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Common baryon source found in proton collisions

7 July 2020

A report from the ALICE experiment

Figure 1

High-energy hadronic collisions, such as those delivered by the LHC, result in the production of a large number of particles. Particle pairs produced close together in both coordinate and momentum space are subject to final-state effects, such as quantum statistics, Coulomb forces and, in the case of hadrons, strong interactions. Femtoscopy uses the correlation of such pairs in momentum space to gain insights into the interaction potential and the spatial extent of an effective particle-emitting source.

Abundantly produced pion pairs are used to assess the size and evolution of the high-density and strongly interacting quark–gluon plasmas, which are formed in heavy-ion collisions. Recently, high-multiplicity pp collisions at the LHC have raised the possibility of observing collective effects similar to those seen in heavy-ion collisions, motivating detailed investigations of the particle source in such systems as well. A universal description of the emission source for all baryon species, independent of the specific quark composition, would open new possibilities to study the baryon–baryon interaction, and would impose strong constraints on particle-production models.

The ALICE collaboration has recently used p–p and p–Λ pairs to perform the first study of the particle-emitting source for baryons produced in pp collisions. The chosen data sample isolates the 1.7 permille highest-multiplicity collisions in the 13 TeV data set, yielding events with 30 to 40 charged particles reconstructed, on average, per unit of rapidity. The yields of protons and Λ baryons are dominated by contributions from short-lived resonances, accounting for about two thirds of all produced particles. A basic thermal model (the statistical hadronisation model) was used to estimate the number and composition of these resonances, indicating that the average lifetime of those feeding to protons (1.7 fm) is significantly shorter than those feeding to Λ baryons (4.7 fm) – this would have led to a substantial broadening of the source shape if not properly accounted for. An explicit treatment of the effect of short-lived resonances was developed by assuming that all primordial particles and resonances are emitted from a common core source with a Gaussian shape. The core source was then folded with the exponential tails introduced by the resonance decays. The resulting root-mean-square width of the Gaussian core scales from 1.3 fm to 0.85 fm as a function of an increase in the pair’s transverse mass (mT) from 1.1 to 2.2 GeV, for both p–p and p–Λ pairs (see figure). The transverse mass of a particle is its total energy in a coordinate system in which its velocity is zero along the beam axis. The two systems exhibit a common scaling of the source size, indicating a common emission source for all baryons. The observed scaling of the source size with mT is very similar to that observed in heavy-ion collisions, wherein the effect is attributed to the collective evolution of the system.

This result is a milestone in the field of correlation studies, as it directly relates to important topics in physics. The common source size observed for p–p and p–Λ pairs implies that the spatial- temporal properties of the hadronisation process are independent of the particle species. This observation can be exploited by coalescence models studying the production of light nuclei, such as deuterons or 3He, in hadronic collisions. Moreover, the femtoscopy formalism relates the emission source to the interaction potential between pairs of particles, enabling the study of the strong nuclear force between hadrons, such as p–K, p–Ξ, p–Ω and ΛΛ, with unprecedented precision.

Further reading

ALICE Collab. 2020 arXiv:2004.08018.

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