Measurements of longitudinal and transverse momentum distributions for neutral pions in the forward-rapidity region with the LHCf detector

TL;DR

The study measures inclusive forward π⁰ production in p+p and p+Pb collisions with LHCf to test fundamental forward-physics hypotheses (limiting fragmentation, Feynman scaling) and to quantify nuclear effects via RpPb. It employs a two-stage MC framework with multiple hadronic generators and dedicated UPC modeling, and corrects for detector effects through Bayesian unfolding and acceptance corrections. The results show general compatibility with scaling hypotheses in the forward region and reveal strong suppression of forward π⁰ production in p+Pb relative to p+p, providing critical benchmarks for air-shower simulations used in cosmic-ray physics. Overall, the work constrains forward hadron production models and underlines the importance of precise forward measurements for high-energy cosmic-ray phenomenology.

Abstract

The differential cross sections for inclusive neutral pions as a function of transverse and longitudinal momentum in the very forward rapidity region have been measured at the Large Hadron Collider (LHC) with the Large Hadron Collider forward detector (LHCf) in proton-proton collisions at $\sqrt{s}=$ 2.76 and 7 TeV and in proton-lead collisions at nucleon-nucleon center-of-mass energies of $\sqrt{s_\text{NN}}=$ 5.02 TeV. Such differential cross sections in proton-proton collisions are compatible with the hypotheses of limiting fragmentation and Feynman scaling. Comparing proton-proton with proton-lead collisions, we find a sizable suppression of the production of neutral pions in the differential cross sections after subtraction of ultra-peripheral proton-lead collisions. This suppression corresponds to the nuclear modification factor value of about 0.1-0.3. The experimental measurements presented in this paper provide a benchmark for the hadronic interaction Monte Carlo simulation codes that are used for the simulation of cosmic ray air showers.

Figures (23)

• Figure 1: (color online). Observation of $\pi^{0}$ decay by a LHCf detector. Left: Type-I $\pi^{0}$ event having one photon entering each calorimeter. Right: Type-II $\pi^{0}$ event having two photons entering one calorimeter, here entering the small calorimeter.
• Figure 2: (color online). Reconstructed invariant mass distributions in $p+p$ collisions at $\sqrt{s}=7TeV$. Left: Type-II $\pi^{0}$ events in the Arm2 small calorimeter. Right: Type-II $\pi^{0}$ events in the Arm2 large calorimeter. The solid curves show the best-fit composite physics model to the invariant mass distributions.
• Figure 3: (color online). Diagrams of all multi-hit events that are rejected. Panels (a) and (b) show the multi-hit Type-I $\pi^{0}$ events and panels (c) and (d) show the multi-hit Type-II $\pi^{0}$ events. Red and green arrows indicate a background particle not originating in a $\pi^{0}$ decay and two photons originating in a $\pi^{0}$ decay, respectively.
• Figure 4: (color online). The acceptance map of $\pi^{0}$ detection by the LHCf detectors in $p_\text{z}$--$p_\text{T}$ phase space: Arm1 Type-I (left top), Arm1 Type-II (right top), Arm2 Type-I (left bottom), and Arm2 Type-II (right bottom). The fiducial area cuts and energy threshold ($E_{\text{photon}}>100GeV$) are taken into account. Dashed curves indicate lines of constant rapidity $\pi^{0}$s, $y$ = 8.8, 9.0, and 10.0 reading from top to bottom.
• Figure 5: (color online). LHCf $p_\text{T}$ distributions (filled circles) in $p+p$ collisions at $\sqrt{s}=7TeV$. Error bars indicate the total statistical and systematic uncertainties. The predictions of hadronic interaction models are shown for comparison: dpmjet (solid red line), qgsjet (dashed blue line), sibyll (dotted green line), epos (dashed-dotted magenta line), and pythia (dashed-double-dotted brown line).