Neutrons quarks

Quarks Subatomic particles that make up protons and neutrons. Quarks have charges of +2/3 or -1/3. 

J. I. Friedman and H. W. Kendall (Massachusetts Institute of Technology) and R. E. Taylor (Stanford) pioneering investigations concerning deep elastic scattering of electrons on protons and bound neutrons, of essential importance for the development of the quark model in particle physics. 

As far as is known, ordinary matter is made of tiny building blocks called elementary particles. For example, an atom is made up of a nucleus surrounded by one or more electrons. As far as scientists have been able to determine, the electrons are elementary particles, not made of anything simpler. Fdowever, an atomic nucleus is not clcmcntai y, but is a composite particle made up of simpler particles called protons and neutrons. (The lightest nucleus is the nucleus of ordinai y hydrogen, which consists of only a single proton.) Today, physicists believe that even protons and neutrons are not elementai y but are composite particles made up of still simpler building blocks called quarks. 

At the present time, quarks are believed to be elementary particles. All the particles in an atom, whether elementary or not, are particles of matter and possess mass. Electrons, protons, and neutrons can also exist outside of atoms. 

This topic is relevant to the physics of neutron stars (nuclei or quark bubbles embedded in a neutron gas), to dilute Bose-Einstein-condensate bubbles inside the background of a Fermi-Dirac condensate, to buckyballs in liquid mercury and to superconducting droplets in a Fermi liquid. 

The investigation of bubbles inside a Fermi gas background is also of relevance for the inner core of neutron stars namely, under the assumption that quark droplets will form, there exists a similar pattern with the quark droplet phase taking over the role of the embedded nuclei. 

After a 20 year break V. H. Ambartsumyan and G. S. Sahakian initiated an intensive research on compact objects during the 1960s in Armenia. In their pioneering work on compact stars they showed, that with increasing density, hyperons appear in nuclear matter and thus a neutron star at high densities consists predominantly of hyperons. Thus, as the density increases more and more heavy particles become stable. After the discovery of quarks as basic constituents of hadrons (including hyperons) the ideas of compact stars with quark cores or stars entirely composed of quark matter were presented. 

Abstract After some historical remarks we discuss different criteria of dynamical stability of stars and the properties of the critical states where the loss of dynamical stability leads to a collapse with formation of a neutron star or a black hole. At the end some observational and theoretical problems related to quark stars are discussed. 

Keywords stellar stability, white dwarfs, neutron stars, quark stars... 

The discovery of the quark structure of matter led to the suggestion of possible existence of quark stars, which are even more compact than neutron stars. In the presence of indefiniteness concerning the quark structure of matter it is not possible now to make definite statements about the existence or nonexistence of stable quark stars, observational and theoretical investigations on this topic are still in progress. 

In this review I first make a historical excursus into the problem, mentioning the results of the key works. Several criteria of stability are discussed, with the main focus on the static criteria, and the energetic method, which permits to obtain conclusions about the stability (sometimes approximate) in a most simple way. Critical states of compact stars at the boundary of the dynamic stability are considered, at which the star is becoming unstable in the process of energy losses, and a collapse begins leading to formation of a neutron star or a black hole. Physical processes leading to a loss of stability are discussed. At the end some observations and theoretical problems connected with quark stars are considered. 

Aguirre, R. M., De Paoli, A. L. (2002). Neutron star structure in a quark model with excluded volume correction. Phys.Rev.C68 055804. 

Berezhiani, Z., Bombaci, I., Drago, A., Frontera, F., Lavagno, A. (2003). Gamma Ray Bursts from delayed collapse of neutron stars to quark matter stars. Astrophys.J., 586 1250-1253. 

Burgio, G. F., Baldo, M., Schulze, H.-J., Sahu, P. K. (2002). The hadron-quark phase transition in dense matter and neutron stars. Phys.Rev., C66 025802-025815. 

Sedrakian, D. M., Blaschke, D. (2002). Magnetic field of a neutron star with color superconducting quark matter core. Astrofiz., 45 203-212. 

Thoma, M.H., Triimper, J., Burwitz, V. (2003). Strange Quark Matter in Neutron Stars - New Results from Chandra and XMM. J.Phys.G30 S471-S478. 

Turolla, R., Zane, S., Drake, J.J. (2003). Bare Quark Stars or Naked Neutron Stars The Case of RX J1856.5-3754. Astrophys. J.603 265-282. 

Neutron stars are important laboratories for the physics of high-density matter. Unlike particles in relativistic heavy-ion colliders, the matter in the cores of neutron stars has a thermal energy that is much less than its rest-mass energy. Various researchers have speculated whether neutron star cores contain primarily nucleons, or whether degrees of freedom such as hyperons, quark matter, or strange matter are prevalent (see Lattimer Prakash 2001 for a recent review of high-density equations of state). 

The commonly accepted pulsar model is a neutron star of about one solar mass and a radius of the order of ten kilometers. A neutron star consists of a crust, which is about 1 km thick, and a high-density core. In the crust free neutrons and electrons coexist with a lattice of nuclei. The star s core consists mainly of neutrons and a few percents of protons and electrons. The central part of the core may contain some exotic states of matter, such as quark matter or a pion condensate. Inner parts of a neutron star cool up to temperatures 108iT in a few days after the star is formed. These temperatures are less than the critical temperatures Tc for the superfluid phase transitions of neutrons and protons. Thus, the neutrons in the star s crust and the core from a superfluid, while the protons in the core form a superconductor. The rotation of a neutron superfluid is achieved by means of an array of quantized vortices, each carrying a quantum of vorticity... 

Neutron stars (NSs) are perhaps the most interesting astronomical objects from the physical point of view. They are associated with a variety of extreme phenomena and matter states for example, magnetic fields beyond the QED vacuum pair-creation limit, supranuclear densities, superfluidity, superconductivity, exotic condensates and deconfined quark matter, etc. 

Bare neutron and quark star emission (Turolla et al. 2004). 

In the simplest approximation we consider an ideal quantum gas of elementary particles such as protons, neutrons, electrons and possibly neutrinos (the quark-gluon substructure will not be considered at densities and temperatures considered). The EOS is found in textbooks and will not be discussed any further here. 

NEUTRON STAR STRUCTURE WITH HYPERONS AND QUARKS... 

Abstract We discuss the high-density nuclear equation of state within the Brueckner-Hartree-Fock approach. Particular attention is paid to the effects of nucleonic three-body forces, the presence of hyperons, and the joining with an eventual quark matter phase. The resulting properties of neutron stars, in particular the mass-radius relation, are determined. It turns out that stars heavier than 1.3 solar masses contain necessarily quark matter. 

Keywords Neutron Star, Brueckner-Hartree-Fock, Three-Body Force, Hyperons, Quark... 

Neutron Star Structure with Hyperons and Quarks... 

The results obtained with a purely baryonic EOS call for an estimate of the effects due to the hypothetical presence of quark matter in the interior of the neutron star. Unfortunately, the current theoretical description of quark matter is burdened with large uncertainties, seriously limiting the predictive power of any theoretical approach at high baryonic density. For the time being we can therefore only resort to phenomenological models for the quark matter EOS and try to constrain them as well as possible by the few experimental information on high density baryonic matter. 

For the description of a pure quark phase inside the neutron star, as for neutrino-free baryonic matter, the equilibrium equations for the chemical potentials,... 

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