From the 20th to 25th of July 2009 the International School of Physics entitled “Radiation and Particle Detectors” was held in Varenna, which involved the use of detectors for the research in fundamental physics, astro-particle physics, and applied physics. At the school ten teachers and thirty students were present.

In the context of fundamental physics the High Energy Physics (HEP) plays an important role. In general the HEP experiments make use of sophisticated and massive arrays of detectors to analyze the particles which are produced in high-energy scattering events. This aim can be achieved in a large variety of approaches. Some examples are the following:

– Measuring the position and length of ionization trails. Much of the detection depends upon ionization.

– Measuring time of flight permits velocity measurements.

– Measuring radius of curvature after bending the paths of charged particles with magnetic fields permits measurement of momentum.

– Detecting Cherenkov radiation gives some information about energy, mass.

– Measuring the coherent “transition radiation” for particles moving into a different medium.

– Measuring synchrotron radiation for the lighter charged particles when their paths are bent.

– Detecting neutrinos by steps in the decay schemes which are “not there”, i.e., using conservation of momentum, etc. to imply the presence of undetected neutrinos.

– Measuring the electromagnetic showers produced by electrons and photons by calorimetric methods.

– Measuring nuclear cascades produced by hadrons in massive steel detectors which use calorimetry to characterize the particles.

– Detecting muons by the fact that they penetrate all the calorimetric detectors.

All these types of detectors are used in the largest accelerator ever built: the Large Hadron Collider (LHC). LHC is a proton-proton (also ions) ring, 27 km long, 100 m underground, with 1232 superconducting dipoles 15 m long at 1.9 K producing a magnetic field of 8.33 T. The figures of merit, for proton-proton operations, are beam-energy 7 TeV (7 × TEVATRON), luminosity 10³⁴ cm⁻²s⁻¹ (> 100 × TEVATRON), bunch spacing 24.95 ns, particles/bunch 1.1·10¹¹, and stored emergy/beam 350 MJ. For ion-ion operations we will have energy/nucleon 2.76 TeV/u, and total initial luminosity of 10²⁷ cm²s⁻¹. The main four experiments are two general purpose experiments (ATLAS and CMS), B-physics and CP violation experiment (LHCB), and heavy ions experiment (ALICE).

The international community of physicists hopes that the LHC will help answer many of the most fundamental questions in physics: questions concerning the basic laws governing the interactions and forces among the elementary particles, the deep structure of space and time, especially regarding the intersection of quantum mechanics and cosmology, where current theories and knowledge are unclear or break down altogether. The enormous success of the Standard Model (SM), tested at per mil level with all particles discovered except the Higgs boson, will hopefully be able to build a Cosmology Standard Model.

The issues of LHC physics include, at least:

– Is the Higgs mechanism for generating elementary particles masses via electroweak symmetry breaking indeed realised in nature? It is anticipated that the collider will either demonstrate or rule out the existence of the elusive Higgs boson, completing (or refuting) the SM.

– Is supersymmetry, an extension of the SM and Poincaré symmetry, realised in nature, implying that all known particles have supersimmetric partners?

– Are there extra-dimensions, as predicted by various models inspired by string theory, and can we detect them?

– What is the nature of the Dark Matter which appears to account for 23% of the mass of the universe?

Other questions are:

– Are electromagnetism, the strong force, and the weak interaction just different manifestations of a single unified force, as predicted by various Grand Unification Theories (GUTs)?

– Why is gravity so many orders of magnitude weaker than the other three fundamental interctions (Hierarchy Problem)? For all proposed solutions: new particles should appear at TeV scale or below.

– Are there additional sources of quark flavours, beyond those already predicted within the Standard Model?

– Why are there apparent violations of the symmetry between matter and antimatter (CP violation)?

– What was the nature of the quark-gluon plasma in the early universe (ALICE experiment)?

Obviously, for the construction of a Standard Cosmology Model, the astro-particle experiments are crucial with direct or indirect dark matter measurements. In particular, the Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics (PAMELA) experiment, which went into space on a Russian satellite launched from the Baikonur cosmodrome in June 2006, uses a spectrometer—based on a permanent magnet coupled to a calorimeter—to determine the energy spectra of cosmic electrons, positrons, antiprotons and light nuclei. The experiment is a collaboration between several Italian institutes with additional participation from Germany, Russia and Sweden. 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. These measurements, together with the complementary electromagnetic radiation observation that will be carried out by AGILE and GLAST space missions, will help to unravel the mysteries of the most energetic processes known in the universe. Recently published results from the PAMELA experiment have shown conclusive evidence of a cosmic-positron abundance in the 1.5–100 GeV range. This high-energy excess, which they identify with statistics that are better than previous observations, could arise from nearby pulsars or dark matter annihilation. Such a signal is generally expected from dark matter annihilations. However, the hard positron spectrum and large amplitude are difficult to achieve in most conventional WIMP models. The absence of any associated excess in antiprotons is highly constraining on any model with hadronic annihilation modes. The light boson naturally provides a mechanism by which large cross-sections can be achieved through the Sommerfeld enhancement, as was recently proposed. Depending on the mass of the WIMP, the rise may continue above 300 GeV, the extent of PAMELA's ability to discriminate electrons and positrons. The data presented include more than a thousand million triggers collected between July 2006 and February 2008. Fine tuning of the particle identification allowed the team to reject 99.9% of the protons, while selecting more than 95% of the electrons and positrons. The resulting spectrum of the positron abundance relative to the sum of electrons and positrons represents the highest statistics to date. Below 5 GeV, the obtained spectrum is significantly lower than previously measured. This discrepancy is believed to arise from modulation of the cosmic rays induced by the strength of the solar wind, which changes periodically through the solar cycle. At higher energies the new data unambiguously confirm the rising trend of the positron fraction, which was suggested by previous measurements. This appears highly incompatible with the usual scenario in which positrons are produced by cosmic-ray nuclei interacting with atoms in the interstellar medium. The additional source of positrons dominating at the higher energies could be the signature of dark matter decay or annihilation. In this case, PAMELA has already shown that dark matter would have a preference for leptonic final states. They suggest that the alternative origin of the positron excess at high energies is particle acceleration in the magnetosphere of nearby pulsars producing electromagnetic cascades. The members of the collaboration state that the PAMELA results presented here are insufficient to distinguish between the two possibilities. They seem, however, confident that various positron production scenarios will soon be testable. This will be possible once additional PAMELA results on electrons, protons and light nuclei are published in the near future, together with the extension of the positron spectrum up to 300 GeV thanks to ongoing data acquisition.

S. Bertolucci, U. Bottigli and P. Oliva