Chardonnet, P.

Orloff, Jean

Salati, P.

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The production of anti-matter in our galactic backyardJean Orloff, P. Chardonnet, P. SalatiTo cite this version:Jean Orloff, P. Chardonnet, P. Salati. The production of anti-matter in our galactic backyard.Rencontres de blois : Matter - Anti-matter Asymmetry 14, Jun 2002, Chateau De Blois, France.pp.1-4, 2002. <in2p3-00021537>HAL Id: in2p3-00021537http://hal.in2p3.fr/in2p3-00021537Submitted on 20 Feb 2003HAL is a multi-disciplinary open accessarchive for the deposit and dissemination of sci-entific research documents, whether they are pub-lished or not. The documents may come fromteaching and research institutions in France orabroad, or from public or private research centers.L’archive ouverte pluridisciplinaire HAL, estdestine´e au de´poˆt et a` la diffusion de documentsscientifiques de niveau recherche, publie´s ou non,e´manant des e´tablissements d’enseignement et derecherche franc¸ais ou e´trangers, des laboratoirespublics ou prive´s.THE PRODUCTION OF ANTI-MATTER IN OUR GALACTIC BACKYARDJ. ORLOFFa, P. CHARDONNETb AND P. SALATIba Laboratoire de Physique Corpusculaire, Universite´ Blaise PascalRue des Landais, F-63177 Aubie`re Cedex, Franceb Laboratoire d’Annecy-le-Vieux de Physique The´orique,Chemin de Bellevue B.P.110, F-74941 Annecy-le-Vieux Cedex, FranceWe evaluate the production of anti-deuteron and anti-3He in p-p collisions within a simplecoalescence model, whose validity is checked against existing collider data. We then deducethe minimal fluxes of these species that should be present in cosmic rays due to collisions ofprimary protons on interstellar gas. Thanks to its increased sensitivity, the AMS experimentshould detect a few of these “standard” anti-deuterons.If we ask ourselves what would constitute the most convincing evidence for the existence ofan anti-star, it is clearly the discovery of heavy anti-nuclei coming from space, e.g. in cosmicrays. Indeed, anti-star cores are presumably the only foundries where anti-nuclei can surviveand pile-up into large aggregates. However, it is less clear how many anti-nuclei should we re-quire to see before being convinced, and how heavy should they be. It is generally believed thatthe observation of a single anti-helium or anti-carbon would undoubtedly signal the presence ofstars made of anti-matter. Such a discovery would be of paramount importance as regards theexistence of a baryon symmetry in the universe and has therefore strong cosmological implica-tions. That is why it is crucial to ascertain that cosmic rays do not already contain detectabletraces of anti-nuclei which could have been directly manufactured in our galaxy. We know forinstance that a fraction p¯/p ≈ 2× 10−4 of anti-protons is produced by the spallation of cosmicray protons on the interstellar gas of the galactic disk. We have computed the abundance ofanti-deuterium D¯ and anti-helium 3H¯e produced through the interaction of high-energy protonswith the interstellar hydrogen1. That calculation requires two ingredients.First, we need to evaluate the production cross section of anti-nuclei during the interactionof a high-energy proton with a proton at rest. Let us concentrate on the case of anti-deuteronswhich requires the formation of both an anti-proton and an anti-neutron. The invariant crosssection for the production of anti-protons is experimentally well known and is reasonably fittedby the Tan and Ng’s parameterization6. Neglecting the breaking of isospin symmetry, the anti-neutron production cross section is equal to its anti-proton counterpart. The production of twoanti-nucleons is then proportional to the square of the production of one of them, a hypothesiswhich is reasonably well established at high energies. However, at lower energies, factorizationhas to break down, if only to respect the kinematic constraints that thresholds for one and twopair production are different. We have crudely taken this into account by assuming in additionthat the center of mass energy available for the production of the second anti-nucleon is reducedby twice the energy carried away by the first anti-nucleon. Once the anti-proton and the anti-neutron are formed, they merge together to give an anti-deuteron if the momentum of thecorresponding two-body reduced system is less than some critical value Pcoal. That coalescencemomentum is the only free parameter of our factorization and coalescence model.Data Ref. pl(GeV) pt(GeV)√s(GeV) ED¯d3σD¯dk3(mb/GeV2)1 5 1.44 1.14 11.5 9.1± 1.6× 10−82 5 1.09 1.5 11.5 2.2± 0.8× 10−83 3 4.8 0.16 53 6.8± 2.7× 10−54 3 5.6 0.21 53 5.6± 1.9× 10−55 3 6.6 0.3 53 1.4± 0.6× 10−56 4 0 0.2→ 0.7 53 1.18± 0.4× 10−3×e−(2.6±0.5)ptTable 1: Kinematical regimes where the measured cross-section for production of anti-deuteron has been used toconstrain Pcoal in figure 1:PSfrag replacements0204060801001 2 3 4 5 6Pcoal[MeV]Pcoal bindingPcoal chosenFigure 1: This figure displays various experimental constraints on the coalescence momentum Pcoal, the only freeparameter of the model discussed in the text. Points 1 and 2 are from Serpukhov with√s= 11.5 GeV while allthe other data have been collected at the ISR at√s = 53 GeV. A coalescence momentum Pcoal of order 60 MeVprovides a reasonable fit of all the points but the last one. This value lies reassuringly close of the characteristicmomentum derived from the binding energy of deuteron.Experimental results 3,4,5 have been summarized in terms of Pcoal on figure 1. Given thecrudeness of our model and the wide range of kinematic regimes explored experimentally, itis quite comforting that all these data are compatible within 2 standard deviations with ourpredictions from our single parameter model. The first five data points are all fitted with acoalescence momentum of order 60 MeV. Because the Serpukhov data (points 1 and 2) corre-spond to a low center of mass energy, a kinematic regime where anti-deuteron production isastrophysically predominant, we have decided to fix the value of Pcoal at 58 MeV. Increasingthis value to 75 MeV would simply double the production of anti-deuterium.Stars sow the interstellar medium with their own matter. That processed material mostlycontains hydrogen, helium and, at a lesser level, some carbon, nitrogen and oxygen, i.e., theCNO elements. Then supernovae driven shocks sweep the interstellar gas and accelerate nu-clei to generate the cosmic rays. The sources of the latter are localized in the galactic planeand correspond to supernovae remnants and pulsars. Primary cosmic rays, such as CNO, arethose species already present in space before being accelerated. They subsequently diffuse inthe galactic ridge during ≈ 10 million years where they undergo spallation reactions with theinterstellar medium. The spallation of CNO generates a non negligible flux of lithium, berylliumand boron nuclei (LiBeB) which are orders of magnitude more abundant in cosmic rays than instars. A single value for the column density through which the primary CNO nuclei travel andfragmentise is enough to account for the observed abundances of the secondary LiBeB species.That observation strongly supports the diffusion and spallation scheme according to which thegalactic plane behaves merely as a leaky box. That simple model needs however to be slightlyrefined. Among the secondaries, the 10Be nucleus is unstable with a half-lifetime of 1.6 millionyears. It plays the role of a chronometer which measures the time actually spent by cosmic raysfrom their production to their escape in the intergalactic medium. Its abundance with respectto its stable partner 9Be has been found smaller than expected, hence a residence time in theoverall galaxy of about 100 million years, ten times larger than the diffusion time in the ridgealone. This strongly suggests the existence of thick confinement layers that extend above andbeneath the galactic plane. The naive leaky box has therefore to be refined into an axisymmetrictwo-zone diffusion model7 which we have used in estimating the abundance of anti-deuteriumand anti-tritium.PSfrag replacements10−810−710−610−510−41 10Pcoal [MeV]Momentum per nucleon [GeV]p¯/pD¯/p× 104(ansatz 1)D¯/p× 104(ansatz 2)3H¯e/p× 108(ansatz 1)3H¯e/p× 108(ansatz 2)Figure 2: The fluxes of cosmic ray anti-protons and of anti-deuterium and anti-tritium nuclei, relative to theproton flux, are presented as a function of the momentum per nucleon. To fit on the same diagram, the curveshave been scaled by a factor of 104 for D¯ and of 108 for 3H¯e. The doubling of curves corresponds to differentfactorization schemes, and their closeness supports the factorization hypothesis in the relevant kinematic regime.The flux of cosmic ray anti-protons relative to the flux of protons is plotted (solid line) infigure 2 as a function of the momentum of the particles. The curve drops sharply below a fewGeV. At higher energies, it exhibits a plateau where the ratio p¯/p reaches a value of ≈ 2×10−4,in good agreement with the previous calculation by Gaisser and Schaefer8. The D¯/p and 3H¯e/pcases are also presented on the same plot. The energy behaviour is fairly the same as for theanti-protons. However, the magnitude of the effect is significantly suppressed. We find that theD¯/p ratio exceeds 10−9 above a momentum per anti-nucleon of about 4 GeV/c. It reaches amaximum of 6× 10−9 for a momentum of ≈ 20 GeV/c. During the space station borne stage ofthe experiment, the AMS collaboration2 should measure a flux of anti-nucleons with a sensitivityreaching a level of ≈ 10−9 relative to cosmic ray protons. The corresponding energy per nucleonranges from a GeV up to 20 GeV.We therefore conclude that AMS should detect a few cosmic ray anti-deuterons. Time is ripeto estimate the theoretical error on this prediction. A step in this direction has very recentlybeen made9. This estimate is specially important in view of both the upcoming experimentalmeasurements, and the exotic signals from the annihilation of supersymmetric dark matter10,11,12or the evaporation of black holes13,14 these measurements might uncover.The 3H¯e/p abundance does not exceed ≈ 4 × 10−13. Even allowing for a generous errorof 2 in our estimates, we conclude that the anti-helium nuclei that are manufactured in ourgalaxy will not be detected by AMS. Alternatively, if anti-matter is processed in anti-stars, theabundance of its various components should be quite similar to the conventional stellar yields.It would follow therefore the same relative proportions as in our own interstellar material. Inany case, the detection of a single anti-helium or anti-carbon by the AMS collaboration wouldbe a smoking gun for the presence of large amounts of anti-matter in the universe and for theexistence of anti-stars and of anti-galaxies.References1. P. Chardonnet, J. Orloff and P. Salati, The production of anti-matter in our galaxy, Phys.Lett. B409, 313 (1997).2. S. Ahlen et al., An Antimatter Spectrometer in Space, Nucl. Instrum. Meth. A350, 351(1994).3. M. G. Albrow et al., Search for Stable Particles of Charge ≥ 1 and Mass ≥ DeuteronMass, Nucl. Phys. B97, 189–200 (1975).4. W. M. Gibson et al., Production of Deuterons and Anti-deuterons in Proton-proton Col-lisions at the Cern ISR, Lett. Nuovo Cim. 21(6), 189–194 (1978).5. V. V. Abramov et al., Production of Deuterons and Anti-deuterons with Large p⊥ in ppand pA Collisions at 70 GeV, Sov. J. Nucl. Phys. 45(5), 845–851 (1987).6. L. C. Tan and L. K. Ng, Parametrization of p¯ Invariant Cross Section in p− p CollisionsUsing a New Scaling Variable, Phys. Rev. D26(5), 1179–1182 (1982).7. W. R. Webber, M. A. Lee and M. Gupta, Propagation of Cosmic-ray Nuclei in a DiffusingGalaxy with Convective Halo and Thin Matter Disk, Ap.J. 390, 96–104 (1992).8. T. K. Gaisser and R. K. Schaefer, Cosmic-ray Secondary Antiprotons : a Closer Look,Ap.J. 394, 174–183 (1992).9. R. P. Duperray, K. V. Protasov and A. Y. Voronin, Antideuteron production in protonproton and proton nucleus collisions, arXiv:nucl-th/0209078.10. F. Donato, these proceedings.11. F. Donato, N. Fornengo and P. Salati, Antideuterons as a signature of supersymmetricdark matter, Phys. Rev. D 62, 043003 (2000) [arXiv:hep-ph/9904481].12. E. A. Baltz and P. Gondolo, Improved constraints on supersymmetric dark matter frommuon g-2, arXiv:astro-ph/0207673.13. A. Barreau, these proceedings.14. A. Barrau, G. Boudoul, F. Donato, D. Maurin, P. Salati, I. Stefanon and R. Taillet,Antideuterons as a probe of primordial black holes, arXiv:astro-ph/0207395.

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