Ruprecht Karls Universität Heidelberg

Higgs Couplings from the LHC

The Higgs boson in the Standard Model is not only the last particle to be discovered (essentially triggering the construction of the LHC), it is also unique in many ways. One of its exciting aspects is that for all Standard Model particles the couplings to the Higgs boson are fixed - they have to be proportinal to the particle's mass. Any deviation from this structure points to physics beyond the Standard Model, including predictions of new and unknown physical states.

Higgs physics at the LHC offers two unique opportunities to test the Standard Model. First, the Higgs potential is one of very few windows to couple new particles to the Standard Model without spoiling renormalizability. This effect is called the Higgs portal and would appear as a uniform modification of all Higgs coupling. Second, two of the Higgs work horses at the LHC, the Higgs couplings to gluons and to photons, do not exist at tree level. Both operators are induced through quantum effects. In the Standard Model, they are dominated by effects from the top quark and the W boson, with some contributions from the bottom quark and the tau lepton. New particles which either get their mass through the Higgs mechanism or appear in the scalar Higgs potential will be observable in these two couplings.

If the 126 GeV anomaly in ATLAS and CMS should turn out to be a signal for a scalar resonance one of the main tasks for the LHC will be to study the properties of this particle. A complete analysis of all couplings is a major part of this effort. SFitter unfolds the many correlated ways the different Higgs couplings affect the observed counting rates in the different LHC channels. It includes correlated statistical, systematic, and theory uncertainties. Its output is a global log-likelihood map plus a detailed local study of its maxima. If you are interested in an orthogonal appraoch with a focus on anomalous couplings please check out this nice analysis by our friends in Barcelona and Sao Paulo.

This private homepage collects Fitter results on the Higgs sector. As the two theorists on the SFitter project we (Tilman Plehn and Michael Rauch) would like to thank our ATLAS collaborators Remi Lafaye and Dirk Zerwas as well as our CMS collaborator Markus Klute for the great collaboration. Some of the material shown below is made by theorists for theorists, i.e. it might include published but preliminary experimental results. If you use any of the results shown here, please refer to the paper where the corresponding official result is published and to this website as the actual source.

If you are interested in talking to us or in collaborating with us, please drop us an email. The modified HDECAY version we use for our analyses is available upon request. If you use it, please refer to our first data analysis. So here are some of our main results, updates, and a list of publications.


SFitter-Higgs update after Moriond and Aspen 2013 and two-Higgs-doublet model analysis.

The result includes the new di-photon rate by CMS. As usual, in additional to the universal coupling modification, the second and third entries show the results for a universal gauge boson and fermion coupling. To the right we show the ratio of couplings extracted from data. The paper also gives a detailed analysis for two-Higgs-doublet models.

Sfitter-Higgs extrapolation of Higgs coupling measurements at a high-luminosity LHC and a LC Higgs factory.

The LHC analysis includes a full extrapolation of all experimental and theoretical uncertainties, including the appropriate likelihood treatment of flat theory uncertainties. We also show the crucial combination of LHC and LC measurements for example for the effective Higgs-photon coupling. Note that the main advantage of a linear collider, the actual measurement of the total Higgs width, does not enter this figure.

SFitter-Higgs update, based on 2011 (7 TeV) and 2012 (8 TeV) data as published after ICHEP 2012.

For the first time, it includes all tree level Higgs couplings and an additional variable coupling of the Higgs boson to two photons (light blue). In additional to the universal coupling modification, the second and third entries show the results for a universal gauge boson and fermion coupling.

Projections included in the same publication, based on 2011 data (7 TeV) as published after Moriond 2012.

For all scenarios we show the expected measurements, i.e. assuming measurements of the Standard Model central values but with proper error bars. As a direct measurement of the bottom Yukawa couplings we include the boosted VH analysis as estimated in the ATLAS full simulation alaysis.

First SFitter-Higgs publication, based on 2011 data (7 TeV) as published after Moriond 2012.

It includes the different tree level Higgs couplings as well as some ratios. The first entry corresponds to a universal form factor modification of all Higgs couplings. The three sets correspond to expected measurements (red), observed measurements (dark blue), and observed measurements with equal W and Z couplings (light blue).


SFitter-Higgs publications
2013 update and 2HDM analysis
combination with linear collider
2012 update
2011 data and projections
2009 projections
SFitter setup (SUSY)
Further reading
Higgs and QCD lecture notes

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