Astroparticle Physics (CSN2)

AUGER

 

The Pierre Auger Observatory is the world leading science project for the exploration of cosmic rays. More than 500 scientists from 16 countries work together to study the highest-energy cosmic rays by measuring the properties of the showers produced in the atmosphere.

The Observatory is located near Malargüe (Argentina) and has been detecting ultra-high energy cosmic rays for more than ten years. An international agreement has been signed in November 2015 by the science funding agency representatives to continue the operation until 2025.

The essential feature of the Observatory is its hybrid design, a combination of a large surface array and a fluorescence detector. The surface detector (SD) is composed of 1600 water Cherenkov units, spaced by 1500 m, covering a total area of 3000 km2. The fluorescence detector consists of 24 telescopes overlooking the surface array.

The simultaneous observation of the showers with different techniques enables high-statistics and high-precision studies and the huge extension of the SD array allows to detect the very rare events at ~10^20 eV, whose flux is ~1 particle/km^2/century. The baseline configuration of the detectors has been enhanced with a smaller and denser array of SD units and with high elevation fluorescence telescopes to reduce the minimum detectable shower energy down to ~10^17 eV.

The Pierre Auger Observatory has yielded dramatic advances in the measurements of cosmic rays. The abrubt suppression of the energy spectrum above 5 x 10^19 eV has been unequivocally observed. This was predicted  since a long time as a consequence of the interaction of cosmic-ray particles  with the low energy photons of Cosmic Microwave Background.

The so called ankle, that is the flattening of the spectrum at 5 x 10^18 eV, has been measured with an unprecedented precision. The FD measurements of the longitudinal shower profile has confirmed that the mass composition is mainly made by light primaries around the ankle and have provided evidence of an unexpected shift towards heavier primaries at the highest energies. A result that is consistent with the no evidence of anisotropy or of association with astrophysical sources of the arrival direction of cosmic rays.

Very stringent limits on the flux of photons and neutrinos have allowed to exclude most of the so-called  top-down models, in which the cosmic rays are generated by decay of super heavy dark matter or topological defects or similar exotic particles, favouring scenarios in which the acceleration of the primaries occurs in astrophysical sources.

The three dimensional nature of the SD  units has allowed to study the showers inclined at large zenith angles. In these showers only muons arrive at ground and the comparison of the measurements with the Monte Carlo simulations has provided a powerful test of the hadronic interaction models extrapolated at energies order of magnitudes larger than the one reachable at LHC.

Due to the reduced duty cycle of the FD, the mass composition of the cosmic rays into the suppression region remains unexplored.

To solve this problem the Auger collaboration has planned an upgrade of the SD called AugerPrime. Plastic scintillator detectors will be installed on the top of each SD units. Combining the different responses of the scintillators and of the water Cherenkov detectors to the electromagnetic and muonic component of the shower, it will be possible to estimate the mass composition at the very high energies and to make anisotropy studies of the light primaries.  

 

 

CTA  Cherenkov Telescope Array

 

 

The CTA project is an initiative to build the next generation ground-based very high energy gamma-ray instrument. It will serve as an open observatory to a wide astrophysics community and will provide a deep insight into the non-thermal high-energy universe. A short movie outlining the envisaged arrays is available here. A special edition of the journal Astroparticle Physics with a focus on CTA can be accessed here.

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

2  Understanding the nature and variety of particle acceleration around black holes

Searching for the ultimate nature of matter and physics beyond the Standard Model

http://cta.roma2.infn.it

 

 

DAMA

 

The DAMA project at the Gran Sasso National Laboratories of the I.N.F.N. is an observatory for rare processes thanks to the development and use of large mass highly radiopure scintillator set-ups. The main activity field is the investigation on Dark Matter particles in the galactic halo and the search for several other rare processes (such as bb decay modes in various isotopes, charge-non-conserving processes, Pauli exclusion principle violating processes, nucleon instability, detection of solar axions and search for exotics).

Detectors

The main radiopure experimental set-ups of the DAMA project at LNGS are:

  • the DAMA/NaI set-up (~100 kg highly radiopure NaI(Tl)), which has completed its data taking in July 2002. It had as main aim the investigation of the presence of a Dark Matter particle component in the galactic halo by means of the model independent annual modulation signature. With the 0.29 ton x year total exposure it achieved a 6.3 sigma C.L. model independent evidence [1], [2]. Some of the many possible corollary quests for the candidate particle have also been carried out [1], [2], [3], [4], [5], [6], [7], [8]. The same experimental set-up has also obtained several other results on other approaches and on various other rare processes (e.g. solar axion, possible Pauli exclusion principle violation, cluster decay, possible spontaneous transition on nuclei to a superdense state, Q-balls, possible charge non conserving processes, SIMPs and nuclearities, electron stability);
  • the DAMA/LXe set-up (~6.5 kg liquid Xenon), which can take data filled with Kr-free Xenon enriched either in 129Xe or in 136Xe isotopes. It has investigated several rare processes with time passing in the various configurations and is in operation. The main experimental results have been obtained investigating e.g.: dark matter with various approaches, double beta decay in 134Xe and in 136Xe, possible charge non conserving processes, electron stability, possible nucleon, di-nucleon and tri-nucleon decay into invisible channels;
  • the DAMA/R&D set-up, where tests of prototype PMTs and detectors are carried out as well as small scale experiments mainly devoted to the search for double beta decay processes in various isotopes (e.g. 40Ca, 46Ca, 48Ca, 64Zn, 70Zn, 180W, 186W, 106Cd, 108Cd, 114Cd, 136Ce, 138Ce, 142Ce, 130Ba); other rare processes have been also studied, we recall e.g.: dark matter with CaF2 crystal scintillator, rare alpha decay in 151Eu, rare beta decay of 113Cd. Many other measurements are in progress/preparation;
  • the second generation DAMA/LIBRA set-up (Large sodium Iodide Bulk for RAre processes; ~250 kg more radiopure NaI(Tl)) is continuing the investigations of DAMA/NaI with a larger mass and an increased sensitivity. In particular the data of the first four annual cycles (exposure: 0.53 kg x year) have been released in 2008. The results confirm the model independent evidence for the presence of Dark Matter particle in the galactic halo [1]. In February 2010 the data of two further annual cycles have been released [2]and in 2013 [3] the cumulative model independent result of DAMA/LIBRA-phase1 has been released by adding an additional annual cycle. The cumulative exposure collected, considering the 7 annual cycles of DAMA/NaI and the 7 annual cycles of DAMA/LIBRA-phase1, is 1.33 ton x year. The model independent evidence is observed with a statistical significance of 9.3 sigma C.L.. This model independent result is compatible with various Dark Matter candidate particles, interaction types and scenarios. No experiment whose results can be directly compared in a model independent way with these ones is available in the field;
  • the DAMA/Ge set-up dedicated to measurements on samples' radiopurity and on relatively small scale experiments, is operative at the LNGS STELLA facility; is has obtained several interesting results on rare processes. It is continuosly in operation and many new measurements are in preparation;
  • the DAMA/CRYS set-up is a new small scale facility mainly dedicated to the characterization of new prototype detectors;
  • finally a 3-rd generation R&D toward the creation of a possible 1 ton NaI(Tl) set-up, proposed already in 1996, is in progress.

Dark matter investigation

The DAMA/NaI and DAMA/LIBRA set-ups have been designed and built to exploit the model independent annual modulation signature and to point out the presence of Dark Matter particles in the galactic halo independently on hypothesized theoretical models. This signature originally suggested in the mid ‘80s by Freese et al. requires that many specific peculiarities are simultaneously satisfied in order to provide unequivocal evidence. In fact, as a consequence of its annual revolution around the Sun, which is moving in the Galaxy, the Earth should be crossed by a larger flux of Dark Matter particles around 2 June (when the Earth orbital velocity is summed to the Sun velocity in the Galaxy) and by a smaller one around 2 December (when the two velocities are opposite). Thus, this signature has a different origin and peculiarities than the seasons on the Earth and than effects correlated with seasons (consider the expected value of the phase as well as the other requirements listed below). The DM annual modulation signature is very distinctive since the effect induced by DM particles must simultaneously satisfy all the following requirements: the rate must contain a component modulated according to a cosine function (1) with one year period (2) and a phase that peaks roughly around about 2nd June (3); this modulation must only be found in a well-defined low energy range, where DM particle induced events can be present (4); it must apply only to those events in which just one detector of many actually “fires” (single-hit events), since the DM particle multi-interaction probability is negligible (5); the modulation amplitude in the region of maximal sensitivity must be < 7% for usually adopted halo distributions (6), but it can be larger in case of some possible scenarios. This approach is very competitive and it needs the realization of an experiment with large mass, high radio-purity and high control of the running conditions, as is the case of the former DAMA/NaI and of the present DAMA/LIBRA experiments.
As mentioned before DAMA/NaI and DAMA/LIBRA have measured an annual modulation effect satisfying all the peculiarities of the considered signature; if the total cumulative data of the two experiments are considered, the measurements refer to 14 annual cycles. In each annual cycle an independent effect of this modulation has been observed. The cumulative exposure of about 1.33 tons x years is several orders of magnitude greater than the exposures typically achieved in this field. The observed modulation effect is highly significant. Neither systematic effects nor side reactions able to mimic the signature were found, no systematic effect able to explain the observed modulation amplitude and to satisfy simultaneously all the requirements of the signature has been found or suggested by anyone over more than a decade. The results obtained by DAMA/NaI and DAMA/LIBRA satisfy all the requirements of the signature and indicate the presence of dark matter particles in our galaxy with high significance. The observed effect is also consistent with a wide range of possible scenarios on the nature of these particles, on characteristics of interaction and on the structure of the galactic halo. In particular, many interpretations of the observed effect in terms of several possible candidates and of various scenarios of particle physics, nuclear physics and astrophysics have been performed so far; other interpretations have been proposed and/or planned. It is worth noting that no experiment whose results can be compared in a model independent way with those of DAMA/NaI and DAMA/LIBRA is available in the field. We also note the currently available model dependent negative results obtained in indirect and direct searches are not in robust conflict with the model independent result of DAMA, while compatibility exists with some possible recent positive hints.
The unique properties and characteristics of DAMA/LIBRA (whose first upgrade has been performed in September 2008, while the second more important one has been carried out in Fall 2010) will also allow in the future the further investigation of the features of the candidate particles and of the astrophysical, nuclear and elementary particle aspects related to the signal characteristics and of the possible second order effects.
The DAMA/LIBRA set-up has also obtained several other results on other approaches and on various rare processes. The experiment has been upgraded and at present DAMA/LIBRA-phase2 is running.

For references and results from all the experimental set-ups
see in : http://people.roma2.infn.it/dama

 

 


Fermi Large Area Gamma ray Telescope (formerly GLAST )

 

The FERMI gamma ray satellite,  is in orbit, since June 2008. After a commissioning period is now delivering data. 

The Large Area Telescope (LAT), onboard of FERMI, is the most sensitive gamma-ray detector to date, in the 20 MeV - 300GeV energy band. It provides large effective collection area (>8000cm2@1GeV), wide field of view (>2sr) and good energy resolution (8%@1GeV).  The very large field of view will make it possible to observe 20% of the sky at any instant, and the entire sky on a timescale of a few hours.

Fermi is opening a new and important window on a wide variety of phenomena, including black holes and active galactic nuclei; the optical-UV extragalactic background light, gamma-ray bursts; the origin of cosmic rays and supernova remnants; and searches for hypothetical new phenomena such as supersymmetric dark matter annihilations and Lorentz invariance violation.
 

 

 

GAPS

 

 

he GAPS experiment was designed to study the antimatter component in cosmic rays with a specific focus on antideuterons, antiprotons and low-energy anti-helium nuclei (< 0.25 GeV / n). The first identification of antideuterons in cosmic rays would be a strong hint of new physics, exploring a wide range of theoretical models of Dark Matter (DM) together with more exotic sources. Secondary production of antideuteron and anti-helium by interaction between cosmic rays and interstellar medium is significantly suppressed at low energies with respect to the production of DM candidates.  
 
GAPS was proposed to NASA for a series of long-duration flights (around 100 total days) with stratospheric balloon from Antarctica. In 2019, the start of the integration phase of the various detectors is expected, while in the summer of 2020 everything will be transported to the American base of McMurdo. The launch is scheduled for the austral summer of 2020-2021. 
 
The GAPS antiparticle identification approach is truly innovative. 
In fact, it will provide a nearly background free technique of identifying antideuterons by using three different methods of detection for every event. Initially, a plastic time of flight (TOF) system tags the particle and records the velocity. This will distinguish antideuterons from lighter particles, such as antiprotons. The particle then slows down and stops in the Si(Li) tracker, forming an excited exotic atom. This atom then de-excites and releases both X-rays and a pion star.
Si(Li) wafers (with 50 ns time resolution and 2 keV energy resolution) will be crucial in the process of particle identification; the X-ray energy only depends on the mass and charge of the particle, therefore its signature precisely determine the type of antiparticle detected as it makes transitions to lower states. Finally, the pion star (the third layer of detection) provides greater background suppression.

The INFN (Trieste, Rome Tor Vergata, Bergamo, Florence, Naples, Pavia) contribution to the project concerns the design and implementation of ASICs for the DAQ of silicon detectors. In addition, the Italian groups will contribute to the development of software for simulation, data-structure and analysis, as well as the future data analysis itself and interpretation of results.

LAser RAnged Satellites Experiment (LARASE)

 

Einstein's theory of general relativity (GR) represents the best theory we have at our disposal for the description of the gravitational interaction, both at the high and low energy scales, and it is the pillar of modern cosmology to understand the universe that we observe through a number of different techniques. Indeed, after 100 years, GR has passed a wide number of experimental verifications and it is currently considered the “Standard Model” for  gravitational physics.

The experiment denominated LARASE (LAser RAnged Satellites Experiment) represents a new research program whose main goal is to provide accurate measurements for the gravitational interaction in the weak-field and slow-motion (WFSM) limit of GR by means of the laser tracking of satellites orbiting around the Earth. In fact, thanks to the very precise measurements provided by the powerful Satellite Laser Ranging (SLR) technique, with a precision down to a few mm in root-mean-square of the so-called Normal Points ―which represent the station-to-satellite distances averaged over a suitable time period ―we are able to reconstruct the orbital elements of each satellite with an accuracy of about a cm over weekly arcs.

Among the various ingredients needed, two of them play a very significant role: i) the quality of the tracking observations of the orbit, and ii) the quality of the dynamical models. The dynamical models are implemented in a software code whose goal is to minimize, opportunely, an observable function and solve for the unknowns in which we are interested. These models have to account for both gravitational and non-gravitational forces in such a way to reduce as much as possible the difference between the observed range and the computed (from the models) one. Of course, the better is the minimization process through the orbit data reduction from one side and the better the estimate of the systematic error sources from the other, more precise and accurate will be the a posteriori reconstruction of the satellite orbit.

 

 

The test masses of the LARASE experiment are spherical in shape and fully passive laser-ranged satellites with a generally low area/mass ratio in order to minimize the non-gravitational accelerations.

In this family, the two LAGEOS and the recently launched LARES are the most important to consider because of the high accuracy of their orbit determination thanks to the very precise measurements of the SLR technique. The older LAGEOS (LAser GEOdynamic Satellite) was launched by NASA on May 4, 1976, LAGEOS II was jointly launched by NASA and ASI on October 22, 1992, finally LARES (LAser RElativity Satellite) was launched by ASI on February 13, 2012.

 

Therefore, LARASE aims to improve the dynamical models of the current best laser-ranged satellites, as well as to improve the error budget estimates of the several perturbations, both  gravitational and non-gravitational, that influence their (in principle) geodesic motion around the Earth.

This  will allow to test in a reliable way Einstein's theory of GR with respect to other metric and non-metric theories for the gravitational interaction and to go beyond the present measurements as well as the kind of tests carried out so far. 

 

 

LIMADOU

LIMADOU is a scientific payload for the Chinese Seismological Experiment Satellite (CSES). INFN and the Italian Space Agency (ASI) participate in the project of the Chinese Earthquake Administration (CEA).

CSES mission will study the ionospheric perturbations possibly associated with earthquakes - especially with destructive ones - and explore new approaches for short-term and imminent forecast, as well as will help find a new way for theoretical studies on the mechanism of earthquake preparation processes. 
The program will make use of new techniques and equipments, in order to obtain world-wide data of space environment of the electromagnetic field, plasma and energetic particles. 
 

The satellite is based on the Chinese CAST2000 platform. It is a 3-axis attitude stabilized and will be placed in a 98° Sun-syncronous circular orbit at an altitude of 500 km in September 2016. CSES satellite will be launched in 2017  and inserted into a circular Sun-syncronous orbit with 98 degrees inclination and 500 km altitude. Expected lifetime is 5 years. 

CSES hosts several instruments onboard:  2 magnetometers, an electrical field detector, a plasma analyzer, a Langmiur probe and a High Energy Particle Detector (HEPD). A memorandum of understanding between the Chinese National Space Administration (CNSA) and the Agenzia Spaziale Italiana (ASI) concerning cooperation on the China Seismo-Electromagnetic Satellite (CSES) has been signed on September 25, 2013. The INFN groups are developing prototypes of the Electric Field Detector (EFD) and of the High Energy Particle Detector (HEPD). 

 

The  HEPD consists of two layers of  plastic scintillators for the trigger, and a calorimeter constituted by a tower of plastic scintillator counters and a LYSO plane.  The direction of the incident particle is provided by two planes of double-side silicon microstrip detectors placed in front of the trigger.

HEPD detector will measure electrons (3 - 100 MeV) and protons (30 - 300 MeV) along CSES orbit.  The angular and energy resolution and the detector acceptance are optimized to accurately detect the expected low short-term time variations of the particle flux from the radiation belts.

 

 

LISA-Pathfinder

Gravity is mediated by the deformation of spacetime. Accelerated matter produces gravitational radiation that travels in waves unimpeded throughout the entire universe. A detailed analysis of these waves will bring the next big revelations in astronomy, cosmology, and fundamental physics alike. Future space-based interferometric gravitational wave detectors cover the most rewarding range of frequencies and enable us to directly study black holes, neutron stars, and even the echo of the Big Bang itself. These detectors – like the one planned by the European Space Agency – will measure minute changes in the position of a free-floating test mass caused by passing gravitational waves. This signal will be very tiny, so it is necessary to keep the environment around the test mass free of gravitational disturbances.
 LISA Pathfinder, an ESA mission which contains an additional NASA payload, is designed to demonstrate the technologies necessary for future space-based gravitational wave detectors. Two test masses will be kept in near-perfect gravitational free-fall, and their position will be measured and controlled with unprecedented accuracy. This is achieved through the use of state-of-the-art technology comprising inertial sensors, a laser metrology system, a drag-free control system and an ultra-precise micro-propulsion system.
The LISA Pathfinder satellite will demonstrate that it is possible to maintain a test mass in a drag-free environment, and accurately measure changes in its position.
 
 
LISA Pathfinder is unusual in design. It does not possess a typical payload structure – the spacecraft as a whole is part of the experiment. At the core of the satellite are two drag-free test masses: cubes of gold-platinum alloy whose position relative to each other, and the satellite, will be monitored by a complex laser system. LISA Pathfinder will be able to measure any local disturbances from the spacecraft that act on these test masses. All of the most relevant hardware was designed to meet the requirements for a future space-based gravitational wave detector, including:  
      
    - Gravity Reference Sensor (vacuum chamber, test masses, electrode housing)
      
    - Launch-lock and test-mass-release-into-orbit mechanisms
      
    - Optical Readout (hydroxide-bonded monolithic optical bench)
      
    - Nd:Yag laser and local laser interferometer
      
    - Cold gas micro-thrusters (for drag-free control)
 
 
Learn more about LISA Pathfinder on the:
ESA mission website: http://sci.esa.int/lisa-pathfinder/
eLISA Consortium website: https://www.elisascience.org/articles/lisa-pathfinder/lpf-mission
 
LPF launched successfully on 3rd December 2015 at 4:04 GMT.
 
LISA Pathfinder (LPF) will place two test masses in a nearly perfect gravitational free-fall, and will control and measure their relative motion with unprecedented accuracy. This is achieved through innovative technologies comprising inertial sensors, an optical metrology system, a drag-free control system and micro-Newton thruster system. The test-masses and their environment will be the quietest place in the solar system.
 
All these technologies are not only essential for eLISA, they also lie at the heart of any future space-based test of Einstein's General Relativity.
 
 
 

LSPE

 

The LSPE (Large Scale Polarization Explorer) experiment is a mm-wave polarimeter aboard of a stratospheric balloon and studies the polarization of the cosmic microwave background (CMB), with the aim of measuring the weak signal produced by cosmic inflation, in the first moments of the evolution of the universe, at energies of the order of 1016 GeV, which cannot be reproduced in any laboratory.

The polarization state of the photons of the microwave background is sensitive to scalar (density) and tensor (gravitational waves) perturbations present at the time of recombination. In fact, most of the CMB photons is diffused (due to Thomson scattering) by free electrons for the last time on the last scattering surface. Thus, any quadrupole anisotropy present in the photons incident on the electrons produces a certain degree of linear polarization of the scattered photons. At the recombination time, protostructures (density perturbations), that will form the galaxies and clusters, and gravitational waves (tensor perturbations), produced by the hypothetical inflation, are both present and both have a quadrupole component that generates a linear polarization of the diffused CMB photons. The symmetry properties, however, are different: in case of scalar perturbations an irrotational polarization field (E-modes) is generated, while tensor perturbations produce a polarization field with both irrotational and rotational (the so-called B-modes) components.

 

So far, the detection of the B-modes has been performed by one experiment only (BICEP2), at a single frequency (140 GHz), and on a limited region of the sky (a few %). Moreover, a combined analysis of the BICEP2 and Plank data has shown that the result is most likely polluted by a large contribution from interstellar dust, thus of non-cosmological origin. New experiments are needed, with a larger coverage of the sky and a broader range of observed frequencies.

The LSPE experiment meets these requirements, observing a large fraction of the sky (25%) in a wide frequency range, between 40 and 250 GHz. It will fly on a stratospheric balloon in the polar night, thus overcoming the problem of transmission and noise pollution that prevent ground experiments, such as BICEP2, to operate efficiently at frequencies higher than 140 GHz.

The INFN sections of Genoa (F. Gatti, National), Pisa (G. Signorelli), Roma (P. de Bernardis) and Rome Tor Vergata (A. Rocchi) actively contribute to the experiment.

 

 

WIZARD/PAMELA 

 

The Space Mission Pamela represents a state-of-the-art of the investigation of the cosmic radiation, addressing the most compelling issues facing astrophysics and cosmology: the nature of the dark matter that pervades the universe, the apparent absence of cosmological antimatter, the origin and evolution of matter in the galaxy. 

PAMELA, a powerful particle identifier using a permanent magnet spectrometer with a variety of specialized detectors, is an instrument of extraordinary scientific potential that is measuring with unprecedented precision and sensitivity the abundance and energy spectra of cosmic rays electrons, positrons, antiprotons and light nuclei over a very large range of energy from 50 MeV to hundreds GeV, depending on the species.

PAMELA has been put in an elliptical orbit at an altitude between 350 and 610 Km, on board of the Resurs-DK1 Russian satellite by a rocket Soyuz, on the 15th of June 2006. In September 2010 the orbit was changed to a nearby circular one, at an altitude of about 570 km, and it has not changed since then.

PAMELA results are available in a number of publications, providing new, precise information on the composition and energy spectrum of cosmic rays. 

The matter and anti-matter components of cosmic rays have been extensively explored both in composition and in energy spectrum. Differential energy spectra of particles of galactic and solar origin, as well as trapped secondaries, have been measured. Moreover, over the long PAMELA data taking period, spectral evolution in time is being monitored, and both short and long term effects are being studied. 

Since October 2013 all the published data are public and available at asi data center, accessible for an easy visualization.

The mission continues to take data and has been recently approved by the Russian Space Agency until 2019.

 

Suggested link: http://pamela.roma2.infn.it

 

 

VIRGO 

 

 

 

Virgo is a ground-based laser interferometer designed to detect gravitational waves (GW). It is located in Cascina, a small town near Pisa on the site of the European Gravitational Observatory (EGO). It is a project born from a collaboration between France (CNRS) and Italy (INFN) and is now operated by an international collaboration of scientists from France, Italy, the Netherlands, Poland, and Hungary.

Gravitational waves are predicted by the theory of General Relativity, published by Albert Einstein in 1916. They are ripples in the fabric of the spacetime that propagate at the speed of light, and are produced when huge masses are accelerated or deformed. This happens in many astrophysical scenarios, including supernova explosions or the gravitational interactions between black holes or neutron stars.

Gravitational waves are completely different from light, the main “messenger” used so far to study the Universe, although nowadays scientists have started to exploit other cosmic messengers, like cosmic rays or neutrinos. Catching gravitational waves will therefore open a new window on the Universe, allowing us to probe extreme phenomena driven by gravity, unexplored domains of the physics of matter at supranuclear density and of strong gravitational fields.

 

Virgo consists mainly in a Michelson laser interferometer made of two orthogonal arms, each 3 kilometers long. Multiple reflections between mirrors located at the end of each arm extend the effective optical length of each arm up to 120 kilometers. The frequency range of Virgo extends from 10 to 6,000 Hz. The whole interferometer attains optical perfection and is extremely well isolated from the rest of the world in order to be only sensitive to the gravitational radiation. To achieve it, scientists have developed the most advanced techniques in the field of high power ultrastable lasers, high reflectivity mirrors, seismic isolation and position and alignment control. The initial Virgo detector observed the sky between 2007 and 2011, resulting in no detection, but in a series of excellent upper-limits on the GW emission for several sources.  At present, Virgo is undergoing a major upgrade that will push all the employed technologies to the limit and improve the sensitivity by a factor of ten at all detection frequencies. Advanced Virgo will start to search again for gravitational waves in 2016 with a much better sensitivity than the initial detectors, opening the so-called second generation era. The sensitivity of Advanced Virgo will allow us to observe gravitational wave sources ten times further away and explore a volume 1000 times larger than before. In such a large volume, the detection of a gravitational wave event will be way more probable. In fact, one could expect to detect at least one gravitational wave signal per month or even per week.  In order to achieve this jump in sensitivity, the scientists of the Virgo collaboration have developed frontier technologies that will be crucial to reduce the spurious signals produced by various types of noises.Some of the key improvements of Advanced Virgo are:

-       the laser power will be higher;

-       a sophisticated system of thermal compensation of the optics to avoid optical aberration will be introduced;

-       a new optical layout designed for reducing the effect of mirror thermal vibrations will be used;

-       new optics suitable for shaping the sensitivity curve are foreseen, to optimize the detector performance on a choice of observed sources;

-       the new mirrors of Advanced Virgo have the best quality in the world. They have a reflectivity of 99.999% and the defects on their surface are kept below one nanometer;

-       laser beam will propagate in an ultra-high-vacuum improved by a factor of ten with respect to Virgo: the residual pressure in the vacuum pipes will be a millionth of a millionth of an atmosphere.

 

Advanced Virgo will hear for gravitational waves with frequencies between 10 Hz and 10000 Hz. These are the same frequencies as the sound waves that are audible by humans. Any signal caught by Advanced Virgo could be sent to a loudspeaker: we will hear the symphony of the Universe.