Dilepton Final States Research Articles

W-boson polarization measurement in the [formula omitted] dilepton channel using the CDF II detector

We present a measurement of the W-boson polarization in top-quark decays in tt¯ events with decays to dilepton final states using data corresponding to 5.1 fb−1 of integrated luminosity in pp¯ collisions collected by the CDF II detector at the Tevatron. Assuming a top-quark mass of 172.5 GeV/c2, a simultaneous measurement of the fractions of longitudinal (f0) and right-handed (f+) W-bosons yields the results f0=0.70−0.17+0.18(stat)±0.06(syst) and f+=−0.09±0.09(stat)±0.03(syst). Combining this measurement with our previous measurement based on single-lepton final states, we obtain f0=0.84±0.09(stat)±0.05(syst) and f+=−0.16±0.05(stat)±0.04(syst). The results are consistent with the standard model expectation.

Higgs signatures in inverse seesaw model at the LHC

In the inverse seesaw mechanism where the spontaneously broken B-L symmetry induces tiny B-L violating Majorana masses for right-handed neutrinos, non-standard Higgs signatures can arise due to a possible Higgs doublet and singlet mixing and/or Higgs boson decays to a left- and right-handed neutrino. This leads to a remarkable feature of hadronically quiet di-lepton final states which can exhibit, in particular, lepton flavour violating signatures coming from flavour-dependent neutrino Yukawa couplings. In this process, one lepton coming from the right-handed decay could be soft enough can be missed by the trigger level cuts of CMS and ATLAS for the di-lepton plus missing energy signature. The prospects of such a signature are investigated for 8 TeV and 14 TeV center of mass energy of the LHC, taking the maximum value of the allowed neutrino Yukawa coupling and the right-handed neutrino mass of 100 GeV. A PYTHIA level simulation shows that the integrated luminosity of 10--20/fb and 1.6/fb for 8 TeV is required to observe the inclusive leptonic and lepton flavour violating signatures, respectively. For 14 TeV, the reach is more and a larger parameter space of the inverse seesaw model can be probed.

Searches for Top and Bottom Squarks in pp Collisions at √s= 8 TeV

Supersymmetry is a popular extension to the standard model, which may solve the hierarchy problem without fine-tuning if it introduces top and bottom squarks with masses not larger than several hundred GeV. This note describes three searches for the direct pair production of these particles, based on a sample of pp collisions data corresponding to approximately 10 fb 1 collected by the Compact Muon Solenoid detector at the LHC, at a center-of-mass energy p s = 8 TeV. The searches are performed in the single lepton final state focusing on events with large transverse mass, the same-sign dilepton final state, and the all-hadronic final state using the T quantity. No evidence for the production of top or bottom squarks is observed. The results are used to place stringent constraints on the masses of these particles.

Search for doubly charged Higgs bosons in like-sign dilepton final states at $\sqrt{s} = 7\ \mathrm{TeV}$ with the ATLAS detector

A search for doubly-charged Higgs bosons decaying to pairs of electrons and/or muons is presented. The search is performed using a data sample corresponding to an integrated luminosity of 4.7 fb-1 of pp collisions at sqrt(s) = 7 TeV collected by the ATLAS detector at the LHC. Pairs of prompt, isolated, high-pT leptons with the same electric charge (ee, emu, mumu) are selected, and their invariant mass distribution is searched for a narrow resonance. No significant excess over Standard Model background expectations is observed, and limits are placed on the cross section times branching ratio for pair production of doubly-charged Higgs bosons. The masses of doubly-charged Higgs bosons are constrained depending on the branching ratio into these leptonic final states. Assuming pair production, coupling to left-handed fermions, and a branching ratio of 100% for each final state, masses below 409 GeV, 375 GeV, and 398 GeV are excluded for ee, emu, mumu, respectively.

Search for high-mass resonances decaying to dilepton final states in pp collisions at $ \sqrt{s}=7 $ TeV with the ATLAS detector

Abstract The ATLAS detector at the Large Hadron Collider is used to search for high-mass resonances decaying to an electron-positron pair or a muon-antimuon pair. The search is sensitive to heavy neutral Z′ gauge bosons, Randall-Sundrum gravitons, Z * bosons, techni-mesons, Kaluza-Klein Z/γ bosons, and bosons predicted by Torsion models. Results are presented based on an analysis of pp collisions at a center-of-mass energy of 7 TeV corresponding to an integrated luminosity of 4.9 fb−1 in the e + e − channel and 5.0 fb−1 in the μ + μ −channel. A Z ′ boson with Standard Model-like couplings is excluded at 95 % confidence level for masses below 2.22 TeV. A Randall-Sundrum graviton with coupling $ {k \left/ {{{{\overline{M}}_{\mathrm{Pl}}}}} \right.}=0.1 $ is excluded at 95 % confidence level for masses below 2.16 TeV. Limits on the other models are also presented, including Technicolor and Minimal Z′ Models.

Measuring heavy neutrino couplings at the LHC

The existence of heavy neutrinos mediating neutrino masses via a type-I seesaw can be directly probed at the LHC, or indirectly in rare lepton flavor-violating processes. The synergy between these two approaches requires a direct measurement of the heavy neutrino couplings. We discuss a strategy to perform such measurements at the LHC in the context of left-right symmetric models, which is also applicable to other models implementing a type-I or type-III seesaw. We demonstrate that the ambiguities in the determination of the heavy neutrino mixing parameters can be resolved by performing an exclusive analysis of dilepton final states, discriminated by flavor and missing energy.

Phenomenology of light sneutrino dark matter in cMSSM/mSUGRA with inverse seesaw

We study the possibility of a light Dark Matter (DM) within a constrained Minimal Supersymmetric Standard Model (cMSSM) framework augmented by a SM singlet-pair sector to account for the non-zero neutrino masses by inverse seesaw mechanism. Working within a 'hybrid' scenario with the MSSM sector fixed at high scale and the singlet neutrino sector at low scale, we find that, contrary to the case of the usual cMSSM where the neutralino DM cannot be very light, we can have a light sneutrino DM with mass below 100 GeV satisfying all the current experimental constraints from cosmology, collider as well as low-energy experiments. We also note that the supersymmetric inverse seesaw mechanism with sneutrino as the lightest supersymmetric partner can have enhanced same-sign dilepton final states with large missing transverse energy (mET) coming from the gluino- and squark-pair as well as the squark-gluino associated productions and their cascade decay through charginos. We present a collider study for the same-sign dilepton+jets+mET signal in this scenario and propose some distinctions with the usual cMSSM. We also comment on the implications of such a light DM scenario on the invisible decay width of an 125 GeV Higgs boson.

Evidence for the associated production of a W boson and a top quark in ATLAS at [formula omitted

This Letter presents evidence for the associated production of a W boson and a top quark using 2.05 fb−1 of pp collision data at s=7TeV accumulated with the ATLAS detector at the LHC. The analysis is based on the selection of the dileptonic final states with events featuring two isolated leptons, electron or muon, with significant transverse missing momentum and at least one jet. An approach based on boosted decision trees has been developed to improve the discrimination of single top-quark Wt events from background. A template fit to the final classifier distributions is performed to determine the cross-section. The result is incompatible with the background-only hypothesis at the 3.3σ level, the expected sensitivity assuming the Standard Model production rate being 3.4σ. The corresponding cross-section is determined and found to be σWt=16.8±2.9 (stat)±4.9 (syst) pb, in good agreement with the Standard Model expectation. From this result the CKM matrix element |Vtb|=1.03−0.19+0.16 is derived assuming that the Wt production through |Vts| and |Vtd| is small.

Search for heavy, top-like quark pair production in the dilepton final state in pp collisions at [formula omitted

The results of a search for pair production of a heavy, top-like quark, t′, in the decay mode t′t¯′→bW+b¯W−→bℓ+νb¯ℓ−ν¯ are presented. The search is performed with a data sample corresponding to an integrated luminosity of 5.0 fb−1 in pp collisions at a center-of-mass energy of 7 TeV, collected by the CMS experiment at the LHC. The observed number of events agrees with the expectation from standard model processes, and no evidence of t′t¯′ production is found. Upper limits on the production cross section as a function of t′ mass are presented, and t′ masses below 557 GeV/c2 are excluded at the 95% confidence level.

Search for pair-produced heavy quarks decaying toWqin the two-lepton channel at(s)=7 TeVwith the ATLAS detector

A search is presented for heavy-quark pair production (QQbar) under the decay hypothesis QQbar to WqWq with q = u,d,c,s,b. The search is performed with 1.04 fb^-1 of integrated luminosity from pp collisions at sqrt(s) = 7 TeV collected by the ATLAS detector at the CERN LHC. Dilepton final states are selected, requiring large missing transverse momentum and at least two jets. Mass reconstruction of heavy quark candidates is performed by assuming that the W boson decay products are nearly collinear. The data are in agreement with Standard Model expectations; a heavy quark with mass less than 350 GeV is excluded at 95% confidence level.

LHC and Tevatron bounds on the dark matter direct detection cross-section for vector mediators

We study the interactions of a new spin-1 mediator that connects the Standard Model to dark matter. We constrain its decay channels using monojet and monophoton searches, as well as searches for resonances in dijet, dilepton and diboson final states including those involving a possible Higgs. We then interpret the resulting limits as bounds on the cross-section for dark matter direct detection without the need to specify a particular model. For mediator masses between 300 and 1000 GeV these bounds are considerably stronger than the ones obtained under the assumption that the mediator can be integrated out.

Evidence for Spin Correlation intt¯Production

We present a measurement of the ratio of events with correlated t and tbar spins to the total number of ttbar events. This ratio f is evaluated using a matrix-element-based approach in events with a single lepton (electron or muon) and at least four jets. We analyze ppbar collisions data corresponding to an integrated luminosity of 5.3 fb^-1 collected with the D0 detector at the Fermilab Tevatron collider operating at a center of mass energy $\sqrt{s}=1.96$ TeV. Combining this result with a recent measurement of f in dilepton final states, we find f in agreement with the standard model. In addition, the combination provides evidence for the presence of spin correlation in ttbar events with a significance of more than three standard deviations.

Determination of the top quark mass from the ${\rm{t\bar t}}$ cross section measured by CMS at√s= 7 TeV

Higher-order QCD predictions are used to extract the top quark mass, both in the pole and in the $\bar{\mathrm{MS}}$ scheme, from the top quark pair production cross section measured in the dilepton final state. The analysed dataset corresponds to an integrated luminosity of 1.14 fb$^{-1}$ collected by the CMS experiment in 2011 in proton-proton collisions at $\sqrt{s} = 7$ TeV.

Search for Single Top tW Associated Production in the Dilepton Channel at CMS

We present a first study of the single top quark W-associated production (tW) in proton-proton collisions at the LHC at a centre-of-mass energy of 7TeV, using data collected with the CMS experiment. The search is performed in the dileptonic final states ee/e\mu/\mu\mu\ with a selection based on kinematical properties and b-tagging information. The contribution of the Z+jets processes to the background is estimated from a sideband in data. Two ttbar dominated control regions are used to constrain the normalization of top quark pair production in the signal region. An excess of events over the expected background is observed. Assigning this excess to events from tW production, the extracted tW cross section is in agreement with the Standard Model expectation.

Search for Dilepton Resonances inppCollisions ats=7 TeVwith the ATLAS Detector

This Letter reports on a search for narrow high-mass resonances decaying into dilepton final states. The data were recorded by the ATLAS experiment in pp collisions at sqrt(s) = 7 TeV at the LHC and correspond to a total integrated luminosity of 1.08/fb (1.21/fb) in the ee (mumu) channel. No statistically significant excess above the Standard Model expectation is observed and upper limits are set at the 95% C.L. on the cross section times branching ratio of Z' resonances and Randall-Sundrum gravitons decaying into dileptons as a function of the resonance mass. A lower mass limit of 1.83 TeV on the Sequential Standard Model Z' boson is set. A Randall-Sundrum graviton with coupling k/M_Pl=0.1 is excluded at 95% C.L. for masses below 1.63 TeV.

How to pin down theCPquantum numbers of a Higgs boson in itsτdecays at the LHC

We investigate how the $CP$ quantum numbers of a neutral Higgs boson or spin-zero resonance $\ensuremath{\Phi}$, produced at the CERN Large Hadron Collider, can be determined in its $\ensuremath{\tau}$-pair decay mode $\ensuremath{\Phi}\ensuremath{\rightarrow}{\ensuremath{\tau}}^{\ensuremath{-}}{\ensuremath{\tau}}^{+}$. We use a method [S. Berge and W. Bernreuther, Phys. Lett. B 671, 470 (2009).] based on the distributions of two angles and apply it to the major 1-prong $\ensuremath{\tau}$ 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 collect the event numbers needed to discriminate between a $CP$-even and -odd Higgs boson and/or between Higgs boson(s) with $CP$-conserving and $CP$-violating couplings after several years of high-luminosity runs.

Measurement of the top quark pair production cross section in pp collisions at [formula omitted] in dilepton final states with ATLAS

A measurement of the production cross section of top quark pairs (tt¯) in proton–proton collisions at a center-of-mass energy of 7 TeV recorded with the ATLAS detector at the Large Hadron Collider is reported. Candidate events are selected in the dilepton topology with large missing transverse energy and at least two jets. Using a data sample corresponding to an integrated luminosity of 35 pb−1, a tt¯ production cross section σtt¯=177±20(stat.)±14(syst.)±7(lum.) pb is measured for an assumed top quark mass of mt=172.5 GeV. A second measurement requiring at least one jet identified as coming from a b quark yields a comparable result, demonstrating that the dilepton final states are consistent with being accompanied by b-quark jets. These measurements are in good agreement with Standard Model predictions.

LHC signatures of the constrained exceptional supersymmetric standard model

We discuss two striking Large Hadron Collider (LHC) signatures of the constrained version of the exceptional supersymmetric standard model (cE6SSM), based on a universal high energy soft scalar mass m_0, soft trilinear coupling A_0 and soft gaugino mass M_{1/2}. The first signature we discuss is that of light exotic colour triplet charge 1/3 fermions, which we refer to as D-fermions. We calculate the LHC production cross section of D-fermions, and discuss their decay patterns. Secondly we discuss the E6 type U(1)_N spin-1 Z' gauge boson and show how it may decay into exotic states, increasing its width and modifying the line shape of the dilepton final state. We illustrate these features using two representative cE6SSM benchmark points, including an "early LHC discovery" point, giving the Feynman rules and numerical values for the relevant couplings in order to facilitate further studies.

CP-violatingHWWcouplings at the Large Hadron Collider

We investigate the possibility of probing an anomalous CP-violating coupling in the HWW vertex at the LHC. We consider the production of the Higgs in association of a W and then decay via the $H \rightarrow WW$ channel taking into account the limits on the Higgs production cross section from the Tevatron. We select the same-sign dilepton final state arising from leptonic decays of two of the three Ws and apply cuts required to suppress the standard model background. Several kinematical distributions and asymmetries that can be used to ascertain the presence of a non-zero anomalous coupling are presented. We find that, for Higgs mass in the range 130-150 GeV and anomalous couplings allowed by the Tevatron data, these distributions can be studied with an integrated luminosity of 30-50 fb$^{-1}$ at the 14 TeV run. Attention is specifically drawn to some asymmetries that enable one to probe the real and imaginary parts (as well as their signs) of the anomalous coupling, in a complementary manner. We also explicitly demonstrate that showering and hadronisation do not affect the utility of these variables, thus affirming the validity of parton level calculations.

Measurement of theWboson helicity in top quark decays using5.4 fb−1ofpp¯collision data

We present a measurement of the helicity of the W boson produced in top quark decays using ttbar decays in the l+jets and dilepton final states selected from a sample of 5.4 fb^-1 of collisions recorded using the D0 detector at the Fermilab Tevatron ppbar collider. We measure the fractions of longitudinal and right-handed W bosons to be f_0 = 0.669 +- 0.102 [ +- 0.078 (stat.) +- 0.065 (syst.)] and f_+ = 0.023 +- 0.053 [+- 0.041 (stat.) +- 0.034 (syst.)], respectively. This result is consistent at the 98% level with the standard model. A measurement with f_0 fixed to the value from the standard model yields f_+ = 0.010 +- 0.037 [+- 0.022 (stat.) +- 0.030 (syst.) ].