7 research outputs found
Comment on measuring the t-tbar forward-backward asymmetry at ATLAS and CMS
We suggest a new possibility for ATLAS and CMS to explore the t-tbar
forward-backward asymmetry measured at the Tevatron, by attempting to
reconstruct t-tbar events, with one of the tops decaying semileptonically in
the central region (|\eta| < 2.5) and the other decaying hadronically in the
forward region (|\eta| > 2.5). For several models which give comparable
Tevatron signals, we study the charge asymmetry at the LHC as a function of
cuts on |\eta| and on the t-tbar invariant mass, m_{t-tbar}. We show that there
is an interesting complementarity between cuts on |\eta| and m_{t-tbar} to
suppress the dominant and symmetric gg -> t-tbar rate, and different
combinations of cuts enhance the distinguishing power between models. This
complementarity is likely to hold in other new physics scenarios as well, which
affect the t-tbar cross section, so it motivates extending t-tbar
reconstruction to higher |\eta|.Comment: 6 pages, 3 figures, 3 tables, v2: to match version appearing in PRD,
resolution in figures improve
Variational Autoencoders for Anomalous Jet Tagging
We present a detailed study on Variational Autoencoders (VAEs) for anomalous
jet tagging at the Large Hadron Collider. By taking in low-level jet
constituents' information, and training with background QCD jets in an
unsupervised manner, the VAE is able to encode important information for
reconstructing jets, while learning an expressive posterior distribution in the
latent space. When using the VAE as an anomaly detector, we present different
approaches to detect anomalies: directly comparing in the input space or,
instead, working in the latent space. In order to facilitate general search
approaches such as bump-hunt, mass-decorrelated VAEs based on distance
correlation regularization are also studied. We find that the naive
mass-decorrelated VAEs fail at maintaining proper detection performance, by
assigning higher probabilities to some anomalous samples. To build a performant
mass-decorrelated anomalous jet tagger, we propose the Outlier Exposed VAE
(OE-VAE), for which some outlier samples are introduced in the training process
to guide the learned information. OE-VAEs are employed to achieve two goals at
the same time: increasing sensitivity of outlier detection and decorrelating
jet mass from the anomaly score. We succeed in reaching excellent results from
both aspects. Code implementation of this work can be found at
\href{https://github.com/taolicheng/VAE-Jet}{Github}.Comment: 35 pages, 22 figures. Revised versio
The ABC130 barrel module prototyping programme for the ATLAS strip tracker
For the Phase-II Upgrade of the ATLAS Detector, its Inner Detector,
consisting of silicon pixel, silicon strip and transition radiation
sub-detectors, will be replaced with an all new 100 % silicon tracker, composed
of a pixel tracker at inner radii and a strip tracker at outer radii. The
future ATLAS strip tracker will include 11,000 silicon sensor modules in the
central region (barrel) and 7,000 modules in the forward region (end-caps),
which are foreseen to be constructed over a period of 3.5 years. The
construction of each module consists of a series of assembly and quality
control steps, which were engineered to be identical for all production sites.
In order to develop the tooling and procedures for assembly and testing of
these modules, two series of major prototyping programs were conducted: an
early program using readout chips designed using a 250 nm fabrication process
(ABCN-25) and a subsequent program using a follow-up chip set made using 130 nm
processing (ABC130 and HCC130 chips). This second generation of readout chips
was used for an extensive prototyping program that produced around 100
barrel-type modules and contributed significantly to the development of the
final module layout. This paper gives an overview of the components used in
ABC130 barrel modules, their assembly procedure and findings resulting from
their tests.Comment: 82 pages, 66 figure
The ABC130 barrel module prototyping programme for the ATLAS strip tracker
For the Phase-II Upgrade of the ATLAS Detector [1], its Inner Detector, consisting of silicon pixel, silicon strip and transition radiation sub-detectors, will be replaced with an all new 100% silicon tracker, composed of a pixel tracker at inner radii and a strip tracker at outer radii. The future ATLAS strip tracker will include 11,000 silicon sensor modules in the central region (barrel) and 7,000 modules in the forward region (end-caps), which are foreseen to be constructed over a period of 3.5 years. The construction of each module consists of a series of assembly and quality control steps, which were engineered to be identical for all production sites. In order to develop the tooling and procedures for assembly and testing of these modules, two series of major prototyping programs were conducted: an early program using readout chips designed using a 250 nm fabrication process (ABCN-250) [2,2] and a subsequent program using a follow-up chip set made using 130 nm processing (ABC130 and HCC130 chips). This second generation of readout chips was used for an extensive prototyping program that produced around 100 barrel-type modules and contributed significantly to the development of the final module layout. This paper gives an overview of the components used in ABC130 barrel modules, their assembly procedure and findings resulting from their tests