[b1_ana] (no subject)

Wed May 22 14:40:27 EDT 2013

Hi,

Some important clarifications are in order.

First, although Hoodbhoy, Jaffe and Manohar quantize the spin along the
virtual photon, the equations in sect. 6 apply, as it says there, and
was confirmed by Simonetta, to target helicity along or perpendicular to
the electron beam, not the q vector.

This can be seen, because there are expressions for both longitudinal
and perpendicular target field. If the expressions referred to the
virtual photon, there would be no formula for transverse helicity (it
would be zero).

As JP pointed out, it's exactly like measuring A_para and A_perp to get
A1 and A2. Last time we did propose to get b1 directly from the
difference sigma_para - sigma_perp, which unfortunately, in addition to
time dependent changes, it had the big issue of acceptance changes from
para to perp due to the strong field.

What we'll measure now is b1/F1, which we call Azz, but it's more like
measuring g1/F1 with A_parallel only, than measuring A1 itself, which
does need the field along q or, else, to also have A_perp to solve for A1.

I've posted the derivation of our eq. (19) starting from H-J-M's section
6, which didn't make into the proposal because of an e-mail issue with
drafts and attachments (the UVA online email does not save attachments
with drafts, so one must remember to reattach files every time a saved
draft is opened for editing. Absurd).

It's clear that for no beam polarization, we are following the right
method to get b1, except for the contribution of target-only vector
asymmetry, which is unavoidable, and which does not spoil the
measurement itself, but it could complicate the interpretation in terms
of pure b1.

http://twist.phys.virginia.edu/~or/b1/b1meth2f.pdf

Cheers,

Oscar

Dustin Keller wrote:
> Some thoughts,
> 
> In response to Wallys questions a PVDIS asymmetry contribution as
> described in their paper seems to require some non-zero value to the 5
>  spin-dependent structure functions listed for the PV differential cross
> section which should not show up for the real spin-1
> target.  It can show up in the photon-Z interference but with our energy
> we are safe to say that the PV asymmetry is zero.  His question may have
> more to do with what is the best way to measure the PV asymmetry, not sure.
> 
> The experiment as we have written it up requires an unpolarized beam. 
> If we have decreased our chances of approval by that request I would
> like to understand the details behind that.  Otherwise this is a single
> configuration change for the preparation of the experiment.  We need
> true unpolarized beam in-order for our approach and method to be valid.
> If it turns out that we can not have unpolarized
> beam we can calculate the contribution from the additional background
> but the degree of beam polarization (and error) can easily overwhelm
> what we are trying to look at.
> 
> In general the orientation of the field should always be chosen along
> the q-vector when possible.  We have known and understood this for a
> while. We chose to orient the field along the beam line for simplicity
> and ease of configuration.  This is fine but this leads to issues that
> may be caught by the PAC so having an answer in mind is probably a good
> idea. When its is not possible to orientation the field along the
> q-vector it best to select the closest possible orientation to the
> q-vector with the quantization axis still in the scattering plane.  This
> concern only hold relevance for our highest x points, and even along the
> beam line the theta_d and phi_d are small.
> 
> For Jaffe the virtual photon is set along the quantization axis which is
> chosen to be the z-axis.  This is true throughout the formalism and was
> setup that way for simplicity.  This implies that corrections must be
> made if the virtual photon points anywhere other than along the z-axis. 
> The observable Azz is more robust but still requires corrections.
> 
> The effect on Azz to a first approximation is a reduction of the
> effective polarization under a Wigner rotation of
> (3/2cos^2(theta_d)-1/2).  This
> assumes that for our larger x points the Azz is dominated by
> the T20 contribution.  This approximation is better for smaller theta_d.
> 
> In addition to a correction to the polarization when the momentum
> transfer vector is not aligned parallel to the deuteron polarization axis
>  then phi_d and theta_d are non-zero likely leading to a small
> contribution from the target-only vector asymmetry for our higher x
> points.  Though the Arenhovel formalism is for electro-disintegration if
> we assume a static deuteron the polarized inclusive cross section
>  can be integrated leaving the only kinematic dependence on the out
> going nucleons contained in know form factors.  This leaves us with the
> capacity to estimate the contribution from the target-only vector
> asymmetry. Estimates indicate using the present configuration as
> mentioned in the writeup to be still smaller than the error previously
> outlined.  Clearly it is best to be open to changing configuration and
> modifying run plans to minimize all such effects.
> 
> The systematic error table pointed out by Steve for their proposal was
> taken from a thesis on R.  The way these errors are presented in the
> thesis is quite different then how they are presented in the proposal
> that Steve pointed out.  I think they are referring to these
> contributions as point to point errors only because in the cross section
> it is not critical to separated these types of contribution from those
> that propagate relative to the observable.  In other words it doesn't
> hurt them to over-estimate.
> 
> For us its very important to separate these as we have done in the
> writeup of the proposal.  There are some very small contributions that
> can be added to our relative uncertainties, but they do not add any
> overall uncertainty.  But we can add them to acknowledge the comment.
> The only other contribution I can think of  to the absolute point to
> point effects would be correlation
> terms.  Analytically there is a correlation between effective
> polarization uncertainty and the change in Azz from drifts.  I think
> this type of description is redundant for the systematic uncertainty
> estimates in the proposal.  I have added some relative contributions to
> our table, but I think modifying the proposal
> is optional in this regard, see attached.  We can also split the table
> into different contributions as Oscar suggested.
> 
> A lingering issue has been the contribution to the BCM drift.  The
> number I used in the proposal was just an optimistic guess.  Pengjia
> gave me some data on the calibration over the g2p/gep runs.  There was a
> period of six day for which the calibration drifted 0.02% which is of
> the same order I used in the estimate on the proposal.  All other BCM
> calibration drifts where larger but a least this is an indication that
> ~0.01% is not impossible.
> 
> 
> dustin
> 
> 
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