arXiv subjects

M. C. Gordillo

Publications and source records attributed to M. C. Gordillo.

At least 19 recordsLinked to original sources

arXiv2026

Diffusion Monte Carlo study of deuteron-like fully light hexaquarks

We perform a Diffusion Monte Carlo study of fully light hexaquarks containing three $u$ quarks and three $d$ quarks within a constituent-quark model. Both compact and baryon--baryon-like arrangements were considered separately. All compact hexaquark configurations are found well above their corresponding baryon--baryon thresholds, suggesting that deeply bound compact six-quark states are not favored in the light-quark sector within this model. By contrast, several dibaryon-like configurations lie close, but above, to the $NN$, $NΔ$, and $ΔΔ$ thresholds and show spatial structures compatible with molecular states. One configuration exhibits two well-defined nucleon-like subclusters separated by several femtometers, characteristic of deuteron-like spatial clustering, although its calculated energy remains slightly above the corresponding threshold.

hep-ph↗

arXiv2026

Sexaquarks and $H$ dibaryons in the $uuddss$ system: a comparison within a constituent quark model

We study the $uuddss$ multiquark within a constituent quark model framework, solving the corresponding nonrelativistic Schrodinger equation by means of a diffusion Monte Carlo (DMC) method. The total wavefunction is written as the product of a radial component and an exact spin-color-flavor state, restricted to isospin $I$=0. For this isospin, all allowed flavor wave functions are included. We explore two distinct constructions of the six-quark system. In the first one, corresponding to a sexaquark, all six quarks are treated as indistinguishable and the wave function is fully antisymmetric with respect to the exchange of any two quarks. In the second one, corresponding to the $H$ dibaryon, the system is partitioned into two sets of three quarks, effectively mimicking a baryon-baryon-like configuration including hidden color terms in which antisymmetry is imposed only within each three-quark cluster. Only when the system is forced into a baryon-baryon-like configuration, and for certain values of the spin, color and flavor quantum numbers, do we obtain states with masses close to, but above, the two-baryon threshold. Those states are characterized by two loosely bound three-quark clusters separated from one another by a distance of $\sim$ 2.5 fm. The remaining structures are compact objects irrespectively of their internal wavefunction.

hep-ph↗

arXiv2026

Hidden-charm $uds\,c\bar c$ pentaquarks as flavor eigenstates in a constituent quark model

We use a diffusion Monte Carlo (DMC) algorithm to solve the Schrödinger equation that describes $udsc\bar c$ pentaquarks within the framework of a non-relativistic constituent quark model. We considered only multiquark states with defined values of parity, color, spin and isospin, selected to be compatible with the experimentally favored assignment $J^P=1/2^-$ for one of the candidates, and assumed $I=0$. However, we found that, to explain the existence of the $P_{cs}(4338)$ and $P_{cs}(4459)$ pentaquarks, we need the total wavefunction to be also an eigenvector of the SU(3) {\em flavor} operator. When we impose that condition, we obtain two structures compatible with the masses extracted from the $J/ψΛ$ spectrum. In addition, two states are predicted below the $J/ψΛ$ threshold but above the $η_cΛ$ one that would not appear in that channel. If we only impose the $I=0$ condition, we obtain a {\em single} (not two) structure compatible with the experimental quantum numbers, with a mass below the $J/ψΛ$ threshold.

hep-ph↗

arXiv2025

Diffusion Monte Carlo calculation of compact $T_{cs0}$ and $T_{c\bar{s}0}$ tetraquarks

The LHCb collaboration amplitude analysis of the decays $B^+ \to D^+ D^- K^+$, $B^+ \to D^- D_s^+ π^+$, and $B^0 \to \bar{D}^0 D_s^+ π^-$ suggested the existence of two new resonant $J^P=0^+$ states with minimum quark content $\bar{u} \bar{d} s c$ and $\bar{s} \bar{u} d c$/$\bar{s} \bar{d} u c$, named $T_{cs0}$ and $T_{c\bar{s}0}$, respectively. In this work, we used the diffusion Monte Carlo (DMC) method to compute the masses of those tetraquarks within the framework of the constituent quark model. We describe the systems as compact structures, not as diquark/antidiquark or meson/meson arrangements. In this context, compact means that we used directly the eigenvectors of the spin, color and flavor operators without any splitting and/or (re)combination of smaller units. This allows us to deduce the existence of two distinct bound configurations for each composition, corresponding to ground and excited flavor states. In both cases, the excited flavor states are the ones with masses comparable to the ones observed by LHCb. In addition, our analysis allows us to assign unambiguously an isospin value of $I$=1 to the experimentally obtained $T_{cs0}$ tetraquark.

hep-ph↗

arXiv2024

Unbalanced droplets of one-dimensional mixtures of fermions

By means of a diffusion Monte Carlo technique, we study one-dimensional unbalanced mixtures of fermionic Ytterbium atoms ($^{173}$Yb, $^{171}$Yb). This means clusters in which the total number of $^{173}$Yb particles is different from the sum of all the atoms belonging to the $^{171}$Yb isotope. Our aim will be to check the possibility of having self-bound arrangements beyond the balanced compositions reported in previous literature rather than exploring all the situations in which that could be possible. In that vein, we focused mainly on mixtures in which the atoms belonging to one isotope are spin-polarized, while the spins of the particles in the other isotope are evenly distributed in two sets. What we found was that, even tough self-bound droplets are possible for different compositions, the most stable ones are clusters with a slight excess of attractively interacting $^{171}$Yb particles with different spins with respect to the number of spin-polarized $^{173}$Yb atoms. Clusters in which the number of repulsively interacting unequal-spin $^{173}$Yb atoms are in excess with respect to the spin-polarized $^{171}$Yb particles have a very narrow stability range.

cond-mat.quant-gas↗

arXiv2024

A diffusion Monte Carlo calculation of fully heavy pentaquarks

Using a diffusion Monte Carlo algorithm, we calculated the spectra of all possible $S$-wave fully heavy pentaquarks within the framework of the quark model. Our aim was to compare the masses of different spin-color configurations corresponding to the same spatial wavefunction for each quark composition. In particular, we computed the masses of the configuration with the maximum number of undistinguishable quarks and those of all the possible baryon+meson splittings compatible with the total number of $b$'s and $c$'s for a given pentaquark. What we found was that, in all cases, compact baryon+meson configurations had lower masses than their more symmetric counterparts. Moreover, those "molecular" associations were unstable (with larger masses) that their infinitely separated non-interacting pieces except int the case of $(ccb)(c\bar{b})$ and $(bbc)(b\bar{c})$ pentaquarks. In any case, even for unstable arrangements, the excess masses are so small ($\sim$20-60 MeV) as to make those compact molecular structures serious candidates to be experimentally detected.

hep-ph↗

arXiv2024

3He adsorbed on molecular hydrogen surfaces

Using a diffusion Monte Carlo (DMC) technique, we calculated the phase diagram of 3He adsorbed on a first solid layer of a molecular hydrogen isotope (H2,HD and D2) on top of graphite. The results are qualitatively similar in all cases: a two-dimensional gas spanning from the infinite dilution limit to a second-layer helium density of 0.048 +/- 0.004 Ang^{-2}. That gas is in equilibrium with a 7/12 commensurate structure, more stable than any incommensurate triangular solid of similar density. These findings are in reasonably good agreement with available experimental data.

cond-mat.other↗

arXiv2024

Heavy multiquark systems as clusters of smaller units -- a diffusion Monte Carlo calculation --

Multiquark systems appear less frequently than mesons and baryons despite the enormous world-wide experimental effort that has been made during the last two decades. In this work, we will propose a possible explanation for that fact, restricting ourselves to the case of sets including only $c$ and $\bar{c}$ quarks. We will show that those multiquarks can be thought as different combinations of smaller units that associate together to produce colorless assemblies with a definite value of the total spin. For instance, for the $cccccc$ hexaquark with $S=0$, we have three possibilities: a set of six undistinguishable $c$ quarks, an association of two $ccc$ baryons, or a set of three $cc$ diquarks close together. This means we can have three different values for the mass of an open-charm hexaquark with $S=0$. Using the diffusion Monte Carlo method, we calculate all possible combinations compatible with tetraquark $cc \bar{c} \bar{c}$, pentaquark $cccc \bar{c}$, open-charm $cccccc$ and hidden-charm $ccc \bar{c} \bar{c} \bar{c}$ hexaquark structures with the minimum value of total spin ($S=0$ or $S=1/2$). We consider compact structures with radial wave functions including interactions between all the quarks in the cluster. We find that, in all cases, the mass of the multiquark decreases with the number of small units that conform the set of quarks. For instance, an open charm hexaquark made up of three diquarks has a smaller mass than a set of six of $c$ undistinguishable units. When the pieces that conform the multiquark are themselves colorless with a definite value of the total spin, the cluster splits into those smaller units that separate infinitely from each other.

hep-ph↗

arXiv2023

Antiferromagnetic behavior in self-bound one-dimensional composite bosons

The structure of self-bound one-dimensional droplets containing a mixture of Ytterbium fermionic isotopes ($^{173}$Yb, $^{171}$Yb) is calculated by means of a diffusion Monte Carlo technique. We considered only balanced setups in which all the atoms of one isotope are spin-polarized, while the atoms of the other can have up to three different spin values, that difference being a necessary requirement to achieve stable systems. Our results indicate that these droplets consist of consecutive "molecules" made up of one $^{173}$Yb and one $^{171}$Yb atom. In other words, we have up to three different kinds of composite bosons, corresponding to the number of spin components in the non-polarized isotope. The fermionic nature of those Yb atoms makes pairs with identical spin composition avoid each other, creating a Pauli-like-hole filed by another molecule in which at least one of the Yb atoms has a different spin from that of their closest neighbors. This effective repulsion is akin to an antiferromagnetic short-range interaction between different kinds of composite bosons.

cond-mat.quant-gas↗

arXiv2023

Phases of 4He and H2 adsorbed on doped graphene

The influence of attractive boron impurities, embedded on a graphene sheet, on the phase diagrams of $^4$He and H$_2$ adsorbed on top was studied using the diffusion Monte Carlo method. The doping of graphene was made by distributing the boron atoms following the same pattern found in an experimentally synthesized substrate. Our results show that while the different incommensurate solid phases of both adsorbates remain largely unchanged after doping, the liquid/gas equations of state are significanty different from the ones on pristine graphene. Doping graphene produces new translationally invariant stable phases for $^4$He, depending on the concentration of boron impurities, but makes the H$_2$ ground state to remain solid. In addition, several new registered phases appear for both adsorbates.

cond-mat.other↗

arXiv2023

Asymptotic mass limit of large fully-heavy compact multiquarks

The properties of fully-heavy arrangements including a number of quarks between 5 and 12 were calculated within the framework of a constituent quark model by using a diffusion Monte Carlo technique. We considered only clusters in which all the quarks had the same mass, and whose number of particles and antiparticles were adequate to produce color singlets. All the multiquarks were in their lowest possible values of $L^2$ and $S^2$ operators. This means that we considered only color-spin wavefunctions that were antisymmetric with respect to the interchange of {\em any} two quarks of the same type. We found that in both all-$c$ and all-$b$ multiquarks, the mass per particle levels off for arrangements with a number of quarks larger of equal than six. The analysis of their structure implies that the fully-heavy multiquarks are compact structures.

hep-ph↗

arXiv2023

Self-bound clusters of one-dimensional fermionic mixtures

Diffusion Monte Carlo calculations on the possibility of having self-bound one-dimen\-sional droplets of SU(6) $\times$ SU(2) ultracold fermionic mixtures are presented. We found that, even though arrangements with attractive interactions with only two spin types are not self-bound, mixtures with at least three kinds of fermions form stable small drops. However, that stabilization decreases for very tight confinements, where a universal behavior is found for Fermi-Fermi and Fermi-Boson clusters including attractive and repulsive interactions.

cond-mat.quant-gas↗

arXiv2023

Phases of $^4$He and H$_2$ adsorbed on a single carbon nanotube

Using a diffusion Monte Carlo (DMC) technique, we calculated the phase diagrams of $^4$He and H$_2$ adsorbed on a single (5,5) carbon nanotube, one of the narrowest that can be obtained experimentally. For a single monolayer, when the adsorbate density increases, both species undergo a series of first order solid-solid phase transitions between incommensurate arrangements. Remarkably, the $^4$He lowest-density solid phase shows supersolid behavior in contrast with the normal solid that we found for H$_2$. The nature of the second-layer is also different for both adsorbates. Contrarily to what happens on graphite, the second-layer of $^4$He on that tube is a liquid, at least up to the density corresponding to a third-layer promotion on a flat substrate. However, the second-layer of H$_2$ is a solid that, at its lowest stable density, has a small but observable superfluid fraction.

cond-mat.other↗

arXiv2023

H2 superglass on an amorphous carbon substrate

The phase diagram of a para-H2 monolayer absorbed on a experimentally syntetized amorphous carbon sheet was calculated using a diffusion Monte Carlo technique. We found that the ground state of that system changed drastically from a perfectly flat substrate to a situation in which the carbon atoms were allowed a certain degree of disorder in the $z$ direction. In the first case, at zero pressure we have a glass of density 0.056 $\pm$ 0.003 Å$^{-2}$ in equilibrium with an incommensurate solid of 0.068 $\pm$ 0.002 Å$^{-2}$. At the equilibrium density, the glass was found to have a tiny, but non-negligible superfluid faction of less than 1 \% (0.44 $\pm$ 0.05 \%). In the $z$-disordered substrate, we observe a significant enhancement of the superfluid fraction in the glass phase as well as a smaller but not zero value in the incommensurate crystal.

cond-mat.other↗

arXiv2022

Diffusion Monte Carlo calculations of fully-heavy compact hexaquarks

We used a diffusion Monte Carlo technique to describe the properties of fully-heavy compact arrangements (no dibaryon molecules) including six quarks and no antiquarks within the framewok of a constituent quark model. Only arrangements whose wavefunctions were eigenvectors of $L^2$ with eigenvalue $\ell$ = 0 were taken into account, what means that we only considered the subset of all the possible color-spin combinations that make the total wavefunctions antisymmetric with respect to the interchange of any two quarks of the same type. This means arrangements with spin $S=0$ for $cccccc$, $bbbbbb$, $cccccb$, $bbbbbc$ and $cccbbb$ and spin $S=0,1,2$ for the $ccccbb$ and $bbbbcc$ hexaquarks. In all cases, the masses of the six-quark arrangements are larger that the ones corresponding to the sum of any of the two baryons we can split them into, but smaller that the ones for a set of six isolated quarks, i.e., all them are bound systems. The analysis of their structure indicates that all the hexaquarks considered in this work are compact objects, except the $cccbbb$, that appears to be a loose association of two baryons.

hep-ph↗

arXiv2022

The $\mathbf{X(3872)}$'s excitation and its connection with production at hadron colliders

The LHCb collaboration has found that the production rate of $X(3872)$ in proton-proton collisions decreases as final state particle multiplicity increases. Moreover, the ALICE experiment at CERN has observed that the number of deuterons produced increases with multiplicity, a behavior that is qualitatively different from that of the $X(3872)$. These experimental findings may point to a compact structure for the $X(3872)$ or, at least, that its hadronization could proceed through a charm-anticharm core. We have recently used a diffusion Monte Carlo method to solve the many-body Schrödinger equation that describes the $X(3872)$ as a $c \bar c q \bar q$ tetraquark system with quantum numbers $I^G(J^{PC})=0^+(1^{++})$ and $1^-(1^{++})$. According to our structural analysis, the quark--(anti-)quark correlations resemble light-meson--heavy-meson molecules of type $ωJ/ψ$ and $ρJ/ψ$, rather than the most extended $D\bar D^{\ast}$ interpretation. It was argued that this fact may be the key to make compatible the molecular features of the $X(3872)$ with its production observables. The same formalism allows us to compute the first color excited $c \bar c q \bar q$ tetraquark state with either $I^G(J^{PC})=0^+(1^{++})$ or $1^-(1^{++})$. A bound-state is found in each channel, their masses are around 4.0 GeV which is an energy region where many new exotic candidates have been collected by the Particle Data Group. Concerning their structural properties, these states cluster in a compact diquark-antidiquark arrangement which matches perfectly with a so-called Born-Oppenheimer tetraquark configuration. The promptly production rates of these states in proton-proton, proton-nucleus and nucleus-nucleus collisions should fall off equal to or even faster than those of the $X(3872)$.

hep-ph↗

arXiv2022

Supersolidity in the second layer of $para$-H$_2$ adsorbed on graphite

We calculated the phase diagram of the second layer of $para$-H$_2$ adsorbed on graphite using quantum Monte Carlo methods. The second layer shows an incommensurate triangular crystal structure. By using a symmetric wave function, that makes possible molecule exchanges, we observed that this nearly two-dimensional crystal shows a finite superfluid density around a total density of 0.1650 Å$^{-2}$. The superfluid fraction of this supersolid phase was found to be small, 0.41$\pm$0.05 \%, but still experimentally accessible.

cond-mat.other↗

arXiv2021

The structure of the X(3872) as explained by a Diffusion Monte Carlo calculation

Two decades after its unexpected discovery, the properties of the $X(3872)$ exotic resonance are still under intense scrutiny. In particular, there are doubts about its nature as an ensemble of mesons or having any other internal structure. We use a Diffusion Monte Carlo method to solve the many-body Schrödinger equation that describes this state as a $c \bar c n \bar n$ ($n=u$ or $d$ quark) system. This approach accounts for multi-particle correlations in physical observables avoiding the usual quark-clustering assumed in other theoretical techniques. The most general and accepted pairwise Coulomb$\,+\,$linear-confining$\,+\,$hyperfine spin-spin interaction, with parameters obtained by a simultaneous fit of around 100 masses of mesons and baryons, is used. The $X(3872)$ contains light quarks whose masses are given by the mechanism responsible of the dynamical breaking of chiral symmetry. The same mechanisms gives rise to Goldstone-boson exchange interactions between quarks that have been fixed in the last 10-20 years reproducing hadron, hadron-hadron and multiquark phenomenology. It appears that a meson-meson molecular configuration is preferred but, contrary to the usual assumption of $D^0\bar{D}^{\ast0}$ molecule for the $X(3872)$, our formalism produces $ωJ/ψ$ and $ρJ/ψ$ clusters as the most stable ones, which could explain in a natural way all the observed features of the $X(3872)$.

hep-ph↗