New articles on hep-ex

[1] 2009.08099

Observation of a resonant structure in $e^{+}e^{-} \to ωη$ and another in $e^{+}e^{-} \to ωπ^{0}$ at center-of-mass energies between 2.00 and 3.08 GeV

Born cross sections for the processes $e^+e^- \to \omega\eta$ and $e^+e^- \to \omega\pi^{0}$ have been determined for center-of-mass energies between 2.00 and 3.08 GeV with the BESIII detector at the BEPCII collider. The results obtained in this work are consistent with previous measurements but with improved precision. Two resonant structures are observed. In the $e^{+}e^{-} \to \omega\eta$ cross sections, a resonance with a mass of $(2179 \pm 21 \pm 3)\text{MeV}/c^2$ and a width of $(89 \pm 28 \pm 5)\text{MeV}$ is observed with a significance of 6.1$\sigma$. Its properties are consistent with the $\phi(2170)$. In the $e^{+}e^{-} \to\omega\pi^{0}$ cross sections, a resonance denoted $Y(2040)$ is observed with a significance of more than 10$\sigma$. Its mass and width are determined to be $(2034 \pm 13 \pm 9)\text{MeV}/c^2$ and $(234 \pm 30 \pm 25)\text{MeV}$, respectively, where the first uncertainties are statistical and the second ones are systematic.

[2] 2009.08261

Jet acoplanarity via hadron+jet measurements in Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}} = 5.02$ TeV with ALICE

We present the first fully-corrected semi-inclusive distribution of charged jets recoiling from a trigger hadron in 0-10% Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 5.02 TeV as a function of the azimuthal angle between the trigger hadron and jet, $\Delta\varphi$. This technique provides a precise data-driven subtraction of the large uncorrelated background contaminating the measurement, and enables the exploration of jet acoplanarity. Results for $R=0.2$ recoil jets in the region $30 < p_{\mathrm{T,jet}}^{\mathrm{ch}} < 40~\mathrm{GeV}/c$ are shown, where a suppression and narrowing of the $\Delta\varphi$ distribution is observed in 0-10% Pb-Pb collisions with respect to a PYTHIA pp reference.

[3] 2003.09323

Hadronic weak decays of $Λ_c$ in the quark model

The hadronic weak decays of $\Lambda_c$ are studied in the framework of a constituent quark model. With the combined analysis of the Cabbibo-favored processes, $\Lambda_c\to \Lambda\pi^+$, $\Sigma^0\pi^+$ and $\Sigma^+\pi^0$, we confirm that the non-factorizable transition mechanisms play a crucial role in the understanding of their compatible branching ratios. We emphasize that the SU(3) flavor symmetry breaking effects, which is generally at the order of $1\sim 2\%$, can be amplified by the destructive interferences among the pole terms in the diagrams with internal conversion. Some contributions are sensitive to the spatial distribution of the scalar-isoscalar light-quark sector in the $\Lambda_c$, and its overlap with the light quarks in the final state hyperon. Namely, a compact diquark configuration is disfavored.

[4] 2007.09621

Hyperon Weak Radiative Decay

We revisit the hyperon weak radiative decays in the framework of the non-relativistic constituent quark model. This study confirms the nonlocal feature of the hyperon weak radiative transition operators which are dominated by the pole terms, and an overall self-consistent description of the available experimental data for the Cabibbo-favored hyperon weak radiative decays is achieved. It provides a natural mechanism for evading the Hara theorem, where sizeable parity-violating contributions can come from the intermediate orbital excitations. Cancellations between pole terms also explain significant SU(3) flavor symmetry breaking manifested by the experimental data. We also discuss several interesting selection rules arising from either the electromagnetic or the weak interaction vertices. These features suggest nontrivial relations among different hyperon decays.

[5] 2009.05061

Impact of high energy beam tunes on the sensitivities to the standard unknowns at DUNE

Even though neutrino oscillations have been conclusively established, there are a few unanswered questions pertaining to leptonic Charge Parity violation (CPV), mass hierarchy (MH) and $\theta_{23}$ octant degeneracy. Addressing these questions is of paramount importance at the current and future neutrino experiments including the Deep Underground Neutrino Experiment (DUNE) which has a baseline of 1300 km. In the standard mode, DUNE is expected to run with a {\textit{low energy}} (LE) tuned beam which peaks around the first oscillation maximum ($2-3$ GeV) (and then sharply falls off as we go to higher energies). However, the wide band nature of the beam available at long baseline neutrino facility (LBNF) allows for the flexibility in utilizing beam tunes that are well-suited at higher energies as well. In this work, we utilize a beam that provides high statistics at higher energies which is referred to as the {\textit{medium energy}} (ME) beam. This opens up the possibility of exploring not only the usual oscillation channels but also the $\nu_{\mu} \to \nu_{\tau}$ oscillation channel which was otherwise not accessible. Our goal is to find an optimal combination of beam tune and runtime (with the total runtime held fixed) distributed in neutrino and antineutrino mode that leads to an improvement in the sensitivities of these parameters at DUNE. In our analysis, we incorporate all the three channels ($\nu_{\mu} \to \nu_{e}, \nu_{\mu} \to \nu_{\mu}, \nu_{\mu} \to \nu_{\tau}$) and develop an understanding of their relative contributions in sensitivities at the level of $\Delta \chi^2$. Finally, we obtain the preferred combination of runtime using both the beam tunes as well as neutrino and antineutrino mode that lead to enhanced sensitivity to the current unknowns in neutrino oscillation physics i.e., CPV, MH and $\theta_{23}$ octant.

[6] 2009.07276

SMEFT ATLAS of $ΔF=2$ Transitions

We present a model-independent anatomy of the $\Delta F=2$ transitions $K^0-\bar K^0$, $B_{s,d}-\bar B_{s,d}$ and $D^0-\bar D^0$ in the context of the Standard Model Effective Field Theory (SMEFT). We present two master formulae for the mixing amplitude $\big[M_{12} \big]_\text{BSM}$. One in terms of the Wilson coefficients (WCs) of the Low-Energy Effective Theory (LEFT) operators evaluated at the electroweak scale $\mu_\text{ew}$ and one in terms of the WCs of the SMEFT operators evaluated at the BSM scale $\Lambda$. The coefficients $P_a^{ij}$ entering these formulae contain all the information below the scales $\mu_\text{ew}$ and $\Lambda$, respectively. Renormalization group effects from the top-quark Yukawa coupling play the most important role. The collection of the individual contributions of the SMEFT operators to $\big[M_{12}\big]_\text{BSM}$ can be considered as the SMEFT ATLAS of $\Delta F=2$ transitions and constitutes a travel guide to such transitions far beyond the scales explored by the LHC. We emphasize that this ATLAS depends on whether the down-basis or the up-basis for SMEFT operators is considered. We illustrate this technology with tree-level exchanges of heavy gauge bosons ($Z^\prime$, $G^\prime$) and corresponding heavy scalars.

[7] 2009.07326

Measurement of beam asymmetry for $π^-Δ^{++}$ photoproduction on the proton at $E_γ$=8.5 GeV

We report a measurement of the $\pi^-$ photoproduction beam asymmetry for the reaction $\vec{\gamma} p \rightarrow \pi^- \Delta^{++}$ using data from the GlueX experiment in the photon beam energy range 8.2--8.8 GeV. The asymmetry $\Sigma$ is measured as a function of four-momentum transfer $t$ to the $\Delta^{++}$ and compared to phenomenological models. We find that $\Sigma$ varies as a function of $t$: negative at smaller values and positive at higher values of $|t|$. The reaction can be described theoretically by $t$-channel particle exchange requiring pseudoscalar, vector, and tensor intermediaries. In particular, this reaction requires charge exchange, allowing us to probe pion exchange and the significance of higher-order corrections to one-pion exchange at low momentum transfer. Constraining production mechanisms of conventional mesons may aid in the search for and study of unconventional mesons. This is the first measurement of the process at this energy.

[8] 2009.07581

Light Detection System and Time Resolution of the NA62 RICH

A large RICH detector is used in NA62 to suppress the muon contamination in the charged pion sample by a factor of 100 in the momentum range between 15 and 35 GeV/c. Cherenkov light is collected by 1952 photomultipliers placed at the upstream end. In this paper the characterization of the photomultipliers and the dedicated Frontend and Data Acquisition electronics are described, the time resolution and the light detection efficiency measurement are presented.

[9] 2009.07709

Toward the Frontiers of Particle Physics With the Muon $g\textrm{-}2$ Experiment

The Muon $g\textrm{-}2$ Experiment (E989) at Fermilab has a goal of measuring the muon anomaly ($a_\mu$) with unprecedented precision using positive muons. This measurement is motivated by the difference between the previous Brookhaven $a_\mu$ measurement and Standard Model prediction exceeding three standard deviations, which hints at the possibility of physics beyond the Standard Model. Muons are circulated in a storage ring, and the measurement requires a precise determination of the muon anomalous precession frequency (spin precession relative to momentum) from the resulting decay positron time and energy measurements collected with calorimeters. The average magnetic field seen by the muons needs to be known with high precision, and so the storage ring magnetic field is shimmed to be very uniform and is continually monitored with nuclear magnetic resonance (NMR) probes. Detailed Muon Campus beamline and muon storage ring simulations are also required for quantifying beam dynamics and spin-related systematic effects in the determination of the muon anomalous precession frequency, e.g. muon losses during the measurement window. At the time of the conference, the experiment has recently commenced Run-3, and the release of Run-1 physics results is planned for 2020.

[10] 2009.07763

Heavy-flavor hadro-production with heavy-quark masses renormalized in the ${\overline{\rm MS}}$, MSR and on-shell schemes

We present predictions for heavy-quark production at the Large Hadron Collider making use of the \msbar and MSR renormalization schemes for the heavy-quark mass as alternatives to the widely used on-shell renormalization scheme. We compute single and double differential distributions including QCD corrections at next-to-leading order and investigate the renormalization and factorization scale dependence as well as the perturbative convergence in these mass renormalization schemes. The implementation is based on publicly available programs, ${\texttt{MCFM}}$ and ${\texttt{xFitter}}$, extending their capabilities. Our results are applied to extract the top-quark mass using measurements of the total and differential $t\bar{t}$ production cross-sections and to investigate constraints on parton distribution functions, especially on the gluon distribution at low $x$ values, from available LHC data on heavy-flavor hadro-production.

[11] 2009.07771

NuSTAR observation of GRO J1744-28 at low mass accretion rate

We present the spectral analysis of the LMXB GRO J1744-28 using $\sim$29 ks of NuSTAR data taken in 2017 February at a low luminosity of $3.2\times 10^{36}$ erg/s (3-50 keV). The continuum spectrum is modeled with an absorbed power-law with exponential cut-off, and an additional iron line component. We find no obvious indications for a CRSF and therefore perform a detailed cyclotron line search using statistical methods on the pulse phase-averaged as well as phase-resolved spectra. The previously detected Type II X-ray bursts are absent. Clear pulsations at a period of 2.141124(9) Hz are detected. The pulse profile shows an indication of a secondary peak, which was not seen at higher flux. The 4$\sigma$ upper limit for the strength of a CRSF in the 3-20 keV band is 0.07 keV, lower than the strength of the line found at higher luminosity. The detection of pulsations shows that the source did not enter the "propeller" regime, even though the source flux of $4.15\times 10^{-10}$ erg/cm$^{2}$/s was almost one order of magnitude below the threshold for the propeller regime claimed in previous studies on this source. The transition into the propeller regime in GRO J1744-28 must therefore be below a luminosity of $3.2\times 10^{36}$ erg/s, which implies a surface magnetic field $\lesssim 2.9\times 10^{11}$ G and mass accretion rate $\lesssim 1.7\times 10^{16}$ g/s. A change of the CRSF depth as function of luminosity is not unexpected and has been observed in other sources. This result possibly implies a change in emission geometry as function of mass accretion rate to reduce the depth of the line below our detection limit.

[12] 2009.07795

Coupled-channel interpretation of the LHCb double-$J/ψ$ spectrum and hints of a new state near $J/ψJ/ψ$ threshold

Recently, the LHCb Collaboration reported pronounced structures in the invariant mass spectrum of $J/\psi$-pairs produced in proton-proton collisions at the Large Hadron Collider. In this Letter, we argue that the data can be very well described within two variants of a unitary coupled-channel approach: (i) with just two channels, $J/\psi J/\psi$ and $\psi(2S)J/\psi$, as long as energy-dependent interactions in these channels are allowed, or (ii) with three channels $J/\psi J/\psi$, $\psi(2S)J/\psi$ and $\psi(3770)J/\psi$ with just constant contact interactions. Both formulations hint at the existence of a near-threshold state in the $J/\psi J/\psi$ system with the quantum numbers $J^{PC}=0^{++}$ or $2^{++}$, which we refer to as $X(6200)$. We suggest experimental tests to check the existence of this state and discuss what additional channels need to be studied experimentally to allow for distinctive tests between the two mechanisms proposed. If the molecular nature of the $X(6200)$, as hinted by the three-channel approach, were confirmed, many other double-quarkonium states should exist driven by the same binding mechanisms.

[13] 2009.07819

Improved Neural Network Monte Carlo Simulation

The algorithm for Monte Carlo simulation of parton-level events based on an Artificial Neural Network (ANN) proposed in arXiv:1810.11509 is used to perform a simulation of $H\to 4\ell$ decay. Improvements in the training algorithm have been implemented to avoid numerical instabilities. The integrated decay width evaluated by the ANN is within 0.7% of the true value and unweighting efficiency of 26% is reached. While the ANN is not automatically bijective between input and output spaces, which can lead to issues with simulation quality, we argue that the training procedure naturally prefers bijective maps, and demonstrate that the trained ANN is bijective to a very good approximation.

[14] 2009.07852

Unraveling the Dirac Neutrino with Cosmological and Terrestrial Detectors

We propose a method for testing the Dirac neutrino hypothesis by combining data from terrestrial neutrino experiments, such as tritium beta decay, with data from cosmological observations, such as the cosmic microwave background and large scale structure surveys. If the neutrinos are Dirac particles, and if the active neutrinos' sterile partners were once thermalized in the early universe, then this new cosmological relic would simultaneously contribute to the effective number of relativistic species, $N_\text{eff}$, and also lead to a mismatch between the cosmologically-measured effective neutrino mass sum $\Sigma m_\nu$ and the terrestrially-measured active neutrino mass sum $\Sigma_i m_i$. We point out that specifically correlated deviations in $N_\text{eff} \gtrsim 3$ and $\Sigma m_\nu \gtrsim \Sigma_i m_i$ above their standard predictions could be the harbinger revealing the Dirac nature of neutrinos. We provide several benchmark examples, including Dirac leptogenesis, that predict a thermal relic population of the sterile partners, and we discuss the relevant observational prospects with current and near-future experiments. This work provides a novel approach to probe an important possibility of the origin of neutrino mass.

[15] 2009.07909

Direct Detection Limits on Heavy Dark Matter

Multiply-interacting massive particles (MIMPs) are heavy (>10^10 GeV/c^2) dark matter particles that interact strongly with regular matter, but may have evaded detection due to the low number density required to make up the local dark matter halo. These particles could leave track-like signatures in current experiments, similar to lightly-ionizing particles. We show that previously calculated limits from the MAJORANA Demonstrator on the flux of lightly-ionizing particles can be used to exclude MIMP dark matter parameter space up to a mass of 10^15 GeV/c^2. We also calculate limits from the standard XENON1T analysis in this high-mass regime, properly taking into account flux limitations and multi-scatter effects. Finally, we show that a dedicated MIMP analysis using the XENON1T dark matter search could probe unexplored parameter space up to masses of 10^18 GeV/c^2.

[16] 2009.08126

Z production in pPb collisions at LHCb

This article presents results of the Z boson production in the proton-lead collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV and $\sqrt{s_{\rm NN}}= 8.16$ TeV collected in 2013 and 2016, respectively, by the LHCb detector at the LHC in forward and backward rapidity. The great precision of the 2016 data are found compatible with nPDFs theoretical predictions within large theoretical uncertainties, and can be useful to constraint new predictions.

[17] 2009.08139

First NNLO fragmentation functions of $K_S^0$ and $Λ/\barΛ$ and their uncertainties in the presence of hadron mass corrections

The current paper presents a determination of $K^0_S$ and $\Lambda/\bar{\Lambda}$ fragmentation functions (FFs) from QCD analysis of single-inclusive electron-positron annihilation process (SIA). Our FFs determinations are performed at next-to-leading order (NLO), and for the first time, at next-to-next-to-leading order (NNLO) accuracy in perturbative Quantum Chromodynamics (pQCD) which is designated as {\tt SAK20} FFs. Each of these FFs is accompanied by their uncertainties which are determined using the `Hessian' method. Considering the hadron mass corrections, we clearly investigate the reliability of our results upon the inclusion of higher-order QCD correction. We provide comparisons of {\tt SAK20} FFs set with the available analysis from another group, finding in general a reasonable agreement, and also considerable differences. In order to judge the fit quality, our theoretical predictions are compared with the analyzed SIA datasets. {\tt SAK20} FFs at NLO and NNLO accuracy along with their uncertainties are made available in the standard {\tt LHAPDF} format in order to use for predictions of present and future measurements in high-energy collisions such as LHC and RHIC.

[18] 2009.08264

Large-P_T inclusive photoproduction of J/psi in electron-proton collisions at HERA and the EIC

We study the inclusive J/psi production at large transverse momenta at lepton-hadron colliders in the limit when the exchange photon is quasi real, also referred to as photoproduction. Our computation includes the leading-P_T leading-v next-to-leading alpha_s corrections. In particular, we consider the contribution from J/psi plus another charm quark, by employing for the first time in quarkonium photoproduction the variable-flavour-number scheme. We also include a QED-induced contribution via an off-shell photon which remained ignored in the literature and which we show to be the leading contribution at high P_T within the reach of the EIC. In turn, we use our computation of J/psi+charm to demonstrate its observability at the future EIC and the EIC sensitivity to probe the non-perturbative charm content of the proton at high x.

[19] 2009.08303

Detailed $β$ spectrum calculations of $^{214}$Pb for new physics searches in liquid Xenon

We present a critical assessment of the calculation and uncertainty of the $^{214}$Pb $\to$ $^{214}$Bi ground state to ground state $\beta$ decay, the dominant source of background in liquid Xenon dark matter detectors, down to below 1 keV. We consider contributions from atomic exchange effects, nuclear structure and radiative corrections. For each of these, we find changes much larger than previously estimated uncertainties and discuss shortcomings of the original calculation. Specifically, through the use of a self-consistent Dirac-Hartree-Fock-Slater calculation, we find that the atomic exchange effect increases the predicted flux by $10(3)\%$ at 1 keV relative to previous exchange calculations. Further, using a shell model calculation of the nuclear structure contribution to the shape factor, we find a strong disagreement with the allowed shape factor and discuss several sources of uncertainty. In the 1-200 keV window, the predicted flux is up to 20$\%$ lower. Finally, we discuss omissions and detector effects in previously used QED radiative corrections, and find small changes in the slope at the $\gtrsim 1\%$ MeV$^{-1}$ level, up to $3\%$ in magnitude due to omissions in $\mathcal{O}(Z\alpha^2, Z^2\alpha^3)$ corrections and $3.5\%$ uncertainty from the neglect of as of yet unavailable higher-order contributions. Combined, these give rise to an increase of at least a factor 2 of the uncertainty in the 1-200 keV window. We comment on possible experimental schemes of measuring this and related transitions.

[20] 2009.08345

Deciphering the mechanism of near-threshold $J/ψ$ photoproduction

The photoproduction of the $J/\psi$ off the proton is believed to deepen our understanding of various physics issues. On the one hand, it is proposed to provide access to the origin of the proton mass, based on the QCD multipole expansion. On the other hand, it can be employed in a study of pentaquark states. The process is usually assumed to proceed through vector-meson dominance, that is the photon couples to a $J/\psi$ which rescatters with the proton to give the $J/\psi p$ final state. In this Letter, we provide a compelling hint for and propose measurements necessary to confirm a novel production mechanism via the $\Lambda_c \bar D^{(*)}$ intermediate states. In particular, there must be cusp structures at the $\Lambda_c \bar D^{(*)}$ thresholds in the energy dependence of the $J/\psi$ photoproduction cross section. The same mechanism also implies the $J/\psi$-nucleon scattering lengths of order 1 mfm. Given this, one expects only a minor contribution of charm quarks to the nucleon mass.

[21] 2009.08376

Exploring the Di-$J/ψ$ Resonances around 6.9 $\mathrm{GeV}$ Based on {\it{ab initio}} Perturbative QCD

We propose an {\it{ab initio}} method to explore the nature of the newly discovered particle $X$(6900). We find that there should exist another state near the resonance at around 6.9 $\mathrm{GeV}$, and the ratio of production cross sections of $X$(6900) to the undiscovered state is very sensitive to the nature of $X$(6900), whereas is almost independent of the transverse momentum and rapidity. This behavior is unlikely changed by higher order corrections. Therefore, the nature of $X$(6900) can be uncovered by experimental facts in the near future. If there is another state near $X$(6900) with cross section larger than half of that of $X$(6900), $X$(6900) should be a tetraquark state. Otherwise, it should be a molecule-like state.

[22] 2009.08450

Fragmentation production of fully-charmed tetraquarks at LHC

The $X(6900)$ resonance, very recently discovered in the double-$J/\psi$ channel at LHCb experiment, has spurred intensive interest in unravelling the nature of the fully charmed tetraquark state. The aim of this paper is to present a model-independent theoretical framework to study the inclusive production of this novel species of exotic hadrons, the resonances composed of four heavy quark (commonly referred to as $T_{4c}$), at large $p_T$ in hadron collision experiments. Appealing to asymptotic freedom and the fact $m_c\gg \Lambda_{\rm QCD}$, we propose that the nonpertubative yet universal gluon-to-$T_{4c}$ fragmentation function, can be decomposed into the product of the perturbatively calculable short-distance coefficient and the long-distance NRQCD matrix elements. We compute the short-distance coefficient at lowest-order in $\alpha_s$ and velocity expansion. Adopting the diquark ansatz to roughly estimate those not-yet-known NRQCD matrix elements, together with the standard QCD factorization theorem, we predict the differential production rates for the $T_{4c/4b}(0^{++})$ and $T_{4c/4b}(2^{++})$ at large $p_T$ in $pp$ collision, which eagerly awaits the confrontation with the future LHC experiments.