Prestigious Discoveries at CERN: 1973 Neutral Currents 1983 W & Z Bosons

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In , Rubbia also speculated that the HPWF experiment had showed evidence for a couple of dimuonic events—reactions in which a pair of muons is produced—that could be interpreted as evidence of a new hadronic quantum number. However, the number of events was too low to be persuasive. After the discovery was announced, it was deduced that Rubbia and his collaborators had observed the same particle in their neutrino experiments.

Ting and B. The Proton-Antiproton Collider After the discovery of neutral currents and of the charm quark, one of the more urgent experimental endeavours became to provide direct evidence for the existence of the three intermediate vector bosons—the W and Z particles. The detection of these massive bosons was, however, extremely complex. Calculation in the framework of the Weinberg-Salam theory predicted that the mediating bosons were highly massive between 50 and GeV. While they naturally existed in the virtual state as mediators of the weak force, in order to produce them in a real state it an extremely high amount of energy was necessary —much higher than the upper limit reached by the accelerators available in the mids.

Since , the Fermilab proton synchrotron accelerated protons up to GeV, but the energy available for the production of new particles did not reach 50 GeV. Because of the conservation of the total linear momentum, most of the kinetic energy of the accelerated particles remains in the form of linear momentum of the particles emitted after the collision.

To reach a greater energy it was more efficient to employ colliders that accelerated particles in counter-rotating rings. After the frontal collision the total momentum is very low because the interacting particles had approximately the same velocity in opposite directions. Most of the energy of the two colliding particles was, then, available for the creation of new particles. In the mids, however, no collider had the centre-of-mass energy sufficient to the production of the W and Z particles. To solve this issue, Rubbia elaborated an idea initially proposed by Peter McIntyre to convert high-energy proton synchrotrons in proton-antiproton colliders.

A device of this kind would have permitted reaching very high energies and produced collisions that were in principle able to generate the intermediate vector bosons. When Rubbia conceived the project, the SU 2 xU 1 gauge theory of electroweak interactions and the SU 3 gauge theory of strong interactions of quarks and gluons, called quantum chromodynamics QCD , were gaining momentum.

In the framework of these theories, the collision between one proton and one antiproton was a very complex interaction of many different particles.

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Among this range of possibilities, there were some collisions able to produce intermediate vector bosons in case they were energetic enough. Theoretically, the neutral Z boson could be produced by a collision between a quark and an antiquark of the same flavour, while the charged W bosons by a collision between a quark and an antiquark of two different flavours. Although quarks antiquarks carry only a fraction of the total energy of the accelerated protons antiprotons , it was calculated that centre-of-mass energies around GeV would have been sufficient to make the intermediate vector bosons detectable.

That same year, Rubbia, along with Cline and McIntyre, put forward a proposal for the conversion of the existing synchrotrons to proton-antiproton colliders. In principle, the idea was rather simple. It did not require a radical modification of the facilities because the mechanism that accelerated protons in one direction also accelerated antiprotons in the opposite direction. The major difficulty of the entire scheme was to produce a beam of antiprotons compact enough to produce the required amount of collisions.

High-energy antiprotons could be generated by the collisions of proton beams on a fixed target. In , the Russian physicist Gersch Budker had developed a method for cooling protons by employing electrons to reduce their momentum variation. The method was tested successfully only in and soon the Fermilab began testing whether it was possible to utilize the electronic cooling to transform the proton synchrotron into a proton-antiproton collider in the near future.

However, the Fermilab management had already decided to embark on the ambitious project to increase the maximum energy of the facility from GeV to 1 TeV, employing giant superconducting magnets. The directorship of the Fermilab decided that priority should be given to this project, before any further transformation was done. One of the major reasons underlying the risky attitude of CERN directors was that CERN had not made until then any fundamental discovery in the field of particle physics.

In spite of the many efforts undertaken, the major discoveries had been made in American laboratories, apart from the discovery of neutral currents. The latter discovery, however, had not been awarded with a Nobel Prize, which was considered the major recognition for scientific achievements. CERN directors hoped that the discovery of the intermediate vector bosons would be the major discovery they had been waiting for since the CERN was established twenty years earlier.


After the project led to the understanding that the stochastic cooling could be indeed practicable, the CERN management decided to start off the collider project in June The UA1-UA2 Competition Rubbia projected a giant machine to detect the particles emitted from the head-on collisions of protons and antiprotons. The project was accepted and started under the name Underground Area 1 UA1.

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  5. Rubbia became the spokesperson of the UA1 collaboration, which comprised about physicists in different groups, coming from Austria, France, Germany, Italy, the United Kingdom, and the United States. The various groups of the UA1 collaboration took the responsibility to build different sections of the detector.

    Rubbia headed the efforts of the CERN group and was actively in charge of developing the complex electronics of the CD.

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    Two different experiments would have allowed a rapid confirmation of the findings made by the other detector or, at worst, provided some evidence in case one of the two detectors did not work properly. A rather traditional experiment was chosen at least with respect to the ambitious innovations of the UA1 project , called UA2. After the two projects were approved a race between them began in order to produce as quickly as possible persuasive results and, consequently, to claim priority for the discovery of the sought-after particles.

    At this early stage, the proton beam contained many more particles than the antiproton beam. The major difficulty was to distinguish a real proton-antiproton interaction from the background constituted by proton-gas collisions. A way to do that was to focus on the time delay between the detected events in different directions. A proton-antiproton collision was expected to produce events that would be detected at the same time by two scintillation counters positioned at the front and the back of the detector. Employing this criterion, the first proton-antiproton collision was detected at CERN after two days of operation with counter-rotating beams.

    The intensity of the antiproton beam was however too low by a factor of to allow the detection of intermediate vector bosons. Moreover, the UA1 collaboration had not completed its own detector. Both the UA1 and UA2 collaborations as well as the CERN scientists working at improving the intensity of antiproton beams worked frenetically to solve the various technical issues. To complete the necessary improvements took about one year, also because of some incidents. In October , a new run began. Both the UA1 and the UA2 had been completed and successfully tested.

    The intensity of the antiproton beams was great enough to allow in principle the detection of a few decays of W and Z particles.

    The History of CERN: Discoveries and Experiments

    The investigators decided to begin looking for evidence for the W bosons, which were produced more abundantly than the Z boson. Although charged, the lifetime of the W particles is so short that they cannot be directly detected. Among the various decays of the W boson the investigators were hoping to detect a characteristic decay—often called signature—that could signal the presence of a W.

    They were looking for a small number of golden events that could be explained only as decays of W bosons. The preferred one was the emission of one electron positron and the related electron antineutrino neutrino with elevated transverse energies. Neutrinos, however, could not be directly detected. Skickas inom vardagar. Skickas inom vardagar specialorder. They established the validity of the electroweak theory and convinced the physicists of the importance of renormalizable non-Abelian gauge theories of the fundamental interactions. The articles collected in this book have been written by distinguished physicists who contributed in a crucial way to these developments.

    The book is a historical account of those discoveries and of the construction and the testing of the standard model.

    Prestigious Discoveries at CERN

    It also reports on the future of particle physics and provides an updated status report on the LHC and its detectors being currently built at CERN. Altarelli, G. Witten, E. Hooft, G.

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    The discovery of the weak neutral currents – CERN Courier

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