Wilfried Buchmuller

Effective Lagrangian analysis of new interactions and flavour conservation

Abstract New interactions with a scale Λ larger than the Fermi scale G F − 1 2 will manifest themselves at energies below Λ through small deviations from the standard model, which can be described by an effective lagrangian containing non-renormalizable SU(3) × SU(2) × U(1) invariant operators. We construct the first two terms of this lagrangian in an expansion in powers of 1/Λ and study systematically possible effects of new interactions such as anomalous magnetic moments, deviations from universality in weak interactions and rare processes. Among the flavour conserving processes the universality of the charged current weak interactions yields the strongest bound on the new interaction scale, Λ > 5 TeV, whereas flavour non-conserving processes imply the bound Λ > 3000 TeV. We derive conditions for natural flavour conservation. Although it cannot be excluded that all flavour changing operators are dynamically suppressed, this appears difficult to understand without a symmetry for which the standard electroweak theory seems to provide no hint. We emphasize the importance of searching for rare decays of D-mesons.

LEPTOGENESIS AS THE ORIGIN OF MATTER

▪ Abstract We explore in some detail the hypothesis that the generation of a primordial lepton-antilepton asymmetry (Leptogenesis) early on in the history of the Universe is the root cause for the origin of matter. After explaining the theoretical conditions for producing a matter-antimatter asymmetry in the Universe we detail how, through sphaleron processes, it is possible to transmute a lepton asymmetry—or, more precisely, a (B – L)-asymmetry—into a baryon asymmetry. Because Leptogenesis depends in detail on properties of the neutrino spectrum, we review briefly existing experimental information on neutrinos as well as the seesaw mechanism, which offers a theoretical understanding of why neutrinos are so light. The bulk of the review is devoted to a discussion of thermal Leptogenesis, and we show that for the neutrino spectrum suggested by oscillation experiments, one obtains the observed value for the baryon to photon density ratio in the Universe, independently of any initial boundary conditions. In ...

Leptoquarks in lepton-quark collisions

Abstract Low-energy experiments permit the existence of leptoquarks with masses of order 100 GeV and couplings to quark-lepton pairs as large as gauge couplings. We study systematically the signatures of all possible scalar and vector leptoquarks in electron (positron)-proton collisions. Clear evidence for leptoquarks would be narrow peaks in the x -distributions of inclusive neutral and charged current processes. At HERA one will be able to explore the mass range up to 300 GeV through direct production, and even somewhat beyond the CM energy of 314 GeV through virtual effects. Conversely, leptoquarks with masses of 200 GeV can be discovered for couplings as small as 10 −3 α em .

Supersymmetric standard model from the heterotic string.

We present a [FORMULA: SEE TEXT] orbifold compactification of the E8xE8 heterotic string which leads to the (supersymmetric) standard model gauge group and matter content. The quarks and leptons appear as three 16-plets of SO(10), whereas the Higgs fields do not form complete SO(10) multiplets. The model has large vacuum degeneracy. For generic vacua, no exotic states appear at low energies and the model is consistent with gauge coupling unification. The top quark Yukawa coupling arises from gauge interactions and is of the order of the gauge couplings, whereas the other Yukawa couplings are suppressed.

Gravitino dark matter in R-parity breaking vacua

We show that in the case of small R-parity and lepton number breaking couplings, primordial nucleosynthesis, thermal leptogenesis and gravitino dark matter are naturally consistent for gravitino masses m3/25 GeV. We present a model where R-parity breaking is tied to B-L breaking, which predicts the needed small couplings. The metastable next-to-lightest superparticle has a decay length that is typically larger than a few centimeters, with characteristic signatures at the LHC. The photon flux produced by relic gravitino decays may be part of the apparent excess in the extragalactic diffuse gamma-ray flux obtained from the EGRET data for a gravitino mass m3/2 ~ 10 GeV. In this case, a clear signal can be expected from GLAST in the near future.

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 mixing and masses for leptons and quarks, as suggested by SO(10) unification. This implies that B - L is broken at the unification scale ΛGUT ∼ 1016 GeV, if mνμ ∼ 3 · 10−3eV as preferred by the MSW explanation of the solar neutrino deficit. The observed value of the baryon asymmetry, nBs ∼ 10−10, is then obtained without any fine tuning of parameters.

Constraints on SU(5)-type leptoquarks

Abstract Low energy effects of scalar leptoquarks in SU(5) unification schemes are analyzed. Flavour changing couplings of leptoquarks with masses MLQ=O (100 GeV) have to be strongly suppressed. Flavour diagonal couplings may be as large as λ= O ( 1 10 ) .

Gamma-rays from decaying dark matter

We study the prospects for detecting gamma-rays from decaying dark matter (DM), focusing in particular on gravitino DM in R-parity breaking vacua. Given the substantially different angular distribution of the predicted gamma-ray signal with respect to the case of annihilating DM, and the relatively poor (of order 0.1°) angular resolution of gamma-ray detectors, the best strategy for detection is in this case to look for an exotic contribution to the gamma-ray flux at high galactic latitudes, where the decaying DM contribution would resemble an astrophysical extragalactic component, similar to the one inferred by EGRET observations. Upcoming experiments such as GLAST and AMS-02 may identify this exotic contribution and discriminate between it and astrophysical sources, or place significant constraints on the mass and lifetime of DM particles.

Baryon asymmetry and dark matter

Abstract We study the implications of a large baryogenesis temperature, T B = O (10 10 GeV), on the mass spectrum of superparticles in supersymmetric extensions of the standard model. Models with a neutralino as lightest superparticle (LSP) are excluded. A consistent picture is obtained with the gravitino as LSP, followed by a higgsino-like neutralino (NSP). Gravitinos with masses from 10 to 100 GeV may be the dominant component of dark matter.

Supergravity at colliders

We consider supersymmetric theories where the gravitino is the lightest superparticle (LSP). Assuming that the long-lived next-to-lightest superparticle (NSP) is a charged slepton, we investigate two complementary ways to prove the existence of supergravity in nature. The first is based on the NSP lifetime which in supergravity depends only on the Planck scale and the NSP and gravitino masses. With the gravitino mass inferred from kinematics, the measurement of the NSP lifetime will test an unequivocal prediction of supergravity. The second way makes use of the 3-body NSP decay. The angular and energy distributions and the polarizations of the final state photon and lepton carry the information on the spin of the gravitino and on its couplings to matter and radiation.