Status of Beam

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Nov 26, 2013 (3 years and 8 months ago)

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Status of Beam
Halo/Gas Simulation

A.Stradling(Wisconsin)


P. Steinberg(BNL)

G. Usai (Chicago)


Standard Model Group


6 December 2006

1

Intro

Review of the practical issues surrounding
beam halo and beam gas

Simulation progress

Initial plots of unweighted beam halo events

2

Beam 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

3

Beam 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

4

Beam 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possibly outdated collimator configuration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

5

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

6

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

7

Beam 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

8

Beam 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

9

Beam Halo
Particles (1)


Kinetic energy of beam halo particles

MeV

Ratio
-

Weighted/
Unweighted

Weighted

Unweighted

Beam Halo
Particles (2)


Initial radius of halo particles on the scoring
plane

Ratio
-

Weighted/
Unweighted

Weighted

Unweighted

cm

Beam Halo
Particles (3)


PDG ID histogram of halo particles

1:1

Ratio
-

Weighted/
Unweighted

Weighted

Unweighted

Beam 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

13

Beam Gas Questions

The accelerator group did their work
assuming that the only important gases in
the beampipe were H
2
, CO
2
, CO, CH
4

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?

14

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

15

Beam Halo Timing

The 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

16

Beam 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
.

17

Next

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

18

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