[Paper Review] Diffraction at HERA, Tevatron and LHC

C. R. Royon|ArXiv.org|Dec 12, 2006

Particle physics theoretical and experimental studies3 citations

TL;DR

This paper reviews diffractive physics at HERA, Tevatron, and the LHC, focusing on exclusive diffractive events and the need for forward proton detectors at ATLAS. It proposes a trigger strategy using silicon detectors at 220 m and 420 m to tag protons and identify exclusive processes like Higgs boson production via double pomeron exchange, enabling precision QCD studies and searches for anomalous couplings at high luminosity.

ABSTRACT

In this short review, we describe some of the results on diffraction from the Tevatron and give some prospects for the LHC. In particular, we discuss the search for exclusive events at the Tevatron and their importance for the LHC program/ We finish by presenting the project of installing forward detectors in the ATLAS collaboration at 220 and 420 m.

Motivation & Objective

• To summarize inclusive and exclusive diffractive measurements from HERA and Tevatron, emphasizing the role of the Pomeron and diffractive PDFs.
• To motivate the installation of forward detectors at the LHC, particularly in ATLAS, to study exclusive diffractive processes.
• To develop a trigger strategy for detecting exclusive events such as Higgs boson decays to bb̄ using proton tagging at 220 m and 420 m.
• To enable precision QCD studies and searches for anomalous couplings via exclusive processes at high-luminosity LHC.
• To assess the feasibility of maintaining low L1 trigger rates while achieving high efficiency for exclusive event selection using kinematic and topological cuts.

Proposed method

• Use of rapidity gap selection and proton tagging via forward silicon detectors at 220 m and 420 m to identify diffractive events.
• Application of kinematic variables ξ (proton momentum fraction loss) and β (parton momentum fraction in Pomeron) to describe diffractive processes.
• Implementation of a two-level trigger system: Level 1 (L1) uses 3D silicon layers in forward detectors to identify proton hits and estimate ξ; Level 2 (L2) combines forward and central ATLAS data.
• Design of L1 triggers based on proton tag presence (ξ < 0.05), jet energy thresholds (ET > 40 GeV), and topological exclusivity cuts (e.g., (E_jet1 + E_jet2)/E_calo > 0.9).
• Use of timing information and compatibility checks between forward proton pseudorapidity and central jet rapidity to suppress background.
• Integration of forward detector data with central ATLAS trigger decisions, enabling exclusive event reconstruction even without direct central triggers.

Experimental results

Research questions

• RQ1 Can diffractive PDFs extracted at HERA be reliably used to describe hard diffractive processes in hadron-hadron collisions at the Tevatron?
• RQ2 What is the survival probability of diffractive events in hadron collisions, and does it depend on the type of hard interaction?
• RQ3 How can exclusive diffractive events such as Higgs boson production be efficiently triggered and reconstructed at the LHC?
• RQ4 What are the achievable trigger rates and reduction factors for exclusive processes at high luminosity (up to 1×10^34 cm⁻²s⁻¹) using forward proton tagging?
• RQ5 Can the use of silicon layers and timing information improve the efficiency and reduce background in exclusive event triggers?

Key findings

• The gluon density in the Pomeron is found to be significantly higher than the quark density, indicating a gluon-dominated Pomeron structure.
• At the Tevatron, factorisation breaking occurs due to soft interactions in initial and final states, affecting the survival probability of diffractive events.
• The survival probability for diffractive processes is expected to be approximately independent of the hard interaction type, supporting universal modeling.
• With a 220 m proton tag and jet exclusivity cuts, the L1 trigger rate for 2-jet events with ET > 40 GeV can be reduced to ~8.8 kHz at 1×10^34 cm⁻²s⁻¹ luminosity.
• Adding a 420 m proton tag and timing compatibility cuts can further reduce the rate to a few Hz at L2, enabling efficient exclusive event selection.
• The proposed trigger strategy allows for a few Hz event rates at L2 while maintaining high efficiency for Higgs boson decays to bb̄ and other exclusive processes.