Theorists’ hydrodynamic flow calculations accurately describe data from collisions of photons with lead nuclei at the ATLAS experiment

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 ‘strongly interacting’ particles.

The researchers show that calculations describing such a system match up with data collected by the ATLAS detector at Europe’s Large Hadron Collider (LHC).

The calculations are based on the hydrodynamic particle flow seen in head-on collisions of various types of ions at both the LHC and the Relativistic Heavy Ion Collider (RHIC), a DOE Office of Science user facility for nuclear physics research at Brookhaven Lab.

With only modest changes, these calculations also describe flow patterns seen in near-miss collisions, where photons that form a cloud around the speeding ions collide with the ions in the opposite beam.

Brookhaven Lab theorist Bjoern Schenke, a co-author of the paper, said: “The upshot is that, using the same framework we use to describe lead-lead and proton-lead collisions, we can describe the data of these ultra-peripheral collisions where we have a photon colliding with a lead nucleus.

“That tells you there’s a possibility that, in these photon-ion collisions, we create a small dense strongly interacting medium that is well described by hydrodynamics – just like in the larger systems.”

See also: Geometry, and the origin of mass

fluid signatures

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

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

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 out along the short axis than perpendicular to it.

This ‘elliptic flow’ pattern was one of the earliest hints 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.

changing the projectile

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

Schenke explained: “When two of these ions pass each other very closely without colliding, you can think of one as emitting a photon, which then hits the lead ion going the other way. Those events happen a lot; it’s easier for the ions to barely miss than to precisely hit one another.”

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

Blair Seidlitz, a Columbia University physicist who helped set up the ATLAS trigger system for the analysis when he was a graduate student at the University of Colorado, Boulder, said: “We had to set up special data collection techniques to pick out these unique collisions.

“After collecting enough data, we were surprised to find flow-like signals that were similar to those observed in lead-lead and proton-lead collisions, although they were a little smaller.”

Schenke and his collaborators set out to see whether their theoretical calculations could accurately describe the particle flow patterns.

They used the same hydrodynamic calculations that describe the behaviour of particles produced in lead-lead and proton-lead collision systems. But they made a few adjustments to account for the ‘projectile’ striking the lead nucleus changing from a proton to a photon.

accounting for energy

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

Schenke advised: “The emitted photon that’s colliding with the lead won’t carry the entire momentum of the lead nucleus it came from, but only a tiny fraction of that. So, the collision energy will be much lower.”

That energy difference turned out to be even more important than the change of projectile.

In the most 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 you look from the collision point along the beamline (in the longitudinal direction).

But when Schenke and colleagues modelled the patterns of particles expected to emerge from lower-energy photon-lead collisions, it became apparent that including the 3D details of the longitudinal direction made a difference.

The model showed that the geometry of the particle distributions changes rapidly with increasing longitudinal distance; the particles become “decorrelated.”

Schenke explained: “The particles see different pressure gradients depending on their longitudinal position. So, for these low-energy photon-lead collisions, it is important to run a full 3D hydrodynamic model (which is more computationally demanding) because the particle distribution changes more rapidly as you go out in the longitudinal direction.”

He added: “From this result, it looks like it’s conceivable that, even in photon-heavy ion collisions, we have a strongly interacting fluid that responds to the initial collision geometry, as described by hydrodynamics.

“If the energies and temperatures are high enough, there will be a quark-gluon plasma. It’s conceivable that, in photon-heavy ion collisions, we have a strongly interacting fluid.”

The paper is published in Physical Review Letters.

Image: The energy density at different times during the hydrodynamic evolution of the matter created in a collision of a lead nucleus (moving to the left) with a photon emitted from the other lead nucleus (moving to the right). Yellow represents the highest energy density and purple the lowest. © Brookhaven National Laboratory.