1. General guidelines for SixTrack The code options aim at being general with no implicit assumptions to avoid misunderstanding (overhead for the user) and to avoid frequent recompilations (overhead for LHC@home team). The official repository should be used for production, while private copies should be limited to code developments. The documentation effort started recently and already helped many users and developers. It should be completed (missing elements to document are beam-beam and wire). In particular the manuals have the targets: – The user manual solves any ambiguities on units and sign conventions.user manual – The physics manual allows to understand the physics in the code and generate correct results. The equations should be as complete as needed to reproduce SixTrack results.physics manual – The developer manual clarifies the definitions, unit conventions of the variables used in the code and the overall structure of the code.developer manual

2. Beam Beam in MadX MadX implements a 4D kick in Twiss and Track module. label: BEAMBEAM, SIGX=real,SIGY=real, XMA=real,YMA=real,CHARGE=real, BBSHAPE=(circular, trapezoidal, hollow), WIDTH=real,BBDIR=int; XMA, YMAL: displacement, BBSHAPE: transverse distribution, WIDTH: length of the transverse edge, BBDIR: direction of the beam The mask file has the macros that extract position, beta from the strong beam, calculate the positions and parameters of the lens and install the bb elements. The SixTrack converter translates the MadX lattice into Sixtrack input (sufficient for 4D). The SixTrack user edits the fort.3 to enable 6D beam beam kicks.

3. SixTrack Beam Beam as present Fort.2 contains element and lattice definitions: Fort.2 – Has 3 blocks: single element list, block of linear elements and lattice. – For each element type one has 3 attributes per element, except beam-beam. Beam-beam element: name1 type h-sep v-sep strength-ratio σ_ hor σ_ ver σ_ lon name2 type h-sep v-sep strength-ratio σ_ hor σ_ ver σ_ lon Fort.3: contains most of the other input including beam information: Fort.3 BEAM partnum emitnx emitny sigz sige ibeco ibtyp lhc ibbc name1 ibsix xang xplane name2 … Ibeco: adding the dipole kick, ibtyp: algorithm for erfc, lhc: optics symmetries, ibbc: coupling effects, ibsix: number of slices, xang: crossing angle (what exactly?), xplane: plane of crossing For 6D, the slicing is calculated internally using the SixTrack optics module based on assumed symmetries between weak and strong optics that are not sufficiently general; Only 4D has the option to use the external input.

4. Beam-Beam lens update proposal Target: – Drop the LHC option since it already led to confusion and it is not general enough even for the LHC; – define explicitly in the input (fort.3) all the input parameters used by the beam-beam lens 4D and 6D. Steps: – Complete in the developer manual the definitions and units of the variables used by the beam-beam code (e.g. bbcu(1..10) ) related to the 6d slicing in order to understand the quantities the beam-beam routine use. – Complete the full equations of the kick (partially done) and the one used for the computations of the slices (not done) in the physics manual. – Update Sixtrack code and document the changes occurred in the input files. – Add missing 6D fields in the MadX bb element, change the Sixtrack converter and update the mask macros to compute the missing fields.

5. Example possible new input Fort.2 – Drop the recently added columns and keep name1 20 h-sep v-sep strength-ratio name2 20 h-sep v-sep strength-ratio Fort.3: add all relevant parameters (e.g.) – 4D lens name1 0 σ xx σ yy σ xy – 6D lens name1 ibsix strong_angle xplane