# Presentation: 926444

## W Boson production 

- W Boson production

- and properties at CDF

- University College London

- Emily Nurse

- EPS, Manchester, July 2007

## W production at the Tevatron

- The electron and muon channels are used to measure W properties, due to their clean experimental signature.
- The large mass (~80 GeV ) of W  bosons gives their decay products large pT.

- W events can be used to:
-  constrain the QCD part of the production mechanism: W charge asymmetry.
-  measure W properties: W mass (see talk by Oliver Steltzer-Chilton) and W width.

- C.O.M energy = 1.96 TeV

## Detecting W decay particles at CDF

- e

- 

**S**
**I**
**L**
**I**
**C**
**O**
**N**

**DRIFT CHAMBER**

**S**
**O**
**L**
**E**
**N**
**O**
**I**
**D**

**EM**

**HADRONIC**

**MUON**
**CHAMBERS**

**Electrons:** detected in central trackers (p measurement) and  EM calorimeter (E measurement).
**Muons:** detected in central trackers  (p measurement), calorimeter (MIP signal) and muon chambers.
**Neutrinos**** ****:** not detected! Vector sum the total *transverse energy (E**T**=Esin****)* in calorimeters. “Missing ET”
- inferred as neutrino pT.

**TRACKERS**

**CALORIMETERS**

- 

## W charge asymmetry: introduction

- W charge asymmetry: introduction

-  xf(x,Q2)

-  x

-  Q2 = 100 GeV2
- MRST2004NLO

-  u

**p**

**W****+**

**u**

**d**

**u**

**u**

**u**

**d**

**p**

**e****+**

******e**

-  d

- Parton Distribution Functions describe the momentum distribution of partons in the (anti-)proton.
- They are constrained by fits to data.

- Fraction of proton momentum carried by quark

- 10-3         10-2          10-1               1

**<footer>****   W production and properties at CDF**

- (helps constrain PDFs!)

## W charge asymmetry: from e+(-) asymmetry 

-   Since pL**** is not known the W**** rapidity cannot be reconstructed  traditionally the *electron charge asymmetry* is measured.
-   The V-A structure of the W**+(-)** decay favours a backward (forward) e**+(-)** “diluting” the W charge asymmetry.

## W charge asymmetry: direct measurement

- This CDF measurement (performed in We channel):
-         **p****L****** determined by constraining MW = 80.4 GeV two possible yW solutions. Each solution receives a weight probability according to:
**V-A decay structure**
**W cross-section:  ******** (y****W****)**
- (Process iterated since  (yW) depends on asymmetry)

## W width : introduction

- Predicted very precisely within the Standard Model by summing the leptonic and hadronic partial widths: WSM = 2091 ± 2 MeV **[PDG: **J. Phys. G **33, **1]
- Deviations from this prediction suggest non-SM decay modes.
- W  is an input to the W mass measurement: MW ~ W / 7

- W

- Ideally we would reconstruct the invariant mass of the decay products to measure W, since  isn’t detected we reconstruct the transverse mass:

- MW

## W width : Analysis strategy

- W width : Analysis strategy

-  Simulate mT distribution with a dedicated *fast* parameterised MC (using a Breit-Wigner lineshape).
-   MC simulates QCD (RESBOS Balazs et.al. PRD56, 5558) and QED (Berends & Kleiss Berends et.al. ZPhys. C27, 155) corrections.
-  Utilise Zll (and W  l) data  to calibrate the detector to a high precision.
-  Fit mT templates (with **W** varying) to the data,  fit range: 90-200 GeV

- optimised to reduce total uncertainty

## W width : Lepton pT 

-  momentum measured in central tracker - scale and resolution calibrated using Z resonance.

- e energy measured in EM calorimeter - scale and resolution calibrated using Zee resonance and E/p in We data.

## W width : Neutrino pT 

- W width : Neutrino pT

- pT inferred from ETmiss
- U=(Ux, Uy)=towersEsin (cos, sin)
-       vector sum over calorimeter towers
-  Excluding those surrounding lepton
**p****T******** = E****T****miss**** = -(U + p****T****lep****)**
- U mostly comes from gluon radiation from incoming quarks (also Underlying Event (UE), photons from bremsstrahlung).

- Accurate predictions of U (QCD radiation, UE, hadronisation and detector response to hadrons) is difficult (and slow) from first principles.
- Devise an ad-hoc parameterised model of the recoil in terms of the boson pT, tuned to the recoil in Zll events (where the Z pT can be reconstructed).

## W width: results

******W**** = 2032 ******** 71 (stat + syst) MeV**

## W width: systematic uncertainties

- W width: systematic uncertainties

## W width: world average

- Central value decreases by 44 MeV:
**2139 ******** 2095 MeV**

- Uncertainty reduced by 22%:
**60 ******** 47 MeV**

- World’s most precise single measurement!

## Indirect Width Measurement

- R =

- W

- Z

- (Zll)

- (Wl)

- (W)

- (Z)

- X

- X

- Rexp =

-   BR (Wl)

-   BR (Zll)

- Precision LEP Measurements

- SM Calculation

- NNLO Calculation

## Summary

- W charge asymmetry measurement - will help constrain PDFs (due to be updated this summer).
- W width measurement (world’s most precise measurement!) - consistent with the Standard Model prediction and indirect measurement.

## Back-up slides

## e asymmetry

## Hadronic Recoil: U

**U****1**

**U****2**

-  Accurate predictions of U is difficult (and slow) from first principles.
-  U simulated with ad-hoc parameterised model, tuned on Zll data.
-  U split into components parallel (U1) and
-  perpendicular (U2) to Z pT
-  7 parameter model describes the response and resolution in the U1 and U2 directions as a function of the Z pT.
-   Systematic comes from parameter uncertainties (limited Z stats).

- Zee

- Zee

- response

- resolution

- Data
- MC

## Hadronic Recoil: U (pT)

- We

- Hadronic Recoil: U (pT)

**U****||**

** ****U******

-  U split into components parallel (U||) and
-  perpendicular (U) to charged lepton.
-  Many distributions used to cross check the model in Wl data:

- Data
- MC

- We

## Backgrounds

- muon channel only:

-  Jet fakes/contains a lepton
-  ETmiss from misconstruction

- electron *and* muon channel:

**Z********ll**

-  One lepton lost
-  ETmiss from
-   missing lepton

**W****

-  *** *decays to e/
-  intrinsic ETmiss

**Decay In-Flight**

-  Kaon/pion decays “In-Flight”
-    to a .
-  Kink in track gives high-pT
-   measurement.
-  ETmiss from mis-measured
-   track pT.

**Need the m****T**** distributions *****and *****the **
**normalisations!**

## Backgrounds