On Apr 19, 2013, at 5:51 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.
I presume the statistics required to make the 1/Q (and maybe 1/Q^2 since a total of 3 mass values are available) non-perturbative corrections are much smaller that the statistics needed at the Z pole for the "primary" measurement? Because 250 GeV and 350 GeV will have much lower statistics than Z pole. Plus, they are on the "wrong" side - the high-mass points will be less sensitive to non-perturbative…PEP/PETRA were on the "better" side being more sensitive to non-perturbative.
Indeed, it will be nice if someone like Iain would work out the basic statistical analysis of alphaS and its non-perturbative correction.
The other direction of thought is to consider the theoretical invention of a new "event shape" variable which has weaker dependence on non-perturbative contributions (i.e. "more infrared safe" ?) while sacrificing statistical sensitivity to alphaS
I mean, invent some variable X = f(alphaS) + g(non-perturbative) where f has weaker dependence (the sacrifice) but g is MUCH smaller than variables used in the past. Then one could still win with super-high statistics.
> 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?
sure!
I think this program adds a good bit of richness and diversity of physics topics to both ILC and TLEP. Having more options and measurements one could make, provides a sense of "contingency" (to put it bluntly) which is always good when large investments are involved…
regards,
Ashutosh
> 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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