The Large Hadron Collider -Exploring a New Energy Frontier Undergraduate Physics Colloquium T. Ferguson February 13, 2009

Outline Short history of particle physics, accelerators, and the Standard Model CERN and the LHC accelerator CMS Experiment A few hoped-for physics discoveries Status and plans

Two Different Types of Accelerators Until about 1970, all accelerators produced a beam of either high-energy electrons or protons, which were then extracted from the accelerator and directed onto a target. The “fixed-target” experiments were built around the target. Starting in the 1970’s, colliding-beam accelerators started in be built, in which 2 beams of particles travel in opposite directions in a circular ring, colliding head on. The “colliding-beam” experiments are built around the collision points.

Advantages of Colliding-Beam Accelerators All of the beam energy can be used for particle production. More “violent” collisions. The particles in the beams can be used over and over again. More efficient. The detectors are in the center-of-mass frame so particles are produced more uniformly in all directions. Easier to detect.

Why do we always want higher energy? The higher the energy accelerator, the larger the mass of particles that can be produced. Just energy conservation and . To probe the structure of a particle to a certain scale, D, the wavelength of the probe must be less than D. The wavelength is inversely proportional to the energy:

Colliding-Beam Accelerators Today Tevatron e+(3.1 GeV) on e- (9 GeV) PEP-II p anti-proton collisions at 1.96 TeV p (920 GeV) on e (27 GeV) BELLE HERA

The Periodic Table – D. Mendeleev (1869)

The “Newer” Periodic Table

What do we know about Particles and their Interactions? “Standard Model” Well-understood theory of forces/particles with 18 particles and 2 forces: the electroweak force and the strong force. Higgs 1960’s EWK We have used the results from these machines to develop 1970’s QCD

LHC Experiments ATLAS CMS LHC-b

LHC Magnets in the Tunnel

Path of the beams in the LHC 20

1232 magnets required. Cooled to 1. 9 K. Have current of 12,000 amps 1232 magnets required. Cooled to 1.9 K. Have current of 12,000 amps. Produces 8.6 Tesla field.

Compact Muon Solenoid (CMS) Detector

Ordered by size: USA (525 collaborators), Italy (398), Russia (326), CERN (204), France (146), UK (117), Germany (116)

One CMS Endcap disk being lowered down the 100 m shaft, with another disk already in place.

Cosmic-ray Muon Reconstruction First 3T magnet test underground in August 2008. 3.1 GeV 6.2 GeV 27 38.1GeV 8.1 GeV 12.6GeV

Higgs Boson In 1960’s, Glashow, Weinberg, Salam showed that the E&M and weak forces could be combined into an overall electroweak interaction. But masses of force carriers are quite different: m(photon) = 0, m(W) = 80 GeV, m(Z) = 91 GeV. What breaks the symmetry? (1964) Peter Higgs and others proposed the Higgs mechanism: a Higgs field which couples differently to the photon, W, and Z. This field breaks the symmetry. Implications: 1) This field gives mass to ALL massive particles. So this is how mass is generated in the Standard Model. 2) Predicts the existence of a massive, spin-0 (boson) Higgs particle – the last undiscovered particle in the Standard Model.

Search for the Higgs Particle, H The Higgs has been searched for ever since its inception. Lower limit on its mass is now m(Higgs) > 114 GeV. Theory start to break down if m(Higgs) > 600 GeV. This is the perfect range for discovery at the LHC. Theoretical production cross section is very low – so expect few events. “Needle in a haystack”. The best Higgs decay mode to use depends critically on the unknown Higgs mass. For a mass below 130 GeV, H -> 2 photons is the best mode. For masses above 130 GeV, H -> ZZ -> 4 leptons dominates.

Higgs Decay to 4 Muons

Beyond the Standard Model The LHC is a “discovery” accelerator. It can produce any new particle with a mass up to about 2 TeV – 7 times higher than the present highest-energy accelerator. There are a multitude of theories predicting a huge variety of possible new particles that could be produced at the LHC. One of the most popular theories is called super-symmetry (SUSY).

Supersymmetry (SUSY) Standard-Model calculations of several processes give infinities at energies around 1 TeV. Supersymmetry fixes this problem and allows the Standard Model to be extended up to the Planck scale of 10^19 GeV. Theory – for every known particle there is a supersymmetric partner whose spin differs by ½ unit. quark (s = ½)  squark (s = 0) lepton (s = ½)  slepton (s = 0) photon (s = 1)  photino (s = ½) etc. This idea is included in most Grand Unified Theories and in all string theories.

Problem: no supersymmetric particle has ever been seen Problem: no supersymmetric particle has ever been seen! Searches have put lower limits on their masses in the range 80 – 150 GeV. So there must be a large symmetry breaking. Theory can’t predict their exact masses or even the mass hierarchy. But for the theory to make sense, they must have masses below about 1 TeV – exactly in the range for discovery at the LHC. So if true, there will be a whole zoo of new supersymmetric particles to discover.

Dark Matter/ Dark Energy Big bang nucleosynthesis, light element abundances, and CMB all say only 4% of the Universe is ordinary matter Should be a particle with mass around a GeV, weakly interacting, electrically neutral, not colored; from WMAP data and cosmological constraints There is no good candidate among the known particles for “Dark Matter”. Possible candidate is the lightest super-symmetric particle.

LHC Status April 2007 - final LHC dipole magnet lowered into the tunnel. June 2008 – last LHC sector cooled to 1.9 K. August 2008 - vacuum pipe closed. September 13, 2008 – Begin injection studies at 400 GeV. September 19, 2008 – While testing last sector of magnets at full current, electrical arc causes damage to about 25 magnets. Repairs will be finished in time for Summer 2009 start-up. First collisions by September 2009.

First Events: Collimators Closed ~2.109 protons on collimator ~150 m upstream of CMS ECAL- pink; HB,HE - light blue; HO,HF - dark blue; Muon DT - green; Tracker Off

Summary After 25 years of planning and 15 years of R&D and construction, the LHC and its 4 experiments will begin running this year. With an energy 7 times larger than the present highest-energy accelerator, the LHC has the possibility to discover a host of new particles and answer fundamental questions of physics.