arXiv subjects

Manuel Toharia

Publications and source records attributed to Manuel Toharia.

36 records · Page 2Linked to original sources

arXiv2010

Flavor Bounds and Phenomenology in the Scalar Sector of RS Scenarios

In the context of a warped extra-dimension with Standard Model fields in the bulk, we obtain the general flavor structure of the couplings to fermions of both the Higgs scalar and the radion graviscalar. In the Flavor Anarchy paradigm, these couplings are generically misaligned with respect to the fermion mass matrix and moreover the off-diagonal couplings can be estimated parametrically as a function of fermion masses and the observed mixing angles. One can then study the flavor constraints and predictions arising from these couplings.

hep-ph↗

arXiv2010

Patterns in the Fermion Mixing Matrix, a bottom-up approach

We first obtain the most general and compact parametrization of the unitary transformation diagonalizing any 3 by 3 hermitian matrix H, as a function of its elements and eigenvalues. We then study a special class of fermion mass matrices, defined by the requirement that all of the diagonalizing unitary matrices (in the up, down, charged lepton and neutrino sectors) contain at least one mixing angle much smaller than the other two. Our new parametrization allows us to quickly extract information on the patterns and predictions emerging from this scheme. In particular we find that the phase difference between two elements of the two mass matrices (of the sector in question) controls the generic size of one of the observable fermion mixing angles: i.e. just fixing that particular phase difference will "predict" the generic value of one of the mixing angles, irrespective of the value of anything else.

hep-ph↗

arXiv2010

Scalar Kinks in Warped Extra Dimensions

We study the existence and stability of static kink-like configurations of a 5D scalar field, with Dirichlet boundary conditions, along the extra dimension of a warped braneworld. In the presence of gravity such configurations fail to stabilize the size of the extra dimension, leading us to consider additional scalar fields with the role of stabilization. We numerically identify multiple nontrivial solutions for a given 5D action, made possible by the nonlinear nature of the background equations, which we find is enhanced in the presence of gravity. Finally, we take a first step towards addressing the question of the stability of such configurations by deriving the full perturbative equations for the gravitationally coupled multi-field system.

hep-ph↗

arXiv2009

Higgs Mediated FCNC's in Warped Extra Dimensions

In the context of a warped extra-dimension with Standard Model fields in the bulk, we obtain the general flavor structure of the Higgs couplings to fermions. These couplings will be generically misaligned with respect to the fermion mass matrix, producing large and potentially dangerous flavor changing neutral currents (FCNC's). As recently pointed out in [arXiv:0906.1542], a similar effect is expected from the point of view of a composite Higgs sector, which corresponds to a 4D theory dual to the 5D setup by the AdS-CFT correspondence. We also point out that the effect is independent of the geographical nature of the Higgs (bulk or brane localized), and specifically that it does not go away as the Higgs is pushed towards the IR boundary. The FCNC's mediated by a light enough Higgs (specially their contribution to $ε_K$) could become of comparable size as the ones coming from the exchange of Kaluza-Klein (KK) gluons. Moreover, both sources of flavor violation are complementary since they have inverse dependence on the 5D Yukawa couplings, such that we cannot decouple the flavor violation effects by increasing or decreasing these couplings. We also find that for KK scales of a few TeV, the Higgs couplings to third generation fermions could experience suppressions of up to 40% while the rest of diagonal couplings would suffer much milder corrections. Potential LHC signatures like the Higgs flavor violating decays $h\toμτ$ or $h\to tc$, or the exotic top decay channel $t\to c h$, are finally addressed.

hep-ph↗

arXiv2008

Radion Mediated Flavor Changing Neutral Currents

In the context of a warped extra-dimension with Standard Model fields in the bulk, we obtain the general flavor structure of the Radion couplings to fermions and show that the result is independent on the particular nature of the Higgs mechanism (bulk or brane localized). These couplings will be generically misaligned with respect to the fermion mass matrix when the fermion bulk mass parameters are not all degenerate. When the Radion is light enough, the generic size of these tree-level flavor changing couplings will be strongly constrained by the experimental bounds on $ΔF=2$ processes. At the LHC the possibility of a heavier Radion decaying into top and charm quarks is then considered as a promising signal to probe the flavor structure of both the Radion sector and the whole scenario.

hep-ph↗

arXiv2008

Higgs-Radion Mixing with Enhanced Di-Photon Signal

In the context of warped scenarios in which Standard Model (SM) fields are allowed to propagate in the bulk, we revisit the possible mixing between the IR localized Higgs field and the Radion graviscalar. The phenomenology of the resulting mostly-Higgs field does not suffer important deviations with respect to the case in which all the SM is localized in the IR brane (original Higgs-Radion mixing scenario). On the contrary, the phenomenology of the mostly-Radion field can present important differences with respect to the original scenario. At the LHC, the most striking effect is now the possibility of sizeable Radion decays into photons in a mass range well beyond the ZZ and WW thresholds, not due to dramatically enhanced couplings to photons but to suppressed couplings to massive fields.

hep-ph↗

arXiv2008

Odd Tachyons in Compact Extra Dimensions

We consider a real scalar field with an arbitrary negative bulk mass term in a general 5D setup, where the extra spatial coordinate is a warped interval of size $πR$. When the 5D field verifies Neumann conditions at the boundaries of the interval, the setup will always contain at least one tachyonic KK mode. On the other hand, when the 5D scalar verifies Dirichlet conditions, there is always a critical (negative) mass $M_{c}^2$ such that the Dirichlet scalar is stable as long as its (negative) bulk mass $μ^2$ verifies $M^2_{c}<μ^2$. Also, if we fix the bulk mass $μ^2$ to a sufficiently negative value, there will always be a critical interval distance $πR_c$ such that the setup is unstable for $R>R_c$. We point out that the best mass (or distance) bound is obtained for the Dirichlet BC case, which can be interpreted as the generalization of the Breitenlohner-Freedman (BF) bound applied to a general compact 5D warped spacetime. In particular, in a slice of $AdS_5$ the critical mass is $M^2_{c}=-4k^2 -1/R^2$ and the critical interval distance is given by $1/R_c^2=|μ^2|-4k^2$, where $k$ is the $AdS_5$ curvature (the 5D flat case can be obtained in the limit $k\to 0$, whereas the infinite $AdS_5$ result is recovered in the limit $R\to \infty$).

hep-th↗

arXiv2007

Existence and Stability of Non-Trivial Scalar Field Configurations in Orbifolded Extra Dimensions

We consider the existence and stability of static configurations of a scalar field in a five dimensional spacetime in which the extra spatial dimension is compactified on an $S^1/Z_2$ orbifold. For a wide class of potentials with multiple minima there exist a finite number of such configurations, with total number depending on the size of the orbifold interval. However, a Sturm-Liouville stability analysis demonstrates that all such configurations with nodes in the interval are unstable. Nodeless static solutions, of which there may be more than one for a given potential, are far more interesting, and we present and prove a powerful general criterion that allows a simple determination of which of these nodeless solutions are stable. We demonstrate our general results by specializing to a number of specific examples, one of which may be analyzed entirely analytically.

hep-ph↗

arXiv2007

Metastable Kinks in the Orbifold

We consider static configurations of bulk scalar fields in extra dimensional models in which the fifth dimension is an $S^1/Z_2$ orbifold. There may exist a finite number of such configurations, with total number depending on the size of the orbifold interval. We perform a detailed Sturm-Liouville stability analysis that demonstrates that all but the lowest-lying configurations - those with no nodes in the interval - are unstable. We also present a powerful general criterion with which to determine which of these nodeless solutions are stable. The detailed analysis underlying the results presented in this letter, and applications to specific models, are presented in a comprehensive companion paper.

hep-ph↗

arXiv2006

Connecting (Supersymmetry) LHC Measurements with High Scale Theories

If supersymmetry is discovered at the LHC, the measured spectrum of superpartner masses and couplings will allow us to probe the origins of supersymmetry breaking. However, to connect the collider-scale Lagrangian soft parameters to the more fundamental theory from which they arise, it is usually necessary to evolve them to higher scales. The apparent unification of gauge couplings restricts the possible forms of new physics above the electroweak scale, and suggests that such an extrapolation is possible. Even so, this task is complicated if the low scale spectrum is not measured completely or precisely, or if there is new physics at heavy scales beyond the reach of collider experiments. In this work we study some of these obstacles to running up, and we investigate how to get around them. Our main conclusion is that even though such obstacles can make it very difficult to accurately determine the values of all the soft parameters at the high scale, there exist a number of special combinations of the soft parameters that can avoid these difficulties. We also present a systematic application of our techniques in an explicit example.

hep-ph↗

arXiv2006

Lepton Flavor Violation and Supersymmetric Dirac Leptogenesis

Dirac leptogenesis (or Dirac neutrinogenesis), in which neutrinos are purely Dirac particles, is an interesting alternative to the standard leptogenesis scenario. In its supersymmetric version, the modified form of the superpotential required for successful baryogenesis contributes new, generically non-flavor-diagonal terms to the slepton and sneutrino mass matrices. In this work, we examine how current experimental bounds on flavor-changing effects in the lepton sector (and particularly the bound on Mu -> e Gamma) constrain Dirac leptogenesis and we find that it is capable of succeeding with superpartner masses as low as 100 GeV. For such light scalars and electroweakinos, upcoming experiments such as MEG are generically expected to observe signals of lepton flavor violation.

hep-ph↗

arXiv2005

Phenomenology of Dirac Neutrinogenesis in Split Supersymmetry

In Split Supersymmetry scenarios the possibility of having a very heavy gravitino opens the door to alleviate or completely solve the worrisome "gravitino problem'' in the context of supersymmetric baryogenesis models. Here we assume that the gravitino may indeed be heavy and that Majorana masses for neutrinos are forbidden as well as direct Higgs Yukawa couplings between left and right handed neutrinos. We investigate the viability of the mechansim known as Dirac leptogenesis (or neutrinogenesis), both in solving the baryogenesis puzzle and explaining the observed neutrino sector phenomenology. To successfully address these issues, the scenario requires the introduction of at least two new heavy fields. If a hierarchy among these new fields is introduced, and some reasonable stipulations are made on the couplings that appear in the superpotential, it becomes a generic feature to obtain the observed large lepton mixing angles. We show that in this case, it is possible simultaneously to obtain both the correct neutrino phenomenology and enough baryon number, making thermal Dirac neutrinogenesis viable. However, due to cosmological constraints, its ability to satisfy these constraints depends nontrivially on model parameters of the overall theory, particularly the gravitino mass. Split supersymmetry with m_{3/2} between 10^{5} and 10^{10} GeV emerges as a "natural habitat" for thermal Dirac neutrinogenesis.

hep-ph↗

arXiv2005

Is it SUSY?

If a signal for physics beyond the Standard Model is observed at the Tevatron collider or LHC, we will be eager to interpret it. Because only certain observables can be studied at a hadron collider, it will be difficult or impossible to measure masses and spins that could easily establish what physics was being seen. Nevertheless, different underlying physics implies different signatures. We examine the main signatures for supersymmetry, with some emphasis on recognizing supersymmetry in parts of parameter space where generic signatures are reduced or absent. We also consider how to distinguish supersymmetry from alternatives that most closely resemble it, such as Universal Extra Dimensions (UED). Using the robust connection between spins and production cross section, we think it will not be difficult to distinguish UED from supersymmetry. We expect that by considering patterns of signatures it is very likely that it will not be difficult to find a compelling interpretation of any signal of new physics.

hep-ph↗

arXiv2005

Gluino decays with heavier scalar superpartners

We compute gluino decay widths in supersymmetric theories with arbitrary flavor and CP violation angles. Our emphasis is on theories with scalar superpartner masses heavier than the gluino such that tree-level two-body decays are not allowed, which is relevant, for example, in split supersymmetry. We compute gluino decay branching fractions in several specific examples and show that it is plausible that the only accessible signal of supersymmetry at the LHC could be four top quarks plus missing energy. We show another example where the only accessible signal for supersymmetry is two gluon jets plus missing energy.

hep-ph↗

arXiv2004

Precision Electroweak Constraints on the mixed Radion-Higgs sector

Adding radion perturbations (up to second order) to the static (RS) metric allows us to calculate the general first and second order interactions of the radion field with the electroweak vector bosons. We use these interactions to compute precision electroweak observables in the case of Higgs-radion mixing and compare with experiment.

hep-ph↗

arXiv2004

Low-energy effective theory from a non-trivial scalar background in extra dimensions

Consequences of a non-trivial scalar field background for an effective 4D theory were studied in the context of one compact extra dimension. The periodic background that appears within the (1+4)-dimensional $ϕ^4$ theory was found and the excitations above the background (and their spectrum) were determined analytically. It was shown that the presence of the non-trivial solution leads to the existence of a minimal size of the extra dimension that is determined by the mass parameter of the scalar potential. It was proved that imposing orbifold antisymmetry boundary conditions allows us to eliminate a negative mass squared Kaluza-Klein ground-state mode that otherwise would cause an instability of the system. The localization of fermionic modes in the presence of the non-trivial background was discussed in great detail varying the size of the extra dimension and the strength of the Yukawa coupling. A simple exact solution for the zero-mode fermionic states was found and the solution for non-zero modes in terms of trigonometric series was constructed. The fermionic mass spectrum, which reveals a very interesting structure, was found numerically. It was shown that the natural size of the extra dimension is twice as large as the period of the scalar background solution.

hep-ph↗

arXiv2003

The radion and the perturbative metric in RS1

We calculate the linearized metric perturbations in the five dimensional two-brane model of Randall and Sundrum. In a carefully chosen gauge, we write down and decouple Einstein equations for the perturbations and get the final and simple perturbative metric ansatz. This ansatz turns out to be equal to the linear expansion of the metric solution of Charmousis et al. \cite{rubakov}. We show that this ansatz, the metric ansatz of Boos et al. \cite{boos} and the one of Das and Mitov \cite{das} are not incompatible, as it appears on the surface, but completely equivalent by an allowed gauge transformation that we give.

hep-th↗

arXiv2002

Higgs-Boson Interactions within the Randall-Sundrum Model

The scalar sector of the Randall-Sundrum model is discussed. The effective potential for the Standard Model Higgs-boson (h) interacting with Kaluza-Klein excitations of the graviton and the radion (phi) has been derived and it has been shown that only the Standard Model-like vacuum is allowed. The theoretical and experimental consequences of the curvature-scalar mixing xi RHH introduced on the visible brane are considered and simple sum rules that relate the couplings of the mass eigenstates (h) and (phi) to pairs of vector bosons and fermions are derived. The sum rule for the ZZh and ZZ phi couplings in combination with LEP/LEP2 data implies that not both the h and phi can be light. We present explicit results for the still allowed region in the (mh,mphi) plane that remains after imposing the LEP upper limits for non-standard scalar couplings to a ZZ pair. The phenomenological consequences of the mixing are investigated and, in particular, it is shown that the Higgs-boson decay h -> phi phi would provide an experimental signature for non-zero xi and can have a very substantial impact on the Higgs-boson searches, having BR(h->phi phi) as large as 30 - 40 %.

hep-ph↗