118619574
Exploring the heavy quark sector of the Bestest Little Higgs model at the LHC
A bstractWe present discovery limits for heavy quarks in the Bestest Little Higgs model via pair production at the LHC running at $ \sqrt {s} = 7\;{\text{TeV}} $. We study pair produced heavy toplike quarks decaying to $ b\overline b {W^{ + }}{W^{ - }} $ and $ t\overline t ZZ $ final states and singly produced heavy top-like quarks via t-channel W exchange. These results are compared to currently available limits on heavy top-like quark cross sections from CMS (with 1.14 fb−1 integrated luminosity) for two scenarios of Yukawa couplings. We find that the CMS data limits the mass of the lightest top partner to larger than 413 GeV in the first scenario, where the two lightest top-partners have a small mass splitting, and 364 GeV in the second scenario, where the mass splitting between the two lightest top-partners is larger.
119224346
Using final state pseudorapidities to improve s-channel resonance observables at the LHC
We study the use of final state particle pseudorapidity for measurements of s-channel resonances at the LHC. Distinguishing the spin of an s-channel resonance can, in principle, be accomplished using angular distributions in the center-of-mass frame, possibly using a center-edge asymmetry measurement, A{sub CE}. In addition, forward-backward asymmetry measurements, A{sub FB}, can be used to distinguish between models of extra neutral gauge bosons. In this article we show how these measurements can be improved by using simple methods based on the pseudorapidity of the final state particles and present the expected results for A{sub FB} and A{sub CE} for several representative models.
19324611
Identification of extra neutral gauge bosons at the LHC using b and t quarks.
New neutral gauge bosons (Z' 's) are predicted by many models of physics beyond the standard electroweak theory. It is possible that a Z' will be discovered by the Large Hadron Collider program. The next step would be to measure its properties to identify the underlying theory that gave rise to the Z'. Heavy quarks have the unique property that they can be identified in the final states. In this Letter we demonstrate that measuring Z' decays to b- and t-quark final states can act as an effective means of discriminating between models with extra gauge bosons.
118404330
Unravelling an extra neutral gauge boson at the LHC using third generation fermions
We study the potential to use measurements of extra neutral gauge bosons (Z') properties in pp collisions at the Large Hadron Collider to unravel the underlying physics. We focus on the usefulness of third generation final states (tau, b, t) in distinguishing between models with non-universal Z'-fermion couplings. We present an update of discovery limits of Z's including the 2010-2011 LHC run and include models with non-universal couplings. We show how ratios of sigma(pp -> Z' -> ttbar), sigma(pp -> Z' -> bbbar), and sigma(pp -> Z' -> tau^+tau^-) to sigma(pp -> Z' -> mu^+mu^-) can be used to distinguish between models and measure parameters of the models. Of specific interest are models with preferential couplings, such as models with generation dependent couplings. We also find that forward-backward asymmetry measurements with third generation fermions in the final state could provide important input to understanding the nature of the Z'. Understanding detector resolution and efficiencies will be crucial for extracting results.
118575499
Discovery and Identification of Extra Neutral Gauge Bosons at the LHC
We present the discovery prospects for a Z' resonance and some expected results for Z' diagnostic measurements at the LHC. The discovery reach is highly dependent on the energy and luminosities that may be attained at the LHC, and a number of energy and integrated luminosity scenarios are presented. In addition, the use of third generation quark final states for distinguishing between models and for measuring a forward-backward asymmetry is explored.
118438101
Constraining the Little Higgs model of Schmaltz, Stolarski, and Thaler with recent results from the LHC
In this paper, we use the latest Higgs measurements from ATLAS and CMS to constrain the parameter space of the model of Schmaltz, Stolarski and Thaler, a Little Higgs model with two Higgs doublets, which we will refer to as the BLH model. We account for all production and decay modes explored at ATLAS and CMS in two scenarios: a general case, which assumes the $h_0$ state is light ($m_{h_0} \approx 125$ GeV) and the masses of the other neutral scalars ($H_0$ and $A_0$) are allowed to vary, and a case with a near-degeneracy between the masses of the $h_0$ and $A_0$ and, for some choices of parameters, the $H_0$ states. The near-degeneracy scenario can result in an enhanced diphoton rate, as measured by ATLAS, but is largely ruled out by a combination of the $h_0 \rightarrow \tau^+\tau^-$ and the heavy $H_0 \rightarrow W^+W^-$ measurements. In the general case, we find large regions of parameter space that are in better agreement with either the ATLAS or CMS results than is the SM. However, a significantly enhanced diphoton rate is only possible through large contributions to the $h_0 \gamma \gamma$ effective coupling from charged Higgs bosons in a region of parameter space that borders on violation of perturbativity in the scalar sector.
123039432
Development and online operation of minimum bias triggers in ATLAS
Since the restart of the LHC in November 2009, ATLAS has recorded pp collisions to investigate new phenomena at the high energy frontier. One early measurement is of charged particle multiplicities in inelastic interactions, which was performed on data sets of about 360,000 and 10,000,000 minimum bias event at (s)1/2 = 900 GeV and 7 TeV respectively. This measurements will help to constrain various phenomenological models describing inelastic interactions. Understanding trigger efficiencies for different event types is crucial to minimize any possible bias in the event selection. ATLAS has a three level trigger system and utilises two main minimum bias triggers. A primary scintillator based trigger situated in the forward regions of 2.1 < |η| < 3.8 was proven to select pp collisions with high efficiency. A complementary control trigger runs on unbiased filled proton bunches and utilizes space points reconstructed in the silicon tracking volume to trigger on inelastic interactions. Both triggers were essential for the analysis of charged particle multiplicities. Their performance and trigger efficiency measurements as well as studies on possible bias sources are presented. The triggers are shown to form an effective and inclusive triggering solution for early LHC physics.
126082272
Measurement of hard double-parton interactions with the ATLAS detector
The production of a W boson in association with two jets was studied in proton-proton collisions at a centre-of-mass energy of √ s = 7 TeV with the ATLAS detector at the LHC. Data corresponding to an integrated luminosity of 36 pb−1 were analysed for the presence of doubleparton interactions. The fraction of events arising from double-parton interactions was extracted using the momentum balance of the two jets, and was evaluated to be f Detector DP = 0.08± 0.01(stat.)± 0.02(syst.) for jets with transverse momentum pT > 20 GeV and rapidity |y| < 2. This is expressed as a measure of the effective area parameter for hard double-parton interactions, σeff = 15±3(stat.)+5 −3(syst.) mb.
59363644
Towards a Level-1 tracking trigger for the ATLAS experiment at the High Luminosity LHC
At the high luminosity HL-LHC, upwards of 140 individual proton-proton interactions (pileup) are expected per bunch-crossing at luminosities of around 5×1034 cm-2s-1. A proposal by the ATLAS collaboration to split the ATLAS first level trigger in to two stages is briefly detailed. The use of fast track finding in the new first level trigger is explored as a method to provide the discrimination required to reduce the event rate to acceptable levels for the read out system while maintaining high efficiency on the selection of the decay products of electroweak bosons along with other high pT physics signatures at HL-LHC luminosities. It is shown that the available bandwidth in the proposed new strip tracker is sufficient for a region of interest based track trigger given certain optimisations. Further methods for improving upon the proposal are discussed.