Tag: mesons

The hunt for supersymmetry: Is a choke on the cards?

The Copernican
April 28, 2014

“So irrelevant is the philosophy of quantum mechanics to its use that one begins to suspect that all the deep questions are really empty…”

— Steven Weinberg, Dreams of a Final Theory: The Search for the Fundamental Laws of Nature (1992)

On a slightly humid yet clement January evening in 2013, a theoretical physicist named George Sterman was in Chennai to attend a conference at the Institute of Mathematical Sciences. After the last talk of the day, he had strolled out of the auditorium and was mingling with students when I managed to get a few minutes with him. I asked for an interview and he agreed.

After some coffee, we seated ourselves at a kiosk in the middle of the lawn, the sun was setting, and mosquitoes abounded. Sterman was a particle physicist, so I opened with the customary question about the Higgs boson and expected him to swat it away with snowclones of the time like “fantastic”, “tribute to 50 years of mathematics” and “long-awaited”. He did say those things, but then he also expressed some disappointment.

George Sterman is distinguished for his work in quantum chromodynamics (QCD), for which he won the prestigious J.J. Sakurai Prize in 2003. QCD is a branch of physics that deals with particles that have a property called colour charge. Quarks and gluons are examples of such particles; these two together with electrons are the proverbial building blocks of matter. Sterman has been a physicist since the 1970s, the early years as far as experimental particle physics research is concerned.

The Standard Model disappoints

Over the last four or so decades, remarkable people like him have helped construct a model of laws, principles and theories that the rigours of this field are sustaining on, called the Standard Model of particle physics. And it was the reason Sterman was disappointed.

According to the Standard Model, Sterman explained, “if we gave our any reasonable estimate of what the mass of the Higgs particle should be, it should by all rights be huge! It should be as heavy as what we call the Planck mass.”

But it isn’t. The Higgs mass is around 125 GeV (GeV being a unit of energy that corresponds to certain values of a particle’s mass) – compare it with the proton that weighs 0.938 GeV. On the other hand, the Planck mass is 10^19 GeV. Seventeen orders of magnitude lie in between. According to Sterman, this isn’t natural. The question is why does there have to be such a big difference in what we can say the mass could be and what we find it to be.

Martinus Veltman, a Dutch theoretical physicist who won the Nobel Prize for physics in 2003 for his work in particle physics, painted a starker picture, “Since the energy of the Higgs [field] is distributed all over the universe, it should contribute to the curvature of space; if you do the calculation, the universe would have to curve to the size of a football,” in an interview to Nature in 2013.

Evidently, the Standard Model has many loose ends, and explaining the mass of the Higgs boson is only one of them. Another example is why it has no answer for what dark matter is and why it behaves the way it does. Yet another example is why the four fundamental forces of nature are not of the same order of magnitude.

An alternative

Thanks to the Standard Model, some mysteries have been solved, but other mysteries have come and are coming to light – in much the same way Isaac Newton’s ideas struggled to remain applicable in the troubled world of physics in the early 20th century. It seems history repeats itself through crises.

Fortunately, physicists in 1971-1972 had begun to piece together an alternative theory called supersymmetry, Susy for short. At the time, it was an alternative way of interpreting how emerging facts could be related to each other. Today, however, Susy is a more encompassing successor to the throne that the Standard Model occupies, a sort of mathematical framework in which the predictions of the Model still hold but no longer have those loose ends. And Susy’s USP is… well, that it doesn’t disappoint Sterman.

“There’s a reason why so many people felt so confident about supersymmetry,” he said. “It wasn’t just that it’s a beautiful theory – which it is – or that it engages and challenges the most mathematically oriented among physicists, but in another sense in which it appeared to be necessary. There’s this subtle concept that goes by the name of naturalness…”

And don’t yet look up ‘naturalness’ on Wikipedia because, for once, here is something so simple, so elegant, that it is precisely what its name implies. Naturalness is the idea that, for example, the Higgs boson is so lightweight because something out there is keeping it from being heavy. Naturalness is the idea that, in a given setting, the forces of nature all act in equal measure. Naturalness is the idea that causes seem natural, and logically plausible, without having to be fine-tuned in order to explain their effects. In other words, Susy, through its naturalness, makes possible a domesticated world, one without sudden, unexpected deviations from what common sense (a sophisticated one, anyway) would dictate.

To understand how it works, let us revisit the basics. Our observable universe plays host to two kinds of fundamental particles, which are packets of some well-defined amount of energy. The fermions, named for Enrico Fermi, are the matter particles. Things are made of them. The bosons, named for Satyendra Bose, are the force particles. Things interact with each other by using them as messengers. The Standard Model tells us how bosons and fermions will behave in a variety of situations.

However, the Model has no answers for why bosons and fermions weigh as much as they do, or come in as many varieties as they do. These are deeper questions that go beyond simply what we can observe. These are questions whose answers demand that we interpret what we know, that we explore the wisdom of nature that underlies our knowledge of it. To know this why, physicists investigated phenomena that lie beyond the Standard Model’s jurisdiction.

The search

One such place is actually nothingness, i.e. the quantum vacuum of deep space, where particles called virtual particles continuously wink in and out of existence. But even with their brief life-spans, they play a significant role in mediating the interactions between different particles. You will remember having studied in class IX that like charges repel each other. What you probably weren’t told is that the repulsive force between them is mediated by the exchange of virtual photons.

Curiously, these “virtual interactions” don’t proliferate haphazardly. Virtual particles don’t continuously “talk” to the electron or clump around the Higgs boson. If this happened, mass would accrue at a point out of thin air, and black holes would be popping up all around us. Why this doesn’t happen, physicists think, is because of Susy, whose invisible hand could be staying chaos from dominating our universe.

The way it does this is by invoking quantum mechanics, and conceiving that there is another dimension called superspace. In superspace, the bosons and fermions in the dimensions familiar to us behave differently, the laws conceived such that they restrict the random formation of black holes, for starters. In the May 2014 issue of Scientific American, Joseph Lykken and Maria Spiropulu describe how things work in superspace:

“If you are a boson, taking one step in [superspace] turns you into a fermion; if you are a fermion, one step in [superspace] turns you into a boson. Furthermore, if you take one step in [superspace] and then step back again, you will find that you have also moved in ordinary space or time by some minimum amount. Thus, motion in [superspace] is tied up, in a complicated way, with ordinary motion.”

The presence of this dimension implies that all bosons and fermions have a corresponding particle called a superpartner particle. For each boson, there is a superpartner fermion called a bosino; for each fermion, there is a superpartner boson called a sfermion (why the confusing titles, though?).

Physicists are hoping this supersymmetric world exists. If it does, they will have found tools to explain the Higgs boson’s mass, the difference in strengths of the four fundamental forces, what dark matter could be, and a swarm of other nagging issues the Standard Model fails to resolve. Unfortunately, this is where Susy’s credit-worthiness runs into trouble.

No signs

“Experiment will always be the ultimate arbiter, so long as it’s science we’re doing.”

— Leon Lederman & Christopher Hill, Beyond the Higgs Boson (2013)

Since the first pieces of the Standard Model were brought together in the 1960s, researchers have run repeated tests to check if what it predicts were true. Each time, the Model has stood up to its promise and yielded accurate results. It withstood the test of time – a criterion it shares with the Nobel Prize for physics, which physicists working with the Model have won at least 15 times since 1957.

Susy, on the other hand, is still waiting for confirmation. The Large Hadron Collider (LHC), the world’s most powerful particle physics experiment, ran its first round of experiments from 2009 to 2012, and found no signs of sfermions or bosinos. In fact, it has succeeded on the other hand to narrow the gaps in the Standard Model where Susy could be found. While the non-empty emptiness of quantum vacuum opened a small window into the world of Susy, a window through which we could stick a mathematical arm out and say “This is why black holes don’t just pop up”, the Model has persistently puttied every other crack we hound after.

An interesting quote comes to mind about Susy’s health. In November 2012, at the Hadron Collider Physics Symposium in Kyoto, Japan, physicists presented evidence of a particle decay that happens so rarely that only the LHC could have spotted it. The Standard Model predicts that every time the B_s (pronounced “Bee-sub-ess”) meson decays into a set of lighter particles, there is a small chance that it decays into two muons. The steps in which this happens is intricate, involving a process called a quantum loop.

What next?

“SUSY has been expected for a long time, but no trace has been found so far… Like the plot of the excellent movie ‘The Lady Vanishes’ (Alfred Hitchcock, 1938)”

— Andy Parker, Cambridge University

Susy predicts that some supersymmetric particles should show themselves during the quantum loop, but no signs of them were found. On the other hand, the rate of B_s decays into two muons was consistent with the Model’s predictions. Prof. Chris Parkes, a British physicist, had then told BBC News: “Supersymmetry may not be dead but these latest results have certainly put it into hospital.” Why not: Our peek of the supersymmetric universe eludes us, and if the LHC can’t find it, what will?

Then again, it took us many centuries to find the electron, and then many decades to find anti-particles. Why should we hurry now? After all, as Dr. Rahul Sinha from the Institute of Mathematical Sciences told me after the Symposium had concluded, “a conclusive statement cannot be made as yet”. At this stage, even waiting for many years might not be necessary. The LHC is set to reawaken around January 2015 after a series of upgrades that will let the machine deliver 10 times more particle collisions per second per unit area. Mayhap a superpartner particle can be found lurking in this profusion by, say, 2017.

There are also plans for other more specialised colliders, such as Project X in the USA, which India has expressed interest in formally cooperating with. X, proposed to be built at the Fermilab National Accelerator Laboratory, Illinois, will produce high intensity proton beams to investigate a variety of hitherto unexplored realms. One of them is to produce heavy short-lived isotopes of elements like radium or francium, and use them to study if the electron has a dipole moment, or a pronounced negative charge along one direction, which Susy allows for.

(Moreover, if Project X is realised it could prove extra-useful for India because it makes possible a new kind of nuclear reactor design, called the accelerator-driven sub-critical reactor, which operates without a core of critical-mass radioactive fuel, rendering impossible accidents like Chernobyl and Fukushima, while also being capable of inducing fission reactions using lighter fuel like thorium.)

Yet another avenue to explore Susy would be looking for dark matter particles using highly sensitive particle detectors such as LUX, XENON1T and CDMS. According to some supersymmetric models, the lightest Susy particles could actually be dark matter particles, so if a few are spotted and studied, they could buffet this theory’s sagging credence.

… which serves to remind us that this excitement could cut the other way, too. What if the LHC in its advanced avatar is still unable to find evidence of Susy? In fact, the Advanced Cold Molecule Electron group at Harvard University announced in December 2013 that they were able to experimentally rule out that they electron had a dipole moment with the highest precision attained to date. After such results, physicists will have to try and rework the theory, or perhaps zero in on other aspects of it that can be investigated by the LHC or Project X or other colliders.

But at the end of the day, there is also the romance of it all. It took George Sterman many years to find a theory as elegant and straightforward as Susy – an island of orderliness in the insane sea of quantum mechanics. How quickly would he give it up?

O Hunter, snare me his shadow!
O Nightingale, catch me his strain!
Else moonstruck with music and madness
I track him in vain!

— Oscar Wilde, In The Forest

Where’s all the antimatter? New CERN results show the way.

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.

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This post originally appeared in The Copernican science blog at The Hindu on April 25, 2013.

On meson decay-modes in studying CP violation

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:

  1. s* → u* + W+ (weak interaction)
  2. W+ → d* + u
  3. u → g + d + d* (strong interaction)
  4. 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 all weak interactions, parity is not conserved. The interaction itself acts only on left-handed particles and right-handed anti-particles, and was parametrized in what is called the V-A Lagrangian for weak interactions, developed by Robert Marshak and George Sudarshan in 1957.

Prof. Robert Marshak

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.

[youtube http://www.youtube.com/watch?v=KDkaMuN0DA0?rel=0]

(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.

  1. Baryon-number violation
  2. Departure from thermal equilibrium
  3. C- and CP-symmetry violation

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

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Equations generated thanks to the Daum equations editor.