Higgs Boson the road to discovery

susy-cern

 

In July 4 physicists announced a discovery that appears to be “like the Higgs boson“, two indipendent experiments ATLAS and CMS acquired and analyzed independent data.
Now CERN confirms the discovery of Higgs boson in the annual winter conference Rencontres de Moriond. They’ve a found it finally !

High energy physics is searching the elementary building blocks of matter and the forces between them. Despite their name (Democrito please excuse me), atoms are not the smallest constituents of matter. Atoms are formed from electrons orbiting around a nucleus, made of protons and neutrons. Protons and neutrons are called nucleon and they’re bound together in the nucleus by a force called strong interactions. But it is not finished yet. Nucleons contain quarks, called up and down. Quarks and electrons are considered “point-like” particle, I mean they haven’t substructure (yet).

disegni

Nucleons are made of quarks up (u) and down (d).

The fundamental forces of the Universe are: gravity, electromagnetism, weak force and strong force. The electromagnetic field consist of packet of energy (“quanta”) we call them photons. Einstein won the Nobel Prize explaining the photoelectric effect, in which he introduced the concept of quanta. All force fileds have energy quanta, they’re also known as “carriers” of forces. Photons are very similar to our ordinary concept of “particle”, but they haven’t mass.

In 1896 Bequerel discovered the radioactivity when high speed electrons, coming from a sample of uranium salt, hit a photographic plate during a rainy day in Paris. This was the evidence of a new force of nature, the weak force. Beta decay is a spontaneous phenomenon in which a neutron became a proton emitting an electron and neutrino, causing the transmutation of the element.

neutron_decay

Neutron beta – decay. Due to the weak force, a neutron became a proton emitting an electron and a neutrino

If we look closely at the phenomen, we discover that beta – decay has an intermediate “virtual step”, in which weak force changes d quark in u quark by emitting W bosons, the quanta of the weak force.

Weak force changes a neutron in positron emitting a W boson, the carrier of the weak force.

Weak force changes a neutron in positron emitting a W boson, the carrier of the weak force.

The first theory of weak interaction was developed by Enrico Fermi in 1933, in order to explain the beta – decay. Enrico Fermi developed a theory very similar to the electromagnetic one with a fundamental difference: weak interaction could change the electrical charge of the particle, as we have seen in the proton beta – decay.
Weak force is essential for our life on the planet Earth, since it transforms hydrogen in helium so the Sun can shine !

Photons are the quanta of electromagnetic force and W are the quanta of weak interactions. Both of them are “carrier” of forces but they have difference. Photons is massless and W boson has a great mass ! An enormous amount of energy was necessary in 1983 in order to discover W bosons. One year later Carlo Rubbia and Simon Van der Merr won Nobel prize “for their decisive contributions to the large project, which led to the discovery of the field particles W and Z, communicators of weak interaction”.
Suddenly a question arises among the physicists, why does W boson has any mass at all ? in an another way “why is the weak force is so weak ?

The problem leading to the Higgs boson was firstly about the masses of the quanta of weak force. Physicists love unification and have a natural disgust for division. James Clerk Maxwell brought together electric and magnetic phenomena in the electromagnetic theory. In a same way, Glashow, Salam and Weinberg developed the theory of the unified weak and electromagnetic interaction between elementary particles. In our world electromagnetic force and weak interactions are very different, but they’re the same force at high energy. The fundamental hypothesis of electroweak theory started form nothing, the vacuum ! In quantum theory the vacuum is full of matter and quanta force! In the early time after the big -bang a new force field, the Higgs fied, arises from the vacuum. Higgs field has broken the simmetry between the electromagnetic and weak force, giving the mass to the carrier of weak interaction, W and Z bosons. The quanta of Higgs field is of course the Higgs bosons.

The existence of Higgs boson is fundamental in order to verify the unified electroweak theory, because without Higgs boson is not possible to derive the spontaneous division of electroweak force in electromagnetic and weak force and also is not possible to explain the non zero mass of the quanta of weak interactions. In addition Higgs boson is the fundamental concept in Standard Model, the most powerful theory of  elementary particle physics. In spite of decades of attempts there was no direct experimental proof the Higgs boson. Until now.

LHC is the acronym for Large Hadronic Collider, in which hadron is another funny way to define particle like proton. LHC is the largest particle accelerator ever built, it consists in a ring of 27 kilometers at a depth of about 100 meters under Geneva. Proton beams are accelerated to speed almost equal to speed of light and then they are made to collide . The energy of the collision is transformed into particles that scientists reveal. ATLAS and CMS are the two detectors which have seen the Higgs. The Higgs Boson is an unstable particle, living for only the tiniest fraction of a second before decaying into other particles, so experiments can observe it only by measuring the products of its decay.

The four main LHC experiments. This diagram shows the locations of the four main experiments (ALICE, ATLAS, CMS and LHCb) that will take place at the LHC. (credit CERN/LHC Photo #: 9906026)

The four main LHC experiments. This diagram shows the locations of the four main experiments (ALICE, ATLAS, CMS and LHCb) that will take place at the LHC. (credit CERN/LHC Photo #: 9906026)

The ATLAS is a multi purpose detector consists of a series of ever-larger concentric cylinders around the interaction point where the proton beams from the LHC collide. It can be divided into four major parts:
1) the inner detector:  it identifies the particle emerging from the collision, mesuring the momentum and charge
2) the calorimeters: their purpose is to measure the energy from particles by absorbing it.
3) muon spectrometer: it  is an extremely large tracking system to accurately measure the momentum of muons.
4) magnet system made of two large superconducting magnet systems to bend charged particles so that their momenta can be measured.
ATLAS has concentrated its efforts to detect Higgs boson looking at two complementary channels: Higgs decays to either two photons or to four leptons.

atlas

ATLAS detector. left) Computer generated cut-away view of the ATLAS detector showing its various components right) look into the ATLA’s eyes !

The CMS detector is designed as a general-purpose detector. The CMS detector is built around a huge solenoid magnet It contains subsystems which are designed to measure the energy and momentum of photons, electrons, muons, and other products of the collisions. It can be divided into major parts:
1) interaction point:  the centre of the detector at which proton-proton collisions occur;
2) the tracker: it is the inner most layer of the detector, it measures the momentum of a particle tracking its path through a magnetic field;
3) calorimeters: it is designed to measure the enery of hadrons, elctrons and photons;
4) magnet:
it is a large solenoid magnet and it’s the central device around which the experiment is built. The job of the big magnet is to bend the paths of particles emerging from high-energy collisions in the LHC;
5) muon detector: it is the last stage of CMS, its purpose is to reveal the elusive muon particle
CMS detect the Higgs boson product of decay into two photons and two muons

CMS detector. left) Computer generated cut-away view of the CMS detector showing its various components right) look into the CMS' eyes !

CMS detector. left) Computer generated cut-away view of the CMS detector showing its various components right) look into the CMS’ eyes !

CERN and ATLAS are two independent experiments hunting the Higgs boson and finally they conclude observation of a new particle at a mass of 125 GeV compatible with the production and decay of the Higgs boson. They have observed for the first time the particle confirms thestandard model of elementary particles.

CMS and ATLAS experimental data, confirming an excees of mass near 125 GeV, compatible with Higgs boson

CMS and ATLAS experimental data, confirming an excees of mass near 125 GeV, compatible with Higgs boson

And now so many questions arise, because the search must go on ! Is there more than one Higgs boson? Is it point-like or composite ? Are all probabilities as predicted ?

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