Observation of (anti)hypernuclei in Pb-Pb collisions with ALICE at the LHC

Observation of (anti)hypernuclei in Pb-Pb collisions with ALICE at the LHC Paolo Camerini * on behalf of the ALICE Collaboration * Dipartimento di Fisica-Universita di Trieste and INFN Trieste NUFRA201 Fourth International Conference on Nuclear Fragmentation 29 September - 6 October, 201, Kemer (Antalya), Turkey

Outline Introduction The ALICE detector performances Detection of light (anti)nuclei Η and Η : study and results Search for n bound states and H-dibaryon Summary and outlook 10/02/201 - NUFRA201 Paolo Camerini

The search for Hypernuclei - How Since 1952 a variety of reactions used to create hypernuclei. Two basic techniques to measure their mass: 1. Missing mass hypernuclear spectroscopy (via measurement of prompt emitted meson) A i Missing mass f YA Strangeness exchange reaction A(K -,π - ) Associated production reaction A(π +,K + ) Photo - production A(γ,K + ), A(e,e K + ) Stable nuclear Targets hypernuclei near stability valley no mass limitation Access to excited states single hyp. 10/02/201 - NUFRA201 Paolo Camerini

The search for Hypernuclei - How 2. Invariant mass Spectroscopy of (ground state) hypernuclei via measurement of weak decay products Nuclear reactions (cosmic rays induced, hadron induced ) Rel. heavy Ion collisions, Access to multistrangeness Access to high isospin A i f 1 f n YA decay Central ultrarelativistic Heavy Ion Collisions (UrHIC) Access to (light) antihypernuclei, exotica fire ball YA decay H wwwa1.kph.uni-mainz.de 10/02/201 - NUFRA201 Paolo Camerini Despite 60 years of activity hypernuclear physics is witnessing a renaissance with several running or planned experiments (GSI,J-PARC, J-LAB, MAMI, LHC, RHIC, ) capable of enriching the hypernuclear chart

The search for Hypernuclei-why Astrophysics Dense astrophysical objects Y-N, Y-Y interaction EoS Dense nuclear matter Cosmology, QCD phase diagram Nuclear matter under extreme conditions, QGP, Confinement Early universe Nuclear - Particle physics Y-N interaction, Y-Y interaction Hadron structure Weak interaction N-N interaction Nuclear structure Many body systems QCD based effective field theories 10/02/201 - NUFRA201 Paolo Camerini

The Quark Gluon Plasma Lattice QCD: at high enough temperature nuclear matter undergoes a Phase Transition to a state where quarks and gluons are no longer confined to volumes of hadronic dimensions: the Quark Gluon Plasma (QGP) ε:energy density Phase of matter of first instants of the universe Can we recreate the QGP phase in the lab? 10/02/201 - NUFRA201 Paolo Camerini Experimental access to QGP would help understanding Quark confinement Hadronization-hadron masses Chiral symmetry restoration QCD phase diagram Evolution of early universe Matter/anti-matter asymmetry

Heavy-Ion Collisions and the QGP What s needed: o High temperature - QGP is expected to set in at T c ~ 170MeV. o Extended, long-lived system: the system must have time to thermalize How? Ultrarelativistic Heavy Ion Collisions: to create and study QGP properties and evolution LHC: s NN 2.76 TeV RHIC: s NN 200 GeV AGS, SPS: s NN few GeV 10/02/201 - NUFRA201 Paolo Camerini

Heavy-Ion Collisions and the QGP t = 0 t = - fm/c Quarks and gluons confined inside hadrons. 10/02/201 - NUFRA201 Paolo Camerini

Heavy-Ion Collisions and the QGP t = 0 t = - fm/c Quarks and gluons confined inside hadrons. Hard collisions 10/02/201 - NUFRA201 Paolo Camerini

Heavy-Ion Collisions and the QGP t = 0 t = - fm/c Quarks and gluons confined inside hadrons. Hard collisions Thermalization 10/02/201 - NUFRA201 Paolo Camerini

Heavy-Ion Collisions and the QGP t = 0 t = - fm/c Quarks and gluons confined inside hadrons. Hard collisions Thermalization Expansion 10/02/201 - NUFRA201 Paolo Camerini

Heavy-Ion Collisions and the QGP t = 0 t = - fm/c Quarks and gluons confined inside hadrons. Hard collisions Thermalization Expansion Chemical freezeout (abundances) 10/02/201 - NUFRA201 Paolo Camerini

Heavy-Ion Collisions and the QGP t = 0 t = - fm/c Quarks and gluons confined inside hadrons. Hard collisions Thermalization Expansion Chemical freezeout (abundances) Kinetic freezeout 10/02/201 - NUFRA201 Paolo Camerini

Probing the QGP Study of fireball evolution and characteristics hard probes (particles produced in the early stage): Heavy-quarks, jet-quenching, photons, quarkonia,... t Soft probes (particles produced in the late stage): hadrons yields, hadrons distributions, strangeness enhancement, low-mass resonances, flow, At LHC energies uu, dd, ss pairs can be easily excited from the quantum vacuum abundant production of strangeness and antimatter. (light) nuclei and hypernuclei: heavy, weakly bound systems very sensitive to fireball characteristics sensitive to late stage of fireball evolution 10/02/201 - NUFRA201 Paolo Camerini

(Hyper)nuclei production in UrHIC Statistical Thermal Model Hadrons emitted from region at statistical equilibrium when fireball reaches limiting temperature Abundances fixed at chemical freeze-out Model reproduces abundances at RHIC energy Andronic et al, PLB697,20(2011) Pb-Pb, s NN = 2.76 TeV,0-10% J.Rafelsky et al.,arxiv:10.2098v1 A.Andronic, private communication A.Andronic et al., PLB697,20(2011) 10/02/201 - NUFRA201 Paolo Camerini (Hyper)nuclei production: Direct thermal production of heavy objects Yields very sensitive to model parameters Low yields Testing ground for models (chemical non equilibrium SHM, UrQMD,DCM,coalescence, ) Sensitive to late stage of fireball evolution (small Binding Energy-coalescence)

(Hyper)nuclei production in UrHIC Production by coalescence If baryons at freeze-out close enough in Phase Space an (anti-)(hyper)nucleus can be formed Coalescence vs. thermal model: different approach but similar yields predicted Symbols: coalescence DCM Lines: thermal UrQMD Pb-Pb/Au+Au; 0-5% y <0.5 Pb-Pb/Au+Au, 0-10% Steinheimer et al, PLB714(2012)85 Predictions for mixed ratios differ sensibly Andronic et al, PLB697,20(2011) DCM (Dubna Cascade Model) UrQMD:Steinheimer et al, PLB714(2012)85 HRG: Subatra Pal PRC87(201) 054905 10/02/201 - NUFRA201 Paolo Camerini

(Hyper)nuclei production in UrHIC Characterizing the medium Correlation between Strangeness and baryon number expected to be very different in QGP and hadron gas scenarios Hypernuclei production sensitive to the Baryon-Strangeness correlation S H p = He Strangeness Population Factor H, He, p and : total production yields S sensitive to local correlation between baryon number and strangeness [Zhang et al.,plb684(2010)], i.e. to the medium characteristics selective with respect to different theoretical approaches deconfined Hadron gas 10/02/201 - NUFRA201 Paolo Camerini

(Anti)(Hyper)nuclei production at LHC Production yield estimate (thermal model) of (anti)(hyper)nuclei in central heavy ion collisions at LHC energy: Yield* ~ 0 p ~ 40 π ~800 Yield He ~ 0.01 H ~ 0.00 Yield/event at mid-rapidity Statistical Thermal Model Light nuclei Hypertriton Search for n, di-baryons LHC Andronic et al., PLB697,20(2011) A. Andronic, private communication 10/02/201 - NUFRA201 Paolo Camerini

A Large Ion Collider Experiment to study QGP ALICE particle identification capabilities are unique. Many different techniques are exploited: de/dx, Time Of Flight, Transition Radiation, Cherenkov Radiation, calorimetry Very thin Silicon tracker (~8%X0), moderate field (B=0.5 T) Excellent vertexing capability Inner Tracking System (ITS): Primary vertex Tracking PID via de/dx 10/02/201 - NUFRA201 Paolo Camerini

A Large Ion Collider Experiment to study QGP ALICE particle identification capabilities are unique. Many different techniques are exploited: de/dx, Time Of Flight, Transition Radiation, Cherenkov Radiation, calorimetry Very thin Silicon tracker (~8%X0), moderate field (B=0.5 T) Excellent vertexing capability Muon spectrometer (-4.0<η<-2.5) for muon ID 10/02/201 - NUFRA201 Paolo Camerini ACORDE (cosmics) V0 scintillator (centrality) η: -1.7.7, 2.8 5.1 ZDC (centrality) FMD (Nch -.4<h<5) PMD (Ng, Nch)...

Collision geometry Central collisions Peripheral collisions Participants b b Spectators Nuclei are extended objects Geometry not directly measurable Centrality (percentage of the total cross section of the nuclear collision) connected to observables via Glauber model Aamodt et al.,prl106(2011)0201 10/02/201 - NUFRA201 Paolo Camerini

Performance ITS TPC ITS vertexing Excellent de/dx performance of TPC 7.2% (2011) resolution in central Pb-Pb collisions Light nuclei/anti-nuclei 10 anti alfa identified out of 2x10 6 events 10/02/201 - NUFRA201 Paolo Camerini

Light nuclei: deuterons Deuterons: Identification via TPC +TOF 1. TPC cut 2. Raw yield extraction via fit p T distribution measured for different centrality classes (blast wave fits superimposed) 10/02/201 - NUFRA201 Paolo Camerini

H detection with Pb-Pb at s NN = 2.76 TeV 2010 Pb-Pb 2.76 TeV ~10 μb -1 2011 Pb-Pb 2.76 TeV~ 150 μb -1 Type Events (x10 6 ) Central ~24 Semi-Central ~21 Events used for the analysis How to detect H (Anti) hypertriton decay channels: 2011 Data taking H H He + π H + π H d + p + π H d + n + π 0 - - 0 H H He + π H + π H d + p + π H d + n + π 0 + + 0 BR= 0.25 (*) (*) Kamada et al., PRC57(1998)4 H search via two-body decays into charged particles: Two body decay: lower combinatorial background Charged particles: ALICE acceptance for charged particles higher than for neutrals 10/02/201 - NUFRA201 Paolo Camerini

The experimental challenge The challenge: extract the H signal from an overwhelming background Centrality dn ch /dη 0-5 % 1601 ± 60 0-80% 546 ± 0 PRL 106, 0201 (2011) He - π 10/02/201 - NUFRA201 Paolo Camerini

H signal extraction H H - He + π He + π + Identify He and π Evaluate ( He,π) invariant mass Apply topological cuts in order to identify secondary decay vertex and reduce combinatorial background Extract signal MAIN APPLIED CUTS: Cos(Pointing Angle) > 0.99 DCA pion track to PV > 0.4 cm DCA between tracks < 0.7 σ ( He,π) p T > 2 GeV/c cτ > 1 cm 10/02/201 - NUFRA201 Paolo Camerini

H signal extraction [( He,π - )+ ( He,π + )] Invariant Mass spectrum Semi-Central Events (10-50%) p T integrated invariant mass spectrum Background evaluation via fitting of like-sign ( He,π) distribution µ = 2.999 ± 0.002 GeV/c 2 σ= (2.08 ± 0.50)x10 - GeV/c 2 To be compared to literature value: µ= 2.9911 ± 0.00005 GeV/c 2 [Jurich NPB52 (197)] 10/02/201 - NUFRA201 Paolo Camerini

H signal extraction vs. p T Central Collisions (0-10%) 2 < p T < 4 GeV/c 4 < p T < 6 GeV/c 6 < p T < 10 GeV/c Data Like-sign Background Combined Fit (pol+gauss) ( He,π - ) + ( He,π + ) Efficiency corrected p T spectra determined in 2 < p T <10 GeV/c interval. Blast wave fit used to evaluate contribution in unmeasured region and determine integrated Yield 10/02/201 - NUFRA201 Paolo Camerini

(Anti)hypertriton Yields dn/dy x B.R. ( H He π) for Central (0-10%) and Semi-central (10-50%) events for H and H separately dn/dy =1/( y N events ) Yield dn/dy xb.r. x 10-5 H:.06 ± 1.01 ± 0.49 Central H:.02 ± 1.15 ± 0.58 Central H: 1.40 ± 0.47± 0.22 Semi-central H: 0.92 ± 0.1 ± 0.17 Semi-central 10/02/201 - NUFRA201 Paolo Camerini Yield Ratios: R= H H / ALICE: R=0.99 ± 0.5 Central R=0.66 ± 0. Semi-Central STATISTICAL-THERMAL MODEL: R=0.95 (Cleymans et al, PRC84(2011) 054916) At s NN =200GeV it is R~0.5 STAR: R=0.49 ± 0.18 ± 0.07 STATISTICAL-THERMAL MODEL : R=0.48

(Anti)hypertriton Yields: centrality scaling Assuming particle production scales with centrality, yields were renormalized by dn ch /dη N ch :number of charged particles Corrected Yields scaled for dn ch /dη 1/ <dn ch /dη> dn/dy x B.R. x 10-9 H: 21.1 ± 6.98 ±.8 Central H: 20.9 ± 7.9 ±.97 Central H: 24. ± 8.11 ±.89 Semi-central H: 16.0 ± 5.4 ±.04 Semi-central Central and semi-central yields consistent after scaling 10/02/201 - NUFRA201 Paolo Camerini

Abundances and the thermal model T ch =164 MeV predictions: extrapolation from lower energies (RHIC) T ch = 156 MeV: best value for LHC H (and K *0 ) not used in global fit. H value normalized by BR( H He+π) =0.25; Thermal model: Andronic et al, PLB 67, 142 Deuteron and H yields: model predictions OK Model assuming thermal equilibrium reproduces abundances at 10-20% level He data ready soon 10/02/201 - NUFRA201 Paolo Camerini

Comparison with theoretical predictions Theoretical Predictions plotted as a function of BR( H He+π ) after being multiplied by BR. Great sensitivity to theoretical models parameters Non equilibrium SHM model (Petran- Rafelsky) provides better global fitting (χ 2 1) to lower mass hadrons but misses H and light nuclei Experimental data closest to equilibrium thermal model with lower T (156 MeV) 10/02/201 - NUFRA201 Paolo Camerini

n and H-dibaryon search Hypothetical udsuds bound state First predicted by Jaffe (Jaffe, PRL 8, 195617 (1977)) Several predictions of bound and also resonant states. Recent Lattice models predict weakly bound states [Inoue et al., PRL 106, 162001 (2011), Beane et al., PRL 106, 162002 (2011)] A. Andronic, private communication Bound: m H < threshold weakly bound: measurable channel H pπ 2.2 GeV/c 2 < m H < 2.21 GeV/c 2 Schaffner-Bielich et al., PRL 84, 405 (2000) 10/02/201 - NUFRA201 Paolo Camerini

n and H-Dibaryon search DATA SAMPLE: Pb-Pb at s NN = 2.76 TeV (2010 ALICE runs) ~14 x 10 6 Minimum bias events (0-80%) No signal visible Obtained upper limits: Strongly bound H(m=2.21 GeV/c 2 ): dn/dy 8.4x10-4 (99% CL) Lightly bound H: dn/dy 2x10-4 (99% CL) 10/02/201 - NUFRA201 Paolo Camerini

n and H-dibaryon search Upper limit set on weakly bound state production is significant within thermal model predictions (factor ~10 lower than prediction) 10/02/201 - NUFRA201 Paolo Camerini

n and H-dibaryon search Upper limit set on weakly bound state production is significant within thermal model predictions (factor ~10 lower than prediction) A similar search for a n bound state (*) was performed in the n dπ + decay channel. Upper limit determined: dn/dy 1.5x10 - (99% CL). Significant within thermal model predictions. (*) Possible signal seen by HypHI experiment at GSI ( 6 Li beam on 12 C 2 AGeV) in the n d π - channel 10/02/201 - NUFRA201 Paolo Camerini

H Lifetime determination ( He,π - ) + ( He,π + ) invariant mass Direct decay time measurement difficult (~ps), but excellent determination of decay vertex allows measurement of lifetime via N(t)=N 0 *exp(-t/τ) t = l/(βγc), βγc = p/m; l: decay distance 10/02/201 - NUFRA201 Paolo Camerini

H Lifetime determination ( He,π - ) + ( He,π + ) invariant mass cτ=5.5 ± 1.4 ± 0.8 cm τ = 185 ± 48 ± 29 ps 10/02/201 - NUFRA201 Paolo Camerini

H Lifetime determination Alice value: τ = 185 ± 48 ± 29 ps Existing measurements cannot distinguish between models Weighted Mean: τ = 169 ± 15 ps 10/02/201 - NUFRA201 Paolo Camerini

H Lifetime determination Weighted mean Alice value: τ = 185 ± 48 ± 29 ps Existing measurements cannot distinguish between models Weighted Mean: τ = 169 ± 15 ps 10/02/201 - NUFRA201 Paolo Camerini

Summary Excellent ALICE performance allows detection of light (anti)nuclei, (anti)hypernuclei and other exotica. ( He,π - ) and ( He,π + ) invariant mass distributions studied in Pb-Pb collisions at s NN = 2.76 TeV H and H : signals extracted for central and semi-central events H and H : similar yields and scale according to multiplicity H and H yields reproduced by equilibrium thermal model with T=156 MeV H lifetime determined Deuteron: Yield reproduced by equilibrium thermal model with T=156MeV H-dibaryon and -n Upper limit determined He: to be released soon 10/02/201 - NUFRA201 Paolo Camerini

Outlook After the 2018 upgrade, ALICE will be able to collect data with better performance at higher luminosity. Expected Integrated Luminosity: ~10 nb -1 ( ~ 10 10 Central collisions) Expected yields will allow detailed study of hypertriton characteristics Performed analysis relevant for future study of strange and multi strange states Particle Expected yields Yield H.0 x 10 5 4 H 8.0 x 10 2 4 H.4 x 10 1 5 H.0 5 H 0.2 Expected yields per 10 10 central collisions computed in the framework of the statistical hadronization model. Predictions done assuming 8% as efficiency per detected baryon. Letter of Intent for the Upgrade of the ALICE Experiment CERN-LHCC-2012-012 ; LHCC-I-022 10/02/201 - NUFRA201 Paolo Camerini