Elastic and Proton-Dissociative Photoproduction of J/psi Mesons at HERA (1304.5162v1)

Abstract: Cross sections for elastic and proton-dissociative photoproduction of J/psi mesons are measured with the H1 detector in positron-proton collisions at HERA. The data were collected at $ep$ centre-of-mass energies sqrt{s} approx 318 GeV and sqrt{s} approx 225 GeV, corresponding to integrated luminosities of L = 130 pb^{-1} and L = 10.8 pb^{-1}, respectively. The cross sections are measured as a function of the photon-proton centre-of-mass energy in the range 25< Wgp < 110 GeV. Differential cross sections $\mathrm{d}\sigma / \mathrm{d}t$, where $t$ is the squared four-momentum transfer at the proton vertex, are measured in the range $|t| < 1.2 \, \gevsq$ for the elastic process and $|t| < 8 \, \gevsq$ for proton dissociation. The results are compared to other measurements. The $\Wgp$ and $t$-dependences are parametrised using phenomenological fits.

• The paper provides robust cross section measurements for both elastic and proton-dissociative J/ψ photoproduction, highlighting distinct t-dependence behaviors.
• It employs detailed event selection and Monte Carlo simulations to accurately separate and analyze elastic and proton-dissociative processes.
• Results validate QCD models of two-gluon exchange and offer key insights into gluon density variations at low Bjorken x for future collider experiments.

Elastic and Proton-Dissociative Photoproduction of J/ψ Mesons at HERA: An Analytical Overview

The paper "Elastic and Proton-Dissociative Photoproduction of J/ψ Mesons at HERA" presents a comprehensive analysis of J/ψ photoproduction using data collected by the H1 detector at the HERA collider. This paper focuses on both elastic and proton-dissociative processes, offering a detailed examination of cross sections as a function of key variables, including the photon-proton center-of-mass energy ($W_{\gamma p}$) and the squared four-momentum transfer ($t$) at the proton vertex.

Methodology and Measurement

Data were obtained from positron-proton collisions at center-of-mass energies of approximately 318 GeV and 225 GeV, with respective integrated luminosities of 130 pb$^{-1}$ and 10.8 pb$^{-1}$. The cross sections were measured in the range $25 < W_{\gamma p} < 110$ GeV and $|t| < 1.2$ GeV$^2$ for elastic processes, with an extended $|t| < 8$ GeV$^2$ for proton dissociation.

The paper employs a robust experimental approach involving event selection criteria that distinguish between the elastic and proton-dissociative regimes. Monte Carlo simulations were utilized to optimize trigger efficiencies, acceptance calculation, and the separation of signal from background noise.

Results and Analysis

Significant findings include a marked difference in the $t$-dependence between elastic and proton-dissociative processes. The elastic differential cross section exhibits an exponential fall-off typical of diffractive processes, parameterized as $$\mathrm{d}\sigma / \mathrm{d}t \propto e^{- b_{el} |t|}$$. On the other hand, the proton-dissociative cross section shows a slower decline, aligning with a power-law fit. Such distinctions highlight variations in the transverse interaction zone size corresponding to each process.

The analysis also explores the $W_{\gamma p}$-dependence of the cross sections, with elastic production ($\delta \approx 0.7$) rising more sharply compared to the proton dissociative component. This deviation from a universal pomeron model underscores the complex dynamics at play, attributed to the increasing gluon density at low Bjorken $x$.

Theoretical and Practical Implications

This paper's outcomes contribute to our understanding of diffractive vector meson production at the energy frontier. The results substantiate theoretical predictions on diffractive processes in perturbative QCD frameworks, particularly validating the model of two-gluon exchange. Furthermore, insights into the gluonic structure and intrinsic differences between elastic and inelastic channels could guide future theoretical explorations and refine existing models.

From a practical standpoint, these findings might inform future experimental setups and analytical techniques at upcoming facilities or current high-energy colliders, where extending predictions to higher energies or precision measurements becomes feasible.

Conclusion and Future Directions

In conclusion, this paper makes substantial contributions to the field of diffractive photoproduction, accurately mapping out phenomenological relations and providing data integral for QCD-based modeling. The research lays groundwork for future investigations, potentially expanding into more complex interactions or uncovering new phenomena in diffractive physics. As collider technologies progress, subsequent studies might aim to explore these interactions at novel energies or integrate advanced machine learning techniques for enhanced data interpretation and modeling.