[b1_ana] [Fwd: Re: PR12-13-011]

[O. A. Rondon]

Hi JP,

Thank you for the explanation. I had two concerns:

- bucket counting when beam trips, etc. What is the chance that somehow
buckets get miscounted? What happens when the helicity is being flipped
between buckets? Is there beam still coming?

- if the beam has some definite small effective polarization, how
precisely can it be known? e.g. can we say that dP_b/P_b ~ 2% when P_b ~
0.01, so dP_b = 2%*0.01 = 2E-4?

The second question is related to the error propagation of P_b, which
has a factor of P_z in front, so even if P_b ~ very tiny, dP_b may not
be very tiny relative to P_b itself, i.e. dP_b ~ P_b at small P_b.

I understand that there are ways to have an effective unpolarized beam.
But is it too demanding to get true unpolarized beam?

Cheers,

Oscar

J. P. Chen wrote:
> Hi, Oscar,
> 
> We have done this for the transversity experiment where
> our main goal is single target spin asymmetry (beam unpolarized)
> and our secondary goal is to get double spin asymmetry as well.
> So the beam polarization was high (typically 80%).
> We do have beam helicity information in the data such that we can
> have the buckets count exactly equal between two beam helicity states.
> Due to the parity beam feed back system, we got the charge asymmetry
> between the two states to be at 10-5 level (<50 ppm), much better
> than we need. (Without the feedback system, we can typically only control
> the charge asymmetry to ~1x10^-3.) For transversity, the systematics
> due to false asymmetry from beam not totally unpolarized is completely
> negligible.
> 
> For our low current running with polarized proton/deutron target, we
> will need to
> rely on feedback system to be run from the other hall. As in g2p case,
> we rely on
> QWeak feedback, which provided charge asymmetry in Hall A at the 10^-4
> level.
> 
> Karl and I can get our g2p students to provide some plots.
> 
> Cheers.
> 
> Jian-ping
> 
> 
> On 5/14/2013 5:58 PM, O. A. Rondon wrote:
>>
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