The ATLAS experiment reports the fundamental properties of the strong interactions between protons at extremely high energies

Elastic scattering measurements at hadron colliders allow a unique experimental approach to non-perturbation dynamics, which cannot be computed from first principles. In a remarkable run of the LHC, proton–proton elastic scattering events were recorded with integrated luminance using ATLAS’s ALFA detector.

As part of the ATLAS Collaboration, physicists from the Institute of Nuclear Physics of the Polish Academy of Sciences measured elastic scattering in proton-proton collisions at the LHC accelerator at 13 center of mass energies. TeV. The measurements require a specialized system due to the small scattering angles in such interactions (less than one thousandth of a degree).

Its main component is a set of detectors located more than 200 meters from the impact site that can measure protons dispersed just a few millimeters from the accelerator beam.

This is made possible by a technique called Roman pots. This technique allows detectors to be placed inside the beam tube of the accelerator and close to the beam during data acquisition. An essential contribution of scientists is to the study of the activation and data collection systems, which are necessary for data to be recorded.

The specific arrangement of the magnetic field forming the LHC accelerator beam is the second essential component of the experimental setup. Maximizing beam focus is the goal in most measurements to increase the frequency of interesting interactions. However, the very large angular divergence of the closely focused beams makes the elastic scattering measurement extremely difficult. This difference is reduced by the unique magnet design, which also ensures accurate measurements.

The distribution of the scattering angle, or more precisely the distribution of the variable t, proportional to the square of that angle, is a direct result of the measurement, reported in the European Journal of Physics C. Shape of the distribution allows inferences about the fundamental characteristics of the strong nuclear interactions between protons at very high energies. This method of gathering information relies on the quantum properties of elastic scattering, which are not present in the game of snooker.

The so-called optical theorem, which is the result of the conservation of probability in quantum processes, is the first of these features. It compares elastic interactions with inelastic interactions, or interactions that lead to the creation of new particles. Because of the high energies of the protons involved in the collisions being studied, inelastic events often occur. The entire cross-sectional value can be calculated using the data of unique elastic interactions thanks to the optical theorem.

In particle physics, cross-section is a quantity used to represent the likelihood of a certain reaction. The probability of a proton-proton collision is described by the total cross-sectional area, which correlates with the proton size. The result of the ATLAS Collaboration is the most accurate measurement of this parameter at the energy level of 13 TeV. Precise detector positioning, within the scope of the IFJ PAN group, allows for high accuracy. The study confirms an important feature of strong interactions: an increase in the total cross-section with an increased collision energy. This increase can be explained by the increase in the size of the proton as the energy increases.

A precise understanding of the entire cross-section is important for the study of the strong interaction and other areas of particle physics. Strong interactions play an important role in many different fields, including the search for new physics, cosmic ray research, and experiments at the Large Hadron Collider (LHC), where they act as background and cause cosmic air showers. Precise measurements of elements such as the overall cross-section make it possible to accurately model these processes.

Elastic scattering in proton-proton collisions can occur through two different mechanisms: strong nuclear interactions and Coulomb contacts, or the attraction of opposite charges. The interference between both mechanisms is a second effect of the quantum characterization of the analyzed process. Their scattering amplitude determines the interference.

In quantum physics, the scattering amplitude is a measure of probability. Its value is a complex number, not a real number, unlike normal probability. As a result, its magnitude and phase or its real and imaginary components are used to describe it. We can learn more about the actual and hypothetical components of the nuclear amplitude by measuring the noise since Coulomb interactions are well understood and their scattering amplitudes can be estimated.

When measured experimentally, the ratio between the real and imaginary parts of the nuclear amplitude is much smaller than predicted by pre-LHC theoretical models. These models are the result of assumptions about specific strong interaction characteristics. These assumptions are put to the test by the observed difference.

The initial assumption was that the proton-antiproton collision properties are identical to the proton-proton and antiproton-antiproton collisions at very high energies. This is because high-energy collisions occur only frequently between gluons, even though quarks and gluons make up protons. The intuitive assumption is that the interactions in different systems are the same because protons and antiprotons have the same gluon structure. Theoretical models represent experimental data by allowing for differences, which is possible due to the quantum nature of interactions.

The growth of the total cross-section with energy is the second basic assumption of the theoretical models. It is believed to exhibit similar characteristics at higher energies than those measured at the LHC accelerator. Another explanation for the observed disparity is the retardation of this growth at energies that exceed the LHC energies.

The fundamental characteristics of strong interactions at high energies are the subject of both proposed hypotheses. Whatever is true, the reported measurements have improved our knowledge of fundamental interactions between particles.

The detectors used in the described studies are being prepared for further measurements of elastic scattering at even higher energies. The Institute of Nuclear Physics of the Polish Academy of Sciences is also working on other processes in which both the strong interaction and the electromagnetic interaction play an important role. The technique of Roman vases played an important role in these studies.

Reference magazine:

  1. Aad, G., Abbott, B., Abbott, DC et al. Measurement of total cross-section and magnetic elastic scattering parameter in pp collision at s√=13 TeV using ATLAS detector. EH. physics. JC 83, 441 (2023). DOI: 10.1140/epjc/s10052-023-11436-8

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