"As a layman, I think we have it", said R. Heuer, the Director General of the European Organization for Nuclear Research (CERN), on Wednesday during the seminar that was webcast live on internet for everyone. 'Higgs Boson' is one of the 10 'Hot Searches' in US on Google Trends right now. It wouldn't be an exaggeration to say that it's the first time in the history, when the entire world, and not just scientific community, is reading, talking and following, in every possible way, an ongoing research project in the fundamental sciences, the search for the Higgs Boson. So, for a physicist like me, who is pursuing research related to the Higgs Boson(s), what would be a better topic to start a blog with than the Higgs Boson itself! And hence, here's an introduction to what the Higgs Boson is all about.
What is it?
Higgs is a (or may be many) particle(s) of 'Scalar' type. 'Scalar' means it has 'spin quantum number' zero. In Quantum Field Theory a 'scalar field' is associated with a Higgs Boson. Since its spin is an integer and not a half-odd-integer (1/2, 3/2, 5/2, ....), it is a 'Boson'. Hence, the name 'Higgs Boson'. In reality their could be only one Higgs or multiple Higgs particles. Which of these possibilities is the reality is what Physicists working at the Larger Hadron Collider (LHC) at CERN are trying to find out experimentally.
Is it the God particle?
Well, what's meant by the God particle? Generally people say that Higgs Boson is responsible for mass of everything in the universe and that's why it is the God particle. Is it?
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| Figure 1: Standard Model particle content. Source: Wikipedia. |
Standard Model (SM) is the theory of Electromagnetic and Weak nuclear and Strong nuclear interactions between elementary particles in the nature and other particles they constitute. As of today this theory stands to be the most successful experimentally tested and verified theory of the aforesaid fundamental interactions. SM says that the 'visible' universe is composed of 12 fundamental particles shown in the figure to the left. It is the Higgs Boson that is theoretically found to be responsible for giving all of these particles except photons and gluons their respective masses. BUT about 99.9% the visible mass of the universe is made up of protons and neutrons, each of which is made up of quarks. 99% of the mass of a proton or a neutron is not accounted for by masses of the free quarks, but it comes from the energy of strong nuclear interaction, that keeps these quarks together in a proton or a neutron, manifesting itself as mass. So Higgs does not account for 99% of the mass of the universe. And that makes the term 'God particle' a misnomer to describe the Higgs Boson.
How does it do it?
10 out of 12 particles in figure 1 get their masses through their 'interaction' with the Higgs field that is spread everywhere in space (remaining 2 particles, the photon and gluon, are massless). A Higgs field is spread all over the space. The most stable state of the Higgs field is to be spread all over the space with a non-zero average value. It acquires this value through a process called the 'Electroweak Symmetry Breaking'.
Next to understand the 'interaction' part, let's consider an analogy. Imagine a room crowded with journalists. If a not-at-all popular person enters the room, no journalist is interested in 'interacting' with him. So the person passes through the room like massless photons and gluons do through a Higgs field with the speed of light in vacuum (c = 300,000 km/sec). Now if a somewhat-popular person enters the room, then a few journalists will try to interact with him to get a news-bite. So, this person cannot move as fast as the first one. Similarly a few fundamental particles get small masses, because their coupling with a Higgs field is not very strong and these particles cannot move with the speed of light. On the other hand, if a very popular celebrity enters the room then he/she has to interact with a lot of journalists. For those, who watched the live webcast of the press conference from CERN on July 4, recall when Peter Higgs entered the press conference room (refer to the photo above). In the same way the heavy particles get their masses through stronger coupling with the Higgs field. These particles cannot move with the speed of light either. Quad erat demonstrandum that the Higgs field is responsible for masses of the fundamental particles in the visible universe.
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| Figure 2: Peter Higgs had to interact with a lot of journalist, like heavy fundamental particles do with the Higgs field and get their masses (At the press conference after the seminar at CERN on July 4, 2012). Source: CERN |
What about the mass of Higgs?
The existence of the Higgs Boson(s) was first theoretically predicted, in 1964 through three research papers by Peter Higgs and Robert Brout, François Englert and Gerald Guralnik, Richard Hagen, Tom Kibble. But theoretical predictions could not lock on the mass of Higgs. Standard Model predicts only a single Higgs Boson. That's what physicists at LHC are mainly looking for right now. So when we hear or read news about the Higgs particle written by the press, most of the times they are talking about the Standard Model (SM) Higgs Boson, unless otherwise is stated. Although, many models have been proposed to answer questions, which standard model cannot and many such model predict existence of more than one Higgs Boson particles. When the LHC was turned on, physicists only knew that SM Higgs is at least about 100 times heavier than a proton (mass of proton is about 1 GeV/c2) and that its not heavier that about 1 TeV that is 100,000 times heavier than a proton. Over the period of 2 years LHC has lead us to experimentally narrow down this wide range with great certainty to few GeV/c2's (between about 120-130 GeV/c2). Its exact value will be known if and when SM Higgs Boson can be claimed to has been discovered. (I'll come to why I didn't say that the Higgs Boson is already discovered, shortly).
How do physicists look for the Higgs Boson(s)?
Higgs Boson(s) cannot be found to exist freely in nature today. All the Higgs particles created right after the beginning of the universe shortly decayed into lighter or more stable particles. So physicist need to replicate the very highly energetic scenario that existed at the beginning of the universe to detect Higgs Boson(s). At LHC this is done by smashing together two protons travelling at 99.9999991% of speed of light with particle detectors built around the collision-region. These collisions create several energetic particles. They interact with each other and go through numerous processes creating other particles before decaying into stable particles. Higgs Boson(s) is expected to be involved during these processes. Thus detecting which particles and how many of them reach the detectors physicist can estimate if any Higg-like particles were created as a result of the collisions. Although, Higgs is so short-lived a particle that it doesn't even last long enough to reach these most sophisticated detectors ever built. That makes searching for the Higgs Boson(s) further difficult. An example of particles detected after one such collision is shown in figure 3.
How close are we to discovering the Higgs Boson(s)?
After about 2 years of elucubration by about 8000 physicists from around the world and after analyzing tens of millions of Gigabytes of data at CERN and at more than 170 other grid-computing centers around the world, finally physicists are in a position to claim that they have observed a new Boson. This announcement by CERN at the seminar on July 4 truly triggered Higgsomania all over the world! But is it the Standard Model Higgs Boson? We don't know..... yet. Apart from the Standard Model Higgs Boson there are numerous other models of Higgs Boson(s), which, theoretically, not only achieve what SM Higgs Boson can do, but also claim to solve few other problems which SM alone cannot solve. For example, two-Higgs-doublet model, the popular supersymmetry model etc. Several of these models involve more than one and at times more complicated Higgs Bosons. Until physicists match all the physical properties of this newly observed Boson with the theoretically predicted characteristics of Higgs Boson(s) in one or more of these models, that too, with convincingly maximized certainty and convincingly minimized error in the measurements and the data analysis, they cannot claim that they have discovered 'the' Higgs Boson(s). Terms like 3σ, 5σ or statistical significance, which are mentioned frequently in scientific literature and seminars related to experimental Physics basically talk about minimizing these errors and maximizing certainty. As physicists continue getting more data from LHC they should be able to narrow down which of these models the new Boson belongs to or whether it belongs to any one of those at all. To finally lock on to 'the' Higgs Boson(s) it might take a few months or longer, but we, physicists, will surely find out what this new Higgs-like Boson is.
How useful is this research in our life?
The Higgs Boson has successfully drawn the attention of the entire world to ongoing research in fundamental Physics. Since my work as a theorist Ph.D. student is closely related to the Higgs-Boson-Physics, this is the most common question I get from people. If I just say that it's the quest of knowledge or it's the pursuit of mankind to know better the universe we live in, the answer wouldn't be complete. I find it essential to add that the research in fundamental Physics has always been driving the inventions in applied sciences in future. Think of the research in Nuclear Physics in first half of the 20th century. Because of that research today cleaner power-generation using Nuclear Power is possible. And what many people don't know is that even the World-Wide Web (WWW) was invented at CERN to facilitate collaboration between physicists sitting across continents. And when the idea was implemented for the use of rest of the world, it revolutionized the world.
So even if it is difficult to foresee the application of this research in our life now, be assured that this research has a potential to trigger another era of scientific revolution in future. Therefore, I conclude this post hoping that we, physicists, will continue to fascinate people with our research and that the encouragement we're receiving from the world due to the Higgs-Boson-related-research now will also continue in the future.
How do physicists look for the Higgs Boson(s)?
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| Figure 3: A collision at LHC as detected by the CMS detector. Source: CERN |
How close are we to discovering the Higgs Boson(s)?
After about 2 years of elucubration by about 8000 physicists from around the world and after analyzing tens of millions of Gigabytes of data at CERN and at more than 170 other grid-computing centers around the world, finally physicists are in a position to claim that they have observed a new Boson. This announcement by CERN at the seminar on July 4 truly triggered Higgsomania all over the world! But is it the Standard Model Higgs Boson? We don't know..... yet. Apart from the Standard Model Higgs Boson there are numerous other models of Higgs Boson(s), which, theoretically, not only achieve what SM Higgs Boson can do, but also claim to solve few other problems which SM alone cannot solve. For example, two-Higgs-doublet model, the popular supersymmetry model etc. Several of these models involve more than one and at times more complicated Higgs Bosons. Until physicists match all the physical properties of this newly observed Boson with the theoretically predicted characteristics of Higgs Boson(s) in one or more of these models, that too, with convincingly maximized certainty and convincingly minimized error in the measurements and the data analysis, they cannot claim that they have discovered 'the' Higgs Boson(s). Terms like 3σ, 5σ or statistical significance, which are mentioned frequently in scientific literature and seminars related to experimental Physics basically talk about minimizing these errors and maximizing certainty. As physicists continue getting more data from LHC they should be able to narrow down which of these models the new Boson belongs to or whether it belongs to any one of those at all. To finally lock on to 'the' Higgs Boson(s) it might take a few months or longer, but we, physicists, will surely find out what this new Higgs-like Boson is.
How useful is this research in our life?
The Higgs Boson has successfully drawn the attention of the entire world to ongoing research in fundamental Physics. Since my work as a theorist Ph.D. student is closely related to the Higgs-Boson-Physics, this is the most common question I get from people. If I just say that it's the quest of knowledge or it's the pursuit of mankind to know better the universe we live in, the answer wouldn't be complete. I find it essential to add that the research in fundamental Physics has always been driving the inventions in applied sciences in future. Think of the research in Nuclear Physics in first half of the 20th century. Because of that research today cleaner power-generation using Nuclear Power is possible. And what many people don't know is that even the World-Wide Web (WWW) was invented at CERN to facilitate collaboration between physicists sitting across continents. And when the idea was implemented for the use of rest of the world, it revolutionized the world.
So even if it is difficult to foresee the application of this research in our life now, be assured that this research has a potential to trigger another era of scientific revolution in future. Therefore, I conclude this post hoping that we, physicists, will continue to fascinate people with our research and that the encouragement we're receiving from the world due to the Higgs-Boson-related-research now will also continue in the future.
References
- Latest press release about update on the Higgs Boson search by the CMS experiment at LHC, July 4, 2012. Link
- Facts and Figures about the LHC. Link
- Worldwide LHC Computing Grid (WLCG) Project. Link
- Broken symmetries and the masses of gauge bosons, Peter Higgs, Physics Review Letters, Vol.13, 508-509 (1964). Link
- Basis-independent two-Higgs-doublet model, S. Davidson, H. Haber, Phys. Rev. D, Vol.72, 035004 (2005). Link, another ink
- A supersymmetry primer, S. P. Martin. Link
