Higgs mass

May 18, 2015

Scientists from the CDF and

A new value for the Higgs boson mass will allow stronger tests of the standard model and of theories about the Universe’s stability.

Figure caption M. Baak et al. (Gfitter Group) Figure 1: Values of the top quark and W boson masses measured in experiments (green) and inferred from calculations (blue). The inner and outer ellipses represent 68% and 95% confidence levels, respectively, for the measured and inferred values. Within current experimental and theoretical uncertainties, the two ways of determining the top quark and W boson masses agree. A more precise value of the Higgs mass would narrow the width of the blue ellipses, whereas improved measurements of the top quark and W boson masses would shrink the green ellipses, making for a more incisive test for new physics. (Note, the calculations assume the Higgs mass has a central value of 125.14GeV, which differs insignificantly from the new measurement by ATLAS and CMS, but does not affect the width of the blue ellipses.) [Credit: M. Baak (Gfitter Group) ]×

A great insight of twentieth-century science is that symmetries expressed in the laws of nature need not be manifest in the outcomes of those laws. Consider the snowflake. Its structure is a consequence of electromagnetic interactions, which are identical from any direction, but a snowflake only looks the same when rotated by multiples of 60∘ about a single axis. The full symmetry is hidden by the particular conditions under which the water molecules crystallize. Similarly, a symmetry relates the electromagnetic and weak interactions in the standard model of particle physics, but we know it must be concealed because the weak interactions appear much weaker than electromagnetism. To learn what distinguishes electromagnetism from the weak interactions was an early goal of experiments at CERN’s Large Hadron Collider (LHC). A big part of the answer was given in mid-2012, when the ATLAS and CMS Collaborations at the LHC announced the discovery of the Higgs boson in the study of proton–proton collisions . Now the discovery teams have pooled their data analyses to produce a measurement of the Higgs boson mass with 0.2% precision . The new value enables physicists to make more stringent tests of the electroweak theory and of the Higgs boson’s properties.

The electroweak theory is a key element of the standard model of particle physics that weaves together ideas and observations from diverse areas of physics . In the theory, interactions are prescribed by gauge symmetries. If nature displayed these symmetries explicitly, the force particles would all be massless, whereas we know experimentally that the weak interactions must—because they are short-ranged—be mediated by massive particles. The so-called Higgs field was introduced to the electroweak theory to hide the gauge symmetry, leading to weak force particles (W± and Z0) that have mass but a photon that is massless.

The Higgs boson is a spin-zero excitation of the Higgs field and the “footprint” of the mechanism that hides the electroweak gauge symmetry in the standard model. The Higgs boson’s interactions are fully specified in terms of known couplings and masses of its decay products, but the theory does not predict its mass. Instead, experimentalists must measure the energies and momenta of the Higgs boson’s decay products and determine its mass using kinematical equations. Once that mass is known, the rates at which the Higgs boson decays into different particles can be predicted with high precision, and compared with experiment. For a mass in the neighborhood of 125 giga-electron-volts (GeV), the electroweak theory foresees a happy circumstance in which several decay paths occur at large enough rates to be detected.

ATLAS and CMS are large, broad-acceptance detectors located in multistory caverns about 100 meters below ground . In the discovery run of the LHC, the ATLAS and CMS Collaborations searched for decays of a Higgs boson into bottom-quark–antiquark pairs, tau-lepton pairs, and pairs of electroweak gauge bosons: two photons, W+W-, and Z0Z0. The actual discovery was based primarily on mass peaks associated with either the two-photon final states or Z0Z0 pairs decaying to four-lepton (electrons or muons) final states. These channels, for which the ATLAS and CMS detectors have the best mass resolution, form the basis of their new report.

Source: physics.aps.org
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What is the mass of the Higgs boson.

Nobody is really quite sure yet. The existence of the Higgs boson is predicted by the Standard Model of quantum mechanics, but nobody has yet been able to experimentally detect one, so a lot of the details of it are still unknown.
The Standard Model does not predict what mass the Higgs boson would have, so it could be anything, really, though it's generally assumed that its mass is somewhere between 115 and 180 GeV/c2, because if it is that will make all the equations we have work properly for pretty much all cases. It is possible, however, that we'll find out that it isn't in this ran…

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