Transcript Title Sans - UC Davis Physics

Searches for New Physics
in the CDF Top Sample
Robin D. Erbacher
Fermi National Accelerator Laboratory
(for the CDF Collaboration)
SUSY 2004 -- Tsukuba, Japan
Saturday June 19, 2004
Elucidating the Top Quark in Run 2
Vtb
Top pairs: s(tt) ~ 7 pb
•Top production rate
•Mass of top
•W helicity in top events
•QCD tests
•New
Newphysics!
physics in X tt
•Anomalous couplings,
new particles
Single top: s(tb) ~ 3 pb
•|Vtb|
•QCD tests
•New physics!
physics?
2
Top Physics at the Tevatron
tWb
Main “usable” top event topologies:
•tt  lnlnbb
di-lepton
5% e+m
•tt  lnqqbb lepton+jets 30% e+m
•tt  qqqqbb all hadronic 45%
3
ttbar Production Cross Section
~85%
~15%
q-qbar
glue-glue
Measuring the top cross section precisely is important
for exploring the possibilities for new physics:
 Test of QCD: Higher cross section could be a sign of
non-Standard Model top production mechanisms
Resonant state X tt OR Anomalous couplings in QCD?
 It could also mean new physics is entering into the
top sample!
4
Cross Section Summary
We have many different
approaches to measuring
the top cross section,
allowing us to carefully
cross check the results-and look for anomalies.
5
Run 1: Anomalies in the Top
Di-lepton Sample?
Four di-lepton
candidates with
very high MET.
?
6
Run 2: Di-lepton Kinematics
7
Distributed as expected? More data to come…
PT(lepton)
Analysis of SM Agreement
Probability Using Kinematics
a = 1.6%
Topological weighting parameter (TW)
Probability(a) of CDF outcome with SM hypothesis 8
using four kinematic variables [MET, Tw, PT, Df(MET,l)] ~1.6%
Run 1: Excess in the b-tagged
Lepton + Jets Sample?
Observed excess of
b-tags in the 2 jet bin
Too many SVX double
tags (more than one btagged jet/event)
Too many multiple tags
(more than one b-tag/jet)
A lot of speculation,
but nothing solid.
9
Run 2: Lepton + Jets Events with at
Least One b-tag
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Run 2: Lepton+Jets Events with Two b-tags
Very Clean Sample: Number of Jets for Double-tagged Events
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Lepton+Jets Cross Sections
Using Event Kinematics
W+  3 Jets Sample
Composition:
W+Jets
~ 75%
QCD fakes ~ 6%
ttbar
~ 19%
3 component likelihood fit:
•ttbar shape from MC
•W+jets shape from MC
•QCD shape from data
QCD from non-iso leptons
Pre-tagged sample:
two times the efficiency for ttbar but more background
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Lepton+Jets Cross Section
Using Many Kinematics: Neural Network
s(tt)=6.7 ±1.1 ±1.5 pb
Output of a 7-Input neural network, choosing both
Adding
more
allows
Fit to neural
network
outputevent
for
topinformation
and
W+jets background:
Looks
like
top!
shape
and
energy
variables
toofdiscriminate
top from
bkg
better
discrimination
top
events.
Sensitivity
similar
to
b-tagged
analysis,
but larger
sample
used 13
Search for High ET Events
HT distribution for W+4p, ttbar, and t' where M(t')=225 GeV
We can set limits on new physics processes in top sample
14
Can a t' Exist?
•Z width measurement rules out a fourth generation with a
light neutrino m(n4)<m(Z)/2
•Even if m(n4)>m(Z)/2, precision electroweak data restrict
masses and couplings of new quarks and leptons (PDG)
•He/Polonsky/Su (hep-ph/0102144): a generic 4th chiral
generation is consistent with EWK data; can accommodate
a heavy Higgs (500 GeV) without any other new physics
(similarly with 2HDM)
•N=2 SUSY requires three more “mirror” generations – the
SUSY breaking mechanism can induce couplings of the
mirror quarks with the known ones
•Other models (eg: “Beautiful Mirrors” hep-ph/ 0109097)
include possibilities of a new heavy up-type quark
decaying to Wb
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Does t' decay to Wq ?
The most natural decay would be t'  W b'.
This is suppressed if M(t') < M(b') + M(W)
•There is reason to believe (from precision ewk data)
that the mass splitting between a t' and a b' quark
should be relatively small (PDG). So:
If M(t') < M(b') + M(W), then….
– t'  W b (Cabibbo suppressed)
– t'  Ws, Wd (doubly/triply Cabibbo suppressed)
– t'  l n b' (virtual W)
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HT Plot with t' Signal, M(t') =225 GeV
Plot for fit result with t‘ signal included at 95% CL limit
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[s(ttbar) 6.12 pb in this fit]
Result: Limits (pb) Versus M(t')
Constraints
vary with
assumed
top mass, but
not by much.
Mtop
sconstraint *
170
7.8  1.0 pb
175
6.7  0.9 pb
180
5.75  0.7 pb
* Taken from
hep-ph/0303085
Mtop=180
Mtop=170
Expected
1s sensitivity
Limits on sBR(t'Wq)2
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Projected Limits with Higher Luminosity
This assumes no improvement in systematic errors 19
W Helicity in Top Events
SM: Only longitudinal and
left-handed W’s can be
produced in the top rest
frame.
Use lepton pT spectra to
determine the fraction
F0 of longitudinally
polarized W’s.
F0 = 0.7 in the Standard Model
The combined dilepton and
lepton+jets b-tagged events
plotted against the best fit.
Result: F0 = 0.27+ 0.35 – 0.21
Or
F0 < 0.88 @ 95% CL
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W Helicity in Top Dileptons
The dilepton events
plotted against the best fit.
Here we see a slight 2s effect.
Result: F0 = -0.54 + 0.12 – 0.47
Or
F0 < 0.52 @ 95% CL
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Summary
Other analyses hunting for hints of new physics in the
top quark datasets:
•W helicity from Run 1 about to be published (fit to
V+A fraction directly); Another Run 2 analysis: cosq*
•Tests of top quark branching ratios and Standard
Model consistency
•Ongoing search for single top in lepton + jets events
•Tests of consistency of kinematics with the SM in the
top quark sample
•Search for top decaying to charged Higgs
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Conclusions
As we increase our datasets at the Tevatron in Run 2,
CDF will have much to say about the possibility of
new physics in our top quark samples.
Stay tuned:
CDF top quark publications are beginning to roll…
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Searches for Single Top
Combined Channel Search:
s(s+t) < 13.7 pb @ 95% C.L.
t-Channel Search:
s(t-chan) < 8.5 pb @ 95% C.L.
We are still looking for single top!
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New Top Mass World Average
•New DØ combined mass:
Mt = 179.0 ± 5.1 GeV/c2
•New world average:
Mt = 178.0 ± 4.3 GeV/c2
•Global fit to electroweak data
using this top mas
Method of LEPEWWG (hep-ex 0312023)
–Best-fit MH  113 GeV/c2
–95% C.L. upper limit 237 GeV/c2
Yellow region excluded:
MH < 114.4 GeV/c2 @ 95% CL
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Current Top Mass Results
Lepton+Jets Mass using
SecVTX b-tagging
L+J: Dynamic Likelihood
Method (Kondo et al)
mt = 174.9 +7.1-7.7 (stat) ± 6.5 (sys) GeV
mt = 177.8 +4.5-5.0 (stat) ± 6.2 (sys) GeV
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Current CDF best