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ALICE looks to the skies

18 July 2012

One of big experiments at the LHC, and its intriguing observations.

 

ALICE is one of the four big experiments at CERN’s LHC. It is devoted mainly to the study of a new phase of matter, the quark–gluon plasma, which is created in heavy-ion collisions at very high energies. However, located in a cavern 52 m underground with 28 m overburden of rock, it can also detect muons produced by the interactions of cosmic rays with the Earth’s atmosphere.

The use of high-energy collider detectors for cosmic-ray physics was pioneered during the era of the Large Electron–Positron (LEP) collider at CERN by the L3, ALEPH and DELPHI collaborations. An evolution of these programmes is now possible at the LHC, where the experiments are expected to operate for many years, with the possibility of recording a large amount of cosmic data. In this context, ALICE began a programme of cosmic data-taking, collecting data for physics for 10 days over 2010 and 2011 during pauses in LHC operations. In 2012, in addition to this standard cosmic data-taking, a special trigger now allows the detection of cosmic events during proton–proton collision runs.

A different approach

In a typical cosmic-ray experiment, the detection of atmospheric muons is usually done using large-area arrays at the surface of the Earth or with detectors deep underground. The main purpose of such experiments is to study the mass composition and energy spectrum of primary cosmic rays in an energy range above 1014 eV, which is not available through direct measurements using satellites or balloons. The big advantages of these apparatuses are the large size and, for the surface experiments, the possibilities for measuring different particles, such as electrons, muons and hadrons, created in extensive air showers. Because the detectors involved in collider experiments are tiny compared with the large-area arrays, the approach and the studies have to be different so that the remarkable performances of the detectors can be exploited.

The first different characteristic for experiments at LEP or the LHC is the location, being some 50–140 m underground. These are in an intermediate situation between surface arrays – where all of the components of the shower can be detected – and detectors deep underground, where only the highest-energy muons (usually of the order of 1 TeV at the surface) are recorded. In particular for ALICE, all of the electromagnetic and hadronic components are absorbed by the rock overburden and apart from neutrinos only muons with an energy greater than 15 GeV reach the detectors. The special features that are brought by ALICE are the ability to detect a clean muon component with a low-energy cut-off, allowing a larger number of detected events compared with deep underground sites, combined with the ability to measure a greater number of variables, such as momentum, arrival time, density and direction, than was ever achieved by earlier experiments.

The tradition in collider experiments, and also in ALICE, is to use these muons mainly for the calibration and alignment of the detectors. However, during the commissioning of ALICE, specific triggers were implemented to develop a programme of cosmic-ray physics. These employ three detectors: A COsmic Ray DEtector (ACORDE), time-of-flight (TOF) and the silicon pixel detector (SPD).

ACORDE is an array of 60 scintillator modules located on the three upper faces of the ALICE magnet yoke, covering 10% of its area. The trigger is given by the coincidence of the signals in at least two different modules. The TOF is a cylindrical array of multi-gap resistive-plate chambers, with a large area that completely surrounds the time-projection chamber (TPC), which is 5 m long and has a diameter of 5 m. The cosmic trigger requires a signal in a read-out channel (a pad) in the upper part of the TOF and another in a pad in the opposite lower part. The SPD consists of two layers of silicon pixel modules located close to the interaction point. The cosmic trigger is given by the coincidence of two signals in the top and bottom halves of the outer layer.

The track of an atmospheric muon crossing the apparatus can be reconstructed by the TPC. This detector’s excellent tracking performance can be exploited to measure the main characteristics of the muon – such as momentum, charge, direction and spatial distribution – with good resolution, while the arrival time can be measured with a precision of 100 ps with the TOF. In particular the ability to track a high density of muons – unimaginable with a standard cosmic-ray apparatus – together with the measurement of all of these observables at the same time, permits a new approach to the analysis of cosmic events, which has so far not been exploited. For these reasons, the main research related to the physics of cosmic rays with the ALICE experiment has centred on the study of the muon-multiplicity distribution and in particular high-density events.

The analysis of the data taken in 2010 and 2011 revealed a muon multiplicity distribution that can be reproduced only by a mixed composition. Figure 1 shows the multiplicity distribution for real data taken in 2011, together with the points predicted for pure-proton and pure-iron composition for the primaries. It is clear from the simulation that the lower multiplicities are closer to the pure-proton points, while at higher multiplicities the data tend to approach the iron points. This behaviour is expected from a mixed composition that on average increases the mass of the primary when its energy increases, a result confirmed by several previous experiments.

High-multiplicity events

However, a few events found both in 2010 and in 2011 (beyond the scale of figure 1) have an unexpectedly large number of muons. In particular, the highest multiplicity reconstructed by the TPC has a muon density of 18 muons/m2. Figure 2 shows the display of this event and gives an idea of the TPC’s capabilities in tracking such high particle densities without problems of saturation, a performance never achieved in previous experiments.

The estimated energy of the primary cosmic ray for this event is at least 3 × 1016 eV, assuming that the core of the air shower is inside ALICE and that the primary particle is an iron nucleus. Recalling that the rate of cosmic rays is 1 m–2 year–1 at the energy of the knee in the spectrum (3 × 1015 eV), and that over one decade in energy the flux decreases by a factor of 100, an event with this muon density is expected in ALICE in 4–5 years of data. Since other events of high multiplicity have been found in only 10 days of data-taking, further investigation and detection will be necessary to understand whether they are caused by standard cosmic rays – and if the high multiplicity is simply a statistical fluctuation – or whether they have a different production mechanism. A detailed study of these events has not shown any unusual behaviour in the other measured variables.

For all of these reasons it is important to see whether other unexpected high-multiplicity events are detected in future and at what rate. To this end, in addition to standard cosmic runs, a special trigger requiring the coincidence of at least four ACORDE modules has been implemented this year to record cosmic events during proton–proton collisions, and so increase the time for data-taking to more than 10 times that of the existing data.

It is interesting to note that the three LEP experiments – L3, ALEPH and DELPHI – also found an excess of high-multiplicity events that were not explained by Monte Carlo models. The hope with ALICE is to find and study a large number of these events in a more quantitative way to understand properly their nature.

Bruno Alessandro, INFN Torino, and Mario Rodriguez, Autonomous University of Puebla, Mexico.

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