Energy scan of isobar collisions probes charge transport in nuclear matter

In a development that could reshape how physicists probe the innermost workings of matter, a team of researchers has proposed and validated a method to measure how electric charge travels through the hottest matter ever created on Earth: the quark-gluon plasma formed in high-energy nuclear collisions. The study, published in The European Physical Journal C, demonstrates that a clever comparison between two nearly identical atomic nuclei — ruthenium-96 and zirconium-96 — can strip away overwhelming experimental noise and reveal, with unprecedented precision, how electric charge is redistributed when atomic nuclei smash into each other at nearly the speed of light.

The work addresses one of the most stubborn gaps in the experimental program at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory and its European counterpart, the Large Hadron Collider. When two heavy nuclei collide at ultrarelativistic energies, the protons and neutrons inside them melt into a state of deconfined quarks and gluons known as the quark-gluon plasma, or QGP. A central goal of these experiments is to map the phase diagram of Quantum Chromodynamics, the theory of the strong force, and to characterize the properties of this exotic state of matter. To do that, physicists track conserved quantities — baryon number and electric charge — that enter the collision locked inside the incoming nuclei and then get carried and spread through the fireball as it forms and cools.

Baryon-number transport has been measured extensively at RHIC and the LHC over the past two decades. Electric-charge transport, however, has remained largely unmeasured, and the reason is simple: the fireball produces enormous numbers of particle-antiparticle pairs, which create positive and negative charges in nearly equal measure. Around the so-called midrapidity region — perpendicular to the beam direction — the medium is almost perfectly charge symmetric, so the net-charge signal carried by the original valence quarks drowns in a sea of freshly created pions. Extracting it with meaningful precision has seemed close to impossible.

The new study, led by Wendi Lv of the University of Science and Technology of China together with Niseem Magdy, Rongrong Ma, Prithwish Tribedy, Zhangbu Xu and colleagues at Brookhaven National Laboratory, Texas Southern University and Kent State University, circumvents this problem with an isobar trick. Ruthenium-96 and zirconium-96 are isobars: nuclei with the same mass number, 96, but different atomic numbers, 44 and 40 respectively. That four-proton difference means the two collision systems carry different total electric charges while being otherwise nearly identical in size and mass. By defining the charge difference between the two systems, ΔQ, as the net charge of one system minus the other, the technique isolates exactly the small excess charge that distinguishes the nuclei — and cancels out the huge, nearly identical pair-production background that plagues every direct measurement.

The researchers go a step further with a double-ratio method that suppresses nearly all experimental systematic uncertainties. Because pions, kaons and protons dominate the charged hadron yields, the net charge can be reconstructed from the yields of positively and negatively charged pions, kaons and protons. Rather than measuring those yields absolutely — a procedure fraught with detector acceptance and efficiency corrections — the team compares the ratios of positive to negative particles within each isobar system and then takes the ratio of those ratios between the two systems. Most detector effects act almost identically on the two systems and cancel in the double ratio. The charge difference then reduces to a simple expression involving the average particle yields multiplied by small deviations of these double ratios from unity. The approach has already been applied successfully to the isobar collisions recorded at RHIC’s top energy, and the new simulations confirm that it reproduces direct charge counts almost exactly.

But knowing how much charge arrives at midrapidity is only half the story; physicists also want to know how far that charge has traveled. This is where the beam-energy scan enters. The rapidity gap, Δy, defined as the difference between the beam rapidity and the rapidity at which the charge is measured, sets the distance over which electric charge has been transported from the incoming projectiles toward the center of the collision. In symmetric collisions at a fixed energy, the accessible range of this gap is limited by detector coverage. By scanning the beam energy — in this study from 19.6 to 200 GeV per nucleon pair — while measuring charge always at the same midrapidity point, the researchers effectively sweep across about two and a half units of beam rapidity, mapping charge transport over a wide range of distances with a single, well-defined observable.

To test the concept, the team ran extensive simulations using two very different event generators: UrQMD, a microscopic transport model in which hadrons propagate along classical trajectories and interact through stochastic scatterings and string fragmentation, and Pythia8 with the Angantyr extension, which builds heavy-ion collisions as a Glauber-geometry superposition of nucleon-nucleon sub-collisions governed by multi-parton interactions, parton showers and Lund-string hadronization. In both models, electric charge and baryon number are carried solely by valence quarks, making the comparison between the two conserved quantities particularly instructive.

The simulations produced a strikingly clean result: the charge difference measured at midrapidity decreases exponentially with increasing rapidity gap, following the form of an exponential decay characterized by a slope parameter. The steeper the slope, the less efficiently charge makes the long journey from beam rapidity to midrapidity. In central collisions, where many nucleon-nucleon interactions occur and multiple scatterings are enhanced, the slope is smaller — meaning charge is transported more effectively. In peripheral, glancing collisions, a large fraction of the initial charge stays locked in the beam remnants and never reaches midrapidity at all, so ΔQ becomes sharply peaked near the beam direction.

Notably, the two models disagree quantitatively: Pythia8 predicts slope parameters roughly twice as large as those from UrQMD, revealing how sensitive the observable is to the microscopic details of charge redistribution. Adding dynamical baryon-junction formation during hadronization in Pythia8 slightly reduces the slope, implying that junction-like mechanisms can also enhance the long-range transport of electric charge.

The most conceptually rich part of the study, however, lies in the comparison between electric charge and baryon number. Electric charge is unambiguously carried by valence quarks. Baryon number, by contrast, is a mystery: it may simply ride along with valence quarks, or it may be traced by Y-shaped gluonic configurations known as baryon junctions, topological structures in the color field proposed decades ago and still unconfirmed experimentally. Because junctions are composed of low-momentum gluons, theory predicts they should be more readily stopped than high-momentum valence quarks — meaning baryon number should spread across rapidity more efficiently, or at least differently, than electric charge.

The team constructed a discriminating ratio, R(Isobar), which compares the average baryon number between the isobar systems to their charge difference, scaled by the ratio of atomic-number difference to mass number. If baryon number and electric charge travel together on the same valence quarks, this ratio should hover near unity. In both models, the ratio’s slope with rapidity gap is always negative — baryon number’s rapidity slope is consistently larger than that of electric charge. That is exactly the opposite of what genuine baryon-junction transport predicts, in which a flatter baryon distribution would emerge because easily-stopped gluonic junctions carry baryon number deep into midrapidity. Intriguingly, the measured value of this ratio from STAR’s analysis of 200 GeV isobar collisions is significantly larger than either model predicts, hinting that nature’s carriers of baryon number may involve physics beyond both frameworks.

The timing of this work is significant. RHIC has completed its final runs, and the isobar concept has been included in the facility’s final Beam Use Request before permanent shutdown, ensuring that data exist across the scan energies. The authors point to future measurements at the Electron-Ion Collider and in the LHC fixed-target program as further opportunities to constrain charge-redistribution models. What the study delivers now is a recipe: a realistic, experimentally feasible method for measuring electric-charge transport with precision comparable to baryon measurements, turning one of heavy-ion physics’ most elusive observables into a sharp tool for testing the microscopic mechanisms that govern how conserved quantities move through QCD matter. In the search to understand what, exactly, carries baryon number through the primordial soup of quarks and gluons, physicists now have a second, independent flashlight to shine into the dark.