7,300 research outputs found
ATLAS silicon module assembly and qualification tests at IFIC Valencia
ATLAS experiment, designed to probe the interactions of particles emerging
out of proton proton collisions at energies of up to 14 TeV, will assume
operation at the Large Hadron Collider (LHC) at CERN in 2007. This paper
discusses the assembly and the quality control tests of forward detector
modules for the ATLAS silicon microstrip detector assembled at the Instituto de
Fisica Corpuscular (IFIC) in Valencia. The construction and testing procedures
are outlined and the laboratory equipment is briefly described. Emphasis is
given on the module quality achieved in terms of mechanical and electrical
stability.Comment: 23 pages, 38 EPS figures, uses JINST LaTeX clas
LHC Coverage of RPV MSSM with Light Stops
We examine the sensitivity of recent LHC searches to signatures of
supersymmetry with R-parity violation (RPV). Motivated by naturalness of the
Higgs potential, which would favor light third-generation squarks, and the
stringent LHC bounds on spectra in which the gluino or first and second
generation squarks are light, we focus on scenarios dominated by the pair
production of light stops. We consider the various possible direct and cascade
decays of the stop that involve the trilinear RPV operators. We find that in
many cases, the existing searches exclude stops in the natural mass range and
beyond. However, typically there is little or no sensitivity to cases dominated
by UDD operators or LQD operators involving taus. We propose several ideas for
searches which could address the existing gaps in experimental coverage of
these signals.Comment: 41 pages, 12 figures; v2: included new searches (see footnote 10),
minor corrections and improvement
Measurement of the rapidity-even dipolar flow in Pb-Pb collisions with the ATLAS detector
The rapidity-even dipolar flow v1 associated with dipole asymmetry in the
initial geometry is measured over a broad range in transverse momentum 0.5
GeV<pT<9 GeV, and centrality (0-50)% in Pb-Pb collisions at sqrt(s_NN)=2.76
TeV, recorded by the ATLAS experiment at the LHC. The v1 coefficient is
determined via a two-component fit of the first order Fourier coefficient,
v_{1,1}= cos \Delta\phi, of two-particle correlations in azimuthal angle
\Delta\phi=\phi_a-\phi_b as a function of pT^a and pT^b. This fit is motivated
by the finding that the pT dependence of v_{1,1}(pT^a,pT^b) data are consistent
with the combined contributions from a rapidity-even v1 and global momentum
conservation. The magnitude of the extracted momentum conservation component
suggests that the system conserving momentum involves only a subset of the
event (spanning about 3 units in \eta in central collisions). The extracted v1
is observed to cross zero at pT~1.0 GeV, reaches a maximum at 4-5 GeV with a
value comparable to that for v3, and decreases at higher pT. Interestingly, the
magnitude of v1 at high pT exceeds the value of the v3 in all centrality
interval and exceeds the value of v2 in central collisions. This behavior
suggests that the path-length dependence of energy loss and initial dipole
asymmetry from fluctuations corroborate to produce a large dipolar anisotropy
for high pT hadrons, making the v1 a valuable probe for studying the jet
quenching phenomena.Comment: 9 pages, 6 figures. Proceedings for the 28th Winter Workshop on
Nuclear Dynamics, Dorado Del Mar, Puerto Rico, United States Of America, 7 -
14 Apr 201
Signals from R-parity violating top quark decays at LHC
We evaluate the potential of the CERN LHC collider to observe rare decays of
the top quark in channels involving R-parity violating (RPV) interactions. We
stress the importance of calculating top quark production and decay
simultaneously as a true 2->4 process.
The process of tt-bar pair production followed by RPV decay of one of the top
quarks is analyzed with fast detector simulation. We show that intermediate
supersymmetric particles can be observed as resonances even if they are heavier
than the top quark due to the significant off-shell top-quark mass effects. The
approach where the top quark is produced on-mass-shell and then decays into 2-
or 3-body final state would in general lead to incorrect kinematical
distributions and rates. The rates of the 2 -> 4 process with top quark
production and RPV 3-body decay depend on the total width of the heavy
intermediate sfermion which could,therefore, be measured indirectly.
We find that the LHC collider offers a unique potential to study rare top
quark decays in the framework of supersymmetry with broken R-parity for
branching fractions of RPV top decays as low as 10^{-6}Comment: 23 pages, 22 figure
Macroscopic Strings and "Quirks" at Colliders
We consider extensions of the standard model containing additional heavy
particles ("quirks") charged under a new unbroken non-abelian gauge group as
well as the standard model. We assume that the quirk mass m is in the
phenomenologically interesting range 100 GeV--TeV, and that the new gauge group
gets strong at a scale Lambda < m. In this case breaking of strings is
exponentially suppressed, and quirk production results in strings that are long
compared to 1/Lambda. The existence of these long stable strings leads to
highly exotic events at colliders. For 100 eV < Lambda < keV the strings are
macroscopic, giving rise to events with two separated quirk tracks with
measurable curvature toward each other due to the string interaction. For keV <
Lambda < MeV the typical strings are mesoscopic: too small to resolve in the
detector, but large compared to atomic scales. In this case, the bound state
appears as a single particle, but its mass is the invariant mass of a quirk
pair, which has an event-by-event distribution. For MeV < Lambda < m the
strings are microscopic, and the quirks annihilate promptly within the
detector. For colored quirks, this can lead to hadronic fireball events with
10^3 hadrons with energy of order GeV emitted in conjunction with hard decay
products from the final annihilation.Comment: Added discussion of photon-jet decay, fixed minor typo
Absorbing systematic effects to obtain a better background model in a search for new physics
This paper presents a novel approach to estimate the Standard Model
backgrounds based on modifying Monte Carlo predictions within their systematic
uncertainties. The improved background model is obtained by altering the
original predictions with successively more complex correction functions in
signal-free control selections. Statistical tests indicate when sufficient
compatibility with data is reached. In this way, systematic effects are
absorbed into the new background model. The same correction is then applied on
the Monte Carlo prediction in the signal region. Comparing this method to other
background estimation techniques shows improvements with respect to statistical
and systematical uncertainties. The proposed method can also be applied in
other fields beyond high energy physics
A Search for Vector Diquarks at the CERN LHC
Resonant production of the first generation vector diquarks at the CERN Large
Hadron Collider (LHC) is investigated. It is shown that the LHC will be able to
discover vector diquarks with masses up to 9 TeV for quark-diquark-quark
coupling alpha_(D)=0.1 and 4 TeV for alpha_(D)=5x10^(-4).Comment: 9 pages, 4 tables, 4 figure
On the active site of elastase: Partial mapping by means of specific peptide substrates
AbstractRNase-S peptide as well as some related octa- and hexapeptides were found to be highly, reactive substrates of porcine elastase (e.g. Ala4-Lys-Phe: Km = 4500 Mâ1, kcat = 32 secâ1, C = 1.4 Ă 105 Mâ1 secâ1). Comparison of the various peptides led to the conclusion that the active site of porcine elastase is composed of 6â7 subsites (c.f. [1]). Preliminary mapping shows that subsites S2, SâČ1 and SâČ2 have hydrophobic character. Occupation of subsite S4 by the substrate is important for efficient hydrolysis. Binding at this subsite was found to be stereospecific
Dark matter searches at LHC
Besides Standard Model measurements and other Beyond Standard Model studies,
the ATLAS and CMS experiments at the LHC will search for Supersymmetry, one of
the most attractive explanation for dark matter. The SUSY discovery potential
with early data is presented here together with some first results obtained
with 2010 collision data at 7 TeV. Emphasis is placed on measurements and
parameter determination that can be performed to disentangle the possible SUSY
models and SUSY look-alike and the interpretation of a possible positive
supersymmetric signal as an explanation of dark matter.Comment: 15 pages, 14 figures, Invited plenary talk given at DISCRETE 2010:
Symposium On Prospects In The Physics Of Discrete Symmetries, 6-11 Dec 2010,
Rome, Ital
An Empirical Charge Transfer Potential with Correct Dissociation Limits
The empirical valence bond (EVB) method [J. Chem. Phys. 52, 1262 (1970)] has
always embodied charge transfer processes. The mechanism of that behavior is
examined here and recast for use as a new empirical potential energy surface
for large-scale simulations. A two-state model is explored. The main features
of the model are: (1) Explicit decomposition of the total system electron
density is invoked; (2) The charge is defined through the density decomposition
into constituent contributions; (3) The charge transfer behavior is controlled
through the resonance energy matrix elements which cannot be ignored; and (4) A
reference-state approach, similar in spirit to the EVB method, is used to
define the resonance state energy contributions in terms of "knowable"
quantities. With equal validity, the new potential energy can be expressed as a
nonthermal ensemble average with a nonlinear but analytical charge dependence
in the occupation number. Dissociation to neutral species for a gas-phase
process is preserved. A variant of constrained search density functional theory
is advocated as the preferred way to define an energy for a given charge.Comment: Submitted to J. Chem. Phys. 11/12/03. 14 pages, 8 figure
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