1. Search for rare decays of W bosons
(A proposal)
Andrea Bocci
Theory expectations
Experimental Signature
Prospects and Conclusions
Disclaimer: I didn’t do any preliminary study, all estimations I report here is just my personal (educated) guesses to be taken with a grain of salt.

2. The Radiative Decay W   + 
Not very much literature on these decays, everybody refers to this seminal work:

3. More from that paper…
Motivations

4. More from that paper…
Calculations

5. A lot of potential to improve substantially the PDG limit
Theory Expectation vs. Experimental Limit
Final Expectations
UA1: 0.7 pb-1 at 630 Gev
Limit <
UA2: 13 pb-1 at 630 Gev
Limit <
CDF Run 0: 4.2 pb-1 at 1800 Gev
Limit <
CDF Run Ia: 16 pb-1 at 1800 Gev
Limit <
CDF Run Ib: 83 pb-1 at 1800 Gev
Limit <
A lot of potential to improve substantially the PDG limit
PDG 2006

6. A more recent intriguing paper

7. The Experimental Picture
Why I believe we can greatly improve the actual limit:
A lot more integrated luminosity (80 -> 1600 pb, a factor 4.5 in sensitivity)
Better triggers (sharper turn-on, lower pt)  higher signal efficiency
Better simulation (tuned on single pions ! )  better background rejection
Extended pion acceptance with the forward tracking
Better analysis techniques (counting experiment -> binned likelihood)
All the aspects of this analysis are well within the expertise of our group and I think that with the data available now we can already hit the 10^-6 limit

8. The Experimental Picture - II
Signature: High pt photon + one high pt (isolated) track, back-to-back
Spectacular W peak !!
(sigma ~ 2-3 GeV)
Dataset: Two triggers can be used. The entire photon spectrum is covered:
PHOTON_DIJET: Used for the  + W/Z(qq) analysis but suitable for this search as well (pt>12 GeV, 1 single tower Et>3 GeV, sumEt>20+Pt_gamma)
PHOTON_25: Inclusive photon pt>25 GeV (but fully efficient at ~30 GeV)

9. The Experimental Picture - III
Background: Essential to understand it well since signal c.s. is very small
Possible source:
 + jet where the jet fragment in a single  (irreducible)
jet() + jet(): same size as 1) but we know how to treat jet faking photons
Ze() + e( ): should be small and we have several handle for the e/  separation
W + jet  e() + jet(): should be negligible
Note that single pions properties in the calorimeter (energy deposition in the EM, CPR pulse, CES signal, etc..) has been study in great detail using MB events:

10. The Experimental Picture - IV
Signal Simulation: I didn’t find a MC that has this W decay in the table.
Naively not different from We (two body decay) although there are differences in cos* ( not (1+ cos*)2 but (1+ cos2*) ). Wqq better?
Improve limit calculation: In Run I they used a simple counting experiment in the signal region. The sharp W peak was not exploited. A binned likelihood can improve the sensitivity

11. Conclusions
The W   +  is an interesting channel because of the theoretical uncertanties on its BR calculation and for possible evidence of BSM physics
If the BR is really around 10^-6 or so this would open a completely new way to measure the W mass at LHC
In the worse scenario the actual PDG limit can be lowered by more than one order of magnitude, with just the data we have
This analysis tailors perfectly to our group. We have experience on all the aspect of this analysis and it a natural extension of our EW line of research