60 Years
of experiments and discoveries at CERN
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The journey in search for the Higgs boson with the ATLAS and CMS experiments at the LHC started more than two decades ago. The discovery of a heavy scalar boson was announced on 4th July 2012, and subsequent data point strongly to the properties as expected for the boson associated with the Brout-Englert-Higgs mechanism. This discovery may well open a window onto physics beyond the standard model.

The understanding of flavour dynamics is one of the most important aims of particle physics. The last 15 years have witnessed the triumph of the CKM mechanism, which describes all flavour changing transitions of quarks in the Standard Model. This key milestone has been reached owing to a series of experiments, in particular to those operating at the so-called B-factories, at the Tevatron, and now at the LHC. We briefly review status and perspectives of flavour physics, highlighting the results where the LHC has given the most significant contributions, notably including the recent observation of the Bs0→μ+μ → decay.

We recall the development of research at CERN devoted to the phase structure of matter governed by the the strong fundamental force. Lattice QCD had predicted in 1984 a deconfinement phase transition from hadronic to partonic matter, to occur at an energy density of about 1 GeV per cubic fermi. This was discovered in experiments colliding heavy nuclei (Pb208) at relativistic SPS energies. Continuing to the present LHC experiments, partonic matter (the so-called "Quark-Gluon Plasma") has been characterized as a near-ideal QCD liquid, with collective properties (viscosity, specific heat, transport coefficient) that can perhaps be understood from fundamental field and string theory. Such matter has governed the cosmological evolution from the attosecond era onwards.

Within weeks of the start of data taking at LEP, experiments were able to confirm the existence of just three neutrino species by relying on the relation between the invisible width of the Z boson and its hadronicdecay width. The full data sample collected at and around the Z resonance, allows to reach an extremely high-precision, determining the number of light neutrino species as Nν = 2.9840 ± 0.0082. LEP experiments also confirmed the result through the elegant observation of events with Z bosons decaying into neutrinos, by detecting events with a single photon in the detector, radiated by the incoming electron or positron. This result is in agreement with expectations from the existence of three species of charged leptons, and is of high importance for the fields of astrophysics and cosmology.

The Large Electron Positron Collider (LEP) allowed to establish the Standard Model of particle physics with unprecedented precision, including all its radiative corrections. These led to predictions for the top quark and Higgs masses, which were beautifully confirmed later on. After these precision measurements the 1999 Nobel Prize in Physics was awarded jointly to Gerardus 't Hooft and Martinus J.G. Veltman "for elucidating the quantum structure of electroweak interactions in physics“. In addition, the precise measurements of the gauge coupling constants excluded unification of the forces within the SM, but allowed unification within the supersymmetric extension of the SM, thus paving the way for a Grand Unified Theory (GUT).

The W and Z bosons are the elementary particles that mediate the weak interaction. In the early 1970s their masses were predicted to be in the range 70 – 100 GeV, too high to be produced at any existing accelerator. It was in this context that the proposal was made to modify an existing high-energy proton synchrotron and transform it into a proton-antiproton collider capable of reaching the required centre-of-mass energy. This article describes the CERN project and the experiments that led to the first detection of the W and Z bosons in 1983. The importance of this discovery was acknowledged by the 1984 Nobel Prize for Physics.

In the neutrino experiment at CERN with the heavy liquid bubble chamber Gargamelle a new process was discovered in 1973 consisting of neutrino induced events without a charged lepton in the final state. This process has been interpreted as a "weak neutral current" interaction and is today a cornerstone of the unified electroweak theory. The discovery will be described in its historical context.

High energy neutrino beams in the late 1970s allowed early quantitative tests of the standard model. Results from experiments with the CERN neutrino beams are reviewed. Studies of the nucleon quark structure and of the weak current together with the precise measurement of the Weinberg angle did establish a new quality of the tests of the electroweak model. The measurements of the nucleon structure functions in neutrino deep inelastic scattering allowed first quantitative tests of QCD.

While the violation of CP symmetry in the mixing of neutral K-mesons had been discovered at Brookhaven in 1964, the hunt for CP violation in the K-meson decay itself ("direct CP violation") prompted two consecutive experiments at CERN since the early 1980's. The first one, NA31, announced in 1993 the first evidence for the existence of direct CP violation with a significance of 3.5 standard deviations. The following experiment, NA48, was designed to improve further the statistical and systematic precision, and gave its final result in 2002, consolidating the observation to a level of almost 7 standard deviations. This article describes the challenges that the two experiments had to face in the scientific context of their times, and reviews their legacy to nowadays physics.

The CPLEAR experiment at the antiproton storage ring LEAR studied in detail symmetries which exist between matter and antimatter. It measured with high precision the time evolution of initially strangeness-tagged neutral kaon states to determine the size of T, CPT and CP violations. Additional studies concerning quantum-mechanical predictions (EPR, coherence of the wave function) or the equivalence principle of general relativity have been made. The article will also include the basic formalism necessary to understand the time evolution of a neutral kaon state.

The Intersecting Storage Rings (ISR) were the first hadron collider ever built, providing proton-proton collisions at centre-of-mass energies as high as 62 GeV, almost five times higher than any previous accelerator. When the ISR began operation in 1972, the proton-proton total cross-section was expected to have already reached a finite asymptotic value, as hinted by previous, lower energy experiments. However, as described in this article, ISR experiments found that the cross-section was rising between 22 and 62 GeV. As also explained in the article, these measurements required the development of new experimental methods better adapted to the environment of a hadron collider.

We review results from deep-inelastic muon scattering experiments at the SPS which started in 1978, and are still actively pursued today. Key results include the precision measurement of scaling violations and of the strong coupling constant, nuclear effects, spin-dependent structure functions, and studies of the internal spin structure of protons and neutrons. These experiments have revealed fundamental insight into the internal structure of nucleons in terms of quarks and gluons.

We recall decisive contributions made by the CERN Intersecting Storage Rings and, later, by the proton-antiproton SPS Collider to revealing the parton structure of hadrons, studying the dynamic of interactions between partons and offering an exclusive laboratory for the direct study of gluon interactions.

The existence of long-lived particles and even stable antiparticles allows constructing completely novel atoms, such as antihydrogen, muonic or pionic atoms or antiprotonic helium. Precision studies of these exotic atoms allow to shed light on the properties of their components and thus carry out highly sensitive tests of fundamental symmetries.

This article highlights the inventive steps needed to measure the quantum correction, g-2, to the gyromagnetic ratio of the muon; first at the CERN synchrocyclotron and then at the PS with the first and second muon storage rings. Theorists at CERN helped to calculate the predictions of quantum electrodynamics. Theory and experiment agreed to 7 parts per million.

In 1957 the CERN 600 MeV Synchrocyclotron was there, ready to work, at the right time to take the lead of weak interactions over laboratories with similar machines of comparable energy. The two still missing β – decays of the pion were indeed first detected and measured at CERN, providing crucial verifications of the V – A coupling: π+ → e+ ν in 1958; and π+ → π0 e+ ν in 1962.

The on-line isotope separator ISOLDE is a dedicated facility for the production of a large variety of radioactive ion beams. The ISOLDE project was approved at CERN on December 17, 1964, and has since then developed into a major international installation for production and acceleration of exotic isotopes. Experiments in the fields of nuclear and atomic physics, solid-state physics, materials science and life sciences are performed at ISOLDE. Some of the highlights from the almost 50 years of experiments at ISOLDE are given, including shape coexistence, changes in shell structure, the nuclear halo phenomenon and weak interaction studies.