the future of particle physics in outer space

ELSEVIER Nuclear Physics B (Proc. Suppl.) 66 (1998) 571-582

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PROCEEDINGS SUPPLEMENTS

The Future of Particle Physics in Outer Space P. Salati a

aLaboratoire de Physique Th~orique ENSLAPP Chemin de Bellevue, LAPP B.P. 110, 74941 Annecy-le-Vieux Cedex, France

Particle physics started as the study of cosmic rays. After having settled on the ground for half a century, it is about to conquer the outer space with satellite and space shuttle borne instruments. Particle physics has actually strong connections with astrophysics and cosmology. This contribution deals with a few of these ties. The exploration of the gamma ray sky and the study of cosmic rays are just two examples of the bright future awaiting high energy physics in space.

1. The connection between particle physics and ou te r space

Ties are strong between astrophysics and particle physics. To commence, there is an historic re- lationship between these two fields. In its infancy, particle physics has started as a hunt for cosmic rays penetrating into the atmosphere. Remember that Anderson discovered the anti-electron, the so-called positron, in an emulsion exposed to the cosmic radiation. The muon has also been found to be an important component of the showers that are generated by high energy particles impacting on the top of the atmosphere. Before trying to synthetize the various species of the subnuclear realm, particle physicists have actually used what outer space provided them.

Then, as we look deeper and deeper in space, more and more exotic objects are unravelled. These distant sources are extremely luminous and appear to be powerful sources of high energy particles. Understanding the mechanisms which trig- ger the jets of blazars for instance and the sub- sequent interactions of the latter with the interstellar medium is of paramount importance for at least two reasons. It is of course fundamental to study those sources from an astrophysical point of view. In addition, the hope may come alive to partly mimic such sources and to accelerate particles in the laboratory up to much higher energies than those currently obtained today. How- ever, particle physi~ will have at some point to go

0920-5632/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII S0920-5632(98)00110-8

back to outer space. Ground based instruments cannot reach exceedingly large energies. From a mere practical point of view, outer space acts as a huge accelerator where high energy particles are naturally present for free.

There is therefore a necessity to go in orbit around the Earth. The atmosphere acts as a buffer that shields the surface of our planet from the most dangerous cosmic radiations. In order to study the latter and to take advantage of them re- quires actually to go in outer space, above the atmosphere, where absorption becomes negligible. Space based detectors are for instance mandatory in order to pursue the study of such sources as pulsars, gamma-ray bursters or active galactic nuclei. Another illustration is provided by the anti- proton and positron fluxes at the Earth. They only can be measured accurately from a satellite or space shuttle borne device as discussed below.

Finally, most of the matter in the universe has not yet been observed directly. Its influence on the self-gravity of our own galaxy or on the dy- namics of clusters of galaxies is well known. How- ever, its nature is still a puzzle. That astronomical missing mass could well be made of elemen- tary particles with very weak interactions with their surroundings. One of the favoured candidates is a neutral species such as the lightest supersymmetric state which supersymmetric mod- els predict to be stable when the R-parity is con- served, ff present in the galactic halo, these particles would still undergo some mutual annihila-

572 P. Salati/Nuclear Physics B (Proc. Suppl.) 66 (1998) 571-582

tion. They would produce a detectable flux of cosmic radiations like gamma-rays, positrons and anti-protons. Another connection between particle physics and cosmology lies in the possible existence of topological defects pervading the intergalactic medium. Those are classical field con- figurations that naturally arise in theories where the gauge symmetries are spontaneously broken and where some scalar Higgs field acquires a non-vanishing vacuum expectation value. Cos- mic strings are just an example of such defects. The decay of the former may be the source of ultra-high energy (UHE) cosmic rays whose energies exceed 10 2° eV. It has even been suggested that UHE particles are mere vortons, i.e., cosmic string loops, that annihilate on top of the atmosphere [1].

Particle physics therefore has a bright future in store in outer space. It has strong connections with astrophysics. This paper deals just with a few of these ties. Section 2 is devoted to the study of gamma-rays. In section 3, the production and the propagation of cosmic-ray nuclei throughout our galaxy is presented. Section 4 deals with the puzzling question of UHE particles. Finally, section 5 is a little bit more speculative and addresses the question of a Moon observatory.

2. The g a m m a ray sky

The instruments on board the Compton Gamma Ray Observatory (CGRO) rely on the well-known technology which has been used for several decades to built the detectors of particle physics. The EGRET experiment, for instance, collects high energy photons from 30 MeV up to 10 GeV. The energy and direction of the inci- dent gamma-ray are determined from the tracks left by the converted electron-positron pair in a stack of spark chambers. CGRO has observed the gamma-ray sky with unprecedented accuracy. The point-like sources can be disentangled from the more extended background of the diffuse emission.

2.1. The local s o u r c e s

Above 100 MeV, a few dozens of sources have been detected by CGRO, yet not all identified.

As shown in figure 1, they fall in two catagories. Pulsars are rotating neutron stars and are mostly distributed in the plane of the Milky Way. High energy cosmic rays are produced by the intense electric fields that exist on the surface of the compact remnants which are highly magnetized. Neu- tron stars are excellent conductors and may house magnetic fields as large as ,,~ 1012 Gauss. Their fast rotation translates the magnetic field into an intense electric field that accelerates charged par- tides. The high energy electrons resulting from this acceleration produce a cascade of secondary particles and gamma rays in the magnetosphere. Another possibility is the existence of a compan- ion whose gas is extracted and flows onto the neutron star. Gravitational energy is released when matter falls into the potential well of the compact object. The neutron star surface may be heated up to ,-, 109 K, hence a copious emission of X and gamma rays. There is a strong correlation among the various electromagnetic signals emit- ted by pulsars. In the case of the Crab pulsar, each period of 33 milliseconds is characterized by two strong spikes that appear both in the radio, optical, X ray and gamma ray bands.

The point-like gamma ray sources that are isQkropically distributed are active galactic nuclei (AGN).'The latter are believed to be galaxies hosting at their centers a massive black hole with mass ranging from 106 up to 10 s Mo. Remem- ber that while stars shine, the pressure of hot gases resulting from thermonuclear burning supports them against gravitational collapse. When they die, stars collapse forming white dwarfs or neutrons stars, depending on whether their inner pressure is due to degenerate electrons or neutrons. However, degeneracy pressure can sustain the pull of gravity only in objects of lim- ited mass. The so-called Chandrasekhar limit is ~ 1.4 M o for white dwarfs and a few M o for neutron stars. As regards black holes, there is no such upper limit and systems may be much heavier. Since the 40's when Carl Seyfert discovered galaxies with an anomalous intense blue emission, an entire bestiary of very exotic objects has been unravelled. That zoo comprises Seyfert galaxies, quasars and blazars. Their luminosity is very large, ranging from 1041 to 1046 ergs s -1. We

P. Salati/Nuclear Physics B (Proc. Suppl.) 66 (1998) 571-582 573

+ I#0

Second EGRET Catalog E > 100 MeV

"! ,o • •

• Active Galactic Nuclei • Pulsars Unidentified EGRET sources • LMC

Figure 1. The gamma ray local sources.

now understand that it is essentially the same object that is seen under various perspectives. A few percents of all galaxies are AGNs and host a giant black hole at their centers. Gas flows in the very deep potential well and forms an accretion disk with temperature ranging from 10 to 40 keV. The blackbody X ray emission of the infalling matter is processed, mostly through Compton scattering, by an extended toms of dust and gas which lies in the equatorial plane of the active nucleus. The resulting spectrum exhibits a characteristic bump in the UV region, hence the excess of the luminosity in the blue optical range that Carl Seyfert discovered. A fraction ~ 95% of all AGNs are radio quiet. The host galaxy is a spiral. When it is seen from cosmological distances, the galaxy

becomes a quasi stellar object (QSO). The rest of the active sources, i.e., one AGN in twenty, is radio loud. In that case, the host galaxy is in general an elliptical, a system known to be formed when two parent galaxies merge together. That fusion induces a more disturbed state than in the radio quiet case. The gas supply of the central black hole is therefore abundant, hence a powerful nucleus which also generates a jet of high energy particles along its polar axis. The accretion disk of the rotating Kerr black hole supplies a magnetic field through a dynamo mechanism. As discussed by Blandford and Znajek [2], the hori- zon of the Kerr black hole behaves like a perfect conductor. The magnetic field is therefore converted by rotation into an electric field that lies


along the polar axis with potential differences as large as 102° volts. Charged particles are accelerated along that electric field and generate a powerful ultra-relativistic jet. The latter is responsi- ble for the radio emission through the synchroton radiation of the accelerated electrons. When the system is seen edge-on or at least when the angle between the line of sight of the observer and the jet axis is large, it behaves just as a radio-galaxy. If located at remote distances, the host galaxy is not resolved and becomes a QSO with radio emission, i.e., a so-called quasar. Because of the geometry, its radio spectrum is steep and rapidly decreases with the frequency. On the contrary, when the observer nearly looks in the direction of the jet, the object is known as a blazar. The latter category comprises quasars whose radio spectrum is flat and BL Lac sources. Blazars are puzzling objects. Because the jet axis nearly points in the direction of the oberver, those sources are rapidly varying in time, with extreme luminosi- ties. The gamma ray spectrum extends up to the TeV range and has been studied in some cases with atmospheric air shower detectors. CGRO has shown for instance that blazar 3C 279 has a flat gamma ray spectrum, with no large variations of dL/d(lnu) from 10 keV up to 10 GeV.

Finally, CGRO has detected a large number (,,~ 700) of gamma ray bursts. The latter were pre- viously linked to the neutron stars of the galactic plane. CGRO has shown that their distribution is on the contrary very isotropic about the Earth's position itself. This suggests that the sources are at cosmological distances. The energetics of the events require that at least the coalescence of two compact objects is involved if bursts radiate isotropically.

2.2. The g a m m a ray diffuse emission High energy protons propagate in our galaxy,

the Milky Way. The latter may be pictured as a central buldge surrounded by a flat disk of stars and gas. Cosmic ray protons interact with the interstellar diffuse gas, mostly hydrogen, and undergo proton-proton collisions. Such reactions are well-known in high energy accelerators where they are artificially induced in order to be studied. In astrophysics, the collision of a high energy

species with a particle at rest is called a spallation reaction. The spallation of cosmic ray protons with the interstellar material is known to generate, among a variety of products, lr ° pions that subsequently decay into photons. This leads to a diffuse gamma ray emission which traces both the gas distribution as seen from the Earth as well as the way high energy protons propagate in the galaxy. The differential energy gamma ray flux at the Earth is actually a convolution along the line of sight of the gas density nH with the gamma ray emissivity dIH/dE.y of the hydrogen atoms embedded inside the cosmic radiation

d~.~ f dlH, , nil(8) ds

dE.y - J -~-~ts) . (1)

The gamma ray emissivity depends in turn on the energy spectrum ep(Ep) of the cosmic ray protons that hit the hydrogen atoms of the interstellar material

dill f d ~ -- - ~ ( E p --+ E.y) '~p(Ep) dEp . (2) dE.y In that relation, de% denotes the cross section with which a proton with energy Ep generates a gamma ray whose energy is in the range between

and + dE.. 4~.,he strong relation between the gamma ray

diffuse emission, the interstellar gas distribution and the cosmic ray proton flux throughout the galaxy has been carefully analyzed. The former has been observed with great accuracy by CGRO. The distribution of atomic HI is directly measured from radio surveys monitoring the famous 21 cm line transition which characterizes that el- ement. More involved is the case of molecular hydrogen H2 whose structure prevents any electric dipole emission. It is traced through the carbon monoxide CO which its clouds contain. The CO molecule is detected through its emission line at a few microns. The conversion factor between the CO line intensity and the column density of molecular hydrogen is unknown and could even depend on the galactocentric radius. However, a recent study performed in the direction of the Perseus arm (b = 0 ° , l - 115 °) shows that 80% of the gamma ray diffuse signal arises there from the pure HI component. The CO to H2 conversion factor is not relevant in that system, hence

B Salati/Nuclear Physics B (Proc. Suppl.) 66 (1998) 571-582 575

the solid conclusion that the cosmic ray proton flux is a factor -~ 1.7 -4-0.2 smaller at the Perseus arm than at the solar circle. The decrease of the cosmic ray density in the outer parts of the galactic plane is in good agreement with the diffusion model exposed in section 3.

Hidden cold gas has been suggested recently as a possible explanation of the galactic dark matter. De Paolis et al. [3] have outlined a scenario in which dark clusters of compact objects pervade the halo together with clouds of molecular hydrogen, at distances larger than 10 to 20 kpc. Pfenniger et al. [4] have also suggested that a flat rotation curve beyond the solar circle could be ex- plained by a thin disk of cold molecular hydrogen, widening at large distances from the center. If un- seen gas was present in the galactic halo, it would be impacted by cosmic rays originating from the disk. This would lead to a strong gamma ray signal showing up as a new component in the galactic diffuse radiation. Analysis of the latter shows no such exotic component. The residual emission left over after the known HI and H2 distributions have been taken into account is consistent with a faint isotropic component. Its integral flux above 100 MeV is 1.5 to 2.5 × 10 -s photons cm -~ s -1 sr -1 , with a spectral index of--- -2. That residual emission translates into a tight constraint on the abundance of gas potentially concealed in the halo. By using a diffusion model which correctly reproduces the radial distribution of cosmic ray protons in the galactic disk, an upper limit of 2 to 4% is inferred [5] on the fraction of dark matter in the form of gas, either diffuse or clumped in cold clouds. The flatter the halo, the stronger the bound. Note that beyond the region where cosmic rays diffuse, no limit may be inferred from the CGRO observations. There, the gamma ray hydrogen emissivity actually vanishes. This naturally leads to the possibility that gas might only be in the outer parts of the halo, turning into massive compact objects in the inner galaxy. Alter- natively, the galactic dark matter could be made of weakly interacting massive particles, such as the LSP discussed above.

2.3. H o t gas in c l u s t e r s o f galaxies The question of the baryonic nature of the as-

tronomical missing mass has actually found a new perspective. It has been recently analyzed in the light of the X ray observations of a few clusters of galaxies. The latter, such as the Virgo or the Fomax clusters, are known to be the largest viri- alized systems in the universe, with typical exten- sions reaching at most ~ 10 h -1 Mpc 1. Those clusters contain a lot of hot gas with temperature T ,,, 107- 108 K as inferred from their X ray spectrum. Because the X rays originate from the bremsstrahlung emission of free electrons with ions, the surface luminosity leads to the density ne of the gas and to its distribution as a function of the distance r to the cluster center. The hot X ray gas has been found to be a major component of those clusters. In the case of Fornax, the gas comprises a third of the total baryonic mass. Furthermore, stars and gas come out to be just a fraction of the total cluster mass, hence the presence of an important dark matter component.

Assuming hydrostatic equilibrium, the pressure gradient d P g / d r and the density pg of the hot gas are both related through

dPg _ G M p g dr *. , r ~ ' (3)

to the-total mass M, whatever its nature, con- tained within a distance r from the cluster center. Even if spherical symmetry is assumed, the problem is further complicated by the necessity to convert the X ray surface luminosity of the cluster into the gas density pg, i.e., into a mere number of free electrons per unit volume. Recent studies [6] have shown that stars and gas contribute on average a fraction

F s = (0.060- 0.003) h , (4)

to the total mass of clusters. The hot gas induces in addition a Sunyaev-Zeldovitch effect on the photons of the cosmic background radiation (CBR). Frequencies are boosted up through Thomson scattering on the hot electrons. The first results indicate a fraction in gas and stars

FB = (0.061+ 0.011) h -1 (5)

1The quantity h is the Hubble expansion constant expressed in units of 100 km/s/Mpc

576 R Salati/Nuclear Physics B (Proc Suppl.) 66 (1998) 571-582

in excellent agreement with the X ray determina- tions. Note finally that these values of the baryon fraction FB of clusters are compatible with the ratio ~ B / ~ M of the cosmological baryon abundance - as determined by primordial nucleosynthesis - to the total abundance of non-relativistic matter. The latter includes of course the baryons but any species that contributes to the overall mass density of the universe. There is no discrepancy between the cluster and the cosmological baryon fractions provided that h -¢ 0.6, ~ M ~ 0 . 5 - 0.6 and ~B "~ 0.08 -- 0.10 [7]. That latter value is actually favoured by the low deuterium measure- ments in some absorption systems lying against distant quasars.

2 . 4 . T h e g a m m a r a y s i g n a l o f s u p e r s y m -

m e t r i c d a r k m a t t e r

As inferred from the cluster X ray analysis, the astronomical dark matter could be mostly made of non-baryonic species. A favoured can- didate is the lightest state X of supersymmetric theories. That particle must be neutral, hence the generic name of neutralino. A major con- stituent of the cosmological missing mass, that species should also pervade the galactic halo. It would still undergo today some annihilation there. Remember that the neutralino X is in general a Majorana fermion and may self-annihilate. During the big bang, when the temperature of the primordial plasma dropped below their mass, neutralinos were depleted precisely through their annihilations into lighter species. That process stopped when its probability became negligible with respect to the expansion rate. Since then, neutralinos have pervaded the intergalactic medium. Present annihilations of galactic neutralinos do not jeopardize their population and generate a detectable signal of gamma rays. Ac- tually, neutralino pair annihilation mostly produces a quark-antiquark pair. The corresponding coloured string fragmentates and subsequently hadronizes. Neutral ~r ° pions are created that decay into photons. The corresponding signal is characterized by an energy spectrum and an an- gular distribution in the sky that differ from the galactic diffuse gamma ray emission generated by the impact of high energy protons on the inter-

stellar gas. However, the gamma ray signal of the galactic neutralinos is mostly swamped in that diffuse emission and will be hard to disentangle. An exciting possibility is the direct annihilation into a photon pair

, (6)

through a loop diagram. The gamma ray line that obtains may be more easily detected provided sig- nificant energy resolution is reached. Figure 2

10000

~ 1000

m

z H

100

Z

.1

I ' ' 1 ' ' " 1 I ' ' I ' ' ' t .

f lh I = 1 / 4 0 t ffi 1 year. S ffi 1 m ~ s r

Spect ra l resoluUon IF

2 5 10 20 50 100

P h o t i n o m a s s i n GeV

Figure 2. For three values of the photino cosmological abundance f ~ h 2 = 1/40, 1/4 and 1, the signal is contrasted with the background noise. The photino mass MS covers the range 1-100 GeV. The signal is the number of gamma ray line photons received by a 1% spectral resolution detector of aperture 1 m 2 sr during an entire year.

shows such a signal in the case of photinos as the

P. Salati/Nuclear Physics B (Proc. Suppl.) 66 (1998) 571-582 577

missing mass [8]. The number of gamma rays which a 1 m s sr detector would collect during one year of operation is presented as a function of the photino mass (solid curves). It is compared to the background noise (dashed line) assuming an energy resolution of 1%. As the sfermion mass goes down, the annihilation cross section increases as well as the gamma ray line signal. However, the cosmological photino abundance is depleted, hence the three curves which are shifted down as f~ ~ h 2 increases. Detection of the gamma ray signal of supersymmetric dark matter may be achieve in the near future with telescopes of the 1 m 2 sr caliber such as the Gamma Ray Large Area Space Telescope (GLAST).

3. Cosmic rays and t he hun t for ant i - mat ter

Stars sow the interstellar medium with their own matter. That processed material mostly con- tains hydrogen, helium and, at a lesser level, some carbon, nytrogen and oxygen, i.e., the CNO ele- ments. Then supernovae driven shocks sweep the interstellar gas and accelerate nuclei to generate the cosmic rays. The sources of the latter are localized in the galactic plane and correspond to supernovae remnants and pulsars.

Parker has studied the propagation of cosmic rays inside our galaxy as a consequence of their scattering by the irregularities of magnetic fields. The presence of the latter is now firmly estab- lished by synchrotron radiation far above the galactic plane as mentioned by Badwar [9]. Mag- netic fields are also detected in other galaxies. So cosmic ray transport crucially depends on their diffusion across the erratic magnetic fields of the Milky Way. Primary cosmic rays, such as CNO, are those species already present in space before being accelerated. They subsequently diffuse in the galactic ridge during ~ 10 million years where they undergo spallation reactions with the interstellar medium. As already mentionned, this leads to gamma rays and also, when a heavy nucleus breaks down after an encounter with an hydrogen atom, to the production of lighter el- ements. The spallation of CNO generates a non negligible flux of lithium, beryllium and boron nu-

clei (LiBeB) which are orders of magnitude more abundant in cosmic rays than in stars. A single value for the column density through which the primary CNO nuclei travel and fragmentate is enough to account for the observed abundances of the secondary LiBeB species. That observation strongly supports the diffusion and spMlation scheme according to which the galactic plane behaves merely as a leaky box. That simple model needs however to be slightly refined. Among the secondaries, the 1°Be nucleus is unstable with a half-lifetime of 1.6 million years. It plays the role of a chronometer which measures the time actually spent by cosmic rays from their production to their escape in the intergalactic medium. Its abundance with respect to its stable partner abe has been found smaller than expected, hence a residence time in the overall galaxy of ~ 100 million years, ten times larger than the diffusion time in the ridge alone. This strongly suggests the existence of thick confinement layers that extend above and beneath the galactic plane. The naive leaky box has therefore to be refined into an ax- isymmetric two-zone diffusion model.

First, cosmic rays are accelerated within a thin disk with radius 0 < r < R = 20 kpc and thick- ness IzJ< h = 100 pc, where they diffuse and interact on atomic and molecular hydrogen. Then, this gaseous plane is sandwiched by extended do- mains containing irregular magnetic fields, with same radial extension and ]z] < L = 3 kpc. These thick layers play the role of confinement reser- voirs. The diffusion coefficient is given by the empirical value

K 6x1023m2s-1 ( 1 + 3 - - - ~ ) °6 = , (7)

where 7¢ = p c / Z e is the particle rigidity. Assum- ing that steady state holds and following Webber, Lee and Gupta [10], the distribution of cosmic ray protons may be derived from the diffusion equa- tion

6Onp 0 (8)

= K A n p -- 2h$(z)rpnp + 2 h $ ( z ) Q p ( r ) ,

where np(Ep, r, z) is the density of protons with energy Ep at radius r and height z. In the right-

578 P. SalatilNuclear Physics B (Proc. Suppl.) 66 (1998) 571-582

hand side of relation (8), the first term describes the diffusion of particles. The second term ac- counts for the destruction of protons by their interactions with the interstellar medium of the galactic plane. The disk is only 200 pc thick, hence our approximation of an infinite thinness and the term 2h ~(z). The collision rate of pro- °-~' tons with the interstellar hydrogen is derived assuming that nH = 1 cm -3 is the average hydro- ,.~ gen density in the thin matter disk. The last term in relation (8) deals with the sources of high energy protons. It matches the distribution of su- '.~ pernovae remnants and pulsars as measured by Lyne, Manchester and Taylor [11] ,.~

Qp(Ep,r) = Q°(Ep) p°6 e-3p , (9)

with p = f i R . ' " ~

The distribution n× of anti-protons and, more generally, of the secondary anti-nucleus species X, follow the same diffusion relation as for the protons up to the production term

Q×(E×, r) = rp np(Ep, r, z = 0) x (10)

x { d N x "E d--E~xt p - + E × ) } dEp .

The latter obtains now from the convolution of the proton energy spectrum np with the cross section for production of an anti-nucleus X in a proton-proton interaction. The differential energy distribution d.IV'x/dE x corresponds to the anti-nucleus X with energy E× that is produced when a proton with energy Ep collides on an hydrogen atom at rest. As a matter of fact, an effective anti-nucleus multiplicity may eventually be defined as

Afffr(Ex) = (11) +oo I,,,,,.,.{o,>(.,,,,>)

The differential flux of protons with energy Ep is denoted by

1 ep(Ep) = ~-~npvp, (12)

and is expressed in units of cm -~ s -1 sr "1 GeV -1, As shown in

figure 3 [12], the ~ /p ratio exhibits a characteristic plateau with a magnitude of .-. 2 x 10 -4 in the GeV region.

I //I / /' i7 I /i !/

J l . . . . . . . . . . . . , , e

Figure 3. The fluxes of cosmic ray anti-protons and of anti-deuterium and anti-tritium nuclei, relative to the proton flux, are presented as a function of the momentum per nucleon. To fit on the same diagram, the curves have been scaled by a factor of 104 for anti-deuterium and of 108 for aatjrhelium aide. The doubling of curves corresponds to different factorization schemes.

If a neutral,no species pervades the galaxy, its annihilation produces an additional flux of anti- protons and of positrons that is superimposed to the conventional spallation productions discussed above. In figure 4, the anti-proton signature 15/p of neutral,no pair-annihilations in the halo is compared to the standard spallation ~/p ratio. Three species of neutralinos are presented : gaugino, higgsino, and a mixture [13]. The lightest Higgs mass is mn = 55 GeV while the sfermion mass is ~ = 500 GeV and tan fl = 8. At low energy, the SUSY anti-proton production may exceed the standard signal in the pure gaugino and higgsino cases. In the intermediate situation, it is well below the spallation anti-proton flux because the neutral,no relic density is f~x h2 ___ 3 x 10 -8, only a tenth of the critical value below which neutralinos are diluted in the galactic dark matter.

B Salati/Nuclear Physics B (Proc. Suppl.) 66 (1998) 571-582 579

O.(X)I

i O.(X]OI

"5

$ le-05

c .[

le-07

m ~ m m - - - -

........ ::::: ::::),. / . " 1 . " " , ,,

y..'* #

"/~" !ii

0. | | I0 10[) Kinetic E [GcV]

Figure 4. The ~/p ratio is featured as a function of energy, for various anti-proton sources: standard spallations (solid line) assuming a spectral index of 2.7 for the primary proton distribution, and neutralino pair annihilations, for three different neutralino compositions : gaugino, higgsino, and a mixture.

The experimental status regarding the detection of cosmic rays is rapidly evolving. Compe- tition is harsh between the balloon borne experiments and the space projects. The IMAX instrument has recently collected 16 anti-protons during a stratospheric flight over Manitoba" in Canada [14]. That small number may be compared to the complete world statistics of only ,-~ 100 events so far collected in the past forty years. The balloon borne experiment BESS [15]aims to reach in 1998 a sensitivity of 10 -7 on the abundance of the 4I~e anti-nuclei relative to protons. The BESS apparatus, like IMAX, is a high energy detector where the magnetic field is generated by

superconducting currents. Those devices are sent in the stratosphere where, in spite of an eleva- tion of ,,~ 40 kin, the column density above the instruments is still ~ 5 g cm -2. That residual atmospheric depth is to be compared to the ,-~ 15 g cm -~ through which cosmic rays propagate during their 10 million years journey inside the galactic ridge. Balloon borne experiments suffer actually from that residual atmospheric opacity which is enough to degrade cosmic ray primaries into showers of secondary species, generating additional anti-protons and positrons.

The necessity to observe cosmic rays from space borne devices is evident. However, two problems are still awaiting the experimentalist. First, the solar wind exerts pressure on the particles that penetrate the solar system from the interstellar medium. The incoming flows vary in time on an eleven year cycle that characterizes the activity of the Sun. The cosmic ray fluxes that are measured at the Earth need therefore to be corrected in order to infer their values far away from the Sun. That so-called solar modulation is a quite intricate process and is still under investigation. Then, the magnetic field of the Earth itself acts as a shield that prevents low energy species from reaching,its inner regions. At distance R from the Earth center, cosmic rays whose momenta make an angle 6 with the direction joining West to East are completely screened below the critical rigidity

/~ cos 4 L

~x-min : ~-~ (I+x/1WEOSOEos3L)2 , (13)

where the geomagnetic latitude is denoted by L and where p is the Earth magnetic dipole mo- ment. The value of the ratio p/R~ is 60 GV on the ground. Therefore, in order to observe low energy cosmic rays, equatorial orbits are prohib- ited. Remember that IMAX and BESS fly over Manitoba, i.e., close to the north magnetic pole, in a region where the geomagnetic suppression effect does not operate.

Two space projects are in competition. PAMELA [16], a satellite borne instrument, should reach the same sensitivity of 10 -7 as BESS in its search for the 4I~e nucleus. In spite of its small acceptance, PAMELA will benefit from a polar orbit, hence from a weak geomagnetic sup-


pression. It is expected to be launched from Baikonur in 1999. Its competitor is the Anti- Matter Spectrometer (AMS) instrument [17]. A permanent magnet will be sent in space in 1998 on board the space shuttle and, later on, as part of the International Space Station Alpha. The magnet is quite large with BL 2 = 0.15 T m 2 and an effective area of ~ 1 m 2 sr. During the two weeks flight of next year, the acceptance should reach ,-~ 3 x 105 m 2 sr s, three orders of magnitude above the IMAX level of 260 m 2 sr s. Between 500 MeV and 3 GeV, 800 anti-protons are expected to be collected. Because the space shuttle orbits near the equator, the Earth magnetic field supresses by a factor of ,-, 20 the number of anti- protons naively inferred from a direct scaling to the IMAX yield. During the space station stage, AMS is expected to reach a level of 10 -9 on the ratio of anti-nuclei to protons. As is clear in figure 3, the D/p ratio exceeds 10 -9 above a momentum per anti-nucleon of ,-, 4 GeV/c. The AMS collaboration should be able to detect a few anti- deuterons. However, the 3He/p abundance does not exceed ,-, 4 x 10 -13. Heavier anti-nuclei are even further suppressed. Therefore, the detection of a single anti-helium or anti-carbon would be a smoking gun for the presence of large amounts of anti-matter in the universe and for the existence of anti-stars and anti-galaxies.

4. Ul t ra high energy species

Particles with energies exceeding ~ 1020 eV are one of the recent mysteries of astrophysics. The nature as well as the source of these species is actually a puzzle. In the last 30 years, experiments such as fly's eye or Agasa have collected ,-, 70 events above 40 EeV 2 and a bunch of ,,, 8 events above 100 EeV. At such high energies, statistics are poor because the incoming fluxes are exceedingly small. Above 50 EeV, the number of particles with energy exceeding E0 is approximatively given by the power law

• (E > E0) -- (14)

(1 EeV 2 (100 particles km -2 yr -1) \ " - ~ o ]

2By definition, 1 E e V = 1018 eV.

Therefore, above 100 EeV, one expects a single event per km 2 and per century.

The sources of the UHE particles are unknown. Potential differences of order ,-~ 102° volts may exist along the jets of powerful blazars. The latter could well be the accelerators of the UHE species. If so, they would be located at cosmological distances and connected to the population of quasars discussed in section 2.1. Another option is an incremental acceleration through gigantic shocks developing in the intergalactic medium. A completely different scenario is provided by the possible decay of topological defects remaining from a false vacuum transition in the early universe. Kinks propagating along cosmic strings have been suggested as the sources of high energy particles. Alternatively, an early phase transition could also produce very massive species at the grand uni- fied scale. These particles would decay in turn into UHE hadrons, neutrinos, charged leptons or gamma rays. Remember that the UHE particles generate an extended air shower when they interact with the Earth atmosphere. Observation of these showers have not allowed so far to determine the nature of the incoming UHE objects. All the known particles are still potentially plausible candidates.

The mystery deepens when the mean free path of these UHE particles is taken into account. Above ,-~ 1015 eV, a photon starts to interact with the CBR and pair produces an electron and a positron. Above ~ 30 EeV, protons photo- produce pions on the CBR through the mechanism P-t-7 + P+~r °. This is the Greisen, Zatsepin and Kuz'min (GZK) cutoff [18] above which the universe becomes opaque to the UHE species. At such high energies, the mean free path does not exceed 100 Mpc. The UHE particles cannot originate more than 100 Mpc away under the penalty of getting their energy degraded through their interactions with the CBR. No plausible source has been identified this close. Note that blazars are located well beyond the GZK path length.

To increase the statistics of UHE particles, a large surface of detection is required. The Obser- vatoire Pierre Auger (OPA) plans to monitor an effective area of 6,000 km 2 with 1,600 automated stations on the ground. Its space competitpr is

P. Salati/Nuclear Physics B (Proc. Suppt) 66 0998) 571-582 581

the Orbiting Wide-angle Light-collectors (OWL), an Earth orbiting experiment. The OWL project is the observation of the atmospheric fluorescence light of the giant air showers produced by the UHE species. Like OPA, the Earth atmosphere is used as a giant calorimeter. OWL follows the development of the cascade in order to determine the nature and the energy of its progenitor. Two satellites should allow for a stereoscopic view of the UHE induced showers. The arrival direction will therefore be determined with an accuracy of ,~ 1 °. While more expensive than OPA, the OWL project should benefit from its orbital location. From space, a vast portion of the Earth surface can be observed. If an area of 106 km 2 is moni- tored, the statistics should reach up to ,-~ 10,000 events each year above 100 EeV.

5. Back to the Moon ?

There are strong arguments in favour of a Moon observatory. To commence, there is no atmosphere to distort the light or to absorb high energy particles. The Moon has no magnetic field so that its surface is not shielded against cosmic rays. The latter can reach directly the ground. Then, the night lasts 350 hours to be compared with a mere 45 minutes for an instrument like the Hubble Space Telescope (HST) in orbit around the Earth. The accumulation time is therefore quite long, allowing for an interesting reduction of the noise when pictures are collected. The Moon surface offers a very stable platform. Maneuvers in orbit, such as the pointing of a telescope to- wards a star, are quite difficult as a result of the lack of gravity. On the Moon, the weight of the instrument itself provides a convenient hold. Fur- thermore, the Moon has no seismic activity. That stability is essential for sensitive interferometers. Finally, the round trip light travel time is about 2.5 seconds, allowing for a convenient monitoring of the instruments from the Earth.

While not insuperable, two serious problems await a Moon mission. First, the cost of sending material and maintaining it there is prohibitive. To reduce the cost, a first step would be an un- manned robotic experiment. In that spirit, the Lunar Ultraviolet Telescope Experiment (LUTE)

is designed to send 2,500 kg for a mere 500 million dollars. Remember that the recent Mars Path Lander project has successfully explored the Mar- tian ground with a robot. Then, any instrument lying on the Moon surface has to withstand a strong radioactivity. Actually, the Moon has no magnetic field or atmosphere to protect against radiation. Sensitive electronic components, such as the charge coupled devices (CCD), are highly vulnerable to the damages induced by the solar flare radiations. The electronics of the detectors can be-shielded but the payload increases. Spe- cial electronics need to be developed such as the charge injection devices (CID) that are highly re- sistant to space radiations. The light falling on a pixel is converted into an electrical charge signal. The charges stay at that pixel and are read out by directly addressing the pixel row and column addresses. CIDs can sustain more than a hundred times the lethal dose for an ordinary CCD. They still work after an exposure to ,,~ l0 T fads.

The future of particle physics in outer space may well lie in observing the gamma ray sky and detecting cosmic rays from a permanent lunar base. A few years ago, the Bush administration suggested that NASA look into putting a man on Mars. One step in this process would be to produce ~4unar station as a testing ground. Assum- ing that this lunar station would happen, many astronomers discussed building observatories on the moon. In their calculations, this would be cost-effective only because the prior infrastruc- ture would already be there. As it happened, it was later decided that manned missions would be too expensive, so the lunar station concept was abandoned, at least for the near future. Such a project could nevertheless be resurrected.

Acknowledgements : PS would like to thank the organizers of the meeting for their kind wel- come as well as for their financial help.

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