exploring the interface of fundamental and effective approaches to extreme matter
Recommendations
This meeting brought together WG1, WG2 and the GDRE "Heavy Quarks" at
the Instituto Superior Técnico, Universidade de Lisboa, from Monday,
June 11, till Thursday, June 14, 2018.
This meeting followed the recommendations of the first WG1 and WG2
meetings to bring together the theoretical (in particular lattice QCD)
and phenomenological researchers represented in WG1 and WG2. Because
experimentalists are interested in this discussion, which has an
immediate influence on the interpretation of their experimental results,
the GDRE "Heavy Quark" joined this meeting as well. As a result the
second meeting had a larger scope than the first WG1/WG2 meetings and
covered the whole variety of topics treated by members of these WGs.
Besides the common parts, there were also parts dedicated to WG1 and WG2
only.
In this joint meeting of WG1 and WG2 we discussed topics of mutual
interest, with input from fundamental (lattice QCD, diagrammatics),
effective (effective field theory, transport theory, models) and
phenomenological approaches, as well as experimental data. The emphasis
for WG1 was on understanding the phase structure of QCD and related
theories, especially in non-standard scenarios, such as in the presence
of external magnetic fields or at nonzero baryon density, where standard
numerical algorithms break down. QCD-like theories are being employed to
investigate parts of the phase diagram not accessible in QCD itself and
new algorithms, especially for QCD at nonzero baryons, are critically
assessed.
The specific recommendations for WG1 that arose out of this meeting is
to further develop the study of QCD-like theories. This should be done
for a number of reasons, namely to
(a) gain insight into QCD itself;
(b) test and verify methods and effective theories as an approximation to QCD;
(c) study these theories in their own right, as they are of interest for
other phenomena in Nature.
A prime example for the latter is the theory of graphene, which can be
investigated using standard lattice QCD methods, but is of course highly
relevant in the context of materials.
In the joint session of WG1+WG2, this was supplemented by a review of
the theoretical approaches which relate properties of the QGP with
experimental observables, also beyond those involving heavy quarks.
The questions which we addressed included
(a) thermal crossover at small chemical potential - role of strange
hadrons? freeze-out patterns? lattice view? spectral modifications?
(b) transport coefficients - what is known theoretically? from the
lattice? experimentally?
(c) heavy quarks - stability of hidden charm
and bottom mesons? Evidence for survival of hidden charm mesons in a
QGP?
(d) role of fluctuations: what is relation between fluctuations
of conserved quantities (baryon number, strangeness, charge) obtained in
lattice calculations and the data observed in experiments;
(e) are
there other observables (dileptons, polarized Lambdas....) which may
carry information on the QGP and how we can interpret the experimental
results;
(f) how can the present theoretical models become more
realistic by including for example magnetic fields or new approaches to
the fragmentation function.
As far as heavy quarks are concerned, there
was a long discussion about what the lattice can provide and what the
numerical programmes, which simulate heavy-ion reactions in order to
interpret the experimental results, need to make more realistic
predictions. Especially the need of transport coefficients, not only at
zero momentum but as function of the particle momentum has been
discussed but one has also realised that to obtain these or similar
quantities on the lattice a time scale beyond the present COST
action is necessary.
For the physics not related to heavy quarks the meeting was a first
exchange of ideas which have been extensively discussed and criticised.
There, the recommendations have been made to include the elements of
criticism when advanced approaches will be presented at the next WG2
meeting.