QCD Corrections and Anomalous Couplings in $Zγ$ Production at Hadron Colliders

TL;DR

The paper addresses the search for non-standard $ZZ\gamma$ and $Z\gamma\gamma$ couplings in hadronic Zγ production by computing ${\cal O}(\alpha_s)$ QCD corrections for both leptonically and invisibly decaying Z channels. It extends previous NLO Zγ calculations to general anomalous couplings, using a Monte Carlo/NLO framework with the Z decay in the narrow width approximation and a dipole form-factor to ensure unitarity. The results show that NLO corrections reshape the photon $p_T$ distributions, especially at the LHC, which can diminish sensitivity to anomalous couplings in inclusive analyses but can be mitigated by a 0-jet strategy. The findings provide more robust limits on anomalous couplings and guidance for experimental analyses at Tevatron and LHC.

Abstract

The processes $pp/ pbar p \to Z γ+ X \to \ell^+ \ell^- γ+ X$ ($\ell=e, μ$) and $pp/ pbarp \to Z γ+ X \to \barννγ+ X$ are calculated to ${\cal O}(α_s)$ for general $ZZγ$ and $Zγγ$ couplings. The impact of ${\cal O}(α_s)$ QCD corrections on the observability of $ZZγ$ and $Zγγ$ couplings in $Zγ$ production at the Tevatron and the Large Hadron Collider (LHC) is discussed.

Figures (15)

• Figure 1: Feynman diagrams for the Born level process $q \bar{q} \rightarrow Z \gamma$ in the Standard Model.
• Figure 2: Additional Feynman diagrams which contribute to the Born level process $q \bar{q} \rightarrow Z \gamma$ when non-standard model $ZZ\gamma$ and $Z\gamma\gamma$ couplings are introduced.
• Figure 3: Feynman rule for the general $Z\gamma V$ ($V= Z, \gamma$) vertex. The factor $e$ is the charge of the proton. The vertex function $\Gamma^{\alpha \beta \mu}_{Z\gamma V} (q_1, q_2, P)$ is given in Eq. (\ref{['EQ:GENVERTEX']}).
• Figure 4: Differential cross sections versus $p_T^{}(\gamma)$ for (a) $p\bar{p} \to Z\gamma +X \to \ell^+ \ell^- \gamma + X$ at ${\sqrt{s}=1.8}$ TeV, and (b) $p p \to Z\gamma +X \to \ell^+ \ell^- \gamma + X$ at ${\sqrt{s}=14}$ TeV in the SM. The jet-inclusive cross sections are shown at the Born level (dashed curves) and with the NLO corrections (solid curves). The cuts imposed are summarized in Sec. IIIB.
• Figure 5: The differential cross section for the photon transverse momentum in the reaction $p\bar{p} \to Z\gamma +X \to \ell^+ \ell^- \gamma + X$ at ${\sqrt{s}=1.8}$ TeV, (a) in the Born approximation and (b) including NLO QCD corrections. The curves are for the SM (solid), $h_{30}^Z=1.0$ (dashed), and $h_{40}^Z=0.05$ (dotted). The cuts imposed are summarized in Sec. IIIB.