arXiv subjects

Xue-Qian Li

Publications and source records attributed to Xue-Qian Li.

At least 73 records · Page 4Linked to original sources

arXiv2013

The CDEX-1 1 kg Point-Contact Germanium Detector for Low Mass Dark Matter Searches

The CDEX Collaboration has been established for direct detection of light dark matter particles, using ultra-low energy threshold p-type point-contact germanium detectors, in China JinPing underground Laboratory (CJPL). The first 1 kg point-contact germanium detector with a sub-keV energy threshold has been tested in a passive shielding system located in CJPL. The outputs from both the point-contact p+ electrode and the outside n+ electrode make it possible to scan the lower energy range of less than 1 keV and at the same time to detect the higher energy range up to 3 MeV. The outputs from both p+ and n+ electrode may also provide a more powerful method for signal discrimination for dark matter experiment. Some key parameters, including energy resolution, dead time, decay times of internal X-rays, and system stability, have been tested and measured. The results show that the 1 kg point-contact germanium detector, together with its shielding system and electronics, can run smoothly with good performances. This detector system will be deployed for dark matter search experiments.

physics.ins-det↗

arXiv2013

Testing lepton flavor universality in terms of data of BES III and charm-tau factory

The recent measurements on $R_K$ and $R_π$ imply that there exists a possible violation of the leptonic flavor universality which is one of the cornerstones of the standard model. It is suggested that a mixing between sterile and active neutrinos might induce such a violation. In this work we consider the scenarios with one or two sterile neutrinos to explicitly realize the data while the constraints from the available experiments have been taken into account. Moreover, as indicated in literature, the deviation of the real PMNS matrix from the symmetric patterns may be due to a $μ-τ$ asymmetry, therefore the measurements on $R_{D(D_s)eμ} = Γ(D(D_s)\rightarrow e^+ν_e)/Γ(D(D_s)\rightarrow μ^+ν_μ)$ and $R_{D(D_s)μτ} = Γ(D(D_s)\rightarrow μ^+ν_μ)/Γ(D(D_s)\rightarrow μ^+τ_τ)$ (and for some other heavy mesons $B^{\pm}$ and $B_c$ etc.) may shed more light on physics responsible for the violation of the leptonic flavor universality. The data of BES III are available to test the universality and that of the future charm-tau factory will provide more accurate information towards the aspect, in this work, we will discuss $R_{D(D_s)eμ}$ and $R_{D(D_s)μτ}$ in all details and also briefly consider the cases for $B^{\pm}$ and $B_c$.

hep-ph↗

arXiv2013

Study on perturbation schemes for achieving the real PMNS matrix from various symmetric textures

The PMNS matrix displays an obvious symmetry, but not exact. There are several textures proposed in literature, which possess various symmetry patterns and seem to originate from different physics scenarios at high energy scales. To be consistent with the experimental measurement, all of the regularities slightly decline, i.e. the symmetry must be broken. Following the schemes given in literature, we modify the matrices (9 in total) to gain the real PMNS matrix by perturbative rotations. The transformations may provide hints about the underlying physics at high energies and the breaking mechanisms which apply during the evolution to the low energy scale, especially the results may be useful for the future model builders.

hep-ph↗

arXiv2013

The radiative decays of $0^{++}$ and $1^{+-}$ heavy mesons

The radiative decay is believed to be an ideal lab to study hadronic structure of newly observed resonances because the reactions are governed by only the electromagnetic interaction (tree level). However, to obtain correct theoretical values, one has to properly deal with the non-perturbative QCD effects in the wavefunction and hadronization. In this work we derive the formulas for the radiative decays of $0^{++}$ and $1^{-+}$ heavy mesons in the light front quark model (LFQM). Because $\mathcal{B}(χ_{c0}\rightarrow J/ψγ)$ is well measured, the theoretical evaluation of the transition rate can be used to test our approach. Within this theoretical framework, the width of $χ_{b0}\rightarrow Υ(1S)γ$ is evaluated. The formulas can be applied to identify the inner structures of new resonances, for example the isospin of $h_{c(b)}$ and the structure of $D_s(2317)$, via processes $h_c\rightarrow η_cγ$, $h_b\rightarrow η_bγ$ and $D_s(2317)\rightarrow D_s^*+γ$.

hep-ph↗

arXiv2013

Introduction of the CDEX experiment

Weakly Interacting Massive Particles (WIMPs) are the candidates of dark matter in our universe. Up to now any direct interaction of WIMP with nuclei has not been observed yet. The exclusion limits of the spin-independent cross section of WIMP-nucleon which have been experimentally obtained is about 10^{-7}pb at high mass region and only 10^{-5}pb} at low mass region. China Jin-Ping underground laboratory CJPL is the deepest underground lab in the world and provides a very promising environment for direct observation of dark matter. The China Dark Matter Experiment (CDEX) experiment is going to directly detect the WIMP flux with high sensitivity in the low mass region. Both CJPL and CDEX have achieved a remarkable progress in recent two years. The CDEX employs a point-contact germanium semi-conductor detector PCGe whose detection threshold is less than 300 eV. We report the measurement results of Muon flux, monitoring of radioactivity and Radon concentration carried out in CJPL, as well describe the structure and performance of the 1 kg PCGe detector CDEX-1 and 10kg detector array CDEX-10 including the detectors, electronics, shielding and cooling systems. Finally we discuss the physics goals of the CDEX-1, CDEX-10 and the future CDEX-1T detectors.

physics.ins-det↗

arXiv2013

The FSI contribution to the observed $B_s$ decays into $K^+K^-$ and $π^+K^-$

Because at the tree level $B_s\rightarrow K^+K^-$ is Cabibbo-triple suppressed, so its branching ratio should be smaller than that of $B_s\to π^+ K^-$. The measurements present a reversed ratio as $R=\mathcal B(B_s\rightarrow π^+K^-)/\mathcal B(B_s\rightarrow K^+ K^-)\sim{4.9/33}$. Therefore, It has been suggested that the transition $B_s\to K^+K^-$ is dominated by the penguin mechanism, which is proportional to $V_{cb}V^*_{cs}$. In this work, we show that an extra contribution from the final state interaction (FSI) to $B_s\to K^+K^-$ via sequential processes $B_s\to D^{(*)}_{(s)} \bar D_s^{(*)}\to K^+K^-$ is also substantial and should be superposed on the penguin contribution. Indeed, taking into account of the FSI effects, the theoretical prediction on $R$ is well consistent with the data.

hep-ph↗

arXiv2012

Study on the effects of the light CP-odd Higgs via the leptonic decays of pseudoscalar mesons

To explain the anomalously large decay rate of $Σ^+\to p+μ^+μ^-$, it was proposed that a new mechanism where a light CP-odd pseudoscalar boson of $m_{A_1^0}=214.3$ MeV makes a crucial contribution. Later, some authors have studied the transition $π^0\to e^+e^-$ and $Υ\to γA_1^0$ in terms of the same mechanism and their result indicates that with the suggested mass one cannot fit the data. This discrepancy might be caused by experimental error of $Σ^+\to p+μ^+μ^-$ because there were only a few events. Whether the mechanism is a reasonable one motivates us to investigate the transitions $π^0\to e^+e^-; η(η^\prime)\to μ^+μ^-; η_c\to μ^+μ^-; η_b\toτ^+τ^-$ within the same framework. It is noted that for $π^0\to e^+e^-$, the standard model (SM) prediction is smaller than the data, whereas the experimental central value of $η\to μ^+μ^-$ is also above the SM prediction. It means that there should be extra contributions from other mechanisms and the contribution of $A_1^0$ may be a possible one. Theoretically calculating the branching ratios of the concerned modes, we would check if we can obtain a universal mass for $A_1^0$ which reconcile the theoretical predictions and data for all the modes. Unfortunately, we find that it is impossible to have such a mass with the same coupling $|g_\ell|$. Therefore we conclude that the phenomenology does not favor such a light $A_1^0$, even though a small window is still open.

hep-ph↗

arXiv2012

$Σ_{b}\toΣ_c$ and $Ω_b\toΩ_c$ weak decays in the light-front quark model

The successful operation of LHC provides a great opportunity to study the processes where heavy baryons are involved. {In this work we mainly study} the weak transitions of $Σ_b\to Σ_c$. Assuming the reasonable quark-diquark structure where the two light quarks constitute an axial vector, we calculate the widths of semi-leptonic decay $Σ_{b}\toΣ_c eν_e$ and non-leptonic decay modes $Σ_{b}\toΣ_c +M$ (light mesons) in terms of the light front quark model. We first construct the vertex function for the concerned baryons and then deduce the form factors which are related to two Isgur-Wise functions for the $Σ_{b}\toΣ_c$ transition under the heavy quark limit. Our numerical results indicate that $Γ(Σ_{b}\toΣ_c eν_e)$ is about $1.38\times10^{10}{\rm s}^{-1}$ and $Γ(Σ_{b}\toΣ_c +M)$ is slightly below $1\times10^{10}{\rm s}^{-1}$ which may be accessed at the LHCb detector. By the flavor SU(3) symmetry we estimate the rates of $Ω_b\toΩ_c$. We suggest to measure weak decays of $Ω_b\toΩ_c$, because $Ω_b$ does not decay via strong interaction, the advantage is obvious.

hep-ph↗

arXiv2012

Effects of nuclear deformation on the form factor for direct dark matter detection

For direct dark matter detections, to extract useful information about the fundamental interaction from data, it is crucial to properly determine the nuclear form factor. The form factor for spin-independent cross section of collisions between dark matter particle and nucleus is thoroughly studied by many authors. When the analysis was carried out, the nuclei are always supposed to be spherically symmetric. In this work, we investigate the effects of deformation of nuclei from a spherical shape to an elliptical shape on the form factor. Our results indicate that as long as the ellipticity is not too large, such effects cannot cause any substantial effects, especially as the nuclei are randomly orientated in a room temperature circumstance one can completely neglect them.

hep-ph↗

arXiv2012

Is $Z_b(10610)$ a Molecular State?

Whether molecular states indeed exist in nature has been disputed for a long time. Several new resonances have been observed in the recent experiments and they seem to be of exotic structures and some of them have been proposed to be molecular states. The very recent observation of $Z_b(10610)[(10608.4\pm 2.0)$ MeV] and $Z_b(10650)[(10653.2\pm 1.5)$ MeV] encourages the interpretation of multi-quark states. In the Beter-Salpeter (BS) approach, we study the possibility if two heavy mesons can form a molecular state by exchanging light mesons. Our results indicate that two heavy mesons can form an isospin singlet (I=0) bound state but cannot form an isospin triplet (I=1) when the contribution of $σ-$ exchange is reasonably small, i.e. as the coupling of $σ$ with mesons $g_σ$ takes the value given in previous literatures. Thus we conclude that the newly observed $Z_b(10610)$ should not be a molecular state, but a tetraquark state instead, at most, the fraction of the molecular state in the physical resonance $Z_b(10610)$ is tiny.

hep-ph↗

arXiv2012

Study of Doubly Heavy Baryon Spectrum via QCD Sum Rules

In this work, we calculate the mass spectrum of doubly heavy baryons with the diquark model in terms of the QCD sum rules. The interpolating currents are composed of a heavy diquark field and a light quark field. Contributions of the operators up to dimension six are taken into account in the operator product expansion. Within a reasonable error tolerance, our numerical results are compatible with other theoretical predictions. This indicates that the diquark picture reflects the reality and is applicable to the study of doubly heavy baryons.

hep-ph↗

arXiv2012

Discussions on Stability of Diquarks

Since the birth of the quark model, the diquark which is composed of two quarks has been considered as a substantial structure of color anti-triplet. This is not only a mathematical simplification for dealing with baryons, but also provides a physical picture where the diquark would behave as a whole object. It is natural to ask whether such a structure is sufficiently stable against external disturbance. The mass spectra of the ground states of the scalar and axial-vector diquarks which are composed of two-light (L-L), one-light-one-heavy (H-L) and two-heavy quarks (H-H) respectively have been calculated in terms of the QCD sum rules. We suggest a criterion as the quantitative standard for the stability of the diquark. It is the gap between the masses of the diquark and $\sqrt{s_0}$ where $s_0$ is the threshold of the excited states and continuity, namely the larger the gap is, the more stable the diquark would be. In this work, we calculate the masses of the type H-H to complete the series of the spectra of the ground state diquarks. However, as the criterion being taken, we find that all the gaps for the various diquaks are within a small range, especially the gap for the diquark with two heavy quarks which is believed to be a stable structure, is slightly smaller than that for other two types of diquarks, therefore we conclude that because of the large theoretical uncertainty, we cannot use the numerical results obtained with the QCD sum rules to assess the stability of diquarks, but need to invoke other theoretical framework.

hep-ph↗

arXiv2011

Determining $1^{--}$ Heavy Hybrid Masses via QCD Sum Rules

The masses of $1^{--}$ charmonium and bottomonium hybrids are evaluated in terms of QCD sum rules. We find that the ground state hybrid in charm sector lies in $m_{H_c}=4.12\sim 4.79$ GeV, while in bottom sector the hybrid may situated in $m_{H_b} = 10.24\sim 11.15$ GeV. Since the numerical result on charmonium hybrid mass is not compatible with the charmonium spectra, including structures newly observed in experiment, we tempt to conclude that such a hybrid does not purely exist, but rather as an admixture with other states, like glueball and regular quarkonium, in experimental observation. However, our result on bottomonium hybrid coincide with the "exotic structure" recently observed at BELLE.

hep-ph↗

arXiv2011

What do the radiative decays of X(3872) tell us

Since the discovery of X(3872), its structure has been in ceaseless dispute. The data of $X(3872)\rightarrow π^+π^-π^0 J/ψ$ suggest that X(3872) may be a high-spin charmonium-like of $2^{-+}$. In terms of the light front quark model (LFQM) we calculate the rates of the radiative decays $X(3872)\rightarrow J/ψ(ψ')γ$ supposing X(3872) to be a $2^{-+}$ charmonium. Within this framework, our theoretical prediction on $\mathcal{BR}(X(3872)\rightarrowψ(1S)γ)$ is at order of $10^{-3}$ which is slightly lower than the Babar's data but close to the Belle's. Our prediction on $\mathcal{BR}(X(3872)\rightarrowψ'γ)$ is at order of $10^{-5}$ if $ψ'$ is a pure 2S state or $10^{-4}$ if $ψ'$ is a $2S-1D$ mixture, which does not conflict with the upper bound set by the Belle collaboration, but is much lower than the Babar's data. Thus if the future measurement decides the branching ratio of $\mathcal{BR}(X(3872)\rightarrowψ'γ)$ to be much larger than $10^{-4}$, the $2^{-+}$ assignment for X(3872) should be ruled out.

hep-ph↗

arXiv2011

Production of charmed baryon $Λ_c(2940)^+$ at PANDA

In this work we evaluate the production rate of the charmed baryon $Λ_c(2940)^+$ at PANDA. For possible assignments of $Λ_c(2940)^+$: $J^P=1/2^\pm$, $3/2^\pm$ and $5/2^\pm$, the total cross section of $p\bar{p}\to \barΛ_c Λ_c(2940)^+$ is estimated, which may exceed 1 nb. With the designed luminosity ($2\times10^{-32}$cm$^{-2}$/s) of PANDA, our estimate indicates that ten thousand events per day if $Λ_c(2940)^+$ is of $J^P=1/2^+$ or $10^8$ per day if it is of $J^P=5/2^+$ can be expected. Those values actually set the lower and upper limits of the $Λ_c(2940)^+$ production. In addition, we present the Dalitz plot and carry out a rough background analysis of the $Λ_c(2940)^+$ production in the $p\bar{p}\to D^0 p\barΛ_c$ and $p\bar{p}\to Σ_c^{0,++}π^{+,-}\barΛ_c$ processes, which would provide valuable information for accurate determination of the $Λ_c(2940)^+$ identity.

hep-ph↗

arXiv2011

Anomalous dipion invariant mass distribution of the $Υ(4S)$ decays into $Υ(1S) π^{+} π^{-}$ and $Υ(2S) π^{+} π^{-}$

To solve the discrepancy between the experimental data on the partial widths and lineshapes of the dipion emission of $Υ(4S)$ and the theoretical predictions, we suggest that there is an additional contribution which was not taken into account in previous calculations. Noticing that the mass of $Υ(4S)$ is above the production threshold of $B\bar B$, the contribution of the sequential process $Υ(4S)\to B\bar B\to Υ(nS)+S\to Υ(nS)+π^+π^-$ ($n=1,2$) may be sizable, and its interference with that from the direct production would be important. The goal of this work is to investigate if a sum of the two contributions with a relative phase indeed reproduces the data. Our numerical results on the partial widths and the lineshapes $dΓ(Υ(4S)\to Υ(2S,1S)π^+π^-)/d(m_{π^+π^-})$ are satisfactorily consistent with the measurements, thus the role of this mechanism is confirmed. Moreover, with the parameters obtained by fitting the data of the Belle and Babar collaborations, we predict the distributions $dΓ(Υ(4S)\to Υ(2S,1S)π^+π^-)/d\cosθ$ which have not been measured yet.

hep-ph↗

arXiv2011

Geometrical Model for Non-Zero theta_13

Based on Friedberg and Lee's geometric picture by which the tribimaximal Pontecorvo-Maki-Nakawaga-Sakata leptonic mixing matrix is constructed, namely, corresponding mixing angles correspond to the geometric angles among the sides of a cube. We suggest that the three realistic mixing angles, which slightly deviate from the values determined for the cube, are due to a viable deformation from the perfectly cubic shape. Taking the best-fitted results of $θ_{12}$ and $θ_{23}$ as inputs, we determine the central value of $\sin^22θ_{13}$ should be 0.0238, with a relatively large error tolerance; this value lies in the range of measurement precision of the Daya Bay experiment and is consistent with recent results from the T2K Collaboration.

hep-ph↗

arXiv2011

Vertex functions for d-wave mesons in the light-front approach

While the light-front quark model (LFQM) is employed to calculate hadronic transition matrix elements, the vertex functions must be pre-determined. In this work we derive the vertex functions for all d-wave states in this model. Especially, since both of $^3D_1$ and $^3S_1$ are $1^{--}$ mesons, the Lorentz structures of their vertex functions are the same. Thus when one needs to study the processes where $^3D_1$ is involved, all the corresponding formulas for $^3S_1$ states can be directly applied, only the coefficient of the vertex function should be replaced by that for $^3D_1$. The results would be useful for studying the newly observed resonances which are supposed to be d-wave mesons and furthermore the possible 2S-1D mixing in $ψ'$ with the LFQM.

hep-ph↗