The Roper resonance and kin

TL;DR

This paper reviews the properties of the Roper resonance N(1440), discussing its complex structure, experimental findings, and recent theoretical advances that identify it as a three-quark state with meson cloud dressing.

Contribution

It highlights recent developments using AdS/QCD and functional methods that improve understanding of the Roper resonance as a three-quark excitation.

Findings

Electroproduction experiments reveal the Roper as a three-quark core with meson cloud.

Recent models reconcile quark predictions with experimental data.

The Roper belongs to a family of N* states with similar properties.

Abstract

The properties of the Roper resonance N(1440) are reviewed. Quark models have long struggled to reproduce its mass relative to its negative-parity partner N(1535). This discrepancy motivated interpretations of the Roper as a dynamically generated meson-baryon state. Including its isospin partners Delta(1600) and Delta(1700) further accentuates the tension between quark-model predictions and experiment. Recent developments based on AdS/QCD and functional methods achieve much improved agreement, identifying the Roper as an ordinary three-quark excitation. Electroproduction experiments at Jefferson Lab have now resolved this long-standing question, revealing the Roper as a qqq core dressed by a substantial meson cloud. The Roper resonance belongs to a family of four N* states with JP = 1/2+; the highest-mass member, N(2100), likely represents a Roper-like excitation in the fourth shell.