The REDTOP detector is a high granularity, fast and covering almost the entire sold angle. Therefore, it can be used for several physics runs, eventually with some modifications, and/or with different particle beams. At present, the Collaboration has identified at least two physics program that can be exploited at Fermilab.
Run-Ia and Run-Ib: Untagged η/η’ factory
The initial run for REDTOP aims at hadro-producing η and η’ meson from the nuclear scattering of a proton beam on a nuclear target. The production mechanism is based on the decay of intra-nuclear baryonic resonances which are generated for beam energies above approx. 1.5 (3.0 for the η’-meson). The advantage of the method is that the production cross-section is relatively large (order of several mbarn) a relatively low-power beam (30-50 W) is sufficient to reach the goal (see also, The accelerator complex page).
The disadvantage of the proposed method is that the mesons are accompanied by a variety of other particles, including nuclear fragments, and no tagging mechanism cam be devised to identify interesting events.
For the Run-Ia (η-factory), the beam energy being chosen is 1.8 GeV, while for Run-Ib (η’-factory), the optimal beam energy is 3.5 GeV. Both energy and power requirements can be easily met with the Delivery Ring accelerator complex.
Run-II: Tagged η factory
An upgraded version of the REDTOP, t-REDTOP, could be run in a later stage of the experiment at the PIP-II facility, currently under construction at Fermilab. At the 800-920 MeV, high intensity (200KW-1MW), CW proton beam, the production mechanism of the η-meson is substantially different. The nuclear process providing the η-meson would be:
p + De → η + 3He+
Therefore, t-REDTOP requires a gaseous Deuterium target and an extra detector to tag the 3He+ ion. The production cross section for the process is approximately five orders of magnitude smaller than at 1.8 GeV and the gaseous target reduces further the luminosity of the beam. However, the large intensity of PIP-II more than compensate for that and the number of η − mesons produced at t-REDTOP is expected to be in the range [1013/year − 1014/year]. The biggest advantages of a tagged η-factory are the following:
- By tagging the production of the η-meson via the detection of the 3He+ ion, the combinatorics background from non − η events, is greatly diminished. Consequently, the sensitivity of the experiment to New Physics is increased by a factor proportional to the square root of that reduction;
- By measuring the momentum of the 3He+ ion in the final state, the kinematics of the reaction is fully closed. That portends to a better measurement of the kinematics of the particles detected since a 4-C kinematic fit could be applied;
- Since the kinematics is closed, any long-lived, dark particle escaping detection could be identified using the missing 4-momentum technique. The latter is considerably more powerful than the, 1-, missing pt or missing energy proposed by some recent experiments searching for dark matter.
Run-III: Tagged η’ factory
The energy of PIP-II complex will not be sufficient to produce η’ mesons with the nuclear process described in the above section. At least 1.7 GeV are required for the process:
p + De → η’ + 3He+ to occur.
However, the production threshold will be obtainable with the future PIP-III accelerator complex. A high-intensity (> 1MW) CW beam (or, a slow-extracted beam) will be necessary to compensate for the low cross-section of the process. Neither the detector nor the target employed for Run-II needs modifications for the run as η’-factory;