Search for a Standard Model Higgs boson in the decay channel H to ZZ(∗) to 4l

This is a condensed description with plots and additional supporting material for the analysis described in the CMS Physics Analysis Summary (PAS) paper: CMS-HIG-11-015

The analysis which was used for a previous CMS public release in the CMS-HIG-11-004 PAS presented on Hig11004TWiki is repeated here with a higher integrated luminosity of 1.66 fb-1. The results presented here updates and replaces those previous results.

Abstract

A search for a Higgs boson in the decay channel H → ZZ(∗) with each Z boson decaying to an electron or muon pair is presented using pp collisions from the LHC at sqrt(s)= 7 TeV. The data analyzed correspond to an integrated luminosity of 1.66 ± 0.07 fb−1 recorded by the CMS detector in 2010 and 2011. The search covers Higgs boson mass (mH) hypotheses of 110 < mH < 600 GeV/c2 with an expected sensitivity varying between one and five times the standard model cross section at 95% CL over most of the mass range. Twenty one events are observed, while 21.2 ± 0.8 events are expected from standard model background processes. Six of the events are below the kinematic threshold of two on-shell Z's (m_H < 180 GeV/c2), while 2.8 ± 0.2 background events are expected. The events are not clustered in mass excluding interpretation as the standard model Higgs boson. The distribution of events is consistent with the expectation of standard model continuum production of ZZ(∗) pairs and the production cross section for 60 < mZ < 120 GeV/c2 is measured to be in agreement with the predicted value. Upper limits at 95% CL on the cross section×branching ratio for a Higgs boson with standard model-like decays exclude cross sections from about one to two times the expected standard model cross section for masses in the range 150 < mH < 420 GeV/c2. Reinterpreted in the context of the standard model with four fermion families a Higgs boson with a mass in the range 120-520 GeV/c2 is excluded at 95% CL.

Main Results

Simple sequential sets of lepton reconstruction, identification and isolation cuts and a set of kinematic cuts have been introduced to define a common baseline for the search at any Higgs boson mass mH in the range 100 < mH < 600 GeV/c2. The instrumental background from Z+jets and the reducible backgrounds from Zb\bar{b} and t\bar{t}, with misidentified primary leptons, are shown to be negligible over most of the mass, with a small contamination remaining at low mass.

Twenty one events are observed in the 2e2mu, 4e and 4mu channels for an integrated luminosity of 1.66 ± 0.07 fb−1, while 21.2 ± 0.8 events are expected from standard model background processes. The distribution of events is compatible with the expectation from the standard model continuum production of Z boson pairs from q\bar{q} annihilation and gg fusion. No clustering of events is observed in the measured m4l mass spectrum. Six of the events are below the kinematic threshold of two on-shell Z's, while 2.8 ± 0.2 background events are expected. The probability that the background fluctuates to the observed number if events is 6.5%. Using the high-mass selection which contains eight events, a total cross section for a pair of Z bosons in the mass range 60 < m_Z < 120 GeV/c2 has been measured to be in agreement with the predicted value.

Upper limits obtained at 95% CL on the cross section×branching ratio for a Higgs boson with standard model-like decays exclude cross sections from about one to two times the expected standard model cross section for masses in the range 150 < mH < 420 GeV/c2. Upper limits obtained in the context the standard model with a fourth fermion generation, exclude a Higgs boson with a mass in the ranges 120 - 520 GeV/c2 at 95% CL.

Figures from HIG-11-004

Figure	Label	Description
	Figure 5 a) pdf, eps, png	The mean expected and the observed upper limits at 95% C.L. on σ(pp → H + X) × B(ZZ → 4l) for a Higgs boson in the mass range 120-600 GeV/c2, for an integrated luminosity of 1.13 fb−1 using the CLs approach. The expected ratios for the SM is presented. The results are obtained using a shape analysis method.

Figure 5 b)pdf, eps, png

The mean expected and the observed upper limits at 95% C.L. on σ(pp → H + X) × B(ZZ → 4l) for a Higgs boson in the mass range 120-600 GeV/c2, for an integrated luminosity of 1.13 fb−1 using the CLs approach. The expected ratios for the SM with a fourth-fermion family (SM4) is presented. The results are obtained using a shape analysis method.

Figure 3 d) pdf,eps,png

Distribution of the four-lepton reconstructed mass for the baseline selection in the sum of the 4l channels. Points represent the data, shaded histograms represent the signal and background expectations. The samples correspond to an integrated luminosity of L = 1.66 fb−1

Figure 4 pdf,eps,png

Distribution of the four-lepton reconstructed mass for the sum of the 4l channels in the high-mass selection. Points represent the data, shaded histograms represent the signal and background expectations. The samples correspond to an integrated luminosity of L = 1.66 fb−1.

Figure 3 a) pdf,eps,png

Distribution of the four-lepton reconstructed mass for the baseline selection in the 4e channel. Points represent the data, shaded histograms represent the signal and background expectations. The samples correspond to an integrated luminosity of L = 1.66 fb−1

Figure 3 b) pdf,eps,png

Distribution of the four-lepton reconstructed mass for the baseline selection in the 4mu channel. Points represent the data, shaded histograms represent the signal and background expectations. The samples correspond to an integrated luminosity of L = 1.66 fb−1

Figure 3 c) pdf,eps,png

Distribution of the four-lepton reconstructed mass for the baseline selection in the 2e2mu channel. Points represent the data, shaded histograms represent the signal and background expectations. The samples correspond to an integrated luminosity of L = 1.66 fb−1

Figure 2 pdf,eps,png

Distribution of the best reconstructed Z candidate invariant mass for the events in the four-lepton background control region defined by a pair of identified leptons with opposite charge and matching flavour and another pair of leptons with isolation cut relaxed, flavour and charge requirements removed and large impact parameters. Solid points represent the data, shaded histograms represent the MC expectations, the signal and the ZZ background contribute negligibly. The samples correspond to an integrated luminosity of L = 1.66 fb−1

Figure 6 a) pdf,eps,png

Event yields in the 4e channel as a function of the event selection steps. Black points with uncertainties represent the data, other symbols represent the MC expectations The samples correspond to an integrated luminosity of L = 1.66 fb−1

Figure 6 b) pdf,eps,png

Event yields in the 4mu channel as a function of the event selection steps. Black points with uncertainties represent the data, other symbols represent the MC expectations The samples correspond to an integrated luminosity of L = 1.66 fb−1

Figure 6 c) pdf,eps,png

Event yields in the 2e2mu channel as a function of the event selection steps. Black points with uncertainties represent the data, other symbols represent the MC expectations The samples correspond to an integrated luminosity of L = 1.66 fb−1

Figure 1 a) pdf,eps,png

Comparison between data and MC at an early stage of the event selection where the four lepton combination is chosen, and the lepton pairs are assigned to the Z1 and Z2 bosons. The full selection requirements are applied to the leptons assigned to the Z1. The plots show from left to right the reconstructed mass mZ1, the reconstructed mass mZ2 and the reconstructed four-lepton invariant mass for the 4e channel. Points with statistical uncertainties represent the data. Shaded histograms represent the MC expectations. The samples correspond to an integrated luminosity of L = 1.66 fb−1

Figure 1 b) pdf,eps,png

Comparison between data and MC at an early stage of the event selection where the four lepton combination is chosen, and the lepton pairs are assigned to the Z1 and Z2 bosons. The full selection requirements are applied to the leptons assigned to the Z1. The plots show from left to right the reconstructed mass mZ1, the reconstructed mass mZ2 and the reconstructed four-lepton invariant mass for the 4mu channel. Points with statistical uncertainties represent the data. Shaded histograms represent the MC expectations. The samples correspond to an integrated luminosity of L = 1.66 fb−1

Figure 1 c) pdf,eps,png

Comparison between data and MC at an early stage of the event selection where the four lepton combination is chosen, and the lepton pairs are assigned to the Z1 and Z2 bosons. The full selection requirements are applied to the leptons assigned to the Z1. The plots show from left to right the reconstructed mass mZ1, the reconstructed mass mZ2 and the reconstructed four-lepton invariant mass for the 2e2mu channel. Points with statistical uncertainties represent the data. Shaded histograms represent the MC expectations. The samples correspond to an integrated luminosity of L = 1.66 fb−1

Table 1 pdf, eps, png

Event yields in the (a) 4e, (b) 4mu and (c) 2e2mu channel for the trigger and the seven event selection steps (see text) with steps three and four regrouped as "4l" for the choice of the best four leptons. The samples correspond to an integrated luminosity of L = 1.66 fb−1

Table 2 pdf, eps, png

Number of ZZ background events and relative uncertainties in the signal region in a mass range from 100 to 600 GeV/c2, estimated from normalization to the measured Z rate and from Monte Carlo simulation, for baseline and high-mass event selections.

Table 3 pdf, eps, png

Number of background events and relative uncertainties for baseline and high-mass event selections in the signal region in a m4l range from 100 to 600 GeV/c2, estimated from data as described in the text. Upper three rows: tt and Z bb /cc estimated from the control region with inverted SIP3D, relaxed isolation, charge and flavour requirements for two leptons. Lower three rows: Z+jets estimated from the control region with relaxed isolation and identification requirements for two leptons.

Table 4 pdf, eps, png

Summary of the magnitude of systematic uncertainties in percent. The uncertainties assigned for the lepton reconstruction, identification and isolation apply to the event yields. The uncertainty assigned to the electron energy scale is further propagated through the shape of the expected signal and background reconstructed mass distributions.

Table 5 pdf, eps, png

Number of events observed, background and signal rates for each final state in a mass range from m1 = 100 GeV/c2 to m2 = 600 GeV/c2 both for the baseline and high-massselections. For ZZ, Z+jets, tt and Zbb /cc the data driven estimations are used, for WZ the Monte Carlo estimation is used.

Table 6 pdf, eps, png

Properties of the four-lepton combinations satisfying the baseline selection for the Higgs boson search.

Additional Figures

Figure	Label	Description
	pdf, eps, png	The mean expected and the observed upper limits at 95% C.L. on σ(pp → H + X) × B(ZZ → 4l) for a Higgs boson in the mass range 120-600 GeV/c2, for an integrated luminosity of 1.13 fb−1 using the CLs approach. The results are obtained using a shape analysis method.

pdf, eps, png

The mean expected and the observed upper limits at 95% C.L. on σ(pp → H + X) × B(ZZ → 4l) for a Higgs boson in the mass range 110-200 GeV/c2, for an integrated luminosity of 1.16 fb−1 using the CLs approach. The results are obtained using a shape analysis method.

pdf, eps, png

Comparison between data and MC for the reconstructed mass mZ1. The full set of selection requirements is applied to the leptons of the pair assigned to the Z1 boson and the other pair of leptons is left entirely opened, i.e. no flavour or charge requirements and no vertex or isolation constraints is applied to the pair of leptons assigned to the Z2 boson. The samples correspond to an integrated luminosity of L = 1.66 fb−1

pdf, eps, png

Comparison between data and MC for the reconstructed mass mZ1. The full set of selection requirements is applied to the leptons of the pair assigned to the Z1 boson and the other pair of leptons is left entirely opened, i.e. no flavour or charge requirements and no vertex or isolation constraints is applied to the pair of leptons assigned to the Z2 boson. The samples correspond to an integrated luminosity of L = 1.66 fb−1

pdf, eps, png

Comparison between data and MC for the reconstructed four-lepton invariant mass. The full set of selection requirements is applied to the leptons of the pair assigned to the Z1 boson and the other pair of leptons is left entirely opened, i.e. no flavour or charge requirements and no vertex or isolation constraints is applied to the pair of leptons assigned to the Z2 boson. The samples correspond to an integrated luminosity of L = 1.66 fb−1

pdf, eps, png

Comparison between data and MC for the impact parameter significance lowest SIP3D between the additional leptons in a Z1+2l sample. The full set of selection requirements is applied to the leptons of the pair assigned to the Z1 boson and the other pair of leptons is left entirely opened, i.e. no flavour or charge requirements and no vertex or isolation constraints is applied to the pair of leptons assigned to the Z2 boson. The samples correspond to an integrated luminosity of L = 1.66 fb−1

pdf, eps, png

Comparison between data and MC for the impact parameter significance highest SIP3D between the additional leptons in a Z1+2l sample. The full set of selection requirements is applied to the leptons of the pair assigned to the Z1 boson and the other pair of leptons is left entirely opened, i.e. no flavour or charge requirements and no vertex or isolation constraints is applied to the pair of leptons assigned to the Z2 boson. The samples correspond to an integrated luminosity of L = 1.66 fb−1

pdf, eps, png

Comparison between data and MC for the sum of the combined relative isolation for the additional leptons in a Z1+2l sample. The full set of selection requirements is applied to the leptons of the pair assigned to the Z1 boson and the other pair of leptons is left entirely opened, i.e. no flavour or charge requirements and no vertex or isolation constraints is applied to the pair of leptons assigned to the Z2 boson. The samples correspond to an integrated luminosity of L = 1.66 fb−1

pdf,eps,png

Distribution of the best reconstructed Z candidate invariant mass for the events in the four-lepton background control region defined by a pair of identified leptons with opposite charge and matching flavour and another pair of leptons with isolation cut relaxed, flavour and charge requirements removed and large impact parameters. Additional requirements on the invariant masses as defined in the ”best 4l candidate” step of the preselection. Solid points represent the data, shaded histograms represent the MC expectations, the signal and the ZZ background contribute negligibly. The samples correspond to an integrated luminosity of L = 1.66 fb−1

Event Displays

Figure	Label	Description
	png, pdf	Candidate C 2e2u
	png, pdf	Candidate D 4e
	png, pdf	Candidate F 4u