Spin-independent Research Articles

Constraints from the neutron EDM on subleading effective operators for direct Dark Matter searches

Interactions between Dark Matter (DM) and nucleons relevant for direct search experiments can be organised in a model independent manner using a Galiliean invariant, non-relativistic effective field theory (NREFT). Here one expands the interactions in powers of the momentum transfer q→\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\overrightarrow{q} $$\\end{document} and DM velocity v→\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\overrightarrow{v} $$\\end{document}. This approach generates many operators. The potentially most important subleading operators are odd under T, and can thus only be present in a theory with CP violating interactions. We consider two such operators, called O\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\mathcal{O} $$\\end{document}10 and O\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\mathcal{O} $$\\end{document}11 in the literature, in simplified models with neutral spin−0 mediators; the couplings are chosen such that the coefficient of the leading spin independent (SI) operator, which survives for v→\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\overrightarrow{v} $$\\end{document} → 0, vanishes at tree level. However, it is generically induced at the next order in perturbation theory. We perform a numerical comparison of the number of scattering events between interactions involving the T−odd operators and the corresponding loop induced SI contributions. We find that for “maximal” CP violation the former can dominate over the latter. However, in two of the three models we consider, an electric dipole moment of the neutron (nEDM) is induced at two-loop order. We find that the experimental bound on the nEDM typically leads to undetectably small rates induced by O\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\mathcal{O} $$\\end{document}10. On the other hand, the model leading to a nonvanishing coefficient of O\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\mathcal{O} $$\\end{document}11 does not induce an nEDM.

Axionlike-particle contributions to the μ→e conversion

We study the μ→e conversion process in nuclear targets arising in models of axionlike particles (ALPs) with hadronic and charged lepton flavor violating (CLFV) interactions. Contributions to this process generally fall into two categories: spin-independent (SI) and spin-dependent (SD). While the SI contribution can be generated by a dipole operator through purely leptonic ALP interactions, the SD contribution can also be present through ALP-quark interactions at tree-level. It is naively anticipated that the SI contribution would be dominant due to its coherent enhancement. In this paper, we show that is not generically the case; in particular, for naturally sized ALP couplings to quarks of order ∼mq/fa, the SD interaction induced by ALP-π0 mixing turns out to be the leading contribution to μ→e conversion. Intuitively, this stems from the suppressed dipole contribution by the QED one-loop factor which counters the effect of SI coherent enhancement. Our study highlights the importance of μ→e conversion searches in exploring the parameter space of generic ALP models, and demonstrates the competitiveness of these searches in probing the CLFV ALP parameter space in the heavy mass range of ma≳mμ. Published by the American Physical Society 2024

Cosmic-ray boosted dark matter in Xe-based direct detection experiments

LUX-ZEPLIN (LZ) collaboration has achieved the strongest constraint on weak-scale dark matter (DM)-nucleon spin-independent (SI) scattering cross section in a large region of parameter space. In this paper, we take a complementary approach and study the prospect of detecting cosmic-ray boosted sub-GeV DM in LZ. In the absence of a signal for DM, we improve upon the previous constraints by a factor of ∼2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sim 2$$\\end{document} using the LZ result for some regions of the parameter space. We also show that upcoming XENONnT and future Darwin experiments will be sensitive to cross sections smaller by factors of ∼3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sim 3$$\\end{document} and ∼10\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sim 10$$\\end{document} compared to the current LZ limit, respectively.

Elusive muonic WIMP

The Weakly Interacting Massive Particle (WIMP) paradigm is one of the most popular scenarios for Dark Matter (DM) theories that however is strongly constrained, in particular by direct detection experiments. We stick with the WIMP hypothesis and consider a Dirac fermion candidate for DM that interacts with the Standard Model (SM) via a spin-1 $Z'$, arising from the spontaneous breaking of an Abelian $U(1)'_{\mu}$ gauge symmetry, under which only second generation leptons and the DM are appropriately charged. Due to the charge assignment, the model is gauge anomalous and can only be interpreted as an effective field theory (EFT) at low energy. The $Z'$ couples at tree level only to the vector DM current, to the axial muon current and to left-handed muonic neutrinos, so the WIMP-nucleon cross section is beyond the experimental reach of spin-independent (SI) direct detection searches. We study the current bounds on this model coming from direct and indirect detection of DM, collider searches, contributions to $(g-2)_{\mu}$ and to neutrino trident production. We find that large regions of the parameter space remains to be explored. In the context of LHC searches, we study the impact of a muon-exclusive signal region for the $3\mu$ + ${E}^{{\rm miss}}_T$ channel with an invariant mass window around $m_{Z'}$. We show that this search can significantly improve the current collider bounds. Finally, from the anomalous nature of our EFT, there remain at low energy triboson anomalous interactions between the $Z'$ and the electroweak (EW) SM gauge bosons. We explore the possibilities of probing these interactions at the LHC and at a 100 TeV proton collider finding it extremely challenging. On the other hand, for a muon collider the resonant channel $\mu^{+}\mu^{-}\to Z'\to ZZ$ could be discovered in the most promising scenario with luminosity of $\mathcal{O}({\rm few}\; 10)$ ${\rm fb}^{-1}$.

Spin-dependent dark matter-electron interactions

Detectors with low thresholds for electron recoil open a new window to direct searches of sub-GeV dark matter (DM) candidates. In the past decade, many strong limits on DM-electron interactions have been set, but most on the one which is spin-independent (SI) of both dark matter and electron spins. In this work, we study DM-atom scattering through a spin-dependent (SD) interaction at leading order (LO), using well-benchmarked, state-of-the-art atomic many-body calculations. Exclusion limits on the SD DM-electron cross section are derived with data taken from experiments with xenon and germanium detectors at leading sensitivities. In the DM mass range of 0.1 - 10 GeV, the best limits set by the XENON1T experiment: $\sigma_e^{\textrm{(SD)}}<10^{-41}-10^{-40}\,\textrm{cm}^2$ are comparable to the ones drawn on DM-neutron and DM-proton at slightly bigger DM masses. The detector's responses to the LO SD and SI interactions are analyzed. In nonrelativistic limit, a constant ratio between them leads to an indistinguishability of the SD and SI recoil energy spectra. Relativistic calculations however show the scaling starts to break down at a few hundreds of eV, where spin-orbit effects become sizable. We discuss the prospects of disentangling the SI and SD components in DM-electron interactions via spectral shape measurements, as well as having spin-sensitive experimental signatures without SI background.

Magnetic moments of leptons, charged lepton flavor violations and dark matter phenomenology of a minimal radiative Dirac neutrino mass model

In a simple extension of the standard model (SM), a pair of vector like lepton doublets (L1 and L2) and a SU(2)L scalar doublet (η) have been introduced to help in accommodating the discrepancy in determination of the anomalous magnetic moments of the light leptons, namely, e and μ. Moreover, to make our scenario friendly to a Dirac like neutrino and also for a consistent dark matter phenomenology, we specifically add a singlet scalar (S) and a singlet fermion (ψ) in the set-up. However, the singlet states also induce a meaningful contribution in other charged lepton processes. A discrete symmetry {mathcal{Z}}_2times {mathcal{Z}}_2^{prime } has been imposed under which all the SM particles are even while the new particles may be assumed to have odd charges. In a bottom-up approach, with a minimal particle content, we systematically explore the available parameter space in terms of couplings and masses of the new particles. Here a number of observables associated with the SM leptons have been considered, e.g., masses and mixings of neutrinos, (g − 2) anomalies of e, μ, charged lepton flavor violating (cLFV) observables and the dark matter (DM) phenomenology of a singlet-doublet dark matter. Neutrinos, promoted as the Dirac type states, acquire mass at one loop level after the discrete {mathcal{Z}}_2^{prime } symmetry gets softly broken, while the unbroken {mathcal{Z}}_2 keeps the dark matter stable. The mixing between the singlet ψ and the doublet vector lepton can be constrained to satisfy the electroweak precision observables and the spin independent (SI) direct detection (DD) cross section of the dark matter. In this analysis, potentially important LHC bounds have also been discussed.

Spin-Dependent Scattering of Scalar and Vector Dark Matter on the Electron

The property of dark matter remains to date unknown. However, a model-independent classification of dark matter candidates can be achieved by using various symmetries, as performed in the standard model. Fermionic dark matter has been extensively researched, and one favored candidate is the neutralino in the Minimal Supersymmetric Standard Model, which is required by fermion–boson symmetry and the preservation of R-parity. Bosonic dark matter has not been sufficiently studied, especially the scenario of dark matter with a mass of sub-GeV. In this paper, we consider the effect of spin-dependent (SD) on scalar and vector dark matter, which are mediated by pseudoscalar and axial-vector, and evaluate the effect on the dark matter–electron scattering cross-section. We list all the interactions and form factor of dark matter–electron SD scattering, and use XENON10/100/1T experiment data to derive the exclusion limit of the SD cross-section. We find that the SD scattering of scalar and vector dark matter can be three orders of magnitude stronger than spin-independent (SI) scattering due to the p-wave scattering.

Loop induced top quark FCNC through top quark and dark matter interactions

We present a comprehensive analysis of the loop induced top quark FCNC signals at the LHC within one class of the simplified model. The loop level FCNC interactions are well motivated to avoid the hierarchy of the top quark couplings from the new physics and standard model. Such a theory will posit a Majorana dark matter candidate and could be tested through dark matter relic density, direct detection experiments (the scattering between dark matter and heavy nuclei), and the collider signals at the LHC. We find that the spin-independent (SI) scattering between Majorana dark matter and nuclei will vanish at the leading order, while the next-to-leading order correction to the SI scattering becomes significant to constrain the parameter space of the model. A detailed comparison between direct detection experiments and LHC searches is also discussed and both of them are very important to fully constrain the model.

Higgsino asymmetry and direct-detection constraints of light dark matter in the NMSSM with non-universal Higgs masses * *Supported by the National Natural Science Foundation of China (11605123)

In this study, we analyze the direct-detection constraints of light dark matter in the next-to minimal supersymmetric standard model (NMSSM) with non-universal Higgs masses (NUHM); we specially focus on the correlation between higgsino asymmetry and spin-dependent (SD) cross section. We draw the following conclusions. (i) The SD cross section is proportional to the square of higgsino asymmetry in dark matter in the NMSSM-NUHM, and hence, it is small for highly singlino-dominated dark matter. (ii) The higgsino-mass parameter is smaller than approximately in the NMSSM-NUHM due to the current muon g-2 constraint, but our scenario with light dark matter can still be alive under current constraints including the direct detection of dark matter in the spin-dependent channel. (iii) With a sizeable higgsino component in the light dark matter, the higgsino asymmetry and SD cross section can also be sizeable, but dark matter relic density is always small; thus, it can escape the direct detections. (iv) Light dark matter in the - and Z-funnel annihilation channels with sufficient relic density can be covered by future LUX-ZEPLIN (LZ) 7-ton in SD detections. (v) The spin-independent (SI) cross section is dominated by - and -exchanging channels, which can even cancel each other in some samples, leaving an SI cross section smaller by a few orders of magnitude than that of one individual channel.

Study of sensitivity to dark matter scattering for liquid argon detectors in EFT framework

The framework of nonrelativistic effective field theory (NREFT) aims to generalize the standard analysis of direct detection experiments in terms of spin-dependent (SD) and spin-independent (SI) interactions. Here we performed EFT analysis and obtained sensitivity curves for liquid argon detectors to scattering of dark matter (DM) particles in case of different interaction models. DM velocity distribution and experimental dependencies are also described.

Prospect of dark matter searches in split SUSY models

The results from the XENON1T/Panda-X and IceCube experiments have set severe upper limits on spin-independent (SI) and spin-dependent (SD) scattering cross section of weakly interacting massive particles on nuclei. In the framework of split supersymmetry scenario supported by the LHC constraints, we investigate the prospect of dark matter (DM) direct and indirect detection experiments. Assuming a relation among gaugino masses at the grand unification scale, we have specified the parameter regions that predict the right DM relic abundance as well as satisfy the constraints on SI and SD scattering cross sections of DM. In the case of universal gaugino masses, the XENON1T/Panda-X results have ruled out completely the well-tempered neutralino region. We have found that 200 times smaller than the current IceCube bound is the sensitivity limit that a future DM indirect detection experiment should have at least in order to have a certain impact beyond the current XENON1T/Panda-X results. The future results from XENON1T will be able to test a signification portion of the Higgsino-like region. Although there are points in the Higgsino-like region that cannot be reached by future DM detection experiments, direct production channels of neutralino/chargino can be used to test the model at the LHC/HL-HLC. In the case of non-universal gaugino masses, beside showing the allowed parameter space, we have specified the regions that can be tested at both future DM direct and indirect detection experiments. One of them are Higgsino-like region, while the others two predict a well mixed bino-wino DM. The DM properties in each region have been examined and demonstrated with a benchmark of-ino spectrum. Interestingly, points in the mixed bino-wino region with rather light chargino can be tested the LHC 14 TeV.

Indirect Searches for Dark Matter Signatures at INO

Neutrino fluxes could arise due to annihilation of Weakly Interactive Massive Particles (WIMPs) in the center of the sun. We study the prospects of search for muon events due to such neutrinos at the upcoming Iron CALorimeter (ICAL) detector to be housed at India-based Neutrino Observatory (INO). Although the atmospheric neutrinos will pose a serious background to the signal neutrinos produced through WIMP annihilation, the former could be supressed significantly by using the directional property of signal neutrinos. For 50kt×10 years of ICAL running and WIMP masses (mχ) between 3-100 GeV, we perform a χ2 analysis and present expected exclusion regions in the σβख़ — mx and σβι — mx plane, where σβख़ and σβι are the WIMP-nucleon Spin-Dependent (SD) and Spin-Independent (SI) scattering cross-sections, respectively. For mx = 25 GeV, the expected 90 % C.L. exclusion limit on σβख़ are σβॄ < 7.82 x 10−41 cm2 for τ+τ− channel and σβख़ < 1.23 10−39 cm2 for 66 channel. For same m×, the expected 90 % C.L. exclusion limits on σβι are σβι < 8.97 × 10−43 cm2 for τ +τ− channel and σβι < 1.43×10−41 cm2 for 66 channel.

Comparison between DAMA/LIBRA and COSINE-100 in the light of quenching factors

There is a long standing debate about whether or not the annual modulation signal reported by the DAMA/LIBRA collaboration is induced by Weakly Interacting Massive Particles (WIMP) in the galaxy's dark matter halo scattering from nuclides in their NaI(Tl) crystal target/detector. This is because regions of WIMP-mass vs. WIMP-nucleon cross-section parameter space that can accommodate the DAMA/LIBRA-phase1 modulation signal in the context of the standard WIMP dark matter galactic halo and isospin-conserving (canonical), spin-independent (SI) WIMP-nucleon interactions have been excluded by many of other dark matter search experiments including COSINE-100, which uses the same NaI(Tl) target/detector material. Moreover, the recently released DAMA/LIBRA-phase2 results are inconsistent with an interpretation as WIMP-nuclide scattering via the canonical SI interaction and prefer, instead, isospin-violating or spin-dependent interactions. Dark matter interpretations of the DAMA/LIBRA signal are sensitive to the NaI(Tl) scintillation efficiency for nuclear recoils, which is characterized by so-called quenching factors (QF), and the QF values used in previous studies differ significantly from recently reported measurements, which may have led to incorrect interpretations of the DAMA/LIBRA signal. In this article, the compatibility of the DAMA/LIBRA and COSINE-100 results, in light of the new QF measurements is examined for different possible types of WIMP-nucleon interactions. The resulting allowed parameter space regions associated with the DAMA/LIBRA signal are explicitly compared with 90% confidence level upper limits from the initial 59.5 day COSINE-100 exposure. With the newly measured QF values, the allowed 3σ regions from the DAMA/LIBRA data are still generally excluded by the COSINE-100 data.

Accidental scale-invariant Majorana dark matter in leptoquark-Higgs portals

We study a classically accidental scale-invariant extension of the Standard Model (SM) containing three additional fields, a vector leptoquark (Vμ), a real scalar (ϕ), and a neutral Majorana fermion (χ) as a dark matter (DM) candidate. The scalar ϕ (scalon) and Majorana fermion χ are both singlets under the SM gauge group, while Vμ has (3, 1, 2/3) quantum numbers under the SU(3)c×SU(2)L×U(1)Y. The Majorana DM couples to the SM sector via both Higgs and leptoquark portals. We perform a scan over the independent parameters to determine the viable parameter space consistent with the Planck data for DM relic density, and with the PandaX-II and LUX direct detection limits for the spin-independent (SI) and spin-dependent (SD) DM-nucleon cross section. The model generally evades indirect detection constraints while being consistent with collider data.

Prospects of indirect searches for dark matter annihilations in the earth with ICAL@INO

We study the prospects of detecting muon events at the upcoming Iron CALorimeter (ICAL) detector to be built at the proposed India-based Neutrino Observatory (INO) facility due to neutrinos arising out of annihilation of Weakly Interactive Massive Particles (WIMP) in the centre of the earth. The atmospheric neutrinos coming from the direction of earth core presents an irreducible background. We consider 50kt × 10 years of ICAL running and WIMP masses between 10-100 GeV and present 90% C.L. exclusion sensitivity limits on σSI which is the WIMP-nucleon Spin Independent (SI) interaction cross-section. The expected sensitivity limits calculated for ICAL for the WIMP annihilation in the earth are more stringent than the limits obtained by any other neutrino detector. For a WIMP mass of 52.14 GeV, where the signal fluxes are enhanced due to resonance capture of WIMP in earth due to Fe nuclei, the sensitivity limits, assuming 100% branching ratio for each channel, are: σSI = 3.43 × 10−45 cm2 for νμ {overline{nu}}_{mu } channel, σSI = 1.02 × 10−44 cm2 for τ+τ− channel and σSI = 5.36 × 10−44 cm2 for b;overline{b} channel.

Selecting μ → e conversion targets to distinguish lepton flavour-changing operators

The experimental sensitivity to μ→e conversion on nuclei is set to improve by four orders of magnitude in coming years. However, various operator coefficients add coherently in the amplitude for μ→econversion, weighted by nucleus-dependent functions, and therefore in the event of a detection, identifying the relevant new physics scenarios could be difficult. Using a representation of the nuclear targets as vectors in coefficient space, whose components are the weighting functions, we quantify the expectation that different nuclear targets could give different constraints. We show that all but two combinations of the 10 Spin-Independent (SI) coefficients could be constrained by future measurements, but discriminating among the axial, tensor and pseudoscalar operators that contribute to the Spin-Dependent (SD) process would require dedicated nuclear calculations. We anticipate that μ→econversion could constrain 10 to 14 combinations of coefficients; if μ→eγ and μ→ee¯e constrain eight more, that leaves 60 to 64 “flat directions” in the basis of QED × QCD-invariant operators which describe μ→e flavour change below mW.

Casting a wide signal net with future direct dark matter detection experiments

As dark matter (DM) direct detection experiments continue to improve their sensitivity they will inevitably encounter an irreducible background arising from coherent neutrino scattering. This so-called “neutrino floor” may significantly reduce the sensitivity of an experiment to DM-nuclei interactions, particularly if the recoil spectrum of the neutrino background is approximately degenerate with the DM signal. This occurs for the conventionally considered spin-independent (SI) or spin-dependent (SD) interactions. In such case, an increase in the experiment's exposure by multiple orders of magnitude may not yield any significant increase in sensitivity. The typically considered SI and SD interactions, however, do not adequately reflect the whole landscape of the well-motivated DM models, which includes other interactions. Since particle DM has not been detected yet in laboratories, it is essential to understand and maximize the detection capabilities for a broad variety of possible models and signatures. In this work we explore the impact of the background arising from various neutrino sources on the discovery potential of a DM signal for a large class of viable DM-nucleus interactions and several potential futuristic experimental settings, with different target elements. For some momentum suppressed cross sections, large DM particle masses and heavier targets, we find that there is no suppression of the discovery limits due to neutrino backgrounds. Further, we explicitly demonstrate that inelastic scattering, which could appear in models with multicomponent dark sectors, would help to lift the signal degeneracy associated with the neutrino floor. This study could assist with mapping out the optimal DM detection strategy for the next generation of experiments.

Prospects of indirect searches for dark matter at INO

The annihilation of Weakly Interactive Massive Particles (WIMP) in the centre of the sun could give rise to neutrino fluxes. We study the prospects of searching for these neutrinos at the upcoming Iron CALorimeter (ICAL) detector to be housed at the India-based Neutrino Observatory (INO). We perform ICAL simulations to obtain the detector efficiencies and resolutions in order to simulate muon events in ICAL due to neutrinos coming from annihilation of WIMP in the mass range mχ = (3−100) GeV . The atmospheric neutrinos pose a major background for these indirect detection studies and can be reduced using the fact that the signal comes only from the direction of the sun. For a given WIMP mass, we find the opening angle θ90 such that 90 % of the signal events are contained within this angle and use this cone-cut criteria to reduce the atmospheric neutrino background. The reduced background is then weighted by the solar exposure function at INO to obtain the final background spectrum for a given WIMP mass. We perform a χ2 analysis and present expected exclusion regions in the σSD−mχ and σSI−mχ, where σSD and σSI are the WIMP-nucleon Spin-Dependent (SD) and Spin-Independent (SI) scattering cross-section, respectively. For a 10 years exposure and mχ=25 GeV, the expected 90 % C.L. exclusion limit is found to be σSD < 6.87× 10−41 cm2 and σSI < 7.75× 10−43 cm2 for the τ+ τ− annihilation channel and σSD < 1.14× 10−39 cm2 and σSI < 1.30× 10−41 cm2 for the b b̄ channel, assuming 100 % branching ratio for each of the WIMP annihilation channel.

\u201cSpin-dependent\u201d varvec{mu rightarrow e} conversion on light nuclei

The experimental sensitivity to mu rightarrow e conversion will improve by four or more orders of magnitude in coming years, making it interesting to consider the “spin-dependent” (SD) contribution to the rate. This process does not benefit from the atomic-number-squared enhancement of the spin-independent (SI) contribution, but probes different operators. We give details of our recent estimate of the spin-dependent rate, expressed as a function of operator coefficients at the experimental scale. Then we explore the prospects for distinguishing coefficients or models by using different targets, both in an EFT perspective, where a geometric representation of different targets as vectors in coefficient space is introduced, and also in three leptoquark models. It is found that comparing the rate on isotopes with and without spin could allow one to detect spin-dependent coefficients that are at least a factor of few larger than the spin-independent ones. Distinguishing among the axial, tensor and pseudoscalar operators that induce the SD rate would require calculating the nuclear matrix elements for the second two. Comparing the SD rate on nuclei with an odd proton vs. odd neutron could allow one to distinguish operators involving u quarks from those involving d quarks; this is interesting because the distinction is difficult to make for SI operators.

Limits on light WIMPs with a 1 kg-scale germanium detector at 160 eVee physics threshold at the China Jinping Underground Laboratory**Supported by the National Key Research and Development Program of China (2017YFA0402200, 2017YFA0402201), the National Natural Science Foundation of China (11175099, 11275107, 11475117, 11475099, 11475092, 11675088), the National Basic Research Program of China (973 Program) (2010CB833006). We thank the support

We report results of a search for light weakly interacting massive particle (WIMP) dark matter from the CDEX-1 experiment at the China Jinping Underground Laboratory (CJPL). Constraints on WIMP-nucleon spin-independent (SI) and spin-dependent (SD) couplings are derived with a physics threshold of 160 eVee, from an exposure of 737.1 kg-days. The SI and SD limits extend the lower reach of light WIMPs to 2 GeV and improve over our earlier bounds at WIMP mass less than 6 GeV.