While dual-phase xenon time projection chambers (TPCs) have driven the sensitivity towards weakly interacting massive particles (WIMPs) at the GeV/c^2 to TeV/c^2 mass scale, the scope for sub-GeV/c^2 dark matter particles is hindered by a limited nuclear recoil energy detection threshold. One approach to probe for lighter candidates is to consider cases where they have been boosted by collisions with cosmic rays in the Milky Way, such that the additional kinetic energy lifts their induced signatures above the nominal threshold. In this Letter, we report first results of a search for cosmic ray-boosted dark matter (CRDM) with a combined 4.2 tonne-year exposure from the LUX-ZEPLIN (LZ) experiment. We observe no excess above the expected backgrounds and establish world-leading constraints on the spin-independent CRDM-nucleon cross section as small as 3.9 * 10^{-33} cm^2 at 90% confidence level for sub-GeV/c^2 masses.
90% CL CRDM-nucleon cross sections
We report results of a search for nuclear recoils induced by weakly interacting massive particle (WIMP) dark matter using the LUX-ZEPLIN (LZ) two-phase xenon time projection chamber. This analysis uses a total exposure of $4.2\pm0.1$ tonne-years from 280 live days of LZ operation, of which $3.3\pm0.1$ tonne-years and 220 live days are new. A technique to actively tag background electronic recoils from $^{214}$Pb $\beta$ decays is featured for the first time. Enhanced electron-ion recombination is observed in two-neutrino double electron capture decays of $^{124}$Xe, representing a noteworthy new background. After removal of artificial signal-like events injected into the data set to mitigate analyzer bias, we find no evidence for an excess over expected backgrounds. World-leading constraints are placed on spin-independent (SI) and spin-dependent WIMP-nucleon cross sections for masses $\geq$9 GeV/$c^2$. The strongest SI exclusion set is $2.1\times10^{-48}$ cm$^{2}$ at the 90% confidence level at a mass of 36 GeV/$c^2$, and the best SI median sensitivity achieved is $5.0\times10^{-48}$ cm$^{2}$ for a mass of 40 GeV/$c^2$.
90% CL WIMP SI cross sections, including sensitivities
90% CL WIMP SDn cross sections, including sensitivities and nuclear structure uncertainties
90% CL WIMP SDp cross sections, including sensitivities and nuclear structure uncertainties
We report the differential yields at mid-rapidity of the Breit-Wheeler process ($\gamma\gamma\rightarrow e^{+}e^{-}$) in peripheral Au+Au collisions at $\sqrt{s_{_{\rm{NN}}}} = $ 54.4 GeV and 200 GeV with the STAR experiment at RHIC, as a function of energy $\sqrt{s_{_{\rm{NN}}}}$, $e^{+}e^{-}$ transverse momentum $p_{\rm T}$, $p_{\rm T}^{2}$, invariant mass $M_{ee}$ and azimuthal angle. In the invariant mass range of 0.4 $<$$M_{ee}$$<$ 2.6 GeV/$c^{2}$ at low transverse momentum ($p_{\rm T}$$ < $0.15 GeV/$c$), the yields increase while the pair $\sqrt{\langle p_{\rm T}^{2} \rangle}$ decreases with increasing $\sqrt{s_{_{\rm{NN}}}}$, a feature is correctly predicted by the QED calculation. The energy dependencies of the measured quantities are sensitive to the nuclear form factor, infrared divergence and photon polarization. The data are compiled and used to extract the charge radius of the Au nucleus.
'54.4 GeV, Centrality: 40-60%'
'54.4 GeV, Centrality: 60-80%'
'54.4 GeV, Centrality: 80-100%'
We report multi-differential measurements of strange hadron production ranging from mid- to target-rapidity in Au+Au collisions at a center-of-momentum energy per nucleon pair of $\sqrt{s_{\rm NN}}=3$ GeV with the STAR experiment at RHIC. $K^0_S$ meson and $\Lambda$ hyperon yields are measured via their weak decay channels. Collision centrality and rapidity dependences of the transverse momentum spectra and particle ratios are presented. Particle mass and centrality dependence of the average transverse momenta of $\Lambda$ and $K^0_S$ are compared with other strange particles, providing evidence of the development of hadronic rescattering in such collisions. The 4$\pi$ yields of each of these strange hadrons show a consistent centrality dependence. Discussions on radial flow, the strange hadron production mechanism, and properties of the medium created in such collisions are presented together with results from hadronic transport and thermal model calculations.
dN/dy of lambda for different centrality bins.
dN/dy of Ks0 for different centrality bins.
Rapidity dependence of Lambda/p for different centrality bins.
In heavy-ion collision experiments, the global collectivity of final-state particles can be quantified by anisotropic flow coefficients ($v_n$). The first-order flow coefficient, also referred to as the directed flow ($v_{1}$), describes the collective sideward motion of produced particles and nuclear fragments in heavy-ion collisions. It carries information on the very early stage of the collision, especially at large pseudorapidity ($\eta$), where it is believed to be generated during the nuclear passage time. Directed flow therefore probes the onset of bulk collective dynamics during thermalization, providing valuable experimental guidance to models of the pre-equilibrium stage. In 2018, the Event Plane Detector (EPD) was installed in STAR and used for the Beam Energy Scan phase-II (BES-II) data taking. The combination of EPD ($2.1 <|\eta|< 5.1$) and high-statistics BES-II data enables us to extend the $v_{1}$ measurement to the forward and backward $\eta$ regions. In this paper, we present the measurement of $v_{1}$ over a wide $\eta$ range in Au+Au collisions at $\sqrt{s_{NN}}=$ 19.6 and 27 GeV using the STAR EPD. The results of the analysis at $\sqrt{s_{NN}}=$19.6 GeV exhibit excellent consistency with the previous PHOBOS measurement, while elevating the precision of the overall measurement. The increased precision of the measurement also revealed finer structures in heavy-ion collisions, including a potential observation of the first-order event-plane decorrelation. Multiple physics models were compared to the experimental results. Only a transport model and a three-fluid hybrid model can reproduce a sizable $v_{1}$ at large $\eta$ as was observed experimentally. The model comparison also indicates $v_{1}$ at large $\eta$ might be sensitive to the QGP phase transition.
Directed flow vs pseudorapidity.
Directed flow vs pseudorapidity.
Directed flow vs pseudorapidity.
Weakly interacting massive particles (WIMPs) may interact with a virtual pion that is exchanged between nucleons. This interaction channel is important to consider in models where the spin-independent isoscalar channel is suppressed. Using data from the first science run of the LUX-ZEPLIN dark matter experiment, containing 60 live days of data in a 5.5~tonne fiducial mass of liquid xenon, we report the results on a search for WIMP-pion interactions. We observe no significant excess and set an upper limit of $1.5\times10^{-46}$~cm$^2$ at a 90% confidence level for a WIMP mass of 33~GeV/c$^2$ for this interaction.
WIMP-Pion interaction cross section at the 90% CL
The first science run of the LUX-ZEPLIN (LZ) experiment, a dual-phase xenon time project chamber operating in the Sanford Underground Research Facility in South Dakota, USA, has reported leading limits on spin-independent WIMP-nucleon interactions and interactions described from a non-relativistic effective field theory (NREFT). Using the same 5.5~t fiducial mass and 60 live days of exposure we report on the results of a relativistic extension to the NREFT. We present constraints on couplings from covariant interactions arising from the coupling of vector, axial currents, and electric dipole moments of the nucleon to the magnetic and electric dipole moments of the WIMP which cannot be described by recasting previous results described by an NREFT. Using a profile-likelihood ratio analysis, in an energy region between 0~keV$_\text{nr}$ to 270~keV$_\text{nr}$, we report 90% confidence level exclusion limits on the coupling strength of five interactions in both the isoscalar and isovector bases.
Isoscalar interaction coupling limit for Lagrangian 1
Isovector interaction coupling limit for Lagrangian 19
Isoscalar interaction coupling limit for Lagrangian 19
A combination of fifteen top quark mass measurements performed by the ATLAS and CMS experiments at the LHC is presented. The data sets used correspond to an integrated luminosity of up to 5 and 20$^{-1}$ of proton-proton collisions at center-of-mass energies of 7 and 8 TeV, respectively. The combination includes measurements in top quark pair events that exploit both the semileptonic and hadronic decays of the top quark, and a measurement using events enriched in single top quark production via the electroweak $t$-channel. The combination accounts for the correlations between measurements and achieves an improvement in the total uncertainty of 31% relative to the most precise input measurement. The result is $m_\mathrm{t}$ = 172.52 $\pm$ 0.14 (stat) $\pm$ 0.30 (syst) GeV, with a total uncertainty of 0.33 GeV.
Uncertainties on the $m_{t}$ values extracted in the LHC, ATLAS, and CMS combinations arising from the categories described in the text, sorted in order of decreasing value of the combined LHC uncertainty.
Searches for dark matter with liquid xenon time projection chamber experiments have traditionally focused on the region of the parameter space that is characteristic of weakly interacting massive particles, ranging from a few GeV/$c^2$ to a few TeV/$c^2$. Models of dark matter with a mass much heavier than this are well motivated by early production mechanisms different from the standard thermal freeze-out, but they have generally been less explored experimentally. In this work, we present a re-analysis of the first science run (SR1) of the LZ experiment, with an exposure of $0.9$ tonne$\times$year, to search for ultraheavy particle dark matter. The signal topology consists of multiple energy deposits in the active region of the detector forming a straight line, from which the velocity of the incoming particle can be reconstructed on an event-by-event basis. Zero events with this topology were observed after applying the data selection calibrated on a simulated sample of signal-like events. New experimental constraints are derived, which rule out previously unexplored regions of the dark matter parameter space of spin-independent interactions beyond a mass of 10$^{17}$ GeV/$c^2$.
Upper limit on the WIMP-nucleon scattering cross section from the multiple-scatter analysis.
Upper limit on the WIMP-nucleus scattering cross section from the multiple-scatter analysis.
Upper limit on the WIMP-nucleon scattering cross section from the single-scatter analysis.
Based on a data sample of 10 billion $J/\psi$ events collected with the BESIII detector, improved measurements of the Dalitz decays $\eta/\eta'\rightarrow\gamma e^+e^-$ are performed, where the $\eta$ and $\eta'$ are produced through the radiative decays $J/\psi\rightarrow\gamma \eta/\eta'$. The branching fractions of $\eta\rightarrow\gamma e^+e^-$ and $\eta'\rightarrow\gamma e^+e^-$ are measured to be $(7.07 \pm 0.05 \pm 0.23)\times10^{-3}$ and $(4.83\pm0.07\pm0.14)\times10^{-4}$, respectively. Within the single pole model, the parameter of electromagnetic transition form factor for $\eta\rightarrow\gamma e^+e^-$ is determined to be $\Lambda_{\eta}=(0.749 \pm 0.027 \pm 0.007)~ {\rm GeV}/c^{2}$. Within the multi-pole model, we extract the electromagnetic transition form factors for $\eta'\rightarrow\gamma e^+e^-$ to be $\Lambda_{\eta'} = (0.802 \pm 0.007\pm 0.008)~ {\rm GeV}/c^{2}$ and $\gamma_{\eta'} = (0.113\pm0.010\pm0.002)~ {\rm GeV}/c^{2}$. The results are consistent with both theoretical predictions and previous measurements. The characteristic sizes of the interaction regions for the $\eta$ and $\eta'$ are calculated to be $(0.645 \pm 0.023 \pm 0.007 )~ {\rm fm}$ and $(0.596 \pm 0.005 \pm 0.006)~ {\rm fm}$, respectively. In addition, we search for the dark photon in $\eta/\eta^\prime\rightarrow\gamma e^{+}e^{-}$, and the upper limits of the branching fractions as a function of the dark photon are given at 90% confidence level.
The binned invariant mass spectrum of $e^+e^-$ pairs produced from the $\eta\to \gamma e^+e^-$ transition. The efficiency-corrected and background-subtracted data are binned in M($e^+e^-$) and the number of events in each bin is shown.
The binned invariant mass spectrum of $e^+e^-$ pairs produced from the $\eta^\prime\to \gamma e^+e^-$ transition. The efficiency-corrected and background-subtracted data are binned in M($e^+e^-$) and the number of events in each bin is shown.
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