Transcript The Physics of Flavor: Half a Billion b Quarks for BaBar Jeffrey Berryhill

TM
The Physics of Flavor:
Half a Billion b Quarks
for BaBar
Jeffrey Berryhill
University of California, Santa Barbara
For the BaBar Collaboration
Texas A&M Physics Colloquium
November 22, 2004
Quarks, Flavor Violation,
and CP Violation
TM
Quarks and the problem of mass
Standard Model “explanation” of quark mass:
Six quark species with unpredicted masses
Spanning almost six orders of magnitude
Up type
(q=+2/3)
Up
u
Charm c
Top
t
Mass
(GeV/c2)
10-3
1
175
Down type
(q=-1/3)
Down
d
Strange s
Bottom b
Mass
(GeV/c2)
5 10-3
10-1
5
The origin of different fermion generations, masses, flavor
violation, and CP violation are all arbitrary parameters of
electroweak symmetry breaking.
A comparative physics of the quark flavors directly probes this
little-known sector.
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Quarks and their Strong Interactions
Quarks cannot be detected in isolation, only as bound states
A quark/anti-quark pair forms a bound state (mesons)
Flavor
u,d
s
c
b
t
spin 0 p+ (ud)
mesons p0 (uu-dd)
K+ (us)
KS0 (ds+sd)
D+ (cd)
D0 (cu)
B0 (db)
B+ (ub)
none
spin 1 r+ (ud)
mesons r0 (uu-dd)
w (uu+dd)
K*+ (us)
K*0 (ds)
f (ss)
D*+ (cd) U (bb)
D*0 (cu)
J/y (cc)
none
b quarks are the heaviest flavor with measureable bound states→
B mesons are a natural starting point for studying the other flavors
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Quarks and Flavor Violation
Photon, gluon or Z boson: quark flavor conserving interactions
W boson: changes any down type flavor to any up type flavor
d
u
qI = s
b
qJ = c
Vij
t
 Vud
V   Vcd

V
 td
Vus Vub 
Vcs Vcb 

Vts Vtb 
The (Cabibbo-Kobayashi-Maskawa) CKM matrix: complex amplitude of
each possible transition
Conservation of probability → CKM matrix is unitary
3X3 unitary matrix has (effectively) four degrees of freedom:
3 angles + 1 complex phase
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Quarks and Flavor Violation: Mixing
Pairs of down (or pairs of up) type quarks can spontaneously
swap flavor for anti-flavor via two flavor-violating exchanges
“Meson Mixing” aka “Flavor oscillation”
Sensitive to Vtd, top quark!
B
0
B
0
Prob(B0 → B0) ≈ exp( -G t)/2 * ( 1 – cos(Dm t) )
Similar to neutrino oscillation, except decay term added
Mixing time ~few ps
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Quarks and CP Violation
For a particle(s) f with momentum p and helicity l
C : Charge conjugation operator C f(p, l) = f(p, l)
P : Parity reversal operator
P f(p, l) = f(-p, -l)
CP f(p, l) = f(-p, -l)
CP eigenstate: particle = anti-particle (Ex: qq mesons)
CP conservation → left-handed particles have the same
physics as right-handed anti-particles
Obviously violated for our (baryon-asymmetric) local universe!
In the Standard Model CP violation originates from
complex phase in CKM matrix → in general, Vij ≠ Vij*
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Three Paths to CP Violation
CP violation → an observable O of particles (f1,f2,…) such that
O(f1,f2,…) ≠ O(CP(f1,f2,…)) = O(f1, f2, …)
1. CP violation in meson mixing
f
B
0
B
0
2
f
≠
B
0
B
2
0
Mixing rate of meson to final state f not the same
as
Mixing rate of anti-meson to same final anti-state
In the Standard Model, very small ~10-3
CP violation in K0 decays first observed through
this path forty years ago!
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Three Paths to CP Violation
2. CP violation in meson decay → “Direct CP violation”
f
B
2
0
f
≠
B
Decay rate of meson to final
state f not the same as
Decay rate of anti-meson to
same final anti-state
0
BABAR
Recently observed in B0
→K+ p- at the 10% level!
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Jeffrey Berryhill (UCSB)
Candidate mass
Blue
Red
mB
B 0  K p 
B 0  K p 
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Three Paths to CP Violation
3.Time dependent asymmetry of meson/anti-meson decay rate to a
common final state
fCP
B
0
B
2
fCP
≠
B
0
2
0
B
0
If B0 and B0 decay to the same final state fCP, there is interference between
amplitude of direct decay
and
amplitude of mixing
followed by decay
In the presence of CP violating phases in these amplitudes, can induce large
time-dependent asymmetry with
frequency equal to mixing frequency: AfCP  C fCP cos(Dmt )  S fCP sin(Dmt )
C fCP  0 implies Direct CP Violation
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CKM Unitarity
Inner product of first and third columns of CKM matrix is zero:
Rescale, rotate and reparameterize to describe a
Unitarity Triangle in the complex plane

 r, 

VudVub
VcdVcb  f2 
0, 0
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 f3 
VtdVtb Measure sides (decay rates)
VcdVcb and angles (CP violation)
to test unitarity
 f1 
Jeffrey Berryhill (UCSB)
1, 0 r
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b Quarks and CKM Unitarity
B decay rates, CP asymmetries measure the entire triangle!
Triangle sides:
B+→ r0 l- n decay rate measures b→u transition (|Vub|)
B0 mixing rate measures |Vtd|
Angles:
B0 → r+ r- time dependent CP asymmetry measures sin 2
B0→ J/y KS0 time dependent CP asymmetry measures sin 2
B+→ D(*)K+ decay rates measure 

 r, 
VudVub
VcdVcb  f2 
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0, 0
 f3 
Need large sample
of B0, B+ mesons
produced under
controlled conditions

td tb

cd cb
V V
V V
 f1 
Jeffrey Berryhill (UCSB)
1, 0 r
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B Factory Experiments
TM
Asymmetric B Factories
S:B = 1:4
e+
3.1 GeV
B0
e9 GeV
Y(4S)
Y(4S)
B0
Y(4S) meson: bb bound state with mass 10.58 GeV/c2
Just above 2 x mass of B meson → decays exclusively to B0 B0 (50%) and B+ B- (50%)
B factory: intense e+ and e- colliding beams with ECM tuned to the Y(4S) mass
Use e beams with asymmetric energy → time dilation due to relativistic speeds
keeps B’s alive long enough to measure them (decay length ~0.25mm)
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PEP-II at the Stanford Linear Accelerator Center
Linac
PEP-II Storage Rings
SF Bay View
BaBar detector
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PEP-II performance
PEP-II top luminosity:
9.2 x 1033cm-2s-1
(more than 3x design goal 3.0 x 1033)
1 day record : 681 pb-1
About 1 Amp of current per beam,
injected continuously
Run1-4 data: 1999-2004
On peak
205 fb-1
s(e+e- →Y(4S)) = 1.1 nb →
227M Y(4S) events produced
454 million b quarks produced!
Also ~108 each of u, d, s, c, and t
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USA
[38/300]
INFN,
INFN,
INFN,
INFN,
The BABAR
Collaboration
California Institute of Technology
UC, Irvine
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[4/20]
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[1/5]
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[5/51]
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LAPP, Annecy
SLAC
LAL Orsay
11 Countries
80 Institutions
593 Physicists
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Norway
[5/31]
Ruhr U Bochum
U Dortmund
Technische U Dresden
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Italy
[1/3]
U of Bergen
LPNHE des Universités Paris
VI et VII
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Laboratoire Leprince-Ringuet
CEA, DAPNIA, CE-Saclay
Germany
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[12/101]
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INFN, Pisa & Univ &
ScuolaNormaleSuperiore
Jeffrey Berryhill (UCSB)
Russia
[1/11]
Spain
[2/2]
Budker Institute, Novosibirsk
IFAE-Barcelona
IFIC-Valencia
United Kingdom
[10/66]
U of Birmingham
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Queen Mary , U of London
U of London, Royal Holloway
U of Manchester
Rutherford Appleton Laboratory
May 3, 2004
The BaBar detector
Electromagnetic Calorimeter
6580 CsI crystals
e+ ID, p0 and  reco
Instrumented Flux Return
19 layers of RPCs
m and KL ID
e+ [3.1 GeV]
Cherenkov Detector
(DIRC)
144 quartz bars
K,p separation
Drift Chamber
40 layers
Tracking + dE/dx
Silicon Vertex
Tracker
e- [9 GeV]
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5 layers of double
sided silicon strips
Jeffrey Berryhill (UCSB)
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Our Friendly Competitors
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TM
Measuring the
Standard Model
CP Violating Phase
B0 → J/y KS0 and sin 2
Decay dominated by a single “tree-level”
Feynman diagram: b → ccs
J/y identified cleanly by decay to a lepton pair;
KS identified cleanly by decay to pion pair.
Both particles are CP eigenstates → both B0 and B0 decay to them
Time-dependent CP violation has amplitude sin 2 and frequency Dm
Works for several other b →ccs decays as well; results can be combined
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Time-Dependent CP Violation: Experimental technique
µ+
µ-
J/ψ
Fully
reconstruct
decay to CP
eigenstate f
π+
π-
KS
B0
e-
e+
B0
Asymmetric energies
produce boosted
Υ(4S), decaying into
coherent BB pair
Δz=(βγc)Δt
Determine time
between decays
from vertices
s(Dt) ≈ 1 ps
-
K
l-
Determine flavor and vertex
position of other B decay
Compute CP violating asymmetry A(Dt) = N(f ; Dt) – N(f ; Dt)
N(f ; Dt) + N(f ; Dt)
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sin 2 fit results
Raw asymmetry A(Dt) ≈ ( 1 – 2w ) sin 2 sin DmDt
(cc) KS (CP odd) modes
J/ψ KL (CP even) mode
Signal yield, background yield, sin 2, flavor tagging, Dt resolution function all
from simultaneous maximum likelihood fit to signal+control samples
sin2β = 0.722  0.040 (stat)  0.023 (sys)
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Consistency checks
Cleanest
charmonium
sample
Χ2=11.7/6 d.o.f.
Prob (χ2)=7%
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J/ψKS(π+π-)
Purest flavor
tagging mode
Lepton tags
ηF=-1 modes
Χ2=1.9/5 d.o.f.
Prob (χ2)=86%
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CKM Unitarity Triangle: Experimental Constraints
Constraints from
Decay rates and
Mixing Rates
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CKM Unitarity Triangle: Experimental Constraints
Constraints from
Decay rates and
Mixing Rates
CP violation in
Kaon decay
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CKM Unitarity Triangle: Experimental Constraints
Constraints from
Remarkable
validation of
the CKM
mechanism
for both flavor
violation and
CP violation!
Decay rates and
Mixing Rates
CP violation in
Kaon decay
CP violation in
b→ ccs
aTm 22 Nov 04
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CP Violation Redux:
Trees vs. Penguins
TM
A Third Path to Flavor Violation
1.
Tree diagram decay: down → up
2.
Box diagram: neutral meson mixing
3. Penguin diagram: down-type changes
to down-type via emission & reabsorption
of W; top-quark couplings Vtd, Vts dominate
SM penguins are suppressed; new physics can compete directly!
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B0 → f KS0 and sin 2
Decay dominated by a single “gluonic penguin”
Feynman diagram: b → sss
f identified cleanly by decay to a kaon pair;
KS identified cleanly by decay to pion pair.
Both particles are CP eigenstates
b
B0
W
u,c , t
g
d
Decay rate 100X smaller than J/y KS
→ small signal, large background
s
s
f , ,( KK )C
s 0
K
d S
114 ± 12 B0 → f KS events out of ½ billion
b quarks produced!
Time-dependent CP violating asymmetry A
can be measured in the same way as J/y KS
Same combination of CKM complex phases as
J/y KS → same relation between A and sin 2
aTm 22 Nov 04
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B0 → f K0 and sin 2: Fit Result
B0fKS
SfK 0  0.29  0.31
S
B0fKL
SfK 0  1.05  0.51
L
0
Btag
fK  1
0
S
0
Btag
0
Btag
fK  1
0
L
0
Btag
sin 2 = S(f K0) = +0.50 ± 0.25 ±0.07
vs.
S(y K0) = +0.72 ± 0.04 ±0.02
Consistent with tree decays, about 1 s low
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Trees(green) vs. Penguins(yellow): BaBar Data
Averaging over
many penguin
decays:
BaBar discrepancy
with tree decays
= -2.7
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s
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Trees(green) vs. Penguins(yellow): World Average
Averaging over
many penguin
decays:
World discrepancy
with tree decays
= -3.5
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s
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Trees(green) vs. Penguins(yellow): World Average
Averaging over
many penguin
decays:
World discrepancy
with tree decays
= -3.5
s
Red boxes:
estimate from
theory of errors
due to neglecting
other decay
amplitudes
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New Physics Scenarios
•New physics at the electroweak scale generically introduces new large
flavor-violating or CP-violating couplings to quarks
→Existing flavor physics measurements severely limit types of new physics!
• The great number of possible new couplings can give rise to many
different combinations of effects
Ex: Right handed (b →s) squark mixing in gluino penguins could introduce a
new phase in b → sss penguins without affecting B mixing nor b → ccs nor b → s
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Future and Follow-up Measurements
•Both B factories hope to collect 4-5 X more data over the next 4-5 years
•Significance of the penguin problem could double and unambiguously
falsify the Standard Model!
•Improved measurements of rates and asymmetries in other penguin decays
(b →s , b→ d , b→ s l l, B → f K*, ……)
•Fermilab Tevatron can measure Bs , Lb decays
•LHCb, BTeV: scheduled to produce billions of B’s in pp collisions
•Super B Factory: 50X version of B factories
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Summary
•The physics of quark flavor, as seen through the b quark,
is a rich area of study with wide-ranging implications
•The Standard Model CKM theory of flavor and CP violation holds up well
for tree-level processes
•Penguin processes, which are especially sensitive to new physics,
could prove to be the lever which cracks the Standard Model wide open
TM
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Backups
TM
Direct CP Violation: BaBar Data
Direct CP
violation
consistent with
0 for all modes
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Direct CP Violation: World Average
aTm 22 Nov 04
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b  s  Asymmetries: Summary
•BaBar measurements on 82 fb-1
K*, K2* preliminary; Xs published
•CP asymmetries consistent with SM (0.4%) at the ~5% level
•K* isospin asymmetry D0- consistent with C7 < 0
•Statistics limited up to ~1 ab-1
sgn D0- = -sgn C7
Can we exclude C7 > 0 ?
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CKM Constraints
aTm 22 Nov 04
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CKM matrix constraint
Ali et al. hep-ph/0405075
SU(3) breaking of
form factors z2= 0.85 ± 0.10
weak annhilation
correction DR = 0.1 ± 0.1
(z2,DR) = (0.75,0.00)
theory error
(z2,DR) = (0.85,0.10)
no theory error
Penguins are starting to
provide meaningful CKM
constraint
Reduction of theory errors
necessary to be competitive
with Bd,Bs mixing
r 95% C.L. BaBar allowed region (inside the blue arc)
aTm 22 Nov 04
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Direct CP Asymmetry: b →s and B  K* 
< 1% in the SM, could receive ~10% contributions from new EW physics
Either inclusive or exclusive decays could reveal new physics
B or K charge tags the flavor of the b quark with ~1-2% asymmetry systematic
Sum of 12 exclusive,
self-tagging
B→Xs  final states
b→s
b→s
Xs = K/Ks + 1-3 pions
E > 2.14 GeV
b →s ACP = (N – N)/(N + N) = 0.025 ± 0.050 ± 0.015
PRL 93 (2004) 021804, hep-ex/0403035
Asymmetries also measured precisely in exclusive K* decays:
B →K*
D0- =
G(K*0 )
ACP = -0.013 ± 0.036 ± 0.010
–
G(K*-
submitted to PRL, hep-ex/0407003
) = 0.050 ± 0.045 ± 0.028 ± 0.024
preliminary
G(K*0 ) + G(K*- )
aTm 22 Nov 04
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Time-Dependent CP Asymmetry in B →K* (113 fb-1)
As in B0→J/y KS, interference between mixed and
non-mixed decay to same final state required for CPV.
In the SM, mixed decay to K* requires wrong photon
helicity, thus CPV is suppressed:
In SM: C = -ACP ≈ -1%
S ≈ 2(ms/mb)sin 2 ≈ 4%
Measuring Dt of K*(→KSp0)  events requires novel beam-constrained vertexing techinque:
Vertex signal B with intersection of K S trajectory and beam-line
Usable resolution for KS decaying inside the silicon tracker
Validated with B0→J/y KS events
aTm 22 Nov 04
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Time-Dependent CP Asymmetry in B →K* (113 fb-1)
Likelihood fit of three components
(qq, BB, K*)
to 5D data
(mES,DE,Fisher,mK*,Dt)
preliminary
K* signal = 105 ± 14 events
S = +0.25 ± 0.63 ± 0.14
C = -0.57 ± 0.32 ± 0.09
submitted to PRL, hep-ex/0405082
Consistent with SM
For C fixed to 0, S = 0.25 ± 0.65 ± 0.14
First ever measurement of time-dependent CP asymmetries in radiative penguins!
aTm 22 Nov 04
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preliminary
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