PHYSICAL REVIEW D 103, 033007 (2021) - Exploring the double-slit interference with linearly polarized photons
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PHYSICAL REVIEW D 103, 033007 (2021) Exploring the double-slit interference with linearly polarized photons Wangmei Zha ,1 James Daniel Brandenburg ,2,3,* Lijuan Ruan,3 and Zebo Tang 1,† 1 University of Science and Technology of China, 230026 Hefei, China 2 Shandong University, 250100 Jinan, China 3 Brookhaven National Laboratory, New York 11973, USA (Received 2 July 2020; revised 5 November 2020; accepted 19 January 2021; published 26 February 2021) The linearly polarized quasireal photons from the highly Lorentz-contracted Coulomb fields of relativistic heavy ions can fluctuate to quark-antiquark pairs, scatter off a target nucleus, and emerge as vector mesons. In the process, the two colliding nuclei can serve as slits, forming a Young’s double-slit experiment. In addition to the well-known double-slit interference pattern in position space, a similar interference pattern may be expected in polarization space due to the linear polarization of the colliding photons. In this paper, we investigate the interference effect in polarization space as revealed by the asymmetries of the decay angular distribution for vector meson photoprodution in heavy-ion collisions. We find a periodic oscillation pattern with transverse momentum, which can reasonably explain the second-order modulation in azimuth for the ρ0 decay observed by the STAR Collaboration. DOI: 10.1103/PhysRevD.103.033007 The double-slit interference experiment plays an essen- decay products in an entangled state, which is an illus- tial role in expressing the central puzzle of quantum tration of the Einstein-Podolsky-Rosen paradox. The dou- mechanics—wave-particle complementarity. Such wave- ble-slit interference effect of photonuclear vector meson particle duality continues to be challenged [1–5] and production in momentum space has been proposed by explored in a broad range of entities [6–11]. The dou- Klein and Nystrand [13] and verified by the STAR ble-slit experiment could also be performed in relativistic Collaboration through measurements of the ρ0 in ultra- heavy-ion collisions utilizing the vector meson from peripheral collisions (UPCs) [14]. In Refs. [15,16], we coherent photoproduction. The highly Lorentz-contracted extended the scenario to hadronic heavy-ion collisions and electromagnetic field accompanied by the heavy nuclei can proposed to exploit the strong interactions in the overlap be viewed as a spectrum of quasireal photons [12]. The region to test the observation effect on the interference. photon emitted from one nucleus can fluctuate to a quark- The photons generated from the highly Lorentz- antiquark pair which then scatters off the other nucleus, contracted electromagnetic field are expected to be fully emerging as a real vector meson. In the production process, linearly polarized. It has been suggested by Li et al. [17] the elastic scattering occurs via Pomeron exchange, which that the collisions of the linearly polarized photons imposes a restriction on the production site within one of (γ þ γ → lþ þ l− ) result in the second-order and fourth- the two colliding nuclei. This indicates that the production order modulations in azimuth (in the plane perpendicular to source consists of two well-separated nuclei (two slits). the beam direction) between the pair momentum and the There are two possibilities: Either nucleus 1 emits a photon lepton momentum for dilepton production. The experimen- and nucleus 2 acts as a target, or vice versa. The two tal signature was confirmed by the STAR Collaboration possibilities are indistinguishable, which present the pro- for the dielectron measurements [18]. The vector meson duction process as a perfectly double-slit experiment. production from linearly polarized photons also possesses Furthermore, for the short-lived vector meson (e.g., ρ0 ), a distinctive signature in the asymmetries of the decay it decays before the wave functions from the two slits can angular distributions. Continuous efforts [19–26] has been overlap, and the interference entity is the complete set of made for more than 50 years to utilize the linearly polarized photons in vector meson photoproduction as a parity filter * dbrandenburg.ufl@gmail.com for the exchange of particles in the t channel, which is an † zbtang@ustc.edu.cn effective tool to separate the natural from unnatural parity exchange in the t channel. Since the photons are in a linear Published by the American Physical Society under the terms of polarization state, the interference can reveal itself in the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to polarization space, inducing an asymmetry in the decay the author(s) and the published article’s title, journal citation, angular distributions. In this paper, we exploit the linearly and DOI. Funded by SCOAP3. polarized photons to investigate the interference pattern in 2470-0010=2021=103(3)=033007(6) 033007-1 Published by the American Physical Society
ZHA, BRANDENBURG, RUAN, and TANG PHYS. REV. D 103, 033007 (2021) z z' V rest frame π θ ϕ Φ x y x' FIG. 1. The coordinate system for the measurement of a two- FIG. 2. Schematic diagram for the direction of electric vector of body decay angular distribution in the vector meson rest frame. the photons, which hit on the target nuclei, in ultraperipheral The definition of the x, y, and z axes is described in the text. The heavy-ion collisions. z0 axis denotes the beam direction, and the x0 axis represents the linear polarization vector. and to spin 1=2 products (e.g., ρ0 → eþ þ e− ) gives d2 N 3 2 sin2 θ polarization space for the vector meson photoproduction in ¼ ð1 þ cos θÞ 1 − cos 2ðϕ − ΦÞ : heavy-ion collisions and demonstrate that the signature is a d cos θdϕ 16π 1 þ cos2 θ periodic oscillation in asymmetries of the decay angular ð2Þ distributions with transverse momentum. Hereinafter, we take the process of γ þ A → V þ A → As revealed in Eqs. (1) and (2), the linearly polarized states dþ þ d− þ A in heavy-ion collisions to illustrate the result in the second-order modulations in azimuth, and the interference effect on the asymmetries of the decay. The strength of the second-order modulation is given by decay distribution depends on the choice of the coordinate system, with respect to which the momentum of one of the 2hcosð2ϕÞi ¼ cosð2ΦÞ ð3Þ two decay products is expressed in spherical coordinates. Herein, the right-handed coordinate system for vector and meson decay is built up as follows: The z axis is chosen to the direction of flight of the vector meson in the photon- sin2 θ 2hcosð2ϕÞi ¼ − cosð2ΦÞ; ð4Þ nucleon center of mass frame; the y axis is normal to the 1 þ cos2 θ photoproduction plane; and the x axis is given by y × z. As illustrated in Fig. 1, the decay angles θ and ϕ are the polar for the ρ0 → π þ þ π − and ρ0 → eþ þ e− cases, respec- and azimuthal angles, respectively, of the unit vector π̃, tively. The modulation strength is determined by the which denotes the direction of flight of one of the decay direction of the linear polarization, and the orientation of particles in the vector meson rest frame. In experiment, the the photon polarization is determined by the direction of the direction of the z axis is approximated by that of incoming electric field vector. As demonstrated in Fig. 2, the electric beams (shown as the z0 axis in Fig. 1). It has been verified field vector is parallel to the impact parameter at leading by Monte Carlo calculations that this is a good approxi- order. That is to say that the modulations of the decay mation. Here, we adopt the approximation in our calcu- distribution for vector meson are determined by the lation for direct comparisons with experimental results. anisotropy in its two-dimensional transverse momentum The angle Φ shown in Fig. 1 denotes the angle between distribution with respect to the impact parameter at leading the photon polarization plane and vector meson produc- order. The variation of polarization direction due to the tion plane. finite size of nuclei should be small in ultraperipheral Under the helicity no-flip assumption, the vector meson collisions, which could be investigated in future work. inherits the photon polarization state, which is fully linearly The two-dimensional transverse momentum distribution polarized. The helicity conservation assumption has of the vector meson from coherent photoproduction can be been investigated by various experimental measurements obtained by performing a Fourier transformation of the [20–25]. Following Ref. [27] and the derivation in the coordinate space amplitude: Appendix, the decay angular distribution of vector meson Z 2 d2 P 1 to two spinless products (e.g., ρ0 → π þ þ π − ) is ¼ 2 d x⊥ ðA1 ðx⊥ Þ þ A2 ðx⊥ ÞÞeip⊥ ·x⊥ ð5Þ dpx dpy 2π ; d2 N 3 where A1 ðx⊥ Þ and A2 ðx⊥ Þ are the amplitude distributions in ¼ sin2 θ½1 þ cos 2ðϕ − ΦÞ; ð1Þ d cos θdϕ 8π the transverse plane for the two colliding nuclei. Consider, 033007-2
EXPLORING THE DOUBLE-SLIT INTERFERENCE WITH … PHYS. REV. D 103, 033007 (2021) (a) (b) (c) A 1 = A2 A1 = 0.5A2 0.4 0.4 0.2 0.2 2〈cos2φ〉 2〈cos2φ〉 0 0 −0.2 −0.2 −0.4 −0.4 0 0.05 0.1 0.15 0.2 0.25 0.3 0 0.05 0.1 0.15 0.2 0.25 0.3 p GeV/c p GeV/c T T FIG. 3. (a) Two-dimensional transverse momentum distribution pattern with pointlike assumption at midrapidity. (b) The modulation strength 2hcos ϕi as a function of transverse momentum at midrapidity. (c) 2hcos ϕi as a function of transverse momentum at forward rapidity (A1 ≠ A2 ). to leading order, we can approximate the production photon approximation, the spatial photon flux generated by over the two nuclei as two point sources: A1;2 ðx⊥ Þ ¼ the heavy nuclei can be written as A1;2 δðx⊥ 12 bÞ, where A1 and A2 are the relative ampli- Z tudes for the two colliding nuclei, δ is the Dirac δ function, 4Z2 α d2 k⃗ γ⊥ ⃗ Fγ ðk⃗ γ Þ i⃗x ·k⃗ 2 nðω; x⃗ ⊥ Þ ¼ k e ⊥ γ⊥ ; b is the impact parameter, and the minus sign in A2 ðx⊥ Þ ωγ ð2πÞ2 γ⊥ jk⃗ γ j2 results from the negative parity of the vector meson. With the interference effect, as demonstrated in Fig. 3(a) for the ⃗kγ ¼ k⃗ γ⊥ ; ωγ ; 1 ωγ ¼ MV ey ; ð6Þ case with b ¼ 20 fm at midrapidity (A1 ¼ A2 ), the trans- γc 2 verse momentum reveals a typical Young’s double-slit interference pattern with a series of alternating light and where x⃗ ⊥ and k⃗ γ⊥ are two-dimensional photon position and dark fringes, which corresponds to significant asymmetries momentum vectors, respectively, perpendicular to the beam in azimuth. According to Eq. (3), the modulation strength direction, ωγ is the energy of the emitted photon, Z is the factor of vector meson decay to two spinless products, electric charge of the nucleus, α is the electromagnetic 2hcosð2ϕÞi, versus transverse momentum (pT ) can be coupling constant, γ c is the Lorentz factor of the beam, MV extracted from the two-dimensional distribution. The result and y are the mass and rapidity, respectively, of the vector is shown in Fig. 3(b). The modulation strength shows a meson, and Fγ ðk⃗ γ Þ is the nuclear electromagnetic form periodic oscillation like the hyperbolic Bessel functions factor. The form factor can be obtained via the Fourier with the transverse momentum. The oscillation cycle is transformation of the charge density in the nucleus. The determined by the distance between the two source (impact charge density profile of the nucleus can be parametrized parameter). For production at forward rapidity in heavy-ion by the Woods-Saxon distribution: collisions, the relative amplitudes from the two nuclei are not equal. Figure 3(c) shows the modulation strength as a a0 function of transverse momentum with unequal amplitudes ρA ðrÞ ¼ ; ð7Þ 1 þ exp½ðr − RWS Þ=d from the two sources. Since the interference is not complete in this case, the oscillation level is weaker than that at where the radius RWS and skin depth d are based on fits midrapidity. Furthermore, the modulation strength is 0 at to electron-scattering data [28] and a0 is the normali- pT ¼ 0, while it is maximum for the case at midrapidity. zation factor. As demonstrated in Eq. (6), the polarization This results since the interference contribution is close to 0 direction for the quasireal photons follows the position for pT ¼ 0. vector x⃗ ⊥. As mentioned herein before, we have qualitatively The scattering amplitude ΓγA→VA with the shadowing demonstrated the interference effect on the asymmetries effect can be estimated via the Glauber [29] þ vector meson of the decay angular distributions for vector meson photo- dominance (VMD) approach [30]: production. The quantitative calculations are described hereinafter. Following Ref. [15], the production amplitude f γN→VN ð0Þ σ VN 0 distribution for vector meson photoproduction is deter- ΓγA→VA ð⃗x⊥ Þ ¼ 2 1 − exp − T ð⃗x⊥ Þ ; σ VN 2 mined by the spatial photon flux and the corresponding γA scattering amplitude ΓγA→VA . According to equivalent ð8Þ 033007-3
ZHA, BRANDENBURG, RUAN, and TANG PHYS. REV. D 103, 033007 (2021) where f γN→VN ð0Þ is the forward-scattering amplitude for where σ NN is the total nucleon-nucleon cross section. Then, γ þ N → V þ N and σ VN is the total VN cross section. To the probability of having no hadronic interaction is account for the coherent length effect, the modified thick- exp½−PH ðbÞ. In experiment, the UPC events are usually ness function T 0 ð⃗x⊥ Þ is defined as triggered or selected by the mutual Coulomb dissociation of the nuclei, in which the number of emitted forward Z þ∞ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi M V ey neutrons can be detected by zero degree calorimeters. T 0 ð⃗x⊥ Þ ¼ ρ x⃗ 2⊥ þ z2 eiqL z dz; qL ¼ ; −∞ 2γ c Following the EPA approach, the Coulomb excitation probability of an ultrarelativistic nucleus can be determined ð9Þ by the photon flux accompanied with the nucleus and the appropriate photon-absorption cross section of nuclei. The where qL is the longitudinal momentum transfer required lowest-order probability for an excitation of nucleus which to produce a real vector meson. The f γN→VN ð0Þ can be emits at least one neutron (Xn ) is determined from the measurements of forward-scattering dσ Z cross section γN→VN jt¼0 , which is well parametrized in dt mXn ðbÞ ¼ dωnðω; bÞσ γA→A ðωÞ; ð14Þ Ref. [31]. Using the optical theorem and VMD relation, the total cross for VN scattering can be given by where nðω; bÞ is photon flux described by Eq. (6) and σ VN f ¼ pVffiffiffi f γV→VN ; ð10Þ σ γA→A ðωÞ is the photoexcitation cross section with incident 4 αC energy ω. The photoexcitation cross section σ γA→A ðωÞ can be extracted from the experimental measurements [33–39]. where f V is the V-photon coupling and C is a correction The probability of mutual dissociation for the two nuclei factor for the nondiagonal coupling through higher mass with at least one neutron emission for each beam (XnXn) is vector mesons. Finally, the production amplitude can be then given by given by pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi PXnXn ðbÞ ¼ ð1 − exp½−mXn ðbÞÞ2 : ð15Þ Aðx⃗ ⊥ Þ ¼ ΓγA→VA nðω; x⃗ ⊥ Þ: ð11Þ As described in our recent work [40], the mutual dissoci- In addition to the coherent production with interference, ation probability with any number of neutron emission can there are also an incoherent contribution in heavy-ion be estimated in a similar way. collisions, which is significant at relative high transverse With the inputs described before, the cross section and momentum. The incoherent production should have no differential distributions for vector meson photoproduction contribution to the asymmetries of decay distribution, can be calculated and can reasonably describe the mea- which would weaken the overall modulation strength. surements from STAR [24,41] and ALICE [42,43]. The Similarly to the case of coherent vector meson photo- estimated modulation strength 2hcosð2ϕÞi of ρ0 → π þ þ production on nuclei, the incoherent cross section σ γA→VA0 π − and ρ0 → eþ þ e− as a function of pT in ultraperipheral could be related to the exclusive γp cross section σ γp→Vp pffiffiffiffiffiffiffiffi Au þ Au collisions at sNN ¼ 200 GeV for mutual dis- via the Glauber þ VMD approach. Here, A0 is the final sociation mode XnXn are shown in Fig. 4. The modulation nuclear state containing products of the nuclear disintegra- strength shows a periodic oscillation with the transverse tion. The approach gives momentum. The amplitude of the second oscillation period Z is weaker than that of the first oscillation period, which is σ γA→VA0 ¼ σ γp→Vp d2 x⃗ ⊥ Tð⃗x⊥ Þe−ð1=2Þσ VN Tð⃗x⊥ Þ ; in ð12Þ due to a larger fraction of incoherent production at higher pT . The oscillation feature completely disappears for pT > 0.16 GeV=c, where the incoherent production plays where σ in 2 VN ¼ σ VN − σ VN =ð16πBV Þ is the inelastic vector the dominate role in the photoproduction. As expected in meson-nucleon cross section and BV is the slope of the t Eqs. (3) and (4), the modulation strength for ρ0 → eþ þ e− dependence of the γp → Vp scattering [31]. is weaker and possesses opposite sign in comparison with Here, we focus on the ultraperipheral collisions, where that for ρ0 → π þ þ π −. Furthermore, it depends on the there is no nuclear overlap to reject hadronic background. acceptance, and the calculations for ρ0 → eþ þ e− are According to the optical Glauber model [32], the mean performed with STAR acceptance as listed in the figure. number of projectile nucleons that interact at least once in The results for ρ0 → π þ þ π − can reasonably describe A þ A collisions with impact parameter b is the transverse momentum dependence of the second- Z order modulation in azimuth observed by the STAR PH ðbÞ ¼ ⃗ d2 x⃗ ⊥ Tð⃗x⊥ − bÞf1 − exp½−σ NN Tð⃗x⊥ Þg; ð13Þ Collaboration [44]. The predictions for ρ0 → eþ þ e− call for further experimental testing. 033007-4
EXPLORING THE DOUBLE-SLIT INTERFERENCE WITH … PHYS. REV. D 103, 033007 (2021) 0.5 jVi ¼ aþ1 j þ 1i þ a−1 j − 1i þ a0 j0i: ðA1Þ 0.4 photoproduction ρ0 → π+ + π- Au + Au 200 GeV ρ0 → e+ + e- 0.3 The calculation is performed in the vector meson rest XnXn |y|0.2 GeV/c |ηe|
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