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Preliminary Observations on Beam Scans Using the Van Der Meer Method

This study examines the effective overlap area determined by scans, analyzing intensity, lifetime, emittance blow-up, hysteresis effects, and comparisons between ATLAS and CMS. Results highlight systematics, consistency, and reproducibility.

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Preliminary Observations on Beam Scans Using the Van Der Meer Method

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  1. S. White, LBS 17 May 2010 Van Der Meer Scans: Preliminary Observations

  2. The Van Der Meer Method The effective overlap area can be determined by scans in separation: • Perfect Gauss : • Double Gauss:

  3. Intensity and Lifetime • Intensity and lifetime over the • fill 1089: • Intensity very stable, lifetime • of several hundred hours. • Scan ~20 minutes. Intensity • variations of the order of 0.1% • over the duration of the scan. • Luminosity lifetime clearly • dominated by emittance blow-up. • Absolute error on the intensity • measurement: see talk by J. J. Gras.

  4. Emittance VDM VDM • Emittance blow up during the scan (left) : the effective beam size (right) measured • during the scan is also affected. • The duration of a scan is about 20 minutes: the growth of the effective beam size during • one scan is ~1% is the worst case: full calibration 1-2%. • Looking at the trend over the fill there could be a small blow-up due to the scan itself. • If significant this effect has to be added to the systematics. A possible solution would be to • scan faster less points / shorter steps.

  5. Effect on the Rates • History of the rates without separation during • the scans (top) and effect of the emittance • variations on the rates (bottom): • There seem to be some correlation for CMS. • No apparent correlation for ATLAS. • The large fluctuations in ATLAS are not • explained by emittance variations. • The overall slope is consistent with emittance • variations.

  6. Hysteresis check - CMS • For each plane scan in opposite direction to check for consistency and hysteresis: if hysteresis • is relevant the overlap profile should be shifted.

  7. Hysteresis Check - ATLAS

  8. Hysteresis Check - Summary • Hysteresis check performed both for ATLAS and CMS: • Largest shift seen in ATLAS vertical ~0.002 mm: negligible effect on the rates. • Effective beam size measurement very consistent from one scan to the other. • Largest growth seen in IP5 vertical and consistent with emittance blow-up (~1%). • Hysteresis effects can be considered negligible. • The fluctuations in the rates shown before cannot be explained by hysteresis / orbit drifts. • Any other ideas??

  9. Comparison between ATLAS and CMS • ATLAS and CMS see the same bunch colliding and should get the same luminosity, unless the • b* are different. • According to b* measurement CMS should have ~3% more luminosity than ATLAS. • Large error bars. • Use values from previous table to compute the effective area: • ATLAS : Aeff= 0.0230 +/- 0.0004 mm2 • CMS : Aeff= 0.0208 +/- 0.0002 mm2 • About 10% difference between the two. Correcting for emittance blow-up between the two • measurements by 2% in horizontal and 4% in vertical we get a difference of 4%. • Very good agreement with b* measurements. • Preliminary absolute luminosity (only statistical error shown): • ATLAS : L= (1.9 +/- 0.032) x1028 cm-2 s-1 • CMS : L= (2.12 +/- 0.02) x1028 cm-2 s-1

  10. Fill to Fill Consistency (Estimates) Should be constant regardless of beam conditions • Data for two fills in ATLAS and CMS. Compare the ratio Nmax/ L0. • ATLAS: sending calibrated values, should get 1 if MC is right. • Fill 1059: Nmax/ L0 = (0.98 +/- 0.027) x 10-30 • Fill 1089:Nmax/ L0 = (1 +/- 0.016) x10-30 • Consistent within error bars. Seems like we are right on spot. • CMS: in principle sending rates should get the effective cross section • Fill 1058: Nmax/ L0 = (1.14 +/- 0.023) x 10-25 • Fill 1089: Nmax/ L0 = (1.04 +/- 0.01) x 10-25 • 10% difference not explained by statistical errors bars. Fill 1089 error of about 2% from • emittance. Fill 1058 parasitic collisions, error on the rates??? • Is this number consistent with a cross section??? (seems large). • Tried to look at HF but max rates go from ~650 to ~4150. Not consistent with beam parameters. • Beam size from HF smaller for fill 1058 : NmaxBCS/ L0HF= (1.08 +/- 0.018) x 10-25

  11. BRAN Data – IP5 • Two sets of BRANS present in IP1 and IP5 (BRANA, BRANP): • BRANA very noisy – low efficiency : only look at BRANP for now.

  12. BRAN Data – IP1

  13. BRAN Data - Summary • Constant added to the fit parameters to get a reasonable Chi2. Effect larger in IP5. • Take only the best fit for each scan. • Excellent agreement on the optimum : initial purpose of the BRANs. • Large error bars on the effective beam size. Still consistent with experiments data. • In general good agreement with experiment, large error bars due to background component.

  14. Summary • Focused on fill 1089. • Significant emittance blow-up during the measurement: to be included in the systematics. • Check for hysteresis effects: non significant effects seen during this measurement. • Excellent reproducibility. statistical error are not an issue. • Comparison between ATLAS and CMS in good agreement with optics measurements. • Fill to fill consistency good for ATLAS. Some differences in CMS need to be understood. • BRAN data in good agreement with experiments. • Need to perform the same analysis for all scans. • Calibration factor should not be derived from a single measurement: repeat scans in all IPs.

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