Such particles could be spotted at the LHC, or near it. In that case, an electron-like particle in the hidden sector would appear as a millicharged one in the Standard Model sector. Hidden sectors and portals may sound suspiciously like science fiction, devised by physicists driven to the sort of desperation Sundrum confessed to, but we already know of at least one instance where nature has played this game.
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Dark matter, no matter how little we know about it, presents strong evidence that another sector exists beyond the Standard Model. In fact, physicists have invoked long-lived particles and hidden sectors to address virtually all the fundamental problems currently plaguing physics, including issues such as dark matter and the matter-antimatter asymmetry of the universe.
It will just go off, and it will decay whenever the hell it wants. And you have to be right there when it decays. The ensuing discussion ultimately resulted in the Mathusla collaboration. After months of meetings, Skype calls and simulations, the three worked out a preliminary design: a meter-tall building that would cover an area larger than seven football fields.
Five layers of particle trackers would hang from the ceiling, allowing researchers to reconstruct the subatomic debris of particle decays that happen in the massive space. MilliQan is a smaller experiment that will require more finesse. MilliQan therefore configures its detectors in three stacked tubes, each of them 1 meter long and all pointed at the spot where protons collide. In Stock.
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The Making of the Atomic Bomb. Large Hadron Collider Phenomenology. The Particle Century. Neutrinos in Particle Physics, Astrophysics and Cosmology. The Nature of Light What is a Photon? One of the key ingredients of modern realizations of CHM is the hypothesis of partial compositeness originally proposed by D. Every SM particle has a heavy partner that can mix with it. In practice the SM particles are linear combinations of elementary and composite states:. For fermions it is an assumption that in particular requires the existence of heavy fermions with equal quantum numbers to SM quarks and leptons.
These interact with the Higgs through the mixing.
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One schematically finds the formula for the SM fermion masses,. The composite particles are multiplets of the unbroken symmetry H.
Composite fermions often belong to representations larger than the SM particles. Partial compositeness ameliorates the phenomenology of CHM providing a logic why no deviations from the SM have been measured so far. In the so-called anarchic scenarios the hierarchies of SM fermion masses are generated through the hierarchies of mixings and anarchic composite sector couplings. The light fermions are almost elementary while the third generation is strongly or entirely composite.
This leads to a structural suppression of all effects that involve first two generations that are the most precisely measured. In particular flavor transitions and corrections to electro-weak observables are suppressed.
Other scenarios are also possible  with different phenomenology. Supersymmetric models also predict that every Standard Model particle will have a heavier partner. However, in supersymmetry the partners have a different spin : they are bosons if the SM particle is a fermion, and vice versa. In composite Higgs models the partners have the same spin as the SM particles.
Higgs Potential and Naturalness After the Higgs Discovery
The mixing of the SM particles determines the coupling with the known particles of the SM. The detailed phenomenology depends strongly on the flavor assumptions and is in general model-dependent. The Higgs and the top quark typically have the largest coupling to the new particles.
For this reason third generation partners are the most easy to produce and top physics has the largest deviations from the SM. Top partners have also special importance given their role in the naturalness of the theory. After the first run of the LHC direct experimental searches exclude third generation fermionic resonances up to GeV.
Deviations from the SM couplings is proportional to the degree of compositeness of the particles.
Fine Tuning Is Just Fine
For this reason the largest departures from the SM predictions are expected for the third generation quarks and Higgs couplings. The first have been measured with per mille precision by the LEP experiment. The hypothesis of partial compositeness allows to suppress flavor violation beyond the SM that is severely constrained experimentally. Nevertheless, within anarchic scenarios sizable deviations from the SM predictions exist in several observables. Overall flavor physics suggests the strongest indirect bounds on anarchic scenarios.
This tension can be avoided with different flavor assumptions.
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