The Large Hadron Collider doesn't just smash protons: it also collides entire atomic nuclei. When two nuclei smash together at tremendous speed, they create a tiny blob of matter so hot that protons and neutrons melt into a 'soup' of quarks and gluons. Fast particles have to plow through this soup, like bullets through water.
Each particle loses energy along the way, just like a bullet flying through water: the longer the path, the stronger the drag. Physicists from the CMS collaboration at the LHC have for the first time measured how this drag depends on the size of the colliding nuclei. They compared collisions of neon, oxygen, xenon, and lead nuclei.
The energy loss grows smoothly with the size of the nucleus, as if the particle were pushing through a longer and longer stretch of thick 'soup.' Simple models that only consider the initial state of the nuclei cannot explain these losses. But models where the particle loses energy along the way describe the data beautifully at high particle energies.
This result is another step toward understanding the properties of the hottest matter that can be created on Earth. It resembles the medium that filled the Universe in the first microseconds after the Big Bang.
as written by the authors arXiv:2602.21325
High-energy partons lose energy while propagating through the hot, strongly interacting medium produced in ultrarelativistic nucleus-nucleus collisions, leading to a suppression of particle production at high transverse momentum ($$p_\mathrm{T}$$). The dependence of this energy loss on the size of the colliding nuclear system has yet to be firmly established experimentally. This Letter presents a systematic study of charged-particle suppression across four different nucleus-nucleus collision systems using nuclear modification factors ($$R_\mathrm{AA}$$) measured by the CMS Collaboration at the CERN LHC. Previous CMS measurements of $$R_\mathrm{AA}$$ in oxygen-oxygen, xenon-xenon, and lead-lead collisions are recast with identical $$p_\mathrm{T}$$ intervals and are complemented by the first measurement of the charged-particle $$R_\mathrm{AA}$$ in neon-neon collisions at $$\sqrt{s_\mathrm{NN}}$$ = 5.36 TeV. The neon-neon data correspond to an integrated luminosity of 0.76 nb$$^{-1}$$. The $$R_\mathrm{AA}$$ in all collision systems examined show similar qualitative trends as a function of $$p_\mathrm{T}$$, but have a magnitude which is ordered with the nucleon number A. The $$R_\mathrm{AA}$$ feature a downward slope at low $$p_\mathrm{T}$$, a local minimum at around 5$$-$$7 GeV, and an upward slope with increasing $$p_\mathrm{T}$$. The $$R_\mathrm{AA}$$ are also compared in terms of A$$^{1/3}$$, which is proportional to the nuclear radius. Models including only initial-state nuclear effects fail to reproduce the observed trends, whereas energy loss models reproduce the trends in the region $$p_\mathrm{T}$$ $$\gt$$ 9.6 GeV.
A lead nucleus is 208 times heavier than a proton, but in collisions at the Large Hadron Collider it is accelerated to 99.999999% of the speed of light.
In Liu Cixin's novel 'The Three-Body Problem,' aliens use subatomic particles to transmit signals — much like the particles physicists use to probe the quark-gluon plasma.