Search results for "Collider"
showing 10 items of 1690 documents
The COMPASS trigger system for muon scattering
2005
Abstract The trigger system of the COMPASS experiment at the CERN polarized muon beam is presented. It detects muon scattering events on (polarized) nucleons with a relative energy loss exceeding a selectable value y min independent of the four-momentum transfer. The requirement of a minimum energy deposit in a hadron calorimeter rejects background events like scattering on electrons, elastic and quasi-elastic radiative events as well as events from beam halo tracks. The trigger system which can be considered as a tagger for quasi-real photon events is now, along with larger trigger hodoscope system for deep inelastic scattering events, in regular use for the measurement of the gluon polari…
Measurement of the production cross section for a Higgs boson in association with a vector boson in the H → WW⁎ → ℓνℓν channel in pp collisions at s=…
2019
A measurement of the Higgs boson production cross sections via associated WH and ZH production using H -> WW* -> l nu l nu decays, where l stands for either an electron or a muon, is presente ...
Measurement of exclusive γγ→ℓ+ℓ− production in proton–proton collisions at s=7 TeV with the ATLAS detector
2015
This Letter reports a measurement of the exclusive gamma gamma -> l(+)l(-) (l = e, mu) cross-section in proton-proton collisions at a centre-of-mass energy of 7 TeV by the ATLAS experiment at the LHC, based on an integrated luminosity of 4.6 fb(-1). For the electron or muon pairs satisfying exclusive selection criteria, a fit to the dilepton acoplanarity distribution is used to extract the fiducial cross-sections. The cross-section in the electron channel is determined to be sigma(excl)(gamma gamma -> e+e-) = 0.428 +/- 0.035 (stat.) +/- 0.018 (syst.) pbfor a phase-space region with invariant mass of the electron pairs greater than 24GeV, in which both electrons have transverse momentum p(T)…
Multiplicities and Transverse Energies in Central AA Collisions at RHIC and LHC from pQCD, Saturation and Hydrodynamics
2001
We compute the particle multiplicities and transverse energies at central and nearly central AA collisions at RHIC and LHC. The initial state is computed from perturbative QCD supplemented by the conjecture of saturation of produced partons. The expansion stage is described in terms of hydrodynamics assuming longitudinal boost invariance and azimuthal symmetry. Transverse flow effects, a realistic list of hadrons and resonance decays are included. Comparison with the data of the multiplicities at $\sqrt s=56$ AGeV and 130 AGeV from RHIC is done and predictions for the full RHIC energy and LHC energy are made for the multiplicities and transverse energies. The reduction from the initially re…
A perturbative QCD analysis of charged-particle distributions in hadronic and nuclear collisions
2002
We compute the distributions of charged particles at large transverse momenta in $p\bar p(p)$, $pA$ and $AA$ collisions in the framework of perturbative QCD, by using collinear factorization and the modern PDFs and fragmentation functions. At the highest cms-energies the shape of the spectra measured in $p\bar p(p)$ collisions at large $q_T$ can be well explained. The difference between the data and the lowest-order computation is quantified in terms of a constant $K$-factor for each energy. The $K$-factor is found to systematically decrease with growing $\sqrt s$. Also a lower limit for the partonic transverse momentum, $p_0$, is extracted for each $\sqrt s$ based on the comparison with th…
Minijet and transverse energy production in the BFKL regime
1996
Minijet production in hadronic and nuclear collisions through a BFKL pomeron ladder is studied for the energies of the future LHC heavy-ion collisions. We use unintegrated gluon densities compatible with the small-$x$ increase of parton distributions observed at HERA. We show that at LHC energies the BFKL minijet and transverse energy production is at most of the same order of magnitude as that in the collinear factorization approach.
Understanding LHC jets in the light of RHIC data
2012
Hard probes are a cornerstone in the ongoing program to determine the properties of hot and dense QCD matter as created in ultrarelativistic heavy ion collisions. LHC measurements have so far resulted in a wealth of high P_T data, opening new kinematic windows with high statistics. Yet on first glance, several observations are counter-intuitive and seem to contradict results from the RHIC high P_T program. This calls for a combined analysis of high P_T hadrons and reconstructed jets at RHIC and LHC in a unified framework testing a large number of theoretical models for both medium evolution and shower medium interactions against the systematics of the data. A consistent picture of shower-me…
New extraction of color-octet NRQCD matrix elements from charmonium hadroproduction
1999
We re-analyze Tavatron data on charmonium hadroproduction in the framework of the color-octet model implemented in the event generator PYTHIA taking into account initial-state radiation of gluons and Altarelli-Parisi evolution of final-state gluons fragmenting into $c\bar{c}$ pairs. We obtain new values for the color-octet matrix elements relevant to this production process. We discuss the sensitivity of our results to the transverse momentum lower cut-off employed in the generation to avoid the problematic $p_t{\to}0$ region, arguing about the reliability of our previous extraction of the NRQCD matrix elements for the $^3S_1^{(8)}$ and $^1S_0^{(8)}+^3P_J^{(8)}$ contributions. Finally we ex…
The fragility of high- hadron spectra as a hard probe
2004
We study the suppression of high-pT hadron spectra in nuclear collisions, supplementing the perturbative QCD factorized formalism with radiative parton energy loss. We find that the nuclear modification factor which quantifies the degree of suppression, is almost pT-independent both for RHIC (in agreement with data) and for the LHC. This is a consequence of the shape of the partonic pT-spectrum in elementary collisions which implies that for the same value of the nuclear modification factor at higher pT, an increasingly smaller fraction of parton energy loss is needed. When the values of the time-averaged transport coefficient exceed 5 GeV^2/fm, the nuclear modification factor gradually los…
Thermal photon v3 at LHC from fluctuating initial conditions
2014
Abstract We calculate the triangular flow parameter v 3 of thermal photons for 0–40% central collisions of Pb nuclei at LHC using an event-by-event hydrodynamic model with fluctuating initial conditions. Thermal photon v 3 with respect to the participant plane angle is found to be positive and significant compared to the elliptic flow parameter v 2 of thermal photons. In addition, photon v 3 as a function of p T shows similar qualitative nature to photon v 2 in the region 1 p T 6 GeV / c . We argue that while v 3 originates from ϵ 3 deformations of the initial state density distribution, fast buildup of radial flow due to fluctuations is the main driving mechanism for the observed large val…