[Paper Review] Status of the Forward Physics Projects in ATLAS

Stefan Ask|ArXiv.org|Jun 5, 2007

Particle Detector Development and Performance4 citations

TL;DR

This paper details the status and design of forward physics detectors in the ATLAS experiment at the LHC, including LUCID for luminosity monitoring, ZDC for neutral particle detection, and ALFA for measuring elastic proton scattering. The ALFA system achieves a luminosity precision of 2% (stat) ± 2% (syst), enabling high-precision measurements of total cross section, nuclear slope, and ρ parameter with sub-1% to 4% precision.

ABSTRACT

The ATLAS experiment at the LHC is building several detector systems for forward physics studies and to determine the luminosity. The main forward systems consist of a Cerenkov detector called LUCID, a Zero Degree Calorimeter (ZDC) and Roman Pots which will house a scintillating fiber tracker system called ALFA. Here we report some of the forward physics activities that are foreseen in ATLAS together with the status of the related detector systems.

Motivation & Objective

• To develop and deploy forward detector systems for precise luminosity determination in ATLAS during LHC operations.
• To enable high-precision measurements of elastic proton scattering at low angles using the ALFA scintillating fiber tracker in Roman Pots.
• To study forward particle production and diffractive physics using the ZDC and LUCID systems.
• To achieve absolute luminosity calibration using ALFA as a reference, improving on machine parameter-based estimates.
• To support the ATLAS heavy ion program through ZDC-based centrality and luminosity measurements.

Proposed method

• ALFA uses scintillating fiber trackers in Roman Pots at 240 m from the IP to measure elastic proton scattering at angles down to 3 μrad.
• Luminosity is measured via LUCID using Cerenkov light from forward inelastic pp interactions, with three methods: zero counting, hit counting, and particle counting.
• The ZDC employs tungsten/quartz calorimeter modules and transverse quartz rods to measure neutral particles at 0° and reconstruct shower positions.
• The differential cross section for elastic scattering is modeled using the formula: |dN/dt| = L·π|−2α/|t| + σ_tot/(4π)(i+ρ)e^{−B|t|/2}|², with t = −(p·sinθ)².
• Luminosity calibration is performed by cross-calibrating LUCID with ALFA measurements under high-β* optics, reducing systematic uncertainty.
• Systematic errors from beam properties, detector alignment, acceptance, and background are studied to ensure luminosity precision.

Experimental results

Research questions

• RQ1 Can the ALFA system achieve the required spatial resolution of ~30 μm to measure elastic scattering at angles as low as 3 μrad?
• RQ2 What is the achievable precision of luminosity measurement using the ALFA system and how does it compare to alternative methods like the optical theorem?
• RQ3 To what extent can the ZDC contribute to luminosity and centrality measurements in heavy ion collisions at the LHC?
• RQ4 How accurately can LUCID measure instantaneous and integrated luminosity using Cerenkov light and multiple counting techniques?
• RQ5 Can the forward detector systems improve the overall hermeticity and background suppression in diffractive physics studies?

Key findings

• The ALFA system is designed to achieve a luminosity measurement precision of 2% (statistical) ± 2% (systematic) using elastic scattering fits.
• The simulation shows ALFA’s acceptance covers the Coulomb interference region, enabling measurement of the real-to-imaginary amplitude ratio ρ with ~4% precision.
• The total proton-proton cross section (σ_tot) and nuclear slope parameter (B) can be measured with precisions of ~1% and ~0.5%, respectively.
• LUCID achieves time resolution sufficient to resolve individual bunch crossings and uses three methods—zero counting, hit counting, and particle counting—for luminosity monitoring.
• The ZDC achieves near-100% acceptance for heavy ion collisions and enables luminosity determination with better than 5% precision.
• Simulations show clear meson peaks in the ZDC invariant mass spectrum, confirming its capability to reconstruct forward mesons and baryons in pp collisions.