Machine Induced Background in the ALICE Muon Trigger System in pp Data taking

YERMIA Frederic Vietri sul mare 2006 1

Machine Induced Background in the ALICE Muon Trigger System

in pp Data taking

Yermia FrédéricINFN Torino

Secondo Convegno Nazionale sulla Fisica di ALICE30 Maggio - 1 Giugno 2006 –

Vietri sul Mare (SA) -


Overview

• Introduction:

Sources of beam-related background

Simulation environment

• Scoring plane (simulation Input)

• Fluxes in trigger chambers

• Strategy and conclusions


ALICE experiment

Central detectors(identification of hadrons,electrons and photons)

Forward muon spectrometeridentification of muons forheavy flavour study

Muon trigger system :

● 2 stations (MT1 & MT2) of 2 planes each

● 72 Resistive Plate Chambers (RPC) ~2.7 m 0.7 m ( 144 m2)

● 20992 electronics channels


Introduction

beam protons may undergo elastic and inelastic scattering with the residual gas nuclei (mainly O & C) in the LHC long straight sections (20< |z| <270 m) => induce fluxes of secondary particles in ALICE Affect the ALICE radiation environment increase of the detector background Machine induced background is proportional to the beam current (Intensity) while particles fluxes produced in pp collisions scale with luminosity at IP MIB be crucial in pp collisions depends on machine operating conditions is different for different run scenarios


Introduction

Among the ALICE detectors, the muon trigger system is one of the most sensitive to the machine induced background.

•The Resistive Plate Chamber’s (RPC’s) rate capability might be saturated by a too high background level.

• Detector lifetime.

PURPOSE: Evaluate the hit rate on the muon trigger detector due to the beam-gas background in pp mode

(update of early studies performed 2 years ago in ALICE-INT-2003-041)


Details of the simulation of the p-A collisions

• Original proton interactions with the nucleus of the residual gas were simulated along the whole length of the LHC Ring sectors.

• Beam–gas interactions in the experimental aera not been into account.• New gas pressure estimates (LHC Project Report 674) (see next slide)

• High energy hadrons and muons are considered – main component (critical to the detector performance)– component (and as well) not studied

– Secondary particles from Ring 1 & 2 beam losses were transported up to 2 planes located at |z|= 22 m from IP2:

e

SCORING PLANE (input of transport simulations)

•Secondary particle cascades can be initiated in the SS2 and transport through the LHC tunnel.

•Interactions in machine elements

•residual gas in the vacuum pipe (H, O and C)


Previous pressure calculations (LHC Project Note 273)

Previous MIB studies (ALICE-INT-2003-041)

New pressure calculations (LHC Project Report 674)

A factor 5 in less in mean.

Details of the simulation of the p-A collisionsUpdate and study of the beam-gas background environment

Arb

itrar

y un

its

Arb

itrar

y un

its


ALICE Simulation Framework

• AliRoot HEAD February 2006 with ROOT v5-09-01

• AliGenHaloProtvino event generator (Interface between the scoring plane and ALICE experimental region)

– input file in ASCII format containing the result of the calculation of the particle beam halo at z=+-22 m (scoring plane)

– particles coming from both sides

• Set-up configuration – L3 magnet– exp. hall– central detectors : ITS, TPC– muon spectrometer (detectors and shieldings)

http://www.to.infn.it/~yermia/indexfisica.html


Muon Spectrometer geometry

19Z (m)

1816

YTrigger stations (16 & 17 m)

Muon filter (14.7-15.9 m)

beamshield

Plug (18-18 m & R=1.1 m)

X22


Scoring Plane

Particles Muons Hadrons Total Protons Neutrons Pions // Kaons

Mean number by second

4.9 e+04 8.4 e+05 8.9 e+05 1.0 e+05 4.0 e+053.1 e+05 //2.8 e+04

Mean number by bunch (40 MHz)

0.0012 0.0210 0.0223 0.0025 0.0100 0.0085

A factor 20 more for hadron contributions w.r.t. muons


pA collision Origins in the SS2

All particles on the scoring plane

Muons HadronsSimilar structures of the vaccum quality

Muons are produced further than hadrons


Scoring Plane: Muons kinematics

X (m) E (TeV)

Theta (deg) E (TeV)

• Uniformity

• Some high energetic muons

• Peaked at small emission angle

R (m

)


Hadronic background

X (m) E (TeV)

Theta (deg) E (TeV)

• Uniformity

• Quasi beam

• Machine effect (material)

• Energetic hadrons at R= 0.5 m

• Small emission angle


Flux on scoring plane weighted by EX-Y coordinates (Hz.GeV/cm²)

h+ h-

Hot Spot


Fluxes on MT11 & MT22X-Y coordinates (Hz./cm²)

Hot spot max. 40 Hz/cm²



Fluxes on MT22Ring 1 and 2 contributions



Main contribution from Ring 2


Fluxes on MT22 X-Y coordinates (Hz./cm²)

MUONS HADRONS ELECTRONS

Hot spot max. 60 Hz/cm² Hot spot max. 20 Hz/cm² Hot spot max. 1 Hz/cm²


Scoring plane conditioned by the hits on MT22 (Hz./cm²)

Muons are the main source of hits on the MT22 but distributed uniformly

Hadron hot spot which corresponds to energetic particle hot spot


Scoring plane: hits from hot spot MT22

High energetic hadron particles


Hit creation vertex from MT22

Hadronic particles interact in the plug

XY projection of the hit creation vertex at z= - 18 m (Hz./cm²)

Tunnel entrance

Scoring plane

plug

Hot spot in the plug due to hadronic

particles

X (m)

Y (m)


Hot spot MT22 from R2:

p-A collisions Vertex

Hadronic particles are created in the lhc tunnel at

120< z < 130 m


120 mDipole D2

IP

LHC Tunnel (IR8)

The Dipole D2 makes positive particles converge towards IP and makes negative particles diverge

Negative particle hot spot on scoring plane


Summary & conclusion• Mean rate on trigger stations: 1-10 Hz/cm²• Hot spot: 40-80 Hz/cm² => Concentrated on 1 RPC per plane• RPC ageing test in maxi avalanche mode: carried out successfully up to 500

Mhits/ cm²

However the results presented here should be quite pessimistic.

Not included in this simulation:• Compensation magnet, at -20 > z > -21m, 0.2 < R < 0.6m• LEP shielding in the tunnel

• Further shielding could be foreseen if needed• Increase the transverse side of the plug (agreed)• Dedicated (small shielding for hot spot)

• Close contributions not included

Strategy


• BACKUP SLIDES


•Resistive Plate Chambers (RPCs)Small (150 m2) and accessible system, based on single-gap RPCs with x-y readout.

-Low resistivity bakelite ( 109 .cm)(Frati laminati)

-Two linseed oil layers to smooth the bakelite surface

-Copper strips (1 cm, 2 cm or 4 cm width)

Trigger detector II

2 trigger stations:2 detection planes per station

18 RPCs per plane

The chambers are read-out on both sides by means of 2 planes of orthogonal strips oriented along the horizontal (X) and vertical (Y) directions (perpendicular to the beam axis) and arranged in projective geometry.

Y

Z

X

(General Tecnica production)


➢ Two bakelite planes of 2 mm ( 109 .cm)➢ High voltage of about 8 kV ➢ Gas gap of 2 mm (51% Ar + 7% iC4H10 + 41% C2H2F4 + 1% SF6

➢ Two perpendicular planes of strips (1 cm, 2 cm or 4 cm width) ➢ Signal picked-up at the extremity of the strips with specific connectors➢ Read-out by a dedicated FEE

RPC working in streamer mode for heavy ions collisionsstrips +

strips -

highvoltage

spacerbakelite

gasgraphite

plasticinsulation

Alternative working mode for pp data taking:Maxi avalanche mixture:

10% iC4H10 + 89.7% C2H2F4 + 0.3% SF6

HV: 10 kV

avalanche-like mode with our FEE developed for streamer mode i.e. without an amplification stage.

Trigger detector


Scoring PlaneMUON Origins Hadron Origins

Weighted by energy Weighted by energy


Hot spot MT11 from R1: Interaction Vertex from R1

Particles interact in the front absorber and the iron wall


Hit densities & background composition

Particles

e-/e+ pi+/pi- K+/K- proton

muon

78 % 2.8 % 0.01 %

16.4 %

2.8 %

Chambers MT11 MT12 MT21 MT22

Hits/second (x 100 000)Without trigger time cut

0.77 0.84 1.16 1.14

Hits/second (x 100 000)Wit trigger time cut

0.10 0.12 0.18 0.18

•Max hit density: 0.3 10-5 hits/cm2

w.r.t. 2 10-3 hits/cm2 in Pb-Pb


Scoring plane: hits from hot spot MT22

Condition: hits inside hot spot

Confirmation of the hot spot position on scoring plane


Hit creation vertex from MT22

Hit particles are created in the whole trigger region

R vs z of the hit creation vertex (Hz./cm²)

plug

Trigger stations

beamshield

Iron wall

cavern

Scoring plane


IP

Proton beam

+ Positive particles

D2

D1

Secondary particles

Proton beam

Negative particles

Machine Induced Background in the ALICE Muon Trigger System in pp Data taking

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