Low Mass Dimuon Production in Proton-Nucleus Collisions at 400 GeV/c with the NA60 Experiment. Antonio Uras for the NA60 Collaboration

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1 STRANGENESS in QUARK MATTER Cracow, Poland September 2011 Low Mass Dimuon Production in Proton-Nucleus Collisions at 400 GeV/c with the NA60 Experiment Antonio Uras for the NA60 Collaboration INFN Cagliari & IPNL Lyon

2 Outline The NA60 experiment Electromagnetic form factors for the η and ω mesons Line shape of the ρ meson Transverse momentum spectra of light neutral mesons Particle ratios Nuclear dependence of the production cross sections 2/22

3 The NA60 Experiment at the CERN SPS NA60 data taking: (2002) 400 GeV/c (2003) 158 GeV/c (2004) 158, 400 GeV/c Present analysis: (2004) p-a 400 GeV/c p-be, p-cu, p-in, p-w, p-pb, p-u 3/22

4 Available Data Samples Description in terms of the expected sources: 2-body and Dalitz decays of the neutral mesons η, ρ, ω, η', φ + Open Charm Total data sample: 180'000 matched muon pairs without target ID (form factors, ρ line shape) 80'000 matched muon pairs with target ID (pt spectra, cross sections) 4/22 WITHOUT target ID 400 GeV/c Events per 20 MeV/c2 Events per 20 MeV/c2 Combinatorial background (< 10% of signal) estimated via an event mixing technique WITH target ID 400 GeV/c

5 Electromagnetic Transition Form Factors and ρ Line Shape

6 The Form Factor in the Dalitz Decays Single Dalitz decays: virtual photon (converting into a lepton pair) + third body lepton pair mass distribution: QED Form Factor 2 Vector Meson Dominance model (VMD): F (M ) = 6/22 P Photon-hadron interactions proceed via a transition to a vector meson 1 2 V =½;!;Á [gabv =2gV ] mv m2v M 2 i V mv P V =½;!;Á [gabv =2gV ] " ¼ 1 2 M 2 # 1

7 Extracting Form Factors and ρ Temperature 0 All contributions are subtracted except:! ¹¹,!! ¹¹¼, ½! ¹¹ Correction for acceptance efficiency vs mass 2 are free parameters 2 and! First measurement of the ρ line shape in proton-nucleus collisions dn / dm ³ 1 4m2¼ M2 3=2 q 1 2 4m2¹ M2 ³ 1+ (m20 M 2) + m20 2 (M ) 2m2¹ M2 ρ temperature: Tρ = 161 ± 7 ± 7 MeV In agreement with the NA60 measurement in peripheral In-In: Tρ = 170 ± 19 ± 3 MeV 7/22 M (2¼MT )3=2 e T

8 Disentangling the Form Factors New theoretical prediction for the ω form factor Terschlusen & Leupold, Phys. Lett. B691, 191 (2010) VMD still close to data for the eta meson! Theory still fails close to the kinematic cut-off! 2 = ± ± [GeV/c2] 2 8/22 2 = ± ± [GeV/c2] 2!

9 Transverse Momentum Spectra

10 Uncorrected Mass Spectra (200 MeV/c slices of pt) GeV/c GeV/c GeV/c GeV/c GeV/c GeV/c GeV/c GeV/c GeV/c Mass spectra extracted target by target and projected in slices of pt Examples shown for the uranium target Small acceptance at low masses at low pt GeV/c 10/22

11 Acceptance Efficiency vs pt ω and φ mesons: good acceptance down to zero pt Target dependence: combination of (mass-dependent) geometrical acceptance and reconstruction efficiency effects Uranium Indium Lead w.r.t. full phase space 11/22 Uranium Indium Lead w.r.t. full phase space

12 Fit of the Corrected Spectra (ω Meson) ³ m 1 dn T / exp mt dmt T dn / dp2t High pt tails not properly described 12/22 µ p2t 1+ 2 p0 Target integrated fit: p02 = 0.76 ± 0.03 ± 0.05 [GeV/c]2 β = 3.31 ± 0.07 ± 0.13

13 Fit of the Corrected Spectra (φ Meson) ³ m 1 dn T / exp mt dmt T dn / dp2t High pt tails not properly described 13/22 µ p2t 1+ 2 p0 Target integrated fit: p02 = 1.27 ± 0.10 ± 0.07 [GeV/c]2 β = 3.81 ± 0.17 ± 0.07

14 Nuclear Dependence of pt No significant trend of pt as a function of A. NA60 result: pt ω < pt φ Agreement with HERA-B p-a measurements (φ meson) Disagreement with the NA27 p-p measurement stating pt ω > pt φ All targets pt ω = 0.61 ± 0.03 ± 0.03 GeV/c 14/22 All targets pt φ = 0.70 ± 0.04 ± 0.02 GeV/c

15 pt Spectrum of the η Meson Previous measurements well described by the power law We have almost no acceptance below 0.6 GeV/c: up to 15% uncertainty when extrapolating η yield in full pt 15/22

16 Particle Ratios & Nuclear Dependence of Cross Sections

17 Events per 20 MeV/c2 Uncorrected Mass Spectra (target by target) 17/22

18 Particle Ratios as a function of A ρ/ω is compatible with ρ/ω = 1 from p-p to p-u. No evidence of dependence on the mass number of the target nucleus φ/ω shows a rising trend as a function of A ( 20% from light to heavy nuclei) Evidence of strangeness enhancement. p-a values significantly higher than p-p η/ω has large systematic uncertainties, due to the extrapolation to full-p T Rising trend recognizable as a function of A ( 30% from light to heavy nuclei) 18/22

19 Production Cross Sections: Nuclear Dependence Several targets available accurate study of σ(a) σ(a) usually parametrized as a power-law function: ¾(A) = ¾0 A Both σ0 and α depend on the considered particle α 2/3 : black disk cross section (soft processes) α 1 : cross section scales with the number of nucleons (hard processes) Be nucleus might be too light to follow the power law trend because of the low A Normalization to both Beryllium and Copper targets 19/22

20 Fit with the Power-law Aα Good description in term of the power law for the ω and φ mesons Removing the Be target: softer dependence for the ω, harder for the φ Hint for different production mechanisms? Softer for the ω, harder for φ (different quark composition) α(ω) = ± ± α(ω) = ± ± α(φ) = ± ± α(φ) = ± ± ω φ Alpha decreases removing Be 20/22 Alpha increases removing Be

21 pt Dependence of the α Parameters Clear rising trend of α vs pt (Cronin Effect) η meson: agreement with previous measurement from CERES-TAPS ω meson: first measurement of α(pt) α φ meson: agreement with previous measurement from HERA-B η 21/22 ω φ

22 Conclusions New high-precision measurement of the η and ω e.m. transition form factors. The ω form factor close to the kinematic threshold is still not explained by theory Study of the ρ line shape: first measurement of the ρ temperature in proton-nucleus collisions Accurate description of the low mass dimuon spectrum in terms of the known sources: in-medium effects not necessary to explain the data. Studies ongoing in connection with possible ρ/ω interference effects pt spectra for the ω and φ mesons studied down to zero pt. Power law is needed to describe the high-pt tails Mean pt larger for the φ than for the ω. No evidence of nuclear dependence ρ/ω = 1 independent of A. Rising trend as a function of A for φ/ω and η/ω Production cross sections scale according to Aα: Dependence is stronger for the φ meson than for the ω For the η, ω and φ mesons: α parameter increases as a function of pt (Cronin effect) 22/22

23 Backup Slides

24 The NA60 Collaboration R. Arnaldi, K. Banicz, J. Castor, B. Chaurand, W. Chen, C. Cicalò, A. Colla, P. Cortese, S. Damjanovic, A. David, A. De Falco, A. Devaux, L. Ducroux, H. En yo, J. Fargeix, A. Ferretti, M. Floris, A. Förster, P. Force, N. Guettet, A. Guichard, H. Gulkanian, J. M. Heuser, M. Keil, Z. Li, C. Lourenço, J. Lozano, F. Manso, P. Martins, A. Masoni, A. Neves, H. Ohnishi, C. Oppedisano, P. Parracho, P. Pillot, T. Poghosyan, G. Puddu, E. Radermacher, P. Ramalhete, P. Rosinsky, E. Scomparin, J. Seixas, S. Serci, R. Shahoyan, P. Sonderegger, H. J. Specht, R. Tieulent, A. Uras, G. Usai, R. Veenhof, H. K. Wöhri 24/22

25 Why tracking before the absorber? Hadron Absorber Beam + Target Vertex Spectrometer Muon Spectrometer 25/22

26 Tracking before the absorber improves resolution η ρ/ω ϕ J/ψ 26/22

27 Combinatorial Background Estimated with a mixed-event technique. Normalized to the Like-Sign component REAL MIXED 27/22

28 Combinatorial Background Combinatorial background accounts for less than 10% of the low-mass spectrum Normalization: uncertainty on the charge-correlation factor at low multiplicities it is more likely to produce opposite sign than like-sign muon pairs 28/22

29 MC Simulations Mass line shapes are mass are generated: Narrow resonances: Gounaris-Sakurai parametrization (similar to a BreitWigner) with mass poles and widths from literature (PDG) Broad resonances (rho): Gounaris-Sakurai broadened by additional phase space factors accounting for the large width Dalitz decays: QED line shapes multiplied by the corresponding form factors Rapidity parametrization: Gaussian-like dn 1 / dy cosh2 (a(y y0 )) Width parameter a is function of the particle mass. It is fixed for the pion mass and decreased for heavier masses proportionally p cm to the maximum rapidity ymax = log( s=mpart ) 29/22 η ρ ω η' φ

30 Form Factors: Isolating the Processes of Interest Eta Dalitz and Omega Dalitz processes are isolated, together with the 2-body decay of ρ 30/22

31 Form Factors: Evaluate Acceptance Correction The correction is evaluated: for each process, separately for the sum of the three processes, according to the normalization extracted from the fit The correction for Rho dominates for mass > 0.6 GeV/c2 The correction for Eta dominates for mass < 0.5 GeV/c2 31/22