Scales of mass generation for Majorana neutrinos (as well as quarks and leptons) can be probed from high energy 2 → n inelastic scattering involving a multiple longitudinal gauge boson final state. We demonstrate that the unitarity of 2 → n scattering puts the strongest new upper limit on the scale of fermion mass generation, independent of the electroweak symmetry breaking scale [Formula: see text]. Strikingly, for Majorana neutrinos (quarks and leptons), we find that the strongest 2 → n limits fall in a narrow range, 136 - 170 TeV (3 - 107 TeV ) with n = 20 - 24 (n = 2 - 12), depending on the observed fermion masses. Physical implications are discussed.

The scale of mass generation for fermions (including neutrinos) and the scale for electroweak symmetry breaking (EWSB) can be bounded from above by the unitarity of scattering involving longitudinal weak gauge bosons or their corresponding would-be Goldstone bosons. Including the exact n-body phase space we analyze the 2{yields}n (n{>=}2) processes for the fermion-(anti)fermion scattering into multiple gauge boson final states. Contrary to naieve energy power counting, we demonstrate that as n becomes large, the competition between an increasing energy factor and a phase-space suppression leads to a strong new upper bound on the scale of fermion mass generation at a finite value n=n{sub s}, which is independent of the EWSB scale, v=({radical}(2)G{sub F}){sup -1/2}. For quarks, leptons, and Majorana neutrinos, the strongest 2{yields}n limits range from about 3 TeV to 130-170 TeV (with 2 < or approx. s < or approx. 24), depending on the measured fermion masses. Strikingly, given the tiny neutrino masses as constrained by the neutrino oscillations, neutrinoless double-beta decays and astrophysical observations, the unitarity violation of {nu}{sub L}{nu}{sub L}{yields}nW{sub L}{sup a} scattering actually occurs at a scale no higher than {approx}170 TeV. Implications for various mechanisms of neutrino mass generation are analyzed. On the other hand,more » for the 2{yields}n pure Goldstone boson scattering, we find that the decreasing phase-space factor always dominates over the growing overall energy factor when n becomes large, so that the best unitarity bound on the scale of EWSB remains at n=2.« less

We provide an analytic solution to the short wave length limit of the integro-differential equation describing the damping of the tensor modes of gravitational waves.

We show that a virtual graviton has a J=0 component, which serves to cancel the J=2,Jz=0 component when the graviton is on shell. In contrast, a massive graviton has no J=0 component either on or off shell. This difference is responsible for the van Dam–Veltman–Zakharov discontinuity.

The cosmological matter-antimatter asymmetry can originate from CP-violating interactions of seesaw Majorana neutrinos via leptogenesis in the thermal phase of the early universe. Having the cosmological CP-phase for leptogenesis requires at least two right-handed Majorana neutrinos. Using only the low energy neutrino observables, we quantitatively reconstruct a minimal neutrino seesaw. We establish a general criterion for minimal seesaw schemes in which the cosmological CP-phase is completely reconstructed from the low energy CP-phases measured by neutrino oscillation and neutrinoless double-beta decay experiments. We reveal and analyze two distinct classes of such minimal schemes that are shown to be highly predictive. Extension, of our reconstruction formalism to a three-heavy-neutrino seesaw is discussed. (C) 2004 Published by Elsevier B.V.

Measurements suggest that our universe has a substantial dark energy component. The most recent data on type Ia supernovae give a dark energy density which is in good agreement with other measurements if the dark energy is assumed to be a cosmological constant. Here we examine to what extent that data can put constraints on a more general equation of state for the dark energy.

The light gluino (12-16 GeV) and light sbottom (2-6 GeV) scenario has been used to explain the apparent overproduction of b quarks at the Fermilab Tevatron. This scenario also predicts the decay Z-->b (b) over bar(g) over tilde(g) over tilde where the gluinos subsequently decay into b quarks and sbottoms. We show that this can contribute to Gamma(4b)=Gamma(Z=b (b) over barb (b) over bar) since most of the sbottoms and b quarks arising from gluino decay have a small angular separation. We find that while no excess in Gamma(4b) is observable due to large uncertainties in experimental measurements, the ratio Gamma(Z-->b (b) over bar(g) over tilde(g) over tilde)/Gamma(Z-->b (b) over barb (b) over bar) can be large due to sensitivity to b-quark mass, the sbottom mixing angle and the gluino mass. We calculate it to be in the range 0.05-0.41 inclusive of the entire parameter space.

We study the unitarity of the standard model (SM) in higher dimensions. We show that the essential features of SM unitarity remain after compactification, and place bounds on the highest Kaluza-Klein (KK) level N_KK and the Higgs mass m_H in the effective four-dimensional (4d) low-energy theory. We demonstrate these general observations by explicitly analyzing the effective 4d KK theory of a compactified 5d SM on S^1/Z_2. The nontrivial energy cancellations in the scattering of longitudinal KK gluons or KK weak bosons, a consequence of the geometric Higgs mechanism, are verified. In the case of the electroweak gauge bosons, the longitudinal KK states also include a small mixture from the KK Higgs excitations. With the analyses before and after compactification, we derive the strongest bounds on N_KK from gauge KK scattering. Applying these bounds to higher-dimensional SUSY GUTs implies that only a small number of KK states can be used to accelerate gauge coupling unification. As a consequence, we show that the GUT scale in the 5d minimal SUSY GUT cannot be lower than about 10^{14} GeV.

Recent measurements suggest our Universe has a substantial dark energy component, which is usually interpreted in terms of a cosmological constant. Here we examine how much the form of this dark energy can be modified while still retaining an acceptable fit to the high redshift supernova data. We first consider changes in the dark energy equation of state and then explore a model in which the dark energy is interpreted as a fluid with a bulk viscosity.

Approximately forty years ago it was realized that the time development of decaying systems might not be precisely exponential. Rolf Winter (Phys. Rev. {\bf 123}, 1503 (1961)) analyzed the simplest nontrivial system - a particle tunneling out of a well formed by a wall and a delta-function. He calculated the probability current just outside the well and found irregular oscillations on a short time scale followed by an exponential decrease followed by more oscillations and finally by a decrease as a power of the time. We have reanalyzed this system, concentrating on the survival probability of the particle in the well rather than the probability current, and find a different short time behavior.

Compactified five-dimensional Yang–Mills theory results in an effective four-dimensional theory with a Kaluza–Klein (KK) tower of massive vector bosons. We explicitly demonstrate that the scattering of the massive vector bosons is unitary at tree-level for low energies, and analyze the relationship between the unitarity violation scale in the KK theory and the nonrenormalizability scale in the five-dimensional gauge theory. In the compactified theory, low-energy unitarity is ensured through an interlacing cancellation among contributions from the relevant KK levels. Such cancellations can be understood using a Kaluza–Klein equivalence theorem which results from the geometric "Higgs" mechanism of compactification. In these theories, the unitarity violation is delayed to energy scales higher than the customary limit through the introduction of additional vector bosons rather than Higgs scalars.

Existing oscillation data point to non-zero neutrino masses with large mixings. We analyze the generic features of the neutrino Majorana mass matrix with inverted hierarchy and construct realistic minimal schemes for the neutrino mass matrix that can explain the large (but not maximal) nu(e)-nu(mu) mixing of MSW-LAM as well as the nearly maximal nu(mu)-nu(tau) mixing and the small (or negligible) nu(e)-->nu(tau) transition. These minimal schemes are quite unique and turn out to be extremely predictive. Implications for neutrinoless double beta decay, tritium beta decay and cosmology are analyzed. (C) 2002 Elsevier Science B.V. All rights reserved.

Unitarity relates the total cross section for neutrino-nucleon scattering to the neutrino-nucleon forward scattering amplitude. Assuming the validity of the perturbative expansion of the forward amplitude in the weak coupling constant, we derive a unitarity bound on the inelastic cross section. The inelastic cross section saturates this bound at a typical neutrino energy E-v similar or equal to 10(8) GeV. This implies that calculations of the inelastic cross section that use current parton distribution functions and lowest order weak perturbation theory are unreliable above this energy. (C) 2001 Elsevier Science B.V. All rights reserved.

The Drell-Hearn-Gcrasimov-Iddings (DHGI) sum rule for electrons is evaluated at order alpha (3) and shown to agree with the Schwinger contribution to the anomalous magnetic moment, (C) 2001 Published by Elsevier Science B.V.

Evidence for neutrino oscillations points to the existence of tiny but finite neutrino masses. Such masses may be naturally generated via radiative corrections in models, such as the Zee model, where a singlet Zee scalar plays a key role. We minimally extend the Zee model by including a right-handed singlet neutrino nu(R). The radiative Zee mechanism can be protected by a simple U(1)(X) symmetry involving only the nu(R) and a Zee scalar. We further construct a class of models with a single horizontal U(1)(FN) (à la Froggatt-Nielsen) such that the mass patterns of the neutrinos and leptons are naturally explained. We then analyze the muon anomalous magnetic moment (g(mu)-2) and the flavor changing mu-->egamma decay. The nu(R) interaction in our minimal extension is found to induce the BNL g(mu)-2 anomaly, with a light charged Zee scalar of mass 100-300 GeV.

We compute the cross section for neutrino-photon scattering taking into account a neutrino mass. We explore the possibility of using intense neutrino beams, such as those available at proposed muon colliders, together with high powered lasers to probe the neutrino mass in photon-neutrino collisions.

We compute the contribution of Kaluza-Klein graviton exchange to the cross section for photon-neutrino scattering. Unlike the usual situation where the virtual graviton exchange represents a small correction to a leading: order electroweak or strong amplitude, in this case the graviton contribution is of the same order as the electroweak amplitude, or somewhat larger, inclusion of the graviton contribution is not sufficient to allow high energy neutrinos to scatter from relic neutrinos in processes such as nu<(nu)over bar> --> gamma gamma, but the photon-neutrino decoupling temperature is substantially reduced.

Recently, Barger et al. computed energy losses into Kaluza Klein modes from astrophysical plasmas in the approximation of zero density for the plasmas. We extend their work by considering the effects of finite density for two plasmon processes. Our results show that, for fixed temperature, the energy loss rate per cm3 is constant up to some critical density and then falls exponentially. This is true for transverse and longitudinal plasmons in both the direct and crossed channels over a wide range of temperature and density. A difficulty in deriving the appropriate covariant interaction energy at finite density and temperature is addressed. We find that, for the cases considered by Barger et al., the zero density approximation and the neglect of other plasmon processes is justified to better than an order of magnitude.

Previous calculations of photon-neutrino scattering in a constant background magnetic field are extended and corrected.

We present collider tests of the recent proposal for weak-scale quantum gravity due to new large compact space dimensions in which only the graviton $(G)$ propagates. We show that the existing high precision LEP-I $Z$-pole data can impose nontrivial constraints on the scale of the new dimensions, via the decay mode $Z\ensuremath{\rightarrow}f\overline{f}+G$ $(f\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}q,\ensuremath{\ell})$. These bounds are comparable to those obtained at high energy colliders and provide the first sensitive probe of the scalar graviton. We also study $W(Z)+G$ production and the anomalous $\mathrm{WW}(\mathrm{ZZ})$ signal from virtual $G$-states at the Fermilab Tevatron and compare them with the LEP-I bound and those from LEP-II and future linear colliders.