Search

The presence of massive sterile neutrinos $N$ mixed with the active ones induces flavor violating processes in the charged lepton sector at the loop level. In particular, the amplitude of ${H}^{0}\ensuremath{\rightarrow}{\overline{\ensuremath{\ell}}}_{i}{\ensuremath{\ell}}_{j}$ is expected to be proportional to the product of heavy-light Yukawa couplings ${y}_{i}{y}_{j}=2{s}_{{\ensuremath{\nu}}_{i}}{s}_{{\ensuremath{\nu}}_{j}}{m}_{N}^{2}/{v}^{2}$, where ${s}_{{\ensuremath{\nu}}_{i,j}}$ express the heavy-light neutrino mixings. Here, we revisit these Higgs decays in the most generic extension of the neutrino sector, focusing on large values of ${y}_{i}$. We show that decoupling effects and a cancellation between the two dominant contributions to these processes makes the amplitude about 100 times smaller than anticipated. We find that perturbative values of ${y}_{i}$ giving an acceptable contribution to the $\ensuremath{\rho}$ parameter imply $\mathcal{B}({H}^{0}\ensuremath{\rightarrow}{\overline{\ensuremath{\ell}}}_{i}{\ensuremath{\ell}}_{j})l{10}^{\ensuremath{-}8}$ for any lepton flavors, a rate that is not accessible at current colliders.

In supersymmetric models extended with a gauge singlet, the mass of the lightest Higgs boson has contributions proportional to the adimensional coupling $\ensuremath{\lambda}.$ In minimal scenarios, the requirement that this coupling remains perturbative up to the unification scale constrains $\ensuremath{\lambda}$ to be smaller than $\ensuremath{\approx}0.7.$ We study the maximum value of $\ensuremath{\lambda}$ consistent with a perturbative unification of the gauge couplings in models containing nonstandard fields at intermediate scales. These fields appear in scenarios with gauge mediation of supersymmetry breaking. We find that the presence of extra fields can raise the maximum value of $\ensuremath{\lambda}$ up to 19%, increasing the limits on the mass of the lightest Higgs boson from 135 GeV to 155 GeV.

Numerical correlations between fermion masses and mixings could indicate the presence of a flavor symmetry at high energies. In general, the search for these correlations using low-energy data requires an estimate of leading-log radiative corrections. We present a complete analysis of the evolution between the electroweak and the grand unification scales of quark mass parameters in minimal supersymmetric models. We take $M_t=180$ GeV and consider all possible values of $\tan \beta$. We also analize the possibility that the {\it top} and/or the {\it bottom} Yukawa couplings result from an intermediate quasifixed point (QFP) of the equations. We show that the quark mixings of the third family do {\it not} have a QFP behaviour (in contrast to the masses, the renormalization of all the mixings is linear), and we evaluate the low-energy value of $V_{ub}$ which corresponds to $V_{ub}(M_X)=0$. Then we focus on the renormalization-group corrections to {\it (i)} typical relations obtained in models with flavor symmetries at the unification scale and {\it (ii)} a superstring-motivated pattern of quark mass matrices. We show that in most of the models the numerical prediction for $V_{ub}$ can be {\it corrected} in both directions (by varying $\tan\beta$) due to {\it top} or {\it bottom} radiative corrections., Comment: LaTeX, 14 pages, 4 figures, 3 tables. Uses epsf.sty. PostScript available at http://deneb.ugr.es/papers/ugft60.ps.gz Plain LaTeX, Positions of the figures corrected again

We use heavy quark effective theory (HQET) and perturbative QCD to study the heavy-meson--light-vector-meson transitions involved in D and B decays. HQET is used to relate the measured D\ensuremath{\rightarrow}${\mathit{K}}^{\mathrm{*}}$l\ensuremath{\nu} vector and axial-vector form factors at four-momentum transfer ${\mathit{q}}^{2}$=0 to the B\ensuremath{\rightarrow}${\mathit{K}}^{\mathrm{*}}$\ensuremath{\gamma} tensor and axial-tensor form factors at ${\mathit{q}}^{2}$=16.5 ${\mathrm{GeV}}^{2}$. Perturbative QCD is then used to find matching conditions for the B-meson form factors at ${\mathit{q}}^{2}$=0. A five parameter ``vector-dominance''-type fit of the two HQET form factors, consisting of single pole, double pole, and subtraction terms, is used to match the data at ${\mathit{q}}^{2}$=16.5 ${\mathrm{GeV}}^{2}$ to QCD at ${\mathit{q}}^{2}$=0. The values at ${\mathit{q}}^{2}$=0 are compared with recent data on the exclusive rate for B\ensuremath{\rightarrow}${\mathit{K}}^{\mathrm{*}}$\ensuremath{\gamma} decay to extract a value for the Cabibbo-Kobayashi-Maskawa matrix element \ensuremath{\Vert}${\mathit{V}}_{\mathit{t}\mathit{s}}$\ensuremath{\Vert}=0.026, with 23% experimental uncertainty, and 29% theoretical uncertainty from higher order QCD effects and violations of heavy quark symmetry.

We investigate the presence of discrete gauge symmetries in Grand Unification models based in $SO(10)$ and $E_6$. These models include {\it flipped} and {\it unflipped} $SU(5)$, $SU(4)\!\times\! SU(2)_L\!\times\! SU(2)_R$, $SU(3)_C\!\times\! SU(3)_L\!\times\! SU(3)_R$, and $SU(6)\!\times\! SU(2)$. Using the Dynkin formalism we find the $U(1)$ subalgebras contained in the unified groups, give an expression for the Higgs fields that preserve each discrete symmetry, and determine the low-energy matter content implied by chirality. We discuss two $Z_2$ and three $Z_3$ nonequivalent cases. Among the possibilities found, only the usual $Z_2$ matter parity (R-parity) of supersymmetric extensions is consistent with a minimal matter content with no right- handed neutrinos, extra Higgs doublets, or nonstandard {\it down}-type quarks., Comment: 20 pages, plain TeX, UFIFT-HEP-93-11

We argue that a relatively light massive gluon with mass $\ensuremath{\lesssim}1\text{ }\text{ }\mathrm{TeV}$, small purely axial couplings to light quarks and sizable vector and axial couplings to the top quark can reproduce the large forward-backward asymmetry observed at the Tevatron without conflicting with the $t\overline{t}$ and the dijet invariant-mass distributions measured at the Tevatron and the LHC. We show that realistic Higgsless models with warped extra dimensions naturally fulfil all the necessary ingredients to realize this scenario. While current data is unable to discover or exclude these heavy gluons with masses $\ensuremath{\approx}850\text{ }\text{ }\mathrm{GeV}$, they should be observed at the (7 TeV) LHC with a luminosity $\ensuremath{\gtrsim}300\text{ }\text{ }{\mathrm{pb}}^{\ensuremath{-}1}$.

Charmed mesons may be produced when a primary cosmic ray or the leading hadron in an air shower collide with an atmospheric nucleon. At energies \ge 10^8 GeV their decay length becomes larger than 10 km, which implies that they tend to interact in the air instead of decaying. We study the collisions of long-lived charmed hadrons in the atmosphere. We show that (\Lambda_c,D)-proton diffractive processes and partonic collisions of any q^2 where the charm quark is an spectator have lower inelasticity than (p,\pi)-proton collisions. In particular, we find that a D meson deposits in each interaction just around 55% of the energy deposited by a pion. On the other hand, collisions involving the valence c quark (its annihilation with a sea cbar quark in the target or c-quark exchange in the t channel) may deposit most of D meson energy, but their frequency is low (below 0.1% of inelastic interactions). As a consequence, very energetic charmed hadrons may keep a significant fraction of their initial energy after several hadronic interactions, reaching much deeper in the atmosphere than pions or protons of similar energy., Comment: 13 pages, version to appear in PRD

We analyze a scenario for baryogenesis at low temperature ([similar to]1 TeV) in supersymmetric extensions of the standard model. These extensions contain gauge singlets and discrete symmetries that suppress unwanted baryon- and lepton-number violation, and offer a framework to accommodate nonstandard neutrino physics. In our scenario, the decay of the lightest supersymmetric particle generates a net lepton number that is subsequently converted, via anomalous electroweak processes, into baryon number. We find that this possibility could be realized in models incorporating a [ital Z][sub 3] discrete symmetry, although it requires a scalar singlet as the lightest supersymmetric particle and a somewhat complicated pattern of couplings and fields in the singlet sector.

The top quark has a large Yukawa coupling with the Higgs boson. In the usual extensions of the standard model the Higgs sector includes extra scalars, which also tend to couple strongly with the top quark. Unlike the Higgs, these fields have a natural mass above $2{m}_{t}$, so they could introduce anomalies in $t\overline{t}$ production at the LHC. We study their effect on the $t\overline{t}$ invariant mass distribution at $\sqrt{s}=7\text{ }\text{ }\mathrm{TeV}$. We focus on the bosons $(H,A)$ of the minimal supersymmetric model and on the scalar field ($r$) associated with the new scale $f$ in Little Higgs (LH) models. We show that in all cases the interference with the standard amplitude dominates over the narrow-width contribution. As a consequence, the mass difference between $H$ and $A$ or the contribution of an extra $T$-quark loop in LH models becomes an important effect in order to determine if these fields are observable there. We find that a $1\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$ luminosity could probe the region $\mathrm{tan} \ensuremath{\beta}\ensuremath{\le}3$ of supersymmetry and $v/(\sqrt{2}f)\ensuremath{\ge}0.3$ in LH models.

We discuss the generation of large intermediate scales of symmetry breaking in grand unification models suggested by the heterotic string. We analyze on dimensional grounds a particular scenario where one flat direction in the effective potential defines two different scales of gauge-symmetry breaking at very high energies ({approx}10{sup 15} GeV). This mechanism implies the presence of one light ({similar to}1 TeV) nonchiral neutrino. The size of the observable low-energy effects seems, however, quite model dependent.

We discuss a Little Higgs scenario that introduces below the TeV scale just the two minimal ingredients of these models, a vectorlike T quark and a singlet component (implying anomalous couplings) in the Higgs field, together with a pseudoscalar singlet \eta. In the model, which is a variation of Schmaltz's simplest Little Higgs model, all the extra vector bosons are much heavier than the T quark. In the Yukawa sector the global symmetry is approximate, implying a single large coupling per flavour, whereas in the scalar sector it is only broken at the loop level. We obtain the one-loop effective potential and show that it provides acceptable masses for the Higgs h and for the singlet \eta with no need for an extra \mu term. We find that m_\eta can be larger than m_h/2, which would forbid the (otherwise dominant) decay mode h -> \eta\eta., Comment: 16 pages. References added, fine tuning analysis included. Version to appear in PRD

The presence of an extra dimension of size R\equiv M_c^{-1} introduces corrections of order (\mu/M_c)\alpha to the gauge and Yukawa couplings and accelerates their running at scales \mu larger than M_c. This could result in a grand unification scale M_X\approx 20 M_c. We study the corrections at the two-loop level. We find corrections of order (\mu/M_c)\alpha^2 for the gauge couplings and of order (\mu/M_c)^2\alpha^2 for the Yukawa couplings. Therefore, in the Yukawa sector one and two-loop contributions can be of the same order below M_X. We show that in the usual scenarios the dominant gauge and Yukawa couplings are decreasing functions of the scale, in such a way that (\mu/M_c)\alpha becomes approximately constant and two-loop contributions introduce just a 30% correction which does not increase with the scale., Comment: 14 pages, added references, corrected typos

The presence of an extra dimension of size $R\ensuremath{\approx}{\mathrm{TeV}}^{\ensuremath{-}1}$ introduces a tower of Kaluza-Klein gauge boson excitations that affects the standard model relations between electroweak observables. The mixing of the W and Z bosons with their excitations changes their masses and couplings to fermions. This effect depends on the Higgs field, which may live in the bulk of the extra dimension, on its boundary, or may be a combination of both types of fields. We use high-precision electroweak data to constrain $1/R$. We find limits from 1 to 3 TeV from different observables, with a model-independent lower bound of 2.5 TeV.