Transcript Form Factor Dark Matter - Kavli Institute for the Physics

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 from
arxiv: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 out
by Xenon10 (see talks by Peter Sorensen).
- Inelastic Dark Matter?
- More exotic alternatives...
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!
- But.. purely electronic interactions don’t work..
 Scattering gives a bad spectrum..
-arxiv:0907.3159
 Absorption gives negligible annual modulation.
-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 dipole
interaction, 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 in
usual inelastic dark matter.
,
Simplifying assumptions...
- Composition of the Earth..
- Angular (in)dependence of the scattering..
- true cross sections are angular independent
at threshold anyway..
- assume nuclei are infinitely heavy..
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 the
annual modulation fraction.
- XENON100, in particular, is fairly constraining.
- XENON100 has low electromagnetic background..
XENON10: ~300kg days:
XENON100: ~400kg days:
- 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..
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.
-arxiv:0811.1444
 Requires decay lengths less than ~1000km.
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
- Galli, Iocco, Bertone, Melchiorri
1 GeV dark matter with thermal relic annihilation
cross section to photons seems ruled out..
but…
 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)
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.
- X-ray line emission:
 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.
- 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.