Physics Studies in ATLAS-Toronto Group

We are carrying on: the search for Higgs with VBF H-->tau + tau channel and ttH, H-->bb channel; the measurement of Higgs mass with VBF H-->ZZ*-->bbll; and study on Jet(s) + Etmiss signature. Brief introduction is listed as below.

• VBF H-->tau + tau, Rachid Mazini

At the LHC, the production cross-section for a Stand Model (SM) Higgs boson is dominated by gluon-gluon fusion. The second largest comes from the fusion of the vector bosons (VBF) radiated from initial-state quarks. For m_H < 2*m_Z, vector boson fusion amounts in leading order to about 20% of the total production cross-section and becomes more important with increasing Higgs mass. In VBF channel, the Higgs boson is accompanied by two jets in the forward regions of the detector. In addition, central jet activity is suppressed due to the lack of color exchange between the quarks. Therefore jet tagging in the forward region of the detector together and a veto of jet activity in the central region are useful tools to enhance the signal-to-background ration.

According to the decays of tau pairs, the final state can be qqll+neutrinos (leptonic), qqlh + neutrinos (semi-letponic), and qqhh + neutrinos (hadronic). The study on VBF channel, can provide additional potential for Higgs discovery, measurement of Higgs boson parameters like couplings to fermions, total width and so on, a chance to detection of an invisible Higgs. Here are some reports on this study from Rachid:

1. Prospects of Higgs Physics with early ATLAS data, 18 April, 2006
2. Progress on tau fake rate from QCD jets, 22 February, 2007
3. More information about the study on this VBF channel is available at Web-page of Higgs group's meeting



• ttH, H-->bb, Bin Guo, Lianliang Ma, Pekka K. Sinervo

The associated production of a Higgs boson (both SM-like and MSSM) with a top-antitop pair, is one of the most promising reactions to study both top quark and Higgs boson physics at the LHC. Associated ttH production will furthermore provide the first direct determination of the top quark Yukawa coupling, allowing to discriminate, for instance, a SM-like Higgs from a more general MSSM Higgs.

The pp-->ttH channel can be used in the difficult search for an intermediate mass Higgs (100 - 130 GeV). In this mass region, the associated top production cross section is quite high but still smaller than the leading gg--> H and qq-->Hqq cross sections by two orders and on order of magnitude, respectively. However, since the final state ttH signature is extremely distinctive, especially if identifying the Higgs through its dominant H-->bb decay become realistic. We are studying the semi-leptonic decay of WW pair, so there are one lepton (electron, muon, not studying tau now), six jets ( 4 b-jets, 2 light jets), and missing ET. The analysis result heavily depends on the b-tagging, and the correction combination of jets for mass reconstruction is also another trouble.
1. ttbar Higgs, 17, April, 2007
2. For more information about this channel, please visit Web-page of ttH, H-->bb group



• VBF H-->ZZ*-->bbll, Christophe Le Maner
In the mass region below 160 GeV, the Higgs boson has a probability of a few percents to decay to a pair of a virtual and an on shell Z boson. If both of the Z bosons decays to a pair of muons or electrons, one gets a very clear signature which is considered to be the golden signature for the discovery of a light Higgs boson. However, the branching fraction of a Z boson to a pair of electrons or muons is about 3.4% while for a pair of down-type quarks it is about 15%. So with this channel, we can take the advantages of this relatively high decay probability, of avoiding saturation by light quarks QCD backgrounds, and of the special characteristic of VBF channel. Report on this study:
1. Observability of a light VBF higgs in the H-->ZZ*-->bbll channel, 12, December, 2006



• Jet(s) + Etmiss, Pierre Savard, etc.
This topic is the study of one working-group, coordinated by Professor Pierre Savard. The physics topics with jets + Etmiss can include:
1. Large Extra Dimensions (LED) (one jet)
2. Supersymmetric LED (2 jets)
3. Leptoquarks (2 jets)
4. Split Supersymmetry (one jet)
5. Z boson branching ratio to neutrinos (one jet)
6. Little Higgs (2 jets)
7. Supersymmetric Dark Matter searches
• mSUGRA
• MSSM
• nMMSM
• ?
Its Twiki page is providing you the latest information about this study.