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

Abstract

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$<\Vert q\Vert <$ 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.

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Cited by in corpus (10)

• A new evaluation of the $HZZ$ coupling: direct bounds on anomalous contributions and $CP$ violating effects via a new asymmetry
• Chromomagnetic and chromoelectric dipole moments of quarks in the reduced 331 model
• Bounds on the absorptive parts of the chromomagnetic and chromoelectric dipole moments of the top quark from LHC data
• Polarized and unpolarized off-shell $H^{\ast }\to ZZ\to 4\ell$ decay above the $2m_Z$ threshold
• $H\to Z\gamma$ decay and $CP$ violation
• Non-diagonal contributions to $Z\gamma V^{\ast }$ vertex, polarizations and bounds on $Z\overline{t}q$ couplings
• $CP$ violation in the $HZZ$ vertex and left-right asymmetries
• The top quark chromomagnetic dipole moment in the SM from the 4-body vertex function
• Off-shell chromomagnetic dipole moments in the SM at and beyond the $Z$ gauge boson mass scale
• $CP$-odd form factor in the $HWW$ vertex