We present results of the search for the super-symmetric partner of the top
quark, the stop quark, decaying to a b-quark and chargino with the subsequent
chargino decay into a neutralino, lepton and neutrino. Using the data sample
corresponding to 2.7 inv fb of integrated luminosity, collected with the CDF
Detector of the Tevatron collider, we reconstruct the stop mass of candidate
events and set 95% C. L. upper limits on masses of the stop quark, chargino and
neutralino and the branching ratio of chargino to neutralino+lepton+neutrino.Comment: ICHEP08 poster, Philadelphia, USA, July 2008. 3 pages, LaTeX, 3
  figure

Ivanov, Andrew G.

English

arXiv

UNT Digital Library

34th International Conference on High Energy Physics, Philadelphia, 2008
Search for Pair Production of Supersymmetric Top Quarks
Mimicking Standard Model Top Event Signatures at CDF
A. Ivanov (for the CDF Collaboration)
University of California, Davis, CA, 95616, USA
We present results of the search for the super-symmetric partner of the top quark, the stop quark
(
t˜1
)
, decaying to a
b-quark and chargino
(
χ˜±1
)
with the subsequent χ˜±1 decay into a neutralino
(
χ˜01
)
, lepton (`), and neutrino (ν). Using
the data sample corresponding to 2.7 fb−1 of integrated luminosity, collected with the CDF Detector of the Tevatron
pp¯ collider, we reconstruct the stop mass of candidate events and set 95% C. L. upper limits on masses of the stop
quark, chargino and neutralino and the branching ratio B(χ˜±1 → χ˜01`±ν).
1. INTRODUCTION AND MOTIVATIONS
Supersymmetry (SUSY) is one of the most plausible extensions to the Standard Model (SM) of particle physics.
It naturally solves the problem with quadratically divergent quantum corrections contributing to the Higgs mass.
It predicts unification of gauge coupling constants at a common GUT scale, and provides a natural dark matter
candidate. SUSY postulates that each of the fundamental SM fermions (bosons) has a boson (fermion) super-partner.
To reconcile the super-symmetry with experimental data, SUSY must be broken, and sparticles are expected to be
much heavier than their SM partners. Perhaps with an exception of the partner of the top quark (t), the stop quark,
whose low mass eigenstate (t˜1) could be actually lighter than the top quark. It is interesting, that the mass inequality
mt˜1
<∼ mt is demanded in the supersymmetric electroweak baryogenesis scenarios [1], which attempt to provide an
explanation for the origin of the baryon asymmetry in the Universe.
If the chargino (χ˜±1 ) happens to be lighter than the stop quark, the decay t˜1 → bχ˜±1 opens and becomes dominant.
At the Tevatron stop quarks, if exist, would be produced in pairs with the cross section a factor of ∼ 10 smaller
than that for the top quarks of the same mass. Assuming that both charginos decay as χ˜±1 → χ˜01`±ν, cascade decays
of stop quarks would produce experimental event signatures with two high-pT , oppositely charged leptons, two and
more jets, and a large missing transverse energy /ET . It is interesting, that this event signature is identical to the
dilepton final state of tt¯ decays. Therefore a small admixture of stop events to the tt¯ dilepton events would impact
measurements of the properties of the top quark. In particular, it would bias the top mass measurements in the
dilepton channel towards lower value relative to those measured in other channels of the tt¯ decays. Intriguingly, it
is exactly what had been observed in Tevatron experiments. The mass measurements in the lepton+jets and the
dilepton channel based on the datasets corresponding to 1 fb−1 of integrated luminosity had only 7% chance to yield
results more discrepant than those observed [2]. Using a conjecture above this feature could be interpreted as a sign
of super-symmetry and it has served as a motivation for the following search.
2. PRELIMINARY EVENT SELECTION AND STOP MASS RECONSTRUCTION
We use data collected with the CDF detector corresponding to 2.7 fb−1 of integrated luminosity. Events with a
high-pT (≥ 18 GeV/c) e or µ candidate are identified using the high speed trigger electronics. In the preliminary
event selection stop candidate events are required to have two leptons (e or µ), two or more jets with ET > 12 GeV
and missing transverse energy /ET > 20 GeV. Events are also classified according to availability of a secondary vertex
tag (b-tag) [3]. Optimized event selection criteria are determined at the last stage of the analysis.
The kinematic reconstruction of stop events is a challenging task, since in each t˜1¯˜t1 event there are only four
particles (2 leptons and 2 b-jets), four-momenta of which are actually measured, while four other particles (2 ν’s and
2 massive χ˜01’s) escape detection, and their existence can only be inferred by an imbalance of transverse energy in
1
FERMILAB-CONF-08-514-E
34th International Conference on High Energy Physics, Philadelphia, 2008
the detector. In addition, masses of χ˜±1 and χ˜
0
1 are unknown, and therefore the kinematics of stop events is severely
unconstrained. The t˜1¯˜t1 event reconstruction is performed by constructing χ2-term, which represents the quadratic
sum of the differences between the true particle masses and invariant masses of their decay products as measured
in the detector and divided by the respective width of the particle. To overcome problems due to unconstrained
kinematics, several useful simplifications are made. First, we use mχ˜±1 as a parameter in the fit and reconstruct
stop mass in each event for several values of mχ˜±1 . Second, to avoid the two-fold ambiguity in assigning a b-jet to
the respective lepton, we chose the pairing that yields the smallest sum of invariant masses
∑
mb`. This approach
identifies the correct pairing in ∼ 90% cases. Third, the pair χ˜01 + ν corresponding to each t˜1 leg is treated as one
massive particle with a large width. Our Monte Carlo studies showed that for a large range of neutralino masses
mχ˜01 ≈ 46 − 90 GeV, the choice of mχ˜01+ν = 75 GeV and Γχ˜01+ν = 10 GeV works reasonably well. Using all of
these assumptions the sums of four-momenta χ˜01 + ν corresponding to each stop leg are still not uniquely identified.
Therefore we integrate over the phase space of all possible solutions weighted by the χ2-term. The reconstructed
mass of the stop quark is then given by mrec
t˜1
=
∫
mrec,i
t˜1
e−χ
2
i dSi/
∫
e−χ
2
i dSi
3. BACKGROUNDS MODELING, SYSTEMATICS UNCERTAINTIES AND LIKELIHOOD FIT
The dominant SM process that contributes to the dilepton + jets event signature is tt¯. Other SM processes include
Z/γ∗+ jets, diboson production, and W+ jet events, where one jet is misidentified as a lepton. We use the pythia [4]
Monte Carlo (MC) event generator to simulate t˜1¯˜t1, tt¯ and diboson processes. Z/γ∗+ associated jet production is
simulated with the alpgen [5] matrix element generator followed by parton fragmentation and hadronization by
pythia. To model W+ jets events we exploit data events with one fully identified lepton plus a lepton-like candidate
that must fail certain lepton ID requirements. A fake rate probability for such an object to be identified as a lepton
is estimated from a large jet-triggered data sample, and then applied to each data lepton + fake event. We validate
the modeling of dilepton events in the control regions corresponding to low /ET , zero and one jet bins and same-sign
charged leptons.
Imperfect knowledge of various experimental and theoretical parameters leads to systematic uncertainties which
degrade our sensitivity to t˜1¯˜t1 signal. The dominant systematic effect is due to the uncertainties in the NLO theoretical
cross sections for t˜1¯˜t1 and tt¯ production. These uncertainties come from two sources: due to renormalization and
factorization scale (11% and 7% for t˜1¯˜t1 and tt¯ respectively) and due to parton density functions (14% and 7%) [6, 7].
We assume that the first one is uncorrelated between two sources, while the latter one is fully correlated. The tt¯
background is normalized to the theoretical cross section value at the top mass world average of 172.5 GeV/c2 [8]
that is dominated by the measurements in the lepton + jets channel of tt¯ decays.
The experimental uncertainties include those due to jet energy scale (3%), b-tagging probability (5%), lepton ID
and trigger efficiencies (1%), initial/final state radiation, and integrated luminosity (6%) which is applied to MC-
based background estimates. Normalization of W and Z+ jets events is determined from data. Uncertainty on
W+jets is driven by the uncertainties in the fake rate predictions (30%), while uncertainty on Z+ jets is due to
mis-modeling of the high- /ET and jet multiplicity distributions and heavy-flavor corrections (16%).
We employ the modified frequentist method, CLs[9], that represents binned likelihood fits to the reconstructed
stop mass of data under the hypothesis of the SM background only and the hypothesis of signal plus background.
The fits are performed simultaneously in the channel with at least one b-tagged jet and the channel with no b-tags.
The systematic uncertainties for both signal and background, described above, enter the fit as Gaussian constrained
nuisance parameters. The shape uncertainties are accounted by allowing templates shapes to change (“morph”)
according to the values of the nuisance parameters.
The sensitivity of the likelihood fit (including all of the systematic uncertainties) to the stop signal is tested for
various event selection criteria imposed separately for b-tagged and non-tagged channel. These criteria are allowed
to vary using an algorithm based on biological evolution. Selection cuts yielding poor sensitivity to the signal are
culled, while those improving sensitivity are bred together until reaching a plateau. Optimizing directly for the best
95% C.L. limit has advantages with respect to event cuts selected based on some intermediate figure of merit.
2
34th International Conference on High Energy Physics, Philadelphia, 2008
 Mass (GeV)t~
100 150 200 250 300 350
Ev
en
ts
/2
0 
Ge
V
0
5
10
15
20
25
Ev
en
ts
/2
0 
Ge
V
data
Stop (DIL BR = 0.18)
=172.5 GeV)tTop (M
Z + Heavy Flavor
Z + Light Flavor
Dibosons
Fakes
=115 GeVt~M
=105.8 GeV±χ∼M
=43.9 GeV0
χ∼
M
)-1CDF Run II Preliminary (2.7 fb
Reconstructed Stop Mass, B-Tagged Channel
Events per 2.7 fb−1 in the signal region with ≥ 1 b-tag.
Source ee µµ eµ ``
top 11.3 ± 1.8 10.4 ± 1.6 26.7 ± 3.8 48.4 ± 7.0
Z/γ∗+HF 1.3 ± 0.3 0.9 ± 0.2 0.4 ± 0.1 2.6 ± 0.5
Z/γ∗+LF 0.9 ± 0.1 0.5 ± 0.1 0.3 ± 0.1 1.7 ± 0.1
diboson 0.2 ± 0.1 0.1 ± 0.1 0.3 ± 0.1 0.6 ± 0.1
fake lepton 0.5 ± 0.2 0.5 ± 0.1 1.8 ± 0.5 2.8 ± 0.8
Total 14.2 ± 2.0 12.4 ± 1.6 29.4 ± 3.8 56.0 ± 7.3
stop 1.1 ± 0.3 1.4 ± 0.4 3.0 ± 0.7 5.5 ± 1.2
Data 15 12 30 57
Table I: The reconstructed stop mass distribution (left) and the table of expected event yields (right) from Standard Model
sources and t˜1
¯˜t1 at the 95% C.L. exclusion limit(right).
2) GeV/c1t
~M(
120 130 140 150 160 170 180
2
) G
eV
/c
0 1χ∼
M
(
45
50
55
60
65
70
75
80
85 2)=105.8 GeV/c±1χ
∼M(
b)=11
±χ∼→1t
~BR(
Observed 95% CL
)-1CDF Run II Preliminary (2.7 fb
l)=0.25ν1
0χ∼→1
±χ∼(2BR
l)=0.50ν1
0χ∼→1
±χ∼(2BR
l)=1.0ν1
0χ∼→1
±χ∼(2BR
Excluded by LEP
2) GeV/c1t
~M(
135 140 145 150 155 160 165 170 175 180 185
2
) G
eV
/c
0 1χ∼
M
(
45
50
55
60
65
70
75
80
85 2)=125.8 GeV/c±1χ
∼M(
b)=11
±χ∼→1t
~BR(
Observed 95% CL
)-1CDF Run II Preliminary (2.7 fb
l)=0.25ν1
0χ∼→1
±χ∼(2BR
l)=0.50ν1
0χ∼→1
±χ∼(2BR
l)=1.0ν1
0χ∼→1
±χ∼(2BR
Excluded by LEP
Figure 1: Observed 95% C.L. limits on the mχ˜01
and mt˜1 for several values of B(χ˜±1 → χ˜01`±ν), and assuming mχ˜±1 =105.8
GeV/c2 (left) and m
χ˜±1
=125.8 GeV/c2 (right). Universality of e, µ, and τ in the χ˜±1 decays is assumed.
4. RESULTS
As can be seen from Table I, the data is consistent with the Standard Model. The fit to the reconstructed stop
mass distribution reveals no evidence of t˜1¯˜t1 production, and thus we proceed and place 95% CL limits on mχ˜01 and
mt˜1 for several values of branching ratio B(χ˜±1 → χ˜01`±ν) and mχ˜±1 . The results are presented in Figure 1.
References
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2 F. Canelli et al., Fermilab-TM-2380-E, TEVEWWG/top 2007/01, [arXiv:hep-ex/0703034].
3 D. Acosta et al., Phys. Rev. D 71, 052003 (2005).
4 T. Sjo¨strand et al., Comput. Phys. Commun. 135, 238 (2001).
5 M. L. Mangano et al., J. High Energy Phys., 0101, 10 (2001).
6 W. Beenakker et al., Nucl. Phys. B515 (1998) 3-14, [arXiv:hep-ph/9810290].
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Search for Pair Production of Supersymmetric Top Quarks Mimicking Standard Model Top Event Signatures at CDF

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Search for Pair Production of Supersymmetric Top Quarks Mimicking Standard Model Top Event Signatures at CDF

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