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Early results from experiment UA1 at the CRN pp# collider
Anne Kernan
Citation: AIP Conference Proceedings 85, 325 (1982);
View online: https://doi.org/10.1063/1.33563
View Table of Contents: http://aip.scitation.org/toc/apc/85/1
Published by the American Institute of Physics
Chapter VI
Early Data from CERN
pp Collider
325
EARLY RESULTS FROM EXPERIMENT UAI AT THE
CERN pp COLLIDER
Anne Kernan
University of California, Riverside,
CA
92521
(Aaohen - A n n e c y (LAPP) - B i r m i n g h a m - CERN - Q u e e n M a r y College,
L o n d o n - P a r i s (College de F r a n e e ) Riverside - Rome - Rutherford
A p p l e t o n L a b o r a t o r y - S a c l a y (CEN) - Vienna C o l l a b o r a t i o n )
The UAI detector is a general purpose 47 apparatus for the measurement of hadron and lepton
momenta at p~ collider energies.
The performance
of the detector and first results from 1981 running
are discussed.
I.
INTRODUCTION
The design of the UAI detector evolved from the p-~ study weeks
organized by C. Rubbia at CERN in March and July of 1977.
A proposal [i] was submitted in January 1978 and was approved in Summer 1978.
The detector was installed in the SPS tunnel in early July 1981 and
recorded proton-antiproton interactions at 540 GeV for the first time
on July 17, 1981 [2].
The results presented here are based on data taken during accelerator development operations in October and November 1981 with luminosity in the range 1025cm-2sec -I.
II. ~ ~IE UAI DETECTOR
The detector has three components:
(i)
the large angle detector covering the angular range 5~
175 ~ with respect to the circulating beams,
(ii) the forward detectors at 0.3~
~ and
(iii) the luminosity monitors at 0.2<e<2 mrad.
i.
The Large Angle Detector
This apparatus (Fig. i) covers the rapidity range -3<Y<3 at cm
3
energy 540 GeV.
It is dominated by the dipole magnet, 7.2x3.5x3.5 m ,
with internal magnetic volume 80 m J, and uniform horizontal field 0.7T.
Each half of the magnet is composed of 8 independently movable C-shaped
sections.
As shown in Fig. 2 these sections are instrumented with
scintillator, providing 16 samples of 5 cm iron with i cm of scintillator (5.5 interaction lengths).
Endcaps of similar construction, with
23 samplings, cover the angular range 5o<0<25 ~ .
The electromagnetic calorimeter inside the magnet consists of 48
units covering the region 25~
~
Each unit (Fig. 3) is made of
26 radiation lengths of 1.2 mm lead sheets interleveaved with 1.5 E~
scintillator sheets.
The endcap electromagnetic calorimeters
(50<8<250) have 27 radiation lengths of lead-scintillator structure.
0094-243X/82/850325-2253.00
Copyright 1982 American Institute of Physics
326
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O
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327
Fig. 2.
One of the 16 "C" models comprising the
magnet yoke and hadron calorimeter.
328
Each is segmented into 32 identical radial pet&Is.
Fig. 3.
Two units of the large angle electromagentic
calorimeter.
The heart of the large angle apparatus is the 6 m long x 2.4m diameter central detector
(Fig. 4) which surrounds the beam pipe [3,4].
This drift chamber system has a total of 6200 sense wires, with maximum drift distance 18 cm. The "image" readout records the complete
time structure of the incoming pulse, providing a bubble-chamber like
picture of the event.
The 2-track resolution is 3 mm and dE/dx resolution is +6%. The coordinate along the wire is obtained by current
division.
Fig. 5 shows the image read-out for events recorded with a
"minimum bias" trigger, (section III), with 80% of the chamber volume
329
Fig. 4.
The Central Detector "image" drift chambers.
330
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331
instrumented at half density and a field of 0.28T.
The exterior of the magnet and endcaps is covered down to i0 ~ by
eight planes of staggered drift tubes for muon detection (Fig. 6) [5].
These contain a total of 5200 sense wires with wire lengths up to 6 m.
Fig. 6.
2.
Muon chamber setup.
The Forward Detector
The forward detector (Fig. 7) extends to within 5 mrad of the
coasting beams the UAI philosophy of complete coverage with image
chambers, and hadronic/electromagnetic calorimetry.
The compensator
dipole in the forward arm is calorimetrized and (on the outgoing
side) contains a 3 meter-long image chamber with 600 sense wires transverse to the beam direction.
3.
The Luminosity Monitors
For luminosity monitoring the small angle elastic scattering
rate is measured by a set of 4 drift chamber telescopes which for
this run were symmetrically arranged at •
m from the crossing point.
Fig. 8 shows the arrangement on one side.
In order to access angles
in the range 0.2<8<2 mrad (0.003<Iti<0.29 GeV 2) the drift chambers
and triggering scintillators are placed in movable sections of the
vacuum tube ("Roman pots") which can be positioned within a few mms
of the coasting beams.
The drift chambers (Fig. 9) have a resolution
i00 ~m in the drift plane and multihit (up to 16) capability.
332
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OJ
333
(a)
Roman pot
Beam
axis
~HI IFZ
//
~ nP
~ ' " Beam
p,pe
//
lator
Drift chamber
Fig. 8: (a) one arm of the SPS-UAI luminosity
monitoring drift chamber system
(b) luminosity monitor in the SPS beam line.
334
The second co-ordinate is obtained by current division with resolution about 2 mm.
176 mm
,_ 19 _,
Steel
Supp
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Sense Wire
9
Wind
/Field
Wire
...._
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i~
44
::
""Thin
woll
"
BEAM
Fig. 9.
Luminosity drift chamber, vertical cross section (mm units).
Fig. i0 shows (a) the ~ versus p angle in the drift time plane
and (b) its projection for candidate elastic scattering events.
The
corresponding plots for the charge division plane are shown in
Fig. ii. The almost complete absence of background is striking.
III.
THE TRIGGER
The design of the trigger is governed by the bunched operation
of the accelerator which is designed to give beam crossings of a
few nsec duration at 3.8 ~sec intervals.
At high luminosity the
trigger must be quite selective.
Thus at the expected maximum
luminosity of L = 1029 cm-2 sec -I the interaction rate will be about
5 x 103 per sec whereas the data acquisition rate is about i0 events
per second.
The main trigger incorporates three separate triggers as shown
in Fig. 12; the pre-trigger, (or minimum bias trigger), the calorimeter trigger and the muon trigger.
The first two and the first
level muon trigger given an accept-reject answer in time to clear the
system before the next beam crossing occurs.
335
Fig. i0. (a) @~ versus @p in the drift time
plane for candidate elastic
scattering events,
Fig. ii. Same for the charge division
coordinate plane.
336
C~,~d I
SIGNALS
MUON '.HAMBER
SIGNALS
EM CENTRAL
AND ENDCAPS
FORWARD ~ VERYFORWARO
(ALREADY ADDED)
I ANALOGUE J
ON ,AORON I
~L~_(!
CH
jAB
RG
~AOC'
TES) sJ
PRETRIGGER
~UON
TRIGGER
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|LEVEL ONE
r
PROCESSOR ;
(EI~qGV)
I PROCESSORZ
I (TRANSVF~_
J\ ENERGY J
t_
FINAL LEVEL LOGIC
I ACCEPT1REJECT
Fig. 12.
The UAI main trigger logic.
1
/
J
337
i.
The Pre-Trigger
The pre-trigger aims to select relatively unbiased beam-beam
interactions while rejecting background such as beam-gas collisions.
For the data reported here the pre-trigger was implemented by a •
nsec coincidence, centered around the crossing time, between hodoscopes at •
m on t h e p r o t o n and antiproton arms.
These hodoscopes
covered the angular range 0.8<e<3.4, (3.5<n<5.0); the minimum angle
of 0.8 ~ (t = 13 GeV2), excludes single diffraction dissociation
events.
2.
Calorimeter Trigger
The calorimeter trigger is based on the pattern of energy deposition in the various calorimeters.
Two custom-built digital processors
convert the ADC imputs to energy and transverse energy (E sinS) respectively.
In November, December 1981 data was recorded with a
range of E T thresholds up to 40 GeV.
3.
Muon Trigger
Prompt high energy muons are identified by the fact that their
tracks point back to the crossing region.
The first level muon trigger uses the pattern of wire hits, in combination with a signal in
the corresponding hadron calorimeter block, to select muon candidates
which point within i00 mrad of the interaction vertex.
The second
level trigger uses microprocessors to reconstruct muon chamber tracks
with i0 mrad accuracy (i00 ~sec per track).
IV.
PHYSICS
The data presented here was obtained with the pre-trigger (section III). This is a "minimum bias" trigger except that single diffraction dissociation events are excluded by the minimum angle (0.8 ~ )
of the triggering hodoscopes.
All this data comes from the largeangle detector (section I.l) which covers the em angular range 5~
175 ~ with respect to the circulating beams.
All energy measurements
have been made with calorimeters.
i.
General Features of pp Interactions at 540 GeV
Figs. 13 through 16 survey the global properties of the interactions.
Fig. 13 is the distribution in total transverse energy ~ E sin8
measured by the electromagnetic and hadronic calorimeters for the
angular range 5~
~
(The energy scale has not been corrected
for the lower response of the EM calorimeter to hadrons as compared
to photons.)
In agreement with the results obtained by the BariKrakow-Liverpool - MPI Munich - Nijmegen collaboration in pp and ~-p
interactions at 300 GeV/C [6], we observe a high probability for the
occurrence of events with large total E T.
Our multiplicity measurements (section (IV.2) suggest that high E T events are primarily due to
"soft" collisions of high multiplicity rather than to hard scattering
of constituents.
338
I000
9 lO 9
u
n
i
I
i
i
i
5"<8<
O9 o
J75 ~
11042 events
Oo
00
I00
"E
o
E
Z
I0
|
0
Fig. 13.
I
20
I
I
I
I
i
I
40
60
80
@ansverse energy (GeV)
Transverse energy distribution for minimum bias events.
Fig. 14 plots the number of segments (maximum 64) hit in the
endcap Em calorimeters (5~
~ versus the number of hits (maximum
48) in the central EM calorimeter (25~
~
A hit may be due to
a photon from n ~ decay or to a charged hadron. We note a strong correlation between the multiplicities recorded in each calorimeter.
Fig. 15 shows the number of charged tracks versus the number of
EM calorimeter hits for the angular range 25~
~
The line indicates the approximate correlation for equal production of ~+, ~-,
n ~ wit~ each ~= giving rise to two calorimeter hits.
339
p~
0
H"
0
fD
r?
i-~.
R
m
i~.
Ln
oO
A ~ 0
E ) i~. t-h
^~
Lne
~
~ e
R
R
i~.
v
340
Fig. 15 has some relevance to the possible existence of
"Centauro" events in the collider energy range [ 7 ] . These events
which have been reported at cm energies ~ i000 GeV in cosmic ray
studies are characterized by an anomalously low T ~ component.
In this
plot such events would appear close to a line of unit slope passing
through the origin.
Because of the saturation of the calorimeter
at 48 hits, and the need for a detailed understanding of relative
calibrations for charged and neutral particles in the calorimeter, no
conclusions can be drawn at this time.
Fig. 16.
Hadronic versus electromagnetic energy in the angular range
25~
~ . The inset indicates how these energies are
measured.
Fig. 16 also shows how the UAI detector may be used to search for
Centauro events.
"Hadronic" energy is plotted versus "Electromagnetic" energy for the angular range 25~
~
Attempting a correspondence
between UAI and the cosmic ray detectors we define electromagnetic
energy as the energy deposited in the first l0 radiation lengths of the
electromagnetic calorimeter; the remaining energy deposited in the
rear EM calorimeter and the hadronic calorimeter is taken as hadronic.
341
2.
Charged Particle Multiplicity in the Central Region [8]
At the ISR it was found that the height of the rapidity plateau
increased by 40% over the energy range ~ss = 24-63 GeV [9]. The degree
of violation of Feynman scaling between 63 GeV and the 540 GeV energy
of the p~ collider is therefore of considerable interest.
We have measured the charged particle multiplicity n• in the
angular range 30~176
the corresponding range in pseudorapidity
q = -log tan 0/2 is lqI<l.3.
The data comes from a run without magnetic field in October, November 1981 with the pre-trigger described
in section III.l.
For this run approximately 50% of the central detector drift chambers were instrumented at half density.
About 90% of
the triggers are p~ collisions, the remaining 10% being due to beam
gas interactions.
A total of 789 events were scanned by physicists on a Megatek
display (Fig. 5). To obtain the mean charged particle multiplicity,
corrections were applied for electrons from Dalitz decay and conversion in the beam pipe and surrounding material (-6% • 2%) and nuclear
interactions in the same material (1.5% • 1%). No correction was applied for K~ or A ~ charged decays which would normally be counted as
two charged tracks. We obtain a mean charged particle multiplicity of
3.9 • 0.3 per unit of q at lql<l.3.
The quoted error includes an allowance for the uncertainties in the correction terms.
In order to
compare our data with those from Thom~ et al. [9, Fig. 17] we have included only events having at least one track in our fiducial region.
If events with zero central tracks are included the quoted numbers is
reduced by ~ 6% to 3.6 • 0.3.
Eiob. (GeV)
i0 3
~A
+1
tO 4
10 5
4
2
e~
o~--e
~
~
~
~
et
~-~-~176
al
xTasoka et el
oSoto
el el
ISR
pp(9)
pp (ll)
pp (12)
IIThis experiment
p~
I
I
I
l
I
30
50
I00
300
500
V-~ ( GeV )
Fig. 17.
The mean charged particle multiplicity per unit of q for
Inl<l.5 as a function of centre-of-mass energy for events
with at least one charged particle in the fiducial region.
The line is the linear fit of Thome et al. to their data.
342
Because thegminimum angle of the triggering hodoscopes is = 14
mrad (t = 13 GeV-) single diffraction dissociation events are excluded
from the sample.
At the ISR these constitute ~ 14% of all inelastic
events [i0]. However these events should negligibly populate the
region of JnJ < 1.3, and their loss should not affect the estimate of
n• for events with at least one track in this interval.
Fig. 17 shows our result together with data from the ISR and
results from two cosmic ray experiments [11,12].
These cosmic ray results are from balloon experiments with nuclear emulsions.
100,
/,
\
J
,
744 + 45
I
'
o-prong events
/~
,t,*
r.
: 6.54
10
@
:J
0
z
011
0
L
5
I
10
CHARGED
Fig. 18.
a
15
I
20
I
25
30
MULTIPLICITY
Charged particle multiplicity
distributions
for Jnl< 1.3.
Fig. 18 shows that the charged particle multiplicity distribution
for JNJ<I.3 is significantly braoder than Poissonian.
In Fig. 19
the normalized charged particle multiplicity is plotted in terms of the
KNO scaling variable z = n•
The distribution argues well
with those measured over the ISR energy range for approximately the
same rapidity range, indicating that KNO scaling holds over the cm
energy range f~s = 50-500 GeV.
343
<n>+
o
O
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0
0
I
I
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Pn
I
0
0
I
I
I
I
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i
I
i
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0
--4>---
--41--
N
II
k~.
i+
A
,+v
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OJ
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01+
~:~'I-IZ0
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po0.10"l
-~
-~
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04
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l>
~ - ~
'8
I>
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,
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!
344
The long tail in Figs. 18, 19 is very likely correlated with the
realtively large cross section observed for events with large E T
(Fig. 13).
It has been suggested by Lattes et al. [7] that the C-jet events
of the Chacaltaya experiment, with an average incident energy of
~130 TeV, show a significant difference from the extrapolation of accelerator events both in their multiplicity and their transverse
energy.
They propose a correlation between E T and multiplicity, suggesting that events fall into different classes.
One such class has
a low value of average E T per secondary particle (gamma rays in their
case) and a low average number of gamma rays per unit of rapidity.
Another class has a high average value of ET per secondary and also a
high multiplicity.
The SPS collider at ~s = 540 GeV has a laboratory
equivalent energy of ~155 TeV for fixed target collisions and so it
is meaningful to examine the data for these effects.
!
|
!
L~
v
O)
(I)
- '15
I0-
o)
I/)
+§
t-
.l,=-
§
++
I0
5
-5
G)
. m
(/)
0
r
O)
O
0
Observed
Fig. 20.
I
I
i
5
I0
15
charged
track
0
multiplicity
Calorimeter transverse energy as a fonction of observed
charged particle multiplicity.
Fig. 20 shows the calorimeter transverse energy as a function of
observed charged track multiplicity.
The left-hand vertical scale
shows the visible energy measured in the electromagnetic and hadron
calorimeters.
The right-hand scale has been corrected by a factor
1.35 to take account of the lower response to hadrons of the electromagnetic calorimeter.
An absolute scale uncertainty of •
still
345
remains, as a precise application of this factor requires knowledge
of the momentum of the incident hadrons.
The average value of the
transverse energy per event divided by the charged particle multiplicity seen in the central detector does not appear to depend on the
multiplicity for these events.
Assuming a ratio 2 for charged/neutral particle production Fig.
20 implies an average E T per secondary particle of 0.50 • 0.i0 GeV.
ACKNOWLEDGEMENTS
I wish to thank my colleagues in UAI for asking me to represent
them at this conference.
This work was supported in part by the United States Department
of Energy.
3~.6
RE FERENCE S
i.
Aachen-Annecy (LAPP)-Birmingham-CERN-London (Queen Mary College)Paris (Coll~ge de France)-Riverside-Rutherford-Sacley (CEN)Vienna Collab., A 4 solid-angle detector for the SPS used as a
proton-antlproton collider at a centre-of-mass energy of 540 GeV,
Proposal CERN/SPSC/78-O6/P92 (1978).
2.
C. Rubbia, Proceedings of the EPS Conference, Lisbon, July 1981.
3.
M. Barranco Luque et al., Nucl. Inst. 176 (1980) 175.
4.
M. Calvetti et al., Nucl. Instr. 176 (1980) 255.
5.
K. Eggert, et al., Nucl. Inst. 176 (1980) 217.
6.
K. Pretzl, this conference.
7.
C . M . G . Lattes, Y. Fujimoto and S. Hasegawa, Phys. Rep. 65 (1980)
151, and references therein.
8.
G. Arnison et al., Phys. Lett. I07B (1981) 320.
9.
W. Thom~ et al., Nucl. Phys. B129 (1977) 365.
i0. M. G. Albrow et al., Nucl. Phys. BI08 (1976) i.
ii. S. Tasaka et al., Proc. 17th Intern. Cosmic-ray Conf. (Paris, 1981)
(CEN, Saclay, 1981) Vol. 5, p. 126.
12. Y. Sato et al., J. Phys. Soc. Japan 41 (1976) 1821.
13. Z. Koba, H. B. Nielsen and P. Olesen, Nucl. Phys. B40 (1972) 317.
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