Moving on to BSM physics

Example of how PDF uncertainties matter for BSM physics– Tevatron jet data were originally taken as evidence for new physics--

iThese figures show inclusive jet cross-sections compared to predictions in the form (data - theory)/ theory

Something seemed to be going on at the highest E_T

And special PDFs like CTEQ4/5HJ were tuned to describe it better- note the quality of the fits to the rest of the data deteriorated.

But this was before uncertainties on the PDFs were seriously considered

WHAT DO WE NOT KNOW WELL?

Today Tevatron jet data are considered to lie within PDF uncertainties. (Example from CTEQ hep-ph/0303013)

We can decompose the uncertainties into eigenvector combinations of the fit parameters-the largest uncertainty is along eigenvector 15 –which is dominated by the high x gluon uncertainty

And we can translate the current level of PDF uncertainty into the uncertainty on LHC jet cross-sections. This has consequences for any new BSM physics which can be described by a contact interaction-consider the case of extra dimensions

S.Ferrag

MJJ (GeV)

d/

dM

Such PDF uncertainties in the jet cross sections compromise the LHC potential for discovery of any new physics which can written as a contact interaction E.G. Dijet cross section potential sensitivity to compactification scale of extra dimensions (Mc) reduced from ~6 TeV to 2 TeV.

And what consequences might this have?

S. Ferrag + A Djouadi

2XD

4XD

6XD

SM

M c = 2 TeV,no PDF error

M c = 6 TeV,no PDF error

M c = 2 TeV,with PDF error

And will we be able to use LHC data itself to improve the situation?

Recently grid techniques have been developed to input jet NLO cross-sections in PDF fits (e.g FastNLO, Applgrid)

This technique can be used for LHC high-ET jet cross-sections

Use data at lower PT and higher η-where new physics is not expected

The reduced gluon uncertainties can then be used in background calculations for new physics signals

Can we know the high-x gluon better?

?

Impact of increasing statistics

Impact of decreasing experimental

systematic uncertainty

Impact of decreasing experimental correlated systematic uncertainty

Challenging!– currently ~5% spring 2011

Can we decrease Jet Energy Scale systematic

to 1%?

Arxiv:0911.0985

M Fiascaris, H Beauchemin

W + jets is a channel which bridges from SM to BSM physics.

In W+>~4 jets Supersymmetric signals could be present. Looking at ratios likeW+n-jets/W+(n-1) jets or Z+n-jets/W+n-jetsis good way to search for BSM signals while controlling the Jet Energy Scale systematicarXiv 1012.5382 looks at W+jets using 1.3pb-1

Of course it’s too early for BSM observations yetBut here are the preliminary plots from the full 2010 stats– statistical errors only

Jet energy scale also a problem in W/Z+jets channel, where SUSY signals may show up – Jet Energy Scale of 5% gives uncertainties 5-12% on the W + (1-6) jet cross-sections. This is larger than the PDF uncertainty (3-8%)

M Fiascaris

Illustrated is MSugra SU(4) compared to Standard Model for 200pb-1 of data in the W/Z +2 jets channel

JES of 5% gives < 5% uncertainty on the ratio –very much less than the statistical error

H Beauchemin

However BSM signals can show up in the R=(W+n jet) / (Z+n jet) ratio and the jet energy scale is less of a problem in the ratio

1 year (10 fb-1)

ATLAS TDR

PDF Uncertainty in High-mass Drell-Yan- won’t stop us seeing Zprimes

Gluons dominant

7 – 9 % Uncertainty

d-Valence dominant

Sea dominant

PDF uncertainties don’t affect the Higgs discovery

potential too badly… but will affect limits..recent updates

For what discoveries do PDF uncertainties not hamper us (much)

F Heinemann

S. Ferrag + A Djouadi

ATLAS pseudo-data

Higgs physics outlook 2011

Can also achieve 3 evidence with 1fb-1 for Higgs masses between 139 < MH < 180 GeV

With 1 fb-1 can exclude a Higgs boson with 129 < MH < 460 GeV

Higgs production production depends on g-g luminosity . These also differ between PDFs.At low-scale some of the same reasons for the differences as for q-qbar apply.At high-scale there are big differences.It has been suggested that fits with no Tevatron jet data do not have a hard enough gluon. Maybe so for HERAPDF, ABKM but not for GJR– reasons for differences are not clear.

But at 90%CL there is considerable overlap

Perhaps G-G luminosity is a better thing to compare for high-Et jets?G=g+4/9(q+qbar)

Spread in Higgs production cross-sections is now > 15%Dependence on αS(MZ) is also increased- use of a common value would certainly decrease discrepancy..But there are disagreements as to the correct value NNPDF, CTEQ, HERAPDF, MSTW all provide PDFs at a series of αS(MZ) values ~0.114-0.122PDF4LHC recommends adding ΔαS(MZ)~0.0012 uncertainty (at 68%CL) in quadrature with PDF uncertaintyHe band of NNPDF2.0, MSTW08, CTEQ6.6 may not be big enough

For Higgs we really need NNLO: arXiv 1101.1832 suggests that the MSTW error estimates used for setting the current Higgs limits may not be large enough... i.e. There are no limits yet!

What consequence would new low-x physics have at the LHC ?BFKL ln(1/x) resummation would change the deduced shape of the gluon –Thorne and White -And has the added benefit of improved χ2 for global PDF fitFor other work on NLL BFKL see also Ciafaloni, Colferai, Salam, StastoAltarelli, Ball, Forte

MRST02

MRST03

Central rapidity

Drell-Yan M(ee) = 4GeV

MRST03 PDFs were a TOY PDF which distrusted all x < 10-3. This would affect the central region for W production.

High density non-linear effects may induce gluon saturation this

also affects the deduced shape of the gluon

Far forward

Thorne and

White

Eskola et al

2011: we already know things are not that bad!

But the TOY PDFs are unlikely to be realistic - a better way could be to look at pt spectra for W and Z production

Lack of pt ordering at low-x is a further consequence BFKL resummation AND most non-linear treatments. This would affect the pt spectra for W and Z production at the LHC (See hep-ph/0508215)

Conventional

Unconventional

Pt spectra are also used to measure MW -- dMW from PDF uncertainties, using pt(e), is ~20 MeV

So we’d better be sure we’ve got the calculations for Pt spectra right

< pT(W) >

MW(fit)

Same pattern

End lecture 9

Current Z-pt measurement doesnt quite have this resolution