Summary of ALICE upgrade workshop

Urs Achim WiedemannCERN TH

Geneva, 13 July 2011

VHMPID

newITS MFT

Thanks to

Andrea Beraudo (CERN TH)

and

Thomas Peitzmann

the (main) organizers of this workshop

The value of real estate in Geneva…Houses are extremely expensive and well-equipped Constructible land

is extremely rare and precious

ALICE has the unique opportunity to built substantial upgrades to address fundamental questions in AA (and in response to insights from first AA@LHC)

Upgrades of ATLAS and CMS will be determined mainly by the needs of high-luminosity pp@LHC

Basic criteria for a good upgrade

• Fundamental motivation Does one get novel access to fundamental issues of QCD at high temperature and/or density? (ideally, there is a credible model-independent link between proposed measurements and first principles of QCD) • Complementarity/competivity - w.r.t. existing ALICE detectors - w.r.t. existing detectors of other LHC experiments - w.r.t. upgrades of other detectors

- w.r.t. other experimental programs

• Technical feasibility/Cost/Timescale (here, task of theorists is limited to understand basic constraints, not to contribute to their elaboration)

QCD saturation physics

- fundamental arguments exist that small-x evolution results in dense (saturated) hadronic wave function and a dynamically generated saturation scale Qs

- undoubtedly a qualitatively novel phenomenon of fundamental interest

- scale at which saturation becomes perturbative not known from 1st principles, needs to be determined phenomenologically

QCD saturation physics … cont’dFeatures in RHIC d+Au and Au+Au have been interpreted in termsof non-linear small-x evolution of saturated initial conditions, aka CGC. (e.g. single inclusives and correlations d+Au @ forward rapidity,

bulk properties in Au+Au, dNch, its centrality dependence …)

If RHIC small-x paradigm holds, then LHC has access to “CGC” in wide kinematic range But - this is ‘mainly’ a p-A program (should upgrades focus on A-A core

competence of ALICE, or on p-A?)

- first pA@LHC will provide highly non-trivial test of RHIC small-x paradigm (ATLAS & CMS have already access to significant fwd physics capabilities, strength of upgrade case based on CGC search is ultimately related to what ATLAS&CMS find in 1st pA@LHC)

How to test saturation physics?- Look for characteristic CGC-like features

A.H. Rezaeian and A. Schafer, PRD81 (2010) 114032

Suggestive models, e.g. Suggestive data, e.g.

But: modeling plays an important and a priori unclear role in interfacing such data to fundamental properties of QCD => upgrade arguments somewhat weak if built solely on model-specific details/interpretations.

Such models and data extrapolated can play an important role in initiating upgrade ideas

Is there better strategy @ small-x?Ideal Example: illustrating capabilities of ep @ LHeC

a.) Generate pseudo-data with some saturation modelb.) Test whether pseudo-data could be accounted for by linear DGLAP-evolution (via nnpdf-analysis)

Discrepancy between pseudo-data (here for F2 and FL) and nnpdf-fit signals characteristic features of non-linear evolution

Similar analysis tools can be/have been developed and should be used to explore relevance of upgrades for CGC search strategies in pA@LHC(will ease discussion with LHCC-referees …)

Albacete & Rojo, LHeC CDR

My opinion on testing small-x in pA@LHCa.) Main motivation should be based on most robust data accessible (elmag, hadronic, heavy flavor single inclusives, jets …)

b.) there should be exp. cross checks of the non-trivial working hypothesis that production factorizes (npdf) x (hard) x (fragm)

c.) measured data should access npdfs over wide fwd kinematics => test that factorization into (n)pdfs holds => accuracy to establish deviations from linear QCD evolution

d.) many correlation measurements may be additional assets for CGC searches (but unlikely to be the main fundamental driver for upgrades since interpretation more model-dependent).

Either: support for this strategy comes from 1st pA@LHCOr: reconsider

(1st CMS&ATLAS pA data will be crucial input for ALICE upgrades)

What about AA and mid-rapidity?

Low-mass dimuons, D/B separation, …

• Measurements address fundamental issues in physics of heavy ion collisions (chiral symmetry restoration, mechanism of high-pt suppression, …)

- ALICE has unique access to this low-mass dimuons at LHC

• Main issue: - feasibilility? - degree of improvement over existing ALICE capabilities?

PID PID PID PID PID PID…- A core competence of ALICE- obvious interest in heavy ion collisions (thermal model, PID dependence of flow, open heavy flavor, …)

Main question: How can improved PID in year > 2016? lead to decisively improved understanding of

fundamental open issues?

No crisp/unique answer yet, but a few basic insights:

- PID as a crucial dimension of jet quenching - PID (open charm/beauty) as a baseline for Quarkonia physics - PID as a tool for improved nPDF

PID and jet quenching …Agnostic argument for PID at high pt:- microscopic models of parton energy loss affect color flow even at high pt- color flow affects hadronization- hadronization can be accessed by PID

S. Sapeta and UAW, 2008

In models, hadonization affects RAA at low pt (coalescence)

at high pt (cluster hadr. + color flow)

A. Beraudo et al, 2011, in prep

Constraining hadronization at high pt crucial for a understanding parton e-loss potential complementary role of ALICE in the high-pt domain where ATLAS/CMS have excellent coverage

Hees-Mannarelli-Greco-Rapp, PRL100 (2008)

See also Beraudo,this workshop

Statistics and Systematics Limitations of current PID capabilities…

I (theorist) understood that the systematic errors of this plot are not yet limited by detector resolution.I do not know the ultimate limitation for high-pt PID. Do you? (dE/dx of K and p in relativistic rise are roughly parallel,

so opportunity for statistical separation at high pT>10 GeV ) Quantifying systematics limitations of current PID capabilities crucial for substantiating case for PID upgrades

Slide of A. Beraudo, ALICE upgrade workshop

• Look at open heavy flavor to see hidden heavy flavorArgument of V. Greco to make progress on quarkonia production in AA

• Clearly this is ALICE (low-pt) core business, of fundamental interest main issue: how far can one go beyond current ALICE capabilities? what are the quantitative gains? can they give rise to qualitative advances?

Options for improved PID …• Small acceptance, high pt (VHMPID 5<pt<10 GeV?, FARICH pt < 14 GeV?)

• Clearly of interest for understanding RAA

(PID-dependence constrains microscopic dynamics and gives access to role of hadronization in jet quenching, constrains one/the main model uncertainty of interpreting high-pt suppression)

STAR prelim, QM09

• Addresses central open question raised by current data (RAA and v2)

• Only limited interest for jet-like correlations if acceptance is small.

PID: small acceptance, large pt …cont’d

+ fragm.

Coal. only

What can be said already:

Motivation from data and models to extend PID to pT > 10 GeV

Plots from talk of V. Greco

- Is flow universal at high pt?- Does RAA depend on parton ID only at high pT?

Main issue: Where is PID needed on an EbyE basis? And where is statistical separation sufficient?

EbyE PID clearly needed for PID-dependent correlation studies.But: correlation measurements require acceptance. Need of specifying physics-dependent acceptance needs to motivate PID upgrades

Instead of a summary

Motivations for upgrades are not simply there, but they evolve:

- as data come in (AA@LHC & pA@LHC) (e.g. pushing for an early pA@LHC run would clarify in a timely manner the case for and the scope of ALICE upgrades focussing on small-x.)

- as novel concepts emerge (e.g. QM11 paradigm that flow&fluctuations are two aspects of the same dynamical evolution results in rethinking role of initial conditions, may change view on PID-dependence of flow, e.g. Do coalesence models and fragmentation models propage

fluctuations in the same way to the hadronic level?) - as theory advances (watch out: jets, vniscometry, small-x) (e.g. significant recent advances in formulating QCD small-x evolution, should result step by step in realistic assessment of experimental capabilities)