Searching for the Next Yukawa Phase of QCD ∗

QCD predicts that the interactions between quarks and gluons change from a confining to a screened Yukawa form above a critical temperature Tc ∼ 150 MeV. In this talk, I review some of the key observables in heavy ion reactions which are being used to search for this new partonic Yukawa phase at SPS and RHIC. These include collective observables such as dE⊥/dyd p⊥, meson interferometry, jet quenching, and J/ψ suppression. ∗ Talk to be published in the Proc. of 14th Nishinomiya-Yukawa Memorial Symposium Nov. 1999, Japan; updated with a critique of the CERN press release Feb. 8, 2000 1 typeset using PTPTEX.sty §1. The Ubiquitous Yukawa In 1935 H. Yukawa 1) proposed a theory of nuclear forces based on the exchange of a massive boson VY (r,m) = αeff e r (1.1) He estimated that m ∼ 100 MeV to account for the short range ∼ 2 fm, and in 1947 Powell discovered the pion and confired this theory. An important theoretical precursor was the Klein-Gordon equation , whose static Green’s function is (1.1). Elaboration of Yukawa’s meson theory since then (including spin, orbit, isospin vertex factors) forms the basis for the present effective theory of nuclear forces 3), . In the unrelated field of electrolytes, Debye and Hückle had also come across the Yukawa potential in another context. It emerged from solving the Poisson equation in a conducting medium . Assuming that local charge density fluctuations occur with a Boltzmann probability, exp(−qφ(x)/T ), the polarizability of the medium in the presence of an external charge density, leads to a non-linear self consistent equation ∇φ(x) = −4π(ρex(x) + ∑ q qnqe −qφ(x)/T ) . (1.2) In the linearized approximation, the solution for a point charge is again eq. (1.1), but in this case the effective mass is the Debye electric screening mass