Frequently asked questions about colliders.

Why should we build bigger colliders?

Colliders are the best microscopes we have. By building more powerful colliders, we will observe nature with ever finer resolution. What may be waiting for us in this unknown territory is limited only by our imaginations.

Colliders are among the most advanced technological achievements and the most impressive feats of engineering. A collider brings unquestionable prestige to its host country and is a gateway to the most state of the art technology. Needless to say, the brightest minds follow as well.

Has the LHC already told us that the particle physics is finished?

Not even close. The LHC has discovered the Higgs boson. With this discovery, we have found all of the particles in the Standard Model. As well, we can make an approximate measurement of the Higgs couplings with the LHC. The Standard Model itself, however, already leaves us with many unanswered questions. The discovery of the Higgs only sharpens these questions.
The answers to all of these questions lie beyond the Standard Model and at energies higher than we have ever reached.

The Standard Model has more than 20 parameters, none of which are explained.

The electroweak phase transition is a central piece of the Standard Model, yet we do not know anything about its nature. We also do not know why the electroweak interaction is so much stronger than gravity. Underlying these mysteries is the fact that the Higgs boson, being the only spin-zero elementary particle, is very special.

There are phenomena which cannot be explained by the Standard Model. For instance, the energy of the universe is dominated by dark matter and by dark energy, neither of which can be accounted for by the Standard Model. Similarly, the origin of the matter-antimatter asymmetry in the universe requires new physics beyond the Standard Model.

Will the next run of the LHC deliver all the answers?

That would be great, but extremely unlikely, primarily because we have not discovered any new physics yet at the 8 TeV run. In going from a center of mass energy of 8 TeV to 14 TeV, the reach for new physics is expected to increase by a factor of 1.5 to 2 in terms of mass scale. This allows us to reach roughly 1 TeV beyond our current limits. This is a very modest increase. On the other hand, almost all new physics scenarios predict a set of new particles. The mass spectrum of these particles span several times the mass of the lightest new particles or more. Therefore, it is impossible for the 14 TeV run to discover all of the new particles. A plausible scenario would be that we discover some of the lighter new particles. Given this glimpse of new physics, we will certainly wish to move forward with larger colliders to unveil the full pictures.

Even if we barely manage to discover all the new particles, it will be impossible to understand their properties well enough to fully characterize what new physics is responsible. We will certainly need to continue onwards to higher energies.

What should the energy of the next collider be so that we can guarantee to discover new physics?

Such a guarantee can never be made. With the discovery of the Higgs boson, we have entered a new era full of the unknown. We know that the answers to the mysteries facing fundamental physics lie beyond the energy frontier. While we do not know exactly where the new physics is, any new discoveries will bring revolutions and paradigm shifts, much like those at the beginning of the last century. We will simply build the next machine which allows us to explore the unknown, motivated solely by our curiosity. The same sense of curiosity compelled Galileo to point his telescope towards Jupiter, without knowing whether or not something new awaited. Forays into the unknown are the way most scientific discoveries are made.

Do we expect to learn something from the next generation of colliders?

A lot. In the past couple of decades, a plethora of theoretical ideas have been invented to address the open questions in the Standard Model. They represent our best theoretical insight. Most of them can be fully tested with the next generation of colliders. Either verifying of falsifying them will lead to quantum leaps in our understanding of nature.

Why isn't the United States pursuing an aggressive collider program?

In the past several decades due to various unfortunate political reasons, the US has been losing its edge regarding the pursuit of fundamental physics. This is particularly true after the cancellation of the SSC. As a result, they have missed some of the most important discoveries in the Standard Model, such as the W boson, the Z boson, and the Higgs boson. This is not due to a lack of scientific vigor in the physics community. In fact, high energy physicists in the US are still playing very active roles in LHC experiments. They have also been very enthusiastic about the prospect of the next generation of circular colliders.

At the same time, CERN has taken over as the world center of the experimental program of high energy physics. Recently, CERN has shown a strong interest in pursuing the planning of future circular colliders, considering it a crucial component of maintaining its leadership in this area.

Instead of continuing searching for SUSY at the next generation of colliders, it is time to give up?

It is certainly not time to give up on SUSY yet. The 8 TeV run of the LHC has set interesting limits on SUSY. It has, however, not qualitatively changed the status of SUSY as a compelling scenario for TeV-scale new physics. There are certainly large classes of possible SUSY scenarios that still remain unexplored. A much more decisive test will require a higher energy collider, such as a 100 TeV pp collider. The upcoming 14 TeV run of the LHC also has the exciting opportunity of catching a glimpse of SUSY by discovering some of the lower lying new physics states. If so, we will have to move to a higher energy collider to discover the full set of new particles predicted by SUSY.

We should also stress that as compelling as SUSY is, it is still just one of the many possible new physics scenarios. We certainly should not base our decision on whether to build a collider or not on the fate of one hypothetical scenario. Independent of the existence of SUSY, our goal is going to the energy frontier, probing the next layer in the microscopic world, and see what is there.

Should we build a Higgs factory or a higher energy pp collider?

Both are powerful tools to looks for new physics, with complementary strengths. Their comparison is analogous to LEP vs. the LHC. A Higgs factory is ideal to study in detail the newly discovered Higgs boson, and look for deviations from the Standard Model predictions as indirect signals of new physics. It can achieve a precision much beyond the capability of the LHC. A higher energy pp collider, on the other hand, will search for new physics particles which are directly prodcued. The improvement in mass reach is proportional to the increase in energy.

Prioritizing among these two options requires considering many factors, including technological feasibility, cost, and the timing of physics opportunities. Presently, a sensible plan is to build a Higgs factory first, followed by a high energy pp collider. This is akin to the LEP+LHC sequence, which has proven to be very successful.


If Japan decide to go forward with the ILC, does it still make sense for China to build the Higgs factory?

If CERN is planning on building a large circular collider, can China even compete?

Have we learned anything from the past collider experiments?

Almost everything. Most of the ingredients of the Standard Model have been discovered by collider experiments. Moreover, collider experiments provide a controlled environment where we can carefully measure study the physics. Indeed, most of the key elements of the Standard Model, such as the theory of strong and electroweak interactions, have been carefully tested by collider experiments.

Is it very difficult to build a collider?

How much is a collider?