[Clas12_verystrange] PAC comments first round response

[Lei Guo]

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/
>