120160020
Interference effects between the initial and final state radiation in a QCD medium
Coherence between multiple emitters is one of the most remarkable properties of QCD jet physics in vacuum. In the presence of a QCD medium one expects that the coherence pattern between multiple emitters is affected. In this work we calculate the gluon emission spectrum off an asymptotic quark traversing a dilute QCD medium. When interferences between initial and final quark are included, the medium induced gluon distribution gets modified. The coherent, incoherent and soft limit of the medium induced spectrum are studied. In the soft limit we find an elegant and intuitive probabilistic interpretation. We comment on possible phenomenological applications of this setup for studying coherence effects on observables studied in high energy nuclear collisions.
113397726
Interference between initial and final state radiation in a QCD medium
Abstract We investigate the color coherence pattern between initial and final state radiation in the presence of a QCD medium. We derive the medium-induced gluon spectrum of an “asymptotic” parton which suffers a hard scattering and subsequently crosses the medium. The angular distribution of the induced gluon spectrum is modified when one includes interference terms between the incoming and the outgoing parton at finite angle between them. The coherent, incoherent and soft limits of the medium-induced gluon spectrum are studied. In the soft limit, we provide a simple and intuitive probabilistic picture which could be of interest for Monte Carlo implementations. The configuration studied here may have phenomenological consequences in high-energy nuclear collisions.
56356571
Medium-induced soft gluon radiation in DIS
We study color coherence effects on the medium-induced soft gluon radiation off an asymptotic quark hit by a virtual photon traversing a hot and dense QCD medium. The transverse momentum spectrum of the emitted gluon is computed at 1st order in the opacity expansion. The interference effects between the initial and final state radiation modify the soft gluon spectrum when a finite angle between the incoming and outgoing quarks is considered, presenting a soft divergence. We comment on possible implications on observables in eA collisions.
14470209
Coherence effects and broadening in medium-induced QCD radiation off a massive $ q\bar{q} $ antenna
A bstractStudies of medium-induced QCD radiation usually rely on the calculation of single-gluon radiation spectrum off an energetic parton traversing an extended colored medium. Recently, the importance of interference effects between emitters in the medium has been explored. In this work we extend previous studies by calculating the single-gluon coherent spectrum off an antenna consisting of a massive quark-antiquark pair. Interferences dominate the spectrum of soft gluons, which are mainly emitted outside of the cone made by the antenna opening angle, while the antenna results in a superposition of independent emitters above a critical gluon energy scale. We study the interplay between the dead-cone effect and medium-induced jet broadening in both cases of soft and hard gluons and present results on energy loss distributions.
119472264
TESTS OF THE STANDARD MODEL USING MUON POLARIZATION ASYMMETRIES IN KAON DECAYS
We have examined the physics and the experimental feasibility of studying various kaon decay processes in which the polarization of a muon in the final state is measured. Valuable information on CP violation, the quark mixing (CKM) matrix, and new physics can be obtained from such measurements. We have considered muon polarization in KL→μ+μ- and K→πμ+μ- decays. Although the effects are small, or difficult to measure because of the small branching ratios involved, these studies could provide clean measurements of the CKM parameters. The experimental difficulty appears comparable to the observation of . New sources of physics, involving nonstandard CP violation, could produce effects observable in these measurements. Limits from new results on the neutron and electron electric dipole moment, and in neutral kaon decays, do not eliminate certain models that could contribute to the signal. A detailed examination of muon polarization out of the decay plane in K+→μ+π0ν and K+→μ+νγ decays also appears to be of interest. With current kaon beams and detector techniques, it is possible to measure the T-violating polarization for K+→μ+π0ν and with uncertainties approaching ~10-4. This level of sensitivity would provide an interesting probe of new physics.
118920095
New, high statistics measurement of the K+ ---> pi0 e+ nu (K+(e3)) branching ratio
E865 at the Brookhaven National Laboratory AGS collected about 70 000 K + e3 events to measure the K + e3 branching ratio relative to the observed K + → π + π 0 , K + → π 0 μ + ν, and K + → π + π 0 π 0 decays. The π 0 in all the decays was detected using the e + e - pair from π 0 → e + e - γ decay and no photons were required. Using the 2002 Particle Data Group branching ratios for the normalization decays, we obtain BR(K + e3(γ)) = (5.13 ′ 0.02 s t a t ′ 0.09 s y s t ′ 0.04 n o r m )%, where K + e3(γ) includes the effect of virtual and real photons. This result is 2.3σ higher than the current Particle Data Group value. Implications for the V u s element of the CKM matrix, and the matrix's unitarity are discussed.
14834850
Architecture of the ATLAS online physics-selection software at LHC
Given the extremely high bunch crossing rate foreseen at the Large Hadron Collider (LHC) and the general-purpose nature of the ATLAS particle physics experiment, after the hardware-based first level trigger, an efficient and flexible trigger software is needed for the online selection of events. This filtering of events is organized in two levels: the second level trigger and the event filter. Both levels are referred together as High Level Trigger (HLT). A coherent approach to event selection across the HLT has been taken. Thus a common core software framework has been designed to maximise the usage of offline interfaces and software components, whilst allowing sufficient flexibility to meet the different interfaces and requirements of the two different levels, notably those of performance and robustness. This paper describes the architecture and high level design of the selection software and shows how the implementation meets the challenges of the ATLAS environment.
118168382
First observation of the isospin violating decay $J/\psi\rightarrow \Lambda\bar{\Sigma}^{0}+c.c.$
Using a sample of $(225.2\pm 2.8)\times 10^6$ $J/\psi$ events collected with the BESIII detector, we present results of a study of $J/\psi\rightarrow \gamma\Lambda\bar{\Lambda}$ and report the first observation of the isospin violating decay $J/\psi\rightarrow\Lambda\bar{\Sigma}^{0}+c.c.$, in which $\bar{\Sigma}^{0}$ decays to $\gamma \bar{\Lambda}$. The measured branching fractions are $\mathcal{B}(J/\psi\rightarrow\bar{\Lambda}\Sigma^{0}$) = $(1.46\pm0.11\pm0.12) \times10^{-5}$ and $\mathcal{B}(J/\psi\rightarrow\Lambda\bar{\Sigma^{0}}$) = $(1.37\pm0.12\pm0.11) \times10^{-5}$. We search for $\Lambda(1520) \rightarrow \gamma \Lambda$ decay, and find no evident signal, and an upper limit for the product branching fraction $\mathcal{B}(J/\psi\rightarrow\Lambda\bar{\Lambda}(1520)+c.c.)\times \mathcal{B}(\Lambda(1520)\rightarrow\gamma\Lambda)<4.1 \times10^{-6}$ is set at the 90% confidence level. We also report the observation of $\eta_{c}\rightarrow\Lambda\bar{\Lambda}$ in $J/\psi \rightarrow \gamma \eta_{c}$, $\eta_{c}\rightarrow\Lambda\bar{\Lambda}$ and measure the branching fraction $\mathcal{B}(\eta_{c}\rightarrow\Lambda\bar{\Lambda}) =(1.16\pm0.12 (stat)\pm0.19(syst)\pm0.28(PDG))\times10^{-3}$.
117048162
Muon Polarization Working Group Report
We have examined the physics and the experimental feasibility at the AGS of various kaon decay processes in which the polarization of a muon in the final state is measured. Valuable information on CP violation, the CKM matrix or new physics can be obtained with these measurements and therefore they are well motivated. In particular, models of non-standard CP violation that produce the baryon asymmetry of the universe could also produce effects observable in these measurements. Limits from measurements such as the neutron and electron electric dipole moment, and epsilon-prime over epsilon in neutral kaon decays do not eliminate all of these models. We have made a more detailed examination of the measurement of the out of the plane muon polarization in Kmu3 decays. With our current knowledge of the AGS kaon beams and detector techniques it is possible to measure this polarization with an error approaching 10**-4. Such an experiment would be well justified since the sensitivity is well beyond the current direct experimental limit (5.3 times 10**-3) and the projected sensitivity (10**-3) of the currently running experiment at KEK in Japan.