New results on $φ$(1020) production from the NA61/SHINE experiment at CERN SPS

TL;DR

The paper addresses how phi(1020) production behaves in intermediate-size nuclear systems at SPS energies and what this implies for strangeness production and deconfinement. It employs the NA61/SHINE detector to measure double-differential $d^2n/(dy\, dp_T)$ and $d n/dy$ for central Ar+Sc collisions at $\sqrt{s_{NN}} = 8.8$, 11.9, 16.8$ GeV through the $\phi\to K^+K^-$ channel using a tag-and-probe PID approach, and compares the results to UrQMD, UrQMD with hydrodynamics, and Angantyr. The findings show significant phi enhancement relative to $p+p$ and behavior similar to Pb+Pb when scaled to pions, while the tested models fail to reproduce the measured spectra and yields, highlighting a nontrivial system-size dependence of strangeness production at SPS energies. These results constrain hadron production models in this energy regime and underscore the need for refined theory to describe intermediate-size collision systems.

Abstract

NA61/SHINE is a multipurpose, fixed-target hadron spectrometer at the CERN SPS. Its research program includes studies of strong interactions as well as reference measurements for neutrino and cosmic-ray physics. A significant advantage of NA61/SHINE over collider experiments is its extended coverage of phase space available for particle production. This includes the entire projectile hemisphere of the collision, with no low-pT cut-off. The energy and system-size dependence of strangeness production plays an essential role in studies of the transition from confined to deconfined matter. With its zero net strangeness and its valence structure composed predominantly of s and $\bar{s}$ valence quarks, the $φ$(1020) meson will not be sensitive to strangeness-related effects in a purely hadronic scenario, but will behave like a doubly strange particle in a partonic system. This contribution presents the first-ever results on $φ$(1020) meson production in intermediate-size systems at the CERN SPS, that is, central Ar+Sc collisions at $\sqrt{s_\text{NN}}$ = 8.8, 11.9, and 16.8 GeV. The presented results include double-differential rapidity-transverse momentum (y-pT) distributions, transverse mass (mT) spectra at midrapidity, pT-integrated rapidity spectra, mean multiplicities (4$π$ yields), and particle ratios. These are compared to data on Pb+Pb and p+p collisions. A discussion of open and hidden strangeness production enhancement is included. Finally, a comparison with three microscopic models is shown, demonstrating their overall failure in describing these new measurements.

Figures (6)

• Figure 1: Schematic layout of the NA61/ SHINE detector system (horizontal cut in the beam plane, not to scale) showing the state of the detector during $\text{Ar}$+$\text{Sc}$ data taking in 2015.
• Figure 2: Illustration of the tag-and-probe method (see text) on one of analysis bins for $\text{Ar}$+$\text{Sc}$ collisions at $\sqrt{s_\text{NN}}\xspace$$=$ 16.8. Left: invariant mass spectrum for the tag sample where at least one of kaon candidates in the pair needs to pass the strict PID cut. Right: invariant mass spectrum for the probe sample where both kaon candidates in the pair need to pass the strict PID cut. Red curves are fitted background contributions, while blue curves show the sum of background and signal components.
• Figure 3: Double-differential ($y$, $p_\mathrm{T}$ ) spectra of $\phi$(1020) produced in the 10% most central $\text{Ar}$+$\text{Sc}$ collisions at $\sqrt{s_\text{NN}}\xspace$$=$ 16.8. Vertical bars represent statistical uncertainties, red rectangles systematic uncertainties, and horizontal bars depict $p_\mathrm{T}$ bin sizes. Blue curves are fits used to obtain the integrals of the unmeasured tails of the $p_\mathrm{T}$ spectra to calculate $p_\mathrm{T}$-integrated $\dv*{n}{y\xspace}$ distribution.
• Figure 4: Transverse mass spectra of $\phi$(1020) mesons produced at midrapidity in the 10% most central $\text{Ar}$+$\text{Sc}$ reactions at three collision energies indicated in the plots. Vertical bars represent statistical uncertainties, red rectangles systematic uncertainties, and horizontal bars depict $p_\mathrm{T}$ bin sizes. Blue curves are exponential fits to obtain inverse slope parameters (see text).
• Figure 5: Rapidity distributions of $\phi$(1020) mesons produced in the 10% most central $\text{Ar}$+$\text{Sc}$ collisions measured by the NA61/ SHINE experiment (red circles), compared to model predictions. Vertical bars represent statistical uncertainties, red rectangles systematic uncertainties, and horizontal bars depict $p_\mathrm{T}$ bin sizes. Model calculations were performed by S. Veli (Technical University of Munich) and T. Janiec (The University of Manchester).