Direct photon cross section with conversions at CDF

TL;DR

The study measures the isolated direct photon cross section in $p\bar{p}$ collisions at $\sqrt{s}=1.8$ TeV using photons that convert to $e^+e^-$ pairs in the CDF inner detector, enabling a tracking-based background subtraction. A new $E/p$-template technique, informed by Monte Carlo and double-conversion data, provides background control and extracts the signal across multiple $p_T$ bins from two independent datasets. The results reproduce the CES-CPR cross section but show a steeper $p_T$ dependence than NLO QCD predictions with CTEQ5M PDFs, highlighting a persistent tension that challenges current theory and can inform future PDF constraints. The method demonstrates a robust and cross-checkable approach for high-$p_T$ photon measurements, with the dominant normalization uncertainty stemming from the photon conversion probability.

Abstract

We present a measurement of the isolated direct photon cross section in p-pbar collisions at sqrt(s) = 1.8 TeV and |eta| < 0.9 using data collected between 1994 and 1995 by the Collider Detector at Fermilab (CDF). The measurement is based on events where the photon converts into an electron-positron pair in the material of the inner detector, resulting in a two-track event signature. To remove pi0 -> gamma gamma and eta -> gamma gamma events we use a new background subtraction technique which takes advantage of the tracking information available in a photon conversion event. We find that the shape of the cross section as a function of pT is poorly described by next-to-leading-order QCD predictions, but agrees with previous CDF measurements.

Figures (6)

• Figure 1: Vertexed and un-vertexed radius of conversion distribution in 8 GeV electron data. The peak at $r = 0$ labelled 'Dalitz' is due to $\pi^0 \rightarrow e^+e^-\gamma$ decays and $\gamma^{*} \rightarrow e^+e^-$ events. The 2 cm $< r <$ 30 cm data selection requirement has been released in this plot.
• Figure 2: The diphoton mass spectrum of double conversion events in the data. The data is fit to two Lorentzians plus a third order polynomial. The polynomial is shown as the dotted line. The $\pi^0$ and $\eta$ peaks are visible at 0.135 GeV and 0.547 GeV respectively. The ratio of the areas of the two peaks, along with the Monte Carlo prediction for the ratio of acceptances, gives an $\eta / \pi^0$ production ratio of $0.687 \pm 0.076$.
• Figure 3: An example of the $E/p$ background subtraction fit in two $p_T$ bins. Left: the 16.5 to 18 GeV bin from the 8 GeV electron (2-tower) data. Right: the 30 to 34 GeV bin from the 23 GeV photon (1-tower) data.
• Figure 4: The total acceptance times efficiency for both conversion datasets. The decrease in the acceptance at high $p_T$ for the 8 GeV electron dataset is due to the 2-tower requirement, which becomes geometrically disfavored. The total conversion probability is not included here.
• Figure 5: The isolated direct photon cross section. The result of the conversion technique is compared with CES-CPR and theory. For the conversion measurement the 8 GeV electron data is shown below 30 GeV, and the 23 GeV photon data above. The theory curve is from the authors of Reference Gluck:1994iz, and uses the CTEQ5M parton distribution functions with the all scales set to the $p_T$ of the photon. Only the statistical error bars are shown here.