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L. H. Orr

Publications and source records attributed to L. H. Orr.

At least 19 recordsLinked to original sources
arXiv2008

Searching for t-bar t Resonances at the Large Hadron Collider

Many new physics models predict resonances with masses in the TeV range which decay into a pair of top quarks. With its large cross section, t-bar t production at the Large Hadron Collider (LHC) offers an excellent opportunity to search for such particles. We present a detailed study of the discovery potential of the CERN Large Hadron Collider for Kaluza-Klein (KK) excitations of the gluon in bulk Randall-Sundrum (RS) models in the t-bar t -> ell^+/- nu b-bar bq-bar q' (ell=e, mu) final state. We utilize final states with one or two tagged b-quarks, and two, three or four jets (including b-jets). Our calculations take into account the finite resolution of detectors, the energy loss due to b-quark decays, the expected reduced b-tagging efficiency at large t-bar t invariant masses, and include the background originating from Wb-bar b+jets, (Wb+W-bar b)+jets, W+jets, and single top + jets production. We derive semi-realistic 5 sigma discovery limits for nine different KK gluon scenarios, and compare them with those for KK gravitons, and a Z_H boson in the Littlest Higgs model. We also analyze the capabilities of the LHC experiments to differentiate between individual KK gluon models and measure the couplings of KK gluons to quarks. We find that, for the parameters and models chosen, KK gluons with masses up to about 4 TeV can be discovered at the LHC. The ability of the LHC to discriminate between different bulk RS models, and to measure the couplings of the KK gluons is found to be highly model dependent.

hep-ph↗
arXiv2007

High p_T Top Quarks at the Large Hadron Collider

Many new physics models predict resonances with masses in the TeV range which decay into a pair of top quarks. With its large cross section, t\bar t production at the Large Hadron Collider (LHC) offers an excellent opportunity to search for such particles. The identification of very energetic top quarks is crucial in such an analysis. We consider in detail the t\bar t\to\ell^\pmνb\bar bq\bar q' (\ell=e, μ) final state for high p_T top quarks. In this phase space region, two or more of the final state quarks can merge into a single jet due to the large Lorentz boost of the parent top quark. As a result, a large fraction of t\bar t\to\ell^\pmνb\bar bq\bar q' events with an invariant mass in the TeV region contains less than four observable jets. Requiring one or two tagged b-quarks, we calculate the W+jets, Wb+jets, Wb\bar b+jets, Wbt, and single top plus jets backgrounds for these final states, and identify cuts which help to suppress them. In particular, we discuss whether a cut on the jet invariant mass may be useful in reducing the background in the \ellν+2 jets channel. We also investigate how next-to-leading order QCD corrections affect high p_T top quark production at the LHC. We find that the \ellν+2 jets and \ellν+3 jets final states with one or two $b$-tags will significantly improve the chances for discovering new heavy particles in the t\bar t channel at the LHC.

hep-ph↗
arXiv2005

Improved Measurement of ttZ Couplings at the LHC

We consider QCD tt~Z production at the LHC with Z->\barννand all-hadronic tt~ decays, i.e. pp -> p_T(miss)bb~+4 jets, as a tool to measure ttZ couplings. This channel has a significantly larger cross section than those where the Z boson decays leptonically. However, tt~, bb~+4 jet, tt~j and tt~jj production give rise to potentially large backgrounds. We show that these processes can be suppressed to an acceptable level with suitable cuts, and find that adding the p_T(miss)bb~+4 jet channel to the final states used in previous ttZ couplings analyses will improve the sensitivity by 10-60%. We also discuss how the measurement of the ttZ couplings may constrain Little Higgs models.

hep-ph↗
arXiv2004

Probing Electroweak Top Quark Couplings at Hadron Colliders

We consider QCD t\bar{t}γand t\bar{t}Z production at hadron colliders as a tool to measure the ttγand ttZ couplings. At the Tevatron it may be possible to perform a first, albeit not very precise, test of the ttγvector and axial vector couplings in t\bar{t}γproduction, provided that more than 5 fb^{-1} of integrated luminosity are accumulated. The t\bar{t}Z cross section at the Tevatron is too small to be observable. At the CERN Large Hadron Collider (LHC) it will be possible to probe the ttγcouplings at the few percent level, which approaches the precision which one hopes to achieve with a next-generation e^+e^- linear collider. The LHC's capability of associated QCD t\bar{t}V (V=γ, Z) production has the added advantage that the ttγand ttZ couplings are not entangled. For an integrated luminosity of 300 fb^{-1}, the ttZ vector (axial vector) coupling can be determined with an uncertainty of 45-85% (15-20%), whereas the dimension-five dipole form factors can be measured with a precision of 50-55%. The achievable limits improve typically by a factor of 2-3 for the luminosity-upgraded (3 ab^{-1}) LHC.

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arXiv2004

Physics Interplay of the LHC and the ILC

Physics at the Large Hadron Collider (LHC) and the International e+e- Linear Collider (ILC) will be complementary in many respects, as has been demonstrated at previous generations of hadron and lepton colliders. This report addresses the possible interplay between the LHC and ILC in testing the Standard Model and in discovering and determining the origin of new physics. Mutual benefits for the physics programme at both machines can occur both at the level of a combined interpretation of Hadron Collider and Linear Collider data and at the level of combined analyses of the data, where results obtained at one machine can directly influence the way analyses are carried out at the other machine. Topics under study comprise the physics of weak and strong electroweak symmetry breaking, supersymmetric models, new gauge theories, models with extra dimensions, and electroweak and QCD precision physics. The status of the work that has been carried out within the LHC / LC Study Group so far is summarised in this report. Possible topics for future studies are outlined.

hep-ph↗
arXiv2003

Theoretical progress for the associated production of a Higgs boson with heavy quarks at hadron colliders

The production of a Higgs boson in association with a pair of top-antitop or bottom-antibottom quarks plays a very important role at both the Tevatron and the Large Hadron Collider. The theoretical prediction of the corresponding cross sections has been improved by including the complete next-to-leading order QCD corrections. After a brief introduction, we review the results obtained for both the Tevatron and the Large Hadron Collider.

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arXiv2003

NLO QCD corrections to hadronic Higgs production with heavy quarks

The production of a Higgs boson in association with a pair of top-antitop or bottom-antibottom quarks plays a very important role at both the Tevatron and the Large Hadron Collider. The theoretical prediction of the corresponding cross sections has been improved by including the complete next-to-leading order QCD corrections. After a brief description of the most relevant technical aspects of the calculation, we review the results obtained for both the Tevatron and the Large Hadron Collider

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arXiv2003

Associated Higgs production with top quarks at the Large Hadron Collider: NLO QCD corrections

We present in detail the calculation of the O(alpha_s^3) inclusive total cross section for the process pp -> t-tbar-h, in the Standard Model, at the CERN Large Hadron Collider with center-of-mass energy sqrt(s_H)=14 TeV. The calculation is based on the complete set of virtual and real O(alpha_s) corrections to the parton level processes q-qbar -> t-tbar-h and gg -> t-tbar-h, as well as the tree level processes (q,qbar)g -> t-tbar-h-(q,qbar). The virtual corrections involve the computation of pentagon diagrams with several internal and external massive particles, first encountered in this process. The real corrections are computed using both the single and the two cutoff phase space slicing method. The next-to-leading order QCD corrections significantly reduce the renormalization and factorization scale dependence of the Born cross section and moderately increase the Born cross section for values of the renormalization and factorization scales above m_t.

hep-ph↗
arXiv2003

Associated Higgs boson production with heavy quarks

The production of a Higgs boson in association with a pair of t-tbar quarks will play a very important role at both hadron and lepton colliders. We review the status of theoretical predictions and their relevance to Higgs boson studies, with particular emphasis on the recently calculated NLO QCD corrections to the inclusive cross section for p-pbar,pp -> t-tbar-h. We conclude by briefly discussing the case of exclusive b-bbar-h production and the potential of this process in revealing signals of new physics beyond the Standard Model.

hep-ph↗
arXiv2002

Associated Top Quark-Higgs Boson Production at the LHC

We compute the O(alpha_s^3) inclusive cross section for the process pp -> t-tbar-h in the Standard Model, at sqrt(s)=14 TeV. The next-to-leading order corrections drastically reduce the renormalization and factorization scale dependence of the Born cross section and increase the total cross section for renormalization and factorization scales larger than m_t. These corrections have important implications for models of new physics involving the top quark.

hep-ph↗
arXiv2001

Direct Measurement of the Top Quark Charge at Hadron Colliders

We consider photon radiation in tbar-t events at the upgraded Fermilab Tevatron and the CERN Large Hadron Collider (LHC) as a tool to measure the electric charge of the top quark. We analyze the contributions of tbar-t-gamma production and radiative top quark decays to p-p, pbar-p -> gamma l^+/- nu bbar-b jj, assuming that both b-quarks are tagged. With 20~fb^{-1} at the Tevatron, the possibility that the ``top quark'' discovered in Run I is actually an exotic charge -4/3 quark can be ruled out at the 95% confidence level. At the LHC, it will be possible to determine the charge of the top quark with an accuracy of about 10%.

hep-ph↗
arXiv2000

Supersymmetry without R-parity : Constraints from Leptonic Phenomenology

R-parity conservation is an {\it ad hoc} assumption in the most popular version of the supersymmetric standard model. Most studies of models which do allow for R-parity violation have been restricted to various limiting scenarios. The single-VEV parametrization used in this paper provides a workable framework to analyze phenomenology of the most general theory of SUSY without R-parity. We perform a comprehensive study of leptonic phenomenology at tree-level. Experimental constraints on various processes are studied individually and then combined to yield regions of admissible parameter space. In particular, we show that large R-parity violating bilinear couplings are not ruled out, especially for large $\tanβ$.

hep-ph↗
arXiv1998

Gluon Radiation in Top Mass Reconstruction: Effect of Hadronic W Decays

Top quark events in hadron collisions often contain additional hadronic jets from gluon bremsstrahlung off the quarks and gluons in the hard subprocesses. Such extra jets must be taken into account in attempts to reconstruct the momentum of the top quark from those of its decay products. We have performed a complete calculation of gluon radiation in top production and decay at the Fermilab Tevatron including hadronically decaying W bosons. In this talk we discuss the effect of gluon radiation on the reconstructed mass of the top quark, using various top mass reconstruction scenarios. Implications for the LHC are briefly discussed.

hep-ph↗
arXiv1998

Gluon Radiation in Top Quark Production and Decay at an e^+ e^- Collider

We study the effects of gluon radiation on top production and decay processes at an $e^+e^-$ collider.The matrix elements are computed without any approximations, using spinor techniques. We use a Monte Carlo event generator which takes into account the infrared singularity due to soft gluons and differences in kinematics associated with radiation in the production versus decay process. The calculation is illustrated for several strategies of top mass reconstruction.

hep-ph↗
arXiv1998

A BFKL Monte Carlo approach to jet production at hadron-hadron and lepton-hadron colliders

The production of a pair of jets with large rapidity separation in hadron-hadron collisions, and of forward jets in deep inelastic scattering, can in principle be used to test the predictions of the BFKL equation. However in practice kinematic constraints lead to a strong suppression of BFKL effects for these processes. This is illustrated using a BFKL Monte Carlo approach.

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