Echoes s s s    

of a

Hidden Valley

at Hadron Colliders

  

 


Hidden Valley -- the Short Story

Hidden Valley Papers

Hidden Valley Talks

Experimental SubPages

Benchmark Models

Monte Carlo Programs

Dumb Jokes about Hidden Valleys

 

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 presence may 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. The first 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.]

 

Hidden Valley Papers

  • 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 and v-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, hep-ph/0511250, and more recently, Carpenter, Kaplan and Rhee, hep-ph/0607204)
  • 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.

 

 

Hidden Valley Talks

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

 

 

Hidden Valley Experimental SubPages [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 Experimental Study

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 experimental signatures;
  • select benchmarks for which well-studied theoretical models are known, with minimal theoretical uncertainties;
  • select benchmarks for which Monte Carlo programs are most reliable

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 current benchmarks:

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 of v-quarks,[with 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 quarks/leptons, and
      • 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.

 

 

Caveats [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!*** since h0 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 good one
  • Thank you Steve Ellis -- yes it used to be the name of a nudist colony