0.3 Overview of CMS Physics Goals and Detector

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Goals of this page

This page provides an introductory description of the CMS physics goals and detector. It is intended as a prelude to the rest of the workbook which discusses the CMS offline software. The material presented here is covered in far more depth and detail in other places, and some useful links for finding further information are provided.

The job of the CMS software (online and offline) is to select and process physics interactions which occur in the CMS detector. During this process tools will be formulated not only to recognize these physics events but also help to find them within the data.


Physics Goals of CMS at the LHC

The first goal of the Large Hadron Collider (LHC) experiment is to search for the elusive Higgs boson. After 1 year of operation at just below design luminosity (at 1033 cm-2s-1, which will give 10 fb-1 of data after a year) the Standard Model (SM) Higgs boson should be either discovered at the LHC experiments, or excluded. Which within two years of reduced luminosity the Higgs boson was found. It currently has all the properties you would expect from a SM Higgs boson.

Although the SM of particle physics has so far been tested to exquisite precision, it is considered to be an effective theory up to roughly one TeV. The prime motivation of the LHC is to elucidate the nature of electroweak symmetry breaking for which the Higgs mechanism is presumed to be responsible. The experimental study of the Higgs mechanism can also shed light on the mathematical consistency of the SM at energy scales above about 1 TeV scale.

There are alternative models to describe our world, for example models that invoke more symmetry such as supersymmetry or invoke new forces or constituents such as strongly-broken electroweak symmetry, technicolour, etc. A yet unknown mechanism is also possible. Furthermore there are high hopes for discoveries that could pave the way toward a unified theory. These discoveries could take the form of supersymmetry or extra dimensions, the latter often requiring modification of gravity at the TeV scale. Hence there are many compelling reasons to investigate the TeV energy scale.

Hadron colliders are well suited to the task of exploring new energy domains, and the region of 1 TeV constituent centre-of-mass energy can be explored if the proton energy and the luminosity are high enough. (Remember that the collisions will take place between constituent particles, i.e. quarks and gluons, within the proton, hence partons carrying just a fraction of the full proton energy.)

The 27km circumference Large Hadron Collider (LHC) has been constructed in a tunnel 100 m underground which straddles the Swiss and French borders. It will deliver proton beams with energy 7 TeV and at a design luminosity of L = 1034 cm-2 s-1. These parameters have been chosen in order to study physics at the TeV energy scale, and provide a seven-fold increase in energy and a hundred-fold increase in integrated luminosity over the current hadron collider experiments. This enables physicists to study a wide unexplored region of particle physics but also requires a very careful design of the detectors to achieve these conditions.

The LHC will also operate in Heavy Ion mode, with design luminosity of L = 1027 cm-2 s-1 and centre-of-mass energy of 1312 TeV, providing high energy heavy-ion beams with energies over 30 times higher than what the present day accelerators have seen so far. This will enable us to further extend the range of the heavy-ion physics programs to include studies of hot nuclear matter. The heavy ion program will also seek to produce quark-gluon plasma - a state of matter in which quarks and gluons behaved like free particles, and which is believed to have existed during the very early times of the universe.

The Compact Muon Solenoid (CMS) experiment is one of four detectors built at crossing sites of the LHC beams, and is one of two general purpose detectors (the other being the ATLAS detector) which have been designed to exploit the physics opportunities presented by the LHC. Thus, the initial goal of the CMS detector is to study several Higgs boson production modes which can be explored with the detector.

higgs production
Simulated production of Higgs to 4 muons in the CMS detector

With a beam spacing of 25 ns, beam crossings occur in the CMS detector at a rate of 40 million per second (40MHz). An additional complication is the approximately 25 interactions which occur with each beam crossing - thus giving 1 billion events occurring in the CMS detector every second. In order to extract physics from these interactions it is vital to have fast electronics and very good resolution (proton-proton interactions are very messy and produce hundreds or thousands of particle candidates) and, because these events occur far too quickly to all be recorded and would take up vast amounts of disk space to store what are, for the majority, uninteresting events, very precise "triggering" is required. The CMS Data Acquisition System and Triggering are introduced briefly below.

CMS Detector Components

The symmetrical CMS detector is built in a traditional barrel design with two endcaps to provide nearly 4π coverage. As its name suggested, the CMS detector is a compact design, and is specifically built to provide good muon detection and resolution. Particle identification is aided by the superconducting solenoid which provides a 4 Tesla magnetic field.

The modular design of the detector is shown in the image below. In this section the components of the CMS detector are briefly described from the inside out. For further introductory information on each component of the CMS detector, see the CMS detector design The main CMS page contains links to more detailed information on each subsystem.

detector picture

Inner Tracker

The inner tracker sits around the LHC beampipe. It is cylindrical in shape, of length 5.8 m, and has a diameter of 2.6 m, and consists of 25,000 silicon strip sensors. The system will provide analogue data from ~10 million channels of the microstrip tracker. The CMS tracking system is designed to reconstruct high-pT muons, isolated electrons and hadrons with high momentum resolution and an efficiency better than 98% in the range |η|<2.5.

The inner-most tracking material consists of 3 layers of silicon pixel detectors which are placed close to the interaction region to improve the measurement of the impact parameter of charged-particle tracks, as well as the position of secondary vertices. In order to deal with high track multiplicities, CMS employs 10 layers of silicon microstrip detectors, which provide the required granularity and precision.

Accurate measurement of secondary vertices is required to study b quark decays - an area of LHC physics which provides rich possibilities to search for new physics interactions. Additionally, detection of particle jets arising from b quark decays, b-tagging, is a very important tool for many other physics studies, from the search of low-mass Higgs bosons to studies of the top quark. Other interactions involving secondary vertex production include the decay of short-lived neutral kaons, KS.

Combined with measurements from the dedicated muon detection system, the tracking system contributes to high resolution muon measurements in CMS.


Electrons, photons and hadrons will deposit their energies in the calorimeters allowing their energy to be measured. The first calorimeter layer or Electromagnetic Calorimeter (ECAL) is designed to measure the energies of electrons and photons with high precision. Strongly-interacting particles, hadrons, deposit most of their energy in the second layer, the hadron calorimeter (HCAL). Muons and tau leptons deposit only a very small fraction of their energy in the calorimeters, and are detected by consulting also with tracking and muon detector subsystems. Neutrinos escape detection, but their presence can be inferred from an apparent energy imbalance in the interaction.

Electromagnetic Calorimeter (ECAL)

CMS has a very compact scintillating crystal calorimeter which offers excellent performance for energy resolution since almost all of the energy of electrons and photons is deposited within the crystal volume. The Electromagnetic calorimeter (ECAL) uses about 75,000 lead-tungstate (PbWO4) crystals with coverage in pseudorapidity up to |η| < 3.0. The crystals convert energy into light, and the scintillation light is detected by silicon avalanche photodiodes (APDs) in the barrel region and vacuum phototriodes (VPTs) in the endcap region.

A preshower system is installed in front of the endcap ECAL for pi-zero rejection and the detection of photons. This is designed to detect the slightly-overlapping pair of photons from a π0 decay and to distinguish these from the single shower produced from a single photon interacting with the calorimeter material. .

One of the main physics challenges which sets design requirements for the resolution and efficiency of the EMC is the decay of a low-mass Standard Model Higgs boson into two photons.

Hadronic Calorimeter (HCAL)

The ECAL is surrounded by a brass/scintillator sampling Hadron Calorimeter (HCAL) with coverage up to |η| < 3.0.

The HCAL plays an essential role in the identification and measurement of quarks, gluons, and neutrinos by measuring the energy and direction of jets and of missing transverse energy flow in events. Missing energy forms a crucial signature of new particles, like the supersymmetric partners of quarks and gluons.

The scintillation light is converted by wavelength-shifting (WLS) fibers embedded in the scintillator tiles and channeled to photodetectors via clear fibres. This light is detected by novel photodetectors (hybrid photodiodes, or HPDs) that can provide gain and operate in high axial magnetic fields. While most of the HCAL is contained inside the CMS magnet, there are several additional layers outside the magnet to detect particles from high energy showers.

The central calorimetery is complemented by a "tail-catcher" in the barrel region - ensuring that hadronic showers are sampled with nearly 11 hadronic interaction lengths. This "depth" of the HCAL is needed to contain high-energy jets and also to filter out hadrons from jets involving muons. Accurate measurements of high-energy jets is important for searches for high mass Standard Model and Supersymmetric Higgs bosons.

The hadron calorimeter has both a barrel and endcap component which are sampling calorimeters with 50 mm thick copper absorber plates interleaved with 4 mm thick scintillator sheets. There are also "forward" HCAL's installed at each end of the CMS detector which provide coverage up to a pseudorapidity of 5.0. Here steel absorber plates are used in the harsher radiation environment of the forward systems and hadronic showers are seen by radiation-resistant quartz-fibers. The Cerenkov light emitted in the quartz fibers is detected by conventional photomultiplier tubes. The forward calorimeters ensure full geometric coverage for the measurement of the transverse energy in the event.

Wide rapidity coverage and accurate energy measurements are the main requirements of the hadron calorimetry system as several Higgs modes, as well as other important modes, require good missing transverse energy measurements.

Superconducting Magnet

At the heart of CMS sits a 13-m-long, 5.9 m inner diameter, 12,000 ton, 4 T superconducting solenoid which in part gives CMS its name. The energy stored in the magnet, if liberated, is large enough to melt 18 tons of gold. The CMS magnet system consists of a superconducting coil, the magnet yoke (barrel and endcap), a vacuum tank and ancillaries such as cryogenics, power supplies and process controls. Inside the bore of the magnet sit the inner tracker and the calorimetry, while outside is the flux return system and muon detector.

In order to achieve good momentum resolution with a compact spectrometer it is necessary to have a combination of a high magnetic field and high precision on the spatial resolution and alignment of the detectors. The momentum measurement of charged particles in the detector is based on the bending of their trajectories. The 4T magnetic field also bring benefits for calorimetry - it enables the detection of many isolated electrons produced by the decays of W's, Z's and b quarks. Using these electrons, the CMS crystal electromagnetic calorimeter can be calibrated to an accuracy of a fraction of a percent.

The magnetic flux is returned via a 1.5 m thick saturated iron yoke instrumented with four stations of muon chambers.

Muon detection

Muons are expected to provide clean signatures for a wide range of physics processes.

The return field of the solenoid is large enough to saturate 1.5 m of iron, allowing 4 muon "stations" to be interleaved with the iron return yoke plates of the magnet system. This provides a muon system with reconstruction efficiency better than 98% over the full pseudorapidity range.

Each muon station consists of several layers of aluminum drift tubes (DT) in the barrel region and cathode strip chambers (CSCs) in the endcap region, complemented by resistive plate chambers (RPCs). The muon detectors are arranged in concentric cylinders around the beam line in the barrel region, and in disks perpendicular to the beam line in the endcaps. Muon identification is ensured by the large thickness of the absorber material (iron), which cannot be traversed by particles other than neutrinos and muons. The DT and CSC detectors are used to obtain a precise measurement of the position and thus the momentum of the muons, whereas the RPC chambers are dedicated to providing fast information for the Level-1 trigger.

Muon measurements taken in the muon stations outside the solenoid are used for fast triggering and standalone muon reconstruction, but can also be combined with information from the inner tracking detectors for more precise reconstruction and momentum measurement.

muon stations

Summary of Particle Detection

particle detection summary

The figure shows a summary of the particle detection process in CMS. Charged particles leave signatures in the inner tracking system, and the vertices from decaying short-lived particles like b quarks and short-lived kaons can be identified. Photons, electrons, neutral pions and kaons are stopped in the crystals of the electromagnetic calorimeter (ECAL) and the scintillation light is used to determined the deposited energy. Hadrons punch thru further and are generally stopped by the hadronic calorimeter (HCAL) where jets are confined and only the highest-energy hadrons and muons pass through the superconducting solenoid into the outer regions of the CMS barrel. Finally, muons are detected in the various muon detectors which interleave the return yoke of the magnet. Neutrinos escape from the CMS detector and are inferred from an imbalance of energy in the reconstructed event called missing transverse energy.

Due to the powerful magnetic field, the trajectories of charged particles are bent in a direction perpendicular to the magnetic field. The direction of bending depends on the charge of the particle, while the amount of bending is a function of particle momentum - this is a central feature of particle identification in modern particle physics detectors. After particles pass through the solenoid, their bending is in the opposite direction, as indicated by the picture in this section. This is due to the fact that the magnetic field is in the opposite direction.

Trigger, Data Acquisition and Offline Computing

The Trigger and Data Acquisition System, TriDAS, is designed to select, out of the 1Bn interactions per second produced by 40MHz proton-proton beam crossings, the most interesting hundred or so events for storage and further analysis.

The first level of triggering, Level 1 (L1), is a hardware trigger, using hardware processors to rapidly select or reject events based on information from the ECAL and muon RPCs. Calorimeter triggering is based on a first measurement of energy deposited in the crystals, whereas the muon triggering centers around transverse momentum measurements of muons. The Level 1 trigger reduces the event rate from 40MHz to 100kHz.

An event passing the L1 trigger decision is transmitted to the Data Acquisition System where information from the 16M channels in the CMS subdetector systems is combined by the event builder into a single event. This event is then challenged by the next level of triggering.

The High Level Trigger (HLT) is a software-based trigger which performs a more sophisticated analysis of the event, including applying fast pattern recognition algorithms and event reconstruction. The main reduction factor for events passing the Level 1 triggering is obtained by using tracker information. The HLT decision reduces the event rate to about 100Hz for storage and later offline analysis. This corresponds to about 12 Petabytes of data to be recorded by CMS annually when it is running at design luminosity. The average processing time for the high level trigger is 40ms per event.

Information Sources

Review status

Reviewer/Editor and Date Comments
Main.Aresh - 19 Feb 2008 change in chapter "MuonDetection" (table tags inserted into TWIKI conditionals for printable version)
JennyWilliams - 10 Oct 2007 essentially rewritten page, using mainly Outreach information as source

Responsible: KatiLassilaPerini
Last reviewed by: ScarletNorberg - 22 November 2015

Topic attachments
I Attachment History Action Size Date Who Comment
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GIFgif CMSParticleDetectionSummary.gif r1 manage 108.4 K 2007-10-11 - 02:28 JennyWilliams CMS Particle Detection Summary
JPEGjpg HiggsProduction.jpg r2 r1 manage 83.3 K 2007-10-11 - 00:40 JennyWilliams Higgs Production Monte Carlo, from della Negra CSS2007 tutorial
GIFgif MuStations.gif r1 manage 20.7 K 2007-10-11 - 05:38 JennyWilliams Muon Stations
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