According to the standard model of particle physics, matter is composed of three "families" quarks and leptons .
Each family has four types of particles: two quarks and two leptons. Ordinary matter consists mainly of particles of the first family: up and down quarks, which are tightly bound inside protons and neutrons, the electrons and electron neutrinos, which are abundantly produced by fusion reactions inside stars. For all we know, quarks and leptons of the second and third family are identical copies of the first family, but have heavier masses. These heavy quarks and charged leptons are unstable particles that can be produced in high energy collisions, but extremely fast decay through the weak interaction , particles of the first family.
This description of the matter is consistent with experiments that have been observed, but why do we have three almost identical replica of the quarks and leptons (also known as the three "flavors" of quarks and leptons), but with different masses, is one of the great open questions in particle physics. In the limit of no electroweak symmetry breaking, any standard model particles should have a mass. The problem of the masses of quarks and leptons, therefore, is intimately connected with the other major open question in particle physics: why carriers of the weak force the W and Z bosons-have mass? In the standard model , these two problems are solved by what is called the mechanism Higgs : quarks and leptons, as well as W and Z bosons have mass because they interact with a new field, called the Higgs field. In this table, is the Higgs field who electroweak symmetry breaking . If high-energy experiments at the Large Hadron Collider (LHC ) at CERN could detect a Higgs field excitation-(Higgs boson), would be proof that this mechanism is the basis of the masses existing particles.
Higgs mechanism postulated in the standard model, however, is not satisfactory for various reasons, mostly because it becomes unstable at high energies. Instead, it is likely that the idea of \u200b\u200ba Higgs field is really just a low energy approximation of a more fundamental theory. The signal from this "true" theory to higher energies appear as new degrees of freedom in the form of new heavy particles. Attempts to find evidence of this new theory can be divided into two main categories: (i) direct search for the new degrees of freedom through experiments in high energy frontier, and in particular in the LHC, and (ii ) indirect search for new particles through precision studies at low energies, the so-called border high intensity. The latter is particularly relevant in determining the structure of taste of new degrees of freedom, or how these new fields are coupled to different families of quarks and leptons. It is in this context that Joachim Brod, Gorbahn Martin, and Emmanuel Stamou Technical University of Munich, Germany, have developed a better theoretical description of a rare particle decay to give experimenters a powerful tool to look for signs a more fundamental theory. Their calculations, specifically details the probability of a K meson (kaon) decays into a pion (π ) neutrino (ν ) and antineutrino (ν -) ( K → π ν ν -) (see Fig), are presented in the journal Physical Review D.
Assuming that the image is correct, the presence of the Higgs field mixes different families of quarks in the weak interaction and establishes a hierarchy in the various modes of decay of heavy quarks in the lightest. In particular, the interaction between the Higgs and weak interactions implies that the processes where a quark changes flavor, but not its cargo (for example, the strange quark is a second family becomes the bottom of the quark first family, which has a different mass but the same electrical charge) can occur only in higher orders of electroweak interactions and are strongly suppressed. These processes, called transitions changing neutral current flavor ( FCNC) are an ideal place to search for new physics: the signal (of the new physics) excel in the narrow context of what already predicted by the standard model.
up the image the disintegration of K → π ν ν is mediated by the transition from neutral current flavor changing s d → ν ν (where syd reprentaciones the strange quark and down respectively). Below is a diagrammatic representation of a loop contribution the extent of s → d ν ν within the standard model on the left and within a standard model extension to the right. Credit: Alan Stonebraker.
decay K → π ν ν -on study by Joachim Brod is a particularly interesting example of FCNC transitions: the likelihood that these processes occur ( call branching ratios ) can be calculated to an exceptionally high degree of precision, not compared to any other FCNC process involving quarks. The exceptionally clean theoretical decay K → π ν ν - in the standard model is a very simple: the probability of decay is dominated by the exchange of heavier particles the standard model (the top quark and W and Z bosons) and these contributions can be calculated reliably in perturbation theory . Since the early nineties, theorists began to make these calculations more and more accurate [more here, here and here ]. Joachim Brod further strengthens the theoretical cleanliness of these modes with a full evaluation of the two-loop electroweak contributions (ie, including contributions to the sixth order in the coupling constant weak.) As was discussed in the present study irreducible theoretical error of the ratios are at 4% and 3% for the modes ( + K Π → + ν ν -) and ( K L → π 0 ν ν -) respectively.
There is a price to pay for this high theoretical cleaning. The ratios are small, for every ten billion K-mesons, only one, on average, should decline in the final state π ν ν - and the experimental signature (two neutrino states end) is difficult detect because they are very weak interacting neutrinos. This is the reason why the experimental search for these processes is a very difficult task. So far, only a few events ( + K → π + ν ν - ) have been observed by the E787 experiment [see here] and E949 [see here ] the Brookhaven National Laboratory . Their combined data lead to a decay rate of which is compatible with the standard model, but has an error too large to provide a very stringent test of the model. In other words, the theory is more accurate than the experimenters are now able to measure. The good news is that new pilot programs to substantially improve the measurement accuracy of these rare decays have been or are being discussed in several laboratories: the experiment NA62 at CERN aims to collect about 50 events + K → π + ν ν - per year, with 20% of fund from 2012. In a similar time scale, the experiment KOTO in J-PARC aims to collect some events as neutral, assuming the standard model is correct. On a longer timescale, experiments related kaon plan Project-X at Fermilab may be able to reach a level of accuracy of a few percentage points in the two decay modes. Thanks to the great cleansing theoretical amplitudes K → π ν ν - , these future experiments will provide valuable information on the flavor structure of physics beyond the standard model.
read the study HERE
source of information:
http://physics.aps.org/articles/v4/15
0 comments:
Post a Comment