Hidden Valley -- the Short Story
Hidden Valley Papers
Monte Carlo Programs
Dumb Jokes about Hidden
Hidden Valley --- The Short Story [The
Long Story is under construction]
What is a hidden valley? A hidden valley has a few
key features, but in simple (and simplistic) language, it is a hidden
sector that is about to be found. It contains relatively light
particles that one might think would already have been seen, but some
barrier (perhaps energetic) has prevented us from finding these particles
up to now. This barrier is about to be breached.
Hidden sectors with a visible effect are of course not new as an idea; the
oldest example is probably ''mirror matter'', initially just a dark version of the
standard model but further extended over time. String theory produces sectors of this type very
easily. But somehow these ideas have not been mainstream, and the Tevatron and LHC phenomenology had not been fully worked out. The
result is that the Tevatron and the LHC detectors have not been designed
to find some of the phenomena that are typical in these models, and that
few studies of how to find signals of hidden valleys have been performed.
Some of this phenomenology is distinct from, or at least extends somewhat
beyond, the well-studied physics of minimal, or even exotic,
supersymmetric models, strongly-interacting electroweak symmetry-breaking
sectors, little-Higgs, extra dimensions, etc. In particular, the
standard searches for deviations from the standard model using isolated
high-pT jets, isolated leptons, missing transverse momentum, and so forth
do not work well in many hidden valley models. It is for this reason
that I find these models so interesting to explore --- to detect their
require experimental and/or theoretical innovations.
I originally started thinking about this in
2001 after informal discussions with Paul Langacker and Mirjam Cvetic
at the University of Pennsylvania about string theory model building.
talk on the subject was given in 2004, at an "ATLAS Muon Workshop"
held at the University of Washington. [The talk is a 10 MB pdf file,
scanned from transparencies. There are a number of other subjects
covered first; hidden valleys, called "veiled sectors" at this time,
form the last section of the talk.]
- PAPER I (hep-ph/0604261,
written with Kathryn Zurek) explores q qbar ==> Z'
==> v-quarks in detail, and mentions several other qualitatively
different models, including the strange and as yet uncalculated
phenomenology of models with heavy particles charged under both color
- PAPER II (hep-ph/0605193,
written with Kathryn Zurek) is not just
about hidden valleys, though hidden valleys provide one example.
This paper points out that it is quite common for a light Higgs boson,
or a CP-odd Higgs boson, to have decays to new and unknown long-lived
particles, which might be visible in the detector as displaced vertices.
This is an overlooked discovery channel for the Higgs boson, and is
especially important for the Tevatron, whose reach may be extended, and
for the LHCb detector, which could perhaps compete with ATLAS and CMS in
this case. A class of weakly-coupled models is considered, along
with hidden-valley-type models. (Other models with this signature
include Chang, Fox and Weiner,
and more recently, Carpenter, Kaplan and Rhee,
- PAPER III (hep-ph/0607160)
points out that if the LSP in the visible
sector is heavier than the LSP in the v-sector, then the former will
decay to the latter, possibly with a long lifetime. Dramatic
changes to standard supersymmetric phenomenology can result. Some
of the phenomenological signatures have appeared before in the context
of other models (see the references in PAPER III for examples), but
there are quite a few new possibilities as well. These ideas
extend to any model with a new conserved or approximately conserved
global symmetry, such as KK parity or T parity.
- PAPER IV in preparation is planned to take a first
look at the phenomenology of the models of PAPER I, but no promises on
the timing of this paper can be made yet.
Some lectures I have given on Hidden Valley models and
their phenomenology; note this is an evolving story so the older ones are
somewhat out of date. In particular, the current simulation package
only became fully available in September.
Lectures by others on related topics
[nothing proprietary is stored here]
These pages are designed to keep track of issues
relevant to discussions with members of individual detectors
Benchmark Models Appropriate for
This is an evolving story; I have been asked to provide
some appropriate models for study at the various colliders. There
are multiple competing issues that determine the selection criteria for
benchmarks, and it is still not clear what the best criteria are.
Among the choices:
- select benchmarks with the widest variety of v-hadron
lifetimes, to allow for experimental study of highly-displaced vertices
in busy environments;
- select benchmarks with the widest variety of
- select benchmarks for which well-studied theoretical
models are known, with minimal theoretical uncertainties;
- select benchmarks for which Monte Carlo programs are
Also, different benchmarks are appropriate at Tevatron
and LHC, and even within LHC, the LHCb detector is clearly distinct from
ATLAS and CMS. (At some point it may even be interesting to look at
TOTEM, but this is premature.) Because of this, I will be organizing
the benchmark models by detector type.
For the experimentalist (or theorist, for that matter)
wanting to explore hidden valleys, it is important to realize that there
are many distinct classes of hidden valley models. At the moment I
can only provide a partial classification scheme (which is of course not
furnished with sharp dividing lines) that is not complete but can still
serve as useful guidance.
Here are the
Comments on the benchmarks are welcome!!
Hidden Valley Monte Carlo Programs
HV0.4 This program is in design and testing phase; it is
important to understand what it can and can't do. I encourage you to
consult with me before using it; otherwise you're running considerable
risk of error.
Currently this program uses Pythia to simulates v-hadron
production via a Z', as in PAPER I, with a v-sector that is just like QCD
[scaled up in energy, and with all electroweak physics turned off.]
The program will be amended and modified to be
more versatile in the coming months; stay updated.
The program is simple enough [but has many subtleties,
which I'll discuss below]:
- first it simulates q qbar ==> Z' ==> v-quarks;
- then it simulates the v-showering and v-hadronization for three flavors
masses just like QCD, except for an overall rescaling.] This
produces v-pions, v-etas, v-kaons, v-nucleons, etc. In the current
version it is assumed that only two v-flavors couple to the Z', so that only the v-pi-zeros
must decay to standard model particles. A simple variant allows
the v-pi-pluses to decay as well, as can occur when there are
v-flavor-changing neutral currents.
- it then can either
- write out the a Les Houches Accord format event record
(using the new standard) with the
- q qbar in the initial state
- vpions in the final state
- two neutrinos to soak up the energy/momentum of
any invisible v-hadrons which we do not need to keep track of
- or it can
- allow the v-pions to decay (mainly to bottom
quarks, with some taus),
- decay, shower and hadronize the resulting
- generate hadron-level Pythia output
If you use the first approach, your version of Pythia,
or the HV program itself, can read in the above-mentioned Les Houches
Accord format file and do the v-pion decays, ordinary showering and
hadronization, produce the hadron level output. A certain Pythia
setting is necessary to do this correctly, as outline below.
[there are many at this stage...]
- Remember the variety of v-models is enormous.
The current program only does one small subclass. Much more
programming and basic theory remain to be done.
- Currently events are unweighted; no attempt is made
to allow for a computation of a real cross-section. Estimates of
rates for one particular model can be found in PAPER I. The events
are suitable for study of techniques for displaced-vertex
reconstruction, but trigger-efficiency estimates will be rough.
- Currently the charges of the particles under the Z'
are chosen without care; they are the Pythia default values, for no
reason other than simplicity. This means both that rates and
rapidity distributions for the Z' are wrong. I don't think anyone
is yet asking sophisticated enough v-questions for this to be a problem;
it will be fixed over time.
- Currently the Z' production of v-quarks is actually
done with Drell-Yan production, followed by the replacement of the
final-state leptons with v-quarks. This gets v-quark angular
distributions wrong, since the true v-quark charges aren't used.
- FIXED!*** Peter Skands has pointed out an error (which
should not affect results much): the Drell-Yan is being generated with
interference between the Z', photon and Z. This will have small
effects on things, such as trigger efficiency estimates. This
will be fixed in the next version.
- Currently the v-pions, which are new particles not
contained in standard Pythia, are stored in the Les Houches Accord event
record file as Higgs bosons (h0 for v-pi-zero, H0 for v-pi-plus).
As an interim step, this will probably be changed to A0 and H0, ***DONE!***
has many special features in Pythia and in the experiments' software.
As a later step, various options for treating the vpions within Pythia
will be introduced.
- Currently the program produces a LHA event record
with undecayed vpions, or hadron-level Pythia output.
An intermediate step, where the LHC event record
includes decayed vpions and their quark or lepton decay products,
will be added soon. For technical reasons, it is not
possible with the current LHA accord to have an LHA event record
containing only the vpion decay products; to get the decay vertices
right it is necessary to have the vpions in the event record as well.
Dumb Jokes About Hidden Valleys
- Thank you everybody -- yes it is a salad dressing, and not a very
- Thank you Steve Ellis -- yes it used to be the name of a nudist