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Concept# Higgs mechanism

Summary

In the Standard Model of particle physics, the Higgs mechanism is essential to explain the generation mechanism of the property "mass" for gauge bosons. Without the Higgs mechanism, all bosons (one of the two classes of particles, the other being fermions) would be considered massless, but measurements show that the W+, W−, and Z0 bosons actually have relatively large masses of around 80 GeV/c2. The Higgs field resolves this conundrum. The simplest description of the mechanism adds a quantum field (the Higgs field) which permeates all of space to the Standard Model. Below some extremely high temperature, the field causes spontaneous symmetry breaking during interactions. The breaking of symmetry triggers the Higgs mechanism, causing the bosons it interacts with to have mass. In the Standard Model, the phrase "Higgs mechanism" refers specifically to the generation of masses for the W±, and Z weak gauge bosons through electroweak symmetry breaking. The Large Hadron Collider at CERN

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Neutron-deficient Hg177-185 isotopes were studied using in-source laser resonance-ionization spectroscopy at the CERN-ISOLDE radioactive ion-beam facility in an experiment combining different detection methods tailored to the studied isotopes. These include either alpha-decay tagging or multireflection time-of-flight gating for isotope identification. The endpoint of the odd-even nuclear shape staggering in mercury was observed directly by measuring for the first time the isotope shifts and hyperfine structures of Hg177-185. Changes in the meansquare charge radii for all mentioned isotopes, magnetic dipole, and electric quadrupole moments of the odd-A isotopes and arguments in favor of I = 7/2 spin assignment for Hg-177,Hg-179 were deduced. Experimental results are compared with density functional theory (DFT) and Monte Carlo shell model (MCSM) calculations. DFT calculations using Skyrme parametrizations predict a jump in the charge radius around the neutron N = 104 midshell, with an odd-even staggering pattern related to the coexistence of nearly degenerate oblate and prolate minima This near-degeneracy is highly sensitive to many aspects of the effective interaction, a fact that renders perfect agreement with experiments out of reach for current functionals. Despite this inherent difficulty, the SLy5s1 and a modified UNEDF1(SO) parametrization predict a qualitatively correct staggering that is off by two neutron numbers. MCSM calculations of states with the experimental spins and parities show good agreement for both electromagnetic moments and the observed charge radii. A clear mechanism for the origin of shape staggering within this context is identified: a substantial change in occupancy of the proton pi h(9/2) and neutron nu i(13/2) orbitals.

We analyse the process of reheating the Universe in the electroweak theory where the Higgs field plays a role of the inflaton. We estimate the maximal temperature of the Universe and fix the initial conditions for radiation-dominated phase of the Universe expansion in the framework of the Standard Model (SM) and of the νMSM - the minimal extension of the SM by three right-handed singlet fermions. We show that the inflationary epoch is followed by a matter dominated stage related to the Higgs field oscillations. We investigate the energy transfer from Higgs-inflaton to the SM particles and show that the radiation dominated phase of the Universe expansion starts at temperature Tr (3-15) × 1013GeV, where the upper bound depends on the Higgs boson mass. We estimate the production rate of singlet fermions at preheating and find that their concentrations at T r are negligibly small. This suggests that the sterile neutrino Dark Matter (DM) production and baryogenesis in the νMSM with Higgs-driven inflation are low energy phenomena, having nothing to do with inflation. We study then a modification of the νMSM, adding to its Lagrangian higher dimensional operators suppressed by the Planck scale. The role of these operators in Higgs-driven inflation is clarified. We find that these operators do not contribute to the production of Warm Dark Matter (WDM) and to baryogenesis. We also demonstrate that the sterile neutrino with mass exceeding 100keV (a Cold Dark Matter (CDM) candidate) can be created during the reheating stage of the Universe in necessary amounts. We argue that the mass of DM sterile neutrino should not exceed few MeV in order not to overclose the Universe. © 2009 IOP Publishing Ltd and SISSA.

2009We present two different approaches to solve the hierarchy problem of the Standard Model and to provide a consistent dynamical mechanism for electroweak symmetry breaking. As a first scenario, we follow the naturalness paradigm as realized in Composite Higgs theories, which conceive the Higgs particle as a bound state of a new strongly interacting sector confining at the TeV scale. We present a minimal implementation of the model and study in detail the phenomenology of vector resonances, which are predicted as states excited from the vacuum by the conserved currents of the new strong dynamics. This analysis allows us to derive constraints on the parameter space of Composite Higgs models from the presently available LHC data and to confront naturalness with experimental results. Motivated by the rising tension between theoretical expectations and the absence of new physics signals at the LHC, we consider as a second possibility the neutral naturalness paradigm and address the hierarchy problem by posing the existence of a mirror copy of the Standard Model, as realized in Twin Higgs theories. This new color-blind sector is the main actor in protecting the Higgs mass from large radiative corrections and is un-discoverable at the LHC, allowing us to push far in the ultraviolet the scale where the Standard Model effective theory breaks down and colored resonances appear. We present an implementation of the Twin Higgs program into a composite model and discuss the requirements for uplifting the symmetry protection mechanism also to the ultraviolet theory. After introducing a consistent Composite Twin Higgs model, we consider the constraints imposed on the scale where colored resonances are expected by the determination of the Higgs mass at three loops order, electroweak precision tests and perturbativity of the ultraviolet-complete model. We show that, although allowing in principle the new physics scale to lie far out of the LHC reach, these constructions need the existence of light colored top partners, with a mass of around 2-4 TeV, to comply with indirect observations. Neutral naturalness models may then evade detection at the LHC, but they can be probed and falsified at future colliders.