Generation (particle physics)

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In particle physics, a generation or family is a division of the elementary particles. Between generations, particles differ by their flavour quantum number and mass, but their electric and strong interactions are identical.

Contents

There are three generations according to the Standard Model of particle physics. Each generation contains two types of leptons and two types of quarks. The two leptons may be classified into one with electric charge −1 (electron-like) and neutral (neutrino); the two quarks may be classified into one with charge −1⁄3 (down-type) and one with charge +2⁄3 (up-type). The basic features of quark–lepton generation or families, such as their masses and mixings etc., can be described by some of the proposed family symmetries.

Generations of matter
Fermion categories Elementary particle generation
TypeSubtypeFirstSecondThird
Quarks
(colored)		down-type down strange bottom
up-type up charm top
Leptons
(color-free) 		charged electron muon tau
neutral electron neutrino muon neutrino tau neutrino

Overview

Each member of a higher generation has greater mass than the corresponding particle of the previous generation, with the possible exception of the neutrinos (whose small but non-zero masses have not been accurately determined). For example, the first-generation electron has a mass of only 0.511  MeV/c2 , the second-generation muon has a mass of 106 MeV/c2, and the third-generation tau has a mass of 1777 MeV/c2 (almost twice as heavy as a proton). This mass hierarchy [1] causes particles of higher generations to decay to the first generation, which explains why everyday matter (atoms) is made of particles from the first generation only. Electrons surround a nucleus made of protons and neutrons, which contain up and down quarks. The second and third generations of charged particles do not occur in normal matter and are only seen in extremely high-energy environments such as cosmic rays or particle accelerators. The term generation was first introduced by Haim Harari in Les Houches Summer School, 1976. [2] [3]

Neutrinos of all generations stream throughout the universe but rarely interact with other matter. [4] It is hoped that a comprehensive understanding of the relationship between the generations of the leptons may eventually explain the ratio of masses of the fundamental particles, and shed further light on the nature of mass generally, from a quantum perspective. [5]

Fourth generation

Fourth and further generations are considered unlikely by many (but not all) theoretical physicists. Some arguments against the possibility of a fourth generation are based on the subtle modifications of precision electroweak observables that extra generations would induce; such modifications are strongly disfavored by measurements. There are functions used to generalize terms for introduction in a new quark that is an isosinglet and is responsible for generating Flavour-Changing-Neutral-Currents' (FCNC) at tree level in the electroweak sectors. [6] [7] Furthermore, a fourth generation with a 'light' neutrino (one with a mass less than about 45 GeV/c2) has been ruled out by measurements of the decay widths of the Z boson at CERN's Large Electron–Positron Collider (LEP). [8] Nonetheless, searches at high-energy colliders for particles from a fourth generation continue, but as yet no evidence has been observed. [9] In such searches, fourth-generation particles are denoted by the same symbols as third-generation ones with an added prime (e.g. b′ and t′).

The lower bound for a fourth generation of quark (b′, t′) masses is currently at 1.4 TeV from experiments at the LHC. [10]

The lower bound for a fourth generation neutrino (ν'τ) mass is currently at about 60 GeV (millions of times larger than the upper bound for the other 3 neutrino masses). [11]

The lower bound for a fourth generation charged lepton (τ') mass is currently 100GeV and proposed upper bound of 1.2 TeV from unitarity considerations. [12]

If the Koide formula continues to hold, the masses of the fourth generation charged lepton would be 44 GeV (ruled out) and b′ and t′ should be 3.6 TeV and 84 TeV respectively. (The maximum possible energy for protons in the LHC is about 6 TeV.)

Origin

Unsolved problem in physics:
Why are there three generations of quarks and leptons? Is there a theory that can explain the masses of particular quarks and leptons in particular generations from first principles (a theory of Yukawa couplings)?
(more unsolved problems in physics)

The origin of multiple generations of fermions, and the particular count of 3, is an unsolved problem of physics. String theory provides a cause for multiple generations, but the particular number depends on the details of the compactification of the D-brane intersections. Additionally, E8 grand unified theories in 10 dimensions compactified on certain orbifolds down to 4 D naturally contain 3 generations of matter. [13] This includes many heterotic string theory models.

In standard quantum field theory, under certain assumptions, a single fermion field can give rise to multiple fermion poles with mass ratios of around  eπ ≈ 23  and  e2π ≈ 535  potentially explaining the large ratios of fermion masses between successive generations and their origin. [1]

The existence of precisely three generations with the correct structure was at least tentatively derived from first principles through a connection with gravity. [14] The result implies a unification of gauge forces into SU(5). The question regarding the masses is unsolved, but this is a logically separate question, related to the Higgs sector of the theory.

See also

Related Research Articles

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<span class="mw-page-title-main">Grand Unified Theory</span> Comprehensive physical model

Grand Unified Theory (GUT) is any model in particle physics that merges the electromagnetic, weak, and strong forces into a single force at high energies. Although this unified force has not been directly observed, many GUT models theorize its existence. If the unification of these three interactions is possible, it raises the possibility that there was a grand unification epoch in the very early universe in which these three fundamental interactions were not yet distinct.

<span class="mw-page-title-main">Weak interaction</span> Interaction between subatomic particles and one of the four known fundamental interactions

In nuclear physics and particle physics, the weak interaction, also called the weak force, is one of the four known fundamental interactions, with the others being electromagnetism, the strong interaction, and gravitation. It is the mechanism of interaction between subatomic particles that is responsible for the radioactive decay of atoms: The weak interaction participates in nuclear fission and nuclear fusion. The theory describing its behaviour and effects is sometimes called quantum flavordynamics (QFD); however, the term QFD is rarely used, because the weak force is better understood by electroweak theory (EWT).

<span class="mw-page-title-main">Standard Model</span> Theory of forces and subatomic particles

The Standard Model of particle physics is the theory describing three of the four known fundamental forces in the universe and classifying all known elementary particles. It was developed in stages throughout the latter half of the 20th century, through the work of many scientists worldwide, with the current formulation being finalized in the mid-1970s upon experimental confirmation of the existence of quarks. Since then, proof of the top quark (1995), the tau neutrino (2000), and the Higgs boson (2012) have added further credence to the Standard Model. In addition, the Standard Model has predicted various properties of weak neutral currents and the W and Z bosons with great accuracy.

<span class="mw-page-title-main">Lepton</span> Class of elementary particles

In particle physics, a lepton is an elementary particle of half-integer spin that does not undergo strong interactions. Two main classes of leptons exist: charged leptons, including the electron, muon, and tauon, and neutral leptons, better known as neutrinos. Charged leptons can combine with other particles to form various composite particles such as atoms and positronium, while neutrinos rarely interact with anything, and are consequently rarely observed. The best known of all leptons is the electron.

<span class="mw-page-title-main">Charm quark</span> Type of quark

The charm quark, charmed quark, or c quark is an elementary particle found in composite subatomic particles called hadrons such as the J/psi meson and the charmed baryons created in particle accelerator collisions. Several bosons, including the W and Z bosons and the Higgs boson, can decay into charm quarks. All charm quarks carry charm, a quantum number. This second-generation particle is the third-most-massive quark, with a mass of 1.27±0.02 GeV/c2 as measured in 2022, and a charge of +⁠2/3⁠ e.

<span class="mw-page-title-main">Top quark</span> Type of quark

The top quark, sometimes also referred to as the truth quark, is the most massive of all observed elementary particles. It derives its mass from its coupling to the Higgs field. This coupling yt is very close to unity; in the Standard Model of particle physics, it is the largest (strongest) coupling at the scale of the weak interactions and above. The top quark was discovered in 1995 by the CDF and DØ experiments at Fermilab.

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<span class="mw-page-title-main">W and Z bosons</span> Bosons that mediate the weak interaction

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W+
,
W−
, and
Z0
. The
W±
 bosons have either a positive or negative electric charge of 1 elementary charge and are each other's antiparticles. The
Z0
 boson is electrically neutral and is its own antiparticle. The three particles each have a spin of 1. The
W±
 bosons have a magnetic moment, but the
Z0
 has none. All three of these particles are very short-lived, with a half-life of about 3×10−25 s. Their experimental discovery was pivotal in establishing what is now called the Standard Model of particle physics.

<span class="mw-page-title-main">Minimal Supersymmetric Standard Model</span> Simplest supersymmetric extension to the Standard Model

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In particle physics, flavour or flavor refers to the species of an elementary particle. The Standard Model counts six flavours of quarks and six flavours of leptons. They are conventionally parameterized with flavour quantum numbers that are assigned to all subatomic particles. They can also be described by some of the family symmetries proposed for the quark-lepton generations.

Sterile neutrinos are hypothetical particles that interact only via gravity and not via any of the other fundamental interactions of the Standard Model. The term sterile neutrino is used to distinguish them from the known, ordinary active neutrinos in the Standard Model, which carry an isospin charge of ⁠±+1/ 2 ⁠ and engage in the weak interaction. The term typically refers to neutrinos with right-handed chirality, which may be inserted into the Standard Model. Particles that possess the quantum numbers of sterile neutrinos and masses great enough such that they do not interfere with the current theory of Big Bang nucleosynthesis are often called neutral heavy leptons (NHLs) or heavy neutral leptons (HNLs).

Leptoquarks are hypothetical particles that would interact with quarks and leptons. Leptoquarks are color-triplet bosons that carry both lepton and baryon numbers. Their other quantum numbers, like spin, (fractional) electric charge and weak isospin vary among models. Leptoquarks are encountered in various extensions of the Standard Model, such as technicolor theories, theories of quark–lepton unification (e.g., Pati–Salam model), or GUTs based on SU(5), SO(10), E6, etc. Leptoquarks are currently searched for in experiments ATLAS and CMS at the Large Hadron Collider in CERN.

<span class="mw-page-title-main">Physics beyond the Standard Model</span> Theories trying to extend known physics

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In the Grand Unified Theory of particle physics (GUT), the desert refers to a theorized gap in energy scales, between approximately the electroweak energy scale–conventionally defined as roughly the vacuum expectation value or VeV of the Higgs field –and the GUT scale, in which no unknown interactions appear.

<span class="mw-page-title-main">Higgs boson</span> Elementary particle involved with rest mass

The Higgs boson, sometimes called the Higgs particle, is an elementary particle in the Standard Model of particle physics produced by the quantum excitation of the Higgs field, one of the fields in particle physics theory. In the Standard Model, the Higgs particle is a massive scalar boson with zero spin, even (positive) parity, no electric charge, and no colour charge that couples to mass. It is also very unstable, decaying into other particles almost immediately upon generation.

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References

  1. 1 2 Blumhofer, A.; Hutter, M. (1997). "Family structure from periodic solutions of an improved gap equation". Nuclear Physics B . 484 (1): 80–96. Bibcode:1997NuPhB.484...80B. CiteSeerX   10.1.1.343.783 . doi:10.1016/S0550-3213(96)00644-X. (Erratum:  doi:10.1016/S0550-3213(97)00228-9)
  2. ↑ Harari, H. (5 July – 14 August 1976). Balian, R.; Llewellyn-Smith, C.H. (eds.). Beyond charm. Weak and Electromagnetic Interactions at High Energy. Les Houches Summer School Proceedings. Vol. 29. Les Houches, France: North-Holland (published 1977). p. 613. Archived from the original on 12 December 2012.
  3. ↑ Harari, H. (1977). "Three generations of quarks and leptons" (PDF). In van Goeler, E.; Weinstein, R. (eds.). Proceedings of the XII Rencontre de Moriond. p. 170. SLAC-PUB-1974.
  4. ↑ "Experiment confirms famous physics model". MIT Press Office (Press release). Massachusetts Institute of Technology. 18 April 2007.
  5. ↑ Mac Gregor, M.H. (2006). "A 'muon mass tree' with α‑quantized lepton, quark, and hadron masses". arXiv: hep-ph/0607233 .
  6. ↑ Botella, Francisco J. (April 2022). "Decays of the Heavy Top and New Insights on ΕK in a One-VLQ Minimal Solution to the CKM Unitarity Problem". European Physical Journal C. 82 (4): 17.
  7. ↑ Rayner, Mark (22 May 2013). "Chasing new physics with electroweak penguins". CernCourier. Retrieved 17 December 2024.
  8. ↑ Decamp, D.; et al. (ALEPH collaboration) (1989). "Determination of the number of light neutrino species". Physics Letters B . 231 (4): 519–529. Bibcode:1989PhLB..231..519D. doi:10.1016/0370-2693(89)90704-1. hdl: 11384/1735 .
  9. ↑ Amsler, C.; et al. (Particle Data Group) (2008). "b′ (4th Generation) Quarks, searches for" (PDF). Physics Letters B . Review of Particle Physics. 667 (1): 1–1340. Bibcode:2008PhLB..667....1A. doi:10.1016/j.physletb.2008.07.018. hdl: 1854/LU-685594 . S2CID   227119789.
  10. ↑ CMS Collaboration (8 May 2019). "Boosting searches for fourth-generation quarks". CERN Courier. Report from the CMS experiment.
  11. ↑ Carpenter, Linda M.; Rajaraman, Arvind (December 2010). "Revisiting constraints on fourth generation neutrino masses". Physical Review D. 82 (11): 114019. arXiv: 1005.0628 . Bibcode:2010PhRvD..82k4019C. doi:10.1103/PhysRevD.82.114019. S2CID   119175322. ABSTRACT: We revisit the current experimental bounds on fourth-generation Majorana neutrino masses, including the effects of right handed neutrinos. Current bounds from LEP‑II are significantly altered by a global analysis. We show that the current bounds on fourth generation neutrinos decaying to e W and μ W can be reduced to about 80 GeV (from the current bound of 90 GeV), while a neutrino decaying to τ W can be as light as 62.1 GeV. The weakened bound opens up a neutrino decay channel for intermediate mass Higgs, and interesting multi-particle final states for Higgs and fourth generation lepton decays.
  12. ↑ Dighe, Amol; Ghosh, Diptimoy; Godbole, Rohini M.; Prasath, Arun (2012). "Large mass splittings for fourth generation fermions allowed by LHC Higgs boson exclusion". Physical Review D. 85 (11): 114035. arXiv: 1204.3550 . Bibcode:2012PhRvD..85k4035D. doi:10.1103/PhysRevD.85.114035. S2CID   119204685.
  13. ↑ Motl, Luboš (13 July 2021). "The "pure joy" E8 SUSY toroidal orbifold TOE". The Reference Frame (blog). Retrieved 23 August 2021– via motls.blogspot.com.
  14. ↑ van der Bij, J.J. (28 December 2007). "Cosmotopological relation for a unified field theory". Physical Review D. 76 (12): 121702. arXiv: 0708.4179 . Bibcode:2007PhRvD..76l1702V. doi:10.1103/PhysRevD.76.121702.
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