CP Violation in {B_s}^0 -> J/psi.phi Decays: Measured with the Collider Detector at Fermilab

146 Pages·2015·5.958 MB·English

by  Sabato Leo (auth.)

Springer Theses
Recognizing Outstanding Ph.D. Research
Sabato Leo
CP Violation in
B 0  J/ψφ
s
Decays
Measured with the Collider
Detector at Fermilab
Springer Theses
Recognizing Outstanding Ph.D. Research
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Sabato Leo
CP Violation in
0
B ! J=ψφ Decays
s
Measured with the Collider
Detector at Fermilab
Doctoral Thesis accepted by
the University of Pisa, Italy
123
Author Supervisors
Dr. SabatoLeo Prof.GiovanniPunzi
UniversityofIllinoisatUrbana-Champaign Universityof Pisa
Urbana,IL Pisa
USA Italy
Dr. DiegoTonelli
CERN
Geneva
Switzerland
ISSN 2190-5053 ISSN 2190-5061 (electronic)
ISBN 978-3-319-07928-8 ISBN 978-3-319-07929-5 (eBook)
DOI 10.1007/978-3-319-07929-5
LibraryofCongressControlNumber:2014943127
SpringerChamHeidelbergNewYorkDordrechtLondon
(cid:2)SpringerInternationalPublishingSwitzerland2015
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Supervisors’ Foreword
Particle physics is now experiencing exciting and puzzling times. Recently,
a massive spin-0 particle compatible with the Higgs boson predicted by the
standard model (SM) was discovered. In addition, null results from experimental
searches excluded non-SM particles as heavy as 1 TeV. These striking findings
conclusivelyestablishthestandardmodelasanaccurateandconsistentdescription
of the fundamental constituents of matter and their interactions at energies
extendinguptothe‘‘Fermiscale’’(100GeV),atwhichelectromagneticandweak
interactions act as an unified force—and beyond.
However,anumberofexperimentalindicationsandsolidtheoreticalarguments
support the idea that the standard model becomes necessarily incomplete at some
energy higherthan the 1TeV scale directly probedthus far,andshouldbe rooted
in a more general theory. The description of such a theory, and the search for its
experimental signatures, is the chief goal of today’s fundamental physics.
Direct observation offeebly interacting massive particles requires accelerators
capable of producing collisions with multi-MHz rates, at energies a few times
larger than the mass of the sought particles. This is a technologically and eco-
nomically daunting task that requires ever-larger international collaboration
enterprises, enduring for decades.
The study of weak interactions of quarks, or quark-flavor physics, offers an
attractive complementary path, since such interactions are sensitive to energy
scalesfarhigherthanthosedirectlyaccessiblewithcurrentandforeseencolliders.
Thepresenceofnon-SMdynamicscanbeindirectly,butunambiguously,inferred
from quark transitions that allow exchanges of virtual non-SM particles.
Thisrequiresfocusingonprocessesthatareexperimentallyaccessible,sensitiveto
a broad class of SM extensions, and whose phenomenology is accurately predict-
able.DiscrepanciesemergingfromthecomparisonofaccurateSMpredictionswith
equally precise experimental results would reveal the presence of non-SM
dynamics.
Indeed,thediscoverypotentialofthisapproachisnonovelty.Indirectsearches
have been instrumental in directing us toward discovery of some of the most
v
vi Supervisors’Foreword
fundamentalbuildingblocksofthestandardmodelitself.Forinstance,theexistence
ofthecharmedquarkwasconjecturedanditsmassestimated,afewyearspriortoits
directdiscovery,basedontheobservationofananomaloussuppressioninKmeson
decayrates.
Intwodecadesofextensiveexplorations,nosearchinquark-flavorrevealedthe
presence of non-SM physics. Nevertheless, sub-10 % contributions from non-SM
dynamicsarenotruledout,especiallyinprocessesinvolvingtransitionsofbottom-
strange mesons (B0 and B(cid:2)0). These bound states of a bottom and a strange quark
s s
offer a rich phenomenology that, at the time of this work, was only marginally
explored.ThedecaysofB0andB(cid:2)0mesonsintothefinalstateinvolvingaJ/ψanda
s s
φmesonstandoutasoneofthefewgoldenchannelsthatofferthemostpromising
discovery opportunities. Interference between direct decays and decays following
particle–antiparticle transitions, B0 $B(cid:2)0 !J=ψφ, provides experimental access
s s
totheunderlyingmixingphase,aparameterthatisvery sensitivetoabroadclass
of non-SM processes, and challenging to measure otherwise. Experimentally,
a sizable decay rate into fully reconstructed final states, with intermediate
resonances decaying into a muon pair and two charged particles makes the
accuratereconstructionoflarge eventsamplesaccessibleinhadroncollisionsand
aids effective suppression of backgrounds. The precision measurement of the B0
s
mixing phase is one of the key goals of current experimental particle physics.
This thesis reports the final measurement of the mixing phase performed in
proton–antiproton collisions at center-of-mass energy of 1.96 TeV, provided by
the Tevatron collider and recorded by the Collider Detector at Fermilab experi-
ment (CDF). Sifting through 1014 collisions from 10 years of operations, a
low-background sample of about 11,000 B0;B(cid:2)0 !J=ψφ decays is reconstructed.
s s
A sophisticated analysis of the data is then performed, which encompasses in a
single measurement all most advanced experimental techniques employed in fla-
vor-physicsathadroncolliders.Incollisionsthattypicallyyieldtenthstohundreds
ofchargedparticles,thedistancetraveledbytheB0orB(cid:2)0mesonbeforedecayingis
s s
measuredwith30lmprecision.This,combinedwithafewpermilmeasurementof
the meson’s momentum, provides a measurement of decay time with about 90 fs
resolution. A suite of algorithms, specifically optimized for this thesis, is used to
study the particle activity in the whole event searching for tiny correlations
between particle species and their electrical charges, which allow determining
whether the bottom-strange particle was produced as a meson or an antimeson.
In addition, spatial correlations between the directions offinal-state particles are
exploited in determining the value of the orbital angular momentum between
decay products. All these informations are encapsulated into a technically
challenging multivariate likelihood fitwith more than 30 parameterstodetermine
the mixing phase.
The relevance and impact of the results is well worth the effort. The determi-
nation of the mixing phase and several supplementary quantities are among the
most precise available from a single experiment. They have been published in a
highlycitedlettertoPhysicalReview,PRL109,171802(2012),wherethisthesis
Supervisors’Foreword vii
is also cited. No indication of deviations from the SM predictions is found and
significantlyimprovedboundsontheallowedphenomenologyofnon-SMparticles
or interactions are obtained.
The experimental exploration of the bottom-strange mixing phase is rapidly
progressing owing to abundant samples available at the Large Hadron Collider,
and promises many further new insights. But this thesis will remain a valuable
resource, providing a clear and detailed description of a legacy measurement
performed by the CDF experiment, where measurements of the bottom-strange
mixingphasewerefirstperformed,andmanyofthekeytechniquesthathavenow
become standard in hadron-collisions flavor physics had been pioneered and
perfected.
Pisa, 2014 Prof. Giovanni Punzi
Dr. Diego Tonelli
Abstract
We report on the final measurement of the CP-violating B0 mixing phase
p s
βJ=ψφ in ffisffi¼1:96 TeV proton-antiproton collisions collected with the Collider
s
Detector at the Fermilab Tevatron collider. Using a sample corresponding to
9.6 fb-1 of integrated luminosity, we fit the decay-time evolution of B0 ?
s
J/ψ(?μ+μ-)φ(?K+K-)decaysinwhichtheb-quarkcontentatproductionand
the CP parity of the final state are identified. The interference of decays with and
without mixing renders the B0 mixing phase observable. The phase is determined
s
tobe-0.06\βJ/ψφ\0.30at 68% confidencelevel. Thedecay-width difference
s
between heavy and light B0 eigenstates, and the average B0 lifetime are also
s s
determined to be ΔΓ = 0.068 ± 0.027 ps-1 and τ = 1.528 ± 0.021 ps,
s s
respectively. These results are among the world’s most precise from a single
experiment, and compatible with standard model predictions.
ix
Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2 Flavor as a Probe for Non-SM Physics. . . . . . . . . . . . . . . . . . . . . 5
2.1 The Current Landscape . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.2 Flavor as a Probe of Non-SM Physics . . . . . . . . . . . . . . . . . . . 6
2.3 CKM Matrix and CP-violation . . . . . . . . . . . . . . . . . . . . . . . . 9
2.3.1 B0 Oscillations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
s
2.4 Analysis of the Time-Evolution of B0 !J=ψφ Decays . . . . . . . 14
s
2.4.1 Time Evolution of the ðSþPÞ-wave System. . . . . . . . . 21
2.4.2 Likelihood Symmetries. . . . . . . . . . . . . . . . . . . . . . . . 24
2.5 Experimental Status. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
2.6 Experimental Aspects. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
2.7 Analysis Strategy. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
3 The Collider Detector at the Fermilab Tevatron. . . . . . . . . . . . . . 33
3.1 The Fermilab Tevatron Collider . . . . . . . . . . . . . . . . . . . . . . . 33
3.1.1 Proton Production . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
3.1.2 Antiproton Production and Accumulation. . . . . . . . . . . 36
3.1.3 Injection and Collisions . . . . . . . . . . . . . . . . . . . . . . . 36
3.1.4 Run II Performances and Achievements. . . . . . . . . . . . 38
3.2 The CDF II Detector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
3.2.1 Coordinates and Notation. . . . . . . . . . . . . . . . . . . . . . 39
3.2.2 Tracking System . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
3.2.3 Time of Flight Detector . . . . . . . . . . . . . . . . . . . . . . . 45
3.2.4 Calorimeters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
3.2.5 Muons Detectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
3.3 Trigger and Data Acquisition Systems. . . . . . . . . . . . . . . . . . . 48
3.3.1 Dimuon Trigger. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
3.3.2 Reconstruction of Physics Objects. . . . . . . . . . . . . . . . 52
xi