Some physics reasons for a ZDC EM Upgrade

1. Some physics reasons for a ZDC EM Upgrade Ed Norbeck University of Iowa March 18, 2013

2. Samples of interesting physics with an improved ED section • Determination of overall angular momentum direction • Separating photons from neutrons • Counting the number of neutrons • First study of photons from spectators • Looking for photon jets • Identifying other particles such as Λ0

3. Spectator neutrons have a larger transverse energy on the side facing the overlap region.

4. Nucleons come in tightly bound pairs. When a pair is sheared in two at the interface between the spectator and the QGP the nucleon remaining in the spectator has a momentum larger than the usual Fermi momentum. Alvioli M and Strikman M 2011 Phys. Rev. C 83 044905 White S and Strikman M 2009 ArXiv:0910.3205v2 [nucl-ex]

5. If the reaction plane is horizontal the asymmetry of the spectator neutron distribution tells whether the spectator was on the right or left side of the reaction (whether the angular momentum points up of down). The CMS reaction data can be divided into two sets depending on the direction of the angular momentum. Differences between the two sets provide a sensitive measure of the effects of angular momentum on the quark-gluon plasma. Some possibilities: The angles between almost back to back jets. Polarization of reaction products (from the angles of decay products) Small differences in left-right asymmetry

6. The amount of angular momentum in the interaction region is huge! It is maximum at impact parameter of 2.5 fm with a value of 2.1 x 106 ħ That is about 104 ħ per nucleon in the interaction region [using the calculations in F. Becattini, F. Piccinini, and J. Rizzo Phys. Rev. C 77 (2008) 024906] The angular momentum per nucleon is even larger at larger impact parameters. There are a number of papers about effects of a large angular momentum in the QGP For example: Flow Vorticity in Peripheral High Energy Heavy Ion Collisions arXiv:1302.5310v1 [nucl-th] 21 Feb 2013 Quark polarization in a viscous quark-gluon plasma Phys. Rev. C 84 (2011) 054910

7. Distinguish between neutron and photon showers and count the number of each. Assuming that all neutrons have the same energy, the EM neutrons should allow prediction of the number of neutrons in the hadronic section. Any discrepancy requires further analysis, perhaps a different neutral particle

8. Schematic sketch showing the difference of photon and neutron showers in the ZDC EM A charged hadron shows a track before the shower. Photons Neutrons Proton

9. Much interesting physics related to photons from spectators Except for the most peripheral reactions the spectator is disintegrated into protons and neutrons. This can not happen without making photons. The quarks in the spectators see a short but huge electric pulse, which will cause the quarks to radiate EM energy. At RHIC the spectator photons can not be seen because they have too low an energy and are spread over a large solid angle. At the LHC they have 27 times more energy and go mostly into the ZDC. The ratio of the signals in the outer EM sections to those in the inner sections as a function of centrality provided evidence for spectator photons, but might be a difference in the neutron distributions (also interesting).

10. Cosmic ray measurements have seen narrow jets of photons (a dozen or so in a 50 µm circle) These may be from heavy ion spectators (from iron on nitrogen) In our detector such a photon jet would appear with a normal photon shower shape but with a order of magnitude larger signal

11. Other particles Λ0 About one half of the Λ0 decay on the way to the ZDC Charged decay products are not swept away by magnets if the decay occurs after the last magnet. Decays are 64% pπ-̶and 36% nπ0 Charged hadrons make neutron like showers with a single track to the front surface. Half of charged hadrons will pass through EM leaving muon type track.