1. Spectroscopy with Hyperon Beams
James Russ
Carnegie Mellon University
Outline
An old topic revisited – Lc+ semileptonic decay
Diffractive S* Production
Cabibbo-suppressed Charm Baryon Decays
New Results on Double Charm Physics with SELEX
Hadron Frascati
October 11, 2007
James Russ
Spectroscopy from Hyperon Beams

2. Two Active Hyperon Beam Experiments
Two Complimentary Experiments, both with silicon vertex detectors and RICH identification of fast forward particles:
WA89: 350 GeV
large volume magnet with chambers inside
additional chambers before and after magnet
SELEX: 600 GeV
Two stage magnetic spectrometer
Only silicon before first magnet
Exceptional vertex resolution and redundant forward tracking
Excellent V, kink reconstruction
Hadron Frascati
October 11, 2007
James Russ
Spectroscopy from Hyperon Beams

3. New Light on an Old Question – Lc+ Semileptonic Branching Ratio
History:
Mark II (1982): BR( Lc+  e+X ) = 4.5 ± 1.7 %
CLEO (1994): BR( Lc+  Le+n ) = 2.1 ± 0.6 %
What are the rest of the modes?
For D mesons, the ground state and p-wave excited states (K*(892)) account for ~85% of the total semileptonic rate.
The equivalent p-wave state for Lc+ decays is the L(1520)
SELEX has the first observation of this channel
Hadron Frascati
October 11, 2007
James Russ
Spectroscopy from Hyperon Beams

4. Measure G(pK-e+n)/ G(pK-p+)
Exploits all features of SELEX: tracking, RICH, eTRD, Pb Glass
eTRD separates e from hadrons up to 160 GeV
eTRD efficiency measured using Pb glass
Treat 3-prong vertices equivalently: vertex detachment L/s > 8 , RICH identification of p, K.
pKe candidate mass < M(Lc+)
Evaluate combinatoric background using e from one event, pK from another
Hadron Frascati
October 11, 2007
James Russ
Spectroscopy from Hyperon Beams

5. The pK Mass Spectrum
Subtract mixed-event background (blue) to get yield plot.
Fit to 1520 peak with fixed width (15.6 MeV) and MC resolution (3.2 MeV) Yield: 110 ± pKp yield in same solid angle: 1461 ± 83
Hadron Frascati
October 11, 2007
James Russ
Spectroscopy from Hyperon Beams

6. Lc+ Semielectronic Branching Ratios
Correct for eTRD efficiency (~80-85%), relative acceptance (1.2), pK- branching fraction: G(pK-e+n)/ G(pK-p+) = 0.36 ± 0.07 (preliminary)
What is G(pK-p+)/G ? PDG: 0.05 ± 0.013
Using 5%, (G(Le+n) + G(L(1520)e+n))/G = 3.9 ± 1.0 %
Including a 15% increase above this rate, a la the D mesons, gives an inclusive rate of 4.5 ± 1.2 %, right on the Mark II inclusive rate ( 4.5 ± 1.7 % ),
However, if all semileptonic decays factorize, as works very well for Do and D+ semileptonic decays, then we can use the universal rate and the Lc+ lifetime to get Ge+ = 1.65 x 1011 Hz  G(e+ X) /G = 3.3%.
Subtract G(Le+n)  G(L(1520)e+n)/G = 0.8 ± 0.6 %  G(pK-p+)/G = 2.2 ± 1.8 %
Can BES measure Lc+ absolute decay rates ? PANDA?
Hadron Frascati
October 11, 2007
James Russ
Spectroscopy from Hyperon Beams

7. Forward S* Production in WA89
WA89 recently published a pKs mass enhancement at very large xF (> 0.7).
The well-known S(1775) state has a significant NK branching fraction, a width around 100 MeV.
Is this the explanation for the bump? The fit is to an S-wave, not D-wave.
The S-wave S(1760) is another possibility
xF dependence similar to WA89 results for S(1385) and S(1660) production (P-wave states).
Hadron Frascati
October 11, 2007
James Russ
Spectroscopy from Hyperon Beams

8. Is This a S* ? Issues of Interpretation
No hint of this state in the Lp+ channel, but backgrounds are bigger.
Diffractive excitation of the S- beam doesn’t tend to make a S+ state.
Both of the S* states around 1770 have significant KN branching ratios, but there is no pK- signal.
Significant Mass Peak is seen. What is the evidence at higher energy?
SELEX doesn’t do V decays as well as WA89. SELEX data at 600 GeV are inconclusive about this state.
Hadron Frascati
October 11, 2007
James Russ
Spectroscopy from Hyperon Beams

9. Cabibbo-Suppressed Weak Decays
Part of the quark picture of weak decays is the Cabibbo mechanism of strangeness-changing due to (d,s) mixing at a weak vertex. In the cartoons red ( blue ) indicates a Cabibbo-favored (unfavored) transition.
p+  K+
Expect ratios of unfavored/favored decays to depend on tan2qC ~ 0.05 if rescattering effects are not important.
Comparing rates for different channels probes weak decay mechanisms
K-  p-
Hadron Frascati
October 11, 2007
James Russ
Spectroscopy from Hyperon Beams

10. Spectroscopy from Hyperon Beams
Mesonic Melange
D mesons have many well-measured channels … and many different relative BRs  rescattering is generally important.
Some Examples
p+ p+ p- / p+ p+ K- = 0.035(.002)
p+ p+ p- po / p+ p+ K- po = 0.21(0.10)
D+ p+ p+ p- p+ p- / p+ p+ K- p+ p- = 0.29(0.04)
p+ p- / p+ K- = 0.036(.002) K+ K- / p+ K- = 0.100(.003)
K+ p- p+ K- / p+ K- p+ p- = 0.030(0.002)
Do p+ p- p+ p- / p+ K- p+ p- = 0.095(0.003)
Hadron Frascati
October 11, 2007
James Russ
Spectroscopy from Hyperon Beams

11. Cabibbo Suppression for Charm Baryons
Compare Lc+ and Xc+ ; two new preliminary SELEX results for Xc+
Lc+ L K+ / L p+ = 0.075(.016)
S+ K+ p- / S+ p+ p- = 0.047(.019)
p p+ p- / p p+ K- = 0.07(.04)
Lc+
Xc+ p p+ K- / S+ p+ K- = 0.22(.03)
S+ K+ K- / S+ p+ K- = 0.16(.06)
S+ p+ p- / S+ p+ K- = 0.27(.10)
S- p+ p+ / S+ p+ K- = 0.44(.17)
Changing spectator d quark to s quark seems to change decay rate for Cabibbo suppression ????
Hadron Frascati
October 11, 2007
James Russ
Spectroscopy from Hyperon Beams

12. New News from SELEX on Double Charm
Double charm analysis is ‘statistics-challenged’ – 10’s of events. Very clean signals published from Lc+ K-p+ , p D+ K- final states.
Recent tracking improvements raise single charm yields by 50%
Create absolutely normalized combinatoric bkg by event mixing
Select Lc+ reconstructions from events having no Xcc+ candidates
Take opposite-sign track pairs from different events
Build statistics by re-using each Lc+ ( or other charm hadron) N (~25) times and renormalizing output mass plot to reflect this.
This method can be applied to any final state dominated by combinatoric background and always has absolute normalization – no adjustments.
Hadron Frascati
October 11, 2007
James Russ
Spectroscopy from Hyperon Beams

13. A New Mode: Xcc+ (3520)  Xc+ p- p+
Use same reconstruction code as for other Xcc+ (3520) modes. No new cuts on tracks. Vertex significance > 0
Another narrow peak ~ 3520 MeV/c2 Width agrees with MC calculation.
Event-mixed background (black line) describes sidebands nicely.
Now see 3 different final state charm particles from Xcc+(3520) decays: Lc+, Xc+, and D+.
Hadron Frascati
October 11, 2007
James Russ
Spectroscopy from Hyperon Beams

14. Spectroscopy from Hyperon Beams
What About the Q=2 State?
If we have a ccd state, there has to be a ccu state.
Look in Xcc++  Lc+ K- p+ p+
Use same cuts as before
Use same code
Just ask for one more pion
Fit the peak with MC-fixed width Gaussian. Green background is absolutely-normalized mixed event combinatoric.
New Xcc++ state at 3452!
Hadron Frascati
October 11, 2007
James Russ
Spectroscopy from Hyperon Beams

15. Xcc++ in a Second Decay Mode
We saw the Xcc+ in the Xc+p+ p- mode, so we should look for this channel in Q=2 state.
This analysis uses only Cabibbo-suppressed pKp mode for good resolution.
See peak at same mass as for Lc+ K- p+ p+ , fit with fixed-MC-width Gaussian. Add data from both modes:
Signal significance goes up to 6.5s.
Distribution fits MC-shape well.
Mixed event background describes sidebands for both modes.
Hadron Frascati
October 11, 2007
James Russ
Spectroscopy from Hyperon Beams

16. Spectroscopy from Hyperon Beams
Double Charm Status
Now have Xcc+(3520) in three decay modes
See Q=2 state Xcc++(3452) in two decay modes
Decays to Xc+ use only Cabibbo-suppressed mode  double charm branching fraction to this decay channel is significantly enhanced compared to Lc+ channel – agrees with theoretical expectations.
Observation of Xcc++ precludes idea that double-charm states might be decays of anomalously-narrow single charm baryons - Sc++ can’t decay strongly into a Xc+ in the final state.
The Xcc+(3520) and Xcc++(3452) are not isospin partners. They have very different decay angular distributions. There must be more states to be found.
Hadron Frascati
October 11, 2007
James Russ
Spectroscopy from Hyperon Beams

17. Double Charm Summary - II
SELEX has evidence of excited states with cascade pion decay under study.
Double Charm events have been seen only in baryon beam production in SELEX. The cross section is much larger than fragmentation predictions.
Not seen in SELEX pion beam or in photoproduction at FOCUS, BELLE or BABAR.
SELEX events are small pT , large xF (.3-.7).
Can any future hadron machine probe this phase space with baryon-baryon collisions to explore Double Charm?
Hadron Frascati
October 11, 2007
James Russ
Spectroscopy from Hyperon Beams

18. Spectroscopy from Hyperon Beams
Summary - I
New semileptonic data from SELEX brings Lc+ decays into agreement with D meson pattern: approximate saturation of inclusive rate by ground state and p-wave excited hadron channels (K, K*(892); L, L(1520)).
The semileptonic picture now calls into question the size of the Lc+  p K+ K- BR. 5% seems to be too big to match a universal semi-electronic width as expected in the spectator picture of semileptonic decays. Where will we get a new high-statistics absolute measurement of this BR?
WA89 has reported the observation of one or more S*  KS p bumps near 1765 MeV in t-channel production at xF > If these are S(1760) and/or S(1775) resonances, it is surprising that the Q=0 and especially the Q=-1 partners aren’t seen. ???
Hadron Frascati
October 11, 2007
James Russ
Spectroscopy from Hyperon Beams

19. Spectroscopy from Hyperon Beams
Summary - II
Cabibbo-suppressed charm hadron decays probe strong corrections associated with final state quarks. Mesons have long exhibited somewhat complicated patterns. New SELEX data confirm that the Xc+ - Lc+ rates differ much more than expected from a mere spectator sd replacement .
Selex reports first observation of a Q=2 Xcc++ state at 3542 MeV in two decay modes. New state probably not isospin partner of Xcc+(3520) because of decay distribution differences. The experimenters hope to continue unfolding the spectroscopy.
Hyperon Beam Experiments have contributed to hadron spectroscopy in some unexpected ways over the past decade. The data sets are mature, but the end of new physics is not yet in sight. Wait for Hadron 2009.
Hadron Frascati
October 11, 2007
James Russ
Spectroscopy from Hyperon Beams

20. Spectroscopy from Hyperon Beams
A Reminder from 1963
The original S(1775) discovery was a lovely mass plot of the K-p system in a bubble chamber experiment. Many hyperon resonances have been seen in mass plots, contrary to WA89 claims about having seen the first mass peaks for hyperon resonances above the S(1385) .
Note that even in those data isolation of a single hyperon state was tough! The S(1775) had a high mass tail.
Hadron Frascati
October 11, 2007
James Russ
Spectroscopy from Hyperon Beams