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An overview definitions, methods what is a van der Meer scan (VDM),

Thanks to the LHC groups and to the ALICE, ATLAS, CMS and LHCb collaborations for their support. Overview and relative bunch population measurements Massimiliano Ferro- Luzzi , on behalf of the BCNWG. An overview definitions, methods what is a van der Meer scan (VDM),

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An overview definitions, methods what is a van der Meer scan (VDM),

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  1. Thanks to the LHC groups and to the ALICE, ATLAS, CMS and LHCb collaborations for their support Overview and relative bunch population measurementsMassimiliano Ferro-Luzzi, on behalf of the BCNWG An overview definitions, methods what is a van der Meer scan (VDM), what is the beam-gas imaging method (BGI) brief historical reminder of Bunch Current Normalization effort The relative bunch population measurements FBCT , vs others (BPTX, DCCT, LDM, Beam-Gas (BG) rates Effects on cross section Analysis of bunch-by-bunch cross-section data Conclusions Special thanks to VladikBalagura for his detailed work on the bunch-by-bunch cross section analysis

  2. What is luminosity R = L  Rate Luminosity  Cross section L = f  N1  N2 "the beam overlap" revolution bunch population in frequency beam1 and beam2 address with VDM scans or BGI, see later An absolute bunch charge measurement is needed

  3. Van der Meer’s trick See refs. [1,2,3,4] Consider single circulating & colliding bunch pair with zero crossing angle R =   L =   f N1 N2 1(x,y) 2(x,y) dx dy With transverse displacements x , y of one beam w.r.t. the other: R (x , y) =   L(x , y) =   f N1 N2 1(x-x , y-y) 2(x,y) dx dy  R (x , y) dx dy =  f N1 N2 1(x-x , y-y) 2(x,y) dx dy dx dy =  f N1 N2 2(x,y) [ 1(x-x , y-y) dx dy] dx dy =  f N1 N2 2(x,y) dx dy =  f N1 N2 x z =1 =1

  4. Beam-gas imaging method pioneering measurement 900 GeV, in 2009 [6] residual gas Again, luminosity L = fN1N2  "overlap" • Beam interacts with residual gas around the interaction region • Reconstruct beam-gas interaction vertices => sample transverse beam profile measure individually the 1 and 2 and rebuild the overlap • even much better if combined with beam-beam interaction vertex distributions • can measure also , bunch charges, hourglass effect, etc... • Strength with respect to van der Meer method: (a) non disruptive, do not affect the beams! (b) can run fully parasitically during physics running time => potentially smaller systematics uncertainties  • Requires: • vtx detector resolution smaller (or at least comparable) to the beam sizes • residual pressure & acceptance must be adapted to this method See refs [5,6,7]

  5. A bit of history 2009 • 1st lumicalib at Pt8, pioneering BGI at 900 GeV 2010 • 1st flat top energy calibrations (VDM and BG) in Apr-May 2010: BCN systematics dominate the cross section uncertainty ~ 10% itself dominated by DCCT total current normalization itself dominated by DCCT baseline uncertainty  started BCNWG, and a detailed analysis  4-6% • Another lumicalib in Oct 2010 large currents (reduced baseline effects)  down to 3-4 % • And another in Nov 2010 (PbPb) • Much more ghost/satellites !! • Meanwhile, advanced solid ground work to tackle satellites/ghost charge issues, in anticipation of DCCT uncertainty reduction Refer to LUMIDAYS 2011 and refs [8,9,10]

  6. 2011 • Large effort to understand BCN: • DCCT: BE-BI + Colin Barschel (CERN-PH, Aachen Univ.) • Relative bunch populations, satellites, ghost: BCNWG • Three notes (being) written: • DCCT (Total current normalization): "RESULTS OF THE LHC DCCT CALIBRATION STUDIES", CERN-ATS-Note-2012-026 PERF • Relative populations: "STUDY OF THE RELATIVE LHC BUNCH POPULATIONS FOR LUMINOSITY CALIBRATION", CERN-ATS-Note-2012-028 PERF • Ghost and satellites: "STUDY OF THE LHC GHOST CHARGE AND SATELLITE BUNCHES FOR LUMINOSITY CALIBRATION", CERN-ATS-Note-2012-029 PERF The two main topics of this session : I) The results of these BCN studies II) The results of the expts' lumi calibrations

  7. The scan series

  8. Beam and bunch populations N For each beam: (we suppress here the beam index j=1 or 2) Ntot = Nmain + Nghost ( + Npilots ) Nmain =  Ni iM sum over the set of all the main bunches (colliding or not) total beam population (every charge in the ring) Ni Pi =  Nmain (  Pi = 1 ) iM population fractions:

  9. Main devices used in BCN Total current: • DCCT Individual bunch charge: • Fast Bunch Current Transformers (FBCT) • IP1 button pickups (BPTX) • Longitudinal Density Monitor (LDM) • Beam-Gas rates in IP8 See the talk of <= Colin Barschel <= here + Jean-Jacques Gras (session 3) <= Gabriel Anders <= Adam Jeff <= JaapPanman the main device: but also:

  10. Definitions: slots, RF bins and all that LHC = 3564 slots = 35640 RF bins (1 bin contains 1 RF bucket) threshold ... i-2 i-1 i i+1 i+2 ... phase undefined ?? slot i+1 slot i 25ns 25ns RF bin response typical ghost/satellites in protons and in Pb runs 2.5ns RF bin i10-9 • FBCT integrates over ~24ns and applies a threshold (cut noise) • LHCbbeam-gas rate allows us to measure the "ghost" charge in the nominally non-filled slots (40 MHz electronics, integrating over 25ns...), but cannot discriminate in the nominally filled slots

  11. Definition of ghost and satellite charges • Given that: • Total beam population is measured with the DCCTs • The relative populations are measured (mainly) with the FBCTs, which • which have a ~25ns integration time, while LHC RF buckets are 4 ~ 1ns long, contained inside RF bins of 2.5ns • which have a 25ns slot threshold below which the slot is assigned 0 charge • The main tool for measuring the particles invisible to the fBCT has been the LHCb BG rates, which also have a 25ns granularity we choose to define...

  12. ... (among many possible definitions) • Ghost charge: charge in all slots which are not nominally filled (total charge below the FBCT threshold  it is not "seen" by the FBCT ) • can be estimated from LHCb beam-gas rate • caveat: trigger efficiency over 25 ns ! • can be estimated from LDM, <0.1ns time resolution ! • caveat: normalization to nominal bunches! (emittance, position...) • definitions of phase relative to FBCT • Satellites: the charge in those neighbouring RF bins which are within 25ns "across" a nominally filled RF bin • they participate negligibly in luminosity (if crossing angle >0) • are integrated in the "slot population" by FBCT • what is the integration efficiency ? • what is the definition of phase (is the nominal bin centered ?)

  13. The FBCT Block schematics see Ref. [11] • The data for the relative bunch population measurements come from a ‘Slot Sum’ mode of acquisition with a specified number of 900 samplings for averaging (taken in 3600 turns with one sampling every fourth turn). • The Slot Sum returns 3564 bunch slot intensities on the high bandwidth channel each averaged over the specified number of turns. • The electronics were configured such that the gain was switched at around 2.3·1010p per bunch (averaged over all populated bunch slots) • 2011 pp scans = LO GAIN, while PbPb scans = HI GAIN toroid trafo thanks to D. Belohrad/ JJ Gras

  14. FBCT subtleties • FBCT uses two integrators (odd and even slots) • Gain was equalized by BE-BI experts • Possible systematic effects between odd/even channel not further investigated in BCNWG • Phase is regularly adjusted to have an optimum between good signal response and low "spill-over" effect • During VDM scans, this is generally done after reaching flat top (before STABLE BEAMS) signal of the same bunch with the 2 integrators (skipping one clock cycle) signals while change the fine phase delay

  15. Typical example of FBCT measurements (Oct 2011) fill 2234 courtesy of V. Balagura

  16. FBCT vs BPTX comparison fill 1653 mar 2011 fill 2234 oct 2011 • More of this in Gabriel Anders' presentation • Compared also to: • LDM => Adam Jeff • BG rates => Jaap Panman • DCCT , see next BPTX is a completely different device, in a different location, with different electronics

  17. Comparing FBCT sum vs DCCT (Pb-Pb Nov 2010) PbPb Nov 2010 • Total current (DCCT) changes by a factor ~0.8 over the duration of the fill • How about the ratio R ? R = FBCTsum / DCCT R0 = R at time 0 (~arbitrary) DCCT A and B (circles and triangles) 1% sumFBCT (squares) summing signals of all nominally filled slots

  18. Comparing FBCT sum vs DCCT (Pb-Pb Nov 2011) PbPb Nov 2011

  19. Comparing FBCT sum vs DCCT (pp 3.5TeV) pp May 2011 pp Oct 2011

  20. Comparing FBCT sum vs DCCT (pp 1.38TeV) pp Mar 2011 • Here: more pronounced change of R . Why ? • Non-optimized longitudinal plane (RF) • increasing ghost charge (see J. Panman) • Beam2: RF klystron trip 1% Klystron B2 trip Klystron B2 trip FBCT response depends on bunch length !

  21. Fill 1795, check on FBCT bunch length sensitivity • Analyzed the start of ramp • strong change of bunch length • stable orbit • Evaluate the slope (sensitivity) of FBCT vs bunch length DCCT energy DCCT bunchlength bunchlength FBCTsum FBCTsum thanks to N. Bacchetta / JJ Gras

  22. FBCT response sensitivity on bunch length (dN/N) Slope:  dl Slopes : B1 ~ 0.37 % / 0.1ns B2 ~ 0.48 % / 0.1ns around nominal bunch intensity • Future: could be refined: make a bunch-by-bunch analysis l thanks to N. Bacchetta / JJ Gras

  23. Typical bunch length spread during STABLE BEAMS May 2011 VDM scans, fill 1783 • Spread of ~ 0.02 ns (envelope) => expect ~0.1% max effect on FBCT-measured populations • NB: this effect was further reduced in the end-of-June 2011 technical stop by addition of a 70MHz filter thanks to N. Bacchetta / JJ Gras

  24. Position dependence of FBCT response • Special MD (machine development): • 30 June 2011, fill 1910, 12 bunches, 450 GeV only beam2 FBCT beam1 FBCT beam1 X move beam1 Y move

  25. Worst beam/plane combination from this study • Beam 1 horizontal: FBCT slope (sensitivity) = ~ 1%/mm (at ~1e11 p)

  26. Effect on lumi calibration experiments ? • Minimal ! • beams stay within 0.1mm at FBCT during VDM scans => effect must be at most 0.1% • NB: did not measure the individual bunch positions at FBCT (only beam average) Beam2 Y X Beam1 Y X

  27. Bunch-by-bunch cross section analysis • Since Nov 2010, usually, (many) more than one colliding pair per experiment were available during VDM scans => each pair "i" gives one cross section measurement • Inspect the "consistency" between all bunch pairs • If one observes an "inconsistency" (beyond statistics), one may try to model and apply a correction

  28. Response of bunch charge measuring device Ideal Real ?? (overly exaggerated here) underestimation of population fraction Nmeasured Nmeasured overestimation of population fraction pure proportionality line Ntrue Ntrue still, can be considered locally linear (affine) to a good approximation

  29. Simple fit model measured "errors" Define the "correct(ed)" populations: Define the deviations from a local (arbitrary) point: Assume errors are: Because of , one of the parameters can be fixed: => Minimize: beam j=1,2 i= index for BCID set of all Main bunches (1...m) e.g. 1 par (), 3-par (12 ) or 5-par (121 2) set of all Colliding main bunches

  30. Example: LHCb Mar 2011 (1.38 TeV) PRELIMINARY each point is one colliding bunch pair (IP8)

  31. Other Mar 2011 scans PRELIMINARY PRELIMINARY PRELIMINARY PRELIMINARY

  32. ATLAS/CMS May 2011 (pp 3.5 TeV, 1.5m) PRELIMINARY PRELIMINARY PRELIMINARY PRELIMINARY

  33. ALICE/LHCb May 2011 (pp 3.5 TeV) PRELIMINARY PRELIMINARY PRELIMINARY

  34. Table of fit results these are 1 by definition

  35. Observations • Affine fit generally improves the 2/ndf for pp, but it remains large for the higher-stat experiments (ATLAS, CMS) • improves especially in Mar 2011, not so in May 2011 ...nor Nov 2010 (PbPb, CMS) • Quadratic fit improves a further bit the 2/ndf for Mar 2011, but does nothing good for May 2011 • For May 2011: have also a complete data set from BPTX • similar analysis done with BPTX data, similar observations • Cross section results are stable: change by less than 0.4% Questions: • How do the extracted fit parameters (1, 2 for the affine fit) compare from IP to IP ? • Especially: how do they compare between CMS and ATLAS in the same fill ?

  36. Comparing fit parameters... • "moderate" agreement • NB: only IP1 and IP5 use the same bunch pairs! • But interesting observation on the difference FBCT - BPTX when compared with direct FBCT vs BPTX fit ... Hypothesis: slope - 1 FBCT - BPTX shaded bands from direct FBCT vs BPTX fit

  37. Comparing IP1/IP5 • See a correlation between IP1/5 in both Mar and May 2011 • In Mar 2011, the affine (and even quadratic) fits reduce correlation • In May 2011, they don't r = unweighted Pearson’s sample correlation coefficient B1 (B2) bunch collides in IP1&5 and... none IP8 (IP2) IP2 (IP8)

  38. Bunch-by-bunch response Remember: here, for illustration, the effects are disgustingly exaggerated • variations of the individual bunch properties can introduce changes ("fluctuations") in the response (bunch-by-bunch) • In our analysis, we did not (could not) take into account (and even less correct for) variations in bunch length, positions, and ... • This may (?) explain why we can't model perfectly with an affine fit • But the fluctuations could also be unrelated to the population fraction measurements! bunch with different bunch length ? Nmeasured bunch with different position at FBCT ? Ntrue

  39. Conclusions • The FBCT provide accurate measurements of the relative bunch populations • however, scrutinizing the high precision data, we start seeing some systematic effects at the permil level • several small effects observed: dependence on bunch length, beam positions, non-linearity, phase ? (satellites?), ... • Our detailed studies give us confidence that the effect on the cross section normalization are small (<0.5%, see table) • valid for the VDM scans from Nov 2010 onward • Given our current understanding*, we recommend using the "fudged" uncertainties (from the 1-par or 3-par b-by-b fits) as a measure of the systematic uncertainty on the cross section due to the relative bunch population measurements. *we cannot explain convincingly the non-statistical fluctuations of the bunch-by-bunch cross section data

  40. Literature (more here: http://lpc.web.cern.ch/lpc/lumicalib.htm) [1] "Calibration of the effective beam height in the ISR" , S. van der Meer, ISR-PO/68-31, 1968 (CERN). link [2] "Measurement of the luminosity of the p-pbar collider with a (generalized) Van der Meer method", C. Rubbia, CERN p-pbar note 38. link [3] "Absolute Luminosity from Machine Parameters", H. Burkhardt, P. Grafstrom, LHC-PROJECT-Report-1019 ; CERN-LHC-PROJECT-Report-1019; link [4] "Notes on Van der Meer Scan for Absolute Luminosity Measurement", V. Balagura, Nucl. Instr. and Meth. A (2011), doi:10.1016/j.nima.2011.06.007. arXiv:1103.1129 [physics.ins-det] , see http://arxiv.org/abs/1103.1129 [5] "Proposal for an absolute luminosity determination in colliding beam experiments using vertex detection of beam-gas interactions", MFL, Nucl. Instrum. Methods Phys. Res., A 553 , 3 (2005) 388-399. link [6] "Prompt production in pp collisions at s=0.9 TeV", LHCb Collab., R. Aiij et al., Physics Letters B Vol. 693, Issue 2, 27 Sep.2010, Pages 69–80. [7] "Absolute luminosity measurements with the LHCb detector at the LHC", LHCb Collaboration, 2012 JINST 7 P01010 doi:10.1088/1748-0221/7/01/P01010. linkarXiv. [8] "LHC Bunch Current Normalisation for the October 2010 Luminosity Calibration Measurements" A. Alici et al. (BCNWG note2), CERN-ATS-Note-2011-016 PERF link [9] "LHC Bunch Current Normalisation for the April-May 2010 Luminosity Calibration Measurements" G. Anders et al. (BCNWG note1), CERN-ATS-Note-2011-004 PERF link [10] "LHC Lumi Days: LHC Workshop on LHC Luminosity Calibration", CERN-Proceedings-2011-001. link [11] "The LHC Fast BCT system: A comparison of Design Parameters with Initial Performance" D. Belohrad, L.K. Jensen, O.R. Jones, M. Ludwig, J.J. Savioz, CERN-BE-2010-010, link

  41. Backup slides

  42. Error on cross section ought to be small! measured Relative error on cross section due to wrong measurement of population fractions: correct depends on - correlations between errors in one beam and population fractions in the other beam, and - in correlations between errors in population fraction in each beam

  43. some (semi-quantitative) coarse bound Define Then: Using as an indicative value, one can evaluate In case then unlikely and really bad

  44. Other stuff • Also tried to look at bunch families: no convincing structure observed • For example, May 2011 courtesy of V. Balagura group of 6 points: 6 VDM scans offset is related to slope different "families" according to collision schedule

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