STERILE NEUTRINO DARK MATTER MODELS / THEORY BIBHUSHAN SHAKYA SNOWMASS CF1 MEETING SEPTEMBER 11, 2020 1 STERILE NEUTRINO DARK MATTER MODELS / THEORY AN (INCOMPLETE, RAPID) OVERVIEW For greater details, see one of several reviews in the literature, e.g. • Sterile Neutrino Dark Matter; Boyarsky+, 1807.07938 • A White Paper on keV Sterile Neutrino Dark Matter; Adhikari+, 1602.04816 • Sterile Neutrino Dark Matter from Freeze-in; Shakya, 1512.02751 • The Phenomenology of Right Handed Neutrinos; Drewes, 1303.6912 • keV Neutrino Model Building; Merle, 1302.2625 • … 2 WHY STERILE NEUTRINOS (AS DARK MATTER) ? • Neutrino masses require BSM, most straightforward implementations feature right handed/sterile neutrinos • Very weakly coupled to SM sector: small but non-negligible production in early Universe, as well as long lifetime, are “automatic” • Relatively straightforward/predictive phenomenology: depends mostly on the sterile neutrino mass and mixing angle with SM neutrinos. Diverse signatures: extended neutrino sectors generally feature observable signals in indirect detection, cosmology, colliders, low energy (neutrino) experiments • Hints of sterile neutrinos at several experiments (3.5 keV line, short baseline anomalies) 3 STERILE NEUTRINO AS DARK MATTER: CAVEATS (PROBLEMS / OPPORTUNITIES) • MASS SCALE: The natural value of the mass of a singlet is at the cutoff scale of the theory (e.g. Planck or GUT scale), most of the viable DM candidates have much lower masses (sub-electroweak scale) • Not a disaster: singlet fermion mass does not get large quantum corrections (unlike the higgs); Seesaw mechanism works across a wide range of mass scales (from keV to GUT scale) Nevertheless, need an explanation for the mass scale 4 STERILE NEUTRINO AS DARK MATTER: CAVEATS (PROBLEMS / OPPORTUNITIES) • MASS SCALE: The natural value of the mass of a singlet is at the cutoff scale of the theory (e.g. Planck or GUT scale), most of the viable DM candidates have much lower masses (sub-electroweak scale) • Not a disaster: singlet fermion mass does not get large quantum corrections (unlike the higgs); Seesaw mechanism works across a wide range of mass scales (from keV to GUT scale) Nevertheless, need an explanation for the mass scale • MIXING ANGLE: sterile neutrinos that explain the observed neutrino masses via the seesaw mechanism cannot be dark matter for ANY mass - the mixing angles involved are too large. For dark matter, need a sterile neutrino essentially decoupled from the seesaw mechanism • again, not a disaster: vanishing coupling with the SM (small Yukawa) is a technically natural limit where the theory has an enhanced Z2 symmetry. Additional sterile neutrinos can generate neutrino masses via seesaw. • Philosophical conundrum: what is the boundary between a sterile / right handed neutrino and a generic heavy neutral lepton / singlet fermion? 5 STERILE NEUTRINO DARK MATTER: A THEORIST / MODEL BUILDER’S CHECKLIST ✓ Motivate the mass scale ✓ Suitable production mechanism to obtain the correct relic density while ensuring sufficiently long lifetime ( minimal mechanism now ruled out; see next slide) ✓ Acceptable momentum distribution (ie should not be too warm, be consistent with all cosmological data) ✓ Embed the candidate into a broader well motivated framework that addresses the issue of neutrino masses 6 2 6 1000 PeV be obtained by coupling the Ni to other fields charged 10 under the U(1) . Introducing an exotic field φ that car- 0 105 ries the opposite charge under U(1)0, one is allowed the 4 100 PeV following higher dimensional operators in the superpo- L 10 tential: 3 keV 10 H y c x c c s 2 10 PeV W LHuN φ + N N φφ. (3) 10 ⊃ M M m ⇤ ⇤ 101 Here x and y are dimensionless (1) couplings (neglect- O 1 1 PeV ing possible flavor structure for now), and M is the scale 100 TeV at which this e↵ective theory needs to be UV⇤ completed 10-3 10-2 10-1 1 with new physics, such as the scale of grand unification MGUT or the Planck scale MP . Here we have ignored the ma eV 2 (LHu) /M term that is of the same order as it is not large enough⇤ to produce the active neutrino mass scale, FIG. 1: Active and sterile neutrino mass scales for various 0 choices of y φ ,withM = MGUT ,tanβ =2(Hu = but we note that it can provide the dominant contribu- h i ⇤ H L h i tion to the mass of the lightest active neutrino. 155.6GeV),and0.001 <x<2. The dashed vertical line at ma =0.05eV is the active neutrino mass scale necessary If the scalar component of φ obtains a vev at the PeV 2 for consistency with atmospheric oscillation data ∆matm = 3 2 scale, presumably from the same mechanism that breaks 2.3 10− eV . ⇥ supersymmetry, this breaks the U(1)0 and (after Hu also acquires a vev) leads to the following active-sterile Dirac mass and sterile Majorana mass scales DARK MATTER AND COSMOLOGICAL CONSTRAINTS y φ H0 x φ 2 m = h ih ui,m= h i . (4) D M M M Sterile neutrinos are constrained by several cosmolog- ⇤ ⇤ ical and direct observations, which require careful treat- This results in a modified seesaw mechanism, arising en- ment. This section provides a brief overview to demon- tirely from higher dimensional operators. Below the elec- strate consistency with these constraints and the viabil- troweak scale, the e↵ective theory maps onto the ⌫MSM ity of dark matter; a more extensive and comprehensive with the following sterile and active neutrino mass scales: study will be presented in a forthcoming paper. x φ 2 We denote the sterile neutrino dark matter candidate ms = mM = h i , by N . As N couples extremely weakly to the SM fields M 1 1 ⇤ 2 2 0 2 and is never in thermal equilibrium in the early Uni- mD y Hu ma = = h i . (5) verse, its relic abundance is not set by thermal freeze-out. mM xM ⇤ Under various conditions,STERILE our frameworkNEUTRINO allows multiple DARK MATTER: Note that the two scales are related as production mechanisms for N1. Active-sterile mixing: Production through active- 0 2 THE TRADITIONAL (MINIMAL) STORY 1 y φ Hu sterile oscillation at low temperatures, known as the ms = h ih i . (6) • produced in the early Universe through oscillation by virtue of the active-sterile ma M Dodelson-Widrow (DW) mechanism [18], is an inevitable ✓ ⇤ ◆ consequencemixing angle of mixing (Dodelson-Widrow with the active mechanism, neutrinos, and non-resonant is production). Fixing the parameters of the theory also determines the known to produce warm dark matter with relic density • for the right combination of mass and mixing angle, has the correct relic mixing angle between the active and sterile sectors: approximately [18–23] abundance and sufficiently long lifetime 0 2 ma y H sin ✓ m 1.8 ✓ = h ui. (7) s ⌦Ni 0.2 9 . (8) ⇡ ms x φ ⇠ 3 10− 3 keV r h i ✓ ⇥ ◆ ⇣ ⌘ Figure 1 shows possible active-sterile mass scale com- Compared• Combined to WIMP-motivated constraints from cold X-ray dark matter (CDM) binations that result from this framework with M = models,line a searches warm dark from matter dark componentmatter might be favor- 16 0 ⇤ MGUT (=10 GeV), tanβ =2 ( H = 155.6 GeV), and able for a resolutionN → νγ and of recent puzzles such as the core vs. h ui decay Lyman-alpha 0.001 <x<2 for various values of y φ .Thisexer- cuspmeasurements problem and the (free “too streaming big to fail” dark problem [24, 25]. cise suggests that both an active neutrinoh i mass scale A combination of X-ray bounds [26–30] and Lyman-alpha 3 2 matter inconsistent with structure of p2.3 10− eV 0.05 eV, necessary for consis- forest data [23, 31, 32] now rule out the prospect of all of ⇥ ⇠ 2 formation) rules out the entire tency with atmospheric oscillation data ∆matm =2.3 dark matter being made up of N1 produced in this man- 3 2 ⇥ parameter space! from Boyarsky+, 1807.07938 10− eV , and a sterile neutrino mass scale of (keV- ner. However, N1 produced through the DW mechanism GeV), necessary for consistency with dark matterO and can still constitute a significant fraction of the dark mat- • A variant: resonant production (Shi-Fuller mechanism) relies on a large lepton cosmological observations, can emerge naturally in this ter abundance; an analysis in [32] showed that ms 5 framework. keV warmasymmetry component to produce constituting the correct60% abundance of the totalwith≥ a smaller mixing angle, but still suffers (to a lesser degree) from the same constraints as above 7 PARAMETER SPACE FOR STERILE NEUTRINOS: BEYOND THE MINIMAL FRAMEWORK • Plenty of parameter space for sterile neutrino dark matter (does not have to be confined to the keV scale!) with a satisfactory production mechanism 8 STERILE NEUTRINOS: BEYOND THE MINIMAL FRAMEWORK Sterile no more: Sterile neutrino dark matter need not be a complete singlet, could be charged under symmetries beyond the SM! Occurs very naturally in well motivated BSM frameworks, e.g.: • Grand unified theories (essentially, left-right theories) • Theories with gauged (or global) lepton number or B-L • Secluded/hidden/dark sectors with symmetries completely unrelated to the SM • Extended sectors that somehow mimic the SM (e.g. neutral naturalness) Such extensions feature additional particles+symmetries/interactions that can • produce sterile neutrino dark matter in the early Universe • modify observable properties of sterile neutrino dark matter • produce correlated signals at other experiments (colliders, neutrino experiments, cosmological probes) 9 BEYOND MINIMAL: DARK MATTER PRODUCTION FREEZE-OUT • SM dark matter can thermalize with the SM bath and undergo canonical freezeout if it has significant interactions (tends to occur, e.g., in left-right models) • Tends to freeze out while relativistic and overproduce dark matter, requires significant (more than an order of magnitude) entropy dilution.