Search

If the top quark is a composite state made out of some constituents, then its interaction with the gluon will be modified. We introduce the leading effective operators that contribute to the radius and anomalous magnetic moment of the top quark and study their effect on the cross section for tl production at the Tevatron and the LHC. Current measurements of the cross sections set a stringent limit on the scale of compositeness. This limit is comparable to similar limits obtained for light quarks and those from electroweak precision measurements. It can be used to constrain the parameter space of some composite Higgs models. [ABSTRACT FROM AUTHOR]

We discuss two possible extensions to the standard model in which an inert singlet scalar state that only interacts with the Higgs boson is added together with some fermions. In one model, the fermions provide for a seesaw mechanism for the neutrino masses; in the other model, for grand unification of the gauge couplings. Masses and interaction strengths are fixed by the requirement of controlling the finite one-loop corrections to the Higgs boson mass, thus addressing the little hierarchy problem. The inert scalar could provide a viable dark matter candidate. Direct detection of this scalar singlet in nuclear scattering experiments is possible with a cross section within reach of future experiments. [ABSTRACT FROM AUTHOR]

We assume that the Higgs boson or a possible resonance—playing a role in strongly interacting models of electroweak symmetry breaking—has been discovered at the LHC, and we propose a search strategy to determine its spin based on two simple asymmetries in the ZZ, W+W-, and tt¯ decay channels. We consider some benchmark values for its mass (above the ZZ threshold, in the interval from 182 GeV/c2 to 1 TeV/c2) and discuss the relative advantages of the different decay processes. A full analysis, including the background, is given. For a center-of-mass energy of 14 TeV, we find that the lowest integrated luminosity required to discriminate between the different spins is, depending on the process and the resonance mass, between 40 fb-1 and 250 fb-1. [ABSTRACT FROM AUTHOR]

We study the beta functions of the leading, two-derivative terms of the left-gauged SU(N) nonlinear σ model in d dimensions. In d>2, we find the usual Gaussian ultraviolet fixed point for the gauge coupling and an attractive non-Gaussian fixed point for the Goldstone boson coupling. The position of the latter fixed point controls the chiral expansion, unitarity and the strength of the tree-level Goldstone boson scattering amplitudes. For large N the model is weakly coupled, unitary at all energies and well described by the lowest order of chiral perturbation theory. Attention is paid to the gauge and scheme dependence of the results. [ABSTRACT FROM AUTHOR]

The μ+→e+X decay, where X is a dark sector boson, provides one of the strongest available bounds on the scale of dark sector interactions. The X boson can be an axion or a dark photon. We show that the concurrent determination of the antimuon and positron polarizations makes it possible to distinguish with a confidence level of 99% between the two dark sector portals with as few as 6 observed events in the case of the massless dark photon. Instead, the massive spin-1, dimension 4 dark portal cannot be distinguished from the axionlike case. We also discuss the possibility that the X boson be a massive spin-2 particle. [ABSTRACT FROM AUTHOR]

We minimally extend the Standard Model field content by adding new vectorlike fermions at the TeV scale to allow gauge coupling unification at a realistic scale. We embed the model into an SU(5) grand unified theory that is asymptotically safe and features an interacting fixed point for the gauge coupling. There are no Landau poles of the U(1) gauge and Higgs couplings in this new setting. Gauge, Yukawa, and Higgs couplings are retraced from the fixed point and matched at the grand unification scale to those of the Standard Model rescaled up to the same energy. All couplings, their fixed point values, and critical exponents always remain in the perturbative regime. [ABSTRACT FROM AUTHOR]

We study the invisible decay of neutral hadrons in a representative model of the dark sector. The mesons KL and B0 decay into the dark sector with branching rates that can be at the current experimental limits. The neutron decays with a rate that could either explain the neutron lifetime puzzle (although only for an extreme choice of the parameters and a fine-tuned value of the masses) or be just above the current limit of its invisible decay (tinvN≳ 1029 years) if kinematically allowed. These invisible decays of ordinary matter provide a novel and promising window into new physics that should be vigorously pursued. [ABSTRACT FROM AUTHOR]

We identify the differential cross sections for tt production and the total cross section for Higgs production through gluon fusion as the processes in which the two effective operators describing the leading nonstandard interactions of the top quark with the gluon can be disentangled and studied in an independent fashion. Current data on the Higgs production and the dσ/dptT differential cross section provide limits comparable to, but not more stringent than, those from the total tt cross sections measurements at the LHC and Tevatron, where however the two operators enter on the same footing and can only be constrained together. We conclude by stating the (modest) reduction in the uncertainties necessary to provide more stringent limits by means of the Higgs production and tt differential cross section observables at the LHC with the future luminosity of 300 and 3000 fb-1. [ABSTRACT FROM AUTHOR]

Quark contact interactions are an important signal of new physics. We introduce a model in which the presence of a symmetry protects these new interactions from giving large corrections in flavor changing processes at low energies. This minimal model provides the basic set of operators which must be considered to contribute to the high-energy processes. To discuss their experimental signature in jet pairs produced in proton-proton collisions, we simplify the number of possible operators down to two. We show (for a representative integrated luminosity of 200 pb-1 at √s = 7 TeV) how the presence of two operators significantly modifies the bound on the characteristic energy scale of the contact interactions, which is obtained by keeping a single operator. [ABSTRACT FROM AUTHOR]

We study the decay of the Z vector boson into a photon and a massless (invisible) dark photon in high-energy collisions. The photon can be used as a trigger for the event, while the dark photon is detected indirectly as missing momentum in the event final state. We investigate the possibility of searching for such a dark photon at the LHC, HL-LHC, and future lepton colliders, and compare the respective sensitivities. As expected, the best result is found for the lepton colliders running at the Z mass, FCC-ee and CEPC, with a final sensitivity to branching ratios of order O(10-11). We also discuss how to use the photon angular distribution of the events in lepton collisions to discriminate between the dark photon and a pseudoscalar state like the axion. [ABSTRACT FROM AUTHOR]