The ALICE Experiment at LHC: Detector

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The ALICE Experiment at LHCDetector Physics

• V. Manzari
• INFN University of Bari - Italy

XI Frascati Spring School Bruno Touschek LNF,
May 15th 19th, 2006
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Contents

• Nucleus-nucleus collisions at the LHC
• The Alice experiment
• Overview of Alice subsystems
• Physics Examples
• Jet quenching
• Heavy flavours
• Conclusions

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QCD tells us

• Ultimate goal understanding of the QCD phase
  diagram
• - QCD prediction ? the existence of a new state
  of matter at high temperature, the quark Gluon
  Plasma

• Tc ?173 MeV, mq?0, Nf2,3
• Order of the transition?
• ?c ? 0.3-1.3 GeV/fm3

• LHC will allow to go much deeper into a QGP phase
  and to study the QGP equation of state.

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SPS to RHIC to LHC

• QGP is characterized by two qualitatives
  changes
• deconfinement chiral simmetry
  restoration
• Both changes will certainly have consequences in
  the final state observed
• by the experimental apparatus

? Energy per NN LHC 30 x RHIC
? Initial energy density LHC 3 ? 10 x
RHIC
? Volume LHC 3 x RHIC
? QGP lifetime LHC 3 x RHIC
? Formation time LHC 1/3 x RHIC

• Initial condition at LHC different than at RHIC
  hotter bigger longer lived

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Novel aspects soft processes

• The energy increase at LHC will make accessible a
  novel range of Bjorken-x
• - solid lines ? relevant x-M2
• ranges for particle production
• Probe initial partonic state in a new Bjorken-x
  range (10-3 - 10-5)
• - nuclear shadowing
• high-density saturated gluon
• distribution (CGC)

Energy increase ? lower x ? ? RHIC forward
region ? LHC mid rapidity (easier detection)
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Novel aspects hard processes

• Hard processes contribute significantly to total
  AA cross-section
• (shard/stot 98)
• Bulk properties dominated by hard processes
• Hard probes abundantly produced
• Hard processes are extremely useful tools
• Probe matter at very early times
• Hard processes can be calculated by pQCD
• Heavy quarks and Weakly interacting probes become
  accessible (Z0, W)

5500 GeV
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LHC as Ion Collider

• Running conditions
• Expected Pb-Pb luminosity ? rather low
  minimum-bias interaction rate (8kHz)
• - LHC detectors in heavy ion mode ?
  lower rates higher particle density
• other collision systems
• pA, lighter ions (Sn, Kr, Ar, O) energies
  (pp _at_ 5.5 TeV)

ltLgt/L0 ()
Run time (s/year)
?geom (b)
L0 (cm-2s-1)
vsNN (TeV)
Collision system
pp
107
0.07
1034
14.0
PbPb
Lmax (ALICE) 1031
Lint (ALICE) 0.7 nb-1/year
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One dedicated HI experiment ALICE Two pp
experiments with HI program ATLAS and CMS

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AA Physics Menu at LHC

• Global properties
• Multiplicities, ? distributions, zero degree
  energy
• Event history
• HBT
• Resonance decays
• Fluctuations and critical behaviour
• Event-by-event particle composition
  spectroscopy
• Neutral to charged ratio
• Degrees of Freedom vs Temperature
• Hadron ratios and spectra
• Dilepton continuum
• Direct photons
• Collective effects
• Elliptic flow
• Deconfinement, chiral simmetry restoration
• Charmonium, bottonium spectroscopy
• (Multi-)strange particles
• Partonic energy loss in QGP
• Jet quenching, high pT spectra

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ALICE The dedicated HI experiment

• ALICE is a general purpose experiment designed
  to study the
• physics of strongly interacting matter and the
  quark-gluon plasma
• in nucleus-nucleus collisions.
• ALICE will meet the challenge to measure flavour
  content and
• phase-space distribution event-by-event at
  the highest particle
• multiplicities anticipated for Pb-Pb
  collisions
• Most (2? 1.8 units ?) of the hadrons (dE/dx
  TOF), leptons (dE/dx, transition radiation,
  magnetic analysis) and photons (high resolution
  EM calorimetry).
• Track and identify from very low pt (lt 100
  MeV/c soft processes) up to very high pt (gt100
  GeV/c hard processes).
• Identify short lived particles (hyperons, D/B
  meson) through
• secondary vertex detection.
• Identify jets.

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Central Solenoid from L3 (LEP)
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• Central tracking PID system
• ITS
• TPC
• TRD
• TOF

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Central Tracking PID
?lt0.9 B 0.4 T TOF (3.7 4 m) TRD (2.9 -
3.7 m) TPC (85 - 250 cm) ITS (4 -45 cm) with -
Si pixel - Si drift - Si strip
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Combined momentum resolution
at low momentum dominated by - ionization-loss
fluctuations - multiple scattering
at high momentum determined by - point
measurement precision - alignment calibration
(assumed ideal here)
resolution 7 at 100 GeV/c excellent
performance in hard region!
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Inner Tracking System (ITS)
longitudinal coverage ? lt 1 (tracking), ?lt2
(multiplicity)
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ITS Silicon Pixel Detector (SPD)

• 2 layers, r 3.9, 7.6 cm
• sensitive length (in z) 28.6 cm (for
  both layers)
• hybrid (bump-bonded) silicon pixel assemblies
• Pb/Sn bumps
• pixel size 50 425 µm2
• binary r/o
• module size 12.8 69.6 mm2
• 240 modules
• 9.8 M channels

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ITS Silicon Pixel Detector (SPD)

• successful system beam test
• Oct. 04
• including full FEE and DAQ, DCS, ECS
• Combined with the other ITS detector systems
• bump bonding at VTT (Finland)
• series production started (? gt 99)
• low-mass support/cooling sectors
• ready
• assembly sites in Bari and Padova
• Status
• ready for installation Nov 06
• viable schedule, but tight little
• contingency

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SPD
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ITS Silicon Drift Detector (SDD)

• Hybrids
• 520 needed production completed done in
  industry
• Modules
• 260 needed
• assembly completed June 06
• Ladders
• 36 needed production ongoing
• assembly completed July 07
• Mechanics
• Components ready for assembly
• Status
• - ready for integration with SSD Jul 06

View of modules with two hybrids Was used in
2004 beam test
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ITS Silicon Strip Detector (SSD)
Ramping of component delivery and
assembly

• Production
• sensors from three vendors under production
• FEE electronics all chips in production
• micro-cables hybrids (Ukraine)
• very advanced technology
• Assembly
• shared between 4 ( later 5) sites (Finland,
  France, Italy) pre-production validated
• Status
• Ready for integration with SDD Jul 06
• SDDSSD ready for installation Sep. 06

p-Hybrid
Sensor
n-Hybrid
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Time Projection Chamber (TPC)
GAS VOLUME 88 m3 DRIFT GAS 90 Ne 10
CO2 Field cage finished FEE finished Read out
chamber finished At present pre-integration of
field cage into experiment
Readout plane segmentation 18 trapezoidal sectors
each covering 20 degrees in azimuth
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TPC
Mounting the TPC Central Electrode With 10-4
parallelism to readout chambers
Completed Readout chamber installation
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Transition Radiation Detector (TRD)
Pad chambers with a total of 1 200 000 channels
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Time-Of-Flight (TOF)

• Multi-gap RPC
• high performance 50 ps resolution achieved!

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TOF performance and construction

• Detector
• -Strip production 20/week to increase to 40/week
  with 2nd automated assembly line
• -Finished 11/06
• -Module assembly start 06/05 finish 11/06
• Supermodules installation test with mock-up
  done successfully
• Start SuperModules installation
• July/August 06

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ITS/TPC/TRD/TOF Pre-Integration
Pre-Integration of ITS/TPC/TRD/ TOF/vacuum
chamber April 2005
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• Specialized detectors
• HMPID
• PHOS

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High Momentum Particle Identification (HMPID)
- Detector production (7 modules)
finished -CsI-cathode 35/42 ready and
performance better than specified - Ready for
Installation July 06
Sensitivity of cathodes Required gt12
clusters Measured gt18 clusters for
relativistic particles
Cathode uniformity 5
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Photon Spectrometer (PHOS)

• single arm em calorimeter
• photons, ?-jet tagging
• dense, high granularity (2x2x18cm3) crystals
• novel material PbW04
• 18 k channels, 8 m2
• cooled to -25o

PbW04 Very dense X0 lt 0.9 cm Good energy
resolution (after 6 years RD) stochastic 2.7
/ E1/2 noise 2.5 / E constant 1.3
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• MUON Spectrometer
• absorbers
• tracking stations
• trigger chambers
• dipole

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Muon Magnet worlds largest warm dipole
Muon Filter
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Muon Tracking System

• Advanced Pad-chamber system with
• 1.2 106 readout channels
• Sagitta resolution of lt 50 µm for
• Mass resolution of 80 MeV at Upsilon
• Production of chambers in
• France, India, Italy, Russia
• Scheduled to be finished end 2005

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Cosmic rays trigger

• Forward detectors
• PMD
• FMD, T0, V0, ZDC

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Trigger Counters T0/V0/FMD/Accorde
Accorde large area Scintillator PM trigger on
Cosmic rays
V0 Scintillator PM
FMD Si ?-strips
T0 Quartz-C PM
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Physics Examples

• Jet Quenching
• Heavy Flavours

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Jet quenching

• Jets in QCD
• - Cascades of consecutive emissions of partons
  initiated by partons from
• an initial hard scattering
• - Parton fragmentation ? showering and
  hadronization.
• In-medium effects ? modifications of the jet
  structure
• Reduction of single inclusive high pt particles
• - Parton specific (stronger for gluons than
  quarks)
• - Flavour specific (stronger for light quarks)
• ? Measure identified hadrons (?, K, p, ?,
  etc.) heavy
• partons (charm, beauty) at high pT
• Change of fragmentation function for hard jets
  (ptgtgt10 GeV/c)
• - Transverse and longitudinal fragmentation
  functions of jets
• - Jet broadening ? reduction of jet energy,
  dijets, ?-jet pairs
• - Suppression of mini-jets same-side/away-side
  correlations

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Experimentally

• Highest sensitivity to the medium properties
• - modifications of the reconstructed jets
• - partonic energy loss ? decrease particles
  carrying a high fraction of the jet energy and
  appearance of radiated energy via an increase of
  low-energy particles
• - broadening of the distribution of
  jet-particle momenta
• Measurement of Jet Energy
• In present configuration Alice measures only
  charged particles
• ( and electromagnetic energy in PHOS)
• The large EM Calorimeter will improve the jet
  energy resolution, increase the selection
  efficiency and further reduce the bias on the jet
  fragmentation jet trigger capabilities needed
  to increase the statistics at high Et.
• Measurement of Jet Structure very important
• Requires good momentum analysis from 1 GeV/c to
  100 GeV/c
• Alice excels in this domain
• pp and pA measurements essential as reference!

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Energy domains for jet reconstruction
2 GeV 20
GeV 100 GeV 200 GeV
Mini-Jets 100/event 1/event
100k/month

• Event structure and properties
• at p gt 2 GeV/c
• Correlation studies
• Limit is given by underlying event

• Reconstructed Jets
• Event-by-event well
• distinguishable objects

Example 100 GeV jet underlying event
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Reduced cone size...

• Large underlaying hadron background ? reduced
  cone size R

• Central PbPb at vs5.5 TeV
• dNch/dy 2000-8000
• dET/d? 1.5-6 TeV
• Energy in R lt 0.7 0.4 -1.5 TeV
• Problems
• Identification...
• Energy resolution background fluctuations
  comparable to jet energy
• use smaller cone size, R 0.3

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Jet quenching?

• Excellent jet reconstruction but challenging to
  measure global medium modification
• Et100 GeV (reduced average jet energy fraction
  inside R)
• Radiated energy 20
• R0.3 dE/E3

• Most of radiated energy stays within cone
• ?
• Jet quenching rather means a medium-induced
  redistribution of the jet energy inside the jet
  cone

Jet shape average fraction of energy
in a sub-cone of radius R
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Low-pT tracking essential...
Simple quenching model The energy loss of a 100
GeV jet is simulated by reducing the energy of
the jet by 20 and replacing the missing energy
by 1 x 20 GeV gluon 2 x 10 GeV gluons 4 x 5
GeV gluons (Jets simulated with Pythia)

• Summary
• ALICE combines low-pt tracking and PID ? study
  of the jet-structure over a wide range of momenta
  and particle species.
• Jet reconstruction restricted to relatively
  high-energy jets
• (Etgt30-40 GeV) while leading particle correlation
  studies play an important
• role in the low-Et region.

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Heavy Flavours

• LHC is the first machine where heavy quarks will
  be produced abundantly in heavy-ion collisions.
• Heavy flavour production in pp and AA collisions
  to pt ? 0
• - open charm and open beauty
• - mechanism of heavy-quark production,
  propagation and hadronisation (in-medium
  quenching compared to massless partons)
• - cross sections as a reference for quarkonia
  production
• ? excellent impact parameter resolution
  (secondary vertex) and PID capability ?
  wide pt range
• - quarkonia
• - yields and pt spectra of J/?, ?, ?, ? and
  ?
• ? ee- in central region and ??- in forward
  region

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Heavy flavour energy loss?
average energy loss
Casimir coupling factor
distance travelled in the medium
transport coefficient of the medium (? gluon
density ? probe the medium)
? R.Baier et al., Nucl. Phys. B483 (1997) 291
(BDMPS)

• Energy loss for heavy flavours is expected to be
  reduced ?
• harder pt spectra for heavy- wrt light-flavour
  mesons
• i) Casimir factor
• light hadrons originate predominantly from gluon
  jets, heavy flavoured hadrons originate from
  heavy quark jets
• CR is 4/3 for quarks, 3 for gluons
• ii) dead-cone effect
• gluon radiation expected to be suppressed for ? lt
  MQ/EQ
• (heavy quarks with momenta lt 20-30 GeV/c ? v ltlt
  c)
• Dokshitzer Karzeev, Phys. Lett. B519 (2001)
  199 Armesto et al., Phys. Rev. D69 (2004)
  114003

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RAA(D) in ALICE

• The dead cone effect can be studied in the
  pt-dependence of the nuclear modification factor
  RAA

High pT (615 GeV/c) Energy loss can be
studied (it is the only expected effect)
Low pT (lt 67 GeV/c) Nuclear shadowing
Nuclear modification factor production yield in
AA collisions normalized to elementary
pp collisions, scaled with the number of binary
collisions
? good sensitivity for measurement of c quenching
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Detection strategy for D0 ? K-?

• Weak decay with mean proper length c? 124 µm
• Track Impact Parameter (distance of closest
• approach of a track to the primary vertex) of
  the
• decay products d0 100 µm
• STRATEGY invariant mass analysis of
  fully-reconstructed topologies
• originating from (displaced) secondary
  vertices
• - Measurement of Impact Parameters
• - Measurement of Momenta

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D0 ? K-?

• expected ALICE performance
• S/B 10
• S/?(SB) 40 (1 month
  Pb-Pb running)

? similar performance in pp (wider primary vertex
spread)
pT - differential
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Proposed ALICE EMCAL

• To improve the capabilities in triggering
• and measurement of high energy jets
• EM Sampling Calorimeter (STAR Design)
• Pb-scintillator linear response
• -0.7 lt ? lt 0.7
• ?/3 lt ? lt ??(opposite to PHOS)
• Energy resolution 15/vE

• Rails already installed
• Support structure funded by DoE,
• to be installed this summer
• 10 1/21/2 Supermodules (SM)
• 1 SM 24x12 modules
• 1 module 4 channels
• 1.2 x 104 total channels (granularity)

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EMCAL

• Single detector 6x6x25 cm3 shashlik 1.44mm
  Pb/1.76mm scintillator sampling
• 77 layers 20 Xo
• WLS fiberAPD readout
• Front End Electronics mainly developed for TPC
  and PHOS
• First SM to be ready for 2008 run
• Full Calorimeter to be completed for 2010 run

EMCAL Project Italy (LNF, Ct) France US
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Conclusions

• LHC
• the next jump in heavy-ion physics
• it is dangerous to make predictions, especially
  about the future
• significant extension of reach at both soft and
  hard frontiers
• ALICE
• dedicated heavy-ion experiment
• address most relevant observables, from very soft
  to very hard
• novel technologies!
• production well under way
• Busy months ahead ? working detector well on
  track for the start-up of LHC (summer 2007)
• Study collisions of lower-mass ions (varying
  energy density) and protons (pp and pA) as
  reference
• pp data will also allow for a number of genuine
  pp physics studies
• ? we are looking forward towards exciting times!