The modelling of non-perturbative effects is an important part of modern collider physics simulations. In hadron collisions there is some indication that the modelling of the interactions of the beam remnants, the underlying event, may require non-trivial colour reconnection effects to be present. We recently introduced a universally applicable toy model of such reconnections, based on hadronising strings. This model, which has one free parameter, has been implemented in the Pythia event generator. We then considered several parameter sets (`tunes'), constrained by fits to Tevatron minimum-bias data, and determined the sensitivity of a simplified top mass analysis to these effects, in exclusive semi-leptonic top events at the Tevatron. A first attempt at isolating the genuine non-perturbative effects gave an estimate of order +-0.5GeV from non-perturbative uncertainties. The results presented here are an update to the original study and include recent bug fixes of Pythia that influenced the tunings investigated.
I briefly review the progress in perturbative QCD calculations in LEP era and discuss the prospects for the extension of their accuracy to higher orders.
We present a new, universally applicable toy model of colour reconnections in hadronic final states. The model is based on hadronising strings and has one free parameter. We next present an implementation of this model in the Pythia event generator and provide several parameter sets ("tunes"), constrained by fits to Tevatron minimum-bias data. Finally, we consider the sensitivity of a simplified top mass analysis to these effects, in exclusive semi-leptonic top events at the Tevatron. A first attempt at isolating the genuine non-perturbative effects gives an estimate of order delta(m_top) ~ 0.…
We apply Bogoliubov compensation principle to the problem of forward-backward asymmetry in $t \bar t$ production at TEVATRON. The non-trivial solution of compensation equation for anomalous four-fermion interaction effective interaction of doublets of heavy and light quarks leads to possibility of existence of $\bar t u,\, \bar b d$ vector bound states (resonance). The coupling of the bound state to $\bar t,\,u$ pair is calculated. With this result we obtain satisfactory description of totality of data on the effect of FB asymmetry including slope of effect dependence on invariant $t \bar t$ m…
Models with essential non-perturbative QCD dynamics and describing Tevatron data on high-$p_\perp$ charmonium are extrapolated to give predictions of prompt J/psi production at the LHC. Differences of up to an order of magnitude occurs. An important point is here the treatment of higher order perturbative QCD effects.
We consider transverse-momentum (q_T) resummation for Drell--Yan lepton pair production in hadron collisions. At small values of q_T, the logarithmically-enhanced QCD contributions are resummed up to next-to-leading logarithmic accuracy. At intermediate and large values of q_T, resummation is consistently combined with the fixed-order perturbative result. We present numerical results for e^+e^- pairs from the decay of Z bosons produced at Tevatron energies. We perform a detailed study of the scale dependence of the results to estimate the corresponding perturbative uncertainty. We comment on t…
We perform a phenomenological analysis of dijet production in double pomeron exchange at the Tevatron. We find that the CDF Run I results do not rule out the presence of an exclusive dijet component, as predicted by Khoze, Martin and Ryskin (KMR). With the high statistics CDF Run II data, we predict that an exclusive component at the level predicted by KMR may be visible, although the observation will depend on accurate modelling of the inclusive double pomeron exchange process. We also compare to the predictions of the DPEMC Monte Carlo, which contains a non-perturbative model for the central…
Theoretical issues are discussed for the measurement of the top-mass using jets, including perturbative and non-perturbative effects that relate experimental observables to the Lagrangian mass, and appropriate choices for mass schemes. Full account for these issues is given for e+e--> t-tbar using a factorization theorem for event shapes for massive quarks. Implications for the Tevatron top-mass measurement are discussed. A mass-scheme, the "MSR-mass", is introduced which allows for a precise description of observables sensitive to scales R << m, but at the same time does not introduce perturb…
The Tevatron experiments have recently excluded a Standard Model Higgs boson in the mass range 160 - 170 GeV at the 95% confidence level. This result is based on sophisticated analyses designed to maximize the ratio of signal and background cross-sections. In this paper we study the production of a Higgs boson of mass 160 GeV in the gg -> H -> WW -> l nu l nu channel. We choose a set of cuts like those adopted in the experimental analysis and compare kinematical distributions of the final state leptons computed in NNLO QCD to lower-order calculations and to those obtained with the event genera…
We examine the role played in double parton interactions (DPI) by the parton--parton correlations originating from perturbative QCD parton splittings. Also presented are the results of the numerical analysis of the integrated DPI cross sections at Tevatron and LHC energies. To obtain the numerical results the knowledge of the single-parton GPDs gained by the HERA experiments was used to construct the non-perturbative input for generalized double parton distributions. The perturbative two-parton correlations induced by three-parton interactions contribute significantly to resolution of the long…
We review the experimental status of the b-quark production at the Fermilab Tevatron. We compare all available measurements to perturbative QCD predictions (NLO and FONLL) and also to the parton-level cross section evaluated with parton-shower Monte Carlo generators. We examine both the single $b$ cross section and the so called b bar{b} correlations. The review shows that the experimental situation is quite complicated because the measurements appear to be inconsistent among themselves. In this situation, there is no solid basis to either claim that perturbative QCD is challenged by these mea…
We present a detailed study of the total $p p$ cross section for scalar Higgs production to next-to-next-to leading order in $α_s$ at LHC energies, and of the $p\bar p$ cross section at the Tevatron, combining an implementation of the solutions of the parton evolution equations at the 3-loop order with the corresponding hard scatterings, evaluated at the same perturbative order. Our solutions of the DGLAP equations are implemented directly in $x$-space and allow the study of the dependence of the results on the factorization ($μ_F$) and renormalization scales $(μ_R)$ typical of a given process…
We present an implementation of heavy quarkonium production within a perturbative QCD cascade based on the Colour Dipole Cascade model. We consider the processes most relevant in the context of the psi' surplus at the Tevatron; g -> psi' and c -> psi' in the colour-singlet model and g -> psi' through the colour-octet mechanism. Our implementation is, however, easily extendible to other quarkonia and other production mechanisms. Where comparison is possible we find good agreement with analytical calculations. We present some suggestions for measurements at the Tevatron that would be sensitive t…
We present a systematic comparison of jet predictions at the LHC and the Tevatron, with accuracy up to next-to-next-to-leading order (NNLO). The exact computation at NNLO is completed for the gluons-only channel, so we compare the exact predictions for this channel with an approximate prediction based on threshold resummation, in order to determine the regions where this approximation is reliable at NNLO. The kinematic regions used in this study are identical to the experimental setup used by recently published jet data from the ATLAS and CMS experiments at the LHC, and CDF and D0 experiments …
Chromomagnetic and chromoelectric dipole interactions of the top quark are studied in a model independent framework. Limits are set on the scale of new physics that might lead to such contributions using available Tevatron data. Prospects at the LHC are reviewed.
A fully differential calculation in perturbative quantum chromodynamics is presented for the production of massive photon pairs at hadron colliders. All next-to-leading order perturbative contributions from quark-antiquark, gluon-(anti)quark, and gluon-gluon subprocesses are included, as well as all-orders resummation of initial-state gluon radiation valid at next-to-next-to-leading logarithmic accuracy. The region of phase space is specified in which the calculation is most reliable. Good agreement is demonstrated with data from the Fermilab Tevatron, and predictions are made for more detaile…
The 1994 data published by the H1 collaboration are compared with models based on Regge phenomenology. The $x_{\PO}$ dependence of the data can be described in a model based on the exchange of a dominant diffractive (pomeron) trajectory with additional sub-leading reggeon contributions. The dynamics of the Pomeron structure is studied within the framework of perturbative QCD and new parton distributions are obtained. These parton distributions will allow a direct test of factorisation breaking at Tevatron.
Parity-odd asymmetries in the decay angular distribution of a W boson produced with a hard jet in p\bar{p} collisions arise only from QCD rescattering effects. If observed, these asymmetries will provide a first demonstration that perturbative QCD calculation is valid for the absorptive part of scattering amplitudes. We propose a simple observable to measure these asymmetries and perform realistic Monte Carlo simulations at Tevatron energies. It is shown that the Tevatron Run-II should provide sufficient statistics to test the prediction.
The scalar partner of the top quark is relatively light in many models of supersymmetry breaking. We study the production of top squarks (stops) at the Tevatron collider and their subsequent decay through baryon-number violating couplings such that the final state contains no leptons. Performing a detector-level analysis, we demonstrate that, even in the absence of leptons or missing energy, stop masses upto 210 \gev/c^2 can be accessible at the Tevatron.
Predictions for angular distributions of top quark decay products that are sensitive to t tbar spin correlations are presented at next-to-leading order in alpha_s for the Tevatron and the LHC.
我们在Tevatron能量（$\sqrt{s} = 1.8$ ~TeV）下研究了过程$\bar p p \rightarrow J/ψ+ γ+X$.计算了微扰QCD对胶子融合和碎裂机制的贡献。对于整个范围的$p_T$，可以探测在Tevatron，融合的贡献被发现是占主导地位的。因此，QCD对这一过程的预测比$J/ψ$的纯强子产生有更好的精度。在Tevatron上对这一过程的实验研究将有助于澄清大$p_T$上夸克偶素产生的几个方面。
This talk is a brief review of the recent studies on probing new physics through single top quark processes and probing exotic top quark decays at the upgraded Tevatron.
We present results for top quark production at the Tevatron including next-to-next-to-leading order (NNLO) soft-gluon corrections. We show the stability of the cross section with respect to kinematics choice and scale when the NNLO corrections are taken into account.
In order to observe a signal of possible CP violation in top-quark couplings, we study top-quark production and decay under the conditions of the Tevatron upgrade. Transverse energy asymmetries sensitive to CP violation are defined. Applying the recently proposed optimal method, we calculate the statistical significance for the direct observation of CP violation in the production and subsequent decay of top quarks.
The top quark data in the lepton plus $τ$ channel offers a viable probe for the charged Higgs boson signal. We analyse the recent Tevatron collider data in this channel to obtain a significant limit on the $H^\pm$ mass in the large $\tanβ$ region.
New interactions in top-quark production and decay are studied under the conditions of the Tevatron upgrade. Studying the process q qbar --> t tbar --> b mu nu tbar, it is shown how the lepton rapidity and transverse energy distribution are modified by nonstandard modifications of the g-t-tbar and the t-b-W vertex.
This dissertation addresses the question of how to detect light top squarks at the upgraded Fermilab Tevatron collider. After a brief introduction to supersymmetry, the basic phenomenology of the light stop is reviewed and the current experimental situation is surveyed. The analysis presented here is based on collider event simulations. The main decay modes accessible to the Tevatron are studied, feasible discovery channels are identified, and recipes for experimental analysis are proposed. It is found that stops with masses up to the top quark mass are liable to detection under these schemes …
We calculate one-loop corrections to the single-top-quark production via $q\overline{q}' \to t\overline b$ at the Fermilab Tevatron from the Pseudo-Goldstone bosons ( PGBs ) in the framework of one generation technicolor model. The maximum correction to the total cross section for the single-top-quark production is found to reach -2.4% relative to the tree-level cross section, which may be observable at a high-luminosity Tevatron.
Single top-quark production, via weak-interaction processes, is an important test of the standard model, potentially sensitive to new physics. However, this measurement is much more challenging at the Tevatron than originally expected. We reexamine this process and suggest new methods, using shape variables, that can supplement the methods that have been discussed previously.
Spin correlations of top quarks produced in hadron collisions have not been observed experimentally with large significance. In this Letter, we propose a new variable that may enable demonstration of the existence of spin correlations with 3-4 sigma significance using just a few hundred dilepton events both at the Tevatron and the LHC. Such number of dilepton events has been observed at the Tevatron. At the LHC, it will become available once integrated luminosity of a few hundred inverse picobarns is collected.
We present a calculation of the D^{*+}, D^+ and D^0 meson single inclusive production cross section for the Tevatron Run II. We use the FONLL approach in perturbative QCD, which, besides including the known next-to-leading order corrections, also provides for the resummation at the next-to-leading logarithmic level of terms enhanced at large p_T by powers of log(p_T/m), where m is the charm mass and p_T is its transverse momentum. Non-perturbative effects in charm hadronization are extracted, in moment space, from recent ALEPH data for D^* fragmentation in e^+e^- collisions.
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.
Using a simple analytic expression for $q \bar{q}, g g \rightarrow t \bar{t} \rightarrow b W^+ \bar{b} W^- \rightarrow b \bar{l} ν_l \bar{b} l' \bar{ν_{l'}}$ with the interference effects due to the polarizations of the $t$ and $\bar{t}$, we demonstrate how the effects can be measured at Tevatron at $3σ$ level.
I will briefly review the status of higher-order calculations for top-quark observables, comment on the need for improvements, discuss some of the recent theoretical advances, and present a few examples to highlight the role of top-quark observables in searches for signals of physics beyond the Standard Model.
We have calculated the $O(α_{ew} M_t^2/M_W^2)$ supersymmetric electroweak corrections to single top quark production via $q\bar q' \to t\bar b$ at the Fermilab Tevatron in the minimal supersymmetric model. The supersymmetric electroweak corrections to the cross section are a few percent for $tan β> 1$, and can exceed 10% for $tanβ<1$. The combined effects of SUSY electroweak corrections and the Yukawa corrections can exceed 10% for favorable parameter values, which might be observable at a high-luminosity Tevatron.
We investigate the effect that a charged Higgs decay channel for the top quark has on the signature for top quark pair production at the Tevatron. The branching ratios for the various multijet and multilepton final states arising from top pair production are obtained for a wide range of values for tangent beta and the charged Higgs boson mass. In addition, the effect of the charged Higgs decay of the top quark on the 2 b-tagged jets and 1 lepton channel is considered.
New physics associated with the heavy top quark can affect top quark production and the partial decay width of $Z \rightarrow b \bar b$. In this paper, we examine the correlated effects of possible new physics on ${R_b}$ measured at LEP I and the single top quark production rate at Tevatron by using an effective lagrangian technique. We point out that certain operators in the effective lagrangian, constrained by the measured value of $R_b$, can lead to significant and potentially observable effects in single top production.
We show that the recently measured 3.2 $σ$ deviations of the charge asymmetry of like-sign dimuon events from the standard model prediction by the D0 collaboration at Tevatron can be explained by introducing the anomalous right-handed top quark couplings. Combined analysis with the $\bs$ and $\bd$ mixings and $B \to X_s γ$ decays has been performed. We discuss how to discriminate the effects on the dimuon charge asymmetry of the anomalous $\tsW$ and $\tbW$ couplings.
We present a calculation of the single top quark cross section for proton-antiproton interactions with sqrt(s) = 1.8 TeV at the Fermilab Tevatron collider. We examine the effects of top mass, parton distribution functions, QCD scale, and collision energy, on each of the component production mechanisms, and study the kinematic distributions for standard model electroweak production. At the upgraded Tevatron with sqrt(s) = 2.0 TeV and high luminosity, it will be possible to test the nature of the Wtb coupling using single top production. We estimate the sensitivity to measure the single top cros…
We report on the calculation of the next-to-leading order QCD corrections to the production of top--antitop-quark pairs in association with a hard jet at the Tevatron and at the LHC. Results for integrated and differential cross sections are presented. We find a significant reduction of the scale dependence. In most cases the corrections are below 20% indicating that the perturbative expansion is well under control. Moreover, the forward--backward charge asymmetry of the top-quark, which is analyzed at the Tevatron, is studied at next-to-leading order. We find large corrections suggesting that…
An overview of the theoretical status of the top quark and single-jet inclusive production cross sections at the Tevatron is presented. NLO results as well as NNLO-NNLL and higher order threshold expressions derived from resummation calculations are discussed. Numerical results are presented for top quark production at the Tevatron, and it is shown that higher orders contribute sizable corrections to the total cross section and differential distributions, and they also dramatically reduce the factorization scale dependence of the cross section. For jet production, the scale dependence is also …
We show that a top-prime quark as heavy as 600 GeV can be discovered at the Tevatron, provided it is resonantly pair-produced via a vector color octet. If the top-prime originates from a vectorlike quark, then the production of a single top-prime in association with a top may also be observable, even through its decay into a Higgs boson and a top. A color octet with mass of about 1 TeV, which decays into a top-prime pair, may account for the CDF excess of semileptonic (Wj)(Wj) events.
We study and analyze the recent observations of the top pair production $σ(p\bar p\to t \bar t)$ at TeVatron through flavor conserving and flavor violating channels ${\it via}$ vector and tensor unparticles. The unparticle sector is considered with the possibility of being a color singlet or octet. The modified unparticle propagator is used to investigate the contribution of these unparticles to the observed $A_{FB}^{t\bar t}$ (forward backward asymmetry in top pair production) and the spin correlation at TeVatron. We have also studied the impact of the flavor violating couplings of unparticle…
We discuss the decays of the lightest scalar top quark $\tilde{t}_1$ in the Minimal Supersymmetric extension of the Standard Model and show that the final state $bτν_τχ_1^0$, where $χ_1^0$ is the lightest supersymmetric particle, can be dominant in a significant area of the parameter space, in particular in the high $\tan β$ regime. We then analyze the prospects for discovering relatively light scalar top quarks accessible at the Tevatron Run II in the $bτ$ and missing energy channel.
Associated single top production with a Z boson or a photon at large hadron colliders provides a precise determination of top flavour-changing neutral couplings. The best way to measure these couplings with the up quark at Tevatron is to search for events with three jets and missing energy or events with a photon, a charged lepton, a jet and missing energy. Other decay channels are also discussed.
We compute the Next-to-Next-to-Leading Order (NNLO) QCD corrections to the partonic reaction that dominates top-pair production at the Tevatron. This is the first ever NNLO calculation of an observable with more than two colored partons, and/or massive fermions, at hadron colliders. Augmenting our fixed order calculation with soft-gluon resummation through Next-to-Next-to-Leading Logarithmic (NNLL) accuracy, we observe that the predicted total inclusive cross-section exhibits a very small perturbative uncertainty, estimated at +-2.7%. We expect that once all sub-dominant partonic reactions are…
We present the most recent and sensitive measurements of the top quark mass, performed by the Tevatron experiments, D0 and CDF, using ppbar collision data at sqrt(s) = 1.96 TeV. We also present the first measurement of the top quark polarization at Tevatron, obtained by the D0 Collaboration in the dilepton channels.
We calculate all main top-quark pair differential distributions at the Tevatron. For the first time Tevatron predictions for this process include all Standard Model corrections through NLO (referred to as complete-NLO) consistently combined with previously computed NNLO QCD corrections. We also assess, for the first time, the impact of dynamical scales on predictions for the latest Tevatron measurements.
