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Baryon spectroscopy at CLAS and CLAS12. Ken Livingston, University of Glasgow, Scotland For the CLAS collaboration. Baryons’10, Dec 2010, Osaka, Japan. Hadron spectroscopy at CLAS The N* programme CLAS 12 Summary. Hadron spectroscopy at CLAS The N* programme CLAS 12 Summary.

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Baryon spectroscopy at clas and clas12
Baryon spectroscopy at CLAS and CLAS12

Ken Livingston, University of Glasgow, Scotland

For the CLAS collaboration

Baryons’10, Dec 2010, Osaka, Japan

  • Hadron spectroscopy at CLAS

  • The N* programme

  • CLAS 12

  • Summary

  • Hadron spectroscopy at CLAS

  • The N* programme

  • CLAS 12

  • Summary

  • Study of nucleon resonances:

    • Measurement of the transition form factors

    • Search for missing states

  • Strangeness production

    • L and S production

    • Cascade spectroscopy

  • Search for “unconventional states”

    • high statistics search of pentaquark states

    • search for hybrid mesons

      > 100 analyses in progress

Missing baryon resonances

With D13

Without D13

Missing baryon resonances

After Cx, Cy from CLAS, revised as most likely P13.

Eur Phys J A34 (07) 243

Mart & Benhold, Phys. Rev. C 61 012201(R) (1999)

  • Clear indication of resonances in γp cross section for many channels

  • Constituent quark models predict many resonances, but several missing

  • Mostly from πN scattering and single π photoproduction

  • Really missing or undetected since weak coupling to these channels

    • Try other channels. Eg. K photoproduction

  • Eg. Cross sections show some hints of new D13.

    • Better to look at angular distributions and polarization observables.

Polarization observables in pseudoscalar meson production
Polarization observables in pseudoscalar meson production

4 Complex amplitudes: 16 real polarization observables.

Complete measurement from 8 carefully chosen observables.

πN has high statistics but in KY recoil is self-analysing



recoil targγ

γ targ recoil










I. S. Barker, A. Donnachie, J. K. Storrow, Nucl. Phys. B95 347 (1975).

Complete, and


circ polarized photons

longitudinally polarized target

transversely polarized target

linearly polarized photons

Jefferson Lab






  • Continuous

  • Electron

  • Beam

  • Accelerator

  • Facility

  • E: 0.75 –6 GeV

  • Imax: 200mA

  • RF: 1499 MHz

  • Duty Cycle:  100%

  • s(E)/E: 2.5x10-5

  • Polarization: 80%

  • Simultaneous distribution to 3 experimental Halls


Experimental Halls

Cebaf large acceptance spectrometer

Electromagnetic Calorimeter

lead/plastic scintillator, 1296 PMTs

Torus Magnet

6 Superconductive Coils

Target +  start counter e mini-torus

Drift Chamber

35,000 cells

Time of Flight

Plastic Scintillator, 684 PMTs

Cherenkov Counter e/ separation, 256 PMTs

CEBAF Large Acceptance Spectrometer

Tagged photons at clas
Tagged photons at CLAS

  • Tagged bremsstrahlung photons up to 6GeV.

  • Timing resolution < 1 beam bucket.

  • Circularly polarized

    • up to ~80% now standard

  • Linearly polarized

    • coherent bremsstrahlung up to >90%.

Polarization observables at clas

Linear Polarisation

Circular polarisation 

→ Y + 

Nucleon recoil

polarimiter x

Longitudinally polarized nucleon targets p n 2011

Transverse polarized nucleon targets p n 2011

Hyperons are “self analysing” 

Polarization observables at CLAS

  • + N→ N + m

Polarization observables - a simple example, S

  • Systematics of detector acceptance cancel out.

  • “Only” need to know Plin, the degree of linear polarization.

1 st pol observables in k photoproduction on p at clas p c x and c z
1st pol. observables in K photoproduction on p at CLAS.P, Cx and Cz

R. Bradford et al., Phys. Rev. C 75, 035205 (2007).

→ L fully polarised

g8b, July 2005

Lin. polarized photon energy range: 1.3 – 2.1 GeV

preliminary results:p0p, Mike Dugger, ASU

CLAS data


Beam Asymmetry Σ

  • Very high statistics. Good agreement with previous measurement.

  • We have P well determined – expect <5% systematic error in P.

  • ~10MeV Eg bins, and will extend up to 2.1GeV

  • Similar for p+n, hp, h’p

g8b preliminary results - K+L and K+S0Craig Paterson, Glasgow

gp-> K+L -> K+pp-gp-> K+S0 -> K+Lg -> K+pp-g

Single polarization observablesS Photon asymmetryP Recoil polarization (induced pol. along y)T Target asymmetryDouble polarization observablesOx Polarization transfer along xOz Polarization transfer along z


figure by R.Schumacher, CMU

g8b preliminary results - K+L

  • Results compared with previous results from LEPS

    • 6, 100MeV Energy bins

    • 1550 -> 2050MeV

    • More bins for our data

Increase the angular coverage to backward angles

LEPS also recently have some consistent, new points at backward angles.

Hicks et al., PRC 76, 042201(R) (2007).


g8b preliminary results - K+L

Ox/Ozextracted from fit to 2d asymmetry

T Target asymmetry from 2d asymmetry (not shown)

P Recoil pol. from acceptance corrected proton dist.

P Recoil pol.

G8 summary

Several analyses at advanced stage. Only a small sample presented here

K+L , K+S0 :Single (P,T,S) and double (Ox,Oz) polarization observables

p0p, p+n, hp, h’p, wp, p+p-p : polarization photon beam asymmetry, S

…. several publications expected next year



CLAS J.W.C. McNabb, et al. (CLAS) Phys. Rev. C 69, 042201(R) (2004).

g9 FROST – FROzen Spin Target (butanol = C4H9OH)

Meson photoproduction with linearly and circularly polarized photons on polarized target

E02-112: γp→KY (K+Λ, K+Σ0, K0Σ+)‏

E03-105/E04-102: γp→π0p, π+n

E05-012: γp→ηp

E06-013: γp→π+π-p

Racetrack coils for transverse polarization

g9a running conditions

November 3, 2007– February 12, 2008

Longitudianally polarized target

Circularly and linearly polarized photon beam 0.5-2.4 GeV

Trigger: at least one charged particle in CLAS

Target Pol > 80%, Relaxation time > 1600hrs – better that design goals


March – July 2010

Transversely polarized target

G9 frost sample analysis of gp p n
g9 FROST sample analysis of gpp+n

Steffen Strauch, University of South Carolina








Measurements on n (Deuterium) target

  • G10. Unpolarized photons on Deuterium

  • 1st measurement of σ for the γ n → K+Σ-

Sergio Anefalos Pereira, INFN. Phys. Lett. B 688 (2010) 289-293

Edwin Munevar, GWU


  • Detect p- n, K+

  • Σfrom p- n inv. mass

  • CLAS Data ▲ LEPS _____ Gent RPR model

  • Circularly and linearly polarized photons on n (g13, Deuterium target)

  • γ n → K+Σ-analysis for polarization observables underway. Edwin Munevar, GWU

Measurements on n (Deuterium) target

  • G13 Linearly polarized photons on n (Deuterium)

  • Measurement of S for γ n → p- p Daria Sokhan, Edinburgh


Measurements on n (Deuterium) target

G13 1st measurement of Beam Asymmetry gn (p)-> K0s L0(p)

Neil Hassall, Glasgow


Single, and double polarization observables in progress

Measurements on n (Deuterium) target

HDIce Solid Deuterium-Hydride (HD) – a new class of polarized target

  • Spin can be moved between H and D with RF transitions

  • All material can be polarized with almost no background

  • Run in Spring 2011

Search for baryon states in γp➝pω➝pπ+π-(π0)

  • Spin density matrix elements in bins ΔW = 10 MeV, for W = 1.7–2.4 GeV in blue - blueshades.

  • Previous world data in red.

  • Preliminary Σ for γ p →ω p

  • Oh, et al Patrick Collins, ASU

  • Paris

  • Sarantsev, et al

  • The CLAS data are used as input to a single channel event-based, energy independent partial wave analysis (the first ever for baryons).

  • ω production is dominated by the well known F15(1680), D13(1700) and G17(2190), and a predicted “missing” F15(2000).


M. Williams, et al. (CLAS) , Phys.Rev.C80:065208,2009

M. Williams, et al. (CLAS) , Phys.Rev.C80:065209,2009

Construction of the new Hall D

Upgrade of the arc magnets


Upgrade of the instrumentation of the existing Halls


Beam Power: 1MW

Beam Current: 90 µA

Max Pass energy: 2.2 GeV

Max Enery Hall A-C: 10.9 GeV

Max Energy Hall D: 12 GeV

Spectroscopy with clas12

Forward Tagger

Spectroscopy with CLAS12

  • electron scattering at “0” degrees (LowQ, post-target tagging): low-Q2 virtual photon  real photon

  • photons tagged by detecting the scattered electron

  • quasi-real photons are linearly polarized and the polarization can be determined event by event

  • high luminosity on thin (gas)- targets

  • can exploit CLAS12 PiD for final states with strangeness

  • Complementary to GLUEX Coh. Brem. photon beam

  • Parasitic running

The construction of Low Q Tagging Facility or Forward Tagger has been proposed to continue the spectroscopy program with CLAS12 using quasi-real photoproduction

A new physics programme

  • Meson spectroscopy

    • Spectroscopy on H target and search for exotics

    • Spectroscopy on 4He and other gas targets

  • Hadron spectroscopy

    • Heavy mass baryon resonances (Cascades and W-)

  • Compton scattering

  • Meson polarizabilities

  • Large -t physics

  • T-GPDs

< 6 GeV

Meson Spectroscopy at CLAS and CLAS12, Carlos Salgado SP2, Wed 14.00


  • CLAS has large hadron spectroscopy programme

    • Huge effort on N*, missing baryon resonances

      • Complete measurements on p and n soon (KL, KS)

      • Single and double pol observables on many other channels

  • CLAS 12

    • A forward tagger offers a whole new hardon spectrosopy programme

Some related CLAS talks to be give here:

Meson Spectroscopy at CLAS and CLAS12, Carlos Salgado SP2, Wed 14.00

K*L and K*S photoproduction at CLAS, Ken Hicks EM1, Tues 17.45

Extraction of Multipole Amplitudes from Over-Complete Experiments in Meson Photoproduction,

A. Sandorfi MO Thurs 10.10

K production on n. Deuterium target

How good a “free” neutron target is Deuterium ?

G13. Compare photon asymmetry of gp (n)-> K+L0 (n) withgp -> K+L0 (free and bound p)

gp (n)-> K+L0 (n) Russell Johnstone, Glasgow

× Free

● Quasi free


photon asymmetry S



Cosqcm (-1.0 - +1.0)

Each plot is 200MeV photon energy bin


Free and quasi-free proton

Quasi free neutron good approx. to free, here.

K production on n. Deuterium target

G13 gn (p)-> K0sL0(p) gn (p)-> K0s S0(p)

Neil Hassall, Glasgow

K production on n. Deuterium target

G13 1st measurement of Beam Asymmetry gn (p)-> K0sL0(p)

Neil Hassall, Glasgow


Single, and double polarization observables in progress

g8b data. g + p → p+ n with (n,p) scatter (detect p+p)



Angle between Pncalc and Ppmeasured


Recoil polarimetry possibilities ?

n(q,f) =no(q){1+A(q)[Pycos(f)–Pxsin(f)] - How to do this for 4p detector

x and y (transverse) components

of nucleon polarisation

Nucleon polarimeter for [email protected], D. Watts, Edinburgh







Start counter

CLAS coherent bremsstrahlung facility

  • Tagging spectrometer with high rate, good energy and timing resolution

  • High precision goniometer (GWU)

  • High quality, thin diamond (Glasgow)

  • Tight photon beam collimation (ISU)

  • Polarimetry

Photon energy

P > 90%

Peak > 90% pol.

“A device called a goniometer tilts the diamond, much like a lady turning her hand to admire the sparkle of a new ring.” - JLAB On Target Magazine

Measurements with photon beam profile detectorD. Glazier, Glasgow

1st Measurement of 2D photon enhancement for coherent bremsstrahlung (MAMI,Mainz) paper in preparation

below peak

coherent peak

above peak

Coherent peak at 300Mev, MAMI electron beam energy 855MeV

  • Good agreement with coherent bremstrahlung calculations

    • Improvements in incoherent component, collimation + multiple scattering.

  • No evidence of high energy photons from quasi channeling.

  • Investigation of 2D strip detector for polarimetry

g8b preliminary results - K+L

  • K+LPhoton Asymmetry, S, extracted from cos(2f) fit to azimuthal kaon distribution

    • Fits shown for 1 energy bin

    • 340 (20E, 17q) kinematic bins

    • Almost full angular coverage

g8b preliminary results - K+S0

  • Results compared with previous results from GRAAL

    • 7, 50MeV Energy bins

    • 1175 -> 1475MeV

    • Good agreement with previous results


g8b preliminary results - K+S0

  • Results compared with previous results from LEPS

    • 6, 100MeV Energy bins

    • 1550 -> 2050MeV

    • More bins for our data!!!

Increase the angular coverage to backward angles


Peak > 90% pol.


K.Livingston, Glasgow

J.Santoro, CUA

Polarimetry: from pair (e+,e-) productionH.Schmieden, Bonn

  • Well described by QED, but experimentally difficult – small opening angle



  • Pair production simulations by Kharkov group

  • Polarimeter built and tested at Sping8, recently tested at Jlab

  • Polarimeter to be based on Jlab design

  • Microstrip detectors, or pixel detectors (Atlas group)

  • Bonn student completed 10 months exchange in Glasgow now to be full time in Bonn.

Polarimetry: from hadronic reactionR. Beck, Mainz -> Bonn

Use reaction with a known photon asymmetry

  • Can be high statistics

  • Very good relative monitor of polarization

  • Combined beam, target polarization.

  • Non-indpendent – depends on specific expt

  • Need very good systematics or calibration

  • Awaiting MAMI polarized target and polarised photon beam in 2nd half of 2007

  • Recent preliminary results from JLab (g8b)‏

  • Proton target

  • Back to back charge particles in Start Counter

  • Atomic or hardonic ?

  • Asymmetry from ~20mins DAQ data

  • Constant with E from 1.3GeV – 1.9GeV