cO 2013 Sanjay R. Arora ALL RIGHTS RESERVED

174 Pages·2013·6.25 MB·English

(cid:13)c 2013
Sanjay R. Arora
ALL RIGHTS RESERVED
SEARCH FOR PAIR-PRODUCED HEAVY
FOURTH-GENERATION BOTTOM-LIKE
QUARKS DECAYING TO BZ AND TW IN
8 TEV PROTON-PROTON COLLISIONS
WITH MULTILEPTON FINAL STATES
BY SANJAY R. ARORA
A dissertation submitted to the
Graduate School—New Brunswick
Rutgers, The State University of New Jersey
in partial fulfillment of the requirements
for the degree of
Doctor of Philosophy
Graduate Program in Physics and Astronomy
Written under the direction of
Amitabh Lath
and approved by
New Brunswick, New Jersey
May, 2013
ABSTRACT OF THE DISSERTATION
Search for pair-produced heavy
fourth-generation bottom-like quarks decaying
to bZ and tW in 8 TeV Proton-Proton Collisions
with multilepton final states
By SANJAY R. ARORA
Dissertation Director: Amitabh Lath
We present a search for anomalous production of events with three or more iso-
√
lated leptons produced in pp collisions at s = 8TeV collected by the CMS
experiment at the LHC. We analyze 9.2 fb−1 of data collected by the CMS ex-
periment during the 2012 LHC run. We categorize observed multilepton events
into exclusive search channels based on various quantities based on the identity
and kinematics of the objects in the events. The search channels are ordered by
the amount of expected Standard Model background. Explicit use of require-
ments such as missing transverse energy or total hadronic energy is avoided. We
emphasize data-based estimation of the Standard Model backgrounds, but also
use simulation to estimate some of the backgrounds when appropriate. We in-
terpret search results in the context of a model involving the exotic bottom-like
quark b(cid:48) decaying to two different modes (b(cid:48) → bZ and b(cid:48) → tW) with varying
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branching ratios. We derive exclusion limits as a function of the b(cid:48) mass as well
as the branching ratios.
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Acknowledgements
I would like to start my thanking my parents. They have supported and encour-
aged me through the years and given me the freedom to choose my own path.
This would simply not have been possible without them.
Ashlee and Manjul have been best friends and siblings through the roller-
coaster ride of a PhD. I would have given up a long time ago without their
support. They also put up with obnoxious behavior that often accompanies the
author (remember Jaipur?!). I look forward to adding countless hours of fun to
the ones we have already had. Meekah and Sephora contributed immensely by
keeping my survival instincts sharp.
The Geesey family: Mr. and Mrs. John and Donna Geesey, Grandpa Geesey,
John, Ryan, Emi as well as Lucy have always been welcoming. They made New
Jersey home and keep me sane whenever things get too hot at work. A week
without them invariably wears me down and I hope to spend many more years
bothering them.
On the physics front, my advisor Amit has been invaluable. He gave me a
chance at a crucial stage in my graduate career. He has always been supportive
of my wild ideas and even wilder work hours (I never did try getting wine at 8
AM). He not only taught me how an experimental physicist thinks but also how
one can be interested in several things without compromising physics. I’ll always
be grateful to him for his support.
While not officially my advisor, Sunil has played that role too. Initially intim-
idating, especially when he was staring at a plot I made, I soon came to deeply
value his insight into physics. Sunil also gradually takes a graduate student from
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being just that to being a confident, professional scientist and for that I’ll always
be thankful.
Richard has been a role-model through the entire process. I have spent more
time on the details of particle physics with him than anyone else. People tend to
stop surprising you once you get to know them well but Richard still pulls out
awe-inspiring tricks that leave me stunned. Hopefully, we’ll get to work together
again in the future.
Scott has been a constant mentor and friend. As for physics, I have learnt
more physics from him per second than anyone else. At the risk of embarrassing
him, I think he’s easily the most outstanding physicist I know. His intuition as
well as knowledge are at just another level and it’s been an honor to work with
him. I am also very thankful for all his support and entertaining conversations
about space and technology.
The group would not be the same without all the postdocs, graduate and
undergraduate students. Matt has always pushed us by doing everything more
efficienctly and smartly and this has only made us better at what we do. Em-
manuel, Patrick, Peter, and Shruti are fellow multileptoners and my days would
be very boring without our sessions at the white-board (how much of that stuff
have we inhaled by now!). Anthony, Christian, Claudia and Rishi are great com-
pany and always brighten things up when they come to Rutgers. David, Eric,
Julia, Kelvin, Kin, and Mohamed, undergraduates only in name, have been col-
leagues and friends. Each of them has a very bright future given what we have
seen from them and I wish them the best of luck. I would also like to thank Eva,
Bob, John, JP, Pieter, and Steve for their support.
Rutgers has been wonderful over the past six years and both the university
and the department have made me feel very welcome. I made great friends here
and found a relaxing but intense physics environment. If I had to do it all again,
I would be back in a heartbeat.
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Last but definitely not the least, I thank all the engineers, technicians, post-
docsandgraduatestudentswhokeepCERNandtheLHCrunning. Dataanalysis
would be impossible without their efforts which often go unacknowledged. It’s
beenaprivilegetobepartofthisgiantexperimentespeciallyatatimeofdiscovery
and hopefully we’ll have many new discoveries in the coming years.
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Dedication
For friends and family
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Table of Contents
Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ii
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iv
Dedication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii
List of Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xi
List of Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xiii
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2. Model: Heavy Fourth-generation Exotic Quark b(cid:48) . . . . . . . . . 3
1.3. The LHC and the CMS Detector . . . . . . . . . . . . . . . . . . 5
1.4. Outline of the Thesis . . . . . . . . . . . . . . . . . . . . . . . . . 10
2. Analysis Strategy and Selection Criteria . . . . . . . . . . . . . . 13
2.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.2. Event Triggers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
2.3. Object Identification . . . . . . . . . . . . . . . . . . . . . . . . . 16
2.4. Search Strategy . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
2.5. Signal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
2.6. Datasets and Monte-Carlo Samples . . . . . . . . . . . . . . . . . 27
3. Background Estimation Techniques and Controls . . . . . . . . 29
3.1. Monte-Carlo Backgrounds . . . . . . . . . . . . . . . . . . . . . . 30
3.2. Data-driven Backgrounds . . . . . . . . . . . . . . . . . . . . . . . 36
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3.3. A walk through the background prediction methods . . . . . . . . 53
4. Trigger Efficiencies, Selection Efficiencies and Other Corrections
to Monte-Carlo Samples . . . . . . . . . . . . . . . . . . . . . . . . . . 59
4.1. Trigger Efficiencies . . . . . . . . . . . . . . . . . . . . . . . . . . 60
4.2. Lepton Identification and Isolation Efficiencies . . . . . . . . . . . 63
4.3. Other Corrections . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
5. Results: Statistical Techniques, Sources of Systematics and Ex-
clusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
5.1. Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
5.2. Statistical Procedure . . . . . . . . . . . . . . . . . . . . . . . . . 83
5.3. Sources of Systematic Uncertainties . . . . . . . . . . . . . . . . . 84
5.4. Exclusion Limits . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
6. Conclusions and Extensions . . . . . . . . . . . . . . . . . . . . . . 98
6.1. Extensions of this Analysis . . . . . . . . . . . . . . . . . . . . . . 99
Appendix A. MET Resolution Dependence on Pileup and Jet Ac-
tivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
A.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106
A.2. Basic Idea . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
A.3. Removing MET Backgrounds . . . . . . . . . . . . . . . . . . . . 111
A.4. Procedure and Results . . . . . . . . . . . . . . . . . . . . . . . . 120
A.5. Applying MET Corrections . . . . . . . . . . . . . . . . . . . . . 128
A.6. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129
A.7. Modeling of MET by Rayleigh Distributions . . . . . . . . . . . . 130
Appendix B. S Distribution Plots for All Channels . . . . . . . . 135
T
B.1. Plots for 3-lepton and 4-lepton Events . . . . . . . . . . . . . . . 135
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