Spin and Parity Measurements of the Higgs-Like Boson in the H → ZZ → 4l Channel at CMS

The most recent CMS results are presented for an analysis of the spin and parity of the Higgs-like boson near 126 GeV in the ZZ decay channel where the Z bosons decay into two muons or electrons. The analysis utilizes the full dataset recorded by CMS of 5.1 fb−1 and 19.6 fb−1 of pp collisions at center-of-mass energies of √s = 7 and 8 TeV respectively. The Standard Model prediction is compared against six alternate JP hypotheses. In all cases, the data favor the Standard Model prediction.


Introduction
On 4 July 2012, the CMS and ATLAS collaborations announced the discovery of a new boson with a mass near 125 GeV that was consistent with the Higgs boson. Characterization of the properties of this new boson is essential to determine whether or not this new particle is a Higgs boson as predicted in the Standard Model. Presented here are results from the CMS experiment pertaining to spin and parity measurements of this new particle in the H → ZZ → 4l channel.

Event Selection
In the H → ZZ → 4l channel, we consider the following final states: 4e, 4µ, and 2e2µ. We require that events pass either the Double Muon, Double Electron, Triple Electron, or Muon + Electron triggers. The fiducial cuts placed on the leptons are p T > 5 GeV and |η| < 2.4 for muons, and p T > 7 GeV and |η| < 2.5 for electrons.
To select the Z candidates, leptons are combined into opposite-sign same-flavor pairs. The pair closest to the nominal Z mass is selected as Z 1 . The remaining pair with the highest p T scalar sum is selected as Z 2 .
We also include Final State Radiation (FSR) photons. Photons are first assigned to their nearest lepton. If the leptons of a Z candidate have FSR candidates, an FSR photon is selected if it brings the Z candidates mass closer to nominal. At most, one photon may be assigned to a Z candidate.
The leptons are required to have a relative isolation < 0.4 with an isolation cone of ∆R < 0.4. If a lepton's FSR photon was selected, that photon is not included in the lepton's isolation sum.
The following phase-space cuts are placed on the Z masses: 40 < m Z1 < 120 GeV and 12 < m Z2 < 120 GeV. a e-mail: dabelknap@wisc.edu To accommodate the trigger thresholds, at least one lepton should satisfy p T > 20 GeV and another p T > 10 GeV.
The spin-parity analysis considers events in the signal region where 106 < m 4l < 141 GeV as seen in Figure 1.

Spin and Parity Measurements
Determining the spin and parity of the Higgs-like particle is done by testing the Standard Model hypothesis (J P = 0 + ) against alternate spin-parity hypotheses. The six alternate hypotheses considered are Table 1. The spin-parity hypotheses tested are listed in the table below. The expected significance is given with the signal strength calculated from data, and when µ = 1 is assumed. The observed separations show the consistency of the data with the SM 0 + model, or the alternate J P models where the signal strength is calculated from the data.
The production angles Φ 1 and θ * are calculated in the rest frame of X. The decay angles θ 1 and θ 2 are shown in their respective Z rest frames. Φ is given in the X rest frame.
J P = 0 − , 0 + h , 2 + mgg , 2 + mqq , 1 + , and 1 − . We use a loglikelihood ratio test statistic that utilizes the full kinematic information of the event. The kinematics of a H → 4l event can be described fully with the five angles θ * , Φ 1 , θ 1 , θ 2 , and Φ ( Figure 2) with the masses m Z1 and m Z2 . θ * Angle between Z 1 's trajectory and the beam axis Φ 1 Angle between the Z 1 decay plane and the X decay plane θ 1,2 Angle between the negative lepton trajectory and the trajectory of its parent Z Φ Angle between the decay planes of the two Zs In order to more strongly distinguish between the spinparity hypotheses, we wish to discriminate against the backgrounds. Using a matrix element likelihood approach [2], a discriminant is constructed in order to distinguish the possible signal hypotheses from the backgrounds This discriminant utilizes all decay angles, m Z1 , m Z2 , as well as the m 4l distribution for m H = 126 GeV. The shapes of this discriminant for the different signal hypotheses are very similar, but differ considerably from the backgrounds.
To distinguish between the Standard Model and an alternative J P hypothesis, a discriminant is calculated utilizing a matrix element likelihood approach with the observables m Z1 , m Z2 , and the decay angles Ω.
Distributions of the value of D J P seen in Figure 3 show the discrimination between the 0 + and alternate hypotheses.
We then build a 2D log-likelihood ratio test statistic from the discriminants (D bkg , D J P ): The values for the test statistic q (Equation 3) are shown as distributions for the 0 + and alternate J P cases, and the value observed from data is indicated by an arrow in Figure 4. From these distributions, we can see the level of separation the D J P discriminant provides. The expected and observed significance for each of the tests are given in Table 1. It is seen that when compared with the six alternate J P hypotheses, the Standard Model pure scalar hypothesis is favored by the data.  Figure 4. The test statistic q = −2 ln(L J P /L S M ) is shown for the SM 0 + model (blue) and the alternate J P hypothesis (yellow). The expected distributions are generated by generating Monte Carlo experiments assuming m H = 126 GeV. The value observed from the data is indicated by a red arrow. From top to bottom, left to right, the hypotheses tested are J P = 0 − , 0 + h , 1 + , 1 − , 2 + m (gg), and 2 + m (qq).