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Asymmetry in the neutrino and anti-neutrino reactions in a nuclear medium
Physics Letters B 723 (2013) 464–469
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Physics Letters B www.elsevier.com/locate/physletb
Asymmetry in the neutrino and anti-neutrino reactions in a nuclear medium Myung-Ki Cheoun a,∗ , Ki-Seok Choi a , K.S. Kim b , Koichi Saito c , Toshitaka Kajino d,e , Kazuo Tsushima f,g , Tomoyuki Maruyama h a
Department of Physics, Soongsil University, Seoul 156-743, Republic of Korea School of Liberal Arts and Science, Korea Aerospace University, Koyang 412-791, Republic of Korea c Department of Physics, Faculty of Science and Technology, Tokyo University of Science, Noda 278-8510, Japan d National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan e Department of Astronomy, Graduate School of Science, University of Tokyo, 7-3-1 Hongo, Tokyo 113-0033, Japan f CSSM, School of Chemistry and Physics, University of Adelaide, Adelaide, SA 5005, Australia g International Institute of Physics (IIP), Federal University of Rio Grande do Norte (UFRN), Natal/RN 59078-400, Brazil h College of Bioresource Sciences, Nihon University, Fujisawa 252-8510, Japan b
a r t i c l e
i n f o
Article history: Received 5 March 2013 Received in revised form 21 April 2013 Accepted 20 May 2013 Available online 23 May 2013 Editor: W. Haxton Keywords: Neutrino-induced reactions Weak and electro-magnetic form factors Nuclear matter Proto-neutron stars Density dependence
a b s t r a c t We study the effect of the density-dependent axial and vector form factors on the electron–neutrino (νe ) and anti-neutrino (ν¯ e ) reactions for a nucleon in nuclear matter or in 12 C. The nucleon form factors in free space are presumed to be modified for a bound nucleon in a nuclear medium. We adopt the density-dependent form factors calculated by the quark–meson coupling (QMC) model, and apply them to the νe and ν¯ e induced reactions with the initial energy E = 8–80 MeV. We find that the total νe cross sections on 12 C as well as on a nucleon in nuclear matter are reduced by about 5% at the nuclear saturation density, ρ0 . This reduction is caused by the modification of the nucleon structure in matter. Although the density effect for both cases is relatively small, it is comparable with the effect of Coulomb distortion on the outgoing lepton in the ν -reaction. In contrast, the density effect on the ν¯ e reaction reduces the cross section significantly in both nuclear matter and 12 C cases, and the amount maximally becomes of about 35% around ρ0 . Such large asymmetry in the νe and ν¯ e cross sections, which seems to be nearly independent of the target, is originated from the differences in the helicities of ν¯ e and νe . It is expected that the asymmetry influences the r-process and also the neutrino-process nucleosynthesis in core-collapse supernovae. © 2013 Elsevier B.V. All rights reserved.
In the core-collapse supernova explosion, the neutrino (ν ) heating was suggested as one of the main mechanisms for the explosion, leading to the so-called ν driven explosions. The central object formed at the core bounce is expected to be a hot and leptonrich proto-neutron star (PNS). Therefore, the shock propagation by the initial bounce from the contraction of heavy nuclei crosses the ν -sphere, i.e. the neutrino energy- and flavor-dependent sphere, and releases vast numbers of neutrinos [1]. These neutrinos propagate through the PNS, whose density is believed to be of about a few times of the nuclear saturation density, ρ0 ∼ 0.15 fm−3 . During the propagation, the neutrinos interact with nucleons in a dense nuclear medium. For example, the asymmetry in ν scattering and absorption in a magnetized PNS may account for the pulsar kick of neutron stars, according to the
*
Corresponding author. E-mail address: [email protected] (M.-K. Cheoun).
0370-2693/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.physletb.2013.05.050
detailed study of the ν transport in dense matter by a relativistic mean field theory [2,3]. Outside of the PNS, the emitted neutrinos also interact with nucleons and the nuclei already produced by the s-process in the progenitor and/or the r-process in the explosion. Around the Si layer, the anti-neutrino (ν¯ ) absorption in the proton-rich environment may produce neutrons immediately captured by the neutrondeficient nuclei, which affect the proton process, dubbed as the ν p process [4]. In the O–Ne–Mg layer, the ν induced reactions may play an important role for producing some p-nuclei which are odd–odd neutron-deficient nuclei. For example, 180 Ta and 138 La in the cosmos are believed to be produced from the ν process [5,6]. Other light nuclei abundances are also closely associated with the neutrino interactions in the He–C layer [7]. Of course, the density of the medium outside the PNS is not so high compared to that of the PNS. However, the nucleons interacting with the neutrinos are strongly bound in a nucleus, and the interactions should thus be different from those in free space.
M.-K. Cheoun et al. / Physics Letters B 723 (2013) 464–469
465
Fig. 1. (Color online.) Effective nucleon mass M ∗ (ρ ) versus the nuclear density ratio ρ /ρ0 (left), and the in-medium axial coupling constant normalized to that in free space (right), R ( g A ) = g A (ρ , Q 2 )/ g A (ρ = 0, Q 2 ), as a function of Q 2 . In the right panel, from the uppermost (for vacuum), the density increases by 0.5ρ0 in order. The lowermost curve is for ρ = 2.5ρ0 .
Recently, strong evidence for the modification of the nucleon structure in nuclear matter has been reported from the proton ) electromagnetic form factors measured in the polarized ( e, e p scattering off 16 O [8] and 4 He [9–12] at MAMI and Jefferson Lab., and also from the study of the neutron properties in a nuclear ) scattering off 4 He [13]. medium through the polarized ( e, en Therefore, it is quite interesting to investigate the possible change in the ν and ν¯ induced reactions due to the variation of the nucleon properties, in order to pin down the ambiguity inherent in the nucleon and/or nuclear structure in the interpretation of various ν reactions in the cosmos. In this Letter, we adopt the density-dependent weak form factors calculated in the quark–meson coupling (QMC) model [14–16]. The quark mass in a hadron can be related to the quark condensate ¯qq in vacuum. The mass (or ¯qq) in nuclear matter may then be reduced from the value in vacuum because of the condensed scalar (σ ) field depending on the nuclear density ρ , namely the Lorentz-scalar, attractive interaction in matter. The decrease of the quark mass in matter leads to the variation of the baryon internal structure at the quark level. Such an effect is considered selfconsistently in the QMC model. As an alternative approach, Smith and Miller have studied the modification of the nucleon structure in nuclear matter using the chiral quark–soliton model [17]. The QMC model has been successfully applied in studying the properties of hadrons in a nuclear medium [18], finite nuclei [19], quasi-elastic (QE) electron scattering off nuclei [20] and hypernuclei [21]. (For a review, see Ref. [22].) There have been many studies on the neutrino–nucleus interactions so far. In particular, the cross sections in the quasi-elastic (QE) and/or production region, where the energy transfer is more than 100 MeV, have intensively been investigated [23] to analyze the recent MiniBooNE data [24]. Furthermore, the neutrino propagation in hot neutron matter has been studied within the framework of the random phase approximation [25]. But, in this Letter, we focus on the energy region of importance in the supernova neutrinos. Therefore we study the ν reactions relevant to the energy range of the LSND experiments [26], where the initial neutrino energy is less than 80 MeV. An initial study for the effect of the density-dependent weak-current form factors on the neutrino scattering can be found in Ref. [27]. However, in that paper, the simple, relativistic Fermi gas model has been used to study the 12 C(νμ , μ− )X reaction, for which only the ν -flux averaged data have been measured. In contrast, in the present calculation, we
treat the nuclear structure of 12 C in terms of quasi-particle random phase approximation (QRPA) [28,29], which enables us to carry out more realistic calculations. We then compare the result with the observed, energy-dependent cross section for the 12 C(νe , e − )12 N g .s. reaction [26]. For more thorough understanding of the density effect, the νe and ν¯ e reactions on a nucleon in nuclear matter are also examined in detail, as well as the ν¯ e reaction on 12 C, because the elementary process may be convenient and useful to see how the variation of the form factors affects the νe and ν¯ e reactions in matter. The weak current operator W μ for the ν induced reaction takes a form of V μ − A μ in the standard electro-weak theory. For a free nucleon, the weak current operator respectively comprises the vector, the axial vector and the pseudo scalar form factors, F iV=1,2 ( Q 2 ), F A ( Q 2 ) and F P ( Q 2 ):
W μ = F 1V Q 2
+ FA
μ i μν γ + F 2V Q 2 σ qν 2M
F P ( Q 2) μ 5 Q 2 γ μγ 5 + q γ , 2M
(1)
where M is the nucleon mass in free space, and Q 2 (= −q2 ) is the four-momentum transfer squared. Because of the conservation of the vector current (CVC) and nonexistence of the second class current, we here ignore the scalar form factor in the vectorial part and the axial-tensor form factor in the axial part. By the CVC hypothesis, the vector and axial form factors for the charged current (C C ) are respectively expressed as [30,31] V ,C C
Fi
p
Q 2 = F i Q 2 − F in Q 2 ,
F CA C Q 2 = − g A / 1 + Q 2 / M 2A p (n) Fi
2
,
(2)
where is the proton (neutron) form factor, and g A and M A are the axial coupling constant and the axial cut off mass, respectively. Before applying to the ν reaction, we briefly discuss the change of the in-medium nucleon properties calculated by the QMC model [14–16]. In Fig. 1 the effective nucleon mass is illustrated in the left panel, which shows a monotonic decrease of the mass while the nuclear density increases. The modification of the axial coupling constant in nuclear matter is also shown in the right panel as a function of Q 2 . Even in the region of small momentum transfer, where most of the ν reactions expected in the cosmos
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M.-K. Cheoun et al. / Physics Letters B 723 (2013) 464–469
V ,C C
V ,C C
Fig. 2. Modification of the weak form factors, R ( F 1V,2 ) = F 1,2 (ρ , Q 2 )/ F 1,2 (ρ = 0, Q 2 ). From the lowermost (for vacuum), the density increases by 0.5ρ0 in order. The uppermost curve is for ρ = 2.5ρ0 .
Fig. 3. (Color online.) Density dependence of the total cross sections for the ν¯e + p → e + + n (left) and νe + n → e − + p (right) in nuclear matter. The black (solid) curves are for the results in free space. The cross sections in both reactions decrease with increasing the density: ρ /ρ0 = 0.5 (red (dashed)), 1.0 (blue (dotted)), 1.5 (yellow (short dotted)), 2.0 (cyan (dot-long-dashed)) and 2.5 (purple (dot-dashed)).
are assumed to occur, the reduction of the axial coupling constant g A (ρ , Q 2 ) (or equivalent to the axial form factor) amounts to 11% at ρ0 . Fig. 2 presents the density dependence of the weak form V ,C C factors given by Eq. (2). The modification of F 2 at small Q 2 V ,C C F1 .
V ,C C F2
is more significant than that of For example, is enhanced by about 11% at ρ0 . Fig. 3 shows the total cross sections for the ν¯ e + p → e + + n and νe + n → e− + p reactions in nuclear matter. Here, we have used the differential cross section formula with the Sachs form factors, G CMC , G CE C and G CAC (= F CA C ) [32]:
dσ
C C
d Q 2 ν (ν¯ )
=
G 2F cos2 θc 1 2π
×
2
yM + 1− y− 2E
(G CE C )2 + y E (G CMC )2 /2M
+
2
y 2 G CMC
1+ y2 2
yE 2M
+1− y+
∓ 2y 1 −
y 2
yM 2E
G CMC G CAC
G CAC
2
,
(3)
M.-K. Cheoun et al. / Physics Letters B 723 (2013) 464–469
Fig. 4. (Color online.) Difference (left),
467
σ − = σ (νe ) − σ (ν¯ e ), and the sum (right), σ + = σ (νe ) + σ (ν¯ e ), of the total cross sections.
Fig. 5. (Color online.) Density dependence of the total cross sections for 12 C(ν¯ e , e + )12 B g .s.(1+ ) (left) and 12 C(νe , e − )12 N g .s.(1+ ) (right). The result denoted by the black (solid) curve (for vacuum) in the right panel is the same as that of the 1+ transition shown in Fig. 1 of Ref. [29]. The measured cross section [26] is also presented in the right panel. V ,C C
where y = Q 2 /(2p · k), G CMC = F 1 2
Q 4M 2
V ,C C F2 ,
+ F 2V ,C C , G CE C = F 1V ,C C −
and E is the initial neutrino or anti-neutrino en-
ergy. The signs ∓ respectively correspond to the
νe ν¯ e
reactions,
and the cross section is multiplied by the Cabbibo-angle factor, cos2 θc . For both reactions, the present results in vacuum (black (solid) curves) are consistent with the previous results calculated in Ref. [33]. For the ν¯e + p → e + + n reaction in matter, the total cross sections decrease by about 15% per each increase (by 0.5ρ0 ) of the density. In contrast, for the νe + n → e − + p reaction, the variation
of the cross section in matter below E ν ∼ 30 MeV is less than 2% (per each increase of the density). Even around E ν ∼ 80 MeV, the cross section maximally decreases by about 5% at ρ0 , compared to that in free space. This large asymmetry between the ν¯ e and νe reactions in nuclear matter can be explained by the last term in Eq. (3), namely the helicity-dependent (HD) term. In Fig. 4, we show the difference, σ − = σ (νe ) − σ (ν¯ e ), and the sum, σ + = σ (νe ) + σ (ν¯ e ), of the cross sections, which respectively correspond to the HD and non-HD terms. The HD term in the left panel increases with increasing the density, while the non-HD term in the right panel
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M.-K. Cheoun et al. / Physics Letters B 723 (2013) 464–469
Fig. 6. (Color online.) Energy dependence of the ratio of the ν¯ e cross section to the right one is for 12 C.
shows a decrease. For the νe reaction, which is given by a half of σ + + σ − , the HD term plays a counterbalancing role of the density effects, and it thus leads to the smaller density effect (see the right panel of Fig. 3). In contrast, for the ν¯ e reaction, the HD term enhances the density effect on the cross section. Therefore, the large difference in the density effects on the ν and ν¯ reactions is closely associated with the difference in the helicities of the incident ν and ν¯ . The change of the form factors definitely affects the ν¯ e - and νe -nucleon interactions in dense nuclear matter. As a result, around ρ0 , the variation of the ν¯ e (νe )-nucleon cross section maximally amounts to 35 (5)%. The radiative corrections and the Coulomb distortion on the nucleon are not taken into account in this work, because those effects are known to be less than 2% [33]. In order to investigate the asymmetry in the νe and ν¯ e reactions on 12 C, we use the following differential cross section formula, which is explained in detail in Ref. [28],
dσν
dΩ
=
(ν /ν¯ )
G 2F k cos2 θc
π (2 J i + 1)
J f ||M ˆ J || J i 2 (1 + ν · β)
J =0
J f ||Lˆ J || J i 2 + 1 − ν · β + 2(νˆ · qˆ )(ˆq · β)
2 Re J f ||Lˆ J || J i J f ||M ˆ J || J i ∗ − qˆ · (νˆ + β) J f | Tˆ Jel | J i 2 + 1 − (νˆ · qˆ )(ˆq · β) J =1
mag 2 + J f | Tˆ J | J i ±
J =1
| J i J f | Tˆ Jel | J i ∗ ,
mag
2 Re J f | Tˆ qˆ · (νˆ − β) J
(4)
νe one, R ν = σ (ν¯ e )/σ (νe ). The left panel is for a nucleon in nuclear matter, while the
and k are respectively the 3-momenta of the initial where ν , β = k/ with being the energy and final leptons, q = k − ν of the final lepton. The density-dependent form factors are involved in the reduced matrix elements. We here include the Coulomb distortion on the outgoing leptons due to the residual nucleus [29]. Using Eq. (4), we calculate the total cross sections for 12 C(ν¯ e , e + )12 B g .s.(1+ ) and 12 C(νe , e − )12 N g .s.(1+ ) . Within the QRPA framework [29], the reactions can be treated by the J = 1 transition from the 0+ ground state of 12 C to the 1+ ground states of 12 B or 12 N. The numerical results are then presented in Fig. 5. To see the effect of the density-dependent form factors on the cross section, we assume that the nuclear density in the interior of 12 C is constant (ρ /ρ0 = 0, 0.5, 1.0), and calculate the form factors in Eq. (4) at those densities. The large asymmetry in the ν¯ e - and νe -12 C cross sections due to the medium effect appears in a similar fashion to that in the case of the in-medium nucleon shown in Fig. 3. We note that, because we consider only the exclusive reaction to the ground state of the daughter nuclei, the cross sections are smaller than those for the nucleon in Fig. 3. The energy dependence of the measured cross section for 12 C(νe , e − )12 N g .s.(1+ ) [26] is also shown in the right panel of Fig. 5. Contrary to the significant reduction in the ν¯ e -12 C cross section, the νe -12 C cross section around the saturation density is reduced less than 10%, comparing with that in free space. Furthermore, if we take the average Fermi momentum of 12 C, which is k F = 225 MeV (ρ = 0.668ρ0 ) [27], in the form factors, the maximum change due to the bound nucleon is shown to be less than 5%, which is within the experimental error bars. Since the Coulomb distortion effect is known to be of about 5–6% [29], the density effect on the νe -12 C reaction is as large as the Coulomb distortion. Finally, in Fig. 6, we show the initial-energy dependence of the cross-section ratio, R ν = σ (ν¯ e )/σ (νe ), on a nucleon in nuclear matter or in 12 C. The ratio decreases with increasing energy. This can be regarded as a valuable, low-energy extension of the
M.-K. Cheoun et al. / Physics Letters B 723 (2013) 464–469
previous calculation performed by Kuzmin et al. [34], in which E is assumed to be larger than 100 MeV. As for the density effect, the higher the density, the smaller the ratio. It means that the asymmetry between the ν¯ e and νe reactions is expected to become larger in a denser nuclear medium, irrespective of the target. As a summary, we have studied the νe and ν¯ e induced reactions via charged current on a nucleon in nuclear matter or in 12 C, using the density-dependent weak form factors calculated by the QMC model. The present calculation has revealed that the antineutrino cross section is reduced maximally by about 35% at the saturation density, while the neutrino cross section at the same density is reduced by about 5% at most. Therefore, it is expected that this asymmetry in the νe and ν¯ e induced reactions influences the r-process and also the neutrino-process nucleosynthesis in core-collapse supernovae. In particular, the anti-neutrino propagation inside the PNS is significantly affected by the density effect caused by the variation of the nucleon structure in dense matter. The ν - and ν¯ -12 C cross sections also show large asymmetry, namely the anti-neutrino cross section is reduced much, while the reduction of the neutrino cross section is rather small. For instance, the cross section for 12 C(νe , e − )12 N g .s.(1+ ) is reduced by about 5% around ρ0 , which is as large as the amount of the Coulomb distortion effect. In the following, we address possible ambiguities latent on our theoretical results. Coulomb distortion and radiative correction are known as less than 2% [33]. Ambiguity on Coulomb distortion by the outgoing electron beyond l = 0 (s) wave was minimized by using the effective momentum approach (EMA) for the electron’s energy beyond a few tens of MeV [28]. But the uncertainty from choosing a reasonable cut for the EMA can be remained, which may cause a few % ambiguity. Therefore, total ambiguities by both effects are thought to be less than 5%, maximally, of whole results. As for the form factors, since the Q 2 in the E ν (ν¯ ) 100 MeV reaction is less than 0.1 [GeV/c ]2 , possible errors in the form factors do not come from the Q 2 dependence including cut off masses, such as axial mass and vector mass, but stem from the normalized values at Q 2 = 0. But those values are determined from many available experimental data. Therefore, although the errors in vacuum may propagate to the in-medium form factors, they may be negligible at least in the energy region considered in this Letter. Remained part is the density dependence of the form factors. But the error evaluation of the density dependence may have no realistic way to reasonably estimate if one takes a model or an approach, for example, in the QMC model used here. Since the density dependence effect is fully to be determined from experimental data analyzed at the saturation point, once the model parameters in the model are fixed to the data, remained step is almost straightforward. The in-medium effect of the modified form factors in the quasielastic neutrino scattering (QE) [24] is in the progress by using the Distorted Wave Born Approximation (DWBA) successfully applied to the electron and neutrino scattering in the QE region [35,36]. Acknowledgements This work was supported by the National Research Foundation of Korea (Grant No. 2012R1A6A3A01040878 and 2011-0015467). The work of KT was supported by the University of Adelaide and the Australian Research Council through grant No. FL0992247 (AWT).
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Physics Letters B www.elsevier.com/locate/physletb
Asymmetry in the neutrino and anti-neutrino reactions in a nuclear medium Myung-Ki Cheoun a,∗ , Ki-Seok Choi a , K.S. Kim b , Koichi Saito c , Toshitaka Kajino d,e , Kazuo Tsushima f,g , Tomoyuki Maruyama h a
Department of Physics, Soongsil University, Seoul 156-743, Republic of Korea School of Liberal Arts and Science, Korea Aerospace University, Koyang 412-791, Republic of Korea c Department of Physics, Faculty of Science and Technology, Tokyo University of Science, Noda 278-8510, Japan d National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan e Department of Astronomy, Graduate School of Science, University of Tokyo, 7-3-1 Hongo, Tokyo 113-0033, Japan f CSSM, School of Chemistry and Physics, University of Adelaide, Adelaide, SA 5005, Australia g International Institute of Physics (IIP), Federal University of Rio Grande do Norte (UFRN), Natal/RN 59078-400, Brazil h College of Bioresource Sciences, Nihon University, Fujisawa 252-8510, Japan b
a r t i c l e
i n f o
Article history: Received 5 March 2013 Received in revised form 21 April 2013 Accepted 20 May 2013 Available online 23 May 2013 Editor: W. Haxton Keywords: Neutrino-induced reactions Weak and electro-magnetic form factors Nuclear matter Proto-neutron stars Density dependence
a b s t r a c t We study the effect of the density-dependent axial and vector form factors on the electron–neutrino (νe ) and anti-neutrino (ν¯ e ) reactions for a nucleon in nuclear matter or in 12 C. The nucleon form factors in free space are presumed to be modified for a bound nucleon in a nuclear medium. We adopt the density-dependent form factors calculated by the quark–meson coupling (QMC) model, and apply them to the νe and ν¯ e induced reactions with the initial energy E = 8–80 MeV. We find that the total νe cross sections on 12 C as well as on a nucleon in nuclear matter are reduced by about 5% at the nuclear saturation density, ρ0 . This reduction is caused by the modification of the nucleon structure in matter. Although the density effect for both cases is relatively small, it is comparable with the effect of Coulomb distortion on the outgoing lepton in the ν -reaction. In contrast, the density effect on the ν¯ e reaction reduces the cross section significantly in both nuclear matter and 12 C cases, and the amount maximally becomes of about 35% around ρ0 . Such large asymmetry in the νe and ν¯ e cross sections, which seems to be nearly independent of the target, is originated from the differences in the helicities of ν¯ e and νe . It is expected that the asymmetry influences the r-process and also the neutrino-process nucleosynthesis in core-collapse supernovae. © 2013 Elsevier B.V. All rights reserved.
In the core-collapse supernova explosion, the neutrino (ν ) heating was suggested as one of the main mechanisms for the explosion, leading to the so-called ν driven explosions. The central object formed at the core bounce is expected to be a hot and leptonrich proto-neutron star (PNS). Therefore, the shock propagation by the initial bounce from the contraction of heavy nuclei crosses the ν -sphere, i.e. the neutrino energy- and flavor-dependent sphere, and releases vast numbers of neutrinos [1]. These neutrinos propagate through the PNS, whose density is believed to be of about a few times of the nuclear saturation density, ρ0 ∼ 0.15 fm−3 . During the propagation, the neutrinos interact with nucleons in a dense nuclear medium. For example, the asymmetry in ν scattering and absorption in a magnetized PNS may account for the pulsar kick of neutron stars, according to the
*
Corresponding author. E-mail address: [email protected] (M.-K. Cheoun).
0370-2693/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.physletb.2013.05.050
detailed study of the ν transport in dense matter by a relativistic mean field theory [2,3]. Outside of the PNS, the emitted neutrinos also interact with nucleons and the nuclei already produced by the s-process in the progenitor and/or the r-process in the explosion. Around the Si layer, the anti-neutrino (ν¯ ) absorption in the proton-rich environment may produce neutrons immediately captured by the neutrondeficient nuclei, which affect the proton process, dubbed as the ν p process [4]. In the O–Ne–Mg layer, the ν induced reactions may play an important role for producing some p-nuclei which are odd–odd neutron-deficient nuclei. For example, 180 Ta and 138 La in the cosmos are believed to be produced from the ν process [5,6]. Other light nuclei abundances are also closely associated with the neutrino interactions in the He–C layer [7]. Of course, the density of the medium outside the PNS is not so high compared to that of the PNS. However, the nucleons interacting with the neutrinos are strongly bound in a nucleus, and the interactions should thus be different from those in free space.
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465
Fig. 1. (Color online.) Effective nucleon mass M ∗ (ρ ) versus the nuclear density ratio ρ /ρ0 (left), and the in-medium axial coupling constant normalized to that in free space (right), R ( g A ) = g A (ρ , Q 2 )/ g A (ρ = 0, Q 2 ), as a function of Q 2 . In the right panel, from the uppermost (for vacuum), the density increases by 0.5ρ0 in order. The lowermost curve is for ρ = 2.5ρ0 .
Recently, strong evidence for the modification of the nucleon structure in nuclear matter has been reported from the proton ) electromagnetic form factors measured in the polarized ( e, e p scattering off 16 O [8] and 4 He [9–12] at MAMI and Jefferson Lab., and also from the study of the neutron properties in a nuclear ) scattering off 4 He [13]. medium through the polarized ( e, en Therefore, it is quite interesting to investigate the possible change in the ν and ν¯ induced reactions due to the variation of the nucleon properties, in order to pin down the ambiguity inherent in the nucleon and/or nuclear structure in the interpretation of various ν reactions in the cosmos. In this Letter, we adopt the density-dependent weak form factors calculated in the quark–meson coupling (QMC) model [14–16]. The quark mass in a hadron can be related to the quark condensate ¯qq in vacuum. The mass (or ¯qq) in nuclear matter may then be reduced from the value in vacuum because of the condensed scalar (σ ) field depending on the nuclear density ρ , namely the Lorentz-scalar, attractive interaction in matter. The decrease of the quark mass in matter leads to the variation of the baryon internal structure at the quark level. Such an effect is considered selfconsistently in the QMC model. As an alternative approach, Smith and Miller have studied the modification of the nucleon structure in nuclear matter using the chiral quark–soliton model [17]. The QMC model has been successfully applied in studying the properties of hadrons in a nuclear medium [18], finite nuclei [19], quasi-elastic (QE) electron scattering off nuclei [20] and hypernuclei [21]. (For a review, see Ref. [22].) There have been many studies on the neutrino–nucleus interactions so far. In particular, the cross sections in the quasi-elastic (QE) and/or production region, where the energy transfer is more than 100 MeV, have intensively been investigated [23] to analyze the recent MiniBooNE data [24]. Furthermore, the neutrino propagation in hot neutron matter has been studied within the framework of the random phase approximation [25]. But, in this Letter, we focus on the energy region of importance in the supernova neutrinos. Therefore we study the ν reactions relevant to the energy range of the LSND experiments [26], where the initial neutrino energy is less than 80 MeV. An initial study for the effect of the density-dependent weak-current form factors on the neutrino scattering can be found in Ref. [27]. However, in that paper, the simple, relativistic Fermi gas model has been used to study the 12 C(νμ , μ− )X reaction, for which only the ν -flux averaged data have been measured. In contrast, in the present calculation, we
treat the nuclear structure of 12 C in terms of quasi-particle random phase approximation (QRPA) [28,29], which enables us to carry out more realistic calculations. We then compare the result with the observed, energy-dependent cross section for the 12 C(νe , e − )12 N g .s. reaction [26]. For more thorough understanding of the density effect, the νe and ν¯ e reactions on a nucleon in nuclear matter are also examined in detail, as well as the ν¯ e reaction on 12 C, because the elementary process may be convenient and useful to see how the variation of the form factors affects the νe and ν¯ e reactions in matter. The weak current operator W μ for the ν induced reaction takes a form of V μ − A μ in the standard electro-weak theory. For a free nucleon, the weak current operator respectively comprises the vector, the axial vector and the pseudo scalar form factors, F iV=1,2 ( Q 2 ), F A ( Q 2 ) and F P ( Q 2 ):
W μ = F 1V Q 2
+ FA
μ i μν γ + F 2V Q 2 σ qν 2M
F P ( Q 2) μ 5 Q 2 γ μγ 5 + q γ , 2M
(1)
where M is the nucleon mass in free space, and Q 2 (= −q2 ) is the four-momentum transfer squared. Because of the conservation of the vector current (CVC) and nonexistence of the second class current, we here ignore the scalar form factor in the vectorial part and the axial-tensor form factor in the axial part. By the CVC hypothesis, the vector and axial form factors for the charged current (C C ) are respectively expressed as [30,31] V ,C C
Fi
p
Q 2 = F i Q 2 − F in Q 2 ,
F CA C Q 2 = − g A / 1 + Q 2 / M 2A p (n) Fi
2
,
(2)
where is the proton (neutron) form factor, and g A and M A are the axial coupling constant and the axial cut off mass, respectively. Before applying to the ν reaction, we briefly discuss the change of the in-medium nucleon properties calculated by the QMC model [14–16]. In Fig. 1 the effective nucleon mass is illustrated in the left panel, which shows a monotonic decrease of the mass while the nuclear density increases. The modification of the axial coupling constant in nuclear matter is also shown in the right panel as a function of Q 2 . Even in the region of small momentum transfer, where most of the ν reactions expected in the cosmos
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V ,C C
V ,C C
Fig. 2. Modification of the weak form factors, R ( F 1V,2 ) = F 1,2 (ρ , Q 2 )/ F 1,2 (ρ = 0, Q 2 ). From the lowermost (for vacuum), the density increases by 0.5ρ0 in order. The uppermost curve is for ρ = 2.5ρ0 .
Fig. 3. (Color online.) Density dependence of the total cross sections for the ν¯e + p → e + + n (left) and νe + n → e − + p (right) in nuclear matter. The black (solid) curves are for the results in free space. The cross sections in both reactions decrease with increasing the density: ρ /ρ0 = 0.5 (red (dashed)), 1.0 (blue (dotted)), 1.5 (yellow (short dotted)), 2.0 (cyan (dot-long-dashed)) and 2.5 (purple (dot-dashed)).
are assumed to occur, the reduction of the axial coupling constant g A (ρ , Q 2 ) (or equivalent to the axial form factor) amounts to 11% at ρ0 . Fig. 2 presents the density dependence of the weak form V ,C C factors given by Eq. (2). The modification of F 2 at small Q 2 V ,C C F1 .
V ,C C F2
is more significant than that of For example, is enhanced by about 11% at ρ0 . Fig. 3 shows the total cross sections for the ν¯ e + p → e + + n and νe + n → e− + p reactions in nuclear matter. Here, we have used the differential cross section formula with the Sachs form factors, G CMC , G CE C and G CAC (= F CA C ) [32]:
dσ
C C
d Q 2 ν (ν¯ )
=
G 2F cos2 θc 1 2π
×
2
yM + 1− y− 2E
(G CE C )2 + y E (G CMC )2 /2M
+
2
y 2 G CMC
1+ y2 2
yE 2M
+1− y+
∓ 2y 1 −
y 2
yM 2E
G CMC G CAC
G CAC
2
,
(3)
M.-K. Cheoun et al. / Physics Letters B 723 (2013) 464–469
Fig. 4. (Color online.) Difference (left),
467
σ − = σ (νe ) − σ (ν¯ e ), and the sum (right), σ + = σ (νe ) + σ (ν¯ e ), of the total cross sections.
Fig. 5. (Color online.) Density dependence of the total cross sections for 12 C(ν¯ e , e + )12 B g .s.(1+ ) (left) and 12 C(νe , e − )12 N g .s.(1+ ) (right). The result denoted by the black (solid) curve (for vacuum) in the right panel is the same as that of the 1+ transition shown in Fig. 1 of Ref. [29]. The measured cross section [26] is also presented in the right panel. V ,C C
where y = Q 2 /(2p · k), G CMC = F 1 2
Q 4M 2
V ,C C F2 ,
+ F 2V ,C C , G CE C = F 1V ,C C −
and E is the initial neutrino or anti-neutrino en-
ergy. The signs ∓ respectively correspond to the
νe ν¯ e
reactions,
and the cross section is multiplied by the Cabbibo-angle factor, cos2 θc . For both reactions, the present results in vacuum (black (solid) curves) are consistent with the previous results calculated in Ref. [33]. For the ν¯e + p → e + + n reaction in matter, the total cross sections decrease by about 15% per each increase (by 0.5ρ0 ) of the density. In contrast, for the νe + n → e − + p reaction, the variation
of the cross section in matter below E ν ∼ 30 MeV is less than 2% (per each increase of the density). Even around E ν ∼ 80 MeV, the cross section maximally decreases by about 5% at ρ0 , compared to that in free space. This large asymmetry between the ν¯ e and νe reactions in nuclear matter can be explained by the last term in Eq. (3), namely the helicity-dependent (HD) term. In Fig. 4, we show the difference, σ − = σ (νe ) − σ (ν¯ e ), and the sum, σ + = σ (νe ) + σ (ν¯ e ), of the cross sections, which respectively correspond to the HD and non-HD terms. The HD term in the left panel increases with increasing the density, while the non-HD term in the right panel
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Fig. 6. (Color online.) Energy dependence of the ratio of the ν¯ e cross section to the right one is for 12 C.
shows a decrease. For the νe reaction, which is given by a half of σ + + σ − , the HD term plays a counterbalancing role of the density effects, and it thus leads to the smaller density effect (see the right panel of Fig. 3). In contrast, for the ν¯ e reaction, the HD term enhances the density effect on the cross section. Therefore, the large difference in the density effects on the ν and ν¯ reactions is closely associated with the difference in the helicities of the incident ν and ν¯ . The change of the form factors definitely affects the ν¯ e - and νe -nucleon interactions in dense nuclear matter. As a result, around ρ0 , the variation of the ν¯ e (νe )-nucleon cross section maximally amounts to 35 (5)%. The radiative corrections and the Coulomb distortion on the nucleon are not taken into account in this work, because those effects are known to be less than 2% [33]. In order to investigate the asymmetry in the νe and ν¯ e reactions on 12 C, we use the following differential cross section formula, which is explained in detail in Ref. [28],
dσν
dΩ
=
(ν /ν¯ )
G 2F k cos2 θc
π (2 J i + 1)
J f ||M ˆ J || J i 2 (1 + ν · β)
J =0
J f ||Lˆ J || J i 2 + 1 − ν · β + 2(νˆ · qˆ )(ˆq · β)
2 Re J f ||Lˆ J || J i J f ||M ˆ J || J i ∗ − qˆ · (νˆ + β) J f | Tˆ Jel | J i 2 + 1 − (νˆ · qˆ )(ˆq · β) J =1
mag 2 + J f | Tˆ J | J i ±
J =1
| J i J f | Tˆ Jel | J i ∗ ,
mag
2 Re J f | Tˆ qˆ · (νˆ − β) J
(4)
νe one, R ν = σ (ν¯ e )/σ (νe ). The left panel is for a nucleon in nuclear matter, while the
and k are respectively the 3-momenta of the initial where ν , β = k/ with being the energy and final leptons, q = k − ν of the final lepton. The density-dependent form factors are involved in the reduced matrix elements. We here include the Coulomb distortion on the outgoing leptons due to the residual nucleus [29]. Using Eq. (4), we calculate the total cross sections for 12 C(ν¯ e , e + )12 B g .s.(1+ ) and 12 C(νe , e − )12 N g .s.(1+ ) . Within the QRPA framework [29], the reactions can be treated by the J = 1 transition from the 0+ ground state of 12 C to the 1+ ground states of 12 B or 12 N. The numerical results are then presented in Fig. 5. To see the effect of the density-dependent form factors on the cross section, we assume that the nuclear density in the interior of 12 C is constant (ρ /ρ0 = 0, 0.5, 1.0), and calculate the form factors in Eq. (4) at those densities. The large asymmetry in the ν¯ e - and νe -12 C cross sections due to the medium effect appears in a similar fashion to that in the case of the in-medium nucleon shown in Fig. 3. We note that, because we consider only the exclusive reaction to the ground state of the daughter nuclei, the cross sections are smaller than those for the nucleon in Fig. 3. The energy dependence of the measured cross section for 12 C(νe , e − )12 N g .s.(1+ ) [26] is also shown in the right panel of Fig. 5. Contrary to the significant reduction in the ν¯ e -12 C cross section, the νe -12 C cross section around the saturation density is reduced less than 10%, comparing with that in free space. Furthermore, if we take the average Fermi momentum of 12 C, which is k F = 225 MeV (ρ = 0.668ρ0 ) [27], in the form factors, the maximum change due to the bound nucleon is shown to be less than 5%, which is within the experimental error bars. Since the Coulomb distortion effect is known to be of about 5–6% [29], the density effect on the νe -12 C reaction is as large as the Coulomb distortion. Finally, in Fig. 6, we show the initial-energy dependence of the cross-section ratio, R ν = σ (ν¯ e )/σ (νe ), on a nucleon in nuclear matter or in 12 C. The ratio decreases with increasing energy. This can be regarded as a valuable, low-energy extension of the
M.-K. Cheoun et al. / Physics Letters B 723 (2013) 464–469
previous calculation performed by Kuzmin et al. [34], in which E is assumed to be larger than 100 MeV. As for the density effect, the higher the density, the smaller the ratio. It means that the asymmetry between the ν¯ e and νe reactions is expected to become larger in a denser nuclear medium, irrespective of the target. As a summary, we have studied the νe and ν¯ e induced reactions via charged current on a nucleon in nuclear matter or in 12 C, using the density-dependent weak form factors calculated by the QMC model. The present calculation has revealed that the antineutrino cross section is reduced maximally by about 35% at the saturation density, while the neutrino cross section at the same density is reduced by about 5% at most. Therefore, it is expected that this asymmetry in the νe and ν¯ e induced reactions influences the r-process and also the neutrino-process nucleosynthesis in core-collapse supernovae. In particular, the anti-neutrino propagation inside the PNS is significantly affected by the density effect caused by the variation of the nucleon structure in dense matter. The ν - and ν¯ -12 C cross sections also show large asymmetry, namely the anti-neutrino cross section is reduced much, while the reduction of the neutrino cross section is rather small. For instance, the cross section for 12 C(νe , e − )12 N g .s.(1+ ) is reduced by about 5% around ρ0 , which is as large as the amount of the Coulomb distortion effect. In the following, we address possible ambiguities latent on our theoretical results. Coulomb distortion and radiative correction are known as less than 2% [33]. Ambiguity on Coulomb distortion by the outgoing electron beyond l = 0 (s) wave was minimized by using the effective momentum approach (EMA) for the electron’s energy beyond a few tens of MeV [28]. But the uncertainty from choosing a reasonable cut for the EMA can be remained, which may cause a few % ambiguity. Therefore, total ambiguities by both effects are thought to be less than 5%, maximally, of whole results. As for the form factors, since the Q 2 in the E ν (ν¯ ) 100 MeV reaction is less than 0.1 [GeV/c ]2 , possible errors in the form factors do not come from the Q 2 dependence including cut off masses, such as axial mass and vector mass, but stem from the normalized values at Q 2 = 0. But those values are determined from many available experimental data. Therefore, although the errors in vacuum may propagate to the in-medium form factors, they may be negligible at least in the energy region considered in this Letter. Remained part is the density dependence of the form factors. But the error evaluation of the density dependence may have no realistic way to reasonably estimate if one takes a model or an approach, for example, in the QMC model used here. Since the density dependence effect is fully to be determined from experimental data analyzed at the saturation point, once the model parameters in the model are fixed to the data, remained step is almost straightforward. The in-medium effect of the modified form factors in the quasielastic neutrino scattering (QE) [24] is in the progress by using the Distorted Wave Born Approximation (DWBA) successfully applied to the electron and neutrino scattering in the QE region [35,36]. Acknowledgements This work was supported by the National Research Foundation of Korea (Grant No. 2012R1A6A3A01040878 and 2011-0015467). The work of KT was supported by the University of Adelaide and the Australian Research Council through grant No. FL0992247 (AWT).
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