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CAVERN BACKGROUNDCharlie Young (SLAC)

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Outline Reminder of what is cavern background Simulation of cavern background

Physics event generation Tracks at scoring volume surface Detector hits for pile-up digitization Estimated hit rates without detector hits

Status and outlook

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What Is Cavern Background?

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Cavern Background

Cavern Background refers to the typically low energy (MeV and below) predominantly neutral (g and n) long lived (compared with LHC bunch

spacing of 25 nsec and revolution time of ~0.1 msec)

background in the ATLAS cavern coming from p-p collisions.

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Typical Energy Distribution

GeVMe

VKeVeVme

V

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Typical Energy Distribution

70 / 4x106 above 1 GeV

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Particle Type Distribution

Photo

nN

eu

tron

Posi

tronEle

ctro

n

Pro

ton

Muon +

Muon -

Pio

n +

Pio

n -

See http://www.fluka.org/fluka.php?id=man_onl&sub=7 for all FLUKA particle codes

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Typical Time Distribution

~ LHC revolution

25nsec BC

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Typical Time Distribution

~1.5x106 / 4x106 after 1 turn

~ LHC revolution

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Uncertainty in Hit Rate Estimated by Radiation Background Task

Force Three major factors in muon detector hit

rate uncertainty: 1.3x for p-p cross section 2.5x for calculation of background particle flux 1.5x for detector response

Overall uncertainty ~5x

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Simulation of Cavern Background

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Overlay vs Pile-Up Overlay using zero-bias data should be

closest to reality but not predictive Different beam energies Shielding plans LHC and/or ATLAS upgrades

Complementary tool based on simulation Cavern background similar to other sources

such as min-bias, beam gas and beam halo Min-bias and cavern background both come from p-p collisions – more later

Save background event as Geant4 hits Add to signal event during pile-up digitization

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Steps

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p-p Event Using PHOJET

Same as RBTF for comparison Other event generators a very small

differences Not well constrained by data

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Energy is significantly more forward peaked at |h| ~ 7

Physics events studied mostly in |h| < 5

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FLUGG: Background Calculation

FLUGG combines Geant4 and FLUKA Geant4 description of detector

Detector simplified from Athena description for speed E.g. no internal structure in calorimeters

More realistic compared with RBTF Shielding and cavern description more important

than in physics simulation, e.g. curved end walls Release-to-release changes unimportant

FLUKA physics Low energy n and g exiting shielding and

calorimeter just like shielding calculations where FLUKA is the standard

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Some Geometry Updates

Curved end wall

Curved ceiling

Access shafts

Flat barrel chambers

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RBTF vs Current Geometry

Barrel torid

Chamber location

Curved end wall

Missing shielding added

Energy deposition

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FLUGG Output Fluence maps in (r,z) space – see examples later Track every particle, including decay and

interaction products, until it falls below (very low) energy cut.

Define scoring volumes Imaginary surfaces surrounding each region of

interest, e.g. muon station, MBTS and MPX Implemented as cylinders

Record “4-vector” when entering any scoring volume (x,y,z,t) and (px,py,pz,E) Note: a particle may have more than one record

because of multiple crossings into scoring volumes

In practice, direction cosines and kinetic energy

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Athena: Simulated Hits Treat “4-vector” output from FLUGG (after some

massaging… ) as event generator Low energy particles Starting points on scoring volume surfaces near muon

chambers a reasonable program speed Standard Athena/G4 with full detector description to

produce simulated hits Knows detailed detector structure such as number of

detection planes Detector response as coded in Athena Kill when exiting muon volume to avoid double counting

Different double counting from the one on next page Existing cavern background sample affected and patched

Usually re-run in each production cycle with latest Athena updates

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Massaging Kill prompt charged particles

Logically identical particles also present in minimum-bias events – both come from p-p collision

Existing cavern background sample did not do this, leading to double counting of prompt muons and artificially high trigger rates in pile-up samples

Modify hit times to be within 25 nsec of a particle traveling at speed of light Athena/G4 has time cut that would otherwise

remove most hits Effect identical to bunch train with 25-nsec

separation

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Shortcut for Hit Rate Convolute 4-vectors produced by FLUGG with detector

response curves Different responses for CSC, MDT, RPC and TGC Different responses for n and g Taken from RBTF Report – examples on next pages Account for path length due to incident angle

Benefits Much faster Higher statistics from artificially boosted response

Global boost factor a no change in relative numbers Drawbacks

No knowledge of detector structure or granularity Overlap between sectors Number of detection planes

Response curve in general different from Athena implementation Comparisons to date have used this result

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MDT Efficiency for Neutrons

1%

0.1%

MeVKeV

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TGC Efficiency for Photons

1%

0.1%MeVKeV

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Changes from RBTF Geometry

No longer cylindrically symmetric Improved detector and cavern description Many cross-checks with Mike Shupe! Flux incident on muon chambers ~75% of before

Beam energy 7 TeV ~75% of 14 TeV

Overall change roughly factor of two Depends on location

No double counting of prompt muons in pile-up Correlated hits directly affecting trigger rates

No need to patch double counting of low-energy particles Uncorrelated hits

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RBTF vs Current GeometryNeutron Fluence

Now generally lower in barrel and endcap chamber areas

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7 TeV vs 14 TeVFlux of neutron (energy > 100 KeV) in kHz/cm2

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Neutron Flux Ratio: 7 to 14 TeV

Bin-by-bin flux ratio of neutron (energy > 100 KeV)

z

r

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Ratio of 7 to 14 TeV

Average Ratio

Energy deposition 0.75

Neutron 0.78

Neutron (E > 100 KeV) 0.78

Photon 0.75

Dose 0.74

Pion 0.76

Hadron 0.77

Average over histogram bins is not very sensible procedure but indicative of change.

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Status and Outlook

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FLUGG Step Geometry stable for some time Starting production at 7 TeV

Initial plan of 106 events Approximately one month

Need to improve production setup and scripts to utilize grid resources for higher production rate Presently running in batch at SLAC only

What energy in future production? More 7 TeV events to support analysis of 2010 data 8(?) TeV for 2011 run 14 TeV for upgrade studies

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Comparison with Data Started in summer 2010

Identifying “cavern background” in data Bunch structure

See next talk for CSC photon hit rate MDT hit rate in data and simulation RPC HV current vs simulated hit rate MIP rate in MPX

Lots more to be done!

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Other Items Earlier study found ~2x improvement in

background with Beryllium beam pipe (already Be for |z| < 3.5 m) Confirmed in our study Beam pipe made of vacuum is even better, but

would be an engineering challenge Simulation predicted background coming out

of barrel endcap gap region Improving description with an eye to proposing

changes High background for small wheel

Check simulation geometry some more! Possible shielding improvements for the future

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Effect of Beam Pipe Material

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z/cm

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Fluences in the muon chamber locations (RBTF fig.3)

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VAug10 photon

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new photon

muon chamber locations

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Change

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Change

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Gap Region

Information extracted from CATIA by Georgian Team.

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Upgrade Detector coverage and shielding are

correlated Need to break deadlock to move ahead Iterations

Detector technology not important to estimating incident rate because of use of scoring volumes

Manpower limited