Photon and Weak Boson Physics: Part 2

W/Z PT Distributions
Z -> b bbar
W/Z cross section as luminosity monitor
resummation
photon isolation
K_T smearing
direct photons + gc production
NLO QCD corrections to W + b bbar production

I will be introducing the first three topics. There are issues which will be discussed in the Working Group IV session on resummation ( George Sterman), photon isolation (J. Qiu), K_t smearing (Marek Zielinski), photons production (Steve Kuhlmann), and NLO QCD corrections to W + b bbar production (S. Veseli).

First is the motivation for the Z -> b bbar studies. The Tevatron would like to investigate the
Higgs. Minimal supersymmetric models favor a light intermediate mass Higgs in the range
80 < m_H< 130 GeV. The Standard Model accomodates this range of mass, also. Integrated
luminosity thresholds for a Higgs mass go as follows:

60 GeV/c² -> 120 GeV/c²
5 fb^-1 -> 25 fb^-1

Present plans are to have 2 fb^-1for Run 2A, 10 fb^-1for Run 2B and 100 fb^-1 for Run 3. This is 20, 100, and 1000 times L_Run1respectively. Run 2 may increase its integrated luminosity to 20 fb^-1
^{for the reasons stated above.}

^{Which Higgs decay is most accessible at the
Tevatron? The branching fractions for the Higgs}
^{are listed below for some interesting modes.}

^M_H^{80 GeV        120 GeV       160 GeV        >160 GeV}
     H-> b bbar       ~1           0.8         0.03
      H-> gg          ~10^-3 2^.10^-3 7^.10^-3
      H-> W⁺W^-                  ~0.1        ~1.0       ~1.0
      H-> t⁺t^-          0.07        0.06        4^.10^-3

So in the mass range 80-120 GeV G(H-> b bbar ) ~1, G(H-> gg ) ~10^-2, G(H-> t⁺t^- ) ~0.1 .
What's more, since LHC has enormous top backgrounds to the qq'-> WH (H-> b bbar), this
mode isn't viable at the LHC(pp - 14TeV).

Hence , if one understands backgrounds for H-> b bbar then the Tevtraon is a place with excellent potential to discover a law mass Higgs.

As a demonstration of understanding a resonant decay to b bbar at a hadronic collider Z-> b bbar has been observed at the Tevtraon/CDF for the first time.
A data set of inclusive muon (5^.10⁶) is required to have two secondary decay vertices and also pass some cuts on kinematic variables driven by the difference between QCD background (qq->bbbar, gg->bbbar), where there is a color connection between the initial state partons and final state quarks, and an electroweak color singlet resonance final state such as the Z. The final results for the counting experiment is ~70+-20 events, and for the maximum likelihood analysis 91+- 30 events.

The Z PT distribution has an impact on resummation calculations of multiple soft gluon emissions at low PT ( CDF: < 20 GeV). At high PT NLO QCD perturbation theory agrees with CDF data better than 20% for momenta < 100 GeV. The data at low PT is magnified and shown here. The W PT distribution is fit with the Z PT data and additional theory input. It is show in both momentum space and impact parameter space. This distribution introduces a 20 MeV uncertainty into the W mass measurement. The D0 results for W PT including the theoretical calculation of Arnold and Kaufman and Z PT results are shown.

The issue of whether the W/Z cross section will be used as a luminosity monitor for Run 2 will be discussed in detail by Gervasio Gomez in WG IV sessions. One can ask what processes might make good luminosity monitors....y -> mm or ee, Dijets, Y-> mm or ee. It is believed that theoretical calculations on the W and Z cross section are good to 2-3%. However, theory predicts a shift in the cross section upwards by 30% when Tevatron cm energy increases from 1.8 to 2 TeV. Hence this cross section has not yet been measured for Run 2. Also, in the equation for the rate R=LsAe both the acceptance and efficiency enter. After the run starts the acceptances (dead channels) and efficiencies of the detectors involved in the monitoring will change, hence one could imagine several luminosity monitors running on different processes and having none of them agree.
At present the CDF and E710+E811 total cross sections for Run 1 (measured by the extrapolating the slope of the elastic cross section at low -q² down to zero, counting elastic and inelastic events, and using the optical theorem) do not agree. (CDF ~80+-2 mb and E710/E811 ~72+-2 mb).