Abstract
We consider the response of the QCD ground state at finite baryon density to a strong magnetic field . We point out the dominant role played by the coupling of neutral Goldstone bosons, such as , to the magnetic field via the axial triangle anomaly. We show that, in vacuum, above a value of , a metastable object appears—the domain wall. Because of the axial anomaly, the wall carries a baryon number surface density proportional to . As a result, for a stack of parallel domain walls is energetically more favorable than nuclear matter at the same density. Similarly, at higher densities, somewhat weaker magnetic fields of order transform the color-superconducting ground state of QCD into new phases containing stacks of axial isoscalar ( or ) domain walls. We also show that a quark-matter state known as “Goldstone current state,” in which a gradient of a Goldstone field is spontaneously generated, is ferromagnetic due to the axial anomaly. We estimate the size of the fields created by such a state in a typical neutron star to be of order .
- Received 19 October 2007
DOI:https://doi.org/10.1103/PhysRevD.77.014021
©2008 American Physical Society