Context:

The European Organization for Nuclear Research (CERN), which hosts the Large Hadron Collider (LHC).

Recently announced that scientists at the organisation found the first evidence of the rare process by which the Higgs boson decays into a Z boson and a photon.

Introduction

The Standard Model (SM) of particle physics has emerged through both theoretical and experimental discoveries spanning the last five decades. It comprises the building blocks of visible matter. The fundamental fermions (quarks and leptons) and the bosons (photons, W and Z bosons, and gluons) that mediate three of the four fundamental interactions. Photons for electromagnetism, the W and Z bosons for the weak interaction and gluons for the strong interaction . The photon remains massless while the W and Z bosons acquire mass through a spontaneous symmetry-breaking mechanism proposed by three groups of physicists. This is achieved through the introduction of a complex scalar field leading to an additional massive scalar boson, labelled the SM Higgs boson. The fermions acquire mass through a Yukawa interaction of the Higgs boson. Only the gravitational interaction remains outside the SM.

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The year 2013 marked the 30th anniversary of the discovery of the W and Z bosons by the UA1 and UA2 experiments at the proton–antiproton collider at CERN, Geneva, Switzerland.

The discovery of the W and Z bosons in the 1980s directed attention towards the search for the Higgs boson, an elusive particle that plays a crucial role in the mechanism of mass generation.

In the following year, 1984, a workshop was held in Lausanne, Switzerland, where the first ideas were discussed about a possible high-energy proton–proton collider and associated experiments for this search. Among the leading protagonists were the scientists from the UA1 and UA2 experiments. The aim was to re-use the Large Electron Positron collider (LEP) tunnel after the end of the electron–positron programme. An exploratory machine was required to cover the wide range of mass, the diverse signatures and mechanisms thought to be effective for the production of new particles at a centre-of-mass energy 10 times higher than previously had been probed. A hadron (proton–proton) collider is such a machine as long as the proton energy is high enough and the instantaneous proton–proton interaction rate is sufficiently large. The centre-of-mass energy was set at 14 TeV and the rate at 1 billion pairs of protons interacting every second (corresponding to an instantaneous luminosity L=1034 cm−2 s−1). The hadron colliders can provide these conditions, though at the expense of ‘clean’ experimental conditions owing to multiple interactions in every bunch crossing overlaying on the one of interest (labelled pile-up).

Some of the physics questions the particle physics community was pondering over are listed below.

A key aim was to clarify symmetry breaking in the electroweak sector, most likely requiring a search for the SM Higgs boson, among other possibilities.

In the absence of a Higgs boson or equivalent, at very high energies such as at the Large Hadron Collider (LHC), the probability of some fundamental processes such as W_L−W_L scattering violates unitarity, i.e. the probability of occurrence becomes greater than one—which obviously would not make sense. A process involving the exchange of a Higgs boson would be able to ‘regulate’ the process and give a finite answer.

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Furthermore, it was known that the discovery of a fundamental scalar (spin 0) Higgs boson would raise another deep question—why would the mass of such a Higgs boson lie in the range probed by the LHC. With known physics, quantum corrections make the mass of a fundamental scalar particle float up to the next highest physical mass scale that, in the absence of extensions to the SM, could be as high as 10¹⁶ GeV. It is widely believed that the answer to this question would lie in new physics beyond the SM (BSM). One appealing hypothesis, much discussed at the time, and still being investigated, predicts a new symmetry labelled supersymmetry. For every known SM particle there would be a partner with spin differing by half a unit; fermions would have boson partners and vice versa, thus doubling the number of fundamental particles. The contributions from the boson and fermion superpartners, and vice versa, would lead to cancellations and allow the existence of a low mass for the Higgs boson. In the simplest forms of supersymmetry, five Higgs bosons are predicted to exist with one resembling the SM Higgs boson with a mass below approximately 140 GeV. The lightest of this new species of superparticles could be the candidate for dark matter in the Universe that is around five times more abundant than ordinary matter.

Also it was clear that a search had to be made for new physics at the TeV energy scale as the SM is logically incomplete; it does not incorporate gravity. Superstring theory is an attempt towards a unified theory with dramatic predictions of extra space dimensions and supersymmetry.

The LHC and its experiments were designed to find new particles, new forces and new symmetries, among which could be the Higgs boson, supersymmetric particles, Z^′ bosons, or evidence of extra space dimensions. An experiment that could cover the detection of all these ‘known’ but yet undiscovered particles, or phenomena, would also allow discovery of whatever else Nature has in store at the LHC energies.

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About Large Hadron Collider (LHC):

• The LHC is the world’s largest and most powerful particle accelerator.
• Located near Geneva, Switzerland, spanning the border of France and Switzerland.
• Built by CERN, it conducts experiments with highly energized particles.
• It can recreate conditions similar to the early universe moments after the Big Bang.
• Scientists collide high-energy subatomic particles and observe their interactions.
• Notably, the discovery of the Higgs boson in 2012 was a significant breakthrough at the LHC.

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