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How Do We Know Antimatter is Absent?
EGRET, >100 MeV
P.Coppi, Yale
Outline
I. Why do we care about antimatter, or the lack thereof, in our universe?
II. Local constraints on antimatter….III. Aside: why local “secondary” antimatter is still
interesting …IV. So not much is around today, but what about in
the past: cosmological constraints on baryogenesis models
V. Summary
Detection of Annihilation Gamma-Rays
• Release of energy by Matter-Antimatter annihilation very efficient
• Makes lots of gamma-rays• Big problem: even with horrible gamma-ray
detectors of the early 1970s they should have been seen …
Run through Steigman (1976) review, still valid today (constraints are even tighter!)
The Diffuse Extragalactic Background
Henry 1999
EnergeticParticles!
Direct Detection of Antimatter
• Especially interesting is detection of Z>2 anti-nuclei: can’t make by cosmic ray interactions or primordial nucleosynthesis
• Nothing seen yet despite lots of looking …[See Bob Streitmatter’s talk ]
Stay tuned for AMS-02 (if it ever gets onto ISS)
Gal. Diffuse Emission, Accel. Sites, and CR Propagation- Interplay of CR, ISM and B-field -
Escaping electron
Galactic Halo
Galactic Ridge
Trapped proton
Trapped electron
Synch. rad.(radio)
Brems. (X-γ)
IC by elect(X-γ)
π0 prod(γ)
IC at accel site
(X-γ)
Escaping proton
π0 production (direct + decay)
in GMCAccel. sites
(SNR, pulsar)
Star light (rad. field)
Earth
Selected Results (Balloons) - BESS
Antideuteron upper limit1.92 x 10-4 (m2s sr GeV/n)-1
measured during flights from1997-2000
D. Haas La Thuile, March 2004, page 41
Why care about “secondary”antimatter?
Significant antimatter is indeed observed in our Galaxy!
Prior OSSE result: the “fountain of death”
Searching for dark matter• The lightest super-symmetric particle χ is a leading candidate for non-
baryonic CDM• It is neutral (hence neutralino) and stable if R-parity is not violated• It self-annihilates in two ways:
χ χ γγ where Eγ = Mχ c2
χ χ Ζγ where Eγ = Mχ c2(1−Mz2/4Mχ
2)
• Gamma-ray lines possible– 30 GeV - 10 TeV
q
qor γγ or Zγ
“lines”?
X
X
Unresolved AGNs
WIMPs
TotalEGRET
GLAST
Ormes 2002, GLAST presentation
Thermal Relic DM Particles1) Initially, DM is in
thermal equilibrium: χχ ↔⎯ f f
2) Universe cools:N = NEQ ~ e−m/T
3) χs “freeze out”:N ~ const
Exponentialdrop
Freeze out
• Final N fixed by annihilation cross section:
ΩDM ~ 0.1 (σweak/σΑ)Remarkable!
• 13 Gyr later, Martha Stewart sells ImClone stock – the next day, stock plummets
Coincidences? Maybe, but worth serious investigation!
No-Lose Theorem: Loophole• Assume gravitino is LSP. Early
universe behaves as usual, WIMP freezes out with desired thermal relic density
• A year passes…then all WIMPsdecay to gravitinos
WIMP≈G
Gravitinos naturally inherit the right density, but escape all searches – they are superweakly-interacting “superWIMPs”
MPl2/MW
3 ~ year
No-Lose Theorem: Loophole• Assume gravitino is LSP. Early
universe behaves as usual, WIMP freezes out with desired thermal relic density
• A year passes…then all WIMPsdecay to gravitinos
WIMP≈G
Gravitinos naturally inherit the right density, but escape all searches – they are superweakly-interacting “superWIMPs”
MPl2/MW
3 ~ year
Cosmological Constraints:
• Depending on when excess antimatter is present still lying around (reheating, nucleosynthesisepoch, recombination epoch…), there can be significant consequences:
• CMB distortions• BBN anomalies• Excess diffuse background
WMAP: Measurement of 10-5 fluctuations on the entire sky to ~0.25°
Can you separate baryons from anti -baryons in standard large scale structure formation scenarios?
NO!
Silk Damping…. No structure (voids) around recombination atL_today < 15 Mpc(=> w/out CDM,only top-downgalaxy formation)
What if there are a few odd antimatter domains still annihilating
away today that somehow got missed?
Can use CMB again: Sunyaev-Zeldovicheffect!
Sunyaev-Zel’dovich Effect DecrementObservations at 1 cm or 30 GHz
» Cluster appears as a cool spot in CMB» This CMB temperature decrement has an
amplitude of ~1 mK, a few parts in 10,000 of the CMB anisotropies
» This cluster distortion is many times stronger than the primary anisotropies in the CMB
» The decrement is independent of the distance of the cluster
Observing Frequency [GHz]
Cha
nge
in C
MB
Tem
pera
ture
[µK
elvi
n]
Observations made here
Massive cluster
Lower masscluster
Temperature map of the microwave background toward one of the most distant known galaxy clusters
» Coolest temperatures are red» Hottest temperatures are blue» Contours are spaced by 2 sigma» Central decrement is ~1/1,000 Kelvin
Dec
rem
ent Increm
ent
New Galaxy Clusters from the SurveySZE Survey will provide many more distant clusters in one month than all previous work combined
One year survey» 12 deg2 mapped» if favored model correct
– ~500 total clusters– ~400 distant clusters at lookback
times corresponding to half the age of the Universe or more
– ~150 clusters at distances greater than today’s most distant known galaxy cluster
Exceedingly strong test of structure formation models
Cluster Yield from SZE Survey
Distance
Tot
al N
umbe
r of
Clu
ster
s Bey
ond
a D
ista
nce
Figu
re fr
om H
olde
r et
al.
in p
rep
Most distantknown clusters
(Best X-ray survey to date)
tH
2tH
3tH
4Age: tH
Don’t mess with Big Bang Nucleosynthesis!
Very constrained now with WMAP…
Integral Flux (E>100 MeV) cm-2s-1
Blazars=EGRB? Notobvious!
Unpleasant surprisefor GLAST? (source countprediction too optimistic?)
Poh
l et a
l. 19
97
BBN Allowed
BBN Allowed
“Degenerate” Big Bang NucleosynthesisUsing the observed
abundances of D, 4He,
and 7Li alone,
BBN is very pliable to
allow large neutrino
asymmetries
Due to cancelling
effects between
νµ/τ and νe, and baryon
density, Ωbh2
CMB
CMB
Allowed Orito et al ’02astro-ph/0203352
Allowed
Allowed
The Death of Degenerate BBN and New Constraints…
CMB
Allowed
Allowed: BBN + LMA+Atm
Some further comments on the cosmological X-ray/gamma-ray
background
• X-ray background could have been diffuse hot gas (except for CMB problems), but it is now resolved into (many!) AGN (accreting black holes)
• What about gamma-ray background? TBD
The gamma-ray background…
Ormes 2002, GLAST presentation
Not “standard” XRB AGN (seem to have E cutoff too low?).
CDF-N/GOODS, 2Msec(!) Chandra “Diffuse” X-RayBackground
CDF-S
Thousands of AGN sources per square degree!
Proposed EGRB Sources
• Blazar AGN (known)• Cosmic Rays in Galaxies (probably not
enough luminosity, caveat: starbursts)• Converging Velocity Flows/Shocks in
Large Scale Structure Formation (e.g., merging clusters – possible evidence)
• Exotica (topological defects, neutralinos, cascading,…matter-antimatter annihilation?)
Mean free pathfor VHE photons
Absorption(pair production)
andCascadingimportant for cosmologicalVHE sources.
Coppi & Aharonian 1997
The cascadespectrum froma cosmologicalpopulation of VHE sources –independent ofprimary sourcespectrum forE>10 TeV!
Coppi & Aharonian 1997
6( ) (1 )n z z∝ +
3( ) (1 )n z z∝ +
0( ) (1 )n z z∝ +
Response to Change in IR/O Background
GeV background measurement= calorimeter for VHE universe!
Coppi & Aharonian 1997
S.J. Lee, Coppi, & Sigl
[B=0]
Blazar Background Models, a la Stecker & Salamon 1996
Including IR/O absorption
Don’tforgetcascades!
Coppi & Aharonian 1997
Most sources can think of, even decaying/annihilating CDM particles, trace large scale structure…look for clustering signal!
Bromm et al. 2003
Will the true extragalactic gamma-ray background please stand up?
EGRET All-Sky Map, E>100 MeV
The problem: even looking towards the Galactic poles…
SynchrotronHydrogenColumn(NH)
Keshet et al. 2003
…see evidence of energetic particles and target matter!
The big unknown:Inverse Comptonemission from halo cosmicray electrons
Discrepancy!
Strong et al 2000
So, let’s make things fit by jiggling I.C. component…
-5 -2 -1 -1
Keshet, Loeb, & Waxman (2003)I ( 100 MeV) [integral flux]
< 0.5 10 ph cm sec sr (3 )!
Eγ
σ
>
×
Strong et al. , 2003, astro-ph/0306345
July 20, 2004 SLAC Experimental Seminar
An alternative (mundane particle physics) explanation…GeV Excess Explained ? No.1/2
Gamma-rays from π0 alone.L=(-30,30) B=(-5,5)EGRET Intensity Map
What ?
The peak moves to ~800MeV
Model A (MSOP02)
Model A (LIS)
Prediction of Scaling Models
July 20, 2004 SLAC Experimental Seminar
GeV Excess Explained? No.2/2
L=(-30,30) B=(-5,5)EGRET Intensity Map
Model A (MOSP02)
Model A (LIS)
Scaling Models
Explains about 50%of the Excess with LIS.
Explains the Excess fully with MSOP02.
Combined with Brems andInverse-Compton (Galprop)
July 20, 2004 SLAC Experimental Seminar
Anomalies in e+ and p-bar ? No.1/2
de Boer et al. 2003
Neutralino decay to γ-ray? Neutralino decay to p-bar?
July 20, 2004 SLAC Experimental Seminar
Anomalies in e+ and p-bar ? No.2/2
de Boer et al. 2003Measurement by HEAT collaboration(e+ spectrum by a series of balloon exp.)
July 20, 2004 SLAC Experimental Seminar
Model A Prediction on p-bar Spectrum
Exp. data Model A vs. Scaling Model
Scaling modelwith LIS
July 20, 2004 SLAC Experimental Seminar
Model A Prediction on Positrons Spectrum
Diffractive process favorse+ over e-
Non-Diffractive process dominates overall spectrum
e+
e-
July 20, 2004 SLAC Experimental Seminar
1. Accurate modeling of p(α)-ISM interaction is likely to explain the GeV Excess with minor modification in the cosmic proton (α) spectrum.
2. Excess in anti-proton flux will naturally be explained by the scaling violation.
3. Excess in e+ flux for E > 5 GeV may be explained by inclusion of diffractive process.
4. With much improved data from GLAST, we can turn Galaxy to a HEP and Astorphysics laboratory.
Summary:
Like the standard model in particle physics, we have a good cosmological model(inflation + reheating & adiabatic perturbations+ CDM + gravitational instability) that explains a lot of data…
Like the standard model, it’s phenomenological and missing lots of key ingredients, e.g., baryogenesis
But whatever you decide to do fix it, it has to be pretty subtle … lots of constraints!
Occam’s razor: antimatter gone before nucleosynthesis epoch => null searches (but you neverknow…)
