We study, employing an effective approach, the early LHC phenomenology of an isospin-singlet W', focusing on the process pp → W' → Wγ. We discuss how observation of this decay would be a hint of the compositeness of the

resonance, and present an estimate of the experimental reach in the 7TeV LHC run

Salvioni, Ennio

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DOI 10.1393/ncc/i2011-11051-4Colloquia: IFAE 2011IL NUOVO CIMENTO Vol. 34 C, N. 6 Novembre-Dicembre 2011Relevance of the decay W ′ → Wγ at the early LHCE. Salvioni(∗)CERN, Physics Department, Theory UnitCH-1211 Geneva 23, SwitzerlandDipartimento di Fisica, Universita` di Padova and INFN, Sezione di PadovaVia Marzolo 8, I-35131 Padova, Italy(ricevuto il 29 Luglio 2011; pubblicato online il 29 Novembre 2011)Summary. — We study, employing an effective approach, the early LHC phe-nomenology of an isospin-singlet W ′, focusing on the process pp → W ′ → Wγ. Wediscuss how observation of this decay would be a hint of the compositeness of theresonance, and present an estimate of the experimental reach in the 7TeV LHC run.PACS 12.60.-i – Models beyond the standard model.1. – IntroductionIn an effective approach, a W ′ can be broadly defined as a spin-1, color-singlet, unitelectric charge state. If we further require a renormalizable coupling to quarks (neededfor a sizable production cross section at the LHC), only 2 irreducible representationsof the Standard Model (SM) gauge group contain such a state: an isospin singlet withhypercharge Y = 1, and an iso-triplet with Y = 0 [1]. The latter contains also a neutralZ ′, which is almost degenerate with the charged W ′: the strong constraints on Z ′ fromelectroweak precision tests and Tevatron/LHC then imply that, in general, an iso-tripletW ′ needs to be heavier and more weakly coupled than its iso-singlet counterpart, whichhas no associated Z ′(1). In what follows we consider the early LHC phenomenology of a“weakly constrained” iso-singlet W ′, focusing on the W ′ →Wγ decay channel. For moredetails, including a discussion of the “first discovery” channels W ′ → jj, tb, see ref. [2]and references therein.(∗) E-mail: Ennio.Salvioni@cern.ch(1) This needs not be the case if the coupling to leptons of the isospin triplet is suppressed withrespect to its coupling to quarks.c© Societa` Italiana di Fisica 195196 E. SALVIONI2. – Phenomenological LagrangianThe Lagrangian containing all the renormalizable interactions between the extra vec-tor and the SM fields reads L = LSM +LV +LV−SM , where LSM is the SM Lagrangian,andLV = DμV −ν DνV +μ −DμV −ν DμV +ν + M˜2V +μV −μ+g242|H|2V +μV −μ − igBBμνV +μ V −ν ,LV−SM = V +μ(igHH†(DμH˜) +gq√2(VR)ijuiRγμdjR)+ h.c.We denoted the extra vector by V ±μ . The right-handed (RH) quark mixing matrix VR isarbitrary in our effective approach; we choose |VR| = 13, which gives the weakest boundsfrom K and B meson mixing [3]. Then, only few parameters describe the resonance:in addition to the W ′ mass, the couplings gH , cB ≡ gB/g′ and gq will be relevant forour discussion (the coupling g4 only affects, in a subleading way, the decay W ′ → Wh).In the mass-eigenstates basis for both vectors and fermions, the W ′ couples dominantlyto RH quark currents. However, W -W ′ mixing induces couplings of the resonance toWZ,Wγ,Wh and left-handed lepton currents. All these trilinear vertices are propor-tional to the W -W ′ mixing angle θˆ, which is given by tan(2θˆ) = 2Δ2/(m2Wˆ−M2), wherem2Wˆ= g2v2/4, Δ2 = gHgv2/(2√2) and M2 = M˜2 + g24v2/4 (we denote by g the SU(2)Lgauge coupling).3. – W ′ →Wγ decayThe partial decay width for W ′ → Wγ can be written, in the limit M2W ′  MW , asΓ(W ′ →Wγ) ≈MW ′(e2/96π)(cB + 1)2θˆ2(M2W ′/M2W ), and is therefore controlled by theparameters cB and θˆ. Before we discuss the LHC phenomenology, we need to ask whatare the current bounds on these two parameters.3.1. Theoretical constraints on cB . – The coupling cB is not significantly constrainedby current data. However, perturbative unitarity arguments imply that if the W ′ is anelementary gauge boson, then its gyromagnetic ratio gW ′ must be equal to 2 at the treelevel [4]. Since gW ′ = 2− cos2 θˆ (1+ cB), this implies cB = −1 at tree level. Therefore, ifthe W ′ is a gauge boson, the decay W ′ →Wγ is expected to be very suppressed, and outof the LHC reach. On the other hand, if the W ′ is a composite vector, the requirement ofpreservation of perturbative unitarity is relaxed, and cB = −1 can be realized (however,one still needs to check that the cutoff is sufficiently larger than MW ′ , see ref. [2]).3.2. Bounds on θˆ. – The W -W ′ mixing angle is constrained both by the electroweak Tparameter, and by semi-leptonic u→ d, s transitions. A detailed analysis of the boundscan be found in ref. [2]. For example, for a W ′ mass of 800GeV, the bound from T is|θˆ| < 10−3, whereas semi-leptonic processes give −1.6× 10−3 < gq θˆ < 1.7× 10−3 in thelimit of negligible CP phases in VR [5]. If such phases are larger, however, the latterbound gets relaxed: |gq θˆ| < 10−1÷−2 is obtained for maximal CP phases [3].RELEVANCE OF THE DECAY W ′ → Wγ AT THE EARLY LHC 197Fig. 1. – (Colour on-line) 5σ discovery prospects of the 7TeV LHC for the W ′ → Wγ → eνγprocess, for MW ′ = 800GeV, gq = 0.84g. The red curves show the expected number of eventsas a function of the parameters of our phenomenological Lagrangian, whereas the blue flatlines represent the number of events needed for a 5σ discovery, taking into account the SMbackground. The region shaded in grey is excluded at 95% CL by searches for resonancesdecaying into WZ at the Tevatron.4. – Search for W ′ →Wγ at the early LHCTo illustrate the early LHC prospects in the W ′ → Wγ channel, we choose a bench-mark point, namely a W ′ mass of 800GeV and a coupling to quarks gq = 0.84g, which isthe largest value allowed by current bounds [2]. We select decays of the W into an elec-tron and a neutrino, and apply a set of simple cuts on the eγE/ T final state, to enhance thesignal-to-background ratio [2]. The background considered is irreducible Wγ productionin the SM. Our results for the benchmark point chosen are displayed in fig. 1: discov-ery of the W ′ → Wγ decay is possible, with 5 fb−1 of integrated luminosity at a LHCcenter-of-mass energy of 7TeV, for |cB + 1| > 2÷ 3 and few × 10−3 < θˆ < 10−2. Whilesuch values of the mixing angle are in tension with the constraint from the T parameter,it is conceivable that a positive contribution to T from additional new physics (such as,e.g., a Z ′) may relax this tension. On the other hand, measurements of semi-leptonictransitions are compatible with such relatively large values of θˆ, provided the phases inVR are non-negligible.5. – ConclusionWe have presented the prospects for discovering at the early LHC the decayW ′ →Wγ, where the W ′ is an iso-singlet charged heavy vector. This process is ofspecial interest, since it is very suppressed if the W ′ is an elementary gauge boson, butcould be observable if the W ′ is a composite state, as discussed above. To conclude,the decay W ′ → Wγ should be searched for at the early LHC, because its observationwould be a hint of the compositeness of the W ′, and would thus shed some light on thetheoretical origin of the resonance after its first discovery in the jj or tb channels.198 E. SALVIONIREFERENCES[1] Del Aguila F., De Blas J. and Perez-Victoria M., JHEP, 09 (2010) 033,arXiv:1005.3998 [hep-ph].[2] Grojean C., Salvioni E. and Torre R., JHEP, 07 (2011) 002, arXiv:1103.2761 [hep-ph].[3] Langacker P. and Uma Sankar S., Phys. Rev. D, 40 (1989) 1569.[4] Ferrara S., Porrati M. and Telegdi V. L., Phys. Rev. D, 46 (1992) 3529.[5] Buras A. J., Gemmler K. and Isidori G., Nucl. Phys. B, 843 (2011) 107, arXiv:1007.1993[hep-ph].

Relevance of the decay W' → Wγ at the early LHC

We study, employing an effective approach, the early LHC phenomenology of an isospin-singlet W′, focusing on the process pp → W′ → Wγ. We discuss how observation of this decay would be a hint of the compositeness of the resonance, and present an estimate of the experimental reach in the 7TeV LHC run. © SocietàItaliana di Fisica

Salvioni E.

Archivio istituzionale della ricerca - Università di Padova

Relevance of the decay W′ → Wγ at the early LHC

http://eprints.bice.rm.cnr.it/17543/1/ncc10073.pdf

Relevance of the decay W' → Wγ at the early LHC

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