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 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