The theory that light and heavy ion particles can create a fluid of "strongly interacting" particles has been supported by a new analysis. In a paper just published in Physical Review Letters, they show that calculations describe the system in a similar way to the data collected by the ATLAS detector.
The calculations are based on the particle flow seen in the head-on collision of various types of ion at the RHIC. The flow patterns seen in near-miss collisions are described in the calculations with only small changes.
Using the same framework we use to describe lead -lead and proton-lead collisions, we can describe the data of these ultra-peripheral collisions. There is a chance that in these photon-ion collisions, we create a small dense strongly interacting medium that is similar to the larger systems.
Signatures that are fluid.
Observations of particles flowing in different ways have shown that the large collision systems create a perfect fluid. The flow patterns were thought to be the result of a large number of strongly interacting particles.
The high energy density created by smashing these high-energy nuclei together is what makes this stuff behave like a fluid.
A uniform pressure gradient is expected from spherical particles colliding head on. There is a pressure gradient that pushes high-energy particles out of the short axis.
One of the earliest hints that a quark-gluonplasma could be created was the elliptic flow pattern. The scientists were surprised by the behavior of the Q GP. Evidence that the quarks and gluons were still interacting despite being free from confinement was established as a defining feature of Q GP. Particles of quark-gluon soup can be created by the collision of protons and large nuclei.
The new paper is about looking at what happens when a photon and a nucleus come together.
The projectile could be changed.
It has been known for a long time that photon-nucleus interactions can be created by ultra-peripherals. When charged particles are accelerated to high energy, they emit waves of light. Each lead ion is surrounded by a cloud of light.
"When two of these ions pass each other very close, you can think of one as emitting a photon, which then hits the lead ion going the other way," he said. It is easier for the ion to barely miss than it is to hit one another.
There are intriguing flow-like signals from the photon-nucleus collision.
Blair Seidlitz, a Columbia University physicist who was a graduate student at the University of Colorado, Boulder, helped set up the Trigger system for the analysis. We were surprised to find flow-like signals that were similar to those seen in lead-lead and protons-lead collisions.
They wanted to see if their theoretical calculations could accurately describe the flow of particles.
They used the same calculations to describe the behavior of particles. They made a few changes to account for the projectile striking the lead nucleus.
A photon can change into another particle with the same quantum numbers according to the laws of physics. One of the most likely results of those photon fluctuations is a particle made of a quark and antiquark.
The two-quark rho particle is just a step down the complexity ladder compared to the protons.
The two quarks collide with the nucleus instead of the three quarks that are inside a protons.
There is accounting for energy.
The calculation had to account for the big difference in energy between the two systems.
The emitted photon that's colliding with the lead won't carry the entire momentum of the lead nucleus it came from. The collision energy will be less.
The change of projectile was less important than the energy difference.
In the most energetic lead-lead or gold-gold heavy ion collision, the pattern of particles emerging in the plane from colliding to the beams is unchanging. The 3D details of the longitudinal direction made a difference when the patterns of particles were modeled. The model shows that the geometry of the distributions changes rapidly as the distance increases.
The particles have different pressures depending on where they are.
The particle distribution changes more rapidly as you go out in the longitudinal direction so it's important to run a full 3D model.
The data and theory matched up nicely for the most obvious elliptic flow pattern when the theorists compared their predictions using this lower-energy, full 3D, hydrodynamic model.
The implications and the future are important.
It looks like it's possible that we have a strongly interacting fluid that responds to the initial collision geometry. There will be a quark-gluon plasma if the temperatures are high.
The results suggest the formation of a small droplet of quark-gluon plasma and how this theoretical analysis offers concrete explanations as to why the flow signatures are a bit smaller in photon.
More detailed analyses of particles flowing from photon-nucleus collisions will be possible thanks to additional data collected by the RHIC and other experiments. There is a possibility that the flow patterns are not a result of the system's response to the initial geometry.
Experiments at an EIC, a facility planned to replace RHIC in the next decade, could provide more conclusive conclusions.
There is more information about Collectivity in Ultraperipheral Pb + Pb Collisions. YsRevLett.129.252302 was published in the journals.aps.org.
Journal information: Physical Review Letters
All Rights Reserved © 2024
