G. Aglieri Rinella

The ALICE Silicon Pixel Detector: readiness for the first proton beam

The Silicon Pixel Detector (SPD) is the innermost element of the ALICE Inner Tracking System (ITS). The SPD consists of two barrel layers of hybrid silicon pixels surrounding the beam pipe with a total of ≈ 107 pixel cells. The SPD features a very low material budget, a 99.9% efficient bidimensional digital response, a 12 μm spatial precision in the bending plane (r) and a prompt signal as input to the L0 trigger. The SPD commissioning in the ALICE experimental area is well advanced and it includes calibration runs with internal pulse and cosmic ray runs. In this contribution the commissioning of the SPD is reviewed and the first results from runs with cosmic rays and circulating proton beams are presented.

The electro-mechanical integration of the NA62 GigaTracker time tagging pixel detector

The NA62 GigaTracker is a low mass time tagging hybrid pixel detector operating in a beam with a particle rate of 750 MHz. It consists of three stations with a sensor size of 60 x 27mm(2) containing 18000 pixels, each 300 x 300 mu m(2). The active area is connected to a matrix of 2 x 5 pixel ASICs, which time tag the arrival of the particles with a binning of 100 ps. The detector operates in vacuum at -20 to 0 degrees C and the material budget per station must be below 0.5% X-0. Due to the high radiation environment of 2 x 10(14) 1 MeV neutron equivalent cm(-2)/yr(-1) it is planned to exchange the detector modules regularly. The low material budget, cooling requirements and the request for easy module access has driven the electro-mechanical integration of the GigaTracker, which is presented in this paper.

D-meson production in p-Pb collisions at s NN =5.02 TeV and in pp collisions at s =7 TeV

The production cross sections of the prompt charmed mesons D0, D+, D∗+ and Ds were measured at mid-rapidity in p–Pb collisions at a centre-of-mass energy per nucleon pair √ sNN = 5.02 TeV with the ALICE detector at the LHC. D mesons were reconstructed from their decays D0→ K−π+, D+ → K−π+π+, D∗+ → D0π+, Ds → φπ+→ K−K+π+, and their charge conjugates. The pTdifferential production cross sections were measured at mid-rapidity in the transverse momentum interval 1 < pT < 24 GeV/c for D0, D+ and D∗+ mesons and in 2 < pT < 12 GeV/c for Ds mesons, using an analysis method based on the selection of decay topologies displaced from the interaction vertex. The production cross sections of the D0, D+ and D∗+ mesons were also measured in three pT intervals as a function of the rapidity ycms in the centre-of-mass system in −1.26 < ycms < 0.34. In addition, the prompt D0 production cross section was measured in pp collisions at √ s = 7 TeV and p–Pb collisions at √ sNN = 5.02 TeV down to pT = 0 using an analysis technique that is based on the estimation and subtraction of the combinatorial background, without reconstruction of the D0 decay vertex. The nuclear modification factor RpPb(pT), defined as the ratio of the pT-differential Dmeson cross section in p–Pb collisions and that in pp collisions scaled by the mass number of the Pb nucleus, was calculated for the four D-meson species and found to be compatible with unity within experimental uncertainties. The results are compared to theoretical calculations that include coldnuclear-matter effects and to transport model calculations incorporating the interactions of charm quarks with an expanding deconfined medium. ∗See Appendix A for the list of collaboration members ar X iv :1 60 5. 07 56 9v 2 [ nu cl -e x] 2 7 N ov 2 01 6 D-meson production in p–Pb and pp collisions ALICE Collaboration

Test-beam results of a silicon pixel detector with Time-over-Threshold read-out having ultra-precise time resolution

A time-tagging hybrid silicon pixel detector developed for beam tracking in the NA62 experiment has been tested in a dedicated test-beam at CERN with 10 GeV/c hadrons. Measurements include time resolution, detection efficiency and charge sharing between pixels, as well as effects due to bias voltage variations. A time resolution of less than 150 ps has been measured with a 200 μm thick silicon sensor, using an on-pixel amplifier-discriminator and an end-of-column DLL-based time-to-digital converter.

A 9-Channel, 100 ps LSB Time-to-Digital Converter for the NA62 Gigatracker Readout ASIC (TDCpix)

The TDCpix ASIC is the readout chip for the Gigatracker station of the NA62 experiment. Each station of the Gigatracker needs to provide time stamping of individual particles to 200 ps-rms or better. Bump-bonded to the pixel sensor the ASIC serves an array of 40 columns x 40 pixels. The high precision time measurement of the discriminated hit signals is accomplished with a set of 40 TDCs sitting in the End-Of-Column region of the ASIC. Each TDC provides 9 channels per column. For the time-to-digital converter (TDC) a delay-locked-loop (DLL) approach is employed to achieve a constant time binning of 100ps. Simulation results show that an average rms time resolution of 33ps with a power consumption of the TDC better than 33 mW per column is achieved. This contribution will present the design, simulation results and implementation challenges of the TDC.

Prototype readout electronics for the upgraded ALICE Inner Tracking System

The ALICE Collaboration is preparing a major upgrade to the experimental apparatus. A key element of the upgrade is the construction of a new silicon-based Inner Tracking System containing 12 Gpixels in an area of 10 m2. Its readout system consists of 192 readout units that control the pixel sensors and the power units, and deliver the sensor data to the counting room. A prototype readout board has been designed to test: the interface between the sensor modules and the readout electronics, the signal integrity and reliability of data transfer, the interface to the ALICE DAQ and trigger, and the susceptibility of the system to the expected radiation level.

Front end optimization for the monolithic active pixel sensor of the ALICE Inner Tracking System upgrade

ALICE plans to replace its Inner Tracking System during the second long shut down of the LHC in 2019 with a new 10 m2 tracker constructed entirely with monolithic active pixel sensors. The TowerJazz 180 nm CMOS imaging Sensor process has been selected to produce the sensor as it offers a deep pwell allowing full CMOS in-pixel circuitry and different starting materials. First full-scale prototypes have been fabricated and tested. Radiation tolerance has also been verified. In this paper the development of the charge sensitive front end and in particular its optimization for uniformity of charge threshold and time response will be presented.

Review of results for the NA62 gigatracker read-out prototype

The Gigatracker (GTK) is a hybrid silicon pixel detector developed for NA62, an experiment studying ultra-rare kaon decays at the CERN SPS. The main characteristics are a time-tagging resoluion of 150ps, with low material budget per station (0.5% X0) and a fluence comparable to the one expected for the inner trackers of LHC detectors in 10 years of operation. To compensate the time-walk, two read-out architectures have been designed and produced. The first architecture is based on a Constant Fraction Discriminator (CFD) followed by an on-pixel Time-to-Digital-Converter (TDC). The second architecture is based on a on-pixel group shared TDC. The GTK system developments are described: the integration steps (assembly and cooling) and the results obtained from the prototypes fabricated for the two read-out architectures.

Calibration of the Prompt L0 Trigger of the Silicon Pixel Detector for the ALICE Experiment

The ALICE Silicon Pixel Detector (SPD) is the innermost detector of the ALICE experiment at LHC. It includes 1200 front-end chips, with a total of ~10 pixel channels. The pixel size is 50 x 425 μm. Each front-end chip transmits a Fast-OR signal upon registration of at least one hit in its pixel matrix. The signals are extracted every 100 ns and processed by the Pixel Trigger (PIT) system, to generate trigger primitives. Results are then sent within a latency of 800 ns to the Central Trigger Processor (CTP) to be included in the first Level 0 trigger decision. This paper describes the commissioning of the PIT, the tuning procedure of the front-end chips Fast-OR circuit, and the results of operation with cosmic muons and in tests with LHC beam. I. SYSTEM DESCRIPTION ALICE (A Large Ion Collider Experiment) is one of the experiments at the Large Hadron Collider (LHC) at CERN, optimized to study the properties of strongly interacting matter and the quark-gluon plasma in heavy ion collisions [1][2]. The ALICE experiment is designed to identify and track particles with high precision over a wide transverse momentum range (100 MeV/c to 100 GeV/c). ALICE will also take data with proton beams, in order to collect reference data for heavy ion collisions and to address specific stronginteraction topics for which ALICE is complementary to the other LHC detectors. The Silicon Pixel Detector (SPD) is the innermost detector of the ALICE experiment, providing vertexing and tracking capabilities [5][6][7]. As shown in Figure 1, the SPD is a barrel detector with two layers at radii of 3.9 cm and 7.6 cm, respectively, from the beam axis. The minimum distance between the beam pipe and the inner layer is ~5 mm. The SPD consists of 120 detector modules, called half-staves. Each of them includes two silicon pixel sensors, flip chip bump bonded to 10 front-end readout chips realized in a commercial 0.25 μm CMOS process. One front-end chip contains 8192 pixel cells organized in 32 columns and 256 rows. The pixel dimensions are 425 × 50 μm (z × rφ); in total there are 9.83 × 10 pixels in the SPD. In order to maintain the material budget constraint of 1% X0 per layer, the sensor chosen thickness is 200 μm and the pixel chips are thinned to 150 μm. Signal and power connections for the chips are provided by an aluminium multilayer bus, glued on top of the ladders. The 10 front-end chips of each half-stave are connected to a Multi Chip Module (MCM). The MCM contains 4 ASICs and one optical transceiver module: they provide timing, control and trigger signals to the chips. The MCM performs the readout of the front-end chips sending the data to the offdetector electronics in the control room [8]. The MCM is connected to 3 single mode optical fibers; two of them are used to receive the serial control and the LHC clock at 40.08 MHz, and the third is used to send the data to the offdetector electronics. Figure 1: SPD (right) and one half-stave (left) Each of the 1200 front-end chips of the SPD may activate its Fast-OR output every 100 ns when at least one pixel inside the chip is hit by a particle. The 1200 Fast-OR bits are sampled and transmitted to the off detector electronics by the MCM. The Fast-OR generation capability is a unique feature among the vertex detectors of the LHC experiments. It allows the SPD to act also as a low latency pad detector that can be added to the first level trigger decision of the ALICE experiment. The Pixel Trigger (PIT) system [9] was designed to process the Fast-OR bits and produce a trigger output for the Level 0 trigger decision. It is composed of 10 OPTIN boards that receive the data streams coming from the 120 modules of the SPD and extract the Fast-OR bits; the OPTIN boards are mounted on a 9U board, called BRAIN, with a large FPGA (called Processing FPGA, type Xilinx Virtex4) that can apply

Performance of the ALICE SPD cooling system

The new generation of silicon detectors for particle physics requires very reduced mass and high resistance to radiations with very limited access to the detector for maintenance. The Silicon Pixel Detector (SPD) is one of the 18 detectors of the ALICE (A Large Io Collider Experiment) experiment at the Large Hadron Collider (LHC) at CERN. It constitute the two innermost layers of the Inner Tracking System (ITS) and it is the closest detector to th interaction point. An evaporative cooling system, based on C4F10 evaporation at 1.9 bar, was chosen to extrac the 1.35 kW power dissipated by the on-detector electronics. The whole system wa extensively tested and commissioned before its installation inside the ALICE experimenta area. Since then we had to deal with a decrease of the flow in some lines of the system tha imposed severe restrictions on the detector operation. Recently, a test bench has been built in order to carry out a series of tests to reproduce the misbehaviour of the system and investigat proper actions to cure the problem. The performance of the systems and the most interesting results of the above mentioned test will be presented.