Near misses at Large Hadron Collider shed light on the onset of gluon-dominated protons

Near misses at Large Hadron Collider shed light on the onset of gluon-dominated protons
Credit: KU

New findings from University of Kansas experimental nuclear physicists Daniel Tapia Takaki and Aleksandr (Sasha) Bylinkin were just published in the European Physical Journal C. The paper centers on work at the Compact Muon Solenoid, an experiment at the Large Hadron Collider, to better understand the behavior of gluons.

Gluons are that are responsible for "gluing" together quarks and anti-quarks to form protons and neutrons—so, gluons play a role in about 98% of all the visible matter in the universe.

Previous experiments at the now-decommissioned HERA electron-proton collider found when protons are accelerated close to light-speed, the density of gluons inside them increases very rapidly.

"In these cases, gluons split into pairs of gluons with lower energies, and such gluons split themselves subsequently, and so forth," said Tapia Takaki, KU associate professor of physics & 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- protons and as well as in heavy nuclei."

The KU researcher said his team's more recent experimental results at the Relativistic Heavy Ion Collider and LHC seemed to confirm the existence of such a -dominated state. The exact conditions and the precise energy needed to observe "gluon saturation" in the proton or in heavy nuclei are not yet known, he said.

"The CMS experimental results are very exciting, giving new information about the gluon dynamics in the proton," said Victor Goncalves, professor of physics at Federal University of Pelotas in Brazil, who was working at KU under a Brazil-U.S. Professorship given jointly by the Sociedade Brasileira de Física and the American Physical Society. "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."

Although experiments at the LHC don't directly study interaction of the proton with elementary particles such as those of the late HERA collider, it's possible to use an alternative method to study gluon saturation. When accelerated protons (or ions) miss each other, photon interactions occur with the proton (or the ion). These near misses are called ultra-peripheral collisions (UPCs) as the photon interactions mostly occur when the colliding particles are significantly separated from each other.

Near misses at Large Hadron Collider shed light on the onset of gluon-dominated protons
Daniel Tapia Takaki of the University of Kansas at work at the Large Hadron Collider's Compact Muon Solenoid. Credit: Tapia Takaki

"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," Tapia Takaki said. "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's group has played a significant role in the study of ultra-peripheral collisions of ions and protons at two instruments at the Large Hadron Collider, first 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," said 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. 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."

The researchers said the work is significant because it's the first establishment of four measured points in terms of the energy of the photon-proton interaction and as a function of the momentum transfer.

"Previous experiments at HERA only had one single point in energy," Tapia Takaki said. "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. The key point here is that although the energy is much lower in our Rho0 studies, the dipole size is very large."

According to team members, many questions remain unanswered in their line of research to better understand the makeup of protons and neutrons.

"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 is," said Goncalves. "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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More information: A.M. Sirunyan et al. Measurement of exclusive ρ(770)0 photoproduction in ultraperipheral pPb collisions at √sNN=5.02TeVThe European Physical Journal C (2019). DOI: 10.1140/epjc/s10052-019-7202-9
Citation: Near misses at Large Hadron Collider shed light on the onset of gluon-dominated protons (2019, September 10) retrieved 18 September 2019 from https://phys.org/news/2019-09-large-hadron-collider-onset-gluon-dominated.html
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Sep 10, 2019
This answers a question I had on the Higgs boson. They needed gluons to make the Higgs but gluons were not available on their own and they used protons instead. Weird, I thought, sure protons have some gluons, but only a few.
"Gluon saturation", Gluons double, then double again, then double again as they approach lightspeed. The LHC reaches six nines (99.9999%) so the protons end up being mostly gluons instead of just a few.
Quantum effects are non intuitive once again.

Sep 11, 2019
They are messing with Mother Nature. It's no wonder that I am noticing more and more "twinning" of words, even in science articles and papers by scientists that haven't been edited yet. I now have many thousands of examples. Most often the errors are such as: the the; of of; on on; toward toward, and many others, including 3 and 4 word twinnings. I could be wrong but I believe that it has to do with the breaking down of subatomic particles at the LHC and other machines. It happens too often to be accidental and due to carelessness.

Sep 11, 2019
The interesting thing here is to note that simply making protons go faster makes them look like they have more gluons, from the perspective of an oncoming particle.

Might want to think about that a little while.

Sep 11, 2019
They are messing with Mother Nature. It's no wonder that I am noticing more and more "twinning" of words, even in science articles and papers by scientists that haven't been edited yet. I now have many thousands of examples. Most often the errors are such as: the the; of of; on on; toward toward, and many others, including 3 and 4 word twinnings. I could be wrong but I believe that it has to do with the breaking down of subatomic particles at the LHC and other machines. It happens too often to be accidental and due to carelessness.

So your theory is that breaking down of subatomic particles causes humans to do typographical errors? Or is it so that the breaking down of subatomic particles causes interaction with computers so that the computer code changes in a such way that some words double? Either way, I'd like to have a deeper explanation.

Sep 11, 2019
@DaS Note that its the force quanta that are multiplied here, not the colour-charge-particles.
Its the gluon force/effect in space-time caused by the proximity of the colour particles (quarks) which is being multiplied by a relativistic effect.
This says something pretty interesting about spacetime and the quantum fields it embodies; that the fields are essentially relativistic in nature.
We already knew that the colour particles themselves are relativistic in nature too as Dirac put relativity into his equation. But the particles and the field quanta are subtly different somehow and respond to relativity differently.

Sep 11, 2019
@Eye, colored gluons are so called because they can convey color charge changes between quarks. Photons cannot change the polarities of EM charges. This is another thing you might want to keep in mind.

Sep 11, 2019
Are the authors of this article sure that gluons are MULTIPLIED by relativity??
Photons, the EM force quanta, red/blue-doppler-shift classically under relativity.
If EM force quanta and QCD colour force quanta act differently that is interesting: It must be because the gluons have mass but photons don't. But then why do gluons and quarks differ as they both have mass, and carry elements of the same QCD charge??

Sep 11, 2019
Because gluons are bosons, and quarks are fermions.

Sep 11, 2019
"The interesting thing here is to note that simply making protons go faster makes them look like they have more gluons, from the perspective of an oncoming particle.

Might want to think about that a little while. "

Thanks for pointing that out Da Scheib. A bit mind bending. So if I were traveling alongside the proton I would not observe any splitting.

I never considered relativistic effects of quarks before. They have electric charge so it makes sense there would be some relativistic effects there. I suppose it's not too far fetched to assume some kind of effects with the color charge as well.

Very thought provoking.


Sep 11, 2019
So if I were traveling alongside the proton I would not observe any splitting.
At least that's how I interpret it. It's not all that surprising all things considered, though I agree it's pretty mind-bending. If you think about the stress-energy tensor you'll see why it must be so.

Sep 11, 2019
Because gluons are bosons, and quarks are fermions.
How does that explain why gluons multiply but quarks don't; within the same hadron at the same velocity??
Why would their differing spins, and differing relations with the Higgs field, cause such a radically different relativistic effects??

Sep 14, 2019
Because gluons are bosons, and quarks are fermions.
How does that explain why gluons multiply but quarks don't; within the same hadron at the same velocity??
Why would their differing spins, and differing relations with the Higgs field, cause such a radically different relativistic effects??

Does the LHC have the energies to add the requisite quarks to form tetra and pentaquarks? For example, an extra charm and anti-charm, as required by the one described here - https://home.cern...articles . They also appear to be quite elusive, so perhaps they haven't been looking in the way necessary to see them.

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