Jure Drobnak

The anomalous forward-backward asymmetry (A_FB) in t tbar production at the Tevatron is in apparent contradiction with the t tbar charge asymmetry (A_C) measurements at the LHC, which agree well with the standard model predictions. We argue that associated production of a state with [u tbar] flavor quantum numbers can lead to a sizable negative contribution to A_C. Exchange of such a resonance in the t-channel would lead to positive contributions to A_FB and A_C, as has been extensively discussed in the literature. Given the additional negative A_C contribution, both the Tevatron and LHC data can be naturally accommodated within this framework. A simple realization of this setup is the well known example of a Z' with flavor off-diagonal up-top couplings. We provide a detailed study of this model, demonstrating that it indeed reproduces the t tbar asymmetry data, and is compatible with other constraints, e.g. t+jet resonance searches, t tbar inclusive jet multiplicities, and atomic parity violation.

The most general set of new physics effective operators contributing to the decay of an unpolarized top quark into a bottom quark and a W gauge boson are considered at next-to-leading order in QCD. We find that the dipole operator contribution to the transverse-plus W helicity fraction F + is enhanced compared to the leading order result at non-vanishing bottom quark mass. Nonetheless, presently the observable most sensitive to new physics contributions is the longitudinal W helicity fraction F L. We also investigate constraints on tW b couplings coming from precision flavor observables, in particular from ∆B = 1 and ∆B = 2 transitions of B q mesons. We find these to be mostly superior to present direct constraints coming from top decay and single top production measurements at the LHC and Tevatron.

A colored weak singlet scalar state with hypercharge 4/3 is one of the possible candidates for the explanation of the unexpectedly large forward-backward asymmetry in t t production as measured by the CDF and D0 experiments. We investigate the role of this state in a plethora of flavor changing neutral current processes and precision observables of down-quarks and charged leptons. Our analysis includes tree-and loop-level mediated observables in the K and B systems, the charged lepton sector, as well as the Z → b b width. We perform a fit of the relevant scalar couplings. This approach can explain the (g-2) µ anomaly while tensions among the CP violating observables in the quark sector, most notably the nonstandard CP phase (and width difference) in the B s system cannot be fully relaxed. The results are interpreted in a class of GUT models which allow for a light colored scalar with a mass below 1 TeV.

We show that the charge asymmetry in t tbar production at the LHC, A_C, and the forward-backward asymmetry at the Tevatron, A_FB, are in general not tightly correlated. They can even have opposite signs, if the underlying new physics (NP) model is general enough. We demonstrate this using two examples of NP: a light axigluon, and a vector that is a color octet and electroweak triplet. The small value of A_C measured at the LHC is thus shown not to exclude a NP interpretation of the anomalously large A_FB at the Tevatron. We identify two observables where significant NP effects are still expected at the Tevatron and the LHC, the b bbar production forward-backward asymmetry and spin polarizations of the pair-produced tops and anti-tops.

The most general set of new physics effective operators contributing to the decay of an unpolarized top quark into a bottom quark and a W gauge boson are considered at next-to-leading order in QCD. We find that the dipole operator contribution to the transverse-plus W helicity fraction F+ is enhanced compared to the leading order result at non-vanishing bottom quark mass. Nonetheless, presently the observable most sensitive to new physics contributions is the longitudinal W helicity fraction FL. We also investigate constraints on t W b couplings coming from precision flavor observables, in particular from Delta B=1 and Delta B=2 transitions of Bq mesons. We find these to be mostly superior to present direct constraints coming from top decay and single top production measurements at the LHC and Tevatron.

A colored weak singlet scalar state with hypercharge 4/3 is one of the possible candidates for the explanation of the unexpectedly large forward-backward asymmetry in t tbar production as measured by the CDF and D0 experiments. We investigate the role of this state in a plethora of flavor changing neutral current processes and precision observables of down-quarks and charged leptons. Our analysis includes tree- and loop-level mediated observables in the K and B systems, the charged lepton sector, as well as the Z to b bbar decay width. We perform a global fit of the relevant scalar couplings. This approach can explain the (g-2)_mu anomaly while tensions among the CP violating observables in the quark sector, most notably the nonstandard CP phase (and width difference) in the Bs system cannot be fully relaxed. The results are interpreted in a class of grand unified models which allow for a light colored scalar with a mass below 1TeV. We find that the renormalizable SU(5) scenario is not compatible with our global fit, while in the SO(10) case the viability requires the presence of both the 126- and 120-dimensional representations.