What the matter is made of? How can we understand the evolution of very complex systems? Which processes fuel the stars? How is the universe evolving? These are just few of the questions driving the research in the science fields in which I worked for many years.
To begin with, let me state something which today everybody knows: matter is made of atoms. It took about 2.5 millennia before the idea proposed by Democritus (5th century b. C.) that the immense variety of substances, things, and living beings is actually the result of the combination of a relatively small number of different species of atoms. Indeed, the atomic nature of matter is the intimate reason why thermodynamics works, despite from the fact that thermodynamics has been developed before the experimental confirmation of the existence of atoms! The whole field of chemistry also relies on the atomic nature of matter: chemical reactions strongly suggest the existence of about hundred elements that combine among themselves to form molecules with well defined and constant proportions. John Dalton formulated his law of multiple proportions in the early 1800s but this was not yet a striking demonstration of the existence of atoms.
We had to wait until Albert Einstein interpreted in 1905 the Brownian motion (scattering of fine dust grains floating in water observable at the microscope) as the result of chaotic molecular motion, which allowed Jean Perrin to estimate mass and dimensions of the atoms, before the atomistic theory was universally accepted. But soon it became clear that the atoms are not indivisible, as their name implies. In 1909, Hans Geiger and Ernest Marsden, under the direction of Ernest Rutherford, bombarded a metal foil with alpha particles to observe how they scattered. These particles are the result of natural radioactivity, discovered in 1896 by Henri Becquerel, and have positive charge. Geiger and Marsden noticed that most alpha particles could traverse the foil undisturbed, but a small fraction was deviated by large angles (even back scattered). Rutherford interpreted the result as the evidence that atoms have a very small nucleus that occupies a tiny volume, still it carries all the mass of the atom. Because the nucleus must be positively charged to produce the repulsive force that is responsible for the back scattering of the alpha particles, while it was known that the atoms are neutral, the negative charge balancing the nuclear charge must be distributed over the entire volume of the atom. This charge is carried on by the electrons, discovered in 1896 by Sir Joseph John Thomson and his colleagues John S. Townsend and Harold A. Wilson. Chemical properties are explained in terms of the electronic "clouds" that surround the nuclei, and the variety of the elements is the result of the existence of nuclei with 1, 2, 3, etc. unit charges.
Thanks to their electric charge, electrons and alpha particles can be accelerated and directed against some sample, where they interact with the nuclei. It was so discovered that the nuclei are not indivisible, but are made of two types of particles, the positively charged protons and the neutrons, which as their name implies carry no electric charge. Actually the alpha particle is itself made of two protons and two neutrons, hence it is the same as a helium nucleus. By increasing the collision energy it was then discovered that protons and neutrons have an internal structure, interpreted in terms of quarks and gluons. On the other hand, as of today there is no indication that electrons have an internal structure: they are really indivisible. When looking at the results of energetic collisions, something completely new emerges: when the collision energy is high enough new particles are created.
Physicists at CERN study what happens when very energetic particles (usually protons or electrons) collide: their total energy (in their center of momentum reference frame) becomes entirely available for a number of interesting processes, all characterized by the creation of new particles. The creation of new particle is possible because energy and mass can be converted one in the other (this is the meaning of Einstein's most famous formula, E = m c2), but whenever a particle is created it must be accompanied by its antiparticle. Similarly, when particle and antiparticle meet each other, they annihilate, i.e. disappear completely leaving only pure energy (which can be carried away by photons or other newly created particles). Particles and antiparticles have opposite charge, such that there is always a perfect balance.
All particles we know fall in one of few categories. One classification is based on their spin (the intrinsic angular momentum), which can be an integer or half-integer multiple of a fundamental unit (the reduced Planck constant ħ). Particles with integer spin are called bosons (after Satyendra Nath Bose), while the others are called fermions (after Enrico Fermi). At the fundamental level, matter is made entirely of fermions. On the other hand, their interactions are mediated by few bosons: photon, Z and W are responsible of the electromagnetic and weak interactions, while gluons mediate the strong interaction. Another subdivision can be made in terms of the interactions. The quarks have e.m., weak and strong interactions and constitute the hadrons (containing minimum two quarks). However, there are particles that do not interact strongly: they are called leptons. Among the leptons we have charged particles (electron, muon, tau) and neutral particle (electron neutrino, muon neutrino, tau neutrino). The neutrinos have only weak interaction, which is so-called because it's really weak! Most neutrinos cross the entire Earth without interacting! We could safely neglect the weak interaction, if the latter would not cause the spontaneous decay of a particle into a lighter one. Despite from the very low intensity, the weak interaction causes all spontaneous radioactivity, hence it is a very important interaction.
Coming back to CERN and other laboratories, most of the particles created in the collisions have an internal structure and are unstable, i.e. they disappear quickly decaying into lighter particles like electrons. In the past decades a large number of new particles have been discovered, and their internal structure has been interpreted in terms of their quark content. Very noticeably, isolated quarks can never be observed: they immediately "hadronize" creating new quarks to build the observable particles (called hadrons). Today we know that there are 6 types of quarks, called with funny names like up, down, strange, charm, bottom and top, and that their interaction (called strong interaction) is the most intense force in nature. Quarks are very peculiar fermions, because they are the only known objects with charge smaller than the fundamental unit (the electron or proton charges, differing only for the sign): they have fractional charge 2/3 (u, c, t) or -1/3 (d, s, b) . Because the quarks have (vastly) different masses and only u and d are stable (but always hidden inside a hadron, where they continuously interact), the heavier ones decay in the lighter ones. During the decay they have their own preferences, such that we can form three pairs (u,d), (c,s), and (t,b).
It is very suggestive that the leptons can also be assigned to 3 families, each with a charged and a neutral partner. Tau and muon particles are heavy and they decay into lighter particles. Muons decay into electrons and neutrinos (leptons are always created in pair with an antilepton in the same family) but tau leptons are so heavy that may decay also in quarks. Aha! This means that there can be a connection between the lepton and quark families... perhaps. As of today, no such connection is known. But it's a fact that the weak interaction connects quarks and leptons.
Particle accelerators are probing Nature at higher and higher energies, allowing scientists to test their models about cosmic evolution, and suggesting scenarios for the different epochs of the Universe life. In fact, energies as high as those obtained using colliders can be found only in the very early Universe, or in "exotic" astronomic objects, like supernovae (SN), pulsars or active galactic nuclei (AGN), thought to be the sources of cosmic rays. Thus, high energy physics and high energy astrophysics, even though they deal with microscopic scale process and macroscopic systems respectively, are quite close to each other.
Let's consider the present picture of the whole Universe. It's experimentally known that it is expanding, then in the past it had to be smaller. If you went back in the time, you would see the temperature to increase and the distances between objects to shorten: you would see the collapse of the whole visible universe. At a given temperature, the mean speed of the particles is known, and their mean energy is proportional to the temperature [well, in the assumption of a gas in equilibrium]. Thus during our back travel along time we would see an increasing mean energy of matter. Sooner or later the mean energy would overcome the chemical energies range, and no stable molecule could survive. Next it would be greater then the ionization energies range, so that electrons and nuclei would decouple (we arrived to an energy at the level of few tens of keV [= 1000 eV = 1.6 10-16 Joule]). After few tens of MeV [= 1000 keV], the order of magnitude of the nuclei binding energy, protons and neutrons are free. At that time the Universe is a hot gas made of protons, neutrons, electrons and photons. It is expected that for higher energies even the protons and the neutrons have to decouple into a free quarks plasma, but we weren't able to reach this energy level in laboratory, where the highest energies today are around 1-10 TeV [= 1012 eV].
The most interesting aspect of the cosmic evolution is that it is driven by gravitation, so far completely neglected in this page. In particle collisions the gravitational interaction is so faint that it can be completely neglected, even in comparison with the weak interaction. However, all aggregate of particles are subject to gravitation. Even more: any energy density is a source of gravitation. Thus at large scales, where particle collisions may be ignored, matter must follow trajectories that are driven by the gravitational interaction. We can walk because we are attracted by the Earth (and vice versa). The Moon is orbiting around the Earth, and both are orbiting around the Sun, which is one of the hundred billion stars in our Galaxy. Everything is freely falling, in a motion completely driven by gravitation. The evolution of the entire universe is driven by gravitation.
A very important aspect of the gravitational interaction is that it is attractive. Thus, clouds of matter may end up into collapsing into a compact object, like a planet or a star. When the mass of the compact object is large enough, the pressure exerted by the surroundings on its core makes it possible to start nuclear fusion reactions. In these reactions, lighter nuclei (starting with hydrogen, the lightest one, just a proton) merge into heavier ones by releasing a lot of energy (in the form of gamma rays). This increases the temperature and the radiation emitted by these nuclear reactions pushes out the matter, balancing the gravitational attraction. Thus, a star is just an enormous ball of hot gas, which is pushed down by gravity and pushed out by the radiation. This is how our Sun is able to produce all the energy which it irradiates. Our life is possible thanks to the nuclear reactions happening at the core of the Sun.
The equilibrium between the two forces that keep the Sun in constant activity will last for few billion years. But at some point the fuel (the hydrogen nuclei that create helium nuclei via the fusion process) will be over. Then the gravitational attraction will compress the entire star until the pressure at its core is sufficient to start another fusion reaction (in which the helium is the fuel). The helium fusion is so powerful that the radiation will push away the most external layers: the Sun will become a red giant, big enough to destroy the Earth [don't worry, humans risk to disappear much much earlier because of our own stupidity]. Once the helium is no more sufficient, there will be another compression. The Sun has a relatively small mass, hence it won't be able to go through all possible fusion reactions. Heavier stars can produce all the elements up to iron (Fe), which is the element most energetically bound. Adding protons and neutrons to iron nuclei won't cause the emission of any energy. Actually it's the contrary: energy must be provided in order to produce bigger nuclei.
A giant star, which has undergone all fusion reactions, cannot stop gravity any more: the star will collapse in a very short time. The core of the star will become so hard that the external layers bounce back in a tremendous explosion: the star is now a nova or supernova (if big enough). The explosion is so big that elements heavier than iron are created and ejected in the space in all directions. We are made of those elements. The Sun is not a first generation star (which would only consist of hydrogen and helium) and the solar system is rich of heavy elements because they had been produced by supernovae exploded before the formation of the Sun and the planets. We are literally made of stardust.
The visible light emitted by the Sun is remarkably constant. However, in other wavelengths the Sun shows quite a lot of activity. The most spectacular events are the solar flares, in which a huge amount of energy is emitted in the hard X-ray spectrum on the timescale of minutes, often accompanied by the release of particles (coronal mass ejections) carrying away a lot of energy and possibly causing disturbances at the Earth. Indeed, the magnetic connection between Sun and Earth is an important factor on our life, and investigations of this activity attempt to forecast the effects of solar flares on telecommunication, airplane routes, and electric power distribution. This is the so-called space weather.
Cosmic Rays (usually abbreviated in CR) are particles coming to the Earth from every direction. They have a very large energy range, from few MeV to 1014 MeV (where 1 MeV = 1.6 10-13 Joule). In addition, their composition is very interesting: at 90% level CR are protons, followed by helium nuclei, electrons and all the other nuclei, but their quantities are different from those measured in the Solar system.
CR are supposed to be created by supernova (SN) explosions: this is the reason why the abundances of the elements heavier than Fe are present among CR in percentage greater than that observed in the Solar system. SN can also accelerate charged particles to maximum energies of about 10 TeV, which is the energy of protons accelerated by the LHC at CERN. Even more powerful "engines" are pulsars and AGNs. We have detected CR events with energies up to one billion TeV.
CR travel for about 10 million years before reaching Earth, and the different abundances of primary elements (produced by stars nuclear reactions) and secondary ones (consumed in the stars reactions), with respect to the local abundances, can give us informations about the propagation of CR in our Galaxy. The most energetic CR come probably from other galaxies, providing us with important informations about cosmic structure and evolution.
One of the most exciting fields of contemporary fundamental physics is centered around the antimatter. High energy physicists know that matter and antimatter are always created in the same amount: for each electrons, also a positron (i.e. anti-electron) is produced. But the visible universe appears to be quite homogeneus: systems up to the galaxy clusters have to be entirely made of matter (or antimatter). The search for antimatter of cosmic origin is the main goal of the AMS experiment, even if there are other important topics that can be covered by AMS.
The Sun is also a source of CR. Although their energy is usually smaller than the galactic CR, the charged and neutral particles emitted by the Sun affect significantly the Earth and the solar system. The most extreme solar events create bursts of particles which may disturb the telecommunications and create danger for astronauts and airplanes. Hence studying the particles emitted by the Sun is important both for science and society, and is part of the research connected to the space weather. The ESA Solar Orbiter mission aims at providing multi-channel observations of the innermost regions of the solar system over a large range of latitudes. In particular, among its 10 instruments, STIX provides X-ray images of the Sun with spectroscopic and time information, whereas EPD will measure the composition, timing and distribution functions of the particles which will later reach the satellite. STIX will be complemented by MiSolFA, a compact X-ray detector which will orbit around the Earth. Both instruments will be able for the first time to obtain "stereo" observations of the same event, contributing to the understanding of the complicate phenomena that happen in solar flares.