The Compact Muon Solenoid Experiment. Conference Report. Mailing address: CMS CERN, CH-1211 GENEVA 23, Switzerland

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1 Available on CMS information server CMS CR-28/5 he Compact Muon Solenoid Experiment Conference Report Mailin address: CMS CERN, CH-1211 GENEVA 23, Switzerland 14 February 28 Observin Heavy op Quarks with chare 5/3 and Heavy B quarks with the CMS detector: A Feasibility Study. Bose a), M. Narain b) Brown University, Providence, Rhode Island, USA Abstract In this note, we present a feasibility study for searches for two exotic particles - a heavy top quark with a fractional chare of 5/3, 5/3, and its partner, the heavy B quark. hese particles decay to a top quark and a W boson, leadin to very busy events with multi-leptons and multi-jets. We consider the event sinatures where same-sin dileptons are likely to be produced. he backrounds are predominantly from standard model sinatures due to ttw W, ttw and multiple-w +jets production. We conclude that it is possible to observe these exotic particles with masses around 5 GeV, in data samples ranin from a few hundred inverse pico-barns to about 1 fb 1 of interated luminosity. Presented at Physics at ev Colliders, Les Houches 27,11-29 June, 27,Les Houches,France a) bose@hep.brown.edu b) narain@hep.brown.edu

2 l + ν l + ν q q l+ ν 5/3 W + t W + b B W t W + b 5/3 W t W b B W + t W b q q q q l + ν q q Fiure 1. Pair production of 5/3 and B quarks and decay to same-sin dilepton final states. Fiures taken from Ref. [1]. 1 Introduction he Lare Hadron Collider (LHC) with a collision center of mass enery of s = 14 ev will allow the LHC experiments to probe particle interactions up to the ev scale, makin it extremely likely that they will be able to detect some sinatures of new physics. In order to make proress in developin our understandin of particle physics beyond the standard model experimental evidence of such physics is essential. Durin the first year of LHC runnin, we expect to collect about one fb 1 of interated luminosity. Recently, Contino and Servant, have suested a model which predicts the existence of an exotic heavy top quark partner with fractional chare 5/3, 5/3, and a heavy B quark [1]. hese particles are predicted in models where the His is a pseudo-goldstone boson and the 5/3 is the prediction of a LR custodial parity invariance of the electroweak symmetry breakin sector. heir study suests that we can observe these particles, if they exist, with datasets correspondin to relatively small interated luminosity, of the order of a few hundred inverse pico-barns. Both the 5/3 quark and B quark can have masses around 5 GeV to a ev or so and are predominantly pair produced. hey both decay to a top quark and a W boson and hence have final state sinatures based on the decays of the top quarks and the W boson. One of the decay sinatures of both the heavy 5/3 quark and the heavy B quarks is shown is Fi. 1, and involves same sin-dileptons in the final state. In this note, we perform a feasibility study on observin the 5/3 quark and B quark with the CMS detector durin the early runnin of the LHC. We consider the multi-w event sinature with same sin dilepton final states as shown in Fi. 1. he major sources of backrounds that contribute to the same-sin di-leptonic channel are: t tw, t tw W, WWW, and WW in association with jets. able 1 summarizes the data used with cross sections for the sinal and backround events. he cross sections quoted are computed to leadin-order (LO). he masses of 5/3 and B are taken to be 5 GeV for this study. Note that they do not necessarily have to be deenerate. σ(fb) σ BR(l ± l ± )(fb) 5/3 5/3 (M=5 GeV) B B (M=5 GeV) t tw ± +jets W ± W + W t tw ± W ± W ± W ± able 1. Cross Sections for the Sinal and Backround Samples. 2

3 2 Event Samples and Simulation he sinal and backround event samples are enerated usin the MadGraph[2] event enerator. For all these samples PYHIA[3] is used to frament and hadronize quarks and luons. For the backround evnets, a jetmatchin alorithm, followin the MLM prescription[4] is employed to ensure that there is no double countin due to the parton showerin in PYHIA. hese samples are then processed via a dedicated fast simulation processor (FAMOS) of the CMS detector. We use CMSSW version for this purpose. Jets with cone size R =.5 were reconstructed with the iterative cone alorithm. Generic jet enery corrections (jet corrections 16X) are applied to the cone jets. 3 Event Selection he distributions for basic kinematic properties of the sinal and backround events, such as p of the first and second leadin leptons, the p of the first two leadin jets, and the missin E in the events are shown in Fis. 2 throuh 4. hey show that the transverse momenta of the lepton and the jets in the sinal sample are harder compared to those from the backround. he number of jets above p of 3 GeV in both the sinal and backround samples, normalized to unit area, are shown in Fi. 5 (left). he same distributions but with all the backrounds summed toether and weihted by their expected theoretical cross sections are shown in Fi. 5 (riht). One notices a clear separation between the sinal and backround in these distributions. In order to enhance the sinal to backround ratio, we select events with the followin kinematic properties: 1. At least 5 jets with p > 3 GeV. 2. p > 1 GeV for the leadin jet. 3. p > 8 GeV for the second leadin jet. 4. wo same-sin isolated leptons (electron or muons). 5. Electrons or muons with η < p > 5 GeV for the leadin lepton. 7. p > 25 GeV for the second leadin lepton. 8. Missin transverse enery > 2 GeV. racker-based isolation is used for both electrons and muons with the isolation for electrons bein defined as: track ( ptrack ) 2 <.2 p ele where all tracks with p track > 1.5 GeV, within an η φ annular isolation cone centered on the reconstructed electron, are summed. he cone has limits,.2 < R <.6 where R = ( η) 2 + ( φ) 2 and p ele is the momentum of the reconstructed electron. In addition R between the electron and its closest jet is required to be reater than.4. For muon isolation the sum of p of tracks in a cone of radius R =.3 is required to be less than 3.. A distribution of H, computed usin the scalar sum of all jet p s in events passin the selection cuts listed above is shown in Fi. 6. We note that the H of the backround peaks at lower values compared to that expected from 5/3 and B samples with mass of 5 GeV. able 2 list the efficiencies for the above kinematic cuts for the various samples. he last row of the table also corresponds to the number of events expected after the basic selection in our data sample with an interated luminosity of 1. fb 1, which is anticipated to be collected durin the first year of LHC runnin. he H distribution normalized to 1. fb 1 is shown in Fi. 7. In addition to the physics backrounds listed in able 2, we expect instrumental backrounds mainly due to chare mis-identification, leadin to same-sin dileptons in the event. he chare mis-reconstruction probability is expected to be less than.1% in the case of 1 GeV muons, while slihtly larer values are expected for electrons [5]. hus a SM process with lare cross sections, for example tt+ 3 jets, Z+ 5 jets are potential 3

4 5/3 5/3 B B t tw t tww WWW WW (M=5 GeV) (M=5 GeV) Efficiencies (ǫ main ) Expected number of events per fb able 2. Efficiencies for the main kinematic cuts. sources of instrumental backround and may ive rise to same-sin dilepton backrounds at the level estimated from ttw +jets, or ttw W +jets. However, the event selection efficiency of these backround processes are expected to be smaller as the first and second lepton p, as well as jet p distributions are much softer compared to those for ttw, ttw W +jets. Given a couple of percent chare mid-identification probability for electrons in the CMS detector, we estimate this instrumental backround, after the basic selection cuts, to be around 1 to 1.5 events. hey can be further drastically reduced by makin a di-lepton invariant mass cut around the Z mass, or by reconstructin the top quark in the event. Since a realistic estimate of the chare mis-reconstruction probability as a function of the lepton p and pseudo-rapidity is not publicly available, we decide not to include this instrumental backround in our analysis at this time. We note that for the future an accurate estimate of this backround is required to validate our results. 4 Reconstruction of the Exotic op Quark he heavy top quark, 5/3, with fully hadronic decay sinature in the event can be fully reconstructed by reconstructin 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-taed. In order to reconstruct the hadronic 5/3, the b-jet associated with the di-leptonic 5/3 needs to be identified. he closest jet identified as a b-jet to the second leadin lepton in the event is marked as bein 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: Reconstructin the W boson from top decay: wo jets with invariant mass consistent within 2 GeV of W boson are assined to this W boson. he two jets are required to have R < 1.5 he p of the reconstructed W is required to be reater than 5 GeV. We find an efficiency of 84% to et the correct W assinment usin this procedure. Reconstructin the W boson from 5/3 decay: Remove jets associated with the first W boson. wo jets that have an invariant mass consistent with a W boson (within 25 GeV) are assined to the second W. he two jets are required to have R < 2.. he p of the reconstructed W is required to be reater than 3 GeV. Reconstructin the top quark from 5/3 decay: Discard the four jets that were assined to the two W bosons. Combine the b-taed jet with one of the two W bosons to compute the top quark mass. he combination which ives an invariant mass closest to the top quark mass, within 4 GeV, is taken. Finally, the top quark candidate is combined with the other W boson to yield the mass of the heavy top 5/3 in the event. able 3 lists the efficiencies for reconstructin events with two W candidates and a top quark candidate. Fi. 8(left panel) shows the tw invariant mass distributions obtained usin the procedure described above. he same distributions but stacked on top of each other can be seen in Fi. 8(riht panel). 4

5 p of leadin lepton (GeV) p of second leadin lepton (GeV) Fiure 2. Distributions of leadin lepton p (left) and second leadin lepton p (riht) for sinal and backround samples. he distributions are normalized to unit area Leadin jet p (GeV) nd leadin jet p (GeV) Fiure 3. Leadin jet p (left) and second leadin jet p distributions for sinal and backround samples. he distributions are normalized to unit area. 5

6 B ME (GeV) Fiure 4. Missin E distributions for sinal and backround samples. he distributions are normalized to unit area /3 5/ All bck Number of jets (with p > 3 GeV) Number of jets (with p > 3 GeV) Fiure 5. Left panel: Number of jets for sinal and individual backround samples. he distributions are normalized to unit area. Riht panel: Number of jets for sinal and cross-section weihted backround sample. 6

7 CMS Preliminary # of events/ (25 GeV).6.4 5/3 5/3 All bck H (GeV) Fiure 6. H distributions for sinal and backround events. A dataset correspondin to an interated luminosity of 24 pb 1 was used. CMS Preliminary -1 1 fb # of events/(25 GeV) /3 5/3 All bck H (GeV) Fiure 7. H distributions for sinal and backround events. A dataset correspondin to an interated luminosity of 1 fb 1 was used. 7

8 5/3 5/3 B B t tw t tww WWW WW (M=5 GeV) ǫ 2W ǫ top able 3. Efficiencies for reconstructin two W candidates and one top quark candidate. # of entries/2 GeV.8.6 5/3 5/3 W ± W ± ttw ± +jets ± ± ttw W + - W ± W W # of entries / 2 GeV /3 5/3 ± ± W W ttw ± +jets ± ± ttw W ± + - W W W tw Invariant mass (GeV) tw Invariant mass (GeV) Fiure 8. tw invariant mass distributions, correspondin to heavy top quark 5/3 mass distribution, for sinal and backround samples (left panel). Stacked tw invariant mass distributions for sinal and backround samples (riht panel). A dataset correspondin to an interated luminosity of 65 pb 1 was used. 5 CMS Discovery Potential for Exotic op Quark and the Heavy B quark wo techniques are used to estimate the sensitivity for observin a sinal correspondin to the exotic heavy top quark 5/3 and its partner, the heavy B quark. From both ables 1 and 2, we note that the number of backround events expected are relatively small and hence it is easy to control the backrounds. A simple event countin technique (usin S/ S + B) after the first set of event selection cuts leads to an estimate of 23 pb 1 of interated luminosity for a 5σ observation of the 5/3 and heavy B combination. If we include an 1% uncertainty on the estimate of the backround events, the required interated luminosity oes up to 24 pb 1. We can also estimate the sensitivity from sinal and backround yields under the tw mass peak. he efficiency for reconstructin 5/3 needs to be taken into account in this procedure. Followin this prescription, the first evidence at 3σ level for the 5/3 + B combination could come with 25 pb 1 of data and 5σ sinificance can be reached with 65 pb 1. No systematic uncertainties have been applied yet. 6 Conclusion We have performed the first sensitivity study for the search for an exotic heavy top quark with chare 5/3, 5/3 and its partner the heavy B quark. We find that it is possible to observe a low mass exotic top quark (M=5 GeV) and its partner in a data sample correspondin to a few hundred pb 1 of interated luminosity. his analysis can be pursued with the data accumulated durin the first year of LHC runnin. Our conclusion is similar to that reached by Contino and Servant [1], who are the oriinal proposers of this model. We should note that we present a feasibility study usin fast simulation and hence this should be considered a work in proress. Additional analysis improvements are underway: full detector simulations of the sinal and backround samples, better jet enery corrections, jet cleanin, fake removal etc. hese should further enhance the sensitivity. his updated version of the analysis will also include a detailed study of systematic uncertainties. 8

9 7 Acknowledment We thank R. Contino and G. Servant (heory Group, CERN) for helpful discussions and collaboration on this work. hey provided us with all the sinal and backround MC samples enerated usin MadGraph, without which the study would not have been possible. We also thank Albert De Roeck and Sarah Eno for helpful comments and suestions. References [1] Discoverin the top partners at the LHC usin same-sin dilepton final states, Roberto Contino and Geraldine Servant, [CERN-PH-H/27-233; SACLAY-7/149]. [2] See [3]. Sjöstrand, L. Lönnblad, S. Mrenna, P. Skands, PYHIA 6.3: PHYSICS AND MANUAL, [LU-P-3-38, hep-ph/38153] (23). [4] S. Höche et al., Matchin Parton Showers and Matrix Elements, [hep-ph/6231]. [5] G. L. Bayatian et al. [CMS Collaboration], CMS technical desin report, volume II: Physics performance, J. Phys. G 34, 995 (27). 9