Decay of Color Gauge Fields in Heavy Ion Collisions and Nielsen-Olesen Instability

We analyze the behavior of unstable modes in the glasma produced in high-energy heavy-ion collisions, using a simple model with effective homogeneous longitudinal color electric and magnetic fields. The unstable modes are approximately described as Nielsen-Olesen unstable modes under the homogeneous longitudinal gauge fields. We find that the Nielsen-Olesen unstable modes show properties very similar to those of the exponentially increasing unstable modes in the glasma recently demonstrated by Romatschke and Venugopalan. Although initial gauge fields in the glasma are much stronger than those in our model, they decay with the production of Nielsen-Olesen unstable modes. We discuss why we can reproduce the features of the glasma effectively by using homogeneous weak magnetic fields. Our analysis supports the idea that the decay of the gauge fields in the glasma is caused by Nielsen-Olesen instability.