arXiv subjects

Xilin Zhang

Publications and source records attributed to Xilin Zhang.

51 records · Page 3Linked to original sources

arXiv2015

Forward Compton Scattering with weak neutral current: constraints from sum rules

We generalize forward real Compton amplitude to the case of the interference of the electromagnetic and weak neutral current, formulate a low-energy theorem, relate the new amplitudes to the interference structure functions and obtain a new set of sum rules. We address a possible new sum rule that relates the product of the axial charge and magnetic moment of the nucleon to the 0th moment of the structure function $g_5(ν,0)$. We apply the GDH and the finite energy sum rule for constraining the dispersive $γZ$-box correction to the proton's weak charge.

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arXiv2014

Hard Probe of Geometry and Fluctuations in Heavy Ion Collisions at $\sqrt{s_{_{NN}}}=$ 0.2, 2.76, and 5.5 TeV

[Background:] A significant quenching of high energy jets was observed in the heavy ion collisions at the Relativistic Heavy Ion Collider (RHIC) facility, and is now confirmed at the Large Hadron Collider (LHC) facility. The RHIC plus LHC era provides a unique opportunity to study the jet-medium interaction that leads to the jet quenching, and the medium itself at different collision energies (medium temperatures). [Purpose:] We study the azimuthal anisotropy of jet quenching, to seek constraints on different models featuring distinct path-length and density dependences for jet energy loss, and to gain a better understanding of the medium. [Methods:] The models are fixed by using the RHIC data, and then applied to study the LHC case. A set of harmonic (Fourier) coefficients $v_{n}$ are extracted from the jet azimuthal anisotropy on a event-by-event basis. [Results:] The second harmonics $v_{2}$, mostly driven by the medium's geometry, can be used to differentiate jet quenching models. Other harmonics are also compared with the LHC (2.76 TeV) data. The predictions for future LHC (5.5 TeV) run are presented. [Conclusions:] We find that a too strong path-length dependence (e.g., cubic) is ruled out by the LHC $v_{2}$ data, while the model with a strong near-$T_c$ enhancement for the jet-medium interaction describes the data very well. It is worth pointing out that the latter model expects a less color-opaque medium at LHC.

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arXiv2014

Marrying ab initio calculations and Halo EFT: the case of ${}^7{\rm Be} + p \rightarrow {}^8{\rm B} + γ$

We report a leading-order (LO) calculation of $^7\mathrm{Be}(p,γ)^8\mathrm{B}$ in a low-energy effective field theory. $^8\mathrm{B}$ is treated as a shallow proton$+^7\mathrm{Be}$ core and proton$+^7\mathrm{Be}^{*}$ (core excitation) $p$-wave bound state. The couplings are fixed using measured binding energies and proton-$^7\mathrm{Be}$ $s$-wave scattering lengths, together with $^8\mathrm{B}$ asymptotic normalization coefficients from ab initio calculations. We obtain a zero-energy $S$-factor of $18.2 \pm 1.2~({\rm ANC~only})$ eV b. Given that this is a LO result it is consistent with the recommended value $S(0)=20.8\pm1.6$ eV b. Our computed $S(E)$ compares favorably with experimental data on $^7\mathrm{Be}(p,γ)^8\mathrm{B}$ for $E <0.4$ MeV. We emphasize the important role of proton-$^7\mathrm{Be}$ scattering parameters in determining the energy dependence of $S(E)$, and demonstrate that their present uncertainties significantly limit attempts to extrapolate these data to stellar energies.

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arXiv2014

Jet Quenching and Its Azimuthal Anisotropy in AA and possibly High Multiplicity pA and dA Collisions

We report our calculation of jet quenching and its azimuthal anisotropy in the high energy AA and high multiplicity pA and dA collisions. The purpose of this study is twofold. First, we improve our previous event-by-event studies, by properly implementing $p_t$ dependence in the modeling. We show that, within the jet "monography" scenario featuring a strong near-Tc-enhancement of jet energy loss, the computed high-$p_t$ nuclear modification factor $R_{AA}$ and the harmonic coefficients in its azimuthal anisotropy $v_{2,\,3,\,4}$, agree well with the available data from both Relativistic Heavy Ion Collider (RHIC) and Large Hadron Collider (LHC). Second, in light of current discussions on possible final state collective behavior in the high-multiplicity pPb and dAu collisions, we examine the implication of final state jet attenuation in such collisions, by applying the same model used in AA collisions and quantifying the corresponding $R_{pA}$ and $R_{dA}$ and their azimuthal anisotropy. The high-$p_t$ $V_n$ provide a set of clean indicators of final state energy loss. In particular we find in the most central pPb (5.02 TeV) and dAu (200 GeV) collisions, $V_2$ is on the order of $0.01$ and $0.1$ respectively, measurable with current experimental accuracy. In addition, our high-$p_t$ $R_{dA}$ is around $0.6$ which is compatible with preliminary dAu results from RHIC.

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arXiv2013

Marrying {\it ab initio} calculations and Halo-EFT: the case of ${}^7{\rm Li} + n \rightarrow {}^8{\rm Li} + γ$

We report a leading-order calculation of radiative ${}^{7}\mathrm{Li}$ neutron captures to both the ground and first excited state of ${}^{8}\mathrm{Li}$ in the framework of a low-energy effective field theory (Halo-EFT). Each of the possible final states is treated as a shallow bound state composed of both $n+{}^{7}\mathrm{Li}$ and $n+{}^{7}\mathrm{Li}^{*}$ (core excitation) configurations. The {\it ab initio} variational Monte Carlo method is used to compute the asymptotic normalization coefficients of these bound states, which are then used to fix couplings in our EFT. We calculate the total and partial cross sections in the radiative capture process using this calibrated EFT. Fair agreement with measured total cross sections is achieved and excellent agreement with the measured branching ratio between the two final states is found. In contrast,a previous Halo-EFT calculation [G. Rupak and R. Higa, Phys. Rev. Lett {\bf 106}, 222501 (2011)] assumes that the $n$-${}^{7}\mathrm{Li}$ couplings in different spin channels are equal, fits the $P$-wave "effective-range" parameter to the threshold cross section for ${}^7{\rm Li} + n \rightarrow {}^8{\rm Li} + γ$, and assumes the core excitation is at high enough energy scale that it can be integrated out.

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arXiv2013

Photon neutrino-production in a chiral EFT for nuclei and extrapolation to $E_ν\sim$ GeV region

We carry out a series of studies on pion and photon productions in neutrino/electron/photon--nucleus scatterings. The low energy region is investigated by using a chiral effective field theory for nuclei. The results for the neutral current induced photon production ($γ$-NCP) are then extrapolated to neutrino energy $E_ν\sim$ GeV region. By convoluting the cross sections with MiniBooNE's beam spectrum and detection efficiency, we estimate its $γ$-NCP, event number, and conclude that such photon production can not fully explain its low energy event excess in both neutrino and antineutrino runs.

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arXiv2013

Event-by-event azimuthal anisotropy of jet quenching in relativistic heavy ion collisions

In this paper we present with full details a systematic quantification, on an even-by-event basis, of the hard probe response to the geometry and fluctuations of the hot QCD matter created in heavy ion collisions at both the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC). The azimuthal anisotropy of jet quenching is extracted and decomposed as harmonic responses (for n=1,2,3,4,5,6) to the corresponding harmonics in the fluctuating initial condition. We show that such jet response harmonics are sensitive to the jet quenching models as well as to the bulk matter initial compositions. By studying these for all centralities at both RHIC and LHC energies we put a strong constraint on the path-length and medium-density dependence of jet energy loss. We also examine the hard-soft di-hadron correlation arising from the hard and soft sectors' responses to the common initial fluctuations. We demonstrate that the experimentally observed "hard-ridge" can be explained this way and that its dependence on the trigger-azimuthal-angle could also be qualitatively understood.

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arXiv2013

Can neutrino-induced photon production explain the low energy excess in MiniBooNE?

This report summarizes our study of Neutral Current (NC)-induced photon production in MiniBooNE, as motivated by the low energy excess in this experiment [A. A. Aquilar-Arevalo et al. (MiniBooNE Collaboration), Phys. Rev. Lett. 98, 231801 (2007); 103, 111801 (2009)]. It was proposed that NC photon production with two anomalous photon-$Z$ boson-vector meson couplings might explain the excess. However, our computed event numbers in both neutrino and antineutrino runs are consistent with the previous MiniBooNE estimate that is based on their pion production measurement. Various nuclear effects discussed in our previous works, including nucleon Fermi motion, Pauli blocking, and the $Δ$ resonance broadening in the nucleus, are taken into account. Uncertainty due to the two anomalous terms and nuclear effects are studied in a conservative way.

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arXiv2013

Azimuthally fluctuating magnetic field and its impacts on observables in heavy-ion collisions

The heavy-ion collisions can produce extremely strong transient magnetic and electric fields. We study the azimuthal fluctuation of these fields and their correlations with the also fluctuating matter geometry (characterized by the participant plane harmonics) using event-by-event simulations. A sizable suppression of the angular correlations between the magnetic field and the 2nd and 4th harmonic participant planes is found in very central and very peripheral collisions, while the magnitudes of these correlations peak around impact parameter b~8-10 fm for RHIC collisions. This can lead to notable impacts on a number of observables related to various magnetic field induced effects, and our finding suggests that the optimal event class for measuring them should be that corresponding to b~8-10 fm.

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arXiv2012

Coherent Neutrinoproduction of Photons and Pions in a Chiral Effective Field Theory for Nuclei

In this paper we study coherent neutrinoproduction of photons [neutral current (NC)] and pions [charged current (CC) and NC] with $E_ν \leqslant 0.5 $ GeV. The production from nucleons and incoherent production with $E_ν \leqslant 0.5 \mathrm{GeV}$ have been studied in [B. D. Serot and X. Zhang, Phys. Rev. C 86, (2012) 015501; and X. Zhang and B. D. Serot, Phys. Rev. C 86, (2012) 035502]. These processes are relevant to the background analysis in neutrino-oscillation experiments, for example MiniBooNE [A. A. Aquilar-Arevalo et al. (MiniBooNE Collaboration), Phys. Rev. Lett. 100, 032301 (2008)]. We work in the framework of a Lorentz-covariant effective field theory (EFT), which contains nucleons, pions, the Delta (1232) ($Δ$s), isoscalar scalar ($σ$) and vector ($ω$) fields, and isovector vector ($ρ$) fields. The Lagrangian exhibits a nonlinear realization of (approximate) $SU(2)_{L} \otimes SU(2)_{R}$ chiral symmetry and incorporates vector meson dominance. In the calculation, a revised version of the so-called "optimal approximation" is applied, in which one-nucleon interaction amplitude is factorized out and its medium-modifications are included. The distortion of final pion wave function is calculated using the Eikonal approximation. In addition, we briefly mention the ambiguity of off-shell interaction amplitude. To benchmark the approximation scheme, we study coherent pion photoproduction. Our scheme is then applied to study the neutrinoproductions. In the pion photoproduction and NC photon production, we are able to address the contributions of two contact terms that are partially related to the proposed anomalous interactions involving $ω(ρ)$ $Z$ boson and photon. The calculation is focused on ${}^{12}C$ target, and can be applied to other medium-heavy nucleus.

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arXiv2012

Neutrinoproduction of Photons and Pions From Nucleons in a Chiral Effective Field Theory for Nuclei

Neutrino-induced productions (neutrinoproduction) of photons and pions from nucleons and nuclei are important for the interpretation of neutrino-oscillation experiments, as they are potential backgrounds in the MiniBooNE experiment [A. A. Aquilar-Arevalo et al. (MiniBooNE Collaboration), Phys. Rev. Lett. {\bf 100}, 032301 (2008)]. These processes are studied at intermediate energies, where the Δ(1232) resonance becomes important. The Lorentz-covariant effective field theory, which is the framework used in this series of study, contains nucleons, pions, Δs, isoscalar scalar (σ) and vector (ω) fields, and isovector vector (ρ) fields. The lagrangian exhibits a nonlinear realization of (approximate) $SU(2)_L \otimes SU(2)_R$ chiral symmetry and incorporates vector meson dominance. In this paper, we focus on setting up the framework. Power counting for vertices and Feynman diagrams is explained. Because of the built-in symmetries, the vector current is automatically conserved (CVC), and the axial-vector current is partially conserved (PCAC). To calibrate the axial-vector transition current (N $\leftrightarrow$ Δ), pion production from the nucleon is used as a benchmark and compared to bubble-chamber data from Argonne and Brookhaven National Laboratories. At low energies, the convergence of our power-counting scheme is investigated, and next-to-leading-order tree-level corrections are found to be small.

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arXiv2012

Incoherent Neutrinoproduction of Photons and Pions in a Chiral Effective Field Theory for Nuclei

We study the incoherent neutrinoproduction of photons and pions with neutrino energy E_ν $\leqslant$ 0.5 GeV. These processes are relevant to the background analysis in neutrino-oscillation experiments, for example MiniBooNE [A. A. Aquilar-Arevalo \textit{et al.} (MiniBooNE Collaboration), Phys. Rev. Lett. 100, 032301(2008)]. The calculations are carried out using a Lorentz-covariant effective field theory (EFT) which contains nucleons, pions, the Delta (1232) ($Δ$ s), isoscalar scalar ($σ$) and vector ($ω$) fields, and isovector vector ($ρ$) fields, and has SU(2)_{L} $\otimes$ SU(2)_{R} chiral symmetry realized nonlinearly. The contributions of one-body currents are studied in the local fermi gas approximation. The current form factors are generated by meson dominance in the EFT Lagrangian. The conservation of the vector current and the partial conservation of the axial current are satisfied automatically, which is crucial for photon production. The $Δ$ dynamics in nuclei, as a key component in the study, are explored. Introduced $Δ$-meson couplings explain the $Δ$ spin-orbit (S-L) coupling in nuclei, and this leads to interesting constraints on the theory. Meanwhile a phenomenological approach is applied to parametrize the $Δ$ width. To benchmark our approximations, we calculate the differential cross sections for quasi-elastic scattering and incoherent electroproduction of pions without a final state interaction (FSI). The FSI can be ignored for photon production.

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arXiv2012

Jet Tomography of Harmonic Fluctuations in the Initial Condition of Heavy Ion Collisions

In this paper we study the jet response (particularly azimuthal anisotropy) as a hard probe of the harmonic fluctuations in the initial condition of central heavy ion collisions. By implementing the fluctuations via cumulant expansion for various harmonics quantified by $ε_n$ and using the geometric model for jet energy loss, we compute the response $χ^h_n=v_n/ε_n$. Combining these results with the known hydrodynamic response of the bulk matter expansion in the literature, we show that the hard-soft azimuthal correlation arising from their respective responses to the common geometric fluctuations reveals a robust and narrow near-side peak that may provide the dominant contribution to the "hard-ridge" observed in experimental data.

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arXiv2011

Electroweak Interactions in a Chiral Effective Lagrangian for Nuclei

We have studied electroweak (EW) interactions in quantum hadrodynamics (QHD) effective field theory (EFT). The Lorentz-covariant EFT contains nucleon, pion, $Δ$, isoscalar scalar ($σ$) and vector ($ω$) fields, and isovector vector ($ρ$) fields. The lagrangian exhibits a nonlinear realization of (approximate) $SU(2)_L \otimes SU(2)_R$ chiral symmetry and incorporates vector meson dominance. First, we discuss the EW interactions at the quark level. Then we include EW interactions in QHD EFT by using the background-field technique. The completed QHD EFT has a nonlinear realization of $SU(2)_L \otimes SU(2)_R \otimes U(1)_B$ (chiral symmetry and baryon number conservation), as well as realizations of other symmetries including Lorentz-invariance, $C$, $P$, and $T$. Meanwhile, as we know, chiral symmetry is manifestly broken due to the nonzero quark masses; the $P$ and $C$ symmetries are also broken because of weak interactions. These breaking patterns are parameterized in a general way in the EFT. Moreover, we have included the $Δ$ resonance as manifest degrees of freedom in our QHD EFT, with a discussion of the irrelevance of the well-known pathologies involving high-spin fields from the modern EFT perspective. This enables us to discuss physics at the kinematics where the resonance becomes important. As a result, the effective theory uses hadronic degrees of freedom, satisfies the constraints due to QCD (symmetries and their breaking pattern), and is calibrated to strong-interaction phenomena. Applications to (anti)neutrino scattering are briefly discussed.

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arXiv2010

Weak Pion and Photon Production off Nucleons in a Chiral Effective Field Theory

Neutrino-induced pion and photon production from nucleons and nuclei are important for the interpretation of neutrino-oscillation experiments, and these processes are potential backgrounds in the MiniBooNE experiment [A. A. Aquilar-Arevalo \textit{et al.} (MiniBooNE Collaboration), Phys.\ Rev.\ Lett.\ {\bf 100}, 032301 (2008)]. Pion and photon production are investigated at intermediate energies, where the $Δ$ resonance becomes important. The Lorentz-covariant effective field theory contains nucleons, pions, Deltas, isoscalar scalar ($σ$) and vector ($ω$) fields, and isovector vector ($ρ$) fields. The lagrangian exhibits a nonlinear realization of (approximate) $SU(2)_L \otimes SU(2)_R$ chiral symmetry and incorporates vector meson dominance. Power counting for vertices and Feynman diagrams involving the $Δ$ is explained. Because of the built-in symmetries, the vector currents are automatically conserved, and the axial-vector currents satisfy PCAC. The irrelevance of so-called off-shell $Δ$ couplings and the structure of the dressed $Δ$ propagator, which has a pole only in the spin-3/2 channel, are discussed. To calibrate the axial-vector transition current $(N\! \leftrightarrow Δ)$, pion production from the nucleon is used as a benchmark and compared to bubble-chamber data from Argonne and Brookhaven National Laboratories. At low energies, the convergence of our power-counting scheme is investigated, and next-to-leading-order tree-level corrections are found to be very small.

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