Hadronic Final States and QCD at HERA

Transcription

1 Hadronic Final States and QCD at HERA Claire Gwenlan (Oxford, STFC Fellow) a celebration of HERA and the UK role IOP meeting: 25 June 2008 With many thanks to: = This means significant UK effort! Claudia Glasman, Thomas Kluge, Steve Maxfield, Mark Sutton, Dan Traynor

2 Hadronic Final States What? Final states containing hadrons (particles, jets) Why? Study of Hadronic Final States at HERA allows: tests of our understanding of pqcd extraction of QCD parameters (α s, ) in regions where experimental and theoretical uncertainties are small constraints on proton and photon structure highlight areas that require further theoretical input low-x region, non-perturbative hadronisation process, underlying event, UK has played a leading role in all of the above 1/26

3 and How? e± p proton structure γ* e± Hard Scatter (pqcd,α s ) fragmentation proton remnant/ underlying event We Measure: Jets Cross Sections Substructure Forward Jets Particle Production Hadrons Photons Event Shapes to obtain results on: tests of pqcd predictions measurements of α s proton, (photon) structure explore low x region: breakdown of DGLAP? non-perturbative effects (hadronisation, underlying event, ) 2/26

4 and How? e± p proton structure γ* e± Hard Scatter (pqcd,α s ) fragmentation proton remnant/ underlying event We Measure: Jets Cross Sections Substructure Forward Jets Particle Production Hadrons Photons Event Shapes (Rough) outline of talk: tests of pqcd predictions measurements of α s proton, (photon) structure explore low x region: breakdown of DGLAP? non-perturbative effects (hadronisation, underlying event, ) (+ a few extras if I have time ) 2/26

5 Jet Production at HERA in case of resolved γp, need photon PDF as well Observable = Σ = flavours Σ flavours PDF PDF PDF γ matrix-element parton density: describes structure of proton sub-process cross section (calculable in pqcd) HERA: ideal environment for precision studies of QCD tests of hadron-induced reactions (unlike e+e-) but cleaner than pp 3/26

6 Blast from the Past Some FIRST measurements of jet production at HERA (in DIS and γp) from FIRST H1 physics paper in γp in DIS 1992 data (27nb-1) June-July July 1992 (51 events) 4/26

7 Situation Today EXAMPLE: inclusive jets in DIS NOW: many many very high precision measurements in DIS and γp: extending to high Q 2,E T single,double,triple differential inclusive,dijet,multijet (+ using infra-red/collinear safe jet definition) typical experimental precision ~5% C.F. ~50-60% at the Tevatron Small hadronic energy scale uncertainty by energy balance with scattered e 5/26

8 Jets at HERA input from Liverpool Some more examples (in DIS and γp): excellent description by NLO QCD over many orders of magnitude in the cross section (pqcd working well!!!) DIS theory uncertainty often larger than experimental!! also sensitive to strong coupling α s : fundamental parameter of QCD (but has to be extracted from experiment) Glasgow FIRST α s from γp DIS α s γp 6/26

9 α s from HERA extraction of α s : Generate cross section predictions for different α s Parameterise: σ(α s ) = Aα s +Bα s 2 Compare to measured cross section Read off α s and it s uncertainty cross section NLO QCD measured value α s (M Z ) Before looking at set of HERA α s values, take a look at one more measurement 7/26

10 Jet Substructure Why? more stringent tests of pqcd predictions study differences between quark- and gluon-initiated jets determinations of α s γp: Quark and gluon jets different (gluon broader) Direct processes, mostly quarks through gγ qq ( like DIS) Resolved processes give rise to quark and gluon jets through: qg qg, gg gg, Results: consistent with expectations - also method to obtain quark- or gluon-enriched samples Bristol and Glasgow including FIRST measurement in γp resolved photon region 8/26

11 Jet Substructure Why? more stringent tests of pqcd predictions study differences between quark- and gluon-initiated jets determinations of α s γp: Quark and gluon jets different (gluon broader) Direct processes, mostly quarks through gγ qq ( like DIS) Resolved processes give rise to quark and gluon jets through: qg qg, gg gg, Bristol and Glasgow including FIRST measurement in γp DIS: shape well described by NLO QCD, sensitivity to α s 8/26

12 HERA α s HERA α s summary Wealth of precision extractions from HERA from variety of observables Consistent with each other and with world average Dominating uncert.: theory (need NNLO) UK led ** * * Bristol, Glasgow, Oxford, UCL more than 1/3 of these analyses led by UK groups *= mentioned in this talk 9/26

13 HERA Combined α s Wealth of precision extractions from HERA from variety of observables average of HERA determinations [C. Glasman et al. ] (now included in Bethke world average) RECENT IDEA (HERA combined α s ): combined H1+ZEUS fit of one observable: inclusive jets in DIS program towards the HERA α s (+ another example of recent collaborative effort) HERA combined α s (2007): α s (M Z ) = ±0.0019(exp.)±0.0026(th.) input from Liverpool, QMUL [c.f. world average in 1989: α s (M Z )=0.11±0.01 (only at 10%) - now PDG and Bethke world averages constantly updated] 9/26

14 HERA α s HERA jet data Wealth of precision extractions from HERA from variety of observables clear running of α s α s (Q) experimental precision as good as LEP 9/26

15 Final States and the Gluon Gluon in proton mainly constrained by HERA S.F. data only indirectly sensitive to gluon constrains low x region Global fits for proton structure: high x gluon poorly known impact on high energy pp collisions low x: gluon large DGLAP sufficient? LO QCD: σ ~α s.g(x) Jet Jet Several final states directly sensitive to the gluon in the proton jets, prompt photon, heavy flavours, 10/26

16 Gluon PDF HERA Jets: sensitive to gluon (and quark) content of proton γp: also to photon structure led by Oxford and UCL PIONEERING method allowing FIRST rigorous use of jet data in an NLO QCD fit TopCite 50+ included in fit for proton PDFs DIS and γp data used (along with NC/CC inclusive data) 11/26

17 Gluon PDF HERA Jets: sensitive to gluon (and quark) content of proton γp: also to photon structure led by Oxford and UCL PIONEERING method allowing FIRST rigorous use of jet data in an NLO QCD fit Gluon uncerts. substantially reduced!!! (medium to high x) precision extraction of α s also obtained (treat as additional free parameter in fit see backups) Proof of Principle: wealth of other HERA-I and -II data also now available!!! 11/26

18 Prompt Photons Prompt Photons: further constraints on proton and photon structure sensitive/clean probe of QCD less sensitive to hadronisation also background to H γγ FIRST observations in γp and DIS at HERA γp led by Glasgow Group resolved γ DIS ep: Q 2 >> 1 GeV 2 γp: Q 2 ~ 0 GeV 2 Latest Status (more recent H1+ZEUS measurements): SURPRISE: NLO QCD too low at small E T 12/26

19 Prompt Photons data from Glasgow Group Prompt Photons: further constraints on proton and photon structure sensitive/clean probe of QCD less sensitive to hadronisation also background to H γγ ep: Q 2 >> 1 GeV 2 resolved γ γp: Q 2 ~ 0 GeV 2 (pb GeV -1 ) dσ/de T γ extra contribution theory: MRSTQED04 data: ZEUS (hep-ex/ ) (GeV) Interesting Aside: proton PDF containing photons (MRSTQED04) able to describe data? E T γ e prediction only currently available for inclusive prompt γ in DIS plot from James Stirling p 12/26

20 Multijets (and the Underlying Event) expected to contribute at small x γ EXAMPLE: 4-jets at LO: O(αα s 3 ) α s α s α s Rich testing ground: multiparton interactions: secondary interaction(s) between beam remnants multijet production directly sensitive to higher order QCD tests of parton showers in Monte Carlo models Also: sensitive to multiparton interactions /underlying event test/tune models 13/26

21 Multijets (and the Underlying Event) Glasgow; Bristol and UCL Need contribution from multi-parton interactions at low x γ (and low invariant mass) Can tune models to data (HERWIG+MPI is in this case!) input for LHC models!!! FIRST measurement of 3jets in γp; FIRST (and still only) measurement of 4jets in γp NB: HERWIG MPI model (Jimmy) also UK contribution; created for HERA γp precise hadronic data (testing ground for higher order pqcd predictions, as well as hadronisation and MPI models) Since these data published, γp O(α s 3 ) calculation has become available 14/26

22 Gaps Between Jets IDEA: search for strongly interacting hard colour singlet exchange: QCD pomeron (complementary to other diffractive studies) standard QCD:- exchange of coloured quark or gluon hadronic activity between jets 2π φ LOOK at high-p T jets, with large η: [means that s >> -t~ p T 2 >> Λ QCD 2 ] (diffractive limit, yet perturbatively calculable) proton remnant η Gap Jet colour-singlet exchange:- little or no activity in region between jets a Gap 0 Jet photon remnant η excess of Gap events evidence for strongly interacting hard CSE 15/26

23 Gaps Between Jets Manchester; UCL and Oxford IDEA: search for strongly interacting hard colour singlet exchange: QCD pomeron (complementary to other diffractive studies) Fraction of events with a gap (f): measured excess over predictions of standard QCD!!! can be described by models with colour singlet exchange excess of Gap events evidence for strongly interacting hard CSE 15/26

24 Problematic Regions: γp to DIS REMINDER: γp: direct and resolved photon resolved is suppressed as Q 2 increases so not usually considered in DIS calculations BUT: separation between γp and DIS is unclear!!! Results (for low Q 2 DIS): NLO QCD fails at low E T, Q 2 MC with resolved photon OK Need resolved virtual photon? input from Liverpool [Glasgow: similar analysis on ZEUS] 16/26

25 Low x: breakdown of DGLAP? No evidence for breakdown of DGLAP in S.F. data turn to HFS DGLAP: k T increases Probing Parton Dynamics: forward jets used to hunt for signs of breakdown of standard DGLAP evolution and onset of BFKL dynamics expected at low x DGLAP: assumptions mean that large transverse energy jets/particles in forward direction (near proton) are suppressed (i.e. k T1 << k Tn ) BFKL (expected at small x): no such suppression in forward direction HERA: DIS probes down to x Bj ~ /26

26 Forward Jets input from Liverpool DGLAP NLO QCD RG: DGLAP Monte Carlo CDM: unordered k T - Direct DGLAP fails - Resolved photon MC better ( mimics higher orders) NB. same data in both - BFKL-like (CDM) also better BUT (still) no full BFKL calculation Hints of breakdown of DGLAP but status still unclear!!! 18/26

27 Hadronisation Process Hadronic Final State can be characterised by Event Shape observables Event Shape Variables: describe energy and momentum flow (i.e. the shape ) of the final state inspired by e+e- measurements typical example: thrust T = p n i i (measures collimation of hadronic system) i p i thrust: pencil-like 1-T 0 spherical 1-T > 0 also broadening, C-parameter, jet mass, Sensitive to non-perturbative hadronisation process 19/26

28 Event Shapes in DIS ZEUS effort led by Glasgow Hadronisation described by: (usually) phenomenological models as implemented in MCs OR: Power Correction Model [Dokshitzer and Webber] - understand hadronisation from first principles? ZEUS data vs pqcd+pc predictions Good agreement Compare event shape data to combination of NLO QCD and power correction model (F = event shape) ZEUS F = F perturbative + F power-correction pqcd bit hadronisation bit (~1/Q) Depends on new parameter α 0 20/26

29 Event Shapes in DIS can get good description of event shapes using power corrections (added to pqcd) universal non-perturbative parameter: α 0 = 0.5 ±10% can also extract α s (consistent with world average) ZEUS effort led by Glasgow Compare event shape data to combination of NLO QCD and power correction model (F = event shape) F = F perturbative + F power-correction pqcd bit hadronisation bit (~1/Q) Depends on new parameter α 0 20/26

30 There is nothing new to be discovered in physics now. All that remains is more and more precise measurement. Lord Kelvin vs. Dave Milstead 21/26

31 Chasing the Magnetic Monopole monopoles? natural explanation for electric charge quantisation (Dirac) predicted or permitted in unification theories (a bit of novelty value!!!) magnetic charge: g = nhc 2Q when n=1, g = g D (Dirac magnetic charge) Q = electric charge (eg. e for electron) previous experimental searches: no conclusive evidence In some theories, monopoles can have low masses - even O(10 GeV) produced at HERA? e p α em e HERA α g m α g m 22/26

32 Chasing the Magnetic Monopole (a bit of novelty value!!!) First (and only!!!) search at HERA Liverpool and Lancaster magnetic monopoles produced in ep collisions may stop in the beam pipe if stable, they remain permanently trapped H1 beam pipe:- home to monopoles? IDEA: Chop H1 beampipe into strips Pass through superconducting coil expect persistent current from monopoles (but not from dipoles) Performed at Southamption Oceanography Centre, UK 23/26

33 Chasing the Magnetic Monopole (a bit of novelty value!!!) First (and only!!!) search at HERA Liverpool and Lancaster Result look for non-zero persistent currents some significant one-off measurements not reproducible no monopoles harmed (or found!) in this experiment 23/26

34 Pentaquarks at HERA? strangeness state Θ* search for: Θ + K 0 p and Θ + K 0 p (minimum quark content uudds) s s 1522 MeV ZEUS TopCite 100+ Θ*: H1 and ZEUS not incompatible remains to be resolved 24/26

35 Pentaquarks at HERA? charmed state Θc mass: 3099±3±5 MeV width: 12±3 MeV not seen by ZEUS TopCite 100+ H1 observed D*p resonance in DIS and γp with HERA-I data (minimum quark content uuddc) pentaquark candidate 25/26

36 Pentaquarks at HERA? charmed state Θc mass: 3099±3±5 MeV width: 12±3 MeV not seen by ZEUS TopCite 100+ H1 observed D*p resonance in DIS and γp with HERA-I data (minimum quark content uuddc) pentaquark candidate H1: peak not confirmed at HERA-II (also, no evidence for Ξ 5q --(0) at HERA) 25/26

37 Summary HFS studies at HERA are a vital tool for understanding the underlying QCD process both at HERA and elsewhere!!! Many many precision measurements now exist, and improved calculations have become available throughout HERA running With current understanding, QCD works(!), describing HERA data excellent agreement with pqcd over wide range of scales many consistent extractions of α s ; running in accordance with QCD Some regions not so well understood cannot yet conclusively confirm observation of BFKL dynamics Magnetic monopoles ; Pentaquarks??? UK has made a massive and often unique contribution to the study of the Hadronic Final State at HERA + mustn t forget the HFS coordinators, from: Bristol,Glasgow,Oxford,Liverpool,QMUL,UCL 26/26

38 and Finally, Apologies.. For those areas in which the UK has made a huge contribution but which unfortunately I have not had time to even touch on today Fragmentation studies (Glasgow, Lancaster, QMUL) charged particle multiplicities scaled momenta Strange particle production (+ QCD instantons) (Oxford) (Anti-)deuteron production (Lancaster) Azimuthal asymmetries/charged multiplicities (Glasgow) Jets in CC DIS (Glasgow) + other photoproduction studies (at least partially covered by Matthew) (I may have missed others?!!!) (For at least some of these I have put a bit of extra information in the backups )

39 Backups

40 Scaling Violations in γp Glasgow Group Compare cross sections for same reaction ep e+jet+x at different γp centre-of-mass energies (W γp ) FIRST observation of scaling violation in γp scaling: ratio of scaled cross section as a function of x T =2E T jet /W γp should be unity Results show non-scaling behaviour in accordance with QCD

41 Jet Substructure Bristol and Glasgow

42 α s extraction Oxford and UCL Groups NLO QCD fit with HERA jets: jet data sensitive to quark and gluon densities in proton breaks correlation between gluon and α s precision determination of α s possible: α (M )= ± (exp.) s Z ± (model)

43 Scaled Momentum QMUL and Lancaster Groups Charged Multiplicities: HERA agrees with other experiments over wide range in energy scale Supports concept of quark fragmentation universality MC models cannot describe data over all Q2 and xp NLO+FF based on e+edata fail to describe xp distribution

44 Strange Particle Production Oxford Group Measurements of: K s0 and Λcross sections Λ/Λ asymmetry Λ/K s0 production ratio Λ polarisation typical set of results ARIADNE mostly OK Ratio of N(K s0 )/N ch (not shown) suggest value of s/u 0.22 more appropriate? N(Λ)=N(Λ) Also, no evidence for Λ polarisation

45 Charged Particle Asymmetries Glasgow Group angle of particle, φ, in the hadronic centreof-mass frame as a function of lower momentum threshold p c Azimuthal dependence has form: 1 dn = A + Bcosφ + Ccos2φ N dφ predicted by QCD Non-perturbative effects small for cos2φ term At high p c, see FIRST clear observation of cos2φ contribution unambiguous evidence of perturbative QCD contribution

46 HFS analyses with UK input ZEUS High-ET dijet photoproduction (Phys. Rev. D76 (2007) ) Photoproduction of events with rapidity gaps between jets at HERA (Eur. Phys. J. C50 (2007) 283) Measuerment of K0s, Lambda, Antilambda production at HERA (Eur. Phys. J. C51 (2007) 1) Event shapes in DIS (Nucl. Phys. B 767 (2007) 1) The dependence of dijet photoproduction on photon virtuality in ep collisions at HERA (Eur. Phys. J. C35 (2004) 487) Observation of isolated high-et photons in DIS (Phys. Lett. B 595 (2004) 86) Substructure dependence of jet cross sections at HERA and determination of α s (Nucl. Phys. B 700 (2004) 3) Three- and Four-Jet Final States in photoproduction at HERA (Nucl. Phys. B 792 (2008) 1) Jet production in charged current e+p DIS at HERA (Eur. Phys. J. C31 (2003) 149) Scaling violations and determination of as from jet production in γp interactions at HERA (Phys. Lett. B 560 (2003) 7) Measurement of event shapes in DIS at HERA (Eur. Phys. J. C27 (2003) 531) Dijet photoproduction at HERA and the structure of the photon (Eur. Phys. J. C23 (2002) 4) High mass dijet cross sections in photoproduction at HERA (Phys. Letts. B531 (2002) 9)

47 HFS analyses with UK input ZEUS (cont.) Study of the effective transverse momentum of partons in the proton using prompt photons in photoproduction at HERA (Phys. Letts. B 511 (2001) 19) Measurement of azimuthal asymmetries in DIS (Phys. Letts. B 481 (2000) 2 Measurement of inclusive prompt photon photoproduction at HERA (Phys. Letts. B 472 (2000) 1 Angular and current-target correlations in DIS at HERA (Eur. Phys. J. C12 (2000) 1) Measurement of dijet photoproduction at high transverse energies at HERA (Eur. Phys. J. C11 (1999) 1) Measurement of three-jet distributions in photoproduction at HERA (Phys. Letts. B 443 (1998) 394) Dijet cross sections in photoproduction at HERA (Eur. Phys. J. C1 (1998) 1/2, 109) Observation of isolated high ET photons in photoproduction at HERA (Phys. Letts. B 413 (1997) 201) Dijet angular distributions in resolved and direct photoproduction at HERA (Phys. Letts. B 384 (1996) 401) Rapidity gaps between jets in photoproduction (Phys. Letts. B 369 (1996) 55) Dijet cross sections in photoproduction at HERA (Phys. Letts. B 348 (1995) 665) Observation of direct processes in photoproduction at HERA (Phys. Letts. B 322 (1994) 287)

48 HFS analyses with UK input H1 Three- and Four-jet production at low x at HERA (Eur. Phys. J. C54 (2008) 389) Measurement of inclusive jet production in DIS t high Q2 and determination of the strong coupling (Phys. Letts. B653 (2007) 134) Dijet cross sections and parton densities in diffractive DIS at HERA (JHEP 0710:042 (2007)) Charged particle production in high Q2 DIS at HERA (Phys. Letts. B654 (2007) 148) Search for baryonic resonances decaying to Xi pi in DIS at HERA (Eur. Phys. J. C52 (2007) 507) Tests of QCD factorisation in the diffractive production of dijets in DIS and photoproduction at HERA (Eur. Phys. J. C51 (2007) 549) Photoproduction of dijets with high transverse momenta at HERA (Phys. Letts. B 639 (2006) 21) Forward jet production in DIS at HERA (Eur. Phys. J. C46 (2006) 27) A direct search for stable magnetic monopoles produced in positron-proton collisions at HERA (Eur. Phys. J. C41 (2005) 133) Forward pi^0 production and associated transverse energy flow in DIS at HERA (Eur. Phys. J. C36 (2004) 441) Measurement of dijet production at low Q2 at HERA (Eur. Phys. J. C37 (2004) 141) Measurement of anti-deuteron photoproduction and a search for heavy stable charged particles at HERA (Eur. Phys. J. C36 (2004) 413)

49 HFS analyses with UK input H1 (cont.) Measurement of inclusive jet cross sections in photoproduction at HERA (Eur. Phys. J. C29 (2003) 497) Measurement of inclusive jet cross sections in DIS ep-scattering at HERA (Phys. Letts. B542 (2003) 193) Measurement of dijet cross sections in photoproduction at HERA (Eur. Phys. J. C25 (2002) 1) Energy flow and rapidity gaps between jets in photoproduction at HERA (Eur. Phys. J. C24 (2002) 4, 517) Measurement of dijet cross sections in photoproduction and photon structure (Phys. Letts. B 483 (2000) 36) Forward Pi0-meson production at HERA (Phys. Letts. B462 (1999) 440) Measurements of transverse energy flow in DIS at HERA (Eur. Phys. J. C12 (2000) 595) Multiplicity structure of hadronic final states in diffractive deep-inelastic scattering at HERA (Eur. Phys. J. C5 (1998) 3, 439) Evolution of ep fragmentation and multiplicity distributions in the Breit frame (Nucl. Phys. B 504 (1997) 3) A study of the fragmentation of quarks in ep collisions at HERA (Nucl. Phys. B445 (1995) 3) Energy flow and charged particle spectra in DIS at HERA (Z. Phys.C62 (1994) 377) Measurement of hadronic final state in DIS at HERA (Phys. Letts. B298 (1992) 469)

50 UK convenors of HFS (I think!) (alphabetically, not chronologically) H1 Tim Greenshaw Steve Maxfield Dave Milstead Dan Traynor ZEUS Claudia Glasman Nick Brook Peter Bussey Jon Butterworth Mark Sutton Matthew Wing