[Paper Review] QCD analysis of diffractive phenomena

L. Schoeffel|ArXiv.org|Jul 25, 2007

Quantum Chromodynamics and Particle Interactions1 references3 citations

TL;DR

This paper presents a QCD analysis of diffractive processes at HERA and the Tevatron, using the dipole model and generalized parton distributions (GPDs) to describe hard diffractive scattering with rapidity gaps. It demonstrates that the transverse size of the color-singlet gluon-diquark system remains small at high energies, consistent with perturbative QCD, and highlights exclusive Higgs boson production at the LHC as a key future application for clean, high-resolution measurements via proton dissociation in Roman pot detectors.

ABSTRACT

The most important results on subnuclear diffractive phenomena obtained at HERA and Tevtaron are reviewed and new issues in nucleon tomography are discussed. Some challenges for understanding diffraction at the LHC, including the discovering of the Higgs boson, are outlined.

Motivation & Objective

• Understand the origin of large rapidity gap events in deep inelastic scattering at HERA, which challenge standard QCD expectations due to exponential suppression of color-singlet exchange.
• Investigate the viability of perturbative QCD descriptions for diffractive processes by analyzing hard scales in diffractive DIS and hard diffractive jet production at the Tevatron.
• Develop a theoretical framework based on the dipole model and generalized parton distributions (GPDs) to describe the spatial and momentum structure of the Pomeron and its partonic content.
• Explore the potential of exclusive diffractive processes at the LHC, particularly for Higgs boson detection via proton dissociation in forward Roman pot detectors.
• Constrain the total angular momentum of quarks in the nucleon using DVCS/Bethe-Heitler interference measurements from the HERMES experiment.

Proposed method

• Apply the color-dipole model to describe the virtual photon fluctuating into a $q\bar{q}$ pair, with the dipole interacting via two-gluon exchange in a color-singlet state.
• Use the Fourier transform to relate the impact-parameter space distribution $g(x, R_\perp; Q^2)$ to the $t$-dependence of the forward amplitude $F_g(x,t; Q^2)$, extracting the transverse size $\langle r_T^2 \rangle$ from the slope $b$.
• Utilize generalized parton distributions (GPDs) to describe the momentum and angular momentum structure of partons, including skewing effects from unequal momentum transfer.
• Measure the interference between deeply virtual Compton scattering (DVCS) and the Bethe-Heitler (BH) process to access GPDs, using helicity and charge asymmetries in the HERMES experiment.
• Model exclusive Higgs boson production in proton-proton collisions at the LHC via double Pomeron exchange, with both protons remaining intact and detected in forward Roman pots.
• Use the relation $M^2 = s x_{\rm l\!P,1} x_{\rm l\!P,2}$ to reconstruct the Higgs mass with high resolution from measured proton momentum losses.

Experimental results

Research questions

• RQ1 How can the large fraction of diffractive events with rapidity gaps at HERA be explained within QCD, given that such gaps are exponentially suppressed in standard perturbative QCD?
• RQ2 What is the transverse spatial size of the diffractive system at high energy and high virtuality, and does it grow with energy as expected in non-perturbative models?
• RQ3 How do generalized parton distributions (GPDs) provide access to the total angular momentum of quarks in the nucleon, and what constraints can be placed on $J_u$ and $J_d$?
• RQ4 Can exclusive Higgs boson production in diffractive $pp$ collisions at the LHC be used as a clean, high-resolution channel for Higgs boson studies?
• RQ5 What role does the Pomeron's partonic structure play in hard diffractive processes, and how does the resolved Pomeron model compare with data from the Tevatron?

Key findings

• The transverse size of the $q\bar{q}$ dipole system in diffractive DIS is measured to be $\sqrt{\langle r_T^2 \rangle} = 0.65 \pm 0.02$ fm at $x_{\rm Bj} \simeq 10^{-3}$ and high $Q^2$, consistent with perturbative QCD predictions.
• The $t$-slope $b$ of the gluon distribution is found to be $2b = \langle r_T^2 \rangle$, and it does not increase with energy, indicating no expansion of the radiation cloud.
• HERMES data provide the first direct constraint in the $J_u$-$J_d$ plane, showing that the total angular momentum of $u$ and $d$ quarks in the nucleon is consistent with lattice QCD results.
• Exclusive Higgs boson production in $pp$ collisions at the LHC is feasible via double Pomeron exchange, with both protons detected in forward Roman pots and the Higgs mass reconstructed with high resolution from $x_{\rm l\!P,1}$ and $x_{\rm l\!P,2}$.
• The dipole model successfully describes HERA data but fails to account for Tevatron data, indicating that additional dynamics such as gap survival or multiple interactions may be at play.