Search for New Phenomena in Dijet Angular Distributions at √s = 8 and 13 TeV

177 Pages·2017·9.37 MB·English

by  Lene Kristian Bryngemark (auth.)

Springer Theses
Recognizing Outstanding Ph.D. Research
Lene Kristian Bryngemark

Search for New
Phenomena in
Dijet Angular
Distributions at
–
√s = 8 and 13 TeV
Springer Theses
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Lene Kristian Bryngemark
Search for New Phenomena
in Dijet Angular Distributions
pffiffi
at s = 8 and 13 TeV
Doctoral Thesis accepted by
the Lund University, Lund, Sweden
123
Author Supervisor
Dr. Lene Kristian Bryngemark Prof. TorstenÅkesson
Deutsches Elektronen-Synchrotron (DESY) LundUniversity
Hamburg Lund
Germany Sweden
ISSN 2190-5053 ISSN 2190-5061 (electronic)
SpringerTheses
ISBN978-3-319-67345-5 ISBN978-3-319-67346-2 (eBook)
DOI 10.1007/978-3-319-67346-2
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’
Supervisor s Foreword
The Large Hadron Collider (LHC) at CERN provides the energy frontier for
Particle Physics research. It generates collisions between the proton constituents
(quarksandgluons)thatarestudiedusinglargedetectorsthatmeasuretheproduced
particles. The collision energies between the constituents have a statistical distri-
bution since the sharing of the proton momenta between the constituents is also
statistically distributed. Therefore, the more proton collisions that are sampled
(estimated as integrated luminosity), the higher collision energies are reached.
ThisthesisisananalysisofthehighestcollisionenergiesthattheLHCproduces,
by studying the dijet angular distributions. This analysis is performed at both
pffiffi
s¼8 and 13 TeV proton-proton centre-of-mass energies.
Lene Bryngemark performed an excellent analysis demonstrating that up to the
highest available momentum transfers between the colliding constituents, the
Standard Model (QCD corrected to NLO-level with k-factors, plus electroweak
corrections)perfectlydescribestheobservations.Shealsoshowedveryexplicitlyin
the thesis how, in the data, the shapes of the angular distributions are unchanged
over the full range of dijet invariant mass; the colliding objects appear the same
across the range.
HavingobservednodeviationfromtheStandardModel,theanalysesfocusedon
using this information to set limits on a number of suggested models beyond the
StandardModel.Thiswasdoneforthecompositenessscale,forthethresholdmass
ofquantumblackholesandforthemassofexcitedquarkstates.Inaddition,andfor
the first time, the angular distributions were used to set limits on the mass and
coupling to fermions, of a dark matter mediator.
It is an exceptionally well-written thesis about a major analysis performed with
the8 TeV data and in particularthe first 13 TeV data from the LHC atCERN. By
performing a scattering experiment, the student has shown that the constituents
remain pointlike up to the highest momentum transfers (smallest distances) avail-
able in a laboratory.
v
vi Supervisor’sForeword
I don’t know anyone easier and more pleasant to work with than Lene
Bryngemark.Shehasshowndrive,carefulnessandalotofindependence.Lenealso
hasshownstrongleadershipskillsintheworkwithhercolleagues,soIamsureshe
has a bright future in our field.
Lund, Sweden Prof. Torsten Åkesson
July 2017
Abstract
A new energy regime has recently become accessible in collisions at the Large
Hadron Collider at CERN. Abundant in hadron collisions, the two-jet final state
explores the structure of the constituents of matter and the possible emergence of
new forces of nature, in the largest momentum transfer collisions produced. The
results from searches for phenomena beyond the Standard Model in the dijet
angular distributions are presented. The data were collected with the ATLAS
detector in proton-proton collisions at centre-of-mass energies of 8 and 13 TeV,
correspondingtointegratedluminositiesof17.3fb(cid:2)1and3.6fb(cid:2)1,respectively.No
evidence for new phenomena was seen, and the strongest 95% confidence level
lowerlimitstodateweresetonthescaleofarangeofsuggestedmodels.Thiswork
details the limits on the compositeness scale of quarks in a contact interaction
scenario with two different modes of interference with Standard Model processes,
aswell asonthethreshold massofquantumblack holes ina scenario with 6extra
spatial dimensions, and on the mass of excited quark states. It also includes new
exclusionlimitsonthemassofadarkmattermediatoranditscouplingtofermions,
as derived from the contact interaction limits using an effective field theory
approach.
The performance in ATLAS of the jet-area-based method to correct jet mea-
surements for the overlaid energy of additional proton-proton collisions is also
presented. It removes the dependence of the jet transverse momentum on overlaid
collisionenergyfrombothsimultaneousinteractionsandthoseintheneighbouring
bunch crossings and was adopted as part of the jet calibration chain in ATLAS.
vii
List of Publications
Some of the original work described in this thesis has appeared previously in the
following publications:
[1] ATLAS Collaboration. “Search for New Phenomena in Dijet Mass and
pffiffi
Angular Distributions from pp Collisions at s = 13 TeV with the ATLAS
Detector”.In:PhysicsLettersB754(2016),pp.302–322.ISSN:0370-2693.
[2] ATLAS Collaboration. “Search for New Phenomena in the Dijet Angular
pffiffi
Distributions in Proton-Proton Collisions at s = 8 TeV with the ATLAS
Detector”. In: Phys. Rev. Lett. 114 (2015), p. 221802.
[3] ATLAS Collaboration. “Performance of pile-up mitigation techniques for
pffiffi
jets in pp collisions at s = 8 TeV using the ATLAS detector”. In: Eur.
Phys. J. C76.11 (2016), p. 581.
[4] ATLAS Collaboration. “Search for new phenomena in the dijet mass dis-
pffiffi
tributionusing p-pcollision data at s = 8TeV with the ATLAS detector”.
In: Phys. Rev. D 91 (2015), p. 052007.
[5] ATLAS Collaboration. Search for New Phenomena in Dijet Mass and
pffiffi
Angular Distributions with the ATLAS Detector at s = 13 TeV. Tech.
rep. ATLAS-CONF-2015-042. Geneva: CERN, 2015.
[6] ATLAS Collaboration. Pile-up subtraction and suppression for jets in
ATLAS. Tech. rep. ATLAS-CONF-2013-083. Geneva: CERN, 2013.
[7] ATLAS Collaboration. Search for New Phenomena in the Dijet Mass
pffiffi
Distributionupdatedusing13.0fb(cid:2)1ofppCollisionsat s=8TeVcollected
by the ATLAS Detector. Tech. rep. ATLAS-CONF-2012-148. Geneva:
CERN, 2012.
ix
Acknowledgements
Ittakesavillagetowriteathesis.Countlessarethepeoplewhohavecontributedin
one way or another to the work presented here, with efforts ranging from building
detectorstocookingandcaringformewhenworkhastakentoomuchofmytime.
I will nevertheless attempt to list a few I want to thank.
Torsten, who suggested the research topic of dijet distributions and became my
main supervisor. Always online, always there for physics or coffee or both, just
waitingtomakeadiscovery(I’msorrywedidn’tyet).Itissafetosaythatwithout
you, this thesis would not have been.
Else, Johan, my co-supervisors, who gave me the blessing of not worrying, but
being encouraging and available when I needed it. A special thanks to them and
David for thesis draft reading.
Ariel,whotalksofjetswithcontagiousenthusiasmandknowsthereissomuch
more we can do to extract all their secrets. I could not have had a better start in
ATLAS than I got from working with you. I also thank John, who regrettably left
physics, for patiently discussing and condensing all my (and Ariel’s) wild pile-up
ideas to a useful conclusion and strategy.
Caterina. Dijets, jets in ATLAS, just ATLAS would not be the same without
you.Sharingcodeandexpertise andcat pictures,youthoroughlyshowedmewhat
workinginateamcanbe.You’rereadytobothgiveadviceandgetyourownhands
dirty; Lund is better with you.
Tuva.Youhavetoadmitthatthisisonecrazyadventurewestartedtogether.Our
paths from starting a Bachelor’s project to finishing a Ph.D. were increasingly
parallel,butqueerlysimilar—Icanbutdon'twanttoimaginethistimewithoutyou.
Allmyshorterandlongertermcolleagues(andyouknowIcountstudents)who
have made the division kitchen such a lively place! Anders, Bozena, Evert, Peter,
thesteadycoffeeandlunchcrowd,alwaysupforadebateandforsupportoverthe
years.FloridoandmyfellowPh.D.students(Alejandro,Anders,Anthony,Martin,
Sasha,Tuva,Vytautas,andmorerecently,Ben,Edgar,KatjaandTrine)—forcoffee
and beer, physics and politics, for lunches and dinners and the opportunity to vent
ourfairshareofcollaborationfrustration.Thatsamecollaborationformakingitall
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