[Clascomment] OPT-IN:Flavor Dependence of qq-bar Creation Observed in the Exclusive Limit

[Viktor Mokeev]

  Hi Will,

 I have the question to you.

 How simple factorization scheme is consistent with pronounced angular dependencies for Lambda-K/N-pi ratio reported in Fig 1.

 In my view, this dependence clearly demonstrate that for fully exclusive reaction in non-perturbative domain we certainly have the contributing mechanisms beyond the simple factorization scheme for soft and hard pQCD processes. For this reason, currently  I would consider the qq-bar ratio reported in the paper as questionable. In the best case,
the simple factorization model can be applicable to estimate these ratio averaged over the final state kinematics variables. However, even this guess should be justified.

  Best Regards,
                           Victor 

----- Original Message -----
From: "William Brooks" <brooksw at jlab.org>
To: clascomment at jlab.org
Cc: clasmbr at jlab.org, mestayer at jlab.org, mokeev at jlab.org, silvia at jlab.org, parkkj at jlab.org, weinstei at jlab.org
Sent: Monday, June 9, 2014 5:16:01 PM
Subject: Re: [Clascomment] OPT-IN:Flavor Dependence of qq-bar Creation Observed in the Exclusive Limit

Dear Mac and Kijun, and CLAS collaborators, 

 The subject matter of your paper is extremely interesting and it may be noticed by a different segment of the physics community than the typical CLAS paper, broadening the impact. It is particularly of interest to those who are studying topics like precocious factorization or duality, where properties of high energy reactions are in some way mirrored at low energies. And, it will clearly be interesting to people studying hadronization mechanisms, if the case is made convincingly that you are really studying string breaking.

 I gather from recent comments that there are historical controversies within one of the working groups about this. Since I and others have not been present for those discussions, it would be useful for someone to explain the issues people have had, since they don’t seem to have been resolved. To me the essential questions are: 1) is the interpretation of the results as color-string-breaking unambiguous or at least strongly convincing? and 2) is the exclusive limit in which you’re working a natural continuation of multi-hadron (semi-inclusive) production taken to the high-z and one-hadron limit?

Along the lines of (1), for semi-inclusive pion production at JLab energies, Hall C has studied the degree of factorization breaking as a function of the semi-inclusive z variable. PHYSICAL REVIEW C 85, 015202 (2012) and PRL 98, 022001 (2007), particularly the latter, show the level of deviation from the fragmentation ansatz as z approaches 1. They do not go all the way to the exclusive limit in these papers. My conclusion is that from z=0.4 to z=0.7 there is no visible departure from factorization for pions. For z approaching 0.8 and 0.9 there is a strong departure due to resonances and rho production and other effects. Still, the deviation as z approaches 1.0 is becoming smaller. To what extent can it be argued that this is pure Lund-string breaking? Perhaps it can be stated/argued that it is an “effective" measurement of this, or a measurement of the string-breaking assuming a “precocious" or “approximate" factorization, neglecting the effect of resonances, and that the consistency of the results with those of higher energies suggests a potential duality behavior. This would not really weaken your conclusions much, and would reduce the skepticism that naturally arises if all these effects are simply not mentioned in the paper. 

Concerning (2), the ideal would be to study the few-hadron semi-inclusive production to demonstrate that the fragmentation cross section in some way smoothly approaches your result as the number of hadrons produced approaches 1, which is to say as the number of "string breaks" approaches 1. I have done a study like this with the Lepto DIS event generator and it certainly produces single and double pion fragmentation final states when the beam energy is low, which disappear as the beam energy increases. A similar study with Pythia, tuned for lowest energies, would be more convincing, and would not require much effort. A complication is that the exclusive reaction gives a two-body final state, so there is a kinematic difference between SIDIS and exclusive. Nonetheless, the cross section for the kinematics of your interest could be evaluated from experimental data as N_hadrons -> 1 and z -> 1, to see experimentally if there is a reasonable continuity, or if on the contrary there is not, and for whatever reason the result for the exclusive case is radically different. Have you done a study of this type, either in simulation or, preferably, in data?

Best regards, 

 - Will