Large Hadron Collider – Page 4

New results on Higgs bosons’ decay into fermions

For a boson to be the Higgs boson, it has to be intimately related to the physical process it was hypothesized in 1964 to help understand. With new results published on June 22, physicists from CERN, the lab that runs the experiments that first discovered the Higgs boson, have found that to be true, further cementing the credibility of their theories as well as discovering more properties that could guide future experiments.

The Higgs boson is too short-lived to be spotted directly. Its lifetime is 10^-22 seconds. In this period, it quickly decays into groups of lighter particles. The theory called the Standard Model of particle physics predicts how often the Higgs decays into which groups of particles. Broadly, the rate of this decay is guided by how strongly the Higgs couples to each particle, and such coupling gives rise to the particle’s mass (Note: the Higgs decays only into fundamental particles, not composite particles like protons and neutrons, because it gives mass only to fundamental particles).

The June 22 Letter in Nature Physics describes the champagne bottle boson‘s decay into fermions, the particles that make up all matter. By experimentally finding these rates, physicists accomplish two things. One, they assert the strength of whichever theory predicted these rates – the Standard Model, in this case (the Yukawa couplings, to be specific). Two, they establish that the Higgs boson does couple to fermions and gives them mass. The Letter draws its conclusions from experiments performed in 2011 and 2012.

The third generation of fermions

However, there is a limitation. Because the Higgs weighs 125 GeV, it could only have decayed into lighter fermions, not heavier ones. This means physicists have experimental proof for the Higgs giving mass to fermions lighter than itself; in this case, these are the so-called third generation fermions comprising the bottom quark and the tau lepton. Quarks are fundamental particles that come together to compose protons and neutrons. Leptons are some of the lightest of the matter particles, one common example of which is the electron.

In 2011, the Compact Muon Solenoid (CMS) experimental collaboration, which is the group of scientists that runs the CMS detector, had looked for Higgs bosons decaying into bottom quark-antiquark pairs. At this time, the Large Hadron Collider, which produces these particles by smashing protons together at high speeds, was operating at an energy of 7 TeV – i.e. each beam of protons coming into the collision had an energy of 7 TeV. The consequent results were published in January this year. The 2012 results concerned the search for Higgs bosons’ decays into tau lepton-antilepton pairs at 8 TeV. The pre-print paper submitted to arXiv is here (link to published paper).

The search for these particles is compounded by the fact that they aren’t just produced by the decaying Higgs boson but by a profusion of other Standard Model processes. The scientists at CERN use a combination of statistical techniques to single out which processes produced the particles of interest. They also use as many unique signatures as possible to narrow down their search. For example, the search for the bottom quark-antiquark pair of particles is reconstructed based on a Higgs boson being produced together with a W or a Z boson, whose decays have their own signatures.

The significance at which they report each decay process is in this table, picturized below.

Summary of results for the Higgs boson mass hypothesis of 125 GeV.

The Letter, as you can see, is open access, as are all the papers linked to in it.

The hunt for supersymmetry: Reviewing the first run – 2

I’d linked to a preprint paper [PDF] on arXiv a couple days ago that had summarized the search for Supersymmetry (Susy) from the first run of the Large Hadron Collider (LHC). I’d written to one of the paper’s authors, Pascal Pralavorio at CERN, seeking some insights into his summary, but unfortunately he couldn’t reply by the time I’d published the post. He replied this morning and I’ve summed them up.

Pascal says physicists trained their detectors for “the simplest extension of the Standard Model” using supersymmetric principles called the Minimal Supersymmetric Standard Model (MSSM), formulated in the early 1980s. This meant they were looking for a total of 35 particles. In the first run, the LHC operated at two different energies: first at 7 TeV (at a luminosity of 5 fb^-1), then at 8 TeV (at 20 fb^-1; explainer here). The data was garnered from both the ATLAS and CMS detectors.

In all, they found nothing. As a result, as Pascal says, “When you find nothing, you don’t know if you are close or far from it!”

His paper has an interesting chart that summarized the results for the search for Susy from Run 1. It is actually a superimposition of two charts. One shows the different Standard Model processes (particle productions, particle decays, etc.) at different energies (200-1,600 GeV). The second shows the Susy processes that are thought to occur at these energies.

Cross sections of several SUSY production channels, superimposed with Standard Model process at s = 8 TeV. The right-handed axis indicates the number of events for 20/fb.

The cross-section of the chart is the probability of an event-type to appear during a proton-proton collision. What you can see from this plot is the ratio of probabilities. For example, stop-stop* (the top quark’s Susy partner particle and anti-particle, respectively) production with a mass of 400 GeV is 10¹⁰ (10 billion) less probable than inclusive di-jet events (a Standard Model process). “In other words,” Pascal says, it is “very hard to find” a Susy process while Standard Model processes are on, but it is “possible for highly trained particle physics” to get there.

Of course, none of this means physicists aren’t open to the possibility of there being a theory (and corresponding particles out there) that even Susy mightn’t be able to explain. The most popular among such theories is “the presence of a “possible extra special dimension” on top of the three that we already know. “We will of course continue to look for it and for supersymmetry in the second run.”

The hunt for supersymmetry: Reviewing the first run

What do dark matter, Higgs bosons, the electron dipole moment, topological superconductors and quantum loops have in common? These are exotic entities that scientists have been using to solve some longstanding problems in fundamental physics. Specifically, by studying these entities, they expect to discover new ingredients of the universe that will help them answer why it is the way it is. These ingredients could in come in a staggering variety, so it is important for scientists to narrow down what they’re looking for – which brings us to the question of why these entities are being studied. A brief summary:

Dark matter is an exotic form of matter that is thought to interact with normal matter only through the gravitational force. Its existence was hypothesized in 1932-1933. Its exact nature is yet to be understood.
Quantum loops refer to an intricate way in which some heavier particles decay into sets of lighter particles, involving the exchange of some other extremely short-lived particles. They have been theoretically known to exist for many decades.
Topological superconductors are exotic materials that, under certain conditions, behave like superconductors on their surface and as insulators underneath. They were discovered fairly recently, around 2007, and how they manage to be this way is not fully understood.
The Higgs boson‘s discovery was announced in July 2012 (and verified by March-June 2013). Math worked out on paper predicts that its mass ought to have been very high – but it was found to be much lower.
The electron dipole moment is a measure of how spherical the electron is. Specifically, the EDM denotes how evenly the electron’s negative charge is distributed around it. While the well-understood laws of nature don’t prevent the charge from being uneven, they restrict the amount of unevenness to a very small value. The most precise measurement of this value to date was published in December 2013.

Clearly, these are five phenomena whose identities are incomplete. But more specifically, scientists have found a way to use advanced mathematics to complete all these identities with one encompassing theory called Supersymmetry (Susy). Unfortunately for them, the mathematics refuses to become real, i.e. scientists have failed to find evidence of Susy in experiments. Actually, that might be an overstatement: different experiments are at different stages of looking for Susy at work in giving these ‘freaks of nature’ a physical identity. On the other hand, it has been a few years since some of these experiments commenced – some of them are quite powerful indeed – and the only positive results they have had have been to say Susy cannot be found in this or that range.

But if signs of Susy are found, then the world of physics will be in tumult – in a good way, of course. It will get to replace an old theory called the Standard Model of particle physics. The Standard Model is the set of mathematical tools and techniques used to understand how fundamental particles make up different objects, what particles our universe is made of, how quantum loops work, how the Higgs boson could have formed, etc. But it has no answers for why there is dark matter, why the electron is allowed to have that small dipole moment, why topological superconductors work the way they do, why the Higgs boson’s mass is so low, etc.

Early next year, physicists will turn to the Large Hadron Collider (LHC) – which helped discover the Higgs boson in 2012 – after it wakes up from its two-year slumber to help find Susy, too. This LHC of 2015 will be way more powerful than the one that went dormant in early 2013 thanks to a slew of upgrades. Hopefully it will not disappoint, building on what it has managed to deliver for Susy until now. In fact, on April 28, 2014, two physicists from CERN submitted a preprint paper to the arXiv server summarizing the lessons for Susy from the LHC after the first run.

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

Our universe, the poor man’s accelerator

The Hindu
March 25, 2014

On March 17, radio astronomers from the Harvard-Smithsonian Center for Astrophysics, Massachusetts, announced a remarkable discovery. They found evidence of primordial gravitational waves imprinted on the cosmic microwave background (CMB), a field of energy pervading the universe.

A confirmation that these waves exist is the validation of a theory called cosmic inflation. It describes the universe’s behaviour less than one-billionth of a second after it was born in the Big Bang, about 14 billion years ago, when it witnessed a brief but tremendous growth spurt. The residual energy of the Bang is the CMB, and the effect of gravitational waves on it is like the sonorous clang of a bell (the CMB) that was struck powerfully by an effect of cosmic inflation. Thanks to the announcement, now we know the bell was struck.

Detecting these waves is difficult. In fact, astrophysicists used to think this day was many more years into the future. If it has come now, we must be thankful to human ingenuity. There is more work to be done, of course, because the results hold only for a small patch of the sky surveyed, and there is also data due from studies done until 2012 on the CMB. Should any disagreement with the recent findings arise, scientists will have to rework their theories.

Remarkable in other ways

The astronomers from the Harvard-Smithsonian used a telescope called BICEP2, situated at the South Pole, to make their observations of the CMB. In turn, BICEP2’s readings of the CMB imply that when cosmic inflation occurred about 14 billion years ago, it happened at a tremendous amount of energy of 10¹⁶ GeV (GeV is a unit of energy used in particle physics). Astrophysicists didn’t think it would be so high.

Even the Large Hadron Collider (LHC), the world’s most powerful particle accelerator, manages a puny 10⁴ GeV. The words of the physicist Yakov Zel’dovich, “The universe is the poor man’s accelerator”— written in the 1970s — prove timeless.

This energy at which inflation has occurred has drawn the attention of physicists studying various issues because here, finally, is a window that allows humankind to naturally study high-energy physics by observing the cosmos. Such a view holds many possibilities, too, from the trivial to the grand.

For example, consider the four naturally occurring fundamental forces: gravitation, strong and weak-nuclear force, and electromagnetic force. Normally, the strong-nuclear, weak-nuclear and electromagnetic forces act at very different energies and distances.

However, as we traverse higher and higher energies, these forces start to behave differently, as they might have in the early universe. This gives physicists probing the fundamental texture of nature an opportunity to explore the forces’ behaviours by studying astronomical data — such as from BICEP2 — instead of relying solely on particle accelerators like the LHC.

In fact, at energies around 10¹⁹ GeV, some physicists think gravity might become unified with the non-gravitational forces. However, this isn’t a well-defined goal of science, and doesn’t command as much consensus as it submits to rich veins of speculation. Theories like quantum gravity operate at this level, finding support from frameworks like string theory and loop quantum gravity.

Another perspective on cosmic inflation opens another window. Even though we now know that gravitational waves were sent rippling through the universe by cosmic inflation, we don’t know what caused them. An answer to this question has to come from high-energy physics — a journey that has taken diverse paths over the years.

Consider this: cosmic inflation is an effect associated with quantum field theory, which accommodates the three non-gravitational forces. Gravitational waves are an effect of the theories of relativity, which explain gravity. Because we may now have proof that the two effects are related, we know that quantum mechanics and relativity are also capable of being combined at a fundamental level. This means a theory unifying all the four forces could exist, although that doesn’t mean we’re on the right track.

At present, the Standard Model of particle physics, a paradigm of quantum field theory, is proving to be a mostly valid theory of particle physics, explaining interactions between various fundamental particles. The questions it does not have answers for could be answered by even more comprehensive theories that can use the Standard Model as a springboard to reach for solutions.

Physicists refer to such springboarders as “new physics”— a set of laws and principles capable of answering questions for which “old physics” has no answers; a set of ideas that can make seamless our understanding of nature at different energies.

Supersymmetry

One leading candidate of new physics is a theory called supersymmetry. It is an extension of the Standard Model, especially at higher energies. Finding symptoms of supersymmetry is one of the goals of the LHC, but in over three years of experimentation it has failed. This isn’t the end of the road, however, because supersymmetry holds much promise to solve certain pressing issues in physics which the Standard Model can’t, such as what dark matter is.

Thus, by finding evidence of cosmic inflation at very high energy, radio-astronomers from the Harvard-Smithsonian Center have twanged at one strand of a complex web connecting multiple theories. The help physicists have received from such astronomers is significant and will only mount as we look deeper into our skies.

On the shoulders of the Higgs

On July 4, 2012, when CERN announced that a particle that looked a lot like the Higgs boson had been spotted, the excitement was palpable. A multibillion-dollar search for an immensely tiny particle had paid off, and results were starting to come in.

On March 6, 2013, when CERN announced through a conference in Italy that the particle was indeed the Higgs boson and that there were only trivial indications as to otherwise, there was closure. Textbook-writers and philosophers alike could take the existence of the Higgs boson for granted. People could move on.

The story should’ve ended there.

It would’ve, too, but for a theory in particle physics that many physicists aren’t quite fond of, called the Standard Model.

For particle physicists, everything in the universe is made up of extremely tiny particles. Even the protons, neutrons and electrons that make up atoms are made up of tinier particles even though we don’t encounter them or their effects in our daily lives.

And those tiny particles, of tinier particles, until particle physicists are satisfied they’ve hit upon the “stuff” of the universe, as it were.

This “stuff”, physicists have found, comes in three types: Leptons, bosons and quarks. They are the ingredients of the universe and everything within it.

Leptons are the lightest of the lot. One example of the lepton is a neutrino, whose mass is so low that it has no problems travelling at very near to the speed of light.

Bosons are the force-carriers. When two particles exert a force on each other, particle physicists imagine that they’re simple exchanging bosons. What kind of boson is being exchanged throws light on the nature of the acting force. Examples include the massless photon, the W and Z bosons, and gluons.

Quarks are the proverbial building blocks of matter. They come in six types, each called a flavour. Unlike leptons and bosons, quarks can be stuck together using gluons to form heavier composite particles. For example, two up-flavoured quarks and one down-flavoured quark together make up the proton.

In the universe as a cauldron, these ingredients come together to make up different phenomena we perceive around us. While each particle sticks to its properties while mixing with others, its behavior is continuously modified by other particles around it. If there are too many particles in the fray, which is natural, matters can get complex for physicists studying them. But after studying them over long periods of time, they found that there are patterns and a few rules that aren’t ever broken.

The set of all these patterns and rules is called the Standard Model. In fact, the Higgs boson was the last remaining piece of this framework, and now that it’s been found, the Model is good as true.

So far, so good.

Where the Model really flounders is when physicists asked why it was the way it was. Why are there six quarks and not five or seven, why are there three kinds of leptons – electron, muon, tau – and now two or four, why can one quark only always be found in the company of another quark and never alone… such questions were just the beginning.

These are the real questions that physicists want the answers to – the ultimate “why” is the goal of all scientific studies.

Many pinned their hopes on CERN’s Large Hadron Collider (LHC), which led the search for the Higgs boson by smashing protons together and open at high energies to see if a Higgs boson popped out. Based on preliminary calculations and simulations, they speculated that something more significant would pop out with the Higgs boson.

There has been nothing so far, i.e. the frustratingly familiar Standard Model is all we have.

But physicists took heart. “The Model is all we have… for now,” they said. Every time a new ingredient of the universe was hoped for and there was none, physicists only believed the hypothetical particle didn’t exist at the energy they were combing through.

Like this, with only negative results to show over hundreds of trials across a swath of energies, physicists have put together a stack of upper and lower limits between which new particles, crucial to the future of particle physics, can be spotted.

A good place to start was that the conditions for these “new” ingredients all were tied in with the conditions in which the Higgs boson could show itself. So, the converse must also be true: the conditions in which the Higgs boson showed itself could contain traces of the conditions for “new physics” to show itself. All physicists would have to do is ask the right questions.

One example is an instance of the fermiophobic Higgs. Because it’s so heavy and energetic, the Higgs boson quickly breaks down into lighter particles like photons, W and Z bosons, leptons and quarks. Of them, leptons and quarks are collectively titled fermions. True to its name, a fermiophobic Higgs doesn’t decay into fermions.

As a result, it will have to decay into the other kinds of particles more often. While they had the resources, physicists were able to determine that a fermiophobic Higgs didn’t exist in the energy range 110-124.5 GeV, 127-147.5 GeV, and 155-180 GeV with 99 per cent confidence.

Another example, this one more favoured in the scientific community, is called suppersymmetry, SUSY for short. Its adherents posit that for every fermion, there is a heavier partnering boson that we haven’t found yet, and vice versa, too. Thus, the Higgs boson has a hypothetical partnering fermion, tentatively called the Higgsino.

When physicists tried to incorporate the rules of SUSY into the Standard Model, they saw that five Higgs bosons would be necessary to explain away some problems. Of these, three would be neutral, and collectively denoted as Φ, and two would be charged, denoted H+ and H-. Moreover, the Φ Higgs would have to decay into two particular kinds of quarks a whopping 90 per cent of the time.

While running experiments to verify this decay rate, tragedy struck: the Standard Model stood in the way. It predicts that the Higgs will decay into the two quarks only 56.1 per cent of the time… And the results swore by it.

The never-say-die faith persisted: Now, physicists wait for 2015, when the LHC will reawaken with doubled energy, possibly bringing them closer to the very high energies at which SUSY might be thriving.

There are other models like these, such as one that suggests there are hidden fermions we haven’t found yet and one that suggests that the Higgs boson decays to lighter, undetected versions of itself before coming into a form we can study. But until 2015, they will be the stuff of science fiction, the Standard Model will rule as a tolerable tyrant, and we will be no closer to understanding the stuff of the universe.

Higgs boson closer than ever

The article, as written by me, appeared in The Hindu on March 7, 2013.

—

Ever since CERN announced that it had spotted a Higgs boson-like particle on July 4, 2012, their flagship Large Hadron Collider (LHC), apart from similar colliders around the world, has continued running experiments to gather more data on the elusive particle.

The latest analysis of the results from these runs was presented at a conference now underway in Italy.

While it is still too soon to tell if the one spotted in July 2012 was the Higgs boson as predicted in 1964, the data is convergent toward the conclusion that the long-sought particle does exist and with the expected properties. More results will be presented over the upcoming weeks.

In time, particle physicists hope that it will once and for all close an important chapter in physics called the Standard Model (SM).

The announcements were made by more than 15 scientists from CERN on March 6 via a live webcast from the Rencontres de Moriond, an annual particle physics forum that has been held in La Thuile, Italy, since 1966.

“Since the properties of the new particle appear to be very close to the ones predicted for the SM Higgs, I have personally no further doubts,” Dr. Guido Tonelli, former spokesperson of the CMS detector at CERN, told The Hindu.

Interesting results from searches for other particles, as well as the speculated nature of fundamental physics beyond the SM, were also presented at the forum, which runs from March 2-16.

Physicists exploit the properties of the Higgs to study its behaviour in a variety of environments and see if it matches with the theoretical predictions. A key goal of the latest results has been to predict the strength with which the Higgs couples to other elementary particles, in the process giving them mass.

This is done by analysing the data to infer the rates at which the Higgs-like particle decays into known lighter particles: W and Z bosons, photons, bottom quarks, tau leptons, electrons, and muons. These particles’ signatures are then picked up by detectors to infer that a Higgs-like boson decayed into them.

The SM predicts these rates with good precision.

Thus, any deviation from the expected values could be the first evidence of new, unknown particles. By extension, it would also be the first sighting of ‘new physics’.

Bad news for new physics, good news for old

After analysis, the results were found to be consistent with a Higgs boson of mass near 125-126 GeV, measured at both 7- and 8-TeV collision energies through 2011 and 2012.

The CMS detector observed that there was fairly strong agreement between how often the particle decayed into W bosons and how often it ought to happen according to theory. The ratio between the two was pinned at 0.76 +/- 0.21.

Dr. Tonelli said, “For the moment, we have been able to see that the signal is getting stronger and even the difficult-to-measure decays into bottom quarks and tau-leptons are beginning to appear at about the expected frequency.”

The ATLAS detector, parallely, was able to observe with 99.73 per cent confidence-level that the analysed particle had zero-spin, which is another property that brings it closer to the predicted SM Higgs boson.

At the same time, the detector also observed that the particle’s decay to two photons was 2.3 standard-deviations higher than the SM prediction.

Dr. Pauline Gagnon, a scientist with the ATLAS collaboration, told this Correspondent via email, “We need to asses all its properties in great detail and extreme rigour,” adding that for some aspects they would need more data.

Even so, the developments rule out signs of any new physics around the corner until 2015, when the LHC will reopen after a two-year shutdown and multiple upgrades to smash protons at doubled energy.

As for the search for Supersymmetry, a favoured theoretical concept among physicists to accommodate phenomena that haven’t yet found definition in the Standard Model: Dr. Pierluigi Campana, LHCb detector spokesperson, told The Hindu that there have been only “negative searches so far”.

LHC to re-awaken in 2015 with doubled energy, luminosity

This article, as written by me, appeared in The Hindu on January 10, 2012.

—

After a successful three-year run that saw the discovery of a Higgs-boson-like particle in early 2012, the Large Hadron Collider (LHC) at CERN, near Geneva, Switzerland, will shut down for 18 months for maintenance and upgrades.

This is the first of three long shutdowns, scheduled for 2013, 2017, and 2022. Physicists and engineers will use these breaks to ramp up one of the most sophisticated experiments in history even further.

According to Mirko Pojer, Engineer In-charge, LHC-operations, most of these changes were planned in 2011. They will largely concern fixing known glitches on the ATLAS and CMS particle-detectors. The collider will receive upgrades to increase its collision energy and frequency.

Presently, the LHC smashes two beams, each composed of precisely spaced bunches of protons, at 3.5-4 tera-electron-volts (TeV) per beam.

By 2015, the beam energy will be pushed up to 6.5-7 TeV per beam. Moreover, the bunches which were smashed at intervals of 50 nanoseconds will do so at 25 nanoseconds.

After upgrades, “in terms of performance, the LHC will deliver twice the luminosity,” Dr. Pojer noted in an email to this Correspondent, with reference to the integrated luminosity. Precisely, it is the number of collisions that the LHC can deliver per unit area which the detectors can track.

The instantaneous luminosity, which is the luminosity per second, will be increased to 1×10³⁴ per centimetre-squared per second, ten-times greater than before, and well on its way to peaking at 7.73×10³⁴ per centimetre-squared per second by 2022.

As Steve Myers, CERN’s Director for Accelerators and Technology, announced in December 2012, “More intense beams mean more collisions and a better chance of observing rare phenomena.” One such phenomenon is the appearance of a Higgs-boson-like particle.

The CMS experiment, one of the detectors on the LHC-ring, will receive some new pixel sensors, a technology responsible for tracking the paths of colliding particles. To make use of the impending new luminosity-regime, an extra layer of these advanced sensors will be inserted around a smaller beam pipe.

If results from it are successful, CMS will receive the full unit in late-2016.

In the ATLAS experiment, unlike with CMS which was built with greater luminosities in mind, pixel sensors are foreseen to wear out within one year after upgrades. As an intermediate solution, a new layer of sensors called the B-layer will be inserted within the detector for until 2018.

Because of the risk of radiation damage due to more numerous collisions, specific neutron shields will be fit, according to Phil Allport, ATLAS Upgrade Coordinator.

Both ATLAS and CMS will also receive evaporative cooling systems and new superconducting cables to accommodate the higher performance that will be expected of them in 2015. The other experiments, LHCb and ALICE, will also undergo inspections and upgrades to cope with higher luminosity.

An improved failsafe system will be installed and the existing one upgraded to prevent accidents such as the one in 2008.

Then, an electrical failure damaged 29 magnets and leaked six tonnes of liquid helium into the tunnel, precipitating an eight-month shutdown.

Generally, as Martin Gastal, CMS Experimental Area Manager, explained via email, “All sub-systems will take the opportunity of this shutdown to replace failing parts and increase performance when possible.”

All these changes have been optimised to fulfil the LHC’s future agenda. This includes studying the properties of the newly discovered particle, and looking for signs of new theories of physics like supersymmetry and higher dimensions.

(Special thanks to Achintya Rao, CMS Experiment.)

Dr. Stone on the Higgs search

On December 10, 2012, I spoke to a bunch of physicists attending the Frontiers of High-energy Physics symposium at the Institute of Mathematical Sciences, Chennai. They included Rahul Sinha, G. Rajasekaran, Tom Kibble, Sheldon Stone, Marina Artuso, M.V.N. Murthy, Kajari Mazumdar, and Hai-Yang Cheng, amongst others.

All their talks, obviously, focused on either the search for the Higgs boson or the search for dark matter, with the former being assured and celebratory and the latter, contemplative and cautious. There was nothing new left to be said – as a peg for a news story – given that what of 2012 had gone before that day had already read hundreds of stories on the two searches.

Most of the memorable statements by physicists I remember from that day came from Dr. Sheldon Stone, Syracuse University, and member, LHCb collaboration.

A word on the LHCb before I go any further: It’s one of the seven detector-experiments situated on the Large Hadron Collider’s (LHC’s) ring. Unlike the ATLAS and CMS, whose focus is on the Higgs boson, the LHCb collaboration is studying the decay of B-mesons and signs of CP-symmetry violations at high energies.

While he had a lot to say, he also best summed up what physicists worldwide might’ve felt when the theorised set of particles’ rules called the Standard Model (SM) was having its predictions validated one after the other, leaving no room for a new theory to edge its way in. While very elegant by itself, the SM has no answers to some of the more puzzling questions, such as that of dark matter or of mass-hierarchy problem.

In other words, the more it stands validated, the fewer cracks there are for a new and better theory, like Supersymmetry, to show itself.

In Dr. Stone’s words, “It’s very depressing. The Standard Model has been right on target, and so far, nothing outside the model has been observed. It’s very surprising that everything works, but at the same time, we don’t know why it works! Everywhere, physicists are depressed and clueless, intent on digging deeper, or both. I’m depressed, too, but I also want to dig deeper.”

In answer to some of my questions on what the future held, Dr. Stone said, “Now that we know how things actually work, we’re starting to play some tricks. But beyond that, moving ahead, with new equipment, etc., is going to cost a lot of money. We’ve to invest in the collider, in a lot of detector infrastructure, and computing accessories. In 2012, we had a tough time keeping with the results the LHC was producing. For the future, we’re counting on advancements in computer science and the LHC Grid.”

One interesting thing that he mentioned in one of his answers was that the LHC costs less than one aircraft-carrier. I thought that’d put things in perspective – how much some amount of investment in science could achieve when compared to what the same amount could achieve in other areas. This is not to discourage the construction of aircraft carriers, but to rethink the opportunities science research has the potential to unravel.

(This blog post first appeared at The Copernican on December 22, 2012.)

Window for an advanced theory of particles closes further

A version of this article, as written by me, appeared in The Hindu on November 22, 2012.

—

On November 12, at the first day of the Hadron Collider Physics Symposium at Kyoto, Japan, researchers presented a handful of results that constrained the number of hiding places for a new theory of physics long believed to be promising.

Members of the team from the LHCb detector on the Large Hadron Collider (LHC) experiment located on the border of France and Switzerland provided evidence of a very rare particle-decay. The rate of the decay process was in fair agreement with an older theory of particles’ properties, called the Standard Model (SM), and deviated from the new theory, called Supersymmetry.

“Theorists have calculated that, in the Standard Model, this decay should occur about 3 times in every billion total decays of the particle,” announced Pierluigi Campana, LHCb spokesperson. “This first measurement gives a value of around 3.2 per billion, which is in very good agreement with the prediction.”

The result was presented at the 3.5-sigma confidence level, which corresponds to an error rate of 1-in-2,000. While not strong enough to claim discovery, it is valid as evidence.

The particle, called a B_s⁰meson, decayed from a bottom antiquark and strange quark pair into two muons. According to the SM, this is a complex and indirect decay process: the quarks exchange a W boson particle, turn into a top-antitop quark pair, which then decays into a Z boson or a Higgs boson. The boson then decays to two muons.

This indirect decay is called a quantum loop, and advanced theories like Supersymmetry predict new, short-lived particles to appear in such loops. The LHCb, which detected the decays, reported no such new particles.

BsMuMu_decay — The solid blue line shows post-decay muons from all events, and the red dotted line shows the muon-decay event from the B(s)0 meson. Because of a strong agreement with the SM, SUSY may as well abandon this bastion.

At the same time, in June 2011, the LHCb had announced that it had spotted hints of supersymmetric particles at 3.9-sigma. Thus, scientists will continue to conduct tests until they can stack 3.5 million-to-1 odds for or against Supersymmetry to close the case.

As Prof. Chris Parkes, spokesperson for the UK participation in the LHCb experiment, told BBC News: “Supersymmetry may not be dead but these latest results have certainly put it into hospital.”

The symposium, which concluded on November 16, also saw the release of the first batch of data generated in search of the Higgs boson since the initial announcement on July 4 this year.

The LHC can’t observe the Higgs boson directly because it quickly decays into lighter particles. So, physicists count up the lighter particles and try to see if some of those could have come from a momentarily existent Higgs.

These are still early days, but the data seems consistent with the predicted properties of the elusive particle, giving further strength to the validity of the SM.

Dr. Rahul Sinha, a physicist at the Institute of Mathematical Sciences, Chennai, said, “So far there is nothing in the Higgs data that indicates that it is not the Higgs of Standard Model, but a conclusive statement cannot be made as yet.”

The scientific community, however, is disappointed as there are fewer channels for new physics to occur. While the SM is fairly consistent with experimental findings, it is still unable to explain some fundamental problems.

One, called the hierarchy problem, asks why some particles are much heavier than others. Supersymmetry is theoretically equipped to provide the answer, but experimental findings are only thinning down its chances.

Commenting on the results, Dr. G. Rajasekaran, scientific adviser to the India-based Neutrino Observatory being built at Theni, asked for patience. “Supersymmetry implies the existence of a whole new world of particles equaling our known world. Remember, we took a hundred years to discover the known particles starting with the electron.”

With each such tightening of the leash, physicists return to the drawing board and consider new possibilities from scratch. At the same time, they also hope that the initial results are wrong. “We now plan to continue analysing data to improve the accuracy of this measurement and others which could show effects of new physics,” said Campana.

So, while the area where a chink might be found in the SM armour is getting smaller, there is hope that there is a chink somewhere nonetheless.