We studied symmetries of QCD at high temepratures using lattice QCD with dynamical u and d quarks. It is well-known that chiral symmetry SU(2)L × SU(2)R gets restored at above critical temperature Tc ' 175 MeV and we also confirmed the restoration of the chiral symmetry. However, at high temperatures of about T ' 380 MeV we found an indication for even bigger symmetry - so called SU(4). This symmetry has chiral symmetry as a subgroup, but transforms also between left- and right-handed quarks (those are not chiral transformations). The indication for this symmetry came from our study of spatial correlators on JLQCD configurations for T = 220 − 380 MeV, where we found interesting degeneracies of spin-one correlators that are not related by the chiral transformations. The observed degeneracies of correlators were greater than those predicted for free deconfined quarks. Analogous SU(4) symmetry was observed before at zero-temperature ”truncated QCD”, where lowest eigenmodes of the Dirac operator were removed. In the described study we found an indication for this interesting symmetry in the proper (non-truncated) high-temperature QCD.

COBISS.SI-ID: 3147876

We confronted the indications of lepton flavor universality violation observed in semi tauonic B meson decays with new physics searches using high pT tau leptons at the LHC. Using effective field theory arguments we correlated possible non-standard contributions to semi-tauonic charged currents with the di-tau signature at high energy hadron colliders. We found that, in general, di-tau searches pose a serious challenge to new physics explanations of the lepton flavor universality anomaly.

COBISS.SI-ID: 29977127

We have investigated the ability of the S3 leptoquark to address the recent hints of lepton universality violation in B meson decays. The S3 leptoquark with quantum numbers (3⎯⎯⎯,3,1/3) naturally emerges in the context of an SU(5) GUT model without any conflict with the stringent limits from observed nucleon stability. Scalar leptoquark S3 with left-handed couplings to 2nd and 3rd generations of charged leptons and down-type quarks seems well-suited to address both RK(∗) and RD(∗). We quantify this suitability with numerical fits to a plethora of relevant flavor observables. The proposed SU(5) model calls for a second leptoquark state, i.e., R̃ 2 with quantum numbers (3,2,1/6), if one is to generate gauge coupling unification and neutrino mass. We accordingly include it in our study to investigate R̃ 2's ability to offset adverse effects of S3 and thus improve a quality of numerical fits. A global fit of the leptoquark Yukawa couplings shows that large couplings of light S3 to τ leptons are preferred. We furthermore identify B→K(∗)ν¯ν as the most sensitive channel to probe the preferred region of parameter space. Large couplings of S3 to τ leptons are finally confronted with the experimental searches for τ final states at the Large Hadron Collider. These searches comprise a study of decay products of the leptoquark pair production, as well as, and more importantly, an analysis of the high-mass ττ final states.

COBISS.SI-ID: 30917159

We showed that in SO(10) models, a Yukawa sector consisting of a real 10, a real 120 and a complex 126 of Higgs fields can provide a realistic fit to all fermion masses and mixings, including the neutrino sector. Although the group theory of SO(10) demands that the 10 and 120 be real, most constructions complexify these fields and impose symmetries exterior to SO(10) to achieve predictivity. The proposed new framework with real 10 and real 120 relies only on SO(10) gauge symmetry, and yet has a limited number of Yukawa parameters. Our analysis showed that while there are restrictions on the observables, a good fit to the entire fermion spectrum can be realized. Unification of gauge couplings is achieved with an intermediate scale Pati-Salam gauge symmetry. Proton decay branching ratios are calculable, with the leading decay modes being p into antineutrino and charged pion or into positron and neutral pion.

COBISS.SI-ID: 31027751

Collider signals of heavy Majorana neutrino mass origin are studied in the minimal Left-Right symmetric model, where their mass is generated spontaneously together with the breaking of lepton number. The right-handed triplet Higgs boson ∆, responsible for such breaking, can be copiously produced at the LHC through the Higgs portal in the gluon fusion and less so in gauge mediated channels. At ∆ masses below the opening of the VV decay channel, the two observable modes are pair-production of heavy neutrinos via the triplet gluon fusion gg → ∆ → NN and pair production of triplets from the Higgs h → ∆∆ → 4N decay. The latter features tri- and quad same-sign lepton final states that break lepton number by four units and have no significant background. In both cases up to four displaced vertices may be present and their displacement may serve as a discriminating variable. The backgrounds at the LHC, including the jet fake rate, are estimated and the resulting sensitivity to the Left-Right breaking scale extends well beyond 10 TeV. In addition, sub-dominant radiative modes are surveyed: the γγ, Zγ and lepton flavour violating ones. Finally, prospects for ∆ signals at future e+e− colliders are presented.

COBISS.SI-ID: 30464039