  We consider a matching market where buyers and sellers arrive according to
independent Poisson processes at the same rate and independently abandon the
market if not matched after an exponential amount of time with the same mean.
In this centralized market, the utility for the system manager from matching
any buyer and any seller is a general random variable. We consider a sequence
of systems indexed by $n$ where the arrivals in the $n^{\mathrm{th}}$ system
are sped up by a factor of $n$. We analyze two families of one-parameter
policies: the population threshold policy immediately matches an arriving agent
to its best available mate only if the number of mates in the system is above a
threshold, and the utility threshold policy matches an arriving agent to its
best available mate only if the corresponding utility is above a threshold.
Using a fluid analysis of the two-dimensional Markov process of buyers and
sellers, we show that when the matching utility distribution is light-tailed,
the population threshold policy with threshold $\frac{n}{\ln n}$ is
asymptotically optimal among all policies that make matches only at agent
arrival epochs. In the heavy-tailed case, we characterize the optimal threshold
level for both policies. We also study the utility threshold policy in an
unbalanced matching market with heavy-tailed matching utilities and find that
the buyers and sellers have the same asymptotically optimal utility threshold.
We derive optimal thresholds when the matching utility distribution is
exponential, uniform, Pareto, and correlated Pareto. We find that as the right
tail of the matching utility distribution gets heavier, the threshold level of
each policy (and hence market thickness) increases, as does the magnitude by
which the utility threshold policy outperforms the population threshold policy.
