[b1_ana] Fwd: comments/questions to PR12-13-011
J. P. Chen
jpchen at jlab.org
Mon Jun 10 18:08:40 EDT 2013
Hi, Karl and All,
To respond to the comments, I want to first repeat what I stated
several times that at high current (10-15 uA as in the case of
transversity),
all the systematics are included in the yield plot (including detector
drift, acceptance drift, BCM drift and acceptance due to BPM
drift. The 2nd plot from transversity is the plot from lumi, which shows
the charge asymmetry between two helicity states to be at the level of
3.8*10^(-5)
with a width of 2.3*10^{-4}. This plot also gives the measure of
stability/drift of
BCMs at high current.
At high current (50uA) for PREX (parity for lead in Hall A), with
regression,
the position difference kept to ~1 nm while the angle difference was <
1 nrad.
(See page 31 of Mark Dalton's talk @ User meeting
http://www.jlab.org/conferences/ugm/talks/thur/MarkDalton_UserGroup_NeutronRadius.pdf).
QWeak reached similar precisions, which are way better than anything we
need.
(A little explanation of regression: it is kind of feedback, but purposely
make large size deviation in position/energy to find the correction
factors).
So our issue is really how well we can keep and monitor the beam charge and
position with low current (100 nA).
New BCM/BPM electronics were designed for both QWeak and g2p, to
reach a precision of 100 um (in position) (reached 100-200 um) and 1%
(reached
1-2%) in current/charge measurement in a few 10's of ms. This allowed a
slower
feedback (slow-lock) system during g2p operation. We relied on QWeak for
charge feedback and position/energy lock. For b1, if there is no
parasitic high
current parity feedback (from another hall). we will need to make the
parity feedback
system to work for low current. It will work in principle. Will need
actual work to change hardware/software to adapt the parity feedback
system for low current with longer
integration time.
The lumi detectors will also work to provide a continuous monitor of
beam charge * target thickness. The stability of lumi's is similar to other
(integrating) detectors. I checked with Mark Dalton. QWeak lumi data shows
stability at the level of 1% for weeks to months. His feeling is that if
we setup
carefully we probably can do better (reach few*10^(-3)?) in weeks? Then for
12 hours we can monitor to 10^(-4) level. He will check with Mark Pitt who
is the expert on lumi and may give us more information.
Cheers.
Jian-ping
On 6/9/2013 11:07 PM, Karl Slifer wrote:
>
> Hi all,
>
> Below please find a draft response to the readers. I'd appreciate
> any/all feedback.
>
> Thanks to Oscar and Dustin for sending comments already. I hope I
> have incorporated them satisfactorily, but let me know if not.
>
> Dustin : any ETA for the updates to the technote? Ideally we can send
> them this short email and then provide your note for the full details.
> Most important would be to clarify the overall drift numbers, as I had
> the same confusion that Ellie raised.
>
> I'll have time to work on this tomorrow morning and early afternoon,
> but have to leave for DC mid afternoon. I'd love to get a response to
> Ewa before then if possible.
>
> thanks much,
>
> -Karl
>
>
> -----------------------------
>
> The measurement is very sensitive to the systematic effects and good
> control of them is the
> key point. Therefor I would like to know if there are any estimates of
> expected size of effects from:
>
> RESPONSE: We agree with the TAC assessment that systematic errors from
> drifts must be mitigated, but
> that they are manageable with a combination of
> hardware upgrades and a dedicated collaboration
> effort. We note that the recent g2p experiment
> involved a similar situation where a significant
> commitment was made to install and run this
> polarized target, but separate groups were tasked
> with substantial tasks of beamline, DAQ and
> detector upgrades. We are grateful to the TAC for
> pointing out several effects which were not
> explicitly discussed in our submitted proposal. We
> have written a short note discussing each of
> these and conclude that the overall systematic
> uncertainty is still of the same order as
> estimated in the proposal.
>
> 1. beam - one aspect is the stability in terms of position and divergency
> this can change acceptance and produce false asymmetries
>
> RESPONSE: (The TAC report pointed out that the false asymmetries from
> beam position drifts are easily
> removed by "regression". Can someone fill in the
> details? I assume that this just means
> that the parity feedback on position is very good,
> but we need some numbers/examples to
> back this up. This is dealt with in section 1.1.4
> of the note, but there are few details.)
>
> 2 beam polarization - how the unpolarized beam will be obtained what
> kind of effects are
> expected from beam polarization (ie. to which level exact
> averaging of opposite beam polarity
> is needed and how the phase space of the beam is polarization
> dependent)
>
> RESPONSE: JLab E06-010 (Transversity) spin-averaged a highly polarized
> (~80%) beam in order to
> obtain an "unpolarized" beam. The parity
> feedback allowed for knowledge of the residual
> beam polarization at the 2.2*10^{-5} level,
> according to the lumi monitors. Please see attached
> plot, which shows the beam asymmetry from that
> experiment.
>
>
> 3. temperature effects on the efficiency (and stability of the
> detectors allignment) - proposed scheme
> of polarization reversals will give data with target polarization
> during the day and unpolarized
> at night or vice versa. This can introduce false asymmetry
> related to any kind of temperature
> dependence in efficiency or allignment. Was it estimated ? Are
> there any studies of this kind of effects
> in previous experiments?
>
> RESPONSE: (This could be addressed by the transversity slides, but I'm
> not sure if the pion yield plot
> addresses this. The main sensitivity to
> temperature will be the BCMs and Dave has plans
> to isolate them, but I don't have any details of
> that yet.)
>
>
>
> 4. for the drift of efficiency and its time dependence (page 25)
> linear evolution in time is assumed.
> for which effects it is justified? It is clear that for example
> changes in packing factor of dilution factor
> can have "step like" characteristics. Are there any ideas to what
> level such effects can be controlled
> during the run?
>
> RESPONSE : (Linear and sinusoidal drifts are the only type that I've
> seen. Higher orders could theoretically
> be present, but if they were I suppose they would
> become an issue for all experiments, not just ours.)
> "Step-like" changes in the packing factor or
> dilution factor have only been observed once in 700 hours
> of running the polarized target. It was
> immediately obvious from the change in polarization. If it
> occurs during this experiment, it would impact
> only a single pol/unpol cycle, which is either a 12 hour or
> 24 hour portion of data. This data would need to
> be either discarded or handled with care.
>
>
> In the proposal "consistency checks on measured cross section for
> each run" is mentioned.
> What precisely is meant? At what level it can be done for the
> proposed measurement? Please give
> more detail, especially on the precision of such test.
>
> RESPONSE : Typically we can monitor the unpolarized yields to the
> better than 1% level. Luminosity monitors
> installed around the beamline can be monitored
> to the ?? level (J.P.?)
>
>
> The other test mentioned in the proposal, where I would like to have
> some more comments on is "the
> measurements of dilution and packing factor - with carbon target "-
> what exactly is planed and which
> precision can be obtained? is it included in the beam time estimate?
>
> RESPONSE: The polarized target material is deuterated ammonia (ND_3).
> We determine the dilution factor by the ratio of
> simulated radiated rates on D to total
> rates. The pf is calculated by interpolating the
> ND3 measured rates between simulated
> rates for different packing factors, with the
> simulation calibrated by the measured rates
> on a carbon target of known thickness. The
> systematic uncertainty of this process
> is at the 4% level. It is important to note that
> the dilution factor is a scale factor so the
> uncertainty is an overall scale factor. We have 6
> hours assigned to this task in our overhead table 4.
> This is a relatively short time since all that is
> needed is to measure the unpolarized cross section
> from a carbon disk, and the rate is usually quite
> high.
>
>
> It would be interesting to see comparison of expected statistical
> errors in each bin with expected false
> asymmetries from time variation of beam and efficiency/acceptance.
>
> RESPONSE: We now have plots graphically showing the full systematic
> uncertainty, both from the normalization
> dependent factors and the possible drifts. The
> plots are shown in the technote.
>
>
> What are the arguments for proposed binning in x?
> the last bean is clear, as much data in this configuration as
> possible, but splitting of SHMS data taking
> in 3 intervals is not discussed from the optimalization point of view,
> it would be good to have it in the
> presentation.
>
> RESPONSE : (Hmm. Not sure how to answer this. Simple answer is that
> the points represent the largest
> spread in x that allows a reasonable overlap with
> HERMES in a reasonable amount of beam-time.
> Ellie has optimized to avoid large systematics
> from F1, and suppression of rates. )
>
>
>
> In general, also the authors call the measurement "ratio method" it
> is the cross section difference method
> as the two data sets are taken at different time. Advantage of "ratio
> method" can be fully used when two
> target cels are exposed at the same time and next order is reversed.
> Such configuration allows several
> additional cross checks, but requires two cell target.
>
> RESPONSE : We have examined a two cell configuration, and while it is
> attractive for the reason you point
> out, it was not clear at the time of the
> proposal submission that it significantly reduced the
> overall systematic uncertainty. We will
> continue to examine this option and are open to using
> it if we are convinced the systematic
> improvement is significant.
>
>
> test of Close-Kumano sum rule - it is very hard to do such test with
> limited coverage in x, may be some
> estimates of contribution measured/extrapolated can be done for
> specific models. For the models mentioned
> on page 17 - can one get some idea what would be the contribution in
> measured range?
>
> RESPONSE : Yes, this in an excellent suggestion. G. Miller and M.
> Sargian have provided us their curves.
> We need to integrate to see the contributions for
> x<0.15 and x>0.5. (Ellie, I think you have these
> now. Can you look into this?)
>
>
>
>
> _______________________________________________
> b1_ana mailing list
> b1_ana at jlab.org
> https://mailman.jlab.org/mailman/listinfo/b1_ana
-------------- next part --------------
An HTML attachment was scrubbed...
URL: https://mailman.jlab.org/pipermail/b1_ana/attachments/20130610/c4cb7fa8/attachment-0001.html
More information about the b1_ana
mailing list