Developed in the early 1970s, the Standard Model of particle physics successfully describes most of the observed elementary particle interactions and precisely predicts many high-energy physics phenomena. However, the Standard Model is still incomplete and cannot explain the matter-antimatter imbalance, the nature of dark matter and dark energy, and it does not describe gravitational interactions.
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An international team of researchers, part of CERN’s Compact Muon Solenoid (CMS) collaboration, has recently developed a way of studying matter created when photons collide. The results of these experiments can shed some light on the physics beyond the Standard Model.
The idea that matter particles, an electron-positron pair, can be created when light particles or photons collide is central to modern physics. The concept illustrates that energy and mass are two sides of the same coin, immortalized in Einstein’s famous equation E=mc2. However, direct observation of the phenomenon involving just photons has proved elusive, since the colliding photons need to have extremely high energy, still well beyond the current technology.
In 1934, Gregory Breit and John A. Wheeler proposed an alternative to direct photon collisions by suggesting that the colliding photons can originate from the electromagnetic field surrounding relativistic heavy ions that pass each other head-on.
Researchers at Brookhaven National Laboratory recently provided the first clear evidence of a direct, single-step particle interaction where electron-positron (matter-antimatter) pairs were formed as a result of a photon-photon collision, as originally predicted by Breit and Wheeler.
Colliding Clouds of Photons
Brookhaven physicist Daniel Brandenburg and his colleagues used the facility's Relativistic Heavy Ion Collider (RHIC) to accelerate gold ions to relativistic speeds (very close to the speed of light).
When the positively charged heavy ions travel at that speed, they generate a strong electromagnetic field surrounding each ion. If the speed and charge of the ions are just right, the strength of the circular magnetic component of the field can be made equal to the strength of the perpendicular electric component. When that condition is met, the electromagnetic field surrounding each of the relativistic heavy ions can be regarded as a flux of quasi-real photons, where the photon flux is proportional to the ion charge squared (hence the choice of heavy highly charged ions such as gold).
Creating Matter (and Antimatter) from Light
At RHIC, the researchers succeeded to accelerate a pair of gold ion beams to 99.995% of the speed of light. When the ion beams travel in opposite directions with high enough energy, two ions brush past each other without colliding in so-called ultraperipheral collisions (UPCs). In such interactions, the 'clouds' of photons that surround each ion can collide, leading to the formation of electron-positron pairs.
By analyzing the energies and trajectories of more than 6000 electron-positron pairs created during UPC events, the scientists proved that they had observed the creation of matter (and antimatter) from energy in a Breit-Wheeler process for the first time.
Bending Photon Beams in a Vacuum
The Brookhaven team also observed how light interacts with a strong magnetic field in the vacuum.
In the first half of the 20th century, Werner Heisenberg and other fellow physicists predicted that the vacuum could be polarized by a very strong magnetic field and affect the paths of photon beams depending on their polarization.
Since the UPCs took place in the intense magnetic fields of the relativistic gold ions, the experimental results also showed clear evidence for an effect known as vacuum birefringence.
Similar to the conventional birefringence (where a light beam splits into two beams of different polarization when passing through materials such as Iceland spar), the vacuum birefringence changes the light path depending on the photon polarization in the presence of very intense magnetic fields. Previously, vacuum birefringence has been observed only for light beams propagating in the magnetic field of neutron stars, meaning that Brandenburg and his colleagues observed the effect for the first time in an Earth-based experiment.
Probing the Physics beyond the Standard Model
Following in the footsteps of the Brookhaven team, researchers from the Compact Muon Solenoid (CMS) collaboration working at CERN's Large Hadron Collider (LHC) are hoping that studying the energy-to-mass conversions during high-energy collisions of heavy ions can help to understand the physics of exotic states of the matter, such as the quark-gluon plasma (QGP).
The term describes a mixture of quarks (the fundamental building blocks of the matter) and gluons – the carriers of the strong interaction responsible for the formation of particles such as protons and neutrons.
The CMS team recently discovered that the UPCs might be unexpectedly affecting the behavior of the QGP. Since the photon collisions are electromagnetic phenomena and QGP is governed by the strong interaction, the Standard Model does not predict a strong influence of the QGP on the photon collisions and vice versa. However, the experimental data clearly showed deviations from the predictions.
As explained by Shuai Yang and Wei Li, physicists at Rice University and part of the CMS collaboration, most of the observed effects can be explained by quantum interference between the photon clouds prior to the collision (causing the photons to move perpendicular to the ion beams). Such behavior is predicted by the Standard Model of particle physics.
However, according to the researchers, the Standard Model explanation does not rule out possible effects related to the QGP. They are hoping that gathering and analyzing data from many more photon collisions could reveal a link between the observed anomalies and a new physics beyond the Standard Model.
References and Further Reading
J. Adam, et al. (2021) Measurement of e+e− momentum and angular distributions from linearly polarized photon collisions. Phys. Rev. Lett., 127, 052302. Available at: https://doi.org/10.1103/PhysRevLett.127.052302
A. M. Sirunyan, et al. (2021) Observation of Forward Neutron Multiplicity Dependence of Dimuon Acoplanarity in Ultraperipheral Pb-Pb Collisions at sNN=5.02 TeV. Phys. Rev. Lett., 127, 122001. Available at: https://doi.org/10.1103/PhysRevLett.127.122001
P. Genzer (2021) Collisions of Light Produce Matter/Antimatter from Pure Energy [Online] www.bnl.gov Available at: https://www.bnl.gov/newsroom/news.php?a=119023 (Accessed on 19 October 2021).
M. Starr (2021) Physicists Detect Strongest Evidence Yet of Matter Generated by Collisions of Light [Online] www.sciencealert.com Available at: https://www.sciencealert.com/physicists-claim-they-ve-finally-observed-matter-being-made-out-of-colliding-light (Accessed on 19 October 2021).
S. Jarman (2021) Photon–photon collisions could shed light on physics beyond the Standard Model [Online] www.physicsworld.com Available at: https://physicsworld.com/a/photon-photon-collisions-could-shed-light-on-physics-beyond-the-standard-mode (Accessed on 19 October 2021).