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Title: Casimir meets Poisson: improved quark/gluon discrimination with counting observables

Charged track multiplicity is among the most powerful observables for discriminating quark- from gluon-initiated jets. Despite its utility, it is not infrared and collinear (IRC) safe, so perturbative calculations are limited to studying the energy evolution of multiplicity moments. While IRC-safe observables, like jet mass, are perturbatively calculable, their distributions often exhibit Casimir scaling, such that their quark/gluon discrimination power is limited by the ratio of quark to gluon color factors. In this paper, we introduce new IRC-safe counting observables whose discrimination performance exceeds that of jet mass and approaches that of track multiplicity. The key observation is that track multiplicity is approximately Poisson distributed, with more suppressed tails than the Sudakov peak structure from jet mass. By using an iterated version of the soft drop jet grooming algorithm, we can define a “soft drop multiplicity” which is Poisson distributed at leading-logarithmic accuracy. In addition, we calculate the next-to-leading-logarithmic corrections to this Poisson structure. If we allow the soft drop groomer to proceed to the end of the jet branching history, we can define a collinear-unsafe (but still infrared-safe) counting observable. Exploiting the universality of the collinear limit, we define generalized fragmentation functions to study the perturbative energy evolution ofmore » collinear-unsafe multiplicity.« less
Authors:
 [1] ;  [2] ; ORCiD logo [3] ;  [3]
  1. Harvard Univ., Cambridge, MA (United States). Dept. of Physics
  2. Reed College, Portland, OR (United States). Physics Dept.
  3. Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States). Center for Theoretical Physics
Publication Date:
Grant/Contract Number:
SC0013607; SC0012567; SC0015476
Type:
Accepted Manuscript
Journal Name:
Journal of High Energy Physics (Online)
Additional Journal Information:
Journal Name: Journal of High Energy Physics (Online); Journal Volume: 2017; Journal Issue: 9; Journal ID: ISSN 1029-8479
Publisher:
Springer Berlin
Research Org:
Harvard Univ., Cambridge, MA (United States); Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States)
Sponsoring Org:
USDOE Office of Science (SC), High Energy Physics (HEP) (SC-25)
Country of Publication:
United States
Language:
English
Subject:
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS; jets; QCD phenomenology
OSTI Identifier:
1425789

Frye, Christopher, Larkoski, Andrew J., Thaler, Jesse, and Zhou, Kevin. Casimir meets Poisson: improved quark/gluon discrimination with counting observables. United States: N. p., Web. doi:10.1007/JHEP09(2017)083.
Frye, Christopher, Larkoski, Andrew J., Thaler, Jesse, & Zhou, Kevin. Casimir meets Poisson: improved quark/gluon discrimination with counting observables. United States. doi:10.1007/JHEP09(2017)083.
Frye, Christopher, Larkoski, Andrew J., Thaler, Jesse, and Zhou, Kevin. 2017. "Casimir meets Poisson: improved quark/gluon discrimination with counting observables". United States. doi:10.1007/JHEP09(2017)083. https://www.osti.gov/servlets/purl/1425789.
@article{osti_1425789,
title = {Casimir meets Poisson: improved quark/gluon discrimination with counting observables},
author = {Frye, Christopher and Larkoski, Andrew J. and Thaler, Jesse and Zhou, Kevin},
abstractNote = {Charged track multiplicity is among the most powerful observables for discriminating quark- from gluon-initiated jets. Despite its utility, it is not infrared and collinear (IRC) safe, so perturbative calculations are limited to studying the energy evolution of multiplicity moments. While IRC-safe observables, like jet mass, are perturbatively calculable, their distributions often exhibit Casimir scaling, such that their quark/gluon discrimination power is limited by the ratio of quark to gluon color factors. In this paper, we introduce new IRC-safe counting observables whose discrimination performance exceeds that of jet mass and approaches that of track multiplicity. The key observation is that track multiplicity is approximately Poisson distributed, with more suppressed tails than the Sudakov peak structure from jet mass. By using an iterated version of the soft drop jet grooming algorithm, we can define a “soft drop multiplicity” which is Poisson distributed at leading-logarithmic accuracy. In addition, we calculate the next-to-leading-logarithmic corrections to this Poisson structure. If we allow the soft drop groomer to proceed to the end of the jet branching history, we can define a collinear-unsafe (but still infrared-safe) counting observable. Exploiting the universality of the collinear limit, we define generalized fragmentation functions to study the perturbative energy evolution of collinear-unsafe multiplicity.},
doi = {10.1007/JHEP09(2017)083},
journal = {Journal of High Energy Physics (Online)},
number = 9,
volume = 2017,
place = {United States},
year = {2017},
month = {9}
}