Unravelling the sbottom spin at the CERN LHC
Unravelling the sbottom spin at the CERN LHC
Alexandre Alves1, ∗ and Oscar Éboli1, †
Instituto de F́ısica, Universidade de São Paulo, São Paulo, Brazil
(Dated: 02/04/07)
Establishing that a signal of new physics is undoubtly supersymmetric requires not only the
discovery of the supersymmetric partners but also probing their spins and couplings. We show that
the sbottom spin can be probed at the CERN Large Hadron Collider using only angular correlations
in the reaction pp → b̃b̃∗ → bb̄/pT , which allow us to distinguish a universal extra dimensional
interpretation with a fermionic heavy bottom quark from supersymmetry with a bosonic bottom
squark. We demonstrate that this channel provides a clear indication of the sbottom spin provided
the sbottom production rate and branching ratio into bχ̃01 are sufficiently large to have a clear signal
above Standard Model backgrounds.
I. INTRODUCTION
Supersymmetric models are promising candidates for physics beyond the Standard Model (SM), despite the present
lack of direct experimental evidence on supersymmetry (SUSY). The CERN Large Hadron Collider (LHC) has a large
reach for the discovery of SUSY [1] that largely relies on the production and decay of strongly interacting new particles
i.e. squarks and gluinos [2, 3]. Establishing that a signal of new physics at the LHC is indeed supersymmetric requires
not only the discovery of the new supersymmetric partners but also probing their interactions and spins [4, 5, 6, 7].
Previously, the squark spin has been studied through the long decay chain q̃ → χ̃02 → ℓ̃ → χ̃01 which is also used to
measure its mass [4, 6] or in conjunction with the gluino spin analysis in the decay chain g̃ → b̃ → χ̃02 → ℓ̃ → χ̃01 [7].
In this work we probe the potential of the CERN Large Hadron Collider (LHC) for unravelling the bottom squark
spin using angular correlations in the short decay chain pp → b̃b̃∗ → bb̄/pT , analogously to what has been done
for the analysis of the slepton spin [4, 5]. A nice feature of this reaction is that only sbottoms and the lightest
SUSY particle (LSP) takes place in it, providing a further check of sbottom spin obtained in the long decay chain
studies. Unfortunately, this analysis can not be extended straightforwardly to light flavor squarks due to large QCD
backgrounds.
To determine the spin nature of the sbottoms at the LHC we compare the SUSY sbottom production and decay
chain with another scenario where the new intermediate states have the same spin as the SM particles and leads to
the same final state bb̄/pT . Such a model are Universal Extra Dimensions (UED) [8] where each SM particle has a
heavy Kaluza–Klein (KK) partner which can mimic the SUSY cascade decay, provided we employ the mass spectra
extracted from the decay kinematics match [9]. Here we are not focusing on UED searches but we use UED only for
comparison with the SUSY predictions.
There are many observables which we can use to discriminate ‘typical’ UED and SUSY models, like the production
rate or the mass spectrum. Nevertheless, at the LHC we measure only production cross sections times branching
ratios, and the UED as well as the SUSY mass spectra are unlikely to be what we currently consider ‘typical’. On
the other hand, spin information is generally extracted from angular correlations. Therefore, we base our analysis
exclusively on distributions of the outgoing SM b quarks as predicted by UED and by SUSY. We demonstrate that this
final state bb̄/pT provides a clear indication of the sbottom spin provided the sbottom production rate and branching
ratio into bχ̃01 are sufficiently large to have a clear signal above SM backgrounds.
∗Electronic address: aalves@fma.if.usp.br
†Electronic address: eboli@fma.if.usp.br
http://arxiv.org/abs/0704.0254v1
mailto:aalves@fma.if.usp.br
mailto:eboli@fma.if.usp.br
II. UED INTERACTIONS AND PARAMETERS
We assumed one extra dimension with size R, where all SM fields propagate [8, 9], leading to a tower of discrete KK
excitations for each of the SM fields (n = 0). In this scenario, the 5-dimensional wave functions for an SU(2)–doublet
fermion are
dL cos
dR sin
. (1)
On the other hand, the roles of the left and right handed n-th KK excitations are reversed for SU(2) singlets. Just
like in the MSSM, the spinors of the singlet (q) and doublet (Q) KK–fermion mass eigenstates can be expressed in
terms of the SU(2) doublet and singlet fields ψd,s
Q(n) = cosα(n)ψ
d + sinα
(n)ψ(n)s ,
q(n) = sinα(n)γ5ψ
d − cosα
(n)γ5ψ(n)s . (2)
In general, the mixing angle α(n) is suppressed by the SM fermion mass over the KK–excitation mass plus one–loop
corrections except for the top quark due to its large mass.
tan 2α(n) =
Q + δm
The non–degenerate KK–mass terms δm(n) contain tree level and loop contributions to the KK masses, including
possibly large contributions from non–universal boundary conditions.
The neutral KK gauge fields will play the role of neutralinos in the alternative description of the sbottom production
and decay. Just as in the SM, there is a KK–weak mixing angle which for each n rotates the interaction eigenstates
into mass eigenstates
γ(n)µ = cos θ
µ + sin θ
3,µ ,
Z(n)µ = −sin θ(n)w B(n)µ + cos θ(n)w W
3,µ . (4)
The n-th KK weak mixing angle is again mass suppressed
tan 2θ(n)w =
v2 g gY /2
)2 − (δm(n)B )2 + v2 (g2 − g2Y ) /4
where δm(n) contains tree level as well as loop corrections to the KK gauge boson masses. Generally (δm
2 ≫ v2
g2 − g2Y
[9] and the lightest KK partner is the B(1), with basically no admixture from the heavy
We only considered the first set of KK excitations to formulate an alternative interpretation of production and decay
of sbottoms at the LHC, using the UED decay b(1) → bγ(1) to mimic a sbottom decay. The relevant interactions for
this decay are
Lγ1q1q = igY
q̄(0)γµ
Yd cosα
(1)PL + Ys sinα
(1)PR
Q(1) − q̄(0)γµ
Yd sinα
(1)PL + Ys cosα
(1)PR
γ(1)µ . (6)
In general, the KK partners of the SM particles do not have a mass spectrum similar to what we expect in SUSY,
however, we imposed that the first KK excitations have the same mass as the SUSY particles. Assigning the LSP
mass to the Lightest KK Particle (LKP) and the Next-LSP mass to the Next-LKP mass fixes the KK-weak mixing
angle by means of Eq. (5)
θ(1)w =
arctan
ggY v
2(m2NLSP −m2LSP )
. (7)
test point particle mass (GeV) branching ratio
S5 χ̃01 122. stable
S5 b̃1 633. Br(b̃1 → bχ̃
1) = 26.6%
S5 b̃2 663. Br(b̃2 → bχ̃
1) = 78.8%
SPS1a χ̃01 97. stable
SPS1a b̃1 517. Br(b̃1 → bχ̃
1) = 4.4%
SPS1a b̃2 547. Br(b̃2 → bχ̃
1) = 29%
L1 χ̃01 97.8 stable
L1 b̃1 280. Br(b̃1 → bχ̃
1) = 40%
L1 b̃2 354. Br(b̃2 → bχ̃
1) = 20%
Table I: Masses of sbottoms, the lightest neutralino, and branching ratios of b̃1,2 → bχ̃
1 for the test points considered in our
numerical simulations.
III. EVENT SIMULATION AND TEST POINTS
We considered three scenarios for the new particle spectrum. Our first test point is the LHC point 5 (S5) [10] that
exhibits rather heavy sbottoms with sizeable decays into bχ̃01; see Table I. Our second reference point is SPS1a [11]
where the sbottom masses are close to ones in the first scenario, however the decays b̃1,2 → bχ̃01 are suppressed. The
third test point exhibits a somewhat light squark spectrum which could be eventually produced at a 1 TeV International
e+e− Linear Collider; we denote this point by L1. The masses for the L1 parameter point are mb̃1 = 280 GeV,
mb̃2 = 354 GeV, mt̃1 = 339 GeV, mt̃2 = 371 GeV, and mχ̃01 = 97.8 GeV. For the SPS1a and S5 parameter choices
the lighter of the two sbottoms is almost entirely a left state b̃1 ∼ b̃L while for L1 it is approximately a maximal
mixture of left and right states b̃1 ∼
2/2b̃L +
2/2b̃R. The salient features of these reference points is summarized
in Table I.
In order to correctly treat all spin correlations we performed a parton–level analysis including the UED interactions
in MADGRAPH [12] and using SMADGRAPH [13] for the SUSY simulation. In our calculations we used CTEQ6L1
parton distribution functions [14] with the factorization and renormalization scales are fixed by µF = µR = (mb̃1 +
mb̃2)/2 for the signal and µF = µR = mV +
i pTi for the SM backgrounds, where mV is the mass of the relevant
electroweak gauge boson and
i pTi is the sum of the transverse momentum of additional jets.
We simulated experimental resolutions by smearing the energies (but not directions) of all final state partons
with a Gaussian error. We considered a jet resolution ∆E/E = 0.5/
E ⊕ 0.03 and σ/ET = 0.46
ET for the
missing transverse energy where
ET is the sum of the jet transverse energies. In addition to that we included a
b-tagging efficiency of ε = 60% with a mistagging probability of 1/200 for light jets [15]. No K-factors were applied
to signals or backgrounds since we do not expect QCD corrections to change significantly the shape of kinematical
distributions [2, 3]. Nevertheless, given the increasing interest in studying spins correlations at the LHC by means of
long and short decay chains, it is important to check if this is indeed the case.
IV. ANALYSIS AND RESULTS
At the LHC we analyzed the production of sbottom pairs followed by their decay in a b and the LSP
pp→ b̃1,2b̃∗1,2 → bb̄χ̃01χ̃01 → bb̄/pT , (8)
i.e. the signal is characterized by two b-tagged jets and missing transverse energy. One feature of the reaction (8) is
that the s–channel subprocesses qq̄ → b̃b̃∗ present the well–known angular distribution
d cos θ∗
∝ 1− cos2 θ∗ (9)
where θ∗ is the polar angle of produced scalar particles in their center-of-mass frame. However, this clean signature
of the sbottom spin is contaminated by the subprocess gg → b̃b̃∗ which contains t-channel diagrams and quartic
couplings; see Fig. 1 left panel. Therefore, it is easier to decipher the new state spin if we enhance the importance of
the qq̄ s–channel subprocesses via a judicious choice of cuts. Notwithstanding, the cuts to isolate the signal must not
introduce bias in the angular distributions used to study the sbottom spin.
-1 -0.5 0 0.5 1
gg+qq
no cuts
cosθ*
-1 -0.5 0 0.5 1
gg+qq
S5 no cuts
cosθ*
Figure 1: Left panel: cos θ∗ distribution for the production of sbottoms coming from qq̄ and gg fusions. Right panel: same
distribution as in the right panel but for the production of b(1). We used the test point S5 spectrum.
On the other hand, the center–of–mass angular distribution of KK bottoms in UED produced by qq̄ fusion is
d cos θ∗
∝ 1 +
E2b1 −m
E2b1 +m
cos2 θ∗ , (10)
where Mb1 and Eb1 are the mass and energy respectively of the b
(1) in the center–of–mass frame. This distribution
peaks in the forward and backward directions being quite distinct of the SUSY prediction. Moreover, we must also
include the gg → b(1)b̄(1) which contains t- and s-channel contributions which present a peak towards the forward and
backward regions as well; see Fig. 2 right panel.
At the LHC we can not reconstruct the the polar angle (θ∗) of produced particles in their center-of-mass frame
due to the presence of undetected χ̃01 or γ
(1). Therefore, we must use an alternative variable that retains part of the
information carried by θ∗. A convenient variable to use in our analysis is [5]
cos θ∗bb ≡ tanh
where ∆ηbb is the rapidity separation of the b-tagged jets. Notice that ∆ηbb is invariant under boosts along the
collision axis, and consequently, cos θ∗bb is invariant under boosts as well. The angle θ
bb is the polar angle between
each reconstructed bottom jet direction in the longitudinally boosted frame in which the rapidities of the bottoms are
equal and opposite.
In order to cos θ∗bb carry information of the produced particle spin, the flight directions of sbottoms and bottoms
must be correlated. We depict in the left panel of Fig. 2 the cosine of the opening angle between the bottom and the
sbottom flight directions in the b̃b̃∗ center–of–mass system for the S5 and L1 spectra. Clearly, the bulk of the signal
is concentrated in region of small opening angles as a consequence of the large energy of the sbottoms after cuts.
Fig. 3 shows that cos θ∗bb is indeed strongly correlated to the cosine of production polar angle (θ
∗) of the sbottoms
and b(1)’s in their center–of–mass system. Therefore, we must expect the shape of cos θ∗bb distributions to resemble the
polar angle spectra of sbottoms and KK bottoms apart from some smearing effects due to non-perfect correlations
between the bottoms and sbottoms (KK bottoms) flight directions. Nevertheless, a clear separation between SUSY
and UED distributions should be possible as was demonstrated in the case of smuon pair production at the LHC [5].
Taking a closer look at Fig. 3 we already realize that UED events are slightly more concentrated near cos θ∗bb = ±1
while SUSY events are homogeneously distributed along the direction cos θ∗bb = cos θb̃.
In our analysis, we included the following backgrounds for the process pp→ bb̄/pT :
• SM QCD and electroweak production of bb̄Z with Z → νν̄ that accounts for ≃ 91% of the total background
after cuts for the S5 and SPS1a test points and ≃ 72% for the L1 scenario.
0.5 0.6 0.7 0.8 0.9 1
normalized distributions
cosθsb/b
Figure 2: Cosine of the opening angle between the bottom and the sbottom in center–of–mass system for two different mass
spectra considered here.
-1 -0.5 0 0.5 1
cosθ*
-1 -0.5 0 0.5 1
cosθ*
Figure 3: In the left (right) panel we display a scattered plot cos θ∗bb ⊗ cos θ
∗ for sbottoms (b(1)) production. Here we chose the
parameter point S5.
• Reducible electroweak and QCD backgrounds like jjZ, jjW , jτντ , bb̄j, jj, jjj, and jjb where some of the
decay products evade detection and j stands for a light jet that might be mistagged as a b jet. These dangerous
backgrounds are efficiently reduced by the requirement of two b-tagged jets. The second largest background
after cuts is bbW where the lepton from the W boson has a large rapidity |ηℓ| > 2.5.
• SUSY processes, excluding b̃b̃∗ production, that lead to the final state bb̄χ̃01χ̃01.
In order to properly trigger [16] the event and tag the b-jets [15] the following acceptance cuts were imposed in all
cases
|ηb| < 2.5 , pbT > 100 GeV , ∆Rbb > 0.7 , /pT > 100 . (12)
A potentially large background is the QCD production of dijets once we take into account that mismeasurements
of the jets properties can lead to missing transverse momentum and that this process has a huge cross section. This
background can be efficiently reduced by the missing transverse momentum cut Eq. (12) and by requiring that the
azimuthal angle between the jets and the missing transverse momentum satisfy
|∆Φ(/pT , pTj)| > 0.3 . (13)
In the L1 test point simulations we applied the following cuts not only to enhance the signal and deplete the
background but also to augment the importance of the qq̄ s-channel subprocess
Mbb̄ > 300 GeV , Meff > 600 GeV , |∆Φ(/pT , pTjsoft)| < 2.4 , (14)
where Meff is the sum of all jet and missing transverse momenta and ∆Φ(/pT , pTjsoft) the azimuthal angle between
the missing transverse momentum and the softest jet. After cuts the SUSY signal cross section is 38.1 fb while the
background cross section is 2.4 fb, leading to S/B ≃ 16.6.
For the parameter point S5 we used the following cuts to optimize the signal
Mbb̄ > 600 GeV , Meff > 1 TeV , |∆Φ(/pT , pTjsoft)| < 2.4 . (15)
The SUSY signal cross section at this test point is 4.55 fb with a background of 1.24 fb and S/B ≃ 3.7 while the
qq̄ fusion accounts for ≃ 40% of the signal events. On the other hand, ≃ 45% of the UED signal stems from qq̄
fusion. The cuts employed in this case were harder for two reasons: first, the bottoms are harder in this case once
the sbottom is much heavier than the neutralino compared to L1 case; second, the signal cross section is significantly
smaller than in the L1 case which required a deeper suppression of backgrounds to avoid a severe bias on the signal
angular distributions.
Finally, we imposed the following cuts for the test point SPS1a
Mbb̄ > 600 GeV , Meff > 1 TeV , (16)
which lead to a signal (background) cross section of 1.07 (1.46) fb and S/B ≃ 0.73.
In the left panel of Fig. 4 we show the impact of cuts on the dσ/d cos θ∗bb distributions for SUSY and UED predictions
assuming the S5 spectrum. We normalized the UED signal cross section to the SUSY one. As the background events
populate the bins of larger | cos θ∗bb|, the selection cuts tend to suppress that region enhancing the central region
for both SUSY and UED. The variable ∆Φ(/pT , pTj) is efficient in rejecting dangerous SM backgrounds like dijet
production, however, it has a potential to bias the distributions as we can see in the left panel of Fig. 4. Therefore, a
harder cut on this variable is not recommended in the study of spin correlations.
A natural question at this point is whether we can mimic the SUSY signal by varying the UED mixing angles.
According to Section II there is not very much room to modify the UED Lagrangian to bring kinematical correlations
closer to the SUSY prediction. The KK weak mixing angle θ
w in Eq. (5) is fixed by the KK masses, so we can
not change it while keeping the masses fixed. The same limitations hold when we try to adjust the mixing between
the singlet and doublet KK fermions, described by the angle α(n); see Eq. (2). In contrast to the 3rd–generation
sfermion sector in the MSSM, the UED mixing angle is not a (third) free parameter, even if we move around the
masses invoking boundary conditions. Notwithstanding, for illustration purpose we vary α(1) in the right panel Fig. 4
to check whether the SUSY cos θ∗bb can be reproduced by a UED decay chain with different couplings to the fermions.
From Eq. (6) we see that varying α(1) effectively enhances the left or right couplings of the KK bottom decay into
bottom plus LKP. This figure allows us to see that the changes in the UED parameters are not sufficient to mimic
the SUSY predictions.
We depict in Fig. 5 the cos θ∗bb spectrum with/without adding the background for the S5 test point, an integrated
luminosity of 300 fb−1, and after applying cuts (12) and (15). To avoid using any information but the spin we assume
the S5 spectrum for the UED particles and normalize their production cross section times branching fractions to the
SUSY rate. From Figure 5, we can easily see that the production of fermionic strongly interacting states (UED)
favor large separations (cos θ∗bb) between the b-tagged jets while the production of scalar particles (SUSY) leads to
a rather constant distribution. Note that these distributions indeed resembles the distributions of the production
angles in the center–of–mass system which reflects the correlation between these observables. Moreover, the UED
cos θ∗bb distribution is similar to the SM background one since they take place through similar diagrams containing
KK partners or SM particles with the same spin quantum numbers.
For the assumed integrated luminosity it is rather easy to distinguish between the two models. These two possibilities
can be disentangled, for instance, through the asymmetry:
σ(| cos θ∗bb| < 0.5)− σ(| cos θ∗bb| > 0.5)
σ(| cos θ∗bb| < 0.5) + σ(| cos θ∗bb| > 0.5)
. (17)
-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1
S5 Spectrum
300 fb-1
(a) SUSY all cuts
(b) SUSY basic cuts
(c) UED all cuts
(d) UED basic cuts
-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1
L = 300 fb-1
S5 Spectrum
UED: α(1)  = 0
UED: α(1)  = π/4
UED: α(1)  = π/2
cosθ* bb
Figure 4: Left panel: impact of cuts on the dσ/d cos θ∗bb distributions for SUSY and UED signals. We normalized the UED
signal cross sections to the SUSY ones using the S5 spectrum. Right panel: SUSY and UED dσ/d cos θ∗bb distributions without
backgrounds but varying the KK bottom mixing angle α(1). The basic cuts are the acceptance defined in Eq. (12) plus
Mbb̄ > 600 GeV.
-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1
Total Bckg
300 fb-1
cosθ* bb
-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1
total bkg
others
300 fb-1
cosθ* bb
Figure 5: In the left (right) panel we plot the cos θ∗bb distribution without (with) the addition of the background contribution.
Here we used the parameter point S5 and assumed an integrated luminosity of 300 fb−1. The error bars indicate the expected
statistical uncertainties.
This asymmetry is +0.238 ± 0.023 for the UED spin assignment while the SUSY interpretation it is significantly
larger +0.373± 0.022 where the quoted errors are statistical. We estimate that an integrated luminosity of 300 fb−1
is needed to reach a 5σ level signal for the S5 spectrum.
The determination of the sbottom spin is much easier for the test point L1 due to the large b̃ → bχ̃01 branching
ratio and production cross section. We depicted in Fig. 6 the cos θ∗bb distribution and without/with adding the SM
background. In this case a mere integrated luminosity of 15 fb−1 is enough to discriminate at the 5σ level UED
and SUSY. The asymmetries are given by +0.565± 0.019 for SUSY and +0.365± 0.021 for UED for this integrated
luminosity. Therefore, there is a clear distinction between the two spin assignments (UED × SUSY) for both S5 and
L1 mass spectra and the above integrated luminosities.
The results for the reference point SPS1a are quite different from the S5 and L1 ones. Due to small b̃ → bχ̃01
branching ratio, the SM backgrounds play a major role. We can see in the left panel of Fig. 7 that the pure SUSY
and UED cos θ∗bb spectrum are quite different. However, once we add the SM background, which has a shape similar
to UED, it is no longer easy to separate the SUSY from UED, even for an integrated luminosity of 1 ab−1; see the
-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1
Total Bckg
50 fb-1
cosθ* bb
-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1
total bkg
others
50 fb-1
cosθ* bb
Figure 6: Same as Fig. 5 but for the parameter point L1 and an integrated luminosity of 50 fb−1.
-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1
naive background subtraction
SPS1a
1 ab-1
cosθ* bb
-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1
total bkg
others
SPS1a
1 ab-1
cosθ* bb
Figure 7: Same as Fig. 5 but for the parameter point SPS1a and an integrated luminosity of 1 ab−1.
right panel of Fig. 7. For instance, the asymmetry (17) is 0.282± 0.019 for SUSY and 0.200± 0.019 for UED and this
extremely large luminosity. In this case SUSY and UED can be discriminated only at at ∼ 4σ level. Therefore, it is
important for this point to subtract the SM background which can be estimated, for instance, from the measurement
of the bb̄µ+µ− cross section. If we neglect systematic errors associated to this extraction from data, the sbottom
spin can be determined at 5σ level for an integrated luminosity of ≃ 500 fb−1. The left panel of Fig. 7 displays the
distributions of SUSY and UED after a naive background subtraction where we just did not add the background to
the signals and the error bars calculated as
S +B. A dedicated study is necessary to determine the actual impact
of the statistical and systematic errors [17].
In general, we noticed that the expected shape of distributions in cos θ∗bb are similar to the ones in smuon pair
production and decay to µχ01 [5], which shows the universality and robustness of the method. Moreover, in both cases
(b̃∗sbx and µ̃∗µ̃) the S5 spectrum seems to be more promising as compared to the SPS1a spectrum, for example,
SUSY versus UED discrimination of smuon spin assignments is possible in the case of S5 (SPS1a) for an integrated
luminosity of 200 fb−1(500 fb−1).
As a final remark note that, apart from negligible effects from bb̄ initiated contributions which include interactions
with gluinos and electroweak interactions with Z bosons, the production rate does not depend upon the left or right
nature of the sbottom since the QCD interactions to the gluons are blind to these details. On the other hand, the
sbottom–bottom–neutralino vertex is sensitive to the relative content of mass eigenstates in terms of left and right
states which by its turn depend upon the mass spectrum. As we have pointed out in the Section III the lightest
sbottom is almost entirely a left state in the S5 mass spectrum, while it is an equal mixture of left and right states
in the L1 mass spectrum. As can be seen in the Figs. 5 and 6, it seems plausible to conclude that the distributions
are not sensitive to the particular mixtures of left and right states of the sbottom mass eigenstates, however a more
detailed study including more test points is necessary to confirm this indication.
V. CONCLUSIONS
In the near future the LHC will start its endeavor in searching for new physics signals. Once those signals have been
identified as the production of new states the next logical step will be the determination of the underlying model among
all candidates by measuring the properties and interactions of the new particles. The size of production cross sections
and mass spectra might provide valuable hints about the underlying model, nevertheless, an undisputed discrimination
will only be possible after determination of the spins of the new states. We showed that the discrimination between
a SUSY spin interpretation against an UED one is possible in the case of scalar bottom (fermionic KK bottom) pair
production in the reaction pp → bb̄/pT . Using the variable proposed in Ref. [5], see Eq. (11), we demonstrated that a
clear determination of the spin of the decaying strongly interacting particle is possible provided the production cross
sections and branching ratios into bχ̃01 are sufficiently large.Even in the worst scenario studied, the SPS1a spectrum,
the determination of spins might be possible after background subtraction, however, a dedicated study of the impact
of statistical and systematic errors on the spin determinations will be necessary in this test point.
Acknowledgments
We would like to thank Tilman Plehn for insightful comments and Fabio Maltoni and the CP3 team for their help
with the implementation of UED model into Madgraph. This research was supported in part by Fundação de Amparo
à Pesquisa do Estado de São Paulo (FAPESP) and by Conselho Nacional de Desenvolvimento Cient́ıfico e Tecnológico
(CNPq).
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http://arxiv.org/abs/hep-ph/0505105
http://arxiv.org/abs/hep-ph/9709356
http://arxiv.org/abs/hep-ph/9809319
www.ph.ed.ac.uk/~tplehn
http://arxiv.org/abs/hep-ph/0605067
http://arxiv.org/abs/hep-ph/0510418
http://arxiv.org/abs/hep-ph/0601186
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http://arxiv.org/abs/hep-ph/0510204
http://arxiv.org/abs/hep-ph/0201195
	Introduction
	UED interactions and parameters
	Event simulation and test points
	 Analysis and results
	Conclusions
	Acknowledgments
	Bibliography
