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J. Matias

Publications and source records attributed to J. Matias.

At least 37 records · Page 2Linked to original sources
arXiv2004

MSLED: A Minimal Supersymmetric Large Extra Dimensions Scenario

We propose a framework for the low-energy realization of supersymmetry which is very predictive, but differs radically in its phenomenological implications from the supersymmetric Standard Model (minimal or otherwise). The proposal consists of a supersymmetric version of the Large-Extra-Dimensions scenario, with the Standard Model living on a 3-brane, coupled to a bulk sector consisting of six-dimensional supergravity. This picture is motivated by a promising recent attempt (hep-th/0304256) to naturally understand the observed dark energy density, and this connection with dark energy prevents making the extra dimensions smaller than of order 5 $μ$m. The resulting inability to change this size makes the model very predictive, and easily falsifiable within the near future. Being supersymmetric, it may plausibly be embedded into a more fundamental theory such as string theory, in which case an additional 4 compact dimensions may also be present having inverse radii at the TeV scale or higher. The model is close to, but consistent with, current experimental constraints. We outline possible phenomenological implications for particle physics (both at accelerators and elsewhere), for precision tests of gravity, for astrophysics and for cosmology.

hep-ph↗
arXiv2002

A Higgs or Not a Higgs? What to Do if You Discover a New Scalar Particle

We show how to systematically analyze what may be inferred should a new scalar particle be discovered in collider experiments. Our approach is systematic in the sense that we perform the analysis in a manner which minimizes apriori theoretical assumptions as to the nature of the scalar particle. For instance, we do not immediately make the common assumption that a new scalar particle is a Higgs boson, and so must interact with a strength proportional to the mass of the particles with which it couples. We show how to compare different observables, and so to develop a decision tree from which the nature of the new particle may be discerned. We define several categories of models, which summarize the kinds of distinctions which the first experiments can make.

hep-ph↗
arXiv2002

Loop-Generated Bounds on Changes to the Graviton Dispersion Relation

We identify the effective theory appropriate to the propagation of massless bulk fields in brane-world scenarios, to show that the dominant low-energy effect of asymmetric warping in the bulk is to modify the dispersion relation of the effective 4-dimensional modes. We show how such changes to the graviton dispersion relation may be bounded through the effects they imply, through loops, for the propagation of standard model particles. We compute these bounds and show that they provide, in some cases, the strongest constraints on nonstandard gravitational dispersions. The bounds obtained in this way are the strongest for the fewest extra dimensions and when the extra-dimensional Planck mass is the smallest. Although the best bounds come for warped 5-D scenarios, for which the 5D Planck Mass is O(TeV), even in 4 dimensions the graviton loop can lead to a bound on the graviton speed which is comparable with other constraints.

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arXiv2000

A Higgs or Not a Higgs?

This talk summarizes a method for analyzing the properties of any new scalar particle, which is systematic in the sense that it minimizes apriori theoretical assumptions about the properties of the scalar particle, leading to very model-independent results. This kind of analysis lends itself to systematic survey through the terrain of candidate theories, which we find has vast unpopulated areas. It is also useful for quantifying the comparison of the goodness of fit of competing descriptions of data, should a new scalar be found.

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arXiv1999

Anatomy of Mixing-Induced CP Asymmetries in Left-Right-Symmetric Models with Spontaneous CP Violation

We investigate the pattern of CP violation in K, B_d and B_s mixing in a symmetrical SU(2)_R x SU(2)_L x U(1) model with spontaneous CP violation. We calculate the phases of the left and right quark mixing matrices beyond the small phase approximation and perform a careful analysis of all relevant restrictions on the model's parameters from Delta m_K, Delta m_B, epsilon, epsilon'/epsilon and the CP asymmetry in B->J/psi K_S. We find that, with current experimental data, the mass of the right-handed charged gauge boson, M2, is restricted to be in the range 2.75 to 13 TeV and the mass of the flavour-changing neutral Higgs boson, MH, in 10.2 to 14.6 TeV. This means in particular that the decoupling limit M2, MH -> infinity is already excluded by experiment. We also find that the model favours opposite signs of epsilon and sin 2beta and is excluded if sin 2beta > 0.1.

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arXiv1999

Low-energy neutrino-photon inelastic interactions

The computation of the polarized amplitudes and cross section of the processes $γν\toγγν$, $γγ\to γν\barν$ and $ν\barν\to γγγ$ is described. We used an effective lagrangian approach for energies below the threshold for $e^+e^-$ pair production and the complete computation at higher energies for application in supernova dynamics. Leading contributions of physics beyond the SM are also commented.

hep-ph↗
arXiv1999

Bs decays: CP asymmetries in left-right models with spontaneous CP violation

We present the contributions of new CP phases in CP asymmetries of two-body neutral $B_s$ decays coming from a left--right model with spontaneous CP violation. Large deviations from the Standard Model predictions can be accommodated in a natural way by this type of models. The new physics effects on the mixing, width difference and decays are analysed. In particular, we show how the measurement of the angle $γ$ in electroweak penguin-dominated processes can be largely affected.

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arXiv1999

CP Asymmetries in B_s Decays and Spontaneous CP violation

We study possible effects of new physics in CP asymmetries in two-body $B_s$ decays in left-right models with spontaneous CP violation. Considering the contributions of new CP phases to the $B_s$ mixing as well as to the penguin dominated decay amplitudes we show that, with the present constraints, large deviations from the standard model predictions in CP asymmetries are allowed in both cases. Detection of the new physics can be done by measuring non-zero asymmetries which are predicted to vanish in the standard model or by comparing two measurements which are predicted to be equal in the standard model. In particular, we show that the measurement of the CKM angle $γ$ in electroweak penguin dominated processes $B_s^0\toρ^0η^{(')}, ρ^0ϕ$ can largely be affected by the new physics.

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arXiv1998

Inelastic photon-neutrino interactions using an effective Lagrangian

We justify the feasibility of substituting a photon leg by a neutrino current in the Euler-Heisenberg Lagrangian to obtain an effective Lagrangian for the process $γν\toγγν$ and its crossed reactions. We establish the link between these processes and the four-photon scattering in both the Standard Model and the effective theory. As an application, we compute the processes $γν\toγγν$ and $γγ\toγν\barν$, give their polarized cross sections, and show how to use the $γγ\toγγ$ results as a check. We settle the question about the disagreement between two computations in the literature concerning the reaction $γγ\to γν\barν$.

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arXiv1998

Five-leg photon-neutrino interactions

In a first part, we justify the feasibility of substituting a photon leg by a neutrino current in the Euler-Heisenberg Lagrangian to obtain an effective Lagrangian for the process $γν\toγγν$ and its crossed reactions. We establish the link between these processes and the four-photon scattering in both the Standard Model and the effective theory. As an application, we compute in this effective theory the processes $γν\toγγν$ and $γγ\toγν\barν$ and show how to use the $γγ\toγγ$ results as a check. We settle the question of the disagreement between two computations in the literature concerning the reaction $γγ\to γν\barν$. In the second part, we present results of the direct computation of the photon-neutrino five-leg processes in the Standard Model, discuss possible astrophysical implications of our results, and provide simple fits to the exact expressions.

hep-ph↗
arXiv1998

Direct computation of inelastic photon-neutrino processes in the Standard Model

In this paper, we compute the Standard Model polarized amplitudes and cross sections of the processes $γν\toγγν$, $γγ\to γν\barν$ and $ν\barν\to γγγ$, for centre-of-mass energies $ω$ within the range of validity of the Fermi theory. By using a large electron-mass expansion of the exact result, we also derive the first correction term to the effective, low-energy ($ω< m_e$) formula for $γν\toγγν$. Finally, we discuss possible astrophysical implications of our results and provide simple fits to the exact expressions.

hep-ph↗
arXiv1997

The $b\to sγγ$ transition in softly broken supersymmetry

We study the effect of supersymmetric contributions to the effective quark transition $b\to sγγ$, including leading order QCD effects. We apply the discussion to the decay $B_s\toγγ$. Even though one-particle irreducible contributions could play a role, numerical cancelations make the amplitude for the two-photon emission strongly correlated to the $b\to sγ$ amplitude which is sharply constrained by experiment. A quite general statement follows: as long as non-standard physics effects appear only in the matching of the Wilson coefficients of the standard effective operator basis, the deviations from the standard model expectations of the decay rates induced by $b\to sγγ$ are bound to follow closely the corresponding deviations on $b\to sγ$. Effects of new physics are therefore bound to be small.

hep-ph↗
arXiv1997

Phenomenological aspects of a Fermiophobic SU(2)xSU(2)xU(1) extension of the Standard Model

We consider an extension of the standard electroweak theory with gauge group $SU(2)_L \times SU(2)_R \times U(1)_{\tilde{Y}}$, where the gauge bosons of the extra $SU(2)_R$ factor do not couple to ordinary fermions. We show that precision electroweak data and flavour physics provide quite stringent indirect constraints on its parameter space, but still allow for relatively light non-standard gauge and Higgs bosons. We then consider the model phenomenology at high-energy colliders, and observe that in the gauge boson sector present bounds and possible future signals are dominated by $Z'$ production. In summary, indirect constraints on the charged gauge boson sector are so tight that observable new effects must be connected either with the neutral gauge boson sector or with the extended Higgs sector of the model.

hep-ph↗
arXiv1997

A fermiophobic SU(2)xSU(2)xU(1) extension of the Standard Model

A new type of gauge extension of the SM is proposed. It is based on a SU(2)xSU(2)xU(1) group with the peculiarity that the gauge bosons of the extra SU(2) do not couple to fermions. This feature relaxes some of the constraints on the masses of the new gauge bosons, leaving the possibility of having lighter masses than in traditional extensions. The model exhibits several interesting properties, it is anomaly free and at tree level it does not have FCNC while loop induced effects are strongly suppressed. Also, from the analysis of $Δρ$ at one loop, two configurations for the vevs giving rise to a screening phenomenon are identified. One of these configurations can also be related with the Bess model. A tree level fit to the most recent electroweak data is performed confirming the possibility of having new light gauge boson masses. The constraints coming from different FCNC processes like $b \to s γ$, $B_{0}-{\bar B}_{0}$ and $K_{0}-{\bar K}_{0}$ mixing are also taken into account. Finally, a generalization of this model for the case of having several extra copies of SU(2) groups is commented, focusing on the presence of screening configurations and the corresponding mass spectrum.

hep-ph↗
arXiv1996

The effective chiral lagrangian of the MSM at one and two loop order

The formalism of the matching conditions between transverse connected Green functions is extended to include the two next to leading corrections, namely the two-loop $M_{H}^{2}$ and the one-loop $1/M_{H}^{2}$ contributions to the coefficients of the electroweak chiral lagrangian which are relevant to the LEP1 physics: $a_{0}$, ${\hat a}_{1}$ and ${\hat a}_{8}$. We derive general expressions for these three coefficients in terms of just bare gauge boson self-energies. By means of the screening theorem, it is shown that the same expressions can be used to get directly from a MSM calculation, the leading Higgs mass contribution to these coefficients at each loop order. In a more general framework, we solve the problems concerning the loss of gauge invariance and the inclusion of only gauge invariant operators by proposing a new formulation of the matching conditions at two and higher loop order. As an example of the usefulness of using an electroweak chiral lagrangian to parametrize the MSM, we will give a simple proof of an extra screening for the renormalized photon self-energy in the on-shell scheme at all orders. In addition it is shown the automatic cancellation of the unphysical $M_{H}^4$ terms in the other gauge boson self-energies at two-loops in this scheme. Finally we will apply the obtained electroweak chiral lagrangian to compute the different Higgs mass contributions to the bosonic part of $Δρ$, $Δr$ and $Δκ$, analyzing carefully the hierarchy between corrections.

hep-ph↗
arXiv1995

RENORMALIZATION and the EQUIVALENCE THEOREM: ON-SHELL SCHEME

We perform an exhaustive analysis of the Equivalence Theorem both in the minimal Standard Model and in an Effective Electroweak Chiral Lagrangian up to ${\cal O}(p^4)$. We have considered the leading corrections to the usual prescription consisting in just replacing longitudinally polarized $W$ or $Z$ by the corresponding Goldstone bosons. The corrections appear through an overall constant multiplying the Goldstone boson amplitude as well as through additional diagrams. By including them we can extend the domain of applicability of the Equivalence Theorem, making it suitable for precision tests of the symmetry breaking sector of the Standard Model. The on-shell scheme has been used throughout. When considering the Equivalence Theorem in an Effective Chiral lagrangian we analyze its domain of applicability, as well as several side issues concerning gauge fixing, Ward identities, on-shell scheme and matching conditions in the effective theory. We have analyzed in detail the processes $W^+ W^- \to W^+W^-$ and $W^+W^+\to W^+W^+$ to illustrate the points made.

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