arXiv subjects

P. I. Porfyriadis

Publications and source records attributed to P. I. Porfyriadis.

At least 19 recordsLinked to original sources

arXiv2006

Neutralino pair production at CERN LHC

We consider the production of neutralino pairs $\tchi^0_i\tchi^0_j$ at a high energy hadron collider, putting a special emphasis on the case where one of them is the lightest neutralino $\tchi^0_1$, possibly constituting the main Dark Matter component. At tree level, the only relevant subprocess is $q\bar q\to \tchi^0_i\tchi^0_j$, while the subprocess $gg\to \tchi^0_i\tchi^0_j$ first appears at the one loop level. Explicit expressions for the $q\bar q$-helicity amplitudes are presented, including the tree level contributions and the leading-log one loop radiative corrections. For the one-loop $ gg\to \tchi^0_i\tchi^0_j$ process, a numerical code named PLATONggnn is released, allowing the computation of $dσ/d \that$ in any MSSM model with real soft breaking parameters. It turns out that acceptable MSSM benchmark models exist for which the $q\bar q $ and the gluonic contributions may give comparable effects at LHC, due to the enhanced gluonic structure functions at low fractional momenta. Depending on the values of the MSSM parameters, we find that the LHC neutralino pair production may provide sensitive tests of SUSY models generating neutralino Dark Matter.

hep-ph↗

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

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.

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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.

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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.

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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.

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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.

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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.

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arXiv2001

The processes gamma gamma to Z H in SM and MSSM

The process gamma gamma to ZH first arises at the 1-loop level, and as such it provides remarkable tests of the structure of the electroweak Higgs sector. These tests are complementary to those in the gauge sector involving gamma gamma to gamma gamma, gamma Z, ZZ. We show that in the Standard Model (SM) where H=H_{SM}, as well as in the supersymmetric case where H=h^0, H^0 or A^0, observables exist (like e.g. the energy dependence, angular distribution, photon polarization dependence or final Z polarization) which present rather spectacular properties. Such properties involve strong threshold effects with steps, bumps or peaks, reflecting the type of Higgs and heavy quarks and chargino masses and couplings predicted by the SM and supersymmetric models.

hep-ph↗

arXiv2001

The heavy neutral Higgs signature in the gamma gamma to Z Z process

If the Standard Model (SM) Higgs particle is sufficiently heavy, then its contribution to gamma gamma to Z Z should be largely imaginary, interfering with the also predominantly imaginary SM "background" generated by the W-loop. For standard model Higgs masses in the region 200 \lsim m_H \lsim 500 GeV, this interference is found to be constructive and increasing the Higgs signal. In the minimal SUSY case an interference effect should also appear for the contribution of the heavier CP-even neutral Higgs boson H^0, provided it is sufficiently heavy. The effect is somewhat reduced though, by the smallness of the H^0 width and the gamma gamma and ZZ branching ratios. The interference is again found to be constructive for part of the parameter space corresponding to sfermion masses at the TeV scale and maximal stop mixing. For both the SM and SUSY cases, regions of the parameter space exist though, where the interference may be destructive. It is therefore essential to take these effects into account when searching for possible scalar Higgs-like candidates. To this aim, we present the complete analytic expressions for both resonance and background amplitudes.

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arXiv2000

The $γγ\to A^0 A^0$ process at a $γγ$ Collider

The helicity amplitudes for the process $γγ\to A^0 A^0$ are studied to 1-loop order in the minimal SUSY (MSSM) model, where $A^0$ is the CP-odd Higgs particle. Simple exact analytic formulae are obtained, in terms of the $C_0$ and $D_0$ Passarino-Veltman functions; in spite of the fact that the loop-diagrams often involve different particles running along their sides. For a usual mSUGRA set of parameters, $σ(γγ\to A^0 A^0) \sim (0.1-0.2)\rm fb $ is expected. If SUSY is realized in Nature, these expressions should be useful for understanding the Higgs sector.

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arXiv1999

The gamma gamma to Z Z process and the search for virtual SUSY effects at a gamma gamma Collider

We study the helicity amplitudes of the process gamma gamma to ZZ in the Standard Model at high energy. These amplitudes receive contributions from the W and charged quark and lepton loops, analogous to those encountered in the gamma gamma to gamma gamma, gamma Z cases studied before. But gamma gamma to ZZ also receives contributions from the Higgs s-channel poles involving the effective Higgs-gamma gamma vertex. At energies of about 300 GeV, the amplitudes in all three processes are mainly helicity-conserving and almost purely imaginary; which renders them a very useful tool in searching for New Physics. As an example, a SUSY case is studied, and the signatures due to the virtual effects induced by a chargino-, charged slepton- or a lightest stop-loop in gamma gamma to ZZ, are explored. These signatures, combined with the analogous ones in gamma gamma to gamma gamma and gamma gamma to gamma Z, should help identifying the nature of possible New Physics particles.

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arXiv1999

Light by Light Scattering at High Energy: a Tool to Reveal New Particles

We point out a few remarkable properties of the $γγ\toγγ$ process at high energy. They should allow to search for effects of new particles and interactions. We give illustrations with threshold effects due to pairs of new charged particles (charginos, charged Higgses, sfermions), resonance effects due to s-channel production of neutral scalars (standard or supersymmetric Higgs particles, technipions) and unitarity saturating amplitudes due to a strongly interacting sector. The use of polarized photon beams is also briefly discussed.

hep-ph↗

arXiv1999

The gamma gamma to gamma Z process at high energies and the search for virtual SUSY effects

We study the helicity amplitudes and the observables of the process $γγ\to γZ$ at high energy. As in the case of the $γγ\to γγ$ process studied before, the relevant diagrams in the standard model (SM) involve $W$, charged quark and lepton loops; while in SUSY we also have contributions from chargino, and charged sfermion or Higgs loop diagrams. Above 250 GeV, the dominant SM amplitudes are themselves dominated by the $W$ loop, and as for $γγ\to γγ$, they are helicity conserving and almost purely imaginary. We discuss the complementary information provided by $γγ\to γZ$ for the identification of possible non standard effects. This process, together with $γγ\to γγ$, should provide very useful information on the nature of possible new physics particles, above the threshold of their direct production.

hep-ph↗

arXiv1999

The gamma gamma to gamma gamma process in the Standard and SUSY models at high energies

We study the helicity amplitudes of the process γγ\to γγat high energy, which in the standard and SUSY models first arise at the one-loop order. In the standard model (SM), the diagrams involve W and charged quark and lepton loops, while in SUSY we also have contributions from chargino, charged sfermion and Higgs loop diagrams. The SUSY contributions are most important in the region above the threshold for producing the supersymmetric partners; since there, they interfere most effectively with the primarily imaginary SM amplitudes. Simple expressions for the relevant 1-loop functions are given, which provide a direct overview of the behaviour of the helicity amplitudes in the whole parameter space at high energies. The various characteristics of a large set of observables are studied in detail.

hep-ph↗

arXiv1998

String Tension and the Generation of the Conformal Anomaly

The origin of the string conformal anomaly is studied in detail. We use a reformulated string Lagrangian which allows to consider the string tension $T_{0}$ as a small perturbation. The expansion parameter is the worldsheet speed of light c, which is proportional to $T_{0}$ . We examine carefully the interplay between a null (tensionless) string and a tensionful string which includes orders $ c^{2} $ and higher. The conformal algebra generated by the constraints is considered. At the quantum level the normal ordering provides a central charge proportional to $ c^{2} $. Thus it is clear that quantum null strings respect conformal invariance and it is the string tension which generates the conformal anomaly.

hep-th↗

arXiv1997

Null Strings in Kerr Spacetime

The null string's equations of motion and constraints in the Kerr spacetime are given. We assume a generic ansatz for the null strings in the Kerr spacetime and we present the resulting solutions in quadratures. Some specific string configurations, that follow from the generic one, are considered separately. In each case we also extract the corresponding solutions in the Schwarzschild spacetime.

hep-th↗

arXiv1996

The complete Faddeev-Jackiw treatment of the $U_{EM}(1)$ gauged SU(2) WZW model

The two flavour, four dimensional WZW model coupled to electromagnetism, is treated as a constraint system in the context of the Faddeev-Jackiw approach. No approximation is made. Detailed exposition of the calculations is given. Solution of the constraints followed by proper Darboux's transformations leads to an unconstrained Coulomb-gauge Lagrangian density.

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