Unveiling the Giants: Discoveries and Breakthroughs from CERN's North Area.

 

This image showcases spectrometers from the EMC and NMC collaborations on the left, the NA31 experiment in the middle, and the BCDMS apparatus on the right. Credit: CERN-PHOTO-2392918, NA31 Collaboration, bul-pho-2004-031.

In the 1980s, under the leadership of Herwig Schopper during the LEP era, CERN's North Area became a hub for groundbreaking physics experiments. The European Muon Collaboration (EMC) and the NA31 collaboration conducted significant experiments, contributing to lasting impacts in particle physics. Noteworthy discoveries include the EMC effect, the proton spin crisis, and the first evidence of direct CP violation. Initiated in the late 1970s, EMC's experiment aimed to confirm the quark picture and quantum chromodynamics through high-energy muon scattering to measure nucleon-structure functions. However, this endeavor led to the profound reassessment of our understanding of protons and neutrons, challenging conventional textbook knowledge.

Structure functions provide insights into the momentum distributions of nucleon's constituent quarks. Experimentalists studied these distributions by scattering the M2 muon beam off a fixed target, allowing them to unravel the quark composition within the nucleon. In 1983, they made a surprising discovery—the momentum distribution of quarks varied between a nucleon within an atomic nucleus and a free nucleon, leading to the identification of the EMC effect. Contrary to initial models predicting minimal influence from the nuclear environment on nucleon structure, this phenomenon challenged expectations. Although various explanations have been proposed, the true origin of the EMC effect remains elusive.

Using a similar experimental setup, the BCDMS collaboration employed muon scattering at the NA4 experiment to investigate quark distributions. At SPS energies, this scattering involved the exchange of either photons or Z-bosons. BCDMS uncovered an interference effect between a photon and a Z boson during their measurements.

In a subsequent groundbreaking discovery, EMC utilized one of the first polarized targets and the polarized M2 muon beam to measure proton spin by assessing quark-spin contributions. Initial expectations assumed proton spin as a sum of three quarks' spins, resulting in a total spin of ½. However, in 1988, the collaboration revealed that only a small percentage of the proton's spin originates from its constituent quarks under high-momentum transfer conditions. This revelation, termed the proton spin crisis, continues to be a subject of investigation in contemporary particle physics.

In the same year, the NA31 experiment achieved a milestone by providing the first evidence of direct CP violation in kaon decays. Direct CP violation manifests when the decay products exhibit different symmetry properties compared to the original particle, as observed in the kaon decay into two pions. This phenomenon holds significance in understanding the origins of our universe. The collaboration utilized secondary and tertiary kaon beams generated from the 450 GeV SPS proton beam to measure a double ratio of decay rates for long-lived neutral kaons (K0L) and their short-lived counterparts (K0S) into two neutral and two charged pions.

To account for the distinct decay lengths of K0L and K0S, the team implemented a mobile target system called XTGV, moving through predefined stations inside a vacuum tube. This innovative approach reduced reliance on Monte Carlo simulations. Approximately a decade later, NA48, the successor to the NA31 experiment situated in CERN's North Area ECN3 hall, also confirmed direct CP violation.

In 1991, EMC's successor, the New Muon Collaboration, made significant contributions to the field of particle physics. Analyzing data collected between 1986 and 1989, they unveiled an uneven distribution of anti-d and anti-u quarks within the nucleon, expanding the insights offered by Quantum Chromodynamics (QCD). Simultaneously, experiments like NA34, initiated in the EHN1 and ECN3 experimental halls during the 1980s, laid the groundwork for CERN's ultra-relativistic heavy-ion program.

This program aimed to investigate nuclear matter under extreme conditions, with the overarching goal of identifying the hypothesized quark-gluon plasma—a state where quarks and gluons exist as free particles.The 1980s marked the inception of the North Area's diverse physics program, setting the stage for continued exploration anticipated to endure for decades to come.

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