Constraints on the Higgs boson width from off-shell production and decay to ZZ→4l or 2l2ν

Table of contents:

Abstract

We constrain the total Higgs boson width, ΓH, using off-shell production and decay to four leptons, 4l, or two leptons plus two neutrinos, 2l2ν with l = e,μ. The measurement is based on the data collected in 2012 by the CMS experiment at the LHC, corresponding to an integrated luminosity L=19.7 fb-1 at √s = 8 TeV. The 4l analysis makes use of the ZZ invariant mass distribution and of a matrix element likelihood discriminant to separate the ZZ components originating from gluon- and quark-initiated processes, while the 2l2ν analysis relies on the transverse mass distribution or missing transverse energy distribution in jet categories. An unbinned-maximum likelihood fit of the above distributions combined with the 4l measurement near the resonance peak leads to an upper limit on the Higgs boson width of ΓH < 4.2*ΓH(SM) at the 95% confidence level, assuming ΓH(SM) = 4.15 MeV. This result considerably improves over previous experimental constraints from the measurement near the resonance peak.

ZZ→4l

Diboson Invariant Mass Distributions

Figure Description
Distribution of the four-lepton reconstructed mass in full analysis mass range for the sum of the 4e, 4μ, and 2e2μ channels. Points represent the data, shaded histograms represent the background and dotted-shaded histogram the gg+VV→ZZ expectations. The expected distributions are presented as stacked histograms. The measurements are presented for of the data collected at √s = 8 TeV. [100-800] GeV range and with variable bin width.
Distribution of the four-lepton reconstructed mass in full analysis mass range for the sum of the 4e, 4μ, and 2e2μ channels. Points represent the data, shaded histograms represent the background and dotted-shaded histogram the gg+VV→ZZ expectations. The expected distributions are presented as stacked histograms. The measurements are presented for of the data collected at √s = 8 TeV. [220-1600] GeV range and with variable bin width. Last bin includes overflow events
Distribution of the four-lepton reconstructed mass in full analysis mass range with a requirement on the MELA discriminant Dgg > 0.65 to reject the ZZ background for the sum of the 4e, 4μ, and 2e2μ channels. Points represent the data, shaded histograms represent the background and dotted-shaded histogram the gg+VV→ZZ expectations. The expected distributions are presented as stacked histograms. The measurements are presented for of the data collected at √s = 8 TeV. [220-1600] GeV range and with variable bin width. Last bin includes overflow events.
Distribution of the four-lepton reconstructed mass in full analysis mass range with a requirement on the MELA discriminant Dgg < 0.5 to reject the gg+VV→ZZ signal for the sum of the 4e, 4μ, and 2e2μ channels. Points represent the data, shaded histograms represent the background and dotted-shaded histogram the gg+VV→ZZ expectations. The expected distributions are presented as stacked histograms. The measurements are presented for of the data collected at √s = 8 TeV. [220-1600] GeV range and with variable bin width. Last bin includes overflow events.
Distribution of the expected four-lepton reconstructed mass in full analysis mass range for the sum of the 4e, 4μ, and 2e2μ channels and for gg+VV→ZZ processes. The expected distribution for a scenario corresponding to a scaling of the width by 25 together with mu=1 is also shown. This illustrates the expected change in gg+VV→ZZ production when changing the Higgs width and at the same time constraining the peak cross section to the SM expectation.
Distribution of the generated four-lepton cross-section in mass range [100, 500] GeV for the gg→ZZ2e2μ channel with different couplings and width scaling. Under a scaling of the width only, the peak yield decreases by the scaling factor while the high mass part of the distribution stays unchanged. When scaling both the width and the coupling product squared, the yield in the peak is kept unchanged while an increase of the yield in the off-shell region by this scaling factor is expected.

MELA Discriminant Distributions

Figure Description
Distribution of the MELA discriminant Dgg in full analysis mass range for the sum of the 4e, 4μ, and 2e2μ channels. Points represent the data, shaded histograms represent the background and dotted-shaded histogram the gg+VV→ZZ expectations. The expected distributions are presented as stacked histograms. The measurements are presented for of the data collected at √s = 8 TeV. [220-1600] GeV range and with 0.05 bin width.
Distribution of the MELA discriminant Dgg in [330-1600] mass range for the sum of the 4e, 4μ, and 2e2μ channels. Points represent the data, shaded histograms represent the background and dotted-shaded histogram the gg+VV→ZZ expectations. The expected distributions are presented as stacked histograms. The measurements are presented for of the data collected at √s = 8 TeV. [220-1600] GeV range and 0.05 bin width.
Distribution of the MELA discriminant Dgg in [220-330] GeV mass range to reject the gg+VV→ZZ signal for the sum of the 4e, 4μ, and 2e2μ channels. Points represent the data, shaded histograms represent the background and dotted-shaded histogram the gg+VV→ZZ expectations. The expected distributions are presented as stacked histograms. The measurements are presented for of the data collected at √s = 8 TeV. [220-1600] GeV range and 0.05 bin width.

Yields

Expected and observed number of events for m4l > 220 GeV per channel and for the sum of the 4e, 4μ and 2e2μ channels. The numbers for the gg signal and the total gg contribution are given for μ=1 (SM). The column ``signal enriched'' has instead cuts of m4l > 330 GeV and Dgg > 0.65. The numbers correspond to 8 TeV data only. VBF errors are negligible with respect to the corresponding gg sources, so they are not reported.

Distributions of the inputs to the MELA discriminant

Figure Description
Distribution of the production angle cos\theta* defined in the ZZ center-of-mass frame, used as input to the MELA discriminant Dgg. Left plot shows full selected sample with mZZ >220 GeV while the right plots shows restricted range mZZ >330 GeV.
Distribution of the two Z boson invariant masses m1 and m2, summed together. These observables are used as input to the MELA discriminant Dgg. Left plot shows full selected sample with mZZ >220 GeV while the right plots shows restricted range mZZ >330 GeV.
Distribution of the Z decay helicity angles cos\theta1 and cos\theta2, summed together, defined in the each Z center-of-mass frame. These observables are used as input to the MELA discriminant Dgg. Left plot shows full selected sample with mZZ >220 GeV while the right plots shows restricted range mZZ >330 GeV.
Distribution of angle Phi between the two Z decay planes defined in the ZZ center-of-mass frame, used as input to the MELA discriminant Dgg. Left plot shows full selected sample with mZZ >220 GeV while the right plots shows restricted range mZZ >330 GeV.
Distribution of angle Phi1 defined in the ZZ center-of-mass frame, used as input to the MELA discriminant Dgg. Left plot shows full selected sample with mZZ >220 GeV while the right plots shows restricted range mZZ >330 GeV.

2D event distributions

Figure Description
Distribution of Dgg vs mZZ with points representing the data. Shaded distribution represents the expected event density in the SM on the left plot, while this density is normalized to unity at each value of mZZ on the right plot.
Distribution of Dgg vs mZZ in a restricted range to enhance the background with points representing the data. Shaded distribution represents the expected event density in the SM on the left plot, while this density is normalized to unity at each value of mZZ on the right plot.
Distribution of Dgg vs mZZ in a restricted range to enhance the signal with points representing the data. Shaded distribution represents the expected event density in the SM on the left plot, while this density is normalized to unity at each value of mZZ on the right plot.

1D results

  • Note that the plots that were previously posted, indicating also the expected result with "no systematics", have been removed as the "w/o systematics" curves were
obtained fixing the on-peak μ to 1 with no statistical uncertainty either.

Figure Description
Likelihood scan using Dgg information only. black solid observed with μ=0.93, black dashed expected with μ=1, red expected with μ=0.93. The yellow (green) band show the 95% (68%) CL on the expected distribution with mu=0.93 from toys.
Likelihood scan using m4l information only. black solid observed with μ=0.93, black dashed expected with mu=1, red expected with μ=0.93. The yellow (green) band show the 95% (68%) CL on the expected distribution with μ=0.93 from toys.

2D results

  • Note that the plots that were previously posted, indicating also the expected result with "no systematics", have been removed as the "w/o systematics" curves were obtained fixing the on-peak μ to 1 with no statistical uncertainty either.

Figure Description
Likelihood scan of the Γ/ΓSM. Black solid observed with μ=0.93, black dashed expected with μ=1, red expected with μ=0.93. The yellow (green) band show the 95% (68%) CL on the expected distribution with μ=0.93 from toys. The p-value for the observation of a limit as tight as the one observed under the assumption μ=0.93 is 0.13
Likelihood scan of the Γ/ΓSM. Black solid observed with μ=0.93, black dashed expected with μ=1. The yellow (green) band show the 95% (68%) CL on the expected distribution with μ=0.93 from toys. The p-value for the observation of a limit as tight as the one observed under the assumption μ=0.93 is 0.13
channel splitting of the observed (solid) and expected (dashed) Γ/ΓSM with μ=0.93. black is with μ=0.93 for the combination of all channels, Red for the 2e2μ channel, blue for 4e, green for 4μ.
channel splitting of the observed Γ/ΓSM with μ=0.93. black is with μ=0.93 for the combination of all channels, Red for the 2e2μ channel, blue for 4e, green for 4μ.
channel splitting of the expected Γ/ΓSM with mu=0.93. black is with μ=0.93 for the combination of all channels, Red for the 2e2μ channel, blue for 4e, green for 4μ.

ZZ→2l2ν

Yields

Figure Description
Yields variation with respect to the SM r=1 senario for each component entering the signal model. The absolute variation of each component and of the total expectations is shown for the gluon-gluon (left) and VBF (right) production modes.

Event yields expected for the different signal components after baseline selection assuming r = 1.


Event yields expected for background processes and observed data. Statistical uncertainties are shown for all backgrounds. The systematic uncertainty is shown for backgrounds derived from data. Processes which correspond with < 0.1% to the total event yields expected are omitted for simplicity.


Event yields in a signal-enriched region expected for the signal and background processes and observed in the data for the ee and μμ channels. The abbreviation gg is used to denote the process gg→ZZ→2l2ν. The abbreviation VBF is used to denote the process qq→ZZqq→2l2νqq. The signal-enriched region is defined by ETmiss > 100 GeV and mT > 350 GeV.


Discriminant Distributions

Comparison between the expected and observed mT and ETmiss distributions in the different event categories. The expected distributions from the different background processes are stacked on top of each other. The shapes of inclusive process (SBI) of gg→ZZ→2l2ν and qq→ZZqq→2l2νqq for a r=10 scenario are superimposed. Dielectron (dimuon) events are shown on top (bottom). 0-jet, at least 1 jet and VBF events are shown on the left, middle and right.

Systematics

Summary of all systematic uncertainties on final event yields. The input uncertainty is quoted.


Results

Figure Description
Scan of the negative log-likelihood, as a function of r = Γ/ΓSM for the 2l2ν analysis. The green and yellow bands correspond repectively to 68% and 95% quantiles of the distribution of the negative log-likelihood at the corresponding value on the dashed red line in MC pseudo-experiments.
Results of shape analysis using ee events only. (μobs is used)
Results of shape analysis using μμ events only. (μobs is used)
Results of shape analysis using 0-jet category events only. (μobs is used)
Results of shape analysis using at least 1 jet category events only. (μobs is used)
Results of shape analysis using VBF category events only. (μobs is used)
Results of cut-and-count analysis using signal-enriched region events. (μobs is used)

Combined results

  • Note that the plots that were previously posted, indicating also the expected result with no systematics, have been removed as the "w/o systematics" curves were obtained fixing the on-peak μ to 1 with no statistical uncertainty either.

Figure Description
Likelihood scan of the Γ/ΓSM. Black solid observed with μ=0.93, black dashed expected with μ=0.93, red dashed expected with μ=1. The yellow (green) band show the 95% (68%) CL on the expected distribution with μ=0.93 from toys. The p-value for the observation of a limit as tight as the one observed under the assumption μ=0.93 is 0.02

Expected and observed 95% CL limits for the 4l and 2l2ν analyses and for the combination. For the observed results, the central fitted values and the 68% CL total uncertainties are also quoted. All quoted values are obtained using the observed value of μ.


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Topic revision: r3 - 2014-04-23 - JimOlsen
 
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