The main challenge for the REDTOP experiment is represented by the rejection of the background since, at the energy where the experiment will run, the ratio of η-production to inelastic proton scattering is about 1:200. Of course, the background events will look very different from the interesting events, both from the kinematic and topological point of view. In order to make the signal-to-noise more favorable, the experiment has been designed with the following two peculiarities:
- The target is sparsified, with multiple elements along the beam axis. This will help to reduce the pile-up since the signal event and the background event will likely occur in different elements
- Since the final events of interest in general contain fast leptons, the apparatus will use the Cerenkov process to detect those particles. By tuning appropriately the refractive index of the radiators, the background events, with only hadrons in the final states, will be mostly invisible to the experiment.
The Target Systems
The target systems (the red circle in the picture above) for REDTOP is composed of ten round foils of beryllium, each about 1/3 mm thick. The diameter of the foil is about 1cm. The target systems are held inside a beam pipe made of either carbon-fiber or beryllium: the pipe will also help in maintaining the vacuum and support the aerogel on its external wall. A proton with 1.8 GeV of kinetic energy has a 0.5% probability to make an inelastic scattering in any of the foils. The probability that one such scattering would produce an η meson is about 0.4%. Since the average intensity of the beam is 1011 protons/sec, the expected number of η mesons produced in one year in the target systems is about 2×1012.
The Optical TPC (OTPC)
The OTPC (the yellow dodecagon) works on the same principles as a conventional TPC. Namely, it will measure the momentum and the position in space of a charged track by using a deflecting solenoidal field. However, rather than using the ionization process to detect those particles, one uses the Cerenkov effect. The momentum of that particle is, then, reconstructed by the photons emitted inside the volume of the OTPC and detected by an array of photo-sensors surrounding the radiator. About 500,000 SiPM’s will be needed to cover the inner walls of the device. The pattern of detected photons will provide also the position in space of the initial particle. The advantage of an OTPC over a conventional TPC is that it will be sensitive only to those fast particles (leptons and fast pions ) that are generated in the decays of the η meson we are interested in (fully described in the REDTOP Physics page). On the other side, hadrons and slower pions will be under Cerenkov threshold and, therefore, will not contribute to recorded hits. A prototype of an OTPC has been built and tested by the T1059 Collaboration at Fermilab.
The Cerenkov Radiators
Two Cerenkov radiators are present in the OTPC:
- A double aerogel cylinder, about 3 cm thick at the inner wall, supported by the beam pipe (the dark green ring in the picture above) . The innermost aerogel has nD =1.22 while the outermost has nD = 1.3;
- low-pressure nitrogen gas filling the rest of the volume of the OTPC. The pressure is adjusted in order to have nD = 1.000145.
Muon particles with momentum larger than 160 MeV will radiate only in the aerogel. The radius, the center and the skewness of the ring will help to determine its velocity. The dual refractive systems will help in discriminating the pions from the muons. On the other side, electrons and positrons will radiate in aerogel as well as in the gas. The ring produced by such light particles has a radius mostly independent from their velocity and much larger than that of the muons. That feature will provide a particle identification for electrons vs. muons. Furthermore, the Cerenkov photons radiated from the gas will generate a characteristic pattern in the photo-sensors. From the analysis of that pattern, one can estimate the momentum of the electron and its position in space. The picture below shows what happens in REDTOP’s OTPC when a 100 MeV electron (red track) travels trough the gas in a 0.6 T magnetic field. Several Cerenkov photons (cyan tracks) are generated and detected by the optical sensors surrounding the gas.
The ADRIANO Dual-Readout Calorimeter
Dual-readout calorimetry is a novel detector technique which has received good attention in the last few years. One version of the latter, named ADRIANO (A Dual-Readout Integrally Active Non-segmented Option), is currently under development at Fermilab by the T1015 Collaboration. The picture below shows three ADRIANO prototypes tested at FTBF in 2015.
It is based on the two simultaneous measurements of the energy deposited by a hadronic or electromagnetic shower into two media with different properties. The first medium is usually a plastic scintillator (or any substance producing scintillating light). Consequently, any charged particle depositing energy in that medium will produce scintillation and will make a signal in the corresponding electronics. The second medium is usually a heavy glass with high refractive index (nD >1.8) and high density (ρ >1.8). This medium will only be sensitive to charged particles with large velocity, such that light is emitted via the Cerenkov effect. Consequently, only the charged electromagnetic component of the shower (namely: electrons and positrons) or photons producing pairs will produce a signal in the corresponding electronics. Furthermore, the high density of the medium will make it an ideal integrally active absorber for all particles impinging on the detector. Summing the Cerenkov (C) and the scintillation (S) signal one has the total energy of the detected particle. Furthermore, by comparing the scintillation (S) vs. the Cerenkov (C) signals one also has information about the ID of the particle which generated the shower. This is shown, for example, in the plot below for simulated particles with Ekin=100 MeV.
The main advantage of using an ADRIANO calorimeter for detecting the photons from the η decay is that the nasty background from most neutrons entering the detector could be easily rejected. Furthermore, the different behavior in terms of S vs. C of muons and pions will help the double aerogel systems in disentangling the signals from the two different types of particles. The ADRIANO calorimeter corresponds to the outermost blue dodecagon in the sketch of the detector at the beginning of this page.
The Muon Polarimeter
The muon polarimeter (the green bars in the detector sketch) is an array of plastic scintillators inserted between the inner and the outer shells of ADRIANO. They have the task of counting the number of electrons and positrons emitted when a muon is stopped inside the ADRIANO calorimeter. This happens in a short range of depths after the muon has lost all of its energy. The initial polarization of the muon is not lost in this process since the ADRIANO lead-glass is homogenous and isoscalar. If the muon carries a non-null polarization, the electrons (or positrons) are emitted non-isotopically in the solenoidal magnetic field. The muon polarimeter will count those electrons: any unbalance in the left-right or front-backward counting is associated with a non-null transverse or longitudinal polarization of the initial muon.
The Photon Polarimeter
The photon polarimeter (the purple ring in the sketch of the detector) works on the same principles as the muon polarimeter. A 5 mm thick scintillating plate is inserted inside the volume of the OTPC. When a photon hits that material it has a finite probability of converting into a e+e- pair. If the circular polarization of the photon is zero, then the decaying pair has an isotropic distribution inside the 0.6 T solenoidal magnetic field. Otherwise, the asymmetry of the direction of the e+e- pair will provide an estimate of the polarization of the initial photon.