The standard model of particle physics has been repeatedly confirmed in measurements, but has no answer to some fundamental questions. This could possibly provide the supersymmetry. Therefore, the LHC experiments aim to search for supersymmetric particles. The analyzes are extremely diverse and sensitive to other extensions of the standard model. In recent years, very difficult searches have been made for the first time, for example for the supersymmetric partners of the Higgs boson.

The interplay of experimental results and theoretical concepts resulted in more than 50 Years the standard model of particle physics, which shows the structure of matter and the fundamental interactions on microscopic length scales of up to 10 – 19 describes meters or energies of up to one teraelectron volt (TeV) – one electron volt (eV) corresponds to the energy that an electron gains when it accelerates over a distance of one meter at a potential of one volt becomes. A central building block is the Higgs mechanism, through which the elementary particles get their mass. With the discovery of a Higgs particle at the Large Hadron Collider (LHC) at CERN in the year 2012 and the possible exploration of the Higgs -Mechanism started a new chapter of basic research.

This article was first printed in the Physik Journal 11, 2015.

Michael Krämer studied physics at the University of Mainz and did his doctorate. He was a research assistant at DESY in Hamburg, at the Rutherford Appleton Laboratory in Oxfordshire and at CERN in Geneva as well as a lecturer and reader at the University of Edinburgh. Since 2004 he has been Professor of Theoretical Physics at RWTH Aachen University.

Jeanette Lorenz (FV Particle Physics) studied physics in Erlangen and at the LMU Munich and completed his doctorate and habilitation at the LMU Munich. Since 2015 she has been leading a junior research group at the LMU Munich. She is a member of the ATLAS collaboration and since 2015 has been leading the activities for the interpretation of the searches for supersymmetry, previously the searches for gluinos and squarks.

Isabell Melzer-Pellmann (FV Particle Physics ) studied physics in Münster and did his doctorate at the University of Mainz. From … to 2014 she led a junior research group at DESY and at the U Hamburg, since then she has been part of the DESY staff. She is a member of the CMS collaboration, where she has led various activities on current and future SUSY searches since 2013.

The standard model is a quantum field theory, the structure of which is determined by symmetries: the Poincaré symmetry, which follows as space-time symmetry from the special theory of relativity, as well as symmetries of the inner degrees of freedom of elementary particles, the gauge symmetries. The predictions of the Standard Model have been impressively confirmed in numerous experiments, but it cannot answer some fundamental questions in physics: What is the nature of dark matter? How did the matter-antimatter asymmetry come about? What is the origin of the neutrino masses? These questions are at the center of current basic research and require new theoretical concepts.

One of the most interesting approaches to extend the standard model is the supersymmetry. It implies a fascinating expansion of space and time by linking them with the quantum property spin and thus creating a relationship between matter particles with half-integer spin (fermions) and force particles with integer spin (bosons). This connection represents the only possible extension of the Poincaré symmetry.

In supersymmetric theories every fundamental particle has a supersymmetric partner whose spin differs by 1/2. A supersymmetric electron with spin 0 (usually denoted by ẽ) belongs to the electron, and a supersymmetric photon with spin 1/2 (ỹ) belongs to the photon. Even a minimal supersymmetric extension of the standard model predicts a multitude of new particles.

As a general theoretical concept, supersymmetry can be implemented in various ways in specific models. The supersymmetric extension of the Standard Model with the smallest number of new particles is the Minimal Supersymmetric Standard Model (MSSM). The standard model is expanded by a further Higgs doublet and each particle is assigned a super partner.

The preservation of the baryon number, a quantum number proportional to the difference in the number of quarks and antiquarks, leads to the Standard model for the stability of protons. In supersymmetric extensions of the Standard Model, processes that violate the baryon number are not necessarily suppressed and can lead to rapid proton decay. In the MSSM, the proton decay is suppressed by the postulate of an additional symmetry – the R parity. Their preservation influences the phenomenology of the MSSM: On the one hand, supersymmetric particles can only arise in pairs, on the other hand, the lightest supersymmetric particle is stable and therefore a candidate for dark matter. In supersymmetric models without preservation of R-parity, the lightest supersymmetric particle decays, which suggests other experimental signatures. In this post we focus on the search for supersymmetry within the framework of the MSSM.

If the supersymmetry were exact, every supersymmetrical particle would have to have the same mass as its standard model partner. Since it has not yet been possible experimentally to prove a supersymmetrical particle with the mass of the electron, the supersymmetry can only be approximately realized. Therefore, supersymmetric particles have to be significantly heavier than their standard model partners. It is unclear which mechanism can break the supersymmetry and on which mass scale supersymmetric particles can be found.

Can the detection of supersymmetric particles succeed with energies close to the TeV scale that can be achieved at the LHC? Two aspects in particular make this seem plausible: the naturalness problem and the existence of dark matter.

In the standard model, quantum fluctuations ensure that the Higgs mass depends on the square of a new fundamental energy scale, for example the Planck scale, E Planck ≈ 10 19 GeV, where the standard model would have to be replaced by a quantum theory of gravity. Many physicists consider the sensitivity of the experimental to M Higgs = 125 GeV determined Higgs mass on the (unknown) physics close to the Planck scale many orders of magnitude higher than unnatural . The principle of naturalness runs through all known physics: A theory is considered natural if the laws of nature do not affect the physical processes at very much larger distances or at very much lower energy scales at small distances or at high energy scales. So it is not necessary to know how quarks interact in atomic nuclei when it comes to describing the orbit of the moon.

The supersymmetry modifies the quantum vacuum so that the Higgs mass no longer depends on the unknown physics close to the Planck scale and naturally lies in the experimentally observed range – if the masses of the supersymmetrical particles are to be found close to the TeV scale and thus in principle within range of the LHC. Whether the naturalness problem actually implies new physics at the LHC, whether it has a different solution or whether it is completely irrelevant for the formation of theories is currently a controversial discussion.

Supersymmetric theories also say the Existence of neutral, weakly interacting particles, which are stable in many models and therefore good candidates for dark matter. Due to the expansion of the universe, the dark matter that was created immediately after the Big Bang freezes out. This leads to the current density of dark matter determined from the cosmic background radiation, if it interacts weakly and its mass is on the TeV scale.

Supersymmetry is a fascinating theoretical concept and has established itself as a phenomenologically attractive model. Due to the many interesting and sometimes experimentally challenging signatures, supersymmetrical extensions of the standard model are an excellent blueprint for the search for new physics at the LHC.