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c Physics at the Energy Threshold
John YeltonU. 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
+,c0, 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 CERN4 from Serpukhov1 from SLAC, e+e- at threshold
c Mass Measurements
The c was discovered in 1974. 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:
(c0) < (c
0) < (c+) < (c
+)(based upon relative contributions of W-exchange,
spectator and interference effects).
These are now measured to be:
(6420 < 9819 < 2006 < 44226) x 10-15 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 knownb) It generally has high efficiencyHowever, it is rather unfortunate that this is the “best”a) It is theoretically a mess as it decays via many
decay mechanisms, andb) 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 (Bc 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 bc fraction.
At the Z0 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 5.01.3% (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 Ecm just above 2 x 2.285 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 RunningHow many do we expect?
MARK II found a .B(cpK-+) of 0.0370.012 nb
This implies, for each 1 fb-1 of luminosity,37000 produced cpK-+ 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 RunningSome 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 GeVOnly 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 pK0
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 cl- 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.
ConclusionEven a modest run of 1 fb-1 running at E=4.6
GeVShould yield the definitive studies ofa) The c massb) The c absolute branching fractionsc) The c inclusive decay fractionsd) The c semi-leptonic decay ratesThis will enable us to understand the c to the
same degree as charmed mesons are understood today.