arXiv:hep-ex/0302037v1 25 Feb 2003
SEARCH FOR NARROW WIDTH t¯ t RESONANCES IN p¯ p
COLLISIONS AT√s = 1.8 TEV.
SUPRIYA JAIN*, NABA MONDAL, AND DHIMAN CHAKRABORTY
(FOR THE DØ COLLABORATION)
We present a preliminary result on a search for narrow width resonances that decay
into t¯t pairs using 130 pb−1of lepton plus jets data in p¯ p collisions at√s = 1.8
TeV. No significant deviation from Standard Model prediction is observed. 95%
C.L. upper limits on the production cross section of the narrow width resonance
times its branching fraction to t¯t are presented for different resonance masses MX.
We also exclude the existence of a leptophobic topcolor particle, X, with MX <
560 GeV/c2for a width ΓX= 0.012MX.
Particles with narrow width that decay to t¯t pairs are predicted by sev-
eral non Standard Model theories1,2. For instance, in one of the scenarios
of the topcolor-assisted technicolor model in Ref. , a heavy Z′is pre-
dicted, that couples preferentially to the third quark generation.
At present, direct searches for these heavy particles or resonances are
possible only at the Tevatron, the 1.8 TeV p¯ p collider located at the Fermi
National Accelerator Laboratory. Experiments seek an excess, beyond that
predicted by the Standard Model (SM), in the distribution of the invariant
mass of the t¯t decay products. Previous searches3from the Tevatron have
limited a leptophobic Z′to a mass higher than 480 GeV/c2. In this paper
we present a preliminary result based on a direct search for t¯t narrow width,
heavy resonances in the inclusive decay modes t¯t → ℓ ν + 4 (or more)
jets, where ℓ = e or µ, using 130 pb−1of data recorded from 1992 to 1996
by the DØ experiment at the Tevatron.
We consider two orthogonal classes of events for this analysis, whose se-
lection is based on: a) a purely topological selection of lepton+jets events
which we denote as e + jets and µ + jets, and b) a selection based pri-
marily on the presence of a non-isolated, soft muon (µ tag) from b and c
quark semileptonic decays, with additional selections on the topology of the
event. These events are denoted as e+jets/µ and µ+jets/µ. The principal
sources of background are due to SM t¯t production, production of W(→ lν)
+ ≥ 4 jets, and production of multijets (Nj∼ 5), in which one of the jets
is misidentified as a lepton, and instrumental effects simulate sufficient E /T
satisfying the neutrino requirement. The selection criteria used to reduce
the contribution from non-t¯t sources are summarized in Table 1.
Table 1.Summary of event selections.
LeptonET>20 GeVpT>20 GeV/cET>20 GeVpT>20 GeV/c
|η| <2|η| <1.7
E /T>20 GeV
|η| <2|η| <1.7
E /T>20 GeVE /T>20 GeVE /T>20 GeV
Jets≥ 4 jets, |η| <2
≥ 4 jets, |η| <2
≥ 4 jets, |η| <2
≥ 4 jets, |η| <2
µ tag NoNo YesYes
>60 GeVE /T>35 GeV, if∆φ(E /T,µ) < 170◦, if
|ηW| <2|ηW| <2∆φ(E /T,µ) < 25◦
selected 42 4143
We consider the resonance signal (X → t¯t) at nine different masses
MX between 400-1000 GeV/c2, with a natural width ΓX = 0.012MX. We
perform a three constraint kinematic fit to the t¯t → l + jets, decay hy-
pothesis4, and apply a cut of χ2<10 to further reduce non-t¯t background,
whereupon 41 events are left in the data sample of which 4 are µ-tagged.
We then use Bayesian statistics5to fit the data mt¯ tdistribution to a
three-source model comprising signal (X → t¯t) and the SM backgrounds4.
No significant deviation is seen in the data mt¯ tdistribution from SM ex-
pectations for any of the resonance masses considered.
To conclude, after investigating 130 pb−1of data, we find no statisti-
cally significant evidence for t¯t resonance, and so establish upper limits on
σXB(X → t¯t) at 95% confidence for MX between 400 and 1000 GeV/c2.
These limits, as shown in Fig. 1, are used to constrain a model of topcolor
assisted technicolor and exclude at 95% confidence level, the existence of a
leptophobic Z′ 2with mass MX<560 GeV/c2for a width ΓX= 0.012MX.
400 450 500 550 600 650 700 750 800 850
σ(X) x B(X → tt
Leptophobic topcolor X
with Γ = 0.012M(X)
D0 95% C.L. upper limits
for Γ = 0.012M(X)
onance mass MX. Included for reference are the predicted topcolor assisted technicolor
cross sections for a width ΓX= 1.2% MX.
The DØ Run I 95% confidence level upper limits on σXB as a function of res-
We thank the staffs at Fermilab and collaborating institutions, and
acknowledge support from the Department of Energy and National
Science Foundation (USA), Commissariat ` a L’Energie Atomique and
CNRS/Institut National de Physique Nucl´ eaire et de Physique des Partic-
ules (France), Ministry for Science and Technology and Ministry for Atomic
Energy (Russia), CAPES and CNPq (Brazil), Departments of Atomic En-
ergy and Science and Education (India), Colciencias (Colombia), CONA-
CyT (Mexico), Ministry of Education and KOSEF (Korea), CONICET and
UBACyT (Argentina), The Foundation for Fundamental Research on Mat-
ter (The Netherlands), PPARC (United Kingdom), Ministry of Education
(Czech Republic), A.P. Sloan Foundation, and the Research Corporation.
1. C. T. Hill and S. Parke, Phys. Rev. D49, 4454 (1994)
2. R. M. Harris, C. T. Hill and S. Parke, Cross section for Topcolor Z′
to t¯t, arXiv:hep-ph/9911288 (1999)
3. CDF Collaboration, T. Affolder et al., Phys. Rev. Lett. 85 , 2062 (2000).
4. DØ Collaboration, V.M. Abazov et al., Search for narrow width t¯t resonances
in p¯ p collisions at√s = 1.8 TeV, (to be submitted to Phys. Rev. Lett.).
5. P. C. Bhat, H. B. Prosper, and S. Snyder, Phys. Lett. B407 , 73 (1997).