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Studying the Higgs -Intro, Tutorial Study, etc.

version 0.20 John Yoh, 12/21/06
Note : The Intro section is designed for Students

We provide here a brief introduction to the Physics of Higgs particle, especially w.r.t. what can be done at the ILC--along with some example Physics performance studies on the Higgs, including one step-by-step Tutorial using the JAS3 package (in progress).


  1. Introduction to Higgs, and ILC capabilities
  2. Example Step-by-step Tutorial on Higgs recoil mass off the Dimuon in ZH into MuMubb
  3. >Some Physics Studies on the Higgs at ILC

Introduction to Higgs, and ILC capabilities

Note : Introductory material suitable for students

The Higgs particle, the only remaining un-discovered particle in the SM (Standard Model), is a particle which we need to discover and understand well. It is the particle that is responsible for the mass of the quarks, etc. Without the Higgs, there is no reason why any particles have any mass !!!!

From previous HEP phsyics results from the measurements of the Top quark and W mass, the simplest SM predict a Higgs mass of below 200 GeV (in fact, near 100 GeV, well below the 114 GeV lower limit set by the LEP experiments--though the uncertainties does allow a Higgs mass up to close to 200 GeV), and thus will be found at the forthcoming LHC collider (schedule to start in Nov. 2007) within the next 5 years. However, just the observation of the Higgs is not sufficient to rule out other models, extension of the SM (such as MSSM--Minimal Supersymmetric extension to the Standard Model---, etc.) To complement what can be studied at the LHC, the ILC is needed to provide much more information on the Higgs, to much better understand the situation.

For example, while the LHC can discover the Higgs --only a few of the cleanest modes can be observed over the huge hadronic backgrounds from the billions of interaction per sec at the LHC.

By contrast, the ILC can produce comparable number of Higgs, but at a much cleaner environment--only a few interactions per sec. In particular, by tagging events with a Dimuon or dielectron from the Z decay, one could reconstruct a recoil mass--and observed a Higgs mass peak--then, by looking at the rest of the particles on events in the Higgs mass peak, one could get an almost-unbiased sample of Higgs, including ALL the decay modes. Such is not possible at the LHC. Thus, a full understanding of the Higgs require the ILC. Of course, there will be competing processes that will give us a Z with similar recoil mass--so, we would need to do a background subtraction to isolate what the Higgs is doing.

Many other phsyics processes are also better studied at the ILC. For example, if Supersymmetry exists, only certain SUSY particles, in a few modes, can be observed at the LHC above the huge background. By contrast, many more SUSY particles can be produced cleanly at the ILC, and studied in more detail.

ZH Recoil mass analysis tutorial is now available as a beta --please check it out an report any problems

So, the current tutorial involves the following steps for each event:

  1. Read the event, unpack the MCParticle
  2. Loop over all the particles, selecting the mu+ and the mu-
    One could set up an array to store the muons momentum, etc.
    Be careful that you only loop over each particle once (the particle list include both initial, intermediate, and final state particles--so, if we count both intermediate and final, we donble-count)
  3. Once the event is identified as one with 2 muons, do some kinematic manipulations to calculate
    first, the mass of the dimuon to make sure it's a Z (e.g. require that the dimuon mass be between 85 to 95 GeV)
    Then calculate the recoil mass off the Z using the Root(S) of the interaction --Use the Conservation of Energy and Momentum to provide 4 equations, with one unknown (mass of the recoil particle)--we can assume an initial Energy of 500 GeV and initial Px, Py, and Pz of 0.
  4. Plot the recoil mass in a histogram
  5. (to be done) Now, do it again, with some Gaussian smearing on the muon momentum measurement---add a smearing of Delta(Pt)/Pt of 2 x 10**-5 (i,e., for a particle with Pt (Transverse Momentum) of 100 GeV/c, the measurement error would be .002 (note there is also an adidtional error that depends on Sin Theta, but let's ignore it for the moment).

This beginner tutorial produces a mass plot of the recoil mass off the dimuon in the generated list --which should be the Higgs--with or without the muon momentum smearing. Note that there is a tail above the Higgs mass peak, due to beamstrahlung (initial or final state emission of additional photons)--in fact in the datafile used, only roughly 1/3 of the events show up at the 120 Gev mass peak. Note that we will not in this tutorial use the simulated event, which would require reconstruction (a far more comlicated process). More realistic physics studies would require such a step.

Future option including FASRMC and perhaps even full simulation/reconstruction.

HIGGS --> DIPHOTONS Tutorial --a 2nd tutorial is now available for beta testing--analysis of ZH --> ZH(-->diphotons), where the recoil mass off the diphoton is now Z, rather that the Higgs (although some of the modes also have other processes, such as WW fusion for v(e)v(e)AA (A=photon) and ZZ fusion for eeAA. The ...AA1... tutorial program in fact provides 11 separate recoil mass histogram, one for each f (fermion) type in the Signal ffHAA and background ffAA.

Some Physics Studies on the Higgs at ILC

Beyond the simple recoil mass tutorial above, we can envisage many follow-up exercises --here's a few examples mostly using the same datafiles. Some of these might be written up as future tutorials (e.g., on how to use the reconstruction moduled)

  1. Do the same recoil mass plot--but, this time, reconstruct the events, and use the reconstructed muon tracks instead. This would require using the reconstruction modules--Note, as of now, the "Production Reconstruction" does not yet exist.
  2. Mass plot using the 2 b quarks (generation level)
  3. Mass plot using 2 reconstructed b jets (single tag, or double tag) and determine the efficiency as well as the mass resolution
  4. Look into possible backgrounds (such as WW, WWj, WW2j, etc.)

Further studies --there are endless variety of processes which can be studied --see 2001 Physics at the ILC Snowmass resource guide. Many of the simple studies mentioned in the "LC Physics resource guide" need to be studied more accurately, with full detector simulation and realistic reconstrucion--as well as to understand better the various backgrounds.

In particular, many backgrounds for the interested physics processes have not been fully understood--or, in some case, even estimated. Backgrounds for a target process includes

  1. Mismeasured target process
  2. Other physics backgrounds other physics processes (such as Zjj, WWjj for ZH into MuMubb).
  3. backgrounds from Gamma-Gamma, e-gamma, etc (which has a much later interaction rate then ee, but most of these should produce only a few particles-p-and not many in the large-angle region.
  4. Backgrounds from machine related processes---beam-gas, due to beam focussing and beam-line magnets, etc.
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