Time-Dependent CP Violation Measurements: Analyses of Neutral B Meson to Double-Charm Decays at the Japanese Belle Experiment

205 Pages·2014·7.966 MB·English

by  Markus Röhrken (auth.)

Springer Theses
Recognizing Outstanding Ph.D. Research
Markus Röhrken
Time-Dependent
CP Violation
Measurements
Analyses of Neutral B Meson to
Double-Charm Decays at the
Japanese Belle Experiment
Springer Theses
Recognizing Outstanding Ph.D. Research
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Markus Röhrken
Time-Dependent CP
Violation Measurements
Analyses of Neutral B Meson
to Double-Charm Decays at the Japanese
Belle Experiment
Doctoral Thesis accepted by
Karlsruhe Institute of Technology, Germany
123
Author Supervisor
Dr. MarkusRöhrken Prof.Dr. Feindt Michael
InstituteforExperimental Nuclear Physics InstituteforExperimental Nuclear Physics
Karlsruhe Instituteof Technology Karlsruhe Instituteof Technology
Karlsruhe Karlsruhe
Germany Germany
ISSN 2190-5053 ISSN 2190-5061 (electronic)
ISBN 978-3-319-00725-0 ISBN 978-3-319-00726-7 (eBook)
DOI 10.1007/978-3-319-00726-7
SpringerChamHeidelbergNewYorkDordrechtLondon
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InmemoriamtoMargareteandBerndRöhrken
Supervisor’s Foreword
CP violation is a subtle quantum effect that makes the world look different when
simultaneously left and right and matter and antimatter are exchanged. Since
Sakharov we know that CP violation is a prerequisite of our present universe
(containing more matter than antimatter) having been developed from the (sym-
metric)bigbang.Thereareonlyafewphysicssystemsthatallowdetectionofsuch
effects, and these require quite involved experimental techniques.
The discovery of CP violation in B meson decays has led to the 2008 Nobel
PrizeinPhysicstoKobayashiandMaskawawhopredictedtheseeffectsalreadyin
the nineteen-seventies. All findings up to now are compatible with the Standard
Modelofparticlephysics,althoughexperimentalistshavepushedthesensitivityby
orders of magnitude in the last decades.
Thisthesisdescribesahighqualityhighprecisiondataanalysisofaninteresting
elementary particle reaction. The data have been taken at the Japanese B-meson
factory KEKB, the world highest luminosity electron positron collider, with the
Belledetector,oneofthemostsuccessfullarge-scaleexperimentsworldwide.The
datahavebeentakenforabout10yearsbyaninternationalcollaborationofabout
400 scientists.
Markus Röhrken’sthesis describesingreat detailhis work onthe byfar world
bestmeasurementsofbranchingratiosandCPviolationparametersintwospecial
reactions with two charm mesons inthe final state. When he started his work, the
experimental status of this reaction showed one of the very few small deviations
from the Standard Model predictions. This was the motivation of a careful
reanalysis with more data and improved techniques - and different results,
resolving the old discrepancy in favor of the Standard Model.
Markusgivesadeepandwell-readableoverviewabouttheory,phenomenology,
experimental setup,data taking, MonteCarlosimulations, psychologicalbias-free
blind statistical data analysis, and systematic uncertainty studies in a superb sci-
entific manner.
Karlsruhe/Germany, 15 May 2013 Prof. Michael Feindt
vii
Abstract
In the Standard Model of electroweak interactions, the effect of CP violation is
causedbyasingleirreduciblecomplexphaseintheCabibbo-Kobayashi-Maskawa
quark-mixing matrix of the three fundamental fermion families. The Belle and
BaBar Collaborations experimentally established the existence of CP violating
phenomena in the B meson system. In particular, the parameter sinð2bÞ, where
b¼arg(cid:2)(cid:2)V V(cid:3) =V V(cid:3)(cid:3), is determined precisely by the measurements of mix-
cd cb td tb
ing-induced CP violation in b!c(cid:2)cs transitions.
The decays of B0 !DþD(cid:2) and B0 !D(cid:3)(cid:4)D(cid:5) are caused by b!c(cid:2)cd transi-
tions. In lowest order perturbation theory, the transitions are described by Cabi-
bbo-disfavored amplitudes. In this order, the mixing-induced CP violation is
sinð2bÞ and no direct CP violation is possible. Additionally, higher order b!d
loop diagrams, so-called penguin-diagrams, can contribute to the decays. The
amplitudes corresponding to these diagrams may contain weak phases that are
different from the amplitudes at lowest order. The theoretical considerations with
respect to possible deviations of the mixing-induced and the direct CP violation
due to the penguin contributions are associated with significant, in particular
hadronic uncertainties, but predict corrections that are negligibly small.
Intheyear2007,theBelleCollaborationusingadatasetof535(cid:6)106 BBpairs
found evidence of an extraordinary large direct CP violation in B0 !DþD(cid:2)
decays: Cdir ¼(cid:2)0:91(cid:4)0:23ðstat:Þ(cid:4)0:06ðsyst:Þ, corresponding to a 3:2r
DþD(cid:2)
deviation from zero. This deviation was neither confirmed by BaBar, nor it was
observed in other B0 !D(cid:3)(cid:4)Dð(cid:3)Þ(cid:5) decays.
In this Ph.D. thesis, the measurements of the branching fraction and the time-
dependent CP violation in B0 !DþD(cid:2) decays based on the final data set of the
Belleexperimentarepresented.Furthermore,thethesisincludesthecorresponding
measurementsofB0 !D(cid:3)(cid:4)D(cid:5) decaystoprovideadirectcomparisontoarelated
decay.
The final Belle data set contains 772(cid:6)106 BB pairs recorded on the !ð4sÞ-
resonanceattheasymmetric-energyeþe(cid:2)-colliderKEKB.The!ð4sÞ-resonanceis
produced with a Lorentz boost of bc¼0:425 along the e(cid:2)-beam. The boosted
center-of-massframeallowsthemeasurementsofproperdecaytimedifferencesof
theproducedBmesons.TheneutralBmesonpairsoriginatingfromdecaysofthe
ix
x Abstract
!ð4sÞ-resonance are quantum-mechanically entangled and evolve coherently. The
measurement of their time evolution allows the experimental determination of
time-dependent CP violating asymmetries.
The measurements in B0 !DþD(cid:2) and B0 !D(cid:3)(cid:4)D(cid:5) decays are performedas
blindanalyses.Onlyafterthevalidationofthetime-integratedandtime-dependent
measurements by a wide variety of cross-checks using simulations and control
samples, the B0 !DþD(cid:2) and B0 !D(cid:3)(cid:4)D(cid:5) data sets are unblinded and the
measurements are performed.
The results of the measurements of branching fractions are
B(cid:4)B0 !DþD(cid:2)(cid:5)¼ð2:12(cid:4)0:16ðstat.Þ(cid:4)0:18ðsyst.ÞÞ(cid:6)10(cid:2)4;
B(cid:4)B0 !D(cid:3)(cid:4)D(cid:5)(cid:5)¼ð6:14(cid:4)0:29ðstat.Þ(cid:4)0:50ðsyst.ÞÞ(cid:6)10(cid:2)4:
Theresultsofthemeasurementoftime-dependentCPviolationinB0 !DþD(cid:2)
decays are
S ¼(cid:2)1:06þ0:21ðstat.Þ(cid:4)0:08ðsyst.Þ;
DþD(cid:2) (cid:2)0:14
C ¼(cid:2)0:43(cid:4) 0:16ðstat.Þ (cid:4) 0:05ðsyst.Þ:
DþD(cid:2)
This measurement excludes the conservation of CP symmetry in B0 !DþD(cid:2)
decays,equivalenttoS ¼C ¼0,ataconfidencelevelof1(cid:2)2:7(cid:6)10(cid:2)5
DþD(cid:2) DþD(cid:2)
corresponding to a significance of 4:2r.
The results of the measurement of time-dependent CP violation in B0 !
D(cid:3)(cid:4)D(cid:5) decays are
AD(cid:3)D ¼þ0:06 (cid:4) 0:05ðstat.Þ (cid:4) 0:02ðsyst.Þ;
SD(cid:3)D ¼(cid:2)0:78(cid:4) 0:15ðstat.Þ (cid:4) 0:05ðsyst.Þ;
CD(cid:3)D ¼(cid:2)0:01(cid:4) 0:11ðstat.Þ (cid:4) 0:04ðsyst.Þ;
DSD(cid:3)D ¼(cid:2)0:13(cid:4) 0:15ðstat.Þ (cid:4) 0:04ðsyst.Þ;
DCD(cid:3)D ¼þ0:12(cid:4) 0:11ðstat.Þ (cid:4) 0:03ðsyst.Þ:
ThismeasurementexcludestheconservationofCPsymmetryinB0 !D(cid:3)(cid:4)D(cid:5)
decays, equivalent to AD(cid:3)D ¼SD(cid:3)D ¼CD(cid:3)D ¼0, at a confidence level of 1(cid:2)
6:8(cid:6)10(cid:2)5 corresponding to a significance of 4:0r.
In both decay modes, the CP violation is driven by mixing-induced CP viola-
tion. The results of the mixing-induced CP violation in B0 !DþD(cid:2) and B0 !
D(cid:3)(cid:4)D(cid:5) decays are consistent with the precision measurements in b!c(cid:2)cs tran-
sitions. The current world average is (cid:2)g S ¼sinð2bÞ¼0:68(cid:4)0:02.
CP
IncomparisontothepreviousB0 !DþD(cid:2) analysisbytheBelleCollaboration
in 2007, the center value of the direct CP violation reduces by a factor of 2 and
deviates instead of 3:2r approximately 2:0r from zero. In B0 !D(cid:3)(cid:4)D(cid:5) decays,
Abstract xi
nodirectCPviolationisobserved.Theresultssuggestthattheextraordinarylarge
direct CP violation in the previous B0 !DþD(cid:2) analysis was caused by a statis-
tical fluctuation in the data. The current experimental precision does not allow to
prove significantly a non-zero direct CP violation at small center values. There-
fore, further measurements are necessary to probe for possible penguin contribu-
tions in b!c(cid:2)cd transitions.
The presented measurements in B0 !DþD(cid:2) and B0 !D(cid:3)(cid:4)D(cid:5) decays
supersede the previous measurements by the Belle Collaboration and are more
precisethanallrelevantpreviousmeasurementsperformedbytheBelleandBaBar
Collaborations.
The results of this Ph.D. thesis are published in
M. Röhrken et al. (Belle Collaboration),
‘‘Measurements of Branching Fractions and Time-Dependent
CP Violating Asymmetries in B0 !Dð(cid:3)Þ(cid:4)D(cid:5) Decays’’,
Physical Review D 85, 091106 (2012).