How neutron stars 'playing it cool' could unlock exotic physics

Scientists have found that three neutron stars, born in the fires of other exploding stars, have cooled off surprisingly quickly, bringing us closer to understanding the exotic nature of matter within the cores of these extreme objects.


The discovery was made by a Spanish team led by Alessio Marino of the Institute of Space Sciences (ICE–CSIC) in Barcelona, using European and American space telescopes that work with X-ray light.


A neutron star is the collapsed core of a massive star that has gone supernova, and can contain up to nearly three times the mass of our sun in a spherical volume just about 6.8 miles (11 kilometers) across. All that matter compacted into such a small area means neutron stars are among the densest concentrations of matter in the known universe, second only to black holes. To make that statement more relatable, consider how a tablespoon of neutron-star material would be comparable to the mass of Mount Everest.



This extreme nature also means the physics that governs neutron stars' interiors remains murky. These objects are called neutron stars to begin with because their matter has been crushed to such a degree that negatively charged electrons and positively charged protons get smushed together, overcoming the electrostatic force between them to form an object full of just neutral neutrons. Deeper in the core of a neutron star, matter may be crushed to an even greater extent, forming exotic, never-before-seen particles such as hypothetical hyperons. Perhaps, scientists believe, or neutrons themselves could be popped apart within a neutron star, creating a soup of the universe's most fundamental particles: quarks.

Related: Weird cosmic object identified as the remains of an exploded dead star


What happens inside a neutron star is governed by the neutron star equation of state. Think of this as a playbook that determines a neutron star's internal structure and composition based on things like its mass, temperature, magnetic field and so on. The trouble is, scientists have literally hundreds of options for what this equation of state could be. Since we cannot replicate on Earth the conditions inside a neutron star, testing which model is the right one is highly dependent on matching them to what astronomical observations tell us.

Now, however, the discovery of three neutron stars with substantially lower surface temperatures compared to other neutron stars of similar age has provided a big clue, allowing researchers to rule out three-quarters of the possible models for the neutron star equation of state in one stroke. Two of the neutron stars are pulsars, which are rapidly spinning neutron stars that fire beams of radio jets toward us. The third neutron star, in the Vela Jr supernova remnant, doesn't display pulsar behavior, but that may just be because its radio jets do not point in our direction.

SOURCE: by Keith Cooper

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A black hole made from pure light is impossible, thanks to quantum physics

Black holes can’t be formed from pure light. Quantum physics would curb their creation under any foreseeable conditions, a new study suggests.

Typically, matter is responsible for black holes. They’re often formed when a star’s core implodes at the end of its life. But matter isn’t necessarily required to form a black hole. According to Einstein’s general theory of relativity, black holes could form from concentrated energy alone.

A black hole formed from electromagnetic energy — that is, light — is called a kugelblitz. That concept has been jangling around in physicists’ brains for decades. But actually producing a kugelblitz seems to be a no-go, theoretical physicist Eduardo Martín-Martínez and colleagues report in a paper accepted to Physical Review Letters. “No known source in the current universe would be able to produce it, neither artificial or natural,” says Martín-Martínez, of the University of Waterloo in Canada.

In recent years, science fiction writers have picked up the kugelblitz mystique and run with it. Fans of the Netflix show Umbrella Academy may be familiar with the term, which is German for “ball lightning.” In season 3, a kugelblitz obliterates large swaths of existence.

In general relativity, gravity results from matter curving spacetime. If enough mass is packed into one region, the spacetime can curve so dramatically that it forms a region within which it’s impossible to escape — a black hole. But in general relativity, energy and mass are equivalent. That means energy can curve spacetime just as matter can, suggesting the wild idea that a black hole could form with no matter at all.

That concept is “a very interesting thought,” says theoretical physicist Juan García-Bellido of Universidad Autónoma de Madrid, who was not involved in the new study, “especially if we want to produce something like this in the laboratory.” Scientists have previously considered whether futuristic lasers might one day form a black hole in a lab, and even proposed using a kugelblitz to power a spacecraft.

Alas, calculations suggest that any attempt at a kugelblitz would result in failure, Martín-Martínez says. “You are not going to get even close. You’re not going to get even something that starts attracting you like Earth would.”

That’s because of a quantum effect that occurs when electromagnetic energy is highly concentrated. According to the well-verified theory of quantum electrodynamics, when light reaches those extremes, pairs of particles and antiparticles begin to form. Those particles — electrons and their positively charged antimatter partners, positrons — would escape the region, taking energy with them. That prevents the energy from reaching the levels needed to form a black hole.

Forming a kugelblitz in a laboratory would require light intensities more than 1050 times that of the state-of-the-art laser pulses, the team calculated. (That’s a mind-bogglingly large factor — a 1 with 50 zeroes after it.) And in nature, the brightest quasars — brilliantly luminous centers of active galaxies — are likewise vastly too dim.

The kibosh on kugelblitzes applies across a huge range of scales. It rules out itty-bitty kugelblitzes with a radius as small as a hundredth of a quintillionth of a nanometer all the way up to 100 million meters. Even outside that range, Martín-Martínez says, a kugelblitz would still be very unlikely.

García-Bellido, however, notes a possible loophole: “It’s much more likely that things like this might have happened in the early universe.”

Just after the Big Bang, the universe is thought to have expanded extremely rapidly, a process known as inflation. That inflation may have imprinted fluctuations in the curvature of spacetime that could cause light to collapse into what’s known as a primordial black hole (SN: 8/7/16). So while light won’t form black holes under its own gravity, that preexisting curvature, García-Bellido says, could have allowed something akin to a kugelblitz.

SOURCE: by Emily Conover

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Demystifying Vortex Rings in Nuclear Fusion, Supernovae

Better understanding the formation of swirling, ring-shaped disturbances—known as vortex rings—could help nuclear fusion researchers compress fuel more efficiently, bringing it closer to becoming a viable energy source.

The model developed by researchers at the University of Michigan (U-M) could aid in the design of the fuel capsule, minimizing the energy lost while trying to ignite the reaction that makes stars shine. In addition, the model could help other engineers who must manage the mixing of fluids after a shock wave passes through, such as those designing supersonic jet engines, as well as physicists trying to understand supernovae.

"These vortex rings move outward from the collapsing star, populating the universe with the materials that will eventually become nebulae, planets, and even new stars—and inward during fusion implosions, disrupting the stability of the burning fusion fuel and reducing the efficiency of the reaction," said Michael Wadas, a doctoral candidate in mechanical engineering at U-M and corresponding author of the study.

"Our research, which elucidates how such vortex rings form, can help scientists understand some of the most extreme events in the universe and bring humanity one step closer to capturing the power of nuclear fusion as an energy source," he said.

Nuclear fusion pushes atoms together until they merge. This process releases several times more energy than breaking atoms apart, or fission, which powers today's nuclear plants. Researchers can create this reaction, merging forms of hydrogen into helium, but at present, much of the energy used in the process is wasted.

Part of the problem is that the fuel can't be neatly compressed. Instabilities cause the formation of jets that penetrate into the hotspot, and the fuel spurts out between them—Wadas compared it to trying to squish an orange with your hands, how juice would leak out between your fingers.

Vortex rings that form at the leading edge of these jets, the researchers have shown, are mathematically similar to smoke rings, the eddies behind jellyfish and the plasma rings that fly off the surface of a supernova.

Perhaps the most famous approach to fusion is a spherical array of lasers all pointing toward a spherical capsule of fuel. This is how experiments are set up at the National Ignition Facility, which has repeatedly broken records for energy output in recent years.

The energy from the lasers vaporizes the layer of material around the fuel—a nearly perfect, lab-grown shell of diamond in the latest record-setter in December 2022. When that shell vaporizes, it drives the fuel inward as the carbon atoms fly outward. This generates a shock wave, which pushes the fuel so hard that the hydrogen fuses.

While the spherical fuel pellets are some of the most perfectly round objects humans have ever made, each has a deliberate flaw: a fill tube, where the fuel enters. Like a straw stuck in that crushed orange, this is the most likely place for a vortex-ring-led jet to form when the compression starts, the researchers explained.

"Fusion experiments happen so fast that we really only have to delay the formation of the jet for a few nanoseconds," said Eric Johnsen, an associate professor of mechanical engineering at U-M, who supervised the study.

The study brought together the fluid mechanics expertise of Wadas and Johnsen as well as the nuclear and plasma physics knowledge in the lab of Carolyn Kuranz, an associate professor of nuclear engineering and radiological sciences.

"In high-energy-density physics, many studies point out these structures, but haven’t clearly identified them as vortex rings," said Wadas.

Knowing about the deep body of research into the structures seen in fusion experiments and astrophysical observations, Wadas and Johnsen were able to draw on and extend that existing knowledge rather than trying to describe them as completely new features.

Johnsen is particularly interested in the possibility that vortex rings could help drive the mixing between heavy elements and lighter elements when stars explode, as some mixing process must have occurred to produce the composition of planets like Earth.

The model can also help researchers understand the limits of the energy that a vortex ring can carry, and how much fluid can be pushed before the flow becomes turbulent and harder to model as a result. In ongoing work, the team is validating the vortex ring model with experiments.

The research is funded by Lawrence Livermore National Laboratory and the Department of Energy, with computational resources provided by the Extreme Science and Engineering Discovery Environment through the National Science Foundation and the Oak Ridge Leadership Computing Facility.

This press release was provided by the University of Michigan

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A Recent Look at Molecular Spectroscopy

Spectroscopy magazine publishes a column titled, “Molecular Spectroscopy Workbench” that focuses on molecular spectroscopic techniques. Many of these articles are designed to be tutorial in nature, allowing readers an inside look at the interactions between electromagnetic waves and matter.



Below is a compilation of four of our most recent Molecular Spectroscopy Workbench columns. Happy reading!

Monitoring Chemical Changes by Raman Spectroscopy

This article discusses the application of Raman spectroscopy in real-time monitoring of chemical reactions, with a particular focus on polymerization. Polymerization involves the loss of a carbon double bond as the polymer chain lengthens, and the strong signals from π electrons make it easy to track this reaction to completion (1). Despite its usefulness, setting up polymerization experiments for demonstration can be hazardous. The article aims to illustrate the chemical and spectral changes that take place during the curing process of a commercial epoxy (1).

Raman Spectra Used to Understand the Origins of Banding in Spherulites

This article examines the formation and characteristics of spherulites in polymers, which form crystal lamellae radiating from a nucleation site when the polymer is crystallized from the melt. When observed under a microscope with crossed polarizers, these spherulites display a distinctive Maltese cross pattern with banding (2). The lit regions, resulting from crystals growing in directions not parallel to the polarizers, often show banding because of rotations of the crystal lamellae (2). Given the sensitivity of polarized Raman spectra to crystal orientation, the article explores the relationship between the observed banding pattern and Raman polarization/orientation behavior (2). The investigation focuses on spherulites of poly(hydroxybutyrate-co-hydroxyhexanoate) (PHBHx) with different compositions (2).

Calibrating the Composition of a Copolymer

This article details the use of Raman spectroscopy in collaboration with Isao Noda to study the properties of the bioplastic polyhydroxybutyrate hexanoate (PHBHx). The properties of PHBHx, such as optical clarity, dyeability, flexibility, and thermal characteristics, depend on the percentage of hexanoate, which introduces propyl side branches to the polymer chain (3). These percentages influence the polymer's maximum crystallinity and, consequently, its physical and chemical properties (3). The article highlights the usefulness of Raman spectroscopy in accurately determining the composition of PHBHx, demonstrating a higher-than-expected precision in measuring hexanoate percentages (3).

Measuring the Crystallinity of PHBHx with Varying Amounts of Sidechains on a Benchtop Instrument

This article explores the use of current benchtop instruments with edge filters that provide Raman spectra down to 50 cm-1 for studying the crystallization of polyhydroxybutyrate-hexanoate (PHBHx). These compact instruments simplify the process compared to larger setups. By averaging the signals from spherulites, the orientation effects are mitigated, facilitating the use of multivariate techniques to compare different samples (4). PHBHx polymers are gaining commercial interest due to their biodegradable nature and production via fermentation, positioning them as sustainable alternatives to non-biodegradable, petroleum-derived polymers (4). The article also explains that controlling the polymer's physical properties, such as crystallinity, is achievable by adjusting the molecular weight and the percentage of sidechains. Removing the effects of polymer chain orientation through spectrum averaging is essential for accurate spectral comparison across samples (4).

SOURCE: by Will Wetzel

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Quantum computer photons create a vortex when they collide

Scientists have stumbled upon a remarkable discovery that challenges our understanding of the quantum world. New research revealed the existence of a previously unknown type of vortex that emerges when photons, the elusive particles of light, engage in a mesmerizing dance of interaction.

The implications of this finding extend far beyond the realm of pure science, holding the potential to revolutionize the field of quantum computing.

The research team, led by a brilliant quartet of scientists — Dr. Lee Drori, Dr. Bankim Chandra Das, Tomer Danino Zohar, and Dr. Gal Winer — embarked on this journey of discovery in the hallowed halls of Prof. Ofer Firstenberg’s laboratory at the Weizmann Institute of Science’s Physics of Complex Systems Department.

The research team, led by a brilliant quartet of scientists — Dr. Lee Drori, Dr. Bankim Chandra Das, Tomer Danino Zohar, and Dr. Gal Winer — embarked on this journey of discovery in the hallowed halls of Prof. Ofer Firstenberg’s laboratory at the Weizmann Institute of Science’s Physics of Complex Systems Department.

Their initial goal was to explore efficient ways of harnessing the power of photons for data processing in quantum computers.

Little did they know that their quest would lead them down an unexpected path, into a world where the rules of classical physics are bent and the secrets of the quantum realm are laid bare.

Secret lives of photons


Photons, the fundamental particles of light, are known for their wave-like behavior. However, getting them to interact with each other is no easy feat. It requires the presence of matter that acts as an intermediary.

To create the perfect environment for photon interactions, the researchers designed a unique setup: a 10-centimeter glass cell containing a dense cloud of rubidium atoms, tightly packed in the center.

As photons passed through this cloud, the researchers closely examined their state to see if they had influenced one another.

“When the photons pass through the dense gas cloud, they send a number of atoms into electronically excited states known as Rydberg states,” Prof. Firstenberg explains.

He goes on to describe how, in these Rydberg states, a single electron within the atom begins to orbit at an astonishing distance, up to 1,000 times the diameter of an unexcited atom.

This electron, with its vastly expanded orbit, generates an electric field so powerful that it envelops and influences countless neighboring atoms, effectively transforming them into what Prof. Firstenberg poetically refers to as an “imaginary ‘glass ball.'”
Quantum vortex rings and colliding photons

As the researchers delved deeper into the interactions between photons, they stumbled upon something extraordinary.

When two photons passed relatively close to each other, they moved at a different speed than they would have if each had been traveling alone. This change in speed altered the positions of the peaks and valleys of the waves they carried.

In the ideal scenario for quantum computing applications, the positions of the peaks and valleys would become completely inverted relative to one another, a phenomenon known as a 180-degree phase shift. However, what the researchers observed was even more fascinating.

When the gas cloud was at its densest and the photons were in close proximity, they exerted the highest level of mutual influence.

But as the photons moved away from each other or the atomic density around them decreased, the phase shift weakened and disappeared.

Instead of a gradual process, the researchers were surprised to find that a pair of vortices developed when two photons were a certain distance apart.
Mysteries of photon vortices

To visualize photon vortices, imagine dragging a vertically held plate through water. The rapid movement of the water pushed by the plate meets the slower movement around it, creating two vortices that appear to be moving together along the water’s surface.

In reality, these vortices are part of a three-dimensional configuration called a vortex ring.

The researchers discovered that the two vortices observed when measuring two photons are part of a three-dimensional vortex ring generated by the mutual influence of three photons.

These findings showcase the striking similarities between the newly discovered vortices and those found in other environments, such as smoke rings.
Photon vortex: Quantum computing’s new ally

While the discovery of photon vortices has taken center stage, the researchers remain dedicated to their original goal of advancing quantum data processing.


SOURCE: by Eric Ralls

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Reinterpreting the Higgs mechanism: Decay and fission of 'magnetic quivers' could clarify quantum structures

A simple concept of decay and fission of "magnetic quivers" helps to clarify complex quantum physics and mathematical structures.




An international research team led by Marcus Sperling, a project leader at the Faculty of Physics, University of Vienna, has sparked interest in the scientific community with pioneering results in quantum physics. In their current study, the researchers reinterpret the Higgs mechanism, which gives elementary particles mass and triggers phase transitions, using the concept of magnetic quivers.

The work is published in Physical Review Letters.

The foundation of Sperling's research, which lies at the intersection of physics and mathematics, is Quantum Field Theory (QFT)—a physical-mathematical concept within quantum physics focused on describing particles and their interactions at the subatomic level.

Since 2018, he has developed the so-called magnetic quivers along with colleagues—a graphical tool that summarizes all the information needed to define a QFT, thus displaying complex interactions between particle fields or other physical quantities clearly and intuitively.
Metaphorical magnetic quivers

A quiver consists of directed arrows and nodes. The arrows represent the quantum fields (matter fields), while the nodes represent the interactions—e.g., strong, weak, or electromagnetic—between the fields. The direction of the arrows indicates how the fields are charged under the interactions, e.g., what electric charge the particles carry.

Sperling explains, "The term 'magnetic' is also used metaphorically here to point to the unexpected quantum properties that are made visible by these representations. Similar to the spin of an electron, which can be detected through a magnetic field, magnetic quivers reveal certain properties or structures in the QFTs that may not be obvious at first glance."

Thus, they offer a practical way to visualize and analyze complex quantum phenomena, facilitating new insights into the underlying mechanisms of the quantum world.


Supersymmetric QFTs

For the current study, the stable ground states (vacua)—the lowest energy configuration in which no particles or excitations are present—in a variety of "supersymmetric QFTs" were explored. These QFTs, with their simplified space-time symmetry, serve as a laboratory environment, as they resemble real physical systems of subatomic particles but have certain mathematical properties that facilitate calculations.

Sperling said, "Our research deals with the fundamentals of our understanding of physics. Only after we have understood the QFTs in our laboratory environment can we apply these insights to more realistic QFT models."

The concept of magnetic quivers—one of the main research topics of Sperling's START project at the University of Vienna—was used as a tool to provide a precise geometric description of the new quantum vacua.
Decay and fission: Higgs mechanism reinterpreted

With calculations based on linear algebra, the researchers Antoine Bourget (University Paris Saclay), Marcus Sperling, and Zhenghao Zhong (Oxford University) demonstrated that—analogous to radioactivity in atomic nuclei—a magnetic quiver can decay into a more stable state or fission into two separate quivers. These transformations offer a new understanding of the Higgs mechanism in QFTs, which either decay into simpler QFTs or fission into separate, independent QFTs.

Sperling stated, "The Higgs mechanism explains how elementary particles acquire their mass by interacting with the Higgs field, which permeates the entire universe. Particles interact with this field as they move through space—similar to a swimmer moving through water."

A particle that has no mass usually moves at the speed of light. However, when it interacts with the Higgs field, it "sticks" to this field and becomes sluggish, leading to the manifestation of its mass. The Higgs mechanism is thus a crucial concept for understanding the fundamental building blocks and forces of the universe.

Mathematically, the "decay and fission" algorithm is based on the principles of linear algebra and a clear definition of stability. It operates autonomously and requires no external inputs. The results achieved through physics-inspired methods are not only relevant in physics but also in mathematical research: They offer a fundamental and universally valid description of the complex, intertwined structures of the quantum vacua, representing a significant advance in mathematics.

SOURCE: by University of Vienna

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Nuclear Physics from Particle Physics

A new theoretical analysis connects the results of high-energy particle experiments at the Large Hadron Collider with three-proton correlations inside nuclei.




How triplets of protons and neutrons behave is an important ingredient in theories of dense nuclear matter. Directly observing that behavior is beyond the reach of terrestrial labs. However, Alejandro Kievsky of Italy’s National Institute of Nuclear Physics and his collaborators have now demonstrated that it can be inferred from particle collisions recorded by the ALICE experiment at CERN’s Large Hadron Collider in Switzerland [1].

Last year, the ALICE experiment reported the results of smashing together beams of protons at an energy of 13 TeV. The kaons, protons, and deuterons that reached ALICE’s detectors carried with them correlations that arose when the particles sprang to life from a volume a few femtometers across. To determine what happens in that tiny volume, Kievsky and his collaborators analyzed the case of three squished-together nucleons (neutrons and protons) and worked out what detectable correlations would ensue after they flew apart. The effort required determining the source function, which described the initial state of the three nucleons, and the scattering wave function, which described their ensuing spatial distribution.

The researchers modeled the three-body source function in terms of the hyperradius, a generalized coordinate depending on the three nucleon–nucleon distances. Calculating the scattering wave function entailed casting the problem in terms of a generalization of spherical harmonics known as hyperspherical harmonics. By expanding the scattering wave function in partial waves, the researchers could simultaneously handle the short-range strong nuclear force and the long-range Coulomb force. Accurately treating the latter force was crucial in describing the asymptotic behavior in the most challenging case of all three nucleons being protons.

Kievsky and his collaborators’ analytical framework can cope with triplets of protons, neutrons, and combinations of the two. It can also handle deuterons and mesons. “We have opened the door to a new way of studying three-body systems,” he says. Indeed, he and his collaborators are looking forward to analyzing the correlations among two protons and a Λ baryon already measured by ALICE.

–Charles Day

Charles Day is a Senior Editor for Physics Magazine.


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CERN’S CMS experiment unlocks a new era of precision physics

Following the recent measurement of a fundamental parameter of the Standard Model, CERN’s Aleko Khukhunaishvili spoke with The Innovation Platform Editor Maddie Hall about its significance.

In March 2024, at the Moriond Winter Conference, CERN announced that the CMS experiment had achieved the most precise measurement at a hadron collider to date and that this result aligned with the Standard Model (SM).

The effective leptonic electroweak mixing angle, a fundamental parameter of the SM, had been previously measured. However, earlier results were in disagreement, leading to inconclusive support for the SM prediction. This increased precision measurement not only provides stronger support for the SM theory but also unlocks a new level of precision physics at the facility.

To elaborate on the significance of this new parameter, The Innovation Platform Editor, Maddie Hall, spoke with CERN’s Aleko Khukhunaishvili about the CMS experiment and the Standard Model.
Can you briefly summarise the CMS experiment and the purpose of the programme?

The Compact Muon Solenoid (CMS) is a general-purpose, nearly hermetic detector that detects the particles produced in high-energy collisions at one of the high-luminosity interaction points at the LHC. It comprises layers of tracking detectors and calorimeters placed around the interaction point, which are used to identify and reconstruct the energies and directions of nearly all particles produced in the collisions. A standout feature of CMS is its large magnetic field of four tesla, generated by a superconducting solenoid. This substantial magnetic field allows very precise measurements of charged particles and facilitates detailed studies of a large variety of processes occurring during the collisions.

The scientific programme of the CMS experiment is incredibly broad, encompassing the study of the SM processes that involve the newly discovered Higgs-Boson, top quark, and electroweak bosons, among others. With the capacity to collect massive amounts of data, we can execute precision tests to examine theoretical relations between various precise experimental observables predicted in the SM. The new measurement of this electroweak mixing angle falls into this category.

Moreover, CMS is a platform for exploration, used to search for ‘new physics’ event topologies that are not predicted or described by the SM. This includes specific signatures from certain models, like supersymmetry or extra dimensions, that could, for example, provide viable dark matter candidates. Thus, CMS offers a vast potential for discovery and breakthroughs in the understanding of the fundamental laws of nature.
What is the electroweak mixing angle, and how was it measured?

The electroweak mixing angle is one of the key parameters in the SM. It was introduced in the Higgs mechanism, which, in a process known as spontaneous electroweak symmetry breaking, gives mass to the W and Z bosons, the mediators of the weak interaction. This weak mixing angle – or the Weinberg angle relates the masses of the W and Z bosons and also defines the couplings i.e. the strength of the interaction of the Z-boson to quarks and leptons. These couplings are used to define what we call the effective mixing angle, which also depends on the fermion flavour due to radiative corrections.

The experiment used a large number of dilepton events produced in proton-proton collisions to measure the effective leptonic mixing angle. In most of these events, a quark from one of the protons interacts with its anti-quark in the other, producing the Z-boson, which subsequently decays to a lepton pair. If we look at the distribution of the angle of the produced negatively charged lepton relative to the original quark, it has an asymmetry, which is strongly dependent on the value of the leptonic effective mixing angle. We use this measurement of the forward-backward asymmetry to extract the value of this parameter.
Can you explain the Standard Model and its importance?

The Standard Model is the most successful model of particle physics that describes all observed matter and three types of fundamental interactions (electromagnetic, weak, and strong) using only a few building blocks of matter particles and mediators of these interactions. The final formulation was completed in the ‘70s in a combined framework of strong interactions (quantum chromodynamics) and the unified electroweak theory.

Since then, one by one, all of its major predictions have been successfully verified in the experiment – including the discovery of the W and Z bosons at CERN in the early ‘80s and the top quark at Fermilab in 1995. Most recently, the discovery of the Higgs boson at CERN in 2012 was a major triumph, completing all the pieces of the SM.

The obvious question was, what next? When we talk about the SM and all of its successes, there is always an appendix. It is not a complete theory because some questions or phenomena cannot be described, such as the asymmetry between matter and antimatter and the presence of dark matter in the Universe. These phenomena could be explained by a more complete theory or some extension of the SM. Finding these new physics signatures or their indications is now the main goal of the experiments at CERN, and these efforts are actively ongoing.

How is the recent measurement significant for CERN and for particle physics in general?
One reason for its significance is that high-energy hadron colliders and detectors are normally designed for discoveries. They normally probe high-energy regions of phase space not explored before with enormous amounts of hadron collisions. Unlike electron colliders, these produce very crowded events with hundreds or even thousands of background particles emerging from the interaction point: From multiple interactions between the constituents of the protons and from many colliding protons in each event.

This measurement demonstrates that thanks to the powerful CMS detector and particle reconstruction techniques, combined with the dedicated analysis techniques developed for this measurement, we could turn those enormous rates of collisions to our advantage since they also produce a huge number of signal events. These precision measurements are crucial, unlocking a whole new front of indirect searches for new physics beyond the Standard Model at the LHC.
What are the implications of the measurement for our understanding of the Standard Model?

The short answer is that the SM remains successful and resilient.

One motivating factor for this new measurement was that the two most precise previous results obtained in the electron-positron colliders were in disagreement. They were about three standard deviations apart, with each being about two standard deviations up or down from what the Standard Model would predict.

In the subsequent Fermilab Tevatron measurements, the CDF and D0 experiments showed very similar, slightly discrepant results. However, the uncertainties were larger, and so the tension was not significant. The first LHC measurement of this parameter had larger uncertainties but seemed to prefer the lower deviation.

Resolving this discrepancy is very important because if the truth is in the middle, which this most recent measurement prefers, it would mean that the SM prediction is correct. If not, it would indicate new physics. The fact that the new result is in agreement with the SM is significant, and it is also a step towards resolving the conflict presented by the earlier results.

There is definitely still room for improvement and increased precision. As such, we can still only talk about preferences since the uncertainties are not yet small enough to definitively rule one way or another. Planned upgrades to the detector, along with a huge amount of data, will significantly increase the precision of this measurement. Therefore, it is incredibly significant that the experiment has demonstrated the capability to perform this precise measurement, even with the very crowded events from the high-luminosity proton-proton operations.
Could variations in the electroweak mixing angle shed light on the existence of new particles or interactions?


The SM predicts very precise relations between the different experimental observables. As such, the main motivation for this measurement is the potential to shed light on possible new particle interactions. Using various precise experimental inputs, such as the Z-boson mass and width, the Fermi constant from the muon lifetime, the top quark mass, the Higgs-boson mass, etc., the SM can predict the effective leptonic mixing angle very precisely.

Any significant deviation from this prediction would indicate that the SM needs to be altered somehow. Technically, this could be explained by new particles beyond the SM contributing to the radiative corrections of various observables – for example, this parameter or the W boson mass.



These two variables are closely related, and their measurements, which are the active subjects of the LHC programme, should be interpreted together. Concerning the mass of the W boson, until recently, there was a good agreement between theory and various measurements. In 2022, the CDF very precisely measured the mass of the W boson, a result that shook the community because it disagreed with both the previous measurements and the SM prediction. The relation between the two parameters is such that the value of the W boson mass measured by CDF would prefer a lower value of the leptonic effective weak mixing angle.
What are the challenges in running an experiment such as this, and how did you mitigate or address these?

The main challenge of this measurement in proton-proton collisions is that we have to rely on the knowledge of the distribution of the valance quark contributions to the proton momentum. While this also results in degraded statistical uncertainty, the huge amount of data used overcomes this.

The large uncertainty from the parton distribution functions (PDF) is partially mitigated by measuring the observed forward-backward asymmetry as a function of the dilepton’s mass and rapidity and using this measurement to simultaneously constrain the PDFs in situ.

Additionally, for the first time for CMS, this measurement includes the central-forward dielectron pairs, which are less affected by the PDF uncertainties. This required many dedicated developments for the forward electron reconstruction and identification, which were implemented specifically for this analysis.

A considerable amount of effort went into precise, dedicated measurements of lepton selection efficiencies, lepton momentum scale and resolution calibration, background studies, charge misidentification studies, theory models, and PDF studies. It took more than five years of dedicated effort to work out all the fine details of these various components and estimate the corresponding systematic uncertainties.
How will this parameter benefit and improve future experiments, and how will it be used to reexamine past measurements and experiments?

What we have measured now, this electroweak mixing angle, is already the most precise result achieved at a hadron collider. It is already precise enough to have an important impact on the future global electroweak fits.

In addition, what we measure and publish are the unfolded results of the forward-backward asymmetry. This is important because it will allow anyone to simply reinterpret this measurement without the need to know detector-related effects. For example, an improved value can be obtained from these unfolded results once the new improved PDFs are released. The unfolded measurements will also allow for a simple combination with future weak mixing angle and other electroweak measurements (e.g. the W boson mass) at the LHC.

We also need to make some improvements in preparation for future measurements. This will involve addressing the main limiting factor mentioned earlier, the uncertainties of the PDFs. In this regard, it is important for all LHC experiments to continue providing high-precision differential measurements to be included in the global PDF analyses.

On the experimental side, work will be mainly directed into efficiently collecting huge amounts of dilepton events of various categories during the high-luminosity LHC operation. Far from a trivial task, this will likely require dedicated developments at both the high-level and level-one triggers. There remains some work to be done in order to prepare for the LHC’s ultimate measurement of this parameter.
What’s next for the CMS experiment?

As mentioned, for the high-luminosity LHC operation, the accelerator complex will be upgraded to provide a huge increase in the luminosities of the colliding protons at the interaction points. Accompanied also by significant upgrades of the CMS subdetectors and trigger systems, it will enable a huge increase in the amount of interesting data produced, as much as a factor of 20 compared to now. In addition, for this measurement in particular, extending the pseudorapidity coverage of the tracking detectors will be crucial and significantly improve its precision.

With all these upgrades, physics’ reach in all areas will increase dramatically – precision Higgs Boson and other SM measurements, as well as searches for new physics. It’s safe to say the most exciting times are ahead of us.

Please note, this article will also appear in the 18th edition of our quarterly publication.

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