Ultra high energy cosmic rays

Donna KubikSpring, 2005

Cosmic accelerators• Thanks to Angela Olinto for

invaluable references and toJeff Wilkes for getting me intoall this in the first place!

Injection, acceleration, propagationin SN1987A

References1. “Ultra High Energy Cosmic Rays: The theoretical challenge”, A. V.

Olinto, Phys.Rept.333 (2000) 329-348astro-ph/0002006

2. “Origin and Propagation of Extremely High Energy Cosmic Rays”,Pijushpani Bhattacharjee and Guenter Sigl, Phys.Rept. 327(2000) 109-247astro-ph/9811011

3. Introduction to Ultrahigh Energy Cosmic Ray Physics, PierreSokolsky, Addison-Wesley, 1989

4. High Energy Cosmic Rays, Todor Stanev, Springer, 2004

Cosmic accelerators• Notice the similar “geometry”

of cosmic ray accelerators, likeSN1987A, and the manmadeaccelerator at Fermilab ;) !

Injection, acceleration, propagationin SN1987A

Injection, acceleration, propagation in a manmade accelerator

Cosmic accelerators• Energy spectra of cosmic rays

have been shaped by acombination of injection,acceleration and propagation

Injection, acceleration, propagationin SN1987A

Injection, acceleration, propagation in a manmade accelerator

Cosmic accelerators• These processes are modified

by energy losses, collisions,and perhaps leakage from thegalaxy, and they all depend onparticle energies.

Synchrotron loss in manmade accelerators

Synchrotron emission from high energyelectrons in M87

Cosmic ray detection• The cosmic ray spectrum has

been measured over a widerange of energy

• The method of CR detectiondepends on the energy of theparticles

Direct detection• <0.1x1015 eV

– Flux is high enough fordirect detection

• Calorimeters, emulsionstacks, TR detectors inballoons, satellites,space station

• A collision between ahigh energy cosmic rayand an atom inphotographic emulsionviewed through amicroscope

Indirect detection• > 0.1x1015 eV

– Flux is too low for directdetection

• Extensive air showers(EAS)

– Ground arrays

– Atmospheric light» Cherenkov» Nitrogen

scintillation(UV)

EAS• The atmosphere above the

area of the EAS array servesas a natural transducer andamplifier for the detector.

• The primary energetic cosmicray collides with a nucleus inthe air, creating manysecondary particles whichshare the original energy.

EAS• The secondary particles also

collide with nuclei in the air,creating a new generation ofstill more particles thatcontinue the process.

• This cascade, called an"extensive air shower”, arrivesat ground level with billions ofenergetic particles that can bedetected over approximately10 square kilometers.

Auger Observatory• Above 1019 eV, the arrival rate

is only 1 particle per squarekilometer per year.

• Cosmic rays with energiesabove 1020 eV have anestimated arrival rate of only1/km2 per century!

• Need huge area

• i.e. Auger Observatorydetection area of 3000 squarekilometers Click on image to see EAS animation of

Pierre Auger Cosmic Ray Observatory Array(Must be online to view animation)

Animation from Auger website http://www.auger.org/photos/AnimGif.html

Composition• The particle identity is

determined by the muoncontent of the shower inground arrays and the depth ofshower max in fluorescencedetectors

• But this is difficult analysis athigh energies

• Need more data

The cosmic ray spectrum Part 1• < 1 GeV

– Temporally correlated withsolar activity – solar origin

• 1 GeV to Knee– Galactic origin (SNR

shocks)

• Knee to Ankle– Larger shocks (galactic

winds) and SN explosionsinto stellar winds

Graph from [2]

The cosmic ray spectrum Part 2• Ankle (>1019.5)

– Drastic change is slope

– Crossover from Galactic toextragalactic origin?

– 100s of events >1019 eV

– 20 events above 1020 eV• AGASA• Fly’s Eye• HIRES

– Highest-energy event is3.2x1020 eV

Graph from [2]

GZK cutoff Part 1• The many events that have been

observed above 7 x 1019 eV withno sign of the Greisen-Zatsepin-Kuzmin (GZK) cutoff was notexpected

• If the Ultra High Energy CosmicRays (UHECRs) are protons,nuclei, or photons fromextragalactic sources, an energycutoff should be present

• Protons with an initial E > 1020 eVfrom >100 Mpc should appear at alower energy due to energy loss tothe CMB.

Graph from [2]

GZK cutoff Part 2• The CMB has a blackbody spectrum with T=2.7 K corresponding to

mean energy of 6.34x10-4 eV [4]. A proton of high enough energy(> 7x1019 eV) will interact inelastically with CMB photons producingpions via

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GZK cutoff Part 3• With each collision, the proton would lose roughly 20% of its energy

• This only happens for cosmic rays that have at least 6 x 1019 eV ofenergy, and this is the predicted GZK cutoff.

• So if cosmic rays were given an initial energy greater than that, theywould lose energy in repeated collisions with the cosmic microwavebackground until their energy fell below this cutoff.

• However, if the source of the cosmic ray is close enough, then it willnot have made very many collisions with microwave photons, and itsenergy could be greater than the GZK cutoff.

• This distance is about 150 million light years.

Calculation of the required UHECRproton energy for pion photoproduction

• The approach is to calculate theproton energy, Ep, required forpion photoproduction usingconservation of 4-momenta, P.

• Considering the left hand side inthe lab frame and the right handside in the center-of-mass frame,where– Ep = UHECR proton energy

(the unknown)– Eγ = average CMB photon

energy = 6.34x10-4 eV [4]– mp = 938.27 MeV/c2– mπ0 = 134.97 MeV/c2– P = 4-momentum

• Conclusion: Ep~2x1020 eV

• See Ref 4 p221 (Stanev)

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• The approach is to calculate thephoton energy, Eγ , required fore+e- production on the CMB usingconservation of 4-momenta, P.

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• Conclusion: Eγ~8.2x1014 eV

• See Ref 4 page 264 (Stanev)

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GZK cutoff Part 4• Above the threshold, multiple pion production becomes important.

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GZK cutoff Part 5• The presence or absence of the GZK cutoff is fundamental to

understanding the sources and nature of the propagation of UHECR

Possible sources• Sources of high energy CR are

thought to be powerful radiogalaxies

• But they are located>>150 Mpc from Earth

• This is a major problem if theparticles are conventionalparticles as nucleons or heavynuclei

• They should lose energy viathe GZK effect The neighboring superclusters

(Earth is at center of image, in the Virgo Supercluster)

100 MLY30.6 Mpc

Possible sources• Mean free path ~ few Mpc

• This process limits CRsources to <150 Mpc

• But then what is acceleratingthe cosmic rays?

Virgo Supercluster(Earth is at center of image, in Local Group)

10 MLY3.06 Mpc

Isotropy• Regardless of whether the

source of the UHECR isGalactic or extragalactic, atthese high energies, theparticle paths should not besignificantly affected bymagnetic fields, so their arrivaldirection should point back totheir source

• But the arrival directions areisotropic

• Note, B= uG in Galaxy, <<uGextragalactic

Graph from [2]

The Challenge• The absence of a GZK cutoff and the isotropy of arrival directions

are two of the main challenges that models for the origin of UHECRsface

• Maybe the source resides in an extended Galactic halo

• That eases the difficulty of the lack of GZK cutoff but is a challengeto acceleration mechanisms

• There are 2 scenarios popularly proposed for the origin of UHECRs:

– Bottom-up

– Top-down

Two scenarios Part 1• Bottom-up

– Astrophysical acceleration

• Top-down

– Physics beyond the standardmodel

Two scenarios Part 2• Bottom-up

– Main hurdle is the need tofind a way to produce thehigh energies

• Top-down

– Main hurdle is to accountfor the high flux

Two scenarios Part 3• Bottom-up

– Pro: The highest energyevent (3.2 x 1020 eV)argues for the existence ofZevatrons (ZeV=1021 eV),

– Con: But even the mostpowerful astrophysicalobjects, as radio galaxiesand AGNs, can barelyaccelerate chargedparticles to 1020 eV

• Top-down

– Pro: Decay of very highmass relics from the earlyuniverse; no accelerationmechanism needed

– Con: The dynamics oftopological defectgeneration and evolutiongenerally selects thepresent horizon scale asthe typical distancebetween defects whichimplies a very low flux

Two scenarios Part 4• Bottom-up

• Statistical acceleration– Fermi acceleration

• Acceleration in shocksassociated with SNremnants, AGNs, radiogalaxies

• Direct acceleration– Assumes existence of a

strong electromagnetic field• Acceleration in strong E

fields generated byrotating neutron starswith high surface Bfields

• Top-down

• Relics of the very earlyuniverse (topological defectsor superheavy relics) producedafter the end of inflation candecay today and generateUHECR

• ZeV energies are not achallenge since symmetry-breaking scales at the end ofinflation are >> 1021 eV (typicalparticle masses between 1022 -1025 eV)

Hillas plot Part 1• Acceleration of UHECR in

astrophyiscal plasmasoccurs when large-scalemacroscopic motion, suchas shocks ad turbulentflows, is transferred toindividual particles

• The max energy can beestimated by the gyroradiusof the particles contained inthe acceleration region

Graph from [1]

Hillas plot Part 2• For a given strength, B,

and coherence length L, ofthe field embedded in theplasma, Emax=ZeBL

• Hillas plot shows knownastrophysical sources withreasonable BL

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Graph from [1]

GRB• Can GRBs be a source of

UHECR?

• GRB and UHECR are bothdistributed isotropically inthe sky

• Average rate of gamma rayenergy emitted by GRBs iscomparable to the energygeneration rate of UHECRof energy > 1019 eV

• GZK still a problem

Isotropic distribution of GRB

Hybrid Models• Uses physics beyond the standard model in a bottom-up approach,

hence termed “Hybrid”

• Uses Zevatrons to generate UHECR that are particles other thanprotons, nuclei, and photons,

• One example: Neutrino masses ~ 1eV, the relic neutrinobackground will cluster in halos of galaxies and clusters of galaxies.High energy neutrinos accelerated in Zevatrons can annihilate onthe neutrino background and form UEHCR through the hadronic Zdecay.

• Neutrinos do not suffer from GZK losses, so the Zevatrons can belocated much farther away

Power law• Power law exists over many

decades

• Source must generate a powerlaw spectrum

Graph from [2]

Composition• The composition of the

UHECR could helpdiscriminate betweenproposed sources

• Galactic disk models need toinvoke heavier nuclei (Fe) tobe consistent with the isotropicdistribution

• Extragalactic models favorproton primaries

• Photon primaries morecommon among top-downscenarios

Graph from [2]

The future• The new experiments, HIRES,

the Auger Observatory, andthe OWL-Airwatch satellite willimprove data on the spectrumand spatial distribution, andcomposition of UHECR.

• The lack of a GZK cutoffshould become apparent withAuger and, if so, mostextragalactic Zevatrons maybe ruled out.

Graph from [2]

The future• The observed spectrum will

distinguish Zevatrons or top-down model by testing powerlas vs. QCD fragmentation fits

• The correlation of arrivaldirections of UHECR withsome known structure as theGalaxy, Galactic halo, LocalGroup, or Local Superclusterwould be key in differentiatingbetween different models

Graph from [2]