Direct Photons are Seen in Heavy-Ion Collisions

In high energy heavy-ion collisions heated nuclear matter, both the original protons and neutrons (or maybe even their constituent quarks) as well as additional particles created out of the excess energy, can be thought of as a hot gas. High energy gamma photons have been observed, as expected, from the decay of particles exiting the hot nuclear gas. But gammas are also expected to be emitted at a lower rate as a kind of thermal glow from the interactions of the particles in the gas.

Such "direct photons" have now been seen for the first time in an experiment conducted at CERN, where lead ions smashed into a stationary lead target (contact Terry Awes, Oak Ridge Natl.Lab, 865-574-4587, awes@mail.phy.ornl.gov). Some theorists believe that direct photons, like the suppression of psi mesons or an enhancement in the production of strange mesons (Update 470), might constitute evidence for the production of quark gluon plasma. (Aggarwal et al., Physical Review Letters, 2 Oct; Select Articles.)