pion physics in the meson factory era r. p. redwine · 2006-11-27 · meson factories the primary...

September 29, 2006September 29, 2006 Bates SymposiumBates Symposium 11

PionPion Physics in the Meson Factory EraPhysics in the Meson Factory Era

R. P. RedwineR. P. Redwine

Bates Linear Accelerator CenterBates Linear Accelerator CenterLaboratory for Nuclear ScienceLaboratory for Nuclear Science

Department of PhysicsDepartment of PhysicsMassachusetts Institute of TechnologyMassachusetts Institute of Technology


Meson FactoriesMeson Factories

Los Alamos Meson Physics Facility (LAMPF)Los Alamos Meson Physics Facility (LAMPF)800 800 MeVMeV Proton Proton LinacLinacLos Alamos, New MexicoLos Alamos, New Mexico

Swiss Institute for Nuclear Research (SIN), now Paul Swiss Institute for Nuclear Research (SIN), now Paul ScherrerScherrer Institute (PSI)Institute (PSI)

600 600 MeVMeV Proton Coupled CyclotronsProton Coupled CyclotronsVilligenVilligen, Switzerland, Switzerland

TriTri--Universities Meson Facility (TRIUMF)Universities Meson Facility (TRIUMF)560 560 MeVMeV Proton CyclotronProton CyclotronVancouver, CanadaVancouver, Canada



The primary goal of the meson factories was to The primary goal of the meson factories was to produce copious quantities of produce copious quantities of pionspions to use in to use in nuclear structure and reaction studies.nuclear structure and reaction studies.Beams of protons, Beams of protons, muonsmuons, and neutrinos were , and neutrinos were also available. also available. Fundamental symmetry tests played a large role Fundamental symmetry tests played a large role at the meson factories (and still do to an at the meson factories (and still do to an extent).extent).I will concentrate on I will concentrate on pionpion--nucleus reactions.nucleus reactions.



LAMPF produced its first beam in 1973.LAMPF produced its first beam in 1973.

SIN and TRIUMF followed very soon after.SIN and TRIUMF followed very soon after.


From Boston, it is equally difficult to get to Los From Boston, it is equally difficult to get to Los Alamos, Vancouver, and Alamos, Vancouver, and VilligenVilligen


PionsPions

The The pionpion is a spinis a spin--0 boson with 0 boson with isospinisospin 1.1.

It has three charge states (+, 0, It has three charge states (+, 0, --).).

Pi Pi –– nucleon interactions involve singlenucleon interactions involve single--chargecharge--exchange as well as elastic scattering.exchange as well as elastic scattering.

As a boson, the As a boson, the pionpion can be absorbed, although can be absorbed, although absorption on a single nucleon in the nucleus is absorption on a single nucleon in the nucleus is very unlikely.very unlikely.


PionPion nucleus total cross sectionsnucleus total cross sections

Throughout most of the Throughout most of the energy region of interest, the energy region of interest, the Delta (1232) piDelta (1232) pi--nucleon nucleon resonance plays a dominant resonance plays a dominant role.role.The Delta is a spin 3/2, The Delta is a spin 3/2, isospinisospin3/2 resonance with a width of 3/2 resonance with a width of about 120 about 120 MeVMeV..The pi+ is sensitive primarily The pi+ is sensitive primarily to protons, the pito protons, the pi-- to neutrons.to neutrons.The The pionpion--nucleus interaction is nucleus interaction is very strong very strong –– most of the most of the action is on the nuclear action is on the nuclear surface.surface.


PionPion nucleus cross sectionsnucleus cross sections

The Delta resonance The Delta resonance appears strongest in the appears strongest in the absorption channel.absorption channel.

This is still not well This is still not well understood.understood.


Mass dependence of cross sectionsMass dependence of cross sections


Comparison of proton spectra following Comparison of proton spectra following pionpion and and photon absorptionphoton absorption

220 220 MeVMeV incident incident pionspions(points)(points)

343 343 MeVMeV incident photons incident photons (histogram) (X 55)(histogram) (X 55)

a) 45 dega) 45 degb) 65 degb) 65 degc) 120 degc) 120 deg


Theoretical ApproachesTheoretical Approaches

Many calculations using multiple scattering to Many calculations using multiple scattering to describe describe pionpion--nucleus interactions.nucleus interactions.

The DeltaThe Delta--hole model explicitly took into account hole model explicitly took into account the fact that the the fact that the pionpion in this energy region was in this energy region was likely to excite a Delta resonance, leaving a hole likely to excite a Delta resonance, leaving a hole in the nucleon states.in the nucleon states.


Elastic ScatteringElastic Scattering

The data are highly The data are highly diffractive in nature.diffractive in nature.

Optical potential Optical potential calculations show strong calculations show strong sensitivity to the radius of sensitivity to the radius of the nucleus.the nucleus.


Delta Delta –– Hole ModelHole Model

The agreement of the DeltaThe agreement of the Delta--hole model with hole model with data is improved if a “Spreading Potential”, data is improved if a “Spreading Potential”, taking into account taking into account pionpion absorption and the absorption and the spinspin--orbit interaction of the Delta, is included.orbit interaction of the Delta, is included.


Delta Delta –– hole model calculations hole model calculations –– with and without with and without the spreading potentialthe spreading potential

pi elastic scattering from 16O at 240 pi elastic scattering from 16O at 240 MeVMeV


Inclusive pi scattering on 16OInclusive pi scattering on 16O

Solid curve includes medium effects on Delta Solid curve includes medium effects on Delta propagationpropagation


Inelastic scattering to discrete statesInelastic scattering to discrete states

DWIA calculationsDWIA calculations


Pi Pi –– Carbon inelastic scatteringCarbon inelastic scattering

Delta Delta –– hole model hole model calculations. calculations. The solid curves include The solid curves include medium effects on the medium effects on the Delta propagator.Delta propagator.


PionPion singlesingle--chargecharge--exchangeexchange

Angular distributions Angular distributions integrated over the integrated over the outgoing outgoing pionpion energyenergyThe lines represent the The lines represent the angular distribution for angular distribution for pipi--nucleon SCXnucleon SCX


PionPion SCX to discrete statesSCX to discrete states

LAMPF pi0 spectrometer LAMPF pi0 spectrometer datadataThis is an isobaric This is an isobaric analogue transition.analogue transition.


PionPion Double Charge ExchangeDouble Charge Exchange

This was the first This was the first observation of observation of pionpion DCX DCX to a discrete state.to a discrete state.It also is the largest It also is the largest single pi DCX cross single pi DCX cross section to a discrete state section to a discrete state ever measured!ever measured!The hope for pi DCX was The hope for pi DCX was that it would show great that it would show great sensitivity to twosensitivity to two--nucleon nucleon correlations.correlations.


PionPion Double Charge ExchangeDouble Charge Exchange


Inclusive Inclusive PionPion DCXDCX

Inclusive data are well Inclusive data are well described by a model described by a model which includes successive which includes successive SCX reactions in SCX reactions in competition with other competition with other important reactions.important reactions.


PionPion DCX on 4HeDCX on 4He

Data at 25 degData at 25 dega) 270 a) 270 MeVMeV pi+pi+b) 240 b) 240 MeVMeV pi+pi+

The curves represent a The curves represent a sequential model of DCX.sequential model of DCX.


PionPion Absorption in NucleiAbsorption in Nuclei

This was a major area of investigation.This was a major area of investigation.

PionPion absorption involves dumping a large absorption involves dumping a large amount of energy in the nucleus.amount of energy in the nucleus.

PionPion absorption was expected to be sensitive to absorption was expected to be sensitive to twotwo--nucleon correlations.nucleon correlations.

The prototype reaction is pi+ d The prototype reaction is pi+ d --> pp.> pp.

The data turned out to be very interesting.The data turned out to be very interesting.


PionPion AbsorptionAbsorption

((pi+,pppi+,pp) reactions on Ni ) reactions on Ni at 165 at 165 MeVMeVThere is a clear indication There is a clear indication for twofor two--nucleon nucleon absorption, but there is absorption, but there is evidence for a lot of cross evidence for a lot of cross section involving more section involving more than two nucleons.than two nucleons.


PionPion absorption on 3Heabsorption on 3He


PionPion absorption at 160 absorption at 160 MeVMeV


PionPion Absorption in 4HeAbsorption in 4He


What do we know about What do we know about pionpion absorption in nuclei?absorption in nuclei?

It is a major reaction channel.It is a major reaction channel.It resonates more than the other channels.It resonates more than the other channels.TwoTwo--nucleon absorption is important, but so is nucleon absorption is important, but so is multimulti--nucleon.nucleon.There is some evidence for coherent absorption There is some evidence for coherent absorption involving more than two nucleons that may be involving more than two nucleons that may be new physics, but it is not definitive.new physics, but it is not definitive.The breakup into separate channels in 4He is The breakup into separate channels in 4He is hard to understand and may be an important hard to understand and may be an important clue.clue.


SummarySummary

The meson factory era was a rich and exciting The meson factory era was a rich and exciting one, and I have only been able to touch on one, and I have only been able to touch on some important aspects.some important aspects.The special properties of The special properties of pionspions do give one do give one access to some important states and reactions access to some important states and reactions which are not available with other probes.which are not available with other probes.The difficulty in describing precisely the The difficulty in describing precisely the interactions of interactions of pionspions in nuclei ultimately limited in nuclei ultimately limited the utility of the utility of pionspions as a probe of nuclear as a probe of nuclear structure.structure.Understanding the important reactions of Understanding the important reactions of pionspionswith nuclei remains an important challenge.with nuclei remains an important challenge.


Large Acceptance Detector System (PSI)Large Acceptance Detector System (PSI)


Delta Delta –– Hole model fits to elastic scattering and Hole model fits to elastic scattering and total cross sectionstotal cross sections

pion physics in the meson factory era r. p. redwine · 2006-11-27 · meson factories the primary...

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