11433826
Isolated Leptons with missing transverse momentum and Search for single top production
Recent results of searches for production of isolated leptons in events w ith large missing transverse momentum in e±p collisions at HERA are reviewed. The H1 collaboration observes an exce ss of these events over the, W-production dominated, Standard Model (SM) predictions in e+ p collisions, this excess is not confirmed by ZEUS. In e−p collisions the rate of these events observed by the H1 collaboration is consistent with th e Standard Model predictions. A potential new-physics source of isolated leptons in events with large missing transve rse momentum at HERA is single top production via flavour changing neutral currents (FCNC). The most rece nt results of searches for this process from the H1 collaboration are presented including limits on the FCNC couplings. The HERA limits remain the strongest on photonic FCNC. International Europhysics Conference on High Energy Physics 2005 Lisboa, Portugal Table 1: The numbers of observed events in different PX T ranges in the H1 isolated high pT lepton search compared to the SM expectations. The fraction of singleW production included in the expectation is indicated in pare nth ses. The statistical and systematic uncertainties, added in quadra ture, are also indicated. Electron Muon Combined H1 Preliminary obs./exp. ( W ) obs./exp. ( W ) obs./exp. ( W ) 1994-2004 Full Sample 19/14.6±2.0(70%) 9/3.9±0.6(84%) 28/18.5±2.7(73%) 158 pb−1 e+ p pT > 25 GeV 9/2.3±0.4(80%) 6/2.3±0.4(84%) 15/4.6±0.8(82%) 1998-2005 Full Sample 6/5.8±0.9(62%) 0/1.5±0.5(76%) 6/7.3±1.4(65%) 53 pb−1 e−p pT > 25 GeV 2/0.9±0.2(71%) 0/0.9±0.2(73%) 2/1.8±0.3(72%) 1994-2005 Full Sample 25/20.4±2.9(68%) 9/5.4±1.1(82%) 34/25.7±4.0(71%) 211 pb−1 e±p pT > 25 GeV 11/3.2±0.6(77%) 6/3.2±0.5(81%) 17/6.4±1.1(79%) Table 2: The numbers of observed events in the ZEUS isolated l epton search, compared to the SM expectations. The fraction of singleW production included in the expectation is indicated in pare nth ses. The statistical and systematic uncertainties added in quadrature are also indicated. Isolatede candidates 12< PX T < 25 GeV P X T > 25 GeV ZEUS (prel.) 99-00 (66 pb −1) 1/1.04±0.11(57%) 1/0.92±0.09(79%) ZEUS (prel.) 03-04 (40 pb −1) 0/0.46±0.10(64%) 0/0.58+0.08 −0.09(76%) 1 Isolated leptons in events with missing transverse momentum The observation of anomalous events with an isolated high en ergy lepton and missing transverse momentum in ep collisions at HERA was reported by the H1 collaboration at the end o f the 1994-2000 running period [1]. Production of single W bosons with subsequent leptonic decay is the only process wi th a measurable cross section at HERA which leads to this type of events. The dominant leading order Feynman diag ram for W production is shown in Fig.1(a); the total cross section is about 1 pb, evaluated including O(ααs) QCD corrections [2]. New results in searches for isolated electrons and muons fro m H1 for the 1994-2005 running period are summarised in Table 1 [3]. An excess of observed data over the SM expectat ions can be seen in the H1 results, this is illustrated in Fig.1(b). This excess is actually observed only in the e+ p data (see Table 1), it is not confirmed in a similar search by th e ZEUS collaboration [4], which is summarised in Table 2. The e xcess over the SM prediction at high values of hadronic PT is suggestive of a heavy particle decay process. More specia lis d searches have been applied to look for high pT leptons arising from the leptonic decay channel of single-top produ ction, as discussed in section 2. A slight excess in theτ channel was reported by ZEUS for 130 pb −1 of data from the 1994-2000 running period [5]. The results of the latest H1 search for isolated τ leptons using 108 pb −1 of data from the 1996-2000 running period [6] are given in Table 3. No excess over the SM is observed.
119312790
Study of taupair production at HERA
A study of events containing two tau leptons with high transverse momentum has been performed with the ZEUS detector at HERA, using a data sample corresponding to an integrated luminosity of 0.33 fb−1. The tau candidates were identified from their decays into electrons, muons or hadronic jets. The number of tau-pair candidates has been compared with the prediction from the Standard Model, where the largest contribution is expected from Bethe-Heitler processes. The total visible cross section was extracted. Standard Model expectations agree well with the measured distributions, also at high invariant mass of the tau pair.
59418732
HERA signature-based searches I: events with isolated leptons and missing transverse momentum at HERA
Recent results of searches for events containing isolated leptons and missing transverse momentum at HERA are presented. Searches in this final state have yielded notable excesses over Standard Model expectations in the past. Searches for isolated leptons in channels corresponding to all three generations of leptons over the full HERA running period are now available. The combined H1+ZEUS results for searches for electrons and muons are compatible with the Standard Model.
113198029
Triggering on Long-Lived Neutral Particles in the ATLAS Detector
Neutral particles with long decay paths that decay to many-particle final states represent, from an experimental point of view, a challenge both for the trigger and for the reconstruction capabilities of the ATLAS apparatus. The Hidden Valley scenario serves as an excellent setting for the purpose of exploring the challenges to the trigger posed by long-lived particles.
96422847
Exclusive electroproduction of two pions at HERA
The exclusive electroproduction of two pions in the mass range 0.4<Mππ<2.5 GeV has been studied with the ZEUS detector at HERA using an integrated luminosity of 82 pb−1. The analysis was carried out in the kinematic range of 2<Q2<80 GeV2, 32<W<180 GeV and |t|<0.6 GeV2, where Q2 is the photon virtuality, W is the photon-proton centre-of-mass energy and t is the squared four-momentum transfer at the proton vertex. The two-pion invariant-mass distribution is interpreted in terms of the pion electromagnetic form factor, |F(Mππ)|, assuming that the studied mass range includes the contributions of the ρ, ρ′ and ρ′′ vector-meson states. The masses and widths of the resonances were obtained and the Q2 dependence of the cross-section ratios σ(ρ′→ππ)/σ(ρ) and σ(ρ′′→ππ)/σ(ρ) was extracted. The pion form factor obtained in the present analysis is compared to that obtained in e+e−→π+π−.
119163803
Inclusive-jet photoproduction at HERA and determination of alpha(s)
Abstract Inclusive-jet cross sections have been measured in the reaction e p → e + jet + X for photon virtuality Q 2 1 GeV 2 and γp centre-of-mass energies in the region 142 W γ p 293 GeV with the ZEUS detector at HERA using an integrated luminosity of 300 pb − 1 . Jets were identified using the k T , anti- k T or SIScone jet algorithms in the laboratory frame. Single-differential cross sections are presented as functions of the jet transverse energy, E T jet , and pseudorapidity, η jet , for jets with E T jet > 17 GeV and − 1 η jet 2.5 . In addition, measurements of double-differential inclusive-jet cross sections are presented as functions of E T jet in different regions of η jet . Next-to-leading-order QCD calculations give a good description of the measurements, except for jets with low E T jet and high η jet . The influence of non-perturbative effects not related to hadronisation was studied. Measurements of the ratios of cross sections using different jet algorithms are also presented; the measured ratios are well described by calculations including up to O ( α s 2 ) terms. Values of α s ( M Z ) were extracted from the measurements and the energy-scale dependence of the coupling was determined. The value of α s ( M Z ) extracted from the measurements based on the k T jet algorithm is α s ( M Z ) = 0.1206 − 0.0022 + 0.0023 ( exp . ) − 0.0035 + 0.0042 ( th . ) ; the results from the anti- k T and SIScone algorithms are compatible with this value and have a similar precision.
119195324
Positioning systems in Minkowski space-time: Bifurcation problem and observational data
In the framework of relativistic positioning systems in Minkowski space-time, the determination of the inertial coordinates of a user involves the {\em bifurcation problem} (which is the indeterminate location of a pair of different events receiving the same emission coordinates). To solve it, in addition to the user emission coordinates and the emitter positions in inertial coordinates, it may happen that the user needs to know {\em independently} the orientation of its emission coordinates. Assuming that the user may observe the relative positions of the four emitters on its celestial sphere, an observational rule to determine this orientation is presented. The bifurcation problem is thus solved by applying this observational rule, and consequently, {\em all} of the parameters in the general expression of the coordinate transformation from emission coordinates to inertial ones may be computed from the data received by the user of the relativistic positioning system.
14983479
On the geometry of Killing and conformal tensors
The second order Killing and conformal tensors are analyzed in terms of their spectral decomposition, and some properties of the eigenvalues and the eigenspaces are shown. When the tensor is of type I with only two different eigenvalues, the condition to be a Killing or a conformal tensor is characterized in terms of its underlying almost-product structure. A canonical expression for the metrics admitting these kinds of symmetries is also presented. The space–time cases 1+3 and 2+2 are analyzed in more detail. Starting from this approach to Killing and conformal tensors a geometric interpretation of some results on quadratic first integrals of the geodesic equation in vacuum Petrov-Bel type D solutions is offered. A generalization of these results to a wider family of type D space–times is also obtained.
119251566
A note on static metrics: the degenerate case
We give the necessary and sufficient conditions for a 3-metric to be the adapted spatial metric of a static vacuum solution. This work accomplishes for the degenerate cases the already known study for the regular ones (Bartnik and Tod 2006 Class. Quantum Grav. 23 569–71).
118482874
Positioning in a flat two-dimensional space-time: The delay master equation
The basic theory on relativistic positioning systems in a two-dimensional space-time has been presented in two previous papers [B. Coll, J. J. Ferrando, and J. A. Morales, Phys. Rev. D 73, 084017 (2006); ibid.74, 104003 (2006)], where the possibility of making relativistic gravimetry with these systems has been analyzed by considering specific examples. Here, generic relativistic positioning systems in the Minkowski plane are studied. The information that can be obtained from the data received by a user of the positioning system is analyzed in detail. In particular, it is shown that the accelerations of the emitters and of the user along their trajectories are determined by the sole knowledge of the emitter positioning data and of the acceleration of only one of the emitters. Moreover, as a consequence of the so-called master delay equation, the knowledge of this acceleration is only required during an echo interval, i.e., the interval between the emission time of a signal by an emitter and its reception time after being reflected by the other emitter. These results are illustrated with the obtention of the dynamics of the emitters and of the user from specific sets of data received by the user.
122117703
Newtonian and relativistic emission coordinates
Emission coordinates are those generated by positioning systems. Positioning systems are physical systems constituted by four emitters broadcasting their respective times by means of sound or light signals. We analyze the incidence of the space-time causal structure on the construction of emission coordinates. The Newtonian case of four emitters at rest is analyzed and contrasted with the corresponding situation in special relativity.