particle physics (6)

Dielectric Laser Accelerators...


Figure 1. The size contrast between conventional accelerator facilities and chip-based accelerators is dramatic. (a) The Next Linear Collider Test Accelerator facility at SLAC was used for early laser-acceleration experiments in 2012–15. (Image courtesy of the Archives and History Office/SLAC National Accelerator Laboratory.) (b) The first dielectric laser accelerator chips demonstrated at SLAC were made of fused silica and were each the size of a grain of rice. (Image courtesy of Christopher Smith/SLAC National Accelerator Laboratory.)

Topics: Applied Physics, Modern Physics, Particle Physics

Physics Today 74, 8, 42 (2021);

Particle accelerators are among the most important scientific tools of the modern age. Major accelerator facilities, such as the 27-km-circumference Large Hadron Collider in Switzerland, where the Higgs boson was recently discovered, allow scientists to uncover fundamental properties of matter and energy. But the particle energies needed to explore new regimes of physics have increased to the TeV scale and beyond, and accelerator facilities based on conventional technologies are becoming prohibitively large and costly. Even lower-energy, smaller-scale accelerators used in medicine and industry are often cumbersome devices; they can weigh several tons and cost millions of dollars.

Efforts are consequently underway to develop more compact, less expensive accelerator technologies. One approach, a dielectric laser accelerator (DLA), uses an ultrafast IR laser to deliver energy to electrons inside a microchip-scale device. Efficient, ultrafast solid-state lasers and semiconductor fabrication methods developed over the past two decades have enabled a new breed of photonic devices that can sustain accelerating fields one to two orders of magnitude larger than conventional microwave-cavity accelerators.

The approach has the potential to dramatically shrink particle accelerators, thereby enabling ultrafast tabletop electron diffraction and microscopy experiments and tunable x-ray sources. An international effort is now underway to develop a laser-driven accelerator integrated on a silicon photonics platform: an “accelerator on a chip.”

Microchip accelerators, Joel England, Peter Hommelhoff, Robert L. Byer, Physics Today

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Image Source: Link below

Topics: Particle Physics, Quantum Computer, Quantum Mechanics, Theoretical Physics

Flatland: “The book used the fictional two-dimensional world of Flatland to comment on the hierarchy of Victorian culture, but the novella’s more enduring contribution is its examination of dimensions.” Source: Wikipedia

After decades of exploration in nature’s smallest domains, physicists have finally found evidence that anyons exist. First predicted by theorists in the early 1980s, these particle-like objects only arise in realms confined to two dimensions, and then only under certain circumstances — like at temperatures near absolute zero and in the presence of a strong magnetic field.

Physicists are excited about anyons not only because their discovery confirms decades of theoretical work, but also for practical reasons. For example, Anyons are at the heart of an effort by Microsoft to build a working quantum computer.

This year brought two solid confirmations of the quasiparticles. The first arrived in April, in a paper featured on the cover of Science, from a group of researchers at the École Normale Supérieure in Paris. Using an approach proposed four years ago, physicists sent an electron gas through a teeny-tiny particle collider to tease out weird behaviors — especially fractional electric charges — that only arise if anyons are around. The second confirmation came in July when a group at Purdue University in Indiana used an experimental setup on an etched chip that screened out interactions that might obscure anyon behavior.

MIT physicist Frank Wilczek, who predicted and named anyons in the early 1980s, credits the first paper as the discovery but says the second lets the quasiparticles shine. “It’s gorgeous work that makes the field blossom,” he says. Anyons aren’t like ordinary elementary particles; scientists will never be able to isolate one from the system where it forms. They’re quasiparticles, which means they have measurable properties like a particle — such as a location, maybe even a mass — but they’re only observable as a result of the collective behavior of other, conventional particles. (Think of the intricate geometric shapes made by group behavior in nature, such as flocks of birds flying in formation or schools of fish swimming as one.)

The known universe contains only two varieties of elementary particles. One is the family of fermions, which includes electrons, as well as protons, neutrons, and the quarks that form them. Fermions keep to themselves: No two can exist in the same quantum state at the same time. If these particles didn’t have this property, all matter could simply collapse to a single point. It’s because of fermions that solid matter exists.

The rest of the particles in the universe are bosons, a group that includes particles like photons (the messengers of light and radiation) and gluons (which “glue” quarks together). Unlike fermions, two or more bosons can exist in the same state at the same time. They tend to clump together. It’s because of this clumping that we have lasers, which are streams of photons all occupying the same quantum state.

Physicists prove the existence of two-dimensional particles called 'anyons', Stephen Omes, Astronomy (December 2020)

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Collider Neutrinos...


New territory Two candidate collider-neutrino events from the FASERν pilot detector in the plane longitudinal to (top) and transverse to (bottom) the beam direction. The different lines in each event show charged-particle tracks originating from the neutrino interaction point. Credit: FASER Collaboration.

Topics: CERN, High Energy Physics, Particle Physics, Research

Think “neutrino detector” and images of giant installations come to mind, necessary to compensate for the vanishingly small interaction probability of neutrinos with matter. The extreme luminosity of proton-proton collisions at the LHC, however, produces a large neutrino flux in the forward direction, with energies leading to cross-sections high enough for neutrinos to be detected using a much more compact apparatus.

In March, the CERN research board approved the Scattering and Neutrino Detector (SND@LHC) for installation in an unused tunnel that links the LHC to the SPS, 480 m downstream from the ATLAS experiment. Designed to detect neutrinos produced in a hitherto unexplored pseudo-rapidity range (7.2 < 𝜂 < 8.6), the experiment will complement and extend the physics reach of the other LHC experiments — in particular FASERν, which was approved last year. Construction of FASERν, which is located in an unused service tunnel on the opposite side of ATLAS along the LHC beamline (covering |𝜂|>9.1), was completed in March, while installation of SND@LHC is about to begin.

Both experiments will be able to detect neutrinos of all types, with SND@LHC positioned off the beamline to detect neutrinos produced at slightly larger angles. Expected to commence data-taking during LHC Run 3 in spring 2022, these latest additions to the LHC experiment family are poised to make the first observations of collider neutrinos while opening new searches for feebly interacting particles and other new physics.

Collider neutrinos on the horizon, Matthew Chalmers, CERN Courier

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Illustration of the FASER experiment. Image Credit: FASER/CERN.

Topics: CERN, Dark Matter, High Energy Physics, Neutrinos, Particle Physics

Neutrinos are ubiquitous and notorious. Billions are passing through you at this moment. Occasionally described as a “ghost of a particle,” neutrinos are nearly massless, thereby making them extremely difficult to detect experimentally (“Neutrino,” meaning “little neutral one” in Italian, was first used by Enrico Fermi in the early 1930s). Neutrinos were first confirmed in 1956 (thanks to a nearby nuclear reactor), and they’ve since been detected from different sources, including the Sun and cosmic rays, but not yet in a particle collider. Their elusiveness has been the source of much intrigue (and, of course, research funding) within the particle physics community since.

What else makes them so curious? Neutrinos come in three flavors — electron neutrino, muon neutrino, and tau neutrino — and may switch between them through the process of oscillation. Neutrino oscillations have been experimentally confirmed only in the past decade at the Super-K Detector in Japan (physicists Takaaki Kajita and Arthur B. McDonald shared the 2015 Nobel Prize in Physics for it). This discovery signified an important direction in the search for physics beyond the Standard Model because the longstanding theory does not explain neutrino oscillations and describes them as completely massless particles. Something isn’t quite adding up.

Enter: FASER. Initially proposed in 2018, the ForwArd Search ExpeRiment (FASER) is CERN’s newest experiment poised to detect neutrinos, potentially up to 1300 electron neutrinos, 20,000 muon neutrinos, and 20 tau neutrinos. Constructed in an unused service tunnel located about 500 meters from an Atlas experiment interaction point, FASER and its corresponding sub-detector, FASERν, have been designed to probe interactions of high-energy neutrinos (predicted to be between 600 GeV and 1 TeV).

FASER Poised to Further Our Understanding of Neutrinos, Dark Matter, Hannah Pell, Physics Central Buzz Blog

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Muon g-2...


Feynman QED Diagram: Fermilab

Topics: Modern Physics, Particle Physics, Quantum Mechanics

Solving a mystery

More than 200 scientists from around the world are collaborating with Fermilab on the Muon g-2 physics experiment which probes fundamental properties of matter and space. Muon g-2 (pronounced gee minus two) allows researchers to peer into the subatomic world to search for undiscovered particles that may be hiding in the vacuum.

Residing at Fermilab's Muon Campus, the experiment uses the Fermilab accelerator complex to produce an intense beam of muons traveling at nearly the speed of light. Scientists will use the beam to precisely determine the value of a property known as the g-2 of the muon.

The muon, like its lighter sibling the electron, acts like a spinning magnet. The parameter known as "g" indicates how strong the magnet is and the rate of its gyration. The value of the muon's g is slightly larger than 2. This difference from 2 is caused by the presence of virtual particles that appear from the quantum vacuum and then quickly disappear into it again.

In measuring g-2 with high precision and comparing its value to the theoretical prediction, physicists aim to discover whether the experiment agrees with the theory. Any deviation would point to as yet undiscovered subatomic particles that exist in nature.

An experiment that concluded in 2001 at Brookhaven National Laboratory found a tantalizing 3.7 sigma (standard deviation) discrepancy between the theoretical calculation and the measurement of the muon g-2. With a four-fold increase in the measurement's precision, Muon g-2 will be more sensitive to virtual or hidden particles and forces than any previous experiment of its kind and can bring this discrepancy to the 5 sigma discovery level.

The centerpiece of the Muon g-2 experiment at Fermilab is a large, 50-foot-diameter superconducting muon storage ring. This one-of-a-kind ring, made of steel, aluminum, and superconducting wire, was built for the previous g-2 experiment at Brookhaven. The ring was moved from Brookhaven to Fermilab in 2013. Making use of Fermilab's intense particle beams, scientists will be able to significantly increase the science output of this unique instrument. The experiment started taking data in 2018.

U.S. Department of Energy - Fermilab: Muon g - 2


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Right-Handed Photons...


Topics: Modern Physics, Particle Physics, Quantum Mechanics, Quarks

Note: A primer on quarks at Hyperphysics</a>

On 17 January 1957, a few months after Chien-Shiung Wu’s discovery of parity violation, Wolfgang Pauli wrote to Victor Weisskopf: “Ich glaube aber nicht, daß der Herrgott ein schwacher Linkshänder ist” (I cannot believe that God is a weak left-hander). But maximal parity violation is now well established within the Standard Model (SM). The weak interaction only couples to left-handed particles, as dramatically seen in the continuing absence of experimental evidence for right-handed neutrinos. In the same way, the polarisation of photons originating from transitions that involve weak interaction is expected to be completely left-handed.

The LHCb collaboration recently tested the handedness of photons emitted in rare flavor-changing transitions from a b-quark to an s-quark. These are mediated by the bosons of the weak interaction according to the SM – but what if new virtual particles contribute too? Their presence could be clearly signaled by a right-handed contribution to the photon polarization.

In pursuit of right-handed photons, A report from the LHCb experiment, CERN Courier

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