vasundhara chetluru december 17, 2015 university of illinois, chicago antiparticle to particle...

Vasundhara Chetluru

April 21, 2023

University of Illinois, Chicago

Antiparticle to particle ratios measurement using the PHOBOS detector

2Vasundhara Chetluru04/21/23

Contents Motivation for studying particle ratios is heavy ion

collisions

PHOBOS @ RHIC Detector description

Like particle ratios analysis Analysis details Results & discussion

Motivation for studying particle ratios is heavy ion collisions


Geometry Production Formation Freezeout Freezeout

0 fm/c ~2 fm/c ~7 fm/c >7fm/c

Time

Antiparticle to particle ratios probe hadron formation & chemical freeze-out stages.

In p+p & d+Au collisions little re-interaction is expected thus the ratios should reflect the initially produced yields.

Do these different conditions influence the measured particle ratios in Cu+Cu and Au+Au?

Relativistic Heavy Ion Collider

2.5 miles circumference

4 Experiments

5 years and more of running

Relativistic Heavy Ion Collider


Au+Au: 19.6, 56, 62.4, 130, 200 GeV

p+p:200, 410 GeV

Cu+Cu:22, 62.4, 200 GeV

d+Au:200 GeV

PHOBOS Experiment

PHOBOS


p+p d+Au Cu+Cu Au+Au410 20200 100 150 400 250130 4.362.4 110 2255.9 1.822.5 2019.6 ~1

GeVsystem

UIC has led the effort of

Building of Octagon, Vertex and the Ring detectors.

Designing and maintaining of the trigger and its electronics, from 2000 forward.

Scintillator Paddles + Zero Degree Calorimeter for triggering TOF wall for high-momentum PID

96000 Silicon Pad channels 4π Multiplicity Array Mid-rapidity Spectrometer

Millions of events to tape

Run V (2005)Cu+Cu data

joined the group

Onsite Trigger support

Calibrating and maintaining the T0 vertex trigger

Collision Centrality


Magnitude of signals in paddle counters determines centrality

Negative

Paddles

Positive Paddles

ZDC N

ZDC PAu Au

x

z

PPPN

Paddle signal (a.u.)

Data

Co

un

ts

Larger signal = more central collision.

Central Collision: Large Npart

Peripheral Collision: Small number of participating nucleons

“side” view of colliding nuclei “side” view of colliding nuclei

Slide from David Hofman’s talk

Like antiparticle to particle ratios

Analysis description

Definition


)(*)(* sec,,, hChhh absfdtrkacctedreconstruc

h

h

Particles

lesAntiparticratio particle like Identified

Identified anti-particle/particle count per event

efficiency trackingand acceptance lgeometrica for theaccount toapplied is )(,htrkacc

resonancesdecay and effectsdetector todue yield

ison modificati for theaccount toapplied is )(sec,,hC absfd

Definition


)(*)(* sec,,, hChhh absfdtrkacctedreconstruc

h

h

Particles

lesAntiparticratio particle like Identified

Identified anti-particle/particle count per event

)(

)(*

)(

)(*

sec,,

sec,,

,

,

hC

hC

h

h

h

h

h

h

absfd

absfd

trkacc

trkacc

tedreconstruc

tedreconstruc

As a function of centrality of the collisions and transverse momentum of the particles.

PHOBOS Spectrometer


Schematic Diagram

near mid-rapidity

ZB1 Cartoon

•Two symmetric spectrometer arms give two independent measurements.

•Outer 9 layers of the 15 layers are located in 2T magnetic field

•Coverage near mid-rapidity and Tracking within 10 cm of interaction point.

• PHOBOS magnet polarity is changed every couple of days.

• Two independent measurements are taken for each polarity.

h

h

Acceptance

Z Z

04/21/23 14Vasundhara Chetluru

Cu+Cu 200 GeV data

Magnetic field settings

)()(21 ,,

BtrkaccBtrkacc hh

For a given bending direction and opposite field settings

Ratios are measured independently for different bending directions.


near mid-rapidity

ZB1

near mid-rapidity

ZB2

)()(12 ,,

BtrkaccBtrkacc hh

2 Arms X 2 Bending-directions = 4 Measurements

h

h

h

h

Particle ratios and acceptance


)(

)(*

)(

)(*

sec,,

sec,,

,

,

,

,

1

2

1

2

hC

hC

h

h

h

h

h

h

absfd

absfd

trkacc

trkacc

Btedreconstruc

Btedreconstruc

B

B

Forward Bending

)(

)(*

)(

)(*

sec,,

sec,,

,

,

,

,

2

1

2

1

hC

hC

h

h

h

h

h

h

absfd

absfd

trkacc

trkacc

Btedreconstruc

Btedreconstruc

B

B

Backward Bending

)()(21 ,,

BtrkaccBtrkacc hh

)()(12 ,,

BtrkaccBtrkacc hh

Tracking – momentum determination.

Particle identification.

Measuring particle ratios


Tracking in the PHOBOS Spectrometer

1. Road-following algorithm finds

straight tracks in field-free

region

2. Curved tracks in B-field found

by clusters in (1/p, ) space

3. Match pieces by , consistency

in dE/dx and fit in yz-plane

4. Covariance Matrix Track Fit

for momentum reconstruction

and ghost rejection





region







and ghost rejection





region


by clusters in (1/p, ∆) space





and ghost rejection





region


by clusters in (1/p, ∆) space





and ghost rejection


Momentum determined with resolution of ~1%

∆

Uses momentum and the energy loss

Particle identification

Particle identification (PID)


Cu+Cu 200 GeV MC

The hits that particles produce provide both momentum information (determined from the position of the hit) and energy loss information (determined from the ionization produced by the particle).

The different energy loss characteristics of pions, kaons, and protons can be used conjointly with momentum to identify the particle type of a track.

Particle identification (PID)


dE/dx slice for Momentum=0.5 Bin

Pions

Kaons

Protons

Cu+Cu 200 GeV MC

PID BandsLimit in momentum is obtained by 3-σ limit of the over lapping bands


Cu+Cu 200 GeV MC

Raw Ratios


)(

)(*

)2(1

)1(2

)2(1

)1(2

)2(1

)1(2

sec,,

sec,,

,

,

Babsfd

Babsfd

Btedreconstruc

Btedreconstruc

B

B

hC

hC

h

h

h

h

Cu+Cu 200 GeV Data

The yield of the produced (primary) particles is changed due to a variety of reasons, by the time they hit detector

material.

Corrections to the measured particles ratios (which are called raw ratios) are

applied to account for this change.

Corrections to the obtained raw ratios

General formula correction


hh

hh

hh

hh

h

h

h

h**

hh

hh

h

hc *

Let h represent the yield and ∆h change in the yield due detector effects or feed-down. This change can be positive or negative.

Then h+ ∆h represents the measured yield (ignoring the efficiency correction).

c represents the correction factor. This is usually obtained using HIJING monte-carlo generator.

hh

hhc

h

h*

absfdBabsfd

BabsfdCCC

hC

hC**

)(

)(sec

sec,,

sec,,

)2(1

)1(2

Feed-down


Accounts for hyperon decay products. Mainly effects the proton ratio. Lambda’s account for the most significant feed-down contribution

to the proton yields. As () = 0.26 ns and c() = 7.89 cm Spectrometer ~10 cm from interaction point. Apply strict distance of closest approach cut to each track.

prim

prim

prim

primfd p

pp

pp

pC

*

protonsfor ,98.0fdC

Cu+Cu 200 GeV MC

MC Feed-down

Secondary Corrections• As the primary collision products pass through the beam pipe and detector

materials, secondary particles are produced. Those which pass through the spectrometer may be reconstructed along with the primary particles.

• The effect of secondaries is negligible is kaons.

• While the protons and pions have 2% and 1% correction effectively.


Absorption Correction As the collision products pass through the detector, some of them are absorbed. This results in

a loss of anti-particles versus particles and a decrease in the anti-particle to particle ratio.

Correction is obtained by studying the effect of hadronic interactions in the detector using HIJING.

5%,0.05% and 1% for protons, pions and kaons respectively.


pT GeV/C

Ab

sorp

tion

co-

effi

cien

t

ProtonsAntiprotons

Cu+Cu 200 GeV MC

Band-width, Beam-Orbit, DCA cut

Paddle-time Difference

Track fit probability, Vertex in Z

Systematic error study

Systematic error study• Systematic uncertainties, which arise from event selection, particle

identification cuts, and the three correction factors are studied.

• No single uncertainty (parameter) dominates the final systematic error, typically the smallest contribution comes from the PID cuts and the largest from either the event selection or, in the case of the proton ratios, the feed-down correction.

• The final systematic uncertainty for a given centrality is determined from the statistically weighted average of the uncertainty determined for each parameter for different arms and bending directions.

• A thorough investigation of the track selection χ2 probability cut has shown a variation independent of the species and arm, but dependent on the bending direction. Hence, this effect yields a scale systematic uncertainty that, for each collision energy, is independent of both centrality and particle species.


Plot with systematic studies


Pions have the smallest systematic variations.

Track-fit probability has the largest contribution for all 3 species.

Systematic errors are studied as a function of centrality bin.

Systematic error

Protons

Kaons Pions

Cu+Cu 62.4 GeV 0.03 0.02 0.03

Cu+Cu 200 GeV 0.04 0.02 0.03

Results

Discussion of results


•No strong dependence on centrality is observed for the Cu+Cu data.

•The final values for the antiparticle to particle ratios of pions, kaons and protons appear to be primarily driven by the collision energy and, within current systematic uncertainties, are largely independent of the colliding system.

•A detailed comparison of the central Cu+Cu results at 200 GeV to results from p+p, d+Au, and central Au+Au collisions at RHIC indicates that the antiparticle to particle ratios are, for the most part, insensitive to the collision species.Average pT

Cu+Cu Proton Kaon Pion

200 GeV 0.31 0.37 0.51

62.4 GeV 0.31 0.36 0.50

Open (closed) circles represent √sNN = 62.4 GeV (200 GeV) data. The error bars represent the combined (1σ) statistical and systematic uncertainties

Discussion of results


•Thermal models• Make the assumption that the

initial state has time to thermalize and this “chemical” thermal nature is preserved during hadronization.

• Can fit each energy with a common chemical “freeze-out” temperature, Tch, and baryon chemical potential mB.

• Suggests a high degree of chemical equilibrium (and thermalization) at the point where particles are “frozen-out” (created).

•Baryon transport• Participating nucleons

experience multiple collisions• Causes loss of incident

momentum and energy• Can lead to “stopping” of

nucleon in CM frame; ie. transport to = 90° w.r.t. beam axis.

Open (closed) circles represent √sNN = 62.4 GeV (200 GeV) data. The error bars represent the combined (1σ) statistical and systematic uncertainties

Backup

Phase diagram


B

Tem

pera

ture

(M

eV)

Abigail Bickley’s talk

Thermal Models


Only µB and T are free parameters if look at production.

Make the assumption that the initial state has time to thermalize and this “chemical” thermal nature is preserved during hadronization.

Have a chemical potential m for every conserved quantum number

Constrain parameters with conservation laws

Grand Canonical Ensemble

Braun-Munzinger, Redlich, Stachel - nucl-th/0304013; Stachel – Trento - 2004

Thermal Model


CTEQ 2006

Phys Lett. B. 518, 41 (2001); J. Phys G28, 1745 (2002)

• Can fit each energy with a common chemical “freeze-out” temperature, Tch, and baryon chemical potential mB.• Suggests a high degree of chemical equilibrium (and thermalization) at the point where particles are “frozen-out” (created).

Baryon transport Proton yield from transport and pair production

Antiprotons generated via pair production


The pair production mechanism is symmetric,

45

Analysis Flow Chart

Vasundhara Chetluru04/21/23


RHIC other experiments

Trigger studies


Charged particle spectra


Au+Au: PRL 94, 082304 (2005), PLB 578, 297 (2004)d+Au: Phys. Rev. Lett. 91, 072302 (2003)

preliminary preliminary

62.4 GeV 200 GeV

Cu+Cu

d+Au

Au+Au

PHOBOScentrality

Flow


Au+Au

19.6 GeV 62.4 GeV 130 GeV 200 GeV

preliminarypreliminary

PHOBOS

Cu+Cu

Au+Au: PRL 94 122303 (2005)

Au+Au

preliminarypreliminarypreliminarypreliminary

PHOBOS


Rapidity & Transport Rapidity:

Longitudinal motion Used if PID and p known

Pseudorapidity: = polar angle to beam axis Used if PID and p not known

Mid-Rapidity: = 90°, p|| = 0

y, 0 @ mid-rapidity Particles measured at mid-rapidity

Generated in collision Transported from beam rapidity

y 1

2ln

E p||

E p||

ln tan 2

Au Au

mid

forward

Beam-orbit study


• Beam-orbit – Mean reconstructed vertex position of the collision in the transverse plane for a given run.

• Steady beam-orbit ensures acceptance and efficiency cancellation for different polarities.

• Data over the whole run range is classified depending on shifts in the beam-orbit. Ratios are calculated independently for each steady beam-orbit region.

Data quality studies Careful checking the data for any kind of anomalous

behavior. Plot below is an example of the average number of tracks

per event study.


Different colors represent different magnetic field

settings

Spectrometer Performance


Data Sample Production Run 2001(200 GeV)• 7.8 M Au+Au Events, Min. Bias Trigger• 32 M reconstructed particles

Acceptance Momentum Resolution

Energy Loss


• The probability of interaction is statistical and can be characterized by the average amount of energy lost per unit path length, dE/dx.

• Experimentally dE/dx is measured in units of minimum ionizing particles, MIPS.

• A MIP is defined as the minimum value of the dE/dx for a given material and is applicable to particles traveling at relativistic velocities, ≥ 0.9c.

• Particles studied have momenta below the “relativistic rise”.

Bethe-Bloch function


β = v / c

v velocity of the particle

E energy of the particle

x distance travelled by the particle

c speed of light

particle charge

e charge of the electron

me rest mass of the electron

n electron density of the target

I mean excitation potential of the target

permittivity of free space

http://en.wikipedia.org/wiki/Electron

http://en.wikipedia.org/wiki/Permittivity

Technique II - For PID band determination


Bethe-Bloch parameterization is used to represent the data.

Technique II - For PID band determination


Correction function to the Bethe-Bloch is obtained

Trigger callibration

TAC Plot


•Triggering on the time difference between the T0 hits on both sides.

•It essentially narrows the collision vertex range, ensuring good data quality.

• Calibration and efficiency studies are the essential parts of running this trigger.

Time T0P (ns)

Tim

e T

0N (

ns)

TAC Plot

60Vasundhara Chetluru

•Triggering on the time difference between the T0 hits on both sides.

•It essentially narrows the collision vertex range, ensuring good data quality.

• Calibration and efficiency studies are the essential parts of running this trigger.

04/21/23

Time T0P (ns)

Tim

e T

0N (

ns)

Triggered hadron correlations

Tracking tuning for heavy ions

CMS Effort


Triggered Hadron Correlatrions Jets are being used as tomographic probes to explore the medium created in ultra relativistic

heavy ion collisions.

At RHIC, tomographic probes provided the evidence of strongly interacting matter. Evolution of triggered correlation functions indicated additional physics phenomenon.

We intend to utilize these techniques to explore the matter properties in the energy domain at LHC where the properties of the created matter remains a mystery.


hadrons

q

q

hadrons

leadingparticle

leading particle

Schematic diagram of a di-jet in a heavy ion collision.

Motivation


4 < pT(trig) < 6 GeV/c pT(assoc) > 2 GeV/c

Away side jet is suppressed for central Au+Au collisions.

Evidence of jet medium interactions, partial thermalization of the medium.

Signal + Background


Signal + Background : Δη-Δφ correlation with respect to the trigger particle (leading particle) in a given event. Normalized by the total triggers.

Background estimated by mixed events technique: Δη-Δφ correlation correlation with respect to the trigger particle from a different event with a similar pT.

Hydjet Pb-Pb 5500GeV , |η| <2.4 , pT > 2.0

0-10 % central collisions. Trigger – 15<pT<20

Tracking tuning for high pT


Algorithmic efficiency

Closed symbols – EffiencyOpen symbols – Fake-rate

Tuning tracking reconstruction algorithm for high pT tracks.

vasundhara chetluru december 17, 2015 university of illinois, chicago antiparticle to particle...

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