Z-boson Exchange Research Articles

Direct detection of electroweak dark matter

TeV-scale dark matter is well motivated by notions of naturalness as the new physics threshold is expected to emerge in the TeV regime. We generalise the Standard Model by including an arbitrary SU(2) multiplet of dark matter particles in non-chiral representation. The pseudo-real representations can be viable DM candidates if one considers a higher-dimensional operator which induces mass-splitting, and avoids the tree-level inelastic scattering through Z-boson exchange. These effective operators give rise to sizable contributions from Higgs mediated dark matter interactions with quarks and gluons. A linear combination of the effective couplings named lambda is identified as the critical parameter in determining the magnitude of the cross-section. When lambda is smaller than the critical value, the theory behaves similar to the known renormalisable model, and the scattering rate stays below the current experimental reach. Nevertheless, above the criticality, the contribution from the higher-dimensional operators significantly changes the phenomenology. The scattering amplitude of pseudo-real models will be coherently enhanced, so that it would be possible for next generation large-exposure experiments to fully probe these multiplets. We studied the parameter space of the theory, taking into account both indirect astrophysical and direct search constraints. It is inferred that multi-TeV mass scale remains a viable region, quite promising for forthcoming dark matter experiments.

Lepton-pair production at hadron colliders at N3LO in QCD

We compute for the first time the complete corrections at N3LO in the strong coupling constant to the inclusive neutral-current Drell-Yan process including contributions from both photon and Z-boson exchange. Our main result is the computation of the QCD corrections to the inclusive production cross section of an axial-vector boson to third order in the strong coupling in a variant of QCD with five massless quark flavours. Since the axial anomaly does not cancel for an odd number of flavours, we also consistently include non-decoupling effects in the top-quark mass through three loops. We perform a phenomenological study of our results, and we present for the first time predictions for the inclusive Drell-Yan process at the LHC at this order in QCD perturbation theory.

Quark-sector Lorentz violation in Z-boson production

Quark-sector Lorentz violation is studied in the context of Drell-Yan dilepton production including effects from Z-boson exchange. We show the chiral nature of the weak interactions enables parity-violating and spin-dependent effects to be studied using unpolarized initial states. Constraints are placed on dimensionless and CPT-even coefficients for Lorentz violation for the first two generations of quarks using measurements from the Large Hadron Collider.

μ→eγdecay in an MSSM extension

An analysis is given of the decay $\ensuremath{\mu}\ensuremath{\rightarrow}e+\ensuremath{\gamma}$ in an minimal supersymmetric standard model extension with a vectorlike generation. Here mixing with the mirrors allows the possibility of this decay. The analysis is done at the one-loop level with the exchange of charginos and neutralinos and of sleptons and mirror sleptons in the loops. A one-loop analysis with W- and Z-boson exchange and mirror leptons and neutrinos is also considered. The effects of $CP$-violating phases from the new sector on the decay $\ensuremath{\mu}\ensuremath{\rightarrow}e\ensuremath{\gamma}$ are analyzed in detail. The constraints arising from the current upper limit on the branching ratio $\mathcal{B}(\ensuremath{\mu}\ensuremath{\rightarrow}e\ensuremath{\gamma})$ from the MEG experiment of $2.4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}12}$ (at 90% C.L.) on the parameter space of supersymmetry (SUSY) models and on vectorlike models are explored. Further, the MEG experiment is likely to improve the upper limit by an order of magnitude in the coming years. The improved limits will allow one to probe a much larger domain of the parameter space of the extended models.

Measurement of high-Q e-p neutral current cross sections at HERA and the extraction of xF3

Cross sections for e-p neutral current deep inelastic scattering have been measured at a centre-of-mass energy of 318 GeV using an integrated luminosity of 15.9 pb-1 collected with the ZEUS detector at HERA. Results on the double-differential cross-section $d^2\sigma / dx dQ^2$ in the range 185 < Q 2 < 50 000 GeV2 and 0.0037 < x < 0.75, as well as the single-differential cross-sections $d\sigma / dQ^2$ , $d\sigma / dx$ and $d\sigma / dy$ for Q 2 > 200 GeV2, are presented. To study the effect of Z-boson exchange, $d\sigma / dx$ has also been measured for Q 2 > 10 000 GeV2. The structure function xF3 has been extracted by combining the e-p results presented here with the recent ZEUS measurements of e+p neutral current deep inelastic scattering. All results agree well with the predictions of the Standard Model.

Parity-violating pion–nucleon coupling hπNN(1) from π+-electroproton production near the threshold

We study the possibility of measuring parity-violating pion–nucleon coupling h πNN (1) from π +-electroproton production in the threshold region. We calculate the electron-helicity asymmetry in terms of h πNN (1) and the Z-boson exchange between the electron and proton in leading-order heavy-baryon chiral perturbation theory. From the result, we demonstrate that the coupling can be determined to an accuracy of ∼2×10 −7 in an experiment with incident electron energy 250 MeV, luminosity 10 38 s −1 cm −2, and a running time of 10 7 s.

Evaluation of beta-decay Part V. The Z-independent outer radiative corrections for allowed decay

Before interpreting nuclear beta-decay in terms of nucleonic wave functions and weak coupling constants allowance must be made for the influence of virtual photon exchange and real photon emission together with Z-boson exchange—the radiative corrections. This paper discusses in detail the lowest order ‘outer’ radiative correction of Sirlin, energy-dependent but weak-interaction-mechanism-independent, and presents graphical and other material to enable its accurate evaluation for allowed decay for all beta-decay transition energies up to 20 MeV and for 0 ≤ | Z| ≤ 100. The need to use electron spectra due to a nuclear charge distribution of finite spatial extension with appropriate convolution of the leptonic and nucleonic wave functions throughout the nuclear volume is emphasized; the variation of these effects between the neutron and proton drip lines is explored. Exponentiation and higher-order Z-independent effects are also considered.

Contribution of terms containing Z-boson exchange to the luminosity measurements at LEP

We have investigated the contribution of terms containing Z-boson exchange to the luminosity measurements at LEP. Comparing the Monte Carlo program BABAMC and the semi-analytical program ALIBABA, we have determined the technical precision of the corresponding O( α) calculation in BABAMC to be 0.03%. Using the ALIBABA program we have assessed the higher-order corrections to these Z-boson exchange contributions to be of the order of 0.1% for the present luminosity measurements. The total theoretical error on the luminosity calculation for LEP experiments is at present not larger than 0.3%.

Transverse spin effect: A means to probe photinos

We consider the e +e − system in storage rings with its natural polarization being transverse to the beam direction. We restrict the analysis to neutrino-like events ν (neutrino), ν (sneutrino), γ (photino) and propose to measure the associated transverse spin asymmetry. We prove that for s ⪢4m ν 2 , photino production is the unique channel which leads to non-vanishing transverse asymmetry. The ν ( ν ) in contrast does not contribute through W − ( W − ) exchange due to chirality while for the Z-boson exchange the ν and its supersymmetric partner ν , contribute equal amounts but with opposite signs in a way similar to that encountered in SUSY-GUTS (hierarchy problem), and hence give a net vanishing result. Using recent limits on the sneutrino mass, we set up the minimum value of √ s above which one may expect the photino to dominate over the neutrino. Also we give a qualitative estimate of the asymmetry and find that it is within the actual precision of such experiments. In this respect transverse asymmetry measurements when restricted to the proposed class of events are a possible probe to new physics beyond the standard model.

Parity violation in deuteron electrodisintegration near threshold with a (0.5-4.0) GeV electron beam.

Making use of a formalism previously developed by Hwang, Henley, and Miller, we obtain numerical predictions for the electron scattering double differential cross section and the parity-violating asymmetry (caused by Z-boson exchange), both defined in the laboratory frame, for the kinematics suitable for the upcoming Continuous Electron Beam Accelerator Facility. For the kinematic regime characterized by ${q}^{2}$\ensuremath{\le}15 ${\mathrm{fm}}^{\mathrm{\ensuremath{-}}2}$ and ${E}_{\mathrm{rel}\mathrm{\ensuremath{\le}}10}$ MeV with ${q}^{2}$ and ${E}_{\mathrm{rel}}$, respectively, the four-momentum transfer squared and the n-p excitation energy, the asymmetry is of order ${10}^{\mathrm{\ensuremath{-}}5}$--${10}^{\mathrm{\ensuremath{-}}4}$ while the cross section remains large, so that a precision test of the standard Glashow-Weinberg-Salam electroweak theory may be attempted. For the kinematic regime characterized by ${q}^{2}$\ensuremath{\ge}30 ${\mathrm{fm}}^{\mathrm{\ensuremath{-}}2}$ and ${E}_{\mathrm{rel}\mathrm{\ensuremath{\le}}10}$ MeV, the asymmetry may be of order ${10}^{\mathrm{\ensuremath{-}}3}$ so that one may attempt an experiment at backward scattering angles to extract interesting information regarding the large ${q}^{2}$ behavior of the nucleon electromagnetic and weak form factors.

Parity violation in pion electroproduction and photoproduction with a proton target

Parity violation in pion electroproduction and photoproduction on a proton target is investigated as a possible means of elucidating the structure of the electronic and non-leptonic neutral weak currents. We calculate both parity-violating effects caused directly by Z-boson exchange, which contributes only to electron scattering, and those due to parity-violating electromagnetic and nuclear interactions. The asymmetry in electroproduction is of order 20 −5, whereas that in photoproduction is ∼10 −7. The sensitivity to the parameter sin 2 θ w is investigated; it is a maximum at backward electron scattering angles. In addition to the Born terms, contributions from the Δ(1232) resonance and the light vector mesons [ϱ(776) and ω(782)] are included in the model. Fits to the observed π 0 and π + electroproduction and photoproduction cross sections indicate that the model accounts for the production data in the medium energy region.

Parity conserving weak neutral current effects in fundamental bound systems

The effects induced by the Z-boson exchange in the hyperfine structure and Lamb shift of some fundamental bound systems is analyzed. The hfs of muonium appears as a promising test of the Weinberg-Salam theiry.

Parity nonconservation in atoms and elastic electron scattering caused by nuclear parity violation

We demonstrate, on the basis of gauge invariance, hermiticity, and time-reversal invariance, that there can be no long range transmission of nuclear parity violation to electrons in an atom or in elastic electron scattering. It follows that the ratio of the nuclear induced parity violation in atoms or in elastic electron scattering to that due to weak neutral currents (Z-boson exchange) remains of the order of the fine structure constant, even in the limit of zero momentum transfer.