[Paper Review] Measurement of hadron composition in charged jets from pp collisions with the ALICE experiment

Benjamin Andreas Heß|arXiv (Cornell University)|Aug 25, 2014

Particle physics theoretical and experimental studies1 references3 citations

TL;DR

This paper presents the first measurement of identified charged hadron composition in jets from pp collisions at √s = 7 TeV using the ALICE experiment at the LHC. By applying advanced particle identification via dE/dx in the TPC with the TPC Coherent Fit method, it extracts pion, kaon, and proton spectra and ratios in jets, revealing increasing strangeness fraction with z^ch and baryon suppression at high momentum fractions, with PYTHIA tunes showing deviations at low pT and low pT,jet.

ABSTRACT

We report on the first measurement of the charged hadron composition in charged jets from pp collisions. The ALICE detector at the LHC was used to study charged pion, kaon and (anti-)proton production in jets. The results were extracted from $2 imes 10^8$ minimum bias events at a centre-of-mass energy of $\sqrt{s} = 7$ TeV. We present the transverse momentum ($p_{\mathrm{T}}$) spectra and reduced momentum spectra ($z^{\mathrm{ch}} \equiv p_{\mathrm{T}}^{\mathrm{track}} / p_{\mathrm{T, jet}}^{\mathrm{ch}}$) of $π$, K and p in charged jets with $p_{\mathrm{T, jet}}^{\mathrm{ch}}$ between 5 and 20 GeV/$c$. The measurements are compared to Monte Carlo calculations.

Motivation & Objective

• To measure the composition of charged hadrons (pions, kaons, protons) in jets from pp collisions at √s = 7 TeV.
• To investigate the fragmentation patterns of identified hadrons as a function of transverse momentum (pT) and momentum fraction (z^ch) in jets.
• To test the performance of Monte Carlo event generators (PYTHIA) in describing jet fragmentation with identified particles.
• To assess the impact of detector effects and systematic uncertainties on particle identification in high-multiplicity jet environments.

Proposed method

• The analysis uses 2×10^8 minimum bias pp events at √s = 7 TeV recorded by the ALICE detector.
• Charged tracks with pT > 0.15 GeV/c and |η| < 0.9 are reconstructed using the ITS and TPC.
• Jets are reconstructed using the anti-kT algorithm with R = 0.4, selecting only |η_jet| < 0.5.
• Particle identification is performed using dE/dx measurements in the TPC with a resolution of ~5%.
• The TPC Coherent Fit method is employed to extract raw yields of π, K, and p differentially in pT and z^ch, using a 2D likelihood maximization with regularization on particle fractions.
• Systematic corrections are applied for detector efficiency, acceptance, momentum resolution, and muon contamination (attributed to pions).

Experimental results

Research questions

• RQ1 How does the relative yield of pions, kaons, and protons vary with transverse momentum (pT) and momentum fraction (z^ch) in charged jets?
• RQ2 To what extent does the jet transverse momentum (pT,jet) influence the fragmentation pattern of identified hadrons?
• RQ3 How well do PYTHIA tunes (Perugia0, Perugia0NoCR, Perugia2011) describe the measured hadron spectra and ratios in jets?
• RQ4 What is the behavior of strangeness (K/π ratio) and baryon (p/π ratio) fractions as a function of z^ch in jets?
• RQ5 What systematic uncertainties affect the particle identification and yield extraction in high-multiplicity jet environments?

Key findings

• The K/π ratio increases monotonically with z^ch, reaching 0.5–0.6 at high z^ch, indicating a rise in strangeness fraction in jet fragments.
• The p/π ratio peaks at z^ch ≈ 0.5–0.6 with a maximum of 0.15–0.2 and decreases toward z^ch = 1, indicating suppression of leading baryons.
• For z^ch > 0.2, the K/π ratio shows scaling across different pT,jet bins, and p/π ratios scale for pT,jet = 10–15 and 15–20 GeV/c.
• PYTHIA Perugia0, Perugia0NoCR, and Perugia2011 tunes describe the data within ~30% for most pT and pT,jet regions, except for protons at pT < 0.5 GeV/c, where deviations exceed 100%.
• The maximum in the proton spectrum at pT ≈ 2 GeV/c is well reproduced by all tunes, but the width and high-pT slope are not accurately described.
• A tension is observed between data and PYTHIA at low pT,jet and for low-pT pions and protons, indicating limitations in current fragmentation models.