Our universe was believed to have been created with equal quantities of matter and antimatter, only for antimatter to completely disappear over time. We know that matter and antimatter can annihilate each other but we don’t know how matter came to gain an upper hand and survive to this day, creating, stars, planets, and – of course – us.
In the theories that physicists have to explain the universe, they believe that the matter-antimatter asymmetry is the result of two natural symmetries being violated. These are the charge and parity symmetries. The charge (C) symmetry is that the universe would work the same way if we replaced all the positive charges with negative charges and vice versa. The parity (P) symmetry refers to the handedness of a particle. For example, based on which way an electron is spinning, it’s said to be right- or left-handed. All the fundamental forces that act between particles preserve their handedness except the weak nuclear force.
According to most particle physicists, matter won the war against antimatter through some process that violated both C and P symmetries. Proof of CP symmetry violation is one of modern physics’s most important unsolved problems.
In 1964, physicists discovered that the weak nuclear force is capable of violating C and P symmetries together when it acts on a particle called a K meson. In the 2000s, a different group of physicists found more evidence of CP symmetry violation in particles called B mesons. These discoveries proved that CP symmetry violation is actually possible, but they didn’t bring us much closer to understanding why matter dominated antimatter. This is because of particles called quarks.
Quarks are the smallest known constituent of the universe’s matter particles. They combine to form different types of bigger particles. For example, all mesons have two quarks each. All the matter that we’re familiar with are instead made of atoms, which are in turn made of protons, neutrons, and electrons. Protons and neutrons have three quarks each – they’re baryons. Electrons are not made of quarks; instead, they belong to a group called leptons.
To explain the matter-antimatter asymmetry in the universe, physicists need to find evidence of CP symmetry violation in baryons, and this hasn’t happened so far.
On December 7, a group of researchers from China published a paper in the journal Physical Review D in which they proposed one place where physicists could look to find the answer: the decay of a particle called a lambda-b baryon to a D-meson and a neutron.
Quarks come in six types, or flavours. They are up, down, charm, strange, top, and bottom. A lambda-b baryon is the name for a bundle containing one up quark, one down quark, and one strange quark. A D-meson is any meson that contains a charm quark. In the process the researchers have proposed, the D-meson exists in a superposition of two states: a charm quark + an up anti-quark (D0 meson) and a charm anti-quark and an up quark (D0 anti-meson).
The researchers have proposed that the probability of a lambda-b baryon decaying to a D0 meson versus a D0 anti-meson could be significantly different as a result of CP symmetry violation.
The proposal is notable because the researchers have tailored their prediction to an existing experiment that, once it’s upgraded in future, will collect data that can be used to look for just such a discrepancy. This experiment is called the LHCb – ‘LHC’ for Large Hadron Collider and ‘b’ for beauty.
The LHCb is a detector on the LHC, the famous particle-smasher in Europe that slams energetic beams of protons together to pry them open. The detectors then study the particles in the detritus and their properties. LHCb in particular tracks the signatures of different types of quarks. Physicists at CERN are planning to upgrade LHCb to a second avatar that’s expected to begin operating in the mid-2030s. Among other features, it will have a 7.5-times higher peak luminosity – a measure of the number of particles the detector can detect.
If the lambda-b baryon’s decay discrepancy exists in the new LHCb’s observed data, the decay proposed in the new study will be one way to explain it, and pave the way for proof of CP symmetry violation in baryons.
The world’s single largest science experiment will restart on March 23 after a two-year break. Scientists and administrators at the European Organization for Nuclear Research – known by its French acronym CERN – have announced the status of the agency’s upgrades on its Large Hadron Collider (LHC) and its readiness for a new phase of experiments running from now until 2018.
Before the experiment was shut down in late 2013, the LHC became famous for helping discover the elusive Higgs boson, a fundamental (that is, indivisible) particle that gives other fundamental particles their mass through a complicated mechanism. The find earned two of the physicists who thought up the mechanism in 1964, Peter Higgs and Francois Englert, a Nobel Prize in that year.
Though the LHC had fulfilled one of its more significant goals by finding the Higgs boson, its purpose is far from complete. In its new avatar, the machine boasts of the energy and technical agility necessary to answer questions that current theories of physics are struggling to make sense of.
As Alice Bean, a particle physicist who has worked with the LHC, said, “A whole new energy region will be waiting for us to discover something.”
The finding of the Higgs boson laid to rest speculations of whether such a particle existed and what its properties could be, and validated the currently reigning set of theories that describe how various fundamental particles interact. This is called the Standard Model, and it has been successful in predicting the dynamics of those interactions.
From the what to the why
But having assimilated all this knowledge, what physicists don’t know, but desperately want to, is why those particles’ properties have the values they do. They have realized the implications are numerous and profound: ranging from the possible existence of more fundamental particles we are yet to encounter to the nature of the substance known as dark matter, which makes up a great proportion of matter in the universe while we know next to nothing about it. These mysteries were first conceived to plug gaps in the Standard Model but they have only been widening since.
With an experiment now able to better test theories, physicists have started investigating these gaps. For the LHC, the implication is that in its second edition it will not be looking for something as much as helping scientists decide where to look to start with.
As Tara Shears, a particle physicist at the University of Liverpool, toldNature, “In the first run we had a very strong theoretical steer to look for the Higgs boson. This time we don’t have any signposts that are quite so clear.”
Higher energy, luminosity
The upgrades to the LHC that would unlock new experimental possibilities were evident in early 2012.
The machine works by using powerful electric currents and magnetic fields to accelerate two trains, or beams, of protons in opposite directions, within a ring 27 km long, to almost the speed of light and then colliding them head-on. The result is a particulate fireworks of such high energy that the most rare, short-lived particles are brought into existence before they promptly devolve into lighter, more common particles. Particle detectors straddling the LHC at four points on the ring record these collisions and their effects for study.
So, to boost its performance, upgrades to the LHC were of two kinds: increasing the collision energy inside the ring and increasing the detectors’ abilities to track more numerous and more powerful collisions.
The collision energy has been nearly doubled in its second life, from 7-8 TeV to 13-14 TeV. The frequency of collisions has also been doubled from one set every 50 nanoseconds (billionth of a second) to one every 25 nanoseconds. Steve Myers, CERN’s director for accelerators and technology, had said in December 2012, “More intense beams mean more collisions and a better chance of observing rare phenomena.”
The detectors have received new sensors, neutron shields to protect from radiation damage, cooling systems and superconducting cables. An improved fail-safe system has also been installed to forestall accidents like the one in 2008, when failing to cool a magnet led to a shut-down for eight months.
In all, the upgrades cost approximately $149 million, and will increase CERN’s electricity bill by 20% to $65 million. A “massive debugging exercise” was conducted last week to ensure all of it clicked together.
Going ahead, these new specifications will be leveraged to tackle some of the more outstanding issues in fundamental physics.
CERN listed a few–presumably primary–focus areas. They include investigating if the Higgs boson could betray the existence of undiscovered particles, the particles dark matter could be made of, why the universe today has much more matter than antimatter, and if gravity is so much weaker than other forces because it is leaking into other dimensions.
Stride forward in three frontiers
Physicists are also hopeful for the prospects of discovering a class of particles called supersymmetric partners. The theory that predicts their existence is called supersymmetry. It builds on some of the conclusions of the Standard Model, and offers predictions that plug its holes as well with such mathematical elegance that it has many of the world’s leading physicists enamored. These predictions involve the existence of new particles called partners.
In a neat infographic by Elizabeth Gibney in Nature, she explains that the partner that will be easiest to detect will be the ‘stop squark’ as it is the lightest and can show itself in lower energy collisions.
In all, the LHC’s new avatar marks a big stride forward not just in the energy frontier but also in the intensity and cosmic frontiers. With its ability to produce and track more collisions per second as well as chart the least explored territories of the ancient cosmos, it’d be foolish to think this gigantic machine’s domain is confined to particle physics and couldn’t extend to fuel cells, medical diagnostics or achieving systems-reliability in IT.
Here’s a fitting video released by CERN to mark this momentous occasion in the history of high-energy physics.
Featured image: A view of the LHC. Credit: CERN
Update: After engineers spotted a short-circuit glitch in a cooled part of the LHC on March 21, its restart was postponed from March 23 by a few weeks. However, CERN has assured that its a fully understood problem and that it won’t detract from the experiment’s goals for the year.
In our universe, matter is king: it makes up everything. Its constituents are incredibly tiny particles – smaller than even the protons and neutrons they constitute – and they work together with nature’s forces to make up… everything.
There was also another form of particle once, called antimatter. It is extinct today, but when the universe was born 13.82 billion years ago, there were equal amounts of both kinds.
Nobody really knows where all the antimatter disappeared to or how, but they are looking. Some others, however, are asking another question: did antimatter, while it lasted, fall downward or upward in response to gravity?
Joel Fajans, a professor at the University of California, Berkeley, is one of the physicists doing the asking. “It is the general consensus that the interaction of matter with antimatter is the same as gravitational interaction of matter,” he told this correspondent.
But he wants to be sure, because what he finds could revolutionize the world of physics. Over the years, studying particles and their antimatter counterparts has revealed most of what we know today about the universe. In the future, physicists will explore their minuscule world, called the quantum world, further to see if answers to some unsolved problems are found. If, somewhere, an anomaly is spotted, it could pave the way for new explanations to take over.
“Much of our basic understanding of the evolution of the early universe might change. Concepts like dark energy and dark matter might have be to revised,” Fajans said.
Along with his colleague Jonathan Wurtele, Fajans will work with the ALPHA experiment at CERN to run an elegant experiment that could directly reveal gravity’s effect on antimatter. ALPHA stands for Anti-hydrogen Laser Physics Apparatus.
We know gravity acts on a ball by watching it fall when dropped. On Earth, the ball will fall toward the source of the gravitational pull, a direction called ‘down’. Fajans and Wurtele will study if down is in the same place for antimatter as for matter.
An instrument at CERN called the anti-proton decelerator (AD) synthesizes the antimatter counterpart of protons for study in the lab at a low energy. Fajans and co. will then use the ALPHA experiment’s setup to guide them into the presence of anti-electrons derived from another source using carefully directed magnetic fields.
When an anti-proton and an anti-electron come close enough, their charges will trap each other to form an anti-hydrogen atom.
Because antimatter and matter annihilate each other in a flash of energy, they couldn’t be let near each other during the experiment. Instead, the team used strong magnetic fields to form a force-field around the antimatter, “bottling” it in space.
Once this was done, the experiment was ready to go. Like fingers holding a ball unclench, the magnetic fields were turned off – but not instantaneously. They were allowed to go from ‘on’ to ‘off’ over 30 milliseconds. In this period, the magnetic force wears off and lets gravitational force take its place.
And in this state, Fajans and his team studied which way the little things moved: up or down.
The results
The first set of results from the experiment have allowed no firm conclusions to be drawn. Why? Fajans answered, “Relatively speaking, gravity has little effect on the energetic anti-atoms. They are already moving so fast that they are barely affected by the gravitational forces.” According to Wurtele, about 411 out 434 anti-atoms in the trap were so energetic that the way they escaped from the trap couldn’t be attributed to gravity’s pull or push on them.
Among them, they observed roughly equal numbers of anti-atoms to falling out at the bottom of the trap as at the top (and sides, for that matter.)
They shared this data with their ALPHA colleagues and two people from the University of California, lecturer Andrew Charman and postdoc Andre Zhmoginov. They ran statistical tests to separate results due to gravity from results due to the magnetic field. Again, much statistical uncertainty remained.
The team has no reason to give up, though. For now, they know that gravity would have to be 100 times stronger than it is for them to see any of its effects on anti-hydrogen atoms. They have a lower limit.
Moreover, the ALPHA experiment is also undergoing upgrades to become ALPHA-2. With this avatar, Fajans’s team also hopes to incorporate laser-cooling, a method of further slowing the anti-atoms, so that the effects of gravity are enhanced. Michael Doser, however, is cautious.
The future
As a physicist working with antimatter at CERN, Doser says, “I would be surprised if laser cooling of antihydrogen atoms, something that hasn’t been attempted to date, would turn out to be straightforward.” The challenge lies in bringing the systematics down to the point at which one can trust that any observation would be due to gravity, rather than due to the magnetic trap or the detectors being used.
Fajans and co. also plan to turn off the magnets more slowly in the future to enhance the effects of gravity on the anti-atom trajectories. “We hope to be able to definitively answer the question of whether or not antimatter falls down or up with these improvements,” Fajans concluded.
Like its larger sibling, the Large Hadron Collider, the AD is also undergoing maintenance and repair in 2013, so until the next batch of anti-protons are available in mid-2014, Fajans and Wurtele will be running tests at their university, checking if their experiment can be improved in any way.
They will also be taking heart from there being two other experiments at CERN that can verify their results if they come up with something anomalous, two experiments working with antimatter and gravity. They are the Anti-matter Experiment: Gravity, Interferometry, Spectrocopy (AEGIS), for which Doser is the spokesperson, and the Gravitational Behaviour of Anti-hydrogen at Rest (GBAR).
Together, they carry the potential benefit of an independent cross-check between techniques and results. “This is less important in case no difference to the behaviour of normal matter is found,” Doser said, “but would be crucial in the contrary case. With three experiments chasing this up, the coming years look to be interesting!”
—
This post, as written by me, originally appeared in The Copernican science blog at The Hindu on May 1, 2013.
If you look outside your window at the clouds, the stars, the planets, all that you will see is made of matter. However, when the universe was born, there were equal amounts of matter and antimatter. So where has all the antimatter gone?
The answer, if one is found, will be at the Large Hadron Collider (LHC), the world’s most powerful particle physics experiment, now taking a breather while engineers refit it to make it even more powerful by 2015. Then, it will be able to spot tinier, much more shortlived particles than the Higgs boson, which itself is notoriously shortlived.
While it ran from 2008 to early 2013, the LHC was incredibly prolific. It smashed together billions of protons in each experiment at speeds close to light’s, breaking them open. Physicists hoped the things that’d tumble out might show why the universe has come to prefer matter over antimatter.
In fact, from 2013 to 2015, physicists will be occupied gleaning meaningful results from each of these experiments because they simply didn’t have enough time to sift through all of them while the machine was running.
They will present their results as papers in scientific journals. Each paper will will be the product of analysis conducted on experimental data corresponding to some experiment, each with some energy, some luminosity, and other such experimental parameters central to experimental physics.
One such paper was submitted to a journal on April 23, 2013, titled ‘First observation of CP violation in the decays of B_s mesons‘. According to this paper, its corresponding experiment was conducted in 2011, when the LHC was smashing away at 7 TeV centre-of-mass (c.o.m) collision energy. This is the energy at the point inside the LHC circuit where two bunches of protons collide.
The paper also notes that the LHCb detector was used to track the results of the collision. The LHCb is one of seven detectors situated on the LHC’s ring. It has been engineered to study a particle known as the beauty quark, which is more than 4.2 times heavier than a proton, and lasts for about one-hundred-trillionth of a second before breaking down into lighter particles, a process mediated by some of nature’s four fundamental forces.
The beauty is one of six kinds of quarks, and together with other equally minuscule particles called bosons and leptons, they all make up everything in the universe: from entire galaxies to individual atoms.
For example, for as long as it lives, the beauty quark can team up with another quark or antiquark, the antimatter counterpart, to form particles called mesons. Generally, mesons are particles composed of one quark and one antiquark.
Why don’t the quark and antiquark meet and annihilate each other in a flash of energy? Because they’re not of the same type. If a quark of one type and an antiquark of another type meet, they don’t annihilate.
The B_s meson that the April 23 paper talks about is a meson composed of one beauty antiquark and one strange quark. Thus the notation ‘B_s’: A B-meson with an s component. This meson violates a law of the universe physicists long though unbreakable, called the charge-conjugation/parity (CP) invariance. It states that if you took a particle, inverted its charge (‘+’ to ‘-‘ or ‘-‘ to ‘+’), and then interchanged its left and right, its behaviour shouldn’t change in a universe that conserved charge and parity.
Physicists, however, found in the 2011 LHCb data that the B_s meson was flouting the CP invariance rule. Because of the beauty antiquark’s and strange quark’s short lifetimes, the B_s meson only lasted for so long before breaking down into lighter particles, in this case called kaons and pions.
When physicists calculated the kaons‘s and pions‘s charges and compared it to the B_s meson’s, they added up. However, when they calculated the kaons‘s and pions‘s left- and right-handednesses, i.e. parities, in terms of which direction they were spinning in, they found an imbalance.
A force, called the weak force, was pushing a particle to spin one way instead of the other about 27 per cent of the time. According to the physicists’ paper, this result has been reached with a confidence-level of more than 5-sigma. This means that some reading in the data would disagree with their conclusion not more than 0.00001 per cent of the time, sufficient to claim direct evidence.
Of course, this wouldn’t be the first time evidence of CP violation in B-mesons had been spotted. On 17 May, 2010, B-mesons composed of a beauty antiquark and a down quark were shown shown to decay at a much slower rate than B-antimesons of the same composition, in the process outlasting them. However, this is the first time evidence of this violation has been found in B_s mesons, a particle that has been called “bizarre”.
While this flies in the face of a natural, intuitive understanding of our universe, it is a happy conclusion because it could explain the aberration that is antimatter’s absence, one that isn’t explained by a theory in physics called the Standard Model.
Here was something in the universe that was showing some sort of a preference, ready to break the symmetry and uniformity of laws that pervade the space-time continuum.
Physicists know that the weak force, one of the fundamental forces of nature like gravity is, is the culprit. It has a preference for acting on left-handed particles and right-handed antiparticles. When such a particle shows itself, the weak force offers to mediate its breakdown into lighter particles, in the process resulting in a preference for one set of products over another.
But in order to fully establish the link between matter’s domination and the weak force’s role in it, physicists have to first figure out why the weak force has such biased preferences.
—
This post originally appeared in The Copernican science blog at The Hinduon April 25, 2013.
At the Large Hadron Collider (LHC) at CERN, near Geneva, Switzerland, experiments are conducted by many scientists who don’t quite know what they will see, but know how to conduct the experiments that will yield answers to their questions. They accelerate beams of particles called protons to smash into each other, and study the fallout.
There are some other scientists at CERN who know approximately what they will see in experiments, but don’t know how to do the experiment itself. These scientists work with beams of antiparticles. According to the Standard Model, the dominant theoretical framework in particle physics, every particle has a corresponding particle with the same mass and opposite charge, called an anti-particle.
In fact, at the little-known AEgIS experiment, physicists will attempt to produce an entire beam composed of not just anti-particles but anti-atoms by mid-2014.
AEgIS is one of six antimatter experiments at CERN that create antiparticles and anti-atoms in the lab and then study their properties using special techniques. The hope, as Dr. Jeffrey Hangst, the spokesperson for the ALPHA experiment, stated in an email, is “to find out the truth: Do matter and antimatter obey the same laws of physics?”
Spectroscopic and gravitational techniques will be used to make these measurements. They will improve upon, “precision measurements of antiprotons and anti-electrons” that “have been carried out in the past without seeing any difference between the particles and their antiparticles at very high sensitivity,” as Dr. Michael Doser, AEgIS spokesperson, told this Correspondent via email.
The ALPHA and ATRAP experiments will achieve this by trapping anti-atoms and studying them, while the ASACUSA and AEgIS will form an atomic beam of anti-atoms. All of them, anyway, will continue testing and upgrading through 2013.
Working principle
Precisely, AEgIS will attempt to measure the interaction between gravity and antimatter by shooting an anti-hydrogen beam horizontally through a vacuum tube and then measuring how it much sags due to the gravitational pull of the Earth to a precision of 1 per cent.
The experiment is not so simple because preparing anti-hydrogen atoms is difficult. As Dr. Doser explained, “The experiments concentrate on anti-hydrogen because that should be the most sensitive system, as it is not much affected by magnetic or electric fields, contrary to charged anti-particles.”
First, antiprotons are derived from the Antiproton Decelerator (AD), a particle storage ring which “manufactures” the antiparticles at a low energy. At another location, a nanoporous plate is bombarded with anti-electrons, resulting in a highly unstable mixture of both electrons and anti-electrons called positronium (Ps).
The Ps is then excited to a specific energy state by exposure to a 205-nanometre laser and then an even higher energy state called a Rydberg level using a 1,670-nanometre laser. Last, the excited Ps traverses a special chamber called a recombination trap, when it mixes with antiprotons that are controlled by precisely tuned magnetic fields. With some probability, an antiproton will “trap” an anti-electron to form an anti-hydrogen atom.
Applications
Before a beam of such anti-hydrogen atoms is generated, however, there are problems to be solved. They involve large electric and magnetic fields to control the speed of and collimate the beams, respectively, and powerful cryogenic systems and ultra-cold vacuums. Thus, Dr. Doser and his colleagues will spend many months making careful changes to the apparatus to ensure these requirements work in tandem by 2014.
While antiparticles were first discovered in 1959, “until recently, it was impossible to measure anything about anti-hydrogen,” Dr. Hangst wrote. Thus, the ALPHA and AEgIS experiments at CERN provide a seminal setting for exploring the world of antimatter.
Anti-particles have been used effectively in many diagnostic devices such as PET scanners. Consequently, improvements in our understanding of them feed immediately into medicine. To name an application: Antiprotons hold out the potential of treating tumors more effectively.
In fact, the feasibility of this application is being investigated by the ACE experiment at CERN.
In the words of Dr. Doser: “Without the motivation of attempting this experiment, the experts in the corresponding fields would most likely never have collaborated and might well never have been pushed to solve the related interdisciplinary problems.”
In particle physics, CPT symmetry is an attribute of the universe that is held as fundamentally true by quantum field theory (QFT). It states that the laws of physics should not be changed and the opposite of all allowed motions be allowed (T symmetry) if a particle is replaced with its antiparticle (C symmetry) and then left and right are swapped (P symmetry).
What this implies is a uniformity of the particle’s properties across time, charge and orientation, effectively rendering them conjugate perspectives.
(T-symmetry, called so for an implied “time reversal”, defines that if a process moves one way in time, its opposite is signified by its moving the other way in time.)
The more ubiquitously studied version of CPT symmetry is CP symmetry with the assumption that T-symmetry is preserved. This is because CP-violation, when it was first observed by James Cronin and Val Fitch, shocked the world of physics, implying that something was off about the universe. Particles that ought to have remained “neutral” in terms of their properties were taking sides! (Note: CPT-symmetry is considered to be a “weaker symmetry” then CP-symmetry.)
Val Logsdon Fitch (L) and James Watson Cronin
In 1964, Oreste Piccioni, who had just migrated to the USA and was working at the Lawrence Berkeley National Laboratory (LBNL), observed that kaons, mesons each composed of a strange quark and an up/down antiquark, had a tendency to regenerate in one form when shot as a beam into matter.
The neutral kaon, denoted as K0, has two forms, the short-lived (KS) and the long-lived (KL). Piccioni found that kaons decay in flight, so a beam of kaons, over a period of time, becomes pure KL because the KS all decay away before them. When such a beam is shot into matter, the K0 is scattered by protons and neutrons whereas the K0* (i.e., antikaons) contribute to the formation of a class of particles called hyperons.
Because of this asymmetric interaction, (quantum) coherence between the two batches of particles is lost, resulting in the emergent beam being composed of KS and KL, where the KS is regenerated by firing a K0-beam into matter.
When the results of Piccioni’s experiment were duplicated by Robert Adair in the same year, regeneration as a physical phenomenon became a new chapter in the study of particle physics. Later that year, that’s what Cronin and Fitch set out to do. However, during the decay process, they observed a strange phenomenon.
According to a theory formulated in the 1950s by Murray Gell-Mann and Kazuo Nishijima, and then by Gell-Mann and Abraham Pais in 1955-1957, the KS meson was allowed to decay into two pions in order for certain quantum mechanical states to be conserved, and the KL meson was allowed to decay into three pions.
For instance, the KL (s*, u) decay happens thus:
s* → u* + W+ (weak interaction)
W+ → d* + u
u → g + d + d* (strong interaction)
u → u
A Feynman diagram depicting the decay of a KL meson into three pions.
In 1964, in their landmark experiment, Cronin and Fitch observed, however, that the KL meson was decaying into two pions, albeit at a frequency of 1-in-500 decays. This implied an indirect instance of CP-symmetry violation, and subsequently won the pair the 1980 Nobel Prize for Physics.
An important aspect of the observation of CP-symmetry violation in kaons is that the weak force is involved in the decay process (even as observed above in the decay of the KL meson). Even though the kaon is composed of a quark and an antiquark, i.e., held together by the strong force, its decay is mediated by the strong and the weak forces.
In fact, even in the case of the KS and KL kaons, their decay into pions can be depicted thus:
KS → π+ + π0 KL → π+ + π+ + π–
Here, the “+” and “-” indicate a particle’s parity, or handedness. When a KS decays into two pions, the result is one right-handed (“+”) and one neutral pion (“0”). When a KL decays into three pions, however, the result is two right-handed pions and one left-handed (“-“) pion.
When kaons were first investigated via their decay modes, the different final parities indicated that there were two kaons that were decaying differently. Over time, however, as increasingly precise measurements indicated that only one kaon (now called K+) was behind both decays, physicists concluded that the weak interaction was responsible for resulting in one kind of decay some of the time and in another kind of decay the rest of the time.
To elucidate, in particle physics, the squares of the amplitudes of two transformations, B → f and B* → f*, are denoted thus.
Here,
B = Initial state (or particle); f = Final state (or particle)
B* = Initial antistate (or antiparticle); f* = Final antistate (or antiparticle)
P = Amplitude of transformation B → f; Q = Amplitude of transformation B* → f*
S = Corresponding strong part of amplitude; W = Corresponding weak part of amplitude; both treated as phases of the wave for which the amplitude is being evaluated
Subtracting (and applying some trigonometry):
The presence of the term sin(WP – WQ) is a sign that purely, or at least partly, weak interactions can occur in all transformations that can occur in at least two ways, and thus will violate CP-symmetry. (It’s like having the option of having two paths to reach a common destination: #1 is longer and fairly empty; #2 is shorter and congested. If their distances and congestedness are fairly comparable, then facing some congestion becomes inevitable.)
Electromagnetism, strong interactions, and gravitation do not display any features that could give rise to the distinction between right and left, however. This disparity is also called the ‘strong CP problem’ and is one of the unsolved problems of physics. It is especially puzzling because the QCD Lagrangian, which is a function describing the dynamics of the strong interaction, includes terms that could break the CP-symmetry.
(The best known resolution – one that doesn’t resort to spacetime with two time-dimensions – is the Peccei-Quinn theory put forth by Roberto Peccei and Helen Quinn in 1977. It suggests that the QCD-Lagrangian be extended with a CP-violating parameter whose value is 0 or close to 0.
This way, CP-symmetry is conserved during the strong interactions while CP-symmetry “breakers” in the QCD-Lagrangian have their terms cancelled by an emergent, dynamic field whose flux is encapsulated by massless Goldstone bosons called axions.)
Now, kaons are a class of mesons whose composition includes a strange quark (or antiquark). Another class of mesons, called B-mesons, are identified by their composition including a bottom antiquark, and are also notable for the role they play in studies of CP-symmetry violations in nature. (Note: A B-meson composed of a bottom antiquark and a bottom quark is not called a meson but a bottomonium.)
The six quarks, the fundamental (and proverbial) building blocks of matter
According to the Standard Model (SM) of particle physics, there are some particles – such as quarks and leptons – that carry a property called flavor. Mesons, which are composed of quarks and antiquarks, have an overall flavor inherited from their composition as a result. The presence of non-zero flavor is significant because SM permits quarks and leptons of one flavor to transmute into the corresponding quarks and leptons of another flavor, a process called oscillating.
And the B-meson is no exception. Herein lies the rub: during oscillations, the B-meson is favored over its antiparticle counterpart. Given the CPT theorem’s assurance of particles and antiparticles being differentiable only by charge and handedness, not mass, etc., the preference of B*-meson for becoming the B-meson more than the B-meson’s preference for becoming the B*-meson indicates a matter-asymmetry. Put another way, the B-meson decays at a slower rate than the B*-meson. Put yet another way, matter made of the B-meson is more stable than antimatter made of the B*-meson.
Further, if the early universe started off as a perfect symmetry (in every way), then the asymmetric formation of B-mesons would have paved the way for matter to take precedence over anti-matter. This is one of the first instances of the weak interaction possibly interfering with the composition of the universe. How? By promising never to preserve parity, and by participating in flavor-changing oscillations (in the form of the W/Z boson).
In this composite image of the Crab Nebula, matter and antimatter are propelled nearly to the speed of light by the Crab pulsar. The images came from NASA’s Chandra X-ray Observatory and the Hubble Space Telescope. (Photo by NASA; Caption from Howstuffworks.com)
The prevalence of matter over antimatter in our universe is credited to a hypothetical process called baryogenesis. In 1967, Andrei Sakharov, a Soviet nuclear physicist, proposed three conditions for asymmetric baryogenesis to have occurred.
The baryon-number of a particle is defined as one-third of the difference between the number of quarks and number of antiquarks that make up the particle. For a B-meson composed of a bottom antiquark and a quark, the value’s 0; of a bottom antiquark and another antiquark, the value’s 1. Baryon-number violation, while theoretically possible, isn’t considered in isolation of what is called “B – L” conservation (“L” is the lepton number, and is equal to the number of leptons minus the number of antileptons).
Now, say a proton decays into a pion and a position. A proton’s baryon-number is 1, L-number is 0; a pion has both baryon- and L-numbers as 0; a positron has baryon-number 0 and L-number -1. Thus, neither the baryon-number nor the lepton-number are conserved, but their difference (1) definitely is. If this hypothetical process were ever to be observed, then baryogenesis would make the transition from hypothesis to reality (and the question of matter-asymmetry become conclusively answered).
The quark-structure of a proton (notice that the two up-quarks have different flavors)
Therefore, in recognition of the role of B-mesons (in being able to present direct evidence of CP-symmetry violation through asymmetric B-B* oscillations involving the mediation of the weak-force) and their ability to confirm or deny an “SM-approved” baryogenesis in the early universe, what are called the B-factories were built: a collider-based machine whose only purpose is to spew out B-mesons so they can be studied in detail by high-precision detectors.
The earliest, and possibly most well-known, B-factories were constructed in the 1990s and shut down in the 2000s: the BaBar experiment at SLAC (2008), Stanford, and the Belle experiment at the KEKB collider (2010) in Japan. In fact, a Belle II plant is under construction and upon completion will boast the world’s highest-luminosity experiment.
The Belle detector (L) and the logo for Belle II under construction
