W Boson Mass and Width Measurements from CDF Run II

1
Weighing Up The Weak Force
W Boson Mass and Width Measurements from CDF Run
II David Waters University College London
and the rest of the UCL CDF group Dan Beecher,
Ilija Bizjak, Mark Lancaster, Sarah Malik, Emily
Nurse, Tom Riddick, Troy Vine
UCL, 30th May 2008
2
Two New Precision Measurements from CDF
Worlds Most Precise Measurement !
W Boson Mass Phys. Rev. Lett. 99, 151801
(2007) Phys. Rev. D. (accepted for publication
2008)
Worlds Most Precise Measurement !
W Boson Width Phys. Rev. Lett. 100, 071801 (2008)
3
Outline

• How Well Do We Know Our Forces ?
• Why Measure the W Mass and Width ?
• The CDF Detector at the Tevatron
• How Do We Make lt1 Precision Measurements ?
• Future Prospects for These Measurements
• First Glimpse at Extended Dataset
• Concluding Thoughts

4
How Well Do We Know Our Forces ?
Electroweak Unification
The symmetry is manifest only if MW MZ M? 0
However the symmetry must be broken since MW
85?mp
Is a Higgs field responsible for this symmetry
breaking ?
5
How Well Do We Know Our Forces ?
Force Carrier Mass Lifetime
EM Photon lt 10-16 eV stable
Strong Gluon lt few MeV stable
Weak NC Z0 91.1876 0.0021 GeV 1/? 2.4952 0.0023 GeV
Weak CC W 80.403 0.029 GeV 1/? 2.137 0.060 GeV
2007
(directly measured)
6
W Mass Playing Precision Catchup

• Electroweak standard model relates precisely
  known parameters and less well known parameters
  through radiative corrections

rearranging
exquisitely well known
what goes into ?
7
W Mass Playing Precision Catchup

• Radiative corrections to MW include those due to
  top and Higgs

• Equivalently, measuring MW and Mtop places
  constraints on the missing piece, MH.

• (LEP EWWG)
• How do MW and Mtop inputs compare ?
• Current top mass precision (amazing !)
• ?(MTOP)?1.4 GeV (0.8)
• Equivalent constraint on MH would come from
• ?(MW)?8 MeV (0.01)
• The most important measurement for us to improve
  now is the W mass !

8
W Mass Triangulating New Physics

• Even after a Higgs discovery at the Tevatron or
  LHC, precision EWK measurements will enable
  powerful Standard Model consistency fits.
• May be possible to distinguish SM from MSSM and
  in general constrain the properties of new
  physics at higher mass scales.

MW, MH,
9
W Width Testing the Standard Model

• The W boson width is comprised of leptonic and
  hadronic partial widths

• Using a precise calculation of
  and assuming
• unitarity (summing over quark flavours),
• no non-Standard Model decay modes of the W

Renton, hep-ph/0206231

• We aim to test this prediction of the Standard
  Model, and make a (small) contribution to the
  global EWK precision data

Equivalent to a 500 MeV MW determination (!)
10
W Width Direct Indirect Measurements

• An important goal is to compare direct and
  indirect width measurements.

DIRECT
INDIRECT
Really measurements of the W leptonic branching
ratio, either directly (LEP-II) or indirectly
(Tevatron)
Literally measure the Breit-Wigner width

• A measurement of far off-shell Ws

Use to test unitarity, extract Vcs etc.
Same answer ???

• A measurement of on-shell Ws

11
The Tevatron
Mode Events/Week/Exp. (before trigger cuts)
50,000
5,000
150
18

• We have a production rate of 1
  Hz at our highest luminosity.

Now operating in precision regime
12

• The Tevatron has delivered gt3.5/fb per
  experiment.
• You will have seen many results based on gt 2/fb.
• Im showing results based on lt 400/pb.
• Yes, were slow . but

EXOTICS
13
The CDF Detector
Drift chamber outer tracker Silicon vertex
detector coverage out to
Central calorimeter Plug calorimeter
coverage out to
Muon chambers coverage out to
14
Measurement Strategy
15
Measurement Strategy

• Fit to MT tail (normalise in peak).
• Focus on
• Resolution tails
• Backgrounds
• Energy momentum scales
• W production modeling

• Fit to MT around the Jacobian peak.
• Focus on
• Energy momentum scales
• W production modeling
• Resolutions
• Backgrounds

16
Measurement Steps

1. Event Selection
2. W Z Production Modeling
3. Determine Momentum Energy Scales
4. Determine Resolutions
5. Measure Backgrounds
6. Fit For the Mass Width

17
Selecting Events

• Careful choice of lepton identification cuts
• Minimal kinematic bias
• Straightforward to simulate
• Tighter cuts for the width since backgrounds are
  more problematic.

18
Measurement Steps

1. Event Selection
2. W Z Production Modeling
3. Determine Momentum Energy Scales
4. Determine Resolutions
5. Measure Backgrounds
6. Fit For the Mass Width

Goals construct as accurate a model of W
production as possible. Determine systematic
uncertainties.
19
W/Z Production Decay Modeling
Corrections Higher orders (EW,QCD) Non-perturb
ative
rapidity distribution
angular mass distributions
20
W Production Modeling pT

• Use the best theoretical model on the market
• RESBOS ? NLO QCD resummation
  non-perturabtive.
• But, we dont trust it blindly ! We constrain
  RESBOS parameters and lineshape using our own Z
  data

Landry et al. (2003)
??W 7 MeV ?MW 3 MeV
21
W Production Modeling pZ

• Different PDFs result in slightly different
  spectra experimental acceptance

• Generate event ensembles using error sets
  provided by PDF fits to world data.

?MW 13 MeV ??W 20 MeV

• Tevatron PDF constraints (mainly from W charge
  asymmetry) will be useful in future.

22
Measurement Overview

1. Event Selection
2. W Z Production Modeling
3. Determine Momentum Energy Scales
4. Determine Resolutions
5. Measure Backgrounds
6. Fit For the Mass Width

Goals measure the momentum scale of the tracker
and the energy scale of the calorimeter to O(1
part per 10,000).
23
Momentum Scale W???

• Start with a detailed cosmic ray internal
  alignment of the COT to 5?m.
• Use precisely known resonances.

Kotwal et al. (2003)
?W
?p/p (J/??Z) 0.00021 ?MW 17 MeV
?p/p (Z) 0.0004 ??W 17 MeV
24
Electron Energy Scale

• How do we precisely determine the electromagnetic
  calorimeter energy scale ?

1
2
Transfer the precise momentum scale to the
calorimeter by fitting the ratio E/p for
electrons.
Extract directly by fitting to precisely known
Z?ee resonance.

• Relatively easy. No tracking.
• Statistically poorer.

• Hard ! Need to understand reconstruction of E and
  p in minute detail.
• Statistically precise.

25
Simulating Electrons (?) Material

• Start from a detailed, tuned material map.
• Very detailed Brem. energy loss treatment.
• Compare E/P tails in data and simulation

No Brem.
Hard Brem.
Determine amount of radiating material to better
than 1
?W
26
Simulating Electrons (??) CAL
Energetic electrons leak into the hadronic
compartment
Soft electrons suffer absorption in the coil
27
Electron Energy Scale

• The results of doing all this

1
2
What does the Z mass come out to be ?
Take scale from E/p fit

• Correct?
• Yes!

• Combine both scale determinations
• ?S/S 0.4 ?MW 30 MeV
• ??W 21 MeV

28
Hadronic Recoil Response

• Calibrate the detector response to the hadrons
  recoiling from the Z pT.

?W
momentum conservation
Z?ee
low pT suppression
linear calorimeter response
??W(e) 38 MeV
?MW(e?) 9 MeV
29
Measurement Overview

1. Event Selection
2. W Z Production Modeling
3. Determine Momentum Energy Scales
4. Determine Resolutions
5. Measure Backgrounds
6. Fit For the Mass Width

Goals determine momentum energy resolutions
as accurately as possible. Non-Gaussian tails are
especially important for the width measurement.
30
Momentum Resolution

• W mass fast simulation is a hit level simulation.
  
• Measure single hit resolution ?h 150 ?m in
  ???? and Z??? events.

• Since ? and Z are produced promptly, also check
  effect of beam constraint.
• The beam is effectively a 39?3 ?m resolution hit
  with long lever arm halves ?(1/P)
• Resolution a very small 3 MeV systematic on MW

31
What About the Tails ?

• Width measurement is additionally concerned with
  non-Gaussian tails

W?e? Tight Calorimeter Cuts

• Tail fraction correct ?
• Fit E/P distribution with cuts designed to reduce
  resolution on E.

• Fit Z??? peak
• Data needs 10?4 more smearing than detailed
  simulation.

??W 26 MeV
32
Electron Energy Resolution
determine ?

• Once again do we measure the same value in Ws
  and Zs ?
• But, careful, there are correlations between Z
  electrons (calibrations, luminosity)

• Consistent ! But vary ? fit over wide range to
  absorb any non-Gaussian tails.

??W 31 MeV
?MW 9 MeV
33
Hadronic Recoil Resolution

• This ones really tough
• Hadronic energy resolution in different parts of
  the calorimeter.
• Effects of underlying event and overlapping
  events.

Tune on Zs and random crossings

• Acid test can we model the W data ?

Good agreement !
??W 40 MeV
?MW 7 MeV
34
Measurement Overview

1. Event Selection
2. W Z Production Modeling
3. Determine Momentum Energy Scales
4. Determine Resolutions
5. Measure Backgrounds
6. Fit For the Mass Width

Goals measure amount of contamination from
backgrounds. Need to know background shapes too.
Always the biggest headache at hadron
colliders !
35
Muon Channel Backgrounds

• Its easy to lose a leg (especially at CDF !)
• But we can estimate this background very reliably.

?W
Ouch ! (for the width)
36
Decay In Flight Background

• A pernicious background very flat in transverse
  mass.
• The handles we have are on track quality ?2 and
  d0

37
Electron Channel Backgrounds
38
QCD Background
QCD template from a background rich
anti-electron sample
39
Measurement Overview

1. Event Selection
2. W Z Production Modelling
3. Determine Momentum Energy Scales
4. Determine Resolutions
5. Measure Backgrounds
6. Fit For the Mass Width

Finally !
40
Results Mass Fits

• Transverse mass fits

Electron MT
Muon MT
MW 80417 ? 48 MeV (stat. syst.) Combination
probability 7
41
Results Mass Fits
Electron ET
MW 80413 ? 48 MeV (stat. syst.) Combination
probability (6 fits) 44
42
W Mass Systematic Uncertainties
43
W Mass World Data
44
W Mass Implications

• A light Higgs is preferred !

Previous World Data
Including New MW
45
W Mass Implications
Including Latest Mtop
March 2008
46
W Mass Prospects
25 MeV MW measurement with data already in the
can ?
47
Results Width Fits

• Transverse mass fits

?W 2032 ? 73 MeV (stat. syst.) Combination
probability 20
48
W Width Systematic Uncertainties
49
W Width World Data
Good agreement with World Average including
INDIRECT 2141 41 MeV
50
W Width Prospects
51
W Mass Full Dataset
NEW !
Z???- mass distribution (for setting p-scale)
52
W Mass Full Dataset
NEW !
Z?ee- mass distribution (for setting E-scale)
53
W Mass Full Dataset
NEW !
W??? transverse mass distribution (for MW fit)
54
W Mass Full Dataset
NEW !
W?e? transverse mass distribution (for MW fit)
55
Precision Measurements _at_ LHC

• Precision measurements of MW etc. will be
  interesting at the LHC.
• More interesting will be to make precision
  measurements of new physics
• New dilepton resonances ?
• New diboson resonances ?

In some models it may be necessary to measure the
mass and width of WZ resonances with high
precision to aid the physics interpretation
Birkedal et al., hep-ph/0412278

• What if the new mass scale is far from
  calibration samples like Zs ?
• Energy loss mechanisms, scale non-linearities,
  etc.
• How to model recoil and reconstruct ET in the LHC
  environment ?
• The role of fast dedicated detector simulations.
• A lot of knowledge can be transferred from
  precision measurements TeV?LHC

56
Summary

• Two new world beating precision measurements from
  CDF

MW 80413 ? 48 MeV
?W 2032 ? 73 MeV

• We know our weak force a little bit better than
  we did a year ago. In fact, these measurements
  add significantly to the global precision EWK
  data.
• The prospects are excellent for further refining
  these measurements.
• Precision measurements of new physics may be
  crucial in understanding what we are seeing at
  the LHC.

57
The End
58
Backup
59
W Width Unitarity Tests

• With the width measured indirectly from the
  branching ratio

• One can go further and use this to constrain the
  least well known CKM element entering the sum

Rept.Prog.Phys.651271-1330,2002
60
R(W/Z) Indirect W Width
SM 3.370 0.024
SM 226.4 0.3 MeV
LEP BR(Z?ll-) 0.033658 0.000023

• Careful propagation of correlated systematics

CDF e?, 72 pb-1 PRL 94, 091803 (2005)
D0 e, 177 pb-1
61
Vcs from R (CDF)
given
world measurements of other CKM matrix
elements.
62
Material Map
63
MW Combination March 08
64
W Mass Full Dataset
NEW !
????- mass distribution (for setting p-scale)