RHIC-PHENIX実験におけるHBDを用いた金金衝突での

電子対生成の測定渡辺陽介 for the PHENIX collaboration

12012年3月25日日曜日

概要• 過去の低質量レプトン対測定 @ PHENIX• PHENIX RHIC Run-10の解析• 二つの要所• Hadron Blind Detector(以下HBD)• (Combinatorial Backgroundの評価)

• まとめ22012年3月25日日曜日

低質量レプトン対測定• 低質量領域はカイラル対称性の物理などに感度が高い

• レプトンは終状態相互作用の影響が少ない

• 左の図はPHENIX, RHIC Run-4の結果

• mee: 0.15-0.75GeV/c2で、予想されるハドロンによる収量より大きい

• その領域でのS/B ~1/200

A. ADARE et al. PHYSICAL REVIEW C 81, 034911 (2010)

)2 (GeV/ceem

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ee! !’ " ee! #

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ee! &J/

ee!’ &

ee (PYTHIA)! cc

ee (PYTHIA)! bb

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sum

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FIG. 25. (Color online) Inclusive mass spectrum of e+e! pairs inthe PHENIX acceptance in p+p collisions compared to the expecta-tions from the decays of light hadrons and correlated decays of charm,bottom, and Drell-Yan. The contribution from hadron decays is in-dependently normalized based on meson measurements in PHENIX.The bottom panel shows the ratio of data to the cocktail of knownsources. The systematic uncertainties of the data are shown as boxes,while the uncertainty on the cocktail is shown as band around 1.

charm cross section, measured in p+p, !cc̄ = 567 ± 57stat ±224syst µb [48], has been scaled by Ncoll (given in Table I).For each centrality class, the data and the cocktail areabsolutely normalized. Each data set is compared with twocorresponding cocktail lines, shown in solid and dotted curves.The difference between the cocktails is due to uncertainty inthe cc̄ contribution (see discussion below).

Unlike the p+p mass spectrum, the Au+Au mass spectrashow enhancement above the cocktail, in particular for theLMR (0.15–0.75 GeV/c2). There is little enhancement for pe-ripheral (60–92%) data, but very strong enhancement for twomost central classes (0–10% and 10–20%). The enhancementincreases rapidly with increasing centrality.

In order to quantitatively describe this enhancement, moreinformation is needed about other components that canpotentially contribute to the LMR, namely the open heavyflavor and internal conversion of real direct photons. Wediscuss them in the next sections.

B. Open heavy flavor contribution

The dilepton yield in the IMR is dominated by semileptonicdecays of charm hadrons correlated through flavor conser-vation. Small contributions also arise from bottom hadronsand Drell-Yan. For p+p data we determine the heavy flavorcontribution by subtracting the hadronic cocktail from the

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-110 = 200 GeVNNsmin. bias Au+Au ee! ! 0"

ee! ! "ee! !' "

ee! #

ee0" ee & ! $ee" ee & ! %

ee! &J/ ee!' & ee (PYTHIA)! cc

sum ee (random correlation)! cc ee (PYTHIA)! bb ee (PYTHIA)!DY

DATA

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> 0.2 GeV/ceT

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)2 (GeV/ceem

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10

FIG. 26. (Color online) Inclusive mass spectrum of e+e! pairsin the PHENIX acceptance in minimum-bias Au+Au compared toexpectations from the decays of light hadrons and correlated decaysof charm, bottom, and Drell-Yan. The charm contribution expectedif the dynamic correlation of c and c̄ is removed is shown separately.Statistical (bars) and systematic (boxes) uncertainties are shownseparately. The contribution from hadron decays is independentlynormalized based on meson measurements in PHENIX. The bottompanel shows the ratio of data to the cocktail of known sources. Thesystematic uncertainties of the data are shown as boxes, while theuncertainty on the cocktail is shown as band around 1.

dilepton data. We integrate the subtracted yield in the IMR,extrapolate to zero e+e! pair mass to get the entire crosssection, correct for geometric acceptance, and convert to aproduction cross section using known branching ratios ofsemileptonic decays [54]. Details of the analysis of the charmcross section are reported in [38].

We find a rapidity density of cc̄ pairs at midrapidity:

d!cc̄

dy

!!!!y=0

= 118.1 ± 8.4stat ± 30.7syst ± 39.5modelµb.

This corresponds to a total charm cross section of!cc̄ = 544 ± 39stat ± 142syst ± 200modelµb, consistent withprevious measurement of single electrons by PHENIX(!cc̄ = 567 ± 57stat ± 224syst µb) [48] and with afixed-order-plus-next-to-leading-log (FONLL) pQCDcalculation (!cc̄ = 256+400

!146µb) [78].In Au+Au the dynamic correlation of c and c̄, which

is essential to determine the mass spectral shape, couldbe modified compared to p+p collisions. The observedsuppression and the elliptic flow of nonphotonic electronsindicates that charm quarks interact with the medium [6],which should change the correlations between the producedcc̄ pairs. We also note that the pT distribution for electronsgenerated by PYTHIA [55] is softer than the spectrum measured

034911-28

PRC 81, 034911(2010)

32012年3月25日日曜日

PHENIX RHIC Run-10• Run-4と同じところ• 荷電粒子飛跡検出• DC, PC1• 電子同定• RICH, EMCal

• Run-4と違うところ• HBD• 磁場 0.0

2000.0

4000.0

6000.0

8000.0

10000.0

0 1 2 3 4X (m) at Y=0

B (Gauss)

Outer

Outer + Inner

Outer - Inner

Outer

Outer + Inner

Outer - Inner

Fig. 4. Total field strength BMod(R) vs. R for + (Outer), ++ (Outer+Inner), and+- (Outer-Inner) field configurations

in the measurement of e+e! pairs produced at RHIC. This measurement is akey ingredient in the PHENIX physics program. Also the ++ field plotted inFig. 4, used with an upgraded charged particle tracking system, will improvethe momentum resolution.

2.2 Fabrication and Assembly

The yoke of the CM (and that of the MMN, discussed below) was fabricatedat the Izhora Steel Works in St. Petersburg from low-carbon steel (the Rus-sian equivalent of 1006 steel) forgings and hot-rolled plate. 1006 steel hasa maximum carbon content of 0.08%. Trial assembly for fit was performedsuccessfully at Izhora, but the yoke was not married to the coils until finalassembly at BNL. Quality Assurance (QA) for permeability and uniformity ofthe steel at the factory in Russia could therefore not rely on magnetic exci-tation tests. QA was done on the magnet components with a combination ofx-ray and ultrasonic inspection. Chemical analysis on melt samples was per-formed at Izhora and at LLNL. The results from these tests were integratedinto the models used to design the magnets to verify that the steel propertieswould meet the design requirements. The results were excellent, as confirmedby powering and mapping the CM at BNL.

The outer coils of the CM were fabricated in Japan at Tokin Corporation [3].Each coil pack comprises 6 bifilar wound double pancakes made with 20.3 mm x20.3 mm copper conductor insulated with fiberglass reinforced epoxy. Theouter coils consist of two assemblies each having 144 turns. The coils arewater-cooled via a 12.8-mm hole in the conductor. The full current testingof two coil assemblies was performed at KEK before the shipment to BNL.Mapping of and performance experience with the CM is all with only theouter coils installed. The inner coils will be fabricated and installed (and the

6

Run10

Run4

42012年3月25日日曜日

HBD：アイデア• なぜRun-4でS/B ~1/200だったのか？

• Combinatorial Background

• πやγからきた電子を除去できればよい

• πやγからきた電子対の開き角は小さい

• 開き角を保存するために磁場なし領域にHBDはおかれる

• バックグラウンドのペアはHBDに二つ重なったヒットをつくる

!"#$%"$&'()*'$%+'$*",-./0%"&.01$-0*23&")/(4$

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! B&'='&>'$%+'$"?'/./3$0/31'$"$*&'0%'$,03/'C*$D'1(67&''$&'3."/$

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52012年3月25日日曜日

HBD：デザイン• 電子同定

• チェレンコフ光検出器

• 4GeV/c以下では電子はチェレンコフ光をだすが、πは出さない

• 位置検出

• ６角形のパッド読み出し(~3cm)

• 物質量は少ないほどいい。γ->eeがバックグラウンドになるので。

• Radiation length 2.4%

• HBDで電子同定される領域には~0.6%

• NIMA 53467(2011)

Fig. 1. Top layout of the inner part of the PHENIX central arm detector showingthe location of the HBD and the inner and outer coils.

tions performed at the ideal detector level aiming at reducing the combina-106

torial background originating from conversions and !0 Dalitz decays by two107

orders of magnitude. At this level of rejection, the quality of the low-mass108

e+e! pair measurement is no longer limited by the background originating109

from these sources, but rather by the background originating from the semi-110

leptonic decay of charmed mesons. The simulations showed that the goal can111

be achieved with a detector that provides electron identification with an ef-112

ficiency of !90%. This also implies a double electron hit recognition at a113

comparable level. The separation between single and double electron hits is114

one of the main performance parameters of this detector. On the other hand,115

a moderate hadron rejection factor of " 50 is su!cient. It is also important116

to have a larger acceptance in the HBD compared to the fiducial central arm117

acceptance to provide a veto area for the rejection of pairs where only one118

partner is inside the fiducial acceptance.119

The requirements on electron identification limit the choice to a Cherenkov-120

type detector. In order to generate enough UV photons in a !50 cm long121

radiator to ensure good distinction between single and double hits, we adopted122

a windowless scheme without mirror and chose pure CF4 as radiator and123

detector gas. The use of a UV transparent window between the radiator and124

the detector element and of a mirror, as commonly done in RICH detectors,125

limits the bandwidth to about 8-9 eV. The choice of CF4 both as the radiator126

and detector gas in a windowless geometry results in a very large bandwidth127

(from !6 eV given by the threshold of CsI to !11.1 eV given by the CF4128

cut-o") and consequently a very large figure of merit N0. The N0 value is129

estimated to be close to 700 cm!1 under ideal conditions with no losses. The130

large value of N0 ensures a very high electron e!ciency, and more importantly,131

4

Fig. 3. Left panel: 3D view of the two arm HBD. Right panel: exploded view of oneHBD arm.

angle ! and ±0.45 units in pseudorapidity ". This extended acceptance with185

respect to the central arms (which cover 90o in ! and ±0.35 units in ") provides186

a very generous veto area for e!cient rejection of close pairs where only one187

track falls inside the fiducial acceptance.188

The right panel of Fig. 3 shows an exploded view of one HBD arm, display-189

ing the various elements of the detector. Each arm consists of a !50 cm long190

radiator directly coupled to a triple GEM photon detector. The latter is sub-191

divided in 12 detector modules, 6 along the ! axis " 2 along the z axis. With192

this segmentation, each detector module is !23 " 27 cm2 in size. In the 2009193

and 2010 RHIC runs, 10 modules were instrumented in each arm covering194

an azimuthal range of 112.5o which is considerably larger than the azimuthal195

range of 90o covered by the central arm detectors.196

3.2 Detector vessel197

The detector vessel has a polygonal shape formed by panels glued together as198

shown in Fig. 3. Eight panels of 63.0 " 23.7 cm2 and two vertical panels of199

63.0 " 54.8 cm2 define the polygonal shape. The panels consist of a 19 mm200

thick honeycomb core sandwiched between two 0.25 mm thick FR4 sheets.201

Six of the eight panels define the HBD active area. The other two panels,202

outside the active area, are service panels. Gas-in and gas-out connections,203

HV connectors serving the GEMs, and a small UV-transparent window are204

located on these two panels.205

Two supporting frames made of FR4, 19 mm thick (dictated by the thickness of206

the honeycomb core of the panels) and 7 mm wide, connect all panels together207

on each side providing mechanical stability and rigidity to the entire box. A208

thin window around the beam pipe is used to further reduce the radiation209

length in the HBD fiducial acceptance. The window is made of a 50 µm thick210

7

62012年3月25日日曜日

HBD：陽子陽子衝突

Charge (p.e.)0 20 40 60 80 100

Yiel

d

0

1000

2000

3000

4000

Open pairs2m < 0.15 GeV/c

Cluster charge

Charge (p.e.)0 20 40 60 80 100

Yiel

d

0

20000

40000

Close pairs2m < 0.15 GeV/c

Cluster charge

Fig. 37. HBD response to single electrons (left panel)and to an unresolved doubleelectron hit (right panel).

background is negligible. This sample is divided into two categories: if both the1034

electron and positron tracks reconstructed in the PHENIX central arms are1035

matched within 3! in both ! and Z directions to two separate HBD clusters1036

we interpret this as the response of the HBD to single electrons. If they are1037

matched to the same HBD cluster we interpret it as the HBD response to a1038

double electron. The HBD single electron response is shown in the left panel1039

of Fig. 37, whereas the HBD double electron response is shown in the right1040

panel. The former is peaked at around 20 photoelectrons, whereas the latter is1041

peaked at about twice that value, at !40 photoelectrons. The mean value of1042

the tagged single electrons is significantly higher, probably reflecting the fact1043

that this sample contains a small fraction of double electron hits. We therefore1044

take the peak values of 20 and 40 photoelectrons to represent the mean HBD1045

response to single and double electrons respectively.1046

The comparison of left panels of Figs. 36 and 37 shows a very good separation1047

between single electrons and hadrons in RB. A large fraction of the hadrons1048

can be rejected by applying a low amplitude cut to the HBD signal.1049

8.6 Figure of merit N0 and photon yield1050

The average number of photoelectrons Npe in a Cherenkov counter with aradiator of length L is given by:

Npe = N0 " L/"2th (5)

where "th is the average Cherenkov threshold over the sensitive bandwidth of1051

the detector and N0 is the figure of merit of the Cherenkov counter.1052

The ideal figure of merit, i.e. in the absence of any losses, is obtained by

44

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! B&'='&>'$%+'$"?'/./3$0/31'$"$*&'0%'$,03/'C*$D'1(67&''$&'3."/$

! :.=C/3).=+$-'%#''/$%+'$=./31'$'1'*%&"/$+.%$7&",$0/$"?'/$?0.&$0/($%+'$(")-1'$'1'*%&"/$+.%$7&",$0$*1"='($@-0*23&")/(A$?0.&$

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11.1.15 5 M. Makek, HP2010

! "

!0 "#

Charge (p.e.)0 20 40 60 80 100

Yiel

d

0

1000

2000

3000

4000

Open pairs2m < 0.15 GeV/c

Cluster charge

Charge (p.e.)0 20 40 60 80 100

Yiel

d

0

20000

40000

Close pairs2m < 0.15 GeV/c

Cluster charge

Fig. 37. HBD response to single electrons (left panel)and to an unresolved doubleelectron hit (right panel).

background is negligible. This sample is divided into two categories: if both the1034

electron and positron tracks reconstructed in the PHENIX central arms are1035

matched within 3! in both ! and Z directions to two separate HBD clusters1036

we interpret this as the response of the HBD to single electrons. If they are1037

matched to the same HBD cluster we interpret it as the HBD response to a1038

double electron. The HBD single electron response is shown in the left panel1039

of Fig. 37, whereas the HBD double electron response is shown in the right1040

panel. The former is peaked at around 20 photoelectrons, whereas the latter is1041

peaked at about twice that value, at !40 photoelectrons. The mean value of1042

the tagged single electrons is significantly higher, probably reflecting the fact1043

that this sample contains a small fraction of double electron hits. We therefore1044

take the peak values of 20 and 40 photoelectrons to represent the mean HBD1045

response to single and double electrons respectively.1046

The comparison of left panels of Figs. 36 and 37 shows a very good separation1047

between single electrons and hadrons in RB. A large fraction of the hadrons1048

can be rejected by applying a low amplitude cut to the HBD signal.1049

8.6 Figure of merit N0 and photon yield1050

The average number of photoelectrons Npe in a Cherenkov counter with aradiator of length L is given by:

Npe = N0 " L/"2th (5)

where "th is the average Cherenkov threshold over the sensitive bandwidth of1051

the detector and N0 is the figure of merit of the Cherenkov counter.1052

The ideal figure of merit, i.e. in the absence of any losses, is obtained by

44

シングルとダブルヒットがきれいにわかれている！

72012年3月25日日曜日

HBD：金金衝突での問題• Occupancyが非常に高い

• 荷電粒子はHBD内でシンチレーション光をだす

• 金金周辺衝突や陽子陽子衝突では荷電粒子の数が少ないので、電子の発するチェレンコフ光に比べて十分小さい

• 金金中心衝突では荷電粒子数が多く、シンチレーション光もシグナルに対して無視できない

• HBDより外でのγコンバージョンはHBDにシグナルを残さないが、Central Armでは電子同定される

• Occupanceyが高いとトラックがたまたまHBD内の偽ヒットにマッチしてしまう(Random matching)

82012年3月25日日曜日

HBD：金金衝突用アルゴリズム•Step 1: バックグラウンドを引く•イベント毎に各パッドの電荷からパッドあたりの平均電荷をひく•Occupancyがだいぶ改善

Efficeincy

1/R

Centrality: 0-10%

Simulation embedded in data

92012年3月25日日曜日

HBD：金金衝突用アルゴリズム

#1

#2

#0#3

0123

45678

9101112131415

•Step 2: クラスタリング•HBDの情報のみでは多くの偽電子ヒットできてしまう•トラックのプロジェクションの位置によって使うパッドの数を事前に決めておく•シミュレーションで最適化

•Step 3:偽ヒットをきりすてる•偽ヒットの電荷は、本物の電子のヒットより少ないはず•Efficiency 80%をたもって、偽ヒットの80%以上を除去できる

Random matchingの除去

Centrality: 0-10%

Simulation embedded in data

3 HBD PATTERN RECOGNITION 47

Figure 46: Rejection of fake hits as a function of electron efficiency for three centrality ranges.

• hbdid>= 5, will reduce the number of the backplane-conversion electrons to ∼ 1/5

• hbdid>= 10, will reduce the number of the backplane-conversion electrons to ∼ 1/10

Table 2: HBD-related variable in PHCentral

hbdid Normalized threshold for cluster charge

hbdcharge Sum of charge inside the cluster

hbdsize Pre-determined cluster size

hbddz Not used

hbddphi Not used

3.4 Track swapping

3.5 Track embedding into HBD

102012年3月25日日曜日

HBD: Combinatorial backgroundの削減

1. HBD cutなし

2. HBDによるγconversionのrejection

3. HBDによるDalitz rejection

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B/&+5CD+>/E>/

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MinBias

112012年3月25日日曜日

まとめと今後• RHIC Run-10ではHBDがインストールされ動作に成功した

• 金金衝突用のHBD解析アルゴリズムの開発を行った

• HBDでCombinatorial Backgroundは大きく減らされる

• Combinatorial Backgroundを高い精度で評価する方法を検討中

122012年3月25日日曜日

BACKUP

132012年3月25日日曜日

Combinatorial Backgroundの評価• S/B~1/200の状況ではバックグラウンドを<<1%のレベルで評価する必要

• 二つのCombinatorial Backgroundの評価法を検討中Mixed Event法

Like-sign pairのアクセプタンス補正法

+: 高い精度でCombinatorial BGの形を決められる-: Nの評価がCorrelated BGがあると難しいCorrelated BGの評価がSimulationに依存する

+: JetなどのCorrelated Backgroundが自動的に考慮される-: αを高い精度で決定するのが難しい

Jet, Double Dalitzなど

142012年3月25日日曜日

HBD：金金衝突での問題

• 荷電粒子はCF4内でシンチレーション光をだす

• 金金周辺衝突や陽子陽子衝突では荷電粒子の数が少ないので、電子の発するチェレンコフ光に比べて十分小さい

• 金金中心衝突では荷電粒子数が多く、シンチレーション光もシグナルに対して無視できない

• HBDより外でのγコンバージョンはHBDにシグナルを残さないはずだが、バックグラウンドの統計的ふらつきによって、あたかもそのトラックがシグナルを残したようにみえてしまう(Random matching)

Number of central arm

Scin

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light

occ

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200GeV

Number of central arm

Ave

rage

cha

rge

per

pad

in a

m

odul

e(p

.e.)

200GeV

152012年3月25日日曜日

HBD：金金衝突用アルゴリズム

#1

#2

#0#3

0123

45678

9101112131415

•Step 1: Underlying event subtraction•各パッドの電荷からパッドあたりの平均電荷をひく•Step 2: クラスタリング•トラックのプロジェクションの位置によって使うパッドの数を事前に決めておく

Random matchingの除去 Dalitzの除去

Efficeincy

1/R 1/R

Efficeincy

Centrality: 0-10% Centrality: 0-10%

Simulation embedded in data Simulation embedded in data

162012年3月25日日曜日