119845469
Expectations for first single-top studies in CMS in proton-proton collisions
The first long physics run of LHC is taking place at a center-of-mass energy of 7 TeV, and is expected to continue until an integrated luminosity of 1 inverse femtobarn will have been collected. We present an analysis technique to measure the t-channel cross section for single top-quark production in CMS, that can confirm the recent observation of single-top quark production by the Tevatron experiments. Events leading to a signature of exactly one muon and two jets are selected and specific data-driven methods have been developed to reduce the sensitivity to the unknown level of background contamination. Single top quarks provide the possibility to study the polarization of an "isolated quark". This feature is exploited in the presented analysis for the measurement of SM-like single top; with more statistics, the chiral structure of the W-t-b vertex could be probed via the study of the top polarization and any sizable deviation from the Standard Model expectation would be a hint to new physics beyond the Standard Model.
118525591
Constraints on Randall-Sundrum model from top-antitop production at the LHC
A bstractWe study the top pair production cross section at the LHC in the context of Randall-Sundrum model including the Kaluza-Klein (KK) excited gravitons. It is shown that the recent measurement of the cross section of this process at the LHC restricts the parameter space in Randall-Sundrum (RS) model considerably. We show that the coupling parameter $ \left( {\frac{k}{{{{\bar{M}}_{{Pl}}}}}} \right) $ isexcludedbythismeasurementabove 0.03to0.23dependingonthemass of first KK excited graviton (m1). We also study the effect of KK excitations on the spin correlation of the top pairs. It is shown that the spin asymmetry in $ t\bar{t} $ events is sensitive to the RS model parameters with a reasonable choice of model parameters.
119234462
Top quark forward-backward asymmetry and W ′-boson with general couplings
A bstractThe measured forward-backward asymmetry in top pair events at the Fermilab Tevatron collider deviates significantly from the standard model expectation. Several models have been proposed to describe the observed asymmetry which grows with the rapidity difference of the top pairs and also with the $ t\overline{t} $ invariant mass. The presence of a heavy charged gauge boson W′ with the coupling W′ − t − d (left-handed, right-handed, and a mixture of left and right-handed couplings) could generate the desired top forward-backward asymmetry keeping the top pair cross section consistent with the standard model prediction. Such W′-boson makes contribution to the electric dipole moment of the neutron through contribution to the d-quark electric dipole moment, recently measured charge asymmetry (AC ) by the LHC experiments, and the total cross section of top pair at the LHC and Tevatron. We show that the upper bounds on neutron and top electric dipole moments disfavour any W′ with a mass below 240 GeV which could explain the Tevatron forward-backward asymmetry. It is shown that the charge asymmetry provides an allowed region in the parameters space with no overlap with the allowed region where the asymmetry could be described.
118609768
Single top quark production in t-channel at the LHC in noncommutative space–time
Abstract We study the production cross section of the t -channel single top quark at the LHC in the noncommutative space–time. It is shown that the deviation of the t -channel single top cross section from the Standard Model value because of noncommutativity is significant when | θ → | ≳ 10 − 4 GeV − 2 . Using the present experimental precision in measurement of the t -channel cross section, we apply upper limit on the noncommutative parameter. When a single top quark decays, there is a significant amount of angular correlation, in the top quark rest frame between the top spin direction and the direction of the charged lepton momentum from its decay. We study the effect of noncommutativity on the spin correlation and we find that depending on the noncommutative scale, the angular correlation can enhance considerably. Then, we provide limits on the noncommutative scale for various possible relative uncertainties on the spin correlation measurement.
121010784
Angular correlations in top quark decays in standard model extensions
The CMS Collaboration at the CERN LHC has searched for the t-channel single top quark production using the spin correlation of the t-channel. The signal extraction and cross section measurement rely on the angular distribution of the charged lepton in the top quark decays, the angle between the charged lepton momentum and top spin in the top rest frame. The behavior of the angular distribution is a distinct slope for the t-channel single top (signal) while it is flat for the backgrounds. In this Brief Report, we investigate the contributions which this spin correlation may receive from a two-Higgs doublet model, a top-color assisted technicolor (TC2) and the noncommutative extension of the standard model.
12405410
Semileptonic decay of a polarized top quark in the noncommutative standard model
In this paper we study the noncommutative effects to the lepton spectrum from the decay of a polarized top quark. It is shown that the lowest contribution comes from the quadratic terms of the noncommutative parameter. The deviations from the standard model are significant for small values of the noncommutative characteristic scale. However, the charged lepton spin correlation coefficient has a remarkable deviation from the standard model from very low values of the noncommutative characteristic scale to 1 TeV.
119292661
Impact of torsion space?time on $t\bar{t}$ observables at hadron colliders
Starting from the effective torsion space-time model, we study its effects on the top pair production cross section at hadron colliders. We also study the effect of this model on top pair asymmetries at the Tevatron and the LHC. We find that torsion space-time can explain forward-backward asymmetry according to measured anomaly at Tevatron. We find an allowed region in the parameters space which
119300457
Angular Correlations in Associated Production of Single Top and Higgs with and without Anomalous Wtb Couplings
We study the angular correlation and the amount of top quark polarization in the production of a higgs boson in association with a single top quark in the t−channel at the LHC. We also study the effect of anomalous Wtb couplings on the angular correlation and on the production cross section of the process. The cross section and angular correlation is almost insensitive to the variation of the Higgs boson mass within 3 GeV. The robustness of the angular correlation against the center-of-mass energy of the proton-proton collision, the variation of parton distribution functions, and the change of factorization scale is investigated. The sensitivity of this process to the anomalous couplings is examined.
119235505
The impact of the Kaluza–Klein excited W boson on the single top at the LHC and comparison with other models
We study the s-channel single top quark production at the Large Hadron Collider (LHC) in the context of extra dimension theories, including the Kaluza–Klein (KK) decomposition. It is shown that the presence of the first KK excitation of the W gauge boson can reduce the total cross section of the s-channel single top production considerably if (3.5 TeV) for 7 TeV (14 TeV) in proton–proton collisions. Then the results will be compared with the impacts of other beyond standard model (SM) theories on the cross section of a single top s-channel. The possibility of distinguishing different models via their effects on the production cross section of the s-channel is discussed.
118539892
PROBING THE ANOMALOUS COUPLINGS OF THE TOP QUARK WITH GLUON AT THE LHC AND TEVATRON
In this paper, we study the sensitivity of the fraction of events arising from gluon–gluon fusion to the chromoelectric and chromomagnetic dipole moments (CEDM and CMDM) as well as the total and differential cross-sections at the LHC and Tevatron. The sensitivity of measured charged asymmetry at the LHC to CEDM and CMDM is also studied. We find that at the Tevatron and the LHC, nonzero values of CMDM could suppress the production rate. It is shown that the ratio of at the Tevatron is more sensitive to CEDM and CMDM than the LHC case. The presence of CEDM always increases the contribution of gluon–gluon fusion process in top pair rate at the Tevatron and LHC. Except for a small range of CMDM, the presence of CEDM and CMDM can increase the fraction of gluon–gluon fusion at the Tevatron and LHC. The measured ratio of at the Tevatron is used to derive bounds on the chromoelectric and chromomagnetic dipole moments as well as the total and differential cross-sections at the LHC and Tevatron, and the measured charged asymmetry at the LHC. The combination of and σLHC provides stringent limits on CMDM and CEDM.
119158715
Probing the top quark chromoelectric and chromomagnetic dipole moments in single top $tW$-channel at the LHC
Abstract We study the effects of chromoelectric and chromomagnetic dipole moments (CEDM and CMDM) on the production cross section of single top tW -channel at the LHC based on the effective Lagrangian approach. We show that the impact of CEDM and CMDM could be large. Using the experimental measurement of the tW -channel cross section, constraints on CEDM and CMDM are extracted. These constraints are comparable with the ones obtained from the top pair analysis.