We perform a model-independent study for top-antitop and top-top resonances in the dilepton channel at the Large Hadtron Collider. In this channel, we can solve the kinematic system to obtain the momenta of all particles including the two neutrinos, and hence the resonance mass and spin. For discovering top-antitop resonances, the dilepton channel is competitive to the semileptonic channel because of the good resolution of lepton momentum measurement and small standard model backgrounds. Moreover, the charges of the two leptons can be identified, which makes the dilepton channel advantageous for discovering top-top resonances and for distinguishing resonance spins. We discuss and provide resolutions for difficulties associated with heavy resonances and highly boosted top quarks.
In the Randall-Sundrum model, we study top-antitop pair production and top spin correlations at the Large Hadron Collider. In addition to the Standard Model processes, there is a new contribution to the top-antitop pair production process mediated by graviton Kaluza-Klein modes in the s-channel. We calculate the density matrix for the top-antitop pair production including the new contribution. With a reasonable parameter choice in the Randall-Sundrum model, we find a sizable deviation of the top-antitop pair production cross section and the top spin correlations from those in the Standard Mode…
We have carried out Monte Carlo calculations on two sets of randomly generated QCD events due to $p\bar{p} \to t \bar{t}$ with top mass $m_t = 170$ GeV, one set leading to $e^+e^-$ or $e^{\pm}μ^{\mp}$ or $μ^+μ^-$ 2jets (dilepton) and the other leading to $e^{\pm}$ or $μ^{\pm}$ 4jets (unilepton) configurations, in order to test the likelihood methods we have proposed for determining the top mass by analyses of these two sets of configurations. For the set of unilepton events, our method gives a very efficient and quite sharp measure of the top mass lying several GeV below the input mass. For th…
A unique parameterization of secondary (thermal) dilepton and photon yields in heavy-ion experiments at CERN-SPS is proposed. Adding those thermal yields to background contributions the spectral shapes of the CERES/NA45, NA38, NA50, HELIOS/3 and WA98 data from experiments with lead and sulfur beams can be well described.
A unique parametrization of secondary (thermal) dilepton yields in heavy-ion experiments at CERN-SPS is proposed. This parametrization resembles a thermal $q \bar q$ annihilation rate. This is inspired by the observation that lepton pair production rates are quantitatively similar, whether expressed in a hadronic or partonic basis. Adding the thermal yield and the background contributions (hadronic cocktail, Drell-Yan, correlated semileptonic decays of open charm) the spectral shapes of the CERES/NA45, NA38, NA50 and HELIOS/3 data from experiments with lead and sulfur beams can be well describ…
We present the results of an analysis for the pair production of scalar leptoquarks at the CERN Large Hadron Collider (LHC) with energy=14 TeV and luminosity=10 fb^{-1} which includes the dominant sources of Standard Model background associated to this process: top-antitop, ZZ, WZ and Z*jj production. The top-antitop process provides the main source of background. We consider leptoquarks introduced in the framework of a superstring-inspired E_6 model. The leptoquark production is found to be dominant in all regions of parameter space for leptoquark masses below 750 GeV. We establish the discov…
We explore the sensitivity of the LHC at 14 TeV centre-of-mass energy (LHC14) to the single production and decay of top-antitop resonances in the four-top final state. We focus on the same-sign dilepton channel, and work within a simplified model with a vector boson coupling to the Standard Model only via its interactions with right-handed top quarks. We find it is possible to discover (exclude) such a vector boson with 300 fb$^{-1}$ of integrated luminosity up to a mass of 1.2 (1.6) TeV for a modest coupling to tops of $g_ρ$=2. We present our results as an exclusion limit on the cross-section…
We have analyzed the available data on $p\bar{p}\,\rightarrow\, t\, \bar{t}$ followed by the decays ($t\rightarrow bW^{+},\: \bar{t}\rightarrow \bar{b}W^{-}$) which lead to $e^{\pm}μ^{\mp}2 \mathrm{jets}$ or $l^{\pm}4 \mathrm{jets}$ configurations, using a likelihood method we proposed earlier. The outcome is compared with the recent CDF analysis. In an appendix, we discuss the nature of the additional ``slow'' $μ^{+}$ observed in one CDF dilepton event, concluding that it is most probably a ``tertiary lepton'' resulting from the decay sequence $b\rightarrow c +\mathrm{hadrons}$, followed by $…
We review recent theoretical progress in low-mass dilepton production at CERN-SpS energies. Various hadronic approaches to calculate the vector correlator in hot/dense hadronic matter are discussed and confronted with each other. Possible consequences for the nature of chiral restoration are indicated.
We explore the discovery potential of doubly charged Higgs bosons (ξ^{\pm\pm}) at the CERN Large Hadron Collider (LHC). For moderate values of the coupling constants in the original Type-II seesaw model, these doubly-charged Higgs bosons are not accessible by any present or near future collider experiments. In a gauged B-L symmetric model we introduce two triplet scalars to execute a variant of type-II seesaw at the TeV scale. This leads to a clear like-sign dilepton signal in the decay mode of ξ^{\pm\pm} for a small vacuum expectation value (\lsim 10^5 \eV) of the triplet scalar ξ= (ξ^{++}, ξ…
Using a rather complete description of the in-medium $ρ$ spectral function - being constrained by various independent experimental information - we calculate pertinent dilepton production rates from hot and dense hadronic matter. The strong broadening of the $ρ$ resonance entails a reminiscence to perturbative $q\bar q$ annihilation rates in the vicinity of the phase boundary. The application to dilepton observables in Pb(158AGeV)+Au collisions - incorporating recent information on the hadro-chemical composition at CERN-SpS energies - essentially supports the broadening scenario. Possible impl…
The significance of thermal dilepton radiation at intermediate invariant masses (1GeV<M<3GeV) in ultrarelativistic heavy-ion collisions is investigated. At CERN-SpS energies, a consistent explanation of the excess observed by NA50 can be given. At RHIC energies the thermal signal is dominated by early emission indicative for QGP formation. Chemical under-saturation effects and the competition with open-charm contributions are addressed.
We present numerical calculations of the production cross section of a heavy Z' resonance in hadron-hadron collisions with subsequent decay into top-antitop pairs. In particular, we consider the leptophobic topcolor Z' discussed under Model IV of hep-ph/9911288 which has predicted cross sections large enough to be experimentally accessible at the Fermilab Tevatron and the Large Hadron Collider at CERN. This article presents an updated calculation valid for the Tevatron and all proposed LHC collision energies. Cross sections are presented for various Z' widths, in p-pbar collisions at \sqrt{s}=…
We investigate in detail the effects of the R-parity lepton number violation in the minimal supersymmetric standard model (MSSM) on the parent process $pp \to e^+ e^- + X$ at the CERN Large Hadron Collider (LHC). The numerical comparisons between the contributions of the R-parity violating effects to the parent process via the Drell-Yan subprocess and the gluon-gluon fusion are made. We find that the R-violating effects on $e^+ e^-$ pair production at the LHC could be significant. The results show that the cross section of the $ e^+ e^-$ pair productions via gluon-gluon collision at the LHC ca…
We study the top quark pair production process p+p(anti-p)-->top+antitop in various kinds of technicolor (TC) models at the Fermilab Tevatron Run II and the CERN LHC. The s-channel neutral pseudo-Goldstone bosons (PGB's) contribute dominately to the production amplitudes from its coupling to the gluons through the triangle loops of techniquarks and the top quark. Cross sections in different TC models with s-channel PGB contributions are calculated. It is shown that the PGB effects can be experimentally tested and different TC models under consideration can be distinguished at the LHC. Therefor…
Top-antitop quark pairs produced at the Tevatron have a sizeable spin correlation. That correlation feeds into the angular distribution of the decay products, particularly in the dilepton channel. Including the expected correlation in an overall analysis of a handful of actual dilepton events continues to favor a lower top mass (centered on 155 GeV) than the single lepton events.
We investigate the significance of thermal dilepton radiation in the intermediate-mass region in heavy-ion reactions at CERN-SpS energies. Within a thermal fireball model for the space-time evolution, the radiation from hot matter is found to dominate over hard 'background' processes (Drell-Yan and open charm) up to invariant masses of about 2 GeV, with a rather moderate fraction emerging from early stages with temperatures $T\simeq 175-200$ MeV associated with deconfined matter. Further including a schematic acceptance for the NA50 experiment we find good agreement with the observed enhanceme…
We study the production of a Higgs boson in association to a top-antitop pair at the Large Hadron Collider. We show how precise predictions for the differential distributions with respect to the transverse momentum of the Higgs boson, to the invariant mass of the top-antitop-Higgs system and to the invariant mass of the top-antitop pair can provide useful information on the possible presence of a pseudoscalar component in the coupling of the top quark with the Higgs boson. We evaluate the production of a top-antitop pair and a Higgs boson to next-to-leading order in fixed order perturbation th…
We show that the dilepton spectra measured by the CERES collaboration in Pb + Au interactions for various charge multiplicities can be reproduced by a hadronic initial state with reduction in the masses of the vector mesons in the thermal bath. Though such an effect may also be achieved by a large broadening of the spectral function we show that the photon spectra is insensitive to this. It is found that for higher multiplicities a good description of the data can also be obtained with quark gluon plasma initial state if the reduction of the vector meson masses in the mixed and hadronic phases…
The requirement of t-b-τYukawa coupling unification is common in simple grand unified models based on the gauge group SO(10), and it also places a severe constraint on the expected spectrum of superpartners. For Yukawa-unified models with μ>0, the spectrum is characterized by three mass scales: {\it i}). first and second generation scalars in the multi-TeV range, {\it ii}). third generation scalars, μand m_A in the few-TeV range and {\it iii}). gluinos in the \sim 350-500 GeV range with chargino masses around 100-160 GeV. In such a scenario, gluino pair production should occur at large rates a…
We use ISAJET to perform a detailed study of the multilepton signals expected from cascade decays of supersymmetric particle produced at the CERN LHC. Our analysis is performed within the framework of the minimal supergravity model with gauge coupling unification and radiative electroweak symmetry breaking. We delineate the regions of parameter space where jets plus missing energy plus 1, 2 (opposite sign and same-sign dileptons), and 3 isolated lepton events should be visible above standard model backgrounds. We find that if any $\eslt$ signal at the LHC is to be attributed to gluino and/or s…
A quantitative evaluation of dilepton sources in heavy-ion reactions is performed taking into account both thermal and non-thermal production mechanisms. The hadronic thermal emission rate is based on an electromagnetic current-correlation function with a low-mass region (LMR, M \lsim 1 GeV) dominated by vector mesons (ρ, ω, ϕ) and an intermediate-mass region (IMR, 1 GeV \le M \le 3 GeV) characterized by (the onset of) a multi-meson continuum. A convolution of the emission rates over a thermal fireball expansion results in good agreement with experiment in the low-mass spectra, confirming the …
The Neutral $Z$ boson pair production due to radion resonance at the Large Hadron Collider (LHC) is an interesting process to explore the notion of warped geometry (Randall-Sundrum model). Because of the enhanced coupling of radion with a pair of gluons due to trace enomaly and top(quark) loop, the radion can provide larger event rate possibility as compared to any New Physics effect. Using the proper radion-top-antitop (with the quarks being off-shell) coupling, we obtain the correct radion production rate at LHC and explore several features of a heavier radion decaying into a pair of real $Z…
A realistic model for the free rho meson with coupling to two-pion states is employed to calculate the rho propagator in a hot and dense hadron gas. The medium modifications are based on hadronic rescattering processes: intermediate two-pion states are renormalized through interactions with surrounding nucleons and deltas, and rho meson scattering is considered off nucleons, deltas, pions and kaons. Constraints from gauge invariance as well as the full off-shell dynamics of the interactions are accounted for. Within the vector dominance model we apply the resulting in-medium rho spectral funct…
Partonic matter produced in the early stage of ultrarelativistic nucleus-nucleus collisions is assumed to be composed mainly of gluons, and quarks and antiquarks are produced at later times. To study the implications of such a scenario, the dynamical evolution of a chemically nonequilibrated system is described by the ideal (2+1)-dimensional hydrodynamics with a time dependent (anti)quark fugacity. The equation of state interpolates linearly between the lattice data for the pure gluonic matter and the lattice data for the chemically equilibrated quark-gluon plasma. The spectra and elliptic flo…
Top-antitop pairs produced in the decay of a new heavy resonance will exhibit spin correlations that contain valuable coupling information. When the tops decay, these correlations imprint themselves on the angular patterns of the final quarks and leptons. While many approaches to the measurement of top spin correlations are known, the most common ones require detailed kinematic reconstructions and are insensitive to some important spin interference effects. In particular, spin-1 resonances with mostly-vector or mostly-axial couplings to top cannot be easily discriminated from one another witho…
We study the same-sign top pair production mediated by the first Kluza-Klein (KK) excitation of the gluon in the Randall-Sundrum (RS) model with flavor violation at the Large Hadron Collider (LHC), in which the nonuniversal couplings between fermions and KK gauge bosons will lead to observable tree level flavor-changing neutral current (FCNC) effects. We find that the same-sign top quarks produced in our case have property of high energy and high transverse momentum, and lead to an observable signal in the same-sign dilepton channel even when the mass of the KK gluon reach up to 3 TeV. We furt…
Heavy neutral resonances appearing in the clean Drell-Yan channel may be the first new physics to be observed at the proton-proton CERN LHC. If a new resonance is discovered at the LHC as a (narrow) peak in the dilepton invariant mass distribution, the characterization of its spin and couplings will proceed via the measurement of production rates and angular distributions of the decay products. We discuss the discrimination of a spin-0 resonance (sneutrino) predicted by supersymmetric theories with R-parity breaking against the spin-1 of Z^\prime bosons and the Randall-Sundrum graviton resonan…
We study the dependence on $\tanβ$ of the event kinematics of final states with $e^+e^- /μ^+μ^- /e^{\pm}μ^{\mp} + E_T^{\rm miss} + jets$, as expected in $pp$ collisions at CERN LHC, within the framework of the minimal supergravity model. With increase of $\tanβ$, the third generation sparticle masses $m_{\tildeτ_1}$ and $m_{\tilde{b}_1}$ decrease due to the increase of the tau and bottom Yukawa couplings. As a result, gluino, stop, sbottom, chargino and neutralino decays to third generation particles and sparticles are enhanced. With $\tanβ$ rising, we observe a characteristic change in the sh…
We study the possibility to test the Type I seesaw mechanism for neutrino masses at the CERN Large Hadron Collider. The inclusion of three generations of right-handed neutrinos (N_i) provides an attractive option of gauging the B-L accidental symmetry in the Standard Model (as well as an extended symmetry X=Y-5(B-L)/4). The production mechanisms for the right-handed neutrinos through the Z' gauge boson in the U(1)_{B-L} and U(1)_X extensions of the Standard Model are studied. We discuss the flavor combinations of the charged leptons from the decays of N_i in the Delta L=2 channels. We find tha…
A study on the possibility of distinguishing new heavy Majorana neutrino models at LHC energies is presented. The experimental confirmation of standard neutrinos with non-zero mass and the theoretical possibility of lepton number violation find a natural explanation when new heavy Majorana neutrinos exist. These new neutrinos appear in models with new right-handed singlets, in new doublets of some grand unified theories and left-right symmetrical models. It is expected that signals of new particles can be found at the CERN high-energy hadron collider (LHC). We present signatures and distributi…
We examine the reach of the CERN LHC pp collider for supersymmetric models where the dominant contribution to soft SUSY breaking parameters arises from the superconformal anomaly. In the simplest viable anomaly mediated SUSY breaking (AMSB) model, tachyonic slepton squared masses are made positive by adding a universal contribution m_0^2 to all scalars. We use the event generator ISAJET to generate AMSB signal events as a function of model parameter space. Assuming an integrated luminosity of 10 fb-1, the LHC can reach to values of $m_{\tg}\sim 2.3$ TeV for low values of $m_0$, where the dilep…
We analyze dilepton emission from hot and dense matter using a hybrid approach based on the Ultrarelativistic Quantum Molecular Dynamics (UrQMD) transport model with an intermediate hydrodynamic stage for the description of heavy-ion collisions at relativistic energies. During the hydrodynamic stage, the production of lepton pairs is described by radiation rates for a strongly interacting medium in thermal equilibrium. In the low mass region, hadronic thermal emission is evaluated assuming vector meson dominance including in-medium modifications of the rho meson spectral function through scatt…
The study of lepton pair production is a powerful test of the Standard Model (SM) and can be used to search for phenomena beyond the SM. New heavy neutral bosons $Z'$ decaying to charged lepton pairs $l^+l^-$ ($l=e, μ$) are predicted by many scenarios of new physics, including models with extended gauge sector. We estimate the LHC $Z'$ discovery potential with Run 2 data comprised of 36.1 fb$^{-1}$ of $pp$ collisions at $\sqrt{s}=13$ TeV and recorded by ATLAS detector at the CERN LHC. The model-independent constraints on the $Z'$ fermion couplings were obtained for the first time for a selecte…
A striking manifestation of CP violation in the electroweak symmetry breaking sector would be the existence of neutral Higgs boson(s) with undefined CP parity. We analyse signatures of such a boson, with a mass of about 300 GeV or larger, produced in high energy proton-proton collisions at LHC energies in its top-quark antitop-quark decay channel. The large irreducible $t\bar t$ background is taken into account. We propose, both for the dilepton and the lepton + jets decay channels of $t\bar t$, several correlations and asymmetries with which (Higgs sector) CP violation can be traced. We show …
We present results for the low mass Drell-Yan (DY) production in proton-proton collisions at the LHC in the color dipole formalism. The DY differential cross sections at 7 TeV as a function of dilepton rapidity, transverse momentum and invariant mass are discussed. We have imposed kinematical cuts related to the low mass DY production investigated by ATLAS and LHCb collaborations.
In the conventional type-(I+II) seesaw model, the effective mass matrix of three known light neutrinos is given by M_nu = M_L - M_D M^{-1}_R M^T_D in the leading-order approximation. We propose an intriguing scenario, in which the structural cancellation condition M_D M^{-1}_R M^T_D = 0 is guaranteed by the A_4 x Z_2 flavor symmetry. As a consequence, neutrino masses are mainly generated by the Higgs triplet M_nu = M_L, while the neutrino mixing matrix is non-unitary and takes on the nearly tri-bimaximal pattern. A discriminating feature of this scenario from the pure type-II seesaw model is t…
The ATLAS and CMS experiments at the CERN LHC have collected about 20 $fb^{-1}$ of data each at the end of their 8 TeV run, and ruled out a huge swath of parameter space in the context of Minimally Supersymmetric Standard Model(MSSM). Limits on masses of the gluino ($\tilde{g}$) and the squarks of the first two generations($\tilde{q}$) have been pushed to the TeV range. Light third generation squarks namely stop and sbottom of sub-TeV masses, on the other hand, are still allowed by their direct search limits. Interestingly, the discovery of a Standard Model(SM) higgs boson like particle with a…
Inclusive Standard Model Higgs boson pair production and subsequent decay to same-sign dileptons via weak gauge W bosons at the CERN Large Hadron Collider has the capability to determine the Higgs boson self-coupling, lambda. The large top quark mass limit is found not to be a good approximation for the signal if one wishes to utilize differential distributions in the analysis. We find that it should be possible at the LHC with design luminosity to establish that the Standard Model Higgs boson has a non-zero self-coupling and that lambda/lambda(SM) can be restricted to a range of 0--3.7 at 95%…
We study the single graviton production at the CERN LHC using the $k_T$-factorization approach of QCD. We consider the Arkani-Hamed-Dimopoulos-Dvali scenario and Randall-Sundrum model with one warped extra-dimension and derive the production amplitudes for spin-$2$ (Kaluza-Klein excitation of the graviton) and spin-$0$ (radion) states, including subsequent graviton decay into the dilepton or diphoton pairs. We use the transverse momentum dependent (unintegrated) parton densities in a proton obtained from Ciafaloni-Catani-Fiorani-Marchesini (CCFM) evolution equation, which resums the leading lo…
We present the calculation of the NLO QCD corrections to the electroweak production of top-antitop pairs at the CERN LHC in the presence of a new neutral gauge boson. The corrections are implemented in the parton shower Monte Carlo program POWHEG. Standard Model (SM) and new physics interference effects are properly taken into account. QED singularities, first appearing at this order, are consistently subtracted. Numerical results are presented for SM and $Z'$ total cross sections and distributions in invariant mass, transverse momentum, azimuthal angle and rapidity of the top-quark pair. The …
We consider top-antitop quark ($t{\bar t}$) production at the Large Hadron Collider (LHC) with subsequent decay into dileptonic and lepton plus jets final states. We present a set of leptonic angular correlations and distributions with which experiments can probe all the independent coefficient functions of the top-spin dependent parts of the $t{\bar t}$ production spin density matrices. We compute these angular correlations and distributions for LHC center-of-mass energies 8, 13, and 14 TeV within the Standard Model at next-to-leading order in the QCD coupling including the mixed QCD-weak cor…
Heavy resonances appearing in the clean Drell-Yan channel may be the first new physics to be observed at the proton-proton CERN LHC. If a new resonance is discovered at the LHC as a peak in the dilepton invariant mass distribution, the characterization of its spin and couplings will proceed via measuring production rates and angular distributions of the decay products. We discuss the discrimination of the spin-1 of Z' representative models (Z'_{SSM}, Z'_{psi}, Z'_{eta}, Z'_{chi}, Z'_{LR}, and Z'_{ALR}) against the Randall-Sundrum graviton resonance (spin-2) and a spin-0 resonance (sneutrino) w…
Motivated by the observed excess of the di-photon signal in Higgs searches, $σ_{γγ}/σ_{\rm SM}\simeq 1.5$, we argue that models with enhanced $Γ(h\to γγ)$ alone are the most favorable scenarios when the latest LHC/Tevatron results are all taken into account. We study the phenomenology of a supersymmetric scenario of light stau first proposed by \textit{Carena et. al. \cite{carlos}} that predicts a 125 GeV SM-like Higgs boson with enhanced diphoton decay through light stau loops. Since it is extremely challenging to search the Drell-Yan stau pair at the LHC due to the small production rate, we …
The resonance production of new chiral spin-1 bosons and their detection through the Drell--Yan process at the CERN LHC is considered. Quantitative evaluations of various differential cross-sections of the chiral bosons production are made within the CalcHEP package. The new neutral chiral bosons can be observed as a Breit--Wigner resonance peak in the invariant dilepton mass distribution, as usual. However, unique new signatures of the chiral bosons exist. First, there is no Jacobian peak in the lepton transverse momentum distribution. Second, the lepton angular distribution in the Collins-So…
The resonance production of new chiral spin-1 excited bosons, Z*, and their detection through the Drell-Yan process in the first physical runs at the CERN LHC are considered. The new neutral chiral bosons can be observed as a Breit-Wigner resonance peaks in the invariant dilepton mass distribution in the same way as the well-known hypothetical gauge bosons, Z'. However, unique new signatures of the chiral bosons exist. These signatures could be very important for the interpretation of the first LHC data. First, there is no Jacobian peak in the lepton transverse momentum distribution at the kin…
We investigate how the CP quantum numbers of a neutral Higgs boson or spin-zero resonance, produced at the CERN Large Hadron Collider, can be determined in its tau-pair decay mode. We use a method developed in an earlier paper based on the distributions of two angles and apply it to the major 1-prong tau decays. We show for the resulting dilepton, lepton-pion, and two-pion final states that appropriate selection cuts significantly enhance the discriminating power of these observables. From our analysis we conclude that, provided a Higgs boson will be found at the LHC, it appears feasible to co…
