NSF-ITP-01-27

UPRF-2001-08

Matrix strings in a B-field

Gianluca Grignani ^{1}^{1}1 Work supported in part by
INFN and MURST of Italy. Email: grignanipg.infn.it

Dipartimento di Fisica and Sezione I.N.F.N., Università di Perugia,

Via A. Pascoli I-06123, Perugia, Italia

Marta Orselli ^{2}^{2}2 Work supported in part by
MURST of Italy. Email: orsellifis.unipr.it

Dipartimento di Fisica and I.N.F.N. Gruppo Collegato di Parma,

Parco Area delle Scienze 7/A I-43100, Parma, Italia

Gordon W. Semenoff ^{3}^{3}3 Work
supported in part by NSERC of Canada and the National Science Foundation
under Grant No. PHY99-07949. Permanent address: Department of
Physics and Astronomy, University of British Columbia,
6224 Agricultural Road, Vancouver, British Columbia V6T 1Z1 Canada. Email:

Institute for Theoretical Physics, University of California, Santa Barbara, California 93106-4030 USA

Abstract

We study the discrete light-cone quantization (DLCQ) of closed strings in the background of Minkowski space-time and a constant Neveu-Schwarz -field. For the Bosonic string, we identify the -dependent part of the thermodynamic free energy to all orders in string perturbation theory. For every genus, appears in a constraint in the path integral which restricts the world-sheet geometries to those which are branched covers of a certain torus. This is the extension of a previous result where the -field was absent [1]. We then discuss the coupling of a -field to the Matrix model of -theory. We show that, when we consider this theory at finite temperature and in a finite -field, the Matrix variables are functions which live on a torus with the same Teichmüller parameter as the one that we identified in string theory. We show explicitly that the thermodynamic partition function of the Matrix string model in the limit of free strings reproduces the genus 1 thermodynamic partition function of type IIA string. This is strong evidence that the Matrix model can reproduce perturbative string theory. We also find an interesting behavior of the Hagedorn temperature.

## 1 Introduction and Summary

It is widely believed that the five different perturbatively consistent superstring theories describe special points in the space of vacua of a single dynamical structure called -theory. However, the nature of the degrees of freedom and a detailed description of the dynamics of -theory are as yet unknown. What has recently become apparent is that there are other points, besides the perturbative string limits, in the moduli space of vacua where -theory could be understood. One of them is the infinite momentum frame which is conjectured to be described by the Matrix model [2, 3]. Others are the various strong field limits which produce field theories with non-commutative geometry [4], or non-commutative open string (NCOS) theory [5],[6] or wrapped and non-relativistic closed string theory models [7], [8],[9]. In this Paper we will compare two limits of -theory in an overlapping domain of validity - the Matrix model in the limit where it should produce perturbative string theory and perturbative string theory in the kinematical context, discrete light-cone quantization (DLCQ), which is described by the Matrix model. We will consider each model in a background of a constant Neveu-Schwarz antisymmetric tensor field, . The coupling of this field to each model is simple and its effect can be taken into account exactly. This will give a one-parameter comparison of the two theories and we will find remarkable agreement.

The Matrix model [2, 3] (for a recent review see [10]) with matrices and gauge group is conjectured to describe -theory on a background of eleven dimensional Minkowski space with a compact null direction, and units of null momentum, .

Type IIA superstrings can be obtained from -theory by compactifying a space direction. This compactification adds a dimension to the matrix model [11]. The resulting Matrix string theory [12], [13] is 1+1-dimensional maximally supersymmetric Yang-Mills theory. With the appropriate identification of degrees of freedom, it should be a non-perturbative formulation of type IIA superstrings on a space with a compact null direction. Dijkgraaf, Verlinde and Verlinde [13] argued that perturbative string theory is described by the moduli space of classical vacua of the 1+1-dimensional Yang-Mills theory. In this moduli space, the degrees of freedom are mutually commuting matrices. The string degrees of freedom are the simultaneous eigenvalues of the matrices. The correct description of the dynamics of these eigenvalues in the limit which produces free strings is a super-conformal field theory on a symmetric orbifold . They also showed that the first correction to the free string Hamiltonian is an irrelevant operator which precisely reproduces the Mandelstam three-string vertex. Elaborations on this limit have been discussed in detail in the literature [14]-[25], [1].

In the following, we will provide further support for these ideas by comparing that limit of matrix string theory which should produce weakly coupled strings with discrete light-cone quantized (DLCQ) perturbative string theory. We will be particularly interested in examining the effect of coupling a constant background to the Matrix string model and comparing the perturbative string limit to the DLCQ type IIA superstring in a background -field. We will find a remarkable agreement between the two. This is an elaboration of our previous results in [21],[1] where a similar comparison was made in the absence of . It provides further evidence in support of the Matrix model conjecture.

We will be interested in the thermodynamic partition function of matrix string theory. To form the thermodynamic partition function, we must identify the energy. In the Matrix model, one of the light-cone momenta is given by where is the compactification radius of the light-cone and the other one is given by , the Hamiltonian of 1+1-dimensional supersymmetric Yang-Mills theory with gauge group . The energy is given by which can be used to form the partition function

(1) |

where is the inverse temperature. The sum over is the trace over the spectrum of . The remaining trace is the thermodynamic partition function of the 1+1-dimensional super Yang-Mills theory with gauge group U(N) and inverse temperature . It is given by the usual finite temperature field theory path integral with compact Euclidean time where the supersymmetry of the model is broken by the anti-periodic boundary conditions for fermions. It will turn out that the matrix string model partition function (1) contains a functional integral over matrix-valued fields which live on a particular 2-torus, characterized by Teichmüller parameter

(2) |

where

(3) |

and is the Euclidean version of a component of the -field. We will find that this underlying torus also emerges in the string theory in an interesting way.

We shall begin by studying the closed bosonic string and the IIA superstring in a constant external . In order to compare with the Matrix model, we do discrete light-cone quantization (DLCQ) of the string. We shall consider the thermodynamic partition function. Studying either string theory or Matrix theory at finite temperature compactifies Euclidean time, . In the IIA string theory, this gives a second compactification of the target space: both the null Minkowski direction and Euclidean time are then compact. In [21],[1] it was seen that this double compactification results in an interesting constraint on geometries of world-sheets in the path integral formulation of the string. Here, we shall see how this constraint is modified by the presence of a -field.The result will be that the integral over all geometries of Riemann surfaces in the string path integral is reduced to an integral over those Riemann surfaces which are branched covers of a torus with Teichmüller parameter given by in (2).

Without compactifications of space-time, and in the absence of D-branes, closed strings would not couple to a constant -field. In fact, a constant -field is gauge equivalent to a constant electromagnetic field and the electric charges that would couple to it live at the ends of open strings. Closed strings are neutral and are normally unaffected by an electromagnetic field. However, when some space-time directions are compact, the states where the closed string wraps the compact dimension can couple to . The coupling amounts to a constant shift of energies and momenta of the wrapped strings. Its origin can intuitively be understood by imagining a closed string as being made from an open string by fusing its ends. If the open string carries charges of opposite sign on its endpoints, the process of creating a small open string, wrapping it around the compact dimension, then fusing the ends together obtains a contribution to its energy from transporting the charged endpoints in a constant electric field. The energy shift is where is the component of electric field in the compact direction, is the compactification radius and is the number of times the resulting closed string wraps the compact direction. A constant shift in energy is a chemical potential and an electric -field should therefore induce a finite density of wrapped closed strings. There would also be a similar shifts in momenta of wrapped states coming from the magnetic components of the -field.If more directions were compact, the presence of the -field would lead to a higher dimensional non-commutative Yang-Mills theory [26], [27], [28].

Consider a closed bosonic string propagating on 26-dimensional Minkowski space with a constant background -field. The action is given by

The -field contributes a total derivative term to the action. The equations of motion do not depend on and have their usual form,

Noether currents can depend on . For example, total momentum which is the Noether charge for space-time translation invariance, has the form

(4) |

and we see that it is modified by only if for some , . This can happen when the string wraps a compactified dimension.

Let us consider the simple case where one of the spatial dimensions is compact, and where the only non-zero component of is . The mass-shell and level matching conditions are

(5) |

(6) |

Here is the quantized momentum in the compact dimension. The integer is the number of times the string wraps the compact dimension. The shift of the energy by the -dependent term is a result of the shift of the momentum in (4). We see that, for fixed , the -field affects the spectrum like a chemical potential, in (5).

The mass-shell condition (5) is solved by light-cone momenta, ,

(7) |

We can obtain a compactified null direction from a compactified spatial direction by an infinite boost along the compact direction together with an infinite rescaling of [29]. We consider the state where the 1-component of the momentum is large and negative and boost in the positive 1 direction. The light-cone momenta in the boosted frame are scaled by the factors

to get

(8) |

Now, is the light-cone compactification radius, . The -field contributes a chemical potential-like shift to the light-cone energy . The only other place that the wrapping number, , appears is in the level matching condition (6) which is unchanged. Note that if we combine (8) and (6), the mass operator has the form

(9) |

In this formula, the string tensions of left and right-movers are shifted by factors of and , respectively.

We are interested in seeing how this spectrum arises in the appropriate limit of Matrix string theory. We shall consider the case where is the only non-zero component of the -field and where it is a constant. To find the action of Matrix string theory in this -field, our starting point is the action for D1-branes on a space-time with the -direction compactified and with a background metric

(10) |

and no -field

The correct D1-brane action can be found by dimensionally reducing 10-dimensional supersymmetric Yang-Mills theory with the above space-time metric to obtain maximally supersymmetric 1+1-dimensional Yang-Mills theory whose 2-dimensional space-time metric is the upper left-hand corner of .

Using the Buscher rules [30, 31] for -duality, this is equivalent to D0-branes on a space with the same compactified direction with dual radius, the Minkowski metric and non-zero -field.

Then, a combination of arguments following Seiberg [29] and Sen [32] and Dijkgraaf, Verlinde and Verlinde [13] can be used to show that, with the appropriate identification of parameters, this D0-brane action is also the Matrix string model which should describe Matrix strings in a -field. Note that, under the boost to the infinite momentum frame which is required to make this identification, the component of the -field that we are interested in, , is invariant.

The linearized coupling of an external -field to both the Matrix model and the Matrix string model has been found before [33, 34]. We find that when is a constant, the full -dependence of the Matrix string model action has linear and quadratic terms in . The linear term agrees with the one found in [33]. From the Matrix string action, we can also deduce how the -field appears in the Matrix model action. There is also a linear and quadratic term and the linear term agrees with the coupling found in [34].

Weakly coupled string theory has a density of states which grows exponentially with energy [35]. This gives rise to a maximum temperature, called the Hagedorn temperature, beyond which a gas of strings cannot be heated. This Hagedorn temperature is also sometimes interpreted as a phase transition [36, 37, 38].

It is interesting to ask the question where a Hagedorn-like behavior could arise in (1). Since the last factor is just the partition sum of a field theory, it is clear that the only possible source of divergence which could give a Hagedorn temperature is in the summation over . The convergence of the sum over is governed by the large limit of the gauge theory. Indeed, there is a rough classification of the behaviors that can occur according to the three possible asymptotic behaviors for this sum, depending on the state of the gauge theory in the large limit. The free energy, , of the gauge theory is defined by

As a function of in the large limit, the free energy can generally be expected to be negative and proportional to a power of ,

There is a simple classification of the behavior of the sum over which depends on the exponent, ,

(11) |

An example of the first case is the gauge theory in a confining phase. The large limit of its free energy is of order one. Then, the sum over in (1) will be convergent and the partition function will be field theory-like with a field theoretical asymptotic density of states.

In the second case in (11), which is the one that we will study in this Paper, the eigenvalues of the matrices dominate and there are of them, thus the free energy of order . This is the limit of free long strings found by Dijkgraaf, Verlinde and Verlinde [13]. We emphasize that, in this Paper, we will not discuss how this limit is obtained or whether it exists as a bona-fide phase of the 1+1-dimensional gauge theory. We will assume that it is obtained in the limit of strong gauge theory coupling and examine the consequences. In this case, the partition sum will diverge when with defining the Hagedorn temperature.

In the third case in (11), , which is what is generically expected in this supersymmetric gauge theory in the large limit. If the theory is in a deconfined or screening phase, the free energy is proportional to in large ’t Hooft limit where is held fixed. Then, the partition sum does not converge for any value of the parameters. We regard this as a reflection of the Jeans instability of hot flat space. States of the Matrix model which have free energy of order in the large limit correspond to string theory black holes. Divergence of the partition sum is due to the nucleation of black holes which dominate the entropy at any temperature [39]. An explicit evaluation of the perturbative limit of the Yang-Mills theory can be found in refs.[40],[41].

### 1.1 Results:

First of all, to illustrate the point, we quantize the Bosonic closed string using DLCQ and operator methods. We find the spectrum and use it to compute the thermodynamic partition function for free strings. This generalizes previous work [21] to a constant -field. We find that, as in [21], the free energy is given by a discrete sum over Teichmüller parameters,

(12) |

where

(13) |

(14) |

and , the parameter in (3).

The integers take values and . Also,

(15) |

is the average of the imaginary parts of and . In spite of the -dependent difference between and , this partition function is real and exhibits the expected modular invariance. It has a symmetry under the replacement

This symmetry is related to a large gauge invariance of the string theory on a toroidal space, , where is the volume of the torus, is a gauge transformation. The free energy exhibits a symmetry under this gauge transformation.

We then show how the same result can be obtained in a covariant quantization using the path integral where the worldsheets have genus 1. In this case the simultaneous compactification of the Minkowski space null direction and Euclidean time involves an periodic identification of coordinates that contains complex numbers. It further requires that we use a Euclidean -field which is related to the physical Minkowski one by . We can re-obtain the Minkowski one by analytic continuation only after the computation of the path integral is done. Our result then reproduces the one that we obtained using operator methods quoted in (12) above.

We then use the covariant path integral formulation to find the -dependence of the free energy of the Bosonic string at all genera. The result is that, at a given genus, appears only in a constraint on the integration over world-sheets in the path integral. At genus , this constraint restricts the integral over world-sheets to those whose period matrix obeys a condition (see Section 4 for notation)

(16) |

where (i=1,…,g) are 4g integers. This is the condition that the Riemann surface with period matrix is the branched cover of a torus with Teichmüller parameter

identical to that in (2). This result, with was found in [1]. The constraint (16) restricts the integration over all Riemann surfaces in the path integral computation of the free energy to an integral over those Riemann surfaces which are branched covers of a particular torus. The only place that the constant -field enters the partition function is in the geometry of the underlying torus.

Note that the two different parameters and defined in (13) and (14), respectively, have underlying -parameters which are obtained by replacing by in and , respectively.

We then show that essentially the same constraint occurs for the Green-Schwarz superstring in the same geometrical setting. For the superstring, we present an operator quantization (DLCQ) which gets the genus 1 contribution to the partition function. The essential formula for the genus 1 contribution is

An investigation of the superstring at higher genera is not given here. Obtaining the effect of compactification of the null direction on the finite temperature partition function similar to (16), but for the super-geometry of the world-sheet should be straightforward.

We then compare our results for the superstring with what we would expect to obtain from the Matrix string model in the same -field. We show that, if we were to compute the finite temperature partition function of the Matrix string model, we must do a path integral for 2-dimensional supersymmetric Yang-Mills theory where the Yang-Mills field variables live on a torus with Teichmüller parameter . This parameter is identical to the which occurred in the string, given in equation (2).

The hypothesis is that the string degrees of freedom are the simultaneous eigenvalues of the matrices. If the matrix elements are doubly periodic functions on the torus, the eigenvalues of the matrices live on branched covers of the torus. We have shown that it is precisely these branched covers which we should expect to become the world-sheets of strings in perturbation theory, where the genus of the branched cover is the genus of the worldsheet. Thus, we can add the conjecture that summing the Matrix string model partition function over the moduli space of branched covers with the appropriate measure should produce string perturbation theory. This has not yet been done in detail beyond genus 1. However, we can check the example of genus 1 explicitly and we find that the partition function of the Matrix string model - where we sum over all genus 1 (unbranched covers) of the basic torus - and the IIA Green-Schwarz superstring are indeed identical. This was done in the absence of -field in [21]. The extension in this Paper to the case with a constant -field is interesting because the -field modifies the geometry of both the Matrix model and the quantized string in a simple way. Seeing that this change maps correctly through the limit of the Matrix model which produces perturbative strings is a non-trivial check of the Matrix string model.

A by-product of our analysis is an expression for the Matrix model action in a constant -field in the direction,

(17) |

Finally, we examine the Hagedorn phase transition in a finite -field. We observe the interesting fact that for the Discrete light-cone quantized closed bosonic string, when one of the directions in which there is electric -field is compactified, the Hagedorn temperature depends on :

This is remarkable in that it doesn’t depend on the compactification radius, so it must hold even if the compactification radius is arbitrarily large. Of course, without the compactification in the first place, would be independent of and would be the usual closed string value . This non-commutativity of compactifying and going to the Hagedorn temperature is a result of the exponential growth of the density of states of the string which is independent of compactification radius.At the Hagedorn temperature, the thermal distribution of string states is unstable and the most favorable configuration is one long string that contains all of the energy. In order to know about the -field, this long string must wrap the compactified light-like direction. Because of this non-extensive behavior, it always has enough energy to do that, no matter how large the radius .

In Section 2 we shall examine the free energy of a Bosonic string in a background -field. We will use discrete light-cone quantization (DLCQ) and compute the thermodynamic partition function using operator quantization in the light-cone gauge.

In Section 3, we compute the same partition function using the covariant path integral. We do this to illustrate a peculiarity of the compactifications that must be implemented in our computation. It is necessary to do a simultaneous compactification of a null direction in Minkowski space in order to get DLCQ and Euclidean time, in order to get a finite temperature path integral. We shall see that the complex identification of time, contained in formulae (66) and (67) indeed produces a partition function which agrees with the one obtained by operator methods.

In Section 4, we use this technique to examine the effect of these simultaneous compactifications on the string path integral at all genera. We show how they lead to a constraint in the path integral measure which restricts the integration over all Riemann surfaces to an integration over the moduli space of branched covers of a particular torus. We find that the external -field enters the partition function only in these constraints.

In Section 5 we extend our genus 1 results for the bosonic string to the type IIA superstring. We obtain a formula for the genus 1 contribution to the thermodynamic free energy of the DLCQ superstring in a constant -field quoted in (153).

In Section 6, we examine the thermodynamic partition function of Matrix string theory in a -field. We first identify the coupling of a constant -field to the Matrix model. Then we examine the matrix string free energy in the limit where we keep only the diagonal components of all of the matrix fields. We give an explicit derivation of the thermodynamic partition function at genus 1. The result is identical to the genus 1 superstring partition function that we derived in Section 5.

In Section 7, we discuss the Hagedorn temperature in the discrete light-cone quantized system. We note a peculiarity of the Hagedorn temperature. It depends on the -field, but not on the light-cone compactification radius.

In Section 8 we give some concluding remarks.

## 2 Free Energy of the Bosonic String in a B-field: DLCQ

### 2.1 Notation

Let us begin by summarizing some of our notation and conventions. We will use the metric of -dimensional Minkowski space given by

(18) |

where is the vector made from the spatial components of , the light-cone coordinates are

(19) |

and . We will always consider the string in critical dimensions, so that for the Bosonic string and for the superstring.

In Euclidean space

consequently

The closed string worldsheet coordinates are denoted by and where as in [42]. We define

When both the target space and world-sheet are Euclidean

The four Jacobi theta functions that we will use are defined by

(20) | |||

(21) |

where

We shall also denote by

the theta function where the term is absent from the sum.

Under the two generators of the modular group, and , the modular transformation properties of the theta functions are

(22) | |||||

(23) |

and

(24) | |||

(25) | |||

(26) | |||

(27) |

They obey Jacobi’s abstruse identity

(28) |

Jacobi’s triple product formulae are

(29) | |||

(30) | |||

(31) | |||

(32) |

The Dedekind eta function is

(34) |

and has the modular transformation properties

(35) |

We shall also use the notation

(36) |

where .

The Poisson re-summation formula is

(37) |

### 2.2 Action, Equations of Motion and Solutions

The action for the closed Bosonic string on 26-dimensional Minkowski space in the presence of a -field is given by

(38) |

Here we have fixed the conformal gauge for the worldsheet metric. When is a constant, the term in the world-sheet Lagrangian density containing it is a total derivative. For this reason, this term does not alter the equations of motion, which will therefore be independent of . These equations of motion and constraints are

(39) | |||

(40) | |||

(41) |

The conserved world-sheet Noether currents which are associated with space-time translation invariance depend on ,

(42) |

In DLCQ a null direction is compactified. We shall consider the case where is identified with . In closed string theory, the boundary conditions on the worldsheet are periodic,

(43) |

where is an integer which counts the number of times the string world-sheet wraps the compact direction . When the string wraps the compact direction, the Noether charges are influenced by as

(44) |

We will consider the situation where the only non-zero component of is . In that case, the only momentum which is influenced by is and

(45) |

The equations of motion have the light-cone gauge solutions

(46) | |||||

(47) | |||||

(48) | |||||

(49) |

where and are the total momentum along the transverse and directions, respectively. As we shall show, in the presence of a -field, does not coincide with the translation generator along the direction, which we will denote by .

Since the canonical commutation relations are

and is a compact variable, the momentum is quantized as

where is any integer. Substituting the solution (46, 48, 49) into the equations of motion, when , we get the explicit solutions for the in terms of the transverse oscillators

(50) | |||||

(51) |

where . Subtracting these equations for we get

(52) |

where

(53) |

This is the level-matching condition, .

### 2.3 Thermodynamic Free Energy

From ref.[21] it is now easy to construct the free energy of the bosonic string in the presence of a constant -field. The only difference lies in the mass spectrum operator which is now given by (56). We use an expression for the free energy of a single relativistic particle and sum it over the mass spectrum of the string. The result is

(57) | |||

(58) |

where denotes the sum over string states with , and the operators given in (56) and (53). In (58), summation over the integer enforces the level matching condition. The summation over comes from expanding the free energy of the relativistic particle in a series of exponentials. The summation over is the sum over light-cone momenta dual to the compact null direction. Some details about how this formula is obtained can be found in [21]. We use the notation for , and given in (13), (14), (15) and (3). In terms of these parameters, the free energy of the Bosonic string is given by

(59) |

The trace over the string states can be easily computed and reads

(60) |

where the Dedekind eta function is defined in equation (34). This is the free energy density of the DLCQ bosonic string in a constant -field. For , equation (60) exactly reproduced the result obtained in ref. [21].

We shall show in the next section how this free energy can be obtained using the covariant Polyakov path integral.

## 3 Covariant Path Integral

In this section we will calculate the free energy of the bosonic string in the presence of a -field at genus 1 using the covariant approach. We will assume that the metrics of both the world-sheet and the target spacetime have Euclidean signatures. The action in Euclidean space is given by

(61) |

Note that we have also Wick rotated the antisymmetric tensor fields, . We shall show that the compactifications of the target space lead to a well-defined path integral also when is complex. To achieve this result we shall use Seiberg approach to the DLCQ of the bosonic string [29]: We shall obtain a compactified null direction from a compactified spatial direction (given by a small circle of radius ) by an infinite boost along the compact direction. At the end of the computation we shall remove the cutoff and we shall analytically continue back to Minkowski . The cutoff is introduced to show which is the correct prescription to give a path integral representation of the DLCQ free energy. Namely we shall show that the result obtained using Seiberg’s approach to DLCQ can be gotten directly compactifying a null direction () but with a Euclidean , which only in the final equation is replaced by a Minkowskian . This is the procedure we shall adopt in the rest of the Paper.

The spacetime is assumed to be flat 26-dimensional Euclidean space. The genus 0 Riemann surface is topologically the 2-sphere. Since it has no non-contractible loops, it cannot depend on , or .

At genus 1 the world-sheet is a torus. We will take the coordinates of the world-sheet, and to lie in the range . Conformal transformations can always be used to put the metric of the torus in the form

(62) |

where then entries are constants given by the complex Teichmüller parameter

(63) |

The determinant of the metric is

(64) |

and the inverse of the metric is

(65) |

With this gauge fixing of the metric, the integration over the metric in the Polyakov path integral becomes an integration over the Teichmüller parameter.

We shall also assume that is a constant. The factor in front of the term containing comes from analytic continuation. The contribution of the -field term to the world-sheet Lagrangian density is proportional to a total derivative and therefore it depends only on the topology of the configuration. We wish to study the situation where the target space has particular compact dimensions. In this case, string configurations which wrap the compact dimensions have non-trivial topology.

Two compactifications will be needed. The first compactifies the light cone Minkowski space by making the identification . In our Euclidean coordinates it becomes the complex identification

(66) |

The second compactification that we shall need is that of Euclidean time which is necessary to produce finite temperature ,

(67) |

With these compactifications the only relevant component of the tensor is .

Following Seiberg [29] we shall consider the light-like compactification as the limit of a compactification on a space-like circle which is almost light-like

(68) |

with . The light-like circle (66) is obtained from (68) as . This compactification is related by a large boost with

to a spatial compactification on

The introduction of the cutoff , which will be removed at the end of the calculation, is used to make the analytic continuation of the field to the Minkowski , well defined at any stage of the computation.

Compactification is implemented by including the possible wrappings of the string world-sheet on the compact dimensions. In general the bosonic coordinates of the string should have a multi-valued part which should take into account the following boundary conditions

(69) | |||||

(70) | |||||

(71) | |||||

(72) | |||||

(73) | |||||

(74) |

where