Welcome to the
Home Page of Zack Sullivan
- Mailing Address:
- Professor Zack Sullivan
- BCPS
- Life Sciences Bldg., Rm. 106f
- Illinois Institute of Technology
- 3101 S. Dearborn St.
- Chicago, IL 60616-3793
- U.S.A.
- Tel#
- (312) 567-3304
- Fax
- (312) 567-3494
- Mail
-
Zack.Sullivan@iit.edu
Here are links to research topics:
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Trilepton Production at the CERN LHC: Standard Model Sources and Beyond
Authors: Zack Sullivan and Edmond L. Berger
Published:
Phys. Rev. D 78, 034030 (2008).
Abstract: Events with three or more isolated leptons in the final state
are known to be signatures of new physics phenomena at high energy collider
physics facilities. Standard model sources of isolated trilepton final states
include gauge boson pair production such as WZ and W\gamma^*, and
tt_bar production. We demonstrate that leptons from heavy flavor decays,
such as b->lX and c->lX, provide sources of trileptons that can be
orders-of-magnitude larger after cuts than other standard model backgrounds to
new physics processes. We explain the physical reason heavy flavor
backgrounds survive isolation cuts. We propose new cuts to control the
backgrounds in the specific case of chargino plus neutralino pair production
in supersymmetric models. After these cuts are imposed, we show that it should
be possible to find at least a 4 sigma excess for supersymmetry parameter
space point LM9 with 30 fb^{-1} of integrated luminosity.
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Missing heavy flavor backgrounds to Higgs boson production
Authors: Zack Sullivan and Edmond L. Berger
Published:
Phys. Rev. D 74, 033008 (2006).
Abstract: We investigate characteristics of the signal and backgrounds
for Higgs boson decay into WW at the Fermilab Tevatron and CERN Large Hadron
Collider. In the lepton-pair-plus-missing-energy final state, we show that the
background receives an important contribution from semileptonic decays of
heavy flavors. Lepton isolation cuts provide too little suppression of these
heavy flavor contributions, and an additional 4 to 8 orders-of-magnitude
suppression must come from physics cuts. We demonstrate that an increase of
the minimum transverse momentum of nonleading leptons in multilepton events is
one effective way to achieve the needed suppression, without appreciable loss
of the Higgs boson signal. Such a cut would impact the efficiency of searches
for supersymmetry as well. We emphasize the importance of direct measurement
of the lepton background from heavy flavor production.
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Angular correlations in single-top-quark and Wjj production at next-to-leading order
Author: Zack Sullivan
Published:
Phys. Rev. D 72, 094034 (2005).
Abstract: I demonstrate that the correlated angular distributions of
final-state particles in both single-top-quark production and the dominant Wjj
backgrounds can be reliably predicted. Using these fully-correlated angular
distributions, I propose a set of cuts that can improve the single-top-quark
discovery significance by 25%, and the signal to background ratio by a factor
of 3 with very little theoretical uncertainty. Up to a subtlety in t-channel
single-top-quark production, leading-order matrix elements are shown to be
sufficient to reproduce the next-to-leading order correlated distributions.
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Understanding Light: Why we need a terascale photon collider
Author: Zack Sullivan
Published:
Proceedings of the 2005 International Linear Collider
Physics and Detector Workshop and 2nd ILC Accelerator Workshop, eConf
C0508141, ALCPG0402 (2005).
Abstract: We do not understand light. I argue that a terascale photon
collider is necessary to determine the structure of the photon at 100
GeV. Uncertainties in photon parton distribution functions lead to cross
section predictions that vary by a factor of 5. This limits our ability to
predict how well we can perform precision measurements, e.g., extracting the
width of Higgs into two photons. These uncertainties will only be resolved by
measuring the gluonic structure of the photon in situ.
- ZTOP: A
program to calculate NLO s-channel and t-channel single-top-quark production
distributions
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Understanding single-top-quark production and jets at hadron colliders
Author: Zack Sullivan
Published:
Phys. Rev. D 70, 114012 (2004).
Abstract: I present an analysis of fully differential
single-top-quark production plus jets at next-to-leading order. I
describe the effects of jet definitions, top-quark mass, and higher
orders on the shapes and normalizations of the kinematic
distributions, and quantify all theoretical uncertainties. I explain
how to interpret next-to-leading-order jet calculations, and compare
them to showering event generators. Using the program ZTOP, I show
that HERWIG and PYTHIA significantly underestimate both s-channel and
t-channel single-top-quark production, and propose a scheme to match
the relevant samples to the next-to-leading-order predictions.
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Fast Evaluation of CTEQ Parton Distributions in Monte Carlos
Author: Zack Sullivan
Published:
Comput. Phys. Commun. 168, 25 (2005).
Abstract: A few changes to the routines that calculate CTEQ
parton distribution functions allow modern compilers to optimize the
evaluations, while having no quantitative effect on the
results. Overall computation time is reduced by a factor of 4-5 in
matrix-element calculations, and by 1.3-2.5 in showering Monte Carlo
event generators. Similar improvements in performance may be expected
in any calculations relying heavily on interpolation or multiple calls
to functions.
Link to Zack Sullivan's Fast CTEQ PDF web page
Earlier Fermilab Note:
Faster Parton Distribution Evaluation in Monte Carlos
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Lower limits on R-parity-violating couplings in supersymmetric models with light squarks
Authors: Edmond L. Berger and Zack Sullivan
Published:
Phys. Rev. Lett. 92, 201801 (2004).
Abstract: We interpret the results of searches for strongly
interacting massive particles to place absolute lower limits on
R-parity-violating couplings for squarks with mass (m_\tilde{q}) below
100 GeV. Recent searches for anomalous isotopes require that there be
a baryon-number-violating or lepton-number-violating coupling larger
than 10^{-22} -- 10^{-21} if m_\tilde{q} > 18 GeV. Using data from
searches for stable particles at the CERN Large Electron Positron
collider (LEP) we demonstrate that this lower limit increases by 14
orders of magnitude, to an R-parity-violating coupling larger than
10^{-8} -- 10^{-7} for any squarks of mass less than 90 GeV. In the
presence of an R-parity-violating coupling of this magnitude,
neutralinos cannot explain the dark matter density in the Universe.
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How to rule out Little Higgs (and constrain many other models) at the LHC
Author: Zack Sullivan
Published: Proceedings of XXXVIIIth Rencontres de Moriond: QCD,
March 22-29, 2003 (The Gioi Publishers, Hanoi, 2003), p. 379.
Abstract: In this talk I describe how to discover or rule out
the existence of W^{prime} bosons at the CERN Large Hadron Collider as
a function of arbitrary couplings and W^{prime} masses. If W^{prime}
bosons are not found, I demonstrate the 95% confidence-level
exclusions that can be reached for several classes of models. In
particular, W^{prime} bosons in the entire reasonable parameter space
of Little Higgs models can be discovered or excluded in 1 year at the LHC.
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Higgs-boson production via bottom-quark fusion
Authors: F. Maltoni, Z. Sullivan, and S. Willenbrock
Published:
Phys. Rev. D 67, 093005 (2003).
Abstract: Higgs bosons with enhanced coupling to bottom quarks
are copiously produced at hadron colliders via b\bar{b} -> h, where
the initial b quarks reside in the proton sea. We revisit the
calculation of the next-to-leading-order cross section for this
process and argue that the appropriate factorization scale for the b
distribution functions is approximately m_h/4, rather than m_h, as had
been previously assumed. This greatly improves the convergence of the
perturbation series, and yields a result with mild factorization-scale
dependence. We also show that the leading-order calculation of gg ->
b\bar{b}h, integrated over the momenta of the final-state particles,
is very sensitive to the factorization and renormalization scales. For
scales of order m_h/4 the gg -> b\bar{b}h cross section is comparable
to that of b\bar{b} -> h, in contrast to the order-of-magnitude
discrepancy between these two calculations for the scale m_h. The
result we obtain improves the prospects for Higgs-boson discovery at
hadron colliders for large values of tan(\beta).
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Fully differential W^{prime} production and decay at next-to-leading order in QCD
Author: Zack Sullivan
Published:
Phys. Rev. D 66, 075011 (2002).
Abstract: We present the fully differential production and
decay of a W^{prime} boson, with arbitrary vector and axial-vector couplings,
to any final state at next-to-leading order in QCD. We demonstrate a
complete factorization of couplings at next-to-leading order in both
the partial width of the W^{prime} boson, and in the full two-to-two cross
section. We provide numerical predictions for the contribution of a W^{prime}
boson to single-top-quark production, and separate results based on
whether the mass of the right-handed neutrino (nu_R) is light enough
for the leptonic decay channel to be open. The single-top-quark
analysis will allow for an improved direct W^{prime} mass limit of 525-550
GeV using data from run I of the Fermilab Tevatron. We propose a
modified tolerance method for estimating parton distribution function
uncertainties in cross sections.
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Fully differential single-top-quark cross section in next-to-leading order QCD
Authors: B. W. Harris, E. Laenen, L. Phaf, Z. Sullivan, and S. Weinzierl
Published:
Phys. Rev. D 66, 054024 (2002).
Abstract: We present a new next-to-leading order calculation
for fully differential single-top-quark final states. The calculation
is performed using phase space slicing and dipole subtraction
methods. The results of the methods are found to be in agreement. The
dipole subtraction method calculation retains the full spin dependence
of the final state particles. We show a few numerical results to
illustrate the utility and consistency of the resulting computer
implementations.
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Heavy-quark parton distribution functions and their uncertainties
Authors: Z. Sullivan and P. M. Nadolsky
Published:
Proceedings of Snowmass 2001: the Future of Particle Physics,
eConf C010630, P511 (2002).
Abstract: We investigate the uncertainties of the heavy-quark
parton distribution functions in the variable flavor number
scheme. Because the charm- and bottom-quark parton distribution
functions (PDFs) are constructed predominantly from the gluon PDF, it
is a common practice to assume that the heavy-quark and gluon
uncertainties are the same. We show that this approximation is a
reasonable first guess, but it is better for bottom quarks than charm
quarks. We calculate the PDF uncertainty for t-channel
single-top-quark production using the Hessian matrix method, and
predict a cross section of 2.12+0.32-0.29 pb at run II of the
Tevatron.
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PDF uncertainties in WH production at Tevatron
Authors: P. M. Nadolsky and Z. Sullivan
Published:
Proceedings of Snowmass 2001: the Future of Particle Physics,
eConf C010630, P510 (2002).
Abstract: We apply a method proposed by members of CTEQ
Collaboration to estimate the uncertainty in associated W-Higgs boson
production at Run II of the Tevatron due to our imprecise knowledge of
parton distribution functions. We find that the PDF uncertainties for
the signal and background rates are of the order 3%. The PDF
uncertainties for the important statistical quantities (significance
of the Higgs boson discovery, accuracy of the measurement of the (WH)
cross section) are smaller (1.5%) due to the strong correlation of the
signal and background.
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Summary: Working Group on QCD and Strong Interactions
Published:
Proceedings of Snowmass 2001: the Future of Particle Physics,
eConf C010630, P5001 (2002).
Abstract: In this summary of the considerations of the QCD
working group at Snowmass 2001, the roles of quantum chromodynamics in
the Standard Model and in the search for new physics are reviewed,
with emphasis on frontier areas in the field. We discuss the
importance of, and prospects for, precision QCD in perturbative and
lattice calculations. We describe new ideas in the analysis of parton
distribution functions and jet structure, and review progress in
small-x and in polarization.
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Young Physicists' Forum
Published:
Proceedings of Snowmass 2001: the Future of Particle Physics,
eConf C010630, I003 (2002).
Abstract: The Young Physicists' Forum was an opportunity for
the younger members of the particle-physics community to gather at
Snowmass 2001 and to study and debate major issues that face the field
over the next twenty years. Discussions were organized around three
major topics: outreach and education, the impact of globalization, and
building a robust and balanced field. We report on the results of
these discussions, as presented on July 17, 2001.
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A supersymmetric solution to the bottom-quark cross section anomaly
Author: Zack Sullivan
Published: Proceedings of the XXXVIth Rencontres de Moriond: QCD,
March 17-24, 2001 (The Gioi Publishers, Hanoi, 2002), p. 93.
Abstract: In this talk, I describe a supersymmetric solution to the
long-standing discrepancy between the bottom-quark production cross section
and predictions of perturbative quantum chromodynamics. Pair production of
light gluinos, of mass 12-16 GeV, with two-body decays into bottom quarks
and bottom squarks, of mass 2-5.5 GeV, yields the correct normalizations
and shapes of the measured bottom-quark distributions. One prediction of
this scenario is that like-sign B mesons, B+B+ and B-B-, should be produced
with a measurable rate at the next run of the Fermilab Tevatron Collider.
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Direct probes of R-parity-violating supersymmetric couplings
via single-top-squark production
Authors: Edmond L. Berger, B. W. Harris, and Z. Sullivan
Published:
Phys. Rev. D 63, 115001 (2001).
Abstract: We study the s-channel production of a single top squark
in hadron collisions through an R-parity-violating mechanism, examining in
detail the case in which the squark decays through an R-parity-conserving
process into a bottom quark, a lepton, and missing energy. We show that
the top squark can be discovered if its mass is less than 400 GeV, or that
the current bound on the size of the R-parity-violating couplings can be
reduced by up to one order of magnitude with existing data and by two
orders of magnitude at the forthcoming run II of the Fermilab Tevatron.
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Low Energy Supersymmetry and the Tevatron Bottom-Quark Cross Section
Authors: E. L. Berger, B. W. Harris, D. E. Kaplan, Z. Sullivan, T. M. P. Tait, and C. E. M. Wagner
Published:
Phys. Rev. Lett. 86, 4231 (2001).
Citations: SPIRES listing
Abstract: A longstanding discrepancy between the bottom-quark
production cross section and predictions of perturbative quantum
chromodynamics is addressed. We show that pair-production of light gluinos,
of mass 12 to 16 GeV, with two-body decays into bottom quarks and light
bottom squarks, yields a bottom-quark production rate in agreement with
hadron collider data. We examine constraints on this scenario from low
energy data and make predictions that may be tested at the next run of the
Tevatron Collider.
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Fully differential QCD corrections to single top quark final states
Authors: B. W. Harris, E. Laenen, L. Phaf, Z. Sullivan, S. Weinzierl
Published:
Int. J. of Mod. Phys. A 16, Suppl. 1A, 379 (2001).
Abstract: A new next-to-leading order Monte Carlo program for
calculation of fully differential single top quark final states is described
and first results presented. Both the s- and t-channel contributions are
included.
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Report of the SUGRA Working Group for Run II of the Tevatron
Authors: The SUGRA Working Group of the ``Physics at Run II workshop:
Supersymmetry and Higgs''
Abstract: We present an analysis of the discovery reach for
supersymmetric particles at the upgraded Tevatron collider, assuming
that SUSY breaking results in universal soft breaking parameters at
the grand unification scale, and that the lightest supersymmetric
particle is stable and neutral. We first present a review of the
literature, including the issues of unification, renormalization group
evolution of the supersymmetry breaking parameters and the effect of
radiative corrections on the effective low energy couplings and masses
of the theory. We consider the experimental bounds coming from direct
searches and those arising indirectly from precision data, cosmology
and the requirement of vacuum stability. The issues of flavor and
CP-violation are also addressed. The main subject of this study is to
update sparticle production cross sections, make improved estimates of
backgrounds, delineate the discovery reach in the supergravity
framework, and examine how this might vary when assumptions about
universality of soft breaking parameters are relaxed. With 30
fb$^{-1}$ luminosity and one detector, charginos and neutralinos, as
well as third generation squarks, can be seen if their masses are not
larger than 200-250 GeV, while first and second generation squarks and
gluinos can be discovered if their masses do not significantly exceed
400 GeV. We conclude that there are important and exciting physics
opportunities at the Tevatron collider, which will be significantly
enhanced by continued Tevatron operation beyond the first phase of Run II.
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Top Quark Physics
Authors: The Top Physics Working Group of the ``1999 CERN Workshop
on SM physics (and more) at the LHC''
Abstract: We review the prospects for studies of the top quark at the
LHC.
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Single-top-squark production via R-parity-violating supersymmetric
couplings in hadron collisions
Authors: Edmond L. Berger, B. W. Harris, and Z. Sullivan
Published:
Phys. Rev. Lett. 83, 4472 (1999).
Abstract: Single-top-squark production via q q' ->
\bar{\tilde{t_1}} probes R-parity-violating extensions of the minimal
supersymmetric standard model through the \lambda''_{3ij}
couplings. For masses in the range 180-325 GeV, and \lambda''_{3ij} >
0.02-0.06, we show that discovery of the top squark is possible with 2
fb^{-1} of integrated luminosity at run II of the Fermilab Tevatron.
The bound on \lambda''_{3ij} can be reduced by up to an order of
magnitude with existing data from run I, and by two orders of
magnitude at run II if the top squark is not found.
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Searching for R-Parity Violation at Run-II of the Tevatron
Authors: R-parity violation subgroup of the Physics at Run II workshop:
Supersymmetry and Higgs
Abstract: We present an outlook for possible discovery of supersymmetry
with broken R-parity at Run II of the Tevatron. We first present a review of
the literature and an update of the experimental bounds. In turn we then
discuss the following processes: 1. Resonant slepton production
followed by R-parity violating decay, (a) via $LQD^c$ and (b) via $LLE^c$.
2. How to distinguish resonant slepton production from $Z'$ or
$W'$ production. 3. Resonant slepton production followed by the decay to
neutralino LSP, which decays via $LQD^c$. 4. Resonant stop
production followed by the decay to a chargino, which cascades to the
neutralino LSP. 5. Gluino pair production followed by the cascade decay
to charm squarks which decay directly via $L_1Q_2D^c_1$. 6. Squark pair
production followed by the cascade decay to the neutralino LSP
which decays via $L_1Q_2D^c_1$. 7. MSSM pair production followed by the
cascade decay to the LSP which decays (a) via $LLE^c$, (b) via $LQD^c$,
and (c) via $U^cD^cD^c$, respectively. 8. Top quark and top squark decays
in spontaneous R-parity violation.
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Higgs-boson production in association with bottom quarks at
next-to-leading order
Authors: D. Dicus, T. Stelzer, Z. Sullivan and S. Willenbrock
Published:
Phys. Rev. D 59, 094016 (1999).
Abstract: We argue that the leading-order subprocess for
Higgs-boson production in association with bottom quarks is (b\bar{b} -> H).
This process is an important source of Higgs bosons with enhanced
Yukawa coupling to bottom quarks. We calculate the corrections to this
cross section at next-to-leading-order in 1/ln(m_H/m_b) and alpha_s
and at next-to-next-to-leading order in 1/ln(m_H/m_b).
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Single-top-quark production at hadron colliders
Authors: T. Stelzer, Z. Sullivan and S. Willenbrock
Published:
Phys. Rev. D 58, 094021 (1998).
Citations: SPIRES listing
Abstract: Single-top-quark production probes the
charged-current weak interaction of the top quark, and provides a
direct measurement of the CKM matrix element V_{tb}. We perform two
independent analyses to quantify the accuracy with which the W-gluon
fusion (gq -> t\bar{b}q) and (q\bar{q} -> t\bar{b}) signals can be
extracted from the backgrounds at both the Tevatron and the
LHC. Although perturbation theory breaks down at low transverse
momentum for the W-gluon fusion \bar{b} differential cross section, we
show how to obtain a reliable cross section integrated over low
\bar{b} transverse momenta up to a cutoff. We estimate the accuracy
with which V_{tb} can be measured in both analyses, including
theoretical and statistical uncertainties. We also show that the
polarization of the top quark in W-gluon fusion can be detected at the
Tevatron and the LHC.
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Testing the Standard Model with Top-Quark Production
Author: Zack Edward Sullivan
Published: University of Illinois, 1998.
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Single-top-quark production via W-gluon fusion at next-to-leading
order
Authors: T. Stelzer, Z. Sullivan and S. Willenbrock
Published:
Phys. Rev. D 56, 5919-5927 (1997).
Citations: SPIRES listing
Abstract: Single-top-quark production via W-gluon fusion at
hadron colliders provides an opportunity to directly probe the
charged-current interaction of the top quark. We calculate the
next-to-leading-order corrections to this process at the Fermilab
Tevatron, the CERN Large Hadron Collider, and DESY HERA. Using a
b-quark distribution function to sum collinear logarithms, we show
that there are two independent corrections, of order 1/ln(m_t^2/m_b^2)
and $\alpha_s$. This observation is generic to processes involving a
perturbatively-derived heavy-quark distribution function at an energy
scale large compared with the heavy-quark mass.
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Supersymmetric QCD correction to top-quark production at the Fermilab
Tevatron
Author: Zack Sullivan
Published:
Phys. Rev. D 56, 451-457 (1997).
Abstract: We calculate the supersymmetric QCD correction to top-quark
production at the Fermilab Tevatron, allowing for arbitrary left-right mixing
of the squarks. We find that the correction is significant for several
combinations of gluino and squark masses, e.g. +33% for m_{\tilde g}=200 GeV,
m_{\tilde t} = m_{\tilde q} =75 GeV.
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Top Quark Physics: Future Measurements
Authors: Top Physics working group at the 1996 DPF/DPB Snowmass
workshop
Abstract: (Report of the Top Physics working group at the 1996 DPF/DPB
Snowmass workshop, to appear in the proceedings). We discuss the study of the
top quark at future experiments and machines. Top's large mass makes it a
unique probe of physics at the natural electroweak scale. We emphasize
measurements of the top quark's mass, width, and couplings, as well as searches
for rare or nonstandard decays, and discuss the complementary roles played by
hadron and lepton colliders.
Zack E. Sullivan,
Illinois Institute of Technology