Strange Particle Production in pp Collisions at sqrt(s) = 0.9 and 7 TeV

TL;DR

This CMS study measures strange-hadron production in pp collisions at $\sqrt{s}=0.9$ and $7$ TeV by reconstructing $K_S^0$, $\Lambda$, and $\Xi^-$ from displaced-vertex decays and reporting $dN/dy$ and $dN/dp_T$ for $|y|<2$. Using MC-based efficiency corrections and lifetime checks, the analysis finds that the $p_T$ spectra are notably broader in data than in PYTHIA predictions and that total yields rise with energy by factors up to $\sim3$, with the largest deficit for $\Xi^-$. Tsallis fits quantify the $p_T$-shape, showing increasing $T$ with particle mass and collision energy, while production ratios show little to no energy dependence. The results provide essential baselines for MC tuning and heavy-ion comparisons, indicating that strangeness production in pp at LHC energies is not yet captured accurately by standard event generators, particularly for multi-strange baryons.

Abstract

The spectra of strange hadrons are measured in proton-proton collisions, recorded by the CMS experiment at the CERN LHC, at centre-of-mass energies of 0.9 and 7 TeV. The K^0_s, Lambda, and Xi^- particles and their antiparticles are reconstructed from their decay topologies and the production rates are measured as functions of rapidity and transverse momentum. The results are compared to other experiments and to predictions of the PYTHIA Monte Carlo program. The transverse momentum distributions are found to differ substantially from the PYTHIA results and the production rates exceed the predictions by up to a factor of three.

Figures (13)

• Figure 1: The $\pi^+\pi^-$ invariant mass distributions from data collected at $\sqrt{s} = 0.9\,\text{Te\spaceV}\xspace$ (left) and 7$\,\text{Te\spaceV}$ (right). The solid curves are fits to a double Gaussian and quadratic polynomial. The dashed curves show the quadratic background contribution.
• Figure 2: The $p\pi^-$ invariant mass distributions from data collected at $\sqrt{s} = 0.9\,\text{Te\spaceV}\xspace$ (left) and 7$\,\text{Te\spaceV}$ (right). The solid curves are fits to a double Gaussian signal and a background function given by $Aq^{B}$, where $q=M_{p\pi^-}-(m_p+m_{\pi^-})$. The dashed curves show the background contribution.
• Figure 3: The $\Lambda\pi^-$ invariant mass distributions from data collected at $\sqrt{s} = 0.9\,\text{Te\spaceV}\xspace$ (left) 7$\,\text{Te\spaceV}$ (right). The solid curves are fits to a double Gaussian signal and a background function given by $Aq^{1/2}+Bq^{3/2}$, where $q=M_{\Lambda\pi^-}-(m_\Lambda+m_{\pi^-})$. The dashed curves show the background contribution.
• Figure 4: Total efficiencies, including acceptance, trigger and event selection, reconstruction and particle selection, and other decay modes, as a function of $|y|$ (left) and $p_{\mathrm{T}}\xspace$ (right) for $\mathrm{K}_\mathrm{S}^0$, $\Lambda$, and $\Xi^-$ produced promptly in the range $|y|<2$. Error bars come from MC statistics.
• Figure 5: $\mathrm{K}_\mathrm{S}^0$ (left), $\Lambda$ (middle), and $\Xi^-$ (right) corrected decay time distributions at $\sqrt{s} =$ 0.9 and 7 TeV. The values of the lifetimes, derived from a fit with an exponential function (solid line), are shown in the legend along with the world-average value. The error bars and uncertainties on the lifetimes refer to the statistical uncertainty only.