Nuclear Physics B89 (1975) 1-18
© North-Holland Publishing Company
D I S C O V E R Y O F THE NEW P A R T I C L E J *
J.J. A U B E R T :~, U. B E C K E R , P.J. BIGGS, J. B U R G E R , M. CHEN,
G. E V E R H A R T , P. G O L D H A G E N :~:~, J. L E O N G , P. M A N T S C H #,
T. M c C O R R I S T O N ##, T.G. R H O A D E S , M. R O H D E ##,
Samuel C.C. T I N G and Sau Lan WU
Laboratory for Nuclear Science and Department of Physics,
Massachusetts Institute of Technology, Cambridge, Mass. 02139~USA
J.W. G L E N N III and Y.Y. LEE
Brookhaven National Laboratory, Upton, Long Island, N Y 11973~USA
Received 3 February 1975
We present a description of an experiment carried out at the 30 GeV Alternating
Gradient Synchrotron of the Brookhaven National Laboratory. The experiment used a
high intensity slow extracted proton beam of between 101° and 2 • 1012 protons per
pulse and measured the e+e - mass spectrum from the reaction
p + B e ~ e + e - + X.
The result of this experiment shows the production of a new type of particle, J, which
decays to e+e - with a width consistent with zero.
1. I n t r o d u c t i o n
In the last decade our group has been working on experiments associated with
e+e - pairs p r o d u c e d f r o m hadron interactions at high energies. One learns three
kinds o f physics f r o m the reaction
h +p ~e+e - +X.
(i) Using a 7.5 G e V bremsstrahlung p h o t o n b e a m one can compare the e+e * Work supported in part by the US Atomic Energy Commission through AEC contract No.
AT (11-1)-3069.
1: Permanent address: Institut de Physique Nucl6aire, Orsay, France.
:~:~Present address: Rutgers University, New Brunswick, NJ 08903.
#Present address: Fermi National Accelerator Laboratory, P.O. Box 500, Batavia, Illinois
60510.
## Deutsches Elektronen-Synchrotron, Hamburg, West Germany.
2
J.J. A u b e r t et al./The n e w particle J
yield with predictions o f QED at large momentum transfer or small distances,
< 10 -14 cm [i].
(ii) One can study the e÷e - decay of photon-like particles with spin 1 and negative parity and charge conjugation, such as the p [2], ~b [3], co [4] and p' [5] and
measure the coupling strengths between photons and massive photon-like particles
[6]. One can also study the production mechanisms of these photon-like particles
produced by photons.
(iii) Search for additional particles which decay to e+e- from pp ~ e+e- + X or
pp ~ #+#- + X.
In this paper, we shall concentrate on an experiment carried out at Brookhaven
using a precision pair spectrometer to detect the e+e - pairs in the mass region 1.5 to
5.5 GeV/c 2 with a mass resolution of---5 MeV/c 2 and a sensitivity of 10 -36 cm 2 in
oB (cross section × branching ratio).
2.
Theory
There have been many theoretical speculations [7] on the existence o f long lived
neutral particles with a mass larger than 10 GeV/c 2 which play the role in weak interactions that photons play in electromagnetic interactions. There is, however, no
theoretical justification, and no predictions exist, for long lived particles in the mass
region 1 - 1 0 GeV/c 2 [5].
There are calculations based on parton models on the production of an e+e - continuum from pp interactions [8]. An early experiment at the AGS [9] studied the
continuum of #+/~- from p +uranium -~/a+# - + X. This experiment gives approximately the size ofg+/a - yield.
Even though there is no strong theoretical justification for the existence of long
lived particles at low masses, there is no experimental indication that they should
not exist. No high sensitivity experiment has been done in this mass region.
3. Design considerations
To perform a high-sensitivity experiment, detecting narrow width particles over
a wide mass range, we make the following four observations.
(i) Since the e+e - come from photon decays, the yield of e+e - is lower than hadron pairs (n+rr - , K+K - , pp, K+p, etc.) by a factor
ct2 F 2 ( m 2 ) ~ 1 0 - 6 .
m4
The factor a 2 c o m e s from the virtual photon decay, m - 4 is the photon propagator and F ( m 2) the form factor of the target proton, where m is the invariant mass of
the e+e - pair.
J.J. A u b e r t et al./The new particle J
3
(ii) Because of(i) one must design a detector which can stand a high flux ofhadrons to obtain a sufficient yield of e+e - pairs.
(iii) The detector must be able to reject hadron pairs by a factor ~ 1 0 6 - 1 0 8 .
(iv) In choosing the best kinematic region to detect the decay of new particles,
one notes that at high energies, inclusive production of p, lr and w from pp interactions [10] can all be described in the c.m. system by a dependence of the form
d3o
J_*2
dp_*IIup±
_ ae-bP~
- - ,
E*
independent o f P~l,
where PlI, P± and E * have their usual meaning.
Thus the maximum yield will occur when the particle is produced at rest in the
centre of mass. If we look at the 90 ° decay of the e+e - pair, we note that they
emerge at an angle 0 = arc tan (1/3') = 14.6 ° in the lab system for an incident proton
energy of 28.5 GeV, independent of the mass of the decaying particle.
4. The beam
The incident beam of intensity 1 0 1 ° - 2 • 1012 protons per pulse was focussed to
a spot size 3 X 6 mm 2. The position of the beam was monitored by a closed circuit
TV. The stability and the intensity of the beam were monitored by a secondary
emission counter and six arrays of scintillation counter telescopes, located at an angle of 75 ° with respect to the beam, and buried behind 12 feet of concrete shielding.
Daily calibrations were made of the SEC with thin A1 and C foils [ 11 ].
5. Experimental set-up
Figs. 1 and 2 show the plan and side views of the spectrometer and detectors.
M0, M 1 and M 2 are dipole magnets. Bending is done vertically to decouple angle (0)
and momentum. The field of the magnets in their final location was measured with
a three dimensional Hall probe at a total of 105 points. CB, C O and Ce are gas threshold ~erenkov counters, C B is Idled with isobutane at 1 arm., C O is filled with hydrogen at I arm. and C e is filled with hydrogen at 0.8 atm. Plan and side view layouts of
both the C B and C O counters are shown in figs. 3 and 4 respectively. A0, A, B and C
are proportional wire chambers with 2 mm spacing and a total of 4 000 wires on
each arm. Behind chambers A and B are situated two planes of hodoscopes, 8 X 8,
for improving timing resolution.
Behind the C chamber there are two orthogonal banks of lead glass counters of
three radiation lengths each, the first containing twelve elements, the second thirteen, followed by one horizontal bank of lead lucite shower counters, seven in number each ten radiation lengths thick, to further reject hadrons from electrons and im-
4
J.J. Aubert et al./The new particle J
C
e
M E ~ 1M1 1 1 1 1 1~~ ~
C
Fig. 1. Plan view of the spectrometer.
B
C
BEAM I
TARGE~T
I
Im 2m
I
1
Fig. 2. Side view of the spectrometer.
prove track identification. During the experiment the outputs from all the counters
were monitored by a PDP 11/45 computer and all the high voltages were checked
every 30 minutes.
The target consists of 9 pieces of 1.78 mm beryllium. This helps us to reject pair
accidentals by requiring the two tracks to come from the same origin.
We list the following unique features of this experiment:
0) To obtain a rejection against hadrons of 108 or better, the two gas ~erenkov
counters in each arm, CO and Ce, are filled with hydrogen and made with thin mylar
windows to reduce knock-on and scintillation effects. The counters are painted black
inside and are decoupled by the strong magnetic fields of M1 and M2, so that knockon electrons produced in CO do not enter Ce and only electrons along the beam trajectory will emit ~erenkov light which is focussed onto the photomultiplier tube.
Special high gain, high efficiency phototubes of the type RCA C3 1000M, were used
to enable us to see the single photoelectron peak (fig. 5).
(ii) To be able to handle a high intensity of 2 × 1012 ppp, with consequent single
arm rates of ~ 20 MHz, there are eleven planes of proportional wires (2 X A0, 3 × A,
J.J. Aubert et al./The new particle J
SIDE V~EW
5
0
0,5
Meters
Fig. 3. Plan and side view of the C B counter shown in its location in the experiment.
~Phototube
Sphericl Mirror
PLAN VIEW
Parabolic
Mirro~
SlOE VIEW
Fig. 4. Plan and side view o f the C o counter shown in its location in the experiment.
'I
6
J.J. A u b e r t et al./The n e w particle J
c
ci) A
a*g
***~
%=,.o=
tn
.==*Q%
eQ~e~olt**oQ!
~d_o
z
Pulse height
Fig. 5. Pulse-height spectrum from the phototube (RCA C31000M) o f the C o ~erenkov counter.
Clearly visible are the one, two or three photoelectron peaks.
-60"
0
+60*
+ 20*
-80 o
. ~ _ ~
-5" +5
3 foreoch
wire # =const
Z
,
Fig. 6. Relative orientation of the planes of wires in the pzoportional chambers.
3 X B, and 3 X C) rotated 20 ° with respect to each other as shown in fig. 6 to reduce multitrack ambiguities.
(iii) To reduce multiple scattering and photon conversion, the material in the
beam is reduced to a minimum. The front and rear windows of C O are 125/~m and
250/~m thick respectively, both the mirrors o f C o and Ce are made o f 3 m m black
lucite and the hodoscopes are 1.6 m m thick.
The thickness of one piece of beryllium target is 1.8 mm and the nine pieces are
each separated by 7.5 cm so that particles produced in one piece and accepted by
the spectrometer do not pass through the next piece.
(iv) To reduce photon and neutron contamination the location of all the hodoscopes and lead glass counters are such that they do not see the target directly. To
further shield the detectors from soft neutrons, 10 000 lbs of borax soap was placed
on top of C O between M 1 and M 2 and around the front part o f Ce behind M 2.
J.J. Aubert et aL/The new particle J
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,
L~_
I00 200
,,, , t , . , !
mm
0
I=
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0
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i 150ram
Fig. 7. Sketch of one o f the M o magnets used for measuring the a m o u n t o f background in the
mass plot coming from coincidences between Dalitz pair electrons.
v
C B Time
of flight
for
r e j e c t i o n of p p - * 7 / ° + X
L~y+e ÷ e- v
-I
] channel
4"CB
L-----~S pectromefer
: 0.2 ns
bJ
Z
I00
Channel
200
Number
Fig. 8. Relative timing between a pulse f r o m the C B counter and an electron trigger from the
same spectrometer arm.
7
8
J.J. Aubert et aL/The new particle J
2o
p-~b acceptance
P 18
(GeV/c)
16
14
12
IC
8
6
4
2
o
-6o
I
-40
I
-co
I
o
I
+zo
(m rad.)
I
+40
+60
Fig. 9. p-q~ acceptance for the medium p setting (mee > 2.2 GeV/c2). This plot, taken from a
Monte-Carlo program, includes the effect of the extended target.
(v) To improve the rejection against n o -* 7e+e - , a very directional ~erenkov
counter C B was placed close to the target and below a specially constructed magnet
M 0 (fig. 7). This counter is painted black inside and is sensitive to electrons o f
10 MeV/c and pions above 2.7 GeV/c, The coincidence between C B and CO, Ce, the
shower counters and the hodoscopes indicates the detection o f an e+e - pair from
the process n o ~ "re+e - , and such events are rejected. A typical plot o f the relative
timing of this coincidence is shown in fig. 8. Conversely, one can trigger on C B and
provide a pure electron beam to calibrate CO, Ce and the shower counters. This is
particularly important in setting the voltage of the C O counters, since the coincidence between C O and C B will ensure that the counter is efficient for a single electron and not a zero degree pair.
(vi) The spectrometer has a very large mass acceptance o f 2 GeV/c 2 and enables
us to study the mass region 1 . 5 - 5 . 5 GeV/c 2 in three overlapping settings. F o r a
point target the acceptance in 0 is + 1°, in ~'it is -+2° and in m o m e n t u m it varies
from 0.6 X.P0 to 1.8 × P0 (where P0 is the principal axis momentum), all in the
lab system. A typical p-q~ acceptance for the experimental situation with an extended target is shown in fig. 9.
J.J. Aubert et aL/The new particle J
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5
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A
A
A
A
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AAA
8
.~N
A
A
A
AA
A AA ~AC
~
c
Fig. 10. Mapping of the efficiency of the Ce counter over its whole phase space. The letters on
the plot refer to efficiencies measured for trajectories between the corresponding points marked
on the grid at each end of the counter.
6. Experimental procedure
Before taking data, approximately 100 hours were spent ensuring that all the detectors were functioning properly and with close to 100% efficiency over the whole
acceptance. We list the following examples:
(i) The efficiency of the ~erenkov counters were measured over the whole phase
space and voltages set.so that everywhere they were close to 100% efficient. A typical result for Ce is shown in fig. 10.
(ii) The voltages and response o f all the lead glass and shower counters were calibrated to ensure that the response did not change with time. A typical pulse-height
spectrum from one of the shower counters is shown in fig. 11.
(iii) The efficiency of the hodoscopes at the far end of the counters was checked
to ensure that they were 100% efficient.
(iv) The timing of the hodoscopes was also checked to ensure that pulses from
each counter arrive simultaneously. A typical result is shown in fig. 14. During the
experiment, the time of flight of each of the 64 hodoscopes and the ~erenkov counters, the pulse heights of the ~erenkov counters and the 78 lead glass and shower
10
J.J. Aubert et aL/The new particle J
tt~
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,
,
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[
,
,
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Ioo
Channel
IL~'~.JI.r.
_..
I
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,
2oo
Number
Fig. 11. A typical pulse-height spectrum for electrons in one of the shower counters. The electron peak is clearly visible. The large number of events at low-pulse height correspond to events
where the energy is shared between two adjacent shower counters. When the pulse heights from
all the counters are summed, this peak disappears.
counters, the singles rates o f all the counters together with the wire chamber signals
were recorded and c o n t i n u o u s l y displayed on a storage/display scope.
(v) To ensure that the PWC's were 100% efficient over their whole area a small
test c o u n t e r was placed b e h i n d the chambers at various positions over the chambers'
area and voltage excitation curves were made at those positions. A typical set of
curves for all the planes is shown in fig. 12.
(vi) To check the timing b e t w e e n the two arms two tests were performed. Firstly,
the test c o u n t e r was physically moved from one arm to the other so that the relative
timing could be compared. Secondly, the e+e - yield was measured at low mass,
rnee < 2 GeV/c 2, where there is an a b u n d a n c e of genuine e+e - pairs, as shown in
fig. 13.
II
ZJ. A u b e r t et al./The new particle J
Z~./
RUN 44
CIIAIqBER CL
SET : low P
TARGET : 9 x Be
PULSE : 4 x 10"
EVENTS : 1000
G2 RATE : IO/PULSE
TRIG : G1 =DX
G2=RI.SI'TI.UI.TEST
AUG 23 1974
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t z)l z-.t, E.~ ~ .112
Fig. 12. E f f i c i e n c y o f all the wire planes as a f u n c t i o n o f the applied voltage. T h e p o s i t i o n w h e r e
this m e a s u r e m e n t w a s m a d e is indicated b y the sketch in the t o p left h a n d corner.
(vii) To avoid systematic errors half of the data was taken at each polarity setting
of the spectrometer.
7. Electronics
In order to handle an intensity of"~ 20 MHz of hadron/arms and "~ 10 KHz of
electrons/arms, we form an initial electron gate, which is a loose coincidence of the
four ~erenkov counters (C O and Ce) , and this is used as a strobe for the hodoscope
discriminator pulses.
The outputs of these gated coincidences are at a "~ KHz rate and they are used as
the stop pulses in the TOF units. The coincidence of these gated hodoscope pulses
form a master trigger for CAMAC gates and for the PWC readout.
12
J.J. A ubert et aL/The n e w particle J
Relative
a r m s for
t i m i n g between
mee<2 G e V / c z
~
~ 1.2 ns
.Q
Z
Channel Number
Fig. 13. Relative timing for events in the mass region below 2 GeV/c 2. This plot serves as a check
on the coincidence between the two arms.
Because of the high rate in the C B counter, 200 MHz circuits were used. All other
circuits were 100 MHz. The efficiency of the ~erenkov and shower counters were
displayed with 100 MHz TSI scalers.
A block diagram of the electronics is shown in fig. 15.
8. Experimental results
Fig. 16 shows the time of flight between the e + and e - arms in the mass region
2.5 to 3.5 GeV/c 2. Fig. 17 shows the total energy spectrum from the lead glass and
shower counters in the energy region 2.5 to 3.5 GeV/c 2. The first data from August
1974 are shown in fig. 18. There is a clear, sharp enhancement at a mass of
3.1 GeV/c 2 [12].
To ensure that the observed peak is a real particle, many experimental checks
were made on the data. We list six examples:
(i) When we decreased the magnet currents by 10%, the peak remains fixed at
3.1 GeV/c 2 (see fig. 18).
(ii) To check second order effects on the target, we increased the target thickness by a factor of two. The yield increased by a factor o f two, not by four.
(iii) To check for pile up in the lead glass and shower counters, different runs
were made with different voltage settings on the counters. No effect was observed
on the yield of J.
(iv) To insure that the peak is not due to scattering from the sides of the magnets,
cuts were made in the data to reduce the effective aperture. No significant reduction
in the J yield was found.
J.J. Aubert et aL/The new particle J
HODOSCOPE
I
S,
13
TIME
OF F L I G H T
CHANNEL
= 0.2ns
~,,............ ~,,,,,,~,,,,,,I
o
I O0
Channel
200
Number
Fig. 14. Time-of-flight spectrum for one of the hodoscope elements using a single arm trigger.
(v) To check the read'-out system of the chambers and the triggering system o f
the hodoscopes, runs were made with a few planes o f chambers deleted and with
sections o f the hodoscopes omitted from the trigger. No effect was observed on the
J yield.
(vi) Runs with different beam intensity were made and the yield did not change.
These and many other checks convinced us that we have observed a real massive
particle J -+ e+e - .
Partial analysis o f the width of the J particle shown in fig. 19 indicates it has a width
smaller than 5 MeV/c 2.
If we assume a production mechanism for J to be
14
J.J. A u b e r t et aL/The n e w particle J
DC stop
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Left Arm R8
Hodoscopes
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(ii) Gote ADC's
(hi) Signet PDP
11/45 to begin
reed cycle
Right Arm
Hodoscopes
E,F,G 8 H
' -[2>
Fig. 15. Block diagram of the electronics.
d3o
dp*2cln*
k re-'l[
_ e - 6 Pxg~
independent o f p ~ ,
E*
'
and an isotropic decay in the rest system o f the J, we obtain a J -~ e+e - yield of
10 -34 cm2/nucleon at 28.5 GeV.
Fig. 20 shows the yield o f e+e - in the region 3.2 to 4.0 GeV/c 2, normalized to
fig. 18. The acceptance in this region is a smooth function and varies at most by a
factor o f two. The observed events are consistent with purely random coincidences.
To a level of 1% of the J yield, with a confidence level o f 95%, no heavier J particles were found. We note that this upper limit is independent o f any production
mechanism. If we assume the same production mechanism as the J, we obtain an upper limit o f 10 -36 cm2/nucleon for the production of heavier J's with a 95% confidence level.
J.J. A u b e r t et al./The n e w particle J
15
50 -
40~
All Events
30
,,>,
_
Events with
I rJ J~'~ 3"0< mee < 3"2 GeV/~2
20
z=
I
II
Z
I0
i
I
L1
, 'ii~:f,
r- 1t3I
L ]-I
r
0
-8
-4
0
4
8ns
Fig. 16. Time-of-flight spectrum between the two arms for events in the mass region
2.5 < mee < 3.5 GeV/c 2. Note that for events within the J peak, the timing spectrum shows a
peak almost completely free of background.
20"
Total Energy Spectre from Pb Gloss
and Shower Counters
5.0 < mee < 3.2 GeV/c z
~15
~s
,,>,
% IO
Right Arm
~
]
~
E
~5
20
15
Ld
"6 I0
Left Arm
E
nnFl--
F3F
1~:,
Pulse Height
Fig. 17. The total energy spectra from the lead glass and shower counters for both arms. The
pulse heights from all the ADC's were added linearly on each arm.
2.5
3.0
I
3.25
3.5
,.... n ra
me* e- [GeV/c 2]
2.75
,o1~~~'~
20-
30-
4O
50
Fig. 18. Mass spectrum for events in the mass range
2.5 < mee < 3.5 GeV/c 2. The shaded events correspond
to those taken at the normal momentum setting, while the
unshaded ones correspond to a momentum setting 10% below normal.
Z
E=
b..
bJ
E
ID
(M
>
%
[ ] normal c u r r e n t
2 4 2 Events -~
6 0 - F I -10% current
70
80
z
.¢1
E
>
uJ
g
t~
U3
>
°Jca
I
+15
I
+10
[
+5
I
I
0
-5
MeV/c 2
Central moss
= 3112 M e V / c 2
I
-I0
1
-15
-20
Fig. 19. Preliminary analysis of the width of the J.
+20
2
4
6
8
i
IO--
I
12--
t~
J.J. Aubert et aL/The new particle J
17
0
t'J
\4
c
w
Y~
z,
:3.2
__~
3.:3
I
3.4
I
3.5
I
3.G
I
3.7
3.8
3.9
4.0
mee ( GeVJc 2 )
Fig. 20. Mass spectrum for events in the mass range 3.2 < mee < 4.0 GeV/c2, normalized to
fig. 18.
9. Impfications
To understand the role of J particles in physics, many more experiments are
needed, in particular we are now searching for long lived particles which decay to
~p, K+K - , K-p, K+~r- and also e+e- at higher and lower masses.
We are also performing excitation curves using a proton target by changing the
energy of the incoming proton beam from 20 to 30 GeV in steps of 2 GeV. This
will give us information on the nature of the production of J's. Attempts will also
be made to see if there are charged J particles [13].
We wish to thank H. Foelsche, L. Smith, D. Berley and the A.G.S. Staff who
have done an outstanding job in setting up this experiment.
We wish to thank Dr. F.J. Eppling, B.M. Bailey, C. Tourtellotte, and the Staff of
the L.N.S. for their help and encouragement.
We also thank Professors E. Lohrmann and G. Weber of the DESY for much help
and encouragement.
We also thank Ms. I. Schulz, Ms. H. Feind, P. Berges, N. Feind, D. Osborne, G.
Krey, R. DiGrazia, J. Donahue, E. Weiner, T. Jones, A. Cook, W. Toki, Dr. T. Nash
and Dr. W. Busza for their help and assistance.
We also thank M. Deutsch, S.D. DreU, T.D. Lee, S. Glashow, R.R. Ran, V.F.
Weisskopf, T.T. Wu, C.N. Yang for interesting discussions.
18
J.J. Aubert et aL/The new particle J
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