Editing UA2 experiment (section)

==Components and operation==
[[File:View from the U2 experimental hall.jpg|thumb|262px|Civil engineering for the underground experimental hall at LSS4]]

The UA1 and UA2 experiments recorded data during proton–antiproton collision operation and moved back after periods of data taking, so that the SPS could revert to fixed-target operation. UA2 was moved on air cushions when removed from the beam pipe of the Sp{{overline|p}}S.<ref name=UA2CERN/>

===Construction===
The UA2 experiment was located some 50 meters underground, in the ring of the [[Super Proton Synchrotron|SPS]]/[[Proton-Antiproton Collider|Sp{{overline|p}}S]] accelerator, and was housed in a big cavern. The cavern was large enough to house the detector, provide room for it to be assembled in a "garage position" without shutting down the accelerator and to where it was also moved back after periods of data taking. The accelerator could therefore revert to fixed-target operation, after periods of operating as a collider.<ref name=UA2CERN/>

===Detectors===
The [[UA1]] and the UA2 experiments had many things in common; they were both operating on the same accelerator and both had the same objective (to discover the [[W and Z bosons]]). The main difference was the detector design; UA1 was a multipurpose [[detector]], while UA2 had a more limited scope.<ref name=UA2CERN/> UA2 was optimized for the detection of electrons from W and Z [[Particle decay|decay]]s. The emphasis was on a highly granular [[calorimeter]] – a detector measuring how much energy particles deposit – with spherical projective geometry, which also was well adapted to the detection of [[hadron]]ic [[Jet (particle physics)|jets]].<ref name=Rubbia/> Charged particle tracking was performed in the central detector utilising a combination of [[multi-wire proportional chamber]]s and [[drift chamber]]s and [[hodoscope]]s.<ref>{{cite web | title = Status and First Results from the UA2 Experiment | date = 17 July 1982 | author = [[Peter Jenni]] on behalf of the UA2 Collaboration | url = https://cds.cern.ch/record/139004/files/198208194.pdf | accessdate = 5 September 2022}}</ref> Energy measurements were performed in the calorimeters. Unlike UA1, UA2 had no [[muon]] detector.

[[File:The UA2 detector after upgrade, improved end-cap calorimeters.jpg|thumb|262px|Detector for the UA2 experiment. The picture shows the detector after the 1985-1987 upgrade, when new end-cap [[calorimeter]]s were added to improve the search for the [[top quark]] and new physics.]]

The calorimeter had 24 slices, each weighing 4 tons.<ref>{{cite web |url=https://cds.cern.ch/journal/CERNBulletin/2015/32/News%20Articles/2038517?ln=en |title=Family reunion for the UA2 calorimeter |author=<!--Not stated--> |date=3 August 2015 |website=CERN Bulletin |publisher=CERN |access-date=28 July 2017 }}</ref> These slices were arranged around the collision point like segments of an orange. Particles ejected from the collision produced showers of secondary particles in the layers of heavy material. These showers passed through layers of plastic scintillators, generating light which was read with [[photomultiplier]] by the data collection electronics. The amount of light was proportional to the energy of the original particle. Accurate calibration of the central calorimeter allowed the W and Z masses to be measured with a precision of about 1%.<ref name=Detector>{{cite web |url= http://cern-discoveries.web.cern.ch/cern-discoveries/Story/UA2.html|title=The UA2 detector |date=2003
|publisher=CERN |access-date=22 June 2017}}</ref>

===Upgrades of the detector===
The 1985-1987 upgrade of the detector was aimed at two aspects: full calorimeter coverage and better electron identification at lower transverse momenta.<ref name=Jenni>{{cite book |last1=Froidevaux |first1=D. |last2=Jenni |first2=P. |year=1989 |chapter=Physics at the Improved CERN ppbar Collider |editor1-last=Altarelli |editor1-first=G. |editor2-last=Di Lella |editor2-first=Luigi |title=Proton-Antiproton Collider Physics |series=Advanced Series on Directions in High Energy Physics |volume=4 |publisher=World Scientific Publishing}}</ref> The first aspect was addressed by replacing the end-caps with new calorimeters that covered the regions 6°-40° with respect to the beam direction, thereby hermetically sealing the detector. The end-cap calorimeters consisted of lead/[[scintillator]] samplings for the electromagnetic part, and iron/scintillator for the hadronic part.<ref name=Jenni/> The performance and granularity of the new calorimeters were set to match the central calorimeter, which was of importance for the triggering system.

The electron identification was improved by the use of a completely new central tracking detector assembly, partly consisting of a pioneering silicone-pad 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 region beam pipe.<ref name=Tech>{{cite book |chapter-url=https://cds.cern.ch/record/2219700?ln=fr |last1=Gößling |first1=Claus |last2=Jarron |first2=Pierre|date=2017  |chapter=A Novel Particle Detector for UA2: The Power of Silicon |title=Technology Meets Research: 60 years of CERN Technology - Selected Highlights |publisher= World Scientific}}</ref> This detector was built as a cylinder, closely surrounding the beam pipe. The detector had to fit into the available space of less than 1&nbsp;cm. It was therefore necessary to miniaturize the components of the detector. This was achieved with two brand new technologies: the silicon sensor and the [[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.<ref name=Tech/>