-
We consider ALPs generated from the breaking of the global Peccei-Quinn (PQ) symmetry [1],
U(1)PQ . Based on the structure of PQ symmetry,a(x)→a(x)+const , the Lagrangian can be written in the formLμALP=∂νaJνPQ,μ . The general muon current is in the formJνPQ,μ=cVμ2Λ¯μγνμ+cAμ2Λ¯μγνγ5μ+cνμ2Λ¯νμγνPLνμ,
(1) where Λ is the new physics scale, and
cVμ ,cAμ , andcνμ are dimensionless couplings. Without the assumption of electroweak invariance, the conditioncνμ=cVμ−cAμ in Eq. (1) can be released1 . After integrating by parts for this Lagrangian,LμALP can be written as [50]a∂νJνPQ,μ=icAμmμΛa¯μγ5μ+αem4πΛ[cVμ−cAμ+cνμ4s2WaW+μν˜W−,μν+cVμ−cAμ(1−4s2W)2sWcWaFμν˜Zμν−cAμaFμν˜Fμν+cVμ(1−4s2W)−cAμ(1−4s2W+8s4W)+cνμ8s2Wc2WaZμν˜Zμν]+igW2√2Λ(cAμ−cVμ+cνμ)a(ˉμγνPLνμ)W−ν+h.c.,
(2) where the symbols
W±μν ,Zμν , andFμν represent the field strength tensors of the massive gauge bosonsW± and Z and the massless photon, respectively, the dual field strength tensor is defined as˜Fμν=12ϵμνρσFρσ ,αem is the fine structure constant,gW is the weak coupling constant, andsW andcW are the sine and cosine of the weak mixing angle, respectively.In Eq. (2), we label the first term as "
\boldsymbol{a\mu\mu} ," which can generate μALPs through muon radiation. However, this term is suppressed bym_{\mu}/\Lambda , necessitating high-intensity experiments to search for light μALPs. The second to fourth terms, labeled as "\boldsymbol{aVV'} ," arise from the chiral anomaly and can produce light μALPs through flavor-changing processes in meson decays [48, 67, 68]. Heavier μALPs can also be produced from these terms through gauge boson fusion and associated gauge boson production processes, despite not being proportional tom_{\mu} , but having\alpha_{\text{em}}/4\pi suppression. The terms in the final line of Eq. (2), labeled as "\boldsymbol{aW\mu\nu} ," are often overlooked in literature [69]. However, they are critical to our study, particularly when searching for μALPs at the GeV scale. This four-point interaction, W-μ-\nu_{\mu} -a, vanishes when the general muon current in Eq. (1) respects the electroweak symmetry. Moreover, this interaction is not related tom_{\mu} and has an obvious\left(\text{energy}/\Lambda\right) enhancement in specific processes. This enhancement behavior is crucial in constraining light μALPs through decays of the W boson and charged mesons [50], as well as in searching for heavier μALPs in t-channel processes such as\mu^{+}\mu^{-}\rightarrow \nu_{\mu} a\overline{\nu_{\mu}} at muon colliders under the EWV scenario, which is defined in the next section.However, searching for μALPs in collider experiments depends on their decay modes. For μALP masses below the electroweak scale (
m_a\lesssim M_W ), their dominant decay modes are to\mu^{+}\mu^{-} and\gamma\gamma [25, 45, 70]. The decay widths are given by\Gamma_{a\rightarrow \mu^+\mu^-} = \frac{(c^A_{\mu})^2 m^2_{\mu} m_a}{8\pi\Lambda^2}\sqrt{1-\frac{4m^2_{\mu}}{m^2_a}} \; ,\; \; \Gamma_{a\rightarrow \gamma\gamma} = \frac{g^2_{a\gamma\gamma}m^3_a}{64\pi} ,
(3) where the coupling constant
g_{a\gamma\gamma} is determined by the chiral anomaly and one-loop triangle Feynman diagrams and can be expressed asg_{a\gamma\gamma} = \frac{\alpha_{\text{em}}}{\pi}\frac{c^A_{\mu}}{\Lambda}\Bigg| 1 - {\cal F} \left(\frac{m^2_a}{4m^2_{\mu}}\right)\Bigg|
(4) and the loop function
{\cal F} (z > 1) = \dfrac{1}{z}\text{arctan}^2\left(\dfrac{1}{\sqrt{1/z -1}}\right) . Here, we only consider the contribution from the muon loop because the contribution from the W boson is highly suppressed and can be safely neglected.Figure 1 shows the branching ratios for
a\rightarrow \mu^{+}\mu^{-} anda\rightarrow \gamma\gamma . Whenm_a\lesssim M_W , the dominant decay mode of μALPs isa\rightarrow \mu^{+}\mu^{-} . Because the partial decay width ofa\rightarrow \gamma\gamma depends slightly on the muon mass and scales withm_a^3 , we can expect the branching ratio ofa\rightarrow \gamma\gamma to increase with the μALP mass. It is important to note that this result is opposite to that of the electrophilic ALPs in Ref. [51] because the muon mass is considerably larger than the electron mass. -
In this section, we focus on distinguishing between different types of ALP-muon interactions at muon colliders. First, we consider the signal process
\mu^{+}\mu^{-}\rightarrow \nu_{\mu}a\overline{\nu_{\mu}} with the relevant Feynman diagrams shown in Fig. 2 and numerically investigate the energy enhancement behavior of this process at muon colliders. To implement{\cal L}_{\mu\text{ALP}} from Eq. (2), we use FeynRules [71] and calculate cross sections for this process using Madgraph5_aMC@NLO [72] while varying the center-of-mass energy. As we know, the conditionc_{\nu_{\mu}} = c^V_{\mu} -c^A_{\mu} is a criterion to determine whether ALP effective field theory is electroweak invariant or not. Therefore, we setc^A_{\mu} / \Lambda = 10 TeV^{-1} andc^V_{\mu} = c_{\nu_{\mu}}=0 as a benchmark point for the EWV scenario. Similarly, we setc^A_{\mu}/ \Lambda = c^V_{\mu}/ \Lambda = 10 TeV^{-1} andc_{\nu_{\mu}}=0 as a benchmark point for the EWP scenario. We vary the center-of-mass energy\sqrt{s} between1-15 TeV withm_a = 10 GeV at muon colliders. Fig. 3 shows the energy enhancement behavior of cross sections in\mu^{+}\mu^{-}\rightarrow \nu_{\mu} a\overline{\nu_{\mu}} , where the full contributions from the EWV and EWP scenarios are depicted by solid lines, and the contributions from\boldsymbol{aW\mu\nu} ,\boldsymbol{aVV'} , and\boldsymbol{aW\mu\nu} in the EWV scenario are depicted by dashed lines.Figure 2. (color online) Feynman diagrams for
\mu^{+}\mu^{-}\rightarrow \nu_{\mu} a\overline{\nu_{\mu}} . Here, the red marker indicates the\boldsymbol{aW\mu\nu} interaction, green indicates the\boldsymbol{aVV'} interaction, and blue indicates the\boldsymbol{a\mu\mu} interaction.Figure 3. (color online) Energy enhancement behavior of cross sections in
\mu^{+}\mu^{-}\rightarrow \nu_{\mu} a\overline{\nu_{\mu}} withm_a = 10 GeV,c^A_{\mu}/\Lambda = 10 TeV^{-1} ,c^V_{\mu}=c_{\nu_{\mu}}=0 (EWV: solid-blue line) andc^A_{\mu} / \Lambda = c^V_{\mu}/ \Lambda = 10 TeV^{-1} ,c_{\nu_{\mu}}=0 (EWP: solid-olive line). In the EWV scenario, the dashed-red, dashed-green, and dashed-black lines are labeled as contributions from the\boldsymbol{aW\mu\nu} ,\boldsymbol{aVV'} , and\boldsymbol{a\mu\mu} interactions, respectively.As shown in Fig. 3, the leading contribution in the EWV scenario originates from the
\boldsymbol{aW\mu\nu} interaction, with the subleading contribution from the\boldsymbol{aVV'} interaction. The contribution from the\boldsymbol{aVV'} interaction is approximately seven orders of magnitude smaller than that from\boldsymbol{aW\mu\nu} , as depicted by the dashed lines in Fig. 3. For\sqrt{s} = 1-2 TeV, the energy enhancement behaviors from these two interactions are evident because the momentum transfer size becomes sufficiently large, making the\boldsymbol{aW\mu\nu} and\boldsymbol{aVV'} interactions important. However, as the energy continues to increase, the growth rate becomes gentler because these two leading contributions steadily increase with the center-of-mass energy as\left(\text{energy}/\Lambda\right) .Our numerical analysis reveals that the contribution from the
\boldsymbol{aW\mu\nu} interaction is considerably greater than those from the\boldsymbol{aVV'} and\boldsymbol{a\mu\mu} interactions because of the novel energy enhancement behavior. Therefore, we show the analytical form for the amplitude square with the average (sum) over initial (final) polarization for the\boldsymbol{aW\mu\nu} interaction in the process\mu^+(p_1)\mu^-(p_2) \rightarrow \nu_{\mu}(q_1)a(q_2)\overline{\nu_{\mu}}(q_3) ,\begin{aligned}[b] \overline{\lvert{\cal M}\rvert ^2} = & \frac{g^4_W\left( c^A_{\mu}-c^V_{\mu}+c_{\nu_{\mu}}\right) ^2}{32\Lambda^2}\left(\frac{1}{k^2-M^2_W}+\frac{1}{k^{\prime 2}-M^2_W}\right) ^2 \\ & \times\left( s-2m^2_{\mu}\right)\left[ s-m^2_a -2q_2\cdot (q_1 +q_3)\right] \,, \end{aligned}
where
s = \left(p_1 +p_2\right) ^2 = \left(q_1 + q_2 + q_3\right) ^2 ,k = p_2 -q_3 , andk' = p_1 -q_1 . This shows that the amplitude square can be enhanced when the momentum transfer in the t-channel process is sufficiently large.In the EWV scenario, the contribution from the
\boldsymbol{a\mu\mu} interaction is negligible, whereas in the EWP scenario, both the\boldsymbol{aVV'} and\boldsymbol{a\mu\mu} interactions exist in\mu^{+}\mu^{-}\rightarrow \nu_\mu a\overline{\nu_\mu} . The cross sections have no obvious change with the increase in the center-of-mass energy in the EWP scenario owing to the lack of the energy enhancement effect. Lastly, the cross sections in the EWV scenario are more than six orders of magnitude larger than those in the EWP scenario for the\mu^{+}\mu^{-}\rightarrow \nu_{\mu} a\overline{\nu_{\mu}} process, as shown in Fig. 3. This is because there is an\boldsymbol{aW\mu\nu} interaction in the EWV scenario but not in the EWP scenario, and this interaction contributes to almost the entire cross-section amount in the EWV scenario. Therefore, this process is powerful in distinguishing μALPs in the EWV scenario from those in the EWP scenario.Next, we discuss four optimal channels for searching for μALPs at muon colliders. Among these channels, we specifically consider the μALP couplings that are independent of the muon mass. These μALP production channels are
\mu^+\mu^-\rightarrow \nu_{\mu}a\overline{\nu_{\mu}} ,\mu^{+}\mu^{-}\rightarrow \gamma a ,\mu^{+}\mu^{-}\rightarrow \mu^{+}\mu^{-} a , and\mu^{+}\mu^{-}\rightarrow Z a . To compare these μALP production channels at a muon collider with\sqrt{s} = 3 TeV, we calculate their cross sections in both the EWV and EWP scenarios with the benchmark pointm_a = 10 GeV andc^A_{\mu} / \Lambda = 10 TeV^{-1} , as shown in Table 1. In the EWV scenario, we find that the\mu^+\mu^-\rightarrow \nu_{\mu}a\overline{\nu_{\mu}} channel has the largest cross section owing to its energy-enhancing behavior caused by the W-μ-\nu_{\mu} -a interaction. The cross section of this channel is approximately six to seven orders of magnitude larger than those of other channels. However, in the EWP scenario, there is no energy-enhancing behavior in the\mu^+\mu^-\rightarrow \nu_{\mu}a\overline{\nu_{\mu}} channel, such that the cross section of this channel becomes smaller than those of other channels. At this point, the cross section of the\mu^{+}\mu^{-}\rightarrow \gamma a channel is the largest, making it the most prospective search channel in the EWP scenario.Production channel Cross section/fb {EWV} {EWP} \mu^{+}\mu^{-}\rightarrow \nu_{\mu} a \overline{\nu}_{\mu} 3.13\times 10^{4} 9.69\times 10^{-3} \mu^{+}\mu^{-} \rightarrow \mu^{+}\mu^{-}a 1.45\times 10^{-2} 1.69\times 10^{-2} \mu^{+}\mu^{-} \rightarrow a \gamma 7.72\times 10^{-2} 8.18\times 10^{-2} \mu^{+}\mu^{-} \rightarrow Z a 3.58\times 10^{-3} 2.54\times 10^{-2} Table 1. Cross sections of different μALP production channels at a muon collider with
\sqrt{s} = 3 TeV and the benchmark pointm_a = 10 GeV andc^A_{\mu} / \Lambda = 10 TeV^{-1} . -
In this section, we conduct a signal and background analysis on three specific search channels: the
\mu^+\mu^-\rightarrow \nu_{\mu}a\overline{\nu}_{\mu} ,\mu^{+}\mu^{-}\rightarrow \gamma a , and\mu^{+}\mu^{-}\rightarrow \mu^{+}\mu^{-} a processes. Our goal is to predict the future bounds for GeV-scale μALPs at muon colliders and compare them with existing bounds. -
To illustrate, we analyze the process
\mu^+\mu^-\rightarrow \nu_{\mu}a\overline{\nu}_{\mu} in the EWV scenario and its relevant SM backgrounds in the context of the popular muon collider proposal with\sqrt{s}=3 TeV [53, 54]. According to Fig. 1, the μALP mainly decays into\mu^+\mu^- whenm_a\lesssim M_W . Hence, we focus on thea\rightarrow \mu^+\mu^- decay mode in our analysis. The μALP becomes highly boosted at the muon collider when it is sufficiently light; hence,\mu^{+} \mu^{-} in the final state may be too collimated to pass the muon isolation criterion at detectors. Taking a cone sizeR=0.1 as the muon isolation criterion at the muon collider, we find that a pair of muons cannot be isolated from each other at detectors whenm_a \lesssim 15 GeV (parton-level), as shown in the left panel of Fig. 4. We can group this type of collimated non-isolated muon as a special signature "muon-jet" (J_{\mu} ), which is a non-QCD jet-like structure that deposits most of its energy in the the muon spectrometer and has a distinct signature from QCD jets. Therefore, we classify the signal signatures into two categories: (1) two isolated muons plus missing energy ({\:/ E} ) form_a\gtrsim 15 GeV, and (2)J_{\mu} plus{\:/ E} form_a \lesssim 15 GeV.Figure 4. (color online) Left panel: Distribution of the opening angle between two muons,
\Delta R_{\mu^{+}\mu^{-}} , from\mu^+\mu^-\rightarrow \nu_{\mu}a\overline{\nu_{\mu}} at a muon collider with\sqrt{s}=3 TeV. Four benchmark mass values of μALP,m_{a}=5 ,20 ,50 ,80 GeV, are displayed. Right panel: Distribution of the μALP lab frame decay length from\mu^+\mu^-\rightarrow \nu_{\mu}a\overline{\nu_{\mu}} at a muon collider with\sqrt{s}=3 TeV. The benchmark pointsm_a =5 ,20 GeV withc^A_{\mu} / \Lambda = 0.1 TeV^{-1} are considered.For the first signal signature, two relevant SM backgrounds,
\mu^{+}\mu^{-}\rightarrow \nu_{\ell}\overline{\nu_{\ell}}\mu^{+}\mu^{-} and\mu^{+}\mu^{-}\rightarrow t\overline{t}\rightarrow (b\mu^+\nu_{\mu}) (\overline{b}\mu^-\overline{\nu_{\mu}}) , are considered. We choose the benchmark pointm_a = 50 GeV withc^A_{\mu} / \Lambda = 0.1 TeV^{-1} to display the signal features. To generate Monte Carlo samples for both signal and background processes, we use Madgraph5_aMC@NLO [72] and pass them to Pythia8 [73] for QED and QCD showering and hadronization effects. We impose pre-selection cuts (P^{\mu}_T > 5 GeV and\lvert\eta_{\mu}\rvert < 2.5 ) at the parton-level for both the signal and backgrounds. To simulate the detector effects, we use the muon collider template in Delphes3 [74], whose muon isolation criterion is consistent with Refs. [75, 76]. We use the Cambridge/Aachen (C/A ) jet clustering algorithm [77, 78] and consider a b-jet tagging efficiency of\epsilon_b = 0.8 with charm-jet and light-jet fake rates ofP_{c\rightarrow b}=0.1 andP_{j\rightarrow b}=10^{-3} , respectively. The following event selections are required to identify the signal signature and suppress background events:● (1)
N(\mu)\ge 2 withP_T^{\mu_1} > 200 GeV,P_T^{\mu_2} > 10 GeV,\lvert\eta_{\mu_{1,2}}\rvert < 1.5 ,● (2)
1500 < {\not E} < 2800 GeV and\lvert\eta_{{\:/ E}}\rvert < 1.8 ,● (3) Veto
N(b)\ge1 GeV withP_T^{b} > 25 GeV,● (4)
{\not E}/M_{\mu_1\mu_2} > 32 ,● (5)
\lvert M_{\mu_1\mu_2}-m_a\rvert < 2 GeV,● (6)
3.0 < \Delta\phi_{\mu_1, {\:/ E}} < 3.3 and2.9 < \Delta\phi_{\mu_2, {\:/ E}} < 3.5 ,where
P_T^{\mu_1} ,P_T^{\mu_2} (\eta_{\mu_1} ,\eta_{\mu_2} ) are the transverse momentum (pseudorapidity) of leading and subleading energetic muons,{\not E} is the missing energy,M_{\mu_1\mu_2} is the invariant mass of a muon pair, and\Delta\phi_{\mu_i, {\not E}} is the azimuthal angle between the i-th muon and{\not E} . The cut-flow table including the signal and backgrounds for each event selection is shown in Table 2, and several kinematic distributions are shown in Fig. A1 of Appendix A.Cut flow in σ/fb Signal \nu_{\ell}\overline{\nu_{\ell}}\mu^{+}\mu^{-} t\overline{t} Generator 2.54 162.70 4.15\times 10^{-2} cut-(1) 1.78 18.60 7.94\times 10^{-3} cut-(2) 1.78 11.28 7.21\times 10^{-3} cut-(3) 1.78 11.27 3.54\times 10^{-4} cut-(4) 1.74 0.15 2.12\times 10^{-5} cut-(5) 1.47 1.23\times 10^{-2} 8.30\times 10^{-7} cut-(6) 1.35 6.17\times 10^{-3} 0 Table 2. Cut-flow table for
\mu^{+}\mu^{-}\rightarrow \nu_{\ell} (a\rightarrow \mu^{+}\mu^{-})\overline{\nu_{\ell}} and relevant SM backgrounds with the signature of two isolated muons plus{\not E} . The benchmark pointm_a = 50 GeV withc^A_{\mu} / \Lambda = 0.1 TeV^{-1} is chosen for the signal. Each event selection is mentioned in the main text. The "Generator" means the cross sections at the parton-level calculated by Madgraph5_aMC@NLO.First, we find two isolated muons and
\not E in the central region of the signal events. To select candidate events, we apply the following trigger criteria:P_T^{\mu_1} > 200 GeV,P_T^{\mu_2} > 10 GeV, and\not E > 1500 GeV. In Fig. A1, the distributions ofP_T^{\mu_1} and\not E show two peaks that correspond to the\mu^{+}\mu^{-}\rightarrow \nu_{\ell}\overline{\nu_{\ell}}\mu^{+}\mu^{-} process. The right peak of the\not E distribution indicates that most of the energy is carried away by the neutrino pair, leaving minimal energy for the two muons, whereas the left peak indicates that each of the two muons and two neutrinos carries almost an equal share of the energy. For the\mu^{+}\mu^{-}\rightarrow t\overline{t}\rightarrow (b\mu^+\nu_{\mu})(\overline{b}\mu^-\overline{\nu_{\mu}}) process, the\not E distribution peak is around2600 GeV, indicating that the two neutrinos take away more energy. As the signal\not E distribution peak is around2000 GeV, we apply\not E < 2800 GeV to reduce these two background events. Moreover, to suppress the b jet background from the\mu^{+}\mu^{-}\rightarrow t\overline{t}\rightarrow (b\mu^+\nu_{\mu})(\overline{b}\mu^-\overline{\nu_{\mu}}) process, we veto events withN(b)\ge1 GeV andP_T^b > 25 GeV. We also apply the ratio\not E/M_{\mu_1\mu_2} as a complementary selection for the μALP mass window, setting\not E/M_{\mu_1\mu_2} > 32 . This selection is based on the observation that the position of the averageM_{\mu_1\mu_2} distribution of the\mu^{+}\mu^{-}\rightarrow \nu_{\ell}\overline{\nu_{\ell}}\mu^{+}\mu^{-} process is larger than that of the signal, and the\not E distribution from this background is relatively small in the range1500 < \not E < 2800 GeV. The μALP mass window selection effectively reduces these two backgrounds while keeping most of the signal events. By applying the cuts of\Delta\phi_{\mu_{1,2}, {\not E}} to reduce some events from\mu^{+}\mu^{-}\rightarrow \nu_{\ell}\overline{\nu_{\ell}}\mu^{+}\mu^{-} , we observe that two isolated muons are well-separated from\not E . In particular, the distribution of\Delta\phi_{\mu_2, {\not E}} is not so large in both\mu^{+}\mu^{-}\rightarrow \nu_{\ell}\overline{\nu_{\ell}}\mu^{+}\mu^{-} and\mu^{+}\mu^{-}\rightarrow t\overline{t}\rightarrow (b\mu^+\nu_{\mu})(\overline{b}\mu^-\overline{\nu_{\mu}}) compared to the signal. Finally, using a benchmark integrated luminosity{\cal L} = 120 fb^{-1} of a muon collider, we define the signal significance Z [79] as\begin{array}{*{20}{l}} Z = \sqrt{2\cdot\left( (N_s + N_b)\cdot ln(1+N_s/N_b)-N_s\right)}, \end{array}
(5) where
N_s andN_b are the relevant signal and background event numbers. Here, the systematic uncertainties are not considered in our simple analysis because the muon collider is still a future collider. After all of the event selections in Table 2, we find that the signal significance can reachZ=38 for our benchmark point of{\cal L} = 120 fb^{-1} , which meansc^A_{\mu} / \Lambda < 0.1 TeV^{-1} is still detectable in the future.In the above analysis, we consider prompt μALP decay with the lab frame decay length
\gamma\beta c\tau_a < 1 mm as a criterion at a muon collider. Here, γ is the Lorentz factor, β is the μALP velocity, and\tau_a is the proper decay time of the μALP. However, as we can expect, the μALP lab frame decay length becomes longer whenm_a ,c^A_{\mu}/ \Lambda are small and β is large. In this situation, μALPs become long-lived particles (LLPs). We take two benchmark points,m_a =5, 20 GeV withc^A_{\mu} / \Lambda = 0.1 TeV^{-1} , to display the\gamma\beta c\tau_a distribution from\mu^+\mu^-\rightarrow \nu_{\mu}a\overline{\nu_{\mu}} at the muon collider in the right panel of Fig. 4. We discuss the situation of μALPs as LLPs in Sec. IV.C.For the second signal signature, possible SM backgrounds originate from
\nu_{\ell}\overline{\nu_{\ell}}b\overline{b} and\nu_{\ell}\overline{\nu_{\ell}}c\overline{c} , where the heavy flavor mesons produced from c, b jets can decay into a collimated muon pair and mimicJ_{\mu} from the signal. The pre-selection cuts (P_T^{\mu} > 5 GeV,\lvert\eta_{\mu}\rvert < 2.5 ) at the parton-level are used for signal and background processes. We take the signal benchmark point asm_a = 5 GeV andc^A_{\mu}/ \Lambda = 0.1 TeV^{-1} . TheC/A jet clustering algorithm forJ_{\mu} with a cone sizeR = 0.1 , which corresponds to the muon isolation criterion at the muon collider, is applied. We set up event selections to identify the signal signature and suppress the background events below:● (1)
N(\mu)\ge 2 withP_T^{\mu_{1,2}} > 5 GeV,\lvert\eta_{\mu_{1,2}}\rvert < 2.5 ,● (2)
N(J_{\mu}) = 1 andP_T^{J_{\mu}} > 20 GeV,\lvert\eta_{J_{\mu}}\rvert < 2 ,● (3)
1500 < {\not E} < 2800 GeV and\lvert\eta_{{\:/ E}}\rvert < 1.4 ,● (4) Veto
N(b)\ge1 GeV withP_T^{b} > 25 GeV,● (5)
\lvert M_{J_{\mu}}-m_a\rvert < 2 GeV.A cut-flow table including the signal and backgrounds for each event selection is shown in Table 3, and several kinematic distributions are shown in Fig. A2 of Appendix A.
Cut flow in σ/fb Signal \nu_{\ell}\overline{\nu_{\ell}}c\overline{c} \nu_{\ell}\overline{\nu_{\ell}}b\overline{b} Generator 2.73 208.20 633.60 \gamma\beta c\tau_a< 1 mm0.52 - - cut-(1) 0.50 4.86\times 10^{-3} 0.17 cut-(2) 0.50 1.39\times 10^{-3} 2.41\times 10^{-2} cut-(3) 0.47 0 6.31\times 10^{-3} cut-(4) 0.47 0 1.27\times 10^{-3} cut-(5) 0.42 0 0 Table 3. Similar to Table.II, but for the signal benchmark point
m_a = 5 GeV andc^A_{\mu} / \Lambda = 0.1 TeV^{-1} as well as the signature of aJ_{\mu} candidate plus\not E .For the μALP prompt decay, we first set
\gamma\beta c\tau_a < 1 mm as a criterion. Then, two muons withP_T^{\mu} > 5 GeV and\lvert\eta_{\mu}\rvert < 2.5 are required to be detectable in the muon spectrometer. We consider aJ_{\mu} candidate withP_T^{J_{\mu}} > 20 GeV and\not E > 1500 GeV as the trigger, which are mainly distributed in the central region. TheJ_{\mu} in the signal events arises from energetic μALPs, whereas in the background events, it arises from the decay of heavy flavor mesons. As shown in Fig. A2,P_T^{J_{\mu}} of the signal is considerably larger than that of the backgrounds, and most of the background events are largely reduced after cut-(2). We further require the selection\not E < 2800 GeV, which retains most signal events while removing significant parts of the background events; in particular, the events from\mu^{+}\mu^{-}\rightarrow \nu_{\ell}\overline{\nu_{\ell}}c\overline{c} are entirely removed. To suppress\mu^{+}\mu^{-}\rightarrow \nu_{\ell}\overline{\nu_{\ell}}b\overline{b} , we vetoN(b)\ge1 GeV withP_T^{b} > 25 GeV. We also require the jet mass ofJ_{\mu} to satisfy the μALP mass window selection, which can entirely remove events from\mu^{+}\mu^{-}\rightarrow \nu_{\ell}\overline{\nu_{\ell}}b\overline{b} . After all event selections in Table 3, we can take this signal benchmark point as background-free. The distribution of the peak ofM_{J_{\mu}} is broader than that ofM_{\mu_1\mu_2} because two muons within aJ_{\mu} cannot pass the muon isolation criteria. The selection of the jet clustering method, in conjunction with the choice of cone sizeR=0.1 , can affect the four-momentum reconstruction ofJ_{\mu} . In some cases, one of the muons is outside the jet cone and cannot be reconstructed, leading to distortions inM_{J_{\mu}} compared toM_{\mu_1\mu_2} . With{\cal L} = 120 fb^{-1} , there are 50 signal events remaining for this benchmark point after all event selections. -
In the EWP scenario, we employ a different approach to search for μALPs compared to the EWV scenario. As explained toward the end of Sec. III, the
\mu^+\mu^-\rightarrow \nu_{\mu}a\overline{\nu_{\mu}} process in the EWP scenario lacks energy-enhancement behavior, leading to a smaller production cross section. For this reason, we focus on the two μALP production channels\mu^{+}\mu^{-}\rightarrow \gamma a and\mu^{+}\mu^{-}\rightarrow \mu^{+}\mu^{-} a , which have larger production cross sections, for the signal-to-background analyses to obtain stronger future bounds. We analyze the process\mu^{+}\mu^{-}\rightarrow \gamma a (a\rightarrow \mu^{+}\mu^{-} ) using the same method as in Sec. IV.A. The details and results are presented below. The signal signatures are first classified into two categories: (1) two isolated muons plus a photon (γ) form_a\gtrsim 15 GeV, and (2)J_{\mu} plus γ form_a \lesssim 15 GeV. To investigate the first signal signature, we consider the relevant SM background\mu^{+}\mu^{-}\rightarrow \gamma\mu^{+}\mu^{-} and choose the benchmark pointm_a = 50 GeV withc^A_{\mu} / \Lambda = 10 TeV^{-1} to display the signal features. The following event selections are required to identify the signal signature and suppress background events:● (1)
N(\mu)\ge 2 withP_T^{\mu_1} > 100 GeV,10 < P_T^{\mu_2} < 500 GeV,\lvert\eta_{\mu_{1,2}}\rvert < 1.5 ,● (2)
E_{\gamma} > 1450 GeV and\lvert\eta_{\gamma}\rvert < 1.6 ,● (3)
2.9 < \Delta\phi_{\mu_1, \gamma} < 3.4 and2.9 < \Delta\phi_{\mu_2,\gamma} < 3.3 ,● (4)
E_{\gamma}/M_{\mu_1\mu_2} > 29 ,● (5)
\lvert M_{\mu_1\mu_2}-m_a\rvert < 2.0 GeV.Here,
E_{\gamma} is the energy of the photon, and\Delta\phi_{\mu_i,\gamma} is the azimuthal angle between the i-th muon and the photon. The cut-flow table detailing the signal and background for each event selection is presented in Table 4, with several relevant kinematic distributions shown in Fig. A3 of Appendix A. First, we find that two isolated muons in the signal are predominantly located in the relatively low transverse momentum regions (as shown in Fig. A3), whereas a number of signal events forE_{\gamma} are mainly located in the relatively higher energy regions. This is because two muons in the signal are produced from the decay of the μALPs, which are secondary particles. By contrast, two muons from the background events mainly originate from the initial particles. Similarly, the photon in the signal is produced from the initial muons and therefore becomes more energetic. However, for the background process, the energy of photons is more divided byP_{z} of muons; hence, the leading photon energy is smaller than that of the signal process, as shown in Fig. A3. To select candidate events, we apply the following trigger criteria:P_T^{\mu_1} > 100 GeV,P_T^{\mu_2} > 10 GeV, andE_{\gamma} > 1450 GeV. To reduce background events, we apply the cut of\Delta\phi_{\mu_{1,2},\gamma} because we observe that two isolated muons are well-separated from the photon. Additionally, we implemented a complementary selection based on the ratioE_{\gamma}/M_{\mu_1\mu_2} to further reduce the contribution from\mu^+\mu^-\rightarrow \gamma\mu^+\mu^- . Specifically, we set the ratioE_{\gamma}/M_{\mu_1\mu_2} > 29 , which effectively reduces background events while retaining most of the signal events. Since the average position of theM_{\mu_1\mu_2} distribution for the\mu^{+}\mu^{-}\rightarrow \gamma\mu^{+}\mu^{-} process is considerably wider than that of the signal, the μALP mass window can further reduce the number of background events. Finally, we consider a benchmark integrated luminosity of a muon collider of{\cal L} = 1000 fb^{-1} to our analysis. After all of the event selections in Table 4, we find that the signal significance can reachZ=6.379 .Cut flow in σ/fb Signal \mu^{+}\mu^{-}\rightarrow \gamma\mu^{+}\mu^{-} Generator 6.84\times 10^{-2} 179.80 cut-(1) 2.70\times 10^{-2} 4.72 cut-(2) 2.69\times 10^{-2} 0.98 cut-(3) 2.48\times 10^{-2} 0.56 cut-(4) 2.03\times 10^{-2} 2.70\times 10^{-2} cut-(5) 1.68\times 10^{-2} 3.06\times 10^{-3} Table 4. Cut-flow table for
\mu^{+}\mu^{-}\rightarrow \gamma a and the relevant SM background with the signature of two isolated muons plus a photon. The benchmark pointm_a = 50 GeV withc^A_{\mu} / \Lambda = 10 TeV^{-1} is chosen for the signal.For the second signal signature, possible SM backgrounds originate from
\gamma b\overline{b} and\gamma c\overline{c} , where the heavy flavor mesons generated from c and b jets can decay into a collimated muon pair and mimicJ_{\mu} from the signal. The pre-selection cuts (P_T^{\mu} > 5 GeV,\lvert\eta_{\mu}\rvert < 2.5 ) at the parton-level are used for the signal and background processes. We still choose the signal benchmark point asm_a = 5 GeV andc^A_{\mu}/ \Lambda = 10 TeV^{-1} . Applying theC/A jet clustering algorithm forJ_{\mu} with a cone sizeR = 0.1 , we implement event selection criteria to isolate the signal and suppress background events as specified below:● (1)
N(\mu)\ge 2 withP_T^{\mu_{1,2}} > 5 GeV,\lvert\eta_{\mu_{1,2}}\rvert < 2.5 ,● (2)
N(J_{\mu}) = 1 andP_T^{J_{\mu}} > 750 GeV,\lvert\eta_{J_{\mu}}\rvert < 1.5 ,● (3)
E_{\gamma} > 1400 GeV and\lvert\eta_{\gamma}\rvert < 1.0 ,● (4)
\lvert M_{J_\mu}-m_a\rvert < 3.0 GeV,● (5)
100 < E_{\gamma}/M_{J_\mu} < 400 .The reasons for implementing these event selection criteria are similar to those described earlier and will not be repeated here. A cut-flow table detailing the signal and backgrounds for each event selection is presented in Table 5, with several relevant kinematic distributions shown in Fig. A4 of Appendix A. Employing all of the event selections listed in Table 3, we find that the signal significance can achieve a value of
Z=20.10 with{\cal L} = 1000 fb^{-1} .Cut flow in σ/fb Signal \gamma c\overline{c} \gamma b\overline{b} Generator 8.03\times 10^{-2} 5.96 8.48 cut-(1) 7.78\times 10^{-2} 4.70\times 10^{-3} 1.78\times 10^{-2} cut-(2) 6.36\times 10^{-2} 7.63\times 10^{-4} 5.49\times 10^{-4} cut-(3) 5.29\times 10^{-2} 1.19\times 10^{-5} 1.50\times 10^{-4} cut-(4) 4.43\times 10^{-2} 0 1.33\times 10^{-4} cut-(5) 3.61\times 10^{-2} 0 4.99\times 10^{-5} Table 5. Similar to Table 4, but for the signal benchmark point
m_a = 5 GeV andc^A_{\mu} / \Lambda = 10 TeV^{-1} as well as the signature of aJ_{\mu} candidate plus a photon.In the subsequent section, we analyze the process
\mu^{+}\mu^{-}\rightarrow \mu^{+}\mu^{-} a (including\mu^{+}\mu^{-}\rightarrow Z a\rightarrow (\mu^+\mu^-) a ) using the same method as in Sec. IV.A. The signal signatures are classified into two categories: (1) four isolated muons form_a \gtrsim 15 GeV, and (2)J_{\mu} plus two isolated muons form_a \lesssim 15 GeV. To investigate the first signal signature, we consider the relevant SM background\mu^{+}\mu^{-}\rightarrow \mu^{+}\mu^{-}\mu^{+}\mu^{-} and choose the same signal benchmark pointm_a = 50 GeV withc_{\mu}^A / \Lambda = 10 TeV^{-1} to display the signal features. The following event selections are required to identify the signal signature and suppress background events:● (1)
N(\mu)\ge 4 withP_T^{\mu_{1,2,3,4}} > 5 GeV,\lvert\eta_{\mu_{1,2,3,4}}\rvert < 2.5 ,● (2)
P_T^{\mu_1} > 200 GeV,P_T^{\mu_{2,3}} > 100 GeV,\lvert\eta_{\mu_{1}}\rvert < 2.0 and\lvert\eta_{\mu_{4}}\rvert < 1.5 ,● (3)
\Delta\phi_{\mu_2,\mu_4} > 0.5 and\Delta\phi_{\mu_3,\mu_4} > 1 ,● (4)
P_T^{\mu_4}/M_{\mu_1 \mu_4} > 0.05 ,● (5)
\lvert M_{\mu_1\mu_4}-m_a\rvert < 5.0 GeV.A cut-flow table including the signal and background for each event selection is shown in Table 6, and several kinematic distributions are shown in Fig. A5 of Appendix A.
Cut flow in σ/fb Signal \mu^{+}\mu^{-}\rightarrow \mu^{+}\mu^{-}\mu^{+}\mu^{-} Generator 1.06\times 10^{-3} 2.82 cut-(1) 1.03\times 10^{-3} 1.26 cut-(2) 6.53\times 10^{-4} 3.26\times 10^{-1} cut-(3) 4.74\times 10^{-4} 1.59\times 10^{-1} cut-(4) 4.35\times 10^{-4} 5.80\times 10^{-2} cut-(5) 3.43\times 10^{-4} 3.67\times 10^{-3} Table 6. Similar to Table 4, but for the
\mu^{+}\mu^{-}\rightarrow \mu^{+}\mu^{-} a channel and relevant SM backgrounds with the signature of four isolated muons.Four isolated muons with
P^{\mu}_T > 5 GeV and\lvert\eta_{\mu}\rvert < 2.5 are applied as trigger criteria. We observed thatP^{\mu_1}_T ,P^{\mu_2}_T , andP^{\mu_3}_T of the signal are more energetic than those of the background. However,\mu_1 and\mu_4 from the signal are distributed in the central regions relative to those from the background, whose\mu_1 and\mu_4 are mainly generated by the initial muons with forward and backward directions. Therefore, we choose cut-(2) in the above to select candidate events. We check all combinations of four muons in the final state to reconstruct a pair of muons that comes from the μALP decay and find that the pair of\mu_1 and\mu_4 is most likely to reconstruct the mass of μALP. To reduce background events, we apply the event selections based on\Delta\phi_{\mu_{2,4}} and\Delta\phi_{\mu_{3,4}} . We observe that the muons produced by the decay of μALPs in the signal are well-separated from\mu_2 and\mu_3 . Furthermore, we incorporate an additional selection criterion involving the ratioP_T^{\mu_4}/M_{\mu_1 \mu_4} to further suppress the contribution from\mu^{+}\mu^{-}\rightarrow \mu^{+}\mu^{-}\mu^{+}\mu^{-} . Specifically, we set the ratioP_T^{\mu_4}/M_{\mu_1 \mu_4} > 0.05 , which effectively reduces background events while retaining the majority of signal events. Since theM_{\mu_{1}\mu_{4}} distribution for the\mu^{+}\mu^{-}\rightarrow \mu^{+}\mu^{-}\mu^{+}\mu^{-} process is almost concentrated in the region greater than85 GeV, the μALP mass window can further reduce the number of background events. Finally, after all of the event selections in Table 6, we find that the signal significance can reachZ=10.44 for{\cal L} = 1000 fb^{-1} in our analysis.For the second signal signature, possible SM backgrounds arise from
\mu^{+}\mu^{-}c \overline{c} and\mu^{+}\mu^{-}b \overline{b} . The pre-selection cuts (P_T^{\mu} > 5 GeV,\lvert\eta_{\mu_{}}\rvert < 2.5 ) at the parton-level are used for the signal and background processes. We still choose the signal benchmark pointm_a = 5 GeV andc^A_{\mu} / \Lambda = 10 TeV^{-1} . Applying the sameC/A jet clustering algorithm forJ_{\mu} with a cone sizeR = 0.1 , we set up event selections to pick up the signal and suppress background events as specified below.● (1)
N(\mu)\ge 4 withP_T^{\mu_{1,2,3,4}} > 5 GeV,\lvert\eta_{\mu_{1,2,3,4}}\rvert < 2.5 ,● (2)
P_T^{\mu_{2,3}} > 100 GeV,\lvert\eta_{\mu_{2,3}}\rvert < 2.0 ,● (3)
N(J_{\mu}) = 1 with300 < P_T^{J_{\mu}} < 1400\; {\rm GeV} ,\lvert\eta_{J_\mu}\rvert < 1.6 ,● (4)
\Delta\phi_{J_{\mu},\mu_2} > 4.5 and1.0 < \Delta\phi_{\mu_{2},\mu_3} < 5.0 ,● (5)
P_{T}^{\mu_2}/M_{J_{\mu}} < 500 ,● (6)
\lvert M_{J_{\mu}}-m_{a}\rvert < 4 GeV.The event selection criteria are implemented for reasons similar to those previously described and will not be reiterated here. The cut-flow table, which includes both signal and background events for each selection, is presented in Table 7, and the corresponding kinematic distributions can be found in Fig. A6 of Appendix A. Upon applying all of the event selections outlined in Table 7, we observe a significant signal significance of
Z=19.86 for our benchmark point with{\cal L} = 1000 fb^{-1} .Cut flow in σ/fb Signal \mu^{+}\mu^{-}c \overline{c} \mu^{+}\mu^{-}b \overline{b} Generator 1.26\times 10^{-3} 52.94 90.18 cut-(1) 1.13\times 10^{-3} 1.98\times 10^{-3} 7.01\times 10^{-2} cut-(2) 7.19\times 10^{-4} 5.82\times 10^{-4} 2.12\times 10^{-2} cut-(3) 6.63\times 10^{-4} 2.65\times 10^{-4} 1.52\times 10^{-2} cut-(4) 6.61\times 10^{-4} 1.06\times 10^{-4} 1.25\times 10^{-2} cut-(5) 6.03\times 10^{-4} 0 8.21\times 10^{-3} cut-(6) 5.39\times 10^{-4} 0 5.41\times 10^{-4} Table 7. Similar to Table 5, but for the
\mu^+\mu^-\rightarrow \mu^+\mu^- a channel and relevant SM backgrounds with the signature of aJ_{\mu} candidate plus two isolated muons. -
The study of signal benchmark points is extended to a wide range of
m_a by employing the search strategies outlined in Sec. IV.A and Sec. IV.B, resulting in the identification of possible future bounds within a95 % confidence level (CL) (Z=1.96 ). To conservatively demonstrate the signal significance of the case without the survival background event after all event selections or the case of a background-free assumption, a minimum of10 signal events is required to be present, and only signal efficiencies larger than10 % are considered in the analysis. Our study is restricted to1 GeV\le m_a \lesssim M_W for μALPs below the electroweak scale. We first summarize our results in Fig. 5 to search the\mu^{+}\mu^{-}\rightarrow \nu_{\mu} a \overline{\nu}_{\mu} channel in the EWV scenario. The dotted lines are used for the case of prompt μALP decay (\gamma\beta c\tau_a < 1 mm) at a muon collider with{\cal L} = 120 fb^{-1} . Note that the lower bound ofm_a originates from technical issues in theJ_{\mu} analysis. Whenm_a < 1 GeV, the μALP mass window selection is no longer sufficiently powerful to distinguish the signal from backgrounds. However, since the event selections for two isolated muons plus{\not E} at the muon collider are sensitive to the values ofm_a , event selections are dynamically optimized for differentm_a to suit each case, as shown in Table 8. The case of prompt decay of μALPs with two isolated muons at the muon collider yields a background-free scenario whenm_a\lesssim 30 GeV, causing a cusp point atm_a = 30 GeV in Fig. 5. Similarly, for the prompt decay of μALPs withJ_{\mu} at the muon collider, SM background events can only survive after all event selections whenm_a = 10 GeV, resulting in a cusp point atm_a = 10 GeV in Fig. 5.Figure 5. (color online) Future bounds on
c^A_\mu/\Lambda of GeV-scale μALPs in the EWV scenario from the muon collider with{\cal L} = 120 fb^{-1} and{\cal L} = 840 fb^{-1} within a95 % CL or 10 survival events for background-free cases (dotted lines for the μALP prompt decay and dashed lines for the μALP as an LLP) as well as existing bounds (bulk regions). Here, we label "2\mu " and "J_{\mu} " to identify two types of signatures at the muon collider.{\cal B}_W\lesssim 10^{-5} represents{\cal B} (W^{\pm}\rightarrow \mu^{\pm}\nu_{\mu}a) < 10^{-5} [50] (blue bulk). For light μALPs, BaBar [49] (hotpink bulk),B^{\pm}\rightarrow K^{\pm}a\rightarrow K^{\pm}(\gamma\gamma) [80] (lightskyblue bulk) are considered. Some other collider bounds are in order: ATLAS2\gamma [32, 33, 35] (magenta bulk), ATLAS3\gamma [34, 35] (red bulk), OPAL3\gamma [30, 35] (green bulk), and ATLAS/CMS (PbPb) [81] (yellow bulk). Finally, the bound from(g-2)_{\mu} [82] is labeled as orange bulk.m_a /GeV\lvert\eta_{\mu_{1,2}}\rvert \lvert\eta_{{\:/ E}}\rvert {\:/ E}/M_{\mu_1\mu_2} \Delta M_{\mu_1\mu_2} \Delta\phi_{\mu_2, {\not E}} 10 < 3.0 < 3.0 > 140 same (2.5,3.6) 20 < 2.0 < 2.0 > 70 same (2.7,3.6) 30 < 1.8 <1.9 > 50 same same 40 < 1.6 same > 40 same same 60 same same > 22 < 1.5 same 70 same same > 22 < 1.5 same 80 same same > 20 < 2.3 (2.8,3.5) 85 same same > 18 < 2.3 (2.8,3.5) Table 8. Changes in several event selections in the
{EWV} scenario with somem_a benchmark points for two isolated muons plus{\not E} at a muon collider, where\Delta M_{\mu_1\mu_2}\equiv \lvert M_{\mu_1\mu_2}-m_a\rvert , and "same" means the same event selection as the benchmark pointm_a = 50 GeV in the main text.For μALPs as LLPs, we first consider the physical size of the radius of the proposed detectors for muon colliders [83]. Several relevant detector parameters for the inner and outer radii of the vertex detector, ECAL, HCAL, and muon system are summarized as follows: (1)
3.0 \leq R_{\text{vertex}} \leq 10.4 cm, (2)150.0 \leq R_{\text{ECAL}} \leq 170.2 cm, (3)174.0 \leq R_{\text{HCAL}} \leq 333.0 cm, and (4)446.1 \leq R_{\text{muon}} \leq 645.0 cm. Therefore, we simply consider the μALP lab frame decay length within10^{-3} \leq\gamma\beta c \tau_a\leq 6.4 m as a detectable LLP with a muon pair displaced vertex and displacedJ_{\mu} signatures at a muon collider. We assume that both a muon pair displaced vertex and displacedJ_{\mu} signatures at muon colliders are background-free after the trigger and μALP mass window selection implementation, as described previously. The analysis of a muon pair displaced vertex and displacedJ_{\mu} signatures at the muon collider is performed using an integrated luminosity of{\cal L} = 840 fb^{-1} . The results of the LLP study to search the\mu^{+}\mu^{-}\rightarrow \nu_{\mu} a \overline{\nu}_{\mu} channel in the EWV scenario are summarized in Fig. 5 with dashed lines. The signal efficiency of the two isolated muon signatures decreases whenm_a\lesssim 30 GeV because these two muons become too close to each other and cannot pass the muon isolation criterion. Similarly, grouping two muons inside aJ_{\mu} candidate is challenging form_a\gtrsim 10 GeV at the muon collider. Therefore, the analysis of signatures with two isolated muons andJ_{\mu} is ideal for μALP searches in the middlem_a range.Several existing bounds are also shown in Fig. 5 for comparison. First, according to the interaction in Eq. (1), there is a new W boson exotic decay channel,
W^{+}\rightarrow \mu^{+}\nu_{\mu}a . The precision measurements of W boson width (\Gamma_W = 2.085\pm 0.042 GeV [84]) can indirectly test μALPs withm_a < M_W in the EWV scenario. Here, we conservatively require the branching ratio ofW^{+}\rightarrow \mu^{+}\nu_{\mu}a to be less than10^{-5} [50] and mark it as the blue bulk in Fig. 5. For lighter μALPs (m_a\lesssim 5 GeV), searching for four muons in the final state [80] (hotpink bulk) andB^{\pm}\rightarrow K^{\pm}a (light skyblue bulk) via BaBar experiments can already constrain some parameter space in the upper-left corner. For heavier μALPs (m_a > 5 GeV), the ATLAS2\gamma [32, 33, 35] (magenta bulk), ATLAS3\gamma [34, 35] (red bulk), OPAL3\gamma [30, 35] (green bulk), and ATLAS/CMS (PbPb) [81] (yellow bulk) can already exclude some parameter space withc^A_{\mu} / \Lambda\gtrsim 10^2 TeV^{-1} . However, the precision measurements of muon magnetic moment can also provide constraints for μALPs. The combined measurement from Fermilab and Brookhaven is reported asa^{\text{EXP}}_{\mu}=116,592,061(41)\times 10^{-11} [85], and if we consider the lattice calculation for hadronic vacuum polarization (HVP), the SM prediction value changes toa^{\text{SM}}_{\mu}=116,591,954(55)\times 10^{-11} [86]. In this situation, the deviation of(g-2)_{\mu} is reported as\Delta a_{\mu} = a^{\text{EXP}}_{\mu} -a^{\text{SM}}_{\mu} = 107(69)\times 10^{-11} , and we consider the\Delta a_{\mu} observation within2\sigma for μALPs in this study. The one-loop contributions from light μALPs to(g-2)_{\mu} are negative and can be written as2 \begin{array}{*{20}{l}} \Delta a^{\rm 1-loop}_{\mu} = \Delta a^{\mu a\mu}_{\mu} +\Delta a^{\mu a\gamma}_{\mu}\,\,\,, \end{array}
(6) where the first term originates from the μ-a-μ loop, and the second term arises from the μ-a-γ loop, as shown in Ref. [82], in the following form:
\begin{align} \Delta a^{\mu a\mu}_{\mu} = & -\left(\frac{c^A_{\mu}m_\mu}{\Lambda}\right)^2\frac{r}{8\pi^2} \int_0^1 {\rm d}x \dfrac{x^3}{1-x+rx^2}\,\,\,, \end{align}
(7) \begin{aligned}[b] \Delta a^{\mu a\gamma}_{\mu} = & -\frac{\alpha_{\rm em}}{4\pi^3} \left(\frac{c^A_{\mu}m_\mu}{\Lambda}\right)^2 \\ &\times \int_0^1 {\rm d}x \Bigg[ (1-x)\left(\ln \dfrac{\Lambda^2_{\rm loop}}{\Delta^2}-\dfrac{1}{2}\right) \\& -3r\left\{x^2 \ln\left(\frac{rx^2 +(1-x)}{rx^2}\right)\right\} \Bigg]\,\,. \end{aligned}
(8) Here,
r=m^2_{\mu}/m^2_a ,\Delta^2 = m^2_{\mu}x^2 +m^2_a (1-x) , and\Lambda_{\rm loop} is the cut-off scale of the loop integration, which is taken to be1 TeV here. The strongest constraint among all of the above is from(g-2)_{\mu} [82] (orange bulk), withc^A_{\mu} / \Lambda\gtrsim 10 TeV^{-1} and extending to a wide range ofm_a . It is important to note that all of the above bounds are rescaled according to our definition of ALP-muon interactions in Eq. (2) and μALP decay branching ratios in Fig. 1. However, several other bounds, such as OPAL2\gamma [30, 35], Belle II [87], and LHCb [88], are so weak that we do not include them here. In comparison to these existing bounds, our proposal to search for μALPs via\mu^+\mu^-\rightarrow \nu_{\mu}a\overline{\nu_{\mu}} at muon colliders remains attractive. Furthermore, the possible future bounds ofc^A_\mu/\Lambda can reach less than0.01-0.1 TeV^{-1} , which opens new doors to explorem_a in the EWV scenario below the electroweak scale.In addition, as discussed in Sec. III, in the EWV scenario, cross sections are more than six orders of magnitude larger than those in the EWP scenario for
\mu^{+}\mu^{-}\rightarrow \nu_{\mu} a\overline{\nu_{\mu}} processes in Fig. 3. Therefore, future bounds from this channel in the EWP scenario are as small as the existing bounds. Additionally, almost the entire cross-section originates from the\boldsymbol{aVV'} interaction in the EWP scenario at the muon collider. Comparing the EWP scenario with the EWV scenario, the longitudinal momentum (P_z ) becomes larger than the transverse momentum (P_T ) for a pair of two isolated muons because the dominant contribution in the signal process is\mu^+\mu^-\rightarrow Za\rightarrow (\nu\overline{\nu})(\mu^+\mu^-) instead of that from the four-point interaction. Whenm_a \gtrsim 30 GeV, the total energy is roughly equally divided into Z and the ALP, resulting in large changes in some kinematic distributions. For comparison, we use the same event selections for both the EWV and EWP scenarios. Most of the signal efficiencies are below10 % in the EWP scenario because the conditionP_T^{\mu_1} > 200 GeV is too stringent in this situation. Meanwhile, the efficiency of the signal is also highly sensitive to{\not E}/M_{\mu_1\mu_2} . Thus, as we can expect, the distributions of two isolated muons in the EWP scenario are distinct from those in the EWV scenario. Eventually, the EWP signal efficiency is approximately10 % to40 % less than that of the EWV efficiency. At the same time, we explore the potential results from the search for μALPs in different channels with{\cal L} = 1000 fb^{-1} . Except for\mu^+\mu^-\rightarrow \nu_{\mu}a\overline{\nu_{\mu}} , we also include\mu^{+}\mu^{-}\rightarrow \gamma a and\mu^{+}\mu^{-}\rightarrow \mu^{+}\mu^{-} a in the EWP scenario. Owing to different generation mechanisms among these channels, the coverage range of the interval ofm_a may vary. The case of the prompt decay of μALPs with two isolated muons in the\mu^{+}\mu^{-}\rightarrow \gamma a channel yields a background-free scenario whenm_a\gtrsim 10 GeV, causing a cusp point atm_a = 10 GeV in Fig. 6. The case of the prompt decay of μALPs with four isolated muons in the\mu^{+}\mu^{-}\rightarrow \mu^{+}\mu^{-} a channel causes a cusp point atm_a = 5 GeV in Fig. 6. Finally, we find that a photon plus a μALP channel has the best potential when searching for μALPs in the EWP scenario. The possible future bounds onc^A_\mu/\Lambda can reach values lower than1-10 TeV^{-1} , which is only slightly greater than existing bounds.Figure 6. (color online) Future bounds on
c^A_\mu/\Lambda of GeV-scale μALPs from the muon collider in the EWP scenario with{\cal L} = 1000 fb^{-1} within a95 % CL or 10 survival events for background-free cases. Only the bounds from the μALP prompt decay are considered (dashed lines for\mu^+\mu^-\rightarrow \nu_{\mu}a\overline{\nu_{\mu}} , solid lines for\mu^{+}\mu^{-}\rightarrow \gamma a , and dotted lines for\mu^{+}\mu^{-}\rightarrow \mu^{+}\mu^{-} a ). The labels "2\mu " and "J_{\mu} " are the same as in Fig. 5. -
In this appendix, we choose several representative kinematic distributions for both signals and backgrounds at a muon collider:
● For the signature of two isolated muons plus
\not E at a\mu^+\mu^- collider,P_T^{\mu_1} ,\eta_{\mu_1} ,\not E ,M_{\mu_1\mu_2} ,\Delta\phi_{\mu_1,\not E} , and\Delta\phi_{\mu_2,\not E} distributions form_a = 50 GeV withc^A_\mu / \Lambda = 0.1 TeV^{-1} are shown in Fig. A1.Figure A1. (color online) Several signal and background kinematic distributions for the signature of two isolated muons plus
{\:/ E} at a\mu^+\mu^- collider.P_T^{\mu_1} ,\eta_{\mu_1} ,{\:/ E} ,M_{\mu_1\mu_2} ,\Delta\phi_{\mu_1,{\:/ E}} , and\Delta\phi_{\mu_2,{\:/ E}} distributions form_a = 50 GeV withc^A_\mu / \Lambda = 0.1 TeV^{-1} .● For the signature of
J_{\mu} plus\not E at a\mu^+\mu^- collider,P_T^{J_{\mu}} ,\eta_{J_{\mu}} ,\not E , andM_{J_{\mu}} distributions form_a = 5 GeV withc^A_\mu / \Lambda = 0.1 TeV^{-1} are shown in Fig. A2.Figure A2. (color online) Several signal and background kinematic distributions for the signature of
J_{\mu} plus\not E at a\mu^+\mu^- collider.P_T^{J_{\mu}} ,\eta_{J_{\mu}} ,\not E , andM_{J_{\mu}} distributions form_a = 5 GeV withc^A_\mu / \Lambda = 0.1 TeV^{-1} .● For the signature of two isolated muons plus γ at
\mu^+\mu^- colliders,P_T^{\mu_1} ,\eta_{\mu_1} ,E_{\gamma} ,M_{\mu_1\mu_2} ,\Delta\phi_{\mu_1,\gamma} , andE_{\gamma}/M_{\mu_1\mu_2} distributions form_a = 50 GeV withc^A_\mu / \Lambda = 10 TeV^{-1} are shown in Fig. A3.Figure A3. (color online) Several signal and background kinematic distributions for the signature of two isolated muons plus γ at a
\mu^+\mu^- collider.P_T^{\mu_1} ,\eta_{\mu_1} ,E_{\gamma} ,M_{\mu_1\mu_2} ,\Delta\phi_{\mu_1,\gamma} andE_{\gamma}/M_{\mu_1\mu_2} distributions form_a = 50 GeV withc^A_\mu / \Lambda = 10 TeV^{-1} .● For the signature of
J_{\mu} plus γ at a\mu^+\mu^- collider,P_T^{J_{\mu}} ,\eta_{J_{\mu}} ,E_{\gamma} ,\eta_{\gamma} ,M_{J_{\mu}} , andE_{\gamma}/M_{J_{\mu}} distributions form_a = 5 GeV withc^A_\mu / \Lambda = 10 TeV^{-1} are shown in Fig. A4.Figure A4. (color online) Several signal and background kinematic distributions for the signature of
J_{\mu} plus γ at a\mu^+\mu^- collider.P_T^{J_{\mu}} ,\eta_{J_{\mu}} ,E_{\gamma} ,\eta_{\gamma} ,M_{J_{\mu}} , andE_{\gamma}/M_{J_{\mu}} distributions form_a = 5 GeV withc^A_\mu / \Lambda = 10 TeV^{-1} .● For the signature of four isolated muons at a
\mu^+\mu^- collider,P_T^{\mu_3} ,\eta_{\mu_2} ,\eta_{\mu_4} \Delta\phi_{\mu_3,\mu_4} ,P_T^{\mu_4}/M_{\mu_{1}\mu_4} , andM_{\mu_{1}\mu_{4}} distributions form_a = 50 GeV withc^A_\mu / \Lambda = 10 TeV^{-1} are shown in Fig. A5.Figure A5. (color online) Several signal and background kinematic distributions for the signature of four isolated muons at a
\mu^+\mu^- collider.P_T^{\mu_3} ,\eta_{\mu_2} ,\eta_{\mu_4} \Delta\phi_{\mu_3\mu_4} ,P_T^{\mu_4}/M_{\mu_{1}\mu_4} , andM_{\mu_{1}\mu_{4}} distributions form_a = 50 GeV withc^A_\mu / \Lambda = 10 TeV^{-1} .● For the signature of two isolated muons plus
J_{\mu} at a\mu^+\mu^- collider,\eta_{\mu_2} ,\eta_{J_{\mu}} ,P_T^{J_{\mu}} ,\Delta \phi_{\mu_{2}\mu_3} ,M_{\mu_{1}\mu_4}/(P_T^{\mu_1}+P_T^{\mu_4}) , andM_{J_{\mu}} distributions form_a = 5 GeV withc^A_\mu / \Lambda = 10 TeV^{-1} are shown in Fig. A6.Figure A6. (color online) Several signal and background kinematic distributions for the signature of
J_{\mu} plus two isolated muons at a\mu^+\mu^- collider.\eta_{\mu_2} ,\eta_{J_{\mu}} ,P_T^{J_{\mu}} ,\Delta \phi_{\mu_{2}\mu_3} ,M_{\mu_{1}\mu_4}/(P_T^{\mu_1}+P_T^{\mu_4}) , andM_{J_{\mu}} distributions form_a = 5 GeV withc^A_\mu / \Lambda = 10 TeV^{-1} .Conversely, we modify the event selections for the detection of two isolated muons plus
\not E with varyingm_a at a muon collider (as shown in Table 8) to optimize the signal efficiency. Specifically, we adjust the ranges of\eta_{\mu_{1,2}} and\eta_{\not E} for small values ofm_a because loosening these criteria can improve signal detection while still eliminating all background events with the current selection criteria. In addition, we adjust\not E/M_{\mu_1\mu_2} based on signal and background distributions because it decreases asm_a increases. Conversely, we do not optimize event selections when detectingJ_{\mu} plus\not E with varyingm_a at a muon collider since the relevant backgrounds are already unlikely to satisfy the conditions of two detectable muons in the muon spectrometer and forming an energeticJ_{\mu} in the central region. As a result, nearly all of these signals are free from background events after the cut-(3) selection in Table 3. Similarly, we fine-tune event selections for the detection of two isolated muons plus a photon and four isolated muons at a muon collider for differentm_a in the EWP scenario, as listed in Tables A1 and A2.m_a /GeVP_T^{\mu_2} \lvert\eta_{\mu_{1,2}}\rvert \lvert\eta_{E_{\gamma}}\rvert E_{\gamma}/M_{\mu_1\mu_2} \Delta M_{\mu_1\mu_2} 30 > 300 same < 1.8 > 46 same 40 > 400 same same > 35 < 1.8 60 same < 1.2 < 1.2 > 24 < 2.2 70 same < 1.2 < 1.2 (20,23) < 2.8 80 same < 1.2 < 1.2 (17,20) < 3.0 85 same < 1.2 < 1.2 (16,19) < 3.0 Table A1. Similar to Table 8, but for the
\mu^{+}\mu^{-}\rightarrow \gamma a channel.m_a /GeV\lvert\eta_{\mu_{1}}\rvert \lvert\eta_{\mu_{4}}\rvert \Delta \phi_{\mu_{3},\mu_4} P_{T}^{\mu_4}/M_{\mu_{1}\mu_{4}} \Delta M_{\mu_1\mu_4} 30 same <1.6 >0.8 same <4.0 40 same same same same <4.0 60 same same same >0.06 same 70 <1.8 < 1.4 >1.2 >0.06 same 80 <1.8 < 1.4 >1.2 >0.08 same 85 <1.8 < 1.4 >1.2 >0.08 < 4.0 Table A2. Similar to Table 8, but for the
\mu^{+}\mu^{-}\rightarrow \mu^{+}\mu^{-} a channel.
Exploring muonphilic ALPs at muon colliders
- Received Date: 2023-03-27
- Available Online: 2023-10-15
Abstract: Axion-like particles (ALPs) are new particles that extend beyond the standard model (SM) and are extensively investigated. When considering ALPs within an effective field theory framework, their couplings with SM particles can be studied independently. It is a daunting task to search for GeV-scale ALPs coupled to muons in collider experiments because their coupling is proportional to the muon mass. However, a recent study by Altmannshofer, Dror, and Gori (2022) highlighted the importance of a four-point interaction, W-μ-