Hi Simon,
On Mar 31, 2006, at 17:34:19, Simon Sirca wrote:
> Hi Chris,
>
> I am neither questioning the Bardin & Shumeiko approach nor
> the physics behind mascarad; I am worried about coupling mascarad
> to our MC. Let me therefore reformulate my simple questions, hoping
> that someone can answer them without pointing just at Afanasev's PRD.
>
> - Does the radiative correction implemented in our simulation
> amount to a single number below the selected cutoff? If yes,
> is there an agreed-upon procedure to determine this cutoff?
Yes. I believe this is universal in all RC's, due to the fact a
infinite pole at W=M (vertex correction, elastic kinematics) cancels
out an infrared divergence in the radiative tail (non-elastic
kinematics). Not only that, but the soft part of the RC (a single
number) is exponentiated to account for multiple photon emission.
The only criterion I know is that you like the cutoff to be smaller
than the energy resolution. If it is too small, multiple photon
emission is not properly accounted for. If it is too large, the
convolution with energy resolution is unrealistic.
>
> - How and why exactly does the large electron energy shift
> that was mentioned come about?
>
It comes from convolution of a non-symmetric tail onto the peak.
(gauss + slope = shift) Thus the shift of the peak itself should be
much less than the energy resolution. The mean could shift much
more, depending on the cutoff. I don't know what the shift is yet,
but it was done wrong in the past (the radiative tail was used
without the elastic peak).
> - Is the XS radiated by mascarad being convoluted with the
> *measured* experimental resolution in the MC?
Yes, that's the only way to calculate the shift. I will just do it
by hand (no MC at all). In BLASTMC, it is done implicitly by
smearing the drift chamber wire resolution.
--Chris
>
> Thanks,
>
> Simon
>
>
>>> up to this relatively high cutoff? I also do not see how the cutoff
>>> relates to a direct *shift* in energy to first order. Finally, I am
>>> not sure the procedure suggested by Chris is optimal. I think that
>>> the sequence of calculating the radiated XS and convoluting it with
>>> the
>>> experimental resolution is incorrect; if I am not mistaken,
>>> this issue was raised already a while ago. Convoluting a radiated
>>> theoretical observable with the corresponding measured spectrum may
>>> imply
>>> double counting, and may just mean the emperor MC's
>>> new clothes!
>
>
> On Fri, 31 Mar 2006, Christopher Crawford wrote:
>
>> Hi Simon,
>> You have to understand how MASCARAD (or POLRAD, the same thing)
>> works. It uses the Bardin & Shumeiko approach, but a cutoff is
>> still required in MC generators. See my talk, at
>> http://blast.lns.mit.edu/PRIVATE_RESULTS/USEFUL/ANALYSIS_MEETINGS/
>> meeting_050630/rc_overview.ppt
>> --Chris
>> _______________________________________
>>
>> TA-53/MPF-1/D111 P-23 MS H803
>> LANL, Los Alamos, NM 87545
>> 505-665-9804(o) 665-4121(f) 662-0639(h)
>> _______________________________________
>>
>>
>> On Mar 31, 2006, at 15:13:52, Simon Sirca wrote:
>>
>>> On Fri, 31 Mar 2006, Christopher Crawford wrote:
>>> > > -Mascarad+Epel issue
>>> > > +Mascarad only produces the radiative tail starting at a cutoff
>>> > > energy for the radiated photon (ad hoc set to 10 MeV).
>>> > > Chi and Vitaliy, is Mascarad implemented in MC this way? I'm
>>> just > checking that both the hard and soft parts have been
>>> integrated out to > the cutoff energy, and that both both parts
>>> are included in the radiative > cross section past that. The
>>> original Mascarad did not generate cross > sections in this manner.
>>> > > > +Electron momentum generated with Mascarad is thus shifted
>>> relative
>>> > > to the unradiated momentum by at least 10 MeV.
>>> > > +Average momentum shift of electrons due to internal radiation
>>> > > convoluted with resolution can only be correctly estimated by
>>> > > Montecarlo if Mascarad is properly combined with the
>>> unradiated
>>> > > yield.
>>> > > We can get this straight from the original Mascarad code, by
>>> calculating > the radiated cross section as a function of cutoff
>>> energy and then taking > the derivative to get the W-spectrum
>>> (and then convoluting with the BLAST > W-resolution). Note that
>>> the momentum shift depends on the cutoff energy > used in the
>>> analysis (not the 10 MeV), and you must be consistent. I'm >
>>> calculating it this way for the geometrical offsets code.
>>> > > > +The proper combination of Mascarad with Epel needs to be
>>> established.
>>> > > This is just a matter of running the original Mascarad to
>>> calculate the > radiated elastic cross section with the cutoff
>>> set to 10 MeV. It is > probably best to add an elastic channel
>>> with the <10MeV > radiation-corrected cross section. Chi, don't
>>> we already have this > channel?
>>> The discussion above sounds a bit funky... Way too complicated for
>>> what a radiation code should do in my opinion. Why should it start
>>> producing the tail only after 10 MeV? So what happened to Bloch-
>>> Nordsieck?
>>> If a cutoff is implemented, it should match the bin size in the
>>> variable
>>> one is trying to correct. And what does "integrated out to the
>>> cutoff
>>> energy mean"? Does it mean that the correction amounts to a
>>> simple factor
>>> up to this relatively high cutoff? I also do not see how the cutoff
>>> relates to a direct *shift* in energy to first order. Finally, I am
>>> not sure the procedure suggested by Chris is optimal. I think that
>>> the sequence of calculating the radiated XS and convoluting it
>>> with the experimental resolution is incorrect; if I am not mistaken,
>>> this issue was raised already a while ago. Convoluting a radiated
>>> theoretical observable with the corresponding measured spectrum
>>> may imply double counting, and may just mean the emperor MC's
>>> new clothes!
>>> Best regards,
>>> Simon
>>> --
>>> Simon Sirca
>>> Dept of Physics, University of Ljubljana Tel: +386 1 4766-574
>>> Jadranska 19 Fax: +386 1 2517-281
>>> 1000 Ljubljana, Slovenia
>>
>>
>
> --
> Simon Sirca
> Dept of Physics, University of Ljubljana Tel: +386 1 4766-574
> Jadranska 19 Fax: +386 1 2517-281
> 1000 Ljubljana, Slovenia
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