1. Looking into the Future: The VHMPID for ALICE
E. García
University of Illinois at Chicago
Septiembre 2007
ICN
Compact muon solenoind

2. Outline
Background to the Very High Momentum Particle Identification Detector (VHMPID)
Detector Characteristics and Simulation
Physics possibilities (work in progress)
Project Status and Plans
Brief introduction, setting the LHS in perspective with RHIC and describing also briefly the detector
Then I am going to talk about different physics programs for heavy ions at LHC, and the capability of the detector to measure these quantities in the light of simulations

3. The reference: RHIC
High energy density
Elliptic Flow
PHOBOS, Nucl. Phys. A757 (2005) 28
Applicability of thermodynamics
Jet Suppression
-In upper left corner is the charged particle multiplicity per participant pair as function of the nucleon nucleon center of mass energy for several systems and energies. The multiplicity seems to scale semi logarithmically with the energy. Using the point 200 GeV we find that at RHIC we have produced a high energy density medium of about .5 GeV/ fm3 (10 times the energy density inside nucleus), this will be about 20 times larger at lhc
- In the top right transparency we present the elliptic flow of charged particles near mid-rapidity as a function of the centrality in Au Au collisions at 200 and 130 GeV. Boxes show the systematic uncertainties.-For central collisions the overlap of the ions colliding is total, so no flow is observed, as the colllisions are more peripheral the flow signal present and increases to a significant value
- From the strength of the elliptic flow signal and the known size of the source it s possible to estimate
that the interactions between the created particles must have occurred in times less that few fm/c.
The interactions at these early times must have been strong. The high density system interacts more like a fluid than like a gas
The medium produced is highly interactive – hydrodynamic limit reached at RHIC
- On the right panel is the anti particle ratio for kaons and protons from BHAHMS.
- What is relevant of this plots is how these experimental variables are well described by relativistic hydrodynamics, represented by the dashed curves on the left and the blue solid curve on the right.
- This gives evidence of certain degree of thermalization of the high energy density system formed at rhic
For this thermalized system, from phobos particle rations, the estimation of the temperature and chemichal potential can be then calculated; T~120 MeV, mB~ 27.2 MeV
-Another signal of the highly interactive medium created at rhic is presented on the right lower panel, where they it was found a suppression of the jet yield on the opposite side on back to back correlations in high pt particles.
- Further more, this shows an important lesson that we have learned from rhic, that is the success in the use of jets and jet correlations to study the highly interactive matter
STAR, PRL 91,
BRAHMS, Nucl. Phys. A757 (2005) 1-27

4. Jet Suppression Jet energy loss deposited in generated medium
Yield suppression  partonic energy loss in medium generated in collision
- One of the most interesting results from rhic is given by the hadron suppression observed at intermediate pt
- Not only that but this suppression behavior behavior seems different for mesons and baryons
- This is attributed to the energy loss in the dense medium generated by the collisions
- This this trend will be also studied at the LHC in a much larger range of pt and at 20 times energy density created at rich, one of the handles for this study will be the transverse momentum particle spectra
Jet energy loss deposited in generated medium

5. Baryon/Meson Anomaly at intermediate pT
RC is the NBIN scaled central to peripheral yield ratio
At intermediate pT baryon to meson splitting independent of the strangeness content
At high pT all particles have similar RCP and appear to show similar suppression

6. Baryon/Meson Puzzle at RHIC
STAR, PRL 97 (152301) 2006
Large enhancement of baryon to meson ratio in A+A collisions
Reaches max. at pT ~ 3 GeV/c
Jet fragmentation not the dominant source of hadronization
Flow effects? Recombination?
p+p: p/ ~ 0.2
Au+Au: p/ ~ 1

7. From RHIC to LHC Factor 28 increase in energy to √sNN=5.5 TeV
Baier,hep-ph/
Accardi, hep-ph/
-Upper left panel are the predicted transverse pt distributions for neutral pions (dashed lines) and charged particles (solid lines) in p+p reactions at typical SPS, RHIC and LHC energies. It is relevant the increase in the pt region that will be available at LHC
-lower right panel is the normalized jet yield as a function of transverse energy for two jet cone radius in Pb+Pb collisions, jets will be at lhc directly identifiable, according to this predictions we will have one jet per 100 events at Et = 100 GeV, or will have
About jets per moth at about Et = 225 GeV
-At least one jet Eti > 20 GeV, or two jets (1,2) with Et1 > 20 GeV and Et2 > 15 GeV per mub and pair of colliding nucleus
access
- In the right panel we have the inclusive upsilon cross section as function of the nucleon-nucleon center of mass energy. Red points: expected values for uplsilon production at RHIC and LHC, black points data . Points are CDF, D0 data at Tevatron, lines are theoretical predictions. Quarkonia states will be available at LHC
Factor 28 increase in energy to √sNN=5.5 TeV
High Luminosity
Large Cross sections
High pT particles
Jets, which are now directly identifiable

8. Baryon/Meson Ratio at the LHC
R.C. Hwa and C.B. Yang, PRL 97, (2006)
Fries and Mueller, EJP C34, S279 (2004)
ξ: suppression factor
ξRHIC = 0.07
ξ LHC =
Γ: overlap factor of shower partons from neighbouring jets
LHC vs RHIC: amplitude of baryon/meson ratio similar, but pushed to larger pT.
Probing baryon/meson differences at the LHC implies particle identification over a large pT range (10 – 30 GeV/c)

9. Challenges for Jet Physics at the LHC
Higher production rates, and the hardening of the spectra may represent a challenge for study of intermediate energy jets.
I may be difficult to separate the leading hadron and the hadrons from the “radiated” energy.
Low signal to background in this region maybe a challenge
The jet correlation studies will require tracking and acuarate PID capabilities (track-by-track)
RHIC
Jet superimposed on 5 TeV Pb + Pb background

10. A Large Hadron Collider Experiment - ALICE
HMPID
PID high pT
TOF
PID
TRD
Electron ID
PMD
γ multiplicity
MUON
μ-pairs
ITS
Low pT tracking
Vertexing
TPC
Tracking, dE/dx
PHOS
γ, π0

11. ALICE PID 3s 2s
(dE/dx)
Existing gap between low and high pT ALICE for detailed (> 3 s) particle identification.
Probing the hadronization mechanisms with ALICE suggests an upgrade for track-by-track PID in the momentum range of 10 – 30 GeV/c

12. VHMPID
The challenge
Relatively small detector covering 5% of acceptance of ALICE’s central barrel
0.5 T magnetic field
PID in the range of 10 – 30 GeV
Good separation resolution ( 3s)
Enough granularity that allows the discrimination of background in a central HI collision

13. Large area CsI photon-to-electron converter
VHMPID Geometry
C5F12 gas radiator (n = )
Large area CsI photon-to-electron converter
Position sensitive charged particle detector
Multi wire proportional chamber (MWPC)
Gas electron multiplier (GEM)
photoelectron
MWPC
Container Filled with gas in such way that any ionizing particle that passes through the tube will ionize surrounding gaseous atoms.
The resulting ions and electrons are accelerated by the potential on the wire
Causing a cascade of ionization which is collected on the wire and results in the flow of current.
GEM

14. Gas Multiplier DetectorDV
CsI
Alternative to the MWPC
Composite grid consisting of two metal layers separated by a thin insulator etched with a regular matrix of open channels (holes)
The metal layers are kept at a suitable difference of potential, allowing the pre-amplification of the charge drift through the channels.
GEM would improve the efficiency of the VHMPID, given the larger multiplication (gain)
The sturdiness of the device when compared to a MWPC is also an advantage.

15. Simulation Based on GEANT 4
The simulations include the CsI quantum efficiency, the gas transmittance and the optical characteristics of the proposed materials.
Neither photoelectron conversion nor the response of the MWPC were included in the simulation at first stage
Photon hit position on the CsI photocathode. The rings are from one event of one incident 16 GeV/c pion (black), one kaon (blue) and one proton (brown).

16. Simulations cont. Identification momentum range Particle With signal
Absence of signal p
3–15 GeV/c k
9–15 GeV/c
18-30 GeV/c
9–18 GeV/c

17. Simulation of Response of MWPC
The response and pixel size of the MWPC’s pads (8 x 8 mm2) was included in the simulation
The simulation was done with chambers of 122 x 120 pads corresponding to a surface of about 1 m2
Further parameters applied were those that reproduce satisfactorily the ALICE High Momentum Particle Identification Detector (HMPID) experimental results
Ring image detected on MWPC. The rings are from one event of one incident 16 GeV/c pion (black), one kaon (blue) and one proton (red).

18. Simulation Cont. pions kaons protons
Cherenkov angle calculated for individual photos using patter recognition algorithm, assuming that the original particle track is known
The points then are given by the average from the N photons from each ring
The design capabilities of the detector using full simulation plus reconstruction are confirmed

19. VHMPID Performance in Beam conditions
The VHMPID detector is simulated at its proposed location with the ALICE detector.
One pion track is embedded in a = 5.5 TeV Pb+Pb HIJING [20] event with a pessimistic charged particle multiplicity dNch/dy ~ 4000 at mid rapidity
Pion trace is clearly detected in the MWPC above the background
Note that the proposed location of the VHMPID is opposite to the EMCAL

20. Physics Possibilities (work in progress)
The proposed location of the VHMPID, opposite to the EMCAL opens the possibility to use both detectors to measure gamma-jet events.
Triggering with the gammas in the EMCAL
Measuring the jet composition in the VHMPID
Study then hadronization, for example: compare the jet multiplicity for proton-leading vs. pion-leading jets
CDF event picture

21. Project Status
Finalize simulations (physics studies) by the end of the year
GEM test planned for this fall
Finalize design by the end of the year
Submit letter of intent to ALICE spring 2008
Start construction of prototype summer 2008