1 Measurement of the cross section and the single transverse spin asymmetry of forward neutrons from p-p collisions at RHIC-PHENIX Manabu Togawa for the PHENIX collaboration. Kyoto University / RBRC

2 Outline Physics motivation Measurement of forward neutron at PHENIX –Experimental setup –Analysis procedure Result of the cross section. –Very forward case. Results of the single transverse spin asymmetry –x F -dependence –Comparing tagged with/without charged particles Summary

3 Motivation In the forward neutron production, there are some interesting behaviors. –In ISR experiment, pp √s=30~63GeV, cross section of forward neutron production at low p T was measured to be larger than at high p T. [left figure] –In the RHIC IP12 experiment, pp √s=200GeV. We found large single transverse-spin asymmetry A N (-10%) [right figure] –PHENIX data can shed new light to understand the forward neutron production mechanism. =  Kinematics ±2.8mrad Forward region xFxF Nucl. Phys. B109 (1976)

4 Does Feynman scaling hold when going to RHIC energy? –From the ISR result, C.S. are well scaling by x F at 30<sqrt(s)<63 GeV.. –Forward neutron C.S. has peak structure and it is well described by pion exchange model. What is the mechanism of neutron asymmetry ? –pion exchange model Asymmetry can be appeared by interference of spin flip. –Twist-3 model, Siverse, Collins… Eur.Phys.J.A7: ,2000 xFxF x F dependence √s >=200 GeV - cross section - asymmetry p T dependence √s >= 200 GeV - asymmetry

5 Setup Collision point BBC 1800 cm Dx magnet Schematic view from simulation. - GEANT3 (Geisha) - From the pythia simulation, Main backgrounds are gamma and proton. ZDC 10*10cm  2.8 mrad 3 module 150X 0 5.1λ I Proton shower event (In case of proton mom = 50GeV/c) Forward counter veto SMD 1 ZDC SMD Neutron position can be decided by centroid method.

6 Data set –In this analysis, √s=200 GeV data which was took from 2003 to 2005 is used. The data is corrected by single neutron trigger with/without MinBias trigger. MinBias is defined as detecting charged particles in BBC(3.0  3.9  Calibration –ZDC : Looking the 1 neutron peak at heavy ion data. –SMD : Relative gain correction is done by cosmic-ray data. Simulation ( GEANT3 with pythia v.6220 or single particle generator ) –Background and efficiency after neutron ID cut. –Unfold the neutron energy. –Smearing effect of asymmetry caused by position cut and resolution. Analysis outline

7 Cross section Integrated p T area : 0<p T <0.11x F GeV/c in each point. Cross section result is consistent with ISR data and x F scaling holds at such higher energy. p T distribution is assuming ISR result

8 Single transverse spin asymmetry Smearing effect is evaluated by simulation. For asymmetry calculation, square root formula is used. xpos ypos Expected mean p T (Estimated by simulation assuming ISR p T dist.) 0.4<|x F |< GeV/c 0.6<|x F |< GeV/c 0.8<|x F |< GeV/c neutron charged particles ANAN Without MinBias -6.6 ±0.6 % With MinBias -8.3 ±0.4 % preliminary * Cut is not same

9 How about charged particles at BBC ? Asymmetry is 0 when data is selected by BBC only. Finite asymmetries are shown tagged with forward neutron. 4  –Opposite direction as neutron (2.28±0.55±0.10)*10 -2 –Same direction as neutron (-4.50±0.50±0.22)*   Neutron analyzing power can be large with charged particles. A N (X) 0,A N (Y ’ ) proton Y polarized proton Y’Y’ n (neutron) Y : charged particles pion exchange model pol direction N*(  *)  n+Y ’ ?? N* ??? Y  will be analyzed with high statistics and good polarization data. Top view preliminary

10 Summary The cross section and the single transverse spin asymmetries of forward neutron production in √s=200GeV polarized p-p collision are measured at RHIC-PHENIX. Cross section –It is consistent with ISR data at p T ~ 0 (GeV/c). x F scaling holds in this high energy. Single transverse spin asymmetry –x F -dependence looks flat, negative polarity. –Analyzing power of forward neutron tagged with charged particles are lager than without them. Prefer the pion exchange model and like diffractive process(?).  Theoretical calculation is very welcome.

11 Future analysis p T dependence –Important parameter same as x F. Charged particle asymmetry tagged with forward neutron. –with high statistics and good polarization data.

12 -5cm5cm Horizontal scan We can see finite asymmetry in other energy RUN ! √s=62.4 GeV√s=410 GeV

13 backup

14 Cross section calculation PHENIX data. ISR data is converted to current cut. ( 0<p T <0.11x F GeV/c )  Input p T shape p T =A*exp(-4.8p T ) Assumed Simulation input comparing The total cross section with PHENIX neutron cut. is estimated. –p T is not evaluated from our data. As simulation input, ISR result is assumed. –In this analysis, radius<2cm from the collision point (1800cm) is calculated.  1.1 mrad : 0<p T <0.11x F GeV/c 。

15 Asymmetry with different trigger Smearing effect is evaluated by simulation. For asymmetry calculation, square root formula is used. neutron charged particles Without MinBiasWith MinBias

16 Summary The cross section and the single transverse spin asymmetries of forward neutron production in √s=200GeV polarized p-p collision are measured at RHIC-PHENIX. Cross section –It is consistent with ISR data at p T ~ 0 (GeV/c). x F scaling holds in this high energy. with 2cm radius cut in ZDC region (~1.1 mrad) Integrated p T range is 0~0.11x F (GeV/c) Single transverse spin asymmetry –x F -dependence looks flat, negative polarity. Integrated p T range is 0.03x F ~0.22x F (GeV/c) –Analyzing power of forward neutron tagged with charged particles are lager than without them. Prefer the pion exchange model and like diffractive process(?).  Theoretical calculation is very welcome.

17 t-range t p (m p,0,0,E p ) p (m n,p T,0,E n ) xFxF (GeV/c)t (GeV 2 ) 0.4~ ~ ~ ISR p T distribution is assuming.

18 Red/Black ~ 0.47Red/Black ~ 0.71 Red/Black ~ 0.75Red/Black ~ 0.73 Mean p T : Mean p T : Mean p T : Mean p T : Mean p T : Mean p T : Mean p T : Mean p T : 0.102

19 Systematic error for C.S. Systematic error in calibration –Energy calibration ~5% –SMD relative calibration (it is including in SMD cut eff) –Trigger bias correction. < 10% Systematic error in simulation –Acceptance few % from pt distribution by SMD pos cut. –SMD cut eff. ~3% –Neutron ID SMD shape match. ~ 3% Forward counter match. ~5% –Background estimation. apply 100% error for BG ratio. Mainly from K0 and proton For cross section 8% –2 particle in 1 event ~ 5% Systematic error in luminosity –Cross section error 13% (22.9 mb * 0.591) for NoVtxCut In total –For shape after cut : ~11.6% For scaling of cross section : 17.1%

20 Systematic error for phi-asym. Measurement : 1% –Bunch shuffling result is ~2% of statistics, RUN5 local pol analysis note. (AN462) –Center scan ~ 1% –Beam gas BG at ZDCN|S trigger : 0.2% Simulation uncertainty 4.1% –Simulation stat 2% –pT match : few% –BG estimation 3% –Center cut scan 2% Total : 4.2% Online polarization 20%  scaling error.

21 Systematic error for x F -asym. Estimation from measurement : 2% –~4% of statistics, by bunch shuffling. –Center scan ~ 2% –Beam gas BG at ZDCN|S trigger : 0.2% –Unfolding : calibration (bin by bin  18p) x F : 0.4~0.6 : 2.1% 0.6~0.8 : 0.3% 0.8~1.0 : 0.3% Simulation uncertainty 7.8% –BG estimation 3% –Position smearing estimated by center cut scan (Differences between PHENIX and flat x F inputs) 7.2% –Unfolding : linearity (bin by bin  17p) x F : 0.4~0.6 : 12.1% 0.6~0.8 : 2.1% 0.8~1.0 : 2.1% Total : 8.1% of asymmetry value (not including blue term)

22 Calibration (ZDC) Neutron cainbration is done by 1 neutron peak from heavy ion run. Select “coulomb” event by BBC ZDC detector 10cm We can see 100 GeV peak in CuCu event Black : all event Red : select center region. pp event after calibrated.

23 Calibration (SMD) Relative gain of each channels are matched by cosmic ray data. Cosmic peak Took by Alexei. Compared with simulation data. See simulation term.

24 nID study using pythia input. pid 1Gamma10K0 long 2Positron11K+ 3Electron12K- 5Mu+13Neutron 6Mu-14Proton 8Pi+15Antiproton 9Pi-25Antineutron Black : sum Green : gamma Blue : neutron Red : proton n p  ZDC threshold is ~5GeV

25 Correction of BG (estimation) With SMD cut and center cut (r<0.5), BG are mainly K0 and proton –From ISR paper, E (GeV)K0 (%)Proton (%) Pythia out. Pythia out is agree K0 ration with ISR data. I used BG distribution after cut from the pythia. Error will be added 100% of this value

26 Neutron measurement stability Stability = e-06/ e-03*sqrt(315.5/95) = 0.3 %.

27 RUN5 pp MB looking 22.9 ± 9.7mb For neutron C.S. correct BBCLL1 cut efficiency. cm Red : BBCLL1(>0 tube) Blue : BBCLL1(noVtxCut) BBCLL1/NoVtxCut ± 0.05 (for maximum error)  8.46% BBCLL1 bias study.

28 Stability of neutron peak after cut This peak

29 NORSH vs. SOUTH (nID num) nID number are agree with SOUTH and NORTH after SMD cut efficiency  Amplitude of C.S. will be same

30 BG estimation from pythia With BBC trigger –K0 : 1.16% proton : 1.60% sum : 2.76% Without BBC trigger –K0 : 0.97% proton : 2.27% sum : 3.24%  correction assuming A of K0 and p = 0. if 3%, asymmetry is smeared as 1.0/1.03 = 0.97

31 Radius distribution for calculating asymmetry Center cut by r>0.5cm Cut radius Real data (1.039) (1.107) sim (center) (1.029) (1.112) sim (flat) (1.018) (1.086) * () means ration to cut radius = 0.5. sim : C.S. is flat sim : C.S. is p T ~ 0 real data Differences of ratio will be systematic error : 2%

32 前方中性子測定

33 ISR 実験 xFxF Nucl. Phys. B109 (1976) ○ 重心系エネルギー 30~63 GeV ○ STAC検出器 40*40*80 cm 3 (iron) 5.5 I res. ○ They were placed in 0 o, 20 o, 66 o, 119 o,(±1)mrad (56m for 0 o ) x F スケーリング 前方ピーク

34 One pion exchange model Eur.Phys.J.A7: ,2000 xFxF メソンクラウドモデル レッジェポール B. Kopeliovich et al. Z.Phys.C73: ,1996

35 ZEUS 測定 Leading neutron spectrometer Beam pipe Neutron exit window 106m Lead plates sampling cal. (7  I ) 3枚の charge veto counter (70*50*2 cm 3 ) 30 GeV 電子 920 GeV 陽子

36 ZEUS results Forward neutron C.S. of neutron-tagged D *± epでの重クォーク生成は  g融合 もし交換されたパイオンと仮想光子との反応な らパイオン内部のグルーオン構造関数が見える はず S. Chekanov et al. physics Letters B 590 (2004) 143–160 S. Chekanov et al. nuclear Physics B 637 (2002) 3–56 Scaling is unreliable

37 生成断面積に関して – 今のところパイオン交 換モデルでよく記述で きている。 横偏極非対称度 – パイオンはスピンフ リップなので、記述で きるだろう。 PHENIX での新しい データが重要だ。 Eur.Phys.J.A7: ,2000 xFxF