Status and Perspectives of Astroparticle Physics in Europe

Astronomical Science

Christian Spiering 1996 Mk (DESY – Deutsches Elektronen-Synchro- n 42 tron, Zeuthen, Germany) 1

Mk n 50 Astroparticle physics has evolved as an 1 interdisciplinary field at the intersec- tion of particle physics, astronomy and cosmology. Over the last two decades, Crab it has moved from infancy to techno Nebula logical maturity and is now envisaging projects on the 100 M€ scale. This price tag requires international coordination, cooperation and convergence to a few flagship projects. The Roadmap Com- mittee of ApPEC (Astroparticle Physics European Coordination) has recently 1E 2006 S Mk H 12 M 87 1 released a roadmap covering the next n 42 14 8+30 26 1 +4 .4 ten years. ApPEC is a corporation of 28 Mk European funding agencies promoting n PG 18 Mk 0 15 astroparticle physics. HE n HE HE 1E 50 53+1 SS S SS 1 SS 11 J1 1 J1 J1 Ve 63 01 1E 82 G0 30 la + 23 S 5– .9 4– 3– Junior 19 1 +0 47 63 2 59 + 37 In 2002, Ray Davis and Masatoshi .1 2 1 LS Te 65 LS GC RX MS PS I + CAS V 0 Ve

J2 5039 J1 H R la Koshiba were awarded the Nobel Prize in 61 B1 Crab 1E A 03 71 15 X 303 3. 25 S 23 2 + –5 Physics for opening the neutrino window 7– 2 9–6 Nebula 41 3946 44 31 3 to the Universe, specifically for the de + 51 PKS 20 tection of neutrinos from the Sun and the 4 H 23 05 Supernova SN 1987A in the Large Magel- PKS –4 50 89 –3 21 09 lanic Cloud. Their work was a unique syn- 55 –3 thesis of particle physics and astrophys- 04 ics. Solar neutrinos also provided the first clear evidence that neutrinos have mass. Figure 1: The TeV AGN Other or unidentified Background colours It is this interdisciplinary field at the in gamma-ray sky as seen Plerion or ambiguous identi- indicate northern (blue)/ in 1996 and 2006. Shell Type SNR fication southern (yellow) sky. tersection of particle physics, astronomy (Graphic courtesy Binary System and cosmology which has been chris- Konrad Bernlöhr, MPIfK) tened astroparticle physics. Basic questions An answer to any of these questions The detection of solar and supernova would mark a major breakthrough in un- neutrinos is not the only new window to Recommendations of the Roadmap com- derstanding the Universe and would the Universe opened by astroparticle mittee (http://www.aspera-eu.org) were open an entirely new field of research on physics. Another one is that of high ener- formulated by addressing a set of basic its own. getic gamma rays recorded by ground- questions: based Cherenkov telescopes. From 1. What are the constituents of the the first source detected in 1989, three Universe? In particular: What is dark Search for Dark Matter sources known in 1996, to nearly 40 matter? sources identified by the end of 2006, the 2. Do protons have a finite life time? The favoured solution to the Dark Matter high-energy sky has revealed a stunning 3. What are the properties of neutrinos? mystery assumes Weakly Interacting richness of new phenomena and puzzling What is their role in cosmic evolution? Massive Particles (WIMPs) produced in details (see Figure 1). Other branches 4. What do neutrinos tell us about the the early Universe. A natural candidate of astroparticle physics have not yet pro- interior of the Sun and the Earth, and for WIMPs is the lightest particle of Mini- vided such gold-plated discoveries, but about supernova explosions? mal SuperSymmetric Models (MSSM), have moved into unprecedented sensitiv- 5. What is the origin of cosmic rays? the neutralino. WIMP searches focus on ity regions with rapidly increasing discov- What is the view of the sky at extreme the detection of nuclear recoils from ery potential – like the search for dark energies? WIMPs interacting in underground detec- matter particles, the search for decaying 6. What will gravitational waves tell us tors (Baudis 2005, Sadoulet 2007). No protons or the attempt to determine about violent cosmic processes and WIMP candidate has been found so far. the absolute values of neutrino masses. about the nature of gravity? Assuming that all Dark Matter is made of these exotic particles, present experi-

The Messenger 129 – September 2007 33 Astronomical Science Spiering C., Status and Perspectives of Astroparticle Physics in Europe

ments with a several kg target mass can 10 24 Figure 2: The ‘grand unified’ neutrino spec- therefore exclude WIMPS with interaction 20 10 trum. cross section larger than ~ 10–43 cm2. Cosmological ν MSSM predictions for neutralino cross 10 16 – 47 – 41 2 Solar sections range from 10 to 10 cm . 10 12 ν Experimental sensitivities will be boosted Supernova burst (1987A) –44 2 10 8 to 10 cm in about a year and may ) –1 Terrestrial anti-ν reach, with ton-scale detectors, 10–46 cm2 10 4 Reactor anti-ν MeV in 7–8 years. Therefore, there is a fair –1 1 Background from old supernova chance to detect dark matter particles in sr –1 –4 the next decade – provided the progress s 10 –2 in background rejection can be realised 10 –8 and provided Dark Matter is made of su- –12 Atmospheric ν persymmetric particles. Presently fa- Flux (cm 10 voured candidate devices are ‘bolomet- –16 10 ν from AGN ric’ detectors operated at a temperature –20 of 10–20 mK which detect the feeble 10 heat, ionisation and scintillation signals 10 –24 GZK ν from WIMP interactions, and noble liq- 10 –28 uid detectors (Xe or Ar) recording ionisa- 10 –6 10 –3 1 10 3 10 6 10 9 10 12 10 15 10 18 tion and scintillation. A variety of present- µeV meV eV keV MeV GeV TeV PeV EeV ly more than 20 Dark Matter experiments Neutrino energy worldwide must, within several years, converge to two or three few ton-scale experiments with negligible background.

AGN) will likely be detected by neutrino Neutrino properties: neutrino-less double Proton decay and low-energy neutrino telescopes in the next decade (see be beta decay astronomy low), no practicable idea exists how to detect 1.9 K cosmological neutrinos, the In the context of astroparticle physics, Grand Unified Theories (GUTs) of particle analogue to the 2.7 K microwave radia- neutrinos – rather than being the subject physics predict that the proton has a tion. of research – mainly play the role of finite lifetime. The related physics may be messengers: from the Sun, from a super- closely linked to the physics of the Big A next-generation proton decay detector nova, from active galaxies. Still, some Bang and the cosmic matter-antimatter could record neutrinos from a galactic of their properties remain undetermined. asymmetry. Data from the Super-Kamio- supernova with unprecedented statis- From the oscillatory behaviour of neu kande detector in Japan constrain the tics: 104–10 5 events, compared to only trinos we can deduce that the masses of proton lifetime to be larger than 1034 years, 20 events for SN 1987A. It would also the three neutrino species differ from tantalisingly close to predictions of vari- allow a precise study of the solar interior each other. But what are the absolute val- ous GUT models. A sensitivity improve- and of neutrinos generated deep in ues of their masses? Further: are neutri- ment of an order of magnitude requires the Earth. Three detection techniques are nos their own antiparticles (‘Majorana detectors on the 105–10 6 ton scale. currently studied: Water-Cherenkov particles’)? Specifically these two ques- detectors (like Super-Kamiokande, see tions could be answered by the obser Proton decay detectors do also detect de Bellefon et al. 2006), liquid scintillator vation of a radioactive decay called neu- cosmic neutrinos. Figure 2 shows a detectors and liquid argon detectors. trino-less double beta decay (Vogl 2006). ‘grand unified neutrino spectrum’. Solar They will be evaluated in the context of a To reach the sensitivity for a mass range neutrinos, burst neutrinos from SN 1987A, common design study which will also of 20–50 meV, as suggested by various reactor neutrinos, terrestrial neutrinos address the underground infrastructure theoretical models, one needs detectors and atmospheric neutrinos have been al- and the possibility of detecting neutrinos with an active mass of the order of one ready detected. They would be also in the from future accelerator beams. This ton, good resolution and very low back- focus of a next-stage proton decay de- design study should converge, on a time ground. Construction of such detectors is tector. Another guaranteed – although not scale of 2010, to a common proposal. envisaged to start in 2013–2015. Different yet detected – flux is that of neutrinos The total cost depends on the method nuclear isotopes and different experi generated in collisions of ultra-energetic and the actual size, and is estimated mental techniques are needed to estab- protons with the 3-K cosmic microwave between 400 and 800 M€. With the start lish the effect and extract a neutrino background (CMB), the so-called GZK of civil engineering in 2012 or 2013, only a mass value. The price tag for one of these (Greisen-Zatsepin-Kuzmin) neutrinos. third of this amount might be due before experiments is at the 50–200 M€ scale, Whereas GZK neutrinos as well as neutri- 2016. with the large range in cost being due to nos from active galactic nuclei (marked the production cost for different isotopes.

34 The Messenger 129 – September 2007 The high-energy Universe 10 4 Figure 3: The spectrum of cosmic rays Fluxes of Cosmic Rays and the domains for various experimental methods. Highest observed 2 Cosmic rays have been discovered nearly 10 energies dwarf the Large Hadron a century ago. Some of these particles (1 particle per m2 and second) Collider at CERN which will accelerate 13 have breathtaking energies – a hundred 10 –1 protons to 10 eV. million times above that of terrestrial accelerators (Olinto 2007; Watson 2005), 10 –4 see Figure 3. How can cosmic acceler ators boost particles to these energies? 10 –7 What is the nature of the particles? The –1 mystery of cosmic rays is going to be 10 –10 solved by an interplay of detectors for sr s GeV) high-energy gamma rays, charged cos- 2 Knee 10 –13 mic rays and neutrinos. (1 particle per m2 and year) Flux (m 10 –16 Charged cosmic rays 10 –19 The present flagship in the search for sources of ultra-high energy cosmic rays 10 –22 Ankle 2 is the Southern Pierre Auger Observatory (1 particle per km and year) in Argentina. This is a 1000-km2 array of 10 –25 water tanks, flanked by air fluorescence 10 –28 telescopes, which measure direction and 1010 10 12 10 14 10 16 10 18 10 20 energy of giant air showers (see Figure 4). Energy (eV) Full-sky coverage would be obtained by a Northern observation site. European groups will play a significant role to es- tablish the scientific case, and after its consolidation make a significant contribu- Figure 4: The Auger detection princi- tion to the design and construction of ples: fluorescence light from air showers is recorded by telescopes, parti- Auger-North. cles at ground level are recorded by Cherenkov water tanks.

TeV gamma rays

European instruments are leading the field of ground-based high-energy gamma-ray astronomy. Most of the new sources in Figure 1 have been estalished by H.E.S.S., an array of four Cherenkov telescopes in Namibia, and MAGIC, a large twin telescope at La Palma. The rich results from current instruments (Aharonian 2007; Voelk 2006) show that high-energy phenomena are ubiquitous in the sky; in fact, some of the objects discovered emit most of their power in the gamma-ray range and are barely visi- ble at other wavelengths (‘dark accelerators’). The need for a next-generation Fluorescence instrument is obvious, and its required telescope characteristics are well understood. CTA, the Cherenkov Telescope Array, could both boost the sensitivity by another order of magnitude and enlarge the usa- Water tanks ble energy range. CTA is conceived to cover both hemispheres, with one site in

The Messenger 129 – September 2007 35 Astronomical Science Spiering C., Status and Perspectives of Astroparticle Physics in Europe

each. The instruments will be prepared by 75˚ Figure 5: Sky map of a common European consortium. 60˚ 4282 events recorded 45˚ by AMANDA in 2000– 2004. 30˚ High-energy neutrinos

The physics case for high-energy neu- 15˚ trino astronomy is obvious: neutrinos can 24 h 0 h provide an uncontroversial proof of the hadronic character of the source; more over they can reach us from cosmic regions which are opaque to other types of radiation (Waxman 2007). European 10 –25 Figure 6: Current and physicists have played a key role in con- Virgo + 2008 LIGO 2005 AURIGA 2005 expected sensitivities for ground-based gravita- struction and operation of the two pio- 10 –20 Virgo Design tional wave detectors. neering large neutrino telescopes, NT200 The solid curves corre- in Lake Baikal and AMANDA at the South 10 –21 spond to existing detec- Pole, and are also strongly involved in GEO-HF tors and their expected 2009 upgrades. Dotted lines AMANDA’s successor, IceCube (Halzen 10 –22 are for new projects. 2007). A complete sky coverage, in DUAL Mo particular of the central parts of the Gal- h (f) [1/sqrt (Hz)] –23 10 (Quantum Limit) axy with many promising source candi- dates, requires a cubic kilometre detector Advanced LIGO/Virgo (2014) 10 –24 in the Northern hemisphere. Prototype installations of AMANDA size are pres- Einstein GW Telescope 10 –25 ently installed at three different Mediterra- 1 10 10 01000 10 000 nean sites (Greece, France, Italy). An Frequency (Hz) EU-funded three-year study (KM3NeT) is in progress to consolidate the scientific case and to work out the technical design of a single, optimised large future re- VIRGO). Predicted event rates, e.g. for European dominance, and for R&D. search infrastructure in the Mediter mergers of neutron star/ black hole sys- Technological innovation has been a pre- ranean, with construction envisaged to tems (BH-BH, NS-NS, NS-BH) are highly requisite of the enormous progress made start in 2011. uncertain and range between 3 and 1000 over the last two decades and enabled for the ‘advanced’ detectors planned to maturity in most fields of astroparticle start data taking in about five years (see physics. It is also a prerequisite for future Gravitational waves Figure 6). This would change dramati- progress towards greater sensitivity and cally with a third-generation underground lower cost and must be supported with Gravitational waves would provide us with interferometer facility (Einstein Telescope, significant funds. information on strong field gravity through E.T.) which would have a guaranteed rate the study of immediate environments of of many thousands of events per year The present ‘first stage’ roadmap will be black holes. The most advanced tools for and move gravitational wave detectors followed by a second stage which will be gravitational wave detection are interfer- into the category of astronomical observ- associated with a detailed census of ex- ometers with kilometre-long arms. The atories. Civil engineering could start in isting budget and human resources avail- passage of a gravitational wave differen- 2012 or 2013. able in the participating agencies. tial contracts space along the two direc- tions of the arms and influences the light travel time (Hong 2005). At present, the The big picture References world’s most sensitive interferometer is Aharonian F. 2007, Science 315, 70 LIGO (USA), the others being GEO600 in Table 1 is based on a scenario where the Baudis L. 2005, astro-ph/0511805 Germany, TAMA in Japan and VIRGO process of cooperation and coordination de Bellefon A. et al. 2006, hep-ex/0607026 in Italy. The research field of Gravitational converges to a few major activities (cost Halzen F. 2007, Science 315, 66 Wave has a huge discovery potential but > 50 M€) between 2010 and 2015. Natu- Hong J., Rowan S. and Sathyaprkash B. 2005, gr-qc/0501007 is still awaiting the first direct detection. rally, there must be room for initiatives Olinto A. 2007, Science 135, 68 In the short term, the European ground below the 50 M€ level. The Roadmap Sadoulet B. 2007, Science 315, 61 interferometers (GEO and VIRGO) should committee suggests that about 15–20 % Watson A. 2005, astro-ph/0511800 turn to observation mode with a fraction of astroparticle funding should be re- Voelk H. 2006, astro-ph/0603501 Vogl P. 2006, hep-ph/0611243 of their time dedicated to their improve- served for smaller initiatives, for participa- Waxman E. 2007, Science 315, 63 ment (GEO-HF, VIRGO+ and Advanced tion in overseas experiments with non-

36 The Messenger 129 – September 2007 Field Experiment Cost scale per Desirable start Remarks Table 1: Future European projects with experiment (M€) of construction > 50 M€ estimated cost. Note that in Dark Matter Low background experi- 60–100 2011–2013 two experiments (differ- most of the cases further R&D efforts, ments with one-ton mass ent nuclei and different or further input from prototype de- techniques) vices, or final confirmation of the physics case, are required before arriving Proton decay and Large infrastructure for 400–800 Civil engineer- – needs huge excava- at a detailed technical proposal. There- low-energy neutrino p-decay and ν astronomy ing: 2012–2013 tion fore the indicated starting dates are astronomy on the 100 kton–1 Mton – most of expenditures termed ‘desirable’. scale likely after 2015 – worldwide sharing Properties of neutrinos Experiments on neutrino- 50–200 2013–2015 two experiments with less double beta decay different nuclei (desira- with one-ton mass bly more worldwide) The high-energy Gamma rays: 100 (South) First site in Physics potential well Universe Cherenkov Telescope Array 50 (North) 2011 defined by rich physics CTA from present gamma ray experiments Charged Cosmic Rays: 85 2010 Confirmation of physics Auger North (1/3 Europe) potential from Auger South results expected in 2007 Neutrinos: 250 2011 Confirmation of physics KM3NeT potential expected from IceCube and gamma ray telescopes. Full Pro- posal expected in 2009. Gravitational waves Einstein Telescope 300 Civil engineer- Conceived as under- ing: 2012 ground laboratory Photo: H.E.S.S. Collaboration

Two of the four H.E.S.S. Cherenkov telescopes for detection of very high energy gamma rays are shown. The H.E.S.S. observatory is situated in Namibia, southern Africa. Each telescope has a mirror diameter of 12 m with a camera consisting of 960 pho- tomultipliers. The four telescopes are coupled and work in stereoscopic mode.

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