The Discovery of the b Quark

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1 The Discovery of the b Quark Bozzetto Elena July 21, 2010 The discovery of the J/ψ and third family of leptons, made respectivetly by Brookhaven National Laboratory-Stanford Linear Accelerator in November 1974 and M. Perl in 1975, has convinced some physists that the hypothesis of an analogous third family of quarks exist. In the Summer 1977, a team of scientists led by Leon M. Lederman at Fermilab in the state of Illinois were about to discover the hypothesis as being correct with the E-288 experiment. The experiment revealed the Upsilon (Υ), a sub-nuclear particle that is the bound state of the bottom quark b and its antiquark b and has a mass of 9.5GeV. The E-288 experiment began studying the couples electron-positron in a proton-nucleon collision. When a substantial amount of dielectron data had been taken, the team observed a J/ψ signal and a clustering of events near 6GeV. This suggested the possibility of a new resonance. A conservative person would not claim this to be a discovery, due to the statistical data for the new resonance being marginal. Dr. J. Weiss, a postdoctoral scientist at the University of Columbia, examined the Greek alphabet and found that the term Upsillon was not yet used. A scientific collaboration decided to call Upsilon if the resonance was real and Oops-Leon if the resonance was false. The result of the experiment was Oops-Leon. The experiment needed to be improved in order to avoid a result of Oops-Leon. The experiment shifted in the spring of 1976 from detecting electrons and positrons to studying pairs of muons µ + and antimuons µ. Muons µ + and antimuons µ produced in collisions of the proton beam from the accelerator on a copper or platinum target. The studied reaction is a follows p + (Cu, P t) µ + + µ + anything. (1) This second phase of the experiment was divided into two sections: µµiphase and µµiiphase. Like the dielectron experiment, in this second phase a doublearm magnetic spectrometer was used. The spectrometer analyzed the production of muon-antimuon pairs. The 400GeV proton beam hit the target 1

2 and each arm measured the trajector and the momentum of µ + and µ. Since the results were considered rara, a high intensity proton beam needed. The problem with the proton beam was that it produced a large number of pions and charged hadrons and it was necessary to reject them. To eliminate the excessive particles, a block of Beryllium (Be) was needed to act as a hadron filter. The Be was placed in each aperture of the double-arm magnetic spectrometer just past the target. This arrangement allowed the Be to absorb the strongly-interacting particles and permitted the muons to pass with minimal scattering. The muons emerging from the filters were then deflected by two large analyzing magnets. The magnets were connected in series and operated with the same polarity so that if one spectrometer arm (the up arm) deflected positive particles upward, the other (the down arm) deflected negative particles downward. In order to avoid low momentum muons from entering the detectors and scattering around through the coils and return yokes of the analyzing magnets, a hodoscope (Greek for path viewer ) was placed in the middle of each analyzing magnet and a scintillator muon detector at the back of each spectrometer arm. These additions are to avoid fake high-mass pairs. Furthermore a threshold Cerenkov counter was installed in each arm. The first phase confirmed there was a cluster of 6 events at 9.5GeV that only John Yoh took seriously. The second phase was an improvement of the first. All the detectors were brought closer to the magnets, there were no detectors upstream of the magnets, and the spectrometer arms were symmetrical with respects to the horizontal median plane in order to detect both µ + and µ in each arm. The team redesigned the beam dump, hadron absorber and shielding so as to minimize fractures, gaps and scattering of muons. The Beryllium was thicker than µµi 2m in order to avoid muons scattering in the aperture. It was placed in each aperture of the double-arm magnetic spectrometer just past the target and packed against steel and tungsten which minimize particle leakage from outside the aperture (the first 30cm of Be could be exchanged for 30cm of copper permitting a corresponding increase in protons on target). Be is very expensive and scarse element. Fortunately for the team, a Wadehouse in Oak Ridge National Laboratory in Tennessee had extra Be. Finally Polyethylene and a steel collimator completed the shielding. In the µµii the team decided to improve the muon identification. This improvement was achieved by the installation of a solid-iron magnet followed by PWC s (proportional wire chamber) in each arm. Additional PWC s were also installed to improve the trajectory of the muon. Now the new apparutus was a double-arm spectrometer with a mass resolution of approximately 0.02 and each arm symmetrically included (Figure 1): eleven PWC s that recorded the trajectory of the muons, refocused

3 Figure 1: E-288 µµii apparatus them partially in a vertical direction and rederminated their momenta; seven scintillation counter hodoscopes that recorded the trajectory of the muons; a drift chamber; a gas filled threshold Cerenkov counter that helped to prevent possible low-momentum muon triggers; an iron magnet to confirm the momentum. Installation of the target box and rigging of the detector and shielding piles were conducted in January and February 1977 with a brief test in April Data collection began on May 15 and after two days of experimentation there were already interesting results. The muons paths, recorded in the PWC s, was coded electronically and written on magnetic tape courtesy of an online system created by Daniel M. Kaplan. These magnetic tapes were then analyzed by a large Control Data Corporation 6600 computers. There were many stages of sorting through the data, controlling whether the electronic detectors were working properly and attempting to identify the few interesting pairs of muons. On May 21, a fire in a device that measured current in a magnet halted the testing temporarily. The fire spread to the wiring and chlorine gas was created by the electrical fire combined with the application of water by firemen, produced a corrosive acid. This acid threatened the electronics of E-288, however only after two days a fire salvage expert arrived with a secret formula from Europe (thanks to the big influence that Lederman had over a U. S. Embassy official) and the team was able to attain 900 electronic circuit boards. A few days after the fire E-288 was back online

4 Figure 2: Measured dimuon production cross section before and after continuum subtraction and on June 15 there was enough data to prove the existence of Upsilon. In fact the team saw evidence on a graph which revealed the Upsilon (only by way of a schematic computer print-out of the electronic detector data) and observed an increase of events near 9.5GeV. The enhancement had a width greater than the apparatus resolution of a full width at half-maximum (FWHM) of 0.5 ± 0.1GeV. Fitting the data minus the continuum fit with a Gaussian of variable width yielded [1]: Mass = 9.54 ± 0.04GeV with FWHM = 1.16 ± 0.09GeV. An alternative fit with two Gaussians whose widths were fixed at the resolution of the apparatus yielded [1]: Mass = 9.44 ± 0.03 and Mass = ± 0.05GeV This suggested that two closely-spaced resonances were being observed (Figure 2). Moreover it is important to note the use of Monte Carlo program carried out

5 Figure 3: The three resonances by Dr. S. W. Herb, a post doctorate from the University of Columbia. He was employed to calculate the acceptance and the resolution of the apparatus thus ensuring that the enhacement was not the result of apparatus or programming artifacts. Many other inspections were conducted to verify the conclusion, for example, the studying of various subsets of the data to search for possible apparatus problems, the combining of muons from different events to check the analysis software or the check that the signal was not caused by double counting of events. Additional data were also taken at a reduce magnet current and showed a consistent enhancement at 9.5GeV mass. The verification of statistically significant effects represents a highly conservative aspect of the culture of high energy physics. Steve Herb announced, on June 30, the discovery of Υ and the possibility that more than one resonance was being observed at a seminar at Fermilab. To verify the presence of a multiple-resonance, the team continued taking data. By the end of August they had accumulate 26, 000 events above 5GeV and approximately 1200 events above continuum [2]. A multiple-resonance interpretation was becoming statistically inescapable. In fact three resonances were being found (Figure 3). They had mass equal to Mass Υ = 9.46 ± 0.02GeV

6 Mass Υ = ± 0.03GeV Mass Υ = ± 0.05GeV respectively. The existence of these levels of Υ states was attributed to the presence of new quark called bottom (b). Therefore physists decided to give a b charge and mass equal to 1/3 and 4.2GeV respectively. The discovery of this quark meant that it had to be a top quark to complete the third generation. This experiment opened up a whole new chapter in the study of the nature of matter. In fact the b quark decay could be fundamental to understanding the problem that involves matter and anti-matter and the existence of the material universe. References [1] Herb et al., Phys. Rev. Lett. 39, 252 (1977). [2] D. M. Kaplan, The Discovery of the Upsilon Family (1994). [3] Fermi News, Vol. 20, July 18 (1997). [4] Fermilab, [5] D. H. Perkins, Introduction to High Energy Physics.