[Paper Review] LHCf Measurements of Very Forward Particles at LHC

T. Sako|arXiv (Cornell University)|Oct 1, 2010

Astrophysics and Cosmic Phenomena4 citations

TL;DR

LHCf measures very forward neutral pions and neutrons at LHC to constrain high-energy hadron interaction models for cosmic-ray air shower simulations. Using two forward calorimeters with high-resolution sampling and position-sensitive layers, it detects gamma-ray pairs from $π^0$ decays, achieving a clear $π^0$ peak at 150 MeV/c² and measuring $π^0$ energies above 3 TeV, providing crucial data for extrapolating models to ultra-high-energy cosmic-ray regimes.

ABSTRACT

The LHC forward experiment (LHCf) is specifically designed for measurements of the very forward ($η$$>$8.4) production cross sections of neutral pions and neutrons at Large Hadron Collider (LHC) at CERN. LHCf started data taking in December 2009, when the LHC started to provide stable collisions of protons at $\sqrt{s}$=900\,GeV. Since March 2010, LHC increased the collision energy up to $\sqrt{s}$=7\,TeV. By the time of the symposium, LHCf collected 113k events of high energy showers (corresponding to $\sim$7M inelastic collisions) at $\sqrt{s}$=900\,GeV and $\sim$100M showers ($\sim$14 nb$^{-1}$ of integrated luminosity) at $\sqrt{s}$=7\,TeV. Analysis results with the first limited sample of data demonstrate that LHCf will provide crucial data to improve the interaction models to understand very high-energy cosmic-ray air showers.

Motivation & Objective

• To measure very forward neutral pion and neutron production cross sections at LHC energies to improve models of high-energy hadron interactions.
• To provide anchor points for extrapolating interaction models into the ultra-high-energy cosmic-ray regime (10^17 eV).
• To test the energy dependence of hadron interaction models using data from multiple collision energies (900 GeV, 7 TeV, and 14 TeV).
• To reduce uncertainties in cosmic-ray air shower simulations by constraining particle production in the forward region.
• To validate and refine Monte Carlo generators such as QGSJET2 and Sybill using experimental data from LHCf.

Proposed method

• LHCf uses two forward calorimeters (Arm1 and Arm2) at ±140 m from IP1, each with 16 layers of plastic scintillators interleaved with tungsten absorbers.
• The detectors employ position-sensitive scintillating fiber (SciFi) and silicon strip sensors to measure shower positions with sub-millimeter resolution.
• Invariant mass reconstruction of gamma-ray pairs from $π^0 \to \gamma\gamma$ decays enables identification of neutral pions.
• Energy spectra are derived from high-energy shower events (>10 GeV) using data from 900 GeV and 7 TeV collisions.
• Data are collected using triggers based on beam position monitors (BPTX) and high-energy shower signals in the calorimeters.
• Contamination from residual gas collisions is suppressed by comparing data from crossing and non-crossing bunches, showing a two-order-of-magnitude lower background.

Experimental results

Research questions

• RQ1 How do the measured forward neutral pion and neutron production cross sections compare with predictions from QGSJET2 and Sybill models at 900 GeV and 7 TeV?
• RQ2 Can the invariant mass reconstruction of gamma-ray pairs from $π^0$ decays be used to identify and measure high-energy $π^0$ candidates with high precision?
• RQ3 What is the energy spectrum of forward photons and $π^0$'s at 7 TeV, and how does it compare to Monte Carlo simulations?
• RQ4 How does the beam-beam collision contamination compare to residual gas background in the forward region?
• RQ5 What is the energy dependence of the forward particle spectrum, and does it show scaling or softening as predicted by different models?

Key findings

• A clear peak in the invariant mass distribution at 150 MeV/c² confirms the detection of $π^0$ decays, with $π^0$ candidates observed at energies above 3 TeV.
• The particle identification parameter L90% shows good agreement with QGSJET2 predictions in both shape and gamma-to-hadron ratio at 900 GeV.
• Energy spectra of gamma-ray-like and hadron-like events at 900 GeV are consistent with Monte Carlo simulations within large statistical and systematic uncertainties.
• The comparison of spectra between the two towers reveals a hard (beamed) spectrum in the small tower covering the zero-degree region, indicating strong forward emission.
• Residual gas background is suppressed by a factor of 100 compared to beam-beam collisions, confirming the dominance of beam-beam events in the data.
• LHCf data at 900 GeV and 7 TeV provide the first direct measurements of very forward $π^0$ production at these energies, with potential to resolve model differences at 14 TeV.