The ATLAS Simulation Infrastructure

Abstract

The simulation software for the ATLAS Experiment at the Large Hadron Collider is being used for largescale production of events on the LHC Computing Grid. This simulation requires many components, from the generators that simulate particle collisions, through packages simulating the response of the various detectors and triggers. All of these components come together under the AT-LAS simulation infrastructure. In this paper, that infrastructure is discussed, including that supporting the detector description, interfacing the event generation, and combining the GEANT4 simulation of the response of the individual detectors. Also described are the tools allowing the software validation, performance testing, and the validation of the simulated output against known physics processes.

Slepton pair production with aNNLO+NNLL precision

“…which close to threshold (z → 1) spoil the convergence of the perturbative series in α s and therefore have to be resummed to all orders [45,46]. After performing a Mellin transformation, 4) and the partonic cross section σ ab can be written in the exponentiated form…”

“…Based on an integrated LHC luminosity of 139 (35.9) fb −1 and for sufficiently large mass differences with the lightest neutralino, the ATLAS (CMS) collaboration has recently excluded left-handed selectrons below 550 (400) GeV. For two (not three, as stated in the ATLAS abstract and conclusion) generations of mass-degenerate sleptons, the limit increases to 700 (450) GeV [6,7]. We therefore adopt for the invariant-mass distributions a default slepton mass of 1 TeV and use 700 GeV as the lower mass limit for the total cross sections.…”

“…Sleptons, the scalar partners of the SM leptons, are usually also among the lightest SUSY particles [3]. While LHC searches already constrain squarks and gluinos, the SUSY partners of quarks and gluons, to the mass range above 1 or 2 TeV [4,5], the limits on left-handed selectron and smuon masses are less stringent and lie at 550 and 560 GeV, respectively [6,7]. Staus can even be as light as 390 GeV [8,9].…”

“…Together with resummation-improved parton density functions (PDFs) [18], also the PDF uncertainty can in principle be reduced [19,20], even though in practice these PDFs must currently be fitted to smaller data sets than global NLO analyses and thus still have larger errors. Similar calculations have been performed for gaugino and higgsino pairs [21][22][23][24][25][26], gluinos and gauginos [27,28], and additional gauge bosons [29][30][31][32], are available within the public code RESUMMINO [33] and are regularly employed in the experimental analyses by ATLAS [34] and CMS [35]. Predictions have also recently been made for the high-luminosity (HL) and high-energy (HE) phases of the LHC [36].…”

Rapidity decorrelation from hydrodynamic fluctuations

“…The factorization ratio was initially proposed as a function of transverse momentum [49] and was later extended in the longitudinal direction as a function of pseudorapidity [50] for the purpose of analyzing the longitudinal dynamics. Since then, the factorization ratios in the longitudinal direction are widely measured in experiments [50][51][52][53][54]. The origin of the rapidity decorrelation is understood from decorrelation of event-plane angle and/or magnitude of flow.…”