1. LHeC and Physics Beyond the Standard Model
Emmanuelle Perez
CERN
LHeC and Physics Beyond the Standard Model
Sensitivity to new physics in ep collisions at 1.4 TeV :
quark radius, electron-quark resonances,
eeqq contact interactions.
LHeC w.r.t. the interpretation of LHC discoveries :
are there limitations due to our limited
knowledge of high x pdfs ?
Crude studies so far, and only a few topics.
3 June 2006
LHeC Steering Committee meeting, 27 Oct. 2007

2. Beyond the Standard Model in ep collisions : introductory remarks
Since ep is not an annihilation machine, the cross-section is “small”
for (heavy) pair-produced new particles.
 Investigate single particle production instead
 The sensitivity depends, in addition to the new particle mass, on
its coupling to SM particles.
 In case of a discovery: information not only on M but also on this
coupling.
Examples : Leptoquarks, Rp-violating SUSY, excited fermions, …
HERA vs. Tevatron showed nice examples of the complementarity between
both facilities, in such model-driven BSM searches.
Once LHC has discovered “something”, additional data from ep and/or ee
may be necessary to tell what the “something” is.
E. Perez
October 07

3. DIS at highest Q2 : towards quark substructure ?- 1 ) f( 6
>
< =
Assign a finite size < r > to the
EW charge distributions :
d/dQ2 = SMvalue x f(Q2)
Global fit of PDFs and < r > using d/dxdQ2
from LHeC simulation, 10 fb-1 per charge,
Q2 up to GeV2 :
< rq > < m
One order of mag. better than current bounds.
At LHC : quark substructure may be seen as a deviation in the dijet spectrum.
Such effects could also be due to e.g. a very heavy resonance.
Could we establish quark substructure with pp data only ?
E. Perez
October 07

4. e+ and e- probe different
Electron-quark resonances
Apparent symmetry between the lepton & quark sectors ?
Exact cancellation of QED triangular anomaly ?
 (unknown) coupling l-q-LQ
“Leptoquarks” (LQs) appear in many extensions of SM
Scalar or Vector color triplet bosons
Carry both L and B, frac. em. charge
difficult, large
backgrounds ep pp eq q
llqq
lqq
qq
NC DIS
CC DIS
Z/DY + jj
QCD
W + jj
W/Z + jj
LQ decays into (lq) or (q) :
ep : resonant peak, ang. distr.
pp : high ET lljj events
Such eq resonances also predicted in Supersymmetry with “R-parity” violation:
’1jk e+
dk ( u j ) _
u Lj ~
( dRk*) Rp
e+ and e- probe different
squarks & couplings
Example: resonant stop production in e+ p,
resonant sbottom in e- p.
E. Perez
October 07

5. “Leptoquarks” : pp versus ep
 = BR( LQ  eq )
Tevatron probes large masses for large
 (LQ  eq) independently of .
But large  (LQ  q) is difficult.
HERA better probes LQs with small 
provided that  not too small
 = BR (LQ  eq)
LHC vs LHeC :
MLQ (GeV)
LHC
pair
prod
A.F. Zarnecki
MLQ (GeV) 
LHeC em
1 TeV
LHC could discover eq resonances
with a mass of up to 1.5 – 2 TeV
via pair production.
Quantum numbers ? Might be
difficult to determine in this
mode.
E. Perez
October 07

6. Determination of LQ properties
pp, pair production
ep, resonant production e+ e-
q or q ? _ +
F = -1
F = +1
e, q
Fermion
number
F=0 LQs : (e+) higher
F=2 LQs : (e- ) higher
(high x i.e. mostly q in initial state) _ _
Scalar or
Vector e q *
qq  g  LQ LQ :
angular distributions
depend on the structure
of g-LQ-LQ. If coupling
similar to WW, vector LQs
would be produced unpolarised…
cos(*)
distribution
gives the
LQ spin.
Chiral
couplings ?
Play with lepton beam
polarisation.
E. Perez
October 07

7. ep : golden machine to study LQ properties
F = 0 or 2 ? Compare rates in e-p and e+p
Spin ? Angular distributions
Chiral couplings ? Play with polarisation of lepton beam
Couples to  ? Easy to see since good S/B in j channel
Classification in the table below relies on minimal assumptions.
ep observables would allow to disentangle most of the possibilities (having
a polarised p beam would complete the picture).
F = 0
F = 2
E. Perez
October 07

8. “LQ spectroscopy” : LHeC versus single LQ production at LHC
Single LQ production at LHC brings information in principle. But cross-section
e.g. at 1 TeV is a factor ~ 400 lower than at LHeC. e+ q g e-
F=0 
Example : Fermion Number F.
Look at signal separately when resonance is formed
by (e+ + jet) and (e- + jet).
Sign of the asymmetry gives F, but could
be statistically limited at LHC. Easier in ep !
LHeC: 10 fb-1 per charge
 = 0.1
Idem for the simultaneous determination
of coupling  at e-q-LQ and the quark flavor q.
Asymmetry
If LHC observes a LQ-like resonance,
M < 1 TeV, with indications (single prod)
that  not too small, LHeC would solve the
possibly remaining ambiguities.
LHC: 100 fb-1
MLQ (GeV)
E. Perez
October 07

9. Beyond LQs… To be further studied :
 SUSY with R-parity conservation ~ e q ~ 0
Pair production via
t-channel exchange of
a neutralino.
Cross-section sizeable when
M < 1 TeV i.e. if squarks
are “light”, could observe
selectrons up to ~ 500 GeV.
Could extend a bit over the
LHC slepton sensitivity
Possible information on
couplings by playing with
e+ / e- / L / R
 in pb, e- p
 in pb, e+ p
 Single production of excited fermions
E. Perez
October 07

10. Esp. models which predict new effects
 Further models …
Esp. models which predict new effects
- “easy” to see in ep because of low, well-understood backgrounds
- more difficult to establish in pp because of large bckgs
Example from HERA experience:
Anomalous top production via
BSM coupling tu.
Single top analysis at Tevatron is
difficult (large W+jets bckgd).
E. Perez
October 07

11. p structure & interpretation of LHC discoveries
The interpretation of discoveries in AA at Alice may require direct
measurements on pdfs in A – not covered here.
Here, focus on ATLAS & CMS discoveries : highest masses  highest x.
Constraints on d and g at high x still limited :
E. Perez
October 07

12. Current high x uncertainties and NP processes at LHC : qq processes_
Current high x uncertainties and NP processes at LHC : qq processes
Example: new W ’, resonant slepton production in RpV SUSY
(reach for a W’ with
SM like couplings)
CMS
Physics
TDR
Vol II
40% uncertainty
on part. lum. For
a 6 TeV W ’.
 g(W ’) ? d u _
W ’ s c ’ _  ~
RpV SUSY : reach would depend on the
strength of the coupling ’ .
With sea quarks involved, uncertainties
large already well below the kinematical limit.
Would make the measurement of the coupling difficult.
E. Perez
October 07

13. Drell-Yan mass spectrum
4 TeV
8 TeV
(shown uncertainties:
from CTEQ 61 sets)
CMS Physics TDR Vol II
Example : new Z’ boson, KK gravitons
in Randall-Sundrum models etc.. Signal = a mass peak.
Partonic luminosities can be “normalised” to the side-bands data if enough stat.
But close to the discovery limit, couplings of a Z’ boson may not be
measured accurately.
E. Perez
October 07

14. DY mass spectrum and “Contact Interactions”
NP in Drell-Yan spectrum might not manifest itself as a mass peak…
e.g. large extra-dimensions, interference with very heavy boson etc…
Effective “contact-term” Lagrangian :
d/ds = SMvalue + …  s /2 + … ( s / 2) 2
LHeC sensitivity
(10 fb-1 e- & e+) :
25 – 45 TeV
 / 2
LHeC
HERA
VV model
VV model
depending on the model
Similar to the
expected sensitivity
at LHC.
(GeV-2)
E. Perez
October 07

15. eeqq Contact interactions in DY and DIS
LL model,  = 30 TeV, sign = -1 :
effect in DY can be “absorbed” in pdf unc.
In some cases, may be difficult to
determine the sign of the interference
of the new amplitude with SM.
LHeC, e- p, LL
At LHeC, sign of the interference
can be determined by
looking at the asym. between
/SM in e- and e+.
Polarisation can further help
disentangle various models.
E. Perez
October 07

16. High x gluon and dijets at LHC
Some NP models predict deviations in dijet mass spectrum at high mass.
Example : some extra-dimension models.
Mc = 4 TeV
S. Ferrag,
hep-ph/
Mc = 2 TeV
Mjj (GeV)
Mjj (GeV)
Due to pdf uncertainties, sensitivity to compactification scales reduced
from 6 TeV to 2 TeV in this example.
[ sensitivity has to be checked taking into account jet energy scale uncert… ]
E. Perez
October 07

17. For “new physics” phenomena “coupling” directly electrons and quarks
Conclusions
For “new physics” phenomena “coupling” directly electrons and quarks
(e.g. leptoquarks, eeqq contact interactions) : LHeC has a sensitivity
similar to that of LHC.
The further study, in ep, of such phenomena would bring important
insights : leptoquark quantum numbers, structure of the “eeqq” new
interaction. These studies may be difficult, if possible at all, in pp.
LHC sensitivity to new (directly produced) particles not much limited
by our pdf knowledge. “Contact-interactions” deviations may be more
demanding.
However, the interpretation of discoveries at LHC may require a better
knowledge of the high x pdfs : e.g. determination of the couplings of a
W ’ or Z ’ if “at the edge” .
E. Perez
October 07

18. Work ahead … (not an exhaustive list)
Determination of LQ quantum numbers at LHC : real MC analysis with realistic
simulation & backgrounds.
Contact interactions : systematic analysis of
- how pp data could discriminate between various models.
- the complementarity between pp, ep, ee (cf A. Zarnecki, Tevatron/LEP/Hera)
Assess the limitations due to our poor knowledge of high-x gluon in
searches with jets.
Further study of the LHeC potential especially in Supersymmetry.
If slepton + squark accessible at LHeC, what additional information do we
learn compared to LHC ?
Need to involve people familiar with analysis in Atlas / CMS.
Establish contacts on BSM at LHeC with theorists / phenomenologists.
E. Perez
October 07

19. Backups
E. Perez
October 07

20. Single LQ production at LHC
 (pb)
Single LQ production also possible at
the LHC. g LQ g LQ  ep  q q e- q e- pp
 ee followed by eq -> LQ not
considered yet. Not expected to change
much the results shown here (Tevatron).
Smaller x-section than at LHeC.
And large background from Z + 1 jet.
200 GeV
1200 GeV
MLQ (GeV)
Can be used in principle to determine the LQ properties in pp.
E. Perez
October 07

21. Current high x uncertainties and NP processes at LHC : quark-quark processes~ g
Example: squark production
(shown uncertainties:
from CTEQ 61 sets)
(pdf) on the relevant partonic luminosities instead of that on the  of a given BSM process.
Estimated
LHC
sensitivity
(SUSY)
But better if couple
to u quarks as well…
For a process involving high x d quarks, pdf uncertainty ~ 20% at the
corner of the LHC phase space.
Could be ~ 50% with extended sensitivity (e.g. LHC upgrade)
E. Perez
October 07