In this note, we present a feasibility study for searches for two exotic particles - a heavy top quark with a fractional charge of 5/3, $T_{5/3}$, and its partner, the heavy $B$ quark. These particles decay to a top quark and a $W$ boson, leading to very busy events with multi-leptons and multi-jets. We consider the event signatures where same-sign dileptons are likely to be produced. The backgrounds are predominantly from standard model signatures due to $t\overline t WW$, $t\overline t W$ and multiple-$W$+jets production. We conclude that it is possible to observe these exotic particles with masses around 500 GeV, in data samples ranging from a few hundred inverse pico-barns to about 1~fb$^{-1}$ of integrated luminosity

Bose, T

Narain, M

CERN Document Server

Available on CMS information server CMS CR-2008/005The Compact Muon Solenoid ExperimentMailing address: CMS CERN, CH-1211 GENEVA 23, SwitzerlandConference Report14 February 2008Observing Heavy Top Quarks with charge 5/3and Heavy B quarks with the CMS detector: AFeasibility StudyT. Bosea), M. Narainb)Brown University, Providence, Rhode Island, USAAbstractIn this note, we present a feasibility study for searches for two exotic particles - a heavy top quark witha fractional charge of 5/3, T5/3, and its partner, the heavyB quark. These particles decay to a top quarkand a W boson, leading to very busy events with multi-leptons and multi-jets. We consider the eventsignatures where same-sign dileptons are likely to be produced. The backgrounds are predominantlyfrom standard model signatures due to ttWW , ttW and multiple-W+jets production. We concludethat it is possible to observe these exotic particles with masses around 500 GeV, in data samplesranging from a few hundred inverse pico-barns to about 1 fb−1 of integrated luminosity.Presented at Physics at Tev Colliders, Les Houches 2007,11-29 June, 2007,Les Houches,Francea) bose@hep.brown.edub) narain@hep.brown.eduq¯ q′ggT¯5/3q′q¯gW−W+ bb¯t¯l+ νl+ νtT5/3W−W+l+ q′ggB¯νq¯gW−W+ bb¯t¯q¯ q′ l+νtBW+W−Figure 1. Pair production of T5/3 and B quarks and decay to same-sign dilepton final states. Figures taken from Ref. [1].1 IntroductionThe Large Hadron Collider (LHC) with a collision center of mass energy of √s = 14 TeV will allow the LHCexperiments to probe particle interactions up to the TeV scale, making it extremely likely that they will be ableto detect some signatures of new physics. In order to make progress in developing our understanding of particlephysics beyond the standard model experimental evidence of such physics is essential.During the first year of LHC running, we expect to collect about one fb−1 of integrated luminosity. Recently,Contino and Servant, have suggested a model which predicts the existence of an exotic heavy top quark partnerwith fractional charge 5/3, T5/3, and a heavy B quark [1]. These particles are predicted in models where the Higgsis a pseudo-Goldstone boson and the T5/3 is the prediction of a LR custodial parity invariance of the electroweaksymmetry breaking sector.Their study suggests that we can observe these particles, if they exist, with datasets corresponding to relativelysmall integrated luminosity, of the order of a few hundred inverse pico-barns. Both the T5/3 quark and B quarkcan have masses around 500 GeV to a TeV or so and are predominantly pair produced. They both decay to a topquark and a W boson and hence have final state signatures based on the decays of the top quarks and the W boson.One of the decay signatures of both the heavy T5/3 quark and the heavy B quarks is shown is Fig. 1, and involvessame sign-dileptons in the final state.In this note, we perform a feasibility study on observing the T5/3 quark and B quark with the CMS detector duringthe early running of the LHC. We consider the multi-W event signature with same sign dilepton final states asshown in Fig. 1.The major sources of backgrounds that contribute to the same-sign di-leptonic channel are: tt¯W , tt¯WW , WWW,and WW in association with jets. Table 1 summarizes the data used with cross sections for the signal and back-ground events. The cross sections quoted are computed to leading-order (LO). The masses of T5/3 and B are takento be 500 GeV for this study. Note that they do not necessarily have to be degenerate.σ(fb) σ ×BR(ℓ±ℓ±)(fb)T5/3 ¯T5/3 (M=500 GeV) 2500 104BB¯ (M=500 GeV) 2500 104tt¯W±+jets 595 18.4W±W+W− 603 18.7tt¯W±W± 121 5.1W±W± 340 15.5Table 1. Cross Sections for the Signal and Background Samples.22 Event Samples and SimulationThe signal and background event samples are generated using the MadGraph[2] event generator. For all thesesamples PYTHIA[3] is used to fragment and hadronize quarks and gluons. For the background evnets, a jet-matching algorithm, following the MLM prescription[4] is employed to ensure that there is no double countingdue to the parton showering in PYTHIA.These samples are then processed via a dedicated fast simulation processor (FAMOS) of the CMS detector. Weuse CMSSW version 1 6 8 for this purpose. Jets with cone size ∆R = 0.5 were reconstructed with the iterativecone algorithm. Generic jet energy corrections (jet corrections 16X) are applied to the cone jets.3 Event SelectionThe distributions for basic kinematic properties of the signal and background events, such as pT of the first andsecond leading leptons, the pT of the first two leading jets, and the missing ET in the events are shown in Figs. 2through 4. They show that the transverse momenta of the lepton and the jets in the signal sample are hardercompared to those from the background. The number of jets above pT of 30 GeV in both the signal and backgroundsamples, normalized to unit area, are shown in Fig. 5 (left). The same distributions but with all the backgroundssummed together and weighted by their expected theoretical cross sections are shown in Fig. 5 (right). One noticesa clear separation between the signal and background in these distributions.In order to enhance the signal to background ratio, we select events with the following kinematic properties:1. At least 5 jets with pT > 30 GeV.2. pT > 100 GeV for the leading jet.3. pT > 80 GeV for the second leading jet.4. Two same-sign isolated leptons (electron or muons).5. Electrons or muons with |η| < 2.4.6. pT > 50 GeV for the leading lepton.7. pT > 25 GeV for the second leading lepton.8. Missing transverse energy > 20 GeV.Tracker-based isolation is used for both electrons and muons with the isolation for electrons being defined as:∑track(ptrackTpeleT)2 < 0.02where all tracks with ptrackT > 1.5 GeV, within an η − φ annular isolation cone centered on the reconstructedelectron, are summed. The cone has limits, 0.02 < ∆R < 0.6 where ∆R =√(∆η)2 + (∆φ)2 and peleT is themomentum of the reconstructed electron. In addition ∆R between the electron and its closest jet is required to begreater than 0.4. For muon isolation the sum of pT of tracks in a cone of radius ∆R = 0.3 is required to be lessthan 3.0.A distribution of HT , computed using the scalar sum of all jet pT s in events passing the selection cuts listed aboveis shown in Fig. 6. We note that the HT of the background peaks at lower values compared to that expected fromT5/3 and B samples with mass of 500 GeV.Table 2 list the efficiencies for the above kinematic cuts for the various samples. The last row of the table alsocorresponds to the number of events expected after the basic selection in our data sample with an integrated lumi-nosity of 1.0 fb−1, which is anticipated to be collected during the first year of LHC running. The HT distributionnormalized to 1.0 fb−1 is shown in Fig. 7.In addition to the physics backgrounds listed in Table 2, we expect instrumental backgrounds mainly due to chargemis-identification, leading to same-sign dileptons in the event. The charge mis-reconstruction probability is ex-pected to be less than 0.1% in the case of 100 GeV muons, while slightly larger values are expected for elec-trons [5]. Thus a SM process with large cross sections, for example tt+ ≥3 jets, Z+ ≥ 5 jets are potential3T5/3 ¯T5/3 BB¯ tt¯W tt¯WW WWW WW(M=500 GeV) (M=500 GeV)Efficiencies (ǫmain) 0.55 0.53 0.15 0.06 0.02 0.03Expected number of events per fb−1 57.2 55.1 2.76 0.30 0.374 0.46Table 2. Efficiencies for the main kinematic cuts.sources of instrumental background and may give rise to same-sign dilepton backgrounds at the level estimatedfrom ttW+jets, or ttWW+jets. However, the event selection efficiency of these background processes are ex-pected to be smaller as the first and second lepton pT , as well as jet pT distributions are much softer comparedto those for ttW, ttWW+jets. Given a couple of percent charge mid-identification probability for electrons inthe CMS detector, we estimate this instrumental background, after the basic selection cuts, to be around 1 to 1.5events. They can be further drastically reduced by making a di-lepton invariant mass cut around the Z mass, or byreconstructing the top quark in the event. Since a realistic estimate of the charge mis-reconstruction probability asa function of the lepton pT and pseudo-rapidity is not publicly available, we decide not to include this instrumentalbackground in our analysis at this time. We note that for the future an accurate estimate of this background isrequired to validate our results.4 Reconstruction of the Exotic Top QuarkThe heavy top quark, T5/3, with fully hadronic decay signature in the event can be fully reconstructed by recon-structing the two W bosons and the top quark in its decay chain.In addition to the basic selection described above, we require that at least two of the jets are b-tagged. In order toreconstruct the hadronic T5/3, the b-jet associated with the di-leptonic T5/3 needs to be identified. The closest jetidentified as a b-jet to the second leading lepton in the event is marked as being part of the di-leptonic decay chain.It is then removed from consideration for the list of jets in the hadronic decay chain.Next the two W bosons and the top quark are reconstructed:• Reconstructing the W boson from top decay:– Two jets with invariant mass consistent within 20 GeV of W boson are assigned to this W boson.– The two jets are required to have ∆R < 1.5– The pT of the reconstructed W is required to be greater than 50 GeV.– We find an efficiency of 84% to get the correct W assignment using this procedure.• Reconstructing the W boson from T5/3 decay:– Remove jets associated with the first W boson.– Two jets that have an invariant mass consistent with a W boson (within 25 GeV) are assigned to thesecond W .– The two jets are required to have ∆R < 2.0.– The pT of the reconstructed W is required to be greater than 30 GeV.• Reconstructing the top quark from T5/3 decay:– Discard the four jets that were assigned to the two W bosons.– Combine the b-tagged jet with one of the two W bosons to compute the top quark mass. The combina-tion which gives an invariant mass closest to the top quark mass, within 40 GeV, is taken.• Finally, the top quark candidate is combined with the other W boson to yield the mass of the heavy top T5/3in the event.Table 3 lists the efficiencies for reconstructing events with two W candidates and a top quark candidate.Fig. 8(left panel) shows the tW invariant mass distributions obtained using the procedure described above. Thesame distributions but stacked on top of each other can be seen in Fig. 8(right panel).4 of leading lepton (GeV)Tp0 50 100 150 20000.020.040.06 T53BBsswwttwttwwwww  of second leading lepton (GeV)Tp0 50 100 150 20000.050.10.15T53BBsswwttwttwwwww Figure 2. Distributions of leading lepton pT (left) and second leading lepton pT (right) for signal and background samples. Thedistributions are normalized to unit area. (GeV)TLeading jet p0 100 200 300 400 500 60000.010.020.03T53BBsswwttwttwwwww (GeV)T2nd leading jet p0 100 200 300 400 500 60000.020.040.06 T53BBsswwttwttwwwwwFigure 3. Leading jet pT (left) and second leading jet pT distributions for signal and background samples. The distributions arenormalized to unit area.5MET (GeV)0 100 200 30000.020.040.060.08T53BsswwttwttwwwwwFigure 4. Missing ET distributions for signal and background samples. The distributions are normalized to unit area. > 30 GeV)TNumber of jets (with p0 2 4 6 8 10 1200.10.20.3T53BBsswwttwttwwwww > 30 GeV)TNumber of jets (with p0 2 4 6 8 10 1200.050.10.150.20.25 5/3T5/3TBBAll bckFigure 5. Left panel: Number of jets for signal and individual background samples. The distributions are normalized to unitarea. Right panel: Number of jets for signal and cross-section weighted background sample.6 (GeV)TH0 500 1000 1500# of events/ (25 GeV)00.20.40.65/3T5/3TBBAll bckCMS PreliminaryFigure 6. HT distributions for signal and background events. A dataset corresponding to an integrated luminosity of 240 pb−1was used. (GeV)TH0 500 1000 1500# of events/(25 GeV)00.511.522.5 5/3T5/3TBBAll bckCMS Preliminary -11 fbFigure 7. HT distributions for signal and background events. A dataset corresponding to an integrated luminosity of 1 fb−1was used.7T5/3 ¯T5/3 BB¯ tt¯W tt¯WW WWW WW(M=500 GeV)ǫ2W 0.64 0.65 0.51 0.63 0.50 0.40ǫtop 0.54 0.53 0.54 0.56 0.13 0.12Table 3. Efficiencies for reconstructing two W candidates and one top quark candidate.tW Invariant mass (GeV)200 400 600 800 1000# of entries/20 GeV00.20.40.60.85/3T5/3TBB± W±W+jets±Wtt±W±Wtt-W+W±W tW Invariant mass (GeV)200 400 600 800 1000# of entries / 20 GeV00.511.5# of entries / 20 GeV 5/3T5/3TBB± W±W+jets±Wtt±W±Wtt-W+W±WFigure 8. tW invariant mass distributions, corresponding to heavy top quark T5/3 mass distribution, for signal and backgroundsamples (left panel). Stacked tW invariant mass distributions for signal and background samples (right panel). A datasetcorresponding to an integrated luminosity of 650 pb−1 was used.5 CMS Discovery Potential for Exotic Top Quark and the Heavy B quarkTwo techniques are used to estimate the sensitivity for observing a signal corresponding to the exotic heavy topquark T5/3 and its partner, the heavy B quark. From both Tables 1 and 2, we note that the number of backgroundevents expected are relatively small and hence it is easy to control the backgrounds.A simple event counting technique (using S/√S +B) after the first set of event selection cuts leads to an estimateof ≈ 230 pb−1 of integrated luminosity for a 5σ observation of the T5/3 and heavy B combination. If we includean 100% uncertainty on the estimate of the background events, the required integrated luminosity goes up to≈ 240 pb−1.We can also estimate the sensitivity from signal and background yields under the tW mass peak. The efficiency forreconstructing T5/3 needs to be taken into account in this procedure. Following this prescription, the first evidenceat 3σ level for the T5/3 +B combination could come with ≈ 250 pb−1 of data and 5σ significance can be reachedwith ≈ 650 pb−1. No systematic uncertainties have been applied yet.6 ConclusionWe have performed the first sensitivity study for the search for an exotic heavy top quark with charge 5/3, T5/3 andits partner the heavy B quark. We find that it is possible to observe a low mass exotic top quark (M=500 GeV) andits partner in a data sample corresponding to a few hundred pb−1 of integrated luminosity. This analysis can bepursued with the data accumulated during the first year of LHC running. Our conclusion is similar to that reachedby Contino and Servant [1], who are the original proposers of this model.We should note that we present a feasibility study using fast simulation and hence this should be considered awork in progress. Additional analysis improvements are underway: full detector simulations of the signal andbackground samples, better jet energy corrections, jet cleaning, fake removal etc. These should further enhancethe sensitivity. This updated version of the analysis will also include a detailed study of systematic uncertainties.87 AcknowledgmentWe thank R. Contino and G. Servant (Theory Group, CERN) for helpful discussions and collaboration on this work.They provided us with all the signal and background MC samples generated using MadGraph, without which thestudy would not have been possible. We also thank Albert De Roeck and Sarah Eno for helpful comments andsuggestions.References[1] “Discovering the top partners at the LHC using same-sign dilepton final states”, Roberto Contino and Geral-dine Servant, [CERN-PH-TH/2007-233; SACLAY-T07/149].[2] See http://madgraph.phys.ucl.ac.be/.[3] T. Sjo¨strand, L. Lo¨nnblad, S. Mrenna, P. Skands, PYTHIA 6.3: PHYSICS AND MANUAL, [LU-TP-03-38,hep-ph/0308153] (2003).[4] S. Ho¨che et al., “Matching Parton Showers and Matrix Elements,” [hep-ph/0602031].[5] G. L. Bayatian et al. [CMS Collaboration], “CMS technical design report, volume II: Physics performance,”J. Phys. G 34, 995 (2007).9

Observing Heavy Top Quarks with charge 5/3 and Heavy $B$ quarks with the CMS detector: A Feasibility Study

Abstract

Similar works

Full text

Available Versions

CERN Document Server

Observing Heavy Top Quarks with charge 5/3 and Heavy BBB quarks with the CMS detector: A Feasibility Study

Abstract

Similar works

Full text

Available Versions

CERN Document Server

Observing Heavy Top Quarks with charge 5/3 and Heavy $B$ quarks with the CMS detector: A Feasibility Study