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DVCS analysis for HERA I

DVCS analysis for HERA I. Laurent Schoeffel (Saclay) and Laurent Favart (ULB). Referees : Beate Naroska and Victor Lenderman. Motivations Status of present measurements and improvements Trigger/Run selections Definition of main physical cuts BH Sample analysis (calibartion of LAR,…)

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DVCS analysis for HERA I

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  1. DVCS analysis for HERA I Laurent Schoeffel (Saclay) and Laurent Favart (ULB) Referees : Beate Naroska and Victor Lenderman Motivations Status of present measurements and improvements Trigger/Run selections Definition of main physical cuts BH Sample analysis (calibartion of LAR,…) DVCS sample Cross sections and discussion 2 main purposes / previous H1 analysis : increase statistics and measure the t slope of the DVCS x-sections => decrease both data and models uncertainties => discrimination between models?

  2. Motivations *+p  +p’ *  with P+=(p+p’)/2 and  ~ xBj/2 Bj (x+)P+ (x-)P+ t=(p-p’)² Factorization With HGeneralized Partons Dist. (GPD)k  k(x+,…) *k(x-,…) d  k  |k(xBj,…)|² d’ for DIS Limit 0 : q(x,bT) = FT [H(x,0,t)] => GPDs  Fourier Transform of impact parameter dependent partons densities => Transverse q/g dist. in coordinate space  longitudinal q/g dist. in momentum space

  3. DGLAP Limit t0 : standard parton densities q-q correlations Meson like Dist. Amplitude (DA) Identical PDF but different correlations ERBL For this analysis : we have written a new DVCS MC which calcultates x-sections+… from GPDs (at NLO ; Q² evolution of GPDs) dDVCS/dxdQ²dt ~ 3x/ Q6 [CFGPD]² ebt ~ 43x/ Q6 ²DIS ebt

  4. Preliminary analysis for EPS03 : 2000 data with 26 pb-1 Status (prel.) and main improvements Improvements : 1996;1997;1999e+;2000 => larger statistics (46.5 pb-1) => two samples Q²=4 GeV² and Q²=8 GeV² => 2 dependences (W) W Better treatment of the Forward detectors (FMD;PRT) : new PRT reweight New treatment of the Zvtx in case of DVCS sample => for the first time : measurement of d/dt  exp(-B|t|) => important gain on the theoretical uncertainty MILOU MC (for DVCS) extended at NLO  treatment of proton dissociation

  5. Analysis strategy DVCS : e+ in SPACAL et (LAr) [Signal Sample] BH with same topology as DVCS BH with e+(LAR) [Control Sample]

  6. Triggers selection 1996 : S3 corrected from ineff.(Q²) // B. Clerbaux Ph.D. Run selection [157877-166249]  B. Clerbaux Ph.D. selection  FMD noise [160123-161157] excluded Lumi = 3.5 pb-1 1997 : S3 ( = 100% after fid. cuts in SPACAL) Run selection // R. Stamen Ph.D. (e.g. FMD readout pb [<184257] excluded) Lumi = 8 pb-1 1999-2000 : Triggers S0 and S3 (S3 : L2 condition SPCL_R30) We use S0 inside SPCL_R30 and S3 otherwise ( = 100% after fid. cuts) Run selection : we reject  noisy PRT periods for 00-99  periods in 1999 with ~25% CJC off [257637-261338] (=> we reject 99 MB period)  high prescales LumiEFF (00) = 26 pb-1 (LTOT = 37 pb-1) LumiEFF (99) = 9 pb-1 (LTOT = 10 pb-1) 96-00 => Lumi = 46.5 pb-1

  7. Run selection : FMD noise/readout 1999e+ : OK 1996 : Runs [160123;161157] noise in L2 Noise in FMD : with random trigger events  no cluster in LAr with E>0.5 GeV… Similar studies for PRT 1997 : Runs <184257 Readout pb in all layers 2000 : OK

  8. 2000 : PRT noise (from X. Janssen)

  9. Forward Detectors quality selection 1996 : FMD noise runs  [160123;161157] => excluded 1997 : FMD readout pb for Runs<184257 and PRT for runs  [190131;193331] 1999e+ : PRT noise for runs  [246729;247590] { periods in 1999 with ~25% CJC off [257637-261338]} 2000 : PRT noise for runs  [263535;263744] and [265400;265900] FMD noise files extracted for this run selection (X. Janssen)

  10. Analysis main selection (0) Fiducial cuts in SPACAL (/year) to ensure trigger efficency ~ 100% (1) 2 EM clusters 96-00 : ESPA>15 GeV and 96/97 : PtLAR> 1 GeV ; 99-00 : PtLAR>1.5GeV (2) E(third cluster in LAR)<0.5 GeV (3) Zlar>-150cm (central or fwd LAR)  LAR  [25;145] deg.  (z/) cuts (/year) (4) FMD and PRT : (FMD01≤1, FMD012≤2) (96 : PRT0126=0 ; 97 : PRT012=0 ; 99/00 : PRT01234=0) Note : noise file for FMD and PRT reweight // X. Janssen analysis (5) Tracks : BH = 1 linked track (DTRA or DTNV if no vertex fitted track is found). For DVCS : no DTRA nor DTNV

  11. Vertex simulations BH SAMPLE BH (elastic+inelastic from COMPTON MC) Normalization/Lumi from COMPTON is correct within 2% ; fluctuations ~ 5% ZVTX is presented after reweighting Dilepton (GRAPE) +  (DIFFVM) ~ 4.5% (99-00) and 6.5% (96-97) of the control sample

  12. Calibrations (LAR) BH SAMPLE EclLAR-Etrack (GeV) after correction before correction (00) MC Calibration EM cluster energy / tracks => agreement cl/tr better than 0.5% well described by the MC EclLAR (GeV) Correction function for energy reconstructed in data and MC / year of analysis

  13. Calibrations LAR Clusters/Tracks The calibration of the LAR Cl/tracks is correct The calibration of SPACAL can be checked with EMPZ and intercalibration with Me

  14. Control plots 96-00

  15. DVCS : vertex treatment No track => no vertex To calculate kinematics we use the nominal value of ZVTX Elastic DVCS MC (MILOU) normalized / nb events Elastic BH contribution (COMPTON) normalized to the lumi of COMPTON justified by the control sample analysis Proton-dissociation DVCS [MILOU] + BH (above the green line) [COMPTON] => important contribution : for BH we keep the normalisation from COMPTON and for MILOU we normalize to a sample of noTAG events Note : at low y (DVCS sample), MILOU (in BH mode) ~ elastic COMPTON

  16. DVCS : Proton Dissociation TAG events Sample => FMD or PRT signal Elastic contribution BH contribution MILOU in proton-dissociation mode is generated with the t slope [d/dt~eBt] B=1.5+/-0.5 GeV-2 The Coplanarity=|e-| ~ flat for DVCS-Pdiss // low “B” value vs elastic nb pair of hits FMD L0+1+2

  17. DVCS : calibrations checks Same conventions as before Fraction of proton-dissociation in the noTAG sample : 16% for 96-97 (+/-8%) 10% for 99-00 (+/-5%)

  18. DVCS : coplanarity =|e-| Broadening of the distribution w.r.t. BH control sample => more events have larger |t| values / BH sample also reflected on Me or |t| spectra BH sample steeper fall for BH

  19. Shapes for the  In the LAR we have a good description of the shape estimators for the real photon => the H1 simulation with the new cluster shape parametrization is doing well !

  20. DVCS 96-00 , backgrounds are taken into account (DIFFVM) : 1% for 99-00 3.5% for 96-97 => 2 samples 96-97 at low Q² 99-00 at larger Q²

  21. 96-97 : 2 GeV² < Q² < 20 GeV² 30 GeV < W <120 GeV |t| < 1 GeV-2 with <Q²> = 4 GeV² <W> = 71 GeV

  22. 99-00 : 4 GeV² < Q² < 80 GeV² 30 GeV < W <140 GeV |t| < 1 GeV-2 with <Q²> = 8 GeV² <W> = 82 GeV

  23. Cross sections determination Note : we come back later on the cross-section in t Cross-section (ep) : 5% inefficency for the BDC in 99e+ for Q² < 20 GeV² ; 3.5% CJC mixing in 97 ; trigger efficency(Q²) for 96 Related to the *p cross-section via the flux factor  : => we can compute *p and determine a and n iteratively… The use of correction factors Nrec/Ngen is jsutified by the good understanding of DATA/MC we have presented (detector resolutions correctly described,…)

  24. Systematics  Substraction of proton dissociation : 8% (96-97) ; 5% (99-00)  Correction factor : <A(Q²;W)/A>~5% [<A(t)/A>~7% with a max of 12% -last bin-] note : <A(Q²;W)/A>~5% = 4% (varying the t-slope)  3% PRT reweighting effect  Determination of a and n : => / = 7% (96-97) ; 5% (99-00)  Substraction of the BH contribution : 5% => / = 3% in the last W bin  Luminosity measurement : 1.5%  Noise from FMD 1.5% (96-97) ; 0.1% (99-00)  CJC noise : 2%  ZVTX position : 1.5 cm (to take into account the treatment of ZVTX) => / < 4.5%  Electron energy : 1% => / < 3%  Photon energy : 2% => / < 3%  Electron angle : 1.3 mrad => / < 4%  Photon angle : 3 mrad => / < 3%

  25. Q² x-sections Correction factors (Q² bins in GeV²) => 99-00 [4;6.5] : 0.16 => 1 without fid. cuts [6.5;11] : 0.34 [11;20] : 0.67 [20;30] : 0.60 [30;80] : 0.58 96-97 [2;4] : 0.08 => 0.96 without fid. cuts [4;6.5] : 0.21 => 0.92 id. [6.5;11] : 0.39 [11;20] : 0.48 We can combine these measurements w.r.t. luminosities of both sample => one global (Q²) for 46.5 pb-1

  26. Q² x-sections Comparisons with previous results Good agreement with EPS03 prel. There’s a difference of about 10% w.r.t. 97 analysis  difference in the calculation of the correction factors. In 97 analysis, Ngenwas calculated with the cut on PT;LAR included, NOT in the present analysis… with the PT;LAR cut

  27. W x-sections Correction factors (W bins in GeV) => 99-00 [30;60] : 0.31 [60;80] : 0.37 [80;100] : 0.47 [100;120] : 0.35 [120;140] : 0.18 => 0.97 without fid. cuts 96-97 [30;60] : 0.16 => 0.94 without fid. cuts [60;80] : 0.23 => 0.95 id. [80;100] : 0.18 => 0.96 id. [100;120] : 0.09 => 1 id. W dependence for the 2 values of Q² Here again, we can combine these measurements w.r.t. luminosities of both sample => (W)

  28. W x-sections Comparions with previous results Good agreement with EPS03 Same comment as before for the comparison with 97 analysis with the PT;LAR cut

  29. Cross section(t) Correction factors (|t| bin in GeV²) 99-00 [0;0.2] : 0.26 => 0.92 without fid. cuts [0.2;0.4] : 0.49 [0.4;0.6] : 0.62 [0.6;1.0] : 0.69 96-97 [0;0.2] : 0.13 => 0.87 without fid. cuts [0.2;0.4] : 0.21 => 0.97 id. [0.4;0.6] : 0.36 [0.6;1.0] : 0.71 For cross section determination, same methode as before with : => two values of B for the 2 samples (different Q²) Note that we can not combine to the same value of Q² as B “may” depend on Q² To determine a global B value, we just combine the 2 samples w.r.t. luminosities => B=5.84 +/- 0.80 GeV-2 at Q² = 6 GeV²

  30. (Q²)/models Error band is calculated with B=5.84 +/- 0.80 GeV-2 QCD predictions : good agreement with CTEQ parametrization for the diagonal part of the input for GPDs => HS,V,g(x,)  QS,V,g(x) : DGLAP the “internal” skewing is generated by the NLO evolution. -in ERBL domain H is continuous and + 2 first “polynomial sum rules” Dipole models : DD overestimates the data ; FM is reproducing nicely the measurements

  31. (W)/models Error band is calculated with B=5.84 +/- 0.80 GeV-2 Again a good agreement is found with CTEQ for QCD models and with FM for dipole inspired models. Note that with the extraction of d/dt we have a stronger constraint on B (compared to previous analysis) => possible discrimination between parametrizations/ models…

  32. Analysis summary Measurement of the DVCS x-sections at HERA I (e+) => L = 46.5 pb-1 => increase of the statistics (factor 4) => 2 bins in Q² as a function of W 2 bins in Q² as a function of t All simulations are done with the new cluster shape parametrization Noise files for FMD and new reweight of PRT Better treatment of the Z vertex Proton dissociation included in MILOU (for DVCS) and SOPHIA for COMPTON => t slope measurement for DVCS process possible (never measured before)

  33. Conclusions On the above plot, the 2 best models (with b(Q²)) => * DVCS is a precise NLO QCD calculation * GPDs models using only dynamicaly generated skewing from classic PDFs leads to very good agreement in shape and normalization with DVCS * Large sensitivity to the ERBL domain (factor 5) DRAFT distributed within 1 week

  34. Comments on BH in MILOU MC Complete formula for BH = d  [A/P()+Bcos()/P()+Ccos(2)/P()] With P() = 1+Xcos() and X~2(2-y)/(1-y)[-t(1-y)/Q²] For integrals convergence we need X<1 which is the case (“most of the time”) due to t/Q²<<1 1. when y->1 (BH sample), X can be very close to 1- (if >1, then the event is not considered) => d  cos(2)/(1+Xcos()) is very unstable numerically => source of the differences between BH new MC and COMPTON => neglecting terms [B] and [C] gives an agreement of up to 5% with COMPTON 2. at low y (for the DVCS sample), X<<1 => very good agreement up to 3%

  35. Calibrations LAR Clusters/Tracks

  36. Effect of fiducial cuts (z/) in LAr Z (LAr)  (LAr)

  37. LAr Control plots Px;Py;Pz

  38. Third Cluster (E3<0.5 GeV) E3 (energy of the third cluster…) Sum of Ecl except the 2 EM clusters

  39. Control plots Lar DVCS “elastic” (noTAG sample)

  40. Control plots Lar DVCS “elastic” (noTAG sample)

  41. Control plots For P-dissociation (TAG sample) : Third cluster E3 in the “elastic” case (noTAG sample) : 96-00

  42. Kinematics determined from DA Q² (GeV²) W (GeV) -t=|PTe+PT|² (GeV²)

  43. Measuring the  Ee(ini)=27.5 GeV Need a boost+rotation in the Belitsky frame => Ei(k) must be known precisely => we take Ei=EMPZ/2 to take into account QED radiation effects Ee(ini)=EMPZ/2 : DVCS generated with MILOU (elastic) : no cut  is calculated With gen. variables DVCS generated with MILOU (elastic) with all cuts applied :  is calculated with rec. variables (EMPZ also…) => LARGE fluctuations (factor ~2)!  (rad)

  44. The behaviour is similar for the BH contribution to the Signal Sample with even larger fluctations => impossible to extract an asymetry [coming from the BH/DVCS interference] ! DATA/MC for 99-00 period [Ee(ini)=EMPZ/2] EMPZ (GeV) GEN/REC for the Signal Sample BH MC (green) and DVCS MC (black) (normalized to nb of events)

  45. The use of BST ? For one part of the 00 sample ~10 pb-1 we can use the vertex reconstructed from BST instead of ZNOM We have checked that it does not bring improvements for DVCS events w.r.t. the present analysis Differences at large t are covered by the systematics on ZVTX and A(t)

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