Physics Beyond the Standard Model: Focusing on the Muon Anomaly

We present a model based on the implication of an exceptional E_{6}-GUT symmetry for the anomalous magnetic moment of the muon. We follow a particular chain of breakings with Higgses in the 78 and 351 representations. We analyse the radiative correction contributions to the muon mass and the effects of the breaking of the so-called Weinberg symmetry. We also estimate the range of values of the parameters of our model.Comment: 14 RevTeX pages, 5 figure

Contributions of order O(mquark2){\cal O}(m_{\rm quark}^2) to Kℓ3K_{\ell 3} form factors and unitarity of the CKM matrix

The form factors for the $K_{\ell 3}$ semileptonic decay are computed to order $O(p^4)$ in generalized chiral perturbation theory. The main difference with the standard $O(p^4)$ expressions consists in contributions quadratic in quark masses, which are described by a single divergence-free low-energy constant, $A_3$. A new simultaneous analysis is presented for the CKM matrix element $V_{us}$, the ratio $F_K/F_{\pi}$, $K_{\ell 3}$ decay rates and the scalar form factor slope $\lambda_0$. This framework easily accommodates the precise value for $V_{ud}$ deduced from superallowed nuclear $\beta$-decays

Baryon chiral perturbation theory with virtual photons and leptons

We construct the general pion-nucleon SU(2) Lagrangian including both virtual photons and leptons for relativistic baryon chiral perturbation theory up to fourth order. We include the light leptons as explicit dynamical degrees of freedom by introducing new building blocks which represent these leptons.Comment: 11 page

Performance of Geant4 in simulating semiconductor particle detector response in the energy range below 1 MeV

Geant4 simulations play a crucial role in the analysis and interpretation of experiments providing low energy precision tests of the Standard Model. This paper focuses on the accuracy of the description of the electron processes in the energy range between 100 and 1000 keV. The effect of the different simulation parameters and multiple scattering models on the backscattering coefficients is investigated. Simulations of the response of HPGe and passivated implanted planar Si detectors to \beta{} particles are compared to experimental results. An overall good agreement is found between Geant4 simulations and experimental data

Measurement of the β\beta-asymmetry parameter of 67^{67}Cu in search for tensor type currents in the weak interaction

Precision measurements at low energy search for physics beyond the Standard Model in a way complementary to searches for new particles at colliders. In the weak sector the most general $\beta$ decay Hamiltonian contains, besides vector and axial-vector terms, also scalar, tensor and pseudoscalar terms. Current limits on the scalar and tensor coupling constants from neutron and nuclear $\beta$ decay are on the level of several percent. The goal of this paper is extracting new information on tensor coupling constants by measuring the $\beta$-asymmetry parameter in the pure Gamow-Teller decay of $^{67}$Cu, thereby testing the V-A structure of the weak interaction. An iron sample foil into which the radioactive nuclei were implanted was cooled down to milliKelvin temperatures in a $^3$He-$^4$He dilution refrigerator. An external magnetic field of 0.1 T, in combination with the internal hyperfine magnetic field, oriented the nuclei. The anisotropic $\beta$ radiation was observed with planar high purity germanium detectors operating at a temperature of about 10\,K. An on-line measurement of the $\beta$ asymmetry of $^{68}$Cu was performed as well for normalization purposes. Systematic effects were investigated using Geant4 simulations. The experimental value, $\tilde{A}$ = 0.587(14), is in agreement with the Standard Model value of 0.5991(2) and is interpreted in terms of physics beyond the Standard Model. The limits obtained on possible tensor type charged currents in the weak interaction hamiltonian are -0.045 $< (C_T+C'_T)/C_A <$ 0.159 (90\% C.L.). The obtained limits are comparable to limits from other correlation measurements in nuclear $\beta$ decay and contribute to further constraining tensor coupling constants

Hadronic Light-by-Light Contribution to Muon g-2 in Chiral Perturbation Theory

We compute the hadronic light-by-light scattering contributions to the muon anomalous magnetic moment, \amulbl, in chiral perturbation theory that are enhanced by large logarithms and a factor of $N_C$. They depend on a low-energy constant entering pseudoscalar meson decay into a charged lepton pair. The uncertainty introduced by this constant is $\pm 60\times 10^{-11}$, which is comparable in magnitude to the present uncertainty entering the leading-order vacuum polarization contributions to the anomalous moment. It may be reduced to some extent through an improved measurement of the $\pi^0\to e^+ e^-$ branching ratio. However, the dependence of \amulbl on non-logarithmically enhanced effects cannot be constrained except through the measurement of the anomalous moment itself. The extraction of information on new physics would require a future experimental value for the anomalous moment differing significantly from the 2001 result reported by the E821 collaboration.Comment: 7 pages, 2 figure

Hadronic light-by-light scattering contribution to the muon g-2: an effective field theory approach

The hadronic light-by-light contribution to a_{mu}, the anomalous magnetic moment of the muon, is discussed from the point of view of an effective low-energy theory. As an application, the coefficient of the leading logarithm arising from the two-loop graphs involving two anomalous vertices is computed, and found to be positive. This corresponds to a positive sign for the pion-pole contribution to the hadronic light-by-light correction to a_{mu}, and to a sizeable reduction of the discrepancy between the present experimental value of a_{mu} and its theoretical counterpart in the standard model.Comment: 4 pages, 1 figure. v2: published versio

Two-loop representations of low-energy pion form factors and pi-pi scattering phases in the presence of isospin breaking

Dispersive representations of the pi-pi scattering amplitudes and pion form factors, valid at two-loop accuracy in the low-energy expansion, are constructed in the presence of isospin-breaking effects induced by the difference between the charged and neutral pion masses. Analytical expressions for the corresponding phases of the scalar and vector pion form factors are computed. It is shown that each of these phases consists of the sum of a "universal" part and a form-factor dependent contribution. The first one is entirely determined in terms of the pi-pi scattering amplitudes alone, and reduces to the phase satisfying Watson's theorem in the isospin limit. The second one can be sizeable, although it vanishes in the same limit. The dependence of these isospin corrections with respect to the parameters of the subthreshold expansion of the pi-pi amplitude is studied, and an equivalent representation in terms of the S-wave scattering lengths is also briefly presented and discussed. In addition, partially analytical expressions for the two-loop form factors and pi-pi scattering amplitudes in the presence of isospin breaking are provided.Comment: 57 pages, 12 figure

Radiative corrections to neutral pion-pair production

We calculate the one-photon loop radiative corrections to the neutral pion-pair photoproduction process $\pi^-\gamma \to \pi^-\pi^0\pi^0$. At leading order this reaction is governed by the chiral pion-pion interaction. Since the chiral $\pi^+\pi^-\to\pi^0\pi^0$ contact-vertex depends only on the final-state invariant-mass it factors out of all photon-loop diagrams. We give analytical expressions for the multiplicative correction factor $R\sim \alpha/2\pi$ arising from eight classes of contributing one-photon loop diagrams. An electromagnetic counterterm has to be included in order to cancel the ultraviolet divergences generated by the photon-loops. Infrared finiteness of the virtual radiative corrections is achieved (in the standard way) by including soft photon radiation below an energy cut-off $\lambda$. The radiative corrections to the total cross section vary between $+2\%$ and $-2\%$ for center-of-mass energies from threshold up to $7m_\pi$. The finite part of the electromagnetic counterterm gives an additional constant contribution of about $1\%$, however with a large uncertainty.Comment: 10 pages, 6 figures, submitted to Eur. Phys. J.

The S11−NS_{11}- N(1535) and −N-N(1650) Resonances in Meson-Baryon Unitarized Coupled Channel Chiral Perturbation Theory

The $s-$wave meson-baryon scattering is analyzed for the strangeness S=0 sector in a Bethe-Salpeter coupled channel formalism incorporating Chiral Symmetry. Four channels have been considered: $\pi N$, $\eta N$, $K \Lambda$, $K \Sigma$. The needed two particle irreducible matrix amplitude is taken from lowest order Chiral Perturbation Theory in a relativistic formalism and low energy constants are fitted to the elastic $\pi N$ phase-shifts and the $\pi^- p \to \eta n$ and $\pi^- p \to K^0 \Lambda$ cross section data. The position of the complex poles in the second Riemann sheet of the scattering amplitude determine masses and widths of the $S_{11}-$ $N$(1535) and $-N$(1650) resonances, in reasonable agreement with experiment. A good overall description of data, from $\pi N$ threshold up to 2 GeV, is achieved keeping in mind that the two pion production channel has not been included.Comment: 35 pages, LaTeX + 7 ps-figure files. Some minor mistakes have been corrected for and a new appendix discussing the matching to HBChPT has been also adde