
META TOPICPARENT 
name="LHCbOperations" 
Nu, Mu and PileUp. The LHCb definitions of what we see and what we don't see. 
 μ (mu): The average number of visible pp interactions per bunch crossing
This is the only thing we can measure at the pit. Hence all numbers coming from the pit should be understood as μ. Until we know more, we will assume this is the fraction 69.9% described above, i.e. μ = 0.699 ν. 

< <  The formula for μ from the instantaneous luminosity and number of bunches is: μ = L_{tot}/N_{bunches}/0.177 10^{30}. For L_{tot}=4 10^{32} and N_{bunches}=1300, μ=1.7. 
> >  The formula for μ from the instantaneous luminosity and number of bunches is: μ = L_{tot}/N_{bunches}/0.177 10^{30}. For L_{tot}=3.5 10^{32} and N_{bunches}=1300, μ=1.52. 

Pileup: the average number of pp interactions in visible events
This is the relevant quantity for the Hlt processing time. Assuming a L0 trigger selecting all visible interactions, we send to the event filter farm all events that contain at least one visible pp interaction. The Poisson distribution for a mean μ is P(n) = μ^{n} exp(μ)/n!. The pileup is the zerosuppressed mean of this distribution = μ/(1P(0)) = μ/(1exp(μ)). At μ=1 this becomes 1.58. For μ=0.7 it is 1.38. 