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:
at high energies / small bjorken - x , qcd predicts that gluons in a hadron wavefunction form a new state , the so - called color glass condensate ( cgc ) @xcite , for a review see @xcite and references therein .
the cornerstone of the cgc is the existence of a hard saturation scale @xmath4 at which nonlinear gluon recombination effects become important and start to balance gluon radiation .
the color glass condensate ( saturation ) approach to qcd at high energy has been very successful to describe a variety of processes at relativistic heavy ion collider ( rhic ) @xcite .
nevertheless , the importance of saturation effects is still disputable given that other approaches offered alternative descriptions . in order to test saturation physics and its relevance
, it seems therefore essential to consider various reactions in different kinematic regions at the large hadron collider ( lhc ) and future collider experiments .
here , we address the role of gluon saturation at lhc energy @xmath5 tev in hadrons and direct photon production in @xmath6 collisions within the light - cone color - dipole approach using various saturation models .
details of calculations can be found in ref .
the concept of saturation and the taming of the power - like rise of the gluon distribution at small bjorken - x was first addressed in ref .
@xcite in the double logarithmic approximation .
the actual calculation of higher - order corrections to the non - linear small - x evolution equations still remains as a challenge @xcite , see also @xcite and references therein .
thus , one may resort to a qcd - like model which incorporates the basic features of gluon saturation into the dipole - proton forward scattering amplitude , and provides predictions which will allow to test the validity of the treatment .
there are several parametrizations proposed in the literature which all give a good description of hera data but predict different saturation scales , see fig .
[ f : sat ] .
the details of saturation models used in fig .
[ f : sat ] can be found in ref .
. the main feature of these models is that for decreasing @xmath7 , the dipole amplitude saturates at smaller dipole sizes .
note that there is no unique definition for the saturation scale in literature .
we define the saturation scale @xmath8 as a energy scale at which the @xmath9 dipole scattering amplitude @xmath10 becomes sizable @xcite .     for various color - dipole models labeled with cgc @xcite , gbw @xcite , klr - ads / cft @xcite and aams - bk1 @xcite .
right : the total dipole - proton cross section @xmath11 at fixed @xmath12 in the various color - dipole models .
, title="fig:",width=6 ]   for various color - dipole models labeled with cgc @xcite , gbw @xcite , klr - ads / cft @xcite and aams - bk1 @xcite .
right : the total dipole - proton cross section @xmath11 at fixed @xmath12 in the various color - dipole models . , title="fig:",width=6 ]
the invariant cross - section of hadron and direct photon production can be calculated via the light - cone color - dipole factorization scheme @xcite , see also ref .
@xcite . in fig .
[ lhc - e1 ] , the differential cross - section of pion @xmath13 and direct photon @xmath14 ( photons radiated in hadronic collisions not via hadronic decays ) production at lhc are plotted versus rapidity at fixed transverse momenta @xmath15 and @xmath16 gev within various color - dipole models ( see fig .  [
f : sat ] ) .
it is seen that the discrepancies among various saturation color dipole model results can be about a factor of @xmath1 at moderate rapidities . at the kinematic limit , i.e. at very forward rapidities and higher @xmath17 where the differential cross - section approaches zero , kinematic constraints limit the parton phase space and saturation effects become less important . it is seen from fig .  [ lhc - e1 ] that for both hadron and photon production , away from the kinematic limit , at not very large @xmath18 and @xmath17 , a color - dipole model with larger saturation scale leads to a stronger peak at forward rapidity ( having in mind that the saturation scale is a dynamical function of @xmath7 , see fig .
[ f : sat ] ) .    in order to understand the _ relative _ importance of saturation effects at various rapidities
, we employ the semi - sat model fitted to hera data with the dipole - proton forward scattering amplitude @xcite : @xmath19^{\frac{1}{2\gamma_s}}. \label{nos}\ ] ] the parameter @xmath20 is defined for @xmath21 as @xmath22 , and for @xmath23 as @xmath24
. the value of other parameters of this model can be found in @xcite .
surprisingly , the fit obtained with such an oversimplified model is as good as for the other models with @xmath25 . in fig .
[ lhc - s1 ] we show , ratio of the two cross - sections for both pions and direct photon at lhc , calculated once with diffusion term and once without , i.e. @xmath26 .
it is seen that at forward rapidities , the diffusion term in the anomalous dimension is not important , since it gives similar results as with a fixed @xmath26 .
the preferred value of anomalous dimension @xmath27 at very forward rapidities is close to the one predicted from the bk equation @xcite .
it is well known that the saturation effects start being essential when the anomalous dimension reaches the value @xmath28 which is the case for forward rapidities ( see refs .
this indicates that direct photon and hadron production at different rapidities at lhc are rather sensitive to saturation .
collisions at lhc as a function of rapidity @xmath18 calculated with various color dipole models for various fixed @xmath17 .
[ lhc - e1],title="fig:",width=6 ]   collisions at lhc as a function of rapidity @xmath18 calculated with various color dipole models for various fixed @xmath17 .
[ lhc - e1],title="fig:",width=6 ]     in the semi - sat dipole model for pion and direct photon production at lhc energy ( @xmath5 tev ) in @xmath6 collisions at various rapidities .
right : the ratio of photon / pion production in @xmath6 collisions at lhc as a function of @xmath17 calculated with the gbw color dipole model .
[ lhc - s1 ] , title="fig:",width=6 ]   in the semi - sat dipole model for pion and direct photon production at lhc energy ( @xmath5 tev ) in @xmath6 collisions at various rapidities .
right : the ratio of photon / pion production in @xmath6 collisions at lhc as a function of @xmath17 calculated with the gbw color dipole model .
[ lhc - s1 ] , title="fig:",width=6 ]    direct photons can only be radiated from quarks , while hadrons can be produced by both gluons and quarks . at the lhc energy at midrapidity gluons
therefore the photon / pion ratio is significantly reduced toward midrapidity . however , at very forward rapidity , valence quarks become important and the photon / pion ratio rises . moreover , at high @xmath17 again valence quarks becomes important and we have a sharp rise of the photon / pion ratio . in fig .
[ lhc - s1 ] right panel , we show the photon / pion ratio @xmath29 as a function of @xmath17 at various rapidities within the gbw model and @xmath6 collisions
. the ratio @xmath29 can be as big as @xmath30 at very forward rapidities @xmath31 at lhc energy .
therefore , direct photon production extends to higher rapidities for a fixed @xmath17 , see figs .
[ lhc - s1 ] .
note that suppression of hadrons at very forward rapidity also ensures significant suppression of radiative decays of those hadrons .
therefore , measurements of direct photons at forward rapidities should be rather clean , as the background from radiative hadronic decays is significantly suppressed .
a.r . acknowledges the financial support from the alexander von humboldt foundation , bmbf ( germany ) , conicyt programa bicentenario psd-91 - 2006 , fondecyt grants 1090312 ( chile ) .
a.  h. mueller and j .- w .
qiu , nucl . phys . * b268 * , 427 ( 1986 ) ; l.  d. mclerran and r.  venugopalan , phys . rev . * d50 * , 2225 ( 1994 ) ; phys . rev . * d49 * , 3352 ( 1994 ) . j.  jalilian - marian and y.  v. kovchegov , prog . part . nucl . phys . * 56 * , 104 ( 2006 ) .
y. v. kovchegov , z. lu and a. h. rezaeian , phys . rev . *
d80 * , 074023 ( 2009 ) [ arxiv:0906.4197 ] .
e. iancu , k. itakura and s. munier , phys . lett . *
b590 * , 199 ( 2004 ) .
k. golec - biernat and m. wsthoff , phys . rev . * d60 * , 114023 ( 1999 ) .
b. z. kopeliovich , a. h. rezaeian , h. j. pirner and i. schmidt , _ phys .
* b653 * , 210 ( 2007)[arxiv:0704.0642 ] ; b.z .
kopeliovich , h. j. pirner , a. h. rezaeian and i. schmidt , _ phys . rev . _ * d77 * , 034011 ( 2008)[arxiv:0711.3010 ] ; b. z. kopeliovich , a. h. rezaeian , i. schmidt , _ phys . rev . _ * d78 * , 114009 ( 2008)[arxiv:0809.4327 ] ; _ nucl .
_ * a807 * , 61 ( 2008)[arxiv:0712.2829 ] ; arxiv:0804.2283
. b. z. kopeliovich , e. levin , a. h. rezaeian , i. schmidt , _ phys .
lett . _ * b675 * , 190 ( 2009)[arxiv:0902.4287 ] ; a. h. rezaeian _
et al . _ , arxiv:0707.2040 ; a. h. rezaeian and z. lu , _ nucl .
phys . _ * a826 * , 198 ( 2009)[arxiv:0810.4942 ] ; b. z. kopeliovich and a. h. rezaeian , int .
e18 * , 1629 ( 2009)[arxiv:0811.2024 ] .
g. watt and h. kowalski , phys .
rev . * d78 * , 014016 ( 2008 ) .
d. boer , a. utermann and e. wessels , phys . rev . * d75 * , 094022 ( 2007)[arxiv : hep - ph/0701219 ] . a. h. mueller and d.n .
triantafyllopoulos , nucl . phys .
* b640 * , 331 ( 2002 ) ; d. n. triantafyllopoulos , nucl . phys . * b648 * , 293 ( 2003 ) .
we investigate hadrons and direct photon production in pp collisions at the lhc energy within the color - dipole approach . 
 we show that greatest sensitivity to gluon saturation effects is reached at very forward rapidities in pp collisions at lhc ( @xmath0 ) . 
 the discrepancies among various saturation models ( fitted to hera data ) results can be about a factor of @xmath1 at forward rapidities . 
 we found that the ratio of direct - photon to pion production can be about @xmath2 at forward rapidities @xmath3 . 
 therefore , direct photon production at forward rapidities should provide a rather clean probe as the background from radiative hadronic decays is significantly suppressed .