K + photoproduction with the Crystal Ball at MAMI T.C. Jude The University of Edinburgh New method of K + detection with the Crystal Ball Extraction

K+photoproduction with the Crystal Ball at MAMI

T.C. JudeThe University of Edinburgh

• New method of K+ detection with the Crystal Ball

• Extraction of (p,K+) cross sections from threshold to 1.4 GeV photon beam

T.C. Jude, K+ photoproduction off the proton. A2 collaboration meeting, 10th March 2010

(p,K+) Cross Section Measurements• Predicted nucleon resonances which have not been observed experimentally may couple to strange decay channels1

• A crucial test of QCD based chiral effective field theories in the strange quark sector2

E [GeV]

[1] S Capstick and W Roberts, Phys. Rev. D58, 074011 (1998)[2] B Borasoy et al. Eur. Phys. J. A 34, 161-183 (2007)[3] R. Bradford et al., Phys Rev. C 73, 035202 (2006)[4] K.H. Glander et al., Eur Phys. J. A 19, 251 (2004)[5] T. Mart arXiv:0803.0601v1 [nucl-th] (2008)[6] T. Nelson and T. Mart. Mod. Phys. Lett. A, 24, 964 (2007)[7] T.S.H Lee et al. Phys. Rev. C. 73, 055204 (2006)[8] T. Van Cauteren et al. Phys. Rev. C 73, 045207 (2006)


• Significant discrepancies exist between cross-sections measurements3,4

• Fits to these data sets reveal differences in nucleon resonances when describing the photoproduction process5,6

• Theoretical models (Coupled channel analysis7, Regge models8) cannot at present discern between data sets.

• No detailed measurements close to threshold energies.

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

n + 0 (~ 36%)

p K+ +

K+ meson detection in segmented calorimeters

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

(~ 21%)

Incident sub-cluster from K+ ~ 3ns

Decay sub-cluster from K+ decay ~ 20ns

• Identify the K+ decay within the crystals of the Crystal Ball and TAPS• Eliminates the need of expensive magnetic spectrometers


Number of crystals in decay sub-cluster

Furthest crystal in decay sub-cluster

Crystal Ball – PID E-E cut(Blue line)


Decay sub-cluster energy

Incident and decay sub-cluster time difference

K+ missing mass


K+ energy loss correction• GEANT4 simulation of K+ phase space distribution in the Crystal Ball.

• The decay of the K+ “switched off”

• Measure K+ energy difference on a NaI crystal by crystal basis


K+ Remnant energy correction• Correction for energy of the K+ decay left in the K+ cluster

• Identify the type of K+ decay (muonic or pionic) from parameters of the decay cluster

Experimental data. 2D cuts to select muonic and pionic events.

Simulated data. K+ weak decay muonic / pionic


K+ Remnant energy correction


GEANT4 simulation• Effect of the ADC integration time interval on the K+ decay cluster energy spectrum.

• Use the Crystal Ball timing of the real data to set the simulated cut off as 600 ns.

• Simulated timing resolution set by measuring time differences of crystals in the same cluster for photon detection.

• Energy resolution set by width of proton mass peak from 0 missing mass spectrum


GEANT4 simulation• Simulation of the Crystal Ball energy sum

• Select (p,K+) events in real and simulated data

• M2 trigger simulated (20 MeV threshold for each trigger section)

• Use cumulative distribution fit to give probability of accepting simulated events


Decay sub-cluster energy.Measured K+ energy < 250 MeV

Decay sub-cluster energy. Measured K+ energy < 100 MeV

90 < cm<1201.0 < E<1.2 GeV

Experimental data (red), simulated data (blue)

Reconstructed to measured K+ energy difference

K+ missing mass


(simulation) (simulation)

Detection of 0 decay• Distinguish between (p, K+) and (p, K+)0 by the identifying the photon from 0 decay


• Identification of the photon from 0 decay

• Use TOF techniques to discriminate between neutrons and photons in TAPS

From the K+ events identified (in simulation):

~ 55% of K+0 events correctly identified by detection of the decay photon~ 5% of K+ events misidentified

Detection of 0 decay


Simulated data

Experimental dataTrain fits to K+ missing mass spectra with simulated data



Fit independent method of yield extraction•2 missing mass histograms per energy and polar angle range:

•Positive and negative identification of 0 decay photon.

•Subtract scaled histogram of decay ID to leave only K+ contribution

~ 12%. SAID distribution of 30 million K+ events

July 2007: 9.62 x 104 K+ events April 2009: 2.82 x 105 K+ events

(p,K+) Cross Section Measurements

July 2007 data set

July 2007: 2.023 x 1027 protons /m2 April 2009: 4.248 x 1027 protons /m2

• July 2007 and April 2009 data sets

• Empty target subtracted, M1 tagger hits included (approximately 4%)

July 2007: 1508.0 MeVApril 2009: 1557.4 MeV



Blue: Yield extraction from fitting to K+ missing mass spectraRed: Yield extraction from identifying 0 decay photon


(p, K+) cross sections with different data sets and methods of yield extraction

July ’07, Fit to missing mass spectra (red)

July ’07, ID 0 decay (blue)

April ’09, Fit missing mass spectra (Green)

April ’09, ID 0 decay (black)


• Cross sections on the next two slides:

• Data points (black), weighted sum of both data sets. Select K+ using decay photon ID.

• Selecting K+ by fitting to missing mass spectra (green)

• Previous SAPHIR1 data points (red). Each angular bin backwards by cos = 0.05.

• Previous CLAS2 data points (blue).

• Kaon-MAID prediction (Magenta). Includes S11(1650), P11(1710), P13(1720) and D13(1900)

• Regge-Plus-Resonance (RPR) model3,4 (light blue). Constrain fits from forward angle cross section data.

[1] K.H. Glander et al., Eur Phys. J. A 19, 251 (2004)[2] R. Bradford et al., Phys Rev. C 73, 035202 (2006)[3] T. Van Cauteren et al. Phys. Rev. C 73, 045207 (2006),[4] Pieter Vancraeyveld, The University of Ghent. Private communication (2010)

JLab notation adopted:Do not integrate out azimuthal angle from differential cross sections (multiply by 2)



Cross section measurements as a function of centre of mass polar angle.

Important constraints on chiral effective field theories near threshold.

Blue line: Cross section extraction from a chiral effective Lagrangian1

2 fits performed to available photoproduction data

[1] B Borasoy et al. Eur. Phys. J. A 34, 161-183 (2007)

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

Black points: This data

Red points: SAPHIR data2


Systematic uncertainties • PID detection efficiency: reconstruction of proton from (p,p)0. (Pauline’s talk.)

• K+ yield extraction: Comparison between methods of yield extraction.

• Detection efficiency and modelling energy sum. Compare SAID and uniform distributions. Extract cross sections with each K+ decay mode.

• K+ hadronic interactions in the NaI crystals1.


Simulated with K+ hadronic interactions

Simulated with no K+ hadronic interactions

Experimental data.1D.H. Wright and A. Heikkinen. Adding Kaons to the Bertini cascade model. Proceedings of CHEP04 (2004)

K+photoproduction with the Crystal Ball at MAMI

T.C. JudeThe University of Edinburgh


Extra slides ....

GEANT4 simulation• Separate energy scale for the K+ decay cluster

• Compare peak position from K+ muonic decay for real and simulated data

• Energy scale varies with K+ energy due to NaI crystal dimensions and light attenuation.


New opportunities with K+ detection in the Crystal Ball

Asymmetry of decay distribution with circular polarised beam

1.0 – 1.2 GeV[deg] cos( )1.0 – 1.2 GeV

• Extraction of the polarisation observables CX and CZ via the distribution of the weak decay products.


100 MeV energy sum in the CB

330 MeV energy sum in the CB

K+ detection in TAPS (simulation)

Energy [MeV]

Energy of decay sub-cluster

Time difference

of incident and decay sub-cluster

Time [ns]

Lab polar angle of K+ detection

[deg]


• Isobar models: describe hadronic interactions from a series of Feynman contributions of resonant and non-resonant meson exchange.

• Coupled channel analysis1. Include multistep processes (N N KY).

• Regge trajectories: constrain the background terms and interpolate into the resonance region superimposing resonance structure (Regge-Plus-Resonance model2).

• Multipole analysis3. Background described by born terms and t-channel (vector meson) exchange. Include “PDG five star” resonances and constrain from experimental data.

Conclusion: K+ resonant structure is poorly understood with the available data to constrain theoretical models.

Theoretical approaches to K+ photoproduction

[1] T.S.H Lee et al. Phys. Rev. C. 73, 055204 (2006)

[2] T. Van Cauteren et al. Phys. Rev. C 73, 045207 (2006)

[3] T. Nelson and T. Mart. Mod. Phys. Lett. A, 24, 964 (2007)


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K + photoproduction with the Crystal Ball at MAMI T.C. Jude The University of Edinburgh New method of K + detection with the Crystal Ball Extraction