recent results from babar adrian bevan on the behalf of the babar collaboration hellenic society for...
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Recent Results from BaBarAdrian Bevan
On the behalf of the BaBar collaboration
HELLENIC SOCIETY FOR THE STUDY OF HIGH ENERGY PHYSICS
Workshop on RECENT DEVELOPMENTS IN
HIGH ENERGY PHYSICS AND COSMOLOGY
Athens, 17-20 April 2003
All results are preliminary unless otherwise stated
Athens, '03 2
Talk Outline
• Theoretical Motivation
• The PEP-II accelerator & BaBar Detector
• Experimental issues• Vertex reconstruction• Flavour tagging• Particle Identification• background suppression techniques
• Results• sin2, sin2, direct CP Violation searches, rare decays
•Summary and Outlook
Athens, '03 3
Physics Background – CP Violation• CP Violation is one of Sakharov’s conditions for obtaining a matter antimatter asymmetry in the universe.
• The observed CP Violation explained by the Standard Model of Particle Physics is 9 orders of magnitude too small to explain our universe
•There must be some kind of new physics to discover in the flavour sector
• Standard Model mechanism for CP Violation in weak interactions is given by a single complex phase in the CKM matrix.
32
4
23
2 2
(1 / 2 )
1 / 2 ( )
(1 ) 1
ub
cd cs
td t
ud u
ts
s
cb
b
A iV V
V V
V
V V
V A
A O
V AV i
CP Violating phase presentlarge uncertainty in magnitude and phase→related to a phase in the unitarity triangle
A. D. Sakharov (1967) JTEP Letters 5, 24
Athens, '03 4
Experimental constraints before the B-factory era
(CP Violation measurements prior to 1999)
• Observed effects have been entirely in the kaon system
• This defined our knowledge of CP Violation in the SM• 1964, Christensen et al. discovered CP Violation in K2 decay
• Following 37 years: measured several CP Violation effects in
• Semi-leptonic kaon decay, ~10-3
• Non-leptonic kaon decay, +-, 00: • CP Violation in mixing: ~10-3
• CP Violation in decay: ’/ ~10-3 (KTeV/NA48)• Observed a CP Violating asymmetry in KL→+-e+e- (13% effect)
(observed a T odd asymmetry in this decay as well)
Athens, '03 5
• CP-LEAR• measured T violation• obtained an upper limit on CPT Violation from
• There are still important measurements to do with kaons
• over-constraining the CKM matrix is the aim
• With the advent of the B factories, we were able to test this description for the first time!
0 0 0 0 0 0, , , L L SK K K e e K e e
0 0
182.1 10K K
m m GeV
Athens, '03 6
Unitarity of the CKM matrix gives 6 triangles in the complex plane; 9 relations in all– few triangles have all sides with the same order in – kaon system is not one of these– interesting one for the Bd/u system is “The Unitarity Triangle”:
| |
| |
itd td
iub ub
V V e
V V e
mixing
The CKM Matrix in terms of Bd/u decay
0 0B B
* * * 0ud cd cub tdb tbV V V VV V
(1,0)(0,0)
(,)*
*td tb
cd cb
V V
V V*
*ub ud
cd cb
V V
V V* , ,
,...
B D DK
K
, ,B
0
* *
/ ,
, ,...
SB J K
D D K
Athens, '03 7
For neutral K, B, D … mesons• strong eigenstates are not CP eigenstates
• mass eigenstates are an admixture of different strong eigenstates
• particle antiparticle (mixing), f = 2
Neutral Meson Phenomenology
0 0
0 0
2 21
H
L
B p B q B
B p B q B
p q
(mixing is very suppressed in the case of D mesons)
0 01 1( ), ( )
2 2H L H LB B B B B Bp q
mass eigenstates=CP eigenstatesif no CP Violation in mixing (q/p=1)
if q/p1 have CP Violation
in mixing; e.g. K=2.3x10-3
CP even
CP odd
Athens, '03 8
Direct CP Violation (f = 1)
• need interference between diagrams with different strong (i) and weak phases (i):
• Direct CPV only seen in ; ' ~ few 10-6
• theory predicts large asymmetries in B+/0 (few to ~80%)
00K K
2 2
,
| | | | 2 sin sinff i i jji ji j
A A A A
2 2| | | | 0ffA A direct CPV
Athens, '03 9
Observing CP violation at the (4S)
• Three observable interference effects:– CP violation in mixing (|q/p| ≠ 1)
– (direct) CP violation in decay (|A/A| ≠ 1)
– CP violation in interference of mixing and decay (Im ≠ 0)
0B
0B
CPf
CP
CP
CP
ff
f
Aqλ
p A
Analyse time evolution of BB system (assume )
CP
CP
2f
f 2f
1 | λ |
1 | λ |C
CP
CP
ff 2
f
2 Im λ
1 | λ |S
direct CP violation→ C ≠ 0
Indirect CP violation → S ≠ 0
0
4
04
( , ) 1 sin( ) cos( )
( , ) 1 sin( ) cos( )
CP
CP
CP
CP
tphys CP d d
tphys CP d
f
f
f
f d
f B f t e m t m t
f
S
SB f t
C
Ce m t m t
( ) ( )
( ) ( )CP
Br B f Br B fA
Br B f Br B f
00
0 0
( ( ) ) ( ( ) )( )
( ( ) ) ( ( ) )
sin( ) cos( )
physphysCP
phys phys
B t f B t fA t
B t f B t f
S m t C m t
Athens, '03 10
s,d
Searching for Direct CP Violation & probing New Physics with Bu/d
• Large ACP requires amplitudes of similar order– b→u: suppressed tree: i.e. charmless decays
• large predicted ACP
– b→s,d: penguins: radiative decays good for constraining BSM
• small predicted ACP
• Understand penguins
• Access to , and
• Sensitive to New Physics effects via loops – minimal SUGRA: B→Xs, K+, K0+ …
– R-parity Violating SUSY: KS …
– SUSY searches – K*
Athens, '03 11
Experimental Issues
• interesting modes have small branching fractions– large light quark continuum background qq– quarks below threshold are u,d,s & c– other B background
• Need good K/ separation
• Need to boost to do time dependent CP measurements
• require good vertex resolution for CP measurements
• need to ‘tag’ the flavour of the B (as a particle/anti particle)
• understand charge bias– detector: trigger, tracking; reconstruction– event selection, particle ID, analysis– physics: differences in (anti)particle interaction in matter (e.g. K)
Athens, '03 12
On the whole - very similar designs with many common features for both Belle and BaBar:
• e+e- colliders
• run on the (4S) resonance – produce ~100% BB pairs
• run 40 MeV below resonance for qq background studies
• To measure CP violation in the B system, need to measure vertex difference of the evolving BB system (t) silicon device
• need asymmetric beam energies
The Accelerator & Experiment
0 0
0.110.10~ 1.09 0.08B B
B B
f
f
(PDG 2002)
Athens, '03 13
E(e-) = 9.0 GeVE(e+) = 3.1 GeV
0.56
Design Achieved
Luminosity (cm-2 s-1) 3 x 1033 5 x 1033
Int. Lum / day (pb-1) 135 341
Int. Lum / month (fb-1) 3.3 7.2
The PEP-II e+e- collider
Most results use: 81 fb-1 on-resonance.
88 million BB events (as for ICHEP02).Most results use: 81 fb-1 on-resonance.
88 million BB events (as for ICHEP02).
Athens, '03 14
Silicon VertexDetector (SVT)
Drift chamber (DCH)
Detector forInternally reflectedCherenkov radiation(DIRC)
ElectromagneticCalorimeter (EMC)
1.5 T Solenoid
InstrumentedFlux Return (IFR)
SVT: 5 layers double-sided Si. Crucial for measuring t.
DCH: 40 layers in 10 super- layers, axial and stereo.
DIRC: Array of precisely machined quartz bars. Excellent Kaon identification.
EMC: Crystal calorimeter (CsI(Tl)) Very good energy resolution. Electron ID, 0 and reco.
IFR: Layers of RPCs within iron. Muon and neutral hadron (KL)
The BABAR experiment
Athens, '03 15
Measure angle of Cherenkov light
– Transmitted by internal reflection
– Detected by~10,000 PMTs
– excellent K/ separation
c
Particle
Quartz bar
Cherenkov light
Active Detector Surface
KDDD 00* ,
Particle Identification (PID)
Detection of Internally Reflected Cherenkov Light (DIRC)
Athens, '03 16
Measuring CP Violation from time dependence1. Start with data sample of BB pairs
2. Reconstruct one B in a CP eigenstate decay mode
3. “Tag” the other B to make the matter/antimatter distinction 4. Determine the time between the two B decay vertices, t
5. Plot t distribution and do CP fit for S and C.0/ SB J K
Athens, '03 17
Event Selection Techniques
Use beam energy to constrain mass & energy difference
*B beamE E E * 2 2( )ES beam Bm E P
MES ~ 3 MeV
B background
signal
E ~ 15-80 MeV; larger with neutrals
qq background
Athens, '03 18
Event Selection Techniques
B events are spherical u,d,s,c is jets
• shape variables
u,d,s,c
B event
Signalu,d,s,cbackground
Fisher Discriminant
Arb
itrar
y U
nits
• Maximum Likelihood fits or cut based analysis off-resonance & E sidebands are used to parameterise light quark background
• flavour-tagging (e, , K, slow from other B)
Athens, '03 19
B Flavour Tagging
• Tagging algorithm with physics-based neural networks– Inputs include leptons, kaons, slow- (from D*), and high-momentum
tracks– Outputs combined and categorized by mistag probability (w)
• 5 mutually exclusive categories:• Lepton – isolated high-momentum leptons
• Kaon I – high quality kaons or correlated K+ and slow--
• Kaon II – lower quality kaons, or slow-
• Inclusive – unidentified leptons, poor-quality kaons, high-momentum tracks
• Untagged – no flavor information is used
b sc
K-
Q = (1-2w)2 = (28.1 0.7)%
clea
ner
sign
alla
rger
m
ista
g p
rob
Athens, '03 20
Tagging: example of Charmless B Decays
• Tagging efficiency is very
different for signal and background
• Strong bkg suppression in categories with the lowest
mistag prob (Lepton/Kaon)
• plots shown are for h+h-,
a rare decay with significant backgrounds.
81/fb B→ h+h- sample split by tagging category
150
100
Athens, '03 21
Vertex Reconstruction
Resolution function parameters obtained from data for both signal and background– Signal from sample of fully reconstructed B
decays to flavor eigenstates: D*(, , a1)– Background from data sideband sample
Beam spot
Interaction Point
BREC Vertex
BREC daughters
Exclusive Brec reconstruction
BTAG direction
TAG tracks, V0s
zBTAG Vertex
c
zt
1
B →
Example in B →
e+e- → qq
t (ps)
z resolution dominated by tag side (other B)
•Average z resolution ~180m
•Average z ~260 m
without the boost would have z~20m …
e.g. CLEO
without the boost would have z~20m …
e.g. CLEO
Athens, '03 22
• Dominant amplitudes for b ccs decay:
• Both amplitudes have the same weak phase:
• For B0 J/Ks we obtain:
Golden Mode for CP Violation in B decay(theoretically clean)
b
d
ccsd
W+
b
d
s
c
c
dW+c
g
Tree Penguin
Measuring
*cb csA V V
VcdVcb arealmost real -only phase isfrom mixing
*2
*
* *
* *icb cs
CP Ctb t
P CPc
d
tb
cs cd
csb csK d cdt
V VV V
V
V V
V V
q A qe
p A p V VV
B0 mixing Decay K0 mixing
Athens, '03 23
sin 2Vtb
*Vtd
Vcb*Vcd
final statesb ccs
CP = +1•B J/ KL
0
CP = -1•B J/ Ks
0, Ks0 +-, 00
•B (2S) Ks0
•B c1 Ks0
•B c Ks0
•B J/ K*0, K*0 Ks0
mostly CP even
E=Eb-Ebeam (GeV)
J/KL signalJ/X backgroundother background
Athens, '03 24
Latest result on 88M BB events
Ks modes KL modes
hep-ex/ 0207042 (PRL)hep-ex/ 0207042 (PRL)hep-ex/ 0207042 (PRL)hep-ex/ 0207042 (PRL)
sin2 = 0.741 0.067 0.034 || = 0.948 0.051 0.030
sin2 = 0.741 0.067 0.034 || = 0.948 0.051 0.030
2641 tagged events (78% purity; 66%
tagged )
tagging efficiency Q=(28.1 0.7)%
Still statistics limited…
Athens, '03 25need more statistics to see if the SM holds out
B0 Ks ( )b sss• pure penguin
• sensitive to new physics
• measured in this mode should
agree with
• In SM:
b ccs
84 million BB pairs 84 million BB pairs (supersedes ICHEP ‘02)(supersedes ICHEP ‘02)
84 million BB pairs 84 million BB pairs (supersedes ICHEP ‘02)(supersedes ICHEP ‘02)
0
0
0.18 0.51 0.07
0.80 0.38 0.12S
S
K
K
S
C
• c.f. world average: sin2 = 0.73 ± 0.06• >2 difference.• Belle measures -0.73±0.64±0.18
sin 2 , 0s sK KS C
Goodness-of-fit: 23%
C
~60 events
Athens, '03 26
Also have results fromD*D*, ’Ks, J/0
but interpretation of the measurement of S and C in terms of the weak phase is plagued by uncertainty …
“reference” sin2
pure penguin
mostly penguin?
colour- & CKM-suppressed tree competing penguin
CKM-suppressed treesmall penguin pollution
0
Everything is consistent
awit nd h / the SMSJ K
Athens, '03 27
Measuring
0B
0B
0B
Interesting modes to measure
need to perform an Isospin analysis of branching ratios of B0 and B0 to 2 final states to determine shift in due to the presence of penguin diagrams
22 eff
doing a ‘quasi 2 body analysis’ – will eventually have to analysethe whole Dalitz plot.
In analogy to these modes one needs to analyse the time dependence of this decay and extract from an isospin analysis of each of the three partial waves (L=0,1,2) in the final state (full angular analysis is required)
The shift in from penguin diagrams is expected to less than that in
Work is now in preparation for analysis of modes
the time evolution of gives the shifted value of
need a neutral B meson decay with several contributing processes that interfere in order to be able to measure the phase
Athens, '03 28
CP Violation in B→
mixingdecay
Tree (T) Level:
)2sin(
0
2
S
C
e i
* *
**tb td
tb td
ud ub
ud ub
V VV V
V V VV
)2sin(1
)sin(
eff2
/1
/12
CS
C
e ii
ii
eeTP
eeTPi
With Penguins (P):
0 0 0 0B & B • Need to measure the weak phase
• can use isospin relations to extract shift
• penguins are significant: P/T~0.2
Athens, '03 29
Can bound the shift on using BR(B→ 0) and upper limit on B0→00
0 0 0
2
0
Bsin ( )
Beff
Br
Br
The Isospin Analysis:
Need to measure decay of Band B to
final states in order to determine the shift
0 0 0, ,
( ),
( ) ( )
A A B f
A CP A A B f
)Br()Br(2
)Br()Br()Br(cos
0
00210
Measure ’ for B(B) from
Already have
Need to measure
Grossman Quinn bound
eff
Athens, '03 30
qq + K
157 19 7 events
)cos()sin(
)(00
00
tmCtmS
BNBN
BNBNtA
dd
tagtag
tagtag
Fit projection in sample of -selected events
0.02 0.34 0.05
0.30 0.25 0.04
S
C
6(4.7 0.6 0.2) 10BR
The first side of the Isospin triangle: B+→-
PRL 2002 89 281802 fit , K simultaneously
K e+e→qq
•large qq background/K separation
Athens, '03 31
The Base of the Isospin Triangle: B→• large qq background /K separation
• Potential background from
– Minimize with tight cut on E
hep-ex/0303028, submitted to PRL
Simultaneously fit for /K
Fit region
ee→ qq
0K
00K
0
ee→ qq
0 1.0 60.9
2321
0.180.17
( ) (5.5 0.6) 10
Yield = 125
0.03 0.02CP
BR B
A
Athens, '03 32
• Small signal; BR few 10-6
background
• Background suppression
– Event shape and flavour
tagging to reduce qq
– Cut on M() and E() to reduce background,
then fix in the fit
Significance including
systematic errors = 2.5
0 0
109
0 0 0 6
23
( ) 3.6 10 (90% . .)
N
BR B C L
The Missing Sides : B→
ee→ qq
hep-ex/0303028, submitted to PRL
Athens, '03 33
Bounding penguin
pollution:
0 0 0
2
0
Bsin ( )
B
0.61@90% . .
eff
Br
Br
C L
0| | 51 @90% . .eff C L
Need ~20 the data to do a better isospin analysis than using a bound in B→
Use Grossman-Quinn Bound:• assumes isospin• most conservative of several bounds in literature• account for correlated systematic uncertainties common to both analyses
0.7 0.6 60.6 0.3(1.6 ) 10
Athens, '03 34
0 0,B B K Not a CP eigenstate
0
| |/(1 ) ( )sin ( )cosXCP
tXB X XX
Xf t e SA Cm t C m tS
related to
direct CPV
K is self tagging
CK, SK, SK =0, CK =-1 •large expected:
ACP() & ACP(K) qq
signalSXF+BBg
6
1.3 61.2
( ) (22.6 1.8 2.2) 10
( ) (7.3 1.3) 10
B B
B B K
Athens, '03 35
B0→+-continuumB-backgroundtotal fit
cos( ) sin( )C m t S m t
~88106 B pairs
Dilution from background
2.1
0.18 0.08 0.03
0.36 0.18 0.04
0.28
0.19 0.24 0.03
0.17 0.08K
S
A
C
A
Athens, '03 36
Direct CP Violation Searches
• Looked for direct CP Violation in many modes
• so far no evidence
• most precise measurement ~4.6%
• charge asymmetry in • is 2.5 effect• is 2.1 effect
• look for updates using more data
Athens, '03 37
Rare Decays:(*)B K ll
• very rare process
• BR ~10-7
• penguin + FCNC process
• consistent with SM
• sensitive to new physics
0.24 0.11 60.20 0.18
* 6
( ) (0.78 ) 10
( ) 3.0 10 (90% C.L.)
BR B Kll
BR B K ll
Athens, '03 38
s,d
(Cabibbo favoured)
* and B K B (Cabibbo suppressed)
• radiative penguins• new physics probes• constrain |Vtd/Vts| (Rt)
Rt
B00
B++
B0
signal region
MES (GeV)
E (
Ge
V)
0 *0 5
* 5
0 *0
0 0 6
6
0 0 6
( ) (4.23 0.40 0.22) 10
( ) (3.82 0.62 0.22) 10
( ) 0.044 0.076 0.012
( ) 1.4 10
( ) 2.3 10
( ) 1.2 10
CP
BR B K
BR B K
A B K
BR B
BR B
BR B
@90% C.L.78fb-1
20.7fb-1
Phys Rev Lett 88 101805 (2002)
Athens, '03 39
Observation of a Narrow Meson Decaying to D+s0 at a Mass of 2.32 GeV/c2
hep-ex/0304021
0
D+s
Athens, '03 40
Other BaBar physics I’ve not covered
•BaBar has also produced results on• B lifetime and mixing; B, m• semi-leptonic B decays• D mixing• fB+B-/fB0B0
to name but a few
• in addition to B; have very large and D samples to study
• produced in excess of 35 publications
• several more preprints have been submitted for publication
http://www-public.slac.stanford.edu/babar/BaBarPublications.htm
Athens, '03 41
Summary• Since the start up of BaBar and Belle we have learnt?
• SM description of CP Violation passed its first real test• Observed CP Violation in the B System • CP Violation in B system consistent with that in kaons• Still not enough CP Violation to explain matter-antimatter
asymmetry in the universe
• Expect ~500 fb-1 by 2006 (5x current data set)
• With this we will:• start to test SM by doing alternate measurements of sin2• start to measure with greater precision in several modes• continue the search for direct CP Violation• work towards over constraining the unitarity triangle
• can we break it?• search for and constrain new physics in the flavour sector
Athens, '03 42
Constraints on (,)Dominated by resultsfrom B physics!
• main constraint: sin2
• fitting using Belle result and indirect constraints ~1.50
• measuring is next test of SM
CKM Fitter Group:
A. Hööcker, H. Lacker, S. Laplace,
F. Le Diberder, Eur. Phys. Jour.
C21 (2001) 225, [hep-ph/0104062]