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CMS-HIG-14-036 ; CERN-PH-EP-2015-159
Limits on the Higgs boson lifetime and width from its decay to four charged leptons
Phys. Rev. D 92 (2015) 072010
Abstract: Constraints on the lifetime and width of the Higgs boson are obtained from HZZ4 events using data recorded by the CMS experiment during the LHC run 1 with an integrated luminosity of 5.1 and 19.7 fb1 at a center-of-mass energy of 7 and 8 TeV, respectively. The measurement of the Higgs boson lifetime is derived from its flight distance in the CMS detector with an upper bound of τH<1.9×1013 s at the 95\% confidence level (CL), corresponding to a lower bound on the width of ΓH>3.5×109 MeV. The measurement of the width is obtained from an off-shell production technique, generalized to include anomalous couplings of the Higgs boson to two electroweak bosons. From this measurement, a joint constraint is set on the Higgs boson width and a parameter fΛQ that expresses an anomalous coupling contribution as an on-shell cross-section fraction. The limit on the Higgs boson width is ΓH<46 MeV with fΛQ unconstrained and ΓH<26 MeV for fΛQ=0 at the 95% CL. The constraint fΛQ<3.8×103 at the 95% CL is obtained for the expected standard model Higgs boson width.
Figures & Tables Summary Additional Figures CMS Publications
Figures

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Figure 1:
The m4 distributions in the off-shell region in the simulation of the gg4 process with the ΛQ (fΛQ=1), a3 (fa3=1), a2 (fa2=1), and Λ1 (fΛ1=1) terms, as open histograms, as well as the a1 term (SM), as the filled histogram, from Eq.(4) in decreasing order of enhancement at high m4. The on-shell signal yield and the width ΓH are constrained to the SM expectations. In all cases, the background and its interference with different signal hypotheses are included except in the case of the pure background (dotted), which has greater off-shell yield than the SM signal-background contribution due to destructive interference.

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Figure 2-a:
Distributions of the four-lepton invariant mass Djet (a) and m4 (b) in the on-shell region of the H boson width analysis. The Djet distributions show events in the dijet category with a requirement 120 <m4< 130 GeV. The m4 distributions combine the nondijet and dijet categories, the former with an additional requirement Dkinbkg> 0.5 to suppress the dominant 4 background. The points with error bars represent the observed data, and the histograms represent the expected contributions from the SM backgrounds and the H boson signal. The contribution from the VBF and VH production is shown separately.

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Figure 2-b:
Distributions of the four-lepton invariant mass Djet (a) and m4 (b) in the on-shell region of the H boson width analysis. The Djet distributions show events in the dijet category with a requirement 120 <m4< 130 GeV. The m4 distributions combine the nondijet and dijet categories, the former with an additional requirement Dkinbkg> 0.5 to suppress the dominant 4 background. The points with error bars represent the observed data, and the histograms represent the expected contributions from the SM backgrounds and the H boson signal. The contribution from the VBF and VH production is shown separately.

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Figure 3-a:
Distributions of the four-lepton invariant mass m4 in the on-shell (a) and off-shell (b) regions of the H boson width analysis for all observed and expected events. The points with error bars represent the observed data in both on-shell and off-shell region distributions. The histograms for the on-shell region represent the expected contributions from the SM backgrounds and the H boson signal with the contribution from the VBF and VH production shown separately. The filled histograms for the off-shell region represent the expected contributions from the SM backgrounds and H boson signal, combining gluon fusion, VBF, and VH processes. Alternative H boson width and coupling scenarios are shown as open histograms with the assumption ϕΛQ= 0 unless specified otherwise, and the overflow bin includes events up to m4= 1600 GeV.

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Figure 3-b:
Distributions of the four-lepton invariant mass m4 in the on-shell (a) and off-shell (b) regions of the H boson width analysis for all observed and expected events. The points with error bars represent the observed data in both on-shell and off-shell region distributions. The histograms for the on-shell region represent the expected contributions from the SM backgrounds and the H boson signal with the contribution from the VBF and VH production shown separately. The filled histograms for the off-shell region represent the expected contributions from the SM backgrounds and H boson signal, combining gluon fusion, VBF, and VH processes. Alternative H boson width and coupling scenarios are shown as open histograms with the assumption ϕΛQ= 0 unless specified otherwise, and the overflow bin includes events up to m4= 1600 GeV.

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Figure 4-a:
Distribution of the four-lepton invariant mass m4 in the off-shell region in the nondijet (a) and dijet (b) categories. A requirement Dgg> 2/3 is applied in the nondijet category to suppress the dominant 4 background. The points with error bars represent the observed data, and the filled histograms represent the expected contributions from the SM backgrounds and H boson signal, combining gluon fusion, VBF, and VH processes. Alternative H boson width and coupling scenarios are shown as open histograms. The overflow bins include events up to m4=1600 GeV, and ϕΛQ= 0 is assumed where it is unspecified.

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Figure 4-b:
Distribution of the four-lepton invariant mass m4 in the off-shell region in the nondijet (a) and dijet (b) categories. A requirement Dgg> 2/3 is applied in the nondijet category to suppress the dominant 4 background. The points with error bars represent the observed data, and the filled histograms represent the expected contributions from the SM backgrounds and H boson signal, combining gluon fusion, VBF, and VH processes. Alternative H boson width and coupling scenarios are shown as open histograms. The overflow bins include events up to m4=1600 GeV, and ϕΛQ= 0 is assumed where it is unspecified.

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Figure 5-a:
Distributions of Dbkg (a) and cΔt (b) in the lifetime analysis with Dbkg> 0.5 required for the latter to suppress the background. The points with error bars represent the observed data, and the filled histograms stacked on top of each other represent the expected contributions from the SM backgrounds. Stacked on the total background contribution, the open histograms show the combination of all production mechanisms expected in the SM for the H boson signal with either the SM lifetime or cτH= 100 μm. Each signal contribution in the different open histograms are the same as the total number of events expected from the combination of all production mechanisms in the SM. All signal distributions are shown with the total number of events expected in the SM. The first and last bins of the cΔt distributions include all events beyond |cΔt|> 500 μm.

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Figure 5-b:
Distributions of Dbkg (a) and cΔt (b) in the lifetime analysis with Dbkg> 0.5 required for the latter to suppress the background. The points with error bars represent the observed data, and the filled histograms stacked on top of each other represent the expected contributions from the SM backgrounds. Stacked on the total background contribution, the open histograms show the combination of all production mechanisms expected in the SM for the H boson signal with either the SM lifetime or cτH= 100 μm. Each signal contribution in the different open histograms are the same as the total number of events expected from the combination of all production mechanisms in the SM. All signal distributions are shown with the total number of events expected in the SM. The first and last bins of the cΔt distributions include all events beyond |cΔt|> 500 μm.

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Figure 6:
Distributions of the four-lepton pT with the selection used in the lifetime analysis and the requirement Dbkg> 0.5 to suppress the backgrounds. The points with error bars represent the observed data, and the filled histograms stacked on top of each other represent the expected contributions from the SM backgrounds. Stacked on the total background contribution, the open histograms show the H boson signal either with the combination of all production mechanisms expected in the SM, or for the VBF or t¯tH production mechanisms. Each signal contribution in the different open histograms is normalized to the total number of events expected from the combination of all production mechanisms in the SM. The overflow bin includes all events beyond pT> 200 GeV.

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Figure 7:
Observed (solid) and expected (dashed) distributions of 2ln(L/Lmax) as a function of the H boson average lifetime cτH.

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Figure 8-a:
Observed distribution of 2 ln(L/Lmax) as a function of ΓH and fΛQcosϕΛQ with the assumption ϕΛQ=0 or π (top panel). The bottom panel shows the observed conditional likelihood scan as a function of fΛQcosϕΛQ for a given ΓH. The likelihood contours are shown for the two-parameter 68% and 95% CLs (a) and for the one-parameter 68% and 95% CLs (b). The black curve with white dots on the bottom panel shows the fΛQcosϕΛQ minima at each ΓH value.

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Figure 8-b:
Observed distribution of 2 ln(L/Lmax) as a function of ΓH and fΛQcosϕΛQ with the assumption ϕΛQ=0 or π (top panel). The bottom panel shows the observed conditional likelihood scan as a function of fΛQcosϕΛQ for a given ΓH. The likelihood contours are shown for the two-parameter 68% and 95% CLs (a) and for the one-parameter 68% and 95% CLs (b). The black curve with white dots on the bottom panel shows the fΛQcosϕΛQ minima at each ΓH value.

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Figure 9-a:
Observed (solid) and expected (dashed) distributions of 2ln(L/Lmax) as a function of ΓH (a) and fΛQcosϕΛQ (b). On the top panel, the fΛQ value is either constrained to zero (blue) or left unconstrained (black, weaker limit), while ΓH=ΓSMH and ϕΛQ=0 or π are assumed on the bottom.

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Figure 9-b:
Observed (solid) and expected (dashed) distributions of 2ln(L/Lmax) as a function of ΓH (a) and fΛQcosϕΛQ (b). On the top panel, the fΛQ value is either constrained to zero (blue) or left unconstrained (black, weaker limit), while ΓH=ΓSMH and ϕΛQ=0 or π are assumed on the bottom.
Tables

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Table 1:
List of observables, x, and categories of events used in the analyses of the H boson lifetime and width. The Djet< 0.5 requirement is defined for Njet2, but by convention this category also includes events with less than two selected jets, Njet< 2.

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Table 2:
Observed and expected allowed intervals at the 95% CL on the H boson average lifetime τH and width ΓH obtained combining the width and lifetime analyses. The constraints are separated into the two conditions used in the width measurement, with either the constraint fΛQ= 0, or fΛQ left unconstrained and ϕΛQ= 0 or π. The upper (lower) limits on H boson average lifetime τH are related to the lower (upper) limits on H boson width ΓH through Eq.(2).

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Table 3:
Observed and expected allowed intervals at the 95% CL on the fΛQ on-shell effective cross-section fraction and its interpretation in terms of the anomalous coupling parameter ΛQ assuming ΓH=ΓSMH. Results are presented assuming either ϕΛQ= 0 or ϕΛQ=π. The allowed intervals on fΛQ are also translated to the equivalent quantity a1ΛQ through Eq.(5), where the coefficient a1 is allowed to be different from its SM value a1= 2.
Summary
Constraints on the lifetime and the width of the H boson are obtained from HZZ4 events using the data recorded by the CMS experiment during the LHC run 1. The measurement of the H boson lifetime is derived from its flight distance in the CMS detector with the upper bound τH< 190 fs at the 95% CL, corresponding to a lower bound on the width ΓH>3.5109 MeV. The measurement of the width is obtained from an off-shell production technique, generalized to include additional anomalous couplings of the H boson to two electroweak bosons. This measurement provides a joint constraint on the H boson width and a parameter that quantifies an anomalous coupling contribution through an on-shell crosssection fraction fΛQ. The observed limit on the H boson width is ΓH< 46 MeV at the 95% CL with fΛQ left unconstrained while it is ΓH< 26 MeV at the 95% CL for fΛQ= 0. The constraint fΛQ<3.8103 at the 95% CL is obtained assuming the H boson width expected in the SM, and the fLQ constraints given any other width value are also presented. Table 2 summarizes the width and corresponding lifetime limits, and Table 3 summarizes the limits on fΛQ under the different ϕΛQ scenarios that can be interpreted from this analysis, and provides the corresponding limits on a1fΛQ.
Additional Figures

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Additional Figure 1:
The m4 distributions in the off-shell region in the simulation of the gg4 process with the ΛQ (fΛQ=1), a3 (fa3=1), a2 (fa2=1), Λ1 (fΛ1=1), and the a1 (SM) terms (solid open histograms) as well as equal on-shell mixtures with the SM term (fΛQ=0.5; fa3=0.5; fa2=0.5 as dashed open histograms). The fa2=0.5 mixture is shown with arg(a2/a1)=ϕa2=π so that the interference has the same destructive interference as in the ΛQ and Λ1 cases. The on-shell signal yield and the width ΓH are constrained to the SM expectations except the case of fΛQ=0.5, which has complete destructive interference in the on-shell signal region and is therefore normalized with half of the factor used in fΛQ=1 for illustration. The on-shell regions is shown in a single bin. The background and its interference with different signal hypotheses are excluded.

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Additional Figure 2-a:
Distributions of cΔt in the lifetime analysis in the on-shell region (a) and the sideband m4 regions (b). No Dbkg requirement is applied. The points with error bars represent the observed data, and the filled histograms stacked on top of each other represent the expected contributions from the SM backgrounds. Stacked on the total background contribution, the open histograms, where present, show the combination of all production mechanisms expected in the SM for the H boson signal with either the SM lifetime or cτH= 100 μm. Each signal contribution in the different open histograms are the same as the total number of events expected from the combination of all production mechanisms in the SM. All distributions are shown with the total number of events expected in the SM. The first and last bins of the cΔt distributions include all events beyond |cΔt|> 1000 μm.

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Additional Figure 2-b:
Distributions of cΔt in the lifetime analysis in the on-shell region (a) and the sideband m4 regions (b). No Dbkg requirement is applied. The points with error bars represent the observed data, and the filled histograms stacked on top of each other represent the expected contributions from the SM backgrounds. Stacked on the total background contribution, the open histograms, where present, show the combination of all production mechanisms expected in the SM for the H boson signal with either the SM lifetime or cτH= 100 μm. Each signal contribution in the different open histograms are the same as the total number of events expected from the combination of all production mechanisms in the SM. All distributions are shown with the total number of events expected in the SM. The first and last bins of the cΔt distributions include all events beyond |cΔt|> 1000 μm.

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Additional Figure 3-a:
Panel a: Distributions of the four-lepton pT with the selection used in the lifetime analysis without the requirement Dbkg> 0.5 to suppress the background. The points with error bars represent the observed data, and the filled histograms stacked on top of each other represent the expected contributions from the SM backgrounds. Stacked on the total background contribution, the open histograms show the H boson signal either with the combination of all production mechanisms expected in the SM, or for the VBF or t¯tH production mechanisms. Each signal contribution in the different open histograms is normalized to the total number of events expected from the combination of all production mechanisms in the SM. The overflow bin includes all events beyond pT> 200 GeV. Panel b: Distributions of cΔt in the lifetime analysis in the on-shell region for the different H boson production mechanisms, area-normalized to illustrate shape differences. Bottom panels: Distributions of cΔt in the lifetime analysis in the on-shell region for different HVV couplings, including SM couplings, in decay for H boson production through gluon fusion (ggH) (a), and in production and decay for H boson production through weak vector boson fusion (VBF) (b). Panel (b) compares the different cΔt distributions to the SM t¯tH distribution (dotted black). The histograms are shown using variable bins up to |cΔt|> 2000 μm, and the first and last bins include all events outside this region. All distributions are area-normalized to illustrate the shape differences.

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Additional Figure 3-b:
Panel a: Distributions of the four-lepton pT with the selection used in the lifetime analysis without the requirement Dbkg> 0.5 to suppress the background. The points with error bars represent the observed data, and the filled histograms stacked on top of each other represent the expected contributions from the SM backgrounds. Stacked on the total background contribution, the open histograms show the H boson signal either with the combination of all production mechanisms expected in the SM, or for the VBF or t¯tH production mechanisms. Each signal contribution in the different open histograms is normalized to the total number of events expected from the combination of all production mechanisms in the SM. The overflow bin includes all events beyond pT> 200 GeV. Panel b: Distributions of cΔt in the lifetime analysis in the on-shell region for the different H boson production mechanisms, area-normalized to illustrate shape differences. Bottom panels: Distributions of cΔt in the lifetime analysis in the on-shell region for different HVV couplings, including SM couplings, in decay for H boson production through gluon fusion (ggH) (a), and in production and decay for H boson production through weak vector boson fusion (VBF) (b). Panel (b) compares the different cΔt distributions to the SM t¯tH distribution (dotted black). The histograms are shown using variable bins up to |cΔt|> 2000 μm, and the first and last bins include all events outside this region. All distributions are area-normalized to illustrate the shape differences.

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Additional Figure 3-c:
Panel a: Distributions of the four-lepton pT with the selection used in the lifetime analysis without the requirement Dbkg> 0.5 to suppress the background. The points with error bars represent the observed data, and the filled histograms stacked on top of each other represent the expected contributions from the SM backgrounds. Stacked on the total background contribution, the open histograms show the H boson signal either with the combination of all production mechanisms expected in the SM, or for the VBF or t¯tH production mechanisms. Each signal contribution in the different open histograms is normalized to the total number of events expected from the combination of all production mechanisms in the SM. The overflow bin includes all events beyond pT> 200 GeV. Panel b: Distributions of cΔt in the lifetime analysis in the on-shell region for the different H boson production mechanisms, area-normalized to illustrate shape differences. Bottom panels: Distributions of cΔt in the lifetime analysis in the on-shell region for different HVV couplings, including SM couplings, in decay for H boson production through gluon fusion (ggH) (a), and in production and decay for H boson production through weak vector boson fusion (VBF) (b). Panel (b) compares the different cΔt distributions to the SM t¯tH distribution (dotted black). The histograms are shown using variable bins up to |cΔt|> 2000 μm, and the first and last bins include all events outside this region. All distributions are area-normalized to illustrate the shape differences.

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Additional Figure 3-d:
Panel a: Distributions of the four-lepton pT with the selection used in the lifetime analysis without the requirement Dbkg> 0.5 to suppress the background. The points with error bars represent the observed data, and the filled histograms stacked on top of each other represent the expected contributions from the SM backgrounds. Stacked on the total background contribution, the open histograms show the H boson signal either with the combination of all production mechanisms expected in the SM, or for the VBF or t¯tH production mechanisms. Each signal contribution in the different open histograms is normalized to the total number of events expected from the combination of all production mechanisms in the SM. The overflow bin includes all events beyond pT> 200 GeV. Panel b: Distributions of cΔt in the lifetime analysis in the on-shell region for the different H boson production mechanisms, area-normalized to illustrate shape differences. Bottom panels: Distributions of cΔt in the lifetime analysis in the on-shell region for different HVV couplings, including SM couplings, in decay for H boson production through gluon fusion (ggH) (a), and in production and decay for H boson production through weak vector boson fusion (VBF) (b). Panel (b) compares the different cΔt distributions to the SM t¯tH distribution (dotted black). The histograms are shown using variable bins up to |cΔt|> 2000 μm, and the first and last bins include all events outside this region. All distributions are area-normalized to illustrate the shape differences.

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Additional Figure 4:
Observed (solid) and expected (dashed) distributions of 2 ln(L/Lmax) as a function of the nuisance parameter ξprod, controlling the full variation of the H boson cΔt parameterization from the two extremes: production completely through gluon fusion (ξprod=1), and production completely through t¯tH (ξprod=1). The SM mixture of the different H boson production mechanisms corresponds to ξprod=0, and the average H boson lifetime cτH is unconstrained in these likelihood scans. Each pure H boson production scenario through the VBF or VH processes with SM HVV couplings are shown with the vertical dot-dashed lines in color.

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Additional Figure 5-a:
Expected distribution of 2 ln(L/Lmax) as a function of ΓH and fΛQcosϕΛQ with the assumption ϕΛQ=0 or π (a). Panel (b) shows the conditional likelihood scan as a function of fΛQcosϕΛQ for a given ΓH. The likelihood contours are shown for the two-parameter 68% and 95% CLs (a) and for the one-parameter 68% and 95% CLs (b). The black curve with white dots on Panel (b) shows the fΛQcosϕΛQ minima at each ΓH value.

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Additional Figure 5-b:
Expected distribution of 2 ln(L/Lmax) as a function of ΓH and fΛQcosϕΛQ with the assumption ϕΛQ=0 or π (a). Panel (b) shows the conditional likelihood scan as a function of fΛQcosϕΛQ for a given ΓH. The likelihood contours are shown for the two-parameter 68% and 95% CLs (a) and for the one-parameter 68% and 95% CLs (b). The black curve with white dots on Panel (b) shows the fΛQcosϕΛQ minima at each ΓH value.

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Additional Figure 6-a:
Observed (solid) and expected (dashed) distributions of 2 ln(L/Lmax) as a function of offshell μggH or μVVH (a), and observed distributions of 2 ln(L/Lmax) as a function of onshell (solid) and offshell (dashed) μggH (blue) or μVVH (red) (b). The fΛQ=0 is assumed, and onshell and offshell μ parameters are alternative parameterizations from the overall μ and ΓH. Panel (c) shows the observed minima of the overall μggH (blue) and μVVH (orange) at each fΛQcosϕΛQ value when the constraint ΓH=ΓSMH is applied.

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Additional Figure 6-b:
Observed (solid) and expected (dashed) distributions of 2 ln(L/Lmax) as a function of offshell μggH or μVVH (a), and observed distributions of 2 ln(L/Lmax) as a function of onshell (solid) and offshell (dashed) μggH (blue) or μVVH (red) (b). The fΛQ=0 is assumed, and onshell and offshell μ parameters are alternative parameterizations from the overall μ and ΓH. Panel (c) shows the observed minima of the overall μggH (blue) and μVVH (orange) at each fΛQcosϕΛQ value when the constraint ΓH=ΓSMH is applied.

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Additional Figure 6-c:
Observed (solid) and expected (dashed) distributions of 2 ln(L/Lmax) as a function of offshell μggH or μVVH (a), and observed distributions of 2 ln(L/Lmax) as a function of onshell (solid) and offshell (dashed) μggH (blue) or μVVH (red) (b). The fΛQ=0 is assumed, and onshell and offshell μ parameters are alternative parameterizations from the overall μ and ΓH. Panel (c) shows the observed minima of the overall μggH (blue) and μVVH (orange) at each fΛQcosϕΛQ value when the constraint ΓH=ΓSMH is applied.
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