Four-top production and top pair production in association with missing energy at e+e− colliders are sensitive probes of beyond-the-Standard-Model physics. We consider Standard Model extensions containing a new U(1)' which couples preferably to the most massive states of the SMsuch as the top quark or Dark Matter but has suppressed couplings to all the light states of the SM, as inspired by

Randall-Sundrum–like setups or theories of partial fermion compositeness. These simple models are poorly constrained by experimental data but lead to striking new signatures at colliders. In this paper we consider Z' �production in association with a top quark pair in 3TeV e+e− collisions at CLIC, leading to interesting four-top final states and tt + Emiss events

Battaglia, M.

Servant, G.

English

arXiv

Four-top production and top pair production in association with missing energy at e+e- colliders are sensitive probes of beyond-the-Standard-Model physics. We consider Standard Model (SM) extensions containing a new U(1)' which couples preferably to the most massive states of the SM such as the top quark or Dark Matter but has suppressed couplings to all the light states of the SM, as inspired by Randall-Sundrum-like setups or theories of partial fermion compositeness. These simple models are poorly constrained by experimental data but lead to striking new signatures at colliders. In this note we consider Z' production in association with a top quark pair in 3 TeV e+e- collisions at CLIC, leading to interesting four-top final states and ttbar+missing energy events

Battaglia, Marco

Servant, Geraldine

CERN Document Server

Four-top production and t tbar + missing energy events at multi TeV e+e- colliders

Scientific Open-access Literature Archive and Repository

DOI 10.1393/ncc/i2010-10611-4Colloquia: LC09IL NUOVO CIMENTO Vol. 33 C, N. 2 Marzo-Aprile 2010Four-top production and tt + missing energy eventsat multi TeV e+e− collidersM. Battaglia(1)(2) and G. Servant(1)(3)(1) CERN Physics Department - CH-1211 Geneva 23, Switzerland(2) Santa Cruz Inst. of Part. Phys. Santa Cruz, University of California - CA 95064, USA(3) Institut de physique the´orique, CEA/Saclay - 91191 Gif-sur-Yvette ce´dex, France(ricevuto il 30 Aprile 2010; approvato il 30 Maggio 2010; pubblicato online il 15 Luglio 2010)Summary. — Four-top production and top pair production in association withmissing energy at e+e− colliders are sensitive probes of beyond-the-Standard-Modelphysics. We consider Standard Model extensions containing a new U(1)′ whichcouples preferably to the most massive states of the SM such as the top quark or DarkMatter but has suppressed couplings to all the light states of the SM, as inspired byRandall-Sundrum–like setups or theories of partial fermion compositeness. Thesesimple models are poorly constrained by experimental data but lead to striking newsignatures at colliders. In this paper we consider Z′ production in association witha top quark pair in 3TeV e+e− collisions at CLIC, leading to interesting four-topfinal states and tt + Emiss events.PACS 12.60.cn – Extensions of electroweak gauge sector.PACS 29.20.db – Storage rings and colliders.1. – Theoretical frameworkWe introduce a very simple effective theory in which the Standard Model is sup-plemented by a spontaneously broken U(1)′ gauge symmetry, the massive gauge bosonof which acts as a portal to the Standard Model (SM) by coupling to the top quark.This type of setup arises naturally in models of “partial compositeness” in which thetop quark acquires its large mass (after electroweak symmetry breaking) through largemixing with composite states in a new strong sector, as in 4d duals to Randall-Sundrum(RS) Models [1]. The effective Lagrangian contains [2],L = LSM − 14F′μνF′μν + M2Z′Z′μZ′μ +χ2Fˆ ′μνFˆμνY + gZ′t t¯γμPRZ′μt(1)+iN¯γμ(∂μ − igZ′N PRZ ′μ)N + MN N¯N,c© Societa` Italiana di Fisica 203204 M. BATTAGLIA and G. SERVANTwhere N is a Dirac fermion which is a singlet under the SM gauge interactions butis charged under U(1)′ and turns out to play the role of a weakly interacting massiveparticle (WIMP). F ′μν (FYμν) is the usual Abelian field strength for the Z′ (hyper-chargeboson), gZ′t is the Z′ coupling to right-handed top quarks, and gZ′N is the coupling toright-handed WIMPs. MN is the WIMP mass. One can easily include a coupling tothe left-handed top (and bottom). Our choice to ignore such a coupling fits well withtypical RS models [1], balancing the need for a large top Yukawa interaction with controlover corrections to precision electroweak observables. The parameter χ encapsulates thestrength of kinetic mixing between the Z ′ and SM hyper-charge bosons.We have included hyper-charge-Z ′ kinetic mixing through the term proportional to χ.Such a term is consistent with the gauge symmetries, and even if absent in the UV, will begenerated in the IR description by loops of top quarks(1). The kinetic mixing parameter χgenerates an effective coupling of SM states to the Z ′, and through electroweak symmetrybreaking, mass mixing of the Z ′ with the SM Z gauge boson resulting in a coupling ofN to the SM Z boson.In this approach, most of the Standard Model is fundamental, but with the WIMP,Higgs, and right-handed top largely composite. The Higgs couples strongly to compositefields, and the amount of admixture in a given SM fermion determines its mass [3]. Inthis picture, the Z ′ is one of the higher resonances, built out of the same preons as theWIMP and tR. RS theories provide a very motivated picture of the UV physics, but moregenerically, in any theory (not necessarily with approximate scale invariance) containingcomposite WIMPs and composite top quarks belonging to a common sector one wouldexpect strong couplings between them as a residual of the strong force which binds them,and perhaps negligible coupling to the rest of the Standard Model.If the Z ′ mixes with the electroweak bosons, this results in strong constraints fromprecision data. We circumvent these constraints by considering a Z ′ whose mixing withthe Z is kinetic. At large Z ′ masses this is not operationally different from the mass-mixing case, but it allows us to consider lower mass Z ′s which are not ruled out byprecision data.2. – Signals at CLICSince the coupling of Z ′ to light SM fermions is suppressed by the small kinetic mixingfactor, the best probe of the dark sector is through the top portal. In particular, theZ ′ can be produced by being radiated from top quarks, as illustrated in fig. 1. In fig. 2,we show the leading-order cross-section at CLIC for ttZ ′ production, as calculated byCalcHep 2.5.4 [4]. Depending on the masses and couplings, the Z ′ will predominantlydecay into tt, NN¯ , or into light fermions. Decays into top quarks lead to four-topevents with a very large cross-section compared to the SM four-top rate, which leads toa characteristic same-sign di-lepton signature [5, 6] (see also [7] for studies of a ttWWfinal state at the LHC). The right-handed nature of the Z ′ coupling to top quarks impliesthat top polarisation also provides an interesting observable. When the Z ′ decays intoWIMPs, a tt + missing energy final state results, which is particularly compelling athigh energy e+e− colliders such as CLIC.We present preliminary results of an analysis aimed at characterising these signalsin multi-TeV e+e− collisions. Signal events are generated using CalcHep, assuming(1) χ can be engineered to vanish in the UV, for example, by embedding U(1)′ into a largergauge group which breaks down at scales of order MZ′ .FOUR-TOP PRODUCTION AND tt + MISSING ENERGY EVENTS ETC. 205Fig. 1. – Z′ mediated production of tttt and tt + Emiss at CLIC.gZ′t = gZ′N = 3, and subsequently hadronised with Pythia 6.125 [8]. We assume thebeams to be unpolarised. Events are processed through full detector simulation usingthe Geant-4-based Mokka [9] program and reconstructed with Marlin-based [10]processors assuming a version of the ILD detector [11], modified for physics at CLIC. Theperformances most relevant to the analysis of these events are a relative parton energyresolution at 1TeV δ90E/E  0.08–0.10, charged particle momentum resolution δpt/p2t =2×10−5 GeV−1, track extrapolation resolution of 5 (μm)⊕ 13(μmGeV−1)pt(GeV) . We perform jetclustering forcing the number of jets to the number of top quarks expected in the signalfinal state of interest. Jet reconstruction is performed with the Durham algorithm [12].In this preliminary study we do not account for machine-induced backgrounds and donot attempt to perform a detailed top reconstruction through the identification of its Wbdecay, which will need to be evaluated in a more refined analysis. We now discuss bothsignatures in turn.2.1. e+e− → tt¯ + Emiss. – The Standard Model background for e+e− → tt¯ + Emissis dominated by e+e− → tt¯ + νeνe, which, at 3TeV, has a cross-section of 4.1 fb. Bycomparison, the signal cross-section for MZ′ = 200GeV, MN = 97GeV, χ = 10−3 is16.5 fb (note that this choice of parameters leads to the correct relic density for darkmatter [2]). For a higher mass MZ′ = 320GeV, the cross-section is 7.6 fb. The eventselection requires 2 ≤ Njet ≤ 6, where Njet is the number of natural jets obtained withthe Durham algorithm for ycut = 0.0025, more than 50 particles with pt > 0.5GeV,Fig. 2. – ttZ′ production at CLIC as a function of the Z′ mass.206 M. BATTAGLIA and G. SERVANTFig. 3. – Signal e+e− → tt¯Z′ → tt¯NN¯ events at 3TeV for MZ′ = 200GeV, MN = 97GeV.Left: display of a reconstructed event. Right: reconstructed Emiss distribution. The points witherror bars show the reconstructed events for 200 fb−1 of statistics and the filled histogram thecontribution of the SM tt¯νν¯ background after event pre-selection.total reconstructed energy in the detector larger than 500GeV, transverse energy largerthan 250GeV, event thrust larger than 0.75 and sphericity lower than 0.30. The missingenergy in the event, Emiss, appears to be the best discriminating variable against theSM tt¯νν¯ “irreducible” background. Figure 3 shows the Emiss distribution for signal andbackground events. We require Emiss < 1900GeV. Events fulfilling these criteria areclustered into two jets. We further require both jets to have | cos θ| < 0.90, where θ isthe polar angle of the jet axis. Next we compute the invariant mass, Mjet of each jet. Werequire the invariant mass of both jets to be in the range 125GeV < Mjet < 225GeV,around the top quark mass. After this selection, there are 648 signal and 22 SM tt¯νeν¯ebackground events, for 200 fb−1 of integrated luminosity at 3TeV. Due to the finiteparton energy resolution and the beamstrahlung effects, it is not feasible to reconstructthe Z ′ mass from the mass of the system recoiling against the tt¯ pair. Instead, we canmeasure the invariant mass of the tt¯ system. The upper endpoint of this distribution iswell preserved after reconstruction (see fig. 4) and can be used to estimate the Z ′ mass.2.2. e+e− → tt¯tt¯. – The four top quark final state represents a spectacular signature,with minimal background from SM processes (see fig. 5). The analysis of the invariantmass of the di-top system offers a mean of measuring the Z ′ mass. The main experimentalchallenge for this analysis is the accurate reconstruction of the twelve parton final state.In this preliminary study, the event selection procedure is similar to that adopted forthe tt¯ + Emiss channel with the exception that in this case there is no missing energyexpected, except through leptonic decays in the top decay chain.The SM cross-section for e+e− → tt¯tt¯ at 3TeV is 0.03 fb while the signal cross-sectionfor MZ′ = 360GeV is 4.2 fb. The dominant SM background will actually come fromtt¯W+jets. The cross-section for tt¯Wjj is estimated to 0.4 fb while that for tt¯WW+N jetsto 0.6 fb(2). We request events to have more 50 particles with pt > 0.5GeV, totalreconstructed energy in the detector larger than 2.5TeV, transverse energy larger than1TeV, event thrust larger than 0.96, sphericity lower than 0.50 and more than four jets(2) We thank Marcel Vos for providing these estimates.FOUR-TOP PRODUCTION AND tt + MISSING ENERGY EVENTS ETC. 207Fig. 4. – (Left) Difference between the energy of the two top quarks. (Right) Invariant mass ofthe tt¯ pair. The points with error bars show the reconstructed events for 200 fb−1 of statisticsand the filled histogram the contribution of the SM tt¯νeν¯e background after event selection. Thecontinuous line is the distribution for signal events at generator level. The right panel showsthe tt¯ invariant mass at generator level for two values of the Z′ mass.using Durham clustering. Events surviving these cuts are forced into four jets and akinematic fit is applied to improve the parton energy resolution. We use a port of thePufitc kinematic fit algorithm [13]. Figure 5 shows the tt¯ invariant mass distributionfor 1 ab−1 of integrated luminosity at 3TeV. The signal is clearly visible over a negligibleSM background. For MZ′ = 360GeV there are 240 signal events in the mass peak aroundthe generated Z ′ mass. The Z ′ mass can be determined with a statistical accuracy of 3.5GeV.To conclude, multi-top production is an expected signature for several processes at,and beyond, the TeV frontier. It has the advantage that the irreducible SM backgroundis small so that multi-top detection can provide clean probes of new physics. It alsocarries experimental challenges from the reconstruction of a large number of partons,with missing energy and leptons. The prospects for studying these signatures at theFig. 5. – Signal e+e− → tt¯Z′ → tt¯tt¯ events at 3TeV. Left: display of a reconstructed event.Right: invariant mass of the pair tt¯ of lowest energy in e+e− → tt¯tt¯ events after kinematic fitwith the SM backgrounds for 1 ab−1 of integrated luminosity and MZ′ = 360GeV.208 M. BATTAGLIA and G. SERVANTLHC are promising [6]. In this contribution, we started investigating these signaturesat e+e− colliders using a template model but it is clear that this work can be useful toother models having similar final states.∗ ∗ ∗We acknowledge useful discussion with J.-J. Blaising, M. Vos and A. Es-pargilie`re. We are grateful to the organisers of the LC09 workshop in Perugia fortheir invitation to an interesting and well-organised meeting. The work of GS is sup-ported by the European Research Council Starting Grant Cosmo@LHC.REFERENCES[1] Randall L. and Sundrum R., Phys. Rev. Lett., 83 (1999) 3370 [arXiv:hep-ph/9905221];Agashe K., Delgado A. and Sundrum R., Ann. Phys. (N.Y.), 304 (2003) 145[arXiv:hep-ph/0212028].[2] Jackson C. B., Servant G., Shaughnessy G., Tait T. M. P. and Taoso M., JCAP,1004 (2010) 004 [arXiv:0912.0004 [hep-ph]].[3] Contino R., Nomura Y. and Pomarol A., Nucl. Phys. B, 671 (2003) 148 [arXiv:hep-ph/0306259]; Agashe K., Contino R. and Pomarol A., Nucl. Phys. B, 719 (2005)165 [arXiv:hep-ph/0412089]; Agashe K. and Contino R., Nucl. Phys. B, 742 (2006) 59[arXiv:hep-ph/0510164]; Contino R., Kramer T., Son M. and Sundrum R., JHEP,0705 (2007) 074 [arXiv:hep-ph/0612180].[4] Pukhov A., arXiv:hep-ph/0412191.[5] Lillie B., Shu J. and Tait T. M. P., JHEP, 0804 (2008) 087 [arXiv:0712.3057 [hep-ph]]; Pomarol A. and Serra J., Phys. Rev. D, 78 (2008) 074026 [arXiv:0806.3247[hep-ph]]; Kumar K., Tait T. M. P. and Vega-Morales R., JHEP, 0905 (2009) 022[arXiv:0901.3808 [hep-ph]].[6] Gauthier L. and Servant G., in preparation.[7] Contino R. and Servant G., JHEP, 0806 (2008) 026 [arXiv:0801.1679 [hep-ph]];Mrazek J. and Wulzer A., arXiv:0909.3977 [hep-ph].[8] Sjostrand T., Mrenna S. and Skands P. Z., JHEP, 0605 (2006) 026 [arXiv:hep-ph/0603175].[9] Mora de Freitas P., in Proceedings of the International Conference on Linear Colliders(LCWS 04), Vol. I (Ed. de l’Ecole Polytechnique, Paris) 2004, p. 437.[10] Gaede F., Nucl. Instrum. Methods A, 559 (2006) 177.[11] Stoeck H. et al. (The ILD Concept Group), The International Large Detector - Letterof Intent, March 2009.[12] Catani S., Dokshitzer Y. L., Olsson M., Turnock G. and Webber B. R., Phys.Lett. B, 269 (1991) 432.[13] Abreu P. et al. (DELPHI Collaboration), Eur. Phys. J. C, 2 (1998) 581.

Four-top production and tt + missing energy events at multi TeV e+e− colliders

Abstract

Similar works

Full text

Available Versions

CERN Document Server

Scientific Open-access Literature Archive and Repository