A new computational analysis by theorists at the US Department of Energy’s Brookhaven National Laboratory and Wayne State University supports the idea that photons (aka particles of light) colliding with heavy ions can create a fluid of “highly interactive” particles. In a paper just published in Physical review lettersthey show that the calculations describing such a system match data collected by the ATLAS detector at the Large Hadron Collider (LHC) in Europe.
As the paper explains, the calculations are based on the flux of hydrodynamic particles seen in head-on collisions of different types of ions at both the LHC and the Relativistic Heavy Ion Collider (RHIC), a user facility of the DOE’s Office of Science for Nuclear Physics Research at Brookhaven Laboratory. With only modest changes, these calculations also describe the flow patterns seen in near-chance collisions, in which photons that form a cloud around the speeding ions collide with ions in the opposite beam.
“The upshot is that, using the same framework that we use to describe lead-lead-proton collisions, we can describe the data for these superliminal collisions where we have a photon colliding with a lead nucleus,” said Bjoern theorist at Brookhaven Lab Schenke, co-author on the paper. “This tells you that there is a possibility that in photon-ion collisions we create a small, dense, strongly interacting medium that is well described by hydrodynamics — just as in larger systems.”
Observations of particles flowing in distinct ways was key evidence that larger colliding systems (lead-proton-lead collisions at the LHC; gold-gold-proton-gold collisions at the RHIC) create an almost perfect fluid. The flow patterns were thought to stem from the enormous pressure gradients caused by the large number of highly reactive particles generated by the interference of colliding ions.
“By smashing these high-energy nuclei together, we create such a high energy density — squeezing the kinetic energy of these guys into a small space — that these things basically behave like fluids,” Schenke said.
Spherical particles (including protons and nuclei) are expected to collide head-on to generate a uniform pressure gradient. But the partially overlapping collisions generate an elongated, almond-shaped pressure gradient that pushes more high-energy particles outward along the short axis rather than perpendicular to it.
This “elliptical flow” pattern was one of the early indications that colliding particles at RHIC could produce a quark-gluon plasma, or QGP – a hot soup of the building blocks that make up protons and neutrons in nuclei/ions. Scientists were initially surprised by QGP’s liquid-like behaviour. But they later established elliptical flow as a defining feature of QGP, and evidence that quarks and gluons still interact strongly, even when free of confinement within individual protons and neutrons. Subsequent observations of similar flow patterns in the collision of protons with intriguingly large nuclei suggest that these proton-nuclei-collision systems can also create small patches of quark-gluon soup.
“Our new paper is about pushing this even further, by looking at collisions between photons and nuclei,” said Schenke.
Change the shell
It has long been known that supramolecular collisions can create photon-nuclei interactions, using the nuclei themselves as the source of the photons. That’s because charged particles accelerated to high energies, such as accelerated lead nuclei/ions at the LHC (and gold ions at the RHIC), emit electromagnetic waves – particles of light. Therefore, every lead ion accelerated at the LHC is essentially surrounded by a cloud of photons.
“When two of these ions pass each other closely without colliding, you can think of one of them as emitting a photon, and then the lead ion hits the other way,” Schenk said. “These events happen so often; it’s easier for the ions to just barely miss it than to hit each other with precision.”
Recently, ATLAS scientists published data on interesting flux-like signals from photon nucleus collisions.
“We had to set up special data-gathering techniques to capture these unique collisions,” said Blair Seidlitz, a Columbia University physicist who helped create the ATLAS Analysis Operating System while a graduate student at the University of Colorado, Boulder. . “After collecting enough data, we were surprised to find flow-like signals similar to those observed in lead-proton collisions, although they were slightly smaller.”
Schenke and his collaborators set out to see if their theoretical calculations could accurately describe particle flow patterns.
They used the same hydrodynamic calculations that describe the behavior of particles produced in lead-lead-proton-lead collision systems. But they made some adjustments to account for the “shell” hitting the lead nucleus changing from a proton to a photon.
According to the laws of physics (specifically, quantum electrodynamics), a photon can undergo quantum fluctuations to become another particle with the same quantum numbers. The rho meson, a particle made of a particular combination of a quark and an antiquark held together by gluons, is one of the most likely outcomes of these fluctuations of photons.
If you go back to the proton – made up of three quarks – the rho particle made up of two quarks is just a step down the ladder of complexity.
“Instead of gluons distributed around three quarks inside a proton, we have two quarks (antiquarks) with gluons distributed around those to collide with the nucleus,” Schenke said.
The calculations also had to take into account the large difference in energy in these photon-nuclei collision systems, compared to proton-lead and lead-lead in particular.
“The emitted photon that hits the lead will not carry the entire momentum of the lead nucleus that came from it, but only a fraction of it. So, the collision energy will be much less,” Schenk said.
This energy difference turned out to be more significant than the shell change.
In the most energetic lead-gold-gold heavy ionic collisions, the pattern of outgoing particles continues in the transverse plane of the colliding beams no matter how far you are from the collision point along the beamline (in the longitudinal direction). But when Schenk and colleagues modeled the particle patterns expected to emerge from low-energy lead-photon collisions, it became clear that including the three-dimensional details of the longitudinal orientation made a difference. The model showed that the geometry of the particle distributions changes rapidly with increasing longitudinal distance; The particles become “interconnected”.
“The particles see different pressure gradients depending on their longitudinal position,” Schenck explained.
“Therefore, for these low-energy photon collisions, it is important to run a full 3D hydrodynamic model (which is more computationally demanding) because the distribution of particles changes more rapidly as they exit in the longitudinal direction,” he said.
When theorists compared their predictions using this low-energy, full-3-D hydrodynamic model with particle flow patterns observed in photon-lead collisions by the ATLAS detector, the data and theory matched up nicely, at least for the more pronounced elliptical flow pattern, Shenke said.
Implications and the future
“From this result, it seems conceivable that even in heavy-photon-ion collisions we have a highly reactive fluid that responds to the initial collision geometry, as described in hydrodynamics,” Schenck said. “If the energies and temperatures are high enough, there will be a quark-gluon plasma,” he added.
Seydlitz, a physicist at ATLAS, commented, “It was very exciting to see these results indicating the formation of a small droplet of quark-gluon plasma, as well as how this theoretical analysis provides concrete explanations for why the flux signatures are slightly smaller in lead-photon collisions.”
Additional data to be collected by ATLAS and other experiments at RHIC and the LHC over the next several years will enable more detailed analyzes of particles streaming from photon-nucleus collisions. These analyzes will help distinguish the hydrodynamic computation from another possible explanation, where the flow patterns are not a consequence of the system’s response to the initial geometry.
In the long-term future, experiments at the Electron-Ion Collider (EIC), a facility planned to replace RHIC sometime in the next decade at Brookhaven Lab, could provide more definitive conclusions.
Wenbin Zhao et al, Aggregation in terminal lead + lead collisions at the Large Hadron Collider, Physical review letters (2022). DOI: 10.1103/PhysRevLett.129.252302. Journal.aps.org/prl/abstract/…ysRevLett.129.252302
Provided by Brookhaven National Laboratory
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