Jorma Tuominiemi
The HIP CMS Programme carries the responsibility for the Finnish participation in the Compact Muon Solenoid (CMS) experiment at the CERN Large Hadron Collider (LHC). The CMS experiment is designed to study proton-proton collisions at 14 TeV collision energy at LHC and also heavy ion collisions. The main scientific goals of CMS are the study of the mechanism of the spontaneous breaking of the electroweak symmetry (search for Higgs bosons) in the Standard Model of the basic structure of matter and the quest for new physics beyond the Standard Model, such as supersymmetry and extra dimensions. It also includes a heavy ion research programme, searching for quark gluon plasma. The CMS detector concept was first proposed in 1990, and the Finnish team has played an important role in its development from the beginning. The HIP CMS team hence has an extensive and thorough knowledge of the key features of the experiment. With the CMS experiment HIP will be in the frontline of High Energy Physics research which will take the next fundamentally important step in understanding the basic structure of matter and the origin of the Universe. The LHC experiments are scheduled to begin in 2008. The year 2007 was the final year for the completion of the CMS subdetectors. The Tracker was successfully integrated and commissioned in the Tracker Installation Facility in Hall B186 in March. To test the apparatus, more than five million cosmic muon events were recorded during the following months. Their analysis has shown that the detector perfectly meets the design specifications. At the end of the year the Tracker was successfully lowered into the underground cave (UXC) and installed in the CMS apparatus. Now all the subdetectors except the end-caps of the electromagnetic calorimeter and the pixel detectors are in place. The data acquisition and the trigger systems have been installed and are being tested with cosmic muons. The Computing, Simulation and Analysis exercise (CSA07) using the CMS Tier0-Tier1-Tier2 system at a level of 50% of the final event rate was done in autumn. The system performed well and tuning to the final performance is advancing. Preparation for the physics analysis continued in many ways. Within CSA07 several analysis projects were done with success. A number of analysis projects aiming at a published paper on the first data to be collected were conducted to test the reconstruction and analysis tools with simulated data. The HIP CMS Programme was divided into two projects: 1) the new CMS Physics Analysis project, the goal of which is to develop simulation and analysis software for the CMS experiment, and to perform the physics analysis once the data taking starts, 2) the CMS Tracker project which has carried responsibilities in the completion of the Outer Barrel part of the silicon tracking detector as well as its testing and research and development on radiation hard silicon sensors for future tracking detectors.
CMS Physics Analysis
Introduction
With the LHC start-up approaching, the work in the project focused more and more on the physics analysis and simulation. Yet a considerable amount of work in 2007 was devoted to the experimental support and services. The physics analysis work concentrated on the studies of MSSM Higgs searches for which HIP was assigned responsibility for the specific channel gb → tH±, H± → τν, with fully hadronic t and τ decays. HIP continues the important co-ordination work on the computing and off-line software support for the CMS users worldwide. The LHC data handling in Finland experienced a major breakthrough in 2007, when the Ministry of Education granted funding for Tier2 computing for the years 2008-2009. The development work of hadronic shower simulation for Geant4 continued. In the field of detector alignment, work using the HIP algorithm made steady progress. The test beam analysis work on the Helsinki Silicon Beam Telescope was recovered in terms of the CMSSW package. A more detailed description of all these activities is presented below.
The project team consisted of seven PhD physicists V. Karimäki, R. Kinnunen, T. Lampén, K. Lassila-Perini, S. Lehti, T. Lindén, J. Tuominiemi, four PhD students A. Heikkinen, M. Kortelainen, L. Wendland, M. Voutilainen (adjoint member) and an undergraduate student P. Kaitaniemi.
Physics analysis and simulation
In 2007 the HIP group took responsibility for performing the detailed preparative analysis for the search of the charged MSSM Higgs bosons in the gb→ tH±, H± → τν channel with fully hadronic final state assuming an integrated luminosity of 100 pb-1. The work published in 2006 in the second volume of the Technical Design Report on Physics serves as a basic guideline for the analysis. The working group was enlarged to contain physicists from TIFR (Mumbai), Imperial College (London), the University of Pisa, the University of Florida and DESY (Hamburg). The HIP group has also responsibility for testing and maintaining the single-Tau trigger aimed to choose the hadronic H± → τν decay mode for the searches of charged Higgs bosons.
The gb → tH±, H± → τν channel is subject to large backgrounds from ttbar, W+jet and QCD multi-jet events. Analysis for measuring these backgrounds exploiting the muons from W decays in ttbar and W+jet events and the production of QCD multi-jet events with the early LHC data was started in 2007. This analysis is very challenging due to uncertainties associated with the missing transverse energy measurement and the possibility of fake energetic tracks in the jets. Further difficulties arise from the many possible sources of missing transverse energy in ttbar events and events from the Breit-Wigner tail of W in W+jet events being a potential background source. Work was started to improve the identification of the energetic τ jet from the H± → τν decay with the Particle Flow methods and with combined calorimetric and tracker methods in collaboration with the CERN Particle Flow group. The effect of misalignment of the tracker modules for τ identification was investigated with different misalignment scenarios. It was found that the impact of the tracker misalignment changes the signal acceptance and background rejection with only a few per cent.
The group also participated in the Higgs boson physics in non-standard scenarios. The production of the MSSM Higgs bosons was investigated in collaboration with the HIP Phenomenology group. The SUSY cascade models with non-universal gaugino masses at the unification scale were used and a CMS analysis note was written on the possible discovery. Prospects for finding the very light NMSSM Higgs bosons were studied with theorists in H → AA→ 4τ final state. The signature for this channel is two τ jets with oppositely charged muons in the τ reconstruction cones, which allows effective suppression of potential backgrounds.
The group was active in the development of new tools and methods for data analysis: a ROOT-based analysis package was written to facilitate the CMS data analysis in remote laboratories. The newly founded Physics Analysis Task Force of CMS is one of the projects. Multivariate analysis tools (TMVA) were tested with b-tagging jets in the backgrounds for the H → ττ channel.The b-tagging efficiency was estimated with several methods available in the ROOT-based TMVA package, and compared with the traditional cut-based track counting b-tagging method.
The results were presented regularly in the working group meetings at CERN and in group meetings and seminars in HIP. The group members participated in international conferences and workshops giving talks on the Higgs boson discovery potential at the LHC.
User support co-ordination
The HIP team continued the all-CMS co-ordination of the user support for physicists using the CMSSW package for data analysis. In 2007, the CMS Software Documentation Policy was written. It defines the different actors and their responsibilities in the CMS Software Documentation and its maintenance. The CMSSW documentation consists of three main elements: (i) the CMS Offline WorkBook, (ii) the CMS Offline Guide and (iii) the CMSSW Reference Manual. The WorkBook contains the instructions on accessing computing resources and using the software to perform analysis within the CMS collaboration. This document has been constantly updated during 2007, and a major review is planned for early 2008.
The Offline Guide gives full details of CMS Software including descriptions of algorithms and software architecture, instructions for analysis and validation and common items such as developers' guide, trouble shooting guide, FAQ lists, and instructions for code optimization. The structure for this document was established in 2007, and most documentation material has been moved in this structure.
The Reference Manual contains the release dependent technical documentation such as C++ class lists, a brief description of contents and purpose of each package and default configuration files and sequences. A documentation plan was drafted to guide the work towards a complete CMSSW documentation suite. As the first step, documentation contact persons were nominated in each off-line and physics subproject, and the priorities were set up. This work will continue in 2008, with the aim to complete the main areas of the CMS Offline Guide.
Computing activities
The major breakthrough in 2007 was that annual funding for LHC computing resources of 800 kEUR/year was granted in 2007 by the Ministry of Education for the years 2008, 2009 and 2010. HIP and CSC signed a contract to formalize the LHC computing project collaboration and the acquisition of computing resources with the 2.4 MEUR funding obtained and to set up a new common project to carry out the needed work. The work on preparing and obtaining LHC computing resources was made in a very close collaboration between the HIP CMS and Technology Programmes, CSC, and NDGF. This close collaboration resulted in several advances in many areas of CMS computing that are summarized here, for further details see also the Technology Programme chapter.
Hardware activities. An agreement was reached between HIP and CSC that the Sepeli cluster at CSC would be turned over gradually to HIP in 2007-2008 for LHC computing usage, so that it would be dedicated for LHC computing by the summer of 2008. CSC started acquisition of a 170 TB dCache disk system to be available in 2008 for LHC computing. CSC and the HIP Technology Programme created each independent dCache test systems. The Ametisti cluster in Kumpula was upgraded with 128 CPUs on loan from CSC from the old part of Sepeli.
Software activities. CMSSW evolved so that it could be installed directly on 64-bit machines on the M-Grid at CSC and in Kumpula. Runtime environments for CMSSW and ROOT were created for enabling M-Grid usage of these applications. An ARC-plugin for the CMS Monte Carlo production tool ProdAgent was created as a contribution by NDGF. PhEDEx was upgraded and configured on the Silo3 file server in Kumpula to use the three available dCache test systems at CSC, at the Technology Programme and in Kumpula (a dCache pool connected to the NDGF test dCache server). The operating system on Mill and Testbed1 was upgraded to Rocks 4.3. A temperature monitoring tool for the Ametisti nodes was developed and taken into usage allowing automatic shutdown in case of machine room cooling problems. The Technology Programme maintained ARC on the Grid resources.
A tutorial on the CMS software framework, CMSSW was organised in Kumpula on the 19th of March 2007 using the Mill cluster with almost 20 participants, including the teachers.
Tracker alignment
The team continued to work on the track-based calibration of the sensor positions in the CMS Pixel and Strip detectors. This calibration task, called detector alignment, aims at finding corrections to sensor positions and orientations to achieve the optimal trajectory reconstruction. It will have a great importance for the physics performance of the CMS. The HIP team has participated actively in the work of the CMS Tracker alignment group, especially in developing alignment methods and strategies for the Pixel detector.
Responsibilities of the team included also a comprehensive revision of the misalignment scenarios for the CMS Tracker. The alignment algorithm implemented by the HIP team has been widely used by the CMS alignment group. In addition to its earlier use, it has now been applied also to the CSA07 exercise as well as to the partial alignment of the CMS with cosmic muon data recorded in the spring and summer of 2007. The HIP team has also applied the algorithm to the data analysis of the upgraded FinnCRack in Helsinki together with the HIP Tracker project.
Geant4 development
The Geant4 version 9.1 was released in December with significant improvements in hadronic physics, such as enhanced modelling of the pre-compound and evaporation phases. Development of the Bertini models included improvements in the cross-section and in the Coulomb barrier treatment. In a systematic validation study of all Geant4 hadronic models applied to a hadron treatment with 60 MeV protons, the Bertini models were found to provide the best agreement with experimental data.
The release also included the first fully functional versions of the INCL hadronic cascade and ABLA evaporation codes. The models passed the Geant4 system tests and a detailed physics validation started. The new simulation features provided by these physics models are needed to ensure accurate hadronic shower calibration for the next generation of calorimeters. Furthermore, INCL extends the intranuclear cascades to ion-ion processes. The development of INCL and ABLA models for Geant4 takes place in a close collaboration with the original developers as a common project between HIP and CEA, Saclay (Commissariat à l'Énergie Atomique). Being theoretically motivated the introduced Geant4 models of hadronic interactions are now more and more frequently used for the CMS detector simulation, replacing a more traditional approach which relies on the parameterization of experimental data.
Test beam off-line data analysis
The test beam activities included code development for the off-line data analysis of the HIP Silicon Beam Telescope (SiBT) in a close collaboration with the CMS Tracker project. The analysis code was successfully implemented in the CMSSW framework. The alignment and a detailed analysis of the reference telescope were successfully achieved.
Outreach activities
The group members were active in introducing CERN and the LHC experiments to visitor groups from Finland. The groups included high school students and members of the Finnish Parliament. The group also approached the general public by writing popular articles on the status and goals of the LHC experiments. A series of popularizing articles about particle physics topics and the construction of the CMS experiment was published on the Internet for high school students. An initiative was realized to form a meeting place for the rapidly growing Geant4 medical user community of Europe. It is an Internet based collaborative platform for "The Geant4 European Medical User Organization", G4EMU.
CMS Tracker
In March 2007, the achievements of the CMS Tracker project 2001-2007 were evaluated by two experts, Prof. Mike Tyndel from Rutherford Appleton Laboratory and Dr. Christian Joram from CERN. The evaluators summarized their observations with the words: "We were impressed by the scientific achievements of the HIP project team over the last 3 years. It can be difficult for a relatively small team to make an impact and achieve visibility in such a large project as CMS and the team should be congratulated on what they have achieved - with leading contributions to the design, construction and assembly of the silicon tracker support structure, the silicon telescope for detailed performance studies, the trigger electronics and the pioneering work on the development of radiation hard silicon sensors". Based on the evaluators' recommendations, the HIP Scientific Advisory Board recommended and the HIP Board accepted the continuation of the Tracker project for the years 2008-2010 under the name "Operation of CMS Tracker".
In 2007, the HIP CMS Tracker project concentrated on three main activities, all related to the development of silicon tracking detectors: 1) commissioning of the CMS Tracker at CERN, 2) testing of silicon detectors using the Finnish Cosmic Muon Rack (FinnCRack) and 3) development of radiation hard silicon detectors for future high energy physics experiments. Among the main achievements during the past year were the final installation of the CMS Tracker inside the CMS and the upgrade and successful beam tests of the Helsinki Silicon Beam Telescope (SiBT).
Commissioning of the CMS Tracker
During 2007, the Finnish Mechanics team at CERN, consisting of one engineer and three technicians, participated in the installation and commissioning of the CMS Tracker. In early 2007 all the Tracker subdetectors were installed inside the Support Tube, the Tracker was sealed and a 15% slice of the detector was connected to cooling and to final hardware for powering and readout. During summer 2007, many scientists from the HIP CMS Tracker project participated in the cosmic muon test runs in the Tracker Integration Facility.
In July 2007, the Tracker slice test was disconnected and preparations for transport and installation started. During the year the CMS YB0 in the CMS cavern was also prepared to receive the Tracker: cables and cooling pipes were installed from outside the CMS to the patch panels inside the magnet and from that point on the empty service channels were installed up to the Tracker level. During the year, studies had been made of the thermal behaviour of the Tracker patch panels and service channels; accordingly a thermal stabilization system had been developed and was installed to maintain good temperatures in them in different working conditions. Also tools, working methods and pre-made components, especially for the cooling pipe work and thermal insulation, were developed to make the coming connection and checkout work efficient.
In December, the CMS Tracker was successfully transported to LHC Point 5, lowered into the CMS cave and fixed to its final position in the CMS.
Finnish Cosmic Rack (FinnCRack)
The Finnish Cosmic Rack (FinnCRack) is a telescope built from eight layers of CMS silicon detectors measuring tracks of cosmic particles. FinnCRack is constructed using components of the CMS Tracker Outer Barrel (TOB), mimicking a six degree (in azimuth) sector of the TOB barrel structure. It is used as a test station providing data for testing of the TOB subsystem level functionality as well as for development of TOB software, e.g., analysis software (CMSSW), software alignment code, on-line software, and run control software. It also provides a platform for the testing of the novel radiation hard detectors for the CMS upgrade and for future collider experiments.
FinnCRack was initially constructed and tested at CERN as a joint effort with the CERN (TOB) group. In 2006 it was transported to the Kumpula Detector Laboratory and recommissioned there. In 2007, the facility for the cooling of the FinnCRack silicon detector modules was finally installed and commissioned. The cooling system includes both a cooling house and cooling-liquid-circulation for the detector modules. In addition, work has been proceeding for the upgrade of the FinnCRack electronics, i.e., trigger system, control and monitoring electronics, and high and low voltage power supplies. FinnCRack also now features accurate flow meters to control the temperature, humidity and coolant flow. Automatic interlog system hardware have been installed to allow continuous operation during nights and weekends.
In 2007, FinnCRack became an operational part of the infrastructure of the Kumpula Detector Laboratory. It is an essential both in the operation of the current CMS Tracker and in the development of novel detectors for the CMS Tracker Upgrade.
Development of Radiation Hard Silicon Detectors, including the Silicon Beam Telescope (SiBT)
During 2007, the research on radiation hard silicon detectors continued in the framework of the CMS Upgrade programme and of the CERN RD39 (60 members, 15 institutes) and RD50 (280 members, 55 institutes) research programmes. This network links together practically all important research groups worldwide in this field and provides access to a wide selection of characterization and simulation tools. In addition, the close collaboration with the University of Helsinki (HU) Accelerator Laboratory continued for detector irradiations and for the development of a simultaneous cryogenic current-capacitance-voltage measurement system. Furthermore, electrical detector characterizations were done at the HIP Detector Laboratory. Some detector characterization was also done in co-operation with the Laboratory of Electrical Engineering of Lappeenranta University of Technology (LUT).
In 2007, the Helsinki Silicon Beam Telescope (SiBT) at the CERN SPS H2 test beam area was completely rebuilt in collaboration with the Fermilab, University of Karslruhe and University of Rochester CMS groups. The SiBT is a reference telescope that accurately measures the tracks of energetic particles for comparison with the measurements from the detector to be tested. The readout electronics and data acquisition (DAQ) of the upgraded SiBT consists of CMS Tracker Outer Barrel hybrids and prototype CMS Tracker data acquisition cards. The telescope contains eight reference detector planes in ±45 degree orientation and has two slots for the devices to be tested. The reference sensors are originally designed for the Fermilab D0 Run IIb. The telescope can be cooled down to a temperature of -20°C, which is very beneficial when studying highly irradiated sensors. The interpolated position resolution of the rebuilt SiBT is 9 μm, the signal-to-noise-ratio (S/N) of the reference detectors is 25 and the active area 4 × 4 cm2.
During the summer 2007 SiBT beam period, several magnetic Czochralski silicon (MCz-Si) sensors were tested. These detectors were manufactured by our group at the Micro and Nanofabrication Center (MINFAB) of Helsinki University of Technology (HUT). The test results were in close agreement with the predictions of the numerical simulations done by our team. The preliminary data analysis indicates a S/N of 41 for a non-irradiated sensor, S/N of 25 for a sensor irradiated with 1E14 cm-2 1 MeV neutron equivalent proton fluence and a S/N of 19.5 with a 5E14 cm-2 1 MeV neutron equivalent proton fluence. These results are measured at 0°C.
Based on the summer 2007 SiBT beam test results, it can be concluded that MCz-Si is a promising material for the Super-LHC Outer Strip Tracker (fluence 2×10E15) but not for the Super-LHC Inner Pixel Tracker (fluence 1×10E16), where a more radiation hard solution should be found. To this end, our group is developing a CID (Current Injected Detector) concept together with the CERN RD39 R&D Programme and St. Petersburg Ioffe Institute. In this R&D work, a cryogenic TCT measurement set-up built by our group for RD39 is an important tool. During 2007, this cryo-TCT was upgraded with adjustable low-noise and high-frequency electronics as well as with new test control and calibration instruments. In the collaboration between Ioffe Institute and HIP, our group has also participated in the dicing of TOTEM edgeless silicon detectors at MINFAB.
One of the consequences of the successful commissioning of the telescope and the first beam tests with proton irradiated MCz-Si detectors, was that SiBT was approved as part of the official CMS Super-LHC Upgrade programme in December 2007.
RPC Trigger
Trigger data Link Boards (LB) for the CMS Resistive Plate Chambers (RPCs) have been designed, constructed and tested by the HIP group at the Microelectronics Laboratory and Electronics Design Center of Lappeenranta University of Technology (LUT). The boards have been assembled at Electro Hill Ltd. Altogether 906 Slave-LBs, 510 Master-LBs (ML), 220 Control Boards (CB) and 220 Front Plane Boards (FB) have been produced.
RPCs will be used in the CMS as dedicated trigger detectors both in the barrel and in the end-caps. There are altogether some 200,000 channels in the CMS RPC system, divided into 96 strip chambers. RPC detectors are very fast and the Link Boards are used to compress and multiplex the data before it is sent to the Level-1 trigger and Data Acquisition (DAS) via 1.6 GB/s optical links.
The last batch of seven boxes consisting Link Boards were delivered to CERN during spring 2007. Each box contained 60 Link Boards. Most of the crates and boards were installed and tested in situ at SX5. The CCU ring originally designed for the CMS Tracker had some ground loop problems which were measured and cured. A special isolation transformer was designed and tested to operate in the high magnetic field.
Design work of the RE1/1 Link Boards was started and is continuing. These boards are needed in the CMS forward direction. The boards have smaller layouts and use liquid cooling. These boards will be mounted into special boxes designed previously by the HIP CMS Tracker Mechanics group. The boxes had prior to this been mounted into CMS due to the mechanical construction of the experiment.