The Mysteries of the Highest Energy Cosmic Rays

0.17 MB·English

by  S. Kovesi-Domokos

The Mysteries of the Highest Energy Cosmic Rays
5
0 S. Kovesi-Domokosa∗ and G. Domokosa†
0
2 aDepartment of Physics and Astronomy
n The Johns Hopkins University, Baltimore, MD
a
J Wegiveasummaryofthetheoretical attemptstomakesenseoftheobservationaldataconcerningthehighest
0 energy cosmic rays, E >3×1019eV.
3

1
1. Preamble cosmic ray flux at 5 1019 8 1019 eV (GZK
v × − ×
1 cutoff) and the pileup [3] (a bump in the spec-
8 It was about forty years ago that Linsley [1] trum) just before the drop3 are called “the GZK
2 presentedanincredulousphysicscommunitywith feature”. The GZKsphere approximatelydefines
1 the 1020 eV event observed at the Volcano
∼ the cosmic neighborhood of sources from which
0 Ranch array. Today ground arrays and fluores-
we expect the arrival of extreme high energy
5
0 cence detectors have collected over a hundred cosmic rays (E 1020 eV and up). We usually
events with energies above 3 1019 eV, and a ≈
/ × putthisparticlehorizonat50–70Mpc. However,
h dozen or so at or above 1020 eV . It is worth
the GZK radius could be considerably less as
p
remembering that the 2 TeV CM energies
- ∼ demonstrated by Stanev et al. [4]. Scattering
p attained at the Tevatron are at the knee of the
in extragalactic random magnetic fields makes
e cosmic ray spectrum. LHC will operate at an
the real distance travelled by the particle longer
h
equivalent fixed target energy falling approxi-
: than the source-Earthdistance; more energy loss
v mately halfway between the knee and the ankle,
processes can take place as a result. Heavier
i aroundthe so-called“second knee” region. Thus,
X nuclei with similar energies also suffer energy
above the second knee, the first interaction in an
r loss through photodisintegration on the cosmic
a extensive air shower involvesphysics untested by
microwaveandontheintergalacticinfraredback-
the well-controlled experimental environments of ground radiations [5]. For 56Fe, for example, the
accelerators.
decrease of the energy loss time is even more
abrupt than for protons, though it starts at a
For many years, the appearance of the GZK
somewhat higher energy.
feature [2] at the highest energies (i.e. above
2 1019 eV) was taken for granted. The pro-
× We will summarize the most important, and
cesses, p+γ π+X, and, of lesser impor-
CMB → often controversial, observational data. Then we
tance, p+γ e+e− +X, are well-studied
CMB → presentthetheoriesformulatedtoexplaincertain
low energy processes in the CM. A proton of
coherent subsets of the data.
energy 1020 eV or above has a mean free path
of 7–8 Mpc in the cosmic microwave background
2. Observational Data
(CMB) radiation. With each interaction event,
the proton loses 20–25% of its energy; after only What follows is a very brief summary of the
a few interactions its energy drops under the most important features of the extensive air
threshold of the pion photoproduction process. shower and EGRET data from a theorist’s point
The expected precipitous drop in the incoming of view. Only ultra high energy cosmic rays
(UHECR) with energies above 3 1019 eV are
∗Speaker. e-mail: [email protected] Invited talk given at ×
XIIIISVHECRI,Pylos,Greece. 3Thisisaconsequenceofthepionphotoproductionbythe
†e-mail: [email protected] abovethresholdprotonsornuclei.
1
2
considered. unlikely that the primary of the “gold-plated”
3 1020 eV Fly’s Eye event was a photon [15].
×
2.1. Spectrum Around 1019 eV, the photon flux is less than
At and above 1020 eV, there are 15 events: 11 40%ofthebaryoniccomponentaccordingtoboth
Haverah Park [16] and AGASA [17]. By search-
observed by AGASA [6] and 2 by HiRes [7] and
ing for deeply penetrating horizontal air show-
one each by Fly’s Eye [8], and Yakutsk [9].
ers, AGASA investigated the UHE neutrino flux
for energies > 3 1017 eV [18]. No significant
There is a substantial disagreement between ×
enhancementabovetheexpectedhadron-induced
AGASA andHiRes atthis time. The HiRes data
flux was reported.
weresuccessfullyfitted[11]withatwo-component
model [10] including primaries with galactic and
2.3. Directionality
extragalactic origins. The Fly’s Eye data sug-
gest a transition from heavy to light (proton) For the energy range we are interested in,
primaries (at energies from 1017 to 1019 eV), the arrival direction distribution is remarkably
which can be interpretedas a sign of the passage isotropic. This could indicate a large number
from galactic to extragalactic sources.The extra- of weak or distant sources. AGASA data show
galactic source model of Berezinsky et al. [12], small-scale clustering [19] (4 doublets and one
which takes into account both uniform source triplet within a separation angle of 2.5o) above
distribution and a maximal energy at 1021 eV, 4 1019eV (57 events). HiRes in stereo mode
×
yields not only the GZK feature but also other above1019eV(162events)claimstoseenosmall-
details of the spectrum, as aresultof considering scale anistropy in the arrival direction distribu-
both energy loss processes. The large number of tion [20]. Recently, it was found [21] that the
events above the GZK cutoff (mostly observed statistical significance here is also very low for
by AGASA) remains unexplained by the two- both the small scale clustering in the AGASA
component models4. data set and the apparent discrepancy between
AGASA and HiRes.
It should be emphasized that even the two
largestexperiments,AGASAandHiRes,havetoo Nevertheless, despite the paucity of data
few events at the highest energies; the discrep- points, statistical analyses have attempted to
ancybetweenthemhaslowstatisticalsignificance discover possible angular correlations between
as a result [13]. the arrival directions of the extremely high en-
ergy cosmic rays (EHECR), and to match these
2.2. Composition in turn with high energy source canditates. The
enticement to carry out an analysis of even the
As mentioned above, fluorescence detectors
small available sample is obvious. Significant
providesomehintsforaFe ptransition. How-
→ angular correlation in the arrival directions of
ever,itisimportanttonotethatthemasscompo-
the EHECR could be the a smoking gun for the
sition results are different for each of the various
existenceofdiscretecompact(repeating)sources.
methods and model calculations used[14]. In ad-
Usingthe AGASAdataofE >4 1019 eV5,the
dition, the shower energy estimates are sensitive
×
presence of clustering can be used to constrain
to the assumption about the nature of the pri-
the number of sources in the nearby Universe
mary. To quote A.A. Watson (loc. cit.), “knowl-
for proton primaries[22] to be 10−5Mpc−3 with
edge about the mass of the primary cosmic rays
at energies above 1017 eV is rudimentary.” As large uncertainties. Tinyakov and Tkachev [23]
claimed to have identified BL Lacertae objects
far as photon primaries are concerned, it is very
as sources generating the anisotropies in the
4Since for both AGASA and HiRes there are systematic
errorsintheenergyassignment,itisdifficulttojudgehow 5At E = 4×1019eV protons have a loss length of ∼
greatthedisagreementbetween themactuallyis. 1000Mpc.
3
AGASA data. Evans et al. [24] and W. Burgett it from the observationaldata, and assignthe re-
et al. [25] questioned this conclusion, however6. mainderto theEGRB.Thisisacomplicatedand
model-dependent process. The EGRB was first
Most authors working in the field agree that discovered with SAS-II [26] and then confirmed
more data from HiRes and from larger experi- by EGRET [27]. A power law energy spectrum
mentsliketheSouthern(andhopefullytheNorth- (E(−(2.1±0.03)) in the range 30 MeV . E .100
ern) Pierre Auger Observatory, EUSO, the Tele- GeV) with no anisotropy was found. Strong et
scope Array, and OWL are necessary to draw al. [28], in a recent reworking of the EGRET
solid conclusions about clustering and the pres- data, substantially improved the determination
ence or absence of the GZK cutoff discrepancies. of the diffuse galactic γ ray flux. They conclude
Until then the BL Lac alignment is merely sug- that in previous analyses the galactic contribu-
gested by the need for objects accelerating par- tion was underestimated; the EGRB must be
ticles to exceptional energies, rather than being revised downward as a result. The new spectrum
a firmly established consequence of the observa- isnotconsistentwithapowerlaw. Onemayinfer
tional data. tentatively that the EGRB arises from a number
of different physical processes.
2.4. Extragalactic Diffuse Gamma Rays
At higher energies (20TeV . E . 500TeV),
Another piece of important data is the ex-
an upper limit on the EGRB can be obtained by
tragalactic diffuse γ ray background (EGRB).
estimatingtheratioofphoton-inducedtohadron-
It is generally believed that unresolved AGN,
induced extensive air showers. The GRAPES
galaxy clusters, distant gamma ray bursts, etc.
collaboration [29] obtained an upper limit of
are the main sources. However, EGRB would
about3 10−4forthefractionofphoton-induced
get a significant contribution from the decay of ×
(muon-poor) air showers.
topological objects, relic particles etc., which are
used by some authors to explain trans-GZK cos-
mic ray events. Comparing that contribution to 3. Theoretical Models
the measured EGRB, one may be able to place
The presence or absence of the GZK feature
upper limits on, or eventually exclude, some of
determines whether it is enough to rely on con-
those schemes. Any electromagnetic (EM) en-
ventional astrophysics to explain the spectrum,
ergy introduced to the EM backgroundradiation
composition, and directionality encrypted in the
(dominated by CMB) produces an EM cascade
data, or if physics beyond the Standard Model is
if the energy is & 1015eV/(1+z) i.e. the pair
needed [30].
production “threshold” on the CMB at redshift
z. The cascade by pair production and inverse 3.1. Traditional Astrophysics
Compton scattering degenerates the injected en-
ergy to under 1015 eV in a couple of Mpc and Thesemodels arealsocalledbottom-up scenar-
∼ ios. At 1019 eV and above, the extragalactic
produces a power-law distribution of GeV-range
component must dominate the energy spectrum.
gamma rays.
Even if there are some objects (neutron stars,
galacticwind,etc)inourgalaxywhichcouldpos-
To obtain the EGRB, one must construct a
sibly accelerate UHECR, it would be impossi-
model of the galactic γ ray background, subtract
ble for them to produce the observed isotropic
6BurgettandO’MalleypartitiontheAGASAsampleinto distribution. For order of magnitude estimates,
three statistically uncorrelated groups by energy: E < the community generally uses Hillas’ formula.
5×1019 eV, 5×1019 ≤ E < 8×1019 eV, and E > Whether the accelerationtakesplaceelectromag-
8×1019eV.Theyfindthemiddlegrouptobepreferentially
neticallyorwiththehelpofdiffusiveshockaccel-
alignedwith the Supergalactic equatorial plane,whilethe
eration (first order Fermi process), the magnetic
othertwogroupsarestatisticallyconsistentwithisotropic
distributions. field must be strong enough to keep the particles
4
intheregionofacceleration,thus,approximately, 3.2. Scenarios Beyond the Standard
Model
E
R &R , 3.2.1. Z-bursts
accelerator gyromagnetic
≈ ZeB
The Z-burst model[33] needs only the inclu-
sion of the neutrino mass in the Standard Model
wherethechargeoftheacceleratedparticleisZe.
(SM) for its attempt to explain the trans-GZK
From this,
events. Among SM particles only the neutrino
canhandlecosmicdistanceswithoutdramaticab-
Emax ZeBRaccelerator. sorption. The assertion is that extreme energy
≈
neutrinos
We are looking for a maximal energy of about
1021 1022 eV. The enormous energy of the Eres =MZ2/2mν =4×1021(mν(eV))−1eV
−
primary singles out exceptional astrophysical
=4 (mν(eV))−1ZeV
sources: radio lobes of powerful radio galaxies, ×
jetsofactivegalacticnuclei,gammaraybursters, forming Z0 bosons resonantly with antineutrinos
and magnetars. There are only a few of these of the relic 1.9 oK thermal neutrino background
possible accleration sites inside the GZK radius could produce enough protons well inside the
for charged hadrons. (For example, the radio GZK radius to account for the EECR. On aver-
galaxy M87, in the center of the Virgo cluster, age a Z0 produces 2 nucleons, 20 photons
∼ ∼
was investigated repeatedly as a candidate[31]. and 50neutrinos. Although the Z-burstmodel
∼
It could provide EHECR at Earth only if the ex- is very economical, but it is likely to have fatal
tragalactic magnetic fields on the way here have problems both because of the required energy
some specific configuration.) Consequently, it is at the source and because of the extreme en-
unlikelythattheGZKcutoffcouldbecompletely ergy neutrino flux required to produce enough
absent in the spectrum. The accelerators are of the trans-GZK events [34]. The new experi-
far beyond the pion photoproduction distance mental limit by WMAP, 2dF and SDSS on the
and the random, poorly known, extragalactic active neutrino mass (Pmνi . 0.72 eV) com-
magnetic fields may cause deflection of charged bined with the neutrino oscillation results gives
particles. In fact, the relevant sources are at cos- the maximal possible neutrino mass mν 0.24
≈
mologicaldistances. Nevertheless there are small eV [35]. This puts the energy of the neutrino for
scale anisotropies and correlations with certain the resonant production at 10 ZeV. If these
∼
sources. These observations indicate that the neutrinos are secondariesof pions, the protons in
cosmic traveler may be a stable, neutral parti- the source must be acceleratedto 103 ZeV. No
∼
cle which interacts very weakly with the cosmic known astrophysical object is capable of acceler-
background radiation fields. This particle could ating protons to such high energies [36]. A large
be a neutrino. enoughfluxbyordinaryastrophysicalsources(or
by beyond-the Standard Model objects [37]) to
Itisgenerallyacceptedthatthedominantform produce a substantial part of trans-GZK events
of acceleration of cosmic rays at the sources is would pump so much EM energy into the uni-
by first order Fermi acceleration at astrophysical verse that the EGRET limits would be violated
shocks. A recent development in the theory of many times over.
shock accelerationisthe introductionofthe back
reaction of the accelerated particles to the shock An overdensity of neutrinos in a halo of our
modifying it strongly[32]. Among other results galaxy or in that of the Local Group could help
the spectrum is claimed to be flatter than in the to reduce the flux requirement. However, these
linear theory; thus, the number of arrivingtrans- neutrino masses are too small to allow any clus-
GZK primaries could be higher. tering of relevance to allow the reduction of the
5
incoming neutrino flux [38], [39]. For a conserva- to the square of the higher dimensional (d=10
tive maximal neutrino mass, mν 0.33 eV, the ?) Schwarzschild radius. Combined with the
≈
neutrino flux required by the Z-burst scenario assumption that m √s, it leads to an un-
bh
∝
is already in marginal conflict with the upper acceptably slow rise of the cross section. The
limit given by the FORTE [40] experiment and published data on near horizontal showers by
in violation of the GLUE [41] limit. A lower AGASA and Fly’s Eye exclude that. By con-
mass makes the problem more severe [39]. This trast, with a string scale of the order 70 TeV one
limitontheZ-burstmodelisvalidevenwhenthe is far below the semiclassical region [49]. The
distant sources emit only neutrinos. growth of the cross section is governed by the
exponentiallyincreasingleveldensityinthetran-
sition region (and only in the transition region)
3.2.2. Strongly Interacting Neutrinos fromthe SMto the stringregime. Inthis scheme
The problem of energy loss during the propa- few near horizontal showers are produced, as a
gation at cosmological distances would not arise result.
if the trans-GZK primaries were neutrinos and
interacted strongly at very high energies [42], Anotherpossibility,suggestedtoachievestrong
causing proton-like air showers. Both the energy cross sections, is the inclusion of instanton in-
and flux reqirements are much lower here than duced processes. There is a detailed discussion
in the Z-burst model. In this case the cosmo- and further referenceson the instantonenhanced
genic neutrino flux (i.e. secondaries from the cross section in [43].
pion photoproduction of extreme energy protons
on the CMB) could account for the trans-GZK It is remarkable that the cosmogenic neutrino
events [43], since all incoming neutrinos produce beam, together with a strongly interacting neu-
anairshower. Fortheprotonstheinjectionspec- trino with sudden onset of large cross section
trum is simple, power law. The onset of strong could give a solution of the trans-GZK events.
interaction must result in a very rapidly rising This is accomplished without the need to as-
cross section; a slow rise would give a large num- sume exotic astrophysical scenarios. There is no
ber of horizontal showers, which are not seen by obvious contradiction with data obtained from
either AGASA or HiRes. For example, in the- FORTE and GLUE, either.
ories of higher than 4 dimensions, Kaluza-Klein
graviton exchange gives a power law increase,
whichisnotonlytooslow,butstronginteraction 3.2.3. Top-down models
cross sections cannot be reached until energies Very heavy topological defects (TD) could
1020 eV [44]. solve both the problem of the long trip for pro-
≈
tons in the CMB and the difficulty to accelerate
The following two schemes also assume some particles to extreme energies [37]. In the early
higher dimensional (string inspired) theory. The universe at the time of phase transition from the
neutrino-nucleon cross section begins to grow at GUT scale (M 1014 GeV) TD (monopoles,
≥
acertainenergy,inessencedue tothe unification cosmic strings, necklaces, textures etc.) could be
of all interactions at a scale of the order of a few formed in localized regions where mass-energy is
tens of TeV or a few TeV in the CMS, producing trapped. In the decay (or annihilation) of these
lepto-quark resonances[45], mini black holes [46] superheavy extragalacticrelics,“X-particles”are
or “p-branes” [47]. The asymptotic (s ) formed which decay trough QCD fragmentation
→ ∞
equivalence of the resonance and mini black hole jets consisting mostly of photons, neutrinos and
schemes was recently demonstrated [48]. The 5% nucleons. These nucleons provide the
∼
onset of the new interaction is very different. trans-GZK events. However, a large fraction of
In the “semiclassical black hole” picture, it is theenergyisinjectedintotheextragalacticspace
assumed that the cross section is proportional via γ-s with E > 1015 eV. Thus, if the TD can
6
generate the flux of EECR, then the EGRET induced.
limit (see 2.4) is always exceeded [50].
3.2.5. Lorentz Invariance Violation
Another type of top-down scheme uses the The idea of small violation of Lorentz invari-
possible existence of superheavy dark matter ance is not new [57]. The idea was reconsidered
particles, which could have been created at the again as a possible explanation for the lack of
endofinflationintheearlyUniverse. Theirdecay the GZK cutoff in the AGASA and Fly’s Eye
or annihilition couldalsoproduce the trans-GZK data[58]aswellasaputativeabsenceofhighen-
spectrum via QCD fragmentation [51]. As cold ergyphotonabsorptionontheinfrared(IR)back-
dark matter, they could have an overdensity in ground [59]7. A review of the various schemes
the halo of our galaxy by a factor of 104. Their can be found in ref. [60]. There is no firmly es-
massmustbe>1012GeVandtheextremeenergy tablished scheme of violating Lorentz invariance
protons will reach the detectors since they are atpresent. The mostpopularscenarioinvokedin
produced closeby. However, there is anisotropy explainingtheexistenceoftrans-GZKcosmicray
in the arrivaldirection because of our position in events follows the paradigm laid out in Ref. [58].
the galaxy. In addition, the fragmentation pro- According to that scheme, Lorentz invariance is
cess produces abouta factor of2 moreultra high violated by assigning different maximal speeds
energyphotonsthanprotons. Asitwasdiscussed to different particle species (for instance, to elec-
in Section 2.2, the experimental photon fraction trons and photons), none of them or some of
is certainly <1. them being equal to what is known to be the
speed of light in vacuo.
3.2.4. Stable Light Neutral Particles In this manner, the rate of pair production
The idea of light, supersymmetric, stable or by photons and the rate of photoproduction by
long-livedhadronsinsteadofnucleonsasUHECR protons can be suppressed by invoking a rather
was entertained for some time. A mass larger small amount of Lorentz invariance violation.
than the mass of the nucleon ( 2 GeV) would Conversely, the absence of trans-GZK flux in
≈
increase the threshold energy of the π photo- future experimentscouldplacesevereconstraints
production and push the GZK cutoff to higher on Lorentz invariance violation. It is important
energies. to point out that the same set of assumptions
leads to some rather dramatic changes in the
One hypothetical particle in this class [52] re- development of cascade showers, see ref. [61]. A
quired the existence of a light gluino. Precision violation of Lorentz invariance as an explana-
measurements at LEP and SLC closed the so- tion of trans-GZK event cannot be ruled out at
called “light gluino window” [53]. Shadrons with present.
a light sbottom were investigated as well [54],
motivated by the preliminary result from the
Tevatron indicating an overproduction of b¯b. 4. Conclusion
Futher data taking,however,did not confirmthe
effect [55]. A plethora of particle physics models was cre-
ated to explain the trans-GZK events, which are
The possibility of axion-photon mixing was anomalousaccordingtoexpectationsbasedonas-
considered in Ref. [56]. The hypothesis was that trophysics and on SM. Z-bursts, TD seem to be
the trans-GZK showers were started by regener- ruled out already to a large extent by the extra-
ated photons, while the axions were the long dis- galactic diffuse γ-ray background measurements.
tance travelers. One may object, however, that
7We were informed by Michael Fall (private communica-
accordingtopresentobservationaldatathetrans-
tion) that the uncertainties in our knowledge of the IR
GZK showers look hadronic and not photon- background areconsiderable.
7
Superheavy dark matter is fatally wounded ac- 8. D.J. Bird, et al., Phys. Rev. Lett. 71, 3401
cordingtotheavailableobservationaldataonthe (1993).
ratio of UHECR photons to protons and on the 9. N.N. Efimov et al., in Astrophysical Aspects
lack of directional asymmetry. Strongly interact- of the Most Energetic Cosmic Rays, eds.
ingneutrinosandLorentzinvarianceviolationare M.Nagano and F.Takahara (World Scientific,
still alive. The nature and origin of EHECR is 1991) p20.
still an enigma. If the observational data from 10. J.N. Bahcall and E. Waxman, Phys. Lett.
the next generation of detectors support the ex- B556, 1 (2003).
istence of trans-GZK events then the next great 11. D.R. Bergman, et al.,Proc. of 28th ICRC
leap for particle physics could come from non- (Tsukuba), 683 (2003); J.N. Matthews (for
accelerator experimental physics. It is evident the collaboration)Acta Phys. Polon. 351863
that our physics and astrophysicsideas cantruly (2004).
be changed by the results coming from the con- 12. V. Berezinsky, A.Z. Gazizov, and
tinuing and the newly commissioned detectors: S.I.Grigorieva, hep-ph/0204357, and
HiRes, Telescope Array, P. Auger Observatory, also astro-ph/0210095.
EUSO, OWL, AMANDA, ICECUBE, NESTOR, 13. D. De Marco, et al., Astropart. Phys. 20, 53
NEMO, ANTARES, HESS, VERITAS, MAGIC, (2003).
GLUE, FORTE etc. 14. A.A. Watson, The mass composition of cos-
mic rays above 1017 eV; these Proceedings.
15. F. Halzen, R.A. Vazquez, T. Stanev,
5. Acknowledgements
H.P. Vankov Astropart. Phys. 3, 151 (1995);
We would like to thank V. Berezinsky and W. also an analysis of the 3 1020 eV event was
×
Burgett for valuable discussions. Special thanks givenbyM.Risse,et al.,Astropart.Phys.21,
are due to the organizers of XIII ISVHECRI, in 479 (2004).
particular to Leo Resvanis for hosting such an 16. M. Ave, et al., Phys. Rev. Letters 85, 2244
informativeand stimulating meeting. We further (2000).
thank the organizers for financial support. 17. K. Shinozaki, et al., Astrophys. J. 571, L117
(2002).
18. S. Yoshida, Proc.of 27th ICRC (Hamburg) 3,
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