Is it Higgs or Higgs-like? : CERN’s Higgs quest- Part I


I know, the question may annoy many of my particle-physicist friends. Maybe this is because
the question can sound a bit disrespecting the discovery of a new boson announced (with online broadcast) by CERN on 4th of this month. But that is merely a prejudice. And I promise, I’ll try hard to clarify
my intention here.

Though I am a condensed matter theorist by profession, I was alert about the announcement program from
and also placed my ears on the webcast. As anticipated, I did not understand any of the slides that have been presented. But what I came to know that many results fit quite close to the behavior that was anticipated from the standard model theory, which demands the presence of Higgs bosons and they were quite definite that it must be a new kind of a boson. Also I heard something called 4.9 sigma confidence.

Before I realize anything or start thinking or do a google-search, many social network pages went flooded with the message that the God-particle (Higgs boson’s pop name) has been found finally. This sort of reaction and circulation totally blew me up. The next day or maybe the second next day, I started seeing announcements of seminars in Indian Institute of Science (IISc) and National Centre for Biological
Sciences (NCBS) in Bangalore on the “Discovery of Higgs particle”.

So it sounded to me that the verdict has been given: It was the Higgs! I personally never mind if the newly discovered particle becomes “the long-sought Higgs” and rather will gladly admire the beauty of the standard model.

However, my question was: Is the announcement of the discovery clear to the commonplace people? Note that
I said, the announcement, not the physics.

At least to me, it was very unclear until I attended Prof. B. Ananthanarayan‘s talk in IISc on 12th July.
He briefly explained the theory of the Higgs mechanism, which is also known as Englert-Brout-Higgs-Guralnik-Hagen-Kibble (EBHGHK) mechanism, emerges during a spontaneous symmetry breaking (SSB) of a local gauge symmetry (or in other words, presence of a finite vacuum expectation value). To construct a unified theory of the electromagnetic field and the weak force (responsible for the beta-decay) Abdus Salam, Sheldon Glashow and Steven Weinberg (GSW) came up with a SU(2)xU(1) gauge theory, where they introduced gauge bosons namely the W^\pm and Z, with an equal footing to the photons for the electromagnetic gauge theory (U(1)). However, due to short-range nature of the weak interaction, the gauge bosons had to have masses in contrast to the massless photons
and to do they cleverly used the idea of Higgs mechanism by introducing an extra spinless scalar field:
the Higgs field. The particle excitations of the Higgs field is the so called Higgs particle or the Higgs boson or the God (damn) particle. As an additional benefit, due to the presence of Higgs field the leptons in the beta decay (even the Higgs boson itself) also acquire masses if we start with massless particles in absence of the field. This unification theory remained an important part of the standard model (SM) and GSW won nobel in 1979 for their contribution.

Birth of the Higgs boson: paper display at CERN museum
(Courtesy: Mou Bhattacharya).

Well, let’s come back to the title: Is the new boson the Higgs boson? Ananthanarayn showed two important
results from the Large Hadron Collider (LHC) experiments to us: one from the CMS and the other from the ATLAS.

The Higgs particles are supposed to be very short-lived. They easily decay to other stable particles.
The CMS data shows distribution of mass of the diphoton decay (Higgs converting into two photons).

Feynman diagram of a typical diphoton decay.


One can easily see a significant bump arises at the mass around 125 GeV, which signifies excess photon
production as one can expect that to happen due to Higgs’ decay. However, the confidence level
(the background significance) for the diphoton channel is 4.1 \sigma.
Nevertheless, the CMS spokesperson Joe Incadella summarized on 4th July: “We have observed a new boson with a mass of 125.3\pm0.6 GeV at 4.9 \sigma significance.”

Ananthanarayan showed the ATLAS data as well. Similar to the CMS one again bumps have been observed
around 126 GeV, this time in two photon channels. Though in the upper channel the bumps are quite obvious,
some of the audience objected saying that it’s a very highly fluctuating data and so they can draw bumps
in other places in principle.

Mass distribution data in the two photon channels of the ATLAS.

Then Anantha showed the probability plot, which shows that the probability of background signal excess is
minimum and is above 5 \sigma (However, I didn’t understand the last line of the caption in
the last figure here.)

The probability of background to produce a signal-like excess for all energies in the ATLAS data.

Now what is this sigma business? Certainly people at CERN know it. So better you should not ask them.
Just because it’s so trivial to them. Let me explain. \sigma is generally used to denote a standard
deviation of a statistical data and for a normal (Gaussian) distribution, which is probably a
typical assumption for the distribution of a large number of independent random variables (see Central Limit Theorem). Now \sigma also indicates the central width of such Gaussian distribution. Therefore higher \sigma should mean towards the tail part of the Gaussian, i.e. the lesser chance
to find a variable there. And 5 \sigma is the particle physicists’ strategy to make a standard
for claiming discoveries, i.e. 99.9999426697% no-chance of background contribution (Likewise 6 \sigma is the businessmen’s strategy). I must acknowledge Gaurav from the CHEP, IISc for helping me to understand this tiny concept.

As sigma increases, the probability decreases in a normal (Gaussian) distribution.

I know, the following questions still remain in your mind:

Well, now what about the title topic? It seems to fit to the Higgs story. So it must be “the Higgs”. Why did you say Higgs-like?

I think, I spoke a lot here. And we need a break too. I’ll make an effort to justify myself in my next post.

PS. I gave a small LHC_Talk in a science show called Dhwani, when LHC just started running.

( Please don’t miss the part -II above. )

Useful references:

4th July webcast –
CERN link, youtube link

Anantha’s Current Science article
Anantha’s Blog

Advertisements

6 thoughts on “Is it Higgs or Higgs-like? : CERN’s Higgs quest- Part I

  1. Dear Himadri,

    Thank you very much for sending me the links. I am pleased to know that you found my talk useful and that it helped you. Hope I reached a few others. Nice to know that you have an active blog. I will try and visit it from time to time.

    Regards, Anant

    • Dear Prof. Ananth,

      Thanks a lot to you too. It would be nice if you can place your valuable comments in my post. Specifically in part-II where I have mentioned a paper that appeared in the arxiv on 4th of July.

      Regards,

      – Hbar

      • Dear Himadri,

        Thank you for your mail. As it happens, I am leaving tomorrow for a 3 week
        trip out of Bangalore. It is a hectic trip and I may not have the time to look
        up the paper you mention in any depth. I will try and get to it end of August,
        by which time, my views may be passe.

        Best regards, Anant

        • It’s ok, Sir. I believe, your views will be always useful. You please take your time. Meanwhile I’ll try to put more interesting stuff. Btw can your blog be subscribed? Thanks for your responses. And wish you a happy journey too. – Hbar

          • I have linked your article to my blog. Please have a look. I actually don’t blog very often any longer and am not aware of any special tool for subscribing. Must be through an RSS feed, but I don’t know anything more. Thanks for the best wishes and I hope to see you again soon.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s