Discovering dark matter Di Subir Sarkar University of Oxford & - PowerPoint PPT Presentation

Discovering dark matter Di Subir Sarkar University of Oxford & Niels Bohr Institute, Copenhagen Dark matter from aeV to ZeV: 3 rd IBS-Multidark-IPPP workshop, Lumley Castle, 21-25 Nov 2016

Snowmass CF1 WG summary, 1310.8327 1501.01200 1504.03198 1503.02641 1610.046111

Mass scale Particle Symmetry/ Stability Production Abundance Quantum # τ > 10 33 Λ QCD Nucleons Baryon ‘ freeze-out ’ from Ω B ~ 10 -10 cf. observed number yr thermal equilibrium Ω B ~ 0.05 We have a good theoretical explanation for why baryons are massive and stable Bethke, 1210.0325 Durr et al , Science 322 :2224,2008 We understand the dynamics (QCD) … and can even calculate the mass spectrum

Chemical equilibrium is maintained Zeldovich, 1965; Wolfram, 1979 as long as annihilation rate exceeds Nucleons (actual) ➛ the Hubble expansion rate Nucleons (actual) ➛ ‘ Freeze-out ’ occurs when annihilation rate: becomes comparable to the expansion rate Nucleons (predicted) ➛ Nucleons (predicted) ➛ where g ~ # relativistic species i.e. ‘ freeze-out ’ occurs at T ~ m N /45, with: However the observed ratio is 10 9 times bigger for baryons , and there seem to be no antibaryons , so we must invoke an initial asymmetry : Why do we not call this the ‘baryon disaster’? cf. ‘WIMP miracle’!

Although vastly overabundant compared to the natural expectation, baryons cannot close the universe (BBN ✜ CMB concordance) Fields, Molaro & Sarkar, Review of Particle Properties , 2016 … the dark matter must therefore be mainly non -baryonic

Ø B -number violation Ø CP violation Ø Departure for thermal equilibrium The SM allows B -number violation (through non-perturbative – ‘sphaleron-mediated’ – processes) … but CP -violation is too weak and SU (2) L x U (1) Y breaking is not a 1 st order phase transition Hence the generation of the observed matter-antimatter asymmetry requires new BSM physics … can be related to the observed neutrino masses if these arise from lepton number violation ➙ leptogenesis ‘See-saw’:

Any primordial lepton asymmetry (e.g. from out-of-equilibrium decays of the right-handed N ) would be redistributed by B+L violating processes (which conserve B-L ) amongst all fermions which couple to the electroweak anomaly – in particular baryons An essential requirement is that neutrino mass must be Majorana … test by Inverted hierarchy detecting neutrino less double beta decay (and Normal hierarchy measuring the absolute neutrino mass scale)

Mass Particle Symmetry/ Stability Production Abundanc scale e Quantum # τ > 10 33 yr Λ QCD Nucleons Baryon Ω B ~10 -10 ‘ freeze-out ’ from number thermal equilibrium cf. observed Asymmetric Ω B ~ 0.05 baryogenesis Λ Fermi ~ Neutralino? R -parity? Violated? (matter Ω LSP ~ 0.3 ‘ freeze-out ’ from parity adequate to thermal equilibrium G F-1/2 ensure B stability) L e ff ⊃ M A A µ A µ + m f ¯ f L f R + m 2 H | H | 2 For (softly broken) supersymmetry we have the ‘ WIMP miracle ’ : g 4 Ω χ h 2 ⇥ 3 � 10 − 27 cm − 3 s − 1 ⇤ 3 � 10 − 26 cm 3 s − 1 χ ⇥ 0 . 1 , since ⌅ σ ann v ⇧ ⇥ 16 π 2 m 2 ⇤ σ ann v ⌅ T = T f χ But why should a thermal relic have an abundance comparable to non -thermal relic baryons?

Mass scale Particle Symmetry/ Stability Production Abundance Quantum # τ > 10 33 yr Λ QCD Nucleons Baryon Ω B ~10 -10 ‘ freeze-out ’ from number thermal equilibrium cf. observed Asymmetric Ω B ~ 0.05 baryogenesis Λ Fermi ~ Neutralino? R -parity? Violated? (matter Ω LSP ~ 0.3 ‘ freeze-out ’ from parity adequate for thermal equilibrium G F-1/2 p stability) Hidden sector (e.g. GMSB) matter also provides the ‘ WIMP less miracle ’ (Feng & Kumar, 0803.4196) … because: g h2 / m h ~ g χ 2 / m χ ~ F /16 π 2 M Such dark matter can have any mass: sub-GeV → ~few TeV g 4 Ω χ h 2 ⇥ 3 � 10 − 27 cm − 3 s − 1 ⇤ 3 � 10 − 26 cm 3 s − 1 χ ⇥ 0 . 1 , since ⌅ σ ann v ⇧ ⇥ 16 π 2 m 2 ⇤ σ ann v ⌅ T = T f χ But why should a thermal relic have an abundance comparable to non-thermal relic baryons?

Mass Particle Symmetry/ Stability Production Abundanc scale e Quantum # τ > 10 33 yr Λ QCD Nucleons Baryon Ω B ~10 -10 cf. ‘ Freeze-out ’ from number (dim-6 OK) thermal equilibrium observed Asymmetric Ω B ~ 0.05 baryogenesis (how?) Asymmetric (like the Λ QCD’ ~ U (1) DB plausible Ω DB ~ 0.3 Dark baryon? observed baryons) 6 Λ QCD Λ Fermi ~ Neutralino? R -parity violated? Ω LSP ~ 0.3 ‘ Freeze-out ’ from thermal equilibrium G F-1/2 τ ~ 10 18 yr Asymmetric (like the Technibaryon? (walking) Ω TB ~ 0.3 observed baryons) e + excess? Technicolour 100 A new particle can naturally share in the B/L asymmetry � n 0 � Χ � � n 0 � B if it couples to the W … linking dark to baryonic matter! 10 So a O (TeV) mass technibaryon can be the dark matter � Χ 1 ¢ ¢ … alternatively a ~few GeV mass ‘dark baryon’ in a ➘ Ω TB / Ω B ≈ 6 ➚ 0.1 hidden sector (e.g. into which the technibaryon decays) 0.01 0.001 0.01 0.1 1 10 m Χ � TeV �

If they mix with the left-handed ‘active’ neutrinos then would behave as super-weakly interacting particles with an effective coupling: q G Fermi | M Dirac | 2 | M Majorana | 2 = M active θ 2 e,µ, τ ≡ M sterile ◆ − 1 ✓ M sterile ≈ 5 × 10 − 5 KeV So they will be created when active neutrinos scatter, at a rate ∝ q 2 G active Hence although they may never come into equilibrium, the relic abundance will be of order the dark matter for a mass of order KeV (however there is no natural motivation for such a mass scale)

L e ff = F 2 + ¯ ΨΨΦ + ( D Φ ) 2 + Φ 2 Ψ 6 D Ψ + ¯ + θ QCD F ˜ F The SM admits a term which would lead to CP violation in strong interactions, hence an (unobserved) electric dipole moment for neutrons → requires θ QCD < 10 -10 θ QCD must be made a dynamical parameter, by introducing a U (1) Peccei-Quinn symmetry which must be broken … the resulting (pseudo) Nambu-Goldstone boson is the QCD axion which acquires a small mass through its mixing with the pion: m a = m π ( f π / f PQ ) Javier Redondo When the temperature drops to L QCD the axion potential turns on and the coherent oscillations of relic axions contain energy density that behaves like cold dark matter with Ω a h 2 ~ 10 11 GeV/ f PQ … however the natural P-Q scale is probably M string ~10 18 GeV Hence QCD axion dark matter would need to be significantly diluted, i.e. its relic abundance is not predictable (or seek anthropic explanation for why θ QCD is small?) Many other possibilities for ‘axion-like particles’ … over a very large range of mass scales

Mass scale Lightest stable Symmetry/ Stability Production Abundance particle Quantum # ensured? Nucleons Baryon τ > 10 33 Ω B ~10 -10 cf. Λ QCD ‘ Freeze-out ’ from number yr equilibrium observed Asymmetric Ω B ~ 0.05 baryogenesis Dark baryon? U (1) DB Λ QCD’ Asymmetric (like Ω DB ~ 0.3 plausible ~ 6 Λ QCD observed baryons) Neutralino? R -parity violated? Ω LSP ~ 0.3 Λ Fermi ‘ freeze-out ’ from (walking) equilibrium ~ G F-1/2 τ ~10 18 yr Technibaryon? Techni- Asymmetric (like Ω TB ~ 0.3 colour observed baryons) τ ≳ 10 18 yr Λ hidden sector Crypton? Discrete Varying gravitational Ω X ~ 0.3? hidden valley? symmetry field during inflation ~ ( Λ F M P ) 1/2 (very model- dependent) Λ see-saw Neutrinos Thermal (abundance Ω ν > 0.003 Lepton Stable . ~ CMB photons) ~ Λ Fermi2 / Λ B-L number Kaluza-Klein ? ? ? ? M string / M Planck states? Peccei- Axions Quinn Stable Field oscillations Ω a » 1!

Di Direct detection ha has s fo focussed on on WIMPs, s, so so is s most ost se sensi sitive at ~ we weak scale Snowmass CF1 WG summary, 1310.8327 Several claims for putative signals have apparently been ruled out by more sensitive experiments … but are we making a fair comparison?

Th There are many ambiguities in interpreting the measured recoil rate: ★ Dark matter interacts differently with neutrons & protons (Giulani, hep-ph/0504157) if the mediator is a (new) vector boson … so e.g. the events seen by CDMS-Si can be consistent with the upper limits set by XENON100 or LUX ★ Moreover different experiments are sensitive to different regions of the (uncertain) dark matter velocity distribution, hence apparently inconsistent results (e.g. CoGeNT and DAMA) can be reconciled by departing from the assumed isotropic Maxwellian form (Fox et al , 1011.1915, Frandsen et al, 1111.0292, Del Nobile et al , 1306.5273) ★ Then there are experimental uncertainties ( instrumental backgrounds , efficiencies, energy resolution) + uncertainties in translating measured energies into recoil energies (channelling, quenching) + uncertain nuclear form factors … No single experiment can either confirm or rule out dark matter (and it is not a good strategy to look just under the WIMP lamp post!)

Ma Many techniques for indirect detection … and many claims! The PAMELA/AMS-02 anomaly ( e + ), WMAP/Planck ‘haze’ (radio), Fermi ‘bubbles’ + Galactic Centre ‘excess’ + 130 GeV line ( γ -ray) … have all been ascribed to dark matter These are probes of dark matter elsewhere in the Galaxy so complement direct detection experiments … but we are just beginning to understand the astrophysical foregrounds!