Background magnetic field stabilizes QCD string against breaking

We argue that there exist a critical background magnetic field eB ≃ 16m 2, above which the string breaking is impossible in the transverse directions with respect to the axis of the magnetic field. Thus, at strong enough magnetic field a new, asymmetrically confining phase may form. The effect – which can potentially be tested at LHC/ALICE experiment – leads to abundance of u-quark rich hadrons and to excess of radially excited mesons in the noncentral heavy-ion collisions compared to the central ones. PACS numbers: 25.75.-q,12.38.Aw Motivation. Noncentral heavy-ion collisions create intense magnetic fields with the magnitude of the order of the QCD scale. According to [1] the strength of the emerging magnetic field B may reach eB max RHIC ∼ m 2 � , eB max LHC ∼ 15m 2 � (1) at the Relativistic Heavy Ion Collider (RHIC) and at the Large Hadron Collider (LHC). Here e = |e| is the absolute value of the electron charge. There are various potentially observable QCD effects associated with the presence of the strong magnetic field background. One can mention a CP-odd generation of an electric current of quarks along the axis of the magnetic field (“the chiral magnetic effect”) [2] and enhancement of the chiral symmetry breaking (“the magnetic catalysis”) [3]. The latter is related to the fact that the background magnetic field makes the chiral condensate larger [4]. Acting through the chiral condensate, the magnetic field also shifts and strengthens the chiral transition [5]. Recently, observation of certain signatures of the chiral magnetic effect was reported by the STAR collaboration at the RHIC experimental facility [6]. Some effects were also found in numerical simulations of lattice QCD. There exists numerical evidence in favor of existence of both the chiral magnetic effect [7] and the enhancement of the chiral symmetry breaking [8]. In addition, a chiral magnetization of the QCD vacuum – discussed first in Ref. [9] – was calculated numerically [10]. The lattice simulations have also revealed that due to CP-odd structure of the QCD vacuum the quark’s magnetic dipole moment in a strong magnetic field gets a large CP-odd piece, the electric dipole moment [11]. In addition to the chiral properties, the background magnetic field should also be important for confining features of QCD despite the photons are not interacting with the gluons directly. A strong enough magnetic field affects the dynamics of the gluons through the influence on the quarks, because the quarks are coupled to the both gauge fields [12]. And, indeed, thermodynamic arguments of Ref. [13] suggest that the background magnetic field should shift and weaken the confinementdeconfinement phase transition in the QCD vacuum. The confining and deconfining regions at zero temperature are separated by a smooth crossover which is located at [13] eBcross[T = 0] ∼ (700MeV) 2 ∼ 25m 2 . (2)