Forward Light Hadrons RHICII Forward and p A

Forward Light Hadrons RHIC-II Forward and p. A Workshop Ramiro Debbe Physics Department RHIC-II 29 -Apr-2005

Outline of the presentation • Brief description of the BRAHMS setup. • Charged particle production as function of rapidity and centrality. • Particle identification: a) Rd. Au for anti-protons and - at =3. 2 b) Composition of h+ at =3. 2 • Results from p+p collisions. • Summary RHIC-II 29 -Apr-2005

The BRAHMS Detector MRS Geometrical Acc. : 0. 8 msr FFS BFS p/p = 0. 0018 p [Ge. V/c] RHIC-II 29 -Apr-2005

Event characterization The centrality of the collision for the results that will be presented is defined as fractions of the total multiplicity measured with the TMA in -2< <2 The centrality of Au-Au collisions can also be defined with the ZDC and BB or TMA correlations. Our triggers are defined with the ZDC and BB, p+p and d+Au collisions were triggered with the INEL detectors. ZDC BB TMA BB INEL RHIC-II 29 -Apr-2005 ZDC

I have a personal preference for the ~direct connection between gluon distributions in the colliding ions and the distribution of produced particles. For d+Au collisions, multiplicity densities show “cascading” in Au side and d. Au = pp at d frag. side. Already seen at lower energies. RHIC-II 29 -Apr-2005

The reach “into” the target wave function p. T From Guzey, Strikman, and Vogelsang. Most of the data collected at 4 degrees would have x 2~0. 01 x 2 Energy and momentum conservation x. L = xa - xb =(2 MT/√s)sinh y ka + kb = k, xaxb = MT 2/s A solution to this system is: xa = (MT/√s) ey xb = (MT/√s) e-y where y is the rapidity of the (x. L, , k) system RHIC-II 29 -Apr-2005

Acceptance and d+Au and p+p Spectra PRL 93 242303 (nucl-ex/0403005) RHIC-II 29 -Apr-2005

Modification of pt spectra as function of rapidity In both p+p and d+Au systems the distributions get softer at higher rapidities, beyond a trivial thermal behavior. RHIC-II 29 -Apr-2005

Rd. Au ratios Cronin like enhancement at =0. Clear suppression as changes up to 3. 2 Rd. A 2 Nd+Au/dp d 1 d T = 2 pp <Ncoll> d N inel/dp. Td where < Ncoll> = 7. 2± 0. 3 7. 2 RHIC-II 29 -Apr-2005 Same ratio made with dn/d follows the low p. T Rd. Au

Rcp ratios At =0 the central events have the ratio systematically above that of semi-central events. We see a reversal of behavior as we study events at =3. 2 Rcp = 1/<Ncoll central> 1/<Ncoll periph> NABcentral(p. T, ) NABperiph(p. T, ) RHIC-II 29 -Apr-2005

d+Au particle abundances The ratios proton/pion from d. Au collisions measured at midrapidity are consistent with “fragmentation in vacuum” RHIC-II 29 -Apr-2005

Using ratios to obtain the Rd. Au of identified negative particles. RHIC-II 29 -Apr-2005

Use of the BRAHMS RICH to identify particles We make use of already produced spectra and Rd. Au factors and use the RICH at high momentum to bypass the long process of invariant yield extraction. 17 Ge. V/c Above the selected momentum threshold the RICH is ~94% efficient. RHIC-II 29 -Apr-2005

Rd. Au for anti-protons and pions (min bias) d Au Preliminary pp RHIC-II 29 -Apr-2005

Difference between h+ and h- in d. Au at ~3 But p. QCD calculations that do well reproducing 0 would have at most a 10% difference not the factors ~3 we measure. • proton Our measurement is showing that the difference is driven by the protons. RHIC-II 29 -Apr-2005 kaon

BRAHMS results from p+p collisions The ratio of particle to anti-particle measured in p+p collisions is remarkably similar to the one we measured in Au+Au collisions. RHIC-II 29 -Apr-2005

Comparison of NLO p. QCD and measurement at = 0 Work done by Marco van Leeuwen STAR Calculations by W. Vogelsang RHIC-II 29 -Apr-2005

Measured h- at 4 degrees and a NLO p. QCD calculation BRAHMS data NLO p. QCD calc. From W. Vogelsang Frag. Function: modified KKP is an attempt to reproduce h- RHIC-II 29 -Apr-2005

Spectra for identified particles at =2. 2 RHIC-II 29 -Apr-2005

Comparison of particle ratios measured in p+p collisions and simulated with PYTHIA For protons we find a remarkable difference that may indicate other processes besides parton fragmentation. =3. 2 We measure a small excess of kaons and see an emerging trend that suffers from low statistics at high pt. RHIC-II 29 -Apr-2005

Summary §Inclusive charged particle production in d. Au compared to scaled pp shows a suppression that depends on rapidity and the centrality of the collisions. §First results that include particle identification in d. Au collisions show particle abundances consistent with “fragmentation in the vacuum”. §Different “Cronin effect” of baryons and mesons is also seen at forward rapidities. §NLO p. QCD describes pp inclusive pt distributions up to the highest rapidities measured. §We see that PYTHIA fails to describe the baryons (protons) at forward rapidity. RHIC-II 29 -Apr-2005