By using the ATLAS detector, observations have been made of a centrality-dependent dijet asymmetry in the collisions of lead ions at the Large Hadron Collider. In a sample of lead-lead events with a per-nucleon center of mass energy of 2.76 TeV, selected with a minimum bias trigger, jets are reconstructed in fine-grained, longitudinally segmented electromagnetic and hadronic calorimeters. The transverse energies of dijets in opposite hemispheres are observed to become systematically more unbalanced with increasing event centrality leading to a large number of events which contain highly asymmetric dijets. This is the first observation of an enhancement of events with such large dijet asymmetries, not observed in proton-proton collisions, which may point to an interpretation in terms of strong jet energy loss in a hot, dense medium.

The status of the research on muon colliders is discussed and plans are outlined for future theoretical and experimental studies. Besides work on the parameters of a 3-4 and 0.5 TeV center-of-mass (COM) energy collider, many studies are now concentrating on a machine near 0.1 TeV (COM) that could be a factory for the s-channel production of Higgs particles. We discuss the research on the various components in such muon colliders, starting from the proton accelerator needed to generate pions from 1098-4402͞99͞2(8)͞081001(73)$15.00 © 1999 The American Physical Society 081001-1 PRST-AB 2 CHARLES M. ANKENBRANDT et al. 081001 (1999) a heavy-Z target and proceeding through the phase rotation and decay (p ! m n m ) channel, muon cooling, acceleration, storage in a collider ring, and the collider detector. We also present theoretical and experimental R&D plans for the next several years that should lead to a better understanding of the design and feasibility issues for all of the components. This report is an update of the progress on the research and development since the feasibility study of muon colliders presented at the Snowmass '96

The χ(b)(nP) quarkonium states are produced in proton-proton collisions at the Large Hadron Collider at sqrt[s] = 7  TeV and recorded by the ATLAS detector. Using a data sample corresponding to an integrated luminosity of 4.4  fb(-1), these states are reconstructed through their radiative decays to Υ(1S,2S) with Υ → μ+ μ-. In addition to the mass peaks corresponding to the decay modes χ(b)(1P,2P) → Υ(1S)γ, a new structure centered at a mass of 10.530 ± 0.005(stat) ± 0.009(syst)  GeV is also observed, in both the Υ(1S)γ and Υ(2S)γ decay modes. This structure is interpreted as the χ(b)(3P) system.

A measurement of the cross section for the inclusive production of isolated prompt photons in pp collisions at a center-of-mass energy ffiffi ffi s p ¼ 7 TeV is presented. The measurement covers the pseudorapidity ranges j j < 1:37 and 1:52 j j < 1:81 in the transverse energy range 15 E T < 100 GeV. The results are based on an integrated luminosity of 880 nb À1 , collected with the ATLAS detector at the Large Hadron Collider. Photon candidates are identified by combining information from the calorimeters and from the inner tracker. Residual background in the selected sample is estimated from data based on the observed distribution of the transverse isolation energy in a narrow cone around the photon candidate. The results are compared to predictions from next-to-leading-order perturbative QCD calculations.

We have measured the effects of high (0-4.5 T) magnetic fields on the operating conditions of 805 MHz accelerating cavities, and discovered that the maximum accelerating gradient drops as a function of the axial magnetic field. While the maximum gradient of any cavity is governed by a number of factors including conditioning, surface topology and materials, we argue that J B forces within the emitters are the mechanism for enhanced breakdown in magnetic fields. The pattern of emitters changes over time and we show an example of a bright emitter which disappears during a breakdown event. We also present unique measurements of the distribution of enhancement factors, , of secondary emitters produced in breakdown events during conditioning. We believe these secondary emitters can also be breakdown triggers, and the secondary emitter spectrum helps to determine the maximum operating field.