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LIB2DHP@LHC (Myths and Reality)

V.A. Khoze (IPPP, Durham)

Main aims: -to quantify the mjor sources of the Lumi-Independent Backgrounds, -to Exclusive Diffractive Higgs Production at the LHC.

(based on works : A. De Roeck, R. Orava and KMR, EPJC 25:391,2002 ; V. Khoze, M. Ryskin and W.J. Stirling, hep-ph/0607134; B. Cox et al. EPJC 45:401,2006;

KMR: EPJC 26:229,2002; C34:327,2004 )

FP-420

Dec. 10th, 2006

☻ If the potential experimental challenges are resolved, then there may be a real chance that for certain MSSM scenarios the CEDP becomes the LHC Higgs discovery channel !

CEDP- Main Advantages: - Measure the Higgs mass via the missing mass technique (irrespectively of the decay channel). -H opens up (Hbb Yukawa coupling); unique signature for the MSSM Higgs sect.

-Quantum number/CP filter/analyzer. -Cleanness of the events in the central detectors.

bb

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Myths

For the channel LIBs are well known and incorporated in the (DPE )MCs:

Exclusive LO - production (mass-suppressed) + gg misident+ soft PP collisions.

Reality

The complete background calculations are still in progress (uncomfortably & unusually large high-order QCD and b-quark mass effects).

About a dozen various sources : known (DKMOR, Andy, Marek) & admixture of |Jz|=2 production.

NLO radiative contributions (hard blob and screened gluons)

③ NLLO one-loop box diagram (mass- unsuppressed, cut-nonreconstructible)

b-quark mass effects in dijet events – (most troublesome theoretically) still incomplete (similar problems in photon-photon collisions).

bb

bb

In the proton tagging mode the dominant H in principle can be observed directly . certain regions of the MSSM parameter space are especially proton tagging friendly (at large tan and M , S/B )

bb

250 20

3

2 2

22

2

2 22

( ,...) ( , ) ( ,.

(

..)

)

harp f

ff

dp ef

e

MM L M y

LM S L M

y M

focus on 2( ,...)bgd

hard M2( , )effL M y the same for Signal and Bgds

effL

contain Sudakov factor Tg which exponentially suppresses infrared Qt region pQCD

new CDF experimental confirmation, 2006

S² is the prob. that the rapidity gaps survive population by secondary hadrons soft physics; S² =0.026 (LHC),

S²/b² -weak dependence on b.

KMR technology (implemented in ExHume)

(Koji’s talk)

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KMR analytical results

( ) 0.8 ( )T TE meas E part

( ) 0.75 ( )T TE meas E part

CDF preliminary

Good ExHume description

(Rjj includes different sources : Centr. Inclus. + soft PP + (rad. ) tail of Centr. Exclus. + experim. smearing)

outside-cone energy (Koji)

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effect. PP lumi 33,( ) 1/SM SMH gg M M

(HKRSTW, work in progress).

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the prolific LO subprocess PPgg gg may mimic bb production

misidentification of outgoing gluons as b –jets

for jet polar angle cut 60 120

(DKMOR WishList)

for forward going protons LO QCD bgd suppressed by Jz=0 selection rule and by colour, spin and mass resol. (M/M) –factors.

RECALL :

bb

forfor reference purps : SM Higgs (120 GeV)reference purps : SM Higgs (120 GeV)

misidentification prob. P(g/b)=0.01 B/S 0.06

23 ( [ ( / ) / 0.01 1](120 / )4 )/ / 4M Ge P gM V b

SM Higgs

(difference in a factor of 2 ? )

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A little bit of (theoretical) jargon

Helicity amplitudes for the binary bgd processes

g gg (

i helicities of ‘active gluons’

q (double) helicities of produced quarks

convenient to consider separately q-helicity conserving ampt (HCA) and q-helicity non-conserving ampt(HNCA)

do not interfere, can be treated independently, allows to avoid double counting (in particular, on the MC stage)

1 2( ) for Jz=0 the Born HCA vanishes,

(usually, HCA is the dominant helicity configuration.)

for large angles HNCA

Symmetry argumts (BKSO-94)

(Jz=2, HCA)

S – Jz=0, LO B- domint. Jz=2

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Jz=0 suppression is removed by the presence of an additional (real/ virtual) gluon (BKSO-94)

in terms of the fashionable MHV rules (inspired by the behaviour in the twistor space)

only (+ - ; + -) J_z=2, HCA (-+ ; -+ /+-)

0

an advantageous property of the PPgg qq large angle amplitudes

sall HNCA (Jz=0, Jz=2, all orders in ) are suppressed by

all HC ampts ((Jz=0, Jz=2, all orders ) are \propto vanish at

/q TEmPPgg cos 90

recall: we require in order to suppress t-channel singl. in bgd processes, also acceptance of the CD.. an additional numerical smallness ( )

60 120

1/ 5

rotational invariance around q-direction (Jz=2, rotational invariance around q-direction (Jz=2, PPPP-case only)-case only)

LOLO HCHCAA vanishes in the Jz=0 case (vanishes in the Jz=0 case (valid only for the Born amplitudevalid only for the Born amplitude))

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Classification of the backgrounds PPgg qq

|Jz|=2 LO production caused by non-forward going protons.

HC process, suppressed by and by

≈ 0.02 * ( ) estimate

NLLO (cut non-reconstructible) HC quark box diagrams.

2cos /1M GeV

≈ ≈ 22 2 22 2// cos32 ( ) sins Fq CNM Cm

result result

dominant contribution at very large masses M

at M< 300 GeV still phenomenologically unimportant due to a combination of small factors

appearance of the factor consequence of supersymmetry2( )F CC N

3 // / SMSM Br BrB S M M

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mass-suppressed Jz=0 contribution

theoretically most challenging (uncomfortably large higher-order effects)

naively Born formula would give 0.06 however, various higher-order effects are essential : running b-quark mass Single Log effects ( )

the so-called non-Sudakov Double Log effects , corrections of order

(studied in FKM-97 for the case of at Jz=0 )

Guidance based on the experience with QCD effects in . DL effects can be reliably summed up( FKM-97 , M. Melles, Stirling, Khoze 99-00 ). Complete one-loop result is known (G. Jikia et al. 96-00 )

-complete calculation of SL effects ( drastically affects the result)

2(8 / ) ln ( / ) 1S bM m F=

5 // / SMSM Br BrB S M M

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-bad news : violently oscillating leading term in the DL non- Sudakov form-factor:

- (≈2.5)

DL contribution exceeds the Born term; strong dependence on the NLLO, scale, running mass…. effects

No complete SL calculations currently available.

2 ....)(1(1 2 / )FC FN C

HNC contribution rapidly decreases with increasing M

Fq=

Currently the best bet: :

Fq ≈

with c≈1/2.

Taken literally factor of two larger than the ‘naïve’ Born term. Cautiously : accuracy, not better than a factor of 5

A lot of further theoretical efforts is needed

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PPgg qqNLO radiation accompanying hard subprocess

+ + radiation off b-quarks

potentially a dominant bgd :( ) strongly exceeding the LO expectation.

2/ ( / )S bm M

only gluons with could be radiated, otherwise cancel. with screening gluon ( ).

t tp Q

KRS-06 complete LO analytical calculation of the HC , Jz=0

in the massless limit, using MHV tecnique.

Hopefully, these results can be (easily) incorporated into more sophisticated

MC programmes to investigate radiative bgd in the presence of realistic expt. cuts.

2| |gg qqgM

3 // / SMq SMq g S Br BrB S M M

,

1/t tg d Q

Large-angle, hard-gluon radiation does not obey the selection rules

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How hard should be radiation in order to override Jz=0 selection rule ?

as well known (classical infrared behaviour)

neglecting quark mass

(a consequence of Low-Barnett-Kroll theorem, generalized to QCD )

the relative probability of the Mercedes –like qqg configuration for Jz=0 radiative bgd process becomes unusually large

marked contrast to the Higgs-> bb (quasi-two-jet- like) events.

charged multiplicity difference between the H->bb signal and the Mercedes like bbg – bgd:

for M 120, N ≈7, N rises with increasing M.

hopefully, clearly pronounced 3-jet events can be eliminated by the CD, can be useful for bgd calibration purposes.

Exceptions: radiation in the beam direction; radiation in the b- directions. could be eliminated DKMOR-02

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beam direction case

if a gluon jet is to go unobserved outside the CD or FD ( )

violation of the equality : (limited by the )

contribution is smaller than the admixture of Jz=2. KRS-06

sinmis g bbM M

b-direction case (HCA)

0.2 ( R/0.5)²

Note : soft radiation factorizes strongly suppressed is not a problem, NLLO bgd numerically small

bbgg

radiation from the screening gluon with pt~Qt :

HC (Jz=2) LO ampt. ~ numerically very small

hard radiation - power suppressed

KMR-02

cos2( / )t tQ p

(R –separation cone size)

bbM

t tM p Q

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Production by soft Pomeron-Pomeron collisions

bb

main suppression : lies within

mass interval

inmiP gP ssM M

bb bbM M

the overall suppression factor :

2 21( (1 )

2/ ) /PP P PPbb bbM g MM M

Background due to central inelastic production

H/bb

mass balance, again

subprocess is strongly suppressed

produces a tail on the high side of the missing mass

PPgg Hgg

(DKMOR-02)

from DKMOR-02 (WishList)

z

PS

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A.Pilkington, CERN27th Sept.2006

1.New MRW(2006) diffractive pdf should be used, which are close to H1 fit B: much lower gluon density at large z (MRW fit available in Durham HEPDATA). 2. Appropriate value of survival factor.3.Proper cuts on low ET. . 4. Possibility to veto the ‘Remnant Pomeron’ jets (CD extra jets, maybe T1, T2 detectors …to be studied) Risto , Marek.

What else should be done/checked?

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Not the end of the story…..

Preliminary estimates :We should be fine : B/S(DPE) <0.1-1.

in terms of Feynman graphs: Central Inelastic Soft PP- Fusion (Pomwig)

Studied by KMR 02-06 formally suppressed by 2S

In ‘theoretical jargon’ : CI - ‘kT- factorization, Soft PP – collinear factorization.

CDF inclusive diffractive dijet data – evidence of manifestation of the CI dijet production (both RunI and RunII).

Currently, in the description of the Rjj <0.8 distr.ibution, normalization to models (Pomwig...) is fixed by the CDF data! Normalization at Rjj>0.8 (Exhume) comes from the theory.

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(recall also Pomwig normal. to CDF Run I Dif. Inel data (0.27-0.1) )

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MAKEDPEMC

HISTORY

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WW mode(detailed studies in B. Cox et al. hep-ph/0505240)

No trigger problems for final states rich in higher pT leptons. Efficiencies ~20% (including Br) if standard leptonic (and dileptonic) trigger thresholds are applied. Further improvements, e.g. dedicated -decay trigger. Statistics may double if some realistic changes to leptonic trigger thresholds are made.

Much less sensitive to the mass resolution.

Irreducible backgrounds are small and controllable.

.

Recall : the h- rate can rise by about a factor of 3.5-4 in some MSSM

models (e.g., small eff scenario).

KRS-05

+ + …….

Currently : trigger situation is more optimistic.

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mode

● Irreducible bgds (QED) are small and controllable. ( >0.1-0.2GeV)

QCD bgd is small if g/- misidentification is < 0.02

(currently ~0.007 for -jet efficiency 0.60)

tp

Lepton Trigger Efficiency Cox et al (e or ) ATALS (e, , 2e, 2, e)

M=120 GeV 8.7% 20.3% M=140 GeV 12.8% 26.9%

M=160 GeV 16.6% 28.8% (Sandra Horwat, MPI)

Lepton trigger efficiency looks optimistic: ATLAS ((e, , 2e, 2, e)

M=120 GeV 22.1% M=140 GeV 25.8% M=160 GeV 28.3%

Pile-Up situation is hopefully bettter

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Approximate formula for the background assuming DKMRO-2002 wishlist inputbb

main uncertn. at low masses

SSM/B1 at ΔM 4 GeV

Four major bgd sources (~1/4 each at M≈120 GeV)

gluon-b misidentification (assumed 1% probability) NLO 3-jet contribution Correlations, optimization -to be studied. admixture of |Jz|=2 contribution + NNLO effectsb-quark mass effects in quasi-dijet events

Recall : large M situation in the MSSM is very different from the SM.

HWW/ZZ - negligible; H bb/- orders of magnitude higher than in the SM

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DKMOR (2002) currently

Four years on

13 GM eV

M=120 GeV

5.76 GeV

WishList

(87% signal catch )

10CDM GeV 30-40 GeV

misidentification prob. P(g/b)=1%

(b-tag efficiency) 0.6

2( )b

2.5% (CMS fast simulation.) Marek1.3% (ATLAS) Andy

2( )b 0.36

no Pile-Up studiesPA (Monika, Marek, Andy, Andrew..)

A.Rozanov (ATLAS)

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MSSM

Recall : large M situation in the MSSM is very different from the SM. HWW/ZZ - negligible;

H bb/- orders of magnitude higher than in the SM detailed studies of statistical significance for the MSSM Higgs signal discovery , based on the CMS Higgs group procedure – in progress (HKRSTW, hopefully first part of 2007.. )

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mhmax scenario, =200 GeV, MSUSY =1000 GeVhbbM. Tasevsky et al. (preliminary)

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Hbb

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small eff scenario

for the SM Higgs at M = 120 GeV = 0.4 fb, at M= 140 GeV = 1 fb

mh 121-123 GeV

hWW PRELIMINARY

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Conclusion

Luminosity Independent Backgrounds to CEDP of Hbb do not overwhelm the signal and can be put under full control especially at M> 120 GeV.

Luminosity Independent Backgrounds to H WW & channels are controllable and could be strongly reduced.

The complete background calculation is still in progress (unusually & uncomfortably large high-order QCD effects).

Further reduction can be achieved by experimental improvements, better accounting for the kinematical constraints, correlations…..

Optimization, complete MC simulation- still to be doneFurther theoretical & experimental studies are needed

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PU (a naïve theorist’s view)

Overlap background: hard event (bb,WW, ...) +2 SD events in the same bunch Xing

2( 1) / 2 ( 1) ( 2) / inhP dU artN N RP RP

(220 ) / 0.03; (420) / 0.01.t tm

Roughly : *( ) 100 ; ( ) (1) .in

hard hardinbb nb WW nb

(Misha’s talk)

we have to achieve reduction of in 6-7 orders:

correlations between measurements in CD and RPs (1, 2, M...),

correlations in azimuthal angles, pt- cuts,

use of fast timing detectors (~40-100) ( Andrew, Monika….)

Nc , cuts (~100, Andy)

veto in T1, T2 detectors - Risto

CN

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Known (un)knowns

The probability to misidentify a gluon as a b-jet P(g/b) and the efficiency of

tagging b.

Does the CEDP environment help ?

Mass window M≈3 (M), any further improvement ?

Correlations, optimisation -to be studied

S² (S²/b²), further improvements, experimental checks.

Triggering issues: Electrons in the bb –trigger ? Triggering on the bb/ without RP condition at M 180 GeV ?

Mass window MCD from the Central Detector only (bb, modes) in the Rap Gap environment?

Can we do better than MCD ~20-30 GeV? Mass dependence of MCD ? (special cases: inclusive bgds, hunting for CP-odd Higgs)

,b b

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BACKUP

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8

(KMR- based estimates)

(more on the pessimistic side, studies based on the CMS Higgs group procedure –still to come)

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PU (a naïve theorist’s view)

PU?

Overlap background: hard event (bb,WW, ...) +2 SD events in the same bunch Xing

2( 1) / 2 ( 1) ( 2) / inhP dU artN N RP RP

(220 ) / 0.03; (420) / 0.01.t tm Roughly : *( ) 100 ; ( ) (1) .in

hard hardinbb nb WW nb