# Presentation: 840403

## d+Au at RHIC:Past and Future

**Carl A. Gagliardi**
- Texas A&M University

- Outline
-   Jet quenching and d+Au as a control experiment
-   Small-x physics and saturation
-   Upgrade plans for the future

## Suppression of high-pT inclusive hadron yields

- Suppression of high-pT inclusive hadron yields

-  central Au+Au collisions:  **factor ~4-5 suppression**
-  pT > 5 GeV/c: suppression ~ independent of pT

- PRL 91, 172302

- Au+Au relative to p+p

- RAA

- PRL 91, 072301

## p+p baseline and NLO pQCD

- PRL 91, 241803

- PHENIX π0

- Calculations by
- W. Vogelsang

- At 200 GeV, pQCD does a very good job describing mid-rapidity yields

- STAR  (h++h-)/2
- BRAHMS  (h++h-)/2

## Jets and two-particle azimuthal distributions

- p+p  di-jet

-   Trigger:  track with pT>4 GeV/c
-    distribution:  2 GeV/c<pT<pTtrigger
-   Normalize to number of triggers

- PRL 90, 082302

- At 200 GeV, p+p collisions show conventional di-jet structure

## Azimuthal distributions in Au+Au

- Au+Au peripheral

- Au+Au central

- pedestal and flow subtracted

- PRL 90, 082302

- Near side:  peripheral and central Au+Au similar to p+p
- Away side:  strong suppression of back-to-back correlations 		         in **central Au+Au**

## Theory vs. data

- PRL 91, 172302

- RCP

- pQCD-I: Wang, nucl-th/0305010
- pQCD-II: Vitev and Gyulassy, PRL 89, 252301
- Saturation: KLM, Phys Lett B561, 93

- pT>5 GeV/c:  well described by pQCD+jet quenching, but also by gluon saturation model (up to 60% central)

- Final state

- Initial state

## Is suppression an initial or final state effect? 

- Initial state?

- gluon saturation

- Final state?

- How to discriminate?  Turn off final state  **d+Au collisions!**

## What STAR found – Jet Quenching 

- What STAR found – Jet Quenching

- PRL 91, 072304

- Inclusive yields and back-to-back di-hadron correlations are **very similar in p+p and d+Au** collisions
- Both are **strongly suppressed** in central Au+Au collisions at 200 GeV

## PHENIX, PHOBOS, BRAHMS found similar results

- PRL 91, 072302/3/5

- PHENIX

- PHOBOS

- BRAHMS

## Phys Rev Lett 91,

- 072302/3/4/5

## Forward particle production in d+Au collisions

- BRAHMS, PRL 93, 242303

- Sizable suppression in charged hadron production in d+Au collisions relative to p+p collisions at forward rapidity

## PHENIX reports a similar effect

- PRL 94, 082302

- Charged mesons are suppressed in the forward direction and enhanced in the backward direction.

## Expectations for a color glass condensate

-  related to rapidity of produced hadrons.

**As y grows**

- D. Kharzeev, hep-ph/0307037

- Iancu and Venugopalan, hep-ph/0303204

- Are the BRAHMS data evidence for **gluon saturation at RHIC energies**?

## One calculation within the saturation picture

- RdAu

- RCP

- Saturation model calculation with additional valence quark contribution (Kharzeev, Kovchegov, and Tuchin, PL B599, 23)

## Pseudo-rapidity yield asymmetry vs pT

- Back/front asymmetry in 200 GeV d+Au consistent with general expectations of saturation or coalescence; doesn’t match pQCD prediction.

- Au direction / d direction

- PRC 70, 064907

## Is saturation really the explanation?

- Difficult to explain BRAHMS results with standard shadowing, but in NLO pQCD calculations **<*****x******g*****> ~ 0.02** is not that small
- (Guzey, Strikman, and Vogelsang, PL B603, 173)

## Comparing d+Au dN/dη to p+emulsion

- PHOBOS attributes effects to limiting fragmentation

- nucl-ex/0409021

## Many (recent) descriptions of low-x suppression...

- A short list (probably incomplete)

-  ...

## Forward physics at a collider Kinematics

- Assume:
- Initial partons are collinear
- Partonic interaction is elastic   *p**T*,  *p**T,*2

- Forward physics at a collider **Kinematics**

- 

- How can Bjorken x values be selected in hard scattering?

**Hard scattering hadroproduction**

## Do we understand forward π0 production in p + p?

- Bourelly and Soffer, EPJ C36, 371:
-     NLO pQCD calculations underpredict the data at low *s* from ISR
-     Ratio appears to be a function of angle and √*s,* in addition to *p**T*

## STAR detector

**Forward ************ ****Detector (FPD) **
** ****Pb-glass EM calorimeter**
** ****Shower-Maximum Detector (SMD)**
** ****Preshower**

-  **TPC: -1.0 ****< ******** < 1.0**
** ****FPD: |********| ******** 3.8 (p + p)**
**          ****|********| ******** 4.0 (p + p and d + Au)**

## ppX cross sections at 200 GeV

- The  inclusive differential cross section for 0  production is consistent with NLO pQCD calculations at 3.3 < η < 4.0
- As η increases, systematics regarding the comparison with NLO pQCD calculations begin to emerge. The data at low pT are more consistent with the Kretzer set of fragmentation functions. PHENIX observed similar behavior at low pT.

- ppX cross sections at 200 GeV

##  dependence of RdAu

- Observe **significant rapidity dependence**, similar to BRAHMS and PHENIX measurements and expectations from saturation framework.

## PYTHIA:  a guide to the physics

-  PYTHIA ***prediction**** *agrees well with the inclusive 0 cross section at 3-4
-  Dominant sources of large xF  production from:
-  q + g  q + g **(2********2)**   + X
-  q + g  q + g + g **(2********3)**   + X

- g+g and
- q+g  q+g+g

- q+g

- Soft processes

- Forward Inclusive  Cross-Section:

- Subprocesses involved:

- STAR FPD

## Constraining the x-values probed in hadronic scattering

- Guzey, Strikman, and Vogelsang, Phys. Lett. B **603**, 173 (2004).

**Measure two particles** in the final state to constrain the *x*-values probed

-  FPD: ||  4.0
-  TPC and Barrel EMC: || < 1.0
-  Endcap EMC:  1.0 <  < 2.0
-  FTPC: 2.8 <  < 3.8

- Collinear partons:
* **x*+ = *p**T*/*s* (e+***1* + e+***2*)
-  *x*** = *p**T*/*s* (e***1* + e***2*)

- Log10(xGluon)

## FPD-TPC correlations in p+p

**PYTHIA (plus  detector sim) predicts**** **
-  **“****S” ****grows with ****<x****F****> and <p****T****,********>**** **
** ****“********s****”**** ****decrease with ****<x****F****> and <p****T****,********>**** **
**PYTHIA prediction agrees with data**
- Larger intrinsic kT required to fit data

- Statistical errors only

**25<E********<35GeV**

**45<E********<55GeV**

**STAR Preliminary**

**STAR Preliminary**

## Any difference between p+p and d+Au?

- Kharzeev, Levin, McLerran gives physics picture (NPA748, 627)

- d+Au: ** “Mono-jet”**

**P****T**** is balanced by many gluons**

- Dilute parton system (deuteron)

- Dense gluon
- field (Au)

- Color glass condensate predicts that the **back-to-back correlation **from p+p** should be suppressed**

- p+p:  **Di-jet**

## Back-to-back correlations with the color glass

- (Kharzeev, Levin, and McLerran, NP A748, 627)

- The evolution between the jets makes the correlations disappear.

## HIJING predicts similar effects in d+Au as seen in p+p

**25<E********<35GeV**

**35<E********<45GeV**

- HIJING predicts **similar effects in d+Au** as seen in p+p

- Are there “trivial” differences between p+p and d+Au?

## Correlations in d+Au

- Statistical errors only

-  **are suppressed at small <x****F****> and <p****T,********>      **
**S****pp****-S****dAu****= (9.0 ± 1.5) %**
**Consistent with     CGC picture**
** ****are consistent in d+Au and p+p at larger <x****F****> and <p****T,********> **
**as expected by HIJING **

**25<E********<35GeV**

**35<E********<45GeV**

**STAR Preliminary**

**STAR Preliminary**

**STAR Preliminary**

**STAR Preliminary**

**Correlations in d+Au**

## FPD Calorimeters

***STAR*** Forward Meson Spectrometer upgrade

## d+Au  ++X, pseudorapidity correlations with forward 

- d+Au  ++X, pseudorapidity correlations with forward 
- HIJIING 1.381 Simulations

-  Increased *p*T for forward  over run-3 results is expected to reduce the background in  correlation
-  Detection of  in interval -1<<+1 correlated with forward  (3<<4) is expected to probe 0.01<*x*gluon<0.1  provides a universality test of nuclear gluon distribution determined from DIS
-  Detection of  in interval 1<<4 correlated with forward  (3<<4) is expected to probe 0.001<*x*gluon<0.01  smallest *x* range until eRHIC
-  At d+Au interaction rates achieved at the end of run-3 (Rint~30 kHz), expect 9,700200 (5,600140)  coincident events that probe 0.001<*x*gluon<0.01 for “no shadowing” (“shadowing”) scenarios.

## PHENIX Nosecone Calorimeter upgrade

**PHENIX Forward Spectrometer:**
**Forward Silicon **
**	****charged particle tracking**
**Forward Calorimeter (em and hadronic)**
**	****W/Si calorimeter**
**	****0.9 < ****η**** < 3**
**	****energy and position measurements**
**	****γ****/e/jet trigger**
**Forward muon system**
**	****muon tracking**
**	****muon trigger**

**Same upgrades on South side**

**NCC**

##  Detection at forward / backward rapidity with  Nosecone Calorimeters -
Direct detection of neutral pions (0.9 < || <3.0).
Large acceptance for high pT pions.
High energy photons.
Determines jet direction plus a rough energy measurement.
γ + jet coincidences and     detection.

- Direct detection of neutral pions (0.9 < || <3.0).
- Large acceptance for high pT pions.
- High energy photons.
- Determines jet direction plus a rough energy measurement.
- γ + jet coincidences and     detection.

- EM

- had

- Side view

- Front view

- χc

## PHENIX Endcap Silicon Vertex upgrade

**Endcaps detect following by **
**displaced vertex of muons :**
**D ******** μ** **+ X**
**B ******** μ** **+ X**
**B ******** J/ ****** **+ X** ****** μ****+ ****μ******

**IP**

**μ**

**B**

**Secondary vertex resolution ~133 ********m (endcap) **

- barrel

- endcap

- endcap

## Gluon shadowing and spin structure function with VTX

**Extracting gluon structure function**
- Vertex detector provides broad range in x in the predicted shadowing region (x <= 10-2) and at larger x
- Measure gluon shadowing in p+A versus p+p,
-     gluon spin in polarized p+p

- Gluon
- Shadowing
- Predictions

- coverage

## Conclusions

- d+Au collisions have played a key role in understanding the physics of Au+Au collisions
- d+Au results provide hints that saturation effects are becoming important
- Both STAR and PHENIX have upgrade plans that will dramatically improve their forward capabilities
- RHIC may be the ideal accelerator to explore the **onset of saturation**

## Slide 38

## Determining the gluon density

- The gluon density is determined by applying QCD evolution equations to account for the *Q*2 dependence (scaling violations) of structure functions measured in DIS.
- At low-x, the full QCD evolution equations can be simplified to approximate the gluon distribution by
*i.e.*, determine *g*(2*x*) by measuring the ln*Q*2 slope of *F*2(*x*,*Q*2) at fixed *x.*
- K. Prytz, Phys. Lett. **B311** (1993) 286

## Gluons in the Proton

-  DIS results from HERA ep collider provide accurate determination of *xg*(*x*) for the proton in the range 0.001<*x*<0.2
-  the low-*x* gluon density is large and continues to increase as *x*0 over the measured range

- J. Pumplin, D.R. Stump, J. Huston, H.L. Lai, P. Nadolsky, W.K. Tung JHEP **0207** (2002) 012.

## Nuclear Gluon Density

- World data on nuclear DIS constrains nuclear modifications to gluon density only for *x*gluon > 0.02

*e.g.*, see M. Hirai, S. Kumano, T.-H. Nagai, Phys. Rev. C**70** (2004) 044905
-  and data references therein

## Simple approximations

**Mid-rapidity particle detection**:
- 0 and <>0
***  **x**q** **** x**g** **** x**T** = *2* p**T** / **s*
**Large-rapidity particle detection**:
- >>
* **x**q** ****  x**T** e****  x**F** *(Feynman *x*), and
-      *x**g* ***  x**F** e*(

- 1.0
- 0.8
- 0.6
- 0.4
- 0.2
- 0.0

- fraction

- 0            10           20          30

- pT,(GeV/c)

*qq*

*qg*

*gg*

-   Large rapidity particle production and correlations involving large rapidity particle probes low-x parton distributions using valence quarks

- NLO pQCD  (Vogelsang)

## Forward π0 production at a hadron collider 

**NLO pQCD**
**S. Kretzer**

- Forward π0 production at a hadron collider

-  **Large rapidity ******** production ************~4******** probes asymmetric partonic collisions**
-  Mostly **high-x valence quark**** + ****low-x gluon**
* ***0.3***** < x*****q****<  0.7**** **
-  **0.001*****< x*****g ****< 0.1**
-  <z> nearly constant and high  ~ 0.8
-  **Large-x quark polarization is known to be large from DIS**
** ****Directly couple to gluons = a probe of low x gluons**

**N**

**N**

********

- q

- g

**E****N**

**x****q****p**

**x****g****p**

- 

********

**E****N**

- (collinear approx.)

## STAR -FPD 

**Preliminary**
**Cross Sections**

- Similar to ISR analysis
- J. Singh, et al Nucl. Phys.
- B140 (1978) 189.

## Back-to-back azimuthal correlationsover a large rapidity interval

**Midrapidity h******** tracks ****in TPC**
**   ****-0.75 < ********< +0.75**
**Leading Charged Particle(LCP)**
**   ****p****T**** > 0.5 GeV/c**

**Trigger by forward ************ **

-  **E****p**** > 25 GeV**
** **************** ******** ****3.8**

**************LCP**

- Coicidence Probability
-  [1/radian]

- S = Probability of “correlated” event under Gaussian
- B = Probability of “un-correlated” event under constant
- s = Width of Gaussian

- Fit LCP normalized distributions and with Gaussian+constant

****

- Beam View

- Top View

## Three  Highlighted Objectives High In FMS Proposal(not exclusive)

- A **d(p)+Au********************+X** measurement of the **parton model gluon density distributions *****xg*****(*****x*****) **in **gold nuclei**** **for** ****0.001< *****x***** <0.1**.  For 0.01<*x*<0.1, this measurement tests the universality of the gluon distribution.
- Characterization of correlated pion cross sections as a function of Q2 (pT2) to search for the onset of **gluon saturation effects** associated with **macroscopic gluon fields. ****(again d-Au)**** **
- Measurements with **transversely polarized protons** that are expected to **resolve the origin of the large transverse spin asymmetries** in  reactions for **forward ************  production. ****(polarized pp)**

## STAR detector layout with FMS

**TPC: -1.0 < ******** < 1.0**
**FTPC: 2.8 < ******** < 3.8**
**BBC : 2.2 < ******** < 5.0**
**EEMC:1 < ******** < 2**
**BEMC:-1 < ******** < 1**
**FPD: |********| ~ 4.0 & ~3.7**

## Pythia Simulation 

**Frankfurt, Guzey and Strikman, **
**J. Phys. G27 (2001) R23 [hep-ph/0010248].**

**Pythia Simulation **

-  constrain *x* value of gluon probed by high-*x* quark by *detection of second hadron* serving as jet surrogate.
-  span broad pseudorapidity range (-1<<+4) for second hadron  span broad range of *x*gluon
-  provide sensitivity to higher *p*T for forward   reduce 23 (inelastic) parton process contributions thereby reducing uncorrelated background in  correlation.

## New FMS Calorimeter

- Lead Glass From FNAL E831

- Loaded On a Rental Truck for Trip To BNL

## Silicon Vertex Detector Upgrade

**Silicon Vertex Detector Upgrade**

- Au 

-  Au

**Endcaps detect following by **
**displaced vertex (********r, ********z) of muons:**
**    ****D (charm) ******** μ**** + X**** **
**    ****B (beauty)******** μ**** + X**
**    ****B ******** J/ ******** + X ******** μ****+ ****μ******

- Si planes constructed of strips 50 m by few mm long

- Pinpoint the decay vertex
- to eliminate backgrounds!

## Design of SVD

** ****4 Tracking stations composed of Si pixels, 50 um by few mm long**
** ****Electronics recently developed by FNAL, ~250,000 channels / pixels. **
**  ****First low power, high speed and high resolution pixel detector**
** ****Can detect large numbers of**** B decays ****and ****D decays per year**** at RHIC**

**Design of SVD**

- x

- x

- x

- x

****

## Dramatic Signal / Background Improvement 

**Dramatic Signal / Background Improvement **
**             ****for Heavy Quarks with SVD**

- _charm_

- beauty

- pT  (GeV/c)

**Simulated Signal to Noise for D********    +X without SVD and with SVD, 10 X improvement is signal / background  ******** accurate  ****yield, slope**** of charm**

- pT  (GeV/c)

- Muon Events

- _charm_

- _Background__ _

- _Background_

- Before vertex cuts

- After vertex cuts

## Endcap performance - Vertex Resolution for Single Muons

- B lifetime is ~ 1 mm at low pT()=1.7 GeV
- c (B+-) = 496 um, c (B0) = 464 um
- c(D+-) = 315 um, c(D0) = 124 um

- Charm: =290 m

- Beauty: =230 m

- Simulated endcap z-vertex resolution for
- D    +X     and
- B    +X

- Z reconstructed

## Simulation of B-> J/-> with Endcap VTX

- Decay Distance (cm)

- 1mm vertex cut eliminates
- >99.95% of prompt J/

- B decays

- Prompt
-    J/

- 1mm cut

- Ratio of B decay
- to prompt J/
- ~ 1%

- σ=133μm

- RHIC 10*L0
- ~400 B-> J/
- per day

- p+p
- √S=200 GeV
- L=1036

- Events