The Roper resonance, also known as P₁₁(1440) or N(1440)1/2⁺, is an unstable nucleon resonance with a mass of about 1,440 MeV/c² and with a relatively wide full Breit-Wigner width Γ ≈ 300 MeV/c². It contains three

quark A quark () is a type of elementary particle and a fundamental constituent of matter. Quarks combine to form composite particles called hadrons, the most stable of which are protons and neutrons, the components of atomic nuclei. All commonly o ...

s (up (u) or down (d)) with total

spin Spin or spinning most often refers to: * Spinning (textiles), the creation of yarn or thread by twisting fibers together, traditionally by hand spinning * Spin, the rotation of an object around a central axis * Spin (propaganda), an intentionally b ...

''J'' = 1/2 and total

isospin In nuclear physics and particle physics, isospin (''I'') is a quantum number related to the up- and down quark content of the particle. More specifically, isospin symmetry is a subset of the flavour symmetry seen more broadly in the interactions ...

''I'' = 1/2. In the

quark model In particle physics, the quark model is a classification scheme for hadrons in terms of their valence quarks—the quarks and antiquarks which give rise to the quantum numbers of the hadrons. The quark model underlies "flavor SU(3)", or the E ...

it is considered to be a radially excited three-quark state with radial

quantum number In quantum physics and chemistry, quantum numbers describe values of conserved quantities in the dynamics of a quantum system. Quantum numbers correspond to eigenvalues of operators that commute with the Hamiltonian—quantities that can be kno ...

''N'' = 2 and positive parity. The Roper Resonance has been a subject of many studies because its mass is actually lower than three-quark states with radial quantum number ''N'' = 1. Only in the late 2000s was the lower-than-expected mass explained by theoretical calculations, revealing a quark core shielded by a dense cloud of mesons.

Discovery

The Roper resonance was discovered in 1963 by a computer fit of particle-scattering theory to large amounts of pion-nucleon scattering data. The analysis was done on computers at

Lawrence Livermore National Laboratory Lawrence Livermore National Laboratory (LLNL) is a federal research facility in Livermore, California, United States. The lab was originally established as the University of California Radiation Laboratory, Livermore Branch in 1952 in response ...

for Ph.D. thesis work of L. David Roper at

Massachusetts Institute of Technology The Massachusetts Institute of Technology (MIT) is a private land-grant research university in Cambridge, Massachusetts. Established in 1861, MIT has played a key role in the development of modern technology and science, and is one of the ...

under the direction of Bernard Taub Feld at MIT and Michael J. Moravcsik at LLNL. The computer code was developed by Richard Allen Arndt and Robert M. Wright.

Decay

Because of the relatively large full width, which according to

uncertainty principle In quantum mechanics, the uncertainty principle (also known as Heisenberg's uncertainty principle) is any of a variety of mathematical inequalities asserting a fundamental limit to the accuracy with which the values for certain pairs of physic ...

means a shorter lifetime, the Roper resonance decays into a system consisting of other

hadron In particle physics, a hadron (; grc, ἁδρός, hadrós; "stout, thick") is a composite subatomic particle made of two or more quarks held together by the strong interaction. They are analogous to molecules that are held together by the ele ...

s with sum of the masses less than the mass of the original state. The Roper resonance decays most of the time via the

strong force The strong interaction or strong force is a fundamental interaction that confines quarks into proton, neutron, and other hadron particles. The strong interaction also binds neutrons and protons to create atomic nuclei, where it is called the n ...

into an ordinary

nucleon In physics and chemistry, a nucleon is either a proton or a neutron, considered in its role as a component of an atomic nucleus. The number of nucleons in a nucleus defines the atom's mass number (nucleon number). Until the 1960s, nucleons were ...

plus a

pion In particle physics, a pion (or a pi meson, denoted with the Greek letter pi: ) is any of three subatomic particles: , , and . Each pion consists of a quark and an antiquark and is therefore a meson. Pions are the lightest mesons and, more gene ...

plus two

pions In particle physics, a pion (or a pi meson, denoted with the Greek letter pi: ) is any of three subatomic particles: , , and . Each pion consists of a quark and an antiquark and is therefore a meson. Pions are the lightest mesons and, more gene ...

, or Δ plus a

Composition

References

* * * * *

Discovery

Decay

Composition

References

See also