120424799
Event reconstruction algorithms for the ATLAS trigger
The ATLAS experiment under construction at CERN is due to begin operation at the end of 2007. The detector will record the results of proton-proton collisions at a center-of-mass energy of 14 TeV. The trigger is a three-tier system designed to identify in real-time potentially interesting events that are then saved for detailed offline analysis. The trigger system will select approximately 200 Hz of potentially interesting events out of the 40 MHz bunch-crossing rate (with 109 interactions per second at the nominal luminosity). Algorithms used in the trigger system to identify different event features of interest will be described, as well as their expected performance in terms of selection efficiency, background rejection and computation time per event. The talk will concentrate on recent improvements and on performance studies, using a very detailed simulation of the ATLAS detector and electronics chain that emulates the raw data as it will appear at the input to the trigger system.
104358197
Measurement of $\cos_{2beta}$ in $B^0 \to D^{(*)}$ 0 h0 decays with a time-dependent Dalitz plot analysis of $D^0 \to K^0_{s} \pi^{+} \pi^{-}$
We report a preliminary measurement of $\cos2\beta$ in $B^0\to D^{(*)0}h^0$ decays with a time-dependent Dalitz plot analysis of $D^0\to K_S\pi^+\pi^-$, where $h^0$ is a light neutral meson such as $\pi^0$, $\eta$, $\eta'$ or $\omega$. The strong phase variation on the Dalitz plot allows the access to the angle $\beta$ with only a two-fold ambiguity ($\beta+\pi$). Using $311\times 10^{6}$ $B\bar{B}$ pairs collected at the Babar detector, we obtain $\cos2\beta = 0.54 \pm 0.54 \pm 0.08 \pm 0.18$, $\sin2\beta = 0.45 \pm 0.36 \pm 0.05 \pm 0.07$, and $|\lambda| = 0.975^{+0.093}_{-0.085} \pm 0.012 \pm 0.002 $, where the first errors are statistical, the second are experimental systematic uncertainties, and the third are the signal Dalitz model uncertainties. This measurement favors the solution of $\beta= 22^\circ$ over $68^\circ$ at an 87% confidence level.
53073574
The ATLAS High Level Trigger Steering
The High Level Trigger (HLT) of the ATLAS experiment at the Large Hadron Collider receives events which pass the LVL1 trigger at ~75 kHz and has to reduce the rate to ~200 Hz while retaining the most interesting physics. It is a software trigger and performs the reduction in two stages: the LVL2 trigger and the Event Filter (EF). At the heart of the HLT is the Steering software. To minimise processing time and data transfers it implements the novel event selection strategies of seeded, step-wise reconstruction and early rejection. The HLT is seeded by regions of interest identified at LVL1. These and the static configuration determine which algorithms are run to reconstruct event data and test the validity of trigger signatures. The decision to reject the event or continue is based on the valid signatures, taking into account pre-scale and pass-through. After the EF, event classification tags are assigned for streaming purposes. Several powerful new features for commissioning and operation have been added: comprehensive monitoring is now built in to the framework; for validation and debugging, reconstructed data can be written out; the steering is integrated with the new configuration (presented separately), and topological and global triggers have been added. This paper will present details of the final design and its implementation, the principles behind it, and the requirements and constraints it is subject to. The experience gained from technical runs with realistic trigger menus will be described.
209834301
Search for supersymmetry in events with photons, bottom quarks, and missing transverse momentum in proton–proton collisions at a centre-of-mass energy of 7 TeV with the ATLAS detector
A search has been performed for the experimental signature of an isolated photon with high transverse momentum, at least one jet identified as originating from a bottom quark, and high missing transverse momentum. Such a final state may originate from supersymmetric models with gauge-mediated supersymmetry breaking in events in which one of a pair of higgsino-like neutralinos decays into a photon and a gravitino while the other decays into a Higgs boson and a gravitino. The search is performed using the full dataset of 7 TeV proton–proton collisions recorded with the ATLAS detector at the LHC in 2011, corresponding to an integrated luminosity of 4.7 fb−1. A total of 7 candidate events are observed while 7.5±2.2 events are expected from the Standard Model background. The results of the search are interpreted in the context of general gauge mediation to exclude certain regions of a benchmark plane for higgsino-like neutralino production.
118500591
Measurement of moments of the hadronic-mass and energy spectrum in inclusive semileptonic anti-B ---> X(c) l- anti-nu decays
125358056
Evidence for the $B^0 \to \rho^0 \rho^0$ decay and implications for the CKM angle $\alpha$
We search for the decays B0->rho0rho0, B0->rho0f0, and B0->f0f0 in a sample of about 348 million Upsilon(4S)->BBbar decays collected with the BABAR detector at the PEP-II asymmetric-energy e+e- collider at SLAC. We find evidence for B0->rho0rho0 with 3.0 sigma significance and measure the branching fraction B = (1.16 +0.37-0.36 +-0.27) 10^{-6} and longitudinal polarization fraction f_L = 0.86 +0.11-0.13 +-0.05. As a consequence, the uncertainty on the CKM unitarity angle alpha due to penguin contributions in B->rhorho decays is estimated to be 18 degrees at 1 sigma level. We also set upper limits on the B0->rho0f0 and B0->f0f0 decay rates. All results are preliminary.
62237714
Partial wave analysis using graphics processing units
Partial wave analysis is an important tool for determining resonance properties in hadron spectroscopy. For large data samples however, the un-binned likelihood fits employed are computationally very expensive. At the Beijing Spectrometer (BES) III experiment, an increase in statistics compared to earlier experiments of up to two orders of magnitude is expected. In order to allow for a timely analysis of these datasets, additional computing power with short turnover times has to be made available. It turns out that graphics processing units (GPUs) originally developed for 3D computer games have an architecture of massively parallel single instruction multiple data floating point units that is almost ideally suited for the algorithms employed in partial wave analysis. We have implemented a framework for tensor manipulation and partial wave fits called GPUPWA. The user writes a program in pure C++ whilst the GPUPWA classes handle computations on the GPU, memory transfers, caching and other technical details. In conjunction with a recent graphics processor, the framework provides a speed-up of the partial wave fit by more than two orders of magnitude compared to legacy FORTRAN code.
119282846
Fundamental Physics at the Intensity Frontier
Particle physics aims to understand the universe around us. The Standard Model of particle physics describes the basic structure of matter and forces, to the extent we have been able to probe thus far. However, it leaves some big questions unanswered. Some are within the Standard Model itself, such as why there are so many fundamental particles and why they have different masses. In other cases, the Standard Model simply fails to explain some phenomena, such as the observed matter-antimatter asymmetry in the universe, the existence of dark matter and dark energy, and the mechanism that reconciles gravity with quantum mechanics. These gaps lead us to conclude that the universe must contain new and unexplored elements of Nature. Most of particle and nuclear physics is directed towards discovering and understanding these new laws of physics. These questions are best pursued with a variety of approaches, rather than with a single experiment or technique. Particle physics uses three basic approaches, often characterized as exploration along the cosmic, energy, and intensity frontiers. Each employs different tools and techniques, but they ultimately address the same fundamental questions. This allows a multi-pronged approach where attacking basic questions from different angles furthers knowledge and providesmore » deeper answers, so that the whole is more than a sum of the parts. A coherent picture or underlying theoretical model can more easily emerge, to be proven correct or not. The intensity frontier explores fundamental physics with intense sources and ultra-sensitive, sometimes massive detectors. It encompasses searches for extremely rare processes and for tiny deviations from Standard Model expectations. Intensity frontier experiments use precision measurements to probe quantum effects. They typically investigate very large energy scales, even higher than the kinematic reach of high energy particle accelerators. The science addresses basic questions, such as: Are there new sources of CP violation? Is there CP violation in the leptonic sector? Are neutrinos their own antiparticles? Do the forces unify? Is there a weakly coupled hidden sector that is related to dark matter? Do new symmetries exist at very high energy scales? To identify the most compelling science opportunities in this area, the workshop Fundamental Physics at the Intensity Frontier was held in December 2011, sponsored by the Office of High Energy Physics in the US Department of Energy Office of Science. Participants investigated the most promising experiments to exploit these opportunities and described the knowledge that can be gained from such a program. The workshop generated much interest in the community, as witnessed by the large and energetic participation by a broad spectrum of scientists. This document chronicles the activities of the workshop, with contributions by more than 450 authors. The workshop organized the intensity frontier science program along six topics that formed the basis for working groups: experiments that probe (i) heavy quarks, (ii) charged leptons, (iii) neutrinos, (iv) proton decay, (v) light, weakly interacting particles, and (vi) nucleons, nuclei, and atoms. The conveners for each working group included an experimenter and a theorist working in the field and an observer from the community at large. The working groups began their efforts well in advance of the workshop, holding regular meetings and soliciting written contributions. Specific avenues of exploration were identified by each working group. Experiments that study rare strange, charm, and bottom meson decays provide a broad program of measurements that are sensitive to new interactions. Charged leptons, particularly muons and taus, provide a precise probe for new physics because the Standard Model predictions for their properties are very accurate. Research at the intensity frontier can reveal CP violation in the lepton sector, and elucidate whether neutrinos are their own antiparticles. A very weakly coupled hidden-sector that may comprise the dark matter in the universe could be discovered. The search for proton decay can probe the unification of the forces with unprecedented reach and test sacrosanct symmetries to very high scales. Detecting an electric dipole moment for the neutron, or neutral atoms, could establish a clear signal for new physics, while limits on such a measurement would place severe constraints on many new theories. This workshop marked the first instance where discussion of these diverse programs was held under one roof. As a result, it was realized that this broad effort has many connections; a large degree of synergy exists between the different areas and they address similar questions. Results from one area were found to be pertinent to experiments in another domain.« less
120101281
The Mu3e Experiment
Abstract The Mu3e experiment is designed to search for charged lepton flavour violation in the process μ + → e + e − e + with a branching ratio sensitivity of 10 − 16 . This requires a detector capable of coping with rates of up to 2 ⋅ 10 9 muons/s whilst being able to reduce backgrounds from accidental coincidences and the process μ + → e + e − e + ν ¯ μ ν e to below the 10 − 16 level. The use of 50 μ m thin high-voltage monolithic active pixel sensors in conjunction with an innovative tracking concept provide the momentum and position resolution necessary, whilst the timing resolution is provided by a combination of scintillating fibres and tiles. The recently approved Mu3e experiment will be located at the Paul Scherrer Institute in Switzerland and is currently preparing for detector construction.