Particle Physics

Cracks in a model

Print edition : May 26, 2017

In a tunnel of the Large Hadron Collider at CERN, the European Organisation for Nuclear Research, in Meyrin, Switzerland, a worker cycles along during maintenance work. A file photograph. Photo: FABRICE COFFRINI/AFP

B meson decay. Depicted here is the b meson decay in two modes, and both should be equally probable.

The mass puzzle. Why fundamental particles have varied masses is something that is not understood fully.

Inside an atom. Just as an atom can be split, physicists found that even protons and neutrons can be split.

Layers of matter. Matter is layered with molecules, atoms, subatomic particles and the world of quarks. Are they the final stage of matter?

The startling LHC “beauty” experiment discovery hints at a tantalising new physics that is a significant modification of the Standard Model of particle physics.

IN a coup d’etat of sorts against the Standard Model of Physics, that is the current theory of “ what-are-we-made-up-of”, on April 18 the LHCb (Large Hadron Collider-beauty) experimental team announced live on web streaming that it had found “indication” of a difference in the behaviour of electrons and muons, signalling the onset of the next revolution in physics.

Akin to the revolution set off by Ernest Rutherford substituting John Dalton’s atomic model and Albert Einstein’s theory of relativity replacing Newtonian physics, the results, if proven, could lead to the demise of the tried-and-trusted “Standard Model” of particle physics, and taking its place will be a brand new model that may explain many of the yet unsolved mysteries of the universe, such as dark matter, and perhaps reveal new particles and even new forces lurking behind the veil of nature.

The Standard Model

Simply put, the Standard Model is the best answer the physicist has today to the question “what are we made up of?”. This theory tries to explain what the world is made up of; how the various elements interact with one another to produce a variety of materials, from atoms of gold; and why water is less dense when frozen.

Like the famous Russian nesting doll, a set of wooden dolls of decreasing size placed one inside another, physicists show that all the things around us are made up of thousands of molecules, which in turn are made up of 118 types of atomic elements. Atoms are made of subatomic particles such as protons, neutrons, mesons, and so on. Now physicists show that even protons and neutrons are composites, made of even smaller particles such as quarks and gluons. The question is, are these quarks and other particles predicted under the Standard Model the terminal building blocks?

According to the Standard Model, the ultimate building blocks of matter are six “flavours” of quarks and six leptons and 13 force carrier particles, such as the photon (see box). When Dmitri Mendeleev arranged all the then known atoms into a periodic table, many elements such as scandium, gallium, technetium and germanium were unknown. However, the periodic table predicted that elements with specific characteristics should exist to fill in the gaps in it. In a similar story, all the particles now forming part of the Standard Model portrait were not known when it was formulated. Experiments conducted at accelerators and colliders confirmed the existence of these fundamental particles. If the discovery of W+, W- and Z0, gauge bosons that mediate weak nuclear force, in 1983 gives credence to the Standard Model, the discovery of the top quark in 1995 confirmed the prediction of the six flavours of quarks by the Standard Model. The last piece of this jigsaw puzzle, the Higgs boson, proved to be elusive for more than 50 years until it was finally discovered in 2012.

The ‘Beauty’ experiment

The LHCb experiment is one of the seven particle physics detector experiments forming part of the LHC, the world’s biggest atom smasher at CERN, the European particle physics laboratory near Geneva, Switzerland. The LHCb, established in 1995 at point 8 on the LHC tunnel, has been exploring the decays of B0 mesons (massive particles containing a bottom quark a.k.abeauty quark”), and the current results come from data collected during its Run I from 2009 to 2013.

“The B meson is made of b quark which was called the beauty quark.... Hence the name beauty stuck,” says Rahul Sinha, a faculty member at The Institute of Mathematical Sciences, Chennai. About 840 scientists from 60 scientific institutes from 16 countries constitute the collaboration that conducts the experiments.

Just as buildings, trees and everything around us decay with time, subatomic particles spontaneously mutate into more stable ones in a process called particle decay. Thus, B0 mesons, a type of subatomic particle, last only for a thousandth of a nanosecond and fall apart into more stable particles. Often the B0 meson decays into a K meson and a pair of oppositely charged muons decays into a K meson and an electron-positron pair. Heavier muons are similar to an electron, except for mass, which is 200 times more.

While deer and moose have antlers and zebras and horses do not, the anatomical structure of all these is closely related as they evolved from a common ancestor. Similarly, a muon in the Standard Model and an electron belong to a class of particles called leptons. These two and the third type of lepton, the tau particle, are supposed to make up “three generations” and are hence expected to behave in the same way, in particular when they interact via electromagnetic force or the weak force in a property known as lepton universality. The Standard Model predicts that the chance that the B0 meson decay will result in a pair of electrons should be about the same as the chance that it will end up as a pair of muons, “up to a small and calculable effect due to the mass difference”.

What was discovered

One of the LHCb experiments counted the number of times the B0 meson decayed into the K meson and a pair of oppositely charged muons or into a K meson and an electron-positron pair. While the expected ratio was around 1, the actual experimental result was 0.7, with more electrons than muons being produced.

Although this discrepancy from the predicted value is tantalising, the statistical deviation was only 2.2 to 2.5 sigma. In contrast, the Higgs boson discovery had a five sigma deviation. Besides, in particle physics the results need to pass through at least three standard deviations before they are considered firm findings. Nevertheless, physicists are excited, given similar indications from some of the earlier studies—for example, in 2013 researchers examined the angles at which particles emerge in such B meson decays and found that they did not entirely match the predictions.

If this finding is substantiated by further data, it will imply a striking breakdown of the Standard Model and a new kind of interaction that violates lepton universality. “Whether this would result in dramatic change or alteration... this result means significant modification of the Standard Model,” says Rahul Sinha. Significantly, it was Rahul Sinha and his collaborators who, way back in 2000, computed this rare and very sensitive decay mode to discover physics beyond the Standard Model. To determine whether this is a definite hint of cracks in the Standard Model or just a statistical fluctuation, we may have to await follow-up analyses and further studies, in particular by the upcoming Belle II experiment in Japan.

Gaps in Standard Model

Until now the Standard Model has been successful in describing how protons and neutrons are formed, how atoms stick together, the mechanism of radioactivity, why the proton is stable, and many other features of physics. A large number of experiments have verified it to a great extent. However, one of the gaping holes in the theory is that it does not incorporate gravity and hence is not compatible with the general theory of relativity.

Ashok Sen, a renowned theoretical physicist from Harish Chandra Research Institute, Allahabad, says: “The reason gravity has not bothered us so much is that it is very weak. But, one can imagine an experiment which will involve accelerating particles to an energy that is much higher than what we have achieved today and colliding them together where gravity will be as strong as the other forces, and in that context if you want to know what the result of the experiment would be, you cannot explain it without understanding gravity.”

Further, the “flavour problem” (why there are six types of quarks) and the “hierarchy problem” (why the coupling strength of gravity is incredibly tiny, just 10-39 of the electromagnetic force) are big mysteries. Further, the Standard Model seems to have no way of accounting for the ever-mounting evidence for dark matter, which makes up more than 96 per cent of the matter in the universe.

Is it revolution?

Does the startling announcement made on April 18 imply the imminent fall of the Standard Model? Are we on the verge of another “revolution” in physics?

In the history of science, when the planet Uranus was discovered by William Herschel in 1781, astronomers carefully observed its path around the sun. To their astonishment, the orbital path showed a mysterious perturbation. This could imply one of two things: that Isaac Newton’s universal law of gravity was faulty or that there was a planet beyond Uranus that was exerting a gravitational pull, resulting in an anomaly. The discovery of Neptune put the controversy to rest. However, a similar perturbation observed in the orbital path of Mercury was resolved only with Einstein’s revolution replacing Newton’s physics.

Which way will this go? Rahul Sinha says: “Given the data reported today and the corresponding mode measured earlier, we should be prepared that change will be needed; whether it would be dramatic like the Rutherford model replacing the earlier indivisible idea of the atom we cannot say at this point.”

Physicists are engaged in collecting more data. After the upgrade, the LHC is doing the Run 2, which will last until 2018. If data from this corroborate the Run 1 results, then it certainly means the demise of the Standard Model. Data from the Belle II experiment at Tsukuba, Japan, in which an Indian group led by the Tata Institute of Fundamental Research (TIFR) has built a layer of the silicon vertex detector, are also keenly awaited.

This experiment specifically produces B mesons from a high-intensity electron-positron collision, providing crucial data on the rare and very sensitive decay mode. “The important point is that LHCb and Belle II will both produce enough B mesons that we should have a better reach (through loop effects) for new physics compared with the CMS and ATLAS experiments at CERN,” says Prof Sinha.

Will this be a failed putsch or a revolution in the offing? Physicists have started taking sides.

T.V. Venkateswaran is a scientist with Vigyan Prasar, Department of Science and Technology, New Delhi, and his email is tvv123@gmail.com.

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