temperature-chemical potential plane where the distinction vanishes is called the tricritical point. It is associated with a second-order phase transition, and critical fluctuations. There is some chance that the effects of such fluctuations could lead to observable consequences in heavy ion collisions.

The challenge for lattice gauge theory is very clear and concrete: locate this point! It is a notable landmark in the QCD phase diagram, and knowing its location would be very helpful for organizing the experimental exploration of that diagram.

There is probably a close relationship between this physical tricritical point and a more theoretical tricritical point implicit in our earlier discussions. That one arises in considering behavior in the temperature-$m_s$ (strange quark mass) plane. For very small $m_s$ one expects a first-order transition, which weakens as $m_s$ increases, and vanishes altogether at some critical $m_s^{\rm crit.}$. A very interesting possibility is that the theoretical tricritical point evolves into the physical one, moving off into nonzero values of $\mu$ above $m_s^{\rm crit.}$. If so, it ought to be possible to locate the physical tricritical point for values of $m_s$ just above $m_s^{\rm crit.}$ by exploiting small $\mu/T$ techniques.

### Measure Critical Behavior

The renormalization group analysis of QCD 