Luminosity monitor and LHC operation H. Burkhardt AB/ABP, TAN integration workshop, 10/3/2006 Luminosity monitor and LHC operation Thanks for discussions and input from Enrico Bravin, Ralph Assmann, Jörg Wenninger, Werner Herr, Roger Bailey and others. Apologies for any omissions (prepared on rather short notice)

Outline Importance of luminosity monitoring in operation and commissioning Operational use in (early) operation

Introduction Luminosity and background signals are the main figures of merit in operation. Fast, robust, relative luminosity is important to help to bring beams into collision, for tuning and optimisation LHC luminometer spec. : beam lumi every sec, bunch-by-bunch lumi every 10 sec, backgrounds every sec. Absolute lumi and vertex position every minute from experiments. (see LHC-B-ES-0007 , Table 5) Some redundancy, particularly in commissioning is essential to understand what we measure and to distinguish signal, background, geometry etc. Needs good communication and signal exchange with experiments

LEP status page

Luminosity and beam parameters For head-on collisions of round beams and N particles / bunch nb bunches (nb = 1 for bunch Lumi) Reduction factor due to the crossing angle qc is the full crossing angle of ~ 300 mrad only really significant (~ 20%) at 7 TeV squeezed sz is the rms bunch length 7.55 cm at 7 TeV

Luminosity and collision rates per bunch crossing According to lum. spec : dynamic range 1026 - 4x1034 cm-2 s-1 What about 450 GeV - pilot beams ? ( pile-up ) Simple estimate, using spp = 100 mbarn Checks with first collisions at injection energy and optics (450 GeV, b*=18 m) at very moderate intensity (2x1010 , L = 2.5 1026 cm-2 s-1) should be ~ easy and are within the specified dynamic range. Generally : rates in machine luminometer high, statistics no problem.

Coincidences, Pile - up. Nominal conditions : many collisions at each crossing, and only few, ~ O ( 10-3 ) beam-gas interactions / crossing. Left / right coincidences will be of little use in this case. For commissioning, early running ( + special conditions high-b ) : Pile-up ok up to some L = 1029 cm-2 s-1 / bunch Beam-gas background instead maybe much higher than nominal in commissioning and early operation. Coincidences (or some other robust reliable method) needed to allow for reliable, background subtracted luminosity determination.

Staged commissioning plan for protons Stage I II III IV Hardware commissioning Machine checkout Beam commissioning 43 bunch operation ? 75ns ops 25ns ops I Install Phase II and MKB 25ns ops II No beam Beam Beam Pilot physics run First collisions 43 bunches, no crossing angle, no squeeze, moderate intensities Push performance (156 bunches, partial squeeze in 1 and 5, push intensity) Performance limit 1032 cm-2 s-1 (limited by event pileup) 75ns operation Establish multi-bunch operation, moderate intensities Relaxed machine parameters (squeeze and crossing angle) Push squeeze and crossing angle Performance limit 1033 cm-2 s-1 (limited by event pileup and *) 25ns operation I Nominal crossing angle Push squeeze Increase intensity to 50% nominal Performance limit 2 1033 cm-2 s-1 (limited by *) 25ns operation II Push towards nominal performance Need to revisit 75ns operation Strategy needed for ion runs Strategy needed for TOTEM runs

Operational use in (early) operation Bring beams into collision Centre collisions in x and y Luminosity / tuning Monitoring of Lumi / Background

Bringing beams into collision Initial beam finding and overlap optimization BPM’s at IP: BPM resolution Dy(IP) in sigma b* 200 mm 283 mm 18 s 0.5 m 200 mm 283 mm 3 s 18.0 m 50 mm 70 mm 4.4 s 0.5 m 50 mm 70 mm 0.7 s 18 m (S. Fartoukh) From R.A. LCC 11/6/2003 Roughly: 0. 3 mm resolution anticipated from BPMs. Beam sizes ~ 0.4 mm at 450 GeV, 0.1 mm at 7 TeV at b* = 18m BPM resolution should be sufficient to get beams sufficiently close to already see some beam-beam effects (collisions, modified tune signal. )

Centre collisions Separation scans in two dimensions ( for LEP only needed in y ) This can be done manually with a set of steering knobs and when safe and reliable enough using a semi - automatic procedure.

Continued tuning Continuous lumi / background monitoring needed for optimisation (orbit, tune, waist, …) Monitor and optimise the bunch-by-bunch specific luminosity (divided by I x I ) In the LHC, the two beams may drift apart and require further separation scans.

Interference with LHCf To my knowledge so far : Running with LHCf instead of the converter in front of the machine luminometer may significantly interfere with the luminosity / vertex / background measurements. To be clarified ! Concern : sensitivity on beam angles and positions at IP ! Angular acceptance rather small : 80 mm / 143 m = 0.56 mrad

My preliminary conclusions Continuous, robust luminosity and background monitoring will be essential for operations. We have to be able to clearly distinguish between luminosity, background, and acceptance effects. Some redundancy and robust background subtraction (including coincidences) will be required, particularly at commissioning.