Two Important Long Range Programs for RHIC

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Two Important Long Range Programs for RHIC

• In addition to the many important RHIC research
  programs that are currently underway or
  projected. There are two programs which are not
  so well known but are of high significance and
  which are not likely to be done at the LHC and,
  in any case, need to be done at the RHIC
  energies.

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A. Photon-Photon HBT

• Only direct photons and weakly interacting
  leptons can be emitted from the quark gluon phase
  of the collision prior to hadronization.
• A unique property of the HBT is that only direct
  photons will display an HBT correlation (due to
  the long life of the p0 meson).
• The ?-? HBT can be studied as a function of pt,
  yielding unique information on space time
  evolution of the early, deconfined, phase.

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Feasibility of ?-? HBT

• Our group has studied this, including many
  increasingly sophisticated simulations, (for the
  past 3 years).
• We have concluded that the measurement is both
  feasible and very informative.
• It is necessary to use a thin (.1 rl) converter
  together with the calorimeter so each detector
  observes one ?, and ?s that have vanishing
  momentum difference can be reliably detected.

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Challenges of the ?-? HBT

• The p0 photon flux, while not making an HBT
  correlation, does lower the size of the signal
  observed by a large factor (of order 103).
• Very careful analysis, which we believe can
  certainly be done is needed. And
• Large event samples are needed, event numbers of
  1-2 billion. With DAQ1000 this is, however just
  a few weeks of running.

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Conclusion

• To carry out the ?-? HBT measurement, long,
  dedicated runs will be needed along with the
  commitment of the serious analysis efforts.
• This is a long range program for RHIC, for
  example one can anticipate different collision
  energies, etc.
• It provides information which cannot be obtained
  by other means and is one of the strong arguments
  for a strong continuing RHIC deployment.
• Some more detailed discussion of the experiment
  can be found at lthepwww.physics.yale.edu/star/gg
  -hbt.pdfgt, a proposal to STAR (not yet acted
  upon).

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B. Violation of P,T by the Strong Interaction

• There is a good possibility that parity (P) and
  time reversal symmetry (T) are violated in
  collisions that produce a QGP state.
• This violation is a spontaneous effect and the
  P,T effects will be randomly different in each
  event. This greatly complicates the experimental
  study of the effect.

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References

• It is not possible here to explain the underlying
  theory. However this effect deals with the most
  fundamental aspects of the basic field theory of
  QCD. Some references are
• D. Kharzeev,R.D. Pisarski, M.H.G.Tytgat Phys.
  Rev. Lett. 81, 512 (1998)
• D. Kharzeev, Phys. Lett.B663, 260-264 (2006)
• Lattice calculations support the existence of the
  effect (show a distribution of topological
  winding numbers).

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Experimental Aspects

• Since the effect (spontaneous symmetry breaking)
  varies randomly in each event, any P (T) odd
  observable will average to zero over many events.
• Thus the effect must be seen in correlations. We
  have made some estimates (with the help of D.
  Karzeev) and the effects, e.g. in the observable
  helicity correlations of ? pairs (via the parity
  violating decay of the ?).

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What is needed?

• Large samples. The ?-? correlation would take
  several billion STAR events. Other correlations
  will be similar.
• Further theoretical as well as experimental work
  because these correlations do not by themselves
  prove the effect. They could be caused by
  ordinary strong interactions.
• The behaviour of the effects must be compared
  with theoretical estimates of that predicted by
  the P,T effect and by the usual strong
  interaction.

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Summary

• These two programs offer unique qand fundamental
  information about the new state of matter
  produced in high energy heavy ion colissions.
• They are difficult, but clearly possible.
• It is my hope that the long range plan will
  recognize the importance and requirements of
  these studies.