New dark matter direct detection signals Josef Pradler Pacific 2018 conferernce Feb 15 2018
A (partial) summary of 2 decades of experimental effort lower detector thresholds more exposure (kg days) CF1 Snowmass report, Ruppin et al 2014
Direct Detection of light DM Nuclear kinetic recoil energy E R q2 2m N µ2 N v2 m N p1 cos q q => A given recoil, demands a minimum relative velocity v min d m N E R 2µ 2 N» ˆ ER 1{2 1 GeV 0.5 kev m ˆ # 1700 km{s Xenon 600 km{s Oxygen => if m < 1 GeV, then there are no particles bound to the Galaxy that could induce a 0.5 kev nuclear recoil on a Xenon atom! kinematical no-go theorem
Direct Detection of light DM Nuclear kinetic recoil energy experimental alternatives: Dark Matter-electron scattering E R q2 2m N µ2 N v2 m N p1 cos q Intensity frontier searches (e.g. electron beams on fixed target) new detection methods q (many examples mentioned already) => A given recoil, demands a minimum relative velocity v min d m N E R 2µ 2 N» ˆ ER 0.5 kev 1{2 1 GeV m ˆ # 1700 km{s Xenon 600 km{s Oxygen => if m < 1 GeV, then there are no particles bound to the Galaxy that could induce a 0.5 kev nuclear recoil on a Xenon atom! kinematical no-go theorem
1 DM-electron scattering DM-electron scattering is a promising experimental avenue for detecting sub-gev DM Essig et al. 2012 & 2017 Ordinary detectors lose sensitivity below few MeV mass E kin 5 ev ˆ mdm 10 MeV => if m < 10 MeV, then there are no particles bound to the Galaxy that could ionize an outer shell Xenon electron! kinematical no-go theorem #2
DM-electron scattering 10 34. galactic 10 35 XENON100 XENON10 σe (cm 2 ) 10 36 10 37?? Many ideas for new detector technology, but what can we say *now*? 10 38 χ freeze out 0.01 0.1 1 10 m DM (MeV) 100 1000
1 The sun as particle accelerator Maxwell-Boltzmann distribution with v max 600 km{sec Elastic scattering on electrons in the sun s interior lifts DM kinetic energy into (sub-)kev regime (process most efficient for m e m DM ) v max a 2T {m 10 4 km{sec ˆ a1 MeV{m reflected spectrum An, Pospelov, JP, Ritz arxiv:1708.03642
The sun as particle accelerator galactic velocity distribution 10 2 σ e =10 37 cm 2 m χ =2MeV. 10 3 reflected spectrum from the sun dn/dv 10 4 10 5 10 6 10 100 1000 velocity (km/sec) 10000 MC simulation
The sun as particle accelerator maximal flux from the sun is a factor pr d {1A.U.q 2» 10 5 smaller than the galactic DM flux sun becomes optically thick for ( - ) - - e " 10 36 cm 2 ( ) NB: our Monte Carlo simulation included gravitational focussing and ISW after reflection when particles travels to earth
Any influence on the structure of the sun? Simple estimate show that that energy loss from / heat transfer inside the sun is very small (for the parameter region of interest) Radius of reflection can be estimated from the optical depth (z refl. )= Z zrefl. 0 dz e n e (R z) =1 Maximum heat flow estimated as J. T (R refl. ) DM v Rrefl. 2 10 16 W MeV T (R refl.) m DM m DM kev Rrefl. R 2 This is to be compared to the total solar luminosity J ' 10 26 W
How does an electron recoil signal look in a LXE detector? N Q E R W n ion ` n ex gas phase W» 13.7 ev n ex {n ion few % Given energy deposition number of quanta, a is produced, distributed in electron-ion pairs and excited atoms N Q n ex E R n ion n ex E R liquid phase
How does an electron recoil signal look in a LXE detector? N Q E R W n ion ` n ex n ` n e gas phase n e n ion p1 rq, n n ion r ` n ex n e r n Measurable: de-excitation photons from initial and recombined excitons n and electrons that escape recombination n e n ion E R n ex liquid phase
How does an electron recoil signal look in a LXE detector? p surv» exp ˆ z v d gas phase n surv v d» 1.7mm{µs 1s n e n Electrons are drifted in the electric r field towards the liquid-gas interface; depending where they are created, n ion attenuation occurs n ex p surv 0.6 0.9 E R liquid phase
How does an electron recoil signal look in a LXE detector? S2 S1 N Q n ion ` n ex gas phase n ` n e n surv S1 g 1 ` S2 g 2 g 1» 0.1, g 2» 10 50 n e r n An electron reaching the liquid-gas n ion interface creates about O(10) PE (S2); it takes on average 10 scintillation photons to collect 1 PE (S1) liquid phase E R n ex
How does an electron recoil signal look in a LXE detector? { detector specific fluctuates gas phase S2 S1 fluctuates n surv n e n r LXE universal (given E-field) { fluctuates fluctuates n ion n ex E R => derive PDF(S1,S2 ER ) liquid phase
How does an electron recoil signal look in a LXE detector? e.g. PandaX N Q n ion ` n ex n ` n e S1 g 1 ` S2 g 2 note the anti-correlation between S1 and S2
Electron vs. nuclear recoils S2 S1 N ER Q E R,e W gas phase n surv In electron recoils, heat losses are negligible but not so in nuclear recoils: n e r n N NR Q E R rl y pe R q`q y pe R qs N NR Q N ER Q n ion n ex NR signal is quenched; additional source of fluctuations heat liquid phase E R
Current sensitivity to sub-mev DM Ionization spectrum from DM electron scattering in XENON10 events/e 5 4 3 2 m χ =0.1MeV,σ e =5 10 36 cm 2 E dep. > 0.19 kev XENON10 observed 1 0 0 20 40 60 80 100 n e
Current sensitivity to sub-mev DM Ionization spectrum from DM electron scattering in XENON10 events/e 5 4 3 2 m χ =0.1MeV,σ e =5 10 36 cm 2 E dep. > 0.19 kev Charge yield XENON10 measured observed to 0.19 kev for expt. limits: demand minimum deposited energy of 0.19 kev => data driven approach 1 0 0 20 40 60 LUX ICHEP 2016 n e 80 100
Current sensitivity to sub-mev DM XENON100 (2016) ionization spectrum events/pe 100 80 60 40 m χ =0.1MeV,σ e =5 10 36 cm 2 E dep. > 0.19 kev XENON100 S2 observed for expt. limits: demand minimum deposited energy of 0.19 kev => data driven approach 20 0 0 200 400 S2 (PE) 600 800 1000
Current sensitivity to sub-mev DM Technicality: kev-scale reflected particles are relativistic ( T core kev) 1. relativistic velocity average pe min q ª E min pqq de m2 E p dn de 2. although xp 1 e nlmy 0 by definition, if plane or Coulomb waves are used for p 1 ey, numerically, the overlap with HF-bound w.f. can be non-zero => we subtract the unity operator in the atomic form factor when the relativistic limit when q r! 1 fpqq xp 1 e e iq r nlmy Ñ f sub pqq xp 1 e e iq r 1 nlmy
Current sensitivity to sub-mev DM S1-scintillation spectrum in XENON100 35 30 25 m χ =1MeV, σ e =10 35 cm 2 XENON100 S1 observed (detected S1 reduces backgrounds through volume fiducialization) events 20 15 10 5 0 0 20 40 60 80 100 S1 (PE) reflected spectrum extends into kev-range => scintillation detectable (benefit of background suppression)
Current sensitivity to sub-mev DM first results from XENON1T 10 8 m χ =1MeV,σ e =2 10 36 cm 2 XENON1T observed events 6 4 2 [Aprile et al 2017] 0 0 10 20 30 40 S1 (PE) 50 60 70
Direct Detection of sub-mev DM 10 34. reflected galactic σe (cm 2 ) 10 35 10 36 10 37 XENON1T PandaX-II LUX (2013) XE100 (S1) XE100 (S2) XENON10 LZ SENSEI SuperCDMS 10 38 stellar constraints χ freeze out 0.01 0.1 1 10 m DM (MeV) 100 1000 => First limit on sub-mev DM-electron scattering!
Direct Detection of sub-mev DM σe (cm 2 ) 10 34 10 35 10 36 10 37 reflected galactic XENON1T PandaX-II LUX (2013) XE100 (S1) XE100 (S2) XENON10 LZ SENSEI SuperCDMS. data-driven ionization/ scintillation yield: minmum energy deposit of 0.19 kev required unlike galactic DM-electron scattering, incoming DM has kev-kinetic energy; ionization from n=4 important 10 38 stellar constraints χ freeze out limits may be improved by from PDF(S1,S2 E) [work in progress] 0.01 0.1 1 10 m DM (MeV) 100 1000 => First limit on sub-mev DM-electron scattering!
Direct Detection of sub-mev DM Example of a successful model (UV completed through Z ) L int = G e (ē µ e)(i @ µ i @ µ ) relic density is set via p-wave annihilation => safe from CMB constraints on energy injection; Neff contributions are model dependent annv = v 2 G2 e 12 (m2 e +2m 2 ) s => First direct test of such DM model 1 m 2 e m 2 e = 1 G2 eµ 2,e! (8 9) 10 35 cm 2 2µ 2,e (2m 2 + m 2 e)v e
2 Non-gravitational signatures of a relativistic cosmic background Cui, Pospelov, JP, 1711.04531 Baryons 5% Dark Energy 69% Cold Dark Matter 26 % CMB N e =3.04 ± 0.33 ) DR / < 0.15 Planck 2015
2 Non-gravitational signatures of a relativistic cosmic background Cui, Pospelov, JP, 1711.04531 Baryons 5% Baryons 5% Dark Energy 69% Dark Energy 69% Cold Dark Matter 26 % => Cold Dark Matter (26 - X) % Dark Radiation X % CMB N e =3.04 ± 0.33 ) DR / < 0.15 Planck 2015 Low redshift Universe Dark radiation (DR): boosted states in the hidden sector see, e.g. Agashe et al. 2014
Late Dark Radiation (DR) Late DR can be sourced by the decay or annihilation of DM. Here we consider DM decay (=most efficient progenitor for a relativistic flux) of a sub-dominant species with lifetime (broadly log-centred) around H0 1 10 Gyr General setup: n DR n, E DR E CMB constraint from late-time ISW and lensing DR < 0.1 DM e.g. Poulin, Serpico, Lesgourges 2016 NB: there are also constraints on structure formation with residual kicked DM state in place e.g. Wang, Peter at al. 2014
Maximum fluxes of DR Galactic and extragalactic contributions to the flux galactic extragalactic. dφ/deν (1/MeV/cm 2 /sec) 10 4 10 3 1 eg. (m ν =0) eg. (m ν =10MeV) gal. (5% smearing) 10 E ν (MeV) τ X =10Gyr m X =50MeV κ =0.1 100 example for 2-body injection
Maximum fluxes of DR Galactic and extragalactic contributions to the flux 10 3 10 2 10 1 1 Fluxes Φ gal, Φ eg. 10 3 10 4 10 5 Maximum flux much in excess of atmospheric nu-flux and DSNB at ~ 10-100 MeV τx/t0 10 1 10 2 10 3 sum 10 4 gal. eg. 10 5 10 κ =0.1 m ν =5MeV 100 m X (MeV) 1000 here: 10% decaying DM component compare with DSNB (Beacom 2010)
Late Dark Radiation in SM neutrinos Option 1: DR are Standard Model neutrinos Benefits: no Neff constraints for direct decay, interactions within SM are known, minimal setup Decaying progenitor motivated by certain neutrino mass generation mechanism Majoron! ( ) breaks global lepton number, Goldstone mode is => Chikashige, Mohapatra, Peccei 1981 Mass of as pseudo-goldstone uncertain, with contributions from Planck-scale suppressed operators; we take it noting a non-standard thermal history e.g. Berezinsky, Valle 1993
Late Dark Radiation in SM neutrinos Measurements / Constraints: E < 16 MeV: signal dominated by solar neutrinos (8B flux) in CC and NC scattering on electrons 16 MeV < E < 30 MeV: inverse beta decay with large visible energy 30 MeV < E < 150 MeV: reactions with neutrons inside nuclei no longer kinematically suppressed, e.g. E > 150 MeV: atmospheric neutrino flux well measured and concordant 16 MeV 30 MeV 150 MeV E solar target for DSNB atmospheric
Late Dark Radiation in SM neutrinos Option 1: DR are Standard Model neutrinos Opportunity: Injection of neutrinos at few 10 s of MeV poorly constrained A 30 MeV neutrino gives signals in direct detection right in the region of largest sensitivity. Neutrino floor can be raised in models that inject but not? excessively
Late Dark Radiation in new physics Option 2: DR are new (semi-)relativistic states that interact with SM Benefits: more possibilities, stronger signals are possible (here we restrict ourselves to the MeV-scale again). For example, X, Y =DM =DR NB: can be a sterile neutrino mixing with, recovering Option 1 Option 2.1: boson => absorption signals standard cases include being a dark photon or axion-like particle; absorption signals have been worked out for direct detection It turns out that it is difficult to detect bosonic DR that is sourced by sub-kev progenitors, as severe astrophysical constraints apply
Late Dark Radiation in new physics Option 2: DR are new (semi-)relativistic states that interact with SM Benefits: more possibilities, stronger signals are possible (here we restrict ourselves to the MeV-scale again). For example, X, Y =DM =DR NB: can be a sterile neutrino mixing with, recovering Option 1 Option 2.2: fermion => scattering signals E.g. well motivated and studied case: Much milder astro-constraints; Neff can be better avoided when coupled to baryons
Direct detection sensitivity to DR 10 4 95% C.L. G B =10G F. τx/t0 10 3 10 2 10 1 1 m ν =10MeV SM ν <= Option 2.2. scattering with stronger than SM interactions <= Option 1. Decay into SM neutrinos 10 1 10 2 XENON1T LUX 10 3 10 100 m X (MeV) 1000 Neutrino experiments will provide constraints
Constraints from neutrino expts. e.g. recasted Super-Kamiokande search for DSNB neutrinos Super-K collaboration 2011 20 15 m X =130MeV τ/t 0 =10 4 SK-II sum DR SM-ν ν µ CC ν e CC π/µ, NC elastic. => events 10 5 sideband search-region with fitted bkg. sideband 0 e.g. 20 30 40 50 60 energy (MeV) 70 80
Summary - DR in DM neutrinos 10 6 10 5 DR = SM ν. Option 1 DR in SM neutrinos τx/t0 10 4 10 3 10 2 10 1 1 10 1 10 2 XENON1T LUX 10 3 10 3 SK(dsnb) SK(sol) 100 m X (MeV) SK(atm) 1000 => if flux is saturated then neutrino floor ~2 orders of magnitude away from current direct detection sensitivity => neutrino floor is raised to by ~2 orders of magnitude for a 30 GeV WIMP (Nikolic, JP in prep)
Summary - DR in a new species 10 5 10 4 10 3 DR = χ G B =10G F. Option 2 new neutrino interacting with baryonic current τx/t0 10 2 10 1 1 m χ =10MeV Borexino Borexino limit derived from elastic scattering on protons 10 1 XENON1T LUX 10 3 10 10 2 100 m X (MeV) 1000
A (partial) summary of 2 decades of experimental effort lower detector thresholds more exposure (kg days) CF1 Snowmass report, Ruppin et al 2014
How can we make progress in the sub-gev region today? 10 38. 10 39 σn (cm 2 ) 10 40 10 41 10 42?? CRESST-II (2015) LUX (2015) XENON100 (2013) SuperCDMS (2014) CDMSlite (2015) 10 43 10 44 10 45 10 46 0.001 0.01 0.1 1 m χ (GeV) 10 100 1000 light Dark Matter WIMPs
3 Gaining access to sub-gev Dark Matter through nuclear recoils Kouvaris, JP, PRL 2017 Inelastic channel of photon emission from the nucleus! Maximum photon energy! max» µ N v 2 {2» m v 2 {2» 0.5 kev m 100 MeV q Key I: Key II: 0.5 kev nuclear recoil is easily missed, 0.5 kev photon is never missed!
Gaining access to sub-gev Dark Matter through nuclear recoils events/kg/day/kev 10 6 10 4 10 2 1 10 2 10 4 10 6 elastic Xenon σ n =10 35 cm 2 m χ =1GeV Bremsstr. 10 8 0.01 0.1 1 nuclear recoil or photon energy (kev) 10
Atomic physics picture of photon-emission The naive treatment of Bremsstrahlung scales as 1/ω all the way to lowest energies _ + Polarized Atom => After the nucleus gets a kick, in the limit that the DM-nucleus interaction time R N {v is fast compared to the orbital time of electrons, r {v, the Atom becomes polarized for inner shell electrons {» 10 4 A 1{3 Z 2
Atomic physics modification => QM calculation V fi 2 2! M el 2 ˇˇˇˇˇ ÿ n i,f «pd fn ê qxn e i m e m N q r iy! ni! ` pd ni ê qxf e i! ni `! me m N q r ny ˇ 2 dipole matrix element for emission of photon boost of the electron cloud
Atomic physics picture of photon-emission dipole emission polarizability of the atom For f=i: d d!de R 9! 3 ˆ p!q 2 ˆ ER m N ˆ d de R Ñ Z2! ˆ ER m N ˆ d de R for large ω naive result is recovered
Gaining access to sub-gev Dark Matter through nuclear recoils including atomic physics modification events/kg/day/kev 10 6 10 4 10 2 1 10 2 10 4 10 6 elastic dr{de R Xenon σ n =10 35 cm 2 m χ =1GeV Bremsstr. dr{d! 10 8 0.01 0.1 1 nuclear recoil or photon energy (kev) 10 => importantly, we can draw from atomic data listings for atom polarizabilities!
Current limits + projections 10 28 XQC. 10 30 m.f.p. σn (cm 2 ) 10 32 XENON1T (2017) 10 34 LUX (2013) XENON100 (S2) 10 36 XENON10 CRESST-III (projected) CDF (m V =300MeV) 10 38 100 m χ (MeV) CRESST-II 1000 CDMSlite => First limit on sub-500 MeV DM-nucleon scattering!
Conclusions existing direct detection experiments are already sensitive to sub-gev DM mass in DM-nucleus, and to sub-mev DM mass in DMelectron scattering Nuclear recoils => if MeV-scale DM decays into dark radiation states, direct detection can become competitive with neutrino experiments in the new physics sector; neutrino floor can be raised => break the no-go theorem from kinematics by going to the inelastic channel of photon emission with higher endpoint energies Electron recoils => break the no-go theorem from kinematics by using the sun as particle accelerator