Standard candle central exclusive processes at the Tevatron and LHC

TL;DR

This work provides a comprehensive, quantitatively uncertainty-aware assessment of central exclusive production for heavy quarkonia and diphoton final states at hadron colliders. It develops a detailed $Q_\perp$-factorised framework with skewed unintegrated gluon densities, Sudakov suppression, and both eikonal and enhanced soft survival corrections, applying it to $\chi_{qJ}$, $\eta_q$, and $\gamma\gamma$ CEP. The study demonstrates good agreement with Tevatron measurements for standard-candle processes and furnishes predictions for RHIC and LHC, highlighting how forward proton detectors and spin–parity analyses can validate the CEP formalism and inform searches for new physics. A key finding is the sizable impact of enhanced absorptive effects at high energies, which must be included to obtain reliable cross sections and distributions for CEP at the LHC.

Abstract

Central exclusive production (CEP) processes in high-energy proton -- (anti)proton collisions offer a very promising framework within which to study both novel aspects of QCD and new physics signals. Among the many interesting processes that can be studied in this way, those involving the production of heavy (c,b) quarkonia and gamma gamma states have sufficiently well understood theoretical properties and sufficiently large cross sections that they can serve as `standard candle' processes with which we can benchmark predictions for new physics CEP at the CERN Large Hadron Collider. Motivated by the broad agreement with theoretical predictions of recent CEP measurements at the Fermilab Tevatron, we perform a detailed quantitative study of heavy quarkonia (chi and eta) and gamma gamma production at the Tevatron, RHIC and LHC, paying particular attention to the various uncertainties in the calculations. Our results confirm the rich phenomenology that these production processes offer at present and future high-energy colliders.

Figures (8)

• Figure 1: The perturbative mechanism for the exclusive process $pp \to p+\chi+p$, with the eikonal and enhanced survival factors shown symbolically.
• Figure 2: Ratio of the cross sections for the CEP of a system of mass $M_X$, evaluated with $\Delta=k_\perp/(k_\perp+M_X)$ and $\Delta=k_\perp/(k_\perp+0.62M_X)$, using a modified form of Eq. (26) of Ref. Martin01ms, as described in the text.
• Figure 3: Distribution (in arbitrary units) within the perturbative framework of the outgoing proton ${\bf p}_{1_\perp}^2$, integrated over the second proton ${\bf p}_{2_\perp}$ and at rapidity $y_X=0$, for the CEP of different $J^P$$c\overline{c}$ states at $\sqrt{s}=14$ TeV. The solid (dotted) line shows the distribution including (excluding) the survival factor, calculated using the two channel eikonal model of Ref. KMRsoft, while the dashed line shows the distribution in the small $p_\perp$ limit, using the vertices of Eqs. (\ref{['R0p']}--\ref{['R0m']}) and excluding the survival factor.
• Figure 4: Distribution (in arbitrary units) within the perturbative framework of the difference in azimuthal angle of the outgoing protons for the CEP of different $J^P$$c\overline{c}$ states at $\sqrt{s}=14$ TeV and rapidity $y_X=0$. The solid (dotted) line shows the distribution including (excluding) the survival factor, calculated using the two channel eikonal model of Ref. KMRsoft, while the dashed line shows the distribution in the small $p_\perp$ limit, using the vertices of Eqs. (\ref{['R0p']}--\ref{['R0m']}) and excluding the survival factor.
• Figure 5: Centre-of-mass scattering angle dependence of the hard subprocess $gg\to\gamma\gamma$ cross section, averaged over incoming gluon polarizations at the amplitude level, for a $J_z=0$ and $|J_z|=2$ incoming $gg$ system. The continuous curve represents production for a fixed $M_{\gamma\gamma}=10$ GeV, and the dashed for fixed $E_{\perp_\gamma}=5$ GeV.