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Title: Colloquium: Roper resonance: Toward a solution to the fifty year puzzle

Abstract

Discovered in 1963, the Roper resonance appears to be an exact copy of the proton except that its mass is 50% greater and it is unstable. These features of the Roper have been very difficult to explain so that for half a century this lightest excited state of the proton has defied understanding. The last decade has presented a new challenge, viz. precise information on the proton-to-Roper electroproduction transition form factors. Reaching to momentum transfer Q 2 4.5 GeV 2 , the data probe a domain within which hard valence-quark degrees of freedom could be expected to determine form factor behavior. An explanation of the Roper should combine an understanding of all these things. This is a prodigious task, but a ten-year international collaborative effort, involving experimentalists and theorists, has presented a candidate solution to the puzzle. Namely, the observed Roper is at the heart of the proton’s first radial excitation, consisting of a dressed-quark core augmented by a meson cloud that reduces the core mass by approximately 20% and materially alters its electroproduction form factors on Q 2 < 2 m N 2 , where m N is the proton’s mass. This Colloquium describes the experimental developments which enabled electroproduction data to be procured within a domain that is the purview of strong quantum chromodynamics, thereby providing challenges and opportunities for modern theory, and surveys the developments in reaction models and QCD theory that have enabled this picture of the Roper resonance to be drawn.

Authors:
 [1];  [2]
  1. Thomas Jefferson National Accelerator Facility (TJNAF), Newport News, VA (United States)
  2. Argonne National Lab. (ANL), Argonne, IL (United States). Physics Division
Publication Date:
Research Org.:
Thomas Jefferson National Accelerator Facility (TJNAF), Newport News, VA (United States); Argonne National Lab. (ANL), Argonne, IL (United States)
Sponsoring Org.:
USDOE Office of Science (SC), Nuclear Physics (NP) (SC-26)
OSTI Identifier:
1501553
Alternate Identifier(s):
OSTI ID: 1499832; OSTI ID: 1510476
Report Number(s):
JLAB-PHY-18-2; arXiv:1710.02549; DOE/OR/23177-4244
Journal ID: ISSN 0034-6861
Grant/Contract Number:  
AC05-06OR23177; AC02-06CH11357
Resource Type:
Accepted Manuscript
Journal Name:
Reviews of Modern Physics
Additional Journal Information:
Journal Volume: 91; Journal Issue: 1; Journal ID: ISSN 0034-6861
Publisher:
American Physical Society (APS)
Country of Publication:
United States
Language:
English
Subject:
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS; particle interactions; quantum field theory; form factors; lepton-hadron colliders

Citation Formats

Burkert, Volker D., and Roberts, Craig D. Colloquium: Roper resonance: Toward a solution to the fifty year puzzle. United States: N. p., 2019. Web. doi:10.1103/RevModPhys.91.011003.
Burkert, Volker D., & Roberts, Craig D. Colloquium: Roper resonance: Toward a solution to the fifty year puzzle. United States. doi:10.1103/RevModPhys.91.011003.
Burkert, Volker D., and Roberts, Craig D. Thu . "Colloquium: Roper resonance: Toward a solution to the fifty year puzzle". United States. doi:10.1103/RevModPhys.91.011003.
@article{osti_1501553,
title = {Colloquium: Roper resonance: Toward a solution to the fifty year puzzle},
author = {Burkert, Volker D. and Roberts, Craig D.},
abstractNote = {Discovered in 1963, the Roper resonance appears to be an exact copy of the proton except that its mass is 50% greater and it is unstable. These features of the Roper have been very difficult to explain so that for half a century this lightest excited state of the proton has defied understanding. The last decade has presented a new challenge, viz. precise information on the proton-to-Roper electroproduction transition form factors. Reaching to momentum transfer Q2≈4.5 GeV2, the data probe a domain within which hard valence-quark degrees of freedom could be expected to determine form factor behavior. An explanation of the Roper should combine an understanding of all these things. This is a prodigious task, but a ten-year international collaborative effort, involving experimentalists and theorists, has presented a candidate solution to the puzzle. Namely, the observed Roper is at the heart of the proton’s first radial excitation, consisting of a dressed-quark core augmented by a meson cloud that reduces the core mass by approximately 20% and materially alters its electroproduction form factors on Q2<2mN2, where mN is the proton’s mass. This Colloquium describes the experimental developments which enabled electroproduction data to be procured within a domain that is the purview of strong quantum chromodynamics, thereby providing challenges and opportunities for modern theory, and surveys the developments in reaction models and QCD theory that have enabled this picture of the Roper resonance to be drawn.},
doi = {10.1103/RevModPhys.91.011003},
journal = {Reviews of Modern Physics},
number = 1,
volume = 91,
place = {United States},
year = {2019},
month = {3}
}

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