Download Measuring Light Neutrino Families

Measuring Light Neutrino
Families at LEP
Siemen Meester
OUTLINE:
• Introduction
• LEP
– Goal
– Peak fit
– Z line shape fit
– One photon event
• Conclusion
Introduction
• Fundemental question: How many families of particles
are there?
q
e
• Answer: Nv = 2.9841 ± 0.008
Z0
• How:

–
–
–
–
Go to Zo resonance peak.
Use formula:       
Z

l
q
Look for missing stuff in detector
inv  l 
Use Formula:
N 
 
l    SM
e
q
LEP
• e+ e- collider
• Produciton of Z0 particles
• precision measurements
of the Standard Model (1989-2000).
s  90  200GeV
Opal detector, one of the four
detectors at LEP
Decay modes
e
e

q
e

Z0
+
q
q
e
q
First order QED correction.
Well known process can correct for this.

1
s
Z resonance peak
Peak fit
At Zo peak Z mode higly favored
f 
se  f
12 ( c)
M z2 ( s  M z 2 ) 2  s 2 z 2 / M 2 z
12 ( c) 2  e  f

M z2
z
with
s  Mz
Decay width
 z  3l  had  N 
If Nv increases so does total decay width.
 f  Bf z
Ratio only depends on hadronic peak
cross section and Γhad/Γl and is
independent of ΓZ.
 12Rl




R

3
l
 M 2 Peak

z had


inv  l 
N 
 
l    SM
Partial width defined as:

N  l

had
Rl 
l
Parameter Fit
Because measured cross section depend
on products of the partial widths and also
on total width, the widths constitute a highly
correlated parameter set.
Explicitly calculate:
 Z     l   q
q   q
q t
Define new parameters:
M z and z
Hadronic pole cross section:

12 ( c) 2  e  f

M z2
z
Ratios
Rl 
had
l
Pole asymmetries:
0 ,l
AFB
The Experiment
Experimental demands:
The principle of the analysis: all visible
channels are detected by large
detectors and classified according to
four categories:
• Hadrons
Very important to measure
luminosity.
( N  N bg )

L
Done using bhabha
scattering
• Electrons
• Muon pairs
• Tau pairs
Calculable cross section with
low angle scattered electrons
Particle Identification
JET me
Electron
Photon
•Different particles leave different signals in the
various detector components allowing almost
unambiguous identification.
•e : EM energy + track
 : EM energy, no track
m : track + small energy deposit + muon
t : decay, observe decay products
 : not detected
Quarks: seen as jets of hadrons
Muon
Pion
Neutrino
Jet
Event display ALEPH:
Four possible Z decays
• top left two electrons
• top right two muons
• bottom left τ decaying
to electron and three
other particles
• bottom right quark
pair to hadrons
Results
 Z  N   l   q
or
N 
inv  l 
 
l    SM
l
 1.9912  0.0012

inv
 5.942  0.0083
ll
N  2.9841.0.0083
Common Uncertainties
• Calibration of beam energy
• Theoretical error on small angle Bhabha
cross section
• Theoretical uncertainties in QED radiative
effects.
• Small uncertainties in the parameterization
of the of the EW cross section near the Z
resonance peak
Higher energies
Lose energy go back to
Z resonance.
Look for one photon event and
missing energy
Select events with certain photon
energy to get good idea about recoil
mass
Results
All in good agreament with 3
neutrino generations
Result from fit:
N  2.95  0.08(stat )  0.03(syst )  0.03(theory)
Conclusion
• 3 light neutrino generations
• There could be of course a sterile neutrino
which doesn’t couple to the Z boson.
• Z characteristics have been precisely
measured by LEP for high energies.