The Left-right Asymmetry

CiteSeerX

I  c 
SLAC-PUB-6444 
LBL-35168 
February  1994 
WI 
e+e- Collisions  at the  SLC  -  the  Left-Right  Asymmetry* 
Gilbert  Shapiro 
representing  the  SLD  collaboration 
Stanford  Linear  Accelerator  Center 
Stanford  University,  Stanford,  CA  94309 
University  of  California 
Lawrence  Berkeley  Laboratory 
Berkeley,  CA  94720 
ABSTRACT 
The  Stanford  Linear  Collider  has  operated  successfully  to  produce 
over  50,000  Zo’s  with  polarized  electron  beams.  The  asymmetry  in 
the  production  of  Zo’s  with  respect  to  left  and  right  handed  electron 
beams  has  been  measured,  and  a preliminary  result  is presented. 
Presented  at the 5th  Workshop  on High-Energy  Spin Physics, 
Protvino,  Russia,  September  2&24,  1993 
* * Word  supported  by  Department  of Energy  contract  DE-AC03-76SF00515. 1. THE  SLC  PERFORMANCE 
The  SLC  was  proposed  in  the  early  1980’s  as  a  major  upgrade  of  the  SLAC 
linear  accelerator  to  develop  ideas  and  techniques  for  an  e+e-  linear  collider,  and 
to  conduct  experiments  at the  Z pole.  This  idea,  proposed  by  Burton  Richter,  was 
intended  to  demonstrate  the  feasibility  of  high  energy  e+e-  colliders  using  linac 
technology.  In  the  SLC  design,  both  the  electron  and  positron  beams  are 
accelerated  in  the  same  linac  structure,  shown  in  Figure  1.  To  bring  these 
bunches  into  head-on  collisions,  a dipole  magnet  at the  end  of the  linac  separates 
the  e+  and  e-  beams,  and  a pair  of  transport  arcs  bring  the  beams  to  a collision 
point.  The  experimental  detector  sits  at  the  collision  point  where  the  arcs  meet. 
The  capability  for  polarized  electron  beams  was  incorporated  into  the  SLC  design 
from  the  beginning. 
The  SLD  detector  was  installed  into  the  interaction  point  in  1991,  following  an 
earlier  run  by  the  Mark  II  detector.  There  was  an  engineering  run  that  year  that 
produced  400  Zo’s with  unpolarized  beams.  In  1992  the  SLC  began  to  accelerate 
polarized  electrons.  That  year  the  SLD  recorded  over  11,000  Zo’s,  with  an average 
electron  polarization  of 22%.  The  results  of the  first  Am  measurement,  using  that 
data,  have  already  been  published  [l]. 
In  1993  the  SLD  recorded  nearly  50,000  Zo’s,  with  an  average  longitudinal 
polarization  of  62%  for  the  electrons  at  the  intercation  point.  The  positrons  are 
unpolarized. 
Figure  2  summarizes  the  improving  SLC  performance  since  1991.  The  peak 
luminosity  so  far,  achieved  in  May  1993,  was  as  high  as  50  calculated  Zo’s  per 
hour.  The  cross-section  at  the  Z  pole  is  30  nanobarns.  So  the  best  luminosity, 
averaged  over  several  seconds  at  120  Hertz  accelerator  repetition  rate,  would 
amount  to 5 $ lo29  per  square  centimeter  per  second. 
-  _ 
2 2. POLARIZATION 
The  components  specific  to  the  polarized  electron  beam  consist  of  the 
polarized  electron  source,  the  spin  rotation  solenoids  at  the  e-  damping  rings,  a 
Moller  polarimeter  at  the  end  of the  linac,  and  the  Compton  polarimeter  placed 
33 meters  after  the  SLD  detector. 
The  polarized  e-  source  consists  of  a  YAG-pumped  Ti:Sapphire  laser 
operating  at  865  nm,  a  series  of  optical  elements  to  control  the  intensity,  pulse 
length,  circular  polarization,  and  steering.  The  laser  beam  passes  through  a 
mirror  box,  and  onto  the  photo-  cathode  of  the  gun  structure.  The  polarized 
electron  gun  operates  at  -120  KV  potential  on  the  cathode.  The  photo-emitting 
surface  consists  of a 14 mm  diameter  wafer  of gallium  arsenide  (GaAs)  which  has 
been  prepared  with  a clean  surface  by  heating,  and  then  coating  with  cesium  and 
fluorine  (approximately  one  atomic  layer  in  thickness).  Several  variations  on  the 
type  of  GaAs  material  have  been  used  in recent  accelerator  runs.  Figure  3 shows 
the  polarization  versus  laser  wavelength  for  three  such  materials  [2].  The  most 
recent  runs  (1993)  used  strained  GaAs  material.  In a fixed-target  run  (not  SLD)  in 
late  1993,  beams  in excess  of 80%  polarization  have  been  reliably  delivered  to the 
experimental-area. 
Figure  4 shows  the  measured  polarization  at the  SLD  detector  during  the  runs 
of  1992  and  1993.  The  horizontal  coordinate  is the  ordinal  number  of the  detected 
Zo events.  The  marked  improvement  in the  polarization  with  the  introduction  of 
the  strained  GaAs  photo-cathode  at  the  beginning  of  1993  is  seen  after  event 
number  11,000.  Soon  thereafter,  after  an  additional  5,000  events,  the  wavelength 
of  the  laser  was  optimized  at  865  nm.  For  the  rest  of  the  1993  run  the  beam 
polarization  averaged  62%. 
The  Compton  polarimeter,  the  principle  instrument  with  which  the  electron 
-beam  polarization  was  measured,  is described  in another  talk  by  the  same 
3 I  . 
speaker  at this  conference  [3]. A discrepancy  between  the  polarization  measured 
by  the  Compton  polarimeter,  which  is close  to the  e+e-  interaction  point,  and  the 
Mdller  polarimeter,  at the  end  of the  linac  before  the beams  enter  the  arcs,  has 
now  been  resolved  by  correcting  a systematic  error  in Moller  polarimeters 
pointed  out by  L. G. Levchuk  [4]. There  is a small  loss  in polarization,  under  376, 
as the beam  goes  around  the  arcs. 
4 TABLE  I: COMPTON  POLARIMETER  SYSTEMATIC  ERROR 
Source  At Present  After  Complete  Expected  For  1994 
Analysis  1993  Run 
Laser  Polarization  1.0  %  0.8  7%  0.5  % 
Detector  Linearity 
Interchannel 
Consistency 
Electronic  Cross - 
talk 
Analyzing  Power 
Calibration 
1.0  0.7  0.5 
0.5  0.5  0.3 
0.2  0.2  0.2 
0.5  0.5  0.2 
TOTAL  1.6  %  1.3  %  0.8  % 
3. THE  LEFT-RIGHT  ASYMMETRY 
The  left-right  asymmetry  ALR  is an  important  electroweak  parameter.  ALR  is 
defined  as 
OL-OR 
ALR=a  +o  L  R 
wheret3L  (is~)  is  the  visible  cross-section  for  the  left-handed  (right-handed) 
incident  electrons.  Simply  stated,  A LR is the  spin-flip  asymmetry  in the  total  cross 
section.  It  is  independent  of  the  final  state  detected,  so  all  final  states  are  used, 
except  e+e-  ;i’M  Z”  TM  e+e-  .  Forward  Bhabha  scattering,  dominated  by  the  t- 
channel,  being  mostly  an  electromagnetic  process,  is  expected  to  be  parity- 
conserving,  and  thus  to  exhibit  not  asymmetry.  Observation  of  the  small 
asymmetry  in  the  forward  Bhabha  channel  is  in  fact  a  systematic  check  that  no 
false  asymmetries  are  introduced  by  the  beam  or the  apparatus. 
The  experiment  consists  of  reversing  the  electron  helicity  frequently,  randomly 
on a 120  Hz basis,  counting  the  Z@s  produced,  and  forming  the  asymmetry 
A 
lN,-NR 
meaS  = P,N  L  +N  R This  measured  asymmetry  Ameas  is,  within  small  corrections,  equal  to  Am 
provided  Pe, the  electron  beam  polarization,  is well  measured,  and  the  integrated 
luminosity  in each  helicity  is equal. 
ALR is a sensitive  electroweak  parameter.  It is sensitive  to the  heavy  masses  of 
the  top  quark  and  the  Higgs  boson,  plus  any  new  objects  which  couple  couple  to 
the  photon  or the  Zo,  such  as those  in supersymmetric  models. 
ALR  can  be  measured  using  any  Zo  decay  channel,  except  Bhabhas.  The 
strategy  is  to  use  all  visible  decays,  since  the  value  measured  is  independent  of 
the  decay  channel,  and  the  statistical  error  will  decrease  in  proportion  to  the 
inverse  square  root  of the  total  number  of  decays.  In  addition,  ALR is insensitive 
to  initial  state  radiation,  and  independent  of  QCD  corrections.  Because  of  the 
frequent  spin  reversals  at  the  source,  drifts  in  the  detector  efficiencies  cancel  in 
the  asymmetry.  The  only  significant  systematic  error  in  ALR  lies  in  the 
measurement  of Pe. 
TABLE  II: PRELIMINARY  CORRECTIONS  TO  ALR 
ERROR  SOURCE  CORRECTION  d(ALR)/ALR 
Background  Fraction  0.6 2 0.3%  +0.4  f  0.2  % 
Polarization  Asymmetry  0.5%  +0.02% 
Energy  Asymmetry  6 $10-6  -0.01  % 
Efficiency  Asymmetry  0  0 
Luminosity  Asymmetry  (-1.0  f  0.2)  $10-2  +O.lO f  0.2  % 
TOTAL  +0.5  k 0.2  % 
Event  selection  for  the  Zo candidates  is currently  based  on  SLD’s  calorimeters 
only.  The  analysis  requires  the  total  energy  deposited  to  exceed  20  GeV.  Events 
with  energy  greater  than  12  GeV  in  the  calorimeter  end  caps  are  excluded  to 
remove  machine  generated  backgrounds.  An  energy  imbalance  cut  is  imposed, 
-that  the  sum  of the  energy  vectors  in the  towers  must  be  less  than  0.8  of the  total 
6 I 
c 
energy  seen.  These  cuts  cuts  remove  most  of  the  two-gamma  events  and  beam- 
related  backgrounds.  The  remaining  sample  contains  92  2  2  %  of  the  hadrons, 
30%  of  the  tau  pairs  that  are  produced,  beam  backgrounds  c  0.3%,  two-gamma 
events  c  O.l%,  and  Bhabha  contamination  c  0.3%.  In  addition,  a valid  Compton 
polarimeter  measurement  is required  within  one  hour  of the  event  candidate  if it 
is to be  used. 
- 
Table  I lists  the  systematic  errors  in the  polarization  measurement  for  the  1993 
running,  and  those  expected  for  next  year.  Table  II  lists  the  preliminary 
systematic  errors  from  sources  other  than  the  polarization  measurement.  Table 
III  gives  the  preliminary  results.  For  the  1993  running  the  center  of  mass  energy 
was  91.26  +/-  .02 GeV,  compared  to the  cross  section  peak  at 91.28  GeV. 
TABLE  III:  PRELIMINARY  RESULTS  FOR  ALR 
Number  Events  Left  Beam  Right  Beam  Raw  Asymmetry 
BHABHAS:  125375  62656  62719  -.0005  + /  - .0028 
Zo’s: 47492  26195  21297  +.1031  f  .0046 
POLARIZATION  OF BEAM  I’,:  .626  + .012 
Am  (1993  data  only)  = .1656  k.0073  (stat)  2  .0032  (sys) 
ALR (combined  92-93)  = .1635  *  .0072  (stat)  2 .0032  (sys) 
From  the  combined  preliminary  value  of Am,  we  can  calculate  a value  of the 
weak  mixing  angle: 
sin%ew  = .2290  2  .OOlO. 
The  errors  will  continue  to  diminish  in  future  SLC  runs,  and  Am  will  likely 
become  the  most  precise  single  asymmetry  measurement  bearing  on  the 
electroweak  theory  parameters.  The  1994  run  is  expected  to  produce  100,000  to 
150,000  Zo’s  with  beam  polarization  exceeding  75%.  Further  runs  to bring  the  Zo 
total  close  to  one  million  events  are  planned  for  the  succeeding  years. 
-  _ 
7 REFERENCES 
[l]  K. Abe  et al.,Phys.Rev.Lett.  70,2515  (1993) 
[2]  T. Maruyama  et al.,  Phys.Rev.Lett.  66,2351  (1991) 
[3]  G. Shapiro,  “The  Compton  Polarimeter  at the  SLC,”  paper  presented  at 
SPIN-93,  Protvino,  Russia. 
[4]  L. G. Levchuk,  “The  Intra-Atomic  Motion  of Bound  Electrons  as a Possible 
Source  of a Systematic  Error  in Electron  Beam  Polarization  Measurements  by 
Means  of a Moller  Polarimeter,”  Kharkov  Institute  of Physics  and 
Technology,  KFTI-92-32 FIGURE  CAPTIONS 
Figure  1. 
Figure  2. 
Figure  3. 
Figure  4. 
Polarization  in the  Overall  SLC  Layout 
The  improving  performance  of the  SLC  is shown  for  the  past  three 
years.  The  rate  of Zo production  per  week  is shown  on the  left  hand 
side,  and  the  total  integrated  Z@s  for  each  year  is shown  on the  right 
hand  side.  The  electron  beams  were  unpolarized  up  to  April  1992. 
The  polarization  versus  wavelength  for  three  cathode  materials  used 
on the  accelerator. 
Electron  beam  polarization  versus  time,  measured  in ordinal  number 
of detected  Zo. I  . 
e+  Source 
.-  -l-l 
Spin  Rotation 
e+  Return 
Line 
Damping 
Ring 
Electron  Spin 
ghf?;;lnic*  FzEz  e-  Source 
POLARIZATION  IN THE  OVERALL  SLC  LAYOUT 
9-92  722OA5 
Fig.  1 I 
c 
6000 r 
1991  - 1993  SLC  Luminosity 
1 
60000 
5000 
4000 
3000 
2000 
1000 
0 
B  SLC  zjweek 
n  SLC  Z total 
5 
-?  Y’ 
a  Lt 
I  E  %  5 
(D  A  7  z  I  3  Lt-  7 
ui  Gi 
0) 
_  Unpolarized  =!=  Pilarked  * 
1991  I  1992  I  1993 
50000 
0 
9-93 
754SAl 
Fig. 2 . 
100 
80 
-g 
-  60 
20 
0 
6-93 
I  I  I  I  0 
l Strained  GaAs 
-  0 AlGaAs 
A Bulk GaAs 
0 
0 
(50% )  __--_______-___--_--___ 
88 
. . . . . ..i---- 
,‘pb* 
l 
8” 
l 
l 
0 
l 
0 
00 
0  P-AA. 
.-A.. 
600  700  800 
h  b-0  7472Al 
Fig.  3 u  --  .  . 
l  * 
-  9 
.r, 
. 
￿-  . 
. 
72 
si 
/  % 
z  E  .-￿ 
. . 
C&p+- 
3 
3 
2 
4 
WEa8  UOlJ3ajTJ  JO  UO~)li?Z$U?~Od 

WI e+e- Collisions at the SLC-

http://www.slac.stanford.edu/pubs/slacpubs/6250/slac-pub-6444.pdf

WI e+e- Collisions at the SLC-

Abstract

Similar works

Full text

Available Versions

CiteSeerX