sign in sign up
<<
Home , Parity (physics)

Parity Violation in Nuclear Physics

signature of the weak

Gerald T. Garvey and Susan J. Seestrom

he elegant and concise negative sign. This result has impli- coordinate axes is inverted, such as description of the physical cations for the well-known solar- in the mirror reflection shown in Tworld contained in the stan- puzzle. The findings of Figure 1. In the second type of pari- dard model of electromagnetic, Laboratory scientists support the ty transformation, all three axes are weak, and strong interactions (see and have been simultaneously inverted (x goes to “Unification of Nature’s Fundamental among the most sensitive tests of its −x, y goes to −y, and z goes to −z), ”) is supported by a vast body validity. However, since aspects of and again the description of the of experimental data. Often only the this research have been reported in familiar forces remains unchanged. first or the most precise experiments previous editions of Los Alamos Such interactions are said to be pari- are cited as providing the requisite Science, it seems opportune to dis- ty-conserving. For a long body of supporting data. However, cuss another area of fundamental physicists thought that all basic the information and techniques research in nuclear and particle interactions must be parity-conserv- developed by the world community physics at the Laboratory. This tale ing. But if an interaction depends of experimental particle and nuclear involves the measurement of the on the “screw”-like behavior of par- physicists has provided the broad strength of the parity-violating inter- ticles, its description will not be base on which this powerful model actions between strongly interacting under a parity transforma- has been constructed. particles—for example, between two tion. Consider the screw in Figure Los Alamos experimentalists, or a and a . 1; it has right-handed threads, so particularly in the Physics and Before 1956 physicists believed when it is rotated as shown, it Medium- Physics Divisions, that all the fundamental interactions advances in the +z direction (up, in along with university users of the in nature would be unchanged by a the figure). Its mirror image, how- Los Alamos Physics Facility mirror reflection (or parity inver- ever, has left-handed threads. (LAMPF), have played a role in sion). Imagine a basic interaction Further, as the screw rotates, its mir- building that base. Many of the between two particles described in ror image rotates in the same direc- major contributions by Laboratory the orthogonal (x, tion but advances in the −z direction. scientists have been in the realm of y, and z) as shown in Figure 1. A Screws can be machined with neutrino physics. For example, mirror reflection that inverts the z- either right-handed or left-handed they have established the most accu- axis (z goes to −z) results in the con- threads, so their handedness (the rate upper limit on the of the figuration shown in the mirror relationship between the direction of antineutrino from very image. The familiar interactions, , or , and the direction of careful measurements of tritium beta such as and the interaction motion) is not an intrinsic property decay. They also made the first between electric charges, depend of nature. However, if an elemen- measurements of the scattering of only on the distance d between the tary particle with intrinsic spin has a electron from , interacting particles, and so a fixed handedness (a fixed relation- which showed that the interference description of those forces is com- ship between its spin direction and between the charged and neutral cur- pletely unchanged by a parity trans- its direction of motion), the descrip- rents of the has a formation in which any one of the tion of the particle will change

156 Los Alamos Science Number 21 1993

Garvey fig1 4/7/93 Parity Violation in Nuclear Physics: Signature of the Weak Force

z Motion B of screw A d

Sense of rotation

Right-handed under a parity transformation and it REALITY screw Right hand is said to violate parity conserva- y tion. Likewise if a basic interaction between particles involves only the Mirror left-handed or only the right-handed “screw”-like behavior of the parti- x cles, the interaction is said to violate parity conservation. Mirror image of right-handed screw looks like a left-handed screw In 1957 it was demonstrated that MIRROR IMAGE the interaction responsible for the Left hand of a neutron into a proton, Sense of an electron, and a neutrino violates rotation parity conservation. Specifically, when -60 nuclei, spinning in the same direction around the z-axis A in the presence of a in d B the +z direction, underwent beta Motion of screw image decay, they emitted more electrons with a component of in ∝ 1 Fgravity and is therefore unaffected by mirror reflection. the −z direction than in the +z direc- d2 tion. This result is not invariant ∝ 1 Felectrostatic and is therefore unaffected by mirror reflection. under a mirror reflection, indicating d2 that the interaction responsible for the decay process, called the weak Figure 1. Effects of a Mirror Reflection interaction, does not conserve parity. A mirror reflection is one type of parity transformation. In the figure the reflection Further, the direction and amount of inverts the z axis. The distance between points A and B is unchanged in the mirror indicated that the weak image, so that the descriptions of the gravitational and electrostatic forces between, interaction is left-handed and vio- say, two electrons located at A and B would would also be unchanged. In contrast, lates parity in a maximal way. The the mirror image of the right-handed screw is a left-handed screw. When turned in reason that the weak interaction is the direction indicated by the red arrows, the screw advances in the +z direction left-handed is because the carriers of whereas its mirror image advances in the °z direction. Note that if you the fingers the weak force (the particles that are of your right hand along the red arrow, your thumb points up, in the direction of motion exchanged in weak processes), name- of the right-handed screw. Alternatively, if you curl the fingers of your left hand along − ly the W+, W , and Z0 bosons, inter- the red arrow, your thumb points down, in the direction of motion of the screw’s left- act with the left-handed component handed mirror image. Forces that depend on the relationship of spin rotation to direc- of particles and the right-handed tion of motion violate parity conservation. component of . (It is interesting to note that although the massless particles with intrinsic spin ton plus an electron and antineutrino W+, W− , and Z0 bosons were postu- show a fixed handedness. A neutri- means that the neutron and proton, lated much earlier to unify the no always appears to spin clockwise which are known to interact through description of the electromagnetic when it is coming toward the the strong force, must also interact and weak interactions, they were first observer and is therefore a left- through the weak force; otherwise directly created and observed at the handed particle, whereas the anti- they could not be involved in weak European Center for Nuclear neutrino appears to spin counter- decay processes. One can therefore Research (CERN) in 1982.) clockwise when it is coming toward ask how the weak force affects the Neutrinos and antineutrinos are the observer and is therefore a right- interaction between two particles that participate in, as far as handed . (the neutron and the proton look the we know, only weak interactions. The fact that the neutron decays same to the strong force and are Thus, it is not a surprise that these via the weak interaction into a pro- both called nucleons). The strong

1993 Number 21 Los Alamos Science 157 Parity Violation in Nuclear Physics: Signature of the Weak Force

Garvey fig2 4/5/93

(a) Mirror Reflection of Particle with Intrinsic Spin force dominates the interaction Particle is spinning clockwise between two nucleons; specifically, from observer's viewpoint the ratio of the strength of the weak interaction to that of the is about 1 to 10 million. Typically the effects of such a small Mirror interaction would be next to impos- sible to detect, but the weak interac- Observer tion has a unique signature in that it Mirror Image is the only interaction in the stan- Mirror image of particle is spinning counter- dard model that violates parity. clockwise from observer's viewpoint Hence measurement of the amount of parity violation in a given process is a direct measure of the role (b) Definition of Spin Polarization played by the weak interaction in Right-handed that process. = As was mentioned earlier, parity violation was discovered in 1957, but Particle is spinning counter- Spin is said to be polarized it was not until seven years later that clockwise from observer's along the direction the first clear parity-violating effect viewpoint of motion was measured in processes other than weak decays of nuclei. In 1964 a Left-handed headed by Yuri G. Abov in the = Observer then Soviet Union observed parity violation in the capture of polarized Particle is spinning clockwise Spin is said to be polarized neutrons by the nucleus cadmium- from observer's viewpoint opposite to the direction of motion 113. The gamma rays emitted fol- lowing the neutron capture were Direction of motion Sense of rotation Spin polarization emitted preferentially in the direction of the neutron polarization, which indicated that parity was violated. Thus the weak interaction between the nucleons within the nucleus was Figure 2. Mirror Reflection of a Particle with Intrinsic Spin producing measurable effects. (a) A proton is moving toward the observer and is spinning around the direction of Unfortunately the complexity of the motion as indicated by the red arrow. The mirror image of the proton is also relative motions of the nucleons in shown. To the observer, the proton spins clockwise, whereas the mirror image of the cadmium nucleus made it impos- the proton spins counterclockwise as both are moving toward the observer. sible to determine the strength of the Therefore if parity is conserved, the probability that a proton is scattered by a tar- weak interaction between pairs of get should be independent of its spin direction, provided that the target nuclei are nucleons from that experiment. spinning in random directions. (b) The direction of spin is often represented by a In 1970 a Los Alamos group led vector that is along the axis of spin rotation. Here the axis of spin rotation of a pro- by Hans Frauenfelder, Dick Mishke, ton is parallel to its direction of motion. According to convention, when the relation and Darrah Nagle began investigat- between the rotation and the motion is like that of a right-handed screw, the spin ing parity violation in the scattering vector points in the same direction as the direction of motion and the proton is said of from protons. For their to have its spin polarized along the direction of motion. When the relation between first experiments they used the the rotation and the motion is like that of a left-handed screw, the spin vector points polarized ion source installed by Joe opposite to the direction of motion and the proton is said to have its spin polarized McKibben in the tandem Van de opposite to the direction of motion. Graaff accelerator. The polarized

158 Los Alamos Science Number 21 1993 Parity Violation in Nuclear Physics: Signature of the Weak Force

Garvey fig3 4/5/93 ion source and its subsequent ver- 104 sions were essential to the experi- 232 ment because they produce a beam l = 0 Th of protons all of which are spinning in the same direction. The proton has intrinsic spin but 103 s)

no intrinsic handedness, so its spin rn direction can be changed relative to its direction of motion. Figure 2 illustrates that the ability to manipu- late the proton’s spin direction in a 102 known and controlled way is valu- able in investigating the degree of l = 1 parity violation in scattering two protons from one another. Figure 2a 101 depicts a fast-moving proton such as Neutron total cross section (ba would be found in a proton beam from an accelerator, as well as its mirror image. The proton is moving to the right and appears to be spin- 100 ning clockwise to the observer at 50 100 150 200 250 300 350 400 450 right. (It is behaving like a left- Neutron energy (eV) handed screw.) In the mirror image the proton is again moving to the Figure 3. Total Cross Section for Scattering and Absorption of right but appears to be spinning Neutrons by 232Th counterclockwise to the observer. The total cross section (or probability) for the interaction of neutrons with 232Th is plot- (It is behaving like a right-handed ted as a function of neutron energy. The many sharp peaks in the cross section are screw.) Thus, if the principle of called resonances and occur when the neutron energy equals the energy of an excited parity conservation applies, protons state of the compound nucleus 233Th and can therefore be absorbed by 232Th. The tall rotating clockwise or counterclock- peaks occur at of nuclear states with orbital equal to zero wise relative to their direction of (l = 0) and the small peaks (lower by two orders of magnitude) occur at energies of motion (or, as defined in Figure 2b, nuclear states with l = 1. Both types of resonances can be studied with great sensitivity with their spins polarized either for the parity-violation effects, which are expected for l = 1 but not for l = 0 resonances. along or opposite the direction of motion) should be scattered identi- was measured in each case. The Van tons scatter mainly as a result of cally from a target composed of pro- de Graaff experiment began in the strong interactions, so the difference tons that are spinning in random early seventies and was not conclud- (resulting from the weak interac- directions. ed until the end of the decade. It tions) between the fraction of parti- A container of hydrogen provides was the first scattering experiment cles scattered for two different spin a suitable target because the average anywhere in the world in which par- polarizations was only 2 parts in 100 spin of the protons (hydrogen ity violation was observed. Protons million. Thus a variety of new tech- nuclei) in the target is zero. In the with spins polarized along their niques had to be invented to make Van de Graaff experiment protons direction of motion were scattered the measurement possible. After the polarized along the direction of slightly more often than those with Van de Graaff experiment was com- motion were scattered from the tar- opposite polarization. The reason pleted, the research group carried get, and then protons polarized in the experiment took so long to carry out a further measurement of parity the opposite direction were scat- out was that the measured asymme- violation using a much-higher-ener- tered. The total scattering cross sec- try between the two neutron polar- gy polarized proton beam available tion, or probability of scattering, izations was very small. The pro- at LAMPF. The observed effects

1993 Number 21 Los Alamos Science 159 Parity Violation in Nuclear Physics: Signature of the Weak Force

Garvey fig4 4/7/93

Figure 4. Setup for Parity- Tungsten spallation target 800-MeV protons Violation Experiment at LANSCE 0 Neutrons are produced by interaction of Neutron moderator 800-MeV protons with a split tungsten target shown at the top of the figure. 4.6 Radiation shield The energies of the neutrons so pro- Unpolarized neutron beam duced range from almost zero to nearly

Flux monitor 800 MeV. The neutrons pass first through a moderator that reduces the

energy of the neutrons to the eV or keV 6.0 Spin filter

range. Because the neutrons are pro- Polarized neutron beam duced in pulses and because the time required to produce and moderate the Shielded cave neutrons is small compared to their time of flight to a detector 56 meters away, the energy of each detected neutron can Nuclear

sample be measured from its measured time of 12 flight. The beam of moderated neutrons

passes through a spin filter—a material Distance from spallation target (meters) in which the proton spins have been aligned in the same direction as the Vacuum beam pipe direction of motion of the neutrons (large red arrows). Those neutrons whose

spin directions are opposite that of the 56 Detector protons in the spin filter are absorbed or

scattered out of the beam. The neutrons with the same spin direction as the pro- Data-acquisition tons interact more weakly with the pro- system tons and remain in the beam. The neu- tron beam emerging from the spin filter were only somewhat larger, at the motion (see Figure 2). The cross contains neutrons with only one spin level of a few parts in 10 million, sections differed depending on the direction (small red arrows) rather than which is near the expected value at polarization of the incident neutrons, both and is thus polarized. As the polar- that energy. However, because the indicating parity violation. It was ized neutron beam passes through the parity-violating effect was so small, again, however, impossible to deduce sample, its intensity is reduced as neu- experimental errors were about the the strength of the weak interaction trons are absorbed by or scattered from same size as the effect and so the between two nucleons from the the nuclei in the sample. A detector precise strength of the weak force observed degree of parity violation in measures the number and the of between two nucleons could not be these experiments because even arrival of the neutrons that are transmit- determined. though the effect of the weak force ted through the sample. The polariza- In the meantime research groups was amplified many times by nuclear tion of the neutron beam can be in the Soviet Union were reporting motions, the amount of amplification reversed (by reversing the polarization parity-violating effects one million could not be quantified. of the protons in the spin filter) and the times larger in the absorption of Figure 3 shows the measured prob- experiment repeated. If the measured very-low-energy polarized neutrons ability of a low-energy neutron inter- fraction of neutrons transmitted through by certain heavy nuclei. Neutrons acting with a thorium-232 nucleus as the sample at a given resonance energy carry the same amount of intrinsic a function of energy of the incident is different for one neutron polarization spin as protons do and, like the pro- neutron. The series of large bumps than for the other, then that resonance ton, their spins can be polarized evident in the data appear at the ener- exhibits parity violation. along or opposite to the direction of gies of quantum states of the com-

160 Los Alamos Science Number 21 1993 Parity Violation in Nuclear Physics: Signature of the Weak Force

Garvey fig5 4/5/93 pound thorium-233 nucleus. When the energy of the incident neutron 232Th corresponds to the energy of one of these states, the neutron is said to be at a resonance and the incident neu- tron can readily share its energy with the neutrons and protons in the thori- um target nuclei. In other words, at a resonance there is a large probability l = 1 for the neutron to be absorbed into a

thorium nucleus. At other energies Neutron transmission l = 0 the neutron is much less likely to interact, and when it does interact, it is simply deflected from the 232Th nucleus without sharing its energy. If the probability of absorption at a 0 100 200 300 400 resonance depends on the spin polar- Neutron energy (eV) ization of the incident neutron, then the resonance process exhibits parity Figure 5. Neutron Transmission Spectrum of 232Th violation. Shown here are the measured values for the fraction of neutrons transmitted through The members of the Triple a 232Th sample as a function of neutron energy. The fraction of neutrons transmitted Collaboration* realized that if parity decreases sharply when the neutron energy is at a resonance of 232Th. That is, the violation could be measured at sev- neutrons can be absorbed by 232Th to form an excited state of the compound nucleus eral resonances of the same nucleus, 233Th. The large dips are at the energies of the l = 0 states of the compound nucleus one could determine an average 233Th; these resonances do not exhibit parity violation. Smaller dips, such as the value of the magnitude of the parity three between 80 and 110 eV, are at the energies of the l = 1 states of the compound violation that would be independent nucleus 233Th, which can exhibit parity violation and are therefore of interest in these of the statistical properties of the experiments. nuclear motions and therefore could be used to estimate the strength of cation depends on statistical proper- the principal advantages of the the underlying -nucleon ties of the quantum states of the LANSCE neutron source over reac- weak interaction. As indicated nucleus and therefore the amplifica- tor neutron sources is that the range above, the effect of parity violation tion has a random distribution. By of available neutron energies is on nucleon-nucleon interactions is averaging the parity-violating much greater. A typical reactor neu- very small. However, the effect is effects observed at many reso- tron source has a very limited flux amplified by a factor of about 1 mil- nances, the average value of the of neutrons with energies above 10 lion by the motion of many nucleons amplification can be determined. eV, whereas Figure 3 shows that in the nucleus to yield the large par- Our theoretical models of the nucle- measuring the parity violation at a ity violations observed for the scat- us are sufficiently detailed to deduce number of resonances of 232Th tering of a neutron by a nucleus at a from the observed average amplifi- requires the availability of neutrons resonance. The size of the amplifi- cation in a nucleus containing many at energies ranging up to several nucleons a fairly good estimate of hundred eV. *The Triple Collaboration is a collaboration between the strength of the weak interaction Since the features of the LANSCE Los Alamos National Laboratory, North Carolina between two nucleons. facility are essential to the successful State University, Duke University, Triangle Universities Nuclear Laboratory, TRIUMF (Canada), The Triple Collaboration also real- experimental program undertaken by University of Technology at Delft (The Netherlands), ized that the Los Alamos Neutron the Triple Collaboration, we describe KEK (Japan), and the Joint Institute for Nuclear Scattering Center (LANSCE) was an it briefly. The major elements of the Research (Russia). The collaboration was formed to study fundamental symmetries using polarized neu- ideal facility at which to carry out facility are LAMPF, which provides trons at LANSCE. the necessary measurements. One of 500- to 700-microsecond-long trains

1993 Number 21 Los Alamos Science 161 Parity Violation in Nuclear Physics: Signature of the Weak Force

Garvey fig6 4/5/93

× × × Figure 4 shows a schematic setup × of the experiment and indicates how × n+ a beam of polarized neutrons is pro- 232Th × Ð × duced. The neutrons produced in the n tungsten target are not polarized; × that is, the spin of each has an equal probability of pointing along or × opposite its direction of motion. They are passed through a spin filter, × × a special material in which the pro- × × tons in water (or hydrocarbon) mole- × cules are polarized along the direc- Neutron transmission × tion of motion of the incoming neu- × × tron beam. The neutrons with spins polarized opposite to those of the × × protons in the spin filter interact × × most strongly with those protons and are therefore scattered out of the 38.0 38.1 38.2 38.3 38.4 38.5 beam. Thus the neutrons that pass Neutron energy (eV) through the spin filter are those with spins polarized along the direction of Figure 6. Parity Violation in a Neutron Resonance of 232Th motion. In this way a polarized neu- Transmission data for two different neutron polarizations are shown near the l = 1, J tron beam is produced. By changing 1− 232 = Ð2 resonance of Th at 38.2 eV. Results for neutrons polarized along the direction the direction of proton polarization of motion (n+) are designated by circles. Results for neutrons of the opposite polar- in the spin filter, neutrons can be ization (n−) are designated by crosses. The circles and crosses are close together polarized either along or opposite over most of the energy range shown. At the resonance (the dip in the transmission their direction of motion. spectrum) the circles clearly fall below the crosses; that is, more neutrons polarized A beam of neutrons polarized in along the direction of motion are absorbed by 232Th than neutrons polarized opposite one direction is passed through a to the direction of motion. Thus this resonance exhibits parity violation. particular nuclear sample (for exam- ple, 232Th), and the experimenters of 250-nanosecond pulses of 800- takes the neutrons to travel to a measure the fraction that are trans- MeV H− ions; the Proton Storage detector some 50 meters away from mitted through the sample. The Ring, which combines the many short the area where they are produced. experiment is repeated with neutrons H− pulses in each train into a single, Therefore the measurement of a neu- polarized in the opposite direction. intense 250-nanosecond proton pulse; tron’s time of flight over the known If parity is conserved, the fraction and finally the LANSCE spallation distance from source to detector transmitted through the target sam- source, in which the incredibly gives a direct measurement of the ple would be the same for both intense pulse of protons is converted neutron’s speed and hence its energy. experiments. Figure 5 shows neu- to neutrons through the process of Thus LANSCE not only generates tron transmission through a 232Th spallation. That is, when the 800- large numbers of neutrons over the sample as a function of energy. The MeV protons impact the tungsten tar- necessary range of energies but also neutron transmission is reduced get, each proton liberates about 20 produces neutrons whose energies when the neutrons have the same neutrons from the neutron-rich tung- can be accurately measured. Only energy as a resonance because then sten nuclei. As a result each intense one requirement for studying parity the probability of neutron absorption pulse from the storage ring creates violation is missing: the neutrons by the nuclei is greatest. Figures 6 about 1013 neutrons all within a quar- coming from the spallation source do and 7 show examples of the data ter of a microsecond. This time is not have a definite polarization, or obtained for the transmission of neu- very short compared to the time it spin direction. trons with opposite polarizations

162 Los Alamos Science Number 21 1993 Parity Violation in Nuclear Physics: Signature of the Weak Force

Garvey fig7 4/5/93

232 238 through a Th target and a U l = 1 target, respectively. Figure 6 shows 238 0.1 U the transmission for the two neutron 1− polarizations for a J = Ð2 , l = 1 reso- nance of 232Th at 38.2 eV, where J is the total angular momentum of the l = 0 (percent) resonance, l is the orbital angular ε 0.0 momentum, and the minus sign denotes the parity of the resonance. (The parity of a resonance can be + or −, depending on whether the wave function of the resonance remains -0.1 unchanged or reverses sign under a parity inversion. Because the sign of the wave function is unmeasur- able, negative-parity wave functions 110 l = 1 do not violate parity conservation.) The top graph in Figure 7 is a plot 90 of the asymmetry parameter ε as a function of neutron energy for trans- 70 mission through 238U. The asymme- try ε is a measure of the difference in transmission for the two neutron 50 polarizations and makes small dif-

Neutron transmission (arbitrary units) Asymmetry l = 0 ferences easier to detect. The the 30 figure shows a small asymmetry for 62 64 66 68 70 72 1− 238 Neutron energy (eV) the J = Ð2 , l = 1 resonance of U at 64 eV and no asymmetry at the large 1+ 238 J = Ð2 , l = 0 resonance at 66 eV. Figure 7. Parity Violation in a Neutron Resonance of U This dependence of neutron trans- The parity-violating effect at the 64-eV, l = 1 resonance of 238U is smaller than that mission on the total angular momen- shown in Figure 6 and is more easily detected by plotting the results for the two neu- tum of the resonance and its parity tron polarizations in terms of the asymmetry parameter ε ≡ (T + − T −)/(T + + T −), where is a well-understood feature of the T + is the transmission for neutrons polarized along the direction of motion and T − is strong force, but beyond the scope the transmission for neutrons polarized opposite to the direction of motion. The of this article to explain. asymmetry parameter ε plotted in the top graph has statistical fluctuations over the The first experiment by the Triple energy range shown except at the 64-eV resonance where the asymmetry between Collaboration was performed on ura- the two neutron polarizations is 0.1 percent. The sum of the transmissions for both nium-238. Seventeen resonances polarizations (T + + T −) is shown in the bottom graph. The parity-violating resonance were examined and five showed mea- at l = 1 appears as a small dip in transmission at 64 eV, whereas an l = 0 resonance surable indicating parity at 66 eV appears as a large dip in transmission. As expected the plot of the asymme- violation. The experiment on 238U try parameter ε shows no asymmetry at the energy of the l = 0 resonance. was the first in which a research team had seen parity violation in more ranging between 1 and 10 percent. knowledge of the weak interaction than one resonance of a single nucle- This data sample is sufficiently large between two nucleons and should us. A later experiment on the isotope that a value can be extracted for the prove far more useful as experimen- 232Th (whose resonances are depict- average strength of the weak interac- tal techniques are improved and addi- ed in Figures 3, 5, and 6) studied tion between a single nucleon and all tional data are taken. The weak twenty-three resonances of which the nucleons in the 232Th nucleus. interaction between nucleons will seven had measurable asymmetries This result is serving to refine our eventually be understood in terms of

1993 Number 21 Los Alamos Science 163

Parity Violation in Nuclear Physics: Signature of the Weak Force

Garvey fig8 4/7/93

(a) Nucleon-Nucleon Weak Interaction (b) Process in (a) Shown at the Level. Showing Meson (M) Exchange. N ′ N

N N ′

M Quark Weak M Strong g interaction interaction W

N N ′

Gerald T. Garvey, currently a Laboratory Senior N ′ Fellow, was director of LAMPF from November N 1985 to August 1990. Among his past positions are directorship of Argonne National Laboratory’s Figure 8. The Nucleon-Nucleon Weak Interaction at the Quark Level Physics Division and professorships at the Universtiy of Chicago and Princeton University. (a) The weak interaction between nucleons N and N' is often described as an exchange He is an American Physical Society Fellow, a of the meson M, in which the open circle is a strong interaction meson-nucleon vertex, recipient of a Humboldt Award, and editor of and the solid circle is a weak interaction meson-nucleon vertex. (b) This cartoon of the Comments on Nuclear and . He weak interaction between nucleons N and N' shows the possible interactions that might received a B.S. from Fairfield University in 1956 and a Ph.D. from Yale Univeristy in 1962. take place involving the three composing each nucleon and the quark-antiquark pair composing the meson. The dotted line is a W boson, which carries the weak force. The spiral lines are , the carriers of the strong force. interactions among the quarks com- Further Reading posing the nucleons and the carriers of the weak force (Figure 8). C. M. Frankle, J. D. Bowman, J. E. Bush, P. P. J. Delheij, C. R. Gould, D. G. Haase, J. N. Knudson, One feature of the observed results G. E. Mitchell, S. Penttilä, H. Postma, N. R. is difficult to understand. There Roberson, S. J. Seestrom, J. J. Szymanski, S. H. seems to be a mysterious preference Yoo, V. W. Yuan, and X. Zhu. 1992. Parity non- conservation for neutron resonances in 232Th. for a positive sign to the asymmetry. Physical Review C 46: 778. That is, neutrons with spins polarized along the direction of motion tend to J. D. Bowman, C. D. Bowman, J. E. Bush, P. P. J. Delheij, C. M. Frankle, C. R. Gould, D. G. Haase, be scattered more readily than neu- J. Knudson, G. E. Mitchell, S. Penttilä, H. Postma, trons with opposite polarization. In N. R. Roberson, S. J. Seestrom, J. J. Szymanski, V. fact, in the case of 232Th, all seven W. Yuan, and X. Zhu. 1990. Parity nonconserva- tion for neutron resonances in 238U. Physical Susan J. Seestrom is a staff member in the Neutron observed asymmetries showed this Review Letters 65: 1192. and Nuclear Science Group of the Laboratory’s preference. Several papers dealing Physics Division, where she has been studying funda- with this issue have been published, V. W. Yuan, C. D. Bowman, J. D. Bowman, J. E. mental and applied nuclear physics with neutrons since Bush, P. P. J. Delheij, C. M. Frankle, C. R. Gould, 1986. She received her B.S. and Ph.D. degrees from but as yet no satisfactory explanation D. G. Haase, J. Knudson, G. E. Mitchell, S. the University of Minnesota. She was a Director’s has emerged. Penttilä, H. Postma, N. R. Roberson, S. J. Seestrom, Postdoctoral Fellow at Los Alamos from 1981 to 1983 It is clear that the facilities and J. J. Szymanski, and X. Zhu. 1991. Parity noncon- and a Research Associate at the University of servation in polarized-neutron transmission through Minnesota from 1983 to 1986, working in medium- personnel at Los Alamos in conjunc- 139La. Physical Review C 44: 2187. energy nuclear physics using and protons as tion with the world scientific com- probes of and reaction dynamics. munity continue to contribute to the store of scientific knowledge that will represent one of the great lega- cies of the last half of this century.

164 Los Alamos Science Number 21 1993