Higgs Boson Discovered: What Today’s CERN Conference Really Meant
By on July 4th, 2012

To be proved wrong has never felt so good!

If you’ve been following our blog for the last few days and been interested in the science posts and those on the Higgs, you’ll know that I was very skeptical about the discovery of the Higgs Boson by the time this presentation comes about. And boy, am I proved wrong! And I am elated about it.

Found!

Let’s be honest – it isn’t the Higgs

Okay, to be honest, the exactly correct statement would be this: There is a particle, hitherto unknown, having mass between 125 and 126.5 GeV (giga electronvolt), which has been discovered with 99.999997% certainty. We still don’t know whether this is the Higgs Boson or not – it could just be another particle.

It’s not the Standard Model Higgs

In fact, to make things interesting, this is really not the Standard Model Higgs. So people claiming that this ‘completes’ the Standard Model are, at least partially, incorrect. While the Standard Model predicts – and requires – the Higgs Boson, there is nothing that we know right now that says that this discovered particle is the Higgs.

Forget the details. Look at the red peak. Look at what it means in the legends. Appreciate and bask in the glory.

Furthermore, the Standard Model predicts a Higgs with the mass of about 140 GeV or thereabout. What we have got is something with a mass in the ballpark of 125-126 GeV. Even if this is the Higgs, this is NOT the Standard Model Higgs.

What we really have!

So what have we really got on our plates? What we do know is that there is a particle whose mass is 125 GeV or so and how it decays. We also know how much it decays through the various decay channels – the so-called ‘branching ratios’. We are yet to know the charge of this particle and its parity. We do not know whether this is a fundamental particle (like the electron, with no ‘sub-parts’) or a composite one (i.e. made up of more elementary particles like quarks).

As Fabiola Gianotti, ATLAS spokesperson, said:

We are entering an era of Higgs measurement.

That is a lot of work left. We need to figure out what we are looking at really.

Rolf Heuer, CERN Director General, said that this is like looking at someone from far away and recognizing him/her immediately to be your best friend. But you’re not quite sure. You want that person to be closer to you so that you can make sure that it is indeed your best friend and not his/her twin.

Quite true. We need to take a better look. Translated into the language of high energy physics, that means ‘We need more data’. The saga will continue till the end of this year, then there will be a break and will continue again in 2014.

So, what can it be, if it’s not the Higgs Boson? It can be one of the supersymmetric partners of some already known particle. We don’t really know anything about the energy scale at which super-symmetry sets in or when supersymmetry breaks, but there is a fair possibility that the LHC might be detecting tantalizing hints in the next three months. When it comes back in full force in 2014, running at a higher energy of 14 TeV, compared to 8 TeV currently, we will definitely rule out or embrace supersymmetry at the 5 TeV energy scale.

Don’t worry, if you don’t get this – it’s futuristic talk. We want to talk more about today’s conference and that is what we will do!

Today’s conference: what they really said!

So this particle we are seeing today – let’s just call it Particle X till CERN says that it is indeed the Higgs boson – decays via different modes. A particle decays if it is heavy and there is no law or conservation principle preventing it from decaying. And it can decay via different end products – two Z-bosons, W-bosons, photons, four leptons etc – and all of the decay channels have some probability. One can be more probable than the other, and some channels can have more background noise than others. This happens if, say, a decay product can come from more than one source.

For example, bottom quarks can be produced from a lot of different sources, like all the so-called QCD processes. This masks the signal coming from the Higgs decay. The subtraction of background often leads to subtraction of the signal itself.

Notice the broken yellow lines emerging out of the detector. Those are photon lines. Actually the broken line is a way to represent the fact that photons are invisible and not caught in the tracker. Then they are detected in the electromagnetic calorimeter, where green lines represent energy dumps.

Clean channels

In order to cut through this mess, it is imperative that one identifies ‘clean channels’. Two such channels for this particle X are lepton channels and the di-photon channels. Particle X can decay into two muons or two electrons accompanied by the respective anti-neutrinos (don’t bother about those) through the lepton channel and, as the name suggests, into two photons in the di-photon channel. And lo, the signal is the strongest in these two channels. CMS and ATLAS (the two detectors at LHC searching for the Higgs) both have been extremely diligent and successful in looking for signals in these two channels. Both have scored grand success.

The Higgs decay modes and how they contribute to the total cross-section

Look at the green squiggle dipping down right above the black continuous squiggle. That, the legend on the left reveals, is the signal from the di-photon channel. Look how strong that signal is! This immediately suggests that the particle is a boson (otherwise, if it were a fermion, it would violate fermion number conservation) and that the particle cannot be a spin-1 particle (as the photon is a spin-1 particle and we can either have spin-0 or spin-2 for the initial particle). The Standard Model Higgs is a spin-0 object.

What about the charged lepton channel? The two lepton channel is an indirect way to infer the presence of the ZZ or the WW channel. The neutral Z-boson or charged W-bosons are formed from the decay of the Higgs boson. These then decay into muons and electrons, which are then detected. It turns out that our particle X mimics the Higgs Boson quite closely.

What does CMS say about the different channels?

For the gamma-gamma channel (another name for the diphoton channel), the surety is about 4.1 sigma for the particle X having a mass of 125 GeV.What about Z-channel? It spits out 3.2-sigma confidence level for X having a mass near 125 GeV.

Add these two in quadrature (square each, then add and then take the square root of the sum) and you get 5.2-sigma!! This is winning, as the confidence level required for announcing a discovery is 5-sigma!

More data is expected to bump up the confidence level even further. Particle X could be as certain as 7-sigma by the end of December.

It’s not worth repeating the story for ATLAS as it is very similar.

Being cautious, still

CMS hit it just right when they cautiously put this up as the defining slide, saying “We have observed a new boson with a mass of 125.3 +/- 0.6 GeV at 4.9-sigma significance”.

Yep, it’s that simple.

The conservative 4.9-sigma, instead of a two-channel combined 5.2-sigma is typical amongst high energy physicists. This takes into account other channels and the so-called ‘look-elsewhere effect’. We need not get into that for our purposes here.

Point of disagreement – a potential for trouble?

Now let’s come to the discrepancy between the two collaborations – CMS says that the boson is at 125.3 +/- 0.6 GeV, while ATLAS says that it is at 126.5 GeV. The ATLAS collaboration hasn’t put in the error bars. So what are we supposed to make of this? What about the 1 GeV discrepancy?

We don’t know right now. Rolf Heuer made light of the incident:

We have the Higgs, but which one?

Rolf Heuer’s bag of quotes doesn’t end there, and so we would like to end with one of his gems – one signifying finality:

I said we will have a discovery this year. DONE!

Done, indeed. Congrats to everyone on the CERN team and the worldwide collaborations!

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Author: Debjyoti Bardhan Google Profile for Debjyoti Bardhan
Is a science geek, currently pursuing some sort of a degree (called a PhD) in Physics at TIFR, Mumbai. An enthusiastic but useless amateur photographer, his most favourite activity is simply lazing around. He is interested in all things interesting and scientific.

Debjyoti Bardhan has written and can be contacted at debjyoti@techie-buzz.com.
 
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