Transcript PPT

Subnuclear Physics in the
1970s
IFIC Valencia. 4-8 November 2013
Lecture 9
SPS collider and UA1
The discovery of the intermediate vector bosons
1-Apr-16
A. Bettini LSC, Padova University and INFN
1
Masses and widths of the vector bosons
The electroweak theory predicts all the properties, masses, total and partial widths, spin and
parity, of the IVBs, W and Z˚, in terms of two quantities, the Fermi constant GF and the weak
mixing angle W
 g2 2 
MW  
 8G 
1/2

F
Giving
MW
 cos W
MZ

1
37.3

GeV
2GF sin W sin W
MW ; 80 GeV
M Z ; 91 GeV
The partial and total widths of the W (calculations are exact, approx values here for simplicity)
     ; 225 MeV
   W  cd   33 MeV ud   W  ud   640 MeV
e


cs   W  cs  660 MeV
cd
us   W  us  35 MeV
The partial and total widths of the Z
Z charges are
 ee   µµ    ; 83 MeV
W  2.04 GeV
gZ 

g
I 3W  Q sin 2 W
cosW
 inv  3 ll   Z   l l   495 MeV
u   Z  uu   c   Z  cc   280 MeV
d  s  b  370 MeV
 h  2 u  3 d  1.67 GeV
Z  inv  3l  h  2.42 GeV
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
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2
The Rubbia et al. proposal
In the 1970s, once sin2W was known from several different experiments, everybody new were to
search for the IVBs. But how?
The two highest energy proton accelerators, at CERN and Fermilab, could accelerate protons to a
few 100 GeV, below threshold to produce the IVBs on fixed target.
In 1976, Cline, Rubbia and Mc-Inteyre proposed to transform (one of) such machines in a
proton anti-proton collider. As in the case of AdA, the same magnetic structure can be used for
particle and antiparticles circulating in opposite direction
C. Rubbia, P. McIntyre and D. Cline; Proc. Int. Neutrino Conf., Aachen 1976. Vieweg, Braunscweig, 1977.
To have the necessary luminosity it was however necessary to be able to “cool” the random
motion of the particles in the packets circulating in the accelerator. Fermilab tried the “electron
cooling”, in which an electron (positron) packet circulates together with the proton (antiproton)
one and cools it. CERN had the technology of stochastic cooling developed by S. van der Meer
in 1972 for the ISR. And CERN wan.
S. van der Meer, Intern Report CERN ISR-PO/72-31 (1972)
D. Möhl, G. Petrucci, L. Thorndal and S. Van der Meer, Phys. Rep. 58 73 (1980)
By 1978, Rubbia has guided an international collaboration in preparing the proposal for the
UA1 detector
A. Astbury et al. A 4π solid angle detector for the SPS used ad a proton-antiproton collider at a centre of
mass energy of 540 GeV, Proposal CERB-SPSC 78-6/P92 (1978)
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The Z resonance
W and Z can be produced in a “formation” process by colliding quarks and antiquarks
But quarks are never free  proton-antiproton collider
 UA1 (CERN). Discover in 1983
Z can be produced in a formation experiment with electron-positron collider
 precision studies at LEP (CERN) and SLC (SLAC) 1989-2001
Quark-antiquark collisions
Energy in the quarks CM
ŝ  xq xq s
Process we want to observe
u  d  e   e
Must have the same colour
Negative chirality
u  d  e   e
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W and Z resonances
u  d  e   e
Near to the resonance  Breit e Wigner
 ud  e e 
1 3
9 ŝ

 ud  e
ŝ  M W
  
2
/ 2
2
W
Probability two colours be equal
 max ud  e e   max u  d  e   e 

4 1  ud  e 4 1 0.640  0.225
GeV-2   388  µb/GeV-2   8.8 nb

2
2
2
2
3 M W W
3 81
2.04
Small <<< tot100 mb. Weak interactions are really weak!!
For the Z
u  u  e  e ;
d  d  e  e
4 1  uu  ee 4 1 0.280  0.083

 388 µb  0.8 nb
3 M Z2  2Z
3 912
2.42 2
4 1  dd  ee

 1 nb
2
2
3 M Z Z
 max uu  e e 
 max dd  e e 
An order of magnitude smaller than for W
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Cross sections
Beam of p = wide band beam of partons (q, g, and a few antiquarks)
Beam of anti p = wide band beam of partons (anti q, g, and a few quarks)
Consider the annihilation of valence quark and antiquark
At √s=630 GeV, the momentum fraction a quark should have to be at resonace is
 x 
MW M Z

 0.15
s
s
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OK. There are many
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6
Production
in
pp
collisions
The width of the partons energy band>> widths of the W and Z resonances
The collisons happens in the proton antiproton CM frame, not in the quark antiquark one. The
latter, and the W or Z they produce have a longitudinal momentum, different from event to event
ŝ  xd xu s
plus analogous ud
ŝ  xu xu s
plus analogous dd
A QCD calculation, with the measured structure functions, gave at s=630 GeV
170
17
 pp  W  e e   530
pb
 pp  Z  e e–  35
pb
90
10
An order of magnitude smalle MZ>MW and for the interplay of the weak charges


Exercise. How many events We and Z e+e– can be detected in one year with L=1032 m–2 s–1
and 50% efficiency?
NW  L    N sec   10 32  530 1040 10 7  0.5  26
1-Apr-16
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NZ  2
7
The Super Protn Synchrotron CERN SPS)
Once AA is full antiprotons are
extracted, brought back in the
PS, accelerated to 26 GeV and
transferred to SPS
Protons accelerated to 26
GeV in the PS are used to
produce antiprotons in a Co
target
The accumulator ring AA accepts a batch of
these with momenta about 3,5 GeV every 2.4 s
accumulating up to 1011 antiprotons in a day
Protons from PS at 26 GeV are injected in the SPS just before the antiprotons in opposite
direction. Protons and antiprotons are then accelerated (up to 270-310 GeV) and remain stored
for hour (coasting). They are bunched in 3 bunches of 4 ns duration each, so that crossings
happen in 6 locations, in two of which UA1 and UA2 experiments were installed
1-Apr-16
from S. van der Meer Nobel lecture
A. Bettini LSC, Padova University and INFN
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Basic accelerator physics concepts
In an accelerator particles can be considered as a statistical system, following the statistical
laws. Canonical phase space is 6D space of the coordinates and their time derivatives
Phase space in accelerator physics: 6D space of x, y, z, x’, y’, z’. (x’= dx/ds, where s is the
coordinate along the orbit, is the beam divergence in the x direction)
Neglect the coupling between the coordinates and define a longitudinal (or horizontal) and a
transverse (vertical) beam emittance
Longitudinal beam emittance:
x = area in x, x’ plane containing 95%
of the particles
Similarly for the other pairs
Liuville theorem. Volumes in the phase space are invaraiant
Acceptance of an accelerating structure = maximum beam emittance it can accept
1-Apr-16
from S. van der Meer Nobel lecture
A. Bettini LSC, Padova University and INFN
9
Stochastic
cooling
The AA must have the largest possible acceptance to be able to accept as many antiprotons as
possible. It is much larger than the SPS acceptance. Phase space volume must be reduced and
particle density in it increased by 109.
Liouville violation? Fortunately there is a trick
Particles are points in the phase space, with empty space between them. Push particles
towards the centre of the distribution squeezing out the empty space. The small-scale density
is conserved, but macroscopically density increases.
Process is called cooling because it reduces
the relative motion of the particles
Cooling can be done only if we have
information of the position in phase space of
the individual particles, and if we can direct
the pushing action towards individual
particles: push forward the slower, push
backward the faster
If only one particle, it is simple. Particle
performs betatron oscillations around the
ideal orbit
Transverse pick-up device gives a signal proportional to the distance from ideal orbit,
which is transmitted, through a chord, shorter than the arc, to the kicker that gives the right
kick when the particle arrives
from S. van der Meer Nobel lecture
1-Apr-16
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Stochastic cooling
In the beam there are some 1012 particles, rather than one
Even with the fastest electronics signals overlap
Nevertheless, signals from the individual particles are present
However, we must reduce the gain of the system. Consider one of the particles. The signals
from all the others in the resolution time of the system have phases distributed at random and
would give a perturbing effect on the one under consideration. This means heating the bunch
Fortunately, the perturbing effect averages
to zero, but second order term heats (i.e.
increases the mean square oscillation
amplitude).
This effect is proportional to the square of
the gain, while cooling effects each
particle linearly with the gain
We can always choose a gain so that
cooling predominates
D. Mohl, G. Petrucci, L. Thorndahl, and S. van der Meer,
Phys. Rept. 58 (1980), 76.
1-Apr-16
from S. van der Meer Nobel lecture
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AA the Antiproton Accumulator
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Inside the vacuum tank
Ferrite frames of the
kickers for pre-cooling.
space for the
antiprotons stack
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Cooling
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Experimental signals
The IVB is rare 10–8 -- 10–9 (tot 60 mb = 61010 pb )
Rejection power must be > 1010
Most frequent final states are quark antiquark, but in a very high backgound
Experimentally q  jet
Huge background from gg  gg,
  B W  qq   3  B W  l l  3=nunber of colurs
gq  gq,
gq  gq ,
qq  qq
Important measured quantity: transverse momentum pT = momentum component
perpendicular to beams

Leptonic finl states are rarer but have a favourable S/N
W
W
Z
Z
 e e
 µ µ
 e– e+
 µ–µ+
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e
µ
2e
2µ
isolated, high pT
isolated, high pT
2 isolated, high pT
2 isolated, high pT
}+
 at high pT = large missing pT
Hermetic detector (UA1 measured missing
pT with a few GeV resolution)
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Identify and measure leptons & hadrons
CD. Tracking central detector in magnetic
field B normal to the beam. Momenta
measurement
EM calorimeters. Electrons and photons
energy measurement
Hadronic calorimeters. Hadron
identification and energy measurement
Fe filter with trakers for µ (DC =, later,
Limited streamer tubes)
Trasversal hermeticity. Missing momentum
= neutrino
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Identify and measure leptons &
hadrons
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Building UA1
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Central detector goes to museum
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The first W
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We
In an electromagnetic calorimeter the W appear as
a localised energy deposit in direction opposite to
missing momentum
Elimination of tracks with pT< 1 GeV leaves
the event completely clean. Only electron
and “neutrino” survive
The tracking central detector in magnetic field
measures momentum and sign of the tracks.
Calorimeters measure the energy
The electronic nature of the track is established by
E=p
The hermeticity allow determining the “transverse
missing momentum” = transverse momentum of 
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21
Measuring MW
Wl l
pTe
The transverse momenta of the initial
quarks and antiquarks are small; hence
is so the one of W
Neglecting it
e
e
pTe
W
*
W
e
pe = mW/2
e
LAB
pTe is the same in the two frames = (mW/2) sin *
CM. W
The CM angular distribution is known
dn transform coordinates dn
dn d *
 
 *
*
d
dpT d dpT
dn
1
dn

*
dpT
d

mW 2
2
   pT
 2 
“Jacobian” peak at pTe = mW/2
“Jacobian” peak at pTmissing = mW/2

The transverse motion of the W (pT
widens the peak but does not cancel it. The measurement
of mW is based on the measurement of the energy of the point half way on the trailing edge
W≠0)
1-Apr-16
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Transverse energy distributions
UA1
UA2
1-Apr-16
MW= 82.7±1.0(stat)±2.7(syst) GeV W<5.4 GeV
MW= 80.2±0.8(stat)±1.3(syst) GeV W<7 GeV
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Twenty years later
Electron trasverse momentum distribution from D0 experiment at Tevatron
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W
spin
and
helicity
@ s=630 GeV <x> = M /√s0.15 valence quarks dominate over sea
W
Direction of the q = direction of the p; Direction of the anti q = direction of the anti p
In the W CM frame, the electron energy >> me.
chirality  helicity
W   e e
V–A W couples only with Fermions with
helicity –
Antifermions with helicity +
Tot. angular mom. J=SW=1
Jz (iniz.) = l = –1
Jz’ (fin.) = l’ = –1
2
2 1
d
1
* 
 d1,1   1 cos 
2

d




N.B. If it were V+A

d
1
 d1,1
d
 
2
2
 1
* 
 – 1 cos 
 2



Forward-backward asymmetry is a consequence of P
violation

To distinguish V–A from V+A electron polarisation
must be measured
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The first Z
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Ze+ e–
In the electromagnetic calorimeters the Z
appears as two localised energy deposits in
opposite direactions
Elimination of tracks with pT< 1 GeV leaves
the event completely clean. Only electron
and positron survive
The tracking central detector in magnetic field
measures momentum and sign of the tracks.
Calorimeters measure the energy of the
electrons
The electronic nature of both tracks is
established by E=p
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Measuring MZ
 
Z e e
0
E1 (e–, µ–)
r r 2
2
m 2  E1  E2   p1  p2   E12  E22  2E1E2  p12  p22  2 p1 p2 cos
 2E1E2 (1 cos )
m  4E1E2 sin  /2
2
2
  100Þ
tan

2
 m
2
m2
 O(1)

  E1    E2      
 E    E    tan  / 2 
1
2
2
E2 (e+, µ+)
 E  20%
E

E
 E 
 4  6%
E
2
 m  1  m

 2  3%
m
2 m2
m2
 2
2
 determined by the measured tracks     10 –2
Main uncertainty due to energy measurement
 m 2 
2

 
statistical error on one measurement (m)2-3 GeV
scale uncertainty 3.1 GeV (UA1); 1.7 GeV (UA2)
UA1 (24 Zee) MZ=93.1±1.0(stat)±3.1(syst)
GeV
UA2
MZ=91.5±1.2(stat)±1.7(syst)
GeV
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1-Apr-16
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1984. The Nobel Prize
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