Search results for "quantum chromodynamics"
showing 10 items of 1027 documents
Influence of a temperature-dependent shear viscosity on the azimuthal asymmetries of transverse momentum spectra in ultrarelativistic heavy-ion colli…
2012
We study the influence of a temperature-dependent shear viscosity over entropy density ratio $\eta/s$, different shear relaxation times $\tau_\pi$, as well as different initial conditions on the transverse momentum spectra of charged hadrons and identified particles. We investigate the azimuthal flow asymmetries as a function of both collision energy and centrality. The elliptic flow coefficient turns out to be dominated by the hadronic viscosity at RHIC energies. Only at higher collision energies the impact of the viscosity in the QGP phase is visible in the flow asymmetries. Nevertheless, the shear viscosity near the QCD transition region has the largest impact on the collective flow of t…
Transverse spectra of hadrons at RHIC
2002
We present results on spectra of pions, kaons and (anti)protons from a study of heavy ion collisions using the perturbative QCD + saturation model to calculate the production of initial (transverse) energy and baryon number followed by a hydrodynamic description of the expansion of produced matter. In particular, we study how the hadron spectra and multiplicities depend on the decoupling temperature $\Tdec$ when the low temperature phase contains all hadrons and and hadron resonances with mass below 2 GeV. We show that the spectra and multiplicities of pions, kaons and (anti)protons measured at RHIC in central Au+Au collisions with $\sqrt s=130$ GeV can be obtained with a single decoupling …
Factorization and N3LLp+NNLO predictions for the Higgs cross section with a jet veto
2013
We have recently derived a factorization formula for the Higgs-boson production cross section in the presence of a jet veto, which allows for a systematic resummation of large Sudakov logarithms of the form alpha_s^n ln^m(p_T^veto/m_H), along with the large virtual corrections known to affect also the total cross section. Here we determine the ingredients entering this formula at two-loop accuracy. Specifically, we compute the dependence on the jet-radius parameter R, which is encoded in the two-loop coefficient of the collinear anomaly, by means of a direct, fully analytic calculation in the framework of soft-collinear effective theory. We confirm the result obtained by Banfi et al. from a…
Measurement of transverse single-spin asymmetries forJ/ψproduction in polarizedp+pcollisions ats=200 GeV
2010
We report the first measurement of transverse single-spin asymmetries in J/psi production from transversely polarized p + p collisions at root s = 200 GeV with data taken by the PHENIX experiment in 2006 and 2008. The measurement was performed over the rapidity ranges 1.2 < vertical bar y vertical bar < 2.2 and vertical bar y vertical bar < 0.35 for transverse momenta up to 6 GeV/c. J/psi production at the Relativistic Heavy Ion Collider is dominated by processes involving initial-state gluons, and transverse single-spin asymmetries of the J/psi can provide access to gluon dynamics within the nucleon. Such asymmetries may also shed light on the long-standing question in QCD of the J/psi pro…
Measurement of the hadronic form factor inD0→K−e+νedecays
2007
We present a preliminary measurement of the q2 dependence of the D0 --> K- e+ nu_e decay rate. This rate is proportional to the hadronic form factor squared, specified by a single parameter. This is either the mass in the simple pole ansatz m_pole = (1.854 +- 0.016 +- 0.020) GeV/c2 or the scale in the modified pole ansatz alpha_pole = 0.43 +- 0.03 +- 0.04. The first error refers to the statistical, the second to the systematic uncertainty.
Search forB0→ϕ(K+π−)decays with largeK+π−invariant mass
2007
Motivated by the polarization anomaly in the B→ (1020)K*(892) decay, we extend our search for other K* final states in the decay B0→ (1020)K*0 with the K*0→K+π- invariant mass above 1.6 GeV. The final states considered include the K*(1680)0, K3*(1780)0, K4*(2045)0, and a Kπ spin-zero nonresonant component. We also search for B0→ D0 decay with the same final state. The analysis is based on a sample of about 384×106 BB pairs recorded with the BABAR detector. We place upper limits on the branching fractions B(B0→ K*(1680)0)<3.5×10-6, B(B0→ K3*(1780)0) <2.7×10-6, B(B0→ K4*(2045)0)<15.3×10-6, and B(B0→ D0)<11.7×10-6 at 90% C.L. The nonresonant contribution is consistent with the measurements in …
Isospin breaking effects in the X(3872) resonance
2009
In this paper we study the effects of isospin breaking in the dynamical generation of the X(3872) state. We also calculate the ratio of the branching fractions of the $X$ decaying into $J/\psi$ with two and three pions, which has been measured experimentally to be close to unity. Together with the X(3872), of positive C-parity, we predict the existence of a negative C-parity state and we comment on which decay channel is more promising to observe this state.
Branching fraction measurement ofB+→ωℓ+νdecays
2013
We present a measurement of the B^+ → ωl^+ν branching fraction based on a sample of 467 million BB pairs recorded by the BABAR detector at the SLAC PEP-II e+e- collider. We observe 1125±131 signal decays, corresponding to a branching fraction of B(B^+ → ωl^+ν)=(1.21±0.14±0.08)×10^(-4), where the first error is statistical and the second is systematic. The dependence of the decay rate on q^2, the invariant mass squared of the leptons, is compared to QCD predictions of the form factors based on a quark model and light-cone sum rules.
Measurement of the relative branching fractions ofB¯→D/D*/D**l−ν¯ldecays in events with a fully reconstructedBmeson
2007
We determine the relative branching fractions of semileptonic B decays to charmed final states. The measurement is performed on the recoil from a fully reconstructed B meson in a sample of 362×106 BB pairs collected at the Υ(4S) resonance with the BABAR detector. A simultaneous fit to a set of discriminating variables is performed on a sample of B→DX-ν decays to determine the contributions from the different channels. We measure Γ(B-→D-ν)/Γ(B-→DX-ν)=0.227±0. 014±0.016, Γ(B-→D*-ν)/Γ(B-→DX- ν)=0.582±0.018±0.030, and Γ(B-→D **-ν)/Γ(B-→DX-ν)=0.191±0. 013±0.019 for the charged B sample, and Γ(B0→D- ν)/Γ(B0→DX-ν)=0.215±0.016±0. 013, Γ(B0→D*-ν)/Γ(B0→DX- ν)=0.537±0.031±0.036, and Γ(B0→ D**-ν)/Γ(B0→…
τ→πππντ decays and the a1(1260) off-shell width revisited
2010
Abstract The τ → π π π ν τ decay is driven by the hadronization of the axial-vector current. Within the resonance chiral theory, and considering the large- N C expansion, this process has been studied in Ref. [1] (D. Gomez Dumm, A. Pich, J. Portoles, 2004). In the light of later developments we revise here this previous work by including a new off-shell width for the lightest a1 resonance that provides a good description of the τ → π π π ν τ spectrum and branching ratio. We also consider the role of the ρ ( 1450 ) resonance in these observables. Thus we bring in an overall description of the τ → π π π ν τ process in excellent agreement with our present experimental knowledge.