[Clas12_verystrange] PAC comments first round response

[Igor Strakovsky]

Lei,

I like all what you wrote

Igor

On Thu, 14 Jun 2012 19:11:40 -0400, Lei Guo <lguo at jlab.org> wrote:

> Hi, Dear all,
> 
> Below is my attempt at answering the PAC comments. I will gather the  
> input from all of you, and have a nice-looking pdf file when we all  
> agree, hopefully, by the end of Friday or saturday.
>>
>> 1. The Theory TAC raised some serious questions about the way the  
>> proposal is written and also about the contents. Please make sure  
>> that all these questions are addressed before the PAC presentation  
>> next tuesday. In particular the question of the u/d mass difference,  
>> about which the theory reviewers seem to disagree about the way you  
>> plan to measure it from the data.
> 
> We understand the concern of the theory reviewers. Of course, the u/d  
> mass difference can not be easily obtained from the isospin multiplets  
> mass splitting, which is also related to the electro-magnetic  
> corrections. Our goal is to measure the splitting for the ground state  
> and other excited states (Xi(1530) for sure) in the same experiment.  
> To our best knowledge, the significant measurement of the ground state  
> splitting are done at different facilities. This measurement is a  
> fundamental observable directly related to u/d mass difference, and it  
> can be compared with future Lattice QCD calculations which we expect  
> to be available soon from the JLab lattice group. It is also important  
> to point out that although this is a fundamental measurement, it is  
> not the driving motivation of our proposal. It is however, a free  
> measurement that we certainly wish to make.
> 
> 
> 
> 
> 
>>
>> 2. It is not clear to me what physics conclusions can be drawn from  
>> the measurement of the photoproduction cross section. As you point  
>> out the four predictions in Fig. 15 are all consistent with each  
>> other, so it is not clear to me how effective the measurements will  
>> be in distinguishing between the various models, with the exception  
>> of the dashed red curve (by the way, what model does it correspond  
>> to ? Is it the VMD calculation ?). Also if I look at Fig. 18, which  
>> shows calculations for various cascade production channels, I note  
>> that for the only channel for which there are experimental data they  
>> agree pretty well with the theory calculation: what are you  
>> expecting to find when you measure the other channels ?
> 
> 
> The Omega- cross section measurement will be the first in  
> photoproduction. In all likelyhood, the models from our theoretical co- 
> authors could also be wrong. For example, we do not know how many  
> intermediate N*/Y*/Xi* contribute to the production of Omega-. Any  
> information we can get in terms of the cross section (total and  
> differential cross section), will be brand-new information and help us  
> understand the production mechanism of the Omega-. In the case of the  
> cascade channel, it is important to point out the existing model used  
> a limited number of intermediate hyperons to reproduce the data. But  
> there is no reason to believe that other higher mass hyperons would  
> not contribute at higher beam energies. The fact that the predicted  
> cascade cross section plateaus around 5GeV is a direct result of not  
> including other hyperons at higher masses, this can directly be  
> checked at CLAS12. Further more, we expect the production mechanism to  
> gradually move from hadronic degree of freedom to partonic degree of  
> freedom at higher beam energies, which should also affect the  
> polarization observables.
> 
> 
> 
> 
> 
> 
>>
>> 3. What level of simulation was used in the estimation of  
>> backgrounds (pp. 41-43) ? You mention fast simulation (with  
>> parametrized detector response): was a full event reconstruction  
>> performed or was the Monte Carlo truth used in the background  
>> estimations ? Is a full simulation foreseen ?
> 
> As it is now, CLAS12 does not have the software ready for full event  
> reconstruction. In estimating the background, we use pythia for known  
> hadronic physics process. This should be sufficient since we are  
> looking at rare channels, and the background should be dominated by  
> the hadronic processed included in Pythia. We do believe the full  
> simulation will be ready some time before the upgrade is completed.
> 
> 
> 
>>
>> 4. For the determination of the spin/parity quantum numbers (which  
>> is a crucial measurements if one aims at having a complete picture  
>> of the cascade spectrum) the DMA method is illustrated, but there is  
>> no quantitative discussion of the actual number of events needed to  
>> make this measurement in CLAS12, beyond the rather generic statement  
>> that large statistics will be collected in the relevant cascade  
>> production channels. I think that this measurement proposal should  
>> be supported by a more quantitative discussion and possibly a full  
>> simulation to demonstrate its feasibility in CLAS12. Also: have  
>> other methods to measure J^P been considered ? (e.g. Dalitz analysis).
> 
> We have performed double moments analysis on simulated data, and used  
> it to estimate the uncertainty of the measurement. The uncertainty, as  
> expected, is about a factor of 10 smaller than the previous  
> measurement. As for other methods to measure J^P, we can use the  
> single moments (for the Xi(1820)-->Lambda K- decay) to determine the  
> spin of the excited cascade. In fact, this was how Xi(1820)'s spin was  
> determined from about 200 events in K^+ p scattering data (Teodoro et  
> al., Phys Lett. 77B, 451, 1978). We would obviously deploy that as  
> well. The double moments advantage was so that we can also determine  
> the parity, which can not be achieved in single moments analysis, due  
> to Minami ambiguity. If we do get lucky and observe the decay of XI*-- 
> >Xi pi pi, then it is possible to use Dalitz analysis to pull out the  
> information of the parent Xi*. However, such as decay channel has not  
> been seen as dominant for any known cascades. But obviously, we would  
> not rule it out. In any case, a full-partial wave analysis can always  
> be employed.
> 
> 
> 
>>
>> 5. Is there another advantage in the increased photon energy, beyond  
>> the broader kinematical range mentioned at several points in the  
>> proposal ? (e.g. observables which have a significant dependance on  
>> the photon energy).
> 
> As was discussed in point 2, we do expect the cross section to  
> increase dramatically from threshold, particularly for the Omega- and  
> excited cascades. Even for the ground state, there is reason to  
> believe that the cross section will continue to increase at higher  
> photon energies, when other intermediate hyperons are allowed to  
> contribute. The behavior of differential cross sections for Xi- ground  
> state, in the cm frame, also changed from near flat to much more  
> backward peaking. It will be interesting to see whether it is similar  
> for Omega-. Furthermore, we also expect the polarization of the Xi- to  
> change as a function of energy. Such a behavior has already been seen  
> in Lambda, attributed to different production mechanisms at different  
> energies. Although the Xi- might have a different polarization  
> mechanism, it should also be sensitive to the production mechanism.
> 
>>
>> 6. A minor point: on page 12 you write that "it seems plausible that  
>> the one NA48 high-statistics measurement of the Csi_0 mass could be  
>> too low". Why is that so ? Since it is the only high-statistics  
>> measurement isn't it plausible that it be the most reliable ?
> 
> We should have stated that the NA48 high statistics data for Xi0 mass  
> should be corroborated. If might be totally correct. But we will be  
> the first experiment to be able to perform the measurement of the Xi-  
> and Xi0 mass in the facility with very high statistics.
> 
> 
> 
>>
>> 7. Do I understand correctly that this proposal does not impose any  
>> further requirements on the CLAS12 detector ?
> Our proposal does not impose any further requirements on the CLAS12  
> detector, other than the forward tagger which is already under  
> construction.
> 
>>
>> This is all for the moment. I am looking forward to your  
>> presentation at the PAC next tuesday.
>>
>> best regards
>> diego
>>
>>
>> -- 
>>
>> Diego Bettoni
>> INFN Ferrara, Director
>> Via Saragat, 1
>> I-44100 FERRARA
>>
>> tel: +39-0532-974275
>> tel: +39-0532-974332
>> fax: +39-0532-974300
>>
>> web: http://www.fe.infn.it/~bettoni/