The diphoton resonance and Higgs results from the CMS Detector of the LHC

Despite the discovery of the Higgs boson, which contributed to the success of the Standard Model, there are at least 60 different analyses carried out in parallel with the CMS detector looking for new physics. In this talk the current status of the Higgs analysis and the search for physics beyond the Standard Model in the two photons final state are presented.


Introduction
During the Run I of the CERN LHC, a new particle was discovered by both ATLAS [1] and CMS [2] collaborations. Subsequent studies using the full LHC Run I data set in various decay channels and production modes and combined measurements from ATLAS and CMS [ [3]- [19]] showed that the properties are so far consistent with expectations from the Standard Model Higgs boson.
In this paper, the results with the Run II of the most sensitive channels with a mass around 125 GeV: H → ZZ → 4l [6] and H → γγ [7] are presented, including the search for resonant production of high mass photon pairs [8]. The analyses followed the strategy described in [9], [10] and [11] respectively. The data collected by CMS in 2016 so far corresponds to 12.9 f b −1 of integrated luminosity of pp collisions at √ s = 13 T eV .

CMS Detector
The Compact Muon Solenoid (CMS) [12] is a multipurpose detector operating at the Large Hadron Collider (LHC) at CERN, which has been successfully collecting data since 2009. The central feature is a superconducting solenoid, 13m in length and 6m in diameter, which provides an axial magnetic field of 3.8 T. CMS detector subsystems relevant for these searches are the tracker (silicon pixel and strip tracker), the electromagnetic calorimeter (ECAL), the hadronic calorimeter, and the muon spectrometer. The physical objects used in these analyses are built as follows: Muons are reconstructed independently in the tracker and the muon spectrometer, and are combined using statistical combination; electrons are reconstructed from clusters of ECAL cells with a matched track in the tracker; and photons are directly obtained from the ECAL measurement. A particle-flow event algorithm [ [13], [14]] reconstructs and identifies each individual particle with an optimized combination of information from the various elements of the CMS detector.

Data Sets
These analyses makes use of an integrated luminosity of 12.9 f b −1 of pp collisions collected by the CMS experiment at √ s = 13 TeV in 2016. The main triggers for the H → ZZ → 4l analysis select either a pair of electrons or muons, or an electron and a muon. While a diphoton trigger with asymmetric transverse energy (ET) thresholds is used for the H → γγ analysis, and the high mass photon pairs search. The trigger selection is fully efficient for resonance masses above 500 GeV.
All signal and background generators are interfaced with PYTHIA 8 [15] to simulate the multiparton interaction and hadronization effects. The generated events go through a detailed simulation of the CMS detector based on GEANT4 [16] and are reconstructed with the same algorithms that are used for data. The simulated events include pp interactions and have been reweighted so that the distribution of the number of interactions per LHC bunch crossing matches that observed in data.

Diphoton resonances
The diphoton channel provides fully reconstructed resonances with a good mass resolution, an opportunity to discover new particles. It is an experimentally robust channel with small systematic uncertainties and relatively low background at hadron colliders. The event selection looks for final states with two high-p T isolated photons passing the selection of p T > 75 GeV, |η| < 2.5, excluding the transition barrel-encap region 1.44 < |η| < 1.57. Two event categories are defined EBEB, and EBEE. The background model is a parametric fit to data with an empirical function f (m ( γγ)) = m a+b * log(mγγ ) γγ . The mass spectra is shown in Figure 1 for both categories.
The signal shape is a convolution of intrinsic line-shape of the resonance, derived using TeV data [8].
PYTHIA, and detector resolution. The signal mass resolution, quantified through the ratio of the full width at half maximum of the distribution, divided by 2.35, to the peak position, is roughly 1% and 1.5% for the EBEB and EBEE categories respectively. Nine mass values in the range 500-4500 GeV with spin−0 and spin−2 with the value of the width Γ X /m X = 1.4 × 10 −4 , 1.4 × 10 −2 , 5.6 × 10 −2 were tested. The observed p-value obtained for selected width is shown in Figure 2. No significant excess in proximity of 750 GeV.

Diphoton
The event selection requires two photon candidates with p 1 T > 30 GeV and p 2 T > 20 GeV. Both photons must satisfy the requirement |η| < 2.5, excluding the transition region between the barrel and the endcap 1.44 < |η| < 1.57. Additionally the energy sum of 3X3 crystals centered on the most energetic crystal in the candidate electromagnetic cluster divided by the energy of the candidate (R 9 variable) should be larger than 0.8, or the charged hadron isolation < 20 GeV , or the charged hadron isolation relative to p T < 0.3.
In order to increase the signal background ratio and improve the analysis sensitivity the events are divided into eight categories, labelled as: ttH Leptonic Tag, ttH Hadronic Tag, VBF Tag 0, VBF Tag 1, Untagged 0, Untagged 1, Untagged 2 and Untagged 3. Figure 3 shows the multivariate discriminator, Boosted Decison Tree (BDT) as described in [17], used to define the Untagged categories, labelled from best resolution to worst (left) and the shows the invariant mass for all categories (right). The dominant background to H → γγ consists of the irreducible prompt diphoton production, and the reducible backgrounds from γ + jet and QCD multijet, where the jets are misidentified as isolated photons. The model used to describe the background is extracted from data with the discrete profiling method [18].

Four leptons
This analysis relies on the efficiency to select leptons, the large signal to background ratio, excellent resolution, and the complete reconstruction of the final state. It is called the golden channel for discovery and property measurements. The event selection requires at least 4 leptons in the event, Z candidates are then built with pairs of leptons of the same flavor and oppositecharge (e + e − , µ + µ − ) and required to have 12 < m l + l − < 120 GeV. Then they are combined into ZZ candidates, we denote as Z 1 the Z candidate with an invariant mass closest to the nominal Z boson mass. The flavors involved define three subchannels: 4e, 4µ and 2e2µ.
The events are separated in 6 mutually exclusive categories in order to increase sensitivity, relying on the number of jets, b-tags, additional leptons, and cuts on the 4 production discriminants. The Figure 5 shows the signal fraction for each category. The main and irreducible background is the ZZ production, the reducible background coming from Z + X is data driven estimated. Two independent methods are applied : Same-sign (SS) and Opposite-sign (OS), using orthogonal control regions. The Figure 5 shows the reducible background estimation using both methods.
The results of the event selection show a good agreement over the whole range of m 4l in the three final states (4e, 4µ and 2e2µ). The highest-mass candidate has a mass of 802 GeV. Figure  6 shows the invariant mass combined. The observed significance from simultaneous fit of the 2D likelyhood in 3 final states by 6 categories is 6.2 σ (6.5σ expected) at 125.09GeV , the mass value obtained in the combination of Run1 [19]. Figure 6 shows the minimum of p-value gotten at m H = 124.3 GeV, a significance of 6.4 σ is observed (6.3 σ expected).
The combined signal strength at m = 125.09 isμ = 0.99 +0.33 −0.26 . The signal strength is also measured for the bosonic and fermionic componentsμ V BF,V H = 0.91 +1.56 −0.91 andμ ggH,ttH = 1 +0.39 −0.32 . Figure 7 shows the signal strength per production processes. The production mechanism signal strengths are compatible with the SM.