Physics potential for the measurement of \({\sigma (H\nu \bar{\nu })\times \text {BR}(H\rightarrow \mu ^+\mu ^)}\) at the 1.4 TeV CLIC collider
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Abstract
The future compact linear collider (CLIC) offers a possibility for a rich precision physics programme, in particular in the Higgs sector through the energy staging. This is the first paper addressing the measurement of the standard model Higgs boson decay into two muons at 1.4 TeV CLIC. With respect to similar studies at future linear colliders, this paper includes several novel contributions to the statistical uncertainty of the measurement. The latter includes the equivalent photon approximation employed to describe \(e^+e^\) and \(e\gamma \) interactions whenever the virtuality of the mediated photon is smaller than 4 GeV and realistic forward electron tagging based on energy deposition maps in the forward calorimeters, as well as several processes with the Beamstrahlung photons that results in irreducible contribution to the signal. In addition, coincidence of the Bhabha scattering with the signal and background processes is considered, altering the signal selection efficiency. The study is performed using a fully simulated CLIC_ILD detector model. It is shown that the branching ratio for the Higgs decay into a pair of muons BR(\({H\rightarrow \mu ^+\mu ^}\)) times the Higgs production crosssection in WWfusion \(\sigma (H\nu \bar{\nu })\) can be measured with 38 % statistical accuracy at \({\sqrt{s} =\text {1.4 TeV}}\), assuming an integrated luminosity of 1.5 ab\(^{1}\) with unpolarised beams. If 80 % electron beam polarisation is considered, the statistical uncertainty of the measurement is reduced to 25 %. Systematic uncertainties are negligible in comparison to the statistical uncertainty.
Keywords
Higgs Boson Higgs Production Higgs Coupling Background Process Time Projection Chamber1 Introduction
Measurements of Higgs branching ratios, and consequently Higgs couplings, provide a strong test of the standard model (SM) and possible physics beyond. Models that could possibly extend the SM Higgs sector (Two Higgs Doublet model, Little Higgs models or Compositeness models) will require Higgs couplings to electroweak bosons and Higgsfermion Yukawa couplings (couplingmass linearity) to deviate from the SM predictions [1, 2].
The compact linear collider (CLIC) represents an excellent environment to study properties of the Higgs boson, including its couplings, with a very high precision [3, 4]. Measurements of rare \({H\rightarrow \mu ^+\mu ^}\) decays are particularly challenging because of the very low branching ratio of \(2\times 10^{4}\) predicted in the SM [5] for a Higgs mass of 126 GeV. Current results indicate that the LHC was not able to access Higgs coupling to muons (\(g_{H{\mu \mu }}\)), based on the runs at 7 TeV and 8 TeV centreofmass (CM) energies [6]. Projections for the HLLHC, assuming 300 fb\({^{1}}\) and 3 ab\({^{1}}\) of data, predict uncertainties of 23 and 8 % respectively for the \(g_{H{\mu \mu }}\) coupling [7]. In order to provide the best physics reach in the shortest time and for an optimal cost, the operation of the CLIC accelerator is foreseen in energy stages of 350 GeV, 1.4 and 3 TeV [8]. At 1.4 and 3 TeV, sufficiently large Higgs boson samples can be produced to allow studies of rare Higgs decays. A sample of \(3.7 \times 10^{5}\) Higgs bosons can be produced at 1.4 TeV CM energy, for an integrated luminosity of 1.5 ab\({^{1}}\) with unpolarised beams. With the expected instantaneous luminosity of \(3.2\times 10^{34}\,\)cm \({^{2}}\) s\({^{1}}\) this can be achieved in approximately five years of detector operation, with 200 running days per year and an effective uptime of 50 % [9]. The signal sample size will be doubled at 3 TeV CM energy due to rising crosssection for WWfusion [10].
A similar study has been performed at 3 TeV CM energy [10]. Compared to the study at 3 TeV, several challenges for the measurement of \(H\rightarrow \mu ^+\mu ^\) at CLIC are discussed for the first time in this paper. First, background processes with photons in the initial state simulated using both the expected Beamstrahlung spectrum at CLIC and the equivalent photon approximation (EPA) [11, 12], were considered. Forward electron tagging (Sect. 5) leads to a rejection of 48 and 42 % of the \(e^e^+\rightarrow e^e^+\mu ^+\mu ^\) and \({e^\pm }\gamma \rightarrow {e^\pm }\mu ^+\mu ^\) background events, respectively. The impact of Bhabha scattering events on the rejection of events with forward electrons is investigated.
The paper is organised as follows. In Sect. 2 the simulation tools used for the analysis are listed and in Sect. 3 the CLIC_ILD detector model is briefly described. Signal and background processes and event samples are discussed in Sect. 4. Tagging of background highenergy electrons is described in Sect. 5. Event preselection and the final selection based on a multivariate analysis (MVA) approach are described in Sect. 6. The dimuon invariant mass fit and the extraction of the statistical uncertainty of the measurement are described in Sect. 7. In Sect. 8 the impact of electron polarisation on the statistical uncertainty of the \({\sigma (H\nu \bar{\nu })\times \text {BR}(H\rightarrow \mu ^+\mu ^)}\) measurement is described. Systematic uncertainties are discussed in Sect. 9, followed by the conclusions in Sect. 10.
2 Simulation and analysis tools
Higgs production through WWfusion is simulated in Whizard 1.95 [13, 14] including a realistic CLIC beam spectrum and initial state radiation. The generator Pythia 6.4 [15] is used to simulate the Higgs decay into two muons. The CLIC luminosity spectrum and beaminduced processes are obtained by GuineaPig 1.4.4 [16]. Background events are also generated with Whizard using Pythia 6.4 to simulate the hadronization and fragmentation processes. Simulation of tau decays is done by Tauola [17]. The CLIC_ILD detector simulation is performed using Mokka [18] based on Geant4 [19]. Before digitisation of the detector signals, pileup from \(\gamma \gamma \rightarrow \text {hadrons}\) interactions is overlaid on the physics events. The particle flow algorithm, PandoraPFA [20, 21] is employed in reconstruction of the finalstate particles within the Marlin reconstruction framework [22]. The TMVA package [23] is used to separate signal from background by MVA of signal and background kinematic properties.
3 The CLIC_ILD detector model
The ILD detector concept [24] is modified for CLIC according to the specific experimental conditions at higher energies [3]. The subsystems of particular relevance for the presented analysis are discussed here. A complete description of the CLIC_ILD detector can be found in [25] .
The main tracking device of CLIC_ILD is the Time Projection Chamber (TPC) providing a point resolution in the \(r\phi \) plane better than 100 \({\upmu }\)m, for charged particles in the detector angular acceptance [3]. Additional silicon trackers cover polar angles down to 7°. They have a single point resolution of 7 \({\upmu }\)m, and together with the TPC improve the tracking accuracy in the \(r\phi \) plane. In order to provide precision tracking and vertexing closer to the beampipe, a Vertex Detector capable of an impact parameter resolution of 3 \({\upmu }\)m [26] is foreseen. Calorimetry at CLIC is based on finegrained sandwich calorimeters optimized for particleflow analysis (PFA). PFA is based on reconstruction of fourvectors of visible particles, combining the information from precise tracking with highly granular calorimetry. The detector comprises a central solenoid magnet, with a field of 4 T. High muon reconstruction efficiency of 99 %, for muons above 7.5 GeV, is achieved by combining information from the central tracker (TPC plus silicon tracker) with information provided by the iron yoke instrumented with the 9 layers of resistive plate chamber detectors.
In principle, hadrons produced in the interaction of the beaminduced photons affect the TPC occupancy and consequently the muon reconstruction efficiency. However, in the studied sample of muons from \({H\rightarrow \mu ^+\mu ^}\) decays, muon reconstruction efficiency is above 99 % in the barrel region, in the presence of \(\gamma \gamma \rightarrow \text {hadrons}\).
The average muon transverse momentum resolution for the signal sample is \(\Delta (1/p_\text {T}) = 3.3\times 10^{5}\, \text {GeV}^{1}\) in the barrel region. The impact of transverse momentum resolution on the statistical uncertainty of \({\sigma (H\nu \bar{\nu })\times \text {BR}(H\rightarrow \mu ^+\mu ^)}\) measurement is discussed in Sect. 9.
In the very forward region of the CLIC_ILD detector, below \(\theta \) \(=\) 8°, no tracking information is available. The region between 0.6° and 6.3° is instrumented with the two silicontungsten sampling calorimeters, LumiCal and BeamCal [27], for the luminosity measurement, beamparameter control, as well as for the tagging of highenergy electrons escaping the main detector at low angles. Together with the very forward segments of the electromagnetic calorimeter covering the polarangle region between 6.3°and 8°, it is possible to suppress the fourfermion SM background with the characteristic lowangle electron signature. The simulation of the veryforward electron tagging is described in Sect. 5.
4 Event samples
List of considered processes with corresponding crosssections. Crosssection values marked by * are generated with the additional kinematic requirements: 100 GeV \(< m_{\mu \mu }<\)150 GeV, and 8°\(<\theta _{\mu }<\)172°, where \(m_{\mu \mu }\) stands for dimuon invariant mass and \(\theta _{\mu }\) is the polar angle of the reconstructed muon. The crosssections for all processes with photons in the initial state include interactions with Beamstrahlung photons (\(e\gamma _\text {BS}\) and \(\gamma _\text {BS}\gamma _\text {BS}\)) as well as \(e^+e^\) and \({e^\pm }\gamma _\text {BS}\) processes modified with EPA when appropriate. Crosssections for processes \({e^\pm }\gamma \rightarrow {e^\pm }\mu ^+\mu ^\) and \({e^\pm }\gamma \rightarrow {e^\pm }\nu _{\mu }\bar{\nu }_{\mu }\mu ^+\mu ^\) represent the sum of crosssections for the processes with both initial states \({e^}\gamma \) and \({e^+}\gamma \)
Process  \(\sigma (fb)\) 

\(e^+e^\rightarrow H\nu \bar{\nu }, H\rightarrow \mu ^+\mu ^\)  0.0522 
\(e^+e^\rightarrow \nu \bar{\nu }\mu ^+\mu ^\)  129 
\(e^e^+\rightarrow e^e^+\mu ^+\mu ^\)  24.5\({^*}\) 
\({e^\pm }\gamma \rightarrow {e^\pm }\mu ^+\mu ^\)  1098\({^*}\) 
\({e^\pm }\gamma \rightarrow {e^\pm }\nu _{\mu }\bar{\nu }_{\mu }\mu ^+\mu ^\)  30 
\(\gamma \gamma \rightarrow \nu _{\mu }\bar{\nu }_{\mu }\mu ^+\mu ^\)  162 
\(e^+e^\rightarrow {e^+e^}\nu _{\mu }\bar{\nu }_{\mu }\mu ^+\mu ^\)  1.6 
In addition to the processes listed in Table 1, we have considered schannel \(e^+e^\rightarrow \mu ^+\mu ^\) production, as well as several processes with tau pair in the final state \(e^+e^\rightarrow \tau ^+\tau ^\), \(e^+e^\rightarrow \nu _{\tau }\bar{\nu _{\tau }}\tau ^+\tau ^\), \(e^+e^\rightarrow e^e^+\tau ^+\tau ^\). Tau decays become relevant if both taus decay into two muons which happens in \(\sim \)3 % of cases [28]. However, the invariant mass of the dimuon system will not match the Higgs mass window considered in this analysis (see Sect. 6.1). The same holds for \(e^+e^\rightarrow \mu ^+\mu ^\) production. Misidentification of a pion as a muon is negligible, due to the fact that muon identification is performed not only by the muon detector but the central tracker, as well. In addition, hadronic events involving pions are further suppressed to a negligible level by kinematical selections.
\(e^+e^\) interactions as well as electron interactions with Beamstrahlung photon (\(e\gamma _\text {BS}\)) are simulated using EPA, whenever the virtuality of the mediated photon is smaller than 4 GeV. In these cases \(e^+e^\) interaction can be described as interaction of quasireal photons \(\gamma _\text {EPA}\) (\({e^\pm }\gamma _\text {EPA}, \gamma _\text {EPA}\gamma _\text {EPA}\)). Similarly, \(e\gamma _\text {BS}\) can be described as \(\gamma _\text {EPA}\gamma _\text {BS}, \gamma _\text {EPA}\gamma _\text {EPA}\) when EPA is applicable. In this analysis, such events are grouped together with the processes involving Beamstrahlung photons. In this way, processes with roughly similar kinematic characteristics are grouped together , as shown in Table 1. The notation \({e^\pm }\gamma \) represents the sum of crosssections for the processes with either \({e^}\gamma \) or \({e^+}\gamma \) in the initial state.
At \({\sqrt{s} =1.4}\) TeV, the Higgs boson is also produced via ZZfusion, with a crosssection of about 10 % of the Higgs production crosssection in WWfusion. However, on a test sample of 300 ZZfusion events followed by the Higgs decay to a pair of muons, not a single event passed the selection described in Sects. 6.1 and 6.2. This implies an efficiency smaller than 1.2 % (95 % CL) for this channel equivalent to less than 0.1 events passing the final selection. Therefore, the Higgs production through ZZfusion is not considered relevant for this analysis.
Photons, dominantly emitted by Beamstrahlung, produce incoherent pairs deposited mainly in the lowangle calorimeters. On average, 1.3 twophoton interactions producing hadronic final states occur per bunch crossing [29] which may affect the muon reconstruction in the tracking detectors. These hadrons are included in the analysis by overlaying 60 bunch crossings in the simulation, before the digitisation and event reconstruction phase. These events, as well as other physics events, are passed trough the full detector simulation [30].
5 Tagging of EM showers in the very forward region
In the polar angle region below \(\theta \) \(=\) 8°, tracking information and hadronic calorimetry are not available. The fourfermion background \(e^+e^\rightarrow e^+e^\mu ^\mu ^+\) of multiperipheral type and similar processes like \({e^\pm }\gamma \rightarrow {e^\pm }\mu ^\mu ^+\) can fake the missing energy signature of the signal if the final state electrons (spectators), emitted at the polar angles smaller than \(\theta \) \(=\) 8°, escape undetected.
Electron detection in the very forward region involves the reconstruction of electromagnetic showers in the presence of intense beaminduced background depositing in the very forward calorimeters a large number of lowenergy particles, mostly incoherent pairs from Beamstrahlung [31]. This deposition amounts to several hundred thousand of \(e^+e^\) pairs per bunch crossing [32].
Furthermore, Bhabha events where one or both electrons are detected in the very forward calorimeters may occur in coincidence with either signal or background, even within the 10 ns time stamp. Tagging of such Bhabha electrons will result in the rejection of signal (or background). In order to prevent significant loss of signal statistics, the electron tagging was optimized to identify showers with energy higher than 200 GeV and a polar angle above 1.7° only.^{1} Under these requirements, the loss of the number of signal events due to tagging of Bhabha electrons amounts to 7 %. Out of these 7 %, in slightly more than a half of events one electron is added to the final state and, in the remainder two Bhabha electrons are added. Table 2 shows rejection rates for signal and background obtained by the forward electron tagging due to Bhabha pileup.
Rejection rates for signal and background by the forward electron tagging
Process  Rejection rate (%) 

\(e^e^+\rightarrow e^e^+\mu ^+\mu ^\)  48 
\({e^\pm }\gamma \rightarrow {e^\pm }\mu ^+\mu ^\)  42 
Signal  7 
6 Event selection
The event selection is done in two steps. First, a preselection is performed aiming to suppress background originating from beamstrahlung as well as the processes with spectator electrons described in Sect. 5. The final event selection uses a multivariate classifier based on boosted decision trees (BDT) to suppress remain background processes on the basis of their kinematic properties.
6.1 Preselection
In order to suppress the impact of the beaminduced background, only reconstructed particles with transverse momenta \(p_\text {T}>\) 5 GeV are used in the analysis. Furthermore, the preselection of events was made by requiring a reconstruction of exactly two muons in the event, with an invariant mass of the dimuon system in the window centered around the Higgs mass 105–145 GeV. In addition, the absence of tagged electrons with energy above 200 GeV and polar angle above 1.7° is required in order to suppress background with spectator electrons emitted in the very forward region of the detector.
6.2 MVA selection

visible energy of the event excluding the energy of the dimuon system, \(E_\text {vis}\),

transverse momentum of the dimuon system, \(p_\text {T}(\mu \mu )\),

scalar sum of the transverse momenta of the two selected muons, \(p_\text {T}(\mu _1)+ p_\text {T}(\mu _2)\),

boost of the dimuon system, \(\beta _{\mu \mu } = \left p_{\mu \mu } \right / E_{\mu \mu }\),

polar angle of the dimuon system, \(\theta _{\mu \mu }\),

cosine of the helicity angle, \(\cos \theta ^*\).
7 Dimuon invariant mass fit
In order to estimate the statistical uncertainty of the signal count, 5000 toy Monte Carlo (MC) experiments are performed, where pseudodata are obtained by randomly picking the signal \(m_{\mu \mu }\) values from the fully simulated signal sample, while background \(m_{\mu \mu }\) values are randomly generated from the total background PDF \(f_b(m_{\mu \mu })\). The size of the signal sample \(N^\prime _s\) and sample sizes \(N^\prime _{b,i}\) of individual backgrounds considered, are obtained from the Poisson distribution for the integrated luminosity of 1.5 ab\({^{1}}\), taking into consideration corresponding crosssections \(\sigma \) and the selection efficiencies \(\varepsilon \) (\(\langle N^\prime _{s}\rangle =L\cdot \sigma _{s}\cdot \varepsilon _{s}\), \(\langle N^\prime _{b,i}\rangle = L \cdot \sigma _{i} \cdot \varepsilon _{i}\), where i is indexing the different background processes listed in Table 1).
For each toy MC experiment, the \(m_{\mu \mu }\) distribution is fitted by the function \(f(m_{\mu \mu })\) given in Eq. 2, and the standard deviation of the resulting distribution of \(N_s\) over all toy MC experiments is taken as the estimate of the statistical uncertainty of the \({\sigma (H\nu \bar{\nu })\times BR(H\rightarrow \mu ^+\mu ^)}\) measurement.
As will be discussed in Sect. 9.1, the dimuon invariant mass distribution is sensitive to the detector \(p_\text {T}\) resolution, while the Higgs width \(\Gamma _{H}\) can be considered negligible in comparison to the detector energy resolution.
7.1 Signal and background PDFs
Fully simulated samples of signal and background (Table 1) are fitted to extract the PDFs. The sizes of the samples vary from several tens of thousands of events for the signal, up to a few million of events for various background processes.
7.2 Distribution of the signal count
The overall function \(f(m_{\mu \mu })\) (Eq. 2) is fitted to the pseudodata of each toy MC experiment using the unbinned likelihood fit. An example of a toy MC fit is given in Fig. 8.
The Higgs coupling to muons, \(g_{H{\mu \mu }}\), is optimally extracted in a global fit procedure taking into account all Higgs measurements at the 350 GeV, 1.4 and 3 TeV stages. The global fit serves to extract Higgs couplings from all measurements, as well as the experimental Higgs width \({\Gamma _{H}}\). Because \({\sigma (H\nu \bar{\nu })\times BR(H\rightarrow }\) \({\mu ^+\mu ^)} \propto \frac{{g^2_{HWW} g^2_{H{\mu \mu }}}}{\Gamma _{H}}\) and having access to \({\Gamma _{H}}\) and \(g_{HWW}\) from other measurements, extraction of \(g_{H{\mu \mu }}\) is possible solely from the measurement presented here. An example of a minimal set of measurements giving a modelindependent access to \({\Gamma _{H}}\) and \(g_{HWW}\) is the following: the \(H\rightarrow b\bar{b}\) measurements at both 350 GeV and 1.4 TeV give access to the ratio \(\frac{g_{HWW}}{g_{HZZ}}\), the recoil mass measurement at 350 GeV CM energy gives access to \(g_{HZZ}\), and the \(H \rightarrow W^+W^\) measurement at 1.4 TeV gives access to the ratio \(\frac{g^4_{HWW}}{\Gamma _{H}}\) [4]. The contributions of these measurements towards the final \(\Delta g_{H{\mu \mu }}\) is negligible at the second significant digit.
The dominant contribution to the \(g_{H{\mu \mu }}\) coupling uncertainty is the statistical uncertainty of the measurement presented here. Systematic uncertainties affect the total uncertainty of \(g_{H{\mu \mu }}\) determination only at the third significant digit, and thus can be neglected (Sect. 9). Under these assumptions, the relative uncertainty of \(g_{H{\mu \mu }}\) is approximated to be 19 %.
8 Impact of electron polarization
Summary of the \({\sigma (H\nu \bar{\nu })\times BR(H\rightarrow \mu ^+\mu ^)}\) measurement at 1.4 TeV CLIC with unpolarised and 80 % polarized electron beams. All uncertainties are statistical
Unpolarised  Polarised (80 %, 0 %)  

\(N_s\)  \(19.3 \pm 0.1\)  \(35 \pm 9\) 
\(\varepsilon _s\)  24 %  25 % 
\(\frac{\delta (\sigma (H\nu \bar{\nu })\times BR(H\rightarrow \mu ^+\mu ^) )}{\sigma (H\nu \bar{\nu })\times BR(H\rightarrow \mu ^+\mu ^)}\)  38 %  25 % 
\(\delta (g_{H_{\mu \mu }})/g_{H_{\mu \mu }}\)  19 %  13 % 
9 Systematic uncertainties
From Eq. 1 it is clear that uncertainties of the integrated luminosity and muon identification efficiency influence the uncertainty of the \(H\rightarrow \mu ^+\mu ^\) branching ratio measurement at the systematic level. It has been shown that at 3 TeV CLIC [35], where the impact of the beaminduced processes is the most severe, the luminosity above 75 % of the nominal CM energy can be determined at the permille level, using lowangle Bhabha scattering. Below 75 % of the nominal CM energy, the luminosity spectrum can be measured with a precision of a few percent using wideangle Bhabha scattering [36]. About 17 % of all Higgs production events occur at a CM energy below 75 % of the nominal CM energy. Having in mind the intrinsic statistical limitations of the signal sample, this source of systematic uncertainty can be considered negligible.
On the detector side, an important systematic effect is the uncertainty on the transverse momentum resolution, because it directly influences the expected shape of the signal \(m_{\mu \mu }\) distribution. The sensitivity of the signal count to the accuracy of the knowledge of the \(p_\text {T}\) resolution \(\sigma _{p_\text {T}}\) has been studied by performing the analysis with an artificially introduced uncertainty of an exaggerated magnitude on the assumed \({p_\text {T}}\) resolution used to extract the signal PDF. Results of the relative shift in signal counts w.r.t. the relative shift of \(\sigma _{p_\text {T}}\) are shown in Fig. 10. The relative bias in signal counting per one percent change of \(\sigma _{p_\text {T}}\) is 0.35 %.
The uncertainty of the muon identification efficiency will directly influence the signal selection efficiency. In addition, the uncertainty of the muon polar angle resolution impacts the \(m_{\mu \mu }\) reconstruction. Based on the results of the LEP experiments [37], it can be assumed that these detector related uncertainties are below a percent.
The systematic uncertainty of the signal count caused by the fit with \(f_{m_{\mu \mu }}\) defined in Eq. 2, was found to be about 1 % which is small compared to the statistical error.
9.1 Benefit of a improved \(p_\text {T}\) resolution
10 Conclusions
It has been shown that the measurement of the crosssection times the branching ratio for the SM Higgs decay into two muons can be performed with a relative statistical uncertainty of 38 % at 1.4 TeV CLIC, assuming 1.5 ab\(^{1}\) integrated luminosity with unpolarised beams. The result is dominated by the limited signal statistics and the irreducible background. The systematic uncertainties are negligible in comparison to the statistical one. This translates into a relative uncertainty of the coupling of Higgs to muons \(g_{H{\mu \mu }}\) of approximately 19 %. If the same integrated luminosity is collected with 80 % lefthanded polarisation for the electrons, the relative statistical uncertainty improves to 25 and 13 % for \({\sigma (H\nu \bar{\nu })\times BR(H\rightarrow \mu ^+\mu ^)}\) and \(g_{H{\mu \mu }}\), respectively.
Footnotes
Notes
Acknowledgments
The authors would like to thank the members of the analysis working group in the CLICdp collaboration for useful discussions. Konrad Elsener, Aharon Levy, Sophie Redford and Eva Sicking are especially acknowledged for carefully reading the manuscript. The production of the investigated event samples would not have been possible without the support from Stephane Poss and Andre Sailer. We acknowledge the support received from the Ministry of Education, Science and Technological Development of the Republic of Serbia within the project OI171012.
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