Search For New Particles Research Articles

Search for new particles at HERA

Recent results on the search for physics beyond the Standard Model at HERA are presented. The data show a general agreements with the SM and therefore limits on specific processes are reported.

Search for new particles, including the Higgs bosons, at LEP

Search for new particles, including the Higgs bosons, at LEP

Search for new particles at LEP

Search for new particles at LEP

Confidence level computation for combining searches with small statistics

This article describes an efficient procedure for computing approximate confidence levels for searches for new particles where the expected signal and background levels are small enough to require the use of Poisson statistics. The results of many independent searches for the same particle may be combined easily, regardless of the discriminating variables which may be measured for the candidate events. The effects of systematic uncertainty in the signal and background models are incorporated in the confidence levels. The procedure described allows efficient computation of expected confidence levels.

Searches with taus at the Tevatron

We discuss tau identification techniques at hadron colliders, and present the measurements and the searches performed so far. We report on top quark pair production in the decay channel containing at least one tau lepton. Also, we present results dedicated to searches for new particles, with taus in the final state. We present a search for the charged Higgs boson in the tau decay channel, as well as for the leptoquark family containing tau leptons. Finally, we indicate the capabilities of detecting and triggering on tau leptons in the future collider run.

High Q2 physics at HERA and searches for new particles

Preliminary results from H1 and ZEUS on e+p scattering at high momentum transfer squared, Q2, are presented, using all available data accumulated by the H1 and ZEUS experiments between 1994-97, corresponding to integrated luminosities of 37 pb-1 and 47 pb-1, respectively. The anomalies observed at high Q2 in the 1994-96 data remain, though with less significance. Since this high Q2 domain represents a new frontier in deep inelastic scattering (DIS), these data are used to search for new particles possessing direct couplings to lepton-quark pairs. Assuming that the slight excess of events observed in neutral current DIS is due to a statistical fluctuation, preliminary limits on the production of leptoquarks and of squarks in R-parity violating the minimal supersymmetric standard model are presented.

Search for exotic phenomena at HERA with ZEUS

Results on the search for exotic physics with the ZEUS detector at HERA are reviewed. We present limits on new physics from direct searches for new particles. Also, an excess in ZEUS data with respect to Standard Model expectation for e + p→ e + X at high x and Q 2 is reported.

Search for new particles decaying to dijets at CDF

We have used 106 pb^-1 of data collected with the Collider Detector at Fermilab to search for new particles decaying to dijets. We exclude at the 95% confidence level models containing the following new particles: axigluons and flavor universal colorons with mass between 200 and 980 GeV/c, excited quarks with mass between 80 and 570 GeV/c^2 and between 580 and 760 GeV/c^2, color octet technirhos with mass between 260 and 480 GeV/c^2, W' bosons with mass between 300 and 420 GeV/c^2, and E_6 diquarks with mass between 290 and 420 GeV/c^2.

Coloron phenomenology

A flavor-universal extension of the strong interactions was recently proposed in response to the apparent excess of high-${E}_{T}$ jets in the inclusive jet spectrum measured at the Fermilab Tevatron. This paper studies the color octet of massive gauge bosons (``colorons'') that is present in the low-energy spectrum of the model's Higgs phase. Constraints from searches for new particles decaying to dijets and from measurements of the weak-interaction $\ensuremath{\rho}$ parameter imply that the colorons must have masses greater than 870--1000 GeV. The implications of recent Tevatron data and the prospective input from future experiments are also discussed.

STUDY OF e+e− INTERACTIONS AT CENTER-OF-MASS ENERGIES OF 130 AND 136 GeV

At the end of 1995, the LEP collider at CERN was operated at center-of-mass energies of 130 and 136 GeV and data corresponding to about 6 pb−1 were collected by each of the four LEP experiments. The cross-sections for fermion-pair production processes and the forward-backward asymmetries for charged lepton pairs were measured and compared to the standard model predictions. Events containing only energetic photons in the final state were used to look for effects arising from new physics. Direct searches for new particles predicted by various models beyond the standard model were performed. Searches for pair or singly produced excited leptons, for unstable charged and neutral heavy leptons, and for supersymmetric particles (chargino, neutralino, scalar leptons and scalar top quark) resulted in new exclusion limits. The ALEPH collaboration reported an excess of four-jet events in its data, which was not confirmed by the other LEP experiments.

Search for new particles in hadronic events with isolated photons

A search for the neutral Higgs boson in the processes e +e − → Z → H 0γ → q q γ and e +e − → Z → Z ∗H 0 → q q γγ has been performed using 2.8 million hadronic Z decays collected with the L3 detector at LEP from 1991 through 1994. No evidence for these processes has been observed. Upper limits at 95% confidence level for the corresponding cross sections have been set and the results have been compared with theoretical predictions beyond the Standard Model.

Recent results of searches for new particles at fermilab

I review recent results from particle searches at Fermilab, with particular emphasis on the searches for the top quark at the Collider by DO and CDF.

Searches for new particles at HERA

The electron-proton collider HERA is a unique tool for the discovery of particles and interactions beyond the Standard Model. Results from the two experiments, H1 and ZEUS, are reported which have been obtained from data collected within the first two years of operation. Excited electrons, neutrinos and quarks, leptoquarks, leptogluons, squarks in a class of supersymmetric models, and also the search for contact interactions are covered.

Searches for new particles at LEP

The most recent developments in the domain of new particle searches at LEP are reviewed. In particular, the updates of the searches for the standard model Higgs boson and for the Higgs bosons of the MSSM are summarized. A new method is proposed to derive a 95% C.L. limit on the standard model Higgs boson mass from the combinations of the analyses of the four LEP experiments. Finally, two monojet events from ALEPH with unexpectedly large transverse momenta and invariant masses are presented.

Neutralino mass bounds in the Next-To-Minimal Supersymmetric Standard Model

We analyze the experimental data from the search for new particles at LEP 100 and obtain mass bounds for the neutralinos of the Next-To-Minimal Supersymmetric Standard Model (NMSSM). We find that for tan β ≳ 5.5 a massless neutralino is still possible, while the lower mass bound for the second lightest neutralino corresponds approximately to that for the lightest neutralino in the Minimal Supersymmetric Standard Model (MSSM).

Searches for new particles using cone and cluster jet algorithms: a comparative study

We discuss the reconstruction of the hadronic decays of heavy particles using jet algorithms. The ability to reconstruct the mass of the decaying particles is compared between a traditional cone-type algorithm and a recently proposed cluster-type algorithm. The specific examples considered are the semileptonic decays of a heavy Higgs boson at $$\sqrt s = 16$$ TeV, and of top quark-antiquark pairs at $$\sqrt s = 1.8$$ TeV. We find that the cluster algorithm offers considerable advantages in the former case, and a slight advantage in the latter. We briefly discuss the effects of calorimeter energy resolution, and show that a typical resolution dilutes these advantages, but does not remove them entirely.

A search for leptoquarks, leptogluons and excited leptons in H1 at HERA

A direct search for new particles in the H1 experiment at HERA is presented for masses ranging from 35 GeV up to ∼ 250 GeV. The data sample of 24 nb −1 accumulated during the first year of operation was analysed for signatures of scalar and vector leptoquarks, leptogluons, excited electrons and excited neutrinos with flavours of the first generation. No evidence for the production of such particles was found in various possible decay channels. Rejection limits are derived.

NEURAL NETWORKS FOR PHYSICS ANALYSIS IN DELPHI

The use of Artificial Neural Networks in physics analysis by the DELPHI experiment at LEP is reviewed. DELPHI has used Neural Networks to tag the primary flavour of hadronic Z0 decays, to enhance the sensitivity of the searches for new particles, to measure properties of quark and gluon jets, and to identify final state particles.

New particle searches at LEP

Considerable effort has been expended by the LEP collaborations since the start of data taking on the search for new particles and phenomena. Two early results provided evidence that only three families of quarks and leptons exist in nature: the width of the Z established to high precision the existence of only three neutrinos and a direct search for particles belonging to a fourth family proved negative. The search for the mass mechanism of the Standard Model has taken up much effort. Different models predict the existence of one or more Higgs bosons, of supersymmetric particles or of effects arising from compositeness; so far no evidence has been found for any such particles. The paper describes the principles, techniques and results of these searches by the four LEP collaborations. Improvements to some of the published limits, obtained by combining the separate results of the collaborations, are presented.

Rare decays of the Z 0 and searches for new particles

A summary is presented of the extensive number of particle searches performed in the last few months by the experiments at LEP and SLC. The measurements of (and limits obtained from) two-photon production are also reviewed.