The heavy ion physics approach to global event characterization has led us to
instrument the forward region in the PHENIX experiment at RHIC. In heavy ion
collisions this coverage yields a measurement of the "spectator" energy and its
distribution about the beam direction. This energy flow is the basis of
event-by-event determination of the centrality and reaction plane which are key
to analyzing particle production in heavy ion collisions. These same tools have
also enabled a unique set of measurements on inelastic diffraction with proton,
deuteron and gold ion beams in the PHENIX experiment. We present first new
results on this topic and discuss briefly the opportunity for diffractive
physics with Heavy Ion beams at the LHC.Comment: RHIC overview talk presented at "Diffraction 2004" in Dorgali,
  Sardegna, Ital

Adler

Appelshauser

Baltz

Baur

Chiu

Klein

Kopeliovich

Sebastian N. White

Silvermyr

Snellings

White

English

arXiv

Crossref

Inelastic Diffraction at Heavy Ion Colliders

The heavy ion physics approach to global event characterization has led us to instrument the forward region in the PHENIX experiment at RHIC. In heavy ion collisions this coverage yields a measurement of the ''spectator'' energy and its distribution about the beam direction. This energy flow is the basis of event-by-event determination of the centrality and reaction plane which are key to analyzing particle production in heavy ion collisions. These same tools have also enabled a unique set of measurements on inelastic diffraction with proton, deuteron and gold ion beams in the PHENIX experiment. We present first new results on this topic and discuss briefly the opportunity for diffractive physics with Heavy Ion beams at the LHC

WHITE, S.

UNT (University of North Texas) Digital Library

      PX # 295      BNL-75180-2005-CP        Inelastic Diffraction at Heavy Ion Colliders  S. White    2004    Physics Department/PO/2-182   Brookhaven National Laboratory P.O. Box 5000 Upton, NY 11973-5000 www.bnl.gov        Notice: This manuscript has been authored by employees of Brookhaven Science Associates, LLC under Contract No. DE-AC02-98CH10886 with the U.S. Department of Energy. The publisher by accepting the manuscript for publication acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes.            DISCLAIMER  This report was prepared as an account of work sponsored by an agency of the United States Government.  Neither the United States Government nor any agency thereof, nor any of their employees, nor any of their contractors, subcontractors, or their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or any third party’s use or the results of such use of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof or its contractors or subcontractors.  The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.                                arXiv:nucl-ex/0501004 v1   10 Jan 2005Inelastic Diffraction at Heavy Ion CollidersSebastian N. WhiteaaBrookhaven National Laboratory,Upton, N.Y. 11973, USAThe heavy ion physics approach to global event characterization has led us to instrument the forward region inthe PHENIX experiment at RHIC. In heavy ion collisions this coverage yields a measurement of the “spectator”energy and its distribution about the beam direction. This energy flow is the basis of event-by-event determinationof the centrality and reaction plane which are key to analyzing particle production in heavy ion collisions. Thesesame tools have also enabled a unique set of measurements on inelastic diffraction with proton, deuteron andgold ion beams in the PHENIX experiment. We present first new results on this topic and discuss briefly theopportunity for diffractive physics with Heavy Ion beams at the LHC.1. IntroductionLike the LHC, the RHIC collider is a dual func-tion machine providing collisions of protons aswell as heavy ions. Unlike the LHC, the exper-iments at RHIC have been designed completelywith the goals of heavy ion physics in mind andthen subsequently used also for measurementswith (polarized) proton colliding beams. In manycases the choice of parameters in detector designhave had interesting consequences for the typesof measurement that can be carried out with ppcollisions but nowhere is this so clear as in thecase of forward instrumentation.1.1. Heavy Ion GoalsThe primary goals of forward instrumentationat RHIC are• to measure absolute luminosity• to provide a truly minimum bias trigger• to characterize the initial conditions of in-dividual events(reaction plane orientation,impact parameter) outside the mid-rapidityregionThe interaction region geometry, depicted inFig. 1, provides a location at z=±18 m of max-imum momentum dispersion where “spectatorfragments” from the colliding nuclei are resolvedprimarily according to their charge-to-mass ratio(Z/A). An intermediate impact parameter eventis shown in Fig. 2. RHIC experiments were de-signed to characterize the reaction plane and im-pact parameter of individual events. The numberof forward going “spectator nucleons” leaving thecollision is anti-correlated with the number of par-ticipants:Nspectator +Nparticipant = 2 ×A. (1)In neutron rich ion species stored at RHIC (Au)and LHC (Pb) most of the information is carriedby neutrons which are detected in the RHIC ZeroDegree Calorimeters (ZDC’s) [2] that were in-stalled in each experiment. Protons are also mea-sured in the PHENIX experiment using a largearea hadron calorimeter recycled from AGS ex-periment E865.The centrality measurement by spectators ispartially diluted by a significant component ofrecombination/coalescence of the spectator nucle-ons (ie n+ p → d) which are neither detected inthe ZDC or the FCAL. Even for central collisions∼ 1/2 of the protons coalesce to higher mass frag-ments [4]. So in PHENIX the participant numberis sampled by the multiplicity in the 3 ≤ |η| ≤ 4region and combined with ZDC energy to formthe centrality algorithm.Early studies of RHIC collisions [3] showedthat spectator neutrons are detected in the ZDC12Figure 1. RHIC interaction region layout.with effZDC ≥ 99.5% in nuclear interactions atall centralities. Even in UltraPeripheral Colli-sions (UPC) where Rimpact ≥ 2 × RAu , beamenergy neutrons are evaporated (and detected inthe ZDC’s) due to the intense electromagnetic in-teractions between high-Z nuclei [1]. The ZDC istherefore the ideal minimum biased event trig-ger for nuclear interaction physics. For UPC, theZDC trigger (“tagged” UPC events-see below) isused to form “rapidity gap” triggers and is theonly practical way to access this physics.Because the dominant UPC process -mutualcoulomb dissociation - is calculable with an ac-curacy of ∼ 5% [1] and cleanly measured [3], itis used as the basis for all absolute luminositymeasurements in RHIC.The reaction plane orientation is best measuredin the ZDC using a position sensitive scintillatorarray at a depth of 2 × λInt in the calorimeterbecause of the large directed flow, v1, among nu-cleons at this rapidity [5]. Since the orientationis measured independently in both ZDC arms(leftand right) the resolution can be directly derivedand we find0.3 = < cos(ψmeas − ψtrue) >=√2· < cos(ψright − ψleft) > (2)Figure 2. Collision geometry in heavy ion colli-sions. Both magnitude and direction of the im-pact parameter are measured by forward “spec-tator” remnants.2. EM Interactions of Heavy IonsUltraperipheral collisions become an importantaspect of heavy ion physics at the large c.m. en-ergies available at colliders. At the LHC, withPb+Pb collisions at√SNN = 5.5 TeV, processescan be studied that are equivalent to 100 TeVquasi-real photon collisions with a stationary nu-clear target [6] opening up a wide range of oppor-tunities [7].The principal method for analyzing such col-lisions, the method of equivalent quanta, origi-nated with Fermi [8] who showed that interactionsof electrons and α−particles with atoms can bereduced to a convolution of an equivalent photonspectra and the known γ−A interaction cross sec-tions. The method was extended to the extremerelativistic case by Weizsacker and Williams andhas found many applications in high energy andnuclear physics.At ion colliders the characteristic strength ofinteraction ∼ Z1 × Z2 × α is large so that multi-ple photon exchanges lead to the phenomenon oftagging. Additional photon exchanges can causephoto-nuclear breakup of one nucleus which emitsa soft neutron [9] detected in the ZDC. Origi-3Cross Section STAR (mb) ref. (mb)σρxn,xn 28.3±2.0±6.3 27σρ1n,1n 2.8±0.5±0.7 2.6σρ(inc.overlap)xn,xn 39.7±2.8±9.7 −σρxn,0n 95±60±25 −σρ0n,0n 370±170± 80 −σρtotal 460±220±110 350Table 1ρ photoproduction results reproduced fromRef. [10].nally the tagging requirement was thought of as apenalty factor required to access this physics witha clean experimental trigger. However, as shownin Ref. [9] the tagging fraction is correlated withimpact parameter and strongly biases the taggedphoton distribution toward higher equivalent en-ergies. We now calculate tagged and untaggedphoton spectra separately, using this nomencla-ture below.2.1. Diffractive vector meson productionat RHICDuring the√SNN = 130 GeV initial Au+Aurun, STAR recorded data with a two-prong trackrequirement (consistent with diffractive ρ →π+π− decays) as well as a minibias ZDC trig-ger [10]. The two data samples were examinedfor diffractive ρ production as well as low massγγ → e+e−candidates.The results are summarized in Table. 1, repro-duced from Ref. [10] where results are also com-pared with a calculation of the ρ production crosssections for both the tagged and untagged cases[11]. The cross sections (subscripts 0n and Xn de-note 0 and 1-or more neutron tags, respectively)are in reasonable agreement with the calculation.Note that the experimental errors are smallest inthe tagged sample.During the most recent (2004) Au+Au run,PHENIX implemented a diffractive trigger sensi-tive to high mass e+e− pairs using a ZDC tag andvetoing on events with activity in the 3 ≤ |η| ≤ 4(=BBC) region. The PHENIX experiment hasexcellent identification capabilities for high mo-mentum electrons due to the highly segmented(∆η × ∆Φ = 0.01 × 0.01) Electromagnetic (EM)Calorimeter and Ring Imaging Cerenkov coun-ters. The diffractive e+e− trigger also requiredat least one cluster of calorimeter energy withgreater than 0.8 GeV. The full trigger isUPC = ZDC(or) · BBC(notand) · EM . (3)This loose rapidity gap trigger (ie a leadingneutron and no activity in at least one BBC arm)is very effective in heavy ion collisions and hada rate of ≤ 0.5% of the min bias rate. A totalof 8.5 M UPC triggers were recorded. Electroncandidates were reconstructed requiring consis-tent EM calorimeter energy and magnetic spec-trometer momentum as well as a Cerenkov match.The resultant e+e− invariant mass spectrum [12]is shown in Fig. 3. All candidates have a net ptof ≤ 100 MeV, consistent with diffractive photo-production and the expected Au nuclear formfactor. This should be compared to the pt ≥ 1GeV/c we find in non-diffractive p+ p→ J/ψ+Xdata .It should be noted that this spectrum is highlypreliminary since, at the time of this initialpass through the data, there has been very lit-tle prior experience in PHENIX reconstructingevents with such low multiplicities (ie where theinteraction vertex is not well constrained). Ourcalculation of the expected J/ψ and e+e− con-tinuum yield is higher than observed by about afactor of 2. Nevertheless, even at this very earlystage in the analysis, it is clear that a diffractivesignal of high mass pairs has been seen and the ca-pability for small cross section diffractive physicshas been demonstrated.3. Au+d→ Au+n+ pImmediately following the first high luminosityAu+Au run RHIC operated as a d+Au collider(at√SNN = 200 GeV). The primary purpose ofthis run was to directly measure the contributionof initial state nuclear effects on inclusive particlespectra observed in d-Au→ π0+X for example.In order to measure inclusive particle cross sec-tions, it is critical to calibrate the d-Au luminos-ity. Also, since particle spectra are compared for4Invariant Mass (GeV/c^2)1 1.5 2 2.5 3 3.5 4 4.5 5012345678 UnlikeLikeFigure 3. Preliminary PHENIX diffractive e+e−mass spectrum.an equivalent number of binary collisions, derivedfrom a Glauber model calculation, it is useful tovalidate this calculation by confirming the corre-sponding minimum bias cross section.During the d+Au run STAR and PHENIX ranwith different min.bias triggers:min.bias(STAR) = ZDCAu−beam(≥ 15GeV)min.bias(PHENIX) = BBCleft · BBCright (4)which, as pointed out in Ref. [14], have differentacceptances for NN collisions with a rapidity gapand hence different cross sections.In PHENIX we have measured directly the in-elastic d+Au cross section by normalizing to thetheoretical calculation [13] for deuteron photodis-sociation of σdiss = 1.38 ± 0.07 b. PHENIXrecorded 12 M events with three different triggersenabled: the 2 inelastic triggers of eqn. 4 in “or”with a trigger requiring ZDCd−beam ≥ 15 GeV.This last trigger is 100% efficient for deuteronbreakup events since, with a reasonable deuteroninput wavefunction, all neutrons fall within theZDC aperture. Dissociation events have a par-ticularly clean signature in the PHENIX forwarddetectors since, in the d beam direction, there isan easily resolved 100 GeV energy peak simulta-neously in both the ZDC and the proton calorime-ter. Also there is no activity in the central detec-tor or the Au beam ZDC.Ref. [14] predicts a factor of 0.83 suppressionof the min.bias(PHENIX) cross section relativeto the naive Glauber value of σGlauber = 2.33 b.Instead we measure, applying a small correctionfor BBC efficiency:σPHENIX(min.bias) = 2.39 ± 0.24b, (5)which differs by ∼ 2 st.dev. from Ref. [14].The error on this preliminary result will probablyreach ≤ 5% after further study.1234567891020304050607080-5 -4.5 -4 -3.5 -3 -2.5 -2 -1.5 -1 -0.5 010-410-310-210-111010 210 310 410 510 6ATLAS dijet photoproduction, taggedFigure 4. Rates of photo-produced jet pairs froma 1 month run in ATLAS.4. pp→ n+XLarge xF inclusive cross sections are relevantfor analysis of super-GZK cosmic ray candidatesand also for LHC luminometry, where this pro-cess will be used for beam diagnostics. At RHICit has a special role in polarimetry since there is5a large (∼ 10%) left-right asymmetry of the neu-tron angular distribution relative to the up-downpolarization of the incident proton. Also a large(∼ 1%) pp → nn+X signal is observed which isnot reproduced by most event generators (ie DP-MJETIII). This signal is the basis of all RHICpp luminosity monitoring. Further study of theseforward physics processes is now under way.5. Photoproduction with Heavy Ions atthe LHCThe ATLAS experiment is now constructingZDC’s to be installed in the TAN absorber at±140 m from the IP1 interaction region. Fullgeant simulation of this detector with√SNN =5.5 TeV Pb+Pb collisions has demonstrated asimilar energy resolution to that observed atRHIC.The ATLAS ZDC’s will be the primary triggerfor diffractive physics and will be combined withlow-pt jet or secondary vertex triggers. A likelyarea of research is the study of photo-produced di-jets, γ−jet pairs and heavy quarks (QQ) and alsothe corresponding diffractive photo-productionprocesses (which have a rapidity gap in both beamdirections). We have carried out [15] a calcu-lation of rates for the above processes and thejet-jet case is presented in Fig. 4 for a 1 monthrun at nominal LHC Pb-Pb luminosities. Evenwith a jet threshold of ≥ 10 − 15 GeV the totalevent rate exceeds ∼ 105. We are now studyingthe ATLAS capability for accessing low pt jetsin events with the very low underlying multiplic-ities expected for these diffractive processes. Itis likely that soft heavy quark production canbe tagged through a secondary vertex at lowerpt. This would probe the lowest x2 parton dis-tributions(abscissa in Fig. 4)- possibly down tox2 ∼ 10−4. We also indicate in Fig. 4 theequivalent incident photon energy and the aver-age rapidity(yjet) corresponding to each bin. Forthe most part, the produced jets are very centraland it should be possible to require a large rapid-ity gap along the direction of the beam emittingthe photon. The ATLAS forward calorimeters areunique in providing sensitivity up to |η| < 5.0 forET ≥ 2.0 GeV and this makes it the most promis-ing experiment to carry out rapidity gap physicswith heavy ions.6. AcknowledgementsI am grateful to the organizers of “Diffraction2004” for a very stimulating and pleasant meet-ing.REFERENCES1. A. J. Baltz, C. Chasman and S. White, Nucl.Instr. And Meth. A417, 1 (1998).2. C. Adler et al., Nucl. Instr. And Meth. A470,488 (2001).3. M. Chiu et al. Phys. Rev. Lett. 89 (2002)012302, nucl-ex/0109018.4. H. Appelshauser et al.[NA49 Collaboration],Eur. Phys. J. A2, 383 (1998).5. R. Snellings et al. Phys. Rev. Lett. 84 2803(2000).6. S. White “Applications of the EquivalentPhoton Approximation to Heavy Ion Colli-sions” in “Electromagnetic Probes of Funda-mental Physics” W. Marciano and S. White,eds. World Scinetific (2003).7. G. Bauer et al. “Hot Topics in UltraperipheralHeavy Ion Collisions”, ibid.8. E. Fermi (translation by M. Gallinaro and S.White) “On the Theory of Interactions be-tween Atoms and Electrically Charged Parti-cles”, ibid.9. G. Baur et al. Nucl.Phys.A729(2003) 787 andnucl-th/0307031.10. C. Adler et al. Phys.Rev.Lett. 89 (2002)272302 and nucl-ex/0206004.11. A. J. Baltz, S. R. Klein, and J. Nystrand,Phys. Rev. Lett. 89 012301 (2002).12. For details see D. Silvermyr inwww.phenix.bnl.gov/∼silvermy/dnp04.pdf.13. S. Klein and R. Vogt, Phys. Rev. C 68 (2003)017902. and nucl-ex/0303013.14. B. Kopeliovich, Phys. Rev. C 68 (2003)044906 and nucl-th/0306044.15. M. Strikman, R. Vogt and S. Whitemanuscript in preparation. To be published inCERN yellow report on UltraPeripheral Col-lision physics and R. Vogt, hep-ph/0407298.

INELASTIC DIFFRACTION AT HEAVY ION COLLIDERS.

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Inelastic Diffraction at Heavy Ion Colliders

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