physics performance and status of the alice zero degree
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IntroductionCentrality measurementStatus of the detector
Physics performance and statusof the ALICE Zero Degree Calorimeters
Grazia Luparelloon behalf of the ALICE Collaboration
Università di Torino and INFN Torino
XLVII International Winter Meeting on Nuclear PhysicsBormio (Italy), 26-30 January 2009
Grazia Luparello 1
IntroductionCentrality measurementStatus of the detector
Detector description
Outline
1 IntroductionDetector description
2 Centrality measurementCentrality in Pb-Pb collisionsCentrality in p-A collisions
3 Status of the detector
Grazia Luparello 2
IntroductionCentrality measurementStatus of the detector
Detector description
ALICE Experiment
ALICE is a dedicated heavy ion experiment. It will study strongly interactingmatter and the Quark Gluon Plasma (QGP).QGP formation is expected at high T values reached in central collisions ⇒centrality trigger: ZDCs + ZEM
Grazia Luparello 3
IntroductionCentrality measurementStatus of the detector
Detector description
ALICE ZDC
Detect the energy carried at 0°by spectatornucleons.
EZDC → Nspec → Npart → Impact parameter.
2 sets of calorimeters located at oppositeside w.r.t. the IP ∼114 m away
Spectators neutrons and protons areseparated by LHC beam optics
Each set consists of:1 neutron calorimeter (ZN)1 proton calorimeter (ZP)
Grazia Luparello 4
IntroductionCentrality measurementStatus of the detector
Detector description
ALICE ZDC
Detect the energy carried at 0°by spectatornucleons.
EZDC → Nspec → Npart → Impact parameter.
2 sets of calorimeters located at oppositeside w.r.t. the IP ∼114 m away
Spectators neutrons and protons areseparated by LHC beam optics
Each set consists of:1 neutron calorimeter (ZN)1 proton calorimeter (ZP)
Grazia Luparello 4
IntroductionCentrality measurementStatus of the detector
Detector description
Detection Technique
ZDC are �spaghetti� calorimeters with quartz �bers (active material)embedded in a dense absorber.
The principle of operation is based on the detection of Cherenkov lightproduced in the �bers by charged particles of the shower produced by thespectator nucleons.
The technique ful�ls various requirements:⇒ reduced transverse size of the shower (Imposed by geometrical constraint)
⇒ resistance to radiation⇒ fast response
ZN ZP
Technique already used in NA50 by the Turin group
Grazia Luparello 5
IntroductionCentrality measurementStatus of the detector
Detector description
Segmentation
Quartz �bers are placed at 0° with respect to the incident particle direction,come out from the rear face of the calorimeter and bring the light to 5 PM
One out of two �bers is viewed by an common PMT.
The others are collected in bundles and sent to 4 PMTs forming 4indipendet towers.
ZN ZP
Grazia Luparello 6
IntroductionCentrality measurementStatus of the detector
Detector description
Nuclear fragmentation
In nucleus-nucleus collisions nuclear fragments are produced with charge tomass ratio similar to that of the beam⇒ remain in the beam pipes and are not detected by the ZDCs.
For a �xed EZDC there are two possiblevalues for the impact parameter, onecorresponding to central and one toperipheral collision.
A forward electromagnetic calorimeter (ZEM) is used.⇒ �spaghetti� calorimeter with quartz �bers tilted at45° w.r.t. incident particles⇒ measures energy carried by produced particles inpseudorapidity range 4.8 < η < 5.7⇒ the measured energy increases monotonically withcentrality
Grazia Luparello 7
IntroductionCentrality measurementStatus of the detector
Centrality in Pb-Pb collisionsCentrality in p-A collisions
Outline
1 IntroductionDetector description
2 Centrality measurementCentrality in Pb-Pb collisionsCentrality in p-A collisions
3 Status of the detector
Grazia Luparello 8
IntroductionCentrality measurementStatus of the detector
Centrality in Pb-Pb collisionsCentrality in p-A collisions
Centrality in PbPb collisions
Use the correlation between the reconstructed EZDC and EZEM values.
Determination of centrality classescorresponding to well de�ned percentiles of thetotal hadronic cross section:R
EZEM,i
REZDC,i
d2σdEZEMdEZDC
=R bmaxbmin
db dσdb
= xi · σtot
Model independent: it is based only on experimental quantities and doesnot depend in the particular model used for simulation.
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IntroductionCentrality measurementStatus of the detector
Centrality in Pb-Pb collisionsCentrality in p-A collisions
Simulation results
10 centrality classes have been de�ned
Reconstructed mean values reproduce quite well the simulated ones.
The separation between adjacent reconstructed mean values is larger thanthe resolution.
The resolution weakly depends on the centrality.
Grazia Luparello 10
IntroductionCentrality measurementStatus of the detector
Centrality in Pb-Pb collisionsCentrality in p-A collisions
Centrality in pA collision
Centrality in pA collisions de�ned through thenumber of NN collisions.Centrality measured detecting the slownucleons emitted by the excited nucleus
Slow Nucleons β (c units) p (MeV/c) Ekin (MeV)
Black 0÷ 0.25 0÷ 250 0÷ 30Gray 0.25÷ 0.70 250÷ 1000 30÷ 400
Gray ⇒ soft nucleons knocked out by wounded nucleonsBlack ⇒ nucleons emitted during the nucleus de-excitation processes.
At collider slow nucleons are Lorentz-boosted ⇒ detected by ZDC.
Simulations have been done to evaluate the ZDC response for slownucleon detection.
Grazia Luparello 11
IntroductionCentrality measurementStatus of the detector
Centrality in Pb-Pb collisionsCentrality in p-A collisions
Slow particle detection
Narrower spatial distribution for black nucleonsFull acceptance for black nucleons and for grey neutronsA few percent of grey protons are outside ZDC acceptanceDi�cult to separate gray from black particles
Grazia Luparello 12
IntroductionCentrality measurementStatus of the detector
Centrality in Pb-Pb collisionsCentrality in p-A collisions
Centrality selection in pA collisions
Select centrality by cutting the energy spectrum in classes corresponding todetermined fractions of the total pA cross sectionEvents from each centrality class correspond to a Ncollisions distribution
Adjacent classes arewell-separated (distancebetween mean values >resolution)
The accuracy depends on thedetector energy resolutionand is 20% averaging overall centralities
% σinel EZDC (TeV) <Ncoll>(RMS)0÷ 5 >163 14.2 (2.9)5÷ 25 123÷ 163 10.8 (2.8)25÷ 50 80÷ 123 7.2 (2.4)50÷ 100 0÷ 80 2.1 (1.3)
Grazia Luparello 13
IntroductionCentrality measurementStatus of the detector
Outline
1 IntroductionDetector description
2 Centrality measurementCentrality in Pb-Pb collisionsCentrality in p-A collisions
3 Status of the detector
Grazia Luparello 14
IntroductionCentrality measurementStatus of the detector
ZDCs platform
ZN and ZP are �xed on a movable platform equipped with:
Laser Diode
Scintillators (to trigger cosmic rays)
⇓It is possible to monitor the stabilityduring data taking in 2 ways:
Laser light ⇒ monitor �PMT +�bers� radiation damage
Absolute PMT gain measurementby means of single phe signal ⇒disentangle PMT from �berradiation damage
Grazia Luparello 15
IntroductionCentrality measurementStatus of the detector
ZDC installation in LHC tunnel
Since summer 2007 the 2 ZDC systems are installed in the tunnel
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IntroductionCentrality measurementStatus of the detector
ZDC commissioning on surface (I)
Measurement of absolute gain at di�erent HV for all PMTs
G =(µ1 − ped) 25 fC/ch
At · e(e = 1.6 · 1019C , At=cable attenuation)
Good agreement betweencosmics rays, �ltered laserlight measurements andprevious PMTcharacterization in lab.
Grazia Luparello 17
IntroductionCentrality measurementStatus of the detector
ZDC commissioning on surface (I)
Measurement of absolute gain at di�erent HV for all PMTs
G =(µ1 − ped) 25 fC/ch
At · e(e = 1.6 · 1019C , At=cable attenuation)
Good agreement betweencosmics rays, �ltered laserlight measurements andprevious PMTcharacterization in lab.
Grazia Luparello 17
IntroductionCentrality measurementStatus of the detector
ZDC commissioning on surface (II)
In PbPb collisions PMTs will work at G∼ 105
PMT response to laser light measured in the range ∼ 1200− 2300 V,using neutral �lters to attenuate the laser light (I = I0 10−D).
Grazia Luparello 18
IntroductionCentrality measurementStatus of the detector
Commissioning in the LHC tunnel
Linearity measurements
Runs standalone in theframework of ALICE DAQsystem performed �ashing allthe PMTs with 2 di�erentlaser light intensities (ratio4÷ 1).
PMT charge ratios measuredat di�erent laser lightintensities, inserting �lters(at �xed HV).
Good linearity(within 6%).
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IntroductionCentrality measurementStatus of the detector
So far
A-A: Main purpose of the ALICE ZDC is to provide a centralitymeasurement in A-A collisions: reliable method to estimate centrality
p-A: ZDCs will measure centrality detecting slow particles emitted by thenucleus by means of the ZDC
Commissioning have been performed
PMT and �bers damage can be monitored during the experiment
Grazia Luparello 20
IntroductionCentrality measurementStatus of the detector
Open issues: ZDCs in p-p collisions
It is possible to tag di�ractive events
Di�ractive interactions are characterized by the exchange of an objectreferred to as the pomeron (P), which has the quantum numbers of thevacuum.
A large region in pseudorapidity space empty of particles; such a region iscalled a rapidity gap.
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IntroductionCentrality measurementStatus of the detector
Single Di�ractive (AB → AX)
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Backup slides
Backup Slides
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Backup slides
Detector description
ZN
Dimensions 7.2 · 7.2 · 100 cm3
Filling Ratio 1/22Absorber W-AlloyDensity 17.6 g/cm3
Number of slabs 44Slab thickness 1.6 mmNumber of �bers 1936Fiber spacing 1.6 mmFiber diameter 365 µm
ZP
Dimensions 22.8 · 12 · 150 cm3
Filling Ratio 1/65Absorber brassDensity 8.48 g/cm3
Number of slabs 30Slab thickness 4 mmNumber of �bers 1680Fiber spacing 4 mmFiber diameter 550 µm
Grazia Luparello 24
Backup slides
Glauber Model
Npart correlated to the impact parameter of the collision. It is a purelygeometric model. At a �xed impact parameter b, the number of partecipantscan be evaluated through the relation:
Npart(~b) =
Zd2s
nATA(~s)
h1− [1− σNTB(~b −~s)]B
i+BTB(~b −~s)
h1− [1− σNTA(~s)]A
io
A,B are mass numbers of the colliding nuclei.
TA,B =RdzρA,B(z ,~s): nuclear thickness functions
σN is the nucleon-nucleon inelastic cross-section
Grazia Luparello 25
Backup slides
Model for slow nucleons production
The used model consists in a parametrization of the experimental results.
slow particles production is indipendent from projectile energy in the rangefrom 1 GeV to 1 TeV⇒ slow nucleons emission dictated by nuclear geometry
kinematical distribution described by indipendent statistical emission froma moving frameblack nucleons emitted from a stationary sourcegray nucleons from a frame moving slowly along the beam direction(βgrey ∼ 0.05)
the number distribution of gray and black particles follow binomialdistribution
Ngray ∝ Ncoll
Nblack ∝ Ncoll
Grazia Luparello 26
Backup slides
Model for slow nucleons production
Experiments with lighter ions (O, S) report a saturation e�ect.
Saturation values:< Nblack >= 12 for Ngray > 7
No experimental data for Pb nuclei existSupposing the number of emitted slownucleons proportional to the target nucleusthickness ⇒ rescaled values for Pb nucleus:< Nblack >= 28 for Ngray > 15
Angular distribution forblack particles is �at whilefor gray particles is forwardpeaked.
Grazia Luparello 27
Backup slides
Model for slow nucleons production
Experiments with lighter ions (O, S) report a saturation e�ect.
Saturation values:< Nblack >= 12 for Ngray > 7
No experimental data for Pb nuclei existSupposing the number of emitted slownucleons proportional to the target nucleusthickness ⇒ rescaled values for Pb nucleus:< Nblack >= 28 for Ngray > 15
Angular distribution forblack particles is �at whilefor gray particles is forwardpeaked.
Grazia Luparello 27
Backup slides
Slow particle detection
Narrower spatial distribution for black nucleons
Full acceptance for black nucleons and for grey neutronsA few percent of grey protons are outside ZDC acceptance.
Hardly separate gray from black particles
Grazia Luparello 28
Backup slides
ZDCs Stability Monitoring
Laser diodeIt lights a few �bers that accross the detector →ADC spectrum
Single phe signal
Cosmic raystrigger requires coinc. of the 2 scintillators(rates: 1.5 ev./s in ZN, 10 ev./s in ZP onsurf.; ∼30 times less in the tunnel)
Laser light attenuated by neutral�lters
G =(µ1 − ped) 25 fC/ch
At · e
(e = 1.6 · 1019C , At=cable attenuation)
Grazia Luparello 29
Backup slides
ZDCs Stability Monitoring
Laser diodeIt lights a few �bers that accross the detector →ADC spectrum
Single phe signal
Cosmic raystrigger requires coinc. of the 2 scintillators(rates: 1.5 ev./s in ZN, 10 ev./s in ZP onsurf.; ∼30 times less in the tunnel)
Laser light attenuated by neutral�lters
G =(µ1 − ped) 25 fC/ch
At · e
(e = 1.6 · 1019C , At=cable attenuation)
Grazia Luparello 29
Backup slides
Double Di�ractive (AB → X1X2)
Grazia Luparello 30
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