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Study Offers New Information Related to Gluon Dynamics in Proton

New outcomes of a study by experimental nuclear physicists Daniel Tapia Takaki and Aleksandr (Sasha) Bylinkin from The University of Kansas have recently been reported in the European Physical Journal C.

Daniel Tapia Takaki of The University of Kansas at work at the Large Hadron Collider’s Compact Muon Solenoid. (Image credit: Tapia Takaki)

The study focuses on research at the Compact Muon Solenoid, an experiment at the Large Hadron Collider, to gain better insights into the behavior of gluons.

Gluons are elementary particles that act to “glue” quarks and anti-quarks together to form neutrons and protons—therefore, gluons have a vital role to play in nearly 98% of all the visible matter in the universe.

Earlier experiments performed at the HERA electron-proton collider, which is now decommissioned, discovered that upon accelerating protons close to the speed of light, the density of gluons within them increases very quickly.

In these cases, gluons split into pairs of gluons with lower energies, and such gluons split themselves subsequently, and so forth,” stated Tapia Takaki, KU associate professor of physics and astronomy. “At some point, the splitting of gluons inside the proton reaches a limit at which the multiplication of gluons ceases to increase. Such a state is known as the ‘color glass condensate’, a hypothesized phase of matter that is thought to exist in very high-energy protons and as well as in heavy nuclei.”

The KU researcher stated that the more recent experimental results achieved by his team at the Relativistic Heavy Ion Collider and LHC appeared to confirm the occurrence of such a gluon-dominated state. The precise conditions and the accurate energy required to notice “gluon saturation” in the proton or in heavy nuclei are still obscure, he added.

The CMS experimental results are very exciting, giving new information about the gluon dynamics in the proton. The data tell us what the energy and dipole sizes are needed to get deeper into the gluonic-dominated regime where nonlinear QCD effects become dominant.

Victor Goncalves, Professor of Physics, Federal University of Pelotas, Brazil

Goncalves was working at KU under a Brazil-U.S. Professorship collaboratively given by the Sociedade Brasileira de Física and the American Physical Society.

Experiments at the LHC do not perform a direct investigation of the interaction of the proton with elementary particles, for example, those of the late HERA collider. However, it is feasible to use an alternative technique to analyze gluon saturation.

Photon interactions with the proton (or ion) take place when accelerated protons (or the ions) miss each other. These near misses are known as ultra-peripheral collisions (UPCs) since the photon interactions largely take place if the colliding particles are separated from each other considerably.

The idea that the electric charge of the proton or ions, when accelerated at ultra-relativistic velocities, will provide a source of quasi-real photons is not new. It was first discussed by Enrico Fermi in the late 1920s. But it’s only since the 2000s at the RHIC collider and more recently at the LHC experiments where this method has been fully exploited.

Tapia Takaki, Associate Professor of Physics and Astronomy, The University of Kansas

Tapia Takaki’s team has had a vital role to play in analyzing ultra-peripheral collisions of protons and ions at two instruments at the Large Hadron Collider, initially at the ALICE Collaboration and more recently with the CMS detector.

We have now a plethora of interesting results on ultra-peripheral heavy-ion collisions at the CERN’s Large Hadron Collider,” stated Bylinkin, an associate researcher in the group. “Most of the results have been focused on integrated cross-sections of vector mesons and more recently on measurements using jets and studying light-by-light scattering.”

Bylinkin continued, “For the study of vector meson production, we are now doing systematic measurements, not only exploratory ones. We are particularly interested in the energy dependence study of the momentum transfer in vector meson production since here we have the unique opportunity to pin down the onset of gluon saturation.”

According to the researchers, the study is crucial since it is the first one to establish four measured points with respect to the energy of the photon-proton interaction and as a function of the transfer of momentum.

Previous experiments at HERA only had one single point in energy. For our recent result, the lowest point in energy is about 35 GeV and the highest one is about 180 GeV. This does not sound like a very high energy point, considering that for recent J/psi and Upsilon measurements from UPCs at the LHC we have studied processes up to the 1000s GeV.

Tapia Takaki, Associate Professor of Physics and Astronomy, The University of Kansas

Takaki added, “The key point here is that although the energy is much lower in our Rho0 studies, the dipole size is very large.”

The researchers said that several questions are still unsolved in their line of study to gain better insights into the composition of neutrons and protons.

According to Goncalves, “We know that at the HERA collider there were already hints for nonlinear QCD effects, but there are many theoretical questions that have not been answered such as the onset of gluon saturation, and there are at least two main saturation models that we don’t know yet which one is the closest to what nature says the proton is.”

Goncalves continued, “We’ve used the latest results from the CMS collaboration and compared them to both the linear and nonlinear QCD-inspired models. We observed, for the first time, that the CMS data show a clear deviation from the linear QCD model at their highest energy point.”


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