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large extra dimensions have recently attracted a lot of interest . they provide many new theoretical ideas to explore questions in particle physics . most excitingly , they also predict signals which will be tested at the upcoming collider experiments . in this talk we focus on the scenario of universal extra dimensions ( ueds )  @xcite . in ueds , all standard model ( sm ) fields propagates in extra dimensions of size @xmath1 . there are many theoretical motivations for this scenario , such as electroweak symmetry breaking  @xcite , proton decay  @xcite , the number of generations  @xcite , neutrino masses  @xcite , etc . the current experimental bounds allow kaluza - klein ( kk ) states in ueds to be as light as a few hundred gev  @xcite . such light kk states can be copiously produced at the current or future colliders with center of mass energies @xmath2 tev . it is therefore interesting to study their collider phenomenology , and one might expect that it would be easy to discover these kk states . however , although their production cross section can be very large , their subsequent detection is non - trivial because they decay nearly invisibly . the phenomenology of ueds shows interesting parallels to supersymmetry . every standard model field has kk partners . the lowest level kk partners carry a conserved quantum number , kk parity , which guarantees that the lightest kk particle ( lkp ) is stable . heavier kk modes cascade decay to the lkp by emitting soft standard model particles . the lkp escapes detection , resulting in missing energy signals .    given that the signals are very similar to supersymmetry ( susy ) , it is important to distinguish them by studying the properties of the new states produced in colliders in details . linear colliders often provide a clean environment for such kind of studies . however , in @xmath3 collisions , the signals will be plagued by the huge two photon backgrounds due to the softness of the observable decay products . on the other hand , the @xmath0 mode , as we will see , stands as a unique opportunity for a detailed study of the kk excitations of the electrons . this talk was based on work in collaboration with konstantin t. matchev and martin schmaltz  @xcite . let us first describe the model . the simplest ued scenario has all of the standard model fields ( no supersymmetry ) propagating in a single extra dimension . in 4 + 1 dimensions , the fermions [ @xmath4 , where upper ( lower ) case letters represent @xmath5 doublets ( singlets ) ] are four - component and contain both chiralities when reduced to 3 + 1 dimensions . to produce a chiral 4d spectrum , we compactify the extra dimension on an @xmath6 orbifold . fields which are odd under the @xmath7 orbifold symmetry do not have zero modes , hence the unwanted fields ( zero modes of fermions with the wrong chiralities and the 5th component of the gauge fields ) can be projected out . the remaining zero modes are just the standard model particles in 3 + 1 dimensions .    the full lagrangian of the theory comprises both bulk and boundary interactions . gauge and yukawa couplings and the higgs potential are contained in the bulk lagrangian in one - to - one correspondence with the couplings of the standard model . the boundary lagrangian interactions are localized at the orbifold fixed points and do not respect five dimensional lorentz invariance .    ignoring the localized terms for the moment , the tree - level mass of the @xmath8-th kk mode is @xmath9 where @xmath10 is the radius of the compact dimension , and @xmath11 is the zero mode mass . the spectrum at each kk level is highly degenerate except for particles with large zero mode masses ( @xmath12 ) . the bulk interactions preserve the 5th dimensional momentum ( kk number ) . the corresponding coupling constants among kk modes are simply equal to the sm couplings ( up to normalization factors such as @xmath13 ) . the feynman rules for the kk modes can easily be derived ( e.g. , see ref . @xcite ) . in contrast , the coefficients of the boundary terms are not fixed by standard model couplings and correspond to new free parameters . in fact , they are renormalized by the bulk interactions and hence are scale dependent  @xcite . one might worry that this implies that all predictive power is lost . however , since the wave functions of standard model fields and kk modes are spread out over the extra dimension and the new couplings only exist on the boundaries , their effects are volume suppressed . we can get an estimate for the size of these volume suppressed corrections with naive dimensional analysis by assuming strong coupling at the cut - off . the result is that the mass shifts to kk modes from boundary terms are numerically equal to corrections from loops @xmath14 . we will assume a symmetry which exchanges the two orbifold fixed points . the boundary terms on the two fixed points are equal under this symmetry . it is anomaly free and preserved by the radiative corrections of the bulk interactions . most relevant to the phenomenology are localized kinetic terms for the sm fields , such as @xmath15,\ ] ] where the dimensionless coefficients @xmath16 and @xmath17 are arbitrary and not universal for the different standard model fields . these terms are important phenomenologically for several reasons : ( _ i _ ) they split the near - degeneracy of kk modes at each level , ( _ ii _ ) they break kk number conservation down to a kk parity ( due to the symmetry between the two fixed points , ) under which modes with odd kk numbers are charged , ( _ iii _ ) they may introduce new flavor violations .    for simplicity and definiteness , we will concentrate on the minimal universal extra dimensions ( mueds )  @xcite , where all boundary terms are assumed to vanish at some cutoff scale @xmath18 . at a lower scale , they will be generated by radiative corrections from the bulk interactions which are calculable . this is analogous to the case of the minimal supersymmetric standard model ( mssm ) where one has to choose a set of soft supersymmetry breaking parameters at some high scale in order to study its phenomenology . in some sense , our choice of the boundary terms may be viewed as analogous to the simplest minimal supergravity boundary condition  universal scalar and gaugino masses .    given this choice of the boundary condition , we can calculate the corrections to the kk spectrum , which depends on the boundary terms at low scales . since the corrections are small one can use the one - loop leading log approximation . in addition to the boundary terms , the kk spectrum also receives corrections from pure bulk effects . loops wrapping around the compact extra dimension with nonzero net winding numbers break the 5-dimensional lorentz invariance and therefore also correct the kk mass formula [ tree - spectrum ] . the bulk corrections are finite and exactly calculable due to their non - local nature . all these corrections , including both bulk and boundary contributions , were computed at one - loop in ref . @xcite . an example of the spectrum for the first level kk modes is shown in fig . [ fig : spectrum ] . typically , the corrections for kk modes with strong interactions are @xmath19 while those for states with only electroweak interactions are a few percent . we find that the corrections to the masses are such that @xmath20 . the lightest kk particle @xmath21 , is a mixture of the first kk mode @xmath22 of the @xmath23 gauge boson @xmath24 and the first kk mode @xmath25 of the @xmath26 @xmath27 gauge boson . we will usually denote this state by @xmath21 . however , note that the corresponding `` weinberg '' angle @xmath28 is much smaller than the weinberg angle @xmath29 of the standard model  @xcite , so that the @xmath21 lkp is mostly @xmath22 and @xmath30 is mostly @xmath25 . the mass splittings among the level 1 kk modes are large enough for the prompt decay of a heavier level 1 kk mode to a lighter level 1 kk mode . but since the spectrum is still quite degenerate , the ordinary sm particles emitted from these decays will be soft , posing a challenge for collider searches . the terms localized at the orbifold fixed points also violate the kk number by even units . however , assuming that no explicit kk - parity violating effects are put in by hand , kk parity remains an exact symmetry . the boundary terms allow higher ( @xmath31 ) kk modes to decay to lower kk modes , and even level states can be singly produced ( with smaller cross sections because the boundary couplings are volume suppressed ) . thus kk number violating boundary terms are important for higher kk mode searches . once the radiative corrections are included , the kk mass degeneracy at each level is lifted and the kk modes decay promptly . the collider phenomenology of the first kk level is therefore very similar to a supersymmetric scenario in which the superpartners are relatively close in mass  all squeezed within a mass window of 100 - 200 gev ( depending on the exact value of @xmath10 ) . each level 1 kk particle has an exact analogue in supersymmetry : @xmath32 bino , @xmath33 gluino , @xmath34 left - handed ( right - handed ) squark , etc . the decay cascades of the level 1 kk modes will terminate in the @xmath21 lkp ( fig .  [ fig : transitions ] ) . just like the neutralino lsp is stable in @xmath10-parity conserving supersymmetry , the @xmath21 lkp in mueds is stable due to kk parity conservation and its production at colliders results in generic missing energy signals .    at hadron colliders , the largest cross sections for kk states come from strongly interacting kk quarks and gluons because of the approximately degenerate kk spectrum . the level 1 kk states have to be pair produced due to kk parity conservation . their cross sections at the tevatron run ii and the lhc can be found in ref . @xcite and  @xcite . in mueds , the heaviest kk state at level 1 is the kk gluon @xmath35 . it decays to a kk quark and an ordinary quark . the @xmath5 singlet kk quarks will decay to @xmath21 directly . the signature is jets@xmath36 , similar to the traditional squark and gluino searches  @xcite . using the existing studies of the analogous susy studies , one might expect that run ii can probe @xmath37 gev while the lhc reach for @xmath38 can be @xmath39 tev  @xcite . however , the jets will be relatively soft in the case of mueds due to the approximate degeneracy of the kk initial and final states , and the measured missing energy is correlated with the energy of the soft recoiling jets hence also small , even though the total missing mass is large . they may cause more difficulties in triggering and separation of backgrounds . further studies in the context of mueds are needed to obtain a better estimate of the reaches with the jetty signatures . the @xmath5 doublet kk quarks will mostly decay to kk @xmath40 and @xmath41 bosons , then subsequently decay to kk leptons and @xmath21 . they give rise to the much cleaner signatures with multi - leptons . using the very clean 4 lepton channel in ref . @xcite we estimate that run iib can go slightly beyond the current indirect bounds ( @xmath42 gev ) from precision data , and lhc can extend the reach to @xmath43 tev . although lhc will be able to extend the reach much beyond the current bounds , to identify the extra - dimensional nature of the new physics once some signals are found will be rather challenging . all signals from level 1 kk states look very much like supersymmetry  all sm particles have `` partners '' with similar couplings . of course , the spins of these sm particle partners are different in susy and ueds , but this difference will most likely escape detection at a hadron collider . in addition , all the observed jets , leptons , and @xmath44 are relatively soft . it would be difficult to know exactly the mass scale of these kk states . additional experimental information will be needed to identify which scenario of new physics is realized and the mass scale associated with it . linear colliders provide a clean environment for precise studies of particle properties . taking the slepton production in supersymmetry as an example , one can get a precise measurement of the slepton mass and the lsp mass from the end points of the final state lepton energy distribution  @xcite . the flat energy distribution between the end points will also tell the scalar nature of the sleptons produced . therefore , a linear collider can be a great tool to distinguish ueds from susy . given the bounds on the masses of the level 1 kk modes , the required energy for pair producing them is probably too high for the first stage of the next generation linear colliders , but may be reachable with energy upgrades or at clic . however , in @xmath3 collisions there are huge backgrounds from the two photon processes for very soft final state leptons or jet ( fig .  [ fig:2photon ] ) . soft leptons or jets are produced by the collisions of the two soft photons from the initial state radiation , while the initial @xmath45 and @xmath46 go down the beam pipe , resulting in large missing energies . the two photon background for slepton search at a 500 gev linear collider is studied in details in ref . it was shown that for very soft final state leptons ( @xmath47 ) , the two photon background is still several order magnitude larger than the signal even after the cuts . in the case of mueds , the mass splittings between the kk leptons and @xmath21 are only about 13% . therefore the leptons coming from decays of the kk leptons will be completely buried in the two photon background . kk quarks receive larger mass corrections and the jets coming from their decay would be harder . their detection in the @xmath3 collisions is more promising .    on the other hand , only kk electrons can be produced in the @xmath0 collisions ( if there is no flavor violation from the boundary terms ) . the signal will be two soft electrons of the _ same _ charge plus large missing energy , while the two photon processes produce fermions of the opposite charges . by charge identification one can remove the problematic two photon background . therefore , the @xmath0 mode of a linear collider stands as a unique opportunity for kk electron studies . all particle properties of the kk electrons should be able to be measured unambiguously , providing valuable information for identifying the corresponding new physics . it would also be useful to study possible lepton flavor violations if they are present as in the case of supersymmetry  @xcite . universal extra dimensions with compactification radius near the tev scale promise exciting phenomenology for future colliders . all standard model particles have kk partners which can be produced with enormous cross sections at the lhc . however , the subsequent decays produce very soft standard model particles which can be difficult to see above the backgrounds , hence their detection and identification are rather non - trivial . the @xmath3 collider has one more piece of information as the total missing energy can be measured , but also suffers from the huge two photon background if the sm particles from the decays of the kk states are too soft . the @xmath0 collider provide a unique clean environment for the detailed studies of the kk electrons . as different possible new physics scenarios often give rise to complicated but sometimes similar collider phenomenologies , it is essential to have as many tools as possible to provide enough experimental information for our understanding of any new physics . i would like to thank k.  t. matchev and m.  schmaltz for collaboration on this work . i also thank n.  arkani - hamed , m.  chertok , a.  cohen , b.  dobrescu and b.  schumm for useful discussions . this work is supported by the department of energy grant de - fg02 - 90er-40560 . t.  appelquist , h .- c . cheng and b.  a.  dobrescu , phys . d * 64 * , 035002 ( 2001 ) , hep - ph/0012100 . n.  arkani - hamed , h .- c . cheng , b.  a.  dobrescu and l.  j.  hall , phys . d * 62 * , 096006 ( 2000 ) , hep - ph/0006238 . t.  appelquist , b.  a.  dobrescu , e.  ponton and h.  u.  yee , phys .  rev . lett .   * 87 * , 181802 ( 2001 ) , hep - ph/0107056 . b.  a.  dobrescu and e.  poppitz , phys . lett .   * 87 * , 031801 ( 2001 ) , hep - ph/0102010 . t.  appelquist , b.  a.  dobrescu , e.  ponton and h.  u.  yee , hep - ph/0201131 . k.  agashe , n.  g.  deshpande and g.  h.  wu , phys .  lett . b * 514 * , 309 ( 2001 ) , hep - ph/0105084 . t.  appelquist and b.  a.  dobrescu , phys .  lett . b * 516 * , 85 ( 2001 ) , hep - ph/0106140 . cheng , k.  t.  matchev and m.  schmaltz , hep - ph/0204342 . cheng , k.  t.  matchev and m.  schmaltz , hep - ph/0205314 . c.  macesanu , c.  d.  mcmullen and s.  nandi , hep - ph/0201300 . d.  a.  dicus , c.  d.  mcmullen and s.  nandi , phys . d * 65 * , 076007 ( 2002 ) , hep - ph/0012259 . h.  georgi , a.  k.  grant and g.  hailu , phys . b * 506 * , 207 ( 2001 ) , hep - ph/0012379 . t.  g.  rizzo , phys . d * 64 * , 095010 ( 2001 ) , hep - ph/0106336 . b.  abbott _ et al . _ [ d0 collaboration ] , phys . lett .   * 83 * , 4937 ( 1999 ) , hep - ex/9902013 ; 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universal extra dimensions ( ueds ) with compactification radius near the tev scale provide interesting phenomenology at future colliders . 
 the collider signals of the first kaluza - klein ( kk ) level are very similar to those of a supersymmetric model with a nearly degenerate superpartner spectrum . 
 the heavier first level kk states cascade decay to the lightest kk particles ( lkp ) , which is neutral and stable because of kk - parity . 
 the signatures involve missing energy and relatively soft jets and leptons which can be difficult for detection . 
 the kk electron signal in @xmath0 collisions is free from the problematic two photon background therefore provides a unique opportunity for a detailed studies of the kk electrons in the universal extra dimension scenario . 
 june 3 , 2002    efi-2002 - 86 +    * universal extra dimensions + at the @xmath0 colliders *    .5 in    * hsin - chia cheng *    .5 in    _ enrico fermi institute , the university of chicago , chicago , il 60637 , usa _    .5 in