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Beam Sizes measured by BaBar

Beam Sizes measured by BaBar. J. Thompson SLAC PEP-II MAC Review, 19 January 2006. Beam Sizes. HER: 9 GeV e-. LER: 3.1 GeV e+. Bunch length s z (~RF). Angular spread s x’ 2 = e x / b x (z). Transverse size s x 2 = e x b x (z). Similarly for y HER and LER are independent

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Beam Sizes measured by BaBar

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  1. Beam Sizes measured by BaBar J. Thompson SLAC PEP-II MAC Review, 19 January 2006

  2. Beam Sizes HER: 9 GeV e- LER: 3.1 GeV e+ Bunch length sz (~RF) Angular spread sx’2 = ex / bx(z) Transverse size sx2 = exbx(z) • Similarly for y • HER and LER are independent • 10 parameters so far need multiple measurements

  3. In collision • BaBar sees the beams where they are colliding (use 2 prong events) • Luminous region (“beamspot”) size observables: • dL/dz – longitudinal luminosity distribution, online and offline • measure effective b*y • measure Sz; get sz,HER, sz,LER when RF voltage changes • sLx(z) – beamspot x size: good measurement online, great measurement offline • sLy(z) – beamspot y size: difficult measurement offline • due to hourglass effect, measure effective b*y • beamspotqxy, qx’y’ – coupling • sLx’ • sLy’(z) – measure vertical emittances and effective b*y • Crossing angles and tilts

  4. Ongoing Analyses • B. Viaud, C. O’Grady: online monitoring of beamspot location and sizes; dL/dz fits • J. Thompson, A. Roodman: measurement of transverse beamspot sizes • M. Weaver, A. Cunha: angular distribution measurements

  5. dL/dz Longitudinal luminosity distribution (assuming b*y,HER = b*y,LER): Si2 = si,HER2 + si,LER2 Hourglass effect gives a z dependence to sy: Fit the distribution to b*y and Sz • Questions: • b*y ~ 1.3 -1.5 cm is higher than expected • Clear change at ~run 45000 (February ’04) βy* [cm] This fit is now done online, with results updated once per BaBar run Run4 Run Number

  6. dL/dz Longitudinal luminosity distribution (assuming b*yHER = b*yLER): Confirmed by independent fits I have done; true for both Run 4 and Run 5. Results shown here are the best validated to date. Si2 = si,HER2 + si,LER2 Hourglass effect gives a z dependence to sy: Fit the distribution to b*y and Sz • Questions: • b*y = 1.3 -1.5 cm is higher than expected • Clear change at ~run 45000 (February ’04) βy* [cm] This fit is now done online, with results updated once per BaBar run Run4 Run Number

  7. New Online Measurements:β*y & Σz RFV(LER) = 4.5 -> 4.05 MV

  8. New Online Plot: Phase Transient 1 Run! Abort gap Bucket number calculated with the “precise” t0 : σ=2 ns Not thoroughly tested though…

  9. Transverse beamspot sizes • sLx (~60mm) is larger than resolution (~30mm) • can be extracted with good accuracy online • sLy (~5mm) is much harder • Requires offline analysis: • Offline analysis aims to measure sLx and sLy with a method that removes resolution from the measurement • Measure as a function of z

  10. Lumi loss along minitrains Late Run 4 Early Run 5 5% loss 15% loss ~Luminosity ~Luminosity sLx (mm) sLx (mm)

  11. sLx(z), sLy(z) sLx in bins of z: <sLx> = 65.5 ± 0.2 mm d(sLx)/dz = (7.2 ± 2.3) x 10-5 Sample Results: 2 days starting July 2 Global qxy = -10 ± 2 mrad sLy in bins of z: sLy(z=0) = 5.2 ± 0.4 mm b*y = 1.5 ± 0.4 cm

  12. Run 5: x Results <sLx> (cm) • 2-day bins of time (some failed fits removed from plots) • Non-zero dsLx/dz first noticed in late Run 4 • Origin is an open question Date in Run 5 Date in Run 5

  13. Run 5: y Results b*y (cm) No trend obvious; Average for all points: b*y = 1.40 ± 0.06 cm Date • 2-day bins of time • From fits to sy(z) sLy,z=0 (cm) Date

  14. Bins of Specific Luminosity Integrating over Run 5 through August sLx (cm) b*y (cm) Specific luminosity Specific luminosity sLx(z) slope sLy,z=0 (cm) Specific luminosity Specific luminosity

  15. z-dependence of angular spread fH = EHER/(EHER-ELER) ~ 1.5 fL = ELER/(EHER-ELER) ~ 0.5  HER dominates measurement

  16. Sample angular spread fits

  17. Angular spread: Data Results 14 ey,LER b*y (mm) 4 ey,HER 3.8 ey,HER 2.6 ey,LER bias is attributed to ISR (from simulation) zw (mm) b*y (mm) • Boost method results • Central values: • b*y = 14.6 mm • ey,HER = 3.2 nm rad • ey,LER ~ 9 nm rad • sLy(z=0) ~ 5-6 mm ey,HER zw (mm) zw (mm) ey,LER 14 15 b*y (mm)

  18. x beam size determination -20.6 xz angle (mrad) -22.4 0 xz crossing angle (mrad) Individual x beam sizes! -1 This study done with quantities available online (BBR:BBRPEP:SVTDXDZ, BBR:BBRPEP:SVTXP) HER IP x angle change, Aug 23 2005

  19. Publications • PAC05 papers: • BV, CO’G, WK, IN, AP: http://accelconf.web.cern.ch/AccelConf/p05/PAPERS/TPAT076.PDF • JT, AR, WK: http://accelconf.web.cern.ch/AccelConf/p05/PAPERS/MPPE057.PDF • Plans for: • EPAC papers this year • BV, CO’G, et al • MW et al • NIM paper bringing all the results together

  20. Plans • dL/dz: • More bunch length measurements (December 9) • Add hadronic events to increase statistics • Transverse beam size fit: • Why is dsLx/dz non-zero? Does qxy have a z dependence? • Understand/reduce systematic error on sLy • Boost analysis • Work on bias due to ISR • Measurement of qx’y’ • Overall • Compare to simulation • How to best use the 3 methods together? • Interpret results: why is the b*y we observe so large? (Dispersion ruled out)

  21. Extra Slides • Measurement methods: • Transverse beam sizes • Angular spread • Bunch length measurements • Additional transverse beam size results

  22. sy(z) method • Each muon track is reconstructed independently, with a point-of-closest-approach (poca) to the beam axis • d measures production point • m measures the reconstruction resolution • Also use the estimated error on d, d (from the track fit) • 2 types of fit: • To m distribution, using d (determine resolution correction) • To d distribution, using d and corrected resolution: • using f, extract sLx, sLy • Also measure z for each event An e+e-  m+m- event as reconstructed True production point m1 m2

  23. Angular studies: Boost method

  24. Bunch Length Measurement Combine dL/dz fit with known scaling of bunch length with RF voltage Fit determines: Sz,HER = 11.1 ± 1.1 mm Sz,LER = 11.1 ± 1.1 mm

  25. Bins of Beam Current: HER Integrating over Run 5 through August sLx (cm) b*y (cm) mA mA sLx(z) slope sLy,z=0 (cm) mA mA

  26. Bins of Beam Current: LER Integrating over Run 5 through August sLx (cm) b*y (cm) mA mA sLx(z) slope sLy,z=0 (cm) mA mA

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