The Higgs Signatures at the CEPC CDR Baseline

As a Higgs factory, The CEPC (Circular Electron-Positron Collider) project aims at the precise measurement of the Higgs boson properties. A baseline detector concept, APODIS (A PFA Oriented Detector for the HIggS factory), has been proposed for the CEPC CDR (Conceptual Design Report) study. We explore the Higgs signatures at this baseline setup with nunuHiggs events. The detector performance of the charged particles, the photons, and the jets are quantified with Higgs ->mumu, gammagamma, and jet final states respectively. The accuracy of reconstructed Higgs boson mass is comparable at different decay modes with jets in the final states. We also analyze the Higgs ->WW* and ZZ* decay modes, where a clear separation between different decay cascades is observed.


Introduction
After the Higgs boson discovery [1,2] at the LHC (Large Hadron Collider), the precise measurements of the Higgs boson properties become vital for the experimental particle physics. The CEPC, a future high energy collider project based on a 100 km circumference main ring [3], is therefore proposed. Operating at 240 GeV center of mass energy, the CEPC has an instant luminosity of ∼3×10 34 cm −2 s −1 and can deliver 10 6 Higgs bosons [4]. The CEPC can determine the absolute Higgs boson couplings to the relative accuracy of 0.1% -1%, roughly one order of magnitude superior to the Higgs signal strength measurements at the HL-LHC [5,6].
At the CEPC, the Standard Model (SM) Higgs bosons are produced mainly through the Higgsstrahlung process ( e + e − → ZH) and the vector boson fusion processes (the Z fusion process e + e − → e + e − H, and the W fusion process e + e − → ννH), see Figure 1. The corresponding cross sections with a 125 GeV SM Higgs boson using non-polarized beam at different center of mass energy is shown in Figure 2. At the CEPC, roughly a quarter of the Higgs boson is generated in association with a pair of neutrinos (ννH), including both the W fusion events and the ZH events with Z decays to νν. The Higgs boson is responsible for almost all the detector signals in these ννH events, providing benchmark samples for the CEPC detector performance study. This manuscript presents the performance analysis of the CEPC baseline detector geometry APODIS (a.k.a. the CEPC v4) [7]. Using full simulated ννH samples, we analyze a set of Higgs signal distributions that covers all the major SM Higgs decay modes. In section 2, we Received

Baseline detector and CEPC software chain
The APODIS is optimized from the CEPC v1 [8], the reference detector design for the CEPC Pre-CDR [3] study. Comparing to CEPC v1, APODIS maintains the same level of performance for the Higgs signals (the lepton identification and the reconstruction of photon and jets), and enhances significantly the performance on the charged Kaon identification [7]. Meanwhile, APODIS has significantly reduced the number of readout channels, the total weight, and the solenoid B-field.
In terms of the software, a full simulationreconstruction toolkit has been established and optimized for the APODIS geometry. The information flow consists of three basic modules: the Generator, the Simulation, and the Reconstruction, see Figure 3. For the Generator, Whizard [10] is used to generate final state particles for given physics process. The samples are then fully simulated using the MokkaPlus [11,12] and reconstructed using Arbor [13] as the core Particle Flow [14] reconstruction.
We simulate and reconstruct the ννHiggs, Higgs→ γγ, µµ, gg, bb, cc, WW * , ZZ * samples, each with ∼50000 events statistic. Most of the Standard Model Higgs boson decay modes are included except the Higgs → τ τ , which has been extensively studied in the reference [15]. The information flow of the CEPC software chain.

Benchmark distributions of the Higgs signals at ννH events
The different decay modes can be classified into three classes according to their dependence to the detector performance. The invariant mass reconstructed from Higgs→ µµ events represents the tracking reconstruction performance, and that from Higgs→ γγ represents the performance of photon reconstruction. The other events mostly depend on the jet reconstruction performance. The muons and photons are single particle level physics objects that can be directly identified from the reconstructed particles. For the other decay mode events (Higgs→ bb, cc, gg, WW * , ZZ * ), we looked into the total invariant mass of the events. In the full reconstructed events, the total invariant mass distributions not only represent the detector performance but also include other physics effects: • The ISR (initial state radiation) photons.
• The neutrinos generated from the Higgs bosons.
• The direction of the jets at the Higgs→di-jets events, due to the acceptance of the detector. Figure 4 shows the correlation between reconstructed Higgs boson mass and the sum of the transverse momentum of the ISR photons (Pt ISR), the sum of the transverse momentum of the neutrinos generated from Higgs boson (Pt neutrino), and the minimum angle between the jets and the beam pipe (|Cos(Theta Jet)|), at the Higgs → di-gluon events. Clearly a strong correlation is observed when these effects are significant. In order to disentangle these effects from the detector performance at jet reconstruction, a monte calo truth level event selection is applied to the events with jets in the final states. The standard cleaning procedure is set up as the Pt ISR < 1 GeV, the Pt neutrino < 1 GeV and the |Cos(Theta Jet)| < 0.85, with the selection efficiency shown in Table 1.    The distribution of reconstructed Higgs boson mass at Higgs → γγ events, parameterized by the half-width of the narrowest interval containing 68.3% of the distribution (σ ef f ).

Higgs → µµ
The Higgs boson decay into µ + µ − is a rare process with a branching ratio of 0.022% for an 125 GeV SM

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Higgs boson [16]. From the reconstructed particles, we select a pair of muons with opposite charge and both have energy higher than 20 GeV. The invariant mass of this pair of muons is reconstructed as the Higgs boson mass. The distribution fitted to a crystal-ball function is shown in Figure 5. The tracking performance is characterized by the resolution of the Higgs boson mass, which reaches 0.20% (σ/Mean) in this benchmark channel.

Higgs→ γγ
The performance of the ECAL (electromagnetic calorimeter) is characterized by the reconstruction of the Higgs → γγ events. We select two most energetic pho-tons with energy higher than 10 GeV and calculate their invariant mass as the Higgs boson mass, see Figure 6. The tail at the left side is caused by the geometry defects and the material before the calorimeter. The width of the distribution is parameterized by the half-width of the narrowest interval containing 68.3% of the distribution, σ ef f [17]. The σ ef f of the distribution is 3.24 GeV and the resolution (σ/Mean) is 2.59%.
At a simplified geometry free of the geometry defects, the reconstructed Higgs boson mass resolution can reach 1.64% [18]. This significant degradation at the APODIS indicates the geometry based correction of the photon energy reconstruction is mandatory.

Higgs→ bb, cc, gg
Roughly 70% of the 125 GeV SM Higgs bosons decay into a pair of jets (bb, cc, and gg). For these Higgs → di-jets events, we collect all the visible final state particles and calculate their invariant mass. The distributions of reconstructed Higgs boson mass before event selection are shown in Figure 7 and the Figure 8 shows the results after applying the event selection. After the cleaning, the resolutions of Higgs boson mass at different Higgs → dijets events are almost consistent, which are 3.63%(bb), 3.82%(cc), and 3.75%(gg).

Higgs→ W W *
The 125 GeV SM Higgs boson has a probability of 21.4% to decay into a pair of W bosons, making this measurement a sensitive probe to the New Physics. Limited by the Higgs boson mass, one of the W boson is off-shell (W * ).
The reconstructed total visible invariant mass distribution is shown in Figure 9. Depending on the decay 010201-4 modes of W and W * (leptonic or hadronic), the total invariant mass distribution is decomposed into different sub-distributions. The main peak at 125 GeV corresponding to the events that both W and W * decay into two quarks. The other two stacks corresponding to the events that W or W * decay into a pair of lepton and neutrino. Fig. 9.
The distribution of reconstructed total visible invariant mass at Higgs → W W * events. Depending on the decay modes of W and W * (leptonic or hadronic), the total invariant mass distribution is decomposed into different subdistributions.
The event selection procedure is also applied. After the event cleaning, the events that W or W * decay into leptons and neutrinos are excluded, as shown in Figure 10. This cleaned invariant mass distribution can fit to a Gaussian function with σ=4.77. The fit result is comparable with the Higgs → di-jets events. The distribution of reconstructed total visible invariant mass at Higgs → W W * events after the event cleaning, fitted to a Gaussian function.
3.5 Higgs→ ZZ * About 2.6% of the 125 GeV SM Higgs boson decay into ZZ * . Similar to Higgs→ W W * channel, one of the Z boson is off-shell. Most of the Z bosons then decay into qq (∼70%), ll (∼10%) or νν (∼20%). Figure 11 shows the reconstructed total invariant mass. With the monte calo truth information, the events are classified depending on the decay modes of Z and Z * (visible or invisible). There are four peaks in the distribution. The peak at zero is corresponding to the events that both Z and Z * decay into neutrinos, which contains about 4% of all the events. The main peak at the expected Higgs boson mass (∼125 GeV) is corresponding to the events with all the final state particles are visible. The other two peaks are corresponding to the conjugation case that Z → visible, Z * → invisible and Z * → visible, Z → invisible. The distribution of reconstructed total invariant mass at Higgs → ZZ * events, classified depending on the decay modes of Z and Z * (visible or invisible).
By using the event selection procedure, the events Z or Z * decay into neutrinos are excluded, as shown in Figure 12. Determined by the Z decay modes, most of the remaining events contain 4 or 2 quarks as final state particles. This cleaned invariant mass distribution can fit to a Gaussian function with σ=4.68. The resolution is also comparable with the Higgs → di-jets events. The distribution of reconstructed total visible invariant mass at Higgs → ZZ * events after the event selection, fitted to a Gaussian function.

Conclusion
Based on the APODIS detector design, we characterize the Higgs signatures at the e + e − → ννHiggs events with Higgs decaying to γγ, µµ, gg, bb, cc, W W * and ZZ * . With the Higgs → µµ, γγ, and jet final states events, we quantify the detector performance, as shown in Table 2. Comparing to the results at LHC, the re-construction accuracy at Higgs → µµ events is improved by about one magnitude, and that at Higgs → di-jets events is improved by about 3 times. The resolution at Higgs → γγ events degrades by roughly 60%, limited by the absence of geometry based correction and fine-tuned calibration, and the sampling fraction of ECAL.
To describe the jet reconstruction performance, a standard event cleaning procedure has been designed. After the event cleaning, the accuracy of the reconstructed Higgs boson mass at different Higgs decay modes with jets as final state particles are comparable, as shown in Table 3.
For the Higgs → WW * and ZZ * events, the total invariant mass distribution is composed of multiple components depending on the decay modes of W and Z bosons. For the WW * events, the classification is based on the leptonic or hadronic decay mode of W and W * . For the ZZ * events, the reconstruction result is sensitive to the visible or invisible decay mode of Z and Z * . The distribution of H → ZZ * is clearly separated with four peaks corresponding to the Z and Z * decay modes. The standard cleaning procedure could efficiently veto the events with significant neutrinos generated from the Higgs boson cascade decay. After the event cleaning, the Higgs boson mass resolutions (σ/Mean) at Higgs → WW * and ZZ * events are comparable with Higgs → di-jets events, see Table 3.
We would like to thank Gang Li and Xin Mo for the physics event generator files.