First Observation of an Anti-Hypernucleus

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

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Λ3 H


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


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.


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)


Relativistic Heavy Ion Collider (RHIC)

STAR


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


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


)/

/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


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


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


Signal counts: 70±17

Mass: 2991±1±2 MeV

Width: 2.5 MeV

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Λ3 H Signal


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(Λ3H) =182−45

+89 ± 27 psWe measure Λ = 267±5 ps PDG value is Λ = 263±2 ps

Lifetime Measurement


Production Rate

)/)(n/n)(p/p(HH33 / ΛΛ∝ΛΛ

Coalescence =>

0.45 ~ (0.77)3

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3H e/3

He ∝ (p / p)2(n / n )

Favoring coalescence


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.

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=182−45+89 ± 27€

Λ3 H

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Λ3H

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Λ3 H

Conclusions

These ratios favor coalescence production of light (hyper-)nuclei.


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-


BackUp


Hypernuclei and Neutron stars

S=-1

S=-2

S=0

J.M. Lattimer and M. Prakash, Science 304, 536 (2004)


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


Combined hypertriton and anti-hypertriton signal : 225±35;

It provides a >6 significance for discovery.

Combined Signal


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.

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S3 = Λ3H

3He ×Λ

p


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First Observation of an Anti-Hypernucleus