16.9 Ten Years of Operation and Physics Analysis in a Nutshell
16.9.3 Discovery and Measurements of the Higgs Boson
Fig. 16.33 Integrated fiducial cross sections times leptonic branching fractions,σWf idversusσZf id, as measured with ATLAS 7 TeV data. The data ellipses display the 68% confidence level coverage for the total uncertainties (full green) and total excluding the luminosity uncertainty (open black).
Theoretical predictions based on various parton distribution function (PDF) sets are shown with open symbols of different colours. The uncertainties of the theoretical calculations correspond to the PDF uncertainties only
production cross-section through gluon-gluon fusion, to simple combinations of the most sensitive channels, and finally to the reduced 7 TeV centre-of-mass energy of the initial run-1 data. These updated expectations, leading to poten- tial discovery with as little as 5–10 fb−1 of integrated luminosity, resulted in a period of great excitement within the ATLAS and CMS experiments, but also in the community at large, from summer 2011 (with 1 fb−1 collected by the experiments) to summer 2012 when the Higgs boson was officially announced as having been discovered by each of the two experiments. The evolution of the Higgs-boson signal significance over this period is illustrated in Fig.16.34.
In summer 2011, as shown in Fig.16.34a, there were no indications of any signal yet and the fluctuations observed as a function of mass were compatible with background fluctuations. At the end of 2011, however, both experiments had excluded a Standard Model Higgs-boson signal over a mass range extend- ing from the LEP limit of 114 to 600 GeV, except for a narrow mass range around 125 GeV in which the largest deviation from background expectations was observed around 125 GeV and corresponded to approximately three standard deviations in each experiment, as shown in Fig.16.34b. Finally, Fig.16.34c,d shows the observed significance in summer 2012 when the discovery was claimed and subsequently published by both experiments [42,43] for 10 fb−1 of data at 7 and 8 TeV.
The four-lepton and diphoton channels have always been rightly considered as the two best channels for Higgs-boson discovery, since they both provide a clear and narrow peak for the Higgs-boson signal in the invariant mass distribution of the final state particles on top of a continuous background. In addition the four-lepton channel can be observed above a much smaller continuum background, consisting predominantly of continuumZZ∗ →4lfinal states. These features can be seen in Figs.16.35and16.36taken from the ATLAS discovery publication [42]. In contrast, the third channel which contributed to the discovery, namely theH → W W∗ → lνlνchannel, has a poor mass resolution because of the presence of neutrinos in the final state, as shown in Fig.16.37.
After the discovery, measurements of the properties of the Higgs boson were performed in successive stages, first focusing on its spin, then on its couplings to bosons and fermions and on possible non-SM contributions to its width. At the end of run-1, ATLAS and CMS produced a combined paper on the Higgs- boson couplings [44], leading to the conclusion that in all production modes and decay channels which had been measured at the time, the Higgs-boson properties were compatible with what one would expect from the SM. More recently, each experiment has produced updated results based also on a large fraction of the run-2 data. This is illustrated in Fig.16.38, which is based on the most recent run-2 ATLAS Higgs combination results [45] and shows that the strength of the measured Higgs-boson couplings to fermions and bosons follows the expectations from the SM, in which for example the Yukawa fermion coupling is expected to be proportional to the fermion mass. Finally, based on the most recent results from the combined run-1 and run-2 datasets from ATLAS and CMS [46], Table16.17shows
Fig. 16.34 Evolution of the combined significance of the Higgs-boson signal in the ATLAS and CMS experiments from exclusion limits in summer 2011 to discovery in summer 2012
[GeV]
m4l
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4l
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→ H Data
Background ZZ(*)
t Background Z+jets, t
=125 GeV) Signal (mH
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Fig. 16.35 Distribution of the four-lepton invariant mass for the selected candidates in theH→ ZZ∗→4lchannel, as observed by ATLAS at the time of discovery in summer 2012. The expected signal formH=125 GeV is shown stacked on top of the overall background prediction
weights / 2 GeVΣ
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H γγ
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=126.5 GeV) (mH
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γ
mγ
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weights - BkgΣ -8 -4 0 4 8
Fig. 16.36 Distribution of the invariant mass of diphoton candidates in theH t oγ γ channel, as observed by ATLAS at the time of discovery in summer 2012. The expected signal for mH=125 GeV is shown stacked on top of the overall background prediction. The residuals of the weighted data with respect to the fitted background is displayed in the bottom panel
that the Higgs couplings to charged third-generation fermions are now all clearly observed unambiguously and measured to be compatible with SM expectations.
In contrast to the channels used for the discovery, the vast majority of the signals
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eνμν/μνeν + 0/1 jets
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WW H
Fig. 16.37 Distribution of the transverse mass of the Higgs boson candidates in theH →W W decay channel, as observed by ATLAS at the time of discovery in summer 2012. The expected signal formH=125 GeV is shown stacked on top of the overall background prediction
Fig. 16.38 Reduced coupling strength modifiers κFmF/v for fermions (F = t , b, τ, μ) and
√κVmV/v for weak gauge bosons (V = W, Z) as a function of their massesmF and mV, respectively, where the vacuum expectation value of the Higgs fieldv =246 GeV. The results are obtained from ATLAS 13 TeV data and the SM prediction is also shown (dotted line). The coupling modifiersκFandκVare measured assuming that there are no beyond-SM contributions to the Higgs-boson decays or production processes. The lower inset shows the ratios of the measured values to their SM predictions
Table 16.17 Summary of direct measurement of all Yukawa couplings of the Higgs boson to third- generation charged fermions (τlepton, bottom quark, and top quark) shown for the ATLAS and CMS experiments
τlepton Bottom quark Top quark
ATLAS Observed significance 6.4σ 5.4σ 6.3σ
Expected significance 5.4σ 5.5σ 5.1σ
Measured to predicted yield ratio 1.09±0.35 1.01±0.20 1.34±0.21
CMS Observed significance 5.9σ 5.5σ 5.2σ
Expected significance 5.9σ 5.6σ 4.2σ
Measured to predicted yield ratio 1.09±0.29 1.04±0.20 1.26±0.28 The expected and observed signal significances are listed, together with the ratios of the observed yields to those predicted by the SM
explored in these cases are among the most difficult Higgs-boson measurements due to the diverse and potentially large backgrounds and to the fact that the signal does not yield a narrow peak above the background.