T.C. Jude D.I. Glazier, D.P. Watts

The University of Edinburgh

Strangeness Photoproduction: Polarisation Transfer & Cross-Section

Measurements

Hyperon Photoproduction

p K+ + p + - (~ 64%)

n + 0 (~ 36%)

p K+ +

• Important for investigations of nucleon resonances

• A crucial test of QCD based chiral perturbation theories in the strange quark sector

T C Jude, The University of Edinburgh, Strangeness Photoproduction A2 Collaboration meeting, Mainz, September 2008

Threshold Energy ~ 911 MeV

Threshold Energy ~ 1046 MeV

Polarisation observables• Spin observables – constrains reaction models and baryon resonance structure

• Circularly polarised photons - polarisation transferred to the recoiling hyperon

• CX and CZ: PX = -PCX, Py = P, PZ = -PCz

• decay is a weak decay (parity violating) – measure the polarisation by the distribution of the decay particles (proton or neutron)

• Previous measurements found them to be close to one and zero respectively for all measured kinematics [1]

T C Jude, The University of Edinburgh, Strangeness Photoproduction A2 Collaboration meeting, Mainz, September 2008

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

Cross-Section Measurements

• Significant discrepancies between cross-sections measured at JLab [2] and ELSA [3].

T C Jude, The University of Edinburgh, Strangeness Photoproduction A2 Collaboration meeting, Mainz, September 2008

E [GeV]

[2] R. Bradford et al., Phys Rev. C 73, 035202 (2006)

[3] K.H. Glander et al., Eur Phys. J. A 19, 251 (2004)

[4] T. Mart arXiv:0803.0601v1 [nucl-th] (2008)

The large angular coverage and intense photon beam over the range ~ 0.9 to 1.4 GeV make the Crystal Ball ideal for strangeness measurements at threshold

• No detailed measurements of p K+ at threshold

Identifying strange mesons

K+ (~ 63%) Mean lifetime of K+ ~ 12 ns

(~ 21%)

First cluster from K+ < 3ns

Secondary cluster from K+ decay > 10ns

• A new method of tagging a strange meson reaction channel• Identify the K+ decay within the crystals of the Crystal Ball

T C Jude, The University of Edinburgh, Strangeness Photoproduction A2 Collaboration meeting, Mainz, September 2008

K+ ID in the Crystal Ball

T C Jude, The University of Edinburgh, Strangeness Photoproduction A2 Collaboration meeting, Mainz, September 2008

K+ ID in the Crystal Ball

• E-E analysis:

K+

• Crystal Ball – Identifies ~ 8% of all K+

• Misidentification of non-strange channels ~ 10-5

T C Jude, The University of Edinburgh, Strangeness Photoproduction A2 Collaboration meeting, Mainz, September 2008

K+ ID in TAPS

T C Jude, The University of Edinburgh, Strangeness Photoproduction A2 Collaboration meeting, Mainz, September 2008

Kinematic corrections

• Distinguishing between K+0 and K+ events by:

(a) Differences in missing mass and reconstructed energies

(b) Identification of the photon from the decay: 0

(c) Direct detection of the neutron and momentum measurement

T C Jude, The University of Edinburgh, Strangeness Photoproduction A2 Collaboration meeting, Mainz, September 2008

• Correcting for the vertex position of the decay: n + 0

Before Correction

After Correction

Invariant mass of two photons in coincidence

K+K+

Missing mass from K+ and photon in coincidence (simulation)Reconstructed K+ energy vs. difference between measured and reconstructed energy

K+0

K+

z Asymmetry Measurements• Measuring the polar angle of the neutron from the decay of the Reference frames consistent with [1]

• Can extract CZ and CX

• Can include more data from the p 0 decay

[1] R. Bradford et al., Phys Rev. C 75, 035205 (2007)T C Jude, The University of Edinburgh, Strangeness Photoproduction A2 Collaboration meeting, Mainz, September 2008

p K+ Cross-Section Measurements

T C Jude, The University of Edinburgh, Strangeness Photoproduction A2 Collaboration meeting, Mainz, September 2008

• 5M p K+ generated events for the detection efficiency, det

binned into E and .

• For preliminary measurements, assume a tagging efficiency, tagg ~ 0.6 [5]

• target density, ~ 2.023E(27) protons m-2

• Use the JLab convention [2]: (integrate over the part of the solid angle only)

[2] R. Bradford et al., Phys Rev. C 73, 035202 (2006)

[3] K.H. Glander et al., Eur Phys. J. A 19, 251 (2004)

[5] V.L. Kashevarov, A2 Collaboration meeting, Dubrovnik (2008)

Next slide: CB data

JLab data[2] (2006)

SAPHIR data[3] (2004)

T C Jude, The University of Edinburgh, Strangeness Photoproduction A2 Collaboration meeting, Mainz, September 2008

-5.5 < cos(K+) < -4.5 -3.5 < cos(K+) < -2.5

-0.5 < cos(K+) < 0.51.5 < cos(K+) < 2.5

Preliminary

PreliminaryPreliminary

Preliminary

Summary

• First detection of kaons with the Crystal Ball at MAMI-C

• A new technique of strangeness identification – has application for calorimeters elsewhere

• Preliminary asymmetry measurements with a circular polarised photon beam – will be used to extract CX and CZ

• Preliminary cross-section measurements close to threshold

T C Jude, The University of Edinburgh, Strangeness Photoproduction A2 Collaboration meeting, Mainz, September 2008