A A Watson- Detection of Neutrino-Induced Air Showers

8/3/2019 A A Watson- Detection of Neutrino-Induced Air Showers

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Detection of Neutrino-Induced Air Showers

A A Watson

School of Physics and Astronomy

University of LeedsLeeds LS2 9JT, UK

[email protected]

PACS Numbers: 96.40-z; 96.40.Tv; 96.40.Pq; 98.70.Sa

Contribution to the Proceedings of the Nobel Symposium on Neutrino Physics, August 2004

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mailto:[email protected]:[email protected]


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Abstract

High energy neutrinos can arise from a variety of processes. Interactions of ultra high energy

cosmic rays with radiation and matter lead to secondary particles, some of which create neutrinos.

Interactions of the 2.7 K CMB radiation with protons gives rise to pions and neutrons, and neutrons

produced in the photodisintegration of heavy nuclei are also a source of neutrinos. There is

speculation that super-heavy relic particles with masses of ~1021 eV are created in the early

Universe: if such particles exist then their decay channels are expected to contain neutrinos. The

different sources of neutrinos are summarised. High energy neutrinos can be detected through the

extensive air showers that they create in the atmosphere and the potential of the Pierre Auger

Observatory, now nearing completion, and of the planned EUSO and ASHRA instruments, to detect

neutrino-induced air showers will be described.

1. Introduction

It is now known that cosmic rays of energy above 10

19

eV exist. These are commonly called UltraHigh Energy Cosmic Rays (UHECRs): the highest energy detected thus far is 3 x 10

20eV [1]. The

UHECR beam is widely thought to be dominated by baryonic primaries (from protons to iron nuclei)

and consequently, as explained below, is expected to contain neutrinos with characteristic energies

of ~1016

eV and ~1018

eV. The energy and number of these neutrinos depend upon the mode of

production and on the distribution of the sources. There is now a realistic possibility of detecting

neutrinos of ~1018

eV using instruments designed to detect giant extensive air showers whilst

neutrinos of 1016

eV may be detectable by the ICECUBE instrument as set out in an accompanying

paper by C Spiering. In this paper I will describe where neutrinos with energies above 1016

eV

might come from and explain the principles behind the detection of the more energetic ones. I will

use the Pierre Auger Observatory, presently under construction and taking data in Argentina, as an

example, but also briefly describe the potential of the EUSO instrument planned for the International

Space Station and a ground-based device, ASHRA, planned for the observation of neutrinos thatinteract in mountains in Hawaii.

2. Sources of very high energy neutrinos

2.1. Proton primaries as a neutrino source

If UHECRs are protons, produced as a result of some electromagnetic acceleration process in a

distant source (Active Galactic Nuclei or AGNs may be such sources), then these protons can

interact with matter or radiation close to the AGN, or with radiation in the inter-galactic or

interstellar space that they cross en route to earth, giving rise to neutrinos through the following

reactions:

pcr+ p p + p + N(+

+ -+

0) (1a)

and

pcr+ + p +

0or n +

+(1b)

In these equations pcr represents a cosmic ray proton. In reaction (1a) N denotes the multiplicity of

pion production. Charged pions and muons are sources of electron and muon neutrinos and near to

the sources the ratio of muon to electron neutrinos will be 4:2. However, because of neutrino mass

and the associated oscillations, and the immense distances from the sources, the neutrino flux at the

earth is expected to consist of nearly equal numbers of electron, muon and tau neutrinos. Reaction

(1a) can take place with any protons or nuclei that are encountered as targets but the matter density

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of intergalactic and interstellar space is so low that material close to sources is the most probable

target for what is a sort of cosmic beam dump. The photon field in reaction (1b) can either be close

to the source or be one of the radiation fields that the proton traverses while travelling to earth. In

particular, the 2.7 K radiation field becomes very important when the proton energy is in the region

of 1020

eV. Neutrinos can arise from the decay chains of the charged pions of reaction (1a) and from

the second decay mode of (1b), from both the +

decay chain or from the decay of the neutron. Note

that the mean travel distance before decay of a neutron of 1020 eV is only about 1 Mpc.

The number of neutrinos created can be predicted with reasonable accuracy if it is assumed that the

primary particles are protons produced throughout the Universe. Of course, it is not known whether

this is the case as there remain uncertainties about the origin of the very highest energy cosmic rays

and the existence or otherwise of the Greisen-Zatsepin-Kuzmin (GZK) cut-off. The muon and

electron neutrinos coming from the charged pion decay chain carry about 5% of the pion energy and

so are in the range 3 x 1018

< E < 2 x 1020

eV. The electron neutrinos from neutron decay carry only

about 4 x 10-4

of the neutron energy so that these neutrinos are created in the range ~2 x 1016

< E 70) from the vertical. A neutrino can interact anywhere in the atmosphere with

equal probability. However, if one restricts a search to large zenith angles then it should be possible

to identify occasions when the neutrino has interacted deep in the atmosphere. The mode of

identification depends on the detection technique.

3.1 Prospects for detection with the Pierre Auger Observatory

A neutrino-induced shower arriving at a large zenith angle has distinctive characteristics that make

it possible to envisage detecting it with a conventional, ground-based, air shower array as discussed

in [9,10]. Most showers detected at large zenith angles will have been produced by baryonic

primaries. The vast majority of the particles detected in such events will be high energy muons as at

>70 the large atmospheric thickness of more than 2500 g cm-2

(for the depth of the AugerObservatory) filters out the electromagnetic radiation that arises from neutral pion decay. The

muons are accompanied by a small fraction of electromagnetic component (around 20%) that is in

time and spatial equilibrium with the muons. This electromagnetic component has its origin in

muon bremsstrahlung, pair production, knock-on electrons and muon decay. These showers have a

large radius of curvature as the source of the muons is far from the shower detector. The particles in

the shower disc arrive tightly bunched in time and the distribution of the signal size is rather flat

across the array. Such events have been known about for some time and studied in some detail [11,

12] using data from the Haverah Park array. By contrast, a shower produced by a neutrino, if it

interacts in the volume of air over the detector, will have a curved shower front, a steep fall-off of

particle signal with distance from the shower core and a distinctively broad time spread of the

particles at the detectors. The only instrument which is currently large enough to have any prospect

of detecting neutrinos, and with the ability to exploit these characteristics, is the Pierre AugerObservatory.

The Pierre Auger Observatory is planned as an instrument with sites in the Northern and Southern

Hemisphere. Each site will contain 1600 Cherenkov detectors holding 12 tonnes of water spread

out over 3000 km2. The water tanks will be overlooked by a set of 4 fluorescence detectors each

capable of detecting the faint light produced from N2 molecules excited by the shower particles as

they traverse the atmosphere. This hybrid method is very powerful for understanding the shower

development and for making a model-independent estimate of the primary energy. The southern

part of the Observatory, sited in Pampa Amarilla near Malarge, Argentina, is nearing completion.

As at December 2004, 2 of the 4 fluorescence detectors have been completed and are overlooking

557 water tanks. All these devices are taking data. The Pierre Auger Observatory is now the largest

shower detector ever constructed and by March 2005 will have an achieved an exposure that

exceeds the famous AGASA array. A description of the prototype instrument has been given in

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[13]. Distinctive and novel features of the water detectors of the Auger Observatory are the FADC

records that are obtained from each of the three 9 photomultipliers that view the individual water

volumes. In figure 3 the FADC records in a near vertical shower (13) are compared with those in

one at 76 from the zenith where the shower penetrated ~ 4 times as much matter. These events

were registered with the prototype array. The broad FADC traces, the curved shower front (4 km as

compared with 27 km) and the steep fall-off of signal size with distance from the shower core

evident in the near vertical event are the signatures that will be sought in inclined events and, if any

are found, used to assess the events as neutrino candidates [14].

Although the air volume up to 1 km above the 3000 km2

contains about 3 km3

water equivalent, it is

not clear that any neutrinos will be detectable. The and e from the GZK processes probably have

too low a flux though some , expected in the primary beam because of neutrino oscillations, may

be discovered [15] as might be neutrinos from some models for AGNs [16]and GRBs [17].

The sensitivity of the Auger Observatory to tau neutrinos of > 1018

eV is particularly interesting as

here one can take advantage of interactions in the rock or the Andes Mountains in the vicinity of

the detector. At 1018

eV the mean free path in rock is about 300 km and taus will travel about 50 km,

escaping from the rock to create showers over the surface detector. At 10

18

eV at least 5 water-tankswill be struck. Only a few such events are expected each year but these will be readily detectable

because of their distinctive signature and direction [15].

3.2 Prospects for detection with EUSO, the extreme Universe Observatory

Attaining the ability to monitor an air mass greater than is possible with the Auger Observatory has

led to the concept of observing the fluorescence light produced by showers from a detector in space.

A promising line is the development of an idea due to Linsley [18] in the form of the EUSO

instrument that has been conceived to fly on the International Space Station. This instrument would

have the capability to monitor ~105

km2

sr (after allowing for an estimated on-time of 8%). The

neutrino events will be identified as developing deep in the atmosphere at large angles from the

zenith and the threshold will correspond to an energy of 5 x 1019

eV. An Italian-led collaborationhas driven the design [19]. Unfortunately, at the time of writing (December 2004), the future of this

imaginative project is unclear because of uncertainties about funding for the Space Station. This is

disappointing as the technique offers one of the only ways to push to energies beyond whatever

limits are found with the Auger instruments.

3.3 Prospects for detection with the ASHRA Instrument

A ground based experiment, ASHRA, is planned for Hawaii, where interactions of tau neutrinos in

mountains will be sought using fluorescence telescopes with very high angular resolution. This is a

novel device which will consist of 12 light collection detectors covering 2 steradians visible from

one site with an 80 mega pixel array of CMOS sensors. The current candidate station sites are

locations near the summits of the three mountains of Mauna Loa, Hualalai, and Mauna Kea on theHawaii Big Island. These have been chosen after taking into account the need for redundant

observation with stereo aperture for UHECRs, the atmospheric purity, the rate of fine weather, low

light pollution and the accessibility. Details of the project, which is Japanese led, with support from

scientists in Taiwan and the USA, can be found in [20]. In addition to searching for neutrino

interactions in the nearby mountains, the ASHRA detector will be used for more conventional

studies of UHECRs and for ground based gamma ray astronomy about 100 GeV. A particular

feature of the instrument is the very fine pointing accuracy, claimed to be 1 arc min of resolution. If

a neutrino signal is detected this angular resolution will be of great value in locating the sources of

the neutrinos and of some of the charged cosmic rays, which are expected to be spread over more

than a degree because of intergalactic and galactic magnetic fields.

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4. Conclusions

The detection of high energy neutrinos by the extensive technique has still to be demonstrated but

with the Pierre Auger Observatory there seems some prospect of discovering, at the very least, tau

neutrinos from cosmogenic processes. What will be seen depends on such unknowns as the neutrino

cross-section, the nature of UHECR sources and the nature of the baryonic primaries. Indeed, as is

clear from figures 1 and 2, if the neutrino energy spectrum could be measured then there would be

important insight into the nature of the primary particles as iron primaries will produce fewer high

energy neutrinos above 1018

eV than will protons. However, the present challenge is to detect any

neutrinos at all: the measurement of the spectrum with the detail required to aid mass resolution will

take some decades.

Acknowledgements: I would like to thank the organisers for the invitation to participate in the

Nobel Symposium on Neutrino Physics. Many enlightening discussions with my co-authors of

reference [3] and with Enrique Zas are gratefully acknowledged. Work on ultra high energy cosmic

rays at the University of Leeds is supported by PPARC.

References:[1] Bird, D., et al., Phys. Rev. Lett., 71 3401 1993

[2] Engel, R., D. Steckel and T Stanev, Phys Rev D64 09301 2001

[3] Ave, M., N Busca, A Olinto, A A Watson, T Yamamoto, Proceedings of CRIS04, Catania,

Nuclear Physics B Proceedings Supplement vol 136 2004

Ave, M., N Busca, A Olinto, A A Watson, T Yamamoto, Astroparticle Physics (in press), astro-

ph/0409316

[4] Hooper, D., A. Taylor and S Sarkar, Astroparticle Physics (in press), astro-ph/0407618

[5] Waxman, E., Ap J 452 L1 1995

[6] Waxman, E. and J Bahcall, Phys Rev D 59 023002 1999

[7] Berezinsky, V., Kachelreiss, M. and Vilenkin, A., Phys Rev Letters 22 4302 1997,

Benakli, K., Ellis, J. and Nanopolous, D.V., Phys Rev D59 047301 1999,

Birkel, M and Sarkar, S., Astroparticle Physics, 9, 297 1998,Chisholm, J.R. and E W Kolb, astro-ph/0306288, submitted to Phys Rev D,

Chung, D J H., E W Kolb and A Riotto, Phys Rev Letters 81 4048 1998,

Rubin, N. A., M Phil Thesis, University of Cambridge, 1999,

Sarkar, S., and Toldra, R., Nuclear Physics B 621 495 2002

[8] Ave, M et al., Phys Rev Lett 85 2244 2000

[9] Berezinsky, V S., and A Yu Smirnov, Astrophysics and Space Science 32 461 1975

[10] Capelle, K.S., et al., Astroparticle Physics 8 321 1998

[11] Andrews, D., et al., Proc 11th ICRC (Budapest) Acta Phys Acad Sci Hung 29 Suppl 3 337 1970

Hillas, A M., et al., Proc 11th ICRC (Budapest) Acta Phys Acad Sci Hung 29 Suppl 3 533 1970

[12] Ave, M, et al., Astroparticle Physics 14 109 2000

[13] Auger Collaboration : Abraham, J., et al., NIM A 523 50 2004

[14] Cronin, J W., Proceedings of the TAUP Conference, Seattle 2003, astroph/0402487[15] Bertou, X., et al. Astropart Phys 17 183 2002

[16] Mannheim K., Astropart Phys 3 295 1995

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[19] www.euso-mission.org

[20] www.icrr.u-tokyo.ac.jp/~ashra

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Figure 1: The neutrino yield for a proton primary (left) and an iron primary (right). In each case the

initial energy was chosen as 1021.5

eV and the propagation distance was 300 Mpc. The different

origins of the neutrinos are shown. The dotted line in the right-hand diagram shows the neutrino

flux that arises from the decay of neutrons from photodisintegration processes. The figure is from[3].

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Figure 2: Electron and muon neutrino fluxes obtained from the nominal choice of astrophysical and

cosmological parameters used in [3] and taken [5]. The protons (left) and iron (right) primaries were

assumed to have a maximum energy at production of 4Z x 1020

eV. The proton flux from the

Waxman and Bahcall model [6] is represented by a solid line. The figure is from [3].

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Figure 3: The upper plot shows FADC traces from a 13 shower: the radius of curvature was 4 km, and thedensity ratio 134 over a distance ratio of 3.7. By contrast the lower figure shows a shower of 76 which has aradius of curvature of 27 km and a density ratio of 7.5 for a distance ratio of 3.5. Picture from Auger

Collaboration data: from J W Cronin [14].

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A A Watson- Detection of Neutrino-Induced Air Showers

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