Discussion on the hypertriton lifetime measurements
Jinhui Chen Shanghai Institute of Applied Physics, CAS
l Why we revisit the lifetime l New measurement from STAR – Signal in separate bins – Updated results and world data – Discussions l Conclusion and Outlook Why hypernuclei?
QCD theory development
• Micro-lab. with proton, neutron and hyperons; • YN & YY interaction (strangeness sector of hadronic EOS)
Astrophysics
• Hyperons are important for cosmology, physics of neutron stars…
Nuclear Physics • Phenomenology: extension of nuclear charts into strangeness, exotic nuclei… • Structure theory: nuclear dof for investigating interaction of baryons in nuclei (hyperons – w/o Pauli blocking) • Reaction theory: new probe for fragmentation of nuclei, phase transition and EOS in hypermatter and finite hypernuclei
2 The first hypernucleus?
The 1st hypernucleus was observed in a stack of photographic emulsions exposed to cosmic rays at about 26 km above the ground. M. Danysz & J. Pniewski, Phil. Mag. 44, 348 (1953)
• Incoming high energy proton from cosmic ray, colliding with a nucleus of the emulsion, breaks it in several fragments forming a star.
• All nuclear fragments stop in the emulsion after a short path
st • From the 1 star, 21 tracks à 9α + 11H + 1 ΛX • The fragment ΛX disintegrates later, makes the bottom star. Time takes~10-12 sec (typical for weak decay)
• This particular nuclear fragment, and the others obtained afterwards in similar conditions, were called hyperfragments or hypernuclei
3 Hypernuclei production in HI Collisions
Λ,Σ,π,Κ,...
Κ Λ Λ Λ,Σ,π,Κ,... π π Λ Λ Κ Production of hypermatter in relativistic HI and hadron collisions
- Production of strange particles and hyperons by “participants”, - Rescattering and absorption of hyperons by excited “spectators” - Will focus on the hypertriton in this talk 3 Λ H (n + p + Λ) 3 Λ H (n + p + Λ) 4 Relativistic Heavy Ion Collider (RHIC)
PHOBOS RHIC BRAHMS PHENIX STAR
AGS
TANDEMS
5 The STAR detector
Magnet MTD BEMC TPC TOF BBC EEMC
STAR: a complex set of various detectors, a wide range of measurements and a broad coverage of different physics topics. STAR-TPC: NIMA 499 (2003) 659 STAR-detector: NIMA 499 (2003) 624 6 Event display
!
A beautiful event in the STAR TPC that includes the production and decay of a antihypertriton candidate. (Data taken from Run4 Au+Au 200GeV MB collision)
7 The mesonic decay of hypertriton 1598 KAMADA, GOLAK, MIYAGAWA, WITAŁA, AND GLO¨ CKLE 57 Kamada et al., PRC 57, 1595(1998) TABLE I. Partial and total mesonic and nonmesonic decay rates and corresponding lifetimes.
21 21 Channel G@sec #G/ GL t5G @sec# 3He 1p2 and 3H 1 p0 0.146 31010 0.384 0.684 31029 d1p 1p2 and d1n1p0 0.235 31010 0.619 0.425 31029 p 1 p 1 n 1 p2 and p 1 n 1 n 1p0 0.368 3108 0.0097 0.271 31027 All mesonic channels 0.385 31010 1.01 0.260 31029 d1 n 0.67 3107 0.0018 0.15 31026 p 1 n 1 n 0.57 3108 0.015 0.18 31027 All nonmesonic channels 0.64 3108 0.017 0.16 31027 All channels 0.391 31010 1.03 2.56 310210 Expt. @6# 2.64 10.92 20.54 310210 Expt. ~averaged!@11# 2.44 1 0.26 20.22 310210
In @1# the hypertriton state has been determined in a par- application of the O matrix onto the wave function compo- tial wave representation and we refer to @1# for the details of nent of the hypertriton. This is given in Appendix B. Though the mesonic decays are Pauli blocked in heavier hypernuclei, they are the our notation. Here we need only the form Once the amplitudes N^pqauU& are determined, the ma- dominant channels in hypertriton. trix element in ~19! is evaluated by quadrature in the manner 2 2 described in @17# and references therein. The first matrix el- uC 3 H&5( dpp dqq upqa&Ca~pq!, ~21! LIn experiment,a E E the 2-body helium3 channelement and inthe Eq. 3-body~19!, the deuteron plane wave channel impulse are approximation more easy to access. with respect to the nucleons, is also evaluated by the same where p,q are the magnitudes of the Jacobi momenta ~9! and techniques via partial wave decomposition. a denotes the following set of discrete quantum numbers: 8 1 a[~ls!j l I~ jI!J~t0!T. ~22! III. RESULTS S 2D We used a hypertriton wave function based on the Here (ls) j describes the coupling of orbital angular momen- Nijmegen 93 NN potential @18# and the Nijmegen YN inter- tum l and total spin s to the total two-body angular momen- action @2#, which include the L2S transitions. The number 1 of different a quantum numbers ~22!, usually called chan- tum j of the NN subsystem, (l 2 )I the corresponding cou- pling of orbital and spin angular momentum of L to its total nels, used in the solution of the corresponding Faddeev equa- angular momentum I,(jI)J, the resulting jI coupling to the tion is 102. This leads to a fully converged state, which has total angular momentum J, and finally the isospin coupling the proper antisymmetrization among the two nucleons built of the two-nucleon isospin t50 and the isospin zero of the L in. Also the NN and YN correlations are exactly included as particle to total isospin T50. generated by the various baryon-baryon forces ~see @1#!. The Also for the evaluation of the matrix elements in ~19! and SNN part of the state has a probability of 0.5% and will be the solution of the Faddeev equation ~20! we work in a par- neglected. tial wave representation, using a complete set of basis states Let us first regard the decay channel: now for three nucleons. They are again denoted as upqa&N but adding a subscript N to indicate that the Jacobi momenta 3 H p213He. are now from ~8!. Furthermore one has to note that this is a L ! subset of states antisymmetrized in the subsystem of par- 1 1 ticles 1 and 2, thus (l s t) has to be odd. In that channel we use the 3He wave function generated by Now projecting the Faddeev equation into the basis the Faddeev equation with the Nijmegen 93 NN interaction upqa&N and inserting appropriate decompositions of the @18#. The kinematically fixed value of the pion momentum is unity one gets kp5117.4 MeV/c. From ~4! and ~10! we obtain the total decay rate GHe50.973 3 10 9 sec 21 as our theoretical pre- 9 N^pqauU&5( ( N^pqautG0~11P!up8q8a8&NN diction. It should be compared to the value 1.066 0.41310 E E 21 3 sec which we estimate from the total LH decay lifetime 210 3^p8q8a8uOup9q9a9&Ca9~p9q9! t5~ 2.44 1 0.26 20.22! 310 @11#~neglecting the non- mesonic piece!, using the factor 3/2 of Eq. ~3! from isospin 1 ^pqautG Pup8q8a8& ^p8q8a8uU&. and the ratio R ~see the Introduction!. The decay rates and (E N 0 NN lifetimes for both mesons are given in Table I. ~23! Let us now ask more detailed questions in relation to that 3He channel. For the free L decay into p1p2 the proton This is a coupled set of integral equations, with a kernel part, turns out to be polarized as a consequence of the interference which is well known @17# from 3N scattering, and an inho- between the two operators in ~13!. For unpolarized L’s the mogeneous term, whose part left of O is also familiar from polarization of the proton in the direction of the outgoing p2 electron scattering @15#. What is left as a new structure is the is easily evaluated as Focus on the hypertriton lifetime (1)
The lifetime measurements are interest especially in view of the short values from early experiments :
st +0.19 -10 The 1 measurements is (0.95 -0.15)*10 s from helium bubble chamber, by Block et al., presented in the proceeding of Conference on Hyperfragments at St, Cergue, 1963, p.62
+2.2 -10 Results from AGS nuclear-emulsion experiments: (0.9 -0.4)*10 s, Phys. Rev.136 (1964) B1803, from Bevatron and AGS: Phys. Rev.139 (1965) B401 +1.9 -10 2-body (3 in flight, 4 at rest) (0.8 -0.3)*10 s +8.2 -10 2-body combined with 3-body (5 in flight, 18 at rest) (3.4 -1.4)*10 s
Nuclear-emulsion with maximum likelihood procedure, Nucl. Phys. B 16 (1970) 46, +0.35 -10 (1.28 -0.26)*10 s,
9 Focus on the hypertriton lifetime (2)
But NEW measurements gave different values:
Helium bubble chamber from Argonne ZGS: +0.45 -10 (2.32 -0.34)*10 s, PRL 20(1968)819 +0.84 -10 (2.64 -0.52)*10 s, PRD 1(1970)66 +0.62 -10 (2.46 -0.41)*10 s, NPB 67(1973)269
Nuclear-emulsion from Bevatron: +1.10 -10 +2.40 -10 2-body is (2.00 -0.64)*10 s and 3-body (3.84 -1.32)*10 s, +1.10 -10 and a combined of (2.74 -0.72)*10 s PRL20(1968)1383
How about the theoretical understanding of these experimental results?
10 Focus on the hypertriton lifetime (3)
The hypertriton being a loosely-bound nuclear system, its mean lifetime should not be significantly different from that of the free Lambda
Theoretical calculations from Dalitz et al., initially gave a short value and updated later on a larger value close to the free Lambda’s Phys. Lett. 1 (1962) 58 and Nuovo Cimento A 46,786 (1986)
The calculations based on modern 3-body interaction force, the total lifetime is predicted to be 2.56*10-10s, Phys. Rev. C 57 (1998) 1595
The hypertriton lifetime data are not sufficiently accurate to distinguish between model, more precise measurements are needed.
11 Figure 4, x28p6 picas/Science
Results from RHIC-STAR Col.
Previous measurement (before 1973) Previous measurement Use nuclear emulsion or bubble chamber STAR Collaboration, Science 328 (2010)58 AcceptedA events: less than 80 450 2.5 B PR180,1307(1969)
2 cT = 5.5 p 2.7 cm 400 1.4 PRD1, STAR 2010 measurement1.5 2 66(1970) 2 C = 1.08 350 C 1 NPB67, Run4 200GeV MB 22M PRL20, 269(1973) 0.5 2 = 0.08 300 819(1968) C STAR Run4 200GeV3 Central0 23M 10 250 3 4 567 8 9 10 11
Run7Count 200GeV MB cT (cm)68M 200 NPB16, 46(1970) H lifetime (ps)
3 , 150 STAR 2012 measurement free , (PDG) , 100 STAR free , Run10 200GeV3 MB ~223M Dalitz, 1962 102 50 ,H PR136, 6B(1964) Run10 200GeV Central ~199M Glockle, 1998 0 Run10&11 low5 1MB0 15 ~213M20 25 energies decay-length/(B G ) (cm) World data
It is promising to obtain an improved lifetime measurement using the new data
12 Datasets and track selection
Datasets and event statistics
Run10 7.7GeV MB ~4M Run10 11.5GeV MB ~11M Run11 19.6GeV MB ~31M Run11 27GeV MB ~49M Run10 39GeV MB ~118M Run10 200GeV MB ~223M Run10 200GeV Central ~199M Run7 200GeV MB ~56M
Analysis method: secondary vertex finding technique • Identify 3He and π candidate • Find the V0 position from daughters pairing • Plot the invariant mass dis. of daughters • Combinatorial background analysis
13 The largest hypertriton sample
Statistics: Run7+Run10+Run11 MB+Central, ~610M events
bin width 4MeV bin width 2MeV
STAR preliminary STAR preliminary
Signal observed from the data (bin-by-bin counting [2.986,2.996]GeV): 602±63, the largest hypertriton sample ever created Background estimation: rotated background
14 Signal in separate bins −L N(t) = N(0)× e−t/τ = N(0)× e βγcτ
STAR preliminary STAR preliminary STAR preliminary
Measure the hypertriton signal in different L/βγ bins: 2-5-8-11-41cm Combined datasets: run10 7.7,11.5,39,200GeV MB, run10 200GeV central, STAR preliminary run11 19.6, 27GeV MB
15 New hypertriton lifetime result
103 450 PR180,1307(1969) free Λ (pdg) STAR Preliminary Λ(STAR Preliminary) 400 STAR(Science 328 (2010) 58)
Counts PRD1, STAR 2010+2012 combined fit
H 66(1970) H lifetime(ps) 3 Λ 350
STAR 2012 Λ lifetime fit 3 Λ NPB67,
PRL20, 269(1973) H+ 3
3 Λ 0.77 300 819(1968)
cτ=3.69± cm
Counts 0.65 H 102 3 Λ H+
3 Λ 2 STAR 250 χ2=1.58 SCIENCE 328,58(2010) 1 200 χ2=0.58 NPB16, 46(1970) 0 2.5 3 3.5 4 4.5 5 5.5 150 τ(Λ) = 260±1ps decay-length/βγ(cm)
100 10 STAR Preliminary 50 STAR Preliminary PR136,6B(1964) STAR 2012 Result STAR preliminaryDalitz, 1962 0 5 10 15 20 25 30 Y.H. Zhu for STAR Col., QM2012, USA decay-length/βγ(cm) J.H. Chen for STAR Col., HYP2012, Spain 26 STAR 2012 preliminary result: τ =123±22 ±10ps STAR 2010+2012 combined fit: 23 τ =138±20 ps 16 C. Rappold et al. / Nuclear Physics A 913 (2013) 170–184 181 Measurement from HpyHI project
C. Rappold et al. / Nuclear Physics A 913 (2013) 170–184 181 Lambda Hypertriton 4H(Lambda)
3 4 Fig. 9. (Color online.) Profiled likelihood ratio for interval estimation of Λ-hyperon (a1), ΛH(b1),andΛHhypernuclei (c1). Interval estimation for 1 standard deviation is shown on each profiled likelihood ratio. Binned decay length distri- 3 4 butions of the signal region of Λ-hyperon (a2), ΛH(b2),andΛHhypernuclei(c2)withthefittedmodel,whichincluded the exponential function resulted from the unbinned maximum likelihood fit and the background contribution estimated by the sidebands. The black line represents the fitted model, while the blue dotted line represents the contribution of the exponential function.
Table 3 Contributions to the systematic errors. 3 4 Contribution to the systematic Λ ΛH ΛH Vertex Z pos (%) 8 18 14 Primary Vertex (%) 4 4 8 Scaling (%) 11 6 17 Sideband (%) 8 6 10 Total (%) 17 20 25 6 12 3 4 HypernuclearFig. 9. (Colorspectroscopy online.) Profiled likelihood at GSI: ratio forLi interval projectiles estimation of Λon-hyperon C (a1), targetΛH(b1),and at 2Λ HhypernucleiA GeV 4.1.(c1). Study Interval of the systematic estimation uncertainty for 1 standard deviation is shown on each profiled likelihood ratio. Binned decay length distri- presents the hypertriton lifetime measurement3 4 from 2-body channel: butions of the signal region of Λ-hyperon (a2), ΛH(b2),andΛHhypernuclei(c2)withthefittedmodel,whichincluded theThe exponential systematic uncertainty function resulted in deducing from the the lifetimeunbinned was alsomaximum investigated, likelihood as summarized fit and the in background contribution estimated42 Table 3.First,theboundaryrangeofthelongitudinalvertexpositioncutwassettoseveralvalues Nucl. Phys.by the sidebands. A 913(2013)170 The black line represents the fitted model, while the blue dotted line representsτ the= contribution183± of the±37ps in order to deduce the effect of the cut condition. The resultant systematic uncertainty is listed 32 inexponential the second row function. of Table 3 (Vertex Z pos). It is mainly due to the change in the data sam- ple’s size, which influences the fitting procedure. In the lifetime estimation, the primary vertex position is not perfectly defined. Variations on the primary vertex can be studied to determine 17 possible systematic influenceTable to 3 lifetime uncertainty. As explained the primary vertex position is restrained by the beamContributions hit position on to the the TOF-start systematic detector, errors. however it does not define well enough the vertex position in 3-dimensions. By means of Monte Carlo simulations for the miss- 3 4 Contribution to the systematic Λ ΛH ΛH Vertex Z pos (%) 8 18 14 Primary Vertex (%) 4 4 8 Scaling (%) 11 6 17 Sideband (%) 8 6 10 Total (%) 17 20 25
4.1. Study of the systematic uncertainty
The systematic uncertainty in deducing the lifetime was also investigated, as summarized in Table 3.First,theboundaryrangeofthelongitudinalvertexpositioncutwassettoseveralvalues in order to deduce the effect of the cut condition. The resultant systematic uncertainty is listed in the second row of Table 3 (Vertex Z pos). It is mainly due to the change in the data sam- ple’s size, which influences the fitting procedure. In the lifetime estimation, the primary vertex position is not perfectly defined. Variations on the primary vertex can be studied to determine possible systematic influence to lifetime uncertainty. As explained the primary vertex position is restrained by the beam hit position on the TOF-start detector, however it does not define well enough the vertex position in 3-dimensions. By means of Monte Carlo simulations for the miss- 3 Hand3 HproductioninPb–Pbcollisions ALICECollaboration Λ Λ¯
detecting their mesonic decay (3 H 3He +EUROPEAN)and(3 H 3He + +)viathetopologicalidentification ORGANIZATION FOR NUCLEAR RESEARCH Λ π− Λ¯ π → → 3 3 + of secondary vertices and the analysis of the invariant mass distributions of ( He+π−)and(He+ π ) pairs. The analysis was done using Pb–Pb collisions at √sNN =2.76TeVtakenin2011.Theeventswere collected with an interaction trigger requiring a signal in both V0-A and V0-C. Only events with a primary vertex reconstructed within 10 cm, along the beam axis, from the nominal position of the ± interaction point were selected. The analysed sample, collected with two different centrality trigger configurations corresponding to the 0–10% and 10–50% centrality intervals, contained approximately 20 106 and 17 106 events, respectively. ×3 × The ΛHcanbeidentifiedviatheinvariantmassofitsdecayproductsand,sinceithasalifetimesimilar to the free Λ (cτ 8cm),inmostcasesitispossibletoidentifyitsdecayuptoafew cm away from ∼ the primary vertex. The decay vertex was determined by exploiting a set of geometrical selections: i) the CERN-PH-EP-2015-105 Distance of Closest Approach (DCA) between the two particle tracks identified using dE/dx in the TPC 30 April 2015 3 as He and π,ii)theDCAoftheπ± tracks from the primary vertex, iii) the cosine of the angle between the total momentum of the decay pairs at the secondary vertex and a vector connecting the primary vertex and the secondary vertex (pointing angle), and iv) a selection on the proper lifetime (cτ)ofthecandidate. An additional selection on the 3 H(3 H) rapidity ( y <0.5)3 was applied. 3 Λ Λ¯ | | 3 H and H3 production+ in Pb–Pb collisions at √sNN =2.76TeV Figure 1 shows the invariant mass distribution of ( He,Λπ−)ontheleftand(Λ¯ He,π )ontherightfor events with 10–50% centrality in the pair transverse momentum range 2 p < 10 GeV/c.Inorder ≤ T to estimate the background, for each event the π track detected at the secondary vertex was rotated 20 times by a random azimuthal angle. The shape of the corresponding (3He, π)invariantmassdistribution was found to reproduce the observed background outside the signal region. The data points were fitted ALICE Collaboration∗ with a function which is the sum of a Gaussian and a third degreepolynomial,usedtodescribethe signal and the background, respectively. The background wasnormalizedtothemeasuredvaluesinthe 3.01 – 3.08 GeV/c2 region. The fit to the background distribution was used to fix the parameters of the polynomial in the combinedMeasurement fit. from LHC-ALICE Abstract
3 3 ) )
Hand HproductioninPb–Pbcollisions2 ALICECollaboration2 Λ Λ¯ c c 3 3 Data The production of the hypertritonData nuclei Hand HhasbeenmeasuredforthefirsttimeinPb–Pb 90 90 Λ Λ¯ ALICE 10-50% Background ALICE 10-50% Background 80 80 Pb-Pb s = 2.76 TeV collisions at √sNN =2.76TeVwiththeALICEexperimentatLHCenergies.Thetotalyield, SourceNN Combined Value Fit Pb-Pb sNN = 2.76 TeV Combined Fit 70 70 5 Signal extraction method 9% dN/dy B.R. 3 H 3He, = (3.86 0.77(stat.) 0.68(syst.)) 10− in the 0–10% most central col- 3 3 - (Λ π−3) 3 + ALICE Collaboration 60 ΛH → He + π ×60 → H → He + π ± ± × Tracking Efficiency 10% lisions, is consistent withΛ the predictions from a statistical thermal model using the same temperature 50Absorption 12% 50 arXiv:1506.08453 Entries/(2.5 MeV/ Entries/(2.5 MeV/ 40Total 18% as for the40 light hadrons. The coalescence parameter B3 shows a dependence on the transverse momen- Table 2: Summary of systematic30 uncertainties for the determination of the proper lifetime30 of 3 H+3 H. 3 3 3 tum, similarΛ Λ¯ to the B2 of deuterons and the B3 of He nuclei. The ratio of yields S3 = ΛH/( He Λ/p) 20 20 × was measured to be S3 =0.60 0.13 (stat.) 0.21 (syst.) in 0–10% centrality events; this value is 10 10 2 ≤ p < 10 GeV/c 2 ≤ p < 10 GeV/c ± ± systematic uncertainties. T compared to differentT theoretical models. The measured S3 is fully compatible with thermal model 0 +1.6 3 Hand3 HproductioninPb–Pbcollisions0 ALICECollaboration The slope of the fit results in a proper2.98 decay 2.99 length 3 of c 3.01τ= 5 3.02.4 1. 3.032(stat 3.04.) 1.0(Λsyst.) cm.Λ¯ To2.98 determine 2.99 3 3.01 3.02 3.033 3.04 +54 3 - ±2 predictions. The measured3 + Hlifetime,2 τ = 181 (stat.) 33(syst.) ps is compatible within 1σ the statistical error the Minuit processor MINOSInvariantembedded mass ( inHe, the−π ROOT)(GeV/ frameworkc ) [29]wasused.Invariant mass ( ,He π Λ)(GeV/c ) 39 ! with" the world average value. − ± 3 3 + Figure 1: Invariant mass of ( He,π−)(left)and(He,π )(right)foreventswith10–50%centralityinthepair
2 ) pT < 10 GeV/c interval. The data points are shown as full circle, while the squares represent the background -1 ≤ 500 Free Λ (PDG) distribution as described in the text.ALICE The curve represents the function used to performR. E. Phillips the and fit J. and Schneps used to evaluate (cm the background) and the raw signal. PR 180 (1969) 1307 3 Pb-Pb s = 2.76 TeV H World Average N ct NN G. Keyes et al. Λ
d 400 d( 102 PRD 1 (1970) 66 In the 0–10% most central collisions, a signal was extracted in three transverse momentum intervals STAR Collaboration (2 p < 4GeV/c,4 p < 6GeV/c,6 p < 10 GeV/c),forboth3 Hand3 H. In the 10–50% cen- Science 328 (2010)58 ≤ T ≤ T ≤ T 300 Λ Λ¯ ALICE 3
Hypertriton Lifetime (ps) 200 G. Keyes et al. 10 NPB 67(1973)269 HypHI Collaboration 1.6 100 cτ = (5.4 ± ± 1.0) cm G. Bohm et al. NPA 913(2013)170 1.2 NPB 16 (1970) 46 R. J. Prem and P. H. Steinberg 0 5 10 15 20 25 PR 136 (1964) B1803 ct (cm) 0
Figure 3: Measured dN/d(ct)distributionandanexponentialfitusedtodeterminethelifetime. The bars and boxes 18 3 are the statistical and systematic uncertainties, respectively. Figure 4: ΛHlifetime(τ)measuredbyinthisanalysis(reddiamond)comparedwithpublished results. The band 3 +18 represents the world average of ΛHlifetimemeasurements τ = 215 16 ps, while the dashed line represent the −
arXiv:1506.08453v1 [nucl-ex] 28 Jun 2015 lifetime of as reported by the Particle Data Group [30]. The lifetimes of light Λ-hypernuclei (A 4) are expected to be very similar to that of theΛ free Λ,ifthe ! " ≤ 3 Λ in the hypernucleus is weakly bound [31]. The measured lifetimes of light hypernuclei such as ΛH[9, charged-particle densities dN /dη .Forcentral(0–10%)collisions dN /dη =1447 39, while for 32–38]arenotknownaspreciselyastheΛ lifetime, and theoretical predictions [31,39–46]arescattered⟨ ch ⟩ ⟨ ch ⟩ ± semi-central (10–50%) dN /dη =575 12. The ratio over a large range, too. Recently, a statistical combinationoftheexperimentallifetimeestimationsof⟨ ch ⟩ ± 3 Havailableinliteraturewaspublished,resultinginanaverage value τ = 216+19 ps [47]. Λ 18 3 H+3 H +54 − (Λ Λ¯ )(0 10%) With the present data, a lifetime of = 181 (stat.) 33(syst.) ps has been obtained. It is compared − τ 39 3 H+3 H − ± ! " (Λ Λ¯ )(10 50%) with the previously published results in Figure 4.Ourresult,togetherwiththepreviousones,wasused# − $ = 1.34 0.35(stat.) 0.24(syst.) (1) ! " dNch/dη (0 10%) ⟨ ⟩ − ± ± to re-evaluate the world average of the existing results using the same procedure as described in [47]. dNch/dη (10 50%) +18 ⟨ ⟩ − The obtained value, τ = 215 16 ps ,isshownasabandinFigure4.Theresultobtainedinthisanalysis% & is compatible with the computed− average. is compatible with unity within 1 σ.The3 H(3 H) production scales with centrality like the charged- ! " Λ Λ¯ particle production. 4Comparisonbetweenexperimentalyieldsandtheoreticalmodels 3 5 3 5 Centrality dNch/dη ΛH B.R. 10− ¯ H B.R. 10− 3 3 3 3⟨ ⟩ × × Λ × × The product of the total integrated yields and the B.R. of the H ( He + π−)decayfor Hand Hfor Λ → 0–10%Λ Λ1447¯ 39 3.86 0.77(stat.) 0.68(syst.) 3.47 0.81(stat.) 0.69(syst.) two centrality classes (0–10% and 10–50%) are reported in Table 3.Thesystematicuncertaintiesalso± ± ± ± ± include the contribution due to the low pT extrapolation as describedc in Section20153. CERN10–50% for575 the12 benefit1.31 of0.37 the(stat. ALICE) 0.23(syst Collaboration..) 0.85 0.29(stat.) 0.17(syst.) It is possible to compare the production yields at different centralities⃝ by scaling them according to the± ± ± ± ± Table 3: Total integrated yields times the B.R. of the 3 H (3He + π )decay,for3 Hand3 HinPb–Pbcollisions Reproduction of this article or parts ofΛ → it is allowed− asΛ specΛ¯ ified in the CC-BY-4.0 license. at √s =2.76TeVfordifferentcentralityclassesin y < 0.5. For each centrality interval the average dN /dη NN | | ⟨ ch ⟩ 6 is also reported [18]. ∗See Appendix A for the list of collaboration members
4.1 Comparison between thermal models and experimental yields Since the decay branching ratio of the 3 H 3He + π was estimated only relative to the charged-pion Λ → − channels [37], the corresponding value (B.R.=35%) provides an upper limit for the absolute branching ratio. On the other hand, a theoretical estimation for the 3 H 3He + π decay branching ratio, Λ → − which also takes into account decays with neutral mesons decays, gave a B.R.=25% [31]. Assuming apossiblevariationontheB.R.intherange15–35%,weshowinFigure5 acomparisonofourresult with different theoretical model calculations [1, 48, 49]. The measured dN/dy B.R. is shown as a ×
7 The physics for a short lifetime?
Revisit the previous theoretical calculation:
A statistical combination analysis of the experimental lifetime data give average of +19 216 -18 ps for hypertriton, indicated that the lifetime of light hypernuclei can be shorter than the free Lambda’s Rappold, Saito et al., PLB 728 (2013) 543
Revisit binding energy (130 ± 50 kev), STAR exp. 3-body analysis have significant progress.
Physics implication:
19 RAPID COMMUNICATIONS
RAPID COMMUNICATIONS
3 C. RAPPOLD et al. SEARCH FOR EVIDENCE OF !n BY ... PHYSICAL REVIEW C 88, 041001(R) (2013)PHYSICAL REVIEW C 88, 041001(R) (2013) and identify hypernuclei by means of the invariant mass reasonable, and the 3He contamination in the 4He identifica- method [22]. tion was estimated to be 1.7%, while the contamination of 4He WeperformedanexperimentwithThe6Li projectiles second point at 2A wasGeV also investigated,into the identification i.e., whether of 3He the was 1.8%two-body [28]. Additionally, decay. Therefore, for we calculated the contribution of 6 4 5 with an intensity of 3 10 ion per second bombarding on a the hydrogen isotopes and π − mesons the correlation between 12 × observed d π − and t 2π − signals can be originated from !He and !He by estimating the upper limit of their production carbon ( C) graphite target with a thickness+ of 8.84 g/cm+ . the estimated3 momentum4 4 5 and the velocity β calculated from The data collection occurred duringthe athree-body period of 3.5 and days four-body with decaysthe time-of-flight of H, H, measurementHe, He, wascross employed, sections as from detailed the experimental data. Then we assessed 1 ! ! ! ! an integrated luminosity of 0.066 pb−6 .Themaingoalofthe in Ref. [28]. The selection cuts used for the deuteron and experiment was to identify and studyand !He, the production that are likely and to the be producedtriton determination in the reaction. required A mis- theiran respective order of magnitude momenta higher to to their production cross sections decay of light hypernuclei, 3Hand 4H, as well as ! hyperons fall in the ranges 4.3 6.5 and 6.5 10.0 GeV/cand ! reconstruction! of these decay modes may occur by observing∼ to estimate∼ the worst-case scenario. One can remark that the in order to demonstrate the feasibility of such hypernuclear their respective masses to be within 0.935
Counts / (2.8 MeV) 20 measurements of charged particles across a large acceptance Counts / (2.8 MeV) dipole magnet. The tracking system(a forb vertexingc then wasb placedd e)inwhichthesecondtwo-body S 1dibaryon,excluding(Data!p) from possibility. HIRES In Col. the review → + → + 0 0 = − in front of the dipole magnetdecay around has the a expected small relative decay momentum2.04 transfer, 2.06 i.e.,2.08 small 2.1 from2.04 Dalitz 2.06 and 2.08 Downs 2.1 [25Excluded], a !-nucleon the (Lp) bound state is volume of hypernuclei. Behind the magnet, two separated Mass (GeV) Mass (GeV) candidates arms of the detection apparatus were situated in such a way Q value, the invariant mass of two out of three(b1) particles t+π− discarded. Those(b2) considerations t+π− may PLB exclude 687, 31(2010) the hypothesis 10 < Z < 30 cm 60 2 < Z < 30 cm to measure disjointedly positively and negatively charged 80 − − PRD 84, 032002(2011) particles. (c d or c e)doesproduceapeaksignalintheobservable 50that the d π − final state originates from a n! bound The four-vectors of the detected particles and fragments 60 40 + + + A possible interpretation were determined after the particlespectrum. identification Because based of the on small Q value in the second decay, 30state. 40 might be the two- and tracking across the magnet as well as measurements of the 20 the daughter particles or fragments20 (d and e) have very similar Apossibleinterpretationfortheobservedthree-body decayst of πan− and Counts / (2.8 MeV) time-of-flight and the energy deposit with the hodoscope MeV) (2.8 / Counts 10 unknown bound state+ of 2 walls. After the decay vertexmomentum finding, the directions invariant mass as their mother0 (b)inthelaboratory 0d π − final states might be the two- and three-body decays of final states of interest was calculated, and a lifetime 2.98 3 3.02 3.04 +2.98 3 3.02 3.04 neutrons associated with a estimation was inferred based onframe the observed after the Lorentz decay vertex boost of the center-of-massMass (GeV) frame of the of an unknownMass (GeV) bound state ofLambda: two neutrons associated with position. The feasibility of the experimental method was 3 3 3 FIG. 1. (Color online) Invariant mass distributions of d π − already demonstrated by observingmother!, (3bH,). and 4H, whose !, !n,via!n t +π − and !n t∗ π − d n π −, ! ! final-state candidate in panels (a1) and (a2) and of t π − in panels physics results are discussed in Ref. [28]. In this Rapid + → + → + → + + The possible contributions(b1) to the and (b2).d π Panels− and (a1)t andπ (b1)− arerespectively. for 10 cm 041001-5 Concluding remarks Measurements of hypertriton lifetime have be interesting project in the field. Independent exp. present different results. Several theoretical interpretations have achieved in the field and tent to conclude a value close to the free Lambda’s New and precise measurements (>600 signals) from STAR Col. in the Relativistic Heavy-Ion Collider give a short value: 26 τ =123±22 ±10ps Data from HypHI from GSI fixed target exp. and from LHC-ALICE exp. show the short lifetime value: 42 54 τ =183±32 ±37ps τ =181±39 ±33ps The discrepancy among different exp. is still there, the hypertriton lifetime is still a puzzle. New measurements from STAR Col. should shed new light on the puzzle. 21 Topological Map Λ (V)rtual Topological3 Map Topological MapH d + p + ⇡ ! d Outlook⇤ " p Topological Map Λ (V)rtual ! Λ (V)rtual v0123 : Mid-point of DCA 1 to 2 Λ (V)rtual 3v012 :H Mid-pointd of+ DCAp 1+ to 3⇡ ! d H⇤ d + p + ⇡ " p ! d ⇤v0123 : Mid-point! of DCA 2 to 3 p " d ⇤H! d + p + ⇡ ! ! v012 : Mid-point of DCA 1 to 2 " p v012 : Mid-point! of DCA 1 to 2 Wev012 assume : Mid-point the three of DCA points 1 toabove 3 to be v012thev012 :three Mid-point : Mid-point vertexes of DCAof ofDCA 1a to triangle, 1 3 to 2 then: v012 : Mid-point of DCA 2 to 3 v012v012 : Mid-point : Mid-point of DCAof DCA 2 to 1 3 to 3 ! ! v012v0123 : Mid-point : Centre of of DCA gravity 2 to of 3 the triangle Lengt( We assume the three points above to be ! We assume the three points above to be the Thenthreethe three vertexeswe have:vertexes of a triangle, of a triangle, then: then: dca1 We assume the three points above to be Decay dca3 theI. threeYifei Xuvertexes (SINAP/BNL) of a triangle, : ongoing then: effort of v0123dca1,v0123 : Centredca2, : Centre dca3,of gravity ofDecay gravity of the Length oftriangle the triangle Lengt( Lengt( reconstructing hypertriton via 3-body Thendca1to2,v0123 we have: : Centre dca2to3, of gravity dca1to3...... of the triangle dca1 Thendecay, we have:lifetime, branching ratio etc. Decay Lengt( dca1 Decay dca3 Then we have: dca3 dca1 dca1, dca2, dca3, Decay Length Decay II.dca1, Topical dca2, sessions dca3, Decay on “Hypertriton Length lifetime” dca1to2, dca2to3, dca1to3...... 4 th Yifeith Xu dca3 dca1,dca1to2,will dca2, be dca2to3,held dca3, during Decay dca1to3...... the Length 12 HYP (Sep. 7 th dca1to2,-12 ,dca2to3, 2015, Tohoku dca1to3...... Uni. Japan) Primary Vertex 4 Yifei Xu 4 Yifei Xu 4 Yifei Xu 22