Instability of colliding metastable strings
Abstract
The breaking of symmetry plays a crucial role in modeling the breaking of supersymmetry (SUSY). In the models that possess both SUSY preserving and SUSY breaking vacua, tubelike cosmic strings called Rtubes, whose surfaces are constituted by domain walls interpolating a false and a true vacuum with some winding numbers, can exist. Their (in)stability can strongly constrain SUSY breaking models theirselves. In the present study, we investigate the dynamical (in)stability of two colliding metastable tubelike strings by fieldtheoretic simulations. From them, we find that the strings become unstable, depending on the relative collision angle and speed of two strings, and the false vacuum is eventually filled out by the true vacuum owing to rapid expansion of the strings or unstable bubbles created as remnants of the collision.
pacs:
11.27.+d, 98.80.Cq, 98.80.kI introduction
Global symmetry breaking plays a crucial role in many aspects of particle physics and field theories. Cosmic strings are generated associated with the symmetry breaking in the cosmic history and hence are the key solitons to probe information of the early universe or to prove (or disprove) a phenomenological model Vilenkin .
One of the most important global symmetries is the symmetry in supersymmetric theories. Since supersymmetry (SUSY) is broken at lowenergy scales, we must live in a vacuum with nonvanishing potential energy, which requires exact symmetry if the vacuum is the global minimum. On the other hand, exact symmetry prohibits nonzero Majorana gaugino mass. In this reason, relatively complicated vacuum structure that contains both true SUSY vacua and false vacua with approximate symmetry (broken explicitly or spontaneously) is now energetically studied for lowenergy model building in the phenomenological point of view. (See SUSY1 ; SUSY2 for reviews.) When a false vacuum is selected in such models after inflation, the symmetry is spontaneously broken and it gives rise to cosmic Rstrings, which cannot be avoided in a high scale inflation or a high reheating temperature scenario.
Cosmic strings in some models of this class have a peculiar feature. If a lower preserving vacuum exists, the field configuration in the string core falls down to the lower vacuum. Therefore, the strings have inner structure that is tubelike domain wall configuration. Note that since the lower vacuum is energetically favored, the interior of the string core possibly starts to expand Steinhardt:1981ec ; Hosotani ; Yajnik:1986tg ; Eto:2012ij ; Kamada:2013rya , and the Universe is eventually filled by the true vacuum, where SUSY is recovered. This situation is inconsistent with our Universe.
Besides this extreme case, it has been also reported that there is parameter space where the metastable tubelike strings exist Eto:2012ij . However, naively thinking, such an object would be unstable against any disturbance, and follow the same fate as the above case. Actually, once multiple strings are formed by the breaking of in the cosmic history, they are inevitably affected by the violent dynamical processes like collision and reconnection. Taking such kinds of dynamical processes of metastable Rstrings into account, it is nontrivial whether the tubelike strings become unstable or keep their stability throughout their collision process. Thus, it is very important to study the collision dynamics and figure out the (in)stability of the tubelike Rstrings, which has a tight relationship with the possibility to constrain the model building for realistic SUSY breaking and breaking.
In this paper, we investigate the dynamical stability/instability for the strings under the reconnection. For the sake of this study, we perform the fieldtheoretic simulations of the collision of two metastable strings described as a solitonic solution in the model of a classical complex scalar field with a potential including a true vacuum and a metastable vacuum. For the simulations, we particularly focus on the two physical parameters characterising the collision process; the relative velocity and the angle of two colliding strings. As a result, we find that the stability of the system strongly depends on these parameters and that surprisingly there is a wide parameter space where the system is stable against the collision. This work implies the importance of taking into account the dynamics of the metastable objects generated in the SUSY breaking and breaking models. Although this system cannot produce the cosmic string network via the Kibble mechanism, the process of the string collision would catch up the feature of more realistic situations. At any rate, it is very useful to reveal novel phenomena by exploiting a simple model which would be shared by a wide class of theories.
The organization of this paper is as follows. In section II, we set up the model and show numerical solutions for static metastable strings. In section III, by using approximations, we estimate the maximum winding number of a metastable string and study analytically instability of colliding two strings. In section IV, we investigate the dynamics of colliding strings by threedimensional simulations. We survey parameter dependence of instability by varying the collision angle and the relative speed of strings. Section V is devoted to conclusions and discussions. In Appendix, we briefly explain our scheme for numerical studies.
Ii setup of model and global string
To illustrate growing instability of metastable strings under reconnections and show generality of such phenomena, we consider a simple single field model with false and true vacua. This is an ideal example to demonstrate various aspects of the collision dynamics of two metastable solitons which would be common in a wide class of models. The Lagrangian of a complex scalar field which carries a charge of global symmetry is given by
(1) 
We engineer a false vacuum and a true vacuum by employing the following sixth order of the potential,
(2) 
where is a dimensionless constant determining the amplitude of a local minimum of at . This potential has the global minimum at where , a local maximum at , and a local minimum at , which are given by
(3)  
Here we introduced several dimensionless quantities, , and . Figure 1 shows the schematic picture of this potential. The EulerLagrange equation we solve in section IV for simulations of colliding strings is given by
(4) 
In the false vacuum, the global symmetry is spontaneously broken. We simply assume that a global string is formed. Consider a static cylindrical solution of the field equation in the cylindrical coordinate, . First we decompose into the radial and angular parts,
(5) 
Then the explicit form of the equation of is obtained from Eq. (4)
(6) 
where . We look for the solution where the scalar field in the interior of the string stays in the true vacuum, and that stays in the metastable vacuum at the exterior. That is, such a solution should satisfy at and at .
Here we emphasize that there can be a (meta)stable tubelike solution. Since the string core has lower energy density than that of its exterior, larger string radius seems to be favorable for the system, which predicts the rollover process. However, there must be the domain wall between its core and exterior, whose energy becomes larger for larger string radius. As a result, there arises a (meta)stable tubelike solution, depending on the parameters. We will see it in more detail in the next section.
The static axialsymmetric solutions can be obtained by solving Eq. (6) with the successive overrelaxation method with the relaxation factor and for , respectively. See the Appendix for details of our numerical schemes. Using the boundary conditions and , we find stable solutions. In Fig. 2, we plot the field configurations (left panel) and the potential energies (right panel) of the numerical solutions with and for . We confirmed that the field configurations are insensitive to the position of boundary, , as long as it is sufficiently far from the domain wall. At the region where the potential energy becomes a peak, there is a domain wall which is the surface of the tube/cylinder. It is found that the radius of the tube/cylinder depends on the winding number, , and the higherwinding solution tends to be thicker. Moreover, the thickness of the domain wall is quite insensitive to the winding number.
Iii Analytic estimations
iii.1 A schematic illustration of stability of the tubelike string and bubble
In the previous section, we demonstrate numerical solutions for the static tubelike string. Before we consider the detailed estimation of their structure, we here give a rough approximation and clarify the stability of those solutions by using a simple thinwall approximation. It is obvious that the stability of the tubelike string depends on the difference in the energy density of the true and false vacua
(7) 
When , the vacua inside and outside the string have the same energy. Then the tubelike string is stable because it is supported by the topological reason. On the other hand, when is large enough, the core energy density wins out and the string grows and expands. Thus, one expects that there exists a metastable tubelike string in an intermediate range for . Let us clarify this by using the thinwall approximation where the wall of the string is thin compared to the core and the exterior. The thin wall’s contribution to the energy density can be approximated by the constant surface energy, . Then, the tension of the string with radius is estimated by
(8) 
where the first term is the contribution of the flux of the global current with being a cutoff scale and being a constant of a mass dimension 2, the second term is of the wall and the last term is of the core of the tubelike string. Here we assume , and are independent of .
The dependence of the string energy on is shown in Fig. 3. When , as we explained, there exists a local minimum, which implies that there exists a stable tubelike string. In the region with , there exist a local minimum and a global maximum at
(9) 
In this case the tubelike strings exist but they are metastable. The tubelike string whose core radius is larger than is unstable. When exceeds the critical value, , there are no local minima at all. Namely, the string is unstable and the string core expands infinitely. In this paper the metastable strings are concerned. Although they are metastable as static configurations, they will become unstable in some dynamical processes such as string collision, annihilation and reconnections.
From Eq. (8) with being zero, the fate of the twodimensional bubble can be also clarified. As shown in Fig. 3, the bubble never grows if . Once the positive is turned on, the bubble becomes metastable. The critical radius of the bubble is . The bubble lager than expands infinitely and the true vacua inside the core wins out. There are no metastable tubelike solutions.
The thinwall approximation explained above is simple and gives an insight about the stability problem. Nevertheless, it is of only limited accuracy and we need a better analysis to get more quantitative results beyond the qualitative properties. To go beyond the thinwall approximation, we will work out another analytical study in the next subsections.
iii.2 Thickness of walls
Now let us go beyond the thinwall approximation and consider the stability of the tubelike solution in more detail. From the numerical calculations of stable static solutions, we found that the tubelike solution is well characterized by the radius of cylinders, , the winding number, and the thickness of the walls, . Since the thickness of the wall is quite insensitive to other two parameters, it is possible to estimate from the variation of the total energy with respect to , and it can be done by considering only the case of .
We here simply approximate the solution with as the following piecewise linear function
(10) 
where we set , see Fig. 2. Then, we approximate the volume integral of the potential energy and gradient energies as
(11)  
(12)  
(13) 
The derivative of the summation of these energy components with respect to gives the desired value of ,
(14) 
iii.3 Upper bound of winding number
We consider how the radius of the cylinder is determined and the upper limit of the winding number with which the static solution exists. As shown in Fig. 2, for , there are two parameters characterizing a cylinder, the radius and the thickness of the surface. However, as shown in the previous subsections, the latter one is not sensitive to the winding number and is determined from . Hence we here focus on the radius of the cylinder while the width of the wall is assumed to be the same as one given in Eq. (14).
We simply approximate the solution and the potential as
(15) 
The volume integral of the potential energy is approximated as
(16) 
where we considered only the deviation from . On the other hand, that of the gradient energy can be divided into the radial and angular parts,
(17)  
(18) 
where is the cutoff length, , and it should be properly regularized. Then the total energy becomes
(19) 
Let us look for the values of to minimize the total energy. One can easily see that there is no global minimum of since the term proportional to in Eq. (16) implies that for . Instead, we investigate whether there is a local minimum in the region, . The derivative of with respect to is calculated as
(20)  
where
(21) 
What we have to do is to find to satisfy for . We plot and with specific values of and in Fig. 4. From the right panel of this figure, it is found that there are two zeropoints for , and the smaller one is the desired value of , at which is locally minimized, and the other zeropoint gives the local maximum of (see the corresponding lines in the left panel). Note that the local minimum of is , which is consistent with the numerical solution in Fig. 2.
In order for these zeropoints to exist, the local maximum of should be positive. Actually, for is always negative, and in the left panel of Fig. 4 the line for with has no local minimum, which indicates the cylinder solution with is unstable. To proceed this calculation, we assume to expand the logarithm function. Then we find that has a local maximum,
(22) 
Therefore the condition on the winding number required for the local minimum of is
(23) 
iii.4 Instability of spherical bubble and critical volume
In subsection III.1 we have seen that the twodimensional bubble is unstable and it shrinks (expands) when its radius is smaller (larger) than the critical value. Here, we study the spherically symmetric configuration (threedimensional bubble) of the scalar field separated by the domain wall. It is also expected to be always unstable. After two cylinders collide with each other, there appears a spherical object at the impact point. To study the instability of such an object, we consider the following ansatz in the spherical coordinate,
(24) 
with assuming that is given by Eq. (14). We simply assume that the field is homogeneous in the azimuthal and the polar directions^{2}^{2}2Strictly speaking, the separation of variables is not justified, since it is impossible to expand in the spherical harmonics because of the nonlinear terms in the potential.. The potential and radial gradient energy are approximated as
(25)  
(26) 
and thus the total energy becomes
(27) 
where we used Eq. (14) to eliminate . The function has the only peak at in , and the critical radius is given by solving ,
(28) 
where is defined in Eq. (21). For the radius of the sphere starts to shrink since . On the other hand, for the radius goes to the positive infinity since . That is, if the volume of the spherical object is larger than the critical volume , this grows infinitely.
Now we give an analytical estimate of the (in)stability of the colliding cylinders. Let us consider the situation that two cylinders collide with the collision angle as shown in Fig. 5. Assuming that a spherical object is produced via the fusion of two cylinders after collision and its volume is equal to the total volume of the two colliding segments of cylinders, , the initial volume of the spherical object is calculated as
(29) 
Then the condition for the sphere to grow infinitely is given by
(30) 
where is obtained by solving in Eq. (20), and hence assuming , this is calculated as
(31) 
As a result, we obtain the upper limit of the collision angle so that the spherical object created at the impact point can grow infinitely,
(32) 
For example, and give the upper limit, , for , and for .
Iv Simulation setup
We explore the (in)stability of collision processes of two metastable strings. The individual string is given by the axialsymmetric solution which is numerically obtained by solving Eq. (4) without the timederivative term, corresponding to Eq. (6). This procedure is nothing but the onedimensional boundary value problem, and is carried out with the GaussSeidel method. Each of the resultant strings is put on the xz surface parallel to each other separated a bit. The strings are Lorentzboosted to collide with each other at an angle with measured on the xz surface including the origin (to be the impact point when colliding). The free model parameters are and , which control the strength of the selfcoupling and the energy scale of the phase transition, respectively. The fiducial parameters are listed in Table 1. Note that we choose for numerical simulations, while has been used so far, to enhance the instability.
The initial separation between the strings is fixed to be , while the radius of the string is about for the fiducial choice of parameters. According to Ref. Vilenkin , the superposition of the two strings is given by
(33) 
where the denominator is determined by the dimension analysis and the fact that far away from two strings.
Then we solve Eq. (4) in the threedimensional cartesian coordinate with the Neumann conditions on the boundaries, , where is the normal vector to each boundary. We use the Leapfrog method for the time domain and approximate the spatial derivatives by the 2ndorder central finite difference. The simulations are stopped at , when the information at the impact point arrives at the nearest boundary of the computational domain.
model parameters  

0.1  
2.2  
1.0  
1  
1 
simulation parameters  

grid size  
box size  240  
time interval  
total steps  600  
simulation time 
V Results
Through the numerical simulations, we find that the collision processes end up with either simply reconnecting and going away from each other, or creating unstable objects with a higher winding number at the impact point. Figs. 611 show the snapshots of the computational domain during simulations, the leftmost panel is at the initial time , and the rightmost one at the final time . The surfaces in them represent given in Eq. (3), and hence in the interior of them the field lies in the true vacuum. The first four figures, Figs. 69, are the cases of parallel strings with relatively small , and the remaining ones, Figs. 10 and 11, are those of antiparallel strings with . Moreover, we plot the field configurations and those phases on the surface at and in Figs. 12 and 13. In the unstable cases, P1, P3, P4 and A1, we find that the true vacuum homogeneously spreads over the interior of the bubbles. As for the phases, one can observe that the winding number is conserved. For the parallel pairs, one can find that the total winding number is (two sets of bluegreenredblue), if one follows the trajectory around the bubble. In the interior, there seems to be a large number of points around which the phase is rotated, although the winding vanishes if one follows the trajectory around the impact point. On the other hand, for the antiparallel pairs, the total winding number around the bubble becomes (greenbluegreenredblueredgreen.)
The (in)stability of a resultant string after collision depends on the velocity of two cylinders, , and the collision angle between them, . We surveyed the parameter space to check the stability. Fig. 14 shows the stability of parallel string pairs, and Fig.15 that of antiparallel pairs where is close to . In order to systematically judge the resultant stability, we calculate two quantities and set a criterion for each. One is the growth rate of the volume of the true vacuum region during simulation,
(34) 
and the other is the ratio of time when the true vacuum region grows to the total simulation time,
(35) 
where denotes the number of elements, is the time interval, and represents the discrete time, . If and only if both and are satisfied, we decide that the resultant strings become unstable, and thus the true vacuum region grows exponentially. We set and . The second quantity, , measures how monotonic the instability grows. After all, however, most of (in)stabilities can be determined by the first one, .
In Figs. 14 and 15, we plot red filled circles for unstable pairs, and green crosses for stable pairs. We find that the slow collision with small results in the failure of the reconnection and thus being unstable, and quite highspeed collision can also make the system unstable in both cases with small (parallel pair) and large (antiparallel pair). In these figures, we also plot black circles labelled as P1,2,3,4 and A1,2. The corresponding figures were already shown in Figs. 611, which are the typical examples of several regions leading to stable and unstable processes in Figs. 14 and 15.
We explain in detail the mechanism of stable/unstable reconnection processes using Figs. 611. Firstly, for the parallel pairs, our findings are followings.

(relatively slow collision) : Two strings merge at the instance of impact and cannot be separated from each other because of a small collision velocity. Then the effective volume of the merged strings around the impact point exceeds the critical volume mentioned in Sec.III, thus making the system unstable.

(moderatespeed collision) : Two strings can safely reconnect with each other. As shown in the middle panel and its right neighbour, a small bubble is created at the impact points of the strings. However, since the reconnected strings rapidly go away from the bubble and the volume of the bubble is relatively small, the bubble finally shrinks. As a result, the system becomes stable.

(highspeed collision) : Because of the large collision velocity, the kinetic energy induced to the bubble at the impact point is large enough to inflate the bubble until its volume exceeds the critical one. Finally the system becomes unstable.

(large angle highspeed collision ) : Since the collision angle is large, the initial impact of strings cannot create a larger bubble at the impact point than the critical volume. However, kinetic energy induced to the bubble is too large, and then the bubble can rapidly expand (the third panel). At the same time, the reconnected strings also rapidly go away from the impact point. As a result, the bubble is about to break up (forth panel). Finally, the expansion rate of the bubble overcomes (last panel).
As for the antiparallel pairs, they annihilate with each other in most of cases owing to the cancellation of their winding numbers [A1]. It is, however, surprising that the highspeed collision makes the system unstable. Actually, after the collision, a bubble with no windings is excited at the impact point, and it grows exponentially.
Vi Conclusion
We have studied the dynamical (in)stabilities of metastable strings after their collision in the model where the potential of a complex scalar field has a false vacuum state corresponding to a SUSY breaking vacuum in realistic SUSY models.
Before performing numerical simulations for the collision, we analytically investigated in a simplified model the thickness of the domain wall constituting the surface of strings, and the existence of static solutions with large winding numbers from the viewpoint of energetics. As a result, we found that the thickness of wall is determined from the shape of given potential, being independent of the winding number of strings, and there exists the upper bound of winding number for the stable solutions. Furthermore, in the same manner, we investigated the collision process of two metastable strings with some approximations neglecting the details of dynamics. Then we found that there is the critical volume of the true vacuum region in a string as indicated in Eq. (28) so that the instability grows, and thus in some cases with a small collision angle, the reconnection is failed.
After that, we performed threedimensional fieldtheoretic simulations of two colliding metastable strings. The initial condition is given as the superposition of two Lorentzboosted strings obtained in another numerical way. We surveyed the (in)stabilities of the collision processes on the parameter space , where is the velocity of strings and is the collision angle.
Consequently, we found that the instability cannot always be observed for the stringstring pairs. We fixed the winding number of strings as and tuned the parameters controlling the shape of potential, and , so that the static solution with does not exist. Nevertheless the strings with most of combinations lead to successful reconnection, or, in other words, the parameter region in the space where the instability grows is highly restricted.
We briefly explain the unstable cases. For small , we confirmed that the reconnection is failed owing to the rapid expansion of the overlap region just after the collision, shown in Fig. 6 [P1] and Fig. 8 [P3], as discussed in the static analysis mentioned above. In addition to this case, we found that the kinetic energy injected into the impact point can leads to failure reconnection. In fact, although the strings with a large velocity seem to succeed in reconnecting safely, the whole system becomes eventually unstable owing to the rapid growth of a true vacuum bubble created at the impact point after the collision, as shown in Fig. 8 [P3] and Fig. 9 [P4]. Surprisingly, this is also the case for the antiparallel pair, as shown in Fig. 11 [A2], although one would envisage that they can pairannihilate. This result would be explained by the shorter time scale of the expansion of the zerowinding bubble created at the impact point than that of pairannihilation.
At first, we had a naive expectation such that the instability would grow because of the temporal formation of unstable strings at the moment of impact, and the antiparallel pairs always annihilate after collision. Our numerical studies, however, clarified that this is not true story. Instead of the winding number iteself, it is confirmed that the volume of true vacuum at the impact point and the collision veclocity are responsible for the (in)stability of the colliding strings. Particularly, it should be stressed that the numerical studies presented here are crucial to find the latter fact, dependence on the velocity or kinetic energy of strings.
Our numerical study has shown the relatively complicated parameter dependence on the stability of tubelike strings against collisions, and will help to constrain the viable parameter space on concrete SUSY breaking models with spontaneous symmetry breaking and also their cosmological history.
Acknowledgements.
T. H. was supported by JSPS GrantinAid for Young Scientists (B) No.23740186, and by MEXT HPCI Strategic Program. The work of M. E. is supported by GrantinAid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology, Japan (No. 23740226) and Japan Society for the Promotion of Science (JSPS) and Academy of Sciences of the Czech Republic (ASCR) under the Japan  Czech Republic Research Cooperative Program. T. K. is supported in part by the GrantinAid for the Global COE Program ”The Next Generation of Physics, Spun from Universality and Emergence” and the JSPS GrantinAid for Scientific Research (A) No. 22244030 from the Ministry of Education, Culture,Sports, Science and Technology of Japan. Y. O.’s research is supported by The Hakubi Center for Advanced Research, Kyoto University.Appendix A numerical schemes
To find the static configuration of an axially symmetric metatable string, we solve Eq. (6) with the succesive overrelaxation method.
First of all, we discretise the coordinate such as for , and represent the discretised as a vector . The spatial derivatives in the lefthand side of Eq. (6) is replaced by the corresponding 2ndorder finite differences,
(36) 
We denote the other terms depending on in Eq. (6) by and those evaluated at by . Then the equation to be solved becomes
(37) 
Solving this equations with respect to in the first term, we find trial values of denoted by ,
(38) 
Notice that in is not . On each stage of iterations, we update so that
(39) 
This scheme with is equivalent to the GaussSeidel method. When we solve Eq. (6) for in Sec. II, we set and for , respectively. For in Sec. IV, we set .
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