Anomalous Couplings of the Gauge Bosons
Introduction
The trilinear gauge boson couplings are a feature of the Standard Model. They
can be measured using gauge boson pair final states. Deviations from the
predictions of the standard model are signs of new physics at a large mass
scale. Two main signatures are an increased cross-section at large values of
s in the parton frame, and an increased cross-section for high
Pt gauge bosons.
Current status of limits on anomalous couplings
The best limits on the W+gamma channel (the one we are evaluating in
the context of CMS) come from the
Tevatron. The
experiments D0 and
CDF have both published recent
limits. The
D0 data set the tightest limits to date on the dimensionless constants
lambda (< 0.3) and delta-kappa (<0.94).
Studies for CMS
Although ATLAS
studied the W+gamma and W+Z channels for the
Technical Proposal, Kate and I are among the first to
look at them in CMS.
W+Gamma signal
The basic cuts are rather straight-forward. You look for an isolated lepton
from the semi-leptonic decay of the W (only clean signature), and an isolated
high Pt photon. To ensure the lepton is associated with the W one also
cuts on missing Pt.
QCD corrections at LHC energies are large, and thus we need to accept
events with the topology W+gamma+0jet and W+gamma+1jet. Only in the exclusive
0-jet channel does any remnant of the Born level radiation zero remain.
Backgrounds
The main backgrounds are from
- Radiative W
- W + jet
- b+b-bar
- t+t-bar
Radiative W decays
To reduce the background from FSR radiation from
the lepton one demands that the minimum invariant mass (a function of the
lepton and photon 4-vectors and the missing Pt) is greater than the
W mass, and that the lepton and photon are widely separated in y-phi
space.
W+jet
The problem here, of course, is the problem of the jet hadronizing as a hard
pi-zero thus faking a photon.
Kate has been working on this! Ask her if you want to know the details.
b b-bar (+gamma)
From this channel, we get real leptons (from the b quarks) and real jets which
once again can fake a photon. There is also a possibility of producing a real
high Pt photon at the parton level.
Peter is looking at this channel, at first sight (no pile-up) our standard
cuts reduce this background to <10% of the signal.
Future work
There is still a lot to do, especially on making sure that the signal
acceptance cuts are still sensible when a realistic amount of pile-up has been
included. We haven't looked at the top quark channel, although this is quite
significant. Currently all of our
W+gamma (+jet) simulation uses the Baur and Ohnemus
generator (includes NLO QCD corrections which are significant at the LHC). We
need to see the effect of changing some "internal" parameters which affect the
relative production of 0-jet and 1-jet events.
Our CMS-Week (Madison, September 1997) talk!
The colour PostScript version of the overheads is
available (zipped postscript, 0.5Mb).
We shared the work, but Kate made the actual presentation.
More Information, feedback etc.
Please EMAIL us to comment, correct, suggest, volunteer etc! We can also
provide a list of relevant publications (look out for Kate's thesis in late
1998).
Produced on 18/09/1997 by Dr Peter R Hobson
Senior Lecturer, Institute for Physical & Environmental Sciences
Brunel University, Uxbridge UB8 3PH G.B.
Telephone: +44(0)1895 274000 FAX: +44(0)1895 272391
EMAIL: Peter.Hobson@brunel.ac.uk