Forward QCD studies and prospects at the LHC

Transcription

1 Forward QCD studies and prospects at the LHC (*) INT Workshop on Perturbative and Non-Perturbative Aspects of QCD at Collider Energies 1 Seattle, September 13-18, 2010 (*) As CMS member, I acknowledge the contributions of other LHC experiments presented in this talk

2 Outline Why Forward Physics? LHC experiments First LHC results Prospects Conclusion 2 2/30

3 Why Forward Physics? 3 3/30

4 QCD processes in pp collisions elastic soft elastic scattering inelastic double diffractive dissocation single diffractive dissocation minimum bias inelastic scattering central exclusive production hard high-t elastic scattering hard diffraction jet production 4 4/30

5 QCD processes in pp collisions elastic double diffractive dissocation single diffractive dissocation minimum bias inelastic scattering ent tion central exclusive production g ev in erly rrec g co n owi und d sha soft elastic scattering inelastic s hard high-t elastic scattering hard diffraction jet production 5 P. Skands: Soft physics became viewed as a non-perturbative quagmire, into the depths of which ventured only fools and old men... 5/30

6 Measuring the underlying event Inclusive distributions: multiplicity, pt,... distributions particle correlations,... Exclusive observables: the underlying event can be studied with well-defined observables and in a modelindependent way Rick Field plots 6 6/30

7 Multi-parton interaction (MPI) tunes Implementation in PYTHIA 6 Perturbative 2-to-2 partonic cross-section is regularized by a cutoff pt0 pt0 governs the description of the amount of MPI: larger MPI activity for smaller values of pt0 Energy dependence of pt0 given by MPI may account for both shadowing corrections and underlying event activity! 7 Cannot be disentangled: MPI tunes to inclusive (non diffractive) data suffer from uncertainty on the diffractive component (background)... measurements needed! 7/30

8 Going forward on the kinematic plane Forward production of particles/jets: Collision of a low and high x parton X p x1 x2 p Collision of 2 low x partons + QCD evolution X forward jet 8 p x1 x2 p 8/30

9 DGLAP Going forward on the kinematic plane sa tu ra t io n BK JIMWLK BFKL Parton densities increase towards low x described by DGLAP predicted by BFKL 9 non-linear evolution equations saturation scale (can be bigger than ΛQCD) 9/30

10 LHC experiments 10 10/30

11 Acceptance of LHC experiments tracker counter ECAL HCAL μ det particle ID ATLAS CMS LHCb ALICE TOTEM tracking telescopes with 3 < η < 5 and 5 < η < 6.5 roman pots 11Upgrade projects: AFP/HPS (formerly FP420) Forward scintillators for CMS LHCf zero degree calorimeter 11/30

12 First LHC results 12 12/30

13 Observation of diffraction at CMS +z E±pz -method: Minus side proton dissociates rapidity gap on plus side HF+ HF- Plus side proton dissociates rapidity gap on minus side Double dissocation/inelastic scattering central or no rapidity gap 13 13/30

14 Observation of diffraction at CMS 2.36 TeV 0.9 TeV Σ E+pz ~ ξ Excess over non-diffractive MC prediction at low ξ and with forward rapidity gap E3<η<5 Σ E+pz ~ ξ E3<η< TeV Note: event selection requires primary vertex, and therefore tracks with η < 2.5 acceptance dies out at small ξ 14/30

15 Study of diffraction-enhanced event sample by ATLAS MBTS Enhancement of diffractive contribution: using Minimum Bias Trigger Scintillators (MBTS) with acceptance for 2.09 < η < 3.84 selecting events with hits on exactly one side of the MBTS MC models predict increase of diffractive contribution from 12-20% to 85-98% Note: basic selection includes track with η < 2.5 acceptance cut at low ξ (large model variation) Ratio of single-sided events to single- and double-sided MBTS events (detector level): 15 SD+DD / SD+DD+ND ratio ~ 30%, independent of model diffractive contribution in PHOJET needs to be increased, PYTHIA 6/8 are fine within uncertainties 15/30

16 Study of diffraction-enhanced event sample by ATLAS 16 PYTHIA 6 has not enough particles pt of particles in PYTHIA 6 is too soft 16/30

17 Study of diffraction-enhanced event sample by ATLAS No surprise... PYTHIA 6 has no hard diffraction! 17 Decomposition in subprocesses: high pt tail in PYTHIA 6 completely due to ND PYTHIA 8 also produces high pt particles in SD and DD (as does PHOJET) 17/30

18 Measurement of forward energy flow by CMS Motivation sensitive to parton radiation and to multi-parton interactions complementary to measurements in central region Strategy: measurement of forward energy flow in HF (3 < η < 5.2) minimum bias events and events with a hard central dijet system with η < 2.5, Δφ(j1,j2) π < 1.0, pt > 8 (20) GeV at s = 0.9, 2.36 (7) TeV CASTOR (5.2 < -η < 6.6) 18 MPI sensitivity enhanced in dijet events! 18/30

19 Measurement of forward energy flow by CMS central dijet minimum bias uncorrected distributions! 0.9 TeV 2.36 TeV 7 TeV 0.9 TeV 2.36 TeV 7 TeV 19 min. bias: increase of energy flow vs. s stronger in data than in MC (true for all tunes) central dijet: ProQ20 (similar to PYTHIA 8) is the best tune (cf. DW best tune for charged particle spectra at central rapdity) 19/30

20 Measurement of forward energy flow by CMS CASTOR (5.2 < -η < 6.6) calibration suffering from remnant field in forward region caused by CMS solenoid only raw, uncalibrated data available CASTOR data confirm strong rise of forward energy flow with s! none of the MPI tunes can reproduce this arbitrary normalization! 0.9 TeV 7 TeV TeV 20/30

21 Forward jets in CMS CMS Experiment at the LHC, CERN Date Recorded: :52:12 CEST Run/Event: / Candidate dijet event at 900GeV ET (GeV) Jet 1 η Jet 2 φ 21 1 jet with pt > 10 GeV and 3.0 < η < 5.0 Jet 1: pt = 13.4 GeV, η = 4.10 and φ = 1.34 ET cut on CaloTowers displayed > 0.3 GeV Jet 2: pt = 13.8 GeV, η = and φ = /30

22 Forward jets in CMS First measurement of forward jets in the range 3.2 < η < 4.7! 22 7 TeV data, detector level distributions (no unfolding, no systematic uncertainties) PYTHIA 6, tune D6T gives good description of data 22/30

23 Jets at large rapidity separation from ATLAS Motivation: Search for colour-singlet exchange, BFKL dynamics, wide angle soft gluon radiation Strategy: Look for jet activity in η range between dijet system two ways to define boundary jets: highest pt jets are boundary jets (selection A) most forward/backward jets are boundary jets (selection B) Δη Event and jet selection: s = 7 TeV boundary jet pt > 30 GeV; average pt > 60 GeV gap fraction defined by third jet veto scale Q0 = 30 GeV 23 23/30

24 Dijets at large rapidity separation from ATLAS no third jet veto! Selection A Selection B Selection A Selection B 24 data/mc discrepancy within energy scale uncertainty 24/30

25 Dijets at large rapidity separation from ATLAS third jet veto pt3 < 30 GeV Gap fractions Selection B Selection A Selection B Selection A 25 good description by PYTHIA (w/o any BFKL/colour singlet exchange) analysis will be improved: larger Δy, lower Q0, smaller systematic uncertainty 25/30

26 Prospects 26 26/30

27 Establishing hard diffraction at the LHC First diffractive dijet candidates in 7 TeV data (HLT_JET15U, EHF < 4 GeV) Selection will be done using forward detectors to tag rapidity gaps (e.g. HF, CASTOR for CMS) single diffractive POMWIG MC non-diffractive MADGRAPH MC 27 27/30

28 Low-x parton density constraints by LHCb % uncertainty Forward spectrometer with acceptance of 1.9 < η < 4.9 Measurement of forward Drell-Yan photons and Z, W± is sensitive to parton density at low x (down to 10-6) Significant improvements to gluon PDF possible in the near future MSTW08 current with 0.1 fb-1 of LHCb data with 1 fb-1 of LHCb data 28 x 28/30

29 Search for saturation effects in p-pb collisions by ALICE Heavy flavour production at forward rapidity Colour Glass Condesate (CGC) predicts depletion of charm yield at low pt and forward rapidity (2.5 < η < 4) Detection of μ's (from D0 decays) is challenging (mainly a trigger issue) J/ψ simpler to measure more promising channel RPb-p for c- and s = 8.8 TeV 29 CGC gluon distr. in a saturated nucleus 29/30

30 Conclusion Forward physics at the LHC is related to underlying event and multi-parton interactions studies allows to investigate parton saturation effects First Forward physics measurements at the LHC observation of diffraction study of diffractive-enhanced particle spectra forward energy flow forward jets jets at large rapidity separations 30 More to come soon! suggestions on what else to measure are most welcome! 30/30

31 Backup slides 31 31/30

32 MPI tunes Energy evolution of pt0 cut-off in perturbative 2-to-2 partonic cross section 32 32/30

33 LHC luminosity 33 33/30