# Presentation: 537201

## An experimental perspective on first jet measurements at LHC: Lessons from RHIC

**An experimental perspective on first jet measurements at LHC: Lessons from RHIC**

**Dan Magestro, The Ohio State University**

- ALICE-USA Collaboration Meeting
- October 15-16, 2005

## Jets in hadronic collisions

**High-energy hadronic collisions: collisions of constituent partons**
- Each parton carries fraction (x) of hadron’s momentum
- “Hard-scattered” outgoing partons back-to-back in azimuth (),
- not back-to-back in pseudorapidity () due to different x

**p**

**p**

- _Transverse plane_

- _Side view of collision_

## Jets as probes of nuclear collisions - 1

**Partons in vacuum fragment characteristically ****→**** **_**calibrated**_** probe**
- Can it be calibrated at LHC?

- parton

- Particle Data Group, PLB 592 (2004)

- = pT(hadron) / pT(parton)

## Jets as probes of nuclear collisions - 2

**Jets as probes of nuclear collisions - 2**

- , K, p spectra

- Momentum-space anisotropy

- Particle ratios

- Resonance production

- HBT interferometry

- Nuclear coalescence

- Strange baryon spectra

- QCD hard scattering, jets

- Heavy particles (charm)

- Nuclear modification

- Direct photons, leptops

**Hard parton scatterings early in collision ******** **_**direct**_** probe**
- ALICE is the only LHC detector that will measure all HI observables

## Observable #1: Inclusive spectra

**RHIC: Initial + final state effects ******** R****AB**** ~ constant**

- STAR, Phys. Rev. Lett. 91 (2003) 072304

- B. Cole, QM2005

_**RHIC measurements**_

## Observable #1: Inclusive spectra

**Significant hardening of spectra**

_**LHC expectation**_

- Reduced sensitivity to initial-state kinematic effects
- Larger variation of energy-loss with pT

## Observable #1: Inclusive spectra

**R****AA**** exhibits strong p****T**** dependence**

- I. Vitev

_**LHC expectation**_

- 5x change!

**Note: collisional energy loss not considered in this model ******** even MORE suppression?**

## Jet and dijet measurements at LHC

**Jet and dijet measurements at LHC**

**Calorimetry, jet-cone algorithms**
- Try to measure full jet energy
- Difficult for lower energy jets dueto background

**Tracking, 2-particle correlations**
- Correlate high-pT _trigger_ particles with _associated_ particles
- Sample fragmentation function, neutral energy (~1/3) lost

**E****T**** < 20 GeV**

**E****T**** >(>) 20 GeV**

- Background contains jets at LHC!

## Jet correlations and the underlying event

**Spatial anisotropy in collision induces 2****nd****-order harmonic**

**p+p**

**Au+Au, 20-40%**

## Observable #2: Back-to-back dijets

_**RHIC results (3)**_

- Higher pT → Away-side suppression

- STAR, PRL 91 (2003) 072304

- Lower pT → Away-side enhancement

**p****T****(assoc) > 2 GeV/c**

**p****T****(assoc) > 0.15 GeV/c**

**4 < p****T****(trig) < 6 GeV/c**

- 

- 

- STAR, PRL 95 (2005) 152301

## Observable #2: Back-to-back dijets

_**RHIC results (3)**_

- Higher pT → Away-side peak emerges: **Dijets**

- 

**8 < p****T****(trig) < 15 GeV/c**

- DM (STAR), nucl-ex/0510002

## Aside: At RHIC we’ve now reached pQCD regime

- O. Barannikova (STAR), QM2005

**At RHIC, pQCD fragmentation dominates hadron production above ~6 GeV/c**

- R. Fries

**At LHC, pQCD dominance predicted to set in a bit higher **

## Observable #2: Back-to-back dijets

_**LHC expectation**_

**Strong p****T**** dependence of energy loss at LHC requires dijet analyses over full p****T**** range!!**

**Low-E****T**** jets: Two-particle correlations**
**High-E****T**** jets: Jet reconstuction above background**

## Observable #2: Back-to-back dijets

**Observable #2: Back-to-back dijets**

_**LHC expectation **_**– ****Low-E****T**

**Two-particle correlations necessary**
**Difficult: large suppression + poor acceptance for away-side jet at low-E****T**

## Observable #2: Back-to-back dijets

_**LHC expectation **_**– ****High-E****T**

**Multiple jets + background reduces purity at “intermediate” E****T**
**Trigger capabilities NEEDED to access high E****T**** **

## LHC operations plan

**LHC operations plan**

**Main objectives:**
- Terminate installation in Febr. 2007
- First pp collisions in summer 2007, √s = 14 TeV
- First long heavy ion (Pb+Pb) run end of 2008, √s = 5.5 TeV

**Higher jet rates: first jet-quenching questions can be answered with ~10****4****-10****5**** events, e.g. prediction for ****strong p****T**** dependence of energy loss**
**Parallel efforts statistical & reconstructed jets needed to explore full p****T**** range**

## An opportunity for early Pb beam? 

**2005 LHC Project Workshop (Chamonix XIV)**
**Pb ions may be available at very early stage of LHC operation**
- Jowett (CERN): Likely short Pb “Pilot Run” end of p+p Pilot Run in late 2007
- Depends on ability to commission ion cycle in SPS in 2006
- Luminosity = (few) x 1024 cm-2s-1  **~10****5**** events in < 1 day**

**This could be a great opportunity to answer first (zeroth) energy loss questions with a one-day pilot run**