Deciphering How the Universe Works
The need for a BIG Electron Collider

 

In the fall of 2000, the world’s most powerful electron collider ceased operation at the CERN laboratory in Geneva, Switzerland. This leaves Fermilab’s Tevatron proton collider (near Chicago) and DESY's HERA electron-proton collider (in Hamburg, Germany) as the only particle accelerators probing for new physics at the energy frontier until CERN’s new LHC proton collider turns on in 2005. No new electron colliders are approved, but several teams of physicists around the world are finalizing designs for a bold new machine that many believe is needed to solve some of the key problems of particle physics and cosmology.

     Currently, we have a successful Standard Model of Particles and Forces, which describes the fundamental particles and the laws of physics that govern their motion and evolution in time. We also have a successful Standard Model of Cosmology that describes the structure and composition of today’s Universe and how it evolved from a Big Bang and a very fast expansion period (Inflation). These Standard Models are a culmination of great achievements of physics in the 20th century, incorporating the revolutionary ideas of relativity, quantum mechanics and gauge theories. Powerful particle accelerators, particle detectors and astronomical telescopes have provided the data necessary for testing and developing these models. Yet many mysteries remain, such as "What mechanism generates the mass of particles?" and "What is the dark matter of the Universe?"

     At the start of the 20th century we learned that Newton’s laws of motion had a limited range of validity, and that Einstein’s theory of special relativity was needed to explain motion of particles traveling near the speed of light. At the start of the 21st century, we know that today’s Standard Models are also incomplete and have limited applicability. Today’s particle physicists and astrophysicists strive to complete our answers to the fundamental questions of particle physics and cosmology. Three fundamental questions the particle physicists must answer are:
                                       "What is the geometry of the Universe?"
                                       "What are the fundamental particles?"
                                       "What are the laws of motion and evolution for fundamental particles?"
Three fundamental questions the astrophysicists must answer are:
                                       "What were the initial conditions of the Universe?"
                                       "How does the Universe evolve?"
                                       "What is the ultimate fate of the Universe?"
These questions could have been asked (and presumably were) by Newton four centuries ago.   They are as fundamental today as they were then.

     New tools (particle accelerators, particle detectors, and telescopes) and a wealth of new data are needed to progress towards fully answering these fundamental questions. In the summer of 2001, a community of 500 particle physicists will gather at Snowmass in the Colorado Rockies to evaluate current data and today’s Standard Models. They will work to forge a plan for what new tools and new measurements should be given priority for the next 20 years. One constituent of this community will argue that a powerful new electron accelerator and collider is needed to progress towards fully answering these fundamental questions and to solve some of the current mysteries. Such a machine is needed to complete the exploration of the energy range at which the weak and electromagnetic forces become comparable in strength, and where they fully exhibit their unification as the electroweak force. In this energy range we expect to find the elusive Higgs particle that can explain why particles have mass; and we hope to understand the wide range of particle masses that exist, from the nearly massless neutrinos to the ultra-heavy top quark. We also hope to find the particles predicted by a new symmetry of the Universe, something known as Supersymmetry (SUSY). This symmetry could explain the wide range of particle masses, and be instrumental in unifying the electroweak force with the strong force and gravity. The lightest SUSY particle, the neutralino, is a leading candidate for the so-called dark matter of the Universe that is needed to reconcile observations of the gravitational motion of stellar systems with observations of the radiation they emit. Understanding the matter density and energy density of the Universe will tell us whether the Universe will continue to expand forever, eventually collapse on itself into a Big Crunch, or have the critical density of a flat Universe as predicted by the Standard Model of cosmology. A consequence of SUSY is a doubling of the number of fundamental particles, just as Dirac’s theory for the dynamics of electrons postulated the existence of an anti-electron and led to a doubling of the known particles at that time. Without an electron collider to probe the electroweak unification energy range, it will not be possible to form a complete picture of particles and forces. Its precise measurements are likely to yield critical information for cosmology, and for formulation of models to explain physics at the highest energy scales, where the gravitational force becomes as strong as the other forces. This powerful electron machine would be of a comparable scale and cost (multi-billion) as the giant LEP and LHC machines at CERN in Geneva, Switzerland. It will therefore have be part of a large international effort.

 

The Historical Role of Accelerators & Colliders

     Many major discoveries have occurred at particle accelerators in the last 40 years.  Key particle discoveries at proton accelerators include the charm and bottom and top quarks, the tau neutrino and the W and Z. Key particle discoveries at electron accelerators include the discovery of quarks inside the proton, the charm quark, the tau lepton, and the gluon. Precise measurements of  Z production at the SLAC and CERN electron colliders in 1989 determined that only 3 types of neutrino exist, implying as well that only 3 families of leptons and quarks exist. This was very important for both particle physics and cosmology, and confirmed an earlier cosmological prediction that only 3 neutrino types could exist in order to explain the relative abundance of light elements in the universe.

     Some of these discoveries have been made in fixed target experiments, where the high energy accelerated beam of protons or electrons hits a stationary target and the detectors sift for tell-tale signs (signatures) of interesting physics in the debris of particles that burst forth from the target. The discovery of quarks at SLAC, the charm quark at Brookhaven, and the bottom quark and tau neutrino at Fermilab, occurred in fixed target experiments. But many of the discoveries have required the higher energies achievable in the violent collision of two opposing accelerated beams. Colliding beam experiments have discovered the charm quark and tau lepton at SLAC, the gluon at the DESY accelerator in Hamburg Germany, the W and Z bosons at CERN, and the top quark at Fermilab. The charm quark is an especially interesting case since it was jointly discovered in a proton fixed target experiment at Brookhaven and a colliding electron beam experiment at SLAC. That exciting development in 1974 became known as the November Revolution in particle physics, convincing the last doubters of the validity of the quark model and paving the way for acceptance of the Standard Model.   Burton Richter and Samuel Ting shared the 1976 Nobel prize in physics for the discovery of the charm quark, while Sheldon Glashow, Abdus Salam and Steven Weinberg shared the 1979 Nobel prize in physics for developing the unified theory of electromagnetic and weak forces, the cornerstone of today’s Standard Model.

 

Today’s Colliders and their Capabilities

     The Tevatron proton-antiproton (pp) collider at Fermilab near Chicago, the LEP electron-positron (ee) collider at CERN near Geneva Switzerland, and the HERA electron-proton (ep) collider at DESY in Hamburg Germany are the facilities probing for new physics at the energy frontier today.

     The Tevatron, with its collision energy of 2 TeV, is the highest energy collider and is the only machine today that can produce the very heavy top quark (discovered at the Tevatron in 1994). It will operate until it is surpassed by the new LHC pp collider at CERN, scheduled to begin operation in 2005. The Tevatron has a good chance to find signals for the Higgs or SUSY particles before the LHC; but if they exist, a detailed studied of their properties (spin, lifetime, decay modes etc.) requires the new LHC machine and a future electron collider.

     The LEP machine operated in 2000 at a collision energy up to 210 GeV and reached the limit of its capabilities for energy reach and new discoveries. This machine (together with SLAC’s SLC ee collider, which ceased operation in 1998) succeeded in demonstrating that there are only 3 types of neutrinos, and also gave an accurate prediction of the top quark’s mass before the top quark was found at Fermilab. Results from LEP and SLC give the tantalizing prediction that the Higgs particle should be close at hand, just beyond the reach of the LEP collider but within grasp for the Tevatron. LEP has now begun  decommissioning  in preparation for installation of the new LHC machine in a 27-kilometer tunnel that circles underneath Geneva and the Jura mountains near the border of France and Switzerland.

     The HERA machine collides 800 GeV protons with 30 GeV electrons.  It extends the energy reach of the classic scattering studies begun at SLAC in the 1960s that demonstrated the existence of quarks inside the proton. It is expected to operate for another 5-10 years.

 

The Next-generation Electron Collider

     Except for the SLC ee collider at SLAC, all particle colliders have been built as circular machines. In circular machines, two beams of particles travel in opposite directions in storage rings, and are brought to a fiery collision in the middle of a large detector. But accelerating particles lose energy due to synchrotron radiation, and a particle that is bending in the magnetic field of a circular accelerator has a large acceleration towards the center of the ring. The amount of synchrotron radiation scales as the 4th power of the beam energy and inversely as the 5th power of the beam particle’s mass. For electrons, which are 2000 times lighter than protons, this becomes a severe constraint and dictates that high energy electron circular colliders must have a very large radius, with the radius scaling roughly as the square of the beam energy. (High energy proton colliders must also go to large radius, but for a different reason. It takes stronger magnetic fields to bend higher energy particles and there are limits to how strong we can make the magnets. Superconducting magnet technology is used to reach the highest bend fields possible. But once the limits are reached with this technology, the only solution for going to ever higher energies is to increase the radius of the collider ring. The 7 TeV beams at CERN’s LHC will travel around a 27-kilometer ring. The 20 TeV beams that were planned for the ill-fated SSC machine in Texas would have circled around a 90-kilometer ring.) The cost for such a machine would also scale as the square of the beam energy. The 100 GeV beams at the LEP ee collider travel around a circle, which is 27 kilometers in circumference. The LEP machine is the last of the high-energy circular colliders for electrons due to this limitation.

     A Linear Collider is the natural solution to the scaling problems of a circular collider. But this solution presents many challenges. First, high acceleration gradients are needed to achieve the full beam energy in a single pass. In a circular machine, the beam energy can be ramped up as the beam circulates many times through the accelerating rf cavities. Second, the beams in a linear collider must have high intensity and be focused to very small spotsizes to achieve high collision rates, since the beams only collide once before they are dumped. For a circular machine, the beam particles can be reused many times for collisions, and so smaller beam intensities with less focusing are needed.

     Linear Collider technology has successfully been pioneered at SLAC with its SLC (SLAC Linear Collider) machine. This machine was a prototype for new ee colliders that will succeed the LEP machine. The SLC’s success also provided a remarkable physics program that made very precise measurements of properties of the Z particle. The SLC prototype has led to designs for the next generation of electron colliders with beam energies of 250 GeV and collision energies of 500 GeV. Future energy upgrades of these machines to collision energies of 1 TeV and higher will also be possible. Design efforts for these Linear Colliders are being led by teams of physicists in the United States, Japan, Germany and at the international CERN laboratory in Geneva.

 

 

Nlc.gif (19046 bytes)

Schematic Layout for SLAC’s NLC design of a BIG Electron Collider,
together with SLAC’s SLC ee collider for comparison of scale

 

Last updated:  04-09-2001 by M. Woods