Light particles can create a fluid stream, the data theory comparison suggests

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 light particles) colliding with heavy ions can create a fluid of particles “strongly interacting”. In an article just published in Physical Review Lettersshow that 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 hydrodynamic particle flux observed in head-on collisions of various ion types at both the LHC and the Relativistic Heavy Ion Collider (RHIC), a DOE Office of Science user facility for research of nuclear physics at the Brookhaven laboratory. With only modest modifications, these calculations also describe the flux patterns observed in near-miss collisions, where photons forming a cloud around speeding ions collide with ions in the opposite radius.

“The upshot is that using the same framework that we use to describe lead-lead and proton-lead collisions, we can describe data from these ultra-peripheral collisions where we have a photon colliding with a lead nucleus,” he said. Brookhaven Lab theorist Bjoern. Schenke, co-author of the paper. “This tells you that there is a possibility that in these photon-ion collisions we create a small, strongly interacting dense medium that is well described by hydrodynamics, just like in larger systems.”

Fluid signatures

Observations of particles flowing in characteristic ways have been key evidence that the largest collision systems (lead-lead and proton-lead collisions at LHC; and gold-gold and proton-gold collisions at RHIC) create a nearly perfect fluid . The flow patterns were thought to arise from the huge pressure gradients created by the large number of strongly interacting particles produced where the colliding ions overlap.

“By putting these high-energy nuclei together we’re creating such a high energy density, compressing the kinetic energy of these guys into such a small space, that this stuff essentially behaves like a fluid,” Schenke said.

Spherical particles (including protons and nuclei) colliding head-on are expected to generate a uniform pressure gradient. But partially overlapping collisions generate an oblong, almond-shaped pressure gradient that pushes more high-energy particles along the short axis than perpendicular to it.

This “elliptical flow” model was one of the first clues that particle collisions at RHIC could create a quark-gluon plasma, or QGP, a hot soup of the fundamental building blocks that make up the protons and neutrons of nuclei/ions. Scientists were initially surprised by QGP’s liquid-like behavior. But they later determined that elliptical flux is a defining feature of QGP and evidence that quarks and gluons still interact strongly, even when free from confinement within single protons and neutrons. Subsequent observations of similar flow patterns in collisions of protons with large nuclei intriguingly suggest that these proton-nucleus collision systems can also create tiny specks of quark-gluon soup.

“Our new paper is about pushing the extremes even further by looking at collisions between photons and nuclei,” Schenke said.

Change the bullet

It has long been known that outermost collisions could create photon-nucleus interactions, using the nuclei themselves as the source of the photons. This is because charged particles accelerated to high energies, such as accelerated lead nuclei/ions at the LHC (and gold ions at RHIC), emit electromagnetic waves, particles of light. Thus, each lead ion accelerated at the LHC is essentially surrounded by a cloud of photons.

“When two of these ions cross very close without colliding, you can think of one emitting a photon, which then hits the lead ion going in the opposite direction,” Schenke said. “These events happen often; it’s easier for the ions to barely miss than to precisely hit each other.”

ATLAS scientists recently published data on intriguing flux-like signals from these photon-nucleus collisions.

“We had to set up special data collection techniques to find these unique collisions,” said Blair Seidlitz, a Columbia University physicist who helped create the ATLAS trigger system for analysis when he was a graduate student at the University of California. Colorado, Boulder. . “After collecting enough data, we were surprised to find flux-like signals similar to those observed in lead-lead and proton-lead collisions, although they were a bit smaller.”

Schenke and his collaborators set out to see if their theoretical calculations could accurately describe particle flux patterns.

They used the same hydrodynamic calculations that describe the behavior of particles produced in lead-lead and proton-lead collision systems. But they made some changes to explain the “bullet” hitting the lead nucleus going from a proton to a photon.

According to the laws of physics (in particular, quantum electrodynamics), a photon can undergo quantum fluctuations to become another particle with the same quantum numbers. A rho meson, a particle made up of a particular combination of quarks and antiquarks held together by gluons, is one of the most likely results of those photonic fluctuations.

If you think back to the proton, made up of three quarks, this two-quark rho particle is just one rung down the ladder of complexity.

“Instead of having a distribution of gluons around three quarks inside a proton, we have the two quarks (quark-antiquark) with a distribution of gluons around those that collide with the nucleus,” Schenke said.

Energy accounting

The calculations also had to take into account the large energy difference in these photon-nucleus collision systems, compared to proton-lead and especially lead-lead.

“The emitted photon colliding with lead will not carry the full momentum of the lead nucleus it came from, but only a small fraction of that. So, the collision energy will be much lower,” Schenke said.

That energy difference proved to be even more important than the bullet swap.

In more energetic lead-lead or gold-gold heavy ion collisions, the pattern of particles emerging in the plane transverse to the colliding beams generally persists no matter how far one looks from the collision point along the beam line (in the longitudinal direction) . But when Schenke and collaborators modeled the patterns of particles expected to emerge from low-energy photon-lead collisions, it became apparent that including the 3D details of the longitudinal direction made all the difference. The model showed that the geometry of particle distributions changes rapidly with increasing longitudinal distance; the particles become “decorrelated”.

“The particles see different pressure gradients depending on their longitudinal position,” Schenke explained.

“So, for these low-energy photon-lead collisions, it is important to perform a full 3D hydrodynamic model (which is more computationally demanding) because the particle distribution changes faster the further you go out in the longitudinal direction,” he has declared.

When the theorists compared their predictions using this low-energy, fully 3D hydrodynamic model with particle flux patterns observed in photon-lead collisions from the ATLAS detector, the data and theory matched well, at least for the more obvious model. of elliptical flow. Schenke said.

Implications and future

“From this result, it seems conceivable that even in photon heavy-ion collisions, we have a strongly interacting fluid that responds to the geometry of the initial collision, as described by hydrodynamics,” Schenke said. “If the energies and temperatures are high enough,” she added, “there will be a quark-gluon plasma.”

Seidlitz, the ATLAS physicist, commented: ‘It was very interesting to see these results suggesting the formation of a small droplet of quark-gluon plasma, as well as how this theoretical analysis offers concrete explanations as to why the flux signatures are somewhat smaller in photon-lead collisions.”

Additional data to be collected by ATLAS and other experiments at RHIC and the LHC in the coming years will enable more detailed analyzes of the particles flowing from photon-nucleus collisions. These analyzes will help distinguish the hydrodynamic calculation from another possible explanation, where the flow patterns are not the result of the system’s response to the initial geometry.

In the longer-term future, experiments at an Electron-Ion Collider (EIC), a facility planned to replace RHIC over the next decade at the Brookhaven Lab, could provide more definitive conclusions.