A low mass pixel detector upgrade for CMS

The CMS pixel detector has been designed for a peak luminosity of 10^34cm-2s-1 and a total dose corresponding to 2 years of LHC operation at a radius of 4 cm from the interaction region. Parts of the pixel detector will have to be replaced until 2015. The detector performance will be degraded for two reasons: radiation damage of the innermost layers and the planned increase of the LHC peak luminosity by a factor of 2-3. Based on the experience in planning, constructing and commissioning of the present pixel detector, we intend to upgrade the whole pixel detector in 2015. The main focus is on lowering the material budget and adding more tracking points. We will present the design of a new low mass pixel system consisting of 4 barrel layers and 3 end cap disks on each side. The design comprises of thin detector modules and a lightweight mechanical support structure using CO2 cooling. In addition, large efforts have been made to move material from the services out of the tracking region.Comment: 10 pages, 10 figures, Proceeding of RD09 - 9th International Conference on Large Scale Applications and Radiation Hardness of Semiconductor Detectors 30 September - 2 October 2009, Florence, Ital

Radiation hardness of CMS pixel barrel modules

Pixel detectors are used in the innermost part of the multi purpose experiments at LHC and are therefore exposed to the highest fluences of ionising radiation, which in this part of the detectors consists mainly of charged pions. The radiation hardness of all detector components has thoroughly been tested up to the fluences expected at the LHC. In case of an LHC upgrade, the fluence will be much higher and it is not yet clear how long the present pixel modules will stay operative in such a harsh environment. The aim of this study was to establish such a limit as a benchmark for other possible detector concepts considered for the upgrade. As the sensors and the readout chip are the parts most sensitive to radiation damage, samples consisting of a small pixel sensor bump-bonded to a CMS-readout chip (PSI46V2.1) have been irradiated with positive 200 MeV pions at PSI up to 6E14 Neq and with 21 GeV protons at CERN up to 5E15 Neq. After irradiation the response of the system to beta particles from a Sr-90 source was measured to characterise the charge collection efficiency of the sensor. Radiation induced changes in the readout chip were also measured. The results show that the present pixel modules can be expected to be still operational after a fluence of 2.8E15 Neq. Samples irradiated up to 5E15 Neq still see the beta particles. However, further tests are needed to confirm whether a stable operation with high particle detection efficiency is possible after such a high fluence.Comment: Contribution to the 11th European Symposium on Semiconductor Detectors June 7-11, 2009 Wildbad Kreuth, German

Measurement of D* Meson Cross Sections at HERA and Determination of the Gluon Density in the Proton using NLO QCD

With the H1 detector at the ep collider HERA, D* meson production cross sections have been measured in deep inelastic scattering with four-momentum transfers Q^2>2 GeV2 and in photoproduction at energies around W(gamma p)~ 88 GeV and 194 GeV. Next-to-Leading Order QCD calculations are found to describe the differential cross sections within theoretical and experimental uncertainties. Using these calculations, the NLO gluon momentum distribution in the proton, x_g g(x_g), has been extracted in the momentum fraction range 7.5x10^{-4}< x_g <4x10^{-2} at average scales mu^2 =25 to 50 GeV2. The gluon momentum fraction x_g has been obtained from the measured kinematics of the scattered electron and the D* meson in the final state. The results compare well with the gluon distribution obtained from the analysis of scaling violations of the proton structure function F_2.Comment: 27 pages, 9 figures, 2 tables, submitted to Nucl. Phys.

Technical design of the phase I Mu3e experiment

The Mu3e experiment aims to find or exclude the lepton flavour violating decay $\mu \rightarrow eee$ at branching fractions above $10^{-16}$. A first phase of the experiment using an existing beamline at the Paul Scherrer Institute (PSI) is designed to reach a single event sensitivity of $2\cdot 10^{-15}$. We present an overview of all aspects of the technical design and expected performance of the phase~I Mu3e detector. The high rate of up to $10^{8}$ muon decays per second and the low momenta of the decay electrons and positrons pose a unique set of challenges, which we tackle using an ultra thin tracking detector based on high-voltage monolithic active pixel sensors combined with scintillating fibres and tiles for precise timing measurements.Comment: 114 pages, 185 figures. Submitted to Nuclear Instruments and Methods A. Edited by Frank Meier Aeschbacher This version has many enhancements for better readability and more detail

Technical design of the phase I Mu3e experiment

The Mu3e experiment aims to find or exclude the lepton flavour violating decay μ→eee at branching fractions above 10−16. A first phase of the experiment using an existing beamline at the Paul Scherrer Institute (PSI) is designed to reach a single event sensitivity of 2⋅10−15. We present an overview of all aspects of the technical design and expected performance of the phase I Mu3e detector. The high rate of up to 108 muon decays per second and the low momenta of the decay electrons and positrons pose a unique set of challenges, which we tackle using an ultra thin tracking detector based on high-voltage monolithic active pixel sensors combined with scintillating fibres and tiles for precise timing measurements

Radiation tolerance of the readout chip for the phase I upgrade of the CMS pixel detector

For the Phase I upgrade of the CMS pixel detector a new digital readout chip (ROC) has been developed. An important part of the design verification are irradiation studies to ensure sufficient radiation tolerance. The paper summarizes results of the irradiation studies on the final ROC design for the detector layers~$2-4$. Samples have been irradiated with 23\,MeV protons to accumulate the expected lifetime dose of 0.5\,MGy and up to 1.1\,MGy to project the performance of the ROC for layer~1 of the detector. It could be shown that the design is sufficiently radiation tolerant and that all performance parameters stay within their specifications. Additionally, very high doses of up to 4.2\,MGy have been tested to explore the limits of the current chip design on 250\,nm CMOS technology. The study confirmed that samples irradiated up to the highest dose could be successfully operated with test pulses

Development of an Indium Bump Bond Process for Silicon Pixel Detectors at PSI

The hybrid pixel detectors used in the high energy physics experiments currently under construction use a three dimensional connection technique, the so-called bump bonding. As the pitch below 100um, required in these applications, cannot be fullfilled with standard industrial processes (e.g. the IBM C4 process), an in-house bump bond process using reflown indium bumps was developed at PSI as part of the R&D for the CMS-pixel detector. The bump deposition on the sensor is performed in two subsequent lift-off steps. As the first photolithographic step a thin under bump metalization (UBM) is sputtered onto bump pads. It is wettable by indium and defines the diameter of the bump. The indium is evaporated via a second photolithographic step with larger openings and is reflown afterwards. The height of the balls is defined by the volume of the indium. On the readout chip only one photolithographic step is carried out to deposit the UBM and a thin indium layer for better adhesion. After mating both parts a second reflow is performed for self alignment and obtaining a high mechanical strength. For the placement of the chips a manual and an automatic machine was constructed. The former is very flexible in handling different chip and module geometries but has a limited throughput while the latter features a much higher grade of automatisation and is therefore much more suited for producing hundreds of modules with a well defined geometry. The reliability of this process was proven by the successful construction of the PILATUS detector. The construction of PILATUS 6M (60 modules) and the CMS pixel barrel (roughly 800 modules) will start in 2005