Measurement of pion, kaon and proton production in proton–proton collisions at \(\sqrt{s} = 7\) TeV

Abstract

The measurement of primary \(\pi ^{\pm }\), \(K^{\pm }\), \(p\) and \({\overline{{p}}}\) production at mid-rapidity (\(|y| <\) 0.5) in proton–proton collisions at \(\sqrt{s}\) \(=\) 7 TeV performed with a large ion collider experiment at the large hadron collider (LHC) is reported. Particle identification is performed using the specific ionisation energy-loss and time-of-flight information, the ring-imaging Cherenkov technique and the kink-topology identification of weak decays of charged kaons. Transverse momentum spectra are measured from 0.1 up to 3 GeV/\(c\) for pions, from 0.2 up to 6 GeV/\(c\) for kaons and from 0.3 up to 6 GeV/\(c\) for protons. The measured spectra and particle ratios are compared with quantum chromodynamics-inspired models, tuned to reproduce also the earlier measurements performed at the LHC. Furthermore, the integrated particle yields and ratios as well as the average transverse momenta are compared with results at lower collision energies.

A preprint version of the article is available at ArXiv.

Introduction

The majority of the particles produced at mid-rapidity in proton–proton collisions are low-momentum hadrons not originating from the fragmentation of partons produced in scattering processes with large momentum transfer. Their production, therefore, cannot be computed from first principles via perturbative quantum chromodynamics (pQCD). Currently available models describing hadron-hadron collisions at high energy, such as the event generators PYTHIA6 [1], PYTHIA8 [2, 3], EPOS [4, 5] and PHOJET [6], combine pQCD calculations for the description of hard processes with phenomenological models for the description of the soft component. The measurement of low-momentum particle production and species composition is therefore important as it provides crucial input for the modelling of the soft component and of the hadronisation processes. Furthermore, it serves as a reference for the same measurement in Pb–Pb collisions to study the properties of the hot and dense strongly interacting medium with partonic degrees of freedom, the quark–gluon plasma, which is created in these collisions. In this paper, the measurement of primary \(\pi ^{\pm }\), \(K^{\pm }\), \(p\) and \({\overline{{p}}}\) production at mid-rapidity in proton–proton collisions at \(\sqrt{s}\) \(=\) 7 TeV using the ALICE detector [710] is presented. Primary particles are defined as prompt particles produced in the collision including decay products, except those from weak decays of light flavour hadrons and muons. Pions, kaons and protons are identified over a wide momentum range by combining the information extracted from the specific ionisation energy loss (d\(E\)/d\(x\)) measured in the inner tracking system (ITS) [11] and in the time projection chamber (TPC) [12], the time of flight measured in the time-of-flight (TOF) detector [13], the Cherenkov radiation measured in the high-momentum particle identification detector (HMPID) [14] and the kink-topology identification of the weak decays of charged kaons. Similar measurements in proton–proton collisions at \(\sqrt{s}\) \(=\) 900 GeV and 2.76 TeV are reported in [1517] and are included, together with lower energy data [1824], in the discussion of the evolution of particle production with collision energy. Similar measurement at the LHC have also been performed in the forward region [25].

The paper is organised as follows. In Sect. 2 the ALICE experimental setup is described, focusing on the detectors and the corresponding particle identification (PID) techniques relevant for the present measurement. Details of the event and track selection criteria and the corrections applied to the measured raw yields are also presented. In Sect. 3 the results on the production of primary \(\pi ^{\pm }\), \(K^{\pm }\), \(p\) and \({\overline{{p}}}\) are shown. These include the transverse momentum (\(p_\mathrm{{T}}\)) distributions and the \(p_\mathrm{{T}}\)-integrated production yields of each particle species and the K/\(\pi \) and p/\(\pi \) ratios. The evolution with collision energy of the \(p_\mathrm{{T}}\)-integrated particle yields, of their ratios and of their average transverse momenta \(\langle p_\mathrm{T} \rangle \) is also presented. In Sect. 4 particle spectra and their ratios (K/\(\pi \) and p/\(\pi \)) are compared with models, in particular with different PYTHIA tunes [13, 25, 26], EPOS [4,