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Test of CPT with antiprotons
Volume 77B, number 3
PHYSICS LETTERS
14 August 1978
TEST OF CPT WITH ANTIPROTONS H. POTH 1 Institut ff~r Kernphysik, Kernforschungszentrum and University, Karlsruhe, West Germany
Received 29 May 1978
It is proposed to study the particle-antiparticle symmetry in a ring accommodating protons and antiprotons simultaneously. Experiments to measure a possible proton-antiproton mass difference, and to test the Dirac theory and CPT theorem, are described.
Only a little more than 20 years after their discovery [1 ] antiprotons are going to become one of the most engrossing particles in accelerator physics [2]. They can be copiously produced by means of highenergy accelerators, and with the recent advent of novel phase-space cooling techniques large numbers of them could be accumulated and stored. From the accelerator point of view the antiproton is just a negatively charged proton. However, its quark content makes it superior to the proton for the production of new particles and resonances. Dirac's theory created the particle and antiparticle picture and stimulated the discovery of the positron and the antiproton. The properties of particles and antiparticles are related by the fundamental principle of CPT invariance [3]. The combined operation of charge conjugation, parity, and time reversal converts a particle into its antiparticle. As a consequence, they have in common the absolute values of their characterizing properties, whereas the sign may be opposite owing to the different behaviour of polar and axial vectors under parity and time reversal. For instance, the sign of the magnetic moment is inverted for the antiparticle. These theories are one of the bases of modern physics, and the particle-antiparticle picture is widely applied in elementary particle physics. Although it was discovered that P [ 4 ] , CP [5], and T [6] invariances are broken separately, it is commonly assumed that 1 Visitor at CERN, Geneva, Switzerland.
the CPT theorem holds. Experimentally its validity can be tested only to certain limits given by the experimental technique, rather than be confirmed in general. This has been done by comparing the properties of particles and antiparticles such as the mass, lifetime, magnetic moment, and g-factors. Table 1, which is not meant to be complete, shows the precision achieved for the different particles [ 7 - 1 5 ] . It can be seen that the precisions for leptons are in general higher than they are for hadrons. There is a non-vanishing value (2o effect) for the mass difference between proton and antiproton. The antiproton mass and magnetic moment have been deduced from the energies and the fine-structure splitting of X-ray transitions in antiprotonic atoms. The errors come mainly from statistics and from the energy resolution of the semiconductor X-ray detectors used. With an intense antiproton beam comparable to pion beams at meson factories (e.g. 106 particles/s), and a crystal spectrometer, a precision a little below 10 -5 in the X-ray energies could be reached. This would result in a similar precision for the mass and about two orders of magnitude less precision for the value of the magnetic moment, which would be a considerable improvement. To achieve a much higher precision is difficult since, in order to derive the mass from the transition energies, the contributions of strong interaction and of higher order electromagnetic effects have to be ascertained in detail. Such contributions are: radiative corrections, electron screening, 321
Volume 77B, number 3
PHYSICS LETTERS
14 August 1978
Table 1 Experimental results comparing the properties of charged particles and antiparticles (m is the mass, r the lifetime, u the magnetic moment, and g the g-factor of the particles). The charge is indicated by the positive and negative sign. Particle
Fractional difference m+- m_ Am
=
-
#+-~_
T+--T_ AT
m
= -
A~
-
7"
--
~r K p-
-(2.4 -+4.7) × 10 -5 [121 b) -(1.3_+ 1.l)× 10 -4 [13] c) (7.6 _+3.9) X 10 -5 [14]
gg
< 1.2 X 10 -8 [7] (2.6_+ 1.7)X 10 -8 [11]
e
- ( 2 + 5) X 10 -6 [8]
g+ ,5,g = "
#
(7.9 -+ 10) × 10 -4 [9] a) -(4 -+9) X 10-4 [10] (5 -+ 7) × 10 -3 [13] (1.1_+0.9)× 10-3 [13] r~-> 1.2 × 10-4 s [15]
-(0.4-+ 7.2) X 10 -3 [14a]
l
a) In ref. [9], the muon lifetimes were measured at 3' = 29.33. b) Using the averaged ~r- mass (139 568.2 -+ 1.5) keV from the measurements of ref. [ 12a] and, assuming a vanishing V# rest mass, the 7r+ mass is calculated to be (139 564.9 -+6.4) keV from the measurement of ref. [12b]. c) Using the 7r mass difference of ref. [ 12 ]. cascade effects, nuclear polarization, and resonance between orbital and nuclear states. The magnitude of these effects is difficult to calculate at the above mentioned level of precision. Experimentally the level of accuracy is also limited by the precision of the energy standards, and systematic errors are difficult to estimate. Since the difference between two large numbers always suffers from the composite r.m.s, error, the ~ atom method has the principal disadvantage that the antiproton mass is measured absolutely and then compared to the proton mass. The aim of this paper is to propose a method whereby the mass difference between proton and antiproton can be measured and the CPT theorem can be tested directly. The principle is outlined in the description of a few experiments. A violation of CPT can as well be provoked by a difference in the absolute 'value of the electrical charge of particle and antiparticle. This will be taken up again at the end of this paper, whilst in the following the equality of these quantities is assumed. The basic idea of this method is to use an accelerator or storage ring as a symmetric mass separator, and to apply the fact that the antiproton is a proton with opposite charge which runs backward in time. This means that in practice it follows the same path as 322
the proton in any magnetic field but in the opposite direction of motion. This principle is used, for instance, in the CERN Super Proton Synchrotron (SPS) and the Fermilab accelerator for the envisaged simultaneous acceleration and storage of antiprotons and protons. In order to measure rest mass differences, it is necessary to accumulate and store sufficient antiprotons and to work at non-relativistic energies. At high energies, a small change in m o m e n t u m will change the mass considerably; however, this may be overcome as shown later. At low energies the antiproton mass is measured in a mass spectrometer which is calibrated with protons. Basically every mass spectrometer consists of a magnetic and an electric field. In our case the magnetic field could be provided by the field of a low-energy storage ring and would fix the m o m e n t u m of the antiprotons. If these are ejected with a defined m o m e n t u m and sent through an electric field perpendicular to their trajectory, they will be deflected according to their mass. This deflection can then be compared to the deflection of protons which have been stored in the same ring and passed through the same field. To reach the ppm level seems to be very difficult, since the particle m o m e n t u m has to be defined with even higher precision, and the beam size has to be very
Volume 77B, number 3
PHYSICS LETTERS
14 August 1978
In each turn this would cause a phase shift 6 of the antiproton with respect to the proton:
on identical paths, they will intersect on each turn. Some o f them will then collide and annihilate. The annihilation produces many pions, which mark the collision point. If there is a difference in revolution frequency, this collision point will circulate around the ring, and this is indicated by the pions. The circulation frequency corresponds to the magnitude of the mass difference, and the direction to its sign. However, the revolution frequency can be measured even for unbunched beams. This can be achieved by picking up the Schottky noise signal of the beams * 1. This signal has a certain frequency distribution, centred around the average beam frequency, and a width characterizing the momentum spread of the beam. The technique is commonly used for beam control. An absolute precision at the ppm level can be reached. Even at highly relativistic energies there exists a possibility of measuring a rest mass difference* 2. By imposing a common radio frequency on bunches of protons and antiprotons, these can be forced to circulate in a ring, with the same revolution time. A possible mass difference will appear as a slight difference in momentum A p , and thus as a difference AR in the average radius of the orbit:
8 = Aco. T
ARIR = (%IR)(Aplp) ,
small in order to measure the deflection accurately. There exists, however, a completely different method for measuring the mass difference directly. Again, use is made o f the fact that protons and antiprotons may be stored in the same ring at low energies. The orbiting particle is characterized by its revolution frequency: = coclT,
w c = e B / m O, 3' = l/x/1 _ / 3 2 ,
where e is the charge and m 0 the rest mass of the particle circulating in a magnetic field B with the fraction /3 of the velocity o f light. The important fact is that for non-relativistic energies this frequency is independent o f the radius of the orbit and is inversely proportional to the rest mass of the particle. It is only for relativistic velocities that a variation in 3' may cover rest mass differences. This means that a difference in mass would show up as a difference Aco in the cyclotron frequency: COp = COp + ACO .
(T is the revolution time). Although not measurable for one cycle, it would grow with the number n of revolutions: fin = n" Aco" T = n A w l w .
If, for instance, the masses are different by 10 - 6 , the antiproton would have completed one more cycle than the proton after 106 revolutions or vice versa. For a I0 m ring and a momentum of 100 MeV/c, this would take only some seconds. There are several possibilities for measuring such a phase shift or beat frequency. It is assumed that the beams are bunched in the initial state o f measurement. Then, one way is to measure the time between subsequent beam pick-up signals (one from the protons, the other from the antiprotons). This time is constant if the revolution frequencies are equal; if there is a difference, it oscillates according to the beat frequency. However, the fact that antiprotons and protons may annihilate during collisions offers another possibility of measuring a difference in the revolution frequencies. Since the bunches of protons and antiprotons travel
~p: momentum compaction, p: momentum o f the particle, R: radius of the orbit, where A p / p is determined by the rest mass difference Am 0 and the imposed frequency: A p _ Am0 P
mo
1 1 _3"20Lp/R
Thus the effect is AR R
Am0 m 0 ,),2 T
1 ,),2 '
"),2 = R / ~ p : gamma of transition energy. If the particles approach the transition energy of the machine, the effect will be magnified considerably owing to the resonant denominator. For instance, in the SPS the transition energy corresponds to 3'T = 24. Working at an energy with 3' = 23.9, a possible mass difference of +1 This was brought to my attention by D. M6hl. ,2 This was pointed out to me by S. van der Meer.
323
Volume 77B, number 3
PHYSICS LETTERS
10 - 5 would cause a beam separation o f 2.3 mm. With a suitable position pick-up system looking at alternating p and ~ bunches, a space resolution of this order should be feasible and could possibly still be increased by an order of magnitude. Up to now, we have not gone into much experimental detail here. It is clear that the precision of the measurement depends strongly on the quality and intensity of the beams. But this is in any case one of the basic premises for experiments with high-energy antiprotons, as planned for instance for the SPS. The last experiment described here would then be straightforward. On the other hand, low-energy rings for the accumulation and storage of antiprotons are needed as an antiproton source, and the deceleration of antiprotons for experiments at lower energies is also discussed [16]. The low-energy experiments would need a ring able to hold a lower momentum of 50 MeV/c, and to accomodate protons and antiprotons simultaneously. The confinement of the beam into an area of less than 1 cm 2 would be required. Recently, phasespace cooling has made considerable progress. Either of the proposed cooling schemes [17,18] could be applied. The Initial Cooling Experiment (ICE) at CERN with stored protons of 1.73 GeV/c has shown that stochastic cooling of horizontal and vertical betatron oscillations and the reduction of the momentum spread can be achieved and that the beam lifetimes can be increased by a factor of 40 [19]. With electron cooling a momentum spread of 10 - 5 - 1 0 - 6 and a beam cross section of 1 mm 2 were obtained [18]. If these methods work equally well for antiprotons, a precision below the ppm level should be achievable. For the determination of the collision point a reasonable luminosity is needed, and for a long beam lifetime a high vacuum ( ~ 10 - 1 0 Torr) is required. It should be noted that the sensitivity of the proposed experiments can be tested independently if the antiproton is replaced by an H - , which can be abundantly produced. Here, a real mass difference between the proton and the H - , of the order of 10 - 3 , is present, which should show a large effect. Comparing the antiproton mass to the H - mass in a mass spectrometer could also be a first-step experiment. The possibility to collect and store antiprotons permits high-precision measurements on other p a r t i c l e antiparticle properties. Recently the limits o f the anti324
14 August 1978
proton lifetime were comprehensively discussed [20] and possible decay modes were proposed. Decay rates accessible to measurements were calculated in detail. It was estimated [20] that with a ring storing 1012 ~ / h a search for an antiproton decay in the lifetime region up to 1014 s should be feasible. As mentioned before, the equality of the absolute value of charge for particle and antiparticle was assumed so far. A deviation from this could also be studied with the help of a storage ring. This asymmetry is favourably investigated by forming a ~p system and searching for a residual charge, or by examination of the charges o f its decay modes. For the comparison o f the magnetic moments, polarized beams are needed. Whilst this can be achieved for protons, it is difficult to pile up a reasonable amount of polarized antiprotons. In general, a low-energy, high-intensity antiproton ring would open up a new dimension for an ample variety of experiments for testing fundamental principles. I am very grateful to T.E.O. Ericson, S. van der Meer, D. M6hl and L. Tauscher for illuminating discussions and suggestions.
References [ 1] O. Chamberlain, E. Segr6, C.E. Wiegand and T. Ypsilantis, Phys. Rev. 100 (1955) 947. [2] F. Bonaudi, S. van der Meer and B. Pope, Antiprotons in the SPS, CERN/DG-2 (1977); P. Strolin, A. Donnachie, K. Htibner, G. Matthiae, M. Braccini, K. Hansen, F. Vannucci and U. Gastaldi, pF at the ISR, CERN-ISR-Workshop 2-9 (1978); G. Backenstoss, Atomic systems of muons and hadrons, CERN-PS-CD1-77-44 (1977); B. Povh, Unconventional tools for physics; beams of antiprotons and relativistic heavy ions, CERN-PS-CDI-77-49 (1977). [3] G Liiders, Ann. Phys. (NY) 2 (1957) 1. [4] C.S. Wu et al., Phys. Rev. 105 (1957) 1413. [5] J.H. Christenson, J.W. Cronin, V.L. Fitch and R. Turlay, Phys. Rev. Lett. 13 (1964) 138. [6] K.R. Schubert et al., Phys. Lett. 31B (1970) 662. [7] S.I. Serednyakov et al., Phys. Lett. 66B (1977) 102. [8] V.W. Hughes, Proc. 6th Intern. Conf. on High-energy physics and nuclear structure (Santa Fe, 1975) eds. D.E. Nagle, A.S. Goldhaber, C.K. ttargrove, R.L. Burman and B.G. Storms, AIP Conf. Proc. No. 26 (New York, 1975) p. 515; D.E. Casperson et al., Phys. Rev. Lett. 38 (1977) 956.
Volume 77B, number 3
PHYSICS LETTERS
[9] J. Bailey et al., Nature 268 (1977) 301. [10] M.P. Baladin et al., Sov. Phys. JETP 40 (1974) 811; M.E. Nordberg et al., Phys. Lett. 24B (1967) 594. [1l] J. Bailey et al., Phys. Lett. 67B (1977) 225. [12] (a) V.N. Marushenko et al., JETP Lett. 23 (1976) 72; A.L. Carter et al., Phys. Rev. Lett. 37 (1976) 1380; (b) W. Daum et al., Phys. Lett. 60B (1976) 380. [13] Particle Data Group, Rev. Mod. Sci. 48 (1976) 1. [14] (a) E. Hu et al., Nucl. Phys. A245 (1975) 403; (b) P. Roberson et al., Phys. Rev. C161 (1977) 1945. [15] S.N. Ganguli et al., Phys. Lett. 74B (1978) 130. [16] K. Kilian, U. Gastaldi and D. M6hl, Deceleration of antiprotons for physics at low energies, CERN/PS/DL77-19 (1977).
14 August 1978
[17] S. van der Meer, CERN-ISR-PS/72-31 (1972). [18] G.T. Budker et ah, Novosibirsk preprint IAF 76-32 (1976); see also: Studies on electron cooling of heavy particle beams made by the VAPP-NAP group at the Nuclear Physics Institute of the Siberian branch of the USSR Academy of Science at Novosibirsk, report CERN 77-08 (1977). [19] S. van der Meer and G. Petrucci, private communication. [20] D. Cline, P. Mclntyre and C. Rubbia, Phys. Lett. 66B (1977) 429.
325
PHYSICS LETTERS
14 August 1978
TEST OF CPT WITH ANTIPROTONS H. POTH 1 Institut ff~r Kernphysik, Kernforschungszentrum and University, Karlsruhe, West Germany
Received 29 May 1978
It is proposed to study the particle-antiparticle symmetry in a ring accommodating protons and antiprotons simultaneously. Experiments to measure a possible proton-antiproton mass difference, and to test the Dirac theory and CPT theorem, are described.
Only a little more than 20 years after their discovery [1 ] antiprotons are going to become one of the most engrossing particles in accelerator physics [2]. They can be copiously produced by means of highenergy accelerators, and with the recent advent of novel phase-space cooling techniques large numbers of them could be accumulated and stored. From the accelerator point of view the antiproton is just a negatively charged proton. However, its quark content makes it superior to the proton for the production of new particles and resonances. Dirac's theory created the particle and antiparticle picture and stimulated the discovery of the positron and the antiproton. The properties of particles and antiparticles are related by the fundamental principle of CPT invariance [3]. The combined operation of charge conjugation, parity, and time reversal converts a particle into its antiparticle. As a consequence, they have in common the absolute values of their characterizing properties, whereas the sign may be opposite owing to the different behaviour of polar and axial vectors under parity and time reversal. For instance, the sign of the magnetic moment is inverted for the antiparticle. These theories are one of the bases of modern physics, and the particle-antiparticle picture is widely applied in elementary particle physics. Although it was discovered that P [ 4 ] , CP [5], and T [6] invariances are broken separately, it is commonly assumed that 1 Visitor at CERN, Geneva, Switzerland.
the CPT theorem holds. Experimentally its validity can be tested only to certain limits given by the experimental technique, rather than be confirmed in general. This has been done by comparing the properties of particles and antiparticles such as the mass, lifetime, magnetic moment, and g-factors. Table 1, which is not meant to be complete, shows the precision achieved for the different particles [ 7 - 1 5 ] . It can be seen that the precisions for leptons are in general higher than they are for hadrons. There is a non-vanishing value (2o effect) for the mass difference between proton and antiproton. The antiproton mass and magnetic moment have been deduced from the energies and the fine-structure splitting of X-ray transitions in antiprotonic atoms. The errors come mainly from statistics and from the energy resolution of the semiconductor X-ray detectors used. With an intense antiproton beam comparable to pion beams at meson factories (e.g. 106 particles/s), and a crystal spectrometer, a precision a little below 10 -5 in the X-ray energies could be reached. This would result in a similar precision for the mass and about two orders of magnitude less precision for the value of the magnetic moment, which would be a considerable improvement. To achieve a much higher precision is difficult since, in order to derive the mass from the transition energies, the contributions of strong interaction and of higher order electromagnetic effects have to be ascertained in detail. Such contributions are: radiative corrections, electron screening, 321
Volume 77B, number 3
PHYSICS LETTERS
14 August 1978
Table 1 Experimental results comparing the properties of charged particles and antiparticles (m is the mass, r the lifetime, u the magnetic moment, and g the g-factor of the particles). The charge is indicated by the positive and negative sign. Particle
Fractional difference m+- m_ Am
=
-
#+-~_
T+--T_ AT
m
= -
A~
-
7"
--
~r K p-
-(2.4 -+4.7) × 10 -5 [121 b) -(1.3_+ 1.l)× 10 -4 [13] c) (7.6 _+3.9) X 10 -5 [14]
gg
< 1.2 X 10 -8 [7] (2.6_+ 1.7)X 10 -8 [11]
e
- ( 2 + 5) X 10 -6 [8]
g+ ,5,g = "
#
(7.9 -+ 10) × 10 -4 [9] a) -(4 -+9) X 10-4 [10] (5 -+ 7) × 10 -3 [13] (1.1_+0.9)× 10-3 [13] r~-> 1.2 × 10-4 s [15]
-(0.4-+ 7.2) X 10 -3 [14a]
l
a) In ref. [9], the muon lifetimes were measured at 3' = 29.33. b) Using the averaged ~r- mass (139 568.2 -+ 1.5) keV from the measurements of ref. [ 12a] and, assuming a vanishing V# rest mass, the 7r+ mass is calculated to be (139 564.9 -+6.4) keV from the measurement of ref. [12b]. c) Using the 7r mass difference of ref. [ 12 ]. cascade effects, nuclear polarization, and resonance between orbital and nuclear states. The magnitude of these effects is difficult to calculate at the above mentioned level of precision. Experimentally the level of accuracy is also limited by the precision of the energy standards, and systematic errors are difficult to estimate. Since the difference between two large numbers always suffers from the composite r.m.s, error, the ~ atom method has the principal disadvantage that the antiproton mass is measured absolutely and then compared to the proton mass. The aim of this paper is to propose a method whereby the mass difference between proton and antiproton can be measured and the CPT theorem can be tested directly. The principle is outlined in the description of a few experiments. A violation of CPT can as well be provoked by a difference in the absolute 'value of the electrical charge of particle and antiparticle. This will be taken up again at the end of this paper, whilst in the following the equality of these quantities is assumed. The basic idea of this method is to use an accelerator or storage ring as a symmetric mass separator, and to apply the fact that the antiproton is a proton with opposite charge which runs backward in time. This means that in practice it follows the same path as 322
the proton in any magnetic field but in the opposite direction of motion. This principle is used, for instance, in the CERN Super Proton Synchrotron (SPS) and the Fermilab accelerator for the envisaged simultaneous acceleration and storage of antiprotons and protons. In order to measure rest mass differences, it is necessary to accumulate and store sufficient antiprotons and to work at non-relativistic energies. At high energies, a small change in m o m e n t u m will change the mass considerably; however, this may be overcome as shown later. At low energies the antiproton mass is measured in a mass spectrometer which is calibrated with protons. Basically every mass spectrometer consists of a magnetic and an electric field. In our case the magnetic field could be provided by the field of a low-energy storage ring and would fix the m o m e n t u m of the antiprotons. If these are ejected with a defined m o m e n t u m and sent through an electric field perpendicular to their trajectory, they will be deflected according to their mass. This deflection can then be compared to the deflection of protons which have been stored in the same ring and passed through the same field. To reach the ppm level seems to be very difficult, since the particle m o m e n t u m has to be defined with even higher precision, and the beam size has to be very
Volume 77B, number 3
PHYSICS LETTERS
14 August 1978
In each turn this would cause a phase shift 6 of the antiproton with respect to the proton:
on identical paths, they will intersect on each turn. Some o f them will then collide and annihilate. The annihilation produces many pions, which mark the collision point. If there is a difference in revolution frequency, this collision point will circulate around the ring, and this is indicated by the pions. The circulation frequency corresponds to the magnitude of the mass difference, and the direction to its sign. However, the revolution frequency can be measured even for unbunched beams. This can be achieved by picking up the Schottky noise signal of the beams * 1. This signal has a certain frequency distribution, centred around the average beam frequency, and a width characterizing the momentum spread of the beam. The technique is commonly used for beam control. An absolute precision at the ppm level can be reached. Even at highly relativistic energies there exists a possibility of measuring a rest mass difference* 2. By imposing a common radio frequency on bunches of protons and antiprotons, these can be forced to circulate in a ring, with the same revolution time. A possible mass difference will appear as a slight difference in momentum A p , and thus as a difference AR in the average radius of the orbit:
8 = Aco. T
ARIR = (%IR)(Aplp) ,
small in order to measure the deflection accurately. There exists, however, a completely different method for measuring the mass difference directly. Again, use is made o f the fact that protons and antiprotons may be stored in the same ring at low energies. The orbiting particle is characterized by its revolution frequency: = coclT,
w c = e B / m O, 3' = l/x/1 _ / 3 2 ,
where e is the charge and m 0 the rest mass of the particle circulating in a magnetic field B with the fraction /3 of the velocity o f light. The important fact is that for non-relativistic energies this frequency is independent o f the radius of the orbit and is inversely proportional to the rest mass of the particle. It is only for relativistic velocities that a variation in 3' may cover rest mass differences. This means that a difference in mass would show up as a difference Aco in the cyclotron frequency: COp = COp + ACO .
(T is the revolution time). Although not measurable for one cycle, it would grow with the number n of revolutions: fin = n" Aco" T = n A w l w .
If, for instance, the masses are different by 10 - 6 , the antiproton would have completed one more cycle than the proton after 106 revolutions or vice versa. For a I0 m ring and a momentum of 100 MeV/c, this would take only some seconds. There are several possibilities for measuring such a phase shift or beat frequency. It is assumed that the beams are bunched in the initial state o f measurement. Then, one way is to measure the time between subsequent beam pick-up signals (one from the protons, the other from the antiprotons). This time is constant if the revolution frequencies are equal; if there is a difference, it oscillates according to the beat frequency. However, the fact that antiprotons and protons may annihilate during collisions offers another possibility of measuring a difference in the revolution frequencies. Since the bunches of protons and antiprotons travel
~p: momentum compaction, p: momentum o f the particle, R: radius of the orbit, where A p / p is determined by the rest mass difference Am 0 and the imposed frequency: A p _ Am0 P
mo
1 1 _3"20Lp/R
Thus the effect is AR R
Am0 m 0 ,),2 T
1 ,),2 '
"),2 = R / ~ p : gamma of transition energy. If the particles approach the transition energy of the machine, the effect will be magnified considerably owing to the resonant denominator. For instance, in the SPS the transition energy corresponds to 3'T = 24. Working at an energy with 3' = 23.9, a possible mass difference of +1 This was brought to my attention by D. M6hl. ,2 This was pointed out to me by S. van der Meer.
323
Volume 77B, number 3
PHYSICS LETTERS
10 - 5 would cause a beam separation o f 2.3 mm. With a suitable position pick-up system looking at alternating p and ~ bunches, a space resolution of this order should be feasible and could possibly still be increased by an order of magnitude. Up to now, we have not gone into much experimental detail here. It is clear that the precision of the measurement depends strongly on the quality and intensity of the beams. But this is in any case one of the basic premises for experiments with high-energy antiprotons, as planned for instance for the SPS. The last experiment described here would then be straightforward. On the other hand, low-energy rings for the accumulation and storage of antiprotons are needed as an antiproton source, and the deceleration of antiprotons for experiments at lower energies is also discussed [16]. The low-energy experiments would need a ring able to hold a lower momentum of 50 MeV/c, and to accomodate protons and antiprotons simultaneously. The confinement of the beam into an area of less than 1 cm 2 would be required. Recently, phasespace cooling has made considerable progress. Either of the proposed cooling schemes [17,18] could be applied. The Initial Cooling Experiment (ICE) at CERN with stored protons of 1.73 GeV/c has shown that stochastic cooling of horizontal and vertical betatron oscillations and the reduction of the momentum spread can be achieved and that the beam lifetimes can be increased by a factor of 40 [19]. With electron cooling a momentum spread of 10 - 5 - 1 0 - 6 and a beam cross section of 1 mm 2 were obtained [18]. If these methods work equally well for antiprotons, a precision below the ppm level should be achievable. For the determination of the collision point a reasonable luminosity is needed, and for a long beam lifetime a high vacuum ( ~ 10 - 1 0 Torr) is required. It should be noted that the sensitivity of the proposed experiments can be tested independently if the antiproton is replaced by an H - , which can be abundantly produced. Here, a real mass difference between the proton and the H - , of the order of 10 - 3 , is present, which should show a large effect. Comparing the antiproton mass to the H - mass in a mass spectrometer could also be a first-step experiment. The possibility to collect and store antiprotons permits high-precision measurements on other p a r t i c l e antiparticle properties. Recently the limits o f the anti324
14 August 1978
proton lifetime were comprehensively discussed [20] and possible decay modes were proposed. Decay rates accessible to measurements were calculated in detail. It was estimated [20] that with a ring storing 1012 ~ / h a search for an antiproton decay in the lifetime region up to 1014 s should be feasible. As mentioned before, the equality of the absolute value of charge for particle and antiparticle was assumed so far. A deviation from this could also be studied with the help of a storage ring. This asymmetry is favourably investigated by forming a ~p system and searching for a residual charge, or by examination of the charges o f its decay modes. For the comparison o f the magnetic moments, polarized beams are needed. Whilst this can be achieved for protons, it is difficult to pile up a reasonable amount of polarized antiprotons. In general, a low-energy, high-intensity antiproton ring would open up a new dimension for an ample variety of experiments for testing fundamental principles. I am very grateful to T.E.O. Ericson, S. van der Meer, D. M6hl and L. Tauscher for illuminating discussions and suggestions.
References [ 1] O. Chamberlain, E. Segr6, C.E. Wiegand and T. Ypsilantis, Phys. Rev. 100 (1955) 947. [2] F. Bonaudi, S. van der Meer and B. Pope, Antiprotons in the SPS, CERN/DG-2 (1977); P. Strolin, A. Donnachie, K. Htibner, G. Matthiae, M. Braccini, K. Hansen, F. Vannucci and U. Gastaldi, pF at the ISR, CERN-ISR-Workshop 2-9 (1978); G. Backenstoss, Atomic systems of muons and hadrons, CERN-PS-CD1-77-44 (1977); B. Povh, Unconventional tools for physics; beams of antiprotons and relativistic heavy ions, CERN-PS-CDI-77-49 (1977). [3] G Liiders, Ann. Phys. (NY) 2 (1957) 1. [4] C.S. Wu et al., Phys. Rev. 105 (1957) 1413. [5] J.H. Christenson, J.W. Cronin, V.L. Fitch and R. Turlay, Phys. Rev. Lett. 13 (1964) 138. [6] K.R. Schubert et al., Phys. Lett. 31B (1970) 662. [7] S.I. Serednyakov et al., Phys. Lett. 66B (1977) 102. [8] V.W. Hughes, Proc. 6th Intern. Conf. on High-energy physics and nuclear structure (Santa Fe, 1975) eds. D.E. Nagle, A.S. Goldhaber, C.K. ttargrove, R.L. Burman and B.G. Storms, AIP Conf. Proc. No. 26 (New York, 1975) p. 515; D.E. Casperson et al., Phys. Rev. Lett. 38 (1977) 956.
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[9] J. Bailey et al., Nature 268 (1977) 301. [10] M.P. Baladin et al., Sov. Phys. JETP 40 (1974) 811; M.E. Nordberg et al., Phys. Lett. 24B (1967) 594. [1l] J. Bailey et al., Phys. Lett. 67B (1977) 225. [12] (a) V.N. Marushenko et al., JETP Lett. 23 (1976) 72; A.L. Carter et al., Phys. Rev. Lett. 37 (1976) 1380; (b) W. Daum et al., Phys. Lett. 60B (1976) 380. [13] Particle Data Group, Rev. Mod. Sci. 48 (1976) 1. [14] (a) E. Hu et al., Nucl. Phys. A245 (1975) 403; (b) P. Roberson et al., Phys. Rev. C161 (1977) 1945. [15] S.N. Ganguli et al., Phys. Lett. 74B (1978) 130. [16] K. Kilian, U. Gastaldi and D. M6hl, Deceleration of antiprotons for physics at low energies, CERN/PS/DL77-19 (1977).
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[17] S. van der Meer, CERN-ISR-PS/72-31 (1972). [18] G.T. Budker et ah, Novosibirsk preprint IAF 76-32 (1976); see also: Studies on electron cooling of heavy particle beams made by the VAPP-NAP group at the Nuclear Physics Institute of the Siberian branch of the USSR Academy of Science at Novosibirsk, report CERN 77-08 (1977). [19] S. van der Meer and G. Petrucci, private communication. [20] D. Cline, P. Mclntyre and C. Rubbia, Phys. Lett. 66B (1977) 429.
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