The gluon exchange model simplifies the internal structure of protons and their collisions.

When a proton collides with a proton, gluons emitted from one valence quark can interact with virtual quarks from quark-antiquark pairs in the other proton. According to the GEM model, the result of such an interaction is a fast proton with an intact structure of valence quarks, and other particles (enclosed in white) created by the process occurring in the interaction region. Become. Credit: IFJ PAN / Dual Color

Inside each proton or neutron are three quarks bound by gluons. In the past, it was often assumed that two of them would form a “stable” pair called a diquark. But it seems to be the end of the physics diquark path. This is one of the conclusions of a new model of proton-proton or proton-nucleus collisions, taking into account the interaction of gluons with seas of virtual and antiquarks.

In physics, the emergence of new theoretical models is often severely exacerbated by old concepts. This also applies to the explanation of the collision of protons with protons or nuclei, proposed by scientists at the Institute of Nuclear Physics at the Polish Academy of Sciences (IFJ PAN) in Krakow. In modern models, the interaction of gluons emitted by one proton with a sea of ​​virtual or antiquarks plays an important role, appearing and disappearing inside another proton or neutron.

Gluons are strong force carriers that are one of the four fundamental forces of nature. This causes quarks to bind to composite structures such as protons and neutrons. In many respects, strong force is unlike any other. For example, it does not weaken, but grows with the distance between the particles. Moreover, unlike photons, gluons can carry certain types of charges (known as picturesque colors) and interact with each other.

Collision of a proton and two protons / neutrons in a carbon nucleus

According to the GEM model, one of the possible collisions between a proton and two protons / neutrons in a carbon nucleus. Two valence quarks of one proton exchange gluon and two valence quarks of protons / neutrons of carbon nuclei. The three incoming proton quarks are no longer neutral in color. These quarks, shown in red on the right, need to catch other particles from the interaction area (shown in white), resulting in secondary particles. Credit: IFJ PAN / Dual Color

Most nuclear reactions, including most of the collisions between protons and protons or nuclei, are processes in which particles “collide” with each other only by exchanging gluons. This type of collision, called soft by physicists, causes considerable problems because the theory that explains them cannot be calculated from first principles. Therefore, inevitably, all models of soft processes today are more or less phenomenological.

“At first, we just wanted to know how an existing tool, known as the dual parton model, handles more accurate experimental data on proton-proton and proton-carbon nuclei collisions,” says Professor Marek Jezabek. (IFJ PAN) recalls. “I quickly realized that I wasn’t dealing with it well, so let’s build on an older model that has been developed for over 40 years, one that is more accurate on the one hand and one that is closer to the nature of the phenomenon described on the other. I decided. “

The Gluon Exchange Model (GEM) built with IFJ PAN is also phenomenological. However, it is not based on similarity to other physical phenomena, but directly on the existence of quarks and gluons and their basic properties. In addition, GEM takes into account the presence of oceanic protons and neutrons, where virtual quark and antiquark pairs are constantly being generated and extinguished, as well as triplets of major (valence) quarks. In addition, it takes into account the limitations due to the principle of baryon number preservation. Simply put, the number of baryons (ie protons and neutrons) that exist before and after the interaction must remain unchanged. Since each quark has its own baryon number (equal to 1/3), this principle allows us to draw more reliable conclusions about what is happening with the quarks and the gluons exchanged between them. I can do it.

“GEM allowed us to explore new scenarios for a series of events involving protons and neutrons,” emphasizes Dr. Andrzej Rybicki (IFJ PAN) and explains in more detail. -Proton collision. One of the protons emits a gluon, which collides with the other protons. It is not the valence quark, but the quark from the virtual sea that exists for a moment. When such gluons are absorbed, the paired sea quarks and antiquarks are no longer virtual and become other particles in a particular final state. Note that in this scenario, new particles are formed even though one valence quark of the proton remains intact. “

The Cracow gluon model leads to interesting insights, two of which are particularly noteworthy. The first concerns the origin of diffracted protons observed in proton-proton collisions. These are fast protons that emerge from the collision site at a small angle. Previously, they could not be produced by the color change process, and it was believed that some other physical mechanism was involved in their production. Here, it was found that the existence of diffracted protons can be explained by the interaction of gluons emitted from one proton with quarks of another proton.

Another observation is not very interesting. Previously, when describing soft collisions, it was assumed that two of the three valence quarks of protons or neutrons would combine to form a “molecule” called a diquark. The hypothesis that the existence of diquarks is not indiscriminately guaranteed by all physicists, but the concept was widely used. This is currently subject to change. The GEM model faced experimental data explaining how protons collide with carbon nuclei and interact with more than one proton / neutron along the way. It turns out that in order to be consistent with the measurements, the new model should assume diquark decay in at least half of the cases.

“Therefore, there are many signs that proton or neutron diquarks are not strongly bound objects. Diquarks can only exist effectively as a random composition of two quarks forming so-called color antitriplets. It’s sexual, and whenever possible, it collapses quickly, “says Dr. Rybicki.

The Krakow model of gluon exchange describes a wide class of phenomena in a simpler and more consistent way than existing tools for explaining soft collisions.Current results presented in an article published in Physics Letter BHas an interesting effect on the phenomenon of antimatter-antimatter annihilation, where antiprotons can be extinguished by multiple protons / neutrons in the nucleus. Therefore, the authors have already developed the first preliminary proposal for making new measurements at CERN using antiproton beams.

See: “Gluon Exchange Model for Diffraction and Inelastic Collisions”, Marek Jeżabek and Andrzej Rybicki, March 15, 2021 Physics Letter B..
DOI: 10.1016 / j.physletb.2021.136200 The gluon exchange model simplifies the internal structure of protons and their collisions.

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