We are made up of atoms. Atoms, however are made up of sub atomic particles like proton, neutron and electron. These sub atomic particles are again composed of fundamental particles, which are quarks. I think many of us know that there are six quarks. Everyday objects, more specifically ordinary matter like rock, bottle, bag e.t.c are composed of these quarks. If a rock has some mass and it is composed of quarks, it's not false to say that the rock gets its mass from these quarks. Fair enough, sounds very straightforward. We also know that fundamental particles are elementary but still carry mass. But, have we ever pondered about how the quarks and other fundamental particles get their masses ? It has something to do with another particle, 'The Higgs Boson' and I know many of you are familiar with it too. So, let's dive in.
Higgs Boson
The Higgs boson is the recent addition to the Standard Model. It is named after the person who discovered it, Peter Higgs. It is not a gauge boson as it doesn't carry any force. It is a fundamental particle like the electron, neutrino, up quark. This particle is generally produced or formed in the lab experiments like at the LHC(Large Hadron Collider). It has a very short lifetime which is one ten-billionth of a trillionth of a second. The mass of this Higgs particle has been measured to be 125Gev. It is a neutral particle meaning not charged. This particle interacts with massive particles not massless particles like photon. However, it can decay into two photons (It requires understanding of the Feynman diagrams). Higgs is mainly known for its action of giving mass to other particles, mainly fundamental particles and we assume that the total mass of everyday objects like human body, bottle, rock is the result of Higgs but that is wrong. This only accounts for a small fraction of the mass while much of it comes from the strong force(keep that in mind). Higgs also helps physicist to unlock the deepest mysteries of the universe, even at a time as early as the big bang.
Higgs field
According to Quantum Field Theory, vibration in the field gives rise to particles. So, HB is the product of vibration in the Higgs Field. But it is not the sole effect of the Higgs field, other fields also contribute to that process. But what is a field? A field is something that has value at every point in the space, that is the literal definition of a field. And, what are the fields made up of? The field themselves aren't made of anything, fields are what the world is made of.
Unlike other field, Higgs Field is a strange kind of field. It is non zero even in empty space. That is why empty space is filled with Higgs. To move HF to 0, enormous energy is required but it is believed to happen to be a fraction after the big bang. The moment at which the Higgs went from being 0 to non zero is called the electroweak phase transition. If the HF value is reduced to 0 in a region of like a golf ball, it'd have same mass as the earth and if we were to make it much bigger than that, there would be so much mass in a small space that the volume would collapse to form a black hole.
How are Higgs Bosons produced and detected ?
On July 4, 2012 the announcement of the detection of the Higgs boson was made at CERN(European Organization for Nuclear Research). The Large Hadron Collider is the world's biggest particle accelerator located at the border between Switzerland and France. At the LHC, the protons are circulated in a closed loop of tunnel, accelerated by electric and magnetic fields closer to the speed of light, and smashed into each other. When LHC is going full steam, a total of 500 trillion protons circulate in two beams. The energy of the collision creates new particles. This process is based on the famous Einstein's mass energy equivalence. An array of particles are formed and sometimes a HB also pops out.
We can't see Higgs because it's lifetime is ridiculously less as I have mentioned above. So, as soon as the Higgs is produced, it quickly decays into other particles like bottom quark, gluons, photons. At LHC there are these huge giant detectors that are filled with materials in which particles leave track as they pass through. And by tracing these tracks, scientists there find out which particle left that track. Our theory predicts what the HB should decay into and if the decayed particle matches the theory, we can tell it is a HB without even seeing it.
As HB doesn't interact with massless particles, how are gluons the product of HB decay? It is because Higgs sets vibration in the Higgs Field which again sets vibration to a massive particle field that it couples to but those vibrations can't give rise to particle so it sets up vibration to gluon field and give rise to gluons.
Higgs boson can also be formed by these processes:
- Fusion of 2 gluons
- Fusion of w+ and w- boson
- Fusion of two z boson
How does Higgs gives mass to particle?
We know that even particles as small as an electron has a mass. For a proton it is in the order of negative 27 while for an electron it is in the order of negative 31 in kilogram units. The mass of a particle can be calculated by multiplying the value of HF at empty state with the interaction strength of particle with Higgs. The particles acquire the mass by the interaction with the Higgs Field. The stronger the interaction the greater the mass and the weaker the interaction lesser is the mass. Quarks interact with the HF strongly so they have greater mass while the photon doesn't interact with the HF so it is massless.
The Higgs also breaks the weak symmetry. The symmetry of weak interaction relates pair of particles in this way
Up ↔ down
Charm ↔ Strange
Top ↔ Bottom
Electron ↔ Electron neutrino
Tau ↔ Tau neutrino
Muon ↔ Muon neutrino
Once the Higgs fills the space, weak symmetry is broken and observed fermions are massive. Now let's introduce a term, helicity. Helicity is the projection of spin operator to the momentum operator. There is a strange property of weak interaction which is that it only couples to left handed particles(Spin points opposite to the direction of motion) and not right handed particles and couples to right handed anti fermions and not to left handed anti fermions. If the weak force couples to only left handed particles, it must be true that such particles have only one helicity and it can happen only if they move at the speed of light, which means they are massless. But Higgs breaks this condition and allow particles to have multiple helicities. If the weak symmetry weren't broken, helicity would be a fixed property of each fermions, which means they all would be massless moving at the speed of light. So particles have mass as a result of symmetry breaking phenomena of the Higgs.
Higgs boson's discovery has been called a revolutionary achievement in the field of physics. This particle is central to the Standard Model. To better understand it and study it, we need better accelerators and detectors. So in future, we could know more and more about this particle as bigger and better accelerators will get built. And Physicist hope that with better understanding of this particle, more mysteries of this vast universe could be unlocked.
The reference of this post is a book by popular physicist Sean Carroll 'The Particle at the End Of The Universe'.
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