Dark Matter and Collider Physics in Split-UED
Seong Chan Park, Jing Shu
Institute for the Physics and Mathematics of the Universe, University of Tokyo, Kashiwa, Chiba
277-8568, JAPANAbstract. Kaluza-Klein dark matter is an attractive weakly interacting massive particle in universal
extra dimension model. In the recent extension "split-UED", annihilation of Kaluza-Klein dark
matter with a mass range 600 − 1000 GeV provides excellent fits to the recently observed excesses
in cosmic electron and positron fluxes of Pamela, ATIC and Fermi-LAT experiments. The cosmic
gamma-ray flux in the same process can be significant around 300 GeV, thus can be observed orconstrained by the forthcoming Fermi-LAT diffuse gamma-ray data. The collider signal at the LHC
is the resonance in the dijets channels and the large missing energy in the missing energy plus jets.
Keywords: dark matter, cosmic ray, extra dimension, Kaluza-Klein parity, LHC
PACS: 11.10.Kk, 95.35.+d, 98.70.Rz

MODEL: UED AND SPLIT-UED
Universal Extra Dimension (UED) [1] and its recent extension split-UED [2] are flat
extra dimension models based on a S1 /Z2 orbifold. All standard model fields are universally propagating through the 5D bulk so that their Kaluza-Klein (KK) excited states
are phenomenologically important if the compactification scale is low (1/R ∼ TeV).
The model has an underlying symmetry, called the KK parity, that prevents the lightest
KK odd state to decay. If such a state is neutral, then it provides a viable dark matter candidate. In the minimal setup, the gauge group is the same as the standard modelone, G = SU(3)c × SU(2)W × U(1)Y . Quarks and leptons are also alleviated to higher
dimensional ones and their KK spectra are doubled. One chirality of their zero modes
is projected out by the orbifold condition, leaving the other one as the standard modelfermions. The most prominent feature of split-UED is the presence of (double) kinkmasses for fermions while keeping the KK parity. The five dimensional action of splitUED is the same as the minimal UED (MUED) except the 5D bulk mass terms:
Ssplit−UED = SmUED −Z4d xZ L−L√ijg m5 (y)Ψ̄i Ψ j(1)where Ψi are the bulk Dirac spinors containing quarks and leptons in their zero modes
ijij
and the kink mass, m5 (y) = µ5 θ (y), is introduced with a step function defined asθ (y > 0) = 1 and θ (y < 0) = −1. Here ± signs depends on the the chirality of thezero mode.
Once 5D bulk mass is introduced, KK fermions get additional masses and becomeheavierm2n = m20 + kn2 + µ52(2)where the first term, second term and third term are coming from the ordinary SM
Yukawa interaction, the momentum of the extra dimension and the 5D bulk mass,respectively. kn is determined by kn L = nπ for KK even modes and the n-th solutionof µ5 = ±kn cot kn L for KK odd modes.
Having the generic idea of split-UED, we can control the KK spectra by turning onsome bulk mass parameters. In order to suppress the hadronic annihilation cross section
and avoid the unnecessary flavor problems, we first choose our 5D bulk masses forij
quarks to be universal and larger than the typical KK scale, which is µq = µ5 δi j . For thecharged leptons, in order to control their annihilation cross section ratio among different
flavors which are dominantly coming from the right-handed components, we choose
separate 5D bulk masses µeR , µµR and µτR to achieve that. We left the bulk masses ofthe left-handed leptons to be small to evade the experimental constrains from the lepton
flavor violation and four fermion operators.

FITTING PAMELA, ATIC AND FERMI-LATThe KK dark matter (B1 ), mainly annihilates into fermion pairs among which charged
lepton pair is dominant. Stable particles, like proton (p), electron (e− ) and photon (γ ),and their antiparticles are generated from the casecade decay of hadrons and heavycharged leptons in the B1 B1 annihilation process and propagate to the Earth. In [3, 4]
we calculate the electron, positron and gamma-ray flux from the annihilation of B1 andcompare them with the recent observations from PAMELA [5], ATIC [6] and FermiLAT collaborations [7].
In Fig 1, we fit the positron fraction (e+ /(e− + e+ )) in the Pamela energy range (left)and electron plus positron flux (e+ e− ) in the Fermi-LAT range (right) with mB1 = 900
GeV. For fitting the ATIC "peak and sharp drop-off pattern", a lower mass mB1 = 620
GeV is preferred as shown in [3]. The anti-proton flux is not a problem in split-UED
because the hadronic branching fraction is suppressed by heavy KK quark masses. In
the figures we took mq1 = 3mB1 so that the hadronic branching fraction is less than 10%.

Mass = 900 GeV, propagation: M1
mq ~ 3mΒ

Mass = 900 GeV, propagation: M1
mq ~ 3mB

1E J (e + e ) (GeV m s sr )1-1

1-2 -1

1HESS syst. uncertainties

100background

0.01

Torii et al. 2001
DuVernois et al. 2001
Torii et al 2008.
ATIC
HESS 2008
HESS 2009
Fermi

+HEAT
AMS 01PAMELAe : µ : τ = 1 : 1 : 1 BF = 320

background

3+-+-e /(e + e )20.1

e : µ : τ = 0.3 : 1 : 0.3 BF = 550
110100E (GeV)1000

1010e : µ : τ = 1 : 1 : 1 BF = 320
e : µ : τ = 0.3 : 1 : 0.3 BF = 550

1001000

E (GeV)FIGURE 1. (Left) The positron fraction (e+ /(e+ + e− )) with mB1 = 900 GeV, compared with PAMELA
data. The mass of KK quark is taken to be about 3mB1 . The black curve is for e : µ : τ = 1 : 1 : 1 case andthe blue curve is for 0.3 : 1 : 0.3 case. (Right) The total flux of positron and electron with mB1 = 900 GeV.

In Fig. 2, we show the gamma-ray signal for Inner Galactic plane region and theregion of intermediate galactic latitudes. The black curve is the sum of primary and ICScontributions, the blue and red lines are the gamma-ray from τ ± and hadrons from B1
dark matter annihilation, respectively, and the magenta lines are contributions of threecomponents of ICS. In the region of low energy (E <∼ 10 GeV), the dark matter signal
is much smaller than the observed data, however, starting from few tens of GeV, thesignal is about a factor of 2 ∼ 3 smaller. A characteristic of our model is that a bumpat E ≈ 300 GeV can be seen, due to the main contributions from primary τ ± and also
the ICS from the star light in a subleading way, which can be checked soon if higher
energy of gamma ray data is available. We also show the case of universa KK lepton
in dashed line for reference. The behavior is similar to the non-universal case, but withmore energetic gamma-ray due to more hard e± and more τ ± .
1e-05
oprimary -- Lepton
primary -- Hadron
total (primary + ICS)

0.25 <_l<_ 29.75
o

1e-05

primary -- Lepton
primary -- Hadron
total (primary + ICS)

o

1:1:1

0.3 : 1 : 0.3
CMB
1e-06
IR2SLo330.25 <_l<_ 360-1-2CMB2oo

1e-06

IRo

0.25 <_|b|<_ 4.75

0.0001-2-1-1

Fermi syst. uncertainties

E dJ/dE (GeV cm sr sec )Fermi preliminaryo-1o

E dJ/dE (GeV cm sr sec )o0 <_l<_ 360 , 10 <_|b|<_ 20SL

1e-07
0.1

1

10100

1000

1e-07
0.1

1E (GeV)

10100

1000

E (GeV)FIGURE 2. The gamma-ray signal from B1 dark matter annihilation for the regions of Inner Galactic
plane (left) and intermediate galactic latitudes (right). The black solid and dashed curves are the sum of
primary and Inverse Compton Scattering (ICS) contributions when the final state charged lepton branchingratios are (0.3 : 1 : 0.3) and (1 : 1 : 1) respectively. The blue and red lines are the gamma-ray from τ ± andhadrons from B1 dark matter annihilation, respectively, and the magenta lines are contributions of threecomponents of ICS in the (0.3 : 1 : 0.3) case.

COLLIDER PHYSICSAt the LHC, the 1st KK colored particles (KK quarks and gluons) in split-UED are
copiously produced and further decay into hard jets, soft jets/leptons and dark matter(missing energy), see Fig. 3 (left). Because of the large mass splitting between the 1stKK quark q1 and 1st KK gluon g1 , we can separate out the production of g1 g1 , g1 q1
and q1 q1 by using the Meff cuts. We focus on the q1 q1 production, which has two hardjets so that we can distinguish the signals from the SM background. The split-UED q1
pair signal is simulated by scaling the SUSY signatures with the same mass spectrum.
After the appropriate cuts, our signal is well above the SM background. We calculate
the MT 2 distribution of the q1 pair signals as shown in [3], and its end point reflects theinformation for combination of q1 and g1 masses. Although we are unable to determine
the mass of individual particle in split-UED, its predictions of the signatures of q1 q1
production at the LHC, e.g. Meff and MT 2 , can be examined whether they are consistent
with the cosmic-ray signatures in the near future, and vice versa.

1.61σ(u1u1)

SM dijet backgroundMg2 = 1.58TeVMg2 = 2TeV

1.2σ(u1d1)

1d σ/dMJJ (pb/GeV)

cross section (pb)

1.4

0.8
0.6
0.4
σ(d1d1)

0.2

Mg2 = 3TeV
10-1

10-200500

1000µ (GeV)

1500

2000

1000

1500

2000

2500

3000

3500

4000

MJJ (GeV)FIGURE 3. (Left) The cross sections as functions of the bulk mass µ with fixing 1/R = 900 GeV. Here,u1 (d1 ) includes both uL1 and uR1 (dL1 and dR1 ) contributions. (Right) The invariant mass distribution ofthe dijet signal. The resonance peaks appear at the Mg2 for 1.58 (for ATIC/PPB-BETS), 2 and 3 TeV.

Because of the KK number violation at the tree level, the KK even gauge bosons,especially the 2nd KK gluon (g2 ), can be singly produced at a large rate. We expect todiscover the resonance in the dijet channels against the large QCD background [8] evenin the early stages of LHC (L =100 pb−1 ). To explore this possibility, in Fig. 3 (right),
we plot the invariant mass distribution of QCD dijets simulated by Madgraph [9] (withrough acceptance cuts |η | < 2.5 and pT > 500 GeV to reduce the SM background). We
have found that even at the early stage, one can discover the g2 just below 4 TeV. For theLHC running time, a g2 with its mass up to more than 6 TeV can be discovered [2].

ACKNOWLEDGMENTS
This paper is based on works [2, 3, 4] with C.-R. Chen, M. Nojiri and M. Takeuchi.
These works were supported by the World Premier International Research Center Initiative (WPI initiative) by MEXT, Japan. SCP and JS were supported by the Grant-inAid for scientific research (Young Scientists (B) 21740172) and (Young Scientists (B)21740169) from JSPS, respectively.

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