Dark matter, electrons and anti-protons
Transcript of Dark matter, electrons and anti-protons
Peter Fisher - MIT
Dark matter, electrons andanti-protons
Peter FisherOct. 29, 2005
Peter Fisher - MIT
Maybe one of the lastantiprotons observed at theBevatron
Peter Fisher - MIT
Outline
•The electron, positron, antiproton’sroles in the search for dark matter
•AMS-01- a first try
•A search for dark matter
•The AMS-02 - a bridge too far?
Peter Fisher - MIT
Origins of Dark Matter
Fritz Zwicky (1933) applied Virial Theoremto motions of “nebulea” (galaxies) in theComa cluster and showed the motionimplied gravitational potential far in excessof the amount of matter measured based onthe light output of the nebulea.
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G t( ) =r p i •
r r i
i∑
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dG t( )dt
= 0 = 2 T + V
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MComa >35
Rv 2
G
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Mgalaxy ~MComa
1000= 4.5 ×1010MSun
L ~ 8.5 ×107LS
→γ = 500
R~600 kpc,characteristic radius
v2=5 x 1015cm2/s, time& space averagedvelocity
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Contemporarypicture:
Halo surroundingbaryonic disk
•May be largevariations of DMdensity
•May be bulkmotion of DM inhalo
•May be satellitehalos
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Three methods of looking for evidence of dark matter
Nuclear recoilAnnihilation inour galaxy
Capture followedby annihilation
Recoiling nucleus with ~10 keVin matter Excess electrons, positrons ,
gammas or antiprotons incosmic rays
Neutrinos from the Earth, Sun orgalactic center
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SuperKAMANDAICECube
AMS,HEAT,GLAST,
CAPRICE,PAMELA
CDMSCRESSTZEPLIN
Experiments
Flux integratedover lifetime of
galaxy
Flux in local3kpc now
Flux at Earth nowSampling
Partial suppressionfor Majorana
Majorana notsuppressed
Majoranasuppressed by
N2
Majorana/Diracsuppression
(g2g2q/M4)(g2g2
B/M4) g2g2B/M4g2g2
q/M4Rate
n2n2nDensity
Process
Capture andAnnihilation
AnnihilationScattering
g
gq
M2g gB
M2
+
Peter Fisher - MIT
Three methods of looking for evidence of dark matter
Nuclear recoilAnnihilation inour galaxy
Capture followedby annihilation
Recoiling nucleus with ~10 keVin matter
Excess electrons, positrons ,gammas or antiprotons incosmic rays
Neutrinos from the Earth, Sun orgalactic center
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DM neutral current interactionsχ, p’ Neutral current:
Jµ=ψ(p’)(CVγµ-CAγµγ5)ψ(p)
J0=CVψ(p’)ψ(p)
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dσdTR
=GF2Mc2
8πv 2N 2 exp −MTRR
2
3h2
J= CAψ(p’) S ψ(p)
VectorAxial-vector
p 0
Quarkcontent
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dσdTR
=2GF
2
πh2TRmax µ2λ2J J +1( )
qΣTq3Δq
Nuclearphysics
Nuclearspin
Z0
χ, p Jµ
Dirac, scalarDirac,ScalarMajorana
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DM neutral current interactionsDirac (Spin Independent, SI)•Elastic cross section proportional to N2 (~1600 for Ge)•Independent of nuclear spin•Simple nuclear physicsMajorana (Spin Dependent, SD)•No enhancement from coherence•Proportional to J(J+1)•Complicated nuclear physics, QCDIf a recoil signal is observed, do not know if it is from SI orSD, sSI~N2sSD
The annihilation channel is more complex (depends onneutral scalars) and Majorana is not obviously suppressedrelative to Dirac.
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γ
e+/e-p/pχDM
χDM
W+
W-
q
q
Fragmentation anddecay
Typical processes:
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νDM + νDM → W + + W −
νDM + νDM → Z 0 + Z 0
νDM + νDM → b + b
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Galprop - I.Moskolenko and A.Strong - cosmic raypropagationproblem fit to all CRknown data
Green’s functions -expected flux onEarth for uniformmonoenergeticsource of electrons
Notnormalized!
Propagation
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Charged particles followmagnetic field lines
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rL =p
0.3 GeVT −m
B≈ 7AU
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v = c
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v|| = c3
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r B o
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r ′ B
Magnetic turbulence - averagevariation of magnetic field:
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η =
r ′ B
r B o +
r ′ B ≈10−4
Mean time between scatteringfrom inhomogenieties:
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τ s =1
ηωL
≈10y
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Electron lifetime determined by time τo topropagate one Xo=65 g/cm2 in hydrogen
For a proton, the relevant scale isλN=51g/cm2
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v = c
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v|| = c3
1 proton/cm3 in ISM Xo=1.3 x 1013 kpc
τo=45 My
30 GeV electron:v=c, gives averagevelocity along fieldc/31/2
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⇒
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⇒
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Number of scatterings: N=τo/τs
Random walk diffusion distance
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d = v|| τ s N =13c 2τ sτ o ≈ 3kpc
Advanceeach step RMS
number ofsteps
Diffusioncoefficient
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Integrated positron signalabove 8 GeV for 100 GeV(solid line) and 30 GeV(dotted line). The Earth islocated at 8.5 kpc radius.
Earth
Contribution of DMoutside of plane ofgalaxy difficult tounderstand - magneticfield structure notwell known
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Propagation makes amess!
During 3 kpc transit,DM annihilationproducts go through~1 Xo or 1 λN ofmaterial
e+/e- spectrum atpoint ofannihilation
e+/e-
spectrumat Earth
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“Typical” signal - neutralino annihilation
Moral - in cosmic rays everything looks like
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dNdE
∝ E −(2.5 to 3.2)
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For a search for dark matter annihilation products, oneshould measure the spectra of
•Electrons - sharp cutoff from prompt decays in somemodels
•Positrons - low background rate, high signal rate, sharpcutoff from prompt decays in some models
•Antiprotons - low background rate, moderate signal rate
•Photons - directionality, but possibly large backgrounds
Ideally, one would have a single detector for all four andthen carry out a simultaneous fit to a series of models.
HEAT and AMS-01 have begun with electrons and positrons.
There are interesting results from HEAT…
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This first problem is that thepropagation (especially solarmodulation) is not wellunderstood…
…the second is that HEATruns out of sensitivity at ~50GeV…
Fit with expected (smooth)normalization with signal (bump)gives 55 times higher relicdensity than observed
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AMS-01 - June 1998
•~100 h data taking at 400 km
•200 M triggers
•6 plane silicon tracker in 1500 Gfield
•4 plane TOF system
•Aerogel thershold Cerenkovcounter (~5 GeV)
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AMS Collaboration
Peter Fisher - MIT
AMS-01 had no particle ID above a few GeV, so
•Select clean Z=-1 events
•Compute all Z=-1 backgrounds from expected CR source
•Use PYTHIA to compute signal spectra for electrons andanitprotons from W, Z and b decays at center of massenergies from 100-1000 GeV (Note: we do not yet considerSUSY models; our limits will be applicable for any processwhich has ZZ, WW or bb final states.
•GALPROP finds signal spectra at Earth
•Signal and background fit to data
How well can this work?
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Mar.2008
(Hah!)
FlewFlewStatus
8-240.4-1.51FOM forDM e-
3,000360170MDR (GV)
26,00023945Exposure(h)
0.50.140.05Aperture(m2-str)
AMS-02AMS-01HEAT
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FOM = 0.1 to 0.01( ) Ω0.05m2 − str
τ45h
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AMS-01MDRHeat
MDR
HighestHEATDatapoint
Preliminary AMS-01 Z=-1selection:
•Downward going
•|Q|=1 from both trackerand TOF
•Well fit track with 4 hits
•Not docked to MIR, not overSAA
•Good match between TOFand track
A major difficulty is mis-reconstructed protons in theZ=-1 signal. This backgroundis calculated by Monte Carlo(200 M events)
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The primary background is mis-reconstructed protons (shown in red), whichdominates for p>50 GeV. Owing to this, thefit is largely determined by the slope atlower energy rather than the spectral cutoffat higher energy.
Fit for the normalization of•Expected electrons•Expected antiprotons•DM signal (electrons, antiprotons)from W+W- fragmentation
Troublehere!
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Not a result! Expectedsensitivity will lie in thisrange
Taking the 90% c.l.upper limit for the DMnormalization from thefit gives the exclusionplot shown. The limitis in an interestingregion.
Clearly, more work isneeded to understandthe instrumentaleffects.
Need something better!
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Improvement requires•Much higher statistics - 3 years inorbit, 3x larger aperture•Higher momentum reach - 7x strongermagnet, two more tracker planes•Particle ID
AMS-02
See: http://ams.cern.ch
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AMS-02 is under construction. All major subsystems arecomplete or will be complete by Jan. 2006, at which timeintegration will start.
20 layer Xe:CO2transition Radiation
Detector
0.7 Tsuperconductingmagnet
8 plane silicontracker
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AMS is currently in the NASA manifest for UF-4 which will bein 18 flights. The President’s space exploration visionforesees retiring the shuttle in 2010. You do the math…
…all options are being explored.
Peter Fisher - MIT
Summary•Elucidation of the dark matter problem will eventuallyrequire detection or very stringent limits on annihilation inour galaxy
•Detecting the decay products is a tough problem:
•High statistics requires a large detector (~1 m2-str),long duration (months)
•Backgrounds are high
•Propagation effects are tricky to assess
•Complex instrument needed for electrons, positrons,protons and anti-protons
For myself, I’m thinking about nuclear recoils