[Olympus] Fwd: BPMs, magnetic field, MIE, etc

[Alexander Kiselev]

   Axel,

   honestly I was expecting a private answer from you first. Since you 
posted my e-mail to the list, let me rephrase the question:

   - if (hypothetically) one measures (0,0) offsets in BPM#1 and BPM#2
for both e+ and e- beams it does *not* really mean, that both beams had 
zero offsets and slopes in case there was a noticeable B*dl inbetween the 
BPMs; it is easy to estimate, that given residual fields of an order of 
~1mT (see fig.4-11 in your thesis, though it is Bx), ~1.5m distance and
2 GeV/c beam energy we are talking about several dozens of either 
microradians or microns; this does not look negligible to ~10um accuracy 
in relative e+ vs e- beam line position determination we claim;

   -> if the above consideration makes sense at all (I do not insist it
does; just asking), was this effect estimated, is it small (cancells?) and 
if not: was it accounted for either in calculating e+ & e- beam line 
parameters (and then in MIE normalization itself) or in 0.27% MIE 
normalization systematics estimate?;

   Thank you,
     Alexander.

PS: personally I doubt, that SyMB *alone* can ever give better luminosity 
normalization than 12 degree monitor with full tracking capability (and 
yes, I carefully read respective chapters in both Axel's and Brian's 
theses); since two systems provide statistically consistent instantaneous
measurements they probably should enter the paper this way (i.e. on equal 
footing for luminosity determination, with all due comments about 
possible magnitude of TPE effects at 12 degrees);

PS: I vaguelly remember, there was a discussion once on how to 
*cross-calibrate* two luminosity systems using any 4 small sets of data
{e+-,f+-}; was about "native" SyMB usage mode at that time (not MIE)
though; based on a simple observation, that flipping magnetic field affects 
charged tracks in two luminosity systems in a known (and different) way;


On Thu, 1 Sep 2016, Axel Schmidt wrote:

>
>
> Begin forwarded message:
>
>> From: Alexander Kiselev <kisselev at mail.desy.de>
>> Subject: BPMs, magnetic field, MIE, etc
>> Date: August 30, 2016 at 17:25:56 EDT
>> To: <schmidta at mit.edu>
>>
>>  Hi Axel,
>>
>>  first, congratulations with your defence. Second (being myself a 12 degree monitor person), let me challenge your 0.27% MIE method systematic
>> uncertainty estimate. Starting with a simple question: was BPM data
>> "corrected" in some sort for the non-vanishing magnetic field along the beam line between BPM#1 and BPM#2?
>>
>>  I mean ~10-20um match between e+ and e- data sounds a bit of a stretch to me, far beyond the capability of a rather simplistic detector we had in hands. With ~1mT residual fields (perhaps not precisely accounted), ~1.5m BPM#1->BPM#2 distance and 2 GeV/c energy you can easily get far off beyond
>> that when comparing electron and positron *beams*. Which IMHO happens *before* the situation you considered in section 6.4 (fig.6-15).
>>
>>  By the way, MIE method itself with all the machinery you developed looks fine to me. I just doubt the absolute luminosity ratio normalization
>> accuracy one can get from it.
>>
>>  Unrelated, concerning fig.8-9: does it mean, that you had to waste ~1/4 or so of the raw data (or do I misinterpret the plot)?
>>
>>  Cheers,
>>    Alexander.
>>
>
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