論文 / 著書情報
Article / Book Information
題目(和文)

LHC-ATLAS実験における多数の粒子飛跡を伴い衝突点から離れた崩壊点を用いた長寿命超対称性粒子の探索

Title(English)

Search for long-lived supersymmetry particle by signature of a hightrack-multiplicity displaced vertex using the LHC-ATLAS Experiment

著者(和文)

PetterssonNora Emilia

Author(English)

Nora Petterson

出典(和文)

学位:博士（理学）,学位授与機関:東京工業大学,
報告番号:甲第10023号,
授与年月日:2015年12月31日,
学位の種別:課程博士,
審査員:陣内 修,久世 正弘,柴田 利明,岡 眞,Todd Tilma

Citation(English)

Degree:,Conferring organization: Tokyo Institute of Technology,
Report number:甲第10023号,
Conferred date:2015/12/31,
Degree Type:Course doctor,Examiner:,,,,

学位種別(和文)

博士論文

Category(English)

Doctoral Thesis

種別(和文)

論文要旨

Type(English)

Summary

Powered by T2R2 (Tokyo Institute Research Repository)

（博士課程）Doctoral Program論 文 要 旨ＴＨＥＳＩＳ ＳＵＭＭＡＲＹ

専攻：
Department of

学生氏名：
Student’s Name

基礎物理学

専攻

Pettersson Nora Emilia

申請学位（専攻分野）： 博士
Academic Degree Requested

指導教員（主）：Academic Advisor(main)

指導教員（副）：Academic Advisor(sub)Doctor of（理学陣内修

久世正弘）

要旨（英文 800 語程度）
Thesis Summary （approx.800 English Words ）This thesis treats the experimental search for new physics conducted at theLHC-ATLAS Experiment. The analysis described within looks to reconstructdisplaced vertices (DVs) arising from long-lived particles (LLPs) postulated bysupersymmetry (SUSY), decaying within the tracking volume of the ATLAS
detector. Several charged daughter particles are expected from LLP decay,
yielding reconstructable tracks from which a vertex is fit. A new DV+jets channel
has been developed, where 20.3 fb-1 data collected at a centre of mass energy of 8
TeV in 2012 are scrutinised for events containing a DV and at least four hightransverse momentum (pT) jets. Solely requiring jets and a DV present in anevent provides a generic and model independent search strategy. The results cantherefore be interpreted on any model yielding a fairly massive LLP decaying
within the given fiducial volume. In this thesis, R-Parity violating (RPV) scenario
of SUSY is put under the test. The targeted signal production process from theproton-proton collision is the following; pair production of prompt decaying
gluinos, where the gluino decays to a quark and a long-lived neutralino. After agiven lifetime, the neutralino decays through the RPV LiQjDk-coupling to a lepton
or lepton-neutrino and a pair of quarks. The indices i, j, and k correspond to thegenerations of the lepton (L) and the quarks (Q and D) in the decay of theneutralino (values of 1, 2 or 3). In total, nine different combinations of thegenerators are considered. One RPV coupling is set to non-zero values, all the restof the SUSY couplings are set to decay promptly. The value of the RPV couplingmust remains relatively small to suppress the neutralino decay, giving it asignificant lifetime.
The displaced vertex analysis is from an experimental view-point quiteunique. Mostly owning to the fact that secondary decays, or displaced decays, are
not favoured by the detector design. This introduces difficulties and typically low
reconstruction efficiencies, and in turn low signal efficiencies for LLP searches.
Specially modified version of the track reconstruction is used to counter-act thisinherited inertia and low performance for non-promptly decaying particles.
Moreover, no standard model (SM) background is to be expected, looking for ahigh track-multiplicity displaced vertex. There are no known massive particles
that could produce the targeted signature. Instead, what is expected is down todetector inefficiencies. For instances, say a long-lived Kaon (497 MeV) get crossed
by a particle track originating from an independent source, perhaps a nuclearinteraction, if this additional track gets fit to the Kaon vertex. The vertex mass
could then potentially become high enough to imitate the signal of a LLP andwould become a source of background vertices. A major part of the thesis has been
dedicated into developing a new and significantly improved technique to estimate
the background down to detector effects and mis-reconstructions. The systematic
uncertainty has been reduced from 100% down to 14%, using the new method
compared to what has been used in the past. Yielding an invaluable improvementof the analysis as the new DV+jets channel is more likely to have a larger
background due to the generic selection criteria; giving the channel a background
100 times larger than for the older channels of DV+muon (which is not a part ofthis thesis).

In the absence of any signal events, upper limits on the production
cross-sections are set. Both in a model independent way through limits on thevisible cross-section and in a model dependent way. The model dependent limits
are set on the decays through one of the nine RPV scenarios of SUSY. Here theupper limits on the cross-sections are evaluated for a range of proper decay length(cτ) of the LLP, going from 1 mm to 1000 mm. The limit on the visible cross-section
is set to 0.14 fb given the full 20.3 fb-1 of data, resulting in an improvement fromearlier reported exclusions of 5.4 fb. Along with a range of values dependent on
proper decay lengths, in the best case for one RPV coupling with ijk = 211, where
the neutralino decays to a muon and light quarks (up and down) the exclusions
are made for 4 mm < cτ < 100 mm at 0.5 fb while the interval outside cτ > 100 mmand cτ < 4 mm are less restricted with exclusion down to 100 fb.
An second analysis is included in the thesis and has been performed, dealing
with the performance of the ATLAS tracking, attempting to quantify the amountof material in the inner detector to reduce the uncertainties related to thematerial descriptions for track reconstruction.
The thesis is structured in two parts, Part I and Part II. The first part of thethesis includes a review of the theoretical background related and necessary tounderstand the displaced vertex analysis. As well as an overview of the LHC
accelerator and the ATLAS Experiment. The second part includes every elementof the displaced vertex analysis; event selection criteria, physics object
reconstruction, background estimation and the results and limits. This part also
contains a chapter dedicated to the material study - the hadronic interaction
analysis.

備考 : 論文要旨は、和文 2000 字と英文 300 語を 1 部ずつ提出するか、もしくは英文 800 語を 1 部提出してください。
Note : Thesis Summary should be submitted in either a copy of 2000 Japanese Characters and 300 Words (Engｌish) or 1copy of 800
Words (English).
注意：論文要旨は、東工大リサーチリポジトリ（T2R2）にてインターネット公表されますので、公表可能な範囲の内容で作成してください。
Attention: Thesis Summary will be published on Tokyo Tech Research Repository Website (T2R2).