Exclusive physics at the LHC with SuperChic 2

TL;DR

Central exclusive production at the LHC is studied with a new Monte Carlo, SuperChic 2, which implements a fully differential treatment of soft survival factors and a broadened set of CEP final states. It combines the Durham model for QCD-CEP and photon-mediated CEP within a unified framework, presenting predictions for exclusive jets, vector meson photoproduction, two-photon processes, and heavy quarkonia, and demonstrates the significant impact of differential survival on kinematic distributions. The work also documents public code availability and discusses implications for Run II measurements with forward proton tagging, emphasizing improved data agreement when survival is modeled differentially. Together, the results provide a versatile, validated tool for precise CEP predictions and for guiding experimental CEP programs at the LHC.

Abstract

We present a range of physics results for central exclusive production processes at the LHC, using the new SuperChic 2 Monte Carlo event generator. This includes significant theoretical improvements and updates, most importantly a fully differential treatment of the soft survival factor, as well as a greater number of generated processes. We provide an overview of the latest theoretical framework, and consider in detail a selection of final states, namely exclusive 2 and 3 jets, photoproduced vector mesons, two-photon initiated muon and $W$ boson pairs and heavy $χ_{c,b}$ quarkonia.

Figures (7)

• Figure 1: The perturbative mechanism for the QCD--induced exclusive process $pp \to p\,+\, X \, +\, p$, with the eikonal and enhanced survival factors shown symbolically.
• Figure 2: Schematic diagrams for the exclusive photoproduction process $pp\to pVp$ with (a) and without (b) screening corrections included.
• Figure 3: Parton--level distributions for 2 and 3--jet CEP with respect to the system invariant mass $M_X$ at $\sqrt{s}=13$ TeV, using MMHT14 LO PDFs Harland-Lang:2014zoa. The final--state partons are required to lie in the pseudorapidity region $-2.5<\eta<2.5$ and have transverse momentum $p_\perp>20$ GeV (leading to minimum invariant masses, $M_X$, of 40 and 60 GeV in the two and three jet cases, respectively), while the three--jet events are defined using the anti--$k_t$ algorithm with $R=0.6$. Distributions are shown for massless quarks, $q\overline{q}$, as well as for $b\overline{b}$ production. Soft survival effects are included using model 4 of Khoze:2013dha.
• Figure 4: Differential cross sections (in pb) at $\sqrt{s}=13$ TeV for exclusive $ggg$ jet production at parton level, with respect to (left) the separation in azimuthal angle $\Delta\phi_{ij}$ for gluon pairs $ij$ passing the cut $0.9<A_{ij}<1.1$, where $A_{ij}$ is defined in (\ref{['aij']}), (right) the rapidity differences $|y_{i,j}-y_X|$ for which ${\rm cosh}\Delta_{ij}>4$ and $\Delta\phi_{ij}<10^\circ$, where $y_X$ is the rapidity of the 3--jet system and the $y_{i,j}$ is the rapidity of parton $i,j$, both of which are binned. All partons are required to have $p_\perp>20$ GeV and $|y|<5$.
• Figure 5: Distributions with respect the $J/\psi$ rapidity $y_\psi$ at $\sqrt{s}=7$ TeV, compared to the LHCb data points from Aaij:2014iea. Theory curves corresponding to the 'bare' and 'screened' cross sections, i.e. excluding and including soft survival effects, respectively, are shown, and the integrated cross sections are normalised to the data for display purposes. The correlated systematic errors are not shown.