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A zero-degree tagging system used for photon-photon experiments at DCI
Nuclear Instruments and Methods 174 (1980) 157-165 © North-Holland Publishing Company
A ZERO-DEGREE TAGGING SYSTEM USED FOR PHOTON-PHOTON EXPERIMENTS AT DCI *
M. BROSSARD, A. FALVARD, J. JOUSSET, B. MICHEL, G. MONTAROU, J.C. MONTRET, P. REICHSTADT, P.Y. BERTIN, G. FOURNIER, M.C. VIALATTE Laboratoire de Physique Corpusculaire, Universitd de Clermont II, BP 45, 63170 A UBIERE, France
J. BUON, A. COURAU, J. HAi'SSINSKI, J.P. MARX and R. SOUCHET Laboratoire de l'Accdldrateur Lin(aire, Centre d'Orsay, Bdt. 200, 91405 ORSA Y, France
Received 16 November 1979 and in revised form 4 March 1980
We describe a zero-degree tagging system designed to study photon-photon collisions with DCI at Orsay. This system consists of hodoscopes of drift chambers, which are placed behind the two bending magnets located at the ends of the interaction region of the ring. It analyses electrons which, in this interaction region, have undergone a fractional energy loss between 0.2 and 0.5, and a trajectory deflection less than 10 mrad.
1. Introduction
1.1. Zero-degree tagging
The improved performances of electron colliding beam machines now make the study of 77 collisions [ 1 - 3 ] possible:
The angular distribution of the virtual photons involved in 3'7 processes is sharply peaked at zero degrees. Most o f the photons are emitted at an angle 0.r ~ me/E, where E is the stored e ± beam energy. Such photons are quasi-real. It follows that an essential goal of a tagging system is to accept electrons scattered at very small angles, down to zero degrees if possible. But most storage rings have a structure and/or a duty cycle such that we cannot observe or select electrons involved in p h o t o n - p h o t o n processes which have lost a fraction o f their energy without undergoing a 1 - 2 ° deflection. This is not the case at DCI. Our set-up does provide an efficient zero-degree tagging o f the quasi-real photons.
e ± + e +-~ e ± + e ± + X .
(1)
The X-state can be either of leptonic or hadronic nature. It consists o f particles which can be seen in a "central" detector when emitted at wide angles, whereas the two scattered electrons are emitted at very small angles with respect to the beams: their detection requires a special apparatus. By measuring the energies and angles o f these outgoing electrons, we can determine the kinematic characteristics o f the virtual photons which collided to produce the X-state. Such a "tagging" o f the virtual photons, combined with the observation o f the X-particles in the central detector, provides the best possible identification o f process (1), since it enables us to verify that the events satisfy the appropriate kinematical constraints. Tagging is thus extremely useful for background rejection. We stress that single tagging b y itself, i.e. the detection o f only one o f the two scattered electrons, can provide the grounds for identifyhag process (1), when carded out with sufficient accuracy.
2. Magnet optics - acceptance and resolution considerations
DCI [4] has two tings, one on top o f the other, with a common interaction section. Fig. 1 shows this interaction section with its adjacent elements. The two Y bending magnets located at the ends o f the interaction region are used as spectrometers which analyse the energies and angles o f the scattered electrons in reaction (1). Figure 2 shows the track projections o f scattered electron, on the vertical and the horizontal planes.
* Work supported by the "Institut National de Physique Nucldaire et de Physique des Particules". 157
M. Brossard et al. / A zero-degree tagging system
158 "V" m a g n e t
Vacuum
chamber
"Y" m a g n e t s
quadrupolus
"\\
-
~-1
'/
'\
I11
/ \
/
~\
0
/
/
lm
\\ \ place
/ of
tagging
hodoseopes'
Fig. 1. Interaction region of DCI.
This trajectory is compared with the stored beam orbit. The latter is deflected in the vertical plane b y an angle ao = 10 ° in the Y magnets. The Y magnet field has been measured at the crossings of a 2 × 2 × 2 cm 3 network. The analysis o f the field map has shown that, in the medium (vertical) magnet plane, the optics differs b y less than 1% from that o f an ideal magnet with a uniform field, 72
Yi
Trajectory c,f a scattered e l e c t r o n /J
cm long. As far as the horizontal projection of the track is concerned, the magnetic field introduces a deflection which is smaller than 1 mrad for electrons in the angle and m o m e n t u m ranges we are concerned with. Such an effect can be neglected altogether. In this ideal magnet approximation, the equations which relate the kinematic characteristics p and 0v o f the scattered electron to the intercept Yd and slope 0v of its trajectory measured in a frame located after the Y magnet (cf. fig. 2) are as follows, where Po is the m o m e n t u m o f the beam's electrons: l Yd = L tg 0v + ..... (cos 0v - cos 0v) p , sin Cto Po
(2)
sin 0'v = sin 0v + (Po/P) sin ao .
(3)
Beam t r a j e c t o r y jB,
i
....
Ov
-
- ..... x
!
Our apparatus measures Yd, 0v and Za [Zd = (L + / ) OH].
2.1. Acceptance and resolution considerations [ = h a i l - l e n g t h o[ t i e
interaction
~ect[on of
D(I
i = equivalent magneti~ lcmgth of the Y magm[
z~ !
! 3' magnetic
....
_~
~-]
pc,le
" - - ~
_
~
~y
__
~rai~ct,~ry c~f a slatttred electron
Zd~_ Beam~.: r a j e c t o r y
Fig. 2. Trajectory projections on: (a) the vertical plane, and (b) the Horizontal plane.
Our goal is to detect electrons scattered at zero degrees, with an acceptance as extended as possible toward small x values, where x = E~/E is the fractional energy loss o f these electrons. Special vacuum chamber sections had to be built to implement our 4 tagging hodoscopes. These sections have 50/am titane windows with an area o f 400 cm 2 , to minimize multiple Coulomb scattering. The shape o f the vacuum chamber allows us to place our detectors as close as possible to the beam orbit, namely at a distance o f only 4 cm. There is 40 cm o f free space available for the detectors, between the titane windows and the adjacent quadYupoles (fig. 1). Our design achieves an x acceptance as follows:
159
M. Brossard et al. / A zero-degree tagging system
0.2 ~
planes 4 and 5 are grounded planes the role of which is to contain the electric field and to shield the detector from external pick-up signals. The length of the wires is 10 cm. Other dimensions are given in fig. 3. One particular feature of our chambers is that all their cells do not have the same drift length. Indeed, in .order to keep the counting rates of the various cells at a similar level, we had to reduce the drift length of the cells which are close to the beam. More precisely, each drift chamber is composed of four (12 mm + 12 mm) cells, one (12 nun + 8 mm)cell, four (8 mm + 8 mm) cells, one (8 mm + 4 mm) cell and four (4 mm + 4 mm) cells. In the first and the last pair of chambers, all the wires are horizontal, and we have displaced one drift plane with respect to the other by 4 mm in order to remove the u p - d o w n ambiguity. In the central pair, the wires are tilted at an angle of +2.3 ° with respect to the horizontal plane: we can thus measure the track distance to the vertical plane which contains the beam orbit, and thereby the angle OH of the track projection on the horizontal plane.
0scattering ~
3. Description of the hodoscopes The track measurement is made by hodoscopes each of which consists of 3 pairs of drift chambers. Fig. 3 shows one of these pairs. Each chamber has 14 cells and each cell consists of 5 parallel planes: planes 1 and 2 contain 100/am copper wires which provide the high voltage gradient, i.e. the drift electric field; plane 3 contains 20 /am sense wires in tungsten with 100/am copper wires on each side which limit the drift space;
3.1. Mechanical construction
In order to detect electrons as close as possible to the stored beam, we use mechanical frames which have only 3 sides to hold the chamber wires, so that the firstdrift cell can be placed almost in contact
0
1 cm
I
i
5 2 ...~.. ...................................................
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......................... ° .................................................................................... °~.o.~ o.: o°~ o ~°o°°°~...o°..~..°o.°°. °°o...~°°.o°°.~°°°o ..... ~ .....o ..... ~ ..... o ..... • .....o ..... ~ .... ¢.°°
5
Copper o
tungsten
wires sense
wires
Fig. 3. A paix of drift chambers.
6mm 6
M. Brossard et al. / A zero-degree tagging system
160
Drift
chamht, r s
Y magnet
iS('
i n t i [ ] :~ t i on C'(~unter
scatcered \~ electron
i• I
Beams
[
quadrupole
I
synchrotron light absorbers
Thin
Ti window
Vacuum
(50 um)/
chamber
Fig. 4. Schematic view of the experimental set-up.
with the beam pipe of the ring. We have applied a mechanical pre-constraint to these frames to ensure the right tension of the wires. No deformation was observed when we wound the sense wires and the high voltage wires. The accuracy of the positioning of the sense wires with respect to each other is 0.1 mm. Furthermore, all 6 chambers composing each hodoscope are placed in a metallic box the position in space of which is known to within 0.1 ram. Such an accuracy is required to obtain the desired momentum precision. Figure 4 shows half of the entire set-up. Each hodoscope is completed by a scintillator which gives the fast signal used in the trigger logic.
0 Kv
•
Copper wires
X
Tungsten
sense
-O,3Kv
wire -0,6Kv
.
-O.9Kv
I.Skv
- t 5 Kv
0 Kv
0,3 Kv
O,6Kv
0 , 9 Kv
4000
4. Electric field map and cell gains We have studied the drift chamber behaviour with a mixed gas: 31% isobutane and 69% argon at atmospheric pressure. The drift velocity [6] is obtained for this mixed gas when the electric field reaches 1000
300G
:p &aJ
2000
V/cm. The electric field map in the drift cells has been determined with a computer routine, and also with an Fig. 5. (a) Equipotential lines in a 8 mm half cell. (b) Electric field in the same haft cell, as a function of the distance to the sense wire.
-1,2 Kv
1000
Distance to
the
sense w i r e
i,2Kv
161
M. Brossard et al. / A zero-degree tagging system
electrolytic cell. We found, in particular, that no significant deviation of the equipotential line distribution appears in any of the cells, not even in those which have an asymmetric geometric structure. This feature makes it possible to saturate the drift velocity in all parts of our chambers. Fig. 5 presents the equipotential lines and the values of the electric field in a 8 mm half cell. The values of the high voltage applied are: Vsense = +1500 V in all cells, Vdrift =--1800, -1200, - 6 0 0 V in the 12, 8, 4 mm half-cells respectively. The relative gains of the various cells of the six chambers which consitute one hodoscope changes by a factor 2. By a suitable choice of the input resistors of the amplifiers, we have compensated for the slight dispersion of the gains. Typical values of the sense wire signals are around 25 mV.
of which are set at 5 mV. These circuits have a differential logic output which allows the transmission of the pulses by fiat cables to LeCroy 2770 A TDC units [7]. Each TDC unit has 96 channels, out of which 84 are used by one of the hodoscopes. The sense wire signals start the TDC conversion process. All channels have a common "stop" signal provided by a pick-up electrode of the storage ring. The calibration of the conversion slope and of the pedestal of each TDC channel is made by a special CAMAC unit which controls a pulse generator. Calibration pulses are applied to all high voltage wires simultaneously. By a capacitive effect, they induce 84 signals on the sense wires of each hodoscope. A series of eight successive tests is made which differ by the delay of the "stop" signal. This delay is changed, step by step, by quantities which are known exactly and which cover the whole TDC conversion range. 5.1. Acquisition o f events
5. Operation of the tagging system on DCI
The sense wire signals are discriminated and amplified by LeCroy DC 201 hybrid circuits the threshold DM I Pick-up electrode"
PM
Figure 6 displays scheme of the logic of acquisition. This system: (i) controls the transmission of the "stop" signals which are sent to the TDCs; (ii) rejects
~=J "
"Read" word
~4)
i F
c.~
"Status"
H1
TDCI
'
I" i.
///.
-%_
'--'~
~
Stop
CTOC)
I1 i"
H2
"status" H3 Bus CAMAC
" S t . glt llS r H4 WC.
SPI'~CIAI, CAMAC UNIT
Fig. 6. Scheme of the logic of acquisition.
162
M. Brossard et al. / A zero-degree tagging system
some of the background events; and (iii) informs the on-line calculator when to read the TDCs. In order to perform these 3 functions, we have: an S signal induced by the bunch passage on a pick-up electrode located inside the ring vacuum chamb er; an L signal generated whenever the central detector * is triggered. three signals, Ca, C2 and Ca, for each hodoscope, which correspond to the 3 pairs of drift chambers. Each of these signals is present if and only if, at least one of the 28 cells which compose a pair of chambers is hit; one C4 signal provided by the scintillator located behind each hodoscope. The "stop" signal is transmitted if and only if there is an (S, L) coincidence. For each horoscope, a (S, L, C1, C5, Ca, C4) coincidence corresponds to the detection of a particle which occurs at the right time with respect to the bunch-bunch crossing, and which is in coincidence with an event recorded by the central detector. The acquisition of 3'3' events and their recording on magnetic tape is performed by the on-line calculator (VARIAN 620 plus a microprocessor) of the DMI group, via a special CAMAC unit. The various signals of the tagging system are entirely processed by this unit which, in particular, generates a "read" word, and four "status" words, one for each hodoscope. The reading of these words indicates to the on-line calculator which of the TDC memories it has to read.
bers the wires of which are tilted with respect to the horizontal direction. In addition to the conversion slope and pedestal of each TDC channel, we have to determine the electron drift velocity v and the delay A introduced by the electronics in the transmission of the "stop" signal to the TDCs. These two quantities o and A are obtained by minimizing the ×5 value which comes out of the linear fits to a large number of electron trajectories.
6.1. Detector performances The operation characteristics of our detector have been obtained by studying b e a m - b e a m and b e a m gas bremsstrahlung events. A slight modification of the logic of acquisition allowed us to take this data independently of the central detector operation. We have registered a sample of l0 s electron tracks. The efficiency of the hodoscopes has been determined from the analysis of these bremsstrahlung events. Its value averaged over all ceils is 0.996. All single cell efficiencies are higher than 0.98. The drift velocity measured is o = (5.32 + 0.05) cm/t~s. In the determination of the space resolution o of the chambers, multiple Coulomb scattering does not play any role when a straight line is fitted to the co-ordinates measured in the first and last pairs of our chambers, without including the central pair. In these conditions, the quantity X2 = ZIG 2
where
6. Track reconstruction and detector performances 4
Four parameters are necessary to calculate an electron trajectory. We shall use two quantities already defined in section 2 above - the intercept Yd and the slope 0 v of the projection of the track on the vertical plane - and similar parameters, z d and 0H, for the horizontal projection. The values of Yd and O'v are computed from the altitude (v co-ordinate) of the hits in the 6 chambers (cf. fig. 2). Since the location of the interaction vertex is given by the central detector, the two other parameters (z a and OR) can be deduced from the z co-ordinate of the point at which the electron crosses the pair of cham* The central detector presently used with our tagging system is the DM1 apparatus [8].
i=1
(.Vc is the calculated value of the co-ordinate and Ym is the measured value of the co-ordinate) has a probability distribution whic~ can be identified with a ×5 • 7 shows the d i s t r i b u d i s t r i b u t i o n l a w : A e - x 12. Fig. t i o n t h a t we have o b t a i n e d . I t leads to o = 0.21 ram.
Figure 8 is a scatter plot of the momentum p versus the angle 0v deduced from the analysis of a sample of b e a m - b e a m and beam-gas bremsstrahlung events. The observed distribution is compared in this figure with the one expected from a Monte Carlo simulation. The concentration of points at the center of the acceptance region indicates that the angular distribution of the inelasticaUy scattered electrons is very
M. Brossard et al. / A zero-degree tagging system
163
0 v :mrad ~, ~15 +10
"I'' i; .1 "~ .'1 ?•...'• .=.' '• '.
'1 ~,'." : Y
+5 0
/.%
-5
...........
i" .: :':; • "li'l.',i! .... 17'.
-10 f(Z)
]~
H
~ =
= Ae
20
15
6,~ tara ,
70
L
,
,
,
i
n
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.,
,.
.*..
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5
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1
°
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.
*
.
.
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. 1
,,
° ..
*.
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);
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~' .° • .2
.: /
•
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, ~.
'. =
,~
-
• •~
JO -15 :
"l
0.4 o ,1
,2
,3
Zlmm
2)
Fig. 7. x 2 distribution o f t h e reconstructed trajectories showing the spatial resolution o f the drift chambers.
sharply peaked at 0 °. Fig. 9 shows the angular distribution in both the vertical and horizontal planes. Their standard deviations are o%= 1.5 mrad and O0H = 0.75 mrad respectively• In fact, bremsstrahlung events have a 1/0 ~ distribution the width of which can be neglected in this analysis. It follows that the width which is observed characterizes the resolution of our apparatus. 6.2. The tagging system used as a mass spectrometer
Bremsstrahlung events cannot be used to determine the momentum resolution of the tagging system. However, a preliminary study of ~,'yevents leading to two and only two particles in the final state has shown us that this resolution is high enough to separate e e ,/~/a , rr*rt- pairs as can be seen in fig. 10. Such a separation is based on a kinematical reconstruction which leads to a value of the mass of the fmal particles. For the singly tagged events, we assume that the missing momentum has been carried away by the undetected electron. We can thus obtain the total energy of the pair of particles the momenta of which +
_
+
-
.... " 0.5
06
07
0.8
09
:I x
Fig. 8. Tagging acceptance in the p, 0 v plane (p is t h e particle m o m e n t u m and 0 v is the deflection of the particle trajectory projected on the vertical plane)• (a) Monte-Carlo predictions for bremsstrahlung events. (b) Observed events. This acceptance is limited by the s y n c h r o t r o n light absorbers (lines I and II), and by the vertical size o f the drift chambers (lines III and IV). A few observed events, which seem to be o u t of t h e tagging angular acceptance, are background events which do n o t come from t h e interaction region of the storage ring.
are measured by DM1, and thereby calculate the mass of these particles (for the doubly tagged events, the total energy of the final particles is given directly by the tagging system). This mass determination has an accuracy which depends on both the tagging system resolution and the DM1 resolution. Taking the latter into account, we find that our tagging system measures the momenta of the scattered electrons with an accuracy of about 1%. More details on this tagging system can be found in ref. [9] and [10].
7. Conclusions We have installed a tagging system on DCI which takes advantage of the fact that the first magnetic elements next to the interaction region are bending
M. Brossard et al. / A zero-degree tagging system
164
ao,
@
-8
-4
0
@
+4
+8
~ mrad.
uov:ts,d.
-lO
-5
0
-+5
+10
PVmrad.
Fig. 9. Angular distributions of the tracks projected: (a) on the horizontal plane; and (b) on the vertical plane.
.1; m'.
15
s
l 2
.
0
t
1
2
3
4
m'(IO~MeV ~)
Fig. 10. Distribution of the reconstructed masses of the particles of the pairs created in -y7 eonisions.
magnets. It consists of hodoscopes of drift chambers with a spatial resolution of 210 tam. This system has a momentum resolution high enough to ensure the identification of 77 events on kinematical grounds even in the case o f single tagging. Furthermore, the fact that it detects electrons at zero degrees with a 10 mrad polar angle acceptance and full azimuthal acceptance over a wide range of fractional energy losses leads to a high tagging efficiency. For instance, our system tags, singly or doubly, 26% of the 77 -->e +eevents, 52% of the 77 -~ P ~ - events and 58% of the 77 ~ n+n- events which are detected in DM1. These tagging efficiencies are computed for an energy of 1 GeV per beam and the DM1 acceptance is characterized by the following cuts: (i) transverse momentum P r greater than 80 MeV/c, (ii) polar angle 0 between 45 ° and 135 °. This system has been in operation since January 1979. Up to now, 250 tagged photon-photon events have been recorded. 25 of them were doubly tagged. Their analysis is in progress.
M. Brossard et al. / A zero-degree tagging system
References [1] N. Arteaga-Romero, A. Jaccarini, P. Kessler andJ. Parisi, Nuovo Cim. Lett. 4 (1970) 993; Phys. Rev. D3 (1971) 1569. [2] S. Brodsky, T. Kinoshita and H. Terazawa, Phys. Rev. D4 (1971) 1532. [3] V. Budnev and I.F. Ginsburg, Phys. Lett. 37B (1971) 320. [4] The Orsay Storage Ring Group, Proc. 8th Int. Conf. on High Energy Accelerators, CERN (1971) 79. [5] G. Charpak et al., Nucl. Instr. and Meth. 108 (1973) 413.
165
[6] A. Breskin et al., Nucl. Instr. and Meth. 119 (1974) 9. [7] S. Mc Laughlin and A. Michalowski, CAMAC Model 2770A, Technical information manual, LeCroy Research Systems, Spring Valley (1977). [8] J. Jeanjean et al., Nucl. Instr. and Meth. 117 (1974) 349. A. Cordier et al., Nucl. Instr. and Meth. 133 (1976) 237. [9] G. Fournier, Th~se de 3~me cycle no. 538, Universit~ de • Clermont II (1977). [10] A. Falvard, Th~se de 3~me cycle no. 551, Universit6 de Clermont II (1978).
A ZERO-DEGREE TAGGING SYSTEM USED FOR PHOTON-PHOTON EXPERIMENTS AT DCI *
M. BROSSARD, A. FALVARD, J. JOUSSET, B. MICHEL, G. MONTAROU, J.C. MONTRET, P. REICHSTADT, P.Y. BERTIN, G. FOURNIER, M.C. VIALATTE Laboratoire de Physique Corpusculaire, Universitd de Clermont II, BP 45, 63170 A UBIERE, France
J. BUON, A. COURAU, J. HAi'SSINSKI, J.P. MARX and R. SOUCHET Laboratoire de l'Accdldrateur Lin(aire, Centre d'Orsay, Bdt. 200, 91405 ORSA Y, France
Received 16 November 1979 and in revised form 4 March 1980
We describe a zero-degree tagging system designed to study photon-photon collisions with DCI at Orsay. This system consists of hodoscopes of drift chambers, which are placed behind the two bending magnets located at the ends of the interaction region of the ring. It analyses electrons which, in this interaction region, have undergone a fractional energy loss between 0.2 and 0.5, and a trajectory deflection less than 10 mrad.
1. Introduction
1.1. Zero-degree tagging
The improved performances of electron colliding beam machines now make the study of 77 collisions [ 1 - 3 ] possible:
The angular distribution of the virtual photons involved in 3'7 processes is sharply peaked at zero degrees. Most o f the photons are emitted at an angle 0.r ~ me/E, where E is the stored e ± beam energy. Such photons are quasi-real. It follows that an essential goal of a tagging system is to accept electrons scattered at very small angles, down to zero degrees if possible. But most storage rings have a structure and/or a duty cycle such that we cannot observe or select electrons involved in p h o t o n - p h o t o n processes which have lost a fraction o f their energy without undergoing a 1 - 2 ° deflection. This is not the case at DCI. Our set-up does provide an efficient zero-degree tagging o f the quasi-real photons.
e ± + e +-~ e ± + e ± + X .
(1)
The X-state can be either of leptonic or hadronic nature. It consists o f particles which can be seen in a "central" detector when emitted at wide angles, whereas the two scattered electrons are emitted at very small angles with respect to the beams: their detection requires a special apparatus. By measuring the energies and angles o f these outgoing electrons, we can determine the kinematic characteristics o f the virtual photons which collided to produce the X-state. Such a "tagging" o f the virtual photons, combined with the observation o f the X-particles in the central detector, provides the best possible identification o f process (1), since it enables us to verify that the events satisfy the appropriate kinematical constraints. Tagging is thus extremely useful for background rejection. We stress that single tagging b y itself, i.e. the detection o f only one o f the two scattered electrons, can provide the grounds for identifyhag process (1), when carded out with sufficient accuracy.
2. Magnet optics - acceptance and resolution considerations
DCI [4] has two tings, one on top o f the other, with a common interaction section. Fig. 1 shows this interaction section with its adjacent elements. The two Y bending magnets located at the ends o f the interaction region are used as spectrometers which analyse the energies and angles o f the scattered electrons in reaction (1). Figure 2 shows the track projections o f scattered electron, on the vertical and the horizontal planes.
* Work supported by the "Institut National de Physique Nucldaire et de Physique des Particules". 157
M. Brossard et al. / A zero-degree tagging system
158 "V" m a g n e t
Vacuum
chamber
"Y" m a g n e t s
quadrupolus
"\\
-
~-1
'/
'\
I11
/ \
/
~\
0
/
/
lm
\\ \ place
/ of
tagging
hodoseopes'
Fig. 1. Interaction region of DCI.
This trajectory is compared with the stored beam orbit. The latter is deflected in the vertical plane b y an angle ao = 10 ° in the Y magnets. The Y magnet field has been measured at the crossings of a 2 × 2 × 2 cm 3 network. The analysis o f the field map has shown that, in the medium (vertical) magnet plane, the optics differs b y less than 1% from that o f an ideal magnet with a uniform field, 72
Yi
Trajectory c,f a scattered e l e c t r o n /J
cm long. As far as the horizontal projection of the track is concerned, the magnetic field introduces a deflection which is smaller than 1 mrad for electrons in the angle and m o m e n t u m ranges we are concerned with. Such an effect can be neglected altogether. In this ideal magnet approximation, the equations which relate the kinematic characteristics p and 0v o f the scattered electron to the intercept Yd and slope 0v of its trajectory measured in a frame located after the Y magnet (cf. fig. 2) are as follows, where Po is the m o m e n t u m o f the beam's electrons: l Yd = L tg 0v + ..... (cos 0v - cos 0v) p , sin Cto Po
(2)
sin 0'v = sin 0v + (Po/P) sin ao .
(3)
Beam t r a j e c t o r y jB,
i
....
Ov
-
- ..... x
!
Our apparatus measures Yd, 0v and Za [Zd = (L + / ) OH].
2.1. Acceptance and resolution considerations [ = h a i l - l e n g t h o[ t i e
interaction
~ect[on of
D(I
i = equivalent magneti~ lcmgth of the Y magm[
z~ !
! 3' magnetic
....
_~
~-]
pc,le
" - - ~
_
~
~y
__
~rai~ct,~ry c~f a slatttred electron
Zd~_ Beam~.: r a j e c t o r y
Fig. 2. Trajectory projections on: (a) the vertical plane, and (b) the Horizontal plane.
Our goal is to detect electrons scattered at zero degrees, with an acceptance as extended as possible toward small x values, where x = E~/E is the fractional energy loss o f these electrons. Special vacuum chamber sections had to be built to implement our 4 tagging hodoscopes. These sections have 50/am titane windows with an area o f 400 cm 2 , to minimize multiple Coulomb scattering. The shape o f the vacuum chamber allows us to place our detectors as close as possible to the beam orbit, namely at a distance o f only 4 cm. There is 40 cm o f free space available for the detectors, between the titane windows and the adjacent quadYupoles (fig. 1). Our design achieves an x acceptance as follows:
159
M. Brossard et al. / A zero-degree tagging system
0.2 ~
planes 4 and 5 are grounded planes the role of which is to contain the electric field and to shield the detector from external pick-up signals. The length of the wires is 10 cm. Other dimensions are given in fig. 3. One particular feature of our chambers is that all their cells do not have the same drift length. Indeed, in .order to keep the counting rates of the various cells at a similar level, we had to reduce the drift length of the cells which are close to the beam. More precisely, each drift chamber is composed of four (12 mm + 12 mm) cells, one (12 nun + 8 mm)cell, four (8 mm + 8 mm) cells, one (8 mm + 4 mm) cell and four (4 mm + 4 mm) cells. In the first and the last pair of chambers, all the wires are horizontal, and we have displaced one drift plane with respect to the other by 4 mm in order to remove the u p - d o w n ambiguity. In the central pair, the wires are tilted at an angle of +2.3 ° with respect to the horizontal plane: we can thus measure the track distance to the vertical plane which contains the beam orbit, and thereby the angle OH of the track projection on the horizontal plane.
0scattering ~
3. Description of the hodoscopes The track measurement is made by hodoscopes each of which consists of 3 pairs of drift chambers. Fig. 3 shows one of these pairs. Each chamber has 14 cells and each cell consists of 5 parallel planes: planes 1 and 2 contain 100/am copper wires which provide the high voltage gradient, i.e. the drift electric field; plane 3 contains 20 /am sense wires in tungsten with 100/am copper wires on each side which limit the drift space;
3.1. Mechanical construction
In order to detect electrons as close as possible to the stored beam, we use mechanical frames which have only 3 sides to hold the chamber wires, so that the firstdrift cell can be placed almost in contact
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Fig. 3. A paix of drift chambers.
6mm 6
M. Brossard et al. / A zero-degree tagging system
160
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Fig. 4. Schematic view of the experimental set-up.
with the beam pipe of the ring. We have applied a mechanical pre-constraint to these frames to ensure the right tension of the wires. No deformation was observed when we wound the sense wires and the high voltage wires. The accuracy of the positioning of the sense wires with respect to each other is 0.1 mm. Furthermore, all 6 chambers composing each hodoscope are placed in a metallic box the position in space of which is known to within 0.1 ram. Such an accuracy is required to obtain the desired momentum precision. Figure 4 shows half of the entire set-up. Each hodoscope is completed by a scintillator which gives the fast signal used in the trigger logic.
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4. Electric field map and cell gains We have studied the drift chamber behaviour with a mixed gas: 31% isobutane and 69% argon at atmospheric pressure. The drift velocity [6] is obtained for this mixed gas when the electric field reaches 1000
300G
:p &aJ
2000
V/cm. The electric field map in the drift cells has been determined with a computer routine, and also with an Fig. 5. (a) Equipotential lines in a 8 mm half cell. (b) Electric field in the same haft cell, as a function of the distance to the sense wire.
-1,2 Kv
1000
Distance to
the
sense w i r e
i,2Kv
161
M. Brossard et al. / A zero-degree tagging system
electrolytic cell. We found, in particular, that no significant deviation of the equipotential line distribution appears in any of the cells, not even in those which have an asymmetric geometric structure. This feature makes it possible to saturate the drift velocity in all parts of our chambers. Fig. 5 presents the equipotential lines and the values of the electric field in a 8 mm half cell. The values of the high voltage applied are: Vsense = +1500 V in all cells, Vdrift =--1800, -1200, - 6 0 0 V in the 12, 8, 4 mm half-cells respectively. The relative gains of the various cells of the six chambers which consitute one hodoscope changes by a factor 2. By a suitable choice of the input resistors of the amplifiers, we have compensated for the slight dispersion of the gains. Typical values of the sense wire signals are around 25 mV.
of which are set at 5 mV. These circuits have a differential logic output which allows the transmission of the pulses by fiat cables to LeCroy 2770 A TDC units [7]. Each TDC unit has 96 channels, out of which 84 are used by one of the hodoscopes. The sense wire signals start the TDC conversion process. All channels have a common "stop" signal provided by a pick-up electrode of the storage ring. The calibration of the conversion slope and of the pedestal of each TDC channel is made by a special CAMAC unit which controls a pulse generator. Calibration pulses are applied to all high voltage wires simultaneously. By a capacitive effect, they induce 84 signals on the sense wires of each hodoscope. A series of eight successive tests is made which differ by the delay of the "stop" signal. This delay is changed, step by step, by quantities which are known exactly and which cover the whole TDC conversion range. 5.1. Acquisition o f events
5. Operation of the tagging system on DCI
The sense wire signals are discriminated and amplified by LeCroy DC 201 hybrid circuits the threshold DM I Pick-up electrode"
PM
Figure 6 displays scheme of the logic of acquisition. This system: (i) controls the transmission of the "stop" signals which are sent to the TDCs; (ii) rejects
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Fig. 6. Scheme of the logic of acquisition.
162
M. Brossard et al. / A zero-degree tagging system
some of the background events; and (iii) informs the on-line calculator when to read the TDCs. In order to perform these 3 functions, we have: an S signal induced by the bunch passage on a pick-up electrode located inside the ring vacuum chamb er; an L signal generated whenever the central detector * is triggered. three signals, Ca, C2 and Ca, for each hodoscope, which correspond to the 3 pairs of drift chambers. Each of these signals is present if and only if, at least one of the 28 cells which compose a pair of chambers is hit; one C4 signal provided by the scintillator located behind each hodoscope. The "stop" signal is transmitted if and only if there is an (S, L) coincidence. For each horoscope, a (S, L, C1, C5, Ca, C4) coincidence corresponds to the detection of a particle which occurs at the right time with respect to the bunch-bunch crossing, and which is in coincidence with an event recorded by the central detector. The acquisition of 3'3' events and their recording on magnetic tape is performed by the on-line calculator (VARIAN 620 plus a microprocessor) of the DMI group, via a special CAMAC unit. The various signals of the tagging system are entirely processed by this unit which, in particular, generates a "read" word, and four "status" words, one for each hodoscope. The reading of these words indicates to the on-line calculator which of the TDC memories it has to read.
bers the wires of which are tilted with respect to the horizontal direction. In addition to the conversion slope and pedestal of each TDC channel, we have to determine the electron drift velocity v and the delay A introduced by the electronics in the transmission of the "stop" signal to the TDCs. These two quantities o and A are obtained by minimizing the ×5 value which comes out of the linear fits to a large number of electron trajectories.
6.1. Detector performances The operation characteristics of our detector have been obtained by studying b e a m - b e a m and b e a m gas bremsstrahlung events. A slight modification of the logic of acquisition allowed us to take this data independently of the central detector operation. We have registered a sample of l0 s electron tracks. The efficiency of the hodoscopes has been determined from the analysis of these bremsstrahlung events. Its value averaged over all ceils is 0.996. All single cell efficiencies are higher than 0.98. The drift velocity measured is o = (5.32 + 0.05) cm/t~s. In the determination of the space resolution o of the chambers, multiple Coulomb scattering does not play any role when a straight line is fitted to the co-ordinates measured in the first and last pairs of our chambers, without including the central pair. In these conditions, the quantity X2 = ZIG 2
where
6. Track reconstruction and detector performances 4
Four parameters are necessary to calculate an electron trajectory. We shall use two quantities already defined in section 2 above - the intercept Yd and the slope 0 v of the projection of the track on the vertical plane - and similar parameters, z d and 0H, for the horizontal projection. The values of Yd and O'v are computed from the altitude (v co-ordinate) of the hits in the 6 chambers (cf. fig. 2). Since the location of the interaction vertex is given by the central detector, the two other parameters (z a and OR) can be deduced from the z co-ordinate of the point at which the electron crosses the pair of cham* The central detector presently used with our tagging system is the DM1 apparatus [8].
i=1
(.Vc is the calculated value of the co-ordinate and Ym is the measured value of the co-ordinate) has a probability distribution whic~ can be identified with a ×5 • 7 shows the d i s t r i b u d i s t r i b u t i o n l a w : A e - x 12. Fig. t i o n t h a t we have o b t a i n e d . I t leads to o = 0.21 ram.
Figure 8 is a scatter plot of the momentum p versus the angle 0v deduced from the analysis of a sample of b e a m - b e a m and beam-gas bremsstrahlung events. The observed distribution is compared in this figure with the one expected from a Monte Carlo simulation. The concentration of points at the center of the acceptance region indicates that the angular distribution of the inelasticaUy scattered electrons is very
M. Brossard et al. / A zero-degree tagging system
163
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sharply peaked at 0 °. Fig. 9 shows the angular distribution in both the vertical and horizontal planes. Their standard deviations are o%= 1.5 mrad and O0H = 0.75 mrad respectively• In fact, bremsstrahlung events have a 1/0 ~ distribution the width of which can be neglected in this analysis. It follows that the width which is observed characterizes the resolution of our apparatus. 6.2. The tagging system used as a mass spectrometer
Bremsstrahlung events cannot be used to determine the momentum resolution of the tagging system. However, a preliminary study of ~,'yevents leading to two and only two particles in the final state has shown us that this resolution is high enough to separate e e ,/~/a , rr*rt- pairs as can be seen in fig. 10. Such a separation is based on a kinematical reconstruction which leads to a value of the mass of the fmal particles. For the singly tagged events, we assume that the missing momentum has been carried away by the undetected electron. We can thus obtain the total energy of the pair of particles the momenta of which +
_
+
-
.... " 0.5
06
07
0.8
09
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Fig. 8. Tagging acceptance in the p, 0 v plane (p is t h e particle m o m e n t u m and 0 v is the deflection of the particle trajectory projected on the vertical plane)• (a) Monte-Carlo predictions for bremsstrahlung events. (b) Observed events. This acceptance is limited by the s y n c h r o t r o n light absorbers (lines I and II), and by the vertical size o f the drift chambers (lines III and IV). A few observed events, which seem to be o u t of t h e tagging angular acceptance, are background events which do n o t come from t h e interaction region of the storage ring.
are measured by DM1, and thereby calculate the mass of these particles (for the doubly tagged events, the total energy of the final particles is given directly by the tagging system). This mass determination has an accuracy which depends on both the tagging system resolution and the DM1 resolution. Taking the latter into account, we find that our tagging system measures the momenta of the scattered electrons with an accuracy of about 1%. More details on this tagging system can be found in ref. [9] and [10].
7. Conclusions We have installed a tagging system on DCI which takes advantage of the fact that the first magnetic elements next to the interaction region are bending
M. Brossard et al. / A zero-degree tagging system
164
ao,
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-8
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@
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.1; m'.
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s
l 2
.
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t
1
2
3
4
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magnets. It consists of hodoscopes of drift chambers with a spatial resolution of 210 tam. This system has a momentum resolution high enough to ensure the identification of 77 events on kinematical grounds even in the case o f single tagging. Furthermore, the fact that it detects electrons at zero degrees with a 10 mrad polar angle acceptance and full azimuthal acceptance over a wide range of fractional energy losses leads to a high tagging efficiency. For instance, our system tags, singly or doubly, 26% of the 77 -->e +eevents, 52% of the 77 -~ P ~ - events and 58% of the 77 ~ n+n- events which are detected in DM1. These tagging efficiencies are computed for an energy of 1 GeV per beam and the DM1 acceptance is characterized by the following cuts: (i) transverse momentum P r greater than 80 MeV/c, (ii) polar angle 0 between 45 ° and 135 °. This system has been in operation since January 1979. Up to now, 250 tagged photon-photon events have been recorded. 25 of them were doubly tagged. Their analysis is in progress.
M. Brossard et al. / A zero-degree tagging system
References [1] N. Arteaga-Romero, A. Jaccarini, P. Kessler andJ. Parisi, Nuovo Cim. Lett. 4 (1970) 993; Phys. Rev. D3 (1971) 1569. [2] S. Brodsky, T. Kinoshita and H. Terazawa, Phys. Rev. D4 (1971) 1532. [3] V. Budnev and I.F. Ginsburg, Phys. Lett. 37B (1971) 320. [4] The Orsay Storage Ring Group, Proc. 8th Int. Conf. on High Energy Accelerators, CERN (1971) 79. [5] G. Charpak et al., Nucl. Instr. and Meth. 108 (1973) 413.
165
[6] A. Breskin et al., Nucl. Instr. and Meth. 119 (1974) 9. [7] S. Mc Laughlin and A. Michalowski, CAMAC Model 2770A, Technical information manual, LeCroy Research Systems, Spring Valley (1977). [8] J. Jeanjean et al., Nucl. Instr. and Meth. 117 (1974) 349. A. Cordier et al., Nucl. Instr. and Meth. 133 (1976) 237. [9] G. Fournier, Th~se de 3~me cycle no. 538, Universit~ de • Clermont II (1977). [10] A. Falvard, Th~se de 3~me cycle no. 551, Universit6 de Clermont II (1978).