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Associated Charged Higgs Boson and Squark Production in the NUHM Model

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Associated Charged Higgs Boson and Squark Production in the NUHM Model UPTEC F10 010 Examensarbete 30 hp Februari 2010 Associated charged Higgs boson and squark production in the NUHM model Gustav Lund Abstract Associated charged Higgs boson and squark production in the NUHM model Gustav Lund Teknisk- naturvetenskaplig fakultet UTH-enheten Conventional searches for the charged Higgs boson using its production in association with Standard Model (SM) quarks is notoriously weak in the mid-tanB range. Hoping Besöksadress: to find an alternate channel to fill this gap, the production of the charged Higgs boson Ångströmlaboratoriet Lägerhyddsvägen 1 in association with supersymmetric squarks is studied. Using Monte Carlo generators Hus 4, Plan 0 the production at the LHC is simulated within the non universal Higgs mass model (NUHM). If the six parameters of the model (m0, m1/2, A0, tanB, u, mA) induce small Postadress: masses of the stop, sbottom and charged Higgs, the production cross section can be Box 536 751 21 Uppsala of the order pb. Through scans of the input parameter the cross section is maximized, with the requirement that the stop decays directly to a neutralino - Telefon: simplifying detection, in the point (m0, m1/2, A0, tanB, u, mA) = (190, 187, -1147, 018 – 471 30 03 179, 745, 13.2) where the cross section is 559 fb. Telefax: 018 – 471 30 00 The production is compared to the irreducible backgrounds stop, stop, t, tbar and t, tbar + 2 jets. The former poses no severe constraints and can be easily removed Hemsida: using appropriate cuts. The latter, SM background, has a cross section almost 1000 http://www.teknat.uu.se/student times larger and strong cuts must be imposed to suppress it. Neglecting hadronization and systematic effects, we show that a 5 sigma discovery is possible at 133 fb-1. In this range, mH+ = 194 GeV and tanB = 13.2, other channels have little or no prospects of detecting the charged Higgs and the studied process shows good prospects for complementing charged Higgs searches at the LHC in the mid-tanB range. Handledare: Johan Rathsman Ämnesgranskare: Gunnar Ingelman Examinator: Tomas Nyberg ISSN: 1401-5757, UPTEC F10 010 1 Popul¨arvetenskaplig sammanfattning p˚asvenska F¨or100 ˚arsedan utf¨ordes experimentell partikelfysik p˚aett radikalt annorlunda s¨att ¨anidag. Exper- imenten, som studerade till exempel elektroner och radioaktiv str˚alning och hur dessa v¨axelverkade med materia, kunde utf¨oras p˚aett skrivbord i ett vanligt laboratorium. Detta eftersom elektroner ¨ar stabila och finns ¨overallt i oss och omkring oss och radioaktiv str˚alning bildas i en stadig takt som till˚ater oss att studera den i lugn och ro. De nya, tyngre och instabila partiklar som uppt¨acktes under seklets f¨orsta h¨alft, till exempel elektronens storebror muonen som uppt¨acktes i kosmisk str˚alning, kr¨avde dock mycket st¨orre maskiner. Detta eftersom energin till deras stora massa m˚aste tas fr˚an r¨orelseenergin hos andra snabbt f¨ardandes partiklar och deras snabba s¨onderfall kr¨avde att m˚anga partiklar hela tiden kunde produceras p˚anytt. P˚amitten av 1900-talet ins˚agsbehovet av gemensamt sammarbete f¨oratt ha r˚admed de dimensioner som kr¨avdes av dessa nya maskiner och tolv Europeiska l¨ander, inklusive Sverige, gick ihop och grun- dade CERN(europeiska centret f¨ork¨arnforskning) i Gen`eve. Under slutet av 2009 startades d¨arLHC, the Large Hadron Collider, och data f¨orv¨antas b¨orja str¨ommain under 2010. LHC accelererar tv˚a str˚alar av protoner i motg˚aende banor i en 27 km underjordisk tunnel intill Gen`evesj¨oni Schweiz. Hundratusentals g˚anger per sekund, n¨arprotonstr˚alarna till˚ats korsa varann, kolliderar protoner med varandra i ljusets hastighet och enorma m¨angder av partiklar bildas och skjuts ut ˚atalla h˚all. Endast ett f˚atal av dessa kollisioner ¨arav intresse och den st¨orsta utmaningen ¨aratt identifiera dessa ¨over det brus som bildas av alla andra partiklar. Den teori som f¨orn¨arvarande beskriver vad vi vet om partikelfysik kallas Standard Modellen och utvecklades p˚a60- och 70-talet. Den beskriver alla k¨anda partiklar och krafter, med undantag f¨or gravitationen, med otrolig precision. Dess enda f¨oruts¨agelse som ¨annu inte bekr¨aftats ¨arexsistensen av den s˚akallade Higgspartikeln som fysiker hoppas kunna p˚avisa med hj¨alp av LHC. Higgspartikeln beh¨ovs f¨oratt teoretiskt kunna f¨orklara varf¨ormassiva kraft¨overf¨oringspartiklar har massa och f¨oratt generera massa till all materia som simmar runt i en slags ”Higgs-v¨atska” och d¨armed blir olika tr¨oga eller massiva beroende p˚ahur h˚art de binder sig till Higgs-v¨atskan. Med denna sista bekr¨aftelse tycks d˚aStandard Modellen vara komplett. Vi tror oss dock veta att den inte kan beskriva allt, f¨orutom sin of¨orm˚agaatt beskriva gravitationen verkar ocks˚an˚agra av Standard Modellens massl¨osapartiklar faktiskt ha massa och tydliga tecken finns att den inte kommer kunna beskriva fysik vid de h¨ogre energier som LHC skall utforska. D¨arf¨orhar en uppsj¨oav andra teorier och utvidgningar formulerats, alla med olika f¨oruts¨agelser och mer eller mindre potential att kunna f¨orklara de fenomen som hittils undg˚att alla f¨ors¨oktill f¨orklaring. Denna rapport ¨arf¨orfattad inom ramen av en av dessa teorier kallad Supersymmetri. Den f¨orutsp˚ar en ny, supersymmetrisk partner till alla existerande partiklar och dubblar s˚aledes partikelinneh˚allet i Standardmodellen. Detta leder till matematiska korrektioner till olika massor som f˚arStandard Mod- ellen att l¨opa amok, perfekt tar ut varandra. Dessutom ger den en l¨amplig kandidat till den m¨orka materia som kosmologer och astronomer uppt¨ackt existerar ¨overallt i Universum. M˚anga av dessa nya partiklar hoppas Supersymmetri-f¨orespr˚akare hitta med hj¨alp av LHC de kommande ˚aren. I till¨agg till att dubbla alla vanliga partiklar f¨orutsp˚arSupersymmetri fem Higgs partiklar ist¨allet f¨orStandard Modellens enda. Tv˚aav dessa ¨arelektriskt laddade och det ¨arm¨ojligheten att uppt¨acka dem denna rapport skall fokusera sig p˚a. 2 Inom en Supersymmetrisk modell, NUHM, som genom olika antaganden blivit starkt begr¨ansad, un- ders¨oks m¨ojligheten att observera och massbest¨ammaden laddade Higgs partikeln vid LHC. Ett flertal datorprogram, som implementerar alla de antaganden som gjorts, anv¨ands f¨oratt simulera produktio- nen av laddad Higgs vid LHC. Vi f¨ors¨oker avg¨ora hur signalen f¨orladdad Higgs skulle komma att se ut i detektorerna beroende p˚a hur den produceras och s¨onderfaller. Denna signal j¨amf¨ors sedan med n˚agra av de bakgrunder som utg¨ors av produktionen av andra partiklar. P˚as˚avis avg¨ors om produktionen av laddad Higgs kan s¨arskiljas och hur l˚ang tid det skulle ta att f˚aett statistikt s¨akerst¨allt resultat. Den normala statistiska variationen av bakgrunden som vi vet existerar, kan annars tolkas som signal om m¨attiden ¨arf¨orkort. Resultatet ¨aratt under vissa speciella antaganden, som starkt begr¨ansar den redan begr¨ansade NUHM modellen, kan den laddade Higgs partikeln detekteras och dess massa best¨ammasp˚aungef¨arett till tv˚a˚arn¨arv¨alLHC kommit upp i sin t¨ankta produktionshastighet. J¨amf¨orelsevis skulle det ta avsev¨art mer ¨antre ˚aratt g¨ora samma sak genom att studera andra processer inom ramen f¨ormer allm¨anna teorier. 3 Contents 1 Popul¨arvetenskaplig sammanfattning p˚asvenska 2 2 Introduction 5 2.1 The Standard model . 5 2.1.1 Gauge invariance . 5 2.1.2 The massive bosons . 6 2.1.3 Strong interaction . 7 2.1.4 Problems with the Standard Model . 8 2.2 Supersymmetry . 9 2.2.1 Why supersymmetry . 9 2.2.2 The Wess-Zumino model . 10 2.2.3 Soft symmetry breaking . 12 2.2.4 Higgs sector and the NUHM . 12 2.2.5 Particle content of the MSSM and NUHM . 13 2.3 Project outline and motivation . 14 3 Simulation tools 15 3.1 Spectrum calculators . 15 3.1.1 SoftSUSY . 15 3.1.2 Susyhit ......................................... 16 3.2 Monte Carlo generators . 16 3.2.1 MadGraph/MadEvent . 16 3.2.2 Pythia.......................................... 16 3.3 ROOT............................................. 17 4 Analysis 17 4.1 Initial scans over parameter space . 17 4.1.1 H-t1-b1 coupling and squark masses . 17 4.1.2 Cross section . 25 4.1.3 Decays and detection . 25 4.2 Signal and background . 30 4.2.1 MSSM background . 32 4.2.2 SM background . 35 4.3 Comparison with other channels . 38 5 Conclusions, Outlook 39 6 Acknowledgments 39 4 2 Introduction The Standard model (SM) of physics was developed during the 60s and 70s and is one of the greatest achievements of modern physics. Precise measurements have been done to confirm its predictions to 3 the 10− level [1]. No significant deviations have been observed between theoretical predictions and experimental results, yet we expect the SM to fail at higher energies than those probed to date. At these energies several new models await experimental data to test their new predictions. Section 2.1 gives an introduction to the SM, the gauge principle and the Higgs mechanism. In section 2.2 the basic idea of Supersymmetry is introduced and finally the rest of the project is outlined in section 2.3. The reader is assumed to be familiar with basic Quantum field theory. If well aquainted with supersymmetry he or she can can skip to section 2.3. Throughout this paper units of ~ = c = 1 will be used. 2.1 The Standard model 2.1.1 Gauge invariance The SM treats three of the four known forces; the strong, the weak and the electromagnetic force.
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