Measurement of the Top Quark Mass with the CMS Experiment

The standard model of particle physics describes an astonishing number of phenomena. Yet at the same time it is incomplete: it does not describe e.g. gravitation. Finding explicit weaknesses in the standard model predictions has proved to be difficult, and hence precision measurements are currently one of the most promising methods towards this goal. One of the most intriguing precision measurements is that of the top quark mass (mt), which is connected for instance to the question about the meta-stability of the universe. This thesis strives multilaterally towards a more precise measurement and interpretation of the top quark mass. The work begins with efforts towards a more precise jet calibration at the CMS. Then, the possible weaknesses of a D0 mt analysis are reviewed. Finally, a mt measurement at the CMS is constructed for the legacy 2017--2018 datasets. The jet energy corrections are the most important experimental factor in the uncertainties of the top quark mass. Hence, they are closely linked with the mt measurement. The work on jets in this thesis aims for an exceptionally precise jet energy calibration for the CMS Run 2 legacy datasets. The author has made several important contributions towards the jet energy corrections in the Run 2 legacy reconstruction. The re-assessment of a D0 top quark mass measurement is performed outside of the CMS and D0 affiliations. The D0 top quark mass value is an important outlier in the top quark mass world combination, and a better understanding of the reasons behind this is desirable. In an earlier study it was shown that there are possible discrepancies in the flavor-dependent jet energy corrections at D0. In this thesis we demonstrate that these discrepancies (if they can be confirmed) shift the D0 top quark mass measurement to a value that is more in line with the other major measurements from CMS, ATLAS and CDF. The work culminates in the design and validation of the first direct CMS lepton+jets mt measurement on the 2017--2018 datasets. The analysis is executed using a new profile likelihood method, where the collected data can constrain systematic uncertainties in situ. Agreement between data and simulation is verified within the systematic uncertainties using control plots. The impact of an extensive set of systematic uncertainties on the mt measurement is assessed using simulations. Also the full effects of limited statistics in simulations are demonstrated using toy experiments. It is confirmed that the limiting systematic uncertainty in the current mt measurements is the modelling of b quark jets. This challenge can be encountered either by enhancing the b jet energy corrections or by performing the measurement on a larger amount of data. In profile likelihood analyses, the latter is also a valid approach.Hiukkasfysiikan standardimalli kykenee ennustamaan uskomattoman määrän ilmiöitä. Silti se on vajavainen, eikä huomioi esimerkiksi painovoimaa. Selkeiden puutteiden paikantaminen standardimallin ennusteissa on kuitenkin hankalaa, ja siksi tarkkuusmittaukset ovat osoittautuneet parhaiksi instrumenteiksi puutteiden etsinnässä. Yksi mielenkiintoisimmista tarkkuusmittauksista koskee t-kvarkin massaa (mt), joka kytkeytyy esimerkiksi kysymykseen universumin metastabiiliudesta. Tämä väitöskirja pyrkii monin keinoin kehittämään t-kvarkin massan mittaustarkkuutta. Työ alkaa CMS:n jettien eli energisten hiukkasryöppyjen kalibraation parantamisella. Keskivaiheilla työtä tutkitaan D0-kollaboraation tarkimman mt-mittauksen mahdollisia heikkouksia. Väitöskirjan loppupuolella rakennetaan CMS-ilmaisimella vuosina 2017–2018 kerättyihin protoni–protoni-törmäyksiin perustuva mt:n tarkkuusmittaus. Jettien energian korjauskertoimet ovat yksi tärkeimmistä kokeellisista tekijöistä mt:n mittaustarkkuudessa. Väitöskirjassa esitellään CMS:n vuosina 2016--2018 keräämille hiukkastörmäyksille käytettyjä menetelmiä erityisen tarkkaan jettienergian kalibrointiin. Kirjan tekijä on merkittävästi myötävaikuttanut kyseisten menetelmien kehitystyöhön. D0:n tarkimman mt-mittauksen tarkastelu on tehty irrallaan sekä D0- että CMS-kollaboraatioista. D0:n mittaama t-kvarkin massan arvo poikkeaa huomattavasti muista tärkeimmistä mittaustuloksista, minkä ymmärtäminen olisi suotavaa. Aiemmassa tutkimuksessa näytettiin, että D0:n jettienergian kalibroinnissa on todennäköisesti ongelmia. Tässä työssä näytetään näiden ongelmien numeerinen vaikutus t-kvarkin massan mitattuun arvoon. Sikäli jos jettienergian kalibroinnissa on epäiltyjä ongelmia, D0:n mt-mittauksen tuloksen pitäisi todella olla lähempänä muita tärkeimpiä mittaustuloksia. Työ huipentuu CMS:n ensimmäisen yhden varatun leptonin sekä jettejä sisältävässä lopputilassa tehtävän mt-mittauksen kehittämiseen vuosina 2017–2018 kerätyille hiukkastörmäyksille. Analyysi toteutetaan uudella tekniikalla, jossa systemaattiset epävarmuuslähteet on sisällytetty mittauksen tekoon käytettävään matemaattiseen malliin. Tämä mahdollistaa systemaattisten epävarmuuksien rajoittamisen mittauksen yhteydessä. Simulaatiot sekä CMS-ilmaisimella kerätty data osoitetaan yhteneviksi arvioitujen epävarmuuksien rajoissa. Lisäksi tulevaa mt:n mittaustarkkuutta arvioidaan simulaatioiden pohjalta. Saadaan vahvistettua, että tärkein epävarmuuslähde mittauksessa ovat b-kvarkeista syntyviin jetteihin liittyvät epävarmuudet, joita on mahdollista rajoittaa joko kehittämällä jettien energiakorjauksia tai keräämällä lisää dataa

Jettien maut: Referenssi-prosesseista top-kvarkin massaan

The LHC began its second run in 2015 with upgraded hardware and software and an increased collision energy. With the higher energy scale the importance of jet physics has grown even larger than before. Jets are collimated sprays of hadrons that are produced in high-energy particle collisions. Understanding jets makes it possible to analyze the proton-proton collisions occurring at the LHC. This work studies jet flavors and their definitions in the context of the CMS experiment. The motivation for this is that the jet energy corrections applied to the CMS data depend on the jet flavors. A jet flavor is typically understood as the flavor of the quark or gluon from which the jet originated. In other contexts, e.g. b-tagging, the meaning of a jet flavor can be slightly different. Focus is given to the study of jet flavor definitions in simulations of proton-proton collisions. Due to the structure of simulations the flavor definitions have an algorithmic form. The flavor studies begin by inspecting the robustness of a previously favored jet flavor definition between three different simulation software packages. Here the robustness of a flavor means that the physical properties of each flavor are the same in three different collision event types (standard candle events). Good robustness properties are observed between the software packages, but an excessive amount of jets is left without any flavor tag. A solution for this problem is sought for by developing enhanced flavor definitions. Two prominent new flavor definitions are found in the studies. The knowledge gained in the flavor studies is then applied to the studies of top quark production. The abundance of jets is particularly high in collisions that produce top quarks, so jet-related knowledge is important. It turns out that here the jet flavor properties are somewhat similar to those observed in the standard candle collision events. However, there are some differences that require further study. To conclude, a simulated measurement of the top quark mass is made. This provides valuable understanding of the practical issues related to a top mass measurement.LHC aloitti toisen toimintajaksonsa vuonna 2015 kasvatetulla törmäysenergialla ja päivitetyllä laitteistolla. Energiaskaalan kasvun myötä jettien fysiikan merkitys on entistäkin suurempi. Jetit ovat kapeita hadroniryöppyjä, joita syntyy korkean energian hiukkastörmäyksissä. LHC:lla tapahtuvien protoni–protoni-törmäysten analysointi on mahdollista jettien ymmärtämisen ansiosta. Tässä työssä tutkitaan jettien makuja ja niiden määritelmiä CMS-kokeen kontekstissa. Tutkimusta motivoi se, että CMS-kokeen tuottaman datan jettien energia-korjauksissa jettien mauilla on suuri merkitys. Jetin maulla tarkoitetaan yleensä sen kvarkin tai gluonin tyyppiä, josta jetti on peräisin. Merkitys voi kuitenkin vaihdella hiukan eri yhteyksissä, kuten b-jettien tutkimuksessa. Työ keskittyy jettimakujen tutkimukseen protoni–protoni-törmäyksien simulaatioissa. Simulaatioiden rakenteen vuoksi jettimaku määritellään algoritmien avulla. Makujen tutkimus alkaa aiemmin hyväksi todetun määritelmän vakauden vertailulla kolmen simulaatio-ohjelmiston välillä. Vakaudella tarkoitetaan sitä, että kunkin jettimaun fysikaaliset ominaisuudet pysyvät samoina kolmessa eri törmäystyypissä (referenssi-prosessit). Simulaatio-ohjelmien väliset vakausominaisuudet todetaan hyviksi, mutta samalla havaitaan, että tilastollisesti liian monen jetin maun määritys epäonnistuu. Tähän ongelmaan haetaan ratkaisua kehittämällä parannettuja maun määritelmiä. Kaksi uutta määritelmää osoittautuu toimiviksi. Makututkimuksessa kerättyä tietoa sovelletaan edelleen top-kvarkkeja tuottavien törmäysten tutkimiseen. Jettejä esiintyy erityisen paljon tällaisissa törmäyksissä, minkä vuoksi niiden tuntemus on tärkeää. Ilmenee, että jettimakujen ominaisuudet ovat jokseenkin samanlaisia kuin referenssi-prosesseissa. Jotkin poikkeavuudet kuitenkin vaativat jatkotutkimusta. Työn päätteeksi suoritetaan simulaatiopohjainen top-kvarkin massan määritys. Tämä tarjoaa hyvää ymmärrystä käytännön hankaluuksista, joita esiintyy top-kvarkin massan mittauksessa

Search for supersymmetry using Higgs boson to diphoton decays at s√ = 13 TeV

A search for supersymmetry (SUSY) is presented where at least one Higgs boson is produced and decays to two photons in the decay chains of pair-produced SUSY particles. Two analysis strategies are pursued: one focused on strong SUSY production and the other focused on electroweak SUSY production. The presence of charged leptons, additional Higgs boson candidates, and various kinematic variables are used to categorize events into search regions that are sensitive to different SUSY scenarios. The results are based on data from proton-proton collisions at the Large Hadron Collider at a center-of-mass energy of 13 TeV collected by the CMS experiment, corresponding to an integrated luminosity of 77.5 fb−1. No statistically significant excess of events is observed relative to the standard model expectations. We exclude bottom squark pair production for bottom squark masses below 530 GeV and a lightest neutralino mass of 1 GeV; wino-like chargino-neutralino production in gauge-mediated SUSY breaking (GMSB) for chargino and neutralino masses below 235 GeV with a gravitino mass of 1 GeV; and higgsino-like chargino-neutralino production in GMSB, where the neutralino decays exclusively to a Higgs boson and a gravitino for neutralino masses below 290 GeV.Peer reviewe

The MetNet vehicle : a lander to deploy environmental stations for local and global investigations of Mars

Investigations of global and related local phenomena on Mars such as atmospheric circulation patterns, boundary layer phenomena, water, dust and climatological cycles and investigations of the planetary interior would benefit from simultaneous, distributed in situ measurements. Practically, such an observation network would require low-mass landers, with a high packing density, so a large number of landers could be delivered to Mars with the minimum number of launchers. The Mars Network Lander (MetNet Lander; MNL), a small semi-hard lander/penetrator design with a payload mass fraction of approximately 17 %, has been developed, tested and prototyped. The MNL features an innovative Entry, Descent and Landing System (EDLS) that is based on inflatable structures. The EDLS is capable of decelerating the lander from interplanetary transfer trajectories down to a surface impact speed of 50-70 ms(-1) with a deceleration of <500 g for <20 ms. The total mass of the prototype design is approximate to 24 kg, with approximate to 4 kg of mass available for the payload. The EDLS is designed to orient the penetrator for a vertical impact. As the payload bay will be embedded in the surface materials, the bay's temperature excursions will be much less than if it were fully exposed on the Martian surface, allowing a reduction in the amount of thermal insulation and savings on mass. The MNL is well suited for delivering meteorological and atmospheric instruments to the Martian surface. The payload concept also enables the use of other environmental instruments. The small size and low mass of a MNL makes it ideally suited for piggy-backing on larger spacecraft. MNLs are designed primarily for use as surface networks but could also be used as pathfinders for high-value landed missions.Peer reviewe

Identification of heavy-flavour jets with the CMS detector in pp collisions at 13 TeV

Many measurements and searches for physics beyond the standard model at the LHC rely on the efficient identification of heavy-flavour jets, i.e. jets originating from bottom or charm quarks. In this paper, the discriminating variables and the algorithms used for heavy-flavour jet identification during the first years of operation of the CMS experiment in proton-proton collisions at a centre-of-mass energy of 13 TeV, are presented. Heavy-flavour jet identification algorithms have been improved compared to those used previously at centre-of-mass energies of 7 and 8 TeV. For jets with transverse momenta in the range expected in simulated $\mathrm{t}\overline{\mathrm{t}}$ events, these new developments result in an efficiency of 68% for the correct identification of a b jet for a probability of 1% of misidentifying a light-flavour jet. The improvement in relative efficiency at this misidentification probability is about 15%, compared to previous CMS algorithms. In addition, for the first time algorithms have been developed to identify jets containing two b hadrons in Lorentz-boosted event topologies, as well as to tag c jets. The large data sample recorded in 2016 at a centre-of-mass energy of 13 TeV has also allowed the development of new methods to measure the efficiency and misidentification probability of heavy-flavour jet identification algorithms. The heavy-flavour jet identification efficiency is measured with a precision of a few per cent at moderate jet transverse momenta (between 30 and 300 GeV) and about 5% at the highest jet transverse momenta (between 500 and 1000 GeV)

Measurement of the top quark forward-backward production asymmetry and the anomalous chromoelectric and chromomagnetic moments in pp collisions at √s = 13 TeV

Abstract The parton-level top quark (t) forward-backward asymmetry and the anomalous chromoelectric (d̂ t) and chromomagnetic (μ̂ t) moments have been measured using LHC pp collisions at a center-of-mass energy of 13 TeV, collected in the CMS detector in a data sample corresponding to an integrated luminosity of 35.9 fb−1. The linearized variable AFB(1) is used to approximate the asymmetry. Candidate t t ¯ events decaying to a muon or electron and jets in final states with low and high Lorentz boosts are selected and reconstructed using a fit of the kinematic distributions of the decay products to those expected for t t ¯ final states. The values found for the parameters are AFB(1)=0.048−0.087+0.095(stat)−0.029+0.020(syst),μ̂t=−0.024−0.009+0.013(stat)−0.011+0.016(syst), and a limit is placed on the magnitude of | d̂ t| &lt; 0.03 at 95% confidence level. [Figure not available: see fulltext.

Measurement of t(t)over-bar normalised multi-differential cross sections in pp collisions at root s=13 TeV, and simultaneous determination of the strong coupling strength, top quark pole mass, and parton distribution functions

Peer reviewe

An embedding technique to determine ττ backgrounds in proton-proton collision data

An embedding technique is presented to estimate standard model tau tau backgrounds from data with minimal simulation input. In the data, the muons are removed from reconstructed mu mu events and replaced with simulated tau leptons with the same kinematic properties. In this way, a set of hybrid events is obtained that does not rely on simulation except for the decay of the tau leptons. The challenges in describing the underlying event or the production of associated jets in the simulation are avoided. The technique described in this paper was developed for CMS. Its validation and the inherent uncertainties are also discussed. The demonstration of the performance of the technique is based on a sample of proton-proton collisions collected by CMS in 2017 at root s = 13 TeV corresponding to an integrated luminosity of 41.5 fb(-1).Peer reviewe