STANDARD MODEL

WHAT WE ARE MISSING

Το Καθιερωμένο Πρότυπο (Standard Model) είναι μια φυσική θεωρία που περιγράφει τα δομικά συστατικά της ύλης και τις μεταξύ τους ισχυρές, ασθενείς και ηλεκτρομαγνητικές αλληλεπιδράσεις. Δεν περιλαμβάνει καμία περιγραφή των βαρυτικών αλληλεπιδράσεων.

Πρόκειται για μια πολύ καλά θεμελιωμένη θεωρία που έχει προβλέψει πολλά πειραματικά αποτελέσματα, όπως την ύπαρξη πολλών σωματιδίων και έχει αντεπεξέλθει σε πάρα πολλούς πειραματικούς ελέγχους. Το βασικό κομμάτι που λείπει στη θεωρία αυτή για να συμπληρωθεί είναι το μποζόνιο Χιγκς του οποίου η ύπαρξη, πιθανολογούνταν με αρκετή βεβαιότητα και πλέον έχει επιβεβαιωθεί. Παρ' όλες τις επιτυχίες της, η θεωρία αυτή δεν μπορεί να εξηγήσει την ύπαρξη σκοτεινής ύλης, τις ταλαντώσεις νετρίνων και την ύπαρξη σωματιδίων με πολύ διαφορετικές μάζες.

Η θεωρία είναι στην πραγματικότητα μια σύνθεση θεωριών που βασίστηκε στις ανακαλύψεις νέων πειραμάτων και τις εξελίξεις της θεωρίας. Ήταν μια συλλογική προσπάθεια στην ευρύτερη έννοιά της, που γίνονταν από ερευνητές σε διαφορετικές ηπείρους επί δεκαετίες. Η σημερινή σύνθεση ολοκληρώθηκε στα μέσα της δεκαετίας του 1970 μετά από πειραματική επιβεβαίωση της ύπαρξης των κουάρκ.

The Standard Model of particle physics is a theory concerning the electromagnetic, weak, andstrong nuclear interactions, as well as classifying all the subatomic particles known. It was developed throughout the latter half of the 20th century, as a collaborative effort of scientists around the world.^[1] The current formulation was finalized in the mid-1970s upon experimental confirmation of the existence of quarks. Since then, discoveries of the top quark (1995), the tau neutrino (2000), and the Higgs boson (2012) have given further credence to the Standard Model. Because of its success in explaining a wide variety of experimental results, the Standard Model is sometimes regarded as the "theory of almost everything".

Although the Standard Model is believed to be theoretically self-consistent^[2] and has demonstrated huge and continued successes in providing experimental predictions, it does leave somephenomena unexplained and it falls short of being a complete theory of fundamental interactions. It does not incorporate the full theory of gravitation^[3] as described by general relativity, or account for the accelerating expansion of the universe (as possibly described by dark energy). The model does not contain any viable dark matter particle that possesses all of the required properties deduced from observational cosmology. It also does not incorporate neutrino oscillations (and their non-zero masses).

Matter particles

Fermions

The Standard Model includes 12 elementary particles of spin  ¹⁄₂ known as fermions. According to the spin-statistics theorem, fermions respect the Pauli exclusion principle. Each fermion has a corresponding antiparticle.

The fermions of the Standard Model are classified according to how they interact (or equivalently, by what charges they carry). There are six quarks (up, down, charm, strange, top,bottom), and six leptons (electron, electron neutrino, muon, muon neutrino, tau, tau neutrino). Pairs from each classification are grouped together to form a generation, with corresponding particles exhibiting similar physical behavior (see table).

The defining property of the quarks is that they carry color charge, and hence, interact via thestrong interaction. A phenomenon called color confinement results in quarks being very strongly bound to one another, forming color-neutral composite particles (hadrons) containing either a quark and an antiquark (mesons) or three quarks (baryons). The familiar proton and theneutron are the two baryons having the smallest mass. Quarks also carry electric charge andweak isospin. Hence, they interact with other fermions both electromagnetically and via theweak interaction.

The remaining six fermions do not carry colour charge and are called leptons. The threeneutrinos do not carry electric charge either, so their motion is directly influenced only by theweak nuclear force, which makes them notoriously difficult to detect. However, by virtue of carrying an electric charge, the electron, muon, and tau all interact electromagnetically.

Each member of a generation has greater mass than the corresponding particles of lower generations. The first generation charged particles do not decay; hence all ordinary (baryonic) matter is made of such particles. Specifically, all atoms consist of electrons orbiting around atomic nuclei, ultimately constituted of up and down quarks. Second and third generation charged particles, on the other hand, decay with very short half lives, and are observed only in very high-energy environments. Neutrinos of all generations also do not decay, and pervade the universe, but rarely interact with baryonic matter.

Gauge Bosons

In the Standard Model, gauge bosons are defined as force carriers that mediate the strong, weak, and electromagnetic fundamental interactions.

Interactions in physics are the ways that particles influence other particles. At a macroscopic level, electromagnetism allows particles to interact with one another via electric and magneticfields, and gravitation allows particles with mass to attract one another in accordance with Einstein's theory of general relativity. The Standard Model explains such forces as resulting from matter particles exchanging other particles, generally referred to as force mediating particles. When a force-mediating particle is exchanged, at a macroscopic level the effect is equivalent to a force influencing both of them, and the particle is therefore said to havemediated (i.e., been the agent of) that force. The Feynman diagram calculations, which are a graphical representation of the perturbation theory approximation, invoke "force mediating particles", and when applied to analyze high-energy scattering experiments are in reasonable agreement with the data. However, perturbation theory (and with it the concept of a "force-mediating particle") fails in other situations. These include low-energy quantum chromodynamics, bound states, and solitons.

The gauge bosons of the Standard Model all have spin (as do matter particles). The value of the spin is 1, making them bosons. As a result, they do not follow the Pauli exclusion principlethat constrains fermions: thus bosons (e.g. photons) do not have a theoretical limit on their spatial density (number per volume). The different types of gauge bosons are described below.

·         Photons mediate the electromagnetic force between electrically charged particles. The photon is massless and is well-described by the theory of quantum electrodynamics.

·         The 
W+
, 
W−
, and 
Z
 gauge bosons mediate the weak interactions between particles of different flavors (all quarks and leptons). They are massive, with the 
Z
 being more massive than the 
W±
. The weak interactions involving the 
W±
 exclusively act on left-handed particles and right-handed antiparticles. Furthermore, the 
W±
 carries an electric charge of +1 and −1 and couples to the electromagnetic interaction. The electrically neutral 
Z
 boson interacts with both left-handed particles and antiparticles. These three gauge bosons along with the photons are grouped together, as collectively mediating the electroweak interaction.

·         The eight gluons mediate the strong interactions between color charged particles (the quarks). Gluons are massless. The eightfold multiplicity of gluons is labeled by a combination of color and anticolor charge (e.g. red–antigreen).^{[nb 1]} Because the gluons have an effective color charge, they can also interact among themselves. The gluons and their interactions are described by the theory of quantum chromodynamics.

The interactions between all the particles described by the Standard Model are summarized by the diagrams on the right of this section.