Heavy ion Physics with LHC

Gopika Sood

Contents

Brief History Summary of RHIC results Future : LHC (2007) Physics prospects with LHC Disoriented Chiral Condensates Centauro-Anticentauro events Chebhshev expansion : A measure of fluctuations Analysis Summary

Brief History… The long standing main objective of heavy ion physics is to explore the phase diagram of strongly interacting matter, to study the QCD phase transition and the Physics of Quark Gluon Plasma (QGP) state.

Chiral symmetrybroken

Chiral symmetryrestored

Past : 15yrs at SPS

Lattice calculations have shown that, for vanshing baryon density, Tc 150-180MeV, strongly interacting matter undergoes a transition from a medium of color singlet hadronic constituents to deconfined Quarks and Gluons.

To test this phase experiments are designed

AGS @ Brookhaven (1986 - 1998)

Beam : Elab < 15 GeV/N, Ös ~ 4 GeV/

Experiments: 4 big, several small

SPS @ CERN (1986 - 2003)

Light Ions(O, S) : 1986 – 1992

Heavy Ions (Pb): 1994 - 2003

Beam: Elab =40, 80, 160, 200 GeV/N,

Ös < 20 GeV/N

Experiments: 6-7 big, NA49, WA98, NA57 etc. and several small

. On the basis of combined results of seven experiments, CERN announced (2000) that a state similar to QGP is created at SPS.

Summary of RHIC results

RHIC CONCLUSIONS : Quark Gluon Plasma goes liquid

An international team of physicists working at the Relativistic Heavy Ion Collider (RHIC) at the Brookhaven National Laboratory in the US says it has found strong evidence for the "quark-gluon plasma" -- the state of matter thought to exist in the first millionth of a second after the Big Bang. However, the researchers found that instead of behaving like a gas of free quarks, antiquarks and gluons as expected, the matter behaves more like a liquid. The results were presented at the April meeting of the American Physical Society and at the Quark Matter 2006.

Main Summary : White papers by RHIC experiments

Experimental and theoretical challenges in the search for the quark gluon plasma: The STAR Collaboration's critical assessment of the evidence from RHIC collisions.By STAR Collaboration Nucl.Phys.A757:102-183, 2005.

The Phobos perspective on Discoveries at RHIC. By Phobos Collaboration. Nucl. Phys. A 757, 28 , 2005.

Formation of Dense partonic matter in relativistic nucleus –nucleus collisons at RHIC : Experimental evaluation by PHENIX collaboration. By PHENIX Collaboration. Nucl. Phys. A757 (2005) 184-283.

Quark Gluon Plasma and the Color Glass Condensate at RHIC ? The perspective from the

BRAHMS experiment Nucl. Phys. A757 (2005) 1-27.

Lots of results (flow and jet physics) defining QGP as a liquid, were presented in recent (Quark Matter 2006) in Shanghai.

Future Plans : RHIC II (Detector upgrades) and e-RHIC

LHC : The future (2007) The nucleon-nucleon centre-of-mass energy for collisions of the heaviest ions at the LHC ( s = 5.5 TeV) will exceed that available at RHIC by a factor of about 30, opening a new physics domain.

Heavy ion programme : pp (14 TeV) Pb-Pb physics pilot run (5.5 TeV) 1-2 yrs Pb-Pb 1 yr pPb-ike collisions (pPb,dPb or Pb) (8.8 TeV) 1-2 yrs. Ar-Ar (6.3TeV)

QGP: LongerLHC Energy = 30 RHIC Energy

Latest status and Physics of LHC experiments : Excellent talks at QM2006, Shanghai.

Heavy ion Physics with CMS : Russell Betts (QM2006)

Heavy ion Physics with ALICE: Federico Antinori (QM2006)

Heavy ion Physics with ATLAS : Peter Steinberg (QM2006)

Major experiments at LHC

ATLAS

CMS

ALICE

Soft Physics and Global Event Characterization Centrality and good Event Selection – Scaling with Npart, Ncoll

Charged Particle Multiplicity – Initial State Gluon Densities Azimuthal Asymmetry (Flow) – Equation of State Disoriented Chiral Condensates – Centauro-Anticentauro events Fluctuations in particle species Spectra + Correlations – Sources, Radial Flow, dE/dx and Quenching

High pT Probes High pT Particles and studies of Jet Fragmentation – Energy Loss and

Modification of Fragmentation Functions. Flavor and Geometry Dependence. Quarkonia (J/, ) and Heavy Quarks – Suppression and Recombination High Energy Photons, Z0, Jet-, Jet-Z0, Multijet Events – Calibrated Measure

of dE/dx

Forward Physics Limiting Fragmentation – Saturation, Color Glass Condensate Ultra Peripheral Collisions – PDF in New Regions of x and Q2

Exotica -

Physics prospects at LHC

Disoriented Chiral Condensate (DCC)

Disoriented Chiral Condensate : Centauro/Anticentauro phenomena…

Chiral symmetry restored phase

Chiral symmetry broken phase

Theory for DCC formation DCC is the bubble of misaligned vacuum which is formed as a result of spontaneous breakdown of Chiral Symmetry. The study of this phenomena can provide important information regarding the vacuum state of system.

The centauro and anticentauro events reported by cosmic ray experiments can be due to formation of DCC.

J.D. Bjorken, Int. J. of Mod. Phys. A7, 4189 (1992). J.D Bjorken, K.L. Kowalski and C. C

Taylor, SLAC-PUB-6109 (1993).

Main experimental signatures :1. Fluctuations in neutral pion fraction2. Anomalous Energy ratio fluctuations

Generic Distribution(Gaussian peaking at

0.33)P(f) = 1/2f

N > Nch,

Anticentauro eventNch,> N , Centauro

event

JACEE prototype

Mysterious Anti-centauroevent JACEE 4LIIG-27

The cosmic ray evidence on “anti-centauro events” serve as a great motivation for performing the exotic event search in the accelerator based experiments at the Large Hadron Collider (LHC). The energies at LHC will be huge and it offers excellent opportunities for pursuing this study and discovering more events of this nature.

Centauro Signal

“Centauro Events”

Centauros were first reported in 1980 on Mt. Chacaltaya in the Bolvian Andes (X-ray emulsion chambers at 5200m) by Brazil-Japan Collaboration. These events contain almost entirely hadrons, with very few photons.

Nh vs Qh for Centauro event observed at Mt. Chacaltaya

CASTOR detector for CMS@LHC has plans to study exotic physics of Centauro events.

(Edwin Norbeck, QM2006, poster)

Ongoing analysis

|η| < .35 φ<1/2π (×2 arm)

√s=130 GeV (Au+Au)

PHENIXRHIC2001

Research in progress (hopefully +ve)

|η| < .65√s=200 GeV (Au+Au)

STARRHIC2004

Nothing +ve2.80 < η <3.75 φ<π

√s=158 AGeV (Pb+Pb)

WA98SPS1998

Nothing +ve3.4 < |η| < 4.2 √s=1.8 TeVMINIMAXTEVATRON1997

Nothing +ve|η|<4.2φ<2π√s=1.8 TeVCDFTEVATRON1996

Nothing +ve|η| < 5.0√s=900 GeVUA5pp1986

Nothing +ve|η| < 3.1√s=540 GeVUA1pp1983

Nothing +ve|η| < 5.0√s=540 GeVUA5pp1982

1 anti-centauro event reported

5.0<η<9.0 φ<2π

--------JACEEBalloon Based Experiment

1992

5 centauro events reported

……..√s = 1.7 TeVBrazil-JapanMt. Chalcataya1980

Remarks - CM EnergyCollaborationExperimentYear

Summary of DCC searches…

Anticentauro Type events

The STAR Experiment

EEMC

Study the energy ratio, i.e., the ratio of the fraction of observed hadronic energy to the electromagnetic energy,

Qh = Ehad/(E e-m + Ehad )

DCC with Chebyshev Polynomials at LHC

DCC in LHC scenario LHC offers a unique possiblility to pursue DCC search in the highest energy domain. The experiments, ALICE, ATLAS and CMS have great potential to look for Centauro, Anti-Centauro events. The importance of such a study lies in the fact that :

It provides important information regarding the vacuum structure of the matter.

It is a strong signal for the Quark Gluon Plasma formation.

Possibilities with CMS : The energy ratio signal

1. ECAL and HCAL (Barrel and Endcap (Cu+Scint)

2. Forward Calorimeter (Fe+Quartz

3. CASTOR Calorimeter (5.2 ≤ ≤ 6.5)

(See Edwin Norbeck’s Poster in QM2006)

Possibilties with ATLAS : The energy ratio signal Forward and LAr Hadronic Calorimeters

-5 ≤ ≤ 5

The pseudorapidity range for Calorimeters is huge and the DCC search an be made in larger - region.

Possibilities with ALICE : The particle ratio signal

1. Multiplicity detectors

2. TPC (to study the charged particle scarcity) -2 ≤ ≤ 2

SIMULATION AND ANTICENTAURO MODELLING

Main experimental signatures :1. Fluctuations in neutral pion fraction2. Anomalous Energy ratio fluctuations

A data set of 10K events is generated using HIJING model for Pb-Pb collisions at LHC energies. Another equivalent sample of events is generated from this data set. Here in this sample, the Anti-Centauro signal is artificially incorporated in the events. All the characteristics of typical JACEE event are considered in this procedure. The extreme value of neutral pion fraction, f ≥ 0.9 is used. In an event, for a randomly selected - region, the charged pions are flipped to the neutral pions until the desired value of f ≥ 0.9 is attained. This transformation is done only for the low pt pions.

Event Classes Impact parameter (fm)

Central events 0 ≤ b ≤ 1

Semi central events 3 ≤ b ≤ 15

Peripheral events 15 ≤ b ≤ 20

Pseudorapidity selection : -10 ≤ ≤ 10

Analysis tool : The coefficients of Chebyshev expansion can be an important tool for measuring fluctuations. This technique is applied to the generated data sets and Chebyshev coefficients for the neutral pion fraction, f , are calculated for various azimuthal bins, 2/18, 2/9, 2/6, 2/4, 2/3, and 2.

Method of Analysis : Chebyshev polynomials

Neutral Pion fraction distribution for the artificially generated centauro events.

Azimuthal distribution of the region where the anti-centauro signal is added.

Extreme case of Neutral Pion fraction (f ≥ 0.9) has been chosen, i.e. very strong anticentauro signal is incorporated.

The added anti-centauro signal is uniform in azimuth.

CHEBYSHEV POLYNOMIALSThe Chebyshev polynomial of degree n, denoted by Tn is given by the explicit formula :

Tn(x) = cos(n arc(cos x))

These polynomials satisfy a discrete orthogonality relation as well as continuous one. If (xk, (k=1,…,m) are the zeros of Tm(x), given by the equation above, and if i, j < m, then,

0 i ≠ jm

k=1 Ti(xk) Tj(xk) = m/2 i = j ≠ 0 m i = j = 0

If f (x) is an arbitrary function in the interval -1 to 1 then the coefficients, cj, j= 0, N-1 are defined as :

Cj = 2/N N k=1 f (xk) Tj-1 (xk) 2/N N k=1 f [ (cos (k-1/2)/N) cos ( (j-1) (k-1/2)/N) ]

For any quantity, f() = dN/d, the Chebyshev series expansion can be written as :

f() = (c0/2) L-1k=1 ck Tk (z)

This equation is valid for the interval -1 to 1, and it can be generalised for any arbitrary range a to b using the transformation :

y = x – (½ (b+a))/ (½ (b-a))

These coefficients carry information about the fluctuation of a function about the mean value. There is a strong tendency that high ranked coefficients lie in low k region and can be an indication to a transition to an exotic phenomena.

F. Takagi, Phys. Rev. C Vol. 32, 1799, (1985).

ANALYSIS AND RESULTSThe Chebyshev coefficients, Cj (for N = 1, 100) are calculated for the generated data sets for all the classes. The distribution of all the coefficients (C0 to C99) is Gaussian. Few coefficients are displayed below :

The distributions of coefficients, C30, C40, C50 and C90 for the generated HIJING events (mb).

The distributions of coefficients, C30, C40, C50 and C90 for the generated HIJING events (mb).

First Coefficient, C0, of the Chebyshev expansion for HIJING (left plot) and artificially produced Anti-centauro events (right plot).

The First Chebhshev coefficient C0

Correlation plots for various Chebyshev coefficients

Correlation between first (C0) and second coefficient (C1).

Correlation between second (C1) and third coefficient (C2).

The variation of coefficients for various patch sizes, viz, 2/18, 2/9, 2/6, 2/4, 2/3, and 2, for exotic (circle) and HIJING events (square).

The first coefficient, C0, varying with centrality for different azimuthal bins.

Centrality and Azimuthal dependence

Mean C0 Percentage of events

Azimuthal size

Simulated Exotic events

Normal HIJING events

ProbabilityStatistical Probability from

Normal HIJING events

2/18 124.8 45.0 P(C0≥ 50) 0.024

2/9 63.2 23.0 P(C0≥ 50) 0.0

2/6 42.7 15.4 P(C0≥ 20) 0.005

2/4 28.5 10.2 P(C0≥ 15) 0.001

2/3 21.4 7.7 P(C0≥ 10) 0.0

14.3 5.2 P(C0≥ 10) 0.0

2 7.1 2.5 P(C0≥ 5) 0.0

Mean value of the first coefficient for the exotic and normal HIJING events, for various azimuthal bins.

SUMMARY AND CONCLUSIONS

The energy factor from RHIC to LHC will be much greater (~30 times) than that of SPS to RHIC and it offers a great potential to discover new physics.

The experiments at LHC are well suited to measure global event properties and identified hadron spectra on a wide momentum range in Pb-Pb and pp collisions.

The nature of the bulk and the influence of hard processes on its properties will be studied via chemical composition, collective expansion, momentum correlations and event by event fluctuations and Disoriened Chiral Condensates.

In this study, Anticentauro events similar to those observed in cosmic ray experiments are generated artificially at LHC energies.

The coefficients of Chebyshev expansion can be a useful tool to study fluctuations in the neutral pion fraction. This technique is applied to the generated data sample for various azimuthal bins.

The coefficients of all ranks are Gaussian in nature. For the exotic events, the magnitude of first few coefficients is strongly enhanced (~3 times) in comparison with the corresponding C j’s obtained from simulation.

The value of coefficients decreases with decrease in centrality and increase in patch size.

The percentage of events having high values of high ranked coefficients is much greater than expected statistically from normal HIJING events.

Finally, lots of exciting and challenging physics expected from LHC experiments … Best summarised by the following :

“Traveller, road is nothing more than your footprints; Traveller, there is no road, you make it as you go”.

H.Satz (hep-ph/0209181)