Observing the First Stars and Black Holes

The high sensitivity of JWST will open a new window on the end of the cosmological dark ages. Small stellar clusters, with a stellar mass of several 10^6 M_sun, and low-mass black holes (BHs), with a mass of several 10^5 M_sun should be directly detectable out to redshift z=10, and individual supernovae (SNe) and gamma ray burst (GRB) afterglows are bright enough to be visible beyond this redshift. Dense primordial gas, in the process of collapsing from large scales to form protogalaxies, may also be possible to image through diffuse recombination line emission, possibly even before stars or BHs are formed. In this article, I discuss the key physical processes that are expected to have determined the sizes of the first star-clusters and black holes, and the prospect of studying these objects by direct detections with JWST and with other instruments. The direct light emitted by the very first stellar clusters and intermediate-mass black holes at z>10 will likely fall below JWST's detection threshold. However, JWST could reveal a decline at the faint-end of the high-redshift luminosity function, and thereby shed light on radiative and other feedback effects that operate at these early epochs. JWST will also have the sensitivity to detect individual SNe from beyond z=10. In a dedicated survey lasting for several weeks, thousands of SNe could be detected at z>6, with a redshift distribution extending to the formation of the very first stars at z>15. Using these SNe as tracers may be the only method to map out the earliest stages of the cosmic star-formation history. Finally, we point out that studying the earliest objects at high redshift will also offer a new window on the primordial power spectrum, on 100 times smaller scales than probed by current large-scale structure data.Comment: Invited contribution to "Astrophysics in the Next Decade: JWST and Concurrent Facilities", Astrophysics & Space Science Library, Eds. H. Thronson, A. Tielens, M. Stiavelli, Springer: Dordrecht (2008

Structural alterations in a type IV pilus subunit protein result in concurrent defects in multicellular behaviour and adherence to host tissue

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/72384/1/j.1365-2958.2001.02629.x.pd

Search for lepton-flavor violation at HERA

A search for lepton-flavor-violating interactions $e p \to \mu X$ and $e p\to \tau X$ has been performed with the ZEUS detector using the entire HERA I data sample, corresponding to an integrated luminosity of 130 pb^{-1}. The data were taken at center-of-mass energies, $\sqrt{s}$, of 300 and 318 GeV. No evidence of lepton-flavor violation was found, and constraints were derived on leptoquarks (LQs) that could mediate such interactions. For LQ masses below $\sqrt{s}$, limits were set on $\lambda_{eq_1} \sqrt{\beta_{\ell q}}$, where $\lambda_{eq_1}$ is the coupling of the LQ to an electron and a first-generation quark $q_1$, and $\beta_{\ell q}$ is the branching ratio of the LQ to the final-state lepton $\ell$ ($\mu$ or $\tau$) and a quark $q$. For LQ masses much larger than $\sqrt{s}$, limits were set on the four-fermion interaction term $\lambda_{e q_\alpha} \lambda_{\ell q_\beta} / M_{\mathrm{LQ}}^2$ for LQs that couple to an electron and a quark $q_\alpha$ and to a lepton $\ell$ and a quark $q_\beta$, where $\alpha$ and $\beta$ are quark generation indices. Some of the limits are also applicable to lepton-flavor-violating processes mediated by squarks in $R$-Parity-violating supersymmetric models. In some cases, especially when a higher-generation quark is involved and for the process $e p\to \tau X$, the ZEUS limits are the most stringent to date.Comment: 37 pages, 10 figures, Accepted by EPJC. References and 1 figure (Fig. 6) adde

Measurement of (anti)deuteron and (anti)proton production in DIS at HERA

The first observation of (anti)deuterons in deep inelastic scattering at HERA has been made with the ZEUS detector at a centre-of-mass energy of 300--318 GeV using an integrated luminosity of 120 pb-1. The measurement was performed in the central rapidity region for transverse momentum per unit of mass in the range 0.3<p_T/M<0.7. The particle rates have been extracted and interpreted in terms of the coalescence model. The (anti)deuteron production yield is smaller than the (anti)proton yield by approximately three orders of magnitude, consistent with the world measurements.Comment: 26 pages, 9 figures, 5 tables, submitted to Nucl. Phys.

Multijet production in neutral current deep inelastic scattering at HERA and determination of alpha_s

Multijet production rates in neutral current deep inelastic scattering have been measured in the range of exchanged boson virtualities 10 < Q2 < 5000 GeV2. The data were taken at the ep collider HERA with centre-of-mass energy sqrt(s) = 318 GeV using the ZEUS detector and correspond to an integrated luminosity of 82.2 pb-1. Jets were identified in the Breit frame using the k_T cluster algorithm in the longitudinally invariant inclusive mode. Measurements of differential dijet and trijet cross sections are presented as functions of jet transverse energy E_{T,B}{jet}, pseudorapidity eta_{LAB}{jet} and Q2 with E_{T,B}{jet} > 5 GeV and -1 < eta_{LAB}{jet} < 2.5. Next-to-leading-order QCD calculations describe the data well. The value of the strong coupling constant alpha_s(M_Z), determined from the ratio of the trijet to dijet cross sections, is alpha_s(M_Z) = 0.1179 pm 0.0013(stat.) {+0.0028}_{-0.0046}(exp.) {+0.0064}_{-0.0046}(th.)Comment: 22 pages, 5 figure

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

Measurement of charm fragmentation ratios and fractions in photoproduction at HERA

The production of D^*+, D^0, D^+, D_s^+ and Lambda_c^+ charm hadrons and their antiparticles in ep scattering at HERA was measured with the ZEUS detector using an integrated luminosity of 79 pb^-1. The measurement has been performed in the photoproduction regime with the exchanged-photon virtuality Q^2 < 1 GeV^2 and for photon-proton centre-of-mass energies in the range 130 < W < 300 GeV. The charm hadrons were reconstructed in the range of transverse momentum p_T(D, Lambda_c) > 3.8 GeV and pseudorapidity |eta(D, Lambda_c)| < 1.6. The production cross sections were used to determine the ratio of neutral and charged D-meson production rates, R_u/d, the strangeness-suppression factor, gamma_s, and the fraction of charged D mesons produced in a vector state, P_v^d. The measured R_u/d and gamma_s values agree with those obtained in deep inelastic scattering and in e^+e^- annihilations. The measured P_v^d value is smaller than, but consistent with, the previous measurements. The fractions of c quarks hadronising as a particular charm hadron, f(c -> D, Lambda_c), were derived in the given kinematic range. The measured open-charm fragmentation fractions are consistent with previous results, although the measured f(c -> D^*+) is smaller and f(c -> Lambda_c^+) is larger than those obtained in e^+e^- annihilations. These results generally support the hypothesis that fragmentation proceeds independently of the hard sub-process.Comment: 29 pages, 5 figures, 6 tables; minor text revision

Observation of isolated high-E_T photons in deep inelastic scattering

First measurements of cross sections for isolated prompt photon production in deep inelastic ep scattering have been made using the ZEUS detector at the HERA electron-proton collider using an integrated luminosity of 121 pb^-1. A signal for isolated photons in the transverse energy and rapidity ranges 5 < E_T^gamma < 10 GeV and -0.7 < eta^gamma < 0.9 was observed for virtualities of the exchanged photon of Q^2 > 35 GeV^2. Cross sections are presented for inclusive prompt photons and for those accompanied by a single jet in the range E_T^jet \geq 6 GeV and -1.5 \leq eta^jet < 1.8. Calculations at order alpha^3alpha_s describe the data reasonably well.Comment: 16 pages, 5 figure

Measurement of beauty production in deep inelastic scattering at HERA

The beauty production cross section for deep inelastic scattering events with at least one hard jet in the Breit frame together with a muon has been measured, for photon virtualities Q^2 > 2 GeV^2, with the ZEUS detector at HERA using integrated luminosity of 72 pb^-1. The total visible cross section is sigma_b-bbar (ep -> e jet mu X) = 40.9 +- 5.7 (stat.) +6.0 -4.4 (syst.) pb. The next-to-leading order QCD prediction lies about 2.5 standard deviations below the data. The differential cross sections are in general consistent with the NLO QCD predictions; however at low values of Q^2, Bjorken x, and muon transverse momentum, and high values of jet transverse energy and muon pseudorapidity, the prediction is about two standard deviations below the data.Comment: 18 pages, 4 figure

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