Chromomagnetic Dipole Moment Research Articles

Probing the top-quark chromomagnetic dipole moment at next-to-leading order in QCD

We present predictions at NLO accuracy in QCD for top-quark pair production induced by an anomalous chromomagnetic dipole moment of the top quark. Our results are obtained for total as well as fully differential cross sections, including matching to parton shower simulations. This process is expected to provide the most stringent direct limits on top-quark chromomagnetic dipole moment. We find that NLO corrections increase the contribution from the dipole moment by about 50\% at the LHC, and significantly reduce the renormalization and factorization scale dependence. Using the NLO prediction, we update the current limit from the Tevatron and the LHC measurements. Apart from total cross section, we also study other observables relevant for LHC phenomenology.

Direct photon production as a probe of quarks chromoelectric and chromomagnetic dipole moments at the LHC

In this paper we show that the $\gamma+$jet invariant mass distribution in proton-proton collisions at the LHC is significantly sensitive to the quarks chromoelectric ($\kappa$) and chromomagnetic ($\tilde{\kappa}$) dipole moments. It is shown that the presence of $\kappa$ or $\tilde{\kappa}$ leads to an increment of the cross section of $\gamma+$jet process in particular in the tail of $\gamma+$jet invariant mass distribution. Using the measured $\gamma+$jet invariant mass distribution by the CMS experiment at the center-of-mass energy of 8 TeV, we derive bounds on the quarks chromoelectric and chromomagnetic dipole moments. In extraction of the limits, we consider both theoretical and systematic uncertainties. The uncertainties originating from variation of the renomalization/factorization scales and the choice of proton parton distribution functions are taken into account as a function of the $\gamma+$jet invariant mass. We exclude $\kappa$ or $\tilde{\kappa}$ above $10^{-5}$ at 95$\%$ confidence level. This is the most stringent direct upper limit on $\kappa$ or $\tilde{\kappa}$.

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 [Formula: see text] events arising from gluon–gluon fusion to the chromoelectric and chromomagnetic dipole moments (CEDM and CMDM) as well as the total and differential [Formula: see text] 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 [Formula: see text] production rate. It is shown that the ratio of [Formula: see text] 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 [Formula: see text] at the Tevatron is used to derive bounds on the chromoelectric and chromomagnetic dipole moments as well as the total and differential [Formula: see text] cross-sections at the LHC and Tevatron, and the measured charged asymmetry at the LHC. The combination of [Formula: see text] and σ LHC provides stringent limits on CMDM and CEDM.

Probing the top quark chromoelectric and chromomagnetic dipole moments in single top tW-channel at the LHC

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.

Probing top-Higgs non-standard interactions at the LHC

Abstract Effective interactions involving both the top quark and the Higgs field are among the least constrained of all possible (gauge invariant) dimension-six operators in the Standard Model. Such a handful of operators, in particular the top quark chromomagnetic dipole moment, might encapsulate signs of the new physics responsible for electroweak symmetry breaking. In this work, we compute the contributions of these operators to inclusive Higgs and $ t\overline t h $ productions. We argue that: i) rather strong constraints on the overall size of these operators can already be obtained from the current limits/evidence on Higgs production at the LHC; ii) $ t\overline t h $ production will provide further key information that is complementary to $ t\overline t $ measurements, and the possibility of discriminating among different contributions by performing accurate measurements of total and differential rates.

Constraining the dipole moments of the top quark

We investigate the direct and indirect bounds on dipole operators involving the top quark. A careful analysis shows that the experimental upper limit on the neutron electric dipole moment strongly constrains the chromo-electric dipole of the top. We improve previous bounds by 2 orders of magnitude. This has significant implications for new physics models and it also means that $CP$ violation in top pair production mediated by dipole operators will not be accessible at the LHC. The $CP$ conserving chromo-magnetic dipole moments are constrained by recent measurements of the $t\overline{t}$ spectrum by the ATLAS collaboration. We also update the indirect constraints on electric and magnetic dipole moments from radiative $b\ensuremath{\rightarrow}s$ transitions, finding that they can be considerably larger than their colored counterparts.

CONSTRAINTS ON UNPARTICLE PHYSICS FROM THE $gt\bar t$ ANOMALOUS COUPLING

We study the impact of unparticle physics to the chromomagnetic dipole moment (CMDM) of the top quark. We compute the effect induced by unparticle operators of scalar and vector nature coupled to fermions on the CMDM. We find that this dipole moment is sensitive to the scale dimensions du of the unparticle and the new couplings of the respective effective operators. Using the bounds imposed on the CMDM by low-energy precision and Tevatron measurements we derive indirect limits on the unparticle parameter space. In particular, we find that the scalar-unparticle operator contribution fulfills both constraints for most of the unparticle parameter space, while the low-energy precision bound on the CMDM excludes a vector-unparticle contribution for low values of respective scale dimension du.

Top Quark Chromomagnetic Dipole Moment in the Littlest Higgs Model with T-Parity

The littlest Higgs model with T-parity, which is called LHT model, predicts the existence of the new particles, such as heavy top quarks, heavy gauge bosons, and mirror fermions. We calculate the one-loop contributions of these new particles to the top quark chromomagnetic dipole moment (CMDM) ΔK. We find that the contribution of the LHT model is one order of magnitude smaller than the standard model prediction value.

Tests of Higgs and top effective interactions

We study the possibility to detect heavy physics effects in the interactions of Higgs bosons and the top quark at future colliders using the effective Lagrangian approach, where the SM Lagrangian is modified by non-renormalizable operators that are invariant under the full strong and electroweak group. The modification of the interactions may enhance the production of Higgs bosons at hadron colliders through the mechanisms of gluon fusion and associated production with a W boson or t t pairs. The most promising signature is through the decay of the Higgs boson into two photons, whose branching ratio is also enhanced in this approach. As a consequence of our analysis we get a bound on the chromomagnetic dipole moment of the top quark.

Using the radiative decayb→sγto bound the chromomagnetic dipole moment of the top quark

Using the CLEO data for $b\ensuremath{\rightarrow}s\ensuremath{\gamma}$, we constrain the chromomagnetic dipole moment of the top quark within the context of a decoupling effective Lagrangian approach. Our results are in agreement with the results obtained for the ${e}^{+}{e}^{\ensuremath{-}}$ and hadron colliders.

Effects of anomalous couplings of quarks on prompt photon production

Prompt photon production is sensitive to the anomalous couplings of gluons to quarks, because it is mainly produced by quark-gluon scattering. We will examine the effects of the anomalous chromoelectric and chromomagnetic dipole moment couplings of quarks on prompt photon production. Using the data collected by CDF and D0 at the Fermilab Tevatron we put a bound on these anomalous couplings. We also estimate the sensitivity of various future high energy collider experiments to these anomalous couplings.

Top-quark production in hadron-hadron collisions and anomalous top-quark-gluon couplings.

We discuss the influence of anomalous tbar-t-G couplings on total and differential tbar-t production cross sections in hadron-hadron collisions. We study in detail the effects of a chromoelectric and a chromomagnetic dipole moment, d' and \mu', of the top quark. In the d'-\mu' plane, we find a whole region where the anomalous couplings give a zero net contribution to the total top production rate. In differential cross sections, the anomalous moments have to be quite sizable to give measurable effects. We estimate the values of d' and \mu' which are allowed by the present Tevatron experimental results on top production. A chromoelectric dipole moment of the top violates CP invariance. We discuss a simple CP-odd observable which allows for a direct search for CP violation in top production.

Probing the chromoelectric and chromomagnetic dipole moments of the top quark at hadronic colliders.

We study the effects of the anomalous chromoelectric and chromomagnetic dipole moments of the top quark on the top-pair production, on the top-pair plus 1jet production, and on their ratios, as well as their dependence on the transverse momenta of the top quark and the jet. We also construct CP-odd and $\hat T$-odd observables to probe the dispersive part of the chromoelectric form factor of the top quark, and compare their sensitivities in both top-pair and top-pair plus 1 jet production.