How Much Better Will Top and Higgs Mass Measurements Get?

[ohwilleke]

The ATLAS and CMS experiments at the Large Hadron Collider (LHC) recently released joint annual review papers regarding their measurements of the properties of the top quark and the Higgs boson, including their masses, respectively.

The most recent mass measurement of the top quark are 172.51 ± 0.51 GeV from combined Run 1 ATLAS data (from all channels measured) and 172.44 ± 048 GeV from combined Run 1 CMS data (from all channels measured). In addition, one CMS result in one channel from Run 2, which is not included in the combined CMS number above, of 172.25 ± 0.63 GeV, has also been released.

The Particle Data Group reports that the global average value for the top quark mass (including measurements from Tevatron as well as the LHC and also the one CMS Run 2 result) is 173.0 ± 0.4 GeV.

The most recent current combined LHC mass measurement of the Higgs boson I have see in most sources is 125.09 ± 0.24 GeV, which is based upon all measurements in all channels at ATLAS and CMS combined, in Run 1. But, the Particle Data Group reports a more precise figure of 125.18 ± 0.16 GeV, which includes one Run 2 measurement in one channel from CMS.

I have three related questions (obviously, answers to some and not others are better than no answers at all):

1. Does anyone known what the expected reduction in the margin of error for these respective mass measurements at the LHC will be once all of the Run 2 data are included?

2. Does anyone know when these mass measurements made that include Run 2 data are likely to be released?

3. Does anyone know how much more precise these measurements are expected to become at any (or all) of the proposed post-LHC colliders (assuming that they are built)?

 

[mfb]

Let's first have a look at something between questions 2 and 3: For the HL-LHC, CMS expects about 0.2 GeV uncertainty in the top mass for lepton+jets (by far the most sensitive channel): Study. ATLAS should reach something similar. Systematic uncertainties will become more important and their future evolution is hard to predict. The question "which top mass are we measuring" will become more important, too.

I didn't find Higgs mass projections but they will be dominated by systematic uncertainties unless people find a clever new way to measure the mass in new channels. A 0.1% to 0.2% uncertainty on energy scales is great already.

Run 2 studies will be somewhere between Run 1 and the HL-LHC. Maybe 0.4 GeV for top per experiment. Higgs might go down to 0.15 GeV to 0.2 GeV per experiment or something like that. Muons give a great energy resolution as you can use the Z for calibration, I don't expect that much improvement compared to H->4l CMS published already, just a bit more statistics.

 

[Reggid]

I can only comment on question 1 and 3 concerning the top mass measurements.

1) The projections for the uncertainty of the top mass measurements at the LHC are around 200 MeV [https://cds.cern.ch/record/1605627/files/FTR-13-017-pas.pdf, http://cds.cern.ch/record/2262606/files/FTR-16-006-pas.pdf], but note that this only relates to the so-called "Monte-Carlo-mass" that is measured. There is an additional ambiguity that can be of order 1 GeV, that is not included in these uncertainties (also not in the ones that you are quoting) that arises because it is not known how to relate this to a well defined field theoretical mass scheme (though there is some progress [https://arxiv.org/pdf/1807.06617.pdf, https://arxiv.org/pdf/1608.01318.pdf])

There are measurements in which one can determine e.g. the pole-mass directly, but those currently have uncertainties of 1.8 GeV [https://arxiv.org/pdf/1603.02303.pdf], with projections for the future at the LHC down to 1 GeV.

3) At a lepton collider with sufficient energy for a threshold scan the top quark mass could be measured with an uncertainty of below 50 MeV [https://arxiv.org/pdf/1604.08122.pdf]