Do Weak Interactions Become Strong at High Energy?

Author(s):  
R. D. Peccei
2019 ◽  
Author(s):  
Gabriel Lopez Castro

Originally thought as clean processes to study the hadronization of the weak currents, semileptonic tau lepton decays can be useful to set constraints on non-standard (NS) weak interactions. We study the effects of new interactions in \tau^- \to (\pi^-\eta,\pi^-\pi^0)\nu_{\tau}τ−→(π−η,π−π0)ντ decays and find that they are sensitive probes of these New Physics effects in the form of scalar and tensor interactions, respectively. Further improved measurements at Belle II will set limits on these scalar interactions that are similar to other low and high energy processes.


The reasons for carrying out experiments with high energy neutrinos are fairly obvious. Until two years ago, our knowledge of weak interactions came only from the study of processes like nuclear B decay, n decay, y decay, u capture, etc., involving energy transfers of the order of 100MeV or less. A fairly comprehensive theory of B decay, capable of accounting for the great bulk of the experimental data, was built up over the years which, however, left open a number of questions which could only be answered by looking at weak interactions at much higher energies, in the multi-GeV region. In principle, these high energy weak processes might be studied experimentally using a variety of beams of energetic particles (such as protons, muons, electrons). In practice, however, a beam of neutrinos, which are unique in that they can undergo only the weak interaction, offer tremendous advantages over other types of particle, since only then are the events to be studied not swamped by an enormous background due to the effects of strong or electromagnetic interactions. The possibility of carrying out experiments with high energy neutrino beams from accelerators was discussed several years ago by many people, notably by Schwartz, Pontecorvo and Lee & Yang. The cross-sections for the interaction of neutrinos with matter was known to be extremely low, of the order of 10 -38 cm 2 /nucleon for a neutrino energy of 1 GeV. If the neutrino beams were to be produced by the decay in flight of pions or kaons produced in conventional proton synchrotrons, the fluxes were such that one might reasonably expect to observe about one interaction per day per ton of detector, and it seemed doubtful if this event rate would be sufficient for quantitative experiments. In fact, the first successful, large-scale neutrino experiment, carried out in 1962 by a group at Brookhaven from Columbia University (Danby et al. 1962) yielded a striking and definite result from only a handful of events; namely, that there are two types of neutrino, v e and v fl , associated with the electron (e) and the muon (y) respectively. More recently (during 1963), major technical advances, particularly at C. E. R. N. have increased the available neutrino fluxes by between one and two orders of magnitude. These developments have been, first, a considerable increase in proton beam intensity, from the region of a few times 10 11 protons/pulse to nearly 10 12 protons/pulse; secondly, the successful extraction of nearly the entire proton beam from the synchrotron, so that one is able to take advantage of the higher intensities of pions and kaons emitted in the forward direction from an external target, instead of taking the beam off at a considerable angle (5° to 10°) from an internal target; thirdly, the development of magnetic focusing devices (‘horns’) which collimate the pion beams from the target into a narrow forward cone. The corresponding neutrino event rates are then of the order of one per ton per hour, rather than one per ton per day, and, with the certainty of further order-of-magnitude increases in proton beam currents (for example at the Argonne ZGS accelerator), the possibility of quantitative neutrino experiments looks very good indeed. Certainly, the prospects are much brighter than anyone thought a few years ago.


1. The basis of our present understanding of weak interactions is a new version of the classic theory of Fermi of 1934. Fermi considered the nucleon reactions, e - + p ⇄ n + v . According to the arrow direction, this represents either electron K -capture or neutron β-decay. He treated the interaction as a point-like scattering process, that is as a process in which the range of the potential V(r a — r b ) vanished. This is equiva­lent to converting the potential interaction energy, H int. = ∫ ψ ' b ψ ' a V (r a – r b ) ψ a ψ b dr a dr b , where ψ a , ψ b are the waves representing the incoming particles and ψ ' a , ψ ' b , the outgoing ones, into the limit H int. = G ∫ ψ ' a ψ ' b ψ a ψ b dr = G ∫ ( ψ ' a ψ a ) ( ψ ' b ψ b ) dr, where r is the volume variable.


2013 ◽  
Vol 2013 ◽  
pp. 1-16 ◽  
Author(s):  
Wen-Yin Ko ◽  
Kuan-Jiuh Lin

Metallic nanoparticles decorated on MWCNTs based transparent conducting thin films (TCFs) show a cheap and efficient option for the applications in touch screens and the replacement of the ITO film because of their interesting properties of electrical conductivity, mechanical property, chemical inertness, and other unique properties, which may not be accessible by their individual components. However, a great challenge that always remains is to develop effective ways to prepare junctions between metallic nanoparticles and MWCNTs for the improvement of high-energy barriers, high contact resistances, and weak interactions which could lead to the formation of poor conducting pathways and result in the CNT-based devices with low mechanical flexibility. Herein, we not only discuss recent progress in the preparation of MNP-CNT flexible TCFs but also describe our research studies in the relevant areas. Our result demonstrated that the MNP-CNT flexible TCFs we prepared could achieve a highly electrical conductivity with the sheet resistance of ~100 ohm/sq with ~80% transmittance at 550 nm even after being bent 500 times. This electrical conductivity is much superior to the performances of other MWCNT-based transparent flexible films, making it favorable for next-generation flexible touch screens and optoelectronic devices.


1968 ◽  
Vol 46 (17) ◽  
pp. 1945-1956 ◽  
Author(s):  
G. Leibbrandt

It is demonstrated within the framework of the intermediate vector boson theory that the interference between purely weak interaction graphs of different orders in g can lead to a seeming violation of time reversal T. This seeming violation is implied by the presence of triple scalar products in experimental cross sections, [Formula: see text], and may occur even if the primary weak interaction Hamiltonian is T invariant. The total scattering amplitude for a set of Feynman graphs is characterized by coefficients Gi. A necessary condition for the appearance of time-odd correlations is that the ratio Gi/Gj be complex.The theory is applied to the scattering of a neutron by an incident high-energy neutrino: νc + n → p + e−. Triple scalar products are shown to arise specifically from the interference between real and imaginary components of the total amplitude, constructed here from second- and sixth-order uncrossed ladder graphs. The essential imaginary components are divergence-free and emerge from poles in the integrand of the eightfold integral (sixth-order graph).Triple scalar products in dσ are proportional to m−6g8 (ln s)/s; there is no cancellation from crossed graphs of order six. Ladder graphs treated on an individual basis do not give rise to J ∙ p1 × p2 terms.


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