Chromomagnetic Dipole Moment Research Articles

Chromomagnetic dipole moments of light quarks in the bestest little Higgs model

In this study, we investigated the anomalous Chromomagnetic Dipole Moment (CMDM), denoted as , of light quarks within the framework of the Bestest Little Higgs Model (BLHM) as an extension of the Standard Model (SM). Our investigation encompassed novel interactions among the light quarks, heavy quark B, and heavy bosons , incorporating the extended Cabibbo-Kobayashi-Maskawa (CKM) matrix characteristic of the BLHM. We thoroughly explored the permissible parameter space, yielding a spectrum of CMDM values ranging from to .

Two-loop contributions to the anomalous chromomagnetic dipole moment of the top quark in two-Higgs-doublet models

We present the results for the anomalous chromomagnetic moment of a quark at the one-loop and two-loop levels through model-independent parameterizations, considering the external gluon to be off-shell. Then, we consider different types of two-Higgs-doublet models with the momentum transfer of the external gluon, q2=±MZ2, and 0, for the numerical evaluations thereof for the top quark. We find that the contributions in the two-Higgs-doublet models are on the order of O(10−3) at the one-loop level and O(10−4) at the two-loop level, where the one-loop and two-loop contributions interfere destructively, resulting in the overall contributions being on the order of O(10−3) with a positive sign.

The top quark chromomagnetic dipole moment in the SM from the 4-body vertex function

A new proposal to compute the anomalous chromomagnetic dipole moment of the top quark, , in the Standard Model is presented. On the basis of the five-dimensional effective Lagrangian operator that characterizes the quantum-loop induced chromodipolar vertices and , the anomaly is derived via radiative correction at the 1-loop level from the non-Abelian 4-body vertex function . We evaluate as a function of the energy scale s = ±E 2, for E = [10, 1000] GeV, taking into account the running of the quark masses and alpha strong through the scheme. In particular, we find that at the typical energy scale E = m Z for high-energy physics, similarly to , and , the spacelike evaluation yields = −0.025 + 0.00384i and the timelike = −0.0318 − 0.0106i. This Re = −0.025 from is even closer to the experimental central value −0.024, than that coming from the known 3-body vertex function , −0.0224. Once again, the Im part is due to the contribution of virtual charged currents, just like in the case. We can infer that the spacelike prediction is the favored one.

The top quark chromomagnetic dipole moment in the SM

We present our results on the top quark chromomagnetic dipole moment, which is based on the 5-dimension effective Lagrangian operator that characterizes the chromodipolar vertex functions gtt and ggtt. The chromomagnetic dipole mu_t is derived via quantum fluctuation at the 1-loop level. We evaluate µˆt(s) as a function of the energy scale s=+-E2, for E=[10,1000] GeV. In particular we focus on the conventional high-energy scale E=mZ, analogously as with alphas(mZ2) and sW(mZ2). The spacelike evaluation matches quite well with the experimental central value, while the timelike case deviates from it.

Off-shell chromomagnetic dipole moments of light quarks in the SM at and beyond the Z gauge boson mass scale

The anomalous chromomagnetic dipole moment, with the gluon off-shell, of the Standard Model (SM) light quarks ([Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text] and [Formula: see text]), at the [Formula: see text] gauge boson mass scale, is computed by using the [Formula: see text] scheme. The numerical results disagree with all the previous predictions reported in the literature and show a discrepancy of up to two orders of magnitude in certain situations.

Chromomagnetic and chromoelectric dipole moments of quarks in the reduced 331 model

The one-loop contributions to the chromomagnetic dipole moment and electric dipole moment of the top quark are calculated within the reduced 331 model (RM331) for non-zero . It is argued that the results are gauge independent and thus represent valid observable quantities. In the RM331, receives new contributions from two heavy gauge bosons, namely and , and one neutral scalar boson , along with a new contribution from the standard model's Higgs boson via flavor changing neutral currents. The latter, which is also mediated by the gauge boson and the scalar boson , can provide a non-vanishing if there is a -violating phase. The analytical results are presented in terms of both Feynman parameter integrals and Passarino-Veltman scalar functions, which are useful to cross-check the numerical results. Both and are numerically evaluated for parameter values still allowed by the constraints from experimental data. It is found that the new one-loop contributions of the RM331 to the real (imaginary) part of are of the order of ( ), which means at least three orders of magnitude smaller than the standard model prediction but larger than the predictions of other models of new physics. In the RM331, the dominant contribution arises from the gauge boson for in the 30-1000 GeV interval and a mass of the order of a few hundreds of GeV. As for , it receives its largest contribution from exchange and can reach values of the order of , i.e., smaller than the contributions predicted by other standard model extensions.

New estimate of the chromomagnetic dipole moment of quarks in the standard model

A new estimate of the one loop contributions of the standard model to the chromomagnetic dipole moment (CMDM) $\hat \mu_q(q^2)$ of quarks is presented with the aim to address a few disagreements arising in previous calculations. We consider the most general case with an off-shell gluon with transfer momentum $q^2$ and obtain analytical results in terms of Feynman parameter integrals and Passarino-Veltman scalar functions, which are then expressed in terms of closed form functions when possible. The calculation is done via a renormalizable linear $R_\xi$ gauge and the background field method, which allows one to corroborate that the resulting $\hat \mu_q(q^2)$ is gauge independent and thus a valid observable quantity. It is found that the QCD contribution from a three-gluon Feynman diagram has an infrared divergence, which agrees with a previous evaluation and stems from the fact that the static CMDM [$\hat\mu(0)$] has no sense in perturbative QCD. For the numerical analysis we consider the region 30 GeV$<\|q\|<$ 1000 GeV and analyze the behavior of $\hat \mu_q(q^2)$ for all the standard model quarks. It is found that the CMDM of light quarks is considerably smaller than that of the top quark as it is directly proportional to the quark mass. In the considered energy interval, both the real and imaginary parts of $\hat\mu_t(q^2)$ are of the order of $10^{-2}-10^{-3}$, with the largest contribution arising from the QCD induced diagrams, though around the threshold $q^2=4m_t^2$ there are also important contributions from diagrams with $Z$ gauge boson and Higgs boson exchange.

Revisiting the top quark chromomagnetic dipole moment in the SM

We revisit the anomalous chromomagnetic dipole moment in the Standard Model and show that its triple gluon vertex contribution, with the on-shell gluon ($q^2=0$), generates an infrared divergent pole. Consequently, the chromomagnetic dipole should not be perturbatively evaluated at $q^2=0$. Focusing on this top quark anomaly, denoted as $\hat{\mu}_t(q^2)$, we compute it with the off-shell gluon with a large momentum transfer, just as the $\alpha_s(m_Z^2)$ convention scale, for both spacelike $q^2=-m_Z^2$ and timelike $q^2=m_Z^2$ cases. We found that $\hat{\mu}_t(-m_Z^2)$ $=$ $-0.0224$ $-0.000925i$ and $\hat{\mu}_t(m_Z^2)$ $=$ $-0.0133$ $-0.0267i$. Our $\mathrm{Re}\thinspace\hat{\mu}_t(-m_Z^2)$ matches well with the current experimental value $\hat{\mu}_t^\mathrm{Exp}=-0.024_{-0.009}^{+0.013}(\mathrm{stat})_{-0.011}^{+0.016}(\mathrm{syst})$, and the $\mathrm{Im}\thinspace\hat{\mu}_t(-m_Z^2)$ part is induced by flavour changing charged currents.

Probing the nonstandard top-gluon couplings through tt¯γγ production at the LHC

In this paper, we investigate the anomalous chromoelectric and chromomagnetic dipole moments of the top quark through top pair production in association with two photons at the LHC. We first present the strategy to reconstruct this process assuming a different source for background processes. Then, we focus on the existing constraints from inclusive top-pair production from the Tevatron and LHC, adding the new LHC measurement. Afterwards, we introduce the new cross section ratio $R_{2\gamma/\gamma}=\sigma_{t\bar{t}\gamma\gamma}/\sigma_{t\bar{t}\gamma}$ and show the usefulness of this ratio in canceling most of the systematic uncertainties and its special functionality to constrain dipole moments. Finally, we use the scalar sum of the transverse momentum of jets, $H_{T}$, in order to define a signal-dominated region and obtain limits on these anomalous top couplings using different amounts of expected data from the LHC.

New probes for axionlike particles at hadron colliders

Axion-like particles (ALPs) appear from spontaneous global symmetry breaking in many extensions of the Standard Model (SM). In this paper, we find bounds on ALP ($a$) model parameters at the LHC from the ALP production associated with a photon and a jet ($j+\gamma+a$) as well as single top and top quark pairs ($t+j+a$, $t\bar{t}+a$) in a model independent approach. In particular, it is shown that the ALP production associated with a photon plus a jet at the LHC is a promising channel with significant sensitivity to probe the ALP couplings to gluons and electroweak gauge bosons. The prospects are presented at the High Luminosity LHC including a realistic detector simulation and pile up effects. Furthermore, the ALP model is examined through its contributions to the top quark (chromo)magnetic dipole moments. It is shown that the top quark magnetic and chromomagnetic dipole moments enable us to probe the ALP couplings to top quark and gauge bosons at a time. The constraints are complementary to those obtained from direct searches, as they are sensitive to light ALPs.

Relating Hadronic CP Violation to Higher-Twist Distributions.

The nucleon sigma term of the isoscalar and isovector quark chromomagnetic dipole moments are essential inputs for the determination of the CP-odd pion-nucleon couplings induced by quark chromoelectric dipole moments. We demonstrate that the former can be mapped to the third moment of the nucleon twist-three chiral-odd distribution functions e^{q}(x) which are in principle measurable in semi-inclusive deep inelastic scattering processes. We perform a survey on existing model calculations as well as experimental data on e^{u}(x)+e^{d}(x) and derive a predicted range for the isoscalar chromomagnetic dipole moment sigma term.

Measurements of mathrm{t}overline{mathrm{t}} differential cross sections in proton-proton collisions at sqrt{mathrm{s}}=13 TeV using events containing two leptons

Measurements of differential top quark pair mathrm{t}overline{mathrm{t}} cross sections using events produced in proton-proton collisions at a centre-of-mass energy of 13 TeV containing two oppositely charged leptons are presented. The data were recorded by the CMS experiment at the CERN LHC in 2016 and correspond to an integrated luminosity of 35.9 fb−1. The differential cross sections are presented as functions of kinematic observables of the top quarks and their decay products, the mathrm{t}overline{mathrm{t}} system, and the total number of jets in the event. The differential cross sections are defined both with particle-level objects in a fiducial phase space close to that of the detector acceptance and with parton-level top quarks in the full phase space. All results are compared with standard model predictions from Monte Carlo simulations with next-to-leading-order (NLO) accuracy in quantum chromodynamics (QCD) at matrix-element level interfaced to parton-shower simulations. Where possible, parton-level results are compared to calculations with beyond-NLO precision in QCD. Significant disagreement is observed between data and all predictions for several observables. The measurements are used to constrain the top quark chromomagnetic dipole moment in an effective field theory framework at NLO in QCD and to extract mathrm{t}overline{mathrm{t}} and leptonic charge asymmetries.

Top quark flavor changing neutral currents and dipole moments through three and four top-quark productions at the LHC

In this paper, the top quark flavor changing neutral current (FCNC) interaction is studied through the process of the three-top quark production at the LHC for center-of-mass energy of 14 TeV. We also investigate the anomalous top quark chromoelectric and chromomagnetic dipole moments through the four top-quarks production signal at the center-of-mass energy of 13 TeV. We demonstrate that these processes are powerful tools to constrain the top quark FCNC couplings as well as the top dipole moments.

Flavor violation in chromo- and electromagnetic dipole moments induced by Z′ gauge bosons and a brief revisit of the Standard Model

The electromagnetic dipole moments of the tau lepton and the chromoelectromagnetic dipole moments of the top quark are estimated via flavor-changing neutral currents, mediated by a new neutral massive gauge boson. We predict them in the context of models beyond the Standard Model with extended current sectors, in which simple analytic expressions for the dipole moments are presented. For the different $Z^\prime$ gauge boson considered, the best prediction for the magnetic dipole moment of the tau lepton, $|a_\tau|$, is of the order of $10^{-8}$, while the highest value for the electric one, $|d_\tau|$, corresponds to $10^{-24}$ $e\,$cm; our main result for the chromomagnetic dipole moment of the top quark, $|\hat{\mu}_t|$, is $10^{-6}$, and the value for the chromoelectric one, $|d_t|$, can be as high as $10^{-22}$ $e\,$cm. We compare our results, revisiting the corresponding Standard Model predictions, in which the chromomagnetic dipole moment of the top quark is carefully evaluated, finding explicit imaginary contributions.

Dilepton azimuthal correlations in toverline{t} production

The dilepton azimuthal correlation, namely the difference ϕ between the azimuthal angles of the positive and negative charged lepton in the laboratory frame, provides a stringent test of the spin correlation in toverline{t} production at the Large Hadron Collider. We introduce a parameterisation of the differential cross section dσ/dϕ in terms of a Fourier series and show that the third-order expansion provides a sufficiently accurate approximation. This expansion can be considered as a ‘bridge’ between theory and data, making it very simple to cast predictions in the Standard Model (SM) and beyond, and to report measurements, without the need to provide the numbers for the whole binned distribution. We show its application by giving predictions for the coefficients in the presence of (i) an anomalous top chromomagnetic dipole moment; (ii) an anomalous tbW interaction. The methods presented greatly facilitate the study of this angular distribution, which is of special interest given the 3.2(3.7)σ deviation from the SM next-to-leading order prediction found by the ATLAS collaboration in Run 2 data.

Chromomagnetic and chromoelectric dipole moments of the top quark in the fourth-generation THDM

The chromomagnetic dipole moment (CMDM) and chromoelectric dipole moment (CEDM) of the top quark are calculated at the one-loop level in the framework of the two-Higgs doublet model with four fermion generations (4GTHDM), which is still consistent with experimental data and apart from new scalar bosons (${H}^{0}$, ${A}^{0}$, and ${H}^{\ifmmode\pm\else\textpm\fi{}}$) and quarks (${b}^{\ensuremath{'}}$ and ${t}^{\ensuremath{'}}$) predicts new sources of $CP$ violation via the extended $4\ifmmode\times\else\texttimes\fi{}4$ CKM matrix. Analytical expressions for the CMDM and CEDM of a quark are presented both in terms of Feynman parameter integrals, which are explicitly integrated, and Passarino-Veltman scalar functions, with the main contributions arising from loops carrying the scalar bosons accompanied by the third- and fourth-generation quarks. The current bounds on the parameter space of the 4GTHDM are discussed and a region still consistent with the LHC data on the 125 GeV Higgs boson and the oblique parameters is identified. It is found that the top quark CMDM, which is induced by all the scalar bosons, can reach values of the order of ${10}^{\ensuremath{-}2}--{10}^{\ensuremath{-}1}$. As for the top quark CEDM, it only receives contributions from the charged scalar boson and can reach values of the order of ${10}^{\ensuremath{-}20}--{10}^{\ensuremath{-}19}\text{ }\text{ }\mathrm{ecm}$ for relatively light ${m}_{{H}^{\ifmmode\pm\else\textpm\fi{}}}$ and heavy ${m}_{{b}^{\ensuremath{'}}}$, with the dominant contribution arising from the $b$ quark. The CEDM would be the most interesting prediction of this model as it can be larger than the value predicted by the usual THDMs by one order of magnitude.

Top quark electric and magnetic color dipole moments in a Two Higgs Doublet Model with CP violation

In this work we study the anomalous top quark-gluon couplings Chromoelectric Dipole Moment and Chromomagnetic Dipole Moment in a general THDM with CP violation also known as THDM Type III. We include the contribution to the color electric and magnetic couplings arising from the Ytt Yukawa coupling. We provide phenomenological restrictions to chromoelectric and chromomagnetic anomalous couplings within the model.

Measurements oftt¯spin correlations and top quark polarization using dilepton final states inppcollisions ats=8 TeV

Measurements of the top quark-antiquark (t t-bar) spin correlations and the top quark polarization are presented for t t-bar pairs produced in pp collisions at sqrt(s) = 8 TeV. The data correspond to an integrated luminosity of 19.5 inverse femtobarns collected with the CMS detector at the LHC. The measurements are performed using events with two oppositely charged leptons (electrons or muons) and two or more jets, where at least one of the jets is identified as originating from a bottom quark. The spin correlations and polarization are measured from the angular distributions of the two selected leptons, both inclusively and differentially, with respect to the invariant mass, rapidity, and transverse momentum of the t t-bar system. The measurements are unfolded to the parton level and found to be in agreement with predictions of the standard model. A search for new physics in the form of anomalous top quark chromo moments is performed. No evidence of new physics is observed, and exclusion limits on the real part of the chromo-magnetic dipole moment and the imaginary part of the chromo-electric dipole moment are evaluated.

Top quark chromoelectric and chromomagnetic dipole moments in a two Higgs doublet model withCPviolation

In this work we study the anomalous top quark-gluon couplings Chromoelectric Dipole Moment (CEDM) and Chromomagnetic Dipole Moment (CMDM) in a general THDM with CP violation. We find that this model provides an important contribution from the $Y_{tt}$ Yukawa coupling that needs to be taken into account. The CMDM and CEDM obtained are $-0.03<\Delta\tilde{k} < 0.005}$ and $|\Delta \tilde{d}| < 0.005$, respectively.

Anomalous chromomagnetic moment of quarks

We do a perturbative calculation of the anomalous chromomagnetic dipole moment (CMDM) of quarks at one-loop, considering the effect of a small mass of the gluon. We find partial agreement with a previous calculation, as well as a divergence. We explain these results by noting that perturbation theory is not valid at the energy scales where these calculations were done, and proceed to give the results at the [Formula: see text] scale. We find significant variation, of the anomalous moment of the light quarks, as a function of gluon mass.