New Particles in the Strange Charm System.

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New Particlesin the Strange Charm System.

• Brian Meadows
• University of Cincinnati

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Outline

• Introduction to Particle Physics
• Brief History
• Forces of nature
• Modern (standard model) view quarks, leptons
  and gauge bosons.
• How are new particles found ?
• The BABAR Experiment
• The discovery of an new kind of particle?
• DsJ ! Ds?0
• What is Interesting about this?
• Other new particles

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A Brief History

• Discovery of electron (Thompson)
• Currant Bun model e-s spread uniformly
  through atomic volume?
• Discovery of atomic nucleus (Rutherford). p is
  H nucleus.
• Are there electrons in the nucleus as well as in
  the outer atom?
• Quantum mechanics suggested this was not so.
• Discovery of anti-electron, e (Anderson)
• Discovery of the neutron, n (Chadwick, 1932)
• Almost done? Just a few details and we
  understand how all 92 elements are built ??

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A Brief History

• Particle-wave duality -gt Gauge bosons
• -gt Photons (?s) exist quanta of
  electromagnetic force.
• -gt Yukawas meson (mass between me0.51 MeV and
  mp938 MeV) are quanta of nuclear force.
• Discovery of ?
• (V. Telegdi who ordered that!)
• m 208 MeV, but no interaction with nuclear
  matter!
• Yukawa meson, ? was found (also ?0, later)
• Many more particles follow !!
• A problem then arises! 92 elements but MANY more
  new particles!!

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Forces of Nature

• Particle physics is all about fundamental forces
• Electromagnetic force
• Holds atoms together (and apart!)
• Stops us falling through the floor.
• ! Long range / 1/r2
• Gravity
• Dominates on astrophysical scales.
• Holds our feet ON the floor!
• ! Long range / 1/r2

? photon
G graviton
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Forces of Nature

• Strong force
• Holds protons (p) and neutrons (n) together in
  nuclei.
• Holds quarks together inside neutrons, protons
  and all hadrons.
• Contributes to hadron decays, e.g.
• r ! pp-
• ! Short range (nuclear diameter 10-15 m)
• Weak force
• Causes radioactive decay e.g.
• n ! p e- ne
• Not really weak but just rare.
• ! Very short range ( 10-18 m).

g gluon
W, Z 0 vector bosons
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The Force Scales

• Particles that leave tracks either
• are stable OR
• decay by weak interaction

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Quarks and Flavors

• Just 6 quarks are building blocks for all
  strongly interacting particles (hadrons)
• They come in two charges
• u c t - charge 2/3 e
• d s b - charge -1/3 e
• Each has a unique flavor
• Isospin u (I ½ ,I3 ½ ) d (I ½
  ,I3 ½ )
• Strangeness s (S -1)
• Charm c (C 1)
• Beauty b (B 1)
• Top t (T 1)
• Baryon number (B) of each is 1/3.
• Antiquarks have opposite charges, flavors and B.

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Quarks and Hadrons

• Hadrons are particles that feel the Strong Force.
• Baryons - p, n, ?, ?, ?, ?, ?c,
• Basic composition - 3 quarks
• p uud, n udd, ? uus, p uud, etc.
• Mesons - ?, K, D, Ds, B, ?, ?, ?, f, a,
• Basic composition - quark-antiquark pairs
• ? ud, ?- ud, K- su, D cd
• Ds cs , B0 bd, etc.
• Additional quark-antiquark pairs are not excluded.

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Quarks and Decay Diagrams

• Strong decay D ! D0 ?
• Weak decay Ds ! K0 K

c
D0
c
u
All flavors conserved
D
u
?
s
K0
c
Flavors NOT Conserved (c ! s)
Ds
W
u
s
K
s
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How to Find a New Particle

• IF a) It is stable OR b) It decays by weak
  interaction
• can observe it directly as a track in a set of
  detectors.
• Its mass is the effective mass M of the decay
  products.
• For example Ds ! K- K ?
• of the indicated decay products.
• (Note units are such that c1!)

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How to Find a New Particle

• IF it decays by Strong or EM forces
• Its lifetime ? is too short for a track of
  visible length.
• BUT its decay products (usually) do leave tracks.
• Measure 4-momenta of decay products and compute
  their effective mass M as before to determine
  particles mass.
• Uncertainty Principle relates lifetime (?) and
  the precision (?M) to which the particles mass
  can be determined.
• ?M ? ¼ h (6 10-27 Jsec)
• We expect a peak in the M distribution with
  width
• ?M 100 MeV (Strong), 10 eV (EM), 0.01 eV for
  Weak decays.

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• The BaBar Detector
• At Stanford Linear Accelerator Center
• (SLAC)

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The BaBar Detector at SLAC (PEP2)

• Asymmetric ee- collisions at (4S).
• ?? 0.56 (3.1 GeV e, 9.0 GeV e-)
• Principal purpose study CPV in B decays

1.5 T superconducting field. Instrumented Flux
Return (IFR) Resistive Plate Chambers
(RPCs) Barrel 19 layers in 65 cm
steel Endcap 18 60 cm
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Electromagnetic Calorimeter

• CsI (doped with Tl) crystals
• Arranged in 48(?) 120(?)
• 2.5 gaps in ?.
• Forward endcap with 8 more ? rings (820
  crystals).

g
g
?
BABAR
?
?0! gg
?0! gg
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Particle ID - DIRC

• Measures Cherenkov angle in 144 quartz bars
  arranged as a barrel.
• Photons transported by internal reflection
• Along the bars themselves.
• Detected at end by 10,000 PMTs

Detector of Internally Reflected Cherenkov light
PMTs
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Particle ID - DIRC
It Works Beautifully!
10 8 6 4 2 0
BABAR
K/? separation (?)
Provides excellent K/? separation over the whole
kinematic range

• 2.5 3 3.5 4
• Momentum (GeV/c)

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Drift Chamber
dE/dx Resolution 7.5
Mean position Resolution 125 ?m

• 40 layer small cell design
• 7104 cells
• He-Isobutane for low multiple scattering

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Silicon Vertex Tracker (SVT)

• 5 Layers double sided AC-coupled Silicon
• Rad-hard readout IC (2 MRad replace 2005)
• Low mass design
• Stand alone tracking for slow particles
• Point resolution ?z 20 ?m
• Radius 32-140 mm

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A Typical Event
? clusters
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• Surprising Discovery of New Particle

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Data Selection

• We looked for decays of well known particles
• Ds ! K-K?
• and
• ?0 ! ??
• The Ds decays are weak
• So it leaves a track.
• The ?0 decays are EM
• So there is no track.
• ?0s could come from
• either end of the Ds track.

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KK-? Effective Mass Spectrum

• Effective mass for
• Ds ! K-K?
• Can also see another well known particle
• D ! K-K?
• Define signal and background (sideband) regions

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The Ds?0 Effective Mass !!see PRL 90, 242001
(2003)

• A striking signal observed in the Ds?0 system.
• Signal clearly associated with both Ds and ?0
• Is not a reflection of any other known state (MC)

Ds
D
Ds(2112) (known)
Ds(2112) (known)
?0
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The Signal is Very Narrow
Ds(2112)
Fit to polynomial and a single Gaussian. N 1267
53 Events m 2316.8 0.4 GeV/c2 ? 8.6
0.4 MeV/c2 (errors statistical only).
Measurement Resolution curve.
? is compatible with detector resolution.
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It Also Behaves Like a Particle ShouldCMS
Momentum (p) Dependence

• Signal seen in all p ranges.
• Background less significant at higher p values
• Yield maximum at 3.9 GeV/c

• Excitation curve appears to be compatible with
  charm fragmentation process.

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Multiple Ds Modes

• Separate Ds! K-K? into ?? and K0K
  subsamples
• Ds(2112) and signal at 2.317 GeV/c2 present in
  both channels with roughly equal strength.

p gt 3.5 GeV/c
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Search for Other DsJ(2317) Decay Modes

• We also looked at effective mass spectra for
• Ds ?0 ?0
• Ds ?
• Ds ? ?
• Ds(2112) ?
• Ds ?0 ?
• In all cases, we required that
• The ?s are not part of any ?0 candidate.
• The combination has p gt 3.5 GeV/c.

None of these found
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Ds?, Ds??, Ds(2112)?

• No evidence for DsJ(2317) in any of these decays.
• Absence of Ds? weakly suggests J 0
• However other two modes would be expected for a
  JP 0.

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Ds??0, Ds(2112)?0 - Other Possibilities

• No evidence for DsJ (2317) either of these
  modes
• BUT
• There seems to be a second state at 2460 MeV/c2
  !

Events / 7 MeV/c2
Ds(2112)?0
A second, new state Ds(2460) ! Ds(2112)p0
m(Ds?0?)
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What is Interesting About New Dss?

• Ds mesons hitherto thought of as cs states.
• Two problems for the new states
• a) cs states have no isospin (I 0)
• The p meson has I1 (triplet of charges).
• p, p0, p-
• So where does the isospin come from in the
  decay
• Ds(2317) ! Ds p0 ??
• b) Other problem has to do with the fact that
  this new state does not fit in with models of
  quark-antiquark mesons.
• Some physicists think it may have an additional
  q-qbar pair!

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Heavy-Light Quark Systems areLike the Hydrogen
Atom

• c quark (Q) much heavier than s quark (q)
• When mQ ! 1, sQ is fixed.
• So jq L sq is separately conserved
• Total spin J jq sQ
• Ground state (L0) is doublet with jq1/2
• Orbital excitations (Lgt0) two doublets
  (jql1/2 and jql-1/2).
• Energy levels can be computed correctly
  predicts where at least 27 Qq and QQ particles
  are found to within 10 MeV.
• The new Ds states have masses too low by 140 MeV
  !

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Heavy-Light Systems (2)
2jqLJ
Width
JP
jq 3/2
2
3P2
small
large
1
1P1
L 1
1
3P1
small
jq 1/2
0
1P0
large
tensor
spin-orbit
jq 1/2
1-
1S1
small
L 0
small
0-
1S0

• Narrow states are easy to find.
• Two wide states are harder.
• Since charm quark is not infinitely heavy, some
  jq1/2, 3/2 mixing can occur for the JP1
  states.

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Charmed Meson Spectroscopy c. 1995
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Charmed Meson Spectroscopy pre 2003
D0K threshold
D0K threshold
BABAR may have found these but below threshold.
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We Seem to have Started Something!

• Our competitor the BELLE experiment in Japan
  has seen a new, massive state
• X(3872) ! J/? ??-
• Again, its mass profile is narrow (width
  comparable to resolution).
• Its existence has been confirmed in the CDF
  experiment at FNAL in proton-antiproton
  collisions at 1 TeV.
• It is also seen in the BaBar experiment.
• What is interesting
• This lies just 100 MeV below D(2112) D
  threshold.
• Ds (2317) lies just 40 MeV below DK threshold
• Ds (2460) lies just 40 MeV below D(2112) K
  threshold

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Yet Another New Narrow State!
BELLEs X
CDF Confirms X

• BELLE m 3872.0 0.6 0.5 MeV/c2
• CDF m 3871.4 0.7 (stat.) MeV/c2
• ? compatible with resolution.

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Unusual Baryons Also Being Seen

• Various peaks have been reported in effective
  mass spectra of exotic systems such as
  strangeness S 1 baryons
• (Cannot be three quark systems because s quark
  has strangeness S -1).
• If confirmed, these signals could be regarded as
  pentaquarks three quark baryons with an
  additional quark-antiquark pair.

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New, Narrow S 1 Baryon!
CLAS hep-ex/0307018
DIANA hep-ex/0304040
Spring-8 hep-ex/0301020 PRL 91 012002 (2003)
?d ! KK-pn
KXe ! K0pXe'
? n ! KK-n
MM(K)

• CLAS m 1542 5 MeV/c2
• DIANA m 1539 2 MeV/c2
• Spring-8 m 1.54 .01 GeV/c2

? ¼ resolution
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Conclusions

• New, narrow (ie width consistent with mass
  resolution) states are being found after the
  discovery by BaBar of
• DsJ(2317) ! Ds p0
• The D_s states have masses inconsistent with
  spectroscopic models.
• There is conjecture that mesons (and baryons)
  with additional quark-antiquark pairs may finally
  be seen.

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Particle ID - DIRC
D0
D0
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The Ds(2317) Appears !!see PRL 90, 242001 (2003)

• When Antimo Palano plotted Ds?0 effective masses
  he found a huge, unexpected peak. A new
  particle!!

There is no signal from Ds sidebands. The (well
known) Ds(2112)! Ds?0 signal is clear too. How
did CLEO miss it?!
CLEO discarded All these events.