Showing papers on "Elementary particle published in 1991"

Three exceptions in the calculation of relic abundances

Abstract: The calculation of relic abundances of elementary particles by following their annihilation and freeze-out in the early Universe has become an important and standard tool in discussing particle dark-matter candidates. We find three situations, all occurring in the literature, in which the standard methods of calculating relic abundances fail. The first situation occurs when another particle lies near in mass to the relic particle and shares a quantum number with it. An example is a light squark with neutralino dark matter. The additional particle must be included in the reaction network, since its annihilation can control the relic abundance. The second situation occurs when the relic particle lies near a mass threshold. Previously, annihilation into particles heavier than the relic particle was considered kinematically forbidden, but we show that if the mass difference is \ensuremath{\sim}5-15%, these "forbidden" channels can dominate the cross section and determine the relic abundance. The third situation occurs when the annihilation takes place near a pole in the cross section. Proper treatment of the thermal averaging and the annihilation after freeze-out shows that the dip in relic abundance caused by a pole is not nearly as sharp or deep as previously thought.

Particles with small violations of Fermi or Bose statistics.

Abstract: I discuss the statistics of "quons" (pronounced to rhyme with muons), particles whose annihilation and creation operators obey the $q$-deformed commutation relation (the quon algebra or $q$-mutator) which interpolates between fermions and bosons. Topics discussed include representations of the quon algebra, proof of the $\mathrm{TCP}$ theorem and clustering, violation of the usual locality properties, and experimental constraints on violations of the Pauli exclusion principle (i.e., Fermi statistics) and of Bose statistics.

Double beta decay

Abstract: Developments in the theoretical investigation of nuclear double-beta decay are reviewed. In particular, the neutrinoless mode is discussed in detail, since it is sensitive to lepton number violation as predicted by gauge theories beyond the standard model and it is expected to give important information on the nature of the neutrinos and the weak interaction. Various approximations made in the theoretical treatment of neutrinoless and two-neutrino double beta decay are examined, and the limits on the effective Majorana mass of the electron neutrino as well as the coupling constants of the right-handed leptonic current are presented.

Quarks and Leptons: An Introductory Course in Modern Particle Physics

Abstract: A pedogogical contribution which introduces the current experimental assault upon the nature of matter and provides an appreciation of contemporary theoretical speculations. Quarks and leptons are discussed and explanations are given on how they react through the exchange of gauge field quanta (photons, gluons, and weak bosons). Assumes basic knowledge of non-relativistic quantum mechanics and the theory of special relativity. Many chapter exercises, some with answers and commentary.

Left-right-symmetric electroweak models with triplet Higgs field

Abstract: We examine the predictions of the conventional SU(2${)}_{\mathit{L}}$\ensuremath{\bigotimes}SU(2${)}_{\mathit{R}}$\ensuremath{\bigotimes}U(1${)}_{\mathit{B}\mathrm{\ensuremath{-}}\mathit{L}}$ left-right-symmetric model in the case where the minimal Higgs sector (containing one bidoublet, one L-triplet, and one R-triplet Higgs field) and the standard lepton representations (incorporating right-handed partners for the observed neutrinos) are adopted. We show that a complete analysis of spontaneous symmetry breaking for the Higgs sector leads to a highly restrictive range of possibilities for global minima that are simultaneously consistent with all experimental observations (such as lepton masses, ${\mathit{K}}_{\mathit{L}}$-${\mathit{K}}_{\mathit{S}}$ mixing, etc.). As a result, the possible phenomenologies for the gauge and Higgs bosons of the model are very limited. For instance, we demonstrate that in the absence of explicit CP violation in the Higgs potential, spontaneous CP violation does not arise and the fermion couplings exhibit ``manifest'' left-right symmetry. Further, we find no entirely natural solutions other than ones in which all of the extra (non-standard-model) gauge and Higgs bosons associated with the left-right-symmetric extension are extremely heavy (typically, more massive than ${10}^{7}$ GeV). Only by ``fine-tuning'' certain parameters of the Higgs potential is it possible to bring these extra particles down to an observable mass scale. Alternatively, symmetries can be introduced to eliminate the terms in the Higgs potential associated with these parameters, but only at the sacrifice of introducing undesirable consequences for fermion masses. Many of the pitfalls and problems are illustrated using a simplified model. Overall, we emphasize the necessity of performing a complete minimization of the Higgs sector before extracting phenomenology.

Theoretical nuclear physics

Abstract: This report summarizes progress during the past year in the following areas of research: Pion double charge exchange, including studies of the contribution of meson exchange currents and the roles of chiral symmetry and of vector mesons. The possibility of bound states of two hyperons and the H dibaryon. Many body theory, including studies of nuclear scattering in a quark cluster model, nuclear structure calculations, and a one-dimensional model with charged'' bosons. Statistical nuclear spectroscopy, including formalism and programs for state densities and partition functions, single-state occupancies and spin-cut off factors, measures for symmetry breaking, and the relationship between statistical and shell-model spectroscopy.

Limits on neutral light scalar and pseudoscalar particles in a proton beam dump experiment

Abstract: A search has been performed for weakly interacting neutral light scalar and pseudoscalar particles in a proton beam dump experiment. No positive signal is observed. Limits on the mass and life-time of these particles are set in the frame of the Standard Model and its minimal supersymmetric extension. The Higgs particle of theSU2L×U1 Standard Theory is excluded for masses in the range 1 MeV

Two Lectures on Fermions and Gravity

Abstract: In these two lectures we want to provide information on the behavior of elementary particles in special relativity (SR), such as electrons, protons, or neutrons, in order to get ideas of the underlying principles of the gravitational interaction of fermions. For tangible matter and for the electromagnetic field, Einstein’s gravitational theory, general relativity (GR), describes all phenomena very well and has been verified experimentally with ever increasing accuracy. In contrast therefrom, not too much is known experimentally for fermions and their gravitational interaction, apart from the celebrated Colella-Overhauser-Werner (or COW) experiment [21] using a neutron interferometer in a gravitational field.

Instability of hot electroweak theory: Bounds on mH and mt.

Abstract: The electroweak vacuum need not be absolutely stable. For certain top-quark and Higgs-boson masses in the minimal standard model, it is instead metastable with a lifetime exceeding the present age of the Universe. The decay of our vacuum may be nucleated at low temperature by quantum tunneling or at high temperature by thermal excitation. We show that the requirement that the vacuum survive the high temperatures of the early Universe places the strongest constraints from vacuum stability on the top-quark and Higgs-boson masses in the minimal standard model. If a single Higgs boson is found experimentally, these constraints may place an upper bound on the scale of new physics beyond the minimal standard model. In contrast with other work, we examine temperatures very large compared to the scale of weak symmetry restoration and find much stronger bounds. We also present a simple analytic approximation that directly relates the bounds to the running coupling constants of the minimal standard model.

Naturalness problems for rho = 1 and other large one loop effects for a standard model Higgs sector containing triplet fields

Abstract: Extensions of the Higgs sector of the standard model (SM) that employ only doublet and singlet Higgs-field representations are not the only ones that guarantee \ensuremath{\rho}=1 at the tree level. Higgs sectors containing triplet (and higher) representations can be constructed in such a way that there is a tree-level custodial SU(2) symmetry yielding \ensuremath{\rho}=1. However, this custodial SU(2) is inevitably violated at the one-loop level. We explore the implications of this violation in the context of a Higgs sector containing triplet fields. In particular, we show that it leads to one-loop corrections to \ensuremath{\rho} and to certain mixings among the Higgs bosons and gauge bosons of the model that are quadratically divergent, thereby creating a new naturalness problem for \ensuremath{\rho} and for certain Higgs-boson couplings. This new class of naturalness problems first arises for a Higgs sector with triplet representations, making such a sector an interesting case study. A priori, deviations from \ensuremath{\rho}=1 induced at one loop are of arbitrary magnitude and sign. We demonstrate that the fine-tuning required to keep one-loop corrections to \ensuremath{\rho} and the Higgs-boson couplings small is similar in nature to that required in the SM to keep the SM Higgs-boson mass in the perturbative regime.

Higgss-channel production ine + e − collisions below theW + W − threshold

Abstract: The cross section for direct neutral Higgs production in the reaction\(e^ + e^ - \to H^0 \to f \bar f\) is calculated in theECM energy range of 40 to 160 GeV and compared to the corresponding Electro-Weak (EW) process\(e^ + e^ - \to (Z,\gamma ) \to f \bar f\). Neglecting radiation effects, a signal of the order of 10−4 to 10−3 over the EW can be expected outside theZ0 region for the Minimal Standard Model Higgs in its decay to a\(b\bar b\) state. For\(M_H > \sqrt s - M_Z \), thes-channel Higgs formation can surpass the Bjorken Bremsstrahlung process and thus may afford at LEP 2 a realistic search method for high mass, say ≧100 GeV Higgs, given enough luminosity. For a non-Standard Model Higgs, in some cases, significantly higher signals are expected. The effects of initial state radiation and-machine energy resolution are evaluated and the gain in using longitudinal polarized electron beams is discussed.

Model dependence of the cosmological upper bound on the Higgs-boson mass.

Abstract: We consider an electroweak scenario for baryon asymmetry generation in an extended model with two Higgs doublets. The upper bound on the mass of the lightest Higgs boson is obtained. It is shown to coincide with that of the theory with one Higgs doublet, namely, mH

Final-state interactions and CP violation in weak decays.

Abstract: The $\mathrm{CP}$-violating difference between the partial decay rates of a particle and antiparticle depends on final-state interactions. A general formalism is presented for calculating this difference based on $\mathrm{CPT}$ invariance and unitarity. Applications are given to $B$ decays and the formalism is compared to the standard method using penguin graphs.

Sum rules for Higgs bosons.

Abstract: We exhibit the sum rules for Higgs-boson couplings required by unitarity. We give explicit results and applications for an SU(2){times}U(1) electroweak gauge theory with arbitrary Higgs multiplets. As an example, we examine the constraints on nonminimal Higgs sectors should a single neutral Higgs boson with the standard-model coupling strength to the {ital ZZ} channel be observed in {ital Z} decays.

Charged-particle Multiplicity Distributions In Z0 Hadronic Decays

Abstract: This paper presents an analysis of the multiplicity distributions of charged particles produced inZ0 hadronic decays in the DELPHI detector. It is based on a sample of 25364 events. The average multiplicity is =20.71±0.04(stat)±0.77(syst) and the dispersionD=6.28±0.03(stat)±0.43(syst). The data are compared with the results at lower energies and with the predictions of phenomenological models. The Lund parton shower model describes the data reasonably well. The multiplicity distributions show approximate KNO-scaling. They also show positive forward-backward correlations that are strongest in the central region of rapidity and for particles of opposite charge.

Solar-neutrino solutions with matter-enhanced flavor-changing neutral-current scattering.

Abstract: We explore quantitatively recent suggestions that the solar-neutrino puzzle may be explained by matter enhancement of nonstandard neutral-current scattering of neutrinos on $d$ quarks. These neutral currents introduce two new parameters, relating to flavor-diagonal and off-diagonal $\ensuremath{ u}\ensuremath{-}d$ scattering, which affect the dynamical mixing of two neutrino flavors in the Sun. We consider this mixing in three alternative scenarios: (a) massless neutrinos, (b) massive neutrinos with off-diagonal but no new diagonal currents, and (c) massive neutrinos with both off-diagonal and new diagonal currents. We determine the regions in parameter space that give consistency between the standard solar model and existing solar-neutrino data from the Homestake and Kamiokande-II experiments, and give the corresponding predictions for future experiments with gallium and superfluid helium detectors. We also discuss the more general situation, with nonstandard neutral-current scattering from $u$ quarks and electrons as well as from $d$ quarks, and consider the effects of transmission through the Earth. Finally, we examine the bounds on nonstandard neutral-current neutrino scattering imposed by other experiments; some particular classes of solution are thereby excluded, but a variety of possibilities remains.

Negative contributions to radiative-correction parameter S from Majorana particles.

Abstract: Oblique corrections from heavy fermions are studied. For certain mass ranges we find negative contributions to the S and T parameters from Majorana particles.