Quark-hadron phase transition in a magnetic field

Quark-hadron phase transition in QCD in the presence of magnetic field is studied. It is shown that both the temperature of a phase transition and latent heat decrease compared to the case of zero magnetic field. The phase diagram in the plane temperature--magnetic field is presented. Critical point, T_\ast=104 MeV, \sqrt{eH_\ast}=600 MeV, for which the latent heat goes to zero, is found.


Introduction
QCD is essentially nonperturbative at low temperatures T < T c (T c is a quark-hadron phase transition temperature) and is characterized by confinement and spontaneous chiral symmetry breaking. In the hadron phase, at low temperature, the dominating contribution to the partition function of the system is given by lightest particles in the physical spectrum. In the case of QCD these particles are π-mesons, which in the limit of massless quarks are Goldstone excitations in the chiral condensate. It is a common method in the low temperature physics of QCD to use effective chiral theory [1,2,3], often called chiral perturbation theory (ChPT).
One of the important questions is the phase structure of vacuum in the external magnetic field H. In the recent paper [4] it is argued that high magnetic fields eH ∼ 10 2 ÷ 10 4 MeV 2 are created in heavy ion collisions. Such magnetic fields should lead to effects that can be experimentally observed at RHIC. Also high magnetic fields eH ∼ Λ 2 QCD could exist in the early universe at the scale of strong interactions. Such high field strength can lead to new interesting phenomena at the stage of quark-hadron phase transition. At the same time it is interesting to study the influence of the external magnetic field on the dynamics of strong interactions from purely theoretical point of view. Different nonperturbative phenomena in abelian magnetic fields were previously studied in [5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20].
In this paper, we study the quark-hadron phase transition in QCD in abelian magnetic field. The physics of the considered phenomenon is the following. The plasma of hot quarks and gluons at T > T c in the magnetic field is a thermodynamic system in a paramagnetic phase. On the other hand, at low temperature, T < T c , hadron matter, which mainly consists of scalar π-mesons, is in diamagnetic phase. The paramagnetic phase is preferable thermodynamicly, because it minimizes the free energy (maximizes pressure). Therefore the temperature of the transition from hadron phase to the quarkgluon phase decreases as compared to the case of zero magnetic field H = 0. Thus, there is the analogy with the physics of condensed matter: confinement phase corresponds to the diamagnetic gas of scalar π-mesons (we neglect the contribution of heavier hadrons), and deconfinement phase corresponds to the paramagnetic phase of quarks and gluons.
2 Free energy of the QCD vacuum at T = 0 and H = 0 The partition function of QCD in euclidian formulation in the presence of external abelian field A µ can be written in a form (here T = 1/β is temperature) where QCD Lagrangian in the background field is here Q q is the charge matrix of quarks with flavor q = (u, d), and we omitted ghost and gauge fixing terms for simplicity. Free energy density is given by the expression βV F (T, H, m q ) = − ln Z. Let us consider the hadron phase. At low temperature, T < T c , (T c is the temperature of chiral phase transition) and at weak external field, H < µ 2 hadr ∼ (4πF π ) 2 , the characteristic momentum in the vacuum loops is small, and the theory is described by the effective low-energy Lagrangian L eff [2,3].
L eff can be represented as a decomposition in a series of powers of momentum (derivatives) and masses L eff = L (2) + L (4) + L (6) + ...
The leading term in (4) is a Lagrangian of nonlinear σ-model in external field V µ Tr(∇ µ U + ∇ µ U) + ΣRe Tr(MU + ), Here U is unitary SU(2) matrix, F π = 93 MeV is a pion decay constant, and the parameter Σ has the meaning of quark condensate, Σ = | ūu | = | d d |. External abelian magnetic field H, directed along z axis, corresponds to V µ (x) = (τ 3 /2)eA µ (x), and vector-potential A µ is taken in the form A µ (x) = δ µ2 Hx 1 . We will furhter neglect the breaking of isospin symmetry of strong interactions and consider masses of light u-and d-quarks equal, m u = m d = m q , thus the mass matrix diagonal,M = m qÎ . In one-loop approximation it is sufficient to use the decomposition of L eff up to quadratic over pion field terms. In exponential parametrization of the matrix U(x) = exp{iτ a π a (x)/F π } we find that where we introduced the fields of charged π ± and neutral π 0 mesons The QCD partition function (1) in one-loop approximation of the effective chiral theory takes the form 1 .
Here the partition function is normalized to the case of perturbation theory at T = 0, Integrating (8) over π-fields one gets where D µ = ∂ µ − ieA µ is a covariant derivative, and the subscript T means that the determinant is evaluated at finite temperature T using standard Matsubara rules. Using (9) and regrouping multipliers in (10) we arrive at the following expression for Free energy then takes the form [7] 1 The partition function Z R eff describes charged π ± and neutral π 0 Bose gases in magnetic field Here F π 0 is the free energy of massive scalar boson F π ± is Schwinger's result for the vacuum energy density of charged scalar particles in magnetic field Next, it is technically not hard to generalize the case of the vacuum H = 0, T = 0 for charged π ± -mesons F = Tr ln(p 2 4 + ω 2 0 (p)) (15) to the case of H = 0, T = 0. Omitting the details, we will note that it corresponds to the following substitutions , and where the degeneracy multiplicity of Landau levels eH/2π is taken into account. Summing over Matsubara frequencies, one obtains the following result for the diamagnetic part of free energy of charged Bose gas here ω n are Landau levels of π ± -mesons in a constant magnetic field H. Expanding ln(...) in the integrand of (13), (17) in a series, one gets the following expressions: and where K n is the Macdonald function.

Quark-hadron phase transition in magnetic field
In order to determine the temperature of phase transition we will write down the pressure in two phases. At zero chemical potential pressure is equal to minus free energy.
In the confinement phase pressure of π-mesons in magnetic field takes the form (see (12)) where the pressure of neutral gas of π 0 -mesons is P π 0 (T ) = −F π 0 (T ). Renormalized contribution of vacuum polarization of charged π ± -mesons to the pressure, which does not depend on temperature (Schwinger polarization) is Diamagnetic term in the pressure, which comes from charged π ± mesons is P dia (T, H) = −F dia (T, H). Fermion (quark) determinant at finite temperature in the magnetic field can be considered in a similar way. Then the pressure in the quark-gluon plasma phase has the form The term proportional to ∝ T 4 comes from the gas of hot gluons, P q (H) is the contribution from vacuum polarization of quarks to the pressure and P para is a paramagnetic term An important physical property of the phase transition is the rearrangement of the nonperturbative QCD vacuum. Due to the scale anomaly in the trace of the energymomentum tensor the new dimensional quantity, gluon condensate G 2 ≡ (gG a µν ) 2 , emerges in QCD. Nonperturbative energy density of the vacuum is related to G 2 : where b = (3N c − 2N f )/3, N c = 3 is the number of colors, and b = 29/3 for N f = 2. Energy density of vacuum is the negative quantity, and the state with G 2 = 0 turns out to be thermodynamically advantageous. Theoretical studies [21,22] and numerical computations in the lattice approximation of QCD [23] show that at the point of phase transition T c one part of the condensate (chromoelectric part) turns to zero, while the chromomagnetic condensate remains almost unchanged as compared to the case T = 0. In the vacuum at T = 0 (E a i ) 2 = (H a i ) 2 , and therefore vacuum energy density above phase transition appears to be less in magnitude than below phase transition, and the difference is approximately (26) Taking this into account, the quantity −∆ε v should be added to the equation of state in the plasma phase. Thus, the pressure in the quark-gluon plasma state is given by the expression Equation (27) Let us now consider "weak" magnetic field, eH ≪ T 2 c . Then the Schwinger contribution to the pressure can be neglected. In the weak field one can use Euler-Maclaurin formula for P dia and (17) can be rewritten in the form For the paramagnetic term P para we find in the weak field where m u = m d = m and Q 2 = (e 2 u + e 2 d )/e 2 = ( 4 9 + 1 9 ) = 5 9 , and pressure P 0q (T, H = 0) is given by 2 2 Using In the absence of magnetic field and in the chiral limit one finds [21] T c = ∆ε v (γ − 3)(π 2 /90) 1/4 . (33) Here γ = 2 · (N 2 c − 1) + (7/8) · 2 · 2 · N c · N f is the number of independent degrees of freedom of quarks and gluons, and γ = 37 for N c = 3, N f = 2. Lattice calculations give the value G 2 = 0.87 GeV 4 , and one finds from (33) phase transition temperature T c ≃ 177 MeV at H = 0.
The influence of magnetic field can be taken into account in the first approximation by redefining vacuum energy density where V = 6.1 · 10 −2 for M π = 140MeV and m ≈ 5MeV .
Considering the term (eH) 2 V as a perturbation, one finds from (33) and (34) that the relative shift of the deconfinement phase transition temperature in the presence of magnetic field is and V /4∆ε v ≃ 9.2GeV −4 . Thus, we see that the presence of a magnetic field leads to a decrease of the quarkhadron phase transition temperature, and ∆T /T c ≈ 10 −2 (eH) 2 /∆ε v .

Results of numerical simulations
Equations (20) and (27) allow to evaluate the pressure in both phases, and to numerically find the dependence of phase transition temperature on the magnetic field from (28). Results of numerical calculations for N c = 3, N f = 2 are presented in Fig. 1. Phase transition temperature, as discussed above, decreases with increasing external magnetic field.
Thermodynamics in each phase is defined by the pressure, and we can evaluate energy density and latent heat in both phases. Energy density is given by Latent heat equals to the difference of energy densities of two phases in the point of phase transition: The dependence of ∆ε(H) is plotted in Fig. 2. The value of the magnetic field √ eH * = 600 MeV where latent heat turns to zero corresponds to the critical point, at which first order phase transition changes to the crossover.

Conclusion
We have studied the quark-hadron phase transition in QCD in the presence of external magnetic field, and have shown, that the temperature of the phase transition decreases in comparison to the case of zero magnetic field. Equation (28) was solved numerically, the phase diagram in the plane temperature-magnetic field and critical point were found.
As was shown above, there are two phases in the presence of magnetic field: diamagnetic phase below T c and paramagnetic above T c . Correspondingly magnetic susceptibility, χ = −∂ 2 P/∂H 2 , changes it's sign at the critical temperature. Thus, magnetic susceptibility may be considered as the order parameter of the model of thermal QCD in the presence of magnetic field.
It is known from lattice calculations that there is a crossover for finite temperature QCD with physical quark masses. In the presence of magnetic field there are additional magnetic terms in the pressure, which give different contribution to the energy density in two phases. Thus we expect that a crossover is replaced by a first order phase transition. Analogous phenomenon was found in [24], where it was shown that chiral transition changes from crossover to the weak first order transition in the linear sigma model in a magnetic field. The following remark should be made. From [7] it is known that the chiral phase transition temperature grows with the magnetic field. As it follows from lattice calculations, deconfinement and chiral phase transitions take place at the same temperature T c in case of zero magnetic field H = 0. On the other hand, as was shown above, quarkhadron phase transition temperature at nonzero magnetic field is lower than in case of H = 0. Thus, chiral and quark-hadron phase transition temperatures are separated in the presence of magnetic field. Correspondingly, there appears a temperature interval, where the quark-hadron phase transition is already passed, but the chiral symmetry is still broken. This phenomenon may be important for the consideration of quark-hadron phase transition in heavy ion collisions and in the early universe.
Authors are grateful to Yu.A. Simonov and A.B. Kaidalov for stimulating discussions and useful comments.