Michele Rosin, University of Wisconsin, Madison Thesis Defense, Jan 27th 2006 1
University of Wisconsin, Madison
Thesis Defense, Madison WIJan. 27th, 2006
Energy Dependence of the Mean Charged Multiplicity in Deep
Inelastic Scattering with ZEUS at HERA
Energy Dependence of the Mean Charged Multiplicity in Deep
Inelastic Scattering with ZEUS at HERA
•Michele Rosin
Michele Rosin, University of Wisconsin, Madison Thesis Defense, Jan 27th 2006 2
Standard ModelStandard Model
Quarks (colored)
Flavor Mass (GeV/c2)
Charge (Q/e)
u 0.003 +2/3
d 0.006 -1/3
c 1.3 +2/3
s 0.1 -1/3
t 175 +2/3
b 4.3 -1/3Leptons (not colored)
Flavor Mass (GeV/c2)
Charge (Q/e)
νe< 1x 10-8 0
e 5.11 x 10-3 -1
νμ<0.00002 0
μ 0.106 -1
ντ<0.02 0
τ 1.7771 -1
Fermions
Matter made of fermions:
quarks or leptons
Each particle has anti-particle with opposite quantum numbers
Quarks carry color “charge”
Four fundamental forces
Electromagentic (EM) force
Weak force
Strong force
Gravity
Michele Rosin, University of Wisconsin, Madison Thesis Defense, Jan 27th 2006 3
Standard Model (II)Standard Model (II)
Boson Force Types Mass(GeV) Charge (Q/e) Color
γ (photon)
Electromagnetic 1 0 0 No
W± Weak 2 80.4 ±1 No
Z0 Weak 1 91.187 0 No
g (gluon) Strong 8 0 0 Yes
Strength of forces determined by coupling constant (αEM and αs)
forces mediated by exchange of bosons: γ, W±, Z0,g
Gravity described at macroscopic scale by general relativity.
very weak, neglected in high energy particle physics
Quantum Electrodynamics (QED): theory of EM, combined with weak Electro-weak theory
Quantum Chromodynamics (QCD): theory of strong interaction
Combined theories Standard Model
Bosons
Michele Rosin, University of Wisconsin, Madison Thesis Defense, Jan 27th 2006 4
Particle ScatteringParticle Scattering
•Study structure of proton•Scattering via probe
Wavelength
•Deep Inelastic Scattering (DIS) – Q2 largeHigh energy lepton transfers momentum to a nucleon via probe
Size of proton ~ 1 fm HERA Q2 Range ~ 40,000GeV2 HERA can probe to ~ 0.001 fm
probe
lepton
Q
h=λ h : Plank’s Constant
Q2: related to momentum of probe
Michele Rosin, University of Wisconsin, Madison Thesis Defense, Jan 27th 2006 5
Kinematic VariablesKinematic Variables
kp
qpy
⋅⋅
=Inelasticity: 0 y 1
( )222 kkqQ ′−−=−=
Virtuality of exchanged photon
22 )( pqW +=Center of mass energy of the *P system
remnant
e(k) e’(k’)
p(P)
(q)
q’
Fraction of proton momentum carried by struck parton 0 x 1 pq
Qx
⋅=2
2
Only two independent quantities
sxyQ =2
peEEkps 4)( 2 ≅+= = Center of mass energy of the ep systems
Michele Rosin, University of Wisconsin, Madison Thesis Defense, Jan 27th 2006 6
History of DIS, QPMHistory of DIS, QPM
Michele Rosin, University of Wisconsin, Madison Thesis Defense, Jan 27th 2006 7
QCD TheoryQCD Theory•QCD Quantum Chromodynamics
Strong force couples to color and is mediated by the gluon
Gluon permits momentum exchange between quarksGluons create quarks through pair productionColor confinement: free partons are not observed, only colorless objects -hadrons
Parton distribution function (PDF) gives probability of finding parton with given momentum within proton (experimentally determined)
Michele Rosin, University of Wisconsin, Madison Thesis Defense, Jan 27th 2006 8
QCD ScaleQCD Scale
•Running of SAs scale increases, S() decreases (= ET or Q)
•Perturbative QCD (pQCD)Small S (hard scale)Series expansion used to calculate observables
•Nonperturbative QCDLarge S (soft scale)Series not convergent
Leading Order (LO) Next to Leading Order (NLO)
HERA DIS Data: Running of S() A = A0 + A1S + A2S2 +...
Michele Rosin, University of Wisconsin, Madison Thesis Defense, Jan 27th 2006 9
pQCDpQCD
Michele Rosin, University of Wisconsin, Madison Thesis Defense, Jan 27th 2006 10
From Partons to HadronsFrom Partons to Hadrons
hard scattering parton showers hadronization
Hard scattering: large scale (short distance) perturbative process
Parton showers: initial QCD radiation of partons from initial partons
Hadronization: colorless hadrons produced from colored partons soft process (large distance) - not perturbatively calculable phenomenological models and experimental input
Michele Rosin, University of Wisconsin, Madison Thesis Defense, Jan 27th 2006 11
The hard scattering process determines the initial distribution of energy
Parton Shower + Hadronization determine the number of charged particles produced
Measure mean number of charged particles produced, (mean charged multiplicity, <nch>), versus the energy available for production of final state hadrons, study the mechanisms of hard scattering, parton showers and hadronization
Multiplicity and Energy Flow
Multiplicity and Energy Flow
Universality of the hadronization process can be tested by comparison with data from e+e- and hadron colliders.
Michele Rosin, University of Wisconsin, Madison Thesis Defense, Jan 27th 2006 12
Hadronic center of mass (HCM) frame
Hadronic center of mass (HCM) frame
Ecms
Nphoton region
=W
E E
Ecms/2 Ecms/2
Nproton region
p
Nphoton region vs W
( )2PqW +=Hadronic center of mass energy is W
0=+qPrr
Definition of HCM frame
Forward moving particles: photon hemisphere
Backward moving particles: proton hemisphere
Incoming photon and proton E= W/2
Final state: both hemispheres E=W/2
Michele Rosin, University of Wisconsin, Madison Thesis Defense, Jan 27th 2006 13
Breit FrameBreit Frame
“Brickwall” frame: incoming quark scatters off photon and returns along same axis
Breit Frame definition:
pZ<0: current region, pZ>0: target region
Current region of Breit Frame is analogous to single hemisphere e+e-: diagrams are similar above dashed line
In e+e- pair of quarks produced back to back with E=√s/2 each of them equiv. to the struck quark of E=Q/2 in DIS.
02 =+qxP
Mean charged multiplicity has been measured for various initial state interactions, e+e-, pp, ep DIS, and fixed target DIS, in both Breit and HCM frames
Michele Rosin, University of Wisconsin, Madison Thesis Defense, Jan 27th 2006 14
Previous Measurements: Multiplicity in e+e- and ppPrevious Measurements:
Multiplicity in e+e- and pp
( )2+−−+ +=eeee
pps
( )2pppp pps +=
( ) ( ) ( )[ ]22211
2 leadingincleadinginchadtot qqqqq −+−=
e+e-
pp
Energy available
for particle production
e+e-
:
pp vs. √q2
had
pp vs. √spp
( )22hadq
Agreement between e+e- and pp plotted vs.
Il Nuovo Cimento 65A N3 (1981) 404
Michele Rosin, University of Wisconsin, Madison Thesis Defense, Jan 27th 2006 15
Previous Measurements: Multiplicity vs. Q in Breit
frame ep DIS
Previous Measurements: Multiplicity vs. Q in Breit
frame ep DIS Current region Breit frame multiplicity vs. Q2 (hemisphere) shown along with e+e- data (whole sphere divided by 2)
Consistent with e+e- data for high Q2 disagreement at Q2< 80 GeV2
ep has gluon radiation whereas e+e- does not– possible source of disagreement at low Q2 ??European Physics Journal C11 (1999)
251-270
Michele Rosin, University of Wisconsin, Madison Thesis Defense, Jan 27th 2006 16
Measurement vs. W in HCM frame
Measurement vs. W in HCM frame
<n>
Zeitschrift für Physik C72 (1996) 573
ep DIS vs. W compared to fixed target DIS experiments and e+e- prediction similar rate of increase with W for ep and e+e-
Michele Rosin, University of Wisconsin, Madison Thesis Defense, Jan 27th 2006 17
Present AnalysisPresent AnalysisStudy energy dependence of <nch> in
photon region of HCM frame
compared to e+e-, pp and previous DIS
Breit Frame: current and target regions
expected to behave similarly to one hemisphere of e+e-: previous results show disagreement at low energies: we use total energy in current region of Breit frame as a scale for comparison with e+e-
Laboratory frame: in bins of X and Q2
Propose an alternative energy scale, the effective mass of hadronic system, Meff
compared <nch> in different regions of Breit and HCM frames for ep DIS
Michele Rosin, University of Wisconsin, Madison Thesis Defense, Jan 27th 2006 18
HERA DescriptionHERA Description
•920 GeV p+
(820 GeV before 1999)
•27.5 GeV e- or e+
•318 GeV cms•Equivalent to a 50 TeV
Fixed Target
•Instantaneous luminosity max: 1.8 x 1031 cm-2s-1
•220 bunches•96 ns crossing time
•IP~90mA p •Ie~40mA e+
DESY Hamburg, Germany
Unique opportunity to study hadron-lepton collisions
ZEUS
H1
Michele Rosin, University of Wisconsin, Madison Thesis Defense, Jan 27th 2006 19
HERA DataHERA Data
ZEUS Luminosities (pb-1) # events (106)
Year HERA ZEUS on-tape Physics
e-: 93-94, 98-99
27.37 18.77 32.01
e+: 94-97, 99-00
165.87 124.54 147.55
Luminosity upgrade
•5x increase in Luminosity
expect 1 fb-1 by end of 2006
Michele Rosin, University of Wisconsin, Madison Thesis Defense, Jan 27th 2006 20
HERA Kinematic RangeHERA Kinematic Range
Q2 = sxy0.1 < Q2 < 20000 GeV2
10-6 < x < 0.9
Equivalent to a50 TeV Fixed Target
Experiment
Michele Rosin, University of Wisconsin, Madison Thesis Defense, Jan 27th 2006 21
ZEUS DetectorZEUS Detector
Electron
27.5GeV
Measure ep final state particles: energy, particle type and direction
General Purpose Detector
Almost hermetic
Michele Rosin, University of Wisconsin, Madison Thesis Defense, Jan 27th 2006 22
Central Tracking Detector
Central Tracking Detector
•Drift Chamber inside 1.43 T Solenoid•Can resolve up to 500 charged tracks•Average event has ~20-40 charged tracks•Determine interaction vertex of the event•Measure number of charged particles (tracks)•Region of good acceptance: -1.75 < η < 1.75
View Along Beam Pipe Side View
e p
Michele Rosin, University of Wisconsin, Madison Thesis Defense, Jan 27th 2006 23
Uranium-Scintillator Calorimeter
Uranium-Scintillator Calorimeter
))2
ln(tan(θη −=
η = -3.0 θ = 174.3o
η = 1.1 θ = 36.7oη = -0.75 θ = 129.1o
η = 0.0 θ = 90.0o
η = 3.0 θ = 5.7o
Depth of FCAL > RCAL due to Ep > Ee
Used for measuring energy flow of particles.
covers 99.6% of the solid angle
alternating uranium and scintillator plates (sandwich calorimeter)compensating - equal signal from hadrons and electromagnetic particles of same energy - e/h = 1Energy resolution e/Ee= 18% / E h/Eh= 35% / E , E in GeV
Positrons27.5 GeV
Protons820 GeV
Michele Rosin, University of Wisconsin, Madison Thesis Defense, Jan 27th 2006 24
ZEUS TriggerZEUS Trigger
First LevelDedicated custom hardwarePipelined without deadtimeGlobal and regional energy sumsIsolated and e+ recognitionTrack quality information
Second Level“Commodity” TransputersCalorimeter timing cutsE - pz cutsVertex informationSimple physics filters
Third LevelCommodity processor farmFull event info availableRefined Jet and electron finding
Advanced physics filters
107 Hz Crossing Rate,105 Hz Background Rate, 10 Hz Physics Rate
Michele Rosin, University of Wisconsin, Madison Thesis Defense, Jan 27th 2006 25
Modeling DIS with Monte Carlo
Modeling DIS with Monte Carlo
Detecto
r Sim
ulatio
n
Parton Level
Hadron Level
Hadronization Models
String Fragmentation (Lund)
Cluster Model
Parton Cascades
LO Matrix Element + Parton Showers (MEPS)
Color Dipole Model (CDM) PDFs
Event generators use algorithms based on QCD and phenomenological models to simulate DIS events
Hard subprocess: pQCD
Parton Cascade
Hadronization
Detector Simulation correct for detector effects: finite efficiency, resolutions & acceptances
Next slide
Next slide
Michele Rosin, University of Wisconsin, Madison Thesis Defense, Jan 27th 2006 26
Monte Carlo models: parton cascades and hadronizationMonte Carlo models: parton cascades and hadronization
Parton Shower Model:
Models for parton cascades:
cascade of partons with decreasing virtuality continuing until a cut-off
Color Dipole Model: Gluons are emitted from the color field between quark-antiquark pairs, supplemented with BGF processes.
Hadronization models:
Lund String Model: Cluster Fragmentation Model: color "string"
stretched between q and q moving apart,
string breaks to form 2 color singlet strings, and so on untilonly on-mass-shell hadrons.
color-singlet clusters of neighboring partons formed
Clusters decay into hadrons
ARIADNELEPTOHERWIG
LEPTOARIADNE HERWIG
Michele Rosin, University of Wisconsin, Madison Thesis Defense, Jan 27th 2006 32
1996-97 Data sample1996-97 Data sample
• Event SelectionScattered positron found with E > 12 GeV A reconstructed vertex with |Zvtx| < 50 cmScattered positron position cut: radius > 25cm40 GeV < E-pz < 60 GeV
Diffractive contribution excluded by requiring ηmax> 3.2
• Track SelectionTracks associated with primary vertex |η| < 1.75 pT > 150 MeV
• Physics and Kinematic Requirement Q2
da > 25 GeV2
y el < 0.95
y JB > 0.0470 GeV < W < 225 GeV ( W2 = (q + p)2 )
Michele Rosin, University of Wisconsin, Madison Thesis Defense, Jan 27th 2006 33
Analysis MethodAnalysis Method
This analysis: Look at final state mean charged multiplicity, <nch>, and compare to other experiments to study similarities in particle production from different initial state particles.
Michele Rosin, University of Wisconsin, Madison Thesis Defense, Jan 27th 2006 34
Analysis Methods: Breit Frame
Analysis Methods: Breit Frame
ep: Split into Current and Target Region – one string two segments.
In ep we have a color field between 2 colored objects the struck quark and the proton remnant
When we use Q2 as a scale we are assuming the configuration is as symmetric as it is in e+e-, but it isn’t
This asymmetric configuration leads to migration of particles from the current region to the target region
Breit Frame diagram
Investigate cause of disagreement between ep vs. Q and e+e- at low energies look more closely at comparison of one hemisphere e+e- and current region Breit frame
Michele Rosin, University of Wisconsin, Madison Thesis Defense, Jan 27th 2006 35
Current region Breit Frame Q and 2*EBreit
Current region Breit Frame Q and 2*EBreit
gq
q
g
Soft Contribution
Hard Contribution
QCD ComptonIn hard and soft processes gluon radiation occurs
These gluons can migrate to target region
Total energy in the current region of Breit frame and multiplicity are decreased due to these migrations (Q2 is not)
Effect is more pronounced for low Q2 : more low energy gluons
Must use 2*EBreit instead of Q for comparing with e+e-2
2QEBreit =No
migrations:With migrations:
2
2QEBreit ⟨
N < N expected
e+e-: whole sphere
ep: shaded region: current region of Breit frame
Michele Rosin, University of Wisconsin, Madison Thesis Defense, Jan 27th 2006 36
Monte Carlo study of
2*E/Q as a function of Q.
Lower energies: Q doesn’t accurately reflect actual energy in hemisphere.
Use 2*E as a scale for comparing to e+e-
Michele Rosin, University of Wisconsin, Madison Thesis Defense, Jan 27th 2006 37
Analysis Methods: photon hemishpere HCM frame
Analysis Methods: photon hemishpere HCM frame
2*E/W as function of W
Difference is negligable
Want to measure dependence of <nch> on Minv as was done in Breit frame,
Migrations here are small, so
2*Ephoton = W so use
Minv = W as scale
Michele Rosin, University of Wisconsin, Madison Thesis Defense, Jan 27th 2006 38
Invariant Mass of Hadronic System
Invariant Mass of Hadronic System
CAL within the CTD acceptance
Study: <nch> vs. Meff
CTD
Measure hadronic final state within η for best acceptance in the central tracking detector (CTD)
Measure # charged tracks, reconstruct number of charged hadrons
Measure invariant mass of the system (Meff) in corresponding Δη region.
Energy is measured in the Calorimeter (CAL)
22222 )()()()( ∑∑∑∑′≠′≠′≠′≠
−−−=ei
iz
ei
iy
ei
ix
ei
ieff pppEM
Used as a scale to compare: current and target regions of Breit framecurrent region Briet frame to photon region HCM
Michele Rosin, University of Wisconsin, Madison Thesis Defense, Jan 27th 2006 39
Correction to hadron level:
matrix
Correction to hadron level:
matrix
The matrix relates the observed to the generated distributions by:
Matrix corrects tracks to hadron level
ρ corrects phase space to hadron level
Step 1: Correction Matrix:
∑ ⋅=DET
DETDETGENGENn
nnnn PMP ,
Step 2: Correction for acceptance of event selection cuts in the bins
DET
GENCρρ
=ρGEN :distribution with GEN level cuts
ρDET :distribution with GEN level cuts
MnGEN ,nDET=
No. of events with nchGEN hadrons generated and nch
DET tracks observed
No. of events with nchDET tracks observed
Michele Rosin, University of Wisconsin, Madison Thesis Defense, Jan 27th 2006 40
Correction to hadron level:
matrix
Correction to hadron level:
matrix To illustrate average size of 1st part of correction:
Mean matrix correction factor:
Mean of C distributions:
Example: current region of Breit frame vs. 2*E
correctionmatrix after on distributitrack
ondistributi tracksduncorrecte
∑ ⋅⋅=data
datadataGENcorrectedn
nnnn PMCP ,
range: 0.98-1.78 average: ~1.2
range: 0.92-2.05 average: ~1.02
Michele Rosin, University of Wisconsin, Madison Thesis Defense, Jan 27th 2006 41
Correction to hadron level:modified bin-by-bin
Correction to hadron level:modified bin-by-bin
Part one: correct to hadron level using only hadrons generated with pT > 0.15 GeV (Detector Effects)
Part two: correct for hadrons with lower pT, using ratio of <gen> with pT cut to <gen> no pT cut in each bin.
DETich
GENich
i n
nC
,
15.0,,
,1 =
15.0,,
,
,2 GENich
GENich
in
nC =
iiDATA
ichcorrected
ich CCnn ,2,1,, ⋅⋅=
number
distribution
Michele Rosin, University of Wisconsin, Madison Thesis Defense, Jan 27th 2006 42
Invariant Mass correction: normal bin-
by-bin method:
Average less than 2%
Correction to hadron level:
modified bin-by-bin
Correction to hadron level:
modified bin-by-bin
Average correction for detector effects: <C1,i>
Correction of hadrons of pT > 0.15 GeV to all hadrons
DETich
GENich
i n
nC
,
15.0,,
,1 =
15.0,,
,
,2 GENich
GENich
in
nC =
Example: lab frame vs. Meff
range: 0.96-1.84 average: ~1.1
range: 1.02-1.15 average: ~1.05DET
inv
GENinvDATA
invinvM
MMM =
Michele Rosin, University of Wisconsin, Madison Thesis Defense, Jan 27th 2006 43
Acceptance correction: Current region of Breit frame:
Acceptance correction: Current region of Breit frame:
Breit Frame: 95% of hadrons in current region visible in detector, only 30% of target region hadrons are visible
All Hadrons
Current Region Breit Frame
Visible Part
Proton
remnant
visibleGENch
GENchhadrons
n
nC
,=η
Multiplied by <nch>
Generated in visible part
Michele Rosin, University of Wisconsin, Madison Thesis Defense, Jan 27th 2006 44
Acceptance correction:Photon region HCM
Acceptance correction:Photon region HCM
HCM Frame: 60-80% Photon region HCM frame contained in visible part of detector. larger corrections.
All HadronsProton
Region HCM Frame
Photon Region HCM Frame
Visible Part
Correction factors for <nch> vs. W in photon region HCM frame: 1.78, 1.42, 1.26Additional correction needed for Meff in HCM:calculated in bins of W, similar to hadron acceptance correction:
Correction factors: 2.38, 1.73, 1.45Applied on an event by event basis
visibleGENeff
GENeffM
M
MC inv
,=η
Michele Rosin, University of Wisconsin, Madison Thesis Defense, Jan 27th 2006 45
ResultsResults<nch>
Michele Rosin, University of Wisconsin, Madison Thesis Defense, Jan 27th 2006 46
Comparison to other <nch> measurements at HERA
Comparison to other <nch> measurements at HERA
<nch>
Michele Rosin, University of Wisconsin, Madison Thesis Defense, Jan 27th 2006 47
Measurement of multiplicity dependence on 2*Ecurrent compared to previous ZEUS measurement vs. Q, and to e+e- and pp data (<nch> is multiplied by 2 for comparison)
2*E gives better description of multiplicity at lower energy
Current region understood, would also like to compare the target region of ep to e+e- but…
<nch>
Comparison of ep DIS <nch> to other experiments
Comparison of ep DIS <nch> to other experiments
Michele Rosin, University of Wisconsin, Madison Thesis Defense, Jan 27th 2006 48
binsbins
Michele Rosin, University of Wisconsin, Madison Thesis Defense, Jan 27th 2006 49
Michele Rosin, University of Wisconsin, Madison Thesis Defense, Jan 27th 2006 50
Correlated & Uncorrelated Systematics
Correlated & Uncorrelated Systematics
Systematic
Change
Ee’ ± 1 GeV
Radius Cut
± 1cm
Q2 ± 2.25 GeV2
yJB± .008
yel± .05
Zvtx± 15 cm
W (upper) ± 15 GeV
W (lower) ± 7 Gev
E - pz± 2 GeV
CAL energy scale
± 3 %
Main systematics here
Michele Rosin, University of Wisconsin, Madison Thesis Defense, Jan 27th 2006 51
Summary and ConclusionsSummary and Conclusions
HFS investigated in NC ep DIS in range 25<q2 and 70< W< 225 in terms of <nch> , the center of mass energy, and the invariant mass, Meff
1st time, lower energy data of cr Breit frame shown to agree with e+e- and pp by using 2*E as energy scale
<nch> in photon region HCM agree with e+e-
Total energy region of analysis from 2 to 200
New energy variable used for comparison between diff e regions of ep HFS
<nch> scales with Meff in the same way as 2*E in cr Breit frame, (and therefore also same as e+e-), <nch> in photon region HCM rises faster as a function of Meff than <nch> in current region BF.
<nch> in photon region HCM show no dep. On x or Q