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DARK MATTER PARTICLES 1 Introduction View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by CERN Document Server DARK MATTER PARTICLES V.Berezinsky INFN, Laboratori Nazionali del Gran Sasso, 67010 Assergi (AQ), Italy and Institute for Nuclear Research, Moscow The baryonic and cold dark matter are reviewed in the context of cosmological mo dels. The theoretical search for the particle candidates is limited by sup ersym- metric extension of the Standard Mo del. Generically in such mo dels there are just two candidates asso ciated with each other: generalized neutralino, which comp o- nents are usual neutralino and axino, and axion which is a partner of axino in sup ermultiplet. The status of these particles as DM candidates is describ ed. 1 Intro duction Presence of dark matter (DM) in the Universe is reliably established. DM in the form of compact microlensing ob jects (machos) are directly observed in the halo of our Galaxy byMACHO, EROS and OGLE collab orations. Rotation curves in our Galaxy and in many other galaxies provide evidence for large halos lled by nonluminous matter. The virial (gravitational) mass of clusters of galaxies is ab out ten times larger than their luminous masses. IRAS and POTENT demonstrate the presence of DM on the largest scale in the Universe. The matter density in the Universe is usually parametrized in terms of 29 2 3 == , where 1:88 10 h g =cm is the critical density and h is the c c 1 1 dimensionless Hubble constant de ned as h = H =(100k m:s :M pc ). Dif- 0 ferent measurements suggest generally 0:4 h 1. The recent measurements of extragalactic Cepheids in Virgo and Coma clusters narrowed this interval to 0:6 h 0:9. However, one should b e cautious ab out the accuracy of this interval due to uncertainties involved in these dicult measuremets. Dark Matter can b e sub divided in baryonic DM, hot DM (HDM) and cold DM (CDM). 1 The density of baryonic matter found from nucleosynthesis is given as 2 0:009 h 0:02: The baryonic cosmological density provided by the mass b clust 3=2 2 h 0:05 and references of intracluster gas is very close to this value, b therein. The structure formation in Universe put strong restrictions to the prop er- ties of DM in Universe. Universe with HDM plus baryonic DM has a wrong prediction for the sp ectrum of uctuations as compared with measurements of COBE, IRAS and CfA. CDM plus baryonic matter can explain the sp ectrum 1 of uctuations if total density 0:3. 0 There is one more form of energy density in the Universe, namely the vacuum energy describ ed by the cosmological constant. The corresp onding 2 energy densityisgiven by ==(3H ). Quasar lensing restricts the vacuum 0 3 energy density: in terms of it is less than 0.7 . Contribution of galactic halos to the total density is estimated as 0:03 0:1 and clusters give 0:3. Inspired mostly by theoretical motivation (horizon problem, atness problem and the b eauty of the in ationary scenar- ios) = 1 is usually assumed. This value is supp orted by IRAS data and 0 POTENT analysis. No observational data signi cantly contradict this value. There are several cosmological mo dels based on the four typ es of DM de- scrib ed ab ove (baryonic DM, HDM, CDM and vacuum energy). These mo dels predict di erent sp ectra of uctuations to b e compared with data of COBE, IRAS, CfA etc. They also pro duce di erent e ects for cluster-cluster corre- lations, velo city disp ersion etc. The simplest and most attractive mo del for a correct description of all these phenomena is the so-called mixed mo del or cold-hot dark matter mo del (CHDM). This mo del is characterized by following parameters: =0; = + + =1; 0 b CDM HDM 1 1 H 50 kms Mpc (h 0:5); 0 : : 0:75 : 0:20 : 0:05; (1) CDM HDM b Thus in the CHDM mo del the central value for the CDM density is given by 2 h =0:19, with uncertaities within 0.1. CDM The b est candidate for the HDM particle is -neutrino. In the CHDM mo del with =0:2 mass of neutrino is m 4:7 eV . This comp onent will not b e discussed further. The most plausible candidate for the CDM particle is probably the neu- tralino (): it is massive, stable (when the neutralino is the lightest sup er- symmeric particle and if R-parity is conserved) and the -annihilation cross- 2 section results in h 0:2 in large areas of the neutralino parameter space. In the light of recent measurements of the Hubble constant the CHDM mo del faces the age problem. The lower limit on the age of Universe t > 13 Gyr 0 (age of globular clusters) imp oses the upp er limit on the Hubble constantin 1 1 Mpc . This value is in slight contradiction the CHDM mo del H < 50 kms 0 with the recent observations of extragalactic Cepheids, which can b e summa- 1 1 rized as H > 60 kms Mpc . However, it is to o early to sp eak ab out a 0 serious con ict taking into account the many uncertainties and the physical p ossibilities (e.g. the Universe can b e lo cally overdense - see the discussion in 4 ref. ). 2 The age problem, if to take it seriously, can b e solved with help of another successful cosmological mo del CDM. This mo del assumes that = 1 is 0 provided by the vacuum energy describ ed by cosmological constant and CDM. Using the limit on cosmological constant < 0:7 and the age of Universe one obtains 0:3 and h< 0:7. Thus this mo del also predicts CDM 2 h 0:15 with uncertainties 0.1. Finally, we shall mention that the CDM CDM with = =0:3 and h =0:8, which ts the observational data, 0 CDM 2 2 also gives h 0:2. Therefore h 0:2 0:1 can b e considered as the value 5 common for most mo dels . We shall analyze here the candidates for CDM which naturally arise from elementary particle physics. The b est known solution for strong CP violation implies axion, which can serve as CDM particle. The sup ersymmetrization of the theory, which includes axion, results in generalized neutralino { a linear combination of ve neutral spin 1/2 particles (wino, bino, two higgsinos and axino, the fermionic partner of axion). This generalized neutralino is most natural candidate for CDM particle. And nally some attention will b e given to the baryonic DM in connection with observations of machos. 2 Machos and Baryonic Dark Matter The total numb er of microlensing events observed in the halo during last two 6 years reached 10. Eight of them are observed by MACHO collab oration and two{byEROS. The duration of lensing e ect is determined by the lens 6 mass. The distribution of observed durations yields the macho mass as +0:30 M =0:46 M . However, this value is mo del dep endent. The most likely 0:17 +0:30 6 halo fraction of machos is f =0:50 . The imp ortant result is observation 0:20 7 of 45 microlensing events in Galactic bulge. For a given rotation curve the heavy bulge implies the lighter halo and thus the fraction of machos increases for a given numb er of observed events. The machos with these masses should b e interpreted as white dwarfs. How- ever, to escap e from the Hubble Deep Field Search these ob jects must b e very 8 faint, two magnitudes fainter than the disc white dwarf sequence . The DM in the halo of our Galaxy is found and most probably it is bary- onic. Could b e all DM in the Universe only baryonic? This question is often 9 ). asked nowdays (e.g. see Let us discuss shortly the problems arising in the baryonic-dominated Universe. nucl 2 Nucleosynthesis requires 0:02h . On the other hand the clusters B cl nucl cl provide 0:2. Therefore, the baryonic density is small ( < ), DM B DM unless h<0:3, which contradicts recent observations. If one arbitrary neglects 3 cl this contradiction, the baryonic dominated Universe with = 0:2 B DM can b e considered. Apart from IRAS data and POTENT analysis which give 1, such mo del faces serious cosmological problems, including the horizon and atness problem and observed sp ectrum of uctuations, which is imp os- sible to explain without CDM and HDM. Probably, these problems could b e solved in some arti cial mo dels with term and vacuum defects (e.g. strings), but at present the corresp onding calculations do not exist. As was mentioned ab ove, the baryonic nature of machos have (or can have) the problems. An interesting idea ab out the nature of machos was recently put 10 forward in ref. These ob jects could b e the neutralino stars, the formations 11 pro duced by neutralinos and baryons around singularities in the distribu- tion of neutralino gas. The neutralino stars are pro duced naturally and they do not meet any problems connected with the Hubble telescop e observations. 12 Unfortunatelly,aswas demonstrated in ref. these ob jects pro duce to o high gamma-ray ux due to annihilation of neutralinos. 3 Axion The axion is generically a light pseudoscalar particle which gives natural and 13 b eautiful solution to the CP violation in the strong interaction (for a review 14 and references see ).
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