modern physics (13)

Weighty W...

10336914854?profile=RESIZE_710x

Living on: data taken by the now-defunct CDF experiment has revealed a surprising mass for the W boson. (Courtesy: Fermilab)

Topics: Fermilab, High Energy Physics, Modern Physics, Particle Physics, Steven Weinberg

The most precise measurement to date of the mass of the W boson has yielded a result seven standard deviations away from that predicted by the Standard Model of particle physics. The stunning result was obtained by a painstaking analysis of data taken at the Fermilab Tevatron collider in the US before it closed in 2011. The particle physics community must now study the results carefully to work out whether it is an incredible statistical fluke, an unknown experimental error, a flaw in the Standard Model, or a genuine indication of physics beyond the Standard Model.

The W boson is one of the most intriguing particles described by the Standard Model. Together with the neutral Z boson, the charged W boson mediates the weak interaction, which causes beta decay and several other important processes in particle physics. The weak interaction has long intrigued scientists searching for physics beyond the Standard Model, partly because it is the only force known to violate charge-parity symmetry. If particles in a process are exchanged for their antiparticles and the spatial coordinates are inverted, the weak interaction in this mirror image process is not always identical. This puzzle is not explained in the Standard Model.

W boson mass measurement surprises physicists, Tim Wogan, Physics World

Read more…

Aharonov-Bohm Effect

10048335471?profile=RESIZE_710x

A quantum probe for gravity: Physicists have detected a tiny phase shift in atomic wave packets due to gravity-induced relativistic time dilation – an example of the Aharonov-Bohm effect in action. (Courtesy: Shutterstock/Evgenia Fux)

Topics: General Relativity, Gravity, Modern Physics, Quantum Mechanics

The idea that particles can feel the influence of potentials even without being exposed to a force field may seem counterintuitive, but it has long been accepted in physics thanks to experimental demonstrations involving electromagnetic interactions. Now physicists in the US have shown that this so-called Aharonov-Bohm effect also holds true for a much weaker force: gravity. The physicists based their conclusion on the behavior of freefalling atomic wave packets, and they say the result suggests a new way of measuring Newton’s gravitational constant with far greater precision than was previously possible.

Yakir Aharonov and David Bohm proposed the effect that now bears their name in 1959, arguing that while classical potentials have no physical reality apart from the fields they represent, the same is not true in the quantum world. To make their case, the pair proposed a thought experiment in which an electron beam in a superposition of two wave packets is exposed to a time-varying electrical potential (but no field) when passing through a pair of metal tubes. They argued that the potential would introduce a phase difference between the wave packets and therefore lead to a measurable physical effect – a set of interference fringes – when the wave packets are recombined.

Seeking a gravitational counterpart

In the latest research, Mark Kasevich and colleagues at Stanford University show that the same effect also holds true for gravity. The platform for their experiment is an atom interferometer, which uses a series of laser pulses to split, guide and recombine atomic wave packets. The interference from these wave packets then reveals any change in the relative phase experienced along the two arms.

Physicists detect an Aharonov-Bohm effect for gravity, Edwin Cartlidge, Physics World

Read more…

DUNE...

10045683300?profile=RESIZE_584x

Image Source: Fermilab, and link below

Topics: Fermilab, High Energy Physics, Modern Physics, Neutrinos, Particle Physics

Solving big mysteries

The Deep Underground Neutrino Experiment is an international flagship experiment to unlock the mysteries of neutrinos. DUNE will be installed in the Long-Baseline Neutrino Facility, under construction in the United States. DUNE scientists will paint a clearer picture of the universe and how it works. Their research may even give us the key to understanding why we live in a matter-dominated universe — in other words, why we are here at all.

DUNE will pursue three major science goals: find out whether neutrinos could be the reason the universe is made of matter; look for subatomic phenomena that could help realize Einstein’s dream of the unification of forces; and watch for neutrinos emerging from an exploding star, perhaps witnessing the birth of a neutron star or a black hole.

DUNE at LBNF, Fermilab

Read more…

Tardigrades and Qubits...

9936590894?profile=RESIZE_584x

(Credit: Giovanni Cancemi/Shutterstock) 

Topics: Biology, Condensed Matter Physics, Modern Physics, Quantum Mechanics

Note: After presenting my research proposal and acceptance by my committee, I've been taking a well-needed break from blogging. I'll post on and off until the New Year, which isn't too far off. Happy holidays!

In recent years, evidence has emerged that quantum physics seems to play a role in some of life’s fundamental processes. But just how it might do this is something of a mystery.

On the one hand, quantum phenomena are generally so delicate that they can only be observed when all other influences are damped – in other words in carefully controlled systems at temperatures close to absolute zero. By contrast, the conditions for life are generally complex, warm, and damp. Understanding this seemingly contradictory state of affairs is an important goal.

So physicists and biologists are keen to explore the boundaries of these very different regimes—life and quantum mechanics—to better understand where they might overlap.

Now Rainer Dumke at the Nanyang Technological University in Singapore and colleagues have created an exotic quantum state called entanglement using a superconducting qubit and a microscopic animal called a tardigrade. Along the way, the team has created the most extreme form of suspended animation ever recorded. “The tardigrade itself is shown to be entangled with the remaining subsystems,” they say.

To perform their entanglement experiment, Dumke and co cooled their tardigrade to below 10 millikelvins, almost to absolute zero, while reducing the pressure to a millionth of that in the atmosphere. In these conditions, no chemical reaction can occur so the tardigrade’s metabolism must have entirely halted stopped and the processes of life halted.

“This is to date the most extreme exposure to low temperatures and pressures that a tardigrade has been recorded to survive, clearly demonstrating that the state of cryptobiosis ultimately involves a suspension of all metabolic processes given that all chemical reactions would be prohibited with all its constituent molecules cooled to their ground states,” say the researchers.

In this condition, the tardigrade can be thought of as a purely dielectric element. Indeed, the researchers simulated their experiment by treating the tardigrade as a dielectric cube.

The experimental setup consisted of two superconducting capacitors, which when cooled can exist in a superposition of states called a qubit. They placed the tardigrade between the capacitor plates of one qubit so that it became an integral part of the capacitor. The team was then able to measure the effect of the tardigrade on the qubit’s properties.

How a Tardigrade "Micro Animal" Became Quantum Entangled with Superconducting Qubit, The Physics AriXiv Blog, Discovery Magazine

Read more…

Deux Ex Machina...

9525710099?profile=RESIZE_584x

Quasiparticles in motion: illustration of ghost polaritons in a calcite crystal being “launched” to record distances by a gold microdisk. (Courtesy: HUST)

Topics: Condensed Matter Physics, Modern Physics, Quantum Mechanics

The existence of ghost hyperbolic surface polaritons has been demonstrated by an international collaboration including researchers in China and the US. Based at Huazhong University of Science and Technology (HUST), National University of Singapore (NUS), National Center for Nanoscience and Technology (NCNST), and the City University of New York (CUNY), the team showed that the polariton – a hybrid light-matter quasiparticle – has a record-breaking propagation distance of three times its photon wavelength. This ghost polariton is an exciting discovery that has applications in sub-wavelength, low-loss imaging, sensing, and information transfer. The full study is described in Nature.

Previously, hyperbolic polaritons, which arise from the strong coupling of electromagnetic radiation to lattice vibrations (phonons) in anisotropic crystals, had only been observed in two forms: bulk polaritons and surface polaritons. Bulk, volume-confined, hyperbolic polaritons (v-HPs) have a real out-of-plane wavevector and hence can propagate within the material supporting them. Surface-confined hyperbolic polaritons (s-HPs), however, have an entirely imaginary out-of-plane wavevector, and so decay exponentially away from the crystal surface, a property called evanescence. The hyperbolic dispersion of these polaritons is the result of the crystal’s dielectric anisotropy, which results in hyperbolic isofrequency contours in k-space (momentum space) and concave wavefronts in real space.

Most studies on v-HPs and s-HPs have been performed in thin layers of van der Waals crystals. These crystals comprise stacks of covalently bound 2D layers that are held together by weak van der Waals forces. However, in such crystal layers, there is no control over the optical axis. This is the direction in which propagating light experiences no birefringence and it is typically aligned with the layers.

Ghost surface polaritons seen for the first time, Kirsty McGhee, Physics World

Read more…

Double Slit...

9519837676?profile=RESIZE_710x

Complementarity A new twist on the double-slit experiment. (Courtesy: Shutterstock/Andrey VP)

Topics: Modern Physics, Quantum Mechanics, Theoretical Physics

One of the most counterintuitive concepts in physics – the idea that quantum objects are complementary, behaving like waves in some situations and like particles in others – just got a new and more quantitative foundation. In a twist on the classic double-slit experiment, scientists at Korea’s Institute for Basic Sciences (IBS) used precisely controlled photon sources to measure a photon’s degree of wave-ness and particle-ness. Their results, published in Science Advances, show that the properties of the photon’s source influence its wave and particle character – a discovery that complicates and challenges the common understanding of complementarity.

The double-slit experiment is the archetypal example of complementarity at work. When a single photon encounters a barrier with two thin openings, it produces an interference pattern on a screen placed behind the openings – but only if the photon’s path is not observed. This interference pattern identifies the photon as a wave since a particle would create only one point of light on the screen. However, if detectors are placed at the openings to determine which slit the photon went through, the interference pattern disappears, and the photon behaves like a particle. The principle of complementarity states that both experimental outcomes are needed to fully understand the photon’s quantum nature.

Wave-particle duality quantified for the first time, Karmela Padavic-Callaghan, Physics World

Read more…

Dielectric Laser Accelerators...

9515584883?profile=RESIZE_584x

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); https://doi.org/10.1063/PT.3.4815

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

Read more…

Nearing Ignition...

9438571460?profile=RESIZE_584x

An artist’s rendering shows how the National Ignition Facility’s 192 beams enter an eraser-size cylinder of gold and heat it from the inside to produce x-rays, which then implode the fuel capsule at its center to create fusion.

LAWRENCE LIVERMORE NATIONAL LABORATORY

Topics: Energy, Environment, Modern Physics, Nuclear Fusion, Nuclear Power

More than a decade ago, the world’s most energetic laser started to unleash its blasts on tiny capsules of hydrogen isotopes, with managers promising it would soon demonstrate a route to limitless fusion energy. Now, the National Ignition Facility (NIF) has taken a major leap toward that goal. Last week, a single laser shot sparked a fusion explosion from a peppercorn-size fuel capsule that produced eight times more energy than the facility had ever achieved: 1.35 megajoules (MJ)—roughly the kinetic energy of a car traveling at 160 kilometers per hour. That was also 70% of the energy of the laser pulse that triggered it, making it tantalizingly close to “ignition”: a fusion shot producing an excess of energy.

 “After many years at 3% of ignition, this is super exciting,” says Mark Herrmann, head of the fusion program at Lawrence Livermore National Laboratory, which operates NIF.

NIF’s latest shot “proves that a small amount of energy, imploding a small amount of mass, can get fusion. It’s a wonderful result for the field,” says physicist Michael Campbell, director of the Laboratory for Laser Energetics (LLE) at the University of Rochester.

“It’s a remarkable achievement,” adds plasma physicist Steven Rose, co-director of the Centre for Inertial Fusion Studies at Imperial College London. “It’s made me feel very cheerful. … It feels like a breakthrough.”

And it is none too soon, as years of slow progress have raised questions about whether laser-powered fusion has a practical future. Now, according to LLE Chief Scientist Riccardo Betti, researchers need to ask: “What is the maximum fusion yield you can get out of NIF? That’s the real question.”

Fusion, which powers stars, forces small atomic nuclei to meld together into larger ones, releasing large amounts of energy. Extremely hard to achieve on Earth because of the heat and pressure required to join nuclei, fusion continues to attract scientific and commercial interest because it promises copious energy, with little environmental impact.

With explosive new result, laser-powered fusion effort nears ‘ignition’, Daniel Clery, Science Magazine

Read more…

Muon g-2...

8782485870?profile=RESIZE_400x

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

 

Read more…

Clocking Dark Matter...

8195804064?profile=RESIZE_710x

Clocking dark matter: optical clocks join the hunt for dark matter. (Courtesy: N Hanacek/NIST)

Topics: Dark Matter, Modern Physics, Quantum Mechanics

An optical clock has been used to set new constraints on a proposed theory of dark matter. Researchers including Jun Ye at JILA at the University of Colorado, Boulder, and Andrei Derevianko at the University of Nevada, Reno, explored how the coupling between regular matter and “ultralight” dark matter particles could be detected using the clock in conjunction with an ultra-stable optical cavity. With future upgrades to the performance of optical clocks, their approach could become an important tool in the search for dark matter.

Although it appears to account for about 85% of the matter in the universe, physicists know very little about dark matter. Most theoretical and experimental work so far has been focussed on hypothetical dark-matter particles, including WIMPS and axions, which have relatively large masses.  Alternatively, some physicists have proposed the existence of “ultralight” dark matter particles with extremely small masses that span many orders of magnitude (10−16–10−21 eV/c2).

According to the laws of quantum mechanics, the very smallest of these particles would have huge wavelengths, comparable to the sizes of entire dwarf galaxies – meaning they would behave like classical fields on scales we can easily measure.

Optical clock sets new constraints on dark matter, Sam Jarman, Physics World

Read more…

Diamond Nanoneedles...

8184316268?profile=RESIZE_710x

Credit: Z. Shi et al., Proc. Natl. Acad. Sci. USA 117, 24634 (2020)

Topics: Materials Science, Modern Physics, Nanotechnology, Semiconductor Technology

If you ever manage to deform a diamond, you’re likely to break it. That’s because the hardest natural material on Earth is also inelastic and brittle. Two years ago, Ming Dao (MIT), Subra Suresh (Nanyang Technological University in Singapore), and their collaborators demonstrated that when bulk diamonds are etched into fine, 300-nm-wide needles, they become nearly defect-free. The transformation allows diamonds to elastically bend under the pressure of an indenter tip, as shown in the figure, and withstand extremely large tensile stresses without breaking.

The achievement prompted the researchers to investigate whether the simple process of bending could controllably and reversibly alter the electronic structure of nanocrystal diamond. Teaming up with Ju Li and graduate student Zhe Shi (both at MIT), Dao and Suresh have now followed their earlier study with numerical simulations of the reversible deformation. The team used advanced deep-learning algorithms that reveal the bandgap distributions in nanosized diamond across a range of loading conditions and crystal geometries. The new work confirms that the elastic strain can alter the material’s carbon-bonding configuration enough to close its bandgap from a normally 5.6 eV width as an electrical insulator to 0 eV as a conducting metal. That metallization occurred on the compression side of a bent diamond nanoneedle.

Diamond nanoneedles turn metallic, R. Mark Wilson, Physics Today

Read more…

Right-Handed Photons...

8181973673?profile=RESIZE_710x

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

Read more…

Schrödinger’s Clock...

8074165057?profile=RESIZE_710x

Credit: Getty Images

Topics: Modern Physics, Quantum Mechanics, Theoretical Physics

Albert Einstein’s twin paradox is one of the most famous thought experiments in physics. It postulates that if you send one of two twins on a return trip to a star at near light speed, they will be younger than their identical sibling when they return home. The age difference is a consequence of something called time dilation, which is described by Einstein’s special theory of relativity: the faster you travel, the slower time appears to pass.

But what if we introduce quantum theory into the problem? Physicists Alexander Smith of Saint Anselm College and Dartmouth College and Mehdi Ahmadi of Santa Clara University tackle this idea in a study published today in the journal Nature Communications. The scientists imagine measuring a quantum atomic clock experiencing two different times while it is placed in superposition—a quirk of quantum mechanics in which something appears to exist in two places at once. “We know from Einstein’s special theory of relativity that when a clock moves relative to another clock, the time shown on it slows down,” Smith says. “But quantum mechanics allows you to start thinking about what happens if this clock were to move in a superposition of two different speeds.”

Superposition is a strange aspect of quantum physics where an object can initially be in multiple locations simultaneously, yet when it is observed, only one of those states becomes true. Particles can be placed in superposition in certain experiments, such as those using a beam splitter to divide photons of light, to show the phenomenon in action. Both of the particles in superposition appear to share information until they are observed, making the phenomenon useful for applications such as encryption and quantum communications.

Quantum Time Twist Offers a Way to Create Schrödinger’s Clock, Jonathan O'Callaghan, Scientific American

Read more…