Standard model Higgs boson searches through the 125 GeV boson discovery

The experimental searches for the standard model Higgs boson are reviewed from the 2 TeV run of the Tevatron with $\simeq 10{\mathrm{fb}}^{-1}$ of recorded data, and from the 7 and 8 TeV runs of the LHC, with $\simeq 5$ and $\simeq 6{\mathrm{fb}}^{-1}$, respectively, i.e., until the July 2012 discovery of a new particle by the LHC experiments. The CMS and ATLAS Collaborations observe independently a new boson with mass $\simeq 125\mathrm{GeV}$, mainly through its bosonic decays in $\gamma \gamma$, $ZZ$, and $W^{+}{W}^{-}$, consistent with the standard model Higgs boson. The CDF and D0 experiments combine their results to see evidence of a similar particle produced in association with a vector boson and decaying fermionically in $b\overline{b}$.

Figure 1

SM Higgs boson production cross sections for $p\overline{p}$ collisions at 1.96 TeV ([160]; [169]; [183]; [44], [45]; [200]; [10]; [184]; [99]; [161]; [174]; [199]; [46]; [53]; [97], [98]; [41]; [50]; [54]; [166]; [173]; [175]) as functions of its mass.

Figure 2

SM Higgs boson production cross sections for $pp$ collisions at 7 TeV as functions of its mass. For a compilation of theoretical results for SM and MSSM Higgs cross sections at the LHC see https://twiki.cern.ch/twiki/bin/view/LHCPhysics/CrossSections. For the SM Higgs production via gluon fusion using Renormalization Group improved predictions see http://rghiggs.hepforge.org/ (2012). From [167].

Figure 3

Branching ratios for the main decays of the SM Higgs boson as functions of its mass.

Figure 4

Constraints from the top-quark mass, the $W$ boson mass, and other SM measurements compared to the allowed Higgs search range in March 2012.

Figure 5

Dijet mass distribution in the D0 $WH$ analysis for the $W+2$ jet sample with one or two identified $b$ jets. The data are well described by the sum of all the SM backgrounds. The simulated signal is also represented.

Figure 6

Distribution of the NN discriminant in the CDF ${\cancel{E}}_{T}bb$ analysis for the sample with two or three jets where two of the jets are tagged as $b$ jets by the secondary vertex algorithm. The data are well described by the sum of all the SM backgrounds. The simulated signal is also represented.

Figure 7

The angle between the lepton candidates in three dimensions: The distribution of this angle for background and a $H\to WW$ signal with $m_h=165\mathrm{GeV}$ are compared.

Figure 8

NN discriminant for $ggH\to WW\to \ell \nu \ell \nu +0$ jet at CDF. The distribution for background and a $H\to WW$ signal with $m_H=165\mathrm{GeV}$ (multiplied by 10) are compared.

Figure 9

Invariant mass of diphoton events in the ATLAS experiment. The results are presented with and without event weighting by an expected signal-to-background ratio.

Figure 10

Invariant mass of diphoton events in the CMS experiment with events weighted by an expected signal-to-background ratio and the unweighted distribution shown as an inset.

Figure 11

The four lepton invariant mass distribution from the ATLAS experiment. The data are displayed as points and the background expectation as a histogram. Several SM Higgs boson signal contributions are included for different hypothetical Higgs boson masses. Background $Z+\text{jets}$ and $t\overline{t}$ bottom, background $ZZ$ middle, and Higgs boson signal top.

Figure 12

A two-dimensional plot of four lepton invariant mass versus matrix element likelihood from the CMS experiment. Data are shown with event by event mass uncertainties while the expectation of a 125 GeV SM Higgs boson is superimposed as a temperature plot. The central region around 125 GeV is highest in probability.

Figure 13

Distribution of the transverse mass in the zero-jet and one-jet $WW$ analyses of ATLAS, with both $e\mu$ and $\mu e$ channels combined. The expected signal for $m_H=125\mathrm{GeV}$ is shown stacked on top of the background prediction.

Figure 14

Distribution of the $\tau \tau$ invariant mass in the combined 7 and 8 TeV data sets for the VBF category of the CMS ${\tau }^{+}{\tau }^{-}$ analysis. The expected signal for $m_H=125\mathrm{GeV}$ is shown stacked on top of the background prediction.

Figure 15

The ATLAS experiments combined upper limit as a function of the Higgs boson mass between 100 and 600 GeV. Solid black: observed limit; dashed black: median expected limit in the background-only hypothesis; colored bands: $\pm 1$, $2\sigma$ distributions around the median expected limit.

Figure 16

The CMS experiments combined upper limit as a function of the Higgs boson mass between 100 and 600 GeV. Solid black: observed limit; dashed black: expected limit in the background-only hypothesis; colored bands: $\pm 1$, $2\sigma$ distributions around the median expected limit.

Figure 17

ATLAS local significance ([37]) for each search channel and the combination. The observed significances are shown with solid curves, and the median expected significances assuming a signal is present at the SM strength are shown with dashed curves. A dash-dotted line indicates the $6\sigma$ threshold. The highest local significances of the $ZZ$ and $\gamma \gamma$ channels are $3.4\sigma$ and $4.5\sigma$, respectively, while the combined significance of all channels is $5.9\sigma$.

Figure 18

CMS local $p$ values ([132]).The observed $p$ values are shown with solid curves, and the median expected $p$ value for the combined search assuming a signal is present at the SM strength is shown with a dashed curve. Horizontal lines indicate the $1\sigma –7\sigma$ thresholds. The highest local significances of the $ZZ$ and $\gamma \gamma$ channels are $3.1\sigma$ and $4.1\sigma$, respectively, while the combined significance of all channels is $5.1\sigma$.

Figure 19

ATLAS best-fit signal strength for all SM Higgs boson decays for $m_H=125\mathrm{GeV}/c^2$.

Figure 20

CMS best-fit signal strength for all SM Higgs boson decays for $m_H=125\mathrm{GeV}/c^2$. Untagged refers to the cross section extracted from topologies sensitive to gluon fusion production. The shaded band corresponds to the $\pm 1\sigma$ uncertainty on the full combination.

Figure 21

The ATLAS two-dimensional fit for the cross section and compared to the SM expectation and the Higgs boson mass for highest significance decay channels.

Figure 22

The CMS two-dimensional fit for the cross section and compared to the SM expectation and the Higgs boson mass for highest significance decay channels and the combined fit using those channels.

Figure 23

Observed and expected 95% C.L. upper limits on the ratios to the SM cross section, as functions of the Higgs boson mass for the combined CDF and D0 analyses. The bands indicate the 68% and 95% probability regions where the limits can fluctuate in the absence of signal.

Figure 24

The background $p$ values $1\text{−}{\mathrm{C}\mathrm{.}\mathrm{L}\mathrm{.}}_b$ as a function of the Higgs boson mass (in steps of 5 GeV), for the combination of the CDF and D0 analyses. The shaded bands correspond to the regions enclosing $1\sigma$ and $2\sigma$ fluctuations around the median prediction in the signal-plus-background hypothesis at each value of $m_H$.

Figure 25

The best-fit signal cross section of all CDF and D0 search channels combined shown as a ratio to the SM cross section as a function of the tested Higgs boson mass. The horizontal line at 1 represents the signal strength expected for a SM Higgs boson hypothesis. The shaded band shows the $1\sigma$ uncertainty on the signal fit.

Figure 26

The best-fit cross section times branching ratio ${\sigma }_{WH}+{\sigma }_{ZH}\times \mathcal{B}(H\to b\overline{b})$ as a function of $m_H$ measured at the Tevatron.

Figure 27

Best-fit signal strength for the three most sensitive boson decay modes at the Tevatron for $m_H=125\mathrm{GeV}/c^2$. The shaded band corresponds to the $\pm 1\sigma$ uncertainty on the full combination.