Heavy neutrino searches at future Zfactories
Abstract
We analyze the capacity of future Zfactories to search for heavy neutrinos with their mass from 10 to 85 GeV. The heavy neutrinos N are considered to be produced via the process \(e^+e^\rightarrow Z\rightarrow \nu N\) and to decay into an electron or muon and two jets. By means of Monte Carlo simulation of such signal events and the Standard Model background events, we obtain the upper bounds on the cross sections \(\sigma (e^+e^\rightarrow \nu N\rightarrow \nu \ell jj)\) given by the Zfactories with integrated luminosities of 0.1, 1 and 10 \(\hbox {ab}^{1}\) if no signal events are observed. Under the assumption of a minimal extension of the Standard Model in the neutrino sector, we also present the corresponding constraints on the mixing parameters of the heavy neutrinos with the Standard Model leptons, and find they are improved by at least one order compared to current experimental constraints.
1 Introduction
In the Standard Model (SM), only lefthanded neutrinos are introduced and no mechanism is responsible for the generation of neutrino mass. However, the observation of neutrino oscillations [1, 2] has given the evidence that neutrinos have tiny but nonzero mass, which may have opened a window towards the new dynamics beyond the SM. To explain the origin of neutrino mass and why they are much smaller than other fermion mass, different kinds of seesaw mechanisms [3, 4, 5, 6, 7, 8, 9, 10, 11] were proposed and work effectively as simple and straightforward methods. Among them, the TypeI Seesaw [3, 4, 5, 6] is a quite natural extension of the SM by introducing gaugesinglet righthanded neutrinos without violating the SM gauge symmetries. Originally, it was proposed with the Majorana mass terms of the righthanded neutrinos at the scale of grand unified theories [12], which automatically lead to tiny neutrino mass. Later, it was found that a much lower Majorana mass scale, e.g. \({\mathcal {O}}\)(10) GeV, is also possible to explain the neutrino mass (see e.g. [13, 14]), given small Dirac mass terms or some symmetryprotected cancellations in the neutrino mass matrix [15, 16, 17, 18, 19]. On the way to verify any of the TypeI Seesaw models, the most important evidence would be direct discoveries of heavy neutrinos. In this sense, the lowscale seesaw models are of special interest, because their particle spectra may contain heavy neutrinos with mass at \({\mathcal {O}}\)(10) GeV, within the reach of the colliders running now or in the near further. For example, extending the SM by three righthanded neutrinos with mass smaller than the electroweak scale [13, 14], three heavy neutrinos beyond the SM spectrum are generated, one of which has a mass at keV scale as a darkmatter candidate and the other two at GeV to hundred GeV scale. Another interesting property of this model is that the mixing between the light neutrinos and the heavy neutrinos can be quite large. Therefore, heavy neutrinos in such a model have good opportunities to be detected by collider experiments. Once they are detected, it will also give us hints about leptonic CP violation as discussed in [20].
Experimentally, there have been direct searches for heavy neutrinos with \({\mathcal {O}}\)(10) GeV mass by the DELPHI Collaboration [21]. In their searches, the heavy neutrinos were considered to be produced via the \(e^{+}e^{}\rightarrow Z \rightarrow \nu N\) process and decay into visible final states. Unfortunately, no signals were observed and thus only upper bounds on mixing parameters were given. There has also been a direct search by the CMS collaboration [22]. On the other hand, such heavy neutrinos also receive constraints from indirect searches like neutrinoless doublebeta (\(0\nu 2\beta \)) decays [23, 24, 25], and can be explored in meson decays and \(\tau \) decays [26]. Regarding future, there are several lepton colliders proposed by different communities, including the Circular Electron Positron Collider (CEPC) [27], the International Linear Collider (ILC) [28], the FCCee (formerly known as the TLEP) [29] and the Super Z Factory. While the main target of most of these colliders is precision study of the Higgs boson properties, they will also be capable of searching for some new particles, new dynamics and even of studies of quantum chromodynamics and hadrons (see e.g. [30, 31, 32, 33, 34]). Most importantly here, they will be ideal facilities to search for heavy neutrinos. Such abilities of the CEPC with \(\sqrt{s} = \) 240–250 GeV and of the ILC with \(\sqrt{s} = 1\) TeV have been investigated by [35, 36]. One can refer to e.g. [37, 38, 39, 40, 41, 42, 43] for collider searches for neutrinos with higher masses.
In this work, we will focus on searching for heavy neutrinos N with \({\mathcal {O}}\)(10) GeV mass at future Zfactories with integrated luminosities of 0.1, 1 and \(10\,\hbox {ab}^{1}\). Similar to DELPHI, we will also consider the signal production process \(e^+e^\rightarrow Z\rightarrow \nu N\) of heavy neutrinos, and reconstruct the heavy neutrinos by their visible decaying final states each containing one charged lepton and two quark jets. The background mainly originates from the SM processes \(e^+e^\rightarrow jjjj\), \(\tau ^+\tau ^\) and \(b{{\bar{b}}}\). Compared to a Higgs factory such as the CEPC with \(\sqrt{s} = \) 240–250 GeV, we find that future Zfactories are much more sensitive to heavy neutrinos with mass below 80 GeV, because the N production cross sections at the Zmass pole is typically higher than those at 240–250 GeV by orders. We have noticed that employing the technique of displacedvertex detection [44, 45], a search for heavy neutrinos at a Zfactory is almost free of background and can thus set even more stringent constraints to the mixing parameters between the heavy neutrinos and the SM leptons than a normal search. However, the prerequisite of the displacedvertex technique depends on a strong assumption that the studied heavy neutrinos have lifetimes long enough to fly a detectable distance before decaying, which is not true for many models. For example, if a heavy neutrino has decay channels with large decay widths, then it will have a very short lifetime so that its decay vertex is “not displaced”. Heavy neutrinos in a model with a Majoron J [46, 47], which was introduced to generate the Majorana scale in an ultravioletcomplete way, have such a feature. Typically, they can efficiently decay into the light Majoron via the invisible channels \(N\rightarrow \nu J\) and thus have short lifetimes. Therefore, while the constraints given by [44, 45] do not apply to such heavy neutrinos, those given in this work are still valid. In other words, the results of our work without making use of the displacedvertex technique are more model independent than [44, 45].
The rest of the paper is orgnized as follows. In Sect. 2, we will introduce the general setup of the newphysics scenario that we consider. The simulation of the production and decay processes of the heavy neutrinos and the corresponding background events at Zfactories will be described, and the event selection conditions will be explained in Sect. 3. We will present the results in Sect. 4 and conclude by Sect. 5.
2 General setup of the scenario
From the above interaction terms (3), we read out that a heavy neutrino (with the mass smaller than the Z boson mass) can be produced associated with light neutrinos via \(e^+e^\rightarrow Z \rightarrow \overline{\nu }N(\nu \overline{N})\), which is actually the dominant production process.^{2} We also find that the produced heavy neutrino can decay weakly into one charged lepton and one onshell or offshell W boson up to its mass, \(N\rightarrow \ell ^W^{+(*)}\). Here, to reconstruct the heavy neutrinos, we choose the signal events with the decaying products including one charged lepton and two jets, all of which can be collected by detectors, as displayed in Fig. 1. The kinematics requires that the mass of the heavy neutrinos are smaller than the Z boson mass.
Next, we emphasize that even we switch on the mixing between heavy neutrinos and all the other lepton sectors, the signal cross sections \(\sigma ({e^{+}e^{}\rightarrow \nu N\rightarrow \nu \ell jj})\) at a Zfactory still basically only depend on the one corresponding mixing parameter \(V_{\ell N}^2\). The reason is as follows. The cross section \(\sigma ({e^{+}e^{}\rightarrow \nu N_k})\) summing up three light neutrinos and their antiparticles is proportional to the mixing parameter \(\sum _{i=1}^3(U^\dagger V)_{ik}^2\), and under the limit that the matrix U is almost unitary, we have \(\sum _{i=1}^3(U^\dagger V)_{ik}^2 \approx \sum _{\ell }V_{\ell k}^2\). On the other hand, safely neglecting the difference between the charged lepton mass in \(N_k\) decays, we have that \({\mathcal {B}}(N_k\rightarrow \ell jj)\propto V_{\ell k}^{2}/\sum _{\ell '}V_{\ell 'k}^{2}\). Therefore, the two \(\sum _{\ell }V_{\ell j}^{2}\) factors get cancelled when we multiply \(\sigma ({e^{+}e^{}\rightarrow \nu N_k})\) by \({\mathcal {B}}(N_k\rightarrow \ell jj)\), and the relation \(\sigma ({e^{+}e^{}\rightarrow \nu N_k\rightarrow \nu \ell jj})\propto V_{\ell k}^{2}\) is obtained. Here, we emphasize that unless specially noted, our analysis in this work will not depend on the assumption of this paragraph and the previous paragraph, which means that the validity of the results of this work is not limited to the specific model extending the SM with only the (1) terms.
The small peaks appearing where \(M_N\) is slightly above the W boson mass are due to the opening of N decaying into an onshell W boson. The decline of the cross sections at the mass close to 90 GeV results from the suppression of the phase space.
3 Event simulation and selection
For event simulation, we use MadGraph [49] as the event generator for both the SM background and the signal with the new dynamics implemented via FeynRules [50, 51, 52] as mentioned previously. After that, Pythia8 [53] and Delphes [54] are used for further hadronization and fast detector simulation, respectively. In the detector simulation, the eekt algorithm and exclusive search have been chosen to construct jets, which, compared to the default antikt algorithm, is more efficient for a lepton collider and mitigates energy peak drifts of jets.
The signal events are produced from the processes \(e^+e^\rightarrow \nu N\rightarrow \nu \ell jj\), and in this work we consider the charged lepton \(\ell \) to be an electron or a muon. Therefore, the final states are forced into three jets (one leptonic jet and two hadronic jets for example) with the the eekt algorithm. In both the electron and muon cases, the main background comes from the \(e^{+}e^{}\rightarrow jjjj\), \(b\overline{b}\) and \(\tau ^+\tau ^\) processes. In one jjjj event, if one jet is too soft or collinear to the beam and is thus not detected but identified as missing energy, and simultaneously another jet is misidentified as an electron or muon, such an event may mimic a signal event. In this work, we assume that the misidentification rates of jets as electrons and muons are about \(10^{4}\). As for the \(b\overline{b}\) and \(\tau ^+\tau ^\) events, the fermion pairs further decay into final states containing one charged lepton, two jets and missing energy, Open image in new window , as shown in Fig. 3. It should be pointed out that, although the decay products of a \(\tau \) lepton are usually recognized as one single jet, in the eektexclusive algorithm it is possible that the products are reconstructed into two jets. This is due to that this algorithm automatically cluster nearest objects step by step until the final state of the event is reconstructed to exactly 3 jets. As for the other \(q\overline{q}\) events with q being a lighter quark, production at the Zmass pole makes them highly boosted such that it is more difficult for the decay products of q or \({\bar{q}}\) to form two open jets. Therefore, it is much more difficult for a lighter quark pair \(q{\bar{q}}\) to mimic a signal event than a \(b{\bar{b}}\) pair.
In the following, we will discuss how we choose the event selection conditions to suppress the background and increase the signal significance. The jjjj events that mimic signal events typically have missing energy with small transverse momenta Open image in new window . Therefore, a cut requiring a least Open image in new window can efficiently suppress such background.
For the \(\tau \tau \) background as shown in the left panel of Fig. 3, the two \(\tau \) leptons in an event fly backtoback with a large boost. Therefore, the charged lepton and the total missing energy in the final state are almost collinear or reverse to each other, and the angle between the two jets decaying from the same \(\tau \) are very small. These inspire some effective cuts on the angular distances \(\Delta R_{jj}\) and Open image in new window , where \(\Delta R = \sqrt{\Delta \eta ^2+\Delta \phi ^2}\) with \(\Delta \eta \) and \(\Delta \phi \) being the differences between the pseudorapidities and the azimuthal angles of the two involved objects, respectively.
In a \(b\overline{b}\) background event as shown in the right panel of Fig. 3, the neutrino, the charged lepton and one of the two jets decay from the same bottom quark with a large boost, so not only the angular distance between the charged lepton and the neutrino but also the angular distance between the jet and the neutrino are small. Therefore, we set cuts on Open image in new window and Open image in new window to reduce the background from the \(b\overline{b}\) process.
 The event selection conditions for the smallmass range (\(M_{N} < 65\) GeV):

\(P_{T}^{j} > 5\) GeV, \(\eta _{j} < 2\), \(\Delta R_{jj} > 0.1\), btag < 0.8, TauTag = 0, BTag = 0;

\(P_{T}^{\ell } > 3\) GeV, \(\eta _{\ell } < 1\);

Open image in new window GeV, \(E_\text {rec}  M_Z < 10\) GeV, \(M_{\ell jj}  M_N < \Gamma ^M_{1/2}\);

 the event selection conditions for the middlemass range (\(65< M_{N} < 80\) GeV):

\(P_{T}^{j} > 5\) GeV, \(\eta _{j} < 2\), \(\Delta R_{jj} > 0.4\), btag < 0.8, TauTag = 0, BTag = 0;

\(P_{T}^{\ell } > 3\) GeV, \(\eta _{\ell } < 1\);

Open image in new window GeV, \(E_\text {rec}  M_Z < 10\) GeV, Open image in new window , \(M_{\ell jj}  M_N < \Gamma ^M_{1/2}\);

 the event selection conditions for the largemass range (\(80< M_{N} < 85\) GeV):

\(P_{T}^{j} > 10\) GeV, \(\eta _{j} < 2\), \(\Delta R_{jj} > 0.4\), \(M_{jj} > 55\) GeV, btag < 0.8, TauTag = 0, BTag = 0;

\(P_{T}^{\ell } > 3\) GeV, \(\eta _{\ell } < 1\);

Open image in new window GeV, \(E_\text {rec}  M_Z < 10\) GeV, Open image in new window , \(M_{\ell jj}  M_N < \Gamma ^M_{1/2}\);

4 Results and analysis
The main results of this work, the upper bounds on \(\sigma ({e^{+}e^{}\rightarrow \nu N\rightarrow \nu e jj})\) and \(\sigma ({e^{+}e^{}\rightarrow \nu N\rightarrow \nu \mu jj})\) at 95% CL given by future Zfactories, are shown in Fig. 4, with the heavy neutrino mass varying from 10 to 85 GeV. The curves for the integrated luminosities of \(0.1\,\hbox {ab}^{1}\), \(1\,\hbox {ab}^{1}\) and \(10\,\hbox {ab}^{1}\) are presented. We find that for most of the mass range, i.e. 15 GeV \(<M_N<\) 75 GeV, the upper bounds on the production cross sections are around a few \(10^{4}\) pb to \(10^{5}\) pb in both the electron and muon cases, with the integrated luminosities varying from \(0.1\,\hbox {ab}^{1}\), \(1\,\hbox {ab}^{1}\) and \(10\,\hbox {ab}^{1}\). Also, we assume that a significance s larger than 5 in (5) indicates discovery of heavy neutrinos, and show the corresponding smallest discovery cross sections \(\sigma ({e^{+}e^{}\rightarrow \nu N\rightarrow \nu e jj})\) and \(\sigma ({e^{+}e^{}\rightarrow \nu N\rightarrow \nu \mu jj})\) in Fig. 5.
To have a direct comparison with previous experimental constraints and some relevant future ones, we consider the case in which (1) is the only source of new dynamics beyond the SM and give the Zfactory constraints on the mixing parameters \(V_{eN}^2\) and \(V_{\mu N}^2\) in Fig. 6.^{4} This is achievable because the cross sections are proportional to the corresponding mixing parameters, \(\sigma ({e^{+}e^{}\rightarrow \nu N\rightarrow \nu \ell jj})\propto V_{\ell N}^{2}\), as analysed in the last paragraph of Sect. 2. For both \(V_{eN}^2\) and \(V_{\mu N}^2\), we find a large improvement compared to DELPHI [21] and CMS [22], the upper bounds being decreased typically by two orders of magnitude even with the lowest luminosity setup. For \(V_{eN}^2\), the given upper bounds by the considered Zfactories are lower than that given by the \(0\nu 2\beta \) decay experiments [23, 24, 25] by at least one order of magnitude in most of the mass range of the heavy neutrino. While for \(V_{\mu N}^2\), the upper bounds given by the Zfactories are at least two orders of magnitude lower than that given by the CEPC as a Higgs factory [36] when \(M_N<\) 70 GeV. One may worry that a heavy neutrino with mixing parameters as small as such bounds will be so stable that the detection of its decay is always out of reach by detectors. If this is true, the bounds given in Fig. 6 will not be valid, since all our analyses are based on the assumption that the signal events can be detected within detectors once they happen. To clarify that this will not be a problem, we estimate how far a 10 GeV heavy neutrino can typically fly before its decay, and the flying distances of the other heavier neutrinos are always shorter given the same relevant mixing parameters. Considering a detector having a diameter of \({\mathcal {O}}\)(1) meter, we find that as long as the mixing parameter \(\sum _{\ell }V_{\ell N}^{2}\) is larger than \({\mathcal {O}}(10^{9})\), the 10 GeV heavy neutrino is most likely to decay before it can fly out of the detector. Comparing \({\mathcal {O}}(10^{9})\) with the bounds in Fig. 6, one can find that the bounds in the cases with \({\mathcal {L}}\) = 0.1 and \(1\,\hbox {ab}^{1}\) are not affected by the limited size of the detector, and that the case with \({\mathcal {L}} = 10\,\hbox {ab}^{1}\) is also basically safe.
5 Conclusion
To conclude, we have presented a study of possible heavy neutrino searches at future Zfactories. For different heavy neutrinos with mass ranging from 10 to 85 GeV, we have obtained the expected upper bounds on the production cross sections of their discovery processes \(e^{+}e^{}\rightarrow \nu N\rightarrow \nu e jj\) and \(e^{+}e^{}\rightarrow \nu N\rightarrow \nu \mu jj\) given by Zfactory with \({\mathcal {L}}\) = 0.1, 1 and \(10\,\hbox {ab}^{1}\), respectively. Under the assumption that the interactions between the heavy neutrinos and the SM particles are only induced by the neutrino mixing, the constraints on the cross sections have been translated to the constraints on the corresponding mixing parameters \(V_{eN}^2\) and \(V_{\mu N}^2\), which, depending on the luminosity setup, are typically improved by two to four orders compared to the DELPHI constraints [21] and the CMS constraints [22]. We also find that the future Zfactories will set much more stringent constraints on \(V_{eN}^2\) than the \(0\nu 2\beta \) decay experiments [23, 24, 25] by one to three orders, and on \(V_{\mu N}^2\) than the CEPC as a Higgs factory [36] by two to three orders, given the heavy neutrino mass is smaller than 80 GeV.
Footnotes
 1.
For clarification, here \(N_j\) is not a Majorana neutrino by itself, but the lefthanded component of a Majorana neutrino. It corresponds to \(N_j^c\) in [26].
 2.
Even when \(Z\rightarrow N\overline{N}\) is kinematically allowed, the cross section is doubly suppressed by the tiny mixing matrix elements \(V_{\ell j}^2\), which receive stringent constraints from previous experiments such as the DELPHI [21].
 3.
By \(\nu \ell \) we always sum over all possible leptons and their antiparticles, including both the leptonnumberconserving final states like \({\bar{\nu }}\ell ^\) and the leptonnumberbreaking ones like \(\nu \ell ^\).
 4.
Notes
Acknowledgements
This work is supported by the Fundamental Research Funds for the Central Universities under the Grant no. lzujbky2019it08, the National Natural Science Foundation of China under the Grant no. U1732101, the Gansu Natural Science Fund under the Grant no. 18JR3RA265 and the Deutsche Forschungsgemeinschaft (DFG) within research unit FOR 1873 (QFET). We are grateful to QingHong Cao, Cheng Chen, Min He, Qiang Li, QiShu Yan and YeLing Zhou for useful discussions. In addition, JND would like to thank Ying Guan for her encouragement.
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