New (species of?) Particle Found in the BaBar Experiment.

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New (species of?) Particle Foundin the BaBar
Experiment.

• Brian Meadows
• University of Cincinnati

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The BaBar Experiment

• A small, NSF funded research group in the UC
  physics dept works in an international
  collaboration on an experiment at Stanford Linear
  Accelerator Center (SLAC).
• The main goal of the experiment is to examine the
  origin of particle-antiparticle asymmetry that
  has resulted in the universe being dominated
  (now, but not always) by matter.
• The discovery reported here, a total surprise,
  was not part of that goal.
• The experiment has two major components
• PEP2 A dedicated accelerator in which high
  energy electrons (e-) collide with anti electrons
  (e).
• BaBar A large detector at the collision point,
  built to detect the particles produced in the
  collisions.

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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-)

1.5 T superconducting field. Instrumented Flux
Return (IFR) Resistive Plate Chambers
(RPCs) Detect ? particles.
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The BaBar Detector

• The experiment centers on observation of ee-
  interactions in which particle/antiparticle pairs
  are produced.
• The particles (p/p-, p0/p0, K/K-, p/p-, etc.)
  are detected in BaBar in several systems
• Silicon Vertex Tracker (Si strips giving precise
  position information).
• Drift CHamber (electronically highlights charged
  particle trajectories)
• DIRC (distinguishes K from p from p, etc.)
• ElectroMagnetic Calorimeter (measures neutral
  particles - p0, g, etc.)
• UC participated in the design, prototyping and
  construction of the DIRC particle identification
  system.

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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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Drift Chamber (DCH)
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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The DIRC

• Acronym for Detector of Internally Reflected
  Cherenkov light.
• The device is completely novel, and is the only
  one in existence in the World.
• K, p, etc. of the same momentum have different
  velocities.
• The DIRC works on the principal that Cherenkov
  light, generated by charged particles passing
  through quartz, radiates at an angle relative to
  the particle trajectory that depends on particle
  velocity.

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Particle ID - DIRC

• Measures Cherenkov angle in quartz
• Photons transported by internal reflection within
  the quartz.
• Detected at end by umbrella shaped array of
  10,000 PMTs

Detector of Internally Reflected Cherenkov light
144 quartz bars
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Particle ID - DIRC
It Works Beautifully!
Provides excellent K/? separation over the whole
kinematic range
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Particle ID - DIRC
D0
D0
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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).

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The New Particles

• Meadows leads the group charged with study of
  production of charmed particles.
• One of the group (Antimo Palano of Bari, Italy)
  was looking at events with a Ds and a ?0 in them
• Short lived particles are seen as peaks in the
  effective mass spectrum of their daughters.

The Ds is a particle that decays very quickly to
three others Ds! KK-p ( 10-16 sec) The
p0 also decays very quickly to two photons p0
! Gg ( 10-10 sec)
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New Particle Ds(2317)! Ds?0

• A bump in the Ds?0 mass spectrum indicates
  existence of a hitherto unknown particle
• Signal clearly associated with both Ds and ?0

Ds
D
?0
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A Little Background - Charmed Mesons

• Charmed mesons have been thought of as made from
  a c quark and either a u, d or s anti-quark.
• The c is much heavier than u, d or s
  anti-quarks.
• The conventional view of particle physicists is
  that these particles behave much like the
  hydrogen atom (heavy proton with light electron).
• In this picture the Ds (charmed-strange meson) is
  a cs.
• Excited states of hydrogen can be predicted with
  great accuracy. It was thought that excited
  states of Ds could be too.
• - That is until BaBars discovery of this new
  state!

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Why is the Ds(2317) Interesting?

• The mass of this Ds excited state is far below
  expectations of a hydrogen atom-like calculation.
• Previous states have been predicted to within 10
  MeV/c2 in mass. This state is 170 MeV/c2 too
  low!
• The decay seen is Ds! Dsp0. This decay in
  itself violates one of the rules (isospin
  conservation) for normal Ds decay.
• All known mesons (including ?, K, etc.) have been
  thought to be made from quark anti-quark pairs.
• This state does not fit that pattern so easily.
• Much speculation has followed this discovery
  (about 50 papers so far) on the nature of this
  new Ds state.

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Yet Another State

• Since this result was published in Physical
  Review Letters in May 2003, interest has focussed
  on yet another state seen by BaBar

This state decays to Dsp0g and also has mass 170
MeV/c2 lower than expected.