physics performance and status of the alice zero degree

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

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Detector description

Outline

1 IntroductionDetector description

2 Centrality measurementCentrality in Pb-Pb collisionsCentrality in p-A collisions

3 Status of the detector

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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

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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)

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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

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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

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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

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Centrality in Pb-Pb collisionsCentrality in p-A collisions

Outline




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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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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.

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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.

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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

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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)

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Outline




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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

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ZDC installation in LHC tunnel

Since summer 2007 the 2 ZDC systems are installed in the tunnel

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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.

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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).

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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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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

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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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Single Di�ractive (AB → AX)

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Backup slides

Backup Slides

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Backup slides


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

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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

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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

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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.

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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

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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)

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Backup slides

Double Di�ractive (AB → X1X2)

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physics performance and status of the alice zero degree

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