OPT-IN: New Λp Cross Sections and its Implication for Neutron Star Equation of State

[Mikhail Osipenko]

Dear Joseph et al.,

it is very interesting paper and your method looks promising, but I have few questions/observations:

1) in Fig.1 put "K^+" in brackets, to emphasize that it is not measured (it is explained in text, but who are reading texts nowadays ;-) ;

2) from the description in page 2 it is not clear how do you handle two protons: i.e. do you make missing mass plots for both protons and apply a cut, keeping those events where at least one proton lies within Lambda decay kinematics? What do you do if both protons are within the cut? What do you do if you have >2 protons? Do you have other cuts on protons (minimum momentum, TBT, tracking Chi^2, Z-vertex)?

3) Fig.3 is very impressive, but a simple estimate gives that <2.5% of protons from Lambda+p reaction will undergo elastic p'+p re-scattering. Yet, in Fig.3 the number of events in correspondence to M_Lambda looks very large (30%?). Could this effect have another explanation? A very low pion momentum for example?

You also mention that the same cut is applied to MC. But does MC show the same 2D distribution?

4) the discussion of cross section normalization is complicated: Eq.7 is OK, although you could specify that acceptance A is acceptance for (Lamnda'+p'). Also, since you have enough space, you could write all quantities, except Gamma, as functions of "(E_Lambda)".

I am not sure in Eq.6, however, do you assume 100% acceptance for detection of Lambda here? Or this is applied to the simulations only, but how are they normalized then? Is "N_Lambda" in Eq.6 the same "Lambda flux" from Eq.4? If it is different, how is it defined and how it is measured?

In Eq.5 do you really mean "Z" as Z-axis of CLAS or it is generic "r"?

Is "average pathlength" constant? How good is this approximation?

5) in Fig.5 you claim that the almost factor of 2 disagreement with world (bubble chamber) data is due to cleaner data sample in this analysis (pp-reduction). However, since an additional pp scattering has probability <2.5% in g12 target is it really possible? Moreover, it is not clear why this should happen only at P_Lambda>1.4 GeV? Furthermore, since bubble chamber experiments were measuring tracks inside the target and were fitting them with straight lines they could easily distinguish rescattering by change of direction. Instead, since you observe the overlap of two bands in Fig.3 it is more likely that in this experiment we have "the uncertainty in the vertex and energy resolution". Large bins of bubble chamber experiments are perhaps due to low statistics, not due to bad resolution...

Furthermore, could you prove the statement: "The elastic pp scattering cross sections are
several times larger than the elastic pp cross sections, in the momentum range studied here."? According to PDG:

https://pdg.lbl.gov/2000/hadronicrpp_page14.ps

pp-elastic cross section at 1 GeV = 25 mb. Essentially equal to yours.


Best Regards,
               Mikhail.