HIP Theory Project: QCD
Detailed research topics
QCD renormalization group evolution
Calculating the properties of QCD matter and bound states at rest is a fully
nonperturbative problem. However,
their dependence on the external energy and transverse momentum
in a scattering process
can be computed from renormalization group (RG) evolution equations
derived in weak coupling, starting from a nonperturbative initial condition.
The RG equation for the dependence on transverse momentum is known as the DGLAP equation.
Using it to obtain parton distribution functions (PDF's) for nuclei is
a specialty of the Jyväskylä group. On this front our current focus
includes incorporating neutrino DIS and new LHC proton-lead data into the
global analysis.
Our
EPS09(s) PDF's
presently define the state of the art in the field.
At high energy, or equivalently small x, the phase space density of
gluons becomes so large that the nonlinearities of
QCD dominate its behavior. The relevant RG equations in this regime are
the BK and JIMWLK equations, which have been intensively studied by
both the Helsinki and Jyväskylä groups.
In particular these groups have the most advanced
techniques for numerical solutions
of the JIMWLK equation.
These studies have direct phenomenological applications in calculating
particle production and multiparticle correlations in p+p and
p+Pb collisions at the LHC.
Thermalization of strongly interacting matter
Experimental evidence from ultrarelativistic heavy ion collisions
points strongly towards the creation of a nearly equilibrated
quark gluon plasma. At present there is, however, no consistent quantitative
description in QCD of how the thermalization required for a hydrodynamical
description would occur in the
initial stage of the collision.
Understanding this early thermalization is a major
outstanding open conceptual issue in heavy ion collisions. The time
development of the early universe also has many situations, such
as thermalization after inflation and baryogenesis, that require
calculations in nonequilibrium field theory.
Nonequilibrium phenomena in gauge theory are a recognized area of
expertize of the Helsinki group. Computing the properties of produced
QCD matter in the
initial stage of a heavy ion collision is a central part of the
physics program of the Finnish QCD
groups.
This line of research will be continued towards a better picture
of the initial stage of a heavy ion collision on one hand and exploring the
cosmological applications on the other hand.
Spacetime development and hard probes of a heavy ion collision
Hydrodynamical studies are in practice the most important way to connect
calculated properties of hot QCD matter to experimental measurements
in heavy ion collisions. The Jyväskylä group has pioneered the application
of relativistic hydrodynamics to URHIC phenomenology.
Recently work has concentrated on deriving the correct second order
viscous relativistic hydrodynamical equations of motion from kinetic
theory and on incorporating
event-by-event fluctuations}.
An ongoing effort is to use this theoretical foundation to
develop a full 3+1-dimensional viscous hydrodynamical
model of a heavy ion collision, including event-by-event fluctuations.
We also specialize in calculations of
the production of thermal photons, whose experimentally observed strong
azimuthal anisotropy is among the outstanding problems in the
field.
Hard probes of QCD matter are high transverse momentum processes whose primary
production cross sections can be computed perturbatively. From the measured
cross sections one can then infer properties of the QCD matter through
which these hard partons pass.
We will continue the development of Monte Carlo simulations, in particular
the YaJEM code,
for in-medium QCD parton shower evolution.
The Monte Carlo description is necessary for a meaningful comparison
of theory and experimentally reconstructed jet observables, including
especially correlations between jets, hadrons and electromagnetic
probes.
