S. Majewski

Technical Design Report on the project to provide higher-granularity, higher-resolution and longitudinal shower information from the Liquid Argon calorimeter to the Level-1 trigger processors.

We propose a new precision measurement of parity-violating electron scattering on the proton at very low Q^2 and forward angles to challenge predictions of the Standard Model and search for new physics. A unique opportunity exists to carry out the first precision measurement of the proton's weak charge, $Q_W =1 - 4\sin^2θ_W$. A 2200 hour measurement of the parity violating asymmetry in elastic ep scattering at Q^2=0.03 (GeV/c)^2 employing 180 $μ$A of 85% polarized beam on a 35 cm liquid Hydrogen target will determine the proton's weak charge with approximately 4% combined statistical and systematic errors. The Standard Model makes a firm prediction of $Q_W$, based on the running of the weak mixing angle from the Z0 pole down to low energies, corresponding to a 10 sigma effect in this experiment.

The discovery of the stop — the Supersymmetric partner of the top quark — is a key goal of the physics program enabled by the Large Hadron Collider. Although much of the accessible parameter space has already been probed, all current searches assume the top mass is known. This is relevant for the “stealth stop” regime, which is characterized by decay kinematics that force the final state top quark off its mass shell; such decays would contaminate the top mass measurements. We investigate the resulting bias imparted to the template method based ATLAS approach. A careful recasting of these results shows that effect can be as large as 2.0 GeV, comparable to the current quoted uncertainty on the top mass. Thus, a robust exploration of the stealth stop splinter requires the simultaneous consideration of the impact on the top mass. Additionally, we explore the robustness of the template technique, and point out a simple strategy for improving the methodology implemented for the semi-leptonic channel.

A bstract In this paper, we recast a “stealth stop” search in the notoriously difficult region of the stop-neutralino Simplified Model parameter space for which $$ m\left({\tilde{t}}_1\right)-m\left({\tilde{\upchi}}_1^0\right)\simeq {m}_t $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>m</mml:mi> <mml:mfenced> <mml:msub> <mml:mover> <mml:mi>t</mml:mi> <mml:mo>˜</mml:mo> </mml:mover> <mml:mn>1</mml:mn> </mml:msub> </mml:mfenced> <mml:mo>−</mml:mo> <mml:mi>m</mml:mi> <mml:mfenced> <mml:msubsup> <mml:mover> <mml:mi>χ</mml:mi> <mml:mo>˜</mml:mo> </mml:mover> <mml:mn>1</mml:mn> <mml:mn>0</mml:mn> </mml:msubsup> </mml:mfenced> <mml:mo>≃</mml:mo> <mml:msub> <mml:mi>m</mml:mi> <mml:mi>t</mml:mi> </mml:msub> </mml:math> . The properties of the final state are nearly identical for tops and stops, while the rate for stop pair production is $$ \mathcal{O} $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>O</mml:mi> </mml:math> (10%) of that for $$ t\overline{t} $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>t</mml:mi> <mml:mover> <mml:mi>t</mml:mi> <mml:mo>¯</mml:mo> </mml:mover> </mml:math> . Stop searches away from this stealth region have left behind a “splinter” of open parameter space when $$ m\left({\tilde{t}}_1\right)\simeq {m}_t $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>m</mml:mi> <mml:mfenced> <mml:msub> <mml:mover> <mml:mi>t</mml:mi> <mml:mo>˜</mml:mo> </mml:mover> <mml:mn>1</mml:mn> </mml:msub> </mml:mfenced> <mml:mo>≃</mml:mo> <mml:msub> <mml:mi>m</mml:mi> <mml:mi>t</mml:mi> </mml:msub> </mml:math> . Removing this splinter requires surgical precision: the ATLAS constraint on stop pair production reinterpreted here treats the signal as a contaminant to the measurement of the top pair production cross section using data from $$ \sqrt{s}=7 $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msqrt> <mml:mi>s</mml:mi> </mml:msqrt> <mml:mo>=</mml:mo> <mml:mn>7</mml:mn> </mml:math> TeV and 8 TeV in a correlated way to control for some systematic errors. ATLAS fixed $$ m\left({\tilde{t}}_1\right)\simeq {m}_t\kern0.5em and\kern0.5em m\left({\tilde{\upchi}}_1^0\right)=1 $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>m</mml:mi> <mml:mfenced> <mml:msub> <mml:mover> <mml:mi>t</mml:mi> <mml:mo>˜</mml:mo> </mml:mover> <mml:mn>1</mml:mn> </mml:msub> </mml:mfenced> <mml:mo>≃</mml:mo> <mml:msub> <mml:mi>m</mml:mi> <mml:mi>t</mml:mi> </mml:msub> <mml:mspace /> <mml:mi>and</mml:mi> <mml:mspace /> <mml:mi>m</mml:mi> <mml:mfenced> <mml:msubsup> <mml:mover> <mml:mi>χ</mml:mi> <mml:mo>˜</mml:mo> </mml:mover> <mml:mn>1</mml:mn> <mml:mn>0</mml:mn> </mml:msubsup> </mml:mfenced> <mml:mo>=</mml:mo> <mml:mn>1</mml:mn> </mml:math> GeV, implying that a careful recasting of these results into the full $$ m\left({\tilde{t}}_1\right)-m\left({\tilde{\upchi}}_1^0\right) $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>m</mml:mi> <mml:mfenced> <mml:msub> <mml:mover> <mml:mi>t</mml:mi> <mml:mo>˜</mml:mo> </mml:mover> <mml:mn>1</mml:mn> </mml:msub> </mml:mfenced> <mml:mo>−</mml:mo> <mml:mi>m</mml:mi> <mml:mfenced> <mml:msubsup> <mml:mover> <mml:mi>χ</mml:mi> <mml:mo>˜</mml:mo> </mml:mover> <mml:mn>1</mml:mn> <mml:mn>0</mml:mn> </mml:msubsup> </mml:mfenced> </mml:math> plane is warranted. We find that the parameter space with $$ m\left({\tilde{\upchi}}_1^0\right)\lesssim 55 $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>m</mml:mi> <mml:mfenced> <mml:msubsup> <mml:mover> <mml:mi>χ</mml:mi> <mml:mo>˜</mml:mo> </mml:mover> <mml:mn>1</mml:mn> <mml:mn>0</mml:mn> </mml:msubsup> </mml:mfenced> <mml:mo>≲</mml:mo> <mml:mn>55</mml:mn> </mml:math> GeV is excluded for $$ m\left({\tilde{t}}_1\right)\simeq {m}_t $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>m</mml:mi> <mml:mfenced> <mml:msub> <mml:mover> <mml:mi>t</mml:mi> <mml:mo>˜</mml:mo> </mml:mover> <mml:mn>1</mml:mn> </mml:msub> </mml:mfenced> <mml:mo>≃</mml:mo> <mml:msub> <mml:mi>m</mml:mi> <mml:mi>t</mml:mi> </mml:msub> </mml:math> — although this search does cover new parameter space, it is unable to fully pull the splinter. Along the way, we review a variety of interesting physical issues in detail: ( i ) when the two-body width is a good approximation; ( ii ) how assuming the narrow width approximation affects the total rate; ( iii ) how the production rate is affected when the wrong widths are used; ( iv ) what role propagating the spin correlations consistently through the multi-body decay chain plays in the limits. In addition, we provide a guide to using MadGraph for implementing the full production including finite width and spin correlation effects, and we survey a variety of pitfalls one might encounter.

The recent successful discovery of the Higgs boson at the Large Hadron Collider (LHC) at the European Center for Nuclear Research (CERN) in Geneva, Switzerland provides a long sought-after key to the Standard Model of particle physics. However, the existence (and mass) of this fundamental scalar does not resolve the tension between the electroweak and Planck scales, hinting at the possibility of new physics that may be accessible at the LHC. The objective of this research is to search for signs of such new physics, in particular for the existence of a supersymmetric partner to the top quark, using the ATLAS (A Toroidal LHC ApparatuS) detector at the LHC to reconstruct signatures of top quarks and missing energy from corresponding undetected particles. Furthermore, in order to ensure that the full potential of the LHC is harnessed in future searches for new physics, the project also entails the upgrade of the ATLAS Liquid Argon Calorimeter read-out electronics, thereby improving the performance of boosted objects, jets, and missing energy triggers for planned higher luminosity running conditions.

The BaBar drift chamber (DCH) is used to measure the properties of charged particles created from e{sup +}e{sup -} collisions in the PEP-II asymmetric-energy storage rings by making precise measurements of position, momentum and ionization energy loss (dE/dx). In October of 2005, the PEP-II storage rings operated with a luminosity of 10 x 10{sup 33} cm{sup -2}s{sup -1}; the goal for 2007 is a luminosity of 20 x 10{sup 33} cm{sup -2}s{sup -1}, which will increase the readout dead time, causing uncertainty in drift chamber measurements to become more significant in physics results. The research described in this paper aims to reduce position and dE/dx uncertainties by improving our understanding of the BaBar drift chamber performance. A simulation program --called garfield--is used to model the behavior of the drift chamber with adjustable parameters such as gas mixture, wire diameter, voltage, and magnetic field. By exploring the simulation options offered in garfield, we successfully produced a simulation model of the BaBar drift chamber. We compared the time-to-distance calibration from BaBar to that calculated by garfield to validate our model as well as check for discrepancies between the simulated and calibrated time-to-distance functions, and found that for a 0{sup o} entrance angle there is a very good match between calibrations, but at an entrance angle of 90{sup o} the calibration breaks down. Using this model, we also systematically varied the gas mixture to find one that would optimize chamber operation, which showed that the gas mixture of 80:20 Helium:isobutane is a good operating point, though more calculations need to be done to confirm that it is the optimal mixture.

Abstract Analysing high transverse momentum π − nucleus interactions at 40 GeV/c, a search has been made for the hypothetical dibaryon H 0 with strangeness −2, spin‐parity 0 + and mass below the two lambda mass. The data came from an experiment performed with a streamer chamber which is well suited to find the H 0 through its unique signature of the weak p decay. No H 0 event has been found. The 90% confidence upper limit for the observed frequency of production of the H° dibaryon per inelastic π − A collision amounts to 1.5 ·10 − 4. Cross section upper limits are estimated for C, Cu and Pb targets, also with 90% confidence level.