# In view of the fact that the standard model has remained unchanged for twenty five years, what directions should we be looking in (in terms of experiment and new ideas) in order to go beyond it?

Dissatisfaction with fine-tuning (e.g. higgs).

Possible concrete hint of something beyond standard model — neutrinos appear to have mass (neutrino oscillation experiments).

LHC has put limits on SUSY, and so far it has not been experimentally observed.
Is no SUSY the death of string theory? Could it just be high energy SUSY is needed and the absence of low energy SUSY isn’t a problem.

Do we just need bigger colliders? At what energy do we give up?

Are particle accelerators the only way? E.g. electric dipole of the neutron. Anomalous magnetic moment of the electron. High precision experiments with neutrons.

Could we invest in new accelerator technologies that can get to higher or different energy scales for cheaper.

After the experimental confirmation of the existence of the Higgs boson the final piece of the standard model was introduced. Although the standard model has remained unchanged for twenty five years, there are several directions that can be taken in order to go beyond it. One clear direction is the experimental search for supersymmetry since theoretically (at least) provides a description of still open questions like the so-called hierarchy problem.  The look for supersymmetry is not a new idea and direction, but for sure must be addressed. However, experiments that look for supersymmetry are usually very expensive without a priori guarantee of success. Another direction that can be taken experimentally is to look for the existence of extra dimensions and, even if it sounds unreal, the existence of small  rapidly evaporating black-holes which may offer information about new physical phenomena . Thus, it is necessary to take alternative directions that are cheaper and will also test new physics beyond the standard model. Nowadays there has been several proposed ideas that can be tested with experiments that do not necessarily  require of big accelerators like the LHC.  Theoretically, there is a clear direction to go beyond the standard model and it is inspired on the physical existence of the Higgs boson. This direction corresponds to focus efforts on the study of cosmological systems that introduce within their description the Higgs field.

The problem is related to problem 2. Standard model is Minkowski background. From quantum gravity point of view, we have background dependent, which is not captured by QFT. In QFT, the full diffeomorphism group is only implemented in topological field theory. Taking account of quantum gravity, we have new insights.

Extra dimension models are still quite conventional quantum field theories. The non-trivial curvature in extra dimension model is still not quite background dependent. Its particle spectrum can be mimic by adding additional gauge groups.

In the Standard Model of particle physics, we still have many naturalness problems.

Generally speaking, the QFT formulism that based on local Lagrangian also has problem. Many QFT’s cannot be described by Lagrangians. So it is curious why our universe can be described by Lagrangian and why the Standard Model is so successful. Many parts of QFT are still not well understood, such as the non-perturbative region. There we don’t have very effective tools. In AdS/CFT, there is a whole set of theories we cannot explore if we just consider local Lagrangian. In BSM, we should ask why impose each restriction. In QFT one usually writes down the fundamental degrees of freedom and local Lagrangian. But it is not true that the theory must be described by those fundamental degrees of freedom.

It is still worth to ask if some parts of the Standard Model are truly fundamental. If one keep doing particle physics, it seems just enough to just use the local Lagrangian formulism. But on the other side, S-matrix theory derives from basic principles like analyticity and unitarity. In many calculations, S-matrix theory provide us a better we to calculate scattering amplitudes and cross sections.

It is not clear which is a better approach to reach new physics, by generalizing local Lagrangian formulism or S-matrix theory.

Neutrino mass

Baryon asymmetry

Dark matter: looking at smaller scales and more accurate missing energy experiments.

LHC could help but other tests are needed.

Low energy experiments. Experiments with more handle on the system, not just colliding two hadrons.

We cannot go for very high energies with accelerator experiments (expensive ,...), cosmology can help us in this direction.

Unclear on spirit of question

Neutrino sector

Almost the only thing that you can write down that’s consistent

How many numbers is the standard/benchmark to “get it”

There are some important unsolved questions in the Standard Model. What is the origin of neutrino masses? What is the reason for baryon asymmetry, that is, why is there more matter than antimatter in the universe? Another direction comes from astrophysics and cosmology; if dark matter is a particle, it should be included somehow in the Standard Model. The LHC does not appear to be the best place to look for dark matter; these searches are performed by looking for missing energy in collisions, and this doesn’t capture the entire parameter space. Also, known particles such as neutrinos and Z bosons are responsible for most of the missing energy, so the background is too strong. We can’t just continue using the same kind of accelerators and merely increasing the energy, we have to try other types of experiments.

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