After analysing cosmic muon events, five were found with the highest muon density ever seen.Designed to study man-made electronpositron collisions in CERN’s LEP ring, the ALEPH detector is also ideal for observing complicated natural cosmic-ray events.
High-energy primary cosmic-ray particles crashing into the atmosphere 20 km above our heads initiate large air showers with hadrons, electrons, muons and neutrinos. By the time they reach ALEPH, 125 m underground, all of these particles are absorbed except for neutrinos and muons above 70 GeV.
ALEPH provides high resolution tracking in its central Time Projection Chamber (TPC) in a solenoidal magnetic field and the large hadron calorimeter surrounding the TPC provides further information about cosmic muons.
Precision study of these muons, in particular of muon bundles, gives vital information about the primary cosmic rays and the way in which shower particles are produced in the very forward (downward) direction. The primary cosmic particle composition around the “knee” of the energy spectrum (4 x 1015 eV) is fundamental input for understanding the cosmic acceleration mechanism that pushes particles to these energies.
After analysing cosmic muon events captured by ALEPH in parallel with LEP data-taking and during a special one-week cosmic run, five events were found with the highest muon density ever seen. Some 100 muons hit the sensitive area (16 m2) of the ALEPH TPC. The most crowded event showed about 160 muons in half of the TPC, the other half suffering data overflow.
Extensive simulations using the Corsika program, which was developed by the Karlsruhe group, reproduce the lower muon multiplicities, assuming a primary cosmic particle composition ranging from protons up to nuclei like iron. However, the five spectacular events are an order of magnitude above the simulation prediction. They could come from either unusually large air showers, above 1017 eV, or fluctuations from lower-energy showers, which could hint at new mechanisms for forward particle production.
Charge and momentum determination of the muons in these events, as well as the study of their structure over larger areas, may shed light on their origin. A special cosmic run of ALEPH using the tracking hadron calorimeter extended muon measurements to cover 50 m2. The largest event from this run produced more than 100 muons.
To study these intriguing events the CosmoLEP group proposes placing a 200 m2 array of drift chambers beside the ALEPH experiment. With this large array, the rate of high multiplicity events would be increased by a factor of more than a hundred and the muon patterns would give a window on the energy and composition of the primary cosmic particles. The larger samples could also reveal point sources of cosmic particles in the depths of the universe.
The underground muon showers that have been seen so far extend over, at most, a few hundred metres. The CosmoALEPH effort covers the barrel of ALEPH’s central hadron calorimeter together with several scintillator counter stations installed around the LEP ring near ALEPH, which are up to about 1 km distant. This pilot experiment saw coincident muons in counters that were several hundred metres apart and triggered an idea for a still wider muon search.
The four LEP experiments, equally spaced around the 27 km tunnel, could look for muon correlations over much larger distances. The same approach is being followed at the H1 and ZEUS detectors at the HERA collider at DESY in Hamburg, which are approximately 2 km apart.