Homer A. Neal
Department of Physics, University of Michigan
Ann Arbor, Michigan USA
Talk Presented at the Coral Gables Conference on Cosmology and
Elementary Particle Physics, December 14, 2001
INTRODUCTORY REMARKS
being named the Samuel A. Goudsmit It is with considerable trepidation that I
take the floor to speak to you today about advances in spin physics, because I
recall having been asked to address this same subject at the Coral Gables
Conference some 24 years ago[1]. Clearly, you will have every reason to expect
me to address the question “what has happened in the intervening
years”. In keeping with the request of the conference organizers, my
presentation today will be more of a retrospective one than a detailed
technical review. I will provide references to sources where the original data
may be found and I will give a brief overview of some of my ongoing work in
studying polarization phenomena in a quark-quark scattering model. But mainly
my focus will be on reflections related to the evolution of spin physics
through the window of proton-proton scattering..
Spin is a beguiling and extraordinarily significant quantum number. It is
responsible for the way Nature behaves. Without it, the Universe would be
radically different, if it existed at all. Humans, as we know them, would not
exist. Chemistry would be totally different. But, in spite of spin having this
critical role, we still know so little about it.
In the book by Tomonaga [2] one finds the following reference “It is a
mysterious beast, and yet its practical effect prevails over the whole of
science. The existence of spin, and the statistics associated with it, is the
most subtle and ingenious design of Nature -- without it the whole universe
would collapse...”
For such an important entity it would be helpful to be able to envision it
using all of the imaginative powers available to the human mind -- even those
based on our classical experiences. But then, as pointed out by Landau and
Lifshitz[3], we are cautioned not to try to understand spin from a classical
viewpoint: “ ..this property of elementary particles is peculiar to
quantum theory ... and therefore has in principle no classical
interpretation... In particular, it would be wholly meaningless to imagine the
‘intrinsic’ angular momentum of an elementary particle as being the
result of its rotation about its own axis”. Not only is spin a
complicated concept, but we are even told how not to think about it.
But mathematical physics seems to be telling us that spin-1/2 particles are not
really that strange at all. At the end of one of the Spin Symposia at Argonne
National Laboratory in 1975 Dirac made the following statement about how
spin-1/2 particles emerged from his equation: “ It was really a surprise
to me that the spin should come out this way .. and that a particle with spin
of half a quantum is really simpler than a particle with no spin at all.”
[4]
Against this background, one should not be surprised to find that particle
interactions that involve spin effects might be especially complicated and
difficult to understand. But, as Dirac suggests, there may be spin effects that
are surprisingly simple as well.
Before I turn to the details of my presentation, I must warn the audience that
there will be many references in my talk that seem to be directed toward the
University of Michigan and its role in the evolution of spin physics. That is
no accident, since I am drawing from materials used when I delivered the
inaugural talk on the occasion of my Distinguished University Professor at the
University of Michigan a couple of years ago. I used that occasion to celebrate
the 75 years of excellence of Michigan in spin physics and I want to share
examples of those achievements with the audience here today.
Samuel Goudsmit and George Uhlenbeck, the discoverers of electron spin, spent
the principal part of their professional careers on the faculty of the
University of Michigan. David Dennison, the first to publish the finding that
the proton had spin of 1/2, spent almost all of his career at the University of
Michigan and was, I might add, one of my most revered professors when I did my
graduate work in Ann Arbor. Richard Crane, who is one of our distinguished
Michigan emeritus faculty, made the first measurement of g-2 for the electron -
a measurment that many at the time had labelled as impossible. Oliver
Overseth, a University of Michigan emeritus faculty member, made the first
alpha measurment for the lambda hyperon. Michael Longo, who is here at the
Conference today, and who was my thesis advisor at Michigan, has made a series
of important polarization studies, including his recent studies of the cascade
hyperon. Finally, another Michigan faculty member who is well known to the
Conference attendees is Alan Krisch, who over the decades has led the advances
in both polarized targets and polarized beam technologies that have resulted in
so many of the new precision data that are now available to the physics
community. So, as you can see, celebrating the contributtions of Michigan
researchers to spin physics seems appropriate. However, by no means do I intend
to slight today the enormous contributions made by others.
SPIN: ITS HISTORY
In the reflections of Dirac who, parenthetically, quizzed me vigorously about
some new data I presented at my talk here in 1977, he indicated that one of the
earliest suggestions that electrons had spin came from Arthur Compton in
1921[4]. Compton noted that if electrons were not symmetrical many properties
of magnetism could be explained. He also noted that tracks in a Wilson Cloud
Chamber had unexpected kinks. He reasoned that if the electron had a magnetic
moment it would induce a magnetic moment in the surrounding medium and lead to
the observed helical motion of the electron.
In the following years there were other indications that new quantum numbers
were needed to explain observations. The number of spectral lines observed in
spectroscopy seemed to be double what one would expect from standard Bohr-orbit
calculations. German physicists labeled the problem the “duplexity
problem”: “Whenever one took an atom and brought in a further
electron, the number of states that one got was double what one expected”
[4].
In this environment of puzzlement, it was Kronig in early 1925 who first
suggested more formally that the electron had an intrinsic spin. The idea was
not embraced by Pauli, and Kronig ceased to pursue the concept. [2]
Additional insights on the Pauli-Kronig exchanges can be found in the Pauli
Festschrift volume edited by Fierz and Weisskopf. [5]
Later in 1925, however, two Dutch students, Goudsmit and Uhlenbeck, produced a
short paper and gave it to their advisor, Ehrenfest. They then traveled to
Haarlem and showed the paper to Lorentz. Lorentz argued that the concept was
not sound and could not possibly be correct because of energy considerations.
Goudsmit and Uhlenbeck went back to Leiden and asked Ehrenfest to withdraw the
paper. But it was too late. Ehrenfest had submitted the paper to
Naturwissenshaften [6]. And, as you know, the rest is history.
Surprisingly, a couple of years later, the discovery of the spin of the proton
had a similar tentative emergence. David Dennison had submitted a paper on June
3, 1927 on the specific heat of the hydrogen molecule. Two weeks after that
submission ( June 16, 1927, to be exact), he sent an addendum to the paper in
which he pointed out for the first time that “the mixing ratio of 1/3:1
means that the spin of the proton is 1/2 ....”[2] Dennison’s personal
reflections about this work can be found in his memoirs in reference [7].
THE EXPERIMENTAL CHALLENGES
Spin manifests itself in many ways. We know that no two electrons in an atom
can be in the same orbital and spin state. This simple rule, coupled with the
fact that electrons have spin 1/2, accounts for the basic structure of the
chemical elements.
Nuclei have spin. Indeed, when a MRI photograph is taken of the human body, as
shown in Figure 1, it is the density of nuclear spins that is being sampled
that leads to the miraculous non-invasive imaging now possible with that
technique. On a personal note, I recall the thrill I had when, as Provost of
the State University of New York at Stony Brook, I had the priviledge of being
the offical escort of Paul Lauterbur, one of our Stony Brook faculty members,
to the ceremonies where he was to receive the Lasker Prize for his invention
of the MRI device. I recalled during the pomp and circumstances of the event
that I finally had an answer to the question from my childhood friends in
Southern Kentucky “what good is that thing called spin anyhow…what
can you do with it? “.
At each of the levels of aggregation from molecules to atoms to nuclei there
are interesting questions about spin to be explored -- and are being explored.
In elementary particle physics we are focussed on a more fundamental set of
issues. We want to know what happens when two elementary particles collide and
what part of the scattering process is due to the spins of the colliding
particles. We demand that any rigorous theory of particle interactions also
properly explain the role of spin in the interactions. Spin determines the
statistics that the interacting particles must obey and we are ever alert to
any indication that our understanding of the spin-statistics relationship is
not correct. We are also vigilant in insuring that every opportunity is
exploited to use spin as a vital tool in determining the extent to which many
of our revered conservation laws are obeyed, and these are most directly tested
at the level of the elementary particles.
I should warn the reader that even these two reactions are viewed as being very
complicated by many experts. My position is quite different. I claim that,
within the context of a constituent model, there are regularities that can be
rather easily understood. Indeed, I believe and that p-p elastic scattering
data in the late sixties was trying to tell us that quarks existed -- and were
even telling us the number of quarks that were inside the proton.
Clearly, if one is intent on extracting as much information as possible about
spin from p-p elastic scattering, one needs to know what are the spins of the
incoming protons and what are the spins of the outgoing protons, and how do the
various spin parameters and their correlations change with angle and momenta.
Expermentalists must be careful to insure that the particles being examined
are indeed protons from the interaction in question and that any asymmetries
observed are due to the physics of the interaction and not something else, such
as asymmetries in the detector. I will elucidate the relationship between spin
and asymmetries below.
Measurement Techniques
I will use a set of experiments which I either led or had a major role in over
the past few decades to provide examples of the techniques available for
making spin studies prior to the recent advances in polarized beam and
polarized target technologies. Then I will comment on the increased
sensitivity made possible by these advances.
First, I must remind the reader that a beam of unpolarized protons striking a
target such as carbon can have its scattered parti
cles polarized, as graphically illustrated in Figure 2. While this may seem
magical at first, it is really only a statement that the scattering potential
may have terms in it like s
n
. Indeed, it is almost as if a demon in the target looks at the spin vect
or of each incoming proton and assigns each a differential probability for
scattering to the left vs right depending on the value of the vertical
component of its spin. (This is why experimentalists must be very careful to
insure that any left-right bias in their equipment is known and corrected for).
This “analyzing power” of carbon and other such materials provides
one of the key tools used for decades in polarization studies. If one knows the
analyzing power for a block of carbon (“A”) for a particular
particle type, for a given energy and detector geometry (constituting a
polarimeter), then one can take that block of carbon and place it in a beam for
that particle type and energy, record the left/right asymmetry e = (
(L-R)/(L+R) ) ) as the beam scatters from the block, and then determine the
beam’s polarization via P = (e/A). That has been, and is still, the most
direct way of determining the polarization of a final state proton. There have
been other exciting possibilities raised in recent years about how to improve
polarimetry, including the use of a polarized target as a polarimeter. [8]
When I gave my previous talk at the Coral Gables Conference, the premiere tool
for fixing the spin of target protons were the lanthanum magnesium nitrate
(LMN) and ethylene glycol polarized targets. The operating principle for the
LMN target was to polarized magnetic centers in the target and to stimulate via
microwave radiation those centers to couple to the “free” protons in
the target and flip the proton’s spin. The LaMn centers do this through a
joint spin flip. but then the LaMn relaxes back to its original spin state and
is ready to couple to another “wrong” state proton and flip it.
Meanwhile the flipped protons remain in their polarized state for a long
period, because of their weak magnetic moment and long relaxation times. The
ethylene glycol targets operated on a slightly different principal of so-called
thermal mixing.
In our ZGS intermediate energy polarization study [9] we employed an ethylene
glycol target with a maximum target polarization of approximately 40%. This
experiment was the first to use a polarized proton target in an external proton
beam at high energies. We used a beam of ~ 5 x 109
protons per pulse, limited to avoid target damage. Moreover, we had to contend
with background events because free protons comprised only ~10% of the
target. Today one has targets with polarizations well in excess of 90% and are
much more radiation resistant.. This is a remarkable advance over the time
since my last presentation. And I must give credit for this to the team of
collaborators led by Alan Krisch.[10]
I should note that one of the dreams of a spin physicists is to have a target
that is essentially pure hydrogen where the spins of the protons can be aligned
almost 100% in any direction chosen. What, of course, stands in the way of
realizing this dream is achieving low enough temperatures and high enough
fields to populate the appropriate spin states in the fae of the small magnetic
moment of the proton. But tremendous progress is being made in this area, as
can be seen from the recently published results on the “Michigan
ultra-cold polarized atomic hydrogen jet target” [11]
In 1977 the concept of polarized proton beams was in its infancy. It was also
the determined effort of Alan Krisch and colleagues that has now made it
possible to achieve beams of highly polarized polarized protons at many
accelerators ( the Argonne ZGS, the Brookhaven AGS, RHIC, and the Indiana
Cyclotron). Since there are other talks at this Conference on polarized beams,
and since it is a subject well described in the literature[10], I will not go
into detail here about the design of these beams. I will mention, however, that
one of the great nemeses of polarization beam acceleration is that the beam
can become rapidly depolarized by machine imperfections as a given
particle passes numerous times through the same imperfection. That problem has
been cleverly addressed by two Rusian physicsists (Derbenev and Kondratenko
[12]), who use the machine tune to cause the proton spin to rotate by 180
degrees on each turn around the machine, which causes the effects of the
imperfection to be cancelled. The technology to accomplish this is commonly
referred to as the “Siberian Snake” [13]. By the way, Dr. Derbenev
has been on the Michigan staff for more than a decade.
I have outlined the three common tools that we have available: 1) carbon
analyzers to measure the polarization of scattered beams, 2) polarized targets
to fix the spin of the target proton, and 3) polarized beams to fix the spin of
the incident proton. Advances in the latter two areas over the decades have
been phenomenal -- and that is what has made possible several of the physics
results I will cover in the text below.
In an experiment at the Brookhaven Cosmotron Mike Longo and I made pioneering
measurements of the polarization in p-p elastic scattering at energies of
approximately 1 GeV using only a carbon analyzer and a very sophisticated
polarimeter. An example of the results is shown in Figure 3.[14] The
significance of these measurements is that, for the first time, precision
measurments were made beyond 90 degrees cms for t- values greater than 1
(GeV/c)2
. What we observed was that the polarization assumed a value of zero well
before it needed to in satisfying symmetry conditions at 90 degrees cms. We
believe this was the first observation of the zero in p-p polarizations at -t
=~ 1 (GeV/c)2
, a feature which we now know persists all the way into the hundreds of GeV/c
incident beam momentum. As noted later, I believe this to be due to a
transition from the scattering of one quark off one quark to two quarks off
two. Unfortunately, we did not know that quarks existed at the time of this
measurment. This advance was made simply using a carbon analyzer, with no
polarized targets or beams.
There are many other features of the p-p system we have studied over the years,
ranging from one of the first measurements of the p-p depolarization parameter
where we fixed the polarized of the target proton and then measured its
polarization after scattering (Figure 6), our measurment of the first
polarization effects above 200 GeV/c, studies of inclusively produced lambdas
from electron/positron collisions, and so on. But I believe the above snapshots
given an indication of what has been possible, and they set the stage for the
model I will describe below.
CAN WE SEE QUARK POLARIZATION?
It was in 1973 that I departed for a sabbatical at the Bohr Institute, made
possible by a Sloan Foundation Fellowship. In addition to my normal luggage, I
was burdened by the data that I described above. The cross section data had
shown discontinuities that I did not understand. The polarization data
displayed “bump” structures which seemed strange, but also seemed to
be correlated with the cross section breaks. As I took up these issues with
Holger Nielsen, whom I met early in my stay at the Institute, we rather quickly
became interested in the possibility that the effects were a manifestation of
the quark structure of the proton. Indeed, we had become aware of the work of
Landshoff who seemed to be having success explaining some of the p-p cross
section magnitudes by using an independent quark model.[15]
This further suggested that in the region we ascribed to a single quark
scattering from a single quark ( -t < 1 (GeV/c)2
), the proton polarization observed was actually the polarization in single
q-q scattering. Armed with the knowledge of what the single q-q scattering
polarization was, we could then predict what the polarization would be in
regions where the dominant scattering mechanism was for 2, 3 or more scatters.
Indeed, in our 1974 paper we focussed on using the region #1 data to predict
what would happen to the polarization in region #2 ( -1 < t < -3.5
(GeV/c)2
). We were satisfied with the success of the model in this test. The data
beyond region #2 was so poor at that time that we were content to note that we
expected the polarization to be even larger in that region, but we attempted no
detailed fits.
As the decades passed and the tecnological developments for improved polarized
targets and beams proceeded, the Krisch group began to accumulate high
statistics data in what we call region #3 ( -t > 3.5 (GeV/c)2
), the region where we might expect that three quarks would scatter from
three quarks. [10] The measurements were the subject of much skepticism in the
community in that no mainstream model could explain how spin effects of >
30% could be present at high energies and large |t|. Indeed, most models
predicted that in that regime, the polarization should be zero. Our quark-quark
scattering model was, to our knowledge, alone in predicting that there should
not only be significant spin effects in that region, they should be big, and be
simply a scaled version of the polarization in the previous regions. A fit
from our 1998 Physics Letter publication[17] (Figure 7) shows how the model is
able to account for the significant structure in the existing p-p polarization
data at the energy that, at present, has the largest span of t values in
existence. The number of multiple scatters was allowed to vary when the fit
shown was generated.
The success of this simple model encourages us to believe that it is the
individual quarks inside the proton that are being seen, that there are only
three of them (since there seems to be no significant shape changes in the
cross section that would signal a fourth region), and that even the spin
information in each q-q scattering is preserved and passed on to the scattered
proton.
We are often asked if the so-called "spin crisis" [18], which
suggests that quarks do not carry all of the proton spin, would invalidate our
model. The answer is it does not. The only features of the scattering process
that we have invoked is that there are three “clusters” inside the
proton that are responsible for the scattering. Each cluster could be a valence
quark plus gluons, plus whatever, and the general concept of the model remains
unchanged. The site of the spin is not important.
ARE SPIN CORRELATION EFFECTS MEASUREABLE AT THE QUARK LEVEL
?
The next challenge we subjected the model to was to explain the extraordinary
behavior observed in the p-p cross section when the two initial protons have
their spins parallel vs anti-parallel. What one notices in the beautiful A
nn
data from the Krisch group (Figure 8) is that the two cross sections are
comparable up until t values are reached that represent the entry into our
region #3. In another paper by Neal and Nielsen published just last year in
Physics Letters[ 19], we have argued that, if the basic tenets of the quark
interchange model are assumed to be valid, the only way to have three quarks
scatter from three quarks and not pay the penalty for spin flips of the
protons, is to have the initial state protons have their spins parallel. So our
model even goes some distance in explaining this dramatic effect.
QUARK POLARIZATIONS AND SPIN EFFECTS IN INCLUSIVE LAMBDA PRODUCTION
I now would like to say a few words about some current work I am conducting
that seeks to test the applicability of our model to
L
inclusive polarizations.
At the time of my last appearance at this Conference it had just been reported
that huge polarizations had been observed in
L
hyperons which were inclusively produced when 400 GeV/c protons struck a Be
target.[20] This was very p
uzzling since both the very high emergy and the inclusive nature of the
production should suggest that spin effects would be small, if they existed at
all. To date, this observation, and the many similiar observations for other
hyperons, remains a large mystery.
I have attempted in recent months to determine just how different the
L
polarization is from what one would expect based on p-p data. More precisely,
since our model purports to be able to extract the q-q
polarization parameter, one might simply ask the question “at a given
L
p
t
, what would one expect the
L
polarization
to be if that polarization were due solely to the u,d quarks coming from the
incident proton?” Heretofore, this has not been a question that could be
simply broached, because mainstream models have no way of asserting directly
what the q-q polarization parameter should be. What almost all models have done
so far in dealing with the L
polarization matter is to assume that the polarization comes mainly from the
s-quark, which is produced from the vacuum as part of a s-sbar pair as
illustrated in Figure 9.
Figure 10 below illustrates the remarkable features of the lambda inclusive da
ta, with the prominent features of its large values and a plateau for
different values of the Feynman x [21]. Figure 11 shows the measured lambda
polarization along with values predicted for the lambda polarization using the
300 GeV/c p-p polarization from Synder et al in conjunction with our model[22].
Some simple assumptions have been made about how to scale the momentum of the
proton data before overlaying those data on the lambda plot. In essence we
require that the pt
of the u,d scatters be sufficient to provide the p
t
needed by the u,d, and s quarks in the lambda. The fact that the measured and
predicted results agree so well, further encourages us in believing the general
validity of our model. By allowing some polarization contribution by the
s-quark and allowing it the ability to flip spins with the u,d quarks at low
pt, but then decreasing that option as pt grows, and noting the requirement for
the u,d quarks to ultimately be in a singlet state in the final lambda, one
has the basic ingredients for achieving a plateau in the lambda polarization,
also in keeping with observation. This work is ongoing and additional steps are
required to rigorously underpin or alter the assumptions advanced above.
ANTICIPATED SPIN STUDIES AT LHC ENERGIES
Throughout the last few decades, whenever the high energy physics community has
faced the prospects of moving to the next higher energy range or t-range there
have been many voices that suggest that in that new range spin effects will
have died away. Such persons have always been wrong. As we face the prospects
of running at the LHC in a few years, I fully expect that we will again be in
an interesting arena where spin effects will be measurable and important in
many analyses.
The LHC will produce an enormous number of t-tbar pairs, for example. The
relative helicities of these particles will set considerable contraints on the
decay distributions of these particles, and will provide sensitive tests of the
Standard Model[23]. Also, the
L
b
will be produced in prodigious quantities at the LHC. By using the measured
decay properties of L
b -->
y L
and
y-> m m
, one should be able to determine the polarization of the
L
b
to < 2%. This may be sufficient to gain some additional insights into how
hyperons are inclusively produced, particularly since most analyses suggest
that, given the massive nature of the b-quark, polarization effects associated
with the L
b
should be significantly larger than even those of the
L
.
CONCLUDING REMARKS
In
concluding, I hope I have given you some flavor of the evolution of spin
physics, the role it has played in shaping our thoughts about the interactions
of particles, and our expectation that it will continue to provide insights as
data are acquired at the LHC.
I want to stress that the window I have offered here on spin physics is very
narrow. This subfield has so many other components these days, ranging from
probes in deep inelastic scattering, from e+ e- studies, and polarized
structure functions to anticipated new revelations from RHIC and UNK. Indeed,
the explosion of the richness of spin studies may be one of the most important
developments over the past quarter-century that I have been priviledged to
observe.
I want to again salute the members of our community who have advanced the
technology in this complex area of our field. I believe that such efforts will
eventually allow us to make the measurements necessary to really learn the role
the mysterious spin plays in Nature.
I also wish to acknowledge Jens Zorn for his assistance with the various
historical issues, and the conference organizers for sustaining this very
special set of scientific meetings over the decades.
References:
1) H. A. Neal, Advances in the Study of Spin Effects in Nucleon-Nucleon
Scattering at Small and Intermediate Momentum Transfers in New Frontiers in
High Energy Physics,, Edited by Behram Kursunoglu, Arnold Perlmutter, and Linda
F. Scott (Plenum Publishing Corporation, 1978)
2) The Story of Spin, Sin-Itiro Tomonaga, University of Chicago Press,
1997, pp. 258
3) L. D. Landau and E. M. Lifshitz, 1977 Quantum Mechanics
(Non-relativistic Theory) Thirs Edition p. 198 (New York:Pergamon).
4) Dirac, P. 1975. In Proc. Summer Study High Energy Physics.
Polarized Beams, ed. J. Roberts, Argonne National Lab. Rep. HEP-75-02.
5) "Theoretical Physics in the 20th Century, A Memorial Volume to
Wolfgand Pauli », Edited by M. Fierz and V.F. Weisskopf. New York,
Interscience (1960) 328p
6) Goudsmit, S. , Uhlenbeck, G. 1925. Naturwissenschaften 13:953.
7) D.M. Dennison, “Recollections of Physics and of Physicists During
the 1920s”, Am. J. Phys. 42, 1051-1056 (1974)
8) A.D. Krisch, In Proceedings of Hamburg 1999;Polarized Protons at High
Energies, Hamburg, Germany 17-20 May 1999
9) Abshire, G.W. et al, Physical Review D, 9 1 February 1974 pg. 555
10) R.C. Fernow, A.D. Krisch, High Energy Physics with Polarized Beams, Ann.
Rev. Nucl. Part. Sci. 1981, 31:107-44.
11) B. B. Blinov et al, AIP Conference Proceedings, June 1, 2001 Vol 570,
Issue 1 pp. 856-860
12) Y. S. Derbenev and A. M. Kondratenko, Part. Accel. 8 115 (1978)
13) A. D. Krisch, Physics of Atomic Nuclei, 62, Issue 3, 1999 pp. 479-484
14) H. A. Neal and M. J. Longo, Phys. Rev. 161, 1374 (1967).
15) P.V. Landshoff, Phys. Rev. D 10 (1974) 1024.
16) H.A. Neal, H.B. Nielsen, Physics Letters B {51} (1974) 79.
18) John Ellis, Proceedinsg of the 12th
International Conference on High Energy Spin Physics, pg. 7, 1997, World
Scientific
19) Homer A. Neal and Holger B. Nielsen, Physics Letter B 508 (2001) 251-258
20) G. Bunce et al, PRL36, 1113(1976).
21) J. Lach, Hyperon Polarization: An Experimental Overview,
Fermilab-Conf-92/378, December, 1992
22) B. Lundberg et al., Phys. Rev. D40, 3557 (1989) ; C. Wilkinson et al.,
Phys. Rev. Let. 46, 803 (1981) ; J.H. Synder et al, Phys. Rev. Lett. 41, 781
(1978)
23) “ATLAS Detector and Physics Performance Technical Design Report”
, Vol. II, CERN/LHCC/99-15, ATLAS Collaboration, 25 May, 1999
1
The simple set of studies I will be speaking about in the remaining portion of
my talk is the examination of spin effects in a few well defined scattering
processes. Namely, what is the role of spin in elastic proton-proton scattering
and in inclusive hyperon production.
The challenge for theorists is to develop techniques to extract information
from accessible experimental measurements that will allow essential aspects of
models to be tested. Almost never does a single experiment produce a result
that points to a particular single amplitude of interest. Further, because
experimental measurements are extremely difficult when spin information must be
extracted from multiple particles, the results often are useless in
discriminating between competing models unless extremely precise tools are
employed. The experimental and theoretical challenges are enormous.
Early in my years as a faculty member at Indiana University while preparing for
additional spin studies, I noted that the existing cross section data in p-p
elastic scattering was still of poor statistical quality. It seemed that it
made no sense to try to understand polarization effects in an energy region
where even the cross section was poorly known, so I temporarily refocussed my
efforts toward attaining a precise measurement for the p-p cross sections in
the intermediate energy regime. At that time the technology of magnetostrictive
spark chambers was just being perfected, and we decided to use these chambers
for the cross section measurements. The results are shown in Figure 4. One
special outcome of these measurements was the discovery of the unusual behavior
of the fixed angle cross section, as portrayed in Figure 5. At the time these
findings were just regarded as another bizarre rendering of the stubborn and
complex p-p system. As discussed later, however, my interpretation now of these
breaks is that they are indicative of the quark structure of the proton.
As previously mentioned, in another experiment I led we used for the first time
an ethylene polarized target in an external proton beam to study the
polarization in p-p elastic scattering in the 6- 12 GeV/c range. One of our
findings from that experiment was a “bump” structure in the
polarization.[9]. I will refer to this odd feature in a later section as one
that gives further credence to our picture of the constituent structure of the
proton.
The principal postulate we advanced in a Physics Letter article [16] was that
within t ranges where the fixed angle cross section was smooth and continuous
in slope, the scattering process was dominated by a fixed number of quark-quark
scatters, and as one crossed one smooth region to another the slope change
that occurred was due to a change in the number of q-q scatters involved (see
Figure 5). We also assumed that, to the extent permitted by the quarks
combining to make a spin-1/2 proton , the quarks behaved independently within
the proton, they contributed their own polarizations to make up the overall
polarization of the proton, and the internal processes that occurred after the
quark scattering resulted in the appropriate redistribution of quark momenta
but did not affect the overall proton polarization.
