Robert Richter on behalf of the ATLAS Collaboration

Upgrade of ATLAS for High Luminosity at the Super-LHC (sLHC)

Robert Richter on behalf of the ATLAS Collaboration

Max-Planck-Institut f¨ur Physik, M¨unchen

Abstract

While the LHC is close to its startup, plans are already advancing for an upgrade to about ten times the LHC luminosity in the second half of the next decade. This upgrade project is called the Super-LHC or sLHC.

Coping with the high instantaneous and integrated luminosity will require many changes to the ATLAS detector. R&D work is developing rapidly for an all-new inner tracker, for modifications in the calorimeters and the muon system as well as for an improved trigger. This article summarizes the environment expected at the sLHC and the status of the upgrade concepts for the ATLAS detector.

Key words: LHC luminosity, sLHC, ATLAS upgrade, high luminosity

1. Introduction

The 14 TeV collision energy of the LHC and the luminosity of 10 ³⁴ cm ⁻² s ⁻¹ will open a new field of physics, allowing the study of rare physics processes with higher accuracy than ever before. An upgrade of the LHC to higher luminosity will help to push the discovery limit to even lower cross sections. Increased luminosity, however, leading to higher particle rates, requires improvements of numerous detector properties, like granularity, radiation tolerance of sensors and electronics as well as readout speed and data processing power. Trigger selectivity also needs substantial improvement, as trigger rates beyond the present 100 kHz seem not realistic for the operation at sLHC.

ATLAS upgrade concepts assume the structure of the superconducting magnets and the ar- rangement of the main subdetectors to remain unchanged. R&D programs have been started to address the numerous technical problems caused by the high particle rates. This article summa- rizes the upgrade concepts for the main ATLAS subdetectors. Layout and performance of the present ATLAS detector are given in [1].

Preprint submitted to Elsevier 12 January 2009

2. Upgrade of the ATLAS Subdetectors

High luminosity in p-p collisions leads to high rates of peripheral interactions resulting in large numbers of outgoing particles. The tracks from ‘minimum bias’ events are characterized by low transverse momenta, rarely exceeding 2 GeV. At nominal LHC luminosity, an average of 23 events with about 1000 charged tracks overlay every high-p T event selected by the trigger, see figure 1. The necessity to reconstruct tracks in a dense environment and to efficiently separate the ones with the required signature from underlying tracks called for a high degree of granularity, which, in turn, led to a large number of sensors and electronics channels. The present Inner Detector (ID) thus contains 8 × 10 ⁷ pixels and 6.3 × 10 ⁶ silicon strips.

At sLHC the number of minimum bias events per trigger will increase by a factor of up to 20, requiring even higher granularity for robust track reconstruction and to keep occupancies at an acceptable level. The present and future layouts are shown in Fig. 2. At the innermost radii of the barrel four layers of 50 × 250 µ m ² pixels are foreseen (LHC: three layers of 50 × 400 µ m ² ).

In the outer region five layers of Silicon-strip detectors are replacing the TRT and the four Si- layers of the present system. In each layer two sets of Si-strip sensors, oriented along slightly tilted directions, provide the azimuthal φ - and the longitudinal z-coordinate with 17 and 500 µ m resolution, respectively. To reduce occupancy, the strip length of 10 cm will be reduced to 2.5 cm for the inner two layers. The optimization of the endcap layout is still not finalized. Aim is to assure nine measurements along the trajectory of each track. In total, 3 × 10 ⁸ pixels and 4.5 × 10 ⁷ strips are foreseen for the new, upgraded ID.

Fig. 1. At LHC luminosity tracks of a physics event (marked in red) are overlayed by many tracks from mini-

mum bias events with low p

, see inset. Fig. 2. Layout of the ATLAS Inner Detector at LHC and a possible layout at SLHC.

The increase in granularity by about a factor four will lead to a corresponding increase in power consumption from presently 60 kW, which calls for new powering concepts like serial powering or local DC-DC conversion to keep supply cable cross sections inside available access areas. High performance cooling will be necessary to evacuate the heat, keeping the operating temperature of the Si-sensors below the required -15 ^◦ C. To cope with the increased data volume, new readout concepts based on optical links with up to 5 Gb/s are under development. For the ASICs, Deep Submicron technologies, based on Si or Si-Germanium material, are expected to sustain the required radiation doses.

For the innermost ID-layers, new pixel detector technologies are under study. Thin planar sensors and 3D sensors achieve improved charge collection efficiency after heavy irradiation by

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a reduced distance between electrodes. Diamond detectors showing negligible dark current, even at room temperature, are also considered.

The ATLAS calorimeters use LAr technology for the inner shell and forward region, while Scintillating Tiles are used in the outer region. The Tiles seem to sustain the anticipated radia- tion. The limiting factor for LAr is the buildup of ions in the drift gap, which may modify the signal amplitude, reducing energy resolution. This effect mainly depends on gap size, voltage and track density, but may also be influenced by surface effects on the electrodes for which cal- culations are unreliable. A comprehensive test program has therefore been started, whereby test structures of the different calorimeter modules are exposed to a proton beam, variable over a wide range of intensities. Preliminary results of these measurements at the Protvino synchrotron show signal degradation only at the highest rates, corresponding to the very forward calorimeter at 3.1 ≤ η ^≤ 5 (FCAL). The FCAL will therefore either be replaced by a detector with smaller gap size or shielded by a warm mini-calorimeter placed in front of the current one. Like in the ID, all readout electronics in LAr and Tile calorimetry will have to be replaced by more radiation tolerant devices.

The Muon system forms the outer shell of the ATLAS detector and is exposed to a high level of neutron- and γ -background caused by shower leakage from the calorimeters and limited shielding of the beam pipe, due to space constraints. The corresponding hit rates from converted electrons in the Monitored Drift Tubes (MDT) may go up to 300 kHz per tube at LHC and to an order of magnitude higher at sLHC, resulting in loss of efficiency for muon tracks and reduction of spatial resolution due to space charge in the drift gas.

Current background simulation indicates excessive hit rates only in the forward part of the end- cap, where MDT chambers will have to be replaced by chamber types with higher rate capability.

Candidates are MDTs with reduced tube radius, TGCs, as used in the forward trigger system but optimised for high rate operation as well as precision gas chambers, like Micromegas and GEMs. Present R&D work is evaluating prototypes with respect to high rate behaviour, position and double track resolution as well as neutron sensitivity. As an estimated area of about 160 m ² has to be covered, practical aspects of construction and production have also to be taken into account.

To improve selectivity for high-p T muons ( ≥ 20 GeV), the layout of the muon trigger chambers will have to be reviewed.

3. Summary

The capability of the LHC for a luminosity increase by an order of magnitude represents an unprecedented challenge for ATLAS detector technology. Design work for an improved detec- tor concept is in an advanced state, while a R&D program for innovative technical solutions is making good progress. A Technical Proposal is foreseen for 2010–2011.

References

[1] The ATLAS collaboration, The ATLAS Experiment at the CERN Large Hadron Collider, JINST 3 S08003 (2008)

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