Forward Physics at the LHC

SM discoveries with early LHC data UCL, March 30th - April 1st 2009

1. Exclusive/diffractive Higgs signal: pp p + H + p

Alan Martin, IPPP, Durham

2. Properties of “soft” interactions (forward/diffractive physics at the LHC)

3. Return to the exclusive processes (at the Tevatron and the LHC)

Advantages of pp p + H + p with H bbbar

If outgoing protons are tagged far from IP then (M) = 1 GeV(mass also from H decay products)

Very clean environment, even with pile-up---10 ps timing

Unique chance to study Hbbbar: QCD bbbar bkgd suppressed by Jz=0 selection rule S/B~1 for SM Higgs M < 140 GeV

SUSY Higgs: parameter regions with larger signal S/B~10, even regions where conv. signal is challenging and diffractive signal enhanced----h, H both observable

Azimuth angular distribution of tagged p’s spin-parity 0++

FP420 ATLAS + CMS

Is the cross section large enough ?

How do we calculate (pp p + H + p) ?

What price do we pay for an exclusiveprocess with large rapidity gaps ?

unintegrated skewed gluons fg given interms of g(x,Qt

2) and Sudakov factorwhich exponally suppresses infrared region

no emission when ~ 1/kt) > (d ~ 1/Qt)i.e. only emission with kt > Qt

> 100 fb !! but….can use pQCD

QCD mechanism for pp p+H+p

…but “soft” scatt. can easily destroy the gaps

gap

gap

eikonal rescatt: between protonsenhanced rescatt: involving intermediate partons

soft physics at high energies

H

soft-hardfactorizn

conservedbroken

Model for “soft” high-energy interactionsneeded to ---- understand asymptotics, intrinsic interest ---- describe “underlying” events for LHC jet algorms

---- calc. rap.gap survival S2 for exclusive prodn

Model should:

1. be self-consistent theoretically --- satisfy unitarity importance of absorptive corrections importance of multi-Pomeron interactions2. agree with available soft data CERN-ISR to Tevatron range

3. include Pomeron compts of different size---to study effects of soft-hard factn breaking

(Dark age?)

Optical theorems

High mass diffractive dissociation

at high energyuse Regge

triple-Pomeron diag

but screening important gN3g3P

but screening important so total suppressed

gN2

so (g3P)bare increased

M2

2

elastic unitarity

e- is the probability of no inelastic interaction

diagonal in b ~ l/p

Must include unitarity

DL parametrization:

Effective Pomeron poleP(t) = 1.08+0.25t

KMR parametrization

includes absorptionvia multi-Pomeron effects

Low-mass diffractive dissociation

include high-mass diffractive dissociation

Elastic amp. Tel(s,b) bare amp.

introduce diffve estates i, k (combns of p,p*,..) which only undergo “elastic” scattering (Good-Walker)

multichannel eikonal

(-20%)

(SD -80%)

(-40%)

g3P ?

PPPPPR

PPR

P

RRP

RRR

RRP

P

PPP

triple-Regge analysis of d/dtdincluding screening

fit: 2 = 171 / 206 d.o.f.

TevatronCERN-ISR

(includes compilation of SD data by Goulianos and Montanha)

g3P= gN ~0.2g3P large, need to includemulti-Pomeron effectsLKMR

g3P= gN ~0.2

M2dSD/dM2 ~ gN3 g3P ~el

M2

2

large ?

ln s

so at collider energies SD ~ el

gN

g3P

Multi-compt. s- and t-ch analysis of soft data

3-channel eikonal, i with i=1,3

include multi-Pomeron diagrams

attempt to mimic BFKL diffusion in log qt by including three components to approximate qt distribution –possibility of seeing “soft hard” Pomeron transition

KMR 2008

model:

Use four exchanges in the t channel

a = Plarge, Pintermediate, Psmall, R

3 to mimic BFKL diffusion in ln qt

soft pQCD average qt1~0.5, qt2~1.5, qt3~5 GeV

VRP1 ~ gPPR,gRRP

VPiPj ~ BFKL

solve for aik(y,b)

by iteration

sec. Reggeon

bare pole absorptive effectsevolve up from y=0

evolve down from y’=Y-y=0

(arXiv:0812.2407)

Parameters

All soft data well describedg3P=gN with=0.25

Pi = 0.3 (close to the BFKL NLL resummed value)’P1 = 0.05 GeV-2

These values of the bare Pomeron trajectory yield, afterscreening, the expected soft Pomeron behaviour ---“soft-hard” matching (since P1 heavily screened,….P3~bare)

R = -0.4 (as expected for secondary Reggeon)

Results

multi-Pomeron coupling from dSD/ddt data ( ~0.01)

diffractive eigenstates from SD(low M)=2mb at sqrt(s)=31 GeV, -- equi-spread in R2, and t dep. from del/dt

= (0) - 1

“large”

“small”

~ g, sea

more valence

LHC (x0.1)

elastic differential d/dt

Description of CDF dissociation data

no P no P

total (mb)

pppXparton multiplicity

“soft”, screened,little growth,partons saturated

All Pom. comptshave bare=0.3

“hard” ~ no screeningmuch growth, s0.3

Predictions for LHC

total = 91.7 mb*el = 21.5 mbSD = 19.0 mb

*see also Sapeta, Golec-Biernat; Gotsman et al.

“soft” Pomeron

“hard” Pomeron

Multi-Pomeron effects at the LHC

Each multi-Pomerondiag. simultaneouslydescribes severaldifferent processes

Example

8 different “cuts”AGK cutting rules

Long-range correlations at the LHC

cutting n eikonal Pomerons multiplicity n times that cutting one Pomeron

long range correlation even for large rapidity differences | ya – yb | ~ Y

R2 > 0

without multi-Pomeron exch. R2>0 only when twoparticles are close, e.g. from resonance decays

Calculation of S2eik for pp p + H +p

average over diff. estates i,k

over bsurvival factor w.r.t. soft i-k interaction

hard m.e. i k H

prob. of proton to bein diffractive estate i

S2eik ~ 0.02 for 120 GeV SM Higgs at the LHC

~ 2 - 3 fb at LHC

<Seik>2 ~ 0.02

Watt, Kowalski

from pJ/p

<Senh>2 = ??

0.6 fm

gap

gap

eikonal rescatt: between protonsenhanced rescatt: involving intermediate partons

H

soft-hardfactorizn

conservedbroken

The new soft analysis, with Pomeron qt structure, enables S2

enh to be calculated

Calculation of S2enhanced for pp p + H +p

model has 4 t-ch. exchanges

a = Plarge, Pintermediate, Psmall, R

3 to mimic BFKL diffusion in ln qt

soft pQCD average qt1~0.5, qt2~1.5, qt3~5 GeV

VRP1 ~ gPPR,gRRP

VPiPj ~ BFKL

~ solve with andwithout abs. effects

bare pole absorptive effectsevolve up to y2

evolve down to y1

enh. abs.changesP3 distribn

P3

P1

y2

y1

p1t

p2t

H

<S2eik> ~ 0.02 consensus

<S2enh> ~ 0.01 – 1

controversyKMR 2008 <S2>tot=<S2

eikS2enh> ~ 0.015

(B=4 GeV-2)

<S2>tot<p2t>2 =

0.0015 LHC0.0030 Tevatron

KMR 2000 (no Senh)

KMR 2008 (with Senh)0.0010 LHC0.0025 Tevatron

Survival prob. for pp p+H+p

However enh. abs. changes pt behaviour from exp form, so

see arXiv:0812.2413

1. Enhanced rescattering reduces the signal by ~30%

2. However, the quoted values of S2 are conservative lower limits

3. The very small values of S2enh in recent literature are

not valid

The arguments are given in arXiv:0903.2980

Comments on S2

CDF observation of exclusive processes at the Tevatronoffers the first experimental checks of the formalism

Observation of exclusive prodn, pp p + A + p, by CDF

with A= or A = dijet or A = c J/ +-

Same mechanism as pp p+H+p

tho’ predns become more unreliable as MA becomes smaller,and infrared Qt region not so suppressed by Sudakov factor

KMR cross section predictions are consistent with CDF data

3 events observed (one due to 0)(excl )CDF ~ 0.09pb (excl )KMR ~ 0.04pb(= 10 fb for ET

>14 GeV at LHC

KMR

Observation of exclusive prodn, pp p + A + p, at Tevatron

y=0

The KMRS predn is reducedby S2

enh ~ 1/3 and by 1.45 due to a revised tot(c(0))

b ?

Early LHC runs can give detailed checks of all of the ingredients of the calculation of (pp p + A + p),sometimes even without proton taggers

Early LHC checks oftheoretical formalism for pp p + A + p ?

Possible checks of:

(i) survival factor S2: W+gaps, Z+gaps

(ii) generalised gluon fg : p p, 3 central jets

(iii) soft-hard factorisation #(A+gap) evts (broken by enhanced #(inclusive A) evts absorptive effects) with A = W, dijet, …

(arXiv:0802.0177)

W+gaps with

Even without a proton tag can be measured by

successfully used by CDF

cross section survival fac.

1 pb

S2 large, as large bt (small opacity)

W+gaps

measure: W+gapsW inclusive

W+gaps has S2 large, as large bt for exch (small opacity)

Z+gaps has bt more like excl. Higgs

~0.2pb for i>3 and ET(b)>50GeVbut to avoid QCD bb backgd use Zl+l-

use track counting veto

expect S2~0.3

Exclusive production as probe of fg

x 0.025 (br for )

exch odderon exch

comparable ?can separate by pt if a tagof upper proton is done (odderon has larger pt)

If |y|<2.5, then sample fg(x1,x2) with xi in (10-4, 10-2)

Bzdak, Motyka,Szymanowski,Cudell

3-jet events as probe of Sudakov factor T

T is prob. not to emit additional gluons in gaps: pp p + A + pT=exp(-n), where n is the mean # gluons emitted in gap

3 central jets allow check of additional gluon emissionSystem A must be colourless – so optimum choice is emission of third jet in high ET dijet production

highest

only highest ET jet used –stable to hadronization,final parton radiation…

pp p + jj + p

pp p + jjj + pstudy both and ET

dependence of central3-jet production

(negligible DPEbackground)

“Enhanced” absorptive effects(break soft-hard factorization)

rescattering on an intermediate parton:

can LHC probe this effect ?

inclusive diffractive

A = W or dijet or ….

known from HERA

A = W or …. A = dijet

pp AXpp AX+p

rough estimates of enhanced absorption S2en

Conclusions – soft processes at the LHC

-- screening/unitarity/absorptive corrections are vital

-- Triple-Regge analysis with screening g3P increased by ~3 importance of multi-Pomeron diagrams

-- Latest analysis of all available “soft” data: multi-ch eikonal + multi-Regge + compts of Pom. to mimic BFKL (showed some LHC predictions ….. total ~ 90 mb)

-- LHC can probe “soft” intns i.e. probe multi-Pomeron struct. via long-range rapidity correlations or via properties of multi-gap events etc.

soft-hard Pomeron transition emerges “soft” compt. --- heavily screened --- little growth with s “intermediate” compt. --- some screening “hard” compt. --- little screening --- large growth (~pQCD)

soft analysis allows rapidity gap survival factors to be calculated for any hard diffractive process

Exclusive central diffractive production, ppp+H+p, at LHC hasgreat advantages, S/B~O(1), but ~ few fb for SM Higgs. However, some SUSY-Higgs have signal enhanced by 10 or more.Very exciting possibility, if proton taggers installed at 420 m

Formalism consistent with CDF data for pp(bar) p + A + p(bar) with A = dijet and A = andA c

More checks with higher MA valuable.

Processes which can probe all features of the formalism used tocalculate (ppp+A+p), may be observed in the early LHC runs, even without proton taggers

Conclusions – exclusive processes at the LHC