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RAGGI COSMICI: MESSAGGERI DELLO SPAZIO

Valerio Formato

Università di Trieste / Istituto Nazionale di Fisica Nucleare

Progetto Flash Forward 2013

(2)

QUANTI TIPI DI FISICA ESISTONO?

(quanti ne volete, se avete abbastanza fantasia...)

Astrofisica {

{

Fisica della materia

{

Fisica delle particelle

elementari

(3)

FISICA DELLE

ASTROPARTICELLE

Astrofisica {

{

Fisica della materia

{

Fisica delle particelle

elementari

(4)
(5)
(6)

Prima di raggiungere la terra viaggiano per circa 6 milioni di

anni!

(7)

SI’ MA COSA SONO?

Protoni (~86%) Nuclei di Elio (~10%)

Elettroni (~1%)

Nuclei piu’ pesanti (~1%)

(8)

SI’ MA COSA SONO?

Protoni (~86%) Nuclei di Elio (~10%)

Elettroni (~1%) Nuclei piu’ pesanti (~1%)

Antimateria (<1%)

(9)

ANTIMATERIA?

1926, Paul A. M. Dirac: “Per ogni tipo di particella elementare ne esiste un’altro perfettamente identico ma con tutte le

cariche invertite.”

(e’ una parafrasi molto ardita!)

Elettrone

Massa: 9.10938291(40)×10−31 kg Carica elettrica: -1

Positrone

Massa: 9.10938291(40)×10−31 kg Carica elettrica: +1

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ANTIMATERIA?

1926, Paul A. M. Dirac: “Per ogni tipo di particella elementare ne esiste un’altro perfettamente identico ma con tutte le

cariche invertite.”

(e’ una parafrasi molto ardita!)

Protone

Massa: 1.672621777(74)×10−27 kg Carica elettrica: +1

Anti-protone

Massa: 1.672621777(74)×10−27 kg Carica elettrica: -1

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PERCHE’ COSI’ POCA?

Viene creata nelle collisioni di raggi cosmici con la materia interstellare

Conosciamo bene il meccanismo (facciamo scontrare particelle negli acceleratori

miliardi e miliardi di volte ogni giorno), solo che e’ un processo molto raro...

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PERCHE’ COSI’ POCA?

Viene creata nelle collisioni di raggi cosmici con la materia interstellare

Conosciamo bene il meccanismo (facciamo scontrare particelle negli acceleratori miliardi e miliardi di volte ogni giorno), solo che e’ un processo molto raro...

O nel decadimento di qualche particella non ancora scoperta!

E siccome l’antimateria creata dai raggi cosmici e’ poca... magari riusciamo a

vederla... :-)

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C’E’ ALTRO?

1.5. ENTERING THE SOLAR SYSTEM: SOLAR MODULATION AND COSMIC RAYS 17

Charge number

Relative abundance

Li Be

B C

N O

Sc Ti

V

Fe

10

-7

10

-6

10

-5

10

-4

10

-3

10

-2

10

-1

1 10 10 2 10 3 10 4 10 5

0 5 10 15 20 25 30

Figure 1.6: Chemical composition of the cosmic radiation as measured at 1AU from the Sun compared to the abundances in the solar system for elements with Z = 1°30. Relative abundances are normalized to the Carbon

abundance. Figure adapted from [80].

reduce the theoretical uncertainties on the predictions [23].

1.4.1 Isotopic abundances

Several significant questions about the galactic confinement of cosmic rays require the obser- vation of the secondary cosmic rays generated by interstellar collisions of the most abundant cosmic-ray species i.e. protons and helium nuclei. Observation of deuterium and

3

He are aimed at determining if helium has the same acceleration and propagation history as heavier nuclei and at making detailed measurements of the energy dependence of the confinement time in the Galaxy.

Deuterium and

3

He are very fragile isotopes and even though they partecipate in the p-p chain reactions in the core of young stars they are almost completely destroyed in the process that lead to the formation of

4

He so that their presence in the cosmic radiation is primarily due to nuclear interactions with the ISM. The main production channel for both deuterium and

3

He is the spallation of primary

4

He nuclei, but for deuterium there is also a contribution from the resonant scattering p

+

p

!

d

+ º+

. The cross-sections for deuterium and

3

He formation are well known and shown in Figure 1.7 while for the last process the cross-section is well measured in the lab frame [52] and shows a sharp peak for an incident proton energy of

ª

600 MeV. This means that most of the deuterium produced by resonant proton scattering will be produced in the energy range between 80-250 MeV. This is an interesting feature of deuterium, since it is the only secondary particle, with the exception of antiprotons, positrons and electrons, which is produced by the propagation of primary protons in the ISM.

1.5 Entering the solar system: solar modulation and cosmic rays

The Sun is the central body and energetic engine of our solar system. Its magnetic field, coupled

with the solar wind (SW), is also the source of the heliosphere, and the cause of the periodic

modulation of the galactic cosmic rays. The Sun represents a fairly typical star in our Galaxy,

classified as G2-V spectral type, mainly composed by hydrogen (

ª

71%), helium (

ª

27%) and traces

of heavier elements, with a radius of r

Ø º

700˙000 km, a mass of m

Ø º

2

·

10

30

Kg, a luminosity of

L

Ø º

3.8

·

10

26

W and an age of t

Ø º

4.6

·

10

9

yrs.

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OK, COME SI FA?

Neutron detector

Anti-coincidence

magnet B

X

Z Y

Proton Antiproton

Scintill. S4 Calorimeter Tracking

system (6 planes)

Geometric acceptance

CAT

CAS CARD

Spectrometer TOF (S1)

TOF (S2)

TOF (S3)

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Nello spazio!

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E POI?

E poi si inizia ad analizzare i dati raccolti dallo strumento...

Dal 2006 fino ad oggi PAMELA ha raccolto circa un miliardo di raggi cosmici, per un totale di circa 20 TB di dati!

Non si possono analizzare uno per uno a mano!

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ALLA FINE...

30 CHAPTER 1. COSMIC RAYS

In particular, unlike charged particles, gamma rays are not deflected by magnetic fields, and thus can potentially provide valuable angular information. For example, point-like sources of dark matter annihilation radiation might appear from high density regions such as the Galactic Cen- ter or dwarf spheroidal galaxies. Furthermore, over galactic distance scales, gamma rays are not attenuated, and thus retain their spectral information. In other words, the spectrum that is measured is the same as the spectrum generated in the dark matter annihilations. The Galactic Center has long been considered to be one of the most promising regions of the sky in which to search for gamma rays from dark matter annihilations. The prospects for this depend, however, on a number of factors including the nature of the WIMP, the distribution of dark matter in the region around the Galactic Center, and our ability to understand the astrophysical backgrounds present signal. If the dark matter density in the inner region of the Milky Way is not particularly high, or if the astrophysical backgrounds turn out to be particularly foreboding, the prospects for identifying dark matter annihilation radiation from the Galactic Center may be quite unfavor- able. In this case, regions of the sky away from the Galactic Center may be more advantageous for dark matter searches. In particular, recent observations of the Isotropic Gamma-Ray Back- ground radiation (IGRB) [77] and future analysis of the galactic center emission [81] from the FERMI-LAT telescope may evidence signals of galactic or extragalactic dark matter.

In addition to gamma rays, WIMP annihilations throughout the galactic halo are expected to create charged cosmic rays, including electrons, positrons, protons and antiprotons. Unlike gamma rays, which travel along straight lines, charged particles move under the influence of the Galactic Magnetic Field, diffusing and steadily losing energy, resulting in a diffuse spectrum at Earth. By studying the spectrum of these particles, it may be possible to identify signatures of dark matter annihilations. In fact, multiple experiments have recently announced results which have been interpreted as possible products of WIMPs.

Recent data from the

PAMELA

experiment about antiprotons and positrons [75, 19] are re- ported in Fig. 1.19 and 1.20. They show a steep increase in the energy spectrum of the positron

Energy (GeV)

0.1 1 10 100

))- (eφ)++ (eφ) / (+ (eφPositron fraction

0.01 0.02

0.1 0.2 0.3 0.4

Muller & Tang 1987 MASS 1989

TS93

HEAT94+95 CAPRICE94 AMS98 HEAT00

Clem & Evenson 2007 PAMELA

kinetic energy [GeV]

10-1 1 10 102

/pp

10-6

10-5

10-4

10-3

BESS 2000 (Y. Asaoka et al.) BESS 1999 (Y. Asaoka et al.) BESS-polar 2004 (K. Abe et al.) CAPRICE 1994 (M. Boezio et al.) CAPRICE 1998 (M. Boezio et al.) HEAT-pbar 2000 (A. S. Beach et al.) PAMELA

Figure 1.19: PAMELA positron fraction compared with other experimental data [75].

Figure 1.20: PAMELA antiproton-proton flux ratio compared with previous measurements and theoretical

calculations for a pure secondary production of antiprotons during the propagation of cosmic rays in

the Galaxy [19].

fraction above 10 GeV up to 100 GeV, where previous hints from TS93 and HEAT are compati- ble with

PAMELA

data, while they show no excess in the ¯p/p energy spectrum, compared with the predicted background. As it will be pointed out in the next section, a careful estimate of the expected antiparticle secondary production is very important in order to identify possible pri-

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ALLA FINE...

(19)

???

Materia oscura?

Pulsar?

Altro?

Ancora non si sa...

ma ogni sorpresa e’

una buona sorpresa!

(20)

???

Materia oscura?

Pulsar?

Altro?

Ancora non si sa...

ma ogni sorpresa e’

una buona sorpresa!

!

(21)

MA...

... le leggi della fisica trattano materia e antimateria allo stesso modo.

Ogni volta che viene creata una particelle viene creata anche la sua antiparticella

(altra parafrasi azzardata)

Pero’ se ci guardiamo intorno ci accorgiamo che il nostro mondo e’ fatto interamente da materia.

E i raggi cosmici ci dicono che anche la nostra galassia e’ fatta

principalmente di materia.

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DOMANDA

(QUESTIONE ANCORA APERTA)

Se materia e antimateria vengono prodotte e “distrutte” allo stesso modo, che fine ha fatto tutta l’antimateria?

(questo e’ uno dei quesiti piu’ grandi rimasti in sospeso riguardo il nostro universo.

Ogni idea e’ ben accetta, anche le vostre!)

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