Isospin Violation Effects Research Articles

Isospin-violating dark matter at the LHC

We consider a toy model of dark matter (DM) with a gauge singlet Dirac fermion that has contact interactions to quarks that differ for right-handed up and down quarks. This is motivated by the isospin-violating dark matter scenario that was proposed to reconcile reported hints of direct DM detection with bounds from non-observation of the signal in other experiments. We discuss how the effects of isospin violation in these couplings can be observed at the LHC. By studying events with large missing transverse momentum, we show that the ratio of mono-photon and mono-jet events is sensitive to the ratio of the absolute values of the couplings to the up and down quarks, while a dedicated study of di-jet plus missing transverse momentum events can reveal their relative sign. We also consider how our results are modified if instead of a contact interaction, a particle that mediates the interaction is introduced. Our methods have broad applicability to new physics that involves unequal couplings to up and down quarks.

Isospin breaking in $K \to 3 \pi$ decays III : Bremsstrahlung and fit to experiment

We complete here our work on isospin violation in the $K\to3\pi$ system. We first calculate $K\to2\pi$ to the same order as we did $K\to3\pi$ in papers I and II of this series. This adds effects of order $G_{27} p^2 (m_u-m_d)$ and $G_{27} p^2 e^2$ to earlier work. We calculate also the lowest order Bremsstrahlung contributions, $K\to2\pi\gamma,3\pi\gamma$. With these and our earlier results we perform a full fit to all available CP conserving data in the $K\to2\pi,3\pi$ system including isospin violation effects. We perform these fits under various input assumptions as well as test the factorization and the vector dominance model for the weak NLO low energy constants.

Chiral loop corrections and isospin violation effects in ɛ'/ɛ

A complete analysis of isospin breaking in $K\to 2\pi$ amplitudes, including both strong and electromagnetic corrections at next-to-leading order in chiral perturbation theory, has been achieved recently. We discuss the implication of these effects, together with the previously known chiral loop corrections, on the direct CP-violating ratio epsilon'/epsilon.

Weak isospin violations in charged and neutral Higgs couplings from supersymmetric loop corrections

Supersymmetric QCD and supersymmetric electroweak loop corrections to the violations of weak isospin to Yukawa couplings are investigated. Specifically it involves an analysis of the supersymmetric loop corrections to the Higgs couplings to the third generation quarks and leptons. Here we analyze the SUSY loop corrections to the charged Higgs couplings which are then compared with the supersymmetric loop corrections to the neutral Higgs couplings previously computed. It is found that the weak isospin violations can be quite significant, i.e, as much as 40-50% or more of the total loop correction to the Yukawa coupling. The effects of CP phases are also studied and it is found that these effects can either enhance or suppress the weak isospin violations. We also investigate the weak isospin violation effects on the branching ratio $BR(H^-\to\bar t b)/ BR(H^-\to \bar\nu_{\tau}\tau^-)$ and show that the effects are sensitive to CP phases. Thus an accurate measurement of this branching ratio along with the branching ratio of the neutral Higgs boson decays can provide a measure of weak isospin violation along with providing a clue to the presence of supersymmetry.

Near-threshold production of a 0(980) mesons in πN and NN collisions and a 0-f 0 mixing

We consider near-threshold $a_0(980)$-meson production in $\pi N$ and $NN$ collisions. An effective Lagrangian approach with one-pion exchange is applied to analyze different contributions to the cross section for different isospin channels. The Reggeon exchange mechanism is also evaluated for comparison. The results from $\pi N$ reactions are used to calculate the contribution of the $a_0$ meson to the cross sections and invariant $K \bar K$ mass distributions of the reactions $pp\to pn K^+\bar K^0$ and $pp\to pp K^+K^-$. It is found that the experimental observation of $a_0^+$ mesons in the reaction $pp\to pn K^+\bar K^0$ is much more promising than the observation of $a_0^0$ mesons in the reaction $pp\to pp K^+K^-$. Effects of isospin violation in the reactions $pN \to d a_0$, $pd \to \mathrm{^3He/^3H} a_0$, and $ dd \to \mathrm{^4He} a_0$, which are induced by $a_0(980)$--$f_0(980)$ mixing, are also analyzed.

Isospin violation and the proton’s neutral weak magnetic form factor

The effects of isospin violation on the neutral weak magnetic form factor of the proton are studied using two-flavour chiral perturbation theory. The first nonzero contributions appear at O(p^4) in the small-momentum expansion, and the O(p^5) corrections are also calculated. The leading contributions from an explicit Delta(1232) isomultiplet are included as well. At such a high order in the chiral expansion, one might have expected a large number of unknown parameters to contribute. However, it is found that no unknown parameters can appear within loop diagrams, and a single tree-level counterterm at O(p^4) is sufficient to absorb all divergences. The momentum dependence of the neutral weak magnetic form factor is not affected by this counterterm.

Renormalization of the one-body off-diagonal Coulomb field in nuclei

Isospin violation effects in nuclei are investigated within a microscopic approach which takes into account the influence of the residual strong interaction on the Coulomb interaction. The renormalization of the off-diagonal Coulomb field acting within a nucleus, by the residual strong interaction is calculated analytically in a simplified RPA. From the expression for the suppression coefficient of the isospin violating part of the Coulomb field, the isospin violating spreading widths of isobaric analog states are derived. The resulting reduction of the width is in agreement with the data on the isospin symmetry restoration and with some earlier evaluations of the isospin violating spreading widths.

Isospin violation effects in the extended tree-level model for the pion-nucleon interaction

The extended tree-level model for the πN interaction leads to a successful description of isospin violation effects observed in the p-wave πN scattering at the Δ 33 region. The corrections for the πN scattering lengths including the ϱω mixing effect are estimated too. The calculated pπ − → pπ − scattering amplitude at threshold is found to be in fair agreement with experiment.

Two-photon decays support the (K [formula omitted])-molecule picture of the S ∗(975) and δ(980)

We calculate the rate for S ∗→γγ and for δ 0→ γγ under the assumption that these hadrons are (K K )-molecules. We find that Λ(δ 0(K K)→γγ)≈0.6 keV , which is larger than the Crystal Ball measurement by a factor of ∼. This is a disagreement of ≈ 2 α given current experimental or estimated theoretical errors. In contrast, the assignment δ=q q predicts a rate for and JADE collaborations for Λ(S ∗→γγ) are consistent with our prediction for Λ(S ∗(K K)→γγ) , but are a factor of ∼5 smaller than the quark model prediction for Λ(S ∗(q q)→γγ) . We also discuss important uncertainties in our estimate of Λ((K K)→γγ) specifically (1) the value of ‖ψ K K ( 0)‖, (2) use of the Born approximation to estimate the as yet unmeasured α( K + K -→ γγ) near threshold and (3) the possibility of large isospin violation effects in the S ∗–δ 0 system. Possible ways of improving on our approximations are successed.