[Eic-core-phys] [EXTERNAL] beam energies

[Pawel Nadel-Turonski]

Hello Everyone,

I just wanted to follow up on our interesting discussions this morning with a little more details on beam energies. Some combinations that were used in the Yellow Report are listed on the simulations page,
https://eic.jlab.org/core/index.php/Simulations <https://eic.jlab.org/core/index.php/Simulations>
but these are not the only, or necessarily always best combinations. In addition to cm energy, the choice of beam energies also affects luminosity and the level of asymmetry determines the particle distribution in the lab frame (angles, momenta), which in turn affects acceptance.

Luminosity considerations
For electrons, the luminosity comes from the beam current, which is limited by the synchrotron radiation power (absorption) limit. In the current EIC design, electrons can be run at maximum current in the 5-10 GeV range, and after that at a rapidly falling current (and luminosity) up to 18 GeV. The current limitation is independent of the proton/ion beam.

Proton and ion beams
At the lowest, discrete energy (41 GeV for protons), the luminosity is limited by space charge (self-focusing of the bunches as they more around the ring). This can to some extent be mitigated by stronger focusing (although this has other side effects), which is the reason for the IR-independent low-energy quad design that has been proposed. It is worth noting that the poor emittance at 41 GeV makes this setting less suitable for exclusive measurement. However, in the continuous energy range (100 - 275 GeV for protons), the luminosity is essentially proportional to the beam energy once all other parameters are optimized.

Asymmetry / acceptance
The lab momenta of particles in the endcaps are limited by the beam energy (i.e., a meson in the electron endcap cannot be more energetic than the electro beam, and in the hadron endcap it does not exceed the hadron beam energy). The asymmetry also shifts the angular distribution of scattered particles in the respective direction. In addition, for protons, the pT acceptance for the Roman pots is best at 275 GeV (for ions the high-pT acceptance is not an issue).
In the yellow report, most of the “standard” beam energies (5x41, 5x100, 10x100, and 18x275 GeV) were reduced luminosity options. The top energy (18x275 GeV for protons, or 18x110 GeV/A for heavy ions) is important for certain studies (e.g., gluon saturation), and 5x100 is probably the lowest energy that could be run in practice. But 10x100 is a far from obvious choice.
For protons 10x275 offers a factor 3 higher luminosity than 5 or 10x100, and a much better high-pT forward acceptance. It also pushes hadrons into a region with much better PID (the dual-radiator RICH in the hadron endcap can easily cover momenta up to 50 GeV/c). Thus, all proton physics that can be done at 10x275 should probably be done at 10x275. However, going to lower cm energies, one can consider 5x275 (or possibly 5x200) as an alternative to 10x100. Since electron detection below 1 GeV is challenging, a higher electron energy does have some  benefits, but the lower hadron energy in the 10x100 configuration also comes with some challenges of its own. Thus, for our simulations it may be beneficial to think about which aspects are most important for each process in this intermediate energy range.

Best,

	Pawel

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