We have searched for the signature of 3- and 4-body decays of pair-produced scalar top quarks (stop) in the inclusive final state containing an electron, a muon, and significant missing transverse ...energy using a sample of p p ̄ events corresponding to 108.3 pb −1 of data collected with the DØ detector at Fermilab. The search is done in the framework of the minimal supersymmetric standard model assuming that the neutralino ( χ ̃ 0 1 ) is the lightest supersymmetric particle and is stable. No evidence for a signal is found and we derive cross-section upper limits as a function of stop ( t̃ ) and neutralino masses in different decay scenarios leading to the bℓν χ ̃ 0 1 final state. (Elsevier)
Excellent energy resolution is one of the primary advantages of electroluminescent high pressure xenon TPCs, and searches for rare physics events such as neutrinoless double-beta decay ...(\(\beta\beta0\nu\)) require precise energy measurements. Using the NEXT-White detector, developed by the NEXT (Neutrino Experiment with a Xenon TPC) collaboration, we show for the first time that an energy resolution of 1% FWHM can be achieved at 2.6 MeV, establishing the present technology as the one with the best energy resolution of all xenon detectors for \(\beta\beta0\nu\) searches.
In experiments searching for neutrinoless double-beta decay, the possibility of identifying the two emitted electrons is a powerful tool in rejecting background events and therefore improving the ...overall sensitivity of the experiment. In this paper we present the first measurement of the efficiency of a cut based on the different event signatures of double and single electron tracks, using the data of the NEXT-White detector, the first detector of the NEXT experiment operating underground. Using a \TO\ calibration source to produce signal-like and background-like events with energies near 1.6 MeV, a signal efficiency of \(71.6 \pm 1.5_{\textrm{ stat}} \pm 0.3_{\textrm{ sys}} \%\) for a background acceptance of \(20.6 \pm 0.4_{\textrm{ stat}} \pm 0.3_{\textrm{ sys}} \%\) is found, in good agreement with Monte Carlo simulations. An extrapolation to the energy region of the neutrinoless double beta decay by means of Monte Carlo simulations is also carried out, and the results obtained show an improvement in background rejection over those obtained at lower energies.
Natural radioactivity represents one of the main backgrounds in the search for neutrinoless double beta decay. Within the NEXT physics program, the radioactivity-induced backgrounds are measured with ...the NEXT-White detector. Data from 37.9 days of low-background operations at the Laboratorio Subterráneo de Canfranc with xenon depleted in \(^{136}\)Xe are analyzed to derive a total background rate of (0.84\(\pm\)0.02) mHz above 1000 keV. The comparison of data samples with and without the use of the radon abatement system demonstrates that the contribution of airborne-Rn is negligible. A radiogenic background model is built upon the extensive radiopurity screening campaign conducted by the NEXT Collaboration. A spectral fit to this model yields the specific contributions of \(^{60}\)Co, \(^{40}\)K, \(^{214}\)Bi and \(^{208}\)Tl to the total background rate, as well as their location in the detector volumes. The results are used to evaluate the impact of the radiogenic backgrounds in the double beta decay analyses, after the application of topological cuts that reduce the total rate to (0.25\(\pm\)0.01) mHz. Based on the best-fit background model, the NEXT-White median sensitivity to the two-neutrino double beta decay is found to be 3.5\(\sigma\) after 1 year of data taking. The background measurement in a Q\(_{\beta\beta}\pm\)100 keV energy window validates the best-fit background model also for the neutrinoless double beta decay search with NEXT-100. Only one event is found, while the model expectation is (0.75\(\pm\)0.12) events.
We report new measurements of the drift velocity and longitudinal diffusion coefficients of electrons in pure xenon gas and in xenon-helium gas mixtures at 1-9 bar and electric field strengths of ...50-300 V/cm. In pure xenon we find excellent agreement with world data at all \(E/P\), for both drift velocity and diffusion coefficients. However, a larger value of the longitudinal diffusion coefficient than theoretical predictions is found at low \(E/P\) in pure xenon, below the range of reduced fields usually probed by TPC experiments. A similar effect is observed in xenon-helium gas mixtures at somewhat larger \(E/P\). Drift velocities in xenon-helium mixtures are found to be theoretically well predicted. Although longitudinal diffusion in xenon-helium mixtures is found to be larger than anticipated, extrapolation based on the measured longitudinal diffusion coefficients suggest that the use of helium additives to reduce transverse diffusion in xenon gas remains a promising prospect.
This work investigates the capability of TMA ((CH3)3N) molecules to shift the wavelength of Xe VUV emission (160-188 nm) to a longer, more manageable, wavelength (260-350 nm). Light emitted from a Xe ...lamp was passed through a gas chamber filled with Xe-TMA mixtures at 800 Torr and detected with a photomultiplier tube. Using bandpass filters in the proper transmission ranges, no reemitted light was observed experimentally. Considering the detection limit of the experimental system, if reemission by TMA molecules occurs, it is below 0.3% of the scintillation absorbed in the 160-188 nm range. An absorption coefficient value for xenon VUV light by TMA of 0.43+/-0.03 cm-1.Torr-1 was also obtained. These results can be especially important for experiments considering TMA as a molecular additive to Xe in large volume optical time projection chambers.
Within the framework of xenon-based double beta decay experiments, we propose the possibility to improve the background rejection of an electroluminescent Time Projection Chamber (EL TPC) by reducing ...the diffusion of the drifting electrons while keeping nearly intact the energy resolution of a pure xenon EL TPC. Based on state-of-the-art microscopic simulations, a substantial addition of helium, around 10 or 15~\%, may reduce drastically the transverse diffusion down to 2.5~mm/\(\sqrt{\mathrm{m}}\) from the 10.5~mm/\(\sqrt{\mathrm{m}}\) of pure xenon. The longitudinal diffusion remains around 4~mm/\(\sqrt{\mathrm{m}}\). Light production studies have been performed as well. They show that the relative variation in energy resolution introduced by such a change does not exceed a few percent, which leaves the energy resolution practically unchanged. The technical caveats of using photomultipliers close to an helium atmosphere are also discussed in detail.
We present a search for large extra dimensions (ED) in p-pbar collisions at a center-of-mass energy of 1.8 TeV using data collected by the D0 detector at the Fermilab Tevatron in 1994-1996. Data ...corresponding to 78.8 +/- 3.9 pb^-1 are examined for events with large missing transverse energy, one high-p_T jet, and no isolated muons. With no excess beyond the background prediction from the standard model, we place limits on the fundamental Planck scale of 1 TeV (0.6 TeV) for 2 (7) ED.
The NEXT experiment aims at searching for the hypothetical neutrinoless double-beta decay from the \({}^{136}\)Xe isotope using a high-purity xenon TPC. Efficient discrimination of the events through ...pattern recognition of the topology of primary ionisation tracks is a major requirement for the experiment. However, it is limited by the diffusion of electrons. It is known that the addition of a small fraction of a molecular gas to xenon reduces electron diffusion. On the other hand, the electroluminescence (EL) yield drops and the achievable energy resolution may be compromised. We have studied the effect of adding several molecular gases to xenon (CO\({}_{2}\), CH\({}_{4}\) and CF\({}_{4}\)) on the EL yield and energy resolution obtained in a small prototype of driftless gas proportional scintillation counter. We have compared our results on the scintillation characteristics (EL yield and energy resolution) with a microscopic simulation, obtaining the diffusion coefficients in those conditions as well. Accordingly, electron diffusion may be reduced from about 10 mm/\(\sqrt{\mathrm{m}}\) for pure xenon down to 2.5 mm/\(\sqrt{\mathrm{m}}\) using additive concentrations of about 0.05%, 0.2% and 0.02% for CO\({}_{2}\), CH\({}_{4}\) and CF\({}_{4}\), respectively. Our results show that CF\({}_{4}\) admixtures present the highest EL yield in those conditions, but very poor energy resolution as a result of huge fluctuations observed in the EL formation. CH\({}_{4}\) presents the best energy resolution despite the EL yield being the lowest. The results obtained with xenon admixtures are extrapolated to the operational conditions of the NEXT-100 TPC. CO\({}_{2}\) and CH\({}_{4}\) show potential as molecular additives in a large xenon TPC, CH\({}_{4}\) showing the best performance and stability to be used in the NEXT-100 TPC, with an extrapolated energy resolution of 0.4% at 2.45 MeV for concentrations below 0.4%.
Using the D0 detector, we have observed events produced in pbar-p collisions that contain W or Z bosons in conjunction with very little energy deposition (``rapidity gaps'') in large forward regions ...of the detector. The fraction of W boson events with a rapidity gap (a signature for diffraction) is 0.89\pm^{0.19}_{0.17}%, and the probability that the non-diffractive background fluctuated to yield the observed diffractive signal is 3 x 10^{-14}, corresponding to a significance of 7.5 sigma. The Z boson sample has a gap fraction of 1.44 \pm^{0.61}_{0.52}%, with a significance of 4.4 sigma. The diffractive events have very similar properties to the more common non-diffractive component.
PDF
