[Hadronic Spectra in Year 1 at RHIC]{.underline}

Peter Jacobs, LBNL/STAR

*Thanks to Mark Baker (PHOBOS), Fleming Vidabaek (BRAHMS) and*

*Bill Zajc (PHENIX)*

• Inclusive single particle distributions of hadrons:

> Transverse momentum distributions in narrow rapidity windows
>
> Yields as function of rapidity

• Some early physics from spectra at RHIC:

> Baryon stopping and initial conditions
>
> Entropy production and energy density
>
> Expansion dynamics (flow)
>
> Jets (leading hadrons) as probe of energy density

• Survey capabilities of the RHIC detectors in this area:

> This is not a competition
>
> This is far from the whole capability of any of them
>
> ![](media/image1.png){width="5.997222222222222in"
> height="7.942361111111111in"}

![](media/image2.png){width="5.997222222222222in"
height="7.942361111111111in"}

[Baryon Stopping]{.underline}

-   How is energy and baryon number transferred from the beam? What is
    baryon density in hottest region?

> rapidity loss of incoming baryons in pA collisions ∆y\<2
>
> ⇒ net baryon density at midrapidity at RHIC is zero

-   But perhaps not in heavy ion collisions:

> Kharzeev: Gluon junction carries baryon number
>
> Werner et al., Capella and Kopeliovich: novel diquark breaking
> mechanisms

[Baryon Stopping(cont'd)]{.underline}

-   Why important? (u,ubar), (ddbar) asymmetry affects dynamics, e.g.:

> Plasma: enhanced (s,sbar) production due to higher Fermi levels for
> nonstrange light quarks
>
> Hadronic gas: large baryonic cross sections: lower ρ~B~ ⇒ earlier
> freezeout at higher T
>
> ![](media/image3.png){width="5.997222222222222in" height="7.5125in"}
>
> ![](media/image4.png){width="5.997222222222222in" height="7.5125in"}

![](media/image5.wmf){width="5.991666666666666in"
height="4.7965277777777775in"}

> [Other Baryons in Year 1?]{.underline}

• STAR: Λ−Λbar in \|y\|\<1, 0.6\<pT\<2.5 GeV/c

> Ξ work in progress

• PHOBOS, BRAHMS may have some capabilities

> ![](media/image6.png){width="5.997222222222222in" height="7.5125in"}

[Energy Density]{.underline}

• Deconfinement expected for ε \> 1-2 GeV/fm^3\
^

• Bjorken: ε = (dE~T~/dη) / ( πR^2^τ )

> ≈ \<p~T~\> (dN/dη) / (πR^2^τ )

• Issues:

> equilibration of energy?
>
> what fraction, over what volume?
>
> ⇒ need theoretical guidance, other approaches

[Transverse Momentum Spectra at p~T~\<1.5 GeV/c]{.underline}

• nonperturbative production mechanisms

• QGP signature: softening of p~T~ spectra due to "stalling" in mixed
phase

• Important testing ground for equilibrium models

• Primary region of interest is at midrapidity, but rapidity dependence
of spectra is important:

> e.g. do protons go out of equilibrium at high y?

• Rates: not an issue in most cases

![](media/image7.png){width="5.997222222222222in" height="7.5125in"}

![](media/image8.wmf){width="5.991666666666666in"
height="4.7965277777777775in"}

![](media/image9.wmf){width="5.991666666666666in"
height="4.752083333333333in"}

[Jets and Leading Hadrons]{.underline}

• Wang, Gyulassy, Eskola, et al.:

> Energy loss (dE/dx) of high energy parton traversing QCD medium
> depends upon parton density of medium

• No hope to fully reconstruct jets with good energy resolution in
central Au-Au collisions at 200 GeV (à la UA1 etc.)

⇒ measure leading hadrons and their correlations

• Gluons couple more strongly to medium:

⇒ dE/dx (gluon) ≈ 2 dE/dx (quark)

• K-, pbar have greater contribution from gluon fragmentation than K+,p

⇒ flavour tagging of jets via leading pbar/p, K-/K+

![](media/image10.png){width="5.997222222222222in" height="7.5125in"}

![](media/image11.png){width="5.997222222222222in" height="7.5125in"}

[Proton Rates into STAR RICH at high p~T~]{.underline}

![](media/image12.wmf){width="6.001388888888889in"
height="5.610416666666667in"}

[Event Characterization in Year 1]{.underline}

-   All experiments: ZDC (b\< \~6 fm)

-   STAR: charged particle tracking in TPCs

> \| η\| \< 4.0

-   PHENIX: charged particle multiplicity in MVD (silicon strips and
    pads, 35K channels)

> \| η\| \< 2.5

-   PHOBOS: charged particle multiplicity (1 0K channels)

> \| η\| \< 5.4

-   BRAHMS: charged particle multiplicity in silicon strips (168
    channels)

> \| η\| \< 4.3

• Conclusion:

> Significant overlap in phase space coverage for all detectors
>
> If systematics (esp. backgrounds) understood, direct comparisons are
> possible

[RHIC/Experimental Run Plans]{.underline}

• RHIC will run Au-Au @ 200 GeV for most or all of

year 1

• But how to run it for maximum physics output?

> From current experiments: reference spectra are essential to
> understand the physics
>
> Is non-central Au-Au better than central Si-Si at 200 GeV? How to
> determine N~BINARY~ and N~PARTICIPANT~?
>
> Is lower energy Au-Au running essential for reference?
>
> Machine people want stable operation: changing energy/species
> decreases achievable ℒ

[RHIC/Experimental Run Plans (cont'd)]{.underline}

• Current positions of experiments for year 1 running:

BRAHMS: ℒ-limited ⇒ run Au-Au @ 200 GeV only

> PHENIX: need to push ℒ for rare probes ⇒ run
>
> Au-Au @ 200 GeV only
>
> PHOBOS and STAR: hadronic rates are high ⇒ run lighter beams @ 200 GeV
> and/or Au-Au at lower energy to survey the landscape

• This is a community-wide issue: how to maximize the physics reach of
RHIC

[Unofficial Summary of Experimental Capabilities for Single Particle
Inclusive Hadronic Spectra in Year 1 at RHIC]{.underline}

(do not keep score!!)

Low p~T~ charged π,K,p spectra: all

Large rapidity coverage: BRAHMS

High p~T~ π^0^: PHENIX and STAR

High p~T~ charged tracks: PHENIX, PHOBOS, STAR

Back-to-back high p~T~ correlations: STAR, PHOBOS

High p~T~ identified K, p: PHENIX, STAR

K~0~^s^, Hyperons: STAR, PHOBOS, BRAHMS