Invisible Higgs Boson Decay into Massive Neutrinos of 4th Generation

Results from several recent experiments provide inderect evidences in the favor of existence of a 4th generation neutrino. Such a neutrino of mass about 50 GeV is compatible with current physical and astrophysical constraints and well motivated in the framework of superstring phenomenology. If sufficiently stable the existence of such a neutrino leads to the drastic change of Higgs boson physics: for a wide range of Higgs boson masses the dominant mode of Higgs boson decay is invisible and the branching ratios for the most promising modes of Higgs boson search are significantly reduced. The proper strategy of Higgs boson searches in such a framework is discussed. It is shown that in the same framework the absence of a signal in the search for invisible Higgs boson decay at LEP means either that the mass of Higgs is greater than 113.5 GeV or that the mass difference between the Higgs mass and doubled neutrino mass is small.Comment: 8 pages, 2 figure

Measurement of the open-charm contribution to the diffractive proton structure function

Production of D*+/-(2010) mesons in diffractive deep inelastic scattering has been measured with the ZEUS detector at HERA using an integrated luminosity of 82 pb^{-1}. Diffractive events were identified by the presence of a large rapidity gap in the final state. Differential cross sections have been measured in the kinematic region 1.5 < Q^2 < 200 GeV^2, 0.02 < y < 0.7, x_{IP} < 0.035, beta 1.5 GeV and |\eta(D*+/-)| < 1.5. The measured cross sections are compared to theoretical predictions. The results are presented in terms of the open-charm contribution to the diffractive proton structure function. The data demonstrate a strong sensitivity to the diffractive parton densities.Comment: 35 pages, 11 figures, 6 table

Photoproduction of D∗±D^{*\pm} mesons associated with a leading neutron

The photoproduction of $D^{*\pm} (2010)$ mesons associated with a leading neutron has been observed with the ZEUS detector in $ep$ collisions at HERA using an integrated luminosity of 80 pb$^{-1}$. The neutron carries a large fraction, {$x_L>0.2$}, of the incoming proton beam energy and is detected at very small production angles, {$\theta_n<0.8$ mrad}, an indication of peripheral scattering. The $D^*$ meson is centrally produced with pseudorapidity {$|\eta| 1.9$ GeV}, which is large compared to the average transverse momentum of the neutron of 0.22 GeV. The ratio of neutron-tagged to inclusive $D^*$ production is $8.85\pm 0.93({\rm stat.})^{+0.48}_{-0.61}({\rm syst.})\%$ in the photon-proton center-of-mass energy range {$130 <W<280$ GeV}. The data suggest that the presence of a hard scale enhances the fraction of events with a leading neutron in the final state.Comment: 28 pages, 4 figures, 2 table

Isolated tau leptons in events with large missing transverse momentum at HERA

A search for events containing isolated tau leptons and large missing transverse momentum, not originating from the tau decay, has been performed with the ZEUS detector at the electron-proton collider HERA, using 130 pb^-1 of integrated luminosity. A search was made for isolated tracks coming from hadronic tau decays. Observables based on the internal jet structure were exploited to discriminate between tau decays and quark- or gluon-induced jets. Three tau candidates were found, while 0.40 +0.12 -0.13 were expected from Standard Model processes, such as charged current deep inelastic scattering and single W-boson production. To search for heavy-particle decays, a more restrictive selection was applied to isolate tau leptons produced together with a hadronic final state with high transverse momentum. Two candidate events survive, while 0.20 +-0.05 events are expected from Standard Model processes.Comment: 28 pages, 4 figures, 3 tables, accepted by Phys. Lett. B. Updated with minor changes to the text requested by the journal refere

Search for contact interactions, large extra dimensions and finite quark radius in ep collisions at HERA

A search for physics beyond the Standard Model has been performed with high-Q^2 neutral current deep inelastic scattering events recorded with the ZEUS detector at HERA. Two data sets, e^+ p \to e^+ X and e^- p \to e^- X, with respective integrated luminosities of 112 pb^-1 and 16 pb^-1, were analyzed. The data reach Q^2 values as high as 40000 GeV^2. No significant deviations from Standard Model predictions were observed. Limits were derived on the effective mass scale in eeqq contact interactions, the ratio of leptoquark mass to the Yukawa coupling for heavy leptoquark models and the mass scale parameter in models with large extra dimensions. The limit on the quark charge radius, in the classical form factor approximation, is 0.85 10^-16 cm.Comment: 28 pages, 4 figures, accepted for publication in Physics Letters

Observation of Scaling Violations in Scaled Momentum Distributions at HERA

Charged particle production has been measured in deep inelastic scattering (DIS) events over a large range of $x$ and $Q^2$ using the ZEUS detector. The evolution of the scaled momentum, $x_p$, with $Q^2,$ in the range 10 to 1280 $GeV^2$, has been investigated in the current fragmentation region of the Breit frame. The results show clear evidence, in a single experiment, for scaling violations in scaled momenta as a function of $Q^2$.Comment: 21 pages including 4 figures, to be published in Physics Letters B. Two references adde

D* Production in Deep Inelastic Scattering at HERA

This paper presents measurements of D^{*\pm} production in deep inelastic scattering from collisions between 27.5 GeV positrons and 820 GeV protons. The data have been taken with the ZEUS detector at HERA. The decay channel $D^{*+}\to (D^0 \to K^- \pi^+) \pi^+$ (+ c.c.) has been used in the study. The $e^+p$ cross section for inclusive D^{*\pm} production with $5<Q^2<100 GeV^2$ and $y<0.7$ is 5.3 \pms 1.0 \pms 0.8 nb in the kinematic region {$1.3<p_T(D^{*\pm})<9.0$ GeV and $| \eta(D^{*\pm}) |<1.5$}. Differential cross sections as functions of p_T(D^{*\pm}), $\eta(D^{*\pm}), W$ and $Q^2$ are compared with next-to-leading order QCD calculations based on the photon-gluon fusion production mechanism. After an extrapolation of the cross section to the full kinematic region in p_T(D^{*\pm}) and $\eta$(D^{*\pm}), the charm contribution $F_2^{c\bar{c}}(x,Q^2)$ to the proton structure function is determined for Bjorken $x$ between 2 $\cdot$ 10$^{-4}$ and 5 $\cdot$ 10$^{-3}$.Comment: 17 pages including 4 figure

Measurement of dijet photoproduction for events with a leading neutron at HERA

Differential cross sections for dijet photoproduction and this process in association with a leading neutron, e+ + p -> e+ + jet + jet + X (+ n), have been measured with the ZEUS detector at HERA using an integrated luminosity of 40 pb-1. The fraction of dijet events with a leading neutron was studied as a function of different jet and event variables. Single- and double-differential cross sections are presented as a function of the longitudinal fraction of the proton momentum carried by the leading neutron, xL, and of its transverse momentum squared, pT^2. The dijet data are compared to inclusive DIS and photoproduction results; they are all consistent with a simple pion-exchange model. The neutron yield as a function of xL was found to depend only on the fraction of the proton beam energy going into the forward region, independent of the hard process. No firm conclusion can be drawn on the presence of rescattering effects.Comment: 40 pages, 18 figure

Deep inelastic inclusive and diffractive scattering at Q2Q^2 values from 25 to 320 GeV2^2 with the ZEUS forward plug calorimeter

Deep inelastic scattering and its diffractive component, $ep \to e^{\prime}\gamma^* p \to e^{\prime}XN$, have been studied at HERA with the ZEUS detector using an integrated luminosity of 52.4 pb$^{-1}$. The $M_X$ method has been used to extract the diffractive contribution. A wide range in the centre-of-mass energy $W$ (37 -- 245 GeV), photon virtuality $Q^2$ (20 -- 450 GeV$^2$) and mass $M_X$ (0.28 -- 35 GeV) is covered. The diffractive cross section for $2 < M_X < 15$ GeV rises strongly with $W$, the rise becoming steeper as $Q^2$ increases. The data are also presented in terms of the diffractive structure function, $F^{\rm D(3)}_2$, of the proton. For fixed $Q^2$ and fixed $M_X$, \xpom F^{\rm D(3)}_2 shows a strong rise as \xpom \to 0, where \xpom is the fraction of the proton momentum carried by the Pomeron. For Bjorken-$x < 1 \cdot 10^{-3}$, \xpom F^{\rm D(3)}_2 shows positive $\log Q^2$ scaling violations, while for $x \ge 5 \cdot 10^{-3}$ negative scaling violations are observed. The diffractive structure function is compatible with being leading twist. The data show that Regge factorisation is broken.Comment: 89 pages, 27 figure

Observation of Events with an Energetic Forward Neutron in Deep Inelastic Scattering at HERA

In deep inelastic neutral current scattering of positrons and protons at the center of mass energy of 300 GeV, we observe, with the ZEUS detector, events with a high energy neutron produced at very small scattering angles with respect to the proton direction. The events constitute a fixed fraction of the deep inelastic, neutral current event sample independent of Bjorken x and Q2 in the range 3 · 10-4 \u3c xBJ \u3c 6 · 10-3 and 10 \u3c Q2 \u3c 100 GeV2