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An Algorithm for Invariant Mass Reconstruction in a Search for Light

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An Algorithm for Invariant Mass Reconstruction in a Search for Light HELSINGIN YLIOPISTO — HELSINGFORS UNIVERSITET — UNIVERSITY OF HELSINKI Tiedekunta/Osasto — Fakultet/Sektion — Faculty Laitos — Institution — Department Faculty of Science Institute of Physics Tekijä — Författare — Author Stefan Richter Työn nimi — Arbetets titel — Title Invariant mass reconstruction in a search for light charged Higgs bosons in pp collisions at √s = 7 TeV Oppiaine — Läroämne — Subject Physics Työn laji — Arbetets art — Level Aika — Datum — Month and year Sivumäärä — Sidoantal — Number of pages Master’s thesis July 2013 121 Tiivistelmä — Referat — Abstract An algorithm for invariant mass reconstruction in a search for light charged Higgs bosons (H±) produced in top quark decays and decaying to a tau lepton and neutrino, H τ ν , is pre- ± → ± τ sented. Here, ‘light’ means lighter than the top quark. The algorithm uses the top quark mass as a kinematical constraint to allow the calculation of the longitudinal momentum of the neutrinos. The invariant mass of the tau-and-neutrino system is then calculated using the missing transverse energy, the calculated longitudinal momentum of the neutrinos, and the measured momentum of the visible decay products of the tau lepton. Methods for resolving ambiguities and recovering unphysical results arising in the invariant mass reconstruction are presented. The invariant mass distribution could be used to extract a possible signal, replacing or comple- menting the transverse mass distribution that has been used so far in the analysis. In a preliminary data analysis using pp collision data at √s = 7 TeV corresponding to an inte- 1 grated luminostity of 5.1 fb− recorded by the CMS experiment, it is shown that using invariant mass distribution obtained with the presented algorithm allows to set a more stringent upper limit on the signal branching fraction (t H b) (H τ ν ) than does using the transverse B → ± × B ± → ± τ mass distribution. An expected upper limit at the 95% confidence level between around 0.37% to 2.5% (transverse mass) and 0.13% to 1.9% (invariant mass) is found, depending on the H± mass. These results suggest that using the invariant mass reconstructed with the new algorithm may improve the signal sensitivity of the search. Avainsanat — Nyckelord — Keywords Charged Higgs bosons, very high-energy pp interactions, decays of tau leptons, kinematics Säilytyspaikka — Förvaringsställe — Where deposited Kumpula Campus Library (Gustaf Hällströmin katu 2, 00014 University of Helsinki) Muita tietoja — övriga uppgifter — Additional information Summaries for the general public are available in several languages in the appendix. HELSINGIN YLIOPISTO — HELSINGFORS UNIVERSITET — UNIVERSITY OF HELSINKI Tiedekunta/Osasto — Fakultet/Sektion — Faculty Laitos — Institution — Department Matemaattis-luonnontieteellinen tiedekunta Fysiikan laitos Tekijä — Författare — Author Stefan Richter Työn nimi — Arbetets titel — Title Invariantin massan rekonstruktio kevyiden varattujen Higgsin bosonien etsinnässä pp-törmäyksissä energialla √s = 7 TeV Oppiaine — Läroämne — Subject Fysiikka Työn laji — Arbetets art — Level Aika — Datum — Month and year Sivumäärä — Sidoantal — Number of pages Pro gradu Heinäkuu 2013 121 Tiivistelmä — Referat — Abstract Tässä työssä esitetään algoritmi invariantin massan rekonstruoimiseksi. Sitä käytetään analyy- sissä, jossa etsitään kevyitä varattuja Higgsin bosoneita (H±), jotka syntyvät huippu-kvarkkien hajoamisissa ja jotka hajoavat edelleen tau-leptoniksi ja neutriinoksi, H τ ν . Kevyt tarkoittaa ± → ± τ tässä yhteydessä kevyempi kuin huippu-kvarkki. Algoritmi käyttää huippu-kvarkin massaa kine- maattisena rajoituksena, jonka avulla pystytään laskemaan neutriinojen pitkittäinen liikemäärä. Taun ja neutriinon muodostaman järjestelmän invariantti massa pystytään sen jälkeen laskemaan käyttäen puuttuvaa poikittaista energiaa, neutriinojen laskettua pitkittäistä liikemäärää sekä tau- leptonien suoraan havaittavien hajoamistuotteiden kokeellisesti määritettyä liikemäärää. Invariantin massan rekonstruktiossa saattaa ilmetä monikäsitteisyyksiä tai epäfysikaalisia tulok- sia. Tässä työssä esitetään keinoja näiden haasteiden ratkaisemiseksi. Invariantin massan jakaumaa voidaan käyttää mahdollisen signaalin havaitsemiseen, johon analy- ysissä on tähän asti käytetty ainoastaan poikittaisen massan jakaumaa. Poikittaisen massan sijaan analyysissä voidaan käyttää invarianttia massaa tai kummankin massan yhdistelmää. Työssä on tehty alustava data-analyysi, jossa on käytetty CMS -kokeen tallentamaa dataa protoni– 1 protoni-törmäyksistä. Datan määrä vastaa integroitua luminositeettiä 5.1 fb− , ja massakeskipiste- energia törmäyksissä oli √s = 7 TeV. Data-analyysin tulokset osoittavat, että uuden algoritmin avulla saadun invariantin massan jakauman käyttö sallii tiukemman ylärajan asettamisen signaal- ihajoamissuhteelle (t H b) (H τ ν ) kuin poikittaisen massan jakauman käyttö. B → ± × B ± → ± τ Odotusarvoinen yläraja 95%:n luottamustasolla on tulosten perusteella välillä 0.37–2.5% (poikit- tainen massa) ja välillä 0.13%–1.9% (invariantti massa) varatun Higgsin bosonin massasta riip- puen. Nämä alustavat tulokset viittaavat siihen, että uuden algoritmin avulla rekonstruoidun invari- antin massan jakauman käyttö saattaa parantaa etsinnän herkkyyttä mahdolliselle signaalille. Avainsanat — Nyckelord — Keywords Varatut Higgsin bosonit, korkeaenergiset pp-törmäykset, tau-leptonin hajoamiset, kinematiikka Säilytyspaikka — Förvaringsställe — Where deposited Kumpulan kampuskirjasto (Gustaf Hällströmin katu 2, 00014 Helsingin yliopisto) Muita tietoja — övriga uppgifter — Additional information Yleistajuisia yhteenvetoja usealla eri kielellä on sisällytetty tutkielman liitteisiin. Pro gradu -tutkielma Examensarbete Master’s thesis Invariant mass reconstruction in a search for light charged Higgs bosons in pp collisions at √s = 7 TeV Stefan Richter 27.7.2013 Ohjaaja | Handledare | Advisor Dr. Sami Lehti Tarkastajat | Examinatorer | Examiners Prof. Paula Eerola Dr. Sami Lehti HELSINGIN YLIOPISTO HELSINGFORS UNIVERSITET UNIVERSITY OF HELSINKI FYSIIKAN LAITOS INSTITUTIONEN FÖR FYSIK DEPARTMENT OF PHYSICS Abstract An algorithm for invariant mass reconstruction in a search for light charged Higgs bosons (H ) produced in top quark decays and decaying to a tau lepton and neutrino, H ± ± ! t±nt, is presented. Here, ‘light’ means lighter than the top quark. The algorithm uses the top quark mass as a kinematical constraint to allow the calculation of the longitudinal momentum of the neutrinos. The invariant mass of the tau-and-neutrino system is then calculated using the missing transverse energy, the calculated longitudinal momentum of the neutrinos, and the measured momentum of the visible decay products of the tau lepton. Methods for resolving ambiguities and recovering unphysical results arising in the invari- ant mass reconstruction are presented. The invariant mass distribution could be used to extract a possible signal, replacing or complementing the transverse mass distribution that has been used so far in the analysis. In a preliminary data analysis using pp collision data at ps = 7 TeV corresponding to an 1 integrated luminostity of 5.1 fb− recorded by the CMS experiment, it is shown that using invariant mass distribution obtained with the presented algorithm allows to set a more stringent upper limit on the signal branching fraction (t H b) (H t n ) than B ! ± × B ± ! ± t does using the transverse mass distribution. An expected upper limit at the 95% confidence level between around 0.37% to 2.5% (transverse mass) and 0.13% to 1.9% (invariant mass) is found, depending on the H± mass. These results suggest that using the invariant mass may improve the sensitivity to a possi- ble signal of the search. <3 This master’s thesis was prepared on the basis of research done at the Helsinki Institute of Physics (Helsinki, Finland) and CERN (Geneva, Switzerland) in the spring and summer of 2013. It was a turbulent period of time in my life, as is evidenced by the fact that writing of this thesis was done in such places as airplanes, a student residence at Trinity College Dublin, my parents’ garden, trains, the shore of Lake Geneva, and the old university li- brary in Heidelberg. I would like to express my deep gratefulness to the people who have accompanied me on the way and far beyond it. First of all, I would like to thank my awesome supervisor Dr. Sami Lehti for providing me the opportunity to do this exciting research, providing valuable help and straight feed- back, letting me work independently and try things my own way, reading my thesis and sending me comments even during his holiday, and granting me a lot of flexibility in all practical matters. I also thank Prof. Paula Eerola for always being there to provide help and good advice, reading my thesis, and showing genuine interest in my progress. My warmest regards go to the rest of my fellow charged Higgs hunters: Dr. Alexandros Attikis, Dr. Cristina Ferro, Doc. Ritva Kinnunen, Dr. Matti Kortelainen, and Dr. Lauri Wendland. It was great fun and very instructive to work with you. Thank you for your advice and interest. Special thanks go to Matti for the computing assistance and the excellent tools he provided that made work a lot more efficient and pleasant. I thank all past and present members of the CMS collaboration for building and maintain- ing the experiment, the colleagues in the CERN accelerator departments for the excellent performance of the LHC, and the technical and administrative staffs at CERN. As my studies draw to a close
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