PHYSICAL REVIEW D 101, 035001 (2020) - Gravitational wave signals of pseudo-Goldstone dark matter in the Z3 complex singlet model - Inspire HEP

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PHYSICAL REVIEW D 101, 035001 (2020)

                          Gravitational wave signals of pseudo-Goldstone dark matter
                                        in the Z3 complex singlet model
                                      Kristjan Kannike ,* Kaius Loos,† and Martti Raidal              ‡

                      National Institute of Chemical Physics and Biophysics, Rävala 10, Tallinn 10143, Estonia

                             (Received 7 August 2019; accepted 14 January 2020; published 3 February 2020)

                  We study pseudo-Goldstone dark matter in the Z3 complex scalar singlet model. Because the direct
                detection spin-independent cross section is suppressed, such dark matter is allowed in a large mass range.
                Unlike in the original model stabilized by a parity and due to the cubic coupling of the singlet, the Z3 model
                can accommodate first-order phase transitions that give rise to a stochastic gravitational wave signal
                potentially observable in future space-based detectors.

                DOI: 10.1103/PhysRevD.101.035001

                     I. INTRODUCTION                                     admits two phases that produce a dark matter candidate.
                                                                         Firstly, the unbroken Z3 symmetry stabilizes S as a dark
   One of the best candidates for the dark matter (DM) is a
                                                                         matter candidate. Secondly, with the broken Z3 symmetry,
scalar singlet [1,2]. The properties of singlet DM have been
                                                                         the imaginary part of S, denoted by χ, is still stable due to
studied in detail [3–6] (see [7,8] for recent reviews; see also
                                                                         the S → S† symmetry of the Lagrangian. The main differ-
Refs. therein). The nonobservation of dark matter by direct
                                                                         ence between the Z2 and Z3 pseudo-Goldstone DM models
detection experiments [9–11], however, puts severe bounds
                                                                         is that the potential of the latter contains a cubic S3 term.
on models of DM comprised of weakly interacting massive
                                                                         There are other possible cubic terms [21–23], but they do
particles (WIMP), pushing the mass of the singlet scalar
                                                                         not respect the Z3 symmetry.
DM—except around the Higgs resonance—over 1 TeV.
                                                                            To this date, the Z3 -symmetric complex singlet model
   A way to suppress the direct detection cross section is to
                                                                         has been studied in the unbroken phase. Originally, the
consider as the DM candidate a pseudo-Goldstone with
                                                                         model was proposed in the context of neutrino physics [24].
derivative couplings to CP-even states [12] (the DM
                                                                         Detailed analysis of DM phenomenology was carried out in
phenomenology of the imaginary part of the complex
                                                                         Ref. [25]. Indirect detection of Z3 DM was considered in
scalar singlet was first considered in [3,13], but only in
                                                                         [26,27]. The cubic coupling can contribute to 3 → 2 scat-
the Higgs resonance region). Because the velocity of DM
                                                                         tering for Z3 strongly interacting (SIMP) DM [28–31]. The
particles in the Galaxy is small, the prospective signal is
                                                                         effects of early kinetic decoupling were studied in [32]. The
suppressed by vanishing momentum transfer. This result
                                                                         Z3 symmetry has been considered as the remnant of a dark
remains practically unaffected when loop corrections to the
                                                                         Uð1Þ local [33–35] or global [36] symmetry. In the
direct detection cross section are taken into account                    unbroken phase, the direct detection cross section can also
[14,15]. Despite that, it is possible for pseudo-Goldstone               be suppressed if the DM relic density is determined by
DM to show up at the LHC [16–19]. Pseudo-Goldstone                       semiannihilation processes [37–42]. However, the suppres-
DM with two Higgs doublets was considered in [20].                       sion is not as large as for pseudo-Goldstone DM.
   In this class of models, the global Uð1Þ symmetry is                     The discovery of gravitational waves (GWs) by the
explicitly, but only softly, broken into a discrete subgroup             LIGO experiment [43,44] opened a new avenue to probe
which is then broken spontaneously. In Ref. [12], the Uð1Þ               new physics. First-order phase transitions generate a
group was explicitly broken into Z2 symmetry. We study                   stochastic GW background [45–47], which may be dis-
the consequences of breaking Uð1Þ into Z3 . The model                    coverable in future space-based GW interferometers
                                                                         [48,49]. While the SM Higgs phase transition is a smooth
  *
      kristjan.kannike@cern.ch                                           crossover [50,51] and does not generate a GW signal, in
  †                                                                      models with an extended scalar sector, the first-order phase
      kaius.loos@gmail.com
  ‡
      martti.raidal@cern.ch                                              transition in the early Universe can become testable by
                                                                         observations. For a recent review on phase transitions and
Published by the American Physical Society under the terms of            GWs, see Ref. [52].
the Creative Commons Attribution 4.0 International license.
Further distribution of this work must maintain attribution to              GWs from beyond-the-SM physics with a scalar singlet
the author(s) and the published article’s title, journal citation,       have been studied in detail. These models admit a two-step
and DOI. Funded by SCOAP3.                                               phase transition that can be of the first order [53–60] and

2470-0010=2020=101(3)=035001(10)                                  035001-1                      Published by the American Physical Society
KANNIKE, LOOS, and RAIDAL                                                               PHYS. REV. D 101, 035001 (2020)

can potentially produce a measurable GW signal [61–72].         phase, the complex singlet S is a DM candidate, while in the
However, in the Z2 pseudo-Goldstone model, all phase            broken Z3 phase, it is its imaginary part, the pseudo-
transitions leading to the correct vacuum are of the second     Goldstone χ, which can be the DM. We concentrate on the
order [73], yielding no stochastic GW signal and excluding      latter case. (In order to avoid domain walls [75–79] due to
the additional potentially powerful experimental test of this   the broken Z3 , we can add to the potential a small term that
class of the SM models. The phenomenology of phase              explicitly breaks Z3 , such as the linear term in S, but
transitions and GWs of the Z3 complex singlet model was         otherwise does not change the dark matter phenomenology.)
studied in detail in Ref. [74] for the case where only the         Without a loss of generality, the vacuum expectation
Higgs boson has a VEV in our vacuum. This work,                 value (VEV) of S can be taken to be real and positive,
however, disregarded the singlet as a DM candidate,             because the degenerate vacua, where χ has a nonzero VEV,
because in the unbroken phase, a sizeable GW signal is          are related to the real vacuum by Z3 transitions. This choice
incompatible with the correct relic density.                    corresponds to a negative value of the parameter μ3.
   The goal of this paper is to study the nature of phase          The stationary point conditions are
transitions and GW signals in the complex scalar singlet
model in which a Uð1Þ symmetry is softly broken into its                        hð2λH h2 þ 2μ2H þ λSH s2 Þ ¼ 0;               ð3Þ
Z3 subgroup. Just like in the Z2 case, the elastic scattering
                                                                                    pffiffiffi
cross section pseudo-Goldstone DM with matter can be                    sð4λS s2 þ 3 2μ3 s þ 4μ2S þ 2λSH h2 Þ ¼ 0:            ð4Þ
small enough for the DM to be well hidden from direct
detection. We find, however, that unlike in the Z2 case,        We see that the model admits four types of extrema: the
strong first-order phase transitions can take place because     origin as O ≡ ð0; 0Þ is fully symmetric; the vacuum
the potential contains a cubic term. The resulting stochastic   H ≡ ðvh ; 0Þ, with Higgs VEV only, spontaneously breaks
GW background is potentially discoverable in future space-      the electroweak symmetry; the vacuum S ≡ ð0; vs Þ, with S
based detectors such as LISA and BBO.                           VEV only, spontaneously breaks Z3 ; our vacuum HS ≡
   The paper is organized as follows. We introduce the          ðvh ; vs Þ breaks both symmetries.
model in Sec. II. Various theoretical and experimental             The mass matrix of CP-even scalars in our HS vacuum
constraints are discussed in Sec. III. DM relic density,        is given by
phase transitions, and the predictions for the direct detec-
tion and GW signals are treated in Sec. IV. We conclude in                                                              
                                                                                      2λH v2             λSH vvs
                                                                            2
Sec. V. The details on the effective potential and thermal                M ¼                                             :   ð5Þ
corrections are relegated to the Appendix.                                            λSH vvs      2λS v2s þ p3 ffiffi μ3 vs
                                                                                                             2 2

                                                                  The mass matrix (5) is diagonalized by an orthogonal
        II. Z3 COMPLEX SINGLET MODEL
                                                                matrix,
   The most general renormalizable scalar potential of the                                                      
Higgs doublet H and the complex singlet S, invariant under                                   cos θ     − sin θ
                                                                                   O¼                             ;           ð6Þ
the Z3 transformation H → H, S → ei2π=3 S, is given by                                       sin θ      cos θ

        V ¼ μ2H jHj2 þ λH jHj4 þ μ2S jSj2 þ λS jSj4             via diagðm21 ; m22 Þ ¼ OT M2 O. The mixing angle θ is
                              μ                                 given by
            þ λSH jSj2 jHj2 þ 3 ðS3 þ S†3 Þ;             ð1Þ
                               2
                                                                                               λSH vvs
where only the cubic μ3 term softly breaks a global Uð1Þ                    tan 2θ ¼                                  :       ð7Þ
                                                                                        λH v þ λS v2s − 4p3 ffiffi2 μ3 vs
                                                                                               2
symmetry. Notice that the Z3 symmetry precludes any
quartic couplings that would result in a hard breaking of the
                                                                The mixing of the CP-even states h and s will yield two
Uð1Þ. The potential Eq. (1)—just like in the original Z2
                                                                CP-even mass eigenstates h1 and h2 The mass of the
pseudo-Goldstone DM model—has an additional discrete
                                                                pseudoscalar χ is, taking into account the extremum
Z2 symmetry, S → S† .
                                                                conditions,
  In the unitary gauge, we parametrize the fields as
               0                                                                             9
                                 v þ s þ iχ                                            m2χ ¼ − pffiffiffi μ3 vs ;                   ð8Þ
           H ¼ vþh  ;          S¼ s         :            ð2Þ                                  2 2
                pffiffi                  2
                     2
                                                                which is proportional to μ3 as it explicitly breaks the Uð1Þ
The S → S† is then equivalent to χ → −χ, which makes χ          symmetry.
stable even as the Z3 symmetry is broken. The model thus          We express the potential parameters in terms of physical
admits two different DM candidates: in the unbroken Z3          quantities in the zero-temperature vacuum, that is the

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masses m21 and m21 of real scalars, their mixing angle θ,                  tree-level values. The model is perturbative [86] if
pseudoscalar mass m2χ , and the VEVs vh and vs ,                           jλH j ≤ 23 π, jλS j ≤ π, and jλSH j ≤ 4π.
                                                                              We require that our HS vacuum be the global one: this
                    m21 þ m22 þ ðm21 − m22 Þ cos 2θ                        implies that
             λH ¼                                   ;              ð9Þ
                                 4v2h
                                                                                                             9m21 m22
                                                                                            m2χ <                            ;        ð18Þ
          3ðm21 þ m22 Þ þ 2m2χ þ 3ðm22 − m21 Þ cos 2θ                                               m21   cos θ þ m22 sin2 θ
                                                                                                             2
     λS ¼                                             ;           ð10Þ
                            12v2s
                                                                           which for sin θ ≈ 0 is approximated by mχ ≲ 3m2. This
                                                                           bound appears to be stronger than the constraint on the
                            ðm21 − m22 Þ sin 2θ
                    λSH ¼                       ;                 ð11Þ     cubic coupling from unitarity at finite-energy scattering.
                                  2vs vh                                     If the mass of x ≡ χ or h2 is less than mh =2, then the
                                                                           Higgs invisible decay width into this particle is given by
                 1                1
          μ2H ¼ − ðm21 þ m22 Þ þ    ðm2 − m21 Þ                                                                  sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
                 4               4vh 2
                                                                                                       g                     m2
                × ðvh cos 2θ þ vs sin 2θÞ;                        ð12Þ                        Γh→xx   ¼ hxx       1 − 4 2x ;          ð19Þ
                                                                                                        8π                   mh
            1              1       1
     μ2S ¼ − ðm21 þ m22 Þ þ m2χ þ    ðm2 − m22 Þ                           with
            4              6      4vs 1
          × ðvs cos 2θ − vh sin 2θÞ;                              ð13Þ                                  m2h þ m2χ
                                                                                                   gh1χχ ¼        ;                   ð20Þ
                                pffiffiffi                                                                        vS
                               2 2 m2χ                                                                
                        μ3 ¼ −         :                          ð14Þ                   1   1 2
                                9 vs                                       gh1h2 h2   ¼               2
                                                                                               m þ m2 ðv cos θ þ vS sin θÞ
                                                                                        vvS 2 h
                                                                                                   
Both the Higgs doublet and the singlet will get a VEV, with                              1 2
the Higgs VEV given by vh ¼ v ¼ 246.22 GeV. We                                          þ vmχ cos θ sin 2θ:                           ð21Þ
                                                                                         6
identify h1 with the SM Higgs boson with mass
m1 ¼ 125.09 GeV [80].                                                      The invisible Higgs branching ratio is then given by

     III. THEORETICAL AND EXPERIMENTAL                                                                  Γh→χχ þ Γh→h2 h2
                                                                                         BRinv ¼                              ;       ð22Þ
                 CONSTRAINTS                                                                       Γh1 →SM þ Γh→χχ þ Γh→h2 h2
   We impose various theoretical and experimental con-                     which is constrained to be below about 0.24 at 95%
straints on the parameter space of the model.                              confidence level [87,88] by direct measurements and below
   First of all, the potential (1) is bounded from below if                about 0.17 by statistical fits of all Higgs couplings [89,90]
                                          pffiffiffiffiffiffiffiffiffiffi                      (decays of the resulting h2 into the SM may be visible [16],
     λH > 0;        λS > 0;        λSH þ 2 λH λS > 0:             ð15Þ     but these points are already excluded by direct detection in
                                                                           any case). In extended Higgs models [91], the mixing
   Secondly, we require the couplings to be unitary and                    phenomenology can be more complicated.
perturbative. The unitarity constraints in the s → ∞ limit                     The mixing angle between h and s is constrained from
are given by                                                               the measurements of the Higgs couplings at the LHC to
                                                                           j sin θj ≤ 0.5 for m2 ≲ mh and j sin θj ≤ 0.37 for m2 ≳ mh ;
         jλH j ≤ 4π;        jλS j ≤ 4π;        jλSH j ≤ 8π;       ð16Þ     for the latter case, we have an ever stronger constraint from
                qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi   the mass of the W boson down to j sin θj ≤ 0.2 for larger
j3λH þ 2λS      9λ2H − 12λH λS þ 4λ2S þ 2λ2SH j ≤ 8π; ð17Þ                values of m2 [92].
                                                                               Last, but not least, we require that the relic density of χ
                                                                           be equal to the value ΩDM h2 ¼ 0.120  0.001 from recent
where the last condition, in the λSH ¼ 0 limit, yields jλH j ≤             Planck data [93].
4
3π  and jλS j ≤ 2π. We also calculate unitarity constraints at
finite energy with the help of the latest version [81] of the
                                                                                        IV. DIRECT DETECTION AND
SARAH package [82–85]. Scattering at finite energy allows
                                                                                      GRAVITATIONAL WAVE SIGNALS
us to set a bound on the cubic coupling μ3 .
   To ensure the validity of perturbation theory, loop                       For the scan of the parameter space, we choose mχ , m2 ,
corrections to couplings should be smaller than their                      and sin θ as the free parameters, while vs is used to fit the DM

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relic density. We generate the free parameter values in the                  μ2H ðTÞ ¼ μ2H þ cH T 2 ;        μ2S ðTÞ ¼ μ2H þ cS T 2 ;    ð26Þ
following ranges: mχ ∈½25;1000GeV, m2 ∈½25;4000GeV,
and sin θ ∈ ½−0.5; 0.5. We use the micrOMEGAs package [94]               with the coefficients,
to fit the DM relic density and CosmoTransitions [95] for the                         1
calculation of phase transitions and the Euclidean action to                  cH ¼       ð9g2 þ 3g02 þ 12y2t þ 24λH þ 4λSH Þ;            ð27Þ
                                                                                      48
determine tunneling rates. The stochastic gravitational wave
signals were calculated using the formulas of Ref. [96] for the                                     1
                                                                                                cS ¼ ð2λS þ λSH Þ:                       ð28Þ
nonrunaway case. At the temperature T  at which gravita-                                           6
tional waves are being produced, the spectrum is a function of
                                                                          For numerical calculations of gravitational wave signals,
the parameters,
                                                                          the exact expression of the thermal potential is used.
                                  ρvac                                       We present in Fig. 1, the results of our scans for the direct
                            α¼          ;                      ð23Þ
                                  ρrad                                   detection signal. In the left panel, we present, for illus-
                                                                          tration, the behavior of spin-independent direct detection
where ρrad ¼ g π 2 T 4 =30 and g is the number of relativistic        cross section σ SI as a function of DM mass mχ for fixed
degrees of freedom (d.o.f.) in the plasma at temperature T  ,            values of m2 ¼ 300 GeV and sin θ ¼ 0.05. Red lines show
and                                                                       the current limit from the XENON1T experiment [10] and the
                                                                         future bound from the projected XENONnT experiment [97].
                            β      dS                                   While the usual Z2 scalar singlet DM is excluded below
                              ¼ T  ;                       ð24Þ
                           H      dT T                                  OðTeVÞ masses, except for the Higgs resonance region, the
                                                                          suppression of the pseudo-Goldstone cross section in this
where S is the Euclidean action of a critical bubble.
                                                                          model allows for lighter DM candidates. The two pro-
   The micrOMEGAs is also used to compute predictions for
                                                                          nounced dips in the cross section are given by the Higgs
direct detection signals, to which we add the tiny loop
                                                                          resonance at mh =2 and the h2 resonance at m2 =2.
correction. Unlike in the Z2 pseudo-Goldstone DM model,
                                                                             The direct detection signal varies over a wide range and
the tree-level direct detection DM amplitude contains a
                                                                          is proportional to μ23 . Because the amplitude is proportional
term that does not vanish at zero momentum transfer, so
                                                                          to sin θ cos θ, larger mixing angles also produce larger
                      Add ðt ≈ 0Þ ∝ λSH μ3 :                   ð25Þ       direct detection signal. The right panel in Fig. 1 presents the
                                                                          results of the whole scan, coded in grey-scale by the mixing
For a large part of the parameter space, however, this term is            angle j sin θj; empty points are excluded by the constraint
small enough, which allows one to explain the negative                    on the Higgs invisible branching ratio. The points shown
experimental results from DM direct detection experiments                 satisfy all other constraints with the exception of the
for a wide range of pseudo-Goldstone DM masses.                           collider constraints [92] on j sin θj (as seen, most such
   In the high temperature approximation (A8), the mass                   points are already excluded by direct detection). Note that
terms acquire thermal corrections,                                        the BRinv can exclude points with mχ > mh =2, because the

FIG. 1. Left panel: Spin-independent direct detection cross section σ SI as a function of the DM mass mχ for fixed m2 ¼ 300 GeV and
sin θ ¼ 0.05. The bound from the XENON1T experiment is shown in red and the predicted sensitivity of the XENONnT experiment in
dashed red. Right panel: Scatter plot of the results in ðmχ ; σ SI Þ plane in the grey-scale representing the dependence on j sin θj. The upper
bound in σ SI is given by the requirement that the ðvh ; vs Þ minimum be the global one. In unfilled points, the Higgs invisible width
surpasses the experimental limit.

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FIG. 2. Left panel: Scatter plot for the peak power of GWs vs the frequency together with the sensitivity curves of the LISA and BBO
experiments. Light red points are forbidden and darker green points are allowed by the results of the XENON1T direct detection searches;
both sets of points pass all other constraints, including the relic density. Right panel: The dependence of the peak power of GWs on the
value of cubic coupling for the points that pass all constraints including direct detection. The color code shows the nature of the phase
transition that gives the strongest signal: O → S in orange, S → S in dark blue, S → HS in cyan, HS → HS in yellow, H → HS in
purple, O → H in red.

Higgs boson can decay invisibly also to two h2 if it is light
enough. Early kinetic decoupling [98,99] may additionally
enhance BRinv several times [32], but, in practice, this
would not change the parameter space of GW signals. In
particular, all our points with a potentially measurable GW
signal have mχ , m2 > mh =2. The upper bound on σ SI arises
from the requirement the HS vacuum be global, which
bound is stronger than that from unitarity that also con-
strains the points from above.
   In Fig. 2, we present the predicted stochastic GW signal
together with the predicted sensitivity curves of the future
LISA and the BBO satellite experiments. We used bubble wall
velocity vw ¼ 0.5 and the fraction κ turb ¼ 0.05κ v in a
turbulent motion that is converted into gravitational waves
following [96], where the fraction κ v of vacuum energy
converted into gravitational waves is given in [100]. To avoid
cluttering the plot, we only show the peak power of each GW
spectrum. In the left panel, light red points are excluded by the
XENON1T direct detection constraints, while the darker green
points are still allowed by direct detection; all points not
satisfying the other constraints have been excluded. In the
right panel, we depict the dependence of the GW signal on
the cubic coupling μ3 . The color code shows the nature of the
phase transition that yields the strongest signal in a sequence
of phase transitions. The largest signal is produced by the
O → S transitions, enhanced by a sizable cubic coupling.
Because mχ ∝ μ3 , there is a similar dependence on mχ =m2 .
The strongest signals can be produced at roughly mχ ≈ 2.3m2.
   In the left panel of Fig. 3, we show the correlation               FIG. 3. Left panel: Scatter plot for the peak power of GWs vs
                                                                      direct detection cross section. The points pass all other con-
between the gravitational wave signal and the direct
                                                                      straints. Right panel: Signal-to-noise ratios of the data points for
detection cross section.                                              the BBO experiment. Light red points are forbidden and darker
   We have also calculated the signal-to-noise ratio for the          green points are allowed by the results of the XENON1T direct
BBO experiment, shown in the right panel of Fig. 3 with the           detection searches; both sets of points pass all other constraints.
same color code as Fig. 1. The horizontal thick line shows            The horizontal thick line shows SNR ¼ 1.

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                                                                                      ACKNOWLEDGMENTS
                                                                      We would like to thank Matti Heikinheimo and Christian
                                                                   Gross for useful comments. This work was supported by
                                                                   the Estonian Research Council Grant No. PRG434, the
                                                                   Grant No. IUT23-6 of the Estonian Ministry of Education
                                                                   and Research, and by the European Union through the
                                                                   ERDF Centre of Excellence program Project No. TK133.

                                                                   APPENDIX: THERMAL EFFECTIVE POTENTIAL
                                                                     At the one-loop level, the quantum corrections to the
                                                                   scalar potential in the MS renormalization scheme are given
                                                                   by
                                                                                        X 1                     
FIG. 4. β=H vs α. Light red points are forbidden and darker                                          4   m2i
green points are allowed by the results of the XENON1T direct                  ΔV ¼              ni mi ln 2 − ci ;       ðA1Þ
                                                                                         i
                                                                                           64π 2         μ
detection searches.
                                                                   where ni are the d.o.f. of the ith field, mi are field-
SNR ¼ 1; the actual detection threshold is expected to be          dependent masses, and the constants ci ¼ 32 for scalars
within an order of magnitude of it [96].                           and fermions and ci ¼ 56 for vector bosons. The masses and
   Because the direct detection cross section is proportional      d.o.f. ni of the fields are given in Table I. The field-
to μ23 , the points on the left panel of Fig. 2 with the highest   dependent masses of h and s are given by the eigenvalues
GW signal are excluded. Note, however, that the direct             m21;2 of the mass matrix of the CP-even eigenstates with
detection signal can still be small even for a sizeable GW         elements given by
signal, because the Higgs portal λSH is on the order of (0.01).
This is in contrast with λSH ∼ 1 in Ref. [74] with the                                                        1
                                                                                   ðm2R Þ11 ¼ μ2H þ 3h2 λH þ λSH s2 ;      ðA2Þ
assumption that the Z3 symmetry is unbroken in our vacuum.                                                    2
   Figure 4 shows the range of β=H vs α with the same                                          2
                                                                                            ðmR Þ12 ¼ λSH hs;              ðA3Þ
color code as Fig. 1. While Ref. [101] showed that for α
close to unity the gravitational wave signal is suppressed,                                         3 pffiffiffi      1
                                                                        ðm2R Þ22   ¼ μ2S þ 3λS s2 þ     2μ3 s þ h2 λSH :   ðA4Þ
our values of α are quite smaller than one.                                                         2           2
                                                                   We neglect the contributions of the Goldstone bosons G0
                    V. CONCLUSIONS
                                                                   and G as unimportant numerically. To calculate the
   We have studied the Z3 complex scalar singlet DM                effective potential in the case of negative field-dependent
model, where only the cubic coupling of the singlet                masses, we substitute ln m2i → ln jm2i j, which is equivalent
explicitly breaks a global Uð1Þ symmetry. The model                to analytical continuation [102]. We set the renormalization
has two phases with stable DM. In the phase where the              scale to μ ¼ M t .
Z3 is spontaneously broken, the residual CP-like Z2                   We add a counterterm potential,
symmetry stabilizes the imaginary part of the complex
singlet S as a pseudo-Goldstone DM candidate. The DM               δV ¼ δμ2H jHj2 þ δλH jHj4 þ δμ2S jSj2 þ δλS jSj4
direct detection cross section can be considerably sup-                                        δμ3 3
pressed by the small momentum transfer or the resonance at               þ δλSH jSj2 jHj2 þ       ðS þ S†3 Þ þ δV 0 ;      ðA5Þ
half the mass of the heavy singletlike particle.                                                2
   While we may be unable to discover the pseudo-
Goldstone DM via direct detection, it may be testable by           TABLE I.    Field-dependent masses and the numbers of d.o.f.
other means. In this model, a stochastic gravitational wave
background can arise from the first-order phase transitions        Field i                           m2i                     ni
due to the presence of a cubic coupling for the singlet. The       h                               m21                       1
strongest signals, which can potentially be observed by the        s                               m22                       1
future BBO experiment, are produced in the O → S phase             χ                   μS þ λS s − 3ffiffi2 μ3 s þ 12 λSH h2
                                                                                        2       2 p                          1
transition (if present) at mχ ≈ 2.3m2 . The strength of the        Z0                          1 2        02 2
                                                                                               4 ðg þ g Þh
                                                                                                                             3
gravitational wave signal is anticorrelated with a small           W                              1 2 2                     6
                                                                                                   4g h
mixing angle and is, therefore, greater where the direct           t                               1      2                 −12
                                                                                                   2 yt h
detection cross section is smaller (at fixed cubic coupling).

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in order to fix the VEVs and the mass matrix in our HS                The counterterm potential δV is chosen such as to keep
minimum to their tree-level values.                                 quantum corrections to the masses, to mixing between h
   The thermal corrections to the potential are given by            and s and to the VEVs zero. The counterterms are given by
                                       
                          T4 X         mi                                                       1
                      VT ¼ 2   ni J ∓     ;                 ðA6Þ                     δλH ¼         ð∂ h ΔV − v∂ 2h ΔVÞ;        ðA10Þ
                          2π i         T                                                       2v3
                                                                                   1
                                                                         δλS ¼         ð4∂ s ΔV − vS ∂ 2χ ΔV − 3vS ∂ 2s ΔVÞ;   ðA11Þ
where                                                                             6v3S
            Z     ∞          h        qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi i                                             1
   J∓ ¼              dyy2 ln 1 ∓ exp − y2 þ x2 ;           ðA7Þ                         δλSH ¼ −       ∂ ∂ ΔV;                ðA12Þ
                                                                                                     vvS h s
              0
                                                                               1
with the − sign applied to bosons and the þ sign to                      δμ2H ¼  ð−3∂ h ΔV þ vs ∂ h ∂ s ΔV þ v∂ 2h ΔVÞ;        ðA13Þ
                                                                              2v
fermions. In the high-temperature limit m=T ≪ 1, the
                                                                              1
thermal contributions are given by                                  δμ2S ¼ −     ðv ∂ 2 ΔV þ 8∂ s ΔV − 3vS ∂ 2s ΔV
                                                                             6vS S χ
                                   T2 X                                     − 3v∂ h ∂ s ΔVÞ;                                   ðA14Þ
                       V T ðTÞ ¼        n m2 :              ðA8Þ
                                   24 i i i                                                pffiffiffi
                                                                                          2 2
                                                                                     δμ3 ¼ 2 ðvS ∂ 2χ ΔV − ∂ s ΔVÞ;            ðA15Þ
  The full thermally corrected effective potential is then                                9vS
                                                                    where we take h ¼ v, s ¼ vS , χ ¼ 0 after taking the
                  V ð1Þ ¼ V þ ΔV þ δV þ V T :               ðA9Þ
                                                                    derivatives.

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