# Presentation: 683503

##  W boson mass and width measurements at the Tevatron

**Sarah Malik**
**University College London**

## W

- r:  O(3%) radiative corrections dominated by *tb* and Higgs loops

- M**H** can be constrained by precisely measuring M**W** and M**top ****:**
-                             M**H **= 85+39-28 GeV  (EWWG)       (MH < 166 (95% CL))

- Derive W mass from precisely measured electroweak quantities

- Measured to 0.002%

- Measured to 0.015%

- Measured to 0.0009%

- W is known very precisely in Standard Model (2MeV)
- New physics could be seen as disagreement between precision measurement and theory.

**M****W**

******W**

## W

-  use leptonic decay modes ;
-  transverse quantities used
-  Use MT to extract both mass and width;

-  Fit in region 90 - 200 GeV
-  Exploit the slower fall off of Breit Wigner lineshape compared to gaussian resolution

-  Fit in region 65 - 90 GeV

******W**

- M**W**

**M****W**

******W**

## W

- Detector Calibration:
- Calibrate recoil
- response
- and resolution

-  Combine information into MT,
-  Add backgrounds to MC templates.
-  Fit for Mass/Width

- Detector Calibration : Calibrate momentum scale and resolution

- Detector calibration :  Calibrate calorimeter energy scale and resolution

- Event Generation: W Production and Decay

**p****T**

**E****T**

**U**

- where pT inferred from missing transverse energy,

**M****W**

******W**

## W

- Use RESBOS.
-  Tune RESBOS parameter g2 to our
-  Z*ll** *data
-   theoretical calculation Z pTWpT.

- Also interfaced with the WGRAD(MW) /Berends Kleiss(W) program which adds upto one photon from the charged lepton.

-  Use a LO generator
- interfaced with the PDF set (CTEQ6M)
-  This produces a W with zero pT.

**+**

**+**

**W p****T**

**QED**

- 

**PDF**

- W

**M****W**

******W**

## W

- e

- 

- Energy loss

- Leakage into hadronic calorimeter

- Bremsstrahlung + conversions

- Shower in electromagnetic calorimeter

**M****W**

******W**

## Momentum Scale and Resolution

- Momentum scale set using:
-  ***J*****/******** ******** data ****(****M*****J*****/******** ****~ 3GeV****)**
-  ****** ****data ****(M******** ****~ 10GeV)**
- Cross -checked using:
-  **Z ******** ****data ****(M****Z**** ~ 91GeV)**

******(1S)**

- Z mass in good agreement with world average value (911882 MeV)

- M(GeV)

- M(GeV)

- Momentum resolution obtained from
- width of distributions.

**M****W**

## Energy scale and resolution

- Fit to the E/p distribution in
-             We data

- At low E/p,  E < p  due to energy leakage into hadronic calorimeter

- At high E/p , p < E due to brehmsstrhalung

- Fit to the invariant mass of the Z in
- Zee data
- provides powerful cross-check

- E

**M****ee****(GeV)**

- p

- Z mass in good agreement with world average  (911882 MeV)

- Mee (GeV)

- E/p (We)

**M****W**

## W

- Recoil defined as vector sum over energy
- in all calorimeter towers excluding
- those containing/neighbouring the lepton(s).
- There are 3 main contributions to the recoil;

-   **QCD **
- Gluon jet recoiling off the boson

-   **Underlying energy**
- Multiple interactions, spectator quark interactions and remnants of the ppbar collision.

-    **Bremsstrahlung****  **
- Photons emitted by lepton that do
- not end up in the excluded region

******W**

## W

-  Pythia/Herwig not accurate enough.
-  ad-hoc parametric model.
-  Model recoil using  Z*ll**  *and minimum-bias      data.

- u1

- u2

- Dominated by resolution of underlying event.

- Dominated by jet resolution

- Zee

- Zee

******W**

## Recoil model for Zs is then applied to Ws.
In W events, recoil is resolved into 2 
directions;
U|| : parallel to pT(lepton)
U : perpendicular to pT(lepton)

- Recoil model for Zs is then applied to Ws.
- In W events, recoil is resolved into 2
- directions;
- U|| : parallel to pT(lepton)
- U : perpendicular to pT(lepton)

- Bias in U|| directly biases MT

- We

**U****||**

**U******

******W**

## W

-  decay in flight(DIF), kaon/pion decaying to  pair.

- Flat in the high MT region.

-  multijet, where one of the jets fakes an electron and the other is mismeasured.

- Electron channel

- Muon channel

-  Backgrounds added to MC templates
-  Electroweak backgrounds taken from full MC
-  QCD backgrounds taken from data.

******W**

## W Mass fits

**M****W**** = 80417 ******** 48 (stat + syst) MeV**

- e +  combination P(2) = 7%

- Include fits to ***p******T******l****** ****** ***and* ****p******T********** *:

**M****W**** = 80413 ******** 48 (stat + syst) MeV**

**M****W**

## W

******W****  = 2032 ± 71(stat + syst) MeV**
**Combination p-value = 20%**

******W**

## W

- MW systematics

- W systematics

**M****W**

******W**

## W

- Reduces uncertainty on world average by 22%
**60 ******** 47MeV**

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

- World’s most precise single measurements!

- Results

**M****W**

******W**

## Electroweak fits

**Summer 2006**

**Including ****New**** CDF M****W**** :**

**Including ****New**** M****top**** :**
*Later this session....*

**M****W**

## Summary

-  MW : **80413 ****± ****48***** *****MeV (stat + syst)**
-  W: **2032 ****± ****71***** *****MeV (stat + syst)**
**Both are the world’s most precise single measurements!**
** **New MW further constrained Higgs mass, lighter Higgs is preferred!
**Mass of Higgs has moved further into directly excluded region**
-  Analyses utilised 200 pb-1 (Mw) and 350 pb-1 (W), both CDF and DØ already have ~2 fb-1 on tape.
** ****Expect improved mass/width measurements to further test the SM and constrain Higgs mass.**