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

Sez. di Bari

The ALICE Inner Tracking System:

present and futureVito Manzari – INFN Bari

([email protected])

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

mailto:[email protected]

Sez. di Bari

ALICE experiment Inner Tracking System

Detector overview and Performance

ITS Upgrade: Physics motivations Upgrade strategy R&D activities Timeline

Conclusions

Outline

V. Manzari - INFN Bari HSTD8 – December7th, 2011 2

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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)

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Detector:Size: 16 x 26 metersWeight: 10,000 tons

Central Barrel2 p tracking & PID

Dh ≈ ± 1

The ALICE detector


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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)

Central Barrel

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The ITS tasks: 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)

The ITS plays a key role for the study of yields and spectra of particles containing heavy quarks

The Inner Tracking System (ITS)


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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)

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

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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

The Inner Tracking System in numbers


Layer Det. Radius

(cm)Length

(cm)Surface

(m2)Ch.

Spatial precision

(mm)Cell

(μm2)

Max occupancy

central PbPb (%)

Power dissipation(W) Material

budget(% X/X0)

rf z barrel end-cap

1SPD

3.9 28.20.21 9.8M 12 100 50x425

2.11.35k 30

1.14

2 7.6 28.2 0.6 1.14

3SDD

15.0 44.41.31 133K 35 25 202x294

2.51.06k 1.75k

1.13

4 23.9 59.4 1.0 1.26

5SSD

38.0 86.25.0 2.6M 20 830 95x40000

4.0850 1.15k

0.83

6 43.0 97.8 3.3 0.86

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PbPb event @ 2.76 A TeV


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SPD Vertex built out of tracklets

Same algorithm in pp and PbPb

with different configuration

parameters• e.g.: cut on # clusters on SPD

Vertex diamond information

delivered to LHC

SPD vertex used as input for

offline reconstruction

Online SPD Vertex


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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%) 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)

Track impact parameter


Pb-Pb

Few hundred micron

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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

p-p

p-p

Pb-Pb

Particle IDentification


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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 we want to upgrade the ITS?


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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 RAA vs pT, 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

ITS Upgrade Motivations


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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.)

Upgrade Strategy


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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

Upgrade Strategy


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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

Improve the impact parameter resolution


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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

Improve tracking, triggering and pT resolution


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7 silicon layers (r = 2.2 ÷ 45 cm) or more to cover the region from IP to TPC 3 innermost layers made of pixels, 3 outer layers either pixels or double sided strips Pixel size ~ 20-30 µm (rφ), rφ resolution ~ 4 ÷ 6 µm Material budget 0.3 ÷ 0.5% X0 per 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 x1013 neq over 10 years (safety factor ~2 included)

Upgrade Scenario


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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

Upgrade Scenario

Upgrade

Current


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Impact parameter resolution


ITS standalone tracking

An additional innermost pixel layer would achieve already a substantial

improvement of the pointing resolution (factor 3 at 200 MeV/c)

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

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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

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HSTD8 – December7th, 2011 23V. Manzari - INFN Bari

Physics signal benchmarkD0 Kπ

Increase of the statistical significance

reduction the statistical uncertainty!

pT range not accessible with the current ITS

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Physics signal benchmarkΛc

New measurement!

Important physics reach: barion over meson ratio in heavy-quark sector

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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

R&D activities


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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)

Monolithic Pixel R&D


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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

Hybrid Pixel R&D


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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

Double-sided Micro-strip R&D

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The upgrade should target the 2017-18 shutdown (Phase I)

• Decisions on the upgrade plans in terms of physics strategy, detector feasibility and funding availability will 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

ITS Upgrade Timeline


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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

• A significant extension of the present physics programme with new measurements

that at present are not possible

Several options for the detector technology implementation are being investigated

and developed

Conclusions


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Back-up slides

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“Russian Doll” Installation

SSD barrel

SDD barrel

Inserting the SDD barrel inside the SSD barrel

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Moving of the SDD+SSD barrel over the SPD

SPD half-barrels mounted face to face

around the beam pipe


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


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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

SPD L0 trigger

SPD Half Stave

Half stave

Sensor

Pixel chips

Readout MCM

Sensor141 mm

1

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


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“Global”1. Seeds in outer part of TPC (lower track density)

2. Inward tracking from the outer to the inner TPC radius

3. Matching the outer SSD layer and tracking in the ITS

4. Outward tracking from ITS to outer detectors PID ok

5. 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 )

pt resolution

TPC-ITS track matching

Tracking strategies


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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

Vertexing

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Upgrade Simulation Tools

3 independent simulation tools have been developed

Fast Estimation Tool: “Toy-Model” originally developed by the STAR HFT collaboration which allows to build a simple detector model. The model featured the calculation of the covariance matrix at each step of a measurement (e.g. layer with radius r) including the multiple scattering.

Fast MC Tool: Extension of the FET that allows to disentangle the performance of the layout from the efficiency of the specic track finding algorithm.

Full MC: Transport code (geant3) designed to be flexible : the detector segmentation, the number of layers, their radii and material budgets can be set as external parameters of the simulation.

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Upgrade Simulation Validation

Fast MC shows the same perfomance as the FET

The 3 simulation tools reproduce the current ITS performance

Fast Estimation Tool (pions) Full MC

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Pointing Resolution

Effects of the innermost layer L0• No vertex resolution

Radial Distance

Material Budget

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Pointing Resolution

Spatial Resolution

Configuration design for better pointing resolution performances:

Improvement at low pT:• Smallest radial distance to the beam line• Smallest material budget

Improvement at high pT: • Smallest cell size

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Red : proton/Kaon separationBlack : kaon / pion separation

Particle Identification

Different configurations are being studied

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Each urrent SPD layer

• Carbon fiber support: 200 μm

• Cooling tube (Phynox): 40 μm wall 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: silicon and interconnect structure (bus)

Hybrid pixel material budget


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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)

How material budget can be reduced


Documents

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