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The particle detector, the newest experiment at the Relativistic Heavy Ion Collider () at the U.S. Department of Energy's (DOE) Brookhaven National Laboratory, has released its first physics results: precision measurements of the number and energy density of thousands of particles streaming from collisions of near-light-speed gold ions.

As described in two papers recently accepted for publication in ÌÇÐÄÊÓƵical Review C and the Journal of High Energy ÌÇÐÄÊÓƵics, these measurements lay the foundation for the detector's detailed exploration of the quark–gluon plasma (QGP), a unique state of matter that existed just microseconds after the Big Bang some 14 billion years ago. Both studies are available on the arXiv preprint server.

The new measurements reveal that the more head-on the nuclear smashups are, the more charged particles they produce and the more total energy those firework-like sprays of particles carry. That matches nicely with results from other detectors that have tracked QGP-generating collisions at RHIC since 2000, confirming that the new detector is performing as promised.

"As a new and highly sophisticated experiment that has gone through a decade of planning, construction, and commissioning, the first questions we need to ask are: Is the detector operating properly, is our calibration accurate, and are our data-processing pipelines reliable?" said Jin Huang, a physicist at Brookhaven Lab and co-spokesperson for the sPHENIX Collaboration. "The best way to do that is to go through measurements of the fundamental collision properties and confirm that the detector is measuring them properly."

But beyond those fundamental measurements, sPHENIX is pushing boundaries in ways that will allow a new level of precision, detection of rarer signals, and a higher-resolution exploration of the QGP.

"This initial threshold of experimentally demonstrating that we are getting the number of charged particles right and getting the energy right will allow us to look deeper into the physics goals, deeper into the QGP features, and really bring out the physics capabilities of this detector," said Megan Connors, a physicist at Georgia State University and the other co-spokesperson for sPHENIX.

Innovative sPHENIX capabilities

Among the new detector's cutting-edge features are precision tracking systems for reconstructing particle trajectories, even for rare, important particles that form and decay a few human-hair-widths from the center of the collision. It also boasts a full suite of calorimeters, devices for measuring the energy of particles streaming from collisions.

An electromagnetic calorimeter measures the energy of electrons and photons, or particles of light, while the hadronic calorimeter—the first surrounding the central part of the collision zone at RHIC—measures the energy of hadrons, composite particles made of quarks, as they stream out at all angles.

"The tracking detectors function like a giant 3D camera. They help us to clearly see the paths of charged particles when the heavy ion beams collide, even when thousands of particles are produced in one of the most central head-on collisions," Huang said. "And when the particles fly out, they also carry energy from the collision with them. That's what the calorimeter is built for—to determine how energetic they are."

The combination of components and precision of their measurements allows the scientists to analyze the data in detailed quantitative ways. For example, "We can precisely measure how much more energy is produced as the collision goes from more peripheral, where the ions hit one another at glancing angles, to more central, or head-on," Connors said. The data show that more central collisions release about 10 times more energy. Similarly, the number of charged particles produced in the most central collisions is 10 times that produced in peripheral ones.

The precision will also allow the scientists to tease out rare signals, like the formation of heavy quarks very close to the collision point. They'll also be able to fully reconstruct jets, collimated sprays of particles that emerge from energetic quarks or gluons, by accounting for all the energy transported by the particles in those jets.

"It's very exciting to show that we are able to measure energies over a large dynamic range as well as to show that we have good control of the collision geometry," said Dennis Perepelitsa, a physicist at the University of Colorado Boulder who serves as physics coordinator and deputy spokesperson for the collaboration.

Perepelitsa noted that future studies will use jets "like a microscope to look at the substructure of the QGP." For example, comparisons of how jets generated by heavy and light quarks interact with the plasma may reveal that it isn't a uniform, smooth soup of free quarks and gluons but instead has clumps—"like a chunky soup instead of a smooth purée," he said. This may help the scientists work out how particles in the jets interact with the plasma and sometimes lose energy, or get "quenched," which in turn will reveal how the QGP gets its remarkable properties.

"These first measurements represent the work of more than 300 sPHENIX scientists—including students and postdocs from around the country and the world—who built and ran the detector, monitored its performance during experimental shifts, and did the extensive work to calibrate the detector and analyze the data," said Connors. "They establish the basis of our experimental study of the QGP and mark the transition to the start of a very exciting chapter for the sPHENIX experiment."