Introduction to the Compact Muon Solenoid Experiment for

the LHCDave Barney, CMS Outreach Coordinator

Overview of Seminar

• Brief overview of the CMS experiment– Motivation (not in-depth physics)– Layout– Sub-detectors– The CMS collaboration

• Visiting the construction site at Cessy– Safety– What is in the assembly hall?– What is outside the hall?

• Useful resources• Questions (and hopefully answers too!)

Physics goals of CMS

• We don’t know what we will find at the LHC!• ATLAS and CMS are “general purpose” detectors –

they need to be designed to be able to detect anything!• We believe that the Higgs boson, and/or

Supersymmetric (SUSY) particles exist, and the LHC will provide collisions energetic enough to create them– But we cannot see Higgs/SUSY particles directly as they either

decay to lighter (stable) particles or cannot be seen with any known detector

– We have to design our detector to look for the stable particles and signs of “invisible” particles…..

Detecting signatures of the Higgs boson

• Most likely mass of the Higgs boson (if it exists) is around 115-130 GeV (1GeV = mass of proton)

• If this is the case, the easiest way to detect it is via its decay to two photons– Need an excellent electromagnetic calorimeter - ECAL (to

measure the energy of these photons)

– Need excellent tracker to identify the primary interaction vertex

• If the Higgs is heavier, it may be seen via its decay to electrons and/or muons– Need an excellent ECAL

– Need excellent muon chambers (for muon identification and momentum measurement) and central tracking (for momentum measurement)

Detecting SUSY Signatures

• Many SUSY particles decay to hadronic jets (many charged and neutral particles in a tight bunch)– Need good calorimeters – hadronic (HCAL) and ECAL

• SUSY decays also lead to the production of the “lightest supersymmetric particle” (LSP), which is invisible in any known detector– Need excellent calorimetry coverage in order to detect

“missing” energy (from simple conservation laws)

CMS Design Goals

• A good and redundant muon system (= many layers – if one layer fails we can fall back on the others)

• The best possible electromagnetic calorimeter

• A high quality central tracking

• A hadronic calorimeter that has good energy resolution and that is as hermetic as possible

• Affordable! (= ~500 MCHF)

The CMS Detector

The magnet systems of ATLAS and CMS

ATLAS A Toroidal LHC Apparatus

µ

CMS Compact Muon Solenoid

µ

The CMS Solenoid (1)

• A solenoid is essentially a cylinder of wire. Passing an electrical current down the wire creates a magnetic field

• The CMS solenoid is designed to provide an axial magnetic field of 4T – about 100000 times that of the earth’s magnetic field

• The current required is ~20 kAmps need to use a superconducting wire (zero resistance)

• The superconductor chosen is Niobium Titanium (NbTi) wrapped with copper – needs to be cooled to ~4K

• The CMS solenoid will be 13m long with an inner diameter of 5.9m – the largest superconducting solenoid ever made!

The CMS solenoid (2)

Superconductingcable

Ultra-pure Aluminium - magnetic stabilizer

Aluminium alloy - mechanical stabilizer

Solenoid piece at Cessy

The CMS Solenoid (3)

The solenoid vacuum vessel and return yoke

• Solenoid needs to be maintained at ~4 K• Need to insert the coil into a vacuum vessel (a bit like a

thermos flask)• The vacuum vessel = two concentric steel cylinders

(both of which are at Cessy – see later) that surround the coil

• Return yoke controls the field outside of the coil, and acts as a “filter” for muons (see later)

• Return yoke = 11000 tonnes of steel, built in sections: 5 barrel “rings” and 3+3 endcap “disks”

• Barrel rings are divided into layers, interspersed with muon chambers; muon chambers also on each endcap disk

• All components of the yoke are at Cessy

The return yoke - parameters

Central Ring Outer Rings

Barrel ring 1250 tonnes 1174 tonnes

Vacuum vessel 264 tonnes -

Superconducting coil 234 tonnes -

Support feet 72 tonnes 66 tonnes

Cabling on vacuum vessel 150 tonnes -

Support for racks and cables 10 tonnes 10 tonnes

Total 1980 tonnes 1250 tonnes

Endcap disk 1 (YE1) ~730 (disk) + 90 (cart) tonnes

Endcap disk 2 (YE2) ~730 (disk) + 90 (cart) tonnes

Endcap disk 3 (YE3) ~300 (disk) + 90 (cart) tonnes

Central barrel ring

Outer barrel rings

Endcap disks

Total weight 12500 tonnes

Diameter 15m

Length 21.6m

Magnetic field 4 Tesla

The CMS Detector - Overview

The Tracker

Pixel endcap disks

214m2 of silicon sensors11.4 million silicon strips65.9 million pixels in final configuration!

The Electromagnetic Calorimeter - ECAL

Parameter Barrel Endcaps

Coverage ||<1.48 1.48<||<3.0

x 0.0175 x 0.0175

0.0175 x 0.0175 to 0.05 x 0.05

Depth in X0 25.8 24.7

# of crystals 61200 14648

Volume 8.14m3 2.7m3

Xtal mass (t) 67.4 22.0

Preshowerbased on Si sensors

ECAL Barrel17 xtal shapes

ECAL Endcap1 crystal shape

Preshowerbased on Si sensors

ECAL Barrel17 xtal shapes

ECAL Endcap1 crystal shape Characteristics of PbWO4

X0 = 0.89cm = 8.28g/cm3

RM (Molière radius) = 2.2cm

The Hadron Calorimeter - HCAL

• CMS HCAL is constructed in 3 parts:– Barrel HCAL (HB)

• Brass (laiton) plates interleaved with plastic scintillator embedded with wavelength-shifting optical fibres (photo top right)

– Endcap HCAL (HE)• Brass plates interleaved with

plastic scintillator

– Forward HCAL (HF)• Steel wedges stuffed with quartz

fibres (photo bottom right)

• ~10000 channels total

More photos later in presentation!

The Muon Chambers

superlayer of 4 DT layers

superlayer of 4 DT layers

superlayer of 4 DT layers

195000 DT channels210816 CSC channels162282 RPC channels

Position measurement:Drift Tubes (DT) in barrelCathode Strip Chambers (CSC) in endcaps

Trigger:Resistive Plate Chambers (RPCs) in barrel and endcaps

The Trigger and Data Acquisition System (1)

• Bunches of protons collide in CMS every 25ns (40 million times per second)

• Each bunch crossing will result in ~1 Mbyte of data (after zero suppression)

• Can only possibly write ~100 Mbytes / second to tape

• CMS trigger system will try to decide (in a very short time!) if a bunch crossing has created something interesting– If yes, then the event is saved– If no, then the event is discarded for ever!

The Trigger and Data Acquisition System (2)

• CMS Trigger system has two stages:– Level-1 trigger

• Implemented in hardware

• Uses coarse-grain information from calorimeters and muon chambers to make a quick decision – in <4sec – e.g.

– are there 2 muons with momenta above certain thresholds?

– Is there an electromagnetic energy deposit > 40 GeV?

• Reduces rate from 40 MHz to a maximum of 100 kHz

– High level triggers• 100 kHz data passed through a high bandwidth switching network to a farm

of ~1000 commercial PCs running data selection algorithms – effectively on-line data analysis

• Use fine-grain information from all sub-detectors, e.g.– Is an ECAL energy deposit matched to hits in the pixel detector? (if so, this

signifies the presence of an electron)

• Reduces rate from 100 kHz to 100 Hz, for storage on tape

The Trigger and Data Acquisition System (3)

~same as wholeworld’s telecomnetwork!

Visiting the Cessy Site - Safety First

• Normally you should only go to the visitors gallery and outside areas

• To enter the assembly hall (for private visits) you must:– Contact Jean-Pierre Girod (163703) and request permission

– Wear safety helmets – failure to do so will result in visits to CMS being suspended

In case of an accident etc.

•Call J-P Girod (weekdays)

•Call the Pompier (74444) – but bear in mind they are 15 minutes away….

The CMS Construction Site at Cessy

VG

SX5

Safetyhelmets

PX56PM54

2585

3580

3584

He gastanks

Schematic of the surface buildings at Cessy

The Gas Cylinders

• Will be filled with Helium gas

• Two cylinders will supply He for the CMS solenoid cryogenic system – about 5000 litres of liquid He are required

• The time to cool the CMS solenoid to ~4K is about 3 weeks

• Other 4 cylinders will supply He for the LHC cryogenic system

The Underground Areas

The PX56 Access Shaft

• ~20m diameter

• Pieces of CMS detector will be lowered down this shaft into the UXC5 cavern

• Walls around are to protect neighbours from noise

Problem: When constructing the PX56 shaft, the excavators hit the water table (nappe phreatique) at about 40m deep – and it is not easy to dig through water!

Solution: put small-bore pipes around the shaft (from surface down to below the water level) and circulate salt-water at ~-5oC for several months. Then replace the salt-water with liquid nitrogen to freeze the water in the shaft. Then dig-out the water and concrete the shaft!

Status of Underground Caverns

Adding waterproof lining before final concrete layersCaverns will be completed by middle 2004

Inside SX5 – the barrel yoke rings

Feet: ~35 tonnes each; from Pakistan (outer rings) or Germany (central ring)

Connecting pieces from Czech republic

Main pieces from Russia

Central ring: ~2000 tonnes

Outer rings: ~1250 tonnes

Inside SX5 – the central barrel ringCentral ring supports solenoid

Outer vacuum vessel for solenoid - manufactured in Lons Le Saunierby France Comte Industrie- Transported to CERN in piecesand welded together at Cessy

Air-pads for moving rings etc.-from Noell GmbH, Germany-Use compressed air at 24 atmospheres from cylinders-Each pad can lift ~350 tonnes-4 pads per side-Rails used to guide the movement-Air-powered pistons push the rings

Inside SX5 – inserting the inner vacuum vessel

Inner vacuum vessel-Manufactured by FCI as a singlepiece and transported by road to Cessy-Supported and rotated by platformmade in Korea

Inside SX5 – the endcap yoke disks

Three disks for one endcap One disk loaded with CSCs

- Disks constructed from wedges made in Japan, assembled @ CERN- “Carts” made in China Stabilization bolts from USA

http://bulletin.cern.ch/eng/articles.php?bullno=47/2003&base=art&artno=BUL-NA-2003-141

Inside SX5 – the Hadron Calorimeter

Most of HCAL is in SX5 – two half-barrels and two endcaps (HF is still on the Meyrin site)

Brass for endcap HCAL has an interesting story……

Lowering the pieces of CMS into the cavern

CMS Collaboration (Nov. 2003)2008 scientists and engineers 160 institutes 36 countries

See http://cmsdoc.cern.ch/peopleCMS.shtml

Some Important CMS MilestonesTask Foreseen Date (as of November 2003)

Surface hall (SX5) finished construction 31 January 2000

Assembly of barrel yoke finished in SX5 31 August 2001

Assembly of endcap yokes finished in SX5 30 April 2002

Assembly of barrel HCAL finished in SX5 20 November 2002

Assembly of endcap HCAL finished in SX5 30 September 2003

Solenoid coil segments completed 30 June 2004

Underground experimental cavern completed 15 July 2004

Solenoid inserted into vacuum vessel 15 November 2004

Yoke closed and magnet test started in SX5 30 January 2005

End of magnet test in SX5 30 April 2005

Racks installed into underground service cavern 30 April 2005

Start lowering large pieces into UXC5 30 May 2005

End of lowering of major pieces into UXC5 30 September 2005

End of installation and cabling in UXC5 30 June 2006

CMS ready for circulating beam(including 20% computing capacity)

1 April 2007

Fully operational computing systems 1 April 2009

Full list can be found at http://cmsdoc.cern.ch/~cmstc

CMS Basic Parameters

Parameter Value

Bunch-crossing frequency 40 MHz

Average # of collisions / bunch-crossing 20

“interaction rate” ~109

Level-1 trigger rate 100 kHz

Average event size 1 Mbyte

Event builder bandwidth 100 Gbytes/sec

Event filter computing power required 106 SI95

Event rate saved to mass storage 100 Hz

Data production 10 Tbytes/day

Sub-Detector Number of channels

Pixels 66 x 106

Silicon microstrips 11.4 x 106

ECAL crystals 0.076 x 106

Preshower strips 0.137 x 106

HCAL 0.01 x 106

Muon chambers 0.576 x 106

TOTAL 78.2 x 106

2008 scientists and engineers

160 institutes

36 countries

Channel CountTrigger and Data Acquisition Parameters

Length 21.6m

Diameter 14m

Mass 12500 Tonnes

Magnetic field 4 Tesla

Collaboration (Nov. 2003)Physical Parameters

Puzzle

Find 4 straight tracks.

View along beam line of the inner tracking, with a H 4 event superimposed. The are very high energy, so leave straight tracks originating from the centre and travelling to the outside

Puzzle solution

Make a “cut” on theTransverse momentumOf the tracks: pT>2 GeV