Isospin Violation Research Articles

Branching ratio measurements and isospin violation in B-meson decays

The approximate symmetry of the strong interactions under isospin transformations is among the most precise tools available to control hadronic matrix elements. It is crucial in extracting fundamental parameters, but also provides avenues for the search of phenomena beyond the Standard Model. The precision of the resulting predictions requires special care when determining the quantities they are to be tested with. Specifically, in the extraction of branching ratios often isospin symmetry is assumed at one point or another implicitly, implying a significant bias for precision analyses. We extract a bias-free value for the production asymmetry between charged and neutral B meson pairs at B factories and discuss its consequences for the determination of branching fractions generally, and isospin-violating observables like the rate asymmetries in B→J/ψK or B→K⁎γ decays specifically.

Analysis of two-body charmedBmeson decays in factorization-assisted topological-amplitude approach

Within the factorization-assisted topological-amplitude approach, we study the two-body charmed $B$ meson decays $B_{u,d,s} \to D^{(*)}M$, with $M$ denoting a light pseudoscalar (or vector) meson. The meson decay constants and transition form factors are factorized out from the hadronic matrix element of topological diagrams. Therefore the effect of SU(3) symmetry breaking is retained, which is different from the conventional topological diagram approach. The number of free nonperturbative parameters to be fitted from experimental data is also much less. Only four universal nonperturbative parameters $\chi^C$, $\phi^C$, $\chi^E$ and $\phi^E$ are introduced to describe the contribution of the color suppressed tree and $W$-exchanged diagrams for all the decay channels. With the fitted parameters from 31 decay modes induced by $b\to c$ transition, we then predict the branching fractions of 120 decay modes induced by both $b\to c$ and $b\to u$ transitions. Our results are well consistent with the measured data or to be tested in the LHCb and Belle-II experiments in the future. Besides, the SU(3) symmetry breaking, isospin violation and $CP$ asymmetry are also investigated.

Light stops, blind spots, and isospin violation in the MSSM

In the framework of the MSSM, we examine several simplified models where only a few superpartners are light. This allows us to study WIMP--nucleus scattering in terms of a handful of MSSM parameters and thereby scrutinize their impact on dark matter direct-detection experiments. Focusing on spin-independent WIMP--nucleon scattering, we derive simplified, analytic expressions for the Wilson coefficients associated with Higgs and squark exchange. We utilize these results to study the complementarity of constraints due to direct-detection, flavor, and collider experiments. We also identify parameter configurations that produce (almost) vanishing cross sections. In the proximity of these so-called blind spots, we find that the amount of isospin violation may be much larger than typically expected in the MSSM. This feature is a generic property of parameter regions where cross sections are suppressed, and highlights the importance of a careful analysis of the nucleon matrix elements and the associated hadronic uncertainties. This becomes especially relevant once the increased sensitivity of future direct-detection experiments corners the MSSM into these regions of parameter space.

A1(1420)resonance as a tetraquark state and its isospin partner

We systematically construct tetraquark currents of ${I}^{\mathrm{G}}{J}^{\mathrm{PC}}={1}^{\ensuremath{-}}{1}^{++}$ and classify them into types $\mathbf{A}$ (antisymmetric), $\mathbf{S}$ (symmetric) and $\mathbf{M}$ (mixed), based on flavor symmetries of diquarks and antidiquarks composing the tetraquark currents. We use tetraquark currents of type $\mathbf{M}$ to perform QCD sum rule analyses, and find a tetraquark current ${\ensuremath{\eta}}_{5\ensuremath{\mu}}^{M}$ with quark content $qs\overline{q}\overline{s}$ ($q=u$ or $d$) leading to a mass of $1.44\ifmmode\pm\else\textpm\fi{}0.08\text{ }\text{ }\mathrm{GeV}$ consistent with the ${a}_{1}(1420)$ state recently observed by the COMPASS Collaboration. Our results support tetraquark explanations for both ${a}_{1}(1420)$ and ${f}_{1}(1420)$, assuming that they are isospin partners. We also study their possible decay patterns. As tetraquark candidates, the possible decay modes of ${a}_{1}(1420)$ are $S$-wave ${a}_{1}(1420)\ensuremath{\rightarrow}{K}^{*}(892)K$ and $P$-wave ${a}_{1}(1420)\ensuremath{\rightarrow}{f}_{0}(980)\ensuremath{\pi}$, while the possible decay patterns of ${f}_{1}(1420)$ are $S$-wave ${f}_{1}(1420)\ensuremath{\rightarrow}{K}^{*}(892)K$ and $P$-wave ${f}_{1}(1420)\ensuremath{\rightarrow}{a}_{0}(980)\ensuremath{\pi}$. We speculate that ${a}_{1}(1420)$ is partly responsible for the large isospin violation in the $\ensuremath{\eta}(1405)\ensuremath{\rightarrow}{f}_{0}(980){\ensuremath{\pi}}_{0}$ decay mode, which is reported by BESIII Collaboration in the $J/\ensuremath{\psi}\ensuremath{\rightarrow}\ensuremath{\gamma}3\ensuremath{\pi}$ process.

Charged mediators in dark matter scattering with nuclei and the strangeness content of nucleons

We consider a scenario, within the framework of the MSSM, in which dark matter is bino-like and dark matter-nucleon spin-independent scattering occurs via the exchange of light squarks which exhibit left-right mixing. We show that direct detection experiments such as LUX and SuperCDMS will be sensitive to a wide class of such models through spin-independent scattering. Moreover, these models exhibit properties, such as isospin violation, that are not typically observed for the MSSM LSP if scattering occurs primarily through Higgs exchange. The dominant nuclear physics uncertainty is the quark content of the nucleon, particularly the strangeness content.

Explicit flavor symmetry breaking and holographic compact stars

We study the effects of flavor symmetry breaking on holographic dense matter and compact stars in the D4/D6 model. To this end, two light flavors and one intermediate mass flavor are considered. For two light quarks, we investigate how the strong isospin violation affects the properties of holographic dense matter and compact stars. We observe that quark-antiquark condensates are flavor dependent and show interesting behavior near the transition from dense matter with only one flavor to matter with two flavors. An intermediate mass quark is introduced to investigate the role of the third flavor. The mass-radius relations of holographic compact stars with three flavors show that the mass-radius curve changes drastically at a transition density from which the third flavor begins to appear in the matter.

Problems with the perturbative QCD interpretation of HERMES data on semi-inclusive leptoproduction of pions

A theoretical analysis has been performed of the HERMES semi-inclusive deep inelastic data on the difference between $\pi^{+}$ and $\pi^{-}$ multiplicities. It turns out that the application of a standard perturbative QCD analysis, using, as usual, isospin symmetry for the fragmentation functions, leads to a poor description of the deuteron data at small z values. If one allows a breaking of isospin invariance, a good fit to both proton and deuteron data can be achieved for all measured z, but the level of isospin violation, especially at small values of z, is simply not credible. We suspect that the problem is a consequence of using the factorized QCD treatment in a kinematic region where it is unjustified.

Combined analysis of the decays τ − → K S π − ν τ and τ − → K − ην τ

In a combined study of the decay spectra of $\tau^-\to K_S\pi^-\nu_\tau$ and $\tau^-\to K^-\eta\nu_\tau$ decays within a dispersive representation of the required form factors, we illustrate how the $K^*(1410)$ resonance parameters, defined through the pole position in the complex plane, can be extracted with improved precision as compared to previous studies. While we obtain a substantial improvement in the mass, the uncertainty in the width is only slightly reduced, with the findings $M_{K^{*\prime}}=1304 \pm 17\,$MeV and $\Gamma_{K^{*\prime}} = 171 \pm 62\,$MeV. Further constraints on the width could result from updated analyses of the $K\pi$ and/or $K\eta$ spectra using the full Belle-I data sample. Prospects for Belle-II are also discussed. As the $K^-\pi^0$ vector form factor enters the description of the decay $\tau^-\to K^-\eta\nu_\tau$, we are in a position to investigate isospin violations in its parameters like the form factor slopes. In this respect also making available the spectrum of the transition $\tau^-\to K^-\pi^0\nu_\tau$ would be extremely useful, as it would allow to study those isospin violations with much higher precision.

Higgs mass from compositeness at a multi-TeV scale

Within composite Higgs models based on the top seesaw mechanism, we show that the Higgs field can arise as the pseudo Nambu-Goldstone boson of the broken U(3)_L chiral symmetry associated with a vector-like quark and the t-b doublet. As a result, the lightest CP-even neutral state of the composite scalar sector is lighter than the top quark, and can be identified as the newly discovered Higgs boson. Constraints on weak isospin violation push the chiral symmetry breaking scale above a few TeV, implying that other composite scalars are probably too heavy to be probed at the LHC, but may be within reach at a future hadron collider with center-of-mass energy of about 100 TeV.

Overcoming velocity suppression in dark-matter direct-detection experiments

Pseudoscalar couplings between Standard-Model quarks and dark matter are normally not considered relevant for dark-matter direct-detection experiments because they lead to velocity-suppressed scattering cross-sections in the non-relativistic limit. However, at the nucleon level, such couplings are effectively enhanced by factors of order ${\cal O}(m_N/m_q)\sim 10^3$, where $m_N$ and $m_q$ are appropriate nucleon and quark masses respectively. This enhancement can thus be sufficient to overcome the corresponding velocity suppression, implying --- contrary to common lore --- that direct-detection experiments can indeed be sensitive to pseudoscalar couplings. In this work, we explain how this enhancement arises, and present a model-independent analysis of pseudoscalar interactions at direct-detection experiments. We also identify those portions of the corresponding dark-matter parameter space which can be probed at current and future experiments of this type, and discuss the role of isospin violation in enhancing the corresponding experimental reach.

Pion-assisted charmed dibaryon candidate

The ${\mathrm{\ensuremath{\Lambda}}}_{c}(2286)N$ system is studied in a chiral constituent quark model, and the resulting $s$-wave interaction is used in separable form within three-body models of the $\ensuremath{\pi}{\mathrm{\ensuremath{\Lambda}}}_{c}N$ system with quantum numbers $(C,I,{J}^{P})=(+1,\frac{3}{2},{2}^{+})$. Separable interactions are also used for the dominant $p$-wave pion-baryon channels dominated by the $\mathrm{\ensuremath{\Delta}}(1232)$ and ${\mathrm{\ensuremath{\Sigma}}}_{c}(2520)$ resonances. Faddeev equations with relativistic kinematics are solved on the real axis to search for bound states and in the complex plane to search for three-body resonances. Some of the models considered generate a very narrow bound state, requiring isospin violation for its strong decay. Other models lead to a narrow resonance ($\mathrm{\ensuremath{\Gamma}}\ensuremath{\lesssim}0.4\text{ }\text{ }\mathrm{MeV}$) for a resonance mass below the ${\mathrm{\ensuremath{\Sigma}}}_{c}(2455)N$ threshold. This would be the lowest-lying $C=+1$ dibaryon, with mass estimated as $\ensuremath{\approx}3370\ifmmode\pm\else\textpm\fi{}15\text{ }\text{ }\mathrm{MeV}$.

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.

Probing the nature ofY(4260)in the isospin violating processY(4260)→J/ψηπ0

The isospin violation process $Y(4260) \to J/\psi \eta \pi^{0}$ is studied assuming that $Y(4260)$ is a $D_{1} \bar{D}+c.c.$ hadronic molecule. In association with the production of the $Z_c(3900)$, which is treated as a $D \bar{D}^{*}+c.c.$ hadronic molecule, this process can help us distinguish their molecular natures from other scenarios, since the incomplete cancellation between the charged and neutral--meson loops, which are prominent in the molecular picture only, produces a peak in the $e^+e^-\to Y(4260)\to J/\psi\eta\pi^0$ cross section at the $D_{1} \bar{D}+c.c.$ threshold and a very prominent peak in the $J/\psi \eta$ invariant mass spectrum in between the $D \bar{D}^{*}+c.c.$ thresholds; the latter being much narrower than the corresponding one in the isospin conserving channel, i.e. $J/\psi \pi^+ \pi^{-}$. The partial width of $Y(4260)\to J/\psi\eta\pi^0$ is about $4 \times 10^{-4}$ of that of $Y(4260)\to J/\psi\pi^+\pi^-$. The cross section of $e^+e^-\to Y(4260)\to J/\psi\eta\pi^0$ at the $D_{1} \bar{D}+c.c.$ threshold is about $0.05 \ \mathrm{pb}$ which is much larger than that produced by the nearby resonances. These features are the direct consequences of the assumed nature of these two states which might be accessible at the high-statistics experiments such as BESIII and LHCb.

Isospin violation in the d(n,γ)3H process at energies relevant for big bang nucleosynthesis

The cross section for a neutron-deuteron (nd) radiative capture is calculated using the pionless effective field theory including isospin symmetry breaking (ISB) corrections up to higher order. The triton is studied as a three-body bound state and one has to take into account various ISB effects, relativistic corrections and external electromagnetic currents. The isospin violation in nd radiative capture is improved compared to the one at NLO and N2LO. The cross section is determined to be σtot = [0.505 ± 0.003] mb up to N2LO. A satisfactory agreement between theory and experiment for the calculated cross section has been found by insertion of three-body forces and ISB effects.

Isospin violation in the yield ofS-wave heavy meson pairs near threshold

We consider production of overall neutral pairs consisting of heavy $D^{(*)}$ or $B^{(*)}$ meson and antimeson in the processes, where the pair can be created in an $S$-wave, such as $e^+e^- \to \pi^0 \, D^* \bar D^{(*)}$ and $e^+e^- \to \gamma \, D^{(*)} \bar D^{(*)}$, and similar reactions with $B^{(*)}$ mesons. The ratio $R^{c/n}$ of the yield of pairs of charged mesons to that of neutral mesons near the threshold for the heavy pair is strongly affected by the isospin breaking due to the isotopic mass differences and due to the Coulomb interaction between the charged mesons. The actual behavior of the isospin breaking in $R^{c/n}$ is also sensitive to the strong interaction between the heavy mesons, so that experimental measurement of this ratio can be used as a probe of this strong interaction. We calculate $R^{c/n}$ at and very near the threshold in terms of the isotopic scattering lengths for the meson pairs. In particular we find that the yield of pairs of charged mesons does not go to zero at exactly the threshold, but rather starts with a finite step, whose height depends on the value of the scattering length in a particular channel.

Isospin breaking andf0(980)-a0(980) mixing in theη(1405) →π0f0(980) reaction

We make a theoretical study of the η (1405) → π 0 f 0 (980) and η (1405) → π 0 a 0 (980) reactions to determine the isospin violation and the mixing of the f 0 (980) and a 0 (980) resonances. We make use of the chiral unitary approach where these two resonances appear as dynamically generated by the meson-meson interaction provided by chiral Lagrangians. We obtain a very narrow shape for the f 0 (980) production in agreement with a BES experiment. As to the amount of isospin violation, assuming constant vertices for the primary η (1405) → π 0 K K and η (1405) → π 0 π 0 η production, we find results which are much smaller than found in the experimental BES paper. The problem is solved by using the primary production driven by η' → K *K followed by K ∗ → K π . Thus, we can predict absolute values for the ratio Γ(π 0 ,π + π − )/Γ(π 0 ,π 0 η ) which are in fair agreement with experiment.

Physics and outlook for rare, all-neutral Eta decays

The η meson provides a laboratory to study isospin violation and search for new flavor-conserving sources of C and CP violation with a sensitivity approaching 10−6 of the isospin-conserving strong amplitude. Some of the most interesting rare η decays are the neutral modes, yet the effective loss of photons from the relatively common decay η → 3π0 → 6γ (33%) has largely obscured rare decays producing 3-5γ's. Particularly important relevant branches include the highly suppressed η → π0 2γ → 4γ, which provides a rare window on testing models of O (p 6 ) contributions in ChPTh, and η → 3γ and η → 2π0 γ → 5γ which provide direct constraints on C violation in flavor-conserving processes. The substitution of lead tungstate in the forward calorimeter of the GluEx setup in Jefferson Lab's new Hall D would allow dramatically improved measurements. The main niche of this facility, which we call the JLab Eta Factory (JEF), would be η decay neutral modes. However, this could likely be expanded to rare η′(958) decays for low energy QCD studies as well as η decays involving muons for new physics searches.

X(3872) AND ITS PARTNERS IN HEAVY QUARK QCD

The properties of X(3872) are analyzed starting from a correlation function written in terms of the quark and gluon degrees of freedom. It is shown that in the infinite charm quark mass limit, X(3872) meson should have three other degenerate meson partners. The possible isospin structure of X(3872) is also analyzed and it is concluded that although the observed isospin violation in the decays of X(3872) into the final states J/ψρ and J/ψω is small, X(3872) can contain a sizable I = 1 component.

Isospin asymmetries inB→(K*,ρ)γ/l+l−andB→Kl+l−in and beyond the standard model

We compute the isospin asymmetries in $B\ensuremath{\rightarrow}({K}^{*},\ensuremath{\rho})\ensuremath{\gamma}$ and $B\ensuremath{\rightarrow}(K,{K}^{*},\ensuremath{\rho}){l}^{+}{l}^{\ensuremath{-}}$ for low lepton pair invariant mass ${q}^{2}$, within the Standard Model (SM) and beyond the SM in a generic dimension six operator basis. Within the SM the $CP$-averaged isospin asymmetries for $B\ensuremath{\rightarrow}(K,{K}^{*},\ensuremath{\rho})ll$, between $1\text{ }\text{ }{\mathrm{GeV}}^{2}\ensuremath{\le}{q}^{2}\ensuremath{\le}4{m}_{c}^{2}$, are predicted to be small (below 1.5%) though with significant cancellation. In the SM the non-$CP$-averaged asymmetries for $B\ensuremath{\rightarrow}\ensuremath{\rho}ll$ deviate by $\ensuremath{\approx}\ifmmode\pm\else\textpm\fi{}5%$ from the $CP$-averaged ones. We provide physical arguments, based on resonances, of why isospin asymmetries have to decrease for large ${q}^{2}$ (towards the endpoint). Two types of isospin violating effects are computed: ultraviolet isospin violation due to differences between operators coupling to up and down quarks, and infrared isospin violation where a photon is emitted from the spectator quark and is hence proportional to the difference between the up- and down-quark charges. These isospin violating processes may be subdivided into weak annihilation (WA), quark loop spectator scattering, and a chromomagnetic contribution. Furthermore we discuss generic selection rules based on parity and angular momentum for the $B\ensuremath{\rightarrow}Kll$ transition as well as specific selection rules valid for WA at leading order in the strong coupling constant. We clarify that the relation between the $K$ and the longitudinal part of the ${K}^{*}$ only holds for leading twist and for left-handed currents. In general the $B\ensuremath{\rightarrow}\ensuremath{\rho}ll$ and $B\ensuremath{\rightarrow}{K}^{*}ll$ isospin asymmetries are structurally different yet the closeness of ${\ensuremath{\alpha}}_{\mathrm{CKM}}$ to $90\ifmmode^\circ\else\textdegree\fi{}$ allows us to construct a (quasi)null test for the SM out of the respective isospin symmetries. We provide and discuss an update on $\mathcal{B}({B}^{0}\ensuremath{\rightarrow}{K}^{*0}\ensuremath{\gamma})/\mathcal{B}({B}_{s}\ensuremath{\rightarrow}\ensuremath{\phi}\ensuremath{\gamma})$ which is sensitive to WA.

Scientific reach of multiton-scale dark matter direct detection experiments

The next generation of large scale WIMP direct detection experiments have the potential to go beyond the discovery phase and reveal detailed information about both the particle physics and astrophysics of dark matter. We report here on early results arising from the development of a detailed numerical code modeling the proposed DARWIN detector, involving both liquid argon and xenon targets. We incorporate realistic detector physics, particle physics and astrophysical uncertainties and demonstrate to what extent two targets with similar sensitivities can remove various degeneracies and allow a determination of dark matter cross sections and masses while also probing rough aspects of the dark matter phase space distribution. We find that, even assuming dominance of spin-independent scattering, multi-ton scale experiments still have degeneracies that depend sensitively on the dark matter mass, and on the possibility of isospin violation and inelasticity in interactions. We find that these experiments are best able to discriminate dark matter properties for dark matter masses less than around 200 GeV. In addition, and somewhat surprisingly, the use of two targets gives only a small improvement (aside from the advantage of different systematics associated with any claimed signal) in the ability to pin down dark matter parameters when compared with one target of larger exposure.