At the Large Hadron Collider (LHC), absolute luminosity calibrations obtained by the van der Meer (
vdM
) method are affected by the mutual electromagnetic interaction of the two beams. The colliding ...bunches experience relative orbit shifts, as well as optical distortions akin to the dynamic-
β
effect, that both depend on the transverse beam separation and must therefore be corrected for when deriving the absolute luminosity scale. In the
vdM
regime, the beam–beam parameter is small enough that the orbit shift can be calculated analytically. The dynamic-
β
corrections to the luminometer calibrations, however, had until the end of Run 2 been estimated in the linear approximation only. In this report, the influence of beam–beam effects on the
vdM
-based luminosity scale is quantified, together with the associated systematic uncertainties, by means of simulations that fully take into account the non-linearity of the beam–beam force, as well as the resulting non-Gaussian distortions of the transverse beam distributions. Two independent multiparticle simulations, one limited to the weak-strong approximation and one that models strong-strong effects in a self-consistent manner, are found in excellent agreement; both predict a percent-level shift of the absolute
pp
-luminosity values with respect to those assumed until recently in the physics publications of the LHC experiments. These results also provide guidance regarding further studies aimed at reducing the beam–beam-related systematic uncertainty on beam–beam corrections to absolute luminosity calibrations by the van der Meer method.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Luminosity is a key parameter in any particle collider, and its precise determination has proven particularly challenging at hadron colliders. After introducing the concept of luminosity in its ...multiple incarnations and offering a brief survey of the pp and pp¯ colliders built to date, this article outlines the various methods that have been developed for relative-luminosity monitoring, as well as the complementary approaches considered for establishing an absolute luminosity scale. This is followed by a survey, from both a historical and a technical perspective, of luminosity determination at the ISR, the Spp̄S, the Tevatron, RHIC and the LHC. For each of these, we first delineate the interplay between the experimental context, the specificities of the accelerator, and the precision targets suggested by the physics program. We then detail how the different methods were applied to specific experimental environments and how successfully they meet the precision goals.
The PEP-II B-factory: status and prospects Kozanecki, W
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
2000, Letnik:
446, Številka:
1
Journal Article
Recenzirano
The PEP-II asymmetric electron–positron collider, which was completed in July 1998, has started delivering luminosity to the BaBar experiment at SLAC. This paper summarizes the design issues of this ...novel accelerator complex, the PEP-II performance as achieved during the commissioning runs, and the early running experience with BaBar.
Searches for lepton-flavor-violating decays of a tau lepton to a lighter mass lepton and a photon have been performed with the entire data set of (963+/-7)x10{6} tau decays collected by the BABAR ...detector near the Upsilon(4S), Upsilon(3S) and Upsilon(2S) resonances. The searches yield no evidence of signals and we set upper limits on the branching fractions of B(tau{+/-}-->e{+/-}gamma)<3.3x10{-8} and B(tau{+/-}-->mu{+/-}gamma)<4.4x10{-8} at 90% confidence level.
A precise measurement of the cross section of the process e(+)e(-) --> pi(+)pi(-)(gamma) from threshold to an energy of 3 GeV is obtained with the initial state radiation (ISR) method using 232 ...fb(-1) of data collected with the BABAR detector at e(+)e(-) center-of-mass energies near 10.6 GeV. The ISR luminosity is determined from a study of the leptonic process e(+)e(-) --> mu(+)mu(-)gamma(gamma). The leading-order hadronic contribution to the muon magnetic anomaly calculated using the pipi cross section measured from threshold to 1.8 GeV is (514.1 +/- 2.2(stat) +/- 3.1(syst)) x 10(-10).
The air-core Barrel Toroid of the ATLAS Detector at the CERN Large Hadron Collider, is the largest superconducting magnet ever built. This muon-spectrometer magnet consists of 8 discrete coils, ...arranged symmetrically around the beam axis. The average field is 0.5 T, and the stored energy 1.1 GJ. The windings are 25 m long, 5 m wide and 0.4 m thick. Each coil cold mass consists of 2 double pancakes of aluminum-stabilized NbTi conductor securely fixed in an aluminum alloy casing. Before assembly, each coil is tested individually in the presence of a magnetic mirror to simulate the magnetic forces exerted on the tie rods. This mirror is composed of several iron plates made of magnetically non-linear material. Each coil is enclosed in a separate, stainless-steel cryostat with relative magnetic permeability slightly larger than 1. In this paper the magnetic field distribution and Lorenz forces between the magnetic mirror and the coil in centered and shifted positions are presented. The magnetic influence of the cryostat on the field distribution is also discussed.
We measure branching fractions and integrated rate asymmetries for the rare decays B-->K(*)l+l-, where l+l- is either e+e- or micro+micro-, using a sample of 384x10(6) BB events collected with the ...BABAR detector at the PEP-II e+e- collider. We find no evidence for direct CP or lepton-flavor asymmetries. However, for dilepton masses below the J/psi resonance, we find evidence for unexpectedly large isospin asymmetries in both B-->Kl+l- and B-->K*l+l- which differ, respectively, by 3.2sigma and 2.7sigma, including systematic uncertainties, from the standard model expectations.
We present an updated measurement of time-dependent CP-violating asymmetries in neutral B decays with the BABAR detector at the PEP-II asymmetric B Factory at SLAC. This result uses an additional ...sample of Upsilon(4S) decays collected in 2001, bringing the data available to 32 x 10(6) BB macro pairs. We select events in which one neutral B meson is fully reconstructed in a final state containing charmonium and the flavor of the other neutral B meson is determined from its decay products. The amplitude of the CP-violating asymmetry, which in the standard model is proportional to sin2 beta, is derived from the decay time distributions in such events. The result sin2 beta = 0.59+/-0.14(stat)+/-0.05(syst) establishes CP violation in the B(0) meson system. We also determine absolute value of lambda = 0.93+/-0.09(stat)+/-0.03(syst), consistent with no direct CP violation.
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