Search for: All records

Abstract We evaluate the$$a_1(1260) \rightarrow \pi \sigma (f_0(500))$$ $a_1\left(1260\right)\to \pi \sigma \left(f_0\left(500\right)\right)$decay width from the perspective that the$$a_1(1260)$$ $a_1\left(1260\right)$resonance is dynamically generated from the pseudoscalar–vector interaction and the$$\sigma $$ $\sigma$arises from the pseudoscalar–pseudoscalar interaction. A triangle mechanism with$$a_1(1260) \rightarrow \rho \pi $$ $a_1\left(1260\right)\to \rho \pi$followed by$$\rho \rightarrow \pi \pi $$ $\rho \to \pi \pi$and a fusion of two pions within the loop to produce the$$\sigma $$ $\sigma$provides the mechanism for this decay under these assumptions for the nature of the two resonances. We obtain widths of the order of 13–22 MeV. Present experimental results differ substantially from each other, suggesting that extra efforts should be devoted to the precise extraction of this important partial decay width, which should provide valuable information on the nature of the axial vector and scalar meson resonances and help clarify the role of the$$\pi \sigma $$ $\pi \sigma$channel in recent lattice QCD calculations of the$$a_1$$ $a_1$. 

New results are presented on a high-statistics measurement of Collins and Sivers asymmetries of charged hadrons produced in deep inelastic scattering of muons on a transversely polarized ${}^6\mathrm{LiD}$target. The data were taken in 2022 with the COMPASS spectrometer using the 160 GeV muon beam at CERN, statistically balancing the existing data on transversely polarized proton targets. The first results from about two-thirds of the new data have total uncertainties smaller by up to a factor of three compared to the previous deuteron measurements. Using all the COMPASS proton and deuteron results, both the transversity and the Sivers distribution functions of the $u$and $d$quark, as well as the tensor charge in the measured $x$range are extracted. In particular, the accuracy of the $d$quark results is significantly improved. Published by the American Physical Society2024 

The COMPASS Collaboration performed measurements of the Drell-Yan process in 2015 and 2018 using a $190\mathrm{GeV}/c$ ${\pi }^{-}$beam impinging on a transversely polarized ammonia target. Combining the data of both years, we present final results on the amplitudes of five azimuthal modulations, which correspond to transverse-spin-dependent azimuthal asymmetries (TSAs) in the dimuon production cross section. Three of them probe the nucleon leading-twist Sivers, transversity, and pretzelosity transverse-momentum dependent (TMD) parton distribution functions (PDFs). The other two are induced by subleading effects. These TSAs provide unique new inputs for the study of the nucleon TMD PDFs and their universality properties. In particular, the Sivers TSA observed in this measurement is consistent with the fundamental QCD prediction of a sign change of naive time-reversal-odd TMD PDFs when comparing the Drell-Yan process with deep inelastic scattering. Also, within the context of model predictions, the observed transversity TSA is consistent with the expectation of a sign change for the Boer-Mulders function. Published by the American Physical Society2024 

Abstract We report on a measurement of Spin Density Matrix Elements (SDMEs) in hard exclusive$$\rho ^0$$ ${\rho }^0$meson muoproduction at COMPASS using 160 GeV/cpolarised$$ \mu ^{+}$$ ${\mu }^{+}$and$$ \mu ^{-}$$ ${\mu }^{-}$beams impinging on a liquid hydrogen target. The measurement covers the kinematic range 5.0 GeV/$$c^2$$ $c^2$$$< W<$$ $<W<$17.0 GeV/$$c^2$$ $c^2$, 1.0 (GeV/c)$$^2$$ ${}^2$$$< Q^2<$$ $<Q^2<$10.0 (GeV/c)$$^2$$ ${}^2$and 0.01 (GeV/c)$$^2$$ ${}^2$$$< p_{\textrm{T}}^2<$$ $<p_{\text{T}}^2<$0.5 (GeV/c)$$^2$$ ${}^2$. Here,Wdenotes the mass of the final hadronic system,$$Q^2$$ $Q^2$the virtuality of the exchanged photon, and$$p_{\textrm{T}}$$ $p_{\text{T}}$the transverse momentum of the$$\rho ^0$$ ${\rho }^0$meson with respect to the virtual-photon direction. The measured non-zero SDMEs for the transitions of transversely polarised virtual photons to longitudinally polarised vector mesons ($$\gamma ^*_T \rightarrow V^{ }_L$$ ${\gamma }_T^{\ast }\to V_L^{}$) indicate a violation ofs-channel helicity conservation. Additionally, we observe a dominant contribution of natural-parity-exchange transitions and a very small contribution of unnatural-parity-exchange transitions, which is compatible with zero within experimental uncertainties. The results provide important input for modelling Generalised Parton Distributions (GPDs). In particular, they may allow one to evaluate in a model-dependent way the role of parton helicity-flip GPDs in exclusive$$\rho ^0$$ ${\rho }^0$production. 

Abstract The first measurement of the multiplicity dependence of intra-jet properties of leading charged-particle jets in proton–proton (pp) collisions is reported. The mean charged-particle multiplicity and jet fragmentation distributions are measured in minimum-bias and high-multiplicity pp collisions at center-of-mass energy$$\sqrt{s}$$ $\sqrt{s}$= 13 TeV using the ALICE detector. Jets are reconstructed from charged particles produced in the midrapidity region ($$|\eta | < 0.9$$ $\left|\eta \right|<0.9$) using the sequential recombination anti-$$k_{\textrm{T}}$$ $k_{\text{T}}$algorithm with jet resolution parametersR= 0.2, 0.3, and 0.4 for the transverse momentum ($$p_\textrm{T}$$ $p_{\text{T}}$) interval 5–110 GeV/c. The high-multiplicity events are selected by the forward V0 scintillator detectors. The mean charged-particle multiplicity inside the leading jet cone rises monotonically with increasing jet$$p_\textrm{T}$$ $p_{\text{T}}$in qualitative agreement with previous measurements at lower energies. The distributions of jet fragmentation function variables$$z^{\textrm{ch}}$$ $z^{\text{ch}}$and$$\xi ^{\textrm{ch}}$$ ${\xi }^{\text{ch}}$are measured for different jet-$$p_\textrm{T}$$ $p_{\text{T}}$intervals. Jet-$$p_\textrm{T}$$ $p_{\text{T}}$independent fragmentation of leading jets is observed for wider jets except at high- and low-$$z^{\textrm{ch}}$$ $z^{\text{ch}}$values. The observed “hump-backed plateau” structure in the$$\xi ^{\textrm{ch}}$$ ${\xi }^{\text{ch}}$distribution indicates suppression of low-$$p_\textrm{T}$$ $p_{\text{T}}$particles. In high-multiplicity events, an enhancement of the fragmentation probability of low-$$z^{\textrm{ch}}$$ $z^{\text{ch}}$particles accompanied by a suppression of high-$$z^{\textrm{ch}}$$ $z^{\text{ch}}$particles is observed compared to minimum-bias events. This behavior becomes more prominent for low-$$p_\textrm{T}$$ $p_{\text{T}}$jets with larger jet radius. The results are compared with predictions of QCD-inspired event generators, PYTHIA 8 with Monash 2013 tune and EPOS LHC. It is found that PYTHIA 8 qualitatively reproduces the jet modification in high-multiplicity events except at high jet$$p_\textrm{T}$$ $p_{\text{T}}$. These measurements provide important constraints to models of jet fragmentation.