Suppressed Black Hole Production from Minimal Length

Large extra dimensions lower the Planck scale to values soon accessible. Motivated by string theory, the models of large extra dimensions predict a vast number of new effects in the energy range of the lowered Planck scale, among them the production of TeV-mass black holes. But not only is the Planck scale the energy scale at which effects of modified gravity become important. String theory as well as noncommutative quantum mechanics suggest that the Planck length acts a a minimal length in nature, providing a natural ultraviolet cutoff and a limit to the possible resolution of spacetime. The minimal length effects thus become important in the same energy range in which the black holes are expected to form. In this paper we examine the influence of the minimal length on the expected production rate of the black holes.

The study of models with Large eXtra Dimensions (LXDs) has recently received a great deal of attention. These models, which are motivated by String Theory [1], provide us with an extension to the Standard Model in which observables can be computed and predictions for tests beyond the Standard Model can be addressed. This in turn might help us to extract knowledge about the underlying theory. The models of LXDs successfully fill the gap between theoretical conclusions and experimental possibilities as the extra hidden dimensions may have radii large enough to make them accessible to experiment. The need to look beyond the Standard Model infected many experimental groups to search for such Standard Model violating processes, for a summary see e.g. [2].
Arkani-Hamed, Dimopoulos and Dvali [3] proposed a solution to the hierarchy problem by introducing d additional compactified spacelike dimensions in which only the gravitons can propagate. The Standard Model particles are bound to our 4-dimensional sub-manifold, often called our 3-brane. Due to its higher dimensional character, the gravitational force at small distances then is strengthened as it goes in the radial distance r with the power −d − 1 instead of the usual −1. This results in a lowering of the Planck scale to a new fundamental scale, * sabine@physics.arizona.edu M f , and gives rise to the exciting possibility of TeV scale GUTs [4]. The radius R of the extra dimensions lies in the range mm to 10 3 fm for d from 2 to 7, or the inverse radius 1/R lies in energy range eV to MeV, respectively. Throughout this paper the new fundamental scale is fixed to M f = 1 TeV as a representative value.

II. BLACK HOLES IN EXTRA DIMENSIONS
Using the higher dimensional Schwarzschild-metric [5], it can be derived that the horizon radius R H of a black hole is substantially increased in the presence of LXDs, reflecting the fact that gravity at small distances becomes stronger. For a black hole of mass M one finds The horizon radius for a black hole with mass ≈ TeV is then ≈ 10 −3 fm, and thus R H ≪ R for black holes which can possibly be produced at colliders or in ultra high energetic cosmic rays (UHECRs). Black holes with masses in the range of the lowered Planck scale should be a subject of quantum gravity. Since there is no theory available yet to perform this analysis, we treat the black holes as semi classical objects.
Consider two partons moving in opposite directions. If the center of mass energy of the partons, √ŝ , reaches the fundamental scale, M f ∼ 1 TeV, and if the impact parameter is less than R H , a black hole with mass M ≈ √ŝ can be produced. The total cross section for such a process can be estimated on geometrical grounds [6] and is of order where Θ denotes the Heaviside function and it is assumed that black hole formation is only possible above some minimal mass, M min < √ŝ , which is of order M f . The possibility of forming these TeV-scale black holes in the lab, or in UHECRs respectively, has been examined in a vast number of publications [7][8][9], for only to mention a few. The status has been nicely summarized in [10].
The expression for the cross section contains only the fundamental Planck scale as a coupling constant. We want to mention that the given classical estimate of the black hole production cross section has been under debate [11], but further investigations by [12,13] justify the use of the classical limit. However, the topic is still under discussion [14]. Setting M f ∼ 1TeV and d = 2 one finds σ ≈ 1 TeV −2 ≈ 400 pb. With this it is further found that these black holes will be produced at LHC in number of ≈ 10 9 per year [7].
The above cross section can be derived in String Theory approximations as well as using the Aichelburg-Sexl metric [13]. In the latter case, the Schwarzschild metric is boosted to form two colliding shock-fronts in which trapped surfaces can be calculated, their occurrence depending on the impact parameter. Using this ansatz it is assumed that the shock waves can be boosted to thin fronts, thus neglecting the uncertainty of the quantum particles. This treatment is justified as the particles with energies √ŝ > M f have a position uncertainty smaller than their horizon. We will see that this feature is modified under the assumption of a generalized uncertainty arising from the minimal length.

III. MINIMAL LENGTH
Even if a full description of quantum gravity is not yet available, there are some general features that seem to go hand in hand with all promising candidates for such a theory. One of them is the need for a higher dimensional spacetime, one other the existence of a minimal length scale. As the success of String Theory arises from the fact that interactions are spread out on the world-sheet and do no longer take place at one singular point, the finite extension of the string has to become important at small distances or high energies, respectively. Now, that we are discussing the possibility of a lowered fundamental scale, we want to examine the modifications arising from this as they might get observable soon. If we do so, we should clearly take into account the minimal length effects.
In perturbative String Theory [15,16], the feature of a fundamental minimal length scale arises from the fact that strings can not probe distances smaller than the string scale. If the energy of a string reaches this scale M s = √ α ′ , excitations of the string can occur and increase its extension [17]. In particular, an examination of the spacetime picture of high-energy string scattering shows, that the extension of the string grows proportional to its energy [15] in every order of perturbation theory. Due to this, uncertainty in position measurement can never become arbitrarily small. For a review, see [18,19].
The minimal length scale does not only appear within string theoretical framework but also arises from various approaches, such as non-commutative geometries, quantum loop gravity, non-perturbative implications of T-Duality [20] or an very interesting gedanken experiment using micro black holes as the limiting Planck scale [21].
Naturally, the minimum length uncertainty is related to a modification of the standard commutation relations between position and momentum [22]. With the Planck scale as high as 10 16 TeV, applications of this are of high interest mainly for quantum fluctuations in the early universe and for inflation processes and have been examined closely [23].
In [24,25] we used a model for the effects of the minimal length in which the relation between the wave vector k and the momentum p is modified. We assume, no matter how much we increase the momentum p of a particle, we can never decrease its wavelength below some minimal length L f or, equivalently, we can never increase its wave vector k above M f = 1/L f . Thus, the relation between the momentum p and the wave vector k is no longer linear p = k but a function 1 k = k(p).
For massless particles, m = 0, this function k(p) has to fulfill the following properties: a) For energies much smaller than the new scale we reproduce the linear relation: b) It is an an uneven function (because of parity) and k p.
c) The function asymptotically approaches the upper bound M f .
In general, the above properties have to be fulfilled in the limit m → 0. A particle with a rest mass close to the new scale would experience an additional uncertainty even at rest. However, for all particles of the Standard Model it is m 2 /M 2 f ≪ 1 and these effects can be neglected. The quantization in this scenario is straightforward and follows the usual procedure. The commutators between the corresponding operatorsk andx remain in the standard form. Using the well known commutation relations and inserting the functional relation between the wave vector and the momentum then yields the modified commutator for the momentum This results in the generalized uncertainty principle (GUP) which reflects the fact that by construction it is not possible anymore to resolve spacetime distances arbitrarily good. Since k(p) gets asymptotically constant, its derivation ∂k/∂p drops to zero and the uncertainty in Eq. (5) increases for high energies. The behavior of our particles thus agrees with those of the strings found by Gross and Mende as mentioned above.
The arising modifications derived in [24,25] can be summarized in the effective replacement of the usual measure in momentum space by a modified measure which is suppressed at high momentum where the absolute value of the partial derivative denotes the Jacobian determinant.
In the following, we will use the specific relation [25] for p(k) by choosing whereê µ is the unit vector in µ direction, p 2 = p · p, and ǫ = L 2 f π/4. The factor π/4 is included to assure that for high energies the limiting value is indeed 1/L f . Is is easily verified that this expression fulfills the requirements (a) -(c).
The Jacobian determinant of the function k(p) is best computed by adopting spherical coordinates and can be approximated for p ∼ M f by With this parametrization of the minimal length effects the modifications read

IV. BLACK HOLES AND THE MINIMAL LENGTH
The properties of Planck size black holes raise a bunch of fundamental questions as they exist in a regime where quantum physics and gravity are of equal importance. Even an examination within a not fully consistent treatment can reveal some of the exciting and new issues on the interplay between quantum physics and gravity. One of the features arising is the evaporation of black holes, which has first been derived in a semi classical treatment by Hawking in 1975 [26] and since that time has been reproduced within various approaches.
In particular, the analysis of the last section raises the question for the final state of the black hole. This topic has been discussed in the literature extensively and is strongly connected to the information loss puzzle. The black hole emits thermal radiation whose sole property is its temperature whatever the initial state of the collapsing matter has been. So if the black hole first captures all information behind its horizon and then completely vanishes into thermally distributed particles the basic principle of unitarity can be violated. This happens when the initial state was a pure quantum state and then evolves into a mixed one [27].
When we try to escape the information loss problem we have two possibilities left: the information is released back by some unknown mechanism or a stable black hole remnant is left which keeps the information. Besides the fact that it is unclear in which way the information should escape the horizon [28] there are several more arguments for the black hole relics [29].
The most obvious one is the uncertainty relation. The Schwarzschild radius of a black hole with Planck mass is of the order Planck length. Since the Planck length is the wavelength corresponding to a particle of Planck mass we see that we get in trouble when the mass of the black hole drops below the Planck mass. Then we have a mass inside a volume which is smaller than the uncertainty principle allows [30]. For this reason is was proposed by Zel'dovich that black holes with masses below Planck mass should be associated with stable elementary particles [31]. The question for black holes with regard to the minimal length was also raises by Gross and Mende [15]. They found by an investigation of the spacetime picture for such string scattering that, with an increasing number of the order in perturbation theory, the size of the string decreases relative to the Schwarzschild-Radius of the collision region. The production of black holes thus does not become impossible but increasingly difficult within the minimal length approach.

V. BLACK HOLES AND THE MINIMAL LENGTH IN EXTRA DIMENSIONS
It has been examined which modifications from the GUP arise for the Hawking-Spectrum of the black hole and it has been shown by Cavaglià, Das and Maartens [32] that the black hole is hotter and decays faster into a smaller number of high energetic particles, finally leaving a stable relic. These results agree with our analysis of the Hawking-Spectrum using a geometrical quantization approach [33].
In the following we will examine the production rates for those black holes under the assumption of an minimal length.
For this purpose, consider again two partons with a center of mass energy √ŝ approaching head on in a collision. Now, their modified uncertainty principle will smear out their focussing at energies √ŝ > M f . This will lead to an effective suppression of the black hole for-mation since the probability of the partons to get trapped inside the horizon is diminished.
Using the GUP formalism, we can derive this modification. The cross section Eq.(2) assumes that the black hole captures the total energy of the collision and thus, the mass of the created black hole is highly peaked around M = √ŝ . Due to the high rest mass of the black hole, its remaining momentum is negligible. However, the precise mass of the black hole might be smeared out by a form factor of order one due to energy losses during the formation and modifications of the horizon radius by a non-zero angular momentum [35].
We will neglect this form factor and further assume the distribution which is easily translated into the minimal length scenario by using Eq. (10) This can also be understood by considering the above mentioned picture of the colliding partons. Caused by the impossibility to focus the particles, we would expect the damping to be approximately R H /∆x. With 1/R H ≈ ∆p and Eq. (9) this yields an exponential suppression factor exp(−ǫM 2 ) for the cross section. Thus, agreeing with the result found earlier.
The only colliders which can reach energies above the TeV-scale and therefore potentially produce the discussed black holes are hadron colliders. To obtain the cross section for proton-proton (pp) collisions the partonic cross section Eq.(12) must be integrated over a folding with the parton distribution functions (PDFs) f i (x, Q 2 ). Here, the index i labels the constituent partons of the hadron and s =ŝ/xy is the center of mass energy of the pp-collision.
By definition, the PDFs parametrize the probability of finding a parton i with momentum fraction x of the hadrons momentum at a given inverse length scale Q associated with the scattering process. Usually, this scale is chosen to be the momentum transfer, that is in the s-channel Q 2 ∼ s. Here, investigating the production of black holes, the length scale of the scattering process is limited by the Schwarzschild radius and the generalized position uncertainty 2 , we thus have 1/Q ∼ R H . 2 It turns out numerically that the results do not depend on this distinction.
Further modifications for the PDFs in the GUP scenario, in addition to the modified scaling in Q, are not to be expected. To see this, one has to keep in mind the way in which the experimental data is extracted and further used for the common PDFs, such as the CTEQ4-Tabulars [37].   The non-perturbative physics of an hadron-hadron scattering process can be characterized by functions of x alone at a fixed small Q 0 at which the minimal length effects are negligible. This measured experimental input at small Q 0 is then extrapolated to high Q using the DGLAP 3 equations [36]. The scale dependence goes with ln(Q 2 /Q 2 0 ), signaling that Q−independent Bjorken scaling is violated by QCD-effects at high Q 2 .
An exact analytical examination of the DGLAP is beyond the scope of this paper. However, since the minimal length disables a further resolution of the hadron structure with increasing energy, the effects can effectively be captured in the above assumed Q-definition: above the new fundamental scale, the structure of the hadron is cloaked behind the generalized uncertainty (and it is left to the realm of philosophy to decide whether it would exist at all in that case).
The results for the above derived differential cross section and the integrated total cross-section with use of the CTEQ4-Tabulars [37] are shown in Fig.1 and Fig.2. The calculations for the differential cross section is done for the expected LHC-energies √ s = 14 TeV.
It can be seen that the effect on the production of black holes is noticeable but does not exceed one order of magnitude and thus stays in the range in which several other uncertainties might come into play (such as M min , form factors, energy losses during collapse, numerical factors from the analysis of trapped surfaces, d, angular momentum etc.).

VI. CONCLUSION
In this paper the influence of a minimal length scale on the production of black holes in a model with large extra dimensions was examined. It was found that the finite resolution of spacetime which is caused by the minimal length results in an exponential suppression of the black hole cross-section. Calculation of the total cross-section for LHC-energies in this scenario shows a decrease of the expected number of black holes by a factor ≈ 5.

ACKNOWLEDGMENTS
This work was supported by a fellowship within the Postdoc-Programme of the German Academic Exchange Service (DAAD) and NSF PHY/0301998.