We study the jet associated production of the new quarks predicted by the
$E_{6}$ GUT model at the LHC. Generator level considerations are made for
different mass values of the lightest of the new quarks to investigate its
discovery potential and the prospects for obtaining its mixing angle to the
Standard Model quarks. We find that after 100 fb$^{-1}$ of data taking, it is
possible to discover the new quark with a significance more than 5$\sigma$ up
to a mass of 1500 GeV. If no discovery is made, it is possible to constrain the
mass vs quark mixing angle plane.Comment: uses axodraw.st

Sultansoy, S.

Unel, G.

Yilmaz, M.

English

arXiv

We study the jet associated production of the new quarks predicted by the $E_{6}$ GUT model at the LHC. Generator level considerations are made for different mass values of the lightest of the new quarks to investigate its discovery potential and the prospects for obtaining its mixing angle to the Standard Model quarks. We find that after 100 fb$^{-1}$ of data taking, it is possible to discover the new quark with a significance more than 5$\sigma$ up to a mass of 1500 GeV. If no discovery is made, it is possible to constrain the mass vs quark mixing angle plane.We study the jet associated production of the new quarks predicted by the $E_{6}$ GUT model at the LHC. Generator level considerations are made for different mass values of the lightest of the new quarks to investigate its discovery potential and the prospects for obtaining its mixing angle to the Standard Model quarks. We find that after 100 fb$^{-1}$ of data taking, it is possible to discover the new quark with a significance more than 5$\sigma$ up to a mass of 1500 GeV. If no discovery is made, it is possible to constrain the mass vs quark mixing angle plane

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arXiv:hep-ex/0608041 v1   15 Aug 2006The single production of the lightest E6 isosinglet quark at the LHCS. Sultansoy1,2, G. Unel3,4 and M. Yilmaz217th August 20061Institute of Physics, Academy of Sciences, Baku, Azerbaijan.2Gazi University, Physics Department, Ankara, Turkey.3University of California at Irvine, Physics Department, USA.4CERN, Physics Department, Geneva, Switzerland.AbstractWe study the jet associated production of the new quarks predicted by the E6 GUT model at the LHC. Generator levelconsiderations are made for different mass values of the lightest of the new quarks to investigate its discovery potential andthe prospects for obtaining its mixing angle to the Standard Model quarks. We find that after 100 fb−1 of data taking, it ispossible to discover the new quark with a significance more than 5σ up to a mass of 1500 GeV. If no discovery is made, it ispossible to constrain the mass vs quark mixing angle plane.1 IntroductionOne possible way to unify the electroweak and the strong forces is embedding the Standard Model (SM) into a larger sym-metry group as the Grand Unified Theories (GUTs) suggest. One possible candidate, which is also favored as the low energyeffective theory of super-strings, is the E6 model [1, 2]. The E6 model predicts a number of new particles, among whichthere is an isosinglet quark per SM family. The forthcoming LHC accelerator will be the place to search for the new coloredobjects in general and in particular for isosinglet quarks, if their masses are within the LHC kinematical range. The currentexperimental limit on the mass of an isosinglet quark with a −1/3 electric charge is m > 199 GeV [3]. The distinguishablefeature of the isosinglet quarks is the existence of FCNCs at tree level as opposed to fourth SM family quarks. If a new quarkis observed, this feature can be used to distinguish between different models. The pair production of the isosinglet quarkswas previously studied for the Tevatron [4] and for the LHC [5]. The pair production cross section is practically independentof the mixing between the SM and the isosinglet quarks. This note addresses the discovery of the isosinglet quarks via theirsingle production at the LHC and the measurements of the mixing angle between the new and the SM quarks.2 The ModelWe will denote the isosinglet quarks for the first, second, and the third SM families by letters D,S,B, respectively. Inaccordance with the SM fermion mass hierarchy, we assume that mD < mS < mB and also that the intra-family mixingdominates the inter-family mixing. Further details of the model can be found in [5]. The mixing angle between the d and Dquarks will be represented by the Greek letter φ. Although the current limit on φ is | sinφ| < 0.07 [5], in this note we considera more conservative value, sinφ = 0.045, for the calculation of the cross sections and decay widths. For other values, bothof these two quantities can be scaled with a sin2 φ dependence. The branching ratios are about 67 % forD →W u and about33 % for D → Z d. If the masses of the SM quarks are generated by the Higgs mechanism, the branching ratios will bemodified to incorporate also the D → H d channel. For mH ≪ mD, the branching ratios become 50 % for CC, 25 % for NCand 25% for the Higgs channel. Further study of the Higgs boson in the context of D quark decays is in preparation [6].3 The Production at the LHCWe implemented the Lagrangian for the signal into a tree level Monte Carlo generator, CompHep v4.4.3 [7]. For both thesignal and the background studies, the contributions from sea quarks were also considered. The parton distribution functionutilized was CTEQ6L1 and the QCD scale was set to be the mass of the D quark for both signal and background processes.1Figure 1: Cross section in single D production as a function of D quark mass for different sinφ values.mD (GeV)σ (pb)10-310-210-11400 600 800 1000 1200 1400 1600 1800 2000Figure 2: The main tree level signal processesuuZdDedZdiagr.1u uZdDed Zdiagr.2u dW+uDed Zdiagr.3uDedZW+dudiagr.4ddZdDedZdiagr.5d dZdDed Zdiagr.6d ZDedZd ddiagr.7dDedZZdddiagr.82Figure 3: The transverse momentum distribution of the parton with the highest energy for the signal case.PT (GeV)dσ/dP (pb)10-810-710-610-510-40 200 400 600 800 1000 1200 1400The cross section for single production of the D quark for its mass up to 2 TeV and for various mixing angles is given infigure 1. The main tree level signal processes with their neutral current decays are given in figure 2. The remaining processesoriginating from the sea quarks contribute about 20 percent to the total signal cross section.Although this note considered a generator level study, various parameters of the ATLAS detector [9] such as the barrelcalorimeter geometrical acceptance, minimum angular distance for jet separation and minimum transverse momentum forjets[10] were taken into account. Four mass values (400, 800,1200, 1500 and 2000 GeV) were studied to investigate the massdependence of the discovery potential for this channel. The cuts common to all considered mass values are:PTp > 15GeV|ηp| < 3.2|ηZ | < 3.2Rp > 0.4MZp = MD ± 20GeVwhere p stands for any parton; R is the cone separation angle between two partons; ηp and ηZ are pseudorapidities of aparton and Z boson respectively; and PTp is the parton transverse momentum. The main property that allows discriminatingbetween the signal and the background is the transverse momentum distribution of the parton with the highest energy. Infigure 3, the transverse momentum distributions of the partons that survive the common cuts are shown for the signal case.One can observe that the distribution peaks at about one half of the D quark mass value. The same distribution for thebackground is given in figure 4, which shows rapidly decreasing differential cross section as a function of parton PT sincethe final state partons are originating directly from the initial partons.3.1 The discovery potentialThe results from the scanning of the signal significance for the hard jet PT are given in figure 5 for D quark massvalues under study. For each case, the optimal value is found by maximizing the significance (S/√B) and it is used forcalculating the effective cross sections presented in table 1. To obtain the actual number of events for each mass value, theee¯ and µµ¯ decays of the Z boson were considered for simplicity of reconstruction. Table 2 contains the expected numberof reconstructed events for both signal and background for 100 fb−1of data taking. Although the lepton identification andreconstruction efficiencies are not considered, one can note that the statistical significance at mD =1500 GeV, is above 5σafter one year of nominal luminosity run.3Figure 4: The transverse momentum distribution of the parton with the highest energy for the background case.PT (GeV)dσ/dP (pb)10-710-610-510-410-310-20 200 400 600 800 1000 1200 1400Figure 5: Dependence of the signal significance on hard jet PT for different D quark mass valuesPT jet (GeV)significance123456780 100 200 300 400 500 600 700 800 900Table 1: The signal and background effective cross sections before the Z decay and after the optimal cuts, together with theD quark width in GeV for each considered mass.MD(GeV) 400 800 1200 1500 2000Γ(GeV) 0.064 0.51 1.73 3.40 8.03Signal (fb) 100.3 29.86 10.08 5.09 1.92Background (fb) 2020 144 18.88 6.68 1.36optimal PT cut 100 250 450 550 750Table 2: Expected number of reconstructed events using electron and muon decays of the Z boson and signal significance for100 fb−1 of data taking.MD(GeV) 400 800 1200 1500 2000Signal Events 702 209 71 36 13.5Background Events 14000 1008 132 47 9.5Signal significance 5.9 6.6 6.1 5.2 4.374Figure 6: 3σ exclusion curves for 10, 100, 300, 1000 fb −1 integrated luminosities are shown from top to down.mD (GeV)sinφ0.010.020.030.040.050.060.070.080.090.1400 600 800 1000120014001600180020003.2 Extracting the quark mixing angleAs mentioned before, the pair production cross section is almost independent of the D − d quarks mixing angle, thereforewe will use the single production discovery results given in the previous section to investigate this angle. In the event of adiscovery in the single production case, the mixing angle can be obtained directly. If no discoveries are made, then the limiton the cross section can be converted to a limit curve in the D quark mass vs mixing angle plane. Therefore we calculatedthe angular reach for a 3σ signal by extrapolating to other sinφ values. Figure 6 gives the mixing angle versus D-quark massplane and the 3σ reach curves for different integrated luminosities ranging from 10 fb−1 to 1000 fb−1, which correspond toone year of low luminosity LHC operation and one year of high luminosity super-LHC operation respectively. The hashedregion in the same plot is excluded using the current values of the CKM matrix elements. One should note that, this channelallows reducing the current limit on sinφ by half in about 100 fb−1 run time.4 The Results and ConclusionsThe process of single production of theE6 isosinglet quarks could essentially enhance the discovery potential if sinφ exceeds0.02. For example, with 300 fb−1 integrated luminosity, the 3σ discovery limit is mD = 2000GeV, if sinφ = 0.03. It shouldalso be noted that for pair production the 3σ discovery limit was found to be about 900 GeV, independent of sinφ. If ATLASdiscovers an 800 GeV D quark via pair production, single production will give the opportunity to confirm the discovery andmeasure the mixing angle if sinφ > 0.03. The FCNC decay channel analyzed in this paper is specific for isosinglet downtype quarks and gives the opportunity to distinguish it from other models also involving additional down type quarks, forexample the fourth SM family. Therefore, the possible discovery of isosinglet quarks at the LHC would validate the E6 as aGUT group.AcknowledgmentsS.S. and M.Y. acknowledge the support from the Turkish Atomic Energy Authority and the Turkish State Planning Orga-nization under the contract DPT2006K-120470. G.U.’s work is supported in part by U.S. Department of Energy Grant DEFG0291ER40679. The authors would like to thank B. Golden, M. Karagoz and A. Lankford for useful comments.References[1] F. Gursey, P. Ramond and P. Sikivie, Phys. Lett. B 60, 177 (1976); F. Gursey and M. Serdaroglu, Lett. Nuovo Cimento21, 28 (1978).5[2] J. Hewett and T. Rizzo, Phys. Rep. 183, 193 (1989).[3] S. Eidelmann et al, P. Phys. Lett. B 592, 1 (2004).[4] T. C. Andre and C.L. Rosner, Phys. Rev. D. 69, 035009, (2004).[5] R. Mehdiyev et al., ATL-PHYS-PUB-2005-021 (2005), to be published in Eur. Phys J. C.[6] S. Sultansoy and G. Unel, in preparation.[7] A. Pukhov, [arXiv:hep-ph/0412191]; E. Boos et al. [CompHEP Collaboration], Nucl. Instrum. Meth. A 534, 250 (2004).[8] J. Pumplin, D.R. Stump, J. Huston, H.L. Lai, P. Nadolsky and W.K. Tung, JHEP 0207, 012 (2002)[arXiv:hep-ph/0201195].[9] ATLAS Detector and Physics Performance Technical Design Report. CERN/LHCC/99-14/15.[10] ATLAS Collaboration, Trigger and Data Acquisition Technical Design Report, LHCC-2003-022/TDR-016.6

The single production of the lightest E6 isosinglet quark at the LHC

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The single production of the lightest E6 isosinglet quark at the LHC

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