Same-sign Leptons Research Articles

$$ t\overline t {W^{\pm }} $$ production and decay at NLO

We present results for the production of a top pair in association with a W-boson at next-to-leading order. We have implemented this process into the parton-level integrator MCFM including the decays of both the top quarks and the W-bosons with full spin correlations. Although the cross section for this process is small, it is a Standard Model source of same-sign lepton events that must be accounted for in many new physics searches. For a particular analysis of same-sign lepton events in which b-quarks are also present, we investigate the effect of the NLO corrections as a function of the signal region cuts.

Search for Gluinos in Events with Two Same-Sign Leptons, Jets, and Missing Transverse Momentum with the ATLAS Detector inppCollisions ats=7 TeV

A search is presented for gluinos decaying via the supersymmetric partner of the top quark using events with two same-sign leptons, jets, and missing transverse momentum. The analysis is performed with 2.05 fb(-1) of integrated luminosity from pp collisions at sqrt[s]=7 TeV collected by the ATLAS detector at the LHC. No excess beyond the standard model expectation is observed, and exclusion limits are derived for simplified models where the gluino decays via the supersymmetric partner of the top quark and in the minimal supergravity and constrained minimal supersymmetric standard model framework. In those scenarios, gluino masses below 550 GeV are excluded at 95% C.L. within the parameter space considered, significantly extending the coverage with respect to existing limits. Depending on the model parameters, gluino masses up to 750 GeV can also be excluded at 95% C.L.

ObservingCP-violating Higgs signals with heavy Majorana neutrinos

We demonstrate the ability to measure the CP-phase of a Higgs boson state by looking at the decays of heavy, Majorana neutrinos in the process $pp \to H \to N N$ at the LHC. This is shown for signals with both opposite-sign ($l^{\pm} l^{\mp} W^{\pm} W^{\mp}$) and same-sign ($l^{\pm} l^{\pm} W^{\mp} W^{\mp}$) leptonic final states. These signals are investigated in the general framework of a sequential fourth generation of fermions with an additional right-handed neutrino. Such a scenario would naturally give rise to heavy Majorana neutrinos as well as significant enhancements to Higgs production via gluon fusion due to the contributions of the new, heavy quark states running in the fermion loops. Combined with the low background inherent to a same-sign lepton signal, this could be a useful way to investigate the CP-nature of a Higgs boson state at the LHC.

Precision superpartner spectroscopy in the inclusive same-sign dilepton channel at the LHC

The inclusive same-sign dilepton channel is already recognized as a promising discovery signature for supersymmetry in the early days of the LHC. We point out that it can also be used for precision measurements of sparticle masses after the initial discovery. As an illustration, we consider the LM6 CMS study point in minimal supergravity, where the same-sign leptons most often result from chargino decays to sneutrinos. We discuss three different techniques for determining the chargino and sneutrino masses in an inclusive manner, i.e., using only the two well measured lepton momenta, while treating all other upstream objects in the event as a single entity of total transverse momentum ${\stackrel{\ensuremath{\rightarrow}}{P}}_{T}$. This approach takes full advantage of the large production rates of colored superpartners, but does not rely on the poorly measured hadronic jets, and avoids any jet combinatorics problems.

Strong double higgs production at the LHC

The hierarchy problem and the electroweak data, together, provide a plausible motivation for considering a light Higgs emerging as a pseudo-Goldstone boson from a strongly-coupled sector. In that scenario, the rates for Higgs production and decay differ significantly from those in the Standard Model. However, one genuine strong coupling signature is the growth with energy of the scattering amplitudes among the Goldstone bosons, the longitudinally polarized vector bosons as well as the Higgs boson itself. The rate for double Higgs production in vector boson fusion is thus enhanced with respect to its negligible rate in the SM. We study that reaction in pp collisions, where the production of two Higgs bosons at high pT is associated with the emission of two forward jets. We concentrate on the decay mode hh -> WW^(*)WW^(*) and study the semi-leptonic decay chains of the W's with 2, 3 or 4 leptons in the final states. While the 3 lepton final states are the most relevant and can lead to a 3 sigma signal significance with 300 fb^{-1} collected at a 14 TeV LHC, the two same-sign lepton final states provide complementary information. We also comment on the prospects for improving the detectability of double Higgs production at the foreseen LHC energy and luminosity upgrades.

Lepton number violating signals of the top quark partners in the left-right twin Higgs model

We study the collider signatures of the left-right twin Higgs model in the case that the right-handed neutrino mass is less than the mass of the right-handed gauge boson. In this scenario, new leptonic decay chains open up, allowing the particles which cancel the one-loop quadratic divergences of the Higgs, the right-handed gauge bosons and top-partners, to be discovered. Half of these events contain same-sign leptons without missing energy, which have no genuine standard model background and for which the backgrounds are purely instrumental. These signals may be used to complement other collider searches, and in certain regions of parameter space, may be the only way to observe the particles responsible for natural electroweak symmetry breaking in the left-right twin Higgs model.

Multiple parton interactions inZ+4j,W±W±+0/2jandW+W−+2jproduction at the LHC

The expected rate for Multiple Parton Interactions (MPI) at the LHC is large. This requires an estimate of their impact on all measurement foreseen at the LHC. Conversely it provides new means of studying MPI at the LHC. In this paper we examine the role of MPI at the LHC, with the design energy of 14 TeV, in - Z production in association with four jets, - W\pm W\pm in association with zero or two jets. -W+ W- in association with two jets. In all cases the vector bosons are assumed to decay leptonically. The MPI contribution to Z+4j is dominated by events with two jets with balancing transverse momentum. It is possible to achieve a good signal to background ratio, close to 20%, for MPI compared to Single Interaction processes by selecting events with two jets with large separation in the transverse plane. The corresponding statistical significance for a luminosity of 1 inverse fb is about 6.9 for the mu+mu- channel alone. The final state channel in which only two same-sign high transverse momentum charged leptons are required and additional hard jets are vetoed is dominated by MPI, with an expected yield of 2500 events with the full LHC luminosity.

A possible discovery channel for new charged leptons at the LHC

We propose a channel for the possible discovery of new charged leptons at the Large Hadron Collider. The proposed final state contains three same-sign leptons, making this new channel practically backgroundless. The method is illustrated for two different cases: the 4-family standard model and the grand unified theory based on the E6 gauge group. An example study taking 250 GeV as the charged lepton mass shows that in both models, about eight signal events can be expected at 14 TeV center-of-mass energy with 1 fb−1 of integrated luminosity. Although the event yield might not be sufficient for detailed measurements of the charged lepton properties, it would be sufficient to claim discovery through a counting experiment.

Heavy Majorana neutrinos in the effective Lagrangian description: Application to hadron colliders

We consider the effects of heavy Majorana neutrinos N with sub-TeV masses and argue that the mere observation of these particles would be a signal of physics beyond the minimal seesaw mechanism. We describe the N interactions using an effective Lagrangian and exhibit the complete set of effective operators of dimension ⩽6 involving the N and Standard Model fields. We argue that these interactions can be relatively easy to track at high-energy colliders. For example, we find that new physics (beyond the N) at the TeV-scale can yield thousands of characteristic same-sign lepton number violating ℓ+ℓ+jj events (j = light jet) at the LHC if mN≲600 GeV, which can also have a distinctive forward-backward asymmetry signal; even the Tevatron has good prospects for this signature if mN≲300 GeV.

Exploring compressed supersymmetry with same-sign top quarks at the CERN Large Hadron Collider

In compressed supersymmetry, a light top squark naturally mediates efficient neutralino pair annihilation to govern the thermal relic abundance of dark matter. I study the CERN LHC signal of same-sign leptonic top-quark decays from gluino and squark production, which follows from gluino decays to top plus stop followed by the stop decaying to a charm quark and the LSP in these models. Measurements of the numbers of jets with heavy-flavor tags in the same-sign lepton events can be used to confirm the origin of the signal. Summed transverse momentum observables provide an estimate of an effective superpartner mass, which is correlated with the gluino mass. Measurements of invariant mass endpoints from the visible products of gluino decays do not allow direct determination of superpartner masses, but can place constraints on them, including lower bounds on the gluino mass as a function of the top-squark mass.

Collider signals of maximal flavor violation: Same-sign leptons from same-sign top quarks at the Fermilab Tevatron

In models of maximal flavor violation (MxFV) suggested in [S. Bar-Shalom and A. Rajaraman, Phys. Rev. D 77, 095011 (2008).] there is at least one new scalar ${\ensuremath{\Phi}}_{\mathrm{FV}}$ which couples to the quarks via ${\ensuremath{\Phi}}_{\mathrm{FV}}{q}_{i}{q}_{j}\ensuremath{\propto}{\ensuremath{\xi}}_{ij}$ where ${\ensuremath{\xi}}_{i3},{\ensuremath{\xi}}_{3i}\ensuremath{\sim}{V}_{tb}$ for $i=1$, 2 and ${\ensuremath{\xi}}_{33}\ensuremath{\sim}{V}_{td}$ and $V$ is the CKM matrix. In this article, we explore the potential phenomenological implications of MxFV for collider experiments. We study MxFV signals of same-sign leptons from same-sign top-quark pair production at the Tevatron and at the LHC. We show that the current Tevatron data set has strong sensitivity to this signature, for which there are no current limits. For example, if ${m}_{{\ensuremath{\Phi}}_{\mathrm{FV}}}\ensuremath{\sim}200\text{ }\text{ }\mathrm{GeV}$ and the MxFV coupling $\ensuremath{\xi}$ has a natural value of $\ensuremath{\sim}1$, we expect $\ensuremath{\sim}12$ MxFV events to survive a selection requiring a pair of same-sign leptons, a tagged $b$-jet and missing transverse energy, over a background of approximately 4--5 events.

Discovering the top partners at the LHC using same-sign dilepton final states

A natural, non-supersymmetric solution to the hierarchy problem generically requires fermionic partners of the top quark with masses not much heavier than 500 GeV. We study the pair production and detection at the LHC of the top partners with electric charge Q=5/3 (T_{5/3}) and Q=-1/3 (B), that are predicted in models where the Higgs is a pseudo-Goldstone boson. The exotic T_{5/3} fermion, in particular, is the distinct prediction of a LR custodial parity invariance of the electroweak symmetry breaking sector. Both kinds of new fermions decay to Wt, leading to a t\bar{t}WW final state. We focus on the golden channel with two same-sign leptons, and show that a discovery could come with less than 100 pb^{-1} (less than 20 fb^{-1}) of integrated luminosity for masses M=500 GeV (M=1TeV). In the case of the T_{5/3}, we present a simple strategy for its reconstruction in the fully hadronic decay chain. Although no full mass reconstruction is possible for the B, we still find that the same-sign dilepton channel offers the best chances of discovery compared to other previous searches that used final states with one or two opposite-sign leptons, and hence suffered from the large t\bar{t} background. Our analysis also directly applies to the search of 4th generation b' quarks.

Detection of heavy Majorana neutrinos and right-handed bosons

The SU c (3) ⊗ SU L (2) ⊗ SU R (2) ⊗ U(1) left-right (LR) symmetric model explains the origin of the parity violation in weak interactions and predicts the existence of additional gauge bosons W R and Z′. In addition, heavy right-handed Majorana neutrino states N arise naturally within the LR symmetric model. The states N could be partners of light neutrino states, related to their nonzero masses through the seesaw mechanism. This makes the searches for W R , Z′, and N interesting and important. In the framework of the minimal LR model, we study the possibility to observe signals from N and W R production in pp collisions after three years of running at low LHC luminosity. We show that their decay signals can be identified with a small background, especially in the case of same-sign leptons in the final state. For the integral LHC luminosity of L t = 30 fb−1, the 5σ discovery of W R boson and heavy Majorana neutrinos N e with masses \(M_{W_R } \) up to 4 TeV and \(M_{N_e } \) up to 2.4 TeV, respectively, is found to be possible.

Same-sign top quarks as signature of light stops at the CERN LHC

We present a new method to search for a light scalar top with ${m}_{{\stackrel{\texttildelow{}}{t}}_{1}}\ensuremath{\lesssim}{m}_{t}$, decaying dominantly into a $c$-jet and the lightest neutralino, at the LHC. The principal idea is to exploit the Majorana nature of the gluino, leading to same-sign top quarks in events of gluino-pair production followed by gluino decays into top and stop. The resulting signature is 2 $b$-jets plus 2 same-sign leptons plus additional jets and missing energy. We perform a Monte Carlo simulation for a benchmark scenario, which is in agreement with the recent WMAP bound on the relic density of dark matter, and demonstrate that for ${m}_{\stackrel{\texttildelow{}}{g}}\ensuremath{\lesssim}900\text{ }\text{ }\mathrm{GeV}$ and ${m}_{\stackrel{\texttildelow{}}{q}}>{m}_{\stackrel{\texttildelow{}}{g}}$ the signal can be extracted from the background. Moreover, we discuss the determination of the stop and gluino masses from the shape of invariant-mass distributions. The derivation of the shape formulas is also given.

Four leptons production in [formula omitted] collisions from 3–3–1 model

We study the process e−+e+→e++e++e−+e− for ILC and CLIC energy regimes, in the minimal 3–3–1 model framework. The main contributions, for the production of two bileptons each one decaying into two same-sign leptons, come from s-channel annihilation (via γ,Z and Z′) and t-channel electron exchange. We evaluate the number of events for an extra neutral gauge boson mass MZ′ in the range 1 TeV to 3 TeV. We compare some distributions from 3–3–1 model with the Standard Model background in order to extract a possible signature of the contribution of bilepton and Z′ for the process. From our analysis we conclude that our results give clear signals for physics beyond the Standard Model in e−e+ colliders.

Mixed top-bottom squark production at the CERN LHC

We calculate cross sections for mixed stop-sbottom pair production at the LHC, analogous to single-top production, a weak process involving the $W$-${\stackrel{\texttildelow{}}{t}}_{i}$-${\stackrel{\texttildelow{}}{b}}_{j}$ vertex. While coupling-suppressed relative to QCD same-flavor squark pair production, the signal is distinctive due to heavy-flavor tagging along with a possible same-sign lepton pair in the final state. SUSY backgrounds can often be suppressed many orders of magnitude by taking advantage of distinct kinematic differences from the signal. Measuring the rate of this process would add significant additional information to that gathered from other SUSY processes. If the stop and sbottom mixings can be determined elsewhere, stop-sbottom production would provide for a measurement of the weak squark gauge coupling and super-CKM vertex factor.

Doubly charged Higgs at LHC

We have investigated the production of doubly charged Higgs particles Δ L, R ++ via the WW fusion process in proton-proton collisions at LHC energies in the framework of the left-right symmetric model. The production cross section of the right-triplet Higgs Δ R ++ is for representative values of the model parameters at femtobarn level. The discovery reach depends on the mass of the right-handed gauge boson W R . At best Δ R ++ masses up to 2.4 TeV are achievable within a one-year run. For Δ L ++ the corresponding limit is 1.75 TeV which depends on the value of the left-triplet vev ν L . Comparison with Drell-Yan pair production processes shows that studies of the WW fusion processes extend the discovery reach of LHC roughly by a factor of two. The main experimental signal of a produced Δ L, R ++ would be a hard same-sign lepton pair. There will be no substantial background due to the Standard Model (SM) interactions, since in the SM a same-sign lepton pair will always be associated with missing energy, i.e. neutrinos, due to lepton number conservation.

A STRATEGY FOR DISCOVERING HEAVY NEUTRINOS

Same-sign lepton collisions in the TeV energy range may well have the unique capability to search for clear signals for the exchange of heavy Majorana neutrinos. Lepton-flavor-violating transitions e−e−→W−W− can thus contribute to the understanding of two unsolved riddles in particle physics: the neutrino mass problem and the question of the Dirac or Majorana character of heavy neutrinos. This search is not similarly accessible to such effects as neutrinoless double beta decay. The resulting experimental signatures are hard to miss.

A search for doubly charged Higgs production in Z 0 decays

A search for the decay of the Z 0 into doubly charged Higgs bosons (H ±±) decaying to same-sign lepton pairs is presented using data collected with the OPAL detector at LEP, with an integrated luminosity of 6.8 pb −. Four-track final states from prompt decays, and events with at least one highly ionizing track from long-lived H ±± were sought. H ±± are excluded in the mass range from zero to 45.6 GeV/ c 2 and for a coupling constant range that extends down to zero.

Prompt (DI) leptons associated with [formula omitted] collider jets

The large p T jet cross sections as well as the charm (in fact D ∗) content of the jets have been measured in p p interactions with √ s=540 GeV. It is then an (almost) straightforward calculation to predict the rates and properties of events with prompt leptons and multileptons resulting from the leptonic decay of the jets' heavy-quark fragments. We find a source of heavy quarks and prompt leptons produced in association with jets which could totally dominate yields conventionally computed from perturbative QCD and which, more importantly, leads to totally different event characteristics. E.g., events where both jets contain a heavy-quark fragment ( c c + c c , c c + b b or b b + b b ) are the origin of opposite- and same-sign lepton pairs with equal probability. The dileptons are accompanied by four strange particles and occasionally conceal their jet origin by trigger bias. They form a possible source of the “anomalous” dilepton events recently observed by the UA1 experiment.