Understanding photon collisions could aid search for physics beyond the Standard Model

The Compact Muon Solenoid experiment was performed at the European Organization for Nuclear Researchs Large Hdron Collider. Credit: CERN
Rice University physicists have shared their findings with colleagues, following the 87-year-old prediction of matter being generated directly from light.

Wei Li, an associate professor at Rice of Physics and Astronomy and coauthor of the Physical Review Letters study, said that "we are essentially looking into collisions of light."

"We know that energy can be transformed into mass," Li said. Li is a particle physicist and collaborates with hundreds on experiments at high-energy accelerators such as the European Organization for Nuclear Research (LHC), Brookhaven National Laboratory's Relativistic Heavy Ion Collider (RHIC).

RHIC or LHC are accelerators that transform energy into matter. They accelerate atoms at the speed of light and smash them together. A notable example is the 2012 discovery at the LHC of the Higgs particle. The Higgs particle was at the LHC's time the last unobserved particle of the Standard Model. This theory describes fundamental forces and building blocks for atoms.

Although it's impressive, physicists know that the Standard Model only explains about 4% of all the matter and energy in our universe. Li stated that this week's study was led by Shuai Yang (a Rice postdoctoral researcher) and has implications for the search to find physics beyond what the Standard Model can explain.

Li stated that there are papers that predict that you can create particles from these ion collisions, and that we have such high densities of photons within these collisions that these photon photon interactions could create new physics beyond that in the Standard Model.

Yang stated, "To search for new physics one must be able to understand the Standard Model processes very well. This effect was not previously thought of when people suggested photon-photon interactions as a way to search for new physics. It's important to remember this.

Yang and his colleagues described an effect that occurs when scientists accelerate opposite beams of heavyions in opposing directions and point them at each other. Ion accelerators can be used to study the strong force. It binds fundamental building blocks known as quarks in the neutrons, protons, and nuclei of atomic nuclei. To observe quarks and their interactions via strong force, physicists use heavy ion collisions.

Heavy ion accelerators can also collide nuclei. Ion beams can also create electric and magnetic fields, which shroud each nuclei with their own cloud of light. These clouds move with nuclei and individual photons can be merged together when they meet.

Yang and his colleagues published a PRL study in July that used data from RHIC. They showed that photon-photon collisions produce matter from pure energies. The experiments showed that light smashups were also associated with nuclei collisions, which created a primordial soup known as quark-gluon plasma (or QGP).

Yang stated that RHIC allows photon-photon collisions to create their mass simultaneously with the formation of quark gluon plasma. "So, you are creating this new mass within the quark-gluon Plasma."

Yang's PhD thesis on the RHIC data published by PRL 2018 suggested that photon collisions may be affecting plasma in a subtle but discernible way. This was both surprising and intriguing, Li stated. Photon collisions are electromagnetic phenomena and quark-gluon plasmas dominate by the strong force.

Li stated that electric charge alone is not sufficient to interact with quark-gluon Plasma. It is not expected to interact with quark-gluon plasma.

Yang's unanticipated findings were explained by a number of theories, he said.

Li stated that one explanation is that photon-photon interactions will look different because quark-gluon plasma does not exist, but because the ions get closer together. It's due to quantum effects and the way that photons interact one another."

Yang speculated that quantum effects could have caused the anomalies. They could cause interference patterns where ions miss one another, but photons collide from their respective light cloud.

Yang stated that the ions do not strike each others directly. They actually pass by. This is called an ultraperipheral collision, as the photons collide but not the ions.

The theory suggested that quantum interference patterns resulting from ultraperipheral photon–photon collisions should be proportional to the distance between passing ions. Yang, Li, and their colleagues used data from the LHC’s Compact Muon Solnoid (CMS), experiment to determine this distance or impact parameter by measuring something completely different.

The Compact Muon Solenoid experiment was performed at the European Organization for Nuclear Researchs Large Hdron Collider. Credit: CERN

Li stated that the two ions are closer together and there is a greater chance for the ion to get excited and emit neutrons. These go straight down the beamline. CMS has a detector that can detect this.

Ultraperipheral photon/photon collisions produce a pair called muons, which typically fly in opposite directions. Yang, Li, and their colleagues discovered that quantum interference caused a distortion in the departure angle for the muons. This was consistent with theory. The distortion is greater the further apart the near-missions ions are.

Li stated that the motion of colliding photons is what causes the effect. Each photon is moving in the same direction as its host ion but they can also move apart from their hosts.

He said, "The photons also have motion in the perpendicular directions." It turns out that the perpendicular motion becomes stronger as the impact parameter decreases in size.

Li stated that this makes it seem like something is changing the muons. It appears that one is moving at a different angle than the other, but it's not. It is an artifact due to the fact that the photon's motion was changing perpendicularly to the beam direction before the collision that created the muons.

Yang stated that the study has accounted for most of the anomalies he had previously identified. The study also established an experimental tool that can be used to control the impact parameter for photon interactions. This will have wide-reaching effects.

He stated that "we can confidently say that the majority of these effects came from the QED effect." But that doesn't mean there aren't effects that can be attributed to the quarkgluon plasma. Although this work provides a baseline of accuracy, we still need more data. There are still at least 15 years for CMS to collect QGP data, and the accuracy of the data will increase.

Continue exploring Creation, even if you are not in contact with the nuclei of lead or gold.

Further information: A. M. Sirunyan et. al., Observation Of Forward Neutron Multiplicity Dependence on Dimuon Acoplanarity In Ultraperipheral Peb-Pb Collisions At sNN=5.02TEV, Physical Review Letters (2021). Information from the Journal: Physical Review Letters A. M. Sirunyan and colleagues, Observation Of Forward Neutron Multiplicity Dependence on Dimuon Acoplanarity In Ultraperipheral Peb-Pb Collisions At sNN=5.02TEV, (2021). DOI: 10.1103/PhysRevLett.127.122001