Can math be evidence? Not ordinarily, but recent calculations are compelling because they show that particles predicted by the theory of quark-gluon interactions but never observed are being produced in heavy-ion collisions at the Relativistic Heavy Ion Collider (RHIC), located at Brookhaven National Laboratory.

They just need to be detected. These heavy strange baryons, containing at least one strange quark, still cannot be observed directly, but instead are making their presence known by lowering the temperature at which other strange baryons "freeze out" from the quark-gluon plasma (QGP) discovered and created at the RHIC.

This would not be the first time that it was accepted that something must be there, it just needs to be found. Before the LHC discovered the Higgs boson, it was known that if the Higgs exists, it had to have been found at Fermilab first, but that experiment's lower luminosity meant it was taking a lot longer to sort it out. Later, it was shown to be in Fermilab data.

RHIC is one of two experiments where scientists create and study a primordial soup of unbound quarks and gluons-akin to what existed in the early universe some 14 billion years ago. The research is helping to unravel how these building blocks of matter became bound into hadrons, particles composed of two or three quarks held together by gluons, the carriers of nature's strongest force.


BS correlations normalized to the second cumulant of net strangeness fluctuations. Results are from (2+1)-flavor lattice QCD calculations performed with a strange to light quark mass ratio ms/ml = 20 (squares) and ms/ml = 27 (diamonds). The band depicts the improved estimate for the continuum result facilitated by the high statistics Nt= 6 and 8 data. arXiv:1404.6511

"Baryons, which are hadrons made of three quarks, make up almost all the matter we see in the universe today," said Brookhaven theoretical physicist Swagato Mukherjee, a co-author on the paper describing the new results in Physical Review Letters. "The theory that tells us how this matter forms-including the protons and neutrons that make up the nuclei of atoms-also predicts the existence of many different baryons, including some that are very heavy and short-lived, containing one or more heavy 'strange' quarks. Now we have indirect evidence from our calculations and comparisons with experimental data at RHIC that these predicted higher mass states of strange baryons do exist," he said.

Added Berndt Mueller, Associate Laboratory Director for Nuclear and Particle Physics at Brookhaven, "This finding is particularly remarkable because strange quarks were one of the early signatures of the formation of the primordial quark-gluon plasma. Now we're using this QGP signature as a tool to discover previously unknown baryons that emerge from the QGP and could not be produced otherwise."

Freezing point depression and supercomputing calculations

The evidence comes from an effect on the thermodynamic properties of the matter nuclear physicists can detect coming out of collisions at RHIC. Specifically, the scientists observe certain more-common strange baryons (omega baryons, cascade baryons, lambda baryons) "freezing out" of RHIC's quark-gluon plasma at a lower temperature than would be expected if the predicted extra-heavy strange baryons didn't exist.

"It's similar to the way table salt lowers the freezing point of liquid water," said Mukherjee. "These 'invisible' hadrons are like salt molecules floating around in the hot gas of hadrons, making other particles freeze out at a lower temperature than they would if the 'salt' wasn't there."

To find evidence, the scientists performed calculations using lattice QCD, a technique that uses points on an imaginary four-dimensional lattice (three spatial dimensions plus time) to represent the positions of quarks and gluons, and complex mathematical equations to calculate interactions among them, as described by the theory of quantum chromodynamics (QCD).

"The calculations tell you where you have bound or unbound quarks, depending on the temperature," Mukherjee said. They were specifically looking for fluctuations of conserved baryon number and strangeness and exploring how the calculations fit with the observed RHIC measurements at a wide range of energies.

The calculations show that inclusion of the predicted but "experimentally uncharted" strange baryons fit better with the data, providing the first evidence that these so-far unobserved particles exist and exert their effect on the freeze-out temperature of the observable particles.

These findings are helping physicists quantitatively plot the points on the phase diagram that maps out the different phases of nuclear matter, including hadrons and quark-gluon plasma, and the transitions between them under various conditions of temperature and density.

"To accurately plot points on the phase diagram, you have to know what the contents are on the bound-state, hadron side of the transition line-even if you haven't seen them," Mukherjee said. "We've found that the higher mass states of strange baryons affect the production of ground states that we can observe. And the line where we see the ordinary matter moves to a lower temperature because of the multitude of higher states that we can't see."

Preprint: A. Bazavov, H.-T. Ding, P. Hegde, O. Kaczmarek, F. Karsch, E. Laermann, Y. Maezawa, Swagato Mukherjee, H. Ohno, P. Petreczky, C. Schmidt, S. Sharma, W. Soeldner, M. Wagner, 'Additional Strange Hadrons from QCD Thermodynamics and Strangeness Freeze-out in Heavy Ion Collisions', arXiv:1404.6511


Citation: A. Bazavov, H.-T. Ding, P. Hegde, O. Kaczmarek, F. Karsch, E. Laermann, Y. Maezawa, Swagato Mukherjee, H. Ohno, P. Petreczky, C. Schmidt, S. Sharma, W. Soeldner, and M. Wagner, 'Additional Strange Hadrons from QCD Thermodynamics and Strangeness Freezeout in Heavy Ion Collisions', Phys. Rev. Lett. 113, 072001, 11 August 2014, DOI: 10.1103/PhysRevLett.113.072001. Source: DOE/Brookhaven National Laboratory