Svjetlana Fajfer

Implications of Lepton Flavor Universality Violations in B Decays

Present measurements of b→cτν and b→uτν transitions differ from the standard model predictions of lepton flavor universality by almost 4σ. We examine new physics interpretations of this anomaly. An effective field theory analysis shows that minimal flavor violating models are not preferred as an explanation, but are also not yet excluded. Allowing for general flavor violation, right-right vector and right-left scalar quark currents are identified as viable candidates. We discuss explicit examples of two Higgs doublet models, leptoquarks as well as quark and lepton compositeness. Finally, implications for LHC searches and future measurements at the (super-)B factories are presented.

Physics of leptoquarks in precision experiments and at particle colliders

We present a comprehensive review of physics effects generated by leptoquarks (LQs), i.e., hypothetical particles that can turn quarks into leptons and vice versa, of either scalar or vector nature. These considerations include discussion of possible completions of the Standard Model that contain LQ fields. The main focus of the review is on those LQ scenarios that are not problematic with regard to proton stability. We accordingly concentrate on the phenomenology of light leptoquarks that is relevant for precision experiments and particle colliders. Important constraints on LQ interactions with matter are derived from precision low-energy observables such as electric dipole moments, (g-2) of charged leptons, atomic parity violation, neutral meson mixing, Kaon, B, and D meson decays, etc. We provide a general analysis of indirect constraints on the strength of LQ interactions with the quarks and leptons to make statements that are as model independent as possible. We address complementary constraints that originate from electroweak precision measurements, top, and Higgs physics. The Higgs physics analysis we present covers not only the most recent but also expected results from the Large Hadron Collider (LHC). We finally discuss direct LQ searches. Current experimental situation is summarized and self-consistency of assumptions that go into existing accelerator-based searches is discussed. A progress in making next-to-leading order predictions for both pair and single LQ productions at colliders is also outlined.

Vector leptoquark resolution of R K and RD(⁎) puzzles

Abstract We propose that three recent anomalies in B meson decays, R D ( ⁎ ) , R K , and P 5 ′ , might be explained by only one vector leptoquark weak triplet state. The constraints on the parameter space are obtained by considering t → b τ + ν data, lepton flavor universality tests in the kaon sector, bounds on the lepton flavor violating decay B → K μ τ , and b → c μ − ν ¯ decays. The presence of such vector leptoquark could be exposed in precise measurements of top semitauonic decays to b quark. The model predicts enhanced decay rate of B → K ν ¯ ν , approximate equality of lepton flavor universality ratios R K ⁎ , R K , and suppressed branching fraction of B s → μ + μ − .

Light colored scalars from grand unification and the forward-backward asymmetry in t t-bar production

The experimental results on the tt production cross section at the Tevatron are well described by the QCD contributions within the standard model, while the recently measured forward-backward asymmetry is larger than predicted within this framework. We consider light colored scalars appearing in a particular SU(5) grand unified theory model within the 45-dimensional Higgs representation. A virtue of the model is that it connects the presence of a light colored SU(2) singlet ({Delta}{sub 6}) and a color octet weak doublet ({Delta}{sub 1}) with bounds on the proton lifetime, which constrain the parameter space of both scalars. We find that both the total tt production cross section and the forward-backward asymmetry can be accommodated simultaneously within this model. The experimental results prefer a region for the mass of {Delta}{sub 6} around 400 GeV, while {Delta}{sub 1} is then constrained to have a mass around the TeV scale as well. We analyze possible experimental signatures and find that {Delta}{sub 6} associated top production could be probed in the tt+jets final states at Tevatron and the LHC.

On the B to D* tau nu Sensitivity to New Physics

B physics has played a prominent role in investigations of new physics effects at low-energies. Presently, the largest discrepancy between a standard model prediction and experimental measurements appears in the branching ratio of the charged current mediated B to tau nu decay, where the large tau mass lifts the helicity suppression arising in leptonic B decays. Less significant systematic deviations are also observed in the semileptonic B to D(*) tau nu rates. Due to the rich spin structure of the final state, the decay mode B to D* tau nu offers a number of tests of such possible standard model deviations. We investigate the most general set of lowest dimensional effective operators leading to helicity suppressed modifications of b to c (semi)leptonic transitions. We explore such contributions to the B to D* tau nu decay amplitudes by determining the differential decay rate, longitudinal D* polarization fraction, D* - tau opening angle asymmetry and the tau helicity asymmetry. We identify the size of possible new physics contributions constrained by the present B to D(*) tau nu rate measurements and find significant modifications are still possible in all of them. In particular, the opening angle asymmetry can be shifted by almost 30%, relative to the standard model prediction, while the tau helicity asymmetry can still deviate by as much as 80%.

Leptoquark model to explain the B-physics anomalies, RK and RD

We show that a model with a scalar leptoquark of hypercharge

New physics models facing lepton flavor violating Higgs decays at the percent level

Y=1/6

Minimally flavored colored scalar in \overline{B}\to {D^{{\left( * \right)}}}\tau \overline{\nu} and the mass matrices constraints

which includes the light right-handed neutrinos, can successfully describe both of the

Light Higgs and vector-like quarks without prejudice

B

Probing anomalous tWb interactions with rare B decays

-physics anomalies,