Status of Beam Halo/Gas Simulation

Status of Beam Halo/Gas Simulation

A.Stradling(Wisconsin) P. Steinberg(BNL) G. Usai (Chicago)

Standard Model Group

6 December 2006

A.Stradling(Wisconsin) P. Steinberg(BNL) G. Usai (Chicago)

Standard Model Group

6 December 2006

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IntroIntro

Review of the practical issues surrounding beam halo and beam gas

Simulation progress

Initial plots of unweighted beam halo events

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Beam HaloBeam Halo

The LHC’s contribution to background

Results from proton losses, including:

Gases and impurities in the beampipe

Collimator impacts (less important)

Beam configuration and optics are taken into account

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Beam Halo DetailsBeam Halo Details

These effects are simulated in the IHEP MARS package

• I. Azhgirey, I. Baishev, K.M. Potter et al. Methodical Study of the Machine Induced Background in the IR8 of LHC. CERN LHC Project Note 258, Geneva, 2001;• I. Azhgirey, I. Baishev, K.M. Potter, V. Talanov, Cascade simulations for the machine induced background study in the IR1 of the LHC. CERN LHC Project Note 324, Geneva, 2003.

Particles were generated to represent one second of run, totaling 8.2 M particles

Particles are produced all along the accelerator, but most of those we see are produced in the inner triplet magnets just before the cavern

~10x more hadrons than muons, concentrated at a smaller radius

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Beam Halo FilesBeam Halo Files

The resulting file is specific to:

ATLAS, Side A (Beam 1), Optics v6.4, and possibly outdated collimator configuration

Design lumi (14 TeV, β* of 0.5 m and beam current of 0.54 A)

It includes nothing after the “scoring plane”, where the experimental volume is defined as beginning. This plane lies at 23000 mm from the IP, between the concrete wall and the beam shields

• /afs/cern.ch/atlas/project/MachineBackgrounds/Sam

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Beam Halo Scaling (1)Beam Halo Scaling (1)

Many of these previous assumptions have changed

LHC Optics version = 6.5, β* of 10 m or worse for initial runs

Collimators are still being finalized

Beam gas distributions account only for adsorbed gases (no other junk or impurities)

It should also be borne in mind that the LHC is not completely symmetric, and there will be differences in intensity between Beams 1 and 2

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Beam Halo Scaling (2)Beam Halo Scaling (2)

Most of these are relatively simple to handle:

Scaling for changes in beam intensity, β*, optics, beam gas density, beam asymmetries and other similar effects is linear - we just change the number of particles that come in at a time

We have 1.8 M particles per second total for 40 M bunch crossings - most bunch crossings will have little or no beam halo

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Beam Halo Scaling, 900 GeVBeam Halo Scaling, 900 GeV

The 900 GeV run is a problem, however - it requires a redo of the MARS simulation

Simple scaling is impossible - the differences lie not in the particle multiplicity, but in the produced particle energies and species proportions

The machine group is (obviously) fairly occupied now, and taking another crack at the simulation effort will take time

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Beam Halo Work in ProgressBeam Halo Work in Progress

Giulio Usai is working on getting the events unweighted and translated into HepMC format for simulation

Collaboration with the G4 community to get the odd detector volume issues (like having a vertex at 23 m) worked out

MC group is excited to have this in their production system

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Beam Halo Particles (1)Beam Halo Particles (1)

•Kinetic energy of beam halo particles

MeV

Ratio - Weighted/Unweighted

Weighted

Unweighted

Beam Halo Particles (2)Beam Halo Particles (2)

•Initial radius of halo particles on the scoring plane

Ratio - Weighted/Unweighted

Weighted

Unweighted

cm

Beam Halo Particles (3)Beam Halo Particles (3)

•PDG ID histogram of halo particles

1:1

Ratio - Weighted/Unweighted

WeightedUnweighted

Beam Gas GenerationBeam Gas Generation

HIJING (A+A, p+A generator) in Athena

Simulation of various gas species in the beampipe of the detector

Starting simple - will eventually make adjustments for gas density profiles and etc.

Generated events are vertex-shifted from 0,0,0 to other locations during simulation

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Beam Gas QuestionsBeam Gas Questions

The accelerator group did their work assuming that the only important gases in the beampipe were H2, CO2, CO, CH4

Other things may be present (N, Ti, V, Zr from the getter, Be from the beampipe, etc) - what will they look like? Will there be any significant presence?

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Timing Complications

Timing Complications

40 MHz bunch crossings give us a τ of 25 ns

25 ns at c translates to 7.5 m

Lots of bunches in flight

Timing the beam halo and beam gas relative to the physics event is crucial

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Beam Halo TimingBeam Halo TimingThe beam halo comes in with the same timing as the bunches themselves

Offsets of 25 ns

Use digitization to overlay beam halo events onto simulated events, as with pileup events for realistic TRT, LAr and Muon simulation

Differences - we will need to open up the number of bunch crossings to 3 on each side

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Beam Gas TimingBeam Gas Timing

More challenging - does not often happen in sync with the event (can be anywhere in the pipe)

Time offsets in the digitization need to be tunable (but are easy to calculate from the vertex position) - I have not yet determined whether this has been implemented in digi yet. Suggestions and recipes welcome.

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NextNext

Get good jobOptions for generation/simulation of the beam gas and beam halo, validate some output

Examine parameters (scaling of beam halo, proportions of beam gas components)

Pass to ProdSys

Start analyses of the effects of beam gas and beam halo on MinBias event triggering

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