Photon Physics in ALICE from conversion electrons

Transcription

1 Photon Physics in ALICE from conversion electrons A. Marin (for the AliCE Collaboration) Introduction: Photon detection via conversion method Physics results with conversion electrons: CERES, STAR Perspectives in ALICE Conclusions and outlook

2 The Large Hadron Collider CMS LHCb ALICE Palaver SS2009 (Heidelberg) ATLAS 7/15/2009

3 ALICE: The dedicated HI Experiment Photon detectors in ALICE: PHOS, EMCAL, PMD

4 Interaction of photons with matter Photoelectric effect Compton scattering Pair production: Eγ>1.02MeV Electromagnetic Calorimeters: The complete photon energy is deposited in the detector (electromagnetic shower) Photon measurement via conversion electrons: Calculate photon momentum and direction by measuring e + /e - from a single conversion in tracking detectors ALICE ( ITS+TPC+TRD)

5 Photons via conversion electrons Identify photons converting in the beampipe, ITS and TPC Clean photon identification Provide directional information Non vertex background (important source of systematic errors in measurement of direct photons) can be rejected. Good momentum resolution for charged tracks The best resolution at low p T Good resolution also at high p T. Independent measurement of the same quantities, with different systematics compared to PHOS/EMCAL. Main drawback: knowlegde of the material budget in the detector. Exact composition and geometry are needed. Main source of systematic errors in this method

6 Photon Detection in ALICE PHOS (100 0 x 0.24 in η) high resolution (energy and spatial) small coverage EMCal larger coverage: 120 O x 1.4 in η coarser spatial resolution than PHOS available in 2010 γζ e + e - Conversions (Central Barrel) large coverage: 360 O x 1.8 in η low conversion probability (8-12%) Palaver SS2009 (Heidelberg)

7 Momentum resolution comparison Central Barrel, pp PHOS Momentum resolution in central barrel very good at low p T At high p T is <4%

8 Experimental photon results obtained with the conversion method: CERES: R. Soualah, PhD (2009) STAR: PRC70, (2004)

9 The CERES SPS Sidc: vertex reconstruction, angle measurement Rich: electron ID TPC: momentum and electron-id (σ mass ~3.8% at φ ee) 2000 Run: s NN =17.2 GeV 30x10 6 events σ/σ geo =7% 3x10 6 events σ/σ geo =20% RICH2 mirror main conversion point 6 mm thick (5% of radiation length) γz e + e - Z

10 π 0 γγ reconstruction method y RICH2 mirror γ e+ e- TPC x π 0 θ γγ γ θ ee z e+ e (m) π 0 γγ ( m π 0 = GeV/c 2, BR=0.98, cτ =25.1 nm ) M π = E E (1 cos θ ) 0 2 γ1 γ 2

11 Reconstructed γ conversion points in CERES Rich2 Mirror TPC entrance spokes Before opan e+e- cut After opan e+e- cut

12 Spectra of π 0 mesons σ π 0 =(7.7±0.2)MeV R. Soualah PhD Thesis (2009)

13 The STAR Large acceptance: 2π coverage at mid-rapidity

14 γ, π 0 reconstruction in STAR@RHIC J. Johnson, PhD Thesis (2002). PRC70, (2004) γ conversion points Very good mass resolution

15 γ, π 0 spectra in s NN =130GeV STAR Coll. PRC 70(2004)044902

16 Perspectives of photon physics in ALICE using the conversion method

17 ALICE Material budget : γ conversion points K. Koch η = 0.9 η = 1.2 TPC ITS Use TPC up to 160 cm average radiation length ~ 8% X/X 0 for η < 0.9 (6.5% X/X 0 ITS + 1/2 3.5% X/X 0 TPC)

18 γ reconstruction ε γ Global = γ Conversion Probability x ε γ reconstruction

19 Photon physics Why? Do not interact strongly. Carry information about the early state. Calibrated baseline Measurement of π 0 and η mesons in pp@ 10TeV Highest energy available in lab Test of pqcd cross section predictions Reference data for PbPb Main source of background in direct photon measurement Measurement of Direct photons in pp@10tev Highest energy available in lab Test of pqcd cross section predictions Reference data for PbPb γ-jet (γ-hadron): Study quark energy loss in the medium Fragmentation function: E jet ~ E γ

20 How many direct photons? but γ/π 0 = 0,01-0,1 for p T > 20 GeV/c We need a good distinction direct/decay γ

21 γ,π 0 reconstruction π 0 γγ e + e - cm Y γ X

22 γ γ Invariant Mass Spectra (π 0 pt bins) 0<pt< <pt<1 1<pt<1.5 GeV 2.5<pt<3. GeV 4.<pt<4.5 GeV Analysis in η <1.2 5<pt<5.5 GeV 5.5<pt<6.5 GeV

23 Reconstructed π 0 Mass and Width

24 Photon detector resolutions PHOS EMCAL ITS+TPC+TRD σ E /E (%) σ x (mm) E 3.26 E E E E 5.3 E ~1 R ip (cm) < 160 σ π (MeV) p t =1-2 GeV 5.5 MeV 16 MeV 3.3 MeV 0.8<p t <2 GeV D. Peressounko@QM09 24

25 K. Koch π 0, η Expected statistics 1 st ALICE year π 0 NLO calculations from Y. Kharlov,, L. Benhabib E d 3 σ dp = 1 3 2πp T p T y 1 Lt 1 DutyFactor 1 Acc 1 Eff N π 0 Preliminary

26 Expected statistics for direct γ. (INCNLO 1.4: LO(Born) Preliminary LO(Born) 1.6x10 9 pp MinBias collisions p T reach ~20 GeV M. Rammler@Uni-Münster Palaver SS2009 (Heidelberg)

27 γ-hadron and γ-jet correlations s=14tev (ALIROOT v4-05-release) γ-jet: 6x10 5 events in GeV bins [5,10],[10,20],[20,30],[30,40],[40,50],[50,..] in PHOS acceptance; Part of data(3x10 5 ) analysed γ-hadron Isolation Cut: 0.2 cone size and p T >0.7GeV A. Marin, Alice Physics Week, Prague 2008 Study TRD L1Trigger possibilities needed

28 Need for a trigger : TRD L1 Similar statistics PHOS-Central Barrel High p T needs a trigger TRD L1 accept: e + e - pair with high p T Upper limmit to be studied

29 Quarkonia and photon physics J/ψ: The smoking gun signature of deconfinement in the QGP. From suppression (SPS,RHIC) to enhancement (LHC). Feed down contribution from χ C & ψ needs to be known. Measurements in pp provide the baseline R χ c 2 1 = ( ) σ χcj σ J ψ J = 1 ( ) ( ) BR χ cj J ψ γ I. Abt et al. (HERA-B), Phys. Rev. D 79 (2009) A. Andronic et al. ArXiv:

30 χ C detection in ALICE χ C J/ψ + γ e + e - e - e + γ J/ψ e + e - γ e + e - Reconstruct γ's that convert in the Central Barrel material cm

31 χ C reconstruction Special events: 1χ C e + e - γ in each minimum bias, η(e +,e -,γ) < 1.2 Select γ Select primary e + e - Openning angle cut e +- γ >0.05 rad With e-pid using Central Barrel (TPC+18 TRD SM's) χ C1 χ C2 1 χ C e + e - γ per event

32 Feasibility of χ C measurement in pp@14tev χ C1 χ C2 Alice p-resolution allows to separate the two χ C states The χ C can be measured down to p t ~0.25GeV/c Mean efficiency ~0.9% P. Gonzalez et at., EPJC 61 (2009) 899 M=M(e + e γ )-M(e + e - ) How many χ C -> J/ψ γ-> e + e - γ are produced/s? L= cm -2 s -1 ; σ 1 = 31.8 µb; σ 2 = 52.5 µb χ C /s = L*(σ 1 *BR(χ C1 ) + σ 2 *BR(χ C2 ))*BR(J/ψ e + e - ) = 1.3/s Given detection efficiency of 0.9% and acceptance of 6.3% the maximum rate of observable χ C 's is expected to be 7.4x10-4 /s assuming a 100% trigger efficiency #detectable J / ψ #detectable χ C 165 Trigger for χ C measurement under study

33 Conclusions and outlook Conversion method successfully used at SPS and at RHIC Photons can be measured with conversion electrons in the ALICE central barrel (ITS+TPC+TRD). The acceptance x conversion prob. comparable to PHOS acceptance Momentum resolution is very good (σ π 0 =3 MeV at low p T ) Definition/Implementation of a high p T γ-trigger in the central barrel will be study. Lot's of physic studies are being prepeared π 0 /η meson spectra Gamma Jet correlations Direct photons χ C Careful evaluation of systematic errors is ongoing.

34 ALICE Detector Installation Goal 2009 Complete: ITS, TPC, TOF, HMPID, FMD, T0, V0, ZDC, Muon arm, Acorde PMD, DAQ ALICE Status Partial installation: PHOS(3/5) 7/18 TRD 2-4/6 EMCAL ~ 50% HLT Palaver SS2009 (Heidelberg) P. Kuijer 7/15/

35 multi TeV cosmic ray event

36 Circulating Protons in LHC ITS tracks on reconstructed tracks, common vertex Circulating beam 2: stray particle causing an interaction in the ITS

37 TPC commissioning TPC installed in ALICE, running continuously May-October million events (cosmics, krypton, and laser) recorded first round of calibrations completed transverse momentum resolution, B=0.5 T particle identification via de/dx resolution: measured 5.7% design 5.5% A. Kalweit HK84 resolution at 10 GeV: measured 6.0% design 4.5% performance close to design, TPC ready for collisions

38 TRD commissioning 4 supermodules installed in ALICE, cosmic ray data taking in horizontal tracks acquired (TRD L1 trigger commissioned and used) Reconstruction and first iteration for calibration parameters (gain, drift velocity) completed cosmic event triggered by TRD L1 GSI scientific report 2008 uniformity better than 10%

39 Ready for proton/ion beam Palaver SS2009 (Heidelberg) P. Kuijer 7/15/2009 Formal end of ALICE installation: July

40 Backup slides

41 Comparison to other experiments

42 Comparison to h ±

43 The charmonium system The χ C family: 35.6% decay rate 20.2 % 1.3 % has been neglected 5.94 %

44 J/ψ suppression at SPS and RHIC PHENIX Au+Au data shows suppression at mid-rapidity about the same as seen at the SPS at lower energy but stronger suppression at forward rapidity. Forward/Mid R AA ratio looks flat above a centrality with N part = 100 Feed down contribution from χ C & ψ needs to be known Measurements in pp provide the baseline Centrality (N part )

45 J/ψ in Statistical Hadronization Model A. Andronic et al. PLB 652 (2007) 259 Clear signal for generation of charmonia due to statistical hadronization at the phase boundary

46 γ detection with conversion method γz e + e - Z Conversions (Central Barrel) large coverage: 360 O φ x 1.8 in η low conversion probability (8-12%) Identify photons converting in the beampipe, ITS and TPC Clean photon identification Provides the γ p Non vertex background can be rejected. Very good momentum resolution for charged tracks

47 PHOTON SPECTROMETER Dense like lead and transparent like crystal to stop photons PbWO 4 crystal (17920 cristals in total) R M =2.2 cm, X 0 =8.9 mm, ρ=8.28 g/cm 3, n=2.16, size: mm 3 PHOS forsees 5 modules installed at 4.6 m apart from the ALICE interaction point. PHOS cristal PHOS supermodule

48 π 0, γ reconstruction in

49 Radiation length

50 Radiation length Both the geometry of the detector and the exact material composition are needed

51 NLO calculations for π 0 (Y. (Y. Kharlov,, L. Benhabib) run time 200Hz 3 days 1.3nb -1 1 month 13nb -1 3 month 39nb -1

52 The CERN accelerator complex LINAC2- BOOSTER-PS-SPS-LHC