Double Charm Baryon Studies in SELEX: Further Studies and Production Questions James Russ, Carnegie Mellon Outline

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1 Double Charm Baryon Studies in SELEX: Further Studies and Production Questions James Russ, Carnegie Mellon Outline Review of Double Charm Baryon Results Original Observation Lifetime Limit Production Characteristics New Decay Modes Different Final State: pd + K - Broad ccu candidate with chain decay Another Unusual State: D s (2632) Unusual Decay modes Unusual Production Characteristics Conclusions 1

2 Double Charm Baryons Charm baryon spectroscopy doesn t stop at one charm quark. Double charm models agree: m(ccq) ~ 3600 MeV/c 2 The issue is how to make them. 2

3 Suggested Double Charm Production Mechanisms Sequential charm pair production plus coalescence - Very low probability process order fb Charm diquarks in hard scatter - diquark mass requires gluon mass similar to that in bb production Intrinsic Charm - Proposed for leading ψψ production OK for double charm? Experimentalist s idea - We make 600 GeV Σ - hyperons from 800 GeV protons at the part per million level by some unknown mechanism look in the forward direction in baryon-baryon interactions. (SELEX 1988) 3

4 SELEX ccd First Observation ccd candidate at 3520 SELEX PID excellent! Λ c + has RICH p, K -. Vertex Reconstruction very precise (primary, secondary separation > 8 σ) 18 fs σ ct (world s best) Wrong-sign combination with K + shows no peak. Not accidental match. 4

5 Features of First SELEX Ξ cc+ Observation First candidate for new baryon comes from baryon beam expt: (ccd) + Λ c + K - π + Cabibbo-favored spectator mode state seen from Σ, p but not π not seen in photoproduction lifetime is very short disagrees with extrapolation from single charm Phys Rev Lett 89 (2002)

6 Additional Evidence for the Ξ cc+ (3520) Another double charm decay mode is pd + K - (not pd - K + ). [Q value is 3x smaller than Λ c+ K - π +.] Conduct blind search using SELEX D + and D + sample, adding a RICH proton and a negative track. Simulation predicts width of peak to be 4 MeV/c 2, similar to Λ c+ K - π + SELEX absolute mass scale good to 1 MeV/c 2 (all charm hadrons) 6

7 Further Investigations of Background First, look at wrong-sign combinations: good D - with RICH p and another positive track. All combinatoric, smooth distribution Make mixed-event combinatoric background. Take D + from one event and pk - from 25 other events. Make normalized background mass plot. (shaded region) 7

8 Comparison of Ξ cc+ (3520) Signals Fit pd + K - signal to fixedwidth Gaussian plus linear bkg Masses agree well Relative branching ratio of the two modes: 0.36 ± 0.21 (error includes statistical and negligible systematic terms) Now have two independent observations of the Ξ cc+ with mass ± 1.7 MeV/c 2 8

9 Also a Ξ cc ++ Excited State Candidate Independent analysis: look at Q=2 Λ c + K - π + π + final state See broad structure at 3780 MeV/ c 2 Broader than resolution either has natural width or two states (HQET doublet) 260 MeV above 3520 state. Similar to p-wave Λ c + (2593,2632) excitation. (330 MeV) 9

10 Evidence for Chain Decay to Ξ cc+ (3520) Follow single-charm analysis strategy Take away the slower π + in the Λ c + K - π + π + final state to look for Ξ cc ++ Ξ cc+ π + decay 1/3 of the signal events in the 3780 peak have a detected chain decay to Ξ cc+ (3520) 10

11 Status of Double Charm Evidence Original observation was statistically robust, at expected mass New observation in pd + K - final state supports idea Observation of chain decay to Ξ cc+ (3520) gives further support. We can conclude that double charm has been produced in small-p t baryon-baryon collisions at s = 38 GeV. It is not seen a 10x larger sample of Λ c + in photoproduction at s = 18 GeV. Can we understand the physics of double charm decay and production? 11

12 What Might Be Wrong with This Picture? The lifetimes are too short to be Ξ cc. o No known mechanism to speed up weak decay in cc diquark o No PI or WS hierarchy effects The production is extraordinary. o Seen only in hadroproduction but not in photoproduction Ξ cc production accounts for 40% of the Λ c+ yield in SELEX Experimentally all observed states have lifetimes of the order of SELEX resolution (30 fs) SELEX Λ c+ yields have strong production asymmetry (nonperturbative effects from Σ - beam ) not seen in photoproduction SELEX seems to require a new leading production mechanism. 12

13 Another Unusual SELEX State SELEX recently published the first observation of an excited D s meson with a very unusual decay mode into D s η. (PRL 93(2004)242001) Background is well-described by mixing D s candidates from one event with photon pairs from 25 other events, then normalizing. 13

14 More Evidence in Another Decay Mode The well-known D sj (2573) state is seen clearly in its D o K + mode. Mass and width agree well with PDG An additional peak visible at same mass as the D s η. Width consistent with experimental resolution. Wrong-sign channel D o K - shows no enhancement. 14

15 Production Issues with D s (2632) The state is not confirmed by B- factories, despite their excellent η detection. Almost 40% of the D s yield in SELEX comes through this channel. The large branching ratio for the η mode is hard to understand in conventional potential models, but there are other possibilities. B-factory D s yields come largely (~75%) from D s* (2112) production, as expected from spin-counting SELEX sees only ~25% of D s yield from D s* (2112) Is this another example of a new production mechanism for heavy flavor at small p t? 15

16 Can we understand the issues in Double Charm The lifetime seems to be much shorter than predicted ( fs) Data in both decay modes show no events with reduced proper time > 100 fs. Simulation shows good reconstruction efficiency out to long lifetimes. (Expt has measured the D + lifetime to good accuracy.) There are a handful of events it might be a fluctuation. If this is confirmed, it may indicate new physics in ccq system. 16

17 What about the cross section? Τhe Ξ cc+ channel supplies about 25% of the forward Λ c+ cross section from baryon beams for x F > 0.2. Must account for ~100 nb of cross section, all forward Where does the anti-double-charm go? It s not forward. SELEX observes charm pairs at rate expected from independent production model. No enhancement. Is mechanism baryon-beam specific? SELEX sees double charm from Σ -, p beams not π - Is there any connection with the D s (2632) production, which is not baryon-specific? 17

18 Summary and Conclusions SELEX double charm signal seen in 3 different manifestations: 2 decay modes plus sequential π + decay from a Q=2 excitation. Double charm events occur at small p t and large x F ; decay feeddown enhances the overall Λ c+ yield significantly. Production is large (~100 nb). Sequential-interaction calculations underestimate SELEX yields by orders of magnitude. The D s (2632) observation only in hadroproduction and not in photoproduction is another challenge. Related to double charm? We still have more to learn about forward production. At what accelerator of the future will these studies be done? 18