Discovering a boson

posted by @ulaulaman #Higgs #ICHEP2012
I'm partially wrong! Yesterday I write that no Higgs could be announced, but today ATLAS and CMS presented the observation of a new boson, that it's to soon to identify with certainty like the Higgs boson, but it is certainly a new boson to add to the picture of the universe. The next quest is to find its properties and to confront with the theoretical properties of the Higgs boson. It could be that the new boson presented today could be different for a bit of properties from the Higgs, but this is not so incredible: like I write, with a Higgs boson with a mass around 125 GeV, we need other ingredients to complete a picture of our stable universe.

Candidate Higgs decay to four electrons recorded by ATLAS in 2012 (source ATLAS)
But... what did it happen today?
The two experiments, CMS, represented by Joe Incandela, and ATLAS, represented by Fabiola Gianotti, showed the results of their last data elaboration about Higgs research. The results come from the combination of the complete data set from 2011 (see, for example, the ATLAS' preprint) with the first part of data from 2012. The conclusions are: CMS sees an excess with $m_H = 125.3 \pm 0.6 GeV$ with a significance of $4.9 \sigma$; ATLAS sees an excess with $m_H = 126.5 GeV$ with a significance of $5.0 \sigma$, that it means discover!

CMS' final result

ATLAS' final result
Now, in order to explain the story, I use the ATLAS' preprint(1) like guideline.
In the 1960s Glashow, Winberg and Salam constructedthe mathematical structure of the Standard Model (SM) of elementary particles(1). The problem with SM is the absence of a process that it origins particles' masses, in particular the gauge bosons $W$ and $Z$. In order to correct this was introduced in MS the mechanism developed by Englert, Brout, Higgs, Guralkin, Hagen and Kibble in a series of 6 papers.
The history of the mechanism starts in 1960 when Yoichiro Nambu introduced
The idea of a connection between mass generation and spontaneous symmetry breaking(2)
His work lives in the research about superconduction, and in this field the most convenient model is the Ginzburg-Landau model(2). The hamiltonian of the theory involves a scalar order parameter $\phi$, that, in a trivial way, describes the interaction between two electrons. The model describe the transition between two different phases in function of the temperature: nearest to the critical temperature $T_C$, the potential $V$ is described by the following formula: \[V (\phi) = \alpha \phi^* \phi + \frac{1}{2} \beta (\phi^* \phi)^2\] After the change of the temperature, $\alpha$ change sign, and the potential has so the sombrero shape and its minimal occurs around the circle(3)
\[\phi^* \phi = - \frac{\alpha}{\beta}\] and the gauge simmetry is spontaneously broken.
The next step was by Nambu himself with Jona-Lasino: they introduced the same mechanism also in particle physics where
the nucleon mass arises from spontaneous breaking of the chiral symmetry(2)
The theory predicts also the existence of a massless boson, the Goldstone, le (or Mambu-Goldstone) boson. The firts problem is that noparticle could be identified as Goldstone boson. Nambu and Jona-Lasino supposed that the role could be covered by the pions: indeed with the breaking of the symmetry, the Goldstone boson would acquire a small mass.
At this point is really interesting to note that in 1963 Anderson observed that in an electron plasma photons, in order to propagates into the plasma, could really acquire a small mass.
Although Anderson had shown that in the nonrelativistic context the massless gauge bosons and Nambu-Goldstone bosons could combine to form a massive vector particle, most particle theorists believed that could not happen in a relativistic theory.(3)
But this idea was wrong, like founded Englert, Brout, Higgs, Guralkin, Hagen and Kibble with their conclusive papers: with the same mechanism, in a relativistic hamiltonian that try to introduce the mass of the elementary particles, the order parameter $\phi$, after the spontaneous breaking of the local gauge symmetry, acquires a non-zero expectation value, that is connected to the mass acquired by the vector field. And this is the Higgs boson (read more theoretical details on DM).
The research of the Higgs boson is really complicated, first of all for the great number of channells involved in the decay of the boson ($H \rightarrow \gamma\gamma$, $H \rightarrow ZZ^*$, $H \rightarrow WW^*$, $H \rightarrow \tau^+\tau^-$, $H \rightarrow bb$ with subsequent decays of the $W$, $Z$ and $\tau$ leading to different final states)(1). There also some specific problem for every channell, for example in the diphoton channell, like Marco Delmastro writes this morning on twitter,
To see #Higgs in diphoton channel you need good energy resolution, and excelllent photon identification
The problem is that the majority of photons in pp collisions don't came from #Higgs decay!
#ATLAS does not depend on tracking to know where the photons come from. The ECAL is longitudinally segmented, can see the direction!
In order to controll all channels and data, ATLAS team introduced a parameter $\mu$, the signal strength factor that
is defined such that $\mu = 0$ corresponds to the background-only model and that $\mu = 1$ corresponds to the SM Higgs boson signal.(1)
After the collection and the selection of the data the nextstep is the study of the systematic uncertainties: is another important step, because with the uncertanties we can say how the measure is good. But the knowledge of the studied range is represented by the $\sigma$, the standard deviation: only with $5 \sigma$ researchers are able to exclude statistical fluctations. For example, the data observed by ATLAS with the complete set since 2011 produced a mass $m_H = 126 GeV$ with a confidence of $2.9 \sigma$(1).
The upgrade following by the first part of the data since 2012 is really incredible, and showed like LHC is a surprising device, more than the beginning expectations.
Personally the most emotional moment today is when Fabiola Gianotti presented the following slide:
So, today, ATLAS and CMS discovered a boson that seems the Higgs boson (or the first of a series of Higgs, or more correctly, Goldstone relativistic bosons!), so it is need to study a further set of data in orther to understand if we are in front of the SM Higgs or in front of something similar but else (for example: we don't know what is the spin of this new boson).
The discovering of this new boson is, in every case, the second step (after the neutrino's oscillations) in the physics beyond Standard Model.
The importance of the today event is resumed by the following qotation by CERN's Director Rolf Heuer:
This historic milestone is only the beginning, it has global implications for the future
My last words to the researchers, because, like Doktor (((1/f))) remembers
The ATLAS collaboration consists of almost 3,000 physicists from 169 institutions, 37 countries and five continents.
I'm really near to all reserachers of the two collaborations, in particularto ATLAS for a lot of reason, one of this is nameb Marco Delmastro, who confessed:
I think I'm going to cry. Seriously. Most of us have been working all their professional lifes for this
The other reason is very simple: during my PhD all of my friends that worked on ATLAS before the construction of the experiment have a lot of problems because
They don't do physics!
Today is the proof that they have done physics!
Official releases:
CERN | ATLAS | slides
Read also:
viXra blog | physics4me | Cosmic Log | npr
Funny site: Have We Found The Higgs Boson Yet?
The next steps for ATLAS, the LHC and the high-energy physics community are to measure the properties of this particle and compare these measurements with the predicted properties of the Higgs Boson. Already some of these properties match the predictions: the fact that it is seen in the predicted channels and at a mass favoured by other, indirect measurements. In the weeks and months ahead, ATLAS will better measure these properties, enabling a clearer picture to emerge about whether this particle is the Higgs Boson, or the first of a larger family of such particles, or something else entirely.

(1) ATLAS Collaboration (2012), Combined search for the Standard Model Higgs boson in pp collisions at sqrt(s) = 7 TeV with the ATLAS detector (preprint)
(2) Tom W B Kibble, Arttu Rajantie, Riccardo Guida, Eugene M. Izhikevich, Chris Quigg (2009). Englert-Brout-Higgs-Guralnik-Hagen-Kibble mechanism (history) Scholarpedia, 4 (1) DOI: 10.4249/scholarpedia.8741
(3) Tom W B Kibble, Arttu Rajantie, Eugene M. Izhikevich, Nick Orbeck, Riccardo Guida, Carlos Lozano, Nicolau Leal Werneck, Chris Quigg, John C Taylor (2009). Englert-Brout-Higgs-Guralnik-Hagen-Kibble mechanism Scholarpedia, 4 (1) DOI: 10.4249/scholarpedia.6441

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