The NA63 experiment studies radiation processes in strong electromagnetic fields

The NA63 experiment directs beams of electrons and positrons onto a variety of targets to study radiation processes in strong electromagnetic fields. The research is relevant to a range of physics including beam-beam effects in linear colliders and the physics of various astrophysical phenomena.

NA63 is located in CERN's North Area, where the high-energy proton beam from the Super Proton Synchrotron (SPS) is split into several secondary beamlines that provide different particles to experiments. One of these secondary beamlines is H4, which can provide protons, hadrons, electrons or muons on request.

Critical fields

At NA63, electron beams from H4 are directed onto targets made from a variety of elements, ranging from the relatively light silicon, through the heavier iron and tin to tungsten, gold and lead. Some of the targets are amorphous, some monocrystals. When the electrons hit the targets, they cause showers of new particles to be expelled from the other side. In the case of crystalline targets, these penetrating particles experience an electromagnetic field so strong that it is close to the theoretical "critical field".

In a critical field the uncertainty of the exact locations of electrons leads to an energy gain that may produce new particles from the vacuum. Such fields are generally only seen in astrophysical phenomena, such as highly magnetized neutron stars, black holes (where the gravitational field is strong) and, perhaps, in the cosmic accelerators that give rise to cosmic rays of the highest known energies. Using a special approach employing crystalline targets and energetic beams, NA63 has managed to test processes at such fields in the laboratory.

Emission times

Another line of enquiry for NA63 is the effect of strong electromagnetic fields on the timing of photon emission. Specifically, fields of a critical magnitude have an intriguing effect on how long it takes for an electron to emit a photon.

An electron entering a magnetic field is accelerated, and therefore must lose part of its energy to radiation in the form of a photon. By exploiting the relativistic phenomena of time dilation and length contraction, the NA63 experiment has shown that this process of photon emission is not instantaneous, but rather, takes time. Because the process takes time, it can be disturbed experimentally.

In a critical electromagnetic field, for example, electrons are deflected so violently that they don't have enough time to radiate photons. So adjusting the electromagnetic field past a critical level can modify the emerging radiation spectrum of a beam of electrons: increase the field and the relative radiation yield from the beam diminishes. NA63 is investigating such effects, which could lead to new radiation sources and perhaps even to lasing at very high photon energies.

The effects of strong fields and emission times are relevant in many other branches of physics, ranging from the so-called “bubble-regime” in plasma wakefields used for extremely high-gradient particle acceleration, through astrophysical objects such as magnetars (heavily magnetized neutron stars) to intense lasers and heavy-ion collisions. The concepts studied at NA63 even apply in a gravitational analogue – Hawking radiation from black holes  which remains to be detected.