[PEPPo] Weekend Update

[hyde at jlab.org]

Very good proposal,
please add my name
Charles E. Hyde
Old Dominion University
Norfolk VA 23529


 Some suggestions on the proposal.

Section 2.1, first paragraph.
Add a sentance.
"However, for CW acceleration, a damping ring is not feasible"
2nd paragraph,
delete "a"
"These processes are routinely...
to OBTAIN LINEARLY ..."

I might suggest deleting the last sentence of paragraph 3,
because it seems to contradict the entire thrust of this proposal.

Section 2.2.1
subsubsubsection "Generalized parton distributions"
Third paragraph,
I guess the phrase "leading twists", with twists PLURAL is accurate,
although to me a little confusing.
"At leading twists, the partonic structure of the nucleon []
is described by four quark-helicity..."
The H, E, H-tilde, E-tilde are twist-2, whereas the H_T I believe
are twist-3.
You could alter this leading phrase as follows.
"The partonic structure of the nucleon [Die03, Bel05] is described at
twist-2 by four quark-helicity-conserving, chiral even
GPDs, (H...E-tilde) and at twist-3 by four quark-helicity-flipping and
chiral-odd GPDs (H_T...E_T-tilde), toghetehr with eight
similar gluon GPDs.
The twist-3 GPDs generally contribute to higher Fourier moments
in the DVCS cross section than the twist-2 GPDs."

Section 2.2.1
subsubsubsection "Deeply Virtual Compton Scattering"
The multiple parenthetic notation after equation (2) is
very hard to read.  I would write it out:
Put a period at the end of Eq.2, and then start
"The cross sections $\sigma$ and $\Delta\sigma$ are even functons of the
out-of-plane angle between the leptonic and hadronic planes,
whereas the $\widetilde{\sigma}$ and $\Delta\widetilde{\sigma}$
are odd functions of this angle;  $S$ is ...."

DVCS is not indistinguishable from BH, but the final state IS.
Change the next sentence;
"The DVCS and BH final states are indistinguishable, and therefore the two
processes interfere. However, the BH amplitude is
known and exactly calculable from the electromagnetic form factors."


Section 3.2
Third from last paragraph
missing Mott polarimeter reference.

Next to last paragraph,
If you rotate the 5MeV dipole, doesn't the new vacuum chamber
have to have new ports at 0deg upstream, downstream,
-12.5 and -30 deg as well as +25 deg for the PEPPo exp?
Perhaps you should clarify this so that the TAC does not have to
for us.

Section 3.3
page 16, replace "consisting" with "consist" and remove comma as follows:
"The momentum selection jaws consist of two remotely...
located between the two $90^\circ$ bends."

Section 4.2
page 21
missing closing parenthesis
"(several magnets are being re-mapped,the vacuum chamber ..."
Also, no explanation of what the room temperature has
to do with the GEANT4 simulation.

Table 3,
in the Positron block, correct "Babbah scattering" to
"Bhabha scattering"
also
"Optical Potons" heading should be
"Optical Photons"

Text discussion of Table 3 (p. 23) item (1),
replace "mean" with "means" as follows
"(1) it provides a means to (possibly).."

Section 4.2.2
p. 24, after fig. 12
correct "on In fig. 13" and rephrase, as follows:
"The corresponding positron collection efficiency at the
end of the S1 solenoid, for collimator apertures
(just after the production target) of 2, 4, and 6 mm are
shown in Fig. 13."

p.25, the discussion of Fig. 13 and 14 is confusing.
Fig. 14 compares collection efficiency for the S1 entrance
at 10 or 50 mm from the target.  But on the previous page,
it was stated that S1 would be 120 mm from the target.
Is it mechanically feasible to have the solenoid as close as
10 mm? (or even 50 mm?)  What distance was used for Fig. 13.
Fig. 14 says 10^6 electrons.  Fig. 13 says 10^8 positrons.
If this is correct,then 50 mm increases the positron efficiency
by a factor of 200 over 120 mm (910/10^6 vs 409/10^8)
Please clarify.

Fig. 15
[ is x' =dx/dz = tan(theta_x) or is the vertical
axis $\atan(x')$?  The axis says degrees in illegible font,
but the values of 10 deg seem inconsistent with Fig. 16)
Suggested clarification of the caption:
"Positron phase space distributions $x'$ {\em vs.} $x$ (left)
and $y'$ {\em vs.} $y$ (left) at the exit of S1 (top) and
S2 (bottom).  The angles are expressed in degrees
($\atan(x')$ and ($\atan(y')$.
I don't understand  the sentence
"While the angular distribution spans up to 90 deg,
only a very small fraction of it will be detected by the
polarimeter..."
Firstly, is this referring to fig 16 only or Figs 15 and 16?
The positron production efficiency is irrelevant here.
What is relevant, is what fraction of this positron phase space
can be converted into useful photons for the polarimeter.
I think this sentence should be deleted, and replace with a phrase, "The
useful phase space for the positron polarimeter will
be discussed in section 5.

Fig 16 caption
We don't need the possesive 's'
"Positron Phase Space at Reconversion target T2".

Section 4.2.3
I think the background study and the discussion of the Compton asymmetry
belong in separate sections.
In particular, move the asymmetry to section 5.
Without the discussion of section 5, I totally do not understand
what it means to have a crystal dependent analysing power.
Fig. 18 needs to be moved to section 5, and needs much
more discussion, since this is the statistical precision of the
entire experiment.

A few quick comments on Section 5.
Section 4.1, end of page 28
"CsI crystal array"  (not crystals)

Equation 8-10.
Define \omega and \omega_0
I presume \omega is the (unique) photon energy at angle theta
and \omega_0 is the incident photon energy
\omega = m_e / [ m/\omega_0 + 1-\cos\theta]

I will keep reading tomorrow.
Charles







> Folks - an updated version of the proposal, which reflects all edits
received by 8 AM this morning,  is posted at
> http://www.jlab.org/~cardman/ in the Prop-e+26JUN11/ directory. As
before, there is a "master" file (in this case, Proposal_r5.tex) that is
the entire proposal, and a related (Proposal_r5_framework.tex) file that
will call the various subsection files.  If you want to make further edits
(and there are quite a few places where work is needed!!!!) please revise
your section in the section file STARTING WITH THE VERSION POSTED AS NOTED
ABOVE, rather than the master file, and email it to me directly (along
with any comments about other parts of the proposal). Please note that the
finished proposal must be submitted this Friday.  We are
running out of time.  I urge those with incomplete sections to send me
their updates ASAP.  I will post (and send to you all) subsequent
versions each time I receive consequent new material, and would hope that
we truly have a "very very very close to final" draft by our weekly
teleconference on Wednesday.
>
>          Regards,
>
>                Larry
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