Tag: large hadron collider

How Peter Higgs revealed the forces that hold the universe together

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Peter Higgs at the Science Museum in London in 2013

Photo by Andy Rain/EPA/Shutterstock

Peter Higgs lived a singular life. He developed a physics theory that stood a chance of radically advancing our understanding of the universe, and lived to see generations of experimentalists chase after and eventually triumphantly corroborate his work in the lab. He died in his home at age 94.

“Without Higgs’s work, we wouldn’t understand why there are atoms. Some pretty basic features of our world would not be understandable,” says John Ellis at King’s College London.

Higgs started that work at the University of Edinburgh in the UK in the 1960s. He was thinking about a branch of physics called quantum field theory, and in July of 1964, he took about a week to write a short paper on the topic. Physics Letters accepted the study but rejected Higgs’s more detailed follow-up work just a week later. Even though Physical Review Letters eventually published a revised version of the second paper, it received no fanfare and remained overlooked for years.


Ironically, these papers contained a key ingredient that was sorely lacking from the theory of all particles in the universe: the reason why they have mass.

Almost all known particles need some mass in order to bind to each other and form the structures, like atoms, that comprise our physical world. But physicists understand all particles as excitations of invisible fields that permeate everything – electrons, for example, are excitations of the electromagnetic field – and even the best theories at the time could not explain where these masses come from.

Higgs theorised that particles would acquire mass by interacting with a new type of field. That field had a very special excitation of its own, another particle called the Higgs boson. The Higgs field solved a huge question in theoretical particle physics, and the Higgs boson was a tantalising target that experimentalists could hunt for in order to tie theory to reality.

“If you remove everything from the vacuum, all matter or quantum fluctuations, all electromagnetic stuff, all gravity, you will be left with the Higgs field,” says Frank Close at the University of Oxford. “And we need that just like a goldfish needs water. It stabilises empty space.”

Working independently from Higgs, physicists François Englert and Robert Brout reached the same conclusion, also in 1964.

However, according to Close, who wrote a biography of Higgs in 2022, Higgs did not necessarily set out to write a groundbreaking paper. He simply followed a line of rigorous and often solitary scholarship, which led him to worry deeply about what seemed to be a technical issue that plagued quantum field theory. Other physicists had previously resolved a similar issue in systems with less cosmic implications, such as perfect conductors of electricity. Higgs figured out how to generalise their mathematics to all of particle physics.

But quantum field theory was unfashionable at the time, and when he lectured about his work at prestigious institutions like Harvard University in 1965, Higgs was largely met with scepticism, says Ellis. In 1976, Ellis and two of his colleagues at the CERN particle physics laboratory in Geneva, Switzerland, wrote a paper drawing attention to how the Higgs boson could show up in some experiments at the facility.

“No one really seemed to care, but to us, [the Higgs boson] was extremely important… And I was absolutely sure that the Higgs boson will be found,” says Dimitri Nanopoulos at Texas A&M University, who co-authored the paper. He was a very young researcher at the time, but that study was prescient about the future of particle physics. By 1984, views among physicists had shifted and they were eager to hunt for the Higgs boson. Leadership at CERN discussed building a new particle collider, in large part to help with the search.

That detector – the Large Hadron Collider (LHC) – found the Higgs boson in 2012. Within the LHC, researchers engineered a careful head-on collision of two incredibly fast protons, a crash capable of producing a Higgs boson. But the boson only lasts for less than a billionth of a billionth of a second before becoming a shower of other particles. Analysis of the collision’s wreckage showed those particles had come from a Higgs boson with such high certainty that the odds of it being a fluke were just 5 in 10 million.

Physicists around the world were rapturous, and Higgs and Englert shared a Nobel prize in physics the next year.

Close and Ellis both say that even before the LHC started to operate, other colliders had obtained less direct evidence vindicating Higgs’s theory, such as very precise measurements of masses of other exotic particles. Higgs was aware of these findings, as he explained to New Scientist in 2012: “I had faith in the theory behind the mechanism as other features of it were being verified in great detail at successive colliders. It would have been very surprising if the remaining piece of the jigsaw wasn’t there.”

Still, the direct search for the Higgs boson at the LHC had a strong influence on particle physics. It bolstered efforts to build new infrastructure like accelerators, and cemented the large collaborations that manage this equipment as a standard approach for conducting scientific research.

Since 2012, the LHC has been upgraded to produce even more energetic collisions, and researchers have set out to answer lingering questions about not only particles, including the Higgs boson itself, but also dark energy and dark matter, the unexplained phenomena that make up most of the universe.

Higgs himself was interested in some of those questions and kept working on them even after he retired in 1996. “The machine at Geneva – which was not designed just to discover the Higgs boson, though sometimes you get that impression – is expected to go on and improve our understanding of the links between particle physics and what happened in the early universe,” he told New Scientist in 2013.

Finding the Higgs boson was the end of one chapter, but not the whole book, says Nanopoulos.

After his retirement, Higgs kept working on his own research. He was particularly interested in supersymmetry, which is a theory that posits the existence of heavy counterparts for every particle that we have detected already. Physicists who share this interest and want to find its experimental signatures hope that the LHC will discover dozens of new particles.

In addition to the Nobel prize, Higgs received several other accolades, including the Paul Dirac Medal and Prize, the Wolf Prize in Physics and the American Physical Society J. J. Sakurai Prize. In 1999, he turned down a knighthood, an act that fit his general rejection of fame. He did not want titles and was embarrassed by the media attention his work garnered over the years, particularly disliking the Higgs boson’s sensational nickname, the “God particle”.

The story of how Higgs even tried to evade the call from the Royal Swedish Academy of Sciences informing him of his Nobel win – by leaving his home without a cell phone – is well-known lore among physicists. Ellis also recalls that Higgs initially turned down the invitation to come to CERN for the official announcement of the discovery of his eponymous boson. But colleagues eventually convinced him to attend the festivities.

Close titled his biography of Higgs “Elusive”, which he says described both the man and the boson. Physicists widely agree that he was one of a kind and respected him for it.

Higgs died in his home in Edinburgh on 8 April after a short illness. He leaves behind two sons, a reinvigorated field of particle-seeking physicists and a clearer understanding of the forces that hold the universe together.

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The ultra-careful quest to find the shape of the electron’s charge

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Studies that test some physical property to an extreme precision are gaining in popularity these days because many physicists are intently looking for small chinks – too small for them to have noticed without a closer look – in a theory that is both powerful yet incomplete. This is the Standard Model of particle physics.

It predicts the existence of different particles; the last of them to be found was the Higgs boson, in 2012. But while the Model is incomplete, its zoo of particles and their combined interactions haven’t been able to explain many things about nature and the universe. For example, the Model doesn’t say what dark matter is and can’t explain dark energy. It doesn’t know why the Higgs boson is so heavy or why gravity is so much weaker than the other fundamental forces.

Where did the antimatter go?

The Model also predicts that when the universe was created, it should have had equal quantities of matter and antimatter – which is clearly not the case.

The equal quantities of the two substances would have annihilated each other, releasing energy in the form of light, so the universe should have been full of light. Yet today, the universe has large amounts of matter and no antimatter. This is one important line of inquiry in the quest to find a flaw in the Standard Model, an edge that is incomplete and could lead the way to a ‘new physics’ to resolve some or all of these mysteries.

In a new study published in Science, researchers from the University of Colorado, Boulder, have reported that they couldn’t find evidence of certain kinds of such ‘new physics’ in an experiment with electrons. This experiment looked for the evidence at the highest precision to date.

The negative result is important because it will tell physicists which alternative theories are feasible. For example, if a theory predicts that an electron would do X in the presence of a very strong electric field, but the new study’s results disagree, then physicists now know to modify their theory to prevent this possibility. The previous such result from a different experiment told physicists that the evidence they were looking for wouldn’t be found at the Large Hadron Collider in Europe.

The Sakharov conditions

In 1967, the Soviet physicist (and Nobel Peace Prize laureate) Andrei Sakharov considered the matter-antimatter asymmetry problem and came up with a set of conditions that, if they’re met, would allow the universe to produce more matter and antimatter. These are (i) baryon number violation, (ii) C- and CP-symmetry violation, and (iii) baryon production rate must be slower than the universe’s expansion rate.

One of the fundamental particles that makes up matter is the quark. A baryon is a particle made up of three quarks. Examples include the proton and the neutron. Every baryon is assigned a baryon number: the number of quarks minus the number of anti-quarks, divided by 3. When a baryon interacts with another particle according to the rules of the Standard Model, the baryon number is conserved, i.e. the total baryon number at the start of the interaction should equal the number at the end.

But Sakharov’s first condition is that for matter to gain an upper hand over antimatter, this rule should be broken in an interaction. That is, this interaction should produce more baryons than anti-baryons (i.e. a baryon made of anti-quarks).

C-symmetry is short for ‘charge conjugation symmetry’. Charge conjugation is a process that replaces a particle with its anti-particle, and as a result flips its charge (positive to negative or negative to positive). If C-symmetry is violated, then there will also be more processes that produce baryons than those that produce anti-baryons.

Like C-symmetry, P-symmetry refers to parity symmetry: if a particular interaction between particles is valid, then its mirror-image – i.e. how you might see the interaction in a mirror – should be equally valid. CP-symmetry refers to an interaction violating C-symmetry and P-symmetry together.

The final Sakharov condition is that the rate at which baryons and anti-baryons are produced should be outpaced by the universe’s expansion. This arises from a simple principle. Consider a hypothetical chemical reaction: A + B → C + D. As the reaction proceeds, the quantity of A + B will dwindle while the quantity of C + D will accumulate. This could cause the reaction to reverse itself: C + D → A + B. To prevent such a reversal, the simplest thing to do is to identify some condition that allows A + B → C + D but not C + D → A + B, like, for example, maintaining a high temperature, and then apply that condition.

Similarly, the third Sakharov condition stipulates that the universe should expand faster than the rate at which baryons are produced, so that a compensatory reverse process doesn’t arise that increases the number of anti-baryons.

So far, physicists have discovered C- and CP-symmetry violation, but only in particles that have quarks. The resulting matter-antimatter asymmetry is insufficient to explain matter’s dominance in the universe today. This means there should be some ‘new physics’, i.e. an extension of the Standard Model, that allows more CP-symmetry violation.

The electron dipole moment

CP-symmetry is a dyadic symmetry – it has two parts – that is actually part of a larger triadic symmetry called CPT. ‘T’ is for time, and T-symmetry means that a particle interaction in one direction that is favoured in forward time should be favoured in the reverse direction when time flows backwards. That is, the laws of physics are the same forward and backward in time. CP-symmetry violation is considered to be equivalent to T-symmetry violation.

In their new study, the University of Colorado researchers checked whether the electric charge of an electron is located at its centre or is slightly off to one side. If it is indeed off, the electron would have a dipole: more negative charge on one side of the particle and more positive charge on the other. And such a dipole will defy T-symmetry.

The dipole has a strength, called the dipole moment, depending on how off-centre the electron’s charge is. “If time were reversed, [an electron’s spin] would flip and the [electric dipole moment] would not, looking fundamentally different from before time-reversal,” independent physicists Mingyu Fan and Andrew Jayich, of the University of California, Santa Barbara, wrote in a commentary accompanying the new paper in Science.

The Standard Model allows the electron to have an electric dipole moment of up to 10-38e cm (e is the electron’s charge). Anything more than this and the Model will break, signalling the effect of some ‘new physics’.

The experiment to look for the electron electric dipole moment (eEDM) measured the energy difference between two states of an electron – one when its spin is in the direction of an external electric field and the other when its spin is aligned opposite to that of the field. In the absence of an eEDM, the energy difference should be zero. If an eEDM is present, one of the electron states should have slightly more energy, and the difference can be used to calculate its value.

Sophisticated techniques

The difference is more pronounced when the external electric field is stronger. Technology has advanced to the extent that physicists can apply extremely powerful fields in their labs, but the most powerful still exist in nature. In the new study, the physicists studied valence electrons in molecules of hafnium fluoride (HfF), which exerted an electric field of around 23 billion V/cm – more than 10,000-times stronger than what researchers can create in the lab, albeit over shorter distances.

The study is easier explained than done, requiring a suite of sophisticated instruments and techniques – some to make the measurements, others to reduce noise and uncertainty in the resulting data, given the smallness of values involved. The research team ionised thousands of HfF molecules and held them in a trap, using lasers to bring them to particular energy states. An external magnetic field was applied to negate noise in parts of the trap. A small electric field was also applied to orient the molecules.

Once the setup was ready, the team ‘created’ the electrons in the two energy states and then measured the energy difference between them using a technique called Ramsey spectroscopy.

According to a 2013 paper by Huanqian Loh, then a doctoral student at the University of Colorado, an eEDM measurement is more sensitive if the external electric field is stronger, if the measurement is coherent for longer, and if the signal-to-noise ratio is higher (i.e. if an electron flips more often between the two states). So in order to make a more sensitive measurement, the team had to optimise for all these attributes.

Ultimately, the team estimated that the electron’s eEDM to be lower than 4.1 × 10-30e cm at a 90% confidence. The team’s paper stated that the “result is consistent with zero and improves on the previous best upper bound by a factor of ~2.4.” The measurement is still eight orders of magnitude above the limit that the Standard Model allows, yet it is useful because it steps closer from the previous measurement.

Since the result is “consistent with zero” up to a certain energy level, it also precludes the existence of hypothetical ‘new physics’ particles up to that level. Drs. Fan and Jayich commended this implication when they compared the team’s feat, achieved with an “apparatus that fits on a table”, to that of “the Large Hadron Collider at CERN, which costs about $4.75 billion to build and $1 billion to run annually,” and probes nature up to a lower energy level, albeit differently.

“Knowledge from eEDM measurements across multiple systems would help guide the requirements of a future high-energy particle collider that could create the time-symmetry-violating particles responsible” for the matter-antimatter asymmetry in the early universe,” the commentary noted.

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Explained | A beginner’s guide to the Large Hadron Collider

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A general view of the LHC experiment during a media visit at CERN near Geneva, Switzerland, July 23, 2014.
| Photo Credit: Science-CERN, Reuters/Pierre Albouy

The Large Hadron Collider (LHC) is three things. First, it is large – so large that it’s the world’s largest science experiment. Second, it’s a collider. It accelerates two beams of particles in opposite directions and smashes them head on. Third, these particles are hadrons. The LHC, built by the European Organisation for Nuclear Research (CERN), is on the energy frontier of physics research, conducting experiments with highly energised particles.

Currently, engineers are warming up the LHC for its third season of operations, following upgrades that will have made the collider and its detectors more sensitive and accurate than before. It will start collecting data again from mid-May.

How does the LHC work?

A typical candidate event inside the LHC, ‘seen’ by the CMS detector in which a collision between two beams has produced two high-energy photons (depicted by red towers) and other particles (yellow lines). The pale blue volume depicts the detector volume.

A typical candidate event inside the LHC, ‘seen’ by the CMS detector in which a collision between two beams has produced two high-energy photons (depicted by red towers) and other particles (yellow lines). The pale blue volume depicts the detector volume.
| Photo Credit:
AP Photo/CERN

A hadron is a subatomic particle made up of smaller particles. The LHC typically uses protons, which are made up of quarks and gluons. It energises the protons by accelerating them through a narrow circular pipe that is 27 km long.

Simply put, this pipe encircles two D-shaped magnetic fields, created by almost 9,600 magnets. Say there is a proton at the 3 o’clock position – it is made to move from there to the 9 o’clock position by turning on one hemisphere of magnets and turning off the other, such that the magnetic field acting on the proton causes it to move clockwise. Once it reaches the 9 o’clock position, the magnetic polarity is reversed by turning off the first hemisphere and turning on the second. This causes the proton to move in an anticlockwise direction, from the 9 o’clock back to the 3 o’clock position.

This way, by switching the direction of the magnetic field more and more rapidly, protons can be accelerated through the beam pipe. There are also other components to help them along and to focus the particles and keep them from hitting the pipe’s walls.

Eventually, the protons move at 99.999999% of the speed of light. According to the special theory of relativity, the energy of an object increases with its speed (specifically, through the equation E 2 = p 2c 2 + m 2c 4, where p is momentum, equal to mass times velocity).

What happens when the particles are smashed?

A view of the LHC in its tunnel at CERN, near Geneva, Switzerland.

A view of the LHC in its tunnel at CERN, near Geneva, Switzerland.
| Photo Credit:
Martial Trezzini/Keystone via AP

When two antiparallel beams of energised protons collide head on, the energy at the point of collision is equal to the sum of the energy carried by the two beams.

Thus far, the highest centre-of-mass collision energy the LHC has achieved is 13.6 TeV. This is less energy than what would be produced if you clapped your hands once. The feat is that the energy is packed into a volume of space the size of a proton, which makes the energy density very high.

At the moment of collision, there is chaos. There is a lot of energy available, and parts of it coalesce into different subatomic particles under the guidance of the fundamental forces of nature. Which particle takes shape depends on the amount and flavour of energy available and which other particles are being created or destroyed around it.

Some particles are created very rarely. If, say, a particle is created with a probability of 0.00001%, there will need to be at least 10 million collisions to observe it. Some particles are quite massive and need a lot of the right kind of energy to be created (this was one of the challenges of discovering the Higgs boson). Some particles are extremely short-lived, and the detectors studying them need to record them in a similar timeframe or be alert to proxy effects.

The LHC’s various components are built such that scientists can tweak all these parameters to study different particle interactions.

What has the LHC found?

Fabiola Gianotti, then spokesperson of the ATLAS detector at the LHC announcing the discovery of a particle consistent with the Higgs boson at CERN on July 4, 2012.

Fabiola Gianotti, then spokesperson of the ATLAS detector at the LHC announcing the discovery of a particle consistent with the Higgs boson at CERN on July 4, 2012.
| Photo Credit:
AP

The LHC consists of nine detectors. Located over different points on the beam pipe, they study particle interactions in different ways. The ATLAS and CMS detectors discovered the Higgs boson in 2012 and confirmed their findings in 2013, for example.

Every year, the detectors generate 30,000 TB of data worth storing, and even more overall. Physicists pore through it with the help of computers to identify and analyse specific patterns.

The LHC specialises in accelerating a beam of hadronic particles to certain specifications and delivering it. Scientists can choose to do different things with the beam. For example, they have energised and collided lead ions with each other and protons with lead ions at the LHC.

Using the data from all these collisions, they have tested the predictions of the Standard Model of particle physics, the reigning theory of subatomic particles; observed exotic particles like pentaquarks and tetraquarks and checked if their properties are in line with theoretical expectations; and pieced together information about extreme natural conditions, like those that existed right after the Big Bang.

What is the LHC’s future?

These successes strike a contrast with what the LHC hasn’t been able to find: ‘new physics’, the collective name for particles or processes that can explain the nature of dark matter or why gravity is such a weak force, among other mysteries.

The LHC has tested some of the predictions of theories that try to explain what the Standard Model can’t, and caught them short. This has left the physics community in a bind.

One way forward, already in the works, is to improve the LHC’s luminosity (a measure of the machine’s ability to produce particle interactions of interest) by 10x by 2027 through upgrades.

Another, more controversial idea is to build a bigger, badder version of the LHC, based on the hypothesis that such a machine will be able to find ‘new physics’ at even higher energies.

While both CERN and China have unveiled initial plans of bigger machines, physicists are divided on whether the billions of dollars they will cost can be used to build less-expensive experiments, including other colliders, and with guaranteed instead of speculative results.

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