The PhaseII Upgrades of CMS are being planned for the High Luminosity LHC (HL-LHC) era when the mean number of interactions per beam crossing (in-time pileup) is expected to reach ~140-200. The potential backgrounds arising from mis-associated jets a
nd photon showers, for example, during event reconstruction could be reduced if physics objects are tagged with an event time. This tag is fully complementary to the event vertex which is already commonly used to reduce mis-reconstruction. Since the tracking vertex resolution is typically ~10^{-3} (50 micron/4.8cm) of the rms vertex distribution, whereas only ~10^{-1} (i.e. 20 vs.170 picoseconds (psec)) is demonstrated for timing, it is often assumed that only photon (i.e. EM calorimeter or shower-max) timing is of interest. We show that the optimal solution will likely be a single timing layer which measures both charged particle and photon time (a pre-shower layer).
The ATLAS detector is capable of resolving the highest energy pp collisions at luminosities sufficient to yield 10s of simultaneous interactions within a bunch collision lasting <0.5 nsec. Already in 2011 a mean occupancy of 20 is often found in pp r
unning. In 2004 studies by ATLAS showed that the detector would have excellent performance also for the foreseeable particle multiplicities in the highest energy p-Pb and Pb-Pb collisions that the LHC will produce. These studies resulted in a letter of intent to the LHC committee by ATLAS to do physics with these beams also. In the past 2 years of data taking, ATLAS detector performance studies have confirmed these expectations at the actual multiplicities presented below. The ATLAS program removes an artificial specialization that arose about 30 years ago in high energy physics when the energy and intensity frontier moved to colliders. Before that time, for example, the same experiment that discovered the $Upsilon$ (CFS and E605 at Fermilab) also measured the nuclear modification factor in the production of high $p_T$ identified charged hadrons using nuclear targets from Beryllium through Tungsten.
We present first results from the ATLAS Zero Degree Calorimeters (ZDC) based on 7~TeV pp collision data recorded in 2010. The ZDC coverage of +/-~350 microradians about the forward direction makes possible the measurement of neutral particles (primar
ily pi0s and neutrons) over the kinematic region x_F >~0.1 and out to p_T<~ 1.2 GeV/c at large x_F. The ATLAS ZDC is unique in that it provides a complete image of both electromagnetic and hadronic showers.This is illustrated with the reconstruction of pi0s with energies of 0.7-3.5 TeV. We also discuss the waveform reconstruction algorithm which has allowed good time-of-flight resolution on leading neutrons emerging from the collisions despite the sparse (40 MHz) sampling of the calorimeter signals used.
We analyze the problem of correlating pp interaction data from the central detectors with a subevent measured in an independent system of leading proton detectors using FP420 as an example. FP420 is an R&D project conducted by a collaboration forme
d by members of ATLAS and CMS to investigate the possibility of detecting new physics in the central exclusive channel, PP -> P + X + P,where the central system X may be a single particle, for example a Standard Model Higgs boson. With standard LHC optics, the protons emerge from the beam at a distance of 420m from the Interaction Point, for M_X ~ 120 GeV. The mass of the central system can be measured from the outgoing protons alone, with a resolution of order 2 GeV irrespective of the decay products of the central system. In addition, to a very good approximation, only central systems with 0^++ quantum numbers can be produced, meaning that observation of a SM or MSSM Higgs Boson in this channel would lead to a direct determination of the quantum numbers.
