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Higgs Branching Fraction Study in ILC

Precise measurement of the Higgs boson properties are important issues for the International Linear Collider (ILC) project to understand the particles mass generation mechanism which strongly related to the coupling with the Higgs boson. Large Hadron Collider (LHC) experiments exclude the large area of the predicted Higgs mass region and their results indicate that Higgs boson mass will be light. Even if LHC discovers the Higgs like particle by the end of 2012, Higgs will be identified by the high precision measurement of the Higgs boson properties in ILC and also Higgs measurement verifies the correctness of standard model (SM) or gives some hints toward its beyond. In this study, we evaluate the measurement accuracies of Higgs branching fraction to the H->bb, cc and gg at the center-of-mass energy of 250 and 350 GeV.


Introduction
Higgs branching ratio (BR) measurement is one of the important issue of the International Linear Collider (ILC) project, which is strongly related to the coupling strength with particles and reveal their mass generation mechanism. Even the Large Hadron Collider (LHC) [1] will discover the Higgs boson in a few year, ILC can confirm whether that is the standard model (SM) predicted one or not, and find some hints toward its beyond. LHC experiment accumulate the total integrated luminosity up to 5 fb −1 by the end of 2011, LHC gradually exclude the heavy Higgs mass region and indicates the light Higgs from the combined results of ATLAS [3] and CMS [4] (115 ≤ M H ≤ 135 GeV). In this region, we obtain the maximum production cross section around the center-of-mass energy ( √ s) of 250 GeV and Higgs boson BR significantly varies depending on the Higgs mass, especially main decay channel shifts from H → bb to W W * around the M H = 140 GeV, as shown in Fig. 1 (a) and (b). In this study, we evaluate the measurement accuracies of the Higgs BRs of H → bb, cc and gg with assuming the Higgs mass of 120 GeV and integrated luminosity (L) of 250 fb −1 , using the International Large Detector (ILD) [5] full simulation at √ s = 250 GeV. In addition, we also consider the operation at the CM energy of 350 GeV, which increase the contribution of W/Z fusion process but Z/H will be boosted, for taking into account the energy staging option in ILC project. Considering the LHC results, we also evaluate the Higgs BR accuracy at the indicated mass region by extrapolating the M H = 120 GeV result.   Figure 2: Higgs production processes categorized with Z decay channels: (a) neutrino (ννH), (b) hadronic (qqH) and (c) leptonic (ℓ + ℓ − H). Figure 2 shows the Higgs production diagrams and ZH analysis procedures are categorized with the Z decay channels; ZH → ννH (Neutrino), qqH (Hadronic) and ℓ + ℓ − H (Leptonic) which mainly form di-jet, four-jet and di-lepton+di-jet final states, respectively. We also consider the W/Z-fusion process for the neutrino and leptonic channels which has larger contribution at √ s = 350 GeV. Assuming the M H = 120 GeV, we obtain the maximum production cross section around the √ s = 250 GeV through the Higgsstrahlung (ZH) process and mainly decays to b quarks (H → bb). In order to maximize the Higgs production cross section, we employ the left-handed electron beam polarization; (e + , e − ) = (+30%, −80%), at the CM energies of both 250 and 350 GeV with the integrated luminosity of 250 fb −1 . As background, we consider the following 2f and 4f final state SM Backgrounds: e + e − → W + W − , ZZ and qq. In addition, we also take into account for the e + e − → tt background only for the √ s = 350 GeV.

Analysis Framework
Since ZH final state forms multi-jet, thus jet clustering, jet energy resolution and quark flavor-tagging are crucial for the Higgs hadronic decay channels (H → bb, cc and gg) analysis. In order to achieve the best jet energy resolution, ILD adopt the Particle Flow Algorithm (PFA) [6], which can achieve the best jet energy resolution; charged tracks energy is measured by tracker instead of the calorimeter and only neutral particles energy is measured by calorimeter with avoiding the cluster overlapping and double counting. ILD detector design is well-suitable for the best PFA performance with adopting the finely segmented calorimeter, large tracker radius and strong magnetic field. For the simulation study, we use the ilcsoft v01 06 [7] ILC common software package. At fist we generate the MC event samples with the Whizard [8]. Then we perform the ILD full detector simulation with the Mokka [9] package assuming the ILD detector model (ILD 00). Generated hits are digitized and reconstructed with Marlin [10] package and perform the PFA (PandraPFA [6]). We also employ the LCFIVTX [11] flavor tagging package embedded in Marlin to identify the quark flavor of H → bb and cc. For the mass production of the full simulation samples, we use the GRID [12] resources for the ILD LOI study [5] and finally saved as ILC common file format (LCIO [13]).

Jet Reconstruction
For the ννH channel analysis, we apply the jet reconstruction forcibly merged into the two-jet which comes from H → bb, cc and gg.

Background Reduction
In this channel, ZZ → ννqq and W W → νℓqq will be the main backgrounds. To suppress these backgrounds, at first, we apply the missing mass cut; 80 < M miss < 140 GeV to suppress the ZZ to leptonic or hadronic decay backgrounds, since M miss should be consistent with the Z mass in Z → νν channel. Then we use the following kinematic variables cut: transverse momentum; 20 < P t < 70 GeV, longitudinal momentum; |P ℓ | < 60 GeV and maximum momentum; P max < 30 GeV to suppress the qq background. Number of charged tracks cut; N chd > 10 well reduce the backgrounds including energetic leptons. We also apply the jet clustering y-value cuts; 0.2 < Y 12 < 0.8 and Y 23 < 0.02, which are the y-value thresholds from two-to one-jet or two-to three-jets and different of number of jets backgrounds are reduced with these cut. Finally we apply the di-jet mass cut; 80 < M jj < 130 GeV which correspond to Higgs mass. After applying all the cuts, we apply the likelihood variable cut using following input variables; M miss , number of reconstructed particles (N P F O ), P max , P l and M jj , and we select the LR > 0.375 where we obtain the maximum signal significance. In order to optimize the cut positions for the analysis at √ s = 350 GeV, we change the following variables cut positions: 50 < M miss < 240 GeV, 10 < P t < 140 GeV, |P ℓ | < 130 GeV and LR > 0.15.

Jet Reconstruction and Pairing
For qqH channel, we apply the four-jet reconstruction forcibly. Then we calculate following χ 2 value to determine the Z or H jet pair candidates: where M 12 and M 34 are reconstructed di-jet invariant masses and σ Z/H are the width of Z and H mass distribution. We select the minimum χ 2 jet pairs as the best candidate of Z and H.

Background Reduction
After the jet clustering and pairing, we apply the background reduction. At first we apply the χ 2 cut to reduce the wrong combination pairs; χ 2 < 10. Then we require the following cuts to suppress the leptonic events: the number of charged tracks cut; N chd > 4 and yvalue cut Y 34 cut, which is a y-value threshold from three-to four-jet; − log Y 34 < 2.7. As event shape cuts, we employ the following variables cut: thrust and its cosine of thrust angle; thrust < 0.9, | cos θ thrust | < 0.9 and the angle between the Higgs candidate jets; 105 < θ H < 160 • to suppress the qq backgrounds. Finally we apply the di-jet mass cut M 12 and M 34 and likelihood variable cut which is calculated with following input variables; thrust, M Z , M H , θ H . We select the likelihood variable cut as LR > 0.2. For the √ s = 350 GeV, we optimize the cut position as follows; thrust < 0.85, 70 < θ H < 120 • , 80 < M Z < 100 GeV, 105 < M H < 130 GeV and LR > 0.1.

Leptonic Channel (ℓ + ℓ − H)
For the ℓ + ℓ − H channel analysis, at first we identify the di-lepton, then we apply the di-jet reconstruction forcibly for remaining particles.

Di-lepton Identification
We apply the following di-lepton identification for electrons and muons from the different aspects in the energy deposition in the calorimeter; E ECAL /E T otal > 0.9 and 0.7 < E T otal /P < 1.2 for electrons and E ECAL /E T otal < 0.5 and E T otal /P < 0.4 for muons, where E ECAL , E T otal and P denote the ECAL energy associated with a track, total energy deposit in whole calorimeter and track momentum.

Background Reduction
After the di-lepton identification, we apply the background reduction with following cut variables, which is summarized on the Table 4. At first we apply the di-lepton mass (M ℓℓ ) cut which should be consistent with the Z mass: 70 < M ℓℓ < 110 GeV for electron and 70 < M ℓℓ < 100 GeV for muons, respectively. Then we apply the Z flight direction cut: | cos θ Z | < 0.8 to suppress the forward region backgrounds. Finally we require the di-jet mass (M jj ) and recoil mass (M rec ) cuts to select the Higgs candidate signal: 100 < M jj < 140 GeV, 70 < M rec < 140 GeV for electron; 115 < M jj < 140 GeV and 70 < M rec < 140 GeV for muon, respectively.

Template Fitting
In order to evaluate the measurement accuracies of the Higgs branching fraction (BR), we employ the template fitting method [14]. At first, we prepare the flavor-likeness template samples which is calculated from the LCFIVTX output x 1,2 : where x 1,2 represents the flavor tagging output from LCFIVTX for di-jet. We assume the Poisson statistics (P ink ) for each bin (i, j, k) of the template samples: where N data ijk is the number of entries in (i, j, k) bin. N template ijk represents the sum of the number of entries at bin (i, j, k) in each template sample: where N s ijk represents the number of entries at the (i, j, k) bin in H → bb, cc, gg and background template sample (N bkg ijk ), which includes the number of entries of SM background and Higgs to none hadronic decays. r s represents the fitting parameters of r bb , r cc , r gg and r bkg , where they are the ratios of number of entries in Higgs hadronic decays of H → bb, cc and gg after the background reduction to the entries predicted from the SM Higgs BR. r bkg is a normalization factor for the SM background and other Higgs none hadronic decays, which is fixed to be 1 from the assumption that the SM backgrounds are well understood. Finally we apply the template fitting with minimizing the following log-likelihood variable L calculated from the product of the probability P ijk in each bin:

Measurement accuracies of BR
To evaluate the measurement accuracies of the sigma times BRs for bb, cc and gg, we apply the 1000 times template fitting Toy-MC and obtain the fitted results of r s (s = bb, cc, gg): where σ ZH is a Higgs production cross section, σ ZH SM and BR(H → s) SM are cross section and BR of H → s predicted in SM. From the Eq. 6, we obtain the measurement accuracies of Higgs BR from following equation: here we assume the 2.5% of cross section measurement uncertainty (∆σ ZH /σ ZH ) estimated from the recoil mass study [15]. Summary tables of the measurement accuracies of Higgs BR are shown in Table 5 and 6 for √ s = 250 and 350 GeV, respectively.

LCWS11
where σ MH and BR(s) MH denote the cross section and BR of H → s at M H , as shown in Fig. 3 (a). In addition, we also compile the H → W W * → 4j result [16], even though this study assume the electron right-handed polarization: (e + , e − ) = (−30%, +80%) to suppress the e + e − → W + W − backgrounds. Figure 3 (b) shows the expected measurement accuracies of the Higgs BR at each mass and summarized on the Table 7.

Conclusion
We evaluate the measurement accuracies of the Higgs BR for H → bb, cc and gg channels. With the template fitting analysis, we obtain the measurement accuracies of σ × BR for H → bb, cc and gg as 3%, 9% and 10%, respectively. We also estimate the Higgs BR measurement accuracies at the prospective mass region by LHC with extrapolating the results at the Higgs mass of 120 GeV. Table 7: Summary of the extrapolated accuracies of the Higgs BR from the result at the M H = 120 GeV, √ s = 250 GeV to prospective masses calculated by HPROD [17] and HDECAY [18]. Here 2.5% of the σ ZH uncertainty is also included. 8 Bibliography