NLOJET++

NLOJET++
Name	NLOJET++
Title	NLOJET++
Developer	Zoltán Nagy; Michel Dasgupta; others
Released	1990s–2000s
Programming language	C++
Operating system	Unix-like
License	academic

NLOJET++ is a computer program for computing next-to-leading order perturbative predictions in quantum chromodynamics for collider observables. It provides fixed-order parton-level cross sections and differential distributions, enabling comparisons between theoretical calculations and experimental measurements from facilities and collaborations such as CERN, Fermilab, DESY, KEK, and SLAC. Developers designed it to interface with analyses by groups including ATLAS Collaboration, CMS Collaboration, H1 Collaboration, ZEUS Collaboration, and theorists from institutions like Institute for Advanced Study, Princeton University, and CERN Theory Division.

Overview

NLOJET++ implements next-to-leading order calculations for jet production and related observables in hadron-hadron, lepton-hadron, and lepton-lepton collisions, covering processes relevant to experiments at Large Hadron Collider, Tevatron, HERA, LEP, SuperKEKB, and other accelerators. The code focuses on parton-level event generation, subtraction methods for infrared singularities, and flexible histogramming to support phenomenology by groups at Universidad Autónoma de Madrid, Institut de Physique Théorique, Max Planck Institute for Physics, and CERN. It has been cited in phenomenological studies alongside Monte Carlo and resummation tools developed at places like SLAC National Accelerator Laboratory, DESY Theory Group, and Lawrence Berkeley National Laboratory.

Theoretical Framework and Algorithms

NLOJET++ is grounded in perturbative Quantum chromodynamics techniques and the subtraction formalism for regulating soft and collinear divergences, employing algorithms compatible with the Catani–Seymour dipole subtraction formalism proposed by researchers at Università di Milano, Universidad de Valencia, and INFN. The code computes matrix elements using methods developed in the context of perturbative calculations by teams at CERN Theory, University of Cambridge, Massachusetts Institute of Technology, and Stanford University. It supports renormalization and factorization scale choices used in studies by groups at University of Oxford, École Polytechnique, California Institute of Technology, and Yale University. The program’s numerical integration routines reflect techniques associated with communities from University of Chicago, Columbia University, and Rutgers University.

Features and Capabilities

NLOJET++ provides next-to-leading order predictions for multi-jet cross sections, event shapes, and inclusive observables employed by collaborations such as ALICE Collaboration, LHCb Collaboration, CDF Collaboration, and D0 Collaboration. Features include modular process definitions, colour-ordered tree-level and one-loop amplitude implementations influenced by work at Institute for Particle Physics Phenomenology, Hamburg University, and University of Durham, and user-configurable jet algorithms like those proposed by researchers connected to FastJet development teams at DESY and CERN PH-AT. Output formats and histogramming are compatible with analysis frameworks used at Brookhaven National Laboratory, Oak Ridge National Laboratory, and SLAC. The code supports comparisons with parton distribution functions from global-fit groups including CTEQ, MSTW, NNPDF, HERAPDF, and collaborations at Jefferson Lab.

Validation and Benchmarking

Validation of NLOJET++ predictions has been carried out by comparisons to analytic results and experimental data from ATLAS, CMS, H1, ZEUS, CDF, and D0, and by cross-checks with other NLO tools developed by teams at MadGraph, MCFM, POWHEG, Sherpa, and HERWIG. Benchmark studies often reference theoretical reviews and workshops at Snowmass', Moriond, Les Houches, EPS-HEP, and ICHEP, and involve groups from Universität Zürich, University of Manchester, Kyoto University, and Peking University to ensure numerical stability and agreement within perturbative uncertainties.

Applications and Use Cases

Researchers use NLOJET++ for precision phenomenology in jet physics, strong coupling determinations, parton distribution function constraints, and tuning of higher-order effects for heavy-flavor and electroweak studies performed by teams at CERN, Fermilab, DESY, KEK, and TRIUMF. It has supported analyses informing measurements by the ATLAS Collaboration and CMS Collaboration on jet spectra, event-shape studies by ALEPH, DELPHI, and OPAL at LEP, and deep-inelastic scattering analyses by H1 and ZEUS at HERA. The program is used in comparisons with resummation approaches from groups at Institut de Physique Théorique, CERN, and University of Milano-Bicocca.

Software Implementation and Distribution

Written primarily in C++, the package compiles on Unix-like systems and integrates numerical libraries and build tools common at institutions such as GNU Project affiliates, CERN IT Department, and university computing centers. Distribution historically occurred via academic channels and collaboration webpages maintained by groups at University of Zürich and Eötvös Loránd University, with user communities exchanging patches at workshops held at Les Houches and Hamburg. Documentation and example drivers reflect standards used by software projects associated with HEP Software Foundation and research groups at SLAC and Brookhaven National Laboratory.

Development History and Contributors

The codebase originated from efforts in the late 1990s and early 2000s led by theorists affiliated with Institute of Physics, Eötvös Loránd University, Institute for Theoretical Physics, ETH Zurich, and collaborators from CERN Theory and Universidad Autónoma de Madrid. Key contributors include researchers who have also published with groups at University of Durham, Universität Göttingen, SISSA, and Università di Roma La Sapienza. Development proceeded through collaborative validation exercises at meetings hosted by Les Houches Workshop, Moriond Conference, and EPS-HEP, with users from ATLAS, CMS, H1, and ZEUS providing feedback that shaped later releases.

Category:High energy physics software