It’s time for physics

After just a few weeks of running, the LHC has already provided the experiments with millions of high-energy collisions. Physicists from all over the world are analysing the new data and retracing the particles discovered in past experiments. The W particle, discovered in 1983 by the UA1 and UA2 experiments at CERN, and the B-meson, discovered in 1977 by the E288 experiment at Fermilab, have recently popped up amongst the rich harvest of information.


One of the first attempts to squeeze the beam at the CMS interaction point: at around 17:00 the squeeze started. Once the squeeze was over, a Luminosty scan was performed to reoptimize the beam crossing and the luminosity was indeed 5 times higher, taking into account the small beam losses which had happened during the squeeze.

Retracing and rediscovering known particles is part of the training process that the LHC experiments are diligently going through during these first weeks of operation. “Just as much as for the LHC machine, the performance of the many different parts of the experiments has to be carefully tested and thoroughly understood in order to make sure that the data are correctly interpreted”, says Sergio Bertolucci, Director for Research and Computing.

Many different particles are created in the proton collisions delivered by the LHC at the four experiment points, and the task of the detectors is to recognize them by measuring their mass, their charge and a few other properties. That's why physicists are currently using the signals coming from known particles to verify that their detectors are working as expected.

And they are! What in the past took long years of data acquisition and analysis is nowadays performed within days. “This excellent result is due to the good performance of both the accelerator and the detectors”, says Bertolucci. Indeed, although the LHC operators are still carrying out studies on the ‘quality’ of the beam that often prevents the experiments from collecting usable data, the ‘good data’ is enough to make the first re-discoveries: on 6 April, less than a week after the first high-energy collisions, ATLAS identified two W-boson candidates. A few days later, on 21 April, LHCb reconstructed the track of the first beauty particle.



Cross-section and discovery time
How do physicists calculate the chances they have of making a discovery? The LHC experiments are looking for particles which, because of their high mass or other properties, are only rarely generated from the energy made available in the collisions.

The probability that an event, such as the creation of a given particle, takes place in the collisions is known to physicists as ‘cross-section’. However, as Bertolucci explains: “one single event is usually not meaningful, and scientists need to collect a certain amount of data before really claiming a discovery”. In general terms, the higher the cross-section, the shorter the time needed for the discovery.

As an example, if supersymmetric particles exist at the energies provided by the LHC, their cross-section (that is, the probability of their being formed) is expected to be relatively high and therefore the discovery is within reach in a relatively limited amount of time. On the other hand, the Higgs boson is expected to have a much smaller cross-section and therefore a longer data acquisition period will be needed to confirm its appearance (if it exists!).

by CERN Bulletin