Why do we study hadrons? Why are they so exciting that 2500 researchers participating in HP3 are studying them?

First of all, what do hadrons have in common? They are all composed by quarks and/or anti-quarks and are glued together by another particle, a force carrier, namely the gluon. The force (named in physics interaction) which holds them together is the strong force, one of the four known to be present in Nature.

Now, why is it so important to study them? Where are the mysteries?

In spite of the fact that hadrons have been studied for many years, we still do not have a complete understanding of their structure and of their interactions.

There are two types of hadrons: the  baryons, composed of three quarks, and the mesons, composed of a quark and an antiquark. You will never find in Nature (apart, maybe, in the very first moments after the Big Bang, when the so-called quark-gluon plasma might have existed) a free quark! Why? We discovered, more than 40 years ago, that the quarks, similar to the electrically charged particles, are carrying a type of "charge": for the quarks the charge is called "colour". The quarks can be red, green or blue (of course, the names of the colours have nothing to do with real colours, are names we use for convenience, but, as you'll see, they are very well chosen). The fact is, we have discovered, that Nature does not like "coloured" objects at the level of hadrons; it prefers colourless (white) hadrons. How can this be achieved?

Can you imagine?

Indeed, by mixing colours into a white particle. Mixing red, green and blue – the fundamental colours - we obtain the baryons, while mixing a coloured and an anticoloured quark we obtain the mesons!

How do the quarks stay together in a hadron? By exchanging gluons, the carrier of the strong force/interaction! And this makes the difference, with respect to all other forces! And WHAT a difference!

Differently from the photons, the electromagnetic force carriers, the gluons are carrying themselves colour charges! Which means that they can interact among themselves.

This possibility, namely the gluons interacting not only with quarks, but among themselves, is giving to the strong interaction a completely different behaviour with respect to all others known (gravity, weak and electromagnetic) interactions. While, for example, gravity and electrostatic interactions diminish with the distance, the strong one increases! While gravity and electrostatic interacttions strongly increase by reducing the distance, the strong one goes to.... zero. This character of the strong force, the so-called asymptotic freedom (quarks are free only when they are very near), is well-represented by a string: the more we extend it, the more we feel the force, the less we extend it, more free we feel.

The  behavior of the strong interaction is rather well understood in the high energies regime, where we have the asymptotic freedom, since with high energies the quarks get very near and their interactions is rather weak (and this happens in the high-energy accelerators), in the low-energy regime, where the strong force is REALLY strong, in spite of important progress, we still have a lot to learn: both theoretically and experimentally.

Why should we care?

 Well, if the answer that knowing more because we are curious is not good enough for you (it is for us!) we can however tell you that we, humans, are built of hadrons feeling the strong force in the low-energy regime!!! In our body, as in all objects surrounding us, hadrons are not at extremely high energies (with a few exceptions such as: high-energy accelerators, like the LHC from CERN, or rarely, in cosmic rays), but quite at low energies. Which means that if we really want to understand ourselves and how we are done, we need to understand well the hadrons, and the strong interacttion in detail.

There is, however, more than this: the strong interaction we are studying is a key-feature from the extremely small scales (as quarks) to the very large scales (as stars).

The proton and the neutrons are small Universes: they are built by three valence quarks, but when we consider their masses we discover that the proton mass is about 50 times bigger than the simple sum of the masses of those quarks. What happens?

This is one of the most fascinating aspects of hadron physics today. The phenomenon was called hadronization, and is presently being studied in many experiments and y theoreticians. What is the actual model of a proton (or neutron)? We know that it is indeed composed by the three valence quarks, but they are “swimming” in a soup of gluons, quarks and antiquarks (these last ones in pairs) which are adding up to their final mass. So they are very complicated objects!

Related to this, who tells that the Higgs boson (the so-called God particle) solves the mystery of the mass is only partially right - it does solve it, but only for elementary particles such as quarks, not for the hadrons! Certainly not for protons and not for neutrons! To do that...we need to know how the hadronization occurs, and this is another story. It is one of the stories of HadronPhysics3.

HadronPhysics3 project is dealing with the theoretical and experimental aspects of hadron physics: from the quark-gluon plasma, the only moment when the quarks might have been free a few instants after the Big Bang, to the present days, when quarks are “imprisoned” in hadrons. The hadrons, in turn, are sometimes alone (as the hydrogen nuclei constituting most of the Universe) and some other times not: as in our body, for example, where they are part of nuclei, forged in nuclear processes in stars.

Stars themselves are still mysterious: one simple question, which however is generating many discussions, is related to the possible role of strangeness in the stars, i.e. the possibility that the "strange" quark, having a mass about 20 times bigger than the up and down quarks (those out of which protons and neutrons are built) might play a role in the evolution of the stars.

HadronPhysics3 deals with physics such as: collision of high energy lead ions to generate the quark-gluon plasma; tomography of hadrons, using various types of beams (such as electrons) colliding with protons; study of the strange atoms (where both the nucleus and its counterpart are hadrons) or strange nuclei; creation and study of new types of hadrons (such as those composed only by gluons, with no quark inside) or a better understanding of those which are poorly known, and many other items - as you can learn from Activities.

All the experiments studying this type of physics are performed at accelerators in Europe by broad international collaborations, where researchers from all over the world are working together. In order to be able to perform these measurements scientists do need to have proper "eyes", namely particle detectors. This is another sector where HadronPhysics3 is strongly involved: the design, construction and test of new, innovative types of detectors and related electronics. These studies are going to have a strong impact on society as well, since they are used in many other fields: from health to safety, from industry to communications, just to name a few.

HadronPhysics3 offers the possibility to have a high mobility of scientists in those places, like accelerators, where research is done. HadronPhysics3 is representing a triple integrated initiative, where the activities are grouped in:

  • 9 Networks and Management activities, in the framework of which scientists are collaborating in order to get better theories, to perform more refined calculations and to improve their tools to interpret the experimental results and to propose new measurements;
  • 14 Joint Research Activities, where new concepts for detectors for hadron physics experiments are proposed, prototypes are built and tested;
  • 5 TARI activities under which scientists are supported to come and work in the laboratories offering the best of the European infrastructures in the framework of hadron physics.

It is certainly worthwhile to mention an added value in projects like HadronPhysics3: they are for sure peace-promoters, since they glue (just to use a word from hadronic physics) together people having common aims, motivated by curiosity and desire to learn more about the Universe, from all over the world, sometimes bringing together in the same team people from countries which are in conflict.

HadronPhysics3 pays special attention to the young scientists, offering them unique opportunities and fellowships (such as PhD positions) to work in the best EU laboratories, and to participate in the highest effort ever attempted to tackle the mystery of the strong force and to grab its deepest secrets.

Its understanding might lead not only to a better knowledge of Nature, but to new and unpredictable applications.




The HadronPhysics3 project is supported by the European Union
under the 7th Framework Capacities Programme in the area of Research Infrastructures (RI).