Pentaquarks: Fact or Fiction?

Transcription

1 Pentauarks: Fact or Fiction? Θ + Elton S. Smith, Jefferson Lab Where do we find new types of matter? The force that binds uarks together (the strong interaction) States with 5 uarks Four years of pentauark news Elton S. Smith 1

2 Families within families DNA 10-7 m 10-9 m Molecule Atom m m Nucleus Proton m <10-18 m Quark 2

3 Families of atoms Gaps in table lead to predictions for the properties of undiscovered atoms Mendeelev and Meyer (1869) 3

4 Families of nuclei 4

5 Matter in extreme conditions We have always been interested in matter under extreme conditions Geology under high pressure Hydrogen in the core of stars Trans-uranium elements Matter in neutron stars Short-lived radioactive isotopes t-uark weighing 185 times the mass of a proton Pentauarks, if they exist, decay as soon as they are formed and have special properties that distinguish them from other known forms of matter New types of matter test our understanding of the interactions that create them 5

6 Binding force neutralizes charge Atoms are charge neutral binding positive nuclei to negative electrons Hadrons are color neutral binding together red, blue and green uarks + Quantum Electrodynamics (QED) Quantum Chromodynamics (QCD) 6

7 Calculations of atoms and hadrons The configuration of atoms can be precisely calculated using the known electromagnetic interaction between the nucleus and its surrounding electrons. The properties of hadrons cannot be calculated in many instances because the interaction between uarks is in general very complicated. But the strong interaction simplifies at high energies where accurate calculations can be made 7

8 Interactions understood in terms of uarks Very high energy e e Z + 0 free uarks not found, only particles that contain uarks 8

9 Asymptotic Freedom The 2004 Nobel Prize was awarded for work that lead to our understanding of the theory of one of Nature's fundamental forces, the force that ties together the smallest pieces of matter the uarks. 9

10 E&M force in a dielectric medium V ( r) = vacuum 1 4πε r / ε 0 Bare charge dielectric media 12 V ( r) = 4πε ( r) 1 r intermolecular distance screened charge / ε Separation r between charges Ref: Aitchison and Hey 10

11 Vacuum polarization diagrams QED: positive sum screening γ QCD: negative sum anti-screening g 11

12 Strong coupling constant α s vs energy Confinement Large distance Asymptotic Freedom small distance Coupling constant Energy (GeV) 12

13 Electromagnetic and color forces γ 1 r 2 +/- charges g 3 color charges 13

14 Quarks are confined inside colorless hadrons Quarks combine to neutralize color force mesons baryons Configurations outside the standard uark model molecules pentauark glueball meson hybrid meson 14

15 Properties of uarks Quark Flavor Charge (Q) Strangeness (S) u +2/3 0 d 1/3 0 s S=+1 s 1/3 1 u 2/3 0 d +1/3 0 s +1/3 +1 u d d u S= 0 Protons are made of (uud) Neutrons are made of (ddu) s S= 1 15

16 Families of uarks Particles from K π Particles from K Ω N Σ Ξ Particles from Θ + Ξ 16

17 Families of protons and neutrons neutron proton Spin=3/ , S= Σ, S= m s =150 MeV Ξ, S= Spin=1/2 Ω, S= 3? 17

18 What are pentauarks? Particles with minimum uark content is 4 uarks and 1 anti-uark. General idea of a five-uark states has been around since late 60 s. However, experimental evidence for 5-uarks has been missing. Experimental searches were revived in 1997 when three Russian physicists predicted that a pentauark called the Θ + should have a mass of 1.53 GeV. 18

19 How do we look for the Θ +? γ us } Θ + K us} K + n { ddu ddu} n Θ + is composed of (uudds) uarks 19

20 Unstable particles have a natural line-width The Breit-Wigner distribution is similar to a gaussian near the peak, but the tails of the curve are flatter. 20

21 Japanese group reports first observation Θ + Mass = 1.54±0.01 GeV Phys. Rev. Lett. 91, (2003) 21

22 Search for pentauark at JLab 22

23 CEBAF Large Acceptance Spectrometer Torus magnet 6 superconducting coils Electromagnetic calorimeters Lead/scintillator, 1296 photomultipliers Liuid D 2 (H 2 )target + γ start counter; e minitorus Drift chambers argon/co 2 gas, 35,000 cells Gas Cherenkov counters e/π separation, 256 PMTs Time-of-flight counters plastic scintillators, 684 photomultipliers 23

24 γd p K + K (n) in CLAS K - K + p 24

25 Path to publication After data is reconstructed, it is analyzed for specific physics (~6 months) Students write theses, analysis notes produced (~6 months) Work is presented to Working Group and committee is formed and draft paper written(~3 months) Ad-hoc committee from CLAS collaboration is appointed and reviews draft (month) CLAS collaboration can comment on draft (2 weeks) Paper draft is submitted to journal Two referees are appointed to review paper (months) After addressing all issues raised, paper is published 25

26 nk + invariant mass distribution Phys. Rev. Lett. 91, (2003) Θ + Gaussian background Λ(1520) events Simulated background γ d Θ + n K Re-scattering n K + p K 26

27 Initial evidence for pentauark states Spring8 DIANA JLab-d ELSA 15 Events/(0.02 GeV/c 2 ) MM c γk (GeV/c 2 ) JLab-p ITEP SVD/IHEP HERMES CERN/NA49 Ξ Θ 0 c H1 ZEUS COSY-TOF pp Σ + Θ +. 27

28 Pentauark Publicity

29 Null Results 29

30 Pentauark Publicity

31 Is the signal real? Statistics Correct statistical analysis with a small number of events is tricky Background Underestimate of backgrounds lead to overestimates of signal Corroboration Initially many experiments verified the result, but null results followed Can it be understood? Despite hundreds of theoretical papers, few explanations Predictive power Expect same channel to show up in other experimental situations (such as high energy), but was not Consistency Measured masses ranged from 1521±3.2 to 1555±10 MeV. 31

32 Search for Pentauarks at CLAS A comprehensive program to search for pentauarks in photoproduction experiments at Jeffeson Lab were approved in with the goal of confirming previous results and explore new kinematics with at least a factor 10 increase in statistics. Relevant Publication g10 deuteron E γ ~ GeV Phys. Rev. Lett. 96, (2006) CLAS(d) g Phys. Rev. Lett. 91, g11 eg3 proton E γ ~ GeV Phys. Rev. Lett. 96, (2006) deuteron E γ ~ GeV data taken, analysis in progress SAPHIR 2003 Phys. Lett. B572, 127 NA Phys. Rev. Lett. 92, Super-g proton planned for 2007 E γ ~ GeV CLAS(p) g Phys. Rev. Lett. 92,

33 New high Statistics CLAS(d) result In hindsight, the original signal size estimate of 5.2±0.6σ in our previous publication was due to a significant underestimate of the background. M(nK + )(GeV) Model-independent uppper limit 95% CL for Θ + is < 20nb. 33

34 Pentauark History Particle Data Group 1986 reviewing evidence for exotic baryons states The general prejudice against baryons not made of three uarks and the lack of any experimental activity in this area make it likely that it will be another 15 years before the issue is decided. Number 350 of publications on pentauarks per year PDG dropped the discussion on pentauark searches after <

35 Pentauark History PDG 2004 assigns a 3-star status to the Θ + pentauark It is difficult to deny a status of three stars and a place in the Summary Tables for a state that six experiments claim to have seen. Nevertheless, as discussed in the above note, we believe it reasonable to have some reservations about the existence of this state on the basis of the present evidence. **** Existence certain, properties known *** Existence likely to certain, confirmation needed ** Evidence for existence is fair * Evidence for existence is poor 35

36 Pentauark History PDG 2005 Θ + reduced to 2-star status Since our 2004 edition, there have been several new claimed sightings of the Θ(1540) +, but there have also been several searches with negative results. PDG 2006 Θ + omitted from Summary Table, reduced to 1-star status To summarize, with the exception described in the previous paragraph, there has not been a high-statistic confirmation of any of the original experiments that claimed to see the Θ + The conclusion that pentauarks in general, and the Θ +, in particular, do not exist, appears compelling. 36

37 Summary Protons and neutrons are part of a family of elementary particles composed of 3 uarks which have been known for forty years. Experimental observations at many laboratories appeared to confirm the predictions for new exotic cousins composed of 4 uarks and 1 anti-uark After severe scrutiny by experiments worldwide, doubt was cast on the initial results. High statistics experiments designed to confirm the initial reports have failed to do so. The eulogy for the pentauark is planned 37