The ITk for the Phase-II upgrade of
ATLAS
, as described in the LoI is an all-silicon tracker. It was designed to measure the transverse momentum and direction of isolated particles (in particular muons and electrons), to reconstruct the vertices of pile-up events and associate the vertex with the hard interaction. It is also able to identify secondary vertices in b-jets with high efficiency and purity, measure the tracks in the cores of high energy jets with high efficiency, provide good two- track resolution and ensure a low rate for reconstruction of fake tracks. It can identify the decay of tau leptons, including impact parameter information, and is also able to reconstruct the tracks associated with converted photons.The LoI layout of theITkforms the basis of the designs presented here for the Reference, Mid- dle and Low scoping scenarios. A major change since 2012, when the LoIwas written, is a much improved understanding [3,4] of the importance for the HL-LHCphysics programme of extending the tracking coverage well beyond |η|=2.7, and matching this with improvements to other detector components at high |η|. In what follows, theLoIlayout is described first, however for the reasons just outlined and detailed in Section XI.2, the Reference scenario layout includes additional pixel coverage to provide tracking to |η|=4.0. This is the maximum coverage possible within the con- straints of the current tracker volume in the z direction and a reasonable beam-pipe diameter. This Reference scenario tracker concept, with very forward coverage, is referred to here and elsewhere as theLoI-VFlayout.
IV.1.1
Design, layout and differences with the existing tracker
TheITkwill be immersed in the magnetic field from a 2 Tesla solenoid. The studies presented here use hybrid silicon pixel modules at the inner radii, surrounded by silicon micro-strip modules at larger radii. It is 6 m long (active length) and will fill all the available space in the detector cryostat out to a maximum active radius of 1,000 mm, including the space currently occupied by the straw-tube detector, the Transition Radiation Tracker (TRT). In the central (barrel) region there are four pixel layers followed by five strip layers (three with shorter strips and two with longer strips). To maintain the coverage and optimal performance in the transition region between the end of the barrel and largest radius of the end-cap there is also a short-stub barrel (radius 862 mm) of limited length in z. In thisLoIlayout, the coverage of the forward regions extends only up to|η|=2.7and is provided by a total of six pixel discs and seven strip discs. For an extended η-coverage more pixel discs are needed. TheLoIlayout has been optimised for coverage, and small gaps have been preserved between sub-detectors to allow for supports, services and insertion clearances.
TheITkhas been designed to balance the tracking performance required for the Phase-II physics programme against the cost of construction. The required tracking performance builds on the lessons learnt during Run 1 and the performance of the ITk will be as good, and in most cases better, than the existingIDin an environment with significantly higher pile-up. For the Reference and Middle scenarios, the coverage of theITkextends to higher values ofηrelative to theLoIlayout, to maximise the physics potential at theHL-LHC. This region poses severe challenges to the design, construction, pattern recognition, track-reconstruction software and the robustness of the tracker to detector losses during operation in this very harsh environment. The LoIlayout of the detector is shown in Fig.6and the resulting sensor areas and channel counts are shown in Table11.
The design of the ITk takes advantage of new technology developed since the construction of the existing ID and its performance will be enhanced by a lower mass construction, reducing the effect of multiple scattering, photon conversions and hadronic interactions. The lower mass is achieved by a combination of several advances. More efficient multiplexed designs lead to fewer
1 Pixel Barrel x 4 (r=39mm,78mm,155mm,250mm) Pixel Discs x 6 (z=877mm,1059mm,1209mm,1359mm,1509mm,1675mm) Disc Strips x 7 Stub Layer x 1 (r=862mm)
m
Long (47.8mm) Strips x 2 (r=762mm, 1000mm) (z=1415mm,1582mm,1800mm,2040mm, 2320mm,2620mm,3000mm) Short (23.8mm) Strips x 3 (r=405mm,519mm, 631mm)Figure 6. A cross-section of the ATLASITktracker presented in the LoIshowing the coverage of the pixel detector in red and strip detector in blue. The rapidity coverage extends up to|η|=2.7and is matched to the coverage of the muon system. The blue line outside theITkvolume represents the coil of the solenoid magnet.
Table 11. The surface area and channel count for different parts of theLoIlayout configuration used for the performance and physics simulation of the Pixel and Strip detectors.
Detector Silicon Area [m2] Channels [106]
Pixel barrel 5.1 445 Pixel end-cap 3.1 193 Pixel Total 8.2 638 Strip barrel 122 47 Strip end-cap 71 27 Strip total 193 74
cables for distributing HV and Low Voltage (LV) power to the detector modules. Higher perfor- mance cooling based on evaporative CO2 technology allows the use of smaller diameter cooling pipes. The use of higher thermal performance carbon-composite materials and techniques, such as thermally-conducting carbon-foam and co-cured electro-mechanical assembly, lead to substantial mass reductions. Low-power, small feature size (65 nm and 130 nm)Complementary Metal-Oxide Semiconductor (CMOS) front-end electronics will be used. New readout technologies, combined with a newTDAQarchitecture with much higher bandwidth than the presentID, allow the readout of large amounts of detector information in this very high pile-up environment while keeping mass low. TheITk is designed for rapid installation and ease of access to take account of the activated environment which will be encountered close to the interaction point. The HL-LHC presents new challenges for the component and system reliability which are required to achieve the target lifetime for this complex detector. TheITkhas much more stringent requirements on radiation tolerance than the existingIDand will have to employ more radiation-hard sensor technologies.