Improved sensitivity to charged Higgs searches un top quark decays t bh + b(τ + ν τ ) at the LHC using τ polarisation and multivariate techniques

Transcription

1 Iproved sensitivity to chared is searches un top quark decays t b + b(τ + ν τ ) at the LC usin τ polarisation and ultivariate techniques Universidad Autónoa de Madrid E-ail: fernando.barreiro@ua.es We present an analysis with iproved sensitivity to the liht ( t b ) chared is searches in the top quark decays t b + b(τ + ν τ )+c.c. in the t t and sinle top production processes at the LC. We note that the τ + arisin fro the + (W + ) decay are predoinantly riht (resp. left) hand polarized. We focus on the doinant hadronic one-pron decay τ ± ρ ± ν τ π ± π ν τ. We use distributions as the fractional enery of the chared pion track with respect to the total τ-jet enery and the cosine of the helicity anle ψ to feed and train a BDT classifier within TMVA to separate the sinal for + production fro the doinant SM backround. PoS(CARGED ) Prospects for Chared is Discovery at Colliders - Chared, October 8-, Upssala University, Sweden Speaker. This work, done in collaboration with A. Ali and J. Llorente, is supported by DESY and MICINN (Spain) under contract FPA-8-. F.B. acknowledes partial financial support by Fundacion Cajaadrid. c Copyriht owned by the author(s) under the ters of the Creative Coons Attribution-NonCoercial-ShareAlike Licence.

2 Chared is searches at the LC. Theoretical backround The is sector of the Minial Supersyetric Standard Model (MSSM) is coposed of two is doublets, Φ u, Φ d with different vacuu expectation values, v u, v d respectively, which are related by tanβ = v u /v d. This leads to five different is particles, naely A, h, and ±. On this work, we focus on the last ones, ±, which can be produced at the LC via the top quark decays t + b. The relevant part of the interaction Laranian can be written as: L + = MW + cotβv ij ui ū i P L d j + tanβv ij d j ū i P R d j + tanβ l j ν j P R l j + h.c. (.) Where V ij are the CKM flavour ixin atrix eleents. If we focus on the leptonic ter, we see that + couples to riht-handed leptons, whereas in the Standard Model (SM), the W bosons couple to the leptons via the chared current interaction Laranian, which involves only lefthanded leptons: L CC = W µ + [ ν j γ µ P L l j +V ij ū i γ µ P L d j ]+h.c. (.) MW Where P L and P R are the left-riht chirality operators, defined as P R,L = (±γ ). These differences on the couplins lead to different anular distributions of the τ decay products, which will be exploited on our analysis.. Relevant observables We consider the process pp t t where both top quarks decay via Wb. The electron or uon coin fro the leptonic W decays are used for trierin purposes. The W + is left to decay into a τ + ν τ. This is our irreducible backround process. Our sinal proceeds when the t b +. The sinal and backround events for top pair and sinle top production have been enerated usin Pythia. and they are required to pass standard acceptance and trier cuts, []. See fiure for the relevant Feynan diaras and [] for recent analysis explorin this channel. + τ + W + τ + PoS(CARGED ) ντ ντ t t b b b b t e t e W W νe νe Fiure : Sinal and SM irreducible backround processes for t + b

3 Chared is searches at the LC. elicity anle ψ The helicity anle is defined as the anle between the outoin direction of the top quark and the ρ eson in the reference frae where the W boson (or the + in our case) is at rest. It can be aproxiated by the followin expression: cosψ = p t p ρ p t p ρ ρb t W (.) The distributions for cosψ for + asses of 9,, and GeV, as well as for the SM backround t W + b are shown in fiure dn/d(cos ψ). τ enery ratio = 9 GeV = GeV = GeV = GeV cos ψ dn/d(cos ψ) Fiure : cosψ for different + asses and the In this study, we only allow τ leptons to decay via the -pron channel τ ρν τ π + π ν τ. Thus, another observable which could help in the classification of + events is the enery and p T of the chared pion noralized to those of the τ-jet i.e. λ e = E π E ρ ; λ p = pπ T p ρ T The distributions for these variables are shown in fiure. τ-jet enery ratio to b-jet enery cos ψ (.) PoS(CARGED ) As the + in this study is heavier than the W, τ-jets coin fro assive is bosons are harder than τ-jets coin fro W bosons. On the other hand, the b-jets coin fro the W + production vertex are harder than the ones coin fro + vertices. Therefore, a ood discriinatin variable would be the ratio fro the τ-jet enery and p T to the correspondin b-jet kineatical quantity. The distributions are shown in the riht part of fiure. The identification of this b-jet (there are two in every event) could be done usin topoloical correlations between the + decay products (the τ-jet) with respect to the b-jet, either in R = ( η) +( φ) between the b-jet fro the + production vertex and the τ-jet fro the + decay, or else usin the fact that this b-jet has opposite chare to the chared pion fro the τ decay.

4 p e Chared is searches at the LC dn/dλ e = 9 GeV dn/dλ e = 9 GeV = GeV = GeV = GeV..... e..... E(π)/E(τ-jet) E(π)/E(τ-jet) E(τ-jet)/E(b) E(τ-jet)/E(b) dn/dλ p = 9 GeV p (π)/p (τ-jet) T T dn/dλ p p (π)/p (τ-jet) T T 8 8 = 9 GeV = GeV = GeV = GeV..... p (τ-jet)/p (b) T T p..... Fiure : Enery and oentu ratio fro the pion to the τ-jet. Ratio τ to b enery and p T. + production in the sinle top channel p (τ-jet)/p (b) T T The sinle top channel has the advantae that the ass of the chared is boson can be easured. We define the W transverse ass as W T = p τ T Eiss T ( cos φ) (.) This variable ives a ood discriination between sinal and backround, but does not help to easure the actual + ass. To this purpose, we define (see []) ( T ) = t +(p l T + p b T + p iss T ) p b T ( p l T + p iss T ) (.) The distributions for the variables defined in equations. and. are shown in fiure. TMVA results Once we have these variables siulated for the sinal and backround processes, we use the to train a Boosted Decision Tree () in order to obtain the best separation between the sinal and backround processes. The classification of a iven event would be done attendin to these distributions. We do this separately for the t t and sinle top processes. PoS(CARGED ). Top pair production For the t t channel, TMVA provides a ood discriination between processes ediated by and W chared bosons. Fiure shows the output distributions for the sinal and backround processes in this channel (left) as well as the ROC curves, efficiency versus backround rejection (riht). For the less favorable case ( = 9 GeV), a backround rejection of 9% can be achieved with a % sinal efficiency. Considerin the less preferred case tanβ = for which B(t + b). (see [7]) and takin into account the fact that for a center of ass enery of

5 T T Chared is searches at the LC () dn/d 8 7 = 9 GeV = GeV = GeV = GeV () dn/d T () T [GeV] () T [GeV] (/N)dN/dx (/N)dN/dx 7 = 9 GeV () dn/d = GeV = 9 GeV = GeV = GeV = GeV 8 8 () T [GeV] () dn/d T Fiure : Transverse asses for the W and chared is (/N)dN/dx (/N)dN/dx = GeV = GeV = 9 GeV = GeV Fiure : outputs for the t t channel () T [GeV] = GeV = GeV s = TeV the t t cross section is σ t t 87 pb, with an interated luinosity of fb, one will et ( ) sinal (resp. backround) events. The production via the two possible chare conjuate final states has been taken into account. The expected sinificance therefore would be: PoS(CARGED ) S = N sinal Nbackround = This will be reduced by a factor of when includin efficiencies for trier, b jet tain and τ identification. For a center of ass enery of s = 7 TeV, the cross sections are reduced by a factor of, so that our expected sinificance will be reduced by an additional factor of.

6 Chared is searches at the LC. Sinle top production The sinle top channel achieves an even better discriination, as the ass distributions provide an additional handle for the separation between the two processes. Fiure shows the outputs for each ass (left) and the ROC curves (riht). (/N)dN/dx (/N)dN/dx = 9 GeV = GeV (/N)dN/dx (/N)dN/dx = GeV = GeV = 9 GeV = GeV Fiure : outputs for the sinle top channel = GeV = GeV A siilar calculation for the sinle top channel (σ pb at s = TeV) yields a sinificance of S 8, which will be reduced by trier and acceptance cuts as discussed above. To suarize, we believe that if a liht + as expected in the MSSM exists, we will not iss it, by lookin at the one pron hadronic τ decays, for any value of tanβ.. Acknowledeents I would like to thank the oranizers of Chared is for their friendly hospitality. References [] A. Ali, F. Barreiro, J.Llorente, [Eur. Phys. J. C7 () 77.] [] The ATLAS Collaboration [JEP () 9] [] The ATLAS Collaboration [arxiv:9. (hep-ex)]. [] The ATLAS Collaboration, ATLAS-NOTE ATL-PYS-PUB-- (). [] The ATLAS Collaboration, ATLAS-NOTE ATL-PYS-PUB-- (). [] E. Gross, O. Vitells, Phys. Rev. D8, (). [arxiv:97.7 (hep-ph)]. [7] A. Sopczak, PoS CARGED8, (8). arxiv:97.98 [hep-ph] PoS(CARGED )