[Clas12_verystrange] Xi(1820)

[Michael Dugger]

Hi,

There was some concern regarding Johann's number for the detection 
efficiency of gamma p -> K+ K+ Xi(1820) when K+ K+ K- pi- are detected. 
The concern was that the reaction requires four charged particles but is 
much lower than gamma p -> K+ K+ K0 Omega- when K+ K+ pi+ pi- are 
detected. Both reactions have four charged particles detected but the 
Omega- reaction has a reported efficiency of about 3.85 % and the Xi(1820) 
reaction has a reported efficiency of about 0.3%.

I think I know what the problem is.

For the K+ K+ K0 Omega- reaction, when we see: K+ K+ pi+ pi- there are 
three positive charged particles and one negative. For the K+ K+ Xi-(1820) 
reaction, when we detect K+ K+ K- pi- there are two positive charged 
particles and two negative particles.

The negative particle detection efficiency for CLAS12 is not very good. 
You can see a plot of efficiencies (in percent) versus momentum for pions, 
kaons, and protons at
http://www.jlab.org/Hall-B/secure/clas12/omegaMinus/ASU/efficiencyPlots.gif

When I create gamma p -> K+ K+ Xi-(1820) events (phase space), where 
Xi-(1820) -> Lambda K- (phase space again), and have Pythia decay the 
Lambda, I get an efficiency of 0.2% when K+ K+ K- pi- are seen.

The efficiency I get is actually lower than what Johann sees (0.3%). I 
think that the difference is probably due to the differences in the 
assumed distribution. Since Johann uses a model and I just have a 
simplistic phase space generation, Johann's numbers are probably more 
realistic.

Take care,
Michael,