# UA2 experiment

The UA2 detector shown in open position at the CERN SPS Collider in 1982

The Underground Area 2 (UA2) experiment was a high-energy physics experiment that ran at CERN’s Super Proton Synchrotron (SPS) from 1981 until 1990. [1] Its main objective was to discover the W and Z bosons. UA2, together with the UA1 experiment, succeeded in discovering these particles in 1983, leading to the 1984 Nobel Prize in Physics being awarded to Carlo Rubbia and Simon van der Meer. The UA2 experiment was also involved in the search of the top quark.

## History

Around 1968 Sheldon Glashow, Steven Weinberg, and Abdus Salam came up with a theory that unified electromagnetism and weak interactions, for which they shared the 1979 Nobel Prize in Physics. This theory, known as the electroweak theory, postulated the existence of W and Z bosons. The theoretical discovery put pressure on experimental physicists to show the existence of the W and Z bosons. It was predicted that the W boson had a mass value in the range of 60 to 80 GeV, and the Z boson in the range from 75 to 92 GeV – energies too large to be accessible by any accelerator in operation at that time. [2]

During those years the prime project for CERN was the construction of an electron-positron-collider, known as LEP. Such a machine is ideal to produce and measure the properties of W and Z bosons.[2] But due to the pressure to find the W and Z bosons, the CERN community felt like it could not wait for the construction of LEP.[3] In 1976 Carlo Rubbia, Peter McIntyre and David Cline proposed to modify the existing Super Proton Synchrotron (SPS). [2] The SPS was originally designed and built for fixed-target experiments, meaning it would accelerate one proton beam. The modification meant making it into a two-beam proton-antiproton-collider, which became known as the SppS. [4] In such a scheme, a proton and an anti-proton beam, each of energy E, circulated in the same magnetic field in opposite directions, providing head-on collisions between the protons and the antiprotons at a total center-of-mass energy ${\displaystyle {\sqrt {s}}=2E}$.[2] The scheme was adopted at CERN in 1978. Eventually the SppS could run with a ${\displaystyle {\sqrt {s}}}$ up to 630 GeV.[5] By the end 1982, the machine had reached high enough energies to permit the observation of ${\displaystyle W\rightarrow e\nu }$ decays. The UA2 and UA1 collaboration chose to detect the W boson by identifying its leptonic decay, because even though the W boson predominantly (≈70%) decays to quark-antiquark pairs, which appear as two hadronic jets, such jets are overwhelmed by other jet production.[2]

Like UA1, UA2 was a moveable detector, custom built around the SPS beam pipe, which searched proton-antiproton collisions for signatures of the W and Z particles.[1] The UA1 and UA2 experiments ran during proton-antiproton running and were rolled back after periods of data taking, so that the SPS could revert to fixed-target operation.[1] The UA2 experiment was approved in December 1978, and began operation in December 1981.[1] The initial UA2 collaboration consisted of about 60 physicists from Bern, CERN, Copenhagen, Orsay, Pavia and Saclay.

From 1981 to 1985, the UA1 and UA2 experiments collected data corresponding to an integrated luminosity of approximately ${\displaystyle 0.9pb^{-1}}$.[6] In this period, about 250 W and 30 Z candidates events were found – enough to establish the existence of the particles, but not enough to allow for detailed tests of the theoretical predictions. [6] From 1985 to 1987 the SppS was upgraded, and the luminosity of the machine increased by a factor 10 compared to the previous performance.[6] The UA2 sub-detectors were also upgraded to cope with the higher expected event rate.

The second experimental phase ran from 1988 to 1990. Groups from Cambridge, Heidelberg, Milano, Perugia and Pisa joined the collaboration, which grew to about 100 physicists. During this phase, UA2 accumulated data corresponding to an integrated luminosity of ${\displaystyle 13.0pb^{-1}}$ in three major running periods.[6] After nearly ten years of operation, the UA2 experimental program stopped running at the end of 1990.

## Components

### Construction

The UA2 experiment was located some 50 meters underground, in the ring of the SPS accelerator, and was housed in a deep cavern, excavated from within. The underground cavern was large enough to house the 2000-ton detector, provide room for it to be assembled without shutting down the SPS and space for it to be rolled back after periods of data taking, so that the SPS could revert to fixed-target operation.[1]

### Detectors

The UA1 and the UA2 experiments had many things in common: they were both operating on the SPS accelerator, and both had the same objective (the discovery of the W and Z bosons). The main difference was the detector design: UA1 was a multipurpose detector, while UA2 had a more limited scope. [1] UA2 was optimized for the detection of electrons from W and Z decays. The emphasis was on a highly granular calorimeter – detectors measuring how much energy particles lost while travelling through – with spherical projective geometry, which also was well adapted to the detection of hadronic jets.[2] Charged particle tracking was performed in the central detector, and energy measurements were performed in the calorimeters. Unlike UA1, UA2 had no muon detector. [1]

The calorimeter had 24 slices, each weighing 4 tons.[7] Like segments of an orange, these slices were arranged around the collision point. Particles ejected from the collision produced showers of secondary particles in the layers of heavy material. These showers passed through layers of plastic scintillator, generating light which was read by the data collection electronics. The amount of light was proportional to the energy of the original particle.[8] Accurate calibration of the central calorimeter allowed the W and Z masses to be measured with a precision of about 1%.[8]

In 1983, the UA2 collaboration decided to upgrade its tracking detector by replacing parts of the existing detector with a silicon detector. In 1989, the collaboration pushed this concept even further by developing a Silicon Pad Detector (SPD) with finer pad segmentation to be placed directly around the collision interaction beam pipe. This detector was built as a cylinder, closely surrounding the beam pipe. The detector had fit into the available space with a tolerance of less than 1 cm. It was therefore necessary to miniaturize the components of the detector. This was achieved with two brand new technologies: the silicon sensor and Application Specific Integrated Circuit (ASIC). Existing electronics were too bulky, and therefore a novel ASIC had to be developed. This was the first silicon tracker adapted to a collider experiment, a technology prior to the present silicon detectors.

## Results

### Discovery of the W and Z bosons

On 22 January 1983, UA2 physicist Luigi Di Lella announced that the UA2 detector had recorded four events that were candidates for a W boson. This brought the combined number of candidate events seen by UA1 and UA2 up to 10. [4] Three days later, CERN made a public announcement that the W boson was found.[9]

The next step was to track down the Z boson. However, the theory said that the Z boson would be ten times rarer than the W boson. The experiments therefore needed to collect several times the data collected in the 1982 run that showed the existence of the W boson. With improved techniques and methods, the luminosity was improved by more than a factor of 100 compared to the initial runs of 1981 .[10] These efforts were successful, and on 1 June, the formal announcement of the discovery of the Z particle was made at CERN.[11]

### Search for the top quark

Throughout the 1989-1989 run, the UA2 collaboration was in a kind of race with the CDF collaboration at Fermilab in the US. They both searched for the top quark. Physicists had known of its existence since 1977, when its partner - the bottom quark - was discovered. It was felt that the discovery of the top quark was imminent. In June 1984, Carlo Rubbia at the UA1 experiment expressed to the New York Times that evidence of the top quark "looks really good". [12] Over the next months it became clear that UA1 had overlooked a significant source of background.[13] In the end, neither UA1 nor UA2 discovered the top quark: CDF showed, early in 1989, that UA2 would be unable to find the top quark — the lower limit of the mass of the top quark was too large to be created in significant numbers at the SPS.[14] The top quark was ultimately discovered in 1994-1995 by physicists at Fermilab.

## References

1. "UA2". CERN. Retrieved 21 June 2017.
2. Di Lella, Luigi; Rubbia, Carlo (2015). "The Discovery of the W and Z Bosons". 60 Years of CERN Experiments and Discoveries. CERN Document Server: World Scientific. p. 137. ISBN 978-981-4644-14-3.
3. ^ Darriulat, Pierre (1 April 2004). "The W and Z particles: a personal recollection". CERN Courier. Retrieved 21 June 2017.
4. ^ a b O’Luanaigh, Cian (12 March 2015). "Carrying the Weak Force: Thirty Years of the W boson". CERN. Retrieved 21 June 2017.