PHENIX Request for Additional Run-2 Beam

SUMMARY:

The PHENIX priorities for any additional beam time in Run-2 are1. 

1. Sufficient additional Au-Au and/or polarized-p on polarized-p running at 200 GeV to achieve our stated physics goals for Run-2.
2. A d-Au run at   Ös_NN = 200 GeV.
3. Anything else

These are stated in priority order. Some brief comments are supplied below; additional material will be provided in oral presentation to the PAC. It is hoped that the additional time between the drafting of this request and the PAC meeting will establish a more accurate understanding of the prospects for improving the integrated luminosity, and thus permit a stronger statement of the preferred strategy.

The 12 weeks of the official Au-Au run began on August 17th, as announced by Associate Laboratory Director for HENP Thomas Kirk. In the intervening 3.7 weeks, PHENIX C-A delivered 40M ZDC triggers (roughly 4 inverse microbarns, as compared to an anticipated 12). During this period, the PHENIX up-time was 45%, slightly below our stated (long-term) goal of 50%. However, this calculation includes 2 weeks of dedicated Level-2 trigger development time, during which trigger optimization took priority over recording data. It is of course expected that this investment during a period of relatively low luminosity will be recovered during the period of expected higher luminosity to follow.

For PHENIX to achieve the physics goals stated in our Run-2 Beam Use Proposal it is essential that

• The availability of stores improves from the current value of 30% or less to 50% or more.
• 200 MHz Storage RF commissioning be completed as soon as possible, so that delivered luminosity is equivalent to "useful luminosity". Currently, only 15-20% of the ZDC triggers are within the ±25 cm fiducial cut imposed by the PHENIX geometrical aperture; with storage RF this improves by a factor of 3-5.
• Luminosities at or near design luminosity be developed as soon as possible. We are very appreciative of the efforts of C-A D to deliver 600 inverse microbarns for full energy Au-Au and have found the week-by-week model of integrated luminosity they have provided most useful as a discussion point. At the same time, we are on record as noting that the this model delivers fully have of the integrated luminosity in the final two weeks of the run, delivered at intensities (3x design) that do not permit us to make efficient use of the beam. The PHENIX Run-2 data acquisition and Level-2 triggers have been designed to make full use of design luminosity. While instantaneous rates in excess of design are allowable, they provide no additional physics beyond that available at design luminosity. We therefore are on record as requesting the earliest possible development of design luminosity as a means of providing the maximal useful integrated luminosity recorded by PHENIX.

This information is summarized in the below table, which has been compiled by the PHENIX Run Coordinator (A. Frawley). 

Roser Luminosity Evolution Model vs PHENIX Reality

Notes:

ZDC trigger cross section = 10.7 barns.

Design luminosity = 2 x 10²⁶ cm^-2sec^-1 = 2100 Hz ZDC count rate.

ZDC scaler increases by 10.7M counts for every inverse microbarn.

Weeks in blue have (in the model) average luminosity greater than design luminosity.

The 12 week run officially started on August 17.

PHENIX STRATEGY FOR ADDITIONAL RUNNING:

A. Possibility of continued running with full energy Au-Au and/or 200 GeV protons

Should it become apparent at the time of this year's PAC meeting (or in later developments) that the current period of 12 weeks of Au-Au physics running does not allow us to achieve a substantial fraction of our Run-2 physics goals, PHENIX will request that any additional running be devoted to continued running of full energy Au-Au. A completely equivalent statement can and will be made for the planned 5 weeks of p-p phyics running at 200 GeV. We are fully appreciative of the usual "square-root of N" difficulties in quantifying "substantial fraction", and further note that these statements are reflective of our commitment towards first completing our stated Run-2 goals, and should not be interpreted as a desire to use any additional running to achieve only incremental increases to (then) existing data sets.

We also request that the current Au-Au 12 week run not be extended into the allotted time period for proton-proton running as a means of providing incremental increases in integrated luminosity. The proton-proton running is an essential portion of our Run-2 physics strategy. We believe that optimal scheduling of both PHENIX and C-A D personnel to utilize this period is best achieved via a fixed rather than sliding schedule.

B. d-Au running

After completion of our existing Run-2 goals, PHENIX's highest priority for additional running is a d-Au run of sufficient duration to obtain essential comparison data for high p_T hadrons and J/Psi's. Such a measurement, together with the p-p comparison data set, would complete the initial exploration of Au-Au collisions at 200 GeV. Experimenters would then have in hand data sets to study

• "elementary" cross sections (in p+p)
• "conventional nuclear effects" (in d+Au)
• "medium or volume effects" (in Au+Au)

Taken together, these data sets would then allow systematic determination of those effects unique to heavy ion collisions, thereby hugely strengthening the case for any new effects observed in Run-2.

A superb case in point is provided by the only qualitatively new phenomena observed (to date) at RHIC: the suppression of hadron production at high transverse momenta. While the necessary baseline data will be obtained in the p-p comparison data set, a d-Au measurement is crucial to distinguishing between cold nuclear matter effects (in particular, p_T-broadening and gluon shadowing) and potential new effects ("jet quenching") due to hot and/or deconfined nuclear matter. The same comments apply the expected J/Psi measurements from PHENIX in Run-2: It will be essential to provide the complementary d-Au measurements in order to quantify the suppression pattern observed in cold nuclear matter.

An informal working group within PHENIX has explored in detail the prospects for commissioning and running deuterons in Run-2. Consultation with the Tandem staff have been most productive, and indicate that deuteron beams of the required intensity can be supplied.  Radiation in the transfer line associated with a deuteron beam is not a serious issue if the beam energy is ~ 10 MeV or less and the beam is pulsed (10^-3 duty factor).  The required work - interlocking the facility to prevent higher energy or continuous beam operation and modifying the Accelerator Safety Envelope - is under way.  Machine development studies for asymmetric operation have been scheduled.

The studies to date indicate that the deuteron source and the Tandem capabilities provide a design luminosity well-matched to the other Run-2 species

so that a d-Au  run at Ös_NN = 200 GeV comparable to the p-p run (3 weeks of machine development followed by 5 weeks of physics running) leads to adequate comparison statistics in the channels of interest.

Two issues are worth noting: First, the choice of d-Au rather than p-Au is driven largely by machine rather than physics considerations. That is, the nearly equal rigidities of deuterons and Au  leads to a minimal ( ~1 mr) crossing angle. Second, the modest d-Au running proposed here is not a substitute for a full p-Au or d-Au run that would fully exploit RHIC's unique capabilities in these channels. A complete description of that physics is available in the white paper on proton-nucleus physics from the Nuclear Physics Long Range Planning meeting. Our interest in  these initial d-Au measurements is driven by the opportunity to provide essential and timely baseline data to resolve  existing and anticipated questions as early as possible in the RHIC program.

C. Long Term Strategy

PHENIX has consistently argued for a long-term planning strategy in scheduling RHIC runs and species. (For example, see the concluding slides in our previous Beam Use presentation.) In particular, we have emphasized the role of lighter species in providing the most efficient means of varying the number of participants and the number of collisions. This is demonstrated in the below figure, which shows the yield of J/Psi's as function of the number of binary collisions ("A*B scaling") provided by various species. It is clear that full exploration of this dynamic range requires running with lighter species such as Si+Si.

It should be noted that the relative yields in this figure take advantage of the higher luminosities expected for lighter species. Specifically, all yields for ions lighter than Au are obtained at roughly 5 times the rate of that for Au-Au. Said differently, for a given Au-Au measurement scaling as A², the full spectrum of light species comparison studies can be performed in the same number of RHIC weeks as those required for the Au-Au measurement.

The systematic study of the variation of charged multiplicity, transverse energy and high p_T hadrons with the nuclear overlap (and hence the number of participants and/or collisions), has been the common thread in PHENIX publications (and in nearly all of our additional papers in preparation from Run-1). We prefer to focus on further exploration in these productive direction via species variations, rather than in energy scans to verify well-established linear or constant dependencies in various channels.