3,682 research outputs found
Data-driven estimations of Standard Model backgrounds to SUSY searches in ATLAS
At the Large Hadron Collider (LHC), the strategy for the observation of supersymmetry in the early days is mainly based on inclusive searches. Major backgrounds are constituted by mismeasured multi-jet events andW, Z and t quark production in association with jets.We describe recent work performed in the ATLAS Collaboration to derive these backgrounds from the first ATLAS data
New Developments in Data-driven Background Determinations for SUSY Searches in ATLAS
Any discovery of new physics relies on detailed understanding of the Standard Model background. At the LHC, we expect to extract the backgrounds from the data itself, with minimum reliance on Monte Carlo simulations. We describe new developments in ATLAS on such data-driven techniques, and prospects for their application on first data
Data-driven estimations of Standard Model backgrounds to SUSY searches
Mismeasured multi-jet events and W, Z and top quark production in association with jets constitute a major background to searches for supersymmetry at the LHC. We describe recent work performed in the ATLAS Collaboration to estimate these backgrounds for a basic SUSY selection, and we discuss methods to derive them from the first ATLAS data
The ATLAS distributed analysis system
In the LHC operations era, analysis of the multi-petabyte ATLAS data sample by globally distributed physicists is a challenging task. To attain the required scale the ATLAS Computing Model was designed around the concept of Grid computing, realized in the Worldwide LHC Computing Grid (WLCG),the largest distributed computational resource existing in the sciences. The ATLAS experiment currently stores over 140 PB of data and runs about 140, 000 concurrent jobs continuously at WLCG sites. During the first run of the LHC, the ATLAS Distributed Analysis (DA) service has operated stably and scaled as planned. More than 1600 users submitted jobs in 2012, with 2 million or more analysis jobs per week, peaking at about a million jobs per day. The system dynamically distributes popular data to expedite processing and maximally utilize resources. The reliability of the DA service is high and steadily improving;Grid sites are continually validated against a set of standard tests, and a dedicated team of expert shifters provides user support and communicates user problems to the sites. Both the user support techniques and the direct feedback of users have been effective in improving the success rate and user experience when utilizing the distributed computing environment. In this contribution a description of the main components, activities and achievements of ATLAS distributed analysis is given. Several future improvements being undertaken will be described
Performance of the ATLAS Muon Drift-Tube Chambers at High Background Rates and in Magnetic Fields
The ATLAS muon spectrometer uses drift-tube chambers for precision tracking.
The performance of these chambers in the presence of magnetic field and high
radiation fluxes is studied in this article using test-beam data recorded in
the Gamma Irradiation Facility at CERN. The measurements are compared to
detailed predictions provided by the Garfield drift-chamber simulation
programme
Development of Muon Drift-Tube Detectors for High-Luminosity Upgrades of the Large Hadron Collider
The muon detectors of the experiments at the Large Hadron Collider (LHC) have
to cope with unprecedentedly high neutron and gamma ray background rates. In
the forward regions of the muon spectrometer of the ATLAS detector, for
instance, counting rates of 1.7 kHz/square cm are reached at the LHC design
luminosity. For high-luminosity upgrades of the LHC, up to 10 times higher
background rates are expected which require replacement of the muon chambers in
the critical detector regions. Tests at the CERN Gamma Irradiation Facility
showed that drift-tube detectors with 15 mm diameter aluminum tubes operated
with Ar:CO2 (93:7) gas at 3 bar and a maximum drift time of about 200 ns
provide efficient and high-resolution muon tracking up to the highest expected
rates. For 15 mm tube diameter, space charge effects deteriorating the spatial
resolution at high rates are strongly suppressed. The sense wires have to be
positioned in the chamber with an accuracy of better than 50 ?micons in order
to achieve the desired spatial resolution of a chamber of 50 ?microns up to the
highest rates. We report about the design, construction and test of prototype
detectors which fulfill these requirements
Grid site testing for ATLAS with Hammer Cloud
With the exponential growth of LHC (Large Hadron Collider) data in 2012, distributed computing has become the established way to analyze collider data. The ATLAS grid infrastructure includes more than 130 sites worldwide, ranging from large national computing centers to smaller university clusters. HammerCloud was previously introduced with the goals of enabling virtual organisations (VO) and site-administrators to run validation tests of the site and software infrastructure in an automated or on-demand manner. The HammerCloud infrastructure has been constantly improved to support the addition of new test workflows. These new workflows comprise e.g. tests of the ATLAS nightly build system, ATLAS Monte Carlo production system, XRootD federation (FAX) and new site stress test workflows. We report on the development, optimization and results of the various components in the HammerCloud framework
Photon polarization from helicity suppression in radiative decays of polarized Lambda_b to spin-3/2 baryons
We give a general parameterization of the Lambda_b --> Lambda(1520) gamma
decay amplitude, applicable to any strange isosinglet spin-3/2 baryon, and
calculate the branching fraction and helicity amplitudes. Large-energy form
factor relations are worked out, and it is shown that the helicity-3/2
amplitudes vanish at lowest order in soft-collinear effective theory (SCET).
The suppression can be tested experimentally at the LHC and elsewhere, thus
providing a benchmark for SCET. We apply the results to assess the experimental
reach for a possible wrong-helicity b --> s gamma dipole coupling in Lambda_b
--> Lambda(1520) gamma --> p K gamma decays. Furthermore we revisit
Lambda_b-polarization at hadron colliders and update the prediction from
heavy-quark effective theory. Opportunities associated with b --> d gamma
afforded by high-statistics Lambda_b samples are briefly discussed in the
general context of CP and flavour violation.Comment: elsart, 15 pages, 1 figure; final version as published in Phys. Lett.
Differences in presentation between paediatric- and adult-onset primary Sjögren's syndrome patients
OBJECTIVES: Primary Sjögren's syndrome (pSS) is a rare disease in paediatric patients. Presenting symptoms differ from those in adult patients. The aim of this study was to evaluate presenting symptoms, classification criteria and clinical assessments, including salivary gland ultrasonography (SGUS), at disease onset in paediatric and adult patients with pSS. METHODS: Data of 23 paediatric- and 33 adult-onset patients with pSS were obtained from our standardised multidisciplinary REpSULT and RESULT cohorts, respectively. Clinical, patient-reported, serological, functional, biopsy and SGUS parameters were compared. RESULTS: In paediatric-onset pSS (pedSS) patients, recurrent parotid gland swelling (91% vs. 49%, p<0.001) and fever (30% vs. 3%, p=0.006) were more often present than in adult-onset patients. In contrast, sicca symptoms of mouth (52% vs. 79%, p=0.046) and eyes (26% vs. 73%, p<0.001) were less common in pedSS patients. In paediatric patients, the entry criteria of the ACR/EULAR classification were most often met due to activity in the glandular domain of the ESSDAI. When applying the ACR/EULAR classification criteria, only 78% of pedSS fulfilled these criteria compared to 100% of adult patients. Abnormal glandular function tests had a greater contribution to fulfilling the criteria in adults, while the biopsy had a greater contribution in paediatric patients. Anti-SSA/Ro serology had similar contribution for both cohorts. SGUS Hocevar score was significantly higher in paediatric compared to adult patients (median 25 vs. 18, p=0.004). CONCLUSION: PedSS has a different presentation than adult-onset pSS. Recurrent parotid gland swelling in paediatric patients should alert clinicians to the potential presence of pSS
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