[Eg6_analysis] .....

[Stepan Stepanyan]

Hi Eric,

I am sure there is a t-dependence in Mx2 distribution.
But the point is not that. I just showed that in case of
two body breakup, (3), you can get a bump at Mx2~20.
The difference between (3) and (4) can simply be explained
as a mechanism of real photon production on a nucleon
in 4He with and without FSI with remaining 3H, respectively.
I guess (4) is not happening too often, hard to imagine no
FSI (final state interactions).
The form of the bump (width) at ~20 GeV2 and exact
position will depend on the strength of the FSI, basically
how much energy gets transferred after scattering on a nucleon
to the renant, in this case to 3H. As we can see from (4) if no
energy is transferred, 3H is a spectator, then Mx2 peaks at
~15 GeV2. That peak gets smeared and drifts to higher masses
depending on the transferred energy.

As I mentioned in my email, this is very simple model but
shows how kinematics works. One should develop some thing
like this and run through realistic acceptance functions (or better
through GSIM) to get the real picture. And we should not forget
pi0 production as well, will have the same picture.

Regards, Stepan

On 4/8/11 5:27 AM, Eric Voutier wrote:
>  Dear Stepan,
>
>  Thanks for this demonstration that looks pretty consistent to me. I
>  am still a bit puzzled. In fact, in all the reactions quoted and 
> especially
>  for incoherent DVCS,  I would expect a t-dependent position of Mx2.
>  Beyond this, I dont understand the difference between  3) and 4),
>  kinematically speaking 4)-events should be part of 3)-events, it is just
>  that 4)-events have some additionnal correlation between the proton and
>  the photon.
>
>  I really dont "feel" the origin of the shift, unless there is some 
> hidden
>  t  difference between all the events.  Would if be possible to have a 2D
>  spectra of the same events you show there  with Mx2 in y an t in  x,
>  with t defined from the real photon.
>  If I understand something to this puzzle, the peak at about the helium
>  mass squared should be associated to small t. For the one at 20 Gev2,
>  and just considering kinematics and also the experimental spectra we
>  got, I still dont get  how a 28 MeV binding energy  leads to a 5 GeV2
>  difference in the missing mass, I should do maths...
>
>   Regards & thanks, Eric
>
> Stepan Stepanyan wrote:
>> Hi Eric,
>>
>> Sorry for responding to your question about the form of the Mx2 that 
>> Yohann
>> has this late. I finally found some time and made some simulations, 
>> just the
>> kinematics, of real photon electroproduction reaction with different 
>> hadronic
>> final states. So the reaction I simulated is, e 4^He -> e' \gamma X, 
>> where X was -
>> 1) ppnn (breakup to four nucleons)
>> 2) pnd (breakup to two nucleons and deuterium)
>> 3) p3H (breakup to a proton and triton)
>> 4) p3H-spectator (scattering on a proton and the triton is a spectator)
>>
>> Attached plot shows superposition of missing mass squared of the 
>> final state
>> electron and the photon from all these reactions, assuming that the 
>> target is 4He,
>> basically mass square of system X. I required that the electron and 
>> the photon
>> have momentum and \theta in the CLAS acceptance region, and also 
>> required
>> that none of hadrons had kinematics in the CLAS detection region 
>> (very simple
>> but should be Ok for this studies).
>> In the graph, the forward peak at Mx2~15 GeV2 is from (4), little 
>> bump at ~20 GeV2
>> is from (3). (1) peaks at very large Mx2 and  (2) is distributed in 
>> the middle. As you
>> can see there is a "peak" from two body breakup and looks to me that 
>> is what Yohann
>> sees. I do not have relative weights of these reactions, but I am 
>> sure one can work
>> this out and match experimentally observed spectrum with simulations 
>> (one should
>> include also pi0 production).
>>
>> Regards, Stepan
>>
>> On 3/22/11 10:31 AM, Eric Voutier wrote:
>>> It seems I lost phone connection while I was talking.... It does not 
>>> work at all by now
>>>
>>> My main question was about the position of tyhe small bump we are 
>>> seeing in Mx2,
>>> any idea about the origin of this ??
>>>
>>> Thanks, eric
>>>
>>>
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