Bulk viscosity of interacting strange quark matter

The quarks are regarded as quasiparticles, which acquire an effective mass generated by the interaction with the other quarks of the dense system. The bulk viscosity is re-expressed in the case of interacting strange quark matter. We find that the viscosity is a few ~ tens times larger than that of non-interacting quark gas due to the small correction of medium effect to the equation of state. The limiting spins of strange stars will rise so significantly that the pulsars with shorter period may exist.

As the hypothesis compact objects, strange stars have not been proven by observation yet.Probing strange stars and then identifying them are significant challenges in nuclear physics and astrophysics.It has been shown in a series of papers, that the emission of gravitational radiation due to r-mode instabilities in hot, young neutron stars severely limits the rotation period of these stars [1,2,3,4,5,6].Whereas Madsen found that the r-mode instability does not play any role in a young strange star contrary to young neutron stars [7].The author reasonably concluded that finding a young pulsar with a rotation period below 5-10 milliseconds would be a strong indication of the existence of strange stars.In another article [8], the author also discussed the evolution of pulsars as strange stars due to r-mode instabilities in rapidly rotating stars.The results revealed that strange stars, perhaps better than neutron stars, explained the apparent lack of very rapid pulsars with frequencies above 642Hz sine existing data are consistent with pulsars being strange stars under some circumstances.All above involve the viscosities of strange quark matter contained in the interior of these stars.This is because of viscosity damping the r-modes.As known, the shear viscosity of quark matter [7,9] is roughly comparable to that of neutron star matter for the parameter range of our interest where the bulk viscosity of quark matter, which damps the r-modes, is decisive for the localization of the instability, differs from neutron star matter.The bulk viscosity of strange quark matter mainly arises from the non-leptonic weak interactions [10] u The importance of dissipation due to the reaction (1) was first stressed by Wang and Lu [11] in the case of neutron stars with quark cores.Afterward, Sawyer [12] expressed the damping as a function of temperature and oscillation frequency.Furthermore, Madsen in 1992 [13] considered the non-linearity of the reaction rate and corrected the factor error of the rate in [12] to obtain the exact formula of strange matter.All these works showed a huge bulk viscosity of strange matter relative to nuclear matter.Possible implications of a pulsar being strange star will result from the discrepancy of bulk viscosities between strange quark matter and normal nuclear matter.
However, up to now, the considerations of the viscosity are based on the equation of state(EOS) described as a free Fermi gas of quarks at zero temperature.In fact, the coupling among quarks (i.e. so-called medium effect) due to strong interaction is very important for quark-gluon plasma(QGP).Many investigations devoted to the field.We here hope to uncover the medium effect on the bulk viscosity of strange matter.As known, one of the most important medium effects are effective masses generated by the interaction of the quarks with the system.Schertler with his colleagues had studied the contribution of effective mass effect to EOS of strange matter [14] and applied the EOS to astrophysical aspect for understanding of strange star's structure [15], where the authors applied the idea of an ideal gas of quasiparticles with effective masses to the case of cold strange matter.
We follow ref [14]: At finite chemical potential(µ i ), the effective (quark) mass of quasiparticle is obtained from zero momentum limit of the quark dispersion following from the hard dense loop(HDL) approximation [16] of the quark self energy.In the case of a vanishing current quark mass, as assumed for up and down quarks, the effective mass is given as follows [17] For strange quarks due to non-vanishing current quark masses, we incorporate the current mass into the effective one [18] m where g is coupling constant of strong interaction.An upper limit for the coupling constant is given by g = 7.7 in [14].As was used in [14], we here take g as a parameter ranging from 0 to 5. Furthermore we replace the dispersion relation of the collective quarks branch by Evidently, transport coefficients depend mainly on EOS for given reaction(1) [11,12,13].Medium effects affect the EOS, then the viscosity will be changed.Although effective mass effect on structure(or EOS) of a strange star is small [14], the viscosity is very sensitive to the EOS.In the following, we will show that this is indeed the case.
We continue to adopt the quasiparticle picture to treat an ensemble of quasiparticles as a free, degenerate Fermi gas of particles with a total energy Due to the µ-dependence of m * (µ) the function E * (µ) = B * (µ)V defines a necessary energy counterterm in order to maintain thermodynamic self-consistency [14].d denotes the degree of degeneracy and k n k = N is particle number of system.Thus the partition function has where β = 1 T , T is the temperature.The grand thermodynamic potential Ω in a volume is easily derived.
We immediately get the pressure We also obtain other static quantities such as particle number density and energy density according to thermodynamic relations To maintain these fundamental relations above, we should require constraint condition [14] ∂P ∂m B * will be uniquely defined following from the constraint equation (9).Therefore the particle and energy densities is explicitly expressed as This, combining ( 6)with( 10)and (11), is the EOS of the strange matter derived by Schertler, Greiner and Thoma in quasiparticle description, together with MIT bag constant B 0 , in β-equilibrium [14] Now we start with the bulk viscosity of SQM in quasiparticle description.We follow the method of derivation of the bulk viscosity which was presented by Wang and Lu [11] and then developed by Madsen [13].
We still keep the Wang and Lu's notation for convenience, defining v, the volume per unit mass and n i , the quark numbers per unit mass.Assume the volume vibrates around the equilibrium volume v 0 , with the period τ and the perturbation amplitude ∆v.
When the amplitude is small, i. e. △v v0 ≪ 1, we express the pressure P (t) through a expansion near equilibrium P 0 According to the reaction (1), we shall have If the net rate of reaction (1) dn d dt and EOS in equilibrium state were given, the mean dissipation rate of the vibrational energy per unit mass can be obtain from following expression It has been certified by Wang and Lu [11] that the first two terms in (13) don't contribute to the energy dissipation rate (15).This conclusion is independent of concrete P 0 .The derivative of the third and fourth terms in ( 12) can be obtained from thermodynamical relation We need slightly change Eq (10) in place of ρ with n i It immediately reads where We continue to use the formula of the reactional rate adopted by Madsen [13] dn The chemical potential difference δµ can be derived like the pressure in Eq( 12) Substituting Eqs ( 17) and ( 18) into above formula, we have The definition of the bulk viscosity is from [12,13] Finally, it reads Our derivations are made in quasiparticle description.We find from these equations that B * modifying the EOS due to the medium effect don't contributes to the viscosity, but the effective mass play an important role on it.The prefactors in ( 19), ( 23) and ( 24) differ from that in [7,8,13].They depend not only the chemical potential µ but also coupling constant g due to m * -dependence of k Fi and C i .When taking g = 0, the mass of u,d-quark vanishes, we have and then the results above go back to Madsen's.It has been realized that the medium effect of EOS expressed in Eq (10) and ( 11) is small under the study [14], but the small contribution to EOS greatly influences on the dynamical quantity (24).Hence the viscosity ζ is sensitive to the mass m * although the EOS is weakly mass-dependence under the assumption of weak coupling.
Combining Eq(20) with Eq (22), the quantities δµ, dn d dt can be calculated in principle, but a general solution can't appear analytically because of strong coupling of the two equations.We will give the solution in the high temperature approximation and then the numerical solution for general case similar to that made by Madsen.
When the temperature is high enough, i. e.2πT ≫ δµ, the cubic term δµ 3 in Eq(20) can be neglected.The equations (20) and ( 22) are easily solved analytically.With Madsen's expression, the bulk viscosity reads The symbol * is used to distinguish the our result from Madsen's.Here ω = 2π τ , and These factors depend on not only the current mass m s but also the coupling g when the chemical potential is given.α, β in [13] is independent of the coupling constant.This is because of inclusion of medium dependent mass effect in our study.
As Madsen has pointed out, the dominating term in rate( 20) is proportional to δµ 3 at low temperature (2πT ≪ δµ).Therefore, in general Eqs(20), ( 22) and ( 24) must be solved numerically.The results of such calculations for the given parameters g, m s and µ are shown in figure 1.The viscosity is much larger than that of non-interacting quark gas, where the maximum increase of the viscosity is larger by about two orders of magnitude.To understand deeply the medium effect, we also give the g-dependence of the curves of the viscosity versus ∆v/v, T , shown in Figs2, and 3.In principle, the viscosity become larger with increasing g.Analogous to Madsen's result, the linear effect, i.e. the constant viscosity in Fig 2 , dominates at tiny vibrations while the viscosity is quickly enlarged when there exist the somewhat larger vibrations.Fig 3 shows that the effect of interacting medium on the viscosity is rather huge except in the extreme high temperature limit, over a few ×10 9 K.To understand the observed pulsar's data, the viscosity for the parameter range of interest is a few ×10 5 ∼a few×10 7 K because Madsen find that the viscosity in the range play an important role if pulsars is strange stars [8].Because the medium effect can enlarges the bulk viscosity of SQM, the critical spin frequency curves in frequency-temperature space (see the figures in [8]) will be modified.Fig 3 in [8] shows that a 2-flavor color superconducting phase(2SC) seems marginally ruled out by the observation data, but we can expect that 2SC quark matter stars may be safe absolutely if the medium effect is included.
In summary, the bulk viscosity of SQM including medium effect is derived.The influence of interacting medium on the viscosity shouldn't be negligible.We find that then the maximal discrepancy of about two orders of magnitude