Discovery of antiproton Antonia Mos I chose the article Observation of Antiprotons because, during the course, I was very intersted in the lectures about the first hadron s discoveries, and particularly in positron detection. Although this experiment was rather different from Andersson s one, I thought that it was the natural continuation of that discovery. 1 Introduction Antiprotons are, with positrons, some of the first antiparticles discovered. The existence of positrons was postulated as a consequence of Dirac equation, formulated in 1928, that predicted another particle identical to the electron, except that the charge would be positive. Afterwards, the equation was modified to describe the proton (and other particles), but before physicists obtained the experimental proof of antiproton s exsistence, they were not sure that a proton was a Dirac particle in the same sense as is the electron. The creation of the antiproton would require a lot of energy, and until 1950 there weren t particle accelerators with enough power to study this problem in laboratory. Besides, experimental events recorded in cosmic ray investigation weren t sufficient to provide antiproton existence. In 1954 was constructed one of the first synchrotrons, the Bevatron in Berkeley, and the experimental demonstration of the existence of antiprotons was one of the objects considered in his planning. The antiproton was firstly observed in 1955 in Berkeley, by Owen Chamberlain, Emilio Segrè, Clyde Wiegand and Thomas Ypsilantis (for this discover Chamberlain and Segrè won the 1959 Nobel Prize in Physics). They grounded the experiment also on the informations obtained by the tests with cosmic rays. 1
2 The Apparatus The aim was producing and detecting antiprotons by establishing the mass of negative particles originating at the Bevatron target. To determinate it, they chose to measure the particles momentum and velocity. They planned an apparatus as follows: the proton beam impinged on a copper target, then it described an orbit, going across several counters. It passed through a first magnet M1, a quadrupole focusing magnet Q1, which brought the particles to a scintillation counter S1, then it went through another quadrupole focusing magnet Q2, a second magnet M2, a second scintillation counter S2, a Čerenkov counter C1 and the special Čerenkov counter C2 and, finally, a third scintillation counter S3. Let s examine more carefully the elements of the apparatus. Magnets M1 and M2 were destined to deflect the proton beam (in this experiment, the particles were deflected three times: through an angle of 21 by the field of the Bevatron after the impact with the target, 32 by M1 and 34 by M2). Quadrupole focusing magnets Q1 and Q2 were used to focus the beam. They consisted of three consecutive quadrupole magnets: every quadrupole 2
magnet consisted of four magnets with alternatively reversed pole. S1 and S2 were plastic scintillation counters and measured ionizing radiation. Because they could provide informations very quickly, they were used to signal passage of particles. Čerenkov counters were used for particle identification. These counters are based on transition radiation, that are emitted by particles travelling in a medium with speeds faster than the speed of light. The light emitted by excited atoms forms a wavefront at a fixed angle with respect to the trajectory. If some other property can be measured in addition to the Čerenkov intensity, the particle can be identified. In particular, it is possible to discriminate between two particles of the same momentum and different masses, because that angle provides a direct measurement of the velocity of each particle. C1 is a common Čerenkov counter, C2 has a special design, and counts only particles in a fixed velocity interval. In C2, such as in C1, Čerenkov light was emitted at different angles for particles of different velocity. This angle continued to be determinated by velocity also after the light passed out of Čerenkov radiator. A cylindrical mirror was arranged to reflect only light emitted in the right direction. This light was then refocused within a small region where it could be detected by a photomultiplier. Using a system of plane mirrors, particles with the correct velocity were counted, while the beam didn t traverse the photomultiplier. C2 was very similar to the Lindenbaum and Pevsner counter: in it, a thin cell of the material to be used for a Čerenkov radiator was enclosed with a specularly reflecting envelope and was viewed from the two sides by photomultiplier tubes (quoting [3]). 3
3 The experiment An important difference between this experiment and the other experiments made with the same target in the same period is that this one used the quadrupole focusing magnets. This idea was suggested by Oreste Piccioni, who realized that a greater number of useful particles would pass through the system, if they employed this magnets. 3.1 First method Since the antiprotons must be selected from a pions background, it was necessary to measure the velocity by more than one method. The first method for determinating the velocity of the particles was by observing the time of flight between scintillation counters S1 and S2. They were separated by 40 ft ( 12, 19 m), so mesons of momentum 1, 19 GeV that had β = 0, 99 covered the distance in 40 millimicroseconds, while proton mass particles of the same momentum with β = 0, 78 needed 51 millimicroseconds. 3.2 Second method The second method was based on the properties of C1 and C2: the counter C1 detected all charged particles for which β > 0, 79, while C2 counted only particles in the velocity interval 0, 75 < β < 0, 78. When arrived at S2, a proton mass particle of momentum 1, 196 GeV/c had β = 0, 78, while, after traversing S2, C1 and C2, velocity was reduced to β = 0, 765. These values were such that the particle was counted in the second Čerenkov counter, but not in the first. After this second measurement, the counter S3 verified that the particle traversed C2 along the axis, with a no large-angle scattering. Notice that the second measure of velocity was completely indipendent by the first. 3.3 Importance of C1 Both these methods could accidentally count mesons as antiprotons. In the first case, coincident counts in S1 and S2 meant that a particle of the beam covered the distance in approximately 51 millimicroseconds. But, if a single meson would be completely excluded (because it s too fast), however accidental coincidences of S1 and S2 might provoke the count of some mesons. Also in measuring by Čerenkov counter C2 could be made some mistakes: in fact meson momentum decreased if it suffered a nuclear scattering inside 4
the counter, and, if it happened, the meson could be counted in C2. The coincidence of this two errors would alter the count, but the problem had been eliminated by introducing the counter C1, which indicated if a particle moved too fast to be an antiproton. Counter C1 was made of a material whose index of refraction was such that antiprotons would produce no radiations in it. However, faster particles, such as mesons and electrons, would give rise to a pulse at the output of this counter. In that case, the event would be rejected also if S1 and S2 (and C2) counted it. 3.4 Test of the apparatus A test of the apparatus was obtained by changing the position of the target and inverting the magnetic fields in M1, M2, Q1 and Q2: in this way, the apparatus would count positive protons. Thanks to this check, Segrè, Chamberlain, Wiegand and Ypsilantis discovered that there was an error in their first calibration of the apparatus: they had set it for a mass different from the proton one and so, inverting the fields, they founded very few protons. After the alignment of apparatus was corrected, protons were easily observed and, when the magnetic fields were again reversed, they started to find possible antiproton events. 3.5 Mass measurement A test to verify that the apparatus was sensitive to the mass of particles was made by tuning the system to a mass different from the proton mass, but in the region of the proton mass. The magnetic field values of M1, M2, Q1 and Q2 were modified, but the velocity selection was left unchanged: in this way, the apparatus was set for particles of a different mass, and, if the system was sensitive to the mass, proton-mass particles would not be registered. The result of this check was the one that the team expected, and the observations showed the existence of a peak of intensity at the proton mass. 5
This test permitted to say that the new particle had mass within 5 percent of that of the proton and to confirm the idea that it was the antiproton. 3.6 Lowering energy Another test was made for measuring the ratio of antiprotons to mesons emitted in the forward direction from the target as a function of Bevatron energy. Lowering the energy, the team verified that the antiprotons disappeared when the energy was sufficiently low. In fact, they observed that, at 6, 2GeV, antiprotons appeared to the extent of one in 44000 pions (but, because of the decay of pions, this corresponded to one in 62000, at the target). 6
They didn t observe antiprotons production at the lowest energy: later, calculations confirmed this result and showed that a few antiprotons should be produced at 5GeV. 4 Possible errors The mistakes caused by an erroneous apparatus tuning have already been considered: anyway, they were solved from the beginning. Another problem the team hypothesized was that other particles with characteristics similar to antiproton s ones would be mistaken for an antiproton. In the case of a negative hydrogen ion, they though they can rule out it, because it was extremely improbable that this ion passed through all the counters without the stripping of its electrons. There weren t known heavy mesons with the proper mass to explain their observations. Moreover, such particles had not a mean life sufficently long to pass through the apparatus without a prohibitive amount of decay. However, this could not be completely ruled out; so they considered the possibility of the existence of unknown negative particles of mass very close to 1840 electron masses, which also had to be singly charged. 5 Ending remarks The result found by Chamberlain, Segrè, Wiegand and Ypsilantis in their experiment was confirmed in the same period with a different test. In fact, in conjuction with their work, another team of physicists, including Gerson Goldhaber and Edoardo Amaldi, made a related experiment by using photographic emulsions. Some time after the first counter experiment, there were seen examples of the annihilation phenomenon between antiprotons and protons or neutrons. In their place, were generated about five pions. Afterwards, the antiprotons were identified by numerous other scientist s teams. For example, these particles have recently been detected by experiments like BESS or PAMELA (both dedicated to the detection of antiparticles in cosmic rays). Finally, in addition to their observation in nature, antiprotons are produced for being used at Fermilab s Tevatron, in proton antiproton collisions. They will also be used in the same way for LHC at CERN. 7
References [1] O. Chamberlain, E. Segrè, C. Wiegand, T. Ypsilantis, Observation of Antiprotons, Phys. Rev. 100, (1955), 947 950. [2] O. Chamberlain, The early antiproton work - Nobel Lectures, Physics 1942-1962, Elsevier Publishing Company, (1964), 489 505 [3] J. Marshall Čerenkov Counters, Ann. Rev. Nuc. Sci. 4, (1954), 141 156 [4] D. Perkins, Introduction to High Energy Physics, Cambridge University Press, (2000). [5] E. Segrè, Nuclei e Particelle, Zanichelli, (1982). [6] L. Valentin, Subatomic Physics: Nuclei and Particles, Hermann, (1981), 78 80. [7] C. N. Yang, La Scoperta delle Particelle Elementari, Boringhieri, (1969). [8] A. Sessler, E. Wilson, A Century of Particle Accelerators, World Scientific Publishing, (2007), 56 60. [9] D. Lincoln, Understanding the Universe - from Quarks to the Cosmos, World Scientific Publishing, (2004), 86 88. 8