Narrow Results

The rare t-quark decays , via the scalar leptoquark doublets are investigated in the minimal four-color symmetry model with the Higgs mechanism of the quark–lepton mass splitting. The partial widths of these decays are calculated and the total width of the charged lepton mode and the total width of the neutrino mode are found. The corresponding branching ratios are shown to be

Fermionic decays of the scalar leptoquarks , , S_m and of the scalar gluons F=F₁, F₂ predicted by the four-color symmetry model with the Higgs mechanism of the quark–lepton mass splitting are investigated. Widths and branching ratios of these decays are calculated and analyzed in dependence on coupling constants and on masses of the decaying particles. It is shown that the decays , , , , are dominant with the widths of order of a few GeV for m_S, m_F< 1 TeV and with the total branching ratios close to 1. In the case of m_S<m_t the dominant scalar leptoquark decays are , , , with the total branching ratios , and . A search for such decays at the LHC and Tevatron may be of interest.

The contributions of scalar and gauge leptoquarks into widths of , B⁰→e^∓τ^± decays are calculated in the models with the vectorlike and chiral four-color symmetry and with the Higgs mechanism of the quark–lepton mass splitting. From the current data on and B⁰ decays the mass limits for scalar and chiral leptoquarks and the updated vector leptoquark mass limits are obtained. It is shown that unlike the gauge leptoquarks the scalar leptoquark mass limits are weak, of order or below their direct mass limits. The search for such scalar leptoquarks at LHC and the further search for leptonic decays are of interest.

Decays of B mesons into final states containing a τ lepton are sensitive to new charged current interactions that break lepton-flavor universality. These decays have been studied only at e⁺e^- colliders, where the low-background environment and well-known initial state make it possible to observe small signals with undetectable neutrinos. In particular, the large data samples of the B factories and recent advances in techniques for full-event reconstruction have led to evidence for the decay B⁺→τ⁺ ν_τ and unambiguous observation of the decays . These results exclude large regions of the parameter space for a variety of new physics models. Furthermore, the branching fraction for has been measured to be higher than the Standard Model expectation by more than three standard deviations, making this an interesting topic for further research. This paper reviews the theoretical and experimental status of this topic, summarizing the results at this time and outlining the path for further improvements.

The contributions of the vector leptoquarks of Pati–Salam type to the branching ratios of $K_L^0,B^0,B_s\to l{l}^{\prime }$ decays are calculated with account of the fermion mixing in the leptoquark currents of the general type. Using the general parametrizations of the mixing matrices the lower vector leptoquark mass limit $m_V$$>$ 86 TeV is found from the current experimental data on these decays. The branching ratios of the decays $B^0,B_s\to l{l}^{\prime }$ predicted at $m_V$ = 86 TeV are calculated. These branching ratios for the decays $B^0,B_s\to {\mu }^{+}{\mu }^{-},e\mu$ are close to the experimental data whereas those for the decays $B^0\to e^{+}{e}^{-},e\tau ,\mu \tau$ and $B_s\to e^{+}{e}^{-}$ are by order of 2–4 less than their current experimental limits. For the decays $B_s\to e\tau ,\mu \tau$ these branching ratios are of orders $10^{-10}$ and $10^{-9}$, respectively. The predicted branching ratios will be useful in the current and future experimental searches for these decays.

The contributions of the chiral gauge leptoquarks $V^{L,R}$ induced by the chiral four-color quark–lepton symmetry to the branching ratios of $K_L^0,B^0,B_s\to l_1{l}_2$ decays are calculated and analyzed using the general parametrizations of the fermion mixing matrices in the leptoquark currents. From the current experimental data on these decays under assumption $m_{V^L}\ll m_{V^R}$, the lower mass limit $m_{V^L}cos{\gamma }_L>5.68\mathrm{\text{TeV}}$ is found, which in particular case of equal gauge coupling constants gives $m_{V^L}>8.03\mathrm{\text{TeV}}$. The branching ratios of the decays under consideration predicted by the chiral gauge leptoquarks are calculated and analyzed in dependence on the leptoquark masses and the mixing parameters. It has shown that in consistency with the current experimental data, these branching ratios for $B_s,B^0\to \mu e$ decays can be close to their experimental limits and those for $B_s,B^0\to \tau e,\tau \mu$ decays can be of order of $10^{-7}$. The calculated branching ratios will be useful in the further experimental searches for these decays.

One interesting way to extend the Standard Model is the hypothesis of bifermions which are bosons that couple to pairs of quarks and leptons. We point out that $\mathrm{\text{SU}}(15)$ grand unification gives a natural way to classify bifermions and discuss leptoquarks, biquarks and bileptons. In fact, $\mathrm{\text{SU}}(15)$ provides an ideal covering group as it contains all possible bifermions within a single model.

A model for electroweak unification of quarks and leptons, in a gauge group SU_C(3) × SU(4) × U_X(1) is constructed. The model requires, three generations of quarks and leptons which are replicas (mirror) of the standard quarks and leptons. The gauge group SU(4) × U_X(1) is broken in such a way so as to reproduce standard model and to generate heavy masses for the vector bosons , the leptoquarks and mirror fermions. It is shown lower limit on mass scale of mirror fermions is , E^- being the lightest mirror fermion coupled to Z boson. As the universe expands, the heavy matter is decoupled at an early stage of expansion and may be a source of dark matter. Leptoquarks in the model connect the standard model and mirror fermions. Baryon genesis in our universe implies antibaryon genesis in mirror universe.

We present a summary of results for searches for new particles and interactions at the Fermilab Tevatron collider by the CDF and the D0 experiments. These include results from Run I as well as Run II for the time period up to July 2014. We focus on searches for supersymmetry, as well as other models of new physics such as new fermions and bosons, various models of excited fermions, leptoquarks, technicolor, hidden-valley model particles, long-lived particles, extra dimensions, dark matter particles and signature-based searches.

Final states containing both leptons and jets can be used to probe for physics beyond the Standard Model. Searches for new physics models with these signatures, such as heavy neutrinos or leptoquarks, for example, are performed using the ATLAS experiment at the LHC. The results of the most recent searches on 13 TeV $pp$ data will be presented.

Results from leptoquark and contact interaction searches at HERA, based on the analysis of DIS event samples e^±p → e^±(ν)X, are reported. The data were collected by the H1 and ZEUS experiments from 1994 to 2005, at c.m. energies of 300 and 318 GeV, and correspond to a total integrated luminosity up to ~ 270 pb^-1 for each experiment. No evidence for any signal of physics beyond Standard Model was found. Limits were derived on the leptoquark Yukawa coupling λ as a function of the leptoquark mass, the effective mass scale in eeqq contact interactions, the effective Planck mass scale in models with large extra dimensions, and the effective quark charge radius. The results are compared to those obtained in direct and indirect searches at other colliders.