New data are reported from a second run of the 2-liter PICO-2L C 3 F 8 bubble chamber with a total exposure of 129 kg-days at a thermodynamic threshold energy of 3.3 keV. These data show that measures taken to control particulate contamination in the superheated fluid resulted in the absence of the anomalous background events observed in the first run of this bubble chamber. One single nuclear-recoil event was observed in the data, consistent both with the predicted background rate from neutrons and with the observed rate of unambiguous multiple-bubble neutron scattering events. The chamber exhibits the same excellent electron-recoil and alpha decay rejection as was previously reported. These data provide the most stringent direct detection constraints on weakly interacting massive particle (WIMP)-proton spindependent scattering to date for WIMP masses < 50 GeV=c 2 .

The dimuon invariant mass spectrum is searched in the range between 5.5 and 14 GeV for a light pseudoscalar Higgs boson a, predicted in a number of new physics models, including the next-to-minimal supersymmetric standard model. The data sample used in the search corresponds to an integrated luminosity of 1.3 fb(-1) collected in pp collisions at √s = 7 TeV with the CMS detector at the LHC. No excess is observed above the background predictions and upper limits are set on the cross section times branching fraction σ × B(pp→a→μ(+)μ(-)) in the range of 1.5-7.5 pb. These results improve on existing bounds on the abb coupling for m(a) < m(Υ(1S)) and are the first significant limits for m(a) > m(Υ(3S)). Constraints on the supersymmetric parameter space are presented in the context of the next-to-minimal model.

A measurement of differential cross sections for the production of a pair of isolated photons in proton–proton collisions at is presented. The data sample corresponds to an integrated luminosity of 5.0 collected with the CMS detector. A data-driven isolation template method is used to extract the prompt diphoton yield. The measured cross section for two isolated photons, with transverse energy above 40 and 25 respectively, in the pseudorapidity range , and with an angular separation , is . Differential cross sections are measured as a function of the diphoton invariant mass, the diphoton transverse momentum, the azimuthal angle difference between the two photons, and the cosine of the polar angle in the Collins–Soper reference frame of the diphoton system. The results are compared to theoretical predictions at leading, next-to-leading, and next-to-next-to-leading order in quantum chromodynamics.

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A 30-g xenon bubble chamber, operated at Northwestern University in June and November 2016, has for the first time observed simultaneous bubble nucleation and scintillation by nuclear recoils in a superheated liquid. This chamber is instrumented with a CCD camera for near-IR bubble imaging, a solar-blind photomultiplier tube to detect 175-nm xenon scintillation light, and a piezoelectric acoustic transducer to detect the ultrasonic emission from a growing bubble. The time of nucleation determined from the acoustic signal is used to correlate specific scintillation pulses with bubble-nucleating events. We report on data from this chamber for thermodynamic "Seitz" thresholds from 4.2 to 15.0 keV. The observed single-and multiple-bubble rates when exposed to a 252 Cf neutron source indicate that, for an 8.3-keV thermodynamic threshold, the minimum nuclear recoil energy required to nucleate a bubble is 19 ± 6 keV (1σ uncertainty). This is consistent with the observed scintillation spectrum for bubble-nucleating events. We see no evidence for bubble nucleation by gamma rays at any of the thresholds studied, setting a 90% C.L. upper limit of 6.3 × 10 −7 bubbles per gamma interaction at a 4.2-keV thermodynamic threshold. This indicates stronger gamma discrimination than in CF3I bubble chambers, supporting the hypothesis that scintillation production suppresses bubble nucleation by electron recoils while nuclear recoils nucleate bubbles as usual. These measurements establish the noble-liquid bubble chamber as a promising new technology for the detection of weakly interacting massive particle dark matter and coherent elastic neutrino-nucleus scattering. The detection of single nuclear recoils at the keV scale is the core problem in both direct searches for weakly interacting massive particle (WIMP) dark matter [1] and the detection of neutrinos via coherent elastic neutrinonucleus scattering (CEνNS) [2]. This signal is unique to WIMPs and neutrinos, enabling low-background searches for these extremely rare scattering events via the discrimination of nuclear recoils (signal) from electron recoils (backgrounds). Easily scalable liquid-based technologies with this capability have proven effective in extending sensitivity to WIMPs [3][4][5][6][7][8][9], but the existing techniques are each limited in at least one dimension: xenon time projection chambers (TPCs) have relatively weak (10 −3 ) electron discrimination [10] and are susceptible to beta-decay backgrounds; argon-based detectors have much stronger (10 −8 ) discrimination at high energies but rapidly lose discrimination for recoil energies below ∼45 keV [11]; and bubble chambers, which have the strongest demonstrated electron-recoil discrimination at < 10 −10 , give virtually no event-by-event energy information [12] and must address backgrounds both far above and below the keV scale.The scintillating bubble chamber inherits both the strong electron discrimination of a bubble chamber and the scintillation-based energy reconstruction of a noble liquid. It can be unders...