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Scientists working with the ALICE detector at the Large Hadron Collider report measurements of the spin alignment, or polarization, of Lambda (Λ) and anti-Lambda (Λ̄) particles along the beam direction in lead–lead collisions at a center‑of‑mass energy of 5.36 TeV per nucleon pair. The polarization shows clear sine-shaped variations with angle, and for the first time at the LHC the team observes a polarization pattern tied to the third‑order event plane — a reference direction that comes from how the collision debris flows outward.
The analysis used a large 2023 data set collected in LHC Run 3: roughly 5 billion collisions, corresponding to an integrated luminosity of about 3.2 nb⁻¹. The measurement relied on ALICE subdetectors installed or upgraded for Run 3, in particular the Fast Interaction Trigger (FIT) to find events and estimate collision geometry, and the Inner Tracking System (ITS) and Time Projection Chamber (TPC) to follow charged particles. Λ hyperons were reconstructed through their typical weak decay into a proton and a pion (Λ → p π⁻ and Λ̄ → p̄ π⁺). The team applied standard track and topological cuts — for example, limits on the distance between decay points and the collision point, and on how well the reconstructed momentum points back to the collision — to reduce background.
The basic idea behind the measurement is that a swirling or vortical motion of the quark–gluon plasma (QGP) created in the collision can align particle spins, a process called spin–vorticity coupling. The event planes are angles that reflect the direction of anisotropic flow — the tendency of matter to stream out more in some directions than others — and different “orders” (second, third, etc.) pick out different flow patterns. The polarization along the beam is extracted from how the decay protons are emitted in the Λ rest frame. Because the weak decay violates parity, the proton direction is correlated with the Λ spin with a known decay parameter (αΛ = 0.750 ± 0.010 for Λ and αΛ̄ = −0.758 ± 0.012 for Λ̄). The analysis corrects for detector acceptance and for the limited precision in determining event planes (the event‑plane resolution at 25% centrality is reported as 0.76). Signal and background are separated by fitting the invariant mass and polarization distributions simultaneously; the background polarization is found to be consistent with zero.
These measurements matter because the observed polarization patterns probe the internal motion and transport properties of the QGP. By comparing the azimuthal (angle-dependent) polarization to hydrodynamic model calculations, the authors find sensitivity to the medium’s bulk viscosity — a measure of how the medium resists uniform compression — and to its vortical structure. The polarization measured with respect to the third‑order event plane is highlighted as an additional, independent handle that can help constrain both bulk and shear viscosity (shear viscosity measures resistance to shape‑changing flow). Earlier experiments and models had shown tensions: simple thermal‑vorticity models predicted opposite signs for some patterns, while newer calculations that include shear effects can reproduce the observed behavior. The new LHC data add higher statistics and a fresh constraint at higher collision energy.
There are important caveats. The paper reports careful estimates of systematic uncertainty by varying selections, event‑plane determination, and fitting methods, and by checking feed‑down contributions; the authors use a statistical test (the Barlow test) to judge which variations matter. Systematic uncertainties are relatively large in central collisions, where the polarization signal is close to zero. The excerpt available here is limited and does not list numeric values for the polarization amplitudes or all centrality and momentum dependences, so detailed quantitative conclusions should be read in the full paper.