Searches for Anomalous Top Quark Production at the Tevatron
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Transcript of Searches for Anomalous Top Quark Production at the Tevatron
Searches for Anomalous Top Searches for Anomalous Top Quark Production at the TevatronQuark Production at the Tevatron
Andrew Ivanov University of California, Davis
for the CDF and D0 Collaborations
Top Quark Symposium April 08, 2005
Apr-08-2005 A. Ivanov, Top Quark Symposium 2
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 3
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 4
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
Focus of this talk
Apr-08-2005 A. Ivanov, Top Quark Symposium 5
Outline: Searches for 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 6
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 7
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 jet-
parton assignment detector resolution
B-tagging reduces combinatorics
Select configuration with best 2 fit
Apr-08-2005 A. Ivanov, Top Quark Symposium 8
Run 1: Search for tt Resonances
Dominant background is from W + jets
Likelihood fit is performed using X->tt templates with various masses
CDF
Apr-08-2005 A. Ivanov, Top Quark Symposium 9
No Evidence for tt Resonance in Run 1
Phys.Rev.Lett. 92, 221804 (2004) Phys.Rev.Lett. 85, 2062 (2000)
CDF
Apr-08-2005 A. Ivanov, Top Quark Symposium 10
Run 1: Limits on Mass of the Topcolor Boson
Model independent search for a narrow resonance Xtt exclude a narrow, leptophobic X boson with a natural width of X = 0.012 MX
MX < 560 GeV/c2 and MX < 480 GeV/c2
MX < 780 GeV/c2 for X = 0.04 MX
CDF
CDF
Apr-08-2005 A. Ivanov, Top Quark Symposium 11
Run 2: Search continues …
CDFMX = 500 GeV/c2
X = 0.012 MX = 1pb
Monte Carlo
Fit Mtt distribution to combination of shapes (lepton + jets, use b-tag)
Use Neural Nets trained with different X masses
Use large statistics - lepton + >=3 jets events sample (no b-tag information)
MX = 700 GeV/c2 SM ttbar W + >= 3 jets
Apr-08-2005 A. Ivanov, Top Quark Symposium 12
Run 2: Search for Resonances Using Matrix Element
Employ matrix element (similar to the top mass measurement) information to weigh each parton configuration for direct M tt
reconstruction
CDF
better reconstruction
Reconstructed events
Generator Level
Apr-08-2005 A. Ivanov, Top Quark Symposium 13
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 14
Run 1: Top Quark pT Spectrum
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
Response functions CDF
Apr-08-2005 A. Ivanov, Top Quark Symposium 15
Run 1: Top Quark pT Spectrum
Phys.Rev.Lett. 87, 102001 (2001)
Upper limit:
R4(225<Pt<425GeV/c)<0.16 @ 95% C.L.
True pT Bin (d/dpT) / Measurement SM Expectation
R1 0.41
R2 0.43
R3 0.13
R4 0.025
R1+R2 0.84
GeV 75p0 T ≤≤
GeV 150p75 T ≤≤
GeV 225p150 T ≤≤
GeV 300p225 T ≤≤
GeV 150p0 T ≤≤
)()(. ..
.
. syststat210 100080
220210
+−
+−
)()(. ..
.
. syststat450 040070
230230
+−
+−
)()(. ..
.
. syststat340 070050
140120
+−
+−
)()(. ..
.
. syststat0000 02400000
03100000
+−
+−
)()(. ..
.
. syststat660 070070
170170
+−
+−
CDF Run 1
Top Quark pT Spectrum
CDF
Apr-08-2005 A. Ivanov, Top Quark Symposium 16
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 17
Run 1: Anomalies in the Top Dilepton Sample?
An excess of events with large missing transverse ET
and lepton pT
Flavor asymmetry: an excess of e events
CDF
1 ev.1 ev.7 ev.
Apr-08-2005 A. Ivanov, Top Quark Symposium 18
A Subset of Run 1 Dilepton Events
Two (out of 9) CDFI di-lepton events have unexpectedly large MET+ΣEt
lepton,(bothgiveverypoor“fits”totthypothesis);onesucheventexistsinD0sample.(FlaggedbyHallandBarnettascandidatesforSUSYeventsintheirDPF96’paper).
MEt+Et(l1)+Et(l2) [GeV]
max(MT1,MT2) [GeV]sqrt(MT1*MT2) [GeV]
l [deg]
CDFID0
CDFI
hep-ph/9609313In better agreement with decays of SUSY quarks with masses around 300 GeV-
[Barnett and Hall, Phys. Rev. Lett. 77 3506 (1996)]
tt MCSUSY MC
D0
CDF
Data:
SUSY ???
Stnd Model
Apr-08-2005 A. Ivanov, Top Quark Symposium 19
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?!)
CDF
SUSY fraction Chance to find 3 evidence
50% 50%
30% 25%
10% 7 %
Sensitivity given 13 events
observed(kinematics only)
Apr-08-2005 A. Ivanov, Top Quark Symposium 20
Kinematics in Run 2 Top Dilepton Sample
Missing ET Leading lepton pT
(leading lepton, met) “topness” = ttbar decay goodness-of-fit
Overall agreement of 1.0-4.5% mainly due to an excess at low lepton pT
more top-like
?
CDF
Apr-08-2005 A. Ivanov, Top Quark Symposium 21
Run 2 Dilepton events
Low pT-lepton events are
accompanied with b-jets -
likely being from ttFlavor asymmetry remains?!..
More data to come
“topness”
CDF
Phys.Rev.Lett. 93, 142001 (2004)FERMILAB-PUB-04-396-E( Subm. to Phys.Rev.Lett.) CDF
1 ev.3 ev.9 ev.
Apr-08-2005 A. Ivanov, Top Quark Symposium 22
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 23
Run 2: Search for t’->Wq
Lepton + jets channel
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
CDF
Data does not prefer t’ contribution
Apr-08-2005 A. Ivanov, Top Quark Symposium 24
Projected Limits: Higher Luminosity / New NN Analysis
CDF
Use Neural Nets trained with different t’ masses
Perform likelihood fit with the NN output variable
HT likelihood sensitivity study
W + >= 3 jets ttbar t’->Wb
Apr-08-2005 A. Ivanov, Top Quark Symposium 25
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 b-couplings improve the EW fit (E. Ma et al. , hep-ph/9909537)
Apr-08-2005 A. Ivanov, Top Quark Symposium 26
Top Quark Charge Measurement
bbdduu
B0
-
bbdduu
B0
+
Determine: charge of W (lepton) pairing between W and b flavor of b-jet
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
Expect with 500 pb-1 to rule out q = -4/3
CDF
Apr-08-2005 A. Ivanov, Top Quark Symposium 27
Top Quark Charge Measurement
Jet Charge Algorithm: Qjet = Σqi pTi
a/ Σ pTia
Sensitivity Study: Limit depends on
lepton- b-jet mismatching
(fraction of mismatches)
Apr-08-2005 A. Ivanov, Top Quark Symposium 28
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 29
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