Perspectives for the measurement of beauty production via semileptonic decays in ALICE

Transcription

1 Perspectives for the measurement of beauty production via semileptonic decays in ALICE Rosario Turrisi INFN Padova (Italy for the ALICE collaboration

2 Contents Motivation: energy loss ALICE detector highlights Performances: electron separation and vertexing e+x: attainable statistics and errors e+x: sensitivity of energy loss measurement μ+x: strategy and performance Conclusions with ALICE" 2/16

3 Physics motivation b production cross section transverse momentum spectrum In AA: quarkonia dissociation energy loss not covered here In pp: (pqcd test bench AA, pa baseline Heavy quarks: abundant yield produced early travel ~4 fm in the medium probe the collision dynamics! PbPb (0-5% centr. 5.5 TeV pp 14 TeV (+pa disentangle medium effects ALICE: very low p t explored, complementary to other LHC exps. QQ (NN [mb] N QQ per collision HERA-LHC workshop CERN with ALICE" 3/16

4 High p t suppression at RHIC Method: compare mesons p t distribution in pp and AA: R 1 dn D, D, AA AA ( pt = D, Ncoll dn pp / / dp dp t t =1 if no medium effect Observed at RHIC for light flavors and charm Reproduced by q ^ = 4-14 GeV 2 /fm (see next slide electrons from c/b decay light-flavored hadrons p T (GeV/c Calculations: Armesto, Dainese, Salgado, Wiedemann, PRD71 ( with ALICE" 4/16

5 Possible explanation: gluonsstrahlung interactions may occur by gluon in-medium radiation (quenching the amount of quenching depends on: color charge: C R =4/3, 3 if quark or gluon, resp. (Casimir factor heavy/light probes (b,c vs. direct pions quark mass (beauty/charm comparison dead cone effect path length L Dokshitzer, Kharzeev, PL519 ( Armesto, Salgado, Wiedemann, PRD69 ( ΔE α s C R 2 qˆ L medium dependence mass dependence Q Gluonstrahlung probability 1 2 [ θ + ( m Q / E Q 2 ] 2 aier, Dokshitzer, Mueller, Peigné, Schiff, (DMPS, NP483 ( with ALICE" 5/16

6 Energy loss at LHC A promising strategy: study the p t -dependent ratio R AA of D or mesons produced in AA and pp: quark energy loss R 1 dn D, D, AA AA ( pt = D, Ncoll dn pp / / dp dp t t between R AA and RD AA (beauty/charm ratio: mass dependence R ( p = R between R /D AA and Rh AA (heavy/light probes: color charge dependence R ( p = R ( p ( p R ( p D D( / h t AA t AA t ( p D( h D( / h t AA t AA t R N: study of charm detection performance done! see ALICE Physics Performance Report, J. Phys. G & CERN/LHCC with ALICE" 6/16

7 The ALICE Detector η < 0.9 TPC + silicon tracker (ITS=SSD+SDD+SPD e/π,, K, p, separation in TRD -44 < η < -2.5 muons with ALICE" 7/16

8 Detection strategy: e+x e+x ackground sources: pions misidentified as electrons charm decay electrons Dalitz decays photon conversions strangeness decays signal: <1 electron/ev out of ~10 3 (all p t s! Detection strategy electron ID in TPC + TRD p T cut-off impact parameter cut-off specific for pp: primary vertex optimization cτ~500μm compare with μm from charm m b ~5 GeV hard p t spectrum impact parameter in bending plane with ALICE" 8/16

9 Impact parameter resolution e+x d 0 resolution Silicon Pixel Detector 2 layers, R=4 and 7 cm, ~10 7 channels p t > 1 GeV/c σ < 60 μm (rφ EAM (Z ~12 μm asymptotic pixel size μm d 0 resolution with ALICE" 9/16

10 Electron separation e+x Combined strategy TRD+TPC TRD rejects 99% of pions and 100% of heavier hadrons (90% electron efficiency TPC (via de/dx analysis rejects again 99% of pions at 90% electrons efficiency (at low p t s fraction of misidentified pions with ALICE" 10/16

11 Electron spectra from b e+x Results for electrons detection in: pp, 14 TeV, 10 9 events ( one year run PbPb, one month run 10 7 events ALICE standard underlying event dn CH /dy=6000 systematic and statistical errors studied in detail pp PbPb with ALICE" 11/16

12 -meson level cross section Using electrons in 2 < p t < 20 GeV/c MC-based procedure à la UA1* obtain meson 2 < p t min < 30 GeV/c inner bars: stat. errors outer bars: stat. p t -dep. syst. errors not shown: 9% normalization error quenching curves for illustration * C. Albajar et al., UA1 Coll., Phys Lett 213 ( , Phys Lett 256 ( E Loss Calculation: Armesto, Dainese, Salgado, Wiedemann, PRD71 ( with ALICE" 12/16

13 eauty quenching e+x Reconstruction of meson-level cross section (details on request R AA (RHIC-like analysis sensitivity to quenching/mass R D (pure quark, no quark/gluon effect prefers mass effect R 1 dn D, D, AA AA ( pt = D, Ncoll dn pp / / dp dp t t e R ( p = D t R R e _ from _ AA e _ from _ D AA ( p t ( p N: study of charm detection performance done! see ALICE Physics Performance Report, J. Phys. G & CERN/LHCC t with ALICE" 13/16

14 eauty in the muon channel μ+x Muon spectrometer: pseudorapidity coverage: -4< η<-2.5 absorber + tracking chambers layers + trigger chambers (22 layers 15 interaction lengths, but p t as low as GeV/c p t resolution ~ 2% 5 bb pairs / central Pb-Pb collision (5 % % Α track μ ± 75 μ + μ μ ± μ ± Α geom ε track ε trigger ε trigger % with ALICE" 14/16

15 -meson level cross section μ+x Method: combined fit of 3 muon data samples (singles, low mass OS, high mass OS w/fixed shape and b amplitude as the only free parameter MC to derive with p t as low as 1 GeV/c! σ vs p min t (UA1 method, see C. Albajar et al., UA1 Coll., Phys Lett 213 ( , Phys Lett 256 ( evaluate stat. and syst. errors p T [GeV/c] Signal (fit 4% 4% 3% 3% 2% 2% 3% 4% 8% 12% Efficiency 10% Total p T -dep. 11% 11% 10% 10% 10% 10% 10% 11% 13% 16% Decay of π,k 4% Normalisation 9% Total p T -indep 10% with ALICE" 15/16

16 Conclusions Heavy flavors can play an outstanding role as QCD test bench in AA reactions at LHC: at low p t explore small-x region at high p t probe the QCD extended medium via energy loss The studies outlined in this talk suggest that ALICE has a good potential in this field: semielectronic decays in central barrel (-0.9<η<0.9 semimuonic decays in muon arm (-4< η< -2.5 Same observable (E loss from two different analyses in the same experiment! and a lot has been left out: charm hadronic and semileptonic decays e-μ coincidences indirect J/ ψ b tagging via topological selections (Lot of work in progress in the ALICE Physics Working Group 3 heavy flavors with ALICE" 16/16

17 Errors evaluation on e s spectra e+x Estimation of errors: MC corrections (efficiency, acceptance, etc. fixed at 10%, p t -independent. indetermination on charm subtraction evaluated using as reference our study on hadronic charm detection normalization error not shown with ALICE" 17/16

18 Energy loss at LHC A promising strategy: study the p t -dependent ratio R AA of D or mesons produced in AA and pp: quark energy loss study of charm energy loss done! see between R AA and RD AA (beauty/charm ratio: pure quark analysis Calculation of energy loss at LHC energies: Armesto, Dainese, Salgado, Wiedemann, PRD 71 ( with ALICE" 18/16

19 Charm/light ratio R (Dh mass+color charge effect R ( p = R ( p R ( p D h D/ h t AA t AA t with ALICE" 19/16

20 Extraction of a minimum-p T -differential cross section for mesons Using UA1 MC method (*, also adopted by ALICE μ (thanks to R.Guernane for useful discussions The meson cross section per unit of rapidity at midrapidity with p T > p T min is obtained from a scaling of the electron-level cross section measured within a given electron phase space Φ e dσ dy ( p T > p min T = σ e, beauty ( Φ e meas dσ dy ( p T > σ ( Φ e p min T The semi-electronic.r. is included here The phase space used is e, beauty e = σ ( Φ F meas e Φ Δη = [-0.9, 0.9] and Δd 0 = [200,600] μm e { Δp T, Δη, Δd0} MC where Δp T are the previously used bins, (* C. Albajar et al., UA1 Coll., Phys Lett 213 ( C. Albajar et al., UA1 Coll., Phys Lett 256 ( with ALICE" 20/16

21 Extraction of a minimum-p T -differential cross section for mesons Using UA1 MC method, also adopted by ALICE μ Systematic error for F e - semi-electronic decay.r.: ~ 3 % - dependence on the shape of the meson p T distribution used as input in the MC: can be minimized using a proper choice of p T min for a given phase space Φ e see following slides - Monte Carlo correction for the efficiency of the selection cuts: this is, in principle, depending on the meson p T distribution, and should be then evaluated at this stage of the analysis. For the present feasibility study we account for it with a 10% systematic. with ALICE" 21/16

22 Extraction of a minimum-p T -differential cross section for mesons Using UA1 MC method, also adopted by ALICE μ Evaluation of F e and determination of the optimal p min T 1 we used the e + X decays from PYTHIA. F e is the ratio of the red area to the blue one. here Δp Te = [3,4] GeV/c with ALICE" 22/16

23 Extraction of a minimum-p T -differential cross section for mesons Evaluation of F e and determination of the optimal p min T 2 in the HVQNMR program we changed the theory parameters: a quark mass and scales b nuclear modification of the PDFs c b fragmentation (Peterson d add the quenching (q = 100 GeV 2 /fm (* (* N. Amesto, A. Dainese, C. A. Salgado, U. A. Wiedemann, hep-ph/ with ALICE" 23/16

24 Extraction of a minimum-p T -differential cross section for mesons Evaluation of F e and determination of the optimal p min T ΔF ~ 1 % Can find an optimal p T min for p Te > 2 GeV/c with ALICE" 24/16