Cherenkov Telescope Array—CTA

Gamma-ray astronomy has a huge potential in astrophysics, particle physics and cosmology. Cherenkov Telescope Array (CTA) is an international initiative to build the next-generation ground-based gamma-ray observatory which will have a factor of 5–10 improvement in sensitivity in the 100-GeV–10-TeV range and an extension to energies well below 100 GeV and above 100 TeV. CTA will consist of two arrays (one in the Northern and another in the Southern Hemisphere) for full sky coverage and will be operated as an open observatory. It will provide a deep insight into the non-thermal high-energy Universe. In this talk, we will briefly present the major design concepts of CTA as well as its vast science case.

gamma-ray astronomy for energies above a few tens of gigaelectronvolts. The CTA will extend these efforts on the in-depth exploration of our Universe in very-high-energy gamma-rays and on the investigation of the cosmic processes leading to relativistic particles.
Besides guaranteed high-energy astrophysics results, already advanced by the current facilities, CTA will have a large discovery potential in key areas of astronomy, astrophysics and fundamental physics research. These include the study of the origin of cosmic rays, their sources and their impact on the constituents of the Universe, as well as the examination of the ultimate nature of matter and of physics beyond the standard model. These will in turn be achieved through the investigation of astrophysical particle accelerators; the exploration of the nature and variety of black hole accelerators, via the study of the production and propagation of extragalactic gamma rays; as well as through searches for dark matter and the effects of quantum gravity.
CTA will consist of two arrays of Cherenkov telescopes, which aim to (a) increase sensitivity by an order of magnitude with respect to current installations, for deep observations around 1 TeV; (b) boost significantly the detection area and hence detection rates, particularly important for transient phenomena at the highest energies, such as extreme flaring from active galactic nuclei (AGN); (c) increase the angular resolution and hence the ability to resolve the morphology of extended sources, such as the shells of supernova remnants (SNRs) and identifying the zones of interaction of cosmic-ray particles with molecular clouds; (d) provide continuous energy coverage for photons from some tens of gigaelectronvolts to beyond 100 TeV; and (e) enhance the sky survey capability, monitoring capability and flexibility of operation.
CTA will be operated as a proposal-driven open observatory, with a Science Data Centre providing transparent access to data, analysis tools and user training. Brazil is an official member of CTA since the spring of 2010, expanding along with Argentina the frontiers of the Consortium to the South American continent which is also a potential site to host the array.

The Very-High-Energy Gamma-Ray Sky
Gamma rays have an inherent potential to study highenergy, non-thermal processes. Gamma-ray telescopes complement those at other wavelengths in providing the broad, multiwavelength coverage necessary for modern astrophysics. At the present moment, close to 2,000 sources of gigaelectronvolt gamma rays are detected by the spacebased instruments Fermi and AGILE. In the very-highenergy (VHE) domain, the current generation of groundbased observatories has led the way, contributing to a broad band of studies of well over 100 sources. Both these results show an abundance and ubiquity of cosmic particle accelerators.
About half of the objects detected by the ground-based instruments are of extragalactic nature, the rest being associated with the Galaxy. Among the galactic teraelectronvolt gamma-ray sources are pulsars and pulsar-wind nebulae (PWN), SNR as well as some 15-20 objects whose identification is unclear, either because of a lack of counterparts in other wavelengths or because multiple possible counterparts exist. Despite the much larger number of gigaelectronvolt sources, the number of well-known galactic objects in this energy range is actually very similar to that of the teraelectronvolt range, with pulsars being the dominant class of galactic gamma-ray emitters. Table 1 shows a list of VHE-detected objects.

Imaging Cosmic Particle Accelerators
While the most powerful accelerators on Earth can accelerate particles to teraelectronvolt energy scales, cosmic rays reaching the Earth have been detected with energies above 10 18 eV. Many of the source classes detected in VHE gamma rays are the likely sites of cosmic-ray acceleration, such as SNRs, where shock waves driven into the interstellar medium constitute the locations where particle acceleration by a first-order Fermi process is believed to take place. The difficulty with a direct detection of the particle acceleration sites is that the charged cosmic-ray particles are strongly deflected in the few microgauss interstellar magnetic fields. Therefore, except for energies in the 10 20 -eV range, cosmic rays cannot be traced back to their sources, but they can be imaged using the radiation created during the acceleration process and propagation.
Strong interactions of cosmic-ray protons and nuclei with target matter inside, or in the vicinity of cosmic accelerators, lead to pion production and hence gamma-ray signatures via π 0 → γ γ decay. Protons or nuclei with power-law spectra of index p generate gamma-ray spectra with γ ∼ p , and a cut-off in proton spectra translates into a cut-off in gamma-ray spectra about a decade in energy below than that in the proton spectra. Cosmic-ray accelerators are thus expected to be visible as gamma-ray sources. The intensity and extent of the gamma-ray emission will depend on the density distribution of the target material, as well as on whether accelerated particles are efficiently confined within the accelerator and on how quickly particles diffuse away after escaping from the acceleration region [7].
Cosmic rays can also interact with the interstellar medium (ISM) matter and produce VHE in this environment. However, the much smaller density of the diffuse ISM leads to an energy loss timescale for relativistic protons of ∼10 7 years which is longer than their residence time in the Galaxy, so that only a small fraction of their energy will be converted into radiation. Overall, about 1 % of the energy injected into relativistic hadrons in the Galaxy emerges in the form of photons [1].
For cosmic electrons and positrons, ionisation, bremsstrahlung, synchrotron radiation (due to interaction with the ambient magnetic fields) and inverse-Compton (IC) scattering of ambient radiation compete as the energyloss processes [2,3]. For the highest-energy electrons, at teraelectronvolt energies, synchrotron and IC emission dominate and synchrotron X-rays and IC gamma rays can be used as effective tracers of electron acceleration. The targets for IC scattering are typically the cosmic microwave background radiation (CMB), starlight and reprocessed starlight re-emitted in the far infrared. The typical lifetime of a high-energy electron in the ISM is 5 × 10 5 years, much shorter than propagation timescales in the Galaxy [7].
Electron spectra with power-law index e generate power-law IC and synchrotron spectra with index ( e + 1)/2, and a cut-off energy E c in electron spectra translates into cut-offs ∝ E 2 c,TeV E ph,eV for IC gamma rays, where E ph is the typical energy of the target photons. With the rapid energy loss, emission by electrons is usually concentrated relatively close to the sites of acceleration.

Gamma-Ray Detection
In space, gamma-ray detection at high energies (between 10 MeV and 100 GeV) is based on pair production and subsequent electromagnetic cascading. The most sensitive satellite-based gamma-ray detector currently operating is the Fermi Large Area Telescope (LAT), which has ≈1 m 2 detection area and ≈2.5 steradian field of view (FoV). The LAT combines a silicon-strip tracker for directional reconstruction and an 8.6-radiation-length-thick calorimeter for energy determination [4]. An anticoincidence shield at the top of the detector is used to reject cosmic rays. The angular resolution achievable is strongly energy-dependent: improving from 5 • at 100 MeV to 0.25 • at 10 GeV, where photon statistics becomes very limited for most sources.

Cherenkov Telescopes
The most sensitive ground-based approach for gamma-ray detection in the range 100 GeV-10 TeV (see [5]) is the Imaging Atmospheric Cherenkov Technique (IACT), which uses the Cherenkov light produced by eletromagnetic cascade electrons and positrons in the atmosphere to establish the properties of the primary gamma ray; the gamma-ray direction is determined by imaging the cascade, whereas the gamma-ray energy is derived from the Cherenkov light yield. The technique is, in principle, applicable for photon energies above ∼5 GeV, where the Cherenkov yield becomes significant.

Stereoscopic Imaging of Air Showers
Stereoscopic imaging was a technique introduced by the HEGRA air shower experiment in the mid-1990s, which first allowed for a full 3-D reconstruction of the shower axis [8]. With systems of atmospheric Cherenkov telescopes providing stereo images of air showers, parameters such as direction, core location and height of shower maximum can be determined in an event-by-event basis. Figure 1 shows how the shower core position is determined using the stereoscopic technique; the intersect point of the lines passing through the long axis of the ellipses gives its projected position in the ground. The shower angle in the sky is determined in a similar way. The single-camera images are ellipses whose major axis coincides with the projection of the vertical shower axis. Both the image of the shower and the point where the shower axis intersects the camera plane lie on this line. As a result, the shower direction can be determined by superimposing the images of two cameras.
The angular resolution of the multitelescope system is governed by shower fluctuations and photon statistics, which cause measurement errors in the determination of image centres and of the image orientation; in particular, the angular resolution is poor if the image axes are approximately collinear. In principle, therefore, with a sufficient number of instruments working together in a geometrically adequate arrangement to do the shower imaging, a limiting angular resolution could be achieved at 1 TeV of the order of 0.005 • , imposed by the natural fluctuations in shower development (see [11,12] and references therein).

Part II: The CTA Design
The design of CTA is largely based on current technology and aims to respond to the basic goals outlined in the introductory section. In particular, in order to view the whole sky, two CTA sites are foreseen. The main site will be in the Southern Hemisphere, given the wealth of sources in the central region of our Galaxy and the richness of their morphological features. A second complementary northern site will be primarily devoted to the study of AGN and cosmology. The performance and scientific potential of the array have been studied in significant detail, showing that the goals of the experiment can be reached [13].

The Array Layout
Given the wide energy range to be covered, a uniform array of identical telescopes, with fixed spacing, is not the most efficient solution for the CTA. Therefore, the array design is separated into three energy ranges.
The first is the low-energy range, for photons below 100 GeV. To detect showers down to a few tens of gigaelectronvolts, the Cherenkov light needs to be sampled and detected efficiently, with the fraction of area covered by light detectors being of the order of 10 %, assuming the use of conventional PMT technology. Since event rates are high and systematic background uncertainties are likely to limit the achievable sensitivity, the area of this part of the array can be relatively small, of the order of few 10 4 m 2 . Efficient photon detection can be achieved with few large telescopes or many telescopes of modest size. If smallto-medium-sized telescopes are used in this energy range, the challenge is to trigger the array, since no individual telescope detects Cherenkov photons to provide a reliable trigger signal. Trigger systems which combine and superimpose images at the pixel level in real time, with a time Detection of a teraelectronvolt gamma ray with an array of Cherenkov telescopes, showing an example of the shower images seen in the camera's focal plane and the principle of stereoscopy. From [9] resolution of a few nanoseconds, can address this issue but represent a significant challenge, given that a single 1,000-pixel telescope sampled at (only) 200 MHz and 8 bits per pixel generates a data stream of more than 1 Tb/s. Therefore, the CTA design conservatively assumes a small number (about 4) of very large telescopes (also denominated LSTs or large-sized telescopes), with about 23 m in diameter, to cover the low-energy section. The current preferred design uses parabolic reflectors and achieves a FoV between 4 • and 5 • , for a nominal energy threshold of 10 GeV [10].
The core energy range from about 100 GeV to 10 TeV is where the shower detection and reconstruction are better understood from current instruments, and an appropriate solution for the layout seems to be a grid of telescopes of the 10-15-m class, with a spacing of about 100 m. Improved sensitivity is obtained both by the increased area covered and by the higher quality of shower reconstruction, since showers are typically imaged by a larger number of telescopes than is the case for current few-telescope arrays. For the first time, array sizes will be larger than the Cherenkov light pool, ensuring uniformly sampled imaging and that a number of images are recorded close to the optimum distance from the shower axis, of about 70-150 m, where the light intensity is large and intensity fluctuations are small. It is also in this distance range that the shower axis is viewed under a sufficiently large angle for efficient reconstruction of its direction.
For the case of the core array, the current layout assumes about 20-25 telescopes of 12-m diameter (also denominated MSTs or medium-sized telescopes), built with the Davies-Cotton reflector design for a FoV between 7 • and 8 • [11]. This portion of the array aims to achieve milliCrab sensitivity throughout the critical range 100 GeV-10 TeV.
Finally, for the high-energy range, above 10 TeV, the key limitation becomes the number of detected gamma-ray showers, which implies that the array must cover multisquare-kilometre areas. Since at high energies the light yield is very large, showers can be detected well beyond the 150-m radius of a typical Cherenkov light pool. Therefore, two implementation options have been considered: either a large number of small telescopes with mirror areas of a few square metres and spacing matched to the size of the light pool of 100-200 m or a smaller number of larger telescopes with some 10-m 2 area which can see showers up to a distance of >500 m and, hence, can be deployed with a spacing of several 100 m or in widely separated subclusters of a few telescopes.
The first option, with about 30 × 5-6-m telescopes (also denominated SSTs or small-sized telescopes), is being preferred. There are multiple designs being currently studied for the small telescopes, both with the Davies-Cotton and Schwarzschild-Couder design [11]. The intended FoV is of ∼10 • , with a total covered area in the ground of about 10 km 2 .
The total cost of the layout defined above, to constitute the Southern CTA array, has been estimated in the order of 100 million euros. Figure 2 depicts an artistic view of the final array. Figure 3 shows the simulated point source differential sensitivity of the CTA, in units of the Crab flux, for one of the several proposed array layouts following the main characteristics outlined above (array I). Observe the contribution of each different "subarray" in composing the total performance of the instrument over the entire energy range.

The CTA Performance
We briefly review below the factors limiting the performance of an array of Cherenkov telescopes (see [12] for a detailed discussion). To detect a cosmic gamma-ray source in a given band, three conditions have to be fulfilled: 1. The number of detected gamma rays N γ (given by N γ = φ γ A t) has to exceed a minimum value, usually taken to be between five and ten photons. The number of detected gamma rays and hence the effective area A are virtually always the limiting factor at the high-energy end of the useful energy range.
2. The statistical significance of the gamma-ray excess has to exceed a certain number of standard deviations, usually taken to be 5, approximated by N γ / N bg for background-dominated observations, where N bg = φ bg src A bg bg , where bg is the background rejection factor of the observations and src is the solid angle subtended by the source.
3. The systematic error on the number of excess gamma rays due to uncertainties in background estimates and subtraction has to be sufficiently small. Fluctuations in the background rates due to hardware or analysis factors will result in background systematics. For current instruments, background uncertainties at a level of few percent have been reported [14].
All the sensitivity constraints mentioned above are illustrated in Fig. 4 which shows a toy model reproducing the main features present in the results of the full-featured Monte Carlo simulation in Fig. 3. At high energy, the sensitivity is limited by the gamma-ray backgrounds (green) and at low energies (in the area of high statistics) by systematic background uncertainty (purple). For more details, see [12].
The sensitivity of CTA, of less than 1 % for 50-h integration in the entire range between 20 GeV and 20 TeV and representing a factor at least ten times better than current instruments at any given energy (see Fig. 5), is driven by a rich science case.

PART III: The CTA Science Impact
At low energies, the science case is dominated by high-z AGN and pulsars, the study of the extragalactic background  From [12] light and of dark matter. In the central range around 1 TeV, population studies of various source classes, legacy source catalogues of extragalactic and galactic objects, the study of extended emission and extended sources as well as precise timing measurements are among the driving scientific topics. Finally, in the high-energy end, above 10 TeV, CTA will be able to explore the cut-off regime of the cosmic-ray accelerators, probing the so-called cosmic pevatrons.
The aims of the CTA can be roughly grouped into three main themes, serving as key science drivers: 1. Understanding the origin of cosmic rays and their role in the Universe comprises the study of the physics of galactic particle accelerators, such as pulsars and pulsar wind nebulae, supernova remnants and gamma-ray binaries. It deals with the impact of the accelerated particles on their environment, via the emission from particle interactions with the interstellar medium and radiation fields, and the cumulative effects seen at various scales, from massive star-forming regions to starburst galaxies. 2. Understanding the nature and variety of particle acceleration around black holes concerns particle acceleration near supermassive and stellar-sized black holes. Objects of interest include microquasars at the galactic scale, and blazars, radio galaxies and other classes of AGN that can potentially be studied in high-energy gamma rays. The fact that CTA will be able to detect a large number of these objects enables population studies which will be a major step forward in this area. Extragalactic background light (EBL), galaxy cluster and gamma-ray burst (GRB) studies are also connected to this field. 3. Searching for the ultimate nature of matter and physics beyond the standard model will cover what can be called "new physics", with searches for dark matter through possible annihilation signatures, tests of Lorentz invariance and any other observational signatures that may challenge our current understanding of fundamental physics.
Current instruments have passed the critical sensitivity threshold in all topics within themes 1 and 2 (whereas three remain to be investigated), but even in the rich panorama they revealed, this is clearly only the tip of the iceberg. For example, H.E.S.S. has conducted a highly successful survey of the Milky Way covering about 600 square degrees, which resulted in the detection of tens of new sources. One of the goals of CTA will be to survey areas of the sky such as the galactic plane for faint VHE gamma-ray sources, specially "dark accelerators". Using standard techniques, CTA can carry a survey of the region |l| < 60 • and |b| < 2 • in 250 h (one-fourth of the available time per year) down to a uniform sensitivity of 3 mCrab [24]. Such a "Galactic plane survey" with CTA has the potential to detect hundreds of sources (see Fig. 6).
Below, we will quickly go through the big science questions that remain and which CTA will be tackled.

Cosmic-Ray Accelerators
A tenet of high-energy astrophysics is that shocks of supernova explosions accelerate cosmic rays (CRs). Particle acceleration up to energies well beyond 10 14 eV has now clearly been demonstrated with the current generation of Cherenkov instruments (Fig. 7), and recent observations by the Fermi/LAT instrument have finally provided convincing evidence for the unambiguous detection of characteristic pion-decay signature in supernova remnants [18].
The large sample of SNR that will be observed by CTAexpected to reach several hundreds of objects-and the corresponding increased coverage to lower and higher energies will allow sensitive tests of these acceleration models in a source-population scale. Improved angular resolution of arcmin will help to resolve fine structures in the remnants which are essential for the detailed study of particle acceleration and interactions.
The CR spectrum observed near the Earth can be described by a pure power law up to an energy of a few petaelectronvolt, where it slightly steepens; this feature is called the "knee". The absence of other features in the spectrum suggests that if SNRs are indeed the ubiquitous sources of galactic CRs, they must be able to accelerate particles up to there.
Even if a SNR can be detected by Cherenkov telescopes during a significant fraction of its lifetime (up to several 10 4 years), it can make 10 15 -eV CRs only for a much shorter time (several hundred years), due to the rapid escape of petaelectronvolt particles from a SNR. This implies that the number of SNRs which have currently a gamma-ray spectrum extending up to hundreds of teraelectronvolt is very roughly of ∼10, the actual number of detectable objects depending on the distance and on the density of the surrounding interstellar medium. The detection of such objects would be extremely important, as it would give clear evidence for the acceleration of galactic CRs up to 10 15 eV.

Active Galactic Nuclei and Their Jets
AGN are among the largest storehouses of energy known in our cosmos. At the intersection of powerful low-density plasma inflows and outflows, they offer excellent conditions for efficient particle acceleration in shocks, turbulence and magnetic reconnection sheets. AGN represent onethird of the known VHE gamma-ray sources, with most of the detected objects belonging to the BL Lac class. The fast variability of the gamma-ray flux (down to minute timescales) indicates that gamma-ray production must occur close to the black hole, assisted by highly relativistic jets resulting in time (Lorentz) contraction when observed on Earth. Details of how these jets are launched or even the types of particles of which they consist are poorly known. Multiwavelength observations with high temporal and spectral resolution can help to distinguish between different scenarios, but this is at the limit of the capabilities of current   [17] instruments. The sensitivity of CTA, combined with simultaneous observations in other wavelengths, will provide a crucial advance in understanding the mechanisms driving these sources.
Available surveys of BL Lacs suffer several biases at all wavelengths, further complicated by Doppler boosting effects and high variability. The big increase of sensitivity of CTA will provide large numbers of VHE sources of different types and open the way to statistical studies of the VHE blazar and AGN populations. This will enable the exploration of the relation between different types of blazars and of the validity of the unifying AGN schemes. The distribution in redshift of known and relatively nearby gamma-ray emitting BL Lac objects peaks around z ∼ 0.3 (Fig. 8). The large majority of the population is found within z < 1, a range easily accessible with CTA. CTA will therefore be able to analyse in detail blazar populations (out to z ∼ 2) and the evolution of AGN with redshift and to start a genuine "blazar cosmology" [19].
Several scenarios have been proposed to explain the VHE emission of blazars. However, none of them is fully self-consistent and the current data are not sufficient to firmly rule out or confirm a particular mechanism. In the absence of a convincing global picture, the first goal for CTA will be to constrain model-dependent parameters of blazars within a given scenario. This is achievable due to the wide energy range, high sensitivity and high spectral resolution of CTA combined with multiwavelength campaigns.
Thus, the physics of basic radiation models will be constrained by CTA, and some of the models will be ruled out.
The second more difficult goal would be to distinguish between the different remaining options to firmly identify the dominant radiation mechanisms. Detection of specific spectral features, breaks, cut-offs, absorption or additional components would be greatly helpful for this.
The role of CTA as a timing explorer will be decisive for constraining both the radiative phenomena associated with and the global geometry and dynamics of the AGN engine. Probing variability down to the shortest timescales will significantly constrain acceleration and cooling times, instability growth rates and the time evolution of shocks, turbulence and magnetic reconnection structures. For the brightest blazar flares, current instruments are able to detect variability on the scales of several minutes. With CTA, such flares should be detectable within seconds, rather than minutes [19]. A study of the minimum variability timescale of AGN with CTA would allow the localisation of VHE emission regions (parsec distance scales in the jet, the base of the jet or the central engine) and would provide stringent constraints on the emission mechanisms as well as the intrinsic timescale connected to the size of the central black hole.
Recently, radio galaxies have emerged as a new class of VHE-emitting AGN [21]-see also Fig. 9 for another example. Given the proximity of these sources and the larger jet angle to the line of sight compared to BL Lac objects, the outer and inner kpc jet structures will be spatially resolved by CTA. This will allow precise location of  [20] the main emission site and also searches for VHE radiation from large-scale jets and hot spots, besides the central core and the nuclear jets seen in radio wavelengths, using very long baseline interferometry (VLBI).

EBL and the Star Formation History in the Universe
Star formation history is one of the fundamental aspects of cosmology and Galaxy formation. Part of the starlight within the galaxies is absorbed by dust and re-emitted at higher wavelengths. The resulting diffuse radiation in the ultraviolet to far-infrared wavelengths, commonly referred to as the EBL, is the second largest background field, in terms of its energy content, after the CMB at 2.73 K. The EBL evolves with the redshift, reflecting the evolution of the stars and galaxies. The time-integrated EBL density depends not only on the number and properties of stars and dust in galaxies but also on the cosmological model of the Universe. The direct measurement of the EBL is complicated and has large uncertainties due to the strong foreground emission in our solar system, especially the zodiacal light. However, indirect EBL measurements are possible using the VHE emission of distant AGN. The idea is that the VHE gamma rays produced in these sources are absorbed via interaction with low-energy photons of the EBL if the photon energies involved are above the threshold for electron-positron pair creation. The VHE gamma-ray absorption, which is energy-dependent and increases strongly with the redshift of the AGN, results in a clear imprint of the EBL on the measured VHE spectra. A fundamental difficulty of the method is that one has to distinguish between intrinsic effects in the AGN and the absorption effect due to the EBL. This requires some assumptions about the intrinsic spectra, which can, e.g. be derived from nearby sources or unattenuated parts of the  [22] spectrum.In addition, EBL attenuation should be a cosmological phenomenon affecting the spectra of all sources in a consistent way. A large sample of AGN will allow this to be tested. To accurately model the intrinsic parameters of distant sources, one requires a simultaneous measurement of the EBL attenuation (at high energies), together with the unattenuated intrinsic spectrum (at the lowest energies), and the CTA will have, in principle, the means to perform such studies. The separation between attenuated and unattenuated energy ranges depends on the redshift of the source. While for a source at z = 0.1 the unattenuated part of the spectrum (defined, say, as less than 30 % absorption by the EBL) can be measured for energies E < 400 GeV, this range shifts to energies E < 160 GeV for z = 0.4 and to E < 70 GeV for z = 1.0. Thus, in order to measure an unattenuated part of the spectrum (with a minimum lever arm of half a decade in energy) for sources at a redshift of at least z = 1.0 (which corresponds to about 50 % of the Universe), an energy threshold of 30 GeV or lower is required and CTA is planned to fulfil these constraints. Note that most of the extragalactic sources employed in EBL studies are variable in flux, and therefore, archival Fermi/LAT data at lower energies cannot be used for the purpose of accurate determining the unattenuated part of the spectrum [23].

Fundamental Physics with Gamma Rays
CTA will contribute to a number of topics related to fundamental physics which can be studied with the observations of cosmic gamma -rays. Among them, the areas in which it is expected to impact more are (a) the improvement of current (Fermi) limits on constraints of Lorentz invariance, (b) dark matter searches and (c) axion searches.

Searching for Dark Matter Annihilation
The dominant kind of matter in the Universe is the as yet unknown dark matter, which is most likely to exist in the form of a new class of particles such as those predicted in supersymmetric extensions to the standard model of particle physics. Depending on the model, these DM particles can annihilate or decay to produce detectable standard model particles, in particular gamma rays. Large dark matter densities due to the accumulation in gravitational potential wells lead to potentially detectable fluxes, especially for annihilation, where the rate is proportional to the square of the density.
CTA will be a discovery instrument with unprecedented sensitivity to study the properties of the dark matter particles. If particles beyond the standard model are discovered (at the LHC or underground experiments), CTA will be able to verify whether they actually form the dark matter in the Universe. Slow-moving dark matter particles could give rise to striking, almost mono-energetic, photon emission whose discovery would be conclusive evidence for dark matter. Line radiation from the most popular Minimal Supersymmetric Extension of the Standard Model (MSSM) is not detectable by Fermi, H.E.S.S. II or MAGIC II, unless optimistic assumptions on the dark matter density distributions are made. Among the individual targets for dark matter searches are dwarf spheroidals and dwarf galaxies. They exhibit large mass-to-light ratios and allow dark matter searches with low astrophysical backgrounds. With H.E.S.S., MAGIC and Fermi/LAT, some of these objects were observed and upper limits on dark matter annihilation were calculated, which are currently about an order of magnitude above the prediction of the most relevant cosmological models. CTA will have good sensitivity for Weakly Interacting Massive Particle (WIMP) annihilation searches in the low-and medium-energy domains.
An improvement in flux sensitivity of 1-2 orders of magnitude over current instruments is expected. Thus, CTA will allow tests in significant regions of the MSSM parameter space (see Fig. 10).
Finally, our galactic centre is one of the most promising regions to look for dark matter annihilation radiation due to its predicted very-high-energy dark matter density (Fig. 10). It has been surveyed by many experiments so far (e.g. H.E.S.S., MAGIC and VERITAS) and high-energy gamma emission has been found. However, the identification of dark matter in the galactic centre is complicated by the presence of many conventional source candidates and the difficulties of modelling the diffuse gamma-ray background adequately.
Dark matter would also cause spectral and spatial signatures in extragalactic and galactic diffuse emission. While the emissivity of conventional astrophysical sources scales with the local matter density, the emissivity of annihilating dark matter scales with the density squared, causing differences in the small-scale anisotropy power spectrum of the diffuse emission. Selection of sites is crucial for achieving optimum performance and science output with CTA. Criteria for site selection include, among others, geographical conditions, observational and environmental conditions, logistics, accessibility, availability, stability of the host region as well as local support. Among the basic requirements, we can cite: 1. Reasonably flat area of 10 km 2 for the south and 1 km 2 for the northern site. For best sky coverage, the latitude of the sites should be around 30 • north and south, respectively. 2. Altitude between ∼1,500 and 4,000 m, as this implies an optimum overall performance. Even higher altitudes allow further reduction of the energy threshold [26] at the expense of performance at medium and high energies and might be considered for the northern array.
Desirable is also to have a low component of geomagnetic field parallel to the surface, since such field deflects air shower particles. 3. High fraction of cloudless nights. For good sites, this fraction is well above 60 %, reaching up to 80 % for the very best ones. Artificial light pollution must be well below the natural level of night sky background, which excludes sites within some tens of kilometres from major population centres. Atmospheric transparency should be good, implying dry locations with low amounts of aerosols and dust in the atmosphere. 4. Environmental conditions should allow operation of telescopes without protective enclosures. Wind speeds above 10 m/s may impact observations; peak wind speeds, which may range from below 100 km/h to beyond 200 km/h depending on the site, have a major impact on telescope structure and cost. Sand storms and hail represent a major danger for unprotected mirror surfaces. Snow and ice prevent observations and would influence instrument costs, e.g. by making heating systems necessary and requiring structural stability. Seismic activity would similarly increase requirements on telescope structures and buildings.

CTA Site Candidates
A preliminary selection has looked for sufficiently large and flat areas above 1,500 m above sea level (a.s.l.), with the requirement that artificial background light is minimal, as determined from satellite images. Average cloud coverage was also restricted to be below 40 %. The resulting analysis identified a few locations matching these basic criteria, and from these locations, potentially interesting sites have been selected. Detailed studies are now being conducted in those candidate sites, for a final selection expected for the end of 2013. Among the northern sites, there are (a) the Canary Islands of La Palma and Tenerife, two well-known and wellexplored observatory sites at about ∼26 • N and 2,400 m a.s.l., and (b) San Pedro Martir, in Baja California-a well-established astronomical site that hosts already two observatories run by the Universidad Autonoma de Mexico, situated at about 31 • N and 2,800 m a.s.l.
The southern candidate sites include (a) the Khomas Highland of Namibia, a well-known astronomical site at 1,800 m a.s.l. and 23 • S, home to the H.E.S.S. instrument, offering a range of suitable large, flat areas; (b) the Chilean site of La Silla, one of the world's premier optical observatory locations, located at 29 • S and 2,400 m a.s.l.; (c) and finally, the El Leoncito and Puna Highland sites in Argentina, both around 30 • S and with altitudes in the range 2,500 and 3,500 m a.s.l., with sky quality equivalent to the Chilean sites and easy railway access.
The final decision among otherwise equivalent sites may rely on considerations such as financial or in-kind contributions by the host regions. It is likely that an intergovernmental agreement will be required to assure long-term availability of the site, as well as guaranteed access and free transfer of data. At the same, issues such as import taxes, value added tax and fees, etc. should be addressed. Such agreements already exist for H.E.S.S., Auger and other observatories operated by international collaborations in the candidate sites above.

Concluding Remarks: The Future in VHE Gamma-Ray Astronomy
The CTA project represents a perfect mix of guaranteed science and discovery potential and will provide a huge science return in astrophysics, particle physics, cosmology and fundamental physics, leading to the definitive establishment of ground-based gamma-ray astronomy in the international astronomical community.
CTA is currently finishing its preparatory phase and will soon enter the construction phase (2014-2018) which will pose it many challenges. A mini-array (named ASTRI-CTA) is already in construction (currently involving the CTA country members Italy, Brazil and South Africa) as the first step in this effort [27].
As exposed in this review, CTA is a safe extrapolation of proven technology and has a very predictable performance which guarantees its success. With a wellorganised collaboration formed by the concerted initiative of the three major current ground-based gamma-ray experiments (H.E.S.S., MAGIC and VERITAS), and counting with the addition of a number of scientists from the wider astrophysics, particle physics and cosmic-ray communities, the Consortium numbers 26 countries and over 800 scientists (representing more than 100 institutions), which include all the expertise necessary for the relevant areas of the experiment. CTA is already in the position to build the first prototype telescopes, following the recommendations and characteristics defined in the preparatory phase. Regarding funding, CTA is highly ranked in major science roadmaps (ESFRI, ASTRONET) and, as it exited the preparatory phase, it is currently supported by the European FP7.
CTA aims to be deployed within a 5-year time, in 2018. Brazil is a full member of CTA, being represented by institutions in the states of São Paulo (Universidade de São Paulo and Universidade Federal do ABC) and Rio de Janeiro (Centro Brasileiro de Pesquisas Físicas and Universidade Federal do Rio de Janeiro), counting with a handful of scientists actively involved in the experiment.