After almost two years of intense and fruitful maintenance and upgrade activities at Point2, ALICE started its official 2015 data taking period detecting cosmic events.
Since beginning of February, the ALICE Run Control Centre initiated the first integration tests of a brand new DAQ+CTP+DCS system developed by the corresponding ALICE DAQ, CTP and DCS groups. Most of the ALICE trigger detectors were ready to produce signals, send them to the CTP and generate trigger signals for the readout system to collect cosmic data. ACORDE (ALICE Cosmic ray detector) and TOF (ALICE Time of flight) generate trigger signals when a secondary cosmic particle impacts, at least one scintillator detector (ACORDE) or two cluster pads (TOF). These particles (mostly muons) are produced at the upper atmosphere when a highly energetic (above 1012 eV ) primary cosmic ray particle collides with an air molecule. Then, traveling at relativistic velocities, they reach the earth surface and penetrate approximately 40 m of molasses rock to reach the ALICE spectrometer, passing through the TPC, SPD, TRD tracking detectors.
Figure 1. Angular distribution of atmospheric μ
During cosmic runs, most of the central barrel detectors use these data for alignment and calibration purposes. Up to the first week of March 2015, we had collected 238,176,977 cosmic events, with 56% of the events with the ALICE magnetic field on and 44% without it. It is interesting to notice that, besides the integration test and detector callibration activities, ALICE is already using this data for physics analysis, looking for a kind of intriguing cosmic events called atmospheric muon bundle events. Katherin Shtejer Diaz (PhD student - Torino University) and Emma Gonzalez (M.Sc student - Puebla University) are in charge of analysing the first 2015 ALICE data. In Figure 1 we show the arrival direction of the cosmic particles detected so far while Figure 2 shows a cosmic event with 101 muons crossing the ALICE TPC. For the 2015 cosmic period, it is expected to record approximately 500,000,000 million events (around 70% more statistics that in Run 1) .
Figure 2. A muon bundle event recorded by ALICE
In Figure 3, the multiplicity distribution of atmospheric muons is presented; one event has more than 100 muons. A publication on the rate of such kind of events is currently with the internal review committee.
Figure 3. The atmospheric muon multiplicity distribution