arXiv subjects

A. Ramos

Publications and source records attributed to A. Ramos.

At least 37 records · Page 2Linked to original sources

arXiv2019

Light and strange quark masses from $N_f=2+1$ simulations with Wilson fermions

We present a nearly final analysis of the $u/d$ and $s$ quark masses, extracted using the PCAC quark masses reported in [PRD 95 (2017) 074504]. The data is based on the CLS $N_f = 2 + 1$ simulations with Wilson/Clover quarks and Lüscher-Weisz gauge action, at four $β$ values (i.e. lattice spacings) and a range of quark masses. We use the ALPHA results of [EPJC 78 (2018) 387] for non-perturbative quark mass renormalisation and RG-running from hadronic to electroweak scales in the Schrödinger Functional scheme. Quark masses are quoted both in the $\overline{\rm MS}$ scheme and as RGI quantities.

hep-lat↗

arXiv2019

Charmed nuclei within a microscopic many-body approach

Single-particle energies of the $Λ_c$ chamed baryon are obtained in several nuclei from the relevant self-energy constructed within the framework of a perturbative many-body approach. Results are presented for a charmed baryon-nucleon ($Y_cN$) potential based on a SU(4) extension of the meson-exchange hyperon-nucleon potential $\tilde A$ of the Jülich group. Three different models (A, B and C) of this interaction, that differ only on the values of the couplings of the scalar $σ$ meson with the charmed baryons, are considered. Phase shifts, scattering lengths and effective ranges are computed for the three models and compared with those predicted by the $Y_cN$ interaction derived in Eur. Phys. A {\bf 54}, 199 (2018) from the extrapolation to the physical pion mass of recent results of the HAL QCD Collaboration. Qualitative agreement is found for two of the models (B and C) considered. Our results for $Λ_c$-nuclei are compatible with those obtained by other authors based on different models and methods. We find a small spin-orbit splitting of the $p-, d-$ and $f-$wave states as in the case of single $Λ$-hypernuclei. The level spacing of $Λ_c$ single-particle energies is found to be smaller than that of the corresponding one for hypernuclei. The role of the Coulomb potential and the effect of the coupling of the $Λ_cN$ and $Σ_cN$ channels on the single-particle properties of $Λ_c-$nuclei are also analyzed. Our results show that, despite the Coulomb repulsion between the $Λ_c$ and the protons, even the less attractive one of our $Y_cN$ models (model C) is able to bind the $Λ_c$ in all the nuclei considered. The effect of the $Λ_cN-Σ_cN$ coupling is found to be almost negligible due to the large mass difference of the $Λ_c$ and $Σ_c$ baryons.

nucl-th↗

arXiv2018

Effective Field Theories in a Finite Volume

In this talk I present the formalism we have used to analyze Lattice data on two meson systems by means of effective field theories. In particular I present the results obtained from a reanalysis of the lattice data on the $KD^{(*)}$ systems, where the states $D^*_{s0}(2317)$ and $D^*_{s1}(2460)$ are found as bound states of $KD$ and $KD^*$, respectively. We confirm the presence of such states in the lattice data and determine the contribution of the $KD$ channel in the wave function of $D^*_{s0}(2317)$ and that of $KD^*$ in the wave function of $D^*_{s1}(2460)$. Our findings indicate a large meson-meson component in the two cases.

hep-lat↗

arXiv2018

Triangle singularities in $B^-\rightarrow K^-π^-D_{s0}^+$ and $B^-\rightarrow K^-π^-D_{s1}^+$

We study the appearance of structures in the decay of the $B^-$ into $K^- π^- D_{s0}^+(2317)$ and $K^- π^- D_{s1}^+(2460)$ final states by forming invariant mass distributions of $π^- D_{s0}^+$ and $π^- D_{s1}^+$ pairs, respectively. The structure in the distribution is associated to the kinematical triangle singularity that appears when the $B^- \to K^- K^{*\,0} D^0$ ($B^- \to K^- K^{*\,0} D^{*\,0}$) decay process is followed by the decay of the $K^{*\,0}$ into $π^- K^+$ and the subsequent rescattering of the $K^+ D^0$ ($K^+ D^{*\,0}$) pair forming the $D_{s0}^+(2317)$ ($D_{s1}^+(2460)$) resonance. We find this type of non-resonant peaks at 2850 MeV in the invariant mass of $π^- D_{s0}$ pairs from $B^- \to K^- π^- D_{s0}^+(2317)$ decays and around 3000 MeV in the invariant mass of $π^- D_{s1}^+$ pairs from $B^- \to K^- π^- D_{s1}^+(2460)$ decays. By employing the measured branching ratios of the $B^- \to K^- K^{*\,0} D^0$ and $B^- \to K^- K^{*\,0} D^{*\,0}$ decays, we predict the branching ratios for the processes $B^-$ into $K^- π^-D_{s0}^+(2317)$ and $K^- π^- D_{s1}^+(2460)$, in the vicinity of the triangle singularity peak, to be about $8\times10^{-6}$ and $1\times 10^{-6}$, respectively. The observation of this reaction would also give extra support to the molecular picture of the $D_{s0}^+(2317)$ and $D_{s1}^+(2460)$.

hep-ph↗

arXiv2017

An analysis of the Lattice QCD spectra for $D^_{s0}(2317)$ and $D^_{s1}(2460)$

In this talk I present the results obtained using effective field theories in a finite volume from a reanalysis of lattice data on the $KD^{(*)}$ systems, where bound states of $KD$ and $KD^*$ are found and associated with the states $D^*_{s0}(2317)$ and $D^*_{s1}(2460)$, respectively. We confirm the presence of such states on the lattice data and determine the weight of the $KD$ channel in the wave function of $D^*_{s0}(2317)$ and that of $KD^*$ in the wave function of $D^*_{s1}(2460)$. Our results indicate a large meson-meson component in both cases.

hep-lat↗

arXiv2017

The strong coupling from a nonperturbative determination of the $Λ$ parameter in three-flavor QCD

We present a lattice determination of the $Λ$ parameter in three-flavor QCD and the strong coupling at the Z pole mass. Computing the nonperturbative running of the coupling in the range from $0.2\,$GeV to $70\,$GeV, and using experimental input values for the masses and decay constants of the pion and the kaon, we obtain $Λ_{\overline{\rm MS}}^{(3)}=341(12)\,$MeV. The nonperturbative running up to very high energies guarantees that systematic effects associated with perturbation theory are well under control. Using the four-loop prediction for $Λ_{\overline{\rm MS}}^{(5)}/Λ_{\overline{\rm MS}}^{(3)}$ yields $α^{(5)}_{\overline{\rm MS}}(m_{\rm Z}) = 0.11852(84)$.

hep-lat↗

arXiv2017

On a conjecture in second-order optimality conditions

In this paper we deal with optimality conditions that can be verified by a nonlinear optimization algorithm, where only a single Lagrange multiplier is avaliable. In particular, we deal with a conjecture formulated in [R. Andreani, J.M. Martinez, M.L. Schuverdt, "On second-order optimality conditions for nonlinear programming", Optimization, 56:529--542, 2007], which states that whenever a local minimizer of a nonlinear optimization problem fulfills the Mangasarian-Fromovitz Constraint Qualification and the rank of the set of gradients of active constraints increases at most by one in a neighborhood of the minimizer, a second-order optimality condition that depends on one single Lagrange multiplier is satisfied. This conjecture generalizes previous results under a constant rank assumption or under a rank deficiency of at most one. In this paper we prove the conjecture under the additional assumption that the Jacobian matrix has a smooth singular value decomposition, which is weaker than previously considered assumptions. We also review previous literature related to the conjecture.

math.OC↗

arXiv2017

The $Λ$-parameter in 3-flavour QCD and $α_s(m_Z)$ by the ALPHA collaboration

We present results by the ALPHA collaboration for the $Λ$-parameter in 3-flavour QCD and the strong coupling constant at the electroweak scale, $α_s(m_Z)$, in terms of hadronic quantities computed on the CLS gauge configurations. The first part of this proceedings contribution contains a review of published material \cite{Brida:2016flw,DallaBrida:2016kgh} and yields the $Λ$-parameter in units of a low energy scale, $1/L_{\rm had}$. We then discuss how to determine this scale in physical units from experimental data for the pion and kaon decay constants. We obtain $Λ_{\overline{\rm MS}}^{(3)} = 332(14)$ MeV which translates to $α_s(M_Z)=0.1179(10)(2)$ using perturbation theory to match between 3-, 4- and 5-flavour QCD.

hep-lat↗

arXiv2016

The Aerogel Cherenkov Detector for the SHMS magnetic spectrometer in Hall C at Jefferson Lab

Hadronic reactions producing strange quarks such as exclusive or semi-inclusive kaon production, play an important role in studies of hadron structure and the dynamics that bind the most basic elements of nuclear physics. The small-angle capability of the new Super High Momentum Spectrometer (SHMS) in Hall C, coupled with its high momentum reach - up to the anticipated 11-GeV beam energy in Hall C - and coincidence capability with the well-understood High Momentum Spectrometer, will allow for probes of such hadron structure involving strangeness down to the smallest distance scales to date. To cleanly select the kaons, a threshold aerogel Cerenkov detector has been constructed for the SHMS. The detector consists of an aerogel tray followed by a diffusion box. Four trays for aerogel of nominal refractive indices of n=1.030, 1.020, 1.015 and 1.011 were constructed. The tray combination will allow for identification of kaons from 1 GeV/c up to 7.2 GeV/c, reaching 10^-2 proton and 10^-3 pion rejection, with kaon detection efficiency better than 95%. The diffusion box of the detector is equipped with 14 five-inch diameter photomultiplier tubes. Its interior walls are covered with Gore diffusive reflector, which is superior to the commonly used Millipore paper and improved the detector performance by 35%. The inner surface of the two aerogel trays with higher refractive index is covered with Millipore paper, however, those two trays with lower aerogel refractive index are again covered with Gore diffusive reflector for higher performance. The measured mean number of photoelectrons in saturation is ~12 for n=1.030, ~sim8 for n=1.020, ~10 for n=1.015, and ~5.5 for n=1.011. The design details, the results of component characterization, and initial performance tests and optimization of the detector are presented.

physics.ins-det↗

arXiv2016

The chiral S=-1 meson-baryon interaction with new constrains on the NLO contributions

We present a study of the $S=-1$ meson-baryon interaction, employing a chiral SU(3) Lagrangian up to next-to-leading order (NLO) and implementing unitarization in coupled channels. The parameters of the model have been fitted to a large set of experimental scattering data in different two-body channels, to threshold branching ratios, and to the precise SIDDHARTA value of the energy shift and width of kaonic hidrogen. In contrast to other groups, we have taken into consideration the $K^- p\to K^+Ξ^-, K^0Ξ^0$ reaction data, since we found in a previous work to be especially sensitive to the NLO parameters of the chiral Lagrangian. In the present work we also include the Born terms, which usually have very little effect, and find them to be non-negligible in the $K^- p\to KΞ$ channels, correspondingly causing significant modifications to the NLO parameters. We furthermore show that the importance of the Born terms becomes more visible in the isospin projected amplitudes of the $K^-p \to KΞ$ reactions. The measurement of processes that filter single isospin components, like the $K^0_L p \to K^+ Ξ^0$ reaction that could be measured at the proposed secondary $K^0_L$ beam at Jlab, would put valuable constraints on the chiral models describing the meson-baryon interaction in the $S=-1$ sector.

nucl-th↗

arXiv2016

A hidden-charm $S=-1$ pentaquark from the decay of $Λ_b$ into $J/ψηΛ$

The hidden charm pentaquark state $P_c(4450)$ observed recently by the LHCb collaboration may be of molecular nature, as advocated by some unitary approaches that also predict pentaquark partners in the strangeness $S=-1$ sector. In this work we argue that a hidden-charm strange pentaquark could be seen from the decay of the $Λ_b$, just as in the case of the non-strange $P_c(4450)$, but looking into the $J/ψηΛ$ decay mode and forming the invariant mass spectrum of $J/ψΛ$ pairs. In the model presented here, which assumes a standard weak decay topology and incorporates the hadronization process and final state interaction effects, we find the $J/ψηΛ$ final states to be populated with similar strength as the $J/ψK^- p$ states employed for the observation of the non-strange pentaquark. This makes the $Λ_ b \to J/ψηΛ$ decay to be an interesting process to observe a possible strange partner of the $P_c(4450)$ . We study the dependence of the $J/ψΛ$ mass spectra on various model ingredients and on the unknown properties of the strange pentaquark.

hep-ph↗

arXiv2016

Workshop on Physics with Neutral Kaon Beam at JLab (KL2016) Mini-Proceedings

The KL2016 Workshop is following the Letter of Intent LoI12-15-001 "Physics Opportunities with Secondary KL beam at JLab" submitted to PAC43 with the main focus on the physics of excited hyperons produced by the Kaon beam on unpolarized and polarized targets with GlueX setup in Hall D. Such studies will broaden a physics program of hadron spectroscopy extending it to the strange sector. The Workshop was organized to get a feedback from the community to strengthen physics motivation of the LoI and prepare a full proposal. Further details about the Workshop can be found on the web page of the conference: http://www.jlab.org/conferences/kl2016/index.html .

hep-ph↗

arXiv2016

Sigma term and strangeness content of octet baryons

By using lattice QCD computations we determine the sigma terms and strangeness content of all octet baryons by means of an application of the Hellmann-Feynman theorem. In addition to polynomial and rational expressions for the quark mass dependence of octet members, we use SU(3) covariant baryon chiral perturbation theory to perform the extrapolation to the physical up and down quark masses. Our N_f=2+1 lattice ensembles include pion masses down to about 190 MeV in large volumes (M_πL > 4), and three values of the lattice spacing. Our main results are the nucleon sigma term σ_{πN} = 39(4)(^{+18}_{-7}) and the strangeness content y_{N} = 0.20(7)(^{+13}_{-17}). Under the assumption of validity of covariant baryon χPT in our range of masses one finds y_{N} = 0.276(77)(^{+90}_{-62}).

hep-lat↗

arXiv2015

Prospects and status of quark mass renormalization in three-flavour QCD

We present the current status of a revised strategy to compute the running of renormalized quark masses in QCD with three flavours of massless O(a) improved Wilson quarks. The strategy employed uses the standard finite-size scaling method in the Schrödinger functional and accommodates for the non-perturbative scheme-switch which becomes necessary at intermediate renormalized couplings as discussed in [arXiv:1411.7648].

hep-lat↗

arXiv2015

Symanzik improvement of the gradient flow in lattice gauge theories

We apply the Symanzik improvement programme to the 4+1-dimensional local re-formulation of the gradient flow in pure $SU(N)$ lattice gauge theories. We show that the classical nature of the flow equation allows to eliminate all cutoff effects at $\mathcal O(a^2)$ which originate either from the discretized gradient flow equation or from the gradient flow observable. All the remaining $\mathcal O(a^2)$ effects can be understood in terms of local counterterms at the zero flow time boundary. We classify these counterterms and provide a complete set as required for on-shell improvement. Compared to the 4-dimensional pure gauge theory only a single additional counterterm is required, which corresponds to a modified initial condition for the flow equation. A consistency test in perturbation theory is passed and allows to determine all counterterm coefficients to lowest non-trivial order in the coupling.

hep-lat↗

arXiv2015

The $Λ_b \to J/ψ~ K ~ Ξ$ decay and the higher order chiral terms of the meson baryon interaction

We study the weak decay of the $Λ_b$ into $ J/ψ~ K ~ Ξ$ and $J/ψ~ η~ Λ$ states, and relate these processes to the $Λ_b \to J/ψ~ \bar K ~N$ decay mode. The elementary weak transition at the quark level proceeds via the creation of a $J/ψ$ meson and an excited $sud$ system with $I=0$, which upon hadronization leads to $\bar K N$ or $ηΛ$ pairs. These states undergo final state interaction in coupled channels and produce a final meson-baryon pair. The $K Ξ$ state only occurs via rescattering, hence making the $Λ_b \to J/ψ~ K ~ Ξ$ process very sensitive to the details of the meson-baryon interaction in strangeness $S=-1$ and isospin $I=0$. We show that the corresponding invariant mass distribution is dominated by the next-to-leading order terms of the chiral interaction. The $I=0$ selectivity of this decay, and its large sensitivity to the higher order terms, makes its measurement very useful and complementary to the $K^- p \to K Ξ$ cross section data. The rates of the $Λ_b \to J/ψ~ K ~ Ξ$ and $Λ_b \to J/ψ~ η~ Λ$ invariant mass distributions are sizable compared to those of the $Λ_b \to J/ψ~ \bar K ~N$ decay, which is measured experimentally, thus, we provide arguments for an experimental determination of these decay modes that will help us understand better the chiral dynamics at higher energies.

hep-ph↗

arXiv2015

Reanalysis of lattice QCD spectra leading to the $D_{s0}^(2317)$ and $D_{s1}^(2460)$

We perform a reanalysis of the energy levels obtained in a recent lattice QCD simulation, from where the existence of bound states of $KD$ and $KD^*$ are induced and identified with the narrow $D_{s0}^*(2317)$ and $D_{s1}^*(2460)$ resonances. The reanalysis is done in terms of an auxiliary potential, employing a single-channel basis $KD^{(*)}$, and a two-channel basis $KD^{(*)}, ηD_s^{(*)}$. By means of an extended Lüscher method we determine poles of the continuum $t$-matrix, bound by about 40 MeV with respect to the $KD$ and $KD^*$ thresholds, which we identify with the $D_{s0}^*(2317)$ and $D_{s1}^*(2460)$ resonances. Using a sum rule that reformulates Weinberg compositeness condition we can determine that the state $D_{s0}^*(2317)$ contains a $KD$ component in an amount of about 70%, while the state $D_{s1}^*(2460)$ contains a similar amount of $KD^*$. We argue that the present lattice simulation results do not still allow us to determine which are the missing channels in the bound state wave functions and we discuss the necessary information that can lead to answer this question.

hep-lat↗

arXiv2015

The $\bar{K} N \rightarrow K Ξ$ reaction in coupled channel chiral models up to next-to-leading order

The meson-baryon interaction in s-wave in the strangeness S=-1 sector has been studied, employing a chiral SU(3) Lagrangian up to next-to-leading order (NLO) and implementing unitarization in coupled channels. The parameters of the Lagrangian have been fitted to a large set of experimental data in different two-body channels, paying special attention to the $\bar{K} N \rightarrow K Ξ$ reaction, which is particularly sensitive to the NLO terms. With the aim of improving the model in the $KΞ$ production channels, effects of the high spin hyperon resonances $Σ(2030)$ and $Σ(2250)$ have been taken into account phenomenologically.

nucl-th↗