arXiv subjects

Nicolás A. Neill

Publications and source records attributed to Nicolás A. Neill.

17 recordsLinked to original sources

arXiv2026

Transport phenomena and observables associated with viscous properties of an anisotropic hot QCD medium at finite baryon asymmetry

We have studied the shear viscosity, bulk viscosity and associated observables of baryon asymmetric hot QCD medium subjected to weak momentum anisotropy induced by the asymptotic expansion of the medium during the early stages of ultrarelativistic heavy-ion collisions. The shear and bulk viscosities are evaluated by solving the relativistic Boltzmann transport equation within the relaxation time approximation in a kinetic theory framework. We have further analysed the Prandtl number, the Reynolds number, specific shear viscosity, and specific bulk viscosity, characterizing the sound attenuation, flow properties, fluidity, and conformal behavior, respectively. The interactions among the constituent partons are incorporated through the anisotropic quasiparticle distribution functions, with the medium characterized by finite temperature, momentum anisotropy and baryon asymmetry. Our analysis shows that both shear and bulk viscosities decrease with increasing anisotropy in both baryonless and baryon asymmetric media. In contrast, finite baryon asymmetry enhances both viscosities compared with their baryonless counterparts. The effects of expansion-induced anisotropy remain pronounced in both the baryonless and baryon asymmetric media, leading to substantial modifications of the associated viscous observables. These features are attributed primarily to the squeezing of the parton distribution functions caused by expansion-induced momentum anisotropy, together with the resulting modification of the partonic dispersion relations. Our results demonstrate that the expansion-induced anisotropy significantly influences the sound attenuation, flow characteristics, fluidity and conformal behavior of baryon asymmetric hot QCD medium, providing new insights into the viscous properties of the medium created in ultrarelativistic heavy-ion collisions.

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arXiv2026

Studying the thermoelectric properties of an anisotropic QGP medium

We have studied how the thermoelectric properties of the quark-gluon plasma (QGP) are affected by a weak-momentum anisotropy arising from the asymptotic expansion of matter in the initial stages of ultrarelativistic heavy-ion collisions. The highly energetic medium produced in such collisions exhibits a notable temperature difference between its central and peripheral regions. This temperature gradient induces an electric field whose magnitude per unit temperature gradient, in the limit of vanishing electric current, defines the Seebeck coefficient of the medium. We have calculated the Seebeck coefficient for both individual quark flavors and the entire QGP medium in the presence of expansion-induced anisotropy by solving the relativistic Boltzmann transport equation in the relaxation time approximation within the kinetic theory framework. The partonic interactions are incorporated through their effective thermal masses within the quasiparticle model for an anisotropic QGP medium. We have observed that the magnitude of the Seebeck coefficient for each quark flavor as well as for the entire QGP medium increases in the presence of expansion-induced anisotropy, indicating a stronger induced electric field in the anisotropic medium compared to the isotropic case. Given that an increase in the Seebeck coefficient may lead to observable signatures such as charge asymmetries in particle distributions and to modifications in the transport behavior of the QGP, these results may provide useful input for future phenomenological studies investigating the internal structure and phase properties of the QGP in heavy-ion collisions.

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arXiv2025

Leading large $N_c$ contributions to Lepton Number Violating Meson Decays

Lepton number violating meson decays, such as $M_1^- \to M_2^+\ell_1^-\ell_2^-$, provide constraints on $d=9$ $ΔL = 2$ operators. RGE-improved bounds on the Wilson coefficients of these operators have been presented in the literature, taking into account perturbative QCD one-loop corrections and the corresponding operator mixing. Here, we present for the first time the contribution of connected diagrams to the hadronic matrix elements $\langle M_2 | {\cal O}_h | M_1 \rangle$. These diagrams, usually overlooked under the assumption that $\langle M_2 | {\cal O}_h | M_1 \rangle \sim \langle M_2 | J_{q_3 q_4} | 0 \rangle \times \langle 0 |J_{q_1 q_2} | M_1 \rangle \gg \langle M_2 | J_{q_3 q_2} \times J_{q_1 q_4} | M_1 \rangle$, can give indeed a significant contribution to the matrix element. Including these connected diagrams is but the first step towards a full non-perturbative computation of the long-range QCD effects in these operators, that should be performed using lattice field theory techniques. However, connected diagrams represent the leading order in the $1/N_c$ expansion of the QCD non-perturbative effects and thus our work can be understood as a realistic, first approximation to a complete calculation of the long-range part of the matrix elements.

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arXiv2025

Deciphering the viscous properties and the Bjorken expansion of the QGP medium at finite angular velocity

We have studied the viscous properties as well as the Bjorken expansion of a rotating QGP medium. In the noncentral events of heavy-ion collisions, the produced medium can carry a finite angular momentum with a finite range of angular velocity. This rotation can significantly affect various properties, including viscous properties and the expansion of the QGP medium. Using a novel relaxation time approximation for the collision integral in the relativistic Boltzmann transport equation at finite angular velocity, we have calculated the shear and bulk viscosities and compared them with their counterparts in the standard relaxation time approximation within the kinetic theory approach. Our results show that the angular velocity increases both shear and bulk viscosities, suggesting an enhanced momentum transfer within the medium and greater fluctuations in local pressure. This rotational effect on viscosities is more evident at lower temperatures than at higher temperatures. Our analysis also shows that, compared to the standard relaxation time approximation, the shear viscosity is lower while the bulk viscosity is higher in the novel relaxation time approximation for all temperatures. Additionally, some observables related to the flow characteristic, fluid behavior and conformal symmetry of the medium are markedly impacted due to rotation. We have also studied the hydrodynamic evolution of matter within the Bjorken boost-invariant scenario and have found that the energy density evolves faster in the presence of finite rotation than in the nonrotating case. Consequently, rapid rotation accelerates the cooling process of the QGP medium.

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arXiv2025

Searching for long-lived light neutralinos from $B$-meson decays with baryonic R-parity violation at Belle II

In a supersymmetry scenario with R-parity violation (RPV), neutralinos with GeV-scale mass, which are necessarily bino-like, are allowed by all constraints and can be produced in association with a baryon in $B$-meson decays via certain $\bar U \bar D \bar D$ operators. In this work, we investigate this scenario with two non-vanishing RPV couplings at the low-energy scale. With one RPV coupling governing the neutralino production rate and another determining its lifetime, this scenario can lead to observable signals with displaced-vertex signatures in the tracking volume of $B$-factories. To maximize the sensitivity to such signals, we develop a new partial-reconstruction technique that yields high efficiency and utilizes most of the decays of relatively heavy, long-lived particles, achieving much better sensitivity than standard full reconstruction. We consider potential background sources and devise selection criteria to suppress their event yields to very low levels. Using a parameterized model of the detector, we estimate in detail the displaced-vertex reconstruction efficiency as a function of neutralino lifetime and mass. For squark masses beyond the LHC limits, we calculate the signal sensitivity of Belle~II, showing that the experiment can probe the RPV couplings well beyond the present bounds, obtained from searches for dinucleon decays, baryon-antibaryon oscillations, and $B^+\to p +\text{missing}$.

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arXiv2025

Probing light neutralinos from pair-produced sleptons with displaced vertices at the high-luminosity LHC

We study light neutralinos ($\tilde χ_1^0$) with masses ranging from 10 GeV to several hundred GeV within the framework of R-parity-violating (RPV) supersymmetry. These light neutralinos can be long-lived, decaying with a macroscopic displacement (order cm) inside the LHC main detectors. Complementing previous works on the subject, here we focus on their production through the electroweak pair production of left-chiral sleptons ($\tilde e_{L}$), with the signal process $pp\to \tilde e^+_{L} \tilde e^-_{L} \to e^+ \tilde χ_1^0 e^- \tilde χ_1^0$. In contrast to the previous study with a singly produced slepton, where the RPV coupling $λ'_{111}$ induces both the production and decay of the light neutralino, in our scenario the production proceeds through Drell-Yan-like processes that are essentially independent of RPV couplings. Correspondingly, we implement a displaced-vertex search strategy for which our numerical analysis shows that the high-luminosity LHC can probe $λ'_{111}$ values up to three orders of magnitude smaller, and neutralino masses up to about four times larger than those accesible in the previously studied single-slepton production scenario.

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arXiv2025

QCD Running in Lepton Number Violating Meson and Tau Decays

Below the electroweak scale, new physics that violates lepton number in two units ($ΔL = 2$) and is mediated by heavy particle exchange can be parameterized by a dimension-9 low-energy effective Lagrangian. Operators in this Lagrangian involving first-generation quarks and leptons contribute to the short-range mechanism of neutrinoless double beta decay ($0νββ$) and therefore they are strongly constrained. On the other hand, operators with other quark and lepton families are bounded by the non-observation of different lepton number violating (LNV) meson and tau decays, such as $M_1^- \to M_2^+\ell_1^-\ell_2^-$ and $τ^- \to \ell^+ M_1^- M_2^-$. In this work, we calculate RGE-improved bounds on the Wilson coefficients involved in these decays. We calculate QCD corrections to the dimension-9 operator basis and find RG evolution matrices that describe the evolution of the Wilson coefficients across different energy scales. Unlike the running of operators involved in $0νββ$-decay, the general flavor structure leads to the mixing of not only different Lorentz structures but also of different quark-flavor configurations. Additionally, operators that vanish for the identical lepton case need to be added to the operator basis. We find new constraints on previously unbounded operators and the enhancement of bounds for specific Wilson coefficients. We also find new bounds coming from the mixing between operators with different quark-flavor configurations.

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arXiv2024

Effective Field Theory and Scalar Triplet Dark Matter

We discuss an extension of the standard model with a real scalar triplet, $T$, including non-renormalizable operators (NROs) up to $d=6$. If $T$ is odd under a $Z_2$ symmetry, the neutral component of $T$ is a good candidate for the dark matter (DM) of the universe. We calculate the relic density and constraints from direct and indirect detection on such a setup, concentrating on the differences with respect to the simple model for a DM $T$ with only renormalizable interactions. Bosonic operators can change the relic density of the triplet drastically, opening up new parameter space for the model. Indirect detection constraints, on the other hand, rule out an interesting part of the allowed parameter space already today and future CTA data will, very likely, provide a decisive test for this setup.

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arXiv2024

Searching for light neutralinos with a displaced vertex at the LHC

We study a bino-like light neutralino ($\tilde χ_1^0$) produced at the LHC from the decay of a scalar lepton ($\tilde e_L$) through the process $pp\to \tilde e_{L} \to e\tilde χ_1^0$ in the context of R-parity-violating (RPV) supersymmetry where $\tilde χ_1^0$ is the lightest supersymmetric particle. For small masses and RPV couplings, the neutralino is naturally long-lived and its decay products can be identified as displaced tracks. Following existing searches, we propose a displaced-vertex search strategy for such a light neutralino with a single RPV coupling switched on, $λ'_{111}$, in the mass range $10\,\mbox{GeV} \lesssim m_{\tilde χ_1^0}\lesssim 230\,\mbox{GeV}$. We perform Monte Carlo simulations and conclude that at the high-luminosity LHC, the proposed search can probe values of $λ'_{111}$ down to two orders of magnitude smaller than current bounds and up to 40 times stronger than projected limits from monolepton searches.

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arXiv2023

Probing $R$-parity violation in $B$-meson decays to a baryon and a light neutralino

We propose a search for $B$ meson decays to a baryon plus missing energy at the Belle II experiment to probe supersymmetry with a GeV-scale lightest neutralino ${\tildeχ_1^0}$ and $R$-parity violation (RPV). We perform analytic computations of the signal branching fractions in the framework of effective field theory, with a single nonzero RPV operator $λ''_{ij3}\bar{U}_{i}^c\bar{D}_{j}^c\bar{D}_3^c$, where $i,j=1,2$. The hadronic form factors are calculated using an SU(3) phenomenological Lagrangian approach for the proton, as well as several hyperons and charmed baryons. Since the decay of the neutralino is kinematically and CKM suppressed in this theoretical scenario, it decays outside the detector and appears experimentally only as missing energy. We detail the analysis techniques at the experimental level and estimate the background in the ${B^+ \to p {\tildeχ_1^0}}$ search using published results for $B^+\to K^+ ν\barν$. Our final sensitivity plots are shown for both $λ''_{113}$ versus the squark mass $m_{\tilde{q}}$ and $λ''_{113}/m^2_{\tilde{q}}$ versus the neutralino mass $m_{\tildeχ_1^0}$. We find that the search at Belle II could probe $λ''_{113}/m^2_{\tilde{q}}$ down to the order of $10^{-8}$ GeV$^{-2}$ in the kinematically allowed $m_{\tildeχ_1^0}$ range. We also obtain current limits on $λ''_{123}$ by recasting an existing search interpreted as ${B^0 \to Λ^0 {\tildeχ_1^0}}$, and comment about searches for ${B^+ \to Σ^+ {\tildeχ_1^0}}$, ${B^0 \to Σ^0 {\tildeχ_1^0}}$, ${B^+ \to Λ_c^+ {\tildeχ_1^0}}$, and ${B^+ \to Ξ_c^+ {\tildeχ_1^0}}$. In closing, we briefly discuss potential searches at the LHCb and BESIII experiments.

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arXiv2022

RGE effects on the LFV scale from meson decays

We consider the lepton-flavor violating (LFV) lepton-quark dimension-6 operators and analyze their contributions to the LFV leptonic decays of vector, pseudoscalar, and scalar neutral mesons $M\to \ell_1 \ell_2$ as well as to $μ(τ) \rightarrow \ell ee, \ell γγ$ decays. These operators contribute to the purely leptonic processes via quark loop. On the basis of quark-hadron duality, we relate these loops to the appropriate meson-exchange contributions. In this way, we extract lower bounds on the individual scales of the studied LFV operators from the experimental and phenomenological limits on the leptonic decays of mesons and leptons. As a byproduct, we shall obtain new limits on the LFV leptonic decays of flavored mesons from the experimental bounds on the three-body lepton decays. We study the effects of QED and QCD radiative corrections to the LFV lepton-quark operators in question. We derive for them the one-loop matrix of the RGE evolution and examine its effect on the previously derived tree-level limits on these operators. We show that the QED corrections are particularly relevant due to operator mixing. Specifically, for some of them the limits on their individual LFV scales improve by up to 3 orders of magnitude.

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arXiv2021

Long-lived light neutralinos at Belle II

We consider light neutralinos of mass about 1 GeV, produced from $τ$ lepton rare decays at Belle II, in the context of R-parity-violating (RPV) supersymmetry. With large and clean samples of $τ$ leptons produced at the Belle II experiment, excellent sensitivity to such light neutralinos with the exotic signatures of displaced vertices is expected. We focus on two benchmark scenarios of single RPV operators, $λ'_{311} L_3 Q_1 \bar{D}_1$ and $λ'_{312} L_3 Q_1 \bar{D}_2$, which induce both the production and decay of the lightest neutralino. For the reconstruction of a displaced vertex, we require at least two charged pions in the final states. We perform Monte-Carlo simulations for both signal and background events, and find that Belle II can explore regions in the parameter space competitive with other probes. In particular, for the $λ'_{311}$ scenario, it can put limits up to two orders of magnitude stronger than the current bounds.

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arXiv2020

Probing Exotic Triple Higgs Couplings at the LHC

In extended Higgs sectors that exhibit alignment without decoupling, the additional scalars are allowed to have large couplings to the Standard Model Higgs. We show that current nonresonant di-Higgs searches can be straightforwardly adapted to look for additional Higgses in these scenarios, where pair production of non-SM Higgses can be enhanced. For concreteness, we study pair production of exotic Higgses in the context of an almost inert two Higgs doublet model, where alignment is explained through an approximate $\mathbb{Z}_2$ symmetry under which the additional scalars are odd. In this context, the smallness of the $\mathbb Z_2$ violating parameter suppresses single production of exotic Higgses, but it does not prevent a sizeable trilinear coupling $hHH$ between the SM Higgs ($h$) and the additional states ($H$). We study the process $pp\rightarrow h^* \rightarrow HH$ in the final states $b\bar b b \bar b$, $b\bar bγγ$, and multi-leptons. We find that at the HL-LHC these modes could be sensitive to masses of the additional neutral scalars in the range $130\mbox{ GeV} \lesssim m_H \lesssim 290\mbox{ GeV}$.

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arXiv2020

Low scale type I seesaw model for lepton masses and mixings

In contrast to the original type I seesaw mechanism that requires right-handed Majorana neutrinos at energies much higher than the electroweak scale, the so-called low scale seesaw models allow lighter masses for the additional neutrinos. Here we propose an alternative low scale type I seesaw model, where neither linear nor inverse seesaw mechanisms take place, but the spontaneous breaking of a discrete symmetry at an energy scale much lower than the model cutoff is responsible for the smallness of the light active neutrino masses. In this scenario, the model is defined with minimal particle content, where the right-handed Majorana neutrinos can have masses at the $\sim 50\mbox{ GeV}$ scale. The model is predictive in the neutrino sector having only four effective parameters that allow to successfully reproduce the experimental values of the six low energy neutrino observables.

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arXiv2019

Almost Inert Higgs Bosons at the LHC

Non-minimal Higgs sectors are strongly constrained by the agreement of the measured couplings of the 125 GeV Higgs with Standard Model predictions. This agreement can be explained by an approximate $\mathbb{Z}_2$ symmetry under which the additional Higgs bosons are odd. This allows the additional Higgs bosons to be approximately inert, meaning that they have suppressed VEVs and suppressed mixing with the Standard Model Higgs. In this case, single production of the new Higgs bosons is suppressed, but electroweak pair production is unsuppressed. We study the phenomenology of a minimal 2 Higgs doublet model that realizes this scenario. In a wide range of parameters, the phenomenology of the model is essentially fixed by the masses of the exotic Higgs bosons, and can therefore be explored systematically. We study a number of different plausible signals in this model, and show that several LHC searches can constrain or discover additional Higgs bosons in this parameter space. We find that the reach is significantly extended at the high luminosity LHC.

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arXiv2017

A novel Randall-Sundrum model with $S_{3}$ flavor symmetry

We propose a simple and predictive model of fermion masses and mixing in a warped extra dimension, with the smallest discrete non-Abelian group $S_{3}$ and the discrete symmetries $Z_{2}\otimes Z_{4}$. Standard Model fields propagate in the bulk and the mass hierarchies and mixing angles are accounted for the fermion zero modes localization profiles, similarly to the the Randall-Sundrum (RS) model. To the best of our knowledge, this model is the first implementation of an $S_{3}$ flavor symmetry in this type of warped extra dimension framework. Our model successfully describes the fermion masses and mixing pattern and is consistent with the current low energy fermion flavor data. The discrete flavor symmetry in our model leads to predictive mixing inspired textures, where the Cabbibo mixing arises from the down type quark sector whereas up type quark sector contributes to the remaining mixing angles.

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arXiv2016

Searching for Additional Higgs Bosons via Higgs Cascades

The discovery of a 125 GeV Higgs boson at the Large Hadron Collider strongly motivates direct searches for additional Higgs bosons. In a type I two Higgs doublet model there is a large region of parameter space at $\tanβ> 5$ that is currently unconstrained experimentally. We show that the process $gg \to H \to A Z \to ZZh$ can probe this region, and can be the discovery mode for an extended Higgs sector at the LHC. We analyze 9 promising decay modes for the $ZZh$ state, and we find that the most sensitive final states are $\ell\ell\ell\ell bb$, $\ell\ell jjbb$, $\ell\ellννγγ$ and $\ell\ell\ell\ell +{}$missing energy.

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