Low-x QCD with CMS at LHC

Hot Quarks 2006

Villasimius, Sardegna

May 17th 2006

David d'Enterria

Intro: parton saturation & evolution (CGC) at low-x

Experimental signatures

Results at HERA (proton), RHIC (nucleus)

LHC perspectives (p,A)

CMS experimental capabilities

CERN, Geneva

Parton structure at low-x (proton)

DIS ep collisions probe partonic distributions in the proton:

HERA: strong rise at low-x of F2(x,Q2) ~ sea-quarks, ∂lnF2 /∂lnQ2 ~ gluons

Q2

Q2 = “resolving power”

F1,F2 = proton structure functions , (y = inelasticity).

Bjorken x = momentum fraction carried by the parton

⇒

(x,Q2) parton quantum evolution at low-x

DGLAP (parton branching): F2(Q2) ~ asln(Q2/Q02)n start. at Q02 ~ 1 GeV2 (“std” PDFs)

BFKL (parton emission ordered in pL~x): F2(x) ~ asln(1/x)n (unintegrated PDFs)

Linear equations (single parton radiation). Cannot work at low-x !

(i) Too high gluon density (system not dilute): nonlinear multi-gluon fusion must balance splittings.

(ii) pQCD (collinear & kT) factorization should break (invalid assumption of incoherent parton scatt.)

(iii) Violation of unitarity even for Q2>>Λ2 (too large perturbative cross-sections).

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xG(x) ~ x-λ	,λ=anom.dim

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Kimber et al

Should measure at x=10-5 – 10-7

xG(x,Q2)= dNg/dy

Q2 = 10 GeV2

Pomeron

Reggeon

Mesons

Colour Glass Condensate

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Kimber et al

Pomeron

Reggeon

Mesons

CGC: Effective field theory of QCD in high-energy (high density, small-x) limit.

hadrons = classical fields (saturated gluon wave-function) below new scale: Q2<Qs2

“Sat. momentum”:	A-“amplified”: Qs2 ~ A1/3 ~ 1.5,3 GeV2 RHIC,LHC

Perturbative calculations (strong Fmn field ⇒ weak coupling as<<1).

had.+had: collision (amplitude level) of 1(2) gluon wave function(s): “resums” all mult. scatts.

Color Glass (“static” color sources = quarks) Condensate (high gluon occupation values).

Quantum evolution: BK, JIMWLK: Non-linear, all-twist eqs. in saturation region

Qs2=asNcx(x,Q2).

gluon “wave function”

Colour Glass Condensate

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Kimber et al

Pomeron

Reggeon

Mesons

Illustration only. There is no “PDFs” now

in the sense of pQCD factorization

CGC: Effective field theory of QCD in high-energy (high density, small-x) limit.

hadrons = classical fields (saturated gluon wave-function) below new scale: Q2<Qs2

“Sat. momentum”:	A-“amplified”: Qs2 ~ A1/3 ~ 1.5,3 GeV2 RHIC,LHC

Perturbative calculations (strong Fmn field ⇒ weak coupling as<<1).

had.+had: collision (amplitude level) of 1(2) gluon wave function(s): “resums” all mult. scatts.

Color Glass (“static” color sources = quarks) Condensate (high gluon occupation values).

Quantum evolution: BK, JIMWLK: Non-linear, all-twist eqs. in saturation region

xG(x,Q2)= dNg/dy

Q2 = 10 GeV2

Qs2=asNcx(x,Q2).

Small-x: go to forward rapidities

Parton-parton kinematics:

In addition: Much lower x values allowed in CGC (2  1, “gluon fusion”):

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CGC:

pQCD:

Accardi, nucl-th/0405046

x(y=4) ~ 10-4

x(y=4) ~ 10-2

x1=pT(e1+e2)/s

x2=pT(e-1+e-2)/s

x1s/2

x2s/2

pT

e.g. pT = 10 GeV/c

θ <10-3 (η~7- 8)

x1s/2

x2s/2

Momentum

balanced by

the “medium”

(“gluon ladder”)

y=0: x1~x2 ~ xT = 2pT/s

Forward y:

Every 2-units of y

x decreases by ~10

Experimental access to low-x parton distrib. (I)

High-pT, heavy-Q (ep,eA):

Diffractive QQbar (ep, gp,A):

g,DY,dijets,heavy-Q (pp,pA,AA):

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q,Q

...

Experimental access to low-x parton distrib. (II)

Kinematical (x,Q2) domains covered experimentally:

Most existing low-x nPDFs measurements are in the non-perturbative regime

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e-p, p-p

e-A, p-A

much less nuclear PDF data

available:

Dd'E, nucl-ex/0404018

HERA (ep): indirect evidences for saturation

Good description of F2(x,Q2) in

“transition region” (moderate/low Q2)

(limit of DGLAP applicability)

[Diffractive PDFs ...]

“Geometrical scaling” of F2(x,Q2):

(x,Q2)-dependence of structure function only

via scaled variable: F2(Q2/Q2s(x))

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Golec-Biernat-Wusthoff

PRD60 114023 (1999)

“dipole cross-section” model (derivable within CGC):

sg*p

t = Q2/Q2s

RHIC (dA,AA): indirect evidences for saturation

Total AuAu particle multiplicity

(plus its centrality evolution):

+ assuming “local parton-hadron duality”

d+Au moderate-pT spectra (& y-evolution)

at forward rapidities (suppression):

CGC “plus”: A theory that describes:

- (leading-twist) “shadowing” (ext. scaling)

- Cronin-effect (BK quantum evolution)

However: evidences at ~low pT. Qs @ RHIC/HERA close to non-perturb. range

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1 gluon = 1 final hadron

Armesto, Salgado, Wiedemann

hep-ph/0407018

Low-x at the LHC (proton)

pp @ 14 TeV :

(i) Very high √s ⇒ very small Bjorken x=2pT/√s, ~70 times lower than p+p @ RHIC

(ii) Very large perturbative cross-sections.

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Drell-Yan

For y < 5, M < 10 GeV

 x down to 10-6-10-7 !

Prompt g

Jets

W,Z production

Heavy Flavours

Processes:

A.Roeck, Blois'03

Low-x at the LHC (nucleus)

PbPb @ 5.5 TeV, pPb @ 8.8 TeV:

(i) Very high √s → Bjorken x=2pT/√s, ~30-45 times lower than AuAu,dAu @ RHIC

(ii) Very large perturbative cross-sections.

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Nuclear xG(x,Q2) is basically

unknown for x<10-3 !

Armesto, nucl-th/0410161

The CMS experiment (|η|<5)

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Physics program:

Higgs (EWSB), Beyond SM, Heavy-Ions, low-x QCD …

Silicon Tracker (|η| < 2.5):

Si pixels, Si strips

Good eff. & purity for pT>1 GeV

Pixel occupancy (PbPb): 1-2%

p/p  2% for pT<70 GeV

Calorimeters (|η| < 5):

EM: PbW04

HCAL: Scint.+steel

Magnet :

4 T solenoid

Muon tracking (|η| < 2.5):

 from Z0, J/, 

σm  50 MeV at the  mass

DAQ & Trigger

High rate capability for A+A, p+A, p+p

HLT: real time HI event reco

T2

(Track)

CASTOR

(Cal)

T1

(Track)

HF

(Cal)

ZDC

140 m

CMS-TOTEM forward detectors (|η |~3-6.7)

HF, TOTEM, CASTOR Platforms

HF, TOTEM (T1,T2), CASTOR

..

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Hadronic Forward Calorimeter (2.5 <η< 5)

..

(5.32 << 6.71)

(5.3 << 6.4)

11.2 m from IP5.

Steel+quartz-fibre Cherenkov (EM+HAD) calorimeter

1.65 m absorber depth (~5.6λ0)

900 towers (1200 channels): Δη x ΔΦ ~ 0.18 x 0.18

Hamamatsu R7525 rad-hard PMTs

Low-x measurements with HF

Forward jet production (“Mueller-Navelet” BFKL jets):

jet

jet



Azimuthal decorrelation between

the 2 jets versus rapidity distance

between the 2 jets:

Large Δη range required

J. Stirling & L. Orr, Del Duca et al.

hadron-hadron

DIS:

A.Roeck, Blois'03

HF, TOTEM, CASTOR Platforms

T2 and CASTOR (5.1 <η < 6.7)

..

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T2 telescope

CASTOR

CASTOR forward calorimeter (5.1 <η < 6.6)

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13570

Tungsten plates + quartz fibres

Cherenkov sampling calorimeter

Light-guides + APDs readout

Azimuth segmented (8 octants)

EM section: 11.2 cm ~ 19 X0

HAD+EM sections: 136 cm~ 10 λI

192 channels

T2

TOTEM T2 Tracker (5.3 <η< 6.7)

TOTEM GEM (“Gas Electron Multiplier”) charged particle telescope detector:

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APV

VFAT

HV

Shielding envelope

GEM half telescope

~ 500 mm

Low-x measurements with T2 + CASTOR

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7<||<9

5.5<||<7.8

Log10(x)

Log10(x)

Log10(x)

Log10(x)

5.2 < |η| < 6.6

Kimber

Martin

Ryskin

Drell-Yan:

Inclusive high-pT hadrons likely possible too (under study ...).

E. Sarkysian, CMS PTDR2

Beam pipe splits

~140 m from IR

Beams

Luminosity

Detector

Hadronic

Section

EM Section

9.6 cm

Zero Degree Calorimeter (|η|> 8.1 neutral)

Downstream1st beam separator dipoles (140 m from IP)

beams

ZDC = Tungsten-plates+Quartz-fibres sampling

Cerenkov calorimeter with HAD and EM sections:

ZDC: forward neutral energy (n,) detection:

ZDC

(Follows closely RHIC experience).

EM section: x33 2mm-W cells (~19X0)

HAD sect: x24 15mm-W cells (~5.6λ0)

PMTs: R7525 (as HF)

Rad hard to ~20 Grad (AA, pp low lum.)

Energy resolution (n,g): /E~10%

Position resolution: ~2 mm (EM sect.)

Signals available for L1

g A : LHC as a photon collider

Heavy-ions (w/ charge Z) produce strong

electromag. fields: coherent action of all protons.

Equivalent flux of photons in electromagnetic

(aka. UltraPeripheral, bmin~ 2RA ) A+A :

Photon beams:

Flux ~ Z2 (~7·103 for Pb).

“Coherence condition” : g wavelength > nucleus size

Center of mass-energies: sgA ≈ 0.7 – 2. TeV ≈ (3 – 10) × sgp(HERA)

Bjorken x range:

∝

(soft bremsstrahlung g spectrum)

E

E

Maximum g energy:

~ 80 – 160 GeV (Pb,Ca)

(y=0): x(J/) ~3·10-3 , x(¡)~10-2

(y=3): x(J/) ~ 2·10-5 , x(¡)~10-4

Very large pQCD cross-sections:

QQbar: s (LHC) ~ (40–100)×s(RHIC) J/: 32 Hz (30M/year), ¡(1S): 170000 (1st UPC meas.)

Very large pQCD cross-sections:

QQbar: s (LHC) ~ (40 – 100)×s(RHIC)

J/: 32 Hz (30M/year), ’: ~3M.

¡(1S): 170,000 (1st UPC measurem.)

Dijets: N(ET> 20 GeV) ~ 1/minute

Charm: (charm) ~ 1.8 b ~ 25% of hadr

B-jets: (bottom) ~ 700 b ~ 1/10,000 of hadr

Dd'E, nucl-ex/0601001

FGS shadowing

y

d/dy, mb

[FGS: Frankfurt et al. 2001]

g A : ZDC-tagged hard photoproduction

Quarkonia: g+A J/,¡+A very sensitive to

nuclear gluon density at small-x:

Dijet: via gluon exchange (well described in QCD & tested @ HERA)

Bottom line: all UPC processes only possible w/ ZDC neutron-tagging

~30% reduction of G(x,Q2) Þ 0.5·sJ/,¡

(J/,¡) ~ | xG(x,Q2) | 2

Perturbative process:

Even stronger suppression expected

if gluon saturation (CGC).

Strikman, Vogt, White, hep-ph/0508296

Wider range of Q2 than QQbar.

Photon-jet (~1% of dijet rate) has clear signature

ttbar possible in pA collisions (measure charge of top quark)

Rates in ATLAS or CMS acceptance:

No shadowing

Summary: CMS = low-x QCD experiment at LHC

CMS IP

5.32 ≤ |η| ≤ 6.86

ZDC

RPs@220m

HF,CASTOR,ZDC + TOTEM: Unparalleled forward coverage at LHC

Unique low-x QCD capabilities in pp, pA, AA at LHC.

Hard scattering measurements (jets, high-pT, DY) down to x~10-6

Studies of gluon saturation,CGC, non-linear QCD fully accessible.

Acceptance (neutral):

θ<~(5.cm) /140.m <~400 μrad

|η| > 8.5 , pT < O(2. GeV/c)

RPs@150m

T1/T2, Castor

CASTOR

TOTEM

ZDC

(5.3 < η < 6.7)

(z = 140 m)

( η > 8.1)

HF

Backup slides

...

..

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100 cm of space available

(9.6 x 12.5 x 100 cm)

Quartz fiber/tungsten plates

EM section segmented horizontally,

HAD section longitudinally

Luminosity detector in 2nd 10 cm

Improves resolution at large b

Readout through HF electronics –

signals available for L1 trigger

Beam pipe splits

~140 m from IR

Beams

HAD

EM

Lum

p remnants

jets

p

η

POMWIG 1.2

pp -> p 2j X

Roman pots

jet

jet

g,q

g,q

Diffractive pp collisions

No ZDC activity = large rapidity gap. Complements (trigger & offline)

leading proton detectors e.g. in dijet single diffraction:

ZDC (even w/o leading p detection) can help “calibrate” DPE Higgs prod.:

Bottom line: ZDC reduces to “zero” holes & cracks in CMS (full 4p). Helps all diffractive ( -, g-mediated) analysis in pp,pA,AA.

empty

ZDC

ZDC

1st process measurable w/ ZDC.

Both closely related (QCD part identical).

Measurement of one constrains the other.

[from M.Albrow

CMS, Feb'06]

[from A.Sobol

CMS, Feb'06]

g g , g A : physics topics

Typical diagrams for gg and gA collisions:

Pomeron, gluon,

or meson

Precision QCD:

Hard photo-production

nuclear GA(x,Q2) function, small-x physics, QQbar in cold nuclear matter

QED, precision QCD

l +l -, C-even cc bb, W+W-...

QED in strong regime (Zaem~1),

heavy-Q spectroscopy,

[quartic GC (W Wgg), ...]

Precision QCD

(low bckgd, simpler initial state than p,A+A)

Quarkonia, heavy-Q, jets ...

Nuclear GA(x,Q2), low-x physics,

QQbar in cold nuclear matter

Physics:

Measurements:

Topics:

g + g

g + A

_

_

All UPC measurements at RHIC: ZDC-triggered (neutron tagging) !

Dd'E, nucl-ex/0601001

Example: Quarkonia g-production @ HERA

HERA: gpVp (V=J/,¡) very sensitive to gluon distrib. at small-x:

Discriminates diff.

Ansätze & evolutions

of G(x,Q2)

Perturbative process:

J/

J/

Ryskin et al.

ZPC 76 (1997)231

g g : Discovery physics & others

 Higgs ( bb)

gg  W+W- & photoproduction of W+W- :

Other “discovery” channels:

 production of magnetic monopoles, SUSY, ...

Others:

Rate is very low (better w/ lighter ions, pA or pp)

Establishes the nature of Higgs (JPC, SM vs. MSSM)

Study quartic/triple gWW, WW vertices

Anomalous coupling sensitive to new physics

_

e+e- pair production

Tests QED in very strong fields (grazing b=2R colls ~ 5 e+e- pairs produced). Huge rates: ~13,000 b in ALICE inner Si

Quantum correlations in multiple Vector Meson Production

QQbar spectroscopy: g+g  0+-2++ states (hb, cb0, cb2,...).

p interactions at the LHC

..

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0.01 < x < 0.1,  tagging range

0.005 < x < 0.3, Bjorken-x range

g luminosity spectra

p Physics

Menu:

K Piotrzkowski

• anomalous W and Z production at Wq  1 TeV

top pair production  top charge + threshold scanning?

single top production and anomalous Wtb vertex

SM BEH  for example, b  H b, q H W q

SUSY studies (complementary to the nominal ones) -

H+ t production (and H++), b and t spairs, tpair, ...

Exotics: compositness, excited quarks, .

UHE cosmic-rays via extended air-showers

Cosmic-ray energy spectrum:

Low stat. for Elab > 100 TeV (10-14 eV) :

only “indirect” measurements (EAS) exist

Cosmic-ray energy & mass determined

via Monte Carlo codes.

Shower development dominated by

forward, soft QCD interactions.

Primary interactions extrapolated from

SppS,Tevatron data (Elab <100 TeV)

up to a factor x106

Current uncertainties of hadronic models

Key (Regge-Gribov) QCD models predictions in PeV-range:

Wide range of predictions !

Air showers rather insensitive (correlated cross- section & multiplicity: partial compensation).

Yet ... ~20% energy & composition uncertainties esp. at high-end of spectrum (E >1018 eV)

σinel(p-Air)

<Nch>

<pT>

[T.Pierog, LHC pA workshop '05]

Calibration and tuning of hadronic models

Model predictions of particle multiplicity & energy flow at LHC:

ZDC: Direct measurement of forward leading baryon (neutron) and π0

cross-sections & energy in pp,pA,AA at Elab ~ 100 PeV !

(strong model constraint)

Slide 1

d7o

2003-08-06T23:02:41

David d'Enterria