arXiv subjects

W. Buchmuller

Publications and source records attributed to W. Buchmuller.

At least 37 records · Page 2Linked to original sources

arXiv2001

CP Violation, Neutrino Mixing and the Baryon Asymmetry

We study neutrino masses and mixings based on the simplest SO(10) mass relations and the seesaw mechanism. We find that the requirement of large neutrino mixings determines the relative magnitude of the heavy Majorana neutrino masses in terms of the known quark mass hierarchy. This leads to specific predictions for the structure of the neutrino mixing matrix, the light neutrino masses, CP violation in neutrino oscillations, neutrinoless double b-decay and the baryon asymmetry.

hep-ph↗

arXiv2001

False Vacuum Decay after Inflation

Inflation is terminated by a non-equilibrium process which finally leads to a thermal state. We study the onset of this transition in a class of hybrid inflation models. The exponential growth of tachyonic modes leads to decoherence and spinodal decomposition. We compute the decoherence time, the spinodal time, the size of the formed domains and the homogeneous classical fields within a single domain.

hep-ph↗

arXiv1999

Neutrino Mixing and the Pattern of Supersymmetry Breaking

We study the implications of a large $ν_μ- ν_τ$ mixing angle on lepton flavour violating radiative transitions in supersymmetric extensions of the standard model. The transition rates are calculated to leading order in $ε$, the parameter which characterizes the flavour mixing. The uncertainty of the predicted rates is discussed in detail. For models with modular invariance the branching ratio $BR(μ\to e γ)$ mostly exceeds the experimental upper limit. In models with radiatively induced flavour mixing the predicted range includes the upper limit, if the Yukawa couplings in the lepton sector are large, as favoured by Yukawa coupling unification.

hep-ph↗

arXiv1999

Combined analysis of diffractive and inclusive structure functions in the semiclassical framework

Small-x DIS is described as the scattering of a partonic fluctuation of the photon off a superposition of target color fields. Diffraction occurs if the emerging partonic state is in a color singlet. Introducing a specific model for the averaging over all relevant color field configurations, both diffractive and inclusive parton distributions at some low scale Q_0^2 can be calculated. A conventional DGLAP analysis results in a good description of diffractive and inclusive structure functions at higher values of Q^2.

hep-ph↗

arXiv1999

Matter Antimatter Asymmetry and Neutrino Properties

The cosmological baryon asymmetry can be explained as remnant of heavy Majorana neutrino decays in the early universe. We study this mechanism for two models of neutrino masses with a large ν_μ-ν_τmixing angle which are based on the symmetries SU(5) x U(1)_F and SU(3)_c x SU(3)_L x SU(3)_R x U(1)_F, respectively. In both cases B-L is broken at the unification scale Λ_{GUT}. The models make different predictions for the baryogenesis temperature and the gravitino abundance.

hep-ph↗

arXiv1998

Inclusive and Diffractive Structure Functions at Small x

In the semiclassical approach, inclusive and diffractive quark and gluon distributions are expressed in terms of correlation functions of Wilson loops. Each Wilson loop integrates the colour field strength in the area between the trajectories of two fast partons penetrating the proton. We introduce a specific model for averaging over the relevant colour field configurations. Within this model, all parton distributions at some low scale Q_0^2 are given in terms of three parameters. Inclusive and diffractive structure functions at higher values of Q^2 are determined in a leading-order QCD analysis. In both cases, the evolution is driven by a large gluon distribution. A satisfactory description of the structure functions F_2(x,Q^2) and F_2^D(3)(xi,beta,Q^2) is obtained. The observed rise of F_2^D(3) with xi is parametrized by a non-perturbative logarithmic energy dependence, compatible with unitarity. In our analysis, the observed rise of F_2 at small x is largely due to the same effect.

hep-ph↗

arXiv1998

CP Asymmetry in Majorana Neutrino Decays

We study CP asymmetries in lepton-number violating two-body scattering processes and show how they are related to CP asymmetries in the decays of intermediate massive Majorana neutrinos. Self-energy corrections, which do not contribute to CP asymmetries in two-body processes, induce CP violating couplings of the intermediate Majorana neutrinos to lepton-Higgs states. We briefly comment on the implications of these results for applications at finite temperature.

hep-ph↗

arXiv1997

Classical Limit for Scalar Fields at High Temperature

We study real-time correlation functions in scalar quantum field theories at temperature $T=1/β$. We show that the behaviour of soft, long wavelength modes is determined by classical statistical field theory. The loss of quantum coherence is due to interactions with the soft modes of the thermal bath. The soft modes are separated from the hard modes by an infrared cutoff $Ł\ll 1/(\hbarβ)$. Integrating out the hard modes yields an effective theory for the soft modes. The infrared cutoff $Ł$ controls corrections to the classical limit which are $\cO{\hbar\betaŁ}$. As an application, the plasmon damping rate is calculated.

hep-th↗

arXiv1997

Baryon Asymmetry of the Universe and Lepton Mixing

Baryogenesis appears to require lepton number violation. This is naturally realized in extensions of the standard model containing right-handed neutrinos. We discuss the generation of a baryon asymmetry by the out-of-equilibrium decay of heavy Majorana neutrinos in these models, without and with supersymmetry. All relevant lepton number violating scattering processes which can inhibit the generation of an asymmetry are taken into account. We assume a similar pattern of mixings and masses for neutrinos and up-type quarks, as suggested by SO(10) unification. This implies that B-L is broken at the unification scale $Λ_{GUT}\sim 10^{16}$ GeV, if $m_{ν_μ} \sim 3\cdot10^{-3}$ eV, as preferred by the MSW solution to the solar neutrino deficit. The observed baryon asymmetry is then obtained without any fine tuning of parameters.

hep-ph↗

arXiv1997

Gluon Radiation in Diffractive Electroproduction

Order $α_s$-corrections to the diffractive structure functions $F_L^D$ and $F_2^D$ at large $Q^2$ and small $x$ are evaluated in the semiclassical approach, where the initial proton is treated as a classical colour field. The diffractive final state contains a fast gluon in addition to a quark-antiquark pair. Two of these partons may have large transverse momentum. Our calculations lead to an intuitive picture of deep-inelastic diffractive processes which is very similar to Bjorken's aligned-jet model. Both diffractive structure functions contain leading twist contributions from high-$p_{\perp}$ jets.

hep-ph↗

arXiv1997

Soft Colour Interactions and Diffractive DIS

The basic ideas and some results of the semiclassical approach to diffractive DIS are briefly described. In the production of high-$p_{\perp}$ jets boson-gluon fusion is predicted to be the dominant partonic process. The $p_{\perp}$-spectrum and the two-jet invariant mass distribution provide a clear test of the underlying `hard' partonic process and the `soft' mechanism of colour neutralization.

hep-ph↗

arXiv1997

Classical Statistical Mechanics and Landau Damping

We study the retarded response function in scalar $ϕ^4$-theory at finite temperature. We find that in the high-temperature limit the imaginary part of the self-energy is given by the classical theory to leading order in the coupling. In particular the plasmon damping rate is a purely classical effect to leading order, as shown by Aarts and Smit. The dominant contribution to Landau damping is given by the propagation of classical fields in a heat bath of non-interacting fields.

hep-ph↗

arXiv1996

Electroweak Phase Transition and Neutrino Masses

The presently observed cosmological baryon asymmetry has been finally determined at the time of the electroweak phase transition, when baryon and lepton number violating interactions fell out of thermal equilibrium. We discuss the thermodynamics of the phase transition based on the free energy of the SU(2) Higgs model at finite temperature, which has been studied in perturbation theory and lattice simulations. The results suggest that the baryon asymmetry has been generated by lepton number violating interactions in the symmetric phase of the standard model, i.e., at temperatures above the critical temperature of the electroweak transition. The observed value of the baryon asymmetry, $n_B/s \sim 10^{-10}$, is naturally obtained in an extension of the standard model with right-handed neutrinos where $B-L$ is broken at the unification scale $Λ_{GUT}}\sim 10^{16}$ GeV. The corresponding pattern of masses and mixings of the light neutrinos $ν_e$, $ν_μ$ and $ν_τ$ is briefly described.

hep-ph↗

arXiv1996

Baryon Asymmetry and Neutrino Mixing

In theories where $B-L$ is a spontaneously broken local symmetry, the cosmological baryon asymmetry can be generated by the out-of-equilibrium decay of heavy Majorana neutrinos. We study this mechanism assuming a similar pattern of mixings and masses for leptons and quarks, as suggested by SO(10) unification. This implies that $B-L$ is broken at the unification scale $Λ_{\mbox{\scriptsize GUT}}\sim 10^{16}$ GeV, if $m_{\n_\m} \sim 3\cdot 10^{-3}$eV as preferred by the MSW explanation of the solar neutrino deficit. The observed value of the baryon asymmetry, $n_B/s \sim 10^{-10}$, is then obtained without any fine tuning of parameters.

hep-ph↗

arXiv1996

Double-logarithmic Scaling of the Structure Function F_2 at small x

Recent data on the structure function F_2(x,Q^2) at small values of x are analysed and compared with theoretical expectations. It is shown that the observed rise at small x is consistent with a logarithmic increase, growing logarithmically also with Q^2. A stronger increase, which may be incompatible with unitarity when extrapolated to asymptotically small values of x, cannot be inferred from present data.

hep-ph↗

arXiv1995

Semiclassical Approach to Structure Functions at Small x

Inclusive and diffractive structure functions for electron-proton scattering are calculated in a semiclassical approach at large momentum transfer $Q^2$ and small values of the scaling variable $x$. The basic process is the production of a quark-antiquark pair in the colour field of the proton. The structure functions are expressed in terms of Wilson lines along the classical trajectories of quark and antiquark passing through the colour field, and their covariant derivatives. Based on some rather general assumptions on properties of the colour field, inclusive and diffractive structure functions are evaluated in terms of four field dependent constants.

hep-ph↗

arXiv1995

Calculating the diffractive from the inclusive structure function

It is demonstrated that the global properties of the rapidity gap events at HERA can be understood based on electron-gluon scattering and a non-perturbative mechanism of colour neutralization. Using the measured inclusive structure function $F_2$ to determine the parameters of the parton model, the diffractive structure function $F_2^D$ is predicted. The ratio of diffractive and inclusive cross sections, $R_D = σ_D/σ_{incl}\simeq 1/9$, is determined by the probability of the produced quark-antiquark pair to evolve into a colour singlet state.

hep-ph↗

arXiv1995

A PARTON MODEL FOR DIFFRACTIVE PROCESSES IN DEEP INELASTIC SCATTERING

We demonstrate that the global properties of the ``rapidity gap'' events, observed at HERA, can be understood based on electron-gluon scattering as the underlying partonic process. Using the measured inclusive structure function $F_2$ to determine the parameters of the parton model, the diffractive structure function $F_2^D$ is predicted. The ratio of diffractive and inclusive cross sections, $R_D = σ_D/σ_{incl}\simeq 1/9$, is determined by the probability of the produced quark-antiquark pair to evolve into a colour singlet state. This colour singlet cluster may fragment into hadrons independently of the proton remnant, yielding the observed gap in rapidity.

hep-ph↗