1 / 17

Beam-beam & the lum in ous region - update

Beam-beam & the lum in ous region - update. W. Kozanecki 4 March 05. “To do” list left over from BaBar week Understand whether the y-truncation of the luminous region (|y| < 25 m in the present luminous-region analysis) significantly biases the vertical luminous size at high |z|

lyndon
Download Presentation

Beam-beam & the lum in ous region - update

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Beam-beam & the luminous region - update W. Kozanecki 4 March 05 • “To do” list left over from BaBar week • Understand whether the y-truncation of the luminous region (|y| < 25 m in the present luminous-region analysis) significantly biases the vertical luminous size at high |z| • Check z-y correlations • Clarify whether the overestimate of b*y in the luminous-region fits occurs both in “b*” fits [ s2y(z) ] & in “bunch-length” fits [ L(z) ] • Understand the discrepancies between the z-dependence of the vertical luminous size directly obtained from the simulation, and that inferred from the individual, single-beam charge y-z distributions

  2. z ~ 0 z ~ 16 mm L/bunch (1024 cm-2 s-1) z ~ 26 mm Y (m) Any truncation bias at high |y|, high |z| ? Could the systematic discrepancy in syL at high z, be due to y-truncation imposed in the luminosity computation by the simulation?

  3. So far ignored z-slice info in b-b sim output, i.e. assumed x, y, z uncorrelated Now ‘sample’ each z slice as it crosses the IP, i.e. plot y*, yp = f( zslice) (y*) low I e- bunch <-- tail head --> dN/dy* e+ z (mm) y-z correlations in single-beam charge distributions

  4. Single-beam y-z correlations : low vs. high current (y*) low I e- (yp) low I bunch <-- tail head --> e+ (y*) high I • Y. Cai: “pinch effect” ! • akin to what is happening in LC • distinct from dyn.  (yp) High I

  5. z-dependence of effectivey, *y : high current *yeff , high current yeff , high current e+ e+ e- e-

  6. z-dependence of effectivey, *y : low current *yeff , low current yeff , low current e+ e- e+ e- Could the “low current” still be too high?  Ilya will generate a very low current data set

  7. b*y ‘measurements’ (on b-b simulations, no detector effects!) • e, b* = simulation input parameters • Compute effective values of e, b* (LER or HER) from e+/e- charge distributions: sy = e b*y s’y = e / b*y • Fit z-dependence of vertical beam size (LER or HER): s2y (z) = sy2 + s’y2 z2 = (e b*y)+ (e / b*y)z2 • Fit z-dependence of luminous region • z-dependence of vertical luminous size syL (z, by*LER, by*HER) (bunch-length independent) • longitudinal luminosity distribution L(z, by*LER, by*HER, sz, LER, sz, HER)

  8. Single-beam y, b*y fits (updated for actual z-dependence) LER sy2 (z) (High current) LER sy2 (z) (Low current) LER High I Low I HER sy2 (z) (High current) HER sy2 (z) (Low current) HER

  9. These results updated for y-z correlations Single-beam b*y fits(high/lowx)

  10. z-dependence of vertical spot size

  11. Ly2 ~ z2 (2 parameters), but we need 4: yLER, yHER , yLER , yHER Several possibilities, e.g.: Fix yLER, yHER , yHERFit yLER Fix yHER / yLER , yHERFit yLER, yLER Fitting the z-dependence of the vertical luminous size syL (z) (m) syL (z) (m) Low current Low current z (mm) z (mm)

  12. b*y fits(high/lowx) using the vertical -beam or -luminous size

  13. Fixed -normalization method Fitting code from B. Viaud Bunch length fits to L(z) distribution (high/lowx)

  14. Fit z+ only Fit z+, y+ Why is the fitted bunch length so stable? L (arb. units) L(z) appears insensitive to * for |z| < 20 mm Fit z+ only Ratio of fitted functions Fit z+ only Fit z+, y+ Fitted / ‘mrsd’ Fit z+, y+ z (mm) z (mm)

  15. Summary of b*y fits(high/lowx)

  16. Summary (I) • A beam-beam ‘pinch effect’ is apparent in the z-slice dependence of the vertical beam sizes, resulting in large variations in effective vertical emittance & -function along the bunch. • the effect is spectacular at nominal bunch current • it may still be significant at low (10%) bunch current, and may be responsible for the small bias observed in the single-beam *y fits. • The bunch length fit [ L(z) ] and the fit to the vertical luminous size [ yL(z) ] return b*y values consistent with each other, but overestimated by ~ 4-5 mm (as suggested by real data). This bias may be due to the above-mentioned pinch effect. • Additional beam-beam simulations at very low current (1% nominal) are in progress to verify this interpretation. • Bunch-length fits of the longitudinal luminosity distribution • return the correct (MC truth) bunch length within < 5%, at both low & high x, under all considered b*y scenarios: • both b*y’s fixed to true (input) values • one or both b*y’s floated in the fit The robustness of the bunch length fit is attributed to the fact that *y does not significantly affects the L(z) distribution until |z| > 20 mm.

  17. However... • Still open / to be understood in the simulation • is the pinch effect really the culprit, i.e. will we get the correct * from the luminous-region analyses at very low current (1% nominal) ? • what is the physics of the pinch effect? • how is it different from the dynamic- effect? • Is it effectively a steady-state phenomenon? • on what time scale (# turns) does it stabilize? • what diagnostics can we run on the simulation to understand it better? • why does the ‘pinch effect’ (if it really is the culprit) induce similar * distortions at nominal and at low (10%) bunch current? • the error treatement is not correct in the (simulated) luminous-region analyses, in that it ignores the peculiar statistical-fluctuation mechanism: in the simulation, fluctuations are driven by the # of macroparticles in each bin, not by the luminosity as in the real world). Could the bias be worse than the present studies suggests? (The statistical treatement of the single-beam simulations IS correct, though.) • ...and in the data • why does floating y change the fitted z+ value? • why does the data fit @ fixed y look bad, while the same fit on the simulation looks decent (up to clarifying the stat. error issues above) ?

More Related