Invariant Mass Spectrum Research Articles

Using spin alignment of inelastically excited nuclei in fast beams to assign spins: The spectroscopy of O13 as a test case

Excited states in 13O were investigated using inelastic scattering of an E/A=69.5-MeV 13O beam off of a 9Be target. The excited states were identified in the invariant-mass spectra of the decay products. Both single proton and sequential two-proton decays of the excited states were examined. For a number of the excited states, the protons were emitted with strong anisotropy where emissions transverse to the beam axis are favored. The measured proton-decay angular distributions were compared to predictions from distorted-wave born-approximation (DWBA) calculations of the spin alignment which was shown to be largely independent of the excitation mechanism. The deduced $^{13}$O level scheme is compared to ab initio no-core shell model with continuum (NCSMC) predictions. The lowest-energy excited states decay isotropically consistent with predictions of strong proton 1s1/2 structure. Above these states in the level scheme, we observed a number of higher-spin states not predicted within the model. Possibly these are associated with rotational bands built on deformed cluster configurations predicted by antisymmetrized molecular dynamics (AMD) calculations. The spin alignment mechanism is shown to be useful for making spin assignments and may have widespread use.

Simulation-assisted decorrelation for resonant anomaly detection

A growing number of weak and unsupervised machine learning approaches to anomaly detection are being proposed to significantly extend the search program at the Large Hadron Collider (LHC) and elsewhere. One of the prototypical examples for these methods is the search for resonant new physics, where a bump hunt can be performed in an invariant mass spectrum after applying a classifier to enhance the presence of a potential signal. A significant challenge to methods that rely entirely on data is that they are susceptible to sculpting artificial bumps from the dependence of the machine learning classifier on the invariant mass. We explore two solutions to this challenge by minimally incorporating simulation into the learning. In particular, we study the robustness of simulation assisted likelihood-free anomaly detection to correlations between the classifier and the invariant mass. Next, we propose a new approach that only uses the simulation for decorrelation but uses the classification without labels approach for achieving signal sensitivity. Both methods are compared using a full background fit analysis on simulated data from the LHC Olympics and are robust to correlations in the data.

On the structure in the ΛN cross section at the ΣN threshold * *Supported in part by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) and the NSFC through the funds provided to the Sino-German Collaborative Research Center CRC 110 “Symmetries and the Emergence of Structure in QCD” (DFG Project-ID 196253076 - TRR 110, Grant No. 12070131001) and by the EU

The complexity of threshold phenomena is exemplified on a prominent and long-known case - the structure in the cross section (invariant mass spectrum) at the opening of the channel. The mass splitting between the baryons together with the angular momentum coupling in the - partial wave imply that, in principle, up to six channels are involved. Utilizing hyperon-nucleon potentials that provide an excellent description of the available low-energy and scattering data, the shape of the resulting cross section is discussed and the poles near the threshold are determined. Evidence for a strangeness dibaryon is provided, in the form of a (unstable) bound state in the vicinity of the threshold. Predictions for level shifts and widths of atomic states are given.

Observation of a New Excited Beauty Strange Baryon Decaying to Ξ_{b}^{-}π^{+}π^{-}.

The Ξ_{b}^{-}π^{+}π^{-} invariant mass spectrum is investigated with an event sample of proton-proton collisions at sqrt[s]=13 TeV, collected by the CMS experiment at the LHC in 2016-2018 and corresponding to an integrated luminosity of 140 fb^{-1}. The ground state Ξ_{b}^{-} is reconstructed via its decays to J/ψΞ^{-} and J/ψΛK^{-}. A narrow resonance, labeled Ξ_{b}(6100)^{-}, is observed at a Ξ_{b}^{-}π^{+}π^{-} invariant mass of 6100.3±0.2(stat)±0.1(syst)±0.6(Ξ_{b}^{-}) MeV, where the last uncertainty reflects the precision of the Ξ_{b}^{-} baryon mass. The upper limit on the Ξ_{b}(6100)^{-} natural width is determined to be 1.9 MeV at 95%confidence level. The low Ξ_{b}(6100)^{-} signal yield observed in data does not allow a measurement of the quantum numbers of the new state. However, following analogies with the established excited Ξ_{c} baryon states, the new Ξ_{b}(6100)^{-} resonance and its decay sequence are consistent with the orbitally excited Ξ_{b}^{-} baryon, with spin and parity quantum numbers J^{P}=3/2^{-}.

Analysis of the [formula omitted] hexaquark molecular state with the QCD sum rules

In this work, we construct the color-singlet-color-singlet type six-quark pseudoscalar current to investigate the Ξ‾ccΞcc hexaquark molecular state with the QCD sum rules, the predicted mass MX∼7.2GeV supports assigning the X(7200) to be the Ξ‾ccΞcc hexaquark molecular state with the quantum numbers JPC=0−+. The X(7200) can decay through fusions of the c¯c and q¯q pairs, we can search for the X(7200) and explore its properties in the J/ψJ/ψ, χc1χc1, D⁎D¯⁎ and D1D¯1 invariant mass spectrum in the future.

Search for trilepton resonances from chargino and neutralino pair production in s=13 TeV pp collisions with the ATLAS detector

A search is performed for the electroweak pair production of charginos and associated production of a chargino and neutralino, each of which decays through an $R$-parity-violating coupling into a lepton and a $W$, $Z$, or Higgs boson. The trilepton invariant-mass spectrum is constructed from events with three or more leptons, targeting chargino decays that include an electron or muon and a leptonically decaying $Z$ boson. The analyzed dataset corresponds to an integrated luminosity of 139 fb$^{-1}$ of proton-proton collision data produced by the Large Hadron Collider at a center-of-mass energy of $\sqrt{s}$ = 13 TeV and collected by the ATLAS experiment between 2015 and 2018. The data are found to be consistent with predictions from the Standard Model. The results are interpreted as limits at 95% confidence level on model-independent cross sections for processes beyond the Standard Model. Limits are also set on the production of charginos and neutralinos for a Minimal Supersymmetric Standard Model with an approximate $B$-$L$ symmetry. Charginos and neutralinos with masses between 100 GeV and 1100 GeV are excluded depending on the assumed decay branching fractions into a lepton (electron, muon, or $\tau$-lepton) plus a boson ($W$, $Z$, or Higgs).

Measurement of proton electromagnetic form factors in the time-like region using initial state radiation at BESIII

The electromagnetic process e+e−→pp¯ is studied with the initial-state-radiation technique using 7.5 fb−1 of data collected by the BESIII experiment at seven energy points from 3.773 to 4.600 GeV. The Born cross section and the effective form factor of the proton are measured from the production threshold to 3.0 GeV/c2 using the pp¯ invariant-mass spectrum. The ratio of electric and magnetic form factors of the proton is determined from the analysis of the proton-helicity angular distribution.

Producing fully charm structures in the J/ψ -pair invariant mass spectrum

Very recently, the LHCb Collaboration reported the observation of several enhancements in the invariant mass spectrum of a $J/\psi$ pair between 6.2 and 7.4 GeV. In this work, we propose the dynamical mechanism to mimic the experimental data of a di-$J/\psi$ mass spectrum given by LHCb, which is based on the reactions, where all the possible combinations of a double charmonium directly produced by a proton-proton collision are transferred into a final state $J/\psi J/\psi$. We find that the LHCb experimental data can be well reproduced. We further extend our framework to study a di-$\Upsilon(1S)$ system, and give the line shape of a differential cross section of a partner process in a $b\bar{b}$ system on the invariant mass of $\Upsilon(1S)\Upsilon(1S)$, which shows that there should exist possible enhancements near $m_{\Upsilon(1S)\Upsilon(1S)}=$19.0, 19.3, 19.7 GeV in the $\Upsilon(1S)$-pair invariant mass spectrum. These predictions can be tested in LHCb and CMS, which can be as a potential research issue in near future.

Fully heavy pentaquark states

Developing the calculation techniques to fivefold heavy hadrons, we perform the study of novel fully-heavy $QQQQ\bar{Q}$ pentaquark states by the QCD sum rule approach that is firmly based on the QCD basic theory. Numerically, masses of fully-heavy pentaquark states are calculated to be $7.41^{+0.27}_{-0.31}~\mbox{GeV}$ for $cccc\bar{c}$ and $21.60^{+0.73}_{-0.22}~\mbox{GeV}$ for $bbbb\bar{b}$, respectively. In experiment, these predicted all-heavy pentaquark states could be searched for in the $\Omega_{QQQ}\eta_{Q}$ invariant mass spectrum.

Particle production in AgAg collisions at EKin=1.58A GeV within a hadronic transport approach

Heavy-ion collisions at low beam energies explore the high density regime of strongly-interacting matter. The dynamical evolution of these collisions can be successfully described by hadronic transport approaches. In March 2019, the HADES collaboration has taken data for AgAg collisions at $E_{\rm Kin}=1.58A$ GeV and in this work, we provide predictions for particle production and spectra within the Simulating Many Accelerated Strongly-interacting Hadrons (SMASH) approach. The multiplicities and spectra of strange and non-strange particles follow the expected trends as a function of system size. In particular, in AuAu collisions, much higher yields of double-strange baryons were observed experimentally than expected from a thermal model. Therefore, we incorporate a previously suggested mechanism to produce $\Xi$ baryons via rare decays of high mass $N^*$ resonances and predict the multiplicities. In addition, we predict the invariant mass spectrum for dilepton emission and explore the most important sources of dileptons above 1 GeV, that are expected to indicate the temperature of the medium. Interestingly, the overall dilepton emission is very similar to the one in AuAu collisions at $1.23 A$ GeV, a hint that the smaller system at a higher energy behaves very similar to the larger system at lower beam energy.

Fully heavy pentaquarks

Very recently, the LHCb Collaboration reported a fully charmed tetraquark state $X(6900)$ in the invariant mass spectrum of $J/\psi$ pairs. If one $J/\psi$ meson is replaced with a fully charmed baryon, we obtain a fully charmed pentaquark candidate. In this work, we perform a systematic study on the mass spectra of the S-wave fully heavy pentaquark $QQQQ\bar{Q}$ in the framework of the chromomagnetic interaction model. Based on our results in two different schemes, we further investigate the decay behaviors for them. We hope that our study will be helpful in searching for such types of exotic pentaquark states in experiments in the future.

Investigation of varXi _c^0 in a chiral quark model

Recently, three new states of varXi _c^0 were observed in the invariant mass spectrum of varLambda ^+_cK^- by LHCb collaboration. In this work, we use a chiral quark model to investigate these three exited states with the help of Gaussian expansion method both in three-quark structure and in five-quark structure with all possible quantum numbers IJ^P=frac{1}{2}(frac{1}{2})^-, frac{1}{2}(frac{3}{2})^-, frac{1}{2}(frac{5}{2})^-, frac{3}{2}(frac{1}{2})^-, frac{3}{2}(frac{3}{2})^- and frac{3}{2}(frac{5}{2})^- . The calculations shows that the masses of 2S and 1D states of varXi _c are comparable to experimental results; In addition, the resonance states of five-quark configuration are possible candidates of these new states with negative parity by using the real scaling method and their decay width is also given.

Coupled-Channel Interpretation of the LHCb Double-J/ψ Spectrum and Hints of a New State Near the J/ψJ/ψ Threshold.

Recently, the LHCb Collaboration reported pronounced structures in the invariant mass spectrum of J/ψ 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 coupled-channel approach employing T matrices consistent with unitarity: (i)with just two channels, J/ψJ/ψ and ψ(2S)J/ψ, as long as energy-dependent interactions in these channels are allowed, or (ii)with three channels J/ψJ/ψ, ψ(2S)J/ψ, and ψ(3770)J/ψ with just constant contact interactions. Both formulations hint at the existence of a near-threshold state in the J/ψJ/ψ 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 X(6200), as hinted by the three-channel approach, is confirmed, many other double-quarkonium states should exist driven by the same binding mechanism. In particular, there should be an η_{c}η_{c} molecule with a similar binding energy.

Phenomenology of τ−→π−ντγ using light cone sum rules

We present the study of radiative tau decay (${\ensuremath{\tau}}^{\ensuremath{-}}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{\ensuremath{-}}{\ensuremath{\nu}}_{\ensuremath{\tau}}\ensuremath{\gamma}$), computing the structure dependent contribution using light cone sum rules. This decay includes the same form factors as the radiative pion decay with the crucial difference that the momentum transfer squared, $t$, between the pion-photon system is positive, which makes these form factors timelike and also as $t$ can now take values up to ${m}_{\ensuremath{\tau}}^{2}$, it can produce real hadronic resonances. The analytical form for these form factors has been calculated using the light cone sum rules method and the invariant mass spectrum in the $\ensuremath{\pi}\ensuremath{-}\ensuremath{\gamma}$ system and the decay width are presented. The structure dependent parameter, $\ensuremath{\gamma}$, the ratio of the axial vector to vector form factor is found to be in good agreement with the experimental determination.

Fully-heavy structures in the invariant mass spectrum of J/ψψ(3686), J/ψψ(3770), ψ(3686)ψ(3686), and J/ψϒ(1S) at hadron colliders

In this work, we predict the line shape of potential fully-heavy structures existed in the invariant mass spectrum of J/ψψ(3686), J/ψψ(3770), ψ(3686)ψ(3686), and J/ψϒ(1S) at high energy proton-proton collisions based on a dynamical rescattering picture, whose verification in experiments should be helpful to further clarify the dominant role in producing fully-charm structure X(6900) recently observed by LHCb. The above final states of vector heavy quarkonia can be experimentally reconstructed more effectively by a μ+μ− pair in the muon detector compared with QQ¯ meson with other quantum numbers. Furthermore, the corresponding peak mass positions of each of predicted fully-heavy structures are also given. Our theoretical studies here could provide some valuable information for the future measurement proposals of LHCb and CMS, especially based on the accumulated data after completing Run III of LHC in the near future.

Study of e+e−→2(pp¯) at center-of-mass energies between 4.0 and 4.6 GeV

Using data taken at 23 center-of-mass energies between 4.0 and 4.6 GeV with the BESIII detector at the BEPCII collider and with a total integrated luminosity of approximately $15$ fb$^{-1}$, the process $e^+e^-\to 2(p \bar{p})$ is studied for the first time. The Born cross sections for $e^+e^-\to 2(p \bar{p})$ are measured, and no significant structure is observed in the lineshape. The baryon pair ($pp$ and $\bar{p}\bar{p}$) invariant mass spectra are consistent with phase space, therefore no hexaquark or di-baryon state is found.

Pair invariant mass spectrum and polarization asymmetry in the event generation of gamma-ray conversions

We examine the dielectron invariant mass spectrum and the variation of the polarization asymmetry as a function of that mass in the event generator of the five-dimensional Bethe–Heitler differential cross section of the conversion of linearly polarized photons that we have implemented recently as a Geant4 Physics Model. We compare the results obtained with simulated samples to analytical expressions.Studies of b→sγ decays at LHC and Belle II experiments could provide hints of physics beyond the standard model in flavor-changing neutral current processes; issues have been reported in the simulation of the properties of the background induced by genuine γ-ray conversions in their detectors. Our results demonstrate that using the five-dimensional model would help solve these issues.

Fully strange tetraquark sss¯s¯ spectrum and possible experimental evidence

In this work we construct 36 tetraquark configurations for the $1S$-, $1P$-, and $2S$-wave states, and make a prediction of the mass spectrum for the tetraquark $ss\bar{s}\bar{s}$ system in the framework of a nonrelativistic potential quark model without the diquark-antidiquark approximation. The model parameters are well determined by our previous study of the strangeonium spectrum. We find that the resonances $f_0(2200)$ and $f_2(2340)$ may favor the assignments of ground states $T_{(ss\bar{s}\bar{s})0^{++}}(2218)$ and $T_{(ss\bar{s}\bar{s})2^{++}}(2378)$, respectively, and the newly observed $X(2500)$ at BESIII may be a candidate of the lowest mass $1P$-wave $0^{-+}$ state $T_{(ss\bar{s}\bar{s})0^{-+}}(2481)$. Signals for the other $0^{++}$ ground state $T_{(ss\bar{s}\bar{s})0^{++}}(2440)$ may also have been observed in the $\phi\phi$ invariant mass spectrum in $J/\psi\to\gamma\phi\phi$ at BESIII. The masses of the $J^{PC}=1^{--}$ $T_{ss\bar{s}\bar{s}}$ states are predicted to be in the range of $\sim 2.44-2.99$ GeV, which indicates that the $\phi(2170)$ resonance may not be a good candidate of the $T_{ss\bar{s}\bar{s}}$ state. This study may provide a useful guidance for searching for the $T_{ss\bar{s}\bar{s}}$ states in experiments.

Detecting the pure triangle singularity effect through the ψ(2S)→pp¯η/pp¯π0 process

In this work, the triangle singularity mechanism is investigated in the $\psi(2S) \to p \bar{p} \eta / p \bar{p} \pi^0$ process. The triangle loop composed by $J/\psi$, $\eta$ and $p$ has a singularity in the physical kinematic range for the $\psi(2S) \to p \bar{p} \eta / p \bar{p} \pi^0$ process, and it would generate a very narrow peak in the invariant mass spectrum of $p\eta (\pi)$ around $1.56387$ GeV, which is far away from both the threshold and relative resonances. In these processes, all the involved vertices are constrained by the experimental data. Thus, we can make a precise model independent prediction here. It turns out that the peak in the $p\eta$ invariant mass spectrum is visible, while it is very small in the $p\pi^0$ invariant mass spectrum. We expect this effect shown in $p \bar{p} \eta$ final state can be observed by the Beijing Spectrometer (BESIII) and Super Tau-Charm Facility (STCF) in the future.

0+ fully-charmed tetraquark states

Motivated by the LHCb's new observation of structures in the $J/\psi$-pair invariant mass spectrum, for which could be classified as possible $cc\bar{c}\bar{c}$ tetraquark candidates, we systematically study $0^{+}$ fully-charmed tetraquark states through QCD sum rules. Making the development of calculation techniques to fourfold heavy hadronic systems, four different configuration currents with $0^{+}$ are considered and vacuum condensates up to dimension $6$ are included in the operator product expansion (OPE). Finally, mass values acquired for $0^{+}$ $cc\bar{c}\bar{c}$ tetraquark states agree well with the experimental data of the broad structure, which supports that it could be a $0^{+}$ fully-charmed tetraquark state.