Transcript Searches for Anomalous Top Quark Production at the Tevatron Andrew Ivanov University of California, Davis for the CDF and D0 Collaborations Top Quark Symposium April 08,

Searches for Anomalous Top
Quark Production at the Tevatron
Andrew Ivanov
University of California, Davis
for the CDF and D0 Collaborations
Top Quark Symposium
April 08, 2005
Why the top quark is so special …
Mass of order
electroweak scale
May play a special
role in the dynamics
of EWSB
Serves as a powerful
probe of physics
beyond the SM
Apr-08-2005
A. Ivanov, Top Quark Symposium
2
Is the top quark ordinary or exotic ?
Exotic in some way ?! New Physics might
reveal itself through
Oblique radiative corrections
Exotic top quark decay modes (scalar bosons, FCNC)
Exotic intermediate heavy states – new gauge bosons or
resonances
Ordinary ? Becomes a major background to
new physics
Top sample might contain an admixture of exotic processes
Apr-08-2005
A. Ivanov, Top Quark Symposium
3
Is the top quark ordinary or exotic ?
Exotic in some way ?! New Physics might
reveal itself through
Focus of
Oblique radiative corrections
this talk
Exotic top quark decay modes (scalar bosons, FCNC)
Exotic intermediate heavy states – new gauge bosons or
resonances
Ordinary ? Becomes a major background to
new physics
Top sample might contain an admixture of exotic processes
Apr-08-2005
A. Ivanov, Top Quark Symposium
4
Outline: Searches for
Resonance in mass of ttbar system
Top quark pT
Anomalous kinematics in top dilepton sample
Anomalous kinematics in lepton + jets sample
Top Charge
Apr-08-2005
A. Ivanov, Top Quark Symposium
5
Why search for structure in the
Mtt spectrum?

Various exotic models predict the existence of
particles decaying to tt
Topcolor-Assisted Technicolor
(Hill, Phys Lett. B345, 483 (1995); Hill and Parke Phys. Rev. D49, 4454 (1994))

Extends technicolor models and attempts to
explain EWSB by introducing a new strong
interaction

Predicts new massive bosons “topgluons” and
a topcolor Z’
Apr-08-2005
A. Ivanov, Top Quark Symposium
6
Run 1: Search for tt Resonances

Each lepton+jets event fit to
ttbar hypothesis by
constraining W and top
masses

Distribution smeared by:
 ambiguity in correct jetparton assignment
 detector resolution

B-tagging reduces
combinatorics

Select configuration with
best 2 fit
Apr-08-2005
A. Ivanov, Top Quark Symposium
7
Run 1: Search for tt Resonances
Dominant background is from W + jets
CDF
Likelihood fit is performed using X->tt templates with various masses
Apr-08-2005
A. Ivanov, Top Quark Symposium
8
No Evidence for tt Resonance in Run 1
CDF
Phys.Rev.Lett. 92, 221804 (2004)
Apr-08-2005
Phys.Rev.Lett. 85, 2062 (2000)
A. Ivanov, Top Quark Symposium
9
Run 1: Limits on Mass of the Topcolor
Boson
Model independent search for a narrow resonance Xtt exclude a
narrow, leptophobic X boson with a natural width of GX = 0.012 MX
MX < 560 GeV/c2
and
MX < 480 GeV/c2
CDF
MX < 780 GeV/c2 for
GX = 0.04 MX
Apr-08-2005
A. Ivanov, Top Quark Symposium
CDF
10
Run 2: Search continues …
CDF
GeV/c2
MX = 500
GX = 0.012 MX
 = 1pb
MX = 700 GeV/c2
SM ttbar
W + >= 3 jets
Monte Carlo
Use Neural Nets trained with
different X masses
Fit Mtt distribution to
combination of shapes
(lepton + jets, use b-tag)
Apr-08-2005
Use large statistics - lepton +
>=3 jets events sample (no b-tag
information)
A. Ivanov, Top Quark Symposium
11
Run 2: Search for Resonances
Using Matrix Element
Generator Level
Reconstructed
events
CDF
better reconstruction
Employ matrix element (similar to the top mass
measurement) information to weigh each parton
configuration for direct Mtt reconstruction
Apr-08-2005
A. Ivanov, Top Quark Symposium
12
Why search for structure in top
quark pT spectrum ?
Many exotic models predict sizeable
enhancements in the tt cross section at high
transverse momenta pT > 200 GeV/c
(T.G. Rizzo , hep-ph/9902273; K. Lane, Phys. Rev. D52, 1546 (1995))
e.g. Anomalous chromomagnetic moment
Apr-08-2005
A. Ivanov, Top Quark Symposium
13
Run 1: Top Quark pT Spectrum
Response functions
CDF
Use an iterative
procedure to unsmear the
distribution of
reconstructed top pT
momentum
Perform a likelihood fit to
extract true top pT
Use superposition of
response functions in 4
bins of pT and background
template
Apr-08-2005
A. Ivanov, Top Quark Symposium
14
Run 1: Top Quark pT Spectrum
True pT Bin
0  pT  75 GeV
(d/dpT) / 
R1
Measurement
SM Expectation
0.10
0.2100..22
21 (stat) 0.08 (syst)
0.41
75  pT  150GeV
R2
0.04
0.4500..23
23 (stat) 0.07 (syst)
0.43
150  pT  225GeV
R3
0.07
0.3400..14
12 (stat) 0.05 (syst)
0.13
225  p T  300GeV
R4
0  pT  150GeV
R1+R2
Top Quark
pT Spectrum
0.031
0.024
0.025
0.07
0.6600..17
17 (stat) 0.07 (syst)
0.84
0.0000.000 (stat) 0.000 (syst)
CDF
Upper limit:
CDF Run 1
Phys.Rev.Lett. 87, 102001 (2001)
Apr-08-2005
R4(225<Pt<425GeV/c)
<0.16 @ 95% C.L.
A. Ivanov, Top Quark Symposium
15
Run 2 Search: Top Quarks Kinematics
Variables of interest:
 hadronic/leptonic top pT,
 ttbar system pT and,
 of the two top quarks, …
Additional jets are source of
combinatorial background
Apr-08-2005
A. Ivanov, Top Quark Symposium
16
Run 1: Anomalies in the Top Dilepton Sample?
CDF
1 ev.
1 ev.
7 ev.
An excess of events with
large missing transverse ET
and lepton pT
Apr-08-2005
Flavor asymmetry:
an excess of em events
A. Ivanov, Top Quark Symposium
17
A Subset of Run 1 Dilepton Events
- agreement with decays of SUSY quarks with masses
hep-ph/9609313
In better
around 300 GeV
[Barnett and Hall, Phys. Rev. Lett. 77 3506 (1996)]
tt MC
SUSY MC
CDFI CDFI
lepton
Two (out of 9) CDFI di-lepton events
have
unexpectedly
large
MET+E
, (both
t
D0
give very poor “fits” to tt hypothesis); one such event exists in D0 sample. (Flagged
by Hall and Barnett as candidates for SUSY events in their DPF 96’ paper).
Data:
Stnd
Model
ql [deg]
MEt+Et(l1)+Et(l2) [GeV]
D0
CDF
sqrt(MT1*MT2) [GeV]
Apr-08-2005
SUSY
???
max(MT1,MT2) [GeV]
A. Ivanov, Top Quark Symposium
18
SM Kinematic Test with Run 2 Dilepton Events
Stay model-independent
Choose a-priori potentially
sensitive to new physics kinematic
variables
Perform Kolmogorov-Smirnov
consistency test between data and
Monte Carlo expectation
Search for and isolate a subset
with the most non-SM features
(SUSY events?!)
Sensitivity
given 13 events
observed
(kinematics only)
Apr-08-2005
CDF
SUSY fraction
Chance to find
3 evidence
50%
50%
30%
25%
10%
7%
A. Ivanov, Top Quark Symposium
19
Kinematics in Run 2 Top Dilepton Sample
Missing ET
Leading lepton pT
CDF
?
(leading lepton, met)
“topness” = ttbar decay
goodness-of-fit
more top-like
Overall agreement of 1.0-4.5% mainly due to an excess at low lepton pT
Apr-08-2005
A. Ivanov, Top Quark Symposium
20
Run 2 Dilepton events
FERMILAB-PUB-04-396-E
( Subm. to Phys.Rev.Lett.)
CDF
Phys.Rev.Lett. 93, 142001 (2004)
1 ev.
3 ev.
9 ev.
CDF
“topness”
Low pT-lepton events are
accompanied with b-jets likely being from tt
Apr-08-2005
Flavor asymmetry remains?!..
More data to come
A. Ivanov, Top Quark Symposium
21
New Particles Decaying to Wq?
Can be a fourth generation up-type quark
He/Polonsky/Su (hep-ph/0102144)
a generic 4th chiral generation is consistent with EWK data;
accommodates a heavy Higgs (500 GeV) without any other
new physics
“Beautiful Mirrors” model
Wagner et al (hep-ph/ 0109097)
predicts a new heavy up-type quark decaying to Wb;
naturally accomodates the LEP b forward-backward
asymmetry results
From the precision EWK data the mass splitting
between a t' and a b' quark is relatively small.
Therefore if Mt’ < Mb’ + MW: t' -> l  b' (virtual W)
Apr-08-2005
A. Ivanov, Top Quark Symposium
22
Run 2: Search for t’->Wq
Lepton + jets
channel
CDF
Use the variable HT =
sum of transverse
momenta of all
objects in the event
Sensitive to any new
high-pT physics
Perform a likelihood
fit and set a limit on
non-SM processes
Apr-08-2005
Data does not prefer
t’ contribution
A. Ivanov, Top Quark Symposium
23
Projected Limits: Higher
Luminosity / New NN Analysis
HT likelihood
sensitivity study
Use Neural Nets trained with
different t’ masses
Perform likelihood fit with the
NN output variable
CDF
Apr-08-2005
t’->Wb
ttbar
W + >= 3 jets
A. Ivanov, Top Quark Symposium
24
Top Charge
Is it the Standard Model top ?
W.-F. Chang et al.,hep-ph/9810531 proposes an
exotic doublet of quarks (Q1, Q4) with charges
(-1/3,-4/3) and M ~ 175 GeV/c2
while Mtop ~ 274 GeV/c2
q = -4/3 is consistent with EW data, new bcouplings improve the EW fit (E. Ma et al. , hepph/9909537)
Apr-08-2005
A. Ivanov, Top Quark Symposium
25
Top Quark Charge Measurement
Determine:
 charge of W (lepton)
 pairing between W and b

flavor of b-jet
CDF
b
Use both dilepton and lepton + jets
samples
Variables to use for flavor b-tagging:
 sign of max pT track in b-jet cone
 jet charge - sum of charge for tracks
inside of the cone
 sign of min pT track
 sign of fragmentation track
b
b B0
d
d u p
u
b B0
d
d +
u p
u
Expect with 500 pb-1 to rule out q = -4/3
Apr-08-2005
A. Ivanov, Top Quark Symposium
26
Top Quark Charge Measurement
Jet Charge Algorithm:
= qi pTia/  pTia
Qjet
Sensitivity Study:
Limit depends on leptonb-jet mismatching
(fraction of mismatches)
Apr-08-2005
A. Ivanov, Top Quark Symposium
27
Near Future is Very Exciting
...
Current Run 2 top analyses in progress utilize datasets of
integrated luminosity ~340 pb-1
Results with this amount of data are expected to be reported
at the Summer conferences
A lot of work was performed on reducing systematic
uncertainties in the past months
Expect better limits and more precise measurements very
soon
Many new analyses testing various top properties are in
preparation
Apr-08-2005
A. Ivanov, Top Quark Symposium
28
More Distant Future is Even More
Exciting…
More data is on
tape
Peak Luminosity
beats new records
every day
Improved trigger
system guarantees
a higher purity data
Stay Tuned
Apr-08-2005
A. Ivanov, Top Quark Symposium
29