D-wave Annihilation in Dark Matter
- D-wave annihilation is defined as a process where the annihilation cross section scales as (v/c)^4 due to the ℓ=2 partial wave dominance.
- Astrophysical analyses incorporate generalized J-factors that fold in velocity distributions, shifting the sensitivity from cold dwarf galaxies to high-velocity massive halos.
- Observational studies using data from clusters, the LMC, and local large-scale structures have set stringent limits that highlight the model-dependent nature of dark matter indirect detection.
Searching arXiv for papers on d-wave annihilation and related usages of the term. {"12query12 annihilation\"12 OR \12"d wave annihilation\"12 OR \12"velocity-dependent annihilation\"12 dark matter d-wave)12"," {"12query12 evidence for p- or d-wave dark matter annihilation from local large-scale structure\" OR 12ti:\12 in velocity-dependent dark matter annihilation in Galactic subhalos\" OR 12ti:\12 J-factors for Milky Way dwarf spheroidal galaxies with velocity-dependent annihilation\"","12max_results12 D-wave annihilation most commonly denotes an annihilation channel whose nonrelativistic cross section is dominated by the partial wave with orbital angular momentum PRESERVED_PLACEHOLDER_12query12. In the velocity expansion used in indirect dark-matter phenomenology, this is written as PRESERVED_PLACEHOLDER_12all:(\12, so the d-wave case obeys PRESERVED_PLACEHOLDER_12 OR \12. The defining consequence is a very strong weighting toward environments with large relative velocities: signals are suppressed in dynamically cold systems such as dwarf spheroidals and are enhanced in massive halos, the smooth Galactic halo, and perturbed regions with an enlarged high-speed tail (&&&12query12&&&, &&&12all:(\12&&&, &&&12 OR \12&&&). The expression also appears in other areas of physics with different meanings, including d-wave cold-atom scattering, annihilation of d-wave gap nodes in superconductors, and weak-annihilation topologies in heavy-flavor decays; those usages are conceptually distinct (&&&12 OR \12&&&, &&&12 dark matter d-wave)12&&&, &&&12max_results12&&&).
12all:(\12. Formal definition and astrophysical signal formalism
For velocity-dependent annihilation models, the basic parametrization used in several dark-matter analyses is
PRESERVED_PLACEHOLDER_12 OR \12^
with PRESERVED_PLACEHOLDER_12 dark matter d-wave)12^ for d-wave annihilation, or equivalently
PRESERVED_PLACEHOLDER_12max_results12^
with PRESERVED_PLACEHOLDER_12sort_by12^ (&&&12sort_by12&&&, &&&12query12&&&). In this notation, PRESERVED_PLACEHOLDER_12relevance12^ is the velocity-independent normalization multiplying PRESERVED_PLACEHOLDER_12query12^ (&&&12query12&&&). The formalism immediately shows why d-wave annihilation differs sharply from PRESERVED_PLACEHOLDER_12ti:\12-wave, PRESERVED_PLACEHOLDER_12all:(\12query12-wave, or Sommerfeld-enhanced cases: the rate is strongly weighted toward the high-velocity tail of the pairwise relative-velocity distribution.
Because the cross section depends explicitly on velocity, the usual density-squared PRESERVED_PLACEHOLDER_12all:(\12all:(\12-factor is not sufficient. The relevant quantity is an effective or generalized PRESERVED_PLACEHOLDER_12all:(\12 OR \12-factor that folds the local velocity distribution into the line-of-sight integral. One form used for subhalos and the Milky Way halo is
PRESERVED_PLACEHOLDER_12all:(\12 OR \12^
while a moment-based form introduces
PRESERVED_PLACEHOLDER_12all:(\12 dark matter d-wave)12^
so that the d-wave signal is governed by PRESERVED_PLACEHOLDER_12all:(\12max_results12^ (&&&12ti:\12&&&). In the dwarf-spheroidal literature, the same idea appears as an effective PRESERVED_PLACEHOLDER_12all:(\12sort_by12-factor,
PRESERVED_PLACEHOLDER_12all:(\12relevance12^
with PRESERVED_PLACEHOLDER_12all:(\12query12^ for d-wave annihilation (&&&12all:(\12&&&).
The corresponding gamma-ray flux retains the standard factorized structure once the generalized PRESERVED_PLACEHOLDER_12all:(\12ti:\12-factor is defined: PRESERVED_PLACEHOLDER_12 OR \12query12^ What changes is entirely in the astrophysical weighting. This is the central methodological distinction between d-wave and velocity-independent annihilation analyses (&&&12ti:\12&&&).
12 OR \12. Dwarf spheroidal galaxies and the first indirect limits
The first systematic effective-PRESERVED_PLACEHOLDER_12 OR \12all:(\12^ treatment of d-wave annihilation for Milky Way dwarf spheroidal galaxies computed effective PRESERVED_PLACEHOLDER_12 OR \12 OR \12-factors for 12 OR \12max_results12^ dSphs under an NFW density profile, Eddington inversion for the dark-matter velocity distribution, and a spherical Jeans analysis of stellar kinematics (&&&12all:(\12&&&). The halo parameters PRESERVED_PLACEHOLDER_12 OR \12 OR \12, PRESERVED_PLACEHOLDER_12 OR \12 dark matter d-wave)12, and PRESERVED_PLACEHOLDER_12 OR \12max_results12^ were inferred using a Plummer stellar density profile, constant stellar anisotropy, Gaussian priors on the half-light radius, ellipticity, and distance, and Jeffreys priors on the halo parameters. The gamma-ray limits were then obtained with the MADHAT framework using nearly 12all:(\12all:(\12^ years of Fermi-LAT Pass 12query12R12 OR \12^ data in the 12all:(\12–12all:(\12query12query12^ GeV range (&&&12all:(\12&&&).
For the d-wave case, this analysis explicitly presented the first indirect-detection bound on a PRESERVED_PLACEHOLDER_12 OR \12sort_by12-suppressed annihilation cross section from dwarf gamma-ray data (&&&12all:(\12&&&). Its physical interpretation was equally important: because dwarf spheroidals are low-velocity systems, their effective PRESERVED_PLACEHOLDER_12 OR \12relevance12-factors are much smaller than in the PRESERVED_PLACEHOLDER_12 OR \12query12-wave case, and changing the annihilation model can alter the inferred PRESERVED_PLACEHOLDER_12 OR \12ti:\12-factor by orders of magnitude. The paper therefore established that the astrophysical ranking of indirect-detection targets is model dependent once velocity weighting is included.
This conclusion is reinforced by the scaling relation used in the dwarf analysis,
PRESERVED_PLACEHOLDER_12 OR \12query12^
which for PRESERVED_PLACEHOLDER_12 OR \12all:(\12^ adds two more powers of the characteristic velocity scale than the PRESERVED_PLACEHOLDER_12 OR \12 OR \12-wave case and four more than the PRESERVED_PLACEHOLDER_12 OR \12 OR \12-wave case (&&&12all:(\12&&&). A direct implication is that dense but dynamically cold systems can be much less constraining for d-wave models than for velocity-independent annihilation.
12 OR \12. Local large-scale structure as the dominant target class
A major revision of the target hierarchy came from a full-sky search for velocity-dependent annihilation in local large-scale structure, based on the CSiBORG suite of 12all:(\12query12all:(\12^ constrained PRESERVED_PLACEHOLDER_12 OR \12 dark matter d-wave)12-body simulations derived from the BORG Bayesian reconstruction of the 12 OR \12M++ galaxy catalogue (&&&12query12&&&). The analysis constructed gamma-ray templates for galaxy- and cluster-mass halos within PRESERVED_PLACEHOLDER_12 OR \12max_results12^ Mpc, resolved the local halo field out to PRESERVED_PLACEHOLDER_12 OR \12sort_by12^ Mpc, and compared the resulting templates with Fermi-LAT data from mission weeks 12ti:\12–12sort_by12 OR \12 dark matter d-wave)12^ in the 12max_results12query12query12^ MeV–12max_results12query12^ GeV range, using PS12 OR \12^ SOURCEVETO events, 12ti:\12^ energy bins, HEALPix maps with PRESERVED_PLACEHOLDER_12 OR \12relevance12, and a Galactic-plane mask PRESERVED_PLACEHOLDER_12 OR \12query12^ (&&&12query12&&&).
The d-wave-specific astrophysical factor in that work required the fourth velocity moment,
PRESERVED_PLACEHOLDER_12 OR \12ti:\12^
implemented by assuming an ergodic distribution function with PRESERVED_PLACEHOLDER_12 dark matter d-wave)12query12^ and an NFW profile, and then coding the analytic result in CLUMPY (&&&12query12&&&). The likelihood marginalized both over reconstruction uncertainties by averaging across all 12all:(\12query12all:(\12^ CSiBORG realizations and over the non-dark-matter templates, namely isotropic background, Galactic diffuse emission, and point sources.
The result was a null detection: the inferred template amplitudes were consistent with zero in every energy bin, and there was no evidence for d-wave annihilation for any channel over the mass range PRESERVED_PLACEHOLDER_12 dark matter d-wave)12all:(\12–PRESERVED_PLACEHOLDER_12 dark matter d-wave)12 OR \12^ (&&&12query12&&&). For the benchmark PRESERVED_PLACEHOLDER_12 dark matter d-wave)12 OR \12^ channel at PRESERVED_PLACEHOLDER_12 dark matter d-wave)12 dark matter d-wave)12, the bound was
PRESERVED_PLACEHOLDER_12 dark matter d-wave)12max_results12^
and the paper concluded that the d-wave limits are about seven orders of magnitude tighter than dwarf-spheroidal limits (&&&12query12&&&). The analysis further found that the constraints are dominated by the most massive halos, roughly PRESERVED_PLACEHOLDER_12 dark matter d-wave)12sort_by12, because d-wave annihilation benefits from the large velocity dispersions of cluster-mass objects. The same work estimated the thermal-relic coefficient for d-wave annihilation as PRESERVED_PLACEHOLDER_12 dark matter d-wave)12relevance12, so the observational upper limits still do not exclude a thermal relic with d-wave annihilation (&&&12query12&&&).
The resulting observational hierarchy is summarized below.
| Target class | Analysis | D-wave outcome |
|---|---|---|
| 12 OR \12max_results12^ Milky Way dSphs | Effective PRESERVED_PLACEHOLDER_12 dark matter d-wave)12query12-factors + Fermi-LAT Pass 12query12R12 OR \12^ | First indirect-detection bounds (&&&12all:(\12&&&) |
| Local large-scale structure within PRESERVED_PLACEHOLDER_12 dark matter d-wave)12ti:\12^ Mpc | CSiBORG + BORG + Fermi-LAT | About seven orders tighter than dSph limits; null detection (&&&12query12&&&) |
| Large Magellanic Cloud | Auriga MW–LMC analogue + 12all:(\12sort_by12.12max_results12relevance12 years Fermi-LAT | PRESERVED_PLACEHOLDER_12max_results12query12–PRESERVED_PLACEHOLDER_12max_results12all:(\12 orders more stringent than previous dwarf bounds (&&&12ti:\12&&&) |
12 dark matter d-wave)12. Smooth halo dominance, subhalo suppression, and host-halo overlap
Cosmological simulations of Milky Way-like halos showed that d-wave annihilation is the most strongly smooth-halo-dominated of the commonly studied partial-wave models. Using six Auriga Milky Way analogues, each with hydrodynamical and dark-matter-only realizations, the generalized annihilation luminosity was written as
PRESERVED_PLACEHOLDER_12max_results12 OR \12^
with PRESERVED_PLACEHOLDER_12max_results12 OR \12^ for d-wave annihilation (&&&12 OR \12&&&). Subhalo velocities were found to be well approximated by a Maxwell-Boltzmann form, and for d-wave emission the relevant factor is the fourth velocity moment PRESERVED_PLACEHOLDER_12max_results12 dark matter d-wave)12^ (&&&12 OR \12&&&).
The main qualitative conclusion was that the smooth halo dominates the annihilation luminosity at all radii within PRESERVED_PLACEHOLDER_12max_results12max_results12^ for PRESERVED_PLACEHOLDER_12max_results12sort_by12-wave, PRESERVED_PLACEHOLDER_12max_results12relevance12-wave, and d-wave models, with d-wave subhalo emission being the weakest of all four cases considered (&&&12 OR \12&&&). In the d-wave subhalo luminosity function, the fitted power-law indices lie in the ranges PRESERVED_PLACEHOLDER_12max_results12query12–PRESERVED_PLACEHOLDER_12max_results12ti:\12^ for Auriga hydrodynamical runs and PRESERVED_PLACEHOLDER_12sort_by12query12–PRESERVED_PLACEHOLDER_12sort_by12all:(\12^ for dark-matter-only runs, implying that the total resolved subhalo luminosity is dominated by the brightest resolved subhalos rather than by a large population of faint objects (&&&12 OR \12&&&). For Au12sort_by12, the fraction of d-wave subhalo luminosity coming from subhalos above PRESERVED_PLACEHOLDER_12sort_by12 OR \12^ is 12all:(\12.12query12query12query12^ in the hydrodynamical run and 12query12.12ti:\12ti:\12ti:\12^ in the dark-matter-only run, so extrapolating to much smaller unresolved masses has little effect (&&&12 OR \12&&&).
A later Auriga-based study refined this picture by including the contribution of unbound dark-matter particles from the smooth Galactic halo that spatially overlap with subhalos (&&&12sort_by12&&&). The enhancement factor was defined as PRESERVED_PLACEHOLDER_12sort_by12 OR \12, comparing bound-plus-unbound to bound-only emission, and the maximal enhancement factor for d-wave models was found to be approximately PRESERVED_PLACEHOLDER_12sort_by12 dark matter d-wave)12^ (&&&12sort_by12&&&). Across six Milky Way-like hosts, about 12sort_by12^ d-wave subhalos were above the smooth foreground when both bound and unbound particles were included, compared with only 12 OR \12^ when only bound particles were used (&&&12sort_by12&&&). The strongest boosts occurred in lower-mass subhalos closer to the Galactic center; representative examples include Au12all:(\12sort_by12, 12ti:\12^ with d-wave boost PRESERVED_PLACEHOLDER_12sort_by12max_results12, Au12all:(\12sort_by12, 12all:(\12 dark matter d-wave)12query12^ with d-wave boost PRESERVED_PLACEHOLDER_12sort_by12sort_by12, Au12 OR \12 dark matter d-wave)12, 12 OR \12 OR \12all:(\12^ with d-wave boost PRESERVED_PLACEHOLDER_12sort_by12relevance12, and Au12 OR \12 OR \12, 12relevance12^ with d-wave boost PRESERVED_PLACEHOLDER_12sort_by12query12^ (&&&12sort_by12&&&).
These two results concern different observables rather than incompatible global conclusions. The 12 OR \12query12 OR \12 OR \12^ Auriga study compared the total annihilation flux from subhalos with that from the smooth halo within the virial radius, whereas the 12 OR \12query12 OR \12max_results12^ analysis isolated individual subhalos and quantified how host-halo overlap modifies their PRESERVED_PLACEHOLDER_12sort_by12ti:\12-factors (&&&12 OR \12&&&, &&&12sort_by12&&&). A plausible implication is that large line-of-sight boosts for selected inner subhalos need not overturn the broader smooth-halo dominance of the total d-wave luminosity.
12max_results12. The Large Magellanic Cloud and dynamical reshaping of the Milky Way halo
A further extension of the velocity-dependent framework examined the Large Magellanic Cloud using a Milky Way–LMC analogue drawn from the Auriga magneto-hydrodynamical simulations (&&&12ti:\12&&&). The analogue system consisted of Auriga halo 12 OR \12max_results12^ / re-simulated halo 12all:(\12 OR \12, with an LMC analogue of halo mass at infall PRESERVED_PLACEHOLDER_12relevance12query12, present-day-like separation PRESERVED_PLACEHOLDER_12relevance12all:(\12^ kpc, speed PRESERVED_PLACEHOLDER_12relevance12 OR \12^ km/s, and a Milky Way virial mass of PRESERVED_PLACEHOLDER_12relevance12 OR \12^ (&&&12ti:\12&&&). The d-wave model again took
PRESERVED_PLACEHOLDER_12relevance12 dark matter d-wave)12^
so the signal was especially sensitive to the high-speed component of the distribution.
The study found that the gamma-ray signal from the LMC analogue rises above the Milky Way foreground by more than a factor of 12all:(\12query12query12^ not only for PRESERVED_PLACEHOLDER_12relevance12max_results12-wave annihilation but also for Sommerfeld, PRESERVED_PLACEHOLDER_12relevance12sort_by12-wave, and d-wave models (&&&12ti:\12&&&). Using 12all:(\12sort_by12.12max_results12relevance12^ years of Fermi-LAT data in a PRESERVED_PLACEHOLDER_12relevance12relevance12^ region of interest centered on the LMC, and taking the simulation-derived PRESERVED_PLACEHOLDER_12relevance12query12-map as the dark-matter template, the authors derived upper limits on the annihilation cross section for all four velocity dependences. For d-wave annihilation, the LMC-derived upper limits were stated to be PRESERVED_PLACEHOLDER_12relevance12ti:\12–PRESERVED_PLACEHOLDER_12query12query12^ orders of magnitude more stringent than previous dwarf-galaxy bounds (&&&12ti:\12&&&).
The LMC also alters the Milky Way halo itself. According to the simulation analysis, it both brings in its own fast dark-matter particles and gravitationally accelerates native Milky Way halo particles, producing outer-halo overdensities and shifting the relative-speed distribution toward higher speeds (&&&12ti:\12&&&). For the d-wave case, the Milky Way PRESERVED_PLACEHOLDER_12query12all:(\12-factor can be boosted by up to a factor of about 12sort_by12^ in the outer halo (&&&12ti:\12&&&). This matters because d-wave annihilation weights the high-speed tail very strongly. The paper therefore elevated the LMC from a mere target to a dynamical agent that changes the indirect-detection interpretation of the Milky Way outskirts.
12sort_by12. Other meanings of “d-wave” and “annihilation”
Outside dark-matter phenomenology, the same words label different physical mechanisms. In ultracold atoms, the relevant topic is control of d-wave scattering rather than particle annihilation. A magneto-optical theory with two strong photoassociation lasers and, optionally, an PRESERVED_PLACEHOLDER_12query12 OR \12-wave magnetic Feshbach resonance predicts that coherence between two excited ro-vibrational bound states can enhance elastic d-wave scattering and suppress inelastic scattering (&&&12 OR \12&&&). For PRESERVED_PLACEHOLDER_12query12 OR \12Yb at PRESERVED_PLACEHOLDER_12query12 dark matter d-wave)12K, the paper estimated an inelastic scattering rate reducible to PRESERVED_PLACEHOLDER_12query12max_results12^ and an elastic rate about two orders of magnitude larger (&&&12 OR \12&&&). This usage concerns PRESERVED_PLACEHOLDER_12query12sort_by12^ scattering in a continuum-bound optical control problem, not annihilation into final particles.
In iron-based superconductors, “annihilation” refers to the merger and disappearance of displaced Dirac gap nodes in a d-wave superconducting state (&&&12 dark matter d-wave)12&&&). For two PRESERVED_PLACEHOLDER_12query12relevance12-centered hole pockets derived from PRESERVED_PLACEHOLDER_12query12query12^ and PRESERVED_PLACEHOLDER_12query12ti:\12^ orbitals, the d-wave order parameter in the orbital basis becomes a mixture of intra-band and inter-band pairing in the band basis,
PRESERVED_PLACEHOLDER_12ti:\12query12^
which shifts the nodal points away from the normal-state Fermi surfaces (&&&12 dark matter d-wave)12&&&). If the two pockets are sufficiently close, the shifted nodes merge at a critical PRESERVED_PLACEHOLDER_12ti:\12all:(\12^ and annihilate below some PRESERVED_PLACEHOLDER_12ti:\12 OR \12, yielding a fully gapped but still d-wave state (&&&12 dark matter d-wave)12&&&). This is a topological nodal annihilation, not a dark-matter annihilation process.
In charm physics, annihilation usually means weak annihilation or PRESERVED_PLACEHOLDER_12ti:\12 OR \12-annihilation. The BESIII observation of the pure PRESERVED_PLACEHOLDER_12ti:\12 dark matter d-wave)12-annihilation decay PRESERVED_PLACEHOLDER_12ti:\12max_results12^ and the evidence for PRESERVED_PLACEHOLDER_12ti:\12sort_by12^ concern valence-quark annihilation through a virtual PRESERVED_PLACEHOLDER_12ti:\12relevance12^ boson in hadronic charm decays (&&&12max_results12&&&). Inclusive semileptonic analyses likewise use “weak annihilation” for the dimension-12sort_by12^ four-quark contribution
PRESERVED_PLACEHOLDER_12ti:\12query12^
which is enhanced by PRESERVED_PLACEHOLDER_12ti:\12ti:\12^ but unrelated to partial-wave d-wave annihilation (&&&12 dark matter d-wave)12query12&&&). A separate terminological caution is that D-Wave quantum annealers use “D-Wave” as a hardware name rather than as a symmetry or partial-wave label; in that context the literature concerns defect production in quantum Ising-chain benchmarks (&&&12 dark matter d-wave)12ti:\12&&&).
The shared vocabulary can therefore be misleading. In current arXiv usage, “d-wave annihilation” most often refers to velocity-dependent dark-matter annihilation with PRESERVED_PLACEHOLDER_12all:(\12query12query12, whereas in other fields the phrase may denote either d-wave symmetry combined with a different annihilation mechanism or an unrelated proper noun.