Abstract
SuperWIMPs are extremely weakly interacting massive particles that inherit their relic abundance from late decays of frozen-out parent particles. Within supersymmetric models, gravitinos and axinos represent two of the most well motivated superWIMPs. In this paper we revisit constraints on these scenarios from a variety of cosmological observations that probe their production mechanisms as well as the superWIMP kinematic properties in the early Universe. We consider in particular observables of Big Bang Nucleosynthesis and the Cosmic Microwave Background (spectral distortion and anisotropies), which limit the fractional energy injection from the late decays, as well as warm and mixed dark matter constraints derived from the Lyman-\(\alpha \) forest and other small-scale structure observables. We discuss complementary constraints from collider experiments, and argue that cosmological considerations rule out a significant part of the gravitino and the axino superWIMP parameter space.
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1 Introduction
The pursuit of signatures of beyond-the-Standard-Model (BSM) physics and an explanation for the dark matter of the Universe has been the holy grail for particle physicists for over three decades. To this end, the Large Hadron Collider (LHC) has probed large swathes of parameter space in a variety of well motivated BSM models. These include Supersymmetry (SUSY), the leading BSM theory that not only solves the hierarchy problem but also provides a slew of particle candidates for the dark matter. Within the context of specific SUSY breaking scenarios as well as simplified models, null measurements at the LHC have translated into constraints on a significant chunk of SUSY particles in the GeV-to-TeV mass range [1,2,3].
However, SUSY/BSM searches at the LHC rely primarily on prompt decays or, at best, decays with proper lengths of \(\mathcal {O}(10-100)\) m in the so-called long-lived particle (LLP) searches [4, 5]. These searches are further subject to the constraints that the produced particles are within kinematic reach of the LHC, i.e., their masses are at most a few TeV, and that they are produced with a cross-section sufficient to generate a detectable signal over the enormous Standard-Model background. Extremely weakly-interacting particles and/or those with very long lifetimes – many of which also reside in well-motivated SUSY/BSM model and parameter spaces – are thus inherently out of the LHC’s reach, even if their masses lie within the conventional GeV-to-TeV collider window.
Interestingly, when the proper decay lengths/lifetimes of these particles exceed \(\mathcal {O}(10)\) m, a second, albeit less conventional, window to explore their properties opens up. Disregarding concerns of naturalness, scenarios of extremely long particle lifetimes could be easily realised in a wide range of BSM theories by particle masses and couplings spanning orders of magnitude (e.g., \(m \sim \mathcal {O}(1)~\textrm{MeV}-\mathcal {O}(100)~\textrm{TeV}\)). In the context of SUSY, these scenarios fall under the superWIMP class of models [6, 7], wherein quasi-stable particles can be efficiently produced in the early Universe and decay at a very late time, i.e., \(t \gg {{\mathcal {O}}}(1)\) s post-Big Bang, during the standard cosmological history. Regardless of whether these quasi-stable particles can account for all of the observed dark matter abundance of the Universe at early times, late-decaying particles leave potentially observable signatures in the cosmic microwave background radiation (CMB), as well as the light element abundances from Big Bang Nucleosynthesis (BBN) and the large-scale matter distribution, particularly the Lyman-\(\alpha \) (Ly\(\alpha \)) forest. Measurements of these observables can in turn be used to probe and constrain regions of SUSY/BSM parameter space inaccessible to collider searches.
As a concrete example, consider the following: in R-parity-conserving SUSY, the conventional lightest SUSY particle (LSP) dark matter candidate is the lightest neutralino, superpartner of the electroweak gauge particles. With masses \(m_{\textrm{LSP}} \sim {{\mathcal {O}}}(0.1\)–1) TeV and weak-like interactions with the Standard Model (SM), the neutralino easily satisfies the observed relic abundance of the Universe and has a range of signatures at collider physics experiments, as well as at direct and indirect dark matter searches [8, 9]. However, in models of Supergravity or in SUSY models extended to include the axion, the lightest neutralino may not be the LSP but can decay to lighter particles of these theories, such as the gravitino \(\tilde{G}\) or the axino \(\tilde{a}\). That is, the neutralino is now the next-to-lightest supersymmetric particle (NLSP), while the gravitino or the axino serves as the LSP [6, 7, 10,11,12].
In the latter scenarios, the decay widths of the NLSP neutralino to \(\tilde{G}\) and \(\tilde{a}\) are generally suppressed, either by the Planck mass \(m_{\textrm{Pl}}\) or by the axion decay constant \(f_a\), such that the lifetime of the NLSP can be much longer than its freeze-out time scale. In this case, the decay of the NLSP can also generate an axino or gravitino population. Provided that the reheating temperature is low enough to avoid significant production of \(\tilde{G}\) or \(\tilde{a}\) from thermal scattering in the very early Universe [13], it is the late-time NLSP-to-LSP decay process that dominates the final \(\tilde{G}\) or \(\tilde{a}\) abundance.
Then, the relic LSP production can be thought of as a two-stage process. First, a neutralino NLSP population is produced by interactions with the SM.Footnote 1 Such a neutralino population with the right observed relic density can build up either via the usual thermal freeze-out mechanism through annihilations with SM particles, or via freeze-in if extremely-weakly coupled to the parent SUSY and other SM particles. Irrespective of the details of this first step, at very late times the neutralino NLSP decays into the gravitino or axino LSP, constituting the second step of the LSP production process. The gravitino or axino LSP thus generated – dubbed as SUSY superWIMPs in the literature – can provide part or all of the observed dark matter in the Universe [6, 16,17,18].
Because superWIMPs are extremely weakly coupled – the interactions of the gravitino and axino are \(m_{\textrm{Pl}}\)- or \(f_a\)-suppressed – prompt searches at colliders are insensitive to a large part of their parameter space. Only a small sliver can potentially be probed [11, 12, 18,19,20,21,22,23,24,25,26, 26,27,28,29], via searches for long-lived particles by ATLAS, CMS, or future experiments such as FASER [30] and Mathusla [31].Footnote 2 In a similar vein, bounds on the superWIMP parameter space from direct and indirect dark matter searches are practically non-existent. Our best prospects for probing and constraining superWIMP scenarios lie in cosmological observations.
A number of early studies have considered how gravitino superWIMPs can be probed cosmologically [6, 7, 22, 34,35,36], based primarily on the premise that electromagnetic and/or hadronic energy released from the NLSP-to-LSP decay has consequences for the light element yields from BBN and the CMB black-body energy spectrum. Using measurements of the Deuterium and Helium-4 abundances, as well as the COBE-FIRAS constraint on \(\mu \)-type spectral distortions, stringent constraints on the gravitino superWIMP parameter space can be set for NLSP lifetimes in the range \(t_{\textrm{NLSP}} \sim 10^4 - 10^8\) s. Similar considerations have also been applied to the axino superWIMP scenario, wherein a frozen-out neutralino or stau decays to an axino accompanied by an SM particle [10, 11, 24,25,26,27, 36, 37]. Independently of whether a particular axion-axino superfield realisation solves the strong CP problem, if the axino is the LSP, cosmological data can be expected to constrain a large part of the parameter space.
In this work, we extend these early analyses to include constraints from the CMB temperature and polarisation anisotropies from the Planck CMB mission [38]. Just as they impact on the light elements and the CMB energy spectrum, electromagnetic energy injections from NLSP decay can likewise have drastic consequences for the reionisation history of the Universe. Energy injection over NLSP lifetimes of \(t_{\textrm{NLSP}} \sim 10^{10}\)–\(10^{24}\) s, in particular, can significantly modify the evolution of the free-electron fraction in the cosmic plasma, altering the CMB anisotropies in ways that are strongly disfavoured by current anisotropy measurements [38, 39]. As we shall demonstrate, this in turn allows us to place stringent constraints on large swathes of superWIMP parameter space previously considered viable, providing a powerful complement to conventional SUSY dark matter searches at colliders as well as at direct and indirect dark matter detection experiments.Footnote 3
The paper is organised as follows. We begin in Sect. 2 with a summary of some well-motivated SUSY superWIMP scenarios amenable to the observational and experimental constraints of this work. In Sect. 3 we describe how these constraints can be applied to derive limits on the superWIMP parameter space, starting with cosmological observations and concluding with collider searches. Sect. 4 summarises the limits thus derived on the gravitino and axino superWIMP parameter space, assuming the initial NLSP abundance matches the observed DM abundance. We conclude in Sect. 5. Appendix Aoutlines the derivation of the LSP momentum distribution expected from NLSP-to-LSP decay, while Appendices B and C discuss scenarios in which the NLSP population is under- or over-produced.
2 SUSY superWIMPs
The general mechanism of superWIMP production in the early Universe is straightforward. Heavier SUSY particles undergo cascade decays to lighter SUSY particles and eventually to the NLSP. The NLSP then freezes out, typically at \(x_{\textrm{f}} \equiv m_{\textrm{NLSP}}/T\sim 25\)–30, with a yield \(Y_{\textrm{NLSP}} \equiv n_{\textrm{NLSP}}/s\), where \(n_{\textrm{NLSP}}\) is the NLSP number density and s is the entropy. Long after freeze-out, the NLSP decays to the LSP. Assuming the decay is complete, \(Y_{\textrm{LSP}} \simeq Y_{\textrm{NLSP}}\), and the final LSP abundance is simply given by \(\Omega _{\textrm{LSP}} h^{2} \simeq (m_{\textrm{LSP}}/m_{\textrm{NLSP}})\, \Omega _{\textrm{NLSP}} h^{2}\), where h is the reduced Hubble parameter. In most superWIMP scenarios, \(m_\textrm{NLSP}\simeq m_{\textrm{LSP}}\), such that LSP inherits the same abundance as the NLSP. A large mass difference is however not precluded and can be a means to relax constraints on the LSP parameter space from relic density considerations.
For SUSY superWIMPs, if there exists a thermal production mechanism in the early Universe generating an abundance proportional to the reheating temperature, then the total superWIMP abundance today is simply the sum of the thermal population and the population arising from NLSP decay (“non-thermal”), i.e.,
In general, however, to generate by thermal means a GeV-to-TeV-mass SUSY superWIMP population to match the observed dark matter abundance requires a reheating temperature in excess of \(T_{\textrm{rh}} \sim 10^{10}\) GeV [13]. Thus, if the reheating temperature turns out to be low, the production of a sizeable population of superWIMP relics must rely entirely on NLSP-to-LSP decay. For axinos in the mass range \(\mathcal {O}(\mathrm{MeV-GeV})\), thermal scatterings can account for the correct relic density for reheating temperatures (\(T_{rh}\)) of about \(10^{4}\) GeV or lower [10].Footnote 4
At this stage it is important to emphasise that, within the general Minimal Supersymmetric Standard Model (MSSM), the mechanism of thermal neutralino freeze-out that generates the right relic abundance is quite restricted given collider and electroweak precision observables, as well as constraints on the Higgs and Z-boson invisible widths [14, 15, 48,49,50,51]. The mechanism of thermal freeze-out for a relic neutralino depends on the nature of the gauge composition of the neutralino; for a comprehensive recent summary, see, e.g., [15]. If the neutralino is light, i.e., \(\textrm{m}_{\chi _{1}^{0}}\lesssim 100 ~\textrm{GeV}\), limits on the charged components of the neutralino sector demand that the light neutral component \(\chi _{1}^{0}\) be predominantly bino.
Then, imposing the Planck-inferred dark matter density, \(\Omega _{\textrm{DM}} h^{2}\lesssim 0.12\) [38], on the neutral relic density leads immediately to a lower limit of \(m_{\chi _{1}^{0}}\gtrsim 34\) GeV on the neutralino mass [14]. Note however that for a decaying neutralino the lower limit is relaxed due to the dilution of the relic abundance. The precise lower limit will depend on the amount of dilution, along with the underlying mechanism and abundance of the relic neutralino. We use this bound as an indicative limit, but do not strictly enforce it. Thus, on the light neutralino side, assuming a thermal freeze-out mechanism the two places with maximally efficient enhancements in the annihilation cross-section so as not to overclose the Universe are at the Z-funnel and the Higgs funnel regions [14].Footnote 5 For a predominantly bino LSP, the cross-section \(\sigma \) for annihilation for light neutralinos to fermions f for the Higgs/Z funnel region is given by
where s represents the centre-of-mass energy, \(m_{Z/h}\) the mass the Z/h boson, \(\Gamma _{Z/h}\) the partial width of Z/h boson to fermions, and \(N_{\chi _{1}^{0}-Z/h}\) denotes the coupling of neutralinos to Z/h bosons. Direct detection constraints however rule out a significant part of the neutralino parameter space in the 10 GeV-to-1 TeV mass range [9, 14, 33, 53], with limits depending on the specifics of the model parameters. In general spin-independent limits from the Xenon-nT/LZ direct detection experiments [49] are quite constraining in the light dark matter scenario (\(\textrm{m}_{\chi _{1}^{0}}\lesssim 200~\textrm{GeV}\)), leaving viable the Z/H funnel regions.Footnote 6
At higher masses, depending on the gauge content of the neutralino and the SUSY mass spectrum, a variety of new annihilation mechanisms can open up. Given the strong limits from collider searches, the most promising scenarios proceed through co-annihilations with sleptons and squarks. If light staus \(\tilde{\tau }\) or stops \(\tilde{t}\) appear in the t-channel, the annihilation cross-section for a predominantly bino neutralino scale as
where \(g_{W} = (8/\sqrt{2}) G_F m_W^2\), with Fermi constant \(G_F\) and W-boson mass \(m_W\).) In this case, co-annihilations aided by Sommerfeld enhancements can lead to the correct relic density [9, 54].
We also add that if the neutralino has a sizable Higgsino component, a TeV scale Higgsino can generate the relic abundance of the Universe through co-annihilation with nearly mass-degenerate charginos [55]; this scenario requires the so-called “well-tempered” neutralino, a right admixture of bino and Higgsino for efficient annihilation [55, 56]. Since the charginos are TeV scale electroweak gauginos, collider limits can be evaded if the rest of the SUSY spectrum is decoupled as in the Split SUSY cases [57]. These considerations are generally encoded within the idea of the so-called relic neutralino surface [9]. General phenomenological and simplified MSSM model studies for the electroweakino sector using LHC data have shown that large swathes of parameter space are allowed within the gaugino sector, implying that there is no generic model-independent lower bound on the light neutralino [33]. The situation is relaxed further in non-minimal models like the next-to-Minimal Supersymmetric Standard Model (NMSSM) or non-universal Gaugino Models (NUGM).
Lastly, we note that, in models with over-abundant dark matter (e.g., models involving a light bino-like neutralino), the superWIMP mechanism is a way to dilute the final relic abundance. We emphasise, however, that, unless otherwise specified, our assumption throughout the present analysis is that the NLSP is always produced with the correct relic abundance prior to decay. That is, had the NLSP been stable, its present-day reduced energy density would be equal to \(\Omega _{\textrm{DM}}\). This assumption also implies that, in those scenarios where a large mass gap exists between the NLSP and the LSP, the latter would constitute only a fraction of the observed dark matter today; the remainder would have to be explained by some other physics.
In what follows, we briefly describe two well-motivated SUSY superWIMPs, the gravitino \(\tilde{G}\) and the axino \(\tilde{a}\). As we shall see in Sect. 4, irrespective of the freeze-out/freeze-in mechanism that produces the NLSP neutralino, energy injection constraints of PMSSM and BBN, coupled with free-streaming bounds from the Ly\(\alpha \) data will constrain the bulk of their parameter spaces.
2.1 Gravitino superWIMPs
Gravitinos \(\tilde{G}\) are spin-\(\nicefrac {3}{2}\) superpartners of gravitons. Depending on the SUSY breaking mechanism, the gravitino mass – given approximately by \(m_{\tilde{G}} \simeq \langle F \rangle /m_{\textrm{pl}}\), where \(\langle F \rangle \) is the SUSY breaking scale – can range from keV to TeV and is thus essentially a free parameter in this study. Because interactions of the gravitino are \(m_{\textrm{Pl}}\)-suppressed, we do not expect them to be efficiently produced via scattering in the early Universe unless the reheating temperature is high [13].
Production from NLSP decay can proceed via the decay of the lightest neutralino \(\chi _{1}^{0}\). Stringent BBN constraints on hadronic energy injection from the decays \(\chi _{1}^{0}\rightarrow \tilde{G} h/Z\) essentially rules out a predominantly wino- or Higgsino-like neutralino [6, 16]. Then, what remains is a bino-like neutralino, which decays into a gravitino predominantly via the two-body decay \(\chi _{1}^{0}\rightarrow \tilde{G} \gamma \), whose width is given by [6]
where \(m_{Pl}\) is the reduced Planck mass, and \(\theta _{\textrm{W}}\) is the weak mixing angle.
Assuming decay at rest and that the energy carried by the photon, \(E_{\gamma }= (m_{\chi _{1}^{0}}^{2} - m_{\tilde{G}}^{2})/(2m_{\chi _{1}^{0}})\), is injected entirely into the cosmic plasma, it is convenient to recast the width (4) as
with
denoting the fraction of the neutralino mass released as electromagnetic energy. Where kinematically allowed, the additional decay channels \(\chi _{1}^0 \rightarrow \tilde{G}Z/h\) are also available. But, as said above, these channels are suppressed for a bino-like \(\chi ^{0}_{1}\). Note that maximal energy injection is represented by \(\epsilon _{\textrm{em}}\rightarrow 0.5\), which occurs as \(m_{\tilde{G}}\rightarrow 0\).Footnote 7
2.2 Axino superWIMPs
Axinos \(\tilde{a}\) are the supersymmetric partners of the axion – the dynamic field expected to solve the strong CP problem – and appear in the axion supermultiplet after the Peccei-Quinn (PQ) symmetry breaking in the form \(A = (s +i a)/\sqrt{2} + \sqrt{2}\theta a + \theta ^{2}F\), where a is the axion, s the saxion,Footnote 8F the auxiliary superfield, and \(\theta \) is the Grassmanian coordinate. The axion couples derivatively to quarks and to the gauge bosons with interactions suppressed by the PQ breaking scale \(f_{a}\); the accompanying SUSY interactions can be found by simply supersymmetrising the effective SM-axion interactions, i.e., the axion supermultiplet A couples to the vector supermultiplet \(V_{a}\). The axion supermultiplet acquires a mass after SUSY is broken. While the saxion mass is roughly set by the the soft SUSY breaking scale, the axino mass depends on the superpotential. For the purposes of this work, we will take the axino mass to be a free parameter, and note that its mass can range from eV to TeV scales.
Like gravitinos, axinos can be produced in the early Universe in abundance via thermal scattering if the reheating temperature is large [59]. However, if the axino is the LSP, production from the decay of a NLSP neutralino population is also possible. Assuming a (pure) bino decay, the decay width is given by [11, 12]
Here, \(f'_{a} \equiv f_{a}/N\), where the factor \(N=1\) and \(N=6\) applies to the KSVZ and DFSZ axion, respectively; the coefficient \(C_{aYY}\) is a model-dependent \(\mathcal {O}(1)\) number [\(\sim 100\) MeV in the case of BBN and \(\sim 1\) MeV in the case of CMB. To illustrate the power of these constraints, the black dashed lines indicate predictions for the gravitino superWIMP scenario, based on Eq. (5), for several neutralino mass values, assuming that the neutralino population has been produced at the correct relic abundance prior to decay
3.1 Light element abundances
The main effect of electromagnetic energy injection into the plasma on the abundances of the light elements (Deuterium \(^2\)H, Helium-3 \(^3\)He, and Helium-4 \(^4\)He) from BBN is photo-dissociation, provided that the injected energy exceeds the reaction threshold (typically \({{\mathcal {O}}}(2\)–30) MeV; see, e.g., Table 1 of Ref. [\(\tau \simeq 10^{12}\) s in the figure is simply due to the lack of calculations in this parameter region in the literature; we are otherwise not aware of any fundamental reason why BBN constraints could not be extended to longer decay lifetimes.
To illustrate the power of energy injection constraints, we have also plotted in Fig. 1 predictions for the gravitino superWIMP scenario, based on Eq. (5), for a number of \(\chi _{1}^{0}\) masses from \(m_{\chi _1^0}=1\) GeV and up to the unitarity limit of \(m_{\chi _1^0}=100\) TeV.Footnote 10 We note however that the unitarity bound is indicative, since the bound will be relaxed due to the decaying neutralino. We emphasise that for a given mass hierarchy between the neutralino and the gravitino, the amount of energy released and the lifetime are fixed by Eq. (5). That is, there are no further variables that can affect the gravitino superWIMP predictions in Fig. 1. As mentioned earlier, the maximal fractional energy released can never be exactly equal to 0.5, which would correspond to a massless gravitino.
3.2 CMB spectral distortions
Electromagnetic energy injection in the early Universe must also perturb the Planck blackbody energy spectrum of the CMB photons, creating a spectral distortion. If both photon number-changing processes, (e.g., double Compton scattering and Bremsstrahlung) and energy-changing processes (e.g., Compton scattering) occur efficiently, then these distortions are quickly wiped out, leaving no trace of the decay in the CMB energy spectrum besides a temperature shift (which is unobservable by spectral measurements, but may be detectable in the anisotropies via \(N_{\textrm{eff}}\)). However, should these processes be inefficient at the time of energy injection and remain inefficient until the present time, the spectral distortions they cause may freeze in and become observable.
A detailed review of CMB spectral distortions can be found in, e.g., [69, \(t \gg 10^{-13}\) s. Nonetheless, increased ionisation of the intergalactic medium between the epochs of recombination (\(z \sim 1100\)) and reionisation (\(z \sim 10\)) raises the optical depth, which manifests itself most prominently in a stronger E-polarisation signal at multipoles \(\ell \sim 20\) [\(\sim 1\) MeV and that the energy is deposited in the plasma instantly upon injection.
3.4 Lyman-\(\alpha \) forest
The production of superWIMPs via NLSP-to-LSP decay is always accompanied by a finite momentum for the LSP, given at the production time \(t_{\textrm{prod}}\) by \(p_{\textrm{LSP}}(t_{\textrm{prod}})= p_\star = (m^2_{\chi _1^0} - m_{\textrm{LSP}}^2)/(2 m_{\chi _1^0})=\epsilon _{\textrm{em}} m_{\chi _1^0}\), assuming decay at rest. Universal expansion causes \(p_{\textrm{LSP}}\) to redshift subsequently as \(p_{\textrm{LSP}}(t)= p_\star R_{\textrm{prod}}/R(t)\), where R(t) is the scale factor and \(R_{\textrm{prod}}\equiv R(t_{\textrm{prod}})\); at a later time t the corresponding LSP velocity therefore reads
where
is the present-day (\(R(t_0)=1\)) LSP velocity, under the assumption that \(m_{\textrm{LSP}} \gg p_{\textrm{LSP}}(t_0)\) holds.
Because the NLSP-to-LSP decay is isotropic, the overall effect of a finite \(v_{\textrm{LSP}}\) is that of isotropic LSP free-streaming. Furthermore, because NLSP-to-LSP decay is a continuous process, together with a redshifting \(p_{\textrm{LSP}}\) we can expect a present-day comoving LSP number density \(n_{\textrm{LSP}}(t_0)\) to be composed of a distribution of particles in momentum space, given approximately by
where the prefactor assumes the \(\chi _1^0\) population was produced with the correct relic abundance, \(R_\Gamma \) is the scale factor corresponding to NLSP lifetime \(t=\tau \equiv 1/\Gamma \), and \(R_\textrm{eq}\) is the scale factor at matter-radiation equality. See Appendix A for the derivation of Eq. (10). Thus, phenomenologically, the LSP population today is dispersive and akin to a warm dark matter (WDM) with a present-day characteristic velocity given approximately by \(v_0 \sim p_\star R_\Gamma /m_{\textrm{LSP}}\). This also means that limits on thermal WDM properties from small-scale fluctuation measurements such as the Lyman-\(\alpha \) forest can be reinterpreted to constrain properties of the LSP and the corresponding superWIMP parameter space [74].
Several recent studies have investigated how to map thermal WDM bounds from the Lyman-\(\alpha \) forest to constraints on WDM from particle decays, e.g., [75, 76]. Roughly, one would compute the linear transfer function of the WDM given the daughter particle’s phase space distribution, and match it to the thermal WDM transfer function that exhibits free-streaming suppression at approximately the same location in wave number k. Here, however, we adopt the simpler approach of estimating the comoving particle horizon – also called the free-streaming horizon – of the LSP population.
The free-streaming horizon arises naturally in semi-analytical solutions of the Vlasov equation that governs the evolution of the linear transfer function. Its inverse correlates with the location in k-space of power suppression due to free-streaming, and can therefore be seen as a proxy for the transfer function. For a LSP produced at a fixed time \(t_{\textrm{prod}}\) and observed later at a time \(t_{\textrm{obs}}\), the free-streaming horizon is given by
where \(t_{\textrm{obs}}\) corresponds typically to a low redshift of \(z \sim 2\), \(t_{\textrm{prod}}\) to some time prior to matter-radiation equality, and \(H\equiv (1/R)(\textrm{d} R/\textrm{d} t)\) is the Hubble expansion rate. Because NLSP decay is a continuous process, in principle we must calculate \(\lambda _{\textrm{FS}}\) for all possible production times \(t_{\textrm{prod}}\) and then average it over the momentum distribution of Eq. (10). For simplicity, however, we assume all LSPs to be produced at \(t_\textrm{prod}=1/\Gamma \); this is a reasonable approximation given that, as shown in Eq. (10), LSPs produced at \(t_{\textrm{prod}}\sim 1/\Gamma \) – whose characteristic momentum is \(\sim p_\star R_\Gamma \) – in any case dominate the LSP distribution today. Then, using Eq. (8) and setting \(t_{\textrm{prod}} = 1/\Gamma \), Eq. (11) can now be rewritten as
valid across the radiation and matter domination epochs, where the subscript “eq” denotes matter-radiation equality, \(\Omega _{\textrm{m}}\) is the present-day reduced total matter density, and h is the reduced Hubble parameter defined via \(H_0 = 100 \, h~\textrm{km} \, \textrm{s}^{-1} \, \textrm{Mpc}^{-1}\).
The typical Ly\(\alpha \) WDM bound in the literature is presented as a lower limit on the WDM mass \(m_{\textrm{WDM}}\), assuming that the WDM constitutes the entirety of the dark matter abundance of the Universe and that the WDM population follows a relativistic Fermi-Dirac distribution, with a temperature linked to the dark matter abundance, i.e.,
where \(T_\nu \) is the temperature of the neutrino background (\(T_{\nu ,0} = 1.95\) K). Fixing \(\Omega _{\textrm{WDM}} h^2 = 0.12\) and using the current best limit \(m_{\textrm{WDM}} \gtrsim 5.3\) keV (95% C.I.) [77], we find an upper bound on the present-day WDM temperature of \(T_{\textrm{WDM},0} \lesssim 2.2 \times 10^{-5}\) eV, or equivalently, a bound on the present-day average WDM velocity of \(v_{\textrm{WDM,0}} \simeq 3\, T_x/m_x \lesssim 1.2 \times 10^{-8}\). Following the steps outlined above and letting \(R_\textrm{prod} \rightarrow 0\), it is straightforward to show that the equivalent upper limit on the comoving WDM free-streaming horizon at \(z_\textrm{obs}=2\) is
This limit can in principle serve as an upper bound on \(\lambda _\textrm{FS}(z_{\textrm{obs}}=2,z_\Gamma )\) for those superWIMP scenarios wherein the LSP explains all of the observed dark matter abundance.Footnote 12
A more versatile analysis of the Ly\(\alpha \) data could however also consider the possibility of a mixed cold+warm dark matter cosmology and vary as part of the fitting procedure the fraction of the total dark matter in the form of WDM, \(f_{\textrm{WDM}} \equiv \Omega _\textrm{WDM}/\Omega _{\textrm{DM}}\). Reference [79] has provided such an analysis and presented the outcome as two-dimensional constraints in the \((f_{\textrm{WDM}},m_{\textrm{WDM}})\)-plane and in the \((f_{\textrm{WDM}},v_0)\)-plane (see Fig. 12 of [79]). We have translated these constraints to constraints in the \((f_{\textrm{WDM}},\lambda _{\textrm{FS}}^{\textrm{WDM}})\)-plane. See a representative set in Table 1. Observe that the upper limit on \(\lambda _{\textrm{FS}}^{\textrm{WDM}}\) deteriorates as we decrease the WDM fraction \(f_{\textrm{WDM}}\); at \(f_{\textrm{WDM}} \lesssim 0.15\), no limit can be set on \(\lambda _\textrm{FS}^{\textrm{WDM}}\). In our analysis of superWIMPs, we take the WDM energy density to be \(\Omega _{\textrm{WDM}} \simeq (m_{\textrm{LSP}}/m_\textrm{NSLP}) \Omega _{\textrm{DM}}\), in accordance with our assumption that the NLSP population is always produced with the correct abundance prior to decay. It then follows simply that \(f_{\textrm{WDM}} = m_\textrm{LSP}/m_{\textrm{NLSP}}\), with the proviso that the remaining \(1-f_\textrm{WDM}\) of the dark matter is cold and explained by some other physics. We apply the Ly\(\alpha \) constraints only to those cases where production takes place before matter-radiation equality, i.e., \(1/\Gamma \le t_{\textrm{eq}}\), because using \(\lambda _{\textrm{FS}}\) as a proxy for small-scale suppression is likely unreliable if the bulk of the NLSP decay happens deep in matter domination.Footnote 13
3.5 Collider constraints
We assume from the outset that the neutralino is not ruled out by conventional jets/leptons + missing energy searches. In principle, within the scope of specific mass spectra, part of the parameter space can indeed be ruled out; however, this requires a larger global fit within specific simplified or full SUSY models. The GAMBIT collaboration has performed a global fit of the pMSSM within a reduced 7-parameter space – the relevant parameters being the trilinear couplings, Higgsino mass parameters, diagonal sfermion masses, and \(\tan \beta \) – taking into account collider, direct detection and relic density observables [33]. They conclude that a large volume of the pMSSM parameter space remains unconstrained, with the neutralino mass ranging from the Z/H funnel region to the mulit-TeV scale. In an extended set-up that includes gravitinos, for a gravitino mass fixed at \(m_{\tilde{G}}=1\) eV, best-fit points that take into account collider searches for the rest of the electroweak gaugino sector indicate that both light and heavy neutralinos remain unconstrained within a variety of simplified-model scenarios [80]. Scenarios like split SUSY models [57, 81] also predict mass spectra that are unconstrained by LHC searches. Thus, we re-emphasize that collider bounds on neutralinos are model independent and depends on the specifics of the mass spectrum and neutralino composition.
Bearing the above in mind, we summarise in this subsection collider constraints on the gravitino and the axino LSP originating from neutralino decay. The neutralino proper decay length to gravitino can be expressed following Eq. (5) as a function of the fractional energy \(\epsilon _{\textrm{em}}\) and the neutralino mass \(m_{\chi _{1}^{0}}\):
Collider searches – including searches of prompt decays that occur at the interaction vertex and are hence sensitive to photons plus missing energy signatures, and LLP searches sensitive to delayed decays – probe proper decay lengths of about 100 m. Thus, in order for colliders to be sensitive to the gravitino superWIMP scenario, it is a priori clear that a large mass hierarchy must exist between the neutralino and the gravitino.
The LEP experiment has placed a lower bound on the gravitino mass from the process \(e^{+}e^{-} \rightarrow \tilde{G}\tilde{G}\gamma \) of \(m_{\tilde{G}}\gtrsim 1.09 \times 10^{-5}\) eV [82]. Furthermore, under the assumption that the rest of the SUSY spectrum is decoupled apart from the selectron and the neutralino \(\chi _{1}^{0}\), the LEP searches exclude a neutralino mass of up to \(m_{\chi _{1}^{0}}\simeq \) 200 GeV for a gravitino mass of \(m_{\tilde{G}}\lesssim 10^{-5}\) eV. At the LHC, gravitino searches have been conducted within the context of gauge mediated supersymmetry (GMSB) breaking models [83]. These searches look for displaced photons assuming a SUSY topology that yields the neutralino NLSP. Assuming a decay channel with maximal production cross-section in the \(pp \rightarrow \tilde{q}\tilde{q}\rightarrow qq\chi _{1}^{0}\chi _{1}^{0}\) followed by the displaced photon signature of \(\chi _{1}^{0}\rightarrow \tilde{G}\gamma \), the ATLAS experiment rules out neutralino masses in the range \(\sim \) 100–400 GeV for \(c\tau \sim 10\)–\(10^{4}\) cm [84]. The latest CMS result [85] at 13 TeV with 78 \(\mathrm fb^{-1}\) luminosity excludes within these scenarios a neutralino mass in the range \(m_{\chi _1^0}\sim 200\)–550 GeV, for \(c\tau \sim 10\)–\(10^{4}\) cm. We will use these experimental bounds for illustrative purposes, but emphasise that they are model-dependent.
Similar considerations apply to the axino superWIMP, in which case we have an additional degree of freedom in the neutralino decay width, namely the axion decay constant \(f_a'\equiv f_a/N\). The proper decay length for the axino follows simply from Eq. (7):
As with the gravitino, collider constraints on the axino superWIMP scenario depend in general on the model specifics. Independently of the specifics, however, in order for colliders to be sensitive to the parameter space, the decay length should be \(\lesssim \mathcal {O} (100)\) m. Within the context of specific models, estimates have been made on the capability of the LHC to probe axinos from neutralino decays in prompt and LLP searches [28]. In this work we will reinterpret existing LLP search results to put limits on this decay process.
Finally, since we are dealing with long-lived neutralinos, if they are light (GeV/sub-GeV), there is potentially sensitivity at fixed-target experiments such as CHARM [86, 87], NA62 [88], NOMAD [89], SHiP [90] and SEAQuest [91], as well as at forward physics facility experiments like FASER [30].
The production of neutralinos in fixed-target experiments depend on the specifics of the model. Typically in such experiments, a beam of particles with energies ranging from \(\sim 100\) GeV (SEAQuest) to \(\sim 450\) GeV (SHiP, NOMAD, NA62) collides with the target, thereby producing hadrons and weakly-interacting particles that could be captured in a far detector. If the rest of the SUSY spectrum is decoupled and heavy, neutralino production proceeds primarily from the decay of pseudo-scalar and vector mesons. These neutralinos then decay to gravitinos/axinos with decay widths given by Eqs. (4) and (7). Thus, the number of long-lived neutralino decays within a fixed-target experiment depends on the decay lifetime, the Lorentz boost factor, and the energy spectrum of the mesons specific to the experiment. While a full analysis for each experiment is beyond the scope of this work, a crude estimate suggests that the NOMAD experiment can exclude up to \(m_{\chi _{1}^{0}}\simeq 300\) MeV for a fixed \(m_{\tilde{a}}\simeq 20\) MeV, assuming \(f_{a}\simeq 10^{3}\) GeV. For the same fixed value of \(m_{\tilde{a}}\), a future experiment like SHiP can rule out \(f_{a}=10^{4}\) GeV for \(m_{\chi _{1}^{0}}\simeq 300\) MeV.Footnote 14
In the case of FASER, a recent assessment of its feasibility to probe light axinos and gravitinos found that, for \(m_{\tilde{a}}\simeq 10\) MeV, it is possible to rule out \(f_{a} \simeq 10^{2}\)–\(10^{3}\) GeV for \(m_{\chi _{1}^{0}}\simeq 300\) MeV [93].