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Technische Universität München Beam Simulation at COMPASS Gentner Day Karl A. Bicker

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Technische Universität München 2/ Karl A. Bicker - Beam Simulation at COMPASS Fixed target Two-stage spectrometer 160/190 GeV muon/hadron beams The COMPASS Spectrometer

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Technische Universität München 3/ Karl A. Bicker - Beam Simulation at COMPASS Beam Simulation at COMPASS Beam Simulation package should generate vertices and beam vector 5-dimensional distribution in [ v x, v y, b x, b y, b z ] (z: beam direction, y: up) Complex correlations between all 5 variables →Impossible to do analytically

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Technische Universität München 4/ Karl A. Bicker - Beam Simulation at COMPASS Idea Use real events for beam simulation → [x, y, p x, p y, p z ] from every event → Beamfile Problem Need more Monte Carlo than real data events → The same vertex/beam is used several times Proposal Smear events Use mean distance between two events as sigma for the smearing (different for every event!)

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Technische Universität München 5/ Karl A. Bicker - Beam Simulation at COMPASS Defining the Mean Distance Basic idea: –Divide the 5d distribution into bins of equal content –For every bin, calculate the mean volume per event –The 5 mean distances will be the edge lengths of the hypercuboid with the same edge ratios as the bin itself, and a volume that is equal to the mean volume per event mean distance ≙ “sigma”

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Technische Universität München 6/ Karl A. Bicker - Beam Simulation at COMPASS Binning

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Technische Universität München 7/ Karl A. Bicker - Beam Simulation at COMPASS Edge Bins are a Problem Single event can artificially enlarge bin Average over all neighboring bins which are not on the edge But: in 5d, most bins are on the edge Even worse: most of them have only neighbors which are also on the edge Solution: use sigmas from neighboring bins

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Technische Universität München 8/ Karl A. Bicker - Beam Simulation at COMPASS What if there are only edge bins? 1.For every event… 2.Find a hypersphere containing 15 events around it 3.Calculate the CoG and find a new sphere containing all events 4.Use the volume of the sphere to calculate the mean volume per event

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Technische Universität München 9/ Karl A. Bicker - Beam Simulation at COMPASS Post Processing Sphere-procedure still produces big sigmas, cap them by hand Binning introduces artifacts: The bin boundary of the first division goes through the whole volume Solution: Produce 5 beamfiles, starting binning with a different coordinate each time For every coordinate, take the sigmas from the beamfile where the coordinate was divided last and merge them to form one beamfile

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Technische Universität München 10/ Karl A. Bicker - Beam Simulation at COMPASS Results: Vertex X Position

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Technische Universität München 11/ Karl A. Bicker - Beam Simulation at COMPASS Results: Vertex Y Position

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Technische Universität München 12/ Karl A. Bicker - Beam Simulation at COMPASS Results: Beam Momentum X

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Technische Universität München 13/ Karl A. Bicker - Beam Simulation at COMPASS Results: Beam Momentum Y

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Technische Universität München 14/ Karl A. Bicker - Beam Simulation at COMPASS Results: Beam Momentum Z

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Technische Universität München 15/ Karl A. Bicker - Beam Simulation at COMPASS Small Binning Artifacts Remain

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Technische Universität München 16/ Karl A. Bicker - Beam Simulation at COMPASS Conclusion A new beam simulation package was developed and tested The vertex and beam distributions of generated events reproduce the data reasonably well First test with physics analyses have been started

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