[Paper Review] Central Diffraction in ALICE

R. Schicker|ArXiv.org|May 11, 2012

High-Energy Particle Collisions Research4 citations

TL;DR

This paper presents the first observation of central meson production in double gap events from minimum-bias proton-proton collisions at √s = 7 TeV using the ALICE experiment at the LHC. By applying a double gap trigger and analyzing invariant mass distributions of pion pairs, the study identifies a significant enhancement of f₀(980) and f₂(1270) resonances, providing strong evidence for double Pomeron exchange dominance in central diffractive processes.

ABSTRACT

The ALICE experiment at the Large Hadron Collider (LHC) at CERN consists of a central barrel, a muon spectrometer and of additional detectors for trigger and event classification purposes. The low transverse momentum threshold of the central barrel gives ALICE a unique opportunity to study the low mass sector of central production at the LHC. I will report on first analysis results of meson production in double gap events in minimum-bias proton-proton collisions at sqrt{s} = 7 TeV, and will describe a dedicated double gap trigger for future data taking.

Motivation & Objective

• To study central diffraction in proton-proton collisions at √s = 7 TeV using the ALICE detector at the LHC.
• To identify and isolate double gap events—characterized by central activity and no forward activity—using trigger and offline selection criteria.
• To measure the invariant mass spectrum of pion pairs in double gap events to probe the quantum numbers and exchange mechanisms in central diffractive production.
• To distinguish between Pomeron, Reggeon, and photon exchange contributions by analyzing the enhancement of specific meson resonances.

Proposed method

• A double gap trigger was implemented at L0 level using the absence of signals in V0A and V0C scintillator arrays to define gaps in pseudorapidity.
• Offline analysis selected events with no signals in V0A, V0C, FMD, and SPD, while requiring central activity in ITS, TPC, and TOF.
• Pion identification was performed using dE/dx from TPC and time-of-flight information for tracks with pT ≥ 300 MeV/c.
• Invariant mass distributions of like- and unlike-sign pion pairs were constructed to estimate and subtract background from detector acceptance and low-pT effects.
• The background contribution was estimated to be less than 5% from the like-sign pion pair distribution.
• The normalized, background-corrected invariant mass spectrum was compared between double gap and no-gap events to identify enhancements linked to specific quantum numbers.

Experimental results

Research questions

• RQ1 What is the fraction of double gap events in minimum-bias pp collisions at √s = 7 TeV in the ALICE experiment?
• RQ2 Which meson resonances are preferentially produced in double gap events, and what does this imply about the underlying exchange mechanism?
• RQ3 To what extent is the f₀(980) and f₂(1270) enhancement in double gap events due to quantum number selection (JPC = 0++, 2++)?
• RQ4 How does the double gap topology suppress states like K⁰ₛ and ρ⁰, and what does this indicate about the exchange mechanism?
• RQ5 What is the role of detector acceptance and pT thresholds in shaping the observed invariant mass spectra?

Key findings

• The fraction of double gap events in the minimum-bias sample was measured to be approximately 2 × 10⁻⁴.
• The track multiplicity in the central pseudorapidity range |η| < 0.9 was significantly lower in double gap events compared to no-gap events.
• The background contribution from like-sign pion pairs was estimated to be less than 5% in the invariant mass spectrum.
• The f₀(980) and f₂(1270) resonances showed a strong enhancement in double gap events compared to no-gap events.
• The enhancement of JPC = 0++ and 2++ states, while K⁰ₛ and ρ⁰ were suppressed, provides clear evidence for double Pomeron exchange dominance.
• The observed mass spectrum features, particularly the suppression of ρ⁰ and K⁰ₛ and enhancement of f₀(980) and f₂(1270), are consistent with quantum number selection rules for double Pomeron exchange.