APS : Parton Showers beyond Leading Logarithmic Accuracy

Dasgupta, Mrinal; Salam, Gavin P.; Soyez, Gregory; Monni, Pier Francesco; Dreyer, Frédéric A.; Hamilton, Keith

doi:10.1103/PhysRevLett.125.052002

Article
Report number	arXiv:2002.11114 ; CERN-TH-2020-026
Title	Parton Showers beyond Leading Logarithmic Accuracy
Related title	Parton showers beyond leading logarithmic accuracy
Author(s)	Dasgupta, Mrinal (U. Manchester (main)) ; Dreyer, Frédéric A. (Oxford U., Theor. Phys.) ; Hamilton, Keith (U. Coll. London) ; Monni, Pier Francesco (CERN) ; Salam, Gavin P. (Oxford U., Theor. Phys.) ; Soyez, Gregory (IPhT, Saclay)
Publication	2020-07-29
Imprint	2020-02-25
Number of pages	6
Note	6 pages, 2 figures, plus supplemental material; v2 brings extra references and clarifications, as accepted by Physical Review Letters
In:	Phys. Rev. Lett. 125 (2020) 052002
DOI	10.1103/PhysRevLett.125.052002
Subject category	hep-ph ; Particle Physics - Phenomenology
Abstract	Parton showers are among the most widely used tools in collider physics. Despite their key importance, none so far has been able to demonstrate accuracy beyond a basic level known as leading logarithmic (LL) order, with ensuing limitations across a broad spectrum of physics applications. In this letter, we propose criteria for showers to be considered next-to-leading logarithmic (NLL) accurate. We then introduce new classes of shower, for final-state radiation, that satisfy the main elements of these criteria in the widely used large-$N_C$ limit. As a proof of concept, we demonstrate these showers' agreement with all-order analytical NLL calculations for a range of observables, something never so far achieved for any parton shower.
Copyright/License	publication: © 2020-2024 authors (License: CC-BY-4.0), sponsored by SCOAP³ preprint: (License: arXiv nonexclusive-distrib 1.0)

$Left: ratio of the cumulative $y_{23}$ distribution from several showers divided by the NLL answer, as a function of $\as \ln y_{23}/2$, for $\as \to 0$. Right: summary of deviations from NLL for many shower/observable combinations (either $\Sigma_\text{shower}(\as\to 0,\as L=-0.5)/\Sigma_\text{NLL}-1$ or $(N^\text{subjet}_\text{shower}(\as \to 0, \as L^2 = 5 )/ N^\text{subjet}_\text{NLL} - 1)/\sqrt{\as}$). Red squares indicate clear NLL failure; amber triangles indicate NLL fixed-order failure that is masked at all orders; green circles indicate that all NLL tests passed.$ $Left: ratio of the cumulative $y_{23}$ distribution from several showers divided by the NLL answer, as a function of $\as \ln y_{23}/2$, for $\as \to 0$. Right: summary of deviations from NLL for many shower/observable combinations (either $\Sigma_\text{shower}(\as\to 0,\as L=-0.5)/\Sigma_\text{NLL}-1$ or $(N^\text{subjet}_\text{shower}(\as \to 0, \as L^2 = 5 )/ N^\text{subjet}_\text{NLL} - 1)/\sqrt{\as}$). Red squares indicate clear NLL failure; amber triangles indicate NLL fixed-order failure that is masked at all orders; green circles indicate that all NLL tests passed.$ $Figure showing emission $p_1$ in the left hemisphere and emissions $p_2$ and $p_3$ in the right hemisphere. On the left we show region a) where $\eta_3>\eta_2$ while on the right we show region b) where $\eta_2>\eta_3>0$. In region b) the condition for $p_3$ to give recoil to $p_2$ depends on $p_{\perp,1}$ and $\eta_1$, which gives rise to a mismatch with the case where $p_1$ is virtual, resulting in a super-leading logarithm as explained in the text.$ Show more plots

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