tevatron, CDF and DØ

**Tevatron**

**Main Injector**

**D-Zero**

**CDF**

**Chicago**

**proton anti-proton collider**
**1.96 TeV, 396 ns bunch spacing**
**Design lumi: 8fb****-1**** by 2008**

- Delivered luminosity ~ 0.8fb-1

- CDF

- DØ

## Motivation

- Precise measurements allow tests of Standard Model predictions

- Ratio of cross-sections give indirect measurement of W width

- Can be used as a “standard candle” to measure or cross-check luminosity

- NNLO @ √s = 1.96 GeV: (Stirling, van Neervan)
- WBr(W→l) = 2687 ± 54 pb
- ZBr(Z→ll) = 251.3 ± 5.0 pb
- R(W Br(W→l) /Z Br(Z→ll) ) = 10.69 ± 0.08

- SM: 3.361±0.024

- LEP

- SM: 226.4±0.3MeV

## PDF Uncertainties

**CDF Preliminary**

-   Generally arise in the estimation of the acceptance for these measurements
-   Involves counting those events you didn’t see
-   Need good Monte-Carlo description of the boson production and decay and detector effects

- DØ uses a brute force approach
- Generate lots of MC for each CTEQ eigenvector set
- Calculate error on observable using prescription in:
-      **J.Pumplin et al, JHEP 310 046 (2003**)

- CDF uses re-weighting technique
- Parameterize acceptance
- Generate large ensemble of MC
- Re-weight events to correspond to different PDF eigenvector sets

## W and Z Boson Production

- Look at the leptonic decays of W and Z

- Hadronic decays too difficult to pick out from the background

- Select W bosons with:
-    single electron/muon triggers
-    standard lepton identification cuts
-    cut on lepton transverse momentum ~20GeV
-    cut on missing transverse momentum  ~20 Gev

- Select Z bosons with:
-    single electron/muon triggers
-    require two reconstructed leptons
-    cut on lepton transverse momentum ~ 20 GeV

## W Production DØ

- WBr(W→e) = 2865.2 ± 8.3(stat) ± 62.8(sys) ± 40.4(pdf) ± 186.2(lum) pb

- ∫L = 177pb-1

- ∫L = 96pb-1

- WBr(W→) = 2989 ± 15(stat) ± 81(sys) ± 194(lum) pb

- For W→e large contributions to systematics in acceptance calculation
- MC-tuning: 1.13%, ID-Eff: 1.43 %, PDFs: 1.41%

- For W→ large contributions to systematics from
- Acceptance (excl PDF): 1.7%, PDFs: 0.9%

- W→e

- W→

## W Production at CDF

- WBr(W→) = 2768 ± 14(stat) ± 60(sys) ± 167(lum) pb

- WBr(W→e) = 2768 ± 14(stat) ± 60(sys) ± 167(lum) pb

******W****(e+********)**** = 2775 ± 10****(stat)**** ± 53****(sys)**** ± 167****(lum)****pb**

- Dominant contributions to systematics from acceptance (PDFs) and the efficiencies

**PRL 94 091803 (2005)**

- ∫L = 72pb-1

- W→e

- W→

## Z Production at DØ

- Z Production at DØ

- ZBr(Z→) = 291 ± 3.0(stat) ± 6.9(sys) ± 18.9(lum) pb

- ZBr(Z→ee) = 264.9 ± 3.9(stat) ± 8.5(sys) ± 5.1(pdf) ± 17.2(lum) pb

- ∫L = 177pb-1

- ∫L = 148pb-1

- Large contributions to systematics from acceptance (PDF 1.7%) and
- understanding detector efficiencies

- Z→

- Z→ee

## Z Production at CDF

**PRL 94 091803 (2005)**

- ZBr(*/Z→) = 248.0 ± 5.9(stat) ± 7.6(sys) ± 14.9(lum) pb

- ZBr(*/Z→ee) = 255.8 ± 3.9(stat) ± 5.5(sys) ± 15.4(lum) pb

******Z****(e+********) = 254.9 ± 3.3****(stat)**** ± 4.6****(sys)**** ± 15.2****(lum) ****pb **

- Dominant contributions to systematics from acceptance (PDFs) and the efficiencies

- ∫L = 72pb-1

- Z→

- Z→ee

- Mass window: 66 ≤MZ≤ 116 GeV

## Cross sections Summary

- Systematics limited measurements ~2-3% level (excl luminosity)
- Dominant contributions from acceptance (large contributions from PDF uncertainties) and efficiencies

- Benchmark analyses for all high pT lepton analyses

## Cross-section Ratios

- Ratio of cross-sections provides an indirect measurement of the W width

- Luminosity essentially cancels in ratio
- Efficiencies and acceptance also cancel to a degree → reduced errors

- R(e) = 10.82 ± 0.16(stat) ± 0.25(syst) ± 0.13(pdf)

**D****Ø**** preliminary**:

**CDF PRL 94 091803 (2005)**

- R(e+) = 10.92 ± 0.15(stat) ± 0.14(syst)

- W = 2.079 ± 0.042 GeV

## Systematics For R(W/Z) at CDF

- K.Copic (UMich), V.Martin, M.Schmitt (Northwestern)
- Presented recently at APS (K.Copic) and joint CTEQ/CDF/DØ W/Z Workshop (D.Waters)
- (http://www.uic.edu/~varelas/wz_workshop.html)

| category | Electrons | muons |
| --- | --- | --- |
| Central value | 10.82 ± 0.16 | 11.12± 0.18 |
| PDF | 0.07 | 0.09 |
| Material | 0.03 | 0.00 |
| Recoil | 0.03 | 0.04 |
| Efficiency | 0.12 | 0.11 |
| Backgrounds | 0.04 | 0.09 |

- acceptance

## A New Way to Measure R at CDF

- Select W and Z events with identical cuts
- Require a single lepton passing trigger and full lepton ID selection
- Fit the transverse momentum (for ) or transverse energy (for e) spectra to determine relative fraction of W or Z in sample

- W+

- Z

- e+

- 

- e+

- e-

## A New Way to Measure R at CDF

- Efficiencies now cancel almost exactly in the ratio
- Construct samples carefully → acceptances very similar for W and Z
- Evaluate PDF uncertainties on the acceptance using 40 CTEQ PDF eigenvector sets and re-weighting method

- CDF Preliminary

## A New Way to Measure R at CDF

- Removing cuts to make selections identical → increased backgrounds

- Cut on hadronic recoil
- softer for W and Z events compared with QCD
- Works well with muons
- For electrons need to be careful to avoid biases
- Need well understood background shapes for template fit

## Acceptance revisited PDF uncertainties

- Only sensitive to the difference in the acceptance for W and Z
- For a typical pair of PDF error sets the difference is very small
- Though for a couple the differences are significant (eg 37,38)
- Estimated systematics from PDFs on R ≤ 0.5%

- CDF Preliminary

- Eigenvector 37

- W

- Z

## A New Way to Measure R at CDF

- Initial studies of the sensitivity estimate with 400-500 pb-1 can achieve similar statistical power to current measurement (72pb-1) but with significantly reduced systematics!

- Trading one set of systematics:
- Efficiency
- PDFs in acceptance
- For:
- statistical errors in the template fit
- Systematics from quality of Monte-Carlo description

## Summary

- WBr(W→e) = 2865.2 ± 8.3(stat) ± 62.8(sys) ± 40.4(pdf) ± 186.2(lum) pb
- WBr(W→) = 2989 ± 15(stat) ± 81(sys) ± 194(lum) pb
- ZBr(Z→ee) = 264.9 ± 3.9(stat) ± 8.5(sys) ± 5.1(pdf) ± 17.2(lum) pb
- ZBr(Z→) = 291 ± 3.0(stat) ± 6.9(sys) ± 18.9(lum) pb
- R(e) = 10.82 ± 0.16(stat) ± 0.25(syst) ± 0.13(pdf)

- DØ preliminary

- Presented results on the precise measurement of W and Z boson production

- W(e+) = 2775 ± 10(stat) ± 53(sys) ± 167(lum)pb
- Z(e+) = 254.9 ± 3.3(stat) ± 4.6(sys) ± 15.2(lum) pb
- R(e+) = 10.92 ± 0.15(stat) ± 0.14(syst)
- W = 2.079 ± 0.042 GeV

- CDF **PRL 94 091803 (2005)**

- Results systematics limited at 2-3% level (+6.5% lumi)

- Dominant systematics come from acceptance and efficiency – includes large contributions from PDF uncertainties

- Which underlying physical aspects of the PDFs contribute to uncertainties?
- Which measurements ( e.g. W charge asymmetry, inclusive jets) at the Tevatron can help?

- Though we may come up with clever ideas to reduce systematics, PDF and other production uncertainties could still play a significant role both in these measurements and other precision measurements such as the W boson mass

- ?