The BABAR EXPERIMENT
Physics motivation: CP violation
The accelerator: PEPII at SLAC
The detector: BABAR
Physics results
Richard Kass
OSU Feb 10 2010
1
C and P Symmetry
Continuous symmetries have been key in our understanding and
discovery of the laws of nature:
WikipediA:
“Noether’s theorem is a
central result in theoretical
physics that shows that conservation
laws can be derived from any
continuous symmetry.”
Symmetry Operation
Conserved Quantity
Translation in space
Linear momentum
Rotation in space
Angular momentum
Translation in time
Energy
Change of phase
Electrical charge
Discrete Symmetries are important also:
Parity: (x, y, z) ↔ (-x, -y, -y)
vectors (mom.) change sign but axial vectors (ang. mom.) do not
Charge Conjugation: particles turn into anti-particles (and visa versa)
proton ↔ anti-proton, electron ↔ positron
C and P are good (conserved) symmetries for EM and the nuclear force.
So, they must be good for all the forces……right?
Richard Kass
OSU Feb 10 2010
2
P and CP Violation
WRONG Parity is violated by Weak Interaction (e.g. b-decay)
Discovered in 1957 (Wu, Co60) Big effect, maximal violation!
Even though Parity was violated it was thought that the
combination of Parity & Charge Conjugation would be
conserved in Weak Interaction.
C. S. Wu
1964 Cronin and Fitch discovered the violation of CP in the decay of the
long-lived, CP-odd neutral K meson into a CP-even final state:
Br(KL→π+π-) ~ 0.2% instead of zero.
The laws of physics are different for matter and anti-matter!
Cronin
Fitch
For ~ 40 years the only way to study CP violation was to use KAONS
We now can study CP violation with B-MESONS
Richard Kass
OSU Feb 10 2010
3
What is CP Violation??
Are the laws of physics the same for particles & anti-particles?
IS “CP” conserved in nature?
P=parity operation: (x,y,z)-> (-x,-y,-z)
C=charge conjugation: turn a particle into anti-particle
ALMOST YES for the weak force
CP violation in kaons ~2/1000
So the answer is NO…
BUT this is a very small effect.
So, what’s the problem?
Richard Kass
OSU Feb 10 2010
4
The Early Universe had lots of matter
and anti-matter….
We all know about matter since it is the stuff we are made of.
But what is anti-matter?
Einstein (1905)
Matter and energy are equivalent
and can transform into each other.
Dirac (1928)
Invents relativistic quantum mechanics
Has extra solution and predicts anti-matter
Anti-matter is like matter but opposite electric charge
e.g. a negatively charged proton…
Ideas of Einstein and Dirac lead to lots of possibilities for anti-matter!
Why not an anti-electron (= positron =e+)?
Richard Kass
OSU Feb 10 2010
5
Anti-Matter Found!
The positron (e+) was discovered in 1932
in cosmic rays by Carl Anderson at Caltech
The photograph shows how positrons were first
identified in cosmic rays using a cloud chamber,
magnetic field and lead plate
C.D. Anderson, Phys. Rev. 43, 491 (1933).
e+ bending in B-field
g
e
-
Why not a photon converting
into matter + anti-matter?
eg
e+
A bubble chamber photo showing
examples of γ→e+e-
Anti-proton found in 1955…
e- e+
Richard Kass
OSU Feb 10 2010
6
Matter-AntiMatter Symmetry
In our current view of nature the fundamental building blocks are quarks and leptons:
An electron is a lepton and a proton is a bound state of 3 quarks (2 u’s and a d)
There is symmetry between building blocks:
For every type of quark/lepton there is an anti-quark/anti-lepton
anti-proton = uud
Bound states of quark anti-quark pairs are MESONS
lots of mesons are possible:
π+= ud, K+=us, B+=ub
Anti-matter is routinely produced on Earth!
Accelerator laboratories:
Fermilab: anti-protons
Cornell/SLAC/KEK: e+
3 generations of quarks & leptons
Hospitals: Positron Emission Tomography
Richard Kass
Looks good on earth,
what about rest
of the universe?
OSU Feb 10 2010
7
Anti-Matter in the Universe
When we look into the night sky
we only see MATTER!
Anti-proton/proton ratio~10-4 in cosmic rays
No evidence for annihilation, e+e-→γ,
from intergalactic clouds
In the Big Bang particle-antiparticle pairs were created from pure
energy in a spontaneous explosion
BUT today we cannot detect significant amounts of antimatter in
the universe - why not?
Since matter and antimatter can annihilate into photons how did an
amount of matter survive?
Predict:
nMatter/nPhoton~ 0
Experiment: nb/ng~ (6.1 ± 0.3) x 10-10 (WilkinsonMicrowaveAnisotropyProbe)
Richard Kass
OSU Feb 10 2010
8
CP Violation in the Standard Model
In the SM a quark turns into another quark by coupling to a W-boson
e.g. a neutron (udd) decays to proton (uud) via: d→uW-
Under a CP Operation we have:
coupling
CP(
g
q
q’
) =
W-
g*
q
q’
W+
Mirror
To incorporate CP violation: g ≠ g*
(coupling has to be complex)
It turns out that with 3 generations of quarks we can easily
incorporate CP violation into the Standard Model:
The Kobayashi-Maskawa Matrix (1973)
Richard Kass
OSU Feb 10 2010
9
CP violation & B mesons
By ~1987 there was enough info known about the KM matrix
and the standard model for theorists to make believable
predictions about observing CP violation with B-mesons.
5 quarks were observed, top quark thought to be “heavy”
W and Z bosons observed & masses measured
lifetime of B mesons measured
mixing in B mesons measured (a B0 can turn into a B0)
some B meson branching fractions measured
THEORY IS IN GOOD SHAPE
KM model is believable…
Richard Kass
OSU Feb 10 2010
10
CP violation & B mesons
EXPERIMENTS are in BAD SHAPE
Theory says we will need ~108 B mesons to observe CPV
the decay modes that exhibit CPV don’t occur very often
Current (1987) experiments can collect~105 B mesons/year
Theory says that the “easiest” way to observe CPV involves
measuring the distance (~30mm) a B meson travels before
it decays.
But the best an experiment can do is ~100mm
Also, since 99.9999% of B meson decays are NOT useful for
studying CPV the detector must be highly efficient at
eliminating these unwanted decays.
Richard Kass
OSU Feb 10 2010
11
Steps to observing CPV with B mesons
Produce B-mesons pairs using the reaction e+e- (4S)  B0B0
(must build an asymmetric energy collider)
Reconstruct the decay of one of the B-mesons’s into a CP eigenstate
example CP: B0 KS and B0 KS
Reconstruct the decay of the other B-meson to determine its flavor (“tag”)
use high momentum leptons: B0 e+ or m+ )X and B0 e- or m- )X
flavor of CP eigenstate also determined at time of the “tag” decay.
Measure the distance (L) between the two B meson decays and convert to proper time
must reconstruct the position of both B decay vertices
t=L/(bgc)
Fit the decay time difference between B0’s and B0’s to the functional form:
dN/dte-G|t| [1±hcp(sin2b)sin(Dmt)]
-B0,+B0
hcp = ±1
CP of final state
= -1for B0 KS
CP violating phase
Dm=difference
between B mass
eigenstates
Dm=0.47x1012h/s
Determine sin2b
Richard Kass
OSU Feb 10 2010
12
Example of “Golden” Signature
of CPV with B mesons
Decay rate is not the same for B0 and B0 tag.
Richard Kass
OSU Feb 10 2010
13
How to Measure Time Dependent Decay Rates
t =0
We need to know the flavor of the B at a reference t=0.
Dz = Dt gbc
0
At t=0 we
B0
know this
meson is B0
B
rec
K s
(4S)
bg =0.56
B0
The two mesons oscillate
coherently : at any given
time, if one is a B0 the
other is necessarily a B0
Richard Kass
tag
W
l - (e-, m-)
In this example, the tagside meson decays first.
It decays semi-leptonically
and the charge of the
lepton gives the flavour of
the tag-side meson :
l -= B0
l += B 0.
Kaon tags also used.
OSU Feb 10 2010
B0
-
l-
l
b
d
Dt picoseconds
later, the B 0 (or
perhaps it is now a
B 0) decays.
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How to Get the Data Sample
Use e+e- annihilations at Y(4S) to get a clean sample of B mesons
At Y(4S) produce B-/B+ (bu/bu) and B0B0 (bd/bd) mesons
BB Threshold
mB0 ~ mB- ~ 5.28 GeV
 )
 bb
 0.28
 hadr )
The Y(4S) - a copious, clean source of B meson pairs
1 of every 4 hadronic events is a BB pair
No other particles produced in Y(4S) decay
Equal amounts of matter and anti-matter produced
Richard Kass
OSU Feb 10 2010
15
Why Do We Need an Asymmetric Energy
e+e- Collider?
In order to measure time we must measure distance: t=L/v.
How far do B mesons travel after being produced by the Y(4S) (at rest)
at a symmetric e+e- collider?
tB=1.6x10-12 sec
At a symmetric collider we have for the B mesons from Y(4S) decay:
plab =0.3 GeV, mB=5.28 GeV
Average flight distance <L>= (bg)ctB= (p/m)(468mm)=(0.3/5.28)(468mm)=(27mm)
This is too small to measure!!
If the beams have unequal energies then the entire system is Lorentz Boosted:
bg= plab /Ecm=(phigh-plow)/Ecm
SLAC: 9 GeV+3.1 GeV bg = 0.55 <L>= 257mm
KEK: 8 GeV+3.5 GeV bg = 0.42 <L>= 197mm
We can measure these decay distances !
Because of the boost and the small plab the time measurement is a z measurment.
0
0
B
B
 200 mm
e+
ee+ e
symmetric
CESR
Richard Kass
z-axis
 30 mm
asymmetric
0
B
B0
SLAC, KEK
OSU Feb 10 2010
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PEPII-Asymmetric e+e- Collider
PEP-II Peak Luminosity 1.2 x 1034 cm-2s-1 (about 50X better than previous accelerators)
BaBar recorded 424 fb-1 at Y(4S) ~4.65 x 108 U(4S)→BB events
Stanford Linear Accelerator Center,
Stanford, California
PEPII is an asymmetric e+e− collider: 9 GeV (e-)/3.1 GeV (e+)
A B-meson travels a measurable distance before decay: bg=0.56 → <bgct>~260mm
Richard Kass
OSU Feb 10 2010
17
Detector Requirments-I
Measure momentum of charged particles
charged particle bend in B-field
Measure the energy of neutral particles (mostly g-rays)
electromagnetic calorimeter
Measure the decay length of unstable particles
decay lengths vary from ~100mm to 10 cm
Measure the identity of produced particles
tell protons from kaons from pions from muons
Trigger the experiment on (almost) every type of event
some events have only 2 particles, some have 20….
NO deadtime
want to collect data whenever the accelerator is running
Detector related effects understood at ~1% level
e.g. do K+’s behave differently than K-’s? (could fake CPV!)
Custom Electronics
Can’t just use commercially available stuff, must design chips, etc
Richard Kass
OSU Feb 10 2010
18
Detector Requirments-II
Useable software….
~million lines of code, hundreds of users distributed over the world
need a realistic computer model of how the detector will work
take “raw” data and turn it into 4-vectors…
Must be able to repair and maintain detector components
things break, wear out, accidents….
Must take 5-6 years to design and construct
Must be designed, built, & operated within a budget
~100 Million $$$$
Must find several hundred physicists to work for > 10 years
collaboration formed in 1994
physicists from North America, Europe, Asia….
Richard Kass
OSU Feb 10 2010
19
BaBar Detector
Electromagnetic
Calorimeter
(EMC)
1.5 T Solenoid
Detector of
Internally
Recflected
Cherenkov
Light (DIRC)
Drift Chamber
(DCH)
Instrumented
Flux Return
(IFR)
Silicon Vertex
Tracker (SVT)
SVT, DCH: charged particle tracking: vertex & mom. resolution, K0s/Λ
EMC: electromagnetic calorimeter: g/e/π0/η
DIRC, IFR, DCH: charged particle ID: π/μ/K/p
Highly efficient trigger for B mesons
Richard Kass
OSU Feb 10 2010
20
The BaBar SVT
 5 Layers of double-sided, AC-coupled silicon
 0.94 m2 of Si
 Φ and z strips
 Inner 3: Precision Vertexing
 Outer 2: Pattern recognition, Low Pt tracking
 Custom rad-hard readout IC (the AToM chip). 140k channels
 Low-mass design (Kevlar/carbon fiber mechanical support)
Richard Kass
OSU Feb 10 2010
Magnet
Be Beam
Pipe
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SVT Performance
z-side
upilex fanout
upilex fanout
φ-side
Average hit efficiency 97%
Slow pion efficiency 70% for PT>50 MeV
Average z hit resolution 10 - 40 μm
Richard Kass
OSU Feb 10 2010
22
The BaBar Drift Chamber
40-layer small-cell chamber
7104 drift cells formed from hexagonal field wire pattern
80 & 120 mm Aluminum field wires and 20 mm tungsten sense wires
Layers organized into superlayers
Wire directions in axial-u-v pattern
Allows fast Level 1 trigger based on segments
80:20 helium:isobutane gas mixture
Low-mass gas to minimize multiple scattering
Small Lorentz angle results in simple t-to-d relation
Mechanical Structure
Thin aluminum endplates
30K precision holes locate feedthroughs with crimp pins
Forward endplate reduced to 12 mm thickness in acceptance region
Load-bearing cylindrical walls
1-mm thick beryllium inner wall (40% load)
Nomex-carbon fiber composite outer wall assembled in two halves
Measures position (relative to wire)
Measures ionization which helps ID pions, kaons, protons
Richard Kass
OSU Feb 10 2010
23
Display of an Event Display
Richard Kass
OSU Feb 10 2010
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The DIRC
PID: Need to tell a pion from a kaon from a proton
Richard Kass
OSU Feb 10 2010
25
The DIRC-Performance
3 S.D. means the
probability of calling
a kaon a pion is ~1 in 300.
Richard Kass
OSU Feb 10 2010
26
Electromagnetic Calorimeter
Measure the energy of photons and electrons/positrons
CsI(TL) scintillates: Energy in => Light out
Richard Kass
OSU Feb 10 2010
27
Muon detector
Anything that goes through the entire detector is a muon
The magnet iron is filled with charged particle
tracking devices
Use special type of proportional chambers
Resistive Plate Chambers (RPC)
Limited Streamer tubes (LST)
Richard Kass
OSU Feb 10 2010
28
Muon Detector
full scale LST
We have some
LSTs in PRB.
Inside an LST
8 cells per LST
Worked in Smith Lab
Me installing last LST into BABAR
Richard Kass
OSU Feb 10 2010
29
Analysis Technique
Threshold kinematics: we know the initial energy (E*beam) of the Y(4S) system Therefore we
know the energy & magnitude of momentum of each B meson
*2
mES  Ebeam
- p*B2
Signal
*
DE  E B* - E beam
Event topology
Signal
(spherical)
Background
Background
(jet-structure)
Also, use neural networks + unbinned maximum likelihood fits
Richard Kass
OSU Feb 10 2010
30
CPV Results for Sin2b
Measurement of sin2β with:
B → J/ψ K0, J/ψ K*, ψ(2S)KS, ηcKS, & χc1KS
Summer 2009
HUGE success,
Just as theorists predicted
Richard Kass
OSU Feb 10 2010
31
BaBar Status
Data taking with BaBar ended April 2008
We are currently in the “intensive analysis” phase
> 400 published articles to date, lots more to come.
BaBar was more successful than anyone imagined..
Discovered and measured CPV using B mesons
First observation of mixing with D mesons
Discovered several new particles (e.g. eta_b)
Limits on existence/mass of SUSY particles
New software tools for data analysis
BaBar (and Belle) showed that the KM model works really well!
Richard Kass
OSU Feb 10 2010
32
BaBar & Belle confirm
matter-antimatter asymmetry;
leads to 2008 Nobel Prize in Physics
Makoto Kobayashi
Toshihide Maskawa
But CPV is still a big mystery/problem
The CPV in the KM model is way too small to explain the
matter-anti-matter asymmetry we see in the universe…
Richard Kass
OSU Feb 10 2010
33
Extra Slides
Richard Kass
OSU Feb 10 2010
34
How Can This Happen?
In 1967 Sakharov showed that the
generation of the net baryon
number in the universe requires:
1.
Baryon number violation
(Proton Decay)
2.
Thermal non-equilibrium
3.
C and CP violation
(Asymmetry between particle and
anti-particle)
Richard Kass
OSU Feb 10 2010
35
Visualizing CKM information from B-meson decays
The Unitarity Triangle
The CKM matrix Vij is unitary with 4
independent fundamental parameters
Unitarity constraint from 1st and 3rd columns:
i V*i3Vi1=0
d
s
b
u
Vud
Vus
Vub
c
Vcd
Vcs
Vcb
t
Vtd
Vts
Vtb
CKM phases
(in Wolfenstein convention)
To test the Standard Model:
Measure angles, sides in as many ways possible
Area of triangle proportional to amount of CP violation
Richard Kass
OSU Feb 10 2010
 1 1 e-iγ 


 1 1 1 
 e-iβ 1 1 


36
Three Types of CP Violation
I) Indirect CP violation/CP violation in mixing
KKlexpected to be small (SM: 10-3) for B0’s
II) Direct CP violation: Prob(Bf) Prob(Bf)
Only CP violation possible for
 in K
charged B’s
Br(B0-+) Br(B0+-)
III) Interference of mixing & decay: Prob(B(t)fCP) Prob(B(t)fCP)
B0s
B0+-
(CKM angle b)
(CKM angle a)
B
B
0
0
f CP
Due to quantum numbers of
Y(4S) and B meson we must
measure time dependant
quantities to see this CP violation
In this talk we will be discussing type III) CP violation
Richard Kass
OSU Feb 10 2010
37
CP Violation at the Y(4S)
CPV from the interference between two decay paths: with and without mixing
AfC P
mixing
|BL>=p|B0>+q|B0>
|BH>=p|B0>- q|B0>
B0
q/p
B
t
fCP
AfCP
0
Measure time dependent decay rates
&
Dm from B0B0 mixing
t 0
ACP (t ) 
G ( B 0 (Dt )  f ) - G( B 0 (Dt )  f )
G ( B 0 (Dt )  f ) + G( B 0 (Dt )  f )
 S f sin (DmDt ) - C f cos (Dm Dt )
Cf 
Sf 
Richard Kass
1- |  f |
2
1+ |  f |2
- 2 Im  f
1+ |  f |2
q Af
f  
p Af
Direct CP Violation: C
|Af/Af|≠1→ direct CP violation
|q/p|≠1→ CP violation in mixing
Sf and Cf depend
on CKM angles
OSU Feb 10 2010
38
Why do we need an
asymmetric collider?
N=number of events
fCP= CP eigenstate (e.g. B0KS)
ffl= flavor state (particle or anti-particle) (e.g. B0e+X)
A
N ( B1  f CP )( B2  f fl ) - N ( B1  f CP )( B2  f fl )
N ( B1  f CP )( B2  f fl ) + N ( B1  f CP )( B2  f fl )
0
The source of B mesons is the U(4S), which has JPC =1--.
The U(4S) decays to two bosons with JP =0-.
Quantum Mechanics (application of the Einstein-Rosen-Podosky Effect) tells us
that for a C=- initial state (U(4S)) the rate asymmetry:
However, if we measure the time dependence of A we find:
N (t1, t2 )( B1  f CP )( B2  f fl ) - N (t1, t2 )( B1  f CP )( B2  f fl )
A(t1, t2 ) 
 sin 2CP
N (t1, t2 )( B1  f CP )( B2  f fl ) + N (t1, t2 )( B1  f CP )( B2  f fl )
Need to measure the time dependence of decays to “see” CP violation using the
B’s produced at the U(4S).
Richard Kass
OSU Feb 10 2010
39