Feb 20091 CMS experiment at LHC Geoff Hall Imperial College London Geoff Hall

Feb 2009 1

CMS experiment at LHC

Geoff Hall

Imperial College London

Geoff Hall

Feb 2009 Geoff Hall2

Large Hadron Collider

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 Beams

7 TeV protons => 14 TeV energy

also ions (eg Pb)

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

Feb 2009 Geoff Hall 3


Experiment by collisions

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

uu

d

uu

d

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,…)

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



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


Design philosophy


Muon System

195k DT channels210k CSC channels162k RPC channels

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

YE+3 Nov 2006


YB0 Feb 2007


December 2007


YE-1 Jan 2008


Geoff Hall13

CMS August 2008

Feb 2009

T. Virdee CMS Week Dec0814

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

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


improve monitoringrepair magnetsrestart summer 2009

Since September, impressive diagnosis of what happened…so:


Discoveries…

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


Physics requirements (I)

Mass peak one means of discovery

=> small (pT)

eg H => ZZ or ZZ* => 4l±

typical pT(µ) ~ 5-50GeV/c

Background suppression measure lepton charges good geometrical acceptance - 4 leptons background channel t => b => l

require m(l+l-) = mZ Z ~ 2.5GeV precise vertex measurement identify b decays, or reduce fraction in data

m2 = Ei2

i∑ −pi

2

tt


Physics requirements (II)

ΔpT

pT

≈0.15pT (TeV)⊕ 0.5%

p resolution

large B and L

high precision space points detector with small intrinsic meas

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

σ(pT )pT

~pT

σmeas

B.L2 Npts


What we hope to find at LHC

Higgs discovery and measurementeg. simplest SM variant several detectable decay channels but, ultimately, modest numbers of

events are expected at LHCp pH

µ+

µ-

µ+

µ-

Z

Z

plus much possible new physics eg SUSY, extra dimensions,…

H-> 4µ

30fb-1

2000

1500

1000

500

Expected number of events

600500400300200mH [GeV/c

2]

Signal Background

H -> 4l

300fb-1

Slide 20

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()=µ2++(+)2

= (v+H)/√2

mH2=2v2=-2µ2

Feb 2009 Geoff Hall


NLO

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.


H -> ZZH -> ZZ(*) (*) ->4->4ll - golden mode - golden mode

H->ZZ->ee

Background: tt, ZZ, Background: tt, ZZ, llllbb (“Zbb”)bb (“Zbb”)

Selections :Selections :- lepton isolation in tracker and calolepton isolation in tracker and calolepton impact parameter, lepton impact parameter, , ee vertex , ee vertex - mass windows Mmass windows MZ(*)Z(*), M, MHH


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


TOBTOB

TIDTIDTIBTIB

TECTEC

PDPD

Tracker system

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


Microstrip Tracker automated module assemblyOuter barrel

3.1M channels

Inner barrel 2.4M channels

Endcaps3.9M channels


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


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


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

Tracker not part of L1 trigger Data volume enormous Technically not possible for LHC


LHC Trigger Levels

Snapshot of work in progress

Feb 2009 Geoff Hall 30

Supersymmetry

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


Tom Whyntie

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

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

Tom Whyntie IC CMS Meeting, 22nd October 2008 33

Overview

• 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 = 10, sign() = +


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 + jets• tt + jet(s), W + jet(s), etc.

q

q

q

LSP

LSP

q

q

q

+ similar


Results for the Dijet System

Documents

Feb 20091 CMS experiment at LHC Geoff Hall Imperial College London Geoff Hall