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X. Dong 1May 10, 2010 NSD Monday Morning Meeting
First Observation of an Anti-Hypernucleus
Xin Dongfor the STAR Collaboration
Science 328, 58 (2010)
May 10, 2010 NSD Monday Morning Meeting X. Dong 2
Introduction
Measurements on (anti-)Hypernuclei at STAR
• and signal
• Lifetime
• Production rate
Conclusions and Outlook
Outline
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Λ3H
€
Λ3 H
May 10, 2010 NSD Monday Morning Meeting X. Dong 3
Hypernucleus
Hypernucleus production in the laboratory is an Ideal probe to study the Y-N interaction; provides information on Equation-of-State of neutron stars.
Configurations of neutron stars: (depending on the Y-N interaction length)Hyperons / Meson condensates / Strange quark matter
J.M. Lattimer and M. Prakash, Science 304, 536 (2004)
What is a hypernucleus?
Nucleus which contains at least one hyperon in addition to nucleons
Hypernucleus of lowest AHypertriton
May 10, 2010 NSD Monday Morning Meeting X. Dong 4
In high energy heavy-ion collisions: (AGS/BNL … )
– (anti-)nucleus/hypernucleus production by coalescence
(wave functions overlap between two or more particles/nuclei in phase space and they re-combine into one particle/nucleus)
At low energies (cosmic ray / Kaon beam stopped in nuclear emulsion or bubble chamber)
hypernucleus production via
– Λ or K capture
– direct strangeness exchange reaction.
How are (anti-)hypernuclei produced
First hypernucleus discovered by Danysz and Pniewski in 1952 from a cosmic ray interaction in nuclear emulsion.
M. Danysz and J. Pniewski, Phil. Mag. 44 (1953) 348
No anti-hypernucleus has been observed before.
May 10, 2010 NSD Monday Morning Meeting X. Dong 5
Particle/Nucleus production at RHIC
Anti-particles and particles are nearly equally produced.Chance to find anti-hypernuclei at RHIC!
3He/3He ratio
10-11 (AGS)
10-3 (SPS)
0.5 (RHIC)
STAR white paper, NPA 757, 102 (2005)
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Relativistic Heavy Ion Collider (RHIC)
STAR
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Solenoidal Tracker At RHIC (STAR)
Time Projection Chamber
BEMC
BEMC
Time Projection Chamber (TPC): ||<1, 2 in azimuthTracking, particle identification via dE/dx
Full Barrel TOFin year2010
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3ΛH mesonic decay,
B.R. 25%, c ~ 7 cm
Data-sets used, Au+Au 200 GeV
~ 67M year 2007 minimum-bias
~ 22M year 2004 minimum-bias
~ 23M year 2004 central,
|VZ|<30cm
−Λ
+Λ
+→
+→
HeH
eHH33
33
Secondary vertex finding technique
Topological Recon. of (anti-)Hypernuclei
TPC track projection error ~ 0.5 cm
Secondary decay
May 10, 2010 NSD Monday Morning Meeting X. Dong 9
)/
/ln( th
dxdE
dxdEz =
Select pure 3He sample: 3He: 5810 counts
anti-3He: 2168 counts
Selection condition: |z|<0.2, p>2 GeV/c
3He & anti-3He selection
K
pd
t
3He
May 10, 2010 NSD Monday Morning Meeting X. Dong 10
refmult = 217vertexX = -0.17vertexY = 0.46vertexZ = -21.60
mass = 2.99164 pt = 4.60He3pt = 4.34pionpt = 0.28
runid = 5017004 eventid = 1844
decayL = 21.25 dca = 0.50 dca1to2 = 0.48 ……
A Candidate Event Display
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Signal counts: 157 ± 30
Mass: 2989 ± 1± 2 MeV
Width: 2.5 MeV
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Λ3H Signal
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Λ3 H =Λ
3H×3 H e/3 HeExpected anti-hypertriton yield: = 59±11
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Signal counts: 70±17
Mass: 2991±1±2 MeV
Width: 2.5 MeV
€
Λ3 H Signal
May 10, 2010 NSD Monday Morning Meeting X. Dong 13
€
(Λ3H) =182−45
+89 ± 27 psWe measure Λ = 267±5 ps PDG value is Λ = 263±2 ps
Lifetime Measurement
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Production Rate
)/)(n/n)(p/p(HH33 / ΛΛ∝ΛΛ
Coalescence =>
0.45 ~ (0.77)3
€
3H e/3
He ∝ (p / p)2(n / n )
Favoring coalescence
May 10, 2010 NSD Monday Morning Meeting X. Dong 15
The measured lifetime is ps, consistent with free Λ lifetime (263 ps) within uncertainty.
First observation of an anti-hypernucleus with 70 candidates, with significance ~4.
The measured / ratio is 0.49±0.18±0.07, and
3He / 3He is 0.45±0.02 ±0.04.
€
=182−45+89 ± 27€
Λ3 H
€
Λ3H
€
Λ3 H
Conclusions
These ratios favor coalescence production of light (hyper-)nuclei.
May 10, 2010 NSD Monday Morning Meeting X. Dong 16
Outlook
Hypertriton Data samples with larger statistics to allow precision measurements on
• Lifetime - binding energy• Production rate - baryon-strangeness correlation
Other channels, e.g. Λd+p+
Search for other hypernuclei: 4ΛH, double Λ-hypernuclues
Search for anti-
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BackUp
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Hypernuclei and Neutron stars
S=-1
S=-2
S=0
J.M. Lattimer and M. Prakash, Science 304, 536 (2004)
May 10, 2010 NSD Monday Morning Meeting X. Dong 19
Topological Recon. of Weak Decays
TPC track projection error ~ 0.5 cmWeak decay particles c ~ 2 - 10 cm
Ks0 Λ
STAR, PRC 77 (2008) 54901
May 10, 2010 NSD Monday Morning Meeting X. Dong 20
Combined hypertriton and anti-hypertriton signal : 225±35;
It provides a >6 significance for discovery.
Combined Signal
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In the coalescence model:
A=2: baryon density <>
A=3: <2>, <Λ>
H. Sato Phys. Lett. B 98 (1981) 153
Yield to measure B/S Correlation
Baryon-strangeness correlation via hypernuclei:
a viable experimental signal to search for the onset of deconfinement.
model: PLB 684 (2010) 224
Baryon-strangeness correlation:
PRL 95 (2005) 182301,
PRC 74 (2006) 054901,
PRD 73 (2006) 014004.
€
S3 = Λ3H
3He ×Λ
p
May 10, 2010 NSD Monday Morning Meeting X. Dong 22
World Map of Hypernucleus Labs