Beyond leading order effects in photon pair production at the Tevatron

TL;DR

The paper investigates beyond-leading-order effects in di-photon production at the Tevatron, emphasizing the fragmentation component's impact under realistic isolation criteria. Using the DIPHOX NLO framework, it demonstrates that additional final-state configurations at NLO enable a region where fragmentation is collinearly enhanced, producing a shoulder in the $q_T$ spectrum and distortions in angular distributions. Although stringent isolation largely suppresses fragmentation, the shape shifts are physically meaningful and could be tested with higher-statistics data, offering a precision test of QCD and informing Higgs searches via H→γγ backgrounds. The work highlights the necessity of including fragmentation at NLO for accurate di-photon observables.

Abstract

We discuss effects induced by beyond leading order contributions to photon pair production. We point out that next to leading order contributions to the fragmentation component of the signal lead to a change in the shape of distributions. This is already mildly visible in present Tevatron data though stringent isolation criteria tend to suppress the fragmentation component considerably. We expect the effect to be experimentally confirmed in future data samples with higher statistics which would serve as a precision test for QCD.

Figures (6)

• Figure 1: $q_{T}$ distribution of photon pairs. Black dots: D0 data d0; white diamonds: average values of the NLO calculation (DIPHOX code) in the corresponding experimental bins. The curve is a spline interpolation between the theoretical average.
• Figure 2: Origin of the $q_{T}$ shoulder in the theoretical calculation. Plot (a) shows the direct component (open squares) split into the phase space regions $\phi_{\gamma \gamma}< \pi/2$ (full triangles) and $\phi_{\gamma \gamma}> \pi/2$ (open triangles) for the experimentally used value $R_{min} = 0.3$. Plot (b) is the same as (a) but for $R_{min} = 0.01$ to show the sensitivity of the effect to this collinear cut. Plots (c) and (d) show the corresponding histograms for the fragmentation component where the enhancement due to photon collinearity is clearly visible.
• Figure 3: Kinematical configuration for which the fragmentation contribution is collinearly enhanced when the two photons become close to each other.
• Figure 4: Top: the $q_{T}$ spectrum (open squares) split into direct (full triangles) and fragmentation part (inversed triangles) for $R_{min} = 0.3$ (a) and $R_{min} = 0.01$ (b). Bottom: the ratio of the total $q_{T}$ distribution divided by the direct one for $R_{min} = 0.3$ (c) and $R_{min} = 0.01$ (d).
• Figure 5: $\phi_{\gamma \gamma}$ distribution of photon pairs. Black dots: D0 data d0, white diamonds: average values of the NLO calculation (DIPHOX code) in the corresponding experimental bins. The curve is a spline interpolation between the theoretical average.