Theorists and scientists conducting experiments that recreate matter as it existed in the very early universe are gathered in Chicago this week to present and discuss their latest results. These experiments, conducted at the world's premier particle colliders - the Relativistic Heavy Ion Collider (RHIC) at the U.S. Department of Energy's Brookhaven National Laboratory, and the Large Hadron Collider (LHC) at the European Center for Nuclear Research (CERN) - are revealing intriguing information about the building blocks of visible matter and the force that holds them together in the universe today.
The Quark Matter 2017 conference brings together nuclear physicists seeking to understand the force that binds the building blocks of visible matter. (Credit: Anjali Chandrashekar)
The Quark Matter 2017 conference (QM17) will feature new results describing the particles created as atomic nuclei smash into one another at nearly the speed of light at RHIC and the LHC. These "ultrarelativistic heavy-ion collisions" melt ordinary protons and neutrons, momentarily setting free their inner constituents - quarks and gluons - so scientists can study their behavior and interactions. The physicists want to sort out the detailed properties of the hot "quark-gluon plasma" (QGP), and understand what happens as this primordial soup cools and coalesces to form the more familiar matter of today's world.
The two scientific collaborations conducting nuclear physics research at RHIC-STAR and PHENIX, named for their house-sized detectors-will present findings that build on earlier discoveries at this DOE Office of Science User Facility. The two collaborations perform cross-checking analyses to verify results, while also exploiting each detector's unique capabilities and strengths for independent explorations. The QM17 presentations will showcase precision measurements made possible by recent detector upgrades.
"These results illustrate how a global community of dedicated scientists is taking full advantage of RHIC's remarkable versatility to explore in depth the structure of nuclear matter over a wide range of temperatures and densities to better understand the dynamic behavior of quarks and gluons and the strong nuclear force," said Berndt Mueller, Associate Laboratory Director for Nuclear and Particle Physics at Brookhaven Lab. "The latest RHIC findings indicate that RHIC sits at the 'sweet spot' for probing the most interesting questions about the quark-gluon plasma and its transition to matter as we know it."
The meeting will also feature talks on the planned upgrade of the PHENIX experiment to a new RHIC detector known as sPHENIX, which will have greatly increased capabilities for tracking subatomic interactions. In addition, at least one talk will focus on the scientific rationale for building an Electron-Ion Collider, a proposed future facility that would enable an in-depth exploration of gluons in protons and other nuclei, opening a new frontier in nuclear physics.
Select QM 2017 Highlights from RHIC
Does size really matter?
Before RHIC began operations in 2000, nuclear physicists suspected it would take collisions of large nuclei such as gold to produce enough heat to create quark-gluon plasma. Since then, RHIC's gold-gold smashups (and later collisions of lead nuclei at the LHC) have reliably recreated a soup of quarks and gluons that flows like a nearly "perfect" liquid with extraordinarily low viscosity. Scientists detect the flow by observing correlations in certain characteristics of particles streaming from the collisions even when they are relatively far apart. More recently, smashups of smaller nuclei such as helium and even single protons with the large nuclei have produced correlation patterns that suggest that smaller drops of QGP might be possible. The latest results, to be presented by PHENIX, come from collisions of protons with aluminum nuclei, and also from deuteron-gold collisions over a range of collision energies. Lowering the energy changes how long the QGP phase lasts, which should change the strength of the correlations. The new results also include the first analysis of particles emerging closest to the colliding beams in the forward and rearward directions, as tracked by the recently installed Forward Silicon Vertex Tracker. Adding this tracker to detector components picking up particles emerging more centrally, perpendicular to the colliding beams, gives the physicists a way to test in three dimensions how the correlations vary with the pressure gradients created by the asymmetrical collisions.
Discerning differences among heavy quarks
PHENIX's Central Barrel and Forward Silicon Vertex Tracker and STAR's high precision Heavy Flavor Tracker (HFT) give RHIC physicists access to studying the behavior of so-called heavy quarks, which go by the exotic names of "charm" and "bottom." These particles, produced in the QGP, start to decay into other particles a short distance from the collision zone, but those decay products eventually strike the trackers. By tracing their tracks, scientists can identify precisely where the decay took place. And since charm and bottom quarks have slightly different lifetimes before decaying, and therefore different travel distances, this method gives the scientists a way to tell them apart.
Going with the flow
One way scientists will use this data is to see how heavy quarks are affected by the QGP, and whether there are differences among them. Earlier indirect findings by PHENIX, later confirmed by STAR, already indicated that heavy quarks get swept up in the flow of the QGP, somewhat like a rock getting pulled along by a stream instead of sinking to the bottom. These observations formed part of the motivation for the construction of the STAR HFT. New data from the HFT to be presented by STAR provide the first direct evidence of heavy quark flow, and show that the interactions of these heavy particles with the QGP medium are strong. STAR's HFT is the first application of the silicon based Monolithic Active Pixel Sensor technology in a collider environment. The measurements show that the flow of a type of heavy particles called D0s, which contain a charm quark, follows the same trend as seen for lighter particles and can be described by the same viscous hydrodynamics. The unprecedented precision in this measurement will pave the path towards precisely determining one of the intrinsic transport properties of the QGP and tell us how quarks interact with it.
PHENIX will present precision results from its Central Barrel Vertex Detector showing that some heavy quarks are more affected by the QGP than others. The results show that charm quarks lose more energy in the QGP than heavier bottom quarks. With this high statistics data set, PHENIX will now be able to study how the energy-loss is affected by how central, or head-on, the collisions are. PHENIX will also present its first heavy-quark result from the Forward Silicon Vertex Tracker, measuring the total cross section of bottom quarks emerging in the forward and rearward directions in collisions between copper and gold ions.
Learning how particles grow
The STAR HFT has also made it possible to make the first measurements of a particle called Lambda c emerging from RHIC collisions. Lambda c is made of three quarks-just like protons and neutrons-but with one of the three being a heavy quark. These Lambda c particles are extremely difficult to tease out from the data. But because they can only be created in energetic particle collisions, they carry unique information about the conditions within. Studying this "sentry" information carried by the Lambda c should help scientists learn how relatively "free" quarks that populate the early-stage QGP eventually coalesce and combine to form the more familiar composite particles of ordinary matter.
Tracking high-momentum jets
Observing how jets of particles springing from individual quarks or gluons lose energy, or get "quenched," as they interact with the medium has been one major sign that RHIC's energetic collisions of gold on gold were forming QGP. STAR will present several new jet studies that provide further insights into both how this quenching occurs and how the lost energy re-emerges, In addition, PHENIX will present new results exploring the question of whether collisions of smaller particles with gold, which appear to create the flow patterns of QGP, also show evidence of jet quenching. Their results include data on jet energy loss in a variety of collision systems, both large and small. The method uses photons emitted opposite the jet to calibrate how much energy the jet should have to determine whether or not there was quenching. The data show some modifications to the jet structure and the yield of high-momentum particles inside the jets, but it is not yet clear how to interpret these results.
Taking the QGP's temperature
Tracking heavy quarks and particles made from them gives RHIC physicists a new way to zero in on a more precise temperature of the QGP-already known to be more than 250,000 times hotter than the center of the sun. The new precision comes from measuring how different bound states of heavy quark-antiquark pairs, held together with different amounts of energy, melt in the plasma. STAR counts up different types of these particles (for example, Upsilons, pairs of bottom and anti-bottom quarks, that come in several binding varieties) using another recently upgraded detector component called the Muon Telescope Detector. Muons are the decay products of the Upsilons. STAR uses these counts to look for a deficit of one type of Upsilon relative to another to set boundaries on the QGP temperature. The physicists are eager to compare their results with those from the LHC, where with higher collision energies, they expect to see higher temperatures.
PHENIX's measurements of temperature have relied on tracking photons, particles of light, emitted from the hot matter (think of the glow of an iron bar in a blacksmith's fire, where the color of the light is related to how hot the iron is). But PHENIX's photon data have uncovered something unusual: While collisions initially emit photons equally in all directions, fractions of a second later the emitted photons appear to have a directional preference that resembles the elliptical flow pattern of the perfect liquid QGP. This is intriguing because photons shouldn't interact with the matter-or even be produced in such measurable quantities as the matter produced in the collisions cools and expands. To explore this mystery, PHENIX measured thermal direct photons at different gold-gold collision energies (39, 62, and 200 billion electron volts, or GeV), as well as in the smaller collision system. The results they present will shed light on the sources of these direct photons.
Disentangling the effects of cold nuclear matter
RHIC physicists are also learning more about "cold" nuclear matter-the state of the nucleus, filled with a field of gluons, before it collides-and how to account for its effects when studying the hot QGP. In order to disentangle the effects of cold nuclear matter, PHENIX is comparing the suppression of the excited state of the bound charm-anti-charm particle known as Psi to its ground state. They are studying collisions of protons and helium with gold or aluminum-small systems where cold nuclear matter predominates-and will use these as a baseline for better understanding the sequential melting of the bound states in the hot QGP. Their findings indicate that the less tightly bound version of the Psi is more than twice as susceptible to the effects of cold nuclear matter than the more tightly bound version. This effect must be accounted for in analyzing the data from QGP-creating collisions where the presence of both cold and hot nuclear matter influences the results.
New way to turn down the energy
STAR has exploited RHIC's ability to collide nuclei over a wide range of collision energies, conducting a Beam Energy Scan to explore the creation of QGP and its transition to ordinary nuclear matter over a wide range of conditions. At QM17 they'll present data from collisions at the lowest energy yet. Instead of colliding one beam into the beam coming into the detector from the opposite direction, as occurs in most RHIC experiments, STAR placed a stationary target (a foil of gold) within the beam pipe inside STAR and aimed just one beam at the target. Like a collision in which one moving car crashes into one that is parked, this fixed-target collision lowered the impact compared to what would occur if both beams (or cars) were moving and colliding head on. Data from these low energy collisions will be an integral part of phase two of the Beam Energy Scan, which is enabled by improvements to the RHIC accelerator complex that allow for higher collision rates.