[Paper Review] Charm production in SIBYLL

R. Engel, Anatoli Fedynitch+3 more|arXiv (Cornell University)|Feb 23, 2015

Astrophysics and Cosmic Phenomena4 citations

TL;DR

This paper presents SIBYLL 2.3rc1, an updated version of the SIBYLL hadronic interaction model tuned to LHC data and extended to include a phenomenological model of charm production. The model improves predictions for atmospheric muon and neutrino fluxes by incorporating both perturbative and non-perturbative charm production, with the latter dominating in the forward direction and significantly affecting high-energy lepton fluxes despite limited experimental constraints.

ABSTRACT

SIBYLL 2.1 is an event generator for hadron interactions at the highest energies. It is commonly used to analyze and interpret extensive air shower measurements. In light of the first detection of PeV neutrinos by the IceCube collaboration the inclusive fluxes of muons and neutrinos in the atmosphere have become very important. Predicting these fluxes requires understanding of the hadronic production of charmed particles since these contribute significantly to the fluxes at high energy through their prompt decay. We will present an updated version of SIBYLL that has been tuned to describe LHC data and extended to include the production of charmed hadrons.

Motivation & Objective

• To improve the SIBYLL event generator by tuning it to LHC data, particularly for high-energy proton-proton interactions.
• To extend the model to include the production of charmed hadrons, which are critical for predicting prompt atmospheric muon and neutrino fluxes.
• To address the lack of experimental constraints on large-x_L charm production by developing a phenomenological model that includes both perturbative and non-perturbative contributions.
• To assess the impact of forward charm production on the inclusive fluxes of atmospheric leptons, especially in the context of IceCube's PeV neutrino observations.
• To quantify uncertainties in atmospheric lepton flux predictions arising from limited phase-space coverage of current charm production measurements.

Proposed method

• The model updates the inelastic proton-proton cross section by narrowing the transverse profile function of hard partons to reduce overestimation at high energies, aligning with TOTEM's 73.5 mb measurement.
• The soft cross section parametrization is refit to LHC data, including $\bar{p}$-p and p-p measurements, and extended to meson-nucleon interactions.
• A new charm production model is implemented with a perturbative component based on the MRS calculation and a non-perturbative component to account for forward production not accessible at colliders.
• The model includes energy-dependent $p_\perp$-cutoffs for minijets to account for parton density saturation effects, avoiding unphysical multiplicity tails.
• The fragmentation model is extended to enhance baryon pair production, improving agreement with LHC data on baryon multiplicities.
• The model is validated by comparing the weighted energy spectrum of D mesons with the MRS perturbative calculation at $\sqrt{s} = 7$ TeV, with normalization at $x_F = 0.19$.

Experimental results

Research questions

• RQ1 How can SIBYLL be updated to better describe high-energy proton-proton interactions using LHC data?
• RQ2 To what extent does the inclusion of charm production improve the prediction of atmospheric muon and neutrino fluxes?
• RQ3 What is the relative contribution of perturbative versus non-perturbative charm production to the forward lepton flux?
• RQ4 How do current LHC measurements constrain the forward charm production region, and what are the implications for atmospheric lepton flux predictions?
• RQ5 What is the uncertainty in atmospheric lepton flux predictions due to limited experimental coverage of large-$x_{\rm L}$ charm production?

Key findings

• The updated SIBYLL 2.3rc1 model reduces the inelastic $p$-$p$ cross section to 73.5 mb at $\sqrt{s} = 7$ TeV, matching TOTEM's measurement, by narrowing the transverse profile of hard partons.
• The model's perturbative charm component shows good agreement with the MRS analytic calculation at $x_F = 0.19$, validating its perturbative foundation.
• Non-perturbative charm production dominates in the forward region ($x_{\rm L} \gtrsim 0.5$), contributing significantly to high-energy atmospheric lepton fluxes despite lacking direct experimental constraints.
• At $\sqrt{s} = 7$ TeV, the LHCb detector covers only about 10% of the $Z_{pD}$ contribution from forward charm production, highlighting the limited phase-space coverage.
• The model predicts that atmospheric lepton fluxes are most sensitive to forward charm production, which is currently not well constrained by experiments or theory.
• Future work will explore parameter sets that maximize or minimize forward charm yield while remaining consistent with data, to quantify uncertainty in atmospheric flux predictions.