Topic 2: Small x physics & the Color Glass Condensate
Description and objectives:
The theory called Quantum Chromodynamics (QCD) describes the physics
of quarks and gluons,
which are the building blocks of hadronic matter: protons, neutrons,
etc. In recent years attention
has turned to the behavior of QCD in extreme circumstances, such as
those of high gluon density
ultra-relativistic collisions between
heavy ions. Theory predicts that in such collisions the gluon
density in one heavy ion as
seen by the particle that strikes
it, will saturate forming an unusual state that has been named the Color
Glass Condensate (CGC).
Experiments that could be
sensitive to the presence and properties of the CGC can be performed in
present-day and future
colliders (RHIC, LHC, EIC). Therefore, observable consequences of the
CGC need to be worked
out in detail. The CGC has its own characteristic momentum scale,
called the saturation
but how to extract it unambiguously from experiment? Also, the change
of the CGC with energy is
predicted by theory using various approximations and limits, they all
reach the conclusion that at asymptotically high energies there will be
scaling behavior, geometric scaling.
A main question is: is there scaling or the onset of scaling at
presently accessible energies? The phenomenology of the CGC in high
energy scattering at small
parton momentum fraction x presents many open questions still.
- A personal FOM 'projectruimte' grant, 2004-2009, "Exposing the
color glass condensate"
- Investigation of geometric scaling, a property of gluon
saturation at very small x. A new model for the dipole cross section
was proposed to help answer the question whether geometric scaling
already occurs at LHC.
- Comparison of certain successful phenomenological dipole cross
sections (employed in the description of small x
observables) with the Balitsky-Kovchegov evolution equation that
follows from theory. Notable differences were found and suggestions for
improvements were given.
- First study of small-x effects on single transverse-spin
asymmetries in forward pion production at high energy,
within the color glass condensate formalism. Predictions for RHIC
collisions at 500 GeV were given.
- The first calculation of the consequences of gluon saturation in
proton-heavy ion collisions on a spin dependent observable: polarized
Lambda hyperon production in hadron-nucleus collisions. It was found
that the polarization peaks at a momentum around the saturation scale,
providing a direct probe of saturation.
- Article by DaniŽl Boer, Andre Utermann, Erik Wessels, "The
Saturation scale and its x-dependence from Lambda
polarization studies", published in Physics Letters B 671 (2009) 91-98.
- Ph.D. Thesis of Erik Wessels "Signatures of gluon saturation in
high energy scattering ", June 2009.
- DIS2008 proceedings contribution "About Geometric Scaling and
Small-x Evolution at RHIC and LHC" by Andre Utermann
- Article by DaniŽl Boer, Andre Utermann, Erik Wessels, "Geometric
Scaling at RHIC and LHC", published in
Physics Review D 77 (2008) 054014 (8 pp.)
- Proceedings contribution "Investigating the extended geometric
scaling region at LHC with polarized and unpolarized final states" by
DaniŽl Boer, Andre Utermann, Erik Wessels, published in J. Phys. G 35
- DIS2007 proceedings contribution "x-Evolution of Phenomenological
Dipole Cross Sections" by DaniŽl Boer, Andre Utermann, Erik Wessels.
- Article by DaniŽl Boer, Andre Utermann, Erik Wessels,
"Compatibility of phenomenological dipole cross sections
with the Balitsky-Kovchegov equation", published in Physical Review D
75 (2007) 094022 (11 pp.)
- SPIN06 proceedings contribution "Gluon saturation effects on
single spin asymmetries" by DaniŽl Boer.
- Article by DaniŽl Boer, Adrian Dumitru, Arata Hayashigaki,
"Single transverse-spin asymmetries in forward pion
production at high energy: incorporating small-x effects in the
target", published in Physical Review D 74 (2006) 074018 (11 pp.)
- Article by DaniŽl Boer, Adrian Dumitru, "Polarized hyperons from
pA scattering in the gluon saturation regime",
published in Physics Letters B 556 (2003) 33-40.