fastNLO: Fast pQCD Calculations for PDF Fits

TL;DR

The paper addresses the computational bottleneck of incorporating jet data into global parton density function fits by introducing fastNLO, a method that precomputes universal, perturbative coefficients on interpolation grids and decouples PDFs and αs from the hard scattering. This yields exact, fast pQCD predictions for arbitrary PDFs with near-instantaneous evaluation and high precision (better than 0.1%), including O(αs^4) threshold corrections. The approach enables inclusion of diverse jet data from DIS and hadron-hadron collisions, validated by comparisons to measurements at HERA, RHIC, Tevatron, and projections for the LHC. By enabling efficient use of jet data in PDF fits, fastNLO improves constraints on the gluon density and broadens the data that can be leveraged in global analyses.

Abstract

We present a method for very fast repeated computations of higher-order cross sections in hadron-induced processes for arbitrary parton density functions. A full implementation of the method for computations of jet cross sections in Deep-Inelastic Scattering and in Hadron-Hadron Collisions is offered by the "fastNLO" project. A web-interface for online calculations and user code can be found at http://hepforge.cedar.ac.uk/fastnlo/ .

Figures (5)

• Figure 1: The $k$-factor for the inclusive $p\bar{p}$ jet cross section at $\sqrt{s}=1.96$ TeV as a function of $p_T$ at different rapidities $y$ for the total cross section (solid line) and for different partonic subprocesses: gluon-gluon (dashed), gluon-quark (dotted) and the sum of all quark and/or anti-quark induced subprocesses (dashed-dotted).
• Figure 2: Contributions of different partonic subprocesses to the inclusive jet cross section at RHIC (left), the Tevatron (middle) and the LHC (right) as a function of $p_T$ and $x_T = 2 p_T/\sqrt{s}$. The subprocess $gq \rightarrow {\rm jets}$ has been separated into the contributions (2) and (3) where either the quark- or the gluon momentum fraction is larger.
• Figure 3: Comparison of PDF uncertainties for the inclusive jet cross section at RHIC (left), the Tevatron (middle) and the LHC (right). The uncertainty band is obtained for the CTEQ6.1M parton density functions and the results are shown as a function of $p_T$ and $x_T = 2 p_T/\sqrt{s}$.
• Figure 4: An overview of data over theory ratios for inclusive jet cross sections, measured in different processes at different center-of-mass energies. The data are compared to calculations obtained by fastNLO in NLO precision (for DIS data) and including ${\cal O}(\alpha_s^4)$ threshold corrections (for $p\bar{p}$ data). The inner error bars represent the statistical errors and the outer error bars correspond to the quadratic sum of all experimental uncertainties. In all cases the perturbative predictions have been corrected for non-perturbative effects.
• Figure 5: The phase space in $x$ and $p_T$ covered by the data sets shown in the previous figure.