DOI 10.1393/ncc/i2017-17030-9 Colloquia: IFAE 2016
IL NUOVO CIMENTO 40 C (2017) 30
A discussion of H
→ ZZ → 4 results and analysis strategies
for 13 TeV
Muhammad Bilal Kiani(1)(2)
(1) INFN, Sezione di Torino - Torino, Italy
(2) Dipartimento di Fisica, Universit`a degli studi di Torino - Torino, Italy
received 17 October 2016
Summary. — Studies of Higgs boson production are presented using the H →
ZZ→ 4 ( = e, μ) decay. These studies are performed using a data sample
corre-sponding to an integrated luminosity of 2.8 fb−1of pp collisions at a center-of-mass energy of 13 TeV collected by the CMS experiment at the LHC during 2015. The observed significance for the standard model Higgs boson with mH= 125.09 GeV is
2.5σ, where the expected significance is 3.4σ. The model-independent fiducial cross section is measured to be σfid= 2.48+1.48−1.14(stat⊕ sys)+0.01−0.04(model dep.) fb.
1. – Description
The studies in various Higgs decay channels and production modes with the full LHC Run 1 [1] data set and combined measurements from ATLAS and CMS showed that the properties of the new boson are so far consistent with expectations for the SM Higgs.
The start of the LHC Run 2 in 2015, at an increased center-of-mass energy of√s = 13 TeV, aimed to rediscover the Higgs boson which opens the way for an era of new precision measurements of the Higgs boson, which will involve the observation and study of its rare production modes such as vector boson fusion (VBF) and associated production with a vector boson (WH, ZH) and top pair production ttH.
2. – Results
The H→ ZZ → 4 analysis is based on the reconstruction, identification and isolation of leptons. The event selection is designed to extract signal candidates from events containing at least four well-identified and isolated leptons, each originating from the primary vertex. First, Z candidates are formed with pairs of leptons of the same flavor and opposite-charge and required to pass 12 < ml+l− < 120 GeV. They are then combined
into ZZ candidates, wherein we denote as Z1 the Z candidate with an invariant mass closest to the nominal Z boson mass, and as Z2 the other one. The details of the event Creative Commons Attribution 4.0 License (http://creativecommons.org/licenses/by/4.0) 1
2 MUHAMMAD BILAL KIANI (GeV) l 4 m 80 100 200 300 400 500 Events / 4 GeV 0 2 4 6 8 10 12 14 Data H(125) * γ ZZ, Z → q q * γ ZZ, Z → gg Z+X 600 800 (13 TeV) -1 2.8 fb CMSPreliminary (GeV) l 4 m 80 100 120 140 160 180 Events / 4 GeV 0 2 4 6 8 10 12 14 Data H(125) * γ ZZ, Z → q q * γ ZZ, Z → gg Z+X (13 TeV) -1 2.8 fb CMSPreliminary
Fig. 1. – Left: Distribution of the four-lepton invariant mass m4l in the full mass range and in
the low-mass region. Right: Points with error bars represent the data and stacked histograms represent expected distributions.
selection can be found in [2]. Figure 1 shows the distributions obtained from this selection.
To extract the signal significance for the excess of events observed in the Higgs peak region and estimate its signal strength, a multi-dimensional fit is performed that relies on two variables: the four-lepton invariant mass m4l and the Dbkgkin discriminant (which separates the gluon fusion from other production modes). The results for the significance of excess and the signal strength modifiers are shown in fig. 2.
The fiducial volume is defined in [3]. Integrated fiducial cross section for pp→ H → 4 is obtained by performing a maximum likelihood fit of the signal and background parameterizations to the observed 4 mass distribution. The results are shown in fig. 3.
(GeV) H m 118 120 122 124 126 128 130 -valuep local 5 − 10 4 − 10 3 − 10 2 − 10 1 − 10 1 σ 1 σ 2 σ 3 σ 4 Observed 4e μ Observed 4 μ Observed 2e2 Observed combined Expected (13 TeV) -1 2.8 fb CMSPreliminary H t ggH,t μ 0 0.5 1 1.5 2 2.5 3 3.5 4 VBF,VH μ 0 2 4 6 8 10 12 14 16 18 20 68% C.L. 95% C.L. best fit SM (13 TeV) -1 2.8 fb CMSPreliminary
Fig. 2. – Left: Significance of the local fluctuation with respect to the SM expectation as a function of the Higgs boson mass. Dashed lines show the mean expected significance of the SM Higgs boson for a given mass hypothesis. Right: Result of the 2D likelihood scan for the μF and μV signal-strength modifiers. The solid and dashed contours show the 68% and 95% CL regions, respectively. The cross indicates the best-fit values, and the diamond represents the expected values for the SM Higgs boson.
A DISCUSSION OF H→ ZZ → 4 RESULTS AND ANALYSIS STRATEGIES FOR 13 TeV 3 4l 2e2μ 4μ 4e [fb] fid. σ 0 1 2 3 4 5 (13 TeV) -1 2.8 fb CMSPreliminary sys. unc.) ⊕ Data (stat. Systematic uncertainty Model dependence H (HRes) + XH → gg XH = VBF + VH + ttH (powheg) (TeV) s 7 8 9 10 11 12 13 14 [fb] fid σ 0 1 2 3 4 5 6 sys. unc.) ⊕ Data (stat. Systematic uncertainty Model dependence = 125 GeV) H Standard model (m (13 TeV) -1 (8 TeV), 2.8 fb -1 (7 TeV), 19.7 fb -1 5.1 fb CMSPreliminary 4l) + X → (H → pp
Fig. 3. – Left: Result of measured fiducial cross section in each final state. Right: The measured fiducial cross section as a function center-of-mass energy√s.
REFERENCES
[1] The CMS Collaboration, Measurement of the properties of a Higgs boson in the
four-lepton final state, arXiv:1312.5353v3 [hep-ex].
[2] The CMS Collaboration, Studies of Higgs boson production in the four-lepton final state
at√s = 13 TeV, CMS-PAS-HIG-15-004.
[3] The CMS Collaboration, Measurement of differential and integrated fiducial cross
sections for Higgs boson production in the four-lepton decay channel in pp collisions at √
