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4.2 Phases of Strongly Interacting Matter
where this matter is visible, increases with decreasing
parton momentum fraction x. Saturation effects are
generic phenomena of all hadrons, including protons
and nuclei; however, due to the higher gluon numbers in
nuclei they are enhanced by a factor A 1/3 , and so is the
saturation scale. The experimentally accessible saturation
region increases substantially with centre-of-mass
energy (see Figure 13).
Investigations of this state will enter a completely new,
as yet unexplored regime of quantum field theory. It also
plays an important role in defining the initial conditions
for any high-energy hadronic interaction. Its investigation
will have far-reaching consequences in high-energy
physics.
Generally, saturated gluon matter reveals itself through
two characteristic signatures:
1. a modification of the momentum distributions of
gluons; and
2. a change from a collection of incoherent gluons to a
coherent state.
The standard approach to studying parton distribution
functions (PDFs) is via deep inelastic scattering (DIS) of
leptons. DIS can nicely constrain the parton kinematics
(i.e., Q 2 and x). However, it is only indirectly sensitive
to the gluon distribution since gluons carry no electric
charge. The PDFs can be tested in their entirety by measuring
the quark structure functions and using evolution
equations to extract information on the gluons.
On kinematic grounds, it is expected that the first
important information about the saturation region can
be obtained from proton-nucleus collisions at the LHC.
These studies will significantly reduce uncertainties in the
nuclear PDFs. In addition, p-A collisions are unique for
the study of strong interaction effects of the initial state,
e.g., multiple scattering. It is therefore vital to establish a
proton-nucleus collisions programme at the LHC.
Hadronic reactions directly probe the gluon distributions.
To test gluon saturation against the low-density
picture of point-like parton-parton scatterings, a reasonably
large transverse momentum p T of the produced
particles (optimally significantly larger than 1 GeV/c) is
needed. This will ensure that the momentum transfer Q
is large enough for perturbative QCD to be applicable as
a reference. For a given p T , the lowest value of x that can
be attained decreases with increasing collision energy
and increasing longitudinal momentum (or rapidity y h ) of
the produced hadron. To study saturated gluon matter it
is therefore advantageous to measure particle production
at large y in p+A collisions at the highest beam energies
available – measurements at forward angles 1 in these
reactions at the new LHC accelerator are thus optimally
suited for this purpose.
At hadron colliders, effects of the gluon saturation
can be seen in:
1. a suppression of inclusive hadron yields in a momentum
range where parton scattering is dominant; and in
2. a decrease and/or broadening of the azimuthal correlation
related to recoil jets from parton-parton
scattering.
Qualitatively such effects have been observed at
RHIC, in particular the suppression of hadrons produced
at forward rapidities. However, no consistent calculation
of the nuclear modification factor from a gluon saturation
model has been performed yet. Moreover, it is questionable
whether the small p T range studied at RHIC allows
for a unique interpretation of the suppression.
Measurements at the LHC with its much larger
dynamic range facilitate a breakthrough for such studies.
Figure 13 illustrates the kinematic reach as a function
of hadron p T and y at the RHIC and LHC accelerators.
At LHC, x-values are already smaller at a given rapidity,
such that the saturation region extends out to larger
p T -values. In addition, a much larger range in rapidity is
accessible at LHC. In general, the LHC will give access
to a significantly larger part of phase space dominated
by gluon saturation, and will in particular allow the use
of p T values high enough that perturbative QCD can be
used as a reference.
To measure the effects of gluon saturation in experiments,
one has to be sensitive to relatively low transverse
momenta for centrally produced particles, while at
Figure 13. Accessible phase space for hadron production as
a function of rapidity y h (see text) and transverse momentum p T,
in collider experiments at RHIC and LHC. The red areas indicate
estimates of the region where gluon saturation should be
observable (p T < Q s ).
1. Large rapidity corresponds to small angles relative to the beam
axis, i.e., the forward region.
98 | Perspectives of Nuclear Physics in Europe – NuPECC Long Range Plan 2010