Cms experiment at lhc
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CMS experiment at LHC. Geoff Hall Imperial College London. Latest CERN accelerator started 2008 very high intensity 10 15 collisions per year very high rate beams cross @ 40MHz few “interesting” events ~100 Higgs decays per year Beams 7 TeV protons => 14 TeV energy also ions

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CMS experiment at LHC

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CMS experiment at LHC

Geoff Hall

Imperial College London

Geoff Hall

Latest CERN accelerator started 2008

very high intensity

1015 collisions per year

very high rate

beams cross @ 40MHz

few “interesting” events

~100 Higgs decays per year


7 TeV protons

=> 14 TeV energy

also ions

(eg Pb)

Large Hadron Collider

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

Geoff Hall

Geoff Hall

Colliding beams maximises the energy available to create new particles (compared to shooting at a target)







Experiment by collisions

  • Hadron collisions are actually between their constituent parts…

    •  ~ 1/p ≈ 1/E

  • gluons

  • quarks: both valence and sea (≈ real and virtual)

  • and the particles they exchange (Z, W,…)

Geoff Hall

What do we actually do?

  • We design, build and operate the experiments

    • LHC was & is enormously challenging so it’s taken a long time…

    • some illustrations of how the experiments are built

  • We analyse the data

    • in the LHC energy range, theory eventually fails

    • so something new must be found

    • for the first time in many years, only experiment can tell us what

  • Now the construction has finished most effort will go into looking at data

    • PhD students and young researchers will be doing most of the work

    • Snapshot of some typical work in progress

Geoff Hall


Muon chambers



4T solenoid

CMS Compact Muon Solenoid

Total weight: 12,500 t

Overall diameter: 15 m

Overall length 21.6 m

Magnetic field 4 T

Geoff Hall

high pT lepton and quarks are signatures

of possible new physics

Large solenoidal (4T) magnet

iron yoke - returns B field, absorbs particles

Muon detection –penetration

detectors in yoke measure muons

Electromagnetic calorimeter – absorb E

good energy resolution for e & g

+ Hadronic calorimeter for pions,…

Tracking system – bend in B field

reconstruct trajectories of most charged particles

momentum measurements from bending

observe directly many decays

complement muon & ECAL measurements

Design philosophy

Geoff Hall

Muon System

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

195k DT channels210k CSC channels162k RPC channels

Geoff Hall

YE+3 Nov 2006

Geoff Hall

YB0 Feb 2007

Geoff Hall

December 2007

Geoff Hall

YE-1 Jan 2008

Geoff Hall

CMS August 2008

Geoff Hall

First data

First LHC Beam(10 Sept)

10 September 2008: beams were steered into collimators and secondary particles detected in CMS

before and after September ~ 300 M cosmic ray events recorded

T. Virdee CMS Week Dec08

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

Since September, impressive diagnosis of what happened…so:

improve monitoring

repair magnets

restart summer 2009

Geoff Hall

Look at interactions for

unexpected behaviour

like large energy at large angle to beam

(how Rutherford discovered the atomic nucleus)

evidence of short-lived particles

visible evidence

Indirect, by peaks in mass spectra


Old picture of a charmed particle production and decay in a bubble chamber

Geoff Hall

Mass peak one means of discovery

=> small s(pT)

eg H => ZZ or ZZ* => 4l±

typical pT(µ) ~ 5-50GeV/c

Background suppression

measure lepton charges

good geometrical acceptance - 4 leptons

background channelt => b => l

require m(l+l-) = mZ GZ ~ 2.5GeV

precise vertex measurementidentify b decays, or reduce fraction in data

Physics requirements (I)

Geoff Hall

p resolution

large B and L

high precision space points

detector with small intrinsic smeas

well separated particles

good time resolution

low occupancy => many channels

good pattern recognition

minimise multiple scattering

minimal bremsstrahlung, photon conversions

material in tracker

most precise points close to beam

Physics requirements (II)

Geoff Hall














What we hope to find at LHC

  • Higgs discovery and measurement

    eg. simplest SM variant

    • several detectable decay channels

    • but, ultimately, modest numbers of events are expected at LHC

H-> 4µ


  • plus much possible new physics

    • eg SUSY, extra dimensions,…

Geoff Hall

The Higgs Model

  • The Higgs is different !

  • Higgs is the only scalar particle in the SM

    • All the matter particles are s=½ fermions

    • All the force carriers are s=1 bosons

  • Postulated to give rise to mass throughspontaneous electroweak symmetry breaking

    • Also to neutrinos if Dirac particles

  • It would be the first fundamental scalar ever discovered

  • Frankly, almost nothing is known about the Higgs

    • Nothing is known for the Yukawa-coupling

    • Nothing is known for the Higgs self-coupling

    • Single Higgs? Two Higgs field doublets? Additional singlet?

    • SUSY? MSSM? NMSSM? Extra-dimensions?

    • If the Higgs is discovered, mapping the potential is crucial


= (v+H)/√2


Geoff Hall

Production of the Higgs

The production cross-section is calculable.

It depends on the Higgs mass, and the production mechanisms.

The Higgs mass is not known and there are few theoretical constraints on it.


Geoff Hall

H -> ZZ(*) ->4l - golden mode

  • Background: tt, ZZ, llbb (“Zbb”)

  • Selections :

  • lepton isolation in tracker and calo

  • lepton impact parameter, mm, ee vertex

  • mass windows MZ(*), MH

H->ZZ->ee mm

Geoff Hall

1032 cm-2 s-1




The luminosity challenge

  • HZZ  ee, MH= 300 GeV for different luminosities in CMS

Full LHC luminosity

~20 interactions/bx

Proposed SLHC luminosity

~300-400 interactions/bx

Geoff Hall











Tracker system

  • Two main sub-systems: Silicon Strip Tracker and Pixels

    • as many measurement points as possible with the most precise measurements close to the interaction point

    • ionisation in silicon produces small current pulses

    • silicon sub-divided into small measuring elements: strips or pixels

    • ~14 layers, ~210 m2 of silicon, 9.3M channels

    • 3 layers, 1m2 pixels, 66M channels

Radiation environment

~10Mrad ionising


Geoff Hall

Microstrip Tracker

Outer barrel

3.1M channels

  • automated module assembly


3.9M channels

Inner barrel 2.4M channels

Geoff Hall


Electromagnetic Calorimeter

Scintillating crystals of heavy material – PbWO4

Light produced by electromagnetic showers

Light signal proportional to electron or photon energy

Geoff Hall

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

    • Largereduction of rate from unwanted high-rate processes

    • Decision must be fast

    • Operation should be deadtime free

    • Flexible to adapt to experimental conditions

    • Affordable

Geoff Hall













  • 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

  • Tracker not part of L1 trigger

    • Data volume enormous

    • Technically not possible for LHC

Geoff Hall

LHC Trigger Levels

Geoff Hall

Snapshot of work in progress

Geoff Hall


  • A new symmetry of nature?

    • each fermion has a boson partner (& vice versa)

    • not yet observed! - therefore likely to be heavy

    • SUSY solves some problems with Higgs mass (in GUTs)

  • there is a lightest SUSY state into which others decay

  • it does not interact with ordinary matter

    • could therefore be the explanation for dark matter

  • it would not be directly observed in CMS

    • the signature would be large missing energy

    • – this relies on good hadron calorimetry

    • but it would wise not to depend on a single technique

  • If SUSY exists, it may show up very early at LHC

Geoff Hall

Early SUSY searches with the all-hadronic n-jet channel.

Tom Whyntie

On behalf of the CMS IC SUSY Group (+ friends)


  • Introduction

    • How can we discover SUSY with CMS?

  • The dijet search channel

    • A calo-MET independent SUSY search?

  • The n-jet search channel

    • How do we go from n to 2 jets?

  • A suggested strategy for n-jets

    • S/B ~7 for LM1 SUSY?

  • Conclusions and plans

Introduction: SUSY at CMS

  • Goal: discover SUSY at CMS

    • Early data, L < 1 fb-1;

    • Minimal understanding of the detector.

  • SUSY parameter space considered:

    • CMS benchmarks: LM1-9 (TDR)

    • Low mass MSuGra SUSY

    • e.g. LM1:

      m0 = 60GeV, m1/2 = 250, A0, tan b = 10, sign(m) = +





+ similar





The Dijet Search Channel

Analysis note recently approved: CMS AN-2008/071

(Flaecher, Jones, Rommerskirchen, Stoye)

  • Two high pt jets

  • Large missing energy

Missing energy relies on calorimeter – is there a way of just using the jets?

Is it possible to formulate a discriminating observable based on jet kinematics?

  • Backgrounds

  • QCD dijet events

  • Z nn + jets

  • tt + jet(s), W + jet(s), etc.

Results for the Dijet System

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