1. 1 Charged hadron production at large transverse momentum in d+Au and Au+Au collisions at  s=200 GeV Abstract. The suppression of hadron yields with high transverse momentum in central Au+Au collisions at RHIC energies is well established effect. We present the PHENIX results on a control experiment at RHIC with deuteron-gold collisions. Charged hadron spectra at mid-rapidity and transverse momentum < 8 GeV/c normalized to the number of nucleon-nucleon collision show small enhancement (Cronin effect) for high momentum particles. Effect grows with centrality of the d+Au collision and number of binary collisions per deuteron participant. Data for p+Au and n+Au, based on spectator nucleon tagged events, show the similar behavior. This tendency is opposite to the suppression in Au+Au collisions. Vladislav Pantuev and Sean Leckey for PHENIX Collaboration Stony Brook University, Stony Brook, New York

2. 2 What do we see in Au+Au? R AA Hadron yield normalized to number of collisions and to the yield in p+p. For peripheral AuAu collisions R AA ~ 1 Peripheral are consistent with hard scattering Central R AA suppressed by factor of 4.5 Did we find new physics in hot dense nuclear matter or do we simply not understand cold nuclear matter? PHENIX

3. 3 Why d+Au collisions are in such interest? Theoretical studies of the hadron suppression in AuAu collisions suggest two major explanations: During parton propagation in high density matter it loses a significant fraction of its energy through gluon bremsstrahlung, reducing the marton momentum and depleting the yield of high pt hadrons. This is a final state effect in the spatially extended medium created in A+A collisions Initial-state effects include nuclear modifications to the parton momentum distributions (structure functions), even parton saturation effect, and soft scattering of the incoming parton prior to its hard scattering. These should be presented in p+A, d+A and A+A It is therefore of paramount interest to determine experimentally the modifications, if any, of high pt hadron yields in a system such as d+Au where dense medium is not produced

4. 4 pA and nA collisions at PHENIX DX 18 meters ZDC FCAL deuteron proton-A interactions – Zero Degree Calorimeter sits 18 m downstream from collision and detects spectator neutrons. neutron-A interactions – Forward Calorimeter sits next to ZDC. DX magnets sweep spectator proton into FCAL. N+A – Roughly 40% of minimum bias sample. – Our tagging system is the ideal environment to study the Cronin effect in N+A. p n

5. 5 How PHENIX selects centrality? N.B.D.: P(n) = (n+k+1)!/(n!(k+1)!) (  /k) n /(1+  /k) (n+k)  is the mean k is not an integer (  /k) 2 = 1/k + 1/   is proportional to sqrt(  )  Assume that the number of hits in South BBC (Au seeing) for a given number of participants in Au is distributed according to the Negative Binomial Distribution (N.B.D.)  Assume that the N.B.D. parameters are proportional to N part in Au.  We assume ,k ~ N au part and weight all N.B.D. with Glauber probabilities. PHENIX has Beam-Beam Counters from both sides of the collision point. Plus PHENIX has forward neutron and proton calorimeters for the spectator nucleons.

6. 6 Minimum bias, tagged, efficiency  Get N.B.D. parameters form data using Glauber weights.  Deduce efficiency function from ratio of measured multiplicity to the fit and get for centrality bins.  Efficiency cancels in ratio. Nice check! Gf minbias tagged ratio

7. 7 …and this is what we actually get for Ncoll Centrality % 60-88 40-60 20-40 0-20 Number of collisions, min.bias Number of collisions per d participant Number of collisions in tagged event 3.2(0.3) 2.2 2.1 6.9(0.6) 4.0 4.6 10.4(0.7) 5.6 7.0 9.6 7.515.0(1.) Glauber model

8. 8 Absence of suppression in minimum bias d+Au Min. bias d+Au collisions at RHIC already show absence of suppression at high pt. Phys.Rev.Lett.91(2003)072303- 01 Even more, some enhancement was found - long known Cronin effect How effect depends on centrality and number of collisions?

9. 9 Centrality dependence in d+Au PHENIX preliminary Error bars: thin lines - statistical errors inner bars - correlated systematic uncertainties, common for pt range outer bars - include an additional systematic errors which are independent of pt - uncertainty in number of collisions, reference spectrum absolute normalization, acceptance corrections

10. 10 p+Au and n+Au PHENIX preliminary 60-88 40-60 20-40 0-20 Nucleon-gold collisions show the similar tendency as d+Au Well known Cronin effect is nicely seen The amplitude of the effect rises with centrality There is sort of plateau in R dA in 2-6 GeV/c momentum region R nA

11. 11 More investigation on R nA Yield at low pT roughly scales with Ncoll (or ~Npart for dAu) Cronin enhancement at high pT increases roughly linear with the number of rescatterings Statistically weighted average of RnA in particular momentum range Examine in “plateau region”, 2-4 GeV/c (black) Compare to low pT region, 0.6-1.0 GeV/c (red) Tagged, squares MB, circles PHENIX preliminary 2- 4 GeV/c 0.6-1.0 GeV/c

12. 12 Rcp - ratio of Central to Peripheral PHENIX preliminary In order to study the modification of hadron spactral shape and to exclude systematic uncertainties we take the ratio Rcp of the three central spectra to the spectra of most peripheral bin each scaled by number of collisions. Within the errors Rcp for hadrons is constant for 2-6 GeV/c with amplitude at about 1.5 Tagged and Min. Bias events are very similar For comparison we plot with the results from just one of calculations for pions, nucl-th/0308029

13. 13 Conclusions PHENIX measured charged hadron spectra at mid-rapidity and transverse momentum < 8 GeV/c in d+Au minimum bias and tagged collisions Hadron suppression at high transverse momentum in central rapidity region in AuAu collisions is not an Initial State Effect We observe Cronin effect in d+Au and n(p)+Au collisions There is no Cronin enhancement in peripheral dAu The amplitude of the effect rises with centrality and number of rescatterings We observe sort of plateau for R dA in 2-6 GeV/c momentum region Minimum bias and tagged events show very similar behavior

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