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Chinese Physics C Vol. 41, No. 2 (2017) 023003 Cross section and Higgs mass measurement with Higgsstrahlung at the CEPC * Zhen-Xing Chen ( � � � ) 1 , 2;1) Ying Yang ( � � ) 2 Man-Qi Ruan ( � � � ) 2;2) Da-Yong Wang ( � � � ) 1;3) Gang Li ( � � ) 2 Shan Jin ( � � ) 2 Yong Ban ( � � ) 1 1 State Key Laboratory of Nuclear Physics and Technology, Peking University, Beijing 100871, China 2 Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China The Circular Electron Positron Collider (CEPC) is a future Higgs factory proposed by the Chinese high Abstract: energy physics community. It will operate at a center-of-mass energy of 240–250 GeV. The CEPC will accumulate an integrated luminosity of 5 ab − 1 over ten years of operation, producing one million Higgs bosons via the Higgsstrahlung and vector boson fusion processes. This sample allows a percent or even sub-percent level determination of the Higgs boson couplings. With GEANT4-based full simulation and a dedicated fast simulation tool, we have evaluated the statistical precisions of the Higgstrahlung cross section σ ZH and the Higgs mass m H measurement at the CEPC in the Z → µ + µ − channel. The statistical precision of σ ZH ( m H ) measurement could reach 0.97% (6.9 MeV) in the model-independent analysis which uses only the information from Z boson decays. For the standard model Higgs boson, the m H precision could be improved to 5.4 MeV by including the information from Higgs decays. The impact of the TPC size on these measurements is investigated. In addition, we studied the prospect of measuring the Higgs boson decaying into invisible final states at the CEPC. With the Standard Model ZH production rate, the upper limit of B (H → inv . ) could reach 1.2% at 95% confidence level. CEPC, Higgs mass, cross section Keywords: 13.66Fg, 14.80.Bn, 13.66.Jn DOI: 10.1088/1674-1137/41/2/023003 PACS: 1 Introduction adjustable. Thus, the Higgs production cross section is available with the recoil technique. In this way, a lep- ton collider can provide absolute measurements of Higgs The Higgs boson has been studied extensively since couplings [10–12]. Besides, it is free of the QCD back- its discovery [1, 2] at the LHC. The up-to-date results grounds. Almost every Higgs event can be recorded and indicate that it is highly Standard Model (SM) like [3– reconstructed. Therefore, an electron-positron Higgs fac- 8]. Many new physics models, however, predict the tory is an essential step in understanding the nature of Higgs couplings deviate from the SM at the percent level. the Higgs boson. Thus, percent or even sub-percent level precision be- The Circular Electron Positron Collider is a Higgs comes necessary for the future Higgs measurement pro- factory proposed by the Chinese high energy physics gram. However, this accuracy is difficult to achieve at community [12]. It will operate at a center-of-mass en- the LHC [9]. Moreover, as the Higgs boson can only be ergy of 240–250 GeV with an instantaneous luminosity reconstructed through its decay products at the LHC, it of 2 × 10 34 cm − 2 s − 1 . With two detectors operating over is impossible for the LHC to access the Higgs total width 10 years, the CEPC will accumulate about one million or absolute couplings in a model-independent way. Higgs events, corresponding to an integrated luminosity Compared to a hadron collider, an electron positron of 5 ab − 1 . collider has significant advantages in precision measure- The SM Higgs bosons are produced via the processes ments of the Higgs boson. The beam energy and po- of e + e − → ZH (Higgsstrahlung), e + e − → ν ¯ ν H (WW fu- larization of the initial states are precisely known and Received 21 January 2016, Revised 25 October 2016 ∗ Supported by the Joint Funds of the NSFC (U1232105) and CAS Hundred Talent Program (Y3515540U1) 1) E-mail: zxchen@ihep.ac.cn 2) E-mail: ruanmq@ihep.ac.cn 3) E-mail: dayong.wang@pku.edu.cn Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Article funded by SCOAP 3 and published under licence by Chinese Physical Society and the Institute of High Energy Physics of the Chinese Academy of Sciences and the Institute of Modern Physics of the Chinese Academy of Sciences and IOP Publishing Ltd 023003-1
Chinese Physics C Vol. 41, No. 2 (2017) 023003 sion) and e + e − → e + e − H (ZZ fusion) at the CEPC [13– With the recoil mass method, the ZH events are se- 18], as shown in Fig. 1. The corresponding production lected without using the decay information of the Higgs cross sections for the SM Higgs boson of 125 GeV, as boson. Thus the inclusive ZH cross section σ ZH and the functions of center-of-mass energy, are plotted in Fig. 2. coupling g HZZ can be determined in a model-independent At the center-of-mass energy of 250 GeV, the Higgs manner. The measured g HZZ , combined with exclusive bosons are dominantly produced from the ZH process, Higgs boson decay measurements, could be used to de- where the Higgs boson is produced in association with a termine the Higgs boson width and absolute values of Z boson. couplings between the Higgs boson and its decay final states [19]. Meanwhile, the Higgs mass m H can be ex- tracted from the M recoil distribution. A good knowledge of the Higgs mass is crucial since m H is the only free parameter in the SM Higgs potential and it determines the Higgs decay branching ratios in the SM. Based on the model-independent analysis, the Higgs decay infor- mation can be used to further suppress the backgrounds, Fig. 1. Feynman diagrams of the Higgs production leading to a better m H precision. mechanisms at the CEPC: the Higgsstrahlung, The recoil mass method allows better exclusive mea- WW fusion, and ZZ fusion processes. surement of Higgs decay channels. Many new physics models predict a significant branching ratio of the Higgs boson decaying to invisible products [20–23]. At the 10 2 LHC, the current upper limit of this branching ratio is about 40% [24, 25], which is much larger than the value 10 predicted in the SM ( B (H → inv . ) = B (H → ZZ → νν ¯ ν ¯ ν ) = 1.06 × 10 − 3 ). At the CEPC, this measurement can be 1 significantly improved by using the recoil mass method. σ/fb In this paper, we evaluate the upper limit on the branch- 10 � 1 ing ratio of the Higgs decaying to invisible final states. ZH A series of simulation studies of similar processes have been performed at the International Linear Col- 10 � 2 WW f usion lider (ILC) [10, 26]. Compared to the ILC, the collision ZZ f usion environment of the CEPC is significantly different. The 10 � 3 ILC uses polarized beams while the CEPC has no beam polarization. Besides, the beam spot size of the CEPC 1 5 0 200 2 5 0 3 00 3 5 0 at the interaction point (IP) is much larger than that of √ / GeV s the ILC, leading to a much weaker beamstrahlung effect Fig. 2. (color online) Production cross sections of and a narrower beam energy spread [10, 12, 27]. The de- the Higgsstrahlung, WW fusion and ZZ fusion tails of parameter comparison are listed in Table 1 [27]. processes as functions of center-of-mass energy. Due to the above differences, the cross sections for both The dashed lines (black) give the possible work- signal and backgrounds are different. Therefore, it is ing energy range of the CEPC. necessary to perform a full detector simulation for the CEPC. The branching ratio of the Z boson decaying into a pair of muons is 3.3%. The muons can be easily identi- Table 1. Comparison of machine and beam param- fied and their momentum can be precisely measured in eters between the CEPC and the ILC. the detector. By tagging the muon pairs from the asso- parameters CEPC ILC ciated Z boson decays, the Higgsstrahlung events can be horizontal beam size at IP 73700 nm 729 nm reconstructed with the recoil mass method: vertical beam size at IP 160 nm 7.7 nm µ + µ − − 2( E µ + + E µ − ) √ s , 4.7 × 10 − 4 2.0 × 10 − 2 � beamstrahlung parameter s + M 2 M recoil = beam energy spread 0.16% 0.24% 5 ab − 1 2 ab − 1 integrated luminosity where E µ + and E µ − are the energies of the two muons, M µ + µ − is their invariant mass, and s is the square of center-of-mass energy. Therefore, the ZH (Z → µ + µ − ) This paper is organized as follows. Section 2 describes events form a peak in the M recoil distribution at the the detector model, Monte Carlo (MC) simulation and Higgs boson mass. samples used in the studies. Section 3.1 presents the 023003-2
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