A distinctive observation of particle pair creation in photon-photon collisions

Generation of matter in an conversation of two photons belongs to a class of extremely exceptional phenomena. From the knowledge of the ATLAS experiment at the LHC, collected with the new AFP proton detectors at the optimum energies out there to-day, a more accurate—and far more interesting—picture of the phenomena occurring in the course of photon collisions is rising.

If you point a glowing flashlight in the direction of a different a single, you do not count on any magnificent phenomena. The photons emitted by both flashlights just move by each other. Having said that, in selected collisions involving large-vitality protons the problem is distinct. The photons emitted by two colliding particles may interact and make a pair of make a difference and antimatter particles. Traces of processes these types of as these have just been observed in the ATLAS experiment at the Huge Hadron Collider (LHC) at CERN close to Geneva. Precise observations were carried out working with the new AFP (ATLAS Ahead Proton) spectrometer, developed with substantial participation of researchers from the Institute of Nuclear Physics of the Polish Academy of Sciences (IFJ PAN) in Cracow. The Polish physicists, funded by the Nationwide Science Centre and the Ministry of Science and Greater Training, have been involved in the progress of AFP detectors because the conception of these equipment.

“Observations of the creation of particles of issue and antimatter from electromagnetic radiation go again to the beginnings of nuclear physics,” says Prof. Janusz Chwastowski, head of the workforce of physicists at the IFJ PAN involved in the AFP detectors.

Without a doubt, it was February 1933 when Patrick Blackett (Nobel 1948) and Giuseppe Occhialini claimed an observation of the generation of an electron-positron pair initiated by a quantum of cosmic radiation. The development of subject and antimatter was thus noticed earlier than the reverse process, i.e. the famed and magnificent positron annihilation. The to start with observations of the latter were created in August 1933 by Theodor Heiting, and three months afterwards by Frédéric Joliot.

“In the most normally recorded gatherings of development, one particular photon transforms into a particle and an antiparticle. In contrast, the phenomenon we are learning is of a diverse character. The particle-antiparticle pair arises right here due to the conversation of two photons. The possibility of these kinds of procedures was to start with reported by Gregory Breit and John A. Wheeler in 1934,” proceeds Prof. Chwastowski.

As a billed particle, the proton going within the LHC beam pipe is surrounded by an electric powered area. Given that the carriers of electromagnetic interactions are photons, the proton can be treated as an object surrounded by photons.

“In the LHC beam pipe, protons get to velocities quite close to the pace of light. A proton and the surrounding subject go through the Lorentz contraction together the way of motion. As a result, from our point of view, a proton moving at virtually the speed of gentle is involved with specially violent oscillations of the electromagnetic discipline. When these kinds of a proton techniques a different just one accelerated in the opposite directions—and this is the circumstance we are working with at the LHC—an conversation concerning the photons may possibly arise,” points out Dr. Rafal Staszewski (IFJ PAN).

At the LHC, collisions of highly energetic proton beams happen in various locations, including the a single located inside the large ATLAS detector. If two photons collide, the consequence could be an electron-positron pair or a muon-antimuon pair (a muon is about 200 moments extra enormous than an electron). These particles, which belong to the lepton spouse and children, made at large angles with regard to the proton beams, are recorded inside the principal ATLAS detector. These kinds of phenomena have been observed at the LHC right before.

“The issue is, we have two far more protagonists of two-photon processes! These are, in a natural way, the photon sources, i.e. the two passing protons. Consequently we get to the essence of our measurement,” suggests Dr. Staszewski and describes: “As a result of the photon emission, just about every proton loses some vitality but, importantly, it nearly does not modify the course of its movement. So, it escapes the detector collectively with other protons in the beam. Nonetheless, the proton that emitted the photon has a slightly reduced power than the beam protons. Therefore, the accelerator magnetic subject deflects it more, and this suggests that it gradually moves absent from the beam. These are the protons we are hunting for with our AFP spectrometers.”

Each and every of the four AFP monitoring models consists of four sensors: 16×20 mm semiconductor pixel plates, placed just one behind the other. A proton that passes by the sensors deposits some strength and as a result it activates the pixels on its route. By analyzing all the activated pixels, the proton path and attributes can be reconstructed.

The need to document protons only a bit deflected from the key beam suggests that the AFP spectrometers have to be inserted directly within the LHC beam pipe, just a couple of millimeters away from the circulating beams.

“When you are running so near to a particle beam with these large energies, you have to be conscious of the threats. The smallest error in the positioning of the spectrometer could final result in burning a hole in it. It would be pretty upsetting, but that would seriously be the the very least of our troubles. The resulting particles would contaminate at least a part of the accelerator producing its shut down for some time,” notes Prof. Chwastowski.

The measurements explained right here were carried out with AFP spectrometers positioned at a length of about 200 m from the issue at which the protons collided.

“Protons interact at the LHC in quite a few ways. As a end result, the protons observed in the AFP spectrometers might originate from procedures other than all those affiliated with photon-photon interactions. To research for the proper protons, we desired to have specific information about the homes of each particle,” emphasizes Ph.D. scholar Krzysztof Ciesla (IFJ PAN), who dealt with the initial analysis of the uncooked details collected by the AFP spectrometers in 2017 and converting them into info about the energies and momenta of the registered protons. The benefits of the proton electrical power measurements were then juxtaposed with the energies of the made lepton pair and, based on conservation concepts, it was established regardless of whether the observed proton could be the source of the interacting photon.

The measurements applying the AFP spectrometers proved to be very statistically considerable, at 9 typical deviations (sigma). For comparison, a five-sigma measurement is generally enough to announce a scientific discovery. So, the AFP spectrometers have effectively passed the check, proved the usefulness of the approach and supplied very exciting, although nonetheless unclear, benefits. It turned out that theoretical predictions do not totally agree with the determined attributes of the investigated interactions. Plainly there are hidden nuances in the two-photon processes observed in significant power proton-proton collisions that involve improved knowing and further measurements.

Presented by
The Henryk Niewodniczanski Institute of Nuclear Physics Polish Academy of Sciences