On December 17 at 6PM the machine operators switched off the beam in the LHC
sending us greetings through our communication channel
(you can find here the status of the beam in real time http://op-webtools.web.cern.ch/op-webtools/vistar/vistars.php?usr=LHC1)
It has been an extraordinaty 2009. The LHC is back and it performed beyond expectations in the past couple of weeks (yes, it has been just a couple of weeks). The machine was able to provide stable beam (which means good quality beam with no risk of damaging the detectors) and the highest energy collisions in the World, breaking the Tevatron record with 2.36 TeV. ATLAS successfully collected hundred of thousands of candidate collisions. What you see in the beautiful visual representation of a reconstructed event is the production of “jets”.
Even though the LHC and the experiments have been built with the aim of discovering New Phenomena, standard “strong processes” are those happening more frequently, thus the ones produced and observed right from the beginning of data taking. What does “strong” stand for? The protons in the two circulating beams are made of quarks (the name “quark” was taken by Murray Gell-Mann from the book “Finnegan’s Wake” by James Joyce). 
Quarks are – based on today’s knowledge – elementary particles (they are not composed of other particles), and there are six of them coupled in pairs due to common properties. The lightest quarks are called “up” and “down”, slightly heavier ones are the “charm and strange” and finally the “bottom and top”.
The mass of the quarks ranges from small (0.001 times the mass of the proton) to the largest mass observed in particle physics so far (170 times the mass of the proton).
Each quark is accompanied by an antiquark with a different charge. What is unusual is their fractional electric charge, for instance the top has charge +2/3 while the bottom has -1/3. The youngest quark is the top quark, discovered at the Tevatron in 1995. Quarks interact via quantum chromodynamics (QCD), the theory of the “strong interaction”. In the same way as the electrons interact via quantum electrodynamics (QED) thanks to their electric charge, quarks interact via their “color” charge. In fact, quarks carry colors (red, green and blue) which however has nothing to do with our daily concept of color. One important characteristic of QCD is called the “confinement”: the force between quarks does not decrease as the quarks separate. This results in not being able to see quarks separately, but only bounded to form composite particles (the top is exceptional in this respect). While travelling through the detectors they generate showers of such particles called jets”.
Our collaborators in ATLAS scanned the data collected so far looking for signs of light particles, composed of quarks, being produced. In the first runs of data taking, we could already re-discover three of the quarks. We observed a bound state of light quarks, the u and d quarks, and once more data had been collected the strange quark – in form of resonance (a peak) – appeared on our screeens!
Finally, let me conclude this fantastic year with the words of the Director General at CERN “It has been a fantastic year for the LHC [….] I want to underline the fact that it has been made possible by the unique global collaboration that is particle physics. It has been truly heart-warming to see the community pulling together to achieve its goals”.
See you in 2010!
