Difficult to observe effects confirm exis

Dead corn effect.

Image: A cascade of particles and gluons initiated by a decelerating charm quark. The more the cascade develops, the lower the energy of the secondary particles and the larger the opening angle of the dead cone avoided by the subsequent gluons.
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Credit: Source: CERN

For the first time, a phenomenon that directly proves the existence of quark mass was observed in a very violent collision of lead nuclei. A team of physicists working on the ALICE detector for the Large Hadron Collider can boast of this spectacular achievement, the observation of the dead cone effect.

The objects that make up our physical daily life can have different characteristics. Among these, the basic role is played by the masses. Despite being very basic, mass has a surprisingly complex origin. The main cause is the complex interaction between protons and quark triplets inside neutrons. In modern physics, the mass of the quark itself due to its interaction with the Higgs field (the sign of which is the famous Higgs boson) is assumed to contribute only a few percent to the mass of protons or neutrons. However, this is just a hypothesis. The mass of a single quark has been determined from years of measurement, but only indirect methods have been used. Thanks to the efforts of scientists and engineers currently working in Geneva at the European Hadron Collider (CERN) LHC, it is finally possible to observe a phenomenon that directly proves the existence of one mass of heavy quarks. rice field. ..

“When a lead nucleus collides with an LHC particle accelerator, the energy density becomes very high, and protons and neutrons can decay to form a quark-gluon plasma instantaneously. Then, the internal quarks interact strongly. They begin to lose energy by moving through powerful fields and releasing gluons, but they do this in a fairly unique way, and our team first succeeded in observing it, “Krachu said. Professor Marek Kowalski of the Institute of Nuclear Physics at the Polish Academy of Sciences (IFJ PAN) begins to explain. Professor Kowalski is one of the members of a large international cooperation that makes measurements using the ALICE detector.

Gluons are particles that carry strong interactions between quarks. Therefore, their role is similar to, for example, the role of photons involved in the electromagnetic interaction between electrons. In electromechanics, there is a phenomenon related to electrons that slow down in an electromagnetic field. Electrons lose energy by emitting photons, and the higher the energy of an electron, the more often the photon flies in a direction that more and more coincides with its direction of motion. This effect is the basis of today’s free electron lasers. This is a unique and powerful device that can generate ultrashort pulses for X-rays.

“Electrons slowing down in a magnetic field like to emit” forward “photons in a horned cone. The higher the original energy, the narrower the cone. Quarks have the exact opposite taste. Losing energy in a strong interaction field releases gluons, but the lower the energy and the higher the mass of the quark, the less gluons fly “forward”, “said Professor Kowalski. I am saying. The theory that there should be a cone with a specific angle around the direction of movement of the quark where the gluon does not appear. The greater the divergence of this cone, the lower the energy of the quark and the greater the mass. This is called a dead cone. “

Theorists predicted the phenomenon of a cone that died more than 30 years ago. Unfortunately, so far, its presence in the experiment has only been indirectly noticed. It is very difficult to directly observe both the nature of the phenomenon and the recording process. Decelerating quarks emit gluons, which themselves can emit more gluons at various angles or turn into secondary particles. As these particles have smaller and smaller energies, the gluons they emit avoid the larger and larger dead cones. To make matters worse, individual detectors can only record this complex cascade in the final state, at different distances from the collision point, and therefore at different times. To observe the dead cone effect, the millions of cascades generated by the charm quark had to be reconstructed from fragmentary data. Analysis performed using advanced statistical tools included data collected during the three years that the LHC was in operation.

Experimental confirmation of the existence of the dead cone phenomenon is the achievement of considerable physical significance. This is because the world of quarks and gluons is dominated by strong interactions described by a theory called quantum chromodynamics. Therefore, the current results published in the prestigious journal Nature are the first direct experimental confirmations of the existence of quark mass.

“Following the vast amount of data collected by ALICE detectors during the collision of lead nuclei and protons with known phenomena that can occur in nature only when the quark mass is non-zero. The current measurements do not give us an estimate of the mass of the charm quarks we have observed, nor do we tell us anything about the masses of other types of quarks. Has been a remarkable success, but in reality it is just a prelude to a long study, “emphasizes Professor Kowalski.

The first direct observations of the dead cone effect contained only gluons emitted by the charm (c) quark. Scientists are now looking for quarks with higher mass, especially cones that have died in the process of containing beauty (b) quarks. This is a major challenge, as the heavier the mass of the quark, the less often it occurs during a collision, making it difficult to collect many cases that ensure sufficient reliability of statistical analysis.

The research reported is fundamentally important to modern physics. This is because the Standard Model is the basic tool currently used to describe phenomena involving elementary particles. The quark mass is an important constant here and is responsible for the correspondence between the theoretical description and the physical reality. Therefore, it is not surprising that the observation of dead cones, which raises expectations for direct measurement of quark mass, has attracted such interest to physicists.

HenrykNiewodniczański Institute of Nuclear Physics (IFJ PAN) is currently one of the largest research institutes of the Polish Academy of Sciences. A wide range of research conducted at IFJ PAN ranges from particle physics and astronomical physics to hadron physics, high-energy, medium-energy, low-energy nuclear physics, and condensate physics (including material engineering). It covers basic research and applied research. Applications of nuclear physics in interdisciplinary research covering medical physics, dosimetry, radiation and environmental biology, environmental protection, and other related disciplines. IFJ PAN’s annual average publication output includes over 600 scientific papers in influential international journals. Each year, the Institute hosts about 20 international and national scientific conferences. One of the institute’s most important facilities is the Cyclotron Center Bronovice (CCB), a unique infrastructure in Central Europe, which functions as a clinical and research center in the fields of medicine and nuclear physics. In addition, IFJPAN operates four accredited research and measurement laboratories. IFJ PAN is a member of the Marian Smoluchowsky-Krakow Research Consortium “Material-Energy-Future” and enjoyed the status of the Leading National Research Center for Physics (KNOW) from 2012 to 2017. In 2017, the European Commission awarded the Institute the HR Excellence in Research Award. The Institute holds the A + category (the highest science category in Poland) in the fields of science and engineering.

contact address:

Professor Marek Kowalski

Institute of Nuclear Physics, Polish Academy of Sciences

Phone: +48 12 6628074

Email: marek.kowalski @ cern.ch, marek.kowalski @ ifj.edu.pl

Science Publications:

“Direct observation of dead cone effect in quantum chromodynamics”

ALICE collaboration

Nature 605, 440–446 (2022)

DOI: https://doi.org/10.1038/s41586-022-04572-w



Website of the Institute of Nuclear Physics, Polish Academy of Sciences.


Press release of the Institute of Nuclear Physics of the Polish Academy of Sciences.



HR: http://press.ifj.edu.pl/news/2022/06/09/IFJ220609b_fot01.jpg

A cascade of particles and gluons initiated by a decelerating charm quark. The more the cascade develops, the lower the energy of the secondary particles and the larger the opening angle of the dead cone avoided by the subsequent gluons. (Source: CERN)

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