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At a seminar held at CERN on November 26, Aliaksandr Pranko, a physicist from the ATLAS experiment astride the Large Hadron Collider (LHC), made an interesting announcement on behalf of his colleagues. He said, for the first time, there’s a sign that the Higgs boson gives mass to all fundamental particles.
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Are you asking yourself “Isn’t that what this year's physics Nobel was given for”? As it happens, the Nobel Laureates in question – Peter Higgs and Francois Englert – won the prize for their work in explaining how fundamental particles acquired mass, especially the W and Z bosons .
Yesterday’s CERN announcement goes ahead and proposes that we have is THE Higgs boson, THE particle that gives mass to all fundamental particles. This includes both matter-particles, known as fermions, and force-particles, which are bosons. Essentially, this piece of evidence goes beyond what 'little' was enough to fetch Higgs and Englert the Nobel, and suggests that the particle is the Higgs boson as predicted by the duo in its entirety.
As Rahul Sinha, Professor, Institute of Mathematical Sciences, said, "The observation goes a step further in ascertaining how close the Higgs boson's properties are to the Standard Model's expectations." The Standard Model is a group of theories and principles that describes how different particles, including the Higgs boson, behave. However, the announcement is not yet tantamount to discovery, so the Higgs boson's properties aren't known yet to completely match up with the Model's expectations.
The significant significance
To claim a discovery in experimental particle physics, a conclusion must have been reached at with at most 1 in 3.5-million chances of an error . That is, the scientists must have observed that at most 1 event spotted by their instruments can be an impostor mimicking the signs of what they're looking for in every 3.5 million events. This amount of statistical significance is called a 5-sigma (5σ) reading.
If, on the other hand, an observation's been made at 3σ or above, it's considered to be strong evidence.
As >this stackexchange thread explains:
"Let's take the Higgs search as an example. In this case, the theory is "a particle with a mass of 125 GeV/c2 exists." So the scientists at ATLAS and CMS would calculate something like the following (I'm just making the numbers up):
If there is no particle with a mass of 125 GeV/c2, we expect to see, on average, 511 photon pairs of energy 125 GeV over 3 months of running the LHC. This is the "expected value."
If there is no particle with a mass of 125 GeV/c2, we have a 68% probability of seeing between 506 and 515 photon pairs of energy 125 GeV over 3 months. This is the 1σ confidence interval.
If there is no particle with a mass of 125 GeV/c2, we have a 95% probability of seeing between 501 and 521 photon pairs of energy 125 GeV over 3 months. This is the 2σ confidence interval.
(skip a couple)
If there is no particle with a mass of 125 GeV/c2, we have a 99.9999427% probability of seeing between 487 and 533 photon pairs of energy 125 GeV over 3 months. This is the 5σ confidence interval. "
Each progressive sigma level is harder to get to than the previous one.
The November 26 'announcement' was based on a 4.1σ reading – good enough for claiming evidence, not good enough for claiming discovery.
Decay channels
The event in question involves what is called a decay channel. The Higgs boson, being a heavy and very unstable particle, quickly decomposes into groups of lighter particles moments after it is formed. Sometimes, it decomposes into one set of particles; other times, into a different set. Each mode of decomposition is called a decay channel, and the probability of it occurring over any other channel is given by its branching ratio.
Each of these sets leaves a distinct signature in the mammoth particle detectors used in the LHC. Finding each signature is a sign that those sets of particles were there, and therefore the Higgs boson was there only moments ago.
When the July 2012 announcement was made, it was on the basis of three important decay channels:
Three decay channels of the Higgs boson
The ZZ* and WW* channels are the Higgs boson breaking down into Z and W bosons, and the γγ channel is the Higgs boson breaking down into two gamma-rays, which are basically high-energy photons which are also bosons. Because the Higgs boson has been observed decomposing into other bosons, we know that the Higgs boson couples with them and gives them mass.
Kajari Mazumdar, Professor, Tata Institute of Fundamental Research (TIFR), said in an email, "The discovery was achieved through bosonic decay modes of Higgs bosons. But the Higgs boson of the Standard Model is also supposed to be responsible for fermion masses. Experimental confirmation of the decay of Higgs boson to fermions is thus very important to learn about how Higgs bosons provide mass to fermions."
The leptonic decay channel
It has been previously known, indirectly, that the Higgs boson gives mass to quarks, which makes up the lot of fermions alongside leptons. With Pranko's announcement, we now have some pretty strong evidence that the Higgs boson decays into particles called the τ (tau) leptons.
According to >slides presented at the seminar , the Higgs boson (at mass 125 GeV) has been observed decaying into pairs of τ leptons at 4.1 sigma, as opposed to the expected significance of 3.2 sigma.
This plot from slide #39 shows the agreement between the data points (black dots) and the areas (in red) where the Higgs is predicted to decay into pairs of tau leptons.
This plot from slide #39 shows the agreement between the data points (black dots) and the areas (in red) where the Higgs is predicted to decay into pairs of tau leptons.
Tariq Aziz, Senior Professor, Department of High Energy Physics at TIFR, told me in an email, "If this is the Standard Model Higgs (as proposed by Peter Higgs and others), then there is a well-defined pattern of decay with well defined decay branching ratios to a variety of final states that includes fermions as well."
Physicists now have an idea about where to look for the events that point to a Higgs boson being able to confer mass on fundamental particles other than bosons. They can build on the data presented during the seminar and design focused experiments that better serve their goal. In these experiments, "if there is any deviation from the well-defined pattern, then this [becomes] non-standard stuff," added Prof. Aziz, alluding to physics beyond the Standard Model – a realm that many physicists are >eager to explore .
