Located in a cavern 52 m underground, with 28 m of overburden rock, ALICE can operate not only for proton-proton or Lead-Lead collisions, but also to detect atmospheric muons produced by cosmic ray interactions with the atmosphere. The process is quite simple: when a primary cosmic ray enters the Earth’s atmosphere, it interacts with air molecules to produce a large number of secondary particles. In turn, these secondary particles produce more particles by decaying into them or colliding with further atmospheric air molecules. When the energy of a secondary particle is below a certain critical energy the multiplication process terminates. The number of the particles then increases until it reaches a maximum, after which the number starts to decrease. The development of all these particles is called extensive air shower.
The muons created, also called atmospheric muons, are one of the components of this shower. Some of them reach ground level and pass through the rock above ALICE, arriving at the experiment. The goal for the researchers here is not very easy: to understand the nature of the primary cosmic ray by detecting and measuring the properties of the muons crossing the experiment. The main purpose is to study the mass composition and energy spectrum of cosmic rays in an energy range not available from direct measurements with satellites or balloons; therefore above 100 TeV.
Two main triggers have been implemented in ALICE to detect atmospheric muons and develop a physics programme. One of these triggers is given from the coincidence of the signal in at least two different modules of ACORDE. The second requires a signal in a readout channel (pad) in the upper part of the Time of Flight (TOF) detector and another signal in a pad of the opposite lower part.
In 2010 and 2011, during intermittent pauses of the accelerator, several days of cosmic data were collected and the events reconstructed with the Time Projection Chamber (TPC). The excellent tracking performance of the TPC, unimaginable with previous experiments, along with measuring momentum, charge, direction and spatial distribution at the same time, allows a detailed study on muon bundles. The muon multiplicity distribution, obtained with these data, has revealed some unexpected events with very high muon density. In particular, the event with the highest multiplicity, shown in the figure below, has 276 muons reconstructed by the TPC for a density of 18 muons/m2. The energy of the primary cosmic ray, supposing it is an iron nucleus, is estimated at around 30000 TeV; an energy also very far from the last generation accelerator. In the standard cosmic ray model we expect, in ALICE, an event of this energy in 4-5 years of data.
Bruno AlessandroDisplay of the cosmic event with the highest
muon multiplicity (276), 58 modules of ACORDE are fired (red rectangles)
Actually only three events of very high muon density have been detected, with 276, 181 and 89 muons, and we cannot rule out that they are due to statistical fluctuations of the various processes involved. To exclude or confirm this possibility, a new trigger, requiring the coincidence of four ACORDE modules has been implemented for 2012. In this way, the trigger rate is so low that cosmic events can also be recorded during proton-proton collisions, increasing the amount of the actual data several times.
At the end of this year we hope to find a large number of these events and to be able to study them in detail, to finally understand their nature. If none of them are found, we should believe that the actual high multiplicities found are due to the effects of statistical fluctuations.