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Subatomic Particle Seen Changing to Antiparticle and Back For The First Time (Physics)

Research used a novel technique originally developed at the University of Warwick

A team of physicists, including the University of Warwick, have proved that a subatomic particle can switch into its antiparticle alter-ego and back again, in a new discovery revealed today.

An extraordinarily precise measurement made by UK researchers using the LHCb experiment at CERN has provided the first evidence that charm mesons can change into their antiparticle and back again.

For more than 10 years, scientists have known that charm mesons, subatomic particles that contain a quark and an antiquark, can travel as a mixture of their particle and antiparticle states, a phenomenon called mixing. However, this new result shows for the first time that they can oscillate between the two states.

Armed with this new evidence, scientists can try to tackle some of the biggest questions in physics around how particles behave outside of the Standard Model. One being, whether these transitions are caused by unknown particles not predicted by the guiding theory.

The research, submitted today to Physical Review Letters and available on arXiv, received funding from the Science and Technology Facilities Council (STFC).

Being one and the other

In the strange world of quantum physics, the charm meson can be itself and its antiparticle at once. This state, known as quantum superposition, results in two particles each with their own mass – a heavier and lighter version of the particle. This superposition allows the charm meson to oscillate into its antiparticle and back again.

Using data collected during the second run of the Large Hadron Collider, researchers from the University of Oxford measured a difference in mass between the two particles of 0.00000000000000000000000000000000000001 grams – or in scientific notation 1×10-38g. A measurement of this precision and certainty is only possible when the phenomenon is observed many times, and this is only possible due so many charm mesons being produced in LHC collisions.

As the measurement is extremely precise, the research team ensured the analysis method was even more so. To do this, the team used a novel technique originally developed by colleagues at the University of Warwick.

There are only four types of particle in the Standard Model, the theory that explains particle physics, that can turn into their antiparticle. The mixing phenomenon was first observed in Strange mesons in the 1960s and in beauty mesons in the 1980s. Until now, the only other one of the four particles that has been seen to oscillate this way is the strange-beauty meson, a measurement made in 2006.

The LHCb experiment at CERN.
The LHCb experiment at CERN © University of Warwick

A rare phenomenon

Professor Guy Wilkinson at University of Oxford, whose group contributed to the analysis, said:

“What makes this discovery of oscillation in the charm meson particle so impressive is that, unlike the beauty mesons, the oscillation is very slow and therefore extremely difficult to measure within the time that it takes the meson to decay. This result shows the oscillations are so slow that the vast majority of particles will decay before they have a chance to oscillate. However, we are able to confirm this as a discovery because LHCb has collected so much data.”

Professor Tim Gershon at University of Warwick, developer of the analytical technique used to make the measurement, said:

“Charm meson particles are produced in proton–proton collisions and they travel on average only a few millimetres before transforming, or decaying, into other particles. By comparing the charm meson particles that decay after travelling a short distance with those that travel a little further, we have been able to measure the key quantity that controls the speed of the charm meson oscillation into anti-charm meson – the difference in mass between the heavier and lighter versions of charm meson.”

A new door opens for physics exploration

This discovery of charm meson oscillation opens up a new and exciting phase of physics exploration; researchers now want to understand the oscillation process itself, potentially a major step forward in solving the mystery of matter-antimatter asymmetry. A key area to explore is whether the rate of particle-antiparticle transitions is the same as that of antiparticle-particle transitions, and specifically whether the transitions are influenced/caused by unknown particles not predicted by the Standard Model.

Dr Mark Williams at University of Edinburgh, who convened the LHCb Charm Physics Group within which the research was performed, said:

“Tiny measurements like this can tell you big things about the Universe that you didn’t expect.”

The result, 1×10-38g, crosses the ‘five sigma’ level of statistical significance that is required to claim a discovery in particle physics.

  • The study ‘Observation of the mass difference between neutral charm-meson eigenstates’ is available online as a preprint at: https://arxiv.org/abs/2106.03744

Featured image: The Large Hadron Collider tunnel © University of Warwick

Provided by University of Warwick

Measuring the Invisible (Particle Physics)

Particle physicist Lindley Winslow seeks the universe’s smallest particles for answers to its biggest questions.

When she entered the field of particle physics in the early 2000’s, Lindley Winslow was swept into the center of a massive experiment to measure the invisible.

Scientists were finalizing the Kamioka Liquid Scintillator Antineutrino Detector, or KamLAND, a building-sized particle detector built within a cavernous mine deep inside the Japanese Alps. The experiment was designed to detect neutrinos — subatomic particles that pass by the billions through ordinary matter.

Neutrinos are produced anywhere particles interact and decay, from the Big Bang to the death of stars in supernovae. They rarely interact with matter and are therefore pristine messengers from the environments that create them.

By 2000, scientists had observed neutrinos from various sources, including the sun, and hypothesized that the particles were morphing into different “flavors” by oscillating. KamLAND was designed to observe the oscillation, as a function of distance and energy, in neutrinos generated by Japan’s nearby nuclear reactors.

Winslow joined the KamLAND effort the summer before graduate school and spent months in Japan, helping to prepare the detector for operation and then collecting data.

“I learned to drive a manual transmission on reinforced land cruisers into the mine, past a waterfall, and down a long tunnel, where we then had to hike up a steep hill to the top of the detector,” Winslow says.

In 2002, the experiment detected neutrino oscillations for the first time.

“It was one of those moments in science where you know something that no one else in the world does,” recalls Winslow, who was part of the scientific collaboration that received the Breakthrough Prize in Fundamental Physics in 2016 for the discovery.

The experience was pivotal in shaping Winslow’s career path. In 2020, she received tenure as associate professor of physics at MIT, where she continues to search for neutrinos, with KamLAND and other particle-detecting experiments that she has had a hand in designing.

“I like the challenge of measuring things that are very, very hard to measure,” Winslow says. “The motivation comes from trying to discover the smallest building blocks and how they affect the universe we live in.”

Measuring the impossible

Winslow grew up in Chadds Ford, Pennsylvania, where she explored the nearby forests and streams, and also learned to ride horses, even riding competitively in high school.

She set her sights west for college, with the intention of studying astronomy, and was accepted to the University of California at Berkeley, where she happily spent the next decade, earning first an undergraduate degree in physics and astronomy, then a master’s and PhD in physics.

Midway through college, Winslow learned of particle physics and the large experiments to detect elusive particles. A search for an undergraduate research project introduced her to the Cryogenic Dark Matter Search, or CDMS, an experiment that was run beneath the Stanford University campus. CDMS was designed to detect weakly interacting massive particles, or WIMPS — hypothetical particles that are thought to comprise dark matter — in detectors wrapped in ultrapure copper. For her first research project, Winslow helped analyze copper samples for the experiment’s next generation.

“I liked seeing how all these pieces worked together, from sourcing the copper to figuring out how to build an experiment to basically measure the impossible,” Winslow says.

Her later work with KamLAND, facilitated by her quantum mechanics professor and eventual thesis advisor, further inspired her to design experiments to search for neutrinos and other fundamental particles.

“Little particles, big questions”

After completing her PhD, Winslow took a postdoc position with Janet Conrad, professor of physics at MIT. In Conrad’s group, Winslow had freedom to explore ideas beyond the lab’s primary projects. One day, after watching a video about nanocrystals, Conrad wondered whether the atomic-scale materials might be useful in particle detection.

“I remember her saying, ‘These nanocrystals are really cool. What can we do with them? Go!’ And I went and thought about it,” Winslow says.

She soon came back with an idea: What if nanocrystals made from interesting isotopes could be dissolved in liquid scintillator to also realize more sensitive neutrino detection? Conrad thought it was a good idea and helped Winslow seek out grants to get the project going.

In 2010, Winslow was awarded the L’Oréal for Women in Science Fellowship and a grant that she put toward the nanocrystal experiment, which she named NuDot, for the quantum dots (a type of nanocrystal) that she planned to work into a detector. When she finished her postdoc, she accepted a faculty position at the University of California at Los Angeles, where she continued laying plans for NuDot.

A cold bargain

Winslow spent two years at UCLA, during a time when the search for neutrinos circled around a new target: neutrinoless double-beta decay, a hypothetical process that, if observed, would prove that the neutrino is also its own antiparticle, which would help to explain why the universe has more matter than antimatter.

At MIT, physics professor and department head Peter Fisher was looking to hire someone to explore double-beta decay. He offered the job to Winslow, who negotiated in return.

“I told him what I wanted was a dilution refrigerator,” Winslow recalls. “The base price for one these is not small, and it’s asking a lot in particle physics. But he was like, ‘done!’”

Winslow joined the MIT faculty in 2015, setting her lab up with a new dilution refrigerator that would allow her to cool macroscopic crystals to millikelvin temperatures to look for heat signatures from double-beta decay and other interesting particles. Today she is continuing to work on NuDot and the new generation of KamLAND, and is also a key member of CUORE, a massive underground experiment in Italy with a much larger dilution refrigerator, designed to observe neutrinoless double-beta decay.

Winslow has also made her mark on Hollywood. In 2016, while settling in at MIT, a colleague at UCLA recommended her as a consultant to the remake of the film “Ghostbusters.” The set design department was looking for ideas for how to stage the lab of one of the movie’s characters, a particle physicist. “I had just inherited a lab with a huge amount of junk that needed to be cleared out — gigantic crates filled with old scientific equipment, some of which had started to rust,” Winslow says. “[The producers] came to my lab and said, ‘This is perfect!’ And in the end it was a really fun collaboration.”

In 2018, her work took a surprising turn when she was approached by theorist Benjamin Safdi, then at MIT, who with MIT physicist Jesse Thaler and former graduate student Yonatan Kahn PhD ’15 had devised a thought experiment named ABRACADABRA, to detect another hypothetical particle, the axion, by simulating a magnetar — a type of neutron star with intense magnetic fields that should make any interacting axions briefly detectable. Safdi heard of Winslow’s refrigerator and wondered whether she could engineer a detector inside it to test the idea.

“It was an example of the wonderfulness that is MIT,” recalls Winslow, who jumped at the opportunity to design an entirely new experiment. In its first successful run, the ABRACADABRA detector reported no evidence of axions. The team is now designing larger versions, with greater sensitivity, to add to Winslow’s stable of growing detectors.

“That’s all part of my group’s vision for the next 25 years: building big experiments that might detect little particles, to answer big questions,” Winslow says.

Featured image: MIT particle physicist Lindley Winslow seeks the universe’s smallest particles for answers to its biggest questions. Credits: Image: M. Scott Brauer

Provided by MIT

Researchers Find Evidence of Elusive Odderon Particle (Particle Physics)

For 50 years, the research community has been hunting unsuccessfully for the so-called Odderon particle. Now, a Swedish-Hungarian research group has discovered the mythical particle with the help of extensive analysis of experimental data from the Large Hadron Collider at CERN in Switzerland.

In 1973, two French particle physicists found that, according to their calculations, there was a previously unknown quasi-particle. The discovery sparked an international hunt.

The Odderon particle is what briefly forms when protons collide in high-energy collisions, and in some cases do not shatter, but bounce off one another and scatter. Protons are made up of quarks and gluons, that briefly form Odderon and Pomeron particles.

And now a research team, involving researchers from Lund University, has succeeded in identifying the Odderon in connection with an advanced data analysis study at the particle accelerator CERN.

”This is a particle physics milestone! It feels fantastic to contribute to an increased understanding of matter; the fundamental building blocks of our world”, says Roman Pasechnik, particle physics researcher at Lund University.

Through extensive data analyzes of elastic proton-proton and proton-antiproton collisions, the researchers were able to hone in on the new particle. The analysis took several months, but finally paid off.

”We worked with some of the world’s best particle physicists. They were astonished when we published our results”, concludes Roman Pasechnik.

Featured image: Roman Pasechnik (Photo: Gunnar Ingelman)


Link to the article in The European Physical Journal C: 

Evidence of Odderon-exchange from scaling properties of elastic scattering at TeV energies

Provided by Lund University

New IceCube Detection Proves 60-year-old Theory (Physics)

On December 6, 2016, a high-energy particle called an electron antineutrino was hurtling through space at nearly the speed of light. Normally, the ghostly particle would zip right through the Earth as if it weren’t even there.

But this particle just so happened to smash into an electron deep inside the South Pole’s glacial ice. The collision created a new particle, known as the W boson. That boson quickly decayed, creating a shower of secondary particles.

The whole thing played out in front of the watchful detectors of a massive telescope buried in the Antarctic ice, the IceCube Neutrino Observatory. This enabled IceCube to make the first ever detection of a Glashow resonance event, a phenomenon predicted 60 years ago by Nobel laureate physicist Sheldon Glashow.

This detection provides the latest confirmation of the Standard Model, the name of the particle physics theory explaining the universe’s fundamental forces and particles.

“Finding it wasn’t necessarily a surprise, but that doesn’t mean I wasn’t very happy to see it,” said Claudio Kopper, an associate professor in Michigan State University’s Department of Physics and Astronomy in the College of Natural Science. Kopper and his departmental colleague, assistant professor Nathan Whitehorn, lead IceCube’s Diffuse and Atmospheric Flux Working Group behind the discovery.

The international IceCube Collaboration published this result online on March 11 in the journal Nature.

“Even three years ago, I didn’t think IceCube would be able to make this measurement, or at least as well as we did,” Whitehorn said.

This detection further demonstrates the ability of IceCube, which observes nearly massless particles called neutrinos using thousands of sensors embedded in the Antarctic ice, to do fundamental physics.

Although the Spartans lead the working group, they emphasized that this discovery was a team effort, powered by the paper’s three lead analysts: Lu Lu, an assistant professor at University of Wisconsin–Madison; Tianlu Yuan, an assistant scientist at the Wisconsin IceCube Particle Astrophysics Center, or WIPAC; and Christian Haack, a postdoc at the Technical University of Munich.

“We lead weekly meetings, we talk about how the work is done, we ask hard questions,” said Kopper. “But without the people doing the actual analysis, we wouldn’t have anything.”

“Our job is to be the doubters-in-chief,” Whitehorn said. “The lead authors did a great job convincing everyone that this event was a Glashow resonance.”

The particle physics community has been anticipating such a detection, but Glashow resonance events are extremely rare by nature and technologically challenging to detect.

“When Glashow was a postdoc at Niels Bohr, he could never have imagined that his unconventional proposal for producing the W boson would be realized by an antineutrino from a faraway galaxy crashing into Antarctic ice,” said Francis Halzen, professor of physics at the University of Wisconsin–Madison, the headquarters of IceCube maintenance and operations, and principal investigator of IceCube.

A Glashow resonance event requires an electron antineutrino with a cosmic amount of energy — at least 6.3 peta-electronvolts, or PeV. For comparison, that’s about 1,000 times more energy than that of the most energetic particles produced by the Earth’s most powerful particle accelerators.

Since IceCube started fully operating in 2011, it has detected hundreds of high-energy neutrinos from space. Yet the neutrino in December 2016 was only the third with an energy higher than 5 PeV.

And simply having a high-energy neutrino is not sufficient to detect a Glashow resonance event. The neutrino then has to interact with matter, which is not a guarantee. But IceCube encompasses quite a bit of matter in the form of Antarctic ice.

IceCube sits on the South Pole, waiting to see particles from the cosmos. Credit: Yuya Makino, IceCube/NSF

The observatory’s detector array has been built into the ice, spanning nearly 250 acres with sensors reaching up to about a mile deep. All told, IceCube boasts a cubic kilometer of coverage, watching over a billion metric tons of extremely clear ice.

That’s what it takes to detect neutrinos, along with a team of scientists who have the skill and determination to spot rare events.

IceCube’s more than 5,000 detectors take in a tremendous firehose of light, Whitehorn said. Detecting the Glashow resonance meant researchers had to pick out a handful of telltale photons, individual particles of light, from that firehose spray.

“This is some of the most impressive technical work I’ve ever seen,” Whitehorn said, calling the team unstoppable over the years-long effort to confirm this was a Glashow resonance event.

Making the work even more impressive was the fact that the lead authors — Lu, Yuan and Haack — were in three countries on three different continents during the analysis. Lu was a postdoc at Chiba University in Japan, Yuan was at WIPAC in the U.S. and Haack was a doctoral student at Rheinisch-Westfälische Technische Hochschule Aachen University in Germany.

“It was amazing to me just seeing that that is possible,” Kopper said.

But this is very much in keeping with the ethos of IceCube, an observatory built on international collaboration. IceCube is operated by a group of scientists, engineers and staff from 53 institutions in 12 countries, together known as the IceCube Collaboration. The project’s headquarters is WIPAC, a research center of UW–Madison in the United States.

Flags outside of IceCube represent the international collaboration of the project. Credit: Yuya Makino, IceCube/NSF

To confirm the detection and usher in a new chapter of neutrino astronomy, the IceCube Collaboration is working to detect more Glashow resonances. And they need IceCube-Gen2, a proposed expansion of the IceCube detector, to make it happen.

“We already know that the astrophysical spectrum does not end at 6 PeV,” Lu said. “The key is to detect more Glashow resonance events and to identify the sources that accelerate those antineutrinos. IceCube-Gen2 will be key to making such measurements in a statistically significant way.”

Glashow himself echoed that sentiment about validation. “To be absolutely sure, we should see another such event at the very same energy as the one that was seen,” said Glashow, now an emeritus professor of physics at Boston University. “So far there’s one, and someday there will be more.”

The IceCube Neutrino Observatory is funded primarily by the National Science Foundation and is operated by a team headquartered at the University of Wisconsin–Madison. IceCube’s research efforts, including critical contributions to the detector operation, are funded by agencies in Australia, Belgium, Canada, Denmark, Germany, Japan, New Zealand, Republic of Korea, Sweden, Switzerland, the United Kingdom, and the United States. IceCube construction was also funded with significant contributions from the National Fund for Scientific Research — the FNRS and FWO — in Belgium; the Federal Ministry of Education and Research and the German Research Foundation in Germany; the Knut and Alice Wallenberg Foundation, the Swedish Polar Research Secretariat, and the Swedish Research Council in Sweden; and the Department of Energy and the University of Wisconsin–Madison Research Fund in the U.S.

Featured image: A visualization of the Glashow resonance event detected by IceCube. The event was nicknamed “Hydrangea.” Credit: IceCube Collaboration

Reference: The IceCube Collaboration., Aartsen, M.G., Abbasi, R. et al. Detection of a particle shower at the Glashow resonance with IceCube. Nature 591, 220–224 (2021). DOI : https://dx.doi.org/10.1038/s41586-021-03256-1)

Provided by MSU

Is it Possible To Obtain Cosmic Acceleration From Nothing, But Matter? (Astronomy)


◉ Recai Erdem and colleagues check the possibility of cosmic accelerated expansion by considering a case where matter is converted to radiation (or vice versa by particle physics processes).

◉ They found that cosmic accelerated expansion can be obtained in this way only if an intermediate state with negative equation of state forms during the conversion.

◉ They said it is difficult to obtain present cosmic accelerated expansion wholly through the usual particle physics interactions in this way since the localization scales of corresponding ρR’s for the usual particle physics processes are at the order of atomic scales i.e. at scales much smaller than the cosmological scales.

Even when they have such an effect, these interactions will first accelerate the universe and then decelerate it in the time scale of the interaction time (which is smaller than ∼ 10^–8 sec), hence the net effect would be zero.

◉ Present cosmic accelerated expansion may be obtained in this way only if the life time of the resonance condensate has a cosmologically relevant time scale.

Observations showed that the universe is undergoing accelerated expansion at present, and many theoretical arguments and observational evidence suggest that the universe must have undergone an accelerated expansion period at the early times as well. Although the standard explanations for these accelerated expansions are cosmological constant at present era and inflationary models at early times there are many alternative ways; for example, quintessence, f(R) models, and gravitational particle production. However all these models have some problems. There is a problem associated with cosmological constant called the cosmological constant problem, and it seems that the best way may be the use of some symmetry to make it cancel and seek another method for late time cosmic acceleration. Inflationary models usually employ at least one new postulated scalar, and need special initial conditions, a similar situation (although less severe) is true for quintessence models.

f(R) type modified gravity models use an extension of general relativity, in gravitational particle production the energy density of the universe is an open thermodynamical system that is assumed to acquire energy from gravitational field while the question of if the universe is a closed system in this case is not clear enough. Therefore it is useful to seek the possibility of additional alternative ways for accelerated expansion. In particular, it would be desirable to have a model where the accelerated expansion is achieved with a minimal extension of the standard models of particle physics and cosmology. Recai Erdem and colleagues in their paper, in the light of the fact that, coupling an energy density to another one, modifies its equation of state, they seek if an energy density transfer due to elementary particle processes may have the potential of providing a source for cosmic accelerated expansion.

Although the analysis in their paper, in principle, is applicable to all types of particle physics processes, they specified it to the case of conversion of heavy particles to light particles i.e. to the conversion of matter to radiation. In fact there must be an era of the creation of matter and radiation not only because the ordinary matter and radiation must be produced anyway but also to have a well defined model that may serve at all eras of the universe. Moreover in the standard lore of cosmology the ordinary matter and radiation are assumed to be produced by the decays or the collisions of some other particles such as Higgs particle, curvaton etc. at early times. Particle physics processes ranging from high energies to atomic physics have an important role at present as well. Therefore the possibility of using just matter and radiation (as in this paper) interacting through the particle physics processes for cosmic acceleration with minimal need for exotic matter is interesting. The results of the following analysis shows that obtaining cosmic acceleration through conversion of matter to radiation (or vice versa) seems impossible except through formation of an intermediate state with negative equation of state (e.g. a QCD-like condensate formed by intermediate particles produced in the particle physics processes).

They considered the Robertson-Walker metric:

and for simplicity they take k = 0 which is in agreement with observations of Patrignani and colleagues. For the illustration of the method they considered a simple case; a universe that consists of matter and radiation. They assume that, at some time t1, the energy density of either of matter or radiation starts to be transferred to the other through some particle physics processes such as those given in Figure 1 below.

FIG. 1. The diagram on the left-hand side shows the decay of a particle with momentum p1 into two particles with momenta p2 and p3 e.g. the decay of a non-relativistic particle to two relativistic particles while the diagram on the right-hand side shows the (inelastic) collision of two particles with momenta p1 and p2 into two other particles with momenta p3 and p4 e.g. the collision of two non-relativistic particles into two relativistic particles through formation of an intermediate state

It seems difficult to obtain the present cosmic accelerated expansion wholly through the usual particle physics interactions in this way since the localization scales of corresponding ρR’s for the usual particle physics processes are at the order of atomic scales i.e. at scales much smaller than the cosmological scales. Even when they have such an effect, these interactions will first accelerate the universe and then decelerate it in the time scale of the interaction time (which is smaller than ∼ 10^–8 sec), hence the net effect would be zero.

– said Edrem.

This type of interactions may be relevant cosmologically only at early times (if they involve the usual particles) provided that a significant redshift takes place during their interaction time e.g. during the lifetime of the resonance particle. A very early time acceleration may be induced by fast out of equilibrium processes as those given in Figure 1 provided an intermediate state with ω < 0 forms. Present cosmic accelerated expansion may be obtained in this way only if the life time of the resonance condensate has a cosmologically relevant time scale.

Although they have considered such a toy model in this study, in order to entertain these possibilities in detail one needs to study different specific models in more detail along the lines given in this paper and confront it with observational data which is beyond the scope of this study that aims to seek the degree of possibility of obtaining the late time and the early time accelerated expansions of the universe in this way.

Specific models along these lines where different options for ρR and ρ'(t) / dt (where, ρ'(t) / dt is the rate of the energy density transfer from matter to radiation) are specified and their theoretical origins discussed and whose the results are confronted with observational data may be considered in future.

– Concluded authors of the study

Reference: Recai Erdem, “Is it possible to obtain cosmic accelerated expansion through energy transfer between different energy densities?,” Physics of the Dark Universe, Volume 15, 2017, Pages 57-71, ISSN 2212-6864,

Copyright of this article totally belongs to our author S. Aman. One is allowed to reuse it only by giving proper credit either to him or to us.

Scientists Work To Shed Light On Standard Model of Particle Physics (Physics)

Mapping the magnetic field for Fermilab’s Muon g-2 experiment

As scientists await the highly anticipated initial results of the Muon g-2 experiment at the U.S. Department of Energy’s (DOE) Fermi National Accelerator Laboratory, collaborating scientists from DOE’s Argonne National Laboratory continue to employ and maintain the unique system that maps the magnetic field in the experiment with unprecedented precision.

Typical magnetic field variations as mapped by the trolley at different positions in the Muon g-2 experiment’s storage ring, shown at the parts-per-million level. ©(Image by Argonne National Laboratory.)

Argonne scientists upgraded the measurement system, which uses an advanced communication scheme and new magnetic field probes and electronics to map the field throughout the 45-meter circumference ring in which the experiment takes place.

“There was a large deviation between Brookhaven’s measurement and the theoretical prediction, and if we confirm this discrepancy, it will signal the existence of undiscovered particles.” — Simon Corrodi, postdoctoral appointee in Argonne’s HEP division

The experiment, which began in 2017 and continues today, could be of great consequence to the field of particle physics. As a follow-up to a past experiment at DOE’s Brookhaven National Laboratory, it has the power to affirm or discount the previous results, which could shed light on the validity of parts of the reigning Standard Model of particle physics.

High-precision measurements of important quantities in the experiment are crucial for producing meaningful results. The primary quantity of interest is the muon’s g-factor, a property that characterizes magnetic and quantum mechanical attributes of the particle.

The Standard Model predicts the value of the muon’s g-factor very precisely. “Because the theory so clearly predicts this number, testing the g-factor through experiment is an effective way to test the theory,” said Simon Corrodi, a postdoctoral appointee in Argonne’s High Energy Physics (HEP) division. “There was a large deviation between Brookhaven’s measurement and the theoretical prediction, and if we confirm this discrepancy, it will signal the existence of undiscovered particles.”

Just as the Earth’s rotational axis precesses — meaning the poles gradually travel in circles — the muon’s spin, a quantum version of angular momentum, precesses in the presence of a magnetic field. The strength of the magnetic field surrounding a muon influences the rate at which its spin precesses. Scientists can determine the muon’s g-factor using measurements of the spin precession rate and the magnetic field strength.

The more precise these initial measurements are, the more convincing the final result will be. The scientists are on their way to achieve field measurements accurate to 70 parts per billion. This level of precision enables the final calculation of the g-factor to be accurate to four times the precision of the results of the Brookhaven experiment. If the experimentally measured value differs significantly from the expected Standard Model value, it may indicate the existence of unknown particles whose presence disturbs the local magnetic field around the muon.

Trolley ride

During data collection, a magnetic field causes a beam of muons to travel around a large, hollow ring. To map the magnetic field strength throughout the ring with high resolution and precision, the scientists designed a trolley system to drive measurement probes around the ring and collect data.

The University of Heidelberg developed the trolley system for the Brookhaven experiment, and Argonne scientists refurbished the equipment and replaced the electronics. In addition to 378 probes that are mounted within the ring to constantly monitor field drifts, the trolley holds 17 probes that periodically measure the field with higher resolution.

“Every three days, the trolley goes around the ring in both directions, taking around 9,000 measurements per probe and direction,” said Corrodi. “Then we take the measurements to construct slices of the magnetic field and then a full, 3D map of the ring.”

Fully assembled trolley system with wheels for riding on rails and the new external barcode reader for an exact position measurement. The 50 cm long cylindrical shell encloses the 17 NMR probes and custom-built readout and control electronics. ©(Image by Argonne National Laboratory.)

The scientists know the exact location of the trolley in the ring from a new barcode reader that records marks on the bottom of the ring as it moves around.

The ring is filled with a vacuum to facilitate controlled decay of the muons. To preserve the vacuum within the ring, a garage connected to the ring and vacuum stores the trolley between measurements. Automating the process of loading and unloading the trolley into the ring reduces the risk of the scientists compromising the vacuum and the magnetic field by interacting with the system. They also minimized the power consumption of the trolley’s electronics in order to limit the heat introduced to the system, which would otherwise disrupt the precision of the field measurement.

The scientists designed the trolley and garage to operate in the ring’s strong magnetic field without influencing it. “We used a motor that works in the strong magnetic field and with minimal magnetic signature, and the motor moves the trolley mechanically, using strings,” said Corrodi. “This reduces noise in the field measurements introduced by the equipment.”

The system uses the least amount of magnetic material possible, and the scientists tested the magnetic footprint of every single component using test magnets at the University of Washington and Argonne to characterize the overall magnetic signature of the trolley system.

The power of communication

Of the two cables pulling the trolley around the ring, one of them also acts as the power and communication cable between the control station and the measurement probes.

To measure the field, the scientists send a radio frequency through the cable to the 17 trolley probes. The radio frequency causes the spins of the molecules inside the probe to rotate in the magnetic field. The radio frequency is then switched off at just the right moment, causing the water molecules’ spins to precess. This approach is called nuclear magnetic resonance (NMR).

The frequency at which the probes’ spins precess depends on the magnetic field in the ring, and a digitizer on board the trolley converts the analog radio frequency into multiple digital values communicated through the cable to a control station. At the control station, the scientists analyze the digital data to construct the spin precession frequency and, from that, a complete magnetic field map.

During the Brookhaven experiment, all signals were sent through the cable simultaneously. However, due to the conversion from analog to digital signal in the new experiment, much more data has to travel over the cable, and this increased rate could disturb the very precise radio frequency needed for the probe measurement. To prevent this disturbance, the scientists separated the signals in time, switching between the radio frequency signal and data communication in the cable.

“We provide the probes with a radio frequency through an analog signal,” said Corrodi, “and we use a digital signal for communicating the data. The cable switches between these two modes every 35 milliseconds.”

The tactic of switching between signals traveling through the same cable is called “time-division multiplexing,” and it helps the scientists reach specifications for not only accuracy, but also noise levels. An upgrade from the Brookhaven experiment, time-division multiplexing allows for higher-resolution mapping and new capabilities in magnetic field data analysis.

Upcoming results

Both the field mapping NMR system and its motion control were successfully commissioned at Fermilab and have been in reliable operation during the first three data-taking periods of the experiment.

The scientists have achieved unprecedented precision for field measurements, as well as record uniformity of the ring’s magnetic field, in this Muon g-2 experiment. Scientists are currently analyzing the first round of data from 2018, and they expect to publish the results by the end of 2020.

The scientists detailed the complex setup in a paper, titled “Design and performance of an in-vacuum, magnetic field mapping system for the Muon g-2 experiment,” published in the Journal of Instrumentation.

References: S. Corrodi, P. De Lurgio, D. Flay, J. Grange, R. Hong, D. Kawall, M. Oberling, S. Ramachandran and P. Winter, “Design and performance of an in-vacuum, magnetic field mapping system for the Muon g-2 experiment”, Journal of Instrumentation, Volume 15, November 2020. https://iopscience.iop.org/article/10.1088/1748-0221/15/11/P11008

Provided by Argonne National Laboratory