Looking at cosmic rays with accelerator detectors

26 September 1999

Large detectors constructed at accelerator labs can also be used in parallel for cosmic-ray studies. Effects measured in distant detectors could be correlated to provide a broader view of particles from the cosmos.


When CERN came into being in the early 1950s, a sea change was taking place in particle physics research. Until then, cosmic rays had provided most of the basic particle physics discoveries. Accelerators made their first contributions at Berkeley in 1949, with the Chicago and Carnegie synchrocyclotrons not far behind. In 1953, the Cosmotron, so named because it was the first accelerator to attain cosmic-ray energies, began operation at Brookhaven and provided physicists with the first laboratory-made strange particles.

At the Bagnères conference in 1953, leading cosmic-ray physicist Cecil Powell, who had been awarded the 1950 Nobel Prize for his 1947 discovery of the pion, said: “We have been invaded. The accelerators are here.”

Although many of CERN’s first-generation physicists cut their research teeth on cosmic rays, for more than 40 years CERN went about building and exploiting its large accelerators, and cosmic-ray physics took a back seat.

With cosmic rays reporting particles beyond 1020 eV ­ thousands of times as high as the highest laboratory energies now available ­ particle physicists are rediscovering the attractions of natural sources of high-energy particles. While these natural sources provide energies far beyond those that will be opened up by the next generation of particle colliders, the event rates are puny in comparison.

Cosmic-ray revival

There are two primary motivations for this cosmic-ray revival. First, dramatic results from the Super-Kamiokande underground experiment in Japan, studying neutrinos produced by cosmic-ray collisions in the atmosphere, strongly suggest that the different kinds of neutrinos transform into each other – or “oscillate”. To exploit these neutrino possibilities fully requires accurate knowledge of the cosmic-ray muon spectrum. With existing data samples mutually disagreeing by 20-30%, more accurate data are called for.

Second, direct measurements of primary cosmic rays of energies of greater than 1014 eV (100 TeV) are impractical. For example, above 1016 eV there is only one particle per square metre per steradian per year. However, the primary cosmic-energy spectrum extends beyond 1020 eV and there is great interest in the composition, the energy spectrum and the interaction of the primary cosmic rays with nuclei in the upper atmosphere. Knowledge of the cosmic-particle composition above the “knee” (a few times 1015 eV) could shed light on how particles are accelerated to such high energies.

From ground-based observations of different particles (in extensive air showers) and from studies of atmospheric scintillation and Cherenkov light, the cosmic-ray community has learned a lot, but many mysteries and uncertainties remain. Even with good data, it is difficult to determine simultaneously a unique primary composition and interaction model.

Data collected by large detectors at accelerator laboratories would be a valuable contribution. With a relatively modest investment, these detectors can be exploited for cosmic-ray physics in parallel with, and with no loss in efficiency for, the primary mission.

The cosmic-ray muon threshold for L3 is about 15 GeV.

At CERN’s LEP collider, a subgroup of the L3 collaboration has formed the L3+Cosmics group, using new electronics to enable the muon spectrometer drift chambers to be read out independently of LEP data collection. A blanket of about 200 sq. m of scintillator has been installed over the top three octants of the magnet to provide a reference time signal for cosmic-ray data collection. Under 30 m of rock, the cosmic-ray muon threshold for L3 is about 15 GeV.

Elsewhere in the LEP ring, members of the ALEPH collaboration are running a pilot CosmoALEPH experiment to look for coincident muons over long distances. This group has examined, together with the newly established CosmoLEP group, the cosmic-ray data collected during the ALEPH runs.

Data archives have revealed a substantial collection of cosmic-ray muon events. Although the detector is live for only 10% of the time for cosmic-ray particles, this nevertheless adds up over several years to more than a million seconds (about 12 days).

Muon multiplicities, etc, have been analysed using sophisticated cosmic-ray simulation programs developed by the Karlsruhe group. Intriguing events, producing unprecedented numbers of muons, have sparked a proposal to study these phenomena in more detail.

Topical workshop

A topical workshop in Sodankyla, about 100 km north of the Arctic Circle in Finland, on 24-29 April and organized by Karsten Eggert of CERN, highlighted the resurgence of interest in cosmic-ray muons.

The Karlsruhe group with its KASCADE detector array has, perhaps, the most comprehensive data so far on ground-level air showers, which can be interpreted in terms of the primary cosmic-ray spectrum and its composition.

C Taylor (Case Western Reserve) and R Engel (Bartol) looked at the primary cosmic-ray nuclear interaction, noting the uncertainty in the final state particle production at very small forward angles that dominates cosmic-ray muon production.

More than 300 000 cosmic-ray muon events have been analysed

Even though Fermilab’s Tevatron Collider energy corresponds to a cosmic ray, of about 2 x 1015 eV, interacting with an air nucleon at rest, there are almost no data on forward particle production at energies above the older fixed-target experiments at a few hundred giga electron-volts.

ALEPH and L3 surveyed their respective cosmic-ray muon observations. H Wilkens (Nijmegen) gave an update on the additions to the L3 detector and electronics and the muon programme in progress. S Tonwar (Tata Institute) described the planned addition of a surface air shower array above L3, which will enable the observation of energetic muons together with the related air shower. J Strom (Arcada), A Bruhl (Siegen) and M Schmelling (Max-Planck Institute) presented the current status of the ALEPH cosmic-ray programme. More than 300 000 cosmic-ray muon events have been analysed, and good agreement with the KASCADE Monte Carlo simulations obtained for multimuon events observed in the 16 sq. m time-projection chamber for multiplicities (total number of produced particles) between 2 and 40. However, there are five events with unexpectedly large multiplicities: up to 150 (in some cases with additional muons observed in the hadron calorimeter).

Other ALEPH studies look at the “decoherence” curve ­ the coincidence rate between two muon detectors as a function of their separation, extending beyond 1 km.

Horst Wachsmuth (CERN) proposed looking for muon coincidences between the four LEP detectors, a phenomenon that should not occur for any “ordinary” cosmic-ray interaction in the Earth’s upper atmosphere. Such a coincidence, suggested by some earlier cosmic-ray experiments, would certainly require explanation.

Other major particle physics detector groups are also interested. A De Roeck (DESY) discussed the potential involvement of the big H1 and ZEUS detectors at DESY’s HERA electron­proton collider for studies of coincident cosmic-ray muons. With these detectors only 3 m below ground they could study cosmic-ray muons down to energies of 2 or 3 GeV, which is of great interest to the atmospheric muon neutrino groups.

It is also possible, using satellite (GPS) time recording, to look for time correlations between cosmic-ray muons at DESY and CERN.

Future study

M Vallinkoski (Oulu) described a possible new cosmic-ray muon experiment in a mine in Phyhasalmi, Finland. The Centre for Underground Physics in Phyhasalmi (CUPP) would deploy a seven-detector array at several vertically aligned depths to study the multiplicity, lateral distribution and energy spectrum of cosmic-ray muons.

It was noted that major detectors could also be sensitive to point sources of cosmic-ray muons and, using the Moon as a cheap and efficient absorber and the Earth’s magnetic field as a spectrometer, might set a limit on the relative flux of high-energy primary antimatter nuclei.

The Chinese IHEP group is particularly interested in seeking evidence for possible cosmic-ray associated weakly interacting massive particles (WIMPS) in L3.

Workshop organizer Karsten Eggert discussed other areas of future study, including working with larger and upgraded detectors. As noted by Lawrence Jones (Michigan) in his opening discussion, muon studies with LEP detectors contain both elements of an ideal experimental programme: practical and useful results, such as the absolute inclusive muon spectrum, while at the same time there is sensitivity to new and potentially exciting discoveries, such as unexpected muon multiplicities, cosmic-ray point sources, WIMP discoveries and statistically significant coincidences. Paolo Liperi of Rome summarized the attractions of the proposed new programme.

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