New insights into the strong interaction from elastic proton-proton scattering at the LHC

The ATLAS experiment at the LHC accelerator has conducted a groundbreaking measurement of elastic scattering in proton-proton collisions at ultra-high energies. By analyzing the scattering angles of the particles, physicists gain valuable insights into the spatial structure of the colliding particles and the properties of their interactions.

To carry out these precise measurements, a dedicated system was developed, including detectors placed over 200 meters from the collision point. These detectors, known as Roman pots, were able to capture scattered protons at distances as small as a few millimeters from the accelerator beam. The trigger and data acquisition system, crucial for recording the data, was developed by the Krakow group.

The experimental setup also involved a unique configuration of magnetic fields that shaped the LHC accelerator beam. Typically, beams are tightly focused to increase the frequency of interesting interactions. However, for elastic scattering measurements, a large angular divergence caused by tight focusing makes it practically impossible to obtain accurate data. The specially designed magnet configuration minimized this divergence, ensuring precise measurements.

The key result of the measurement is the distribution of the scattering angle, specifically the variable “t” proportional to the square of that angle. By studying this distribution, researchers drew conclusions about the fundamental properties of strong interactions between protons at very high energies. This analysis relies on the quantum properties of elastic scattering, which are not observed in everyday collisions, such as those in billiard ball games.

One important quantum property utilized in the analysis is the optical theorem, which relates elastic interactions to inelastic ones, where additional particles are produced. The optical theorem allowed the determination of the total cross-section parameter from measurements of only elastic interactions. The total cross-section describes the likelihood of any type of proton-proton collision and is closely related to the size of protons. The ATLAS Collaboration’s published results provide the most precise measurement of the total cross-section at an energy of 13 TeV.

Achieving such high precision required several factors, including the precise determination of the detector position by the IFJ PAN group. The obtained measurement confirmed an important property of strong interactions—the increase of the total cross-section with increasing collision energy. This increase can be interpreted as the size of protons growing with energy.

Accurate knowledge of the total cross-section is not only crucial for studying strong interactions but also plays a role in other areas of particle physics. For instance, in experiments at the LHC, strong interactions act as background in the search for new physics. Additionally, in cosmic ray research, strong interactions are responsible for the development of cosmic air showers. Precise modeling of these processes relies on precise measurements of quantities such as the total cross-section.

In proton-proton collisions, elastic scattering can occur through two mechanisms: strong nuclear interaction and Coulomb interaction, which is the repulsion between electric charges. The interference between these mechanisms is another consequence of the quantum nature of the process. By measuring the interference, insights into both the real and imaginary parts of the nuclear scattering amplitude can be gained.

The scattering amplitude is a quantum measure of probability, described by complex numbers. While the scattering amplitude for Coulomb interactions can be calculated accurately, the measured interference reveals a significant discrepancy compared to predictions from pre-LHC theoretical models. These models are based on assumptions about the properties of strong interactions. The observed deviation challenges these assumptions.

The first assumption is that the properties of proton-antiproton collisions at very high energies are the same as those of proton-proton and antiproton-antiproton collisions. This assumption is based on the fact that high-energy collisions predominantly occur between gluons, despite protons being composed of quarks and gluons. Since the gluon structure is the same in protons and antiprotons, assuming identical interactions in different systems seems natural. However, allowing for differences, made possible by the quantum nature of interactions, enables the theoretical models to describe the experimental data more accurately.

The second assumption concerns the growth of the total cross-section with energy. It was previously assumed that the character of this growth at energies above those currently measured at the LHC would be consistent with observations thus far. However, the observed discrepancy can also be explained by a slowing down of this growth at energies above those achieved at the LHC.

Both hypotheses involve the fundamental properties of strong interactions at high energies. Regardless of which hypothesis proves to be true, the reported measurements contribute significantly to our understanding of particle interactions.

Currently, the detectors used in these studies are being prepared for further measurements of elastic scattering at even higher energies. The Institute of Nuclear Physics Polish Academy of Sciences is also conducting research on other processes where strong and electromagnetic interactions play significant roles. The Roman pots technique will continue to play a crucial role in these ongoing studies.

Source: The Henryk Niewodniczanski Institute of Nuclear Physics Polish Academy of Sciences

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