Model Of Electroweak Interactions Research Articles

SANCscope—v.1.00

In this article we have summarized the status of the system SANC version 1.00. We have implemented theoretical predictions for many high energy interactions of fundamental particles at the one-loop precision level for up to 4-particle processes. In the present part of our SANC description we place emphasis on an extensive discussion of an important first step of calculations of the one-loop amplitudes of 3- and 4-particle processes in QED, QCD and EW theories. Program summary Title of program: SANC Catalogue identifier: ADXK_v1_0 Program summary URL: http://cpc.cs.qub.ac.uk/summaries/ADXK_v1_0 Program obtainable from: CPC Program Library, Queen's University of Belfast, N. Ireland Designed for: platforms on which Java and FORM3 are available Tested on: Intel-based PC's Operating systems: Linux, Windows Programming languages used: Java, FORM3, PERL, FORTRAN Memory required to execute with typical data: 10 Mb No. of bytes in distributed program, including test data, etc.: 3 658 844 No. of bits in a word: 32 No. of processors used: 1 on SANC server, 1 on SANC client Distribution format: tar.gz Nature of physical problem: Automatic calculation of pseudo- and realistic observables for various processes and decays in the Standard Model of Electroweak interactions, QCD and QED at one-loop precision level. Form factors and helicity amplitudes free of UV divergences are produced. For exclusion of IR singularities the soft photon emission is included. Method of solution: Numerical computation of analytical formulae of form factors and helicity amplitudes. For simulation of two fermion radiative decays of Standard Model bosons ( W ± , Z ) and the Higgs boson a Monte Carlo technique is used. Restrictions on the complexity: In the current version of SANC there are 3 and 4 particle processes and decays available at one-loop precision level. Typical running time: The running time depends on the selected process. For instance, the symbolic calculation of form factors (with precomputed building blocks) of Bhabha scattering in the Standard Model takes about 15 s, helicity amplitudes—about 30 s, and bremsstrahlung—10 s. The numerical computation of cross-section for this process takes about 5 s (CPU 3 GHz IP4, RAM 512 Mb, L2 1024 KB).

Development of a low energy particle detector using a superconducting transition edge sensor

It is possible for a superconductive transition edge sensor (TES) to detect directly the low energy proton generated in the neutron beta decay. The precision measurement of the proton spectrum is important to test the prediction of the Standard Model of electroweak interaction. Therefore, we are developing a TES microcalorimeter with a large effective area for a proton detector. Our first TES device consists of four-pixels using a Ti/Au bilayer. All pixels are parallel biased and X-rays from a 55Fe source are detected successfully.

Measurement of the mass and width of the W boson

The mass and width of the W boson are measured using e+e– → W+W– events from the data sample collected by the OPAL experiment at LEP at centre-of-mass energies between 170 GeV and 209 GeV. The mass (m W) and width (Γ W) are determined using direct reconstruction of the kinematics of W+W– → ${\text{q\ifmmode\expandafter\bar\else\expandafter\=\fi{q}}}{\ell }\nu$ and W+W– → ${\text{q\ifmmode\expandafter\bar\else\expandafter\=\fi{q}q\ifmmode\expandafter\bar\else\expandafter\=\fi{q}}}$ events. When combined with previous OPAL measurements using W+W– → $ {\ell }\nu {\ell }\nu $ events and the dependence on of the WW production cross-section at threshold, the results are determined to be $ \begin{array}{*{20}c} {{m_{{\text{W}}} = 80.415 \pm 0.042 \pm 0.030 \pm 0.009\;{\text{GeV}}}} \\ {{\Gamma _{{\text{W}}} = 1.996 \pm 0.096 \pm 0.102 \pm 0.003\;{\text{GeV}}}} \\ \end{array} $ where the first error is statistical, the second systematic and the third due to uncertainties in the value of the LEP beam energy. By measuring in the channel using several different determinations of the direction of jets with differing sensitivities to soft particles, a limit is also obtained on possible final-state interactions due to colour reconnection effects inW+W– → ${\text{q\ifmmode\expandafter\bar\else\expandafter\=\fi{q}q\ifmmode\expandafter\bar\else\expandafter\=\fi{q}}}$ events. The consistency of the results for the W mass and width with those inferred from other electroweak parameters provides an important test of the Standard Model of electroweak interactions.

Electroweak evolution equations

Enlarging a previous analysis, where only fermions and transverse gauge bosons were taken into account, we write down infrared-collinear evolution equations for the Standard Model of electroweak interactions computing the full set of splitting functions. Due to the presence of double logs which are characteristic of electroweak interactions (Bloch-Nordsieck violation), new infrared singular splitting functions have to be introduced. We also include corrections related to the third generation Yukawa couplings.

Measurement of theWboson polarization in top decay at CDF ats=1.8 TeV

The polarization of the W boson in t{yields}Wb decay is unambiguously predicted by the standard model of electroweak interactions and is a powerful test of our understanding of the tbW vertex. We measure this polarization from the invariant mass of the b quark from t{yields}Wb and the lepton from W{yields}l{nu} whose momenta measure the W decay angle and direction of motion, respectively. In this paper we present a measurement of the decay rate (f{sub V+A}) of the W produced from the decay of the top quark in the hypothesis of V+A structure of the tWb vertex. We find no evidence for the nonstandard V+A vertex and set a limit on f{sub V+A} < 0.80 at 95% confidence level. By combining this result with a complementary observable in the same data, we assign a limit on f{sub V+A} < 0.61 at 95% CL. This corresponds to a constraint on the right-handed helicity component of the W polarization of f{sub +}<0.18 at 95% CL. This limit is the first significant direct constraint on f{sub V+A} in top decay.

Constraints on scalar couplings from [formula omitted

New interactions with Lorentz scalar structure, arising from physics beyond the standard model of electroweak interactions, will induce effective pseudoscalar interactions after renormalization by weak interaction loop corrections. Such induced pseudoscalar interactions are strongly constrained by data on π ± → l ± ν l decay. These limits on induced pseudoscalar interactions imply limits on the underlying fundamental scalar interactions that in many cases are substantially stronger than limits on scalar interactions from direct β-decay searches.

Electroweak radiative corrections topp(−)→W±→ℓ±νbeyond the pole approximation

We present a calculation of the complete electroweak O(alpha) corrections to pp/ppbar -> W+/- -> l+/- nu (l=e, mu) in the Standard Model of electroweak interactions, focusing on those corrections which do not contribute in the pole approximation. We study in detail the effect of these corrections on the transverse mass distribution, the W-width measurement, and the transverse mass ratio and cross section ratio of W and Z bosons.

Triple gauge vertices at one-loop level in two-Higgs-doublet model

Renormalized triple gauge vertices (TGV) are examined within the two-Higgs-doublet model of electroweak interactions. Deviations of the TGV from their standard-model values are calculated at the one-loop level, in the on-shell renormalization scheme. As a consistency check, UV divergence cancellations anticipated on symmetry grounds are verified explicitly. Dependence of the TGV finite parts on the masses of possible heavy Higgs scalars is discussed briefly.

A Stueckelberg extension of the Standard Model

An extension of the standard model of electro-weak interactions by an extra abelian gauge boson is given, in which the extra gauge boson and the hypercharge gauge boson both couple to an axionic scalar in a form that leads to a Stueckelberg mass term. The theory leads to a massive Z′ whose couplings to fermions are uniquely determined and suppressed by small mixing angles. Such a Z′ could have a low mass and appear in e+e− collisions as a sharp resonance. The branching ratios into ff̄ species, and the forward–backward asymmetry are found to have distinctive features. The model also predicts a new unit of electric charge e′=Q′e, where Q′ is, in general, irrational, in the coupling of the photon with hidden matter that is neutral under SU(2)L×U(1)Y.

New limits from the Milano neutrino mass experiment with thermal microcalorimeters

In the Standard Model of electroweak interactions an important input parameter is still missing: the absolute value of the mass of one of the neutrinos. In this paper we report the final results of the Milano electron anti-neutrino mass experiment after having measured for about one year the beta spectrum of 187Re with 10AgReO4 microcalorimeters. We describe the experimental set-up, the detector performance and the measuring conditions. We present the updated limit on the electron anti-neutrino mass which is the most stringent so far obtained with thermal detectors. We also give the most precise estimates for the 187Re transition energy and lifetime.

Muon decay: is it really non-neutrino physics?

Muon decay, after more than 60 years of experimental and theoretical investigations, has been established as a firm basis for the standard model of electroweak interactions. The Lorentz structure of the decay interaction has been determined completely from existing experiments in a model-independent way. Neutrino experiments have played a decisive role in that analysis. Precision experiments in muon decay continue to be an important and sensitive testing ground for deviations from the standard model.

Asymmetric effects at 3D Ising-like critical points

The Standard Model of electroweak interactions has a line of first order phase transition in the plane (higgs mass, temperature) that ends in a critical point belonging to the 3D Ising model universality class [K. Rummukainen et al, hep-lat/9805013. Similar critical points are found in finite-temperature QCD [M. Stephanov et al, hep-ph/9806219; F. Karsch et al, hep-lat/0107020. When these critical points are studied by Monte Carlo simulations on the lattice, one observes certain residual deviations from Z 2 symmetry (which is exact for the Ising model). Here we study whether such deviations can be attributed to asymmetric corrections to scaling, which are relatively poorly studied. We compute the critical exponents in the local potential approximation (LPA), that is, in the framework of the Wegner-Houghton equation. We find that the exponent for the leading antisymmetric correction to scaling is approximately 1.691 in the LPA. This high value implies that such corrections cannot explain observed asymmetries.

Process $$\gamma \gamma \to \nu \bar \nu$$ in a strong magnetic field

The three-vertex loop amplitude in a strong magnetic field are analyzed in a general form by using the asymptotic behavior of the electron propagator in an external field. The process \(\gamma \gamma \to \nu \bar \nu\) is studied in terms of the scalar-vector-vector (SVV), pseudoscalar-vector-vector (PVV), vector-vector-vector (VVV), and axial-vector-vector-vector (AVV) combinations of couplings. It is shown that only in the case of the SVV combination does the amplitude grow linearly with increasing magnetic-field strength, the amplitudes evaluated with the other combinations of couplings (PVV, VVV, and AVV) featuring no linearly increasing terms. The process \(\gamma \gamma \to \nu \bar \nu\) is also studied within the left-right model, which is an extension of the Standard Model of electroweak interactions and which may involve an effective scalar ννee coupling. Possible astrophysical manifestations of this process are discussed.

Renormalization of the minimal supersymmetric standard model

The renormalization of the Minimal Supersymmetric Standard Model of electroweak interactions is presented to all orders of perturbation theory using the algebraic method. Special attention is directed to the issues of soft supersymmetry breaking, gauge fixing, and infrared finiteness. We discuss the implications of ℏ-dependent field parametrizations on the counterterm structure and provide a complete set of on-shell normalization conditions.

Leptogenesis, CP violation and neutrino data: what can we learn?

A detailed analytic and numerical study of baryogenesis through leptogenesis is performed in the framework of the standard model of electroweak interactions extended by the addition of three right-handed neutrinos, leading to the seesaw mechanism. We analyze the connection between GUT-motivated relations for the quark and lepton mass matrices and the possibility of obtaining a viable leptogenesis scenario. In particular, we analyze whether the constraints imposed by SO(10) GUTs can be compatible with all the available solar, atmospheric and reactor neutrino data and, simultaneously, be capable of producing the required baryon asymmetry via the leptogenesis mechanism. It is found that the Just-So 2 and SMA solar solutions lead to a viable leptogenesis even for the simplest SO(10) GUT, while the LMA, LOW and VO solar solutions would require a different hierarchy for the Dirac neutrino masses in order to generate the observed baryon asymmetry. Some implications on CP violation at low energies and on neutrinoless double beta decay are also considered.

Electroweak fits at LEP

Abstract High precision electroweak measurements performed over ten years at LEP and SLC have allowed to constrain the Standard Model of electroweak interactions. The model have been used to predict the mass of the top quark and to set limits on the mass of the Higgs boson.

Electroweak radiative corrections to neutral-current Drell-Yan processes at hadron colliders

We calculate the complete electroweak O(alpha) corrections to pp, pbar p -> l+l- X (l=e, mu) in the Standard Model of electroweak interactions. They comprise weak and photonic virtual one-loop corrections as well as real photon radiation to the parton-level processes q bar q -> gamma,Z -> l+l-. We study in detail the effect of the radiative corrections on the l+l- invariant mass distribution, the cross section in the Z boson resonance region, and on the forward-backward asymmetry, A_FB, at the Fermilab Tevatron and the CERN Large Hadron Collider. The weak corrections are found to increase the Z boson cross section by about 1%, but have little effect on the forward-backward asymmetry in the Z peak region. Threshold effects of the W box diagrams lead to pronounced effects in A_FB at m(l+l-) approx 160 GeV which, however, will be difficult to observe experimentally. At high di-lepton invariant masses, the non-factorizable weak corrections are found to become large.

PHOTON-PAIR CONVERSION INTO NEUTRINOS IN A STRONG MAGNETIC FIELD

A general analysis of the three-vertex loop amplitude in a strong magnetic field, based on the asymptotic form of the electron propagator in the field, is performed. In order to investigate the photon–neutrino process [Formula: see text], the vertex combinations of the scalar–vector–vector (SVV), pseudoscalar–vector–vector (PVV), three-vector (VVV), and axial-vector–vector–vector (AVV) types are considered. It is shown that only the SVV amplitude grows linearly with the magnetic field strength, while in the other amplitudes, PVV, VVV and AVV, the linearly growing terms are canceled. The process [Formula: see text] is investigated in the left–right-symmetric extension of the standard model of electroweak interaction, where the effective scalar νν e e coupling could exist. Possible astrophysical manifestations of the considered process are discussed.

Inclusive dileptonic rareBdecays with an extra generation of vectorlike quarks

We investigate the leading effects of extending the Standard Model of electroweak interactions by an extra iso-singlet up- and down- type quark pair on various distributions and total branching ratio of the inclusive B-> X_s l^+ l^- (l =e,\mu) rare B decays. The presence of the extra vector-like down quark $D$ results in the non-unitarity of the extended quark mixing matrix V, which in turn leads to b-> s FCNC at the tree level proportional to (V^\dagger V)_{sb}. On the other hand, the effective penguin and box vertex functions are sensitive to the mass of the extra iso-singlet up quark m_U. The experimental upper bound on BR(B-> X_s \mu^+ \mu^-) is used to constrain the parameters of the model. It is shown that the shapes of the differential branching ratio and forward-backward asymmetry distribution are very sensitive to the value of the model parameters. We also calculate the CP aymmetry distribution of the dileptonic decay in the vector-like quark model. It is shown that, for a typical choice of the model parameters, asymmetries up to around 10% can be achieved for certain values of the dilepton invariant mass.

Measurement of the mass and width of the W boson in e +e − collisions at 189 GeV

The mass and width of the W boson are determined in e +e − collisions at LEP using 183 pb −1 of data recorded at a centre-of-mass energy s =189 GeV with the OPAL detector. The invariant mass distributions from 970 W +W −→qqqq and 1118 W +W −→qqℓ ν ℓ candidate events are used to measure the mass of the W boson, M W=80.451±0.076 (stat.)±0.049 (syst.) GeV. A direct measurement of the width of the W boson gives Γ W=2.09±0.18 (stat.)±0.09 (syst.) GeV. The results are combined with previous OPAL results from 78 pb −1 of data recorded with s from 161 to 183 GeV, to obtain: M W=80.432±0.066 (stat.)±0.045 (syst.) GeV, Γ W=2.04±0.16 (stat.)±0.09 (syst.) GeV. The consistency of the direct measurement of M W with that inferred from other measurements of electroweak parameters provides an important test of the Standard Model of electroweak interactions.