\u00a9 shutterstock\/remotevfx.com<\/figcaption><\/figure>\nThe mean free path is the distance a particle can travel before interacting with another particle.<\/p>\n
\u201cIf you think about trying to walk through a crowd, it\u2019s the typical distance you can get before you bump into someone or have to change your course,\u201d he said.<\/p>\n
The quarks and gluons are able to interact strongly and frequently with a short mean free path.<\/p>\n
The collisions dissipate and distribute the energy of the fast-moving particles. The strongly interacting QGP exhibits collective behaviour, including nearly frictionless flow.<\/p>\n
\u201cIt\u2019s much more difficult to change the momentum of a heavy quark because it\u2019s like a train\u2014hard to stop,\u201d Mukherjee stated. \u201cIt would have to undergo many collisions to get dragged along with the plasma.\u201d<\/p>\n
If the QGP is a perfect fluid, the mean free path for the heavy quark interactions should be short enough to make that possible.<\/p>\n
Calculating the heavy quark diffusion coefficient was a way to check this understanding.<\/p>\n
Supercomputers helped to pave the way for the new calculation<\/h3>\n The calculations needed to solve the equations of quantum chromodynamics (QCD) \u2014 the theory that describes quark and gluon interactions \u2014 are mathematically complex.<\/p>\n
Many powerful supercomputers and advances in theory helped pave the way for the new calculation.<\/p>\n
\u201cIn 2010\/11, we started using a mathematical shortcut, which assumed the plasma consisted only of gluons, no quarks,\u201d said Olaf Kaczmarek of Bielefeld University, who led the German part of this effort.<\/p>\n
The team was able to work out their method using lattice QCD by thinking only of gluons.<\/p>\n
In this method, scientists ran simulations of particle interactions on a discretised four-dimensional space-time lattice.<\/p>\n
They placed the particles in discrete positions on an imaginary 3D grid to model their interactions with neighbouring particles. They then saw how those interactions changed over time.<\/p>\n
The team used many different starting arrangements and included varying distances between particles.<\/p>\n
They then figured out how to add in the complexity of the quarks.<\/p>\n
The team loaded a large number of sample configurations of quarks and gluons onto the 4D lattice. They used repeated random sampling to find the most probable distribution of quarks and gluons within the lattice.<\/p>\n
\u201cBy averaging over those configurations, you get a correlation function related to the heavy quark diffusion coefficient,\u201d said Luis Altenkort, a University of Bielefeld graduate student who also worked on this research at Brookhaven Lab.<\/p>\n\u00a9 Brookhaven National Laboratory<\/figcaption><\/figure>\nAs an analogy, think about estimating the air pressure in a room by sampling the positions and motion of the molecules. \u201cYou try to use the most probable distributions of molecules based on another variable, such as temperature, and exclude improbable configurations\u2014such as all the air molecules being clustered in one corner of the room,\u201d Altenkort said.<\/p>\n
The QGP was simulated at a range of fixed temperatures. The heavy quark diffusion coefficient for each temperature was calculated. This could be used to map out the temperature dependence of the heavy quark interaction strength.<\/p>\n
\u201cThese demanding calculations were possible only by using some of the world\u2019s most powerful supercomputers,\u201d Kaczmarek said.<\/p>\n
As Mukherjee noted, \u201cThese powerful machines don\u2019t just do the job for us while we sit back and relax; it took years of hard work to develop the codes that can squeeze the most efficient performance out of these supercomputers to do our complex calculations.\u201d<\/p>\n
The heavy quark diffusion coefficient is largest at the temperature at which the QGP forms<\/h3>\n The calculations show that the heavy quark diffusion coefficient is the largest at the temperature at which the QGP forms. It then decreases with increasing temperatures.<\/p>\n
The result implies that the QGP comes to an equilibrium very quickly.<\/p>\n
\u201cYou start with two nuclei, with essentially no temperature, then you collide them and in less than one quadrillionth of a second, you get a thermal system,\u201d Petreczky said.<\/p>\n
The heavy quarks also get thermalised.<\/p>\n
For that to happen, the heavy quarks must undergo many scatterings with other particles very quickly. This implies that the mean free path of these interactions must be very small.<\/p>\n
The calculations show that the mean free path of the heavy quark interactions is very close to the shortest distance allowable at the transition to QGP.<\/p>\n
The quantum limit is established by the inherent uncertainty of knowing both a particle\u2019s position and momentum simultaneously.<\/p>\n
The scientists argue that this independent measure provides corroborating evidence for the low viscosity of the QGP.<\/p>\n
Improving understanding of how heavy ion collision systems evolve<\/h3>\n Now that it is confirmed that the heavy quark interactions with the QGP vary with temperature, the team can improve their understanding of how the actual heavy ion collision systems evolve.<\/p>\n
\u201cMy colleagues are trying to develop more accurate simulations of how the interactions of the QGP affect the motion of heavy quarks,\u201d Petreczky said.<\/p>\n
\u201cTo do that, they need to take into account the dynamical effects of how the QGP expands and cools down \u2014 all the complicated stages of the collisions.<\/p>\n
\u201cNow that we know how the heavy quark diffusion coefficient changes with temperature, they can take this parameter and plug it into their simulations of this complicated process and see what else needs to be changed to make those simulations compatible with the experimental data at RHIC and the LHC.<\/p>\n
\u201cWe\u2019ll be able to better model the motion of heavy quarks in the QGP, and then have a better theory to data comparison.\u201d<\/p>\n","protected":false},"excerpt":{"rendered":"
Learn about the new calculation that will help physicists interpret experimental data to better understand the interactions of heavy quarks.<\/p>\n","protected":false},"author":18,"featured_media":33765,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"_monsterinsights_skip_tracking":false,"_monsterinsights_sitenote_active":false,"_monsterinsights_sitenote_note":"","_monsterinsights_sitenote_category":0,"footnotes":""},"categories":[24615],"tags":[21963,17008],"acf":[],"yoast_head":"\n
Why do heavy quarks get caught up in the flow?<\/title>\n \n \n \n \n \n \n \n \n \n \n \n \n \n\t \n\t \n\t \n \n \n \n \n \n\t \n\t \n\t \n