Evaluating the blast-wave model as a description of 5 TeV p-Pb $\bf p_t$ spectra

Abstract

The blast-wave (BW) spectrum model is interpreted to reveal relativistic motion (collective flow) of the hadron emission system relative to the center-of-momentum (CM) frame in high-energy A-B collisions. In essence, any spectrum deviation in the CM frame from a reference distribution (e.g. Boltzmann distribution on transverse mass $m_t$) is interpreted to reveal a flowing particle source. The ALICE collaboration has applied the BW model to identified hadron (PID) spectra for four hadron species from 5 TeV $p$-Pb collisions. From model fits BW parameters $T_{kin}$ (freeze-out temperature) and $\langle \beta_t \rangle$ (transverse speed) are inferred that suggest strong radial expansion in more-central $p$-Pb collisions. Such results from the small $p$-Pb collision system are counterintuitive given that strong radial expansion should be driven by large density gradients. The present study is intended to address that problem. Several methods are employed to evaluate the quality of the BW model data description, including logarithmic derivatives and the Z-score statistic. The stability of the BW model definition across several applications to data is investigated. The BW model data description is compare to that of the two-component (soft+hard) model (TCM) that has been previously applied to the same $p$-Pb PID spectra. The general conclusion is that the BW model is falsified by $p$-Pb PID spectrum data according to standard statistical measures and that the fitted parameter values do not convey the intended meaning. Statistically acceptable data descriptions provided by the TCM indicate that other collision mechanisms (projectile-nucleon dissociation, dijet production), that are consistent with conventional QCD, are more likely responsible for observed spectrum characteristics.