Ring-imaging Cherenkov detector
The ring-imaging Cherenkov, or RICH, detector is a device for identifying the type of an electrically charged subatomic particle of known momentum, that traverses a transparent refractive medium, by measurement of the presence and characteristics of the Cherenkov radiation emitted during that traversal.
RICH detectors were first developed in the 1980s and are used in high energy elementary particle-, nuclear- and astro-physics experiments.
The ring-imaging detection technique was first proposed by Jacques Séguinot and Tom Ypsilantis, working at CERN in 1977.
[1] Their research and development, of high precision single-photon detectors and related optics, lay the foundations for the design[2][3] development [4] and construction of the first large-scale Particle Physics RICH detectors, at CERN's OMEGA facility [5][6][7] and LEP (Large Electron–Positron Collider) DELPHI experiment.
[8] A ring-imaging Cherenkov (RICH) detector allows the identification of electrically charged subatomic particle types through the detection of the Cherenkov radiation emitted (as photons) by the particle in traversing a medium with refractive index
Knowledge of the particle's momentum and direction (normally available from an associated momentum-spectrometer) allows a predicted
vs momentum of the source particle, for single Cherenkov photons, produced in a gaseous radiator (n~1.0005, angular resolution~0.6mrad) is shown in the following Fig.1: The different particle types follow distinct contours of constant mass, smeared by the effective angular resolution of the RICH detector; at higher momenta each particle emits a number of Cherenkov photons which, taken together, give a more precise measure of the average
than does a single photon (see Fig.3 below), allowing effective particle separation to extend beyond 100 GeV in this example.
The essence of the ring-imaging method is to devise an optical system with single-photon detectors, that can isolate the Cherenkov photons that each particle emits, to form a single "ring image" from which an accurate
, producing no radiation in this case (which would also be a very clear signal of particle type = proton, since fluctuations in the number of photons follow Poisson statistics about the expected mean, so that the probability of e.g. a 22 GeV/c kaon producing zero photons when ~12 were expected is very small; e−12 or 1 in 162755).
With a suitably focusing optical system this allows reconstruction of a ring, similar to that above in Fig.2, the radius of which gives a measure of the Cherenkov emission angle
is a measure of the optical response of the RICH; it can be thought of as the limiting case of the number of actually detected photons produced by a particle whose velocity approaches that of light, averaged over all relevant particle trajectories in the RICH detector.
The average number of Cherenkov photons detected, for a slower particle, of charge
Given the known momentum of the emitting particle and the refractive index of the radiator, the expected Cherenkov angle for each particle type can be predicted, and its difference from the observed mean Cherenkov angle calculated.
The following Fig.4 shows the 'number of sigma' deviation of the kaon hypothesis from a true pion ring image (π not k) and of the pion hypothesis from a true kaon ring image (k not π), as a function of momentum, for a RICH with
= 0.64 milliradians; Also shown are the average number of detected photons from pions(Ngπ) or from kaons(Ngk).
One can see that the RICH's ability to separate the two particle types exceeds 4-sigma everywhere between threshold and 80 GeV/c, finally dropping below 3-sigma at about 100 GeV.
It is important to note that this result is for an 'ideal' detector, with homogeneous acceptance and efficiency, normal error distributions and zero background.
No such detector exists, of course, and in a real experiment much more sophisticated procedures are actually used to account for those effects; position dependent acceptance and efficiency; non-Gaussian error distributions; non negligible and variable event-dependent backgrounds.
The related probabilities, which are the usual measures of signal detection and background rejection in real data, are plotted in Fig.5 below to show their variation with momentum (simulation with 10% random background); Note that the ~30% π → k misidentification rate at 100 GeV is, for the most part, due to the presence of 10% background hits (faking photons) in the simulated detector; the 3-sigma separation in the mean Cherenkov angle (shown in Fig.4 above) would, by itself, only account for about 6% misidentification.
More detailed analyses of the above type, for operational RICH detectors, can be found in the published literature.
For example, the LHCb experiment at the CERN LHC studies, amongst other B-meson decays, the particular process B0 → π+π−.
The following Fig.6 shows, on the left, the π+π− mass distribution without RICH identification, where all particles are assumed to be π; the B0 → π+π− signal of interest is the turquoise-dotted line and is completely swamped by background due to B and Λ decays involving kaons and protons, and combinatorial background from particles not associated with the B0 decay.
[9] On the right are the same data with RICH identification used to select only pions and reject kaons and protons; the B0 → π+π− signal is preserved but all kaon- and proton-related backgrounds are greatly reduced, so that the overall B0 signal/background has improved by a factor ~ 6, allowing much more precise measurement of the decay process.
In a focusing RICH detector, the photons are collected by a spherical mirror with focal length
In the more compact proximity-focusing design a thin radiator volume emits a cone of Cherenkov light which traverses a small distance, the proximity gap, and is detected on the photon detector plane.
The image is a ring of light the radius of which is defined by the Cherenkov emission angle and the proximity gap.
An example of a proximity gap RICH detector is the High Momentum Particle Identification (HMPID), one of the detectors of ALICE (A Large Ion Collider Experiment), which is one of the five experiments at the LHC (Large Hadron Collider) at CERN.
The LHCb experiment on the Large Hadron Collider, Fig.9, uses two RICH detectors for differentiating between pions and kaons.
