Misura del feed-down della chib(3P) in Upsilon(3S)

Sa primu evidèntzia de s'esistènzia de su mesone chib(3P) est istada iscoberta dae sa Collaboratziòne ATLAS in su 2011. Su traballu presentadu in custa tesi est a pitzus de sa mesura de sa fratzione de Upsilon (3S), chi fiat pensàda chenze feed-down fintzas a s'iscoberta de sa chib(3P), unu produtu de su decadimentu de sa chib(3P). Custu cuntributu est apretàdu impreende sa fratzione sigma(chib(3P)->Upsilon(3S)+gamma)/sigma(Upsilon^{inc}(3S)), intra sas Upsilon (3S) chi benent de su decadimentu de sa chi_b(3P) e sa produtzione inclusiva, est a ischire totu cantu funti is Upsilon(3S). Sa ricostrutzione est istada fata cun is mesones Upsilon decadende in duos leptones Upsilon->mu+ mu- e, po sa ricostrutzione de su mesone chi_b, cun is fotones cunvertidos. S'efitzèntzia esti istudiada dae sa fingidura Monte Carlo. Custa tesi istudiat su 90% de su campione regòllidu dae s'esperimentu CMS in su 2012 in collisiones pp a sqrt{s}=8 TeV. Su contributu in su tretu chinemàticu |y_{Upsilon}|<1.0, p_{T,Upsilon}>9.5 GeV e p_{T,gamma}> 0.5 GeV est istadu apretadu e bàlet (11.7 +/- 3.5)%, in ue s'errore impreadu est s'istatìsticu.

The first evidence of the existence of the chib(3P) meson has been found by the ATLAS Collaboration in 2011. The work presented in this thesis regards the measurement of the fraction of Upsilon (3S), which was considered feeddown-free until chib(3P) discovery, originating from the chib(3P) decay. This contribution is estimated from the ratio sigma(chib(3P) -> Upsilon(3S)+gamma)/sigma(Upsilon^{inc}(3S)) between the Upsilon(3S) produced from the chib(3P) decay and the inclusive Upsilon(3S) production, that is both the directly produced Upsilon(3S) and those coming from the chi_b(3P) decay. The Upsilon mesons are reconstructed in the leptonic decay Upsilon\->mu+ mu- and converted photons are used to reconstruct the chib candidates. The efficiency is studied via Monte Carlo simulations. This thesis studied approximately 90%of the data collected by CMS in 2012 in pp collisions at sqrt{s}=8 TeV. The feed-down contribution in the kinematic range |y_{Upsilon}|<1.0, p_{T,Upsilon}>9.5 GeV and p_{T,gamma}> 0.5 GeV is measured to be (11.7 +/- 3.5)%, where the reported uncertainty is statistical. At the time of the writing of this thesis, neither previous experimental studies nor theoretical estimations have been done.