Nuclear Theory·nucl-th

Tuesday·January 17, 2017

11 papers—4 primary·7 cross-listed

01
Revealing long-range multi-particle collectivity in small collision systems via subevent cumulants
Jiangyong Jia🇺🇸 · Mingliang Zhou🇺🇸 · Adam Trzupek🇵🇱
Multi-particle azimuthal cumulants, often used to study collective flow in high-energy heavy-ion collisions, have recently been applied in small collision systems such as $pp$ and $p$ +A to extract the second-order azimuthal harmonic flow $v_{2}$ . Recent observation of four-, six- and eight-particle cumulants with "correct sign" $c_{2} {4} < 0, c_{2} {6} > 0, c_{2} {8} < 0$ and approximate equality of the inferred single-particle harmonic flow, $v_{2} {4} \approx v_{2} {6} \approx v_{2} {8}$ , have been used as strong evidence for a collective emission of all soft particles produced in the collisions. We show that these relations in principle could be violated due to the non-Gaussianity in the event-by-event fluctuation of flow and/or non-flow. Furthermore, we show, using $pp$ events generated with the PYTHIA model, that $c_{2} {2 k}$ obtained with standard cumulant method are dominated by non-flow from dijets. An alternative cumulant method based on two or more $η$ -separated subevents is proposed to suppress the dijet contribution. The new method is shown to be able to recover a flow signal as low as 4\% imposed on the PYTHIA events, independently of how the event activity class is defined. Therefore the subevent cumulant method offers a more robust way of studying collectivity based on the existence of long-range azimuthal correlations between multiple distinct $η$ ranges. The prospect of using the subevent cumulants to study collective flow in A+A collisions, in particular its longitudinal dynamics, is discussed.
nucl-thnucl-exPRC(2017)·174 citations
02
$Λ_{c}$ Semileptonic Decays in a Quark Model
Md Mozammel Hussain🇺🇸 · Winston Roberts🇺🇸
Hadronic form factors for semileptonic decay of the $Λ_{c}$ are calculated in a nonrelativistic quark model. The full quark model wave functions are employed to numerically calculate the form factors to all relevant orders in ( $1/ m_{c}$ , $1/ m_{s}$ ). The form factors obtained satisfy relationships expected from the heavy quark effective theory (HQET). The differential decay rates and branching fractions are calculated for transitions to the ground state and a number of excited states of $Λ$ . The branching fraction of the semileptonic decay width to the total width of $Λ_{c}$ has been calculated and compared with other theoretical estimates and experimental results. The branching fractions for $Λ_{c} \to Λ^{*} l^{+} ν_{l} \to Σ π l^{+} ν_{l}$ and $Λ_{c} \to Λ^{*} l^{+} ν_{l} \to N K ˉ l^{+} ν_{l}$ are also calculated. Apart from decays to the ground state $Λ$ (1115), it is found that decays through the $Λ (1405)$ provide a significant portion of the branching fraction $Λ_{c} \to X_{s} l ν_{l}$ . A new estimate for $f = B (Λ_{c}^{+} \to Λ l^{+} ν_{l}) / B (Λ_{c}^{+} \to X_{s} l^{+} ν_{l})$ is obtained.
nucl-thhep-phPRD(2017)·18 citations
03
Nonmesonic Weak Decay of $Λ$ Hypernuclei: The Three--Nucleon Induced Mode
E. Bauer🇦🇷 · G. Garbarino🇮🇹 · C. A. Rodríguez Peña🇦🇷
The nonmesonic weak decay of $Λ$ hypernuclei is studied within a microscopic diagrammatic approach which is extended to include the three--nucleon induced mechanism. We adopt a nuclear matter formalism which, through the local density approximation, allows us to model finite hypernuclei, a one--meson--exchange weak transition potential and a Bonn nucleon--nucleon strong potential. One--, two-- and three--nucleon induced weak decay rates are predicted for $_{Λ}^{12}$ C by including ground state correlations up to second order in the nucleon--nucleon potential and the recoil of the residual nucleus. Three--nucleon stimulated decays, $Λ N N N \to n N N N$ ( $N = n$ or $p$ ), are considered here for the first time. The obtained decay rates compare well with the latest KEK and FINUDA data. The three--nucleon induced rate turns out to be dominated by $nn p$ -- and $n pp$ --induced decays, it amounts to $\sim$ 7\% of the total nonmesonic rate and it is $\sim 1/2$ of the neutron--induced decay rate. The reduction effect of the nuclear recoil is particularly relevant for the three--nucleon induced rates ( $\sim$ 15\%), less important for the two--nucleon induced rates ( $\sim$ 4\%) and negligible for the one--nucleon induced rates. Given the non--negligible size of the three--nucleon induced contribution and consequently its importance in the precise determination of the complete set of decay rates, new measurements and/or experimental analysis are encouraged.
nucl-thPLB(2017)·4 citations
04
Production of $ρ^{0}$ meson with large $p_{T}$ at NLO in heavy-ion collisions
Wei Dai🇨🇳 · Ben-Wei Zhang🇨🇳 · Enke Wang🇨🇳
Production of large transverse momentum $ρ^{0}$ meson in high-energy nuclear collisions is investigated for the first time at the next-leading-order in the QCD improved parton model. The $ρ^{0}$ fragmentation functions (FFs) in vacuum at any scale $Q$ are obtained, by evolving a newly developed initial parametrization of $ρ^{0}$ FFs at a scale $Q_{0}^{2} = 1.5 Ge V^{2}$ from a broken SU(3) model through NLO DGLAP equations. The numerical simulations of $p_{T}$ spectra of $ρ^{0}$ meson in the elementary $p + p$ collisions at NLO give a decent description of STAR $p + p$ data. In $A + A$ reactions the jet quenching effect is taken into account with the higher-twist approach by the medium-modified parton FFs due to gluon radiation in the quark-gluon plasma, whose space-time evolution is described by a (3+1D) hydrodynamical model. The nuclear modification factors for $ρ^{0}$ meson and its double ratio with $π^{\pm}$ nuclear modification in central $Au + Au$ collisions at the RHIC are calculated and found to be in good agreement with STAR measurement. Predictions of $ρ^{0}$ nuclear modification and the yield ratio $ρ^{0} / π^{0}$ in central Pb+Pb at the LHC are also presented. It is shown that the ratio $ρ^{0} / π^{0}$ in central Pb+Pb will approach to that in p+p reactions when $p_{T} > 12$ ~GeV.
nucl-thnucl-exPRC(2018)·10 citations

Affiliations

first authorsco-authorsvia INSPIRE

Revealing long-range multi-particle collectivity in small collision systems via subevent cumulants

Λc​ Semileptonic Decays in a Quark Model

Nonmesonic Weak Decay of Λ Hypernuclei: The Three--Nucleon Induced Mode

Production of ρ0 meson with large pT​ at NLO in heavy-ion collisions

Affiliations

$Λ_{c}$ Semileptonic Decays in a Quark Model

Nonmesonic Weak Decay of $Λ$ Hypernuclei: The Three--Nucleon Induced Mode

Production of $ρ^{0}$ meson with large $p_{T}$ at NLO in heavy-ion collisions