Hi,

   I think there has been considerable confusion in this thread between alpha_s (QCD coupling) and alpha_had (hadronic loop contribution to QED coupling).  In my decidedly non-theorist understanding, it is alpha_had which is the immediate problem here.

   I would caution people from assuming that just because you can produce 10^12 Z bosons at a machine like TLEP that you could immediately turn that into an improvement on sin2theta_eff by another order of magnitude.  There are systematics involved in measuring this asymmetry (even using a Blondel scheme) and at a circular machine you will need to use (very large) spin rotators near the IP to bring the beams into collision with longitudinal polarization.  Because of this, you can't reverse the polarization on a short time scale, and you can't easily reverse the e+ and e- polarization independently.  This seriously impacts the ability to control certain experimental uncertainties.

   From the 2005 Snowmass study, it was challenging enough to consider a GigaZ program at ILC, but it was considered feasible due to the cancellations available at a linear machine.  It really isn't clear to me whether you could even achieve GigaZ precision in a circular machine.  I am not saying it can't be done, but I haven't seen even a conceptual description of how you might approach this.

Regards,
-Eric

On Apr 19, 2013, at 7:17 AM, Jens Erler wrote:

> Hi,
> 
> I also see alpha_s less of a concern.  The alpha_s value from the Z-lineshape has
> currently a negligible theory error, so it is especially here that I would expect great
> improvement.  If one then also gets alpha_s to per mille precision from event shapes, 
> one would even gain another independent EW observable to very high precision. 
> But one needs a plausible avenue to credibly improve Delta alpha (M_Z).
> 
> Jens
> 
> 
> On Apr 19, 2013, at 4:51 AM, "Peskin, Michael E." <[log in to unmask]> wrote:
> 
>> Folks,
>> 
>> I am confused by this thread.
>> 
>> The main barrier to improvement of alphas is the fact that it is difficult to disentangle perturbative and nonperturbative contributions to event shapes in e+e-.   The nonperturbative contributions fall off as 1/Q, so a substantial moment arm in Q is needed.   Currently what is done is to compare Z 
>> data with PEP/PETRA data, although the latter is taken with detectors that are now two generations old.
>> 
>> If TLEP is built, it will be possible to get high-statistics samples of e+e- event shapes in the same, modern detector at 91 GeV, 250 GeV, and 350 GeV.  Then it should be possible to fit out the 1/Q terms and reach per mil precision in alphas.    Probably it is worth bringing in an alphas expert (e.g. Iain Stewart at MIT) to quantify this.
>> 
>> Need I add that, if ILC is built in Japan, we can carry out this program even sooner and extend it to even higher energies?
>> 
>> Thanks,
>> 
>> Michael 
>> 
>> -------------------------------------------------------------------------------------------
>> Michael E. Peskin                           [log in to unmask]
>> HEP Theory Group, MS 81                       -------
>> SLAC National Accelerator Lab.        phone: 1-(650)-926-3250
>> 2575 Sand Hill Road                       fax:     1-(650)-926-2525
>> Menlo Park, CA 94025 USA              www.slac.stanford.edu/~mpeskin/
>> ---------------------------------------------------------------------------------------------
>> ________________________________________
>> From: Ashutosh Kotwal [[log in to unmask]]
>> Sent: Friday, April 19, 2013 12:48 AM
>> To: Michael Schmitt
>> Cc: Sven Heinemeyer; Peskin, Michael E.; snowmass-electroweak; [log in to unmask]
>> Subject: Re: [SNOWMASS-ELECTROWEAK] question from the Capabilities group
>> 
>> On Apr 18, 2013, at 8:08 AM, Michael Schmitt <[log in to unmask]> wrote:
>> 
>>> Hi,
>>> 
>>> I believe Sven is right.    As you say, it is hard to see how to improve alpha_S
>>> without a dedicated facility/program and no such thing is planned as far as
>>> I know.
>> 
>> 
>> it would be interesting to think about alpha_S measurement at GigaZ and TeraZ using event shapes etc. and how statistics and systematics play into that.
>> 
>> Ashutosh
>> 
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