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Sfermion
View on Wikipediafrom Wikipedia
In supersymmetric extension to the Standard Model (SM) of physics, a sfermion is a hypothetical spin-0 superpartner particle (sparticle) of its associated fermion.[1][2] Each particle has a superpartner with spin that differs by 1/2. Fermions in the SM have spin-1/2 and, therefore, sfermions have spin 0.[3][4]
The name 'sfermion' was formed by the general rule of prefixing an 's' to the name of its superpartner, denoting that it is a scalar particle with spin 0. For instance, the electron's superpartner is the selectron and the top quark's superpartner is the stop squark.
One corollary from supersymmetry is that sparticles have the same gauge numbers as their SM partners. This means that sparticle–particle pairs have the same color charge, weak isospin charge, and hypercharge (and consequently electric charge). Unbroken supersymmetry also implies that sparticle–particle pairs have the same mass. This is evidently not the case, since these sparticles would have already been detected. Thus, sparticles must have different masses from the particle partners and supersymmetry is said to be broken.[5][6]
Fundamental sfermions
[edit]Squarks
[edit]Squarks (also quarkinos)[7] are the superpartners of quarks. These include the sup squark, sdown squark, scharm squark, sstrange squark, stop squark, and sbottom squark.
| Squark | Symbol | Associated quark | Symbol |
|---|---|---|---|
| First generation | |||
| Sup squark | Up quark | ||
| Sdown squark | Down quark | ||
| Second generation | |||
| Scharm squark | Charm quark | ||
| Sstrange squark | Strange quark | ||
| Third generation | |||
| Stop squark | Top quark | ||
| Sbottom squark | Bottom quark | ||
Sleptons
[edit]Sleptons are the superpartners of leptons. These include the selectron, smuon, stau, and their corresponding sneutrino flavors.[8]
| Slepton | Symbol | Associated lepton | Symbol |
|---|---|---|---|
| First generation | |||
| Selectron | Electron | ||
| Selectron sneutrino | Electron neutrino | ||
| Second generation | |||
| Smuon | Muon | ||
| Smuon sneutrino | Muon neutrino | ||
| Third generation | |||
| Stau | Tau | ||
| Stau sneutrino | Tau neutrino | ||
See also
[edit]References
[edit]- ^ He-sheng, Chen; Dongsheng, Du; Weiguo, Li (2005). High Energy Physics: Ichep 2004 - Proceedings Of The 32nd International Conference (In 2 Volumes). World Scientific. p. 109. ISBN 9789814481274. Retrieved 30 September 2019.
- ^ Masayuki, Nakahata; Y, Itow; Masato, Shiozawa (2004). Neutrino Oscillations And Their Origin, Proceedings Of The 4th International Workshop. World Scientific. ISBN 9789814485586. Retrieved 30 September 2019.
- ^ Baer, Howard; Tata, Xerxes (2006). Weak Scale Supersymmetry: From Superfields to Scattering Events. Cambridge University Press. p. 129. ISBN 9781139455077. Retrieved 30 September 2019.
- ^ Cline, David B (1997). Flavor-changing Neutral Currents: Present And Future Studies: Proceedings Of The Symposium. World Scientific. p. 229. ISBN 9789814545822. Retrieved 30 September 2019.
- ^ Seamus, Hegarty; Keith, Potter; Emanuele, Quercigh (1992). Joint International Lepton-photon Symposium And Europhysics Conference On High Energy Physics - Lp-hep '91 (In 2 Volumes). World Scientific. p. 500. ISBN 9789814555531. Retrieved 30 September 2019.
- ^ Khalil, Shaaban; Moretti, Stefano (2017). Supersymmetry Beyond Minimality: From Theory to Experiment. CRC Press. ISBN 9781315350875. Retrieved 30 September 2019.
- ^ Khlopov, Maxim Yu. (1999). Cosmoparticle Physics. World Scientific. p. 53. ISBN 978-981-02-3188-0. Retrieved 23 June 2020.
- ^ Masayuki, Nakahata; Y, Itow; Masato, Shiozawa (2004). Neutrino Oscillations And Their Origin, Proceedings Of The 4th International Workshop. World Scientific. p. 442. ISBN 9789814485586. Retrieved 30 September 2019.
- Martin, Stephen, P. (2011). "A Supersymmetry Primer". Perspectives on Supersymmetry. Advanced Series on Directions in High Energy Physics. Vol. 18. pp. 1–98. arXiv:hep-ph/9709356. doi:10.1142/9789812839657_0001. ISBN 978-981-02-3553-6. S2CID 118973381.
{{cite book}}: CS1 maint: multiple names: authors list (link)
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Sfermion
View on Grokipediafrom Grokipedia
In supersymmetric extensions of the Standard Model of particle physics, sfermions are hypothetical spin-0 scalar particles that serve as superpartners to the Standard Model's spin-1/2 fermions, including quarks and leptons.[1] These bosons, which carry the same quantum numbers as their fermionic counterparts except for spin, are predicted to exist in theories like the Minimal Supersymmetric Standard Model (MSSM) to realize the symmetry between bosons and fermions.[1] Sfermions encompass squarks (superpartners of quarks) and sleptons (superpartners of leptons, such as selectrons, smuons, staus, and sneutrinos).[1]
Sfermions are organized into chiral multiplets, with distinct left-handed and right-handed variants (e.g., and for squarks), reflecting the chiral structure of the Standard Model fermions.[1] Their masses arise primarily from soft supersymmetry-breaking terms in the Lagrangian, parameterized by squared-mass values like for left-handed squark doublets, often assumed to be around the electroweak scale or higher to address the hierarchy problem.[1] Mixing between left- and right-handed states can occur, particularly in the third generation due to large Yukawa couplings, leading to mass eigenstates like lighter and heavier stops ().[1]
In supersymmetric phenomenology, sfermions play crucial roles in processes such as gauge coupling unification at high energies, radiative corrections to the Higgs boson mass, and potential signatures at colliders like the Large Hadron Collider, where their production and decays could reveal supersymmetry.[1] Certain sfermions, such as the lightest sneutrino, have been considered as candidates for dark matter if stable or nearly stable.[1] Despite their theoretical importance, no direct evidence for sfermions has been observed, constraining their masses to exceed hundreds of GeV in many models.[1]
Unlike leptons, squarks carry nontrivial color charge as SU(3)_C triplets (or antitriplets for right-handed fields), allowing them to participate in strong interactions mediated by gluons and gluinos, which facilitates their production at hadron colliders.[12]
Theoretical Foundations
Supersymmetry Basics
Supersymmetry (SUSY) is a theoretical framework that extends the Standard Model of particle physics by introducing a symmetry that relates bosons, which have integer spin, to fermions, which have half-integer spin, such that each particle has a superpartner differing by 1/2 unit of spin.[2] In this setup, the superpartners of bosons are fermions, and vice versa, forming supermultiplets that maintain the symmetry under supersymmetric transformations generated by fermionic operators. This symmetry is realized through an extension of the Poincaré algebra, where the supersymmetry algebra is given by with and as the supersymmetry generators, as the Pauli matrices extended to four dimensions, and as the momentum operator, while the anticommutators among the 's and among the 's vanish.[2] The primary motivations for SUSY include resolving the hierarchy problem, where quantum corrections to the Higgs boson mass would otherwise require unnatural fine-tuning to remain at the electroweak scale rather than the Planck scale.[3] Additionally, SUSY facilitates gauge coupling unification by predicting the running of the strong, weak, and electromagnetic couplings to meet at a high energy scale, a feature not present in the Standard Model alone.[4] It also provides natural candidates for dark matter, such as the lightest supersymmetric particle, which remains stable under reasonable assumptions and could account for the observed cosmological dark matter density.[5] SUSY was developed in the 1970s, with the first supersymmetric field theory in four dimensions proposed by Julius Wess and Bruno Zumino in 1974, known as the Wess-Zumino model, which demonstrated the consistency of interacting supersymmetric theories.[6] This was followed in 1976 by the formulation of supergravity, which incorporates local supersymmetry and couples SUSY to gravity, marking a key milestone in unifying supersymmetry with general relativity.[7] Further advancements in the 1980s led to the construction of realistic models incorporating the Standard Model, culminating in the Minimal Supersymmetric Standard Model (MSSM).[8] The MSSM represents the simplest supersymmetric extension of the Standard Model, introducing superpartners for all known particles while preserving the gauge symmetries SU(3)_C × SU(2)_L × U(1)_Y, and it is formulated using chiral superfields for matter and Higgs sectors and vector superfields for gauge bosons.[8] Chiral superfields contain complex scalar fields, Weyl fermions, and auxiliary fields, enabling the description of quarks, leptons, and Higgsinos, whereas vector superfields describe gauginos and their scalar partners.[9] Within this framework, the scalar superpartners of Standard Model fermions are termed sfermions.[10]Definition and Role of Sfermions
In supersymmetry (SUSY), sfermions are the scalar (spin-0) superpartners of the Standard Model fermions, including quarks and leptons.[11] They carry the same quantum numbers as their fermion partners, such as electric charge, color charge for squarks, and lepton number for sleptons.[1] Sfermions are denoted with an "s-" prefix added to the name of their fermion counterpart, for example, the selectron () for the electron or the squark () for a quark.[11] A primary role of sfermions in SUSY models is to resolve the hierarchy problem by canceling the quadratic divergences in radiative corrections to the Higgs boson mass that arise from fermion loops in the Standard Model.[11] In the SUSY framework, the bosonic sfermion loops contribute with opposite sign to the fermionic loops, ensuring that the net correction remains finite and of order the supersymmetric mass scale, thus stabilizing the electroweak scale without fine-tuning.[1] Additionally, sfermion-Higgs interactions, particularly through loops involving third-generation sfermions like stop squarks, contribute to electroweak symmetry breaking by generating negative mass-squared terms in the Higgs potential.[11] In the Minimal Supersymmetric Standard Model (MSSM), left-handed and right-handed sfermions are treated as distinct scalar fields due to the chiral nature of the Standard Model.[1] The left-handed sfermions form SU(2)_L doublets, such as for squarks or for sleptons, while the right-handed sfermions are SU(2)_L singlets, such as , , or .[11] These sfermions participate in the MSSM superpotential through Yukawa couplings that generate fermion masses, given by where the hatted fields denote superfields containing the sfermions as scalar components, and are Yukawa matrices.[1] Unlike gauginos, which are the fermionic partners of gauge bosons arising from vector superfields, or higgsinos, the fermionic partners of the Higgs fields from Higgs chiral superfields, sfermions are purely scalar particles originating from the chiral superfields of the matter sector.[11] This distinction underscores their role in extending the fermion content of the Standard Model with bosonic degrees of freedom necessary for SUSY.[1]Classification
Squarks
Squarks are the scalar superpartners of quarks in supersymmetric extensions of the Standard Model, carrying the same quantum numbers as their fermionic counterparts except for spin, which differs by 1/2 unit.[12] They transform as color triplets under the SU(3)_C gauge group, distinguishing them from colorless sfermions like sleptons.[12] In the Minimal Supersymmetric Standard Model (MSSM), squarks are organized into chiral superfields: left-handed squark doublets under SU(2)_L, and right-handed squark singlets and (often denoted simply as and for the scalar components).[12] There are three generations of squarks, mirroring the quark generations.[12] The first generation consists of up-squarks () and down-squarks (); the second includes charm-squarks () and strange-squarks (); and the third comprises top-squarks (stops, ) and bottom-squarks (sbottoms, ).[12] The third-generation squarks, particularly stops and sbottoms, receive special attention due to the large top and bottom Yukawa couplings, which introduce significant mixing effects in their mass matrices despite being part of the general generational structure.[12] The quantum numbers of squarks are determined by their embedding in the supersymmetric gauge group SU(3)_C × SU(2)_L × U(1)_Y.[12] Left-handed squark doublets transform as (3, 2, 1/3) under (SU(3)_C, SU(2)_L, Y), while right-handed up-type squark singlets are (, 1, -4/3) and down-type are (, 1, 2/3).[12] These assignments ensure consistency with the electric charges of the partner quarks: +2/3 for up-type and -1/3 for down-type squarks.[12] The following table summarizes the symbols for squark fields, their chiralities, and corresponding quark partners across generations:| Generation | Up-type Squark | Down-type Squark | Partner Quark (Up-type / Down-type) |
|---|---|---|---|
| First | , | , | / |
| Second | , | , | / |
| Third | , | , | / |
Sleptons
Sleptons are the spin-0 scalar superpartners of the leptons in supersymmetric extensions of the Standard Model, such as the Minimal Supersymmetric Standard Model (MSSM).[12] As color singlets under SU(3)_C, they do not participate in strong interactions and couple solely through electroweak forces.[12] In the MSSM, sleptons comprise charged sleptons and sneutrinos across three generations, with the left-handed components arising from SU(2)_L doublet superfields and the right-handed charged ones from singlet superfields.[12] The charged sleptons include the selectrons (), smuons (), and staus (), while the sneutrinos are , , and (left-handed in the MSSM, with no right-handed counterparts unless extended).[12] The left-handed sleptons form SU(2)_L doublets with weak isospin and hypercharge , where the third component for and for , yielding electric charges and , respectively (via ).[12] The right-handed charged sleptons are SU(2)_L singlets with , , and .[12] In superfield notation, the right-handed fields correspond to the conjugate superfield with for (carrying ), but the physical inherits the quantum numbers of the lepton partner.[12] These sleptons are organized into three generations mirroring the lepton families: the electron family (selectrons and electron sneutrino), muon family (smuons and muon sneutrino), and tau family (staus and tau sneutrino).[12] Within the charged sector, the stau is often the lightest slepton due to the relatively large tau lepton Yukawa coupling, which enhances the renormalization group evolution of the associated soft SUSY-breaking mass parameters, driving the stau mass lower compared to selectrons or smuons.[12] Sneutrinos in the MSSM are complex scalar fields and behave as Dirac-type particles, but in extended models incorporating right-handed neutrinos (e.g., for neutrino mass generation), they can mix and acquire Majorana masses, rendering them self-conjugate Majorana particles.[12] If stable—such as in R-parity-conserving scenarios where they are the lightest supersymmetric particle—sneutrinos represent viable dark matter candidates due to their weak interactions and relic density compatibility.[12] The following table summarizes the slepton symbols and their corresponding Standard Model lepton partners:| Slepton Symbol | Corresponding Lepton | Type | Generation |
|---|---|---|---|
| Charged | Electron | ||
| Neutral (sneutrino) | Electron | ||
| Charged | Muon | ||
| Neutral (sneutrino) | Muon | ||
| Charged | Tau | ||
| Neutral (sneutrino) | Tau |
Physical Properties
Mass Spectrum
In the Minimal Supersymmetric Standard Model (MSSM), sfermion masses arise primarily from soft supersymmetry (SUSY) breaking terms introduced to lift the degeneracy between fermions and their scalar superpartners, as exact SUSY would require equal masses for particles and sparticles. These soft terms originate from interactions with a hidden sector where SUSY is spontaneously broken, often mediated by gravity, gauge interactions, or anomalies, leading to sfermion masses significantly larger than the corresponding fermion masses , typically to evade direct detection constraints while stabilizing the Higgs hierarchy.[13] The scalar potential governing sfermion masses includes contributions from F-terms, D-terms, and soft breaking terms: , where is the superpotential, the first two terms preserve SUSY, and the soft mass-squared parameters (e.g., , , for squark doublets and singlets, and analogous , for sleptons) generate the dominant mass contributions after electroweak symmetry breaking. For up-type squarks, the tree-level mass-squared matrix in the left-right basis is approximately diagonal: , with D-term shifts of order providing electroweak-scale corrections that split left- and right-handed states. Similar structures apply to down-type squarks, charged sleptons, and sneutrinos (which lack right-handed counterparts and receive masses mainly from -terms).[13] Mass hierarchies among sfermions emerge from the choice of soft terms and renormalization group (RG) evolution from a high unification scale. In universal scenarios like minimal supergravity (mSUGRA), soft masses are flavor- and generation-independent at the boundary (), but QCD-driven RG evolution makes third-generation squarks lighter due to large top Yukawa couplings, allowing the lightest stop mass to be as low as a few hundred GeV while first- and second-generation sfermions remain heavier (TeV scale) to suppress flavor-changing neutral currents. In contrast, general MSSM models permit non-universal soft masses, leading to more varied spectra, such as lighter sleptons than squarks due to weaker hypercharge and weak couplings compared to strong interactions.[13] Radiative corrections from RG evolution and loops further shape the spectrum, with the stop sector playing a pivotal role: large top Yukawa couplings drive significant mixing and mass reductions for third-generation up-squarks, contributing up to to the lightest Higgs mass, enabling GeV for stop masses around 1-2 TeV in viable parameter space. These corrections, computed at one- and two-loop levels, underscore the stop's influence on Higgs phenomenology without requiring fine-tuning.[14][13]Flavor Mixing
In supersymmetry, sfermions exhibit chiral mixing between their left-handed and right-handed components, arising from off-diagonal elements in the scalar mass-squared matrix induced by trilinear soft SUSY-breaking terms (A-terms) and the Higgsino mass parameter μ combined with the tangent of the Higgs mixing angle tan β. For each sfermion flavor, this results in a 2×2 mass matrix structure, where the off-diagonal left-right (LR) term is proportional to the corresponding fermion mass times (A - μ tan β for down-type or charged leptons, or A - μ cot β for up-type). This mixing is diagonalized to yield the physical mass eigenstates, such as and for the selectron sector.[15] A representative example is the selectron mass-squared matrix, where the LR entry is given by with the electron mass, the trilinear coupling, and the diagonal entries dominated by soft masses and plus electroweak contributions. Since is small (~0.511 MeV), chiral mixing is negligible for first- and second-generation sleptons but becomes significant for the stau in the third generation due to the larger tau mass (~1.777 GeV). The mixing angle θ is approximately , determining the admixture of chiral states in the mass eigenstates.[13] Generational mixing among sfermion flavors occurs due to misalignment between the fermion mass basis (aligned with the CKM matrix for quarks) and the sfermion soft mass basis, introducing off-diagonal entries in the 6×6 mass matrices for squarks or sleptons. This leads to CKM-like mixing angles in the squark sector, parametrized by dimensionless δ_{ij} factors (e.g., δ_{LL_{ij}} for left-left mixing), which quantify the ratio of off-diagonal to diagonal mass terms. In the Minimal Flavor Violation (MFV) framework, such mixing is suppressed and aligned with the underlying Yukawa matrices, minimizing flavor-changing neutral currents (FCNCs).[15] A unique feature is the large left-right mixing in the third-generation sfermions, particularly the stop squark, where the LR term scales as ~m_t (A_t - μ cot β) with m_t ~173 GeV, yielding mixing angles up to ~m_t / m_{\tilde{q}} (~0.1-0.3 for TeV-scale squark masses). This enhances stop decays and impacts Higgs sector phenomenology. Constraints on generational mixing arise from FCNC processes, such as b → s γ, which bound δ_{23}^{d LL, RR} ≲ 0.01 and δ_{23}^{d LR} ≲ 10^{-2} (m_{\tilde{q}}/1 TeV)^2 from gluino-squark loops, as measured by LHCb and other modern experiments.[16] These mixing effects can enhance or suppress sfermion-mediated decays and FCNC rates, but remain minimal in aligned MFV scenarios where off-diagonals are O(λ_CKM^2) ~10^{-3}-10^{-5}. Large mixings are probed at colliders via flavor-violating signatures, though LHC limits (e.g., squark masses > 2 TeV as of 2024) tighten bounds indirectly; slepton masses are constrained to > 300-700 GeV in various models, with no evidence for sfermions observed as of 2025.[16][15]Interactions
Gauge Interactions
In the Minimal Supersymmetric Standard Model (MSSM), sfermions inherit the same gauge quantum numbers as their Standard Model (SM) fermion partners, leading to analogous but scalar couplings to gauge bosons via the supersymmetric extension of the SM gauge interactions. The relevant Lagrangian terms arise from the kinetic part of the scalar superfields, , where the covariant derivative incorporates the gauge fields . For colored squarks, the strong interaction with gluons is described by the vertex factor , where is the strong coupling constant, are the SU(3)_C generators, denotes the up-type squark field, and is the gluon field, mirroring the quark-gluon vertex but without the Dirac structure due to the scalar nature of sfermions. Similarly, electroweak couplings for both squarks and sleptons involve the photon, W, and Z bosons, with strengths proportional to the electric charge , weak isospin , and hypercharge of the corresponding fermion . Sfermions also couple to gauginos and their fermionic partners through supersymmetric gauge interactions originating from the super-Yang-Mills sector of the theory. The general form of these Yukawa-like terms in the Lagrangian is , where is the gauge coupling for the group index , is the sfermion, is the fermion, and is the gaugino (e.g., gluino for SU(3)_C, wino for SU(2)_L, or bino for U(1)_Y). For squarks specifically, the coupling to gluinos takes the form , enabling strong interactions that facilitate processes like squark-gluino pair production. In contrast, sleptons lack color charge and thus couple only to electroweak gauginos, with vertices proportional to or (the SU(2)_L and U(1)_Y couplings, respectively). Neutralino-sfermion-fermion couplings, arising from the neutral gaugino components mixed into the neutralino mass eigenstates , are proportional to the hypercharge for the bino contribution and weak isospin for the wino contribution, modulated by the neutralino mixing matrix elements . The effective Lagrangian term is of the form , where the projection operators reflect the chiral structure of the original gaugino interactions. These couplings determine key decay channels for neutralinos and charginos involving sfermions. Conservation of R-parity in the MSSM, defined as , assigns sfermions (and all superpartners) an odd R-parity, ensuring that sfermion decays proceed to a fermion plus a supersymmetric partner, such as or for squarks, rather than directly to SM particles alone. This structure preserves the stability of the lightest supersymmetric particle and shapes collider signatures through these gauge-mediated decays.[17]Higgs and Yukawa Couplings
In the Minimal Supersymmetric Standard Model (MSSM), the Yukawa couplings of the Standard Model are extended to include interactions involving sfermions, the scalar partners of quarks and leptons, through terms in the superpotential and soft supersymmetry-breaking Lagrangian. The relevant superpotential terms are , where are left-handed quark and lepton superfields, are right-handed ones, and are the up- and down-type Higgs superfields; these generate fermion masses and corresponding sfermion-Higgs interactions upon Higgs vacuum expectation value (VEV) acquisition.[11] In the soft-breaking sector, trilinear scalar couplings, known as A-terms, arise as , where and denote right- and left-handed sfermion fields, respectively; these terms are crucial for sfermion mass generation and mixing.[1][11] Direct tree-level couplings between Higgs bosons and sfermions include contributions from both F-terms and D-terms in the scalar potential. The F-term-derived Higgs-sfermion-fermion vertex takes the form for up-type fields (with analogous forms for down-type), where is the lightest CP-even Higgs boson, is the fermion Yukawa coupling, is the left-handed sfermion, the fermion, and ; for down-type sfermions, this coupling is enhanced by a factor of , where are the Higgs VEVs, making it particularly relevant in large- scenarios.[11] D-term contributions provide quartic Higgs-sfermion interactions proportional to , where is a gauge coupling and the electroweak VEV, arising from the term in the scalar potential with sfermion fields contributing alongside Higgs fields.[11] At loop level, sfermions mediate contributions to Higgs production (e.g., via gluon fusion) and decay processes, such as enhanced rates to bottom quarks in the presence of large .[1] Large trilinear A-terms significantly influence left-right chiral mixing in the sfermion mass-squared matrices, with off-diagonal elements or , particularly pronounced for third-generation sfermions like stops and sbottoms due to large Yukawa couplings.[11] This mixing is especially important for stau-Higgs couplings in large- models, where enhanced interactions can probe supersymmetric contributions to lepton flavor violation.[1] Regarding electroweak symmetry breaking (EWSB), sfermion VEVs are forbidden at tree level by R-parity conservation, which distinguishes Higgs and matter superfields, preventing dangerous flavor-changing neutral currents; however, radiative corrections from sfermion loops can indirectly affect the Higgsino mass parameter through renormalization group evolution of the soft masses.[11]Phenomenology
Production at Colliders
Sfermions can be produced at high-energy colliders such as the Large Hadron Collider (LHC) through various processes governed by their gauge interactions. For squarks, the dominant mechanism is pair production via strong interactions, primarily through gluon-gluon fusion () and quark-antiquark annihilation (), which proceed at leading order in perturbative QCD. These processes have cross sections scaling as , where is the strong coupling constant, and decrease rapidly with increasing squark mass . The leading-order approximation for the squark pair production cross section for light flavors (first and second generations) is given by reflecting the color factors and averaging over initial parton states in proton-proton collisions. Higher-order corrections, including next-to-leading order (NLO) and soft-gluon resummation to next-to-leading logarithmic (NLL) accuracy, enhance the predictions and reduce scale uncertainties, with total cross sections at 13 TeV of around 1 picobarn for squark masses around 500 GeV.[18] Associated production processes for squarks include squark-gluino pairs () via strong interactions, which can dominate if the gluino mass is comparable to or lighter than the squark mass, contributing significantly to multi-jet final states. For third-generation squarks, particularly the lighter stop (), an important electroweak-mediated channel is , arising from mixing in the stop-sbottom sector due to the large top Yukawa coupling; this process allows production even if stops are lighter than other squarks. Slepton pair production, in contrast, occurs via electroweak interactions, such as through s-channel photon or Z-boson exchange and via W-boson exchange, with cross sections suppressed by (where denotes electroweak couplings) relative to squark production. Associated slepton production, like or , further involves neutralino or chargino exchange and remains subdominant. At 13 TeV, slepton pair cross sections are on the order of tens of femtoseconds for masses around 200 GeV and a few femtoseconds for 300 GeV.[19] Kinematic features of sfermion production are influenced by threshold effects near the parton center-of-mass energy , where the cross section vanishes below threshold and rises steeply, modulated by parton distribution functions. Initial-state radiation (ISR) of gluons or quarks can boost the produced pairs, leading to additional jets that aid in reconstructing sfermion masses from visible decay products and missing transverse momentum, though the exact kinematics depend on the supersymmetric spectrum.Decay Modes
The dominant decay modes of sfermions in R-parity-conserving supersymmetric models are two-body processes into the corresponding fermion and the lightest neutralino or a chargino, such as for up-type squarks or for selectrons, provided these channels are kinematically accessible. These gaugino-mediated decays typically saturate the branching ratio at nearly 100% when the sfermion mass exceeds that of the final-state superpartner, as alternative channels like three-body decays or emissions of gauge bosons are suppressed by phase space or additional couplings. The preference for these modes arises from the strong (for squarks) or electroweak (for sleptons) nature of the interactions, which dominate over Yukawa-suppressed alternatives unless significant mixing alters the couplings. In scenarios with mass hierarchies where heavier neutralinos or charginos are accessible, sfermions—especially squarks—undergo cascade decays that chain through intermediate superpartners. For instance, a heavy squark may decay as or , with the intermediate steps involving on-shell gauginos or Higgsinos. Such cascades are prevalent for third-generation squarks due to their larger production rates and masses, leading to multi-jet plus lepton or boson signatures, though the exact topology depends on the superpartner spectrum and mixing angles. The partial width for the canonical two-body decay , assuming a dominant gaugino coupling and negligible fermion mass, is approximated by where denotes the appropriate SU(2)_L or U(1)_Y gauge coupling. This expression highlights the strong dependence on the mass ratio, with the width vanishing as the neutralino mass approaches the sfermion mass, and it provides a benchmark for higher-order corrections that can modify rates by up to 20% in electroweak precision calculations. In gauge-mediated supersymmetry breaking, light sleptons often act as the next-to-lightest supersymmetric particle (NLSP) and predominantly decay to a lepton plus the gravitino lightest supersymmetric particle (LSP), . Flavor-violating sfermion decays, such as , are highly suppressed in minimal models adhering to flavor symmetry but become observable probes of sfermion mixing if non-minimal flavor violation is present, with rates constrained by rare decay experiments.[20] When a slepton serves as the NLSP, its decay to the gravitino can yield long-lived particles due to the feeble gravitational coupling, with lifetimes ranging from picoseconds to meters depending on the SUSY-breaking scale; in such cases, the slepton may appear stable or displaced, though associated photonic signals arise if neutralino admixtures contribute.[21] Flavor mixing from the sfermion sector can subtly alter these branching ratios by redistributing decay probabilities across generations.[20]Experimental Status
Current Constraints
Experimental searches at the Large Hadron Collider (LHC) have imposed stringent lower limits on sfermion masses using data from Run 2 (139 fb⁻¹ at 13 TeV) and Run 3 (additional ~200 fb⁻¹ collected through November 2025), totaling over 340 fb⁻¹.[22] The ATLAS and CMS collaborations have excluded squark masses below approximately 2.2 TeV for first- and second-generation squarks in simplified models where they decay directly to quarks and the lightest neutralino, assuming massless neutralinos.[23] For third-generation squarks, particularly the lighter stop in compressed scenarios with small mass splittings to the neutralino (~10-50 GeV), limits reach above 1.1 TeV, with recent CMS analyses setting exclusions up to 900 GeV in mass-degenerate cases using ~340 fb⁻¹.[24] Gluino-mediated squark production scenarios yield stronger bounds, excluding squark masses below 2.4-2.6 TeV depending on the neutralino mass.[23] Slepton limits are generally weaker due to their electroweak production cross-sections but have been tightened with full Run 3 data through 2025. ATLAS excludes selectron and smuon masses up to 400-450 GeV for left-handed sleptons and 350-400 GeV for right-handed ones in models with nearly massless neutralinos, based on ~340 fb⁻¹.[25] CMS results from recent analyses set limits of 280-320 GeV for mass-degenerate charged sleptons and up to 300 GeV for selectrons at small mass splittings (~5 GeV) to the neutralino. Stau limits are lower, around 250-300 GeV, owing to larger mixing effects and softer kinematic signatures. Sneutrino bounds derive primarily from the Z boson's invisible decay width, excluding masses below ~45 GeV, with collider searches pushing limits to 250-350 GeV in scenarios with associated production.[26] Analyses from 2025 conferences, such as LHCP and Moriond, incorporating up to ~200 fb⁻¹ of Run 3 data, have not yielded any discoveries of sfermions but have significantly constrained phenomenological minimal supersymmetric Standard Model (pMSSM) parameter space, reducing viable regions for light sfermions by factors of 20-50% compared to pre-Run 3 expectations, particularly in electroweakino-slepton co-annihilation scenarios.[27] No excesses beyond Standard Model expectations were observed in CMS SUSY searches targeting sfermion pair production.[27] Additional constraints arise from flavor physics and dark matter searches. Recent measurements of the B_s meson mass difference Δm_s by LHCb and Belle II tightly limit squark flavor mixing in the down-type sector, excluding significant left-right mixing parameters (δ_{23}^d)_{LL,RR} > 0.01-0.05 for squark masses around 1 TeV, as deviations from Standard Model predictions would alter box diagram contributions.[28] For sneutrino dark matter candidates, direct detection experiments like XENONnT and LZ report null results, excluding spin-independent cross-sections above 10^{-47} cm² for sneutrino masses of 10-100 GeV, consistent with Higgs portal couplings below 10^{-3}.[26]| Sfermion Type | Lower Mass Limit (95% CL, GeV) | Scenario/Assumption | Experiment (Luminosity) | Reference |
|---|---|---|---|---|
| Light squarks (ũ, d̃) | >2200 | Decay to jet + neutralino (m_χ=0) | ATLAS/CMS (~340 fb⁻¹) | [23] |
| Gluino-mediated squark | >2400-2600 | m_χ < 500 GeV | ATLAS (~340 fb⁻¹) | [23] |
| Stop (compressed) | >1100-1350 | Δm ~10-50 GeV to neutralino | ATLAS/CMS (~340 fb⁻¹) | [24] [23] |
| Selectron/Smuon (L/R) | 400-450 / 350-400 | Direct production, m_χ~0 | ATLAS (~340 fb⁻¹) | [25] |
| Stau | >250-300 | Mass-degenerate, small Δm | CMS (~340 fb⁻¹) | [25] |
| Sneutrino | >250-350 (collider); >45 (Z width) | Associated production / invisible Z | ATLAS/LEP | [26] |