Flipping t tbar asymmetries at the Tevatron and the LHC

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.Comment: 5 pages, 5 figure

Leptonic Monotops at LHC

We study the possibility of detecting New Physics (NP) phenomena at the LHC through a new search strategy looking at the monotop (top plus missing energy) signature which is common to a variety of NP models. We focus on the leptonic top decay mode and study the discovery or exclusion reach of the 2012 LHC data for three example models. Contrary to the hadronic mode, in this case the problematic QCD multijet background can be safely neglected. We find that the key kinematic variable to suppress most of the remaining SM backgrounds is the transverse mass of the charged lepton and missing energy. In fact, one could expect that the single-top production measurements already address the monotop signature in this mode. This is however not the case because in the SM single-top production the transverse mass has an end point determined by the W mass, while the NP signals typically have an additional source of missing energy. We compare, under the same conditions, our monotop search strategy with existing single-top measurements and find a considerable improvement in the monotop signature reach.Comment: 21 pages, 6 figure

QCD Corrections to Flavor Changing Neutral Coupling Mediated Rare Top Quark Decays

Recently we have presented an analysis of flavor changing neutral coupling mediated radiative top quark decays at next-to-leading order in QCD. In the present paper we provide the details of the calculation of QCD corrections to t-> q gamma and t-> q Z decays within the effective theory approach including operator mixing. In particular, we calculate virtual matrix element corrections and the corresponding bremsstrahlung contributions. In the case of t-> q gamma we study the effects of kinematic cuts on the extracted branching ratios. Analytical formulae are given at all stages of the calculation. We find that the t-> q gamma decay can be used to probe also the effective operators mediating t-> q g processes, since these can naturally contribute 10% or more to the radiative decay, given typical experimental cuts on the decay kinematics at hadron colliders. Conversely, we argue that any positive experimental signal of the t-> q g process would indicate a natural lower bound on t-> q gamma decay rate.Comment: 12 page

Flavor Changing Neutral Coupling Mediated Radiative Top Quark Decays at Next-to-Leading Order in QCD

We compute the branching ratios for the rare top quark decays t to c gamma and t to c Z mediated by effective flavor changing neutral couplings at the next-to-leading order in QCD including the effects due to operator mixing. After re-suming contributions of order [alpha_s log (Lambda / m_t)]^n, where Lambda is the scale at which the effective operators are generated, at leading log level using renormalization group methods, we compute finite matrix element corrections and study the effects of experimental kinematic cuts on the extracted branching ratios. We find that the t to c gamma decay can be used to probe also the effective operators mediating t to c g processes, since these can naturaly contribute 10% or more to the radiative decay. Conversely, any experimental signal of t to c g would indicate a natural lower bound on t to cZ, gamma.Comment: 4 pages, 3 figure

Interplay of t --> b W Decay and B_q Meson Mixing in Minimal Flavor Violating Models

Precise measurements of the top quark decay properties at hadron colliders offer interesting new possibilities of testing the standard model. At the same time, recent intriguing experimental results concerning CP violation in the B_d and B_s systems have stimulated many studies of physics beyond the standard model. We investigate anomalous t W d_j interactions as a possible source of new effects in B_{d,s} - bar B_{d,s} oscillations within a model independent approach based on the assumptions of Minimal Flavor Violation. After matching our effective operators onto the low-energy effective Lagrangian describing B_{d,s} meson mixing and evolving it down to the B-mass scale, we extract the preferred ranges of the anomalous t W d_j interactions at the weak scale. These values are then compared to previously considered constraints coming from the rare radiative B --> X_s gamma decay. Finally, we reconsider the associated effects in the t --> b W decays and find that the W helicity fractions F_{L,+} can deviate by as much as 15%, 30% from their standard model values, respectively. The deviations in F_L in particular, can reach the level of expected precision measurements at the LHC.Comment: 7 pages, 3 figures, published versio

Probing anomalous tWb interactions with rare B decays

Precision studies of top quark properties are currently underway at the LHC and Tevatron colliders with the prospect of probing anomalous t-W -b interactions. In the mean time, recent experimental results for the B_{d,s} - \bar{B}_{d,s} oscillation observables, the branching ratio B_s->\mu^+\mu^-, as well as the forward - backward asymmetry in B->K\starl^+l^-, accompanied by the accurate theoretical predictions for the relevant observables obtained within the SM motivate a combined study of these observables in the presence of anomalous t - W - b vertices. We investigate contributions of such anomalous couplings to the B->X_s l^+l^- decay mode, and combining them with the modifications of the B_{d,s} - \bar{B}_d,s, B->X_s{\gamma} observables, we determine indirect bounds on the real and imaginary parts of the anomalous t - W - b interactions. We find these to be mostly superior to present direct constraints coming from top decay and production measurements at the LHC and Tevatron. Finally, we predict the allowed effects in the branching ratios of the B_s->\mu^+\mu^- and B->K^{(*)}\nu \bar{\nu}, as well as the forward-backward asymmetry in B->K^*l^+l^-. We find that improved knowledge of these observables in the future could further constrain some of the anomalous tWb interactions.Comment: 14 page