Analysis strategy for the SM Higgs boson search in the four-lepton final state in CMS

The current status of the searches for the SM Higgs boson in the $H$$\rightarrow$$ZZ^{(*)}$$\rightarrow$$4\ell$ decay channel with the CMS experiment is presented. The selection cuts for suppressing the backgrounds while keeping very high signal efficiencies are described, along with the data-driven algorithms implemented to estimate the background yields and the systematic uncertainties. With an integrated luminosity of $1.66 \mathrm{fb}^{-1}$, upper limits at 95% CL on the SM-like Higgs cross section $\times$ branching ratio exclude cross sections from about one to two times the expected value from the Standard Model in the range $150<m_{H}<420 \mathrm{GeV}$. No evidence for the existence of the SM Higgs boson has been found so far.


Signal and backgrounds
The H → ZZ → 4 analysis described here considers three final states (4e, 4µ, 2e2µ) over a Higgs boson mass range 100 < m H < 600 GeV/c 2 . The signature of signal events is very clean: two pairs of same-flavour, opposite-charge, high-p T isolated leptons pointing to the same reconstructed vertex are looked for. Mass constraints can be set on the dilepton invariant mass: also at low m H , at least one Z boson is on-shell, therefore at least one pair of leptons has m + − m Z .
The ZZ → 4 background is often referred to as the irreducible one, because of its signal-like kinematics. Since the SM Higgs boson is a scalar particle, angular correlations among the final-state leptons can be exploited to discriminate between the signal and the ZZ background.
The reducible backgrounds are Zbb, Zcc, tt, with heavyflavour quarks decaying semileptonically. Leptons originating from these decays usually point to displaced vertices and they are soft and not isolated. The instrumental backgrounds are processes with jets misidentified as leptons, such as Z + jets, W + jets, QCD events.

Event selection
The analysis consists of a set of cuts aiming to reduce the background contributions by preserving a high signal efficiency, as described in [2] [3] [4].

-Requirements on muons and electrons
First of all, electrons and muons must satisfy some trigger, reconstruction and identification requirements. Depending on the trigger menu deployed during data taking, reconstructed leptons are required to be matched to online trigger objects passing a single-or doublelepton trigger selection. They also have to pass loose p T and isolation cuts. a e-mail: alberto.graziano@cern.ch -First Z candidate selection The first Z candidate is defined as the one with the dilepton invariant mass closest to the nominal Z mass, in the mass window 60 < m + − < 120 GeV/c 2 , after a selection including cuts on the p T of both leptons, on their isolation and on the significance of their 3D impact parameter with respect to the primary event vertex.
The presence of a third high-quality lepton of any flavour and charge is required. At this stage of the selection the phase space of the main reducible backgrounds is preserved for data-driven background estimation and control.

arXiv:1202.1746v1 [hep-ex] 8 Feb 2012
EPJ Web of Conferences The requirement of a fourth lepton with matching flavour and opposite charge with respect to the third one is added.
-'Best 4 candidate' selection The second Z candidate is reconstructed from the two highest-p T leptons not associated to Z 1 and passing m Z 2 > 12 GeV/c 2 . At this stage the ambiguity due to combinatorics in events with extra fake leptons is limited and the 'best 4 candidate' is chosen. The 4 candidate must satisfy m 4 > 100 GeV/c 2 . Moreover, in the 4e and 4µ final states only, at least three out of the four possible + − combinations are required to have m + − > 12 GeV/c 2 in order to reject background events with leptonic J/ψ decays.
-Cut on relative lepton isolation The two leptons with the largest isolation variable, which is the sum of tracker, ECAL and HCAL isolation divided by the lepton p T , are then considered. The sum of their isolation values is required to be lower than a threshold.
-Cut on the 3D impact parameter significance of leptons The displaced vertex of leptons originating from b-quark decays can be a handle for further background rejection. A cut is therefore applied on the significance of the 3D impact parameter of the lepton track with respect to the reconstructed primary vertex. This significance is defined as S IP 3D = IP 3D /σ IP 3D and its absolute value is required to be less than 4 for all selected leptons.
-Cut on Z 1 , Z 2 kinematics Finally, additional constraints are imposed on the p T of the selected leptons (p 1 , 2 ,µ 3 ,µ 4 T > 20, 10, 5, 5 GeV/c for muons, p 1 , 2 ,e 3 ,e 4 T > 20, 10, 7, 7 GeV/c for electrons) and on the invariant mass of the second Z candidate, which is required to be 20 < m Z 2 < 120 GeV/c 2 in the baseline selection and 60 < m Z 2 < 120 GeV/c 2 in the high mass selection. The m 4 distribution for events passing the baseline selection is shown in Fig. 2.

ZZ → 4 cross section measurement
The ZZ → 4 inclusive cross section has been measured after the high mass selection cuts 60 < m Z 1 < 120 GeV/c 2 , 60 < M Z 2 < 120 GeV/c 2 as where the sum runs over the three final states (4e, 4µ, 2e2µ). This result should be compared with the theoretical value:

ZZ background control
The contribution from ZZ background, which is the main one after the whole event selection, can be estimated from the number of Z → 2 events observed in data with the following formula: This method exploits the fact that most Feynman diagrams are shared by the two processes. The results are compatible with those obtained directly from MC, but the systematic uncertainties are smaller because most of them cancel out in the ratio.

Zbb, Zcc, tt background control
In order to perform a data-driven measurement of the Zbb, Zcc, tt background yield, the first Z candidate is defined as in the signal selection, whereas the flavour, charge and isolation requirements on the leptons from Z 2 are relaxed and the cut on their impact parameter significance reversed: |S IP 3D | > 5 (see Fig. 3).
To propagate the event yield from this control region to the signal phase space, correction factors are introduced that account for the relaxed isolation and kinematical cuts, for the reversed impact parameter cut and for the combinatorics related to considering pairs of leptons of any flavour and charge. 6 Z + jets background control

Single lepton fake rate measurement
Prior to measuring the Z+jets background yield, the single lepton fake rate must be evaluated. This is done from a sample of exactly 3 leptons, therefore signal-free, in which the contamination from WZ events is removed with a cut on the missing transverse energy ( E T < 25 GeV/c 2 ). A Z 1 candidate is looked for like in the signal selection and the remaining lepton (a 'fakeable object') is required to pass identification and isolation cuts. The fraction of fakeable objects passing this selection, as a function of lepton p T and pseudorapidity, is the single lepton fake rate: ε(p T , η ) = N(passing ID and isolation cuts) N(fakeable objects) (3)

Definition of the control region
A Z 1 candidate is reconstructed as in the signal selection. The control region is signal-free because the third and fourth leptons are required to have the same flavour and charge, ± 3 ± 4 (the lepton charge misassignment is negligible). No identification and isolation cuts are applied on these two leptons, whereas the kinematical cuts m 3 4 > 12 GeV/c 2 , m 4 > 100 GeV/c 2 are.

Extrapolation to the signal region
The number of Z+jets events in the signal region (SR) can be extrapolated from the one in the control region (CR) by means of the following formula:    5. The mean expected and the observed upper limits at 95% CL on σ(pp → H + X) × BR(ZZ → 4 ) for a Higgs boson in the mass range 120 ÷ 600 GeV/c 2 , for an integrated luminosity of 1.66 fb −1 using the CL s approach. The expected ratio for the SM is presented. The results are obtained using a shape analysis method.