Phenomenologists Are Underrated in Theory-Experiment Debate

Although Norman Ramsey’s letter in the September 2001 issue of Physics Today (page 78 ) claims not to be taking sides in the debate between theory and experiment, his message is clear. We are now back in particle physics with the same need for critical experiments that Ramsey noted for quantum electrodynamics in 1947. Today’s theorists must face present reality with the humility appropriate to the situation.

We have a standard model that everyone admits is incomplete. Experimenters are looking for clues to the new physics beyond the standard model, but no theorist can provide reliable advice on where or how to look. It is all up for grabs.

We have a theory called quantum chromodynamics (QCD) for strong interactions—a theory that everyone believes is correct. But nobody knows how to calculate parameters crucially important for our search for new physics, like the strong interaction phases in weak decay final states. Theorists on the lattice think that they are making great progress when they can get a good approximation to the mass of ρ, which we know already from experiment. But they are useless in getting values of parameters that we don’t know, need to know, and should be given by QCD.

The search for new physics and for critical tests of the standard model in charge conjugation—parity (CP) violation is being carried out at accelerators by large experimental collaborations that have no theory telling them where to look. The one exception is the seminal paper by Ikaros Bigi and Tony Sanda, ¹ which pinpointed the “golden” K_s + J/ψ channel.” Experimenters are guided mainly by their own intuition and by the phenomenologists, who are highly undervalued in this debate between theory and experiment. The phenomenologists analyze the experimental information without too much prejudice from unreliable theories. They then attempt to provide guidance regarding profitable directions in the search for new knowledge. Their papers are generally criticized by nitpicking theorist referees and judged by comparison with published theoretical papers that will probably turn out to be wrong. Phenomenologists can thank Paul Ginsparg, who established the online electronic print archives at http://arXiv.org , for giving them a way to reach the experimenters who need their advice without having to deal with obnoxious referees.

Here is one small example of the problems faced in looking for clues to new physics. In 1998, I noted that decays like D⁺ → K_s + π ⁺ had contributions from Cabibbo-favored and doubly suppressed decays, and that this might lead to a direct CP violation between charge conjugate D⁺ and D⁻ decays. The standard model says that both the favored and doubly-suppressed amplitudes depend on the same Cabibbo-Kobayashi-Maskawa matrix elements and there should be no CP-violating relative phase. But suppose some new physics contribution has a CP-violating phase. This new physics could give a direct CP violation between charge conjugate D⁺ and D⁻ decays. This can be easily verified when data are available. Even if there is no effect, and no new physics theory that predicts an effect, such data provide information that can be used to constrain future new physics models.

When I suggested this idea to my experimental friends, the response was, “Interesting! Too bad you didn’t tell us sooner. It would have been trivial to check this at the early stages of our experiment, but now it will involve remounting many tapes and a lot of work and expense. We cannot justify that without a good theoretical reason.” I wrote up my suggestion and put it on the Web as http://arXiv.org/abs/hep-ph/9810375 . I tried to tell all my experimental friends who were planning future experiments so that they could test this and similar ideas in time. Of course I never sent the paper for publication. At my age I don’t need the credit, and I have no patience for ridiculous arguments with referees.

Today, progress in particle physics depends on the work of postdocs who analyze the tremendous amount of data accumulating at accelerators, and who must establish some record to get future jobs. They need input from good phenomenologists who can point them in profitable directions. Theorists are only marginally useful.