The discovery of W ± and Z 0 vector-bosons

Transcription

1 The discovery of W ± and Z 0 vector-bosons Giulia De Zordo April 15, 2014 Abstract This article is about the discovery of the W ± and Z 0 vector-bosons, the carriers of weak interaction. The discovery happened in 1983 in UA1 experiment at CERN, made by Carlo Rubbia and Simon Van Der Meer. After an historical introduction of weak interaction, we describe the physics and the set of the experiment. From UA1 experiment, physicists select events characterized by a lepton with a large transverse momentum and large missing transverse energy, it was the signature of a W event. Later they select events characterized by a pair of leptons with a large transverse momentum, it was the signature of a Z 0 event. In the last section, we report some measurements of W and Z 0 mass and decay width made by experiments with higher statistics. 1 Introduction The existence of a new type of fundamental interaction, the weak interaction, was first predicted by Enrico Fermi (Rome, 21 September 1901, Chicago, 28 November 1954) in 1933 in order to explain the beta decay n p + e + ν e. In its initial steps, Fermi s theory predicts the existence of two vector bosons W ±, particles of spin 1, which were the carriers of the weak force. Because of the structure of beta decay, the range of interaction was supposed to be very short and so the weak force carriers had to be heavy particles. Weak interactions of this type are called charged current interactions, in these interactions there is a variation of electric charge and of leptons and quarks flavour. For many years, only this type of weak interaction was known. With the introduction of the Electroweak Theory (EWT), the existence of another type 1

2 of weak interaction, the neutral charged interaction was predicted. Neutral charged interactions are mediated by the vector boson Z 0, with no electric charge and spin 1. In these interactions there is no electric charge change and no variation of quarks and leptons flavour. In 1968, Sheldon Glashow (New York, 5 December 1932), Abus Salam (Shaug, 29 January 1926, Oxford, 21 November 1996 ) and Steven Weinberg (New York, 3 May 1933) described how it would be possible to treat electromagnetic and weak interations as different aspects of a single electroweak interaction, with a single coupling constant given by the elementary electric charge e. They predicted that this symmetry between electromagnetic and weak interactions would be manifest at very large momentum transfers (q 2 > 10 4 GeV). At low energies it would be a broken symmetry: of the four mediators involved, one, the photon, would be massless and the others, W +, W, Z 0, would be massive. As a result, compared with electromagnetic interactions, weak interaction would be short-range and apparently feeble. The theory contains an arbitrary constant, the weak mixing angle denoted by sin 2 θ W, which was to determined by experiment (sin 2 θ W = 0.227±0.014, [3]). This value was used to predict the W ± and Z 0 masses M W = 78.3 ± 2.4 GeV/c 2, M Z = 89.0 ± 2.0 GeV/c 2 [3]. In 1979 Salam, Weinberg and Glashow shared the Nobel Prize for the Electroweak Theory even though the weak force carriers had not then been detected. Several years later, towards the end of 1982, the circle was completed when the first signs of W ±, Z 0 were seen at CERN with right masses and other properties as predicted by theory. In this article we will describe the discovery of W ± and Z 0 vector bosons in UA1 experiment made by Carlo Rubbia (Gorizia, 31 March 1934) and Simon Van Deer Meer (Aia, 24 November 1925, Geneva, 4 March 2011). 2 Physics of the experiment The W ± and Z 0 bosons are highly unstable particles, which were first produced in the reactions p + p W + + X, (1) p + p W + X + (2) and p + p Z 0 + X 0 (3) 2

3 where X ± and X 0 are arbitrary hadronic states allowed by the conservation laws. The heavy bosons were detected via their subsequent decays W + l + + ν l (4) W l + ν l (5) and Z 0 l + + l (6) where the charged leptons l ± were either muons or electrons. They are not the only possible decay modes, in fact branching ratios of leptonic decay modes of W are about 11% for each leptonic generation, while branching ratios of leptonic decay modes of Z 0 are around 3.4% for each leptonic generation. The production mechanism that gives rise to the reactions (1), (2), (3) involves quark-antiquark annihilation processes such as u + d W + (7) d + ū W (8) and u + ū Z 0, d + d Z 0 (9) for charged and neutral bosons respectively. In accordance with Electroweak Theory results, in the centre of mass of the q q system, the total energy of the q q pair must be at least 80 GeV or 90 GeV, corresponding to the production of either a W ± or a Z 0 boson at rest. However, the energy of the p p system has to be considerably higher for a reasonable reaction rate to occur, because each quark has only a fraction of the parent proton energy. In order to produce a particle with mass of 80 GeV/c 2 or 90 GeV/c 2, proton and antiproton beams had to have a energy of 270 GeV each. This value of energy was out of reach of particle accelerators until the end of 70. It was clear that the search for the W and Z bosons called for new accelerators, preferably of the collider type. In 1976, Carlo Rubbia and Simon Van Der Meer resolve CERN authorities to change the SPS (proton-proton collider) into a proton-antiproton collider, the Sp ps. In the course of preliminary experiments at CERN, in July 1981, the luminosity was quite low ( cm 2 s 1 ) and no meaningful events could be detected. In the second run, in December 1982, the luminosity was raised to cm 2 s 1 and the computer program aimed at hunting likely events was operate on-line. The number of these events was very small, but in January 1983, the two groups of investigators working independently in U A1 3

4 and UA2 gave their first report on their results. Out of 10 9 p p collisions, 10 6 were recorded. Of this moltitude, only nine events were selected by computer and each of them bore the signature of the W boson. During a third period in 1983, this time with a luminosity six times greater, the Z 0 boson was discovered. Several months later in 1984, Carlo Rubbia and Simon Van Der Meer won the Nobel prize for the discovery of the weak force carriers. 3 Setup The experiment setup is made of a complex of accelerators: a complex of facilities used to produce proton-antiproton collisions in Sp ps (Figure 1) and of two huge detectors called UA1 and UA2 (Underground Area 1 and 2), the first one direct by Carlo Rubbia and the second one by Pierre Darriulat and Luigi Dilella. Placed underground, in two sections of the Sp ps where collisions between the beams take place, the two detectors function independently of each other. Now, we will describe the complex of accelerators and the detectors in more details. 3.1 The CERN proton-antiproton collider The first stage in the sequence of interconnected accelerators rings is the 28 GeV Proton Synchrotron (PS). In order to produce the anti-protons, protons are extracted from the PS and directed at a metal target. In the collisions some anti-protons are produced at about 3.5 GeV, they are collected and transferred to a ring called the Anti-proton Accumulator (AA). When the anti-protons are created, their energies and directions are quite dispersed. Prior to further acceleration they have to be stacked into dense, highly ordered bunches; in order to do this, in 1968 Simon Van Der Meer developed the technique of the stochastic cooling. After the cooling process in AA, the proton-antiproton bunches are injected again into the PS ring and accelerated to 26 GeV. The anti-protons are then sent in the SPS ring (a 6.9 kilometer ring), where protons that were accelerated in the PS to the same energy are already revolving in the opposite direction. In the SPS the proton and anti-protons beams are brought up to 270 GeV (540 GeV in the centre of mass of the p p system). In particular, there were six bunches of protons (each bunch of 20 cm lenght with protons) and six bunches of antiprotons (each bunch of 20 cm of lenght with antiprotons) travelling in opposite direction with exactly equal energy of 270 GeV. The time between two beams crossing is 3.8 micro-seconds. The beams collide in two regions called interaction zones in which the detectors 4

5 Figure 1: The facilities used for p p collisions at the Sp ps collider at CERN ([8]). are set. 3.2 The detectors The two huge detectors are called UA1 and UA2 (Underground Area 1 and 2, since they had be buildt at 20 metres in deep). UA1 is 10 metres long by 5 metres wide, weighting about 2000 tonnes. It contains several layers of detectors in a concentric geometry, all trigged to operate when a p p collision occurs (Figure 2). As one moves out from the centre of the detector there are: a central tracking detector (CD) used to observe charged particles and to measure their momentum from the curvature of the tracks in an applied magnetic field of 0.7 Tesla of intensity, normal to the direction of the beams; a set of electromagnetic shower counters (S) which absorb and detect both electrons, which are also observed in the central detector, and photons, which are not; a set of hadron calorimetres (HC), which absorb and detect both neutral and charged hadrons and a set of conters to identify muons (MD), which are the only charged particles to penetrate the hadron calorimeters. Only neutrinos escape detections, they can be detected as missing energy in calorimeters. The U A2 is a similar but smaller array of detectors, design specifically to detect the intermediate bosons; it has no magnet and weighs only 200 tonnes. 5

6 Figure 2: Schematic diagram showing a cross-section of the U A1 detector seen along the beam direction ([3]). 4 Discovery of W ± and Z 0 bosons 4.1 Discovery of W ± It was assumed that once a W is produced, it schould decay within a very short interval of the order of s to hadrons and leptons. It was therefore decided to look only for the decay modes described in (4) and (5). The instruction for the computer was to select events in which: 1- the leptons arising from the decays have large momenta and are emitted at wide angles to the initial beam direction. In UA1 experiment, the leptons are requested to have a transverse momentum p T greater than 10 GeV/c and to be emitted at more than 5 to the beam direction; 2- there is large missing transverse momentum carried away by neutrino. In summary, an event of W ± is characterized by a charged lepton with a large transverse momentum and by a large missing transverse momentum (Figure 3). In all, a total of 43 events were observed in the original UA1 experiment, where the charged lepton was either an electron or a muon. If we assume that W ± was produced with zero transverse momentum, the momentum conservation would imply that the transverse momentum of 6

7 Figure 3: Graphic visualisation of charged particles tracks under a magnetic field. We can see the track of an e with a energy of 48 GeV, [12]. the e ± produced in the decay would be equal and opposite to the transverse momentum carried away by neutrino. Since the mass of the lepton is negligible E e = (p 2 T c 2 + m 2 c 4 ) 1 2 pt c from which we have that E e E T. This result had been predicted theoretically and is confirmed by the experiment as we could see in Figure 4. From the 43 events, it had been possible to determine mass, decay width and lifetime of the W. The determination of the mass is difficult because of the missing energy of the neutrino. In order to avoid this problem, we can use Monte Carlo methods of simulation. Finally, the experiment hold to these results M W = ± GeV/c 2, Γ W = 2, 141 ± GeV which corresponds a lifetime τ W = s. 4.2 Discovery of Z 0 It was decided to look only for the decay mode describe in (6) and the instruction for the computer was to select events in which the pair of leptons arising from the decays have large momenta and are emitted at wide angles to the initial beam directions; in other words, events in which the pair of leptons have very large transverse momentum. The electron and the positron were both detected in the central detector, where they gave rise to tracks that were almost stright, despite the applied magnetig field. They were identified as electrons, and not as muons or hadrons, from their signal in the 7

8 Figure 4: The missing transverse energy E T plotted as a function of the measured transverse energy E e of the electron for the first W ± production events observed in the UA1 experiment ([4]). Figure 5: In May 1983, the central detector of UA1 experiment at CERN reveals the signature of the Z 0 particle as it decays into an electron-positron pair [12]. 8

9 electromagnetic shower counters: the e + e pair stands out dramatically in the output from them (Figure 6). Figure 6: Output from the electromagnetic shower counters corresponding to a Z 0 event. Each square element corresponds to a particular direction in space and the height of the tower to the energy of the particle detected ([5]). It was also observed that the invariant mass of Z 0 corresponds to the mass of the decaying particles. The Z 0 invariant mass was predicted in the electronweak theory to be about 90 GeV/c 2, this result was confirmed for twelve events observed by UA1 and UA2 collaborations in their first experiments. 5 Mass of W ± and Z 0 with higher statistics After 1983, other experiments tried to measure W and Z 0 mass and width with higher statistics, for example, in Figure 7, we can see e e + mass distributions obtain in an experiment at Fermilab 1. A clear peak is seen in both cases, corresponding to a mass of 90 ± 0.4 GeV/c 2 and a width of order 3 GeV. The parameters have since been measured by other methods, leaing to a mass of M Z = ± GeV/c 2 1 Fermi National Accelerator Laboratory (Fermilab), located just outside Batavia, Illinois, near Chicago, is a US Department of Energy national laboratory, specializing in high-energy particle physics. 9

10 Figure 7: Z 0 peaks observed in the e + e and µ + µ mass distributions from a Fermilab experiment by CDF collaboration ([6]). and a width of Γ Z = ± 0, 0023 GeV corresponding to a lifetime of s. From measurement at the LEP 2 accelerator and at Tevatron, the W mass and width are now estimed to be M W = ± GeV/c 2, Γ W = ± GeV [11]. and M W = ± GeV/c 2, Γ W = ± 0.49 GeV [11], respectively. Combining these results, yields on average W mass and width of M W = ± GeV/c 2 and Γ W = ± GeV, [11]. 2 Electron-Positron Collider (LEP) was one of the largest particle accelerators ever constructed. It was buildt at CERN, it was a circular collider with a circumference of 27 kilometres buildt in a tunnel straddling the border of Switzerland and France. It was used from 1989 until To date, it was the most powerful accelerator of leptons ever buildt. 10

11 References [1] Y. Ne eman and Y. Kirsh, The Particle Hunters, Cambridge University Press, , (1996) [2] D. H. Perkings, Introduction to High Energy Physics, Cambridge University Press, , (2000) [3] B.R. Martin and G. Shaw, Particle Physics, John Wiley and Sons, , (2008) [4] M. Spiro, Proceedings of the 1983 International Symposium on Lepton and Photon Interactions at High Energies, Cornell University [5] B. Sadoulet, Proceedings of the 1983 International Symposium on Lepton and Photon Interactiona at High Energies, Cornell University [6] M. Campbell, Proceedings of the 1989 International Symposium on Lepton and Photon Interactions at High Energies, Stanford University [7] [8] [9] [10] [11] [12] 11