July 2009 1

CMS experiment at LHC

Geoff Hall

Imperial College London

Geoff Hall

July 2009 Geoff Hall2

Large Hadron Collider

Latest CERN accelerator started 2008 26km circumference ring

~100m underground Beams

7 TeV protons or ions, eg Pb

very high intensity 1015 collisions per year

very high rate beams cross @ 40MHz

few “interesting” events ~100 Higgs decays per year

(but a small problem occurred - with a big impact)

July 2009 Geoff Hall3

Particle physics in two slides Matter originated in the Big Bang

LHC energies correspond to tiny fraction of a second in the life of the universe

Make up hadronic matter, eg proton (uud) neutron (udd) mesons (q + anti-quark)

Leptons Families - like quarks e- makes atoms with nuclei µ and are like heavy electrons each has a neutrino partner

All quarks and leptons have mass

July 2009Geoff Hall4

Forces hold matter together

Strong nuclearStrong nuclearElectromagneticElectromagnetic

Weak nuclearWeak nuclear

GluonsGluons

QuarksQuarks

Mesons Baryons

Mesons Baryons

NucleiNuclei

PhotonPhoton

AtomsLightChemistryElectronics

AtomsLightChemistryElectronics

Atomselectrons

Atomselectrons

Neutron decayBeta radioactivityNeutrino interactionsSolar burning

Neutron decayBeta radioactivityNeutrino interactionsSolar burning

W & Z BosonsW & Z Bosons

quarksleptons

neutrinos

quarksleptons

neutrinos

Forces are transmitted by fields, also represented by particles (, W, Z, gluon)The “Standard Model” has unified some of the (4) forces of nature … astonishingly successfully

The most significant missing item is mass

It may be explained by a new field (and particle) - Higgs (boson)

Forces are transmitted by fields, also represented by particles (, W, Z, gluon)The “Standard Model” has unified some of the (4) forces of nature … astonishingly successfully

The most significant missing item is mass

It may be explained by a new field (and particle) - Higgs (boson)

July 2009 Geoff Hall5

Experiment by collisions

Colliding beams maximises the energy available to create new particles

uu

d

uu

d

Actually hadron collisions are between their constituent parts…

gluons quarks and the particles they exchange (Z, W,…)

July 2009 Geoff Hall6

Experiment design

ppT

pL

July 2009 Geoff Hall7

CMS Compact Muon Solenoid

ECAL

Tracker

HCAL

4T solenoid

Muon chambers

Total weight: 12,500 tOverall diameter: 15 mOverall length 21.6 mMagnetic field 4 T

July 2009 Geoff Hall8

Muon System

195k DT channels210k CSC channels162k RPC channels

Gaseous planar ionisation detectors embedded in iron magnet return yoke to measure particle trajectories

YB0 Feb 2007

July 2009 Geoff Hall9

December 2007

July 2009 Geoff Hall10

YE-1 Jan 2008

July 2009 Geoff Hall11

Geoff Hall12

First data

First LHC Beam (10 Sept)

10 September 2008: beams were steered into collimators and secondary particles detected in CMSbefore and after September ~ 300 M cosmic ray events recorded

July 2009

July 2009 Geoff Hall13

The luminosity challenge

10331033

10351035

1032 cm-2 s-1 1032 cm-2 s-1

10341034

Full LHC luminosity~20 interactions/bx

Proposed SLHC luminosity~300-400 interactions/bx

HZZ ee, MH= 300 GeV for different luminosities in CMS

July 2009 Geoff Hall14

TOBTOB

TIDTIDTIBTIB

TECTEC

PDPD

Tracker system

Radiation environment ~10Mrad ionising~1014 hadrons.cm-2

July 2009 Geoff Hall15

Microstrip Tracker

automated module assemblyOuter barrel 3.1M channels

Inner barrel 2.4M channels

Endcaps3.9M channels

July 2009 Geoff Hall16

Electromagnetic Calorimeter

Scine

Preshowerbased on Si sensors

ECAL Barrel17 xtal shapes

ECAL Endcap1 crystal shape

Preshowerbased on Si sensors

ECAL Barrel17 xtal shapes

ECAL Endcap1 crystal shape

Parameter Barrel Endcaps

Depth in X0 25.8 24.7

# of crystals 61200 14648

Volume 8.14m3 2.7m3

Xtal mass (t) 67.4 22.0

Scintillating crystals of heavy material – PbWO4

Light produced by electromagnetic showers

Light signal proportional to electron or photon energy

July 2009 Geoff Hall17

Trigger and DAQ systems

Trigger selects particle interactions that are potentially of interest for physics analysis

DAQ collects the data from the detector system, formats and records to permanent storage

First-level trigger: very fast selection using custom digital electronics Higher level trigger: commercial computer farm makes more sophisticated

decision, using more complete data, in < 40-50 ms

Trigger requirements High efficiency for selecting processes of interest for physics analysis Large reduction of rate from unwanted high-rate processes Decision must be fast Operation should be deadtime free Flexible to adapt to experimental conditions Affordable

July 2009 Geoff Hall18

p pH

jet jet

e+

e-

Z

Z

Triggering

Primary physics signatures in the detector are combinations of: Candidates for energetic electron(s) (ECAL) Candidates for µ(s) (muon system) Hadronic jets (ECAL/HCAL)

Vital not to reject interesting events Fast Level-1 decision (≈3.2 µs) in custom hardware

up to 100kHz with no dead-time Higher level selection in software

July 2009 Geoff Hall19

What we hope to find

Higgs discovery (simplified!) Will be produced with many other particles ~20 events per beam crossing hundreds of secondary particles/25ns

p pH

µ+

µ-

µ+

µ-

Z

Z

Much new physics New forces New particles New symmetries

p p

e- e

q

q

q

q1

-

g~

~

20~

q~

10~

Machine incident

A superconducting cable connecting magnets and carrying ~9kA “quenched” – became resistive - and began to heat up

in < 1s the cable failed and an arc punctured the helium enclosure, releasing gas at high pressure

all the protection systems worked, but the pressure rose higher than expected

July 2009 Geoff Hall20

improve monitoringrepair magnetsrestart summer 2009

Since September, impressive diagnosis of what happened…so: