ATLAS at Uppsala University

The Division of Particle Physics at Uppsala University takes part in research on elementary particle physics in the ATLAS experiment at the CERN Large Hadron Collider (LHC) in Geneva. Currently the main research activity is physics analysis of the proton-proton-collision data successively being accumulated by ATLAS. The plan is to collect an integrated luminosity of about 1 fb-1 by summer 2011 and of 5–10 fb-1 by end 2012, which is enough to enable the search for most of the particles at the TeV scale predicted by Supersymmetry and other theories beyond the Standard Model. The Uppsala ATLAS Group has since long developed an expertise in charged Higgs physics analysis and tau trigger conditions and more recently also in physics analysis for the study of Multi-Parton Interactions. Another important activity of the group is to prepare for the ultimate upgrade of the LHC luminosity (HL-LHC) by developing new semiconductor technology for a track based first level trigger. The Group welcomes new PhD students and Postdocs to join its research program at ATLAS.


Charged Higgs boson searches – contact Arnaud Ferrari

In the Standard Model, one complex scalar Higgs doublet is responsible for the electroweak gauge symmetry breaking, giving rise to only one physical Higgs boson. In so-called Two Higgs Doublet Models (2HDM), such as the minimal supersymmetric extension of the Standard Model (MSSM), two complex scalar Higgs doublets are responsible for the electroweak symmetry breaking. As a result, the Higgs spectrum consists of five physical states: two charged and three neutral bosons.

If the charged Higgs boson is lighter than about 170 GeV, it can be produced in the decay of top quarks, which are being copiously pair produced in pp collisions at the LHC. For a certain region of the Higgs parameter space, the dominating decay mode of charged Higgs bosons is H+→τν, where the tau-lepton may decay either to leptons or hadrons.

The Uppsala ATLAS group is searching for charged Higgs bosons decaying to tau leptons in top-antitop pair events. For the channel where the tau decays leptonically we are studying two discriminating variables.

  • An angular variable, proportional to the invariant mass of the charged lepton and the b-jet coming from the same top quark, which allows to distinguish between direct and indirect (via a tau) decays of a charged (W or Higgs) boson.

  • A generalized transverse mass which is obtained by solving numerically a system of equations describing the kinematics of di-lepton top-antitop events, and which gives a lower bound on the charged Higgs boson mass.

For the channel where the tau decays hadronically we are directly measuring the ratio of lepton+hadronic tau final states to di-lepton final states. An increased ratio compared to Standard Model predictions could indicate the presence of a charged Higgs boson in the top quark decay chain.

Tau trigger efficiency measurements – contact Richard Brenner

The heaviest lepton in the Standard Model is the tau. Because of its high mass and large coupling to the Higgs boson, the triggering and reconstruction of the tau lepton is important at ATLAS and the LHC. The tau lepton has a very short lifetime and decays both to hadrons and leptons. This makes the identification of the tau lepton complicated.

The Uppsala ATLAS group studies the efficiency of the hadronic tau trigger. Measurements are done both by selecting taus from decaying Z-bosons and by reweighting QCD samples. The efficiency measurement based on Z-bosons is superior to the method with reweighted QCD sample in terms of systematic error but the statistics and transverse momentum range of the Z-boson sample is limited. Therefore the two methods are complementary.

Multi-parton interactions – contact Claus Buszello

The collision of two protons is a complex process. In hard collisions, two constituents of the proton can interact to form heavier particles that we seek to find or study, while the rest of the proton breaks up. The remnants of this break up are often referred to as “underlying event” as opposed to the higher energetic “hard event”. Due to the nature of QCD, these remnants can interact with each other as well as with the partons created in the hard scattering. Furthermore, a single collision is not restricted to just one hard interaction. Collisions where two processes with significant momentum transfer take place at the same time – known as double parton interactions – have been observed.

Using the DØ detector at Fermilab and the ATLAS detector at CERN, we study the nature of the underlying event at various energies as well as the rate of double parton interactions in proton proton collisions at the LHC. These measurements lead to an improved understanding of the nature of the proton and the effect of soft color interactions in collisions. The double parton interaction rate for example can tell us how evenly partons are distributed in the proton, of if they are more clumped together. All these findings will make their way into a better description of the proton and thus will also help us reveal and understand new phenomena at the Tevatron or LHC.

The luminosity upgrade of the LHC to a High Luminocity LHC (HL-LHC) – contact Richard Brenner

The Uppsala group has together with groups in Oslo and Begren contributed to the Silicon Central Tracker (SCT) project with the construction and test of 300 SCT barrel Silicon microstrip detector modules for the ATLAS detector. The group also had a central role in the development and construction of the Detector Control System (DCS) now in operation for the SCT.

The group is now participating in the upgrade of the silicon tracker for the HL-LHC. In the current R&D phase, we are in particular studying the implementation of a fast track trigger that would help keep the trigger rates at a manageable level despite the increase in luminosity the HL-LHC would bring. The emphasis of the track trigger project is presently on studying and understanding the physics requirements for a track trigger and its conceptual implementation.

The main challenge in the instrumentation of a track trigger is to handle the large amount of data in real time. Methods of data reduction, data transmission and data processing are in focus. Two main concepts are studied.

  • Self seeded track trigger: substantial data reduction is done on detector level to fit within a manageable data transmission bandwidth.

  • Calorimeter/muon based track trigger: A first level (L0) trigger is produced and only the parts of the tracker inside a Region of Interest is read out.

The group is involved in two hardware developments for the HL-LHC track trigger:

  • Wireless data transmission of data with mm waves: Low feature size, low power and low cost technology that obviates the need for wires and connectors with Gbps capability.

  • 3D packaging: Technology to implement more functionality in front-end circuits, to shorten path lengths in the electronics and to enable mixing of different technologies. Work done in the AIDA project of the EU 7th Framework.