Hadron Collider Hadron Collider PhysicsPhysics

Jay HauserJay HauserUCLAUCLA

Some slides copied from Peter Some slides copied from Peter Richardson (Durham U.)Richardson (Durham U.)

OutlineOutline

1)1) A short physics introductionA short physics introduction Electroweak unification and the Higgs BosonElectroweak unification and the Higgs Boson

2)2) Why Hadron Colliders?Why Hadron Colliders? Going beyond fixed-target and eGoing beyond fixed-target and e++ee-- circular colliders. circular colliders. Constituents of the proton.Constituents of the proton. How to calculate cross-sections in proton collisions.How to calculate cross-sections in proton collisions.

3)3) Current generation of hadron Current generation of hadron colliderscolliders

Tevatron at Fermilab near Chicago.Tevatron at Fermilab near Chicago. Large Hadron Collider at CERN (Switzerland)Large Hadron Collider at CERN (Switzerland)

4)4) Future linear Future linear ee++ee-- and muon colliders and muon colliders

Ideas of Force UnificationIdeas of Force Unification

-

18731967 theory

1983 expt.

1686, 1915

Weak + Electromagnetic Weak + Electromagnetic UnificationUnification

• Energy scale about 100 GeV.

• The theory hinges on the “Higgs” particle, energy<1000 GeV.

• Enigmatic Higgs particle is not yet observed, does it exist?

• If the Higgs doesn’t exist, there is a theorem that there must be some additional force to be discovered, with effects visible in the <1000 GeV range.

• Goal of the “current” generation of colliders is to find the Higgs or its replacement.

• Current “Tevatron” probably can’t find the Higgs, future “LHC” probably will (year ~2009).

Why Hadron Colliders (I)?Why Hadron Colliders (I)?

Before colliders, there was “fixed-target” Before colliders, there was “fixed-target” – a beam of particles hits a block of – a beam of particles hits a block of matter.matter.– Modern era of accelerators started in 1931.Modern era of accelerators started in 1931.– Relativistic disadvantage: ERelativistic disadvantage: ECMCM increases increases

slowly as sqrt(Eslowly as sqrt(Ebeambeam)) Colliders: counter-rotating beams within Colliders: counter-rotating beams within

a vacuum pipe.a vacuum pipe.– Advantage: EAdvantage: ECMCM = 2*E = 2*Ebeambeam increases faster increases faster– Developed around 1970.Developed around 1970.– Need pretty intense beams for a worthwhile Need pretty intense beams for a worthwhile

rate of interactions.rate of interactions.

Accelerators: Bigger, and Accelerators: Bigger, and Stronger Magnets Stronger Magnets Higher Higher

EnergyEnergy

Berkley 11 inch

This is still pre-WW II

1931 Lawrence and Livingston operate the first Cyclotron

Modern Particle Modern Particle AcceleratorsAccelerators

The particles gain energy by surfing on the electric fields of well-timed radio oscillations (in a cavity like a microwave oven)

The particles are guided around a ring by strong magnets so they can gain energy over many cycles and then remain stored for days

Historical Development of Historical Development of CollidersColliders

Beam

More energy: colliding beams

Beam hits matter (fixed-target)

Why Hadron Colliders (II)?Why Hadron Colliders (II)?

Electron-positron (eElectron-positron (e++ee--) colliders ) colliders (>1970) are excellent!(>1970) are excellent!– The Feynman diagrams are simple.The Feynman diagrams are simple.– Positrons circulate in the same set of Positrons circulate in the same set of

magnets as the electrons.magnets as the electrons.– You know the initial state energy and You know the initial state energy and

momentum (zero) precisely by the momentum (zero) precisely by the magnetic field and the radius of the magnetic field and the radius of the electron path.electron path.

Many great things were discovered Many great things were discovered with ewith e++ee--, but…, but…

e-e- e+e+

Why Hadron Colliders (III)?Why Hadron Colliders (III)?

Electrons are by far the lightest particles.Electrons are by far the lightest particles. Therefore, magnetic fields accelerate them a lot, and Therefore, magnetic fields accelerate them a lot, and

acceleration of charges results in electromagnetic acceleration of charges results in electromagnetic “synchrotron” radiation.“synchrotron” radiation.

For highly relativistic particles, this radiation depends on the For highly relativistic particles, this radiation depends on the relativistic relativistic =E/mc=E/mc22 factor as: factor as:

So electrons, having the lowest mass, radiate like So electrons, having the lowest mass, radiate like crazycrazy if E if E (hence (hence ) is high.) is high.

Circular eCircular e++ee-- machines topped out with the LEP accelerator, machines topped out with the LEP accelerator, which reached about 200 GeV energy, which required massive which reached about 200 GeV energy, which required massive amounts of power to keep the electrons going around.amounts of power to keep the electrons going around.

So if you want more center-of-mass energy than that, use So if you want more center-of-mass energy than that, use protons, which are 2000 times more massive than the protons, which are 2000 times more massive than the electron.electron.

Colliding Proton/Antiproton Colliding Proton/Antiproton BeamsBeams

No problem with synchrotron radiation energy loss, but…

Like throwing bags of marbles at each other at high velocity:

Marble-marble collisions are interesting, not bag-bag collisions

Fortunately, the number and arrangements of the “marbles” has been measured by other experiments

Proton Constituent ParticlesProton Constituent Particles

• Proton internal structure is quarks and gluons

• Also, there are 2 up (u) and 1 down (d) “valence” quarks

• There are also gluons, holding them together, that carry 50% of the proton momentum!

• There are also “sea” quarks!

• Argh – at a fundamental level, what is the beam??

Probability FunctionsProbability Functions

• Suppose the beam energy was 1000 GeV, and you knew that each valence quark carried 1/6 of the proton momentum, and there were 5 gluons carrying 1/10 of the proton momentum, and there were no sea quarks.

• Then you would have 2 u and 1 d quarks of 167 GeV, and 5 gluons of 100 GeV in each proton.

• Assuming no multiple scattering (Born approximation of scattering Quantum Mechanics), you could calculate all the scattering.

Real Hadron ScatteringReal Hadron Scattering

• The constituents don’t take on exact fractions of proton momentum, but have probability functions called “parton distribution functions” (PDFs)

• PDFs measured in high-energy electron-proton and neutrino-proton experiments. Good cross-checks between them, so it all makes sense.

• The initial state is a probability function you have to integrate over.

• For example, for quark-antiquark scattering:

= Integral [ (fundamental cross-section) * (prob that beam A has quark with momentum PA) * (prob that beam B has antiquark with momentum PB) ]

Top EventsTop Events

• Let’s start by thinking about how the Let’s start by thinking about how the top quark is producedtop quark is produced

• The top quark is produced by the The top quark is produced by the strong interaction.strong interaction.

Collider ExperimentsCollider Experiments

Particle Physics Particle Physics DetectorsDetectors

• A tracking chamber measures the energies of charged particles (with aid of a big magnet to bend them)

• A calorimeter measures energies of neutral particles

• A muon system sees only penetrating muon particles

• Used to take pictures (bubble chambers), now we use fully electronic readout

Timeline of Proton Collider Timeline of Proton Collider DiscoveriesDiscoveries

1975 1980 1985 1990 1995 2000 2005 2010 2015 2020

W/Z bosons Top quark Higgs Supersymmetry

Proton-Proton-protonproton

Proton-antiprotonProton-antiprotonProton-protonProton-proton

Proton-Antiproton Proton-Antiproton Collisions at Fermilab Collisions at Fermilab

(Chicago)(Chicago)

The Tevatron The Tevatron accelerator, 6 km accelerator, 6 km circumferencecircumference

The CDF (Collider Detector at Fermilab)

experiment

The LHC (Large Hadron Collider) The LHC (Large Hadron Collider) at the CERN Laboratoryat the CERN Laboratory

The CERN The CERN Laboratory near Laboratory near

Geneva, Geneva, SwitzerlandSwitzerland

France

Switzerland

The LHCThe LHC The LHC is built in the old The LHC is built in the old

LEP tunnel at CERN near LEP tunnel at CERN near Geneva.Geneva.

It will collide protons at an It will collide protons at an energy of 14 TeV starting in energy of 14 TeV starting in 2007.2007.

There will be four There will be four experiments.experiments.

Two ATLAS and CMS to look Two ATLAS and CMS to look for the Higgs, LHCB to look for the Higgs, LHCB to look at B physics and ALICE for at B physics and ALICE for heavy ion physics.heavy ion physics.

Here I will concentrate on Here I will concentrate on the physics for ATLAS and the physics for ATLAS and CMS.CMS.

LHC ExperimentsLHC Experiments

The two general The two general purpose LHC purpose LHC experiments ATLAS experiments ATLAS and CMS follow the and CMS follow the general design we general design we have just have just considered.considered.

ATLAS

CMS

The CMS Endcap Muon The CMS Endcap Muon SystemSystem

• Chambers produced at Fermilab

• Equipping with electronics and testing at UCLA

• 300,000 data channels “trigger” electronics built by UCLA

• Support from UC Riverside and UC Davis scientists

Start from:

40 million events/sec

x10 million sec/year (30% run eff.)

x10 years

=4x1015 events

Data AnalysisData Analysis

End result:

Search for Higgs particle

Look for data > background rate

~40 events excess

10-14 factor:

Each Higgs event is like a 1g needle in a 100 million metric ton haystack

How to Find Needles in Large How to Find Needles in Large Haystacks...Haystacks...

Multi-step approach:

I) Special-purpose 40 MHz Electronics

“Level 1 Trigger”

II) Fast “online” Computers

“Level 2 Trigger”

III) “Offline” Analysis

Crunch Petabyte data store

(1 Million Gigabytes)

UCLA

UCSD & UCLA

Caltech

Higgs SignalsHiggs Signals By looking at a large By looking at a large

number of different number of different signals the LHC can signals the LHC can discover the Higgs over discover the Higgs over a wide mass range.a wide mass range.

This range more than This range more than covers the mass covers the mass excepted from the excepted from the precision electroweak precision electroweak data.data.

The LHC should The LHC should discover the Higgs.discover the Higgs.

Hadron vs. Lepton CollisionsHadron vs. Lepton Collisions

The Tevatron is currently running at 1.96TeV The Tevatron is currently running at 1.96TeV centre-of-mass energy colliding protons and centre-of-mass energy colliding protons and antiprotons. Will continue till LHC start up in 2007.antiprotons. Will continue till LHC start up in 2007.

The LHC will start in 2007 and run for about 10 The LHC will start in 2007 and run for about 10 years colliding protons at 14 TeV.years colliding protons at 14 TeV.

People are working hard on the design of electron-People are working hard on the design of electron-positron positron LinearLinear Colliders for the future. Colliders for the future.– No synchrotron radiationNo synchrotron radiation– But have to accelerate the particles But have to accelerate the particles very forcefullyvery forcefully to to

reach high energy in one pass (linear accelerator already reach high energy in one pass (linear accelerator already many km long)many km long)

The Long-Term Future of The Long-Term Future of CollidersColliders

• A future linear collider (ILC for International Linear A future linear collider (ILC for International Linear Collider) operating between 500 GeV and 1 TeV will Collider) operating between 500 GeV and 1 TeV will hopefully be built to start data taking some time after hopefully be built to start data taking some time after LHC start-up. LHC start-up.

• A second generation linear collider operating at energies A second generation linear collider operating at energies of up to 5 TeV could then be built to explore higher of up to 5 TeV could then be built to explore higher

energies (e.g. the CLIC research project at CERN).energies (e.g. the CLIC research project at CERN). • These machines will measure the properties in more These machines will measure the properties in more

detail.detail.• There are other proposals to build a neutrino factory as There are other proposals to build a neutrino factory as

the first step towards a muon collider. This would be on the first step towards a muon collider. This would be on the same sort of timescale as a second generation linear the same sort of timescale as a second generation linear collider.collider.