SUSY at the Pole

0.1 MB·English

by  Joern Kersten

SUSY at the Pole
7
0 J. Kerstena∗
0
2 aThe Abdus Salam ICTP, High Energy Physics Section, Strada Costiera 11, 34014 Trieste, Italy

n
a We study the role neutrino telescopes could play in discovering supersymmetric extensions of the Standard
J Modelwithalong-livedstaunext-to-lightestsuperparticle. Insuchasetup,pairsofstausareproducedbycosmic

5 neutrino interactions in the Earth matter. In optimistic scenarios, one can expect several pair events per year
1 in a cubic kilometre detector such as IceCube. We also show that no significant event rate can be expected for
decaysof staus stopped in thedetector.

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1 1. INTRODUCTION Earthbeforedecayingunlessthegravitinoisvery
2 light. Thus,wecantreatthestauasastablepar-
1 Cosmic neutrinos reach energies of at least ticle in the following.
6 1011GeV. In collisions with nucleons inside the
Cosmic neutrino interactions produce pairs of
0 Earth, the centre of mass energy exceeds 1TeV
SUSY particlesbecauseofR parityconservation,
h/ already for neutrino energies of about 106 GeV. which quickly decay into a pair of staus. Due
p Consequently, supersymmetric particles can be
to the large boost factor, these could then show
- produced,providedthatSUSYexistsclosetothe
p up as upward-going, nearly parallel tracks in a
e electroweak scale. Hence, it is natural to ask neutrino telescope like IceCube [1], as suggested
h whether SUSY could be found in cosmic ray ob-
in [2].
v: servatories. Unfortunately, this is not possible in
i mostscenarios,sincethe producedsuperparticles
X
immediately decay into the lightest one (LSP), 2. STAU PRODUCTION AND PROPA-
r which is electrically neutral and thus not observ- GATION
a
able.
The interactions leading to the production of
This problem can be avoided, if the next-
superparticles are analogous to the charged and
to-lightest superparticle (NLSP) is charged and
neutralcurrentneutrino-quarkinteractionsinthe
long-lived. This is possible, for example, if the
Standard Model (SM). Instead of a W or a Z, a
LSP is the gravitino, the superpartner of the
chargino or a neutralino is exchanged, resulting
graviton, and if R parity is conserved. The most
in a squark and a slepton. We calculated the
natural charged NLSP candidate in this case is
cross section for two different SUSY mass spec-
thelighterstau,usuallycomposedpredominantly
tra [3]. The first one is given by the benchmark
of the scalar partner of the right-handed tau. Its
pointcorrespondingtoSPS7[4]. Thesecondone
decay length L is roughly given by
(denoted by “min m” in the following) consists
−6 2 of squarks at 300 GeV and sleptons, charginos
L ≈ mτ˜ m3/2 Eτ˜ , and neutralinos ate100 GeV. In any case, the
2R⊕ (cid:18)100GeV(cid:19) (cid:18)400keV(cid:19) (cid:18)500GeV(cid:19) SUSYcrosssectionisseveralordersofmagnitude
(1) smaller than its SM counterpart, mainly due to
the much heavier particles in the final state.
where m3/2 is the gravitino mass and R⊕ is the TravellingthroughtheEarth,thestausloseen-
Earthradius. Aswewillonlyconsiderstauswith
ergychieflyduetoradiativeprocessesathighen-
energies above 500 GeV, they cross the whole
ergies. The resulting energy loss scales with the
∗E-Mail:[email protected] inverse particle mass, so that it is much smaller
1
2
for staus than for muons. Hence, while muons
have to be produced not more than a few tens of
kilometres outside the detector to be observable,
staus are visible even if produced much farther
away [2,5]. This may compensate for the smaller
production cross section.
3. STAU DETECTION
For the high-energy cosmic neutrinos, we as-
sumed the Waxman-Bahcall flux E2F(E ) ≈
ν ν
2·10−8cm−2s−1sr−1GeV per flavour [6]. Note
that currentdata allowfor a flux largerby about
an order of magnitude [7], so that we may have
underestimated the number of staus correspond-
ingly. Fig. 1 shows the results for the spectra of
muons and staus, where an improved treatment
oftheenergylossatlowenergies[8]wasemployed
compared to [3]. Note that muons always domi-
nate over staus if one considers the spectrum in
Figure 1. Fluxes of upward-going muons and
terms of the energy deposition in the detector,
staus for two different SUSY mass spectra (see
which is the actual observable. Therefore, the
text for details).
total rate of one-particle events can be used for
reconstructing the neutrino flux without taking
into account the contribution from NLSPs [3].
IceCube will be able to identify the two tracks
fromastaupair,iftheirseparationisgreaterthan
about 50m and less than 1km [9,10]. Assuming
that this separation can be approximately calcu-
lated from the angle between the initial SUSY
particles, we obtained the rates of stau pairs
shown in Fig. 2. The total number of stau pair
events per yearinIceCube is about5 forthe min
m scenario. For the SPS 7 mass spectrum, it de-
creases to 0.07, chiefly due to the larger squark
measses and the accordingly smaller cross section
for the production of superparticles [8,3].
These numbers have to be compared to the
background of muon pairs. While the number
ofupward-goingmuonsarrivingatthesametime
just by coincidence is tiny, a non-negligible num-
ber of muon pairs arises from SM processes, for
exampleiftheinitialneutrino-nucleoninteraction
produces a muon and a hadron which decays to
Figure 2. Rates of parallel stau tracks through
another muon. However, as muons have to be
a detector like IceCube for two different SUSY
produced close to the detector, they will always
mass spectra (see text for details).
be separated by less than 50m in IceCube and
thus not contribute to pair events [11].
3
4. STOPPED STAUS trinos and a SUSY mass spectrum corresponding
to an SPS benchmark point. However, up to 50
Staus with a very small energy will stop in the
events per year are possible, if the neutrino flux
detector and decay after a while, predominantly
is closetoits experimentallimitandifthe super-
into a gravitino and a tau lepton. The latter can
particles are comparatively light.
produce an observable cascade. If this cascade
We have also discussed an alternative signal
can be correlated with an upward-going particle
from the decay of long-lived SUSY particles that
track ending at the same position, which may be
are stopped in the detector. Even in the most
possible in IceCube for stau lifetimes less than
optimistic case, not more than a few events per
a few hours [10], this provides another virtually
decadecanbeexpectedinthedetectorscurrently
background-free signal that does not rely on the
under construction, so that we have to conclude
observation of stau pairs. In addition, the stau
that this signalwillnotbe observableinthe near
lifetime could be measured. Let us therefore es-
future.
timate the rate of such events in the optimistic
min m scenario,
ACKNOWLEDGEMENTS
d3Ne
∆A∆E
dAdtdE(cid:12)E=mτ˜ I would like to thank Markus Ahlers and An-
≈ 10−2(cid:12)(cid:12)km−2yr−1 1km2(2·10−3 GeV 1km) dreas Ringwald for the collaboration leading to
100GeV cm [3],onwhichthistalkisbased,MichaelRatzand
≈ 0.02yr−1 , (2) Christian Spiering for valuable discussions, and
lastbutnotleasttheorganisersofNOW2006for
where the first quantity in the first line is the the marvellous workshop.
flux of (single) staus at the stau rest energy ac-
cording to Fig. 1. Moreover, ∆E is the energy
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