Light Vector Boson Research Articles

Protophobic light vector boson as a mediator to the dark sector

The observation of a protophobic 16.7 MeV vector boson has been reported by a $^8$Be nuclear transition experiment. Such a new particle could mediate between the Standard Model and a dark sector, which includes the dark matter. In this paper, we show some simple models of the dark matter which satisfy the thermal relic abundance under the current experimental bounds from the direct and the indirect detections. In a model, it is found that an appropriate self-scattering cross section to solve the small scale structure puzzles can be achieved.

Nucleon-nucleon bremsstrahlung of dark gauge bosons and revised supernova constraints

We calculate the rate of production of hypothetical light vector bosons (LVBs) from nucleon-nucleon bremsstrahlung reactions in the soft radiation limit directly in terms of the measured nucleon-nucleon elastic cross sections. We use these results and the observation of neutrinos from supernova SN1987a to deduce constraints on the couplings of vector bosons with masses $\lesssim 200$ MeV to either electric charge (dark photons) or to baryon number. We establish for the first time strong constraints on LVB that couple only to baryon number, and revise earlier constraints on the dark photon. For the latter, we find that the excluded region of parameter space is diminished by about a factor of 10.

Lepton flavor violation with light vector bosons

New sub-GeV vector bosons with couplings to muons but not electrons have been discussed in order to explain the muon's magnetic moment, the gap of high-energy neutrinos in IceCube or the proton radius puzzle. If such a light Z' not only violates lepton universality but also lepton flavor, as expected for example from the recent hint for $h\to\mu\tau$ at CMS, the two-body decay mode $\tau \to \mu Z'$ opens up and for $M_{Z'} < 2 m_\mu$ gives better constraints than $\tau\to 3\mu$ already with 20-year-old ARGUS limits. We discuss the general prospects and motivation of light vector bosons with lepton-flavor-violating couplings.

Search for the $U$ Boson in the Process $e^+e^- \to \mu ^+ \mu ^- \gamma $, $U \to \mu ^+\mu ^-$ with the KLOE Detector

We present a search for a new light vector boson, carrier of a "dark force" between WIMPs, with the KLOE detector at \DA$\Phi$NE. We analyzed $e^+ e^- \to \mu^+ \mu^- \gamma$ ISR events corresponding to an integrated luminosity of $239$ pb$^{-1}$ to find evidence for the $e^+ e^- \to U\gamma ,\,\, U\to\mu^+\mu^-$ process. We found no $U$ vector boson signal and set a 90% CL upper limit on the ratio of the U boson and photon coupling constants between 1.6$\times10^{-5}$ to 8.6$\times10^{-7}$ in the mass region $520<M_{\rm U}<980$ MeV. A projection of the KLOE sensitivity for the $\mu\mu\gamma$ and $\pi\pi\gamma$ channels at full statistics and extended muon acceptance is also presented.

Scattering of dark particles with light mediators

We present a treatment of the high energy scattering of dark Dirac fermions from nuclei, mediated by the exchange of a light vector boson. The dark fermions are produced by proton-nucleus interactions in a fixed target and, after traversing shielding that screens out strongly interacting products, appear similarly to neutrino neutral current scattering in a detector. Using the Fermilab experiment E613 as an example, we place limits on a secluded dark matter scenario. Visible scattering in the detector includes both the familiar regime of large momentum transfer to the nucleus ($Q^2$) described by deeply inelastic scattering, as well as small $Q^2$ kinematics described by the exchanged vector mediator fluctuating into a quark-antiquark pair whose interaction with the nucleus is described by a saturation model. We find that the improved description of the low $Q^2$ scattering leads to important corrections, resulting in more robust constraints in a regime where a description entirely in terms of deeply inelastic scattering cannot be trusted.

Search for light vector boson production in [formula omitted] interactions with the KLOE experiment

We have searched for a light vector boson U, the possible carrier of a “dark force”, with the KLOE detector at the DAΦNE e+e− collider, motivated by astrophysical evidence for the presence of dark matter in the Universe. Using e+e− collisions collected with an integrated luminosity of 239.3 pb−1, we look for a dimuon mass peak in the reaction e+e−→μ+μ−γ, corresponding to the decay U→μ+μ−. We find no evidence for a U vector boson signal. We set a 90% CL upper limit for the mixing parameter squared between the photon and the U boson of 1.6×10−5 to 8.6×10−7 for the mass region 520<mU<980 MeV.

Dark matter progenitor: Light vector boson decay into sterile neutrinos

We show that the existence of new, light gauge interactions coupled to Standard Model (SM) neutrinos give rise to an abundance of sterile neutrinos through the sterile neutrinos' mixing with the SM. Specifically, in the mass range of MeV-GeV and coupling of $g' \sim 10^{-6} - 10^{-2}$, the decay of this new vector boson in the early universe produces a sufficient quantity of sterile neutrinos to account for the observed dark matter abundance. Interestingly, this can be achieved within a natural extension of the SM gauge group, such as a gauged $L_\mu-L_\tau$ number, without any tree-level coupling between the new vector boson and the sterile neutrino states. Such new leptonic interactions might also be at the origin of the well-known discrepancy associated with the anomalous magnetic moment of the muon.

Muong−2, rare kaon decays, and parity violation from dark bosons

The muon g-2 discrepancy between theory and experiment may be explained by a light vector boson Z_d that couples to the electromagnetic current via kinetic mixing with the photon. We illustrate how the existing electron g-2, pion Dalitz decay, and other direct production data disfavor that explanation if the Z_d mainly decays into e+e-, mu+mu-. Implications of a dominant invisible Z_d decay channel, such as light dark matter, along with the resulting strong bounds from the rare K -> pi + 'missing energy' decay are examined. The K decay constraints may be relaxed if destructive interference effects due to Z-Z_d mass mixing are included. In that scenario, we show that accommodating the muon g-2 data through relaxation of K decay constraints leads to interesting signals for dark parity violation. As an illustration, we examine the alteration of the weak mixing angle running at low Q^2, which can be potentially observable in polarized electron scattering or atomic physics experiments.

Dark Forces at KLOE/KLOE-2

Searches for dark matter particles in the GeV mass range and for dark forces are strongly motivated by the numerous striking astrophysical observations recently re- ported by many experiments. Flavor factories, like the Frascati Φ-factory DAΦNE, are particularly suited to search for the light gauge vector boson, called U boson, which is thought to mediate an unknown interaction between hypothetical dark matter particles. By using the KLOE detector, limits on U boson coupling factor e 2 of the order of 10 −5 ÷ 10 −7 have been set through the study of the φ Dalitz decay, the Higgsstrahlung process and Uγ events. New experiments with the upgraded KLOE detector and the increased luminosity of DAΦNE are expected to improve the already set upper limits by a factor of two or better.

K+ meson yields by proton beams on low-Z targets and production of U-boson in kaon rare decays

The production of -meson in proton–low-Z targets (12C, 7Li) reactions for beam energies is analyzed on the basis of a relativistic transport (ART) model means. The double differential cross-section and the ratio of production with various targets for different emission angles are calculated. Moreover, the light neutral vector boson U from kaon decays is investigated and the momentum spectra of the U-boson with mass are obtained. It is found that the shapes of U-boson's momentum spectra distributions are practically independent of its mass distributions but sensitive to the emission angle. Furthermore, the decay channel is considered and the energy spectra of positron from U-boson decays are first presented, which will be conduced to search for the U-boson in experiments and understand the light dark-matter particles.

Higgs decays as a window into the dark sector

A light vector boson, ${Z}_{d}$, associated with a ``dark sector'' $U(1{)}_{d}$ gauge group has been introduced to explain certain astrophysical observations as well as low energy laboratory anomalies. In such models, the Higgs boson may decay into $X+{Z}_{d}$, where $X=Z$, ${Z}_{d}$ or $\ensuremath{\gamma}$. Here, we provide estimates of those decay rates as functions of the ${Z}_{d}$ coupling through either mass mixing (e.g., via an enlarged Higgs mechanism) or through heavy new fermion loops and examine the implied LHC phenomenology. Our studies focus on the higher ${m}_{{Z}_{d}}$ case, $\ensuremath{\gtrsim}\text{ }\mathrm{\text{several}}\text{ }\text{ }\mathrm{GeV}$, where the rates are potentially measurable at the LHC, for interesting regions of parameter spaces, at a level complementary to low energy experimental searches for the ${Z}_{d}$. We also show how measurement of the ${Z}_{d}$ polarization (longitudinal vs transverse) can be used to distinguish the physics underlying these rare decays.

New Limit on Possible Long-Range Parity-Odd Interactions of the Neutron from Neutron-Spin Rotation in LiquidHe4

Various theories beyond the standard model predict new particles with masses in the sub-eV range with very weak couplings to ordinary matter. A parity-odd interaction between polarized nucleons and unpolarized matter proportional to g(V)g(A)s · p is one such possibility, where s[over →] and p[over →] are the spin and the momentum of the polarized nucleon, and g(V) and g(A) are the vector and axial vector couplings of an interaction induced by the exchange of a new light vector boson. We report a new experimental upper bound on such possible long-range parity-odd interactions of the neutron with nucleons and electrons from a recent search for parity violation in neutron spin rotation in liquid ^{4}He. Our constraint on the product of vector and axial vector couplings of a possible new light vector boson is g(V) g(A)(n) ≤ 10(-32) for an interaction range of 1 m. This upper bound is more than 7 orders of magnitude more stringent than the existing laboratory constraints for interaction ranges below 1 m, corresponding to a broad range of vector boson masses above 10(-6) eV. More sensitive searches for a g(V) g(A)(n) coupling could be performed using neutron spin rotation measurements in heavy nuclei or through analysis of experiments conducted to search for nucleon-nucleon weak interactions and nuclear anapole moments.

Muon Anomaly and Dark Parity Violation

The muon anomalous magnetic moment exhibits a 3.6σ discrepancy between experiment and theory. One explanation requires the existence of a light vector boson, Z(d) (the dark Z), with mass 10-500 MeV that couples weakly to the electromagnetic current through kinetic mixing. Support for such a solution also comes from astrophysics conjectures regarding the utility of a U(1)(d) gauge symmetry in the dark matter sector. In that scenario, we show that mass mixing between the Z(d) and ordinary Z boson introduces a new source of "dark" parity violation, which is potentially observable in atomic and polarized electron scattering experiments. Restrictive bounds on the mixing (m(Z(d))/m(Z))δ are found from existing atomic parity violation results, δ2<2×10(-5). Combined with future planned and proposed polarized electron scattering experiments, a sensitivity of δ2∼10(-6) is expected to be reached, thereby complementing direct searches for the Z(d) boson.

Dark light, dark matter, and the misalignment mechanism

We explore the possibility that the dark matter is a condensate of a very light vector boson. Such a condensate could be produced during inflation, provided the vector mass arises via the Steuckelberg mechanism. We derive bounds on the kinetic mixing of the dark matter boson with the photon, and point out several potential signatures of this model.

Constraints for weakly interacting light bosons from existence of massive neutron stars

Theories beyond the standard model include a number of new particles some of which might be light and weakly coupled to ordinary matter. Such particles affect equation of state of nuclear matter and can shift admissible masses of neutron stars to higher values. The internal structure of neutron stars is modified provided the ratio between coupling strength and mass squared of a weakly interacting light boson is above $g^2/\mu^2 \sim 25 ~\mathrm{GeV}^{-2}$. We provide limits on the couplings with the strange sector, which cannot be achieved from laboratory experiments analysis. When the couplings to the first family of quarks is considered the limits imposed by the neutron stars are not more stringent than the existing laboratory ones. The observations on neutron stars give evidence that equation of state of the $\beta$-equilibrated nuclear matter is stiffer than expected from many-body theory of nuclei and nuclear matter. A weakly interacting light vector boson coupled predominantly to the second family of the quarks can produce the required stiffening.

Enhanced rare pion decays from a model of MeV dark matter

A model has been proposed in which neutral scalar particles chi, of mass 1-10 MeV, annihilate through the exchange of a light vector boson U, of mass 10-100 MeV, to produce the 511 keV line observed emanating from the center of the galaxy. The chi interacts weakly with normal matter and is a viable dark matter candidate. If the U-boson couples to quarks as well as to electrons, it could enhance the branching ratio for the rare decay pi0 -> e+e-. A recent measurement by the KTeV Collaboration lies three standard deviations above a prediction by Dorokhov and Ivanov, and we relate this excess to the couplings of the U-boson. The values are consistent with other constraints and considerations. We make some comments on possible improvements in the data.

Direct searches for new physics at the Terascale with a high-precision neutrino scattering experiment: NuSOnG

High-precision neutrino scattering at energies of order 100 GeV offers a clean probe of Beyond Standard Model physics at the Terascale. NuSOnG (Neutrino Scattering On Glass) is a high-statistics neutrino experiment which has been developed under that principle and has been proposed to run at the Fermilab Tevatron in year 2015. One of its strongest features is that it offers a unique opportunity for direct searches for new physics at TeV-scale energies, providing information complementary to that of the LHC. NuSOnG is sensitive to an unmatchable spectrum of new physics: from new light neutrino properties, to new interactions manifested through rare events, to new particles such as light neutrissimos, light vector bosons, etc. A particular example which illustrates the experiment's discovery potential, is the search for evidence of ‘matrix freedom’ or non- unitarity in the neutrino sector. Non-unitarity can manifest itself in a number of ways, such as flavor-dependent neutrino couplings, or instantaneous flavor transitions, both of which would cause observable effects at NuSOnG. This poster presents these examples, thus highlighting the truly rich discovery potential of the experiment.

Hadron liquid with a small baryon chemical potential at finite temperature

We discuss general properties of a system of heavy fermions (including antiparticles) interacting with rather light bosons. First, we consider one diagram of Φ. The fermion chemical potential is assumed to be small, μ f ≲ T . Already for the low temperature, T ≪ min ( T bl.f , m b ) , the fermion mass shell proves to be partially blurred due to multiple fermion rescatterings on virtual bosons, m b is the boson mass, T bl.f ( ≪ m f ) is the typical temperature corresponding to a complete blurring of the gap between fermion–antifermion continua, m f is the fermion mass. As the result, the ratio of the number of fermion–antifermion pairs to the number provided by the ordinary Boltzmann distribution becomes larger than unity ( R N > 1 ). For T ≳ m b * ( T ) (hot hadron liquid, blurred boson continuum), m b * ( T ) is the effective boson mass, the abundance of all particles dramatically increases. Bosons behave as quasi-static impurities, on which heavy fermions undergo multiple rescatterings. The soft thermal loop approximation solves the problem. The effective fermion mass m f * ( T ) decreases with the temperature increase. For T ≳ T bl.f fermions are essentially relativistic particles. Due to the interaction of the boson with fermion–antifermion pairs, m b * ( T ) decreases leading to the possibility of the “hot Bose condensation” for T > T cb . The phase transition might be of the second order or of the first order depending on the species under consideration. We study in detail properties of the system of spin 1 2 heavy fermions interacting with substantially lighter scalar neutral bosons (e.g., Nσ system). Correlation effects of higher order diagrams of Φ are evaluated resulting in a suppression of vertices for T ≳ m b * ( T ) . The abundance of high-lying baryon resonances proves to be of the same order, as the nucleon–antinucleon abundance, or might be even higher for some species. Further we discuss the system of heavy fermions interacting with more light vector bosons (e.g., Nω and Nρ) and then, with pseudo-scalar bosons (e.g., Nπ). For the fermion–vector boson system correlation effects are incorporated by keeping the Ward identity. In case of the fermion–pseudo-scalar boson system correlation effects are rather small. Finally, we allow for all interactions. We estimate R N ∼ 1.5 for T ∼ m π / 2 ; T bl.f proves to be near T cb ; both values are in the vicinity of the pion mass m π .

THE RATIOS OF ENERGY LOSSES THROUGH PHOTONEUTRINO PROCESS IN SUPERSTRING INSPIRED E6 MODEL AND IN THE STANDARD MODEL

In general E6 inspired model which has two additional U(1) symmetries at the string scale whose one combination is a light enough vector boson Z(θ), we compute the contribution of Zθ to the [Formula: see text] scattering at the aim of calculating the energy loss. We find that the two U(1)'s must be combined at 90° to have a significant contribution.

Neutral pion radiative decay as an efficient tool for searching for new light gauge bosons

It is shown that the search for the decay π 0→ γ+ “nothing” is an efficient and unambiguous method for looking for new light vector bosons.