CMS is a general-purpose proton-proton detector designed to run at center of mass energies 14 TeV and highest luminosity L=10³⁴ cm^-2 s^-1 at the Large Hadron Collider (LHC). All subdetector systems are currently under construction. CMS will be ready for the first collisions expected in April 2007.The short bunch crossing time at the LHC (25ns) places challenging requirements on the readout electronics.Furthermore, both the detectors and electronics have to withstand high levels of irradiation. Beam tests using prototype detectors and electronics designed in the 0.25μm technology have confirmed the expected performance.

Magnet

   The detector will be built around a long (13 m) and large bore (Φ=5.9 m) high-field (4T) superconducting solenoid leading to a compact design for the muon spectrometer.The magnetic flux is returned through 1.5 m of saturated iron yoke (1.8 T) instrumented with muon chambers.

Inner Tracking

   All high P_t muons, isolated electrons and charged hadrons, produced in the central rapidity region, are reconstructed with a momentum precision of ΔP_t / P_t » 0.005 + 0.15 P_t ( P_t in TeV), a direct consequence of the high magnetic field. The tracking volume is given by a cylinder of length 6 m and diameter 2.6 m. In order to deal with high track multiplicities tracking detectors with small cell sizes are used. Silicon microstrip detectors provide the required granularity and precision in the bulk of the tracking volume. Stereo information is provided by back-to-back microstrip detectors with strips at a small angle. Pixel detectors placed close to the interaction region improve the measurement of the track impact parameter and secondary vertices. High track finding efficiencies are achieved for isolated high P_t tracks and for tracks in jets.

Muon System

   Centrally produced muons are measured three times, in the inner tracker, after the coil and in the return flux. They are then identified and measured in four identical muon stations (MB) inserted in the return yoke. Each muon station consists of twelve planes of aluminium drift tubes designed to give a muon vector in space, with 100μm precision in position and better than 1 mrad in direction. The four muon stations include RPC triggering planes that also identify the bunch crossing and enable a cut on the muon transverse momentum at the first trigger level. The endcap muon system also consists of four muon stations (ME). Each station consists of six planes of Cathode Strip Chambers. The chambers are arranged such that all muon tracks traverse four stations at all rapidities, including the transition region between the barrel and the endcaps. The last muon stations are after a total of ³ 20λ of absorber, so that only muons can reach them. The combined (inner tracker as well as muon chambers) muon momentum resolution is better than 5% at 0.3 TeV in the central rapidity region ½η½< 2, and 10% for P_t= 2 TeV. Low-momentum (P_t< 100 GeV) muons are measured before the absorber with a precision of about 1.5% up to a pseudorapidity of 2.

Calorimetry

   The coil radius is large enough to install essentially all the calorimetry inside and hence avoid the coil-electromagnetic calorimeter interference. A high precision electromagnetic calorimeter (ECAL) using lead tungstate (PbWO₄) crystals has been chosen. Lead tungstate is a dense and relatively easy crystal to grow. The scintillation light is detected by silicon avalanche photodiodes in the barrel region (EB, ½η½< 1.48) and vacuum phototriodes in the endcap region (EE, 1.48 <½η½< 3.0). The expected energy resolution is better than 0.6% for electrons and photons with energies greater than 75 GeV. A preshower system (SE), made of silicon sensors, is installed in front of the endcap calorimeter (1.65£½η½£ 2.6). The ECAL is followed by a copper/scintillator sampling hadronic calorimeter (HB, HE). The light is channelled by clear fibres fused to wave-length shifting fibres embedded in scintillator plates. The light is detected by photodetectors that can provide gain and operate in high axial magnetic fields (proximity focussed hybrid photodiodes). Coverage up to rapidities of 5.0 is provided by a steel/quartz fibre calorimeter (HF). The Cerenkov light emitted in the quartz fibres is detected by photomultipliers. The front-end chain consists of a preamplifier/range selector (FPPA), an ADC and a serializer/optical link. A 0.25μm technology version of the serializer has been chosen. Photon-pizero separation in the forward region requires a preshower detector (SE) in front of the crystals.

Trigger and Data Acquisition

   The trigger and data acquisition consists of four parts: the front-end detector electronics, the calorimeter and muon first level trigger processors, the readout network and an on-line event filter system. The first two parts are synchronous and pipelined with a pipeline depth corresponding to »3μs. The latter two are asynchronous and based on industry standard data communication components and commercial PCs. The resources that would have been required for a hardware second level trigger processors are invested in a high bandwidth (» 500 Gbit/s) readout network and in the event filter processing power (10⁶-10⁷ MIPs), both of which are more suitable for upgrading as commercially available technology develops. The CMS Level-1 trigger decision is based upon the presence of physics objects such as muons, photons, electrons, and jets, as well as global sums of E_t and missing E_t (to find neutrinos). The DAQ system has to assemble the data from the triggered event, contained in about 500 front-end buffers (readout units), into a single processor in a ``farm'' for executing physics algorithms so that the input rate of 100 kHz is reduced to 100 Hz of sustainable physics. A new Event Builder setup has been installed that consists of 64 Intel-PCs interconnected by two networks based on the most advanced technologies: a 64 port Gbit Ethernet (Foundry) and a 128-port Myrinet switch (Myricom).

Computing and Core Software

   For complex systems, such as the CMS detector, an ‘object oriented’ approach, implemented in C++, is now the choice of software developers. C++ releases have been made of the functional prototypes of the software, comprising the framework (CARF), the reconstruction program (ORCA), a basic GEANT4-based simulation program (OSCAR) and an interactive graphics toolkit (IGUANA). The OO technology has been used in the production of Level-1 and High Level Trigger simulation data. ORCA has been used for detector, trigger and physics studies. The data storage, networking and processing power needed to analyze CMS data is well in excess of those of today's facilities. Technological advances will help to make the data analysis possible in a distributed environment, where physicists are scattered all over the world. The optimum mix of storage, networking and processing will change as technology develops. A multi-Tier model, similar to that developed by the MONARC project, underpinned by Grid Technology to provide efficient resource utilization and rapid turnaround time will be prototyped.

Physics Reconstruction and Selection

   Physics simulation work focuses on the development of the eventual reconstruction code. This development is taking place using C++ and object-oriented methods. The first priority is a full understanding and verification of the Higher Level Triggers (HLT). Since CMS does not employ distinct physical intelligences for the would-be Level-2 and Level-3 triggers, but only a single processor farm, the task of selecting events is intimately linked with that of reconstructing the associated detector information online. With this in mind, four ``Physics Reconstruction and Selection'' (PRS) groups started (electron/photon, muon, jet/missing E_t, and b/τ vertexing). The aim of the groups is to develop the reconstruction and selection procedures (algorithms and software) starting from the output of the Level-1 trigger, and aiming ultimately at the full off-line reconstruction. During 2000, the four groups delivered the first algorithms that correspond to a reduction of the event rate after Level-1 by about a factor of 10 using information from single CMS sub-detectors. The activity now continues as a new CMS project, the PRS project, which has close ties with the Computing and Trigger/DAQ projects. The PRS groups are working on reconstructing physics objects using information from multiple CMS sub-detectors.

Physics Performance

   Although high luminosity is essential to cover the entire range of mechanisms of electroweak symmetry-breaking, the LHC machine will start at significantly lower luminosities (L £ 10³³ cm^-2s^-1). The pixel detectors and the PbWO₄ crystal electromagnetic calorimeter considerably enhance the discovery potential of CMS at low luminosities. A Standard Model (SM) Higgs boson with mass between 95 and 150 GeV would be discovered via its two-photon decay after an integrated luminosity of about 3x10⁴ pb^-1. The same integrated luminosity gives a discovery range covering masses from 135 to 525 GeV in the four lepton (e or μ) channel. An integrated luminosity of 10⁵ pb (taken at 10³⁴ cm^-2 s^-1) would allow discovery via these channels over the full range between 85 and 700 GeV. Tagging the events produced by WW- and ZZ-fusion by detecting characteristic forward jets, and using decay modes with larger branching ratios (H ®WW®lνjj, and H® ZZ ®lljj), should allow the discovery range for a SM Higgs boson to be extended up to 1 TeV for the same integrated luminosity. The two photon and four lepton channels are also crucial for the discovery of a Higgs boson in the Minimal Supersymmetric Standard Model (MSSM). Various channels involving the tau lepton (h⁰, H⁰, A⁰ ® τ τ, H ® τ ν) help to cover much of the remaining allowed (m_Α, tanβ) parameter space. Precise impact parameter measurements using the pixel detector play an important role here. Many physics studies have been carried out in the context of supergravity models (SUGRA). A many-point scan of the gaugino / scalar mass parameter space has been conducted. Squarks and gluinos up to 2 TeV can be detected using, as signature, events with one or more charged leptons, missing transverse energy and two or more jets. Sleptons as much as 400 GeV can be found by looking for events without hadronic jets, but with lepton pairs and missing transverse energy with distinctive kinematic characteristics. Three-lepton states are particularly promising for the detection of charginos and neutralinos. In many cascade decays a heavier neutralino is produced that subsequently decays into the lightest one with the emission of a pair of charged leptons. For low to moderate values of tanβ the spectrum of the di-lepton invariant masses shows a strikingly sharp end-point determined by the difference in neutralino masses. This feature can be used to select and almost fully reconstruct some events yielding e.g. the mass of the bottom squark. The above studies of specific SUSY models indicate that it is possible to detect and measure a large fraction of the expected SUSY spectrum in CMS. Within the SUGRA models it should be possible to determine the fundamental parameters at the GUT scale. The copious production of B mesons at LHC opens the way for significant measurements of CP violation effects in the B system. Using the B⁰_s and B⁰_d channels two of the angles in the unitarity triangle can be measured. Furthermore, by observing the time development of B⁰_s oscillations, the mixing parameter x^s can be measured. In addition to running as a proton-proton collider, LHC will be used to collide heavy ions at a centre of mass energy of 5.5 TeV per nucleon pair. A new form of deconfined hadronic matter, the quark-gluon plasma (QGP), should be formed at the resulting high energy densities (4-8 GeV/fm³). The formation of quark-gluon plasma in the heavy ion collisions is predicted to be signaled by a strong suppression of Y¢and Y¢¢ production relative to Y production when compared to pp collisions. The CMS detector is well suited to detect low momentum muons and reconstruct the Y, Y¢and Y¢¢ mesons produced. The good mass resolution (σ=37 MeV at Υ mass) afforded by the 4T field, enables the measurement of the suppression.