A High Luminosity e+e- Collider to study the Higgs Boson

A strong candidate for the Standard Model Scalar boson, H(126), has been discovered by the Large Hadron Collider (LHC) experiments. In order to study this fundamental particle with unprecedented precision, and to perform precision tests of the closure of the Standard Model, we investigate the possibilities offered by An e+e- storage ring collider. We use a design inspired by the B-factories, taking into account the performance achieved at LEP2, and imposing a synchrotron radiation power limit of 100 MW. At the most relevant centre-of-mass energy of 240 GeV, near-constant luminosities of 10^34 cm^{-2}s^{-1} are possible in up to four collision points for a ring of 27km circumference. The achievable luminosity increases with the bending radius, and for 80km circumference, a luminosity of 5 10^34 cm^{-2}s^{-1} in four collision points appears feasible. Beamstrahlung becomes relevant at these high luminosities, leading to a design requirement of large momentum acceptance both in the accelerating system and in the optics. The larger machine could reach the top quark threshold, would yield luminosities per interaction point of 10^36 cm^{-2}s^{-1} at the Z pole (91 GeV) and 2 10^35 cm^{-2}s^{-1} at the W pair production threshold (80 GeV per beam). The energy spread is reduced in the larger ring with respect to what is was at LEP, giving confidence that beam polarization for energy calibration purposes should be available up to the W pair threshold. The capabilities in term of physics performance are outlined.Comment: Submitted to the European Strategy Preparatory Group 01-04-2013 new version as re-submitted to PRSTA

Distinguishing Various Models of the 125 GeV Boson in Vector Boson Fusion

The hint of a new particle around 125 GeV at the LHC through the decay modes of diphoton and a number of others may point to quite a number of possibilities. While at the LHC the dominant production mechanism for the Higgs boson of the standard model and some other extensions is via the gluon fusion process, the alternative vector boson fusion is more sensitive to electroweak symmetry breaking through the gauge-Higgs couplings and therefore can be used to probe for models beyond the standard model. In this work, using the well known dijet-tagging technique to single out the vector boson fusion mechanism, we investigate its capability to discriminate a number of models that have been suggested to give an enhanced inclusive diphoton production rate, including the standard model Higgs boson, fermiophobic Higgs boson, Randall-Sundrum radion, inert-Higgs-doublet model, two-Higgs-doublet model, and the MSSM. The rates in vector-boson fusion can give more information of the underlying models to help distinguishing among the models.Comment: 31 pages, 3 figures; in this version some wordings are change

Excess Higgs Production in Neutralino Decays

The ATLAS and CMS experiments have recently claimed discovery of a Higgs boson-like particle at ~5 sigma confidence and are beginning to test the Standard Model predictions for its production and decay. In a variety of supersymmetric models, a neutralino NLSP can decay dominantly to the Higgs and the LSP. In natural SUSY models, a light third generation squark decaying through this chain can lead to large excess Higgs production while evading existing BSM searches. Such models can be observed at the 8 TeV LHC in channels exploiting the rare diphoton decays of the Higgs produced in the cascade decay. Identifying a diphoton resonance in association with missing energy, a lepton, or b-tagged jets is a promising search strategy for discovery of these models, and would immediately signal new physics involving production of a Higgs boson. We also discuss the possibility that excess Higgs production in these SUSY decays can be responsible for enhancements of up to 50% over the SM prediction for the observed rate in the existing inclusive diphoton searches, a scenario which would likely by the end of the 8 TeV run be accompanied by excesses in the diphoton + lepton/MET and SUSY multi-lepton/b searches and a potential discovery in a diphoton + 2b search.Comment: 42 pages, 19 figure

Search for the Higgs boson in events with missing transverse energy and b quark jets produced in proton-antiproton collisions at s**(1/2)=1.96 TeV

We search for the standard model Higgs boson produced in association with an electroweak vector boson in events with no identified charged leptons, large imbalance in transverse momentum, and two jets where at least one contains a secondary vertex consistent with the decay of b hadrons. We use ~1 fb-1 integrated luminosity of proton-antiproton collisions at s**(1/2)=1.96 TeV recorded by the CDF II experiment at the Tevatron. We find 268 (16) single (double) b-tagged candidate events, where 248 +/- 43 (14.4 +/- 2.7) are expected from standard model background processes. We place 95% confidence level upper limits on the Higgs boson production cross section for several Higgs boson masses ranging from 110 GeV/c2 to 140 GeV/c2. For a mass of 115 GeV/c2 the observed (expected) limit is 20.4 (14.2) times the standard model prediction.Comment: 8 pages, 2 figures, submitted to Phys. Rev. Let

Measurement of the ttbar Production Cross Section in ppbar Collisions at sqrt{s} = 1.96 TeV using Kinematic Characteristics of Lepton + Jets Events

We present a measurement of the top quark pair ttbar production cross section in ppbar collisions at a center-of-mass energy of 1.96 TeV using 230 pb**{-1} of data collected by the DO detector at the Fermilab Tevatron Collider. We select events with one charged lepton (electron or muon), large missing transverse energy, and at least four jets, and extract the ttbar content of the sample based on the kinematic characteristics of the events. For a top quark mass of 175 GeV, we measure sigma(ttbar) = 6.7 {+1.4-1.3} (stat) {+1.6- 1.1} (syst) +/-0.4 (lumi) pb, in good agreement with the standard model prediction.Comment: submitted to Phys.Rev.Let

Measurement of the ttbar Production Cross Section in ppbar Collisions at sqrt(s)=1.96 TeV using Lepton + Jets Events with Lifetime b-tagging

We present a measurement of the top quark pair ($t\bar{t}$) production cross section ($\sigma_{t\bar{t}}$) in $p\bar{p}$ collisions at $\sqrt{s}=1.96$ TeV using 230 pb$^{-1}$ of data collected by the D0 experiment at the Fermilab Tevatron Collider. We select events with one charged lepton (electron or muon), missing transverse energy, and jets in the final state. We employ lifetime-based b-jet identification techniques to further enhance the $t\bar{t}$ purity of the selected sample. For a top quark mass of 175 GeV, we measure $\sigma_{t\bar{t}}=8.6^{+1.6}_{-1.5}(stat.+syst.)\pm 0.6(lumi.)$ pb, in agreement with the standard model expectation.Comment: 7 pages, 2 figures, 3 tables Submitted to Phys.Rev.Let

Search for Third Generation Vector Leptoquarks in p anti-p Collisions at sqrt(s) = 1.96 TeV

We describe a search for a third generation vector leptoquark (VLQ3) that decays to a b quark and tau lepton using the CDF II detector and 322 pb^(-1) of integrated luminosity from the Fermilab Tevatron. Vector leptoquarks have been proposed in many extensions of the standard model (SM). Observing a number of events in agreement with SM expectations, assuming Yang-Mills (minimal) couplings, we obtain the most stringent upper limit on the VLQ3 pair production cross section of 344 fb (493 fb) and lower limit on the VLQ3 mass of 317 GeV/c^2 (251 GeV/c^2) at 95% C.L.Comment: 7 pages, 2 figures, submitted to PR

Observation and Mass Measurement of the Baryon Ξb−\Xi^-_b

We report the observation and measurement of the mass of the bottom, strange baryon $\Xi^-_b$ through the decay chain $\Xi^-_b \to J/\psi \Xi^-$, where $J/\psi \to \mu^+ \mu^-$, $\Xi^- \to \Lambda \pi^-$, and $\Lambda \to p \pi^-$. Evidence for observation is based on a signal whose probability of arising from the estimated background is 6.6 x 10^{-15}, or 7.7 Gaussian standard deviations. The $\Xi^-_b$ mass is measured to be $5792.9\pm 2.5$ (stat.) $\pm 1.7$ (syst.) MeV/$c^2$.Comment: Minor text changes for the second version. Accepted by Phys. Rev. Let

Polarizations of J/psi and psi(2S) Mesons Produced in ppbar Collisions at 1.96 TeV

We have measured the polarizations of \jpsi and \psiprime mesons as functions of their transverse momentum \pt when they are produced promptly in the rapidity range $|y|<0.6$ with \pt \geq 5 \pgev. The analysis is performed using a data sample with an integrated luminosity of about 800 \ipb collected by the CDF II detector. For both vector mesons, we find that the polarizations become increasingly longitudinal as \pt increases from 5 to 30 \pgev. These results are compared to the predictions of nonrelativistic quantum chromodynamics and other contemporary models. The effective polarizations of \jpsi and \psiprime mesons from $B$-hadron decays are also reported.Comment: 8 pages, 7 figures, published in Physical Review Letter