Transcript Direct Detection of Supersymmetric Particles in Neutrino Telescopes Z. Chacko University of Arizona I. Albuquerque & G.

Direct Detection of
Supersymmetric Particles in
Neutrino Telescopes
Z. Chacko
University of Arizona
I. Albuquerque & G. Burdman
Historically cosmic rays have played a very important
role in particle physics. Led to the discovery of
positron, muon and pion.
However there has been very little recent progress in
the direct detection of exotic particles. Why is this?
Four reasons jump out!
1. Require centre of mass energy in collision to exceed the
mass of any exotic particle being produced. However flux
of high energy cosmic rays falls rapidly with energy.
2. Cross-section for heavy particle to be produced falls rapidly
with centre of mass energy.
Therefore number of events falls very quickly with mass of
exotic. Relatively few events!
3. Heavy particles typically have very short lifetimes. A tau is only
20 times heavier than a muon. However,
The short lifetime makes it difficult to characterize exotic particles.
4. How does one distinguish an exotic particle from the
surrounding debris?
These reasons make identification of an exotic very challenging!
An enormous detector can compensate for the reduced number
of events, at least partially.
This is exactly what large neutrino telescopes offer! These
consist of cubic grids of Cerenkov counters placed in ice or
water that can detect charged particles passing through. Each
side of the grid is of order a kilometer across.
Neutrino telescopes offer a new opportunity for the direct detection
of charged,quasi-stable exotics produced by cosmic rays.
There remains the problem of identifying exotics. Are there
interesting candidate theories that predict quasistable charged
particles?
Consider supersymmetry! Supersymmetric theories have a
discrete symmetry called R-parity, under which all Standard
Model particles are + but all supersymmetric particles are -- .
This means the `Lightest Supersymmetric Particle’, the `LSP’,
is stable!
What is the LSP? This is generally either the neutralino or the
gravitino, depending on the supersymmetry breaking scale.
If the LSP is the gravitino, the `Next-to-Lightest Supersymmetric
Particle’, the `NLSP’, is typically the superpartner of the
right-handed tau, the right-handed stau. In a large class of
theories, including gauge mediated models and superWIMP
Models, this particle is long-lived and can be directly detected in
neutrino telescopes.
How are the staus produced? High energy neutrinos passing through
the earth collide with nucleons, resulting in production of a pair of
supersymmetric particles, which decay promptly to staus. This is
the supersymmetric analogue of the weak processes which give rise
to muon events in neutrino telescopes.
The signal is a pair of parallel, upward going charged tracks.
What are the dominant diagrams?
How large is the corresponding cross-section?
We see that cross-section for stau production is 3-4 orders of
magnitude below cross-section to Standard Model particles.
Why is this? For a neutrino of energy E to produce a pair of
particles whose masses sum to M, the parton it interacts with
needs to carry a parton momentum fraction
where m is the mass of the nucleon. Since supersymmetric
particles are heavier, they require bigger values of x for fixed
neutrino energy E. Since parton distribution functions fall with
x, the supersymmetric cross-sections are much smaller.
Naively, this would imply that any signal of supersymmetry
would be swamped. However, this does not happen! The
reason is that the smaller cross-section for supersymmetric
particles is compensated for by their much larger range.
Why does the range matter?
Define P as probability any given neutrino will give rise to an event
in the detector. Then, assuming P is much less than one,
where Pμ is the probability of a muon event, n the number density
of nucleons in the earth, σμ the neutrino-nucleon cross-section to
muons and Lμ the muon range. Similarly, the probability of a
supersymmetric event
Since stau range is larger than muon range as much as three orders
of magnitude, this can partially compensate for reduced cross-section.
Why is the muon range so much smaller? Consider formula for
electromagnetic energy loss,
where a(E) and b(E) are slowly varying functions of energy. While a(E)
represents energy loss due to ionization, b(E) represents energy loss
from radiation. At high energies, radiation losses dominate. Crudely,
b(E) scales as the inverse of the mass of the propagating particle.
Since the stau is much heavier than the muon, it travels much further.
Stau range can be as large as thousands of kilometers.
(careful study by Reno, Sarcevic and Su)
Since stau range is very large, the stau tracks will appear nearly
parallel. What is their separation in detector?
We can estimate the angular separation of the tracks by
where E is the energy of the incident neutrino and M is of order the
masses of the supersymmetric particles. Then track separation
This naïve estimate is justified by a more detailed calculation. The
signal is therefore two parallel charged tracks about 100m apart.
What are the possible backgrounds? Parallel tracks arising from
independent single muon events are very rare. Instead, the largest
background arises from Standard Model processes which result in
two muons. The main source of these di-muons is the production
of a charmed hadron, which subsequently decays semi-leptonically
to a muon.
Here Hc is a charmed baryon and Hx is a strange or non-strange
baryon.
However we expect most of the di-muon background can be
eliminated by making cuts on track separation and angular
distribution. Since the muon range is only of order 10 kilometers,
much smaller than the stau range, the muon tracks, even if parallel,
are only very rarely more than 30m apart. It may also be possible to
distinguish individual muon tracks from stau tracks.
How large are the signals? For this we need to know the incident
neutrino flux. We will use the Waxman-Bahcall and
Mannheim-Protheroe-Rachen bounds as a guide.
Results show that first concrete evidence for supersymmetry may
emerge from neutrino telescopes. See also analyses of Bi, Wang,
Zhang & Zhang, and Ahlers, Kersten & Ringwald.
As explained earlier, the track separations of signal and background
tend to be very different.
Conclusions
Neutrino telescopes may provide the first concrete evidence for
weak scale supersymmetry.
More generally, any theory that predicts quasi-stable charged
particles at the weak scale which can be pair produced through
neutrino-nucleon interactions can be probed in this way.