Experimental Studies of Quark-Gluon Structure of Nucleons, Nuclei

90 Pages·1996·4.321 MB·English

I
30EETR/*8e7--TI
EXPERIMENTA STUDIES
OF THE QUARK-GLUON STRUCTURE
OF NUCLEONS AND NUCLEI
AND OF PION- AND PROTON-NUCLEUS
INTERACTIONS
Progress Report
on DOE Grant No. DEFG03-94ER40847
-
April 1,1994 March 31,1997
New Mexico State University
Las Cruces, NM 88003
October 1,1996

DISCLAIMER
This report was prepared as an account of work sponsored by an agency of the United
States Government. Neither the United States Government nor any agency thereof, nor
any of their employees, make any warranty, express or implied, or assumes any legal liabili-
ty or responsibility for the accuracy, completeness, or usefulness of any information, appa-
ratus, product, or process disdosed, or represents that its use would not infringe privately
owned rights. Reference herein to any specific commercial product, process, or sem'ce by
trade name, trademark, manufacturer, or otherwise does not necessarily constitute or
imply its endorsement, recommendation, or favoring by the United States Government or
any agency thereof. The views and opinions of authors expressed herein do not necessar-
ily state or reflect those of the United States Government or any agency thereof.
.
Contents
1 Introduction 1
2 Experiments on the Quark-Gluon Structure of Nucleons and Nuclei 3
2.1 HERMES: Measurement of Spin-Dependent Structure F'unctions of the
.............................
Nucleon at HERA 4
. . . . . . . . . . . . . . . . . . .
2.1.1 Inclusive Lepton Scattering 5
. . . . . . . . . . . . . . . .
2.1.2 Semi-inclusive Lepton Scattering 7
........................
2.1.3 Preliminary Results 9
. . . . . . . . . .
2.1.4 Cryogenic Polarized Internal 3He Gas Target 11
....................
2.1.5 The HERMES Spectrometer 12
......................
2.1.6 The First-Level Trigger 13
....................
2.1.7 Photoproduction Triggers 18
. . . . . . . . . . . . .
2.1.8 Slow Controls for the HERMES Trigger 20
2.1.9 Forward Trigger Scintillatorsf or Reduction of Proton Background 21
.
2.2 Fermilab Experiment No E866 (NUSEA): Measurement of ;i<z)/Ti(z)
...............................
in the Proton 25
2.2.1 Measurement of Flavor Symmetry Breaking in the Nucleon Sea 25
........................
2.2.2 The Target System 28
. . . . . . . . .
2.2.3 The E866 Database and Monitoring Software 29
. . . . . . . . . . . . . . . . . .
2.2.4 The Data Acquisition System 30
....................
2.2.5 E772 Dump Data Analysis 31
2.3 TJNAF Experiment E89-012: Two-Body Photodisintegration
of the Deuteron at Forward Angles and Photon Energies
.........................
Between 1.5 and 4.0 GeV 34
. . . . . . . . . . . . . . .
2.4 Workshop on the Structure of the q' Meson 36
. . . . . . . . . . . . . . .
2.5 Future Experiments at MIC and Fermilab 37
3 Experiments on Pion Interactions 38
.........................
3.1 Experiments at LAMPF 38
3.1.1 Pion Elastic and Single Charge Exchange Scattering horn
...........................
Polarized 3He 38
. . . . . . . . . . . . .
3.1.2 Exp 1291: Neutron Radii of Ca isotopes 43
. . . . . . . . . . . . . . .
3.2 Experiments at the Paul Scherrer Institute 45
...
111
. .. .+ . ..
3.2.1 Experiment No. R-87-13: A Study of Multi-particle Final States
in Pion-Nuclear Reactions with a Large Acceptance Detector
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
(LADS) 45
3.2.2 Experiment No. R91-11: The 7&(a+,7r+p)6He Reaction . . . 53
,
. . . .
3.2.3 Experiment No. R91-11.2: The 72i(n,pp)5HeR eaction 53
4 Experiments on Proton-Nucleus Interactions 55
4.1 IUCF Experiment No. 371: Cross Section and D"(t9) Measurements
. . . . . . . . . . . . . . . . . . . . . . .
in n) Reactions 1 55
161171180(p,
4.2 LAMPF-WNR Experiment No. 4N0076: n-p Scattering in the 50-250
. . . . . . . . . . . . . . . . . . . . . . . . . . . .
MeV Energy Range 58
"
4.3 TRIUMF Experiment No. 709: ,P Reactions at
. . . . . . . . . . . 9.0 1.9 .21 .94 .1 9.6 2.r (.n .) 9.0 .1 .9 2.1 .9 4. 1.9 .6 Y. . . . .
200 MeV. 59
5 Publications and Presentations 66
. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.1 Published Papers 66
. . . . . . . . . . . . . . . . . . . .
5.2 Papers Submitted for Publication 70
. . . . . . . . . . . . .
5.3 Papers Presented at Meetings and Conferences 70
6 Personnel 79
7 Degrees Awarded 80
iv
1 Introduction
This report summarizes the work on experimental research in intermediate energy
nuclear physics carried out by New Mexico State University from April 1, 1994,
through March 31, 1996 under a grant from the U. S. Department of Energy.
During this period we began phasing out our programs of study of pion-nucleus
and pion-nucleon interaction and of nucleon-nucleus charge-exchange reactions, which
have been our major focus of the past two or three years. At the same time we
continued moving in a new direction of research on studies of the internal structure
of nucleons and nuclei in terms of quarks and gluons. The pion and nucleon work
has been aimed at improving our understanding of the nature of pion and proton
interactions in the nuclear medium and of various aspects of nuclear structure. The
studies of the quark-gluon structure of nucleons are aimed at clarifying such problems
as the nature of the quark sea and the relation of the nucleon spin to the spins of the
quarks within the nucleon, questions which are of a very fundamental nature.
The experimentalw ork on pion and nucleon reactions has- been carried out with the
LAMPF accelerator at the Los Alamos National Laboratory and the cyclotrons at the
Paul Scherrer Institute (PSI) near Zurich, Switzerland, and at the Indiana University
Cyclotron Facility (IUCF), as collaborative efforts among several laboratories and
universities. The quark-structure studies are in progress at Fermilab (FNAL), near
Chicago, at the, HEM facility at the Deutsches Elektronen Synchrotron (DESY)
laboratory in Hamburg, Germany, and at the Thomas Jefferson National Accelerator
Facility (TJNAF, formerly CEBAF) in Newport News, Va. Plans are also underway
for projects at the Relativistic Heavy Ion Collider (RHIC) facility under construction
at Brookhaven National Laboratory (BNL) near Upton, NY. These involve larger
group efforts than previously.
The personnel at NMSU supported by this grant included four faculty members
(including limited support of two new faculty who were hired jointly by NMSU and
TJNAF), six postdoctoral research associates (occupying two budgeted positions),
ten graduate students (including two from another university who worked on an
experiment for which we bear much responsibility), and two undergraduate students.
Our pion studies have involved measurements of elastic and inelastic scattering,
single- and double-charge-exchange scattering, quasielastic scattering, and absorp-
tion, using polarized and unpolarized targets. The physics addressed involved studies
of nuclear structure, of the pion-nucleus interaction mechanism, including its spin
dependence, and investigation of the pion-nucleus absorption mechanism, as well
as of charge symmetry and chiral symmetry breaking. The nucleon-nucleus charge-
1
exchange work has been aimed at increasing our understanding of the spin and isospin
dependence of the nucleon-nucleus interaction.
Our studies of the internal structure of nucleons and nuclei in terms of quarks and
gluons currently involve three experiments. Fermilab E866 (NUSEA Collaboration),
a measurement of the Drell-Yan process in the proton'and the deuteron, aimed at
determining the extent of flavor symmetry breaking in the nucleon sea-quark distribu- .
tions, began data taking in August 1996. HERMES, a measurement the deep-inelastic
'scattering of polarized electrons fiom polarized internal targets in the HERA storage
ring at the DESY laboratory in Germany, is presently in the second year of data tak-
ing. This is aimed at understanding the relation between the spin of the nucleon and
the spins of its quarks. TJNAF experiment E89-012, a measurement of deuteron pho-
todisintegration, completed data taking in spring 1996. These data will determine at
what photon energies the reaction mechanism is dominated by the underlying quark
structure of the proton and neutron.
Given the long timescales for mounting large experiments, it is already necessary
to begin preparations for experiments which will start data taking in 1999 or later,
when most of our present involvements will be completed. This year we joined the GO
collaboration at TJNAF and the PHENIX collaboration at RHIC. GO will determine
the flavor singlet form factors of the proton from measurement of parity violating
scattering of polarized electrons from a proton target. Our particular interest at
RHIC is measurement of spin-dependent proton-proton reactions which are directly
sensitive to the sea quark and gluon contributions to the proton spin.
, 'I
2
2 Experiments on the Quark-Gluon Structure of
Nucleons and Nuclei
During this time we have worked on preparations for and running of three experiments
which study the quark-gluon structure of nucleons and nuclei. These are the HERMES
experiment at HEMf acility at the DESY laboratory (Hamburg, Germany), E866 at
Fermilab, and E89-012 at TJNAF.
The HERMES collaboration constructed a new spectrometer and target system
in the HERA East Hall, and the first year of measurements of deep inelastic electron
scattering by a polarized internal 3He target was completed in 1995. Running with a
polarized proton target began in summer 1996, and is presently in progress; the com-
bined proton and 3He (neutron) measurements will permit the first direct quark flavor
decomposition of the nucleon spin structure. The HERMES program is expected to
continue past 1999.
The Fermilab experiment will compare the distributions of the up and down sea
quarks in the nucleon as determined from measurements of the Drell-Yan reaction on
proton and deuterium targets. E866 started data taking in July 1996. The Fermilab
schedule is somewhat uncertain at this time, but running is expected to continue into
1997 and possibly into 1998.
E89-012 at TJNAF, one of the first experiments to run at the newly-commissioned
facility, measured the photodisintegration of the deuteron in order to determine the
photon energies at which scaling according to quark counting rules occurs. This
scaling is an indication that the reaction mechanism is dominated by the underlying
quark structure rather than hadronic degrees of freedom.
The work we have done on these experiments is summarized below.
3
2.1 HERMES: Measurement of Spin-Dependent Structure
Functions of the Nucleon at HERA
Alberta, Argonne National Laboratory, California Institute of Technology, Colorado,
DESY/Zeuthen, Dubna, Edangen-Ncrnberg, LNF &ascatti, Ferrara, Reiburg, 'Gent,
MPI Heidelberg, nhob-Urbana, Liverpool, wbconsin-Madison, Maraurg,
Massachusetts Institute of Technology, Moscow, MCnchen, NIKHEF, Oregon State
University, Pennsylvania, St. Petersburg, INFN Sem'one Sanita (Rome), Simon
&mer University/TRIUM., Tokyo, YeTevan Physics Institute, and NMSU (Chum-
ney, Haas, Kyle, Papavassiliou, Pate, Simon); R. Milner, MIT, Spokesman.
The HERMES project explores the spin structure of the nucleon, a topic of funda-
mental concern to the nuclear and particle physics community. A series of polarized
deep-inelastic-scattering experiments at SLAC[l, 2, 3,4] and CERN[5, 61 have a pro-
duced a body of inclusive lepton scattering data which show that the quarks carry
much less of the nucleon spin than is supposed in a naive quark model. The goal
of HERMES is both to repeat these inclusive measurements with smaller systematic
errors and, more importantly, also to take semi-inclusive data (e.g. $(< e'?r+)X)
which can reveal more detailed information about the contribution to the nucleon
spin from each quark flavor.
While using the standard technique of deep inelastic lepton scattering to explore
nucleon structure, HERMES drastically departs from the previous spin structure
experiments at CERN and SLAC by making use of an internal polarized gas target
in an electron beam storage ring. The completed (and ongoing) CERN and SLAC
-
experiments use targets with large spin dilution factors that is, the polarized
nucleons represent a small fraction of the total number of nucleons in the target.
At HERMES, the targets are composed of pure polarized gases, greatly reducing the
dilution factor. The spin orientation of the HERMES targets can be flipped every few
minutes, eliminating any systematic errors that might arise from long-term changes
in detector performance.
Starting with an empty hall in April 1994, when the steel experimental plat-
form was delivered to the HERA East Hall and welded together, and ending on
27 November, 1995, the HERMES target and detector systems were fully installed
and commissioned. The first polarized deep inelastic scattering data were taken in
1995. Though this first year of running was not without its problems, overall the
effort was a tremendous success.
During the running period of April-November 1995, an internal target of polarized
4
3He was used (see Section 2.1.4 below describing this target). In addition, unpolarized
hydrogen and deuteriumgas targets were used. For 1996, a polarized hydrogen target,
driven by an atomic beam source, has been installed in place of the 3He target. This
new target can also permit the use of unpolarized gases as targets.
The NMSU group continues to be very active in the HERMES experiment. For
-
example, during running the experiment from Jdy November 1996 two of the eight
period coordinators were from our group. Two important upgrades to the experi-
mental system involving our group occured this year. One of them, the design and
installation of a forward trigger scintillator system, was led by our group and has
enabled the experiment to record much cleaner data, even in the face of worsening
proton background conditions. The other upgrade, involving several groups, was
the introduction of a new kind of trigger in which the scattered positron is not de-
tected. This "hadron trigger" enables HERMES to study polarized photoproduction
. of hadrons. Both of these upgrades are described below.
Here, we briefly describe the physics that HERMES can explore with these various
targets in these first two years of running, some preliminary results, and the outlook
for the future.
2.1.1 Inclusive Lepton Scattering
By inclusive scattering, we meas that only the scattered lepton is detected, so that
all final states of the target system are included. Inclusive scattering has been the
primary source of data on the spin-structure of the nucleon so far. The primary tools
for interpreting these data are various QCD sum rules.
Two types of inclusive asymmetries will be measured at HERMES, one for beam
and target both polarized longitudinally (All), and one for longitudinal beam and
transverse target polarization (Al):
- -
,tl ,tt at-' *t+
&=
= at1 + ,It' at-++ at"
However, the physically more meaningful quantities are the asymmetries in virtual
photon absorption, which are directly related to the structure functions[7]:
5