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The ALICE Inner Tracking System: present and future Vito Manzari – INFN Bari ( ) o n behalf of the ITS Collaboration in the ALICE Experiment at LHC. Outline. ALICE experiment Inner Tracking System Detector overview and Performance ITS Upgrade:

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The ALICE Inner Tracking System: present and future Vito Manzari – INFN Bari

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TheALICE Inner Tracking System: present and future

Vito Manzari – INFN Bari


on behalf of the ITS Collaboration in the ALICE Experiment at LHC



  • ALICE experiment
  • Inner Tracking System
    • Detector overview and Performance
  • ITS Upgrade:
    • Physics motivations
    • Upgrade strategy
    • R&D activities
    • Timeline
  • Conclusions

HSTD8 – December7th, 2011


A Large Ion Collider Experiment

  • ALICE is the dedicated heavy ion experiment at LHC
  • Study of the behavior of strongly interacting matter under extreme conditions of compression and heat in heavy-ion collisions up to Pb-Pb collisions at 5.5 TeV
  • Proton-proton collisions:
    • Reference data for heavy-ion program
    • Genuine physics (momentum cut-off < 100 MeV/c, excellent PID)

HSTD8 – December7th, 2011


The ALICE detector

Central Barrel

2 p tracking & PID

Dh ≈ ± 1


Size: 16 x 26 meters

Weight: 10,000 tons

HSTD8 – December7th, 2011


Central Barrel

  • Tracking
  • Pseudo-rapidity coverage |η| < 0.9
  • Robust tracking for heavy ion environment
    • up to 150 points along the tracks
  • Wide transverse momentum range (100 MeV/c – 100 GeV/c)
    • Low material budget (13% X0 for ITS+TPC)
  • PID over a wide momentum range
  • Combined PID based on several techniques: dE/dx, TOF, transition and Cherenkov radiation
  • Rate capabilities
  • Interaction rates: Pb-Pb < 8kHz, p-p < 200 kHz (~30 events in the TPC)
  • Multiplicities: central Pb-Pb events ~2000, Pb-Pb MB ~ 600

Inner Tracking System (ITS)

HSTD8 – December7th, 2011


The Inner Tracking System (ITS)

  • The ITS plays a key role for the study of yields and spectra of particles containing heavy quarks
  • The ITStasks:
    • Secondary vertex reconstruction (c, b decays) with high resolution
      • Good track impact parameter resolution
      • < 60 µm (rφ) for pt > 1 GeV/c in Pb-Pb
    • Improve primary vertex reconstruction, track momentum and angle resolution
    • Tracking and PID of low pt particles, also in stand-alone
    • Prompt L0 trigger capability (FAST OR) with a latency <800 ns (SPD)

HSTD8 – December7th, 2011


The “current” Inner Tracking System

  • ITS requirements
  • Good spatial precision
  • High efficiency
  • High granularity (≈ few % occupancy)
  • Minimize distance of innermost layer from beam axis (mean radius ≈ 3.9 cm)
  • Limited material budget
  • Analogue information in 4 layers for particle identification via dE/dx
  • The ITS (Inner tracking System) consists of 6 concentric barrels of silicon detectors based on 3 different technologies
    • 2 layers of Silicon Pixel Detector (SPD)
    • 2 layers of Silicon Drift Detector (SDD)
    • 2 layers of Silicon double-sided microStrip Detector (SSD)

HSTD8 – December7th, 2011


The Inner Tracking System in numbers

  • Radial distance defined by beam-pipe (inwards) and requirements for track matching with TPC (outwards)
  • Inner layers: high multiplicity environment (~100 tracks/cm2)  2 layers of pixel detectors

HSTD8 – December7th, 2011


PbPb event @ 2.76 A TeV

HSTD8 – December7th, 2011


Online SPD Vertex

  • SPD Vertex built out of tracklets
  • Same algorithm in pp and PbPb

with different configuration


    • e.g.: cut on # clusters on SPD
  • Vertex diamond information

delivered to LHC

  • SPD vertex used as input for

offline reconstruction

HSTD8 – December7th, 2011


Track impact parameter

  • The transverse impact parameter in the bending plane d0(rφ) is the reference variable to look for secondary tracks from strange, charm and beauty decay vertices
  • Impact parameter resolution is crucial to reconstruct secondary vertices : below 60 µm for pt > 1 GeV/c
  • Good agreement data-MC (~10%)

Few hundred micron

  • The material budget mainly affect the performance at low pt (multiple scattering)
  • The point resolution of each layers drives the asymptotic performance
  • ITS standalone enables the tracking for very low momentum particles (80-100 MeV/c pions)


HSTD8 – December7th, 2011


Particle IDentification

  • dE/dx measurement
    • Analogue read-out of charge deposited in 4 ITS layers (SDD & SSD)
    • Charge samples corrected for the path length
    • Truncated mean method applied to account for the long tails in the Landau distribution
  • PID performance
    • PID combined with stand-alone tracking allows to identify charged particles below 100 MeV/c
    • p-K separation up to 1 GeV/c
    • K- separation up to 450 MeV/c
    • A resolution of about 10-15% is achieved




HSTD8 – December7th, 2011


Intermediate Summary

  • The ITS performance are well in agreement with the design values
  • ALICE has collected p-p and PbPb data at the various energies and the data analysis is progressing very well
  • Many papers are being published containing very relevant results
  • Then …..
  • Why do wewant to upgrade the ITS?

HSTD8 – December7th, 2011


ITS Upgrade Motivations

  • Extend ALICE capability to study heavy quarks as probes of the QGP in heavy-ion collisions
      • Main Physics Topics and Measurements of interest
        • Study the quark mass dependence of the energy loss
          • Measure the Nuclear Modification factor RAAvspT, down to low pT, of D and B mesons
        • Study the thermalization process of heavy quarks in the hot and dense medium formed by heavy ion collisions
          • Measure the baryon over meson ratio (Λc / D or Λb/ B)
          • Measure elliptic flow of charged mesons
    • Exploit the LHC luminosity increase improving the readout capabilities, now limited to ≈1 kHz

HSTD8 – December7th, 2011


Upgrade Strategy

  • Improve the impact parameter resolution by a factor 2÷3
    • How:
      • Reduce of the radial distance of the innermost layer (closest to the IP)
      • Reduce of the material budget
      • Reduce of the pixel size
    • Physics reach:
      • Low pT heavy-flavour mesons
      • v2 of charmed hadrons
      • Heavy flavour baryons (Λc,Λb, …)
      • Better identification of secondary vertices from decaying charm and beauty and increase statistical accuracy of channels already measured by ALICE (e.g. displaced D0, J/Ψ, etc.)

HSTD8 – December7th, 2011


Upgrade Strategy

  • Improve trigger capabilities
    • How:
      • Improve standalone tracking efficiency and pT resolution
      • Selection of event topologies with displaced vertices at Level 2 (~100 μs)
    • Physics reach:
      • Strong enhancement of relevant signals
  • Exploit luminosity increase
    • How:
      • Improve readout time and standalone tracking capability
    • Physics reach:
      • Strong enhancement of relevant signals

HSTD8 – December7th, 2011


Improve the impact parameter resolution

  • Get closer to the IP
    • Radius of innermost Pixel layer is defined by central beam pipe radius
      • Present beam pipe: ROUT = 29.8 mm, ΔR = 0.8 mm
      • New Reduced beam pipe: ROUT = 19 mm, ΔR = 0.5 mm
  • Reduce material budget (especially innermost layers)
      • reduce mass of silicon, electrical bus (power and signals), cooling, mechanics
      • Present ITS Pixel layers: X/X0 ~1.14% per layer
      • Target value for new ITS: X/X0 ~0.3 – 0.5% per layer
  • Reduce pixel size
    • Reduce size of interconnect bumps, monolithic Pixels
      • currently 50μm x 425μm

HSTD8 – December7th, 2011


Improve tracking, triggeringandpTresolution

  • Higher standalone tracking efficiency
    • Increase granularity
    • Increase number of layers in the outer region (seeding) and inner region (occupancy)
  • Extended trigger capabilities
    • High standalone tracking efficiency
    • Low readout time < 50μs for Pb-Pb, ~μs for p-p (current ITS ~1ms in both cases)
  • Increase momentum resolution
    • increase track length
    • increase spatial resolution
    • reduce material budget

HSTD8 – December7th, 2011


Upgrade Scenario

  • 7 silicon layers (r = 2.2÷ 45 cm) or more to cover the region from IP to TPC
  • 3innermost layers made of pixels, 3 outer layers either pixels or double sided strips
  • Pixel size ~ 20-30 µm (rφ), rφresolution ~ 4÷ 6 µm
  • Material budget 0.3 ÷ 0.5% X0per layer
  • Power consumption 250-300 mW/cm2
  • Innermost pixel layer: ultra-light high-resolution high-granularity as-close-as-possible to IP (r ≈ 2.2 cm)
    • Hit density ~ 100 tracks/cm2 in HI collisions
    • Radiation tolerant design (innermost layer) compatible with 2 Mrad / 2 x1013neq over 10 years (safety factor ~2 included)

HSTD8 – December7th, 2011


Upgrade Scenario

  • The new ITS will be based on Pixel and Strip detectors
    • The innermost layers should be mounted on an insertable mechanics and should be served from one side only for a fast replacement in case of reduced efficiency



HSTD8 – December7th, 2011


Impact parameter resolution

  • An additional innermost pixel layer would achieve already a substantial improvement of the pointing resolution (factor 3 at 200 MeV/c)

ITS standalone tracking

  • However, a completely new ITS is mandatory to improve the standalone tracking efficiency at low pT and cope with the increased LHC luminosity
    • New detector technologies for a faster readout

HSTD8 – December7th, 2011


Standalone Tracking Efficiency

  • A factor 2 gain in tracking efficiency at 200 MeV is achieved with the configuration under study
  • Tracking efficiency and an improved d0 resolution allow to detect charmed and beauty hadrons below 2 GeV/c

HSTD8 – December7th, 2011


Physics signal benchmark

D0 Kπ

Increase of the statistical significance 

 reduction the statistical uncertainty!

pT range not accessible with the current ITS

HSTD8 – December7th, 2011


Physics signal benchmark


New measurement!

Important physics reach:

barion over meson ratio in heavy-quark sector

HSTD8 – December7th, 2011


R&D activities

  • Pixel detectors
    • Hybrid pixels with reduced material budget and small pitch
    • Monolithic pixels rad-tolerant
  • Double-sided strip detectors (outer layers)
    • Shorter strips and new readout electronics
  • Electrical bus for power and signal distribution
    • Low material budget
  • Cooling system options
    • air cooling, carbon foam, polyimide and silicon micro-channels structure, liquid vs evaporative
    • low material budget

HSTD8 – December7th, 2011


Monolithic Pixel R&D

  • State-of-the-art architecture (MIMOSA family) uses rolling-shutter readout
  • Pixel size ~20 µm possible
  • Target for material budget < 0.3 % X0(50 µm thick chip)
    • STAR HFT Monolithic: 0.37% X0
  • Ongoing developments:
    • Evaluation of properties of a quadruple well 0.18 CMOS
      • radiation tests structures
      • study characteristics of process using the MIMOSA architecture as reference
      • design of new circuit dedicated to ALICE (MISTRAL)
      • investigation of in-pixel signal processing using the quadruple-well approach
    • Novel high resistivity base material for depleted operation (LePix)

HSTD8 – December7th, 2011


Hybrid Pixel R&D

  • Pixel size limit due to flip chip bonding technology (~30 µm)
  • Target for overall material budget < 0.5 % X0, about 1/3 of silicon
  • (100 µm sensor, 50 µm front-end chip)
    • Present SPD 1.14% X0, silicon 0.38% X0
    • (200 µm sensor, 150 µm front-end chip)
  • Edgeless sensors to reduce insensitive overlap regions
  • High S/N ratio, ~ 8000 e-h pairs/MIP
  • Power/Speed optimization
  • Proven radiation hardness
  • Ongoing developments:
    • Thin and Edgless detectors (FBK, VTT, IZM)
    • Low cost bump bonding, Lower power FEE

HSTD8 – December7th, 2011


Double-sided Micro-strip R&D

  • Sensor layout
    • Strip detector technologies are rather mature
    • Optimize the design to cope the expected higher multiplicity at smaller radius and nominal LHC energy
      • Optimize stereo angle to limit ambiguities in track reconstruction
      • Smaller “virtual” cell to reduce occupancy
  • Front-end electronics
    • Low-momentum PID requires a wide dynamic range
    • Data digitization directly on front-end chip
  • Ongoing developments
    • Sensor layout
    • Fully differential front-end chip
      • ADC or ToT for the digitization of the analogue information

HSTD8 – December7th, 2011


ITS Upgrade Timeline

  • The upgrade should target the 2017-18 shutdown (Phase I)
    • Decisions on the upgrade plans in terms of physics strategy, detector feasibility and funding availabilitywill be taken in 2012
    • The global upgrade may require a two-stage approach with a Phase II in 2020 and beyond.
  • end 2011: Preparation of a Conceptual Design Report
  • 2011-2014: R&D for Phase I
  • 2014-2016: Production and pre-commissioning for Phase I
  • 2017-2018: Installation and commissioning for Phase I

HSTD8 – December7th, 2011



  • The current Inner Tracking System performance is well in agreement with the design requirements and expectations
    • The achieved impact parameter resolution allows to reconstruct the secondary vertices of charm decays
    • Standalone capability allows to track and identify charged particles with momenta down to 100 MeV/c
  • An upgraded ITS will extend the ALICE physics capabilities:
    • Strong increase of the statistical accuracy in the measurements of yields and spectra of charmed mesons and baryons already possible with the present detector
    • Asignificant extension of the present physics programmewith new measurements that at present are not possible
  • Several options for the detector technology implementation are being investigated and developed

HSTD8 – December7th, 2011


Back-up slides

HSTD8 – December7th, 2011


“Russian Doll” Installation

SDD barrel

SSD barrel

  • Inserting the SDD barrel inside
  • the SSD barrel

HSTD8 – December7th, 2011


“Russian Doll” Installation

SPD half-barrels mounted face to face around the beam pipe

  • Moving of the SDD+SSD barrel
  • over the SPD

HSTD8 – December7th, 2011


“Russian Doll” Installation

  • Moving the TPC over the ITS barrel, i.e. SPD+SDD+SSD

HSTD8 – December7th, 2011


SPD L0 trigger

SPD Half Stave

Readout MCM

Half stave

141 mm




  • Overall latency constrain 800 ns (Central Trigger Processor)
  • Key timing processes are data deserialization and Fast-OR extraction
    • Algorithm processing time < 25 ns
  • 10 Algorithms provided in parallel
    • Detectors commissioning, p-p and PbPb physics
    • Cosmic, minimum bias and multiplicity algorithms

Pixel chips

  • The SPD is made of 120 modules, called half-staves
  • Pixel chip prompt Fast-OR
    • Active if at least one pixel hit in the chip matrix
    • 10 signals in each half-stave (1200 signals in total)
    • Transmitted every 100 ns

HSTD8 – December7th, 2011


Tracking strategies

  • “Global”
    • Seeds in outer part of TPC (lower track density)
    • Inward tracking from the outer to the inner TPC radius
    • Matching the outer SSD layer and tracking in the ITS
    • Outward tracking from ITS to outer detectors  PID ok
    • Inward refitting to ITS  Track parameters OK
  • “ITS stand-alone”
    • Recovers not-used hits in the ITS layers
    • Aim: track and identify particles missed by TPC due to pt cut-off, dead zones between sectors, decays
      • pt resolution ≤ 6% for a pion in pt range 200-800 MeV/c
      • pt acceptance extended down to 80-100 MeV/c (for )

TPC-ITS track matching

pt resolution

HSTD8 – December7th, 2011



  • Primary vertex reconstructed with tracklets
  • Tracks reconstruction starts from outside (TPC) towards ITS using the vertex as seed
  • TPC reconstructed tracks are matched with SSD outer layer
  • Once the reconstruction reaches the first SPD layer it is back-propagated
  • Re-fit from outside and the vertex is recalculated using the tracks

HSTD8 – December7th, 2011


Upgrade Simulation Tools

  • 3 independentsimulationtoolshavebeendeveloped
    • Fast EstimationTool: “Toy-Model” originallydeveloped by the STAR HFT collaborationwhichallows to build a simple detector model. The model featured the calculation of the covariancematrixateachstep of a measurement (e.g. layer with radiusr) including the multiple scattering.
    • Fast MC Tool: Extension of the FET thatallowsto disentangle the performance of the layout from the efficiency of the specictrackfindingalgorithm.
    • Full MC: Transport code (geant3) designed to be flexible : the detector segmentation, the number of layers, theirradii and materialbudgets can be set asexternalparameters of the simulation.

HSTD8 – December7th, 2011


Upgrade Simulation Validation

Fast Estimation Tool (pions)

Full MC

  • Fast MC shows the sameperfomanceas the FET
  • The 3 simulationtoolsreproduce the current ITS performance

HSTD8 – December7th, 2011


Pointing Resolution

  • Effects of the innermost layer L0
    • No vertex resolution

Radial Distance

Material Budget

HSTD8 – December7th, 2011


Pointing Resolution

Spatial Resolution

  • Configuration design for betterpointingresolutionperformances:
    • ImprovementatlowpT:
      • Smallestradialdistance to the beam line
      • Smallestmaterialbudget
    • Improvementat high pT:
      • Smallestcellsize

HSTD8 – December7th, 2011


Particle Identification

  • Different configurations are being studied

Red : proton/Kaonseparation

Black : kaon / pionseparation

HSTD8 – December7th, 2011


Hybrid pixel material budget

  • Each urrent SPD layer
    • Carbon fiber support: 200 μm
    • Cooling tube (Phynox): 40 μmwall thickness
    • Grounding foil (Al-Kapton): 75 μm
    • Pixel chip (Silicon): 150 μm 0.16%
    • Bump bonds (Pb-Sn): diameter ~15-20 μm
    • Silicon sensor: 200 μm 0.22%
    • Pixel bus (Al+Kapton): 280 μm 0.48%
    • SMD components
    • Glue (Eccobond 45) and thermal grease

Two main contributors: siliconand interconnect structure (bus)

HSTD8 – December7th, 2011


How material budget can be reduced

  • How can the material budget be reduced?
    • Reduce silicon chip thickness
    • Reduce silicon sensor thickness
    • Thin monolithic structures
    • Reduce bus contribution (reduce power)
    • Reduce edge regions on sensor
    • Review also other components (but average contribution 0.1-0.2%)
  • What can be a reasonable target
    • Hybrid pixels: ~0.5%X0
      • silicon: 0.16% X0(present SPD 0.38%)
      • bus: 0.24% X0(present SPD 0.48%)
      • others: 0.12%X0 (present SPD 0.24%)
    • Monolithic pixels: 0.37% X0(as for STAR HFT)

HSTD8 – December7th, 2011