1. L*(1520) Photoproduction in gppK+K- from the g11 data set
Raffaella De Vita and Alex Kubarovsky
Physics motivations
Final results for the differential cross section
First results for the decay angular distribution
Summary

2. Physics Motivation g K+ ? p L*
L* production mechanism is still poorly understood due to the lack of experimental data
Existing data from photo- and electro-production suggest dominance of t-channel processes and in particular of K* exchange
Several model predictions for total and differential cross section available
J. M. Laget
V. Yu. Grishina et al.
L. Roca et al.
S. Nam et al. (last paper hep-ph )
...
Precise measurement of cross section and decay angular distribution can provide constraints on model prediction and insight on strangeness production g K+ ? p L*

3. Existing data
First photoproduction measurement were performed at SLAC and Daresbury
Daresbury measured differential, total cross section and decay angular distribution in the energy range GeV
First look at the decay angular distribution showed dominance of mz=±3/2 spin projection  K exchange
Process was found to be identical to gpKL
Limited statistics

4. Existing data
First photoproduction measurement were performed at SLAC and Daresbury
Daresbury measured differential, total cross section and decay angular distribution in the energy range GeV
First look at the decay angular distribution showed dominance of mz=±3/2 spin projection  K exchange
Process was found to be identical to gpKL
Limited statistics
Unpublished results from SAPHIR
Recently results for have been published by LEPS gp  KL(1520)  pK+K-
Differerential cross section shows an enhancement close to threshold that is interpreted as due to a resonance
Disagreement with Daresbury is observed

5. Existing data
First photo-production measurement were performed at SLAC and Daresbury
Daresbury measured differential, total cross section and decay angular distribution in the energy range GeV
First look at the decay angular distribution showed dominance of mz=±3/2 spin projection  K exchange
Process was found to be identical to gpKL
Limited statistics
Unpublished results from SAPHIR
Recently results for have been published by LEPS gp  KL(1520)  pK+K-
Differerential cross section shows an enhancement close to threshold that is interpreted as due to a resonance
Disagreement with Daresbury is observed
Significant difference with respect to gpKL production
Different behaviour of decay angular distribution for forward and backward angles is observed

6. Existing data
Electroproduction of L* has been studied at DESY and CLAS
CLAS data (S. Barrow, e1c) showed
Dominance of t-channel process confirmed
Decay angular distribution showed significant contribution from mz=±1/2 spin projection
Experimental situation is still controversial
Discrepancies between measured observables in different energy and angular range may point to different production mechanisms
CLAS-g11 data set can provide data over a broad kinematic region, filling the gaps between existing data and allowing a detailed study of the L* production mechanism

7. Analysis Procedure
The L*(1520) cross section has been extracted from the g11 data set, independently by the Genova and RPI/Moscow/JLab Groups
Analysis of pK- decay mode of L*(1520)
pK+K- final state selected using different event topologies (pK+(K-), pK+K-, pK-(K+)
Exclusivity of final state ensured using the missing mass technique
Data binned in 60 MeV energy bins mateched to tagger E-counters and 20 –(t-tmin) bins
L*(1520) extracted fitting the mass spectra with a BW convoluted with a Gaussian function
Detection efficiency estimated from MC simulation using a realistic event generator
Photon flux normalization from gflux with additional correction for multiple hits
Standard trigger efficiency (15% correction to two-tracks events) and normalization correction (18% correction) developed for g11 data set

8. Event Selection g Eg=(2.44±0.03) GeV p K- L* p K+
3 charged particles in the final state
three event topologies are analyzed
pK+ detected, K- missing
pK+K- detected
pK- detected, K+ missing
the three topologies are affected by different background and have complementary coverage

9. Event Topologies
Eg=(2.44±0.03) GeV

10. Yield extraction
Eg=(2.44±0.03) GeV
-(t-tmin)=(0.58±0.05) GeV2
gppK+ (K-)
The L* yield is extracted as a function of Eg and t bin fitting the mass spectrum with BW function convoluted with a Gaussian (CLAS resolution) + polynomial (background)
Fixed Parameters:
BW width fixed to PDG value
Gaussian sigma fixed to 4 MeV for invariant mass spectra and energy dependent parameterization of missing mass resolution for missing mass spectra
Free Parameters
BW normalization  L* yield
BW mass
polynomial function parameters
gppK+ K-
gppK- (K+)

11. Efficiency
Eg=(2.44±0.03) GeV
CLAS detection efficiency determined from MC simulations based on realistic event generator
+ GSIM + GPP + RECSIS
t and energy dependence of generated events tuned to the real data (three MC iterations)
isotropic L* decay to pK-
efficiency determined as ratio of reconstructed over generated event as a function of Eg and t
Generated events Reconstructed gppK+ (K-)
Reconstructed gppK+ K-
Reconstructed gppK- (K +)

12. Differential Cross Section
Eg=(2.44±0.03) GeV
Efficiency corrected yield divided by bin size and photon flux to extract the differential cross section
All “standard” g11 correction included:
Trigger efficiency correction for two-tracks events
Current-dependent correction to photon flux normalization
Multiple hit correction
gppK+ (K-)
gppK+ K-
gppK- (K +)

13. Differential Cross Section

14. Differential Cross Section

15. Differential Cross Section

16. Differential Cross Section

17. Differential Cross Section

18. Differential Cross Section

19. Differential Cross Section

20. Differential Cross Section

21. G11-G10 Comparison
G11
G10-low field
G10-high field
B. McKinnon

22. pK- and Sp decay modes
K. Moriya’s analysis of LSp decay

23. Comparison with World Data
Daresbury (Eg= GeV)
CLAS-g11 (Eg>2.8 GeV)

24. Comparison with World Data
CLAS-g11
LEPS

25. Comparison with World Data
CLAS-g11
LEPS

26. t slope
The differential cross section at low t shows an exponential behavior
The exponential slope was estimated fitting the first 10 points in each energy bin

27. t slope
The differential cross section at low t shows an exponential behavior
The exponential slope was estimated fitting the first 10 points in each energy bin
CLAS-g11

28. t slope
The differential cross section at low t shows an exponential behavior
The exponential slope was estimated fitting the first 10 points in each energy bin
CLAS-g11
Flattening of the slope at higher energies
Strong change of t slope at low energy

29. Total Cross Section
An estimate of the total cross section was obtained from the measured differential cross section:
The three topologies are combined using the weighted average
The resulting differential cross section is integrated
Systematic error on the analysis procedure evaluated from comparison of event topology
10% systematic error on normalization accounting for photon flux uncertainty and trigger efficiency and normalization corrections
NINA
SAPHIR (unpublished)
CLAS – g11
W(GeV)

30. Decay Angular Distribution (GJ Frame)
The decay angular distribution was extracted in the Gottfried-Jackson frame as a function of the photon energy (integrated in t) K- L* K?
qGJ p p’

31. Summary The gp  KL(1520)  pK+K- was studied in the g11 data set
The final state was indentified in three different event topologies using the missing mass technique  check of systematics associated to yield extraction, acceptance and efficiency evaluation
Differential cross sections from different topologies are in good agreement
Final results was obtained combining the three topologies
Results:
The differential cross section is forward peak, compatibly with dominance of t-exchange processes
Total cross section shows an enhancement close to threshold
Decay angular distribution is close to flat
Write analysis note !!

33. backup

34. Summary and Plans (from last Spectroscopy meeting presentation)
Last months spent in checking and improving analysis procedure:
Absolute normalization checked against g10
Leakage from other decays (Sp) studied and first correction implemented
New event topology added to verify the cross section behavior at small t
Good agreement between event topologies when all g11 corrections are implemented
T-counter dependence of normalized yield?
Things are converging…final results soon

35. Summary and Plans (from last Spectroscopy meeting presentation)
Last months spent in checking and improving analysis procedure:
Absolute normalization checked against g10
Leakage from other decays (Sp) studied and first correction implemented
New event topology added to verify the cross section behavior at small t
Good agreement between event topologies when all g11 corrections are implemented
T-counter dependence of normalized yield?
Things are converging…final results soon
TO BE IMPLEMENTED
ALMOST THERE…

36. G11-G10 Comparison
Comparison of differential cross section for gppK+K- from g11 with gdpK+K-(n) from g10
Same energy and t binning
4 energy bins from 2.0 to 3.6 GeV
Differential cross section as a function of -t

37. Leakage from Sp decay of L*
Contribution to the pK+(K-) topology can come from:
L*  pK- (22.5%)
but also
L*  Sp (42%)
L*  Lp p (10%)
that result in pK+p0p- , pK+p0p-g or pK+p-pp final states

38. Leakage from Sp decay of L*
Contribution to the pK+(K-) topology can come from:
L*  pK- (22.5%)
but also
L*  Sp (42%)
L*  Lp p (10%)
that result in pK+p0p- , pK+p0p-g or pK+p-pp final states
Does not happen for other topologies since the K- is explicitly measured
SOLUTION:
side band subtraction

39. Leakage from Sp decay of L*
Contribution to the pK+(K-) topology can come from:
L*  pK- (22.5%)
but also
L*  Sp (42%)
L*  Lp p (10%)
that result in pK+p0p- , pK+p0p-g or pK+p-pp final states
Does not happen for other topologies since the K- is explicitly measured
SOLUTION:
side band subtraction

40. Energy Binning
Unexpected fluctuations in normalized yield were observed when using a fine energy binning:
60 MeV bins
in GeV range

41. Energy Binning
Unexpected fluctuations in normalized yield were observed when using a fine energy binning:
60 MeV bins
in GeV range
Different possible explanations were investigated:
Binning effect because of finite E-counter width? New energy binning implemented
16 E-counters  1 Energy bin  62 MeV bins

42. Normalized Yield as a function of Energy
Event yield associated to each E-Counter was estimated and normalized to photon flux
Structure are observed in different position for different electron beam energies
4 GeV
5 GeV

43. Normalized Yield as a function of energy: dependence on t-counter?
Event yield associated to each E-Counter was estimated and normalized to photon flux
Structure are observed in different position for different electron beam energies
4 GeV
5 GeV

44. Normalized Yield as a function of energy: dependence on t-counter?
Event yield associated to each E-Counter was estimated and normalized to photon flux
Structure are observed in different position for different electron beam energies
4 GeV
Seem to be associated to T-Counters
Exact origin of the structures not yet understood but corrections are needed for precise cross sections estimates
5 GeV

45. Correction for T-counter dependence

46. Energy Binning
Unexpected fluctuations in normalized yield were observed when using a fine energy binning:
60 MeV bins
in GeV range
With new energy (E-counter based) binning and T-Counter correction, normalized yield becomes smooth