Hi Ashutosh,
I totally agree with you as far as GigaZ is concerned. Something like
GigaZ is an extremely worthwhile effort, and theorists will likely live up
to that challenge during the next 1-2 decades. However, Michael was asking
about a machine that will accumulate 100 times the luminosity of GigaZ
(effectly a TeraZ), and I honestly believe that it is unclear whether we
can even approximately match this level of precision with other
experimental inputs and theory.
Best,
Ayres
On Fri, 19 Apr 2013, Ashutosh Kotwal wrote:
> Dear Michael, all,
> Thanks for this information - I also read the responses. My opinion is different from those stated.
>
> One way to state my opinion is that for almost 2 decades, MZ and GF have been 10 times more precise than sin2theta. MW has now caught up with sin2theta, but for most of this time it was also lagging.
>
> 10 years ago, alphaEM(MZ) was 4 times worse that it is now, and was at the same level then as sin2theta and MW are now.
>
> My point is, despite all this, we never thought that MZ and GF was "wasted precision" for the last two decades while the other measurements slowly caught up, including Mtop.
>
> So I think there is nothing wrong with sin2theta leapfroging to become the pack leader in precision. EWPO have been global program. We will find ways for other EWPOs to catch up.
>
> GigaZ will produce sin2theta precision equal to current MZ and GF precision. I see no reason to stop there.
>
> Another example - when Tevatron MW was 100 MeV error, alphaEM was an equivalent 15 MeV error - this was in year 2000. One could have "given up" on MW ambitions by saying "alphaEM would limit us anyway". Yet here we are, with MW at 15 MeV and alphaEM at 4 MeV equivalent.
>
> I wonder if Lattice QCD can help on the non-perturbative QCD loops in alphaEM running.
>
> regards,
> Ashutosh
>
>
>
>
> On Apr 18, 2013, at 1:09 AM, "Peskin, Michael E." <[log in to unmask]> wrote:
>
>> Dear Colleagues,
>>
>> Last week, the accelerator physicists' group in Frontier Capabilities met to discuss
>> lepton colliders. The following question was raised, and your group should answer it.
>>
>> For a long time, ILC has been contemplating a run at the Z pole with 10^34 luminosity
>> ("Giga-Z"). This would be able to improve most precision electroweak observables by
>> about 1 decimal place. This program is by now well documented.
>>
>> The TLEP proposal includes a program at the Z with luminosity 10^36 ("Tera-Z"). TLEP is
>> limited by the total amount of synchrotron radiation power that has to be carried
>> off. There is less synchrotron radiation per particle at lower energies, so higher
>> beam currents are possible.
>>
>> However, really getting 10^36 luminosity at the Z puts other constraints on the design.
>> The machine physicists remarked that TLEP is much easier to build for a program at
>> 250 GeV with 10^35 if one would back off to 10^35 also at the Z.
>>
>> The question for you is, how much would the extra factor of 10 at the Z pole (or the
>> extra factor of 100 beyond Giga-Z) buy you in terms of the physics? My quick impression
>> is that it is not easy to convert the extra luminosity into physics. GF and MZ must be
>> improved, and NNLO electroweak becomes relevant. The uncertainty in alpha(mZ) also
>> needs improvement, and I do not see a way to do that.
>>
>> However, these are just off-the-cuff remarks. If someone is interested in doing a real
>> analysis of this question for the Snowmass study, I encourage you.
>>
>> Here is a reference on TLEP at the Z:
>>
>> https://indico.cern.ch/conferenceDisplay.py?confId=222458
>> talk of Alain Blondel at the bottom of the page
>>
>> Thank you!
>>
>> 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/
>> ---------------------------------------------------------------------------------------------
>>
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