International Journal of Modern Physics A

I give a brief introduction to central exclusive production in hadron–hadron collisions, the subject of this special issue of the International Journal of Modern Physics A.

Protons may sometimes have the configuration of a compact diquark, separated transversely from the quark by a string. If both protons in a collision have this form, the strings may be parallel, giving final states with high multiplicity, ellipticity and a ridge structure, or transverse, with leading protons and (sometimes) rapidity gaps. Simple considerations lead one to expect, at , central masses in the few GeV/c² range, appropriate for glueball states.

The high-energy behavior of soft scattering observables such as total cross-sections, elastic scattering at small momentum transfer, diffractive dissociation and central production have been described successfully in the context of Regge theory, with the same basic structure holding as energies have increased. For elastic scattering and diffraction dissociation the defining energies were those of the ISR, the collider and the Tevatron. The elastic scattering data from the LHC demonstrate the continuing applicability of Regge theory. Preliminary data on diffraction dissociation promise to add to our understanding and now, for the first time, we can expect to test fully these concepts in central production. Although the latter is the principal objective of this discussion, understanding the first two is an essential prerequisite as they define the formalism and establish parameters.

Selected results from the study of Double Pomeron Exchange with the Split Field Magnet detector at the CERN Intersecting Storage Rings are presented. This concerns various general features of central pionic systems as well as the spectroscopy in the π⁺π^- channel, emphasizing the production properties of the f₂(1270) meson. This summary refers to a series of publications over a time span of 17 years based on the work of the CCHK^1,2 and ABCDHW^3–7 collaborations at the CERN ISR.

The non-Abelian nature of QCD suggests that particles that have a gluon constituent, such as glueballs or hybrids, should exist. This paper presents a study of central meson production in the fixed target experiments WA76, WA91 and WA102 at the CERN Omega spectrometer at centre-of-mass energies of , 23.8 and 29 GeV. A study of the resonance production cross-section as a function of shows which states are compatible with being produced by Double Pomeron Exchange (DPE). In these DPE processes, the difference in the transverse momentum between the exchange particles (dP_T) can be used to select out known states from non- candidates. The distribution of the azimuthal angle (ϕ) between the two exchange particles suggests that the Pomeron transforms like a nonconserved vector current. Finally there is evidence from an analysis of the decay modes of the scalar states observed, that the lightest scalar glueball manifests itself through the mixing with nearby states.

The CERN Intersecting Storage Rings, with from 22 GeV to 63 GeV and , allowed the first observations of p + p → p + X + p with two leading protons (x_F > 0.95) or two rapidity gaps Δy > 3. Studies of the central hadronic system (X) were made to search for glueballs, finding f₀ and f₂ resonances, and to advance our understanding of hadronic diffraction. I review the experiments, not including those at the Split Field Magnet (SFM) facility covered elsewhere in this volume. Some double pomeron exchange studies at the CERN Collider are also covered.

This paper presents a review of the study of Exclusive Central Production at a center-of-mass energy of at the Fermilab Fixed Target program. In all reactions reviewed in this paper, protons with an energy of 800 GeV were extracted from the Tevatron accelerator at Fermilab and directed to a Liquid Hydrogen target. The states reviewed include , ϕϕ and D*^±. Partial Wave Analysis results will be presented on the light states but only the cross-section will be reviewed in the diffractive production of D*^±.

We review some selected results within the Durham model of central exclusive production. We discuss the theoretical aspects of this approach and consider the phenomenological implications for a selection of processes.

The Collider Detector at Fermilab, CDF, observed for the first time in hadron–hadron collisions photon–photon (γ + γ → e⁺e^-, μ⁺μ^-) and photon–pomeron (γ + ℙ → J/ψ, ψ(2S)) interactions, as well as by double pomeron exchange, ℙ + ℙ or DPE. Exclusive π⁺π^- production was also measured at and 1960 GeV; resonance structures are discussed.

We present a review of central exclusive dijet production in collisions, where the proton and antiproton emerge intact, and only two jets of transverse energy above a certain threshold are present in the final state. The results are published in two papers by the Collider Detector at Fermilab (CDF) Collaboration, a PRL (2000) and a PRD (2008), based on data collected at and 1.96 TeV, respectively, and a D0 Collaboration paper from studies at 1.96 TeV. In all three cases predictions for the cross-section of Higgs boson production are discussed, a process that proceeds via a similar mechanism as dijet production. Roman Pot Spectrometers equipped with tracking detectors are used to measure the outgoing antiproton (CDF and D0) and proton (D0), and special forward detectors are employed to help reduce backgrounds and enrich the data in diffractive and exclusive dijet events.

The present status and future plans of the physics program of Central Exclusive Production (CEP) at RHIC are described. The measurements are based on the detection of the forward protons from the Double Pomeron Exchange (DPE) process in the Roman Pot system and of the recoil system of charged particles from the DPE process measured in the STAR experiment's Time Projection Chamber (TPC). The data described here were taken using polarized proton–proton collisions at . The preliminary spectra of two-pion mass reconstructed by STAR TPC in central region of pseudorapidity |η| < 1, are presented. Near future plans to take data with the current system at center-of-mass energy and plans to upgrade the forward proton tagging system are presented. Also a possible addition of the RPs to the sPHENIX detector is discussed.

This paper reviews measurements of exclusive production performed by the CMS experiment in proton–proton collisions during Run 1 of the LHC. The results include photon-induced production of μ⁺μ^-, e⁺e^- and W⁺W^- pairs, and a search for strong Central Exclusive Production of γγ pairs.

The LHCb detector and LHC running conditions are ideally suited to measuring central exclusive production. Several recent measurements of exclusive dimuon, single and double charmonia are reviewed. The potential for future measurements across a broad range of physics channels is discussed.

The LHC detectors have incomplete angular coverage in the forward direction, for example in the region 6 ≲ |η| ≲ 8, which can be improved with the addition of simple scintillation counters around the beam pipes about 50 m to 120 m from the intersection point. These counters detect showers created by particles hitting the beam pipes and nearby material. The absence of signals in these counters in low pileup conditions is an indication of a forward rapidity gap as a signature of diffraction. In addition, they can be used to detect hadrons from low mass diffractive excitations of the proton, not accompanied by a leading proton but adjacent to a rapidity gap over (e.g.) 3 ≲ |η| ≲ 6. Such a set of forward shower counters, originally used at CDF, was used in CMS (FSC) for high-β* running with TOTEM during LHC Run-1. During LS1 the CMS FSC system is being upgraded for future low pileup runs. A similar system, called HERSCHEL is being installed in LHCb. ALICE is implementing scintillation counters, ADA and ADC, with 4.5 ≲ |η| ≲ 6.4.

At CERNs Large Hadron Collider, a unique possibility to study central exclusive processes such as the production of low mass resonances, charmonium states and jets, as well as to search for missing mass or momentum signatures opens up by the detection of both leading protons with the special β* = 90 m optics. At with that optics the leading proton acceptance of the TOTEM Roman Pots covers all diffractive masses, provided that the proton four-momentum transfer |t| ≳ 0.04 GeV². This paper describes the physics potential with an integrated luminosity of 10 pb^-1 and 100 pb^-1 for common CMS and TOTEM data taking at β* = 90 m optics.

The ALICE experiment at the Large Hadron Collider (LHC) at CERN consists of a central barrel, a muon spectrometer and 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 exclusive production at the LHC.

We discuss the photon-induced processes that can be studied at the LHC, leading to intact protons in the final state. We give the SM cross-section for W- and photon-pair production. These processes lead to unprecedented sensitivities on quartic anomalous couplings between photons and W and Z bosons, reaching the values expected for extra dimension models.

We present the project to install new forward proton detectors in the CMS and ATLAS experiments called CT-PPS and AFP respectively.

We review activities in the field of theoretical, phenomenological and experimental studies related to the production of the Higgs boson in central exclusive processes at LHC in models beyond Standard Model. Prospects in the context of the Higgs boson discovery at LHC in 2012 and of proposals to build forward proton detectors at ATLAS and CMS side are summarized.

The holographic approach to double diffractive Higgs production is presented in terms of exchanging the AdS graviton/Pomeron. The goal is to provide a simple framework for central exclusive production from a dual strong coupling perspective.