arXiv subjects

Andreas Warburton

Publications and source records attributed to Andreas Warburton.

8 recordsLinked to original sources

arXiv2015

Measurements of $α_s$ in $pp$ Collisions at the LHC

The coupling of the strong force, $α_s$, is deemed to be a fundamental parameter of Nature, and, beyond the quark masses, constitutes the only free parameter in the QCD Lagrangian. Provided is an overview of CERN Large Hadron Collider (LHC) measurements of $α_s(M_Z)$ evaluated at the $Z$-boson mass and of the running of $α_s(Q)$ as a function of energy-momentum transfer $Q$. The measurements were performed by the ATLAS and CMS Collaborations using proton-proton ($pp$) collisions with centre-of-mass energies of 7 TeV and data samples with time-integrated luminosities up to 5 fb$^{-1}$. Four different categories of observable were used in the described extractions of $α_s$: inclusive jet cross sections, 3-jet to 2-jet inclusive cross-section ratios, 3-jet mass cross sections, and top-quark pair production cross sections. These results, which include the first NNLO measurement of $α_s$ at a hadron collider and the first determinations of $α_s$ at energy scales above 1 TeV, are consistent with each other, with the world-average value, and with QCD predictions of their running with $Q$.

hep-ex↗

arXiv2013

A Novel in situ Trigger Combination Method

Searches for rare physics processes using particle detectors in high-luminosity colliding hadronic beam environments require the use of multi-level trigger systems to reject colossal background rates in real time. In analyses like the search for the Higgs boson, there is a need to maximize the signal acceptance by combining multiple different trigger chains when forming the offline data sample. In such statistically limited searches, datasets are often amassed over periods of several years, during which the trigger characteristics evolve and system performance can vary significantly. Reliable production cross-section measurements and upper limits must take into account a detailed understanding of the effective trigger inefficiency for every selected candidate event. We present as an example the complex situation of three trigger chains, based on missing energy and jet energy, that were combined in the context of the search for the Higgs (H) boson produced in association with a $W$ boson at the Collider Detector at Fermilab (CDF). We briefly review the existing techniques for combining triggers, namely the inclusion, division, and exclusion methods. We introduce and describe a novel fourth in situ method whereby, for each candidate event, only the trigger chain with the highest a priori probability of selecting the event is considered. We compare the inclusion and novel in situ methods for signal event yields in the CDF $WH$ search. This new combination method, by virtue of its scalability to large numbers of differing trigger chains and insensitivity to correlations between triggers, will benefit future long-running collider experiments, including those currently operating on the Large Hadron Collider.

physics.ins-det↗

arXiv2010

QCD Results from the Fermilab Tevatron proton-antiproton Collider

Selected recent quantum chromodynamics (QCD) measurements are reviewed for Fermilab Run II Tevatron proton-antiproton collisions studied by the Collider Detector at Fermilab (CDF) and DZero Collaborations at a centre-of-mass energy of sqrt{s}=1.96 TeV. Tantamount to Rutherford scattering studies at the TeV scale, inclusive jet and dijet production cross-section measurements are used to seek and constrain new particle physics phenomena, test perturbative QCD calculations, inform parton distribution function (PDF) determinations, and extract a precise value of the strong coupling constant, alpha_s(m_Z) = 0.1161^{+0.0041}_{-0.0048}. Inclusive photon production cross-section measurements reveal an inability of next-to-leading-order (NLO) perturbative QCD (pQCD) calculations to describe low-energy photons arising directly in the hard scatter. Events with gamma + 3-jet configurations are used to measure the increasingly important double parton scattering (DPS) phenomenon, with an obtained effective interaction cross section of sigma_{eff} = 16.4 +- 2.3 mb. Observations of central exclusive particle production demonstrate the viability of observing the Standard Model Higgs boson using similar techniques at the Large Hadron Collider (LHC). Three areas of inquiry into lower energy QCD, crucial to understanding high-energy collider phenomena, are discussed: the examination of intra-jet track kinematics to infer that jet formation is dominated by pQCD, and not hadronization, effects; detailed studies of the underlying event and its universality; and inclusive minimum-bias charged-particle momentum and multiplicity measurements, which are shown to challenge the Monte Carlo generators.

hep-ex↗

arXiv2008

Decays of B_s Mesons and b Baryons: A Review of Recent First Observations and Branching Fractions

Recent rate measurements of B_s mesons and Lambda_b baryons produced in sqrt{s} = 1.96 TeV proton-antiproton and Upsilon(5S) electron-positron collisions are reviewed, including the first observations of six new decay modes: B_s -> D_s^+ K^- (CDF), B_s -> D_s^- D_s^+ (CDF), B_s -> D_{s1}^-(2536) mu^+ nu_mu X (DZero), B_s -> phi gamma (Belle), Lambda_b -> p pi^- (CDF), and Lambda_b -> p K^- (CDF). Also examined are branching-fraction measurements or limits for the B_s -> D_s^{(*)} D_s^{(*)} modes (Belle, CDF, and DZero), the B_s -> gamma gamma radiative penguin decay (Belle), and three two-body charmless B_s meson decay channels (CDF). Implications for the phenomenology of electroweak and QCD physics, as well as searches for physics beyond the Standard Model, are identified where applicable.

hep-ex↗

arXiv2004

Branching Fractions and CP Asymmetries in Two-Body Nonleptonic Charmless b-Hadron Decays

Relative branching fractions of B^0_d,s -> h^+ h^'- decays (where h, h' = K or pi) and the direct CP asymmetry A_CP in the B^0_d -> K^+ pi^- mode are measured with 179 +- 11 pb^-1 of data collected at sqrt{s} = 1.96 TeV using the CDF II detector at the Fermilab Tevatron ppbar collider. The first branching-fraction measurement of a B^0_s meson to two pseudoscalars, Br(B^0_s -> K^+ K^-), and a search for the baryon mode Lambda^0_b -> p^+ h^- are also presented, in addition to branching-fraction limits on the rare channels B^0_s -> K^+ pi^-, B^0_d -> K^+ K^-, and B^0_s -> pi^+ pi^-.

hep-ex↗

arXiv2003

|Vcb| and |Vub| from CLEO

I report on studies of inclusive and exclusive semileptonic b -> c l nu and b -> u l nu decays in 9.7 million BBbar events accumulated with the CLEO detector in symmetric e^+e^- collisions produced in the Cornell Electron Storage Ring (CESR). Various experimental techniques, including the study of spectral moments and the inference of neutrino candidates by exploiting the hermeticity of the CLEO detector, are used in conjunction with theoretical calculations to provide estimates of the CKM matrix elements |Vcb| and |Vub|.

hep-ex↗

arXiv2000

First Observation of the Sigma_c^{*+} Charmed Baryon, and New Measurements of the Sigma_c^0, Sigma_c^+, Sigma_c^{++}, and Omega_c^0 Charmed Baryons

Using ~13.7 fb^{-1} of e^+e^- collision data recorded at near the Upsilon(4S) resonance by the CLEO II and CLEO II.V detector configurations on the Cornell Electron Storage Ring, we present the world's most precise measurements of the Sigma_c^0, Sigma_c^+, and Sigma_c^{++} charmed-baryon masses as well as the first measurements of the intrinsic widths of the Sigma_c^0 and Sigma_c^{++} baryons. We also report on the first observation and mass measurement of the Sigma_c^{*+} charmed baryon, M(Sigma_c^{*+}) - M(Lambda_c^+) = (231.0 +- 1.1[stat] +- 2.0[syst]) MeV/c^2, and the first CLEO observation of the Omega_c^0 baryon, for which we measure a mass M(Omega_c^0) = (2694.6 +- 2.6[stat] +- 1.9[syst]) MeV/c^2 from a sample of (40.4 +- 9.0[stat]) candidate events. All new results are preliminary.

hep-ex↗

arXiv1999

CKM Physics from B Decays using the CLEO Experiment

We report on studies of three types of B-meson decay that can contribute to an understanding of fundamental intergenerational quark mixing, charge-conjugation--parity violation, and long-distance quantum chromodynamics. Specifically, we discuss a selection of analyses related to the Cabibbo-Kobayashi-Maskawa (CKM) parameters Vcb, Vub, alpha, and gamma, and the nonperturbative heavy quark effective theory quantities lambda_1 and lambda_bar. We first describe an examination of the first and second moments of the hadronic-recoil mass and charged-lepton energy spectra in inclusive b -> c l nu decays. We also report on the reconstruction, using similar experimental techniques, of the CKM-suppressed decay B -> rho l nu and the extraction of its branching fraction, Br(B0 -> rho- l+ nu) = (2.57 +- 0.29 +0.33/-0.46 +- 0.41) x 10E-4, as well as the value |Vub| = (3.25 +- 0.14 +0.21/-0.29 +- 0.55) x 10E-3, where the uncertainties are statistical, systematic, and due to model dependence, respectively. Finally, we present results on rare two-body charmless hadronic B -> K pi, pi pi, and K K decays and comment briefly on their implications to the geometry of the CKM unitarity triangle, including a bound on gamma.

hep-ex↗