You will be presented with the full text of a science research paper. Please write an abstract for this paper. Your response should include the abstract and no additional text.

Paper text:
when probing small distances inside a hadron or nucleus with a hard process , one resolves their partonic constituents .
increasing the energy of the scattering process at a fixed momentum transfer allows to probe lower - energy partons , with smaller energy fraction @xmath2 as the parton densities in the hadronic / nuclear wavefunction grow with decreasing @xmath3 they eventually become so large that a non - linear ( yet weakly - coupled ) regime is reached , called saturation , where partons do not interact with the probe independently anymore , but rather behave coherently .
the color glass condensate ( cgc ) is an effective theory of qcd @xcite which aims at describing this part of the wave function . rather than using a standard fock - state decomposition , it is more efficient to describe it with collective degrees of freedom , more adapted to account for the collective behavior of the small-@xmath0 gluons .
the cgc approach uses classical color fields : @xmath4\ |{\cal a}\rangle \label{cgc}\ .\ ] ] the long - lived , large-@xmath0 partons are represented by a strong color source @xmath5 which is static during the lifetime of the short - lived small-@xmath0 gluons , whose dynamics is described by the color field @xmath6 the arbitrary separation between the field and the source is denoted @xmath7    the cgc wavefunction @xmath8 $ ] is the fundamental object of this picture , it is mainly a non - perturbative quantity , but the @xmath9 evolution can be computed perturbatively . requiring that observables are independent of the choice of @xmath10 a functional renormalization group equation can be derived . in the leading - logarithmic approximation which resums powers of @xmath11
the jimwlk equation describes the evolution of @xmath12|^2 $ ] with @xmath7 the information contained in the wavefunction , on gluon number and gluon correlations , can be expressed in terms of n - point correlators , probed in scattering processes .
these correlators consist of wilson lines averaged with the cgc wavefunction , and resum powers of @xmath13    the jimwlk equation reduces to a hierarchy of equations for the correlators .
most of the phenomenology uses a mean - field approximation which significantly simplifies the high - energy qcd evolution : it reduces the hierarchy to a single closed non - linear equation for the two - point function @xmath14 the balitsky - kovchegov ( bk ) equation @xcite .
it reads @xmath15 with @xmath16 all the correlators can then be expressed in terms of the solution of this equation . finally
, the fourier transform of the dipole correlator @xmath17 is an ( all - twist ) unintegrated gluon density .
it determines forward particle production @xcite , while more exclusive final states involve more complicated correlators . solving eq .   reveals the existence of an intrisic momentum scale in the nuclear wave function : the saturation scale @xmath18 which characterizes the transition from the dilute regime @xmath19 to saturation regime @xmath20    one of the most important progress is the recent calculation of the next - to - leading evolution equation @xcite .
concerning how the running coupling should be included , two schemes have been proposed by balitsky ( b ) and kovchegov and weigert ( kw ) .
the following substitution should be done in formula , with @xmath21 given in @xcite : @xmath22\ ] ] @xmath23\ .\ ] ] at next - to - leading order , there remains a discrepancy between the linear part of the bk equation and the bfkl equation .
running coupling corrections to particle production have also been investigated @xcite .
another important recent theoretical development is the inclusion of pomeron loops in the evolution @xcite , and the derivation of potential consequences at very high energies @xcite .
concerning phenomenology at present colliders , there was however no significant impact .
forward particle production in pa collisions allows to investigate the non linear qcd dynamics of high - energy nuclei with a probe well understood in qcd .
indeed , while such processes are probing small - momentum partons in the nuclear wavefunction , only high - momentum partons of the proton contribute to the scattering ( @xmath24 and @xmath25 ) .
the dilute hadron contributes via standard parton distribution functions while the cgc is described by its unintegrated gluon distribution .
it was not obvious that the cgc picture , which requires small values of @xmath10 would be relevant at present energies .
however , it has been the case for many observables in the context of hera @xcite and rhic @xcite .
one of the most acclaimed successes is the prediction that the yield of high-@xmath1 particles at forward rapidities in d+au collisions is suppressed compared to a pp collisions , and should decrease when increasing the rapidity .    in fig.1 the @xmath26 @xmath1 spectra computed in the cgc approach @xcite
is compared to rhic data , and the description of the slope is impressive .
the need of k factors to describe the normalization could be expected since this is a leading - order based calculation . improving the calculation with the next - leading evolution
has yet to be done . while the suppression was predicted in the cgc approach , other postdictions later offered alternative descriptions .
the idea is that the value of @xmath0 probed in the deuteron is so high that large-@xmath0 effects could be responsible for the suppression @xcite .
this would not happen in @xmath27 collisions at the lhc , with a smaller @xmath28    while the cgc framework was quite successful in describing single inclusive particle production at forward rapidities , the focus should now shift towards more exclusive observables like two - particle production @xmath29 in particular the correlations in azimuthal angle between the produced hadrons should be suppressed compared to pp collisions @xcite . by contrast with single particle production , in two - particle production
the cgc can not be described only by its unintegrated gluon distribution , the so - called @xmath30-factorization framework is not applicable .
this means that more tests could be done , probing the cgc structure deeper .
the second d+au run at rhic gives the opportunity to carry out such measurements .
mueller - navelet jets @xcite in hadron - hadron scattering are two jets produced in each of the forward directions . in the high - energy regime ,
in which the jets are separated by a large rapidity interval , this process is sensitive to the small-@xmath0 qcd evolution .
an interesting observable is the azimuthal decorrelation of the jets as a function of their rapidity separation @xmath31 and of the ratio of their transverse momenta @xmath32 @xcite .
predictions are shown in fig.2 , for tevatron and lhc kinematics , where @xmath33 is the relative azimuthal angle between the two jets .
the curves are obtained in the linear regime , using next - to - leading logarithmic ( nll ) bfkl evolution . at higher energies
, saturation effects will also be relevant @xcite .
coming back to forward particle production in pa collisions , predictions for the process @xmath34 are shown in fig.3 , for rhic and the lhc @xcite .
@xmath35 @xmath36 and @xmath37 @xmath38 are the transverse momenta and rapidities of the final state hadrons , and the azimuthal angle spectra are displayed .
it is obtained that the perturbative back - to - back peak of the azimuthal angle distribution ( which is recovered for very large momenta ) is reduced by initial state saturation effects . as the momenta decrease closer to the saturation scale ( @xmath39 ) , the angular distribution broadens .
but at rhic energies , saturation does not lead to a complete disappearance of the back - to - back peak .
finally , predictions for the total charged - particle multiplicity in aa collisions at the lhc are shown in fig.4 .
two approaches are compared : in the first , @xmath30-factorization is assumed but the evolution of the unintegrated gluon densities is accurately obtained from the next - leading bk equation @xcite ; in the second , the x evolution is only parametrized but multiple scatterings are correctly taken into account by solving classical yang - mills equation @xcite . while full next - leading treatment of both multiple scatterings and small-@xmath0 evolution is desirable , the numbers obtained are similar , which indicates that the uncertainties in both approaches are under control .
the color glass condensate picture of the nuclear wave function at small-@xmath0 successfully predicted the suppressed production of high-@xmath1 particles at forward rapidities in deuteron - gold collisions at rhic . 
 this triggered more efforts which resulted in theoretical improvements and predictions for different observables which will provide further phenomenological tests . 
 i review recent theoretical developments and discuss the resulting predictions .