Home Page of Zack Sullivan

Here are links to research topics:

• Testing parton distribution functions with t-channel single-top-quark production
  Authors: John Campbell, Tobias Neumann, and Zack Sullivan
  Published: Phys. Rev. D 104, 094042 (2021)
  Abstract: The production of single top-quarks in the t-channel at hadron colliders imposes strong analytic constraints on parton distribution functions (PDFs) through its double deeply inelastic scattering (DDIS) form. We exploit this to provide novel consistency checks between LO, NLO and NNLO PDF fits and propose to include it as a constraint in future PDF fits. Furthermore, while it is well-known that the b-quark PDF is highly sensitive to the b-quark mass, we show that the treatment of this systematic uncertainty is still incomplete, fragmented or outright missing at the moment. Consequently, we conclude that the b-quark mass uncertainty is the dominant but so far broadly neglected theory uncertainty for this process.

• An optimal quantum sampling regression algorithm for variational eigensolving in the low qubit number regime
  Authors: Pedro Rivero, Ian C. Cloet, and Zack Sullivan
  Published: Submitted to Phys. Rev. Research, IIT-CAPP-20-06
  Abstract: The VQE algorithm has turned out to be quite expensive to run given the way we currently access quantum processors (i.e. over the cloud). In order to alleviate this issue, we introduce Quantum Sampling Regression (QSR), an alternative hybrid quantum-classical algorithm, and analyze some of its use cases based on time complexity in the low qubit number regime. In exchange for some extra classical resources, this novel strategy is proved to be optimal in terms of the number of samples it requires from the quantum processor. We develop a simple analytical model to evaluate when this algorithm is more efficient than VQE, and, from the same theoretical considerations, establish a threshold above which quantum advantage can occur. Finally, we demonstrate the efficacy of our algorithm for a benchmark problem.

• Single-top-quark production in the t-channel at NNLO
  Authors: John Campbell, Tobias Neumann, and Zack Sullivan
  Published: JHEP 02 (2021) 040
  Abstract: We present a calculation of t-channel single-top-quark production and decay in the five-flavor scheme at NNLO. Our results resolve a disagreement between two previous calculations of this process that found a difference in the inclusive cross section at the level of the NNLO coefficient itself. We compare in detail with the previous calculations at the inclusive, differential and fiducial level including b-quark tagging at a fixed scale \mu=m_t. In addition, we advocate the use of double deep inelastic scattering (DDIS) scales (\mu^2 = Q^2 for the light-quark line and \mu^2 = Q^2 + m_t^2 for the heavy-quark line) that maximize perturbative stability and allow for robust scale uncertainties. All NNLO and NLOxNLO contributions for production and decay are included in the on-shell and vertex-function approximation. We present fiducial and differential results for a variety of observables used in Standard Model and Beyond Standard Model analyses, and find an important difference between the NLO and NNLO predictions of exclusive t + n-jet cross sections. Overall we find that NNLO corrections are crucial for a precise identification of the t-channel process.

• Off-shell single-top-quark production in the Standard Model Effective Field Theory
  Authors: Tobias Neumann and Zack Sullivan
  Published: JHEP 06 (2019) 022
  Abstract: We present a fully differential and spin-dependent t-channel single-top-quark calculation at next-to-leading order (NLO) in QCD including off-shell effects by using the complex mass scheme in the Standard Model (SM) and in the Standard Model Effective Field Theory (SMEFT). We include all relevant SMEFT operators at 1/Lambda^2 that contribute at NLO in QCD for a fully consistent comparison to the SM at NLO. In addition, we include chirality flipping operators that do not interfere with the SM amplitude and contribute only at 1/Lambda^4 with a massless b-quark. Such higher order effects are usually captured by considering anomalous right-handed Wtb and left-handed Wtb tensor couplings. Despite their formal suppression in the SMEFT, they describe an important class of models for new physics. Our calculation and analysis framework is publicly available in MCFM.

• Harnessing the QCD power spectrum for high energy physics
  Authors: Zack Sullivan and Keith Pedersen
  Published: XLVIIIth International Symposium on Multiparticle Dynamics (ISMD 2018), edited by A. Yang, W.Y. Wang, S.C.C. Ng, A.H. Chan, C.H. Oh, and K.K. Phua, EPJ Web of Conferences 206, 05005 (2019)
  Abstract: We introduce a new framework to encode information in the QCD power spectrum on an event-by-event basis from which jets and their substructure can be extracted. Our new "power jets" scheme requires no jet radius parameter, and allows pileup to be extracted in situ. The result is an accurate kinematic reconstruction of underlying hard physics which is robust to extremely high pileup.

• Are PDFs still consistent with Tevatron data?
  Author: Zack Sullivan
  Published: XLVIIth International Symposium on Multiparticle Dynamics (ISMD 2017)}, edited by A. Ayala Mercado and E. Cuautle Flores, EPJ Web of Conferences 172, 03008 (2018)
  Abstract: As active data taking has moved to the LHC at CERN, more and more LHC data have been included into fits of parton distribution functions. An anomaly has arisen where formerly excellent agreement between theoretical predictions and experiment in single-top-quark production at the Tevatron is no longer quite as good. Is this indicative of a deeper issue?

• Spherical harmonics for multiparticle final states
  Authors: Keith Pedersen and Zack Sullivan
  Published: Proceedings, Meeting of the APS Division of Particles and Fields (DPF 2017), edited by Michael Eads, eConf C170731 (2017).
  Abstract: We examine collider physics via the spectral power H_l, which utilizes much more detector information than sequential jet reconstruction. We use H_l to define a novel jet definition and use it to reconstruct both e+e- > qqg and e+e > tt final states. We find that a significant amount of pileup can be trivially subtracted from H_l jets, provided that the "shape" of pileup can be determined ahead of time. Finally, we find that future progress with H_l jets will require the addition of degrees of freedom accounting for jet "shape" (e.g. radial energy distribution). This should allow the measurement of jet shapes using much more information than traditional techniques.

• Probing the two Higgs doublet wedge region with charged Higgs boson decays to boosted jets
  Authors: Keith Pedersen and Zack Sullivan
  Published: Phys. Rev. D 95, 035037 (2017)
  Abstract: Two Higgs doublet extensions of the standard model, such as supersymmetry, predict the existence of charged Higgs bosons. We explore the reach for TeV-scale charged Higgs bosons through their associated production with top quarks, and their decay to boosted top jets and mu_x tagged boosted bottom jets, at a 14 TeV CERN Large Hadron Collider and at a 100 TeV Future Circular Collider. In particular, we show the moderate tan(beta) "wedge" region of parameter space cannot be probed at the Large Hadron Collider for TeV-scale H^+- because the cross section is too small. However, a 100 TeV future proton collider can close the wedge region below 2 TeV, and search for H^+- up to 6 TeV.

• A priority based noise tolerant jet framework and algorithm
  Authors: Daniel Duffty and Zack Sullivan
  Published: Submitted to JHEP
  Abstract: We introduce a new framework for jet definitions called p-jets that matches the computational speed of the currently used anti-k_T jet algorithm, but avoids combining much of the energy from background pileup events with signal jets. As a first illustration of the p-jet framework, we compare the effectiveness of a p-jet algorithm to the anti-k_T algorithm in reconstructing low energy jets from resonant Z boson production and 50 pileup events.

• Boosted-bottom jet tagging and BSM searches
  Authors: Zack Sullivan
  Published: Proceedings of the 51st Rencontres de Moriond: QCD and High Energy Hadronic Interactions, edited by Etienne Auge, Jacques Dumarchez, and J. Tran Thanh Van (ARISF, Paris, 2016), p. 75.
  Abstract: I present a new scheme for tagging boosted heavy flavor jets called "mu_x tagging" and its application to TeV-scale physics beyond the Standard Model. Using muons from B hadron decay to define a particular combination "x" of angular information, and jet substructure variables, we identify a clean (epsilon_fake/epsilon_b ~ 1/100) good efficiency (epsilon_b = 14%) tag. I demonstrate the usefulness of this new scheme by showing the reach for discovery of leptophobic Z' -> bb_bar and tH^{+-} -> ttb.

• mu_x boosted-bottom-jet tagging and Z-prime boson searches
  Authors: Keith Pedersen and Zack Sullivan
  Published: Phys. Rev. D 93, 014014 (2016)
  Abstract: We present a new technique for tagging heavy-flavor jets with p_T > 500 GeV called "mu_x tagging." Current track-based methods of b-jet tagging lose efficiency and experience a large rise in fake rate in the boosted regime. Using muons from B hadron decay, we combine angular information and jet substructure to tag b jets, c jets, light jets, and "light-heavy" jets (those containing B hadrons from gluon splitting). We find tagging efficiencies of epsilon_b = 14%, epsilon_c = 6.5%, epsilon_{light-light} = 0.14%, and epsilon_{light-heavy} = 0.5%, respectively, that are nearly independent of transverse momentum at high energy. We demonstrate the usefulness of this new scheme by examining the discovery potential for multi-TeV leptophobic Z-prime bosons in the boosted-b-tagged dijet channel at the Large Hadron Collider.

  MuXboostedBTagging: A DELPHES 3 module implementing mu_x boosted boosted-bottom-jet tagging

• Flavor tagging TeV jets for BSM and QCD
  Authors: Keith Pedersen and Zack Sullivan
  Published: Proceedings of the DPF 2015, eConf C150804
  Abstract: We present a new scheme for tagging high-pT bottom and charm jets using energetic muons. Contemporary track-based b tags lose their ability to reject light jet background as jet pT -> O(TeV), where the massive boost exposes fundamental limits in tracking resolution. For our "$\mu_x$" tag, the signal efficiency and light jet rejection is robust versus pT. In the tested regime (jet pT in [0.5,2.1] TeV), $\mu_x$ tags ~14% of b jets, ~6.5% of c jets and ~0.65% of light jets. Since $\mu_x$ tagging should be immediately useful in a searches for heavy resonances, we test it with a typical dijet search - a heavy, leptophobic Z'.

• Improving parton distribution uncertainties in a W mass measurement at the LHC
  Authors: Zack Sullivan and Seth Quackenbush
  Published: Proceedings of the DPF 2015, eConf C150804
  Abstract: We reexamine the dominant contribution of parton distribution function (PDF) uncertainties to the W mass measurement, and determine their contribution is +-39(30) MeV when running the Large Hadron Collider at 7(13) TeV. We find that spurious correlations in older PDF sets led to over-optimistic assumptions regarding normalization to Z observables. In order to understand the origin of the large uncertainties we break down the contribution of the PDF errors into effects at the hard matrix element level, in showering, and in sensitivity to finite detector resolutions.
  Using CT10, CT10W, and charm enhanced PDF sets in comparison to older PDF sets, we develop a robust analysis that examines correlations between transverse mass reconstructions of W and Z decays (scaled by cos $\theta_W$) to leptons. We find that central leptons (|$\eta_l$| < 1.3) from W and Z bosons carry the most weight in reducing the PDF uncertainty, and estimate a PDF error of +10/-12 MeV is achievable in a W mass measurement at the LHC. Further reductions of the W mass uncertainty will require improved fits to the parton distribution functions.

• Parton distributions and the W mass measurement
  Authors: Seth Quackenbush and Zack Sullivan
  Published: Phys. Rev. D 92, 033008 (2015)
  Abstract: We examine the sources of parton distribution errors in the W mass measurement, and point out shortcomings in the existing literature. Optimistic assumptions about strategies to reduce the error by normalizing to Z observables are examined and found to rely too heavily on assumptions about the parametrization and degrees of freedom of the parton distribution functions (PDFs). We devise a strategy to combine measurements as efficiently as possible using error correlations to reduce the overall uncertainty of the measurement, including Z data, and estimate a PDF error of +10-12 MeV is achievable in a W mass measurement at the LHC. Further reductions of the W mass uncertainty will require improved fits to the parton distribution functions.

• LHC and Tevatron constraints on a W' model interpretation of the top quark forward-backward asymmetry
  Authors: Edmond L. Berger, Zack Sullivan, and Hao Zhang
  Published: Phys. Rev. D 88, 114026 (2013)
  Abstract: Aspects of a flavor-changing W' model with right-handed couplings are addressed in this paper in light of Tevatron and LHC data. Our fit to the Tevatron top-quark forward-backward asymmetry and the tt_bar inclusive cross section includes higher-order loop effects in the effective interaction. The higher order corrections change the best fit value of the W' effective coupling strength as a function of the W' mass. The consistency of the model is checked against the shape of the tt_bar invariant mass distribution. We use these updated W' parameters to compute the expected contributions from W't associated production and, for the first time, W'W' pair production at the LHC. We do a full Monte Carlo simulation of the tt_bar+X final state, including interference between the tW' induced tt_bar j process and the standard model tt_bar j process. Interference effects are shown to be quantitatively important, particularly when the W' mass is large. The jet multiplicity distribution in tt_bar+jet production at 8 TeV constrains the W' model severely.

• Next-to-leading order pp -> W' -> tb production at 14 TeV and 33 TeV
  Authors: Zack Sullivan
  Published: Planning the Future of U.S. Particle Physics, econf C1307292, SNOW13-00120
  Abstract: I update the predicted leading order and next-to-leading order cross sections and total widths for W' bosons that decay to top and bottom quarks at 14 TeV and 33 TeV pp colliders (pp -> W' -> tb). Separate tables are included for right- and left-handed bosons. Theoretical uncertainties are completely dominated by parton distribution function uncertainties, and are computed for W'_+- production at a 14 TeV pp collider.

• Reconstructing top quarks at the upgraded LHC and at future accelerators
  Authors: R. Calkins, S. Chekanov, J. Conway, J. Dolen, R. Erbacher, J. Pilot, R. Poschl, S. Rappoccio, A. Schwartzman, Z. Sullivan, and B. Tweedie
  Published: Planning the Future of U.S. Particle Physics, econf C1307292, SNOW13-00075
  Abstract: This report describes the studies performed for the Snowmass "Top algorithms and detectors" High Energy Frontier Study Group.

• Searching for W' bosons through decays to boosted-top and boosted-bottom jets
  Authors: Daniel Duffty and Zack Sullivan
  Published: Phys. Rev. D 90, 015031 (2014)
  Abstract: We propose a novel model-independent method to search for W' bosons at the Large Hadron Collider by looking at dijets where one jet is identified as a boosted-top jet and another jet is tagged as a boosted-bottom jet. Performing a detector simulated signal and background study, we demonstrate that the reach in effective coupling g' is improved over existing analysis methods by up to factor of 2 for W' masses above 1.5 TeV, and the reach in mass is extended by 600 GeV to 2.6 TeV at an 8 TeV Large Hadron Collider. We introduce a method for distinguishing high-energy b jets from light-quark jets, and describe a previously unexplored set of backgrounds to top-jets. We propose a series of data-driven samples that might be used to measure the efficiencies for these new backgrounds in the LHC data.

• Model independent reach for W' bosons at the LHC
  Authors: Daniel Duffty and Zack Sullivan
  Published: Phys. Rev. D 86, 075018 (2012)
  Abstract: The semileptonic decay of single-top-quark production provides a strong probe for W-prime bosons at the CERN Large Hadron Collider. We propose an explicit search strategy for pp -> W-prime -> tb -> l nu b j for use at 7 TeV and 8 TeV collider energies, and integrated luminosities ranging from 5 to 20 fb^-1. Based on detector-simulated results, we predict that a lower bound can be placed on the mass of right-handed W-prime_R with standard model-like couplings of m_{W-prime_R} > 1800 GeV at sqrt{S}=7 TeV with 5 fb^-1, and of m_{W-prime_R} > 2000 GeV at 8 TeV with 20 fb^-1. For left-handed W-prime_L bosons we find a lower bound of 1750-1800 GeV at 7 TeV and 5 fb^-1 depending on the sign of the interference with standard model single-top-quark production. We present effective coupling g^prime dependent limits for accessible masses, and stress the importance of these limits for comparison with theoretical models.

• Constraints on UED from W' searches
  Authors: Thomas Flacke, Arjun Menon, and Zack Sullivan
  Published: Phys. Rev. D 86, 093006 (2012)
  Abstract: We obtain contraints on three Universal Extra Dimensional models utilizing limits from the CMS Collaboration on W' production and decay into a single-top-quark final state. We find a weak constraint on the Minimal Universal Extra Dimensions model due to small Kaluza-Klein number violating terms. In contrast, the W' search puts a strong limit on the size of the Dirac mass term of the quarks in Split Universal Extra Dimension models. In Non-minimal Universal Extra Dimension models the W' search constrains the splitting between the boundary localized kinetic terms of the gauge bosons and the quarks. Each of these bounds can be translated into constraints on the mass splitting between the Kaluza-Klein excitations of the SU(2) charged quarks and the Klauza-Klein excitations of the W boson.

• Associated Higgs plus vector boson test of a fermiophobic Higgs boson
  Authors: Edmond L. Berger, Zack Sullivan, and Hao Zhang
  Published: Phys. Rev. D 86, 015011 (2012)
  Abstract: Production in association with an electroweak vector boson V is a distinctive mode of production for a Higgs boson H without tree-level couplings to fermions, known as a fermiophobic Higgs boson. We focus on HV associated production with H decay into a pair of photons, and V into a pair of jets, with the goal of distinguishing a fermiophobic Higgs boson from the standard model Higgs boson. Performing a simulation of the signal and pertinent QCD backgrounds, and using the same event selection cuts employed by the LHC ATLAS Collaboration, we argue that existing LHC data at 7 TeV with 4.9 fb^{-1} of integrated luminosity may distinguish a fermiophobic Higgs boson from a standard model Higgs boson near 125 GeV at about 1.9 standard deviation signal significance (1.9 sigma) per experiment. At 8 TeV we show that associated production could yield 2.8 sigma significance per experiment with 10 fb^{-1} of data.

• Top quark forward-backward asymmetry and W^prime bosons
  Authors: Daniel Duffty, Zack Sullivan, and Hao Zhang
  Published: Phys. Rev. D 85, 094027 (2012)
  Abstract: The top quark forward-backward asymmetry measured at the Fermilab Tevatron collider deviates from the standard model prediction. A W^prime boson model is described, where the coupling W^prime-t-d is fixed by the tt_bar forward-backward asymmetry and total cross section at the Tevatron. We show that such a W^prime boson would be produced in association with a top quark at the CERN Large Hadron Collider (LHC), thus inducing additional tt_bar+j events. We use measurements of tt_bar+n-jet production from the LHC to constrain the allowed W^prime-t-d couplings as a function of W^prime boson mass. We find that this W^prime model is constrained at the 95% C.L. using 0.7 fb^{-1} of data from the LHC, and could be fully excluded with 5 fb^{-1} of data.

• Higgs exclusion and the H -> WW^* -> l nu c j semi-leptonic channel at the Tevatron
  Authors: Arjun Menon and Zack Sullivan
  Published: Submitted to Phys. Rev. D
  Abstract: We study the Higgs boson decay to W+W-, where one boson decays to leptons, and the other decays to c+jet at Tevatron. Given the current charm tagging acceptances, this channel can help improve and confirm the current combined Tevatron exclusion limit on a standard model-like Higgs boson. If charm acceptance can be improved to at least 24%, this channel could provide the second tightest limits on a Higgs boson mass between 140-190 GeV from a single channel measurement.

• Reconciling the CDF Wjj and single-top-quark anomalies
  Authors: Zack Sullivan and Arjun Menon
  Published: Proceedings of the DPF-2011 Conference, Providence, RI, August 8-13, 2011, edited by Thomas Speer, eConf C110809 (2011)
  Abstract: We demonstrate that there is no evidence of any Wjj excess or deficit within CDF data if a data-derived background estimation that includes single-top-quark production is used instead of a Monte Carlo estimate. Instead, when coupled with the CDF measurement of single-top-quark production, a more interesting anomaly exists within CDF data: namely, there are too many W+0 b-tag and W+2 b-tag events, and too few W+1 b-tag events. As we previously predicted, there is no significant evidence of any of these anomalies in the D0 data set.

• Standard model explanation of a CDF dijet excess in Wjj
  Authors: Zack Sullivan and Arjun Menon
  Published: Phys. Rev. D 83, 091504(R) (2011)
  Abstract: We demonstrate the recent observation of a peak in the dijet invariant mass of the Wjj signal observed by the CDF Collaboration can be explained as the same upward fluctuation observed by CDF in single-top-quark production. In general, both t-channel and s-channel single-top-quark production produce kinematically induced peaks in the dijet spectrum. Since CDF used a Monte Carlo simulation to subtract the single-top backgrounds instead of data, a peak in the dijet spectrum is expected. The D0 Collaboration has a small upward fluctuation in their published t-channel data; and hence we predict they would see at most a small peak in the dijet invariant mass spectrum of Wjj if they follow the same procedure as CDF.

• Charm tagging and the H -> W^+W^- -> l nu c j semi-leptonic channel
  Authors: Arjun Menon and Zack Sullivan
  Published: Submitted to JHEP
  Abstract: We introduce a method to discover the Higgs boson at the Large Hadron Collider (LHC) through its decay to W^+W^-, where one boson decays to leptons, and the other decays to c+jet. This mechanism is complementary to the decay into dileptons, but has the potential to measure the invariant mass peak of the Higgs boson, and to avoid large recently discovered QCD backgrounds from heavy flavor decays. In addition, this mechanism motivates the study and creation of a dedicated charm jet tagger at LHC experiments. Existing charm jet tagging, in the form of fakes to bottom jet tagging, provides sensitivity to a standard model Higgs boson that is comparable to WW fusion. A 50% charm tagging efficiency in the relevant kinematic range could allow an independent 5-sigma discovery of a 165 GeV Higgs boson in 7 fb^-1 of integrated luminosity at a 14 TeV machine, or exclusion with a 7 TeV collider.

• Isolated leptons from heavy flavor decays: Theory and data
  Authors: Zack Sullivan and Edmond L. Berger
  Published: Phys. Rev. D 82, 014001 (2010)
  Abstract: Events with isolated leptons play a prominent role in signatures of new physics phenomena at high energy collider physics facilities. In earlier publications, we examine the standard model contribution to isolated lepton production from bottom and charm mesons and baryons through their semileptonic decays (b, c -> l + X), showing that this source can overwhelm the effects of other standard model processes in some kinematic domains. In this paper, we show that we obtain good agreement with recent Tevatron collider data, both validating our simulations and showing that we underestimate the magnitude of the heavy-flavor contribution to the isolated lepton yields. We also show that the isolation requirement acts as a narrow band-pass filter on the momentum of the isolated lepton, and we illustrate the effect of this filter on the background to Higgs boson observation in the dilepton mode. We introduce and justify a new rule of thumb: isolated electrons and muons from heavy flavor decay are produced with roughly the same distributions as b and c quarks, but with 1/200 times the rates of b and c production, respectively.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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

• 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.

• 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.

• 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).

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.
  

• 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.

• 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).

• 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.

• Testing the Standard Model with Top-Quark Production
  Author: Zack Edward Sullivan
  Published: University of Illinois, 1998.
  

• 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.

• 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.

• 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.

• I wrote "Smashing Particles" for the PBS NOVA show "The Elegant Universe," which first aired in October 2003. This piece explains the significance of several discoveries in particle physics.

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