Lecture on pentaquark searches upcoming

[mormonator_rm]

Here at Penn State University we will be hearing from Stepan Stepanyan of the Jefferson Lab on the subject of "Hunting for Pentaquarks" on Monday, 1/19, at 2:30 PM in 339 Davey Lab. I'm going to take notes and relay the results to all of you out there...

If you have any thoughts on pentaquarks, post 'em here and compare notes just for fun.

 

[mormonator_rm]

Okay folks! The lecture on the search for pentaquarks happenned today (Jan. 19). Here's a brief synopsis;

Through the chiral soliton model published in 1997, the first valid theory for a grouping of qqqqq\bar states was made public. However, even before that, CERN in 1973 found a peak at the resonance in question, and they ignored it on grounds of lack of theoretical support. Well, they blew it, didn't they?

Currently, there are over 80 publications on this topic, many of them having just been published in the last month. Among the models used to describe the observation are the Skyrme Model, where the parity of the so-called \theta⁺ particle and its related partners is positive; the Uncorrelated Quark Model, where the parity is negative and \theta⁺ must be a wide state unless I = 2 is the isospin group for \theta⁺ and its predicted partners; Lattice QCD, where parity is again negative with a K⁺n phase shift and a width of < 5 MeV.

The lecturer, Stepan Stepanyan from Jefferson Lab, then showed us a spectroscopy graphic from the arXiv publication hep-ph/0307341 showing the predicted masses and quantum numbers of \theta⁺ and its derivative particles in the chiral soliton model. He also pointed out the order of the initial publications of the experimental data confirming the existence of this first-seen pentaquark particle; hep-ex/0301020, hep-ex/0304040, hep-ex/0307018, hep-ex/0311046, and so on (that's all I could write down before he changed the slide again).

Review of experiments:

LEPS/Spring-8;
Laser backscattering facility, tested decay paths that are normally associated with \Lambda and \Sigma production, and managed to find \theta⁺(1540) in \gamma n -> K^-K⁺n final state data at about 1.54 GeV.

DIANA/ITEP;
750 MeV K⁺ beam incident on 700-liter xenon bubble chamber, interaction energy was determined by the track length of the kaon. Peak occurred at 1.539 GeV with statistical significance of 4.4 sigma. Width upper limit at 9 MeV.

CLAS/JLAB;
CEBAF/ Hall B CLAS detector was used with Bremsstrahlung photon tagging system. \gamma d exclusive reaction produced final states of pK^-K⁺n in a 10 cm liquid deuterium target using a 3 GeV photon beam. No significant signal was found at the time.

He briefly went over CERN and DESY results as well, then skipped back to CLAS data, pointing out that the same data was re-analyzed using new fits and cuts, resulting in a peak being identified at 1.542 GeV with width < 21 MeV.

CLAS also continued with a \gamma p scatterring experiment to complete a full photon scattering battery of tests. Several new proposed production channels for \theta⁺ were considered, and one by one ruled out. The only decay observed for theta+ was that which ended in a K⁺n final state.

In summary, although \theta⁺ has appeared, one of the other predicted pentaquarks called \theta⁺⁺ has not been observed as yet. The outlook for further experimentation is hopeful, with new projects for CLAS being approved recently. There will be new 4 GeV and 6 GeV beam runs with the deuterium scattering, and searches for \cascade^--, another predicted particle in the group. Plans are to increase the signal statistics by a factor of at least 20.

...sorry folks, but that's all the better my notes get, and I haven't got the time to put evertything in TeX, so it'll have to do as is for now. But I'll try to answer questions if you have any.

 

[selfAdjoint]

Uhh, is this standard practice?

He briefly went over CERN and DESY results as well, then skipped back to CLAS data, pointing out that the same data was re-analyzed using new fits and cuts, resulting in a peak being identified at 1.542 GeV with width < 21

Fudge factor?

 

[mormonator_rm]

Well, he skipped over CERN and DESY so quickly that I did not have time to write any notes on them. I was also confused at the answer to a question I asked him at about that moment, so I was trying to write that down, too. Apparently the first CLAS results were from 1999, and the reanalysis was sparked by interest in the particular region that was ignored the first time around... yeah, beats me. My own work is all about reanalysis of Fermilab pp\bar data from 1994, so I guess I can sort of understand how things get overlooked when you don't think you have a reason to look there (especially when the peak is sufficiently narrow).

 

[selfAdjoint]

Thanks, that gives me some clue as to what they were doing. I can see restudying old results when new studying methods become available.

 

[PRodQuanta]

I have a pretty elementary question here.

I just first heard of the pentaquark last year. I forget where I read about it, but was an interestring read. It stated that they found (or had significant evidence) of the pentaquark. My question: Was that the first sighting of the pentaquark?

Paden Roder

 

[mormonator_rm]

hep-ex 0301020 is the first paper mentioning the pentaquark, so yes that was early last year. \Theta+(1540) was the particle that was seen, and since then it has been determined that they expect it to be a member of an anti-decuplet of pentaquark states. So now one of the things they are looking for is other members of the anti-decuplet to be manifested in the data as well.