(APC – Université Paris Diderot - France)

Ultra-high-energy cosmic-rays

Etienne Parizot (APC – Université Paris Diderot - France)

and the challenge of particle acceleration in the universe

E. Parizot (APC, Paris 7)!Glasgow, 13 Nov. 20012! — IZEST 2012 / UHECRs challenging particle acceleration in the universe —!

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  1785: Coulomb notices the spontaneous discharge of electroscopes

  1895-1900: discovery of the subatomic world: X-rays, electrons, radioactivity…

ionizing radiation !

  1900: Wilson confirms spontaneous discharge of electroscopes in deep underground mines

natural radioactivity (Rutherford)

Cosmic rays timeline (ultra-brief) 3


  1909: Wulf studies electroscopes spontaneous discharge at bottom and top of Eiffel Tower (320 m)

  1910-1911: Pacini studies spontaneous discharge far from Earth crust (lake, sea)

not due to rock radioactivity

  1911-1912: Hess studies spontaneous discharge at different altitudes, with balloon flights up to 5300 m (7 Aug. 2012)

radiation source from above! (+ Gockle, 1909)

anomalously small attenuation of irradiation!



0 km

2 km

4 km

6 km

8 km

10 km

0 20 40 60 80Intensité du rayonnement

Synthèse des mesuresde Hess et de Kolhörster

(1912 - 1914)

radiation intensity

altitude

very penetrating!

  Summary of Hess and Kohlörster observations (1912-1914)

« The result of these observations seems to be explained in the easiest way by assuming that an extremely penetrating radiation enters the atmosphere from above » (V. Hess)



Cosmic rays timeline (ultra-brief)   1912-1929: Millikan believes that “Hess rays” are gamma-rays

gives them the name of “cosmic rays” (1925)

  1927-1929: Experiments with Geiger counters and cloud chambers with magnetic fields show that the particles are charged (Bothe, Kohlörster, Skobeltzyn)

But these are secondary particles (after interaction of primary cosmic rays in the atmosphere)

  1928-1930: flux variation with latitude shows the primaries are charged (Clay, Compton)

  1933: east/west asymmetry shows CRs’ charge is positive (Alvarez & Compton)

  1941: CRs are composed mostly of protons

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Cosmic rays timeline (ultra-brief)   Birth of the science of particle physics

■  Major discoveries ◆  Positron ⇒ antimatter !

◆  Muon

◆  Pions : π 0, π +, π -!◆  Kaons (K)

◆  Lambda (Λ)!

◆  Xi (Ξ)!

◆  Sigma (Σ)!

“strange” particles!

1932

1936

1947

1949

1949

1952

1953

(lifetime is much too long)!

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Key discovery: atmospheric showers   1938: Coincident detection of secondary particles over large areas from the

cascade induced by a single cosmic-ray event (Pierre Auger)

particle shower

atmospheric shower

1 very energetic particle

many secondary particles

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Very high energy cosmic rays   Pierre Auger assesses the existence of cosmic rays of

unconceivably high energy:

  The detection rate decreases rapidly with energy the flux decreases sharply, in E-2.7 or so, but with no evidence for a cutoff…

Lorentz factor: Γ > 106 E > 1015 eV

  2 main reasons for this quest:

  Search for higher and higher energies, with lower and lower fluxes, with larger and larger detectors…

  Try to break through the magnetic mist!   Challenging acceleration processes!

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A wonder of the Physical world!

The cosmic-ray spectrum!

100 MeV

1021 eV

CR flux

Energy

32 o

rder

s of m

agni

tude

12 orders of magnitude

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E. Parizot (APC, Paris 7)!Glasgow, 13 Nov. 20012! — IZEST 2012 / UHECRs challenging particle acceleration in the universe —!100 MeV

1021 eV

~ 1 particle / m2 / second

Energy

Flux

~ 1 particle / m2 / yr

~ 1 particle / m2 / billion years!

The cosmic-ray energy spectrum

32 o

rder

s of m

agni

tude

Out of equilibrium !!!

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The cosmic-ray energy spectrum 12


Georgi Zatsepin! Yakutsk (Sibérie)

Pierre Auger!

58 detectors covering 12 km2

Cherenkov tanks (water), 12 km2

Haverah Park (UK)

The quest for ultra-high-energy CRs 13


John Linsley! Volcano Ranch (New Mexico)

1962: a cosmic ray with E ≥ 1020 eV !!! Several joules = macroscopic energy !

Lorentz factor of 1011 v ≈ 0,99999999999999999999995 × c

1 second 3500 years 1.5 m d(Earth,Sun)



  A 1020 eV atmospheric shower yields ~100 billion particles in the atmosphere!

  By a clear, moonless night, one can detect the induced fluorescence light!

15 october 1993: 3.2×1020 eV !!!!

keeps up the dream of a “cosmic-ray astronomy”!

Fly’s Eye, puis HiRes (Utah)



Which cosmic-ray sources behind the magnetic mist?

  As charged particles, cosmic rays are deflected by magnetic fields

  Larmor radius: rL = E/qBc

Proton

B = 3 µG rL ~ 1/3 pc E = 1015 eV

rL << size of the Galaxy isotropization

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Which cosmic-ray sources behind the magnetic mist?

  No astronomy with cosmic rays sources are still not known!

≠ Source position is known Source position is unknown

Next slide: high resolution image of the sky seen in cosmic rays…

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Breaking the magnetic mist at high E ?

  Larmor radius ∝ E

  Protons with E >> 1018 eV are not confined in the Galaxy

Proton

B = 1 nG rL ~ 100 Mpc E = 1020 eV

rL >> size of the Galaxy pointing astronomy?

increasing energy

larger than the horizon scale!

???

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proton γ-ray photon

e+

e-

UHE proton as seen in the “cosmic frame”

The “GZK effect”   Major prediction by Greisen (1966) and Zatsepin &Kuz’min

(1966) a few weeks after the discovery of the CMB

+ CMB photon

very low energy (T = 2.7 K)

as seen in the “proton rest frame” + or π

  In the proton rest frame, the gamma-ray loses energy to produce the secondary particles

  In the “cosmic frame”, the UHE proton loses energy!

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10-6

10-5

0,0001

0,001

0,01

0,1

1

106 107 108 109 1010

σκ

(mba

rn)

Eγ (en eV)

production de pions

production de paires e+/e-

[cross section] x [inelasticity]

Pion production

e+/e- pair production

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e+e–

π

interaction length

attenuation length

Proton attenuation length 22


0,2

0,4

0,6

0,8

1

10 100 1000

P(d<D)

D(Mpc)

Protons

19.2

19.4

19.6

19.8

20.020.2

20.4

Proton horizons   For different energies, the plot shows the fraction of protons

(ordinate) coming from a distance smaller than the abscissa

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GZK cut-off for nuclei   Photo-dissociation through interactions with CMB photons

in the “nucleus rest frame”

as seen in the “cosmic frame”

+

+

γ-ray photon

CMB photon

energy losses! mass-dependent horizon scale…

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0,2

0,4

0,6

0,8

1

10 100 1000

P(d<D)

D(Mpc)

He

19.2

19.4

19.6

19.8

Helium horizons   For different energies, the plot shows the fraction of He nuclei

(ordinate) coming from a distance smaller than the abscissa

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0,2

0,4

0,6

0,8

1

10 100 1000

P(d<D)

D(Mpc)

CNO

19.2

19.4

19.6

19.8

20.0

“CNO” horizons   For different energies, the plot shows the fraction of C, N or O

nuclei coming from a distance smaller than the abscissa

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10-1

100

101

102

103

104

1019 1020 1021

ProtonHeliumOxygenIron

χ75

(Mpc

)

E (eV)

He OH

Fe

GZK horizons for UHECRs 27


1023

1024

1025

18,4 18,8 19,2 19,6 20 20,4

E3 Φ(E

) (eV

2 m-2

s-1sr

1 )

log10E eV

Pure Proton β=2.3

evolution : (1+z)5

1023

1024

1025

18,4 18,8 19,2 19,6 20 20,4

E3 Φ(E

) (eV

2 m-2

s-1sr

1 )

log10E eV

9 ≤ Z ≤ 11

protons

12 ≤ Z ≤ 19

20 ≤ Z ≤ 26

β = 2.3

Fe only (at sources)Emax= Z × 1020.3 eV

Pure Fe sources Pure proton sources

Fitting the UHECR spectrum 28


UHECR phenomenology

 Modification of the energy spectrum by the GZK effect   Energy-dependent horizon within which the sources must be!

 Modification of the composition

 Modification of the arrival directions:

  Photo-dissociation + magnetic rigidity effects…

  Hopefully limited at UHE, but the deflections depend on particle charge!

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  Energy cut-off confirmed with high statistics!

Main results of the Pierre Auger Observatory

Is it the GZK cut-off (horizon effect) or the end of the acceleration process?

(or both?!)

Can we isolate sources in the sky before the spectrum ends?

Drastic reduction of the flux above ~ 6 1019 eV

(3000 km2 in Argentina)

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  Energy cut-off confirmed with high statistics!

Main results of the Pierre Auger Observatory

Is it the GZK cut-off (horizon effect) or the end of the acceleration process?

(or both?!)

Can we isolate sources in the sky before the spectrum ends?

Drastic reduction of the flux above ~ 6 1019 eV

(3000 km2 in Argentina)

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  Disappointing image of the UHECR sky above 60 EeV

Main results of Auger 32

No obvious accumulation of events in specific arrival directions…!

no source identified! Question still open!


  However, the first evidence for anisotropies has been observed. But not easy to interpret and with moderate significance.

Main results of Auger

Excess of correlation with local matter (~100 Mpc)!

One will need to significantly increase the statistics at the highest energies, where the number of sources within the GZK horizon is very limited, in order to isolate sources in the sky…!

for E ≥ 6 1019 eV!

The deflections are probably large (≥ 10°) !

Major challenge for the coming years!! JEM-EUSO?!

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  Composition features… Main results of Auger

Consistent with large deflections (and weak or no anisotropy)!Transition towards a heavier composition around 1019 eV…!

(But maybe in conflict with other results in Northern hemisphere)!

But relies on extrapolations of hadronic physics models!

constraints for and from high-energy physics!

+ details do not work perfectly well: more muons than predicted!!

cf. LHC results!!

Atmospheric depth of maximum shower development!

AVERAGE! SPREAD!

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UHECRs and high-energy physics - 1

unexplored hadronic physics

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Recent input from LHC results have been implemented to better constrain the models used in atmospheric shower simulations

science in progress…

QGSJET, SYBILL, EPOS…

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Discrepancy between models predictions and observations (number of muons: too many + no feature in energy!)

new physics or new constraints on: -  cross sections -  multiplicity -  rapidity -  etc.


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Experimental challenge: disentangle the muon component from the EM component

under study…

+ independent estimate of the composition

By anisotropy studies? By radio data? By astrophysics?


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  Historically, Cosmic Rays played a key role in High-Energy Physics, giving birth to Particle Physics, and allowing one to explore the physical world at an unprecedented energy scale.

  Then their role decreased because of the poor control on and understanding of the “cosmic beam”, compared to the high-energy beams produced in man-made accelerators…

  Today, UHECRs give access to a new realm of physics, beyond accelerator’s reach. But we still suffer from the poor understanding of the beam (and its extreme rarity!)…

Intermediate comment 39

  Progress in high-energy astrophysics and “astroparticle physics” is a key to progress in high-energy physics, and vice-versa!


  Major question in high-energy astrophysics:

  Energetics arguments indicate a link with supernovæ explosions (~20%-30% of their kinetic power)

Particle acceleration in the universe 40

What is the acceleration mechanism?

Where do the cosmic rays come from?

What are the sources?

(but it could be a coincidence, or it could be an indirect link)

  We do know a mechanism to accelerate particles at the shock wave created by the supersonic (super-Alfvénic) supernova ejecta in the interstellar medium!

Diffusive Shock Acceleration

  We do see energetic particles at the shock supernova fronts!


  X-ray rims from synchrotron emission of TeV electrons in amplified magnetic field

Supernova remnants 41 Chandra (satellite X)

Red 0.95-1.26 keV, Green 1.63-2.26 keV, Blue 4.1-6.1 keV

Tycho (1572)

  TeV gamma-ray emission

broad-band spectrum

π0 decay (hadronic) ?

Inverse Compton scattering (leptonic) ?


Diffusive shock acceleration 42

Chandra (satellite X)


Tycho (1572)

  Supernova explosion (~ 3/century)

supersonic ejecta: V = 104 km/s

  Key aspect of the shock wave = discontinuity in velocity!

super-Alfvénic flow

collisionless shock wave

Vshock



Chandra (satellite X)


Tycho (1572)

  Supernova explosion (~ 3/century)

supersonic ejecta: V = 104 km/s

  Key aspect of the shock wave = discontinuity in velocity!

super-Alfvénic flow

collisionless shock wave

Vshock

+ magnetic turbulence! resonant interaction between energetic particles

and plasma waves



  Reflection off “magnetic walls”

No energy gain, because a B field does not produce any work



elastic bounce unchanged velocity

v

v

  Simple analogy

Tennis ball bouncing off a standing wall



elastic bounce unchanged velocity

v

v

  Simple analogy

Tennis ball bouncing off a standing wall

v V

v + 2V unchanged velocity��with respect to the racket

elastic bounce ball acceleration





V

moving magnetic structure

energy gain!






or energy loss!

( drop shot in tennis!) V





V


energy change

[equivalent to the work of the induced E field…]



  Always head-on interactions across a shock wave! shock front

n1, p1, T1 v1

n2, p2, T2 v2

upstream medium downstream medium

velocity discontinuity: Δv/c

•  In the downstream rest frame, the upstream medium is coming towards the particles that cross the shock

•  In the upstream rest frame, the downstream medium is coming towards the particles that cross the shock



  Always head-on interactions across a shock wave! shock front

n1, p1, T1 v1

n2, p2, T2 v2

upstream medium downstream medium

velocity discontinuity: Δv/c

  Energy gain at each shock crossing!

Balance between exponential energy growth and constant probability of escaping away from the shock (due to the global drift along the flow in the shock rest frame)

compression ratio

universal power law spectrum in E-2 !!


Limitations of shock acceleration 52

  Magnetic turbulence and waves must be present on both sides of the shock

shock front

Vshock

waves resonantly produced upstream by energetic particles themselves tricky!

~ easy downstream (shocked medium)

It works: we do see particle acceleration at collisionless shocks! (supernovæ, extragalactic, interplanetary, etc.)

  important problem for relativistic shocks! Challenging for ultra-high-energy cosmic rays (UHECR)



  Keep the particle inside the accelerator!

Shocks fronts are not infinite planes!

  Key limitation, due to the size of the accelerator The Larmor radius of the particle must be smaller than the size of the accelerator

In fact, diffusion-advection at the shock implies: (“work of an effective induced E field”)!

so-called “Hillas criterion”


“Hillas plot” 54


“Hillas plot” 55



  Hillas criterion not so many candidates for ultra-high-energy cosmic rays (UHECRs)!

  “Optimistic view”:   sources are among the few candidates   the particle acceleration process works

at its maximum possible efficiency   we roughly see the end of the

acceleration spectrum

  “Pessimistic view”:   Adding refinements and taking into account actual conditions will reduce the maximum energy and make the process fail for UHECRs

Optimistic in another way! it just requires other ideas for particle acceleration in the universe!



  Acceleration (energy gain) competes with energy losses!

  The longer the particle stay in the accelerator, the higher the probability to interact with ambient fields or particles

  Problem for large shocks…   Problem for high-power regions…

Can severely challenge the Hillas criterion!

energy losses -  synchrotron radiation -  Inverse Compton scattering -  photo-pion production -  photo-dissociation


New ideas for particle acceleration? 58

  What about wake-field acceleration?

  For instance, gamma-ray bursts are hugely powerful events

See past and coming works by Tajima, Takahashi, Chen, Hillmann, Ebisuzaki…

They emit in a few seconds the total energy radiated by the Sun in 10 billion years! Ultra-relativistic outflows and huge amount of high-energy photons in a small volume (1046 J in a few tens of km… ?)

  In any case, one should think about non linear effects… A new field within astrophysics, very little explored (if at all!)…

  Short timescale acceleration can we avoid losses?

Possible connections with iZEST community…


Other possible connections 59

  Exploration of high-energy physics

  Exploring fundamental physics at 1020 eV

Hadronic physics from UHECR interactions in the atmosphere (shower physics, cross sections, etc.)

Highest-energy particles in the universe can we use them as the cosmic rays were used in the first half of the XXth century to discover new structures and new physics?

  Lorentz Invariance Violation… (“predicted” by most quantum gravity theories…)

  Exploring space-time structure… UHECRs propagate in space-time at an unexplored energy scale may feel small-scale structures


Lorentz Invariance Violation (LIV) 60

  Cf. talk by Professor Tajima this morning

Different propagation timescales for the different photon energy

Constraints from astrophysical observations of the energy/time structures in the light curves of distance sources

Or studies with “infinitesimal” timescales on “human-scale” distance

(Abdo, et al, 2009)

IZEST?


LIV and the GZK cut-off 61

  Lorentz Invariance Violation and the GZK cut-off

The GZK cut-off in the UHECR spectrum is due to energy losses from the interactions between UHECRs and CMB photons

Interaction cross sections:

  Ingredients: well known and measured at the relevant energies

Photon energy distribution: very well-known in the cosmic frame: CMB black body spectrum!

  But the calculation assumes that we know how to make a Lorentz transform with a Lorentz factor of 1011 !

  The energy of the photons may not transform as we think! That would change the effective energy threshold for pion production, and thus the energy scale of the cut-off!


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  Different particles may have different “maximum attainable velocities”!

LIV and the GZK cut-off

Violation of Lorentz Invariance:

Will be modified if c is different for the protons and the pions (simple kinematics!)

  Threshold and elasticity of photo-pion production


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  Energy losses and GZK horizon will thus be different from those calculated in the standard case…


Proton attenuation length

(Stecker-Scully 2009)

Proton energy

no LIV

with LIV


64



UHECR flux (Stecker-Scully 2009)

UHECR energy

no LIV

with LIV

Flux recovery at ultra-high energy!


65



UHECR flux (Stecker-Scully 2009)

Look for larger statistics at higher energy!

no LIV

with LIV

Flux recovery at ultra-high energy!

Current limit


Perspective 66

  Go into space to increase the statistics at UHE energy

  Momentum is building up! 2017 ?

  JEM-EUSO!

Large field-of-view UV telescope on the Kibo module of the International Space Station

Observe 200 000 km2 at once!


Perspective 67

  UHECRs offer an interesting way to explore high-energy physics and fundamental physics at the highest energies known

  The acceleration of particles in the universe is challenging and not well understood

new ideas are welcome!

  There are extreme environments in the universe where non linear electromagnetic effects might be important must be studied !

  This moment is timely for explorative interactions between the IZEST community and the high-energy astrophysics and astroparticle physics communities

(new instruments and capabilities under view)

Documents

(APC – Université Paris Diderot - France)