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: Claudia Glasman, Thomas Kluge, Steve Maxfield, Mark Sutton, Dan Traynor

Hadronic Final States and QCD at HERA

= This means significant UK effort!

http://www.scitech.ac.uk/home.aspx

Hadronic Final States and QCD at HERA Claire Gwenlan25 June'08

Study of Hadronic Final States at HERA allows:• tests of our understanding of pQCD

• extraction of QCD parameters (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, …

Hadronic Final States

Why?

UK has played a leading role in UK has played a leading role in allall of the of the aboveabove

2/26

What? Final states containing hadrons (particles, jets)


… and How?

Particle ProductionParticle

Production

HadronsPhotons

Event Shapes

2/26

We Measure:

eJets Jets

Cross Sections SubstructureForward Jets

proton structureproton structure

e

p

Hard Scatter (pQCD,s)

*

proton remnant/underlying eventproton remnant/underlying event

frag

men

tati

on

frag

men

tati

on

to obtain results on:• tests of pQCD predictions• measurements of s

• proton, (photon) structure • explore low x region:

breakdown of DGLAP?• non-perturbative effects

(hadronisation, underlying event, …)


… and How?

Particle ProductionParticle

Production

HadronsPhotons

Event Shapes

We Measure:

eJets Jets

Cross Sections SubstructureForward Jets

proton structureproton structure

e

p

Hard Scatter (pQCD,s)

*

proton remnant/underlying eventproton remnant/underlying event

(Rough) outline of talk:• tests of pQCD predictions• measurements of s

• proton, (photon) structure • explore low x region:

breakdown of DGLAP?• non-perturbative effects

(hadronisation, underlying event, …)

(+ a few extras if I have time…)

frag

men

tati

on

frag

men

tati

on

2/26


Observable = Σflavours PDF matrix-elementObservable = Σflavours PDF matrix-elementObservable = Σflavours PDF PDF matrix-element Observable = Σflavours PDF PDF matrix-element

Jet Production at HERA

3/26

in case of “resolved” p, need photon PDF as well

HERA: ideal environment for precision studies of QCD tests of hadron-induced reactions (unlike e+e-) but cleaner than pp

parton density: describes structure of proton

sub-process cross section (calculable in pQCD)

Observable = Σflavours PDF matrix-elementObservable = Σflavours PDF matrix-element


Blast from the Past…

4/26

Some FIRST measurements of jet production at HERA (in DIS and p)

in DIS

in p

1992 data (27nb-1) June-July 1992

(51 events)

from FIRST H1 physics paper


Situation Today…

5/26

NOW: many many very high

precision measurements in DIS and p:• extending to high Q2,ET

• single,double,triple differential

• inclusive,dijet,multijet (+ using infra-red/collinear safe jet definition)typical experimental

precision ~5%

EXAMPLE: inclusive jets in DIS

C.F. ~50-60% at the Tevatron

Small hadronic energy scale uncertainty by energy balance with scattered e

Small hadronic energy scale uncertainty by energy balance with scattered e


Jets at HERA

6/26

Some more examples (in DIS and p): excellent description by NLO QCD over many orders of magnitude in the cross section (pQCD working well!!!)

p

DIS

Glasgo

wFIRST s from p

theory uncertainty often larger than experimental !!

input from Liverpool

also sensitive to strong coupling s:

fundamental parameter of QCD (but has to be extracted from

experiment)

s

DIS


s from HERA

0.110 … 0.116 … 0.122

NLO QCD

measured value

s(MZ)

cro

ss

secti

on

7/26

extraction of s:• Generate cross

section predictions for different s

• Parameterise: (s) = As+Bs

2

• Compare to measured cross section

• Read off s and it’s uncertainty

Before looking at set of HERA s values, take a look at one more measurement …


- also method to obtain quark- or gluon-enriched samples

Jet Substructure

Why?• more stringent tests of pQCD predictions• study differences between quark- and

gluon-initiated jets• determinations of s

8/26

Bristol and Glasgow

including FIRST measurement in pp:

• Quark and gluon jets different (gluon broader)

• Direct processes, mostly quarks through g qq (– like DIS)

• Resolved processes give rise to quark and gluon jets through: qgqg, gg gg, …

Results: consistent with expectations

resolved photon regionresolved photon region


Jet Substructure

Why?• more stringent tests of pQCD predictions• study differences between quark- and

gluon-initiated jets• determinations of s

8/26

Bristol and Glasgow

including FIRST measurement in pp:

• Quark and gluon jets different (gluon broader)

• Direct processes, mostly quarks through g qq (– like DIS)

• Resolved processes give rise to quark and gluon jets through: qgqg, gg gg, …

DIS: shape well described by

NLO QCD, sensitivity to s


HERA s HERA s summary

– Consistent with each other and with world average

– Dominating uncert.: theory (need NNLO)

Bristol, Glasgow, Oxford, UCL

more than 1/3 of these analyses led by UK groups…

UK led

• Wealth of precision extractions from HERA from variety of observables

9/26

****

*= mentioned in this talk


• combined H1+ZEUS fit of one observable: inclusive jets in DIS– program towards the HERA s (+ another example of recent collaborative

effort)

average of HERA determinations [C. Glasman et al. ] (now included in Bethke world average)

HERA Combined s


input from Liverpool, QMUL[c.f. world average in 1989: s(MZ)=0.110.01 (only at 10%)- now PDG and Bethke world averages constantly updated]

HERA combined s (2007): s(MZ) = 0.11980.0019(exp.)0.0026(th.)HERA combined s (2007): s(MZ) = 0.11980.0019(exp.)0.0026(th.)

RECENT IDEA (HERA combined s):

9/26


HERA s


clear running of s

experimental precision as good as LEP

HERA jet data

s(Q)

9/26


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?

Several final states directly sensitive to the gluon in the proton jets, prompt photon, heavy flavours, …

Several final states directly sensitive to the gluon in the proton jets, prompt photon, heavy flavours, …

Jet

Jet

LO QCD:

~s.g(x)

10/26


Gluon PDF led by Oxford and UCL

11/26

PIONEERING method allowing FIRST rigorous use of jet data in an NLO QCD

fit

• included in fit for proton PDFs– DIS and p data used(along with NC/CC inclusive data)

TopCite 50+HERA Jets: sensitive to gluon (and quark) content of proton p: also to photon structure


Gluon PDF led by Oxford and UCL

11/26

PIONEERING method allowing FIRST rigorous use of jet data in an NLO QCD

fit

• Gluon uncerts. substantially reduced !!! (medium to high x)

HERA Jets: sensitive to gluon (and quark) content of proton p: also to photon structure

• precision extraction of 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!!!


p: Q2 ~ 0 GeV2ep: Q2 >> 1 GeV2

resolved

Prompt PhotonsFIRST observations in p and DIS at HERA

led by Glasgow Group

p

DIS

12/26

Latest Status (more recent H1+ZEUS measurements): SURPRISE: NLO QCD too low at small ET

Prompt Photons:• further constraints on proton

and photon structure• sensitive/clean probe of QCD

– less sensitive to hadronisation• also background to H


Prompt Photonsdata from Glasgow

Group

ep

extra contribution

plot

fro

m J

ames

Stir

ling

prediction only currently available for inclusive prompt in DIS

Interesting Aside: proton PDF containing photons (MRSTQED04) able to describe data?

12/26

p: Q2 ~ 0 GeV2ep: Q2 >> 1 GeV2

resolved

Prompt Photons:• further constraints on proton

and photon structure• sensitive/clean probe of QCD

– less sensitive to hadronisation• also background to H


Rich testing ground:• multijet production directly sensitive to higher order QCD • tests of parton showers in Monte Carlo models

Rich testing ground:• 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

Multijets

13/26

Multijets (and the Underlying Event)

EXAMPLE: 4-jets at LO: O(s

3) s

s

s

multiparton interactions:secondary interaction(s) between beam

remnants

expected to contribute at small x


Multijets (and the Underlying Event)

14/26

Glasgow; Bristol and UCL FIRST measurement of 3jets in p;

FIRST (and still only) measurement of 4jets in p

• Need contribution from multi-parton interactions at low x (and low invariant mass)

• Can tune models to data (HERWIG+MPI is in this case!)– input for LHC models !!!

• precise hadronic data (testing ground for higher order pQCD predictions, as well as hadronisation and MPI models)

Since these data published, p O(s3)

calculation has become available

NB: HERWIG MPI model (Jimmy) also UK contribution; created for HERA p


JetJet

JetJetproton remnant

photon remnant

0

2

IDEA: search for strongly interacting hard colour singlet exchange: “QCD pomeron” (complementary to other diffractive studies)

Gaps Between Jets

• standard QCD:- exchange of coloured quark or gluon hadronic activity between jets

• colour-singlet exchange:- little or no activity in region between jets a “Gap”

15/26

JetJet

JetJetproton remnant

Gap

photon remnant

0

2

LOOK at high-pT jets, with large : [means that s >> -t~|pT

2| >> ΛQCD2]

(diffractive limit, yet perturbatively calculable)

excess of “Gap” events evidence for strongly interacting hard CSEexcess of “Gap” events evidence for strongly interacting hard CSE


IDEA: search for strongly interacting hard colour singlet exchange: “QCD pomeron” (complementary to other diffractive studies)

Gaps Between Jets

15/26

excess of “Gap” events evidence for strongly interacting hard CSEexcess of “Gap” events evidence for strongly interacting hard CSE

Manchester; UCL and Oxford

Fraction of events with a gap (f):• measured excess over predictions

of standard QCD!!!• can be described by models

with colour singlet exchange


input from Liverpool [Glasgow: similar analysis on

ZEUS]

Need resolved virtual photon?Need resolved virtual photon?

16/26

REMINDER: p: direct and resolved photon• resolved is suppressed as Q2

increases so not usually considered in DIS calculations

Results (for low Q2 DIS):• NLO QCD fails at low ET, Q2

• MC with resolved photon OK

Problematic Regions: p to DIS

BUT: separation between p and DIS is unclear !!!


• 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

Low x: breakdown of DGLAP?

DGLAP: assumptions mean that large transverse energy jets/particles in forward direction (near proton) are suppressed (i.e. kT1 << kTn)

BFKL (expected at small x): no such suppression in forward direction

17/26

HERA: DIS probes down to xBj~10-5

Hadronic Final States and QCD at HERA Claire Gwenlan

No evidence for breakdown of DGLAP in S.F. data turn to HFS

DG

LA

P:

k T incr

ease

s


- Direct DGLAP fails- Resolved photon MC better (‘mimics’ higher orders)- ‘BFKL-like’ (CDM) also better – BUT (still) no full BFKL calculation

Forward Jetsinput from Liverpool

18/26

Hints of breakdown of DGLAP but status still unclear !!!

DGLAP NLO QCD

RG: DGLAP Monte Carlo CDM: unordered kT

NB. same data in both


pencil-like

1-T 0

Hadronisation Process

• Event Shape Variables: describe energy and momentum flow (i.e. the ‘shape’) of the final state– inspired by e+e- measurements– typical example: thrust

(measures collimation of hadronic system)

Hadronic Final State can be characterised by Event Shape observablesHadronic Final State can be characterised by Event Shape observables

also broadening, C-parameter, jet mass, …

i i

i i

|p|

|np|T

i i

i i

|p|

|np|T

thrust:

spherical

1-T > 0

19/26

Sensitive to non-perturbative hadronisation process


Event Shapes in DIS

Hadronisation described by:• (usually) phenomenological

models as implemented in MCs

OR:• ‘Power Correction Model’

[Dokshitzer and Webber]

- understand hadronisation from first principles?

ZEUS effort led by Glasgow

20/26

• Compare event shape data to combination of NLO QCD and power correction model (F = event shape)

F = Fperturbative + Fpower-correction

ZEUS data vs pQCD+PC predictions

Good agreement

pQCD bit Depends on new parameter 0

ZEUS

hadronisation bit (~1/Q)


Event Shapes in DISZEUS effort led by Glasgow

20/26

• Compare event shape data to combination of NLO QCD and power correction model (F = event shape)

F = Fperturbative + Fpower-correction

hadronisation bit (~1/Q)pQCD bit Depends on new parameter

0

• can get good description of event shapes using power corrections (added to pQCD)

• universal non-perturbative parameter: 0 = 0.5 10%

• can also extract s (consistent with world average)


“There is nothing new to be discovered in physics now. All that remains is more and more precise measurement.”

Lord Kelvinvs.

Dave Milstead

21/26


Chasing the Magnetic Monopole (a bit of novelty value!!!)

In some theories, monopoles can have low masses - even O(10 GeV)

produced at HERA?

previous experimental searches: no conclusive evidence

g nc

2Qmagnetic charge:

when n=1, g = gD (Dirac magnetic charge)

Q = electric charge (eg. e for electron)

m

m

e

e

p

g

g

emHERA

monopoles?• natural explanation for

electric charge quantisation (Dirac)

• predicted or permitted in unification theories

22/26


H1 beam pipe:- home to monopoles?

• magnetic monopoles produced in ep collisions may stop in the beam pipe if stable, they remain permanently trapped

Performed at Southamption Oceanography Centre, UK

First (and only!!!) search at HERA

(a bit of novelty value!!!)

IDEA: Chop H1 beampipe into stripsPass through superconducting coil expect persistent current from monopoles (but not from dipoles)

Liverpool and Lancaster

Chasing the Magnetic Monopole

23/26


Result• look for non-zero

persistent currents

• some significant one-off measurements

• not reproducible

First (and only!!!) search at HERA

(a bit of novelty value!!!)

Liverpool and Lancaster

no monopoles harmed (or found!) in this experiment

Chasing the Magnetic Monopole

23/26


*: H1 and ZEUS not incompatible remains to be resolved

ZEUS

strangeness state *

1522 MeV

+ 0 + 0s ssearch for: Θ K p and Θ K p

Pentaquarks at HERA?

24/26

TopCite 100+

(minimum quark content uudds)


charmed state c

H1 observed D*p resonance in DISand p with HERA-I data (minimum

quark content uuddc) pentaquark candidate

mass: 309935 MeVwidth: 123 MeV

not seen by ZEUSnot seen by ZEUS

Pentaquarks at HERA?

25/26

TopCite 100+


Pentaquarks at HERA?charmed state c

H1 observed D*p resonance in DISand p with HERA-I data (minimum

quark content uuddc) pentaquark candidate

mass: 309935 MeVwidth: 123 MeV

not seen by ZEUSnot seen by ZEUS

H1: peak not confirmed at HERA-II(also, no evidence for Ξ5q

--(0) at HERA)

25/26

TopCite 100+


UK has made a massive and often unique contribution to the study of the Hadronic Final State at HERA

• 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 s; running in accordance with QCD

• Some regions not so well understood – cannot yet conclusively confirm observation of BFKL dynamics

• Magnetic monopoles ; Pentaquarks???

Summary

26/26

+ mustn’t forget the HFS coordinators, from: Bristol,Glasgow,Oxford,Liverpool,QMUL,UCL


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…)


Backups


Scaling Violations in p

• Compare cross sections for same reaction epe+jet+X at different p centre-of-mass energies (Wp)

• scaling: ratio of scaled cross section as a function of xT=2ET

jet/Wp should be unity

Glasgow Group

Results show non-scaling behaviour in accordance with QCD

FIRST observation of scaling violation in p


Jet Substructure

Bristol and Glasgow


s extraction

NLO QCD fit with HERA jets: • jet data sensitive to quark and

gluon densities in proton– breaks correlation between gluon

and s

• precision determination of s possible:

Oxford and UCL Groups

s Z(M )=0.1183 0.0028(exp.)

0.0008(model)

s Z(M )=0.1183 0.0028(exp.)

0.0008(model)


Scaled MomentumQMUL and Lancaster

GroupsCharged Multiplicities:• Clear observation of scaling

violation in fragmentation• 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+e- data fail to describe xp distribution

xp =2p(Breit)/Q2


Strange Particle ProductionOxford Group

• Measurements of:– Ks

0 and Λcross sections– Λ/Λ asymmetry– Λ/Ks

0 production ratio– Λ polarisation

typical set of results• ARIADNE mostly OK

– Ratio of N(Ks0)/Nch (not shown)

suggest value of s/u0.22 more appropriate?

• N(Λ)=N(Λ)• Also, no evidence for Λ

polarisation


• Azimuthal dependence has form:

predicted by QCD

• Non-perturbative effects small for cos2 term

Charged Particle Asymmetries

• angle of particle, , in the hadronic centre-of-mass frame as a function of lower momentum threshold pc

Glasgow Group

At high pc, see FIRST clear observation of cos2 contribution unambiguous evidence of perturbative QCD contribution

Ccos2φBcosφAdφdn

N1


HFS analyses with UK input

– High-ET dijet photoproduction (Phys. Rev. D76 (2007) 072011)– 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 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 p 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)

ZEUS


– 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)


ZEUS (cont.)


– 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)


H1


– 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)


H1 (cont.)


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

Documents

Hadronic Final States and QCD at HERA