Minimal Supersymmetric Standard Model
MSSM is the minimal supersymmetrical model as it considers only "the [minimum] number of new particle states and new interactions consistent with "Reality".
[1] Supersymmetry pairs bosons with fermions, so every Standard Model particle has a (yet undiscovered) superpartner.
If discovered, such superparticles could be candidates for dark matter,[2] and could provide evidence for grand unification or the viability of string theory.
The failure to find evidence for MSSM using the Large Hadron Collider[3][4] has strengthened an inclination to abandon it.
[6] The Higgs boson mass of the Standard Model is unstable to quantum corrections and the theory predicts that weak scale should be much weaker than what is observed to be.
In the MSSM, the Higgs boson has a fermionic superpartner, the Higgsino, that has the same mass as it would if supersymmetry were an exact symmetry.
There are five classes of particle that superpartners of the Standard Model fall into: squarks, gluinos, charginos, neutralinos, and sleptons.
To avoid these problems, the MSSM takes all of the soft supersymmetry breaking to be diagonal in flavor space and for all of the new CP violating phases to vanish.
The original motivation for proposing the MSSM was to stabilize the Higgs mass to radiative corrections that are quadratically divergent in the Standard Model (the hierarchy problem).
The Higgs vacuum expectation value (VEV) is related to the negative scalar mass in the Lagrangian.
There are two loop corrections and both TeV-scale and GUT-scale threshold corrections that alter this condition on gauge coupling unification, and the results of more extensive calculations reveal that gauge coupling unification occurs to an accuracy of 1%, though this is about 3 standard deviations from the theoretical expectations.
However, if superpartners are found in the near future, the apparent success of gauge coupling unification would suggest that a supersymmetric grand unified theory is a promising candidate for high scale physics.
This leads to the generic prediction that the MSSM will produce a 'missing energy' signal from these particles leaving the detector.
Because these particles only interact with the weak vector bosons, they are not directly produced at hadron colliders in copious numbers.
Thus a typical decay is Note that the “Missing energy” byproduct represents the mass-energy of the neutralino ( N͂01 ) and in the second line, the mass-energy of a neutrino-antineutrino pair ( ν + ν ) produced with the lepton and antilepton in the final decay, all of which are undetectable in individual reactions with current technology.
Due to phenomenological constraints from flavor changing neutral currents, typically the lighter two generations of squarks have to be nearly the same in mass and therefore are not given distinct names.
The superpartners of the top and bottom quark can be split from the lighter squarks and are called stop and sbottom.
In R-parity conserving scenarios, squarks are pair produced and therefore a typical signal is Gluinos are Majorana fermionic partners of the gluon which means that they are their own antiparticles.
Therefore, pairs of gluinos can decay to This is a distinctive signature because it has same-sign di-leptons and has very little background in the Standard Model.
[citation needed] Because of the high mass of the tau lepton there will be left-right mixing of the stau similar to that of stop and sbottom (see above).
A single Higgsino (the fermionic superpartner of the Higgs boson) would lead to a gauge anomaly and would cause the theory to be inconsistent.
It also contains chiral superfields for the Standard Model fermions and Higgs bosons (and their respective superpartners).
Quartic couplings are not soft supersymmetry-breaking parameters since they lead to a quadratic divergence of the Higgs mass.
This contributes an additional term to the Higgs mass at loop level, but is not logarithmically enhanced by pushing
(known as 'maximal mixing') it is possible to push the Higgs mass to 125 GeV without decoupling the top squark or adding new dynamics to the MSSM.
As the Higgs was found at around 125 GeV (along with no other superparticles) at the LHC, this strongly hints at new dynamics beyond the MSSM, such as the 'Next to Minimal Supersymmetric Standard Model' (NMSSM); and suggests some correlation to the little hierarchy problem.
A large amount of theoretical effort has been spent trying to understand the mechanism for soft supersymmetry breaking that produces the desired properties in the superpartner masses and interactions.
[14] mSUGRA is one of the most widely investigated models of particle physics due to its predictive power requiring only 4 input parameters and a sign, to determine the low energy phenomenology from the scale of Grand Unification.
Typically a hidden sector breaks supersymmetry and communicates it to massive messenger fields that are charged under the Standard Model.
This makes any phenomenological analysis (e.g. finding regions in parameter space consistent with observed data) impractical.
