Zγγproduction with leptonic decays and triple photon production at NLO QCD

TL;DR

Problem: Accurate SM backgrounds for new-physics searches require NLO QCD predictions for triboson processes such as $Z\gamma\gamma$ and $\gamma\gamma\gamma$ with realistic decays and off-shell effects. Approach: a fully exclusive NLO QCD calculation implemented in a VBFNLO-based Monte Carlo, including real and virtual corrections with Catani-Seymour subtraction and photonic isolation via the Frixione prescription. Findings: large NLO corrections with integrated $K$-factors around $1.5$ for $Z\gamma\gamma$ and $2.6$ for $\gamma\gamma\gamma$, accompanied by significant distortions in differential distributions and nontrivial scale dependence not captured by LO uncertainties. Significance: provides essential background predictions for LHC searches, demonstrates the necessity of fully exclusive NLO tools, and completes the NLO treatment of triple-vector-boson production, with results to be included in the next VBFNLO release.

Abstract

We present a calculation of the O(alpha_s) QCD corrections to the production of a Z boson in association with two photons and to triple photon production at hadron colliders. All final-state photons are taken as real. For the Z boson, we consider the decays both into charged leptons and into neutrinos including all off-shell effects. Numerical results are obtained via a Monte Carlo program based on the structure of the VBFNLO program package. This allows us to implement general cuts and distributions of the final-state particles. We find that the NLO QCD corrections are sizable and significantly exceed the expectations from a scale variation of the leading-order result. In addition, differential distributions of important observables change considerably. The prediction of two-photon-associated Z production with Z decays into neutrinos from the charged-lepton rate works well, once we use an additional cut on the invariant mass of the charged-lepton pair.

Figures (10)

• Figure 1: Examples of topologies of Feynman diagrams contributing to the different processes at tree level. Top row:$pp\to Z\gamma\gamma$ + X including $Z$ decays into a pair of charged leptons. Bottom row:$pp\to Z\gamma\gamma$ + X including $Z$ decays into a pair of neutrinos (left) and $pp\to \gamma\gamma\gamma$ + X (right).
• Figure 2: Left: Scale dependence of the total LHC cross section for $p p \to Z \gamma\gamma + X \to \ell^+ \ell^- \gamma\gamma + X$ at LO and NLO within the cuts of Eqs. (\ref{['eq:cuts1']}, \ref{['eq:cuts2']}). The factorization and renormalization scales are together, inversely or independently varied in the range from $0.1 \cdot \mu_0$ to $10 \cdot \mu_0$. We show NLO curves without and including an additional veto on ${p_T}_j$ of 50 GeV. Right: Same as in the left panel without jet veto, but for the different NLO contributions at $\mu_F=\mu_R=\xi\mu_0$ with $\mu_0 = m_{Z\gamma\gamma}$.
• Figure 3: Left: Scale dependence of the total LHC cross section for $p p \to Z \gamma\gamma + X \to \nu \bar{\nu} \gamma\gamma + X$ at LO and NLO within the cuts of Eqs. (\ref{['eq:cuts1']}, \ref{['eq:cuts2']}). The factorization and renormalization scales are together, inversely or independently varied in the range from $0.1 \cdot \mu_0$ to $10 \cdot \mu_0$. We show NLO curves without and including an additional veto on ${p_T}_j$ of 50 GeV. Right: Same as in the left panel without jet veto, but for the different NLO contributions at $\mu_F=\mu_R=\xi\mu_0$ with $\mu_0 = m_{Z\gamma\gamma}$.
• Figure 4: Left: Scale dependence of the total LHC cross section for $p p \to \gamma \gamma\gamma + X$ at LO and NLO within the cuts of Eqs. (\ref{['eq:cuts1']}, \ref{['eq:cuts2']}). The factorization and renormalization scales are together, inversely or independently varied in the range from $0.1 \cdot \mu_0$ to $10 \cdot \mu_0$. We show NLO curves without and including an additional veto on ${p_T}_j$ of 50 GeV. Right: Same as in the left panel without jet veto, but for the different NLO contributions at $\mu_F=\mu_R=\xi\mu_0$ with $\mu_0 = m_{\gamma\gamma\gamma}$.
• Figure 5: Left: Transverse-momentum distribution of the photon with largest transverse momentum in $Z_\ell\gamma\gamma$ production with $Z$ decaying into charged leptons for the LHC. We show LO (dotted blue line) and NLO cross sections without (solid red) and including (dashed black) a jet veto of 50 GeV, using the cuts of Eqs. (\ref{['eq:cuts1']}, \ref{['eq:cuts2']}). Right: Associated K-factor as defined in Eq.(\ref{['eq:kfactor']}) without (solid red) and including (dashed black) the jet veto.