The Compact Muon Solenoid (CMS)
Experiment at LHC
Serguei Ganjour
CEA-Saclay/IRFU, Gif-sur-Yvette, France
CMS Collaboration
Facility and Data Streaming (Lecture I)
Physics Program and Perspectives (Lecture II)
Gomel, Belarus
July 15-26, 2009
The Standard Model
☞ Precision tests of electroweak theory
(LEP)
➠ gauge boson properties;
☞ Top quark discovery (Tevatron)
➠ completeness of the 3-rd generation
☞ CP-violation in B sector (B-Factories)
➠ CKM mechanism drive the quark mixing and generate fermion masses;
➠ predominance of matter over antimatter;
Mass generation mechanism remains unproved element of
the Standard Model, unless Higgs boson is discovered
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The CMS experiment at LHC
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Open Questions to the SM and Beyond
☞ Even if the Higgs exists, the SM remains logically incomplete:
➠ why its mass so low O(1TeV)?
➠ does not explain dark matter mystery;
➠ does not incorporate gravity
☞ Superstring theory invents supersymmetry
(SUSY) and extra space-time dimensions (ED)
☞ SUSY at O(1TeV) stabilizes the Higgs mass
against divergent radiative corrections
➠ solves the hierarchy problem;
➠ provides a candidate for the dark matter;
➠ unify the gauge forces (GUT);
☞ Gravity is a handle to manifest ED
➠ can bring gravity strength at O(1-10TeV);
➠ recast or eliminate the hierarchy problem;
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Five Goals of the LHC
☞ Rediscover the Standard Model at new energy domain
➠ precision test of the gauge boson and top-quark properties
➠ measure the QCD objects - jets (background for searches)
☞ Discover the origin of the Electroweak Symmetry Breaking (EWSB)
➠ search a Higgs boson and check the mass generation mechanism
☞ Produce a Dark Matter candidate
➠ observation of neutral stable particle interacting with a strength of electroweak force;
➠ search SUSY particles at 1 TeV energy scale;
☞ Search for new forces of Nature
➠ new force particle would decay into known particles
➠ new symmetries might guide towards unification of all interactions
☞ Explore space-time structure
➠ evidence of hidden space-time dimensions;
➠ search for graviton and micro black holes;
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The CMS experiment at LHC
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Facility
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The Discovery Machine
☞ Tevatron explored 200÷300 GeV horizon
☞ LHC will probe much beyond - terascale
√
➠ pp-collisions at s = 14 TeV
➔ probing region 2 ÷ 3 TeV
➠ luminosity 1034cm−2s−1
➠ nearly 3000 bunches;
➠ bunch crossing every 25 ns
➠ up to 20 collisions/bunch crossing
➠ σtot = 100mb, 109 interactions/s
☞ Gauge boson factory:
events/s events/year
Process
W → eν
200
2×109
Z 0 → e+ e−
20
2×108
tt̄
8
8×107
Higgs (m = 120 GeV)
0.4
4×106
g̃g̃ (m = 1 TeV)
0.01
1×105
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The CMS experiment at LHC
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The LHC Experiments
27km tunnel
50-175m deep
7000
superconducting
magnets
combines
PS and SPS
4 interaction
regions
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The CMS experiment at LHC
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Detector Requirements
☞ Inner tracker
➠ good pT resolution and high efficiency (high magnetic
field, large volume)
➠ pixel detector to trigger and tag τ ’s and b-jets
☞ Muon detector
➠ large lever-arm for high momentum muons
➠ unambiguous charge measurement up to 1 TeV
☞ Electromagnetic Calorimeter
➠ ∆E/E ∼ 0.5% at 50 GeV
➠ high granularity (separate charged
and neutrals, reject π 0)
☞ Hadron Calorimeter
➠ high hermeticity and coverage (ETmiss
measurement)
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The CMS experiment at LHC
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Detector Layout and Subsystems
ECAL Barrel
PbWO4 cysts
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Silicon Tracker
Microstrips/Pixels
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Superconducting
Coil (4 Tesla)
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¢
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¢
¢
¢
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HCAL Scin./brass
sandwich
¢
´
´
´
´
´
´
´
ECAL Endcap
ES Preshower
´
´
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+́
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µ
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HH
Y
H
HH
HH
HH
HH
Muon Barrel
DT/RPC
The CMS experiment at LHC
HH
Muon Endcap
CSC/RPC
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The CMS detector
Compact, modular
Weight: 12500 t
Diameter: 15 m
Length: 21.6 m
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About 6 CMSs can be placed inside ATLAS!
The CMS experiment at LHC
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CMS Lowering
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The CMS experiment at LHC
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Inner Tracker (Si)
☞ Around 1000 charged particles emerge
from interaction region every 25 ns
10
µ, pt=1GeV
µ, pt=10GeV
t
σ(δ p /p ) [%]
☞ Efficient triggering and tagging of τ and
b−jets requires pixel detector
t
µ, pt=100GeV
Radius
Cell size
Occupancy per
( cm)
LHC crossing
r < 10
100 × 150µm
10−4
20 < r < 50 10cm×80µm
2-3%
r > 55
25cm×180µm
1%
☞ Pixels: S=1m2, 65M pixels, r=4,7,10 cm
1
0
0.5
2
☞ Micro-strips: S=223m , 10M strips, r=20-120 cm
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1.5
2
η
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Pixel Detector
☞ 3 barrel layers (768 modules), 2 endcap disks (672 modules)
☞ Outermost radius is r=10.2 cm, 16k readout chips
☞ Fast track reconstruction with pixel triplets
➠ processing time ≤ 20ms/event
➠ triggering on τ ’s, b-jets, isolated muons
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The CMS experiment at LHC
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Electromagnetic Calorimeter (ECAL)
PWO4 crystal properties
X0
0.89 cm
RM
2.19 cm
Front face
2.2×2.2 cm2
Rare face
2.6×2.6 cm2
Length
23 cm (25.8X0)
Barrel ECAL (EB)
= 1.
9
.4 7
1
=
y
653
Preshower (ES)
= 2.6
Endcap
ECAL (EE)
☞ APD (VPT) photodetectors for EB (EE)
σ E (%)
E
z
= 3.0
1.4
Test-beam EB SM
S = 3.37±0.10 %
1.2
C = 0.25±0.02 %
N = 108 MeV
1
0.8
0.6
0.4
0.2
00
µ
¶
σ 2
E
20
=
40
EB Supermodule (SM)
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µ
¶2
S
√
E
60
+
µ
80
¶
N 2
E
+ C2
100
120
140
Ebeam (GeV)
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☞ Estimated energy in the ECAL:
Ee, γ = F xclusters
Σ G ci Ai
Corrections
Calibration
☞ Energy correction scheme
Number of events
Energy Correction
Fit results:
With correction
1000
m = 120.00
Without correction
σ=
800
0.62
σ / m = 0.51 %
χ2 / Ndf = 0.98
600
400
200
➠ F = 1 for 5x5 crystal sum for the energy of
unconverted photons;
➠ overall containment factor;
➠ local containment and boundaries;
➠ correct for the bremsstrahlung;
➠ crystal transparency (laser monitoring)
0
114
116
118
120
122
124
Energy (GeV)
☞ Calibration and alignment
➠ exploit W + → e+ν, Z 0 → e+e− events
➠ dedicated π 0 calibration
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ECAL Calibration
☞ Start-up calibration precision
➠ test beam calibration only for 9 SM
for EB (500 Xtals for EE)
➠ others have couple % calibration from
cosmics for EB
➠ about 10% lab calibration for EE
☞ Several paths for in-situ calibration
Strategy
Time Precision
Mean energy deposited by jet triggers independent few hours 2-3%
on φ at fixed η (correct for tracker material)
π 0 mass peak (L = 2 × 1033cm−2s−1)
few days
≤1%
100 pb−1
≤1%
Z 0 → e+e− absolute calibration
5 fb−1
≤0.5%
W → eν E/p measurement
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Electrons and Photons
☞ Combination of ECAL and tracker improves
electron and photon measurements
☞ Rejection power of π 0, η, depends on
☞ Preshower detector for EE
(t = 2X0) σ ∼ 0.5mm
☞ About 50% of photons
converts in the tracker
Efficiency
➠ granularity of calorimeter;
➠ identification of converted photons
1
0.9
0.8
0.7
¢
¢
0.6
two tracks
0.5
➠ degrades ECAL energy
resolution
➠ use track information
E/ pT
P
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trk
∼1
¢̧
¢
0.4
one track
0.3
@
0.2
R
@
0.1
0
-2.5
-2
-1.5
-1
-0.5
0
0.5
1
1.5
2
2.5
True conversion eta
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Hadron Calorimeter (HCAL)
Barrel (HB)
|η| < 1.4
Outer (HO)
|η| < 1.26
Endcap (HE) 1.3 < |η| < 3
Forward (HF)
3 < |η| < 5
Brass absorber
Plastic scintillator
with WLS fibers
HPD read out
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HCAL Resolution
σ / Erec
☞ Calibration methods
➠ radioactive source 60Co, 5mCi
➔ uniform detector response
➔ 2% (5%)for HB/HE (HF)
➠ exploit W → τ ν, Z → τ τ , Z/γ+jets
➔ absolute energy calibration
➔ about 2% within one month
☞ Best combination of HCAL and ECAL allows jet
energy measurement
Pions- G4 QGSP, ECAL+HCAL
Pions- G4 LHEP, ECAL+HCAL
0.15
0.1
σ =120% + 6.9%
E
E
0.05
0
30
➠ segmentation ∆η × ∆φ ≤0.1×0.1
1 HCAL tower has 25 ECAL crystals underneath
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Pions- TB data, ECAL+HCAL
0.25
50
2
10
E [GeV]
0.6
0.5
|η|<1.4
0.4
1.4<|η|<3.0
0.3
3.0<|η|<5.0
rec
➠ 65% - charged hadrons, 25% - EM objects
(e/γ), 10% - neutral hadrons
➠ e/π HCAL response is different
Non-compensated resolution
0.2
σ(ET /ETMC)fit/<ETrec/ETMC>fit
☞ Jets of particles comprise
0.3
0.2
0.1
0
0
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100
150
200
250
300
EMC
T , GeV
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Missing Transverse Energy (MET)
☞ Signature of weakly interacting stable particles
➠ identified from energy balance in transverse
plane - missing ET (MET)
➠ computed with calorimeter tower reco objects
➠ need to minimize non-Gaussian tails
☞ ETmiss measurement requires several corrections
➠ Jet Energy Scale (JES)
~ Tcorr
E
~T −
=E
Njets
X
i=1
~
~raw
(pcorr
T − pT )
JES measure from Z 0/γ+jet energy balance
➠ account for muons (non-calo objects)
➠ specific τ -jets (particle flow)
➔ improve charged hadrons using tracker
➠ soft underlying (UE) and pile-up (PU)
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R (c m)
Muon System
800
MB 4
700
MB 3
600
MB 2
500
MB 1
400
300
200
ME 2
100
ME 3
ME 4
ME 1
0
0
200
☞ Choice of the detector technologies has been driven by
➠ large surface
➠ neutron flux
➠ muon rates
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400
600
800
1000
1200
Z (c m)
☞ Exploit three detector types
➠ Drift Tubes (DT) - Barrel
➠ Cathode Strip Chambers (CSC) - Endcap
➠ Resistive Plate Chambers (RPC)
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☞ Require good muon identification and
resolution over wide range
➠ ∆mµµ/mµµ ' 1% at 100 GeV/c
➠ σpT /pT ≤ 10% at 1 TeV/c
∆p/p
Resolution and Alignment
1
0.0<η<0.2
2
☞ Muon chambers must be aligned with
central tracker to within 100-500 µm
10-1
➠ optical-mechanical system;
➔ (r,φ)- 0.2 mm, (z)-0.4mm,
(rotation)-0.04÷0.1 mrad
10-2
➠ algorithms based on muon tracks
crossing the spectrometer
MB CSC Trk.-Muon
displ.(x-y), mm 0.2 0.2
0.2
10-3
0.2
displ. (z), mm 0.2 0.4
rotation, mrad 0.05 0.1
0.04
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Full system
Muon system only
Inner Tracker only
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The CMS experiment at LHC
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103
p[GeV/c]
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The Purpose of Trigger
CMS 2-level Trigger
☞ Level-1 (L1):
➠ rejection 10−4
➠ hardware based
?
?
☞ High-Level Trigger
(HLT):
➠ rejection 10−3
➠ software based
?
?
?
Physics signals
☞ off-line analysis
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Trigger Strategy
☞ L1: custom synchronous processors
1 GHz
100 kHz
1 TB/s
150 Hz
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➠ coarse granularity information from calorimeters and muon chambers
➠ identification: e/γ clusters, µ, jets, ETmiss
➠ local pattern recognition and momentum energy evaluation
➠ processing time O(1µs)
☞ HLT: asynchronous CPU farms
➠ access to full event data
➠ finer granularity, precise measurement
➠ identification: electrons, γ, µ, jets, ETmiss, b,
τ -tagging (matching of different subsystems)
➠ processing time O(1 ÷ 100 ms)
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Trigger Requirements
☞ Basic requirements:
➠ keep high signal efficiency;
➠ apply inclusive selection (discovery of
unexpected physics)
➠ minimize calib./allign. impact
☞ HLT challenges:
➠ limited CPU time (high L1 rate)
➠ high background reduction without
compromising signal efficiency
☞ Consequent filtering:
➠ use muon and calorimeters only
➠ match pixel with external detectors
➠ conditional tracking: stop when given
σ(pT )/pT reached
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Data Streaming
☞ LHC produces more data then can be recorded
➠ up to 20 collisions occurring every 25 ns intervals
➠ trigger systems select events with promising features
☞ Typical collision event at CMS ∼ 1 MB/event
☞ Storage rate is ∼ 109events/year, i.e. ExaBites
☞ Global Network of computers GRID provides:
➠ access to the stored data around the world
➠ processing power to analyze these data
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Data Distribution Model
☞ Tier 0 (CERN)
☞ Tier 1 (7 centers)
☞ Tier 2 (institutes)
➠ accept data from DAQ;
➠ focus on reconstruction
➠ host reconstructed data
➠ distribute to the Tier 1;
➠ form and distribute data
sets
➠ physics skimming
➠ archive on tape;
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The CMS experiment at LHC
➠ data analysis
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Cosmic/Beam Runs
☞ Cosmic RUn at ZEro Tesla (CRUZET)
CRAFT Data Taking Schedule
➠ start 31-Mar 2008
☞ Cosmic Run At Four Tesla (CRAFT)
➠ Ran over 4 weeks continuously
➔ from 13-Oct to 11-Nov 2008
➔ 19 days with B-Field 3.8 T
➠ Recorded 370 M cosmic events
➔ 290 M with B-Field on
➔ 194M with all components in
9
Almost 10 recorded cosmic
events at CMS
Also running couple weeks with single
proton beam of LHC!
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☞ Operational experience
➠ about 70% data taking efficiency
➠ study effects of B-Field
➠ collect data for detector studies
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Muon Multiplicity
Analyzed events spanning 5 orders of magnitude in muon multiplicity!
Cosmic/Beam Halo
O(1) muon
Analysis using: tracks
Charge ratio
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Cosmic Shower
O(10 − 100) muons
... segments
shower origin
Beam “Splash”
O(105) muons
... hits
shower energy and shape
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Cosmic Signal in B-Field
All subdetectors are included in the Global Cosmic Runs
ECAL in magenta, HCAL in blue, tracker and muon hits in green
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Tracker Alignment
☞ Using 4M tracks for alignment and 1M for validation
➠ mean of residuals sensitive to module displacement
➠ include only modules with >30 (200) hits in tracker (pixel)
➠ exploit Kalman fit for algorithm with B-Field
RMS=26µm
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☞ Measure dE/ρdx with pointing muons
➠ muons measured in the tracker
➠ estimate track length form track
propagation inside ECAL crystal
Demonstrates correctness of the tracker
momentum and the ECAL energy scales
☞ High
energy
ECAL events are
due to muon brem
☞ Rate of events
with cluster above
2 GeV is 0.3%
dE/ ρdx (MeV cm2/g)
ECAL Cosmic Analysis
10
Experimental data vs
expected stopping power
for PbWO4
ionization loss
brem radiation
1
1
10
102
p (GeV/c)
CRAFT
200 GeV
@
@
R
@
E=290 GeV
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HCAL Muon Response
☞ Muon track matching in DT and Tracker
➠ 20 GeV/c < pµ <1000 GeV/c
HB energy: signal from HB towers corrected for
muon path length in HB
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Muon Cosmic Analysis
☞ Behavior of detectors in B-Field was verified using
Monte Carlo simulation and data observables
☞ Reasonable agreement between data and MC
Every aspect of CMS from detector to software has
to work to obtain these plots
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First Beam Data
Beam splash event in CMS
Correlation ECAL energy with energy
measured by HCAL for beam splash events
☞ Single beam shoots protons onto closed
collimators 150 m upstream of CMS
☞ Huge energy deposits by secondaries
☞ Around hundreds particles cross one
ECAL crystal
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Summary
☞ The Large Hadron Collider (LHC) will be a unique tool for fundamental physics research
and will be the highest energy accelerator in the world for many years following its
completion.
☞ It was constructed using the frontier technologies and new computing developments of
the last years.
☞ The CMS detector is a general purpose detector ideally suited to identify and reconstruct
particles from pp-collisions
➠ very large tracker volume, high B-Field (4 T);
➠ large lever-arm for muons;
➠ fine granularity, high resolution ECAL;
➠ nearly full solid angle coverage HCAL
☞ Detector has been completed and tested with cosmic ray and single beam events of LHC
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