Luminous Dark Matter

Brian Feldstein

arXiv:1008.1988

-B.F., P. Graham and S. Rajendran

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:

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 fromarxiv:1005.0380

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.

-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!

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...

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 outby Xenon10 (see talks by Peter Sorensen).

- Inelastic Dark Matter?

- More exotic alternatives...

Electromagnetic Energy Deposit

- A tantalizing possibility..

Most experiments discard electromagneticevents as background.. DAMA does not.

DAMA’s annual modulation search isprecisely what allows them to do this!

- But.. purely electronic interactions don’t work..

Scattering gives a bad spectrum..

Absorption gives negligible annual modulation.

-arxiv:0907.3159

-Pospelov, Ritz, Voloshin

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:

- A very simple possibility:

A single magnetic dipole operator.

- Can mediate both the upscattering and the decay.

-Requires only a Dirac fermion with a magnetic dipoleinteraction, plus a Majorana mass splitting.

- We take

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 inusual inelastic dark matter.

Simplifying assumptions...

- Composition of the Earth..

- Angular (in)dependence of the scattering..

- assume nuclei are infinitely heavy..

- true cross sections are angular independentat threshold anyway..

Calculate the Event Rate...

σ ~ e2Z2 / 4πΛ2

Γ ~ δ3 / πΛ2

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 theannual modulation fraction.

- XENON100, in particular, is fairly constraining.

- XENON100 has low electromagnetic background..

XENON10: ~300kg days:

XENON100: ~400kg days:

XENON100 constrains the modulation fraction to be larger than about 50%.

This puts an upper bound on the allowed dark mattermasses.. scattering must be near threshold.

- It is actually relevant that XENON100 has onlypresented data from the winter!

As usual, there may be large experimental uncertainty..

X-Ray Satellites

- Generally, Earth based experiments have large radioactive background... What about satellite experiments?

Potentially dangerous, since they can probe long distances:

- 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.

Compare with the cosmic x-ray background measurements of e.g. the SWIFT or RTXE satellites.

Requires decay lengths less than ~1000km.

-arxiv:0811.1444

Parameter Space

Blue: Xenon100

Red: SWIFT

Yellow: relic density

DM proton cross sections of

Less Important Constraints..

- Collider searches require Λ > TeV.

- CDMS analysis of electromagnetic events requires modulation fractions > 25%.

CMB Constraint

1 GeV dark matter with thermal relic annihilationcross section to photons seems ruled out..

- Galli, Iocco, Bertone, Melchiorri

but…

Luminous dark matter has a built in mechanism to avoid this constraint!

- In the early universe, both the dark matter particle andits excited state are present in the thermal bath.

- Before recombination, however, the excited state is gone…

A single magnetic dipole moment vertexno longer mediates annihilation.

Need two of them… much more suppressed!

(perhaps this is a useful mechanism outside the context of this model)

Other constraints we checked..

.. but which are irrelevant:

- CoGeNT:

Sensitive to electromagnetic events, but their backgroundis ~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.

- X-ray line emission: The dark matter particle can upscatter off of, e.g.,Hydrogen throughout the galaxy. The subsequent decayscontribute to the x-ray background, but are safe by ~7 ordersof magnitude.

- Neutrino detectors, e.g. SuperK:

Trigger thresholds are too high.. ~ MeV.

- Directional dark matter detectors:

Thresholds also currently too high.

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.