arXiv subjects

G. J. Gounaris

Publications and source records attributed to G. J. Gounaris.

At least 37 records · Page 2Linked to original sources

arXiv2006

Neutralino pair production in a $γγ$ Collider

We present a complete 1-loop study of the process $γγ\to \tchi^0_i \tchi^0_j$ and the predicted cross section in a $γγ$ Linear Collider. A suitable numerical code PLATONlc, valid for any set of real MSSM parameters, is released. This study and code are complementary to those suitable for Dark Matter detection through the inverse process $\tchi^0_i \tchi^0_j \to γγ$ describing neutralino-neutralino annihilation at rest, which were presented previously. If SUSY is realized in Nature, both codes should be very helpful in future astrophysical and collider studies of the neutralino sector.

hep-ph↗

arXiv2006

Coherent pion production by neutrinos on nuclei

The main part of coherent pion production by neutrinos on nuclei is essentially determined by PCAC, provided that the leptonic momentum transferred square Q^2 remains sufficiently small. We give the formulas for the charged and neutral current cross sections, including also the small non-PCAC transverse current contributions and taking into account the effect of the μ^- mass. Our results are compared with the experimental ones and other theoretical treatments.

hep-ph↗

arXiv2006

Description of $e^-e^+\toγγ, Z γ, ZZ$ in SM and MSSM

We present a complete analysis of the one loop electroweak corrections to $e^-e^+\toγγ, ~Zγ, ~ZZ$ in the Standard (SM) and the Minimal Supersymmetric Standard Model (MSSM). Analytic expressions are written for the contributions to the helicity amplitudes. Several observables accessible for polarized or unpolarized beams and transverse, longitudinal or unpolarized final states are computed. We show that in the few hundred GeV region, these observables provide a test of the various SM or MSSM components. For the dominant TT amplitude at high energy, the sensitivity to the details of the various sectors disappears, but the energy dependence fixed by leading logarithmic contributions, provides a model independent signature discriminating SM from MSSM. Subdominant TL or LL amplitudes though, remain sensitive to the details of the SM or MSSM sectors. Numerical illustrations are given for energies up to several TeV. The analysis may also be used to search for new physics characterized by anomalously strong interactions among the neutral gauge bosons.

hep-ph↗

arXiv2006

Addendum to "Helicity conservation in gauge boson scattering at high energy"

In a previous paper we have established that for any two-body process involving even numbers of transverse vector bosons and gauginos, the dominant asymptotic amplitudes obey helicity conservation (HC); i.e. the sum of the helicities of the two incoming particles equals to the sum of the helicities of the two outgoing ones. This HC has been proved to all orders in MSSM, provided $(s, |t|,|u|)$ are much larger than all masses in the model; but only to leading 1-loop logarithmic order in SM. In the present addendum, the validity of HC is extended to all two-body processes. Renormalizability is crucial for the HC validity, while all known anomalous couplings violate it.

hep-ph↗

arXiv2006

Neutralino-neutralino annihilation to gammaZ in MSSM

The 1-loop computation of the processes $\tchi_i \tchi_j \to γZ$ has been performed at an arbitrary c.m. energy for any pair of MSSM neutralinos. As an application suitable for Dark Matter (DM) searches, the neutralino-neutralino annihilation is studied at the limiting case of vanishing relative velocity, describing the present DM distribution in the galactic halo; and at a relative velocity of about 0.5, determining the neutralino relic density contributions. The most useful situation is obviously for $i=j=1$, but the case of non-identical neutralinos may also be useful in some corners of the parameter space. Our results are contained in the FORTRAN code PLATONdmgZ, applying to any set of real MSSM parameters. Numerical results are also presented for a sample of 6 MSSM models, describing the various possible neutralino properties. A comparison with other existing works is also made.

hep-ph↗

arXiv2005

Supersymmetry Parameter Analysis: SPA Convention and Project

High-precision analyses of supersymmetry parameters aim at reconstructing the fundamental supersymmetric theory and its breaking mechanism. A well defined theoretical framework is needed when higher-order corrections are included. We propose such a scheme, Supersymmetry Parameter Analysis SPA, based on a consistent set of conventions and input parameters. A repository for computer programs is provided which connect parameters in different schemes and relate the Lagrangian parameters to physical observables at LHC and high energy e+e- linear collider experiments, i.e., masses, mixings, decay widths and production cross sections for supersymmetric particles. In addition, programs for calculating high-precision low energy observables, the density of cold dark matter (CDM) in the universe as well as the cross sections for CDM search experiments are included. The SPA scheme still requires extended efforts on both the theoretical and experimental side before data can be evaluated in the future at the level of the desired precision. We take here an initial step of testing the SPA scheme by applying the techniques involved to a specific supersymmetry reference point.

hep-ph↗

arXiv2005

Contrasting the anomalous and the SM-MSSM couplings at the Colliders

This talk consists of two parts. In the first, the present experimental bounds on the anomalous couplings of the gauge bosons, based mainly on the LEP and Tevatron experiments, are reviewed. In the second part, the theorem of helicity conservation (HC) is presented, which should be valid in either the Standard Model (SM) or MSSM, for any two-body process at high energies and fixed angles. The energy-range for the HC validity is discussed and, under certain conditions, it should well be within the LHC or ILC range. Since all known anomalous couplings violate HC, its testing may provide a way for generically identifying the possible presence of anomalous (non-renormalizable) contributions.

hep-ph↗

arXiv2005

Single Neutralino production at CERN LHC

The common belief that the lightest supersymmetric particle (LSP) might be a neutralino, providing also the main Dark Matter (DM) component, calls for maximal detail in the study of the neutralino properties. Motivated by this, we consider the direct production of a single neutralino $\tchi^0_i$ at a high/energy hadron collider, focusing on the $\tchi^0_1$ and $\tchi^0_2$ cases. At Born level, the relevant subprocesses are $q\bar q\to \tchi^0_i \tilde g$, $g q\to \tchi^0_i \tilde q_{L,R}$ and $q\bar q'\to \tchi^0_i\tchi^\pm_j$; while at 1-loop, apart from radiative corrections to these processes, we consider also $gg\to \tchi^0_i\tilde{g}$, for which a numerical code named PLATONgluino is released. The relative importance of these channels turns out to be extremely model dependent. Combining these results with an analogous study of the direct $\tchi^0_i\tchi^0_j$ pair production, should help in testing the SUSY models and the Dark Matter assignment.

hep-ph↗

arXiv2005

NOSTOS: a spherical TPC to detect low energy neutrinos

A novel low-energy ($\sim$few keV) neutrino-oscillation experiment NOSTOS, combining a strong tritium source and a high pressure spherical Time Projection Chamber (TPC) detector 10 m in radius has been recently proposed. The oscillation of neutrinos of such energies occurs within the size of the detector itself, potentially allowing for a very precise (and rather systematics-free) measure of the oscillation parameters, in particular, of the smaller mixing angle $θ_{13}$, which value could be determined for the first time. This detector could also be sensitive to the neutrino magnetic moment and be capable of accurately measure the Weinberg angle at low energy. The same apparatus, filled with high pressure Xenon, exhibits a high sensitivity as a Super Nova neutrino detector with extra galactic sensitivity. The outstanding benefits of the new concept of the spherical TPC will be presented, as well as the issues to be demonstrated in the near future by an ongoing R&D. The very first results of small prototype in operation in Saclay are shown.

hep-ex↗

arXiv2005

About helicity conservation in gauge boson scattering at high energy

We remark that the high energy gauge boson scattering processes involving two-body initial and final states, satisfy certain selection rules described as helicity conservation of the \underline{gauge boson} amplitudes (GBHC). These rules are valid at Born level, as well as at the level of the leading and sub-leading 1-loop logarithmic corrections, in both the Standard Model (SM) and the Minimal Supersymmetric Standard Model (MSSM). A "fermionic equivalence" theorem is also proved, which suggests that GBHC is valid at all orders in MSSM at sufficiently high energies, where the mass suppressed contributions are neglected.

hep-ph↗

arXiv2004

Electron Spectra in the Ionization of Atoms by Neutrinos

For neutrinos of O(10keV) energies, their oscillation lengths are less than a few hundred meters, thereby suggesting the fascinating idea of oscillation experiments of small geometrical size. To help evaluating this idea we calculate the ionization cross sections of H, He, Ne and Xe, using any neutrino flavor, in the few keV energy range. We find that the atomic ionization cross sections per electron are always smaller than the neutrino cross sections off free electrons, approaching it from below as the energy increases to the 100 keV region. At the 10-20 keV range though, atomic binding effects are very important, particularly for the heavier atoms, inducing a factor of two reduction of the Xe ionization cross section, compared to the free electron one.

hep-ph↗

arXiv2004

Neutralino-neutralino annihilation to photon and gluon pairs in MSSM models

The complete 1-loop computation of the processes $(\tchi^0_i \tchi^0_j \leftrightarrowγγ, ~ gg)$, for any pair of the four MSSM neutralinos, has been performed for an arbitrary c.m energy. As a first application suitable for Dark Matter (DM) searches, the neutralino-neutralino annihilation is studied at relative velocities $v/c\simeq 10^{-3}$ describing the present DM distribution in the galactic halo, and $v/c\simeq 0.5$ determining neutralino relic density contributions. Numerical results are presented for 31 MSSM benchmark models indicating considerable sensitivity to the input parameters. Numerical codes are released, which may be used for the computation of the annihilation of any two neutralinos to two photons or two gluons at the aforementioned $v/c$ values. In the near future, we intend to complement them with codes for the inverse process $γγ\to \tchi^0_i \tchi^0_j$, observable at the future high energy Linear Colliders.

hep-ph↗

arXiv2003

The flavor distribution of Cosmic Neutrinos

Simple approximate expressions for the relative flavor fluxes of energetic cosmic neutrinos detected on Earth, are presented in terms of their initial fluxes at the surface of the producing cosmic sites, assuming the neutrino mixing angles to lie in the experimentally favored region. These expressions highlight the main sensitivities to the initial production fluxes as well as to small variations of the mixing angles within the experimentally preferred region, thus providing simple methods to disentangle these physical quantities from cosmic neutrino data. This is more so, due to some striking mathematical properties of the relative neutrino flavors at $(s_{23}=1/\sqrt{2}, s_{13}=0)$, which somehow characterize the whole experimentally preferred region. We also assess the quality of our approximate expressions through a numerical comparison with the exact results.

hep-ph↗

arXiv2002

The processes $e^-e^+\toγγ, Z γ, ZZ$ in SM and MSSM

We present the results of a complete analysis of the one loop electroweak corrections to $e^-e^+\toγγ, ~Zγ, ~ZZ$ in the Standard (SM) and the Minimal Supersymmetric Standard Model (MSSM). A special emphasis is put on the high energy behaviour of the various helicity amplitudes and the way the logarithmic structure is generated. The large magnitude of these effects, which induce striking differences between the SM and MSSM cases at high energies, offers the possibility of making global tests which could check the consistency of these models, and even decide whether any additional new physics is required.

hep-ph↗

arXiv2002

The neutralino projector formalism for complex SUSY parameters

We present a new formalism describing the neutralino physics in the context of the minimal supersymmetric model (MSSM), where CP violation induced by complex $M_1$ and $μ$ parameters is allowed. The formalism is based on the construction of neutralino projectors, and can be directly generalized to non-minimal SUSY models involving any number of neutralinos. It extends a previous work applied to the real SUSY parameter case. In MSSM, the method allows to describe all physical observables related to a specific neutralino, in terms of its CP eigenphase and three complex numbers called its "reduced projector elements". As the experimental knowledge on the neutralino-chargino sectors will be being accumulated, the problem of extracting the various SUSY parameters will arise. Motivated by this, we consider various scenarios concerning the quantities that could be first measured. Analytical disentangled expressions determining the related SUSY parameters from them, are then derived, which also emphasize the efficiency of the formalism.

hep-ph↗

arXiv2001

The ionization of H, He and Ne atoms using neutrinos or antineutrinos at keV energies

We calculate the ionization cross sections for H, He or Ne atoms using $ν_e$ and $\bar ν_e$ scattering at keV energies. Such cross sections are useful for e.g. $\bar ν_e$-oscillation experiments using a tritium source. Using realistic atomic wave functions, we find that for $E_ν\lsim 10 ~\rm keV $ the atomic ionization cross sections, normalized to one electron per unit volume, are smaller than the corresponding free electron ones, and that they approach it from below as energies of 20 keV are reached.

hep-ph↗

arXiv2001

A description of the neutralino observables in terms of projectors

Applying Jarlskog's treatment of the CKM matrix, to the neutralino mass matrix in MSSM for real soft gaugino SUSY breaking and $μ$-parameters, we construct explicit analytical expressions for the four projectors which acting on any neutralino state project out the mass eigenstates. Analytical expressions for the neutralino mass eigenvalues in terms of the various SUSY parameters, are also given. It is shown that these projectors and mass eigenvalues are sufficient to describe any physical observable involving neutralinos, to any order of perturbation theory. As an example, the $e^-e^+ \to \tilde χ^0_i \tilde χ^0_j $ cross section at tree level is given in terms of these projectors. The expected magnitude of their various matrix elements in plausible SUSY scenarios is also discussed.

hep-ph↗

arXiv2001

Off-shell structure of the anomalous Z and gamma self-couplings

We establish the general off-shell structure of the three neutral gauge boson self-couplings V1*V2*V3*, with applications to the Z*Z*Z*, Z*Z*gamma*, gamma*gamma*Z* cases. New coupling forms appear which do not exist when two gauge bosons are on-shell. We give the contribution arising from a fermionic triangle loop. It covers both the standard model (SM) and possible new physics (NP) contributions like those arising in the MSSM. For what concerns NP contributions with a high scale, we discuss the validity of an effective Lagrangian involving a limited set of parameters. Finally we write the general expression of the V1*V2*V3*-vertex contribution to the e+e- to (f(bar) f)+(f'(bar) f') amplitude.

hep-ph↗