The Copernican: Standard Model

Showing posts with label Standard Model. Show all posts

Wednesday, 21 November 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.

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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.

[caption id="attachment_24512" align="aligncenter" width="584"]

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.[/caption]

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.

Sunday, 1 July 2012

The philosophies in physics

As a big week for physics comes up–a July 4 update by CERN on the search for the Higgs boson followed by ICHEP '12 at Melbourne–I feel really anxious as a small-time proto-journalist and particle-physics-enthusiast. If CERN announces the discovery of evidence that rules out the existence of such a thing as the Higgs particle, not much will be lost apart from years of theoretical groundwork set in place for the post-Higgs universe. Physicists obeying the Standard Model will, to think the snowclone, scramble to their boards and come up with another hypothesis that explains mass-formation in quantum-mechanical terms.

For me... I don't know what it means. Sure, I will have to unlearn the Higgs mechanism, which does make a lot of sense, and scour through the outpouring of scientific literature that will definitely follow to keep track of new directions and, more fascinatingly, new thought. The competing supertheories–loop quantum gravity (LQG) and string theory–will have to have their innards adjusted to make up for the change in the mechanism of mass-formation. Even then, their principle bone of contention will remain unchanged: whether there exists an absolute frame of reference. All this while, the universe, however, will have continued to witness the rise and fall of stars, galaxies and matter.

It is easier to consider the non-existence of the Higgs boson than its proven existence: the post-Higgs world is dark, riddled with problems more complex and, unsurprisingly, more philosophical. The two theories that dominated the first half of the previous century, quantum mechanics and special relativity, will still have to be reconciled. While special relativity holds causality and locality close to its heart, quantum mechanics' tendency to violate the latter made it disagreeable at the philosophical level to A. Einstein (in a humorous and ironical turn, his attempts to illustrate this "anomaly" numerically opened up the field that further made acceptable the implications of quantum mechanics).

The theories' impudent bickering continues with mathematical terms as well. While one prohibits travel at the speed of light, the other allows for the conclusive demonstration of superluminal communication. While one keeps all objects nailed to one place in space and time, the other allows for the occupation of multiple regions of space at a time. While one operates in a universe wherein gods don't play with dice, the other can exist at all only if there are unseen powers that gamble on a secondly basis. If you ask me, I'd prefer one with no gods; I also have a strange feeling that that's not a physics problem.

Speaking of causality, physicists of the Standard Model believe that the four fundamental forces–nuclear, weak, gravitational, and electromagnetic–cause everything that happens in this universe. However, they are at a loss to explain why the weak force is 10³²-times stronger than the gravitational force (even the finding of the Higgs boson won't fix this–assuming the boson exists). An attempt to explain this anomaly exists in the name of supersymmetry (SUSY) or, together with the Standard Model, MSSM. If an entity in the (hypothetical) likeness of the Higgs boson cannot exist, then MSSM will also fall with it.

Taunting physicists everywhere all the way through this mesh of intense speculation, Werner Heisenberg's tragic formulation remains indefatigable. In a universe in which the scale at which physics is born is only hypothetical, in which energy in its fundamental form is thought to be a result of probabilistic fluctuations in a quantum field, determinism plays a dominant role in determining the future as well as, in some ways, contradicting it. The quantum field, counter-intuitively, is antecedent to human intervention: Heisenberg postulated that physical quantities such as position and particle spin come in conjugate quantities, and that making a measurement of one quantity makes the other indeterminable. In other words, one cannot simultaneously know the position and momentum of a particle, or the spins of a particle around two different axes.

To me, this seems like a problem of scale: humans are macroscopic in the sense that they can manipulate objects using the laws of classical mechanics and not the laws of quantum mechanics. However, a sense of scale is rendered incontextualizable when it is known that the dynamics of quantum mechanics affect the entire universe through a principle called the collapse postulate (i.e., collapse of the state vector): if I measure an observable physical property of a system that is in a particular state, I subject the entire system to collapse into a state that is described by the observable's eigenstate. Even further, there exist many eigenstates for collapsing into; which eigenstate is "chosen" depends on its observation (this is an awfully close analogue to the anthropic principle).

[caption id="attachment_23523" align="aligncenter" width="600"]

xkcd #45[/caption]

That reminds me. The greatest unsolved question in my opinion is whether the universe houses the brain or if the brain houses the universe. To be honest, I started writing this post without knowing how it would end: there were multiple eigenstates it could "collapse" into. That it would collapse into this particular one was unknown to me, too, and, in hindsight, there was no way I could have known about any aspect of its destiny. Having said that, the nature of the universe–and the brain/universe protogenesis problem–with the knowledge of deterministic causality and mensural antecedence, if the universe conceived the brain, the brain must inherit the characteristics of the universe, and therefore must not allow for freewill.

Now, I'm faintly depressed. And yes, this eigenstate did exist in the possibility-space.

Wednesday, 11 January 2012

Updating the particles parade: Exotic hadrons!

What better way to ringing in the new year than finding out that another particle has gate-crashed the 43-year old Standard Model party! Last year, superluminal neutrinos sent Nobel Prize winners and other like-minded geniuses scurrying back to their whiteboards in an effort to tweak Einstein's theories of relativity. Today, physicists at Japan's High Energy Accelerator Research Organization (KEK) found conclusive evidence of a strange hadron on the site of the B Factory (KEKB), a high-energy electron-positron collider. The so-called Belle Experiment found conclusive evidence of a hadron that had four quarks instead of the mesonic two or the baryonic three.

Hadrons are the heavier of the fundamental particles, with their lighter counterparts being called the leptons. Together with bosons, quarks and gluons, they comprise the Standard Model. The Model was cemented as a "precise" framework in 1967 when Steven Weinberg and Abdus Salam incorporated the Higgs mechanism into it as a way to explain the origin of mass in the universe. Thereafter, it has been uncannily precise in its prediction of the masses and other properties of various particles in its umbra. However, with the construction of high-energy and high-luminosity colliders such as the LHC, the Tevatron and the KEKB, unanticipated insight has been obtained into the workings of nature with the discovery of particles much outside the ambit of the Standard Model.

Hadrons, by definition, were thought to contain two quarks (the building blocks of matter, as it were) of one colour and its anti-colour or three quarks of the same colour. Essentially, they had to be "white" because of a principle called colour confinement (these aren't actual colours but markers used to identify certain properties of the particles' quantum states). When a hadron contained two quarks, it was a meson; when it contained three quarks, it was a baryon. The hadron discovered today has four quarks.

[caption id="attachment_21244" align="aligncenter" width="253" caption="Image from Wikipedia"]

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Since the colour confinement principle cannot break down under any circumstances, the hadron, despite being "exotic", has to be white. This can be achieved by multiple combinations of the twelve different types of quarks and anti-quarks. In order to further quantify its innards, therefore, physicists studied its decay pattern. They found that the new hadron decayed into a bottomonium and a charged pi meson, π±. A bottomonium is a quark-anti-quark pair and is therefore charge-neutral (the bottom quark is the second-heaviest amongst all the quarks; the heaviest is the top quark). The π meson, on the other hand, was found to be charged, indicating that the hadron, too, must be charged because of charge conservation.

The decay birthed a bottomonium weighing 10,610 MeV/c² and a π± weighing 10,650 Mev/c², placing their individual masses at close to 11 times that of a proton. Because the bottomonium is colour-confined and charge-neutral, the meson too must be colour-confined but could consist of an up and anti-down pair, giving the exotic hadron a charge of +¹/₃. Even though the measurements made by the KEKB are precise enough to merit confirmations, a Belle II Experiment is in the works that will record up to 50 times more data starting early 2015 with an astounding luminosity of 50 attobarn.

Such increasingly frequent additions to the Standard Model's particulate load has sparked concerns that there will come a point where just tweaking its principles will become insufficient. For this reason, research interest in such previously-fringe phenomena is growing because they shed important light on things we never thought could exist. At first, there were the protons, neutrons and electrons, but today, there are more than 50 different particles that make up our universe, defining everything from the strong nuclear force between baryons to the rise and fall of galaxies. It seems such is the material of impossibility: the more we try to look for what may not be there, the more we understand what the hell is going on.

Tuesday, 13 December 2011

With the quarry cornered, the real hunt for the Higgs boson begins!

The CERN has announced that the Higgs boson, a.k.a. the God particle, has been glimpsed with a mass in the vicinity of 126 GeV and with a general restriction between 115.5 GeV and 131 GeV (95% confidence level, or CL) with a 3.6-sigma local precision and a 2.3-sigma global precision at a luminosity of 4.9 inverse-femtobarn (fb^-1) by the ATLAS experiment.

In other words, there does exist a good chance of a quarry, and the quarry has been cornered.

[caption id="attachment_20965" align="aligncenter" width="530" caption="From the presentation by ATLAS's Fabiola Gianotti (unfortunate that she had to use Comic Sans)"]

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This means physicists know that the mass of the Higgs boson is not in the high-mass region (above 200 GeV) but in the low-mass region (<200 GeV). Consequently, the channels that are tracking events in this region will be watched specifically and more carefully in the checks that will be run in 2012. The checks are necessary because the events will now have to be studied with greater luminosity (measured by the inverse-femtobarn) and because the standard deviation of the errors will have to be brought down from 0.54011 GeV/c² (2.3-sigma) precision to 0.00023 GeV/c² (5-sigma).

[caption id="attachment_20968" align="aligncenter" width="530" caption="The yellow box shows what will change during the 2012 checks"]

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In order to detect the Higgs boson in the vicinity of 125 GeV to 126 GeV at the 5-sigma tolerance level, the luminosity will have to be increased from what is now 4.9 fb^-1 to 20 fb^-1. Also, the low-mass observation channels will each have to have a precision high enough to keep deviations below 0.400138 GeV/c². The Compact Muon Solenoid (CMS) experiment has also done a good job of excluding energy levels in between 127 GeV and 600 GeV with a 95% CL, and from between 117 GeV to 543 GeV with a 99% CL.

[caption id="attachment_20970" align="aligncenter" width="530" caption="The exclusion limits arrived at by the CMS experiment"]

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[caption id="attachment_20969" align="aligncenter" width="530" caption="The center chart shows the broad excess just above the mean-mark, a flat plateau that points at an event "somewhere there", and the chart on the right, corresponding to a high-sensitivity detection channel, shows a sharp peak (toward the left) - a finer image of the plateau."]

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The final results and papers will be published by the end of January, together with an update on the installation of new channels and refined analyses. The high-points of the entire experiment were the 95% CL exclusion of the 127-600 GeV range and the observation of a small excess of events in the 115-127 GeV mass range with a (high) deviation of 0.62009 GeV/c². Because of the low precision, the observation could also have been a result of background fluctuations in the experiments, necessitating a verification in 2012 - which will yield a definite answer.

Friday, 9 December 2011

Looks like we're getting somewhere.

On December 13, scientists working at the Large Hadron Collider (LHC) will make an important announcement. After the emergence of consistent information regarding two independent events at the ATLAS and CMS detectors, physicists at the sophisticated lab are excited because they may or may not have stumbled upon the elusive Higgs boson. Both events in the detectors showed a particle with mass corresponding to 125 billion electron volts that quickly decayed with a curious radiation pattern: both the signature of a Higgs boson particle. Perhaps the only reason—and an important one—is that the finding doesn't conform to the five-sigma statistical tolerance limit: reportedly, the result is 3.5 or 4.5 sigma, giving it a whopping 1-in-370 chance of being a false positive.

Looks like we're getting somewhere.

On December 13, scientists working at the Large Hadron Collider (LHC) will make an important announcement. After the emergence of consistent information regarding two independent events at the ATLAS and CMS detectors, physicists at the sophisticated lab are excited because they may or may not have stumbled upon the elusive Higgs boson. Both events in the detectors showed a particle with mass corresponding to 125 billion electron volts that quickly decayed with a curious radiation pattern: both the signature of a Higgs boson particle. Perhaps the only reason—and an important one—that stakeholders are being circumspect is that the finding doesn't conform to the five-sigma statistical tolerance limit: reportedly, the result is 3.5 or 4.5 sigma, giving it a whopping 1-in-370 chance of being a false positive.

Monday, 26 September 2011

My kingdom for a neutrino

In 1982, when the construction for CERN’s Large Hadron Collider (LHC) experiment was given the go-ahead, physics entered a very exciting period. It promised them the answers to their biggest questions and, in the event that that didn’t happen, it promised them ample evidence to come to a conclusion of their own.

Two decades later, with the device in full operation, results are emerging, some more improbable than the rest. The project was put in place to attempt to create the conditions of the Big Bang so physicists could detect the Higgs boson. However, nobody anticipated such a thing as evidence of super-luminary travel by neutrinos.

The existence of neutrinos was first proposed by Austrian physicist Wolfgang Pauli in 1930 to account for the excess mass and energy left behind after a neutron disintegrated into a proton and an electron. The first direct observation was to be made only in the 1970s, more than 40 years later. The neutrino is one of the many indivisible particles of this Universe, and is of neutral charge and very little mass. In fact, amongst all the particles that have any mass, a neutrino is the lightest. This means that according to Albert Einstein’s theory of relativity, the mass of the particle will hit infinity only when it travels at speeds terribly close to that of light. And by terribly close, I’m talking 99.9999% close.

Such neutrinos were generated by the LHC over the course of some of its experiments and sent to the Gran Sasso National Laboratory in Italy for study. Located 730 km south of the LHC and almost a kilometre under Mt. Gran Sasso, the laboratory receives the particles in a massive tank of ultra-pure water.

Once a neutrino comes in contact with a proton of a water molecule, they react to form a neutron and a positron. The positron collides with an electron, its anti-particle, to annihilate each other, releasing two gamma rays. The neutron is captured by another nucleus to release a third gamma ray. Therefore, the signature of a neutrino capture is the release of three gamma rays.

[caption id="attachment_20371" align="aligncenter" width="439" caption="The Super-KamiokaNDE experiment in Japan contains a tank of 50,000 litres of water, fit with an array of tens of thousands of photo-multiplier tubes (as above) to detect the release of energy in case of a neutrino capture. The cylindrical container has other systems in place to detect the position of a capture, too."]

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At the laboratory, as scientists waited for the neutrinos to arrive and set off the reactions, they were hardly prepared when the release of the gamma rays was detected precisely 60 nanoseconds before it was due. While such a small difference might seem trivial, the implication is that the neutrinos arrived before any other kind of electromagnetic radiation did. Since electromagnetic radiations possess the fastest speed attainable in this Universe according to Einstein, the speeding neutrinos have possibly defied the greatest physicist of the last century.

Where does that leave the world of physics?

Centuries of hypothesizing and experimenting by scientists have ingrained the importance of reasoned scepticism in their minds. While the Gran Sasso National Laboratory has claimed that 16,000 such instances have been recorded and documented, they haven’t ruled out any errors. For now, physicists the world over await similar conclusions, and so confirmations, from the two other colliders capable of replicating such conditions.

One of them is the J-PARC in Japan. Located along the coast of the Tohoku prefecture, the device was damaged and unable to operate for the next 12 months, at least, by the earthquake in March 2011 in nearby Fukushima. The other collider is the Fermilab Tevatron, an atom-smasher of considerable reputation, in the USA. After close to three decades of operation, the facility is scheduled to shut down permanently on September 30, 2011.

Collaboration rather than competition seems to be the emerging mantra. The shadow of CERN is beginning to loom large on most particle physics labs, and they’re finding it difficult to compete with CERN and its flagship project. Further, in these days of ballooning fiscal deficits and bond rating downgrades in the US, funding is hard to come by for such "creamy" projects.

Unsurprisingly, physicists are prepared to wait. Why won’t they when the speed of light – a form of electromagnetic radiation – has been the definitive cornerstone of some of the most important foundations of our understanding of this Universe? By defying that limit, neutrinos have brought upon them the scrutiny of the entire scientific community.

For one, by being faster than light, neutrinos speeding toward Earth from distant stars get here before the image of the star does, making it possible for astrophysicists to peek farther back into history. Second, the particle that carries electromagnetic energy, the photon, was thought to be massless so it wouldn’t violate the theory of relativity. However, the neutrino has mass, and that means all of Einstein’s works will have to be disrobed and studied. The larger consequence of this is that almost all high-energy installations on this planet, ranging from nuclear power plants (NPPs) that power cities to radio-telescopes searching for extra-terrestrial life, become available for changes as much as at the design level.

(In an NPP, the flow of single-phase coolants in pressurized water reactors is assumed to flow not faster than the speed of light. Even if the fluid dynamics of single-phase coolants has already been modelled on luminary principles, how could there be any changes at the design level?

If the speed of the coolants can be increased even higher, then the critical discharge, i.e., the maximum flow rate permissible, will also go higher. This translates into enhanced cooling, and this obviously means that fuel rods can be made even thicker and more power could be generated.*)

Similarly, the way the world works also is not going to change since the neutrino has always been working the same way for billions of years irrespective of how we thought it worked. But what is going to change is the way we understand electromagnetic concepts. The Standard Model of particle physics, the Big Daddy of all the theories of physics, won’t have to be tweaked so much as twisted around to accommodate this yet-unsubstantiated phenomenon.

Physicists will wait, and while they wait, they’ll debate. They’ll conduct more experiments, record more data, hypothesize more, refute more, argue more, all the while hoping that the Tevatron will be fired up for one last battle, that the J-PARC will be resuscitated for a one-of-a-kind challenge. In my opinion, no overbearing modifications will have to be made to the pool of knowledge we possess regarding physics as such. Even if the neutrinos did travel at super-luminary speeds - which I highly doubt and attribute to minute measurement errors adding up to cause the spike - it won't be long before the concept of the phenomenon is quickly subsumed by the search for other even greater truths. Yes, our perspectives are going to change, but more than anything, the discovery's original role as a tool with which to discover more about nature will not.

Let's not get carried away, though. After all, disproving the greatest minds of physics in history requires a future of its own.

*On the other hand, for example, the speed at which the gravitational force acts on any body is limited to the speed of electromagnetic radiations, but that doesn't mean discovery of a higher speed in this Universe is going to change anything. It's only a role reversal at the most (although the massiveness of the neutrino is going to make a difference) because the practically achievable velocity is going to remain the same.