[b1_ana] Fwd: comments/questions to PR12-13-011

[Karl Slifer]

---------- Forwarded message ----------
From: Karl Slifer <karl.slifer at unh.edu>
Date: Fri, Jun 14, 2013 at 1:50 AM
Subject: Re: comments/questions to PR12-13-011
To: Ewa Rondio <Ewa.Rondio at fuw.edu.pl>
Cc: Naomi Makins <gnome at jlab.org>, Juergen Arends <arends at kph.uni-mainz.de>


Dear Ewa, Dear Juergen,

Thank you again for providing your questions.  Please find our brief
response below, with details in the attached note.  I'd very much
appreciate the opportunity to clarify any further issues via email or in
person.  To this end, perhaps we can discuss during Monday's morning coffee
break?

best regards,

 Karl




On Sun, Jun 9, 2013 at 12:00 PM, Ewa Rondio <Ewa.Rondio at fuw.edu.pl> wrote:

> Dear Karl,
>
>     Thank you for well written proposal. I like the summary of theoretical
> approaches.
> Below is the list of comments/questions from the first reading, part could
> be obvious for
> You, but not clear for the PAC members not working at JLab.
> If You need any clarifications, please lat me know.
>
>              best regards
>
>                 Ewa Rondio
>
> The measurement is very sensitive to the systematic effects and good
> control of them is the
> key point.



Response: We agree that good control of systematic effects is the key to
this experiment.  In general,  we were very encouraged by the TAC statement
that mitigation of these effects is possible with a combination of planned
upgrades and sufficient commitment from the collaboration.


Therefor I would like to know if there are any estimates of expected size
> of effects from:
> 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 mentioned that false asymmetries from beam
position drifts can be addressed by regression techniques, as was the
procedure during the Qweak experiment. In fact, Qweak controlled position
differences to the nm(!) level. I should also note that our beam will be
rastered over a 2 cm diameter area, so effects arising from any
sub-millimeter drifts in the beam position will be greatly suppressed, even
before any feedback is implemented.



> 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: We obtain unpolarized beam by counting equal numbers of buckets
in each helicity state.  The parity feedback system typically reduces any
residual beam charge asymmetry to the 10^-5 level.  This worked very well
for E06-010 which was also required "unpolarized" beam, but took data with
a nominally 80% polarized beam.  We've also verified this level of
suppression with helicity data from the g2p 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: Temperature variations are a significant concern, but mostly
impact the BCMs.  This effect was observed during the QWeak experiment.  We
are encouraged from discussions with Hall C staff member Dave Mack that
thermal isolation of the cavities is planned, and is expected to
suppress most of the possible thermal variation of the cavities.

The detector efficiency and alignment is fairly insensitive to thermal
drifts as evidenced by Fig.2 of the attached technote. This plot shows
that the measured yield (which would also reflect any variation in
detectors, charge, target length, beam position, acceptance etc.)
from experiment E06-010 drifted less than 0.355% over 15 days of
continuous running.  This translates to systematic effects at the 10^-4
level over the period of one 12 hour cycle.

One caveat to our conclusions is that E06-010 ran at a larger current.  We
will need to verify that these results also hold for the planned 100 nA
running of this experiment, either with data from a similar experiment or
with parasitic tests during 12 GeV running.



> 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: Step-like changes in the packing fraction or df have only been
observed once in 700 hours of running the polarized target during the SANE
and RSS experiments.  Such a step is immediately obvious from
the associated step in the target polarimetry, which is extremely sensitive
to the material filling factor, since the NMR coil is embedded within.  We
will have additional monitoring of such possible shifts with the new 1.1
kW Faraday cup, and lumi monitors.  We've discussed this with the Qweak
lumi experts (Dalton,Pitts) who expect that we can monitor stability at the
10^-4 level by using the existing lumis in counting mode. Hall C staff
members Dave Mack and Chris Keith also have preliminary plans for an
additional PMT-free temperature stabilized luminosity monitor.

For reference, an estimate of the length change due to the loss/gain of a
single bead can be obtained from the ratio of its 0.008 cm3 volume to that
of the full 6.8 cm3 rastered volume,  which is 1/850.  We note that a
step-like variation would impact only a single pol/unpol cycle.  Any such
cycle would need to be either discarded or treated with extreme care in the
offline analysis.


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 better than
1%.  The Faraday cup and lumis are expected to provide tighter constraints
on the yield at the 10^-4 level.


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:  Full details of this technique are discussed
here<https://userweb.jlab.org/~jones/rss/dfpf_writeup.pdf>.


 To summarize, we determine the dilution factor by the ratio of simulated
radiated yield from D to the total yield.  The packing factor is calculated
by comparing the measured ND3 yield to simulated yields of known packing
factors.  The simulation is calibrated using the measured yield from a
carbon target of known thickness.  The time for these runs (6 hours) is
included in our overhead estimate of Table 4. It is relatively short,
 since the rate from the carbon target is high. This procedure has been
used in many previous polarized target experiments, and typically results
in a systematic uncertainty of about 4%.  It is important to note that the
dilution factor is a scale factor to the asymmetry, so this enters as
a relative error.



> 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: Fig. 1 of the attached note shows the full systematic
uncertainty, which includes estimates of the scale dependent normalization
errors, along with the uncertainty arising from time-dependent drifts.




> 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: In general, we want to cover the largest spread in x that allows
reasonable overlap with Hermes in a reasonable amount of beam time.   The
x=0.49 point was chosen to probe the largest observed Hermes asymmetry, but
is not exactly overlapping since the HMS can not be pushed any lower in x
without sacrificing statistics.  The x=0.16 point was chosen to optimize
the uncertainty in b1 by balancing the statistics against the systematic
contribution from F1, which increases dramatically as x->0.  The middle
two points were chosen to map out any potential zero crossing, (which is
very important to observe in the context of the CK sum rule) with similar
statistics for each bin.



> 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:  The existing target magnet has a uniform field region of about 3
cm length, which precludes using two in-line cells.   We are certainly open
to the possibility of using two cells, since this was demonstrated to work
well in COMPASS.   But we will need full monte-carlo simulation of the
impact on the acceptance, and to investigate the design of a new solenoidal
target magnet.  Neither of these require particularly large effort, but
will not be completed prior to our defense.



> 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?
>


COMMENT: Thank you for this excellent suggestion. I expect we can have this
estimate for the presentation.
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