The Status of CMS

The Status of CMS Joseph R. Incandela Physics Department, University of California, Santa Barbara, California, 93106, USA Abstract. After a brief overview of the Compact Muon Solenoid (CMS) experiment, the status of construction, installation and commissioning is described. Very good progress has been achieved in the past year. Though many significant challenges still lie ahead, CMS should be ready for recording data from first collisions in the Large Hadron Collider (LHC) accelerator complex at CERN in late 2007. Keywords: CMS, LHC, Higgs PACS: 14.80.Bn, 11.60.Jv

INTRODUCTION Although the Standard Model (SM) of particle physics has proven to be extremely successful in its description of nature at the highest energies and smallest distance scales accessible to physics to date, it is by no means a complete theory and leaves many important questions unanswered. A key tenet of the SM is the necessity of electroweak symmetry breaking (EWSB) for which the Higgs mechanism is presumed, but not yet proven, to be responsible. The nature of EWSB is expected to be manifested at the TeV energy scale that will be accessed by the Large Hadron Collider (LHC) complex. There are also many alternative possible sources of EWSB such as Technicolor, and there exists the possibility that the true source of EWSB may differ from anything that has yet been considered. Beyond EWSB, the LHC program may yield discoveries that could elucidate the nature of a unified theory underlying the SM. Examples include the discovery of a spectrum of supersymmetric partners of the usual cast of SM particles, or evidence of extra dimensions resulting from gravity at the TeV scale. The Compact Muon Solenoid (CMS) at the LHC will have very broad capabilities allowing exploration of these phenomena in proton-proton interactions at a center-of-mass energy of 14 TeV – seven times the highest energy achieved previously at the Fermilab Tevatron. In addition, heavy-ion beams at energies over 30 times higher than previously accessible will be provided by the LHC and their collisions will be studied by CMS.

OVERVIEW OF THE CMS DETECTOR [1] A schematic drawing of CMS is shown in Figure 1. The total weight of the apparatus is 12500 tons. The detector, which is cylindrical in shape, has length and diameter of 21.6 m and 14.6 m, respectively. The overall size is set by the muon tracking system which in turn makes use of the return flux of a 13 m-long, 5.9 m-diameter, 4 Tesla superconducting solenoid. This rather high field was chosen to facilitate the construction of a compact tracking system on its interior while also allowing good muon tracking on the exterior without the need for excessive demands being placed on muon-chamber resolution and alignment. The return field saturates 1.5 m of iron into which is interleaved four muon tracking stations. In the central region (pseudorapidity range |η| < 1.2) the neutron induced background, the muon rate and the residual magnetic fields are all relatively small, while in the forward regions (1.2 < |η| < 2.4) all three quantities are relatively high. As a result, drift tube (DT) chambers and cathode strip chambers (CSC), are used for muon tracking in the central and forward regions, respectively. Resistive plate chambers (RPC) with fast response and good time resolution but coarser position resolution are used in both regions for timing and redundancy.

Figure 1. Schematic drawing of the CMS detector with a portion removed to reveal the various subsystems as indicated. The detector is divided into 5 barrel Yoke sections, and 3 endcap yokes per end. The HF is mounted on the stand at far right.

The solenoid is large enough to house the inner tracker and the calorimetry. The sensing elements of the EM calorimeter (ECAL) are PbWO4 (lead tungstenate) crystals. The crystals can tolerate a radiation dose of 10 Mrad and have short radiation (X0 = 0.89 cm) and Molière (2.2 cm) lengths which are ideal for the design of a compact calorimeter with fine granularity. They are also fast, emitting 80% of all scintillation light within the 25 ns spacing between proton bunch crossings at the LHC. This light is detected by Si avalanche photodiodes (APD) and vacuum phototriodes (VPT) in the barrel and endcap regions, respectively. Due to their sensitivity to temperature change, both the crystals and the APD require a temperature stability of ~0.1 C to take full advantage of the excellent inherent energy resolution of the crystals. A preshower system is installed in front of the endcap ECAL to help detect πo→γγ. The CMS central hadronic calorimeter (HCAL) is outside of the ECAL but also inside the magnet coil. As a result of the latter, the absorber material in the HCAL is brass. A fraction of high energy hadronic showers extend beyond the HCAL and are taken into account by means of the signals retrieved from an additional layer of scintillators comprising the hadron outer (HO) detector that line the outside of the coil. The active elements consist of plastic scintillator tiles with embedded wavelength-shifting fibers that are spliced to long attenuation length fibers. The latter carry the light to hybrid photo-diodes which provide signal amplification in high axial magnetic fields. These technologies make it possible to build the HCAL with almost no un-instrumented cracks or dead areas in |η|. The endcap hadron calorimeter (HE) uses the same technology as the HCAL and covers the pseudorapidity region 1.3 < |η| < 3 while the region 3 < |η| < 5 is covered by the Fe and quartz-fiber Hadron Forward (HF) calorimeter. Cerenkov light emitted in the quartz fibers is detected by fast photomultipliers. The HF technology is ideal for the dense jet environment typical of this region as it leads to narrower and shorter hadronic showers. Calorimeter coverage to |η| < 5 is useful for reducing the uncertainty on missing transverse energy. The CMS tracker occupies a cylindrical volume of length 5.8 m and diameter 2.6 m. The outer portion of the tracker is comprised of 10 layers of silicon microstrip detectors and the inner portion is made up of 3 layers of silicon pixels. Silicon provides fine granularity and precision in all regions for efficient and pure track reconstruction even in the very dense track environment of high energy jets. The three layers of silicon pixel detectors at radii of 4, 7 and 11 cm provide 3D space points that are used to seed the formation of tracks by the pattern recognition. The 3D points also enable measurement of the impact parameters of charged-particle tracks with a precision of order 20 μm in both the r-φ and r-z views. The latter allows for precise reconstruction of displaced vertices to yield efficient b tagging and good separation between heavy and light quark jets. In regard to performance, the CMS experiment is designed for: • Good μ and charged particle tracking with momentum resolution over a wide range of momenta in |η|