 c Physics at the Energy Threshold John Yelton U. of Florida CLEO experiment A review of what we know, and what we do not know, about the  c, with an accent on what new knowledge can be gained by running with e + e - annihilations (just) above threshold.

What is a  c ? A  c + is a cud combination in an iso-singlet configuration. The ground state is the lowest mass charmed baryon. The higher  c +,  c 0,  c +,  c ++ states, (10 found so far!) all cascade down to the  c via strong decays, leaving the  c to decay weakly. Thus it is copiously produced in e + e - annihilation, but most of the observed  c baryons do not originate from the primary interaction.

 c Investigations The PDG uses 52 papers in its compilation: 22 from e + e - at B factories (CLEO, ARGUS and BELLE) 19 from electronic fixed target experiments (FNAL and CERN) 6 from bubble chambers at CERN 4 from Serpukhov 1 from SLAC, e + e - at threshold

 c Mass Measurements The  c was discovered in However, its mass was not reliably measured until Abrams et al (1980), measured it in an e + e- threshold experiment. They got it right!

 c Mass Measurements The  c mass still not as accurately measured as mass differences of charmed baryons. The most precise measurement was by CLEO I and was systematically limited by uncertainties in the energy loss of the protons in particular. At threshold, a beam- constrained mass can be calculated, minimizing these uncertainties. Thus, a machine running at threshold should be able to make the definitive measurement.

 c Decay Lifetime Running at low energy e + e - is not the right way to measure the  c lifetime. This has been well measured both by fixed target experiments, and by CLEO and cannot be measured at threshold.

 c Decay Mechanisms The short lifetime is well understood. Charmed baryons can decay via W- exchange diagrams, which are not (unlike for mesons) helicity suppressed. These compete with conventional spectator-type diagrams

Lifetime Hierarchy for Baryons The lifetime hierarchy for charmed baryons was predicted in 1986 by Guberina et al. They expected:  (  c 0 ) <  (  c 0 ) <  (  c + ) <  (  c + ) (based upon relative contributions of W-exchange, spectator and interference effects). These are now measured to be: (64  20 < 98  19 < 200  6 < 442  26) x s

 c  pK -  + Branching Fraction The decay mode pK -  + has long been used as the normalizing mode for  c decays. This is because it is a) The largest decay mode known b) It generally has high efficiency However, it is rather unfortunate that this is the “best” a) It is theoretically a mess as it decays via many decay mechanisms, and b) It is a 3-body decay with resonant substructure, and therefore its efficiency is difficult to determine.

Absolute Branching Fraction Without knowing an absolute branching fraction, we have no means of knowing how many charmed baryons are being produced in a reaction. The absolute branching fraction is a vital engineering number for studies of B mesons. It limits the measurement of B branching fractions (B  c is 6%?)

Absolute Branching Fraction Also in the B region, parameters such as quark masses and the QCD renormalization cut-off scale depend upon the b  c fraction. At the Z 0 higher order corrections can be tested by measuring the number of charm quarks per hadronic event.

 c  pK -  + Measurements Previous methods have included: a)measuring the increase in proton production as one crosses  c threshold b) assuming that baryonic B decays all proceed via B   c (known to be incorrect!) c) using the semi-leptonic b.f. together with a theoretical model. More recent studies have concentrated on correlations of charmed particles and protons. PDG “estimate” is 5.0  1.3% (in 2000) Coincidentally, CLEO measured 5.0  1.3% soon after! BaBar (unpublished) measure 6.12  0.31  0.42%

 c  pK -  + at Threshold The high luminosity of B-factories at SLAC and KEK make it possible to imagine many possible methods for measuring B(  c  pK -  + ) either in continuum or B- decays. They will be systematically limited. Uncertainties, particularly concerning  c production and decay, are difficult to overcome. You can work very hard and still get the answer wrong! If you run at  c +  c - threshold you are free from these uncertainties.

Threshold Running It has been shown by MARK II at SPEAR that running at E cm just above 2 x GeV produces charmed baryon pairs. If you reconstruct one  c there must be another in the event. So we reconstruct one  c and look at the other particles.

Threshold Running How many do we expect? MARK II found a .B(  c  pK -  + ) of  nb This implies, for each 1 fb -1 of luminosity, produced  c  pK -  + decays. The efficiency is large! The particles are of a momentum where they can be easily identified, and yet most of them are above p=100 MeV/c. Efficiency may be 50%.

Threshold Running Some particles have momenta below 100 MeV/c – low momentum tracking, as always, very important.

Threshold Running What energy to run at? We don’t know where will be best cross-section. Ideally: 4.57 GeV < E < 4.71 GeV Only a  c +  c - and no other particles – however is the cross-section big enough? Next threshold is  c at 4.94 GeV pD threshold of 5.08 GeV must be avoided.

Threshold Running Assuming 50% reconstruction efficiency (for pK  ), and 1 fb -1 of data, can expect 500 fully reconstructed, clean events with e + e -   c +  c - (where each  c  pK  ). By itself, this should get a statistical uncertainty in the measurement of 4.5% of itself, and be enough for easily the best measurement in the world.

Threshold Running Can other decay modes used for absolute b.f.? Obvious ones are pK 0 s and  +. Both require detection of secondary particles. Need to make sure that the particle detection system does not overly rely on hits close to the beampipe. These are actually better decays to use for absolute b.f. because they are 2-body.

B Factory Measurement Huge samples of charmed baryons are available for study at the “B factories”. These can be used for spectroscopy and also for measurements of other exclusive hadronic channels. It makes little sense to compete in these fields.

Inclusive Decays By tagging one  c and looking at the rest of the event, we can measure inclusive decay rates.  c  pX,  c   X,  c   X,  c   X etc. These are all very good “engineering” numbers.  c  nX may be possible, using anti-neutron signature. Do they add to 100%? Is there something missing?

Semi-Leptonic Studies The decay  c  l - has been measured and studied, including the rates, form factor studies, and CP violation. It is particularly important because it is theoretically simple (the only pure spectator diagram decay!), No studies done on semi-leptonic decay with anything other than a . Almost impossible to perform an investigation of these decays except at threshold.

Conclusion Even a modest run of 1 fb -1 running at E=4.6 GeV Should yield the definitive studies of a) The  c mass b) The  c absolute branching fractions c) The  c inclusive decay fractions d) The  c semi-leptonic decay rates This will enable us to understand the  c to the same degree as charmed mesons are understood today.