Ultra High Energy Cosmic Rays: a chance for new physics

G. MieleUniversity of Naples29th November 2002

References used to realize the present talk:

• F. Halzen talk at NEUPAST (sept. 2002)

• F. Halzen, astro-ph/9810368

• T.W. Jones, astro-ph/0210477

• G. Sigl, astro-ph/0210049

High energy cosmic rays particles are shielded by Earth’s atmosphere. They reveal their existence by indirect effects:

i) Ionizationii) showers of secondary charged particles covering

areas up to many km2

After almost 90 years of CR research their origin is still an open question

For kinetic energy < 100 MeV

The solar winds shields proton coming from outside the solar system. The sun provides the proton flux.

Above this energy CR spectrum is approximated bya broken power laws ∂ E-g

Cosmic Cosmic RayRay

SpectrumSpectrum

g=2.8

4·1015 eV

g=3.0

5·1018 eV

g=2.7

The bulk of CR’s, up to ~ 4 · 1015 eV is believed to be of galactic origin. The change of slope (E-2.7 ÆE-3.0) is interpreted as a transition to a steeper galactic component.

At energies ~ 5 · 1018

eV the slope changes again (E-2.8) for a softer extragalactic component.

The The ““kneeknee”” and the and the ““ankleankle”” featuresfeatures are are commonly commonly viewed as corresponding to transition either viewed as corresponding to transition either in the nature in the nature of CR of CR accelerator accelerator or in the or in the propagation propagation of of CRsCRs

The The gyroradius gyroradius of a of a particle with charge particle with charge ZZ and and energy energy EEEeVEeV=E/=E/EeVEeV in a in a magnetic field magnetic field BBmmGG isis

Rg ª (EEeV/Z BmG) kpc 1 EeV = 1018 eV

Since the galactic magnetic field is few mG, confinement of CRs within the galaxy is limited to energy below 1 EeV

Over the last few years several giant air showers have been detected, measuring the secondary shower particles directly in water tanks or scintillation counters and in fluorescence telescopes detecting the Nitrogen emission induced by the shower

• Surface Detector (SD) (AGASA)

• Fluorescence Detector (FD) (HiRes)Auger

Even if the existence of few hundreds EeV (~ 50 Joule) events is still an open question, several groups have seen them!

Waiting for Auger

TThe Akeno Giant Air Shower Array (AGASA)he Akeno Giant Air Shower Array (AGASA)

AGASA covers an area of about 100 km2 and consists of 111 detectors on the ground (surface detectors) and 27 detectors under absorbers (m-detectors).

TThe he High High Resolution Fly’s eye Resolution Fly’s eye ((HiResHiRes))

Every clear, moonless night the atmosphere over a 3000 Km2 ground area is monitored from two sites. The passage of a cosmic ray shower causes the atmosphere (actually the nitrogen) to fluoresce, and the faint near-UV light is detected by arrays of large mirrors equipped with fast photomultiplier cameras.

UHECR UHECR flux flux as measured as measured by HiResby HiRes--I and I and HiResHiRes--II II detectors detectors and AGASA and AGASA experimentexperiment

Arrival directions of cosmic rays with energies above 4 x 1019eV. Red squares and green circles represent cosmic rays with energies of > 1020eV , and (4 - 10) x 1019eV , respectively. (DJ < 10)

What is the nature and the origin of these UHECRs?

• Bottom – up acceleration mechanisms:

Astrophysical environments able to accelerate particles at least up to ZeV (1021 eV)

• Top –down model:

Massive relic particles or cosmological defects of primordial origin decaying in UHECRs

Bottom Bottom –– up acceleration mechanismsup acceleration mechanisms

Two main features:

• The Acceleration mechanisms of UHE charged particles

• The Propagation in the cosmo of UHE particles

Propagation Propagation

The most frequently discussed constraint on the origin of UHECRs comes from their large energy loss rates

Nucleons:

• p + gb ö p + e- + e+ proton pair production

The proton energy threshold for this reaction is E± = 103/Eg(eV)GeV. For CMB Eg(eV)~ 10-3 eV ï E± = 0.01 EeV

• N + gb ö N + n p photo-production of single or multiple pions

The nucleon threshold energy for single pion production on the CMB photon is Eth = mp (mN + mp/2)/ Eg ï Eth@ 50 EeV

• n ö p + e- + ne neutron decay

No threshold. A 1 EeV neutron travels 30 Kpc before decaying

__

Greisen Zatsepin Kuz’min

Photons:• g + gb ö e- + e+ pair production

At energies below 1014 eV photons interact mainly with universal IR/O backgrounds. Between 1014 eV and 100 EeV with the CMB, while above ~ 100 EeV the target is the universal radio background (URB).

Electrons:

• e + gb ö e + g inverse Compton scattering

The leading electron (positron) transfers almost all its initial energy to the photon background via inverse Compton scattering. Electrons, differently from protons, are also subject to synchrotron losses in large extragalactic magnetic fields.

Neutrinos:

• n + nb ö x + x neutrino-antineutrino annihilation

This process occurs via W or Z exchange. Neutrino interactions become especially relevant if the relic neutrinos mave masses mn ~ eV (HDM). Z resonance at Eres= 4 · 1021 (eV/ mn) eV. Big drawback:enormous primary neutrino fluxes needed

__ __

How to make a Cosmic Zevatron

Accelerating a particle to ZeV energies is not easy!

The simplest constraint: The accelerated particles must be contained in the accelerator while they are being accelerated. The gyroradius (Rg) < the accelerator size (R)

RBeZE

BeZcpRg <==

E < Z e B R = (0.9 Z BG Rpc) ZeV

Magnetic field cannot accelerate charged particles; only electric fields can do that. Thus if an induction mechanism is at work the above relation becomes

RBeZE accβ<

Where bacc= vacc/c is a characteristic large scale speed

The “The “HillasHillas plotplot” ” showsshowsthe minimum the minimum magnetic magnetic field required to field required to accelerate accelerate particles particles of of charge charge ZeZe to to 100 100 EeVEeV. . Successful accelerators Successful accelerators must fall above must fall above the linethe line

The energetic associated to UHECR phenomenology

Remarkably

The flux at 100 EeV translates into a local energydensity of about 3 · 10-22 J m-3. Taking the loss time as 3 · 108 yr and assuming a steady state, the mean luminosity of UHECRs per unit volume must be about 10-37 W m-3 or about 3 · 1044 erg Mpc-3 yr-1.

This number is comparable with the same estimate for Gamma Ray Bursts.

A fraction of one moderate AGN inside 100 Mpc.

UHECRs & GRBs have the same origin?

Nature’s Particle Accelerators

• Electromagnetic Processes:

- Synchrotron Emission

• Eg ∂ (Ee/me c2)3 B

- Inverse Compton Scattering

• Eg ∂ (Ee/me c2)2 E0g

- Bremmstrahlung

• Eg ~ 0.5 Ee

• Hadronic Cascades

- p + g Ø p≤ + p0 + … Ø e≤ + n + g + …

- p + p Ø p≤ + p0 + … Ø e≤ + n + g + …

AGN with accretion disk and a pair of jets. The galaxy is powered by a central super-massive balck hole (~ 109 MŸ). Particles, accelerated in shocks in the disk or the jets, interact with the high density of ambient photons (~ 1014 cm-3)

Top – Down Scenarios

Relic Density – simple approach

<sann v>

increasing

Neq

X=m/T

Decoupling occurs when

G < H

G= <sann v> Nc

Neq = g c (m T/2 p)3/2 e-m/T

H = 1.66 g*1/2 T2/MP

Violating the Lorentz Invariance

Trying to evade GZK cut off, let us change the dispersion relation.

2

432222 2

PP ME

MEEdmpE ζξ −−−≅−−

( )ε

ππ

422

0,N

thmmmp +

=GZK for photo-pion production. The threshold:

010,

3

0,

4

0,

=−

+

−

−

th

th

th

th

th

th

pp

pp

pp αβbecomes with a(x) and b(z)

For some values of x and z the threshold is never reached

Neutrino Astronomy can solve the controversy!

Cosmogenic neutrino flux per flavour (thick line) produced by primary proton flux from The AGASA and HiRes data. The UHECR sources are assumed to inject a proton spectrum ∂ E-2 up to 1022 eV with luminosity ∂(1 + z)3 up to z = 2.

Predictions for a top-down model with m = 2 1014 GeV

Neutrino Telescope

ANTARES

High Energy Neutrino Physics

Ofelia Pisanti – Dipartimento di Scienze Fisiche and INFN – Napoli (Italy)

The Pierre Auger Giant Array Observatory

• 3000 km2 area at an altitude of ~ 1300 m a.s.l (Mendoza, Argentina);

• 1600 Čerenkov light detectors with a 1.5 km spacing;

• 13000 photomultipliers for 24 fluorescence telescopes located in 4 sites (duty cycle of 10%);

• 40 Čerenkov light detectors and 2 fluorescence telescopes in the Engineering Array (actually in data taking);

• ν acceptance of ~ 15 km3 sr water equivalent;

• 3000 events yr-1 expected with energies above 1019 eV and 30 events yr-1 above 1020 eV;

• more than 250 physicists from 19 countries.

Auger numbers

Neutrinos as universe messengers

• observation of UHE ν induced showers very important for getting information on cosmic ray sources in the universe and on interactions of ν with the matter in presently not accessible ranges;

• neutrino initiated showers have in principle different signatures from thehadronic ones. But on more quantitative basis we need simulations.

Auger Collaboration meeting – Malargue – 14/11/2002

• ν and ν show similar behaviours;

_

• νe showers have a more reach e+e- component with respect to proton showers, implying that the rise in the number of charged particles stops later;

• in νµ showers the outgoing lepton carries away a variable fraction of (hidden) energy.

Charged

p

νe

νµ

νe_

_νµ

Eprimary=1015 eV

Auger Collaboration meeting – Malargue – 14/11/2002

The actual aspect depends on the fraction of neutrino energy carried by the outgoing lepton produced in the FI.

Single showers

El = .99 EνEl = .02 Eν

Auger Collaboration meeting – Malargue – 14/11/2002

Muons

νe

p

νµ

Eprimary=1015 eV

The number of muons-antimuons

is larger in a p than a νe or a νµ

shower

Auger Collaboration meeting – Malargue – 14/11/2002

The threshold for νe shower detection by fluorescence is lower than for p showers

Edep

p

νe

νµ

Eprimary=1015 eVThe e.m. energy deposition is larger in a νethan a p or a νµshower

Conclusions

• UHECRs, if they exist, represent a great challenge for astro-particle physics

• New ideas have been proposed both on the astrophysical and elementary particle physics sides

• New experiments (Auger) are going to solve the controversy

• Neutrino astronomy will represent the new frontiere of astroparticle