![]() They used repeated random sampling to find the most probable distribution of quarks and gluons within the lattice. The team loaded a large number of sample configurations of quarks and gluons onto the 4D lattice. They then figured out how to add in the complexity of the quarks. The team used many different starting arrangements and included varying distances between particles. They then saw how those interactions changed over time. They placed the particles in discrete positions on an imaginary 3D grid to model their interactions with neighbouring particles. In this method, scientists ran simulations of particle interactions on a discretised four-dimensional space-time lattice. The team was able to work out their method using lattice QCD by thinking only of gluons. “In 2010/11, we started using a mathematical shortcut, which assumed the plasma consisted only of gluons, no quarks,” said Olaf Kaczmarek of Bielefeld University, who led the German part of this effort. Many powerful supercomputers and advances in theory helped pave the way for the new calculation. The calculations needed to solve the equations of quantum chromodynamics (QCD) - the theory that describes quark and gluon interactions - are mathematically complex. Supercomputers helped to pave the way for the new calculation If the QGP is a perfect fluid, the mean free path for the heavy quark interactions should be short enough to make that possible.Ĭalculating the heavy quark diffusion coefficient was a way to check this understanding. “It would have to undergo many collisions to get dragged along with the plasma.” “It’s much more difficult to change the momentum of a heavy quark because it’s like a train-hard to stop,” Mukherjee stated. The strongly interacting QGP exhibits collective behaviour, including nearly frictionless flow. The collisions dissipate and distribute the energy of the fast-moving particles. ![]() The quarks and gluons are able to interact strongly and frequently with a short mean free path. “If you think about trying to walk through a crowd, it’s the typical distance you can get before you bump into someone or have to change your course,” he said. The mean free path is the distance a particle can travel before interacting with another particle. Department of Energy’s Brookhaven National Laboratory. “The low viscosity implies that the ‘mean free path’ between the ‘melted’ quarks and gluons in the hot, dense QGP is extremely small,” said Swagato Mukherjee co-leader of the work and member of the nuclear theory group at the U.S. The resulting QGP flowed with little resistance, showing that there are many strong interactions between the quarks and gluons in the hot quark soup. The collisions melt the boundaries of individual protons and neutrons to set the inner quarks and gluons free. The low viscosity of matter generated in RHIC’s collisions of gold ions was a major motivator for the new calculation, explained Petreczky. The low viscosity was a major motivator for the calculation The calculation explains why this surprising image makes sense when thinking about the low viscosity of the QGP. Usually, the water flows, but the rock stays.” “It would be like seeing a heavy rock get dragged along with the water in a stream. “Initially, seeing heavy quarks flow with the QGP at RHIC and the LHC was very surprising,” said Peter Petreczky, co-leader of the work and member of the nuclear theory group at the U.S. Its viscosity is so low that it also approaches the quantum limit. The new study also demonstrates that this matter – the quark-gluon plasma (QGP) – is nearly perfect liquid. The calculation will help explain experimental results that show heavy quarks getting caught up in the flow of matter generated in heavy ion collisions at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven and the Large Hadron Collider (LHC) at Europe’s CERN laboratory. These quarks and gluons have so many short-range, strong interactions with the heavier quarks that they pull the particles along with their flow. The answer: very fast.Īs described in the paper, ‘ Heavy Quark Diffusion from 2 + 1 Flavor Lattice QCD with 320 MeV Pion Mass,’ the momentum transfer from the ‘freed up’ quarks and gluons to the heavier quarks occur at the limit of what quantum mechanics will allow. This number describes how quickly a melted soup of quarks and gluons transfers its momentum to heavy quarks. The team has developed a calculation of the heavy quark diffusion coefficient. A new calculation will help physicists interpret experimental data from particle collisions at the RHIC and LHC to better understand the interactions of quarks and gluons.Ī group of theorists has used some of the world’s most powerful supercomputers to produce a major advance in nuclear physics.
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