Dear Chris,
Many thanks to you and the other Higgs conveners for preparing this comprehensive summary document, which looks very good.
I've enclosed suggestions below on some key points that are important for the facilities comparisons, most of which we discussed in Seattle. I've CC'ed the list so others can object (or support) if they feel strongly.
For completeness, I've also provided below that detailed comments on additional minor issues that can be ignored by everyone else.
Thanks again and hope these suggestions are useful.
Best,
Eric
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*Significant comments*
p. 10, paragraph 3:
- I would suggest to add that the HL-LHC can measure the Higgs coupling to muons to probe differences between the second and third generation leptons. The precision on ratio of partial widths to taus and muons was studied by ATLAS in Fig. 3b and Table 5 of ATL-PHYS-PUB-2012-004:
http://cds.cern.ch/record/1484890/
It can be determined to 67% and 25% with 300 and 300 fb-1 (statistically limited by H->mumu). The corresponding relative precisions on the ratio of tau and muon couplings are half that, i.e. 34% and 13% with 300 and 3000 fb-1.
- It might also be useful to mention that the Higgs total width can be indirectly inferred at the LHC using mild assumptions, namely assuming that the Higgs coupling to vector bosons is less than 1. This assumption is physically motivated to preserve unitarity in high-mass VV scattering. The precision is informed by the precision on b-quark coupling (determines lower bound on total width) and vector bosons (upper bound).
Table 1-14:
- The caption says a 7-parameter coupling fit is desired from lepton colliders to compare against LHC. But as we discussed in Seattle (Markus's question), I would advocate to compare using an 8-parameter fit, namely that shown on Table 1-9 for HL-LHC but with an additional parameter for the muon coupling, kappa_mu. This would represent (almost) the full coupling information the LHC can report.
- It would also be useful to quote the model-independent upper limit on BR_inv for the HL-LHC, determined in the ZH(inv) channel. Again it might also be useful to quote the precision on an indirect constraint of the Higgs total width by the HL-LHC with the mild assumptions previously mentioned.
p. 32, paragraph 6:
- It would be very helpful to state explicitly that the physically motivation for the 10^-5 benchmark chosen for f_CP is that this the necessary precision required to understand if CP violation in the Higgs sector can explain baryogenesis. This connects closely to the ILC precision attainable (see next comment).
p. 24, paragraph 1:
- This states: "An e+e- machine will quickly overrun the LHC measurement of f_CP in H->ZZ decay and may reach an interesting level of precision". I would suggest to rephrase this more strongly as: "... particularly interesting level of precision". Again it is particularly valuable because the ILC expected precision for the CP-odd fraction (0.00005 with 500/fb @ 500 GeV) would hit the same order of magnitude precision required to potentially connect to baryogenesis.
p. 32, bullet 1 of Conclusions:
- The LHC precision on the Higgs mass is stated here as "~100 MeV". But Sec. 1.5.1 quotes the precision as "100 MeV and 50 MeV per experiment at 300 and 3000 fb-1" (see text and Table 1-21), and 25 MeV combined at 3000 fb-1 (in text). Therefore I would suggest to quote the HL-LHC precision as 25 MeV (combined).
- The muon collider precision for the Higgs total width is stated here as "1.9-17%". But Sec. 1.5.3 (paragraph 1 and Table 1-20) quotes the precision as 3.6-8.3%. If these numbers are correct, I would suggest to quote those.
p. 32, bullet 5 of Conclusions:
- Regarding the CP-odd fraction f_a3 measured by the HL-LHC in the H->ZZ->4l channel, it's stated "The e+e- machines can measure this to a greater precision in the ee->ZH mode." This seems significantly understated since the ILC precision (0.00005) is 3 orders of magnitude better than that of the HL-LHC (0.02). Therefore I would suggest to rephrase more strongly as "... significantly greater precision…". (The HL-LHC number does not include H->tautau, but it's best not to speculate on that potential without detailed study.)
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*Detailed (minor) comments*
Table 1-1:
- Convert to same convention, so easy to compare.
Table 1-2:
- Might be useful to quote production modes considered, since that would affect conclusions stated.
p. 7, last paragraph:
- Please cite the ATLAS and CMS conference notes and papers on coupling measurements. For ATLAS, our public results on coupling measurements are:
http://arxiv.org/abs/1307.1427
https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/CONFNOTES/ATLAS-CONF-2013-034/
https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/CONFNOTES/ATLAS-CONF-2012-127/
- Also may be better to call the high luminosity upgrade "proposed" rather than "approved", since it is not approved yet.
ATLAS citations are broken (see above for correct links) in:
- Table 1-7
- p. 9, paragraph 2
- Fig. 1-1 caption
- p. 26, paragraph 4
p. 10, paragraph 2:
- "fast parametric simulations" -> "parametrized detector performance based on fast simulation"
Table 1-12:
- The ATLAS numbers for the expected precision on signal strengths can be found in Fig. 3a and Tables 4-5 of ATL-PHYS-PUB-2012-004:
http://cds.cern.ch/record/1484890/
p. 19, paragraph 3:
- "seriously challenging" -> "very challenging"
Table 1-19:
- Expected precisions on spin & CP seem to be missing many estimates.
- It would be better to quote the 95% CL upper limit on the CP-odd fraction, rather than the precision, since this quantity is expected to be zero in the SM.
p. 24, last sentence:
- Define and state that the mixing angle is the variable that is probed to 5 degrees. It might be best to quote this in radians.
Sec. 1.5.1, paragraph 1:
- The ATLAS reference for its mass measurement is the coupling publication:
http://arxiv.org/abs/1307.1427
p. 24, last paragraph:
- Hyperlink says "Table 1.4", but actually links (correctly) to Table 1-19.
p. 25, second paragraph:
- Same issue.
Conclusions (p. 31-33):
- It might be helpful to group the bullet points together by common topic -- couplings, spin & CP, mass & width, etc. Some points seem unordered.
On Jul 26, 2013, at 12:43 PM, Christopher G. Tully <[log in to unmask]> wrote:
> Dear all,
> The Higgs Snowmass committee would like to invite the community to read over the
> draft report on Higgs studies. We look forward to interesting discussions and feedback
> next week at Minnesota, so that we can form a clear perspective and message on the
> exciting potential for further pursuit of this physics in the decades to come.
> The document is posted on the Snowmass2013 twiki here:
> http://www.snowmass2013.org/tiki-index.php?page=The+Higgs+Boson
> Best,
> Andrei, Sally, Heather, Jianming, Chris, Rick
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__________________________________________________
Eric Feng
Wigner Fellow, Argonne National Laboratory
CERN Office: 40-R-C01 Tel: +41 22 76 71124
Email: [log in to unmask]
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