1. 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)

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

3. 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

4. LEP status page

5. 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

6. Luminosity and collision rates per bunch crossing
According to lum. spec : dynamic range x1034 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 = cm-2 s-1) should be ~ easy and are within the specified dynamic range.
Generally : rates in machine luminometer high, statistics no problem.

7. 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.

8. 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 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

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

10. 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 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: 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. )

11. 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.

12. 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.

13. 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

14. 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.