# Introduction

Short Introduction: Energy and Precision frontiers.

This site is dedicated to the physics of colliding electrons, positrons or gammas at high energy in the hope to bring a comprehensive view of what can be discovered and/or improved in our current understanding of matter and of the Universe.

What is the next step? Following the great progress made at the proton-(anti-)proton colliders (Tevatron and LHC) targeting the “Energy Frontier”, one widely supported option is the short term construction of a large linear or circular electron-positron colliders to tackle the “Precision Frontier”. This is what will be discussed here.

Longer Introduction:

High energy physics proceeds through two complementary tracks: colliding proton against proton (or proton-anti-protons) and colliding e+e-. In the proton case, very high colliding energies are achieved. This is called the “Energy Frontier”. In the second case, due to the more elementary and therefore simpler nature of the electron, e+e- colliders are said to target the “Precision Frontier”.

The proton being a complex system of quarks and gluons (the partons) the actual initial state of the elementary collision is not well defined. Was it a quark colliding another quark? or a gluon, a quark? or a gluon, a gluon? In addition, quarks have different natures or flavors like “up” or “down”. Other favors although not defining the proton like the strange, bottom or top quarks, due to specific quantum effects may be the actual initial colliding elements.

The initial center-of-mass energy is not fixed either. How much momentum of the initial proton is carried by the colliding parton? The detailed study of each event may provide some of this information, but only when the final state  of the collision is fully known.

The analysis of the final state is also complicated by the fact that, aside of the 2 main colliding partons, other components of the same initial protons may have also interacted (creating the so-called underlying events). Moreover in the same bunch of colliding protons, several proton collisions may have been recorded by the detectors (pile-up). The final “picture” is therefore kind of “noisy”.

The Higgs discovery

However, the varying center-of-mass energy can be beneficial and, associated with the highest energy availability, allow proton colliders to cover, at once, a large range of possible new particle masses. This is the way to go when attempting to discover new particles whose masses are not predicted. The pp colliders are, therefore, also called “discovery” machines. This is how, a couple of years ago, the Higgs boson was discovered at a mass of ~ 125 GeV at the CERN proton-proton collider LHC.

New particle Searches

The LHC is making a beautiful work searching for new particles of a different type, called Supersymmetric or, to friends, Susy particles. Supersymmetry is a proposed theory that “symmetrizes” the components of the Standard Model (fig.1). Each spin 1 (boson) gets a spin 1/2 (fermion) superpartner and, similarly, each spin1/2, a spin1 superpartner leading to a gauge coupling<-a> uoinication. It has been shown that supersymmetric models would make possible the unification of three of the 4 known forces of nature: the electro-magnetic, the weak and the strong forces. Gravitation, the forth “force” (in the Newtonian sense) would need an even more unifying theory Ttheory of Everything or ToE) whose most elaborated and advertised example is the superstring or M theory.

The first LHC measurements did not yet show any signal for such particles, putting therefore stringent lower limits on their mass.  LHC is greatly reducing the possible parameter range of these thought particles. The most straightforward version of Susy has now difficulty to stand as being the necessary step toward a unifying theory. However, due the nature of the pp collision as described previously some of these particles may have gone undetected and “cleaner” machines are still needed to either fully reject or to confirm the “attractive” Susy. Higher mass ranges that could be reached by higher energy colliders has also to be investigated to probe Susy in its more elaborated versions or to unveil other unifying theories, but as for today, there is no clue to select the collider energy that should be targeted.

Lepton colliders for precision physics

This is where e+e- collisions enter the game. In this case, the initial state and its energy are precisely fixed (besides small QED/QCD corrections effects and technical beam precision). The nature of the initial colliding bodies is also well known (e-/e+ are elementary particles “point-like” with no substructures, at least at first approximation). To study a given particle to a high precision, it suffices to tune the collider to the precise energy maximizing the particle production. In addition, the final states are usually simpler and cleaner than in pp collision as there are no underlying events. Therefore very precise measurement can be performed that can be matched to theoretical calculations which can also be much more precise than in the pp case.

Other leptons colliders include muon colliders where muons are heavy electrons (~207 times more). But although electrons are stable particles, muons lifetime at rest is only 2.2 $\mu$s. So the whole acceleration process and beam sharpening should be carried out before the muons decay. Thankfully the higher its energy, the longer it survives as described by the theory of relativity (the faster gets older). An important R&D is dedicated to the design of muon colliders. They will certainly be considered in future projects, but probably not for the next generation.

Polarization

Another important way to study the deep structure of the forces is based on the knowledge of the spin state of the initial particles. For a given process1, only some sub-processes are allowed depending on the initial spin combination. Selecting the initial particle spin allows to single out sub-processes that can be study specifically.

For the pp colliders, at high energy, this information is virtually unavailable. In the e+e- collision case, the e- beam polarization2 can be as high a 80% and for the positron e+ around 30% (60% in a possible upgrade). This is therefore a big advantage to disentangle the various processes producing the same final state.

So e+e- machines are for precision/detailed studies. But, do we need this high precision? Yes, definitely because at the current level of our understanding, a small but precisely measured unexpected effect may unveil the existence of a new particle. Such a discovery would sign the existence of a more inclusive theory than the Standard Model, a “Beyond the Standard Model” theory (BSM)3. For example, by measuring with high precision the coupling of the Higgs boson to it-self (self-coupling) one may infer the existence of a new “virtual” particle that could only be “really” produced at a somewhat higher energy. In this sense precision machine are probing nature at energies beyond their maximum operating energy.

After the pp LHC discovery, there is therefore a strong motivations to study in details the Higgs and any other particle that could show up at LHC or even to discover particles that would have escaped the LHC scrutiny.

This blog will focus on the physics studies that can be performed at e+e- machines (also able to collide e- e- or gamma gamma). Four international projects are currently being discussed in the high-energy physics community. Although they are on a quite different state of advancement and able to achieve different performances, they all share a common physics framework: the e+e- physics.

Two linear e+e- colliders:

1. ILC (international Linear Collider) proposed to be hosted in Japan
2. CLIC (Compact Linear International Collider) (CERN)

Two circular e+e- colliders:

1. FCC e+e- (Future Circular Collider e+e-) (CERN)
2. CepC (Circular e+e- Collider) China

This site is open to the discussion of these various projects and their comparative features.