HERA Collider physics - introduction

Workshop on EW and BSM physics at the EIC May 2020 Results from H1 and ZEUS Stefan Schmitt Outline

● Introduction Next talk: E. Gallo about searches and electroweak – The HERA collider and collider experiments physics at HERA – HERA kinematics

● Selected physics results in perturbative QCD Disclaimer: – Inclusive cross sections, structure functions, PDFs – Heavy flavor production this talk is on HERA result, but reflects my personal opinions only.

There is a slight preference in showing results from H1 rather than ZEUS, simply because I know the H1 results better. Please apologize for that.

CFNS workshop, May 2020 S.Schmitt, HERA introduction 2 The HERA collider

● Operated from 1992 to 2007

● Circumference 6.3 km Integrated luminosity: ● Electrons or positrons colliding with protons about 500 pb−1 per collider experiment ● Proton: 460-920 GeV, Leptons 27.6 GeV

● Peak luminosity ~7×1031 cm-2s-1 Two collider experiments: ● Lepton beam polarisation up to 40-60% (Sokolov- H1 and ZEUS Ternov effect, rise-time ~30 minutes) HERA-b & HERMES Fixed-target, not covered in this talk

Straight section Curved section

CFNS workshop, May 2020 S.Schmitt, HERA introduction 3 HERA compared to other colliders

● HERA at construction time: energy frontier (E ~Tevatron, E ~ ½ LEP) Centre-of mass energy p e Detectors were designed for discoveries, not so much for precision

● EIC compared to HERA:

– Reduced center-of-mass energy ×0.3 – Much higher luminosity ×100 Luminosity – Better lepton polarisation – Target polarisation – Heavy targets – Much improved detectors: tracking, acceptance, particle identification, forward detectors, ...

CFNS workshop, May 2020 S.Schmitt, HERA introduction 4 The HERA publication harvest

Status: Feb 2020 H1+ZEUS combined 8 publication ● Top-ten cited (excluding detector papers) JHEP 1001 (2010) 109 H1+ZEUS 1000+ Data combination, PDF Eur.Phys.J. C21 (2001) 33 H1 700+ Low-x, PDF, alpha_s H1 223 publications Nucl.Phys. B470 (1996) 3 H1 500+ Low-x, PDF Eur.Phys.J. C21 (2001) 443 ZEUS 500+ Low-x, PDF Phys.Lett. B315 (1993) 481 ZEUS 500+ Observation of diffraction ZEUS 250 publications Nucl.Phys. B407 (1993) 515 H1 400+ Rise of F2 at low-x Eur.Phys.J. C75 (2015) 580 H1+ZEUS 400+ Data combination, Low-x, PDF Phys.Lett. B316 (1993) 412 ZEUS 400+ Rise of F2 at low-x Z.Phys. C76 (1997) 613 H1 400+ Difffractive PDF ● Both collaborations are still active Z.Phys. C74 (1997) 207 ZEUS 400+ High Q² DIS and open for new members ● 500+ citations: proton at low-x and PDFs

● 250+ citations: total cross-section, diffractive PDF, ● Data are available for analysis at diffractive vector mesons, pentaquark DESY, including computing ● 200+ citations: charm, jets, DVCS infrastructure (batch system) HERA data are used mainly to study: PDFs and perturbative QCD, low-x and diffraction, transition from soft to hard QCD CFNS workshop, May 2020 S.Schmitt, HERA introduction 5 The HERA detectors H1 and ZEUS

Liquid Argon calorimeter σ =0.5/√E, σ =0.11/√E, -1.5<η<3.4 Muon chambers had EM Lead+fiber in backward (electron) direction 4π em+had. [SpaCal] σ =0.07/√E, -4<η<1.4 calorimeters EM

Drift-chambers as main tracking devices + silicon vertex detectors

electron beam proton beam

Asymmetric detectors Uranium-scintillator calorimeter Centre-of-mass system is σ =0.35/√E, σ =0.18/√E, -3.5<η<4 had EM boosted to proton-direction

Ee=27.6 GeV, Ep=920 GeV

CFNS workshop, May 2020 S.Schmitt, HERA introduction 6 HERA boost visualized

“typical” DIS event in H1 detector

Proton beam limits acceptance in target region

E =27.6 GeV E =920 GeV E =54000 GeV e p Ee=160 GeV Ep=160 GeV e

ep center-of-mass system Proton rest-frame Laboratory system c.f. fixed-target experiment CFNS workshop, May 2020 S.Schmitt, HERA introduction 7 Processes studied at the HERA collider

Scattered electron ● Neutral Current DIS (Deep Inelastic Scattering)

– electron in main detector

● Charged current DIS Hadrons collimated in a jet – neutrino with high transverse Side view Transverse view momentum (escapes detection) Large electron scattering angle (rare event)

● Photoproduction Small electron scattering angle (frequent event) – Electron scattered at very low Hadrons are spread all over angle (dedicated low-angle detector or not detected) Electron beam Proton beam

Scattered electron CFNS workshop, May 2020 S.Schmitt, HERA introduction 8 Processes studied at the HERA collider

Scattered electron ● Neutral Current DIS

– electron in main detector See next talk E. Gallo ● Charged current DIS Hadrons collimated in a jet – neutrino with high transverse Side view Transverse view momentum (escapes detection) Neutral current (NC) event

● Photoproduction Charged current (CC) event Imbalance in transverse plane → neutrino – Electron scattered at very low angle (dedicated low-angle detector or not detected)

CFNS workshop, May 2020 S.Schmitt, HERA introduction 9 Processes studied at the HERA collider

● Neutral Current DIS

– electron in main detector Scattered ● Charged current DIS electron in detector – neutrino with high transverse momentum (escapes detection) Neutral current (NC) event

● Photoproduction Photoproduction (most frequent type of event) – Electron scattered at very low angle (not detected or scattered Electron beam Scattered into dedicated low-angle tagger) electron is not detected

CFNS workshop, May 2020 S.Schmitt, HERA introduction 10 Photoproduction and DIS

● Main kinematic variable: negative four-momentum squared Q²= −(e-e')²

● Q² provides a natural hard scale for Scattered perturbative calculations electron in 2 detector ● Deep-inelastic scattering (DIS): Q ≫0 – Perturbative QCD applicable Neutral current (NC) event

Photoproduction (most frequent type of event) 2 ● Photoproduction: Q ∼0 – Perturbative QCD works only if there is another hard scale (jet, Electron beam Scattered electron is heavy quark, etc) not detected

CFNS workshop, May 2020 S.Schmitt, HERA introduction 11 Neutral current DIS kinematics at HERA

● Kinematic variables: Q², x, y, Q²=sxy Kinematic plane Q² vs x: ● Determine from 4-vectors of beam particles e, p, where does HERA measure? scattered electron e' and hadronic final state X Q²~30 GeV² (e' p) ( Xp) x~0.001 ye=1− , yh= (ep) (ep) 2 p Q2 Typical low Q², electron Q2= T , x= low-x event 1− y sy

● “Electron” method: y=y and p =p e T T,e ● At low y, the electron method is limited by energy resolution, initial and final state radiation → use y=y (sigma method) h ● Other methods also in use: double-angle, etc

CFNS workshop, May 2020 S.Schmitt, HERA introduction 12 Neutral current DIS kinematics at HERA

● Kinematic variables: Q², x, y, Q²=sxy Kinematic plane Q² vs x: ● Determine from 4-vectors of beam particles e, p, why is there this funny scattered electron e' and hadronic final state X shape? Limited statistics Q²~30 GeV² (e' p) ( Xp) x~0.001 ye=1− , yh= (ep) (ep) 2 Limited by centre- p Q2 of-mass energy Typical low Q², electron Q2= T , x= low-x event 1− y sy

● “Electron” method: y=y and p =p e T T,e

● At low y, use y=y (sigma method) → hadrons h contributing to y have to be within detector Hadron acceptance h limitation in proton acceptance → low y / high x is not accessible direction, electron method is limited by HERA is “low-x” because of acceptance energy resolution & Angular acceptance in radiation limitations in the forward (proton) direction electron direction HERA = low-x CFNS workshop, May 2020 S.Schmitt, HERA introduction 13 Radiative effects

Kinematics of radiative photons ● Radiation from electron line n o

r – Initial state (ISR) t c e l effectively reduces e

m

a FSR: collinear to beam energy e

b scattered electron

o – Final state (FSR) t

r a

e reduces measured n i l l

o electron momentum → use calorimeter c

: QED Compton: balances P R T – QED Compton S I of scattered electron corresponds to very low “true” Q² at proton vertex

CFNS workshop, May 2020 S.Schmitt, HERA introduction 14 Radiative effects: HERA methods

Kinematics of radiative photons ● Radiation from electron line has three poles: ISR,

n FSR, QED Compton o r t

c ● e

l Radiation has two effects e

m – a FSR: collinear to For a given event, the kinematic e

b scattered electron

reconstruction is distorted o t

r

a → apply cuts, use “robust” kinematic e n i

l reconstruction (Sigma method or similar) l o c

: – QED Compton: balances P For a given class of events, the predicted R T

S cross section changes wrt. the Born level I of scattered electron → radiative corrections, ratio of prediction with and without radiation, model-dependent

CFNS workshop, May 2020 S.Schmitt, HERA introduction 15 HERA physics topics

Electron Hard probes: Jets, heavy – Color-neutral probe, flavour, photons source of virtual or Inclusive quasi-real photons reactions Proton structure Proton and PDF HERA Searches & – Source of quarks and data electroweak gluons Soft QCD particle production, exclusive states, diffraction

CFNS workshop, May 2020 S.Schmitt, HERA introduction 16 HERA physics topics

Electron Hard probes: Jets, heavy – Color-neutral probe, flavour, photons source of virtual or Inclusive quasi-real photons reactions Proton structure Proton and PDF HERA Searches & – Source of quarks and data electroweak gluons Soft QCD particle production, See talk by exclusive states, E. Gallo diffractionRequires a dedicated long talk...

CFNS workshop, May 2020 S.Schmitt, HERA introduction 17 Inclusive cross sections, PDFs

H1+ZEUS Data combination Inclusive cross sections, PDF fit Longitudinal structure function

CFNS workshop, May 2020 S.Schmitt, HERA introduction 18 Deep-inelastic scattering at HERA

● Measure Cross section as a function of Q² Neutral current ● High Q² ~ 104 GeV² probes smallest distances ∼1/Q4 ● Combination of H1 and ZEUS data (41 datasets)

● Neutral Current (NC): Charged current – photon, Z-boson exchange 2 2 2 4 1 Q M – Cross section shape similar to 1/Q ∼ /( + W )

● Charged Current (CC):

– W-boson exchange MW=80.4 GeV

– Cross section shape ~1/(Q²+MW²)² High Q²: rare events → Talk by E. Gallo

EPJ C75 (2015) 85 CFNS workshop, May 2020 S.Schmitt, HERA introduction 19 Proton structure measurement at HERA

● Momentum transfer Q²

● Second variable: Bjorken-x

● Cross sections depend on Q² and Bjorken-x Each curve 2 2 2 Q is for a Q =−(e−e' ) and x= 2p(e−e' ) different x e : beam electron 4-momentum p : beam proton 4-momentum e ' : scattered electron 4-momentum Fixed- target data

● Interpretation in the quark-parton model: x is the momentum fraction of the struck quark

● Scaling violations: for fixed x, the cross section does change with Q²

EPJ C75 (2015) 85 CFNS workshop, May 2020 S.Schmitt, HERA introduction 20 Structure functions and parton densities

● Cross section differential in Q²,x is related to structure functions

2 ± 4 2 d σ NC Q x ± ~ Y − ~ y ~ =σ =F ∓ x F − F 2 2Y r , NC 2 Y 3 Y L dx dQ 2 π α+ + + 2 Difference between inelasticity y=Q /(sx) Suppressed by y²≪ 1 e+p and e−p 2 scattering: xF helicity factors: Y ±=1±(1− y) 3 Visible only at high Q², because m =91.2 GeV ● Structure functions are related to quark and Z Talk by E. Gallo gluon count in the proton ~ F 2∼∑ (xq+x q¯) valence plus sea quarks → dominant 2 ~ Q x F3∼ 2 2 ∑ ( xq−x ¯q) valence quarks → contributes Q +m Z ~ only at high Q² F L∼xg gluons talk by E. Gallo EPJ C75 (2015) 85

CFNS workshop, May 2020 S.Schmitt, HERA introduction 21 DGLAP and PDF fit: from HERA to LHC

● QCD fit: extract PDF (i.e. quark and gluon densities) from structure functions

● Dokshitzer, Gribow, Based on DGLAP equations Lipatow, Altarelli, Parisi

●

Given the PDF as a function of x at one scale Q μ , DGLAP predicts the PDF at another scale μ = 0 1 μ

e l ● Factorisation: PDFs are universal: ep, pp, ... a c 2 S σ ep=PDF(μ , x)⊗∣M∣ +higher twist 2 σ pp=PDF(μ , x1)⊗PDF(μ , x2)⊗∣M∣ +higher twist Q² large enough, DGLAP works ● Measure at HERA → predict LHC cross section Q² too low, DGLAP not applicable ● Within HERA: measurements at different μ²=Q² are fitted to the same PDF using DGLAP

Variable x CFNS workshop, May 2020 S.Schmitt, HERA introduction 22 HERA and EIC kinematic reach

● HERA kinematic reach

– For low Q²<100 GeV²: low-x: 3×10-5

● Nevertheless, HERA data is the fundament of all modern PDF determinations Q² large enough, DGLAP works

Q² too low, DGLAP not applicable ● The EIC is going to improve substantially the precision at high x

CFNS workshop, May 2020 S.Schmitt, HERA introduction 23 HERAPDF NNLO QCD fit

● Proton PDFs at a scale μ²=10 GeV², from NNLO QCD fit of HERA data (14 parameter fit)

● The experimental uncertainties are small

● At low x<10-3, model and parametrization uncertainties dominate Gluon ● The high-x region is not measured well, only scaled 1/20 constrained by sum-rules and choice of parametrization Sea quarks scaled 1/20 → EIC data are needed to improve on this Valence quarks

EPJ C75 (2015) 85 CFNS workshop, May 2020 S.Schmitt, HERA introduction 24 Data at low-x and DGLAP fit

● Zoom in at low-x, low Q²

● Theoretically and experimentally challenging Q² changes between subpanels. ● Experiment: low-y → low For a given electron energy, background panel, the x- dependence is shown ● Theory: higher orders, resummation, …

Turn-over of σr is not well Fit quality described by deteriorates as NNLO QCD fit more data at → structure lower Q² are function F ? included L y2 EPJ C75 (2015) 12, 85 σ r , NC≃F 2− F L Y + CFNS workshop, May 2020 S.Schmitt, HERA introduction 25 Measurement of FL

H1/ZEUS compatible at ● DGLAP fit probes gluon by scaling violations, 1-2σ F probes the gluon more directly L HERA measurements of y 2 F : large range in Q² but F F 2 L σr , NC≃ 2− L y=Q /(s x) Q²/x is fixed. Y + F L∼αs xg(x) direct probe of gluon density EPJC 74 (2014) 2814 PRD 90 (2014) 072002 ● Reasonable agreement of FL and predictions

● HERA: small energy range √s=225-320 GeV,

so only limited range of FL is accessible

● HERA+EIC: possible measurement of FL over a much large range → powerful test of QCD

CFNS workshop, May 2020 S.Schmitt, HERA introduction 26 Hard probes: heavy flavour

H1+ZEUS charm and beauty data combination Charm and beauty masses

CFNS workshop, May 2020 S.Schmitt, HERA introduction 27 Heavy flavor and jet production

● Charm and beauty quarks or extra Example partons are emitted in higher order contribution to QCD processes charm and beauty ● Sensitive to fundamental QCD production parameters: couplings and quark masses

● Measurement presented here

– Charm and beauty structure Example functions contribution to jet production

CFNS workshop, May 2020 S.Schmitt, HERA introduction 28 Charm and beauty data combination

+11 more ● Combination of 13 datasets on charm HERA (H1 and beauty in DIS from H1 and ZEUS or ZEUS) analyses ● Experimental methods Sec.vertex D* mass peak – Fully reconstructed D or D* decays

– High PT leptons from heavy flavor decays Example: charm data – Secondary decay vertices combination at Q²=32 ● Accuracy of the combined data: GeV² ~5-9% for charm, ~15-25% for beauty

EPJC 78 (2018) 473 CFNS workshop, May 2020 S.Schmitt, HERA introduction 29 Structure functions with charm, beauty

● Heavy quarks are produced mainly in boson-gluon fusion c ● Theory is challenging: multiple hard scales, massive particles, etc

Shape in x is not described Q² is changing perfectly. between Could be subpanels related to Each difficulties in subpanel showns the x describing dependence inclusive DIS data (see Charm Ratio to Theory Charm data previous slides) EPJC 78 (2018) 473

CFNS workshop, May 2020 S.Schmitt, HERA introduction 30 Structure functions with charm, beauty

● Probability to produce heavy quark depends on its mass c → can extract quark masses mc and

mb from HERA data alone: +0.077 mc (mc)=1.290−0.053 +0.138 Beauty data mb (mb)=4.049−0.118

Particle data group: EPJC 78 (2018) 473

mc =1.28±0.025 GeV m = 4.18±0.03 GeV b Data are statistically limited → CharmCharm data data looking forward to have precision data from EIC.

CFNS workshop, May 2020 S.Schmitt, HERA introduction 31 Summary

● HERA: operated 1993-2007, still with active analyses

● New members are welcome in H1 or ZEUS, to contribute some of the studies we missed in the past

● Kinematic reconstruction: forward (proton) acceptance and photon radiation from electron line together limit the accessible range in (Q²,x) → improvement at EIC expected from 3× lower centre-of-mass energy (at same Q² access ~10× higher x) and from better detector acceptance

● Some highlights from HERA combined data: inclusive cross sections, PDF fits, charm and beauty production in DIS

CFNS workshop, May 2020 S.Schmitt, HERA introduction 32 Backup slides

CFNS workshop, May 2020 S.Schmitt, HERA introduction 33 Accelerators for particle physics at DESY

● DESY was founded in 1959

● German national laboratory for particle physics, accelerators, synchrotron sources

● Accelerators for particle physics

– DESY 1964-1978 [6 GeV] HERA ring: 6.3 Since 1978: used as pre-accelerator only airport km – DORIS 1974-1992 [e+e− √s=12 GeV] ~5 miles to city center 1992-2012: used as synchrotron source circumference soccer arena horse racetrack – PETRA 1978-1986 [e+e− √s=45 GeV] 1990-2007: pre-accelerator, since 2009 synchrotron source – ± DESY HERA 1992-2007 [e p √s=320 GeV] campus ● DESY accelerators in 2020 → photon science

CFNS workshop, May 2020 S.Schmitt, HERA introduction 34 The Sigma method

● Kinematic reconstruction using four- ● Electron method: 2 E 0−E e (1−cosθe) vectors (neglecting p and e masses): ye= 2 E 0 (e ' p) ( Xp) Q2=2 E E (1+cosθ ) y =1− , y = e 0 e e e (ep) h (ep) 2 2 ● ISR effectively changes beam energy E 2 pT Q 0 Q = , x= 1− y sy → very poor reconstruction of low y ● HERA frame: electron beam along -z ● Sigma method: estimate effective E0 axis → ratios of 4-vector products from data and get y from hadrons result in expressions involving (E-pz) Take Q² from Σ=(E− pz )e+(E− pz)X electron method For electron: (E-pz)e=Ee(1-cos θe) (E− pz )X (eΣ) or use Σ also in yh → yΣ= the Q² calculation Low Q²: (E-pz)e is close to 2Ee Σ

CFNS workshop, May 2020 S.Schmitt, HERA introduction 35 The HERA “discovery”: rise of F2 at low x

● Discovery in the early HERA data: structure

function F2 rises strongly at low x

● At the time: a surprise

● Impressive improvement in precision – it took >20 years to 2015: 1.3% uncertainty achieve this 1993: 25% uncertainty

CFNS workshop, May 2020 S.Schmitt, HERA introduction 36 Towards the best precision: data combination

● Measurements of inclusive processes have been published in a total of 41 H1 and ZEUS papers

● Data have been combined to a uniform HERA dataset: EPJ C75 (2015) 12, 85 – Measurements of NC and CC for both e+p and e-p scattering at √s=318 GeV – Measurements of NC e+p scattering at √s={225,252,300,318} GeV

● Precision better than 1.5% is reached for large parts of the data

CFNS workshop, May 2020 S.Schmitt, HERA introduction 37 The power of combining

● A total of 2927 H1 and ZEUS measurements are averaged to about 1307 combined reduced cross sections

● Up to six measurements contribute to a single point

● Systematic uncertainties and their cross-correlations are handled consistently

● Better than 1.5% precision is reached over a wide kinematic range

● Excellent data consistency: Shown here: e+p data, √s=320 GeV at χ²/ND.F.=1687/1620 selected values of x EPJ C75 (2015) 12, 85 CFNS workshop, May 2020 S.Schmitt, HERA introduction 38 Transition to photoproduction

EPJ C75 (2015) 12, 85

● At HERA, both deep-inelastic scattering (Q²>0) and photoproduction (Q²=0) can be measured

● Can map the transition region, where Q² is very small

● challenge for perturbative calculations

Perturbative QCD Regge theory High Q²»1 GeV² High energy W»Q Q² evolution: DGLAP W-dependence: equations Regge trajectories HERA data at lowest Q² can W =√(q+ p)2=Q2 (1/ x−1) photon-proton energy not be included in PDF fit CFNS workshop, May 2020 S.Schmitt, HERA introduction 39 Fit of photoproduction and HERA DIS data

● Tensor-pomeron model Subset of (Ewerz, Maniatis, HERA Nachtmann) Ann.Phys 342 (2014) 31 DIS data at low Q² ● Coherent description of DIS cross section: wrt. W photoproduction and low-x forward virtual Compton scattering DIS data amplitude

● Fit three trajectories and Q² dependencies

2 0.0088 – ϵ2 + Reggeon ∼W where ϵ2=0.0485−0.0090 PRD 100 (2019) 114007 2 0.0073 – Hard Pomeron ∼W ϵ0 where ϵ =0.3008+ 0 −0.0084 HERA photoproduction 2 0.0076 – ϵ1 + data: W~200 GeV Soft pomeron ∼W where ϵ1=0.0935−0.0064

Compatible with canonical DL intercept α0=1+ε=1.0808 EIC data can bridge the gap to fixed target in Good fit quality: χ²/ND.F.=587.9 / 536 (probability 6%) photoproduction CFNS workshop, May 2020 S.Schmitt, HERA introduction 40 Tensor pomeron fit at larger Q²

● HERA data at √s=318 GeV, Q²≥1.5 GeV²

● Described well by the Tensor pomeron fit

● Soft pomeron contribution is significant even at Q²=10 GeV²

● Could be an indication of sizable non- perturbative contributions to the DIS data relevant for PDF fits (e.g. HERAPDF fit includes data at Q²≥3.5 GeV²)

PRD 100 (2019) 114007 CFNS workshop, May 2020 S.Schmitt, HERA introduction 41 Jet production in the Breit frame

Scattered ● Hadrons are emitted in jets electron

● Interpretation: a parton is scattered off Proton remnants the proton and fragments

● Breit-frame: Lorentz-transformation to Hadrons have proton remnant and current-jet Side view Transverse view well separated collimated in a jet

● Measure the emission of further jets

which have PT>0 wrt expected current jet direction Laboratory frame

Breit-Frame

CFNS workshop, May 2020 S.Schmitt, HERA introduction 42 Jet cross section measurements

● Jet production measured as a function of

Q² and the jet transverse momentum PT

Example graph for jet production

Q² increases Accuracy: 1-2% between plus 2.5% EPJC 76 (2016) 1 subfigures normalisation EPJC 75 (2014) 65 Achieved with sophisticated unfolding technique Each subfigures shows the P dependence T Ratio to NLO theory Steeply falling cross section → look at ratio to theory NNLO also shown – fits much better to data CFNS workshop, May 2020 S.Schmitt, HERA introduction 43 Measurement of αs

● Fundamental parameter in QCD: coupling αs

● Coupling is running with the scale μ²=Q²+PT²

● Probe this running in a single experiment

● Extract the coupling at the conventional

scale μ=mZ

EPJC 77 (2017) 791 CFNS workshop, May 2020 S.Schmitt, HERA introduction 44