Two streams of cars collide at an intersection. In the first moments after the impact, the debris scatters not randomly, but as if briefly enveloped by a common cloud. This is roughly what physicists saw at the Large Hadron Collider when protons smashed into each other at tremendous speed.
They studied particles composed of heavy quarks, such as the Upsilon and Psi. These particles live very briefly and decay into lighter ones. It turned out that the number of such particles depends on how many particles are produced in the collision. The denser the 'crowd', the fewer heavy particles survive. This is strange: if they simply flew apart, the crowd should not interfere.
The scientists tested four different measurement approaches: they looked at how particles scatter in directions, how they cluster, how those born immediately behave, and how those appearing later behave. None of the known models could explain all four results at once. However, all data converge if we assume that at the moment of collision an invisible medium arises that acts on particles uniformly in all directions.
This medium appears very early, almost instantly after the impact, and behaves as a single entity. This resembles the behavior of matter at extreme density, when quarks and gluons are no longer confined inside protons and form a common 'soup'. Perhaps this is exactly the 'soup' we see in the data.
as written by the authors arXiv:2604.17657
The multiplicity-dependent suppression of $$\Upsilon(n\mathrm{S})$$ states measured by CMS in $$pp$$ at $$\sqrt s=7\,$$TeV \cite{CMS2020}, and of $$\psi(2S)\big/J/\psi$$ measured by LHCb at $$\sqrt s=13\,$$TeV \cite{LHCb2024}, is subjected to four multi-differential tests: \emph{cone isolation}, \emph{azimuthal sectors}, \emph{transverse sphericity}, and \emph{prompt vs. non-prompt}. Cone and sphericity close a \emph{scissors constraint}: the local reading of the Comover Interaction Model is in tension with the cone data, its global reading with the sphericity data. The non-prompt flatness forces the mechanism to act at early proper times. None of the considered hadronic or string-based frameworks -- CIM local or global, PYTHIA 8 MPI \cite{Sjostrand2015}, rope hadronisation \cite{Bierlich2015}, CGC \cite{Ma2015}, Trainor TCM \cite{Trainor2008} -- naturally satisfies the four constraints simultaneously in its published form. The surviving class is an early, globally correlated medium consistent with partonic degrees of freedom, co-occurring with the ALICE strangeness enhancement \cite{ALICE_SE}, the long-range ridge \cite{CMS_ridge}, and below the threshold of the partonic baryon-meson $$v_2$$ \cite{ALICE_v2}, in a density window compatible with the Campanini \& Ferri equation of state \cite{Campanini2011}.
Heavy quarks are born in the first moments of the collision, but their behavior depends on what happens around them later. It's as if a newborn's fate depended on the weather outside.
In Liu Cixin's novel 'The Three-Body Problem', aliens use protons as probes, unfolding them into multidimensional structures. Here, colliding protons hint at the existence of a hidden medium that governs particles.