The Large Hadron Collider never runs out of surprises, and this time the surprise wears a tiny, fast-moving cloak: charm quarks. These are heavy enough to be born in the initial hammer blows of a proton–proton collision, and light enough to be cradled into ordinary hadrons within the blink of a detector’s eye. In the ALICE experiment, scientists trace how these quarks exit the collision and decide which particles they become, from D mesons to more exotic charm baryons.
Renu Bala of the University of Jammu, India, leads the ALICE Collaboration’s charm measurements, presenting new results that tease apart two linked ideas: how often charm quarks are produced in proton–proton collisions, and how they choose their final homes among hadrons. The study uses data from 5.02 and 13 TeV collisions to infer the total charm production cross section at midrapidity by summing over all ground-state charm hadrons. It also peels back the curtain on fragmentation—the shuffling of quarks into mesons and baryons—and asks whether the rules that apply in cleaner electron–positron collisions hold when the quark sea is thick with gluons and other quarks. The answer, it seems, is both simpler and messier than we hoped: some rules hold for charm mesons, but charm baryons dance to a different tune when the environment shifts.
In plain terms, this is about following a quark through a crowded nightclub and watching how its choices depend on the crowd. Do quarks always end up as the same kinds of particles regardless of the room? Or does the mood of the crowd—here, the density of partons and the dynamics of the collision—tilt the odds toward certain final states? The ALICE results tilt toward the latter for baryons, nudging theorists to refine ideas about hadronization that go beyond simple fragmentation functions copied from cleaner experiments. The work is a reminder that nature’s recipes are often context-dependent, and that high energy collisions are laboratories not just for finding particles but for testing the rules by which matter assembles itself.
From a practical standpoint, the numbers matter because theorists use fragmentation functions to connect quark-level calculations to what detectors actually see. If universality holds, a calculation done in one kind of experiment can be ported to another. If not, models must accommodate how a parton-rich environment reshapes the final state. This study—led by Renu Bala of the University of Jammu for the ALICE Collaboration—shows both sides of the coin: mesons largely ride the familiar track, while baryons take a detour when the crowd gets dense. That distinction is where the physics gets interesting and where future measurements can pin down the right ideas about hadronization.
Charm production and meson fragmentation stay on familiar ground
In the pp collisions studied, charm quarks are created in the initial hard scatterings, and their fate is traced through the ground-state charm mesons such as D mesons. The ALICE results compare the measured prompt D mesons with the expectations from perturbative QCD calculations, notably FONLL. The data for prompt D mesons, and for non-prompt D mesons that arise from beauty-hadron decays, align with the theoretical framework when fragmentation functions are borrowed from e plus e minus and lepton-hadron collision data. In other words, for mesons, the fragmentation story seems universal enough to cross experimental boundaries. These results reaffirm the universality of meson fragmentation.
Moreover, by summing up the production cross sections of all ground-state charm hadrons at midrapidity, ALICE reconstructs the total charm production cross section and finds it to be consistent with FONLL predictions within uncertainties. The energy dependence between 5.02 and 13 TeV is mild, suggesting that the core production mechanism is stable across the LHC’s pp runs. The meson side is calm and predictable.
Baryon to meson: charm’s surprising preference in a busy environment
The Λc / D0 ratio climbs with pT and is higher than what fragmentation models calibrated in e+e− collisions would predict. The result is a striking sign that charm baryon production is enhanced in the hadronic environment of pp collisions, particularly at low to intermediate pT. This behavior has puzzled theorists for years and is precisely what hadronization models are meant to explain. Baryon production defies universal fragmentation, and the strong departure hints at new hadronization dynamics when the parton soup is dense.
Several theories chase this effect: color reconnection with string junctions in PYTHIA 8; the Statistical Hadronization Model coupled to the Relativistic Quark Model; and coalescence-inspired schemes like the Catania and Quark-Combination models. These approaches can bring the predicted Λc/D0 and Ξc/D0 ratios closer to the data, but none perfectly matches the full pattern, especially for charm-strange baryons. Environment-driven hadronization is real and hard to pin down.
What fragmentation fractions teach us and why it matters
The right-hand panel shows charm quark fragmentation fractions into different hadrons at midrapidity in pp collisions at 5.02 and 13 TeV. The lack of a strong energy trend suggests fragmentation patterns are stable across these energies in pp collisions, but the fractions differ from those measured in electron–positron collisions. Environment matters for fragmentation.
On the other hand, the total charm production cross section at midrapidity, reconstructed by summing the ground-state charm-hadron cross sections, lines up with FONLL calculations within one to one and a half standard deviations across the energies studied. This indicates that, despite the baryon-to-meson deviations, the overall charm yield is governed by perturbative dynamics that are robust to environmental changes. Charm production obeys robust perturbative rules.
With LHC Run 3 on the horizon, ALICE will push to higher precision and to rarer charm baryons, enabling sharper tests of hadronization models. Bigger data sets will tighten the uncertainties, especially for the charm-strange sector, and may reveal whether the baryon enhancement scales with event activity or depends on more subtle aspects of the collision geometry. In the broader arc of high-energy physics, these measurements help bridge the gap between the clean laboratories of e+e− collisions and the messier, lived-in environment of proton collisions, guiding theory toward a unified picture of how quarks become the particles we detect every day.
