660 research outputs found
Experimental evidence of planar channeling in a periodically bent crystal
The usage of a Crystalline Undulator (CU) has been identified as a promising
solution for generating powerful and monochromatic -rays. A CU was
fabricated at SSL through the grooving method, i.e., by the manufacturing of a
series of periodical grooves on the major surfaces of a crystal. The CU was
extensively characterized both morphologically via optical interferometry at
SSL and structurally via X-ray diffraction at ESRF. Then, it was finally tested
for channeling with a 400 GeV/c proton beam at CERN. The experimental results
were compared to Monte Carlo simulations. Evidence of planar channeling in the
CU was firmly observed. Finally, the emission spectrum of the positron beam
interacting with the CU was simulated for possible usage in currently existing
facilities
Steering efficiency of a ultrarelativistic proton beam in a thin bent crystal
Crystals with small thickness along the beam exhibit top performance for steering particle beams through planar channeling. For such crystals, the effect of nuclear dechanneling plays an important role because it affects their efficiency. We addressed the problem through experimental work carried out with 400 GeV/c protons at fixed-target facilities of CERN-SPS. The dependence of efficiency vs. curvature radius has been investigated and compared favourably to the results of modeling. A realistic estimate of the performance of a crystal designed for LHC energy including nuclear dechanneling has been achieved
Top-quark spin correlation at Linear Colliders with anomalous couplings
We investigate the feasibility of probing anomalous top-quark couplings of
, , and in terms of an effective
Lagrangian with dimension-six operators at future linear colliders
with a c. m. energy GeV. We first examine the
constraints on these anomalous couplings from the data at LEP
I and from unitarity considerations. We then consider in detail the effects of
anomalous couplings on spin correlations in the top-pair production
and decay with three spin bases: the helicity, beamline and off-diagonal bases.
Our results show that the polarized beams are more suitable for exploring the
effects of different new operators. For polarized beams, the helicity basis
yields the best sensitivity.Comment: 23 pages, 10 figures, references adde
Single decay-lepton angular distributions in polarized e+e- --> t tbar and simple angular asymmetries as a measure of CP-violating top dipole couplings
In the presence of an electric dipole coupling of t-tbar to a photon, and an
analogous "weak" dipole coupling to the Z, CP violation in the process e+e- -->
t tbar results in modified polarization of the top and anti-top. This
polarization can be analyzed by studying the angular distributions of decay
charged leptons when the top or anti-top decays leptonically. Analytic
expressions are presented for these distributions when either t or tbar decays
leptonically, including O(alpha_s) QCD corrections in the soft-gluon
approximation. The angular distributions are insensitive to anomalous
interactions in top decay. Two types of simple CP-violating polar-angle
asymmetries and two azimuthal asymmetries, which do not need the full
reconstruction of the t or tbar, are studied. Independent 90% CL limits that
may be obtained on the real and imaginary parts of the electric and weak dipole
couplings at a linear collider operating at sqrt{s}=500 GeV with integrated
luminosity 500 fb^{-1} and also at sqrt{s}=1000 GeV with integrated luminosity
1000 fb^{-1} have been evaluated. The effect of longitudinal electron and/or
positron beam polarizations has been included.Comment: 26 pages, latex, figures included as latex files, version appearing
in Pramana - Journal of Physics, minor corrections and note added in proo
Single and Multiple Volume Reflections of Ultra-Relativistic Electrons in a Bent Crystal as Tools for Intense Production of Electromagnetic Radiation
Electromagnetic radiation emitted by ultrarelativistic volume-reflected and multi-volume-reflected electrons in a bent crystal is very intense and takes place over a broad angular range of the incident beam. Such radiation results to be nearly independent from the particles trajectory and charge. This paper describes the possible applications these features allow, from the production of a gamma or a positron source to the crystal-assisted collimation of future linear e+/e− colliders and crystal-based electromagnetic calorimeters
Precise measurement of the top quark mass in the dilepton channel at D0
We measure the top quark mass (mt) in ppbar collisions at a center of mass
energy of 1.96 TeV using dilepton ttbar->W+bW-bbar->l+nubl-nubarbbar events,
where l denotes an electron, a muon, or a tau that decays leptonically. The
data correspond to an integrated luminosity of 5.4 fb-1 collected with the D0
detector at the Fermilab Tevatron Collider. We obtain mt = 174.0 +- 1.8(stat)
+- 2.4(syst) GeV, which is in agreement with the current world average mt =
173.3 +- 1.1 GeV. This is currently the most precise measurement of mt in the
dilepton channel.Comment: 7 pages, 4 figure
Double parton interactions in photon+3 jet events in ppbar collisions sqrt{s}=1.96 TeV
We have used a sample of photon+3 jets events collected by the D0 experiment
with an integrated luminosity of about 1 fb^-1 to determine the fraction of
events with double parton scattering (f_DP) in a single ppbar collision at
sqrt{s}=1.96 TeV. The DP fraction and effective cross section (sigma_eff), a
process-independent scale parameter related to the parton density inside the
nucleon, are measured in three intervals of the second (ordered in pT) jet
transverse momentum pT_jet2 within the range 15 < pT_jet2 < 30 GeV. In this
range, f_DP varies between 0.23 < f_DP < 0.47, while sigma_eff has the average
value sigma_eff_ave = 16.4 +- 0.3(stat) +- 2.3(syst) mb.Comment: 15 pages, 13 figure
Evidence for an anomalous like-sign dimuon charge asymmetry
We measure the charge asymmetry A of like-sign dimuon events in 6.1 fb(-1) of p (p) over bar collisions recorded with the D0 detector at a center-of-mass energy root s = 1.96 TeV at the Fermilab Tevatron collider. From A, we extract the like-sign dimuon charge asymmetry in semileptonic b-hadron decays: A(sl)(b) = -0.00957 +/- 0.00251 (stat) +/- 0.00146 (syst). This result differs by 3.2 standard deviations from the standard model prediction A(sl)(b)(SM) = (-2.3(0.6)(+0.5)) x 10(-4) and provides first evidence of anomalous CP violation in the mixing of neutral B mesons
Dependence of the production cross section on the transverse momentum of the top quark
We present a measurement of the differential cross section for
events produced in collisions at TeV as a function
of the transverse momentum () of the top quark. The selected events
contain a high- lepton (), four or more jets, and a large imbalance
in , and correspond to 1 fb of integrated luminosity recorded
with the D0 detector. Each event must have at least one candidate for a
jet. Objects in the event are associated through a constrained kinematic fit to
the process. Results
from next-to-leading-order perturbative QCD calculations agree with the
measured differential cross section. Comparisons are also provided to
predictions from Monte Carlo event generators using QCD calculations at
different levels of precision.Comment: 8 pages, 6 figures, 4 tables, updated to reflect the published
versio
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