Single Top Production Research Articles

Next-to-leading order QCD corrections to the single top quark production via model-independenttqgflavor-changing neutral-current couplings at hadron colliders

We present the calculations of the complete next-to-leading order (NLO) QCD effects on the single top productions induced by model-independent $tqg$ flavor-changing neutral-current couplings at hadron colliders. Our results show that, for the $tcg$ coupling, the NLO QCD corrections can enhance the total cross sections by about 60% and 30%, and for the $tug$ coupling by about 50% and 20% at the Tevatron and LHC, respectively, which means that the NLO corrections can increase the experimental sensitivity to the flavor-changing neutral-current couplings by about $10%--30%$. Moreover, the NLO corrections reduce the dependence of the total cross sections on the renormalization or factorization scale significantly, which lead to increased confidence on the theoretical predictions. Besides, we also evaluate the NLO corrections to several important kinematic distributions, and find that for most of them the NLO corrections are almost the same and do not change the shape of the distributions.

IsolatingWtproduction at the LHC

We address the issue of single top production in association with a W boson at the Large Hadron Collider, in particular how to obtain an accurate description in the face of the top pair production background given that the two processes interfere with each other. We stress the advantages of an MC@NLO description, and find that for cuts used to isolate the signal, it makes sense to consider Wt as a well-defined production process in that the interference with top pair production is small, and the cross-section of the former is above the scale variation uncertainty associated with the latter. We also consider the case where both Wt and top pair production are backgrounds to a third process (Higgs boson production followed by decay to a W boson pair), and find in this context that interference issues can also be neglected. We discuss the generalization of our results to other situations, aided by a comparison between the MC@NLO approach and a calculation of the WWbb final state matched to a parton shower.

Angular correlations int-channel single top production at the LHC

When a top quark decays there is a large amount of angular correlation, in its rest frame, between its spin orientation and the direction of flight of the charged lepton from its decay. In this paper we investigate the prospects of measuring this angular correlation using the MC@NLO framework. The strength of the correlation is investigated for different spin bases. The robustness against variation of parton distribution function sets and uncertainties, factorization scale dependence, center-of-mass energy, and the jet $R$-parameter is also examined.

NLO predictions for t-channel production of single top and fourth generation quarks at hadron colliders

We present updated NLO predictions for the electroweak t-channel production of heavy quarks at the Tevatron and at the LHC. We consider production of single top and fourth generation t' starting from both 2 to 2 and 2 to 3 Born processes. Predictions for tb' and t'b' cross sections at NLO are also given for the first time. A thorough study of the theoretical uncertainties coming from parton distribution functions, renormalisation and factorisation scale dependence and heavy quark masses is performed.

NLO single-top production matched with shower in POWHEG:s- andt-channel contributions

We present a next-to-leading order calculation of single-top production interfaced to Shower Monte Carlo programs, implemented according to the POWHEG method. A detailed comparison with MC@NLO and PYTHIA is carried out for several observables, for the Tevatron and LHC colliders.

CDF trigger final balance: Offline resolution at low level selections to fight against Tevatron increasing luminosity

The CDF detector at Tevatron collider is at present the most long-lasting high energy physics experiment. Since its first data taking in 1992 it has produced many results of primary importance, such as the discovery of top quark and, more recently, the observations of Bs oscillations and single-top production. None of them would have been possible without a fast and efficient trigger system. Based on a three level architecture, the CDF trigger takes decisions on simple calorimetric and tracking objects and assures both high efficiency on signal events and low dead time. It reduces the data flow rate from 2.53 MHz, the collision rate, to 150 Hz, the current limit on tape writing and is flexible enough to be easily adapted to the continuously growing instantaneous luminosity. In the last years the Tevatron instantaneous luminosity has rapidly increased and is now reaching 4 × 10 32 cm - 2 s - 1 . The CDF trigger system has been widely upgraded to cope with increasing trigger rates. The upgrade result is online reconstruction of missing transverse energy, jets and tracks with a quality comparable to the offline one. Jet energy and direction can be precisely determined and tracks can be subjected to 3-D reconstruction with good resolution. These upgrades reduce high trigger rates to acceptable levels and have provided invaluable tools to increase the purity of the collected samples. They also represent a helpful experience for LHC experiments where background rates will be much more demanding.

Two-Loop Soft Anomalous Dimensions and Next-to-Next-to-Leading-Logarithm Resummation for Heavy Quark Production

I present results for two-loop soft anomalous dimensions for heavy quark production which control soft-gluon resummation at next-to-next-to-leading-logarithm accuracy. I derive an explicit expression for the exact result and study it numerically for top quark production via e(+)e(-)-->tt, and I construct a surprisingly simple but very accurate approximation. I show that the two-loop soft anomalous dimensions with massive quarks display a simple proportionality relation to the one-loop result only in the limit of vanishing quark mass. I also discuss the extension of the calculation to single top and top pair production in hadron colliders.

Squark flavor violation at the LHC

We consider the prospects for measuring squark flavor violation through the signal of single top production at the LHC. We study this signal in the context of R-symmetric supersymmetry, which allows for large flavor violation in the squark sector, however the results can also be generalized to the MSSM. The single top signal arises from squark pair production in which one squark decays to a top and gaugino, whereas the other squark decays to a non-top quark and gaugino. We study three decay patterns: (I) squark decay into a quark and neutralino LSP; (II) squark decay into a quark and neutralino NLSP, with subsequent decay of the NLSP to a photon and gravitino; (III) squark decay into a quark and chargino NLSP, with subsequent decay of the NLSP to a H^\pm/W^\pm and gravitino. Case II is the most promising, when the NLSP decay is prompt, since every event contains two hard photons that can be used to tag the events, reducing the background to a negligible level. Case I is promising if the neutralino LSP is bino-like. We carefully consider large SM backgrounds and identify a series of cuts to isolate the signal. Case III can occur in the MRSSM with Higgsino-like lightest gauginos. Due to the large Higgs coupling, squarks preferentially decay to top quarks, substantially reducing the potential flavor violating signal. Nevertheless, the flavor violating signal might still be identifiable if the chargino NLSP is long-lived.

Top physics studies at the LHC in the standard model and beyond with the ATLAS detector

The LHC will be a top quark factory, producing large numbers of top quarks even at the initial low luminosities. This will enable a rich program of top quark physics to be explored, both within the Standard Model and using top quarks as probes of physics beyond the Standard Model. Recent studies from ATLAS are presented, including prospects for the measurements of the production of top pairs and single top production, angular correlations in the top decay, and the precision measurement of the top quark mass. The search for physics beyond the Standard Model will be illustrated with searches for rare top decays involving Flavour Changing Neutral Currents, and the reconstruction of tt̄ resonances resulting from new heavy particles in various models.

Littlest Higgs model with T-parity and single top production in ep collisions

Based on calculating the contributions of the littlest Higgs model with T-parity (called LHT model) to the anomalous top coupling tqγ (q = u or c), we consider single top production via the t-channel partonic process eq → et in ep collisions. Our numerical results show that the production cross section in the LHT model can be significantly enhanced relative to that in the standard model (SM).

Dimension six flavor changing neutral current operators and top-quark production at the LHC

In this work we calculate the effects of the electroweak flavor changing neutral currents dimension six effective operators on single-top production at the LHC. These results are then combined with previous ones we have obtained for the strong sector. This allow us, for the first time, to perform a combined analysis of flavor changing neutral currents in top production with all contributing dimension six operators. Finally, we study the feasibility of their observation and characterization both at the Tevatron and at the LHC.

TOP QUARK PHYSICS

A review is presented of the current experimental status of the top quark sector of the Standard Model. The measurements summarized include searches for electroweak single top production, the latest results on the [Formula: see text] production cross-section, and searches for new physics in top quark production and decay. In addition, the recent measurement of the top quark mass to a precision of 0.7% is highlighted.

The constraint on FCNC coupling of the top quark with a gluon from ep-collisions

Using the constraint on the single top production cross-section obtained at the HERA collider, σ(ep→etX), we evaluate an upper limit on coupling constant of the anomalous top quark interaction with a gluon via flavor-changing neutral current: |κtgq/Λ|≲0.4 TeV−1, BR(t→gq)<13%.

Single top production associated with a neutral scalar at LHC in topcolor-assisted technicolor

The topcolor-assisted technicolor (TC2) model predicts a number of neutral scalars like the top-pion (πt0) and the top-Higgs (ht0). These scalars have flavor-changing neutral-current (FCNC) top quark couplings, among which the top-charm transition couplings may be sizable. Such FCNC couplings induce single top productions associated with a neutral scalar at the CERN Large Hadron Collider (LHC) through the parton processes cg → tπt0 and cg → tht0. In this paper we examine these productions and find their production rates can exceed the 3σ sensitivity of the LHC in a large part of parameter space. Since in the Standard Model and the minimal supersymmetric model such rare productions have unobservably small production rates at the LHC, these rare processes will serve as a good probe for the TC2 model.

Impact of single-top measurement to Littlest Higgs model with T-parity

We show that a precise measurement of the single-top production cross section at the Tevatron and the LHC can strongly constrain the model parameters of the Littlest Higgs model with T-parity. A reduction in the single-top production rate from the Standard Model prediction implies new physics phenomena generated by the heavy T-parity partners of the top quark. We show that the degree of polarization of the top quark produced from the decay of its heavy T-odd partner (T−) can be utilized to determine the new physics energy scale, and the mass of T− can be measured from the missing transverse momentum distribution in the T−T¯− event.

Single top production and decay at Nlo

Single top production and decay at Nlo

Associated W and Higgs boson photoproduction and other electroweak photon-induced processes at the lhc

Experimental prospects for studying at the LHC photon-proton interactions at center of mass energies up to and above 1 TeV are discussed. Cross sections are presented for many electroweak and beyond the Standard Model processes. Selection strategies based on photon interaction tagging techniques are discussed. Assuming a typical LHC multipurpose detector, the production of single top associated to a W, and anomalous top signals and their irreducible backgrounds are presented after applying detector acceptance cuts. The associated photoproduction of Higgs and W bosons has a typical cross section of 20 fb. The possibility of observing this reaction is investigated for topologies with signal-to-noise ratio close to unity.

Single top quark photoproduction at the LHC

High-energy photon-proton interactions at the LHC offer interesting possibilities for the study of the electroweak sector up to TeV scale and searches for processes beyond the Standard Model. An analysis of the W associated single top photoproduction has been performed using the adapted MadGraph/MadEvent [F. Maltoni and T. Stelzer, JHEP 0302 , (2003) 027; T. Stelzer and W.F. Long, Phys. Commun. 81 , (1994) 357–371] and CalcHEP [A. Pukhov, Nucl. Inst. Meth A 502 , (2003) 596–598] programs interfaced to the Pythia [T. Sjöstrand et al., Comput. Phys. Commun. 135 , (2001) 238] generator and a fast detector simulation program. Event selection and suppression of main backgrounds have been studied. A comparable sensitivity to | V t b | to those obtained using the standard single top production in pp collisions has been achieved already for 10 fb −1 of integrated luminosity. Photoproduction at the LHC provides also an attractive framework for observation of the anomalous production of single top due to Flavour-Changing Neutral Currents. The sensitivity to anomalous coupling parameters, k t u γ and k t c γ is presented and indicates that stronger limits can be placed on anomalous couplings after 1 fb −1 .

Single W and Anomalous Single Top Production at HERA

The search for events containing isolated leptons (electrons or muons) and missing transverse momentum produced in e^{\pm}p collisions is performed individually and in a common phase space with the H1 and ZEUS detectors at HERA in the period 1994--2007. The presented H1+ZEUS data sample corresponds to an integrated luminosity of 0.97 fb^{-1}, and comprises the complete high energy data from the HERA programme. A total of 87 events are observed in the data, compared to a Standard Model prediction of 92.7 \pm 11.2. At large hadronic transverse momentum P_{T}^{X} > 25 GeV in the e^{+}p data, luminosity 0.58 fb^{-1}, 23 data events are observed compared to a SM prediction of 14.6 \pm 1.9. Production cross section measurements of events containing isolated leptons and missing transverse momentum and of single W production are performed by H1, where the measured cross sections are found to be in agreement with SM predictions. A complementary search by H1 for events containing an isolated tau lepton and missing P_{T} is also presented. A measurement of the W polarisation fractions is performed by H1, where the presented results are found to be in agreement with the SM. Finally, the H1 isolated lepton events are examined in the context of a search for anomalous single top production. In the absence of a clear signal, an upper limit on the anomalous top production cross section \sigma_{ep\to etX} < 0.16 pb is established at the 95% confidence level, corresponding to a limit an upper bound on the anomalous magnetic coupling \kappa_{tu\gamma} < 0.14.

Single-top hadroproduction in association with aWboson

We present the calculation of the Wt single-top production channel to next-to-leading order in QCD, interfaced with parton showers within the MC@NLO formalism. This channel provides a complementary way of investigating the properties of the Wtb vertex, with respect to the s- and t-channels. We pay special attention to the separation of this process from top quark pair production.