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Category: particle physics – Page 105

More than 40 years since its discovery, the Z boson remains a cornerstone of particle physics research. Through its production alongside heavy-flavour quarks (bottom and charm quarks), the Z boson provides a unique window into the internal dynamics of a proton’s constituents. Specifically, it allows researchers to probe the heavy-flavour contributions to “Parton Distribution Functions” (PDFs), which describe how a proton’s momentum is distributed among its constituent quarks and gluons. Using the full LHC Run-2 dataset, the ATLAS Collaboration measured Z boson production in association with both bottom (b) and charm © quarks, the latter for the first time in ATLAS. In their new result, physicists studied Z boson decays into electron or muon pairs produced in association with “jets” of particles. They focused on jets arising from the hadronisation of b or c quarks, creating two jet “flavours”: b-jets and c-jets. Physicists developed a new multivariate algorithm that was able to identify the jet-flavour, allowing them to measure the production of both Z+b-jets and Z+c-jets processes. Researchers then took this one step further and applied a specialised fit procedure, called the ‘flavour-fit’, to determine the large background contribution due to Z production together with other flavour jets. This method is driven by data and allows a precise description of the jet flavours for every studied observable. This led to a significant improvement in the precision of the results, allowing a more stringent comparison with theoretical predictions. The Z boson provides a unique window into the internal dynamics of a proton’s constituents. So, what did they find? ATLAS researchers measured the production rates (or “cross sections”) of several physics observables. These results were then compared with theoretical predictions, probing various approaches to describe the quark distributions in protons, the most recent computational improvements in QCD calculations and the effect of different treatments of the quark masses in the predictions. For example, Figure 1a shows the differential cross section for Z+1 b-jet production as a function of the transverse momentum of the most energetic b-jet in the event. Results show that predictions treating the b-quarks as massless (blue squares and red triangles) provide the best agreement with measurements. Z+2 b-jets angular observables are in general well understood, while some discrepancies with data appear in the invariant mass of the 2 b-jets, whose spectrum is not well modelled by the studied predictions. Figure 1: Measured fiducial cross-section as a function of a) leading b-jet pT for Z+b-jets events and b) leading c-jet x_F (its momentum along the beam axis relative to the initial proton momentum) for Z+c-jets events. Data (black) are compared with several theoretical predictions testing different theoretical flavour schemes, high order accuracy calculations and intrinsic charm models. (Image: ATLAS Collaboration/CERN) Studying Z+c-jets production offered a unique possibility to investigate the hypothesis of intrinsic (valence-like) components of c-quarks in the proton. With this result, the ATLAS Collaboration contributes to the long-standing debate on the existence of this phenomenon, currently supported by experimental measurements from the LHCb Collaboration. As shown in Figure 1b, the Z+c-jets results were compared with several hypotheses for intrinsic charm content. Due to the larger experimental and theoretical uncertainty on Z+c-jets processes, the current result makes no strong statement on the intrinsic c-quark component in the proton. However, it does improve physicists’ sensitivity to this effect, as the new data will be used in future by PDF fitting groups to set tighter constraints on the intrinsic charm distribution in the proton. Overall, the new ATLAS result provides deep insights for refining theoretical predictions, thereby fostering a deeper understanding of the dynamics of heavy-flavour quark content in the proton. About the event display: Display of a candidate Z boson decaying to two muons alongside two b-jets, recorded by the ATLAS detector at a centre-of-mass collision energy of 13 TeV. Blue cones indicate the b-jets, and the red lines indicate the muon tracks. Starting from the centre of the ATLAS detector, the reconstructed tracks of the charged particles in the inner detector are shown as cyan lines. The energy deposits in the electromagnetic (the green layer) and hadronic (the red layer) calorimeters are shown as yellow boxes. The hits in the muon spectrometer (the outer blue layer) are shown as light blue blocks. (Image: ATLAS Collaboration/CERN) Learn more Measurements of the production cross-section for a Z boson in association with b-or c-jets in proton-proton collisions at 13 TeV with the ATLAS detector (arXiv:2403.15093, see figures) Measurements of the production cross-section for a boson in association with in proton–proton collisions at 13 TeV (JHEP 7 (2020) 44, arXiv:2003.11960) LHCb Collaboration, Study of Z Bosons Produced in Association with Charm in the Forward Region (Phys. Rev. Lett. 128 (2022) 82,001, arXiv:2109.08084) See also the full list of ATLAS physics results.

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For centuries, goldsmiths have sought ways to flatten gold into ever finer forms. An approach based in modern chemistry has finally created a gold material that literally can’t get any thinner, consisting of a single layer of atoms.

Sticking to the naming conventions of materials science, researchers have named this new two-dimensional material ‘goldene’, and it has some interesting properties not seen in the three-dimensional form of gold.

“If you make a material extremely thin, something extraordinary happens – as with graphene,” explains materials scientist Shun Kashiwaya of Linköping University in Sweden.

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An international research team led by the University of Göttingen has demonstrated experimentally that electrons in naturally occurring double-layer graphene move like particles without any mass, in the same way that light travels.

Furthermore, they have shown that the current can be “switched” on and off, which has potential for developing tiny, energy-efficient transistors – like the light switch in your house but at a nanoscale.

The Massachusetts Institute of Technology (MIT), USA, and the National Institute for Materials Science (NIMS), Japan, were also involved in the research. The results were published in Nature Communications (“Probing the tunable multi-cone band structure in Bernal bilayer graphene”).

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Physicists have just found something no one expected, lurking on the surface of an arsenic crystal.

While undertaking a study of quantum topology – the wave-like behavior of particles combined with the mathematics of geometry – a team found a strange hybrid of two quantum states, each describing a different means of current.

“This finding was completely unexpected,” says physicist M. Zahid Hasan of Princeton University. “Nobody predicted it in theory before its observation.”

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If the effect is confirmed to be happening over WASP-76b, it could reveal a great deal about this strange and extreme exoplanet — a world unlike anything seen in our stellar domain.

Related: Ultra-hot exoplanet has an atmosphere of vaporized rock

“There’s a reason no glory has been seen before outside our Solar System – it requires very peculiar conditions,” Olivier Demangeon, team leader and an astronomer at the Institute of Astrophysics and Space Sciences in Portugal, said in a statement. “First, you need atmospheric particles that are close-to-perfectly spherical, completely uniform and stable enough to be observed over a long time. The planet’s nearby star needs to shine directly at it, with the observer — here CHEOPS — at just the right orientation.”

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Silicon-based electronics are approaching their physical limitations and new materials are needed to keep up with current technological demands. Two-dimensional (2D) materials have a rich array of properties, including superconductivity and magnetism, and are promising candidates for use in electronic systems, such as transistors. However, precisely controlling the properties of these materials is extraordinarily difficult.

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A numerical investigation has revealed a surprising correspondence between a lattice spin model and a quantum field theory.

The search for a quantum spin liquid (QSL) in a real magnetic material has been at the forefront of condensed-matter physics since this exotic quantum state was first proposed over half a century ago. Meanwhile, theorists continue to grapple with understanding what rich physics might emerge from this state. Now Alexander Wietek of the Max Planck Institute for the Physics of Complex Systems in Germany and his collaborators have made a significant advance toward that goal. Through numerical simulations, they have presented a compelling numerical case that the spectrum of elementary excitations of a well-studied QSL has a one-to-one correspondence with the spectrum of excitations of a well-studied quantum field theory [1]. If a real QSL is discovered or fabricated, the correspondence opens the prospect of testing theories from particle physics with condensed-matter systems.

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Peter Higgs, pivotal in the discovery of the “God Particle,” has died at the age of 94. His groundbreaking work, for which he received a Nobel Prize, laid the foundation for understanding the universe’s fundamental structure and continues to guide current and future research in particle physics.

Peter Higgs has passed away at the age of 94. An iconic figure in modern science, Higgs in 1964 postulated the existence of the eponymous Higgs boson. Its discovery at CERN in 2012 was the crowning achievement of the Standard Model ℠ of particle physics – a remarkable theory that explains the visible universe at the most fundamental level.

Alongside Robert Brout and François Englert, and building on the work of a generation of physicists, Higgs postulated the existence of the Brout-Englert-Higgs (BEH) field. Alone among known fundamental fields, the BEH field is “turned on” throughout the universe, rather than flickering in and out of existence and remaining localized. Its existence allowed matter to form in the early universe some 10-11 s after the Big Bang, thanks to the interactions between elementary particles (such as electrons and quarks) and the ever-present BEH field. Higgs and Englert were awarded the Nobel Prize for physics in 2013 in recognition of these achievements.

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