Jet production in (un)polarized pp collisions: dependence on jet algorithm

TL;DR

The paper develops and validates a narrow-jet (NJA) framework to compute NLO single-inclusive jet cross sections in both unpolarized and longitudinally polarized pp collisions, comparing cone and $k_t$-type jet definitions. It extends prior analytic results to $k_t$-type algorithms, elucidating how jet algorithms alter cross sections but largely leave spin asymmetries robust. The approach agrees closely with exact NLO codes at RHIC energies and offers practical inputs for threshold resummation and SCET-related studies. These findings guide experimental jet analyses and enhance theoretical precision in polarized nucleon structure investigations.

Abstract

We investigate single-inclusive high-pT jet production in longitudinally polarized pp collisions at RHIC, with particular focus on the algorithm adopted to define the jets. Following and extending earlier work in the literature, we treat the jets in the approximation that they are rather narrow, in which case analytical results for the corresponding next-to-leading order partonic cross sections can be obtained. This approximation is demonstrated to be very accurate for practically all relevant situations, even at Tevatron and LHC energies. We confront results for cross sections and spin-asymmetries based on using cone- and kt-type jet algorithms. We find that jet cross sections at RHIC can differ significantly depending on the algorithm chosen, but that the spin asymmetries are rather robust. Our results are also useful for matching threshold-resummed calculations of jet cross sections to fixed-order ones.

Figures (7)

• Figure 1: Upper left: Ratio of single-inclusive jet cross sections at RHIC for the cone algorithm, as computed within the NJA and with fastNLOfastnlo. Lower left: Same for the jet cross sections for the $k_t$-type algorithms. Here, the exact NLO calculation was performed with the FastJet code Cacciari:2011magreg. Right: Similar comparisons for Tevatron (upper, $\sqrt{S}=1960$ GeV) and LHC (lower, $\sqrt{S}=7$ TeV) energies. The exact NLO results for Tevatron and for LHC with $R=0.5$ were obtained from fastNLO, the others from FastJet.
• Figure 2: The ratio ${\cal R} (0.2, 0.4)$ as defined in Eq. (\ref{['r2']}) for $pp$ collisions at RHIC at $\sqrt{S} = 200$ GeV. The solid histogram shows our result within the NJA, while the dashed one shows the corresponding result for the $k_t$/anti-$k_t$ algorithms presented in Soyez:2011np.
• Figure 3: Spin-averaged NLO cross sections for single-inclusive jet production at RHIC at center-of-mass energies $200$ GeV (left) and $500$ GeV (right). Results are shown for the cone and $k_t$-type algorithms, for two different values of the jet parameter $R$.
• Figure 4: Scale dependence of the NLO cross sections shown in Fig. \ref{['fig:cross']} for $R=0.4$ for center-of-mass energies $200$ GeV (left) and $500$ GeV (right). For notational convenience we have defined $T(\mu)\equiv d^2\sigma/dp_{T_J}d\eta_J$ at a given scale $\mu=\mu_F=\mu_R$.
• Figure 5: The ratio ${\cal R}_{\mathrm{algo}}$ at RHIC for $\sqrt{S} = 200$ GeV (left) and $\sqrt{S} = 500$ GeV (right), for the spin-averaged case. Results are shown for two different values of the jet parameter $R$.