Luminous Dark Matter

1. Luminous Dark Matter Brian Feldstein arXiv:1008.1988 -B.F., P. Graham and S. Rajendran

2. Dark Matter- The Standard Story -Roughly 23% of the universe seems to consist of some form of non-baryonic dark matter. -A compelling possibility: Weakly Interacting Massive Particles (WIMPs) -Weak Scale cross sections give approximately the right relic abundance:

3. Dark Matter Direct Detection -Look for nuclear recoils due to dark matter scattering. -Many such experiments: CDMS, XENON, CRESST, etc.. -Limits placed on cross section vs mass. -modified from arxiv:1005.0380

4. The DAMA Mystery - DAMA sees an 8.9σ annual modulation in its nuclear recoil events. -arxiv:0804.2741 - Phase is consistent with Dark Matter induced recoils.

5. -There is no recognized standard model explanation for the DAMA signal. -DAMA looked at: Neutron flux, temperature variation, muons, neutrinos, etc.. -All calculated signal rates are much too small to explain the signal. -But: standard WIMPs capable of explaining DAMA also seem completely ruled out!

6. Meanwhile... • CoGeNT reports an excess of events over background predictions.. • CRESST reports an excess of Oxygen band events (not yet published, exposure not specified)... • CDMS-II reports 2 events in signal region with a background of 1 event...

7. Looking for an explanation… -No experiment can rule out a dark matter origin for the DAMA signal in a model independent way. -Many Experimental Uncertainties… Present Status: - Various Light Dark Matter Possibilities.. • May be able to incorporate CoGeNT, but probably ruled out • by Xenon10 (see talks by Peter Sorensen). - Inelastic Dark Matter? - More exotic alternatives...

8. Electromagnetic Energy Deposit - A tantalizing possibility.. • Most experiments discard electromagnetic • events as background.. DAMA does not. • DAMA’s annual modulation search is • precisely what allows them to do this!

9. - But.. purely electronic interactions don’t work..  Scattering gives a bad spectrum.. -arxiv:0907.3159  Absorption gives negligible annual modulation. -Pospelov, Ritz, Voloshin

10. Enter Luminous Dark Matter...  Energy is deposited directly through photons.  Upscatter, and then decay to a ~3keV photon. - A line fits the DAMA spectrum well:

11.  A single magnetic dipole operator. - A very simple possibility: - Can mediate both the upscattering and the decay. • Requires only a Dirac fermion with amagnetic dipole • interaction, plus a Majorana mass splitting. - We take

12. Note: Upscatter and decay do not both have to occur inside the detector! • Excited state can travel a very large distance. • As long as the decay length is << , • Upscatter Rate ≈ Signal Rate. • Signal rates depend only on detector volume... - Can boost the modulation fraction as in usual inelastic dark matter.

13. - Composition of the Earth.. Simplifying assumptions... - Angular (in)dependence of the scattering.. • true cross sections are angular independent • at threshold anyway.. - assume nuclei are infinitely heavy..

14. Calculate the Event Rate... σ ~ e2Z2 / 4πΛ2 Γ ~ δ3/πΛ2

15. Constraints.. • The upscattering events are undetected at direct detection experiments, for dark matter lighter than a couple of GeV.. • But.. it’s no longer really true that experiments other than DAMA are insensitive to electromagnetic events!! • Our only freedom to avoid problems is the • annual modulation fraction. - XENON100, in particular, is fairly constraining.

16. - XENON100 has low electromagnetic background.. XENON10: ~300kg days: XENON100: ~400kg days:

17. It is actually relevant that XENON100 has only • presented data from the winter!  XENON100 constrains the modulation fraction to be larger than about 50%. • This puts an upper bound on the allowed dark matter • masses.. scattering must be near threshold.  As usual, there may be large experimental uncertainty..

18. X-Ray Satellites - Generally, Earth based experiments have large radioactive background... What about satellite experiments?  Potentially dangerous, since they can probe long distances:

19. - The satellites measure the photon flux in terms of photons/ cm2 s sr.  We predict roughly ~ L / 4π. Typical decay length ~ vf / Γ  Essentially limits the allowed decay lengths from above.

20. Compare with the cosmic x-ray background measurements of e.g. the SWIFT or RTXE satellites. -arxiv:0811.1444  Requires decay lengths less than ~1000km.

21. Parameter Space Blue: Xenon100 Red: SWIFT Yellow: relic density  DM proton cross sections of

22. Less Important Constraints.. - Collider searches require Λ > TeV. • CDMS analysis of electromagnetic events requires modulation fractions > 25%.

23. CMB Constraint - Galli, Iocco, Bertone, Melchiorri • 1 GeV dark matter with thermal relic annihilation • cross section to photons seems ruled out.. but…

24.  Luminous dark matter has a built in mechanism to avoid this constraint! • In the early universe, both the dark matter particle and • its excited state are present in the thermal bath. - Before recombination, however, the excited state is gone… • A single magnetic dipole moment vertex • no longer mediates annihilation.  Need two of them… much more suppressed! (perhaps this is a useful mechanism outside the context of this model)

25. Other constraints we checked.. .. but which are irrelevant: - CoGeNT: • Sensitive to electromagnetic events, but their background • is ~10 times too high. (We have nothing to say about a possible signal at CoGeNT.. the energy range is wrong..) - CAST (axion telescope): • Searching for x-rays, but their background is more than • ~100 times too high.

26. The dark matter particle can upscatter off of, e.g., • Hydrogen throughout the galaxy. The subsequent decays • contribute to the x-ray background, but are safe by ~7 orders • of magnitude. - X-ray line emission: - Neutrino detectors, e.g. SuperK:  Trigger thresholds are too high.. ~ MeV. - Directional dark matter detectors:  Thresholds also currently too high.

27. Conclusions • DAMA is still a compelling mystery, but one which is becoming harder to explain as time goes on.. • Unlike most other direct detection experiments, DAMA does not throw away purely electromagnetic events. • Upscattering of dark matter to an excited state which decays via emission of a photon can explain the DAMA result without contradicting other experiments. • Only a single magnetic dipole interaction is needed for both the upscattering and decay. • XENON100 should be able to essentially rule out or confirm the scenario very soon.