[Mott] Run II planning

Wed Aug 12 23:02:30 EDT 2015

Dear Joe,

I had been hoping that the second run would happen no later than 
September.  This is because we are headed out of the country for an 
extended period on October 23rd.  (To trekking east of Kangchenjunga, no 
less - the world's third highest peak, at 28,146'.  We won't go to the 
top!)  As I am sure you appreciate, I am greatly interested in 
participating in the second run, and thus in having a voice in what we 
choose to do with the time.  However, there is realistically NO way I 
could participate for a full week prior to our departure (or at absolute 
max, six days).  Thus I have to be counted out from October 16-17 on 
through our return in late November.  I'm sorry it has come to this, 
but, "you only go around once" as we keep reminding ourselves.

I have to say that I am less than impressed with Bluejeans.  There are 
at least three problems with it (not that I think there is much you can 
do about them, but you should at least know).  First, while our 
two-person dedicated session worked perfectly in allowing me to see what 
you were presenting, it NEVER works at the time of our conference 
calls.  Every time I try to get in, I receive, after a very lengthy 
waiting time, a message that they are sorry, but they are experiencing 
technical difficulties, please try again.  The second, considerably 
worse problem is that there is strong moderately variable intensity 
background noise.  It sort of sounds like one is running a variable (but 
high) power hair dryer all the time - a loud, variable, persistent whine 
all the time.  It is loud enough to make some fraction of the spoken 
word unintelligible.  The third, and worst of all, doesn't happen all 
the time, but is terrible when it does, as it did this morning.  The 
sound keeps dropping out.  I'd estimate the "up" duty factor to be no 
better than 30% or so.  The dropouts are of two kinds - a total loss of 
information, or an enormous reduction in volume.  The last 12-13 minutes 
of today's meeting were basically unintelligible to me, which is why I 
finally gave up.  I could occasionally catch few word stretches of the 
conversation, and guess the remainder in the best of circumstances, and 
would be totally in the dark most of the time.  I'm sure JLab likes to 
use "free" software, but at the same time, they should appreciate that 
it comes with consequences.  Have you ever thought about "Go To 
Meeting"?  Mary has far more conference calls than I do, and it seems to 
work pretty flawlessly for her groups, which are both bi-costal and 
involve a number of participants.  I believe it is still no cost 
software as well.  Is there any chance of switching - we certainly have 
enough experience with Bluejeans from my perspective.

Anyway, on to my comments.  I thought that Dan's analysis of the 
systematic effects was both valuable and interesting, though I had 
already reached the conclusion by the end of the actual beam runs that 
the systematic effects were tiny at best.  I think he could take things 
one step further by doing a few Chi-squared tests.  For example what is 
the Chi-squared probability that the four data points at different 
values of the 0L02 phase are the same value (high, I suspect)?   
Similarly, one could simply make a model for the various beam positions 
on the target foil (by the way, I thought we used 1 (or 2?) mm position 
changes, instead of one "spot diameter", but this is easily checked) 
such that the asymmetry is the central value plus a linear (or more 
likely quadratic) term for left-right displacements and a different one 
for up-down displacements, and then fit the data.  I wouldn't spend a 
lot of time on this, but I believe it would show that there is no 
meaningful dependence of the asymmetry on beam position on the target 
foil for any realistic beam position motion during the run. Recall that 
we ran for hours, and NEVER re-steered the beam from run to run because 
nothing had changed on the scale of a small fraction of the spot size,  
verified by (occasional) observation.  Properly set up, CW SRF 
accelerators are well known for their high quality stability.  I guess 
all this is to say that I think a very small amount of additional work 
would pretty cleanly establish that the systematic effects he looked at 
are tiny compared to our experimental error bars (which are small), and 
thus that we don't really need to spend much time on systematic studies 
in the October run (once we reproduce the January results with 
convincing precision, though not on all foils).

Is seven shifts all we can hope for in October (says he who may not be 
able to attend if its the latter part of the month)?  Or is it just what 
you think will solve all remaining issues??  I personally would like to 
see a couple different energies run, for several reasons.  Beam energy 
meaningfully influences the nuclear size effect on the analyzing power, 
and may also affect the size of the radiative corrections - our greatest 
uncertainty in my opinion. While one might not choose to run as detailed 
a set of measurements at two other energies (meaningfully higher and 
lower), it would be nice to be able to compare the measured asymmetry 
with what we would expect  hopefully again demonstrating that the 
nuclear size effect and the radiative corrections (?) can't be vastly 
larger than what we presently believe.  Perhaps we should ask Xavier 
what he things the energy dependence of the radiative corrections might be??

I also believe there is some merit in rotating the plane of polarization 
by 90 degrees, and demonstrating that the up/down and left/right 
asymmetries are basically statistically equal with pretty good precision 
(and by inference we have built a good instrument). Eliminating the dump 
dipole has great intellectual appeal to me, and a convincing 
demonstration would, I believe, add to our result (and to future use of 
the polarimeter for helping the halls).

I have done a fair bit of playing with the information that Dan has sent 
regarding his many fits, and have reached the conclusion that the only 
factor that influences our measured asymmetry is the energy cut.  I 
think that studies to further illuminate or understand this are the most 
important item we could address in October.  But, what exactly would I 
recommend I cannot exactly say right now.  I do believe that, were funds 
available like in the old days, the one possible improvement to the 
polarimeter would be to go to a considerably better scintillator (i.e. 
considerably improved width of the elastic peak).  I don't know if one 
might hope to scrounge such scintillators from long gone experimental 
equipment or not - there is certainly a lot of stuff out there.  I was 
interested in Marcy's fit to the asymmetry versus thickness, where she 
gets a lambda of 0.316.  All of Dan's many results are consistent with 
lambda being 0.324.  I assume that Marcy's fit is different from Dan's 
but this might be worth a few minutes of someone's looking.

I liked you analysis along the lines of estimating the analyzing power 
of the second scattering.  I had had some thoughts along these lines, 
hoping to do analytical estimates, but your presentation this morning 
was quite nice.  To me, the difference between the Lebow and FESEM 
thicknesses and their effect on the second scattering analyzing power 
was very interesting - they can't both be right, and I thought the FESEM 
results looked more believable, though that's just a qualitative 
impression from this morning.  Clearly more work is needed here, and 
this might be part of the October work.

Anyway, if we are to keep the same foil ladder, then the above 
encompasses pretty much what I think we should focus on in October. I 
personally feel that there is considerable merit in exploring the effect 
of changing Z, but if I am the only advocate...................  I 
certainly DO NOT advocate changing the foil ladder before October.  And, 
personally, I have zero interest in making 499 MHz measurements.  I 
would personally much rather invest in making it "quick and easy" to 
switch to and from 62 or 31 MHz for measurements for the halls.

Maybe I'll have some other thoughts later.  I gather from what you said 
you will be gone for a month.  Mary and I are gone in mid-September for 
a bit.  Among other things we are getting a special tour of the Hanford 
reservation (where they made the plutonium for the bombs).  I'm sure you 
appreciate that I'll have a hard question or two for our poor tour 
guide!  I'm not sure when all this activity morphed from a perceived 
national need to a clearly criminal activity, but it certainly did that 
morph in my mind.  Now we sit on ~ 44 metric tons of declared excess 
plutonium which is beginning to look like it will cost as much to get 
rid of as it cost to make in the first place..............  The 
"cleanup" has been going on for ~ 20 years now, and _*nothing*_ is yet 
cleaned up.   Probably because we are only spending about $2B annually 
on the effort!  See how you become curmudgeonly in old age??

Anyway, enjoyed the part of the program I could hear this morning. More 
later.  I think Dan is doing a fine service to our effort.

Best,
Charlie

On 8/12/2015 1:06 PM, Joseph Grames wrote:
> If you haven't already then please send me your (un)availability for the period Oct. 4-25, the likely window of opportunity to perform a 2-3 day Mott Run 2.
>
> thanks,
> Joe
> _______________________________________________
> Mott mailing list
> Mott at jlab.org
> https://mailman.jlab.org/mailman/listinfo/mott

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