Part V: Experimental Methods
Introduction
Experimental particle physics uses particle accelerators and sophisticated detectors to probe the fundamental structure of matter. Modern experiments at facilities like the LHC, Fermilab, and KEK test theoretical predictions and search for new physics.
Particle Accelerators
Large Hadron Collider (LHC)
The LHC at CERN is the world's largest and most powerful particle accelerator:
- Circumference: 27 km
- Collision energy: $\sqrt{s} = 13.6$ TeV (Run 3, 2022-)
- Luminosity: $\mathcal{L} \approx 2 \times 10^{34}$ cm$^{-2}$s$^{-1}$
- Proton-proton collisions every 25 ns
- ~40 million collisions per second
Other Major Facilities
Fermilab (USA):
Tevatron ($p\bar{p}$, retired 2011)
NOvA, DUNE (neutrino experiments)
KEK (Japan):
Belle II ($e^+e^-$ at $\Upsilon(4S)$)
T2K (neutrino oscillations)
SLAC (USA):
BaBar (retired)
LCLS (X-ray laser)
DESY (Germany):
HERA ($ep$, retired 2007)
European XFEL
Detector Components
Tracking Systems
Track charged particle trajectories using silicon pixels and strips:
- Silicon pixel detectors: spatial resolution $\sim 10 \mu$m
- Drift chambers, straw tubes for outer layers
- Measure momentum: $p_T = 0.3 B R$ (GeV, Tesla, meters)
- Vertex resolution: $\sim 15 \mu$m (crucial for $b$-tagging)
Calorimeters
Measure particle energies by total absorption:
Electromagnetic (ECAL):
Pb-WO$_4$ crystals (CMS)
Liquid argon (ATLAS)
Measure $e^\pm, \gamma$ energies
Resolution: $\sigma_E/E \sim 2-3\%$
Hadronic (HCAL):
Brass/scintillator or steel/scintillator
Measure jet energies
Resolution: $\sigma_E/E \sim 50-100\%/\sqrt{E}$
Muon Systems
Outermost detector layer, identifies muons that penetrate calorimeters:
- Drift tubes, resistive plate chambers (RPCs)
- Muons are minimum ionizing particles (MIPs)
- Combined with tracker for precise momentum measurement
- $p_T$ resolution: $\sim 1-2\%$ at 100 GeV
Trigger and Data Acquisition
Trigger System
Reduces 40 MHz collision rate to ~1 kHz for permanent storage:
Level-1 Trigger (Hardware):
- Custom electronics
- Reduces rate to $\sim 100$ kHz
- Decision in $< 4 \mu$s
- Uses calorimeter and muon info
High-Level Trigger (Software):
- Farm of computers running reconstruction
- Reduces rate to $\sim 1$ kHz
- Full detector information available
- Event size: $\sim 1$ MB → ~1 PB/year
Particle Identification
Signatures
Electrons:
Track + ECAL deposit
$E/p \approx 1$
Photons:
ECAL deposit, no track
Electromagnetic shower
Muons:
Track + muon system hit
Minimal calorimeter energy
Jets (quarks/gluons):
Track + ECAL + HCAL
Hadronic shower
Neutrinos:
Missing transverse energy
$E_T^{\text{miss}} = -\sum \vec{p}_T$
$b$-jets:
Displaced vertex ($c\tau \sim 450 \mu$m)
Soft lepton tag
Key Takeaways
- •LHC: 27 km circumference, $\sqrt{s} = 13.6$ TeV, ~40M collisions/s
- •Detectors: tracking (silicon), calorimeters (EM/hadronic), muon systems
- •Trigger reduces 40 MHz → 1 kHz for storage (~1 PB/year)
- •Particle ID: electrons, photons, muons, jets, neutrinos ($E_T^{\text{miss}}$)
- •$b$-tagging: displaced vertices, lifetime $c\tau \sim 450 \mu$m