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

[Pawel Nadel-Turonski]

Hi Mark,

That is interesting! Thank you for pointing i. out! One thing one needs to keep in mind with such low-energy beams is that the y-coverage could be limited - but it is certainly worth exploring! It could be a good option for spectroscopy, for instance.

Cheers,

	Pawel


> On Aug 13, 2021, at 11:14 AM, mdbaker at mdbpads.com wrote:
> 
> Pawel,
> 
> One comment. If it is useful, electron beams are now planned down to
> 2.5 GeV as well as up to 18. I'm not sure what the luminosity impact is.
> 
> Mark
> 
> On 2021-08-13 09:54, Pawel Nadel-Turonski wrote:
>> 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
>> 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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