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Quarks and nuclear physics
NuclearPhysicsA358(198l)ll3~lZCk.O Not to be reproduced
North-HoUandPublishingCo.,
by photoprint
or microfUm
without
written
Amsterdam
permission
from the publisher.
QUARKS AND NUCLEAR
PHYSICS
B. POVH Max Planck
Institut
fiir Kernphysik,
Germany
Abstract: The introduction of quark concept in nuclear physics may improve our understanding of the nuclear properties just where such improvements are needed most: at small distances. Strange particles, antiprotons and high energy hadron interaction with nucleus are useful sources of information concerning the short range nuclear force. Deep inelastic scattering of electrons off nuclei will eventually tell us if quarks in nuclei feel that they are not in an isolated hadron but rather in hadrons closely packed in a nucleus.
1. Introduction The introduction of quark concept in nuclear physics will certainly not change essentially our present picture of the nucleus. It may, however, improve our understanding of the nuclear properties just where such improvements are needed most: at small distances. It is further a challenge for us to understand the connection between the quark-quark interaction which is presumably responsible for the behaviour of baryons at small relative distances and the boson exchange picture which is very likely valid at distances larger than 1 fm. The connection between these two complementary pictures may turn out to be essential to understand nuclear properties which depend to much the same extent on the long range forces approximated by one boson exchange and short-range forces presumably due to the quark exchange interaction. Thus, when we talk about quarks in nuclear physics we ask the following question. Can the nucleon-nucleon force and nucleon-nucleus one be understood in a simple way by involving the quark substructure of the nucleon. In particular at small internuclear distances, the quark language may be more suitable and straightforward than the language using phenomenological potentials married to the boson exchange models practiced so far in nuclear physics. We can also turn the question around and ask, do the quarks in the nucleus feel that they are not in an isolated hadron but rather in hadrons closely packed in a nucleus. Pirner and Vary'), in a contribution to this symposium, analyse deep inelastic electron scattering2) on aHe. The structure function on 3He differs from the one which one would obtain by summing the structure functions of the three nucleans in the "He. They attribute the difference in the structure function to the contribution of the scattering on the quarks that share their momenta between more than three quarks. According to their analysis, about 16 % of quarks in aHe feel the neighborhood of 5 rather than 2 quarks. If their interpretation of the data will stand further critical tests, experimental and theoretical ones,electronswill turn out to be as good a probe to investigate the quark structure of nuclei as they are useful in investigating hadrons. In my talk, however, I will discuss some alternative approaches on how to attack the quarks in nuclei. In all cases I will assume a probe with strong interaction. Consequently, the interaction of strongly interacting particles with nuclei is more complicated than that of electrons as in this case gluons play as important a role as quarks. 2. Interaction
of strange
particles
with nuclei
The interaction of strange particles with a final nucleus can be described to a good approximation by just two parameters. The depth of the nucleon potential in the nucleus and the strength of the spin-orbit interaction. Both of these two parameters are determined to a large extent by the short range hyperon-nucleon interaction.
113c
114c
B.POVti
In standard shell model approximation we split the nucleus into the core and the nucleon. The core has a zero isospin, spin and strangeness, an approximation that is usually good to about l/A. The long range nucleon-nucleon force is given by one pion exchange. As the pion is an isovector, most of this interaction will be averaged out due to the zero isospin core. Only the exchange terms survive the averaging. The major contribution to the nucleon potential comes thus from the nucleon-nucleon interaction from distances smaller than 1 fm. For nucleons moving in a nucleus the exchange terms which are of long range complicate the picture. In hypernuclei, however, exchange terms result primarily from K and K* exchange,their net contribution being relatively smalls). Therefore, it is justified to exercise quark models when the two parameters describing the hyperon-nucleus interaction are to be determined. Such undertaking gains in importance since the interaction parametersofstrange particles with nuclei are becoming available. It was pointed out by Pirne@) that already the ratios between the parameters determining the interaction for different particles may be a good test for simple quark models. Let me turn now to the h-nucleus interaction. In a recent experiment5) of the Heidelberq-Saclay-Strasbourg collaborations a systematic study of hypernuclei has been performed. The potential depth and the strength of the spin-orbit interaction have been determined independently. Most extensive analysis of data has been performed by 6ouyssy5). Using a shell model potential of the form V(r) = - Vcf(r)
+VLS(_&
ztf@@$Q.Q i7
where
(I) f(r) = {l + exp(r-R)/aI-1
and r. and a have the standard values parameters for nucleons and A are
and
R = ro(A-l)i/a
(r. = 1.1 fm, a = 0.60 fm). The interaction
= 50 MeV
V", c 32 + 2 MeV
IS = 20 MeV vN
VLS n ?- 4 i 2 MeV
"CN
(2)
Pirner pointed out'+) that in interaction of strange particles with a nuclear core, the contribution of the strange quark to the spin-orbit interaction is strongly reduced and can thus be neglected in the first approximation. As in A particle, their contribution to the spin-orbit intethe up and down quarks are antiparallel, raction cancels out. Therefore, one expects that the A particle does not show any spin-orbit splitting in nuclei. Let me shortly sketch the arguments for vanishing spin-orbit coupling in more detail. In the lowest order the contribution to the spin-orbit coupling comes from the interaction schematically shown in fig. 1. The effective coupling of a quark to the core is contributing to the spin-orbit coupling. It is of color-magnetic character and is proportional to VLS
Q-CORE
=
9’ 2m2
eff
(d/d;)f(r)
3 r QQ
The contribution to the spin-orbit coupling of the up and down-quarks vanishes as they are coupled to a singlet state. The contribution of the strange quark is strongly reduced because of the large effective mass of the s quark. These considerations became particularly interesting after it has been found7) that c particles bound to nuclei may live long enough as to also determine the spin-orbit coupling in c hypernuclei. Contrary to the h particle, c has the up and down-quark coupled to a triplet state and the contributions of the two quarks to
QUARKS
AND NUCLEAR
115c
PHYSICS
S
1~0, j=O
CORE
I=J=S=O
Fig. 1. Schematically shown the interaction of the A particle with the nucleon one. a) As the up and down quarks are antiparallel, their contribution to the spin orbit interaction cancels out. b) The contribution of the strange quark to the spin orbit interaction is reduced becauseofitslargemassascompared to the up and down quark. the spin-orbit force added. and nucleons, one obtains
Comparing
VLS c
the spin-orbit
coupling
of the 1 particles
= 4 VLS 3 N'
Let me show our best z-hypernuclear spectra (fig. 2). Statistics of this spectrum are not tremendously good and one still may have some doubts if the structures observed are really due to the c-hypernuclei. But in all spectra we observe a systematic shift of about 3 MeV to higher excitations for c hypernuclei as compared to the A hypernuclei. In a year we may have clean data on the spin-orbit coupling in c hypernuclei and we may be able to make a clear statement in how much the simple quark considerations were right or wrong. But the present data seem to be solid enough and deserve to be used in the present discussion. In particular, the relative shift of 3 MeV between the A and c spectra for the psi2 shell hypernuclei should be checked if it agrees with the expression (4). First, we have to guess the average potential of z particle in a nucleus. By averaging the interaction over the nuclear core and forgetting the exchange contributions,
116~
0. POVH
MA
MHy-
175
%
200
225
250
= 57 MeV/c
(MeV)
275
300
325
1 gBe(KIG)
350
:Be
1
I
300
250
>
200
f hl c
150
z 5
100
50
0
0
-25
-50
-75
-100 -125 -150 -175
B,, (MeV)
1
I
I
25
0
-25
I -50
I
I
-75
-100
B.&MeV) Fig. 2. Spectrum obtained from the (K-,R-) reaction on yBe at a kaon momentum of 720 MeV/c. The spectrum is plotted as a function of the transformation energy MhyMA, that is the difference between the hypernuclear mass and the target mass. Also the n-binding energy scale (Bh) is given. For comparison the IBe spectrum of ref.6) is inserted, which was taken at a kaon momentum of 900 NeV/c. All lines are drawn to guide the eye.
is expected. Taking (4) and (5) seriously, c-hypernuclear states of p3j2 shell should appear more bound than the A hypernuclei. The binding excess is expected to be 2-3 MeV, this is about as much as the energy shift for psi2 nucleon due to the spin-orbit interaction. At first sight it seems, therefore, that the estimates (4) and (5) result in a wrong prediction of about 6 MeV in the p312 shell c hypernuclei. This conclusion is, however, premature. The E particle in a nucleus has an effective mass close to its free mass, contrary to the nucleon effective mass that is about 0.8 of the free nucleon mass. Assuming an effective mass of 0.75 for the bound E particle,thehypernuclear spectra are consistent with (4) and (5).
QUARKSANDNUCLEARPHYSICS
117c
The A potential as well as the spin-orbit coupling are reduced as compared to the nucleon ones in all models. In particular Brockmann and Weisea) have shown that in the framework of a relativistic mean field approach the single particle potentials and spin-orbit interactions for nucleons and A particles can be well reproduced and are in good agreement with the experimental values (2). The difference in the nucleon- and A-nucleus interaction is related to a factor of about three weaker coupling of A particle to the isoscalar-scalar and isoscalar-vector boson than the nucleon. In a preliminary calculation, Brockmann and Weiseg) estimated in a simi ar way th potential parameters for Z hypernuclei. They obtained V = 40MeV and VLi - I/3 VL5 The spin-orbit term is in obvious contradiction with (5). It is super-Flu&s to !ay that this estimate can be easily reconciled with the experimental data. Unfort nately, to get a clear answer we have to wait for a direct measurement of the V$. Encouraging, however, is the fact that the potential parameters for nucleon-, A- and z-nucleus interaction seem to give enough constraintstodistinguish between some predictions of simple quark and boson-exchange models.
3. Antinucleon-nucleon
system
At first sight, the antinucleon-nucleon system combined with the nucleonnucleon one seems to be an excellent case to study the relation between the quark and boson-exchange picture of the nuclear force. The long range forces in the two systems are governed by one pion exchange and must, therefore, be the same. At smaller distances the boson exchange model predicts the potential for both by taking into account the G parity of the exchanged object. In addition, the annihilation is very likely intimately related to the qq dynamics. The annihilation radius is, therefore, expected to be of the same order as the confinement volume of the baryons. Unfortunately, the reality is less simple than this. Most work, experimental kef.la)] and theoretical 11), has been oriented towards baryonium - a baryon-antibaryon bound system. If baryonium spectroscopy turns out to be feasible, in fact all the expectations on the pp system will be fulfilled. From the baryonium spectrum the size of the system and the interaction of constituents could be deduced. The concept of baryonium has been first introduced formally by using duality argumentsls) between the t and s channel. In the s channel the pp interaction can be viewed upon as resulting from diquark-diantiquark intermediate states (fig. 3). III the quark jargon these states are called exotic as they are not of a q4 oraqqq type. So far, exotic states have not been experimentally observed. The central problem of baryonium is whether it can be observed or not. As illustrated in fig. 3, a strong affinity between the q{ pairs may dominate the process so that the baryonium states do not show up as resonances. Or saying it in a simpler way, annihilation may wash out baryonium states completely. Another appealing feature of the baryonium is that its spectrum is also well predictable using the potential model. In all these considerations we neglect the annihilation. The relation between the NN and NN potential can be obtained by the use of the one boson exchange model and considering the G parity of the exchange object. The main difference in the potential comes from the different sign of the contribution of the w exchange. The repulsive core in NN interaction is believed to come from the w exchange. In NN system w exchange gives.astrong attraction at small distances. Therefore, one expects a rich spectrum of NN bound and unbound states contrary to the NN system. It is thus clear that baryonium spectroscopy would be a beautiful method to test the interrelations between the quark and the potential picture of the baryon-baryon interaction. Experimental evidences for narrow baryonium states are rather scarce. In the last few years there have been many announcements of observations of narrow resonances above and below the pp threshold which have been interpreted as baryonium states. In most cases experimental observations could not be established by repeating the measurement or confirming the statistical significance of the first finding. Without going into a critical analysis of experimental data I may summarize the present situation by saying that the experiment searching for narrow baryonium states failed to show evidences for such states. The pp interaction is dominated by
B.POVH
118c
2 qi _ mesons
q q ;i ilj states
new resonances
q4 -
Fig. 3. Schematically
?
shown
?
mesons
pii interaction.
the annihilation. If baryonium states exist, then they are rather weakly populated and do not stick out of annihilation background, or the states are broad. So the first task of future experiments is to look for broad (r > 30 MeV) resonances, and to find out if the broad baryonium states can also be of some use in solving the problem stated above. Even if further search for baryonium states may turn out not to be successful, the i;p system offers a rich source of information on interaction between baryons. From detailed study of pp scattering one may eventually be able to disentangle the imaginary and real part of the pp interaction. The second one can be compared to NN interaction. The study of annihilation will give us a new way to investigate the quark dynamics in baryon interaction. Such a program is, however, much less attractive than the baryonium spectroscopy. Data will be difficult to interpret and many redundent measurements are required. But investigation of strong interaction has always been like that. Because of its intrinsic complexity, understanding of the strong interaction was done by small steps and not by decisive single experiments.
4. Hadron-nucleus
interaction
at high energies
For a long time, hadron-nucleus interaction at high energies has been of great interest for investigation of strong interaction. Before quarks became fashionable the hadron-nucleus interaction was believed to be able to help a lot in clearing up some of the questions of strong interaction. In hadron-hadron collisions, what we observe is the asymptotic state developed after collision. In the nucleus the hadron interacts many times. Because of the long interaction time in hadron-hadron collision, successive collisions take place before the asymptotic state has been fully developed. The typical interaction time is of the order of 1 fm. In the system of the nucleus the interaction time of the hadron is dilated. In this way nucleus as a target adds a new valuable parameter - number of interaction - to the experiments. This new parameter gives a further constraint on the models of strong interaction. With quarks in the hadrons, there are new perspectives in physics of hadronnucleus collision. High transfer momentum events in hadron-hadron collision are the simplest to treat. In close analogy to the lepton-quark collision they may be described as quark-quark collisions rather well in the perturbative treatment of QCO. Just for general interest it should be mentioned that spectator quarks after such hard collision propagate through the nucleus andthequark-nucleus interaction could
QUARKS
AND NUCLEAR
PHYSICS
119c
be studied if one could separate the events with non-overlapping jets of particle emerging from quark hadronization. The hadronization length of quark is of the same order as the earlier introduced interaction time of strong interaction. Our main interest, however, should concern soft collisions. These soft collisions are those which may shed more light in the structure of the strong interaction which is important at small momentum transfers and thus may help to understand strong interaction, also at low energies. The main issue in hadron-nucleus interaction so far is the question whether quarks in the hadrons behave like "independent" constituents or whether the interaction is a result of complicated contribution of the gluon "soup" as a whole. The first possibility, sometimes named as the additive quark model, explains qualitatively and also quantitatively many static properties of hadrons and the total hadron-hadron cross section at high energies. One of the relevant information for us is that the ratio of TI and the proton total cross section on hydrogen are proportional to the quark contents of the hadron, i.e. 2:3. This ratio stays the same also for the successive hadron collisions in the nucleus. In recent experiments, the Heidelberg-Lund-Virginia collaboration13) at CERN measured systematically particle production on nuclei using &, K*, and p,p- hadrons as projectiles. When normalized to the same number of collisions in nucleusin the case of protons and antiprotons the numberofcollisions inanucleus will be largerthanthatforpion and kaons - the particle production behave in the same manner for all incoming hadrons. This suggests that the soft collisions are governed by the interaction of soft gluon not associated directly to single quarks, but rather to the whole hadron. This is not a very surprising result. A typical momentum transfer in soft collisions is about 300 MeV/c. At such momentum transfer the characteristic dimension determining the interaction volume has a value of 0.7 fm, which is just the dimension of the hadron. Selecting inelastic collisions with larger momentum transfer we can investigate when and how individual quarks come into play in strong interaction. Hadron-nucleus collision can, therefore, be very useful to study the role of quarks and gluons in the strong interaction in dependence of the momentum transfer involved in the interaction. Understanding of this dependence will also help us to design models for strong interaction at small energies.
Conclusion Electron has proven to be an excellent probe when the quark structure of baryons is to be studied. Being a point-like particle and interacting only with quarks via their electric charge - gluons are electrically neutral - electron is probably also the best probe if one wants to study the role of quarks in nuclei. With the quark picture one hopes to improve the understanding of the short range nucleon interaction. In particular , one hopes to be able to describe the short range interaction in more simple terms than is presently the case. Strange particles, antiprotons, and high energy hadrons as nuclear probes add many constraints to the models of strong interaction. They may be sufficient to eventually come up with a consistent picture of strong interaction at low energies.
References H.J. Pirner and J. Vary, contribution to this symposium. Cl. Day, J.S. McCarthy, I. Sick, R.G. Arnold, B.T. Chertok, S. Rock, Z.M. Szalata, B.A. Mecking and G. Tamas, Phys. Rev. Lett. 43 (1979) 1143. R. Brockmann and W. Weise, Nucl. Phys. to be published. H.J. Pirner, Phys. Lett. 85B (1979) 190. R. Bertini, 0. Bing, P. Bmen, W. BrUckner, H. Catz, A. Chaumeaux, J.M. Durand, M.A. Faessler, T.J. Ketel, K. Kilian, B. Meyer,J..Niewisch, B. Pietrzyk, B. Povh H.G. Ritter and M. Uhrmacher, Phys. Lett. 83B (1979) 306. A. Bouyssy, Phys. Lett. 848 (1979) 41.
12oc
B. POVH
7) R. Bertini, 0. Bing, P. Birien, W. Briickner, H. Catz, A. Chaumeaux, J.M. Durand. M.A. Faessler. T.J. Ketel. K. Kilian. B. Mever. J. Niewisch. B. Pietrzyk, 8. Povh, H.G. Ritter and M. Uhrmacher, Ph>s.-Lett. m (1980) 375. 8) R. Brockmann and W. Weise, Phys. Lett. 69B (1977) 167. R. Brockmann and W. Weise, private commxcation. Iii L. Montanet, Phys. Reports 63 (1980) 201. 11) G.C. Rossi and G. Veneziano, Phys. Reports 63 (1980) 149. Rosner, Phys. Rev. Lett. 21 (1968) 950. 12) J.L. B. Pietrzyk, B. Povh, M. Schijder, P.C. 13) M.A. Faessler, U. Lynen, 3. Nizisch, Gugelot, T. Siemiarczuk and J.P. Zielinksi, Nucl. Phys. 8157 (1979) 1 and to be published.
North-HoUandPublishingCo.,
by photoprint
or microfUm
without
written
Amsterdam
permission
from the publisher.
QUARKS AND NUCLEAR
PHYSICS
B. POVH Max Planck
Institut
fiir Kernphysik,
Germany
Abstract: The introduction of quark concept in nuclear physics may improve our understanding of the nuclear properties just where such improvements are needed most: at small distances. Strange particles, antiprotons and high energy hadron interaction with nucleus are useful sources of information concerning the short range nuclear force. Deep inelastic scattering of electrons off nuclei will eventually tell us if quarks in nuclei feel that they are not in an isolated hadron but rather in hadrons closely packed in a nucleus.
1. Introduction The introduction of quark concept in nuclear physics will certainly not change essentially our present picture of the nucleus. It may, however, improve our understanding of the nuclear properties just where such improvements are needed most: at small distances. It is further a challenge for us to understand the connection between the quark-quark interaction which is presumably responsible for the behaviour of baryons at small relative distances and the boson exchange picture which is very likely valid at distances larger than 1 fm. The connection between these two complementary pictures may turn out to be essential to understand nuclear properties which depend to much the same extent on the long range forces approximated by one boson exchange and short-range forces presumably due to the quark exchange interaction. Thus, when we talk about quarks in nuclear physics we ask the following question. Can the nucleon-nucleon force and nucleon-nucleus one be understood in a simple way by involving the quark substructure of the nucleon. In particular at small internuclear distances, the quark language may be more suitable and straightforward than the language using phenomenological potentials married to the boson exchange models practiced so far in nuclear physics. We can also turn the question around and ask, do the quarks in the nucleus feel that they are not in an isolated hadron but rather in hadrons closely packed in a nucleus. Pirner and Vary'), in a contribution to this symposium, analyse deep inelastic electron scattering2) on aHe. The structure function on 3He differs from the one which one would obtain by summing the structure functions of the three nucleans in the "He. They attribute the difference in the structure function to the contribution of the scattering on the quarks that share their momenta between more than three quarks. According to their analysis, about 16 % of quarks in aHe feel the neighborhood of 5 rather than 2 quarks. If their interpretation of the data will stand further critical tests, experimental and theoretical ones,electronswill turn out to be as good a probe to investigate the quark structure of nuclei as they are useful in investigating hadrons. In my talk, however, I will discuss some alternative approaches on how to attack the quarks in nuclei. In all cases I will assume a probe with strong interaction. Consequently, the interaction of strongly interacting particles with nuclei is more complicated than that of electrons as in this case gluons play as important a role as quarks. 2. Interaction
of strange
particles
with nuclei
The interaction of strange particles with a final nucleus can be described to a good approximation by just two parameters. The depth of the nucleon potential in the nucleus and the strength of the spin-orbit interaction. Both of these two parameters are determined to a large extent by the short range hyperon-nucleon interaction.
113c
114c
B.POVti
In standard shell model approximation we split the nucleus into the core and the nucleon. The core has a zero isospin, spin and strangeness, an approximation that is usually good to about l/A. The long range nucleon-nucleon force is given by one pion exchange. As the pion is an isovector, most of this interaction will be averaged out due to the zero isospin core. Only the exchange terms survive the averaging. The major contribution to the nucleon potential comes thus from the nucleon-nucleon interaction from distances smaller than 1 fm. For nucleons moving in a nucleus the exchange terms which are of long range complicate the picture. In hypernuclei, however, exchange terms result primarily from K and K* exchange,their net contribution being relatively smalls). Therefore, it is justified to exercise quark models when the two parameters describing the hyperon-nucleus interaction are to be determined. Such undertaking gains in importance since the interaction parametersofstrange particles with nuclei are becoming available. It was pointed out by Pirne@) that already the ratios between the parameters determining the interaction for different particles may be a good test for simple quark models. Let me turn now to the h-nucleus interaction. In a recent experiment5) of the Heidelberq-Saclay-Strasbourg collaborations a systematic study of hypernuclei has been performed. The potential depth and the strength of the spin-orbit interaction have been determined independently. Most extensive analysis of data has been performed by 6ouyssy5). Using a shell model potential of the form V(r) = - Vcf(r)
+VLS(_&
ztf@@$Q.Q i7
where
(I) f(r) = {l + exp(r-R)/aI-1
and r. and a have the standard values parameters for nucleons and A are
and
R = ro(A-l)i/a
(r. = 1.1 fm, a = 0.60 fm). The interaction
= 50 MeV
V", c 32 + 2 MeV
IS = 20 MeV vN
VLS n ?- 4 i 2 MeV
"CN
(2)
Pirner pointed out'+) that in interaction of strange particles with a nuclear core, the contribution of the strange quark to the spin-orbit interaction is strongly reduced and can thus be neglected in the first approximation. As in A particle, their contribution to the spin-orbit intethe up and down quarks are antiparallel, raction cancels out. Therefore, one expects that the A particle does not show any spin-orbit splitting in nuclei. Let me shortly sketch the arguments for vanishing spin-orbit coupling in more detail. In the lowest order the contribution to the spin-orbit coupling comes from the interaction schematically shown in fig. 1. The effective coupling of a quark to the core is contributing to the spin-orbit coupling. It is of color-magnetic character and is proportional to VLS
Q-CORE
=
9’ 2m2
eff
(d/d;)f(r)
3 r QQ
The contribution to the spin-orbit coupling of the up and down-quarks vanishes as they are coupled to a singlet state. The contribution of the strange quark is strongly reduced because of the large effective mass of the s quark. These considerations became particularly interesting after it has been found7) that c particles bound to nuclei may live long enough as to also determine the spin-orbit coupling in c hypernuclei. Contrary to the h particle, c has the up and down-quark coupled to a triplet state and the contributions of the two quarks to
QUARKS
AND NUCLEAR
115c
PHYSICS
S
1~0, j=O
CORE
I=J=S=O
Fig. 1. Schematically shown the interaction of the A particle with the nucleon one. a) As the up and down quarks are antiparallel, their contribution to the spin orbit interaction cancels out. b) The contribution of the strange quark to the spin orbit interaction is reduced becauseofitslargemassascompared to the up and down quark. the spin-orbit force added. and nucleons, one obtains
Comparing
VLS c
the spin-orbit
coupling
of the 1 particles
= 4 VLS 3 N'
Let me show our best z-hypernuclear spectra (fig. 2). Statistics of this spectrum are not tremendously good and one still may have some doubts if the structures observed are really due to the c-hypernuclei. But in all spectra we observe a systematic shift of about 3 MeV to higher excitations for c hypernuclei as compared to the A hypernuclei. In a year we may have clean data on the spin-orbit coupling in c hypernuclei and we may be able to make a clear statement in how much the simple quark considerations were right or wrong. But the present data seem to be solid enough and deserve to be used in the present discussion. In particular, the relative shift of 3 MeV between the A and c spectra for the psi2 shell hypernuclei should be checked if it agrees with the expression (4). First, we have to guess the average potential of z particle in a nucleus. By averaging the interaction over the nuclear core and forgetting the exchange contributions,
116~
0. POVH
MA
MHy-
175
%
200
225
250
= 57 MeV/c
(MeV)
275
300
325
1 gBe(KIG)
350
:Be
1
I
300
250
>
200
f hl c
150
z 5
100
50
0
0
-25
-50
-75
-100 -125 -150 -175
B,, (MeV)
1
I
I
25
0
-25
I -50
I
I
-75
-100
B.&MeV) Fig. 2. Spectrum obtained from the (K-,R-) reaction on yBe at a kaon momentum of 720 MeV/c. The spectrum is plotted as a function of the transformation energy MhyMA, that is the difference between the hypernuclear mass and the target mass. Also the n-binding energy scale (Bh) is given. For comparison the IBe spectrum of ref.6) is inserted, which was taken at a kaon momentum of 900 NeV/c. All lines are drawn to guide the eye.
is expected. Taking (4) and (5) seriously, c-hypernuclear states of p3j2 shell should appear more bound than the A hypernuclei. The binding excess is expected to be 2-3 MeV, this is about as much as the energy shift for psi2 nucleon due to the spin-orbit interaction. At first sight it seems, therefore, that the estimates (4) and (5) result in a wrong prediction of about 6 MeV in the p312 shell c hypernuclei. This conclusion is, however, premature. The E particle in a nucleus has an effective mass close to its free mass, contrary to the nucleon effective mass that is about 0.8 of the free nucleon mass. Assuming an effective mass of 0.75 for the bound E particle,thehypernuclear spectra are consistent with (4) and (5).
QUARKSANDNUCLEARPHYSICS
117c
The A potential as well as the spin-orbit coupling are reduced as compared to the nucleon ones in all models. In particular Brockmann and Weisea) have shown that in the framework of a relativistic mean field approach the single particle potentials and spin-orbit interactions for nucleons and A particles can be well reproduced and are in good agreement with the experimental values (2). The difference in the nucleon- and A-nucleus interaction is related to a factor of about three weaker coupling of A particle to the isoscalar-scalar and isoscalar-vector boson than the nucleon. In a preliminary calculation, Brockmann and Weiseg) estimated in a simi ar way th potential parameters for Z hypernuclei. They obtained V = 40MeV and VLi - I/3 VL5 The spin-orbit term is in obvious contradiction with (5). It is super-Flu&s to !ay that this estimate can be easily reconciled with the experimental data. Unfort nately, to get a clear answer we have to wait for a direct measurement of the V$. Encouraging, however, is the fact that the potential parameters for nucleon-, A- and z-nucleus interaction seem to give enough constraintstodistinguish between some predictions of simple quark and boson-exchange models.
3. Antinucleon-nucleon
system
At first sight, the antinucleon-nucleon system combined with the nucleonnucleon one seems to be an excellent case to study the relation between the quark and boson-exchange picture of the nuclear force. The long range forces in the two systems are governed by one pion exchange and must, therefore, be the same. At smaller distances the boson exchange model predicts the potential for both by taking into account the G parity of the exchanged object. In addition, the annihilation is very likely intimately related to the qq dynamics. The annihilation radius is, therefore, expected to be of the same order as the confinement volume of the baryons. Unfortunately, the reality is less simple than this. Most work, experimental kef.la)] and theoretical 11), has been oriented towards baryonium - a baryon-antibaryon bound system. If baryonium spectroscopy turns out to be feasible, in fact all the expectations on the pp system will be fulfilled. From the baryonium spectrum the size of the system and the interaction of constituents could be deduced. The concept of baryonium has been first introduced formally by using duality argumentsls) between the t and s channel. In the s channel the pp interaction can be viewed upon as resulting from diquark-diantiquark intermediate states (fig. 3). III the quark jargon these states are called exotic as they are not of a q4 oraqqq type. So far, exotic states have not been experimentally observed. The central problem of baryonium is whether it can be observed or not. As illustrated in fig. 3, a strong affinity between the q{ pairs may dominate the process so that the baryonium states do not show up as resonances. Or saying it in a simpler way, annihilation may wash out baryonium states completely. Another appealing feature of the baryonium is that its spectrum is also well predictable using the potential model. In all these considerations we neglect the annihilation. The relation between the NN and NN potential can be obtained by the use of the one boson exchange model and considering the G parity of the exchange object. The main difference in the potential comes from the different sign of the contribution of the w exchange. The repulsive core in NN interaction is believed to come from the w exchange. In NN system w exchange gives.astrong attraction at small distances. Therefore, one expects a rich spectrum of NN bound and unbound states contrary to the NN system. It is thus clear that baryonium spectroscopy would be a beautiful method to test the interrelations between the quark and the potential picture of the baryon-baryon interaction. Experimental evidences for narrow baryonium states are rather scarce. In the last few years there have been many announcements of observations of narrow resonances above and below the pp threshold which have been interpreted as baryonium states. In most cases experimental observations could not be established by repeating the measurement or confirming the statistical significance of the first finding. Without going into a critical analysis of experimental data I may summarize the present situation by saying that the experiment searching for narrow baryonium states failed to show evidences for such states. The pp interaction is dominated by
B.POVH
118c
2 qi _ mesons
q q ;i ilj states
new resonances
q4 -
Fig. 3. Schematically
?
shown
?
mesons
pii interaction.
the annihilation. If baryonium states exist, then they are rather weakly populated and do not stick out of annihilation background, or the states are broad. So the first task of future experiments is to look for broad (r > 30 MeV) resonances, and to find out if the broad baryonium states can also be of some use in solving the problem stated above. Even if further search for baryonium states may turn out not to be successful, the i;p system offers a rich source of information on interaction between baryons. From detailed study of pp scattering one may eventually be able to disentangle the imaginary and real part of the pp interaction. The second one can be compared to NN interaction. The study of annihilation will give us a new way to investigate the quark dynamics in baryon interaction. Such a program is, however, much less attractive than the baryonium spectroscopy. Data will be difficult to interpret and many redundent measurements are required. But investigation of strong interaction has always been like that. Because of its intrinsic complexity, understanding of the strong interaction was done by small steps and not by decisive single experiments.
4. Hadron-nucleus
interaction
at high energies
For a long time, hadron-nucleus interaction at high energies has been of great interest for investigation of strong interaction. Before quarks became fashionable the hadron-nucleus interaction was believed to be able to help a lot in clearing up some of the questions of strong interaction. In hadron-hadron collisions, what we observe is the asymptotic state developed after collision. In the nucleus the hadron interacts many times. Because of the long interaction time in hadron-hadron collision, successive collisions take place before the asymptotic state has been fully developed. The typical interaction time is of the order of 1 fm. In the system of the nucleus the interaction time of the hadron is dilated. In this way nucleus as a target adds a new valuable parameter - number of interaction - to the experiments. This new parameter gives a further constraint on the models of strong interaction. With quarks in the hadrons, there are new perspectives in physics of hadronnucleus collision. High transfer momentum events in hadron-hadron collision are the simplest to treat. In close analogy to the lepton-quark collision they may be described as quark-quark collisions rather well in the perturbative treatment of QCO. Just for general interest it should be mentioned that spectator quarks after such hard collision propagate through the nucleus andthequark-nucleus interaction could
QUARKS
AND NUCLEAR
PHYSICS
119c
be studied if one could separate the events with non-overlapping jets of particle emerging from quark hadronization. The hadronization length of quark is of the same order as the earlier introduced interaction time of strong interaction. Our main interest, however, should concern soft collisions. These soft collisions are those which may shed more light in the structure of the strong interaction which is important at small momentum transfers and thus may help to understand strong interaction, also at low energies. The main issue in hadron-nucleus interaction so far is the question whether quarks in the hadrons behave like "independent" constituents or whether the interaction is a result of complicated contribution of the gluon "soup" as a whole. The first possibility, sometimes named as the additive quark model, explains qualitatively and also quantitatively many static properties of hadrons and the total hadron-hadron cross section at high energies. One of the relevant information for us is that the ratio of TI and the proton total cross section on hydrogen are proportional to the quark contents of the hadron, i.e. 2:3. This ratio stays the same also for the successive hadron collisions in the nucleus. In recent experiments, the Heidelberg-Lund-Virginia collaboration13) at CERN measured systematically particle production on nuclei using &, K*, and p,p- hadrons as projectiles. When normalized to the same number of collisions in nucleusin the case of protons and antiprotons the numberofcollisions inanucleus will be largerthanthatforpion and kaons - the particle production behave in the same manner for all incoming hadrons. This suggests that the soft collisions are governed by the interaction of soft gluon not associated directly to single quarks, but rather to the whole hadron. This is not a very surprising result. A typical momentum transfer in soft collisions is about 300 MeV/c. At such momentum transfer the characteristic dimension determining the interaction volume has a value of 0.7 fm, which is just the dimension of the hadron. Selecting inelastic collisions with larger momentum transfer we can investigate when and how individual quarks come into play in strong interaction. Hadron-nucleus collision can, therefore, be very useful to study the role of quarks and gluons in the strong interaction in dependence of the momentum transfer involved in the interaction. Understanding of this dependence will also help us to design models for strong interaction at small energies.
Conclusion Electron has proven to be an excellent probe when the quark structure of baryons is to be studied. Being a point-like particle and interacting only with quarks via their electric charge - gluons are electrically neutral - electron is probably also the best probe if one wants to study the role of quarks in nuclei. With the quark picture one hopes to improve the understanding of the short range nucleon interaction. In particular , one hopes to be able to describe the short range interaction in more simple terms than is presently the case. Strange particles, antiprotons, and high energy hadrons as nuclear probes add many constraints to the models of strong interaction. They may be sufficient to eventually come up with a consistent picture of strong interaction at low energies.
References H.J. Pirner and J. Vary, contribution to this symposium. Cl. Day, J.S. McCarthy, I. Sick, R.G. Arnold, B.T. Chertok, S. Rock, Z.M. Szalata, B.A. Mecking and G. Tamas, Phys. Rev. Lett. 43 (1979) 1143. R. Brockmann and W. Weise, Nucl. Phys. to be published. H.J. Pirner, Phys. Lett. 85B (1979) 190. R. Bertini, 0. Bing, P. Bmen, W. BrUckner, H. Catz, A. Chaumeaux, J.M. Durand, M.A. Faessler, T.J. Ketel, K. Kilian, B. Meyer,J..Niewisch, B. Pietrzyk, B. Povh H.G. Ritter and M. Uhrmacher, Phys. Lett. 83B (1979) 306. A. Bouyssy, Phys. Lett. 848 (1979) 41.
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7) R. Bertini, 0. Bing, P. Birien, W. Briickner, H. Catz, A. Chaumeaux, J.M. Durand. M.A. Faessler. T.J. Ketel. K. Kilian. B. Mever. J. Niewisch. B. Pietrzyk, 8. Povh, H.G. Ritter and M. Uhrmacher, Ph>s.-Lett. m (1980) 375. 8) R. Brockmann and W. Weise, Phys. Lett. 69B (1977) 167. R. Brockmann and W. Weise, private commxcation. Iii L. Montanet, Phys. Reports 63 (1980) 201. 11) G.C. Rossi and G. Veneziano, Phys. Reports 63 (1980) 149. Rosner, Phys. Rev. Lett. 21 (1968) 950. 12) J.L. B. Pietrzyk, B. Povh, M. Schijder, P.C. 13) M.A. Faessler, U. Lynen, 3. Nizisch, Gugelot, T. Siemiarczuk and J.P. Zielinksi, Nucl. Phys. 8157 (1979) 1 and to be published.