REDTOP: Rare Eta Decays with a TPC for Optical Photons

The experiment

REDTOP is a new Fermilab project in its proposal stage. It belongs to the High Intensity class of experiments as it aims at detecting small variations from the Standard Model by looking at a large number of events produced with very intense beams. REDTOP scientists propose using a 1.9 GeV continuous proton beam impinging on a target made with 10 foils of beryllium to produce about 1012 η mesons in one year of running. The detector surrounding those targets will attempt to capture the decay products of the  η mesons and, in particular, those that are either not expected or are suppressed at the 10-11 level. Read a more detailed discussion of the physics processes of interest for REDTOP.

REDTOP would be located inside the MC-1 experimental hall, presently housing the  Muon g-2 experiment, on the Fermilab’s Muon Campus. A sketch of the detector location along with a portion of the Muon g-2 storage ring is shown in the picture below.

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The accelerator

One of the more challenging aspects of REDTOP is related to the proton beam required to produce and detect the events desired. The kinetic energy of that beam, in fact, needs to be high enough to generate the η mesons inside the target but, at the same time, not too high so that the background is kept at manageable levels.  Monte Carlo simulations indicate that an energy of 1.9 GeV is optimal for the experiment. Furthermore, the beam time structure needs to be as uniform as possible, in order to allow the detector to collect, skim and transfer the information to the data storage.

Fermilab is developing its new Muon Campus to provide beam for the Muon g-2 and Mu2e experiments. At centerstage of the complex is the Delivery Ring (DR).  For Muon g-2, 8 GeV protons will be targeted from the Main Injector/Recycler and the DR will be tuned to collect 3.1 GeV/c momentum muons.  For Mu2e, the 8 GeV protons (8.9 GeV/c) will be transferred directly from the Recycler into the DR.  The ring will have RF cavities for maintaining bunched beams as well as a slow resonant extraction system for use during Mu2e running.  Though both Muon g-2 and Mu2e will run the DR at constant field, an additional RF cavity and improvements to the power supply system of the DR would allow the DR to decelerate beam to lower energies such as those required for REDTOP.  Hence, such enhancements to the DR would open up a new realm of lower-energy, high rate experiments to augment the upcoming Muon program.  Read a more detailed discussion of the accelerator complex for REDTOP.

The detector

Of course, as it is usually the case with High Intensity beams, the rare processes that the scientists want to study are accompanied by a much larger number of background events that can potentially hide or mimic those of interest.detector_sketch

Therefore, in order to keep the background at tolerable levels, the detector must be very fast and sensitive only to those particles expected to be produced in the processes of interest, namely electrons, positrons, muons and photons. This is why the whole apparatus is based on the  detection of prompt Cerenkov light, which is promptly created while the background particles (essentially, all the hadrons) are mostly under the Cerenkov threshold. Read a more detailed discussion of REDTOP detector.

The running plan

The detector for the REDTOP experiment is designed to be almost hermetic and general purpose. This will make it ideal for an extended run, exploiting different beam energies and beam configurations. At present, the experiment is planned to have three runs:

RUN I – η factory

This will be, in fact, where the η meson will be produced, making  REDTOP, in all respects, an η-factory. The beam energy will be approximately 1.8 GeV (see this link for a more detailed description of the beam characteristics). The goal is to collect in excess of 1012 η mesons in one year of running.

RUN II – η’ factory

The beam required for this running phase of the experiment are protons with energy between 3.5 and 4.0 GeV. The minimum required intensity is about 60 W. The baseline detector configuration requires a minor change in order to adequate the refractive index of the aerogel to the faster leptons produced in the η’ decay. The goal is to collect of the order of 1010 η’ mesons in one year of running.

RUN III – muon scattering experiment (optional)

This running phase will use the hermetic REDTOP detector  for exploring the proton radius anomaly with a muon scattering experiment. The experiment will collect a lower statistics than its direct competitor (the MUSE detector at PSI). However, the apparatus and the experimental technique is considerably different, giving to REDTOP the opportunity to contribute to this class of experiments with a different systematic uncertainty. Such contributions are still under investigation; consequently, this running phase is still considered optional. The beam required   for phase III are muons with energy between 0.2 and 0.8 GeV. The baseline detector configuration requires no changes from Phase II. However, a graphite target needs to be added upstream the detector.  The goal is to collect of the order of 1012 muons scattering events in one year of running.

RUN IV – rare kaon decay expeiment

Run-IV represents a considerable departure from the previous three runs, since, in this case, both the detector and the beam have to be re-configured. The main physics topic will be the study of the CP violating process K+ -> pi+ nu nu. The kaon beam will be generated as a secondary from the primary 8 GeV proton beam. The beryllium target needs also to be replaced with an active plastic target. From the detector side, the Optical-TPC needs to be removed and replaced with a low-mass drift chamber and a range stack. The rest of the detector, ADRIANO, the Muon Polarimeter and the solenoid will remain unchanged. The detector concept parallels those of the E949 project at BNL and ORKA at Fermilab. Preliminary Monte Carlo studies, performed with the GenieHad simulation framework, indicate, in fact, that the π/K production ratio from an 8 GeV primary proton beam is about 13  (when the very soft pions are swept away), which is considerably more favorable that the corresponding value at 92 GeV (proposed for the ORKA experiment) which is about 22.