- Email: [email protected]
Latest results on “subthreshold” production of kaons and antiprotons
203~
Nuclear Physics A488 (1988) 203~208~ North-Holland, Amsterdam
LATEST RESULTS ON "SUBTHRESHOLD" PRODUCTION OF KAONS AND AEJTIPROTONS Jim Carroll University of California at Los Angeles, Los Angeles, California 90024* We present our recent results on "subthreshold" production of K-, Kf, and antiprotons in relativistic nuclear collisions. At a NN center-of-mass (cm) angle of O" we have measured: the inclusive cross sections for production of K- from Si+Si collisions at incident energies of 1.0-2.1 A GeV, and from CtC, and Ca+Ca at 2.1 A GeV; as well as the yield of Kf from Si+Si at 1.26 A GeV. At a cm angle of 90° we have measured the inclusive cross section for production of K- in Si+Si collisions at 2.1 A GeV. The systematics of the excitation function, the mass dependence, and the angular dependence are compared with similar data on subthreshold pion yields. The present status of the five antiproton candidate events is discussed.
1
INTRODUCTION
BY 'subthreshold production' we refer to nuclear reactions yielding particles that could not be produced in free nucleonnucleon collisions at the same value of kinetic energy per nucleon. This definition thus implies that some sort of collectivity is required for these reactions to occur. It is useful to distinguish between two types of 'collectivity' - that which is intrinsic to the nuclei singly (such as Fermi motion, clusters of nucleons or quarks, etc); and that which is characteristic of the collision itself (possible equilibration of the various kinetic and chemical degrees of freedom for example).
One expects to be able to measure the
intrinsic nuclear effects by studying q + A + q f X reactions, where q represents a 'non-composite' probe, and it is primarily the collisional.collectivity that is the object of study in nucleusnucleus subthreshold production.
By choosing to detect particles of
various masses (eg pions, kaons, antiprotons) one may study these collisional collective effects in a range of excitation energies, within which the dynamics of the collision process may be expected ~---~~______-~~----~~______ * Supported by USDOE under contract DE-AT03-81ER40027,PA DE AMO3-765SF-00034 0375-9474/88/$03.50 @ Elsevier Science Pub&hersB.V. (North-Holland Physics Publishing Division)
J. Carroll / “Subthreshold”production of kaons and antiprotons
204~
to change significantly.
Much work has been done in the past
few years in studying subthreshold pion productionl.
Our effort has
focussed on studies at higher excitation energies with the aim of understanding how, and how far, the collision dynamics proceeds toward equilibration when the excitation energies are far removed from the realm of conventional nuclear physics.
In the following,
due simply to considerations of time, I will present only our recent results2 and must leave until another occasion interesting comparisons with relevant work of others, both experimental and theoretical. 2. RESULTS Our initial measurements showed that, in Si + Si collisions at 2.1
A GeV, K- are produced at a level more than 10 times higher than
that obtained from a somewhat careful calculation which included the effects of internal nuclear motion, and that the invariant cross section shows an exponential behavior with a slope of about 95 MeV.3 Our program since then has been to outline, within the capabilities of available facilities, the systematics governing this process. The data presented here are still preliminary
in the sense that not
all systematic corrections have been identified and carried out, nor have systematic errors been calculated - all errors shown represent counting statistics only. 2.1. Excitation function for SiJrSi-+ K- +
X
Figure 1 shows invariant cross sections at O" for Si + Si -+ K- + X.
Points belonging to the same incident energy have been connected
by straight lines to indicate the groupings, with the exception of 101
I
’ I L
10-4 / 2 5 KAON & K'I:ETI"C""EN~GY4&") FIGURE 1. CM spectra for Si+Si->K-
-0 2 ---04 -0 6 AVAILABLEEXClTATIONENERGY (Go!) FIGURE Z.Excitationfunction for Si+Si
J. Carroll / ‘Subthreshold”production
the
data at 2.1
combined
A GeV, where the dashes
data sets.
than two orders
in the slope
energy).
There are some indications
cm energy phase
(at least for low values
where the incident
is high.
Although
space limitations,
kinematic
cover more
no large changes
are
of kaon cm kinetic
that the slope increases
energies
for
is low and the kaon center
this behavior
would be expected
from
these data are far from the absolute
boundary.
For the purpose
of estimating
a total cross section,
fitted the data at each incident
A small anisotropy
(to be discussed
angular
integral.
The results
plotted
vs Q (the available
no significant
structure
at the highest
excitation
is visible
energy per NN pair). Although
any simple conclusions channel.
about the size of the
The slope of the exponentially
part of the curve is about 50 MeV. slopes are observed threshold)
the errors
used to arive at this
figure do not permit
experiments
are shown in Fig.1,
at the L threshold,
strangeness
near absolute
kaon cm energies.
later) has been used in the
of this procedure
in the data as well as the approximations
exchange
we have
energy with a single exponential,
and have chosen to omit the points
section
a fit to several
the measurements
in cross section,
apparent
those points
indicate
Note that although
of magnitude
205c
of kaons and antiprotons
In the subthreshold
that range from about 1 MeV
to about 10 MeV at energies
is about 1% of its threshold
value
rising
pion (very
where the cross
(as is the case for the
kaon data). 2.2. Mass dependence. Figure target
3 shows the cm spectra
combinations
for three equal-mass
projectile-
C+C, Si+Si, and Ca+Ca, all at 2.1 A GeV incident 101
CM KE=?lMEV
I
CM KE=ZlBMEV :
400
KAON EM KI:&IC ;&&n&V) FIGURE3.MassdependenceatZ.lAGeV
A OF A + A COLLISION FIGURE4,MassdependenceatZlAGeV
206c
J.Carroll / “Subt~res~o~d’~prod~ct~on of kaons and antiprotons
energy.
The fitted lines show no appreciable change in slope when
going from carbon to silicon to calcium.
This same data is shown in
where the cross section at fixed kaon cm When
another form in Figure 4,
energy is plotted versus the mass number of the target.
interpreted as an Ak dependence the two points at lower mass give while the upper points give k = 1.1. At this stage of
k = 3.5,
analysis we prefer not to assign errors to these derived quantities. The subthreshold pion experiments also find values of k that We were decrease with target mass, but for N+AL they find k = 0.67. unable to obtain a combination of flux and energy large enough to permit the measurement of yields from the Nb+Nb and La+La systems so study of the apparent saturation, or roll-over, of the cross section must await new facilities. 2.3. Slopes of exponential kinetic energy spectra In Figure 5 we show a compilation of slopes obtained by fitting the experimental data. Within the existing statistics there is no obvious trend in the kaon data - all measurements fall between 90-100 MeV.
In particular there is no significant difference
between K- data at 90° and O" cm. The pion slopes are somewhat higher and consistent with other measurments. -T’-
I -
+ i---o-i i-oi
All below
0 deg
cm
0 deg
cm
(Si+Si)K90 deg
cm
0 deg
cm
2.1 A CeV
4 e
1.7 A GeV 1.55
*
0
”
(C+C)K-
is (%+Si)K-
w
-._._
I’
90 deg cm 0 deg cm
(Ca+Ca)K-
k--e-e
M
”
(Sl+Si)n (Si-tSi)?r
A GeV
1.4 A GeV
e-81 -L----L-.-~
1 26 A GeV
5
200 300 EXPONENTIAL SLOPE (MeV) FIGURE 5. Measured slopes. 100
2.4. Subthreshold
K+
400
10-d
l100
300
400
KAON'CM KINETIC p&J?CY MeV) FI~~RE6.Corn~arisonofK and k -data
measurement
Figure 6 contains the first result of our measurement of the cross section for subthreshold production of K', taken at an incident energy (1.26 A GeV) such that the Q for the NN + NM+
J. Carroll / “Subthreshold”production
of kaons and antiprotons
207~
channel was the same as that for the NN 3 NNK'K- channel at 2.1 A GeV.
Note that the K+ cross section is higher than that for the K-,
even though no strangeness exchange channel is available to contribute to this yield.
The observed K'/K- ratio at a fixed Q may
be greater than 1 because the absorbtion of IZ- in nuclear matter is larger than for K', while the final state phase space is larger for the 3-body Kf channel than the B-body K- channel.
The approximate equality of these cross sections suggests that the production mechanism is directly related to the Q of the NN collisions rather than to secondary for tertiary) production mechanisms such as strangeness exchange. 2.5. Angular dependence Several theories4 have suggested that there should be a large angular dependance of the K- yield.
The result of our measurements at O" and 90° cm are shown in Figures 7 and 8. The pion data, shown in Fig 7, show a large asymmetry, consistent with that observed in other inclusive experiments.
The kaon asymmetry (Fig 81, however,
is quite small (ds(90°)/ds(Oo) about 2), in rather strong disagreement with the theoretical calculations.
iI
I
-
i
0
$ ,:
o”
cm
90”
200
cm
400
600
PION CM KINETICENERGY (GeV) PIGLRE 7. Illonangulardependence aI 2.1 A GeV
KAON CM KINETICENERGY (GeV) ~IG~RE8.Kaonan~ulardep~nd~nce at 2.1A GeV
2.6. Unfinished analysis The kaon data which are still being analyzed include cm energy spectra for: subthreshold K'
1.26 GeVtA
Si+Si 3 Kf + X
above threshold
2.1 GeV/A
Si+Si 3 K' + X
target mass dependence
2.1 GeV/A
Si+(Mo,La,Pb) --fK- + X
J. Carroll / ‘Subthreshold”
208c
2.1.
Antiproton
In relativistic process
heavy ion collisions,
should indicate
processes.
accumulated protons:
the existence
During
TOF of protons
(The pion-proton
of the group is consistent under these conditions. high-resolution
with that of protons other events
between
The width
background.
as either pions
Of the 5 events,
(The Smirnov-Kalmogorov
that the 5 events
(p-bar/p-) is about 4 10e7
test indicates
are
Auxillary
'clean', with no
there is strong evidence
that these events are in fact anti-protons, a more quantitative
4 give
than those
come from the pulse height
in question Although
pile up in crucial detectors.
for pions by a third
with that of pions
identified
of the pion sample is only about 8s.)
ways of making
50 ps
the other two, is
and inconsistent
shows that the events
laboratory
is 2900 ps.)
in a lead glass calorimeter
by pions and kaons.
we have
of anti-
(TOF) of the group is within
are clearly
or kaons; there is no detected much larger pulse heights
or
under the same experimental
mid-way
All
apparatus
period,
characteristics
with the 50 ps sigma we measure
consistent
distribution
conditions
For each event, the TOF measured
detector,
that the probability
of extreme
TOF difference
or kaons.
produced
of a
as antiproton
our last data acquisition
the mean time-of-flight
conditions.
the observation
(1 Gev/NN-pair)
5 events with the timing
of the measured
of kaons and antiprotons
production
as far below threshold
production unusual
production
we are still working The observed
statement.
(at 1.89 GeV/c),
corresponding
on
yield
to a
cross section of about 80 nb/sr/(GeV/c).
REFERENCES 1) See Stachel,
et al, Phys. Rev. C 33
(1986) 1420 and references
therein. 2) The collaboration
which has done the work presented
Carlson,
J. Carroll,
J. Gordon,
Ganezer,
and S. Abachi,
Perez-Mendez, Hallman,
Barasch,
J-F Wang, K.
U.; E. Barasch,
et al, Phys. Lett. 161B and Schuermann,
V.
LBL; A. Shor, BNL; P. Kirk, LSU; T. UC Davis
3) Shor, et al, Phys. Rev. Lett. 48 (1982) 1597
4) Zwermann
here: S.
UCLA; B. Keay, G. Krebs, P. Lindstrom,
and T. Mulera,
Johns Hopkins
G. Igo, S. Trentalange,
(1985) 265
Phys. Lett. 145B
Barz and Iwe, Phys. Lett. 153B
(1985) 211
(1984) 315
Nuclear Physics A488 (1988) 203~208~ North-Holland, Amsterdam
LATEST RESULTS ON "SUBTHRESHOLD" PRODUCTION OF KAONS AND AEJTIPROTONS Jim Carroll University of California at Los Angeles, Los Angeles, California 90024* We present our recent results on "subthreshold" production of K-, Kf, and antiprotons in relativistic nuclear collisions. At a NN center-of-mass (cm) angle of O" we have measured: the inclusive cross sections for production of K- from Si+Si collisions at incident energies of 1.0-2.1 A GeV, and from CtC, and Ca+Ca at 2.1 A GeV; as well as the yield of Kf from Si+Si at 1.26 A GeV. At a cm angle of 90° we have measured the inclusive cross section for production of K- in Si+Si collisions at 2.1 A GeV. The systematics of the excitation function, the mass dependence, and the angular dependence are compared with similar data on subthreshold pion yields. The present status of the five antiproton candidate events is discussed.
1
INTRODUCTION
BY 'subthreshold production' we refer to nuclear reactions yielding particles that could not be produced in free nucleonnucleon collisions at the same value of kinetic energy per nucleon. This definition thus implies that some sort of collectivity is required for these reactions to occur. It is useful to distinguish between two types of 'collectivity' - that which is intrinsic to the nuclei singly (such as Fermi motion, clusters of nucleons or quarks, etc); and that which is characteristic of the collision itself (possible equilibration of the various kinetic and chemical degrees of freedom for example).
One expects to be able to measure the
intrinsic nuclear effects by studying q + A + q f X reactions, where q represents a 'non-composite' probe, and it is primarily the collisional.collectivity that is the object of study in nucleusnucleus subthreshold production.
By choosing to detect particles of
various masses (eg pions, kaons, antiprotons) one may study these collisional collective effects in a range of excitation energies, within which the dynamics of the collision process may be expected ~---~~______-~~----~~______ * Supported by USDOE under contract DE-AT03-81ER40027,PA DE AMO3-765SF-00034 0375-9474/88/$03.50 @ Elsevier Science Pub&hersB.V. (North-Holland Physics Publishing Division)
J. Carroll / “Subthreshold”production of kaons and antiprotons
204~
to change significantly.
Much work has been done in the past
few years in studying subthreshold pion productionl.
Our effort has
focussed on studies at higher excitation energies with the aim of understanding how, and how far, the collision dynamics proceeds toward equilibration when the excitation energies are far removed from the realm of conventional nuclear physics.
In the following,
due simply to considerations of time, I will present only our recent results2 and must leave until another occasion interesting comparisons with relevant work of others, both experimental and theoretical. 2. RESULTS Our initial measurements showed that, in Si + Si collisions at 2.1
A GeV, K- are produced at a level more than 10 times higher than
that obtained from a somewhat careful calculation which included the effects of internal nuclear motion, and that the invariant cross section shows an exponential behavior with a slope of about 95 MeV.3 Our program since then has been to outline, within the capabilities of available facilities, the systematics governing this process. The data presented here are still preliminary
in the sense that not
all systematic corrections have been identified and carried out, nor have systematic errors been calculated - all errors shown represent counting statistics only. 2.1. Excitation function for SiJrSi-+ K- +
X
Figure 1 shows invariant cross sections at O" for Si + Si -+ K- + X.
Points belonging to the same incident energy have been connected
by straight lines to indicate the groupings, with the exception of 101
I
’ I L
10-4 / 2 5 KAON & K'I:ETI"C""EN~GY4&") FIGURE 1. CM spectra for Si+Si->K-
-0 2 ---04 -0 6 AVAILABLEEXClTATIONENERGY (Go!) FIGURE Z.Excitationfunction for Si+Si
J. Carroll / ‘Subthreshold”production
the
data at 2.1
combined
A GeV, where the dashes
data sets.
than two orders
in the slope
energy).
There are some indications
cm energy phase
(at least for low values
where the incident
is high.
Although
space limitations,
kinematic
cover more
no large changes
are
of kaon cm kinetic
that the slope increases
energies
for
is low and the kaon center
this behavior
would be expected
from
these data are far from the absolute
boundary.
For the purpose
of estimating
a total cross section,
fitted the data at each incident
A small anisotropy
(to be discussed
angular
integral.
The results
plotted
vs Q (the available
no significant
structure
at the highest
excitation
is visible
energy per NN pair). Although
any simple conclusions channel.
about the size of the
The slope of the exponentially
part of the curve is about 50 MeV. slopes are observed threshold)
the errors
used to arive at this
figure do not permit
experiments
are shown in Fig.1,
at the L threshold,
strangeness
near absolute
kaon cm energies.
later) has been used in the
of this procedure
in the data as well as the approximations
exchange
we have
energy with a single exponential,
and have chosen to omit the points
section
a fit to several
the measurements
in cross section,
apparent
those points
indicate
Note that although
of magnitude
205c
of kaons and antiprotons
In the subthreshold
that range from about 1 MeV
to about 10 MeV at energies
is about 1% of its threshold
value
rising
pion (very
where the cross
(as is the case for the
kaon data). 2.2. Mass dependence. Figure target
3 shows the cm spectra
combinations
for three equal-mass
projectile-
C+C, Si+Si, and Ca+Ca, all at 2.1 A GeV incident 101
CM KE=?lMEV
I
CM KE=ZlBMEV :
400
KAON EM KI:&IC ;&&n&V) FIGURE3.MassdependenceatZ.lAGeV
A OF A + A COLLISION FIGURE4,MassdependenceatZlAGeV
206c
J.Carroll / “Subt~res~o~d’~prod~ct~on of kaons and antiprotons
energy.
The fitted lines show no appreciable change in slope when
going from carbon to silicon to calcium.
This same data is shown in
where the cross section at fixed kaon cm When
another form in Figure 4,
energy is plotted versus the mass number of the target.
interpreted as an Ak dependence the two points at lower mass give while the upper points give k = 1.1. At this stage of
k = 3.5,
analysis we prefer not to assign errors to these derived quantities. The subthreshold pion experiments also find values of k that We were decrease with target mass, but for N+AL they find k = 0.67. unable to obtain a combination of flux and energy large enough to permit the measurement of yields from the Nb+Nb and La+La systems so study of the apparent saturation, or roll-over, of the cross section must await new facilities. 2.3. Slopes of exponential kinetic energy spectra In Figure 5 we show a compilation of slopes obtained by fitting the experimental data. Within the existing statistics there is no obvious trend in the kaon data - all measurements fall between 90-100 MeV.
In particular there is no significant difference
between K- data at 90° and O" cm. The pion slopes are somewhat higher and consistent with other measurments. -T’-
I -
+ i---o-i i-oi
All below
0 deg
cm
0 deg
cm
(Si+Si)K90 deg
cm
0 deg
cm
2.1 A CeV
4 e
1.7 A GeV 1.55
*
0
”
(C+C)K-
is (%+Si)K-
w
-._._
I’
90 deg cm 0 deg cm
(Ca+Ca)K-
k--e-e
M
”
(Sl+Si)n (Si-tSi)?r
A GeV
1.4 A GeV
e-81 -L----L-.-~
1 26 A GeV
5
200 300 EXPONENTIAL SLOPE (MeV) FIGURE 5. Measured slopes. 100
2.4. Subthreshold
K+
400
10-d
l100
300
400
KAON'CM KINETIC p&J?CY MeV) FI~~RE6.Corn~arisonofK and k -data
measurement
Figure 6 contains the first result of our measurement of the cross section for subthreshold production of K', taken at an incident energy (1.26 A GeV) such that the Q for the NN + NM+
J. Carroll / “Subthreshold”production
of kaons and antiprotons
207~
channel was the same as that for the NN 3 NNK'K- channel at 2.1 A GeV.
Note that the K+ cross section is higher than that for the K-,
even though no strangeness exchange channel is available to contribute to this yield.
The observed K'/K- ratio at a fixed Q may
be greater than 1 because the absorbtion of IZ- in nuclear matter is larger than for K', while the final state phase space is larger for the 3-body Kf channel than the B-body K- channel.
The approximate equality of these cross sections suggests that the production mechanism is directly related to the Q of the NN collisions rather than to secondary for tertiary) production mechanisms such as strangeness exchange. 2.5. Angular dependence Several theories4 have suggested that there should be a large angular dependance of the K- yield.
The result of our measurements at O" and 90° cm are shown in Figures 7 and 8. The pion data, shown in Fig 7, show a large asymmetry, consistent with that observed in other inclusive experiments.
The kaon asymmetry (Fig 81, however,
is quite small (ds(90°)/ds(Oo) about 2), in rather strong disagreement with the theoretical calculations.
iI
I
-
i
0
$ ,:
o”
cm
90”
200
cm
400
600
PION CM KINETICENERGY (GeV) PIGLRE 7. Illonangulardependence aI 2.1 A GeV
KAON CM KINETICENERGY (GeV) ~IG~RE8.Kaonan~ulardep~nd~nce at 2.1A GeV
2.6. Unfinished analysis The kaon data which are still being analyzed include cm energy spectra for: subthreshold K'
1.26 GeVtA
Si+Si 3 Kf + X
above threshold
2.1 GeV/A
Si+Si 3 K' + X
target mass dependence
2.1 GeV/A
Si+(Mo,La,Pb) --fK- + X
J. Carroll / ‘Subthreshold”
208c
2.1.
Antiproton
In relativistic process
heavy ion collisions,
should indicate
processes.
accumulated protons:
the existence
During
TOF of protons
(The pion-proton
of the group is consistent under these conditions. high-resolution
with that of protons other events
between
The width
background.
as either pions
Of the 5 events,
(The Smirnov-Kalmogorov
that the 5 events
(p-bar/p-) is about 4 10e7
test indicates
are
Auxillary
'clean', with no
there is strong evidence
that these events are in fact anti-protons, a more quantitative
4 give
than those
come from the pulse height
in question Although
pile up in crucial detectors.
for pions by a third
with that of pions
identified
of the pion sample is only about 8s.)
ways of making
50 ps
the other two, is
and inconsistent
shows that the events
laboratory
is 2900 ps.)
in a lead glass calorimeter
by pions and kaons.
we have
of anti-
(TOF) of the group is within
are clearly
or kaons; there is no detected much larger pulse heights
or
under the same experimental
mid-way
All
apparatus
period,
characteristics
with the 50 ps sigma we measure
consistent
distribution
conditions
For each event, the TOF measured
detector,
that the probability
of extreme
TOF difference
or kaons.
produced
of a
as antiproton
our last data acquisition
the mean time-of-flight
conditions.
the observation
(1 Gev/NN-pair)
5 events with the timing
of the measured
of kaons and antiprotons
production
as far below threshold
production unusual
production
we are still working The observed
statement.
(at 1.89 GeV/c),
corresponding
on
yield
to a
cross section of about 80 nb/sr/(GeV/c).
REFERENCES 1) See Stachel,
et al, Phys. Rev. C 33
(1986) 1420 and references
therein. 2) The collaboration
which has done the work presented
Carlson,
J. Carroll,
J. Gordon,
Ganezer,
and S. Abachi,
Perez-Mendez, Hallman,
Barasch,
J-F Wang, K.
U.; E. Barasch,
et al, Phys. Lett. 161B and Schuermann,
V.
LBL; A. Shor, BNL; P. Kirk, LSU; T. UC Davis
3) Shor, et al, Phys. Rev. Lett. 48 (1982) 1597
4) Zwermann
here: S.
UCLA; B. Keay, G. Krebs, P. Lindstrom,
and T. Mulera,
Johns Hopkins
G. Igo, S. Trentalange,
(1985) 265
Phys. Lett. 145B
Barz and Iwe, Phys. Lett. 153B
(1985) 211
(1984) 315