Electroweak Studies at the Tevatron

1
Electroweak Studies at the Tevatron
P. Grannis Prague
Sept. 12, 2001
One of the major puzzles for particle physics is
the origin of the symmetry breaking of the SU(2)
x U(1) electroweak interaction of the Standard
Model. The LEP, SLC and Tevatron experiments
have shown that the SM is correct to very high
accuracy, but have not yet revealed the
underlying character of the symmetry breaking.
The DØ and CDF experiments at the Fermilab
Tevatron discovered the top quark in 1995, and
have made precision measurements of the top and W
boson masses to give sensitive probes of EWSB.
The Tevatron run now underway will give
significant improvements for these measurements
and should allow the discovery of the Higgs boson
up to a mass of about 180 GeV.
2
Electroweak Symmetry Breaking
b w w0 w-

The basic SM interaction Lagrangian contains
massless isovector and isoscalar gauge bosons
that mediate the SU(2) and U(1) parts of the
unified electroweak interaction
Introduction of the complex Higgs doublet f1 ,
f2 yields symmetry breaking, giving mass to the
gauge bosons, absorbing 3 of Higgs fields to form
the longitudinal polarization components of W and
Z.
f1 , f2
g mg 0 W W- mW
80.45 Z0 mZ 91.187

The current data are in excellent agreement with
the SU(2) X U(1) SM and require the higher order
SM EW corrections.
3
But we dont like the SM Electroweak theory!

• The hierarchy problem the Higgs mass gets
  higher order loop corrections that should drive
  it to the Planck scale of O(1017 GeV) unless
  there is very arbitrary fine tuning. (Why are
  the EW and Planck scales so different?)
• The SM does not allow unification of EW and
  Strong interactions the 3 couplings evolve with
  Q2 but do not meet at a common point.
• There are 26 arbitrary parameters in the SM
  that must be set by hand. (Why are these
  as they are?)
• The CP violation in the SM cannot explain the
  baryon-antibaryon asymmetry seen in the universe.
• The SM cannot incorporate gravity with EW and
  Strong interactions
• The SM does not provide dark matter, and
  cannot explain the dark energy that dominates the
  universe.

Continue to improve precision of EW variables
find the Higgs find new non-SM phenomena that
point the way to a more complete theory.
4
Tevatron Measurements
Two of the most sensitive parameters to constrain
the Higgs mass are the top quark and W boson
masses. For example, the W mass depends
quadratically on Mt through virtual tb loops and
logarithmically on MH through Higgs loops.
MW (pa/ 2 GF)1/2 / sinqW (1-Dr) Dr
(Mt/MW)2 ln(MH/MW)
Tevatron experiments have discovered the top
quark and measured its mass Both LEP and
Tevatron experiments have measured the W mass.
5
The Fermilab Tevatron
p p collisions at 1.8 TeV for Run I in 1992
1996 with integrated luminosity Lt 125 pb-1.
Tevatron upgrades include a new pre-accelerator,
Recycler ring, improved p source Run II now
underway at ECM 2 TeV and expected Lt 15
fb-1 by 2007. Two general purpose detectors
CDF
and DØ
both substantially upgraded for Run II.
6
The DØ Detector
Tracking region r lt 75 cm Uranium/LAr
calorimeter Magnetized iron toroid m detector
h
Calorimeter segmentation 4 sections of EM CAL
and 3-4 sections of fine HAD CAL (Dh Df
0.1) , plus backing coarse hadronic CAL
(steel/copper). Hermetic coverage nearly
compensated hadronic and EM response. Calorimeter
unchanged for RunII new electronics for faster
sampling.
7
DØ Tracking detector
Run II tracking detector
Run I no magnet drift chamber tracking with
TRD for electron ID
8
Collaboration photo
600 people, 73 institutions in 18 countries
Argentina Brazil China Colombia Czech
Republic Ecuador France Germany
India Korea Mexico The Netherlands
Poland Russia Sweden United Kingdom
United States Vietnam
9
Top quark measurements
tt production 90 qq annihilation and 10 gg
fusion.
q
t t
t t
g
g
g
q
g

• Top BRs (SM) t ? W b (100)
• with W ? qq (67)
• ? en , mn
  , tn (11 each)
• So final states of tt ( lepton e or m)
• Dilepton bbl l nn 2 jets, 2
  leptons, (low bknd, small signal)
  (BR 5)
• Leptonjets bbqql n 4 jets, lepton,
  (moderate bknd signal) (BR
  30)
• All jets bbqqqq 6 jets
  (large QCD bknd 106 x signal observed!) (BR
  45)
• In all cases, there is an additional hadronic
  system X formed from gluon radiation, and the
  spectator partons usually recoiling at small
  angles from the tt,.

ET
ET
10
Top quark discovery
In 1994, CDF and DØ reported on the top quark
search using about 23 pb-1 of data CDF had an
excess (probability for background 0.26)
corresponding to a tt cross
section of about 2X the SM
expectation DØ with the same
sensitivity saw no significant excess
of events.
PRL 74, 2632 (1995)
In spring 1995, with 50 pb-1, both experiments
published the top quark observation at 5s
significance.
dilepton
lepton jets
DØ observed 17 events with 3.79 0.55 events
background. Introduce new variables for bknd
suppression.
bknd
top
data
Data have character of mix of bknd and signal
Measure s 6.4 2.2 pb and Mt 199 30 GeV
fitted mass
11
Top event selection for mass analysis
Require n jets, m leptons, missing ET ( )
(cuts at 15 20 GeV) n 4 for lepton
jets n 2 for dileptons m 1 for lepton
jets m 2 for dileptons (detailed
differences in cuts for specific
subchannels) for mass analysis, require c2 for
top hypothesis lt 10
ET
gt
gt
gt
gt
Backgrounds to top pair production

• Lepton jets channel
• W? en production with 4 jets from QCD
  radiation. Model with VECBOS MC parton generator
  and fragmentation with HERWIG. There is a real
  W, but the QCD jets tend to have a back-to-back
  tree structure from the radiative evolution.
• QCD multijets where one jet fakes a lepton.
  These are determined from data, using
  anti-electron/muon cuts. These events retain
  much of the parent back-to-back structure of the
  primary parton scattering.
• Dilepton Channels
• For em, Z?tt ? emX and W?mn jets with jet
  faking e
• For ee, W?en jets with jet faking e WW
  pairs
• For mm, Z? mm jets (due to poor dimuon mass
  resolution)

12
Top event sample
About 125 pb-1 Run I data
m
Lepton jets select 77 candidate events.
5 have jets with b-tag (nearby m) predict 51
10 events background
mjet event with b-tag( m )
ET
m
m
Dileptons select 6 events (3 em, 2ee,
1mm) background is 1.45 0.31 events (0.51
em, 0.21ee, 0.73mm)
Striking dilepton event em 2jet
(ET , pT(e), pT(m) all over 100 GeV!
jets
ET
e
Including the all jets channel, these events
give a cross section for t t (at our measured
mass) of 5.87 1.65 pb in good agreement with
NNLO theory.
13
Top mass measurement (leptonjets)
Have 6 final state objects thus in zero mass
approximation, 18 kinematic quantities needed.
Measure 3-momentum of 4 jets and one lepton 2
components of ET. Constraints from equal t and t
masses, and (qq), (l n) must have W mass, so
have 20 known quantities ? 2C fit. BUT Do not in
general identify the b-jets (only for b?mX in DØ
CDF had separated vertex tag in Run I), so have
12 ways to assign the jets to partons (6 if there
is a b-tag). Initial state radiation can
give extra jet this should be absorbed into the
state X. Final state radiation from top or
jets these should be combined with the parent
jet.
true mass
METHOD Perform a 2C fit for all associations
of the jets to the tt hypothesis choose the
smallest c2 solution. Call the best fit mass the
fitted mass. Do the same for (Herwig) MC events
with a family of known true top mass and get
template distributions of fitted mass for each
true mass. Could also use other kinematic
measured quantities (pT(e), pT(b), etc.) but
these not as accurate.
fitted mass
14
Lepton jets analysis
Example HT distribution

• Form topological variables
• missing transverse energy
• HT S(jets 2-N) scalar transverse energy
  for non-leading jets. Large for top events due
  to large parent top mass, small for dijets.
• A (aplanarity) smallest eigenvalue of
  momentum tensor a measure of jets to be aplanar
  large for top events since the top quarks tend
  to give isotropic decays QCD jet backgrounds
  have parent 2-jet topology
• Dr minimum jet angular separation

data
ET
background
signal
Construct a measure D from variables that
indicates the relative probability for event to
be top vs. background (two variants weighting
technique for low mass bias (DLB) and Neural
Network (DNN)). Use this D to make a cut, or
as a weight for final event selection.
(a)
(b)
(c)
Observed mass vs. DNN for (a) signal, (b)
background (c) data
DNN
fitted mass
15
Top mass in lepton jets channel
Best fits 171.3 6.0 GeV (neural net
analysis) 174.0 5.6 GeV (weight
analysis) (statistical errors) Systematic
errors jet energy scale 4.0 GeV signal
generator 1.9 bknd generator 2.5 multiple
intns 1.3 MC stats 0.9 likelihood
fit 1.0 method diff. 0.8 Combined final result
for lepton plus jets Mt 173.3 7.8 GeV
Bknd (low D) sample
Mfit
Mtrue
Signal (high D) sample
Mfit
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Dilepton channel analysis
For the dilepton channel, there are two missing
neutrinos, so the fit is underconstrained by one.
DØ employs two methods Neutrino weighting
(nWT) and matrix element weighting (MWT). For
nWT, we assign a weight based on how much of the
nn phase space for signal is consistent with the
event kinematics, computed for a set of Mt
values. For MWT, compute a weight based on the
probability to have the lepton energies as
observed, and the product of PDFs required, for a
set of Mt values. After smearing many times with
the known resolution functions, each event has a
weight distribution as a function of Mt
For the MWT analysis, weight distributions for
the 6 observed events
The weight distributions for all events are
averaged, and compared to MC templates for a
known input top mass. A best likelihood fit
gives the top mass.
Average weight averages for 160 and 180 GeV MC
samples
17
Dilepton analysis results
nWT Analysis
MWT Analysis
Averaged dilepton mass (with correlations) Mt
168.4 12.3 3.6 GeV Lepton jets and
dilepton top mass combined Mt 172.1 7.1
GeV
18
CDF top mass determination
CDF relies on the silicon vertex detector to tag
at least one long-lived b-quark this reduces
background. They do not use the topological
variables like HT or A, and do not use neural
network techniques to distinguish likely
background events.
CDF fitted top mass, combined from dilepton,
lepton jets and all jets channels 176.8
6.5 GeV Combine the CDF and DØ results with
correlations accounted for in the error
Tevatron average Mt 174.3 5.1 GeV
19
Top quark spin correlation
20
Charged Higgs search
Study top decay branching ratios seeking
departure from SM values due to H decays.
21
W boson mass
The W boson mass receives corrections from the
Higgs vacuum polarization diagrams (as well as
from the top quarks loops, and possible new
particles). W production by Drell-Yan, with QCD
corrections
g
g
q
q
W
W
W
etc.
q
q
g
Use the decay W ? e ne (m nm) So, reaction
is p p ? e n X, where X is the hadronic
recoil (QCD radiation) and underlying event. DØ
uses both central ( h lt 1.1) and end (1.5 lt
h lt 2.5) electrons. The recoil energy is seen
for h lt 4.5. A very new result uses central
electrons that hit the calorimeter near the
module edges a gain of 14 in statistics over
previous studies. Infer n transverse momentum by
the missing energy in event (cannot get
longitudinal energy due to loss of low pT
particles in beam pipe) pTn - pTe - pTX
- pTe - UT
22
W mass variables
Three (correlated) variables are sensitive to the
W mass Electron pT , Missing ET (neutrino),
Transverse mass mT mT 2pT(e) pT(n)
1 cos(f(e) - f(n)) mT is insensitive to the
W production dynamics (corrections O(pTW/MW)2 )
but requires the inferred neutrino pT, hence is
sensitive to detector response. pT(e) and
pT(n) Jacobian edges depend on pT(W). pT(e)
depends only on well measured electron
kinematics pT(n) also depends on hadronic
response
mT and pT(e) distributions as generated and pT(W)
0 (solid line) correct pT(W) (red points) and
after detector resolution effects (yellow
shading)
23
W production and decay model
d3s d2s
dP dpT2 dy dm dpT2 dy
dm where dP/dm is a mass-dependent
Breit-Wigner with measured GW and convoluted with
the quark momenta within the proton from PDF and
the differential cross-section is taken from NLO
QCD theory with all orders gluon resummation,
convoluted over the parton momenta using a PDF.
Data used to fix QCD parameters. Account is
taken for the mix of scattering by valence and
sea quarks, taken from Herwig MC.
Production

X

M MW
Decay
The V-A coupling of the charged current gives
fully polarized W at lowest order. Corrections
due to the possibility of sea quark production
and higher order QCD radiation are present.
The detector response, resolutions, efficiencies
and backgrounds are taken from fits to auxiliary
data sets. Together with the production and
decay model, these are used in a fast Monte Carlo
program to generate large numbers of W (or Z)
bosons for specific assumed values of MW (MZ),
smeared by detector response functions.
Detector Response
24
Detector response modelling
The detector response functions for electrons,
recoil energy, radiative photons, etc. are all
determined from data distributions.

• Electron response Emeas aEtrue d taken
  from Z?ee and precision LEP Z mass
• Electron resolution s/E c s/ E n/E
  taken from fit to Z lineshape
• Electron directions chamber and calorimeter
  position calibration from muons and Z?ee
• Hadronic response require pT balance of ee-
  from Z and the hadronic recoil
• Hadronic resolution shape of pT(X)
  distributions along/perpendicular to Z?ee
• Trigger efficiencies measured with special data
  sets
• Energy corrections to pT(e) and UT for hadronic
  energy falling into electron window
• Correct selection bias for UT close to pe (loss
  of events due to energy isolation cut)
• Radiative decays (W? eng) taken from theory and
  modelled in MC
• Effect of extra minimum bias events underlying
  the W production taken from special inclusive
  triggers overlay these events at the same
  luminosity as for signal events
• Backgrounds (mainly from QCD jets misidentified
  as electrons at 10-4 level) taken from special
  data sets by selecting bad electrons. W?tn
  ? enn is included in the decay MC

25
Detector fits
The MC, with parameters determined from data, can
be confronted with data to show the validity of
the model. Some examples
Z?ee distributions for central/end and end/end
es showing validity of electron response,
resolution and background determinations
ratio of data h distribution to that of MC (an
important constraint on the PDF)
recoil energy along electron direction
recoil transverse energy
26
Mass fits

• The W mass is obtained by comparing MC templates
  with various assumed MW to the data, and
  performing a likelihood fit. This example shows
  the mT distribution fit for end electrons.
• (similar fits for pT(e) and pT(n) distributions )
• A variety of cross checks are performed
• consistency of mT, pTe, pTn fits
• vary fit region in both mass and h
• bin results as a function of time, thus L
• vary the recoil pT cut
• fit for the Z mass using transverse mass
• compare result from two end calorimeters
• compare for different electron impact position
• compare for different EM energy fractions

mT distribution
c2 distribution
27
Combined W mass fits
Systematic errors arising from uncertainty in
detector or theory parameters are computed the
parameter errors are themselves correlated in
some cases, and when the same data sets are
employed, are correlated because of the data set.
The simultaneous measurement of MW for central
and end electrons is important in reducing the
theoretical error due to uncertainty in the PDF.
The full correlation matrix is determined,
yielding these W mass values
Run Ia (central e) mT 80.35 0.25
GeV Run Ib (central e) mT
80.44 0.12 Run Ib (central e)
pTe 80.48 0.14 Run Ib
(central e) pTn 80.37
0.18 Run Ib (end e) mT
80.76 0.23 Run Ib (end e)
pTe 80.55 0.24 Run Ib
(end e) pTn
80.74 0.35 Run Ib (central e-edge)
mT 80.60 0.44 Run Ib (central
e-edge) pTe 80.73
0.53 Run Ib (central e-edge) pTn
80.51 0.61 Overall DØ average 80.483
0.084 GeV
28
Combined World results
Combining with the CDF result, Tevatron MW
80.454 0.060 GeV . LEP experiments precision
(per experiment) about the same as Tevatron.
LEP MW has increased over the past two years, so
now good agreement between LEP and Tevatron LEP
MW average 80.450 0.039
The indirect MW indication from Z, n, top
measurements is 80.373 0. 023 GeV, nearly 2s
from the measured value.
World Avg MW 80.451 0.033 GeV
29
Constraints from top and W mass
With Tevatron top mass of 174.1 5.1 GeV and W
mass of 80.451 0.033, we get a strong
constraint on the Higgs boson mass in the
framework of the SM. The direct MW, Mt
measurements are shown in the green ellipse.
The red ellipse is the indirect prediction from
the precision LEP/SLC/nN measurements. The
agreement is reasonable (better than 2s), but
with the new higher MW, there is a weak hint of
the effect of new physics. Supersymmetry
would provide new particles whose virtual effects
would predict higher MW.
The combined constraints from Z, top, W, nN now
require SM Higgs mass to be less than 200
GeV. Direct limit from LEP of 113.5 GeV sets
limit somewhat above the best indirect prediction
of the SM Higgs mass.
30
EW measurements in Run II
Now starting Run II with expectation of 15 fb-1
of data per experiment

• Top measurements
• Reduce top mass error to about 1 2 GeV new
  methods using tt matrix elements ratio of Mt/MW
  in top events combined use of lepton jets and
  dileptons to reduce jet energy scale errors. New
  neural network selection will improve signal
  efficiency.
• Single top production measure EW coupling,
  Vtb, Gt
• tt spin correlations as test of SM production
• tt invariant masses to seek new resonances
  (technicolor, topcolor inspired)
• Rare decays t ? c Z, t ? g Z, t ? s W
• charged Higgs search through t ? b H
  
  
  (deviation from SM BRs)
• W/Z measurements
• MW to 20 MeV per experiment
• Direct GW from W line shape
• AFB for Z probes sin2QW from light quarks
• AFB for W gives u/d quark ratio needed for MW
• Trilinear couplings ( Dk and l)

after 2 fb-1
31
Higgs search in Run II
LEP Higgs limit is 113.5 GeV (2.2s hint at 115
GeV). Tevatron will extend the Higgs search
reach. For MH lt 135 GeV, dominant decay is H?bb
and need to use production via WH/ZH. For MH gt
135 GeV, can access H?WW and use dominant gg
production.
SM Higgs Branching ratios
SM Higgs production cross sections
SM Higgs range from EW fits
Susy (h) mass range
32
Higgs studies
MH lt 135 GeV (WH and ZH production with H ?b b
) Use l n (bb), l l (bb), nn (bb), qq
(bb) ??, and l n (tt) channels require
excellent b-tag efficiency and purity good
(di-b-jet) mass resolution Neural network event
selection for high efficiency good control of
initial and final state radiation combine all
channels and both experiments MH gt 135 GeV (gg
production with H ? W W- (W1?l n, W2?qq) Use a
series of cuts on lepton kinematics, event
topology, transverse mass, etc. (A neural
network selection will improve this channel).
33
Higgs reach
Monte Carlo studies using fast parametrization of
detectors combine CDF and DØ data to
obtain the limits for all channels and
plot Integrated luminosity required (for either
experiment) 95 CL limit, 3s
evidence, 5s discovery vs. Higgs mass
Will rule out LEP indication if not true with 2
fb-1 Will see 3s evidence for 115 GeV Higgs with
5 fb-1 Will discover (5s) 115 GeV Higgs with 12
fb-1 See 3s evidence with 15 fb-1 / expt for MH lt
135 GeV and 145 lt MH lt 175 GeV
34
Charged Higgs
Study top decay branching ratios, seeking
departure from SM values due to H ? tn, cs, tb
(Wbb), for a range of MH and tanb. Need
well-determined tt cross section. Limits for 2
fb-1 and 10 fb-1
35
Run II DØ Detector
36
First?pp collisions of Run 2 at DØ
April 3, 2001
Antiproton halo

• Luminosity counters
• timing

Luminosity ( coincidence)
5 ? 1027 cm-2 sec-1
Proton halo
Vertex distribution along z of min bias events
37
First Reconstructed Muon
Two views of the same muon track

• P. Balm, NIKHEF

Hits in A,B,C layer scintillator counters (red
lines)
Hits in A, B, C layers of mini-drift tubes (magent
a dots)
(x,y) view of scintillator hits
P. Balm, NIKHEF
38
Central Fiber Tracking and Preshower
Detector

• Electron candidate

Preshower hits
Fiber tracker hits
D. Alton, Michigan
39
W ? e? candidate
Run 125232 Event 183666 Electron candidate
recorded using EM trigger pT 38 GeV
isolationlt0.2 EM fraction 0.97
Layer 1 8 GeV
Layer 2 14 GeV
hot cell
Layer 3 15 GeV
Layer 4 0.3 GeV
P. Petroff and L. Duflot, Orsay
40
Photon Jet candidate event
?
jet

• Ia IashviliUC Riverside

? candidate
jet
41
K0S ? ??-
A very small fraction of our data
Background sample (same sign pions)
S. Towers, Stony Brook
Signal sample (opposite sign pions)
42
Jet Events

• Calorimeter Level 1 trigger
• Run124640 event 524035

V. Zutshi Brookhaven
43
Triggered Muons
Hits in A, B, C layers of mini-drift tubes shown
as magenta dots
Hits in A,B,C layer scintillator counters shown
as red lines
C. Royon Saclay
44
Global track finding
Track with 5 fiber tracker hits, 5 3D silicon hits
Relative alignment of silicon and fiber
trackers verified to 40 ?m level
D. Whiteson, LBNL
36 ? 36 Store Run 119679, Event 232931Level 3
(software trigger) Global Tracking
45
Primary Vertex Finding

• Analysis of one of the first 36x36 runs
• Primary vertices reconstructed from tracks in
  silicon

ltXvtxgt0.27 cm ltYvtxgt0.32 cm ltZvtxgt7.04 cm
A. Schwartzman Buenos Aires
46
Summary
The CDF and DØ measurements of the top quark and
W boson have given critical new understanding of
the Electroweak interaction. The new Tevatron
run with upgraded accelerator and detectors
should make major advances in illuminating the
nature of Electroweak symmetry breaking before
the LHC begins.
47
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