An experiment to measure CP
violation in B mesons
Colin Jessop
University of Notre Dame
Jan 28th, 2009
Colin Jessop at Notre Dame
Measuring a CP phase by Interference
Interference of two amplitudes is required
initial
final
The two amplitudes must have a relative CP even phase q and relative CP odd phase d
CP even phase:
CP odd phase:
July30th, 2008
(the CP violating phase)
Bigi,Jessop,LoSecco at Notre Dame
Measuring a CP violating Phase
initial
For the
final
CP : M  M (antiparticle) process
Define an Asymmetry:
Acp=0 if q=0 or d=0
July30th, 2008
Bigi,Jessop,LoSecco at Notre Dame
Direct CP Violation
CP even phase from strong interaction.
Real Amplitude
+
Complex: CP odd phase from Vub
CP even phase from gluon
Strong Phase from non-perturbative QCD is not reliably predicted
July30th, 2008
Bigi,Jessop,LoSecco at Notre Dame
Use Mixing to Provide CP even Phase
W
b
B
0
d
u,c,t
B0
u, c, t
s or b
d
W
B B
0
0
Mixing parameters discovered to be favourable to such a measurement in 1980’s
and these parameters are precisely measurable
July30th, 2008
Bigi,Jessop,LoSecco at Notre Dame
Measuring CP odd phase in B Mixing
fcp
(e.g yK0s)
Acp  sin mix t  sin cp 
CP eigenstates
fcp accessible to B0 and B0 are rare
( < 1 in 3000 B decays)
Need to produce millions of B meson decays
Need a way to identify if you started with B0 or B0
Need to measure time dependence of process
July30th, 2008



Bigi,Jessop,LoSecco at Notre Dame
sin mixt dt  0

A Confluence of Nature and Technology
In the 1980’s we found that the parameters of B decays made this measurement
possible but not at existing facilities
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Challenge
Solution
Millions of B
mesons
High Luminosity
e+e- storage rings
Identifying B or B
Produce BB pairs
Measuring time
dependence
Development of
silicon track
detectors
Bigi,Jessop,LoSecco at Notre Dame
An Asymmetric B meson “Factory”
July30th, 2008
Bigi,Jessop,LoSecco at Notre Dame
Two B-Factories: USA and Japan
BELLE at KEKB nr Tokyo, Japan
Both US and Japanese experiments
Parralleled each other 1993-2004
After 2004 funding issues in US slowed
BaBar down and lead to eventual
termination of experiment in 2007
(486 fb-1)
BELLE in Japan is still going strong
(895 fb-1 so far)
At termination BaBar was running at seven times design luminosity.and had published
over 300 papers
July30th, 2008
Bigi,Jessop,LoSecco at Notre Dame
Experimental Technique for B Factories
Btag  B 0 (t )
 Brec  B (t )
0
  4S 
e
e
J/y
0
Brec


KS0

Exclusive
B Meson
Reconstruction

0
Btag
K
z ~ 260  m
(4S) produces
coherent B pair:
t  t

Δ t  Δ z/  βγ  c
Time-integrated asymmetries are zero
0
0
Brec
 Bflav
(flavor eigenstates)
0
Brec
 BCP0 (CP eigenstates)
July30th, 2008
B-Flavor Tagging
lifetime, mixing analyses
CP analysis
Bigi,Jessop,LoSecco at Notre Dame
BABAR Collaboration
Gathering at SLAC, July 2002
9 Countries
76 Institutions
550 Physicists
July 2002
July30th, 2008
Bigi,Jessop,LoSecco at Notre Dame
BABAR Detector
1.5 T solenoid
Electromagnetic Calorimeter
e+ (3.1 GeV)
Cerenkov Detector
(DIRC)
-
e (9 GeV)
Drift Chamber
Silicon Vertex Tracker
Instrumented Flux Return
July30th, 2008
Bigi,Jessop,LoSecco at Notre Dame
Silicon Vertex Detector at BABAR
e- beam
•
•
e+ beam
5 Layer AC-coupled
double-sided silicon detector
Located in high radiation area
– Radiation hard readout electronics (45Mrad)
•
•
97% hit reconstruction efficiency
Hit resolution ~15 μm at 90o
July30th, 2008
Bigi,Jessop,LoSecco at Notre Dame
Completed SVT Detector
B1 bending
magnet
Readout
chips
Beam pipe
rBP,IR = 25 mm
2 x Be walls =
Layer 1,2
0.83+0.53 mm T
Layer 3
1.5 mm H2O
Layer 4
cooling gap
Layer 5
July30th, 2008
Bigi,Jessop,LoSecco at Notre Dame
BABAR Drift Chamber
•
•
•
40 layers of wires (7104 cells) in 1.5 Tesla magnetic field
Helium:Isobutane 80:20 gas, Al field wires, Beryllium inner wall, and all
readout electronics mounted on rear endplate
Particle identification from ionization loss (7% resolution)
16 axial, 24 stereo layers
July30th, 2008
Bigi,Jessop,LoSecco at Notre Dame
An Innovative Detector for Seperating K+/+
July30th, 2008
Bigi,Jessop,LoSecco at Notre Dame
Cerenkov Detector Construction
Projection Screen
July30th, 2008
Quartz bars
Bigi,Jessop,LoSecco at Notre Dame
K+/+ Separation
K+/+ for tagging and CP eigenstate identifiction
July30th, 2008
Bigi,Jessop,LoSecco at Notre Dame
High Resolution Electromagnetic Calorimeter
• 6580 CsI(Tl) crystals with
photodiode readout
•0 reconstruction for rare
CP eigenstates
0
s = 5.0%
July30th, 2008
Bigi,Jessop,LoSecco at Notre Dame
Completed Detectors
First collisions May,
1999
Completed Feb 1999
First collisions May, 1999
July30th, 2008
Bigi,Jessop,LoSecco at Notre Dame