CMS-PAS-HIN-25-020

CMS-PAS-HIN-25-020
Observation of enhanced baryon production using strange hadrons in OO collisions at 5.36$ \mathrm{TeV} $
CMS Collaboration
2026-07-06

Abstract: A hot deconfined medium known as the quark-gluon plasma (QGP) is created in collisions of ultrarelativistic heavy nuclei. In these systems, hadron production is strongly modified relative to proton–proton (pp) interactions. In the transverse momentum region 2 $ < p_\mathrm{T} < $ 5 GeV, collective radial flow and quark coalescence are important mechanisms that can produce such modifications. While these effects are well-established for collisions of large ions, such as lead, it is unknown if they are present in systems produced using smaller ions. To address this question, the $ p_\mathrm{T} $ spectra of two strange hadrons, $ \mathrm{K}^{0}_{\mathrm{S}} $ and $ \Lambda $, are measured in oxygen–oxygen (OO) and pp collisions at a center-of-mass energy per nucleon pair of $ \sqrt{s_{\mathrm{NN}}} = $ 5.36 TeV. The measurements are produced using the CMS detector at the LHC, and the OO and pp datasets correspond to integrated luminosities of 6.8$ \mathrm{nb}^{-1} $ and 1.1$ \mathrm{pb}^{-1} $, respectively. The spectral modification of $ \mathrm{K}^{0}_{\mathrm{S}} $ mesons in OO collisions relative to pp collisions is found to be similar to that of inclusive charged hadrons. Conversely, the relative production of $ \Lambda $ baryons in OO versus pp collisions exhibits a pronounced enhancement when compared to $ \mathrm{K}^{0}_{\mathrm{S}} $. A baryon-to-meson enhancement is observed in OO collisions, suggesting the formation of a deconfined medium, and offering insight into the roles of strangeness, radial flow, and quark coalescence in QGP.
Links: CDS record (PDF) ; CADI line (restricted) ;

Figures	Summary	References	CMS Publications

Figures
png pdf	Figure 1: (Upper panel) Invariant differential cross sections of charged hadrons (blue circles), $ \mathrm{K^0_S} $ (orange squares), and $ \Lambda $ (magenta diamonds) in $ \text{OO} $ (solid markers) and pp (open markers) collisions. The markers show the cross section averaged across the entire bin width. Statistical uncertainties are shown as error bars and are smaller than the marker size. (Lower panels) Systematic uncertainties, with normalization uncertainties excluded, for the corresponding cross sections in OO (colored band) and pp (black band) collisions.
png pdf	Figure 2: The charged-hadron (blue circles), $ \mathrm{K^0_S} $ (orange squares) and $ \Lambda $ (magenta diamonds) nuclear modification factors in OO collisions. Statistical uncertainties are shown as error bars, while boxes show systematic uncertainties after excluding the normalization uncertainty. The normalization uncertainty is shown by the gray band around unity.
png pdf	Figure 3: The ratio of the $ \Lambda $ and $ \mathrm{K^0_S} $ cross sections as a function of $ p_{\mathrm{T}} $ in $ \text{OO} $ (magenta circles) and pp (blue squares) collisions. Statistical uncertainties are shown as error bars and are smaller than the marker size. The boxes show the total systematic uncertainties.
png pdf	Figure 4: Charged-particle nuclear modification factor, $ R_\text{AA} $, measured for $ \text{OO} $, NeNe, XeXe, PbPb, and pPb collisions. The bands around the markers indicate the total systematic uncertainties after excluding the normalization uncertainties, while the error bars show the statistical uncertainties. The boxes on the left, centered at unity, represent the relative normalization uncertainty for each collision system.

Summary

In summary, the invariant differential cross sections of $ \mathrm{K}^{0}_{\mathrm{S}} $ and $ \Lambda $ particles have been measured in oxygen-oxygen (OO) collisions at a center-of-mass energy per nucleon pair of 5.36 TeV. Using a reference measurement performed in proton-proton (pp) collisions at the same collision energy, the nuclear modification factor ($ R_\text{AA} $) of these particles in OO collisions is reported. The $ \mathrm{K}^{0}_{\mathrm{S}} $ $ R_\text{AA} $ is consistent with the $ R_\text{AA} $ of charged hadrons, whereas the $ \Lambda $ $ R_\text{AA} $ shows a significant enhancement relative to charged hadrons in the transverse momentum range $ 2 < p_\mathrm{T} < 8 $ GeV. At higher $ p_\mathrm{T} $, both $ \mathrm{K}^{0}_{\mathrm{S}} $ and $ \Lambda $ $ R_\text{AA} $ values are significantly suppressed, likely indicating the presence of parton energy loss effects in a deconfined QGP medium. An enhancement of the $ \Lambda $/$ \mathrm{K}^{0}_{\mathrm{S}} $ cross section ratio in OO collisions relative to pp collisions is observed for $ 2.3 < p_\mathrm{T} < 5.2 $ GeV, with a significance exceeding five standard deviations. This establishes an observation of a baryon-to-meson enhancement in OO collisions and indicates modified hadron production consistent with quark coalescence and collective radial flow in a strongly interacting deconfined medium.

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