Transverse-Momentum Spectra in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi mathvariant="normal">A</mml:mi><mml:mi mathvariant="normal">u</mml:mi><mml:mo>+</mml:mo><mml:mi mathvariant="normal">A</mml:mi><mml:mi mathvariant="normal">u</mml:mi></mml:math>and<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>d</mml:mi><mml:mo>+</mml:mo><mml:mi mathvariant="normal">A</mml:mi><mml:mi mathvariant="normal">u</mml:mi></mml:math>Collisions at<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msqrt><mml:msub><mml:mi>s</mml:mi><mml:mrow><mml:mi>N</mml:mi><mml:mi>N</mml:mi></mml:mrow></mml:msub></mml:msqrt><mml:mo>=</mml:mo><mml:mn>200</mml:mn><mml:mtext> </mml:mtext><mml:mtext> </mml:mtext><mml:mi mathvariant="normal">G</mml:mi><mml:mi mathvariant="normal">e</mml:mi><mml:mi mathvariant="normal">V</mml:mi></mml:math>and the Pseudorapidity Dependence of High-<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mi>p</mml:mi><mml:mi>T</mml:mi></mml:msub></mml:math>Suppression

Abstract

We present spectra of charged hadrons from $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ and $d+\mathrm{A}\mathrm{u}$ collisions at $\sqrt{{s}_{NN}}=200\text{ }\mathrm{G}\mathrm{e}\mathrm{V}$ measured with the BRAHMS experiment at RHIC. The spectra for different collision centralities are compared to spectra from $p+\overline{p}$ collisions at the same energy scaled by the number of binary collisions. The resulting ratios (nuclear modification factors) for central $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ collisions at $\ensuremath{\eta}=0$ and $\ensuremath{\eta}=2.2$ evidence a strong suppression in the high ${p}_{T}$ region ($&gt;2\text{ }\text{ }\mathrm{G}\mathrm{e}\mathrm{V}/c$). In contrast, the $d+\mathrm{A}\mathrm{u}$ nuclear modification factor (at $\ensuremath{\eta}=0$) exhibits an enhancement of the high ${p}_{T}$ yields. These measurements indicate a high energy loss of the high ${p}_{T}$ particles in the medium created in the central $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ collisions. The lack of suppression in $d+\mathrm{A}\mathrm{u}$ collisions makes it unlikely that initial state effects can explain the suppression in the central $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ collisions.