arXiv subjects

J. Lorenzo Diaz-Cruz

Publications and source records attributed to J. Lorenzo Diaz-Cruz.

At least 19 recordsLinked to original sources

arXiv2023

Solving the Naturalness Problem with Feeble Coupled Sectors

The discovery of a light Higgs boson means that whatever form new physics takes, it should keep stable the Higgs mass. Besides the well-known solutions to the naturalness problem (Supersymmetry, Conformal symmetry, Compositeness, etc), models that include heavy particles with feeble couplings to the Standard Model (SM) can be considered natural, since the corrections to the Higgs mass remains of the order of the electroweak (EW) scale. This solution can be used for model building too, with realizations that include the see-saw mechanism for neutrino masses and FIMP dark matter models, but it also holds for generic sectors that have Planck-suppressed couplings with the SM. One can also incorporate this solution within the SMEFT framework; the corresponding higher-dimensional operators induce small corrections to both the Higgs mass and its self-coupling, a prediction that can be tested at a future Higgs factory. We present a natural extension of the SMEFT that describes corrections to the SM, while also including a Feeble Coupled Sector aimed to account for the dark cosmos, with predictions for new signals that can be tested too.

hep-ph↗

arXiv2020

A Private SUSY 4HDM with FCNC in the Up-sector

We present a SUSY model with four Higgs doublets of the ``private type", where each fermion type (up, down, and charged leptons) obtain their masses from a different Higgs doublet H_f ( f = u_1, d, e). The conditions for anomaly cancellation, imply that the remaining Higgs doublet of the model (H_{u_2}), must have the same hypercharge as H_{u_1}, and thus can only couple to up-type quarks, which opens the possibility to have FCNC's only in this sector. We study the Lagrangian of the model, and in particular the Higgs potential, in order to identify the Higgs mass-eigenstates and their interactions; for the Yukawa matrices we consider the 4-texture case. We obtain constraints on the model parameters by using LHC measurements on the properties of the 125 GeV Higgs boson ($h$), and identify viable regions of parameter space. These constraints are then used to evaluate the prospects to detect the FCNC decay mode t -> ch at the future high-luminosity (HL) option for the LHC, which are compared with current limits from LHC-run2. Then, we also evaluate the FCNC decay of the next heavier Higgs boson H_2 -> tc, which can reach typically BR(H_2 -> tc) = O(10^{-4}-10^{-5} ); the search for the signal at HL-LHC is also studied, finding that it could be detectable for some specific regions of the model parameter space.

hep-ph↗

arXiv2020

Hidden Signals of New Physics within the Yukawa Couplings of the Higgs boson

The Large Hadron Collider (LHC) has measured the Higgs boson couplings with the heavier particles of the Standard Model (SM), and they seem to lay on a single line as function of the particle mass, as predicted in the SM. However a complete test of this property must involve the lighter generations, and the coming measurements at LHC, or future colliders, may reveal hidden patterns associated with physics beyond the SM. In renormalizable multi-Higgs models, the Higgs-fermion couplings could still be linear, but they could lay on multiple lines. Then, the angle subtended by these lines, with respect to the SM line, can be used to characterize different models, as it is shown here for the Two-Higgs doublet Model (2HDM). Models where fermion masses arise from higher-dimensional operators, may also result in large deviations from the SM values for the Higgs couplings, which would show an irregular pattern as a function of the fermion masses. In the case of neutrino masses, when they arise from the see-saw mechanism, one finds that the Higgs Yukawa couplings will show a non-linear mass relation.

hep-ph↗

arXiv2019

Very long-lived Stop NLSP in MSSM scenarios with Gravitino LSP

We present the calculation of the stop ($\tilde{t}$) lifetime that results from its decay into gravitinos ($\tildeΨ^μ$) in the final state, namely $\tilde{t}\to \tildeΨ^μ + X$, where $X=t, bW, b lν_l$ for the two-, three- and four-body decay modes. The full calculation is obtained using the Feynman rules for massive gravitino, which is compared with the results obtained employing the equivalence theorem, where the longitudinal component of the gravitino is replaced by the goldstino. The stop turns out to be very long-lived in these scenarios, with lifetimes of $\mathcal{O}$($10^8$s, $10^{14}$s, $10^{20}$s) for the corresponding 2-,3- and 4-body modes under consideration, and therefore all of them are safe from the big bang nucleosynthesis problem. However, the lifetime for the four body decay mode occurs during the epoch of galaxy formation. When the stop is produce at colliders, in these scenarios, it will hadronize and decay outside of the detector, even for the lowest values of stop lifetime.

hep-ph↗

arXiv2019

The Higgs Profile in the Standard Model and Beyond

We present a review of Higgs physics in the SM and beyond, including the tests of the Higgs boson properties that have been performed at LHC and have permitted to delineate its profile. After presenting the essential features of the BEH mechanism, and its implementation in the SM, we discuss how the Higgs mass limits developed over the years. These constraints in turn helped to classify the Higgs phenomenology (decays and production mechanisms), which provided the right direction to search for the Higgs particle, an enterprise that culminated with its discovery at LHC. So far, the constraints on the couplings of the Higgs particle, point towards a SM interpretation. However, the SM has open ends that suggest the need to look for extensions of the model. We discuss in general the connection of the Higgs sector with some new physics (e.g. supersymmetry, flavor and Dark matter), with special focus on a more flavored Higgs sector. Thus is realized in the most general 2HDM, and its textured version, which we study in general, and for its various limits, which contain distinctive flavor-violating signals that could be searched at current and future colliders.

hep-ph↗

arXiv2017

Linking LFV Higgs decays $h\to \ell_i \ell_j$ with CP violation in multi-scalar models

The study of LFV decays of the Higgs boson, $h\to \ell_i \ell_j$, has become an active research subject both from the experimental and theoretical points of view. Such decays vanish within the SM and are highly suppressed in several theoretical extensions. Due to its relevance and relative simplicity to reconstruct the signal at future colliders, it is an important tool to probe SM extensions where it could reach detectable levels. Here we identify a mechanism that induces LFV Higgs interactions, by linking it with the appearance of CP violation in the scalar sector, within the context of general multi-Higgs models. We then focus on the simplest model of this type to study its phenomenology. The scalar sector of this minimal model consisting of a Higgs doublet and a Froggatt--Nielsen (FN) (complex) singlet is studied thoroughly, including the scalar spectrum and the Yukawa interactions. Constraints on the parameters of the model are derived from low-energy observables and LHC Higgs data, which is then applied to study the resulting predicted rates for the decay $h\rightarrow τμ$. Overall, branching ratios for $h \rightarrow τμ$ of the order $10^{-3}$ are obtained within this approach consistent with all known constraints.

hep-ph↗

arXiv2017

Where have all the large Representations gone?

Gauge theories describe the interactions of the fundamental building blocks of nature with great success. The Standard Model achieves a partial unification of the electromagnetic and weak interactions, and it also acomodates the strong interactions. The known quarks and leptons appear in the fundamental representations (or singlets) of the SU(3)_cxSU(2)_L x U(1)_Y gauge symmetry. However, larger representations (EW triplets, color sextes, etc.) could also occur in principle. Bounds on such exotic states based on electroweak precision tests, unitarity, perturbativity and collider searches, indicate that they should be very heavy or may be non-existent. But why only small representations occur in nature? Several ideas that could give some light into this problem are discussed here, including the approach of Nielsen et al, as well as the possible compositeness of quarks and leptons. Then, we discuss the problem within the context of grand unified theories, where a principle of "minimal complexity" is proposed to restrict the size of large representations, when they are required to form unified multiplets.

hep-ph↗

arXiv2017

Helicity Amplitudes for massive gravitinos in N=1 Supergravity

We develop the formal tools needed to construct helicity amplitudes for massive gravitino in N=1 SUGRA. We start by considering the helicity states for massive spin-3/2 particles, which involves the solutions of Rarita-Schwinger equation. These solutions are written in terms of the helicity bra-ket notation and are used then to derive the interactions of gravitino with matter and gauge supermultiplets within N=1 Supergravity. The corresponding interactions of goldstinos are discussed too, relying on the goldstino-gravitino equivalence theorem. Finally to appreciate the power of our methods we evaluate the decay width chi_0 -> γG, as well as the cross sections for e^+ e- -> GG.

hep-ph↗

arXiv2017

Higgs couplings and new signals from Flavon-Higgs mixing effects within multi-scalar models

Testing the properties of the Higgs particle discovered at the LHC and searching for new physics signals, are some of the most important tasks of Particle Physics today. Current measurements of the Higgs couplings to fermions and gauge bosons, seem consistent with the Standard Model, and when taken as a function of the particle mass, should lay on a single line. However, in models with an extended Higgs sector the diagonal Higgs couplings to up-quarks, down-quarks and charged leptons, could lay on different lines, while non-diagonal flavor-violating Higgs couplings could appear too. We describe these possibilities within the context of multi-Higgs doublet models that employ the Froggatt-Nielsen (FN) mechanism to generate the Yukawa hierarchies. Furthermore, one of the doublets can be chosen to be of the inert type, which provides a viable dark matter candidate. The mixing of the Higgs doublets with the flavon field, can provide plenty of interesting signals, including: i) small corrections to the couplings of the SM-like Higgs, ii) exotic signals from the flavon fields, iii) new signatures from the heavy Higgs bosons. These aspects are studied within a specific model with 3+1 Higgs doublets and a singlet FN field. Constraints on the model are derived from the study of K and D mixing and the Higgs search at the LHC. For last, the implications from the latter aforementioned constraints to the FCNC top decay $t\to ch$ are presented too.

hep-ph↗

arXiv2016

Flavor Violating Higgs signals in the Texturized Two-Higgs Doublet Model (2HDM-Tx)

Flavor violating Higgs signals, such as the top FCNC decay $t\to ch^0$ and the LFV Higgs decay $h^0\to τμ$, have been studied at the LHC. These signals can arise within the general Two-Higgs doublet model (THDM), where each Higgs doublet couples to all fermions types through Yukawa matrices $Y^f_1$ and $Y^f_2$. The Yukawa matrices can be assumed to have the same form or they could have different structures. In this paper we study the case when both $Y^f_1$ and $Y^f_2$ have completely different forms, but in such a way that they complement to produce a specific hermitian mass matrix. We find that for specific four-zero textures, the Flavor Violating Higgs couplings depend only on the free parameters $\tan β$, $γ_f$ and the fermion masses. We use the current bounds on the low energy processes, to derive constraints on the Heavy Higgs boson mass, $\tan β$ and $γ_f$. Then, we use these constraints to evaluate the LFV Higgs decays, which reach Branching ratios that could be tested at the LHC.

hep-ph↗

arXiv2016

An Introduction to the Massive Helicity Formalism with applications to the Electroweak SM

The power of the helicity formalism has been appreciated recently from its application to the massless case, where plenty of formal aspects of multi-legs amplitudes have been derived. However, in order to extend the formalism to the realistic cases, such as the electroweak Standard Model and QCD with massive quarks, some extra inputs are needed. We discuss first the formalism needed to evaluate amplitudes expressed with the modern notation for massive fermions and vectors based on a thorough treatment of Proca vector fields. Then, we present some examples of elementary processes where it is shown how the formalism leads to tremendous simplifications, these include 2-body decays $Z\to ff$, $h\to ff$ and $h\to W^{-}W^{+}$ as well as the 3-body decay $h\to V f'\bar{f}$.

hep-ph↗

arXiv2016

IDMS: Inert Dark Matter Model with a complex singlet

Within the Inert Doublet Model (IDM) there is a viable dark matter candidate. This simple model can provide a strong enough first order phase transition, which is required in order to account for the matter-antimatter asymmetry in the Universe (BAU). However, another necessary ingredient is missing, as there is no additional source of CP violation in the IDM, besides the standard CKM phase from the Standard Model. Additional CP violating phase can appear if a complex singlet of $SU(3)_C \times SU(2)_W \times U(1)_Y$ with a non-zero vacuum expectation value is added to the scalar sector of the IDM. We construct the scalar potential of the inert doublet plus singlet model (IDMS), assuming an exact $Z_2$ symmetry, with singlet being $Z_2$-even. To simplify the model we use a softly broken $U(1)$ symmetry, which allows a reduction of the number of free parameters in the potential. We study the masses and interactions of scalar particles for a few benchmark scenarios. Constraints from collider physics, in particular from the Higgs signal observed at LHC with $M_h\approx 125$ GeV are discussed, as well as constraints from the dark matter experiments.

hep-ph↗

arXiv2015

Weyl spinors and the helicity formalism

In this work we give a review of the original formulation of the relativistic wave equation for particles with spin one-half. Traditionally à la Dirac, it's proposed that the ``square root'' of the Klein-Gordon (K-G) equation involves a 4 component (Dirac) spinor and in the non-relativistic limit it can be written as 2 equations for two 2 component spinors. On the other hand, there exists Weyl's formalism, in which one works from the beginning with 2 component Weyl spinors, which are the fundamental objects of the helicity formalism. In this work we rederive Weyl's equations directly, starting from K-G equation. We also obtain the electromagnetic interaction through minimal coupling and we get the interaction with the magnetic moment. As an example of the use of that formalism, we calculate Compton scattering using the helicity methods.

physics.gen-ph↗

arXiv2015

From the Higgs to the top: Couplings and Rare Decays

Within the Standard Model the Higgs couplings to fermions and gauge bosons, as function of the particle mass, are predicted to lay on a single line. However, new patterns appear in multi-Higgs models that employ the Froggart-Nielsen mechanism, where the diagonal couplings could lay on different lines and flavor-violating Higgs couplings could appear too. These aspects are studied for a specific model with 3+1 Higgs doublets and a singlet FN field. Constraints on the model are derived from the Higgs search at LHC, and some remarks are presented on their implications for the rare top and Higgs decay, t -> ch and h -> τμ.

hep-ph↗

arXiv2014

The 125 GeV Higgs signal at the LHC in the CP Violating MSSM

The ATLAS and CMS collaborations have observed independently at the Large Hadron Collider (LHC) a new Higgs-like particle with a mass $M_h \sim$ 125 GeV and properties similar to that predicted by the Standard Model (SM). Although the measurements indicate that this Higgs-like boson is compatible with the SM hypothesis, however due to large uncertainties in some of the Higgs detection channels, one still has the possibility of testing this object as being a candidate for some Beyond the SM (BSM) physics scenarios, for example, the Minimal Supersymmetric Standard Model (MSSM), in the CP-conserving version (CPC-MSSM). In this paper, we evaluate the modifications of these CPC-MSSM results when CP-violating (CPV) phases are turned on explicitly, leading to the CP-violating MSSM (CPV-MSSM). We investigate the role of the CPV phases in (some of) the soft Supersymmetry (SUSY) terms on both the mass of the lightest Higgs boson $h_1$, and the rates for the processes $gg \rightarrow h_1 \rightarrow γγ$, $gg \rightarrow h_1 \rightarrow ZZ^*\rightarrow 4l$, $gg \rightarrow h_1 \rightarrow WW^*\rightarrow l νl ν$, $pp \rightarrow V h_1 \rightarrow V b\bar b$ and $pp \rightarrow V h_1 \rightarrow V τ^+τ^-$, ($V \equiv W^\pm, Z$) at the LHC, considering the impact of the flavor constraints as well as the constraints coming from the Electric Dipole Moment (EDM) measurements. We find that the imaginary part of the top and bottom Yukawa couplings can take very small but non-zero values even after satisfying the recent updates from LHC. Our study shows that the CPV-MSSM provides an equally potential solution (like its CP-conserving (CPC) counterpart) to the recent LHC Higgs data. Improvement in different Higgs coupling measurements is necessary in order to test the possibility of probing the small dependence on these CPV phases in the Higgs sector of the MSSM.

hep-ph↗

arXiv2014

Lepton pair emission in the top quark decay $t \to bW^+\ell^-\ell^+$

The heaviness of the top quark makes its 2-body decay mode $t \to bW^+$ to be dominant, at such level that hardly any other decay mode reaches a detectable branching ratio (BR) within the SM. Here we study the decay $t \to bW^+\ell^-\ell^+$ ($\ell=e, μ, τ$), which diverges for massless leptons, and it can reach a BR $\sim \mathcal{O}(10^{-5}\sim 10^{-6})$ for reasonable values of the low energy cut in the lepton-pair invariant mass. This rate surpasses almost any other rare decays such as $t \to cX$ ($X=γ,Z,g,H, W^+W^-$), and thus offers the possibility of being detectable. Furthermore, the estimate of this channel is relevant because it can mimic the signal arising from the lepton number violating decay $t\to bW^-\ell^+\ell^+$, when the $W$ boson decays into lepton channels.

hep-ph↗

arXiv2013

Slim SUSY

The new SM-like Higgs boson discovered recently at the LHC, with mass $m_h \simeq$ 125 GeV, as well as the direct LHC bounds on the mass of superpartners, which are entering into the TeV range, suggest that the minimal surviving supersymmetric extension of the SM (MSSM), should be characterized by a heavy SUSY-breaking scale. Several variants of the MSSM have been proposed to account for this result, which vary according to the accepted degree of fine-tuning. We propose an alternative scenario here, Slim SUSY, which contains sfermions with multi-TeV masses and gauginos/higgsinos near the EW scale, but it includes the heavy MSSM Higgs bosons ($H^0$, $A^0$, $H^\pm$) near the EW scale too. We discuss first the formulation and constraints of the Slim SUSY scenario, and then identify distinctive heavy Higgs signals that could be searched at the LHC, within scenarios with the minimal number of superpartners with masses near the EW scale.

hep-ph↗

arXiv2013

An extra Z' gauge boson as a source of Higgs particles

Models with extra gauge bosons often require an extended Higgs sector, which contains a rich spectrum of Higgs bosons with properties that deviate from the Standard Model (SM). Such Higgs bosons could be searched using standard mechanisms similar to the SM. However, the existence of the new gauge bosons could provide new production mechanisms, which could probe the non-standard origin of the Higgs particle. In this paper we study how the Z' from a model with extra U(1)' could be used as a source of Higgs bosons. We study 3-bodies decays of the Z' into a Higgs boson and a top anti-top pair or WW pair, namely Z'\to t\bar{t} h, WW h. We find that it is possible to get {\Br'}s as high as 10^{-2} for these modes, which could be studied at future colliders. We also study the production of the Higgs bosons in association with the Z vector boson at a linear collider, through the reaction e^+ e^- \to Z, Z'\to Z+h, including both the resonant and the non-resonant effects.

hep-ph↗