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Introduction to
Particle Collider
Experiments Suyong Choi
Korea University
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Contents
• http://goo.gl/VlBhfq
• Preliminaries
• Particle Colliders
• Cross Section and Luminosities
• Particle detectors for Particle Colliders
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PRELIMINARIES
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Natural Units
• ℏ = 1, 𝑐 = 1
• 𝐸2 = 𝑚2 + 𝑝 2
• Δ𝑝Δ𝑥 ≥1
2
• Recover correct dimensions later by multiplying suitable
combinations of
• ℏ𝑐 = 197 𝑀𝑒𝑉 ⋅ 𝑓𝑚
• 𝑐 = 3 × 108 𝑚/𝑠
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Four Vector
• Four vector: 𝐸, 𝑝𝑥, 𝑝𝑦, 𝑝𝑧 = 𝑝
• Sum of two four vectors:
• 𝐸1, 𝑝1𝑥, 𝑝1𝑦, 𝑝1𝑧 + 𝐸2, 𝑝2𝑥 , 𝑝2𝑦, 𝑝2𝑧 =
𝐸1 + 𝐸2, 𝑝1𝑥 + 𝑝2𝑥 , 𝑝1𝑦 + 𝑝2𝑦, 𝑝1𝑧 + 𝑝2𝑧
• Lorentz Invariant: 𝐸2 − 𝑝 2 = 𝑝2
• In case of a real particle, this is its mass
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Definitions and Relations
• 𝛽 =𝑣
𝑐= 𝑣
• 𝛾 =1
1−𝛽2
• 𝑝 =𝑚𝛽
1−𝛽2= 𝑚𝛽𝛾
• 𝐸 = 𝑚𝛾
• For 𝛽 → 1, 𝐸 ≈ 𝑝
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PARTICLE COLLIDERS
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LHC
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LHC 가속기 및 실험
지하 100m, 둘레 27km
ATLAS, CMS, ALICE, LHCb, TOTEM, LHCf
CERN 연구소
1954년 설립, 유럽 20 개의 회원국 참가
핵 및 입자 물리 관련 순수 과학 연구
연간 약 1.3 조원 독립 예산 (스페인 1200억 분담)
다수 특허 보유 – World Wide Web의 발상지
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CERN Accelerator
Complex
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Particle Colliders
• Center of mass energy
• 𝑠 = 2𝐸𝑏𝑒𝑎𝑚
• 𝑝𝑝 or 𝑝𝑝 Collider
• 𝑠: 7,8,13,14 TeV for LHC
• Described well by quarks/gluons collisions
• they carry 1/3 of proton momentum on average
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c
쿼크
발견
b
쿼크발견
t
쿼크발견
W,Z
발견
힉스발견
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Accelerator Technologies
• New accelerator
technologies needed
to reach factor of
10~100 in Energy
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LHC Dipole Magnet
• 1232 15 meter dipole
magnets in LHC
tunnel
• 11000 ampere current
max
• 8.5 T max. B field
• 1.9K operating
temperature
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Charged Particle Motion
in Constant B Field
• Motion of charged particle perpendicular to 𝐵
𝛾𝑚𝑣2
𝑟= 𝑞𝑣𝐵 ⇒ 𝛾𝑚𝑣 = 𝑝𝑇 = 𝑞𝐵𝑟
• Practical equation: 𝑝𝑇𝐺𝑒𝑉
𝑐= 0.3 × 𝐵 𝑇 × 𝑟[𝑚]
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Storage Rings
• Storage ring • Probability of interaction
is low at high energies
• Only a small fraction of particles interact
• Need to recycle the beams
• LHC has two vacuum rings that cross at collision points
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Fixed Target vs Collider
• Colliding beam experiment
• First proposed in 1956 by Kerst
• Oppositely directed beam with same energy
• Center of mass energy is 2𝐸 whereas in fixed target it is
2𝑚𝑡𝑎𝑟𝑔𝑒𝑡𝐸
• 1 TeV=1000 GeV energy beam, in a fixed target can only
produce 44 GeV of useful energy
• In collider, it is 2000 GeV
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𝑒+𝑒− and Hadron Colliders
• Electron collider
• Clean
• Precision measurement
• Due to synchrotron radiation, difficult to get to high energies (radiated power ~ 𝛾4/𝑟2)
• Hadron collider
• Easier to accelerate to high energies
• Proton breakup produces many particles
• Better for search for heavy particles – continuous range of E
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CROSS SECTION
AND LUMINOSITIES
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• Cross section for inelastic collision is ~0.1 barn at 10 TeV CM Energy (1 barn=10-24 cm2)
• If two protons are separated by distance less than b, then they collide and break up the proton
• cross section: 𝜎 = 𝜋𝑏2
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b
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Naïve Cross Section
• If beam energy is the only scale in the system, then
𝜎 ∝1
𝐸2
• From dimensional analysis
• If other scale present, then this is not valid
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Cross Section
and Luminosity
• If a certain process has 1 fb=10-15 barn, then probability
for this process to occur is 10-14 times smaller than that to
break up the proton
• In accelerators, you have a “bunch” of particles colliding
• N particles in transverse beam radius of r
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r
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Luminosity
• Probability for interaction?
• Assume particles are uniformly distributed: 𝑁1𝜎
𝜋𝑟2
• By making the beam size r smaller, one can increase probability for interaction
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r
𝑁1
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Luminosity
• Probability of interaction when two bunches collide
𝑃 =𝑁1𝜎
𝜋𝑟2𝑁2
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N1 N2
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Instantaneous
Luminosity
• Rate of interaction and Instantaneous Luminosity
• In an accelerator bunches cross at a fixed frequency
𝑅 = 𝑓𝑁1𝑁2𝜎
𝜋𝑟2= ℒ𝜎
• Unit of instantaneous luminosity is usually in 𝑐𝑚−2𝑠−1
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May 13 2008
24 Collisions at the LHC
Bunch Crossing 4 10 7 Hz
7x10 12 eV Beam Energy 10 34 cm -2 s -1
Luminosity 2835 Bunches/Beam 10 11
Protons/Bunch
7/8/13/14 TeV Proton Proton colliding beams
Proton Collisions 10 9 Hz
Parton Collisions
New Particle Production 10 -5 Hz
(Higgs, SUSY, ....)
p p H
µ +
µ -
µ +
µ -
Z
Z p p
e - n
e
m +
m -
q
q
q
q
c 1
-
g ~
~
c 2 0 ~
q ~
c 1 0 ~
Selection of 1 event in 10,000,000,000,000
7.5 m (25 ns)
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Cross Sections
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Integrated Luminosity
• 𝐿 = ℒ𝑑𝑡
• Unit of integrated luminosity: cm-2
• 𝜎 ℒ𝑑𝑡 : Expected number of events for certain process
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1 b-1 1024 cm-2
1 mb-1 = 103 b-1 1027 cm-2
1 mb-1 = 103 mb-1 1030 cm-2
1 nb-1 = 103 mb-1 1033 cm-2
1 pb-1 ?
1 fb-1 ?
“inverse
picobarn”
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𝐻 → 𝑍𝑍 → 4ℓ 27
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PARTICLE DETECTION IN
COLLIDER EXPERIMENTS
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Particle Detectors at
Coliders
• Symmetric, Layered, Hermetic
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Particle Detectors at
Colliders
• Most collider detectors are “general purpose” detectors
• Sensitive to all kinds of physics processes
• Detect variety of particles
• Large acceptance: from low to high 𝑝𝑇, low to high angle
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Particle Detection
• Energy loss of particles in medium
light, temperature change, ionization
detectable signal
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Particle
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Principle of Particle
Detection
• To detect a particle
• The particle must reach the detector → particle must be long-lived
• AND interact with detector material
→ Ionize the material → light signal or electron signal
→ measurable signal above noise
• To distinguish particle types
• Different types interact differently
• Goal: measure particle type and momentum (or energy)
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General Particle Detector
Concept
• Different particles interact differently with material
• Electron and gamma interact early
• Hadronic particles undergo nuclear interactions with material
• Muons pass through
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EM Interactions
of Particles in Material
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Energy Loss by Charged
Particles Through Ionization
• Minimum ionizing
particle when 𝛽𝛾~3
−𝑑𝐸
𝑑𝑥∼ 2(𝑀𝑒𝑉 ⋅ 𝑐𝑚2/𝑔)
× 𝜌(𝑔
𝑐𝑚3)
• Relativistic rise ~ log 𝛽𝛾
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Radiative Energy Loss of
Relativistic Electrons
• Bremsstrahlung
• Depends on particle mass 1
𝑚2
• Much more important for 𝑒±
• Loss of energy
−𝑑𝐸
𝑑𝑥∝ 𝐸
• Radiation length 𝑋0 length scale where energy reduces to 1/e • Value depends on material
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Electromagnetic Shower
• Ultrarelativistic electron/positron or high-energy 𝛾
• Bremsstrahlung pair production Bremsstrahlung
• Shower of electrons and photons are produced
• Process continues until electron energies are below the
critical energy for Bremsstrahlung
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Hadronic Shower
• Pion, Kaon, proton, neutrons can interact with nucleus
• Nuclear cross sections are much smaller than EM interaction cross sections • Longer and wider showers
• Interaction length (𝜆) • scale of interaction
• Depends on material
• 𝜆 = 18𝑐𝑚 , 𝑋0 = 0.6𝑐𝑚 for lead
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The depths not
drawn to scale
Hadronic Jets
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CMS Detector 39
+z
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CMS Quarter View 40
+z
𝜂 = − ln tan𝜃
2
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CMS Slice 41
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Measuring 𝑝𝑇 of Charged
Particles
• Radius of curvature R 𝑝𝑇 = 0.3𝑞𝐵𝑅
• Momentum resolution depends
• position resolution of each layer
• # of layers and distance between layers
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d
R
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Measuring Energy using
Calorimeters
• Energy in calorimeter ~ # of showered particles
• Fluctuations of showering determines energy resolution
• Example) Electromagnetic Calorimeter
• # of visible photons ~ Energy of stopped electron/gamma
• Fluctuation in # of photons → Energy resolution
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Electromagnetic shower
in CMS PbWO4 crystal
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Dilepton Resonances
• Muon momentum
• Tracker + muon detectors
• Detector alignment
• Electron energy
• Electromagnetic
calorimeter + tracker
• Effects of resolution
and event selection seen
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EVENT RECONSTRUCTION AND
ANALYSIS
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The Data Problem
• Cannot save all collision data
• Inelastic collision cross section is very large
• Trigger
• Select interesting events only
• High-pT objects
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Event Reconstruction
• From electronic signals to particles (physics objects)
• Complex algorithms to reject noise
• Track reconstruction
• Jets
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Hit Positions Track
segments Tracks
Calorimeter Energies
Clustering Jets
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Data Analysis
• Preselection • Background rich sample
• Compare with known SM backgrounds (usually simulation)
• Test understanding of the detector and reconstruction algorithms
• Event selection • Signal rich sample
• Compare with signal+bkgd prediction
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Collected
Data Signal
Region
Collected Data
Background
Preselection
Event Selection
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Cross Section
Measurement
• Cross section formula: 𝜎 =𝑁 𝑜𝑓 𝐸𝑣𝑒𝑛𝑡𝑠−𝐵𝑘𝑔𝑑.𝐸𝑣𝑒𝑛𝑡𝑠
𝐴×𝜖× ℒ𝑑𝑡
• Acceptance (𝐴) – fraction of signal events that can be detected
• Kinematic – Certain momentum needed to reach detector
• Geometric – finite coverage, cracks
• Efficiency (𝜖) – fraction of accepted signal events that pass event selection
• Acceptance and efficiency measured from Simulated Events assuming theory correctly reproduces kinematics of the event
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Parameter Extraction
• For a given theory with {𝜃𝑖} parameters, predict expected
number of signal events
𝑆 = 𝐴({𝜃𝑖}) × 𝜖({𝜃𝑖}) × 𝜎({𝜃𝑖}) ℒ𝑑𝑡
• And compare with number from experiment: 𝑁 − 𝐵
• Uncertainty of 𝑁 − 𝐵 is 𝑁 𝛿𝐵 = 𝑁 + 𝛿𝐵 2
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SELECT CMS RESULTS
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Running 𝛼𝑠
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Cross Sections Measured
at CMS
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Particle Searches
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LHC Long Term Schedule
• Operation up to 2035
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7,8 TeV LS 1 Run 2
13,14 TeV LS 2
Run 3
14 TeV LS 3
~2035
Run 4,…
14 TeV
2013 2010 2015 2018 2020 2023 2025
25 fb-1 100 fb-1 300 fb-1 3000 fb-1
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HANDS ON WITH REAL CMS
EVENT
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CMS Event Display
• http://www.i2u2.org/elab/cms/event-display/
• Click “folder” → Select “collections” to look for various selected real data events
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한국-CMS 실험 사업
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한국 CMS 참여
• 1998-2011년까지 한국은 CMS검출기 제작에 총 20억원을
현금/현물로 지원 초전도자석의 테이블 (한국 중공업)
• 전방 뮤온 검출기 중 RPC 제작
• DAQ시스템 건설에 공헌
• 2007년부터 한-CERN 사업 안에서 진행
• 2014년 현황
• 8개 대학, 교수 14인, 17인 postdoc 포함 총 80 명
• 예산: 14.5억 (체재비, 인건비) + 6.7억 (검출기 R&D) = 21.2억
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1998-2006 : 자석 회전 장치 60
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1998-2006: 전방 뮤온
RPC 검출기 61
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전방 저항판 (RPC) 검출기 RE4
제작 (2013-2014)
• KCMS RPC gap 660장 전량 공급
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BACKUP
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Cyclotron
• 1932 Lawrence and Livingston at Berkeley
• Accelerate with fixed RF frequency and constant B-field
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60 Inch Cyclotron (1939)
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220 ton magnet
16 MeV
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184 inch Cyclotron (1942)
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4000 ton magnet
200 MeV
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Linear Accelerator
• In a uniform wave guide, 𝑣𝑝ℎ𝑎𝑠𝑒 > 𝑐, RF field quickly
becomes out of phase with particles
• We need a disk-loaded wave guide
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time
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Accelerator Structure
• Focusing
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Synchrotron Accelerator
• Particle accelerates in closed path
• RF accelerating field should be synchronized with beam
• B field should be increased as E increases
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Bending Charged
Particles
• Dipole magnetic field is used to bend
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Mandelstam Variables
• Assume process
• In scattering between two particles, one can define 3 variables
𝑠 = 𝑝𝐴 + 𝑝𝐵2 = 𝑝𝐶 + 𝑝𝐷
2
𝑡 = 𝑝𝐶 − 𝑝𝐴2 = 𝑝𝐵 − 𝑝𝐷
2
𝑢 = 𝑝𝐴 − 𝑝𝐷2 = 𝑝𝐶 − 𝑝𝐵
2
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pD
pC
pB pA
𝑝𝐴 + 𝑝𝐵 = 𝑝𝐶 + 𝑝𝐷
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KEKB
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• 1998.12 ~ 2010. 6
• Asymmetric collider
𝐸𝑒− = 8 𝐺𝑒𝑉 𝐸𝑒+ = 3.5 𝐺𝑒𝑉
• Total integrated luminosity: 1052 fb-1
• CKM Matrix and b-quark sector probed to unprecedented degree
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Tevatron
• Proton on antiproton at 1.96 TeV
• Discovered top quark in 1995
• 20 years of running (1989-2009)
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