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 occurring in 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 scale, 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.
Funding:
- A personal FOM 'projectruimte' grant, 2004-2009, "Exposing the color glass condensate"
Main results:
- 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.
Output:
- Article by Daniël 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 Daniël 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 Daniël Boer, Andre Utermann, Erik Wessels, published in J. Phys. G 35 (2008) 054001.
- DIS2007 proceedings contribution "x-Evolution of Phenomenological Dipole Cross Sections" by Daniël Boer, Andre Utermann, Erik Wessels.
- Article by Daniël 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 Daniël Boer.
- Article by Daniël 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 Daniël Boer, Adrian Dumitru, "Polarized hyperons from pA scattering in the gluon saturation regime", published in Physics Letters B 556 (2003) 33-40.