Split supersymmetry
Encyclopedia
In particle physics
, split supersymmetry is a recent proposal for new physics beyond the Standard Model
. It was proposed separately in three papers. The first by James Wells in June 2003 in a more modest form that mildly relaxed the assumption about naturalness
in the Higgs potential. In May 2004 Nima Arkani-Hamed
and Savas Dimopoulos
argued that naturalness in the Higgs
sector may not be an accurate guide to propose new physics beyond the Standard Model and argued that supersymmetry may be realized in a different fashion that preserved gauge coupling unification and has a dark matter
candidate. In June 2004 Gian Giudice and Andrea Romanino argued from a general point of view that if one wants gauge coupling unification and a dark matter candidate, that split supersymmetry is one amongst a few theories that exists.
The new light (~TeV) particles in Split Supersymmetry (beyond the Standard Models particles) are
The Lagrangian for Split Supersymmetry is constrained from the existence of high energy supersymmetry. There are five couplings in Split Supersymmetry: the Higgs quartic coupling and four Yukawa couplings between the Higgsinos, Higgs and gauginos. The couplings are set by one parameter,
, at the scale where the supersymmetric scalars decouple. Beneath the supersymmetry breaking scale, these five couplings evolve through the renormalization group equation
down to the TeV scale. At a future Linear collider, these couplings could be measured at the 1% level and then renormalization group evolved up to high energies to show that the theory is supersymmetric at an exceedingly high scale.
becomes a quasi-stable particle with a lifetime that could be up to 100 seconds long. A gluino that lived longer than this would disrupt Big Bang nucleosynthesis
or would have been observed as an additional source of cosmic gamma rays. The gluino is long lived because it can only decay into a squark and a quark
and because the squarks are so heavy and these decays are highly suppressed. Thus the decay
rate of the gluino can roughly be estimated, in natural units
, as
where 
is the gluino
rest mass and
the squark rest mass. For gluino
mass of the order of 1 TeV
, the cosmological bound mentioned above sets an upper bound of about
GeV
on squarks masses.
The potentially long lifetime of the gluino leads to different collider signatures at the Tevatron
and the Large Hadron Collider
. There are three ways to see these particles:
as supersymmetry does, because the particles which have masses way beyond the TeV
scale play no major role in the unification. These particles are the gravitino
- which has a small coupling (of order of the gravitation
al interaction) to the other particles, and the scalar partners to the standard model fermions - namely, squarks and sleptons. The latter move the beta-function
s of all gauge couplings together, and do not influence their unification, because in the grand unification theory
they form a full SU(5) multiplet
, just like a complete generation
of particles.
Split supersymmetry also solves the gravitino cosmological problem, because the gravitino
mass is much higher than TeV
.
The upper bounds on proton decay
rate can also be satisfied because the squarks are very heavy as well.
On the other hand, unlike conventional supersymmetry
, split supersymmetry does not solve the hierarchy problem
which has been a primary motivation for proposals for new physics beyond the Standard Model
since 1979. One proposal is that the hierarchy problem
is "solved" by assuming fine-tuning
due to anthropic reasons
.
. Often when a new notion in physics is proposed there is a knee-jerk backlash. When naturalness in the Higgs sector was initially proposed as a motivation for new physics, the notion was not taken seriously. After the supersymmetric Standard Model was proposed, Sheldon Glashow quipped that 'half of the particles have already been discovered.' After 25 years, the notion of naturalness had become so ingrained in the community that proposing a theory that did not use naturalness as the primary motivation was ridiculed. Split supersymmetry makes predictions that are distinct from both the Standard Model
and the Minimal Supersymmetric Standard Model
and the ultimate nature of the naturalness in the Higgs sector will hopefully be determined at future colliders.
Many of the original proponents of naturalness no longer believe that it should be an exclusive constraint on new physics. Kenneth Wilson originally advocated for it, but has recently called it one of his biggest mistakes during his career. Steven Weinberg
relaxed the notion of naturalness in the cosmological constant and argued for an environmental explanation for it in 1987. Leonard Susskind
, who initially proposed technicolor
, is a firm advocate of the notion of a landscape and non-naturalness. Savas Dimopoulos
, who initially proposed the supersymmetric Standard Model, proposed split supersymmetry.
Particle physics
Particle physics is a branch of physics that studies the existence and interactions of particles that are the constituents of what is usually referred to as matter or radiation. In current understanding, particles are excitations of quantum fields and interact following their dynamics...
, split supersymmetry is a recent proposal for new physics beyond the Standard Model
Standard Model
The Standard Model of particle physics is a theory concerning the electromagnetic, weak, and strong nuclear interactions, which mediate the dynamics of the known subatomic particles. Developed throughout the mid to late 20th century, the current formulation was finalized in the mid 1970s upon...
. It was proposed separately in three papers. The first by James Wells in June 2003 in a more modest form that mildly relaxed the assumption about naturalness
Naturalness (physics)
Naturalness is the property that all parameters appearing in a theory take values of order 1...
in the Higgs potential. In May 2004 Nima Arkani-Hamed
Nima Arkani-Hamed
Nima Arkani-Hamed is a leading Canadian American theoretical physicist with interests in high-energy physics, string theory and cosmology....
and Savas Dimopoulos
Savas Dimopoulos
Savas Dimopoulos is a Greek particle physicist at Stanford University. He was born in Istanbul, Turkey and later moved to Athens due to ethnic tensions in Turkey during the 1950s and 1960s. Dimopoulos studied as an undergraduate at the University of Houston...
argued that naturalness in the Higgs
Higgs
The term Higgs appears in:* Alan Higgs, English businessman and philanthropist* Sir Derek Higgs, an English business leader and merchant banker* Eric Sidney Higgs, English archaeologist*Griffin Higgs...
sector may not be an accurate guide to propose new physics beyond the Standard Model and argued that supersymmetry may be realized in a different fashion that preserved gauge coupling unification and has a dark matter
Dark matter
In astronomy and cosmology, dark matter is matter that neither emits nor scatters light or other electromagnetic radiation, and so cannot be directly detected via optical or radio astronomy...
candidate. In June 2004 Gian Giudice and Andrea Romanino argued from a general point of view that if one wants gauge coupling unification and a dark matter candidate, that split supersymmetry is one amongst a few theories that exists.
The new light (~TeV) particles in Split Supersymmetry (beyond the Standard Models particles) are
| Field | Spin | Gauge Charges | Name |
|---|---|---|---|
 |  |  | gluino |
 |  |  | wino |
 |  |  | bino |
 |  |  | higgsino |
 |  |  | higgsino |
The Lagrangian for Split Supersymmetry is constrained from the existence of high energy supersymmetry. There are five couplings in Split Supersymmetry: the Higgs quartic coupling and four Yukawa couplings between the Higgsinos, Higgs and gauginos. The couplings are set by one parameter,
, at the scale where the supersymmetric scalars decouple. Beneath the supersymmetry breaking scale, these five couplings evolve through the renormalization group equationRenormalization group equation
Renormalization group equation may refer to:* Beta-function* Callan–Symanzik equation* Exact renormalization group equation...
down to the TeV scale. At a future Linear collider, these couplings could be measured at the 1% level and then renormalization group evolved up to high energies to show that the theory is supersymmetric at an exceedingly high scale.
Long Lived Gluinos
The striking feature of split supersymmetry is that the gluinoGluino
A gluino is the hypothetical supersymmetric partner of a gluon. Gluinos are expected by supersymmetry theorists to be pair produced in particle accelerators such as the Large Hadron Collider if they exist....
becomes a quasi-stable particle with a lifetime that could be up to 100 seconds long. A gluino that lived longer than this would disrupt Big Bang nucleosynthesis
Big Bang nucleosynthesis
In physical cosmology, Big Bang nucleosynthesis refers to the production of nuclei other than those of H-1 during the early phases of the universe...
or would have been observed as an additional source of cosmic gamma rays. The gluino is long lived because it can only decay into a squark and a quark
Quark
A quark is an elementary particle and a fundamental constituent of matter. Quarks combine to form composite particles called hadrons, the most stable of which are protons and neutrons, the components of atomic nuclei. Due to a phenomenon known as color confinement, quarks are never directly...
and because the squarks are so heavy and these decays are highly suppressed. Thus the decay
Particle decay
Particle decay is the spontaneous process of one elementary particle transforming into other elementary particles. During this process, an elementary particle becomes a different particle with less mass and an intermediate particle such as W boson in muon decay. The intermediate particle then...
rate of the gluino can roughly be estimated, in natural units
Natural units
In physics, natural units are physical units of measurement based only on universal physical constants. For example the elementary charge e is a natural unit of electric charge, or the speed of light c is a natural unit of speed...
, as
where is the gluinoGluino
A gluino is the hypothetical supersymmetric partner of a gluon. Gluinos are expected by supersymmetry theorists to be pair produced in particle accelerators such as the Large Hadron Collider if they exist....
rest mass and
the squark rest mass. For gluinoGluino
A gluino is the hypothetical supersymmetric partner of a gluon. Gluinos are expected by supersymmetry theorists to be pair produced in particle accelerators such as the Large Hadron Collider if they exist....
mass of the order of 1 TeV
TEV
TEV may refer to:* TeV, or teraelectronvolt, a measure of energy* Total Enterprise Value, a financial measure* Total Economic Value, an economic measure* Tobacco etch virus, a plant pathogenic virus of the family Potyviridae....
, the cosmological bound mentioned above sets an upper bound of about
GeVGEV
GEV or GeV may stand for:*GeV or gigaelectronvolt, a unit of energy equal to billion electron volts*GEV or Grid Enabled Vehicle that is fully or partially powered by the electric grid, see plug-in electric vehicle...
on squarks masses.
The potentially long lifetime of the gluino leads to different collider signatures at the Tevatron
Tevatron
The Tevatron is a circular particle accelerator in the United States, at the Fermi National Accelerator Laboratory , just east of Batavia, Illinois, and is the second highest energy particle collider in the world after the Large Hadron Collider...
and the Large Hadron Collider
Large Hadron Collider
The Large Hadron Collider is the world's largest and highest-energy particle accelerator. It is expected to address some of the most fundamental questions of physics, advancing the understanding of the deepest laws of nature....
. There are three ways to see these particles:
- Measuring the ratio of momentum to energy or velocity in tracking chambers ( dE/dx in the inner tracking chamber or p/v in the outer muon tracking chamber)
- Looking for excess singlet jet events that arise from initial or final state radiation.
- Looking for gluinos that have come to rest inside the detector and later decay. Such an event may occur if the gluino hadronize to form an exotic hadronHadronIn particle physics, a hadron is a composite particle made of quarks held together by the strong force...
which strongly interactsStrong interactionIn particle physics, the strong interaction is one of the four fundamental interactions of nature, the others being electromagnetism, the weak interaction and gravitation. As with the other fundamental interactions, it is a non-contact force...
with a nucleonNucleonIn physics, a nucleon is a collective name for two particles: the neutron and the proton. These are the two constituents of the atomic nucleus. Until the 1960s, the nucleons were thought to be elementary particles...
in the detector to create an exotic charged hadronHadronIn particle physics, a hadron is a composite particle made of quarks held together by the strong force...
. The latter will decelerate by electromagnetic interaction inside the detector and will eventually stop.
Advantages and drawbacks
Split supersymmetry allows gauge coupling unificationGrand unification theory
The term Grand Unified Theory, often abbreviated as GUT, refers to any of several similar candidate models in particle physics in which at high-energy, the three gauge interactions of the Standard Model which define the electromagnetic, weak, and strong interactions, are merged into one single...
as supersymmetry does, because the particles which have masses way beyond the TeV
TEV
TEV may refer to:* TeV, or teraelectronvolt, a measure of energy* Total Enterprise Value, a financial measure* Total Economic Value, an economic measure* Tobacco etch virus, a plant pathogenic virus of the family Potyviridae....
scale play no major role in the unification. These particles are the gravitino
Gravitino
The gravitino is the supersymmetric partner of the graviton, as predicted by theories combining general relativity and supersymmetry; i.e. supergravity theories...
- which has a small coupling (of order of the gravitation
Gravitation
Gravitation, or gravity, is a natural phenomenon by which physical bodies attract with a force proportional to their mass. Gravitation is most familiar as the agent that gives weight to objects with mass and causes them to fall to the ground when dropped...
al interaction) to the other particles, and the scalar partners to the standard model fermions - namely, squarks and sleptons. The latter move the beta-function
Beta-function
In theoretical physics, specifically quantum field theory, a beta function β encodes the dependence of a coupling parameter, g, on the energy scale, \mu of a given physical process....
s of all gauge couplings together, and do not influence their unification, because in the grand unification theory
Grand unification theory
The term Grand Unified Theory, often abbreviated as GUT, refers to any of several similar candidate models in particle physics in which at high-energy, the three gauge interactions of the Standard Model which define the electromagnetic, weak, and strong interactions, are merged into one single...
they form a full SU(5) multiplet
Multiplet
In group theory, a multiplet is formally a group representation of an algebra.In physics, multiplet can mean:* A group of related spectral lines* A group of related subatomic particles...
, just like a complete generation
Generation (particle physics)
In particle physics, a generation is a division of the elementary particles. Between generations, particles differ by their quantum number and mass, but their interactions are identical....
of particles.
Split supersymmetry also solves the gravitino cosmological problem, because the gravitino
Gravitino
The gravitino is the supersymmetric partner of the graviton, as predicted by theories combining general relativity and supersymmetry; i.e. supergravity theories...
mass is much higher than TeV
TEV
TEV may refer to:* TeV, or teraelectronvolt, a measure of energy* Total Enterprise Value, a financial measure* Total Economic Value, an economic measure* Tobacco etch virus, a plant pathogenic virus of the family Potyviridae....
.
The upper bounds on proton decay
Proton decay
In particle physics, proton decay is a hypothetical form of radioactive decay in which the proton decays into lighter subatomic particles, such as a neutral pion and a positron...
rate can also be satisfied because the squarks are very heavy as well.
On the other hand, unlike conventional supersymmetry
Supersymmetry
In particle physics, supersymmetry is a symmetry that relates elementary particles of one spin to other particles that differ by half a unit of spin and are known as superpartners...
, split supersymmetry does not solve the hierarchy problem
Hierarchy problem
In theoretical physics, a hierarchy problem occurs when the fundamental parameters of some Lagrangian are vastly different than the parameters measured by experiment. This can happen because measured parameters are related to the fundamental parameters by a prescription known as renormalization...
which has been a primary motivation for proposals for new physics beyond the Standard Model
Standard Model
The Standard Model of particle physics is a theory concerning the electromagnetic, weak, and strong nuclear interactions, which mediate the dynamics of the known subatomic particles. Developed throughout the mid to late 20th century, the current formulation was finalized in the mid 1970s upon...
since 1979. One proposal is that the hierarchy problem
Hierarchy problem
In theoretical physics, a hierarchy problem occurs when the fundamental parameters of some Lagrangian are vastly different than the parameters measured by experiment. This can happen because measured parameters are related to the fundamental parameters by a prescription known as renormalization...
is "solved" by assuming fine-tuning
Fine-tuning
In theoretical physics, fine-tuning refers to circumstances when the parameters of a model must be adjusted very precisely in order to agree with observations. Theories requiring fine-tuning are regarded as problematic in the absence of a known mechanism to explain why the parameters happen to...
due to anthropic reasons
Anthropic principle
In astrophysics and cosmology, the anthropic principle is the philosophical argument that observations of the physical Universe must be compatible with the conscious life that observes it. Some proponents of the argument reason that it explains why the Universe has the age and the fundamental...
.
History
The initial attitude of some of the high energy physics community towards split supersymmetry was illustrated by a parody called supersplit supersymmetrySupersplit supersymmetry
Supersplit supersymmetry was conceived as an April Fool's Day joke in 2005 by a group of some of the leading young theoretical high energy physicists. It was meant as a parody of split supersymmetry. In the model, the particles in split supersymmetry are decoupled leaving no trace at low...
. Often when a new notion in physics is proposed there is a knee-jerk backlash. When naturalness in the Higgs sector was initially proposed as a motivation for new physics, the notion was not taken seriously. After the supersymmetric Standard Model was proposed, Sheldon Glashow quipped that 'half of the particles have already been discovered.' After 25 years, the notion of naturalness had become so ingrained in the community that proposing a theory that did not use naturalness as the primary motivation was ridiculed. Split supersymmetry makes predictions that are distinct from both the Standard Model
Standard Model
The Standard Model of particle physics is a theory concerning the electromagnetic, weak, and strong nuclear interactions, which mediate the dynamics of the known subatomic particles. Developed throughout the mid to late 20th century, the current formulation was finalized in the mid 1970s upon...
and the Minimal Supersymmetric Standard Model
Minimal Supersymmetric Standard Model
The Minimal Supersymmetric Standard Model is the minimal extension to the Standard Model that realizes supersymmetry, although non-minimal extensions do exist. Supersymmetry pairs bosons with fermions; therefore every Standard Model particle has a partner that has yet to be discovered...
and the ultimate nature of the naturalness in the Higgs sector will hopefully be determined at future colliders.
Many of the original proponents of naturalness no longer believe that it should be an exclusive constraint on new physics. Kenneth Wilson originally advocated for it, but has recently called it one of his biggest mistakes during his career. Steven Weinberg
Steven Weinberg
Steven Weinberg is an American theoretical physicist and Nobel laureate in Physics for his contributions with Abdus Salam and Sheldon Glashow to the unification of the weak force and electromagnetic interaction between elementary particles....
relaxed the notion of naturalness in the cosmological constant and argued for an environmental explanation for it in 1987. Leonard Susskind
Leonard Susskind
Leonard Susskind is the Felix Bloch Professor of Theoretical Physics at Stanford University. His research interests include string theory, quantum field theory, quantum statistical mechanics and quantum cosmology...
, who initially proposed technicolor
Technicolor
Technicolor is a color motion picture process invented in 1916 and improved over several decades.It was the second major process, after Britain's Kinemacolor, and the most widely used color process in Hollywood from 1922 to 1952...
, is a firm advocate of the notion of a landscape and non-naturalness. Savas Dimopoulos
Savas Dimopoulos
Savas Dimopoulos is a Greek particle physicist at Stanford University. He was born in Istanbul, Turkey and later moved to Athens due to ethnic tensions in Turkey during the 1950s and 1960s. Dimopoulos studied as an undergraduate at the University of Houston...
, who initially proposed the supersymmetric Standard Model, proposed split supersymmetry.
External links
- Implications of Supersymmetry Breaking with a Little Hierarchy between Gauginos and Scalars by James D. Wells
- Supersymmetric Unification Without Low Energy Supersymmetry And Signatures for Fine-Tuning at the LHC by Nima Arkani-Hamed and Savas Dimopoulos
- Split Supersymmetry by G.F. Giudice and A. Romanino
- Authority Articles on Split supersymmetry