Fixed target, striking physics

11 March 2019

A strong tradition of innovation and ingenuity shows that, for CERN’s North Area, life really does begin at 40.

As generations of particle colliders have come and gone, CERN’s fixed-target experiments have remained a backbone of the lab’s physics activities. Notable among them are those fed by the Super Proton Synchrotron (SPS). Throughout its long service to CERN’s accelerator complex, the 7 km-circumference SPS has provided a steady stream of high-energy proton beams to the North Area at the Prévessin site, feeding a wide variety of experiments. Sequentially named, they range from the pioneering NA1, which measured the photoproduction of vector and scalar bosons, to today’s NA64, which studies the dark sector. As the North Area marks 40 years since its first physics result, this hub of experiments large and small is as lively and productive as ever. Its users continue to drive developments in detector design, while reaping a rich harvest of fundamental physics results.

Specialised and precise

In fixed-target experiments, a particle beam collides with a target that is stationary in the laboratory frame, in most cases producing secondary particles for specific studies. High-energy machines like the SPS, which produces proton beams with a momentum up to 450 GeV/c, give the secondary products a large forward boost, providing intense sources of secondary and tertiary particles such as electrons, muons and hadrons. With respect to collider experiments, fixed-target experiments tend to be more specialised and focus on precision measurements that demand very high statistics, such as those involving ultra-rare decays.

Fixed-target experiments have a long history at CERN, forming essential building blocks in the physics landscape in parallel to collider facilities. Among these were the first studies of the quark–gluon plasma, the first evidence of direct CP violation and a detailed understanding of how nucleon spin arises from quarks and gluons. The first muons in CERN’s North Area were reported at the start of the commissioning run in March 1978, and the first physics publication – a measurement of the production rate of muon pairs by quark–antiquark annihilation as predicted by Drell and Yan – was published in 1979 by the NA3 experiment. Today, the North Area’s physics programme is as vibrant as ever.

The longevity of the North Area programme is explained by the unique complex of proton accelerators at CERN, where each machine is not only used to inject the protons into the next one but also serves its own research programme (for example, the Proton Synchrotron Booster serves the ISOLDE facility, while the Proton Synchrotron serves the Antiproton Decelerator and the n_TOF experiment). Fixed-target experiments using protons from the SPS started taking data while the ISR collider was already in operation in the late 1970s, continued during SPS operation as a proton–antiproton collider in the early 1980s, and again during the LEP and now LHC eras. As has been the case with collider experiments, physics puzzles and unexpected results were often at the origin of unique collaborations and experiments, pushing limits in several technology areas such as the first use of silicon-microstrip detectors.

The initial experimental programme in the North Area involved two large experimental halls: EHN1 for hadronic studies and EHN2 for muon experiments. The first round of experiments in EHN1 concerned studies of: meson photoproduction (NA1); electromagnetic form factors of pions and kaons (NA7); hadronic production of particles with large transverse momentum (NA3); inelastic hadron scattering (NA5); and neutron scattering (NA6). In EHN2 there were experiments devoted to studies with high-intensity muon beams (NA2 and NA4). A third, underground, area called ECN3 was added in 1980 to host experiments requiring primary proton beams and secondary beams of the highest intensity (up to 1010 particles per cycle).

Experiments in the North Area started a bit later than those in CERN’s West Area, which started operation in 1971 with 28 GeV/c protons supplied by the PS. Built to serve the last stage of the PS neutrino programme and the Omega spectrometer, the West Area zone was transformed into an SPS area in 1975 and is best known for seminal neutrino experiments (by the CDHS and CHARM collaborations, later CHORUS and NOMAD) and hadron-spectroscopy experiments with Omega. We are now used to identifying experimental collaborations by means of fancy acronyms such as ATLAS or ALICE, to mention two of the large LHC collaborations. But in the 1970s and the 1980s, one could distinguish between the experiments (identified by a sequential number) and the collaborations (identified by the list of the cities hosting the collaborating institutes). For instance CDHS stood for the CERN–Dortmund–Heidelberg–Saclay collaboration that operated the WA1 experiment in the West Area.

Los Alamos, SLAC, Fermilab and Brookhaven National Laboratory in the US, JINR and the Institute for High Energy Physics in Russia, and KEK in Japan, for example, also all had fixed-target programmes, some of which date back to the 1960s. As fixed-target programmes got into their stride, however, colliders were commanding the energy frontier. In 1980 the CERN North Area experimental programme was reviewed in a special meeting held in Cogne, Italy, and it was not completely obvious that there was a compelling physics case ahead. But it also led to highly optimised installations thanks to strong collaborations and continuous support from the CERN management. Advances in detectors and innovations such as silicon detectors and aerogel Cherenkov counters, plus the hybrid integration of bubble chambers with electronic detectors, led to a revamp in the study of hadron interactions at fixed-target experiments, especially for charmed mesons.

Physics landscape

Experiments at CERN’s North Area began shortly after the Standard Model had been established, when the scale of experiments was smaller than it is today. According to the 1979 CERN annual report, there were 34 active experiments at the SPS (West and North areas combined) and 14 were completed in 1978. This article cannot do justice to all of them, not even to those in the North Area. But over the past 40 years the experimental programme has clearly evolved into at least four main themes: probing nucleon structure with high-energy muons; hadroproduction and photoproduction at high energy; CP violation in very rare decays; and heavy-ion experiments (see “Forty years of fixed-target physics at CERN’s North Area”).

Aside from seminal physics results, fixed-target experiments at the North Area have driven numerous detector innovations. This is largely a result of their simple geometry and ease of access, which allows more adventurous technical solutions than might be possible with collider experiments. Examples of detector technologies perfected at the North Area include: silicon microstrips and active targets (NA11, NA14); rapid-cycling bubble chambers (NA27); holographic bubble chambers (NA25); Cherenkov detectors (CEDAR, RICH); liquid-krypton calorimeters (NA48); micromegas gas detectors (COMPASS); silicon pixels with 100 ps time resolution (NA62); time-projection chambers with dE/dx measurement (ISIS, NA49); and many more. The sheer amount of data to be recorded in these experiments also led to the very early adoption of PC farms for the online systems of the NA48 and COMPASS experiments.

Another key function of the North Area has been to test and calibrate detectors. These range from the fixed-target experiments themselves to experiments at colliders (such as LHC, ILC and CLIC), space and balloon experiments, and bent-crystal applications (such as UA9 and NA63). New detector concepts such as dual-readout calorimetry (DREAM) and particle-flow calorimetry (CALICE) have also been developed and optimised. Recently the huge EHN1 hall was extended by 60 m to house two very large liquid-argon prototype detectors to be tested for the Deep Underground Neutrino Experiment under construction in the US.

If there is an overall theme concerning the development of the fixed-target programme in the North Area, one could say that it was to be able to quickly evolve and adapt to address the compelling questions of the day. This looks set to remain true, with many proposals for new experiments appearing on the horizon, ranging from the study of very rare decays and light dark matter to the study of QCD with hadron and heavy-ion beams. There is even a study under way to possibly extend the North Area with an additional very-high-intensity proton beam serving a beam dump facility. These initiatives are being investigated by the Physics Beyond Collider study (see p20), and many of the proposals explore the high-intensity frontier complementary to the high-energy frontier at large colliders. Here’s to the next 40 years of North Area physics!

Forty years of fixed-target physics at CERN's North Area

Probing nucleon structure with high-energy muons

High-energy muons are excellent probes with which to investigate the structure of the nucleon. The North Area’s EHN2 hall was built to house two sets of muon experiments: the sequential NA2/NA9/NA28 (also known as the European Muon Collaboration, EMC), which made the observation that nucleons bound in nuclei are different from free nucleons; and NA4 (pictured), which confirmed the electroweak effects between the weak and electromagnetic interactions. A particular success of the North Area’s muon experiments concerned the famous “proton spin crisis”. In the late-1980s, contrary to the expectation by the otherwise successful quark–parton model, data showed that the proton’s spin is not carried by the quark spins. This puzzle interested the community for decades, compelling CERN to further investigate by building the NA47 Spin Muon collaboration experiment in the early 1990s (which established the same result for the neutron) and, subsequently, the COMPASS experiment (which studied the contribution of the gluon spins to the nucleon spin). A second phase of COMPASS still ongoing today, is devoted to nucleon tomography using deeply virtual Compton scattering and, for the first time, polarised Drell–Yan reactions. Hadron spectroscopy is another area of research at the North Area, and among recent important results from COMPASS is the measurement of pion polarisability, which is an important test of low-energy QCD.

Hadroproduction and photoproduction at high energy

Following the first experiment to publish data in the North Area (NA3) concerning the production of μ+μ pairs from hadron collisions, the ingenuity to combine bubble chambers and electronic detectors led to a series of experiments. The European Hybrid Spectrometer facility housed NA13, NA16, NA22, NA23 and NA27, and studied charm production and many aspects of hadronic physics, while photoproduction of heavy bosons was the primary aim of NA1. A measurement of the charm lifetime using the first ever microstrip silicon detectors was pioneered by the ACCMOR collaboration (NA11/NA32; see image of Robert Klanner next to the ACCMOR spectrometer in 1977), and hadron spectroscopy with neutral final states was studied by NA12 (GAMS), which employed a large array of lead glass counters, in particular a search for glueballs. To study μ+μ pairs from pion interactions at the highest possible intensities, the toroidal spectrometer NA10 was housed in the ECN3 underground cavern. Nearby in the same cavern, NA14 used a silicon active target and the first big microstrip silicon detectors (10,000 channels) to study charm photoproduction at high intensity. Later, experiment NA30 enabled a direct measurement of the π0 lifetime by employing thin gold foils to convert the photons from the π0 decays. Today, electron beams are used by NA64 to look for dark photons while hadron spectroscopy is still actively pursued, in particular at COMPASS.

CP violation and very rare decays

The discovery of CP violation in the decay of the long-lived neutral kaon to two pions at Brookhaven National Laboratory in 1964 was unexpected. To understand its origin, physicists needed to make a subtle comparison (in the form of a double ratio) between long- and short-lived neutral kaon decays in pairs of neutral and charged kaons. In 1987 an ambitious experiment (NA31) showed a deviation from one of the double ratios, providing the first evidence of direct CP violation (that is, it happens in the decay of the neutral mesons, not only in the mixing between neutral kaons). A second-generation experiment (NA48, pictured in 1996), located in ECN3 to accept a much higher primary-proton intensity, was able to measure the four decay modes concurrently thanks to the deflection of a tiny fraction of the primary proton beam into a downstream target via channelling in a “bent” crystal. NA48 was approved in 1991 when it became evident that more precision was needed to confirm the original observation (a competing programme at Fermilab called E731 did not find a significant deviation from the unity of the double ratio). Both KTeV (the follow-up Fermilab experiment) and NA48 confirmed NA31’s results, firmly establishing direct CP violation. Continuations of the NA48 experiments studied rare decays of the short-lived neutral kaon and searched for direct CP violation in charged kaons. Nowadays the kaon programme continues with NA62, which is dedicated to the study of very rare K+ π+νν decays and is complementary to the B-meson studies performed by the LHCb experiment.

Heavy-ion experiments

In the mid-1980s, with a view to reproduce in the laboratory the plasma of free quarks and gluons predicted by QCD and believed to have existed in the early universe, the SPS was modified to accelerate beams of heavy ions and collide them with nuclei. The lack of a single striking signature of the formation of the plasma demands that researchers look for as many final states as possible, exploiting the evolution of standard observables (such as the yield of muon pairs from the Drell–Yan process or the production rate of strange quarks) as a function of the degree of overlap of the nuclei that participate in the collision (centrality). By 2000 several experiments had, according to CERN Courier in March that year, found “tantalising glimpses of mechanisms that shaped our universe”. The experiments included NA44, NA45, NA49, NA50, NA52 and NA57, as well as WA97 and WA98 in the West Area. Among the most popular signatures observed was the suppression of the J/ψ yield in ion–nucleus collisions with respect to proton–proton collisions, which was seen by NA50. Improved sensitivity to muon pairs was provided by the successor experiment NA60. The current heavy-ion programme at the North Area includes NA61/SHINE (see image), the successor of NA49, which is studying the onset of phase transitions in dense quark–gluon matter at different beam energies and for different beam species. Studies of the quark–gluon plasma continue today, in particular at the LHC and at RHIC in the US. At the same time, NA61/SHINE is measuring the yield of mesons from replica targets for neutrino experiments worldwide and particle production for cosmic-ray studies.


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