# Presentation: 669716

## W Mass and Width at the Tevatron

- Sarah Eno, University of Maryland
- For the DØ Collaboration

**Notes:**
Hi! My name is Sarah Eno, and I going to present the results of a direct measurement W width and an update to the measurement of the W mass by the D0 collaboration.

## W Mass and Width at the Tevatron

- A new measurement of the W mass using “edge” electrons from DØ
- A new “direct” measurement of the W width from DØ

- New Results from the Tevatron Run I (1994-1995) Data

**Notes:**
These measurements were done using data taken taking in 1994 and 1995 at the Tevatron.  The W mass is updated to include electrons in a region of the calorimeter that was not used in previous analyses, and the W width measurement is the first direct measurement of this quantity from D0.

## Run I DØ Detector

- Analyses based on 85 pb-1 taken in 1994-1995.

**Notes:**
Here is a picture of the Run I Dector and because the calorimeter is the star of these analyses, I include a little picture here.  The integrated luminosity from the 1994 run was about 85 inverse picobarns.

## W Mass

- One of the best ways we have to constraint the Higgs mass before its discovery

- Highest precision measurement from the Tevatron’s Collider.

**Notes:**
The W mass measurement is of course one of the most interesting measurements to come from the Tevatron, both because it is our most precise measurement, and because when combined with measurements of other EWK observables such as the top mass, it can be used to predict the higgs mass because of the relationships between these measurements caused by diagrams like these.

## Previous Measurements

- Direct
-  80.433 0.079 (CDF)
-  80.447 0.042 (LEP2)
-  80.482 0.091 (DØ)
- Indirect
-  80.136  0.084 (NuTeV)
-  80.380  0.023 (LEP1/SLD/Tevatron top mass)

- For DØ measurement, W statistical error alone is 60 MeV.  Most systematics are also statistics limited.

- LEP EWK working group  Web page, 22 Jul 02

**Notes:**
There exist precise direct measurements of the “W mass from the Tevatron experiments and from LEP II, and also the W mass can be inferred indirectly from measurements from LEP1/SLD and the Tevatron top mass measurement.  All the current measurements are in good agreement.  The uncertainty on the Tevatron measurements still have a large contribution from the statistical uncertainty due to the size of the W sample and also of the size of the Z sample used to calibrate and understand the detector resolutions.

## DØ Calorimeter

- DØ Calorimeter

- most DØ exclude electrons 1.8 cm from module edges (10% of width)
- increase W statistics by 9%
- increase Z statistics by 15% and thus knowledge of energy scale (the leading systematic uncertainty)

- Moliere radius is 1.9 cm

**EM**

**CH**

**FH**

**Cryostat wall**

**Notes:**
So, it is still worth while to try to add more statistics to the samples by using electrons that are regions of the calorimeter where you have larger systematic uncertainties on the detector response.  The D0 central calorimeter, which covers the rapidity region eta<1.1, is made of Uranium and liquid Argon, and is divided into 32 modules in phi.  Near the edge of the modules, the response of the detector to electrons can be less uniform, because the structure of the calorimeter is less uniform.  Most previous analyses did not use electrons within 1.8 cm of the module boundaries.  By including them, we can increase the statistics on the W sample by 9% and the Z by 15%.

## Selection

- C is central, non-edge
- E is endcap
-    is central, edge

**Notes:**
This table shows the cuts that are used to select the sample used in this analysis, and the size of the sample for central, endcap, and these new “crack” electrons.

## Basic Methodology 

- common to all DØ W mass measurements

- Fast Monte Carlo with parameterizations of the electron, MET response used to produce distributions of measured PT(e), PT(), MT as a function of M(W)
- smearing functions determined mostly from Ze+e- data (and other data)
- Mass and statistical error are determined from a binned maximum likelihood fit

**Notes:**
To determine the mass, we use a fast Monte Carlo with parameterizations of the electron, and neutrino response to produce distributions of the measured transverse momentum of the electron, neutrino, and the transverse mass.  These smearing functions are mostly determined from data, and especially from the Z data.  The mass is then determined using a binned maximum likelihood fit between the data and Monte Carlo.

## Parameterization of Resolution

- Key to using these “edge” electrons is understanding their response function

- Z’s with an edge electron look like those without for most of the events.  However, some fraction produce a low edge tail

- Normalized at peak

- Difference between above two histograms

- Points: Z’s with 2 central e’s, one edge.  Histogram: Z’s without edge

**Notes:**
The key to using these “edge” electrons as part of the measurement is understanding their response in the detector.  The top figure here shows the Z mass from events with 2 central electrons, and those with 1 central and 1 edge electron.  You can see that the central-edge sample looks like the central-central sample, except with a low edge tail.  If you take the difference between these two distributions, you can see a sort of low, wide Z peak.

## Response

- Model: shower process is unaffected by module edge.  But, for some fraction f, the electronic response is lowered due to smaller electric drift field (and thus resolution is also worsened)

- Predicted response to 40 GeV electrons from this model

**Notes:**
A model which reproduced this distribution is to assume these edge electrons have the same response as the central electrons some fraction of the time, and a low response with worse resolution the rest of the time.  This could occur, for example, if the response is lowered due to the nonuniformity of the electric field in the Argon near the edges of the calorimeter.  By fitting the Z peak, we find the fraction with “normal” response is about 35%.  For the other, we find the response is consistent with having a much larger constant term in the resolution function and a response that is a few percent lower.  This plot here shows the response to to 40 GeV electrons that we obtain from this model.  

## Systematic Tests

- Z’s with one edge, one endcap electron

- Z’s with two edge electrons

**Notes:**
To check out model, we see if it can correctly predict the Z mass distribution for other kinds of Z’s containing an edge electron, like endcap-edge Z’s and Z’s with 2 edge electrons.

## Results using only the edge electrons

- MW=80.5740.405 GeV

- Uncertainty dominated by statistics and uncertainty in constant term in resolution and in energy scale (all around 230 MeV)

**Notes:**
Here is the transverse mass distribution of W’s with edge electrons, along with the distribution from the fast MC for the best fit mass.  Fitting this distribution, the transverse momentum of the electron, and the transverse momentum of the neutrino, we get xxx.  The uncertainty has 3 approximately equal major contributions: the statistical error, the uncertainty in the edge response, and the uncertainty in the edge resolution.

## Systematic Tests

- Fitted W mass as function of distance from the crack

## Combined Analysis

- Use Z’s with edge electrons to improve understanding of scale in non-edge regions
-    80.4820.091
- 80.481 0.085
- Add edge electron W’s
- 80.482 0.084
- Combine with CDF, LEP2
-  80.4500.034

- Numbers from LEP EWK working group web page, 22 Jul 02

**Notes:**
We can also use Z’s containing these edge electrons to understand better the detector response for other kinds of electrons, and for the neutrino.  Especially, our understanding of the response of the central and endcap electrons improves.  When we use this new parameterizations on the central and endcap W sample, our error improves from 91 to 85 MeV.  When in addition we add the edge W’s, the error improves to 84 MeV.  When we combine this with CDF, we get a combined direct W mass of 80.451+- 0.032.  This figure shows that this is in reasonable agreement with the that inferred indirectly from other EWK measurements.

## Higgs Mass

- LEP EWK working group web page

## W Width

- Standard Model Prediction depends on:
- Number of decay modes available to the W
- coupling of W to the EWK doublets
-  EWK corrections to these couplings
-  QCD corrections to these couplings
-  W mass
- (W) = 2.09210.0025 GeV (0.12% uncertainty!)

**Notes:**
The second measurement is a direct measurement of the W width.  This quanitity can predicted with a .12% accuracy, and depends on 

## Direct Measurement

- Shape of transverse mass distribution for mT90 GeV is affected by the W width

**Notes:**
The value of the W width affects the W transverse mass distribution, especially for transverse masses above 90 GeV.

## Selection

**Notes:**
Here are the W selection criteria we use for this analysis.

## Method

- Basically, same method as W mass.  Maximum likelihood fit to templates generated using a parameterization of the detector response.  Fit range is 90 GeV < mT < 200 GeV

**Notes:**
The method for determining the width is basically the same as was used to determine the mass.  A fast Monte Carlo is used to generate transverse mass distributions for different W widths, and fit to the data using a binned maximum likelihood fit.

## Results

**Notes:**
The result is in good agreement with the standard model prediction.  The uncertainty is currently dominated by the statistical uncertainty.

## Results

- Combine with LEP2 and CDF “direct” results

- Numbers from FERMILAB-FN-716
- May ‘02

**Notes:**
CDF and the LEP2 experiments have also done direct measurements of the width.  When we combine these with this measurement, we obtain xxx, in good agreement with the prediction of the standard model.

## (Wl)

- In SM, depends on:
- Couplings of W to EWK doublets
-  EWK corrections 1/2%
- (We) = 226.50.3 MeV (0.13%)

- Can get this from our (W) measurement along with

## (We)

- In SM, depends on:
- Couplings of W to EWK doublets
-  EWK corrections 1/2%
- (We) = 226.50.3 MeV (0.13%)

- From theory.  Depends on quark couplings to W/Z and QCD corrections.  Many uncertainties cancel in ratio

**Notes:**
We can also use this measurement to get an indirect determination of the partial width of W to electrons.  The ratio of the cross section for W to e nu to Z to ee, which has already been measured at the Tevatron, is related to this partial width thru this expression.  To extract the partial width, we also need the branching fraction of Z->ee, which has been precisely measured at LEPI, the W width, which we have just measured, and the ratio of the production cross section for W’s and Z’s at the Tevatron.  This we get from theory, and this ratio has uncertainties due to uncertainties on the quark couplings to the W and Z, and QCD corrections to the production diagrams.  Many of these uncertainties, though, cancel in the ratio.
Our indirect measurement is in good agreement with the predictions of the standard model. 

Anybody know why this has the same % error as the total width, even though the total width also has QCD corrections

## Summary

- New Measurements from DØ
- M(W)=80.5740.405 GeV
- New Combined DØ Results
- M(W)=80.483 0.084 GeV
- New Tevatron Results
- M(W)=80.456 0.059 GeV
- (W)=21.60 0.047 GeV

- _New World Results_
- M(W)=80.451 0.032 GeV
- (W)=2.158 0.042 GeV