HIGGS-BOSON
[The article below is an unusual piece in INSAF Bulletin but is being produced because of the profound significance of recent proof the existence of Higgs-Boson. The version without photos is included in August INSAF Bulletin and the version with photographs appears as a Supplement. We thank Professor Gurtu for writing this article for the readers of INSAF Bulletin.
Professor Atul Gurtu is an experimental particle physicist who recently retired from the Tata Institute of Fundamental Research (Mumbai). His name appears among the top particle physicists of the world. From 2003-2011 Prof Gurtu was the Leader and Spokesperson of the Indian team in the CMS Collaboration at CERN, Geneva. He is currently an Adjunct Distinguished Professor at King Abdulaziz University, Jeddah. He will be happy to explain and clarify any points on recent discovery of the Higgs-Boson and can be contacted at atul.gurtu@gmail.com
Of course some would claim that Higgs-Boson was known to Indians from Vedic times and its properties are mentioned in Rigveda. Editors ]
HITTING PAY-DIRT: DISCOVERY OF THE HIGGS BOSON
Atul Gurtu
4 July 2012: The most expensive and painstaking hunt in fundamental science is over and a new chapter is beginning. The Higgs boson is finally pinned down; the so-called “God Particle” unveiled. Elation, not only among the practitioners of particle physics, but headlines the world over. But why? What’s the big deal? Discoveries have been made in the past. What’s so special this time? Read more…
Looking at the myriad objects around us in the world, gazing up at the skies, humans have always asked the questions: what is all this made up of? What lies in the sky yonder, the stars, the galaxies; how did it all begin, and where is it all headed? The discovery of the Higgs represents a moment in the history of science when a turning point has been reached in the answer to these basic questions.
So, why is the Higgs boson so important and why did Leon Lederman (Nobel Laureate and former Director of Fermilab, near Chicago) coin the phrase “The God Particle”? It’s because it is the manifestation of a field, now called the Higgs field, which pervades the entire universe and is responsible for giving mass to all the fundamental constituents of matter.
The Higgs boson, the cornerstone of the extremely successful theory, the Standard Model of Particle Physics, was the missing piece, still not experimentally discovered though it had been predicted as early as 1964 when Peter Higgs (and others) had suggested its existence. The discovery is as much a triumph of Man’s intellect in predicting it, as it is of the amazing tools developed to search for it and to discover it: the Large Hadron Collider (LHC) particle accelerator at CERN (Geneva, Switzerland), the two mammoth state-of-the-art particle detectors (ATLAS and CMS), a world-wide computing GRID for extracting sense and science out of the petabytes (mega-giga bytes) of data flowing out of them, and most importantly the ingenuity of the scientists and engineers for implementing the whole project.
A little digression on the Standard Model: matter consists of atoms which give it chemical properties; an atom in turn has a nucleus at the center surrounded by electrons in orbits around it. The 20th century saw unveiling of the structure of the nucleus and today we know that there are twelve fundamental constituents of matter which can be clubbed in three generations. The visible universe we see around us needs only the first generation for its existence: the two quarks called up and down (u d) which make up the protons and neutrons of atomic nuclei, the electrons, which orbit the nuclei, and the neutrino, which is necessary to explain the properties of radioactivity. The other two generations were discovered in the 20th century when scientists studied high energy interactions. 2nd and 3rd generation particles are heavier than their first generation counterparts and have some other unique properties. The 2nd generation consists of the charm and strange quarks, the muon and its neutrino, and the 3rd consists of the top and bottom quarks, the tau and its neutrino. These are the so-called “matter” particles.
But it’s not enough to have only the matter particles; there are forces between them. We are very familiar with the electromagnetic force and gravity. Two other forces discovered in the last century are the strong force, which binds the nuclei (or quarks) and the weak force, responsible for radioactive decays. Following Maxwell’s unification of electricity and magnetism in the late 19th century, scientists have endeavored to unify other forces as well. Einstein failed in his attempt to unify electromagnetism and gravity. In an effort to unify the electromagnetic and weak forces the idea of the Higgs field and generation of mass due to its presence played a key role and led to their successful unification. It is now called the “electroweak” theory. And the Higgs boson is the cornerstone of the theory. Finally, what does “boson” denote? All the sub-atomic particles are characterized by a property called spin. All the force carrying particles have integral spin (0, 1,2 …) and are called bosons after Satyendra Nath Bose, and all the matter particles (quarks, electrons, neutrinos) have half integral spin (1/2, 3/2, …) and are called fermions after Enrico Fermi. The Higgs boson is the only particle in the theory to have spin 0.
So, how was the discovery of the Higgs made? To discover a particle at the LHC it first has to be created in a proton-proton interaction with some of the kinetic energy of colliding protons being converted into its mass (Einstein’s E = Mc2). As it would quickly decay into stable particles, e.g., photons, these have to be detected in the instruments surrounding the interaction point. Being predicted in 1964, the Higgs had been searched for a long time; unsuccessful searches at lower energies at CERN and Fermilab implied it was heavier than 114.4 GeV (Giga electron volt), equivalent to the mass of 120 protons, and also could not be in mass range 145 – 179 GeV. Furthermore, in addition to having sufficient energy to produce the Higgs, a sufficient number of them have to be produced and studied to ensure that other reactions were not mimicking the Higgs signal (this is called the background).
Thus the LHC accelerator had been conceived in the mid-1990’s: it was to have high enough collision energy between protons so that it can easily cover high Higgs masses, and the proton beams would be sufficiently intense so that huge numbers of proton-proton interactions can be studied within a reasonably short time. The currently available energy is 8 Tera electron Volt (TeV, equivalent to 8500 proton masses). The machine is a technological marvel: it consists of two interlaced steel pipes of 27 km circumference in which the protons circulate in opposite directions, being made to collide at 4 intersecting interaction points. In order to accelerate and store these beams the insides of these pipes are the coldest place in the universe(!) being maintained at 1.9 degree Kelvin (outer space is at 2.7 degree), and they are the most empty place in the galaxy, with extremely high vacuum so the beams don’t dissipate. A view of the accelerator inside the tunnel is shown in the picture 1 below. Both rings are encased within the same envelope structure.
To detect the Higgs decays are two experiments, ATLAS and CMS, whose detectors were designed to be sensitive to the predicted Higgs decays, in particular to high energy photons. Each of these detectors is about 20 meter long and 20 meter (CMS) to 40 meter (ATLAS) in diameter. They are packed with state-of-the art electronics instruments to detect the passage of sub-atomic particles flying out from the interactions at almost the speed of light and to pin-point their trajectories to within micrometers at various places in the detectors. They have magnetic fields to bend the trajectories of charged particles, to determine their momentum, and they have energy measuring devices. Each detector costs around 500 Million US$ to build and 150 or so institutions fabricated different parts of each of them. It was an equal marvel of engineering and meticulous planning for it to all come together and function flawlessly.
When protons collide they produce myriad particles which leave their electronic traces in the detectors; these tiny signals are detected and recorded, each interaction leading to many megabytes, or more, of data. This is piped onto the resident computer storage and then farmed out to many tens of dedicated remote computer centers around the world in the collaborating institutions. Computing GRID technology using intercontinental gigabyte/second data transfers was developed to take care of this gargantuan task of computing.
The LHC project is really a worldwide collaborative project with over 40 countries playing a role. India has contributed in all aspects of this project. Significant amount of accelerator hardware and testing and installation expertise was provided, led by the Raja Ramanna Center for Advanced Technology, Indore. Indian software specialists, mainly from BARC, have been instrumental in setting up computing GRID related software.
Indian scientists are participating in two experiments, including contributions of detector hardware. One is the CMS experiment with TIFR, Mumbai as the nodal institution along with BARC, Delhi, Panjab and Viswa Bharati universities. This experiment is one of the two at the LHC (other is ATLAS) which have discovered the Higgs. The second is the ALICE experiment with VECC, Kolkata as the nodal institution. This is a smaller, specialized experiment designed to detect a new phase of matter, the quark-gluon plasma, by a studying collisions of lead nuclei (LHC accelerates lead nuclei too).
By the end of 2011 CMS and ATLAS had studied around half a trillion proton-proton interactions with available energy of 7 TeV (Tera electron Volts) in search for the Higgs (and also doing much other physics). How would the Higgs show up, say in its decay into two photons? In each interaction many photons are created, which are detected and their energies measured. The vast majority of these would be coming from other processes and constitute a “background” for the Higgs decay into two photons. The trick is to take one pair of photons at a time, assume they come from the decay of a particle and calculate what would be the mass of this decaying parent particle. Knowing the energies and directions of the photons this is very simple:
Parent Mass = square root of [2 x E1 x E2 x (1 – cosine theta)]
where E1 and E2 are the energies of the photons and theta is the angle between them.
One then just plots the number of 2-photon combinations versus the calculated mass. The “background” is a smooth shape, whereas, if a Higgs is produced, an excess “peak” shows up in this mass plot. Using such methods, at end-2011 it was observed by both experiments that they detect some excess of events at 120 – 127 GeV effective mass. However, the uncertainty on the number of events was not small enough to rule out that maybe the background had “fluctuated” to mimic the Higgs signal. [A simple analogy is of one trying to make out one voice (signal) amid a crowd of people all of who are conversing, whose voices constitute the background. If one wants to discern the signal distinctly one needs to reduce the background (but one can’t do that beyond a point) and to increase the signal (ask the guy to shout!). Thus acquiring more data is like increasing the signal compared to the background. And someone in the crowd may also shout loudly once in a while, which may mimic the desired signal.]
In 2012 the LHC operated at the slightly higher energy of 8 TeV and by mid-June had already generated data equal to that during the whole of 2011 (this itself is a triumph for the machine). Now the Higgs signal, seen as a peak in the mass plot, became quite clear and a claim of discovery was in order. The discovery was announced in a special day-long seminar organized at CERN on 4th July as well as reported at the International Conference on High Energy Physics at Melbourne which started at that time.
The CMS plot for 2-photon mass combinations is shown below in picture 2. The black points are the data. A peak shows up at a mass of 125 GeV, which is interpreted as a new particle, possibly and most likely the Higgs. We still use the qualification “possibly and most likely” because while a new particle has been discovered which “smells” like the Higgs, and “feels” like the Higgs, but for 100% surety one needs to study its properties further to completely confirm the identification.
Similar results were obtained by the other experiment, ATLAS. Both CMS and ATLAS studied not only the Higgs –> 2 photon decay, but also other decay modes which are consistent with this. Pictures 3 and 4 are digitizations of a Higgs –>2 photon event from CMS and a Higgs –> 4 leptons (electrons and muons) event from ATLAS, as reconstructed from the electronic signals recorded in the detectors. In the CMS event the photons are the two towers shown in the right hand upper and lower quadrants, and in the ATLAS event the 4 leptons are the 4 towers protruding out.
Where do we go from here? Work for the scientists is cut-out: study many more interactions and determine the properties of this new particle to confirm its identity. If indeed it is the Higgs, then its “low” mass (in principle it could have been anywhere up to 650 or 700 GeV) is completely consistent with what a beyond-standard-model theory, SuperSymmetry (SUSY), predicts. But then one should be able to detect SUSY particles at LHC too, and not having seen them yet has started giving head-aches to SUSY proponents!
So, one will continue the search for SUSY particles. One major interest is that SUSY particles could help untangle an 80 year old cosmological puzzle based on astronomical observations. Like the planets revolve around the sun, stars in a galaxy revolve around the center of the galaxy. Gravitation theory being well known, and by observing how the stars are distributed in the galaxy, one can predict the speed of revolution of stars at different distances from its center. When astronomers studied their observations they were astounded to find that the speed of stars far away from the center was far in excess to that expected. One way to explain this was to assume the existence of unseen matter uniformly distributed within the galaxies in addition to the shining (visible) matter – the stars. When the SUSY theory was formulated about 40 years back it was a great bonus to predict the existence of SUSY particles which could easily be the unseen “Dark Matter” needed to explain the galactic motion of stars. One was serendipitously connecting the physics of the micro-world to that of the skies! So, one sees what an exciting program of fundamental science lies ahead of the LHC: from the sub-atomic to the cosmological! Also the high energies obtained in LHC collisions mimic conditions prevailing a millionth of a millionth second after the big bang creation of the universe.
We cannot end without mentioning that even if the LHC helps unravel the mysteries of the visible universe and dark matter, these constitute only about 30% of the known energy in the universe. The remaining 70% is called dark energy, whose presence was deduced from the study of the expansion of the universe, which seems to be accelerating in its expansion.
Truly, a vast frontier in science remains unexplored; but discovery of the Higgs boson has provided an experimental anchor on which to peg future human endeavor in this direction and is a great milestone in the march of human intellect. That’s why the hoopla!!
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