Strange Matter Research Articles

The puzzle of muons: novel particles or novel properties

In order to examine a muon excess observed by the Pierre Auger Observatory, detailed Monte Carlo simulations were carried out, assuming the existence of strangelets (hypothetical stable lumps of strange quark matter) in the primary cosmic rays. We obtain a rough agreement between the simulations and the data for ordinary nuclei without any contribution of strangelets in primary flux of cosmic rays.

Integration and testing of the Mini-EUSO telescope

The Mini-EUSO telescope, approved by ASI (Italian Space Agency) and Roscosmos (Russian Space Agency) as long-term program of space experiments to be launched on the Progress spacecraft during the Expedition 60/61 in 2019, will be placed on a nadir-facing, UV transparent window on the ISS Russian Zvezda module looking down on Earth. As one of the JEM-EUSO collaboration missions, its first objective is to observe UV light produced by UHECR (Ultra High Energy Cosmic Ray) events from upper atmosphere (~ 400 km altitude), moreover, from this point of view, Mini-EUSO will perform, for the first time, high-resolution mapping of night-earth UV (300 - 400 nm) emission, focusing on terrestrial background sources, TLEs (Transient Luminous Events), bioluminescence, meteoroids burning in atmosphere, search for SQM (Strange Quark Matter) signals, as well as detecting and mapping space debris. The integration of the instrument, currently underway at the University of Rome Tor Vergata, is at an advanced stage in order to be compliant with a launch opportunity in June 2019. The on ground test results will be presented.

Exact solutions of a model for strange stars with interacting quarks

The search for the true ground state of dense matter has remained open since Edward Witten proposed the strange matter hypothesis in 1984. In this hypothesis, the strange matter is assumed to be composed of u, d, and s quarks, having an energy per baryon lower than that of quark matter (only u and d), and even lower than that of the nuclear matter. In this sense, neutron stars would be strange stars. Later work showed that a color‐flavor‐locked (CFL) state would be preferred to the one without any pairing for a wide range of the parameters (gap Δ, strange quark mass ms, and bag constant B). We use an approximate, yet very accurate, CFL equation of state (EoS) to obtain exact solutions for the static Einstein field equations that describe a compact relativistic object, providing the first solution directly connected with microphysics.

Can strangelets solve the muon puzzle?

In order to examine a muon excess observed by the Pierre Auger Observatory, detailed Monte Carlo simulations were carried out, assuming the existence of strangelets (hypothetical stable lumps of strange quark matter) in the primary cosmic rays. We obtain a rough agreement between the simulations and the data for ordinary nuclei without any contribution of strangelets in the primary flux of cosmic rays.

Holographic compact stars meet gravitational wave constraints

We investigate a simple holographic model for cold and dense deconfined QCD matter consisting of three quark flavors. Varying the single free parameter of the model and utilizing a Chiral Effective Theory equation of state (EoS) for nuclear matter, we find four different compact star solutions: traditional neutron stars, strange quark stars, as well as two non-standard solutions we refer to as hybrid stars of the second and third kind (HS2 and HS3). The HS2s are composed of a nuclear matter core and a crust made of stable strange quark matter, while the HS3s have both a quark mantle and a nuclear crust on top of a nuclear matter core. For all types of stars constructed, we determine not only their mass-radius relations, but also tidal deformabilities, Love numbers, as well as moments of inertia and the mass distribution. We find that there exists a range of parameter values in our model, for which the novel hybrid stars have properties in very good agreement with all existing bounds on the stationary properties of compact stars. In particular, the tidal deformabilities of these solutions are smaller than those of ordinary neutron stars of the same mass, implying that they provide an excellent fit to the recent gravitational wave data GW170817 of LIGO and Virgo. The assumptions underlying the viability of the different star types, in particular those corresponding to absolutely stable quark matter, are finally discussed at some length.

Equation of State for Hot Quark Matter with Neutrino Confinement

The equation of state and composition of hot strange quark matter that is opaque to neutrinos are determined. This study is based on the MIT quark bag model. Three different variants of the lepton population in hot quark matter are considered. In the first, only e–, e+, νe, and $$ {\overline{\upnu}}_e $$ leptons are present in the material. In the second, μ–, μ+, νμ, and $$ {\overline{\upnu}}_{\upmu} $$ are added to these leptons. And in the third, τ– neutrinos and τ+ antineutrinos are also present and the phenomenon of neutrino oscillations is taken into account. Numerical calculations are done for different temperatures and lepton charge densities. It is shown that when neutrinos exist in hot quark matter (T~1012 K) the number of u quarks is 24-33% and 37-42% greater than the number of d and s quarks, respectively, depending on the baryon charge concentration. When neutrino oscillations are included, these parameters go to 19-27% and 30-37%. When there are no neutrinos, on the other hand, the number of d quarks is less than the number of u quarks by 2-8%. It is shown that for a fixed baryon charge concentration, the pressure of the quark matter depends strongly on temperature. This dependence is especially strong at comparatively low densities. As opposed to this, the pressure has a weak temperature dependence for a fixed energy density.

Transition between general relativity and quantum gravity using quark and strange quark matter with some kinematical test

Contemporary piece of writing devotes to the investigation of plane symmetric cosmological model with quark and strange quark matter in the deformations of the Einstein’s theory of General Relativity (GR). At small or large scales (ultraviolet or infrared gravity), deformations of the Einstein’s theory could provide a better handling of cosmic acceleration without magnetism (along with singularities). In particular, a proper deformation of GR in the ultraviolet regime could play the role of describing the transition between GR and quantum gravity. As a matter of fact, although with a different purpose in mind, it was Einstein himself who proposed in the 30’s the reformulation of GR by taking the field of orthonormal frames or tetrads as the dynamical variable instead of the metric tensor (Einstein, Phys. Math. Kl 217, 401, 1928). As per the observation, pressure and energy density of the model approaches the bag constant in negative and positive ways at $$t\rightarrow \infty $$ , i.e. $$p\rightarrow -B_c $$ and $$\rho \rightarrow B_c $$ , the negative pressure due to the Dark Energy (DE) in the context of accelerated expansion of the universe. So the strange quark matter gives an idea of existence of dark energy in the universe and supports the observations of the SNe-I (Riess et al., Astron. J. 116,1009, 1998; Perlmutter et al., Astrophys. J. 517, 565, 1999). Also these results agree with the study of Aktas and Aygun (Chinese J. Phys. 55, 71, 2017) and Sahoo et al. (New. Astron. 60, 80, 2018).

Interface effects of strange quark matter with density dependent quark masses

We study the interface effects in strangelets adopting mean-field approximation (MFA). Based on an equivparticle model, the linear confinement and leading-order perturbative interactions are included with density-dependent quark masses. By increasing the confinement strength, the surface tension and curvature term of strange quark matter (SQM) become larger, while the perturbative interaction does the opposite. For those parameters constrained according to the 2$M_\odot$ strange star, the surface tension is $\sim$2.4 MeV/fm${}^2$, while unstable SQM indicates a slightly larger surface tension. The obtained results are then compared with those predicted by the multiple reflection expansion (MRE) method. In contrast to the bag model case, it is found that MRE method overestimates the surface tension and underestimates the curvature term. To reproduce our results, the density of states in the MRE approach should be modified by proper damping factors.

Anisotropic pressure in strange quark matter in the presence of a strong nonuniform magnetic field

Thermodynamic properties of strange quark matter (SQM) in a nonuniform magnetic field are studied within the phenomenological MIT bag model under the charge neutrality and beta equilibrium conditions, relevant to the interior of strange quark stars. The spatial dependence of the magnetic field strength is modeled by the dependence on the baryon chemical potential in the exponential and power forms. The total energy density, longitudinal and transverse pressures in magnetized SQM are found as functions of the baryon chemical potential. It is clarified that the central magnetic field strength in a strange quark star is bound from above by the critical value at which the derivative of the longitudinal pressure with respect to the baryon chemical potential vanishes first somewhere in the interior of a star under varying the central field. Above this upper bound, the instability along the magnetic field is developed in magnetized SQM. The change in the form of the dependence of the magnetic field strength on the baryon chemical potential between the exponential and power ones has a nonnegligible effect on the critical magnetic field strength while the variation of the bag pressure within the absolute stability window for magnetized SQM has a little effect on the critical field.

Relationship between magnetic field and anisotropy parameter in gravitation theories

The magnetized strange quark matter (MSQM) solutions are obtained for a Marder type universe using constant deceleration parameter. The exact solutions of field equations are obtained for f(R,[Formula: see text]T) = R + 2f(T) model given by [T. Harko et al., Phys. Rev. D 84, 024020 (2011)] with cosmological term. Also, we have obtained General Relativity (GR) solutions for MSQM distributions. For [Formula: see text], we get the dark energy model, i.e. [Formula: see text], [Formula: see text] and [Formula: see text]. However, for [Formula: see text], we find the cosmological constant [Formula: see text] as negative in f(R,[Formula: see text]T) theory and GR. These results agree with [S. Aygün et al., Astrophys. Space Sci. 361, 380 (2016); C. Aktaş and S. Aygün, Chinese J. Phys. 55, 71 (2017); P. K. Sahoo et al., Mod. Phys. Lett. A 32, 1750105 (2017)] in f(R,[Formula: see text]T) theory. The physical consequences of our obtained models are discussed at the end.

Magnetic moments of octet baryons in strange matter

The modification of masses as well as magnetic moments of octet baryons in the presence of hot and dense isospin symmetric strange baryonic matter has been studied using the chiral SU(3) quark mean field model. Effective masses of octet baryons are found to decrease considerably as a function of baryonic density of the nuclear medium. The strange baryons show a significant decrease in their masses in the higher baryonic density regime as compared to nucleons. Baryonic magnetic moments comprise the explicit contributions from the valence quarks, sea quarks as well as orbital angular momentum of sea quarks. The magnitudes of magnetic moments of octet baryons are found to increase as a function of the density of the nuclear medium, since the quark spin polarizations corresponding to valence quarks, sea quarks and orbital angular momentum of sea quarks vary significantly in the baryonic medium particularly in the low density and temperature region. The magnetic moments of strange baryons show a consistent behavior as a function of the strangeness fraction with the increase of the temperature of the medium, whereas in the case of nucleons the magnetic moments show an opposite behavior at T = 0 and T = 100 MeV.

Anisotropic strange stars in Tolman\u2013Kuchowicz spacetime

We attempt to study a singularity-free model for the spherically symmetric anisotropic strange stars under Einstein’s general theory of relativity by exploiting the Tolman–Kuchowicz (Tolman in Phys Rev 55:364, 1939; Kuchowicz in Acta Phys Pol 33:541, 1968) metric. Further, we have assumed that the cosmological constant varLambda is a scalar variable dependent on the spatial coordinate r. To describe the strange star candidates we have considered that they are made of strange quark matter distribution, which is assumed to be governed by the MIT bag equation of state. To obtain unknown constants of the stellar system we match the interior Tolman–Kuchowicz metric to the exterior modified Schwarzschild metric with the cosmological constant, at the surface of the system. Following Deb et al. (Ann Phys 387:239, 2017) we have predicted the exact values of the radii for different strange star candidates based on the observed values of the masses of the stellar objects and the chosen parametric values of the varLambda as well as the bag constant {mathcal {B}}. The set of solutions satisfies all the physical requirements to represent strange stars. Interestingly, our study reveals that as the values of the varLambda and {mathcal {B}} increase the anisotropic system become gradually smaller in size turning the whole system into a more compact ultra-dense stellar object.

Anisotropic strange stars in the Einstein\u2013Maxwell spacetime

We present here a detailed analysis on the effects of charge on the anisotropic strange star candidates by considering a spherically symmetric interior spacetime metric. To obtain exact solution of the Einstein–Maxwell field equations we have considered the anisotropic strange quark matter (SQM) distribution governed by the simplified MIT bag equation of state (EOS), p=frac{1}{3}left( {rho }-4,B right) , where B is the bag constant and the distribution of the electrical charge is given as q(r)=Qleft( {r}/{R}right) ^3=alpha {r^3}, where alpha is a constant. To calculate different constants we have described the exterior spacetime by the Reissner-Nordström metric. By using the values of the observed mass for the different strange star candidates we have maximized anisotropic stress at the surface to predict the exact values of the radius for the different values of alpha and a specific value of the bag constant. Further, we perform different tests to study the physical validity and the stability of the proposed stellar model. We found accumulation of the electric charge distribution is maximum at the surface having electric charge of the order {{10}^{20}}~C and electric field of the order {10}^{21-22} V/cm.

Quark Matter May Not Be Strange.

If quark matter is energetically favored over nuclear matter at zero temperature and pressure, then it has long been expected to take the form of strange quark matter (SQM), with comparable amounts of u, d, and s quarks. The possibility of quark matter with only u and d quarks (udQM) is usually dismissed because of the observed stability of ordinary nuclei. However, we find that udQM generally has lower bulk energy per baryon than normal nuclei and SQM. This emerges in a phenomenological model that describes the spectra of the lightest pseudoscalar and scalar meson nonets. Taking into account the finite size effects, udQM can be the ground state of baryonic matter only for baryon number A>A_{min} with A_{min}≳300. This ensures the stability of ordinary nuclei and points to a new form of stable matter just beyond the periodic table.

Lepton-rich cold QCD matter in protoneutron stars

We investigate protoneutron star matter using the state-of-the-art perturbative equation of state for cold and dense QCD in the presence of a fixed lepton fraction in which both electrons and neutrinos are included. Besides computing the modifications in the equation of state due to the presence of trapped neutrinos, we show that stable strange quark matter has a more restricted parameter space. We also study the possibility of nucleation of unpaired quark matter in the core of protoneutron stars by matching the lepton-rich QCD pressure onto a hadronic equation of state, namely TM1 with trapped neutrinos. Using the inherent dependence of perturbative QCD on the renormalization scale parameter, we provide a measure of the uncertainty in the observables we compute.

Neutron stars' quark matter not so strange.

Calculation also casts doubt on existence of Earth-gobbling strangelets.

Kicks of magnetized strange quark stars induced by anisotropic emission of neutrinos

We study the anisotropic neutrino emission from the core of neutron stars induced by the star's magnetic field. We model the core as made out of a magnetized ideal gas of strange quark matter and implement the conditions for stellar equilibrium in this environment. The calculation is performed without resorting to analytical simplifications and for temperature, density and magnetic field values corresponding to typical conditions for a neutron star's evolution. The anisotropic neutrino emission produces a rocket effect that contributes to the star's kick velocity. We find that the computed values for the kick velocity lie within the range of the observed values, reaching velocities of the order of $\sim1000$ km s$^{-1}$ for magnetic fields between $10^{15}-10^{18}$ G and radii of 20 to 5 km, respectively.

FLRW cosmological models with quark and strange quark matters in f ( R , T ) $f(R,T)$ gravity

In this paper, we have studied the magnetized quark matter (QM) and strange quark matter (SQM) distributions in the presence of $ f(R,T)$ gravity in the background of Friedmann--Lema\^itre--Robertson--Walker (FLRW) metric. To get exact solutions of modified field equations we have used $f(R,T) = R + 2 f(T)$ model given by Harko et al. with two different parametrization of geometrical parameters \textit{i.e.} the parametrization of the deceleration parameter $ q $, and the scale factor $ a $ in hybrid expansion form. Also, we have obtained Einstein Static Universe (ESU) solutions for QM and SQM distributions in $f(R,T)$ gravity and General Relativity (GR). All models in $f(R,T)$ gravity and GR for FRW and ESU Universes with QM also SQM distributions, we get zero magnetic field. These results agree with the solutions of Akta{\c{s} and Ayg\"un in $f(R,T)$ gravity. However, we have also discussed the physical consequences of our obtained models.

Anisotropic strange stars under simplest minimal matter-geometry coupling in the f ( R,T ) gravity

We study strange stars in the framework of $f\left(R,\mathcal{T}\right)$ theory of gravity where the strange quark matter distribution inside the stellar system is governed by the phenomenological MIT Bag model equation of state (EOS). Further, for a specific value of $B$ and observed values of mass of the strange star candidates we obtain the exact solution of the modified Tolman-Oppenheimer-Volkoff (TOV) equation in the framework of $f\left(R,\mathcal{T}\right)$ gravity and have studied in detail the dependence of the different physical parameters due to the chosen different values of $\chi$. To check the physical acceptability and stability of the stellar system based on the obtained solutions we have performed different physical tests, viz., the energy conditions, Herrera cracking concept, adiabatic index etc. In this work, we also have explained the effects, those are arising due to the interaction between the matter and the curvature terms in $f\left(R,\mathcal{T}\right)$ gravity, on the anisotropic compact stellar system. It is interesting to note that as the values of $\chi$ increase the strange stars become more massive and their radius increase gradually so that eventually they gradually turn into less dense compact objects. The present study reveals that the modified $f\left(R,\mathcal{T}\right)$ gravity is a suitable theory to explain massive stellar systems like recent magnetars, massive pulsars and super-Chandrasekhar stars, which can not be explained in the framework of GR. However, for $\chi=0$ the standard results of Einsteinian gravity are retrieved.

Moment of inertia, quadrupole moment, Love number of neutron star and their relations with strange-matter equations of state

We investigate the impact of strange matter equations of state involving $\Lambda$ hyperons, Bose-Einstein condensate of $K^-$ mesons and first order hadron-quark phase transition on moment of inertia, quadrupole moment and tidal deformability parameter of slowly rotating neutron stars. All these equations of state are compatible with the 2 M$_{solar}$ constraint. The main findings of this investigation are the universality of the I-Q and I-Love number relations, which are preserved by the EoSs including $\Lambda$ hyperons and antikaon condensates, but broken in presence of a first order hadron-quark phase transition. Furthermore, it is also noted that the quadrupole moment approaches the Kerr value of a black hole for maximum mass neutron stars.