Transcript 118
Form Factor Dark Matter
Brian Feldstein
Boston University
In Preparation
-B.F., L. Fitzpatrick and E. Katz
In Preparation
-B.F., L. Fitzpatrick, E. Katz and B. Tweedie
Dark Matter- The Standard Story
-Roughly 23% of the universe seems to consist of an exotic
form of non-luminous, 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.
-Limits placed on cross section vs mass.
-Many such experiments: CDMS, CRESST, XENON, etc..
-arxiv:0809.1829
The DAMA Mystery
- DAMA sees an 8.2σ annual modulation in its nuclear
recoil events.
-arxiv:0804.2741
- Phase is consistent with Dark Matter induced recoils.
-There is no proposed standard model explanation for the
DAMA signal.
-DAMA looked at: Neutron flux, temperature variation,
muons, neutrinos, etc..
-All calculated modulation amplitudes are much too small to
explain the signal.
-But: standard WIMPs capable of explaining DAMA also
seem completely ruled out!
On the other hand…
-No experiment can rule out a dark matter origin for the
DAMA signal in a model independent way.
What distinguishes DAMA from other experiments?
-masses of nuclei used in experiment (NaI)
-range of recoil energies probed (10’s of keV)
-searching for annual modulation
-not vetoing purely electromagnetic events
-spin of nuclei
-crystal structure
Direct Detection Event Rates
-Collisions/time for one target particle:
n v
number density
cross section
relative velocity
-Collisions/time/recoil energy/detector mass:
target nuclei/detector mass
dark matter velocity distribution:
after putting in the cross section …
atomic number
nuclear form factor
Momentum transfer:
Everything depends only on q, except for the
factor of Z2, and the reduced mass in vmin.
-Various proposed explanations for DAMA:
-Light Dark Matter
-Gelmini, Gondolo, 2004
scatter off of Sodium at DAMA
-Light Dark Matter + Channeling…
-Bernabei et. al, 2007
-Drobyshevski, 2007
-Leptophilic Dark Matter
-Fox, Poppitz, 2008
only electromagnetic events
-Spin Dependent couplings
-Savage, Gondolo, Freese, 2004
Light dark matter and Leptophilic dark matter have problems with
the DAMA modulating spectrum.. Spin dependent couplings ruled
out by COUPP..
Inelastic Dark Matter
-Tucker-Smith, Weiner
vmin = q/2 + /q
Presently, Inelastic Dark Matter seems
to be the only viable solution
Form Factor Dark Matter
-Multiply the cross section by a new function F(q) coming
from the dark sector.
-Comes from dark matter with internal structure (c.f. the
nuclear form factor).
-Two goals: Fix the DAMA spectrum and reduce events at
other experiments. Need F(q) to fall towards low q.
-Very general.. but a priori might not even work.
q Overlap
Irreducible prediction for events in the DAMA range of q.
Best Case Scenario
-Choose a form factor by hand...
-Two requirements:
In the DAMA region of q, match the DAMA
spectrum exactly (to within error bars)
Outside the DAMA region of q, set F(q) = 0.
-Are the number of events predicted at other
experiments acceptable?
If no, we can give up on this idea!
Doesn’t look great with the Standard Halo model
On the other hand, halo uncertainties are significant…
Via Lactea Simulations:
-Diemand, Kuhlen, Madau, 2006
-Fairbairn, Schwetz, 2008
Note: the effect of baryons is not included in Via Lactea…
-March-Russell, McCabe, McCullough, 2008
Model Building…
-Need a form factor which dies rapidly at low energies
A simple higher dimension interaction is not sufficient.
-Look for a relatively simple proof of principle.
-Introduce a new mechanism: Interfering gauge bosons.
- Dark gauge forces mix with hypercharge, but with various
signs:
- We assume the dark matter particle is neutral under the
new gauge forces, but that it contains charged
constituents.
- Leading interaction is a higher dimension operator:
(Λ, mi ~ hundreds of MeV, q0 ~ 50MeV)
Results..
2 gauge boson model (99% constraints shown)
3 gauge boson model (95% constraints shown)
The models don’t work with the standard halo
2 gauge boson model
3 gauge boson model
not too far from optimal, but some room for improvement ..
Channeling
- Most experiments do not measure the full recoil energy,
but only a fraction.
- The actual recoil energy is inferred through a “quenching
factor”.
- Sometimes large uncertainties Can be very important.
- Not measured directly at all relevant energies.
- Proposal: The extrapolation of the DAMA quenching
factor down to the energies of its signal are incorrect.
- Usually the Iodine quenching factor is taken to be ~.09
- At low energies, perhaps it can become much higher, at
least for some fraction of events.
- Recoils which travel along a crystal axis at low energy
are expected to have a quenching factor of ~1.
“Channeling”
If Channeling at DAMA is real..
DAMA is sensitive to much lower recoil energies than
was originally thought!
DAMA becomes a much more sensitive detector,
especially for light WIMPS.
-There is a “critical channeling angle”:
Channeling is a real physical phenomenon:
(just not necessarily at DAMA…)
-Lindhard, 1965
Unfortunately, it still seems to not be enough…
Bad spectrum, or too many events at null experiments.
(or so it seems…)
But…
A simple higher derivative interaction works extremely well!
Conclusions
- DAMA presents an exciting experimental puzzle.
- If it is correct, we may be able to learn a lot
about the structure of the dark sector.
- Models with dynamical form factors are a viable
solution, but a certain amount of complexity is
required.
- With channeling included, simple form factor
models work nicely as well.
- New experimental results should be coming
soon…