Combined Higgs Results

Chapter 2 Combined Higgs Results This chapter presents the combined ATLAS search for the Standard Model Higgs boson. The analysis has been performed using 4.7 fb of s = 7 TeV data collected in 2, and 5.8 fb of s =8TeV data collected in the first half of 22. The results of the H WW ( ) lνlν analyses presented in Chapter are combined with searches in the H ZZ ( ) llll [] and H γγ [2] channels, using both the 7 TeV and 8 TeV data sets, and with several other Higgs searches using the 7 TeV data set [3]. Clear evidence for the production of a neutral boson with a mass of around 26 GeV is found [4]. This observation has a significance of 5.9 standard deviations and is compatible with the production and decay of the Standard Model Higgs boson. The remainder of the chapter is organized as follows: Section 2. provides a brief overview of the main Higgs searches used in the combination. Section 2.2 describes the procedure for combining the individual analyses. Section 2.3 presents the combined results. 2. Overview of other Higgs searches at ATLAS 2.. H ZZ ( ) llll The search for the SM Higgs boson through the decay H ZZ ( ) llll, wherel is an electron or muon, provides sensitivity over a wide mass range, from to 6 GeV. The H ZZ ( ) llll analysis selects Higgs boson candidates by selecting two pairs of isolated high p T leptons. Each pair is required to be of same flavor and opposite charge. A visualization of an event selected by the H ZZ ( ) llll analysis is shown in Figure 2.. An excess of observed events is searched for in the four lepton invariant mass distribution, m 4l, which would peak at the value of m h for resonant Higgs production. The largest background is from 296

2. Combined Higgs Results 297 Figure 2.: Event display of a selected H ZZ ( ) llll event with four identified electrons. The electrons are shown in red and correspond to localized high energy deposits in the calorimeter matched to tracks in the inner detector, indicated by the red lines. The electrons have a combined invariant mass of 24.6 GeV. The upper-left panel shows the projection of the detector in the plane perpendicular to the beam line. The lower-left panel shows the projection of the detector along the beam line, running left to right in the panel. The upper-right and lower-right panels are zoom-ins of interesting regions. The middle-right panel depicts the energy deposited in the calorimeters as a function of φ and η.

2. Combined Higgs Results 298 Events/5 GeV 25 2 Data Background ZZ Background Z+jets, tt Signal (m =25 GeV) H Syst.Unc. ATLAS H ZZ 4l 5 s = 7 TeV: Ldt = 4.8 fb s = 8 TeV: Ldt = 5.8 fb 5 5 2 25 m 4l Figure 2.2: The four-lepton invariant mass distribution, m 4l, for selected events in the H ZZ ( ) llll analysis using the combined 7 TeV and 8 TeV data sets. The observed data is compared to the background expectation. The signal expectation for a SM Higgs with = 25 GeV is shown. continuum Z(γ )Z(γ ) production, referred to as SM ZZ. For low masses, there are also important background contributions from Z/γ + jets and t t production, where two of the leptons arise from misidentification. Figure 2.2 shows the four-lepton invariant mass for selected events. The expected background and observed data are shown along with the expected signal for m h = 25 GeV. The SM ZZ background is predicted from MC simulation normalized to the theoretical cross section. The reducible Z/γ +jets and t t backgrounds are estimated using dedicated control regions in data. An observed excess of events over the predicted background is seen in the m 4l distribution in the region corresponding to signal with m h = 25 GeV. The results are interpreted statistically using a fit of signal and background models to the observed m 4l distribution. The fit accounts for the various sources of systematic uncertainty. The significance of an excess is given by p, the probability that the observed data is a result of a fluctuation of the background in absence of signal. Figure 2.3 shows the observed p, as a function of the tested Higgs mass, using the combined 8 TeV and 7 TeV data sets. The results using the individual 7 TeV and 8 TeV data sets are also shown separately. In the combined analysis, the lowest observed p value is at

2. Combined Higgs Results 299 Local p -2-3 -4-5 -6-7 ATLAS 2-22 H ZZ 4l 22 Exp. 22 Obs. 2 Exp. 2 Obs. s = 7 TeV: Ldt = 4.8 fb 2-22 Exp. 2-22 Obs. s = 8 TeV: Ldt = 5.8 fb 2 σ 3 σ 4 σ 5 σ 5 2 25 3 35 4 45 5 Figure 2.3: The observed p as a function of m h for the H ZZ ( ) llll channel. The dashed line shows the corresponding expectation for the signal+background hypothesis at the given value of m h. Results are shown separately for the s= 7 TeV data (dark, blue), the s= 8 TeV data (light, red), and their combination (black). m h = 25 GeV, with a value of 2.9 4, corresponding to a statistical significance of 3.4 standard deviations. As a result of the observed excess of events over the expected background and the consistency of the excess with a potential Higgs signal, the observed data is used to perform a measurement of m h and the production cross section. The cross section is reported in terms of the signal strength parameter, µ. The signal strength parameter multiplies the expected signal yield, acting like a scale factor on the total number of predicted signal events. It is defined such that µ = corresponds to the background-only hypothesis and a value of corresponds to background plus Higgs signal produced with the expected SM cross section. The measurements are made using the profile likelihood ratio, see Section.6 for more details. Figure 2.4 shows the best fit values for µ and,withthe contours that correspond to the 68% and 95% confidence levels. The observed excess at 25 GeV has an observed production cross section consistent with that of the SM Higgs boson, i.e., µ =. The combined 7 TeV and 8 TeV H ZZ ( ) llll analyses shows a 3.4 standard deviation excess at m h = 25 GeV with a production cross section that is consistent with the SM Higgs expectation.

2. Combined Higgs Results 3 Signal strength (µ) 5 4 3 ATLAS s = 7 TeV: Ldt = 4.8 fb s = 8 TeV: Ldt = 5.8 fb 2 + 22 Data H ZZ Best fit 68% CL 95% CL 4l fixed ESS in lighter colors 2 22 24 26 28 3 Figure 2.4: Best fit values for µ and m h in the combined H ZZ ( ) llll analysis. The contours that correspond to the 68% and 95% confidence levels are shown. The lighter lines indicate the effect of important systematic uncertainties effecting the measurements. 2..2 H γγ The search for the SM Higgs boson through the decay H γγ is performed in the mass range between GeV and 5 GeV. The H γγ analysis selects Higgs boson candidates by selecting pairs of isolated high p T photons. A visualization of an event selected by the H γγ analysis is shown in Figure 2.5. An excess of observed events is searched for in the di-photon invariant mass distribution which would peak at the value of m h for resonant Higgs production. The dominant background is from continuum γγ production. Smaller contributions also arise from γ+jet and jet+jet production with one or two jets mis-identified as photons. The level of background is constrained by a fit to the observed data. To improve sensitivity, the selected events are divided into categories according to the expected signal-to-background ratio and mass resolution. Figure 2.6 shows the di-photon invariant mass for the selected events. The upper panel, marked a),

2. Combined Higgs Results 3 Figure 2.5: Event display of a selected H γγ event. The photons are shown in yellow and correspond to localized high energy deposits in the calorimeter not matched to tracks in the inner detector. The photons have a combined invariant mass of 26.9 GeV. The upper-left panel shows the projection of the detector in the plane perpendicular to the beam line. The lower-left panel shows the projection of the detector along the beam line, running left to right in the panel. The lower-middle and lower-right panels are zoom-ins of interesting regions. The middle-right panel depicts the energy deposited in the calorimeters as a function of φ and η.

2. Combined Higgs Results 32 shows the di-photon invariant mass spectrum, m γγ, of all selected events. The fit to the backgroundonly model is shown in the dotted line. The panel marked b) shows the background-subtracted data. The lower-panel, marked c), shows the combined m γγ after properly weighting the different events according to their respective categories. Again, the fit of the background-only model is shown in the dotted line. The bottom panel, marked d), gives the weighted data, after background subtraction. An observed excess of events over the fitted background is seen in the m γγ distribution in the region corresponding to signal with m h = 25 GeV. The fit including signal with m h = 26.5 GeV, shown in the solid lines, accurately models the observed data. The results are interpreted statistically as described above for the H ZZ ( ) llll analysis. Figure 2.7 shows the observed p, as a function of the tested Higgs mass, using the combined 8 TeV and 7 TeV data sets. The results using the individual 7 TeV and 8 TeV data sets are also shown separately. In the combined analysis, the lowest observed p value is at m h = 26.5 GeV, with a value of 2 6, corresponding to a statistical significance of 4.7 standard deviations. As a result of the observed excess of events over the expected background and the consistency of the excess with a potential Higgs signal, the observed data is used to perform a measurement of m h and the production cross section. The measurements are made analogously to those of the H ZZ ( ) llll analysis described above. Figure 2.8 shows the best fit values for µ and,with the contours that correspond to 68% and 95% confidence levels. The observed excess at 26.5 GeV has an observed production cross section consistent with that of the SM Higgs boson, i.e., µ =. 2..3 H WW ( ) The H WW ( ) analysis was described in detail in Chapter. This section provides a brief review with the same level of detail as the other channels described above. The reader is directed to Chapters 8 and for more information. The search for the SM Higgs boson through the decay H WW ( ) lνlν, wherel is an electron or muon, provides sensitivity over a wide mass range, from to 6 GeV, and is particularly important in the region below 2 GeV. The H WW ( ) analysis selects Higgs boson candidates by selecting events with pairs of isolated high p T leptons, with large momentum imbalance, due to the un-detected neutrinos. A visualization of a WW candidate event is shown in Figure 2.9. The H WW ( ) analysis is particularly challenging because the un-detected neutrinos prevent the Higgs invariant mass from being fully reconstructed. Most of the H WW ( ) sensitivity comes from comparing the total event yield with the predicted background. This type of analysis requires a high signal-to-background selection and an accurate modeling of the residual backgrounds. Meeting

2. Combined Higgs Results 33 Events / 2 GeV Events - Bkg weights / 2 GeV Σ 35 ATLAS 3 25 2 5 5 2 - -2 8 6 s=7 TeV, Ldt=4.8fb s=8 TeV, Ldt=5.9fb (a) (b) Data Sig+Bkg Fit (m =26.5 GeV) H Bkg (4th order polynomial) H γγ 2 3 4 5 6 Data S/B Weighted Sig+Bkg Fit (m =26.5 GeV) H Bkg (4th order polynomial) Σ weights - Bkg 4 2 (c) 8 4-4 -8 (d) 2 3 4 5 6 Figure 2.6: The distributions of the invariant mass of di-photon candidates after all selections for the combined 7 TeV and 8 TeV data sample. The inclusive sample is shown in upper figure, marked a). The distribution weighted according to the event categories is shown in the lower figure marked c). The result of a fit using a m h = 26.5 GeV signal component and the background component is superimposed. The excess of data with respect to the background fit are displayed in panels marked b) and d). m γ γ

2. Combined Higgs Results 34 Local p -2-3 -4-5 -6-7 ATLAS 2-22 4 σ 5 σ 22 Exp. 22 Obs. 2 Exp. 2 Obs. s = 7 TeV: Ldt = 4.8 fb s = 8 TeV: Ldt = 5.9 fb H γγ 2-22 Exp. 2-22 Obs. 2 σ 3 σ 5 2 25 3 35 4 45 5 Figure 2.7: The observed p as a function of m h for the H γγ channel. The dashed line shows the corresponding expectation for the signal+background hypothesis at the given value of m h. Results are shown separately for the s= 7 TeV data (dark, blue), the s= 8 TeV data (light, red), and their combination (black). Signal strength (µ) 4.5 4 3.5 3 2.5 2.5.5 ATLAS s = 7 TeV: Ldt = 4.8 fb s = 8 TeV: Ldt = 5.9 fb Best fit 68% CL 95% CL fixed ESS in lighter colors 2 + 22 Data H γ γ 2 22 24 26 28 3 Figure 2.8: Best fit values for µ and m h in the combined H γγ analysis. The contours that correspond to the 68% and 95% confidence levels are shown. The lighter lines indicate effect of the important systematic uncertainties effecting the measurements.

2. Combined Higgs Results 35 Figure 2.9: Event display of a WW event in the eνeν channel. The electrons are shown in yellow and correspond to localized high energy deposits in the calorimeter matched to tracks in the inner detector, indicated by the red and orange lines. The dashed line, label ET miss, indicates the direction of the measured momentum imbalance. The left panel shows the projection of the detector in the plane perpendicular to the beam line. The upper-right panel shows the projection of the detector along the beam line, running left to right in the panel. The lower-right panel depicts the energy deposited in the calorimeters and the direction of the ET miss, as a function of φ and η. this challenge has been the subject of this thesis. The dominant backgrounds are continuum WW production and top-quark production, each of which have real W pairs in the final state. Other important backgrounds include Z/γ and W +jet events. Figure 2. shows the best estimate of the Higgs mass for selected events using the observed leptons and the measured momentum imbalance. The data is shown after subtracting the estimated backgrounds, along with the corresponding signal distribution with m h = 25 GeV. An observed excess of events over the predicted background is seen. The observed excess is consistent with the expectation of a signal with m h = 25 GeV. The results are interpreted statistically as described above. Figure 2. shows the observed p, as a function of the tested Higgs mass, using the combined 8 TeV and 7 TeV data sets. The results

2. Combined Higgs Results 36 Events / GeV 8 6 ATLAS s = 7 TeV, s = 8 TeV, Ldt = 4.7 fb Ldt = 5.8 fb H WW lνlν + / jets Bkg. subtracted Data H [25 GeV] 4 2-2 6 8 2 4 6 8 2 22 24 m T Figure 2.: Estimated Higgs mass distribution, m T, in data with the estimated background subtracted. The predicted signal for m h = 25 GeV is overlaid. using the individual 7 TeV and 8 TeV data sets are also shown separately. A broad distribution of p values, consistent with the poor mass resolution in the H WW ( ) analysis, is observed around 3 3, corresponding to a statistical significance of 2.8 standard deviations. As a result of the observed excess of events over the expected background and the consistency of the excess with a potential Higgs signal, the observed data is used to perform a measurement of the production cross section. Figure 2.2 shows the best fit values for µ and, with the contours that correspond to 68% and 95% confidence levels. The results from the H WW ( ) lνlν analysis are given in green. The large uncertainty on m h in the H WW ( ) lνlν analysis is a result of the poor mass resolution from the un-measured neutrinos. The results from the H ZZ ( ) llll and H γγ analyses shown above are repeated in the figure for comparison. The measured cross sections for the observed excesses at m h = 25 GeV are consistent among the three analyses. The observed excess in H WW ( ) lνlν analysis has a measured production cross section consistent with that of the SM Higgs boson at m h = 25 GeV. 2.2 Higgs Combination The ultimate Higgs sensitivity comes from the combination of all of the individual search channels. The most sensitive channels at low mass, m h < 2 GeV, are the H WW ( ) lνlν, H ZZ ( ) llll, and H γγ channels presented in the previous section. For these channels, the full 7 TeV and 8 TeV

2. Combined Higgs Results 37 Local p -2-3 -4-5 -6 ATLAS 2-22 H WW lνl ν 2 σ 4 σ 22 Exp. 22 Obs. s = 7 TeV: Ldt = 4.7 fb s = 8 TeV: Ldt = 5.8 fb -7 2 Exp. 2-22 Exp. -8 2 Obs. 2-22 Obs. 5 2 25 3 35 4 45 3 σ 5 σ Figure 2.: The observed p as a function of m h for the H WW ( ) channel. The dashed line shows the corresponding expectation for the signal+background hypothesis at the given value of m h. Results are shown separately for the s= 7 TeV data (dark, blue), the s= 8 TeV data (light, red), and their combination (black). Signal strength (µ) 5 s = 7 TeV: Ldt = 4.7-4.8 fb Best fit 68% CL s = 8 TeV: Ldt = 5.8-5.9 fb 95% CL 4 3 ATLAS 2 22 H γ γ H ZZ 4l H WW lνlν 2 2 25 3 35 4 45 Figure 2.2: Best fit values for µ and m h in the H WW ( ) lνlν analysis. The results from the H ZZ ( ) llll and H γγ analyses shown above are repeated for comparison. The contours that correspond to the 68% and 95% confidence levels are shown.

2. Combined Higgs Results 38 Table 2.: Channel m h range Data Set H WW ( ) lνlν - 6 7 TeV and 8 TeV H ZZ ( ) llll - 6 7 TeV and 8 TeV H γγ - 5 7 TeV and 8 TeV H ττ - 5 7 TeV VH Vb b - 3 7 TeV H ZZ llνν 2-6 7 TeV H ZZ llq q 2-6 7 TeV H WW lνq q 3-6 7 TeV Analyses used in the Higgs combination, where l stands for an electron or muon. data sets are used. The full combination includes additional channels that are important in the mass range above 2 GeV and channels that add sensitivity at low mass. The analysis of these additional channels has only been performed using the 7 TeV data set. Table 2. lists the individual search channels that enter the final combination. The additional channels are briefly summarized below. The additional channels designed for the low mass region are H ττ and H b b. These channels have relatively low sensitivity, but are important because they are sensitive to the Higgs couplings to fermions. The H ττ analysis is categorized according to the τ decays [5]. It is performed in the mass range from to 5 GeV. The sub-channels are triggered using electrons or muons, except for the fully hadronic channel, which is triggered using the two hadronic τ decays. The dominant backgrounds are from Z ττ and multi-jet events. The di-tau invariant mass distribution is used as the final discriminating variable. The H b b analyses select Higgs events produced in association with a W or Z boson [6]. These associated production modes are used to improve the signal to background and to provide a source of trigger. The search is performed in the ZH llb b, ZH ν νb b, and WH lνb b channels, where l represents an electron or muon. These analyses are performed for m h between and 3 GeV. The sub-channels are triggered using the leptons, except for the ZH ν νb b channel, which is triggered by the missing transverse momentum. All three analyses require two b-tagged jets. The invariant mass of the two b-jets is used as the final discriminating variable. The remaining additional channels improve the search sensitivity in the high mass range. The H ZZ ( ) llνν analysis is performed for m h between 2 and 6 GeV[7]. The transverse mass, computed from the di-lepton transverse momentum and the missing transverse momentum, is used as the final discriminating variable. The H ZZ ( ) llq q search is also performed in the mass range between 2 and 6 GeV[8].

2. Combined Higgs Results 39 95% CL Limit on σ/σ SM 2 Expected Combined ATLAS 2 + 22 Data L dt ~ 4.6-4.8 fb, s = 7 TeV L dt ~ 5.8-5.9 fb, s = 8 TeV Expected H γ γ Expected H bb Expected H ZZ* llll Expected H ZZ* llν ν Expected H ZZ* llqq Expected H WW* lνlν Expected H WW* lνqq Expected H τ τ 2 3 4 5 6 Figure 2.3: The expected 95% CL cross section upper limits as a function of m h for the individual search channels and their combination. The expected limits are those for the background-only hypothesis, in the absence of a Higgs boson signal. The dominant background arises from Z+jet production. The sensitivity is improved by adding a dedicated llb b sub-channel. The invariant mass of the llq q system is used as the final discriminating variable. The final analysis included in the combined Higgs result is H WW ( ) lνq q. This analysis is performed for m h from 3 to 6 GeV[9]. The mass of the two selected jets are required be consistent with a W boson. This mass constraint allows for an event-by-event estimate of the Higgs mass. The reconstructed Higgs mass is then used as the final discriminating variable. To provide a feeling for the relative sensitivity of the various Higgs searches, in the different mass ranges, Figure 2.3 shows the expected limits of the individual channels as a function of m h. At the lowest masses the sensitivity is driven by the H γγ analysis; the H ττ and H b b analyses provide additional sensitivity. From 25 to 2 GeV, the sensitivity is driven by the H WW ( ) lνlν and H ZZ ( ) llνν analyses. Above 2 GeV, H ZZ ( ) llll is the strongest channel. The H ZZ ( ) llνν, H ZZ ( ) llq q, and H WW ( ) lνq q analyses provide additional sensitivity at high mass. The analyses introduced above are combined using a statistical procedure similar to that described in previous section. Different values of µ are tested with a statistic described in Chapter, based on the profile likelihood ratio. The likelihood function includes all the parameters that describe the

2. Combined Higgs Results 3 systematic uncertainties and their correlations. Uncertainties on background normalizations or background model parameters from control regions or sidebands are uncorrelated among channels. The uncertainties due to: luminosity, lepton efficiency and energy scale, jet energy scale, and theoretical uncertainties on the Higgs production mechanisms are properly correlated among the different analyses. The CL s prescription is used to extract 95% exclusion limits. A SM Higgs boson with mass m h is considered excluded at the 95% confidence level (CL) when µ = is excluded at that mass. The combined significance of an excess in the data is quantified using p, which represents the probability that the background can produce a fluctuation greater than or equal to the excess observed in data. 2.3 Results An excess of events is observed around 25 GeV in both the H ZZ ( ) llll and H γγ analyses. The H WW ( ) lνlν channel saw an consistent excess in this region. The combined p is shown in Figure 2.4. The largest significance for the combination of channels is found at m h = 26.5 GeV, with a p value corresponding to a significance of 6.σ. The expected significance is 4.9σ, with an uncertainty band indicated in the figure. The global significance for an excess of this size to occur anywhere in the mass range -6 GeV is estimated to be 5. σ. This corresponds to the observation of new particle. The mass of the newly observed particle as measured using H ZZ ( ) llll and H γγ channels is 26. ±.4(stat.) ±.4(sys.) GeV. The leading sources of systematic uncertainty come from the electron and photon energy scales and resolutions. A summary of the measured signal strengths in the combined and individual channels is shown in Figure 2.5. The combined signal strength of the observed excess is µ =.4 ±.3 at m h = 26 GeV. The measured cross section is consistent with the SM Higgs boson prediction of µ =. The observed decays to pairs of vector bosons, with zero net charge, indicates that the new particle is a neutral boson. The observation in the di-photon channel disfavors the spin hypothesis [, ]. Figure 2.6 shows the observed p in full search range. The only significant excess is observed at 26 GeV. The data is consistent with the background only hypothesis throughout the rest of the mass range. The observed data is used to set limits on the Higgs production cross section. Figure 2.7 shows the combined 95% CL exclusion limits, in terms of the signal strength parameter, as a function of m h. The expected exclusion region is from GeV to 582 GeV. Apart from the region between 22 and 3 GeV, corresponding to the observed excess, the mass range from to 56 GeV is excluded at the 95% CL level. Figure 2.8 shows the break down of the observed limits according to channel.

2. Combined Higgs Results 3 Local p -2-3 -4-5 -6-7 -8-9 - ATLAS 2-22 s = 7 TeV: Ldt = 4.6-4.8 fb s = 8 TeV: Ldt = 5.8-5.9 fb Obs. Exp. ± σ 5 2 25 3 35 4 45 5 Figure 2.4: The observed and expected p as a function of m h in the low mass range for the combined Higgs search. The dashed curve shows the corresponding expectation for p for the signal+background hypothesis at the given values of m h ; the blue band gives the ± one sigma region. The horizontal dashed lines indicate the p-values corresponding to significances of to 6 sigma. σ σ 2σ 3σ 4σ 5σ 6σ ATLAS 2-22 W,Z H bb s = 7 TeV: Ldt = 4.7 fb H ττ s = 7 TeV: Ldt = 4.6-4.7 fb H WW s = 7 TeV: Ldt = 4.7 fb s = 8 TeV: Ldt = 5.8 fb H γγ s = 7 TeV: Ldt = 4.8 fb s = 8 TeV: Ldt = 5.9 fb H ZZ lνlν 4l s = 7 TeV: Ldt = 4.8 fb s = 8 TeV: Ldt = 5.8 fb = 26. GeV Combined s = 7 TeV: Ldt = 4.6-4.8 fb s = 8 TeV: Ldt = 5.8-5.9 fb µ =.4 ±.3 Signal strength (µ) Figure 2.5: Measurements of the signal strength parameter µ for m h = 26 GeV for the individual channels and their combination.

2. Combined Higgs Results 32 Local p -2-3 -4-5 -6-7 -8-9 ATLAS 2-22 - 5 s = 7 TeV: Ldt = 4.6-4.8 fb s = 8 TeV: Ldt = 5.8-5.9 fb Sig. Expected Observed 2 σ 3 σ 4 σ 5 σ 6 σ 2 3 4 5 Figure 2.6: The observed and expected p as a function of m h for the entire search range for the combined analysis. The dashed curve shows the corresponding expectation for p for the signal+background hypothesis at the given values of m h. The horizontal dashed lines indicate the p-values corresponding to significances of 2 to 6 sigma. 95% CL Limit on µ ATLAS 2-22 s = 7 TeV: Ldt = 4.6-4.8 fb s = 8 TeV: Ldt = 5.8-5.9 fb ± σ ± 2σ Observed Bkg. Expected 5 CL s Limits 2 3 4 5 Figure 2.7: The observed (solid) 95% CL upper limit on the signal strength as a function of m h. The dashed line gives the expected exclusion under the background-only hypothesis. The green and yellow shaded bands give the one and two sigma uncertainties on the expected exclusion.

2. Combined Higgs Results 33 95% CL Limit on σ/σ SM 2 ATLAS 2 + 22 Data L dt ~ 4.6-4.8 fb, s = 7 TeV L dt ~ 5.8-5.9 fb, s = 8 TeV Expected Combined Observed Combined Expected H γ γ Observed H γγ Expected H bb Observed H bb Expected H ZZ* llll Observed H ZZ* llll Expected H ZZ* llν ν Observed H ZZ* llν ν Expected H ZZ* llqq Observed H ZZ* llqq Expected H WW* lνlν Observed H WW* lνlν Expected H WW* lνqq Observed H WW* lνqq Expected H τ τ Observed H τ τ 2 3 4 5 6 Figure 2.8: The observed (solid) and expected (dashed) 95% CL cross section upper limits for the individual search channels and the combination as a function of m h. 2.4 Conclusions Excesses consistent with the production of the Standard Model Higgs boson have been observed in the H WW ( ) lνlν, H ZZ ( ) llll, H γγ decay channels. The combined excess corresponds to a significance of 5.9 standard deviations. These results provide conclusive evidence for the discovery of a new neutral boson with mass of 26. ±.4(stat) ±.4(sys) GeV. The measured signal strength is consistent with the SM Higgs prediction. While all of the current observations are consistent with new particle being the Standard Model Higgs boson, the relatively large uncertainties of the current data set cannot exclude significant deviations from the Standard Model hypothesis. More precise tests the compatibility of the new particle with the Standard Model Higgs boson will be the subject of future work. 2.5 Bibliography [] ATLAS Collaboration, Observation of an excess of events in the search for the Standard Model Higgs boson in the H ZZ ( ) 4 channel with the ATLAS detector, ATLAS-CONF-22-92 (22). http://cdsweb.cern.ch/record/464. 2 [2] ATLAS Collaboration, Observation of an excess of events in the search for the Standard Model Higgs boson in the gamma-gamma channel with the ATLAS detector, ATLAS-CONF-22-9 (22). http://cdsweb.cern.ch/record/464. 2

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