Charged Current Sector Research Articles

Dispersive evaluation of the inner radiative correction in neutron and nuclear β decay

We propose a novel dispersive treatment of the so-called inner radiative correction to the neutron and nuclear $\beta$-decay. We show that it requires knowledge of the parity-violating structure function $F_3^{(0)}$ that arises from the interference of the axial vector charged current and the isoscalar part of the electromagnetic current. By isospin symmetry, we relate this structure function to the charged current inelastic scattering of neutrinos and antineutrinos. Applying this new data-driven analysis we obtain a new, more precise evaluation for the universal radiative correction $\Delta_{R}^{V,\,new}=0.02467(22)$ that supersedes the previous estimate by Marciano and Sirlin, $\Delta_R^V=0.02361(38)$. The substantial shift in the central value of $\Delta_R^V$ reflects in a respective shift of $V_{ud}$ and a considerable tension in the unitarity constraint on the first row of the CKM matrix which is used as one of the most stringent constraints on New Physics contributions in the charged current sector. We also point out that dispersion relations offer a unifying tool for treating hadronic and nuclear corrections within the same framework. We explore the potential of the dispersion relations for addressing the nuclear structure corrections absorbed in the ${\cal F}t$ values, a crucial ingredient alongside $\Delta_R^V$ in extracting $V_{ud}$ from superallowed nuclear decays.

The custodial Randall-Sundrum model: from precision tests to Higgs physics

We reexamine the Randall-Sundrum (RS) model with enlarged gauge symmetry SU(2)_L x SU(2)_R x U(1)_X x P_LR in the presence of a brane-localized Higgs sector. In contrast to the existing literature, we perform the Kaluza-Klein (KK) decomposition within the mass basis, which avoids the truncation of the KK towers. Expanding the low-energy spectrum as well as the gauge couplings in powers of the Higgs vacuum expectation value, we obtain analytic formulas which allow for a deep understanding of the model-specific protection mechanisms of the T parameter and the left-handed Z-boson couplings. In particular, in the latter case we explain which contributions escape protection and identify them with the irreducible sources of P_LR symmetry breaking. We furthermore show explicitly that no protection mechanism is present in the charged-current sector confirming existing model-independent findings. The main focus of the phenomenological part of our work is a detailed discussion of Higgs-boson couplings and their impact on physics at the CERN Large Hadron Collider. For the first time, a complete one-loop calculation of all relevant Higgs-boson production and decay channels is presented, incorporating the effects stemming from the extended electroweak gauge-boson and fermion sectors.

The gluonic contribution to polarized deep-inelastic scattering in the standard model

We study the gluonic contribution to the spin-dependent deep-inelastic proton structure functions g 1 p , g 3 p and g 4 p that appear when full neutral current sector (i.e. γ-, Z 0-exchange and γZ 0-interference) and the charged current sector (W ±-exchange) are taken into account, which is indispensable if polarized lepton-proton experiments at large Q 2 should be carried out in the future. It is shown that the gluonic contribution enters in such a way that experimental results for the first moment of the structure functions will not help us to disentangle the recently proposed scenarios for the proton spin, but that one might obtain information about the polarized valence quark content of the proton from a measurement of the parity violating structure functions g 3 p or g 4 p .

CP-ODD PHENOMENA DUE TO FINITE WIDTH EFFECTS OF THE TOP QUARK

Strong experimental indications exist that the top quark mass in the Standard Model (SM) exceeds the mass of the W gauge boson. Since this opens the decay mode t → Wb a suitable regularization of the top quark propagator in the s-channel is required in many tree reactions. As in the Z0 case this regularization essentially results in an. additional imaginary contribution (iΓtmt) to the propagator. This absorptive phase can combine with complex coupling constants of the charged current sector to give rise to new CP violating effects. Theoretical upper bounds for the size of these CP-odd signals are estimated in various extensions of the SM such as: the four generation SM, left-right symmetric models and models with two or three Higgs doublets.

Calculability of the Flavor‐Mixing Parametersa

Despite the impressive success of the standard model, the repetition of the family‐generation of leptons and quarks persists as one of the most challenging unresolved problems in particle physics. The fermion family problem is to understand the fermion mass hierarchy of the leptons and quarks as observed in family‐generations. Any consistent solution to this problem should also describe the flavor mixings in the charged‐current sector including the CP‐invariance violation as well as the empirical flavor‐conservation up to the αGf order in the neutral weak interaction sector. In the standard model the fermion masses are generated by spontaneous breaking of the electroweak symmetry via the Higgs mechanism, but they are arbitrary free parameters. In other words, the standard model can be adjusted to accommodate the fermion family‐generations but lacks calculability for the fermion mass spectrum. Because there are several fermion generations, the weak interaction eigenstates are not necessarily the mass eigenstates and therefore there will inevitably be mixings between different flavor states in the charged current sector thus introducing further arbitrary parameters to the standard model. However, the neutral‐current sector is flavor diagonal up to αGf order in the standard model because the fermions in each generation are assigned in the same dimensional representations.

Calculability of the Flavor-Mixing Parameters

Despite the impressive success of the standard model, the repetition of the family-generation of leptons and quarks persists as one of the most challenging unresolved problems in particle physics. The fermion family problem is to understand the fermion mass hierarchy of the leptons and quarks as observed in family-generations. Any consistent solution to this problem should also describe the flavor mixings in the charged-current sector including the CP-invariance violation as well as the empirical flavor-conservation up to the αGf order in the neutral weak interaction sector. In the standard model the fermion masses are generated by spontaneous breaking of the electroweak symmetry via the Higgs mechanism, but they are arbitrary free parameters. In other words, the standard model can be adjusted to accommodate the fermion family-generations but lacks calculability for the fermion mass spectrum. Because there are several fermion generations, the weak interaction eigenstates are not necessarily the mass eigenstates and therefore there will inevitably be mixings between different flavor states in the charged current sector thus introducing further arbitrary parameters to the standard model. However, the neutral-current sector is flavor diagonal up to αGf order in the standard model because the fermions in each generation are assigned in the same dimensional representations.

A neutrino with a large magnetic moment and a naturally small mass

We examine the question of having large magnetic moment for the electron neutrino (needed in one proposed solution to the solar neutrino puzzle) while keeping its mass naturally small in unified gauge theories. We present a phenomenologically viable model realizing this idea in an SU(3) L×U(1) extension of the standard model, where one embeds an SU(2) gauge symmetry transforming a neutrino to an antineutrino. The model predicts new physics in the neutral and charged current sector, which are testable in the near future.

Two neutralZbosons in an SO(10) supergravity model

We consider here the supergravity model of the grand unification gauge group SO(10), which breaks to $\mathrm{SU}{(2)}_{L}\ifmmode\times\else\texttimes\fi{}\mathrm{U}{(1)}_{R}\ifmmode\times\else\texttimes\fi{}\mathrm{U}{(1)}_{B\ensuremath{-}L}\ifmmode\times\else\texttimes\fi{}\mathrm{SU}{(3)}_{C}(\ensuremath{\equiv}{G}_{2113})$ at a mass scale of ${10}^{15}$ GeV. The low-energy breaking of the gauge group ${G}_{2113}$ to $\mathrm{U}{(1)}_{Q}\ifmmode\times\else\texttimes\fi{}\mathrm{SU}{(3)}_{C}$ is induced by supergravity. The model has the following distinctive features: (i) There should be another neutral gauge boson, whose mass could be less than 1 TeV. (ii) The proton lifetime for the decay mode $p\ensuremath{\rightarrow}{e}^{+}{\ensuremath{\pi}}^{0}$ is very much enhanced, but, under some circumstances, it could be barely observable. (iii) Weak-interaction phenomenology in the charged-current sector remains unaltered.

Muon Decay and the Majorana Neutrino

Recently, the Berkeley group reported the precise and excellent experimental result on the e+ spectrum end point in /1+ decay.') In order to get the lower limit for the possible heavier (righthanded) gauge boson mass M( Wz»380 GeV, they have used the following assumptions made by Beg, Budny, Mohapatra and Sirlin;Z) (i) the manifest left-right symmetry in the charged current sector, (ii) no mirror leptons, (iii) the CP invariance, (iv) the smallness of all neutrino masses (Ll j =(mjlmp)4{.l), m" being the f.I. mass. In this note, we shall reinterpret this Berkeley result from the viewpoint of the massive Majorana neutrino, and show the information which we can draw from this result for the case where the right-handed Majorana neutrino (Nn+k) has a quite large mass mn+kl m,,~ 1. Namely, the fourth assumption (Ll j 4{.l) is reexamined by taking into account the effect due to the neutrino mixing. In order to be self-contained, we shall start from the effective lepton-hadron weak interaction Hamiltonian to treat the 7[+ decay;

Spectral function sum rules in quantum chromodynamics: (I). Charged currents sector

The Weinberg sum rules of the algebra of currents are reconsidered in the light of quantum chromodynamics (QCD). We derive new finite-energy sum rules which replace the old Weinberg sum rules. The new sum rules are convergent and the rate of convergence is explicitly calculated in perturbative QCD at the first non-trivial order. Phenomenological applications of these sum rules in the charged current sector are also discussed.