# Presentation: 000298

## Measurement of Invariant Differential Cross Sections of Identified Charged Hadrons in p+p Collisions at RHIC

- Mark Harvey
- Brookhaven National Laboratory
- July 18-24, 2004
- Hot Quarks 2004 Workshop
- Taos Valley, New Mexico, USA

## Physics Motivation

- Differential Invariant Cross Section in p+p Collisions
- fundamental for understanding the particle production in
-     p+p collisions
- baseline measurement for disentangling nuclear effects which arise in d+Au and Au+Au collisions
- Hard-scattered partons give rise to hadronic sprays; i.e., jets
- Physics implications:
- Careful examination of the Nuclear Modification factors; e.g., RdAu and RAuAu
- Initial state multiple scattering “broadens” the momentum distribution and enhances the high pT yield compared to point-like scaling from p+p collisions
- Lack of high momentum enhancement  jet suppression

## Differential Invariant Cross Section

**Experimental Definition:**
**Measurement of Luminosity (*****L*****) in p+p collisions ******** Beam-Beam Counter (BBC) triggered events**
**Ed********/d****3****p = 1/2******** 1/p****T**** 1/*****L***** dN/dydp****T**
**where****	**** *****L***** = N****eventsBBC****/********pp****BBC**
**Application to A+A collisions:**
**The differential invariant yield in A+A collisions may be decomposed into a soft phenomenological piece and a hard pQCD component **
**EdN****AB****/d****3****p = N****part**** EdN****soft****/dyd****2****p****T**** + N****coll**** 1/********pp****in**** EdN****hard****/dyd****2****p****T**
**Cross section sensitive to two important pieces of physics!**
**yields described in a two component model  ******** Soft and hard production mechanisms**

## Hadronic Collisions

**D**

**D**

**D**

**D**

**U**

**U**

**U**

**U**

**D**

**D**

**D**

**D**

**D**

**D**

**D**

**D**

**D**

**D**

**D**

**D**

**D**

**D**

**D**

**D**

**U**

**U**

**D**

**D**

**D**

**D**

**D**

**D**

**D**

**D**

**D**

**D**

**D**

**D**

**D**

**D**

**D**

**D**

**U**

**U**

**U**

**U**

**D**

**D**

**D**

**D**

**U**

**U**

**U**

**U**

**U**

**U**

**D**

**D**

**D**

**D**

**U**

**U**

**U**

**U**

**U**

**U**

**D**

**D**

**D**

**D**

**U**

**U**

**U**

**U**

**D**

**D**

**D**

**D**

**D**

**D**

**D**

**D**

**D**

**D**

**D**

**D**

**D**

**D**

**D**

**D**

**U**

**U**

**D**

**D**

**D**

**D**

**D**

**D**

**D**

**D**

**D**

**D**

**D**

**D**

**D**

**D**

**D**

**D**

**U**

**U**

**U**

**U**

**D**

**D**

**D**

**D**

**U**

**U**

**U**

**U**

**U**

**U**

**D**

**D**

**D**

**D**

**U**

**U**

**U**

**U**

**U**

**U**

**Participants**

**Spectators**

**Spectators**

**b**

**Nuclear**
**System A**

**Nuclear**
**System A**

**The number of spectator nucleons **
**  ****may be expressed as  **
**N****spect****= 2A – N****part**

**N****part****: No. of nucleons participating **
**  ****inelastically in interaction process**

**D**

**D**

**D**

**D**

**U**

**U**

**U**

**U**

**U**

**U**

**D**

**D**

**D**

**D**

**U**

**U**

**U**

**U**

**U**

**U**

**D**

**D**

**D**

**D**

**U**

**U**

**U**

**U**

**U**

**U**

**D**

**D**

**D**

**D**

**U**

**U**

**U**

**U**

**U**

**U**

**N****coll****: Binary collisions**
**two particle interactions **

**p**

**p**

**b: Impact Parameter**

**x**

**z**

## PHENIX Detector

_***Identified Charged Hadron Analysis Detection Elements:***_

## Particle Detection and Measurement

- High Resolution Time-of-Flight (TOF)
- detector – TOF time resolution in Au+Au is 115 ps, relative to 2 cut in m2; K/ separation up to 2 GeV/c; p/K separation extends up to pT = 4 GeV/c... Beam-Beam Counter (BBC) provides *start clock* trigger

**In p+p, clean pion and kaon separation out to |p| ~ 1.8 GeV/c; p/K separation **
**beyond 3 GeV/c -- TOF timing resolution ~135 ps**

**m****2 ****vs. momentum **

- p+p @ √200 GeV

## Differential Invariant Cross Section in p+p collisions

**Pion spectra,**
**   ****exhibit concave**
**   ****shape** **well**
**   ****described by**
**   ****gamma-type**
**   ****function; **
**  **
** ****“****Ap****T ****2-****λ****e****-p****T****/T****”**
**   **

**Proton spectra,**
***   *****Characterized **
**   ****by gamma **
**   ****function**

**Kaon spectra,**
**   ****Fall-off**
**   ****exponentially **
**   ****in *****p******T****** ***

- +, K+, p

- -, K-, p

**At low p****T ****(< 1.2 GeV/c), each particle species goes like f(p****T****) = e****-bp****T  ****(b=6); similar **
**to behavior observed at ISR** **energies **

**PHENIX Preliminary**

**PHENIX Preliminary**

## Differential Invariant Cross Section

- Magnitude of the
**charged pion**** ****average**
- cross section is in
- excellent agreement
- with **published pi zero **
**work**

**Phys. Rev. Lett. 91, 241803  (2003)**

**The p+p XSECT for **
**pions measured**
**out to ****10 orders of **
**magnitude as a**
**function of p****T****!**

## Au+Au pT Spectra @ √s = 200 GeV(central vs. peripheral)

**Central **
- low pT; slopes vary
-      w.r.t. mass
- at higher pT, produced (anti) proton yield equivalent to pion yield
**Peripheral **
- dependence in mass is less pronounced
- particle production yield similar to p+p

**Au+Au p****T**** Spectra @ √s = 200 GeV****(central vs. peripheral)**

**Phys.Rev.C69:034909,2004**

## d+Au pT Spectra @ √s = 200 GeV(4 centrality classes)

**d+Au p****T**** Spectra @ √s = 200 GeV****(4 centrality classes)**

- particle production
-   yield decreases with
-   increasing centrality
- pions; power law
-   for all centralities
- kaons and protons; pT
-   exponential in each
-   centrality class

##  p+p Particle Ratios

- K-/K+; 0.90  0.01  0.03

- p/p; 0.72  0.01  0. 02

** ****p+p Particle Ratios**

**pion, Kaon and proton ratios are***** flat in  p******T***

**Shaded boxes **
**represent systematic **
**error estimates**

**PHENIX Preliminary**

**PHENIX Preliminary**

**PHENIX Preliminary**

## Particle Ratios for three Collision Systems

- p

- p

***Particle Ratios for ******three Collision Systems***

-  **slide prepared by Felix Matathias**

## Particle Ratios for three Collision Systems

- d

- Au

***Particle Ratios for ******three Collision Systems***

-  **slide prepared by Felix Matathias**

## Particle Ratios for three Collision Systems

- Au

- Au

***Particle Ratios for ******three Collision Systems***

-  **slide prepared by Felix Matathias**

## Particle Ratios for three Collision Systems

- Au

- Au

**Like particle ratios: agree very well independent of collision system**

***Particle Ratios for ******three Collision Systems***

-  **slide prepared by Felix Matathias**

## K+/+ Ratios; 3 Collision systems

**  ****steadily with **
**  ****increasing p****T **

**No apparent **
**  ****indication of **
**  ****saturation in this **
**  ****limited p****T ****range**

***K******+******/************+****** Ratios; 3 Collision systems***

**All three systems**
**  ****have comparable**
**  ****magnitudes –**
**  ****somewhat enhanced**
**  ****for most central**
**  ****Au+Au collisions**

-  **slide prepared by Felix Matathias**

## p/ Ratios for three Collision Systems

**p/******** Ratios for three ****Collision Systems**

**p+p, min bias **
** ****d+Au and**
** ****peripheral Au+Au**
** ****are directly**
** ****comparable**

**In p+p, p/******** Ratios**
**  ****increase rapidly for **
**  ****p****T**** < 1.4 GeV/c – **
**  ****flattens around **
**  ****0.4 (0.3) for the **
**  ****positive (negative) **
**  ****particles at higher p****T**

**Central Au+Au dramatically higher **
**for p****T ****> 1.4 GeV/c ******** New Physics!**

-  **slide prepared by Felix Matathias**

## A look at RAA 

**R****AA**** considerably less than 1 for**
** ********’****s; implication ******** jet quenching**
** ****due to hard scattered parton**
** ****absorption in hot medium; p’s **
** ****and K’s indicate other  new physics.**

**   ********’****s are suppressed in central **
**      ****collisions; seem to be **
**      ****approaching unity for increasing**
**      ****p****T  ****in peripheral collisions**

**K’s appear to flatten out within limited p****T**** window in both central and peripheral collisions, respectively**

**p’s increase fairly rapidly at low p****T ****;**** ****however, the protons are different and are consistent with no suppression!**

**Work in Progress**

**R****AA**** was computed in the following way:**
**R****AA****Ncoll****  =  Y****AA****/(Ncoll(Y****pp*****(********BBC****/********inelpp****)))**

## Summary

**Differential invariant cross section of identified charged hadrons in p+p collisions ****(paper preparation for publication in process)**
**baseline for d+Au and Au+Au **
**Excellent agreement between charged pion average and published pi zero result**
**High precision measurement of pions out to 10 orders of magnitude!**
**Particle Ratios:**
**Like particles; agree very well, independent of collision system**
**K/******** ratios rise steadily in limited p****T**** range**
**p/******** ratios saturate for all systems except Au+Au central – may be viewed as flow effect since protons are much heavier than pions; ergo, flow contribution to their slope ~ m <u****t****>****2 ****; see**** ****Phys.Rev.C69:034909,2004**

## Summary Continued

**Essentially, R****AA**** pions are suppressed in central collisions – rising steadily peripheral... magnitude slightly increased for Kaons; however, they saturate in limited p****T ****window; Protons are not suppressed at higher p****T **** -- very different from pions!**
**Particle production mechanisms may be described with similar source in soft domain (p****T**** < 1.5 GeV/c) for  ************, K******** and p (pbar), respectively – difference in production probably limited to large p****T**** regime.**
**More work to be done on R****dAu**** and R****AuAu**** before physics result is ready for “prime time”**