LHC

LHC Overview

• The Large Hadron Collider (LHC) is the leading accelerator in the world
• Capable of accelerating protons and heavy ions
• Proton beams at 6.8 TeV
• Lead ion beams at 522 TeV (2.3 TeV per nucleon)
• 17 mile (27 km) long synchrotron
• Up to 574 ft (170 m) underground
• Tilted at a 14 mrad angle due to geological conditions
• Built in pre-existing LEP tunnel
• Located underneath Switzerland and France
• Hadron beams circulate in opposite directions
• Collisions occur at 4 interaction points

LHC magnets

• Complex system of magnets used to steer and control beams
• 1232 main dipoles keep beam along ring
• Superconducting magnets generate a field strength of 8.3 T
• 15 m long and weighs 15 tons
• Sextupole, octupole and decapole magnets help to maintain beam integrity
• Sets of quadrupole magnets guide beams to collision points
• Compress beams to increase proton density (0.2 mm diameter to 20 μm)
• Precisely steer beams into interaction point volume (20 μm x 20 μm x 8 cm)

Accelerating protons

• Protons are sourced from Hydrogen gas
• Electrons are stripped, leaving protons
• Initially accelerated using a linac
• Passed through a series of synchrotrons
• Increasing energy at each stage
• Previous colliders make up acceleration chain
• Heavy ion acceleration is similar

LHC proton beams

• LHC collisions occur in “fills”
• Number of protons decreases throughout a fill
• A fill lasts several hours until the beam is dumped and a new fill is started
• Each beam consists of 2835 “bunches” of protons
• Each bunch begins with ~10¹¹ protons
• Each interaction point has one bunch crossing every 25 ns
• Most protons in a bunch pass without interacting, but a few interact at each crossing
• 600 million collisions per second
• Protons lost continually lost during run

https://op-webtools.web.cern.ch/vistar/vistars.php

Best laid plans

• Originally planned to start running at 14 TeV in 2005
• Budgetary constraints and technical delays pushed start date to 2008
• Initial testing led to quenching and severe damage
• Operations began in 2009 at 2.36 TeV
• Data collected in 2010-211 at 7 TeV
• Currently producing collisions at 13.6 TeV

LHC Run Schedule

Luminosity

Run 1

Run 2

Closer look at hadron collisions

• Hadron collisions are much more complex than lepton collisions
• Two partons can interact in a “hard scatter” event
• Occurs at a primary vertex (PV)
• Initial state radiation and final state radiation
• Typically gluon radiation but can also be photons
• Multiple parton interaction (MPI) hard scatter event can happen
• Remaining partons can scatter elastically into detector

Pileup

• Proton bunch crossings have multiple collisions
• Referred to as in-time pileup
• Quantified using number of primary interaction vertices (N_PV) and average number of interaction per bunch crossing (<μ>)
• Out-of-time pileup also occurs
• Primarily due to collisions in preceding or subsequent bunch crossings
• Additional effects from collisions with beam collimators and stray gas molecules
• Detector latency (read-out time) is generally longer than 25 ns, so additional interactions in that time can result in overlapping detector signals
• Many techniques used to mitigate effects of pileup
• Precise tracking to associate reconstructed particles to interaction vertices
• Other techniques to disentangle pileup calorimeter energy deposits

ATLAS event displays

• Collisions (events) are highly complex
• Can be fully described only as non-human-readable data
• Event displays visually depict reconstructed particles in a collision
• Perspective and transparency optimized for each event

End view

Side view

“Unrolled” view

Pileup - 2 vertices

Pileup - 25 vertices

Pileup - 2, 50, 140 vertices

Pileup profiles

Run 1

Run 2

Main LHC Experiments

ALICE

LHCb

CMS

ATLAS

ALICE

• A Large Ion Collider Experiment
• Specialized for Pb-Pb collisions
• Also uses p-Pb collisions
• Goals are to improve understanding of QCD
• Quark-gluon plasma
• State of matter in which free color charges exist
• Quark deconfinement
• Existence of quarks outside of bound states
• Many discoveries including new tetraquarks and pentaquarks

LHCb

• Large Hadron Collider beauty experiment
• Focused on studying properties of B-hadrons
• Hadrons containing a b quark
• Numerous measurement goals
• B-hadron branching ratios
• Asymmetries in flavor-changing neutral currents
• CP violation in B-hadron decays
• Could explain matter/anti-matter asymmetry
• Other B-hadron decays

ATLAS and CMS

• A Toroidal LHC ApparatuS and Compact Muon Solenoid
• General purpose experiments
• Designed to search for generic particle discovery and measurements
• Similar designs and physics goals
• Primarily focused on pp collisions, but also make use of heavy ion collisions
• Friendly competition for discoveries
• Simultaneous observation of the Higgs boson
• Agreement to inform each other of major discoveries in advance of public announcement
• Harmonization of some techniques to enable comparisons and combinations
• More details next week

Other LHC experiments

• LHCf
• Measurement of particles traveling close to beamline
• MATHUSLA and FASER
• Search for long-lived particles and neutrinos
• Recent detection of neutrinos
• MilliQan
• Search for milli-charged particles
• MOEDAL
• Search for magnetic monopoles and other exotic particles
• TOTEM
• Total cross-section measurements

Future Circular Collider

• Ongoing work towards the Future Circular Collider (FCC) at CERN
• 90-100 km long
• Limited by geography
• ~100 TeV collision energy
• LHC will be in acceleration chain
• At least 30 years expected for design and construction