The Minimal Supersymmetric Standard Model
(MSSM) is the minimal extension to the Standard Model
that realizes supersymmetry
, although non-minimal extensions do exist. Supersymmetry pairs bosons
, therefore every Standard Model particle has a partner that has yet to be discovered. If these supersymmetric partners exist, it is likely that they will be observed at the Large Hadron Collider
, which began operations in 2008. If the superparticles are found, it is analogous to discovering antimatter
and depending on the details of what is found, it could provide evidence for grand unification
and might even in principle provide hints as to how string theory
The MSSM was originally proposed in 1981 to stabilize the weak scale, solving the hierarchy problem. The Higgs 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. Because fermion masses are radiatively stable, the Higgs mass inherits this stability.
The only unambiguous way to claim discovery of supersymmetry is to produce superparticles in the laboratory. Because superparticles are expected to be 100 to 1000 times heavier than the proton, it requires a huge amount of energy to make these particles that can only be achieved at particle accelerators. Currently the Tevatron is the highest energy particle collider and is actively looking for evidence of the production of supersymmetric particles. Most physicists believe that supersymmetry must be discovered at the LHC if it is responsible for stabilizing the weak scale. There are five classes of particle that superpartners of the Standard Model fall into: squarks, gluinos, charginos, neutralinos, and sleptons. These superparticles have their interactions and subsequent decays described by the MSSM and each has characteristic signatures.
The MSSM imposes R-parity to explain the stability of the proton. It adds supersymmetry breaking by introducing explicit soft supersymmetry breaking operators into the Lagrangian that is communicated to it by some unknown (and unspecified) dynamics. This means that there are 120 new parameters in the MSSM. Most of these parameters lead to unnacceptable phenomenology such as large flavor changing neutral currents or large electric dipole moments for the neutron and electron. To avoid these problems, the MSSM takes all of the soft susy breaking to be diagonal in flavor space and for all of the new CP violating phases to vanish.
There are three principle motivations for the MSSM over other theoretical extensions of the Standard Model, namely:
- Gauge coupling unification
- Dark Matter
These motivations come out without much effort and they are the primary reasons why the MSSM is the leading candidate for a new theory to be discovered at collider experiments such as the Tevatron or the LHC.
The original motivation for proposing the MSSM was to stabilize the Higgs mass to radiative corrections that are
quadratically divergent in the Standard Model (hierarchy problem). In supersymmetric models, scalars are related to fermions and
have the same mass. Since fermion masses are logarithmically divergent, scalar masses inherit the same
radiative stability. The Higgs vacuum expectation value is related to the negative scalar mass in the Lagrangian.
In order for the radiative corrections to the Higgs mass to not be dramatically larger than the actual value, the
mass of the superpartners of the Standard Model should not be significantly heavier than the Higgs vev --
roughly 100 GeV. This mass scale is being probed currently at the Tevatron and will be more extensively
explored at the LHC.
Gauge Coupling Unification
If the superpartners of the Standard Model are near the TeV scale, then measured gauge couplings of the three
gauge groups unify at high energies. The beta-functions
for the MSSM gauge couplings are given by
is measured in SU(5) normalization -- a factor of
than the Standard Model's nomalization and predicted by Georgi-Glashow
The condition for gauge coupling unification at one loop is whether the following expression is satisfied
Remarkably, this is precisely satisfied to experimental errors in the values of . 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.
This prediction is generally considered as indirect evidence for both the MSSM and SUSY GUTs. It should be noted that gauge coupling unification does not necessarily imply grand unification and there exist other mechanisms to reproduce gauge coupling unification. 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.
is preserved, then the lightest superparticle (LSP
) of the MSSM is stable and is a weakly interacting massive particle (WIMP
) — i.e. it does not have electromagnetic or strong interactions. This makes the LSP a good dark matter
candidate and falls into the category of cold dark matter
Predictions of the MSSM Regarding Hadron Colliders
have active experimental programs searching for supersymmetric particles. Since both of these machines are hadron
colliders — proton antiproton for the Tevatron and proton proton for the LHC — they search best for strongly interacting particles. Therefore most experimental signature involve production of squarks
. Since the MSSM has R-parity
, the lightest supersymmetric particle is stable and after the squarks and gluinos decay each decay chain will contain one LSP that will leave the detector unseen. This leads to the generic prediction that the MSSM will produce a 'missing energy
' signal from these particles leaving the detector.
There are four Neutralinos
that are fermions and are electrically neutral, the lightest of which is typically stable.
They are typically labeled
. These four states are mixtures of
, neutral Wino
, and neutral Higgsinos
. Because these particles only interact with the weak vector bosons,
they are not directly produced at hadron colliders in copious numbers. They primarily appear as particles in cascade
decays of heavier particles usually originating from colored supersymmetric particles such as squarks or gluinos.
In R-parity conserving models, the lightest neutralino is stable and all supersymmetric cascades decays end up decaying
into this particle which leaves the detector unseen and its existence can only be inferred by looking for unbalanced momentum
in a detector.
The heavier neutralinos typically decay through a to a lighter neutralino or through a to chargino. Thus a typical decay is
- Missing energy +
- Missing energy +
The mass splittings between the different Neutralinos will dictate which patterns of decays are allowed.
There are two Charginos
that are fermions and are electrically charged. The heavier chargino can decay through
to the lighter chargino. Both can decay through a
are the scalar superpartners of the quarks and there is one version for each Standard Model quark.
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
Squarks can be produced through strong interactions and therefore are easily produced at hadron colliders.
They decay to quarks and neutralinos or charginos which further decay. Squarks are typically pair produced
and therefore a typical signal is
- 2 jets + Missing energy
- 2 jets + 2 leptons + Missing energy
Gluinos are Majorana
fermionic partners of the gluon
which means that they are their own antiparticles.
They interact strongly and therefore can be produced significantly at the LHC. They can only decay to a quark
and a squark and thus a typical gluino signal is
- 4 jets + Missing energy
Because gluinos are Majorana, gluinos can decay to either a quark+anti-squark or an anti-quark+squark with equal probability. Therefore pairs of gluinos can decay to
- 4 jets+ + Missing energy
This is a distinctive signature because it has same-sign di-leptons and has very little background in the Standard Model.
Sleptons are the scalar partners of the leptons
of the Standard Model. They are not strongly interacting and therefore are not produced very often at hadron colliders unless they are very light. They will typically be found in decays of a charginos and neutralinos if they are light enough to be a decay product
superpartners, and bosons have fermionic superpartners. For most of the Standard Model
particles, doubling is very straightforward. However, for the Higgs boson, it is more complicated.
A single Higgsino (the fermionic superpartner of the Higgs boson) would lead to a gauge anomaly and would cause the theory to be inconsistent. However if two Higgsinos are added, there is no gauge anomaly. The simplest theory is one with a two Higgsinos and therefore two scalar Higgs doublets.
Another reason for having two scalar Higgs doublets rather than one is in order to have Yukawa couplings between the Higgs and both down-type quarks and up-type quarks; these are the terms responsible for the quarks' masses. In the standard model the down-type quarks couple to the Higgs field (which has Y=-1/2) and the up-type quarks to its complex conjugate (which has Y=+1/2). However in a supersymmetric theory this is not allowed, so two types of Higgs fields are needed.
|SM Particle type
|| -1 |
|| -1 |
|| -1 |
|| -1 |
|| -1 |
|| -1 |
In supersymmetric theories, every field and its superpartner can be written together as a superfield
. The superfield formulation of supersymmetry is very convenient to write down manifestly supersymmetric theories (i.e. one does not have to tediously check that the theory is supersymmetric term by term in the Lagrangian).
The MSSM contains vector superfields
associated with the Standard Model gauge groups which contain the vector bosons and associated gauginos. It also contains chiral superfields
for the Standard Model fermions and Higgs bosons (and their respective superpartners).
The MSSM Lagrangian
The Lagrangian for the MSSM contains several pieces.
- The first is the Kahler potential for the matter and Higgs fields which produces the kinetic terms for the fields.
- The second piece is the gauge field superpotential that produces the kinetic terms for the gauge bosons and gauginos.
- The next term is the superpotential for the matter and Higgs fields. These produce the Yukawa couplings for the Standard Model fermions and also the mass term for the Higgsinos. After imposing R-parity, the renormalizable, gauge invariant operators in the superpotential are
The constant term is unphysical in global supersymmetry (as opposed to supergravity).
Soft Susy Breaking
The last piece of the MSSM Lagrangian is the soft supersymmetry breaking Lagrangian. The vast majority of the parameters of the MSSM are in the susy breaking Lagrangian. The soft susy breaking are divided into roughly three pieces.
- The first are the gaugino masses
Where are the gauginos and is different for the wino, bino and gluino.
- The next are the soft masses for the scalar fields
where are any of the scalars in the MSSM and are hermitean matrices for the squarks and sleptons of a given gauge quantum numbers. The eigenvalues of these matrices are actually the masses squared, rather than the masses.
- Finally there are the and terms which are given by
The terms are complex matrices much as the scalar masses are.
Problems with the MSSM
There are several problems with the MSSM — most of them falling into understanding the parameters.
- The mu problem: The supersymmetric Higgs mass parameter μ appears as the following term in the superpotential: μHuHd. It should have the same order of magnitude as the electroweak scale, many orders of magnitude smaller than that of the planck scale, which is the natural cutoff scale. The soft supersymmetry breaking terms should also be of the same order of magnitude as the electroweak scale. This brings about a problem of naturalness: why are these scales so much smaller than the cutoff scale yet happen to fall so close to each other?
- Flavor universality of soft masses and A-terms: since no flavor mixing additional to that predicted by the standard model has been discovered so far, the coefficients of the additional terms in the MSSM Lagrangian must be, at least approximately, flavor invariant (i.e. the same for all flavors).
- Smallness of CP violating phases: since no CP violation additional to that predicted by the standard model has been discovered so far, the additional terms in the MSSM Lagrangian must be, at least approximately, CP invariant, so that their CP violating phases are small.
More recently physicists have become concerned about the non-discovery of the Higgs boson, or any superpartner at LEP II or the Tevatron; many nevertheless hold out hope on account of the possibility that the Large Hadron Collider due to begin operation at CERN in 2008 will discover it.
Theories of Supersymmetry breaking
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. The three most extensively studied
Gravity Mediated Supersymmetry Breaking
Gravity Mediated Supersymmetry Breaking is a method of communicating supersymmetry breaking to the supersymmetric Standard Model through gravitational interactions. It was the first method proposed to communicate supersymmetry breaking. In gravity mediated supersymmetry breaking models, there is a part of the theory that only interacts with the MSSM through gravitational interaction. This hidden sector of the theory breaks supersymmetry. Through the supersymmetric version of the Higgs mechanism
, the gravitino
, the supersymmetric version of the graviton, acquires a mass. After the gravitino has a mass, gravitational radiative corrections to soft masses are incompletely cancelled beneath the gravitino's mass.
It is currently believed that it is not generic to have a sector completely decoupled from the MSSM and there should be higher dimension operators that couple different sectors together with the higher dimension operators suppressed by the Planck scale. These operators give as large of a contribution to the soft supersymmetry breaking masses as the gravitational loops; therefore, today people usually consider gravity mediation to be gravitational sized direct interactions between the hidden sector and the MSSM.
mSUGRA stands for minimal supergravity.
The construction of a realistic model of interactions within N = 1 supergravity framework where supersymmetry breaking is
communicated through the supergravity interactions was carried out by Ali Chamseddine, Richard Arnowitt and Pran Nath in 1982 .
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.
Gravity Mediated Supersymmetry Breaking was assumed to be flavor universal because of the universality of gravity; however, in 1986 Hall, Kostelecky, and Raby
showed that Planck scale physics that are necessary to generate the Standard Model Yukawa couplings spoil the universality of the supersymmetry breaking.
Gauge Mediated Supersymmetry Breaking (GMSB)
Gauge Mediated Supersymmetry Breaking is method of communicating supersymmetry breaking to the supersymmetric Standard Model through the Standard Model's gauge interactions. Typically a hidden sector breaks supersymmetry and communicates it to massive messenger fields that are charged under the Standard Model. These messenger fields induce a gaugino mass at one loop and then this is transmitted on to the scalar superpartners at two loops.
Anomaly Mediated Supersymmetry Breaking (AMSB)
Anomaly Mediate Supersymmetry Breaking is special type of gravity mediated supersymmetry breaking that results in supersymmetry breaking being communicated to the supersymmetric Standard Model through the conformal anomaly.