An energy threshold of (220±10) eV was achieved at an efficiency of 50% with a four-channel ultra-low-energy germanium detector each with an active mass of 5 g. This provides a unique probe to WIMP dark matter with mass below 10 GeV. With a data acquisition live time of 0.338 kg-day at the Kuo-Sheng Laboratory, constraints on WIMPs in the galactic halo were derived. The limits improve over previous results on both spin-independent WIMP-nucleon and spin-dependent WIMPneutron cross-sections for WIMP mass between 3−6 GeV. Sensitivities for full-scale experiments are projected. This detector technique makes the unexplored sub-keV energy window accessible for new neutrino and dark matter experiments.PACS numbers: 95.35.+d, 98.70.Vc There is compelling evidence from cosmological and astrophysical observations that about one quarter of the energy density of the universe can be attributed to Cold Dark Matter(CDM), whose nature and properties are still unknown [1]. Weakly Interacting Massive Particles (WIMP, denoted by χ) are the leading candidates for CDM. There are intense experimental efforts[2] to look for WIMPs through direct detection of nuclear recoils in χN→χN elastic scattering or in the studies of the possible products through χχ annihilations. The Kuo-Sheng(KS) Laboratory[12] is located at 28 m from a 2.9 GW reactor core with an overburden of about 30 meter-water-equivalence. Limits on neutrino magnetic moments(µ ν )[13] with a 1.06-kg germanium detector(HPGe) at a threshold of 5 keV were reported [14]. These data also allowed the studies of reactor electron neutrinos [15] and reactor axions [16]. A background level of ∼ 1 event kg −1 keV −1 day −1 (cpkkd) at 20 keV, comparable with those of underground CDM experiments, was achieved. The current goal is to develop detectors with kg-scale target mass, 100 eV-range threshold and lowbackground specifications for the studies of WIMPs, µ ν and neutrino-nucleus coherent scatterings [17].Ultra-low-energy germanium detectors(ULEGe) is a matured technique for sub-keV soft X-rays measurements. They typically have modular mass of 5−10 g while detector arrays of up to 30 elements have been constructed. Compared with Al 2 O 3 , Ge provides enhancement in χN spin-independent couplings(σ SI χN ) due to the A 2 dependence [1,18], where A is the mass number of the target isotopes. The isotope 73 Ge (natural isotopic abundance of 7.73%) comprises an unpaired neutron such that it can provide additional probe to the spin-dependent couplings of WIMPs with the neutrons(σ SD χn ). The nuclear recoils from χN interactions in ULEGe only give rise to ∼20% of the observable ionizations compared with electron recoils at the same energy. The suppression ratio is called the quenching factor(QF) [19]. For clarity, all ULEGe measurements discussed hereafter in this article are electron-equivalent-energy, unless otherwise stated.The ULEGe array consists of 4-element each having an active mass of 5 g [20]. Standard ultra-low-background specifications were adopted in its construction and ch...

Neutrino-nucleus elastic scattering provides a unique laboratory to study the quantum mechanical coherency effects in electroweak interactions, towards which several experimental programs are being actively pursued. We report results of our quantitative studies on the transitions towards decoherency. A parameter (α) is identified to describe the degree of coherency, and its variations with incoming neutrino energy, detector threshold and target nucleus are studied. The ranges of α which can be probed with realistic neutrino experiments are derived, indicating complementarity between projects with different sources and targets. Uncertainties in nuclear physics and in α would constrain sensitivities in probing physics beyond the standard model. The maximum neutrino energies corresponding to α>0.95 are derived. PACS numbers: 13.15.+g, 21.10.Ft The elastic scattering of a neutrino with a nucleus [1,2] νA el :where A(Z, N ) denotes the atomic nucleus with its respective atomic, charge and neutron numbers, is a fundamental electroweak neutral current process in the Standard Model (SM) which has never been experimentally observed. It can provide a sensitive probe to physics beyond SM (BSM) [3,4] and plays an important role in astrophysical processes [1,5]. It offers prospects to study neutron density distributions [6], to detect supernova neutrinos [7] and to provide a compact and transportable neutrino detector for real-time monitoring of nuclear reactors [8]. The νA el events from solar and atmospheric neutrinos are the irreducible background [9] to forthcoming generation of dark matter experiments [10].There are active experimental programs to observe and measure the processes with neutrinos from reactors [11] or from decay-at-rest pions (DAR-π) [4] with a spallation neutron source [12]. The νA el reaction provides a unique laboratory to study the quantum mechanical coherency effects in electroweak interactions. At low momentum transfer, the de Broglie wavelength of the neutrinos is large compared with the nucleus, and the scattering amplitude of individual nucleons will coherently add to contribute to the cross-sections. Typically the neutrino energy (E ν ) and the measurable nuclear recoil kinetic energy (T ) are much less than the target nucleus mass (M ) in the discussion of coherency. The three-momentum transfer * Corresponding Author: htwong@phys.sinica.edu.tw (q ≡ | q|) is given by q 2 =2M T + T 2 2M T . Kinematics places constraints to the maximum recoil energy to beMeV is usually taken to characterize the requirement of coherency. The theme of this article is to quantify this transition − the first such investigation in the literature. We parametrize the degree of coherency and study its dependence on E ν , nuclear recoil detection threshold (T min ), and the nucleus A(Z, N ). Potential reaches of the up-coming experimental programs are derived.The differential cross-section of νA el scattering in SM is given by [2]:or, equivalently, in the experimental measurable T aswhere F Z (q 2 ) and F N (q 2 ) are, ...

Neutrino-electron scatterings are purely leptonic processes with robust Standard Model (SM) predictions. Their measurements can therefore provide constraints to physics beyond SM. Thē νe − e data taken at the Kuo-Sheng Reactor Neutrino Laboratory were used to probe two scenarios: Non-Standard Neutrino Interactions (NSI) and Unparticle Physics. New constraints were placed to the NSI parameters (ε eL ee ,ε eR ee ), (ε eL eµ ,ε eR eµ ) and (ε eL eτ ,ε eR eτ ) for the Non-Universal and Flavor-Changing channels, respectively, as well as to the coupling constants for scalar (λ0) and vector (λ1) unparticles to the neutrinos and electrons.

A search of axions produced in nuclear transitions was performed at the Kuo-Sheng Nuclear Power Station with a high-purity germanium detector of mass 1.06 kg at a distance of 28 m from the 2.9 GW reactor core. The expected experimental signatures were mono-energetic lines produced by their Primakoff or Compton conversions at the detector. Based on 459.0/96.3 days of Reactor ON/OFF data, no evidence of axion emissions were observed and constraints on the couplings gaγγ and gaee versus axion mass ma within the framework of invisible axion models were placed. The KSVZ and DFSZ models can be excluded for 10 4 eV ma 10 6 eV. Model-independent constraints on gaγγ · g 1 aNN < 7.7 × 10 −9 GeV −2 for ma 10 5 eV and gaee · g 1 aNN < 1.3 × 10 −10 for ma 10 6 eV at 90% confidence level were derived. This experimental approach provides a unique probe for axion mass at the keV−MeV range not accessible to the other techniques.

Relativistic millicharged particles (χq) have been proposed in various extensions to the Standard Model of particle physics. We consider the scenarios where they are produced at nuclear reactor core and via interactions of cosmic-rays with the earth's atmosphere. Millicharged particles could also be candidates for dark matter, and become relativistic through acceleration by supernova explosion shock waves. The atomic ionization cross section of χq with matter are derived with the equivalent photon approximation. Smoking-gun signatures with significant enhancement in the differential cross section are identified. New limits on the mass and charge of χq are derived, using data taken with a point-contact germanium detector with 500 g mass functioning at an energy threshold of 300 eV at the Kuo-Sheng Reactor Neutrino Laboratory.Big Bang nucleosynthesis [12,13], and direct laboratory experiments [14][15][16].The theme of this article is to report new direct laboratory limits on χ q using data acquired by an n-type Point-Contact Germanium (PCGe) detector with sub-keV sensitivity and excellent energy resolution. The PCGe technology [17] is being used in the studies of coherent neutrino nucleus elastic scattering, as well as searches of "light" WIMPs [18][19][20][21] and neutrinoless double beta decay.This article is organized as follows. Three cases of χ q are selected for investigations, whose origins and flux estimations are given in Section II. The interaction of χ q with matter and the derivation of cross sections are discussed in Section III. Physics constraints on χ q derived from data taken with the PCGe at the Kuo-Sheng Reactor Neutrino Laboratory (KSNL) [17,[22][23][24][25] are presented in Section IV. II. SOURCES OF MILLICHARGED PARTICLES A. Nuclear Reactor-χqNuclear reactor cores are powerful sources of γ-rays and therefore have been used for searches of axions [26,27], dark photons [28,29], and millicharged neutrinos [30]. Approximately half of the γ-radioactivity are of "prompt" origins directly from highly excited fission frag-arXiv:1808.02719v2 [hep-ph]