Observation of quantum entanglement with top quarks at the ATLAS detector

Entanglement is a key feature of quantum mechanics with applications in fields such as metrology, cryptography, quantum information and quantum computation. It has been observed in a wide variety of systems and length scales, ranging from the microscopic to the macroscopic. However, entanglement remains largely unexplored at the highest accessible energy scales. Here we report the highest-energy observation of entanglement, in top–antitop quark events produced at the Large Hadron Collider, using a proton–proton collision dataset with a centre-of-mass energy of √s = 13 TeV and an integrated luminosity of 140 inverse femtobarns (fb)−1 recorded with the ATLAS experiment. Spin entanglement is detected from the measurement of a single observable D, inferred from the angle between the charged leptons in their parent top- and antitop-quark rest frames. The observable is measured in a narrow interval around the top–antitop quark production threshold, at which the entanglement detection is expected to be significant. It is reported in a fiducial phase space defined with stable particles to minimize the uncertainties that stem from the limitations of the Monte Carlo event generators and the parton shower model in modelling top-quark pair production. The entanglement marker is measured to be D = −0.537 ± 0.002 (stat.) ± 0.019 (syst.) for 340 GeV < mtt < 380 GeV. The observed result is more than five standard deviations from a scenario without entanglement and hence constitutes the first observation of entanglement in a pair of quarks and the highest-energy observation of entanglement so far

Neutral Bremsstrahlung Emission in Xenon Unveiled

POPULAR SUMMARY When ionizing radiation interacts with xenon, copious amounts of ultraviolet light are emitted at particular wavelengths—an “electroluminescence” that is leveraged in dark matter searches and neutrino detectors. But researchers were not aware of the presence of another, fainter, light emission over a broader wavelength range, from the ultraviolet to the near infrared. Therefore, scientists explained the corresponding light pulses as being due to impurities in the gas. Here, we show that these pulses are instead signals of a new kind of light emitted in xenon, caused by the scattering of electrons onto neutral atoms. Using studies in a small laboratory system expressly conceived for this purpose, we identify this light—dubbed neutral bremsstrahlung—in the large detector of the NEXT experiment, an underground particle detector in Spain. Given the smallness of our detector, its xenon purity is very well controlled. In addition, we can precisely isolate the scintillation emission from a specific region of the detector and study this emission under very well controlled conditions, both when electroluminescence can and cannot occur. This allows us to observe and study scintillation emission other than electroluminescence. Simultaneously, we implement a robust theoretical model for the neutral bremsstrahlung, which describes the experimental data very well and allows us to unambiguously assign the observed scintillation mechanism to neutral bremsstrahlung. Now, scientists know that discovering dark matter and observing neutrinos requires more than just making the xenon pure inside of large detector systems. Researchers should also separate the light due to neutral bremsstrahlung to optimize the design of the detectors and to improve their sensitivity.The NEXT Collaboration acknowledges support from the following agencies and institutions: the European Research Council (ERC) under Advanced Grant No. 339787-NEXT; the European Unions Framework Program for Research and Innovation Horizon 2020 (20142020) under Grant Agreements No. 674896 No. 690575, and No. 740055; the Ministerio de Economa y Competitividad and the Ministerio de Ciencia, Innovacin y Universidades of Spain under Grants No. FIS2014- 53371-C04 and No. RTI2018-095979, the Severo Ochoa Program Grants No. SEV-2014-0398 and No. CEX2018- 000867-S, and the Mara de Maeztu Program MDM-2016-0692; the Generalitat Valenciana under Grants No. PROMETEO/2016/120 and No. SEJI/2017/011; the Portuguese FCT under Project No. PTDC/FIS-NUC/3933/2021 and under Project No. UIDP/04559/2020 to fund the activities of LIBPhys-UC; the U.S. Department of Energy under Contracts No. DE-AC02-06CH11357 (Argonne National Laboratory), No. DE-AC02-07CH11359 (Fermi National Accelerator Laboratory), No. DE-FG02-13ER42020 (Texas A&M), and No. DE-SC0019223/DE-SC0019054 (University of Texas at Arlington); and the University of Texas at Arlington (USA). D. G.-D. acknowledges Ramón y Cajal program (Spain) under Contract No. RYC- 2015- 18820. J. M.-A. acknowledges support from Fundacin Bancaria la Caixa (ID 100010434), Grant No. LCF/BQ/ PI19/11690012. We would like to thank Lorenzo Muñiz for insightful discussions on the subtleties of electron transport in gase