THE search for the ingredients of

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1 NOBEL PRIZE 2013 Physics COVER STORY K. SMILES MASCARENHAS FRANCOIS ENGLERT AND PETER HIGGS HAVE BEEN JOINTLY AWARDED THE NOBEL PRIZE IN PHYSICS THIS YEAR FOR THE THEORY OF HOW MOST PARTICLES ACQUIRE MASS. THE CERN LABORATORY IN 2012 ULTIMATELY DETECTED THE ELUSIVE HIGGS BOSON. HERE S AN INSIGHT INTO HOW THEY WERE LED TO THE THEORY AND HOW THE HIGGS BOSON WAS DISCOVERED. THE search for the ingredients of matter is ages old. The Chinese had the simplest idea that everything should be made up of just two complementary entities, Yin and Yang. In everything they saw around them, they perceived a combination of just two entities light Illustration of the Universe depicting evolution of matter and darkness, good and bad, heat and cold, etc. The ancient Greeks felt that not just two, but a few more ingredients are required to build our Universe. Air was the chief ingredient, which was complemented by earth, water and fire. At every epoch in time, we can say that there was a Standard Model to satisfy SCIENCE REPORTER, DECEMBER man s quest to build the entire known Universe with the smallest number of ingredients. The existence of unseen particles was suspected for a long time. But many scientists questioned the very existence of Atoms till the end of the nineteenth century and it was finally settled out of any doubt by that remarkable scientist Albert Einstein in his 1905 paper on Brownian motion. In 1800, the British chemist John Dalton showed that all the matter around us is composed of relatively few elements. While some matter was composed of purely one element, others may be termed as compounds since they were made up of a combination of atoms (called molecules) of different elements (for example, water, which is composed of two elements hydrogen and oxygen). His law of multiple proportions was confirmed by many chemists and firmly established his theory. Antoine Lavoisier proved that burning is simply the chemical combination of a combustible material with oxygen in the air. Towards the end of the nineteenth century, the standard model was a collection of atoms of distinct elements and the two fundamental forces in nature, gravity and electromagnetic force (the British physicist James Clerk Maxwell managed to unify two forces electricity and magnetism into one force).

2 Peter Higgs Francois Englert In 1869, Russian scientist Mendeleev placed these elements in a particular order (the periodic table) according to some of their properties. It became clear, that all elements were related to each other in some way and were built in a particular order. Mendeleev left gaps in the periodic table as he suspected that some of the elements were missing and would be discovered in future. But as more and more elements were discovered, people started to suspect that the atoms of these elements were far from being divisible and must themselves be made up of fewer sub-atomic particles. In the beginning of the nineteenth century, the British scientist Michael Faraday, through his experiments on electrolysis, demonstrated that matter is composed of electric charges. He even went on to explain chemical bonding in terms of electrostatic attraction. Towards the end of the nineteenth century, with the discovery of the discharge tube, the existence of a negatively charged subatomic particle called the electron was confirmed and was studied in great detail by J.J. Thompson, the Director of the Cavendish Lab in U.K. The discovery of Radioactivity in some elements also confirmed the existence of charges in an atom. Subjecting a sample of a radioactive substance like Uranium to a magnetic field proved that the radiation from it consisted of positively charged particles and negatively charged particles (later on identified as electrons). The rays were identified later. Probing subatomic particles by bombarding them with other particles was pioneered by the British physicist Ernest Rutherford and his colleagues Geiger and Marsden when they probed the atoms of gold in the form of a thin foil by bombarding them with positively charged particles. The atom which is electrically neutral was found to consist of a positively charged nucleus at the center with a cloud of negatively charged particles called electrons around them. After the successful work of the Danish physicist Niels Bohr in formulating a simple model of the atom, the neutrons were discovered and it appeared that man s quest for describing matter was finally achieved since all known matter could be built by a successful combination of the protons and neutrons in the nucleus, surrounded by electrons hovering like a cloud around the nucleus. The number of protons in the nucleus defined the element and the number of neutrons (which was hypothesized before its actual discovery in 1932) in the nucleus explained the existence of the isotopes of those elements. Isotopes of an element had exactly the same chemical properties but were heavier than the original element. THE INITIAL SUCCESS OF THIS STANDARD MODEL MADE BOHR TO MAKE THIS FOLLOWING IMPETUOUS REMARK IN 1930: PHYSICS, AS WE KNOW, WILL BE OVER IN SIX MONTHS TIME. 13 The standard model now had only three particles and four known forces (two nuclear forces are now added with the discovery of the structure of the Atom, one was the strong nuclear force that packed the protons and neutrons in the nucleus and the other was the weak nuclear force responsible for the rays). The initial success of this standard model made Bohr to make this following impetuous remark in 1930: Physics, as we know, will be over in six months time. He had every reason to believe that man had ultimately conquered every frontier in physics. The success of the discoveries made in the nineteenth century, chiefly classical mechanics, Thermodynamics, Electromagnetic theory, the revolutions caused by the Relativity theory, Quantum mechanics and Astrophysics in the beginning of the twentieth century, and now the explanation of all known matter in the Universe in terms of just three entities. surely it appeared that SCIENCE REPORTER, DECEMBER 2013

3 there was nothing more to be discovered in physics. But alas, his expectation was shortlived. With the invention of the Wilson s cloud chamber and a number of other modern particle detectors many new particles began to pop out. Some were A typical candidate event for the Higgs bosonn found in Cosmic rays (no one is absolutely sure about their origin even today) and some in accelerators designed to tap nuclear energy. The discovery of hitherto unknown particles like the neutrino, positron, muon, kaon, pion, etc. and a zoo of other elementary particles prompted NOBEL PRIZE 2013 Physics scientists to believe that we were nowhere near our goal of describing matter with fewer ingredients. Many experiments seemed to suggest that particles like the electrons were truly elementary, in the sense they are not made up of smaller entities, but the protons and neutrons were actually composite in nature. Particles that were believed to be composite came to be known as Hadrons (Hadrons are further divided into Baryons and Mesons) and particles like the electron that are not made up of any other building blocks were called Leptons. Further, the British physicist P.A.M. Dirac derived a relativistic equation for the electron and concluded that for every particle, an antiparticle exists as well. His prediction was confirmed when the positron, which was identical to the electron in every respect except charge (which was positive), was discovered. Soon many more anti particles were discovered and it is possible to have antihydrogen atom made with the antiproton at the nucleus and a positron orbiting around it. The existence of antimatter is now firmly established and many astronomers believe that some of the ATLAS Experimental Set-up (Source: ucsc.edu/personnel/ atlas.html) Muon Detectors Electromagnetic Calorimeters Solenoid Forward Calorimeters Detector characteristics Width: 44m, Diameter: 22m, Weight: 7000t CERN AC- ATLAS VI 997 End Cap Toroid Barrel Toroid Inner Detector Hadronic Calorimeters Shielding SCIENCE REPORTER, DECEMBER

4 galaxies out there in the Universe could be made up of antimatter. The strong nuclear force is responsible for holding the protons and neutrons together in the nucleus, and the weak nuclear force is responsible for the decay. The decay is triggered by a neutrino to convert neutron to proton by the ejection of an electron. Einstein THE HIGGS BOSON WAS THE ONLY MISSING LINK IN THE JIG-SAW PUZZLE OF THE STANDARD MODEL FOR A LONG TIME. AND IT HAS LANDED FRANCOIS ENGLERT AND PETER HIGGS WITH THE NOBEL PRIZE IN PHYSICS THIS YEAR. believed in the unification of all these forces. His attempts to unify gravitational forces and electromagnetic forces did not bear any fruit. To add to the already existing confusion, a theory developed by Feynman, Schwinger and Tomonaga (called the Quantum Electrodynamics or QED, for short) predicted the existence of force mediating particles to explain the known natural forces. Classical physics describes the force of repulsion between two charges in terms of the electric field they create. QED interprets the force as the exchange of particles (virtual photons) between the charges. The strong nuclear force that keeps proton and neutrons packed in the nucleus was also suspected to be due to the exchange of particles. The theoretical prediction of the Japanese physicist Hideki Yukawa about a particle (pion) that could be responsible for interaction of protons and neutrons in the nucleus was dramatically confirmed with its detection (even though the muon was initially mistaken for the pion). To explain the weak nuclear force (responsible for decay), two particles were postulated by Steven Weinberg, Sheldon Glashow and Abdus Salaam in the year 1968 on the basis of the modern Standard Model the W (which comes in two charges) and the neutral Z particle. Their existence was dramatically confirmed in Particles like the photon, W and Z are called gauge bosons and using symmetry principles one would expect all of them to be massless. But the W and Z bosons have a mass of roughly 100 times the proton mass. All these particles had extremely short lifetime and detecting them was a real challenge. In the middle of the 1920s, two different types of statistics were proposed An internal section view of the Large Hadron Collider (Source: 15 SCIENCE REPORTER, DECEMBER 2013

5 ON 4 JULY 2012, CERN OFFICIALLY ANNOUNCED THE DISCOVERY OF A NEW FUNDAMENTAL PARTICLE WITH PROPERTIES SIMILAR TO THOSE EXPECTED OF A HIGGS BOSON. to describe the behaviour of elementary particles known at that time. The first one was proposed in 1924 by Satyendranath Bose and Albert Einstein and the second one was proposed in 1927 by Enrico Fermi and Paul Dirac. It is remarkable that to this day, all the known particles obey the rules dictated by either of these statistics and it is truly strange that there are only two types of statistics and no other to describe a conglomeration of particles. Therefore, all the known particles fell neatly into two groups. Those that obeyed the first were called Bosons and the ones that obeyed the latter were called Fermions (this nomenclature was suggested by Dirac). The basic difference between these two groups can be appreciated by a simple thought experiment. Imagine two streams of these particles crossing Cern test breaks speed of light seconds time taken by neutrinos each other s path at a point. If they are Fermions, one stream will influence the other and their paths will be disturbed. If they are Bosons, they will simply cross over, oblivious of the other stream. It should be remarked that all particles are endowed with an intrinsic spin very much like a spinning top. In atomic units, if the particle has an intrinsic angular momentum in integral units of h/2π, then it is a boson. For instance, the spin of a photon, being either +1 or -1, makes it a boson. Fermions have multiple half integral spins. Protons and electrons qualify to be fermions. All force mediating particles are bosons. Since all particles are either bosons or fermions, the discovery of any new particle would warrant a Nobel Prize for either Bose or Fermi in addition to the one who discovered it. But the Nobel prizes are not awarded posthumously (except the only case for medicine in 2011, when the Nobel committee was unaware that the recipient, Ralph Steinman, had died a week before the announcement was made) and they are not mentioned in any award. OPERA Experiment seconds faster than the expected time NOBEL PRIZE 2013 Physics The clues to the ultimate t structure t of matter came from two extreme frontiers in physics. One is particle physics that examines the structure of particles in the sub-micro scale by colliding particles at high energies and examining the wreckage. The other is Astrophysics that takes a look at matter in its largest dimension, looking deeper and deeper into the universe to indirectly examine how it was in the beginning. Observations of distant galaxies have confirmed that the universe is expanding from a common point. If we reverse time, it is logical to assume that sometime in the distant past all the known matter in the universe was confined in a singularity (no one knows its real dimension, all we can say is that it had an extremely small dimension). Inside the singularity, energy and temperature must have been enormous and none of the known laws of physics must have been operational. Out of this energy, matter was created. Particles collided and new particles were formed only to dissolve into energy. But how can all the matter be confined in such a small dormitory? No one knows for 732 km distannce travelled through rock SWITZERLAND Geneva ITALY Gran Sasso Cern Geneva Gran Sasso Cern, Switzerland: A beam of neutrino particles is sent through rock towards Italy SCIENCE REPORTER, DECEMBER Gran Sasso, Italy: Bricks with ultrasensitive covering at underground laboratory detect arrival

6 sure but none of the matter at that stage knew that someday they will be referred to by an Indian s name. At some point in time, some of them for some unknown reason assumed an Italian s name (people who had been to Italy will agree that they are not so friendly in general!). They refused to be cuddled up in the dormitory (the degeneracy pressure) and what resulted was a violent explosion that was never witnessed before or will be seen again (Big Bang). Modern theories could not predict the nature of the particles immediately after the Big Bang but speculated they were massless till a strange field now called the Higgs field spread around them. Some of the particles found this field to be sticky and developed an inertia when they tried to move about them and acquired an inertial mass. Others, like the photons and some neutrinos, did not care about this strange field, and managed to remain massless. In 1964 Englert, along with Robert Brout (who died in 2011), and Higgs independently proposed a model, now known as the Brout-Englert-Higgs or BEH mechanism, which links mass to a proposed new particle. This particle, explicitly predicted by Higgs and now known as the Higgs boson plays the same role as a force mediating particle in this Higgs field, very much the same as the virtual photon plays in the Electromagnetic field. Without the Higgs field, everything would have been massless and electrons and protons would have madly rushed through the universe at the speed of light with no chance of any matter to form. Fundamental symmetries This is how the Standard Model looks now Peter Higgs (right) and François Englert (left) at a CERN seminar, 2012 in the laws of Nature dictate that all particles should be massless. The Higgs boson is credited with generating masses for most of the fundamental particles, as some observed that particles eat the Higgs boson to gain weight! But the problem with the standard model was that it could not specifically state the energy of the Higgs boson or its equivalent mass. It was suspected that the Higgs boson must have an equivalent energy in excess of about 120 GeV. To isolate it, protons must be made to collide with energies in excess of a few TeV (1012 ev). Large accelerators were constructed and billions of dollars were spent in a bid to search for the elusive Higgs boson. The Large Hadron Collider (LHC), a circular accelerator 27 km in circumference, was the most ambitious scientific apparatus ever built by man (covered in detail in the July 2012 issue TWO NUCLEAR FORCES ARE NOW ADDED WITH THE DISCOVERY OF THE STRUCTURE OF THE ATOM, ONE WAS THE STRONG NUCLEAR FORCE AND THE OTHER WAS THE WEAK NUCLEAR FORCE RESPONSIBLE FOR THE / RAYS. 17 of Science Reporter). Many countries participated in the search for the Higgs boson and India also got to play a major role (covered in the November 2012 issue of Science Reporter). On 4 July 2012, CERN officially announced the discovery of a new fundamental particle with properties similar to those expected of a Higgs boson. The discovery was made independently by two teams, ATLAS and CMS at the LHC. It took another nine months and the dedicated work of hundreds of experimenters before CERN confirmed that the particle was indeed the longsought Higgs boson. The Standard model that is now available to us unites the fundamental building blocks of nature (quarks and leptons) and three of the four forces known to us (the fourth, gravitation, remains outside the model and probably will never come into its framework) into an edifice that appears to satisfy the ancient quest of man. The Higgs Boson was the only missing link in the jig-saw puzzle of the standard model for a long time. And it has landed Francois Englert and Peter Higgs with the Nobel Prize in Physics this year. Now that it has been discovered, the Standard Model appears to be complete. But will this be the Ultimate Standard Model? Many scientists, who are so used to surprises from Nature, doubt it! Prof. K. Smiles Mascarenhas is currently the Dean (Academic Affairs) at the Coimbatore Institute of Engineering and Technology, Narasipuram post, Coimbatore He was formerly a scientist at the Millimeter wave lab of the Raman Research Institute, Bangalore. smiles51@rediffmail. com. SCIENCE REPORTER, DECEMBER 2013