Origin and evolution of cosmic accelerators - theunique discovery potential of an UHE neutrino

telescope

Astronomy Decadal Survey (2010-2020) Science White Paper

Interested physicists from the IceCube and ANITA Collaborations(Editors: Pisin Chen1, K. D. Hoffman2)

1. Kavli Institute for Particle Astrophysics and Cosmology, SLAC National AcceleratorLaboratory, Stanford, CA 94305

2. Department of Physics, University of Maryland, College Park, MD 20742

Abstract

One of the most tantalizing questions in astronomy and astrophysics, namely theorigin and the evolution of the cosmic accelerators that produce the highest energy cosmicrays (UHECR), may be best addressed through the observation of ultra high energy(UHE) cosmogenic neutrinos. Neutrinos travel from their source undeflected by magneticfields and unimpeded by interactions with the cosmic microwave background. At highenergies, neutrinos could be detected in dense, radio frequency (RF) transparent mediavia the Askaryan effect. The abundant cold ice covering the geographic South Pole, withits exceptional RF clarity, has been host to several pioneering efforts to develop thisapproach, including RICE and ANITA. Building on the expertise gained in these efforts,and the infrastructure developed in the construction of the IceCube optical Cherenkovobservatory, a low-cost array of radio frequency antenna stations could be deployed nearthe Pole to efficiently detect a significant number of UHE neutrinos with degree scaleangular resolution within the next decade. Such an array, if installed in close proximityto IceCube, could allow cross-calibration on a small but invaluable subset of neutrinoevents detected by both the optical and radio methods. In addition to providing criticalinformation in the identification of the source of UHECRs, such an observatory could alsoprovide a unique probe of long baseline high energy neutrino interactions unattainable inany man-made neutrino beam.

1 Neutrinos: a unique astro-nomical messenger

Our knowledge of the universe is derived fromthe observation of fundamental particles thatact as messengers, providing a window intotheir origins. However, photons above 30TeV have horizons that are limited by pairproduction due to their interactions with thegalactic infrared (IR) and cosmic microwavebackground (CMB) photons. Further, pro-tons are bent by inter- and intra- galacticmagnetic fields, making charged particle as-tronomy only possible at energies above 1019

eV. At higher energies, protons interact withCMB photons through the Greisen-Zatsepin-Kuzmin (GZK) [1] process:

p+ γ2.7K → ∆+ → n+ π+

↪→ µνµ

↪→ eνeνµ

thus a large fraction of the cosmic volume isnot accessible with charged particles. Figure1 compares the propagation distance of pro-tons and photons to the horizons required forthe study of some astrophysical objects. Neu-trinos, on the other hand, are only weaklyinteracting and therefore have a mean freepath which exceeds the Hubble radius, al-lowing the possibility that they could reachEarth from deepest space.

The fact that ultra-high energy cosmic rays(UHECR) have been observed, and almostcertainly include a significant proton frac-tion, guarantees the existence of neutrinos at1017−1019 eV, as required by standard-modelphysics (See Fig. 2). In addition to the GZKneutrinos, there may exist additional UHEneutrinos induced by various relic topologi-cal defects. All of these scenarios result in anultra-high energy neutrino spectrum. It is ev-ident that cosmic neutrinos provide a unique

and complementary window into some of thedeepest mysteries of our universe.

Figure 1: The red (blue) shaded regions showthe distances that are inaccessible to protons(photons) as a function of energy. For com-parison, the distance scales required for vari-ous astrophysical studies are also shown.

It has been recognized [10] that the neu-trinos produced in GZK interactions could beused to characterize the UHECR source spec-trum and spatial distribution. A study of theUHE neutrino spectrum with good statisticswill provide an important piece of the puz-zle of the origins of cosmic rays, especially ifpoint sources were resolved. A large and ver-satile UHE neutrino telescope would also besensitive to novel aspects of cosmology (suchas the Big Bang relic topological defects)and fundamental physics at the energy fron-tier (such as non-Standard Model neutrinoflavor physics and electroweak interactions,Lorentz invariance, extra dimensions, space-time foams, superconducting strings, etc.). Itshould also be noted that the study of astro-physical neutrinos in a previously unexploredenergy regime could even reveal completelyunexpected phenomena. UHE cosmic neutri-nos can address a variety of issues in astro-physics and particle physics [11], [12]. Here

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Figure 2: World ultra-high energy cosmicray and predicted cosmogenic neutrino spec-trum as of early 2007, including data fromthe Yakutsk [2], Haverah Park [3] the Fly’sEye [4],AGASA [5], HiRes [6], and Auger[7], collaborations. GZK neutrino models arefrom Protheroe & Johnson [8] and Kalashevet al. [9].

we present the scenarios with the greatest dis-covery potential, including the issue of theorigin and evolution of the “cosmic accelera-tors” that are the most critical to the produc-tion of UHECRs, and we propose a path forexploring this regime that could be realizedwithin the next decade.

2 Cosmic accelerators

One of the 11 Science Questions for the NewCentury put forward by the NRC TurnerCommittee on Connecting Quarks with theCosmos [13] is: “How do cosmic accelera-tors work and what are they accelerating?”Existing models for UHECR can be largelycategorized as top-down or bottom-up sce-narios. The top-down scenarios assume thatthe UHECRs are the decay products of someexotic, non-Standard Model particles, whichcould have been naturally produced at the

post-inflationary stage of the universe withmasses of 1013 − 1014 GeV. The bottom-upscenario, on the other hand, assumes that theUHECRs are ordinary particles, e.g., protons,accelerated at their source to ultra-high ener-gies.

Observations of the UHECR spectrumabove the ankle ( 3× 1018 eV) from AGASA,HiRes and Auger show both a dip caused bye+e− pair production [17, 18] and a bumpconsistent with a GZK accumulation clearlyvisible. In particular, hybrid energy measure-ments from Auger have reconciled the dis-crepancy between the HiRes and AGASA en-ergy scales, favoring the HiRes claim of theobservation of the GZK cutoff. This evidencesupports a simple ansatz where UHECR areaccelerated in extragalactic sources [11] andreaches us over a long baseline, favoring thebottom-up scenario [19].

If the production is indeed bottom-up,what accelerates the cosmic particles andwhere are the sources? A number of bottom-up cosmic acceleration models have been pro-posed, where the most developed, diffusiveshock acceleration [14], could in principle ac-celerate protons to 1021 eV. Above these en-ergies, one may have to invoke more exoticmodels, such as unipolar induction [15] orplasma wakefield acceleration [16].

Whether the origin of UHECR is top-down or bottom-up and whether their sourcesare local AGNs, UHE neutrinos are the in-escapable by-product of propagation throughthe cosmic microwave background. Neutrinoswill be produced in this GZK process withina distance (1+z)−3R0,GZK from the source ofthe parent UHECR, where R0,GZK ∼ 50 Mpcis the local GZK radius. For example, for aUHECR produced by a cosmic accelerator lo-cated at z = 1, the GZK neutrinos would beinduced within 6 Mpc from the source. Thus,neutrinos produced by cosmologically distantsources necessarily point back to the sourcewith sub-degree accuracy. This is in distinc-

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tion to charged particles - it is an open ques-tion if UHECR above the GZK cutoff pointback to local sources, but old UHECR, whichhave interacted, and have almost certainlydiffused from their source position on the sky.

An EHE neutrino observatory large enoughto amass a sample on the order of hundredsof events would provide two unique discoveryopportunities based on the detection of GZKneutrinos. First, the GZK neutrino spectrumand directions would be indispensable in amulti-particle astronomical analysis to deter-mine the sources of the highest energy par-ticles in the Universe. Once the sources forUHECRs are identified, it may be possible tofurther investigate the evolution of the cosmicaccelerators through the redshift dependenceof their host galaxies. Complementing theastrophysical implications, mere detection ofneutrino induced events would extend ourknowledge of neutrino properties. By mea-suring the event rate as a function of nadirangle, the opacity of the Earth can be usedto determine neutrino-nucleon cross sectionsat center of mass energies unavailable to anycurrent or planned laboratory facility. Forexample, a small cross section compared tostandard model extrapolations could indicatenon-perturbative aspects of nucleon struc-ture, while a cross section increasing in en-ergy could be a doorway to multi-dimensionalphysics beyond the standard model.

3 Radio detection of neutrinos

Ironically, the very lack of interactivity thatmakes neutrinos so valuable in probing denseobjects from cosmological distances presentsa major challenge to their detection, evenat GZK energies where their cross sectionsare enhanced. In addition, at GZK ener-gies, the flux will be low (see Fig. 2). Thetechnique used by the current generation ofneutrino observatories relies on the detec-

tion of optical Cherenkov radiation princi-pally from secondary leptons or cascades pro-duced in neutrino-nucleon deep inelastic scat-tering, with large natural reservoirs of wa-ter or ice serving as both a target mediumand a Cherenkov radiator. The absorptionand scattering of light in these media re-quire a density of instrumentation that is toocostly to scale up to an array of the size re-quired for GZK studies. An alternate de-tection mechanism was suggested as early as1962 when Askaryan [20] proposed that highenergy showers might produce coherent ra-dio emission in dense media. These emis-sions would arise as an excess of negativecharge builds up as electrons are swept outalong a relativistically advancing shower front(20% more electrons than positrons when theshower is fully developed). The longer wave-length components of the broadband radia-tion from the motion of this large net negativecharge will add coherently, while for smalleroptical wavelengths the individual particlesources will contribute to the radiation fieldswith essentially random phases. The net re-sult gives rise to a radio frequency (RF) im-pulse. The shower dimensions determine thewavelengths over which these emissions arecoherent, with the amount of power radi-ated in radio exceeding the optical for showerenergies greater than about 1015 eV. The“Askaryan effect” has been demonstrated ina series of experiments at SLAC by direct-ing pulses of electrons into sand, salt andice [21, 22, 23, 24]. The exceptional RF clar-ity of cold ice suggests that a sparse arrayof antennas embedded in the South Polar IceCap may provide a cost effective approach tothe detection of GZK neutrinos.

Any design for a future GZK neutrino ob-servatory will rely heavily on the ongoing pi-oneering efforts to exploit the Askaryan effectfor UHE neutrino detection. The most am-bitious of these is ANITA [27, 28], a balloonborne antenna array which, for a few weeks

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in the austral summers of 2006-2007 [29]and 2008-2009, surveyed the entire continentfor RF emissions emanating from horizontalneutrino-induced showers that are refractedat the surface of the Antarctic ice. WhileANITA may be poised to observe the firstGZK neutrino, their synoptic approach isseason-limited, making it difficult to collectsufficient statistics for study. In addition,their vantage point limits their ability to re-ject surface noise by reconstructing a threedimensional interaction vertex within the vol-ume of the ice. A complementary approach istaken by RICE [25], a small grid of submergedantennas embedded in the South Polar Ice-cap at depths of 100-300m. While this ap-proach delivers a decreased effective volumecompared to ANITA, it has the unique capa-bility of making the in situ measurements ofthe RF properties of the ice (including mea-surements of attenuation length, noise, andbirefringence) that will be essential in the de-sign and simulation of any GZK scale array,as well as the demonstrated ability to recon-struct three dimensional vertices. In addi-tion, RICE has been a testbed for the techni-cal design and analysis techniques that wouldbe employed in any englacial array.

Recently, a unique opportunity has beenpresented by the ongoing construction of theIceCube neutrino observatory, which, with aninstrumented area of a cubic kilometer, is thelargest optical Cherenkov observatory in ex-istence. By installing antenna arrays para-sitically in holes drilled for IceCubes photo-tubes and using IceCubes power and commu-nication infrastructure, the RF properties ofthe South Pole ice are being studied over alonger baseline (up to a kilometer) and atgreater depths (down to 1450m) than allowedby RICE. By marrying the expertise devel-oped for RICE, ANITA, and IceCube, thiswas done quickly, and at minimal cost. Todate, five clusters of receivers containing fourchannels each, and six transmitters have been

deployed within the IceCube footprint [26].These deployments suggest an obvious

path toward a GZK scale array that exploitsthe infrastructure at the Pole, the exceptionalvolume and clarity of the ice there, and a syn-ergy between the well established optical, andthe pioneering radio technique. At UHE, thesecondary leptons that IceCube relies uponfor neutrino detection may propagate for 20-30 km or more (in the case of muons or taus)before reaching the array [30]. This poten-tially long propagation distance leads to anunknown amount of lost energy. The kine-matics of the event is such that the leptontypically carries 75-80% of the primary neu-trino energy, with the remainder being de-posited into a local hadronic cascade initi-ated by debris recoiling from the initiatingcharged current interaction. The energy de-posited in this cascade can only be measuredby IceCube if it is contained within the in-strumented volume. However, while initiatedby hadrons, the cascade rapidly develops intoan e+e−γ shower in ice, which would be de-tectable through Askaryan emission. The si-multaneous detection of an event with bothtechnologies, therefore, is possible, and wouldprovide complementary information.

A concentric array of RF antennas sur-rounding IceCube (Fig. 3) could observe theradio emission from the primary vertex ofsome of the same events that produce de-tectable leptons in IceCube. Even a relativelycoarse array with km scale spacing betweenantenna clusters may detect the strong coher-ent radio impulses from the cascade vertex.Although such hybrid events would make uponly a small fraction of the event sample, evena few cross calibrated events would provide aninvaluable verification of the radio technique.A parallel investigation into the detection ofacoustic signals initiated by thermal energydeposited by UHE neutrino interactions is on-going [33] and may also be deployed, at smallincremental cost, in a hybrid detection sce-

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nario [31]. It should be noted that cold ice isthe only environment where all three of thesemethods could be used simultaneously.

Figure 3: Simulation of an incident 1019 eVneutrino which deposits 35% of its energyinto a shower that was seen by 4 of the sub-surface radio detectors (red dots), while thesecondary lepton passes just outside the Ice-Cube array (blue hexagon) and is detectedwith an energy of 6.5× 1018 eV.

4 A phased approach to an Ice-Cube radio extension

As outlined in section 2, it will be necessaryto collect O(100) GZK neutrino events in or-der to attain adequate statistics to exploit thescience potential offered by GZK neutrinos.In our consideration of the event rates, wehave to be realistic about the confidence withwhich we can anticipate the GZK flux. De-spite significantly improved data on the spec-trum near and beyond the ankle, observationscan still be accommodated with a wide rangeof assumptions regarding the injection slopeat the source, the cosmological evolution of

the sources and the composition at injection.Accounting for AUGER observations, currentbottom up scenarios yield GZK event ratesranging from unobservable (although this re-quires an unlikely conspiracy of parameters;see [32]) to ∼ 0.1 per km2 per year. Experi-mental limits from ANITA and RICE allowevent rates larger by a factor of 10, leav-ing room for enhanced cross-sections or directcontributions from the acceleration source tothe total neutrino flux. To be prudent, weenvision a phased approach to building anRF array, where a modest initial installationcould be expanded after the presence of a sig-nal was confirmed and the flux measured.

Phase-I: Prototype Testbed [34] Con-tinuing the measurements started with RICEand the IceCube codeployments, a prototypeantenna station would provide a comprehen-sive temporal measurement of the detectedpower spectrum in the 30-to-1000-MHz rangedown to power levels of -110 dBm/MHz forboth continuous and episodic events, the ra-dio signal transmission as a function of thesub-surface antenna depth, and long-termmonitoring of RFI backgrounds at the SouthPole.

Phase-II: 50-100 km2 Radio AntennaArray As an intermediate step, one may aimat an event rate of 3-5 GZK neutrinos peryear, perhaps requiring coverage of an areaof 50-100 km2 around IceCube. The guid-ing principle for the design should be that itwould deliver degree-scale precision in the re-construction of the incoming neutrino direc-tions, which should be sufficient for resolvingthe locations of the astrophysical sources forcosmic accelerators.

Phase-III: Full-Scale Array Assumingan event rate of 3-5 GZK neutrinos per year,Phase II should be able to make a definitivestatement about UHECR cosmic acceleratorsassuming 10 years of operation. We note,however, that the antenna array would beextendable. Based on the experience gained

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and better knowledge on the GZK neutrinoflux through Phase-II, the array could be ex-tended to 300-1000 km2, capable of detectingat least 20-50 GZK neutrinos per year. Thiswould not only help to expedite the data col-lection, but also allow the pursuit of addi-tional frontier astrophysics and fundamentalphysics questions.

5 Summary

We point out that one critical question inastronomy and astrophysics, namely the ori-gin and the evolution of the cosmic accelera-tors that produce the highest energy cosmicrays, may be best addressed through the de-tection of ultra high energy (UHE) cosmicneutrinos. Using existing technology, we be-lieve that this can be realized using a low-cost array of radio antennas surrounding Ice-Cube. Such a UHE neutrino telescope wouldextend neutrino astronomy to ExaVolt ener-gies, yielding substantial rates of cosmogenicneutrinos, the so-called GZK or “guaranteed”neutrinos, and determine their direction todegree-scale precision, allowing the identifica-tion of the sources of the highest energy cos-mic rays. Such an observatory has the addedbenefit of providing a probe of neutrino inter-actions at energies unattainable in the labo-ratory. Building such an array near IceCubemay also allow the detection of a small num-ber of hybrid events, yielding a cross calibra-tion of the two techniques.

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