Creation (of particles) without contact in the collisions of lead and gold nuclei


When heavy ions, which accelerate to the speed of light, collide with each other in European and US accelerators, plasma quarks are formed for fractions of a second or even their “cocktail” is mixed with other bodies. According to scientists from the Institute of Nuclear Physics of the Polish Academy of Sciences (Instytut Fizyki Jądrowej Polskiej Akademii Nauk, or IFJ PAN), experimental data show that there are underrated factors on the scene: photons. Their collisions lead to the emission of seemingly surplus particles, the presence of which could not be explained.

The phenomenon, like any phenomenon, reveals the new problem that had to be solved by the scientific community and in this case the scientists of IFJ PAN. Before referring to their work and the new knowledge gained, let’s look at some facts related to the problem. The quark-gluon creature is undoubtedly the most exotic state of matter known to us. At the LHC at CERN in Geneva, this state of matter is formed during central collisions of two lead ions approaching each other from opposite directions, moving at speeds very close to that of light. This quark-gluion soup is sometimes mixed with other bodies. Unfortunately, the theoretical description of the sequence of events that includes the creature and a cocktail of other sources,fails to describe the data collected in the experiments. This discrepancy between theoretical descriptions and experimental data is the new problem that scientists have tried to solve. A solution that will lead to new knowledge that will lead to improvements in the theoretical model.

The team of scientists at the Institute of Nuclear Physics of the Polish Academy of Sciences, which worked to solve the problem, published their study in Physics Letters B and in this publication explained the reason for the theory-experiment discrepancies observed. The data collected during the lead nucleus collisions at the LHC, as well as during the gold nucleus collisions at the RHIC (Relativistic Heavy Ion Collider) at Brookhaven National Laboratory near New York, began to agree with the theory when the process description takes into account the collisions between the surrounding photons and the two ions that interact. This is worth shedding light on a little more.

It could generally be argued that with sufficiently high energies, large mass ions collide not only with their protons and neutrons, but even with their photon clouds. Let’s leave Dr. Mariola Klusek-Gawenda from IFJ PAN to clarify this: “When describing ion collisions at the LHC we already take into account collisions between photons. However, these are considered to be extremely peripheral collisions, in which ions do not strike each other, but pass each other without change, interacting only with their electromagnetic fields. “No one thinks that photon collisions could play a role in violent reactions where protons and neutrons merge into a quark-gluon soup.”

In conditions known from everyday life, photons do not collide with each other. However, when dealing with large mass ions that accelerate to about the speed of light, the situation changes. The gold nucleus contains 79 protons, the lead nucleus up to 82, so the electric charge of each ion is respectively many times greater than the elementary charge. The carriers of electromagnetic interactions are photons, so each ion can be treated as an object surrounded by a cloud of many photons. In addition, in RHIC and LHC, ions move at speeds close to that of light. Therefore, from the observer’s point of view in the laboratory, both – ions and photon clouds that surround them – appear to be extremely thin entities, straightening in the direction of motion.With each pass of such a proton-neutron pancake, there is an extremely violent oscillation of the fields, electric and magnetic.

In quantum electrodynamics, the theory used to describe electromagnetism in relation to quantum phenomena, there is a maximum critical value of the electric field, in the range of 10 to 16 volts / cm. It is applied in static electric fields. In the case of large-mass atomic collisions in RHICs and LHCs, we are dealing with dynamic fields that appear only for milliseconds of one billionth of a billionth of a second. For such an extremely short period of time, the electric fields in ion collisions can still be 100 times stronger than the critical value.

The consequences of such a situation are explained by Dr. Wolfgang Schäfer of IFJ PAN: “In fact, the electric fields of ions colliding at RHIC and LHC are so strong that they create virtual photons and collide with them. As a result of these processes, lepton-antilepton pairs are formed at various points around the ions where there was no matter before. “The bodies of each pair move away from each other in a characteristic way: usually in opposite directions and approximately perpendicular to the initial direction of ion motion.” Noting that the family of leptons includes electrons and the largest mass of muons and tau.

Photon interactions and the production of lepton-antileptone pairs associated with them are critical in peripheral collisions. Conflicts such as those described by Krakow physicists a few years ago. To their surprise, they now have to manage to show that the same phenomena also play an important role in direct nuclear conflicts, even central ones. The data collected for the gold nuclei in the RHIC and the lead nuclei in the LHC show that during such collisions a certain “extra” number of electron-positron pairs appear, which move relatively slowly in directions approximately perpendicular to the ion beams. Their existence could be accurately explained,simply considering the production of lepton-antileptone pairs by colliding photons.

“The real icing on the cake for us,” concludes IFJ PAN Professor Antoni Szczurek , ” Wigner, we could finally explain why the detectors in the experiments of the largest modern accelerators record these types of distributions of leptons and antileptons escaping from the region of nuclear collisions (for a definite centrality of the collision). “Our understanding of the most important processes that take place here has become more complete.”

The model has piqued the interest of physicists working with ATLAS and ALICE detectors at the LHC and will be used in future experimental data analyzes.

Source : IFJ PAN

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