strangeness measurements with the experiment

Gábor Veres Strangeness in Quark Matter ‘06, UCLA, March 27, 2006

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Strangeness measurements with the Experiment

Gábor VeresEötvös Loránd University, Budapest, Hungary

Massachusetts Institute of Technology, Cambridge, USAfor the Collaboration

Strangeness in Quark Matter ’06UCLA, California, March 27, 2006


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Collaboration (March 2006)

Burak Alver, Birger Back, Mark Baker, Maarten Ballintijn, Donald Barton, Russell Betts, Richard

Bindel,

Wit Busza (Spokesperson), Zhengwei Chai, Vasundhara Chetluru, Edmundo García, Tomasz

Gburek, Kristjan Gulbrandsen, Clive Halliwell, Joshua Hamblen, Ian Harnarine, Conor Henderson,

David Hofman, Richard Hollis, Roman Hołyński, Burt Holzman, Aneta Iordanova, Jay Kane,

Piotr Kulinich, Chia Ming Kuo, Wei Li, Willis Lin, Constantin Loizides, Steven Manly, Alice

Mignerey,

Gerrit van Nieuwenhuizen, Rachid Nouicer, Andrzej Olszewski, Robert Pak, Corey Reed,

Eric Richardson, Christof Roland, Gunther Roland, Joe Sagerer, Iouri Sedykh, Chadd Smith,

Maciej Stankiewicz, Peter Steinberg, George Stephans, Andrei Sukhanov, Artur Szostak,

Marguerite Belt Tonjes, Adam Trzupek, Sergei Vaurynovich, Robin Verdier, Gábor Veres,

Peter Walters, Edward Wenger, Donald Willhelm, Frank Wolfs, Barbara Wosiek, Krzysztof

Woźniak, Shaun Wyngaardt, Bolek Wysłouch

ARGONNE NATIONAL LABORATORY BROOKHAVEN NATIONAL LABORATORYINSTITUTE OF NUCLEAR PHYSICS PAN, KRAKOW MASSACHUSETTS INSTITUTE OF TECHNOLOGY

NATIONAL CENTRAL UNIVERSITY, TAIWAN UNIVERSITY OF ILLINOIS AT CHICAGOUNIVERSITY OF MARYLAND UNIVERSITY OF ROCHESTER


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Outline

Identification of strange particles in PHOBOS slow particles stopping in the active part of the detector dE/dx in the Si spectrometer Time Of Flight measurement reconstruction of mesons

Identified particle spectra and ratios in Au+Au at 62.4 GeV

Identified particle spectra and ratios in d+Au at 200 GeV

Connections to net baryons and baryon transport

New techniques to reconstruct the meson at low pT


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The PHOBOS detector

137000 Silicon Pad Channels

1m

Spectrometer

Octagon

Vertex

Ring Counters

Paddle TriggerCounters

Čerenkov Counter

ProtonCalorimeter

Calorimeter

Time of Flight

Spectrometertrigger


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PID Capabilities of PHOBOS

pT (GeV/c)0.03 0.5 5.0

Stoppingparticles dE/dx TOF

Particle ID from low to high pT

Eloss (MeV)1 2 3 4 50

p (GeV/c)

30

40

50

60

70

1/v

(ps/

cm)

PRC 70 (2004) 051901(R)

p+p

K +K + -

++-

p (GeV/c)

K+

K–

mK+K–


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Particle identification at low pT

10 cm

z

-x

70 cm

PHOBOS Spectrometer:• 16 layers of silicon wafers• fine pixelization, precise deposited energy measurement• collision vertex close (10 cm) to spectrometer• near mid-rapidity coverage• dipole magnetic field of 2T at maximum, but small at the first layers

• pT > 0.2 GeV/c track curvature in B field p,charge, dE/dx in Si mass ToF • pT = 0.03 – 0.2 GeV/c low-p particles stop in silicon wafers p, mass

B field negligible no charge identification


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Mass measurement

(‘energy-range’ method):

Cuts on dE/dx per plane ”MASS HYPOTHESIS”

Search for particles ranging out

in the 5th spectrometer plane:

A B C D E

dE

/dx

Ek= 8 MeV

p Ek=21 MeV

K Ek=19 MeV Cuts on Eloss (Ek=kinetic energy)

”MOMENTUM HYPOTHESIS”

• Eloss = dE (kinetic energy)• <dE/dx> Eloss m

(1/2) ( m2)

Corrections• acceptance • efficiency • background

silicon plane

Finding very low pT particles

0 10 20 Z [cm]

X[c

m]

AB

CD

EF

Be pipe .

.


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Test of the method:

Reconstruction of lowmomentum MC particles

Eloss [MeV]

<d

E/d

x>

Elo

ss [

10-3

Ge

V2/c

m]

(+,)

(K+,K–)

(p,p)

MC

Measuring particle mass at low pT

Eloss [MeV]

(+,)

(K+,K–)

(p,p)

Au+Au sNN=200 GeV 15% most central

DATA

PRC 70 (2004) 051901(R)

<d

E/d

x>

Elo

ss

[1

0-3G

eV

2/c

m]


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Medium pT: PID via dE/dx in the Si

0 0.5 1 1.5p [GeV/c]

5

10

15

0

dE

/dx

[M.I

.P.]

dE/dx

• Calibrated and well studied dE/dx measurement in the Spectrometer

• Particles separated in the 1/2 region of the Bethe-Bloch function

• Track-by-track identification: –K: up to p0.4 GeV/c K–p: up to p0.8 GeV/c

• Yields can be extracted using fits up to p1.5 GeV/c for protons (PID in the statistical sense)

A realistic line-shape is used: natural tail to higher dE/dx (Landau-fluctuations)

• the mean positions are given by the Bethe-Bloch formula• the width follows the empirical relation: dE/dx ~ (dE/dx)0.9 • only the amplitudes are free fit parameters

K

p

d


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Medium pT: PID via Time of Flight• Improved time measurement (new T0 detectors, better cables and correction for timing drifts)

• Particles separated: 1/ vs. p (even better constraint than dE/dx)

• Track-by-track identification: –K: up to p1.3 GeV/c K–p: up to p2.3 GeV/c

• Yields can be extracted using fits up to p5 GeV/c for protons (PID in the statistical sense)

0 1 2 3 4 5p [GeV/c]

50

45

40

35

30

60

55

1/v

[ps

/cm

]

K

p

d

A realistic line-shape is used reflecting the time resolution• the mean positions are given by 1/v=m2/p2+1/c2

• the width: same for all species (time resolution does not depend on mass) • only the amplitudes are free fit parameters


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Acceptance of the PID

Time of Flight + tracking

Very low pT particles dE/dx in the Si Spectrometer

p=const.lines

different bending directions


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Synthesis of dE/dx and TOF data

• Data merged from both detectors and bending directions

• Small but finite rapidity range

• Additional error estimated from difference between linear and constant fit

0 0.5 1.0 1.5y0

1

2

3

4

pT [

GeV

/c]

0 0.5 1.0 1.5y

3

2

2.5

d2N

/ 2p

Td

pTd

y [ G

eV–

2c2

]

Proton data pointson the pT-y plane

Invariant yieldsat pT=0.69 GeV/c

PHOBOSpreliminary

PHOBOSpreliminary


13Identified spectra in Au+Au at 62.4 GeV

Corrections:• Acceptance, efficiency• Occupancy in the spectrometer• Feed-down from weak decays (DCA fits and estimates)• Ghosts (fakes), secondaries• Dead channels in the detector• Momentum resolution

Centrality bins:0-15% Npart=294±1015-30% 160±1030-50% 78±8

• Smooth centrality dependence• Large baryon/meson ratios at high pT


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Antiparticle to particle ratios

Results at 62.4 GeV fit smoothly into the energy evolution of the antiparticle/particle ratios

(ratios are integratedover the accessiblepT and y range in centralcollisions)


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Fraction of protons among all hadrons

p>++K+ line

At pT2.5–3 GeV/c, baryons become dominantin central Au+Au collisions at 62.4 GeV

PHOBOS Preliminary

Au+Au 62.4 GeV

(p>– or p>K–) line


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Open symbols: not feeddown-corrected

• central A+A collisions• data from: E802 (4%) NA44 (3.7%) NA49 (5%) STAR (5% and 6%) PHENIX (5%) PHOBOS (15%)

PHOBOS Preliminary

PHOBOS Preliminary

p/p

• p/+ ‘crossing’ is there at all measured energies• ‘crossing’ pT value increases with energy• contribution of ‘pair produced’ protons grows with energy• p/p at high pT grows with energy since pT spectra of p and p are similar

pT where p/+=1, as a function of s


17Low pT spectra of identified particles in Au+Au collisions

at 62.4 GeV

Blast wave fit parameters: 0-15%: Tfo = 99 MeV, <T>=0.5115-30%: Tfo = 98 MeV, <T>=0.5130-50%: Tfo = 97 MeV, <T>=0.49

d2 N

/2p

Td

pTd

y [

Ge

V2

c2 ]

d

2 N/2

pTd

pTd

y [

Ge

V2

c2 ]

d2 N

/2p

Td

pTd

y [

Ge

V2

c2 ]

Fit to the high pT part and extrapolating to low pT


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Low pT identified spectra, Au+Au at 200 GeV

No enhancement for pions at low pT observed Flattening of (p+p) spectra down to very low pT

(consistent with transverse expansion of the system)

• Blast Wave fit: Tfo= 99 MeV <T> = 0.54

T= 229 MeV for (++–) 293 MeV for (K++ K–) 392 MeV for (p + p)

mT =pT2+mh

2

B-E fit

BWF fit

d2N

/2p

Td

pTd

y [G

eV

–2c2

]

d2N/2mTdmTdy=A[exp(mT/T)±1]–1

• Bose-Einstein fit:

PHOBOS, PR C70, 051901 (R) (2004) PHENIX, PR C69, 034909 (2004)


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Low pT identified spectra, d+Au at 200 GeV

pT range:(++-) : > 0.03 GeV/c

(K++ K-): > 0.09 GeV/c

(p + p) : > 0.14 GeV/c

Background corrections: very low-pT:

(++-) : 20%

(K++ K-): 10%

(p + p) : 20% intermediate pT

(p + p) : 20%

Systematic uncertainties: very low pT: 30% intermediate pT : 15%

Event selection: described in PRL 93, 082301

d2N

/2p

Td

pTd

y [

Ge

V

c2]

Blast wave fit:

Tfo = 152 MeV<T>=0.37


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mT-scaling in d+Au collisions at 200 GeV

• mT spectra of (++–), (K++K–) and (p + p) have similar shape

• (K++ K-) yield is smaller

than the other species by a factor of 2 (strangeness suppression)

• Local slopes of mT spectra are similar

Tlo

cal[G

eV/c

2 ]d

2 N/2

pTd

pTd

y [G

eV–2

c2 ]


21mT scaling in d+Au vs. Au+Au at 200 GeVT

loc[

GeV

/c2 ]

d2 N

/2m

Td

mTd

y [G

eV–2

c4 ]

PRC 70 (2004) 051901(R)

• In central Au+Au, larger flattening of p+p mT spectra at small pT

• Low pT spectra of p+p can constrain models describing collective transverse expansion of the system

Tlo

c[G

eV/c

2 ]d

2 N/2

mTd

mTd

y [G

eV–2

c4 ]


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Net proton yield at midrapidity, Au+Au

62.4 GeV: PHOBOS Preliminary200 GeV: PHENIX PRC 69, 024904 (2004) (correlated errors assumed: underestimated errors)

• Net protons: p–p

• Their yield is proportional to Npart within errors!

Really strange result:

• Number of protons‘transported’ to midrapidityper participant pair isindependent of numberof collisions per participant!

PHOBOS

Preliminary


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meson studies

Tracking modified for -s: ● Si layers 0-5 (no mag. field) straight lines● Si layers 8-9 (low mag. field) 3D-, 4D-fits

• extends the PID (strangeness scope) of PHOBOS in a special direction• low pT is an important tool to study the hot medium created

0 0.5 1pT [GeV/c]

0

0.5

1

y

0 0.5 1 1.50

100

200

Single mesonacceptance (MC)

pT [GeV/c]

Kaons in differentspectrometer arms

Kaons in the same arm

MC


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mass resolution at very low pT (MC study)

No background,only mesons

With high multiplicity background ( embedded in Au+Au data)

Single with 0<pT<0.13 GeV

=8.4 MeV

MK+K– [GeV/c2]1 1.02 1.04

2000

1500

1000

500

0

=11.5 MeV

MK+K– [GeV/c2]1 1.02 1.04

100

50

0

Table values: M=1.019 GeV/c2

=4.26 MeV/c2

MCMC


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signal level estimation

BRAHMS (nucl-ex/0403050) K– rapidity distribution: Gaussian, width 2.14

Total # of 's per 10% most central event: 36

STAR (nucl-ex/0406003)dN/dy6.65 for ||<0.5, T=357 MeV

Assumptions for 's: Rapidity: Gaussian, width=2.14 pT:

rapidity distributions:NA49 (nucl-ex/0305017)

K+

K–

pTexp( )T

–pT+M 22


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Expected signal size, no background

Single mesons, 0.0 < pT < 0.13 GeV/c

MK+K– [GeV/c2]

• Expected signal from all our Au+Au events at 200 GeV passing event selection (67 M events)

• ~ Npart assumed

• y distribution shape assumed to be centrality independent

• pT slope vs. centrality from STAR

MC


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Preliminary meson studies

• A new method of meson reconstruction was developed.

• Acceptance of the PHOBOS detector for low pT was studied.

• The method was tested on • single MC • single MC embedded into real data events

• The method is being applied on real data and its performance

is being studied.


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Summary

• PHOBOS developed various PID techniques: stopping low pT particles, dE/dx, TOF, resonance spectroscopy • No evidence for enhanced production of very low pT pions in the most central Au+Au collisions at 62.4 and 200 GeV

• Flattening of p+p mT spectra at low pT in the 15% most central Au+Au collisions at 62.4 and 200 GeV

• Approximate mT scaling of particle spectra for d+Au collisions at low and intermediate pT

• At high pT, baryons become dominant over mesons. Net baryon yield per participant is centrality independent

• Extensive MC studies on sensitivity to mesons


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backups


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PHOBOS magnetic field

Phobos Si Spectrometer Phobos Magnetic Field (Gauss)

0

5000

10000

15000

20000


31PID Measurement in PHOBOS Spectrometer

<d

E/d

x>

pT > 0.2 GeV/c pT = 0.03 - 0.2 GeV/c

Etot = dEi , i=A, ... ,E

Mpi = Ei dEi/dx

Mp = < Mpi >

/K separation: pT < ~0.6 GeV/cp(p) separation: pT < ~1.2 GeV/c

Kp

p+p

K +K + -

++-


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d+Au, Model Comparison


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strangeness measurements with the experiment

Documents

gevc lowp particles

silicon wafers p

eloss ek

b field p

low ptphobos spectrometer

spectrometer plane

kinetic energy eloss

low pttest