1 dark matter indirect detection -...
TRANSCRIPT
Nicole Bell, The University of Melbourne Seminar, The Ohio State University, 13 May 2011
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Dark Matter Indirect Detection: Electroweak Bremsstrahlung
&
Other Topics
Nicole BellThe University of Melbourne, Australia
with
Ahmad Galea, Thomas Jacques, Kalliopi
Petraki
(Melbourne)James Dent, Lawrence Krauss (Arizona)
Tom Weiler
(Vanderbilt)
Nicole Bell, The University of Melbourne Seminar, The Ohio State University, 13 May 2011
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Outline
Introduction
Dark matter annihilation in the Sun
Late decaying dark matter & small scale structure
Enhanced annihilation via internal bremsstrahlung
Nicole Bell, The University of Melbourne Seminar, The Ohio State University, 13 May 2011
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•Evidence for dark matter arises from a wide range of astrophysical observations (galaxy rotation curves, CMB, BBN, large scale structure, lensing, SN1a,…)
•All are sensitive to dark matter’s gravitational influence.
•As yet, very little information about the particle properties of dark matter.
Dark Matter
Nicole Bell, The University of Melbourne Seminar, The Ohio State University, 13 May 2011
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Axions, Neutralinos, Gravitinos, Axinos, Sneutrinos, Kaluza-Klein particles, Heavy Fourth Generation Neutrinos, Mirror particles, superWIMPs, WIMPzillas, Sterile Neutrinos, Light Scalars, Q-Balls, Brane
World
Dark Matter, Primordial Black Holes, …. something we haven’t thought of ….
In other words …
we just don’t know …
Dark Matter Candidate Zoo
Nicole Bell, The University of Melbourne Seminar, The Ohio State University, 13 May 2011
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Produce dark matter in a collider
Direct detection
Indirect detection
Ideal situation would be to obtain positive signals via all approaches and combine information/check for consistency.
Dark matter detection strategies
Nicole Bell, The University of Melbourne Seminar, The Ohio State University, 13 May 2011
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(1) Assume dark matter initially in thermal equilib.:
(2) Universe cools and the non-relativistic DM is Boltzmann suppressed:
(3) “Freeze out”
--
relic density fixed:
vconstN
1.
TmEQ eNN /~
Final dark matter abundance proportional to inverse of the annihilation cross section.
Thermal Relic Dark Matter
ff
Nicole Bell, The University of Melbourne Seminar, The Ohio State University, 13 May 2011
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“WIMP Miracle”
The thermal relic picture sets the “natural scale”
for
the dark matter annihilation cross section:
Correct relic density for:• electroweak strength couplings• GeV
–
TeV
masses
Realistic prospects of detecting annihilation signals!
Indirect detection also sensitive to decaying DM, in some scenarios
implies
Nicole Bell, The University of Melbourne Seminar, The Ohio State University, 13 May 2011
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Annihilation in our Galaxy or in nearby galaxies Photons, antimatter, neutrinos
Annihilations in galaxies throughout the Universe cosmic diffuse fluxes
Annihilation in Sun/Earth Neutrinos only
Indirect DetectionIndirect DetectionSearch for fluxes of DM annihilation or decay products from regions where the DM density is high:
Nicole Bell, The University of Melbourne Seminar, The Ohio State University, 13 May 2011
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Annihilation of dark matter
Baltz
et al., JCAP 2008
Nicole Bell, The University of Melbourne Seminar, The Ohio State University, 13 May 2011
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Annihilation in the Sun
Nicole Bell, The University of Melbourne Seminar, The Ohio State University, 13 May 2011
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Neutrinos from the Sun•
Dark matter accumulates in the centre of Sun (and Earth)
•
Neutrinos are the only annihilation products able to escape •
High energy neutrinos detected by IceCube, SuperK, etc.
•
Capture rate and (if in equilibrium) the annihilation rate controlled by WIMP-nucleon scattering cross section
•
Competitive limits for spin-dependent cross sections
D. Hooper
Nicole Bell, The University of Melbourne Seminar, The Ohio State University, 13 May 2011
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IceCube, PRL 2010; Limits with 22 string detector.
IceCube
limits on Neutralinos
Nicole Bell, The University of Melbourne Seminar, The Ohio State University, 13 May 2011
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Wilkstrom
and Edsjo, JCAP 2009
Full IceCube
Sensitivity -
Forecast
Nicole Bell, The University of Melbourne Seminar, The Ohio State University, 13 May 2011
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Batell, Pospelov, Ritz and Shang, PRD 2010
Annihilation in the Sun –
secluded dark matter
Annihilation via metastable
mediators, V VV SM particles
The mediators propagate before decaying:
Can decay outside the Sun solar gamma rays
Nicole Bell, The University of Melbourne Seminar, The Ohio State University, 13 May 2011
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Schuster, Toro, Weiner and Yavin, PRD 2010
Annihilation to long lived
force carriers which escape the Sun before decaying
Inelastically
interacting
dark matter forms halo of dark matter outside the Sun
gamma rays or charged particles from the Sun Test with Fermi data in near future
Nicole Bell, The University of Melbourne Seminar, The Ohio State University, 13 May 2011
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Neutrino signal enhanced for secluded DM
High energy neutrinos produced at the solar core undergo
significant absorption in the Sun, for E > O(100) GeV.
If mediators propagate out of the dense core before
decaying neutrino signal greatly enhanced
Since the solar density falls exponentially with radius, this can have a large effect, even for short mediator lifetimes.
~100 GeV, x=optical depth in Sun
Nicole Bell, The University of Melbourne Seminar, The Ohio State University, 13 May 2011
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Neutrino evolution in the Sun
Injection by mediator decays Absorption by CC and NC interactions
Regeneration of neutrinos at lower energy (from tau
decay, or from NC scattering) . Flavour
oscillations
Helpfully, the flavour
oscillations and the interactions approximately decouple.
Nicole Bell, The University of Melbourne Seminar, The Ohio State University, 13 May 2011
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Density of SunNucleon density
Note: scale height of approximately 0.1 solar radius
Define (dimensionless) optical depth:
Or, approximately,
Nicole Bell, The University of Melbourne Seminar, The Ohio State University, 13 May 2011
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And, to a very good approximation:
Nicole Bell, The University of Melbourne Seminar, The Ohio State University, 13 May 2011
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Bell and Petraki, JCAP 2011
Bottom to top:mediator lifetimes of
= 0.001 s, 0.1 s, 0.3 s,
1 s, and 10 s.
Note: solar radius ~ 2.2s
Enhanced HE neutrino
fluxes from Sun
Nicole Bell, The University of Melbourne Seminar, The Ohio State University, 13 May 2011
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The ratio of μ±
track events observable at IceCube
in the presence of mediators of lifetime
over those in the absence of metastable
mediators, integrated over muon
energies E > 100GeV
Bell and Petraki, JCAP 2011
Enhancement of the high energy region of the nu flux
big enhancement of event rates, because detection
cross sections grow with energy
Nicole Bell, The University of Melbourne Seminar, The Ohio State University, 13 May 2011
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Late decaying dark matter
Nicole Bell, The University of Melbourne Seminar, The Ohio State University, 13 May 2011
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Structure & decaying dark matter Structure & decaying dark matter
Problems with CMD:CDM simulations correctly reproduce observations at
large
scales, but discrepancies seen at small scales
cuspy
central galactic density profiles (observations prefer cores)overprediction
of satellite galaxies
i.e. too much power on small scales.
Possible resolution:Warm dark matter inhibits growth of structure on small scalesalternatively: Decaying dark matter may alleviate the problem
Nicole Bell, The University of Melbourne Seminar, The Ohio State University, 13 May 2011
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Dark matter decay to almost Dark matter decay to almost degenerate daughter degenerate daughter
• DM energy density approx. unchanged by decays
• Very little energy transferred to radiation
• The daughter DM particle receives a recoil velocity
•
The recoil velocities affect galactic structure if decays occurs at appropriate time. Lifetimes of about 1Gyr are interesting.
Nicole Bell, The University of Melbourne Seminar, The Ohio State University, 13 May 2011
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Dark matter kicksDark matter kicks
Structure formation problems eased:Recoil velocities disrupt small halos (e.g. Abdelqader
& Melia)Heating causes central cores to expand and form cores (e.g. Sanchez-Salcedo)
Early decays (before recombination)Kaplinghat
(2005); Cembranos, Feng, Rajaraman
& Takayama
(2005)
Late decaysSanchez-Salcedo
(2003); Abdelqader
& Melia
(2008); Peter, Moody & Kamionkowski
(2010); Peter and Benson (2010)
Nicole Bell, The University of Melbourne Seminar, The Ohio State University, 13 May 2011
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Adapted from Peter et al. (2010)
Dark matter decay parameter spaceDark matter decay parameter space
Nicole Bell, The University of Melbourne Seminar, The Ohio State University, 13 May 2011
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Dark matter decayDark matter decay--
Constraints from observed fluxes of radiationConstraints from observed fluxes of radiation
If l is a standard model particle, constrain the lifetime using observational measurements of radiation backgrounds
Kuksel
& Kistler
(2007)
Bell, Galea
& Petraki
(2010)
Bell, Galea
& Petraki
(2010)
Nicole Bell, The University of Melbourne Seminar, The Ohio State University, 13 May 2011
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Excluded Excluded lifetimeslifetimes
Nicole Bell, The University of Melbourne Seminar, The Ohio State University, 13 May 2011
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Excluded lifetimesExcluded lifetimes
Bell, Galea, Petraki
(2010)
Nicole Bell, The University of Melbourne Seminar, The Ohio State University, 13 May 2011
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Annihilation to Antimatter
Nicole Bell, The University of Melbourne Seminar, The Ohio State University, 13 May 2011
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Fermi e++e-
excessPhys. Rev. Lett. 102, 181101 (2009)
PAMELA e+ excessNature 458, 607-609
PositronsAnnihilation to Antimatter
Nicole Bell, The University of Melbourne Seminar, The Ohio State University, 13 May 2011
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Antiproton data consistent with theory expectation (for secondary production of antiprotons via cosmic ray propagation in the Galaxy).
AntiprotonsAnnihilation to Antimatter
Nicole Bell, The University of Melbourne Seminar, The Ohio State University, 13 May 2011
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Positrons from astrophysics?Re-examine the expected positron flux from: pulsars supernova remnantsmodified cosmic ray propagation/acceleration
Positrons from Dark Matter? Challenging because: Must produce enough e+e-
without overproducing pbar
or
gamma ray, or radio fluxes Need big cross sections!
Boost via DM clumping/substructure or enhanced cross
sections”Sommerfeld”, non-thermal DM, … But annihilation to leptons is often suppressed…...
Resolution of positron anomalies?
Nicole Bell, The University of Melbourne Seminar, The Ohio State University, 13 May 2011
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Pulsars?
Yuksel, Kistler
and Stanev, PRL 2009
Possible contribution from Geminga
pulsar to positron fraction:
Nicole Bell, The University of Melbourne Seminar, The Ohio State University, 13 May 2011
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Parameterize the annihilation cross section as:
<v> = a + bv2 + …
a --
from s-wave (L=0) annihilationb --
both s-wave and p-wave (L=1) contributions
The Lth partial wave contribution is suppressed as v2L
In galactic halos, v~ 10-3c, so only the s-wave contribution will be significant.
However, in many models, s-wave annihilation to a fermion pair is helicity
suppressed by a factor of 2
DMf mm
Annihilation cross section
Nicole Bell, The University of Melbourne Seminar, The Ohio State University, 13 May 2011
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Majorana
neutralinos
annihilate to a fermion
pair via:
• t-
and u-channel exchange of sfermions helicity
suppressed
• s-channel exchange of Zhelicity
suppressed
• s-channel exchange of higgs suppressed by yukawa
couplings
2 vevfm
Example: SUSY
Nicole Bell, The University of Melbourne Seminar, The Ohio State University, 13 May 2011
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When is annihilation suppressed? For s-channel annihilation:
Suppressed: scalaraxial-vector
Non-suppressed: pseudo-scalar vector (not allowed for Majorana
DM)
tensor
(not allowed for Majorana
DM)
s-channel exchange of a pseudo-scalar is the sole non-suppressed mode for Majorana
DM.
What about t-
and u-channel annihilation?
Fierz
transform to s-channel form. Non-suppressed only if a pseudo-scalar term present
Nicole Bell, The University of Melbourne Seminar, The Ohio State University, 13 May 2011
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Cao, Ma, Shaughnessy, PLB 2009.Dark matter = gauge-singlet Majorana
fermion
=
Example: leptophillic
model
SUSY analogue
Annihilation of bino
dark matter to fermions via exchange of sfermions
Nicole Bell, The University of Melbourne Seminar, The Ohio State University, 13 May 2011
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in CM frame, same helicity
But, opposite chirality!
In massless
limit, helicity
= chirality
not possible!
For small but non-zero masshighly suppressed!
Majoranaopposite
spins
Opposite Spins
Helicity
suppression of s-wave
Nicole Bell, The University of Melbourne Seminar, The Ohio State University, 13 May 2011
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Emission of a photon can lift this suppression: Bergstrom, PLB 225, 372, 1989;Flores, Olive, Rudaz, PLB 232, 377, 1989;Bringmann, Bergstrom, Edsjo, 2008; Barger, Gao, Keung, Marfatia, 2009,…
Lifting the suppression (photons)
The photon carries away a unit of angular momentum no longer helicity
suppressed.
Nicole Bell, The University of Melbourne Seminar, The Ohio State University, 13 May 2011
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Lifting the suppression (photons)
Final state radiation (FSR) “Virtual internal bremsstrahlung”
(VIB) Effect most pronounced for near-degenerate
and
(i.e. the co-annihilation region)
Bringmann, Bergstrom, Edsjo, 2008
Nicole Bell, The University of Melbourne Seminar, The Ohio State University, 13 May 2011
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Bergstrom, Bringmann, Edsjo, PRD 2008
Bringmann, Bergstrom, Edsjo, JHEP 2008
e+e-
signals in SUSY models
Gamma rays Positrons
Nicole Bell, The University of Melbourne Seminar, The Ohio State University, 13 May 2011
43Lifting the suppression: electroweak (W,Z) bremsstrahlung
Radiating a W or Z boson can also lift the suppression Both VIB and FSR are important similar to
brem, except for W/Z mass effects.
distinct phenomenology: W and Z bosons decay to charged leptons, neutrinos, gammas, and hadrons hadron
production even for “leptophillic”
models
Bell, Dent, Jacques & Weiler, 2010.
Bell, Dent, Galea, Jacques, Krauss & Weiler, 2011
Ciafaloni, Cirelli, Comelli, De Simone, Riotto
& Urbano, 2011
Nicole Bell, The University of Melbourne Seminar, The Ohio State University, 13 May 2011
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W bremdiagrams
Nicole Bell, The University of Melbourne Seminar, The Ohio State University, 13 May 2011
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rate larger than photon brem
rate, except near threshold
Bell, Dent, Galea, Jacques, Krauss and Weiler, arXiv:1104.3823
Nicole Bell, The University of Melbourne Seminar, The Ohio State University, 13 May 2011
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W, Z, and
brem
In the limit where the W/Z mass is negligible, we have:
Adding all the bremsstrahlung
processes:
Nicole Bell, The University of Melbourne Seminar, The Ohio State University, 13 May 2011
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Ratio of eW and e+e-
cross sections
• Annihilation to e+e-, e+e-Z & eW dominates over e+e-• Enhancement by up to three orders of magnitude!
(v~10-3c, Galactic halo)
Bell, Dent, Galea, Jacques, Krauss and Weiler, arXiv:1104.3823
Nicole Bell, The University of Melbourne Seminar, The Ohio State University, 13 May 2011
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Ratio of eW and e+e-
cross sections
• Insensitive to the dark matter mass, except near threshold
Bell, Dent, Galea, Jacques, Krauss and Weiler, arXiv:1104.3823
Nicole Bell, The University of Melbourne Seminar, The Ohio State University, 13 May 2011
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Total photon energy spectrumred = totalorange = primary photons from
brem
green = secondary photons from W/Z decay
Nicole Bell, The University of Melbourne Seminar, The Ohio State University, 13 May 2011
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Total electron energy spectrumred = totalgreen = primary electrons from
brem
orange = primary electrons from W/Z bremblue = secondary electrons from W/Z decay
Nicole Bell, The University of Melbourne Seminar, The Ohio State University, 13 May 2011
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Annihilation spectra:
/ W/ Z brem
p
γ
ν
e
Bell, Dent, Jacques & Weiler, 2011
Nicole Bell, The University of Melbourne Seminar, The Ohio State University, 13 May 2011
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Maximum allowed brem
cross sections
Bremsstrahlung
can’t make significant contribution to e+ flux, without overproducing pbar
ee eee
/ppp
Bell, Dent, Jacques & Weiler, 2011
Nicole Bell, The University of Melbourne Seminar, The Ohio State University, 13 May 2011
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Kachelriess, Serpico
and Solberg PRD 2009.
Models with no helicity suppression
EW-brem
still occurs, but is subdominant
W/Z decays ensures there is at least a minimal yield of hadrons, photons, charged leptons and neutrinos.
neutrinos-only
(+ W/Z brem)
electrons-only e+e-
(+ W/Z brem)
Nicole Bell, The University of Melbourne Seminar, The Ohio State University, 13 May 2011
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Electroweak bremsstrahlung, even if a subdominate process, ensures that some minimal hadron production occurs, even for “leptophillic” models.
Obvious exception:If you are below threshold to make W and Z bosons.
--
This includes the light force carrier models (Arkani- Hamed
et al, Weiner et al, etc).
Nicole Bell, The University of Melbourne Seminar, The Ohio State University, 13 May 2011
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Electroweak bremsstrahlung Annihilation to fermions is often helicity
suppressed
Internal bremstrahlung
of , W & Z can lift suppressions, enhance
annihilation yields, and result in interesting multi-messenger signals
Allows indirect detection of processes that would otherwise be too
suppressed to give observable signals. Antiproton limits are important.
Conclusions
Decay Dark matter decay to an almost degenerate daughter might ease small scale structure problems …. but parameter space somewhat constrained
Annihilation in the SunAnnihilation via metastable
mediators can greatly enhance the high
energy neutrino flux produced by dark matter annihilation in the
Sun